Part 1: High temperature and low temperature systems
By Ed Butts, PE, CPI
This edition of Engineering Your Business will begin a two-part series outlining the basic design concepts and pitfalls associated with geothermal and water source (groundwater) heat pump systems.
Introduction to Geothermal Energy
Geothermal energy, simply defined, is thermal energy generated and stored within the Earth. Substantial heat occurs in the center of the Earth as deeper depths result in higher temperatures and more pressure. The combined impact from the heat and pressure inside the Earth is intense enough to melt and liquify solid rock, known in the melted and semi-liquid form as magma. Since magma is less dense than the surrounding rock formations, it can often rise to the surface and flow overland.
Most of the time, though, magma remains confined beneath the surface, generating significant heat in surrounding rock formations along with the groundwater that has become trapped within or adjacent to those rocks. Occasionally, that trapped water under significant pressure also escapes through cracks in the Earth to form isolated pockets or pools of hot water (hot springs) or bursts from the ground through a mixture of hot water and steam (geysers). The rest of this heated water remains contained in underground pools or large water bodies called geothermal reservoirs.
Geothermal energy (Figure 1) in all its forms is used for various purposes such as electrical power generation, comfort heating, and recreational uses. It can be subdivided into high, moderate, and low temperature classifications—which can each involve various types and methods of water supply schemes and designs.
High temperature geothermal supplies are generally classified as steam and are those in which the ambient temperature exceeds 428°F. Moderate geothermal applies to water with a median ambient temperature range between 212° to 428°F. Low temperature water describes subsurface water (groundwater) with an ambient temperature below 212°F—although the most common range of low temperature groundwater typically occurs between 50° and 80°F.
For purposes of this column, geothermal systems will be separated into two basic categories of groundwater or fluid temperature: high temperatures (above 212°F) and low temperatures (212°F and below).
High Temperature Geothermal Systems
High temperature geothermal systems use water or steam above 212°F, primarily for electrical power generation. This directly drives a steam turbine and generator using processes such as: dry steam (Figure 2), flashing steam (Figure 3), or using an alternate third method referred to as a binary cycle (Figure 4), which obtains the required energy from a geothermal source to propel an alternate fluid or mixture to generate the rotational energy through a turbine that also spins a generator.
Worldwide, more than 12,000 megawatts of electrical power is produced using geothermal resources in more than 80 countries.
Power generated from geothermal sources is cost-effective, reliable, sustainable, and environmentally friendly, but has historically been limited to areas in seismic zones, especially those near tectonic plate boundaries.
Technological advances have dramatically expanded the range and size of viable geothermal resources beyond power generation, especially for applications including home or space heating. Although geothermal wells release greenhouse gases from deep within the Earth, these emissions are much lower than those emitted from the use of fossil fuels (coal, natural gas).
Geothermal energy is available in vapor-dominated, liquid-dominated, or combined (two-phase) forms. Vapor-dominated sources offer temperatures from 464° to 572°F that produce superheated steam. Liquid-dominated reservoirs are more common in geothermal sources with temperatures exceeding 392°F. Flash plants are the most common way to generate electricity from these sources.
Most geothermal wells individually generate between 2 to 10 megawatts of electrical equivalent power. Steam is separated from liquid via cyclone separators, while the liquid is returned to the reservoir for subsequent reheating and reuse. In areas where hot springs or geothermal reservoirs are situated near the Earth’s surface, geothermal water can be pumped through a heat exchanger, which transfers the heat from the water into the building’s heating system. The spent water is injected back down a well into the reservoir to be reheated and subsequently reused again.
Design of High Temperature Extraction Wells
Drilling and exploration for deep geothermal resources is expensive. Wells exceeding a depth of 20,000 feet are not uncommon, plus the inherent nature and chemical quality of the superheated fluid and steam requires design and operating considerations far beyond that needed for a typical water well.
Due to the common similar characteristics of temperature and depth to an oil well, a high temperature geothermal supply well is often drilled to these same standards and using the same declining casing string diameters with increased depth (telescoping casing). The primary considerations revolve around the number of proposed casing strings, along with the diameter and depth of each string and the requirement for adequate surface sealing to prevent the migration of heated fluid from the wellbore into surrounding shallow formations.
Occasionally, the thermal expansion or depth characteristics of a geothermal well can result in stresses beyond the safe level of the casing material and associated wall thickness from elevated temperatures or internal stresses, possibly resulting in bursting (from tension stresses) or crushing (from compression stresses) developed within the pipe string. This can present a risky condition when the casing string is much longer than permitted for the material and wall thickness, or a portion is subjected to higher temperatures with the remaining casing out of the hot fluid at a much lower value.
Many high temperature geothermal supplies develop flowing wells or geysers and therefore do not require pumping to the surface. In cases where pumping is required, as with an oil well, design of a pumping system is especially critical for factors including thermal expansion of components, motor cooling (for submersible units), submersible pump cable selection, and potential corrosion from chemistry incompatibility.
The actual design of a high temperature geothermal well or pumping system is beyond the scope of this column and should only be performed by qualified and experienced personnel using recognized engineering criteria.
Enhanced Geothermal Systems (Figure 5), known in some applications as Aquifer Thermal Energy Storage Systems, are similar in concept to the principle of Aquifer Storage and Recovery, in which water is actively injected into wells to be stored and subsequently pumped back from the aquifer and used for heating or cooling.
The water is injected under high pressure to expand (fracture) existing rock fissures. This enables the water to freely flow in and out of the wellbore for a standing column type of installation or repumped from a separate well for a two-well system. The technique was adapted from oil and gas extraction techniques, but the geologic formations are usually deeper in depth and no toxic chemicals are used—reducing the possibility of any surface environmental damage. Drillers can also employ directional drilling to expand the size of the reservoir.
Low Temperature Geothermal Systems
A specific use of geothermal energy, especially in regions with stable lower groundwater temperatures, is vested in water source heat pumps where typical groundwater temperatures average 45° to 80°F. This range is acceptable for use in both comfort heating and air conditioning since this groundwater temperature range is generally within the general criteria for efficiently accomplishing both purposes.
At each building or home, the water is circulated through a heat pump coil, where heat is either extracted from the water (during heating mode) or rejected to the water (during cooling mode). At 15 to 20 feet below ground surface, the soil and water remain within a constant temperature range of 50° to 60°F throughout the year.
Although the mean Earth temperature varies throughout the United States based on latitude (Figure 6), the stability of the groundwater temperature, which generally mirrors the local mean air temperature, provides the ability to design a reliable and predictable heating and cooling system.
For example, although a geothermal source of low temperature water may consist of a fluid at a temperature between 100° to 212°F, a water source heat pump in Oregon extracts or injects the latent heat from or into an ambient source of groundwater within a typical temperature range of 50° to 55°F, while groundwater in Florida or Texas may do the same with water as high as 75°F.
Water source heat pumps are most efficient when the water source is at a stable year-round temperature at or below 50° to 60°F. Use of a water source heat pump can be related to the use of the Earth as a heat source during a heating cycle or as a heat sink (or depository) during a cooling cycle.
Low temperature geothermal projects range from direct use applications (where the geothermal water is used directly to provide heat to a spa or an aquaculture project) to projects that extract heat from or into the water through use of geothermal (water source) heat pumps to provide heating, cooling, and domestic hot water for homes, schools, and buildings.
Low temperature geothermal applications do not generally include projects where the geothermal energy is used to generate electricity. The direct use of low temperature geothermal water resources—including greenhouse heating, warm water aquaculture, space heating, irrigation, swimming pools, and hot spring baths—is generally covered by a separate regulatory process for design and use. Low temperature geothermal projects can be separated into two fundamental types: closed loop (ground-coupled) or open loop (groundwater) with variations of each (Figure 7).
Closed Loop Low Temperature Systems
The closed loop method uses a contained fluid (often an environmentally friendly antifreeze/water solution) that circulates through a series of pipes (called a loop) under the ground or beneath the water of a pond or lake and into a building. In the winter, an electric compressor and heat exchanger pulls the heat from the pipes and sends the warmed air via a duct system throughout the building. In the summer, the process is reversed as the pipes draw heat away from the building and carry it back to the ground or water outside where it is absorbed.
In a ground-coupled or closed loop system, a closed loop of pipe is placed in the ground either horizontally or vertically. The water/antifreeze solution is then circulated through the pipes to either collect heat from the ground in the winter or reject heat to the ground in the summer.
Generally, closed loop projects follow a different regulatory process. In many states, vertical closed loop projects are often regulated as ground source heat pumps and share many of the same regulations as open loop or water well systems.
Conversely, horizontal closed loop projects are often not regulated by state or local water well regulatory agencies, particularly since they do not involve the construction or use of wells. Instead, developers of horizontal closed loop systems are typically required to meet local building, plumbing, and land-use codes and regulations.
Typically, horizontal closed loop installations are simpler to install and are generally more cost-effective for smaller projects. However, a major risk can occur if a leak develops in the loop that releases the solution into the soil, possibly resulting in groundwater contamination. A backhoe or chain trencher usually digs a series of trenches 3 to 6 feet deep. As many as six individual pipes, usually oriented in parallel connections, are buried in each trench, with minimum separations of a foot between each pipe and 10 to 15 feet between trenches. The length of the trenches is determined by the number of pipes installed.
Alternatively, piping can be installed in overlapping loops, similar to that of a Slinky toy. This coil of plastic tubing is spread out and overlapped in a trench and buried. This type of installation can reduce trench length by one-third to two-thirds.
Vertical closed loop systems are generally used for larger buildings and schools. To install a vertical loop, a contractor either installs vertical pipes directly in the ground or bores vertical holes (piles) into the ground. Long, hairpin-shaped loops of pipe are then inserted into the borehole. The hole is subsequently backfilled, plugged, or grouted and the pipes are connected to headers in a trench leading back to the building. This method is less likely to result in groundwater contamination should a leak develop.
Open Loop Low Temperature Systems
Since open loop geothermal projects involve the direct use of low temperature groundwater from wells, this category will be the primary topic. In a typical open loop system, geothermal water is brought up from a well and circulated through a heat exchanger (heat pump). After heat is extracted from or added to the water, the water is then either returned to the underground aquifer or original well by injection or discharged onto the ground or into or under a surface water source.
In some cases, residual geothermal water can also be used for land application through irrigation. This process can involve obtaining water rights along with water quality and well construction permits and often follows similar regulatory and construction processes as conventional water wells. We will outline the basic design concepts for low temperature open loop geothermal well and pumping systems in the next column.
Heat Pump Performance Comparisons
When comparing the relative performance of heating and cooling systems, it is best to avoid the use of the word efficiency, which has a specific thermodynamic connotation. The term Coefficient of Performance (COP) is instead used to define the ratio of useful heat movement per work input. Most vapor-compression heat pumps use electrically powered motors to produce their work input by operating the compressor and fan.
According to the U.S. Environmental Protection Agency, geothermal (water to air) heat pumps can reduce net energy consumption by up to 44% compared with air-to-air heat pumps and up to 72% compared with ordinary or conventional electric resistance heating or furnaces. The COP for heat pumps ranges from 3.2 to 4.5 for air-to-air heat pumps to a high of 4.2 to 5.2 for water source heat pumps, since a well-designed water source heat pump system benefits from the moderate underground water temperatures and the ground acts naturally as a ready storehouse of thermal energy.
During a cooling cycle, a heat pump’s operating performance is described as its Energy Efficiency Ratio (EER) or Seasonal Energy Efficiency Ratio (SEER). As with a COP, a larger EER number indicates better performance. The manufacturer’s literature should provide both a COP to describe performance during a heating mode and an EER or SEER to describe performance in a cooling mode. Actual performance varies and depends on many factors such as manufacturer variances, installation details, temperature differences, and unit maintenance.
This wraps up this basic introduction and overview of geothermal resources. Next month, we will continue our discussion on geothermal system design with a detailed review of the design techniques used for geothermal system well and well pump design.
Until then, work safe and smart.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at firstname.lastname@example.org.