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Direct or non-electric use of geothermal energy refers to the immediate use of the energy for both heating and cooling applications. Geothermal direct use goes back to when people used hot springs for bathing, cooking food, and loosening feathers and skin from game. Today, the primary forms of direct use include heating swimming pools and baths or therapeutic use (i.e., balneology), space heating and cooling (including district heating), agriculture (mainly greenhouse heating, crop drying, and some animal husbandry), aquaculture (heating mainly fish ponds and raceways), and providing heat for industrial processes and heat pumps (for both heating and cooling). In general, the low geothermal fluid temperatures (i.e., <300°F) required for direct heat use are available throughout most of the United States, but are more abundant in the western states.
A low-temperature, geothermal direct-use system is made up of several key components, including:
- A production facility
- Downhole and circulation pumps and wells
- A mechanical system
- Transmission pipelines and distribution networks
- Heat exchangers
- Heat convectors
- Cooling systems (refrigeration)
Peaking or back-up plants
- Disposal system
- Storage pond
- Injection well.
A well is drilled into a geothermal reservoir to provide a steady stream of hot water. A production facility is used to bring up the water, and a mechanical system delivers the heat directly for its intended use. The cooled water is injected underground or into a storage pond via a disposal system.
A downhole pump is a tool used in a well to lift the fluid to the surface. Unless the well is artesian, downhole pumps are needed, especially in a large-scale direct-use system.
The fluid state in transmission pipelines of direct-use projects can be liquid water, steam vapor, or a two-phase mixture (i.e., steam and water). These pipelines carry fluids from the wellhead to either a site of the application, or a steam-water separator. Thermal expansion of metallic pipelines heated rapidly from ambient to geothermal fluid temperatures (which could vary from 120°F to 390°F) causes stress that must be accommodated by careful engineering design. The cost of transmission pipelines 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.
Heat exchangers are commonly used to transfer the heat from the geothermal water to a secondary fluid. The heating of individual rooms and buildings is achieved by passing this heated secondary fluid through heat convectors (or emitters) located in each room. Cooling can occur with an absorption refrigeration system.
A peaking system may be necessary to meet maximum load. This can be done by increasing the water temperature or by providing tank storage. Both options mean that fewer wells need to be drilled. When the geothermal water temperature is warm (below 120°F), heat pumps are often used. The equipment used in direct-use projects represents several units of operations.
Fluid disposal is either surface, in a storage pond, or subsurface, through an injection well.
Types of Technology
Geothermal direct heating systems have a few technology options to consider.
The two most common types of downhole pumps are lineshaft pump systems and submersible pump systems. Both types have been used for many years for cold water pumping and more recently in geothermal wells (for example, lineshafts have been used on the Oregon Institute of Technology campus in 192°F water for 55 years). If a lineshaft pump is used, special allowances must be made for the thermal expansion of various components and for oil lubrication of the bearings. The lineshaft pump is preferred over the submersible pump in conventional geothermal applications because it costs less and has a proven track record. However, for setting depths exceeding 800 ft., a submersible pump is required.
Supply and distribution networks can consist of either a single-pipe or a two-pipe system. The single-pipe system is a once-through system where 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 re-circulated so the fluid and residual heat are conserved.
A two-pipe system must be used when mixing of spent fluids is called for, and when the spent cold fluids need to be injected into the reservoir. Two-pipe distribution systems cost typically 20% to 30% more than single-pipe systems.
Steel piping is used in most cases, but fiberglass reinforced plastic or polyvinyl chloride (PVC) can be used in low-temperature applications. Above-ground pipelines have been used where excavation in rock is expensive and difficult; however, below-ground installations are more common to protect the line 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. The counter-current 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 advantage of occupying less space, they can easily be expanded when additional load is added, and are typically 40% cheaper. The plates are usually made of stainless steel, but titanium can be used when the fluids are especially corrosive. Plate heat exchangers are commonly used in geothermal heating systems in the United States.
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 the larger size as compared to the plate type.
Downhole heat exchangers eliminate the problem of disposal of geothermal fluid, since only heat is taken from the well. However, their use is limited to small heating loads, such as the heating of individual homes, a small apartment, house, or business.
Cooling can be accomplished from geothermal energy using lithium bromide and ammonia absorption refrigeration systems.
The lithium bromide system is the most common because it uses water as the refrigerant. However, it is limited to cooling above the freezing point of water. The major application of lithium bromide units is for the supply of chilled water for space and process cooling in either one- or two-stage units. The two-stage units require higher temperatures (about 320°F), but they also have high efficiency. The single-stage units can be driven with hot water at temperatures as low as 180°F. Lower geothermal water temperatures result in lower efficiency and require a higher flow rate. Generally, a condensing, or cooling, tower is required, which will add to the cost and space requirements.
Direct geothermal resources fill many needs: space heating and cooling, greenhouse heating, industrial processing, and hot water, to name a few. Geothermal district heating systems can save consumers 30% to 50% of the cost of natural gas heating. It is important to consider using a geothermal fluid several times to maximize benefits. For example, the heat could be used for process heat and the resulting lower-temperature fluid could be used for space heating. This multi-stage utilization, where lower water temperatures are used in successive steps, is called cascading or waste heat use.
The temperature and the flow rate of the resource affect the design of the mechanical systems. Two main temperature differences influence the feasibility, flow requirements, and the design of the heating equipment.
The first is the difference between the available geothermal water and the temperature needed for the intended use or process. This difference determines the feasibility of the system. The greater the difference in temperature, the smaller the size and lower the cost of the heat transfer equipment.
The second is the difference between the temperature of the geothermal water entering the system and leaving the system. This temperature difference determines the geothermal water flow rate necessary to provide the heat to the system. Larger temperature differences enable lower flow rates through the system, but the leaving geothermal temperature cannot be lower than the process temperature.
Geothermal projects require a relatively large initial capital investment, with small annual operating costs thereafter. Thus, a district heating project, including production wells, pipelines, heat exchangers, and injection wells, may cost several million dollars. By contrast, the initial investment in a fossil-fuel system includes only the cost of a central boiler and distribution pipelines. The annual operation and maintenance (O&M) costs for the two systems are similar, except that the fossil-fuel system may continue to pay for fuel at an ever-increasing rate; while, the cost of the geothermal fuel is stable. The two systems, one with a high initial capital cost and the other with high annual costs, must be compared. The following table quantifies the cost of various direct-use types based on experiences in the United States.
|Application||Capital $/kW||Cost/year $/kWyr||O&M $/kWyr||Total $/kWyr||Capacity Factor||Unit Cost cents/kWh|
|Residential Space Heating*||800||71.1||7.1||78.2||0.29||3.08|
|Comm./Inst. Space Heating*||500||44.4||4.4||48.8||0.25||2.23|
|Greenhouse Gas Heating||250||22.2||2.2||24.4||0.25||1.11|
|Aquaculture Pond Heating||200||17.8||1.8||19.6||0.69||0.32|
|Geothermal Heat Pumps**||850||75.5||7.6||83.1||0.12||6.78|
Assessing Resource Availability
Most direct-use applications use geothermal fluids in the low-to-moderate temperature range between 120°F and 300°F. In general, the reservoir can be exploited by conventional water well drilling equipment. Low-temperature systems are also more widespread than high-temperature systems (300°F); so, they are more likely to be located near potential users. In the United States, for example, of the 1,350 known or identified geothermal systems, 5% are above 300°F, and 85% are below 195°F. Most geothermal resources are located in the western United States, indicated by the red and orange colors in the map below.
Resource assessment for direct-use depends on the size of the project. For smaller projects such as individual residences, apartments, and businesses, and for small greenhouse and aquaculture projects, only local experience, information on adjacent wells (if any), and data on the local geology are investigated. If this information appears positive, an exploration well is drilled and should be anticipated to serve as the production well.
Standard equipment is used in most direct-use projects, provided that allowances are made for the nature of geothermal water and steam. Temperature is an important consideration, and so is water quality. Corrosion and scaling caused by the unique chemistry of geothermal fluids may lead to operating problems with equipment components exposed to flowing water and steam. In many instances, problems caused by fluids can be designed out of the system.
Relevant Codes and Standards
Design standards for geothermal direct-use systems typically involve two components:
- Below-ground installation such as drilling wells, casing, and pumps
- Above-ground installations such as pipelines, pumps, valves, heat exchangers, in-building heat convectors, refrigeration equipment, and low temperature components such as heat pumps.
The below-ground equipment standards are usually specified for high temperatures (above 212°F or 100°C) resources by state and country regulations and standards that would require special values, such as blow-out preventers and drilling muds. These are usually regulated and inspected by departments of geology and mineral industries or local level organizations. Low temperature resources (below 212°F or 100°C) are usually regulated as standard water wells under the supervision of water resources departments or similar agencies.
The following additional resources can provide further detail on geothermal direct-use technology.
- Geothermal Education Office provides facts about geothermal energy, view maps of geothermal energy use worldwide, and classroom materials.
- Geo-Heat Center offers extensive information about geothermal technical assistance to advance geothermal development and publications available for downloading.
- DOE's Geothermal Energy Technologies Office researches, develops, and validates innovative and cost-competitive technologies and tools to locate, access, and develop geothermal resources in the United States.
- Stanford University Geothermal Program provides a general overview of the Stanford Geothermal Program and the research being done there.
- Geothermal Resources Council
- Geothermal Education Office
- Geothermal Energy Association
- International Geothermal Association
- National Geothermal Academy, University of Nevada