Page 224 - Geothermal Energy Renewable Energy and The Environment

212 Geothermal Energy: Renewable Energy and the Environment

(a) 65°C

In 85°C
Users

Return 40°C
(b) In
60°C

40°C Heat 60°C
exchanger Users

Return 40°C
(c) In
50°C 60°C

50°C
30°C Heat
exchanger Users
Heat pump

Return 40°C
FIGUre 11.6 Schematic representation of three operating modes for district heating systems. (a). Geothermal
resource with high temperature fluid (85°C) and high flow rate. Fluid is directly piped to user sites, after
mixing with return fluid. (b). Moderate temperature (60°C) resource is piped through a heat exchange unit
to transfer heat to a closed-loop system. The closed-loop distributes the heat to user sites. The temperatures
along the flow path are meant only to indicate approximate and relative changes. (c). A low temperature (50°C)
resource is piped through a heat exchange unit to transfer heat to a closed-loop system. The heat is supple-
mented by heat extracted from the return fluid by a heat pump. As with b the temperatures along the flow path
are meant only to indicate approximate and relative changes.

Finally, district heating systems can be developed that essentially rely on large scale ground
source heat pump systems. These systems can utilize fluids at temperatures less than 40°C, as
discussed in the previous chapter. We will not consider these further in this chapter.

manaGinG reTurn TemperaTure
In all operating modes, it is crucial to establish a method for controlling T . There are several ways
R
to do this.