Page 370 - Handbook of Energy Engineering Calculations

Since geothermal power generation depends on naturally occurring
sources, there is a limitation on the number suitable sites. And engineers
cannot create new geothermal sites the way they can design, and build, new
fossil-fueled power plants. While there are many predictions on wider use of
geothermal energy, its ultimate importance is limited by the number of new,

acceptable sites that can be found.
Thus, vapor-dominated systems are those* in which water is vaporized into
steam that reaches the surface in a relatively dry condition at about 400°F

(205°C) and rarely above 100 psig (8 bar). This steam is most suitable for use
in turboelectric power plants, with the least cost. It does, however, suffer
problems similar to those encountered by all geothermal systems, namely the
presence of corrosive gases and erosive material and environmental
problems. Vapor-dominated systems, however, are a rarity; there are only

five known sites in the world to date.
At this writing (2011), New Zealand recently put a 140-MW geothermal
plant online on North Island. As with other geothermal plants, this new plant

required huge equipment that had to be transported through nearby forests.
Other geothermal projects in New Zealand promise to supply 15 percent, and
more, of the nation’s power needs in coming years. One of these future plants
will have a 250-MW output. These, and other installations around the world,
show that engineers will continue to seek viable geothermal sites for many

years to come.

CYCLE ANALYSIS OF A VAPOR-DOMINATED

GEOTHERMAL STEAM POWER PLANT

A 100-MW vapor-dominated steam power plant as shown in Figs. 2 and 3
uses saturated steam from a geothermal well with a shut-off pressure of 400

psia (2,756 kPa). Steam enters the turbine at 80 psia (551 kPa) and condenses
at 2 psia (13.8 kPa). The turbine polytropic efficiency is 0.82, and the
turbine-generator combined mechanical and electrical efficiency is 0.90. The
cooling-tower cooling-water exit temperature is 70°F (21°C). Calculate the

3
necessary steam flow, lb/h and ft /min, the cooling-water flow, lb/h, and the
plant efficiency and heat rate, Btu/kWh, if reinjection occurs prior to the