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Re: [EnergyPlus_Support] Equipment Sizing for best savings - food for thought



Jean,
In thinking about this I suppose one would start by autosizeing, then reducing the loads with a parametric study or optimization routine that uses the amount of hours the building has setpoints not met to be as close to 300 without going over as possible. This would be the inverse of the ASHRAE 90.1 recommendation for addressing load ours not met by adding coil capacity.  Now if this inverse procedure to achieve 299.9 hours of setpoints not met produced a chiller capacity that was 60% of the load, than the chiller ran most efficiently at 40% of the load as you say than would it be running at 40% of the first autosized load or of the new 60% maximum setpoints not met load?

Also in my works I have found it somewhat challenging as a modeler to recommend that an engineer stake their credentials on sizing a system that is less than the design cooling load.  Possibly this is because I am the modeler and because I work for a materials manufacture, but still I think that this is a common risk that engineers may not be willing to take. The common concept is to include a safety factor that is above ASHRAE recommendations of 1.15, but for the sake of conversation lets assume the ASHRAE safety factors. For example, an office building with an autosized sensible design cooling load of 44810[W], and after adding our bioPCM product to the externally adjacent drop ceiling, south and west walls had an autosized sensible design cooling load of 28528[W] The 16,283[W] represents a 36.34% reduction of sensible design load.  I then added the ASHRAE 1.15 saftey factor to get a total load of 51532[W], then subtracted the 16,283[W] load reduction from the PCM for a final design recommendation to the project engineer of 35249[W]. This recommendation was to size their system at 78.66% the autosized load / 68.40% of the ASHRAE 1.15 safety factor load.  The engineer did not feel comfortable recommending a design with a load below the either the autozized load or there own hand calcs even though there was PCM to offset the load in a calculated amount.  

The final design was to "right size" the system using the autosized load of 44,810[W] then adding the 16,283[W] benefits of the PCM and saying that the project had a 21.34% safty factor. Considering that the engineer was planing on a 20% safety factor before I came along I thought that this was good and the project was a success. Here is a visualisation I did of the project>> https://docs.google.com/spreadsheet/ccc?key=0Ag7cD5ZV9QwRdFZxS0VmaXlhQ0ZQUzBidDFhdVNkVkE&usp=sharing

Thanks for starting this thread- I wonder how others approach the concept of sizing a system for lowest first costs and optimal operational efficiency and when we all know that the peak loads seldom happen and is a "safety factor" really worth the added first costs and long term inefficiencies that come with the industry standard of the ASHRAE recommended 1.15 safety factor (or above) cooling system sizing. 

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Jeremiah D. Crossett  | Senior Analyst  |  Phase Change Energy Solutions
120 E. Pritchard St.  | Asheboro, NC 27203  | Mobile 503-688-8951
  





On Sat, Apr 13, 2013 at 4:50 AM, Jean marais <jeannieboef@xxxxxxxxx> wrote:
 

Take chillers...they are often most efficient at about 40% of their full load capacity...ironically in real life projects, I have found that during cooling periods, 40% is about the maximum load they need to deliver. They are off or operating at min. PLR most of the time. Seldom does the demand exceed 60%. This counter intuitively makes sense. The equipment is most efficient for when the most "cooling" power is being delivered.

So, e+ sizes equipment to meet the maximum design loads (which are often somewhat more than what would occur in a typical year). This means that autosizing will "over-size" your equipment by default.

Theoretically, one should do parametric runs at different sizing factors to get the "ideal" equipment sizes. And ideal may in fact mean "over sized".




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