Air conditioning energy consumption

[wilsonudevil]

Hi all,

I'm currently working on the development of a new hybrid ventilation system that has the ability to warm and cool incoming supply air.

Now we want to compare the energy use of the system with that of a 'through the wall' packaged air conditioner. We're using CIBSE weather data that gives an hour-by-hour account of a typical year. I've worked out how much energy our system will use, but am struggling to find any data on how much energy an equivalent a/c system would consume.

Any ideas?

Thanks in advance!

[vijay_study]

Hi,

Can details be provided on method by which air is being cooled / warmed.

Regards

[wilsonudevil]

Our system simply circulates hot or cold water through a heat exchanging copper coil. The air passes over the coil and then into the room.

We 'd like to compare to a compressor-based refrigerant system of around 1kW.

[wilsonudevil]

A bit more detail:

Over the period of one year our unit will use 57kWh to maintain a supply air temperature of 20C at 40l/s over the summer (i.e. when ambient temperature is above 20C).

Over the winter the unit will require 780kWh to supply air at 18C, again at 40l/s whenever ambient temperature drops below 18C.

[nike123]

Energy consumption can be compared only by comparing COP for different weather conditions of your system and given "window shaker".

[Brian_UK]

I think you should also be aware that you are trying to compare two completely different systems.

You are designing a ventilation system
but you are doing a cost comparison with an air recirculation system.

[wilsonudevil]

I think you should also be aware that you are trying to compare two completely different systems.

You are designing a ventilation system
but you are doing a cost comparison with an air recirculation system.
Yes I realised this! My problem is that I work under two supervisors- an academic supervisor who told me that you can't compare the two systems, and an industry supervisor who wants me to compare them nevertheless!

Some might say that I'm stuck between a rock and a hard place.

[wilsonudevil]

Any idea how converting a through the wall air conditioner to use fresh air would affect the COP? I think what I'm being asked to do is not possible :(

[nike123]

Any idea how converting a through the wall air conditioner to use fresh air would affect the COP? I think what I'm being asked to do is not possible :(

Fresh air doesn't affect COP of unit but gives additional heat load to the conditioned space!

Many window shakers have flap to introduce 5-10% fresh air of their circulation rate!

[wilsonudevil]

Fresh air doesn't affect COP of unit but gives additional heat load to the conditioned space!

Many window shakers have flap to introduce 5-10% fresh air of their circulation rate!
Ok I understand. So I need to work out the extra heat load generated by 100% fresh air. Which is obviously dependent on outside air temperature.

Any idea where I can find typical COPs for these window shakers? Manufacturer's literature is extremely vague!

[nike123]

Ok I understand. So I need to work out the extra heat load generated by 100% fresh air.

Why 100%?


Any idea where I can find typical COPs for these window shakers? Manufacturer's literature is extremely vague!

It is usually between 2.5 and 3.5!

Maybe Eurovent (http://www.eurovent-certification.com/en/Certified_Products.php?rub=03&srub=&ssrub=&lg=en) site.
I am sure that you could always send e-mail to manufacturer for that data!

[Gary]

A bit more detail:

Over the period of one year our unit will use 57kWh to maintain a supply air temperature of 20C at 40l/s over the summer (i.e. when ambient temperature is above 20C).


Are you saying you want the temperature of the room air to be 20C?... or the temperature of the air leaving the coil to be 20C?

[wilsonudevil]

Are you saying you want the temperature of the room air to be 20C?... or the temperature of the air leaving the coil to be 20C?
Temperature of the air leaving the coil to be 20C...

[wilsonudevil]

Why 100%?



It is usually between 2.5 and 3.5!

Maybe Eurovent site.
I am sure that you could always send e-mail to manufacturer for that data!
I'm looking at a ventilation unit that uses 100% fresh air, and then trying to compare it to a different sort of unit to see if it offers any advantages from an energy point of view.

Basically trying to find a way to justify the development of the new unit. Which is quite hard!

edit: That eurovent site is brilliant- thanks!

[Gary]

Temperature of the air leaving the coil to be 20C...

A 20C coil is going to remove little if any moisture from the fresh air. You are going to have a very humid room.

For example, if you take 30C fresh air at 50%RH and cool it to 20C, its RH will be 90%.

[nike123]

I'm looking at a ventilation unit that uses 100% fresh air, and then trying to compare it to a different sort of unit to see if it offers any advantages from an energy point of view.


Ventilation unit doesn't cools or heat air, and therefore, cannot be compared with air-conditioning unit!
Or you using wrong terms here?

[The MG Pony]

@ Nike He is stuck between a hard spot as usual industry guy wants to compare two non comparable items!

I feel for the poor guy!

[Gary]

Our system simply circulates hot or cold water through a heat exchanging copper coil. The air passes over the coil and then into the room.


How are you heating/cooling the water?

[wilsonudevil]

Ventilation unit doesn't cools or heat air, and therefore, cannot be compared with air-conditioning unit!
Or you using wrong terms here?
This particular ventilation unit has the ability to temper the air. So during the winter it warms the ventilation air and during the summer it cools it.

I agree it can't be compared with a conventional air conditioning unit that's primary function is to cool the ambient room air!

[wilsonudevil]

How are you heating/cooling the water?
The air is being warmed/cooled using a finned tube heat exchanger with warm/cool water circulating inside it.

[Gary]

The air is being warmed/cooled using a finned tube heat exchanger with warm/cool water circulating inside it.

And that water is being heated/cooled by ______?

[wilsonudevil]

And that water is being heated/cooled by ______?
Cooling/heating the water with one of those combined electric water heater/chillers. Basically a recirculating bath with a heating/cooling element.

[wilsonudevil]

A 20C coil is going to remove little if any moisture from the fresh air. You are going to have a very humid room.

For example, if you take 30C fresh air at 50%RH and cool it to 20C, its RH will be 90%.
I've always wondered about that (well, since I got into the whole thermal comfort/indoor air field). When you cool air it becomes more humid. But the actual water content of the air remains the same. So why does it feel 'wetter' since there isn't actually any more water in the air?

[nike123]

http://hyperphysics.phy-astr.gsu.edu/HBASE/Kinetic/relhum.html

[Gary]

Cooling/heating the water with one of those combined electric water heater/chillers. Basically a recirculating bath with a heating/cooling element.

Therein lies your energy usage... and your basis for comparison.

[Gary]

I've always wondered about that (well, since I got into the whole thermal comfort/indoor air field). When you cool air it becomes more humid. But the actual water content of the air remains the same. So why does it feel 'wetter' since there isn't actually any more water in the air?

A major part of your body's cooling mechanism is the evaporation of moisture from your skin.

Less humid air is more absorbent and helps this process. In very dry climates the moisture evaporates so fast you may not even realize you are sweating.

More humid air is less absorbent and hinders this process, making you feel wetter because you are wetter.