Cooling equipment efficiency transforming with metrics

As with many other types of appliances and equipment installed in buildings, the energy efficiency of ACs currently in use and for sale around the world has been rising in recent years because of incremental improvements in airconditioning technology and shifting demand, though enormous variations remain across countries and regions. The average seasonal energy efficiency ratio (SEER) a commonly used measure of the efficiency of cooling equipment that takes into account changes in operating conditions throughout the cooling season of ACs in the residential sector weighted by sales reached 4.2 in 2016, about 50 Percent higher than in 1990. The average SEER of commercial AC sales improved slightly more by 57 Percent since 1990 to the same global average of around 4.2 in 2016. Because most packaged and split ACs around 50 Percent of installed cooling output capacity globally last on average around 10 to 12 years, the average SEER of the stock of ACs in operation has risen at a similar pace but with a slight lag; ACs in use in 2016 averaged a SEER of around 3.9 in the residential sector and slightly lower at 3.7 in the commercial sector.

Measuring the efficiency of ACs
The energy efficiency of ACs can be measured is several different ways, though all involve some comparison of the amount of energy input required to produce a unit of cooling output (or vice versa). ACs move heat rather than convert it from one form to another, so standard measures of thermal efficiency are not appropriate for describing the performance of these devices. Conventions vary by country. Commonly used metrics differ according to units (metric or imperial), the purpose of measurement (efficiency at full load, at the time of peak demand or across a season) and test conditions (notably indoor and outdoor temperatures). They are sometimes adapted to the conditions in a specific country. The most widely used metrics around the world are as follows:

•  Coefficient of performance (CoP): The ratio used for either heating or cooling equipment to describe the amount of useful energy (i.e. heating or cooling output) delivered as a ratio of the energy input (e.g. electricity) to deliver that useful output. The higher the COP, the more efficient the device. For ACs, the CoP usually exceeds 1, as ACs mechanically transfer more energy from a heat source (indoor air) to a heat sink (the exterior) than the amount of energy that is used in mechanical theprocess.
•  Energy efficiency ratio or rating (EER): A specific ratio used for cooling equipment that, similar to a CoP, is the ratio of the output of cooling energy (measured in British thermal units in the United States and kilowatt-hours [kWh] elsewhere) to input energy (in kWh). It can equally be measured in terms of capacity (for example, Watt of output per Watt of input). In the United States, it is generally calculated using an outside temperature of 95°F (35°C), an inside temperature of 80°F (27°C) and relative humidity of50 Percent.
•  Seasonal energy efficiency ratio (SEER). The EER adjusted for the overall performance of the equipment for the weather over a typical cooling season. It is calculated with the same indoor temperature, but over a range of outside temperatures, with a certain specified percentage of time in each of the temperaturecategories.

Because climatic conditions vary considerably across the world, these metrics have often been adapted to the conditions prevailing in specific regions or countries.

Given differences in test conditions, it is generally not possible to convert between any of these cooling equipment efficiency metrics. As a result, one can have an AC that has a higher EER than another one, but a lower SEER. In addition, the metrics used in practice are not always appropriate to operating conditions. Larger temperature reductions also lower efficiency, as the AC’s compressor has to work harder. This makes it difficult to compare ratings across regions with very different climates, even when using SEERs.

The use of generic terminology has given rise to a great deal of confusion and difficulty in comparing efficiency estimates across countries. Efforts in recent years have tried to increase clarity by introducing more specific metrics, such as the cooling seasonal performance factor, which can be used to measure the efficiency of reversible heat pump ACs or other equipment. Like SEERs, this factor is meant to be the cooling equivalent of the heating seasonal performance factor used to measure the heating performance of a heat pump, representing the range of AC or heat pump operating conditions when in cooling mode. The North American Air-Conditioning, Heating and Refrigeration Institute has developed a similar metric, the integrated part load value, specifically for chillers.

Further efforts are needed to improve the measurement of energy efficiency of all types of cooling equipment in order to assess more accurately their performance under real operating conditions and to facilitate comparisons of performance within and across national and regional markets.

Drivers of energy use for cooling
Obviously, the basic driver of cooling demand is climate, i.e. the temperature of the air and the level of humidity. Air conditioning is commonplace today in countries that experience hot weather for at least several weeks or months of the year. In cool-climate countries, mainly in the northern hemisphere, air conditioning is quite simply unnecessary most of time, with electric fans generally sufficing during heatwaves. But an unpleasantly hot climate is not a sufficient condition for everyone to avail of air conditioning: households and the owners of commercial buildings have to be able afford to buy an air conditioning system (AC) in the first place and then to pay for the electricity to run it. It follows that, in hot countries, rising incomes and population are pushing up demand forcooling.

Climate aside, the amount of energy needed to meet demand for space cooling varies mainly according to the type and efficiency of the equipment used, how it is used and how often it is used, as well as the type and thermal efficiency of buildings. Decisions by occupants about the type of AC, which rooms get cooled and when, and temperature settings can have a considerable impact on cooling energy demand. For example, in many Chinese households today, small mini- split ACs are used in several rooms, but the occupants often turn the equipment on only when they are inside the room and feel hot. This “part-time, partspace” type of cooling demand can use as much as five times less energy compared with running ACs in every room all summer.

The energy consumption per unit of cooling output of ACs currently on sale around the world varies massively. Technological advances mean that new ACs already on the market or that will come to market imminently are a lot more efficient and could hold back the overall growth in energy demand for cooling. Prices can also be expected to come down. In general, more efficient equipment does not automatically cost more to buy than less efficient.

But demand for thermal comfort can be met in different ways, some of which involve no energy at all. For example, a well-designed building in a moderately hot climate may need little or no mechanical cooling. In other words, investments in improving the energy performance of residential and commercial buildings may be more economically attractive over the lifetime of those buildings than simply installing and running ACs.

There is also scope for electricity utilities to manage the cost of electricity by proactively changing the pattern of demand for electricity to power ACs and lowering peak electricity through a set of techniques known as demand-side management. Differentiated pricing involving higher prices of electricity during peak periods can also incentivise changes in behaviour and purchases of more efficient equipment.

In the longer term, emerging technologies, such as solar cooling (either thermal or photovoltaic), battery and thermal storage, and integrated solutions, such as district cooling networks, could also have a major impact on the needs for grid-based electricity capacity. Going one step further, the use of the heat ejected from ACs could also contribute to meeting hot water needs, either within a single building or a larger area through district cooling networks.

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