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Direct Use of Geothermal Resources 211

100,000
Heat
90,000 Pumps
Bathing
80,000
70,000 Space
60,000 heating
TJ/yr 50,000 Greenhouses Industrial
40,000
Aquaculture
30,000 Snow melting and
HVAC
20,000
Agricultural
10,000 drying
Misc.
0
FIGUre 11.5 Energy utilization in direct use applications, as of 2005. (From Lund, J. W., Freeston, D. H.,
and Boyd, T. L., Geothermics, 34, 691–727, 2005.)
is required to be between 30 and 200 kg/s, depending on the size of the district heating system and
the temperature of the resource) and the temperature of the resource. The geothermal power, P ,
G
that must be provided from a resource is
Q Geo > P = m × C × (T − T ), (11.10)
R
p
G
G
where m is the mass flow rate (kg/s), C is the constant pressure heat capacity of the fluid (J/kg-°K),
p
and T and T are the temperature (°K) of the water from the geothermal source and temperature
R
G
of the return water after it has been through the network, respectively. The heat demand, Q , which
L
will be imposed on the system is a complex function of time. During the day the load can vary by
up to a factor of 3, and will be affected by the seasonal climatic variability. From Equation 11.10 it
is evident that the only variable that can be controlled that will affect P is the return temperature,
G
T , since the other variables are established by the properties of the natural system. Maximizing
R
the temperature drop across the network thus becomes a means to increase the power output of the
system. However, how this is addressed depends upon the operating mode that can be employed for
the system (Figure 11.6).
The simplest systems source hot water from a surface hot water spring or shallow well and feed
the produced fluid to a small network of user sites with direct disposal of the fluid, as in Ranga,
Iceland (Harrison 1994). Such systems generally have more heat available than can be used by the
district; that is, Q Geo >> P . Such systems commonly have high temperature fluids (> 65°C). Since
G
management of P is not necessary, the magnitude of T is unimportant. Nevertheless, development
R
G
of such systems must be done in a way that minimizes heat losses from a pipe that conveys the hot
water to the locations in the network where heat is needed. Addressing this factor is important to
assure that the system is operated efficiently and in a way that does not compromise potential future
use of the resource.
Sites with moderate temperature fluids (50–65°C) are capable of operating in a mode in which
the geothermal water passes through a heat exchanger that efficiently allows the heat to be trans-
ferred to a closed-loop heating network. The fluid in the heating network is pumped to the user sites
at which heat is extracted.
Sites with lower temperature fluids available, down to about 40°C, have been developed that
use heat pumps to increase the temperature of the circulating fluid. These systems require input of
some additional energy to supplement that of the geothermal fluid. Some of that additional heat can
be extracted from the fluid that normally would be the fluid exiting the system.