The temperature of the emitters was controlled using heaters (main and guard, like the guarded-hot-plate thermal conductivity setup) at their back side that were regulated using a PID control. Each main heater was connected to a power supply (2425 Keithley and 2440 Keithley) in a four-wire configuration to allow accurate measurement of the heating power at the emitter only. A triple-channel power supply (2230-30-1, Keithley) was also used to power the guard heaters in a two-wire configuration. The data acquisition and PID control were accomplished using LabVIEW. The main heater power and emitter temperature were averaged for 2 min after the initial transient peak in heater power. The uncertainty of the emitter temperature was determined from the thermocouple and ambient temperature sensor accuracy, as well as their fluctuations during the averaging period (2-min average after stabilization of the emitter temperature). The cooling power uncertainty was determined after accounting for the accuracy of the main emitter area measurement and of the power supplies, the main heater power fluctuations during the averaging period, as well as the small parasitic lateral heat transfer between the main and guard emitters. Specifically, we performed an indoor measurement to characterize the lateral heat transfer coefficient between the main and guard emitters and found it to be 24.5 W/m2K, meaning that for every degree in temperature difference, an effective change in cooling power of 24.5 W/m2 was observed at the main emitter. This effect was, however, only found to be important near the stagnation temperature of the devices. Because of higher parasitic heat gain and low cooling power close to the stagnation temperature, the guard temperature was higher than the main emitter, causing heat transfer between the two emitters and giving rise to additional 2D parasitic heat gains at the main emitter. Since all other data points and the model assumed 1D heat transfer at the main emitter, the cooling power data point at the stagnation temperature of both devices was removed.

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