Stirling effort keeps its cool while on the move

Tom Shelley reports on new technologies from aerospace and defence for cooling, deep freezing and keeping things cold or warm during transport At one end of the scale, space and night vision system developed Stirling engine coolers are able to keep human embryos deep frozen without risk of contamination or other possibly horrific consequences for future human beings. At the other end, materials technologies partly derived from NASA programmes are providing the means to make very light weight carrying containers for air cargo and human transport of pharmaceuticals, vaccines and food. While the initial impetus was aerospace and military requirements, beneficiaries of the technologies range from soldiers in the field right through to poor people needing medication in Third World countries, tourists who want a cold drink having hiked to the top of some mountain and sheds full of chickens. The Stirling engine coolers are being used in new embryo freezers developed by small Cambridge company, Asymptote. Department of Health guidelines require that all cryopreservation equipment should be capable of being sterilised and should be so processed. In addition, in order that the embryos should survive undamaged, it is essential that the cooling process follow an exact pre-determined profile. Asymptote has therefore adopted the Thales Stirling Cryocooler, a device more commonly found in missiles, infra red detector cooling systems in satellites and imaging cameras in battle tanks. It consists of two sections, a cold finger and a compressor, with two pistons that are reciprocated towards and away from each other by electromagnet coils. The cold finger has an expansion space inside its tip, below which is a displacer-regenerator on a spring. Initially, the pistons are pushed towards each other, compressing the gas. In this stage, the compression is nearly isothermal, heat output being dissipated via heat sinks around the compressor and the base of the cold finger. In the next stage, the pistons remain stationary, but the displacer moves downwards as gas above it compresses, and gas flows through the regenerator into the space inside the end of the cold finger, giving up heat as it passes through. In the third phase, which like the first, is nearly isothermal, the pistons are driven outwards and the gas expands, drawing in heat from around the cold finger. In the fourth phase, the pistons again remain stationary but the displacer moves upwards under spring pressure because of the lower gas pressure in the expansion space. The gas takes up stored heat from the regenerator and re-enters the compression space at ambient temperature. The Thales coolers typically cost a few thousand pounds each but will run for five years without maintenance. Others commercially available are less expensive but insufficiently reliable for this application. Other refrigeration methods, such as Peltier modules and compressors, lack the cooling ability or are too large and cumbersome to be suitable. Liquid nitrogen cooling is cheap, but relies on deliveries being made and is less than totally convenient, being associated with the risk of cold burns and asphyxiation. Used directly on its own, liquid nitrogen cooling is less than precise. Asymptote's new unit, designated EF500, uses a feedback system to control the cooling process to an accuracy of {{plusminus}} 0.2 deg C. The sample plate design if based on a mathematical model of heat flow through the plate. This ensures that variation in the cooling profile between samples is minimised. While the EF500 unit depends on military and aerospace technology for its cooling, aerospace is the original source of materials for cold (and heat) retaining enclosures developed by Paul Harrison and his company Aerotrim in Warminster. The company's key patented technology is called 'Polarthem', which comprises an inner reflective plastic reinforced foil layer, a filler layer of hollow fibres and a durable outer layer. The result is a very good thermal insulator that is at the same time, flexible, durable and very light weight. Because it is flexible, it can be made into a rucksack for the personal transport of drugs, blood, or ice cream, or into a knock down container for air cargo transport. The military are using the containers for fold down freezers, refrigerators, and for keeping computers cool within EMC shielded enclosures in hot climates. Astra Zeneca pharmaceuticals are transported world wide in air cargo shipping containers that weigh only 20kg. National Blood has undertaken tests that show that the company's bags will keep blood above 18 deg C for more than 9 hours at 2 to 6 deg C ambient. The Ministry of Defence has undertaken tests that show that frozen food at - 22 deg C gains less than 4 deg C over 6 hours in 30 deg C ambient when stored under the company's pallet covers. Tests conducted by Cambridge Refrigeration Technology have show than thermal conductance is 1.19 W/m2.K. Aerotrim makes no pretence to being the only company to make and sell advanced aerospace derived materials, especially polymer reinforced reflecting foils. To mention just one, we encountered a company called Apollo Energy Research at this year's Royal Show. Apollo Thermo-Foil ES looks like aluminium foil, and like it has a thermal emissivity of less than 0.05 and an reflectance of more than 95%, but is laminated with 'Valeron', more information at www.valeron.com and www.apollo-energy.com, in order to prevent the aluminium and anti-tarnish layers from tearing. Valeron is made in Belgium, and is a polyethylene that is stretched and extruded, and three layers of it are cross laminated, so there is stretched polymer in all directions. This is prevents the aluminium element from tearing. The idea comes from NASA, but Apollo managing director Colin Hawkes told Eureka that his material is much tougher. At the Royal Show, he was selling it to line the insides of roofs of warehouses, packing sheds and poultry units to reduce the heat stress of users, both human and animal. It is water resistant, tarnish resistant and can be cleaned with high pressure water sprays. By reducing heat radiated from roofs heated by sunlight, it can be used to save a pile of money otherwise required for air conditioning. Stirling engine cooling may also be appropriate for other refrigeration tasks than cryopreservation. It completely does away with the issue of effects of refrigerants on the environment since it uses none. Asymptote Cryopreservation Aerotrim Apollo Energy Research Valeron Strength Films Eureka says: It is always worth taking a look at all the technologies available to solve a problem rather than just doing things the way people have done them before. Nowhere is this more true than in the case of cryogenics, freezing and refrigeration where there is a host of technologies to choose from Pointers * Using a Stirling engine offers a more convenient way of achieving very low temperatures than just pouring in liquid nitrogen, assuming that deliveries are always on hand. It also does away with the need for refrigerants. * It is possible to make thermal insulation light weight, flexible and durable all at the same time in order to improve portability of things that either have to be kept cold or hot * Polymer reinforced aluminium foil, developed from technologies originated for the NASA space program is a great way of almost eliminating heat transfer by radiation