Cooling Everything Without Electricity

As the world gets warmer, the use of power-hungry air conditioning systems is projected to increase significantly, putting a strain on existing power grids and bypassing many locations with little or no reliable electric power. Now, an innovative system developed at MIT offers a way to use passive cooling to preserve food crops and supplement conventional air conditioners in buildings, with no need for power and only a small need for water. The system, which combines radiative cooling, evaporative cooling, and thermal insulation in a slim package that could resemble existing solar panels, can provide up to about 19 degrees Fahrenheit (9.3 degrees Celsius) of cooling from the ambient temperature, enough to permit safe food storage for about 40 percent longer under very humid conditions. It could triple the safe storage time under dryer conditions.

The findings are reported today in the journal Cell Reports Physical Science, in a paper by MIT postdoc Zhengmao Lu, Arny Leroy PhD ’21, professors Jeffrey Grossman and Evelyn Wang, and two others. While more research is needed in order to bring down the cost of one key component of the system, the researchers say that eventually such a system could play a significant role in meeting the cooling needs of many parts of the world where a lack of electricity or water limits the use of conventional cooling systems.The system cleverly combines previous standalone cooling designs that each provide limited amounts of cooling power, in order to produce significantly more cooling overall — enough to help reduce food losses from spoilage in parts of the world that are already suffering from limited food supplies.

This technology combines some of the good features of previous technologies such as evaporative cooling and radiative cooling,” Lu says. By using this combination, he explains, “we show that you can achieve significant food life extension, even in areas where you have high humidity,” which limits the capabilities of conventional evaporative or radiative cooling systems.

In places that do have existing air conditioning systems in buildings, the new system could be used to significantly reduce the load on these systems by sending cool water to the hottest part of the system, the condenser. “By lowering the condenser temperature, you can effectively increase the air conditioner efficiency, so that way you can potentially save energy,” Lu says. Other groups have also been pursuing passive cooling technologies, he adds, but “by combining those features in a synergistic way, we are now able to achieve high cooling performance, even in high-humidity areas where previous technology generally cannot perform well.”

The system consists of three layers of material, which together provide cooling as water and heat pass through the device. The only maintenance required is adding water for the evaporation, but the consumption is so low that this need only be done about once every four days in the hottest, driest areas, and only once a month in wetter areas.


SuperGrids: How to Join the Solar Power Grids of Entire Continents

India gained notoriety when it finished November’s COP26 climate summit by weakening a move to end the use of coal. Less widely recognised is that the country also started the Glasgow summit in a more positive fashion, with a plan to massively expand the reach of solar power by joining up the electricity grids of countries and even entire continents. Indian prime minister Narendra Modi has talked about the idea before, but the One Sun One World One Grid initiative launched in Glasgow now has the backing of more than 80 countries, including Australia, the UK and the US. The alliance is just one example of a growing movement to create regional and, eventually, globalsupergrids”: long-distance, high-voltage cables linking each country’s growing renewable power output.

The supergrid movement is being driven partly by the need to maintain a smooth flow of power onto electricity grids. Local weather makes the amount of power generated by wind and solar variable, but this becomes less of an issue if the grid is larger and distributed over a wider geographical area. What’s more, supersized green energy projects are often sited far from the cities or industrial areas demanding their energy, be it wind farms in the North Sea or solar farms in the Australian outback. Supergrids offer a solution to this problem by connecting large renewable energy sources with the people who use the power.

The Indian government is keen on links to the Middle East, to help India decarbonise using imported renewable energy,” says Jim Watson at University College London. The UK, one of India’s partners on the One Sun One World One Grid initiative, is also considering new long-distance cables.

Last September, the UK started importing hydropower from Norway via a 724-kilometre subsea cable. In the coming years, the cable is expected to be used mostly to export electricity from the UK’s growing number of offshore wind farms so that it can be stored in hydropower facilities in Norway and released onto grids as needed. In 2022, UK start-up Xlinks will try to persuade the UK government to guarantee a minimum price for electricity generated at a mega wind and solar farm to be built in Morocco that could power UK homes via a 3800-kilometre subsea cable. “I will very confidently predict that over the next 15 years the world will see a huge number of interconnectors,” says Simon Morrish at Xlinks of such cables.

Xlinks is also working with Australian firm Sun Cable on its proposal to build the world’s largest solar farm in the north of Australia and connect it, via Darwin, to Singapore through a 4200-kilometre cable, to supply it with low-carbon electricity. In September, Sun Cable gained approval to route the high-voltage cable through Indonesian waters. 2022 may also see progress on efforts to build an “energy island” in the North Sea, which would act as a vast hub for offshore wind farms that can supply several European countries. UK company National Grid recently told New Scientist it is in talks about the pioneering project.


Ultrathin, Lightweight Solar Panels

A race is on in solar engineering to create almost impossibly-thin, flexible solar panels. Engineers imagine them used in mobile applications, from self-powered wearable devices and sensors to lightweight aircraft and electric vehicles. Against that backdrop, researchers at Stanford University have achieved record efficiencies in a promising group of photovoltaic materials. Chief among the benefits of these transition metal dichalcogenides – or TMDs – is that they absorb ultrahigh levels of the sunlight that strikes their surface compared to other solar materials.

Transition metal dichalcogenide solar cells on a flexible polyimide substrate

Imagine an autonomous drone that powers itself with a solar array atop its wing that is 15 times thinner than a piece of paper,” said Koosha Nassiri Nazif, a doctoral scholar in electrical engineering at Stanford and co-lead author of a study published in the Dec. 9 edition of Nature Communications. “That is the promise of TMDs.”

The search for new materials is necessary because the reigning king of solar materials, silicon, is much too heavy, bulky and rigid for applications where flexibility, lightweight and high power are preeminent, such as wearable devices and sensors or aerospace and electric vehicles.

Silicon makes up 95 percent of the solar market today, but it’s far from perfect. We need new materials that are light, bendable and, frankly, more eco-friendly,” said Krishna Saraswat, a professor of electrical engineering and senior author of the paper. While TMDs hold great promise, research experiments to date have struggled to turn more than 2 percent of the sunlight they absorb into electricity. For silicon solar panels, that number is closing in on 30 percent. To be used widely, TMDs will have to close that gap.

The new Stanford prototype achieves 5.1 percent power conversion efficiency, but the authors project they could practically reach 27 percent efficiency upon optical and electrical optimizations. That figure would be on par with the best solar panels on the market today, silicon included.

Moreover, the prototype realized a 100-times greater power-to-weight ratio of any TMDs yet developed. That ratio is important for mobile applications, like drones, electric vehicles and the ability to charge expeditionary equipment on the move. When looking at the specific power – a measure of electrical power output per unit weight of the solar cell – the prototype produced 4.4 watts per gram, a figure competitive with other current-day thin-film solar cells, including other experimental prototypes. “We think we can increase this crucial ratio another ten times through optimization,” Saraswat said, adding that they estimate the practical limit of their TMD cells to be a remarkable 46 watts per gram.”


Energy Conversion Efficiency of Perovskite Solar Cells could Go Beyond 30%

Solar cells are excellent renewable energy tools that use sunlight to drive an electrical current for power. They’ve been used to power homes since the 1980s, and their performance and production cost have improved dramatically since then. The most common solar cells, based on silicon, work well for a long time. They retain more than 80% of their functionality even after 25 years. However, the efficiency—i.e., how much of the incoming sunlight is converted to electrical power—of commercial-scale silicon solar cells is currently only around 20%.

Maximizing solar cells‘ energy conversion efficiency will improve their competitiveness compared to fossil fuels and help optimize them as a sustainable energy source. Researchers have intensively focused on an alternative to silicon: perovskite materials to enhance solar cells’ efficiency. Designs based on such materials must meet certain requirements, such as ease of fabrication on a large scale, and minimize reflected—i.e., wasted—light.

In a recent study published in Nano-Micro Letters, researchers from Kanazawa University applied a thin metal oxide filmreproducible, uniform, and compact—onto a perovskite solar cell. The researchers used a combination of lab work and computational studies to evaluate their solar cell design performance fairly.

(a) Schematic diagram of the perovskite/perovskite tandem solar cell, and (b)  current–voltage characteristic curves of the best-investigated perovskite/perovskite tandem solar cell. Inset shows quantum efficiency for top perovskite and bottom perovskite.

We used spray pyrolysis to deposit a front contact layer of titanium dioxide onto a perovskite solar cell,” explains Md. Shahiduzzaman, lead and corresponding author. “This deposition technique is common in the industry for large-scale applications.

Upon finding an optimum thickness for the front contact layer, the researchers measured an energy conversion efficiency of 16.6%, assuming typical sunlight conditions. As mentioned, this isn’t quite as good as commercial silicon-based solar cells. Nevertheless, electromagnetic simulations were a powerful tool for predicting the possible energy conversion efficiency limit by optimizing specific parameters.

Computational simulations suggest that the energy conversion efficiency of perovskite/perovskite tandem solar cells could go beyond 30% by a multi-layer front contact,” says Md. Shahiduzzaman, lead and corresponding author. “This is close to the theoretical efficiency limit of silicon-based solar cells.”