IN A laboratory in Italy, 100 fridges sit quietly monitoring their electricity supply. It鈥檚 an odd thing for fridges to do, but these are no ordinary fridges. They are part of an experiment that, if successful, could transform the reliability of supplying electricity from renewable sources.
One of the key problems with renewables is their intermittent availability. You can only generate energy from the wind when it is blowing, or from the sun when it鈥檚 shining. Critics argue this is why we will never be able to rely on renewables for the majority of our electricity generation. But that criticism may soon be silenced. Researchers are developing new ways to balance supply and demand so that interruptions to the supply at a power station are unnoticeable to the consumer.
鈥淚nterruptions in supply will go unnoticed by the consumer鈥�
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For example, the idea behind the fridge experiment exploits the peculiar way the power grid responds when demand exceeds supply. In Europe, the frequency of the alternating current on the grid hovers close to 50 hertz, in North America it is 60 hertz. If demand increases, or the supply drops 鈥� as might happen, for example, if the wind stops blowing at a large wind farm 鈥� the frequency will dip below this level by up to 1 hertz as the remaining generators struggle to keep up. If it dips further, to around 48.8 hertz in Europe, the grid operators must shed some of the load, and parts of the country are disconnected from the grid and blacked out.
The Italian fridges are connected to a network that simulates this kind of power crisis, but instead of relying on a central control room to switch out the load, it is the fridges themselves that respond. As the frequency drops, a built-in controller in each fridge detects the change, checks the temperature of the fridge, and calculates how long it can stay chilled without drawing any power. It then switches the fridge off for as long as is safe. A similar system, developed by the the US Department of Energy鈥檚 Pacific Northwest National Laboratory in Richland, Washington, was successfully tested last year in 150 specially modified tumble-dryers.
If the technology, called dynamic demand, were fitted to enough fridges and air-conditioning units it could go a long way to smoothing out the fluctuations caused by the intermittent nature of renewable energy supplies. A report last year by the UK鈥檚 Department for Business, Enterprise and Regulatory Reform said the dynamic demand controllers would cost no more than 拢4 per appliance, a cost easily offset by the market value of the balancing services each fridge provides, estimated at around 拢30 over its lifetime. Fitting all the UK鈥檚 30 million domestic fridges with dynamic demand controllers would slice 2 gigawatts off peak demand, which could mean that two fewer coal-fired power station would be needed, according to Andrew Howe, CEO of RLTec, the London-based company that developed the dynamic demand software being tested in the Italian fridges. If industrial and commercial fridges were also included, dynamic demand could compensate for the sort of fluctuations expected if 20 per cent of the UK鈥檚 electricity were supplied from renewables, Howe says.
Dynamic demand is not the only way to tackle these fluctuations. In a contrasting approach, known as smart grid systems, an operating system run by the utility company is in two-way communication with controllers in consumers鈥� appliances. Using information fed in by the appliances, combined with predictions of renewable power output based on local short-term weather forecasts, the operating system can balance demand to match supply by telling non-essential appliances to switch themselves off. 鈥淲e can turn off a compressor in somebody鈥檚 air-conditioning system for 15 minutes, and the temperature really won鈥檛 change in the house,鈥� says Karl Lewis, chief operating officer of GridPoint in Arlington, Virginia, a company that designs smart grids.
By providing homes with smart meters to monitor their energy use, such systems can also help smooth out demand by encouraging consumers to set their washing machines for cheaper, off-peak times, for example.
GridPoint is working with Minnesota-based Xcel Energy to test the technology on a city-wide basis in Boulder, Colorado. In August, Xcel began equipping homes in the city with smart meters and remotely controlled devices. Next year, it plans to introduce solar and wind energy generators onto the grid. The hope is that the project will pave the way for cities of the future to be powered largely by electricity from renewables.
Read all the articles in our special issue on renewable energy
Energy and Fuels 鈥� Learn more about the looming energy crisis in our comprehensive special report.
Saving up for a windless day
Keeping the electricity flowing will take a battery the likes of which you鈥檝e never seen
When you need it there鈥檚 not enough, and when you don鈥檛 there鈥檚 too much. All too often, that鈥檚 the story with renewable energy. So finding a way of storing the excess generated when demand is low and releasing it for use at peak times is a priority if renewable electricity is ever going to be as reliable as fossil fuels.
At the moment, the preferred option for handling peak demand is to turn on gas-turbine generators, something that can be done at almost a moment鈥檚 notice. But as gas has become more expensive, electricity companies have become increasingly interested in energy storage as a way of dealing with the peaks, says Dan Rastler, an energy storage specialist at the Electric Power Research Institute (EPRI) in Palo Alto, California.
To store the quantities of electrical energy needed to keep the grid supplied, conventional batteries like the lead-acid cells used in cars just will not do. One of the alternatives being developed is the sodium sulphur battery. Another option is the vanadium flow battery, which can store large amounts of energy in chemical form in electrolyte solutions stored in large tanks. When fed into the battery proper, the electrolyte鈥檚 chemical energy is converted into electricity, and the spent solution is passed onto a holding tank. Then, when surplus power is available, the process can be reversed to regenerate the electrolyte. A 2-megawatt (MW) vanadium flow battery with an energy capacity of 12 megawatt-hours (MWh) is due to be installed next year at the Sorne Hill wind farm at Buncrana in County Donegal, Ireland. It will cost $6.3 million.
Another promising alternative is to use spare capacity when demand is low to store compressed air. Electricity from a wind generator, for example, is used to compress the air, which is stored underground in aquifers or salt domes. When electricity is needed, the compressed air is released and fed into a gas turbine.
The turbine is fuelled with natural gas, so the process is not emissions-free, but because a large proportion of the fuel consumed typically goes into compressing the air before it is mixed with the gas for combustion, the turbine will use up to 50 per cent less fuel. This results in carbon dioxide emissions of 86.5 kilograms per megawatt-hour, compared with 222 kg/MWh for a conventional gas turbine. 鈥淚t is one of the few options we have for storing large amounts of energy, and acting like a shock absorber on the system,鈥� says Rastler.
EPRI plans to build a 300-MW demonstrator plant, and estimates that compressed-air energy storage (CAES) plants like this will cost around $600 or $750 per installed kilowatt to build, compared with $1850 to $2150 per kilowatt for sodium sulphur batteries. Coal-fired power stations cost $476 per kilowatt to build, while more efficient, integrated gasification combined cycle plants, in which coal is converted into synthetic gas and impurities are removed before combustion, cost a whopping $3593 per kilowatt.
A group of municipal utility companies in Iowa is planning to build a $214 million, 268-MW CAES facility that will be used to store electricity from a 75-MW wind farm. It should be operating within five years. The technology is not new 鈥� a CAES plant was first built in Huntorf, Germany, in 1979. But unlike the Huntorf plant, the Iowa installation will capture exhaust heat from the turbine and use it to preheat the compressed air before combustion, increasing efficiency.
Meanwhile, a project funded by the European Commission is developing an advanced CAES plant which will not consume any natural gas, and so emit no CO2. When air is compressed it releases heat, and in CAES plants like that in Iowa, this heat is lost to the environment. In the European system, the heat will be stored in a honeycomb of ceramic bricks. To recover the energy, the compressed air is passed over the bricks to absorb heat, and then used to drive a modified steam turbine.
Helen Knight
