The efficiency of recovering electricity from heat storage is 44%

     University of Michigan (U-M) research indicates that devices capable of converting heat into electricity are approaching the theoretical maximum efficiency, making them increasingly viable for usage on the grid.Thermal solar cells can be used to convert intermittent renewable energy into electricity and store it in heat batteries during peak production hours.

     Andrej Lenert, an associate professor of chemical engineering at U-M and the primary author of the study published in Joule, explains that as we increase the proportion of renewable energy sources on the power grid to achieve decarbonization objectives, it becomes necessary to have energy storage systems that are both more affordable and capable of storing energy for longer periods. This is because the energy generated by solar and wind sources does not always align with the times when it is needed.

     Thermophotovoltaic cells function in a similar manner to photovoltaic cells, which are more often referred to as sun cells. Thermophotovoltaics utilize infrared photons, which have lesser energy, to convert electromagnetic radiation into electricity, as opposed to visible light photons that have more energy.

     The team’s novel gadget demonstrates a power conversion efficiency of 44% at a temperature of 1435°C, which falls within the desired range of 1200°C to 1600°C for high-temperature energy storage. It exceeds the 37% attained by prior designs at these temperatures.

     According to the study’s contributing author, who is the Peter A. Franken Distinguished University Professor of Electrical Engineering at U-M, this is a type of battery that is extremely passive. There is no need to extract lithium through mining, as is the case with electrochemical cells. Consequently, there is no requirement to contend with the electric vehicle industry. Unlike hydroelectric energy storage using pumped water, this method allows for flexibility in location and eliminates the requirement of a nearby water source.

     Thermophotovoltaics would encircle a heated block of material in a heat battery, with the substance being maintained at a minimum temperature of 1000°C. That temperature could be achieved by conducting electricity from a wind or solar farm through a resistor, or by harnessing surplus heat from solar thermal energy or the manufacturing of steel, glass, or concrete.

     The storage medium emits thermal photons with a spectrum of energies. At a temperature of 1435°C, approximately 20-30% of the particles possess sufficient energy to produce electricity in the thermophotovoltaic cells developed by the scientists. The crucial aspect of this work is enhancing the semiconductor material to efficiently catch photons by expanding its range of desired photon energies and matching them with the predominant energies generated by the heat source.

     However, the heat source emits photons with energies that are both higher and lower than what the semiconductor can effectively convert into electrical energy. Without meticulous engineering, those would be forfeited.

     To address this issue, the scientists constructed a narrow air layer within the thermophotovoltaic cell, positioned right after the semiconductor. Additionally, they incorporated a gold reflector outside the air gap, creating a device referred to as an “air bridge”.

     This cavity functioned as a means of confining photons with the appropriate energies, allowing them to enter the semiconductor while reflecting the other photons back into the heat storage material. This provided an additional opportunity for the energy to be re-emitted as a photon that could be captured by the semiconductor.

     The team has submitted an application for patent protection with the help of U-M Innovation Partnerships and is actively looking for partners to commercialize the invention.