Environmental Engineering Reference
In-Depth Information
The result depends on the aquifer transmissivity and the depth to water. In
many cases, the same flow that can be pumped out can indeed be re-injected,
but the hydraulic head available for turbine operation is reduced. In an aqui-
fer with low transmissivity, injection results in a mound that can potentially
reach the surface. In comparing this to the pumping cycle, if the pump depth
below the water datum level is the same as the depth from the surface to the
water datum level, the mound of injection will just reach the surface, negat-
ing the ability to produce power from the injection flow using a turbine.
This modeling exercise indicates that the aquifer UPHES system must be
designed with aquifer transmissivity, injection mound height, and depth
to water as major design parameters. The transmissivity must be relatively
large so that the mound of injection remains low enough to reserve suffi-
cient hydraulic head for turbine power generation. In the case of the 200-foot
(61-meter) water depth example, if the transmissivity of the aquifer is 6.5 ft
2
/
min (100.6 cm
2
/s), then there remain 182 feet (45.7 meters) of head for turbine
operation.
This situation can be modeled as a simple electrical circuit with a volt-
age source representing the total hydraulic head potential and resistances
representing the “head drop” for the turbine and for the aquifer injection
mound. The current in the circuit represents water flow. The resistance asso-
ciated with the turbine correlates to the resistance to water flow in the pipe
and in the turbine. The injection resistance correlates to the transmissivity
(resistance to water flow) encountered in the aquifer. Figure 4.6 shows an
electric circuit model for system head. This electrical model gives accurate
insight into the interactions of the design parameters. Holding the total head
constant, a transmissivity increase correlates to a reduction in the injection
resistance (R
Injection
in the figure). A reduction in the injection resistance is
coupled with an increase in the turbine resistance, keeping total flow con-
stant, but increasing the power output of the turbine. Alternately, decreasing
the flow while holding total head and transmissivity constant will decrease
the injection head. Then, more head (voltage) will drop across the turbine.
Q
Total
+
HEAD
Total
+
-
HEAD
Turbine
R
Turbine
-
+
HEAD
Injection
R
Injection
-
FIGURE 4.6
Electrical circuit model for hydraulic head.
