Environmental concerns about some replacement refrigerants have led to interest in more innovative and potentially less environmentally damaging forms of cooling. The BRE has been investigating and reviewing some of these technologies as part of a DETR-funded research project.
Water vapour-based systems
These systems use water as a refrigerant. As such it is highly effective (better than most cfcs, hcfcs and hfcs), but has the disadvantage of a very low vapour density and evaporating pressure, which make its use in conventional vapour compression systems impracticable.
Recent attempts to overcome these problems have used either ejectors or turbo compressors. Ejectors typically use steam passing through an expansion nozzle to entrain a secondary flow of low pressure water vapour from an evaporator vessel and raise its pressure at the ejector discharge. The driving energy is heat to raise the steam that drives the compression process.
There are few moving parts and the system can be powered by waste heat or even solar energy.
A major disadvantage of these systems is the low efficiency of ejectors which leads to low overall coefficients of performance (cops).
The University of Nottingham has built a prototype system that has been installed in a building in Leicestershire, using solar panels and a back-up gas-fired boiler. Its cop is only around 0·3, but this is insignificant when the system is being powered by solar energy.
More crucial is the high capital cost of the prototype, although the University is investigating how ejectors may be moulded from plastics which should reduce the cost of manufactured systems.
A prototype turbo compressor system has been installed at the LEGO factory in Denmark. This uses a two-stage turbo compressor to compress the very high volume of water vapour needed for its 2 MW cooling capacity.
As a prototype the cost of the system was very high, at around £4650/kW compared to £50-£100/kW for conventional chillers. However, its cop is claimed to be higher than conventional R22 chillers.
It remains to be seen whether the very high capital cost could be reduced by using modern mass production techniques.
Stirling cycle
In principle, the cop of the Stirling cycle should be higher than for vapour compression systems, but there are various technical difficulties that have so far limited its use to small prototype domestic refrigerators.
There is no circulating refrigerant fluid, and the hot and cold heat areas are very small which creates heat exchange difficulties. Heat pipes may be required to transfer heat to and from the system.
Acoustic cooling
Acoustic cooling uses a sound generator inside a closed tube to vibrate a gas and cause alternate compression and expansion, and therefore heating and cooling.
The efficiency of prototypes has not been as good as vapour compression systems and the devices have been physically large for the amount of cooling produced. However, development of acoustic cooling is currently underway in the USA and Japan.
Magnetic cooling
The basis of magnetic cooling is that a metal heats up when it is magnetised and cools when it is demagnetised. Currently, very expensive and rare gaolinium is used for its good magnetocalorific property.
Magnetic cooling could achieve a significantly higher energy efficiency than vapour compression systems. A prototype 500 W system with a superconducting magnet achieved a cop of over five, more than an equivalent vapour compression system.
However, the cost of the prototype was very high and the viability of the technology depends on finding suitable, cheaper materials.
Pulse-tube cooling
Pulse-tube cooling appears to be similar to acoustic cooling, except that a compressor is used instead of a sound generator to induce oscillation and alternate compression and expansion of inert gas.
Thermo-electric cooling
Thermo-electric (Peltier) cooling occurs when a current is passed across the junction of two dissimilar metals – one side of the device becomes hot and the other cold.
Thermo-electric cooling systems of up to about 400 W are available commercially for cooling electronic equipment, at a cost of around £2.50/W.
Although single thermo-electric devices have low cooling capacities they may be connected together electrically to produce more cooling.
As they do not have a circulating fluid, heat transfer is more difficult than for vapour compression systems. For large duties additional heat transfer systems would be required.
Thermionic cooling
Thermionic cooling devices consist of two electrodes separated by a vacuum. Cooling occurs when high energy electrons cross the vacuum from the negative side to the positive side of the device, reducing the temperature of the negative side.
Borealis Technical, one of the companies developing this technology, claims that the devices could be manufactured using semiconductor technology, which could make them very cheap.
Predicted costs are in the region of £0.06 to £0.16/W which is competitive with current systems.
A 100 mm-square device could theoretically provide up to 300 W of cooling with a higher cop than vapour compression systems. Higher cooling duties could be provided by linking several devices together.
Several companies are developing thermionic cooling devices, but because of patent applications little further information has been published.
Air cycle refrigeration
Air cycle refrigeration is a tried and tested technology that has long been the basis of aircraft cabin cooling.
Until now the low energy efficiency and high cost of air cycle systems have prevented its use in buildings. However, recent studies by the BRE and the University of Bristol (Building Services Journal, November 1998) have shown that air cycle systems may well be viable for buildings that require simultaneous heating and cooling.
Although air cycle cooling systems have low cops, they can provide relatively high temperature heat recovery without the efficiency penalty experienced by vapour compression systems.
Mass production techniques could also at least partially overcome the relatively high cost of the systems.
Conclusions
Various alternative cooling technologies that could replace vapour compression systems using conventional refrigerant fluids are being developed or already exist.
Many of these alternatives have fewer or no moving mechanical parts, so in principle ought to be more robust and reliable. However, many currently fail to match vapour compression in both energy efficiency and capital cost, and it remains to be seen whether further development will overcome these drawbacks.
The need to minimise the emission of harmful refrigerant fluids from vapour compression systems will inevitably increase their operating costs, which will also help the economic case of the alternatives.
Source
Building Sustainable Design
