Page 273 - Low Temperature Energy Systems with Applications of Renewable Energy

258 Low-Temperature Energy Systems with Applications of Renewable Energy

Part H. Exergetic efficiency of the preheater-evaporator.
Part I. Overall exergetic efficiency of the geothermal CHP system.
2. A university campus has a small geothermal CHP system. The hot geofluid is received (state
A) at a temperature of 125 C, a pressure of 0.30 MPa, and a mass flow rate of 40 kg/s. All of

it passes first through a binary power plant to generate electricity for the buildings and to run
the geothermal well pumps. Cooling water is available at a temperature of 5 C (state C). The

binary cycle uses R245fa as the working fluid. The pinch-point temperature differences in the

evaporator and condenser are 4 C and 8 C, respectively. The geofluid leaving the power
plant goes directly to the heating plant. The plant specifications are as follows:
Geofluid : T A ¼ 125 C; P A ¼ 0:3 MPa; _ m A ¼ 40 kg=s:

Binary cycle: working fluid ¼ R245fa; evaporator pressure ¼ 1.25 MPa; condenser pres-
sure ¼ 0.25 MPa; turbine inlet (state 1) is saturated vapor; condenser outlet (state 4) is satu-

rated liquid; DT pp,e ¼ 4 C; DT pp,c ¼ 8 C; turbine isentropic efficiency ¼ 0.85; feed pump

isentropic efficiency ¼ 0.75.

Cooling water: inlet temperature T C ¼ 5 C.
Calculate the following:
Part A. Mass flow rate of the R245fa.
Part B. Power generated by the turbine if the generator efficiency is 0.97.
Part C. Power needed to run the feed pump if the drive motor efficiency is 0.95.
Part D. Net power delivered if the well pumps require 125 kW from the power plant.
Part E. Thermal efficiency of the binary cycle.
Part F. Geofluid temperature leaving the power plant, T B .
Part G. Cooling water mass flow rate.
Part H. Cooling water discharge temperature.
Part I. Maximum thermal power delivered to the campus heating system if the geofluid
reinjection temperature must be above 65 C.

3. With reference to Figs. 6.8 and 6.9 for the 220 C geothermal resource, consider a case where
the H-factor is 0.24.
Part A. What separator temperature yields the optimum system design point based on
combined exergy-value output?
Part B. Assuming the plant is set up to operate at the optimum condition, determine the
total combined exergy output for electricity and heat, in kW.
Part C. If the plant costs $5500/kW, where the kW amount includes the combined elec-
trical and heat exergy output, and the plant is assumed to operate at its design capacity for
8,000 h/y (i.e., 91% capacity factor), how many years would be needed to recover the cap-
ital cost (simple pay-back)?
Part D. Assess and comment on the result from Part C, in light of the fact that annual oper-
ating & maintenance costs have not been accounted for.

Acknowledgments

The author of this chapter thanks Anna Suzuki, Assistant Professor, Tohoku University, Sendai,
Japan, for providing information on the Shizukuishi project.