Most larger air-conditioning systems for domestic purposes are reversible in operation, thus providing heating and cooling. Most are electrically powered, and it is to a large extent the drive to reduce CO2 emission that favours systems powered by natural gas.

Natural gas offers an environmentally friendly solution for air-conditioning, often at a lower cost than electrical energy. Phil Kreveld explains.

High-efficiency engines powered by natural gas are now viable replacements for electric power in heating, ventilation and air-conditioning.

There are hundreds of gas-powered systems operating in Australia, mainly in large settings such as commercial buildings, medical centres, supermarkets and educational facilities.

Climate conditioning requires ‘high grade’ (low entropy) energy such as electricity and fuels (coal, oil, etc) to move heat from buildings to a higher temperature outside or to extract heat from a colder environment to heat interiors.

Manufacturing economies dictate that such plant is of high capacity, tens of kilowatts and beyond, and therefore mainly suitable for large buildings. Yanmar systems from Japan cover the range to 90kW.

There is a growing market in the United States for natural gas plant, which is favoured largely due to the cost of peak-demand electricity. In Japan, natural gas climate control technology is well established.

Without a doubt, cooled electrically powered compressors are the most efficient application for a refrigeration or heating cycle – but not necessarily the most cost-effective.

In summer the demand for electricity is high, causing the spot price to shoot up. In Australia, peak demand prices vary widely, but they can seriously affect business operating costs.

A New South Wales supermarket’s power bill doubled because of maximum demand monthly ratchets. Hybrid systems are now in use that employ gas-powered compression whenever electricity prices rise above a predetermined level.

In future there may also be a move to solar-powered climate control. However, with current technology,  such systems are commercially viable only on a whole-of-life basis, as capital costs are far higher than for conventional plant.

Solar energy plants rely on vapour absorption cycles and are also in use for systems driven by waste heat.

Absorption chilling is based on a thermo-chemical process between a refrigerant and an absorber –  commonly ammonia in water as refrigerant and lithium bromide as absorber.

Heat drives the refrigerant out of the absorber and into the evaporator. A pump is necessary to move the fluids, but this uses much less electricity than a compressor.

The overall efficiency of local electricity generation plant, for example ‘tri-state’ generators using waste heat to drive ammonia-water air-conditioning, is thus significantly improved.

The salient advantage for gas-driven systems is a reduction in greenhouse emissions, which needs to be seen in light of several factors.

The basic thermal efficiency of a cooling cycle is unaffected, as this is dictated by the ratio of outside and inside absolute temperatures (see inset). Inverter-driven electric compressors are highly efficient, particularly at full load.

Gas engines are also capable of control, although limited by speed-torque characteristics.

On the other hand, the generation of electricity by thermal power stations is an inefficient process, at best about 30%. CO2 emission is about one tonne per MW/h for coal-fired plant and could be halved by using gas-driven turbines. An even greater reduction would come from avoiding electricity consumption altogether, apart from the power to drive fans.

Maximum efficiency for modern natural gas engines can be as high as 40%. Even on the basis of identical thermal efficiency for gas turbine plant, natural gas systems can be ahead of the game if they achieve less CO2 per kWh of shaft energy.

In the case of a reversible system, the coefficient of performance is enhanced not only by the heat energy available through compression of the refrigerant but also because waste heat in the engine coolant can be used.

When working as a heat pump in ambient temperatures of 5C or less, icing can occur on the external condenser coil (now in effect an evaporator). This is a problem with electrically powered equipment requiring de-icing cycles.

For brief periods, the heat pump reverts to cooling mode, transferring heat from the building’s interior to the external condenser, or resistance heating is used for de-icing the condenser. For gas engine plant, waste heat is available for de-icing.

Typical piston-driven engines compare unfavourably with electric motors. About one-third of the fuel energy disappears out of the exhaust, and a similar portion into the engine coolant, leaving the rest for mechanical energy including oil pumps, alternators, thrashing sump oil, etc.

However, these figures do not reflect the sophistication that is being applied to natural gas engines. These include electronic ignition, coil-on-plug technology and improved cylinder head design.

There are improved engine cycles (eg: the Atkinson cycle as in hybrid cars), and better synthetic oils allowing thousands of hours of operation between oil and filter changes.

The use of natural gas in contrast to propane reduces NOx and CO2 emissions markedly.

Reciprocating compressors are usually chosen for smaller systems (below 100 tonnes cooling capacity).

Multiple reciprocating machines can be installed for higher building loads. Further advantages include simple controls and the ability to manage the speed using belt drives.

Screw-type compressors are used for large-capacity loads. Engines for small-capacity loads are often derived from automotive technology, and diesel engines are used for loads above 500 tonnes.

The noise from gas engines is much greater than from electric drives, and well above the accepted ‘outside environment’ disturbing level of 55bBA, thus necessitating attenuating enclosures.

The Australian gas engine market is confined to smaller units, which can be multiplied in a rooftop installation, for example. This makes a lot of sense, as it is usually possible to divide a building into logical control zones and assign particular compressor units, thereby allowing individual control strategies.

There are other advantages in gas. It can be employed if a central chiller plant’s electrical power demand is greater than the street reticulation’s capacity. Also, gas may be ideal for an older building whose wiring and switchboards are under-rated for the climate control task.

Australia’s greenhouse gas reduction target is likely to open up the market for natural gas plant. Current overall energy savings are substantial, although gas prices vary widely. Based on compressor energy demand (the largest part of climate control demand), energy cost savings of 30-50% can be made.