Split air-conditioning systems move towards greater efficiency

Most people must be noticing that R410A split-system air conditioning is more efficient than previous systems. But the change is not just due to the refrigerant.The latest air-conditioning equipment in the split-systems sector of the market is demonstrating a surge in energy efficiency that sees the cooling achieved per kWh of electricity used approximately doubling. There are two principal factors, says Donald Daw, product and marketing manger with with the air-conditioning division of Mitsubishi Electric in the UK. The first is the use of refrigerant R410A to replace R407C, which has itself replaced R22 in the European market — although not in many other world markets. Technology changes While a small, but useful, increase in efficiency is achieved simply by changing to R410A it is the technology changes that have accompanied the transition to R410A that are much more significant. The change of refrigerant alone improves efficiency by only 5%. What is happening in reality is that full-load efficiency has risen by 30 to 50% in just one generation of split systems. Several technological developments combine to achieve that efficiency boost. Among them are compressor and motor developments. There are also improvements in heat exchangers and an increase in the sensible heat ratio to realise a more energy-efficient fall in dry-bulb temperature. Donald Daw summarises: ‘These developments are about reducing losses, especially in compressors and motors, which consume the most energy in a system. No cost penalty At the the smaller end of the market up to 3 hp, rotary compressors with DC motors are becoming the norm. Motors have become lighter and have lower losses New materials are 10 times more magnetic than iron. What is more, this new generation of compressors imposes no cost penalty. For larger units, from 4 to 6 hp, frame compliant scroll compressors are becoming more widely used, although they add about 15% to the price.
In the new generation of split-system air conditioners,inverter drives make a greater contribution to improving COP than other developments because of the benefits at part-load conditions.
However, these technological developments are not the principal reason for the surge in energy efficiency. It is the use of inverter drives that is making possible such advanced in energy efficiency. While the use of inverter drives to make possible the variable-speed operation of compressor motors offers no benefits at full load, there are substantial gains at part load — which can represent 95% of the operating time of split air-conditioning systems. Indeed, Donald Daw reasons, ‘You could argue that specifiers should sacrifice full-load efficiency for better part-load efficiency.’ Matching Inverter technology offers other benefits. Among them are the better matching of cooling capacity to demand and reduced starting currents — up to 10 times less than fixed-speed compressors. Also, because cooling capacity is better matched to the requirement, compressors start and stop less often — prolonging their life. A more comfortable environment is also achieved. Because temperature variations are less, greater comfort is achieved. Variable-speed systems are forgiving of uncertainties in heating/cooling calculations and can also be run at greater than nominal speed to provide more capacity in extreme conditions. The low starting current of variable-speed air conditioners (which is lower than a 3-phase fixed-speed unit) enables 14 kW units to be run from a single-phase power supply. Since many high-street shops have only a single-phase supply and 3-phase is very expensive to provide, invertor drives open up new market opportunities. Even though a fixed-speed system is more efficient than variable speed at full load, the load profile in the UK is such that the benefits of variable-speed operation can be expected to achieve 45% lower energy consumption over a year. Donald Daw suggests that invertor drives add 15% to equipment costs, but such is the reduction in energy costs that payback can be as little as eight months — or up to two years for the lightest applications. ‘Can any sane person justify fixed-speed equipment?’ he asks. Donald Daw continues, ‘There is no longer any logical reason for choosing fixed-speed equipment instead of invertor driven. In Japan, it is no longer possible to buy fixed-speed equipment, and it can only be a couple of years before the same applies to the UK.’
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