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Efficient air conditioning systems

Cooling is becoming a rapidly emerging demand in developing countries, which signifies that the building design and shell measures need to reduce cooling loads, and the efficiency of air-conditioning will need to be improved. Air conditioning systems are implemented in numerous sectors, namely buildings, industry and transport. They are distinguished in two main categories, room air conditioners and central air conditioners. The efficiency of today’s best available technologies for air conditioners is considerably higher than average installed efficiencies, offering further scope for CO2 emission savings.

See also 'Highly efficient heating, ventilation and air conditioning'.

Introduction top

A main technology applied on a global scale for space cooling is the air conditioning systems. This technology can be applied in numerous sectors, namely buildings, industry and transport. An air conditioner cools the building or transport space with a cold indoor coil called the evaporator. The condenser, a hot outdoor coil, releases the collected heat outside. The evaporator and condenser coils are serpentine tubing surrounded by aluminum fins. This tubing is usually made of copper. A pump, called the compressor, moves a heat transfer fluid (or refrigerant) between the evaporator and the condenser. The pump forces the refrigerant through the circuit of tubing and fins in the coils. The liquid refrigerant evaporates in the indoor evaporator coil, pulling heat out of indoor air and thereby cooling the home. The hot refrigerant gas is pumped outdoors into the condenser where it reverts back to a liquid giving up its heat to the air flowing over the condenser's metal tubing and fins.  Air conditioning systems are distinguished in two main categories, room air conditioners and central air conditioners.

Room air conditioners are the most applied systems worldwide and cool a specific space (room) rather than an entire building or floor. In the case where the required space for cooling is limited, room air conditioners can be less expensive to operate than central units, despite the fact that their overall efficiency is lower than the other two types of air conditioning systems. Small room air conditioners can be plugged into any 15- or 20-amp, 115-volt household circuit that is not shared with any other major appliances and they demand less than 7.5 amps of electricity. Larger room air conditioners require more than 7.5 amps and can function in an own electricity circuit of 115 or more volts.

Central air conditioners can cool a wider surface of a building and function as follows. They circulate cool air through a system of supply and return ducts. Supply ducts and registers (openings in the walls, floors, or ceilings covered by grills) carry cooled air from the air conditioner to the building. This cooled air becomes warmer as it circulates through the home; then it flows back to the central air conditioner through return ducts and registers. A central air conditioner is either a split-system unit or a packaged unit.  In a split-system central air conditioner, an outdoor metal cabinet contains the condenser and compressor, and an indoor cabinet contains the evaporator. In many split-system air conditioners, this indoor cabinet also contains a furnace or the indoor part of a heat pump. The air conditioner's evaporator coil is installed in the cabinet or main supply duct of this furnace or heat pump. If your home already has a furnace but no air conditioner, a split-system is the most economical central air conditioner to install. In a packaged central air conditioner, the evaporator, condenser, and compressor are all located in one cabinet, which usually is placed on a roof or on a concrete slab next to the house's foundation. This type of air conditioner also is used in small commercial buildings. Air supply and return ducts come from indoors through the home's exterior wall or roof to connect with the packaged air conditioner, which is usually located outdoors. Packaged air conditioners often include electric heating coils or a natural gas furnace. This combination of air conditioner and central heater eliminates the need for a separate furnace indoors. 

illustration © climatetechwiki.org

Figure 1: Packaged air conditioner (Source: US DOE 1999)

 

Cooling is becoming a rapidly emerging demand in developing countries, which signifies that the building design and shell measures need to reduce cooling loads, and the efficiency of air-conditioning will need to be improved (IEA 2008). The efficiency of today’s best available technologies for air conditioners is considerably higher than average installed efficiencies, offering further scope for CO2 emission savings (IEA 2010).

Feasibility of technology and operational necessities top

In a thermodynamically closed system, any energy input into the system that is being maintained at a set temperature (which is a standard mode of operation for modern air conditioners) requires that the energy removal rate from the air conditioner increases. This increase has the effect that for each unit of energy input into the system (say to power a light bulb in the closed system) this requires the air conditioner to remove that energy. In order to do that the air conditioner must increase its consumption by the inverse of its efficiency times the input of energy. As an example, presume that inside the closed system a 100 watt light bulb is activated, and the air conditioner has an efficiency of 200%. The air conditioner's energy consumption will increase by 50 W to compensate for this, thus making the 100 W light bulb use a total of 150 W of energy.

Each air conditioner has an energy-efficiency rating that lists how many Btu per hour are removed for each watt of power it draws. For room air conditioners, this efficiency rating is the Energy Efficiency Ratio (EER), while for central air conditioners, it is the Seasonal Energy Efficiency Ratio (SEER). As explained in EnergyStar, the EER is a measure of how efficiently a cooling system will operate when the outdoor temperature is at a specific level (95oF). In normal terms, the higher the EER, the more efficient the system is. In technical terms, EER is the steady-state rate of heat energy removal (i.e. cooling capacity) by the product measured in Btuh divided by the steady-state rate of energy input to the product measured in watts. This ratio is expressed in Btuh/watt. The SEER measures how efficiently a cooling system will operate over an entire season, and the higher the SEER the more efficient the system is. In technical terms, SEER is a measure of equipment the total cooling of a central air conditioner or heat pump (in Btu) during the normal cooling season as compared to the total electric energy input (in watt-hours) consumed during the same period. An important direct measure of efficiency is the coefficient of performance (COP), which demonstrates the cooling power of an air conditioning system divided by fan and compressor power.

Room air conditioners generally range from 5,500 Btu per hour to 14,000 Btu per hour. There are several appliance standards worldwide, which require a minimum EER for room air conditioners (for instance in the USA these must be 8.0 or greater). For milder climates an efficient air conditioning system could have a minimum 9 EER, while for hotter climates this should be over 10. As far as the smaller portable air conditioning units are concerned, the least efficient unit has an EER of less than 1.5. Furthermore, national minimum standards for central air conditioners require a SEER of 9.7 and 10.0, for single-package and split-systems, respectively. Currently, there is a wide selection of units with SEERs reaching nearly 17 sold worldwide. Air conditioning systems for houses and small commercial buildings in general have a COP from 2.2-3.8 in the USA and the EU (IPCC 2007). In Japan, as a producing country, more efficient mini-split systems exist with a higher COP from 4.5 to 6.2 (for a 2.8 kW cooling capacity system). Based on the IEA (2008), there is still significant space for improvement, for instance through using variable-speed drive compressors, improving heat transfer at the heat exchangers, optimising the refrigerant, utilising more efficient compressors, and optimizing controls (IEA 2008).

Both SEER and EER are included in the ENERGY STAR specification because each rating indicates the energy efficiency of the product under different operating modes. SEER rating more accurately reflects overall system efficiency on a seasonal basis and EER reflects the system’s energy efficiency at peak day operations. Both ratings are important when choosing an air conditioning system.

Air conditioning systems are abundant in the market and they are upgrading frequently. Still, users often lack the understanding of the most appropriate technology for their needs. There is a lack of comparative information and consumers need to be aware of the advantages of more energy efficient air conditioning models. In general, new air conditioners with higher EERs or SEERs sport higher price tags, although the higher initial cost of an energy-efficient model can repaid via the energy use reduction several times during its life span. Furthermore, the installation of more advanced systems can carry a higher cost, when for instance improvements in control systems are incorporated in the system (controlling the coolers), which can achieve additional energy savings. The technology development and deployment of air conditioners will depend on whether the increased prices for such techniques are passed on to purchasers of the equipment (IEA 2008).

In countries with their own manufacturers, such as China, financial barriers predominantly inhibit the adoption of more efficient air conditioners. Chinese domestic manufacturers are reluctant to invest in R&D, since R&D resources of medium and small scale domestic manufacturers are low. Making investment in the energy efficiency of appliances is likely to be more demanding for small or medium scale manufacturers than for large scale manufacturers since they face severe sales competition. A similar obstacle was experienced in the DSM project implemented under the GEF in Thailand, where EGAT also tried to develop a labelling scheme for air conditioners. However, in contrast to the small number of fluorescent tube and refrigerator manufacturers, the Thai air conditioner industry was more diverse and fragmented, with more than 55 different manufacturers (Birner and Martinot 2005). The share of efficient air conditioners went from 19 to 38 % in Thailand. In fact, surveys show that a variety of energy-efficient appliances promoted through the Thailand project have sustained markets, although some programs, like the labeling program for air conditioners, appear to have been less effective at achieving sustainable changes. Large changes in consumer awareness and understanding have accompanied these projects (IEA 2007).

Status of the technology and its future market potential top

Energy efficiency of air conditioners in developing countries is currently low compared to that in developed countries. The main use of air conditioning systems in developing countries are mainly in large office buildings, hotels and high-income households. This trend is nevertheless shifting nowadays, as individual apartment and home air conditioning is becoming more common in developing countries and in some cases it can reach higher levels than in the developed ones.

In the near future, energy efficiency of air conditioners is expected to raise, such in the case of China, where the projections estimate an increase of EER to 3.2 from 2.6 in 2005, with an annual improvement of 5.3% (IEA 2007). This increase is similar to the EER improvement of the Japanese air conditioners (3.7% annually), which is due to the Top Runner program. The energy efficiency of these air conditioners went beyond the target spontaneously as more energy efficient products were profitable in Japan in terms of Life Cycle Costs , which has not occurred in the markets of developing countries.

The increase of production of new air conditioners for household use, especially for the room to house sized units, has reached 26% over the last years (IPCC 2007), around 45.4 million units in 2001. In China, the EER of air conditioners is assumed to develop with Chinese domestic technologies and for the low CO2 emission case, EER of air conditioners is assumed to be improved by the adoption of technologies coming from developed countries (IEA 2007). In the Figure below, a comparison of the energy efficiency of the Chinese and Japanese air conditioning systems is presented. 

illustration © climatetechwiki.org

Figure 2: Comparison of EER of Chinese and Japanese air conditioners (Source: IEA 2007)

Several energy labeling programs exist worldwide, which aim at the improvement of energy efficiency of air conditioning systems, such as the Energy Star in the USA, the labeling program of the EU, the Energy Conservation Act in India, the Japanese Top Runner program and others (IEA 2010). In 2005, China launched an energy efficiency labelling program which classifies appliances in 5 grades in order to provide more detailed information to consumers. The grade 1 has an EER1≥3.40 and grade 5 ranges between 2.80 and 2.60 EER (IEA 2007).

illustration © climatetechwiki.org

Figure 3: Distribution of EER in Chinese air conditioning markets (Source: IEA 2007)

New standards in effect in 2006 in the United States call for an improvement of 30% over the previous standard introduced in 1992. Japan’s Top Runner has set far higher performance requirements than those in place in other OECD countries. Most air conditioners are driven by heat pumps. The efficiency of this technology has improved significantly in recent years. For example the COP of heat-pump air conditioners increased from around 4.3 in 1997 to around 6.6 in 2006, while some COPs reach 9.0. Developments are underway to use solar power for cooling purposes. Evaporative coolers also work well in hot, dry climates. These units cool the outdoor air by evaporation and blow it inside the building. Evaporative coolers cost about half as much to install as central air conditioners and use about a quarter as much energy (IEA 2008).

illustration © climatetechwiki.org

Figure 4: Typical evaporative cooler (Source: Alabama Power)

Improvements in air conditioners for cars lack the necessary incentives for further R&D research, as they are not properly captured in fuel economy tests in OECD countries (IEA 2007). Modifications in test procedures, or the introduction of additional test cycles (e.g. with air conditioners and/or lights turned on) could help encourage such improvements

The R&D efforts can increase the technical potential of the air conditions by 1.5 to 1.7 in 2030 and 2050 respectively (IEA 2010). Several innovations are already examined in air conditioning systems, such as for instance with the utilization of minichannels (Kandlikar 2007). Significant refrigerant charge reductions are possible due to the higher surface area-to-volume ratio for the minichannels. Additional advantages include capital cost reductions, reduced environmental impact due to lower refrigerant inventory, and possible improvements in coefficient of performance (COP) of the system. Other innovation techniques for the air conditions in cars involve the utilization of pneumatic variable compressors (Wang et al. 2009). This is especially important for small cars with low displacement engine.

Contribution of the technology to social development top

Efficient air conditioning systems, in conjunction with other technologies as well, can improve local and regional air quality, particularly in large cities, contributing to improved public health (e.g., increased life expectancy, reduced emergency room visits, reduced asthma attacks, fewer lost working days) and avoidance of structural damage to buildings and public works (IPCC 2007). 

Contribution of the technology to economic development (including energy market support) top

Air conditioning systems are substantial energy users and they can increase the running costs of a building by up to 50%. Air conditioning is also the major driver of peak power loads in many OECD countries and the variable demand peaks it creates are very expensive for utilities to serve (IEA 2008).

Currently, energy efficient air conditioners use 30 percent to 50 percent less energy to produce the same amount of cooling as air conditioners made in the mid 1970s. Irrespective of the age of an air conditioning system, an amount of 20-40% of cooling energy costs can be saved by replacing the incumbent with a newer efficient one. If such systems are combined with air-sealing retrofits alone can save an average of 15–20% of annual heating and air conditioning energy use in US houses. Furthermore, if if high-efficiency electric appliances are used, the savings are increased due to reduced electricity demand for air conditioning, especially in commercial buildings (IPCC 2007).

Contribution of the technology to protection of the environment top

Air conditioners need refrigerants to work. Fluorocarbons, widely used as refrigerants, contribute to global warming and conventional fluorocarbons contribute to ozone layer depletion. Although HFCs have been phased in as refrigerants since they do not harm the ozone layer, they are included in the basket of greenhouse gases under the Kyoto Protocol. Between 2008 and 2012, emissions of CO2 and five other greenhouse gases including HFCs must be 5% less than 1990 greenhouse gas emissions. The Kyoto Protocol does not cover CFCs and HCFCs since the manufacture of these was eliminated under the Montreal Protocol

HFCs are currently considered the best refrigerant for air conditioners since they do not harm the ozone layer and offer the same level of performance as conventional refrigerants. These non-CO2 emissions from cooling and refrigeration contribute more than 15% of the 8.6 GtCO2 emissions associated with buildings (IPCC 2007). Furthermore, although the air conditioning systems are designed to reduce their lifecycle emissions, leaks, ranging from 3 to 15%, can occur and additional emissions in the installation, service and disposal of the technology phases can be significant. All these types of emissions depend highly on the practices employed at the installation site. For the time being, the best way to reduce environmental impact is to reduce damage to the ozone layer by switching to HFC while at the same time reducing global warming by recovering refrigerants and making products more energy efficient. Emissions due to these uses are projected to remain about constant until 2015 and decline if effective policies are pursued.

In Europe, a number of countries have existing policies that aim at reducing leakage or discouraging the use of refrigerants containing fluorine. Similarly, in Japan, Australia, Canada and the USA (and other world regions) several regulations for the CFC and HFCFC refrigerants exist. 

Climate top

Lifetime emissions of refrigerants from cooling equipment, expressed as CO2-eq per unit of cooling, have fallen significantly during the past 30 years, but there is still room for improvement. For instance, according to the IEA (2010), in India nowadays the electricity consumption of room air conditioners could be cut by around 10% at a saving of between USD 14/tCO2 and USD 65/ tCO2 saved. But increasing the electricity saving to around 30% would result in costs of between USD 120/tCO2 and USD 170/tCO2 saved. This latter cost range could fall to between USD 50/tCO2 and USD 100/tCO2 saved by 2030, but will still even then be a cost rather than a saving. The electricity generation necessary for air conditioners in China will amount to 674 TWh if the technology will be mainly imported and sold widely, and around 401 TWh if the technology is developed and produced domestically and fewer equipments are sold up to 2010. A feasible CO2 reduction in both these cases can be between 52Mt and 216Mt for each case which would account for 38% and 15% of total potential COemissions in the next 15 years. In developing countries where some minimum performance standards are applied, such CO2 reductions can be significant. As an example, in Ghana, electricity generation by air conditioners would have amounted to 43TWh between 2005 and 2011. The electricity generation could be reduced by 3TWh by proper implementation of minimum energy efficiency standards. Corresponding possible reduction of CO2 emissions would be 3Mt during the same period (IEA 2007). 

Financial requirements and costs top

Well-designed passive solar homes can minimise or eliminate the need for air conditioning. Good “non-passive” building design should, in any case, be able to significantly reduce the need for air conditioning in many climatic conditions. But where air conditioning is deemed necessary, more efficient cooling systems offer the potential for significant energy savings at low cost. More efficient systems, although initially more expensive, can have lower life-cycle costs. However, there is a wide range in terms of costs, from a negative cost of energy saved in the case of replacement systems up to USD 0.03/kWh. Programmable thermostat controls can save energy and money. Shifting to an energy-star rated air conditioning unit can result in negative abatement costs (Seeline Group 2005).

A number of options in the United States for the residential sector exhibit strong negative abatement costs, such as advanced unitary compressors for central airconditioning units at an abatement cost of USD –95/t CO2 saved, and Cromer cycles for humid climates at a negative abatement cost of USD –80/t CO2 saved (Sachs 2004). For the service sector, an advanced roof-top air conditioner unit could save over 4 000 kWh per year at a negative cost of USD –72/t CO2 saved. In the European Union, shifting to least life-cycle costs would reduce the electricity consumption of split air conditioners by 38% at a negative abatement cost of between USD –117 and USD –600/t CO2 saved (Riviere 2008)

In India today, room air conditioner electricity consumption could be cut by around 10% to 11% (for USD –14 to USD –65/t CO2 saved) to around 30% (for USD 120 to USD 170/t CO2 saved). This latter cost range could fall to between USD 50 and USD 100/t CO2 by 2030 (McNeil, et al. 2005 and IEA analysis). Split system heat pump type air conditioning systems could potentially reduce China’s air conditioner electricity consumption by 27% at a cost of USD –20/t CO2 saved (Fridley, et al. 2001 and IEA analysis). In the service sector, higher-efficiency refrigeration units can often achieve significant savings at negative costs (McKinsey, 2007b) (IEA 2008).

A representation of the additional investments in the residential and commercial buildings for cooling according to the IEA scenarios for the period 2005-2020 can be found below. 

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Figure 5: Additional investments for cooling in the residential and commercial buildings in 2005-2020 (Source: IEA 2008)

References top

Birner, S., Martinot, E. 2005. Promoting energy-efficient products: GEF experience and lessons for market transformation in developing countries. Energy Policy 33(14), 1765-1779.

IEA, 2007. Energy Efficiency of air conditioners in developing countries and the role of CDM. International Energy Agency, Paris, France. 

IEA, 2008. Energy Technology Perspectives - Scenarios and Strategies to 2050. International Energy Agency, Paris, France.

IEA, 2010. Energy Technology Perspectives - Scenarios and Strategies to 2050. International Energy Agency, Paris, France.

IPPC, 2001. Best available techniques reference document on the production of Iron and Steel. Integrated Pollution and Prevention Control. 

Kandlikar, Satish G. 2007. A Roadmap for Implementing Minichannels in Refrigeration and Air- Conditioning Systems—Current Status and Future Directions. Heat Transfer Engineering 28 (12), 973-985.

Mingyu Wang, Mark J. Zima, Kadle, S. 2009. Energy-Efficient Air Conditioning Systems Utilizing Pneumatic Variable Compressors. SAE International, Delphi Corporation.

US DOE, 1999. Energy-Efficient Air conditioning. DOE/GO-10099-379 FS 206.

Seeline Group, 2005

McNeil, et al., 2005

Riviere, 2008

(Sachs, 2004)