Exergy analysis of solar energy systems - Solar Energy Sciences and Engineering Applications - page 105

Figure 3.3.4 Effect of pressure at state 4 on energy and exergy efficiencies of the system.
because working fluid passing through the solar collector, as air in this case, has a
certain limit up to which it can get heated and therefore an increase in area does not
necessarily mean better performance of the system from the efficiency perspective.
The pressure of the working fluid entering the turbine at state 4 plays an important
role in the performance of the system as shown by energy and exergy efficiencies
in Figure 3.3.4. The energy and exergy efficiencies are found to be increasing from
16.37% to 18.745 and 17.26% to 19.76%, respectively with increase in pressure at
state 4 from 500 kPa to 1000 kPa. Increase in pressure of the fluid entering the turbine
means that the stream entering the turbine has higher energy content as compared to
the low pressure stream and therefore for the same exit pressure the performance of
the system is enhanced.
Finally, a bar chart is provided to illustrate which component of the system has the
greatest amount of exergy destroyed at a constant ambient temperature and pressure
of 298 K and 101 kPa as shown in Figure 3.3.5. It can be seen that the maximum
amount of exergy is destroyed by solar thermal collector followed by condenser, boiler
and turbine. This bar chart shows that in order to enhance the energy and exergy
efficiencies one should first try to reduce the exergy destruction rate in the solar thermal
collector as it is the source of maximum losses.
3.3.2 Case study 2: Exergy analysis of solar photovoltaic/thermal
(PV/T) system for power and heat production
In this case study, a detailed energy and exergy model of solar PV/T system is presented.
Operating parameters such as ambient temperature and solar flux are varied to see their
effect on energy and exergy efficiencies.