Abstract
The method of detailed balance, introduced by Shockley and Queisser, is often used to find an upper theoretical limit for the efficiency of semiconductor pn-junction based photovoltaics. Typically the solar cell is assumed to be at an ambient temperature of 300 K. In this paper, we describe and analyze the use of radiative cooling techniques to lower the solar cell temperature below the ambient to surpass the detailed balance limit for a cell in contact with an ideal heat sink. We show that by combining specifically designed radiative cooling structures with solar cells, efficiencies higher than the limiting efficiency achievable at 300 K can be obtained for solar cells in both terrestrial and extraterrestrial environments. We show that our proposed structure yields an efficiency 0.87% higher than a typical PV module at operating temperatures in a terrestrial application. We also demonstrate an efficiency advantage of 0.4-2.6% for solar cells in an extraterrestrial environment in near-earth orbit.
Highlights
The detailed balance method used by Shockley and Queisser to calculate the limiting efficiency of ideal solar cells entails balancing of particles entering and exiting the solar cell [1]
At open-circuit conditions, no current is drawn from the device, and the incoming photon flux must be balanced by an outgoing photon flux from radiative carrier recombination, the detailed balance
Zhu et al presented a scheme using radiative cooling to reduce the temperature of a Si solar cell by 18.3 K below its operating temperature; the device would still operate above the ambient, leading to an efficiency decrease compared to the ideal Shockley-Queisser limit [5]
Summary
The detailed balance method used by Shockley and Queisser to calculate the limiting efficiency of ideal solar cells entails balancing of particles entering and exiting the solar cell [1]. The efficiency limit described by Shockley and Queisser is derived for a solar cell in contact with a heat sink at an ambient temperature of 300 K [1] This is an idealization of the best case scenario, and in reality the solar cell heats up considerably above the ambient temperature, when at operating conditions, and leads to a significantly lower efficiency. Zhu et al presented a scheme using radiative cooling to reduce the temperature of a Si solar cell by 18.3 K below its operating temperature; the device would still operate above the ambient, leading to an efficiency decrease compared to the ideal Shockley-Queisser limit [5]. We present a method to increase the photovoltaic power conversion efficiency above the Shockley and Queisser limit calculated at 300 K by lowering the solar cell temperature below the ambient using passive radiative cooling. We analyze the use of radiative cooling for space applications and show that our proposed device improves the efficiency of solar cells operating under standard conditions in low-earth orbit (temperatures typically between 293 and 358 K) [16]
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