Note: FredRosse got the jump on some of this before I was able to post it. I agree with most of his comments. However, there are other forms of ice storage than the ice-slurry scenario he describes. Regardless, the increased temperature differential with ice gives it a technical advantage in heat transfer effectiveness. That comes with significant operational disadvantages, though - so chilled water thermal storage is just as competitive, long term.
johnhjo said:
Well, the idea is to learn how efficient chiller is with ice thermal storage. I would like to know (if possible) formula from strach. I hear a rumor saying ice thermal storage is efficient in concept but not in reality even ice is made during off peak hour.
John,
If that was actually your intent, you asked the wrong question. Efficiencies have only an ancillary contribution to thermal storage. What you are really doing is
time-shifting peak demand - the same as time-shifting a television program with a VCR. Thermal storage allows one to "record" ton-hours of cooling. This is much more than an equivalent substitute of the chillers used to "charge" the storage, though. The tank responds dynamically to load peaks, and may operate as if it were
many more chillers than the numbers used to charge it.
As referenced in my "trick" question post above, one ton of ice possesses 24 hrs x 12,000 Btu/hr heat capacity. That capacity exists - regardless of the time used to melt it. That time is entirely dependent on a variable load. Moreover, the price in efficiency was already paid in freezing it.
Whatever heat transfer efficiency penalty was in effect when freezing the ice, becomes a heat transfer benefit in melting it.
In thermal storage systems, efficiency improvements are ancillary effects that occur by optimizing the run load of the chillers, and the condenser water temperature leverage. The run loads can be optimized to the most efficient loading of the chillers, since the load is entirely dictated by
you in "charging" the storage.
Condenser water system advantages present themselves by more "leverage" with greater temperature gradients - e.g., cooling at night with reduced cooling tower temperatures. Chillers must be carefully selected to take advantage of this quality however - or you may find yourself adding heat to the condenser side of the process just to maintain head pressures. High-pressure chillers, or chillers with variable-speed drives, are more tolerant to lower head pressures, and take advantage of lower condenser water temperatures.
Those two factors have the largest effect on efficiencies with thermal storage. However, those efficiencies are still a net
penalty compared to direct cooling.
The decision about ice versus water (as in a
chilled water storage tank) is mostly an operational one. The ice process will be marginally more efficient in the heat transfer modes due to a higher temperature gradient. (As stated earlier, the efficiency of melting and re-freezing is contained within the process, though.) However, ice production itself contains many physical obstacles that penalize the process. On the other hand, water storage is a volume issue, and the temperature gradient must be consistent through the whole system - including the coils in the using devices. So, it has disadvantages, too.
If you have the space, though, I would vote for chilled water storage in almost every case - not ice.
