The process of saving energy in the refrigeration of a walk-in cooler or freezer often involves the pleasant prospect of saving even more energy that one might expect. Any electrical energy consumed inside a refrigerated space by things like room lights, reach-in door lights and evaporator fans is completely converted to heat that must be removed by the refrigeration system. This heat is part of what is known as the cooling load or refrigeration load. If you add to the cooling load, you suffer what could be considered a “compressor penalty”. If you reduce the cooling load through efficiency measures inside the walk-in you are rewarded with a “Compressor Bonus”.
When electricity is consumed by equipment outside the walk-in, it does not contribute to the heat needing to be removed, so no compressor penalty or bonus applies. In the case of electric heaters for the reach-in doors of a walk-in cooler that warm the exterior of the glass and the doorframes to eliminate condensation from them, an estimated 60% of that heat seeps into the walk-in and becomes part of the cooling load. Thus, a reduction in door heater output or runtime will result in only 60% of the compressor bonus that would come from reducing that electrical energy when used entirely inside the walk-in.
A Freeaire® Cooler Controller involves evaporator fan control, which saves energy by switching off the evaporator fans, after a short delay, whenever the compressor is switched off. This has the effect of even slightly enhancing any compressor bonus through a kind of “virtuous circle”, in which reduced compressor system operation leads to less evaporator fan operation, which in turn leads to less compressor system operation, and so on.
How large a Compressor Bonus is depends on the compressor system’s efficiency. One measure of efficiency is the Coefficient of Performance (COP).
Coefficient of Performance = Energy Output/Energy Input.
A COP of 2.5, a typical value for a 40°F walk-in cooler’s refrigeration system, means that 2.5 units of energy (in the form of heat) are removed from a space for every 1 unit of energy (in the form of electrical energy) used to power the system. A walk-in cooler’s COP of 2.5 leads to a Compressor Bonus of 40% because 40% is the reciprocal of 2.5 (40% =1/2.5). Therefore, 10 kWh/day of electricity saved inside a walk-in cooler by, say, upgrading the evaporator fans to ECMs, actually leads to 14 kWh/day of electrical energy savings due to the multiplying effect of the Compressor Bonus.
The lower the COP is, the higher the Compressor Bonus is. Thus, a walk-in freezer with a typical COP value of only 1.67 would benefit from a Compressor Bonus of 60%, because 60% is the reciprocal of 1.67 (60% = 1/1.67). This is why efficiency measures done inside a freezer are even more productive of savings that those done in a walk-in cooler.
The Compressor Bonus is like compounding interest. When the interest earned by a savings account is re-invested in that account and allowed to accumulate, there is more interest on which to earn interest every year. Investing in refrigeration efficiency leads to saving money on electric bills that can be reinvested in your business. Unlike the paltry interest rates that apply to bank savings accounts these days, however, Compressor Bonuses of 40-60% are a sure-fire way to compound your refrigeration energy savings.
 Commercial Refrigeration Loadshape Project, Northeast Energy Efficiency Partnerships, 10/9/2015 http://www.neep.org/sites/default/files/resources/NEEP%20CRL_Report_FINAL_clean.pdf