Performance Analysis of a RED-MED Salinity Gradient Heat Engine

Abstract

A performance analysis of a salinity gradient heat engine (SGP-HE) is presented for the conversion of low temperature heat into power via a closed-loop Reverse Electrodialysis (RED) coupled with Multi-Effect Distillation (MED). Mathematical models for the RED and MED systems have been purposely developed in order to investigate the performance of both processes and have been then coupled to analyze the efficiency of the overall integrated system. The influence of the main operating conditions (i.e., solutions concentration and velocity) has been quantified, looking at the power density and conversion efficiency of the RED unit, MED Specific Thermal Consumption (STC) and at the overall system exergy efficiency. Results show how the membrane properties (i.e., electrical resistance, permselectivity, water and salt permeability) dramatically affect the performance of the RED process. In particular, the power density achievable using membranes with optimized features (ideal membranes) can be more than three times higher than that obtained with current reference ion exchange membranes. On the other hand, MED STC is strongly influenced by the available waste heat temperature, feed salinity and recovery ratio to be achieved. Lowest values of STC below 25 kWh/m3 can be reached at 100 °C and 27 effects. Increasing the feed salinity also increases the STC, while an increase in the recovery ratio is beneficial for the thermal efficiency of the system. For the integrated system, a more complex influence of operating parameters has been found, leading to the identification of some favorable operating conditions in which exergy efficiency close to 7% (1.4% thermal) can be achieved for the case of current membranes, and up to almost 31% (6.6% thermal) assuming ideal membrane properties.

Thermolytic reverse electrodialysis heat engine: model development, integration and performance analysis

“…In particular, Hu et al [9] reported a maximum exergy efficiency of about 5%, coupling a RED unit with a MED of 10 effects fed with low-grade heat at 95 °C. Palenzuela et al [10] investigated the impact of membrane properties and operating conditions on the integrated system. They reported an exergy efficiency around 7% for the case of real membranes using low-grade heat at 100°C, and up to almost 31% for the case of ideal membranes.…”

Air Gap Diffusion Distillation: A New Method for Solution Separation Technology

“…Hu Junyong [39] , et al analyzed the influence of operating parameters and structural parameters on the system through theoretical models of construction, low temperature multieffect distillation, reverse electrodialysis power generation. Patricia Palenzuela, [40] et al analyzed the performance of RED-MED and Salt Gradient Heat Engine. It is believed that the influence of operating parameters on the overall system is more complex and some favorable operating conditions have been determined.…”

Anomalous Thermo-osmotic Conversion Performance of Ionic Covalent-Organic-Framework Membranes in Response to Charge Variations

“…Thermo-osmotic energy conversion based on reverse electrodialysis (RED) has shown particular promise, as it allows operation at relatively low temperatures compared with those of other thermodynamic cycles and does not have any environmental risk [12][13][14][15][16] . RED-based energy conversion is based on charge separation via ion-permeation membranes to convert the chemical potential/temperature gradient into electricity.…”