Solar power is nice; however, it depends heavily on availability of sunlight, which can be limited in some locations and it’s unavailable at night. There are systems that store energy, but they’re usually expensive and it can drive up the cost of using solar power.
Lucky us, researchers have designed an off-grid, low-cost modular energy source that can efficiently produce power at night. This system uses commercially available technology that can help provide lighting in developing countries as well as meet the need for nighttime lighting in urban areas.
Led by Shanhui Fan from Stanford University, researchers found that radiative cooling can be a less-expensive alternative. This approach uses the temperature difference resulting from heat absorbed from the surrounding air and the radiant cooling effect of cold space to generate electricity.
In The Optical Society (OSA) journal Optics Express, the researchers theoretically demonstrate an optimized radiative cooling approach that can generate 2.2 Watts/sqm with a rooftop device that doesn’t require a battery or any external energy.
This is about 120 times the amount of energy that has been experimentally demonstrated and enough to power modular sensors such as ones used in security or environmental applications.
“We are working to develop high-performance, sustainable lighting generation that can provide everyone — including those in developing and rural areas — access to reliable and sustainable low-cost lighting energy sources,”
“A modular energy source could also power off-grid sensors used in a variety of applications and be used to convert waste heat from automobiles into usable power,” said Lingling Fan, first author of the paper.
Optimizing and maximizing power generation
One of the most efficient ways to generate electricity using radiative cooling is to use a thermoelectric power generator. These devices use thermoelectric materials to generate power by converting the temperature differences between a heat source and the device’s cool side, or radiative cooler, into an electric voltage.
So, the researchers optimized each step of thermoelectric power generation to maximize nighttime power generation from a device that would be used on a rooftop. Energy harvesting becomes more improved so more heat flows into the system and incorporate new commercially available thermoelectric materials.
They also calculated that a thermoelectric power generator covering one square meter of a rooftop could achieve the best trade-off between heat loss and thermoelectric conversion.
Wei Li, a member of the research team said, “One of the most important innovations was designing a selective emitter that is attached to the cool side of the device. This optimizes the radiative cooling process so that the power generator can more efficiently get rid of excessive heat.”
Using computer modeling to simulate a system, the researchers have demonstrated and looked at the result of the new approach. The models showed that the result was what the researchers had calculated.
After the experiments, the researchers have begun to examine optimal designs for operating the system during both day and night, attempting to expand the practical applications of the system.
Desalinating water using solar power
Speaking of solar power, researchers at MIT and in China have developed a completely passive solar powered desalination system. It could provide almost 6 liters of fresh drinking water per hour for every square meter of solar collecting area.
Similar to the radiative cooling system above, this one could potentially serve off-grid arid coastal areas to provide an efficient, low-cost water source.
The system uses multiple layers of flat solar evaporators and condensers, lined up in a vertical array and topped with transparent aerogel insulation. Its efficiency lies in the way it uses each of the multiple stages to desalinate the water.
desalinating water may not need to be this big and the cost can be low with the researchers’ system. photo by Le plombier du désert Wikimedia CommonsAt each stage, heat released by the previous stage is harnessed instead of wasted. This way, the experiment can achieve an overall efficiency of 385% in converting the energy of sunlight into the energy of water evaporation.
Basically, the device used in the demonstration/experiment is a multilayer solar still with a set of evaporating and condensing components like those used to distill liquor.
It uses flat panels to absorb heat and then transfer that heat to a layer of water so that it begins to evaporate. The vapor then condenses on the next panel. That water gets collected, while the heat from the vapor condensation gets passed to the next layer.
Whenever vapor condenses on a surface, it releases heat. In this multilayer evaporator, the released heat flows to the next evaporating layer, recycling the solar heat and boosting the overall efficiency.
“When you condense water, you release energy as heat. If you have more than one stage, you can take advantage of that heat,” said Evelyn Wang, the co-author.
Adding more layers increases the conversion efficiency for producing potable water, but each layer adds cost and bulk to the system. The researchers then settled on a 10-stage system which was tested on an MIT building rooftop.
Their proof-of-concept device delivered pure water that exceeded city drinking water standards, at a rate of 5.78 liters/sqm (about 1.52 gallons per 11 square feet) of solar collecting area.
Wang stated that this result is more than two times as much as the record amount previously produced by any such passive solar-powered desalination system
MIT doctoral students and co-author Lenan Zhang said that with more desalination stages and further optimization, such systems could theoretically reach overall efficiency levels as high as 700 or 800 percent.
And, unlike some desalination systems, this one doesn’t require the disposal of salt accumulation or concentrated brines. According to the researchers, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater.
Low cost desalination system
Innovation is way nicer when it’s low cost. Well, the researchers’ demonstration unit was built mostly from inexpensive, readily available materials such as a commercial black solar absorber and paper towels for a capillary wick to carry the water into contact with the solar absorber.
Wang said that in most other attempts to make passive solar desalination systems, the solar absorber material and the wicking material have been a single component, which requires specialized and expensive materials. This system won’t be like that.
There is an expensive component to this prototype, though, which is a layer of transparent aerogel used as an insulator at the top of the stack. However, there are less expensive insulators which can be used as an alternative.
According to Wang, the team’s key contribution is a framework for understanding how to optimize such multistage passive systems, which they call thermally localized multistage desalination.
The formulas they developed could likely be applied to a variety of materials and device architectures, allowing for further optimization of systems based on different scales of operation or local conditions and materials.
One possible configuration would be floating panels on a body of saltwater such as an impoundment pond. These could constantly and passively deliver fresh water through pipes to the shore, as long as the sun shines each day. Maybe if this system is combined with the one before, we can get a super desalination device?
Other systems could be designed to serve a single household, perhaps using a flat panel on a large shallow tank of seawater that is pumped or carried in. A system with a roughly 1-square-meter solar collecting area could meet the daily drinking water needs of one person. In production, they think a system built to serve the needs of a family might be built for around $100.
Further experiments will be done by the researchers to continue to optimize the choice of materials and configurations, and to test the durability of the system under realistic conditions.
All in all, the team hopes that their desalination system could mitigate or alleviate water scarcity in parts of the developing world where reliable electricity is scarce but seawater and sunlight are abundant.
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