Thermal storage (hot or cold) is a cost saving technique, and in some cases, energy saving technology. Commercial cooling can account for as much as 40% of peak demand on a hot summer day. Cool storage can shift some or all of the on-peak demand to off-peak hours. With cool storage (chilled water or ice storage), cost is saved by:
- Reducing electric demand charges through decreasing or eliminating chiller operation during peak demand periods
- Operating chillers at night and displacing energy use from peak to off-peak periods when the energy is at a lower cost.
Hot water storage for domestic service water is a common example. It assures a supply during peak occupant demand times, and reduces cost in the same manner as cool storage. Storing hot water for space heating is less common, except where electric energy is used for heating.
There are a number of advantages for users, specifiers, and utilities:
- Users gain by reducing their utility bills - largely through peak-shaving; reduced equipment size, space and weight; reduced compressor kW due to operating at more hours at full load and at nighttime lower condensing temperatures; availability of cold air distribution when ice is used; possible backup cooling or heating redundancy in event of power failure; when chilled water storage is used, availability of an added fire-protection water source.
- Specifiers gain from cool storage being a proven technology whose use can differentiate their projects from those of others.
- The utilities can gain from reduced peak use, improved load factors, added off-peak sales, deferred peak-capacity expansion costs, and improved competitive position over gas-fired alternatives.
- May increase first cost of HVAC system,
- More complicated system design,
- Requires well-trained maintenance crew,
- Possible ambient heat gain to storage tanks.
- Specifying engineer has little incentive to use as it costs more to design and the firm may have little or no experience with the technology.
- Some of the risks include night-time loads greater than planned, insufficient storage provided so on hot days demand is not saved, improper controls supplied, operator inattention or unskilled, condensation on ducts with low temperature supply air when a fan is out of service.
Cooling, not heating, is a main concern of commercial building owners. Many buildings are populated with people, heat-emitting office and production equipment, and ample lighting — all generating so much heat that cooling is required during most of the working hours all year. Most of these hours occur during the peak hours defined by the electric rate structure. Cool storage can shift all or most of this use to lower-cost off-peak hours resulting in lower operating cost without sacrificing comfort...and in some cases, increasing comfort.
The best cool storage applications are in any building:
- being charged on a time-of-use electric rate schedule
- having high on-peak demand charges, with relatively low or no off-peak demand charge
- that has peak cooling loads during utility on-peak hours
- has very few comfort or process cooling hours per day, week or month, but with high peak loads during those few hours (such as churches).
- requiring low humidity control where ice storage can be used.
Air conditioning applications that can best benefit are office buildings, schools and college buildings, religious institutions, laboratories, large retail stores, libraries, museums, and the public use areas (meeting rooms, exhibit halls, convention centers) of hotels and public assembly buildings.
Other applications include:
- Industrial processes with batch cooling requirements
- Facilities where low humidity can be achieved with the low water temperature achieved from ice storage
- Buildings where space is at such a premium that the small ducts used with low air temperature distribution are advantageous, such as retrofit of older, historic buildings.
- Facilities where the cold storage can be tied into existing ammonia or other refrigeration systems.
Technology Types (Resource)
Depending on the needs of the building or process and the electric rate structure, there are several types of cool storage designs that may be employed on a given project:
- Full storage (load shifting)- discharging stored capacity without any concurrent chiller operation
- Partial storage (load leveling)- discharging storage to meet cooling loads with concurrent operation of some chiller(s) piped in parallel with storage).
- Full recharge - recharging storage with chiller operation
- Partial recharge - recharging storage with chiller capacity while simultaneously providing capacity to the cooling load.
- Standby - no normal use of storage, with chillers serving the cooling loads as they would in the absence of storage. Storage used when power outages occur.
Storage capacity is usually defined in ton-hours which is the sum of the actual tons required each hour for the design day. It can be achieved using either chilled water storage or ice storage. Chilled water storage typically requires more space (½ to 1 gal per sq ft of conditioned space) than ice storage (1/16 to _ gal).
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