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Images of particles made from a promising battery cathode material called NMC

Solid-state Photon Enhanced Thermionic Emission for Solar Energy Conversion

Global Climate and Energy Project

James Harris, electrical engineering; Nicholas Melosh, materials science and engineering

Global climate imperatives require a worldwide shift away from greenhouse gas- emitting activities such as fossil fuel combustion. Concentrated solar thermal installations are particularly appealing as they can store a fraction of their power in the form of heat to provide baseload power, yet current conversion efficiencies prevent them from reaching competitive market prices. One extremely promising avenue to make these CSP technologies cost competitive adding a topping cycle to increase the total efficiency without increasing the land footprint required. Solar thermal topping cycles operate by receiving the incident solar radiation, converting a portion of it to electricity, and then delivering the excess heat to the CSP system. These two systems in tandem could reach much higher efficiencies than possible with one unit alone. However, increasing the efficiency of these add-on devices, such as thermoelectrics, at the operating temperatures required for steam or molten salt conversion processes is still a significant challenge.

Our group recently described a new physical mechanism for direct conversion of solar energy to electricity called Photon-Enhanced Thermionic Emission (PETE), based on research conducted under a prior GCEP program. PETE operates at elevated temperatures (600 to 900 oC) under high solar concentration (100s to 1000s of suns) appropriate for CSP. The process operates by thermionically emitting photoexcited electrons from a light absorbing p-type semiconductor cathode into vacuum, where they are collected by a lower temperature, low work function anode. Due to the combination of photovoltaic and thermal energy collection processes, PETE has the potential to achieve power conversion efficiencies above 47%, higher than the fundamental limits for single-junction PV. Furthermore, PETE devices integrated with solar thermal converters could achieve efficiencies over 55%.

One of the most challenging aspects to the PETE process is that it requires direct emission of electrons into vacuum. Here, we explore whether this direct emission is necessary, or if it is possible to use a wide-bandgap semiconductor to create a solid-state PETE device (SS-PETE) instead. This device configuration has several advantages. First, the device consists of a monolithic body, and does not require a good quality vacuum between separated plates. Secondly, the operating voltage can be engineered based on the semiconductor band alignment rather than the material workfunctions. Finally, the device form factor would allow simple integration with several different CSP designs. We have evaluated the potential efficiency of these devices and demonstrated some initial device fabrication steps. Theoretically these devices could be quite efficient, yet do require a substantial temperature gradient. We investigate a simple test- collector based on this design.