Refrigerator Heat Pump Efficiencies

There are four different criteria used to describe heat pump efficiency. In all of these criteria, the higher the number the higher the efficiency of the system. Heat pump efficiency is determined by comparing the amount of energy delivered by the heat pump to the amount of energy it consumes. It is important to highlight that efficiency measurements are based on laboratory tests and do not necessarily measure how the heat pump performs in actual use.

Coefficient of Performance (COP)

The COP is the most common measurement used to rate heat pump efficiency. COP is the ratio of the heat pump’s heat output to the electrical energy input, as follows:

COP = Heat Output/Electrical Energy Input

For example, air-source heat pumps generally have COPs of 2 to 4; they deliver 2 to 4 times more energy than they consume. Water and ground source heat pumps normally have COPs of 3 to 5. The COP of air-source heat pumps decreases as the outside temperature drops. Therefore, two COP ratings are usually given for a system: one at 8.3°C (47°F) and the other at -9.4°C (17°F). When comparing COPs, make sure ratings are based on the same outside air temperature. COPs for ground- and water-source heat pumps do not vary as much because ground and water temperatures are more constant than air temperatures.

While comparing COPs is helpful, it does not provide the whole picture. When the outside temperature drops below 4.4°C (40°F), the outdoor coils of a heat pump must be defrosted periodically. It is actually possible for the outdoor coil temperature to be below freezing when a heat pump is in the heating cycle. Under these conditions, any moisture in the air will freeze on the surface of the cold coil. Eventually the frost could build up enough to keep air from passing over the coil and the coil would then lose efficiency. When the coil efficiency is reduced enough to appreciably affect system capacity, the frost must be eliminated. To defrost the coils, the heat pump reverses its cycle and moves heat from the house to the outdoor coil to melt the ice. This reduces the average COP significantly.

In fact, some heat pump units have an energy saving feature that will allow the unit to defrost only when necessary. Others will go into a defrost cycle at set intervals whenever the unit is in the heating mode. Another factor which lowers the overall efficiency of air-to-air heat pumps is their inability to provide enough heat on the coldest days of the winter. This means a back-up heating system is required. This back-up is often electric resistance heat, which has a COP of only one. Whenever the temperature drops into the -3.8°C to -1.1°C range, or whatever its balance point is, and this electric resistance heat kicks in, overall system efficiency drops.

Primary Energy Ratio (PER)

Heat pumps may be activated either electrically or by engines (like internal combustion engines or gas motors). Unless electricity comes from an alternative source (e.g. hydro, wind, solar, etc.), heat pumps also utilize primary energy sources upstream like a thermo-electric plant or on-spot like a natural gas motor. When comparing heat pump systems driven by different energy sources it is more appropriate to use the PER, as defined by Holland et al. (1982), as the ratio of useful heat delivered to primary energy input. So this can be related to the COP by the following equation:

PER = n • COP

where n is the efficiency with which the primary energy input is converted into work up to the shaft of the compressor.

However, due to high COP, the PER, as given below, becomes high relative to conventional fossil fuel fired systems.

In the case of an electrically driven compressor where the electricity is generated from a coal burning power plant, the efficiency r may be as low as 0.25 or 25%. The above equation indicates that gas engine driven heat pumps are very attractive from a primary energy ratio point of view since values for r of 0.75 or better can be obtained. However, heat recovery systems tend to be judged on their potential money savings rather than their potential energy savings.

(EER)potential energy savings.

The EER is used for evaluating a heat pump’s efficiency in the cooling cycle. The same rating system is used for air conditioners, making it easy to compare different units. In practice, EER ratings higher than 10 are the most desirable. EER is the ratio of cooling capacity provided to electricity consumed as follows:

EER = Cooling Capacity/Electrical Energy Input

Heating Season Performance Factor (HSPF)

A heat pump’s performance varies depending on the weather and how much supplementary heat is required. Therefore, a more realistic measurement, especially for air-to-air heat pumps, is calculated on a seasonal basis. These measurements are referred to as the Heating Season Performance Factor (HSPF) for the heating cycle. The industry  standard test for overall heating efficiency provides a rating known as HSPF. Such laboratory test attempts to take into account the reductions in efficiency caused by defrosting, temperature fluctuations, supplemental heat, fans and on/off cycling. HSPF is the estimated seasonal heating output divided by the seasonal power consumption, as follows:

HSPF = Total Seasonal Heating Output/Total Electrical Energy Input

It can be thought of as the ‘average COP’ for the entire heating system. An HSPF of 6.8 corresponds roughly with an average COP of 2. HSPFs of 5-7 are considered good. The higher the HSPF. the more efficient the heat pump. To estimate the average COP, divide the HSPF by 3.4.

Most utility-sponsored heat pump programs require that heat pumps have an HSPF of at least 6.8. Many heat pumps meet this requirement. Some heat pumps have HSPF ratings above 9. In general, more efficient heat pumps are more expensive. Compare the energy savings to the added cost.

Seasonal Energy Efficiency Ratio (SEER)

As explained above, a heat pump’s performance varies depending on the weather and the amount of supplementary heat required. Thus, a more realistic measurement, particularly for air-to-air heat pumps, is calculated on a seasonal basis. These measurements are referred to as the Seasonal Energy Efficiency Ratio (SEER) for the cooling cycle. Therefore SEER is rating the seasonal cooling performance of the heat pump. The SEER is the ratio of the total cooling of the heat pump to the total electrical energy input during the same period.

SEER = Total Seasonal Cooling Output/Total Electrical Energy Input

Naturally, the SEER for a unit will vary depending on where in the country it is located. SEERs of 8-10 are considered good. The higher the SEER the more efficiently the heat pump cools. The SEER is the ratio of heat energy removed from the house compared to the energy used to operate the heat pump, including fans. The SEER is usually noticeably higher than the HSPF since defrosting is not needed and there is no need for expensive supplemental heat during air conditioning weather.

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