Energy savings in air conditioning

[COMAPRO]

See If you can help me out answering following questions I would be very grateful to you : 1. I read somewhere , I do no recall where , that for every degree higher, one sets the temperature of the room the air conditioner has to cool , the electrical energy savings would be 3% , is it true ? and where in the internet can I read about it ? 2. when one designs the size of a coolong air conditioner for a specific space , how much of the time should the compressor as normal be running if the thermostat is set to 78 F ?Remember that if the compressor runs , lets say 50% of the time , one can save a lot of electricity because 80% of the total energy consumption of a cooling air conditioner is done by the compressor . 3. If in the example above one decides to lower the thermostat to 70 F , percentage wise how much longer would run the compressor if the other conditions are exactly alike like people , number of times door opens etc ? 3. Is it true or not that instead of leaving the evaporator fan in the auto position , it should be left in the on position , because that way the compressor would run less time and not like it is recommended to leave in the auto position to save energy ? ( compressor draws 8 - 9 times more power than evaporator fan ) thank you for your promtly reply
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[ChasBean1]

1. 1°F changes between an ~55°F airstream and a 72°F room change heating energy around 5.5 to 7%. Not everything is found on the internet.

2. Under design conditions (max OA enthalpy), I would like to see my compressor run 100% of the time.

3. With the system maintaining constant discharge temperature, you'd raise load 5-12 percent, depending on what the room temperature is. Lowering from 72° to 71° would be about 6%, 69 down to 68° would be about 7%.

Changing design parameters without knowing what you're doing: priceless. You could wreck the building. Just be careful. Remember with the fan, E_in can never be less than E_out. You might find that you run the fan 8-9 times longer than the compressor. There's no magic, so let the system run in auto.

 

[COMAPRO]

Chasbean1, I do not agree with your statement "Under design conditions (max OA enthalpy), I would like to see my compressor run 100% of the time." Compressor running time is controled by the thermostat and if the compressor runs all the time . it means it is not reaching the temperature set by the thermostat, which means either the temperature has been set too low or the design was not right. I think that a good air conditioning design has to take into consideration electrical power consumtion in terms of energy savings, reaching of course the goal to create confort for the people that are in the space that has to be cooled.

 

[mdonner]

Running the compressor 100% of the time at worst case design conditions, maintaining design room temperature, is the best situation. The heat load has to be removed, so a too small compressor will be a poor design, and a too big compressor (large margins) may save you in some cases, but will draw the same amount of energy (or higher, if efficiency drops)and will be more expensive to buy, install and service.

 

[COMAPRO]

hi mdonner , your opinion brings back my second question how long should the compressor be running as optimal time in terms of electrical energy savings , 60 % or what ? I am involved in electrical energy savings in general and I would like to tell my customers the best recommendation for cooling in order to save energy and therefore money , like placing the thermostat in the right place and setting a right temperature etc

 

[mdonner]

Thermostat placement depends on many parameters, and some applications will use more then one thermostat. Usually it will be located near return air grill.
For residential AC, controll will usually be ON/OFF. As I mentioned in my first reply, 100% compressor running time under worst conditions (hot & humid day, with max occupancy) indicates good design. Running 60% (for example) of the time at worst conditions means too wide control band and too big system compared to needs.
Comercial and industrial comfort AC compressor will run constantly with capacity control, usually with PI(D)controller reading chiller water temperature. Room/space thermostat will control the zone valve (on/off or 3 way) rather then compressor.
Since you did not specify the type and size of system, I can not be more specific.

 

[COMAPRO]

Placing the thermostat near the return air grill is ok as long as it is in the space that has to be cooled but I have seen so many times the thermostat near the return grill but in places that do not need to be cooled and space that it is not occupied by a human being. Now back to my question compressor runing time. If the compressor is control only by a thermostat , should not you agree that is nonsense running all time , what is the thermostat then for ?

 

[ChasBean1]

COMAPRO, mdonner's last post was right on. Multistaging of compressors ensures constant cooling without excess on/off cycles.

But let's assume you're dealing with a one-stage setup. I'm afraid that your philosophy is that the less the compressor comes on, the better. If this were true you should install a very large AC unit so the temperature drops as quickly as possible so the compressor spends more time off. This philosophy is poor for two reasons:

1. Thermal comfort. Your occupants are temporarily frozen and the rest of the time it is too muggy.
2. Humidity. During the compressor off cycle, the fan pumps in unconditioned, humid, summer air. Humidity control is awful in the summer during compressor off cycles.

Slightly undersizing a dumb (e.g. one stage) DX cooling system is my preference because the compressor runs more often during humid conditions. Remember also that smaller systems draw less electricity.

In the end, a large system on the roof can maintain an average space temperature by cycling excessively. A smaller rooftop system can often maintain the same average temperature with a much better temperature and humidity profile. A slightly higher space temperature than the thermostat setting can be fine during summer design conditions, as long as dehumidification continues.

 

[COMAPRO]

Hi, Chasbean , let me put it in other words. Lets say that you walk into an office that is cooled with a standard central unit ( one sigle stage compressor ) controled by a regular thermostat that is set to 65 F and lets assume that because of such a low temperature the compressor is running all day long. wouldnt you agree that if I set the temperature to 75 f the compressor would cycle in and off and the kwhr would be less in this case allowing the customer to save electrical energy ? Why is it that you can read that for every degree you set the temperature higher one save 3% energy ?

 

[ChasBean1]

Yes. For a GIVEN AC unit, raising the thermostat setting will raise room temperature and save money. Please read again my 5/19 post. I calc'd that it saves more money than what you posted. Note that to understand my answers, you might have to re-read your original post - my numbered answers correlate with your numbered questions.

 

[mdonner]

Deer COMAPRO
I see now that I probably misunderstood the purpose and meaning of your questions. I give up.