ADVANCES IN SOLAR AIR HEATER

A

Seminar Report

Submitted In Partial Fulfilment of the Requirements

For The Degree Of

Master of Technology in Mechanical Engineering (Thermal Engineering)

Prepared by

HARDIK V.RAMANI

(13MMET16)

Guided by

PROF. S. V. JAIN

DEPARTMENT OF MECHANICAL ENGINEERING

INSTITUTE OF TECHNOLOGY

NIRMA UNIVERSITY

AHMEDABAD-382481

DECEMBER-2013

I

CERTIFICATE

This is to certify that the Seminar Report entitled “ADVANCES IN SOLAR AIR

HEATER” submitted by HARDIK V. RAMANI (13MMET16), towards the partial

fulfilment of the requirements for the degree of Master of Technology in Thermal

Engineering, Institute of Technology, Nirma University of Science and Technology,

Ahmedabad is the record of work carried out by him under my supervision and guidance. In

my opinion, the submitted work has reached a level required for being accepted for

examination. The results embodied in this Seminar work, to the best of my knowledge,

haven’t been submitted to any other university or institution for award of any degree.

Prof. S. V. Jain Dr R. N. Patel

Professor at ME Department, Professor and Head of ME Department,

Institute of Technology, Institute of Technology,

Nirma University, Ahmedabad Nirma University, Ahmedabad

II

APPROVAL SHEET

The Seminar entitled Advances in Solar Air Heater by HARDIK V. RAMANI

(13MMET16) is approved for the degree of Master of Technology in Mechanical

Engineering (Thermal Engineering).

Examiners

____________

--------------------

Date: __________

Place: Nirma University

Ahmedabad

III

ACKNOWLEDGEMENT

It gives me great pleasure in expressing thanks and profound gratitude for the dignitaries who

made this seminar work to complete. I would like to give my special thanks to my Guide,

Prof S. V. Jain, Professor, Department of Mechanical Engineering, Institute of Technology,

Nirma University, Ahmedabad for his valuable guidance and continual encouragement

throughout the Seminar work. I heartily thankful to him for his time to time suggestion and

the clarity of the concepts of the topic that helped me a lot during this study. And I am also

Thankful to Prof. N. K. Shah for their valuable guidance and support. I am also thankful to

Dr. R. N. Patel, Head of Department of ME, Institute of Technology, Nirma University,

Ahmedabad, for his continual kind words of encouragement and motivation throughout the

Project. I am also thankful to Dr K Kotecha, Director, Institute of Technology for his kind

support in all respect during my study.

HARDIK V. RAMANI (ROLL NO.13MMET16)

IV

ABSTRACT

Given that the future of our planet is intricately entwined with the future choices of energy,

effective exploitation of non-conventional energy sources is becoming increasingly essential

for modern world as fossil fuels are hazardous to environment and cannot sustain supply for

long time as they are not renewable. Moreover, demand of energy is increasing rapidly. In

this scenario, solar energy is being seen as potential viable resource for ever increasing

hunger of the energy for the development of nation and by and large globe. In this seminar

work, effort has been made to demonstrate this reality with proof of statistics from reliable

sources. Furthermore numerous new designs of Solar Air Heater are emerging in various

aspects, in different number of roughness, in different cost. Extensive review of research

done in this field in recent past is covered with their design characteristics and their

suitability for specific conditions and applications with respect to their merits and demerits.

V

CONTENTS

CONTENT PAGE NO.

CERTIFICATE .........................................................................................................................II

APPRROVAL SHEET.............................................................................................................III

ACKNOWLEDGEMENT……..…………………………………………….………………IV

ABSTRACT ………………………………………………………………………………….V

CONTENTS………………………………………………………………………………….VI

LIST OF FIGURES...............................................................................................................VIII

LIST OF TABLES....................................................................................................................X

NOMENCLATURE ...............................................................................................................XI

CHAPTER 1: INTRODUCTION...........................................................................................1

1.1. Overview Of Solar Energy..............................................................................................1

1.2. Advantages......................................................................................................................1

1.3. Disadvantages.................................................................................................................1

1.4. Solar Air Heating............................................................................................................2

1.5. Application .....................................................................................................................2

1.5.1 Heating......................................................................................................................3

1.5.2 Cooling......................................................................................................................3

1.5.3 Ventilation & Moisture Control................................................................................3

1.5.4 Filtration....................................................................................................................4

1.6.Saving with Solar Air Heater...........................................................................................4

CHAPTER 2: LITERATURE REVIEW................................................................................5

2.1 Introduction .....................................................................................................................5

2.2 Principle...........................................................................................................................5

2.3 Types of Solar Air Heater................................................................................................6

2.2.1 Porous .......................................................................................................................6

2.2.2 Non-Porous...............................................................................................................7

CHAPER 3: LOW COST SOLAR HEATER.........................................................................8

3.1 Introduction. ....................................................................................................................8

3.2. Experimention ................................................................................................................8

3.2.1. General.....................................................................................................................8

VI

3.2.2 Test at No-Load ........................................................................................................9

3.2.3 Test with Load...........................................................................................................9

3.3 Result and Discussion ..................................................................................................10

3.3.1 Test at No - Load....................................................................................................10

3.3.2 Teat with Load........................................................................................................12

3.3.2.1 During Summer Season .................................................................................12

3.3.2.1(A) Temperatura Rise.............................................................................12

3.3.2.1(B) Thermal Efficiency..........................................................................13

3.3.2.1(C) Instantaneous Thermal Efficiency...................................................15

3.3.2.2 During Winter Season ...................................................................................17

3.3.2.2(A) Temperatura Rise.............................................................................17

3.3.2.2(B) Thermal Efficiency..........................................................................18

3.3.2.2(C) Instantaneous Thermal Efficiency...................................................20

3.3.3 Comparision ..........................................................................................................22

3.4 Conclusion .....................................................................................................................22

CHAPTER 4:EFFICIENCY IMPROVEMENT BY ROUGHNESS..................................25

4.1 General...........................................................................................................................25

4.2 Effective technique to enhance rate of heat transfer......................................................25

4.3 Calculation.....................................................................................................................26

4.4 Experiment ....................................................................................................................28

4.5 Experimentl Result.........................................................................................................28

SUMMARY.............................................................................................................................30

REFERENCES.........................................................................................................................31

VII

LIST OF FIGURES

LIST OF FIGURES PAGE NO

Fig. 1.1 Power of Sun 1

Fig. 1.2 Basic of Solar Air Heater 2

Fig. 1.3 Application 3

Fig. 2.1 Simple Solar Air Heater 5

Fig. 2.2 Principle 6

Fig. 2.3 Porous 6

Fig. 2.4 Non-Porous 7

Fig. 3.1 single glazed solar air heater 8

Fig. 3.2 double glazed solar air heater 8

Fig.3.3 (a) experimental set-up 10

Fig.3.3(b) schematic of experimental set-up 10

Fig.3.4 (a) Variation of ambient temperature, solar radiation intensity and Stagnation temperature during the day at no load in single glazing solar air heater

11

Fig.3.4 (b) Variation of ambient temperature, solar radiation intensity and Stagnation temperature during the day at no load in double glazing solar air heater

11

Fig.3.4 (a) Variation of ambient temperature, solar radiation intensity and Stagnation temperature during the day at no load in packed bed solar air heater

11

Fig 3.5(a) Rise in temperature of air during the day for summerSeason at different air flow rates in single glazing solar air heater

12

Fig 3.5(b) Rise in temperature of air during the day for summerSeason at different air flow rates in double glazing solar air heater

13

Fig 3.5(c) Rise in temperature of air during the day for summerSeason at different air flow rates in packed bed solar air heater

13

Fig 3.6(a) Thermal Efficiency for single glazing solar air heater during the day for summer Season at different air flow rates

14

Fig 3.6(b) Thermal Efficiency for double glazing solar air heater during the day for summer Season at different air flow rates

14

Fig 3.6(c) Thermal Efficiency for packed bed solar air heater during the day for summer Season at different air flow rates

14

Fig 3.7 (a) Instantaneous Thermal Efficiency versus (To-Ta)/I for 16

VIII

single glazed solar air heater at different air flow rates for summer seasonFig 3.7 (b) Instantaneous Thermal Efficiency versus (To-Ta)/I for double glazed solar air heater at different air flow rates for summer season

16

Fig 3.7 (c) Instantaneous Thermal Efficiency versus (To-Ta)/I for packed bed solar air heater at different air flow rates for summer season

16

Fig.3.8 (a) Rise in temperature of air during the day for winterseason at different air flow rates in single glazed solar air heater

17

Fig.3.8 (b) Rise in temperature of air during the day for winterseason at different air flow rates in double glazed solar air heater

18

Fig.3.8 (c) Rise in temperature of air during the day for winterseason at different air flow rates in packed bed solar air heater

18

Fig.3.9 (a) Thermal Efficiency for single glazing solar air heater during the day for winter season at different air flow rates

19

Fig.3.9 (b) Thermal Efficiency for double glazing solar air heater during the day for winter season at different air flow rates

19

Fig.3.9 (c) Thermal Efficiency for packed bed solar air heater during the day for winter season at different air flow rates

19

Fig. 3.10 (a) Instantaneous Thermal Efficiency versus (To-Ta)/I for single glazed solar air heater at different air flow rates for winter season

20

Fig. 3.10 (b) Instantaneous Thermal Efficiency versus (To-Ta)/I for double glazed solar air heater at different air flow rates for winter season

21

Fig. 3.10 (c) Instantaneous Thermal Efficiency versus (To-Ta)/I for packed bed solar air heater at different air flow rates for winter season

21

Fig.3.11 Variation of ambient temperature, solar radiation intensity on aperture and outlet temperature in all three solar air heater for summer season

22

Fig.3.12 Variation of ambient temperature, solar radiation intensity on aperture and outlet temperature in all three solar air heater for winter season

22

Fig 4.1 Absorber Plate Shapes 25

Fig.4.2 Energy Balance 26

Fig.4.3 Reynolds numbers vs Nusselt number 28

Fig.4.4 Reynolds numbers vs Friction factor 29

Fig.4.5 Reynolds numbers vs Thermo hydraulic performance 29

LIST OF TABLE

IX

LIST OF TABLE PAGE NO

Table 1 Uncertainty in measurement of various equipment used during experimentation.

9

Table 2 The average efficiency of each solar air heater during period 11:00–13:00 in summer and winter seasons for each flow rate.

15

Table 3 Heat removal factor based on air outlet temperature (Fo), heat removal factor based on air inlet temperature (FR) and collector efficiency factor (F0) for the three solar air heaters in summer and winter season.

17

Table 4 Details of the cost (US$/m2) (material and fabrication cost) of single glazed, double-glazed and packed bed solar air heaters.

23

Table 5 The Energy gain kJ/US$ of each solar air heater during period 11:00–13:00 in summer and winter seasons for each flow rate.

23

Table 6 Observation Table 28

NOMENCLATURE

Lower Cost Solar Air Heater

X

A aperture area of solar air heater (m2)

Cp specific heat of air (J/kg-_C)

Cd empirical-discharge coefficient (dimensionless)

d orifice diameter (m)

I solar radiation intensity (W/m2)

m mass flow rate of air (kg/s)

P1 upstream pressure of orifice plate (mm of water)

P2 downstream pressure of orifice plate (mm of water)

Ts stagnation temperature (0C)

Ta ambient temperature (0C)

To temperature of air at the outlet of solar air heater (0C)

Ti temperature of air at the inlet of solar air heater (0C)

UL overall heat loss coefficient (W/m2 K)

Gth thermal efficiency of the solar air heater (%)

FR collector heat removal factor depending on air inlet temperature (dimensionless)

Fo collector heat removal factor depending on air outlet temperature (dimensionless)

s transmissivity of glazing (dimensionless)

a absorptivity (dimensionless)

Wxi uncertainty in measurement of independent variable xi

WR uncertainty in result R

XI

CHAPTER 1

Introduction

1.1 Overview Of Solar EnergySolar power is energy from the sun and without its presence all life on earth would end. Solar energy has been looked upon as a serious source of energy for many years because of the vast amounts of energy that are made freely available, if harnessed by modern technology. As shown in fig1.1,simple example of the power of the sun can be seen by using a magnifying glass to focus the sun’s rays on a piece of paper. Before long the paper ignites into flames.

Fig.1.1 Power Of The Sun[1]

The simplest and the most efficient way to utilize solar energy are to convert it into thermal energy for heating applications. The most important and basic components of the system required for conversion of solar energy into thermal energy are called solar collector.

1.2 Advantages[1]1. Solar energy is free although there is a cost in the building of ‘collectors’ and other equipment required to convert solar energy into electricity or hot water.2. Solar energy does not cause pollution. However, solar collectors and other associated equipment / machines are manufactured in factories that in turn cause some pollution.3. Solar energy can be used in remote areas where it is too expensive to extend the electricity power grid.4. Many everyday items such as calculators and other low power consuming devices can be powered by solar energy effectively.5. It is estimated that the world’s oil reserves will last for 30 to 40 years. On the other hand, solar energy is infinite (forever).

1.3 Disadvantages[1]

1. Solar energy can only be harnessed when it is daytime and sunny.2. Solar collectors, panels and cells are relatively expensive to manufacture although prices are falling rapidly.

1

3. Solar power stations can be built but they do not match the power output of similar sized conventional power stations. They are also very expensive.4. In countries such as the UK, the unreliable climate means that solar energy is also unreliable as a source of energy. Cloudy skies reduce its effectiveness.5. Large areas of land are required to capture the suns energy. Collectors are usually arranged together especially when electricity is to be produced and used in the same location.6. Solar power is used to charge batteries so that solar powered devices can be used at night. However, the batteries are large and heavy and need storage space. They also need replacing from time to time.

1.4 Solar Air Heating[1,7]As the name suggests, Solar Air Heating is the conversion of solar radiation to thermal heat. The thermal heat is absorbed and carried by air which is delivered to a living or working space. The transparent property of air means that it does not directly absorb effective amounts of solar radiation, so an intermediate process is required to make this energy transfer possible and deliver the heated air into a living space. The technologies designed to facilitate this process are known as Solar Air Heaters.

Solar Air Heaters operate on some of the most fundamental and simple thermodynamic principles:

Fig.1.2 Basic of Solar Air Heater

Absorption of the solar radiation by a solid body results in the body heating up. In broad terms this solid body is known as the 'collector'. Some bodies are better at absorption than others, such as those with black non-reflective surfaces.

Convection of heat from the heated solid body to the air as it passes over the surface. Typically a fan is used to force the air across the heated body, the fan can be solar powered or mains powered.

Different types of Solar Air Heater Technology achieve this process using the same basic principles but through the use of different solid bodies acting as the collector. The fan that transfers the air across the heated surface is also used as part of a ducting system to direct the heated air into the dwelling space. In addition to heating the air within that space, the heat can further be absorbed by thermal mass such as walls, flooring, furniture and other contents. Such heat is effectively 'stored' and slowly dissipates beyond sunlight hours.

1.5 Applications [1,7]

The benefits of Solar Thermal Air are often more than just heating.

2

Fig.1.3 Application

1.5.1 Heating

The primary function of a solar air heating system is to provide home heating.   The heat generated is extremely energy efficient and as a result there are several significant benefits:

a higher level of comfort lower power bills lower carbon emissions

Space heating accounts for 38% of residential energy usage in Australia, with some states such as Victoria as high as 55% (hot water accounts for 23%).  Although results vary, case studies have shown some homes to reduce the heating component of their annual energy bills by around 50%, resulting in a total household reduction of around 20% per annum.  The amount of savings achieved depends on several factors:

The size and type of Solar Air Heating System installed. The size and type of existing conventional heater. The habits of the occupants (e.g. thermostat setting). The thermal properties of the dwelling (e.g. insulation, draft sealing, size). The local climate.

1.5.2 Cooling

Many solar air heating systems can also be used to help cool homes by:

Transferring cool outside air into the home, especially after sunrise during summer; and/or

Expelling hot air out of the roof cavity to reduce the transfer of heat from ceiling to inside air.

The cooling effect is often likened to an evening breeze bringing in cool air after sunset.  The same fan that is used to transfer warm air into the home for heating can be used to transfer air via one of the methods above.  The same ducting and thermostat control used to control the heating system can also be utilized, often with no or very little additional hardware.

1.5.3 Ventilation & Moisture Control

Modern Australian homes are designed and built to seal tight to minimize the amount of heat loss (or heat gain in summer) for the purpose of energy efficiency.  While this helps with

3

heating and cooling bills, it can also introduce humidity related problems as it restricts the amount of fresh air entering the dwelling.  Moisture from showers, cooking and even breathing can become trapped and lead to condensation on internal surfaces and mound.

1.5.4 Filtration

Many Solar Thermal Air systems incorporate a high grade air filter to ensure that not only is the incoming air fresh, but well filtered from dust, pollen and larger particles from wood fires and transport emissions.  The majority of the air enters the house in this controlled manner, rather than entering randomly through windows and doors.

1.6 Saving With Solar Air Heater[7]

Case studies and university investigations have shown installations to reduce the reliance on conventional heating & cooling systems by over 50%.  Naturally results do vary depending on the local climate, size & type of system installed, size & type of existing heating/cooling, size of dwelling and the thermal properties of the house (insulation, draft protection).

4

CHAPTER 2

Literature review

2.1 Introduction

Solar energy striking the collector(s) passes through the high-transmittance solar glazing and heats the highly efficient absorber plate. When there is heat available in the collector(s) and the building requires heat, simple controls automatically activate the fan. The fan moves air through the collector(s), where it is heated, and then redistributed to the building using conventional, off-the-shelf, HVAC ducting and air handling equipment.

Fig.2.1 Simple Solar Air Heater[7]

Check valves prevent reverse thermo-siphoning and uncalled-for heat, so you get clean, free, solar heat only when you want. Solar air heat is often the most efficient and cost competitive solar technology available in colder climates, saving clients many thousands of dollars and eliminating toxic emissions.

2.2 Principle[5]

A conventional solar air heater is essentially a flat plate collector with an absorber plate. It is a transparent cover system at the top and insulation at the bottom and on the sides. The whole assembly is enclosed in a sheet metal container.

As shown in fig.2.2 cold air of inside home is drawn by fan in to duct which is covered by solar collector. The heat absorbing by solar collector is transfer to the air in duct and so air

5

become warm. Warm air has low density as compare to cold air so it flow to upward and thus natural convection current is being set. Air has low co-efficient of heat convection so for increasing velocity blower is used.

Fig.2.2 Principle[7]

2.3 Types of Solar Air Heater

2.3.1.Porous Type

Fig.2.3 Porous [5]

6

It has porous absorber which may include slit and expanded metal, overlapped glass plate

absorber.

2.3.2 Non-Porous Type

Fig.2.4 Non-Porous [5]

In this type air does not passing through below the absorber plate but air may flow above the plate.

7

CHAPTER 3Low Cost Solar Air Heater

3.1 IntroductionTwo low cost solar air heaters viz. single glazed and double glazed were designed, fabricated and tested. Thermo Cole, ultraviolet stabilised plastic sheet, etc. were used for fabrication to reduce the fabrication cost. These were tested simultaneously at no load and with load both in summer and winter seasons along with packed bed solar air heater using iron chips for absorption of radiation. The initial costs of single glazed and double glazed are 22.8% and 26.8% of the initial cost of packed bed solar air heater of the same aperture area. It was found that on a given day at no load, the maximum stagnation temperatures of single glazed and double glazed solar air heater were 43.5 C and 62.5 C respectively. The efficiencies ofSingle glazed, double glazed and packed bed solar air heaters corresponding to flow rate of 0.02 m3/ s-m2 were 30.29%, 45.05% and 71.68% respectively in winter season. The collector efficiency factor, heat removal factor based on air outlet temperature and air inlet temperature for three solar air heaters were also determined.

3.2. Experimentation3.2.1 GeneralAn experimental set up was constructed and tested in Punjab Agricultural University, Ludhiana (310 N), India. The experiments were conducted at no load and with load in both summer and winter seasons to do the thermal analysis and to compare the performanceof these three solar air heaters.

Fig. 3.1 single glazed solar air heater.[6]

Fig. 3.2 double glazed solar air heater.[6]

8

3.2.2 Test at no loadThe test set-up comprises of single glazed solar air heater, double glazed solar air heater and conventional packed bed solar air heater. Solar irradiation on the aperture of the solar air heaters, ambient temperature and stagnation temperature in the solar air heaters was recorded after every half an hour. The values of overall heat loss coefficient was calculated every half an hour from 11:00 to 14:00 by using the following equation:I(ατ) = UL(Ts-Ta)The transmissivity of glazing of single glazed, double glazed and packed bed solar air heater was 0.86,

3.3.3 Test with loadA photograph and schematic diagram of experimental set up are shown in Fig.3.3 (a) and (b) respectively. The test set-up comprises of single glazed solar air heater, double glazed solar air heater, conventional packed bed solar air heater, electric blower, orifice meters, gate valves and pipe network. To control and measure the flow rate of air in each solar air heater, valve and orifice meter are installed after each solar air heater. Electric blower is used to pass the air in solar air heaters.

Tests were conducted during summer and winter under normal weather conditions. Ambient temperature and temperatures at outlet of each solar air heaters were recorded for each flow rate. Ambient air was sucked into the solar air heaters, so the inlet air temperature was taken equal to ambient temperature. Solar radiation intensity on the aperture was recorded with the Silicon pyranometer by placing it adjacent to the glazing cover of solar air heater, in the same plane i.e. facing due south.

All these observations were recorded every half an hour during the day for each flow rate. All the tests started at 10:30 and ended at 14:30. Observations were started after 30 min when solar air heater reached steady state condition. Wind speed data for the experimental days was recorded from the Department of Agricultural Meteorology, Punjab Agricultural University, Ludhiana, India. The instantaneous thermal efficiency of solar air heaters was calculated from the experimental data for each flow rate from the daytime measurements between 11:00 and 13:00, using following relation:

Solar air heater performance parameters viz. heat removal factor depending on air outlet temperature (Fo), heat removal factor depending on air inlet temperature (FR) and collector efficiency factor (F0) were also determined. These parameters depend on construction materials, flow conditions and design type of the collector. The instantaneous thermal efficiency of a solar air heater, in which useful energy is expressed in the form of the energy gain by the absorber and energy lost from the absorber, can be expressed as

9

Fig.3.3 (a) experimental set-up [2]

Fig.3.3 (b) schematic of experimental set-up [2]

3.3 Results And Discussion

3.3.1. Test At No LoadFig.3.4 shows the variation in ambient temperature, solar radiation intensity and stagnation temperature in the single glazed, double glazed and packed bed solar air heaters, respectively, during the day.

10

Fig.3.4 (a) Variation of ambient temperature, solar radiation intensity on aperture andStagnation temperature during the day at no load in single glazing solar air heater[2]

Fig.3.4(b) Variation of ambient temperature, solar radiation intensity on aperture andStagnation temperature during the day at no load in double glazing solar air heater[2]

Fig.3.4 (c) Variation of ambient temperature, solar radiation intensity on aperture and Stagnation temperature during the day at no load in packed bed solar air heater[2]

11

The maximum stagnation temperature achieved for single glazed, double glazed and packed bed solar air heaters was 43.50C, 62.50C and 85.70C respectively. The corresponding values of solar radiation and ambient temperature were 740 W/m2 and 19 0C respectively for single glazed and double glazed solar air heaters and 560 W/m2 and 35 0C respectively for packed bed solar air heater.

The overall heat loss coefficients of solar air heaters based on aperture area were calculated by putting the experimental data given in Fig.3.5 into Eq. (1). The values of overall heat loss coefficient varied from 23.38 to 28.75 W/m2*K for single glazed,11.23 - 14.68 W/m2*K for double glazed solar air heater and 7.55-9.60 W/m2*K for packed bed solar air heater. Therefore the average overall heat loss coefficient of single glazed solar air heater, double glazed solar air heater and packed bed solar air heater is 25.66 W/m2*K,12.33 W/m2*K and 7.97 W/m2*K respectively.

3.3.2 Test with Load

3.3.2.1 Results of Testing During Summer Season

3.3.2.1(A) Rise in TemperatureThe curves for rise in air temperature from inlet to outlet of each solar air heater for each flow rate are plotted in Fig.3.5. The maximum rise in air temperature in single glazed solar air heater and double glazed solar air heater was found to be 18 0C and 12 0C, respectively, for flow rate of 0.020 m3/s per m2 aperture area while maximum rise in temperature in packed bed solar air heater was 35 0C, for flow rate of 0.011 m3/s per m2 aperture area.

Fig 3.5(a) Rise in temperature of air during the day for summerSeason at different air flow rates in single glazing solar air heater[2]

12

Fig 3.5 (b) Rise in temperature of air during the day for summerSeason at different air flow rates in double glazing solar air heater[2]

Fig 3.5(c) Rise in temperature of air during the day for summerSeason at different air flow rates in packed bed solar air heater[2]

It was observed that as the air flow rate increased the rise in air temperature from inlet to outlet increased in case of single and double glazed solar air heaters whereas it decreased in case of packed bed solar air heaters. This is so because in flat plate absorber, convective heat transfer coefficient increases with increase in flow rate thereby increasing the heat gain of the air. While in case of packed bed absorber turbulence is created even at low air flow velocity resulting in increased value of convective heat transfer coefficient. So increase in velocity does not result in increase in convective heat transfer coefficient. Hence heat gain does not increase and temperature of outlet air decreases.

3.3.2.1(B) Thermal Efficiency The thermal efficiency of the each solar air heater during day for each flow rate is shown in Fig. 3.6 The maximum efficiency of single glazed, double glazed and packed bed solar air heater was 37.45%, 24.07% and 66.23%, respectively, for the flow rate of 0.020 m3/s per m2

aperture area.

An increase in efficiency with increase in flow rate was observed in all solar air heaters because of changes in flow conditions [8,9]. Table 2 shows the average efficiency of each air heater during the period 11:00–13:00 for each flow rate. The maximum average efficiency was observed in packed bed solar air heater for each flow rate because of better heat transfer and lesser thermal losses [9,10].

13

Fig 3.6(a) Thermal Efficiency for three solar air heaters during the day for summerSeason at different air flow rates in single glazing solar air heater[2]

Fig 3.6 (b) Thermal Efficiency for three solar air heaters during the day for summerSeason at different air flow rates in single glazing solar air heater[2]

Fig 3.6(c) Thermal Efficiency for three solar air heaters during the day for summerSeason at different air flow rates in packed bed solar air heater[2]

14

During period 11:00–13:00, the maximum average efficiency in packed bed solar air heater was 50.30, 56.20, 64.56 and 66.23 for the flow rate of 0.011, 0.014, 0.017 and 0.020 m 3/s per m2 aperture area respectively.

It is observed that for all flow rates, the efficiency of single glazed solar air heater is more than double glazed solar air heater as single glazing allows more radiation to pass than double glazing and due to higher temperature in summer there is not much reduction in heat loss. The average thermal efficiency of single glazed solar air heater for 17.5 0C temperature rise was found to be 37.5%, and it was 24.0% for 11 0C temperature rise in double glazed solar air heater.

Whereas for previous designs of low cost solar air heaters i.e. black porous textile absorber solar air heater and plastic wrapping film with air bubbles solar air heater, the thermal efficiency was 18% and 12.5%, respectively for air temperature rise of 10 0C. This indicates that with the present low cost solar air heater higher efficiency can be achieved even for higher air temperature rise.

3.3.2.1(C) Instantaneous Thermal Efficiency

The instantaneous thermal efficiency at noon as a function of temperature parameter (To- Ta)/I for the three solar air heaters at different flow rates are shown in Fig.3.7. The empirical relations and regression coefficient of the best fit line are also shown in Fig.3.7. The scatter of the data around the straight line is mainly attributed to wind speed and the dependence of the heat loss on the data are to be expected [8,9]. It can be seen from Fig.3.7 that the thermal efficiency increases with increase in air mass flow rate as was discussed earlier in Fig.3.7 [8,9]. It can also be seen that the thermal efficiency decreases with increase in temperature parameter (To-Ta)/I. This is because increase in temperature parameter causes increase in absorber temperature that causes increase in heat losses hence decrease in efficiency

15

Fig 3.7 (a) Plot of instantaneous thermal efficiency versus (To-Ta)/I for single glazed solar air heater at different air flow rates for summer season[2]

Fig 3.7 (b) Plot of instantaneous thermal efficiency versus (To-Ta)/I for double glazed solar air heater at different air flow rates for summer season[2]

Fig 3.7 (c) Plot of instantaneous thermal efficiency versus (To-Ta)/I for Packed bed solar air heater at different air flow rates for summer season[2]

16

The heat removal factor based on air outlet temperature (Fo), heat removal factor based on air inlet temperature (FR) and collector efficiency factor (F0) computed from Fig.3.7 respectively for each solar air heater are given in Table 3.

The value of FR, FO and F0 for packed bed solar air heater was the highest followed by single glazed solar air heater and double glazed solar air heater. These results shows that the packed bed is most efficient solar air heater due to higher heat removal factor mainly because of better heat transfer between air and packing material (iron chips) in the packed bed solar air heater, which eventually reduced heat losses [8].

3.3.2.2. Results of testing during winter season3.3.2.2(A) Rise in TemperatureThe curves for rise in air temperature from inlet to outlet of each solar air heater for each flow rate are shown in Fig.3.8. The maximum rise in air temperature in single glazed, double glazed and packed bed solar air heater was observed to be 19.5 0C, 33.5 0C and 50.5 0C respectively at flow rate of 0.011 m3/s per m2 aperture area.

Fig3.8 (a) Rise in temperature of air during the day for summerseason at different air flow rates in single glazed solar air heater[2]

17

Fig.3.8 (b) Rise in temperature of air during the day for winterseason at different air flow rates in double glazed solar air heater[2]

Fig3.8 (c) Rise in temperature of air during the day for winterseason at different air flow rates in packed bed solar air heater[2]

It was observed that as the air flow rate decreased the rise in air temperature in single glazed, double glazed and packed bed solar air heaters increased.

3.2.1.2(B) Thermal Efficiency The thermal efficiency of the each solar air heater during day for each flow rate is shown in Fig.3.9. The maximum average efficiency of single glazed, double glazed and packed bed solar air heater was 30.29, 45.05 and 71.68%, respectively, for the flow rate of 0.020 m 3/s per m2 aperture area. An increase in efficiency with increase in flow rate was observed in all solar air heaters due to change in flow conditions [8,9].

18

Fig.3.9 (a) Thermal Efficiency for single glazing solar air heater during the day for winterseason at different air flow rates[2]

Fig.3.9 (b) Thermal Efficiency for double glazing solar air heater during the day for winterseason at different air flow rates[2]

Fig.3.9 (c) Thermal Efficiency for packed bed solar air heater during the day for winter

19

season at different air flow rates[2]

Table 2 shows the average efficiency of each solar air heater during the period 11:00–13:00 for each flow rate. The maximum average efficiency was observed in packed bed solar air heater for each flow rate because of better heat transfer and lesser thermal losses [8,10]. During period 11:00–13:00, the maximum average efficiency in packed bed solar air heater was 60.46%, 62.02%, 66.21% and 71.68% for the flow rate of 0.011, 0.014, 0.017 and 0.020 m3/s per m2 aperture area respectively. It is observed that during winter for all flow rates, the efficiency of double glazed solar air heater is more than single glazed solar air heater because reduction in input is less than saving in heat loss due to lower ambient temperature in winter.

There is change in trend of air temperature rise in single and double glazed solar air heaters during summer and winter season. This is so because due to increase in air flow rates the value of convective heat transfer coefficient from absorber increases resulting in more heat gain by air irrespective of season. Simultaneously, the hot air flowing in the air heater loses heat to the ambient. This heat loss is more in winter than in summer. Hence during summer heat gain due to higher convective heat transfer coefficient is predominant while during winter heat loss from hot air to ambient becomes predominant.

The thermal efficiency was found to be 30.9% for 12.5 0C temperature rise for single glazed solar air heater. Whereas for double glazed solar air heater the thermal efficiency was 45.1% for 18 0C temperature rise. These efficiency values are higher than the earlier low cost solar air heaters [11].

3.3.2.2(C) Instantaneous Thermal EfficiencyIn Fig.3.10 the plots of instantaneous thermal efficiency at noon as a function of temperature parameter (To-Ta)/I for the three solar air heaters at different flow rates are shown. The trends of variation in thermal efficiency with temperature parameter and flow rate were found to be similar to that of summer season as seen in Fig.3.6

Fig. 3.10 (a) Plot of instantaneous thermal efficiency versus (To-Ta)/I for single glazed solar air heater at different air flow rates for winter season[2]

20

Fig. 3.10 (b) Plot of instantaneous thermal efficiency versus (To-Ta)/I for double glazed solar air heater at different air flow rates for winter season[2]

Fig. 3.10 (c) Plot of instantaneous thermal efficiency versus (To-Ta)/I for Packed bed solar air heater at different air flow rates for winter season[2]

For winter season, the heat removal factor based on air outlet temperature (Fo), heat removal factor based on air inlet temperature (FR) and collector efficiency factor (F0) for each solar air heater are given in Table 3. The values of FR, FO and F0 for packed bed solar air heater were highest followed by double glazed solar air heater and single glazed solar air heater.

The variation of ambient temperature, solar radiation intensity on aperture and outlet temperature in single glazed solar air heater, double glazed solar air heater and packed bed solar air heater for a typical day of summer and winter season at flow rate of 0.011 m 3/s per m3 aperture area are shown in Figs.3.11 and 3.12 respectively.

21

Fig.3.11 Variation of ambient temperature, solar radiation intensity on aperture and outlet temperature in all three solar air heater for day of summer season at flow rates of 0.011 m3/s per m2 aperture area.[2]

Fig.3.12 Variation of ambient temperature, solar radiation intensity on aperture and outlet temperature in all three solar air heater for a typical day of winter season at flow rates of 0.011 m3/s per m2 aperture area.[2]

3.3.3 Comparison of solar air heaters based on energy per unit cost

To compare the thermal performance of solar air heaters, their average thermal efficiencies between 11:00 and 13:00 were calculated and compared in summer and winter seasons for each flow rate. The ratio of average thermal efficiency of single glazed solar air heater to packed bed solar air heater and double glazed solar air heater to packed bed solar air heater during period 11:00–13:00 in summer and winter seasons for each flow rate are shown in Table 2. The ratio of average thermal efficiency remains almost constant at all the flow rates for both the solar air heaters. The ratio of average thermal efficiency of single glazed solar air heater to packed bed solar air heater for summer and winter seasons are 0.5 and 0.4

22

respectively, and the ratio of average thermal efficiency of double glazed solar air heater to packed bed solar air heater for summer and winter seasons are 0.33 and 0.65 respectively.

The cost of single glazed solar air heater, double glazed solar air heater and packed bed solar air heater is estimated. The bill of material of these solar air heaters are given in Table 4. The capital cost of single glazed, double glazed and packed bed solar air heater turns out to be 27.34, 32.11 and 120.00 respectively.

The energy gain kJ per US$ of each solar air heater during period 11:00–13:00 in summer and winter seasons for each flow rate are given in Table 5. Energy gain kJ/US$ for both single glazed solar air heater and double glazed solar air heater is more than packed bed solar air heater.

Thus, for the same money spent, low cost solar air heaters collect more energy than packed bed solar air heater. This shows that one can install large area of single/double glazed solar

air heaters to get the same energy output as of packed bed solar air heater at a lesser cost than packed bed solar air heater. Other advantage of low cost solar air heater is that single person

can easily carry these due to its light weight and these can be stored indoor during off-season.

3.4 Conclusions[2]Single glazed low cost solar air heater gives better thermal efficiency during summer while double glazing is better during winter for all flow rates. For flow rate of 0.020 m3/s per m2

aperture area, the maximum average thermal efficiency was 37.45% for single glazed and 24.07% for double glazed solar air heater during summer. Corresponding figures for winter were 30.29% and 45.05% respectively.

23

For flow rate of 0.020 m3/s per m2 aperture area, the maximum rise in air temperature was 180C for single glazed and 12 0C for double glazed solar air heater during summer. Corresponding figures for winter were 19.50C and 33.50C respectively.

The heat removal factor based on air outlet temperature (Fo), heat removal factor based on air inlet temperature (FR) and collector efficiency factor (F0) were found to be higher for packed bed solar air heater as compared to single and double glazed solar air heaters for summer as well as winter season. These factors for single glazed solar air heater were more during summer, whereas in winter values of double glazed solar air heater were more.

For the same initial investment, low cost solar air heaters collect more energy than packed bed solar air heater. For flow rate of 0.020 m3/s per m2 aperture area, the solar energy gain per unit investment was 0.13 kJ per US$ for single glazed, 0.10 kJ per US$ for double glazed and 0.03 kJ per US$ for packed bed solar air heater during summer. Corresponding figures for winter were 0.08 kJ per US$, 0.07 kJ per US$ and 0.02 kJ per US$ respectively.

24

CHAPTER 4

EFFICIENCY IMPROVEMENT BY ARTIFICIAL ROUGHNESS

4.1 GeneralIt is well known, that, the heat transfer coefficient between the absorber plate and working fluid of solar air heater is low. It is attributed to the formation of a very thin boundary layer at the absorber plate surface commonly known as viscous sub-layer The heat transfer coefficient of a solar air heater duct can be increased by providing artificial roughness on the heated wall (i.e. the absorber plate) The use of artificial roughness on the underside of the absorber plate disturbs the viscous sub-layer of the flowing medium. It is well known that in a turbulent flow a sub-layer exists in the flow in addition to the turbulent core. The purpose of the artificial roughness is to make the flow turbulent adjacent to the wall in the sub-layer region. Experiments were performed to collect heat transfer and friction data for forced convection flow of air in solar air heater rectangular duct with one broad wall roughened by discrete v –groove & v- shape ribs. The range of parameters used in this experiment has been decided on the basis of practical considerations of the system and operating conditions. The range of Reynolds number of 3000-14000, Relative Roughness Height ( eh/D ) of height 0.030 to 0.035, Rib angle of attack 600, heat flux 720 W/m2 and pitch of relative roughness pitch 10 the Result has been compared with smooth duct under similar flow and boundary condition It is found from the investigation that on increasing the roughness of a roughened plate the friction factor and heat transfer performance of solar air heater increase and the rate of increase of heat transfer performance of solar air heater get reduced as the roughness of plate increases.

4.2 Effective technique to enhance the rate of heat transfer• The Thermal efficiency of solar air heater has been found to be poor.• The reason behind it is low heat transfer capability between the absorber and air

flowing in the duct.• So, by providing the artificial roughness on the underside of the absorber plate the

heat transfer coefficient

Fig 4.1 Absorber Plate Shapes[3] A conventional solar air heater generally consists of an absorber plate with a parallel plate below forming a passage of high aspect ratio through which the air to be heated flows. As in the case of the liquid flat-plate collector, a transparent cover system is provided above the absorber plate, while a sheet metal container filled with insulation is 'provided on the bottom and sides. The arrangement is sketched in fig. 4.1 Two other arrangement, which are not so common are also shown in fig 4.1 In the arrangement shown in fig 4.1, the air flows between the cover and absorber plate; as well as through the passage below the absorber plate.

25

However, the value of the heat transfer coefficient between the absorber plate and air is low and this result in lower efficiency. For this reason, the surfaces are sometimes roughened or longitudinal fins are provided in the airflow passage. A roughness element has been used to improve the heat transfer coefficient by creating turbulence in the flow. However, it would also result in increase in friction losses and hence greater power requirements for pumping air through the duct. In order to keep the friction losses at a low level, the turbulence must be created only in the region very close to the duct surface, i.e. in laminar sub layer. 4.3 CalculationSolar air heaters, because of their inherent simplicity, are cheap and most widely used as collection device. The thermal efficiency of solar air heaters has been found to be generally poor because of their inherently low heat transfer capability between the absorber plate and air flowing in the duct. In order to make the solar air heaters economically viable, their thermal efficiency needs to be improved by enhancing the heat transfer coefficient. In order to attain higher heat transfer coefficient, the laminar sub-layer formed in the vicinity of the absorber plate is broken and the flow at the heat-transferring surface is made turbulent by introducing artificial roughness on the surface.

Fig.4.2 Energy Balance[4]

The useful heat gain of the air is calculated as: Qu = m’Cp ( Tfo - Tfi) (4.1)The heat transfer coefficient for the test section is: h = Qu/A ( Tpm - Tfm ) (4.2)Where,Tpm is the average value of the heater surface temperatures,Tfm is the average air temperature in the duct = (Tfi + Tf0)/2

The Nusselt number: Nu = h Dh / Kair (4.3)Where,Dh is hydraulic mean diameter of test duct h is convective heat transfer coefficient Kair is thermal conductivity of airThe friction factor was determined from the measured values of pressure drop across the test length: f =( ΔP)Dh/(2ρairLV2

air) (4.4)Where,ΔP is pressure drop in the test duct ρ is density of airL is test duct lengthV air is average velocity of airThermal Performance (overall enhancement ratio) (Nur/Nus)/ (fr/fs) 1/3 (4.5) Mean Air & Plate TemperatureTile mean air temperature or average flow temperature flow is the simple arithmetic mean of the measure values at the inlet and exit of the test section.

26

Thus Tfav = (ti + toav) /2 (4.6)The mean plate temperature, tpav is the weighted average of the reading of 6 points located on the absorber plate.Pressure Drop Calculation Pressure drop measurement across the orifice plate by using the following relationship: Po = ρm x h x 9.81 x 1 (4.7)Where, Po = Pressure diff. ρm = Density of the fluid (kerosene) i.e. 0.8x103

h = Difference of liquid head in U-tube manometer, mMass Flow Measurement Mass flow rate of air has been determined from pressure drop measurement across the orifice plate by using the following relationship: m = Cd x A0 x [2 ρ0 / (1 - 4)] (0.5) (4.8)Where m = Mass flow rate, kg / sec. Cd = Coefficient of discharge of orifice i.e. 0.62 A0 = Area of orifice plate, m2

ρ0 = Density of air in Kg/m3 r = Ratio of dia. (do / dp) i.e. 26.5/53 = 0.5Velocity MeasurementV=m/ρWH Where, m = Mass flow rate, kg / sec3 H = Height of the duct in m W= Width of the duct, m ρ =Density of the air kg / m3

Reynolds Number The Reynolds number for flow of air in the duct is calculated from: Re Where, of air at tfav in m2/sec Dh = 4WH / 2 (W+H) =0.04444Heat Transfer CoefficientHeat transfer rate, Qa to the air is given by: Qa = m cp (t0 – ti) (4.8)The heat transfer coefficient for the heated test section has been calculated from: h = Qa / Ap (tpav – tfav) (4.9)Ap is the heat transfer area assumed to be the corresponding smooth plate area.Nusselt NumberTile Heat Transfer Coefficient has been used to determine the Nusselt number defined as;Nusselt No. (Nu) = h Dh/ KWhere k is the thermal conductivity of the air at the mean air temperature and Dh is the hydraulic diameter based on entire wetted parameter.

Thermo hydraulic performance

Heat transfer and friction characteristic of the roughened duct shows that enhancement in heat transfer is , in general , accompanied with friction power penalty due to a corresponding increase in the friction faceted. Therefore it is essential to determine the geometry that will result in maximum enhancement in heat transfer with minimum friction penalty. In order to

27

achieve this object of simultaneous consideration of thermal as well hydraulic performance, i.e. thermo hydraulic performance, hp = (Nu /Nus) / (fr/fs)1/3 (4.9)

4.4 Experiment A value of this parameter higher then unity ensure the fruitfulness of using an enhancement device and can be used to compare the performance of a number of arrangement to decide the best among these. The value of this parameter for the roughness geometries are investigated.

Table 6 Observation[4]

S. No.

Reynolds no.(Re)

Inlet temperature of air (ti) OC

Average

outlet

Tempera

ture

(toav)

OC

Average air temperature (tfav) OC

Average

plate

temperat

ure (tpav)

OC

Heat transferQ

(W)

Convectiv

e heat

transfer

coffecient

(h)

W/m2-oK

Nusselt

no.

(Nu)

Friction

Factor

(f)

Thermo hydraulic performance

1 5387 34.00 46.00 40.00 72.28 136.8 14.12 22.57 0.032 0.52 7604 33.50 44.00 38.75 70.48 169.0 17.75 28.37 0.0275 0.693 9315 33.00 42.00 37.50 66.00 178.0 20.81 33.23 0.025 0.8534 10788 33.00 41.00 37.00 63.00 182.0 23.33 37.26 0.023 1.05 12051 32.00 39.50 36.00 61.20 191.3 25.26 40.37 0.022 1.16 13211 31.50 39.00 35.2 60.00 209.0 27.80 44.40 0.021 1.07

4.5 Experimental Results

The effect of various flow and roughness parameters on heat transfer characteristics for flow of air in rectangular ducts of different relative roughness height in the present investigation are discussed below. Results have also been compared with those of smooth ducts under similar flow and geometrical conditions to see the enhancement in heat transfer coefficient.

Fig.4.3 Reynolds numbers vs Nusselt number[4]

28

Figure shows the values of Nusselt Number increases with increases in Reynolds Numbers because it is nothing but the ratio of conductive resistance to convective resistance of heat flow and as Reynolds Number increases thickness of boundary layer decreases and hence convective resistance decreases which in turn increase the Nusselt Number.

Fig.4.4 Reynolds numbers vs Friction factor[4]

Figure shows the plots of experimental values of the friction factor as the function of Reynolds number for smooth plate and rough surface. It is clear that Value of friction factor drop proportionally as the Reynolds number increases due to the suppression of viscous sub-layer with increase in Reynolds number.

Fig.4.5 Reynolds numbers vs Thermo hydraulic performance[4]

Figure shows as Reynolds No. increases Thermo hydraulic performance also increases and it is max. for v groove plate and minimum for smooth plate.

29

SUMMERY

For the same initial investment, low cost solar air heaters collect more energy than packed

bed solar air heater.

In the entire range of Reynolds number, it is found that the Nusselt Number increases, attains

a maximum value for v groove roughened plate and increases with increasing roughness

geometry.

On increasing the roughness on the plate the friction factor also increase.

The value of the friction factor reduces sharply at low Reynolds Number and then decrease

very slightly in comparison to low Reynolds Number.

The experimental values of the heat transfer of the v groove Roughness absorber plate has

been compared with smooth plate. The plate having Roughness geometry v groove, gives the

maximum heat transfer

30

REFRENCES

1. The Solar Thermal Air Heating and Cooling Association (STA),

http://solarairheating.org.au/

2. R.S. Gill, Sukhmeet Singh, Parm Pal Singh, Low cost solar air heater, Energy

Conversion and Management,2012

3. M.K. Mittala, Varuna, R.P. Saini, S.K. Singal, Effective efficiency of solar air heaters

having different types of roughness elements on the absorber plate, Elsevier, Energy 32

(2007) 739–745, September 2005

4. Manash Dey Effect of Artificial Roughness on Solar Air Heater: An Experimental

Investigation, Int. Journal of Engineering Research and Application Vol. 3, Issue 5, Sep-

Oct 2013, pp.88-95

5. Sukhatme S.P., "Solar Energy: Principles of Thermal Collections and Storage", Tata

McGraw-Hill, New Delhi 2003.

6. Rai G.D., "Non-Conventional Energy Sources ", Khanna Publishers Delhi,1999

7. RREA Rural Renewable Energy Alliance,www.rreal.org

8. Akpinar Ebru Kavak, Kocyigit Fatih. Energy and exergy analysis of a new flatplate solar

air heater having different obstacles on absorber plates. Appl Energy 2010;87:3438–50.

9. Akpinar Ebru Kavak, Kocyig˘it Fatih. Experimental investigation of thermal performance

of solar air heater having different obstacles on absorber plates. Int Commun Heat Mass

Transfer 2010;37:416–21.

10. Ramadan MRI, El-Sebaii AA, Aboul-Enein S, El-Bialy E. Thermal performance of a

packed bed double-pass solar air heater. Energy 2007;32:1524–35.

11. Bansal NK, Uhlemann R. Development and testing of low cost solar energy collectors for

heating air. Sol Energy 1984;33:197–208

31