LIQUID nitrogen seems a good place to start if you want to cool something down.

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LIQUID nitrogen seems a good place to start if you want to cool something down. If Peter Dearman, a British inventor, is correct, it might be even better than it looks. Mr Dearman is a man with a bee in his bonnet. He has dreamed, since he was 15, of making a useful motor powered by liquid nitrogen. Now, at 64, he thinks he has done it—not, as he had originally imagined, to run a car, but to run a refrigerator that promises to be more efficient and less polluting than conventional alternatives.

.The Dearman engine works a bit like a steam, petrol or diesel engine. It uses heat to expand a gas (known as a working fluid), which drives a piston. The difference is that instead of starting at room temperature the working fluid ends up there, and starts instead at -196°C, the boiling point of nitrogen. That temperature change, and the expansion of the nitrogen it causes, is enough to do work equivalent to a more conventional heat engine—but not, sadly, much better than such an engine. Nitrogen engines have thus, for most of Mr Dearman’s life, been an idea looking for an application. Now, though, he thinks he has found a niche whose incumbent he can replace—the inefficient, pollution-generating diesel-powered refrigerators that keep the contents of food-delivery lorries cold.

The challenge was to extract more cooling power from liquid nitrogen via the engine than can be garnered by using it directly. Mr Dearman was able to do so by dividing the labour of cooling into two. First, instead of injecting liquid nitrogen directly into the engine’s cylinder, his system boils it by running it through a system of pipes called a heat exchanger. This apparatus is connected to the compartment to be refrigerated, and sucks heat out of that compartment and into the nitrogen, which turns from a liquid into a gas.

This process cools the compartment but does not raise the nitrogen’s temperature, which remains at -196°C. That is because the heat in question, known as latent heat of vaporisation, serves merely to change the nitrogen’s state, not to warm it up. The resultant cold, gaseous nitrogen is now injected into the engine’s cylinder (see diagram) along with a mixture of water and glycol, known as a heat-transfer liquid, that is at room temperature.

The heat-transfer liquid rapidly warms the nitrogen, causing it to expand and drive the piston—which powers a conventional refrigeration circuit that provides a second dose of cooling to the cold compartment. The piston’s return stroke then expels the gas and the now-cooled heat-transfer liquid. The nitrogen is vented to the air, and the heat just extracted from the cold compartment by the conventional refrigeration circuit is used to warm the heat-transfer liquid back to room temperature, ready to be used again.

This double cycle extracts 40% more cooling power from liquid nitrogen than systems which use the stuff directly as a coolant. That makes it competitive with the conventional, diesel-powered refrigerators now used in chiller lorries. Its crucial advantage over diesel, though, is that it is pollution-free. Nitrogen is 78% of air, so a little more will make no difference. The tiny particles of soot that refrigerator diesel engines produce, by contrast, are a health hazard up with which regulators are increasingly unwilling to put. Mr Dearman’s engine, now undergoing trials, provides an alternative. If it proves itself, the bees in his bonnet may at last yield some honey.