Last edited 11 Nov 2016

Coefficient of Performance CoP

The Coefficient of Performance (CoP) is a ratio that describes the efficiency of a system. It is based on the relationship between the power (kW) input to a system compared to the amount of power that is output.

CoP = power output / power input.

The higher the number, the more efficient the system.

In the construction industry, the term CoP is often used in relation to equipment such as heat pumps. Heat pumps transfer heat from a lower temperature source to one of a higher temperature and can be used to provide hot water, space heating, or for other applications such as heating swimming pools.

In the case of heat pumps, the CoP is the relationship between the power that is drawn from the heat pump as cooling or heat, and the power that is consumed by it.

The equation is expressed as follows:

CoP = Q/W

Where:

  • Q is the heat supplied to or removed from the reservoir.
  • W is the work consumed by the heat pump.

Typically a simple electric heater with no moving parts may convert 1 kW of electricity into 1 kW of heat, giving it a COP of 1. Whereas, a heat pump may convert 1 kW of electricity in 3 or 4 kW of heat, giving it a CoP of 3 or 4.

Rather than being generated by the pump, the heat is being moved from outside the structure to inside. The warmer the external heat source, the higher the CoP, and the less electricity required to convert it to heat output. This has advantages for the user over traditional heating systems as it significantly reduces carbon emissions and results in lower operating costs.

The CoP of a ground source heat pump is usually high because of the 'free' heat energy available from the ground through a series of buried pipes. Air source heat pumps draw in air from outside the building, and so the CoP can vary seasonally. In winter, the outside air is much colder so more electricity is required to raise the heat as required. The CoP will usually be relatively low, at around 2.5. In summer, the external air temperature is much warmer so less electricity is required, often resulting in a CoP of 4 or more.

The Seasonal Energy Efficiency Ratio (SEER) value in cooling, and the Seasonal Coefficient of Performance (SCOP) value in heating give an indication of anticipated real-life performance that takes into consideration these changes in energy efficiency over the course of a year.

SCOP is measured in accordance with the procedures in BS EN 14825:2013. The National Calculation Methodology for calculating carbon dioxide emission rates from buildings uses SCOP.

Refrigerators work by extracting heat rather than exhausting it. In this case, the equation is expressed as:

CoP = Qc/W

Where:

  • Qc is the heat extracted.
  • W is the work consumed by the refrigerator.

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