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.
Highlights
Most of the industrial plants continuously discharge large amounts of waste heat, which entails an environmental problem and contributes to an increase in energy costs
The other two membrane properties, i.e., the water permeability, L p, and the salt diffusivity, Ds, were taken equal to zero for the ideal case, since in this way the diffusive flux of the salt from the concentrate to the dilute compartment and the osmotic flux resulted equal to zero. These last two terms are detrimental for the Reverse Electrodialysis (RED) performances as they cause uncontrolled mixing, which leads to a reduction of the driving force and the destruction of part of the exergy contained in the inlet streams [45]
This paper presents a parametric analysis on a close-loop salinity gradient heat engine powered by waste heat, adopting a reverse electrodialysis unit for power generation and a multi effect distillation unit for the regeneration of solutions
Summary
Most of the industrial plants continuously discharge large amounts of waste heat, which entails an environmental problem and contributes to an increase in energy costs. Notwithstanding the RED-MED coupling appears very promising thanks to lower thermal consumption compared to other evaporative technologies [37], the need for a more rigorous approach identifying the real potential of this technology arises With this regard, the exergy efficiency is used to measure the maximum thermodynamic capability of the process to convert heat into power, i.e., how far is the investigated RED-MED HE from the ideal reversible process. To other evaporative technologies [37], the need for a more rigorous approach identifying the real potential of this technology arises
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
