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Energy and Environment

The 2011 Energy & Environment Winner - CES

Highview’s scalable liquefying technology stores excess energy as a cryogenic fluid until it is needed.

Cryogenic energy storage (CES) pilot plant

Highview Power Storage, Scottish and Southern Energy, BOC/Linde, University of Leeds

Energy storage is critical if we are serious about replacing fossil fuels with renewable electricity sources. One of the key problems with wind, wave and solar power is their intermittency, so matching the unreliable supply of green electricity to our demand requires an intermediate step so we can save the energy in some form for when we need it. Additionally, those countries whose traditional infrastructure is struggling to meet demand could benefit hugely from a back-up system to help accommodate peak consumption.

At the moment there are about as many methods of storing renewable energy as there are of generating it and, similarly, each has its own pros and cons. But engineers at UK Highview Power Storage have developed a potential solution to several of the sector’s issues by building the world’s first liquid air energy-storage system.

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Cold storage:Highview’s cryogenic energy storage generates cold as it operates

Working with researchers at Leeds University, the Highview team has used established technology to create a modular, scalable and relatively cheap energy-dense system that can be located easily next to existing infrastructure. They believe it could be a way of capturing as much energy as a small pumped-hydro system, but without the need for reservoirs and mountains.

The company opened a pilot facility near Slough in April 2010 in partnership with Scottish and Southern Energy and industrial gas company BOC, supported by £1.1m from the Department of Energy and Climate Change (DECC). It takes electricity from a nearby biomass plant and uses it to liquefy air by cooling it to -200°C. The energy can then be released and supplied back to the grid by evaporating the cryogenic fluid and using the resulting gas to drive turbine generators. Waste or ambient heat can be used in the evaporation process and the cold energy from the exhaust stored and reused to liquefy more air, making the whole system more efficient.

The company estimates that the capital cost of cryogenic energy storage will be less than $1,000 per kW

’If you want a green grid, you need energy storage,’ said Highview’s chief operating officer, Toby Peters. ’It’s the whole issue of strategic policy for energy security and economics. It’s the time-shifting of energy to support not just intermittent sources but also must-run plants. We think [our] system has some pretty important benefits – one of which is capital costs, which is a very important driver in this market.’

The company estimates that the capital cost of cryogenic energy storage will be less than $1,000 (£635) per kilowatt (kW) when the technology is mature, one quarter of the costs of sodium-sulphur batteries and between half and a quarter of that required to pump water uphill into reservoirs.

’Pumped hydro is the gold standard but there aren’t many mountains close to London,’ said Peters. ’[Our technology is] modular and scalable, and you can move it. Because of the cycle, we can harness waste heat and, specifically, low-grade waste heat. And we generate cold as we operate. When you think about data centres, the application demand for cold is very big.’

The full system returns about 50 per cent of the energy put in, rising to 70 per cent efficiency if it uses waste heat from another source, such as a power station. This is similar to the efficiency of the much less energy-dense compressed-air storage plants and compares with 70-85 per cent for batteries and 65-75 per cent for pumped hydro.

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Good investment:Highview’s system gives back about 50 per cent of the energy put in

But costs and efficiency aren’t the only advantages of using liquid air to store energy, said chief technology officer Rob Morgan. ’These fluids are already shipped around in tankers all around the world. The big issue with hydrogen is that there’s no infrastructure, but for cryogenic fluid there is. Not only are we looking at a storage solution here but, as part of the broader ways of moving energy around, it’s potentially quite an attractive medium.

The other benefit of Highview’s modular design is that it can be tailored to the specific supply-and-demand needs of a particular situation, with separate liquefaction and evaporation systems that work together to maximise the process’s efficiency.

’Generally speaking, what you want to do is charge the unit up at times of low price or of excess wind ability and when you want it back is at times of system stress or high price. There are more low-price, low-demand times than times of system stress and high price,’ said chief executive officer Gareth Brett.

’But the way that works in the UK is different to the continent and the US. So having a unit where you can independently size the charging system compared with the discharge system, as well as what size tank you have, is quite an advantage because you get to tailor-make your system to suit the application without having to re-engineer it from scratch, just by applying more or fewer modules.’

“There are more lowprice, low-demand times than of system stress and high price”


The Highview system stores energy by using electricity to operate a liquefactor, which would typically be used to distil the chemical components of the air, compressing the gas to around 40 bar and passing it through a series of heat exchangers and expansion turbines. To release the energy, the cryogenic fluid is first compressed further to around 70 bar and then evaporated to drive turbine generators. This figure will likely increase to 100-150 bar for commercial operation, but is still well within the pressures used by existing steam turbines.

’The best place to put storage is next to places where people are using power and often producing heat, so you can produce a more efficient cycle and put energy back into the network,’ said Morgan.

’If you can store that cold energy and then use it to help liquefy the gas-recharger cycle, you can significantly improve the efficiency of the process. It takes in liquid cryogenic fluid on the way in and produces power and very cold gas on the way out, so if you can capture that cold energy then you’re most of the way to turning that gas back to liquid for storage.’

The cold storage and its integration into the system are the most novel parts of Highview’s facility and enable a doubling of the electricity-generation process. The cold energy is captured using a specially designed buffer that uses technology similar to that used in the steel and chemical industries, where cold is often stored in beds of sand or gravel.

With the technology proven, Highview is now looking for partners in the energy sector and in the manufacturing supply chain to enable it to roll out its technology on a commercial level. While the system’s potential for enabling better use of renewable electricity is clear, the energy industry may need to generate more of the stuff before it invests more heavily in energy storage, and governments, including the UK’s, are still deciding on how best to proceed. A more immediate path for Highview may come through supporting countries where existing fossil-fuel infrastructure is reaching the limits of its capacity, such as in South Africa and China.

Here, the need for a system to match unequal supply and demand is more immediate and so could be an ideal match for a larger-scale version of cryogenic energy storage.

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The other shortlisted candidates in this category were:

ICARES: Integrated Compressed Air Renewable Energy Systems

University of Nottingham, Thin Red Line Aerospace

The team from Nottingham University and Thin Red Line Aerospace developed an energy-storage system that uses fabric bags of compressed air designed to be held deep underwater as an inexpensive and on-demand method of capturing renewable energy. The hydrostatic pressure from storing compressed air deep underwater provides most of the force needed to keep the air contained and provides an energy-storage solution that can operate very close to offshore wind turbines. The fabric pressure vessels created by Thin Red Line were made from a weight-saving material originally designed for the aerospace industry, while the Nottingham researchers developed powerful analysis tools to determine where the pressure points were in the fabric structure.

Development of widely accessible organic solar-cell technology

University of Warwick, Molecular Solar, Warwick Ventures

The third-generation solar cells that originated in Warwick University’s chemistry department use organic photovoltaic OPVs that offer the prospect of low-cost, lightweight, flexible solar-generating material.

The cells are composed of thin films of a cheap and abundant organic semiconductor material that can harvest light from a large proportion of the solar spectrum.

These are mounted on a flexible substrate developed by Molecular Solar called FLEXIFILM and weigh less than one per cent of the weight of conventional silicon cells. The materials used can be cheaply mass-produced with minimal environmental impact because they are made from non-toxic compounds and are recyclable. Molecular Solar says its technology has the potential to reduce the cost of solar electricity generation and to make it competitive with conventionally generated electricity at the point of use.

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