Page 242 - Low Temperature Energy Systems with Applications of Renewable Energy

Geothermal energy in combined heat and power systems 229

h 3f
h 2 A 1
h 3g h 3f
h 3 ¼ (6.20)
A
1 þ
h 3g h 3f
A ¼ 0:5 h ðh 2 h 3s Þ (6.21)
t;d
(6.22)
s 3s ¼ s 2
x 3s ¼ðs 2 s 3f Þ=ðs 3g s 3f Þ (6.23)

h 3s ¼ h 3f þ x 3s ðh 3g h 3f Þ (6.24)

y ¼ _ m 7= _ m 5 ¼ _ m 7= _ m R ð1 x 1 Þ (6.25)

The term h t,d represents the isentropic efficiency of a steam turbine operating
entirely with superheated (i.e., dry) steam. Owing to the moisture that forms during
the expansion of saturated vapor, the actual geothermal steam turbine efficiency will
be less than h t,d . The Baumann rule was adopted to show this degradation and leads
to Eq. (6.20). For computational purposes, h t,d was taken as 0.88; thus the A-factor
becomes

A ¼ 0:44 ðh 2 h 3s Þ (6.26)

The factor y in Eqs. (6.7)e(6.9) determines what fraction of the separated geofluid
is sent to the heating facility. This must be determined by a detailed assessment of the
reservoir and the reinjection needed to maintain reservoir pressure without excess
cooling of the formation, along with the needs of the heating applications. For the pre-
sent analysis, a value of 2/3 has been arbitrarily assigned to y, i.e., 1/3 of the separated
geofluid is reinjected; thus

_ m 7 ¼ð2=3Þ _ m 5 ¼ð2=3Þ _ m R ð1 x 1 Þ (6.27)

An optimization will be conducted to seek the value of the flash temperature T 1 that
yields the maximum total exergy output of the plant, i.e., the greatest sum of electrical
power and separated brine exergy (which is delivered to the HX):
_ _ _
E out ¼ W e þ E 7 (6.28)

where the electrical output is

_ _
W e ¼ h W T ¼ h _ m R x 1 ðh 2 h 3 Þ (6.29)
G G