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.
http://www.asymptote.co.uk/cryo
sales@aerotrim.fsnet.co.uk
www.apollo-energy.com
www.valeron.com
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
For more technical
developments see www.eurekamagazine.co.uk