The concept of power generation is conversion of heat to electricity, which is the prime form of energy. Until recently, the only methods available to harvest low-grade heat (<160 Fahrenheit) were thermoelectrics and organic rankine cycle, which captured only 2 to 3 percent of waste heat. With collaborators at MIT and Stanford, an engineering professor at the University of Houston has discovered a less expensive, more efficient method, called thermally regenerative electrochemical cycle (TREC), for harvesting these sources of waste energy.
Hadi Ghasemi, assistant professor of mechanical engineering at the UH Cullen College of Engineering, published three papers about this discovery in journals including Nature Communications, Proceedings of the National Academy of Sciences (PNAS) and Nano Letters. Low-grade heat sources are ubiquitous in the environment and are generated in wide ranges of industrial processes. Harvesting these energy sources optimizes energy consumption and minimizes detrimental environmental effects, such as exhaust released into the atmosphere by power plants and industrial factories.
Low-grade energy harvesting
Efficiency of harvesting waste heat is only high with extremely hot gases, and it diminishes as temperatures decrease. Eighty percent of waste energy released into the atmosphere in the United States each year is less than 212 degrees Fahrenheit. At such low temperatures, neither of the existing methods for harvesting energy is effective, despite tremendous amounts of research conducted over the course of two decades, Ghasemi said.
The new method, which harvests at least twice the amount of waste energy as the other methods, is the subject of a paper that recently published in the journal, Nature Communications. Ghasemi and his collaborators found that dependence of batteries’ electrode potential on temperature could be used in a thermodynamic cycle to generate electricity. They named the process Thermally Regenerative Electrochemical Cycle.
For this project, the researchers worked with a small temperature range, from 50 degrees to 140 degrees Fahrenheit, to achieve heat to electricity efficiency of 5.7 percent, which is the highest amount of low-grade waste heat harvested to date. Existing technologies achieve approximately 2 percent efficiency, and the theoretical maximum amount of thermal electrical energy achievable is 15 percent. The method uses heat in addition to electrical energy to charge the batteries.
These new batteries are discharged at room temperature and recharged at a higher temperature. The batteries fully charge with less energy than originally required because a portion of the heat transferred to the battery converts to electricity during the charging process at higher temperature.
Ghasemi also used Thermally Regenerative Electrochemical Cycle technology to develop charging-free batteries, which are especially appealing in remote areas where electricity is not available.
His second paper, which published in the journal, Proceedings of the National Academy of Sciences of the United States of America, expanded on his previous research. Ghasemi and his collaborators eliminated the need for recharging electricity with a type of material that generates energy exclusively with heat. The charging-free process for recharging batteries yields 2 percent efficiency rather than 5.7 percent efficiency available with electricity.
“You can charge the small battery in an oven, or in the exhaust from a motor vehicle, or in any hot environment without electricity,” Ghasemi said.
Membranes are the most expensive components of batteries, so Ghasemi and his collaborators used Thermally Regenerative Electrochemical Cycle technology to develop membrane-free batteries to significantly lower their cost.
His third paper, which published in the journal, Nano Letters, focused on altering the chemistry of batteries to eliminate the need for ion membranes. Ghasemi and collaborators found that new materials – nickel hexacyanoferrate (NiHCF) cathode and a silver/silver chloride anode – eliminate the need for ion membranes and achieve energy efficiency of approximately 3.5 percent. The discovery creates opportunities for new membrane-free electrochemical systems to harvest waste heat.
“Waste energy adversely affects both the economy and the environment,” Ghasemi said. “Three new batteries open windows to harvest waste energy with high efficiency.”