Four alternative ways of generating energy

06 August 2020

Wind turbines, hydroelectric power stations, solar installations on roofs or in fields: The means we will be using to generate the "green electricity" of tomorrow already seem to be a foregone conclusion. And yet, researchers are working on completely different methods. For example, electrical energy could also be generated using waves or even air humidity.

Solar arrays have long been a familiar sight on roofs, house facades, in fields and even on former landfill sites. Recently, the solar industry has also discovered the potential of water. On a quarry pond near Renchen, not far from the French border, Baden-based energy supplier Erdgas Südwest has installed a floating solar array. With an output of 750 kilowatts, its main task is to supply the machinery of the nearby gravel plant with electricity.

From the point of view of Energie Südwest, waterborne photovoltaic systems offer many advantages. Unlike onshore installations, they don’t take up scarce land area, meaning that they don’t compete with agriculture, for instance. Bathing is in any case mostly forbidden in quarry ponds for safety reasons. And, unlike on house roofs, where shading can occur, floating solar panels are not shaded and are even cooled by the water from below. In this way, the modules provide up to 10 percent more energy which can be used directly on site by excavators, vibrators and conveyor belts – thereby also reducing the demand on the power grids. Not only that, but the floating solar arrays also provide ecological relief for the lakes themselves. Because they shield the water beneath them from the sun, they slow down the growth of algae which deprive standing waters and their inhabitants of oxygen in hot summers. 

Energie Südwest has 150 quarry ponds in its catchment area alone. Flooded quarries, reservoirs and open-cast lignite mining lakes are also held to be promising fields of application for floating photovoltaic systems. Singapore's SERIS solar research institute estimates the global potential for power generation on standing waters at 400,000 megawatts – enough to replace hundreds of nuclear or coal-fired power plants.

These photovoltaic systems are still about 20 to 25 percent more expensive than those on field sites. And the benefits of energy yield may be counteracted by an increased need for maintenance due to algae growth and seabird guano. Experts, however, expect installation costs to fall significantly in the near future. Such plants are likely to become significant especially in densely populated countries with a lot of water bodies.

Across the world, however, floating solar power is already gaining momentum. From a starting point of 1.1 gigawatts in mid-2018, the globally installed waterborne solar capacity had almost doubled to 2 gigawatts by the end of 2019. The largest and most efficient plants are currently to be found in China. In a former coal mining and floodplain area near Suzhou, a solar power plant with a nominal capacity of 70 megawatts has been built over an area of 140 hectares.

In Europe, despite its small size, the Netherlands has leapfrogged the UK and France to become a major promoter and pioneer of the technology. Since earlier this year, the largest plant in the world outside China since has been afloat on an 18-hectare quarry pond near Zwolle. It has an output of 27.4 megawatts – enough to supply 7,200 households with electricity. According to the national consortium “Zon op Water”, solar power plants with a total capacity of 2 gigawatts will have taken to the water in the Netherlands by 2023.

Metals can spontaneously develop an electrical charge at high humidity, as a worker found to his painful cost in 1843. He suffered a severe electric shock because a metal surface had become charged in a steam-filled room. In physical terms, what happens here is that ions pass from the surface to the small droplets of water suspended in the air. Depending on the type of metal, a positive or negative charge will occur.

Scientists from Israel's Tel Aviv University have developed a novel battery based on this knowledge. For this purpose, the researchers first searched for two metals whose charging behaviour diverged as much as possible from one another. Precious metal and zinc proved to be the most electrifying pair in these experiments: when the humidity rose to over 60 percent, a voltage of almost 1 volt could be measured. This effect was also recorded during a first practical test on the roof of the institute.

From the scientists’ point of view, this was a promising result. After all, the voltage of the humidity accumulator is close to that of the 1.5-volt batteries which can be used to operate radios, torches, alarm clocks and bicycle lamps. This research could form the basis for the future development of batteries which use humidity to charge themselves. This would mainly benefit people in tropical developing countries with constantly high levels of humidity where the electricity grid is still patchy.

Waves cause a lot of turbulence on the surface of the world's oceans every day. When they hit a coastline, they are estimated to have a capacity of 15 to 30 kilowatts per metre. If this energy were to be completely converted into electricity, a 30 to 60-kilometre stretch of coastline could replace a large coal-fired or nuclear power plant. Around one-sixth of the world's electricity needs could be satisfied in this way.

After the oil crises of the 1970s, researchers made their first attempts to tap the enormous potential of wave energy. And yet, these experiments did not make it out of the starting blocks - the technology used was either unable to withstand the harsh and salt-laden environment or was simply too inefficient. And political interest in alternative energy sources drained away as the price of oil fell. Wave power was only rediscovered at the turn of the millennium. RWE, EnBW, e.on, Siemens and Heidenheim-based turbine manufacturer Voith invested in various technological approaches, the aim of all of which was to use what were in some cases very different methods to transform wave motions on the surface of the oceans into mechanical energy.

Energy company e.on helped fund the huge wave converters that the British Pelamis Wave Power company tested in the turbulent waters of the North Sea off the Scottish Orkney Islands, starting in 2004: The limbs of the 180-metre-long and 1,300-tonne red steel “sea snake” followed the motion of the waves. In doing so, they activated hydraulic pumps at their joints which drove a generator. In 2013, e.on withdrew from the project, citing as its reason the all-too-slow development of the technology. When other investors followed suit, Pelamis filed for bankruptcy in 2014. Other wave-power plants also hit the financial rocks or failed to make it out of the demonstration phase because their efficiency or reliability failed to live up to expectations.

Although the major energy companies have pulled out of wave power, smaller enterprises continue to work away on the idea. Bavarian start-up SINN Power is working on a low-cost wave-power technology based on a modular construction system. The technology was first tested in the Greek port of Heraklion in 2015 and has, according to the company, survived six-metre-high waves. A floating platform equipped with solar cells and small wind turbines is also set to supply small islands in the Caribbean, for instance, with green electricity.

Rain causes plants to grow but could also power LED lamps - using hydroelectric power. When rainwater runs over a roof, an umbrella or a non-conductive surface, it generates an electrically charged trail. The droplets collect the opposite charge. Physicists from the Max Planck Institute for Polymer Research in Mainz have investigated this well-known phenomenon in more detail.

They have developed a method for determining charge generation and also devised a theoretical model to improve understanding of the process. In the next step, the researchers want to develop a surface that will make the best possible use of the charging effect. In this way, it will at least be possible to generate smaller amounts of electricity. This could, for example, benefit the inhabitants of isolated and rainy areas where electricity is not yet available.

Researchers from Hong Kong are taking a different approach. They want to convert the impact energy from raindrops into electricity. For this purpose, they have developed a nano-current generator that can generate energy from the fall of individual droplets. According to their study, a single droplet of water falling from a height of 15 centimetres is sufficient to light up 100 LED lamps.

For this to work, however, the drops must fall as evenly as possible and from the same height onto the surface of the generator. For use in the rain, the researchers adapted the construction so that the rainwater was first collected and then divided into regularly falling droplets via a system of fine capillaries. 

The key to the generation of electrical current is a layer of indium tin oxide and Teflon, which continues to charge electrically as each new droplet falls. As a result, the generator produces a thousand times as much power as it would without the material, even over a small area. From the point of view of the researchers, this technique offers great potential. In all those places in the world where rain falls or water flows, this kind of generator could produce electrical energy – regardless of whether the water droplets come into contact with an umbrella or the hull of a ferry. Before this can be realised, however, scientists all in likelihood still have a lot of research ahead of them. After all, generating a short burst of energy is acknowledged to be comparatively easy, but accumulating enough electricity to continuously supply electrical appliances is widely seen as considerably more difficult.

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