We are considering using ice storage in one of the LEED NC project. The cooling peak load is calculated to be around 550 RT, and by looking at the load profile, we will need a ice tank of 3000 RT-hr to satisfy the peak building cooling load. We are thinking of have a ice storage system that provides a half load capacity, 1500 RT, with an ice charging chiller of 220 RT/350 RT (ice charging mode/normal cooling mode), and have an additional 295 RT cooling-only chiller to meet the other half of peak cooling demand.

However, if I created my system exactly as described above (I am using Trace 700, BTW), it will result cooling unmet hours to be way over 2000, which clearly cannot be accepted in 90.1. And if I were to reduce the unmet cooling hours to below 300, I would have to increase my cooling only chiller from 295 RT to 575 RT, which surely is not a rational system design. I am guessing it is because my cooling load is 550 RT, so the program just requires me to have a cooling capacity big enough to match it (I don't know that much about DOE 2.0 source code, so this is purely my assumption).

So if there is a way to get around this problem? Or do I just need to have a chiller that is as big as Trace 700 simulated?

Also, in 90.1 the paragraph that explains how you deal with unmet hours, it states: "Alternatively, unmet hours exceeding these limits may be accepted at the discretion of the rating authority provided that sufficient justification is given indicating that the accuracy of the simulation is not significantly compromised by these unmet hours". So would this be accepted if we explain to the reviewing body that with the ice storage of half load capacity, we only need a cooling-only chiller of 295 RT then?