Abstract
Whilst the efficiency of reverse electrodialysis (RED) for thermal-to-electrical conversion has been theoretically demonstrated for low-grade waste heat, the specific configuration and salinity required to manage power generation has been less well described. This study demonstrates that operating RED by recycling feed solutions provides the most suitable configuration for energy recovery from a fixed solution volume, providing a minimum unitary cost for energy production. For a fixed membrane area, recycling feeds achieves energy efficiency seven times higher than single pass (conventional operation), and with an improved power density. However, ionic transport, water flux and concentration polarisation introduce complex temporal effects when concentrated brines are recirculated, that are not ordinarily encountered in single pass systems. Regeneration of the concentration gradient at around 80% energy dissipation was deemed most economically pragmatic, due to the increased resistance to mass transport beyond this threshold. However, this leads to significant exergy destruction that could be improved by interventions to better control ionic build up in the dilute feed. Further improvements to energy efficiency were fostered through optimising current density for each brine concentration independently. Whilst energy efficiency was greatest at lower brine concentrations, the work produced from a fixed volume of feed solution was greatest at higher saline concentrations. Since the thermal-to-electrical conversion proposed is governed by volumetric heat utilisation (distillation to reset the concentration gradient), higher brine concentrations are therefore recommended to improve total system efficiency. Importantly, this study provides new evidence for the configuration and boundary conditions required to realise RED as a practical solution for application to sources of low-grade waste heat in industry.
Highlights
20% of the world's population are without power, due to the absence of networked electricity, and the fragility of existing power grids, resulting in frequent large-scale power outages, in low-income countries [1,2,3]
Large concentration gradients required for high power density in single pass
To evidence the maximum achievable power density for the reverse electrodialysis (RED) stack which comprised a fixed membrane area, evaluation was conducted in single pass, as this excludes the cumulative exergetic loss imposed by recycling the feed
Summary
20% of the world's population are without power, due to the absence of networked electricity, and the fragility of existing power grids, resulting in frequent large-scale power outages, in low-income countries [1,2,3]. Waste heat energy is abundant, estimated to be equivalent to 246 PJ globally [4]. Thermalto-electric conversion of this waste heat could provide a reliable source of off-grid power for small-scale applications. 63% of this energy source is classified as low-grade heat below 100 °C [4]. Conventional thermal-to-electric technologies, such as the Organic Rankine Cycle (ORC), are unsuitable for the conversion of low-grade heat as Carnot efficiency is directly proportional to the hot source temperature [5]. The specific cost per kW of small-scale ORCs in the range of 1–100 kW is prohibitively high [6]. Thermoelectric generators have recently been proposed for applications to transportation and manufacturing sectors, widespread use is hindered by high cost and low efficiency (< 10%) [7]
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