An Exergy Assessment of Converting Low-Grade Heat to Energy With a Novel Osmotic Heat Engine

The question of “how to fuel the future” is a growing challenge. Identifying reliable energy resources that promote a clean energy economy is essential to offsetting the demand of non-renewable energy resources. For example, salinity gradients and low-grade heat are two largely untapped sources of potential energy. Advanced membrane-based processes such as pressure retarded osmosis (PRO) and reverse electrodialysis (RED) can convert the salinity gradient between two streams to useful energy/work. Membrane distillation (MD) is a thermally driven membrane process that can use low-grade heat (from power generation or industrial processes) to simultaneously concentrate a saline feed stream and produce distilled water. The current study aims to develop and test these three membrane-based processes (MD, PRO, and RED) that when hybridized in a closed-loop system, can utilize low-grade heat to produce useful work.

In the preliminary stages of this study, a system-based approach was used to evaluate the potential applications of these systems and to develop a model that yields further insight to their Carnot efficiencies. Bench scale experiments will be performed to optimize these processes individually and then combined. Results from these experiments will be used to design and fabricate a pilot scale system that demonstrates the performance of the closed-loop heat engine. A techno-economic and environmental life cycle assessment of the proposed systems against commercial competitors (e.g., Organic Rankine Cycle) will be performed. Results will be used to assess the applicability of this technology to market and its potential impact in securing the nations economy and energy resources.