Dual-temperature Kalina cycle for geothermal-solar hybrid power systems

Abstract

This thesis analyzes the thermodynamics of a power system coupling two renewable heat sources: low-temperature geothermal and a high-temperature solar. The process, referred to as a dual-temperature geothermal-solar Kalina hybrid cycle, is analyzed in detail and then compared to appropriate single-heat source power systems, in order to assess any thermodynamic synergies. With increasing demand for more efficient renewable sources of power generation, a plant design where the working fluid is heated (and partially vaporized) by low- to medium-temperature geothermal brine, before being further vaporized by solar heat, presents an opportunity for efficient operation of the power plant. Given a set of design parameters and the constrained optimization of decision variables, a design basis plant configuration is first chosen. Then, the power output attained by the Kalina hybrid is compared to that attained by a combination of a geothermal organic Rankine cycle and a solar standalone steam cycle, with the same boundary conditions. The Kalina hybrid plant is found to produce 9.5 MW of power, with 100 kg/s of geothermal brine and a solar-to-geothermal heat input ratio constrained to 1. The system performance is increasing in the working fluid low pressure and decreasing in the ammonia molar concentration, at the cost of a corresponding increase in solar-to-geothermal heat input ratio. On a design power comparison basis, the hybrid configuration displays no thermodynamic synergy between geothermal and solar energy modes. Specifically, the hybrid plant produces 29% less net power than the combined single-energy mode plants. No assessment of possible economic synergies is attempted. Potential changes to the current Kalina hybrid cycle that can lead to higher thermodynamic performance include regenerating heat within the cycle; using the solar high quality heat source in alternative locations in the cycle; employing one pressure-turbine loop instead of two; using reheat between the two turbines; and investigating other plausible working fluid mixtures including hydrocarbons and refrigerants.