Environmental Engineering Reference
In-Depth Information
UPHES have considered installations of between 1000 MW and 3000 MW
as the most economical scale, and have analyzed the economics based on a
varying consumer load supplied by conventional coal-fired power plants.
Smaller installations serve single users, small communities, agriculture,
or industrial operations. These smaller installations can utilize an existing
underground water structure and consume no net water (except for evapo-
ration). The economical sizing of a smaller UPHES is a complicated matter;
cost of electricity, geological formations, water table characteristics, existing
infrastructure, user load profiles, and renewable energy source availability
all contribute to optimal sizing of the system.
Design Overview
Generally, a large system requires excavation of the lower reservoir from suit-
able geologic strata. Most studies of UPHES have targeted hard rock (such
as granite) lower reservoir beds for favorable structural properties. Solution
mining of underground salt domes to create a large underground caverns
has also been proposed. A large lower reservoir would be excavated in a
network of extruded, narrow caverns rather than a single large cavern. This
method improves the structural integrity of reservoir excavations and is bet-
ter suited to known excavation techniques. The system can be designed for
single- or double-drop configuration. A single-drop configuration is shown
in Figure 4.1, and a double-drop installation utilizes an intermediate storage
reservoir at half the depth of the main reservoir.
The availability and capability of water turbines and pumps that can oper-
ate at very high pressures contribute to the decision of how deep a lower res-
ervoir is built and whether to use a double-drop system. However, a design
trade-off must be considered for reservoir depth because as depth increases,
the volume of water required to generate the same power decreases.
Equation (4.1) below is the basic power calculation for UPHES and illus-
trates the trade-off between depth and water flow.
In addition, large scale UPHES systems require the main power station to
be located below the lower reservoir to eliminate cavitation issues that could
reduce the lifetime of the machinery. An underground power station calls for
safe personnel access to a great depth below the surface. The obvious major
barrier to deployment of UPHES is the difficulty of excavating a large, stable
water reservoir at a significant depth below the earth's surface, presumably
from hard subterranean rock. An underground power station is not an ame-
nable location for human occupation and must be operated remotely.
η
(4.1)
P Q H
=
⋅
⋅
ρ
⋅
g
⋅
where P = power generated in watts [W]. If horsepower [hp] is used in the
equation, all other variables' units must be changed accordingly. Q = fluid
flow in cubic meters per second [m
3
/s], ρ = water density in kilograms per
