By taking benefit of the temperature difference among a solar panel and ambient air, engineers create solar cells which could produce electricity at night.
Compared to the 100-200 watts per square meter which solar cells produce when the sun is shining, the night production is a trickle at 50 milliwatt per square meter. “But it’s already financially exciting for low-power-density applications such as LED lights, charging a cellphone, or looking to power small sensors,” says Shanhui Fan, a professor of electrical engineering at the Stanford University who had published the work together with co-authors in Applied Physics Letters.
Fan and his colleagues harnessed the idea of radiative cooling, the phenomenon through which materials radiate heat into the sky at night time after absorbing solar energy full day and others tapped before to make cooling paint and energy-efficient air-conditioning. Due to this effect, temperature of a standard solar cell pointing towards sky at night time falls under ambient air temperature. This makes a heat flow from ambient air to solar cell. “That heat flow may be harvested to generate power,” Fan said.
To imply that, researchers included a photovoltaic cell with a commercial Thermo-Electric Generator (TEG) module, that converts temperature difference into electric power. The TEG sits below the solar cell, and an aluminum sheet among the two generate heat from the solar cell to the TEG. The different aspect of the TEG connects through a heat sink to ambient air.
Existing solar panels can be retrofitted with a TEG to generate power at night, Fan said, the important factor for the devices to work nicely together is to have very close thermal contact among solar cells and the TEG, a task that retrofit answers will need to overcome.
The team examined their prototype TEG-integrated solar cell for 3 days in October 2021 on a rooftop in Stanford, Calif. The demonstration showed a night electricity manufacturing of 50 milliwatt per square meter. The team estimates that during a hotter, drier climate, the identical setup could generate as much as 100 milliwatt per square meter.
Fan said that there is huge room for improvement, due to the fact the conventional solar cell they used isn’t designed for radiative cooling. It releases heat waves in the mid-infrared range of wavelengths about 10 micrometers. By tweaking that emission wavelength, solar cell can be made also cooler at night, which might rise the temperature difference, and in the end the power that the TEG produces.
“In principle, it can be feasible to engineer the thermal-emission property of the solar cell to optimize its radiative cooling performance with not affecting solar performance,” Fan said. “Our theoretical calculations factor to the possibility of some hundred milliwatts or perhaps even 1 watt.”
The Stanford team intend to engineer new solar cells to enhance the night power generation and additionally plan to scale-up their prototype. Cost will be one barrier to scaling-up the idea, since TEGs are generally made from costly materials. The team has not completed an in-depth cost analysis, however Fan said that for an apples-to-apples comparison, you’ll must compare their 24-hour solar cell with a night set-up in that you produce solar power during the day after which use a battery to get power at night.
Given the considerably longer life-times of TEG modules over a battery’s 5 years, Fan said that “our initial calculations suggest that there are application scenarios where thermoelectric might have the potential to outperform a battery approach.”
