# Temperature dependent electricity consumption of HVAC System

I am trying to figure out if there is a decrease in electricity consumption of a cooling unit if the ambient temperature decreases from 35°C to 20°C. More detailed, a company is planning to install an HVAC system in a production hall with huge machines. Currently, each machine is cooled by a cooling unit attached to the machine. The question, will the electricity consumption of the cooling units decrease if the temperature in the production hall decreases?

• The energy use will depend on the ambient temperature and also the CoP of the device used and it's capacity : is it sized correctly or is it too large... The electricity consumption, should, logically decrease, but it depends on other factors as well. – Solar Mike Dec 8 '17 at 14:49
• Your question should not be "will it decrease," but "by how much" . If, for example, there's a 50% operational overhead to keep the machine turned on, then a 50% decrease in the power used to cool things is only a 25% decrease in total power consumption. Further, you haven't stated to what temperature you want to maintain the cooled machines at. What if their optimum point is 25 C? – Carl Witthoft Dec 8 '17 at 16:11

A machine consumes energy, part of which end-up as heat in the machine structure itself and in the room.

A cooling unit dissipate heat from a place and "push" this heat somewhere else. In our case, the cooling unit dissipate from the machine and push this heat to the room. The difference of temperature between this other place (the room) and the machine determinate the efficiency of the cooling system.

Considering the OP question, the cooling unit seem active, meaning it consume energy to actively dissipate heat. For this purpose, the energy consumed by the cooling unit will end-up in the room as well as the dissipated heat of the machine.

Any decent cooling unit has a target temperature, and would adjust energy consumption for cooling just enough to get the target temperature (thermostat). If that is the case, lowering the room temperature will increase efficiency of the cooling unit, which will reduce it consumption accordingly. (Yes to your answer)
If the unit is not able to adjust it own consumption, it will consume the same energy to cool the machine to a lower temperature due to the higher efficiency. (No to your answer)

Finally, another alternative would be to conduct the machine heat or the cooling unit heat to the exterior(*) this would reduce the heat generated inside the room and by this way, reduce the room temperature, lower the cooling unit consumption, etc.. without requiring the HVAC. If the outer temperature is low enough, improving ventilation of the room (with exterior) might already have a huge impact.

(*) This is supposing you have a lower temperature outside.

Assuming that the cooling unit attached to the machine is an air-cooled direct expansion unit, the cooling coefficient of performance of the unit (COP), which dictates how much energy the unit will use, is mainly dependent on the dry bulb temperature of the ambient air surrounding the condensing unit of the cooling system and the actual cooling output the system has to provide. We can assume that the cooling output required by the machine will stay the same (even if it will actually probably decrease due to the fact the the temperature of the hall will be reduced, which will reduce further the energy use of the machine cooling system) so the COP will mostly change because of the reduced ambient temperature. The cooling COP of refrigeration cycle increases with the a decrease in temperature of the air surrounding the condensing unit (here the hall but usually outdoors) so the cooling system of the machine will use less energy.

Now, the cooling energy use of the machine cooling system will be reduced but this will be offset by the energy use of the cooling system installed to condition the hall. By how much? We would need a lot more information.

• I have information of the cooling units of the machines (COP, rated voltage, specific cooling capacity, etc.) as well as by how much the ambient temperature is reducing in the production hall. How exactly could I calculate the reduction of energy use? I assume I need more information from the manufacturer of the cooling unit such as COP dependent on Tout and Tin. – Majos Dec 10 '17 at 9:14
• It is not as trivial as it seems. You'll need more information than that, for example: weather data (although if the cooling load is mainly internal gain driven, it probably won't make much of a difference), opaque surfaces (walls, roofs, etc..) and fenestration thermal performance, installed lighting power and operation schedules, installed equipment power and operation schedules, HVAC type and performance and operation schedules, etc... Then you could perform the calculation using the bin method or using a whole-building energy modeling software. – Jeremy L. Dec 11 '17 at 18:09