(747b) Optimal Production of Power from Mid-Temperature Geothermal Sources: Scale and Safety Issues

Geothermal energy is a promising source from within the Earth’s interior. In spite of its potential, it has not been extensively exploited yet. High-temperature reservoirs, with temperatures above 220 ºC, are the ones most suitable for commercial production of electricity. However, the medium- and low-temperature water-dominated systems, with temperatures between 110 and 160 ºC, are the most abundant [1]. So far most of the work focuses on evaluating the lay out of the Organic Rankine Cycles (ORC), to determine their capacity of extracting energy from low temperature sources [2] and the fluid that can be used [3]. However, either simplified cycles are used to compare differ fluids or details analysis of just a particular fluid are presented in the literature.

In this work we use a superstructure optimization approach for the mathematical optimal design and operation of Organic Rankine binary Cycles for the recovery of energy from medium and low geothermal sources. We focus on the three fluids, Benzene, toluene and cyclohexane, that our previous study identified as the most promising from environmental, economic and safety points of view [4].In particular, we develop a superstructure for the heat exchanger network to use the hot brine from a geothermal well allowing double and single extractions to optimally design the best cycle, where the thermodynamics of the fluids is included through surrogate models for the enthalpies and entropies. Two solutions are presented, the first one optimizes the energy output and a second one where economic, environmental and safety considerations are included in a normalized objective function to evaluate their effect on the selection of the cycle and on the operating conditions. Finally, the effect of the scale on the investment and production costs is evaluated.

The economic optimization selects a two expansion cycle using toluene as organic fluid, producing 10.4 MW at 0.075€/kWh with an investment of 102M€. Safety considerations slightly change the operating conditions, reducing the pressures and temperatures, but not the selection of the working fluid. Sustainable and economic terms overcome safety issues. The results are competitive with other renewable-based technologies for thermal power production such as CSP or biomass.

[1] Barbier, E. Geothermal energy technology and current status: an overview. Renew. Sust. Energy Reviews 2002. 6 (1-2): 3-65

[2] DiPippo, R. Geothermal Power Plants. Principles, applications, case studies and environmental impact. Elsevier, 2008. ISBN: 978-00-8055-476-1

[3] Wang, J.; Yan, Z.; Wang, M.; Ma, S.; Dai, Y. Thermodynamic analysis and optimization of an organic Rankine Cycle (Orc) using low grade heat source. Energy 2013; 49: 356-365

[4] Martínez-Gómez, J.; Peña-Lamas, J., Martín M.; Ponce–Ortega, J.M. A Multi-objective optimization approach for the selection of working fluids of geothermal facilities: Economic, environmental and social aspects. J. Environ. Manag. 2017; 203 (3): 962-972