Page 259 - Low Temperature Energy Systems with Applications of Renewable Energy

Geothermal energy in combined heat and power systems 245

6.5.3 Exergy losses in CHP plants

The system shown in Fig. 6.3 will be used to illustrate the exergy losses in a CHP plant.
To facilitate this presentation, Table 6.3 shows all the thermodynamic properties
needed to carry out the exergy efficiency calculations. The optimum point in
Fig. 6.7 for the case of H ¼ 0.5 is chosen for this exercise. Numbers shown in bold,
italic face were assumed; all others were found using Refprop or calculated. A pres-
sure loss of 10% was assumed for fluids passing through heat exchangers. The End
User receives only enough hot water from the distribution system to meet its needs.
The circulating hot water within the End User was taken as 1.2 kg/s; the calculated
hot water distribution rate is nearly identical for this case.
With the exergy rates now determined at all state points and neglecting work re-
quirements for water pumps, the exergy accounting can be carried out. Each compo-
nent will be studied separately.

1. Separator, S

_ E D;S ¼ _ m 1 e 1 _ m 2 e 2 _ m 5 e 5 ¼ 66:42 38:93 27:49 ¼ 0:00 MW

This result says that the separator has no exergy destruction because it merely divides the
two-phase liquid-vapor mixture into its constituent parts, with each part retaining its original
exergy. If we account for pressure losses in the separator, there would be some loss of exergy.

2. Turbine, T

_ E D;T ¼ _ m 2 e 2 _ m 3 e 3 _ W T ¼ 38:93 7:15 52:835 ð2743:5 2244:6Þ=1000
¼ 5:42 MW

This is about 14% of the exergy supplied to the turbine at state 2; in that sense, the turbine
has a functional exergy efficiency of 86%. Viewed as a percentage of the change in exergy by
the steam passing through the turbine, it would be 83% efficient. The turbine delivers
26.36 MW to the generator, of which 25.7 MW is sent out to the grid.

3. Heat exchanger, HX

_ E D;HX ¼ _m 7 e 7 þ _m 9 e 9 _m 8 e 8 _m 10 e 10 ¼ 18:33 þ 0:02 3:24 7:60 ¼ 7:51 MW

This is a significant loss, amounting to about 41% of the incoming exergy. That is, only
59% of the incoming exergy, essentially all of it from the separated brine, is transferred to the
fresh water for use in the hot water distribution system. This component should be examined
to find ways to improve the operation.
4. End user, EU
Various applications may be occurring at the end user such as radiant floor heating of
residences, domestic hot water production, snow melting, greenhouse heating, etc. The
transfer of exergy may happen in a hot water tank, under the surface of a driveway or side-
walk, in an air coil or in soil at a greenhouse, etc. In all cases, at the low temperatures