The fluid state in transmission lines of direct-use projects can be liquid water, steam vapor, or a two – phase mixture. These pipelines carry fluids from the wellhead to either a site of application or a steam – water separator. Thermal expansion of pipelines heated rapidly from ambient to geothermal fluid temperatures (which may vary from 50 to 200°C) causes stress that must be accommodated by careful engineering design.
The cost of transmission lines and the distribution networks in direct-use projects is significant. This is especially true when the geothermal resource is located a great distance from the main load center; however, transmission distances of up to 60 km have proven economical for hot water (e. g., the Akranes project in Iceland, where asbestos cement covered with earth has been successful).
Carbon steel is the most widely used material for geothermal transmission lines and distribution networks, especially if the fluid temperature is higher than 100°C. Other common types of piping material are fiberglass-reinforced plastic (FRP) and asbestos cement. The latter material, used widely in the past, cannot be used in many systems today due to environmental concerns; thus, it is no longer available in many locations. Polyvinyl chloride (PVC) piping is often used for the distribution network and for uninsulated waste disposal lines in which temperatures are well below 100°C. Conventional steel piping requires expansion provisions, either bellows arrangements or by loops. A typical piping installation has fixed points and expansion points approximately every 100 m. In addition, the
piping has to be placed on rollers or slip plates between points. When hot water pipelines are buried, they can be subjected to external corrosion from groundwater and electrolysis. They must be protected by coatings and wrappings. Concrete tunnels or trenches have been used to protect steel pipes in many geothermal district heating systems. Although expensive (generally more than $300 per meter of length), tunnels and trenches have the advantage of easing future expansion, providing access for maintenance and a corridor for other utilities, such as domestic water, waste water, electrical cables, and phone lines.
Supply and distribution systems can be either single-pipe or two-pipe systems. The single-pipe system is a once-through system in which the fluid is disposed of after use. This distribution system is generally preferred when the geothermal energy is
abundant and the water is pure enough to be circulated through the distribution system. In a two-pipe system, the fluid is recirculated so that the fluid and residual heat are conserved. A two-pipe system must be used when mixing of spent fluids is necessary and when the spent cold fluids need to be injected into the reservoir. Two-pipe distribution systems typically cost 20-30% more than single – piped systems.
The quantity of thermal insulation of transmission lines and distribution networks depends on many factors. In addition to minimize the heat loss of the fluid, the insulation must be waterproof and watertight. Moisture can destroy any thermal insulation and cause rapid external corrosion. Aboveground and overhead pipeline installations can be considered in special cases. Considerable insulation is achieved by burying hot water pipelines. For example, burying
bare steel pipe results in a reduction in heat loss of approximately one-third compared to aboveground steel pipes in still air. If the soil around the buried pipe can be kept dry, then the insulation value can be
Transmission length – miles
25 50 75 100 125
Transmission length – km
FIGURE 12 Temperature drop in hot water transmission line.
retained. Carbon steel piping can be insulated with polyurethane foam, rock wool, or fiberglass. Belowground, such pipes should be protected with PVC jacket; aboveground, aluminum can be used. Generally, 2.5-10 cm of insulation is adequate. In two – pipe systems, the supply and return lines are usually insulated, whereas in single-pipe systems only the supply line is insulated.
In flowing conditions, the temperature loss in insulated pipelines is in the range of 0.1-1.0°C/km, and in uninsulated lines the loss is 2-5°C/km (in the approximate range of 5-15 liters/s flow for 15-cm diameter pipe). It is less for larger diameter pipes; for example, a <2°C loss is experienced in the aboveground, 29-km long and 80- to 90-cm-diameter line (with 10 cm of rock wool insulation) from Nesja – vellir to Reykjavik, Iceland. The flow rate is approximately 560 liters/s and 7h are required to cover the distance. Uninsulated pipe costs approximately half that of insulated pipe and thus is used where temperature loss is not critical. Pipe material does not have a significant effect on heat loss; however, the flow rate does. At low flow rates (off – peak), the heat loss is higher than that at greater flows. Figure 12 shows fluid temperatures, as a function of distance, in a 45-cm-diameter pipeline insulated with 50 mm of urethane.
Several examples of aboveground and buried pipeline installations are shown in Fig. 13. Steel piping is used in most cases, but FRP or PVC can be used in low-temperature applications. Aboveground pipelines have been used extensively in Iceland, where excavation in lava rock is expensive and difficult. In the United States, belowground installations are more common to protect the line from vandalism and to eliminate traffic barriers.
The principal heat exchangers used in geothermal systems are the plate, shell-and-tube, and downhole types. The plate heat exchanger consists of a series of plates with gaskets held in a frame by clamping rods (Fig. 14). The countercurrent flow and high turbulence achieved in plate heat exchangers provide for efficient thermal exchange in a small volume. In addition, compared to shell-and-tube exchangers, they have the advantages of occupying less space, are easily expandable when addition load is added, and cost 40% less. The plates are usually made of stainless steel, although titanium is used when the fluids are especially corrosive. Plate heat exchangers are commonly used in geothermal heating situations worldwide.
Shell-and-tube heat exchangers may be used for geothermal applications but are less popular due to problems with fouling, greater approach temperature
(the difference between incoming and outgoing fluid temperature), and larger size.
Downhole heat exchangers eliminate the problem of disposing of geothermal fluid since only heat is taken from the well. However, their use is limited to small heating loads, such as heating of individual homes, a small apartment house, or business. The exchanger consists of a system of pipes or tubes suspended in the well through which secondary
water is pumped or allowed to circulate by natural convection (Fig. 15). In order to obtain maximum output, the well must have an open annulus between the wellbore and casing, and perforations above and below the heat exchanger surface. Natural convection circulates the water down inside the casing, through the lower perforations, up into the annulus, and back inside the casing through the upper perforations. The use of a separate pipe or promoter to increase the vertical circulation has proven successful in older wells in New Zealand.