THERMAL ENERGY STORAGE CUTS PEAK LOAD
(ANCONEWS VOL. 2, NO. 4)

LESS EFFICIENT, BUT BETTER

Doing something less efficiently is usually not a good idea. Running large air-conditioning chillers at night to make and store cold water, or ice, to handle the peak cooling loads of a summer day is less efficient than the traditional handle-the-load-as-it-happens method. But it IS a good idea; and it’s becoming increasingly popular.

We recently asked Dr. Raj Gopal to expand upon this idea. Dr. Gopal has been responsible for much of ANCO’s work in the field of Thermal Energy Storage (TES), and is Vice Chairman of ASHRAE’s Building Operation Dynamic Committee TC4.6. The following is a summary of his comments:

Several electric utilities in the U.S. are aggressively promoting "Cool-Storage" as a means of reducing their peak system demand. They are prompted to these actions by the sharpness of their peak loads. The reward for shifting peak daylight kW’s to "off-peak" periods can be a postponement in the need to construct new generating capacity, the elimination of high cost "peaking" generators, or a reduced need to purchase power from adjacent utilities.

Regardless of the specific method chosen, implementing a Cool-Storage system will increase the total energy (kW-hours) required for a given air-conditioning load. This lowered efficiency is dictated by certain basic thermo-dynamic laws. However, by exercising care in the design and operating scheme for such systems, the loss in the efficiency can be held to within a few percentage points. In fact, the efficiency of the overall societal power system could conceivable increase - if, for example, the need to run an inherently inefficient gas turbine-powered "peaking" generator could be eliminated by shifting sufficient demand away from the utilities’ peak demand period.

COOL-STORAGE METHODS

Cooling capability can be stored either by chilling or freezing water (although glycol and eutectic salts have also been used). Water is the material of choice for a variety of practical and thermodynamic reasons. Its readily available, relative harmlessness, and the wide availability of equipment for its storage and handling are obvious factors dictating the selection. The choices between cooling or freezing this water, the subsequent choices in equipment, use of eutectic salts to raise freezing temperatures, etc., are far from simple. Options are numerous, and answers are not clear cut. Ultimately, the Cool Storage method selected must meet the particular needs and constraints of the facility in which it is installed.

UTILITY REBATE

Cool-Storage systems are designed, evaluated, and approved in a real-world environment where the concerns and preferences of the electric utility must be given considerable weight. Utilities provide financial incentives to promote the inclusion of Cool-Storage methods in new and retrofitted air-conditioning systems. These incentives usually involve payment of a rebate to the end-user/customer. Special off-peak billing rate schedules are also available to the larger customers for whom separate metering of their air conditioning load is practical.

Rebates are normally predicated on the number of kW’s of demand shifted to an off-peak period. Substantial payments are common. Southern California Edison’s (SCE) present limit of TES rebates is $300,000, based on $200 per kW shifted. At Wisconsin Electric (WE), rebates are variable, typically $350 per kW shifted and $0.08 per kWhr shifted during the May to September cooling period. The combined effect of such rebates and the reduced operating costs achieved with a properly designed TES system is typically a 2-5 year simple payback on the cost of its implementation.

A utility is, understandably, concerned with verifying the feasibility of proposed demand-shifting TES projects. Over the past four years, ANCO has been providing third-party evaluations for both SCE and WE. To-date, more than 75 feasibility studies and screening reviews have been performed by ANCO. A typical review process includes: 1) a site visit, 2) a preliminary screening study to see if cool storage is attractive, 3) a detailed feasibility study (if warranted) to determine the benefits to the customer and the utility, and 4) a technical and economic review of the Cool Storage project proposed by consultants and vendors.

In collaboration with SCE, ANCO developed an array of computerized analytical tools which accept as inputs the seasonal and diurnal cooling load profiles, a description of the proposed Cool-Storage system, and a definition of the Baseline system against which the TES system is to be compared. This model computes the technical and financial parameters required to assess feasibility. The key outputs, for both Baseline and TES, are: the annual cost of energy (kW-hr); the annual cost of demand (kW); energy usage and demand by seasonal and diurnal billing rate periods. These outputs, in conjunction with the engineering and operational judgment of ANCO’s professional staff, provide an objective and realistic assessment of each Cool-Storage system proposed to SCE.

For WE, ANCO’s review of over 50 TES proposals has identified a substantial number of unattractive candidate TES proposals - with a resultant savings of about $8,000 each in avoided cost for detailed design studies. Nevertheless, an estimated total of 2 megawatts of peak load was shifted to off-peak by those projects recommended by ANCO.