Environmental Engineering Reference
In-Depth Information
to hydraulic conductivity. Transmissivity has units of area per time, and is
usually calculated as the hydraulic conductivity times the aquifer thickness.
The next step would be to derive an expression for the transmissivity in an
aquifer experiencing recharge flow through a well. Although not included
here, the result of this derivation should match the governing equation above
for hydraulic flow in an aquifer.
Transmissivity (T) is a measure of the volume of water flowing through a
cross-sectional area of an aquifer [for example, 1 foot times the aquifer thick-
ness (b)] under a hydraulic gradient (for example 1 ft /1 ft) in a given amount
of time. Transmissivity is a parameter used to calculate water flow in aqui-
fers, and is equal to hydraulic conductivity (k) times aquifer thickness (b),
as shown in Equation (4.7). Hydraulic conductivity (and therefore transmis-
sivity) depends on the permeability of the medium, specific weight of water,
and dynamic viscosity of water. The equation for hydraulic conductivity is
found from application of Darcy's law.
Transmissivity therefore depends on the above quantities, including
another length dimension, aquifer thickness. In this text, the quantity of
transmissivity is used to evaluate water flow and aquifer performance. It
should be noted that transmissivity can be related back to the basic prop-
erties of aquifer materials. Another common material property, porosity,
is the ratio of the empty space volume to the total volume in a material.
Porosity can change with depth, because the weight of material from
above compresses the voids between particles. While porosity of a mate-
rial can affect the intrinsic permeability, these quantities are not necessar-
ily related.
Water Pump Turbine
The core of the aquifer UPHES system is an integrated pump turbine and
motor generator unit. As the name suggests, this single unit performs the
functions of both pumping water using electrical power and generating elec-
tricity from water power. This type of integrated machine exists commer-
cially for large pumped hydroelectric installations, normally employing a
Francis reaction type turbine coupled to a synchronous AC electric machine.
A unit sized and designed for the proposed aquifer UPHES application is
not yet commercially available. In this section, the important design consid-
erations for the integrated pump turbine and motor generator unit for use in
an aquifer UPHES system are described.
An option for the design of the aquifer UPHES pump turbine is the use of
standard centrifugal or “vertical turbine” well pumps in the forward direc-
tion for pumping and in reverse for turbine operation.
9
Figure 4.7 shows an
example of a submersible vertical turbine pump. This use of the device is
known as a pump-as-turbine (PAT) design. A first order estimation of the
turbine efficiency of a centrifugal pump is that it is the same as the pump
efficiency. Although originally designed as a pump, a centrifugal pump may
