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Direct Use of Geothermal Resources 213

In all cases, user sites employ some form of heat exchanger to extract heat from the circulating
fluid. Radiators, or radiant floor, wall, and ceiling installations can accomplish this. In each case,
the more heat that is extracted by the user, the lower will be the value of T . There are several ways
R
to accomplish this effect. Consider, again, the relationship in Equation 11.1,

Q = k × A × dT/dx
cd
that describes conductive heat transfer. The primary mode for heat to be extracted from the
circulating fluid in the district network is by conduction through the radiator walls. In Equation 11.1
there are two means for increasing Q and thus reducing T . The parameter k is in units of W/m-°K.
R
cd
Since a watt is equivalent to a J/s, the energy extracted from the fluid will depend upon the contact
time of the fluid with the radiator. Hence, the slower the flow through the radiator, the greater will
be the heat extraction. Another means for accomplishing the same thing is to increase the area,
A, of contact. Hence, putting oversized radiators in place will also increase Q and decrease T .
cd
R
Managing such networks can then be accomplished by encouraging efficiency by either charging for
water usage, which would favor slower flow rates through a radiator, or charging by the temperature
drop across the site, which could be done by oversizing the heat extraction units or decreasing flow
rates or both.

pipinG and heaT loss
Efficient and sustainable management requires that heat losses unrelated to the primary purpose
of the system be minimized. For a district heating system this ultimately requires minimal loss
of heat in the piping system between the heat source and the users sites. Insulated pipe is com-
mercially available that can allow transmission of hot water over tens of kilometers with heat
losses of 10–15%, depending upon the flow rate. Sizing the pipe to allow sufficient flow, mini-
mal heat loss, and minimal cost requires careful and thorough analysis of the existing demand,
including the likely maximum daily and seasonal loads. In addition, it is also important to deter-
mine whether growth of the system is to be accommodated by the design, or if the system is
intended to support a fixed market size. Either case will dictate different approaches to sizing
the system.

maTerials compaTibiliTy and fluid chemisTry
Finally, compatibility of materials is an important aspect of a district heating system. If a new
system is being constructed that will provide service to a new community, it can be stipulated that
only compatible materials be used in the piping system. However, it is often the case that a district
heating system will be constructed that will serve both new and old structures. Plumbing materials
are likely to be diverse, from copper and steel pipe to various types of plastic pipe. In such a case, it
is important to assess the chemical aggressiveness of the geothermal fluid and the fluid used in the
closed loop, if the latter is part of the system. In some instances, oxygen diffusion through polypro-
pelene or polybutelene pipe may affect corrosion rates in metal plumbing and the risk of that should
be evaluated (Elíasson et al. 2006).
Geothermal fluids generally contain some component of dissolved gases. Although the con-
centrations can be quite small, over a long period of operation accumulation of exsolved gases can
significantly impact heat transfer processes and fluid flow. It is important to consider the geochem-
istry of the resource fluid and evaluate the extent to which degassing may occur. If the potential for
degassing is at all significant, the ability to de-gas the system should be an integral element of the
system design. Commercially available degassing tanks and facilities can be readily designed into
the network to prevent potential problems.