Page 251 - Low Temperature Energy Systems with Applications of Renewable Energy

238 Low-Temperature Energy Systems with Applications of Renewable Energy

than without a recuperator, and thereby opens more direct heat applications for
possible use.

6.3.3 Cascade geothermal CHP stations
It is common practice in geothermal power plants to link flash-steam plants and binary
plants in a cascade arrangement. This allows the binary cycle, called a bottoming cy-
cle, to capture energy in the separated brine to generate additional electricity without
the need for more production wells. This is similar to the CHP system described in
Section 6.2, except that a binary cycle (see Fig. 6.14) would receive the separated brine
at state 5 in Fig. 6.3 first, followed by the direct use applications. Of course, the type of
direct heat use is more restricted in this case owing to the lower temperature available
after the binary plant.

6.4 Working fluid selection criteria in CHP stations

We now consider the important matter of the selection of the working fluid. This
design decision has great implications for the performance of an organic Rankine cycle
(ORC) binary plant. While there are many choices available for working fluids, there
are also many constraints on that selection that relate to the thermodynamic properties
of the fluids, as well as considerations of health, safety, and environmental impact.

6.4.1 Thermodynamic factors
Table 6.1 lists some candidate fluids and their relevant thermodynamic properties; pure
water is included solely for comparison. Clearly all of the candidate fluids have critical
temperatures and pressures far lower than water. Furthermore, since the critical pres-
sures of the hydrocarbons are reasonably low, it is appropriate to consider the feasi-
bility of supercritical cycles.
Binary mixtures of these fluids have also been studied for use in geothermal binary
plants. In particular, the thermodynamic properties of 90% i-C 4 H 10 and 10% i-C 5 H 12
were determined by the U.S. National Bureau of Standards (predecessor of NIST)
when it was chosen as the working fluid for the Heber Binary Demonstration plant
in California in the 1980s [5]. Mixtures evaporate and condense at variable tempera-
ture, unlike pure fluids that change phase at constant temperature. This means that
subcritical-pressure boilers for mixed fluids can be better matched to the brine cooling
curves, in a manner similar to, but not exactly like, supercritical pure fluids. Water-
ammonia mixtures, commonly used in absorption refrigeration cycles, are used in
Kalina cycles which have been applied to low-temperature geothermal resources [6].
The liquid-vapor saturation pressure-temperature curve is another important prop-
erty for binary cycle working fluids. The curve must be appropriate for the temperature
of the geothermal fluid. Figure 6.18 shows the (P-T)-saturation lines for the substances
in Table 6.1, except for water which is not a candidate for geothermal binary plants.