Standard equipment is used in most direct-use projects, provided allowances are made for the nature of geothermal water and steam. Temperature is an important consideration, as is water quality. Corrosion and scaling caused by the sometimes unique chemistry of geothermal fluids may lead to operating problems with equipment components exposed to flowing water and steam. In many instances, fluid problems can be designed out of the system. One such example concerns dissolved oxygen, which is absent in most geothermal waters except perhaps the lowest temperature waters. Care should be taken to prevent atmospheric oxygen from entering district heating waters, for example, by proper design of storage tanks. The isolation of geothermal water by installing a heat exchanger may also solve this and similar water quality-derived problems. In this case, a clean secondary fluid is then circulated through the user side of the system as shown in Fig. 9.
The primary components of most low-temperature, direct-use systems are downhole and circulation pumps, transmission and distribution pipelines, peaking or backup plants, and various forms of heat-extraction equipment (Fig. 9). Fluid disposal is either surface or subsurface (injection). A peaking
FIGURE 9 Geothermal direct-utilization system using a heat exchanger.
system may be necessary to meet maximum load. This can be done by increasing the water temperature or by providing tank storage (such as is done in most of the Icelandic district heating systems). Both options mean that fewer wells need to be drilled. When the geothermal water temperature is warm (<40°C), heat pumps are often used.
The equipment used in direct-use projects represents several units of operations. In the following sections, the major units are described in the order in which they are used to transform geothermal waters for district heating.