How is the COP of a heat pump calculated and what affects it?

The efficiency of a heat pump is commonly measured using the COP value (Coefficient of Performance), which indicates how much heat energy the device produces relative to the amount of electrical energy it consumes. In its simplest form, COP is the ratio between the heat output and the electricity input:

COP = heat output (kW) / electricity input (kW)

For example, if a heat pump produces 4 kilowatts of heat while consuming 1 kilowatt of electricity, the COP is 4. This means that for every kilowatt of electricity used, four kilowatts of heat are generated.

COP value and real-world performance

The COP listed in technical data reflects a performance value measured in standardized laboratory conditions. These are usually reported according to the EN 14511 standard, which allows a measurement accuracy tolerance of ±5% for brine-to-water heat pumps. Laboratory tests provide reliable information on device performance, but it is important to note that a single COP value only represents efficiency under specific conditions.

In real-world applications and at system level, the actual performance of a heat pump may differ significantly from optimal values due to various factors. The overall system configuration plays a major role in determining efficiency. The length and dimension of piping in the system affect pressure losses, which in turn influence the electricity consumption of circulation pumps. Additional system components such as valves also contribute to pressure drops.

From an efficiency perspective, it also matters whether the heat pump charges only the storage tanks – keeping network pressure losses low – or whether it circulates fluid through the entire heating system. The building’s total electricity consumption remains the same; the difference lies in whether the pump electricity is recorded under the heat pump’s consumption or as part of the rest of the system’s consumption.

Factors Affecting COP

A heat pump is a key component of the heating system, and the operation of the entire system influences the COP value. Several factors affect it. Here are the most significant ones:

System Design

The design of the heat pump system is critical in determining the COP. A carefully planned system accounts for several elements, such as the properties of the heat source and the heating target, proper system sizing, and compatibility of necessary components. For example, properly dimensioned piping minimizes heat losses, and well-selected control devices ensure energy-efficient operation. The effect of the energy source on efficiency is determined by the temperature of the energy supplied to the evaporator and the electricity consumed by the circulation pump.

Heat Pump Technology and Components

Modern heat pumps that use inverter technology can adjust compressor speed continuously. The COP varies depending on the compressor’s rotation speed, and the best efficiency is typically achieved at mid-frequency levels. The type of refrigerant, the sizing of heat exchangers, and control automation also affect the COP. Long operation cycles support efficiency, as frequent starting and stopping consume more energy and reduce performance.

Temperature of the Heat Source

The warmer the heat source, the higher the heat pump’s COP. For instance, a ground source heat pump draws energy from the ground, which maintains a stable temperature year-round. In contrast, air-to-water heat pumps use outdoor air, which can drop significantly during winter. The colder the air, the more work the compressor must do, leading to a lower COP.

Heating System Temperature

The temperature of the supply water also affects COP. If the system has high-temperature radiators (e.g., in older buildings), the heat pump must generate hotter water, increasing compressor workload and reducing efficiency. Underfloor heating systems typically require lower supply temperatures, which helps maintain a higher COP.

Operation of Circulation Pumps

Circulation pumps are a key factor in the heat pump’s overall efficiency. An energy-efficient pump that adjusts flow according to heating demand can significantly improve system performance. Over-dimensioned or continuously running pumps can waste energy and lower the COP. An optimally functioning circulation pump ensures stable heat distribution and reduces system load, helping to maintain a higher COP over the system’s lifetime.

Installation and Maintenance

Even the most efficient heat pump can perform poorly if installed incorrectly or poorly maintained. Long or poorly insulated piping, dirty filters, or incorrect settings can reduce the COP significantly.

Summary

COP is an important indicator of a heat pump’s efficiency, but it alone does not fully reflect the unit’s overall performance. Proper sizing, professional installation, and selecting a heat pump type suited to the conditions are key factors in achieving high efficiency throughout the system’s lifetime. The expertise of HVAC professionals is invaluable when aiming for optimal energy efficiency and user comfort.