Heat to electricity conversion by cold carrier emissive energy harvesters

Abstract

This paper suggests a method to convert heat to electricity by the use of devices called cold carrier emissive energy harvesters (cold carrier EEHs). The working principle of such converters is explained and theoretical power densities and efficiencies are calculated for ideal devices. Cold carrier EEHs are based on the same device structure as hot carrier solar cells, but works in an opposite way. Whereas a hot carrier solar cell receives net radiation from the sun and converts some of this radiative heat flow into electricity, a cold carrier EEH sustains a net outflux of radiation to the surroundings while converting some of the energy supplied to it into electricity. It is shown that the most basic type of cold carrier EEHs have the same theoretical efficiency as the ideal emissive energy harvesters described earlier by Byrnes et al. In the present work, it is also shown that if the emission from the cold carrier EEH originates from electron transitions across an energy gap where a difference in the chemical potential of the electrons above and below the energy gap is sustained, power densities slightly higher than those given by Byrnes et al. can be achieved. V C 2015 AIP Publishing LLC. [http://dx

Radiative sky cooling: fundamental physics, materials, structures, and applications

“…Cold carrier EEHs have a similar device configuration compared to hot carrier solar cells but operates in the completely opposite manner [76]. The working principle of hot carrier solar cells is to reduce thermalization loss by suppress carrier cooling (phonon absorption) and extract those hot carriers through narrow energy bands or discrete states by selective contacts [95].…”

“…Second, a novel emissive energy harvesting (EEH) system has been proposed to harness nighttime radiative cooling power for power generation. We will discuss the fundamental limit of the power output of EEHs and the prospect of adopting long-wave IR rectennas and cold carrier absorbers for these applications [49,76]. Finally, the potential benefits of radiative cooling for IR detectors and electronics will be discussed.…”

“…Hence, there is an optimal temperature of the cold reservoir that maximizes the power at a given T Hot and T Sky . Figure 12B-C summarizes the performance of an ideal EEH (Carnot limited) with different emission bandwidths [76]. The power density can reach up to ~300 W/m 2 (equivalent to a solar cell with 30% efficiency under one sun) with heat sink at a temperature of 500 K. It should be noted that, at high heat sink temperature that gives an optimal T Cold above the ambient, it is not desirable to confine the wavelength range of thermal radiation within the atmospheric window because of the additional outgoing heat from the colder reservoir to the surrounding background.…”

“…However, dominant Auger recombination and generation in mid-IR bandgap semiconductors makes such a design impractical. As shown in Figure 13, much more promising candidates have been proposed, that is, rectenna-based and cold carrier EEHs [49,76], both of which will be discussed in detail as follows.…”

“…However, effective strategies to harness such energy have not been fully explored. Recently, a device named emissive energy harvester (EEH) that can transfer heat from a heat sink to sky through radiative cooling from a cold reservoir and convert part of the heat flow into useful work has been proposed [49,76] (see Figure 12A). Coupled Carnot's law to the steady-state heat flow, the maximum output power from the engine is P Cool ×(T Hot /T Cold − 1), where P Cool is the net radiative cooling power from the cold reservoir to the sky described by Eq.…”