Seasonal energy efficiency ratio
The efficiency of air conditioners is often rated by the Seasonal Energy Efficiency Ratio (SEER) which is defined by the Air Conditioning, Heating and Refrigeration Institute in its standard ARI 210/240, Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment.[1]
The SEER rating of a unit is the cooling output in Btu (British thermal unit) during a typical cooling-season divided by the total electric energy input in watt-hours during the same period. The higher the unit's SEER rating the more energy efficient it is.
For example, consider a 5,000-British-thermal-unit-per-hour (1,500 W) air-conditioning unit, with a SEER of 10 BTU/W·h, operating for a total of 1000 hours during an annual cooling season (e.g., 8 hours per day for 125 days).
The annual total cooling output would be:
- 5000 BTU/h × 8 h/day × 125 days/year = 5,000,000 BTU/year
With a SEER of 10, the annual electrical energy usage would be about:
- 5,000,000 BTU/year / 10 BTU/W·h = 500,000 W·h/year
The average power usage may also be calculated more simply by:
- Average power = (BTU/h) / (SEER) = 5000 / 10 = 500 W
If your electricity cost is 20¢/kW·h, then your cost per operating hour is:
- 0.5 kW * 20¢/kW·h = 10¢/h
Relationship of SEER to EER and COP
The Energy Efficiency Ratio (EER) of a particular cooling device is the ratio of output cooling (in Btu/hr) to input electrical power (in Watts) at a given operating point (indoor and outdoor temperature and humidity conditions). The Seasonal Energy Efficiency Ratio (SEER) has the same units of Btu/W·hr, but instead of being evaluated at a single operating condition, it represents the expected overall performance for a typical year's weather in a given location. The EER is related to the coefficient of performance (COP) commonly used in thermodynamics, with the primary difference being that the COP of a cooling device is unit-less: the cooling load and the electrical power needed to run the device are both measured using the same units, e.g. watts. Therefore a COP is universal and can be used in any system of units. However, the COP is an instantaneous measure (i.e. a measure of power divided by power), whereas both EER and SEER are averaged over a duration of time (i.e. they are measures of energy divided by energy). The time duration considered is several hours of constant conditions for EER, and a full year of typical meteorological and indoor conditions for SEER.
The SEER is calculated at a part loaded standardized ARI test. (Defined on/off cycle) This more closely represents the performance from equipment cycling, instead of the steady state conditions under which the EER is measured.
Typical EER for residential central cooling units = 0.875 × SEER. SEER is generally a higher value than EER for the same equipment.[1]
A more detailed method for converting SEER to EER uses this formula:
- EER = -0.02 × SEER² + 1.12 × SEER[2]
Note that this method is used for benchmark modeling only and is not appropriate for all climate conditions.[2]
A SEER of 13 is approximately equivalent to a COP of 3.43, which means that 3.43 units of heat energy are removed from indoors per unit of work energy used to run the heat pump.
Theoretical maximum
The SEER and EER of an air conditioner are limited by the laws of thermodynamics. The refrigeration process with the maximum possible efficiency is the Carnot cycle. The COP of an air conditioner using the Carnot cycle is:
where is the indoor temperature and is the outdoor temperature. Both temperatures must be measured using a thermodynamic temperature scale based at absolute zero such as Kelvin or Rankine. The EER is calculated by multiplying the COP by 3.413 which is the conversion factor from BTU/h to Watts:
For an outdoor temperature of 100 °F (311 K) and an indoor temperature of 95 °F (308 K), the above equation gives a COP of 103, or an EER of 350. This is about 10 times as efficient as a typical home air conditioner available today.
The maximum EER decreases as the difference between the inside and outside air temperature increases, and vice versa. In desert climates, where the temperature may be as high as 120 °F (49 °C), the maximum COP drops to 13.5, or an EER of 46 (assuming an outdoor temperature of 120 °F (49 °C) and an indoor temperature of 80 °F).
The maximum SEER can be calculated by averaging the maximum EER over the range of expected temperatures for the season.
US government SEER standards
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 products. As of January 2006, all residential air conditioners sold in the United States must have a SEER of at least 13. ENERGY STAR qualified Central Air Conditioners must have a SEER of at least 14.
Today, it is rare to see systems rated below SEER 9 in the United States because aging, existing units are being replaced with new, higher efficiency units. The United States now requires that residential systems manufactured after 2005 have a minimum SEER rating of 13, although window units are exempt from this law so their SEERs are still around 10.
Substantial energy savings can be obtained from more efficient systems. For example by upgrading from SEER 9 to SEER 13, the power consumption is reduced by 30% (equal to 1 − 9/13). It is claimed that this can result in an energy savings valued at up to US$300 per year depending on the usage rate and the cost of electricity.
With existing units that are still functional and well-maintained, when the time value of money is considered, retaining existing units rather than proactively replacing them may be the most cost effective. However, the efficiency of air conditioners can degrade significantly over time.[3] Therefore, maintenance should be performed regularly to keep their efficiencies as high as possible.
But when either replacing equipment, or specifying new installations, a variety of SEERs are available. For most applications, the minimum or near-minimum SEER units are most cost effective, but the longer the cooling seasons, the higher the electricity costs, and the longer the purchasers will own the systems, incrementally higher SEER units are justified. Residential split-system ACs of SEER 20 or more are now available, but at substantial cost premiums over the standard SEER 13 units.
Calculating the annual cost of power for an air conditioner
Air conditioner sizes are often given as "tons" of cooling where 1 ton of cooling is being equivalent to 12,000 BTU/h (3.5 kW). This is approximately the power required to melt one ton of ice in 24 hours. The annual cost of electric power consumed by a 72,000 BTU/h (21 kW) (6 ton) air conditioning unit operating for 1000 hours per year with a SEER rating of 10 and a power cost of 12¢ per kilowatt-hour (kW·h) may be calculated as follows:
- unit size, BTU/h × hours per year, h × energy cost, $/kW·h ÷ SEER, BTU/W·h ÷ 1000 W/kW
Example:
- (72,000 BTU/h) × (1000 h) × ($0.12/kW·h) ÷ (10 BTU/W·h) ÷ (1000 W/kW) = $864 annual cost
As another example, a theoretical residential structure near Chicago with a calculated cooling load of 4 tons:
- (4 tons) × (12,000 BTU/h/ton) = 48,000-British-thermal-unit-per-hour (14,000 W).
The estimated cost of electrical power for the 4 ton unit with a SEER rating of 10 and an energy cost of 10¢ per kilowatt-hour, using 120 days of 8 hours/day operation, would be:
- (48,000 Btu/h) × (960 h/year) × ($0.10/kW·h) ÷ (10 BTU/W·h) ÷ (1000 W/kW) = $461 annual cost
Maximum SEER Ratings
Today there are residential AC units available with SEER ratings up to 26. [4] There are a variety of technologies that will allow SEER and EER ratings to increase further in the near future.[5] Some of these technologies include rotary compressors, inverters, DC brushless motors, variable-speed drives and integrated systems.[5]
Minimum EER Value
When operated in heating mode, a heat pump is more efficient than operating resistance heaters in most cases. This is because a space heater can convert only the input electrical energy directly to output heat energy with none of the efficiency or conversion advantages of a heat pump. Likewise, when a heat pump operates near its most inefficient outside temperature, typically 0 °F, the heat pump will perform close to the same as a resistance heater.
A resistance heater, converting 100% of its input electricity to output heat would have a 3.41214 EER (COP = 1), due to the fact that it takes about 0.293071 watt hours to equal one BTU.
Lower temperatures may even make a heat pump operate below this threshold, which is why conventional heat pumps include heater coils or auxiliary heating from LP or natural gas to prevent low efficiency operation of the refrigeration cycle. To compensate for this inefficiency, "cold climate" heat pumps are designed to operate efficiently below 0 °F for heating in cold climates.
References
- ^ a b "ANSI/AHRI 210/240-2008: 2008 Standard for Performance Rating of Unitary Air-Conditioning & Air-Source Heat Pump Equipment" (PDF). Air Conditioning, Heating and Refrigeration Institute. 2008.
- ^ a b "U.S. DOE Building America House Simulation Protocols, Revised October 2010" (PDF). 2010.
- ^ US Department of Energy Framework Public Meeting for Residential Central Air Conditioners and Heat Pumps (June 12, 2008) at 35– 36 (transcript) [1].
- ^ "Fujitsu Ductless Mini Splits: Wall Mounted Systems". 2010.
- ^ a b "How High Will SEER Go?". 2006.
See also
- Thermal efficiency
- Coefficient of performance
- Air conditioner
- Air conditioning
- HVAC
- Energystar
- Heat pump
- HSPF
- Annual fuel utilization efficiency (AFUE)