Solar Water Pumping[1]

8/3/2019 Solar Water Pumping[1]

1/14

MECH 6009 : Solar Energy Application - Solar Water Pumping Page 1 of 14

SOLAR WATER PUMPING

Mech 6009 Solar Energy Application

Master of Science in Mechanical Engineering

Department of Mechanical Engineering

The University of Hong Kong

Submitted To: Dr. K. Sumathy

Prepared By: Mr. Chan Chun On UID: 2001952494

Mr. Lam Kin Ki UID: 1994595976

Content

1.0 Introduction

2.0 Basic Theory

3.0 Available Technology

4.0 Application

5.0 Summary of Factors Affecting The Performance of Solar Water Pump

6.0 References

Prepared by: Chan Chun On - UID: 2001952494

Lam Kin Ki - UID 1994595976


2/14


1.0 INTRODUCTION

Utilization of solar energy for pumping water can be in general divided into two

categories. The first one is converting solar energy into electric energy by using

photovoltaic, thermoelectric, etc. in which electric induction motor is energized to drivethe conventional pump for pumping water. The second make uses of the solar thermal

energy as the prime mover of the thermodynamic processes of a fluid to achieve

pumping water, is called solar water pump. The fluid undergoes the thermodynamic

process can be either water or other liquid have low boiling point. Since the first

category is only have interest on the electricity generation via solar energy, the rest part

of this report will concentrate on the solar water pump.

2.0 BASIC THEORY

This section discuss the fundamental components and its operating principals that used

as a basis for developing various type of solar water pump through the introduction of

additional component to achieve water pumping. Detailed descriptions of each type of

solar water pump please refer to the next section.

Solar Collector

It is a device to convert solar radiation into thermal energy that absorbed by the working

fluid. It can be a flat plat collector or concentrating collector depending on the system

configuration and required state of working fluid. Normally, the kinetic of the working

fluid relies on the thermosiphon effects induced by temperature and state changes of the

working fluid. But in some circumstances, a circulating pump is used to drive the

working fluid at the liquid state region. Because of the circulating pump requires the

source of energy for its operation, most of the solar water pump is designed to us

thermosiphon as the kinetic to drive the working fluid.

Working Fluid

Throughout the heat is added into the working fluid, the state of working fluid changes

from liquid phase to gas phase or probably a mixture of them. In the flat plate solar

collector system using thermosiphon as the driving force of the working fluid, a mixture

of liquid and gas phases of the working fluid will be produced whilst pure gas phase

can be achieved in the concentrating type collector. Also, the type of working fluid

will depend on the temperature rise through the solar collector and the operating

pressure. The pressure of the working fluid is thus increased and easily explained by

the following Fig. 1 - Temperature-Volume Diagram.




3/14


Figure 2.1 : Temperature Volume Diagram of the Working Fluid.

The analysis of the thermodynamic conversion of energy of working fluid can be

explained by using Rankine Cycle, Brayton Cycle and Stirling Cycle2.

Condenser

Heat of the working fluid is rejected to the thermal sink and thus condensed to liquid

that is used to undergo the next cycle of water pumping action. Normally, water is

used as the thermal sink which is being pumped or discharged through the elevated

reservoir for further usage.

Others

The rest components is make use of the increased pressure of the working fluid through

the solar collector and pump water from the sump to the elevated reservoir, (i.e. Turbine,

Pneumatic Tank, Boiler, Piston Pump, Valves, etc1). Types and numbers of additional

equipment are conducted in the following section that different type of solar water pump

is discussed briefly.




4/14


3.0 Available Technology

3.1 There are two main types of solar thermodynamic pumps - Conventional &

Unconventional systems.

Conventional Solar Pumping Systems

Conventional solar pumping systemsuse solar energy to operate

thermodynamic cycles for (i) a

mechanical power output device to

drive a mechanical pump directly or

via electrical generator, or (ii) a

working fluid pressure differential to

operate a diaphragm pump. Rankine

cycle with low boiling point working

fluid is generally used to drive a

power output expander, e.g. a turbine,

where the total vapour enthalpy is

converted into mechanical energy

output. The working fluid can be

heated directly in a solar collector or

indirectly in a heat exchanger by hot

water/steam produced by a collector.

Arrangements of indirectly heated

primary & secondary working fluid

system are prevailing for better

control. Figure 3.1 shows a typicallarge system.

Fig. 3.1 Schematic of one of theworlds largest conventional solar

water pumping system for irrigation.

A remarkable design of unitary solar

hot engine is shown in Figure 3.2 for

driving small pumps. Air at area A is

heated up by solar radiation and

expands to push piston P down. In

down-stroke of the piston, displacer D

moves to the left. On the up-stroke,

the displacer moves to the right and

heat of the hot air is rejected to the

cooling water at end B. An engine

efficiency of 9% and a power output

of 0.15kW were reported. It was

suggested assemblies of small solar

engines instead of large systems for

the high efficiency.

Fig. 3.2 Schematic Diagram of a

Solar Hot-Air Engine.




5/14


Unconventional Pumps (Specially designed expanders)

Due to high capital cost and maintenance problems for lots of moving parts, and the

number of energy conversion stages involved in conventional pumps, unconventional

pumping are developed for simple and reliable operation. Unconventional systems work

on vapour in, water out principal of vacuum effect due to the volume change when a

working fluid condenses in a tank and create a negative pressure inside to suck water

into the tank. The unconventional systems can be divided into Air and Water

cooled systems.

3.1.1 Air-Cooled Pumps, i.e. working fluid in the pump is cooled by air to achieve the

pumping.

a) Automatic Solar-Powered Savery

Pump - The arrangement is shown in

Figure 3.3. Steam over 100C is

generated at Boiler B and enters tank

D through valve E operated by float F.

As water in tank D gets pumped out

through pipe J, water level descends

and chain hung float G shuts the

steam inlet valve. When steam

condenses, vacuum is created anddraws well water. Required

maintenance is low.

Fig. 3.3 Arrangement of an Automatic

Solar-Power Savery Pump

b) A Practical Design of Air-Cooled

Pump - A typical air-cooled pump

design for practical irrigation lift is

shown in Figure 3.4. It operates with

pentane as the working fluid and

consists of a large closed tank

immersed in the pumping water.

There is no moving part in the pump

except a couple of check valves.

Neither an auxiliary power source nor

high technical skill is required.

Pentane is vaporised in flat-plate

collector under pressure and then is

allowed to pressurise water in the

closed tank effecting water pumping.

Fig. 3.4 Schematic diagram of a Practical

Air-Cooled Pump for Irrigation Lift




6/14


On vapour condensation, partial vacuum is created and water is drawn into the tank. At

night, the vapour condenses through the solar collectors. 120m3

water against a 12m lift

can be pumped per day.

3.1.2 Water-Cooled Pumps, i.e. working fluid in the pump is cooled by water to achieve the

pumping.

a) Maccracken Thermopump - The pump

schematic is shown in Figure 3.5.

Heat is supplied from solar collector

to the generator where the liquid

evaporates, pushes the floater down

and discharges water in the collapsible

rubber lung. When the floater reaches

the bottom of its stroke, vapour

escapes through the vapour tube and

is condensed when it comes on

contact with the liquid, which has

been cooled by the cold water in the

rubber lung. When all the vapour is

condensed, the pressure is decreased,

water is lifted from the reservoir, thefloater moves up to block the vapour

tube entrance, and liquid fills the

cylinder and generator, and the

intermittent pumping action starts

again. Organic working fluid, operates

in the pump with a solar collector,

when it is cooled by the wall, creates

a suction to lift water up form the

supply tank. Maccracken

Thermopump and its variants are only

suitable for shallow water sources but

not lifting water from deep well.

Fig. 3.5 Schematic the Maccracken

Thermopump

b) Bellow Actuated Water pump The advantage of the pump is ease in multi-staging to

develop high pump head. Figure 3.6 shows the arrangement. The system operates with

a close circuit Rankine cycle. The working fluid is separated from water being pumped

to avoid water contamination. Solar heat collected to boil the working fluid in the

collector tube grids and partly in the boiler drum. At the desired pressure at the boiler

drum, the bellow in the bellow chamber is alternatively connected to the vapour

chamber and condenser by a 3-way valve. The bellow expands and contracts in the

confined actuating water chamber, which is initially filled with water. The

pressurisation and rarification of the trapped air in the bellow chamber which, in turn,

acts on the water chamber to effect alternately delivery and suction of water.

Condensate is returned to the vapour chamber by equalizing the pressures in the vapour

chamber and condenser which is water-cooled.

Prepared by: Chan Chun On - UID: 2001952494Lam Kin Ki - UID 1994595976


7/14


Fig. 3.6 Schematic the Bellow

Actuated Water pump

Based on the similar operation, a

simplified system operating with a

diaphragm in lieu of the bellow called

Solar Liquid Piston Pump is designed.

It is also a reliable device but with

relatively low pump head.

c) Brown-Boveri system & the Modified Brown-Boveri System - The Brown-Boveri

System is based on the Bellow Actuated Pumping principle but without diaphragm or bellow. Its schematic arrangement is shown in Figure 3.7. However, water may be

contaminated form the working fluid as there is no separating media in between. It is

not suitable for potable or irrigation uses. A Modified Brown Boveri System, as shown

in Figure 3.8, overcame the disadvantage by adding a few tanks to avoid the direct

contact. The system operation is by thermosiphon with pentane which is heated in the

collector. When pressure in tank S is high enough, vapour in tank S quickly goes into

tank A which contains water. Water in tank A, in turn gets displaced to tank B, which

initially contains air at atmospheric conditions. Water entering vessel B compresses the

air in it to the discharge pressure. This compressed air pushes the water from immersed

vessel C to overhead tank D. Water from overhead tank D flows through the cooling

coils in vessel A on the way to end use. Water flowing through the cooling coil

accelerates the condensation of pentane vapour in vessel A. Because of this condition,

pressure in vessel A decreases. This pressure reduction causes water in vessel B to

return vessel A, thereby bringing the water in vessel C, through 1-way valve 4. The

system is now ready for the next cycle, however, the next cycle cannot be started until

and unless the condensation (i.e. the complete condensation of pentane vapour in A)

and the collector heats the pentane for a longer period than required.




8/14


Fig. 3.7 Schematic of a

Brown-Boveri system.

Fig. 3.8 Schematic of a Modified

Brown-Boveri Solar Water Pumping

System

With the Brown-Boveri System

Modification as a development

platform, many other studies are

flourishing. Researchers also

suggested that insulating the water

tank to minimise the heat losses can

make a difference in system

performance. A scheme proposed by

Kwant et al. is shown in Figure 3.9.

Fig. 3.9 Diagram of the Kwant et al.

Pumping System

d) Sumathy Solar thermal Water Pumping System - A complete analysis of a solar thermal

water pumping system was carried out by Sumathy et al. The system analysed with

pentane as the working fluid is shown in Figure 3.10. The study demonstrated that

inserting a vapour storage tank between the separation tank and water tank to limit the

pentane pressure and temperature can increase pump performance. This is to reduce the

mass flow of pentane per cycle by avoiding sudden gushing of vapour pentane to

entrain liquid pentane into Vessel A. It is crucial that liquid pentane should be drained

from tank A periodically because the accumulated pentane adds to the volume of water

already present in tank A and eventually would flow into tank B at the start of pumping.




9/14


As the volume of the liquid pentane in tank A increases, the initial volume of air in tank

B decreases. Thus it is not possible to compress the air to the required pressure.

Fig. 3.10 Schematic of the Sumathy et

al. solar thermal water pump

3.2 Performances Comparison

3.2.1 Conventional and Unconventional Pumps

Output from conventional solar pumping system is relatively high due to use of large

steam/vapour heat engines. Large solar thermal powered convention turbine pump has

developed power at 37kW and large SOFRETS (proprietary design with turbine

powered electrical generator) system can generate 50kW electrical power. A small hotair engine can develop power output around 0.12kW.

Performances of unconventional solar thermal pumping system are usually expressed in

accumulated capacity of daily water pumped and pump head. Performances up to

120m3

water per day against a head of 12m for an air-cooled pump and 800m3/day

against a head of 9.1m for a water-cooled pump were reported. The daily pumping

capacity of an air-cooled system is limited by the water tank size because the exhaust

vapour needs to be cooled and condensed at night. In lift irrigation for a meaningful

area, the volume of the required water tank will be too large to be economical. Water

cooled pumps have the advantage to overcome the limitation by using water to

condense the spent vapour and are more efficient for quantity of water lift per day.

3.2.2 Ranges of Flow Rate and Pressure Head

a) Flow rate and pump head of conventional solar water pumping systems with

mechanical pumps depend on the power output of the system heat engine and general

characteristics of the mechanical pumps. Under the stable operating range, water flow

rate will decrease as the pump head increases.




10/14


Typical water flows and pump heads of conventional solar thermal pumping systems

are as follows:

Description of Pump/Pumping system Water flow Pump Head

3.68kW solar installation c/w Binary Rankine cycle

with monochlorobenzine vapour turbine to drive amechanical pump

11.3m3/day 45.7m

622m2

parabolic trough collector for a R-113

Rankine cycle and Caloria HT-43 as the primary

working fluid to drive a turbine (36300rpm)

powered pump (1760 rpm)

3744m3/day. 34.0m

6.2m3/day 3.0m1.4m

2flat plate collector, Rankine cycle c/w R113

to connect to a Diaphragm 4.0m3/day 6.0m

SOFRETES, 2499m2 flat plat collector, R-11

vapour turbine powered electrical generator to drive

motorised centrifugal pump

1000m3/day (Developed

electrical

power up to

30kW)

SOFRETS, 330m2 collector area 14.4m3/day 14m

b) For air-cooled unconventional solar water pumps, it has been shown that for the same

effective volume of a tank and the same daily pumping period, the pump head will

increase with the increases of the collector temperature and collector area as per the

energy requirement shown in the Table below:

Energy Requirement of air-cooled pump

Net lift of water, m 9.14 18.29 27.43

Collector temperature, C 62.2 74.4 83.9

Collector area, m2

23.2 37.2 51.1

Energy to preheat pentane and flash tank, MJ 55.8 100.9 136.3

Theoretical energy requirement, MJ 59.9 74.1 94.1

Heat losses to

(a) side wall, MJ 129.9 204.0 206.9

(b) top cover, MJ 16.0 21.0 23.1

(c) to pentane layer on water, MJ 40.4 52.9 58.1

Preheating period 07:00h

10:00h

07:00h

10:00h

07:00h

10:00h

Pumping period 10:00h

15:00h

10:00h

15:00h

10:00h

15:00h

Tank size: diameter 3.05m, height:3.66m, effective volume:24.92m3

Typical water flow and pump head of other air-cooled unconventional pumping systems


Air-cooled, piston hydraulic pump c/w 12m2

collector & methyl chloride as working fluid

1.20m3/day 15m

Air-cooled Solar Water Pump 120 m3/day 12m




11/14


c) For Water-Cooled Unconventional Pumping System, the daily pumping capacity can be

high because of multi-cycle operation during the day. The amount of water pumped in a

day is related to the number of operating cycles per day and the volume of the

immersed water tank. Arrangement of multi-stage water cooled pumps can also be used

to achieve large pump heads.

From the researches of Sumathy et.al on a system with a vapour tank, results showed

that the pump could perform 23 cycles a clear sky day and water lift at 14.6L per cycle

(i.e. daily lift at 336 L) against a head of 6m. The total radiation incident on the

collector was 14.8MJ and the overall efficiency was 0.134%. It took 2 to 3 minutes for

each cycle. The studies revealed the following important relationship of water-cooled

pumps:

i) Relation between Number of cycles per day and discharge head is shown in Figure

3.11.

The number of cycles per day

decreases with increasing discharge

head because the pump requires a

higher starting pressure to pump water

at higher discharge heads, so delaying

the starting time. Hence the operation

period of the pump is reduced. The

number of cycles is also decreased.Furthermore, the time that elapses

between two successive cycles

becomes greater as the discharge

increases.

Fig. 3.11 No. of Cycles per Day

against Discharge Head

ii) Relationship between the water

pumped and discharge head is shown

in Figure 3.12:

Amount of water pumped per day

decreases as the discharge head

increases. The quantity of water lifted

per cycle should theoretically be the

volume of water in vessel C. The

decrease is commensurate with

decrease in the number of cycles

shown in Figure 3.11.

Fig. 3.12 Daily Water pumped

against Discharge Head




12/14


Typical water flow and pump head of other water-cooled unconventional pumping

systems


Solar Liquid Piston Pump (112mm dia. x 75mm

high)

12960L/day. 0.8m

Sudhakars modified pump c/w 93m2

collector

area

800000L/day 9.1m

330L/day 6m

240L/day 8m

Sumathys water cooled pump c/w 1m2

collector area and incorporated with a new vapour

tank 180L/day 10m

3.2.3 Range of Efficiency

The efficiencies of thermal solar water pumps are relatively low in comparison with

electrical/diesel pumping, however, the energy source for thermal solar water pump is a

renewable and is basically free, self-sustainable and suitable for using in rural area

where electrical power distribution is not available. The efficiency of solar thermal

water pumping system is defined as the ratio of the hydraulic workdone by the pump

onto the water being pumped throughout the operation period of operation of the pump

to the total solar radiation incident on the collector during the period of operation of the

pump. For continuous operated systems, e.g. conventional system with centrifugal

pumps, the efficiency can be expressed as the ratio of pump hydraulic power (product

of water flow rate and the total pump head pressure) to the instantaneous solar radiation

incident on the collector. For unconventional systems, the overall efficiency,, can beexpressed by:

=NWh/Htot whereNis the number of operating cycles per day

Whis hydraulic workdone by the pump/cycle (Vwgh, water flow per cycle x total water

pressure increased across the pump)

Htotis total solar radiation incident on collector during pumping period

The overall efficiencies of water cooled systems (as Fig. 3.9) are numerically low and

general between0.05% and 0.10% against a pump head of 10m. Low efficiency is

because there are many stages in converting solar energy to hydraulic work. Use of high

efficiency solar energy collector can certainly improve the efficiency. In the recent

works by Sumathy et al. on a water cooled pump added with a Vapour Tank (Figure

3.10 above), a significant improvement of 30% increase in the overall efficiency from

the general 0.10% to 0.13% was achieved. This is because the Vapour Tank allows the

working fluid vapour to enter tank A quickly, and the successful optimization of the

sizes of various tanks in the system. The study also shows that overall efficiency is

closely related with the pump head (Figure 3.13).




13/14


The overall efficiency decreases

marginally with increasing discharge

head. The decrease can be attributed

to the decrease in the number of

cycles that the pump can perform in a

day. Although the work required to

pump water per cycle increases with

increasing discharge head, the number

of cycles decreases drastically. Hence,

for a given intensity of solar radiation,

the decrease in the number of cycles

with the increasing discharge head

results in lower efficiency.

Fig. 3.13 Overall Efficiency against

Discharge Head.

4.0 APPLICATION

The efficiency of the solar water pump can be as low at the order of 0.1% but the

special significance of application in countries where farming communities are

scattered over large and distant area and where the electricity supply network is not

available. This would include most of the countries in Asia, Africa and Latin

American. Because of the large distances involved and low energy requirements,

transmission of electrical energy from power plant becomes an uneconomic investment.The same is applied on transmission piping network for oil delivery for the operation of

oil engine to drive the pump for water pumping. Also, the availability of the skilled

technician and engineer for operating and maintenance of oil engine, electricity motor

and pump are an important concern. The use of solar water pump in which solar

energy is abundant and rich in region close to the equator become an important

concerns with low demand of energy consumption.

The development of solar water pump using the simplest technology where only solar

collector, liquid separator, pneumatic vessel, valves with least mechanical moving part

that enhance the application of the solar water pump for irrigation purposes. Also, the

required pressure head is not so high for the irrigation with significant amount of daily

consumption, this is the main reasons that the solar water pump can than have the

trends become more and more popular.




14/14


5.0 SUMMARY OF FACTORS AFFECTING THE PERFORMANCE OF SOLAR

WATER PUMP

Since the rate of vapour generated through the flat plate collector is low for obvious

reasons, the water can only be pumped slowly by this vapour.

Because of slow displacement of water by the vapour, it will cause condensation during

the pumping process thus reducing the efficiency of the solar water pump.

When the flat plate collector operate at unnecessary high temperature at some particular

solar time in which the incidence solar radiation is nearly normal to the solar collector,

this reduces the collector efficiencies and thus the overall efficiency of the solar water

pump.

Too many valves are operated manually and causing the low efficiency of the solar

water pump because the pumping process is not instantaneous.

The exposed area of the solar collector is not optimized in the working environment.

After a number of cycle operations, it may require to return the condensed working

fluid in the pressure vessel back to the separating tank of the working fluid.

Environmental factors which affect the intensity of solar radiation reaches the solar

collector, which is outside the scope that we can control.

Angle of latitude and declination also participate part of the factors that affect the

efficiency of the solar water pump. This is not confined in the application of solar

water pump but for all kinds of solar energy application. The same is applied for the

environmental factor as described above. Insulation of the high temperature operating equipment will reduce condensation of

working fluid occurs thus enhance the efficiency of the system. Carefully design and

choose of the insulation material is a great importance of the system design.

Also the condensation of the working fluid before it diverted back for the next cycle of

operation should be carefully controlled to ensure no significant sub-cooling of the

working fluid thus increase the operating efficiency

6.0 REFERENCES

a) Y.W.Wong & K. Sumathy Solar thermal water pumping systems: a review.

Renewable and Sustainable Energy Reviews 3 (1999) 185-217

b) K. Sumathy, A. Venkatesh & V. Sriramulu A solar thermal water pump. Applied

Energy 53 (1996) 235-243



Solar Water Pumping[1]

Documents

Transcript of Solar Water Pumping[1]