Commissioning chilled water TES systems: as we've seen in recent articles, good commissioning experience can make valuable design advice. This recent ASHRAE presenter provides an overview of TES options before getting into details: tank placement relative to coil placement, tips on using pumps and valves, thermocline issues, and multiple tank considerations - and not using

Engineered Systems, Feb, 2004 by Lucas B. Hyman

A goal of the commissioning process is to deliver a project that, at the end of construction, is fully functional and meets the owner's needs. Some of the fundamental objectives of the commissioning process are to:

* Clearly document the owner's project requirements (OPR);

* Provide documentation tools (basis of design, commissioning plan, design, and construction checklists);

* Help with coordination between parties (owner, engineer, and contractor);

* Accomplish ongoing verification that the engineering and construction achieve the OPR;

* Verify that complete O&M manuals are provided to the owner;

* Verify that maintenance personnel are properly trained; and

* Accomplish functional performance tests that document proper operation prior to owner acceptance.

This article highlights the following:

* Key OPR for a stratified CHW TES system;

* Successful CHW TES design strategies of design);

* Caution flags (lessons learned);

* Guidelines of ASHRAE Standard 150, "Method of Testing the Performance of Cool Storage Systems requirements;" and

* Key CHW TES information to obtain during testing.

TES BASICS

TES is a method by which energy (cooling or heating) is produced and stored at one time period for use during a different time period. For cooling applications, using thermal storage can result in the reduction of electricity costs, chiller equipment size, and maintenance costs.

There are two basic concepts in TES and each has different major advantages. The two concepts are partial storage and full storage.

Partial-storage systems use smaller chillers, cooling towers, and a TES system to provide a facility's daily total cooling load needs, with a plant running at a constant load about equal in tons to 1/24 of the daily total ton-hours. The partial-storage system has an advantage by allowing for a smaller, less costly chiller plant than a conventional chiller plant.

Full-storage systems typically require larger storage systems and larger chiller plants than partial storage systems. Full-storage systems hold the chiller plant off during the period of highest energy charges (the on peak period) and meet the cooling load solely from thermal storage during that period.

Typically, the thermal storage capacity is generated at night or in an off-peak period, when electric rates are lower and building cooling requirements are low. Full-storage system chillers, towers, and buildings are often about the same size as conventional chiller plants but this depends on the storage strategy, cooling load profile, and electrical rate structure.

Full-storage TES systems, therefore, gain their major advantage from the difference between on peak and off-peak electric demand charges and energy rates. Partial storage systems also benefit from these factors to a smaller degree. Since TES allows the shift of electrical demand and energy consumption to off-peak periods, users can achieve large electricity cost savings when the central plant uses electric drive chillers.

OWNER TES PROJECT REQUIREMENTS

An owner, of course, wants a facility that is flexible, expandable, and also one that represents a reasonable capital investment. TES systems are often installed because they represent the lowest life-cycle-cost option. In addition, key OPR of a CHW TES system typically include:

* Required thermal storage capacity (often measured in ton-hours);

* Design CHW at;

* Peak cooling day profile; and

* The thermal storage strategy to be employed.

The design CHW [DELTA]T (temperature difference between the CHW supply and return temperatures) is a critical piece of information since with a stratified TES system, the thermal storage capacity is directly proportional to the [DELTA]T (Q = Mc[DELTA]T). Also, as CHW systems operate with relatively small [DELTA]Ts, even a few degrees less than the design at can have a dramatic impact. For example, on a 16[degrees][DELTA]T system, a 1[degrees] drop in [DELTA]T to 15[degrees] represents a 6% loss of TES capacity.

The [DELTA]T of the distribution system and/or the chiller system is not necessarily the same as that in the TES tank. The chillers must, however, produce water at least as cold as that stored in the TES tank. The CHW return temperature from the building(s) coils determines how high the CHW return temperature will be.

The peak day cooling load profile not only shows the facility's peak cooling load, and the shape and nature of the load, but also provides the day's required ton-hours as represented by the area under the cooling load profile curve. Along with the CHW [DELTA]T, the peak day's cooling load in ton-hours and the TES strategy to be employed were key information used by the designer to size the TES tank.

SUCCESSFUL CHW TES DESIGN STRATEGIES (BASIS OF DESIGN)

In order to help ensure that the OPR are met at the end of the project, our experience indicates that successful CHW TES strategies employ:

* High [DELTA]T CHW systems;

* Variable flow with two-way valves--constant [DELTA]T systems;

* Simple controls;

* TES tank at high point (desirable but not necessary); and