By using highly efficient Heatric PCHEs for cold recovery Highview Power Storage has doubled the efficiency of its ingenious Liquid Air Energy Storage (LAES) system.
Ranks of turbines wave their arms on some windy hillside. Hectare upon hectare of solar panels stare blankly back at a relentless desert sun. But what happens to that energy when nobody needs it? And what will replace it when the wind doesn’t blow or after the sun sets?
Collect the sun …
Few national electricity grids have much built-in energy storage capacity and the lack has become a significant impediment to the expansion of renewables. Pumped hydro has historically been used by electricity grids to store energy, but not every country has the mountains for that. Large-scale batteries, meanwhile, are made from expensive and hard-to-come-by materials and their performance degrades over time.
Other mechanical energy storage technologies exist. Compressed Air Energy Storage (CAES), for example, uses surplus power (when demand is low) to compress air and then generates electricity later by expanding it through a turbine.
… and freeze it
Now a UK-based company is taking mechanical energy storage a step further. Highview Power Storage, designer and developer of Liquid Air Energy Storage (LAES), has come up with a truly ingenious approach. By marrying established technologies from the industrial gas sector and turbo machinery from the power generation sector, Highview has built a system that stores energy as super-cooled, liquefied atmospheric air using only readily-available components from mature industrial supply chains and with no geological or geographical constraints.
During the first stage, the process uses surplus or off-peak electricity to power a liquefaction plant, which cleans and cools atmospheric air to minus 196oC. Each 700 litres of gas becomes a single litre of liquid so that very large quantities of air can be stored safely and at low pressure in insulated tanks. This is the Highview energy store. The techniques used to create and maintain it are already used widely in the manufacture and storage of industrial gases.
When electricity is needed liquid air is drawn out of the storage tanks and pumped at high pressure through a heat exchanger. As the liquid warms it evaporates and expands to form a stream of high pressure gas which is then used to drive a turbine.
Waste cold recovery
The economic viability of the Highview system depends on minimising energy losses because making liquid air is a power-hungry process. This is where Heatric’s printed circuit heat exchangers (PCHEs) make a crucial contribution.
During the power recovery stage, as the liquid air warms and evaporates, and before it enters the expansion turbine, Heatric PCHEs capture almost all of the available cold, sending it to a high grade cold store for re-use in liquefaction. This is Highview’s key innovation; a ‘waste cold recovery’ process which significantly reduces the system’s total energy consumption and increases round-trip efficiency.
Highview’s Head of Engineering, Stuart Nelmes, explains:
“We use Heatric units where the thermal exchange is of the highest importance. The higher the grade of cold we are able to put back into our high grade cold store, the more energy we save because the refrigeration compressors at the start of the process need to do less work. Base efficiency of the standard cycle at commercial scale without cold recycle is around 25%. Highview’s cold recycle process increases the round trip efficiency above 60% and the use of Heatric PCHEs helps to achieve these efficiencies. Add waste heat to the warming and expansion phase and that figure rises beyond 70%.”
The unique design of Heatric PCHEs brings the process fluids into very close proximity, enabling exceptionally high energy transfer rates and a very close ‘approach temperature’ (i.e. the difference between the temperatures of the cryogenic liquid and the gas capturing the cold energy). Minimising the approach temperature equates to maximising cold recovery and thus energy saved. It is a key performance parameter for the Highview system.
“Getting the approach temperature as low as possible improves our round-trip efficiency by quite a large percentage. At the pilot plant we recorded steady-state approach temperatures of 1.7 degrees, which is pretty impressive.”
Raising the bar
The Heatric PCHEs must also be able to handle high pressures.
“To get the most energy out of the store we pump the air to high pressure while it is still in liquid form. Our pilot plant operates at 60 bar. This will rise to 120 bar in our next pre-commercial plant but we expect to go as high as 200 bar at full-scale.”
Nelmes acknowledges that shell and tube units could cope with such pressures, but, he says, their sheer size would have ruled them out even if their inability to match PCHE approach temperatures had not already:
“At grid-scale our process stores between 200 and 400 megawatt-hours (MWh) in several thousand tonnes of media. That requires a lot of thermal energy to be exchanged, so these devices can get very large. Creating heat exchangers that are compact and high capacity, that’s where Heatric really excels. We’ve had other manufacturers visiting the pilot plant and they are always taken aback by the size of the Heatric units compared to the work they do. That’s something nobody else can compete with at this point.”
Optimising the system
Clearly, Highview’s decision to go with Heatric technology was driven strongly by performance considerations. Nonetheless, Heatric and Highview have worked closely together, optimising PCHE specification to capture some very significant savings on budget for Highview.
“Heatric saw that our initial process parameters could be tweaked to get more out of the sizing and performance capabilities of the PCHE units. The changes Heatric suggested helped us reduce our original budget for PCHEs by 50%. That’s a considerable saving.”
From 2011 until the end of 2014 Highview operated a grid-connected 350kW/2.5MWh pilot plant at SSE’s (Scottish & Southern Energy) 80MW biomass heat and power plant near London. The pilot plant has now been relocated to the University of Birmingham’s Centre for Cryogenic Energy Storage for further testing. It will be re-commissioned by the end of spring.
In February 2014 the UK government’s Department of Energy & Climate Change (DECC) awarded Highview and site host Viridor more than £8 million to build a new 5MW pre-commercial LAES technology demonstration plant at a landfill gas-to-energy site in Greater Manchester. The facility will for the first time demonstrate the LAES technology at commercial scale, utilising waste heat from the landfill gas engines to improve the efficiency of the LAES system.
Last year Highview also announced two licence agreements. The first, with GE Oil & Gas, is a global licensing and technology collaboration agreement for GE to explore opportunities to integrate LAES technology at ‘peaker’ power plants running GE gas turbines and engines. The second is a licence agreement and investment deal with US clean coal technologist Advanced Emissions Solutions Inc. of Denver Colorado through its subsidiary ADA-ES. The licence is for grid-connected LAES non-peaker plant storage applications covering North America (US, Canada & Mexico).