The Coefficient of Performance (COP) of a heat pump gives you a measure of the efficiency with which the system will extract kinetic energy from the outside ambient air to a refrigerant and then transfer that energy to the air inside the house. To compress the refrigerant, energy must be supplied to a compressor pump forcing the refrigerant gas to condense.
The efficiency of the heat pump design can be determined as follows:
- COP is the ratio of the heat energy generated to the energy required to compress the refrigerant gas
- Refrigerant gas, when condensed to a liquid, requires work
- The compressed refrigerant liquid absorbs heat from the outside air
When buying a heat pump, it is important to check what the COP rating is for the various appliances. The cost of the power required to run the heat exchanger will be far higher than the cost of the equipment. Let’s look at which factors to consider when looking for a heat pump.
How Is COP Calculated And What Does It Mean?
The Coefficient of Performance is a measure of how much heating power is achieved for the amount of power needed by the compressor unit to generate that heat. A COP = 2 means that 2 kW of heating power was generated while the compressor needed 1 kW of power to condense the refrigerant gas during the cycle.
For a COP of 1 or less, it means that the system is not efficient at all. A COP above 3 is considered very efficient, and a COP of 4.5 is considered the best possible COP achievable in practical applications. It would be best to consider a COP of between 2.5 and 3.5 as very good.
Modern heat pumps use a chemical compound called a refrigerant that functions well as it only requires a small amount of energy to change from a liquid to a gas and back again.
This compound is called Freon, and the most common blend used in heat pumps today is designated as R410A.
Heat Exchange
The refrigerant compound is responsible for moving heat from one set of coils in the closed-loop heat pump. The coil that is absorbing the heat from the outside ambient environment is called the heat exchanger, and the coil on the inside of the house where the heat is given off as the gas compound is compressed back to a liquid is called the condenser.
Air Conditioning
An air conditioner does the reverse in the hot summer months extracting the heat from inside the house and releasing it to the outside ambient air.
The refrigerant is not used up in this process as it is inside a closed-loop system. Unless the gas leaks out through a poor seal or a puncture to the system, it can last forever.
The reality is that due to the constant cooling and heating of the aluminum pipes from which the system is made, leaks develop over time to the seal joints and the metal pipes. These can be repaired, and the refrigerant replenished from time to time. The only part of the system that performs physical work is the compressor motor.
Compressor
The amount of work performed by the compressor to force the warm gas back into a cold pressurized liquid requires that the seals of the pump to the heating coils and the cooling coils be without leaks. Aluminum is used for these pipes as it is a light, strong metal that conducts heat very well.
Aluminum is also a pliable metal that can be bent and shaped to form heating and cooling tubes. The metal retains its integrity and does not crack due to the temperature extremes.
To find the leaks in a system, a special UV-colored dye is added to the refrigerant that shows up the leak’s location under UV light.
What Typically Goes Wrong With Heat Pumps?
Although the basic design of heat pumps is quite simple, it requires a trained technician to service and repair them. The refrigerant gasses are not environmentally friendly and must be removed and replaced by a qualified technician. The system’s electronic controls present a significant risk of electrocution to anyone tampering with it.
When heat pumps are properly installed by a qualified installer and undergo annual maintenance checks, there is not much wrong with them, and you can expect at least ten years of hassle-free service. Several things are noteworthy regarding the operation of heat pumps.
The heat pump may frost up and even develop ice buildup on the external heat exchanger coils and housing. This is due to moisture in the air freezing up when the cold liquid turns into a gas in the external coils. Heat is extracted from the air and transferred to the gaseous refrigerant.
Heat pumps are programmed to run in reverse on a set frequency or when a temperature sensor on the coil initiates the defrost cycle. If the defrost cycle is too short, the ice buildup will worsen and require adjustment.
How Is The Operational Efficiency Of A Heat Pump Determined?
The amount of work required by the electrical compressor pump of the heat pump system to convert the warm gaseous refrigerant back to a cold pressurized liquid before absorbing heat from the air and transferring it to the coils in the house is key to determining the efficiency of the system.
The COP is calculated by dividing the amount of heating generated in kW by the amount of energy in kW, required by the compressor to turn the gas back into a liquid. It is a simple ratio between the power output and the power input of the system.
A Coefficient of Performance above 2.5 means that the amount of heat released into the house is 2.5 kW for every 1 kW of power needed by the compressor to return the gas back to a liquid and repeat the process. The COP of the system is determined by how efficiently the compressor can operate.
The closed-loop system of heating and cooling coils linked to a compressor pump must be charged with the correct amount of refrigerant and free of any leaks. Regular maintenance of heat pumps is required to ensure that the COP remains as high as possible.
References:
