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ENERGY - POST 16 - UNI T 3

ENERGY - POST 16 - REVISION 1

DESIGN ING AND MAKING A SOLAR PANEL

CONTENTS

Page No

STUDENT BRIEFING SHEETS

1 Design Problem 86

2 Iden tifyin g a n eed for h ot water an d sit in g th e system 88

3 Design in g th e solar collector 90

4 Design in g th e solar h ot water system 91

5 Design in g a solar pum p con troller 926 Trackin g th e sun 93

7 M an u fact ure an d in st allat io n of t h e so lar co llect or an d syst em 9 4

INFORMATION SHEETS

1 Backgroun d in form ation 95

2 Sitin g th e solar collector 99

3 Types of solar collector 103

4 Design in g th e solar collector 109

5 Th e collector box 115

6 Collector efficien cy 120

7 Con n ectin g th e collectors 124

8 Moun tin g th e collectors 126

9 System Design 128

10 Plum bin g tech n iques 133

11 Design in g with electron ics 134

12 Design option - Th e solar pum p con troller 139

13 Weath er protection 140

14 Trackin g m ech an ism s 142

15 Design option - Trackin g th e sun 144

16 Useful Addresses 147



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STUDENT BRIEFING SHEET 1 - D ESIGN P ROBLEM

ENERGY FROM TH E SUN

In th is coun try our clim ate makes it fairly h ard to u se the sun asa useful source of en ergy. Phot ovoltaic cells can p rodu ceelectricity from sun ligh t, but th e electricity is expen sive and isusually on ly worth con siderin g for an app lication where th ere isn o m ains electricity available. Th is is because of th e h igh in itialcost an d th e relatively poor climate in th is coun try.Developm ents in ph otovoltaic cell techn ology sh ould m aketh em cheaper in th e future. However solar hot water panels canprod uce useful am oun ts of ho t water cost- effectively,particularly if th ought is put into sourcing th e m aterials and th esystem is self built.

D ESIGN BRIEF

To design an d m ake a solar collector to m eet an iden tified n eedfor hot water with in t h e sch ool or for a related application. Thesystem m igh t be fixed or portable depen ding on th e needs of th eapplication identified.

You should take responsibility for the development of the projectand organ ise your tim e so th at th e project is com pleted on tim e

GETTING STARTED

Getting started is one of th e problem s with designing an ythin g.Lookin g at solution s to prob lems devised by ot h ers is always agood place to start. Obtaining produ ct information frommanufacturers will be a useful source of ideas. Visits to seesystem s operatin g will be very h elpful if th ey are possible toarrange.

However a lot can b e worked out from an u n derstand ing of heattransfer, condu ction , convection, an d radiation. Th e collectoritself can be designed primarily with these principles in mind.

You will need to d raw up an out line specification for each part o f th e system. It is imp ortant t o develop an un derstan ding of howthe solar hot water system operates.

ELEMENTS OF TH E SOLAR HOT W ATER SYSTEM

• Collector panel

• Collector casing

• Collector mounting

• Hot water storage tank (if required)

• Pipework

• Control system for hot water flow and for tracking sun (if required)

• Hot water draw off points



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Water flow around th e system is produced solely by the n aturalcirculation of water as it h eats up, expan ds an d is replaced by

h igher den sity colder water . This is called th erm o-siph on ing.This is th e sim plest system t o con struct bu t for it to be effective an um ber of criteria h ave to be satisfied, see Inform ation Sh eet 9.

The collector itself is likely to be eith er som e form of flat p latecollector or a parabo lic reflector. The decision will be guided byth e type of application chosen an d th e m aterials th at are available.

BUDGET AVAILABLE

This has to be discussed with your t eacher. Every scho ol'ssituat ion is different . However, you will see th at th ere arepossibilities with in t h is project for using an d adap tin g 'scrap' or

found materials and items. You will find that it requires moreskill to select u seful items from what you can fin d, th an to orderthem n ew.

TASKS

• Research the backgroun d of solar water heaters and theirapplication.

• Ident ify the budget you h ave ava ilable .

• Develop an outl ine specification for the proposed solar panel.

• When carrying out th is pro ject you need to p lan your t imecarefully. How lon g will you allow for: research, design,m akin g and evaluating?

Expansion pip eand overflow

Cylinder

Coldfeed

Inletheader

Flow

Return

To taps

The following diagram illustrates a basic solar h ot w ater system .



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STUDENT BRIEFING SHEET 2 - IDENTIFYING A NEED FOR

H OT W ATER AN D SITING TH E SYSTEM

IDENTIFYING A NEED

Your d esign m ust be m atched t o a real need you h ave ident ifiedfor ho t water with in t h e sch ool or college. You will need t ocon sider a range of issues when iden tifyin g a particularapplication such as;

• vo lum e and tem perature of w ater required, and therefore

the en ergy requirem ent.

You n eed to be realistic with t h e scale of n eed you in tend tom eet. It is useful to calculate approxim ately th e amo un t of

energy required to m eet this need to give an idea of whatyou will be expecting from your solar collector. Is it feasibleto con sider meeting th is n eed?

• the nature and timin g of w ater deman d.

You n eed to iden tify wh at th e water will be used for andwh en, to h elp gauge th e required storage capacity of th eproposed system an d th e degree to wh ich th e proposedapplication m atches th e variations in solar radiation.

• budget lim itations.

Obviou sly your bud get limit will con strain th e size of th e

proposed system an d will therefore limit th e nu m ber of appropriate applications available to you.

• practical con straints such as any siting restriction s and

the relation ship with the existing ho t water system .

You m igh t n eed to re-examine you r chosen application afteryou h ave con sidered in m ore detail th e siting of th e deviceas th ese two t h ings are closely linked.

SITING YOUR SOLAR COLLECTOR

Your ch oice of site will be depen den t on a ran ge of factors;

• the lev el of solar radiation over a period o f time.

Obviou sly th e high er the levels of solar radiation th e better.You can m easure this over time u sin g a solarimeter asoutlined in un it 1, or you can judge th e best site fromobservation of shadin g, wind speed an d d irection etc.Excessive wind speeds can reduce the collector efficiency.



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• wh ere the h ot w ater will be used.

The th ree elemen ts of your system, th e collector, the storage

tank (if th ere is to be on e) an d th e point of use, should be asclose togeth er as possible to redu ce heat losses from th epipework. Th is h as obvious im plications for the siting of thesolar collector. In ad dition it is n ecessary to con sider thefeasibility of th e pipe run s. Puttin g pipework across th eplaygroun d is an obvious non -starter.

• safety and security.

You will n eed to con sider th e safety of th e collector, keepin g itfree from vandalism and accidental dam age.

• ease of access.It is important to consider the ease of access for maintenancepurposes. In addition you m igh t wish to consider th eeducation al and inform ative value of what you h ave done.Do you p ositively wan t oth er studen ts to be able to see th edevice and un derstan d wh y it h as been installed and h ow itworks? Add ition ally, would you like in som e way to be ableto d isplay h ow well it is ach ievin g its aims wh en it is in u se?

• environ men tal impact.

In th is con text th is refers mainly to visual im pact, alth oughth e collector's im pact on land u se might be an issue

depend ing on th e sitin g ch oice.

Clearly it m igh t be n ecessary to m ake a comprom ise between th eabove factors as it is rarely po ssible to find an ideal all roun d sitebut it is necessary to con sider which are th e m ost impo rtantfactors.

TASKS

• Quan tify as clearly as possible the quan tity of water requiredand th e percentage to be m et from th e solar hot water system .

• Consider the overall type and size of collector that m ight beappropriate to your ap plication.

• On a p lan of the si te mark out the rela t ionship betweenpoten tial sites and point s of use for the h ot water.

• Balance the issues outlined above to identify an appropriateapp lication an d a suitable site.

• Begin sourcing elem ents of th e system, collector panel, storagetank etc.

Risk Assessm en t

Care needs to b e taken in scrapyards as custom ers arefrequ ent ly expected to wo rk in u n safe places. Visits sh ou ldalways be accom pan ied by a mem ber of staff.



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STUDENT BRIEFING SHEET 3 - D ESIGNING TH E SOLAR

COLLECTOR

Having m ade a first assessm ent of th e type of collector th at youwill be design ing you will also n eed to con sider the followingissues in m ore detail;

• wh at size the collector panel should be

It will be very difficult to design th e collector such th at youwill be able to provide all th e ho t water you n eed. Th ereforeth e size of th e collector(s) you d esign will be depen den t on ;

- the e ffic iency of the collector,- the quan t i ty o f ho t water requ ired ,

- the required tempera ture of the water ,- the percentage of the energy demand for the chosenapplication you inten d to m eet with th e solar collector.

The size will also depen d on what m aterials you can obtain,budget limitations and practical siting constraints.

• the co llector design

The collector design should m aximise th e h eat transferbetween th e surface exposed to th e sun 's radiation, an d th ewater circulating aroun d th e system. The efficiency of th e heattransfer will depend on th e m aterials used, the volum e of water h eld by th e collector, th e surface area of the collectorand th e rate of heat loss from th e collector. Durin g operationth e collector efficien cy will also depen d on its orien tation toth e sun , the am oun t of solar radiation, th e water temperatureand t he surround ing air temperature.

• the type of collector box required and its mounting

The boxin g of th e collector pan el if requ ired, needs to bedesigned to b e weath erproof, and t o m inim ise heat loss andm aximise solar gain wh ere appropriate. The m oun ting n eedsto be constructed such t h at th e collector can be m oved in toth e mo st effective position relative to th e sun . Sh ould it befixed or m oveable?

• w hat m aterials are available to yo u no w or as a result of

som e tim e spent researchin g sources

Wh at m aterials you h ave available will h eavily influen ce yourcollector design . With a lim ited bud get you will n eed toprioritise your expend iture. Which is th e key elem ent of yourcollector design in term s of increasin g th e collector efficien cy?

TASKS

• Produce a deta iled design

specification for th ecollector panel, box andmount ing .

• Produce working d rawingsfor th e collector, its casin gand i ts m oun ting.

• Iden t ify sou rces fo rm aterials and com ponen tstakin g into accoun t anybudget limitations.

Risk Assessm en t

Care needs to be taken inscrapyards as custom ers arefrequently expected to workin unsafe places. Visitssho uld always beaccom pan ied by a m emberof sta ff.



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STUDENT BRIEFING SHEET 4 - D ESIGNING TH E SOLAR H OT

W ATER SYSTEM

You will n eed to con sider;

• w heth er the h ot w ater w ill be used as it is heated or w ill it

need to be stored

Most app lication s will require some form o f hot wat er storage,but obviously if you can just use the water as it is h eated th enit m akes th e system design very easy, but n ot very flexible.

• the am ount an d distribution of w ater you n eed to produce

The am oun t of water you n eed to prod uce will affect th e sizeof storage tan k required as well as the collector design . It is

also imp ortant at th is stage to identify th e nu m ber of draw off point s required.

• whether the system needs to be incorporated into the

existing hot water system or will it stand alone

If th e system is isolated from th e existing h ot wat er system itwill on ly produce useful qu antities of h ot water du ring th esumm er mon th s. If h owever th e system is used to pre-h eatwater before th e use of a conven tion al fuel source, th en eventh e sm all amo un ts of solar en ergy during the winter mon th sm igh t be made use of.

Wh en con sidering th is issue it will also be n ecessary tocomp are the distribution of the d raw off points required withth at of th e existing pipework, as well as con sidering th epractical difficulties in volved with incorp oratin g a solarcollector into the existing hot water system.

• w heth er the system requires a pum p for circulating th e

water

A pum p will allow th e flow of water aroun d th e system to beclosely con trolled, and will be required if th e system is to fitinto a space th at will not allow th e necessary arrangem ent of tank an d collector to generate therm o-sipho n ing.

If a pu m p is to be used you w ill h ave to establish h ow it willbe cont rolled. You will also need to con sider h ow to p reventth e collector actin g as a radiator at tim es wh en t h ere is nosun.

• how to protect exposed elements of the system from the

cold

It will be necessary to p rotect th e collector and an y externalpipework from th e cold to preven t freezin g. Th ere are severalways th at th is m igh t be don e dependin g on th e type of solarcollector design you are prod ucing.

TASKS

• Produce a deta iled design

specification for th e solarh ot water system .

• M ap ou t th e pip e ru n s insome detail to ident ify an yproblem areas th at m ightarise.

• Produce work ing d rawingsof th e solar systemidentifying th e m aincompon ents and th eirconnections.

• Iden t ify sou rces fo rm aterials and com ponen tstakin g into accoun t anybudget limitations.



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STUDENT BRIEFING SHEET 5 (OPTIONAL) - D ESIGNING A

SOLAR PUM P CONTROLLER

If you n eed a pum p in your solar system you will need to be ableto cont rol its operation to m aximise the h eat transfer from t h ecollector to th e storage tank. To do t h is th e pum p n eeds tooperate when th e temperature of the water in th e collector ish igh er than t h e water in th e storage tan k.

You will n eed to con sider:

• The spec ificat ion of the pum p and how to supply power to it .Low voltage in -line pu m ps are available from m arineequip m ent supp liers at a reason able price. But it will be

n ecessary to check wheth er they are larger enough to p um pth e volumes of water required within th e solar system.

• How to supply power to the contro lle r.The con troller requires a smoothed 12V DC power sup ply tooperate the relay.

• The design of the so la r contro lle r.The schem atic diagram b elow sh ows th e basic system wh ichcan either be design ed to your own choice of comp on ents, orbuilt straigh t from t he d etailed con struction plan with inInformation Sh eet 12.

• How to ca librate the contro lle r.The controller can be calibrated by setting th e poten tiometerso that th e LED comes on wh en th e therm istors are at th erequired temperature difference.

• The size of the temperature difference.The temp erature differen ce needs to be set h igh enou gh tooffset th e heat losses from t h e pipe run s as well as the p owerloss from t h e pum p. Th is ensures a net en ergy input into th e

storage tank. So th e insulation levels and th e len gth of piperun s are impo rtant in th is assessmen t. However a tem peraturedifference of aroun d 4 or 5 degrees C is com m on .

Thermistoron collector15 K

Thermistoron storagetank 15 K

5 K preset

2

8

3 1

7LM 311N 1 K

0 V

+12 V

500 Ω

2N3053

To pumpcircuit

12 V relay



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STUDENT BRIEFING SHEET 6 (OPTIONAL) -

TRACKING TH E SUN

Th e collector needs to obtain as m uch en ergy from th e sun aspossible. Th e sun m oves across the sky durin g th e day. It is willalso be at differen t h eights in t h e sky depend ing on th e season.This variation will affect t h e efficiency o f th e collector. On eoption th en is to develop m echanisms that will enable thecollector to track th e sun as it m oves in t h e sky.

There are a nu m ber of thin gs to con sider if th is is to beattempted;

• Under what condi t ions do you n eed the collector to move?

• How many axes do you wish th e collector to move in?

• What can you sense to opera te the movement and whatsensor can you use?

• Wha t outpu t dev ice cou ld you use?

• Wh at voltage supply are you going to use? Wh at will governthis decision?

• How will the output device switch on when m ovement i srequired?

• How wil l you ensure enough current to operate the outputdevice?

• Do you need to return the collector to a start ing posit ion atsome point?

A n um ber of specific issues relatin g to th e m echan isms involvedneed to be considered.

• Does your output device provide enou gh force or torque tom ove th e collector? How can you in crease it?

• Do you need to slow down or speed up the mot ion f rom youroutp ut d evice?

• Does your output device move in th e same plane anddirection as your collector n eeds to m ove?

TASKS

• C on sid er wh e th e r it is

advantageous to t h e systemyou are developin g for it totrack th e sun .

• If so you will need todevelop th e designspecification for th eelectron ics and t h em ech anisms required.

• Produce work ing d rawingsof the required con trolsystems.

• Source the ma ter ia ls andcomponents required bythe designs.

Risk Assessm en t

Electrical system s forcontrol will need to run at alow voltage avoidin g anychan ce of shock oroverh eating an d fire fromshort circuits.

1 axis 2 axes 3 axes



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STUDENT BRIEFING SHEET 7 - MANUFACTURE AN D

INSTALLATION OF TH E SOLAR COLLECTOR AN D SYSTEM

Having prod uced det ailed design drawin gs of th e solar collectoran d associated system , you will n eed to:

• produce a production plan

You sh ould draw up a sch edule for produ ction in order toidentify when th ere m igh t be hold up s, for example due toorderin g of materials or as a result of any p rodu ctionprocesses. It will then be possible to m inim ise th e impact of th ese delays.

• order materials

You will already have iden tified th e balance between n ew andfound m aterials. Hopefully sources for the m ain com pon entsof th e collector and solar system h ave already been iden tified.

• produce an y aids fo r manu facture, jigs and formers

• prepare m aterials and m anufacture solar collector

• install solar system

• produce an instruction guide for use and maintenan ce of

system

This is an essen tial stage in th e developm ent of th e finishedprodu ct. You n eed to pass on t o future users of the system th ekey point s that th ey n eed to kn ow about its operation andmaintenance.

• site trial an d ev aluate system

This is th e crunch ; does the finish ed system m eet th e originalspecification? How m uch water does th e system h eat and towhat temperature?

- o n a clo u dy d ay in Ju n e- o n a clear d ay in Ju n e- on a clear day in December

- a s a n an n u a l a verage

Does it satisfy th e need o riginally ident ified in th e sch ool?Does it provide th e amou n t n eeded, when it is needed?

What type of fuel does the system displace? What financialsaving does this represent (a) now and (b) over the next tenyears? How m uch carbon emission will th e system preven t?

Can you iden tify ways in which th e perform ance of thesystem could be im proved? How wou ld you do it differently if you did it again?

Risk Assessm en t

Working at h eights is

h azardous an d should becarefully supervisedSolar pan els can p rodu cevery hot wat er, care n eedsto be taken to avoid burns.Care sh ould be taken wh enh and lin g and storin g glassSoldering and weldingshould be carried out in asafe man n er.



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INFORMATION SHEET 1 - BACKGROUND INFORMATION

Back up to Briefing Sheet 1

Solar inten sity is far easier to estimate t h an wind speed because itis fixed by far fewer, and more certain variables. The level of solarradiation t h at reaches th e earth is governed by th e position o f th e sun in th e sky, a known quan tity, and th e level of cloudcover and atm ospheric dust and pollution. Therefore th e solarradiation received over one clear day in an y m on th of the yearcan be qu ite accurately estim ated . On average th e nu m ber of h ou rs of sun shin e is also fairly well established. This th en is verydifferent from wind en ergy which can vary mu ch m ore widelyand un predictably as a result of qu ite complex atm osphericinteractions.

The intensity of solar radiation above the earth's atmosphere isabout 1,365 W/m 2. However th is figure is redu ced as a result of reflection and absorption by water vapour an d gases in th eatm osphere. The map below sh ows the variation in an nu alaverage daily totals of solar radiation across th e coun try. Solarradiation levels can vary between 100 0 W/ m 2 on a clear summ erday to less than 200 W/m 2 on a cloudy summ er day, and mu chlower still in m idwint er.

8

9

10

9

10

11

9

Daily totals of solar radiation

averaged over the year (MJm -2 )[3.6MJ = 1 k Wh]



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The tot al or 'global' solar radiation levels h ave an elemen t of direct solar radiation an d an elemen t of 'diffuse' solar radiation .The diffuse radiation is caused by scatterin g by the atm osph erean d is at a peak on o vercast days. The graph sho ws th e balan cebetween diffuse and direct solar radiation o ver a whole year. Th efigures are based on south facing surface tilted at 45 degrees tothe horizontal.

Th e amou n t of solar en ergy th at is th en con verted into usefulh eat in th e form o f hot water com ing out of th e tap, isdepend ent on th e conversion efficien cy of the solar hot watersystem itself. Efficiencies range from between 25% for a DIY

system to 60% for an advanced com m ercial system.

The application s for solar collectors vary depen din g on t h edesign. The m ost comm on application for solar collectors in th iscoun try is for dom estic h ot wat er systems, some specialisedsystems provide water to central heating systems but usually thetemp erature required is too h igh . An oth er useful app lication is toh eat swim m ing poo ls wh ere the tem perature level required ison ly a few degrees above air tem peratu re.

In ot h er countries, usually but n ot always the h otter on es, largerscale developm ents h ave been p ut in place where great n um bersof solar collectors are arranged to provide large quan tities of h otwater for wh ole com m un ities. Solar systems can also be used tocool buildin gs.

Th e oth er most im portan t opt ion for utilisin g solar en ergy is th euse of photovoltaic cells that convert solar radiation intoelectricity. They are already com m ercially viable in th is coun tryfor remot e applications wh ere there is no grid electricity. Th em an ufacturin g costs of th ese cells is con stant ly falling an d th em an ufacturers and in du stry experts believe they will be able tocomp ete with convent ional fuel sources within th e next twodecades. They require complex manufacturing processes and arecurren tly out of th e budget of m ost sch ools for m eetin g any real

electricity need th at th e sch ool m igh t h ave. Th ere are man ysmall but useful kits th at are available, at reason able prices, thaten able effective stud y of th e poten tial for ph otovo ltaic cells.

0

50

100

150

200

250

J F M A M J J A S

Solar

IrradiationkW

h.m

- 2



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Solar hot water systems h owever can b e constructed within th eresources th at scho ols have available an d can effectively meetsmall scale needs for hot water th at th e sch ool m igh t h ave.

GLOSSARY

PLATE EFFICIENCY - efficiency of tran sfer of heat from th eabsorber surface to th e fluid passin g th rough th e collector. Awell designed absorber sho uld h ave one of about 90%.

ABSORPTANCE - capacity to absorb solar radiation .

AMBIENT TEMPERATURE - surroun ding air temperature.

ANGLE OF INCIDENCE - angle between a light ray and a no rmal

to th e surface on wh ich th e ray is incident.COEFFICIENT - n um ber expressin g am oun t of some chan ge or

effect un der certain con dition s of tem perature, pressure etc.

DIRECT SYSTEM - the water com ing out of the hot taps hasactually passed through the collectors.

EMITTANCE - ability to em it infra-red radiation .

EMISSIVITY - power of a surface to radiate h eat or ligh t.

FLAT PLATE COLLECTOR - variety of d ifferent collectors th ath ave combination s of flat, grooved an d corrugated shapes as

th e absorbing surface an d various ways of transferring th eabsorbed solar radiation from t h e surface of th e collector toth e heated fluid - no con centration of th e solar radiation .

HEAT LOSS COEFFICIENT - rate of heat loss at a given collectortemp erature - affected by th e cover, the in sulation at th e backand sides of the collector an d th e emittan ce of th e absorbersurface - U(W/ m 2.°C).

INCIDENT SOLAR RADIATION - th e solar radiation falling onth e collector at any poin t in tim e.

INDIRECT SYSTEM - th e fluid passin g th rough th e collector is

n ot th e sam e as th at which will com e out of th e hot tap.OPTICAL EFFICIENCY - m aximum efficiency with which th e

solar radiation can be collected = prod uct of th e transm issionof the cover, the absorptance of the absorber coating.

SYSTEM EFFICIENCY - th e efficien cy of the wh ole system,includes heat losses from th e pipework and th e storage tank.

PANEL LOOP - water in the whole solar system.

PARABOLA - a p articular curve wh ere parallel rays of light



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approachin g it from th e front are focused on to on e point.

SELECTIVE SURFACE - one which absorbs sunlight efficientlybut re-radiates heat as infra-red radiation far less th an a

n orm al black body would do.

SPECIFIC HEAT - energy needed to raise the tem perature of 1kgof a substance by 1°C. (J/kg.°C)

STAGNATION TEMPERATURE - temperature the collectorm igh t reach on a sun ny day wh en n o fluid is flowing in it.

THERMAL CAPACITY - quan tity of heat required to raise thetem perature of a body by 1°C.

THERMAL MASS - an object's capacity for storin g heat (objectswith h igh th ermal mass warm up slowly but store a lot of h eat in t h e process).

THERMAL RADIATION - infra-red radiation .

THERMO SIPHON - the p rocess of water circulation th at ispowered by density changes with in th e pan el loop as a resultof water temperature chan ges in th e collector pan el.

TILT ANGLE - th e angle between th e collector surface and th eh orizon tal (th e groun d).

TRANSMITTANCE - ability to transm it (allow th rough) h eat orlight.



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INFORMATION SHEET 2 - SITING TH E SOLAR COLLECTOR


Assessing th e Dem and fo r Hot Water

It is im portan t to get an overview of h ot water use in th e sch oolbefore deciding what elem ent of th at dem and m ight be able tobe met th rough a solar hot water system .

Possibilities- sin ks - h and wash ing (most frequent in toilets and workshops?)- kitchen s- sh owers- swim m ing pool

- food techn ology facilities - washin g up?To get an idea of the en ergy requirem ent for th ese uses it isnecessary to estim ate both h ow m uch water is used an d wh attemp erature th e water is heated to. Note that it is n ot th etemp erature th e water com es out of th e taps th at is impo rtant inth is context, but th e temperature that th e water is heated up towhen it is stored.

For all the u ses with a water out let (tap o r shower h ead) youcould time h ow long p eople keep it flowin g, note h ow far theyhave turn ed on th e tap, work out th e average for each activityand th en u sed a stopwatch, bucket and calibrated container tocalculate how m uch wat er is n orm ally used. You will h ave todecide h ow big a samp le you n eed in order to ensure that youh ave an accurate eno ugh average figure.

Alternat ively you could use average figures for h ot water u se.

Then you need to know h ow man y t im es this is done in a dayand also when . For examp le almost all th e han d wash ing mightbe between m orning break and th e end of lunchtime.As well as th e time of day u sed you also need t o con siderseason al variations.

You will need t o separate out t h e use accordin g to location if youare goin g to be able to assess wh eth er you will be able to m eet aparticular need in on e part of th e site.

You will also n eed to assess what t em perature th e water needs tobe for each particular use.



100



Th en fill out a table someth ing like th is (add o r removecategories if you fin d th at th is will not provide you with th einformation you n eed) -

Type of Location Litres Storage Tim e Season

use used Tem p used used

How can you display the in form ation you h ave collected in away that m akes it easy to draw some conclusion s about th e mo st

appropriate hot water need to m eet?

Calculating the Energy Required

It is possible to calculate the am oun t of energy requ ired to h eatth e amou n t of water required to a set temp erature usin g thefollowing formula;

Energy Required = M ××××× C ××××× (T1 - T2)

W h ere: M = M ass o f w at er t o b e h e at ed (kg)C = Specific Heat Cap acity of wat er = 4200 J/kg.°CT1 = Tem peratu re required of ho t water (°C)T2 = Tem peratu re water supplied at (°C)

Assum ing th at th e m ains water is at 14°C and th e hot water isrequired at 55°C an d 100 litres of water are requ ired daily:

Daily Energy Requirem ent = M × C × (T1 -T2)= 100 × 4200 × ( 55 - 14)= 17220000 J= 17.22 MJ

Ann ual Energy Requirement = 17.22 × 36 5= 6285.3 MJ

1 MJ = 0.278 kW h3.6 MJ = 1.0 kWh

The average daily global irradiation on a h orizont al surface isapp roxim ately 10 MJ / m 2. However, no t all th is energy m ay beh arnessed towards h eatin g water due to t h e efficiency of th ecollector and th e ho t water system .



101



Matching Energy Requirem ent w ith Collec tor Performance

The following graph outlines the performance of different typesof solar collector. It sho ws the am oun t of energy that m igh t beexpected to be produ ced from th e collector. Using th e graph itshould be possible to draw som e basic conclusions about h owmu ch of the h ot water demand you m ight be able to m eet.

It is impo rtant to remem ber th at one option to con sider might be

to m eet a only a percentage of the h ot water needed for anyapplication.

Matching Supply with D eman d

It is necessary to con sider wh en t h e ho t water will be requiredand th e degree to wh ich th is m atches the variation in solarint ensity on a season al and possibly daily basis.

Season ally it is worth drawin g up a com parison of supp ly anddem and on a graph showing m on th ly figures for en ergy required,calculated as above, and m on th ly figures of solar energy suppliedto th e solar system, calculated from th e table below an d th e

graph ab ove. You sh ould assum e 2 or 3 m2

of collector typ e D(mid way through class).

5000

4500

4000

3500

3000

2500

20001500

1000

500

01 2 3 4 5 6

B - Advanced collectors

C - Evacuated tube collectors

D - Single glazed with selected coating

E - Single glazed with matt black coating

F - Unglazed

Effective collector area in m2 per

100 L/day draw off at 55°C

A n n u a l s o l a r e n e r g y s u p p l i e d

i n M J p e r 1 0 0 L / d a y a t 5 5 ° C

Approximate percentage of the annual solar energy supplied in each month

January 2 July 13February 5 August 13March 6 September 12April 10 October 8May 12 November 4June 13 December 2

Note: these values apply to systems located in the London area, with south facing

collectors tilted at 30°



102



On a daily basis, a system with a storage tan k, particularly if it iswell insulated, will even o ut an y of th e daily variations in solarradiation. However if the system does not have a storage tank,i.e. the h ot water is bein g used as it is prod uced, th en o bviouslyth ere will not be a lot of h ot water first th ing in th e mo rnin g.

Assessing Levels o f Solar Radiatio n

If you h ave a solarimeter, either m anu factured from Unit 1 orborrowed for th e project, it will be possible to m easure theamo un t of solar energy falling on a certain spot.

It will probably no t be practical to take readings with t h esolarim eter all over the site, so you n eed to con sider on whatbasis you will decide wh ere to take d etailed readin gs.

If you d o n ot h ave access to a solarim eter th en estim ates of solarradiation levels can be m ade using data from th e m eteorologicaloffice. However it will still be n ecessary to refine t h e data withreference to t h e actual site.

You on ly need to stop for a mom ent t o realise that you alreadyknow quite a lot abou t th e effect of solar energy on your schoolsite. You will be well aware of wh ich room s becom e into lerablyh ot in June and which n ever get any sunlight, and you willprobably know wh ich areas in th e playgroun d get sunligh t evenin January.

Beware of the effect of shadin g, it cou ld be th at trees, buildingsand oth er obstacles might cast a shadow over th e site. Mightth ere be obstructions at 8am which you will n ot observe between9am an d 4pm ? Does th is m atter? Is th ere any way that you canpredict accurately where th e sunlight will fall at any t im e of day?

Do you n eed to design anydevices to en able you to t rackand record the path of the sunth rough t h e sky at differen ttimes ?

The collector will have to b eposition ed such th at it canabsorb as much radiation aspossible with in its designlimitation s. Clearly th e sundoes n ot remain stationaryan d t h erefore it is necessary toconsider the best orientationand in clination for the

collector. As th e sun d oes no tremain at th e same h eight inth e sky all year th is will alsoneed som e consideration .

It m ight o f course be possibleto m ove the collector in orderto m axim ise th e solarradiation falling on it.

Win d will also h ave a coolingeffect on a collector h en ce it is

n ecessary to cho ose a shelteredsite. Try com parin g windspeeds at different sites.



103



INFORMATION SHEET 3 - TYPES OF SOLAR COLLECTOR

Back up to Briefin g Sheet 2 & 3

There are a ran ge of differen t t ypes of collector, som e of wh ichare far too expen sive to con sider but are worth b eing aware of.

Evacuated tubes

Th e evacuated tube con sists of an inn er tube contain ing arefrigerant th at boils at a low tem perature. In-between th e inn ertub e and out er glass casin g th ere is a vacuum to redu ce heat loss.The fins attached to the inner tube also have a selective coatingon th em th at greatly reduces th e re-radiation of heat. Th erefrigerant absorbs th e sun's energy, rises up th e tub e and is

conden sed by water circulating aroun d t h e system . As th erefrigerant is cond en sed it passes its laten t h eat to th e waterwhich is th en circulated to th e storage tank.

Evacuated t ube systems are com m ercially available, however th eyare very expen sive. Th ey prove useful in situation s where h ightem peratu res are required, e.g. solar coolin g applian ces an dind ustrial app lication s. They are also of use where th ey need to

operate under difficult conditions e.g. space heating where themain d emand is in th e winter.

Trickle Collec tors

This is th e sim plest and cheap est type of solar collector. It h as aperforated water feed pipe runn ing along th e top of some form of corrugated sheet. The water is allowed to t rickle down th e gulleysto be collected by th e gutter pipe at th e bottom . Th e water willbe warmed b y th e sun's radiation directly and by transfer of h eatfrom th e collector itself.

Condenser

Sealing cap

Frost protection ring

Heat pipe

Absorber plate

Absorber support

Bottom support



104



Of th e comm on ly available corrugated m aterials, alum inium isth e best ch oice because of its therm al con du ctivity, which is fourtimes greater than th at of m ild steel.

The surface cont act area can be increased by addin g a detergen tto th e water, wh ich will break the surface ten sion of the wateran d allow it to run over a larger area of the gully.

The collector will have to be well treated to with stan d th ecorrosive effect of air an d water com bin ed. Dry temp eratures of up to 200°C can be reached causing expan sion followed by rapidcon traction as water circulates. A stoved-on finish wou ld be a

good choice.

Alth ou gh a t rickle absorber is the ch eapest it does h ave severaldisadvant ages. It is less efficien t as water can evaporate as it run sdown th e gulleys. Th is increases th e heat loss an d can o bscureth e glazing cover when it conden ses thereby reducing th eamo un t of solar radiation th at can ent er the collector. Th ecollector will require extra m aint enan ce as dirt an d du st will getint o th e system so access to th e collector pipes m ust be po ssiblefor cleanin g. It is also m uch m ore difficult to cont rol the p um p,if th ere is one, because of the wide fluctuation s in th e collectorswet and d ry tem perature.

Flat Plate Co llecto rs

Water is spread between t wo sheets of mat erial, th e upp er of wh ich form s the solar absorbin g surface. Water is in con tact withthe whole or a large proportion of the absorbing surface,th erefore h eat only h as to pass th rough t h e collector plate inorder to heat up t h e water.

An impo rtant consideration wh en u sin g this type of collector isth e volume of water with in it. If th e absorber has a large am oun tof water in it, it will take a lon g time to h eat up as it will h ave ah igh th ermal capacity. Ideally it sh ould h ave a water capacity of less than 2.5 litres/sq. metre, however a compromise may be

n ecessary if cost is a major con sideration .

a

b

c

Trickle absorber:a) perforated feed pipeb) Corruga ted sheetc) gutter pipe



105



Th e therm al con ductivity of the m aterials is n ot so impo rtantwhen th e water is in contact with th e whole absorber surface aslong as the m aterial is not t oo th ick. Modern cen tral heatin gradiators, often u sed as collectors, behave part ly as san dwichcollectors and p artly as tub e and sheet collectors.

Man y flat plate collectors are n ow com m ercially available, thebest also p ossess a selective coating to reduce re-radiation as th ecollector heats up . Th ey also attem pt t o reduce th e ratio of watervolume to surface area to maximise heat transfer.

Tube an d Sheet Collecto rs

The m aterials used within tu be an d sh eet collectors are of greaterimpo rtance as they rely h eavily on th e ability of th e sheet totransfer heat to th e water in th e piping. Th e spacing between th epiping, and th e bon d between p ipe and sh eet are also of crucialimpo rtance. With m aterials th at are poorer con ductors of heat,like steel, it will be necessary to h ave th e pipin g spaced closertogether and to h ave thicker sh eet to reduce the resistance toheat flow to th e pipework.

Coldwa ter in

Glazing

Box

Insulation

Ho twater

out

Black heatabsorbing

surface

Reflectivematerial

W ater in

W ater out

W ater out

W ater in



106



The parallel con figuration is far better for system s th at rely onn atural therm o-siphon ing to encourage water circulation, asth ere are no down ward flow elemen ts that wou ld cause problem sin th is sort of system.

The clip fin collector is a type of tube an d sh eet collector. It ism ade from an alumin ium p late attached to copper piping. Heatis absorbed by th e plates an d tran sferred to water passin gth rough th e pipes. Pipin g is arran ged in a sim ple grid. Cold waterenters the grid at th e bottom , passes through th e grid, where it ish eated, and h ot water leaves the pan el at th e top, diagon allyoppo site the po int at wh ich it en ters.

Heat losses may be reduced by th e addition of glazin g andinsulation . Clearly the m ain advantage of tube and sh eet

collectors over th e majority of sandwich collectors is that t h erewill be a greater surface area exposed to th e solar radiation, inproportion to th e volum e of water passing throu gh it.

W aterout W ater

in



107



Parabol ic reflectors

A parabolic m irror will focus ligh t bein g aimed d irectly at it on toa point , shown as F on th e diagram b elow. Th e focal length canbe altered by chan gin g th e size an d sh ape of th e parabola.

Constructing a Parabola Graphically

Rays

from

sun

Focal length

Focal point (F)

Focus, F, is 2 0 mm from d irectrix DD.

Draw axis AB through F perpendicular to DD.

Bisect FA to give V, the vertex.

Construct lines E, G, H, Jand so on at equal intervals from Vparallel to DD.

W ith centre F in each ca se and radii equal to the

perpendicular di stances of E, G, H , J, etc. from DD, strikearcs to cross the lines in 1, 2, 3, 4, etc.

Draw a curve through 1, 2, 3, 4, etc. to give the requiredparabola.

D D

A

V

F

B

J

H

G

E

4

2

1

3



108



Constructing a Parabola Mathem atically

Con siderin g th e above graph , if th e parabola has a focal point at(a,0) th e parabola m ay be plotted using th e equation:

y2 = 2a x

Try con structin g different shap ed parabolas altering th e focallen gth to see th e affect it has on t h e sh ape.

Points to Con sider in D esigning a Parabolic Reflector

• Wh at advantages or disadvantages are there in h aving a longfocal len gth rath er than a short on e?

• Wh at properties does the m aterial used for the reflector need?

• Is hea t going to cause a problem?

• How are you going to m ake your reflector?

• How are you going to m ove and h old your re flec tor?

It is worth n otin g that p arabolic reflectors will con centrat e directradiation on ly. Therefore careful con sideration n eeds to be givento t h e efficien cy of such a device in o ur climate with a largeamo un t of diffuse radiation. Are there any advan tages that m igh toffset th is?

F (a,0)

y

x

-10

-8

-6

-4

-2

0

2

4

6

8

10

0 1 2 3 4 5 6 7 8 9 10



109



INFORMATION SHEET 4 - D ESIGNING TH E SOLAR COLLECTOR


The collector is the device wh ich h eats the water. It has tocollect the solar en ergy an d tran sfer it int o h ot water. Basicsystem s have sets of pipes which start with cold water in t h emand h eat up in th e sun.

This is clearly n ot d ifficult. It would be d ifficult to p revent wat er-filled pipes from h eatin g up in th e sun . But h ow do you ensurethat this process is as efficient as possible?

Con sider h ow t h e followin g factors will affect th e solar collector'sefficiency:

- surface area of the collector

- volume of water in collector

- water flow rate through the collector

- m aterial of which the collector is m ade

- heat transfer between collector surfaces and circulating water

- surface treatment of the collector

There are a range of collector con figuration s th at will vary inefficiency depen ding o n th e factors outlined above. Consider th econfigurations outlined below bearing in m ind you r needs andth e availability o f m aterials.

Hosepipe solar water heater



110



Th ere are several key poin ts th at shou ld be n oted wh enconsidering the construction of the solar collector:

• th e solar collector should ideally h ave a water capacity of lessth an 2.5 litres/m 2 of absorber surface.

• Flow rates of between 0.01 to 0.02 kg/s per square metre of

collector area have been foun d to be satisfactory.

• If a pipe and plate system is being used then there areoptim um th icknesses for th e plate, and o ptim um distan cesbetween th e pipes, to m aximise heat tran sfer dependen t onth e m aterial being u sed. Minor variation aroun d t hese figureswill n ot cause m ajor problem s.

Material Th ickness (m m ) Spacing (m m )

Copper 0.25 138Alum in ium 0.5 138Steel 1.0 100

• Having a very good bon d between pipes and plates is veryimp ortant t o m aximise the rate of h eat transfer between th etwo (i.e. a good th ermal bon d such as welding, brazin g orh igh tem peratu re solder). Soft solder sh ould be fin e unless

you ach ieve a very h igh absorbency tem perature, in whichcase th e bon d m ight fail. Maximising th e surface area of pipein con tact with th e plate is also imp ortant .

Parallel tubes bonded to uppersurface of absorber plate.

Tubes bonded to lower surfaceof absorber plate.

Tubes bonded to formed absorberplate to give increased contact areafor good thermal conduction.

Integral tube and plate.

Corrugated sheets rivetedtogether to enable passage ofheat transfer fluid between them.

Formed metal sheets spotwelded to produce sandwichconfiguration.

Trickle collector with fluid passingdirectly over absorber plate.

Corrugated sheet spot weldedflat sheet.

SpacingThickness



111



Materials

You should consider the following factors when considering th em aterials to be used for th e collector.

- thermal proper t ies - absorption an d conduct ion ,

- no t ea sily damaged by heat ,

- durable,

- will h o ld wat er,

- can be easily joined to i tself or other m aterials withwaterproof joints,

- compat ib le with the o ther compon ents in th e system andfluids u sed (corrosion )

- a ffo rdable p rice .

Copper h as the best th ermal properties but is also th e m ostexpensive of the com m on ly used m aterials. However th e pipean d fittin gs are readily available and are frequen tly used withalum inium sheet.

Th ere is a dan ger of corrosion when you m ix m etals in a system .Metals have different electro-ch em ical pot en tials an d, as a result,when two different metals are immersed in water, there is anincreased t enden cy for on e of th em to d issolve.

You m ay inad vertent ly be effectively creatin g a batt ery between

the two metals. Dissolved oxygen in the water can increase thisproblem. With a direct system where m ains water is used therewill always be oxygen present.In a sealed, indirect system th ere can be some corrosion in t h eearly stages which redu ces th e oxygen con ten t. Th is isparticularly a problem wh ere water flows from a copper pipe int oalumin ium or galvanised iron.

Manufacturers of aluminium absorber plates stress that theyshould only be used in indirect systems with anti-corrosiveadditives in th e water and th ey sh ould b e electrically isolatedfrom any oth er metallic elemen ts and th ey sh ould n ever beconnected directly to copper pipes.

If aluminium absorber plates are used with copper tubing carem ust be taken to ensure th at m oisture will not be able to collectbetween th e two (for example from cond ensation).



112



Surface Treatm en t

A surface of matt black paint is very good at ab sorbing sun ligh t(it will con vert about 95% of the incident sun light in to h eat)but it is also very good at re-em ittin g h eat. As th e temp eratureincreases so does th e rate of em ission of h eat.

In a single glazed collector the m aximum temp erature mightreach 150°C so pain ts and p rim ers h ave to be temperatureresistant.

However, because th e h eat loss from th e collector increases asth e tem perature o f th e water increases, its efficien cy will bepoo r at h igh tem peratu res. Solar systems will th erefore operatemore efficiently when raising water temperature from 10 to20°C than from 40 to 50°C. The hotter an object becom es, the

greater th e rate at wh ich it loses heat. So, it is most efficient tomake a collector operate at the lowest useful temperature andm inimise the tem perature difference between t he in let an dout let water flows.

On e way of con trolling th e operating tem perature is to speedup th e rate of water flow th rough th e collectors.

This effect can be m inim ised if th e collector has som e form of selective surface. A 'selective' surface will still absorb m ost o f th een ergy, but will also retain it b etter. These surfaces m ay beapp lied by electro-platin g or by dippin g a metal absorber in

appropriate chem icals to produ ce a th in sem i-condu ctin g filmover th e surface. The th in film will absorb sh ort wave solarradiation, but n ot em it lon ger wave th ermal radiation well. Agood selective surface will h ave an absorbtan ce of solarradiation greater than 90% and a low emittan ce of thermalradiation of less th an 20%. Good selective surfaces may reachtemp eratures of 200°C.

Selective surfaces includ e -

- black chromium

- black nickel

- oxidised stainless steel

- black copper oxide

- electro-plated zinc

There is an ad h esive m etal film coated with a selective surfacewh ich can be applied to an absorber. Thin , selectively coatedn ickel foil can be bon ded on to th e absorber.

The followin g table sh ows th e effect of different n um bers of covers an d u sin g a selective surface. It shows th e overall loss of h eat to th e surroun dings from th e top cover, assum ing a meanwind velocity of 5m /s and an amb ien t tem perature of 10°C.Oth er losses from th e back and sides are not included. Th e

long-wave emissivity of 0.1 indicates the selective surface.



113



Overall loss of heat from top co ver in W/ sq.m

Plate tem p. 40°C 80°C

Surface Type Black Selective Black Selective

long-wave emissivity 0.95 0.1 0.95 0.1

1 cover 189 93 525 2632 cov ers 78 57 280 1683 cov ers 63 45 182 119

The reduction in en ergy loss due to th e selective surface becom es

increasin gly significant as th e collector plate gets ho tter, so wh ata selective surface will add t o efficien cy depen ds on h ow oftenthe collector reaches high temperatures.

Water Flow Through the Collecto r

If a pipe an d sheet typ e collector is bein g constructed t h en it isimpo rtant to con sider th e type of water circulation t h at will berequired. If th e circulation is occurrin g th rough th ermo-siph on ing then it is im portan t th at th is flow is n ot h amp ered inany way.

If th e system is pum ped, th en h orizon tal risers will not cause anyproblems. However it is importan t to try to avoid backcirculation as shown below. This can be done either by varyingth e size of riser to artificially increase the resistan ce to flow in th eright h and risers, or by having th e outlet on t h e sam e side as th einlet.

Expected flow patternin a grid of pipes

Actual flow pattern ina grid of pipes withpumped circulation



114



Reflectors

Lenses or reflectors can be u sed to ach ieve h igher tem peratu resby focusing th e solar en ergy. On ly direct sun ligh t can becon centrat ed, so, if you design a system based on reflectors youm ay find th at you can greatly in crease th e temp eratures you canreach in direct sun light, but you n eed to consider how yoursystem will perform wh en t h ere is only diffuse solar energy an dalso h ow it will be affected b y wind chill, or sim ply cold aircondition s. Also, what h appen s when th e device is no t directlyfacin g the sun ?

See in formation sheet 3 for more information on parabolicreflectors.



115



INFORMATION SHEET 5 - THE COLLECTOR BOX


You sh ould con sider what th e collector box n eeds to d o;

- keep heat in ,- keep rain , snow etc. ou t ,- keep out creatures such as small mamm als, large insects and

birds.- a llow condensat ion to e scape ,- contain th e collector and associated pipework- a llow for expansion and contrac tion due to changes in

temperature

Materials

Th e container could be m ade of:

- painted t imber (3 coats and repainted at least every 3 years)- t ana lised t imbe r (and pain ted)- rot resistant wood, such as cedar, European larch or oak (all

air dried an d n ot green). You can treat tim ber yourself an dthere are 'environmentally friendly' treatments available

- g lass reinforced p last ic ,- ga lvan i sed st ee l,- alum in ium .

You sh ou ld consider ways of reducing deterioration of th em aterials over tim e and th erefore th e need for m ainten ance. Forexample as well as weatherproofing the construction materialsand th e join ts, use corrosion resistan t screws and avoid m ixin gm etals, such as m ild steel screws in alum inium boxes.

Efficiency and m inim ising heat loss

On ce the water in th e pipes h as heated up you will need toensure that t h is h eat goes where you wan t it, and d oes not justescape out of th e back of th e panel or heat up th e air around th epipework on its way to be used.

Th e ho t collector can lose heat by con duction into whatever istouch ing it, by con vection u p int o th e air and by radiating itfrom its surface. Heat can th erefore be retain ed by insulation an dth e glazin g.



116



N on-selectiveabsorber

Air

Glass

Solarradiation

Reflectionfrom glass

Reflectionfrom

absorber Convectionlosses

Radiation

loss

Usefulheat

Back losses

Absorbedenergy

Longwave

IRconvection

Energy ba lance of a flat p late collector

Glazing

Th e maxim um spacing between th e glazing and th e absorbershould b e 40mm to p revent excessive heat loss th roughconvection and excessive edge shading. Th e m inim um spacingshould be approximately 25mm to m inim ise heat loss due toconduction.

Th e glazing cover should allow in th e maxim um amo un t of solarradiation wh ilst also acting as a barrier to h eat loss from t h eabsorber. As th e collector's absorber surface h eats up , it radiatesh eat out in th e infra-red range at a wavelength of 3 microns andm ore. Th ese lon g wavelen gths cann ot pass th rough someglazing m aterials and are therefore absorbed by th e glazin g andsom e of th e energy is re-em itted tow ards th e absorber.

Glazing m aterial needs to be ch osen to let th rough visible daylightand ultraviolet but to reflect back an y infra-red radiation from th esurface of th e absorber itself. So it needs to h ave high sh ort-wavetransm ittances (below 3 m icron s) but low lon g-wavetransm ittances (over 3 micron s). About 98% of in comin g solarradiation is at less than 3 m icron s. Optical wavelen gths arem easured in Angstrom Un its (Å). 1000 Å = 1 m icron; 1Å = 10 -10 m .

Ordin ary glass (3mm wind ow glass) is qu ite good for solar pan els- it will transm it th rough about 90% of th e solar radiation fallingon it. Som e of the oth er 10% is absorbed by th e glass and is thenre-radiated from both th e intern al and extern al surfaces.

4mm float glass has a transmittance for solar radiation of about84%.

Water wh ite glasses h ave a low iron con ten t an d are availablewith a h igh er solar tran smittance, but m ay be expensive anddifficult to get h old of.



117



Horticultural glass comes in stan dard sizes and is ch eaper. Som etransparent plastic materials have h igh short-wave transm ittancecharacteristics but also h ave app reciable lon g-wavetransmittances and will therefore allow heat to escape. Withsome p lastics th eir tran smittance deteriorates when th ey areexposed to th e ultra violet light of th e sun for periods of time.Filon - glass fibre in resin with a surface coatin g to p rotect itfrom degrading (sh ould be 90% tran smittance)

Tedlar - thin plastic sheet, a fluorocarbon in the same chemicalfamily as teflon , does n ot d egrade significant ly, very durab le,very difficult to tear, very light weigh t, abou t th e same p rice asglass (sh ould b e 97% tran smitt an ce) (used for drum skin s).

Teflon - usually very thin , very clear film capable of

withstandin g high tem peratures, therefore good n ext to th eabsorber in m ultiple layer system s.

Acrylic sheet - has very good tran smittan ce but wou ld n ot stan dup well to h igh temp eratures. It is used in som e comm ercialcollectors, but h as to be well sup port ed, so as n ot to distort toom uch if it softens when h ot.

Th ere are transparent coatings which can be app lied to th e inn ersurface of a glass cover to increase the reflection of long-waveradiation. Th ese in clude tin oxide an d indium oxide.

The glazing also prot ects the collector from th e chill effect of wind . Glazing will reflect away and absorb som e of th e solarradiation but , by reducing h eat losses, it substan tially increasesthe overall efficiency.

The fixin gs of glass to th e box sh ould be flexible in order to giveth e glass room to m ove. On e option is outlined in th e diagrambelow.

a

d

c

b

Cross-sectional view ofsolar collector.

a) g lazing ang le

b) glazing angle spacerstrip (on bottom sideonly)

c) glass

d) absorber plate



118



Double glazing

Doub le glazin g would lower h eat loss but it also reduces energyinp ut to som e exten t because each layer absorbs an d reflectsaway som e radiation, an d it is expensive. Wh en t h e absorption of energy in to each cover is taken in to account, th e transmittan celosses for in cidence an gles up to 35 degrees, are typically abou t10% for single glazin g, 18% for dou ble glazin g an d 25 % for tripleglazing. Losses can h owever be sub stant ially less for very t h infilm covers.

Two layers of different glazin g m aterials could be u sed tooptimise the characteristics of both.

The inn er layer of dou ble glazing will reach m uch h ighertemp eratures th an th e outer on e an d th e con sequen ces of th is

m ust be considered.

Low tem perature systems, such as tho se h eating water forswim m ing poo ls, often do n ot u se glazing at all.In d irect radiation, tran smittance prop erties vary with th e angleof incidence. Transm ittance does n ot drop very much u p to anangle of about 50° but it th en starts to drop q uite sharply.

However because a large propo rtion of th e energy absorbed isfrom diffuse radiation , and because diffuse radiation is bestcollected on a ho rizon tal surface, th e ideal tilt angle to m aximisetransm ittance will be less th an 50°. See In format ion Sh eet 8.

Insulation

Con sider th e need for in sulation bot h to reduce heat loss and toprotect exposed parts of the system from freezing. Where thenare the key areas th at n eed insulatin g?

In th e solar circuit, is there any advan tage in insulatin g th e pipesreturnin g the cooled water to the pan el?

100

90

80

70

60

50

40

30

20

10

00 10° 20° 30° 40° 50° 60° 70° 80° 90°

Angle of incidence

T r a n s m i t t a n c e %

Single glazing

Fibreglass plastic

Double glazing

A n g

l e

o f i n

c i d

e n c e

Light ray

Collector



119



Wh at about th e effect of high temp eratures in side th e container?An y insulatin g mat erial n eeds to be resistant t o -

- high tem peratures (not melt or give off gases),

- infestation by insects,

- m oisture,

- fire.

Warm cel insulation is m ade from fireproofed, shredded , recycledpaper. It is the most environm entally sound in sulating material,having m uch less energy expend ed in its man ufacture and aclean er prod uction process th an rockwool or fibreglass. It is n on -toxic and a very good insulator (125mm depth of warmcel givesequivalent insulation to 150mm of fibreglass).

Low tem peratu re plastic foam insulation m aterials, likepolystyrene, sh ould n ot be used imm ediately behin d an absorberbecause of th eir reactions in th e high tem peratures th at th ey maybe exposed to. Som e give off vapou rs wh ich can con den se onth e back of the glazin g and interfere with its tran smittan ce.



120



INFORMATION SHEET 6 - CALCULATING COLLECTOR

EFFICIENCY


There are two ap proach es to find ing th e efficien cy of yourcollector:

1. Experimental measurement of the performance of thecom plete collector.

2. Calculation o f the collector efficiency from data obtained onth e constituent parts of your collector.

Expe rim ental Measurem ent of Overall Collecto r Efficiency

The sim plest meth od o f measuring efficiency is to set up aconstant flow experim ent where m easured h eat energy isremoved from t he collector and comp ared with m easured solaren ergy supp lied to t h e collector.

W aterin from

tap

Tin

S o l a r r a d

i a t i o n p o

w e r G w a

t t s / m 2

( m e a s u r e

w i t h s o l a

r i m e t e r )

tout

W aterout

M ass of wa ter(kg) in time t

Efficiency = Useful power output from collectorTotal power input from sun

Now, total power inpu t form th e sun = GA Watt

G = Solar radiation p ower in W /m 2

A = Area of co llector in m 2



121



Usefu l p ow er o u tp u t = p o wer ga in e d b y w at erfrom collector th rough collector

So , u sefu l p o wer ou t pu t = m cw(Tou t - Tin )t

m = mass of water (kg) collected in tim e tc

w= specific heat capacity of water (4200 J/kgoC)

Tou t

= outlet tem peratureT

in= in let tem perature

t = time taken in seconds

The re fo re , e fficiency = mcw(T

ou t- T

in)

GAt

This approach can u sefully measure th e significan ce of variouschan ges to your d esign .

Calculating Collector Efficiency from Constituent Data

By con sidering th e action o f the p arts of a solar collector an dth eir effect on th e overall efficien cy the d esign can be opt im ised.

The relevant equations are:

Efficien cy = Useful power output = Pc

Total power in pu t GA

G = Solar radiation p ower in W /m 2 (measured by

solarim eter or from standard d ata)A = Area of collector in m 2

Pc

= Power outp ut from collector in W

Solarcollectorarea A

Total input power = GA

Solar radiation G W / m2

Thermal power loss tosurroundings = UA(T

s- T

a)

Pow er transmitted throughglazing = GA τ

Power absorbed = GA τα



122



But,Power ou tput = Power absorbed by - th erm al power lostfrom collector th e black surface to th e surroun din gs

or Pc

= GA τα - UA (Ts

- Ta)

τ = t ransmission facto r fo r the glazingα = absorptiv ity of the b lack surfaceU = heat loss factor for the absorber (comm only called

'the U value') in W/ m 2°CT

s= mean t empera tu re o f abso rber

= (input tem p + output temp)/2T

a= am b ien t t em p erat ure

Puttin g th is int o ou r Efficien cy equation gives:

Efficien cy = Pc

= GA τα - UA(Ts-T

a)

GA GA

or Efficien cy = G τα - U(Ts

- Ta)

GNote th at for a given collector, τα

an d U are fairly constan t wh ile

(Ts-T

a) an d G both vary.

It is usual to p resen t th e 'efficiency curve' as a fun ction of th evariable

(Ts-Ta) or ∆TG G

Wh en designing a collector you will want to vary τ an d α

to see

th e effect of using d ifferen t m aterials.

Som e Typical Values Used in Efficiency Calculations

τ, th e tran smission of solar radiation b y th e cover is typically;

0.85 for single 4m m glass or acrylic sheet0.72 for dou ble 4m m glass or acrylic sh eet

0.90 for tedlar thin film0.95 for Teflon th in film

These figures can be redu ced by 1 or 2% due to dirt an d up to 8%du e to du st in very dry areas.

α, th e absorbtan ce of th e collector surface is typically 0.93 for agood m att black pain t.

With collectors having h igh levels of insulation beh ind th eabsorber pan el, th e heat loss factor, U, is determ ined m ainly byth e nu m ber of covers an d th e wind speed. It also increase a little

at h igh absorber temp eratures.Typically: 1 cover, U=5.5; 2 co vers, U=3.0.



123



The graph below illustrates h ow th e efficien cy of different typesof collector vary with th e tem perature conditions.

Th e graph shows th at increasing th e n um ber of glazing coverson ly becom es viable when you n eed higher tem peratures fromthe solar collector. For example, heating swimming poolsrequires only a low tem perature in crease, therefore glazin g th ecollector for this application would be counter productive.

An added factor n eeds to be con sidered to account for theaddition al losses between collector an d tap .

Systems can vary between 25% t o 60% efficient depen din g onth eir design an d m aterial use. Th erefore a solar system willconvert between 25-60% of the solar energy falling on thecollector into h eat energy com ing out of the tap s.

Evacuated tube

S i n g l e c o v e r - b a s i c

D o u b l e g l az e d U n

g l a z

e d

Single cover - selective

E f f i c i e n c y ( % )

Mean temp of collector



124



INFORMATION SHEET 7 - CONNECTING TH E COLLECTORS


Un less the iden tified n eed for h ot water is qu ite sm all, th ere willbe a need for more than one collector. How the collectors areconn ected up could h ave a major impact on th e even flow of water throu gh th e system and th erefore it 's overall performan ce.

Connecting Thermo-Siphoning Systems

System 'a' will cause problem s as it forces the flow d own wards.With in a th ermo-sipho nin g system all pipework sh ould allow anun restricted flow u pwards as in 'b'.

a

b

Both 'a' an d 'b' are arran ged in series, th is will produ ce high ertem peratu res which w ill reduce th e efficien cy as a result of h igher h eat loss. System 'd', arranged in parallel, will usually bepreferable un less the h igh tem peratures are actually requ ired.System 'd' is also slight ly tilted to encou rage m ore efficientup wards flow of water.

cd



125



Connecting Pumped System s

Again p arallel conn ection of collectors is better u n less h ightem peratu res are specifically requ ired.

However within a pu m ped system conn ected in parallel as insystem 'e', th e water flow could be u n even causing n o flow areasan d even reverse flow as illustrated. Th is cou ld greatly reduce th eefficien cy of th e system .

System 'f' th en provides a well balanced circulation patt ern withno flow disturbances.

e

f

It is worth con siderin g the u se of com pression fittin gs as opposedto soldered joint s to allow th e easy remo val of a pan el if necessary.



126



INFORMATION SHEET 8 - MOUNTING TH E COLLECTORS


Having iden tified th e best spot for th e collectors taking int oaccoun t th e factors outlin ed in in formation sheet 2, you willn eed to mo un t th e collectors facin g the righ t direction an d at th eright angle to maximise th e amou n t of solar radiation th at th eyreceive. The tilt angle will increase th e amo un t of absorption of solar radiation , see below. However, th e optim um an gle is acomp rom ise between th e n eed to collect direct radiation, wh ichin win ter requires a very steep angle, an d th e need p articularly inth is clim ate to collect diffuse radiation, wh ich is at a m aximu mwhen th e collectors are horizontal.

Variation of the solar energy supplied by the example system with a class Ecollector, w ith orientation and tilt of the collector

Direct radiation - tilt increases absorption

If th ey are bein g mou n ted on a slopin g roof th en th e choices arelim ited, with a flat roof h owever there is a lot m ore freedom . Th ediagram below illustrates th at th e optim um settings for th ecollector range quite considerably. For example, with the collectorfacin g due south , the an gle from h orizon tal could range from 15to 50 degrees and still produce a very similar amou n t o f energy.

90° 80° 70° 60° 50° 40° 30° 20° 10° 0° 10° 20° 30° 40° 50° 60° 70° 80° 90°

W

SW SE

E

S

-5%

-10%

-20%

Tilt angle Horizontal



127



Mounting on the ground within th e school grounds might m aketh e collectors easier to m aintain, an d will m ake them m oreno ticeable and h igh er profile. Althou gh th ese m igh t beimpo rtant considerations, th e possibility of accidental or n on -acciden tal dam age sh ould be carefully con sidered before groun dlevel m oun ting is decided up on .

Mounting the collectors requires only a triangle frame set at therequired angle, th ough it is im portan t to con sider the weigh t of th e collector full of water when deciding up on th e stren gth of th e frame. In addition th e frame sh ould be secured to t h e surfaceit is restin g on in som e approp riate way, particularly if thecollectors are being fixed to th e roof. It is worth n otin g whererain water collects on th e surface and t ry to avoid th ese places.

Would it be worth considering th e need to allow th e collector tobe mo ved to track th e sun eith er during th e day or over the yearto reflect th e variation in h eight of the sun in t he sky? It could beautomatic or manual control.



128



INFORMATION SHEET 9 - SYSTEM D ESIGN


There are a range of different system design s, the ch oice of whichdepend s on th e particular situation, and t ype of h ot water needth at you are dealing with.

System Layout

Stan d alon e system

Pros

• Easier to set up if only supplying water to on e or two draw off points.

• No need to worry about exist ing system.

• More efficient for smaller hot water deman ds when there is

enough sun , due to lower standing h eat losses from tan ks an dpipes.

Cons

• Produces less useful heat all year round. Som e form of preheat system will supply more useful heat to the whole systemwh en h ot water is required du rin g periods with lowersunshine levels.

• Becomes comp licated to install if water is required at an um ber of existin g draw off point s.

• Might need some form of back-up system

Vent

Cold feed

Coldwatercistern

Domestic hot w ater

High levelimmersion

heater

Pump

Solarprimary

expansiontank

Solar

collector

Checkvalve



129



Pumped System

Solarcollector

Checkvalve

Pump

Domestic hot w ater

Cold feed

Coldwatercistern

Solar primary expa nsion tank(You can use a sealed expa nsion unit. Fillw ith water, anti-freeze, corrosion inhibiter)

Vent for airand steam

Pre-heatcylinder

Existingcylinder

Auxiliary heatfrom

immersionheater orexistingboiler

Pros

• Flexib le system arrangement

• Slightly m ore efficient heat transfer between panel and tank

Cons

• More com plicated system requires solar controller to operatep u m p

• More ma in tenance, h ighe r runn ing cost s



130



Thermo -Sipho nin g System

Coldwatercistern

Domestichot water

Cold feed

Expansion tank.

(Fill with water, anti-freeze, corrosioninhibiter)

Vent

Solarcollector

Existing

cylinder

Pre-heat cylinderVertical distance:M inimum of 1 metre

Pros

• System is a lot sim pler, no n eed for pumps, solar controllersetc, water flow is con trolled auto m atically by chan ges indensity of water due to varying water temperatures .

• System is a lot cheaper for the same reason.

• Requires less maintenance .

• No e lect r ic ity required to run system.

Cons

• There are a nu m ber of system configuration restrict ions thatcould cause problems du ring installation.

- Pipework in solar circuit needs to be at least 22mm ,preferably 28m m .

- The p ipe runn ing from the top o f the panel to the t ankm ust run uph ill all the way.

- The p ipe runn ing back must run downh i ll al l the way.- The tank shou ld be at least 600mm , preferably 1m above

th e top of th e panel. Th is m igh t n eed to be increased if theh orizon tal distance is more th an a few m etres.

- The pipe layout within th e collector shou ld consist of vertical risers as opp osed to h orizont al len gth s (diagram 3).

- It is not possible to have a self draining thermo-siphonsystem.



131



Elem ents o f the System

Cold Water Feed Tan k

Th e cold feed tan k should be conn ected to th e main s supp ly viaa ball cock, similar to t h e valve foun d in W.C. cistern s. Anoverflow p ipe is also required in case th e ball valve sho uld everfail to turn off th e cold water sup ply.

Exp ansion Tank

As water in th e solar tank is h eated it will expand and anydissolved air will com e out o f solution . It is th erefore n ecessaryto h ave an expan sion tank t o allow for pressure ch anges with inth e solar system , and a vent t o allow th e air to escape to h elppreven t blockages. There sh ould be a vent pipe in th e ho t watersupp ly side of the system, u sually back in to t h e cold water feed

tan k, as well as a vent p ipe to a separate expan sion tan k as part of th e solar collector circuit, see diagram s above.

Pump

If a pum p is bein g used th en it will need to be capable of providin g a flow rate of between 0.01 an d 0.02 kg/s per squarem etre of collector area. A n orm al 30 W central h eatin g pum pshou ld be sufficient except for very large systems. A variable headpu m p wou ld be ideal if available, to be able to experimen t withdifferent flow rates. However to avoid 240V applian ces it m ightbe n ecessary to u se a car water pum p or similar.

The pum p sh ould be in stalled wh ere it will rem ain full of wateran d o n a vertical piece of pipe to avoid airlocks. All pipeworkshould be flushed ou t before the installation of the pu m p toensure th at th ere are no loose particles that could clog up t h epum p wh en it starts workin g. Th e pum p sho uld be isolated by agate valve on each side of it in th e circuit to en able easy remo valif n ecessary. A controller sh ould switch t h e pum p on when th etemperature of the collector is about 4°C higher than th at of thesolar tank an d switch it off again when th e temp eraturedifference falls to 1°C. If th e tem peratu re difference is less th an1°C then th e pum p will probably be using more energy than th ecollector is collecting.

Storage Tank

Th e h ot water storage tan k should be sized with th e dailydem and for hot water in m ind. Ideally the tan k sh ould be greaterth an 80% of the daily h ot water deman d, however in reality th echo ice may be limited by availability. Again th e ideal tanks areth e typical hot water cylinders with an integral h eat exch angercoil.

If on e of th ese tanks is not available, then it will be n ecessary toconstruct a heat exch anger to place in an open topp ed tank. Th ebest m aterial to u se is copper pipin g because of its good h eatcondu ction prop erties.



132



The h eat exchan ger size shou ld ideally be in th e region of 0.2-0.3m 2 /square m etre of collector area. Th is equ ates to 4 m etres of 15m m pipe or 3 metres of 22mm pipe. It is best to bend th e pipein slow bend s, see diagram below, as sharp b end s tend t o increaseth e resistan ce to water flow. This is particularly imp ortan t inthermo-siphoning systems where the flow is already quite slow.Remember the flow in these systems should be upwards, inpum ped systems th e direction of flow does n ot m atter.

Heat Exchanger for a thermo-siphoning system

Heat exchanger in apumped system

All pipework and hot water tanks should be well insulated toprevent heat loss.

It is also im portan t to remem ber th at each litre of water weighs1kg, so a full tan k, m aybe 200 litres will be qu ite h eavy. You willn eed to en sure that th is is con sidered when th e tank is installedand structural supp orts for th e tank are provided.



133



INFORMATION SHEET 10 - P LUMBING TECHNIQUES -

W ATER P IPES, FITTINGS AN D JOINTS

Back up to Briefin g Sheets 3, 4 & 7The following is an ext ractadap ted from a 'DO IT ALL'DIY guide (prod uced h ere bykind p erm ission ). A ran ge of such guides is available, and ,of course, consumablem aterials and small equipm entcan easily be obtain ed fromth is source.

1 . TYPES OF P IP E

COPPER PIPE

Th is is comm on ly used fordom estic water and cen tralh eating system s. It is availablein d iameters 8, 10, 12, 15, 22,28 and 32mm . The mostcomm on sizes being 15mmand 22mm . Old copper pipingis in Im perial sizes:

a) 1/2" is the sam e as modern

15m m and all fittingsshould be interchan geable.

b) 3/4" is sligh tly sm aller thanm odern 22m m so anadapter is needed.

Ben dable corrugated copp erpip e is also available. It is easyto bend by h and an d is idealfor use in awkward areas.

PLASTIC PIPEThis is a available in d iamet ersof 15mm and 22mm . It is easyto u se an d is flexible enou ghto be bent through 90° byhan d. It can be used fordom estic water and cen tralheating but m ust not be usedto con n ect directly to a boiler.

2 . CUTTING AN D

P REPARING P IP E

Pipe cut ters are simp le and fastto u se an d m ake a perfectsquare cut. The cutter leaves alip on th e in side of the pipewhich sho uld be removedwith a ream er (part of th e pipecutting tool) or round file, toavoid th e build up of scale.

COPPER PIPE

This can be cut with a h acksaw, but t h e saw will leaveburrs both in side and o utsideth e pipe. Use a flat file tosmooth off the burrs on theoutside and a reamer or roun d

file for th e inside.

PLASTIC PIPE

This can be cut with atrimm ing knife or a hack saw.Wh en joint ing plastic pipe am etal in sert m ust be placedinside the pipe to providesupport so t h e fitting can gripth e pipe better.

Cop per pipe can be easilycrushed in a vice, so wh encutting with a h ack saw, use abench h ook to rest the pipe on.



134



INFORMATION SHEET 11 - D ESIGNING WITH ELECTRONICS

Back up to Briefin g Sheets 4 & 6

Wh en solving a pro blem with electron ics, it is best to break itint o separate blocks. Each circuit will be m ade up of at least th reeblocks:

The Input senses and reacts to changes. Some typical inputdevices are:

1. Switch

A switch m ay be u sed to sense pressure on it. E.g. it can sense if adoo r is open or closed or if som eon e is pressing it.

IN PU T PROCESS OU TPU T

2. Light D epen den t Resistor (LDR)

An LDR's resistance is depen den t o n th e level of light fallingupon it and h ence the voltage drop across it may chan ge.

Magnet

Reed switch

+6V

0V

Tothyristor

100K

Reedswitch

Light dependent resistor SymbolV across

LDR



135



3. Therm istor

A th erm istor acts as a resistor wh ose resistan ce chan gesdepend ing upon its amb ien t tem perature. Th erefore th e voltagedrop across the th ermistor will chan ge with temp erature.

4. Diode

The voltage drop across a diod e will ch an ge slightly as itsambient temp erature ch anges. Th is chan ge in voltage m ay beincreased by u sin g several diodes in series.

V acrossthermistor

Symbol

Generalpurpose diod e Symbol

Th e Process reacts to ch anges of the inpu t an d con trols th eoutp ut. It may be necessary to perform o peration s on th e in putchan ges as th ese chan ges m ay n ot be sufficien t to directlycontrol the outp ut.

The followin g devices m ay be used to am plify inp ut ch an ges:

1. Transisto rA tran sistor m ay be used as a switch an d/ or an am plifier.

Transistor

Base

Collector

Emitter



136



In th e circuit below, the tran sistor needs to be on for the bulb tobe on . Th ere m ust be mo re than about 0.7 volts across th e baseand emitter conn ections of the tran sistor for it to be on .If on e of th e inp ut resistors is replaced with an LDR or ath ermistor, th e circuit could be used to switch a ligh t on (or off)when certain ligh t or h eat con ditions are m et.

2. Operation al Am plifie r (OP AMP)

The operation al amp lifier m ay be used to am plify th e differencebetween t wo voltages. It h as a gain of about 1000 0.

10 kΩ

4.7 kΩ

INPUT OUTPUT

However the out put voltage may not b e outside th e ran ge of thesupply voltage (i.e. +9V to -9V ). In fact in practice it would beabou t 8V to -8V as there is a voltage drop across the Op Am p totake in to accoun t.

As th e gain of th e Op Amp is very large, th e out pu t voltage willbe either +8V wh en V

1is greater th an V

2or -8V wh en V

2is

greater than V1.

Th e outpu t is switch ed on and off by th e process part of thecircuit. It can perform similar functions to what the sensors are

detectin g , e.g. heat, light, m ovemen t.

Output Voltage (Vout

) = Gain ××××× (V1

- V2)

V1

V2

Vout

+9V

-9V

0V

+

-



137



Typical outpu t devices include:

1. Motors

Motors provide a rotary mo tion and , in general, th e greater theirtorque, th e m ore curren t they require.

2. Relay

A solenoid also relies on a m agnetic field being created wh en avoltage is applied across the coil. However, th is field is used to

operate a bolt wh ich is qu ickly drawn in to th e field. A springm ay be used to retu rn it to its "off" position .

A relay is used to separat e two circuits electrically. A coil in t h erelay is energised by a voltage being pu t across it. Th is in turncreates a magnetic field in order to op erate th e switch in t h esecon dary part of th e relay.

3. Solenoid

Enclosed relay withPCB mounting pins

O pen frame relay

Frame

Armature

Switchcontacts

Symbol for relaySolenoid

MCircuit

symbol formotor

Electricmotors



138



4. Speaker

A speaker can p rodu ce a sou n d ou tpu t. However it is necessary to'switch' it on an d off very quickly in order to p rodu ce an aud iblenoise.

Symbol

5. Buzzer

A buzzer can be used to prod uce a soun d an d it on ly requires aD.C. voltage across it to operate.

Symbol

Buzzer



139



INFORMATION SHEET 12 - D ESIGN OPTION - THE SOLAR

PUMP CONTROLLER


Construction Plan

Stripboard is probably the m ost convenient system, with t hefollowin g com pon ents.

Voltage comparator LM311N plus 8 pin socket

Transistor 2N 3053

Potentiom eter Cerm et Pre-set 5K

Relay 12 V m iniature Relay: PCB Mounting

2 bead therm istors 15K LED To show relay operation

Metal film resistors 500 Ω and 1000 Ω

Th e best way to fix the bead th ermistor to th e collector andstorage tank is with silicone sealant to prevent condensationshorting th e con tacts.

The 5K Cerm et preset sh ould be adju sted to set th e requiredtemp erature differen ce between th e two th ermistors to o perateth e relay an d th en th e pum p. Th e thermistors should be put indifferent substances, possibly water, with known temperatures toenable the accurate calibration of the controller.

Thermistoron collector15 K

Thermistoron storagetank 15 K

5 K preset

2

8

3 1

7LM 311N 1 K

0 V

+12 V

500 Ω

2N3053

To pumpcircuit

12 V relay



140



INFORMATION SHEET 13 - W EATHER P ROTECTION

Back up to Briefing Sheet 3, 4 & 7

The exposed collector an d pipework will be vulnerable to th eextrem es of th e weath er, particularly h igh winds and freezin gweather.

Apart from th e extrem es, th e collector should be built to p reventwater penetration , usin g mastic on all join ts and at t h e inlet andoutlet h oles, and th e surfaces treated to reduce the n eed form aint enan ce. Alum inium or fibreglass casings will last years.Tim ber casin g, even if it is well treated, will n eed th e re-app lication of some form of protective coating after a few years.Silicon e sealant will last a lot lon ger th an con vent ion al putt y for

sealing th e glazin g covers to th e fram e.

High w inds

From a structural poin t of view th e forces placed on t h e collectorby high winds should be considered wh en m oun ting thecollector. All fixings shou ld be stron g and secure, h owever itwou ld be best t o avoid wind y sites if at all possible.

Freezing w eather

All extern al pipework shou ld be insulated t o redu ce the risk of freezin g. An ti-freeze can be used as an additive to th e water inth e solar circuit as lon g as th e water that is heated b y th e solarpan els is separated from th e water that com es out of th e taps by ah eat exchan ger (an ind irect system). Car anti freeze is notrecomm end ed because of its toxicity. However if th e system isarran ged such th at th e water level in th e cold feed tan k is h igh erth an t h e water level in th e solar expan sion tan k, this en sures thatth e pressure in t h e solar circuit is lower, so if there is a leakbetween th e two parts of the system, th e water flow will be to th esolar circuit rath er than th e other way roun d.

Th ere sh ould b e approximately 20-25% an tifreeze with in th esolar circuit to effectively prevent freezing. This will probablyn eed replacin g after appro xim ately 5 years.

An insulated cover placed over th e collector at n ight is a simp leoption but it does require a good m emory to remem ber at th ecrucial tim e.

Th e oth er option is a self draining system th at drains th e waterout of th e collector du ring th e winter. A m an ual system relies onmemory; the diagram below illustrates the system layout for anautom atically d rain ing system .

Because the collector an d all exposed pipework are above th elevel of the cold water cistern, wh en th e pum p is not working th ewater falls to th e level of the cold wat er cistern t h ereby

protectin g th e exposed p arts of the system.



141



An open vent is required as sh own in th e diagram an d th e coldwater tank m ust be large enou gh to take all the displaced water.Th e layout of th e pipework sh ould en courage easy draining an dfillin g witho ut developin g air pockets, blockages or pools of water.

There could also be a problem in h ard water areas with scaleform ation. It will not be possible to add an inh ibitor to th e solarcircuit because the water h eated is th e water that comes out of th e taps. Excessive scale form ation in t h e collector could redu ceth e efficiency o f th e collector over tim e, requiring de-scalingevery few years.

Coldfeed

Domestic hot water

Vents

Pump

Pre-heat cylinderExistingcylinder

Openvent

Solar

collector

This connection mustbe above water levelin co ld w ater cistern



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INFORMATION SHEET 14 (OPTIONAL) - TRACKING

MECHANISMS


It m ay be n ecessary to speed up , slow do wn , increase or decreasesa force or torque, or con vert on e type of m otion t o th e other,when considering th e design to m ove your collector.

Th e followin g mech anisms m ay be worth con siderin g.

1. Spur Gears - used t o in crease an d d ecrease rotary speeds an dto d ecrease an d in crease th e torqu e respectively.

spur gears

Speed of = Speed of × n o. of teeth on driverdriven gear drivin g gear n o. of teeth on driven

2. Bevel Gears - are used to transmit a rotary motion th rough 90°

bevel gears



143



worm and wheel

4. Rack and Pinio n - is used to convert a rotary mot ion t o alinear m otion or vice versa.

Speed of wh eel = speed of worm × 1teeth on wheel

Rack

Pinion

3. Worm and Wheel - As the worm on ly has one tooth per turnof the h elix, th is m ech anism slows down a rotary m otion by alarge ratio.



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INFORMATION SHEET 15 (OPTIONAL) - D ESIGN OPTION -

TRACKING TH E SUN


Parabolic m irror mo vem ent circuit

LDR 1 LDR 2 LDR 3

The three LDR's are set up as above. LDR 3 provides the referencevoltage to both op am ps. As the sun m oves, the level of ligh t onth e LDRs alters an d will cause a positive voltage from on e of th eop am ps. This in turn causes th e moto r to m ove in o n e directionor th e oth er. See the m otor cont rol circuit for con n ection t o th iscircuit.

-

+

-

+

12 V

0 V

Base 1

0 V

Base 2

360 Ω

2.2 K

360 Ω

2.2 K

OPAMP1

OPAMP2

LDR1 LDR2 LDR3

VR1 VR2 VR3

Parabolic m irror m ovem ent PCB



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Motor Control Circuit

The circuit is based on two t ran sistor pairs. Each p air is driven b ya th ird transistor. Wh en Q5, see diagram below, is turned on , itturn s on transistor pair Q1 and Q3. Th is m eans curren t flowsfrom th e power rail through Q1, th rough th e moto r from left toright, an d th en t hrou gh Q3 to th e groun d rail. Likewise Q6 drivestransistors Q2 and Q 4 which allows current to flow through th emotor from right to left, therefore reversing the direction of rotation.

Care m ust be taken th at Q5 and Q 6 are n ot on sim ultaneously asth is results in a short circuit between Q1 an d Q2, wh ich shortsout the power rails and will damage the transistors.

In n ormal use Q1 - Q4 will becom e quite ho t an d so should be

con n ected to a suitable heat sink. Th ey are placed in a row onth e PCB to allow conn ection t o th e sam e heat sin k, althou ghth ey mu st be electrically isolated from each ot h er by use of m icaheat sink pads.

The diodes are used to pro tect th e tran sistors from back EMFwhich m ay be caused by th e dc motor.

Th e circuit could be up rated to con trol a higher power m otor bychan gin g th e stated t ransistors to tran sistors with a h igh ercurrent rating.

The weigh t of th e collector is a very im port an t factor wh enconsidering if tracking the sun automatically is worthwhile.

M

+ V

0 V

Base 1

Q1 Q2

Q3 Q4

Q5 Q6

Base 2



146



Motor control PCB



147



INFORMATION SHEET 16 - USEFUL ADDRESSES

INTERNATIONAL SOLAR ENERGY SOCIETY,UK SECTION (UK ISES)

C/o Solar Energy Laboratory, (Dr. Leslie Jesch), Department of Mechan ical En gineering, University of Birmin gham , Edgbaston ,Birmingham, B15 2TT.Tel. 0121 414 3344 Fax. 0121 4143958Aim s to prom ote th e use of all form s of solar en ergy;communicates ideas and developments in all aspects of solarenergy th rough con ferences, meetings, journ als & otherpublications. Membersh ip open to bot h professionals andint erested lay-peop le.

SOLAR TRADE ASSOCIATIONBrackenh urst, Greenh am Com m on South , Newbury, Berkshire.RG15 8HFI.Tel. 01635 46561Aims to m aintain standards within th e solar indu stry, and toprom ote th e use of solar power in general. Member comp aniesare obliged to adh ere to codes of con du ct coverin g advertising,selling, installation , servicing an d repair. Up-to-date list of m em bers is available from th e above address.

CENTRE FOR ALTERNATIVE TECHNOLOGY

Mach yn lleth , Powys, SY20 9 AZ,Tel. 01654 7024 00. Fax. 01654 7 02400,Edu cation al charity with o ver 20 years experience as a workin gdemonstration of renewable energy technologies (solar, wind,water, energy conservation), organic growing and other aspectsof sustain able livin g. Wide ran ge of inform ation sheets andbooks on solar pow er. Weekend courses on solar collectors an doth er topics. Con sultancy an d information departm ents. Schoolvisits. Open to visitors.

RENEWABLE ENERGY ENQUIRIES BUREAU

En ergy Techn ology Sup port Un it, B156 Harwell Laborato ryOxfordshire, OXll ORA. Tel. 01235 432450.Co-ordinat es developm ent of renewable energy in t h e UK, an

beh alf of th e Departm ent of En ergy. Ran ge of free pub lication seg. on solar research p rogram in general ('Using en ergy from t h esun ' ), project pro files of research in progress, sum m aries of research com pleted . Free jour n al 'RE View'.



148



MANUFACTURERS AN D SUPPLIERS OF DIY KIT S

APPROPRIATE ENERGY SYSTEMS LTDThe Park, Fin dh orn Bay, Forres, Moray, Scot lan d IV36 OTZ.Tel. 01309 69013 2 Fax. 01309 690933.Manu facturer of lightweigh t h igh perform ance flat platecollectors. Sup plies h ot wat er kits an d p an els.

DULAS ENGlNEERING LTD

The O Id Sch oo l, Eglwys Fach , Mach yn lleth , Powys SY20 8SX.Tel. 01654 78133 2 Fax. 01654 781390.Complete or part kits supplied.

ENERGY ENGlNEERING

Herons Reach, Cound Moor, Shrewsbury, Shrops. SY5 6BB.Tel. 01694 73164 8 Fax. 01694 731696.Com plete or p art kits. Also solar collectors an d system s forswim m ing pools.

GREENSLADE SOLAR, (A DIVISION OF BRITISH FUELS LTD)

146 Fish erto n Street , Salisbury, Wiltsh ire SP2 7QR.Tel. 01722 42700 0 Fax. 01722 339498.Solar h eating, installers of flat p late and evacuated tu be system s;swimm ing p ool h eating; supplies DIY kits.

R.S.U.

1-3 Cwmffrwd Trading Estate, Cwm ffrwd, Cam arth en , DyfedSA32 8 22.Tel. 01267 22260 6 Fax. 01267 221758.Man ufacturer of Daystar do m estic h ot wat er system s. Systemdesign, installation, advice for DIY.

SENSIBLE ENERGY SYSTEMS

16a Belham Road , Kin gs Lan gley, Herts. WD4 8BY.Tel. 01923 260970 (No fax).

SOLAR ENERGY SERVICES

86 Firle Road, Eastbourne, East Sussex, BN22 8EJ.Tel. 01323 64697 9.

Solar heating,'Thermomax' evacuated tube installers; swimmingpool heating; also supplies DIY kits.

SOLAR STORE

349 Canterbury Road, Densole, Folkestone, Kent CT18 7BE.Tel. 01303 892491 (No fax.)Inst allers of flat p late system s; sup plies DIY kits.



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TIMBER TREATMENT AN D PAINTS

AURO ORGANIC PAINT SUPPLIES LTD

Un it I, Goldston es Farm , Ashd on , Saffron Waldon CBlO 2LZ.Tel: 01799 584888 (No fax.)Man ufactures and supp lies low to xic wood preservatives such asCuprinol and organ ic paints.

BIOFA NATURAL PAINTS

5 Scho ol Road, Kidlingto n , Oxford, Oxfordsh ire OX5 2HBTel. 01867 5500 8 Fax. 0186755008.Supp lier of environm entally frien dly paints an d woo dtreatm ent s. Full ingredient s list.

ENVIRONMENTAL PAINTS LTD

Un it 71, Du n scar In du strial Estate, Blackburn Road, Egerton ,Bolto n BL7 9 PQ.Tel. 01204 5968 54 Fax. 01204 309107.Manu facturers of environm entally friend ly paint.

NUTSHELL SUPPLIES

Newtake Staverton , Devon TQ9 6PE.Tel. 01803 8677 70 Fax. 01803 866650.Manu factures an d supp lies en viron m entally frien dly paints an dwood treatmen ts

ABSORBENT SURFACES

INCO SELECTIVE SURFACES LTD

Wiggen Wo rks, Holm er Rd, Hereford HR4 9SL.Tel. 01432 3822 00 Fax. 01432 264030.Man ufacturers of Maxorb & Maxlam, m aterials designed toabsorb bu t n ot re-radiate solar en ergy, from which you can m akeoth er types of collectors.

PULMAN PANS LTD

Eastfield Industrial Estate, Penicuik, Edinburgh, Scotland, EH268HA. Tel. 0196 8 6783 86'Clip Fin' solar collector plate wh ich can be clipp ed over stan dard

15m m copper pipin g. Also available from th e Centre forAlternat ive Techn ologyPrice: £4.15 per fin or 12 (eno ugh for on e 1m 2 pan el) for £46.75no t including p+p.

INSULATION

Warm cel, m ade from fireproofed recycled newsprin t - h ighlyeffective, no n -toxic, non -irritant . Th is is available in an 8kg bag -enough for four panels - for £13.00, including p+p, from Centrefor Alternative Techn ology



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GLAZING FOR COLLECTORS

APPROPRIATE ENERGY SYSTEMS LTD

See section on DIY.

FORMERTON SHEETS SALES LTD

Unit 39, City Industrial Park, Southern Rd, Southampton SOIOHG. Tel. 01703 332 761 Fax. 01703 225748.Man ufacturers of selective absorption glazin g m aterials, that letin t h e righ t ligh t frequencies and keep in infra red.

FILON PRODUCTS LTD

Aldridge Rd, Streetly, Sut ton Cold field B74 220.Tel. 0121 353 081 4 Fax.0121 352 08 86.Man ufacturer of glass fibre sheet glazing, will advise on local

distributors.

SOLAR ANT I-FREEZE AN D SYSTEM CLEANERS

FERNOX MANUFACTURING CO. LTD.

Britan n ica Wor ks, Clavering, Essex CBll 4QZTel. 01799 55081 1 Fax. 01799 5508 53.Sup pliers of Alph i no n -toxic an ti-corrosion an d an tifreeze, andSuperfloc low toxic system cleanser.

P UMP SUPPLIERS

APPROPRIATE ENERGY SYSTEMS LTDSee DIY section .

PUMPING SERVICES (GB) LTD

Whitehouse Street, Hunslet, Leeds LS1O IAD.Tel. 01532 44611 1 Fax. 01532 465649.

RA-SOLAR

Frogs Hall Farm, Lavenham, Sudbury, Suffolk C070 9QHTel. 01787 24735 9 Fax. 01787 248458.

SOLAR CONTROLLERS

DULAS ENGINEERING LTD

(See Kit Supp liers)Sup plies a com plete kit of parts to b uild th e solar con troller - aswell as th e ready-made art icle.

ELECTROMAIL

P.O. Box 33 , Corby, Nort h an ts. NN17 9EL.Tel. 01536 204555 Fax. 01536 405555. Individual componentsavailable by m ail order.

MAPLIN ELECTRONICS

P.O. Box 3, Rayleigh, Essex SS6 8LR.Tel. 01702 55415 5 Fax. 01702 553935.Supp lies in dividual com pon ents by m ail order.

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