With the help of the University of Birmingham’s energy storage specialists, Professor Yulong Ding and Professor Toby Peters, MechChem Africa takes a look at the use of liquified air energy storage technology and reports on ongoing research initiatives and early implementation successes.
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In 2018, Highview Power, a UK-based energy storage company, opened the world’s first grid-scale cryogenic energy storage plant. With a capacity to deliver 5.0 MW for three hours or 15 MWh of electrical energy, the plant is grid-connected and reliably delivers electricity on-demand, while using excess grid capacity to replenish the liquid air used by the process.
The development began when co-founder, Peter Dearman came up with an idea for a reciprocating engine driven by liquid nitrogen. The volume of nitrogen expands 700 times on evaporation from a liquid, potentially creating high expansion pressures that can be used by an engine or turbine.
In 2005, Dearman met Highview co-founder Toby Peters who saw the potential for energy storage on a very large scale and tested the concept with Professor Yulong Ding, then at the University of Leeds. In 2011, the original engine part of the business branched off to form the Dearman Engine Company, which began exploring cool-power mobile applications with the Universities of Birmingham and Loughborough and industry partners taking them through to commercial trials in UK and European supermarkets.
From 2006, Highview with Professor Ding worked to invent and file patents for cryogenic energy storage and generation. This work resulted in the world’s first demonstrator for cryogenic energy storage, first built at the Slough Heat & Power site by Highview. This plant was later moved to the University of Birmingham.
In 2013, Highview partnered with the University of Birmingham and Professor Ding was appointed as the Highview Power/Royal Academy of Engineering Research Chair in Energy Storage and charged with further developing cryogenic energy storage technology, alongside other thermally based energy storage concepts.
Explaining the underpinning principle, Yulong Ding says that this technology goes back to 1902, when Georges Claude developed a cycle to improve the chilling efficiency of the Linde method for liquefying atmospheric gases. Claude used a turbo-expander to further lower the air temperatures during air separation. “A turbo-expander works in a similar way to a gas turbine, expanding high-pressure gas so it becomes lower in pressure and temperature, while extracting mechanical power. A turbo-expander cools the gas, while a turbine is used when the core purpose is to recover power,” he explains.
Describing how cryogenic energy storage systems work, he lists four main components: a charging device that uses off-peak or excess electricity to power an industrial liquefier to produce liquid air; an energy store where the liquid air is held in an insulated tank at low pressure; a power-recovery unit where regasified liquid air is used to drive a series of turbines to generate electricity; and the clever fourth bit, which is to capture the ‘cold’ as the liquid air is heated back to ambient and regasified for recycling to help drive the liquefaction process. This inventive step significantly increases the overall cycle efficiency.
Ambient air is taken from the surrounding environment. Then, using electricity, it is cleaned, dried and refrigerated through a series of compression and expansion stages until the air liquefies. “Using a modified version of the Claude Cycle, this process can convert 700 litres of ambient air into one litre of liquid air. By keeping this liquid air at -196 °C or below, it can be stored in very well insulated tanks at atmospheric pressure,” he explains.
When power is required, liquid air is withdrawn from the tanks, pumped to high pressure, reheated and then expanded. “The resulting high-pressure gas is then used to drive expansion turbine generators to generate electricity. No fuel is burnt in the process, so the only emission is clean dry air,” says Ding.
The system can be further enhanced by harnessing low grade waste heat from industrial or power generation or high grade waste cold from LNG regasification. As Peters explains: “Globally we currently throw away vast amounts of energy as waste heat and also waste cold. Given that the system uses liquid air as the storage medium, it can exploit low grade waste heat in the regasification process, which is often otherwise thrown away. Likewise, as the first stage is to liquefy air down to -196 °C, the system can be integrated with LNG regasification plants to harness the copious amount of cold that is exhausted as the LNG is regasified before being injected into the gas grid. Coupling liquid air energy storage to cryogenic storage facilities, therefore, brings many unique benefits.”
Energy storage research at Birmingham
Led by Dr Jonathan Radcliffe at the University of Birmingham, MANIFEST (Multi-Scale Analysis for Facilities for Energy Storage) is a £5 million project that taps into Birmingham’s expertise in cryogenic and thermal energy storage. The programme is continuing to investigate improvements in energy storage technologies through integration and exploring potential application scenarios to accelerate the deployment of the technologies.
MANIFEST addresses a number of research questions about how materials can be better used and integrated and how energy storage devices can be best optimised. The project brings together Birmingham’s expertise in liquid air and thermal energy storage with scientists from across the country working on thermo-mechanical and electrochemical storage technologies.
“Modelling energy storage systems is extremely complex and challenging. The MANIFEST programme provides cross-university and cross-discipline collaborations for addressing this challenge,” explains Ding. “Equally important and also of particular interest to us is experimental validation of multiscale modelling through this research programme.
“Technologies such as liquid air and thermal energy storage have great potential to help crack the energy conundrum: how can variable generation from renewables meet the needs of energy users. We have one of the world’s first experimental cryogenic energy storage facilities on campus and have also achieved success with the first commercially available shipping container constructed from cold storage materials that can be charged with cold energy.”
As an additional part of the MANIFEST project, the University of Birmingham is taking a lead role in establishing UKESTO (UK Energy Storage Observatory), a national ‘observatory’ for energy storage that will give scientists online access to data from experimental facilities at partner universities within the consortium.
MANIFEST and UKESTO lead, Jonathan Radcliffe, Reader in Energy Systems and Innovation, says: “MANIFEST is allowing detailed studies of a range of energy storage technologies and their potential impact across the energy system. There is a focus on batteries now, but that is just part of what will be required to integrate renewables at the scale needed to be on track for net-zero. And whilst there are a growing number of energy storage demonstrator sites in the UK and globally, there is little data available on their operations.
“UKESTO will connect energy storage pilot plants on university campuses to create a network of national facilities that establish the UK as an innovation hub – allowing systematic study of energy storage technologies to an extent that is not possible with industrial demonstrators.”
Professor Ding and Professor Toby Peters, who left Highview and is now Professor of Cold Economy at the University of Birmingham, are widely recognised as the founding fathers of liquid air energy storage. Working together, they led the team that invented and proved the idea of cold recycling, which is key to achieving high-levels of efficiency.
In terms of competing technologies, pumped hydro-electric storage still accounts for 95% of global capacity, with lithium-ion batteries, which have been the fastest growing recent years, now accounting for most of the rest of the world’s energy storage. Both have issues. Pumped storage requires two large water reservoirs close to each other with a sufficient height difference between them. The construction of the reservoirs and interconnecting tunnels is expensive and can also impact heavily on the surrounding environment. Lithium is relatively rare and supply is tightly controlled. Cobalt is also needed, which is both toxic and rare. The batteries also degrade and they have a realistic limit of about four hours per day for harnessing large amounts of power.
Cryogenic storage solutions such as those from Highview Power tick many boxes, including sustainability, cost, effectiveness and the fact that they have a small footprint and can be sited anywhere. The only box not yet ticked – until now – has been demonstration at large scale. Now, with plans to begin building two large cryogenic energy storage plants in 2019, that is set to be ticked too.
The most recent of these is a 50 MW (minimum) 400 MWh Highview Power Storage plant, to be built in northern Vermont in the USA for Encore Renewable Energy to provide eight hours of energy storage. The facility will contribute to resolving the longstanding energy transmission challenges surrounding the state’s Sheffield-Highgate Export Interface (SHEI) and enable the efficient transport of excess power from the company’s wind and solar renewable energy sources to help integrate and stabilise the power grid.
“We’re hugely excited at the opportunity to build on the vast experience in cryogenic energy storage at the University of Birmingham and help to unlock the potential of liquid air and other energy storage technologies,” says Peters.
“Liquid air energy storage is a unique solution to provide low-cost, large-scale long duration energy storage with no geographical constraints. It can also harness waste heat or waste cold in the system to further increase the overall efficiency.
“With the demand now for large-scale, long duration energy storage, liquid air can emerge as the serious competitor to lithium-ion in grid-scale-storage,” Peters concludes.
Acknowledgement: Dominic Joyeux: A new contender for energy storage; Ingenia magazine: Royal Academy of Engineering; Issue 78, March 2019.