New photovoltaic device that converts incandescence directly into electricity

Thermophotovoltaic devices are an efficient alternative to thermoelectric generators for converting heat into electricity. However, current systems are only efficient when the thermal source has temperatures above 1200 degrees Celsius, which limits their possible applications.

A researcher at the Institute of Solar Energy of the Polytechnic University of Madrid (UPM) in Spain, proposes a new device that manages to overcome this limitation. Thanks to a new design, based on bifacial thermophotovoltaic cells, it is possible to convert radiant heat into electricity at temperatures lower than those required by conventional monofacial designs. This will allow the development of more efficient and compact heat recovery and energy storage systems that could be used in domestic applications.

Thermophotovoltaic conversion began to be developed in the mid-20th century within the framework of the United States Defense programs. The idea was to come up with an alternative to thermoelectric devices that would convert heat into electricity more efficiently. However, the absence of high-quality materials prevented its development, and thermoelectric devices, with conversion efficiencies of less than 10%, were the ones that ended up prevailing. Since then, thermoelectric converters have been widely used in military and space applications, including the Apollo missions that landed humans on the Moon, but their low efficiency has prevented their widespread use for large-scale electricity generation.

Meanwhile, research into thermophotovoltaic devices continued on its way and little by little, its limitations were resolved. In the early 2000s, a research team in the United States was able to demonstrate for the first time an efficiency greater than 20%, and very recently, other teams also in the United States have been able to demonstrate efficiencies of up to 40%. These results have placed thermophotovoltaic technology at the level of the best thermal machines used today to generate electricity on a large scale. But with a great advantage: thermophotovoltaic cells do not require moving parts, which allows for simpler and more compact systems. For this reason, several research centers and companies have rushed to develop solutions that incorporate these devices. For example, the so-called “thermophotovoltaic batteries” developed by the UPM and its spin-off company Thermophoton, which have recently been awarded the radar prize for European innovation.

However, current thermophotovoltaic devices have an important limitation: they are only efficient at very high temperatures, above approximately 1200 degrees Celsius. At lower temperatures, its efficiency does not exceed 20%. This limitation is due to how current devices are designed. A conventional thermophotovoltaic converter consists of a photovoltaic cell and a mirror placed on its rear face. When the cell is exposed to incandescent radiation, photons with sufficient energy are absorbed in the cell and produce electricity. But low-energy photons (called “infrared”) are transmitted through the cell, reflected by the mirror, and returned to the glowing emitter, overheating it. This last step of “reheating” is vital for the efficiency to be high, since, otherwise, the low-energy photons would be wasted.

The problem is that when the glow temperature decreases, the energy of the photons also decreases, and therefore the mirror is responsible for reflecting more infrared photons. Therefore, the use of very efficient mirrors is imposed, much more efficient than those that currently exist. With current mirrors it is very difficult to achieve high efficiencies at temperatures below 1200 degrees Celsius.

“The thermophotovoltaic bifacial cell proposed and patented by the UPM solves this problem by eliminating the use of mirrors,” says Alejandro Datas, the inventor of said cell. Instead, the cell is inserted into an incandescent cavity to capture radiation on both sides. Unlike conventional cells, low energy photons pass through the cell and are reabsorbed directly in the emitter. In addition, since the cell is illuminated on both sides, the power generated is twice that generated by a conventional cell. “This last aspect is key, since it allows to produce twice the power without increasing the cost of the device, that is, it allows the cost of the power generated to be reduced by half”, explains the researcher.

One of the keys to the design is that the cell is cooled by the edges. The simulations carried out predict that in this way it is possible to keep the cells at a reasonably low temperature, below 80 degrees Celsius, without significant losses in efficiency.

Basic scheme of the new design to make better use of the bifacial thermophotovoltaic cells. (Image: UPM)

The main conclusion of the study is that bifacial thermophotovoltaic cells would allow the efficient conversion of radiant heat into electricity at temperatures lower than those required by conventional monofacial designs. Therefore, they could be used for industrial waste heat recovery or in thermal energy storage systems. The so-called thermophotovoltaic batteries usually store heat at temperatures above 1200 degrees Celsius so that, among other things, the thermophotovoltaic conversion is efficient. “The use of bifacial thermophotovoltaic cells in these systems would allow the operating temperature to be lowered, favoring the development of smaller systems that could be used not only for grid-scale energy storage, but also in domestic applications”, concludes Alejandro Datas.

The study is titled “Bifacial Thermophotovoltaic Energy Conversion”. And it has been published in the academic journal ACS Photonics. (Source: UPM)