20-05-2013, 04:34 PM
Thermodynamic analysis and optimization of a solar thermal water pump
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Abstract
Thermodynamic analysis has been carried out in detail, to predict the performance of a solar thermal
water pump working at dierent discharge heads. It is shown that the heating time of the working ¯uid and
the condensation time of the spent vapour play an important role in determining the number of cycles that
a pump can perform in a day. The heating time in turn depends on the amount of working ¯uid being
loaded in the system initially. Similarly, it is observed that an optimal cooling coil area dictates the eective
condensation time. Hence, in the present work, an optimum design has been outlined for a solar thermal
water pump, both in terms of the amount of the working ¯uid to be loaded in the system and the optimum
cooling coil area.
Introduction
The thermodynamic utilization of solar radiation for water pumping is an inherently sensible
proposition, because the output of a solar water pump will be higher in hot sunny weather, when
demand for water is also high [1]. Most of the solar thermal water pumps [2,3] work on the
principle that there is a change in the volume of a liquid on vapourization and conversely, when
the vapour is condensed. The volume increase at a given pressure is utilized in displacing water to
a higher elevation, while the volume reduction at a lower pressure is used for suction of water for
the next cycle of pumping action.
System description
Fig. 1 shows the schematic of the solar thermal water pump considered in this work. The pump
consists of the following components: (1) ¯at-plate solar collector coupled to an insulated tank S
that contains ethyl ether, (2) insulated vapour storage tank N, (3) insulated vessel A containing
water initially, (4) uninsulated vessel B containing air, (5) vessel C immersed in the well water
containing water initially, (6) overhead water storage tank D, (7) shell-and-coil condenser ®lled
with water initially and (8) a set of valves between the various components listed above which are
operated manually.
Analysis of the system
To start the ®rst cycle of operation of the pump, ethyl ether vapour at state 3 (Fig. 2) in tank N
is assumed to communicate with vessel A by closing valve 1 and opening valve 2. Consequently,
the ethyl ether vapour expands to state 4. The vapour entering vessel A displaces a part of water in
it to vessel B. The water entering vessel B must be able to compress the air in it to the pressure
corresponding to the discharge head. Thereafter, the discharge pressure must be maintained in
vessels B and C until all the water in C is lifted to the overhead tank D. Thus, the minimum
pressure (P1p) required in the vapour storage tank N is determined by the work done by the ethyl
ether vapour in compressing air in vessel B to the discharge pressure, and by the work required
in lifting Vw VC litres of water through the discharge head. The pump can work only if the
pressure in tank N is P1p or higher.
Optimization of heating time Model
The collector and the separation tank S are initially ®lled with m kg of liquid ethyl ether at an
ambient temperature ta. Valve 1 is kept open so that the vapour in N and the liquid in the collector
are in equilibrium and the pressure in the system is the saturation pressure corresponding to the
initial temperature of ethyl ether in the collector. Thus, in the analysis, at any instant, the ethyl
ether in the system is considered to be a mixture of liquid and vapour of a certain dryness fraction.
When the collector is exposed to the sun, the incident solar radiation heats up the liquid ethyl
ether in the collection system by thermosyphon action. As the heating progresses, the temperature
of ethyl ether in the separation tank increases. As stated above, at any instant, the pressure in the
system is the saturation pressure of ethyl ether corresponding to its temperature.
Conclusions
It is shown that the performance of the solar thermal water pump depends mainly on heating
time and condensation time. In order to achieve a continuous operation of the pump (to an extent
possible), the heating time and condensation time should be shorter than the pumping time. In
this study, it is identi®ed that the initial amount of ethyl ether controls the heating time and the
preferred condensation time can be obtained by optimizing the surface area of cooling coil. This
approach can be generalized and extended for any discharge head or any desired amount of water
to be pumped.