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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
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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.
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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.
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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.
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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
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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.
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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.
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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.
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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.
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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
Description of Pump/Pumping system Water flow Pump Head
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
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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
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Typical water flow and pump head of other water-cooled unconventional pumping
systems
Description of Pump/Pumping system Water flow Pump Head
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).
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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.
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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
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