Nanophotonic Solar Thermophotovoltaic Device

Solar photovoltaic (PV) panels have an average efficiency of 15 to 20 per cent, which decreases over the lifetime of a power plant. Due to this, large tracts of land are required for setting up megawatt-scale projects. To overcome these challenges, extensive research is being conducted across nations to increase the efficiency of solar cells.

To this end, a team of MIT research scientists have demonstrated a nanophotonic solar thermophotovoltaic (STPV) device that can help more than double the present theoretical efficiency limits of a normal solar panel. This, in turn, would lead to twice the power generation from a given area of the solar panel, ensuring savings in land area and costs. This is the first time that an STPV device has been showcased, which has higher solar-to-electrical conversion efficiency as compared to a normal PV cell. Moreover, the cell claims to theoretically reach efficiency limits greater than that of an ideal solar cell. This study was supported by the Solid-State Solar Thermal Energy Conversion (S3TEC) Center, which is funded by the U.S. Department of Energy.

A conventional solar cell does not take advantage of all the photons due to which part of the solar energy is dissipated as unusable heat. However, in this new technology, an intermediate component is present which first allows the absorption of heat. Further, the heat is absorbed up to temperatures that enable the intermediate component to emit thermal radiation. This emitted radiation can then be absorbed by the solar cell to generate electricity.

The device consists of a two-layer intermediate absorber material. The outer layer, which faces the sunlight, is composed of an array of multi-walled, vertically aligned carbon nanotubes, which absorb the light’s energy and turns it into heat very efficiently. This layer is bonded tightly to a layer of nanophotonic crystals. These crystals can be modified to emit precisely determined wavelengths of light when heated. The crystals should be heated to an operational temperature of about 1,000°C. In the experiment, it was observed that the crystals continued to emit a narrow band of wavelengths of light, which precisely matched the wavelength that could be captured by an adjacent PV cell for electricity generation. Therefore, the heat which is re-emitted as light can be converted into exactly the colours that match the PV cell’s peak efficiency.

During operation, a conventional solar-concentrating system with lenses or mirrors will be used to focus sunlight in order to maintain high temperatures. In addition, an advanced optical filter that lets through all desired wavelengths of light to the PV cell will be used. Any wavelengths reflected by the optical filter will get re-absorbed, thereby maintaining the heat levels of the photonic crystal.

The research team has already produced an initial test device with a measured efficiency of 3.2 per cent and with further effort, 20 per cent efficiency is expected to be reached, sufficient for a commercially viable product.

The system offers a number of advantages over conventional PV cells. The photonic device produces emissions based on heat rather than light. Therefore unlike PV cells, it remains unaffected by brief changes in the environment such as sudden cloud covers. Moreover, if it is coupled with a thermal storage system, power can be provided around-the-clock. In other words, continuous on-demand power can be made available.

In addition, by absorbing the entire energy that would have been wasted away as heat, the device can prevent damage of solar-concentrating systems that is sometimes caused by excessive heat generation. This was confirmed by running tests in which a PV cell with STPV components was used first under direct sunlight and then with the sun completely blocked such that the cell was illuminated only by the secondary light emissions from the photonic crystal. The results proved that the actual performance matched the theoretical predictions.

The researchers state that the next step would be to find methods to make larger versions of the small, laboratory-scale experimental unit as well as developing ways to commercially manufacture these systems. The initial tests were done on a 1 cm chip, but follow-up tests will be done with a 10 cm chip. Moreover, it would be important to make this technology economically feasible and cost effective for its widespread use and uptake.

In conclusion, this innovation has paved the way to practically utilise the entire spectrum of sunlight for electricity generation, which cannot be currently done through any type of conventional solar cells. Moreover, the technology can be easily implemented as it retains the use of a PV cell with only an addition of a nanophotonic crystal layer. When it reaches commercial operation, it can be extremely beneficial for nations like India, which face scarcity of land acquisition. As the country receives high solar irradiation throughout the year, solar energy is one of the most viable forms of renewable energy available for electricity generation. In addition, the technology could solve the grid integration issues of solar PV that are caused by sudden cloud covers, making solar energy more reliable and less variable.