Coefficient of performance and heat pump efficiency
Generally, “efficiency” is a comparison of output to input. A relatively small input that produces a relatively big output is ideal. This comparison of output to input is generally the best way to look at efficiency for heat pumps, too.
The COP is a unitless number because the output and input are both measured in watts. The larger the COP, the more efficient the unit is. If you have a heat pump with a rated COP of 3.5, you will get 3.5x as much heat from the heat pump as the heat equivalent of the watts you put into it. In other words, for every one watt you put in, you’ll get 3.5 watts out.
To show you how important this concept is, let’s compare this to the efficiency of a baseboard heater. A baseboard heater is entirely enclosed in a room. Everything you put into the baseboard heater is coming back out as heat for the room. Because the heat output of the baseboard heater equals the energy you put into the baseboard heater, the baseboard heater has a COP of 1. The baseboard heater is “perfectly” efficient because it returns all of the energy input as heat, but unlike the heat pump, it can never achieve an efficiency greater than 1.
The fact that you can get more out of the heat pump than what you put in is what makes the heat pump so efficient. Today’s highly efficient heat pumps often mean that homeowners can heat their homes with heat pumps for less than what they would spend on natural gas, currently thought of as the most efficient fuel on the market. Even in cold temperatures, the heat pump can still extract heat out of the outside air. The catch is that it has to work harder, reducing its COP in cold temperatures.
In the summer, the heat pump changes its focus. Rather than extracting heat out of the outside air, it extracts heat out of the inside air, creating a negative temperature difference between the ambient air around the building and the building’s interior. When a heat pump is used to cool a space, its energy efficiency is measured by its Energy Efficiency Ratio (EER) or its Seasonal Energy Efficiency Ratio (SEER).
In the next post, we’ll look at EER and SEER more closely. In the meantime, if you would like more information about how you can heat and cool your home or business with high-efficiency heat pumps, please give us a call at New England Ductless at (781) 995-2665 to set up a consultation.
Photo Credit: Petria Follett, via FreeImages.com