Coefficient of Performance: Heating and Cooling Equipment Efficacy

Air conditioners and space heaters are used to provide comfort when outdoor temperatures are too high or too low, but this comes at an energy cost. The Coefficient of Performance (COP) is a metric used to describe the efficacy of cooling and heating equipment, and it represents a ratio of thermal output to power input. The article discusses what the basics and how it can be usedf for the benefit of your business to find energy efficiency improvement possibilities.

COP Basics

The basic formula to calculate COP is the following1)COP is applied for appliances using electricity. Gas-fired appliances generally use the AFUE (Annual Fuel Utilization Efficiency) metrics.

COP =  Heating or Cooling Output / Power Input

The formula can be adjusted to calculate power consumption, if the unit’s output and COP are known:

Power Input = Heating or Cooling Output / COP

It is important to note that the output and input must use the same measurement units, normally watts or kilowatts. Also, keep in mind that some heat pumps offer reversible operation, capable of providing both air conditioning and space heating, and these have a separate COP for each mode (we will talk about heat pumps in an upcoming article).

The heating or cooling output of equipment is defined by the condition and occupancy of a building. Therefore, reducing the output is a viable energy-saving measure (i.e. in case of excessive use), but only to a certain extent because it also sacrifices comfort. However, upgrading to equipment with a higher COP allows the power input to be reduced while the heating or cooling output stays the same, reducing energy expenses without affecting system performance. Furthermore, an equipment upgrade can be complemented with building insulation and air-tightness upgrades for improved effect, further reducing the heating and cooling load (see below).

The COP is a very useful metric for comparing the efficacy of heating and cooling equipment because the comparison applies even for different system configurations. For example, you can compare two mini-split type air conditioners with each other, but you can also compare a mini-split unit with a window-type air conditioner or with a heat pump in cooling mode.

Energy Savings When Upgrading to a Higher COP

The following example will describe how upgrading to equipment with a higher COP reduces energy expenses. This example is for heating, but the same procedure applies for cooling.

• Assume a room requires , but currently uses an electric resistance heater with a COP of 1. The electric power consumed is 3 kilowatts as well.
• If the resistance heater is upgraded to a heat pump with the same output of 3 kilowatts, but a COP of 3 power input is reduced to only 1 kilowatt.

Electric resistance heaters are generally limited to a COP of 1, while heat pumps normally offer between 2.5 and 3. There are also high-performance models whose COP can exceed 4.

A COP above unity may seem to contradict the law of conservation of energy, but this is not the case: heat pumps can capture outdoor heat with a refrigerant, and then they compress the refrigerant to release the heat inside. In the example above, where 3 kW of heat are provided with 1 kW of electricity, the other 2 kW of heat are absorbed from the outdoor air. The use of a refrigerant makes this possible, even when the outdoor temperature is lower. However, the outdoor heat is not counted when calculating the COP because it is essentially free; only the electricity used to run the compressor has a cost.

If this heater is used for 200 hours each month, total energy consumption would be reduced significantly:

• Consumption before the upgrade = 3 kW x 200 hours = 600 kWh
• Consumption after the upgrade = 1 kW x 200 hours = 200 kWh

Assuming an electricity rate of 0.14 EUR/kWh, this upgrade would yield savings of 56 EUR per month, or 672 EUR per year. Since heat pumps have a service life of around 15 years, the total savings accumulated would exceed 10,000 EUR. This doesn’t come as a surprise though given the radical efficiency improvement.

Improving your Building Envelope for Increased Savings

Before upgrading heating or cooling equipment, it is recommended that you check your property for air leaks or deficient insulation; a basic building energy audit would provide this information. If you fix these issues first, you can reduce the required capacity of the new unit, achieving extra savings because you have a unit that is both smaller and more efficient. A functional building envelope keeps the heat outside during the summer and inside during the winter, providing energy savings all year long.

Lábjegyzetek   [ + ]