The Dearman liquid air engine is an innovative heat engine that uses liquid air (or liquid nitrogen) as a “fuel” and emits cold air as exhaust. (Earlier post.) Air turns to liquid when refrigerated to -196 ˚C, and can be conveniently stored in insulated but unpressurized vessels. Exposure to heat (including ambient) causes rapid re-gasification and a 710-fold expansion in volume. This expansion creates pressure, which can be used to drive an engine piston, and also gives off cold, which can be used to provide refrigeration or air conditioning.

Dearman liquid air engine, Gen 2 design. Click to enlarge.

Engines running on liquid air (or liquid nitrogen, which is already widely available) are zero emissions at the point of use, and can be zero carbon depending on the source of electricity used to make it.

The Dearman Engine operates by the vaporization and expansion of such cryogenic fluids. Ambient or low grade waste heat is used as an energy source with the cryogen providing both the working fluid and heat sink. The Dearman Engine process involves the heat being introduced to the cryogenic fluid through direct contact heat exchange with a heat exchange fluid (HEF) inside the engine.

The HEF facilitates extremely rapid rates of heat transfer within the engine. This allows injection of the liquid cryogen directly into the engine cylinder whereupon heat transfer occurs via direct contact mixing with the HEF. The heat transfer on injection generates very rapid pressurization in the engine cylinder.

Direct contact heat transfer continues throughout the expansion stroke giving rise to a more efficient near-isothermal expansion. With the pressurization process taking place in the cylinder, the amount of pumping work required to reach a given peak cylinder pressure is reduced.

After each expansion cycle the heat exchange fluid is recovered from the exhaust and reheated to ambient temperature via a heat exchanger similar to a conventional radiator.

For application in a refrigeration system:

1. Liquid nitrogen (LiN) is stored at ~3 bar in a cryogenic vessel.

2. LiN is pumped to ~40 bar and transferred to a vaporizing heat exchanger where it provides cooling for the chilled compartment. Approximately two thirds of the total cooling supplied comes from this source.

3. The LiN is fed to the Dearman Engine, where after being combined with the HEF support ancillary systems such as feed pumps, an alternator and fans for air circulation as well as drive the compressor of a vapor compression refrigeration cycle that provides additional cooling. The other one-third of the total cooling supplied comes from this source.

4. The HEF is then reclaimed and used to harvest heat from the condenser of the refrigeration cycle, which has the advantage of approximately doubling its efficiency.

5. The HEF is re-used in the engine. The only emission back to the atmosphere is air or nitrogen.

Working with its technology partner Hubbard Products (earlier post), the entire refrigeration system has also been reconfigured, downsizing a number of components and optimizing them to deliver maximum efficiency and higher levels of performance. Dearman’s Generation 2 system will now be tested with support from HORIBA MIRA, before a commercial, on-road, field trial begins with a UK operator in 2016.

This is another important milestone as we develop our revolutionary clean cold and power technology. We have always planned to evolve and continuously improve our core Dearman engine, but to achieve such rapid improvements in weight and efficiency is a significant achievement. We aspire to offer systems which are not only cleaner, but that are also cheaper and perform better than polluting diesel alternatives. We are another step closer to achieving that goal.

—Michael Ayres, Deputy CEO of Dearman

Transport refrigeration systems are used to keep the cargo in refrigerated trucks cool. They are generally diesel-powered and can be up to 29 times more polluting than modern, Euro VI, truck engines.

There are an estimated one million transport refrigeration systems in operation in the EU, with many more around the world. The number is also growing rapidly and so is their environmental impact.

Dearman is developing its zero-emission technology as an alternative, which will help operators to deliver substantial emissions savings, while also reducing their operating costs.

The zero-emission transport refrigeration system is the first of a portfolio of clean cold and power technologies being developed by Dearman, with eventual applications in public transportation and the built environment, as well as logistics.

This latest testing builds on the Innovate UK-funded Cool-E project conducted earlier this year. It benefits from continued support from project partners, including HORIBA MIRA, Air Products, Loughborough University and Hubbard Products Ltd.