Clean transport refrigeration and the Cold Economy
If the environmental cost of unbridled expansion of diesel-powered cold chains is becoming clearer, so too are some potential solutions. A range of zero tailpipe emission technologies are now being developed, although some appear more promising than others.
Battery-electric TRUs are already commercially available, as are eutectic plates that store cold in a salt solution (similar in principle to a beer cooler cold pack), both of which are quiet and, with fewer moving parts, lower maintenance. But these suffer a number of drawbacks including cost, weight and recharge times, and support only more limited duty cycles. Electric TRUs powered by hydrogen fuel cells are being piloted in California, but seem likely to face the same challenges as other hydrogen technologies, and the costs are unknown at this stage.
Refrigeration powered by evaporation of cryogenic liquids is also commercially available – around 2,000 units operate in Europe, from manufacturers including Frostcruise (Linde, UK), blueeze (Air Liquide, France), Frappa Trailers (France), and Cryotherm (Germany). Typically such systems evaporate liquid nitrogen (LIN) through a heat exchanger in the goods compartment, which removes the need for a TRU engine and compressor altogether – although power is still required for fans. Thermo King offers a system based on evaporation of CO2, but this contains less cold per litre than LIN and so requires a bigger tank, or limits the kind of duty cycles that can be serviced.
Evaporation systems are quiet and low maintenance, but are not as efficient as they could be for two reasons. First, they run on liquid nitrogen rather than liquid air, which performs in the same way but requires 20% less energy to produce. Second, they exploit only the cooling produced by re-gasification, and not the simultaneous and massive expansion in gas volume, which can be used to drive a piston or turbine, meaning they waste around a third of the available energy.
These insights have led to the development of an innovative 'cold and power' approach, which is more efficient because it produces both cooling and power from the same unit of liquid air or nitrogen. First the cryogen is passed through a heat exchanger to cool the goods compartment, and then it expands through a piston engine to produce electricity to power the fans, or shaft power to drive a secondary conventional refrigeration cycle. This means it will use less fuel to produce the same amount of cooling and so reduce operating costs. The Dearman TRU is currently being tested on a vehicle at MIRA, and will go into commercial field trials in 2015 and larger-scale international trials in 2016. Further research and development is being undertaken with The University of Birmingham within the Centre for Cryogenic Energy Storage.