Much has been said about the intermittency of renewables. Solar panels, in particular, are known for only generating electricity during the daytime. While battery storage technology provides a potential long-term solution to this problem, an emerging approach is to develop solar panels that can generate electricity at night.
The concept of capturing the sun’s energy at night may seem contradictory – surely the sun needs to be shining for solar panels to work?
Below we will explore the different ways that solar panels can indeed work at night, including the recent scientific advancements that are opening up new possibilities for solar power generation around the clock.
How traditional solar panels work
There’s no question that traditional solar panels are specifically designed to generate electricity from sunlight. The photovoltaic (PV) cells inside the panel absorb the photons from the sun and use them to create an electrical current. This process is called the photovoltaic effect.
The following is a breakdown of the process in more detail.
When light shines on a photovoltaic (PV) cell – also called a solar cell – that light may be reflected, absorbed, or pass right through the cell.
The PV cell is composed of semiconductor material. When the semiconductor is exposed to light, it absorbs the light’s energy and transfers it to negatively charged particles in the material called electrons. This extra energy allows the electrons to flow through the material as an electrical current. This current is extracted through conductive metal contacts – the grid-like lines on a solar cell – and can then be used to power your home.
The efficiency of a solar cell is determined by the amount of energy it can extract from the light source. This largely depends on the light’s characteristics, such as its intensity and wavelengths. Longer wavelengths have less energy and shorter wavelengths have more.
The “band gap” of a PV semiconductor is a key feature, determining which wavelengths of light it can absorb and convert to power. This will translate to a limited range of wavelengths, with the cell ignoring those that are longer and shorter. If the semiconductor’s band gap matches the wavelengths of light shining on the PV cell, it can efficiently make use of the available energy.
Solar cells have been specifically designed to absorb sunlight. A standard silicon solar cell responds to most of the visible parts of the sun’s light spectrum, roughly half of the infrared light, and a portion of the ultraviolet light (but not much of it, making UV lights some of the least efficient lights to charge a solar light with).
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High-efficiency solar cells
To increase the efficiency of solar cells, there are multi-layer designs that mix the silicon with impurities, each with its own response curve. The top layer absorbs shorter wavelengths and the bottom converts the longer ones. The result is significantly better conversion efficiency and better energy output.
Do solar panels need direct sunlight to charge?
Solar panels do not need direct sunlight to charge. They will still produce electricity on cloudy and overcast days, but the amount of electricity produced will be lower than on bright, sunny days. This is because the clouds block some of the sun’s photons from reaching the panel. Just how much the system’s output is reduced is dependent on the amount and duration of cloud cover.
The angle of the sun, coupled with the strength of the sun’s radiation, means that some solar systems can generate twice the electricity in summer, than in the shorter days of winter (usually well in excess of household requirements).
Fortunately, most modern solar panels are designed to work well in low-light conditions and will still generate adequate electricity on overcast days. As a rule of thumb, a standard solar panel will produce around 25% to 45% of the energy it would normally produce under ideal conditions, while higher-efficiency panels can yield even higher outputs.
Can traditional solar panels charge without sunlight?
This may come as a surprise but, technically, yes. Solar cells can charge with other forms of visible light besides sunlight.
The wavelengths of light that can be converted to solar energy are present in both direct sunlight and artificial light, so artificial lights such as incandescent, fluorescent, LED, and metal halide lamps could be used to charge solar cells provided the light is strong enough.
We can see this in action with solar-powered calculators and watches. The solar cells in these devices are able to convert the light from artificial sources into enough power to run the calculator.
However, when we compare the amount of power generated from artificial light sources versus direct sunlight, there is a big difference. The solar cells in calculators and watches are very small, so they don’t require much power to run. But if we tried to use the same method to power an entire home, it would be a different story.
While charging an entire solar panel or array of panels with artificial light is theoretically possible, it’s not necessarily practical.
Ben Minnaert and Peter Veelaert from Ghent University, Belgium, conducted a research study to discover how different types of traditional solar panels perform when placed indoors under various light sources at 500 lux illumination.
They found that it was indeed possible to harness electricity from artificial light through solar panels. However, efficiency values were well below those achieved with sunlight – too low, in fact, for Standard Test Conditions
They found that the monocrystalline technology and incandescent light were the best possible combination to obtain electricity from artificial light, followed by polycrystalline and CIGS technologies combined with incandescent light.
In short, there’s no real practical or logical reason to try and power solar panels with artificial light. No artificial light can mimic the strength and radiance of true sun rays, and certainly not at the level needed to perform efficiently. Just as you wouldn’t bother using a candle to cook your food (unless you’re on a fondue diet), you’d be wasting your time and literal energy trying to charge your solar panels with artificial light.
So, can solar panels work at night?
From the above information, we can probably agree that the traditional solar panels we have on our roofs cannot practically work at night. However, the formerly theoretical process of generating electricity from solar at night is no longer a pipe dream.
Below are three examples of pioneering research into harnessing solar energy when the sun isn’t shining.
University of New South Wales: Thermoradiative diode
Image: UNSW
In an apparent world first, Australian researchers have demonstrated that solar power can be generated at night.
In May this year, a team from the University of New South Wales (UNSW) was able to achieve the seemingly impossible by harnessing the Earth’s own infrared thermal radiation to generate electricity.
Speaking to ABC RN’s Drive, Associate Professor Ned Ekins-Daukes, who led the research, explained how this process works:
“We get energy from the sun — it arrives, it warms up the Earth but then the Earth actually radiates the exact same amount of energy back out into space,” Professor Ekins-Daukes says.
By using a semiconductor device called a thermoradiative diode composed of materials found in night-vision goggles, the team was able to capture photons leaving Earth along the infrared spectrum and convert them into electricity.
While the methodology used differs considerably from photovoltaics (solar PV), Professor Ekins-Daukes says the process is ultimately still harnessing solar power.
According to him and his team, if a power cell device can be used to capture the radiant heat flow and convert into electricity, there’s “a large and unused spectrum of potential power to be exploited.”
While this technology is still in its early stages, the implications are significant. The team says proving the theoretical process is the first step in making specialised, and much more efficient, devices that could one day capture the energy at much larger scale – perhaps replacing or working alongside battery storage.
“We’ve just demonstrated that this is possible … Right now, the device we’ve made is relatively low power. [But] this is to be expected when you’re at the very early stages,” Professor Ekins-Daukes says.
The amount of energy produced was only extremely small — roughly equivalent to 1/100,000th of a solar powered cell.
“It will take some time … And I have to be honest, we need to find some new materials to achieve [widespread use].”
But he says, in the future it may be possible to combine photovoltaic devices and the thermoradiative diode for “night-time solar” power.
“You could have a panel that generates power during the day, but then could also power the other items that are still running in your house at night.”
Professor Ekins-Daukes likens the new research to the work of engineers at Bell Labs who demonstrated the first practical silicon solar cell in 1954. From only 2 per cent efficiency, research and innovation led to today’s industry-standard silicon cell with an efficiency of around 23 per cent.
And Dr Michael Nielsen, co-author of the paper, said: “Even if the commercialisation of these technologies is still a way down the road, being at the very beginning of an evolving idea is such an exciting place to be as a researcher.
“By leveraging our knowledge of how to design and optimise solar cells and borrowing materials from the existing mid-infrared photodetector community, we hope for rapid progress towards delivering the dream of solar power at night.”
Stanford University: thermoelectric generator
In the United States, scientists at Stanford University have also been working on thermal radiation-based solar cells but with a slightly different approach.
The team, led by Professor Shanhui Fan modified an off-the-shelf solar cell by adding a thermoelectric generator (TEG), a device that produces currents from temperature differences.
When solar panels radiate heat at night, a phenomenon occurs that results in the panels being cooler than the night air. Using the thermoelectric generator, the team was able to exploit this temperature difference to produce electricity.
Image: Stanford University
“The solar panel turned out to be a very efficient thermal radiator,” says Professor Fan. “So, at night, the solar panel can actually reach a temperature that’s below the ambient air temperature, and that’s a rather unusual opportunity for power harvesting.”
The modified solar cell generated a power output of 50 microwatts per square meter when directed towards a clear night sky. This is just 0.04 per cent of the daytime energy production of a standard solar cell. However 50 milliwatts per square meter, could enable low-power devices, such as a phone charger or a low-wattage LED light, to function.
The researchers claim that the current prototype could be improved to generate more power and there are no major barriers in one day scaling the system up to a commercial product.
Soochow University: triboelectric nanogenerator (TENG) technology
This approach, developed by researchers at Soochow University in China, doesn’t necessarily allow solar panels to work consistently at night, but it does broaden the energy-harvesting potential of solar panels by allowing them to capture energy from a wider range of sources, specifically falling rain.
By placing a transparent layer containing a triboelectric (TENG) nanogenerator over a conventional solar panel, the team was able to convert the mechanical energy (friction) generated by raindrops landing on and running off solar panels, into small amounts of electrical power.
Developed over the past several years, TENG technology is still bound by technical limitations, but the potential it offers for solar panels is significant.
As well as boosting power output on rainy days, the friction-powered panels could also produce electricity at night if it rains. The scientists say their solar panels offer “an efficient approach to collect energy from the environment in different weather conditions”.
