Page 175 - Geothermal Energy Renewable Energy and The Environment

Generating Power Using Geothermal Resources 161

FIGUre 9.6 (See color insert following page 17.0..) Geothermal dry steam generating facilities at The
Geysers, California. The steam in the photograph is from the cooling towers of several generators. The generators
are in the buildings adjacent to the cooling towers. (U.S. Geological Survey photograph by Julie Donnelly-Nolan.)

A variety of operational and engineering constraints, beyond the basic thermodynamic limita-
tions of natural systems, must also be evaluated when determining the overall performance of a
generating system. Generating facilities invariably have power demands for monitoring and control,
lighting, environmental mitigation, facility power, and other needs that are parasitic to the generat-
ing capacity. These loads will diminish output and must be rigorously catalogued and quantified in
order to design and construct efficient facilities that are economically viable.
Although rare, useful dry steam systems are a resource that can provide significant power generation.
For example, at The Geysers in California (Figure 9.6), the currently installed capacity is approximately
1400 MW, making it the world’s largest geothermal power generation site, with additional generating
capacity currently under development. Larderello, in Italy, is the only other operating dry steam facility
in the world. An additional dry steam resource has been found in Indonesia but has yet to be developed.
Initial flow tests of the Indonesian site have produced flow rates of about 3.78 kg/s (Muraoka 2003).

hydroThermal sysTems
Most geothermal systems currently producing power are wet steam or hydrothermal systems.
Hydrothermal systems have the common characteristic that their temperature and, hence, enthalpy
conditions are on the low enthalpy side of the critical point in an enthalpy–pressure diagram
(Figure 9.7) and are thus liquid dominated. As such fluids ascend from depth they will flash to
steam. Whether they flash in the well or on their way to the turbine is a matter of engineering and
operational decisions. The pressure and temperature conditions that most commonly are encoun-
tered for hydrothermal geothermal resources are enclosed within the shaded region in Figure 9.7.
For illustration purposes, we will consider a geothermal reservoir at 200 bars pressure and 235°C
(point 1 in Figure 9.7). The enthalpy at this condition is 1018 kJ/kg (Bowers 1995). This fluid will
flash to steam once the pressure has been reduced to 30.6 bars. We will assume that, as before, from
this point on the system behaves isenthalpically. In describing the behavior of this fluid as it moves
up the well, we will follow the approach described by DiPippo (2008).
The movement of the fluid up a well pipe, which is schematically represented in Figure 9.8, is
constrained to follow the law of conservation of momentum, which is basic to hydraulic flow. The
momentum equation is
m × a = ∑ F = –dP – dF /A – (ρ × g × h), (9.7)
b
fl