TSU-M, Ice Chiller® Application Guide

LOAD PROFILE:
The load profile is the hour-by-hour representation of the cooling load over a certain time period, usually one day. In some systems the load profile may be for a longer period, such as a week. The load profile is used to determine the amount of thermal storage required. Whereas in a conventional system the chiller is selected for the maximum or instantaneous peak cooling load at any given time, in a thermal storage system the chiller is selected based on the total ton-hours required for the whole time period. Accurate determination of the systemās load profile is essential when designing an ice thermal storage system.

Load profiles can take many different shapes depending on the application. Figure 1 illustrates a typical HVAC load profile for an office building with a 500-ton peak cooling load and a 12-hour cooling requirement. The shape of this curve is typical of most HVAC applications. For the purpose of preliminary budgeting, BAC can generate a similar load profile based on the ownerās data on peak load and load duration, and select the ice storage equipment accordingly.

The Air-Conditioning and Refrigeration Institute (ARI) has published Guideline T, "Specifying the Thermal Performance of Cool Storage Equipment", to establish the minimum user-specified data and supplier-specified equipment performance data. BACās computer selection program follows ARI Guideline T.

(Figure 1: Typical HVAC load profile for 500-ton load and 12-hour duration)

OPERATING STRATEGIES:
The next step in selecting thermal storage equipment is to define an operating strategy. Choices include "full storage" or "partial storage". Partial storage strategies can be further characterized as "demand-limiting" or "load-levelling". The choice of operating strategy will depend upon the load profile, the utility rate structure, energy costs, and equipment first cost.

Full storage systems assume the goal is to minimize on-peak demand for the lowest operating cost. Full storage eliminates the need to operate the chiller during the utility on-peak period by storing 100% of the required cooling capacity during off-peak periods. This strategy shifts the largest amount of electrical demand and results in the lowest operating cost. It also requires the largest chiller capacity and thermal storage capacity of the three strategies, so its first cost is the highest of the three.

With the partial storage strategies, the chiller will operate during the on-peak period, unlike the full-storage strategy. With partial storage - demand limiting, the goal is to not exceed the non-storage peak loads on the facility ("Non-storage loads" include lights, pumps, appliances, fans, motors, etc.) The non-storage loads establish the peak demand, and the thermal storage equipment is selected so that the chiller operation does not exceed the facilityās non-storage demand. This strategy reduces the chiller size somewhat compared to the full-storage option, at an increase in peak demand and overall energy consumption.

With partial storage-load levelling, the goal is to distribute the cooling load equally over the load cycle, usually a 24-hour period. This reduces the size of the chiller a nd the thermal storage equipment even further compared to the full storage and demand limiting options, for the lowest first cost and shortest payback period. Since the chiller operates fully loaded continuously, operating costs are higher than with either of the other strategies.

Choice of operating strategy is dependent on the relative priorities of first cost, demand charges, and energy consumption charges. An analysis of these factors must be performed on every project in order to make the optimum choice for that project.