Page 226 - Geothermal Energy Renewable Energy and The Environment

214 Geothermal Energy: Renewable Energy and the Environment

aqUacUlTUre
One of the simplest direct use applications for geothermal fluids is aquaculture. Geothermal fluids
have been used to raise carp, catfish, bass, mullet, eels, sturgeon, tilapia, salmon, trout, tropical fish,
lobsters, crayfish, crabs, alligators, algae, prawns, shrimp, mussels, scallops, clams, oysters, and
abalone (Dickson and Fanelli 2006).
Geothermal fluids are used to optimize the temperature for breeding, growth, and health of the
species of interest. The impact can be dramatic. For example, according to Dickson and Fanelli
(2006), alligators bred under natural conditions reach a length of about 1.2 m in three years.
However, if a constant temperature of about 30°C is maintained, alligators will grow to lengths of
about 2 m in the same period of time. Shown in Figure 11.7 are growth curves for other animals as
a function of temperature. It is evident that, at least in terms of controlling growth rates, optimal
temperatures exist that geothermal fluids can maintain.
The design of an aquaculture facility must take into account heat loss from a variety of processes.
If the facility uses open pools, heat losses due to conduction from the pool walls must be taken into
account, as well as heat loss from the surface by convection, radiation, and evaporation. The magni-
tude of the heat losses can be determined by considering an example fish pond. We will consider a
facility that measures 10 × 15 meters and is 1.5 meters deep, and is constructed with concrete walls
10 cm thick in the ground. We will assume that winter air blows over it at a rate of 1.0 m/s at a winter
low temperature of 10°C. We will assume the temperature of the pond is maintained at 27°C. The
3
total volume of the facility is 225 m which is 225,000 liters.
The heat loss by conduction is
Q = k × A × dT/dx
cd
2
Q = 1.4 W/m-K × 225 m × (12 K/0.1 m)
cd
Q = 37,800 J/s.
cd
Silver
100 Trout perch

Percentage of optimal growth rate 60 Shrimp Catfish
80
Barramundi

40

20

0
0 10 20 30 40
Temperature (°C)
FIGUre 11.7 Rate of growth of fish as a function of environmental temperature. The optimal temperatures
for silver perch and barramundi are indicated by the shaded bars. The dependence of growth rate on tempera-
ture for trout, shrimp, and catfish are shown as curves over their growth range. Note the steep drop-off in
growth rate at the high temperature portion of the growth ranges for shrimp and catfish. This phenomenon is
common among aquatic animals. (Data from Beall, S. E. and Samuels, G., The Use of Warm Water for Heating
and Cooling Plants and Animal Enclosures, Oak Ridge National Laboratory Report ORNL + TM + 3381,
1971; Mosig, J. and Fallu, R., Australian Fish Farmer: A Practical Guide, Collingwood, Australia: Landlinks
Press, 2004.)