Due to environmental concerns and dwindling oil and coal reserves, researchers are increasingly looking at sustainable and environmentally friendly energy resources, such as wind and solar energy. Picture: Reuters
Due to environmental concerns and dwindling oil and coal reserves, researchers are increasingly looking at sustainable and environmentally friendly energy resources, such as wind and solar energy. Picture: Reuters

Tech News: Harvesting solar energy at night

By Louis Fourie Time of article published Oct 18, 2019

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CAPE TOWN – The 4th Industrial Revolution (4IR) is driven by a number of innovative technologies. However, most of these technologies need energy or electricity to be developed and to function properly. 

Due to environmental concerns and dwindling oil and coal reserves, researchers are increasingly looking at sustainable and environmentally friendly energy resources, such as wind and solar energy. 

Major strides have been made with regard to solar energy over the past decade and solar cells became much more efficient in generating electricity. However, one of the biggest challenges until now has been that solar systems could not generate power at night since it obviously needs the rays and energy of the sun. 

Users of solar panels usually have to acquire expensive battery packs to store some of the solar energy for the night. Others use huge capacitors to store the energy. Unfortunately battery and other energy storage development followed a much slower innovation trajectory and are thus still expensive and limited in their use whether in electrical cars or part of a solar system at home.

But perhaps there are some solutions on the horizon. Currently many kinds of solar panels exist for the harvesting of energy. The solar panel that is the best known by people and often found in home installations is the solar panel that generates electricity from the sun through a photovoltaic (PV) process. 

Another type of solar device generates electricity from heat through a thermal process. It is this process that interests a team of researchers at Stanford University in Palo Alto California (USA). They have therefore dedicated themselves to develop a new system that can harness energy even in complete darkness.

These researchers under the leadership of Professor Shanhui Fan have developed an inverse version of the solar panel – also called an “anti-solar panel” by some people. This remarkable breakthrough entails that the new system they have built generates electricity when the sun is hotter than the Earth. 

The difference in temperature is converted into usable energy. While a standard solar panel uses the heat difference to generate electricity during the day when the Earth is cooler than the sun, the Stanford device uses the heat difference between the coolness of the night atmosphere and a warmer Earth for the generation of electricity. 

According to an article published by the Stanford team in the scientific journal Joule, the thermoelectric generator-based device harnesses the energy from the difference in temperature between Earth and outer space. The harnessing is done through a passive cooling mechanism called “radiative sky cooling” to maintain the cold side of the generator several degrees below the ambient temperature. 

The air around the device heats the warm side of the generator, which converts the temperature difference into electricity.  In future it my even be possible to reverse the process during the daytime to have a constant stream of electricity.

According to professor Shanhui Fan, the amount of energy coming from the Sun is approximately equal to the amount radiated by the Earth, in order to keep the Earth at a more or less constant overall temperature. The amount of power available for harvesting is thus very large.

The thermoelectric generator-based device has the potential to overcome the biggest drawback of solar energy by harvesting energy during the night and on cloudy days. Unfortunately the research is far from complete and the technology would probably take another few years to become commercially available. The research team has only tested their system with a very small prototype (a 20 cm black aluminium disk) that created enough energy to power a single small LED light while in total darkness.

This may be a small step forward, but it is a step with unimaginable potential since it can produce electricity at night when solar panels do not function. And the wonderful news is that the dish-like device is much cheaper to manufacture than solar panels, which are still rather expensive.

Researchers, together with international renewable energy companies, under the leadership of Professor Craig Buckley from Curtin University (USA) followed a very different approach to generate electricity for use during the night. They developed an innovative thermal battery as part of a Concentrated Solar Power (CSP) system, which are being developed by their commercial partner United Sun Systems. This system requires a battery to store and release energy to ensure continuous solar power generation.

For many years storage has been a major drawback of renewable energy systems. Current battery systems are not only expensive, but even the most high-tech ones are quite inefficient and has a relatively short shelf life depending on how they are used and drained. 

But a new prototype thermal battery, developed by Professor Buckley from Curtin University, can store and release solar energy whenever it is needed. The principle of the process is not complex and involves that the battery uses high-temperature metal hydride or metal carbonate as the heat storage medium and a low temperature gas storage container for storing the hydrogen or carbon dioxide. 

During the absence of sunlight in the night, or times of heavy cloud cover, hydrogen or carbon dioxide is released from the gas storage tank. The higher temperature metal absorbs the gases to form metal hydride/metal carbonate, which generates significant heat. The heat is then used to generate electricity during the day and night when solar power is not available.

While a normal battery stores electrical energy that can be used to provide electricity when the sun is not shining, the thermal battery stores heat from the concentrated solar thermal, which then can be used during the night or cloudy days to run a turbine to produce electricity.

The Curtin research team plans to integrate the thermal battery with its thermochemical energy storage into a dish-Stirling system that can provide up to 46 kW of electricity. Depending on the power requirements of a particular enterprise or housing development, an array of dishes could be erected to deliver a much higher kilowatt output.

According to Professor Moran, the Deputy Vice-Chancellor Research from Curtin University, this cost-effective thermal battery will “revolutionise the landscape of renewable energy production” and would be a worthy and sustainable competitor to fossil fuels.

Professor Jeffrey Grossman and his research team from the Massachusetts Institute of Technology (better known as MIT) have spent many years to develop alternative storage solutions for solar energy. According to them the best technology to harness sunlight energy, appears to be Solar Thermal Fuels (STFs). Their latest technology is an improvement of a previous design entailing solar power to be stored as a liquid substance. They are also working on a solid state STF application that could be implemented in the windows of buildings, car windshields, car tops, and other surfaces constantly exposed to sunlight.

In the past number of years the photovoltaic industry has experienced remarkable growth, but to be a truly sustainable energy source 24 hours a day, it will have to solve the problem of providing electricity during night-time and cloudy days. Current solar batteries are a storage option, but are not really economically viable as a mainstream solution.

We can only hope that the thermal systems described above will come to fruition and that innovative research would continue. For the sake of our planet and the increasingly noticeable global warming we urgently need an alternative method to store solar energy for use during the night. The solution will have to provide sustainable energy that could rival fossil fuels as a viable source of energy for households and commercial enterprises, as well as large electricity users such as heavy industries and the mines.


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