Source:ThinkGeoEnergy – Geothermal Energy News
Original URL:https://www.thinkgeoenergy.com/study-evaluates-the-economics-of-geothermal-hybrid-power-plants/
A new study done by a group of researchers from the National Renewable Energy Laboratory (NREL) and Idaho National Laboratory (INL) looks at the technical and commercial viability of geothermal hybrid power plants. The study specifically analyzes scenarios for a geothermal-natural gas hybrid plant, as wel as a “triple-hybrid” plant that combines natural gas, solar thermal, thermal energy storage, and geothermal.
The full paper, “Techno-Economic Analysis of Greenfield Geothermal Hybrid Power Plants using a Solar or Natural Gas Steam Topping Cycle,” by Wendt et. al. can be accessed via this link: https://doi.org/10.2172/2372872.
Leveraging efficiency and flexibility
Geothermal power plants face several challenges to competitively provide power to U.S. energy markets. One such challenge is the dependence on traditional PPAs, which do not compensate for flexible operation of geothermal plants and limit the ability of the geothermal system to balance electricity supply and demand or provide frequency regulation when other renewable generation decreases.
Geothermal deployment in the United States has also been traditionally limited, typically occurring in the western region where higher temperature hydrothermal resources (e.g. 175–225°C) are most easily accessed.
U.S. geothermal deployment has traditionally occurred in the Western United States where higher temperature hydrothermal resources (e.g. 175–225 °C) are most easily accessed. Low-to-medium temperature resources (e.g. 90–150°C) may be accessed over a larger proportion of the U.S. and provide an opportunity to increase deployment of geothermal. However, lower temperatures result in a lower thermal efficiency and increased generation costs.
The study proposes solar thermal- and natural-gas combustion waste heat recovery-based topping cycle hybridization of geothermal binary power plants. This approach provides several benefits that may allow geothermal power plants to generate power at more competitive costs.
- The addition of heat from solar thermal and natural gas-combustion waste heat to a geothermal power plant provides additional input that can be converted to electricity.
- The temperature of heat from concentrating solar collectors or natural gas combustion is higher than that of geothermal heat from low-to-medium temperature resources, improving the efficiency of conversion of thermal to electrical energy.
- Integration of solar thermal systems with thermal energy storage and natural gas combustion means power generation can occur during peak demand periods.
Hybrid geothermal power plant configurations
Several models for hybrid geothermal operations were proposed by the study.
- Hybrid geothermal – concentrating solar power
A hybrid geothermal-solar power plant configuration with a steam Rankine topping cycle and an organic Rankine bottoming cycle was investigated. Hybrid plant configurations were investigated in which both the geothermal heat and the solar heat rejected from the topping cycle were used to vaporize the ORC working fluid.
Evaluation of the fraction of geothermal heat to be used for ORC working fluid vaporization identified that the hybrid plant performance was maximized (relative to the combined performance of a standalone geothermal plant and standalone solar plant operating off of the same geothermal and solar thermal resources, respectively) when all geothermal heat was used for preheating the ORC working fluid and all solar heat rejected from the topping cycle was used for vaporizing the ORC working fluid (the optimal design point configuration does not use any geothermal heat for vaporizing the ORC working fluid).
- Hybrid geo-gas engine
A hybrid geo-gas plant configuration using a natural-gas reciprocating engine generator set and waste heat to drive an ORC-bottoming cycle. The geothermal heat is used to preheat the ORC working fluid and the waste heat from the reciprocating engine is used to vaporize the ORC working fluid. This allows the ORC heat exchangers to have small MTD values while extracting the maximum quantity of heat from the geothermal fluid.
As with the hybrid geo-solar plant, the ORC-bottoming cycle can continue to operate when the heat from the gas engine is unavailable. This requires the geothermal fluid to supply the heat for both preheating and vaporizing the ORC working fluid. Since the geothermal fluid flow rate is assumed to remain constant, the ORC working fluid flow rate and corresponding ORC net power generation will be reduced when only the geothermal heat source is available.
- Tripe hybrid geo-gas-solar
A hybrid geothermal-gas cycle was considered to evaluate potential performance and CO? emissions reduction benefits. The hybrid geothermal-gas cycle recovers heat from the gas turbine exhaust stream in a manner similar to the conventional NGCC cycle. However, in contrast with the conventional NGCC cycle, the hybrid geo-gas cycle uses a back-pressure steam turbine such that the heat rejected from the steam Rankine cycle can be transferred to an ORC-bottoming cycle.
Similar to the hybrid geo-solar plant, 18 the ORC-bottoming cycle has a design point configuration in which the geothermal resource provides the heat to preheat the ORC working fluid while the heat rejected from the steam Rankine cycle is used to vaporize the ORC working fluid.
Modeling and results
- Hybrid geothermal – concentrating solar power
A hybrid plant design combining an ORC-bottoming cycle and steam-topping cycle tapping a solar thermal resource and low- temperature geothermal resource (<120 degrees C) produces a lower LCOE than a standalone geothermal-only system. This presents a compelling case for a hybrid plant design for the economic development of geothermal resources in locations with low geothermal resource temperatures.
However, in areas with higher geothermal resource temperatures (>120 degrees C), the geothermal-only plant has a lower LCOE than the hybrid cycle and thus could be developed without the need for solar heat addition.
- Hybrid geo-gas engine
The case study analysis for a geothermal-natural-gas reciprocating engine hybrid plant indicates that it can produce power at a lower LCOE compared to a standalone geothermal plant provided that the natural gas engine operates for more than 12 hours per day.
The LCOE is comparable to that of the standalone natural-gas reciprocating engine. However, the hybrid plant has the benefit of also reducing the carbon emissions of power generation relative to the natural-gas reciprocating engine. This may represent a scenario in which the hybrid plant provides an opportunity for the deployment of a low-temperature geothermal resource that otherwise may have an LCOE too high to develop and operate as a standalone resource, while also reducing the carbon intensity of natural-gas generation sources.
- Triple hybrid plant
Analysis of a “triple-hybrid” plant that combines natural gas, solar thermal, thermal energy storage, and geothermal suggest that the model has a significantly higher energy generation and revenue than a standalone NGCT or the original geothermal-solar hybrid.
The triple-hybrid design benefits most from using a smaller solar field so that the solar energy can be dispatched at the most valuable times available. The triple-hybrid plant also has a lower LCOE than the standalone NGCT.
The triple-hybrid plant was evaluated making simple assumptions about the dispatch profile of the gas cycle, and more nuanced and realistic dispatching schedules should be analyzed in future work.
Source: US DOE Office of Scientific and Technical Information
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