Environmental Engineering Reference
In-Depth Information
termediate- and low-temperature geothermal fields can be utilized for both power
generation and the direct use of heat (Tester et al.
2005
).
Geothermal heat pumps utilize relatively low and constant temperature ground
heat and have been increasingly finding applications as efficient home heating and
cooling installations. Hydrothermal power plants are the most typical installation
for electricity generation and include two main types: hot water-dominated and
steam-dominated systems. The temperature range for the hydrothermal systems is
typically 150-300 ºC. Binary-cycle power plants may also provide a useful option
where low reservoir temperatures (approximately 100-150 ºC) are encountered.
Hot dry rocks (HDR) and hot wet rocks (HWR), supercritical rocks and magma
energy systems are at the research or development stage. Supercritical rocks refer
to high temperature rock masses whose temperature and pressure exceed the critical
point of water (374 ºC, 22 MPa). In contrast with naturally occurring hydrothermal
systems, HDR and HWR refer to geothermal reservoir systems created using artifi-
cial methods; these engineered reservoir systems using hydraulic stimulations were
first proposed by Tester et al. (
1989
) and Potter et al. (
1974
). The aim of geothermal
energy extraction using hydraulic stimulations is to mine the heat energy stored in
subsurface rocks by creating a system of open, connected fractures through which
water can be circulated down injection wells, heated by contact with the rocks,
and returned to the surface in production wells to form a closed loop, as illustrated
in Fig.
6.6
. Following the introduction of HDR, the concept of HWR has been
proposed which intends to combine an artificial reservoir system with the existing
hydrothermal system (Takahashi and Hashida
1992
).
EGS is a newly coined terminology defined as engineered reservoirs that have
been created to extract economical amounts of heat from low permeability and/or
porosity geothermal resources (Fig.
6.6
). EGS may refer to geothermal reservoir
systems that have been produced by means of artificial and engineering methodolo-
gies such as hydraulic stimulation methods in currently non-productive formations
and rock masses. In this regard, EGS is an equivalent concept to HWR which aims
to improve and enhance the performance of existing reservoirs by use of hydraulic
stimulation. HDR may be also included in the EGS concept and in this chapter, the
terminology EGS includes both HDR and HWR.
Heat extraction from ultra-high temperature systems such as supercritical rocks
and magma may be feasible and attractive in the future, but this chapter focuses on the
development of hydrothermal and EGS systems, which are current and near-future
heat sources for electricity generation. The reader is referred to Hashida et al. (
2000
),
and Hashida and Takahashi (
2003
) for the development of supercritical rocks, and
Colp (
1982
), and Teplow et al. (
2009
) for the exploitation of magma energy.
6.4
EGS Projects
In order to realise the potential of geothermal energy, the 3 pre-requisites already
mentioned of high temperature rock, water and flow path are needed. Most current
geothermal sources have magma relatively close to the surface and water coming
