Seasonal energy efficiency ratio
From Wikipedia, the free encyclopedia
The efficiency of air conditioners are often, but not always, rated by the Seasonal Energy Efficiency Ratio (SEER). The higher the SEER rating, the more energy efficient is the air conditioner. The SEER rating is the Btu of cooling output during a simulated, typical cooling-season divided by the total electric energy input in watt-hours (W·h) during the same period. <ref>Definition of SEER (scroll down to "Seasonal energy efficiency ratio")</ref>
- SEER = BTU ÷ W·h
For example, a 5000 Btu/h air-conditioning unit, with a SEER of 10, operating for a total of 1000 hours during an annual cooling season (e.g., 8 hours per day for 125 days) would provide an annual total cooling output of:
- 5000 Btu/h × 1000 h = 5,000,000 Btu
With a SEER of 10, the an annual electrical energy usage would be about:
- 5,000,000 Btu ÷ 10 = 500,000 W·h
This is equivalent to an average power usage during the cooling season of:
- 500,000 W·h ÷ 1000 h = 500 W
The average power usage may also be calculated more simply by:
- Average power = (Btu/h) ÷ (SEER, Btu/W·h) = 5000 ÷ 10 = 500 W
Contents |
[edit] Relationship of SEER to EER and COP
SEER is related to the Energy Efficiency Ratio (EER) and also to the coefficient of performance (COP) commonly used in thermodynamics. The EER is the efficiency rating for the equipment at a particular pair of external and internal temperatures, while SEER is calculated over a range of expected external temperatures (i.e., the temperature distribution for the geographical location of the SEER test). Formulas for the approximate conversion between SEER and EER or COP in California are: <ref>SEER conversion formulas from Pacific Gas and Electric</ref>
- (1) SEER = EER ÷ 0.9
- (2) SEER = COP x 3.792
- (3) EER = COP x 3.413
From equation (2) above, a SEER of 13 is approximately equivalent to a COP of 3.43, which means that 3.43 units of heat energy are moved indoors to out per unit of work energy.
The relationship between SEER and EER is relative depending on where you live because equipment performance is dependent of air temperatures, humidities, and pressures. The relationship stated above is typical if you live in the lower-elevation portions of California; however, if you live in Georgia it is better approximated by
SEER = EER ÷ 0.80
due to the much higher humidities. A similar relationship exists in relating SEER and COP, also depending on where you live.
[edit] US Government SEER Standards
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.<ref>Minimum SEER ratings required in the US</ref> 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 when the time value of money is considered, most often retaining existing units rather than proactively replacing them is the most cost effective. 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 18 or more are now available, but at substantial cost premiums over the standard SEER 13 units.
[edit] 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 defined as being equivalent to 12,000 BTU/h. The annual cost of electric power consumed by a 72,000 BTU/h (6 ton) air conditioning unit operating for 1000 hours per year with a SEER rating of 10 and a power cost of $0.12 per kilowatt-hour (kW·h) may be calculated as follows:
- unit size, BTU/h × hours per year, h × power cost, $/kW·h ÷ SEER, BTU/W·h ÷ 1000 W/kW
- (72,000 BTU/h) × (1000 h) × ($0.12/kW·h) ÷ (10 BTU/W·h) ÷ (1000 W/kW) = $864 annual cost
As another example, a 2000 ft2 residential unit near Chicago would require a 4 ton air conditioner based on a location-specific rule-of-thumb that 1 ton is required for each 500 ft2 for a typical house: <ref>How Contractors Really Size Air Conditioning Systems</ref>
- (2000 ft2) ÷ (500 ft2/ton) = 4 tons.
- (4 tons) × (12,000 BTU/h/ton) = 48,000 BTU/h.
The estimated cost of electrical power for the 4 ton unit with a SEER rating of 10 and a power cost of $0.10 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
[edit] Heat pumps
Air conditioners, for cooling, and heat pumps, when in heating mode, both work similarly in that heat is typically transferred from a cooler "heat-source" to a warmer "heat-sink". The smaller the temperature increase between the heat source and sink, the greater the capacity and efficiency of the air conditioner or heat pump. When a reversible heat pump is in its heating mode, heat is absorbed from the cooler outdoors and moved to the warmer indoors of a building.
In heating mode when the heat source (outdoor) temperature falls below about 40 degrees Fahrenheit, a heat pump begins to reach a point called the "balance point" where the system's ability to gather heat is decreased to the point where the machine's effectiveness falls to near COP=1, or EER=3.412; electric resistance heating has a COP of 1.0. Similarly, when the heat-sink (indoor) temperature rises to about 120 degrees Fahrenheit, the system will operate much less effectively. Air-source heat pumps, that gather heat from outdoor air, therefore work very well in mild climates and with normal indoor comfort conditions. However, in very cold climates, heat pump systems often revert to electric resistance heating mode due to the loss of effectiveness during cold periods. But in many climates, and with increasing competing fuel costs, electric heat pumps are cost-effective.
Ground-coupled (geothermal) water-source heat pumps rarely have this problem of reaching a "balance point" because they use the ground as a heat source/heat sink. The ground's high thermal mass typically provides a moderated heat source/sink temperature, as compared to the air above it. In very large ground-coupled systems, however, is it possible to affect the ground temperature somewhat due to unbalanced heat extraction and addition over the years. Overall, ground-source system tend to have better thermal performance than air-source, but at significantly higher initial system cost. An economic comparison of all options is appropriate when making HVAC system selection decisions.
[edit] References
<references/>
[edit] See also
- Thermal efficiency
- Air conditioning
- Energy conservation
- HVAC
- Star rating
- Annual fuel utilization efficiency (AFUE)
