UC Davis

Photovoltaics possess significant potential due to the abundance of solar power incident on earth; however, they can only generate electricity during daylight hours. To produce power after the sun has set, we consider an alternative two-temperature system, consisting of the earth (300 K) as a heat source and outer space (3 K) as a heat sink. Motivated by the concept of radiative cooling, which can be employed to passively cool objects below ambient temperature by exhausting the earth’s heat toward the sky, in this thesis we explore how to convert this infrared flux into power. We first discuss the principles of thermoradiative photovoltaics and the theoretical limits of coupling a device with deep space, followed by a discussion of the practical limits and integrability of this nighttime photovoltaic concept.Second, we demonstrate an alternative approach that generates mechanical power from the earth’s ambient radiation using a Stirling engine. Year-long, outdoor experiments show temperature differences >10 K are sustained during most months, resulting in the generation of >0.1 mW of mechanical power with a potential for >6 W/m^2. We further apply this technique for air circulation, achieving >0.3 m/s with a potential volumetric flow rate that exceeds 5 cfm, which is sufficient for air circulation in greenhouses and residential buildings.