A Fully Regenerable Thermal Silver Ammonia Battery to Convert Low-Grade Waste Heat to Electricity

Tuesday, 3 October 2017: 15:00
Maryland D (Gaylord National Resort and Convention Center)
M. Rahimi, T. Kim, C. A. Gorski, and B. E. Logan (The Pennsylvania State University)
A vast amount of low-grade thermal energy (temperature < 130 ⁰C) is available globally at industrial plants and from solar and geothermal sources. Converting these sources of energy to electricity has drawn increasing attention in recent years. Solid-state thermoelectrics (SSTs) and liquid-based thermoelectrochemical systems (TECs) have been widely investigated as means of converting low-grade waste heat to electrical energy. However, despite much progress in both the methods, these systems have failed to produce high power densities, and have not been cost-effective. A recently developed thermally regenerative ammonia batteries (TRAB) showed a significantly higher power production (~12 times more) than both SSTs and TECs. In a TRAB, electrical power is obtained from the formation of metal ammine complexes, which are produced by adding ammonia to the anolyte, but not to the catholyte. After the cell discharges, ammonia is separated from the anolyte using a conventional technology, such as distillation with low-grade waste heat, and then added to the other electrolyte for the next discharge cycle. The previous TRAB based on copper electrodes and a copper salt electrolyte showed a relatively high power production. However, unbalanced rates of anode dissolution and deposition of copper on the cathode limited the use of copper in closed-loop cycles. To address the reversibility issue, a silver-based TRAB was developed as an alternative to the copper-based TRAB. With silver, the cathodic and anodic coulombic efficiencies of the TRAB were the same (~100%), resulting in a reversible system for converting low-grade waste heat into electricity over many successive cycles. The developed silver system produced a net maximum power density of 30 W m−2-electrode area, with a net energy production of 490 Wh m−3-anolyte in a flow cell with an optimal hydraulic retention time (HRT) of 2 s. Successive deposition and dissolution cycling (i.e., the electrode reversibility) showed the system was stable over a hundred cycles. An initial economic analysis of the system showed that the price of electricity produced based on materials costs was 1.8 times more than the average electricity price is the U.S. ($ 120 MWh−1), due primarily to the cost of the membrane as well as sliver. However, this could be reduced to $ 120 MWh−1 if the cost of a membrane used in the system could be reduced to $10 m–2. Other potential benefits, such as elimination of air pollution, and beneficial issues related to health and climate change were not included in the comparison. Although the cost of building and operation relative to energy production of Ag-TRAB is currently higher than that of conventional technologies, this approach could generate a cleaner method of electrical power generation using a waste source of heat if the commercial cost of ion exchange membranes could be significantly reduced.