Page 271 - Low Temperature Energy Systems with Applications of Renewable Energy

256 Low-Temperature Energy Systems with Applications of Renewable Energy

6.7 Summary

Geothermal combined heat and power (CHP) systems are a very effective way to uti-
lize the available energy in a geothermal resource. CHP systems may be designed with
any of several geothermal power generation schemes including dry steam, flash steam,
binary or hybrid type plants. The fundamental principles of thermodynamics are
applied to CHP systems and working equations are derived that allow analysis to be
carried out to determine optimum performance for various conditions that may be
encountered in a geothermal resource. There is a wide assortment of possible working
fluids for use in geothermal CHP that may be chosen based on technical and environ-
mental factors. Examples are given showing optimum performance results that achieve
the best thermodynamic and economic performance for a range of price ratios for elec-
tricity and heat. Exergy analysis reveals where thermodynamic losses occur and sug-
gests how to improve efficiency by eliminating irreversibilities. Two historically
important case studies are included of CHP systems: a large municipal district heating
system in Japan where a power plant supplied waste hot brine to heat fresh water for
many heating needs for a nearby town, and a unique university campus system in the
United States that provides both electricity and heat for all of the buildings including
some snow-melting for walkways.

Nomenclature

General Note: See Nomenclature for Chapter 1; new symbols for this chapter are
shown below.
A Baumann rule factor, Eq. (6.22)
H exergy value factor
y flow split ratio, Eq. (6.26)

Subscripts

ex exergy
th thermal
U utilization

Review questions

1. List the kinds of geothermal energy conversion systems that may be used in conjunction with
a direct-heat application.
2. What kinds of direct-heat applications are appropriate for geofluid supply temperatures:

(a) less than 40 C; (b) between 80 and 120 C; and (c) between 130 and 150 C?
3. What technical factors would limit the lowest temperature for which a geofluid might be used
for a particular direct-heat application.