when do free cooling modules make economic sense?

[alkirtley]

I'm curious if anyone has put together a payback analysis tool that can be used to determine when adding a free cooling module to a chiller (in my case an air-cooled, modular chiller) makes economic sense. Essentially, I'd like to be able to plug in climate data for various locations along with relevant data on the free cooling module (cost, capacity) and be able to quickly estimate whether the investment would pay off based on typical air temperatures. If anyone has created such an analysis, I would be interested in seeing it.

Thanks,

 

[marcoh]

It called Excel and a couple of hours of your time (or delegate to a keen young graduate engineer)

If you have hourly weather data available it should be relatively simple to calculate free cooling hours, cost if you were to run the compressors etc for every hour of the year (ie 8760 rows in the spreadsheet!). And don't forget the increase fan static and increased fan power!

Just be sceptical with any manufacturers claims. I had a manufacturer show me an incredible payback time on a heat recovery system, turned out he assumed that every hour during summer was at peak design conditions!

 

[gepman]

alkirtley
As marcoh says you can use Excel. The analysis would be very building dependent. Since you have an air cooled instead of water cooled chiller your free cooling will be dependent on the dry-bulb and not the wet-bulb temperature that a water cooled chiller would use. You will need to know the cooling demand of your building hour by hour along with the capacity of your "free cooling" unit. I checked in eQUEST to see if they had this analysis already built in (they have the air system economizer but not a chiller system free cooling) but they don't. You could with more effort model the free cooling system in eQUEST using their powerful "scheduling" feature (you can make schedules for almost anything dependent on almost anything and therefore could schedule the free cooling unit based upon ambient temperature). If you are good with software it would probably take 8 to 16 hours to model your building and system with about 75% of that used to make the free cooling model and schedule. eQUEST is free. I have mentioned eQUEST enough in these forums that you should be able to search for it and find the link to download the software.

 

[Drazen]

without even a simple thermodynamic model, energy analysis makes no sense. just attempting to count free cooling hours is really nothing.

Chapter on energy estimating in ASHRAE Fundamentals can give you an idea of complexity of factors involved, and also links to some simplified commercial software.

gepman gave useful comments, add 20 other important factors to that, the most important being thermal caractheristics of building envelope, load patterns for main power consumers in the system, demanded operating hours etc. etc.

if excell table would suffice, no engineers would be required to make energy analysis! i would lose part of my job, but for the moment i don't feel threatened.

 

[marcoh]

Drazen, Use the modelling software to get an accurate cooling/heating load profile for the building, export it into excel it is possible achieve a lot playing with the numbers, especially if you are trying to model something that your software may not be able to achieve (eg free cooling on an air cooled chiller).

Like anything though: garbage in = garbage out, and you need to know the limitations of your assumptions and calculations.