What is the most efficient central cooling system to install?
Generally, a well-designed central cooling system will provide greater efficiency than individual units, but efficiency and stability of operation of central systems can be compromised when only a small proportion of apartments are using air conditioning. A central system is unlikely to be practical for small developments of less than about 50 apartments.
In general, a central chilled water fan coil unit system using high efficiency water cooled chillers will be the most efficient central cooling system choice. Though common in hotels, it is not as common in residential design, due to the challenge of apportioning the energy cost between apartments. If a source of cogeneration heat is available, a central chilled water fan coil unit system with an absorption chiller would not only be energy efficient but would also have energy costs low enough that apportioning may not be an issue.
Gas-driven options have lower efficiency than their electric alternatives, but may actually score better due to the lower greenhouse potential of gas compared to electricity.
A central system using high efficiency water source packaged unit system has the advantage that the majority of the energy cost can be metered at the apartment. Depending on the climate, such systems may have efficiency better than an equivalent chilled water fan coil system, though high efficiency water source units are not readily available.
Low/medium efficiency systems will not score as well in BASIX as high efficiency options.
Systems using an evaporative process for heat rejection (cooling tower, evaporative fluid cooler or evaporative condenser) generally provide better efficiency than air cooled equipment, though not in all climates.
A low efficiency central water source packaged unit system provides better efficiency than a central fan coil unit system with medium efficiency air-cooled chillers, a central variable refrigerant volume unit system or individual unit system.
The sizing (and capital cost) of a central cooling system is very dependent upon the thermal performance of the building envelope – particularly reduction of solar gain. Smart building design and glazing techniques can cut capital costs of cooling equipment, while large areas of unprotected glass can lead to such high peak solar gains that either the local cooling capacity will be inadequate or capital costs will be high.
When choosing a cooling system, energy efficiency is one of many factors that should be considered. Apart from initial cost, other considerations include maintenance cost, health and safety issues, reliability, maintainability and life expectancy.
As with central hot water systems, effective insulation of chilled water or refrigerant pipes is also critical if high efficiency is to be achieved. The Building Code of Australia (BCA) requires minimum level of insulation for insulating of heating and cooling piping, vessels and tanks.