Converting geothermal to electric power

The type of energy conversion system used to produce electrical power from a geothermal resource depends on the type and quality (temperature) of the resource. Vapor-dominated resources use conversion systems where the produced steam is expanded directly through a turbine. Liquid-dominated resources use either flash-steam or binary systems, with the binary conversion system predominately used with the lower temperature resources.

Direct steam systems/vapor-dominated resources

When the geothermal resource produces a saturated or superheated vapor, the steam is collected from the production wells and sent to a conventional steam turbine (see Fig. 1). It would be unusual to see superheated steam from the source as it arrives at surface. The stream is most oftern saturated as produced. If the steam at the wellhead is saturated, steps are taken to remove any liquid that is present or forms prior to the steam entering the turbine. Before the steam enters the turbine, appropriate measures are taken such as steam separators or scubbers upstream of the turbine to remove any liquids or solid debris from the steam flow. Mitigation of corrosive substances contained in the process stream (typically removed with treated water washing) may be required. Normally, a condensing turbine is used; however, in some instances, a backpressure turbine is used that exhausts steam directly to the ambient.

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  Fig. 1—Schematic diagram of a dry-steam power plant.

The steam discharges to a condenser where it is condensed at a subatmospheric pressure (typically a few inches of Hg). The condenser shown in Fig. 1 is a barometric condenser. In a barometric condenser, the cooling water is sprayed directly into the steam, with the cooling water and condensate being pumped to a cooling tower where the condensing heat load is rejected to the ambient. Some plants use surface condensers where the latent heat from the condensing steam is transferred to cooling water being circulated through the condenser tubes. With a surface condenser, the cooling water and condensate are typically pumped to the cooling tower in separate streams. The steam condensate provides a makeup water source for the evaporative heat rejection system. Any excess condensate, together with the tower blowdown, is injected back into the reservoir.

Hydrothermal resources typically contain varying amounts of dissolved minerals and gases that impact both the design and operation of the energy conversion systems. In power cycles where steam is extracted from the geothermal resource and expanded in a condensing turbine, the cycle design must account for the removal of the noncondensable gases extracted from the resource with the steam. If not removed, these gases accumulate in the condenser, raising the turbine exhaust pressure and decreasing power output. When hydrogen sulfide is present in the process steam, it also accumulates in the condenser, though a portion partitions or dissolves in the condensate or cooling water. When the hydrogen sulfide levels are sufficiently high so that some abatement process of the condensate or cooling water is required, surface condensers are typically used to minimize the quantity of water that has to be treated. In addition, the noncondensable gas stream containing hydrogen sulfide must also be treated prior to being released to the atmosphere.

Flash steam systems/liquid-dominated resources

With few exceptions, the fluid in hydrothermal resources is predominantly liquid. Frequently, the reservoir pressure is insufficient to overcome the hydrostatic head in the wellbore and bring the fluid to the surface as a liquid, at flow rates sufficient for commercial production. Depending on the power cycle used, it may be necessary to use downhole pumps to provide the necessary flow. In instances when the reservoir temperature is sufficiently high, the fluid is allowed to flash in the wellbore. This reduces the hydrostatic head in the wellbore and allows more production flow. When flashing occurs in the well, a two-phase fluid is produced from the well. The conversion systems used with this flow condition are typically flash-steam power cycles. In a single-flash cycle, a separator is used to separate the fluid phases, with the steam phase being sent to a turbine.

Typically, in this cycle, the fluid pressure immediately upstream of the separator is reduced, which results in additional flashing of the liquid phase and produces additional steam flow. This single-flash steam power cycle is depicted in Fig. 2. Once the steam leaves the separator, the cycle is very similar to that for a vapor-dominated resource (Fig. 1). The saturated liquid brine leaving the separator is reinjected along with cooling tower blowdown and excess condensate.

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  Fig. 2—Schematic diagram of a single flash-steam power plant.

The dual-flash steam power cycle adds a second low-pressure flash to the single-flash cycle. In the dual-flash cycle, the liquid leaving the first (high pressure) separator passes through a throttling device that lowers fluid pressure, producing steam as the saturated liquid flashes. The steam from this second flash is sent either to a second turbine or, if a single turbine is used, to the turbine at an intermediate stage. The steam exhausting the turbine(s) is condensed with a heat-rejection system similar to that of the steam plant used with a vapor-dominated resource. In the dual-flash cycle, the optimum pressure of the first separator is higher than the optimum flash/separator pressure in a single-flash cycle. Unless the resource temperature is high, the optimum first-stage pressure can be found using an initial approximation that this separator temperature is at the mid-point of the temperature where flashing starts to occur (liquid reservoir temperature) and 100°C. The second, or low pressure, flash is typically just above atmospheric pressure. As the resource temperature increases, the optimum pressures for the two flash stages increase.

As with the direct steam systems (vapor-dominated resource), flash plants must have provisions to remove noncondensable gases from the heat-rejection system, to remove liquid from the saturated steam before it enters the turbine and, if levels are sufficiently high, remove hydrogen sulfide from the noncondensable gas and condensate streams. In addition, mineral precipitation is generally associated with the flashing processes. This requires the use of chemical treatment in the wellbore, separators, and injection system to prevent the deposition of solids on piping, casing, and plant-component surfaces. The potential for mineral precipitation increases as the fluid is flashed because the dissolved minerals concentrate in the unflashed, liquid phase.

Binary systems/liquid-dominated resources

A binary conversion system refers to a power cycle where the geothermal fluid provides the source of energy to a closed-loop Rankine cycle that uses a secondary working fluid. In this closed loop, the working fluid is vaporized at pressure using the energy in the geothermal fluid, expanded through a turbine, condensed, and pumped back to the heat exchangers, thus completing the closed loop. This type of conversion system is used commercially with liquid-dominated resources where the fluid temperatures are below ~200°C. Typically, this conversion system requires the use of pumped production wells to provide necessary well flow and to keep the fluid in a liquid phase to prevent minerals from scaling of heat exchanger surfaces. The system is depicted schematically in Fig. 3 with an evaporative heat-rejection system.

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  Fig. 3—Schematic diagram of a binary power plant.

In some areas where geothermal resources are found, there is little water available for evaporative heat-rejection systems. In these cases, the cooling tower and condenser, shown in Fig. 3, are replaced with air-cooled condensers. A commercial plant that uses this sensible heat-rejection system is shown in Fig. 4. Typically, all of the geothermal fluid that passes through the binary plant heat exchangers is injected back into the reservoir. This is environmentally desirable, as it effectively eliminates all emissions to the ambient and, more importantly, provides a recharge to the reservoir to maintain its productivity. The working fluids used in these plants are volatile and typically are in a gas phase at room temperature and atmospheric pressure. They liquefy at moderate pressures, and the entire working-fluid system is generally operated at above atmospheric pressure to prevent the leakage of air into the closed loop. Existing plants use isobutane, pentane, or isopentane working fluids.

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  Fig. 4—Picture of the air-cooled binary plant near Mammoth Lakes, California. Photo by Joel Renner, used with permission.

The performance of the binary system depends on a number of factors, including the resource conditions and the selection of the working fluid. These plants are usually used with lower temperature resources because, relative to the flash-steam power cycles, the binary cycle can produce more power from a given quantity of geothermal fluid. Cycles can be designed to maximize the conversion of the geothermal energy to power. In simple cycles, the working fluid is boiled at a single pressure. One method of improving performance is to boil at multiple pressures (the working-fluid flow stream is split into high- and low-pressure stream paths). Another proposed technique is the heating and vaporization of the working fluid above the fluid’s critical pressure. Both of these design strategies attempt to match the working fluid heat addition process to the sensible cooling of the geothermal fluid (as depicted on a plot of temperature vs. total heat transferred). While the supercritical cycle has a higher associated component and pumping costs because of the higher operating pressures, these cycles have fewer components and are less complex than the multiple boiling cycles. They also are more efficient in converting the geothermal energy into electrical power.

Conversion efficiency is maximized by minimizing the temperature differences during the heat-addition and heat-rejection processes. The conversion systems that more efficiently convert the geothermal energy to electrical power also tend to be more equipment intensive, especially with regard to heat-transfer areas. If there is a significant cost associated with the production of the geothermal fluid (resource exploration, drilling, surface piping, etc.), these costs will offset the additional energy-conversion-system cost and the more efficient plants will produce power at lower cost.

Studies have shown that power cycles using working fluids of mixed hydrocarbons have superior performance (in terms of power produced from a unit quantity of geothermal fluid) to those having single-component working fluids. Mixtures have an advantage because their isobaric phase changes (boiling and condensation) are nonisothermal. This allows the vaporization of the mixture to more closely match the sensible cooling of the geothermal fluid. Perhaps more importantly (in terms of reducing cycle irreversibility), this characteristic allows the desuperheating and condensing of the working fluid to more closely approach the sensible heating profile of the cooling fluid (water or air).

A binary cycle is being commercially developed that uses an ammonia-water mixture as the working fluid instead of a hydrocarbon. In this cycle, a great amount of recuperative preheating of the working fluid is accomplished with the superheat in the turbine exhaust. Though the cycle has a more complex heat-exchanger train than indicated by the flow schematic in Fig. 9.16, it is more efficient in converting the geothermal energy into electrical power. The systems using this cycle are called Kalina Cycle^® systems.

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See also

Geothermal energy

Geothermal exploration

Geothermal drilling and completion

Geothermal reservoir engineering

Geothermal production measurement

PEH:Geothermal_Engineering