(183e) Process Synthesis for the Valorisation of Low-Grade Heat: Geothermal Brines and Industrial Waste Streams

With the energy demand reaching maxima every year, hot waste streams and brines are promising resources to increase the share of renewable energy. On the one hand, geothermal energy is becoming of age to produce power and its potential can reach 42 million MW. The reservoirs can be from 110 to 220ºC, however the most abundant are within the range of 110ºC and 160ºC [1]. In addition, industrial hot flue gases that have been used within heat integration schemes may still be available at temperatures around 100ºC [2]. Refrigeration for the disposal of these hot streams results in water or power consumption depending on the technology used, a good example of the water energy-nexus [3]. Alternatively, it is possible to further use any of these streams as hot end for thermodynamic cycles to produce power.

A systematic two-stage hybrid approach is presented for the simultaneous selection of thermodynamic cycle, its configuration, thermal fluid and operating conditions for the valorisation of mild hot streams from industry and natural brines. The heuristic-based stage uses previous studies and literature to narrow down the alternatives among the HTF’s and cycle configurations. Next, to capture the thermodynamics of the fluids rigorously, machine learning combined with optimization is used to develop correlations for the entropy and enthalpy of the preselected fluids. The aim is to achieve models that are easier than equations of state to estimate the properties required to perform mass and energy balances to the cycles. Subsequently, the thermodynamic cycles are modelled using an equation-based approach, and optimized to select the number of stages, split fractions, pressures, and temperatures across the flowsheets. Finally, a detailed economic evaluation is performed involving investment and production costs.

The pre-screening yields three fluids, benzene, toluene and 1,1,1,2,3,3,3-heptafluoropropane (R227ea) and two promising cycles, dual pressure organic Rankine cycle (ORC) and organic flash Rankine cycle (OFRC). The mathematical optimization shows that for higher temperatures, the OFRC with two flash stages using Benzene is the configuration of choice in terms of thermodynamic performance, but the two-pressure level (dual) ORC provides cheaper electricity. For temperatures of the hot resource lower than 120ºC, the efficiency of both cycles converges, but the best fluid turns out to be R227ea alongside the dual ORC cycle showing better performance and cost. The power production costs present a minimum at DT_min equal to 8 ºC.

The results of the process analysis are used to evaluate the type of facility, cost and power that can be produced from the resources in Spain as a case study, using the results of this work. Promising production costs can be achieved for hot resources above 120 ºC. Economics of scale can help but a detailed geological study of each reservoir is required to determine the total production capacity of the resources available in Spain.

References

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

[2] Jouhara H, Khordehagh N, Almahmoud S, Delpech B, Chauhan A, Tassou SA. Waste heat recovery technologies and applications. Thermal Sci. Eng. Progress 6 (2018) 268-289

[3] ] IEA Introduction to the water-energy nexus. 2020

https://www.iea.org/articles/introduction-to-the-water-energy-nexus. Last accessed July 2021