Interview: an introduction to liquid nitrogen – Nick Owen

2017-05-22T16:47:25+00:00 November 5th, 2015|Viewpoints|

When we think about fuel, we generally think about something that burns – wood, coal, diesel. Since the age of steam, and later the advent of the internal combustion engine, power has come from heat. So, it may seem counterintuitive to say that power can be extracted from a liquid that’s stored at -196ºC – but that is exactly what Dearman technology does. Liquid nitrogen is the ‘fuel’ at the heart of Dearman’s cutting-edge technology portfolio. But how does it work? Is it safe? Where does it come from? Nick Owen, Chief Technology Officer at Dearman, explains.

What is liquid nitrogen, and where does it come from?

The air we breathe is made up of a mix of gases. Oxygen is about 21%, while nitrogen makes up 78% of the atmosphere. When you make a gas very cold, it condenses, and becomes a liquid. Different gases condense at different temperatures, so it’s possible to extract individual gases – oxygen, nitrogen, argon etc. – and store them as liquids. This happens on an industrial scale every day at the Air Separation Units around the country. These liquids – or cryogens – are securely stored in large insulated containers and transported to hospitals, factories and industrial facilities. The cryogen we use in the Dearman Engine is liquid nitrogen.

Is it energy intensive to produce liquid nitrogen?

Cooling air to almost -200ºC does require energy – most Air Separation Units use grid electricity. But Air Separation Unitss are generally run at night, when electricity is cheaper and often produced by nuclear power stations that can’t be turned off, or supplemented by renewable energy like wind power, that no-one else wants at that time of day.

As there is four times as much nitrogen as oxygen in the atmosphere, but less demand for it commercially, the industry vents thousands of tonnes of nitrogen gas every day. Dearman technology can make use of this otherwise wasted product, using liquid nitrogen in zero-emission power and cooling applications that deliver substantial CO2 savings.

While it is an energy intensive process, and we don’t yet have a 100% renewable grid, using liquid nitrogen as a fuel still offers substantial CO2 well-to-wheel savings. Of course, as the grid decarbonises in the years to come with more renewable sources in the mix, grid electricity will become even cleaner. Creating liquid nitrogen will in turn become less carbon intensive. And that makes nitrogen an even ‘cleaner’ fuel.

How does it work as a fuel?

When nitrogen gas is cooled to -196ºC is condenses to a liquid and its volume dramatically decreases. Around 700 litres of nitrogen gas condenses to only 1 litre of liquid nitrogen. If you expose the liquid nitrogen to normal air temperature, it boils and becomes a gas again, increasing around 700 times in volume. In a Dearman Engine, we capture this expansion inside a piston chamber. By introducing a little warm water, which acts as a heat exchange fluid, we can make this expansion more efficient. Simply, the liquid becomes a rapidly expanding gas, and this expansion causes a piston to move, generating power.

Is it expensive?

The price of liquid nitrogen will depend on various things, including how much is being ordered and the cost of delivery. But compared to traditional fossil fuels you might put in a car or bus, liquid nitrogen is much cheaper.

The challenge has always been to use liquid nitrogen efficiently. Dearman technology represents a big leap forward in this area by delivering high efficiency cooling and power. In many applications and geographies Dearman can offer a substantial operating saving with comparable capital costs.

Is liquid nitrogen readily available?

With the exception of Aberdeen and Plymouth, all major UK cities are within a short distance of an Air Separation Unit that could provide liquid nitrogen. Storage tanks can be obtained easily as well. I often say, I could make a call today and have several tonnes of liquid nitrogen delivered tomorrow. And it’s true.

In fact, there is a huge opportunity to access the vast amounts of liquid nitrogen which is wasted day-in day-out. Ten European countries have enough spare liquid nitrogen production capacity to fuel some 70,000 Dearman-powered transport refrigeration systems immediately.

Is liquid nitrogen safe? What are the hazards involved in its use?

Nitrogen is classed as an inert gas – it is not toxic or poisonous – it makes up 78% of the air we breathe, after all. When it is a very very cold liquid, it has to be handled with care – but consider that anyone can fill a car with flammable petrol or diesel. An excess of nitrogen in an enclosed, unventilated space acts as an asphyxiant – but so does the exhaust of a car’s combustion engine. Again, it just needs to be managed carefully.

Perhaps surprisingly, you can store liquid nitrogen closer to a building than you can diesel – equally, consider that many of us drive around sitting on dozens of litres of diesel every day without a thought. The hazards involved in the use and transport of liquid nitrogen are well-known and there are established safety protocols around its use to prevent dangers arising – as I mentioned earlier, it is used in many industries already.

Liquid nitrogen should be used sensibly in appropriate applications, and by trained individuals. If it is used in this way, it should be no more hazardous, and even safer, than established fuels such as petrol, electricity or natural gas.

So, what are the benefits? Is this the fuel of the future?

Liquid nitrogen has two properties which make it an excellent ‘fuel’ for processes requiring both power and cooling. It is very very cold and acts as a heat sink, cooling its surroundings, and expands rapidly when it becomes a gas, generating power. Dearman technology harnesses these properties for use in applications such as refrigerated transport, air conditioning, or even as part of a hybrid engine.

It’s not as energy dense as diesel – a litre of liquid nitrogen won’t deliver the same amount of power as a litre of diesel, but it does generate lots of cooling naturally. I think diesel will be the fuel of choice for primary engines for a long time to come, but there are some processes for which it makes sense, economically and environmentally, to use a cryogen like liquid nitrogen.

The only emission from a liquid nitrogen-powered Dearman Engine is nitrogen – no harmful particulate matter, no NOx, and no CO2. The gas is simply and safely reabsorbed into the atmosphere.

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