Page 184 - Geothermal Energy Renewable Energy and The Environment

170 Geothermal Energy: Renewable Energy and the Environment

An important environmental aspect of binary plants is their absence of emissions. The flow
path of the fluid powering the turbine forms a closed loop (Figure 9.12). In other words, the iso-
pentane or other fluid continuously cycles from condenser to heat exchanger to turbine and back
to the condenser. Unlike flash plants that inevitably exhaust some proportion of the geothermal
steam to the atmosphere during the cooling and condensation process, the closed loop design of a
binary plant results in no atmospheric discharge during the equivalent part of the cycle. In addi-
tion, since the boiling point of the fluid in the closed loop is so low, air-cooling often is sufficient
to accomplish the necessary cooling and condensation. Finally, although the geothermal fluid
flows in an open loop from the reservoir through the heat exchanger and back into the Earth, there
is no release to the atmosphere of geothermal fluid. Hence, binary plants produce no atmospheric
discharge.
The absence of water cooling, however, makes binary plants susceptible to changes in ΔT due to
the diurnal fluctuation of the air temperature. In locations where the outside air temperature remains
relatively low, such as during the winter months in most parts of the United States, northern Europe,
Canada, and other moderate- to high-latitude settings, binary plants can generate power close to
their rated outputs. During the summer months, however, high midday temperatures reduce the effi-
ciency of the plant by reducing the ΔT. The result is a reduction in generating capacity by as much as
20%. Research efforts are focusing on the development of cooling systems that can overcome such
effects by allowing for a scaled cooling capacity that fluctuates with the ambient air temperature and
humidity, thus allowing maintenance of a near-constant ΔT.

case sTUdy: The Geysers
As a case study of geothermal power production, we will examine The Geysers in northwestern
California. Although The Geysers is one of the rare dry steam fields and cannot be considered a
typical geothermal resource for that reason, its development history highlights important issues that
affect many geothermal power operations, whether the resource is a dry steam system or a hydro-
thermal system. It is from that perspective that we will use this system for study.

GeoloGy
The Geysers is located in a complex geological environment that remains the topic of considerable
research. The Geysers region lies just south of the landfall of the Mendocino transform fault, which
is part of a triple junction plate boundary involving the Cascadia subduction zone off the coast of
Washington and Oregon, and the San Andreas Fault (Figure 9.14). It has been argued that this triple
junction has migrated northward from Baja California to its present position over the last ~25 mil-
lion years. As it migrated, it allowed the mantle to come in contact with the overlying continent
without an intervening slab of subducting oceanic crust (Furlong, Hugo, and Zandt 1989) at the
edge of this migrating system. The window allowed hot mantle to interact with crustal rocks at a
relatively shallow level, resulting in high heat flow and the formation of a string of volcanic centers
that are progressively younger as one proceeds from south to north in California.
This tectonic framework instigated the development of the thermal anomaly, of which The
Geysers is a manifestation. Today, the region is the site of very recent volcanic activity that formed
eruption centers as recently as 10,000 years ago (Donnelly-Nolan et al. 1981; Hearn, Donnelly-
2
Nolan, and Goff 1995). Heat flow in the area is about 500 mW/m (Walters and Coombs 1989),
which is about 60 times the global average. Steaming ground and hot springs were present in the
area when it was first considered for development in the early 1900s. These surface manifestations
require the presence of magma bodies at a relatively shallow depth below the surface. Modeling of
the thermal structure and evolution of the area, along with analysis of fluid inclusions in rocks from
the region suggest that magma bodies that have solidified are the primary heat source, existing at
depths as shallow as 3 km (Dalrymple et al. 1999).