Page 222 - Geothermal Energy Renewable Energy and The Environment

210 Geothermal Energy: Renewable Energy and the Environment

The heat loss is then equal to

b
Q = [a × (P − P ) × A × H ]/(2.778e−7). (11.7)
w
ev
a
w
esTablIshInG The FeasIbIlITy oF a dIrecT Use applIcaTIon
The above discussion of heat transfer mechanisms indicates that the total heat loss, Q that must be
TL
accounted for when considering an application is the sum of all relevant heat loss mechanisms,
Q = Q + Q + Q + Q . (11.8)
cv
rd
TL
cd
ev
This value represents the heat loss that must be assumed for the operating conditions of the appli-
cation, and does not take into account the actual heat required to do the work that the application
was designed to perform. The amount of heat, Q , required to perform the function of the designed
L
installation will depend upon the specific process and the size of the operation. Assuming that Q
L
is constant over time, then the geothermal resource must be of sufficient temperature and flow rate
to satisfy
Q Geo > Q + Q . (11.9)
L
TL
For most applications it is likely that seasonal variability will influence the value of Q through
TL
changes in air temperature and other seasonal variables. For this reason the concept of a design
load was developed. The design load is the most severe set of conditions a facility is likely to expe-
rience. For our considerations, the design load becomes the most severe set of conditions likely to
be encountered that will maximize heat loss. Hence, when evaluating the feasibility of a potential
direct use geothermal project it is important to establish whether the resource is sufficient to meet
the most demanding conditions that are likely to be encountered.
Strategies for conducting such an analysis are varied. In some instances, where an abundant
resource is available, it may be suitable to size the facility in such a way that the resource meets
all probable demands. In other instances, possibly for reasons of economics, it may turn out to be
sufficient to design the facility such that the geothermal resource will meet the demand of some
maximum percentage of probable events. The remainder, lower probability conditions can then
be addressed with supplemental energy sources. In the remainder of this chapter we will discuss
these considerations in the context of specific examples. The examples chosen reflect the diversity
of the types of applications that can use warm geothermal fluids and the range of issues they must
address.

dIsTrIcT heaTInG

In 2005, space heating accounted for 55,256 TJh/yr of the total 273,372 TJh/yr of energy consumed
through direct use applications. That is the third largest user of geothermal fluids for direct use,
worldwide (Figure 11.5). The vast majority of these heating systems involve district systems in
which multiple users are linked into a network that distributes heat to users.

evaluaTion and operaTion
The basic requirements for district heating systems are a source of warm geothermal fluid, a net-
work of pipe to distribute the fluid, a control system, and a disposal or reinjection system. The design
of such systems requires matching the size of the distribution network to the size of the available
resource. The resource attributes that must be established are the sustainable flow rate (usually this