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I have a system that is using 2.02kWh per day, and I'm trying to figure out its heat output, cooling needs, and cooling cost. I've worked it all out to the point of determining the amount of cooling needed (hopefully that's right), but I don't know how to convert that back to energy needed for the cooling unit, so I was hoping someone could give me some guidance.

$0.08 per kWh

2.02kWh per day * 365 = 737.3kWh per year

737.3kWh * 3,412.14 BTU/kWh = 2,515,772.024 BTU per year

2,515,772.024 / 12,000 BTU per ton = 209.65 tons of cooling per year (seems really high but no matter how I go about calculating it that's the number I get, so I just accepted it for the time being...any help here would be appreciated as well)

This is where I get really stuck. How do you convert tons to kWh for the cooling unit? I feel like it should be a very simple process of reversing these same steps, but I can't grasp it. Then one equation I found from Emerson Network Power to determine cost looked like this, though it appears to ignore actual usage...:

.747 kW per ton * 8760 hours per year * $0.08 per kWh / 0.9 cooling efficiency = $603.50 per ton per year

Does that makes sense? Is that just the cost for one ton of cooling, because at 209 tons of cooling that seems a little ridiculous ($140,000 to cool one system...I don't think so).

If someone could just help me put the pieces together to determine kWh from the cooling that would be great. If i can at least get that then determining cost is easy (multiply yearly kWh by $0.08...seems simpler than the Emerson equation, anyway).

Thanks.

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1 Answer

It sounds like you are sizing your cooling needs incorrectly -- Check out this link, and review your server's specifications for their min/max/average heat dissipation (often provided to you in BTU/Hours) and figure in your duty cycle (how heavily loaded the box will be, which influences how close to maximum thermal dissipation you'll get).

Armed with that information you can then size cooling capacity accordingly -- Remember to account for other sources of heat, such as:

  • UPS systems and PDUs
  • Switches, Routers Fiber Termination equipment, etc.
  • Lighting (even fluorescent lighting puts off heat)
  • People in the room
  • Air flow when doors are opened
  • Seasonal Variation (outside heat getting past insulation)
  • Margin/Overhead (in case your system has a bad day, the heat load goes up, you lose an air conditioner, etc.)

With respect to cost, once you know how many tons of cooling you will need you can determine how much power that will require (based on the specs of the various aircon units you will look at, the expected duty cycle, and local utility costs).
I can't guess at what the figure will be, but I can say that cooling is usually a substantial expense for any large server room.

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Also - you want to make sure that you have more cooling then you think you need. You'll always be adding servers, or having cooling units preforming at less then spec. I've never been bitten by specing 150% cooling. ;-) –  tsykoduk Apr 7 '11 at 6:39
    
@tsykoduk absolutely -- It's better to have more cooling than you need (150-200%) than less: You can always shut down an air conditioner or run at a 30-40% duty cycle instead of 50-60%, but you can't make a 10-ton unit cool a 15-ton heat load. –  voretaq7 Apr 7 '11 at 13:46
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