Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI, USA.
Department of Chemical Engineering, University of Michigan, Ann Arbor, MI, USA.
Nature. 2020 Oct;586(7828):237-241. doi: 10.1038/s41586-020-2717-7. Epub 2020 Sep 21.
Thermophotovoltaic cells are similar to solar cells, but instead of converting solar radiation to electricity, they are designed to utilize locally radiated heat. Development of high-efficiency thermophotovoltaic cells has the potential to enable widespread applications in grid-scale thermal energy storage, direct solar energy conversion, distributed co-generation and waste heat scavenging. To reach high efficiencies, thermophotovoltaic cells must utilize the broad spectrum of a radiative thermal source. However, most thermal radiation is in a low-energy wavelength range that cannot be used to excite electronic transitions and generate electricity. One promising way to overcome this challenge is to have low-energy photons reflected and re-absorbed by the thermal emitter, where their energy can have another chance at contributing towards photogeneration in the cell. However, current methods for photon recuperation are limited by insufficient bandwidth or parasitic absorption, resulting in large efficiency losses relative to theoretical limits. Here we demonstrate near-perfect reflection of low-energy photons by embedding a layer of air (an air bridge) within a thin-film InGaAs cell. This result represents a fourfold reduction in parasitic absorption relative to existing thermophotovoltaic cells. The resulting gain in absolute efficiency exceeds 6 per cent, leading to a very high power conversion efficiency of more than 30 per cent, as measured with an approximately 1,455-kelvin silicon carbide emitter. As the out-of-band reflectance approaches unity, the thermophotovoltaic efficiency becomes nearly insensitive to increasing cell bandgap or decreasing emitter temperature. Accessing this regime may unlock a range of possible materials and heat sources that were previously inaccessible to thermophotovoltaic energy conversion.
热光伏电池类似于太阳能电池，但它们不是将太阳辐射转化为电能，而是设计用来利用局部辐射的热量。高效率热光伏电池的开发有可能使网格规模热能存储、直接太阳能转换、分布式联产和余热收集等广泛应用成为可能。为了达到高效率，热光伏电池必须利用辐射热源的宽光谱。然而，大多数热辐射处于低能量波长范围，无法用于激发电子跃迁并产生电能。克服这一挑战的一种有前途的方法是让低能量光子被热发射器反射和再吸收，在那里它们的能量有机会在电池中再次用于光生。然而，目前的光子恢复方法受到带宽不足或寄生吸收的限制，与理论极限相比，效率损失很大。在这里，我们通过在薄膜 InGaAs 电池中嵌入一层空气（空气桥）来实现低能量光子的近完美反射。与现有的热光伏电池相比，这一结果使寄生吸收减少了四倍。绝对效率的提高超过了 6%，导致超过 30%的高功率转换效率，这是用大约 1455 开尔文的碳化硅发射器测量得到的。随着带外反射率接近 1，热光伏效率对增加电池能带隙或降低发射器温度变得几乎不敏感。进入这个区域可能会解锁一系列以前无法用于热光伏能量转换的可能的材料和热源。
