The intermittent nature of energy supplies from renewable sources is forcing network operators to invest in technologies that can underpin the development of a smarter grid.
The latest evidence of that can be found at a substation at Hemsby in Norfolk, where trials of a pioneering energy storage system that has the potential to advance the integration of power generation from wind and solar farms are being undertaken. The facility has been fitted with a system comprising lithium-ion battery modules operating in combination with fast-acting grid interface electronics which will take energy generated from a nearby windfarm, storing and releasing it into the power network when it is most required. The technology, the first of its kind in the UK, enables dynamic, independent control of both active and reactive power in the electricity distribution system. This improves grid voltage and stability, as well as levelling-out power fluctuations from the intermittent wind supply.
The trials are relatively small-scale – the system comprises eight stacks of 13 lithium-ion battery modules which will be continually charged and discharged, and can store up to 200kWh of electrical energy. But the technology can be scaled-up, say its developers, so having the potential to play a useful role in the development of more flexible, reliable and smarter grids.
The trials are being led by UK Power Networks, which delivers power across London, the South East and East of England. According to Dave Openshaw, head of future networks at the company, the Hemsby project is indicative of how energy storage is becoming an increasingly important technology, as smarter electricity networks are developed to stabilise the integration of power generated from intermittent renewable sources.
“Britain is looking at up to 30GW of wind generation by 2020,” he says. “To put that in context, the maximum demand on the system at the moment is 60GW, but during the summer minimum it can fall below 30GW, so there could theoretically be times when we are generating more wind power than we have demand for at a national level.
“On other days wind output might be nothing like 30GW because of an anticyclone. It’s not unusual for anticyclones to occur on very cold days when the electricity demand is at its highest. That means there would still need to be a certain level of ‘conventional’ baseload generation which could be nuclear, gas, or coal, ultimately with carbon capture and storage.
“The other issue with wind generation is the speed of variation. On any day the ramp rates between low and high generation can be quite steep. Now for National Grid the problem is keeping the system in balance in real time – and one way to do that is by storing electricity. So if wind output is more than needed, it can be stored, but if there is a sudden shortfall of wind power, it would be possible to call on storage to make up the difference. The alternative is keeping conventional fossil-fuelled generation on spinning reserve, or part-loaded. That’s neither efficient nor carbon friendly.”
The technology used at the Hemsby site is based on ABB’s Dynapeaq system which uses static VAR compensators (SVC Light), combined with rack-mounted Li-ion battery storage modules. SVC Light comprises voltage source converters connected in shunt to the grid at transmission as well as sub-transmission and distribution level. At its heart are state-of-the-art insulated gate bipolar transistors that are used as fast switching devices, effectively artificially creating an AC voltage from a DC source. The package also includes a power transformer, control and protection system and auxiliary power equipment.
“The Saft lithium-ion batteries are similar to those used in electric vehicles, but packaged to give much greater capacity,” says Openshaw. “The clever part of the Dynapeaq system is the way it converts the AC voltage, which the network runs on, to DC which is needed to charge the batteries, and then converting it back from DC to AC to feed it on to the grid. That’s done in a swift manner through the use of voltage source converters with insulated gate bipolar transistors. The voltage source converter is also able to control power factor, and therefore the voltage on the network, and also the power quality through the use of harmonic filtering.”
The system operates with an efficiency of 90-95%. Openshaw thinks that’s acceptable, but predicts that the figure will improve as the technology is developed. “At the worse case you might get 10% losses,” he says. “The transistors can get quite warm as a result of the conversion so they have to be water cooled, and that is lost in heat ultimately.
“Actually the losses are a small price to pay when you bear in mind the savings that are being generated by not having to build additional generation, and transmission distribution networks. The technology continues to improve, so the expectation in future is that those losses will be reduced even further.”
The Hemsby trial will continue until 2013. Openshaw says UK Power Networks is keen to see how efficiently the system operates, and to what extent it can offset variability. “The trial will tell us the true value of the system as a new piece of technology,” he says.
The battery storage is relatively small-scale – 200kWh of electrical energy, enough to supply about 200 homes for an hour. But ABB insists that the Dynapeaq system could be scaled up to 50MW for 60 minutes and more.
“There’s absolutely no reason that it couldn’t be scaled up to become a much bigger installation, and this sort of solution could potentially be replicated across many parts of the UK where windfarms connect to the electricity network,” says Openshaw. And UK Power Networks is also interested in looking at other battery technologies. He says: “Sodium Sulphur is very promising, and there’s a more complex battery technology called Vanadium Redox flow which is effectively a very big fuel cell. In essence energy storage is one of the new ways in which we will be able to control variability from wind output both at distribution network level and also at national level.”
Ultimately, Openshaw believes that energy storage will be merely one solution in a suite of technologies coming under the banner of the “smart grid”. “At some time we are all going to be driving around in electric vehicles and heating our homes with heat pumps, so if we don’t do it smartly we are going to spend an awful lot of money reinforcing the network with more copper and you can imagine the cost and disruption that will cause.
“A truly smart grid will combine technologies like energy storage, responsive demand and dispatchable local generation. Incorporating these within our distribution network management strategy will enable us to either reduce the peaks in the daily load profile, which is good for managing networks, or shift the profile to more closely follow availability of wind power, which is good for managing energy prices.”
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