Trying to find the energy

Materials World magazine
,
2 Jun 2015

Alternative sources of power could derive from the sun, sea, sand and space. Glynn Garlick reports.

Fossil fuels still dominate the world energy market, but this cannot last forever. Advanced technologies have allowed access to oil and gas reserves that would not have been possible in the past, but even if new reserves
are tapped into, they are finite and their use could be limited by efforts to cut greenhouse gas emissions.

To counter this, alternative and greener sources of energy are being looked at, including geothermal and biomass. Other possibilities are more ‘far out’ and may not be viable today because of their cost or the limitations of current technology, but they could be used in the future. 

As Barack Obama said when addressing Congress in February 2009, ‘To truly transform our economy, protect our security, and save our planet from the ravages of climate change, we need to ultimately make clean, renewable energy the profitable kind of energy.’

Found in space

One solution being looked at is the final frontier. For example, the Space Solar-Power Systems (SSPS) project headed by the Japan Aerospace Exploration Agency (JAXA) aims to harness solar energy using collectors in outer space. This would be transmitted to Earth as either microwave or laser energy and converted into electricity. They claim the process would be far less affected by weather conditions and natural disasters than ground-based systems, and there would be no price fluctuations caused by supply-and-demand conditions.

However, there are challenges. The cost of transporting and operating the large-scale systems required would need to be reduced to compete with other energy sources. The technology to transport, build, operate and repair them would also be needed, along with safe power generation, transmission and reception systems. Measures to repel space debris and solar flares would also have to be developed.

Another idea is to mine helium-3 on the moon. According to the European Space Agency, ‘The idea of harvesting a clean and efficient form of energy from the moon has stimulated science fiction and fact in recent decades. Unlike Earth, which is protected by its magnetic field, the moon has been bombarded with large quantities of helium-3 by the solar wind. It is thought that this isotope could provide safer nuclear energy in a fusion reactor, since it is not radioactive and would not produce dangerous waste products.’

China is looking at the possibility of mining helium-3 on the moon, but the idea suffers from similar problems to space-based solar power – it would be tremendously expensive, and the country's space programme is a long way from being able to make it happen.

Sea here

The sea offers a more realistic answer to providing cleaner energy than outer space. Ocean Thermal Energy Conversion (OTEC) technology uses the temperature difference in areas with warm surface water and cold deep water to drive a steam cycle that turns a turbine and produces power. This process can produce renewable, non-polluting power 24 hours a day, seven days a week, but early systems operated in countries such as Japan have only produced tiny amounts of energy.

However, US company Lockheed Martin is working on larger-scale projects, including one for a pilot plant off the coast of southern China to supply power at a green resort planned by Chinese company Reignwood. Lockheed Martin says that military shore-side bases and communities in the tropics, many of which are largely dependent on imported fossil fuels for power and transportation, are ideal candidates for such a system.

Again, there are challenges that would need to be overcome, including the cost of building facilities large enough to generate significant power and able to withstand the pounding of the ocean and major storms. OTEC is also relatively inefficient and plants have to work hard pumping huge amounts of water to produce modest amounts of electricity, some of which has to be used to operate the systems. Onshore plants could affect shorelines with ecosystems such as mangroves and coral reefs, while intervening in the temperature balance of the sea could have environmental impacts, particularly as pumping cold water to the surface releases CO2.

Another emission-free source of energy is osmotic power using pressure-retarded osmosis (PRO). This uses membranes to allow river water to flow into seawater. Salt molecules pull the fresh water through the one-way membrane, causing an increase in pressure that can power a turbine. Norwegian company Statkraft opened a prototype facility using the PRO technique at Tofte near Oslo in 2009. It discontinued its efforts in 2013, saying the necessary technology would not be sufficiently developed to become competitive in the foreseeable future.

However, research into osmotic power is continuing in an effort to create state-of-the-art technology that could make the environmentally friendly method work. Dutch company RedStack opened a pilot plant on the Afsluitdijk closure dam in 2014. This uses its reverse electro dialysis (RED) technology, which also uses membranes to produce power. In this case, the electricity production is directly linked to the mixing process, and no additional intermediate conversions such as turbines or electro-chemical reactions are needed.

A less environmentally friendly potential source of energy from below the sea is methane hydrate, which consists of ice crystals containing methane, the chief constituent of natural gas. When methane hydrate is warmed or depressurised, it reverts to water and natural gas. Hydrate deposits usually occur under Arctic permafrost or beneath the ocean floor. The energy content of methane occurring in hydrate form is said to be immense, although according to the British Geological Survey, the amount of methane hydrate in permafrost soils is poorly understood, with some estimates ranging from 7.5–400 gigatonnes of carbon. Countries such as Japan, the USA and China are researching its potential, and samples have been taken from the bottom of the Sea of Japan.

However, there are problems. As with oil and gas tapped through fracking, there is opposition to the idea of accessing a fossil fuel at a time when countries are trying to cut greenhouse gas emissions. While extracting the gas should not be too difficult, preventing the breakdown of hydrates and the release of methane could be a problem. Another concern is that the process could destabilise the seabed and this could lead to submarine landslides.

Seeing the light

Not all sources of alternative energy involve large-scale projects. London-based Pavegen has created a tile that harvests kinetic energy from footfall at busy locations. The energy can power off-grid applications such as pedestrian lighting and advertising signage, or can be stored in a battery. The top surface of the tiles is made from 100% recycled rubber, while the base is constructed from more than 80% recycled materials. The system can be retrofitted or specified for new developments. How the technology works is a secret, but the company has said it involves piezoelectricity. 

Pavegen projects include fitting tiles in Heathrow Airport’s Terminal 3 to power LED lights along a corridor, at a football pitch in Rio de Janeiro to provide lighting, and outside a railway station in Saint Omer, France, to power LED bench lighting and two USB ports within the seating areas. The tiles can also send out wireless data for crowd flow modelling, looking at how people move through cities. Such a system can be a part of the energy mix. Pavegen CEO Laurence Kemball-Cook says, ‘I strongly believe that disruptive technologies are the best way to change people's attitudes to energy and reduce the reliance on fossil fuels and grid power.’

There are numerous other projects, both big and small, aimed at creating alternative sources of power for the future. In Ireland there are plans for a plant that uses a motor-generated flywheel to harness kinetic energy from the grid at times of oversupply. This green energy can then be released from submerged turbines at times of supply shortfalls, rather than having to use fossil fuels.

Work is also under way in the USA and the United Arab Emirates to use sand to store heat from solar energy sources, which can then be used to generate electricity.

Going to waste

Some methods of creating energy are nothing new but are being affected by changing circumstances. Creating energy from waste is an example of this, but the UK Government’s Department for Environment, Food and Rural Affairs, in its Energy from waste: a guide to the debate, says, ‘Energy from waste must at the very least not compete with recycling, reuse and prevention and should ideally support them. At the same time recovery through energy from waste needs to be pulling waste out of less environmentally sound disposal routes, particularly landfill, but also incineration with insufficient energy recovery.’

Methods used in the sector include combustion, in which residual waste is burnt and the energy is recovered as electricity or heat. Another process is gasification and pyrolysis, involving fuel being heated with little or no oxygen to produce syngas that can be used to generate energy. A third method is anaerobic digestion, using micro-organisms to convert organic waste into a methane-rich biogas that can be used to generate electricity and heat, or converted to biomethane.

As recycling becomes economic and practical for a wider range of waste types, the composition of that which remains will change, and the industry has to be able to deal with this, just as the world has to be flexible in its efforts to find alternative energy sources, now and in the future.