Cold storage
Posted on Friday, August 26th, 2011

First published in Energy World, September 2011.

Only connect. In the last twelve months Britain has shelled out £4.3 million pounds to wind farms that were not producing power even though the wind was blowing. Over the same period our power stations and heavy industries chucked 800 gigawatt hours of waste heat up the chimney, about the same as all the heat consumed by the entire country.

Although both statements represent a scandalous waste of energy and money, they are ostensibly unrelated. But now a British start-up company has found an ingenious way to exploit one problem to solve the other. Their technology, which stores energy in the form of liquid air, should help incorporate a far higher proportion of renewables in the grid; balance supply and demand; and cut both emissions and costs.

National Grid has to pay wind farms to turn themselves off when it can’t find a home for their output. That tends to happen when bottlenecks in the transmission system stop the power getting to where it’s needed, or when there is simply too much wind and too little demand. The UK network operator says the problem will worsen as offshore wind capacity soars over the next decade, and that electricity storage will be vital to smooth out renewable production: soaking up excess power when demand is low, releasing it when demand is high.

The problem is, what kind of storage? Pumped storage, like the Dinorwig plant in Snowdonia, is well proven, but most of the suitable sites have already been exploited. Batteries are expensive and their chemical reactions degrade over time and shorten their working lives. Compressed air storage requires huge underground caverns, and in any case is in its infancy. National Grid says there are “significant hurdles around the economics” of the newer technologies.

The solution could lie on a drab industrial estate on the outskirts of Slough, where a pilot plant is being developed by Highview Power, in the shadow of the local biomass power station. Highview’s chief executive Gareth Brett, a sandy-haired electrical engineer who developed a keen appreciation of the importance of storage during a career spanning National Grid, nuclear operator British Energy and Dinorwig, is convinced they have the answer. “Our system is one of the few that is truly scalable to utility scale”, he says. “In economic and efficiency terms it’s comparable to pumped storage but without the geographical constraints”.

Highview Power’s pilot plant in Slough.
Click to enclarge

Highview’s pilot plant consists of a series of shipping containers housing compressors and turbines arranged around a 10m high cryogenic storage tank, and works on a system so simple it seems surprising nobody developed it sooner (see diagram). Electricity from the grid drives industrial refrigeration equipment to cool air to minus 200C, producing liquid nitrogen, which is stored in the tank. When power is needed, the nitrogen is pumped to high pressure and fed into an evaporation unit to produce high pressure gas, which then drives a 4 stage turbine to generate electricity.

Diagram courtesy of Highview Power.
Click to enclarge

If that was all there was to it, the system would return only about a quarter of the energy it consumes, but there are several clever adaptations that improve the energy balance.

The efficiency of the system depends on the size of the gap between the storage temperature of the liquid nitrogen and the temperature of the gas as it enters the turbine: the bigger the difference, the more the gas expands, the more energy is delivered. So the nitrogen is pre-heated to around ambient temperature using exhaust gas from the turbine, and then raised to around 50C using waste heat from the power station next door.

During the process the exhaust gas that has been used to warm the nitrogen cools to around -160C, and is pumped into a cold store made of granite pebbles, where the ‘coolth’ is saved to be used later when the plant is next creating liquid air.

These improvements raise the ‘round trip’ efficiency dramatically, and Highview say the system would be up to 70% efficient if built on industrial sites with a ready source of waste heat at 100C, such as cement factories, waste-to-energy plants and data centres. Given that British industry throws away as much heat as the country consumes, locating enough waste heat shouldn’t be a problem, says the company.

The pilot plant is rated at just 300 kilowatts, with the capacity to store 3 megawatt hours. Once this has been commissioned and proven, the company plans to build a 10MW plant with storage of 60MWh – about enough to keep a small town going for 6 hours – which would sit comfortably on a site of 4000 square metres, or 1 acre. Larger plants are relatively more compact, so space should not be a constraint either, according to Brett.

“There are plenty of brown-field sites”, he says. “We haven’t done an estate agent’s trawl, but we haven’t found any difficulty in finding potential sites for our follow-on projects. The signs are pretty good”.

The Highview system would be easier to install on industrial sites than combined heat and power, because there would be no need to dig up the roads to lay district heating pipes. It would also make better use of low grade (up to 120C) heat, which is not hot enough to provide heating over much distance. Yet far from being mutually exclusive, the system could also help overcome some of the problems associated with existing CHP plants.

“One of the big problems with CHP is being able to modulate the heat output for the CHP power station to suit the heat demand”, says Brett, “and there’s obviously quite a big connection between the amount of electricity the power station produces and the amount of heat it produces”. Having a device that can absorb heat, but can also be switched on and off, he argues, could help stabilize heat demand. “Suddenly the housing estate you’re trying to keep warm doesn’t determine when your crisp factory can make its bags of crisps!” A fully integrated system such as this could provide “a kind of total solution”, helping to balance both local heat loads and supply, and the electricity grid.

Another major advantage is that – unlike other storage technologies – all the components and processes are already employed at scale in the industrial gas and utility industries, and supported by well-established supply chains. Companies like BOC already operate nitrogen liquefaction plants producing 2000 tonnes per day, equivalent to 200MWh of electricity, and the LNG industry uses storage tanks around 100,000 tonnes, or 10GWh – enough to keep the whole country going for about a quarter of an hour. Highview says the existing supply chain would allow it to build plants of 100MW now, and 1GW eventually – the size of a nuclear power station.

Highview’s plants could be built near wind farms to smooth their output, or near towns or industrial centres to shave peak demand and ease grid bottlenecks. They could compete in the Short Term Operating Reserve market, through which the National Grid buys back-up capacity to help balance the system from day to day, or even provide long-term, seasonal storage for the times when a high pressure weather system produces a windless week in February – because storage tanks are cheap. They would cut emissions by soaking up overnight nuclear and wind power and displacing coal and gas plants during the day, and would even cut emissions if they were charged using fossil power stations.

“Even if there is no renewable energy on the grid”, Brett explains, “with a storage device you can charge up using the average efficiency plant, which then gives you lower carbon content electricity at peak than running a highly inefficient peaking plant like an open cycle gas turbine”. The average carbon content of UK electricity is around 500 grams per kilowatt hour, while emissions from peaking plant are about 1000g/kWh.

Because the system is based on standard industrial kit, it should be much cheaper than other storage technologies. Highview claims its system has a capital cost of $140 per kilowatt hour of storage capacity, whereas the US Electric Power Research Institute estimates the figure for pumped hydro is $310 and batteries upwards of $470. However, although the company says its system would earn a return in today’s market, the returns would not be high enough to attract many outside investors. For that reason they argue emerging storage technologies – which unlike renewables and electric vehicles receive no subsidy – need public support until they can survive without it.

In its Electricity Market Reform white paper published last month, the Department of Energy and Climate Change recognized the importance of storage and the need to support it through the proposed capacity mechanism, to be developed as part of an electricity systems policy next year. While they would rather the capacity mechanism were already in place, Highview welcome the announcement. “The fact that it’s headed in the right direction, and there seems to be a decent commitment to the kind of future in which our technology has an important role to play, is very positive”, says Brett.

In order to accommodate the amount of renewable capacity required by Britain’s 2020 targets, National Grid says it will need to raise its operating reserve from about 4.7GW today to around 7.3GW in 2020. In that context, if all goes well, Highview is confident its system to could deliver some 500MW.

One Comment on “Cold storage”

Tali Says:
August 26th, 2011 at 1:32 pm

Rather than subsidizing storage, which we’ve done largely without before planning to deploy renewables, wouldn’t it be a fairer accounting to mandate that renewables projects be required to fund sufficient storage or demand management as their fraction of nameplate capacity rises? We can increase the subsidy if required. Otherwise, the renewable generators are externalising a cost inherent in attempting to provide the 24×7 electricity we’ve come to expect.

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