A liquid nitrogen powered engine could provide an energy storage alternative to batteries and fuel cells. Such an engine is driven by the pressure created as liquid nitrogen (LN2) returns to its gas form and could provide a way of powering a vehicle with a longer range and faster refueling than a battery without producing any noxious tailpipe emissions.
In the UK there are now two groups working to get LN2 market ready,
Engineers at UK Highview Power Storage have developed a potential solution to several issues by building the world’s first liquid air energy-storage system. 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 Highview team 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 process looks quite simple, store 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 (1 bar = 14.5psi) 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 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-sulfur batteries and between half and a quarter of that required to pump water uphill into reservoirs.
Dearman Engine Company (DEC) has plans to develop a commercial prototype engine and is working with engineering consultancy Ricardo and several UK academics to assess its engine’s feasibility. Their proof-of-concept model, which its developers said is far more efficient than any previous design for a LN2 engine, has already been used to power a car at more than 30mph with only cold air in the exhaust.
The engine was invented by company co-founder Peter Dearman and operates in a different way that produces considerable power.
Toby Peters a Dearman Engine Company director explains, “Mr. Dearman invented a process whereby you inject a heat-exchange fluid such as anti-freeze and water into the head of the piston just before you inject the liquid nitrogen. You keep the liquid nitrogen liquid right the way up to the piston. The result of that is that all the expansion takes place inside the cylinder. And because you’ve got this volume of heat-exchange fluid, it’s isothermal expansion, so it keeps the temperature the same, which is far more efficient.”
Peters winds up with a strong consumer outlook. “Our product is likely to be, on a cost-base, more competitive to a piston engine than a battery, and piston engines are massively cheaper.”
LN2 is handy stuff and a lot of it is already around. There is the advantage of relying on an existing distribution infrastructure as many industrial companies use it for cooling. LN2 could also be generated in remote places using renewable energy sources and a small liquefaction plant. Such a system would work well as a solution for remote military bases.
Peters said the key engineering challenge was optimizing the injection of liquid nitrogen into the cylinder but that all the technologies involved, including the LN2 storage tanks, were mature enough not to be showstoppers.
The Brits are doubling up on LN2. Dearman Engine was spun out of Highview Power Storage, who last week won the magazine The Engineer’s Technology and Innovation Awards Grand Prix for its utility-scale liquid-air energy-storage system.
The numbers look pretty decent. Peters takes this up saying, “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 centers, 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.
LN2 can get into this game. The capital cost matter looks to be offering a big advantage and that advantage could lead to an installed base much quicker than the competition.
The most curious thing is explained by Chief Technology Officer Rob Morgan, “ . . . costs and efficiency aren’t the only advantages of using liquid air to store energy. 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 also, 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 maximize the process’s efficiency.
Highview’s CEO, Gareth Brett points up an important point, “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.”
“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, he said.
This idea with its obvious financial advantages could very well evolve into the mass market. While LN2 is non-flammable, practically reacts with essentially nothing in personal transport or homes as a major safety consideration; suddenly freed or spilled it would offer skin contact freeze burns.
The Brits are getting pretty far among with this and the data to date is very encouraging. Lets hope we see more information and early applications soon. LN2 looks very good.