Assessment of a Geothermal Combined System with an Organic Rankine Cycle and Multi-effect Distillation Desalination

Aida Farsi,Marc A Rosen

doi:10.1007/s41748-021-00275-w

Abstract

An analysis is reported of a geothermal-based electricity-freshwater system in which an organic Rankine cycle is integrated with a multi-effect distillation desalination unit. The system is driven by geothermal hot water extracted from the production well. Mass, energy, entropy, and exergy rate balances are written for all system components, as are energy and exergy efficiency expressions for each subsystem. The exergy destruction rate associated with the temperature and chemical disequilibrium of the freshwater and brine with the reference environment are taken into account to reveal accurate results for irreversibility sources within the desalination process. The developed thermodynamic model is simulated using thermodynamic properties of the working fluids (i.e., ammonia, seawater, distillate, and brine) at each state point. A sustainability analysis is performed that connects exergy and environmental impact concepts. That assessment expresses the extent of the contribution of the system to sustainable development and reduced environmental impact, using exergy methods. Results of the sustainability analysis indicate that, with an increase in the reference environment temperature from 20 to 35 ^circ{rm C}, the exergy destruction rate decreases for the multi-effect distillation and organic Rankine cycle systems respectively from 6474 to 4217 kW and from 16,270 to 13,459 kW. Also, the corresponding sustainability index for the multi-effect distillation and organic Rankine cycle systems increases from 1.16 to 1.2 and 1.5–1.6, respectively, for the same increase in reference environment temperature.

Highlights

Increasing freshwater demands in irrigation, industrial, and municipal sectors have placed significant stresses on available freshwater supply options like rivers, lakes, and aquifers to satisfy the global need for freshwater
Integration of organic Rankine cycle (ORC) and multi-effect distillation (MED) systems driven by geothermal energy can potentially increase exergy efficiency, and reduce both exergy destruction and environmental impact
The exergy destruction rates associated with temperature disequilibrium of the freshwater and temperature and chemical disequilibrium of the brine, respectively, are responsible for 6% and 14% of total exergy destruction rate of the MED unit

Summary

Introduction

Increasing freshwater demands in irrigation, industrial, and municipal sectors have placed significant stresses on available freshwater supply options like rivers, lakes, and aquifers to satisfy the global need for freshwater. Karytsas et al (2002) showed that employing low/medium-temperature geothermal heat can potentially offset the consumption of 453,600 kg of oil per year Another geothermal-based MED plant was built at Texas, with an average freshwater production rate of 120 m3/day (Birney et al 2019). It is shown that integration of the ORC and MED systems potentially improves the exergy efficiency, and reduces the exergy destruction rate and environmental impacts For this purpose, mass, energy, entropy, and exergy rate balances are developed for each component of the geothermal combined electricity-freshwater system. A sustainability analysis is performed to show how exergy methods assist in understanding renewable energy technologies and in making energy utilization more efficient and less likely to cause environmental impacts. Relations between exergy efficiency, environmental impact, and sustainability for the combined system are investigated

System Description

System Model and Analysis

Results and Discussion

Conclusions