Power Plant Engineering

Geothermal-Preheat Hydrid Systems. In these systems the low-temperature geothermal en-
ergy is used for feed water heating of an otherwise conventional fossil-fueled steam plant. Geothermal
heat replaces some, or all, of the feed water heaters, depending upon its temperature. A cycle operating
on this principle is illustrated in Fig. 2.37. As shown, geothermal heat heats the feed water throughout
the low-temperature end prior to an open-type deaerating heater. The DA is followed by a boiler feed
pump and three closed-type feed water heaters with drains cascaded backward. These receive heat
from steam bled from higher-pressure stages of the turbine. No steam is bled from the lower-pressure
stages because geothermal brine fulfills this function.
High-pressure
turbine
Low
pressure
turbine
Surface
Condenser
Condenser
pump
Hot
brine
Cold
brine
Geothermal
heat exchanger
Deaerating
heater
Boiler
feed pump
Feedwater heaters
Superheater
Reheater
Evaporator
Economizer
Fig 2.37. Schematic of a Geothermal-Preheat Hybrid System.
Fossil-Superheat Hybrid Systems. In these systems, the vapor-dominated steam, or the vapor
obtained from a flash separator in a high-temperature liquid-dominated system, is superheated in a
fossil fired super heater.
Fig. 2.38 show schematic flow. It comprises a double-flash geothermal steam system. Steam
pro-duced at 4 in the first-stage flash separator is preheated from 4 to 5 in a regenerator by exhaust
steam from the high-pressure turbine at 7. It is then superheated by a fossil fuel fired super heater to 6
and expands in the high pressure turbine to 7 at a pressure near that of the second stage steam separator.
It than enters the regenerator, leaves it at 8, where it mixes with the lower pressure steam produces in
the second stage flash separator at 15, and produces steam at 9, which expands in the lower pressure

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NON-CONVENTIONAL ENERGY RESOURCES AND UTILISATION
95
turbine to 10. The condensate at 11 is pumped and reinjected into ground at 12. The spent brine from
the second stage evaporator is also reinjected in to ground at 16.
Regenerator
Fossil-fired
superheater
High-pressure
turbine
Low-pressure
turbine
10
Condenser
11
12
Reinjection
16
Ground
Well
1
13
2
3
13
14
15
8
4
5
6
7
9
9
Separators