Page 183 - Geothermal Energy Renewable Energy and The Environment

Generating Power Using Geothermal Resources 169

of vaporization of about 344 kJ/kg (Schuler, Daura, and van Gunsteren 2001). Geothermal fluids
with moderate temperatures are thus capable of boiling relatively large masses of isopentane.
In Figure 9.12, the values that are shown for the temperatures along the flow path are more or
less typical of binary plants that use isopentane. These values are useful for evaluating the energet-
ics of such systems. For the flow rate and temperatures shown, and using the constant pressure heat
capacity for isopentane, we can calculate approximately the power transferred to the turbine. The
enthalpy drop per unit time across the turbine during the expansion is given by

Δ H = m × C × ΔT.
p
For the conditions in the figure, this becomes

Δ H = 20 kg/s × 2.29 kJ/kg-K × 50°K = 2288.4 kJ/s = ~2.3 MW.

For the efficiencies of turbines used previously (85%), the maximum possible power output for
this system would be about 1.95 MW. This example provides an indication of the size of a generat-
ing facility that can be built for systems with these characteristics. By combining a variety of engi-
neering enhancements and coupling together several units with generating capacities similar to that
calculated here, facilities that produce 10–20 times that amount of power can be developed. Indeed,
binary plants with power outputs from 2 MW to more than 50 MW are common. Larger generating
facilities are also in place.
Binary generating facilities that are intended to provide power to local industries, facilities, and
communities are also available. A pioneering application of small binary power plants has success-
fully been developed at Chena Hot Springs Resort in Alaska. Two 210 kW generators have been
installed using the local hot spring resource, providing low cost power to a relatively remote setting.
At the Oregon Institute of Technology in Klamath Falls, Oregon, a single unit has been installed
that provides 280 kW (gross power output) from a single binary generator. Figure 9.13 shows the
arrangement of this particular generator and its dimensions.

5.79 m
Cooling
water inlet

Condenser

Evaporator
2.74 m

Geothermal
fluid inlet

FIGUre 9.13 (See color insert following page 17.0..) A 280 kW binary power system as installed at the Oregon
Institute of Technology. The length dimension is from geothermal fluid inlet to geothermal fluid outlet (not shown).
The height dimension is from the base of the unit to the center point for the exit for the cooling water. The maxi-
mum width of the unit is 2.29 m. Total unit weight is 12,519 kg. The power specification is for the gross power
output. The net output is between 225 and 260 kW and depends on the ΔT between the incoming geothermal fluid
and the cooling water. This particular unit is produced by Pratt & Whitney. (Photograph by William E. Glassley.)