The U.K. Institution of Mechanical Engineers recently concluded that one of the most promising technologies for improving the cold chain of developing countries was cryogenics using liquid nitrogen or "liquid air." Liquid air is attractive as it offers the dual benefit of being able to provide refrigeration and to provide energy — thereby proving a beneficial technology to those regions where electricity supplies are intermittent or where diesel generation is expensive.

Now, a coalition of researchers and commercial concerns called the Liquid Air Energy Network has published a report called "Liquid Air on the Highway" to coincide with the Institution of Mechanical Engineers' cold chain report.

The report details how the use of liquid air engine technology could transform transport — not just in refrigerated transport applications but across the whole commercial vehicle fleet — by supplementing the traditional combustion engine. The report assesses the business case for the potential of cryogenic engines in the U.K. over the next decade.

It certainly makes some bold claims. The report states that a projected British commercial vehicle fleet running on liquid air that grows to 36,000 vehicles by 2025 could save more than 1 billion liters of diesel and reduce 1.4 million tons of CO2e (from "well-to-wheel"). Development of the technology, it contends, could also create jobs and reduce particulate pollution.

The key advantage of liquid air arises from its ability to both drive a piston engine and to cool, the LAEN contends.

"Air turns to liquid when refrigerated to around minus-194 degrees C at ambient pressure, and can be conveniently stored in insulated but unpressurised vessels," the report states. "Exposure to heat — even at ambient temperatures — causes rapid regasification and a 700-fold expansion in volume, which can be used to drive a turbine or piston engine to do useful work.

"The main potential applications are in electricity storage and transport. And in both, liquid air can provide the additional benefit of waste heat recovery and/or cooling. Liquid air turns an engine's waste heat into extra power and provides efficient zero-emission refrigeration."

While a number of engine concepts are being developed, the report focuses on the two closest to commercial deployment: a zero-emissions "power and cooling" engine for truck and trailer refrigeration, and a diesel-liquid air "heat hybrid" engine for buses, trucks and other commercial vehicles.

Naturally the version that offers most interest to the cooling sector is the power and cooling engine. The Dearman Engine Company, a U.K. firm, is developing both applications, with its refrigeration engine beginning on-vehicle testing this year and commercial production scheduled from 2016. It is working with a refrigeration specialist on the refrigeration unit that will be cooled by the liquid nitrogen.

The Dearman engine is suitable for use as a zero-emission, highly-efficient transport refrigeration unit (TRU) — for anything from vans to shipping containers "because it extracts both shaft power and cold from the same unit of liquid air or nitrogen, delivering immediate savings in fuel costs and emissions. The potential savings will become increasingly significant since the global refrigerated vehicle market is booming."

The Dearman engine could be used in a number of configurations: on its own, as the principal engine of a zero-emissions vehicle, combined with an internal combustion engine to form a "heat hybrid" or as the "power and cooling" refrigeration unit.

The potential comes from the fact that the world of transport refrigeration is currently dominated by diesel — either through a compressor driven by the vehicle's main engine, or a separate TRU. With refrigeration estimated to consume as much as 20 percent of a vehicle's fuel, it is clear why the refrigerated distribution sector and supermarkets are enthusiastic about the potential for liquid nitrogen.

And that's not to mention its contribution to reducing pollutants and noise — which can often stop trucks from being allowed to make urban or night-time deliveries.

The report notes that a range of vehicle manufacturers and industrial gas producers have begun to offer vehicle refrigeration based on the concept of liquid nitrogen evaporation, with Marks & Spencer and Starbucks among those retailers using the technology in refrigerated trailers.

"Such systems are zero-emission at the point of use and quieter, so useful for making deliveries at night," the report states. "Liquid nitrogen is either sprayed directly into the trailer, where it evaporates and displaces warmer air with inert cryogenic gas, or it is passed through a heat exchanger that cools the air in the compartment indirectly."

Crucially, though, neither of these liquid nitrogen concepts extracts any power from the evaporation process. This is where the refrigeration unit currently being developed by Dearman offers an advance on existing technologies, because it uses liquid air or nitrogen to produce both cooling and shaft power.

The supermarket sector is seen as a fertile ground for the new technology, using fleets of the liquid air trucks and trailers to save fuel and cut emissions as well as cutting local air pollution, the report says.

"Some supermarkets have already trialed vehicle refrigeration based on the simple evaporation of liquid nitrogen, but the liquid air 'cooling and power' approach would be more efficient and cost-effective," the report states. "Vehicles carrying a tank of cryogenic fuel in insulated tanks at approximately minus-200 degrees C could then exploit its cooling potential to further raise fuel economy — for example, by improving the efficiency of processes such as internal combustion engine charge cooling, knock-limit improvement and exhaust gas recycling."

The Liquid Air Energy Network has calculated that owners of "cooling and power" refrigerated trailers, could see a payback in as little as three months. The business case claimed for trailers is the most dramatic, but for rigid "cooling and power" trucks, the payback is still a healthy three years or less.

The report also notes that one of its most striking findings is the disproportionate impact of vehicle refrigeration on greenhouse gas emissions and local air pollution: "In particular, trailer refrigeration units, powered by auxiliary diesel engines can emit many times more NOx and PM than the truck’s main drive engine or a diesel car because they are currently unregulated."

The thrust of the report is that to establish the technology is technically feasible — the liquid nitrogen supply network is already well-established in many countries — but that it requires governments to get behind development, either via funding or incentivizing by removal of green levies or carbon taxes.

Could we see liquid air revolutionizing refrigerated transport and the cold chain beyond? Or is it too big a leap to make for a distribution industry wedded to the conventions of diesel? It is something that the refrigeration industry can certainly play a part in.