Basics of Air Conditioners
An air conditioner (AC) works similar to a refrigerator: it uses a coolant to transfer heat from one place
(inside home) to another space (outside). An AC is made based on two basic laws of physics:
(a) When the coolant turns from liquid into gas, it absorbs heat (in the same way that your
hands feel “cold” when they are wet): this process happens inside the conditioned space to cool
down the space and is called “vaporization”. To help with vaporization process, a set of coils are
used to maximize surface connection of the coolant with conditioned air. A fan may be used to
blow cooled air into the conditioned space.
(b) When the coolant turns from gas into liquid it gives out heat: this happens outside the
conditioned space and is called “condensation”. Again, a fan may be used to blow hot air to
outside. To keep cooling efficiently, coolant’s pressure needs to be controlled. This is partly
done by a compressor outside the conditioned space that compresses the coolant and increases
its pressure.
What parts consume electricity in an AC?
Figure 1 - Major parts of an AC. Other than parts shown, an AC usually has sensors, timers, and valves (source:
HowStuffWorks).
Two major sources of electricity consumption in an AC are the compressor and the fans that blow cold
air into the conditioned space and hot air into the outside space.
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Air conditioner Types
Normal houses use either a room unit or a central AC system. Room air conditioners (like the window
units) package all parts of the AC system in one unit, which sits on a window or in a closet. As they are
very compact, they are good for conditioning small spaces.
Central air conditioning systems separate the compressor and hot coils (outside) from expansion valve
and cold coils (inside). Some larger units may even put the entire AC system outside the conditioned
space and deliver cool air or water through pipes to conditioned spaces. At each space, fans will blow
cold air to cool down the space.
Central ACs offer more comfort than room units. If multiple spaces need to be conditioned, central
systems can be more efficient. However, central systems cost more than room units, especially if
ductwork for air distribution is not already installed in the building. Central AC systems are designed for
multiple spaces. Thus, if they are used in a building that is only partially occupied (such as a residential
building where at any point in time only one or two rooms are occupied), central AC systems may be less
efficient that room AC systems1.
In large multi-family houses and office buildings, the central AC unit may be located far from
conditioned spaces. As a result, cool air generated in the condenser unit may lose its coldness when it
arrives at the conditioned space. In such situations, water can be used to transfer the coldness created
in condenser unit to conditioned spaces. Two common forms of AC systems using water between
condenser and conditioned units are chilled water systems and cooling tower systems.
Coolants
Coolants are evaluated based on their cooling capacity, potential for depleting ozone layer, and global
warming potential.
Destruction of the ozone layer allows more harmful ultraviolet rays to reach the Earth's surface,
resulting in the increase of skin cancer and other problems. Chlorine and bromine, two chemical
elements that exist in some coolants are known to deplete ozone layer. A metric called ODP (Ozone
Depletion Potential) is used to measure the ozone depletion potential of coolants. The potential for a
single molecule of the refrigerant to destroy the Ozone Layer. All refrigerants use R11 as a datum
reference where R11 has an ODP = 1.0. The less the value of the ODP - the better the refrigerant is for
the ozone layer and the environment.
Global warming refers to a phenomenon in which infrared rays are absorbed by molecules in CO2 and
methane, as well as air conditioner refrigerants like CFCs, HCFCs, and HFCs. This prevents heat from
escaping the Earth's surface. A metric called GWP (Global Warming Potential) is used to measure global
warming potential of coolants. GWP is a measurement (usually measured over a 100-year period) of
how much effect a refrigerant will have on Global Warming in relation to Carbon Dioxide. CO2 has a
GWP = 1. The lower the value of GWP - the better the refrigerant is for the environment.
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http://www.greenbuildingadvisor.com/blogs/dept/musings/window-mounted-air-conditioners-save-energy
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Coolants with Global Warming and Ozone Depletion Potential
CFC
Chemical compounds consist of chlorine, fluorine, and carbon. CFCs were widely used in the past as
refrigerant for air conditioners. They were found to deplete the ozone layer and developed countries
stopped manufacturing them in 1995. CFCs are greenhouse gases with a global warming potential three
to 10 times that of HFCs.
HCFC
Chemical compounds consist of hydrogen, chlorine, fluorine, and carbon. They were used to substitute
for CFCs, and have an ozone depletion potential just one-twentieth that of CFCs. Developed countries
will stop manufacturing HCFCs in 2020. The global warming potential is about the same as HFCs.
Example: R22 (or HCFC22)
R22 is a single hydrochlorofluorocarbon or HCFC compound. Low chlorine content and ozone depletion
potential, ODP = 0.05. Modest global warming potential GWP = 1700. R22 can still be used in small heat
pump systems, but new systems cannot be manufactured for use in the EU after 2003. From 2010 only
recycled or saved stocks of R22 can be used. It will no longer be manufactured. Little odor, colorless as
gas or liquid, non toxic, non irritating, non flammable, non corrosive, stable. It is used in Packaged airconditioning units where size of equipment and economy are important. Common uses: Air
Conditioning, Low and Medium Temperature Refrigeration2.
HFC
Chemical compounds consist of hydrogen, fluorine, and carbon. They ozone depletion potential of zero.
HFCs are currently in the process of being substituted for HCFCs. However, HFCs are designated in the
Kyoto Protocol as a greenhouse gas whose emission must be reduced. As well, like CFCs and HCFCs, the
release of HFCs into the atmosphere is forbidden under the Fluorocarbons Recovery and Destruction
Law in Japan.
Example: R134a
R134a is a single hydrofluorocarbon or HFC compound. No chlorine content, no ozone depletion
potential, ODP = 0. Modest global warming potential GWP = 1300. It is used as a replacement for R12 in
cars and stationary AC, as well as Medium Temp Refrigeration3.
Example: R407C
R407C is a ternary blend of hydrofluorocarbon or HFC compounds. 23% of R32, 25% of R125 and 52% of
R134a. No chlorine content, no ozone depletion potential, ODP = 0. Global warming potential GWP =
16104.
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http://www.engineeringtoolbox.com/refrigerants-properties-d_145.html
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Example: R410A
R410A is a binary blend of hydrofluorocarbon or HFC compounds 50% of R32 and 50% of R125. No
chlorine content, no ozone depletion potential. ODP = 0 Global warming potential GWP = 17255.
Global Warming Potential of Major Coolants
Alternative Coolants
Natural refrigerants are substances that exist in nature that can be used as refrigerants. The main
natural refrigerants are ammonia, hydro carbon (propane), and CO2.
Ammonia
Ammonia does not contribute to global warming, and it is a superb refrigerant. Because it is toxic and
slightly combustible, however, it must be carefully controlled. Since we believe ammonia is suitable for
use in places like factories, where it can be strictly controlled, we use it in large-scale refrigeration
equipment.
Propane
Propane has equivalent performance to R22, and isobutane, a substance similar to propane, is used as a
refrigerant in refrigerators. Propane is highly combustible and thus susceptible to explosion in the event
of leakage into the air. Current technology cannot guarantee the safety of propane air conditioners.
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Example: R290
R290 is pure propane, a hydrocarbon and efficient naturally occurring refrigerant with similar properties
to R22. No ozone depletion potential, ODP = 0 Extremely low global warming potential GWP = 3.
Environmentally safe but highly flammable. Used only after careful consideration to safety6.
CO2
CO2 has a lower toxicity rating and is not flammable. However, because it has a low COP, air
conditioners using it require more electricity.
HFO
HFO-1234yf is the first in a new class of refrigerants acquiring a global warming potential (GWP) rating
335 times less than that of R-134a (but still 4 times higher than the alternative substitute R-744) and an
atmospheric lifetime of about 400 times shorter7.
Daikin’s Solutions
Daikin products currently on the market include an ammonia absorption chiller and chilling unit using
ammonia, and a heat pump water heater using CO28. Daikin is phasing out the HCFC (R-22) currently in
use and replacing it with HFCs, which have an ozone depletion potential of zero. Daikin is looking at
HFC32, a refrigerant that is highly efficient yet has little environmental impact and minimum global
warming impact. HFC32 is slightly flammable9. Daikin is looking at a type of HFC, HFC32 has a global
warming potential approximately one-third of HFC410A and HFC407C, and it is more efficient. It can also
be adapted for use in air conditioners in a relatively short time. However, it is slightly combustible. The
other substitute refrigerants of R22 (i.e., HFC410A and HFC407C) have about the same global warming
impact and energy efficiency as conventional R22 but less ozone layer depletion potential.
Godrej’s Solutions
Godrej & Boyce Mfg. Co. Ltd. started a new production line of split and window-type ACs using
hydrocarbon (propane) refrigerant R290 in India. R290 (propane) technology is a very suitable
alternative for climate-friendly cooling even in high ambient temperatures. For example, in the 1.5 T
split category, which is the most common air conditioner segment in India, the Godrej 5-star R290 AC
will consume at least 23% less energy than the current top-of-line 5-star models across brands10.
Cooling Capacity and Total Cooling Output of ACs
The cooling capacity of ACs is rated in British thermal units per hour (Btu/h), which is a unit of power.
Rule of thumb is that an AC generally needs 20-30 Btu per hour for every square foot it supports.
However, AC size needs to be determined as accurately as possible. If the house uses passive cooling
techniques such as shading, ventilation or vegetation, the Btu estimate can be lowered. Likewise, the
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http://www.engineeringtoolbox.com/refrigerants-properties-d_145.html
http://en.wikipedia.org/wiki/2,3,3,3-Tetrafluoropropene
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http://www.daikin.com/csr/environment/activity/index.html#a02
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http://www.daikin.com/csr/information/lecture/04_2.html
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http://www.hydrocarbons21.com/news/view/3087
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Btu requirements are increased by factors such as the size of the household, frequent use of heatproducing appliances, or summer humidity levels.
An over-sized system will cool the space irregularly, leaving the space partially conditioned. It will need
to be turned on and off frequently to avoid over-cooling the space. This results in increased
consumption, and hurts the ability of the AC to condition the space by removing moisture from air while
operating. An AC system that is too small will also need to be operated constantly, adding to the
electricity consumption and costs.
Total cooling output is measured in Btu, which is a unit of energy.
Measuring Efficiency of AC Systems
Two metrics are used to measure the efficiency of AC systems: Energy Efficiency Ratio (EER) and
Seasonal Energy Efficiency Ratio (SEER). Both metrics have units of Btu/W.h.
Energy Efficiency Ratio (EER) of an AC is the ratio of its cooling capacity (in Btu/h) measured at an
outside temperature of 95’F over its current wattage (energy usage). Once the size (Btu/h) of the AC is
determined, the efficiency of the AC system will determine how much electricity it uses. For example, if
system A has a EER of 6, and system, B has a EER of 8, system B will provide the same amount of cooling
as system A while consuming 25% less energy (8 - 6 =2; and 2/8 = 0.25).
Seasonal Energy Efficiency Rating (SEER) is equal to the total cooling output (in Btu) over the cooling
season divided by the total electrical energy input (in watt-hours) over the cooling season. For typical
residential air conditioners, EER equals about 0.875 SEER; however, this ratio varies somewhat for
different air conditioner models.
Central air conditioners are rated by both EER and SEER, while room air conditioners (window units) are
rated only by EER. The idea behind the SEER rating is to allow consumers to compare the seasonal
efficiency (rather than the peak efficiency) of split-system air conditioners. When shopping for a room
air conditioner, look for an Energy Star unit with an EER of 10.7 or more. If you are in the market for a
new central air conditioner, look for an Energy Star unit with a SEER of at least 14.5 and an EER of at
least 12.
Another metric used to measure the efficiency of ACS, Coefficient of Performance (COP) is the energy
consumption efficiency. COP = cooling (heating) capacity (kW) divided by rated power consumption
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