Page 234 - Geothermal Energy Renewable Energy and The Environment

222 Geothermal Energy: Renewable Energy and the Environment

The economics of this direct use complex are positive. In 2005, the savings in propane and
electrical costs for the district heating system alone was $43,355. Given the then-current pro-
pane costs, it was determined that the avoided costs of propane would allow a simple payback
time of just over eight years. The laundry facility replaced an older facility that used more
than 26,700 l of propane per year. With the use of geothermal heat instead of the propane for
clothes washing and drying, the avoided cost is sufficient to allow a simple payback period of
about three years. It is anticipated that additional savings of about $78,000 will be achieved in
2009.

synopsIs

Geothermal resources that have temperatures in the range of 10–150°C are capable of provid-
ing heat energy for a variety of direct use applications. Such resources are currently employed
around the world in a broad range of industries. Total installed capacity for direct use applica-
tions is in excess of 273,000 TJ/yr. Direct use applications are based on the fundamental pro-
cesses that control heat transfer; conduction, convection, radiation, and evaporation. The extent
to which each of these processes affect the performance of a particular direct use application
depends upon the specific needs and design of the application. As a result, each system must be
rigorously evaluated for heat losses, demand load, and the magnitude of the potential geother-
mal heat supply. Efficient use of these low- to moderate-temperature resources can be enhanced
by coupling together several applications that cascade from one to the other. Such systems allow
the maximum amount of heat to be used for useful work. Since geothermal direct-use applica-
tions reduce or eliminate the need for a fuel cycle, have a high capacity factor, and reduce fire
risk by eliminating the need for combustion, they can provide significant benefits over tradi-
tional technologies. By reducing or eliminating the fuel cycle, as well as implicitly reducing the
need for electricity generation, they significantly reduce greenhouse gas emissions (Fridleifsson
et al. 2008).

Problems
11.1 For a direct use fish pond application, what are the two most significant sources of heat
loss?
11.2 What methods or strategies could be employed to reduce the heat losses in Problem 11.1?
11.3 Plot the convective heat loss, as a function of wind velocity from 0 to 10 m/s, for a 5 m
by 5 m pool. Assume that the air temperature is 5°C and the pool temperature is 35°C.
Do the same calculation for an air temperature of 10°C.
11.4 Plot how long it would take for the pool in Problem 11.3 to drop by 5°, as a function of
wind speed, if its depth was 1 m?
11.5 Assume a geothermal water resource of 40°C were available. Plot the flow rate neces-
sary to prevent the pool temperature from dropping more than 5°C.
11.6 Consider the effects of radiation and evaporation in the above calculations and replot
the needed input flow rate determined in 11.5.
11.7 Figure 11.1 indicates that curing concrete blocks requires temperatures between about
65°C and 80°C. If a geothermal resource were available with water temperatures of
50°C, is there any method that could be employed to use it for the curing process?
11.8 Using Figure 11.1, propose a cascaded system consisting of four applications that could
be developed using a geothermal resource with a temperature of 93°C.
11.9 If the Canby geothermal resource were functioning at a flow rate of 5 l/s, what changes
might be possible to increase the use of the available energy.
11.10 Discuss three environmental considerations that must be addressed when developing a
direct use application.
11.11 Describe the considerations that would have to be addressed if a district heating system
were to be developed using ground source heat pumps.