Chilling heat pumps

They can be extremely efficient – multiplying the energy needed to run the compressors so that they deliver as much as six times this value in heating or cooling. This ratio, termed the coefficient of performance (cop), depends on how effectively the system is designed and operated.

Heat pumps have traditionally used chlorofluorocarbons (cfcs) or hydrochloro-fluorocarbons (hcfcs) as refrigerants. Now that both are known to deplete ozone and contribute to global warming, the search is on to find alternatives that are more in line with the post-Montreal environmental agenda.

Heat pumps are often used for domestic heating and cooling in Japan, the United States and Switzerland. Utilities companies in many Swiss cantons strongly promote heat pump use by providing grants and cheaper electricity tariffs.

In the UK, the largest heat pump market is commercial buildings, where they are used mainly for cooling. Systems have traditionally used refrigerants listed on the left hand side of table 1.

Alternative refrigerant fluids

Two families of refrigerants currently dominate when heat pump fluids are replaced: hcfcs and hfcs. However, neither family – or even blended variants – is likely to succeed as a long-term replacement for cfcs. Even hfcs, which are immune from the worries of ozone-depletion, have high global warming potentials.

Instead, natural refrigerants are held up as the most likely long-term replacements for more damaging working fluids. Ammonia, hydrocarbons, carbon dioxide, air and water could all theoretically be used in heat pumps for buildings. These are all free of the ozone depletion problems of hcfcs – although they bring with them new technical challenges.

To date in the UK, ammonia and hydrocarbons have been used more often in refrigeration systems than heat pumps. However, in Germany, 6% of existing heat pumps use hydrocarbons, and they represent the largest share of working fluids for new installations.

Air and water have played very modest roles, particularly in residential and commercial buildings. Air cycle units are being used for air conditioning, eg in aircraft and trains, while water is used in open cycle industrial heat pumps. Carbon dioxide is also receiving considerable attention as a possible heat pump working fluid.

Hydrocarbon refrigerants

Hydrocarbons have been used as working fluids for many years in petrochemical plant. They have not been used extensively in buildings – apart from in domestic refrigerators.

As well as having zero ozone depletion potential and a negligible effect on global warming, hydrocarbons also have attractive thermodynamic properties. Heat pumps and air conditioning units using propane have been shown to use 10-20% less energy than equivalent systems charged with R12, R22 or R134a.

However, the environmental and thermodynamic advantages of hydrocarbons are offset by the problem of flammability. This creates dual barriers of regulation for safety and unclear liability in the event of an accident.

Hydrocarbon heat pumps have been used with some success in Sweden and Norway. In one such scheme, a church near Lillehammer, Norway, was heated with a propane-charged ground source heat pump with a duty of 45 kW. Plate heat exchangers were used as both evaporator and condenser, and linked to a semi-hermetic compressor¹.

For safety reasons the heat pump was housed in a separate building, with two rooms separated by a gas-tight wall. Where possible electrical equipment was located in a separate room, so that sparks could not ignite leaked propane vapour. In addition, the room was equipped with ventilation extracts close to the floor and a gas detector alarm.

Because of perceived flammability hazards, hydrocarbons have tended to be targeted at applications with small inventories or charges of working fluid. Domestic heat pumps, or systems using compact plate heat exchangers, are examples. They are sometimes more efficient than conventional refrigerants. One recent study found heat transfer coefficients up to 25% higher in evaporators when using hydrocarbons rather than R22, and pressure drops that were 25-40% lower.

Carbon dioxide (R744)

Before the advent of cfcs, CO₂ was widely used in marine systems, air conditioning and general refrigeration systems. It is now back in the frame as one of the 'natural' working fluids for heat pumps.

CO₂ is non-flammable, odourless, non toxic, (although it could cause suffocation), has a zero ozone depletion potential and a minor impact on global warming. It is cheaper than hfcs – only half the price of R134a – and it can also be obtained as a waste product.

Further, the refrigeration capacity per unit of volume is much higher than other common working fluids, including hfcs. Compressor, valves and piping can therefore all be smaller than they would otherwise be, trimming installation costs.

For much of the heat pump cycle, CO₂ exists as a gas because its critical temperature is 31·18C. For this reason it is sometimes called a 'trans-critical cycle'.

The theoretical coefficient of performance (cop) of CO₂ in a trans-critical cycle is low – typically only half that for R12. Introducing an internal heat exchange bumps the cop up to 75% of an R12 system.

In practice though, actual cops may be much higher than the theoretical values. The low pressure ratio with a high pressure difference leads to high heat transfer. As a result, practical performances are on a par with R134a in some applications.

International research work using CO₂ in heat pumps has achieved cops as high as 3·8 when producing hot water at 658C². This betters the cop of an R22-charged system (3·3).

One disadvantage of CO₂ heat pumps is the high operating pressures needed: from 70 to 90 bar. This pressure is unlikely to create problems in component design, but shell and tube heat exchangers may have to be used to cool the gas instead of standard plate heat exchangers.

Ammonia (R717)

Ammonia has been used as a working fluid in refrigeration plant for almost 100 years – particularly for food refrigeration and cold storage. It has thermodynamic and transport properties that are suitable for heating and cooling cycles, and does not deplete ozone or add to global warming.

However, less than a hundred ammonia heat pumps have actually been installed². Nevertheless, the popularity of ammonia in heat pumps is increasing, despite safety concerns.

In Norway, ammonia is becoming the dominant working fluid for heat pumps in district heating schemes and large commercial buildings.

But such applications still tend to be directed at the medium to large capacity end of the market, where separate plantrooms and/or extensive safety control systems can be justified.

An 8·1 MW unit at the new Oslo airport has a charge of 2500 kg ammonia (0·31 kg/kW installed heating capacity). The plantroom is 1 km from the airport terminal and is gas-tight with a fail-safe emergency ventilation system. Gas detectors and sprinklers have been installed and the drain is automatically closed if an ammonia leak is detected.

One way towards greater acceptance of ammonia-charged systems is to reduce the charge/unit of heating or cooling capacity. Gram Refrigeration in Denmark appears to have succeeded spectacularly in achieving this, with a system charge of only 0·028 kg/kW cooling capacity, around one tenth that of most competing ammonia plant.

Air (R729)

Air is the cheapest and safest refrigerant available. It has been used as a working fluid in refrigeration/heat pump cycles for many years, and, because systems can be made highly compact, the air cycle has become attractive for aircraft air conditioning.

Now it is being considered for buildings, where both heating and cooling can be provided simultaneously³.

Unlike most working fluids, air can be used in open cycles – the air being collected from outside the heat pump and exhausted to the environment after use. However, because air does not become liquid during the heat pump cycle, no latent heat transfer is involved. This means the cycle uses more energy than vapour compression equivalents.

This raises questions about the overall environmental benefit of using air in heat pumps. This is the main reason that air cycle technology is unlikely to break into the mass market.

Water (R718)

Water is the ideal working fluid in many cycles. It is environmentally benign, and does not lead to ozone depletion or global warming. Its refrigeration or heat pump effect is also good, due to its very high latent heat.

However, the principal factor affecting compressor size is the specific volume of the working fluid. Unfortunately, at temperatures appropriate to heat pumps and air conditioning in buildings, water vapour has an extremely high specific volume. This means the compressor has to handle over 200 times the gas volumetric throughput that would be required in systems with conventional refrigerants, like hcfcs or hfcs.

Some industrial heat pumps use steam/water based systems with a mechanical vapour recompression (mvr) semi-open cycle. Such a cycle could also be appropriate for building applications.

MVR technology differs from the conventional closed vapour compression cycle in that the heat source is also the working fluid. The source could, of course, be used to evaporate water in a heat exchanger, bringing the cycle closer to that of the conventional one.

Industrial experience suggests that high cops can be achieved when the working fluid temperature is less than 808C, with a temperature lift of 208C or less. These technical characteristics make water less attractive for applications in buildings.

However, the Lego factory in Denmark has overcome these technical barriers. It uses a semi-open mvr water cycle which is fitted with a deaerator to remove gases that do not condense. Here, a plant duty of 1·9 MW cooling resulted in a cop reported to be greater than 10.

A schematic of the Lego plant is shown below. Process water at 13·58C is fed to the evaporator where it expands to about 11 mbar, this corresponds to a saturation temperature of 88C. Of this water, 1% evaporates and the remaining 99% is cooled.

The evaporated water – now the working fluid – is compressed in a two stage turbo-compressor. The condensed vapour is fed directly into the stream of injected water from the cooling tower, heating it by 4-58C. Energy use is about 50% of an equivalent R22 plant, however, the capital cost involved is significantly higher.

Looking to the future

There are technical hurdles to large-scale uptake of all natural refrigerants. Innovative design and manufacture could help to overcome the dual problems of safety and cost for ammonia and hydrocarbon systems.

CO₂ is a more likely contender than air or water for the mass market for heat pumps. However, even CO₂ requires considerable development work before it qualifies as an acceptable commercial alternative to conventional refrigerants.

Of course, refrigerants themselves are only one part of the refrigeration jigsaw. In the future new technologies may supersede gas compression heat pumps, opening the door to still more environmentally benign alternatives.