How to use COP
How Coefficient of Performance Can Help You
By Eric A. Woodroof, Ph.D., CEM, CRM, PCF
Buildings Magazine August 2016
Understanding Coefficient of Performance (COP) can help you receive utility rebates and also help you attain the special tax credits (only for 2015-2016) of $1.8 per square foot, which were extended for tax years 2015-2016 (you only have 4 months left). In this article, I will explain COP, Energy Efficiency Ratio (EER) as well as SEER and HSPF. All of these are “performance metrics” of HVAC equipment, similar to how “miles per gallon” is an indicator for car performance under different conditions (highway, city, etc.). We will primarily focus on the “heat pump” portion of the HVAC equipment, and a simple example is a window air conditioner, which is moving heat from the inside to the outside of a room. In the Winter season, the same heat pump could be rotated 180 degrees to move heat from the outside to the inside.
The Definition of COP that I think is easiest to comprehend is:
COP = (Energy moved) / (Energy input)
A heat pump can have a COP greater than 1, because it can move more energy than it takes to operate. Don’t be confused… this is not “breaking a law of physics” because we are not “transforming” energy (like burning methane to liberate heat and CO2)… we are only “moving” energy, similar to how a bicycle can move you farther than walking for the same input energy.
Example- Let's say you have a room that has three incandescent lights in it. To keep the room at constant temperature, you must move the heat from these inefficient lights out of the room. If your heat pump has a COP of 3, then you can move 3 units of heat for every one unit of input energy. If the amount of waste heat you want to move were 3 kWh, then you would only spend 1 kWh on fuel. This sounds pretty impressive and is why heat pumps are used for many air conditioning processes, including heating a building (except when outside temperatures are too low that there isn’t enough heat entrained within the outside air).
Many of my students struggle with energy savings calculations involving COP. Specifically- when calculating energy savings from installing energy-efficient lighting, (or insulation, window films, etc.) in a room. These types of upgrades reduce the amount of heat you have to move to the outside.
Using the previous example, if the 3 incandescent lights are replaced with 3 LEDs, less waste heat will need to be moved out of the room. For simplicity’s sake, let’s say that post-retrofit, the heat pump doesn’t have to move 3 units of heat, but that reduction in energy (on the down-stream side) only saves one unit of fuel (because the Einput= Emoved/COP).
Thus, in most cases where you are saving energy “downstream”, you should divide by the COP to determine the fuel savings (kWh or MMBTU).
COP is elegant, and unit-less. However, in the United States, we like to express “energy moved” in units that are BTUs, “Tons” and/or “Ton-hours” of cooling (1 “ton” is a “rate” of energy flow and equals 12,000 BTUs per hour). This is different than the units we like to use for electric energy input (kWh). Because the units are different (BTUs divided by kWh)- the Energy Efficiency Ratio (EER) was invented and it is the common “performance metric” in the US. Because we are just converting kWhs into BTUs, EER has a linear relationship with COP:
EER = (COP) X (3.412)
So if you are ever given COP, you can always find EER (and vis-versa). In addition, you can also use the equations below to find other performance metrics that may be useful to you:
COP = (Tonscooling) / (kWinput) = (EER)/(3.412)
Note that these performance metrics are dependent on a few variables such as outside and indoor temperatures (it is harder to reject heat to 100oF), which is why “ground source” heat pumps are a great solution. In the Summer, it is easier to reject heat to 60oF ground temperature vs 90oF outside air temperature. During Winter, it is also easier to extract heat from the 60oF ground vs outdoor temperatures.
Because EER and COP are based on “instantaneous” measurements, the industry developed a “Seasonal EER” (SEER), which is an “average” performance metric for a heat pump over a whole summer season (a range of temperatures). Heating System Performance Factor (HSPF) is a similar metric determined by measuring heat transfer rates during the heating season’s temperatures.
All of these performance metrics are useful to know when retrofitting HVAC systems. If you can install a system with a high COP, you are going to save energy (usually more than enough to justify the extra expense). Many utilities will also give rebates if you install systems that have a high EER/SEER/COP, because these energy-efficient models help the utility when it is most constrained by demand. In my area, I can get a $25 per ton rebate if I install a system with a EER greater than 12.5. So, my building has about 20 tons of cooling equipment, which means I can spend an extra $500 to buy the better systems, but I will also save energy (and $) worth far more than that over the equipment’s life.
This brings me to my final point… 2016 is the year to upgrade your HVAC and lighting equipment, because – if you get the retrofit done before December 31, you can get a $1.8 tax deduction (per square foot). This extra benefit can help a 50% Return on Investment become a 100% ROI! Even as a landlord, I have found it economically advantageous to replace systems because I will get such a tax break (and my tenants get the savings… which they appreciate). If you want more information about this special tax benefit, read the Jan Edition of Buildings Magazine, or watch this 3.5 min video. If you miss out and congress doesn’t renew the $1.8/square foot deduction… you lose that money forever.
Fall Season (with reasonable outside temperatures) is a good time to upgrade your HVAC… when you do- make sure you ask about COP/SEER and get the maximum rebates with the lowest life cycle cost.