Thermally regenerative ammonia batteries for converting low-grade waste heat to electricity

Open Access
Rahimi, Mohammad
Graduate Program:
Chemical Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
September 01, 2017
Committee Members:
  • Bruce Ernest Logan, Dissertation Advisor
  • Bruce Ernest Logan, Committee Chair
  • Manish Kumar, Committee Member
  • Michael Anthony Hickner, Committee Member
  • Thomas E Mallouk, Outside Member
  • Christopher Aaron Gorski, Dissertation Advisor
  • electrochemical energy storage
  • battery
  • thermall battery
  • waste heat recovery
  • ion exchange membrane
  • energy conversion
  • electrochemistry
  • flow battery
  • electrodeposition
Significant quantities of low-grade waste heat (temperature <130 °C) are available globally at various industrial plants and from solar or geothermal sources. Converting this heat energy to electricity has drawn increasing attention in recent years. Organic Rankine cycles, solid-state thermoelectrics, liquid-based thermoelectrochemical cells and salinity gradient energy systems have previously been investigated as means of converting low-grade waste heat to electrical energy. Despite much progress, these approaches have not produced high power densities or been cost-effective. Thermally regenerative ammonia batteries (TRAB) using copper electrodes and salts have produced ~12 times higher power densities than these previous approaches. 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. To improve TRAB performance through a reduction in the membrane resistance, a series of quaternary ammonium-functionalized poly(phenylene oxide) anion exchange membranes (BTMA-AEMs) were examined for their impact on performance relative to a commercial AEM (Selemion AMV). The synthesized AEMs had different degrees of functionalization (DF; 25% and 40%), and thicknesses (50, 100 and 150 µm). Power and energy densities were shown to be a function of both DF and membrane thickness. The power density of TRAB was 31% higher using a BTMA AEM (40% DF, 50 µm thick; 106 ± 7 W m–2-electrode area) compared to the Selemion (81 ± 5 W m–2-electrode area). Moreover, the energy density increased by 13% when using a BTMA-based membrane (25% DF, 150 µm thick; 350 Wh m–3) compared to the Selemion membrane (311 Wh m–3). The thermal-electric conversion efficiency improved to 0.97% with the new membrane compared to 0.86% for the Selemion. This energy recovery was 7.0% relative to the Carnot efficiency, which was 1.8 times greater than the highest previously reported value of a system used to capture low-grade waste heat as electricity. A TRAB was adapted and tested as a process for recovering copper from solutions containing high concentrations of copper ions. Copper removal reached a maximum of 77% at an initial copper concentration (Ci) of 0.05 M, with a maximum power density (P) of 31 W m–2-cathode area. Lowering Ci decreased the percentage of copper removal from 51% (Ci=0.01 M, P=13 W m–2) to 2% (Ci=0.002 M, P=2 W m–2). Although the final solution might require additional treatment, the adapted TRAB process removed much of the copper while producing electrical power that could be used in later treatment stages. These results showed that a TRAB might be a useful technology for removing copper ions and producing electricity by using waste heat. To improve the anodic coulombic efficiency of a thermal battery, ethylenediamine was examined as an alternative ligand to ammonia. The power density of the developed thermally regenerative ethylenediamine battery (TRENB) was 85 ± 3 W m−2-electrode area (20 W m−2-membrane area) with 2 M ethylenediamine, and 119 ± 4 W m−2 (27 W m−2-membrane area) with 3 M ethylenediamine. This power density was 68% higher than that obtained using a TRAB in parallel tests, and the energy density of 478 Wh m–3-anolyte was ~50% higher than that produced by TRAB. The anodic coulombic efficiency of TRENB was 77 ± 2%, which was more than twice that obtained using ammonia in TRAB (35%). The higher anodic efficiency reduced the difference between the anode dissolution and cathode deposition rates, resulting in a process more suitable for closed loop operations. The thermal-electrical efficiency, based on ethylenediamine separation using waste heat was estimated to be 0.52%, which was lower than that of TRAB (0.86%), mainly due to the more complex separation process. However, this energy recovery could likely be improved through optimization of ethylenediamine separation process. TRABs based on copper can only be operated for a limited number of recharging cycles due to unbalanced rates of gain and loss of metal on the copper electrodes (i.e., low reversibility). 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 fully 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 and the silver. However, this could be reduced to $120 MWh−1 if the cost of a membrane 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 provide 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.