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Thermal storage systems do not negatively impact the building’s indoor environmental control operations, as the load shedding or some load control programs have done.
As a thermal storage system is a central system, it uses chilled water from the central plant to cool the air in the AHUs. Thermal storage systems are easily adaptable to the water system of a central air conditioning system. As discussed in Sec. 1.5, central systems served 24 percent of the floor space of commercial buildings in 1995.
Real-time pricing and other rate structures are available as the power providers maneuver to offer the most competitive structure in the electric deregulation environment.
Thermal storage systems have longer operating hours of compressors and pumps, and chillers and cooling tower at full-load operation, at lower outdoor temperatures at nighttime, and a backup source for cooling during emergencies.
Drawbacks of thermal storage systems include high initial cost and complicated operation, maintenance, and control.
Full Storage and Partial Storage
The ton-hour, or ton h (kWh), is the unit of stored refrigeration. One ton-hour is the refrigeration or heat absorption of 12,000 Btu (3.516 kWh) performed by a refrigeration system during a 1-h period. The aim of thermal storage strategy is to incur the lowest possible energy cost and initial investment so that life-cycle costs are minimized. The economic benefit of a thermal storage system can also be assessed by calculating the simple payback period of its cost. A simple payback or lifecycle cost analysis of the building load profile, utility electric rate structure, and system characteristics is always necessary.
Determination of the optimum size of a thermal storage system is based on the utility’s electric rate structure (difference between on-peak and off-peak unit charges) and the building refrigeration load profile.
Direct cooling denotes the process by which compressors produce chilled water to cool the building directly. When the cost of the direct cooling by a refrigeration system is lower than the cost of stored energy, the operation of the thermal storage system is said to be at chiller priority. On the other hand, if the cost of the direct cooling is higher than the cost of stored energy, the operation is said to be at storage priority. Construction cost and the utility’s incentive payments should be considered. If the energy cost difference between on-peak and off-peak hours is great, the full use of stored energy during on-peak hours may be most economical.
There are two kinds of thermal storage: full storage and partial storage. Figure 31.1 shows the load-time diagrams for full storage and partial storage. For a full-storage, or load shift thermal storage, system, all refrigeration compressors cease to operate during on-peak hours, and the building refrigeration load during that period is entirely offset by the chilled water supplied from the thermal storage tank, as shown in Fig. 31.1a.
Partial storage, or load leveling, can be either in load-leveling mode, in which refrigeration compressors are operated at full capacity during on-peak hours, as shown in Fig. 31.1b, or in demand limited mode, in which building electric demand limits only part of the refrigeration compressors operated, as shown in Fig. 31.1c.
A utility’s demand charge is the building total demand charge, which is the sum of the demand charges for HVAC&R systems and other uses including the electricity for lighting, escalators, computers, and electric appliances.
The following is a comparison of different storage strategies of a thermal storage system with heat recovery and cold air distribution for an office building in Dallas, Texas, as described by Tackett (1989).
FIGURE 31.1 Full and partial storage: (a) full storage; (b) partial storage, all compressors
operating; (c) partial storage, 50 percent of compressors operating.