Process synthesis for the valorisation of low-grade heat: Geothermal brines and industrial waste streams

The increasing demand of power and the need to reduce our dependency on fossil resources represent an opportunity to valorise low-to-medium grade heat streams such as mild hot streams from industry and natural brines into electricity. A systematic approach is required for the simultaneous selection of thermodynamic cycle which includes its configuration, the thermal fluid, and the optimal operating conditions. A methodology that integrates heuristics, for pre-screening, machine learning, to include rigorous thermodynamics, and mathematical optimization, for process flowsheet design is proposed. 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 temperatures over 120 degrees C, the OFRC using Benzene is the configuration of choice in terms of thermodynamic performance, but the ORC provides the most economical electricity. For hot resources below 120 degrees C, the efficiency of both cycles converges, but the best fluid turns out to be R227ea alongside the dual ORC cycle showing better performance and lower cost. The cooling costs present a minimum at Delta T-min equal to 8 degrees C. The results on process design are used to evaluate a exploitation of geothermal resources across Spain.

Automated summary

The increasing demand for power and the need to reduce dependency on fossil resources create an opportunity to convert low-to-medium grade heat streams into electricity. This study proposes a systematic approach to simultaneously select the thermodynamic cycle, configuration, thermal fluid, and optimal operating conditions. Results show that, for temperatures above 120 degrees C, the organic flash Rankine cycle using Benzene is the best configuration in terms of thermodynamic performance, while the organic Rankine cycle provides the most cost-effective electricity. For temperatures below 120 degrees C, the dual organic Rankine cycle using R227ea fluid shows the best performance and lower cost.