Huge amounts of excess heat are developed by industrial processes and by electric power plants. Researchers around world have spent decades looking methods to control some of such wasted energy. Most such hard work have concentrated on thermoelectric devices – solid-state materials which can create electricity from the temperature gradient – but effectiveness of such devices is limited by availability of materials. Energy is one of those large troubles; in United States, more than half of energy we burn each year gets lost as heat instead of being put to employ.
Now researchers at Stanford University and Massachusetts Institute of Technology have discovered the novel substitute for low-temperature waste-heat conversion into electricity – i.e., in cases where temperature differences are less than 100 degrees Celsius.
Waste heat is by requirement created both by machines which do work and in other processes which utilize energy, for instance in the refrigerator warming room air or a combustion engine releasing heat into environment. The requirement for many systems to reject heat as the by-product of their operation is primary to laws of thermodynamics.
Almost all power plants and manufacturing processes, such as steelmaking and refining, release incredible amounts of low-grade heat to ambient temperatures. The new battery technology is developed to take benefit of this temperature gradient at industrial scale.
Voltage and temperature
The new Stanford-MIT system is based on principle called as thermogalvanic effect that states that voltage of the rechargeable battery is dependent on temperature. "To harvest thermal energy, we subject the battery to four-step process: heating up, charging, cooling down and discharging. First, the uncharged battery is heated by waste heat. Then, as the battery is still warm, the voltage is applied. Just the once completely charged, battery is permitted to cool. As the thermogalvanic effect, voltage increases as temperature decreases. When battery has cooled, it really delivers more electricity than was used to charge it. That extra energy does not emerge from nowhere. It comes from the heat which was added to system. Stanford-MIT system aspires at harvesting heat at temperatures below 100 C that accounts for the main part of potentially harvestable waste heat. One-third of the entire energy consumption in United States ends up as low-grade heat.
In an experiment, a battery was heated to 60 C, charged and cooled. Process resulted in the electricity-conversion efficiency of 5.7%, roughly double efficiency of conventional thermoelectric devices. This heating-charging-cooling approach was first planned in 1950s at temperatures of 500 C or more, noting that most heat recovery systems work greatest with higher temperature differences.
A main advance is using material which was not around at that time for battery electrodes, with proceeds in engineering the system. This technology has extra benefit of using low-cost, abundant materials and manufacturing processes which are already extensively utilized in battery industry.
Clever idea
As the new system has the important benefit in energy-conversion efficiency over conventional thermoelectric devices, it has the much lower power density – i.e., the amount of power which can be carried for the given weight. New technology too will need additional research to guarantee long-term dependability and develop the speed of battery charging and discharging. It will need a lot of work to take next step.
There is at present no good technology which can make effective use of relatively low-temperature differences this system can harness. This has the efficiency we believe is quite attractive. There is so much of this low-temperature waste heat, if the technology can be developed and set up to use it. The outcomes are very hopeful. By studying the thermogalvanic effect, the MIT and Stanford researchers were capable to convert low-grade heat to electricity with honest efficiency. This is a intellectual idea, and low-grade waste heat is all over the place.
The Stanford work was partly funded by U.S. Department of Energy (DOE), National Research Foundation of Korea and SLAC National Accelerator Laboratory. The MIT work was partly funded by the DOE, in part through Solid-State Solar-Thermal Energy Conversion Center.