Flexible Geothermal Power Dispatch Using Storage Tanks

Dispatchable energy resources are key for reliable district heating and power supply. Whereas the development of fossil fuel resources has dominated the supply for dispatchable capacity, this trend is forecasted to slow down due to the growing climate change concerns and transition to renewable resources. Geothermal energy has always been an economical resource for district heating and baseload power. With the decline of fossil fuel plants, geothermal facilities have been expanding beyond baseload to supply flexible heat and electricity. There is only a limited number of studies that investigate approaches to facilitate geothermal heat/power production flexibility, including venting off geofluids to the atmosphere, wellhead throttling, turbine inlet pressure control, and turbine bypass to the condenser. This study aims to techno-economically evaluate the effectiveness of using surface hot water storage tanks in achieving flexible geothermal production to meet power and/or heat demand optimally.

We examine a 150 deg C, liquid-dominated geothermal resource developed with a 30-MW subcritical binary geothermal power plant. We evaluate the techno-economics of upgrading this power plant with storage tank/s to provide flexible capacity to a power grid that suffers from reliability concerns resulting from high penetration rates of intermittent renewable resources. Historical data of net load, power clearing prices, capacity clearing prices are used as inputs to optimize the geothermal hot water tank/s design along with the charge and discharge schedule. The design is constrained to allow for continuous flow in production wells without the need for throttling, where it provides a bypass of geofluid into the storage tank/s during charge hours, and from storage to the power plant during discharge hours. The optimization objective aims at maximizing the profitability of this geothermal power plant while it is exposed to the deregulated and competitive retail energy market. Therefore, it charges during off-peak hours (lower prices) and discharges during on-peak hours (higher prices). Radiative, convective, and conductive heat losses are incorporated in finding the optimal hot water tank/s dispatch schedule.

The result of this work allows for determining the levelized capital cost and internal return of revenue of the project in the presence of a storage facility compared to the baseline alternative where only baseload power is generated. The results are also compared to the scenario where the geothermal power plant operates based on $/MWh price following a power purchase agreement. This aims to evaluate how feasible it is to operate with flexible dispatch in the absence of power purchase agreements.