Thermal storage (hot or cold) is a cost saving technique, but requires more energy consumption. It works by shifting on-peak demand to off-peak hours. Since commercial coolingcan account for as much as 40% of peak demand on a hot summer day, this can be an effective option depending on prevailing electric rates.
- Reducing electric demand charges through decreasing or eliminating chiller (or water heater) 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, because it doesn’t displace energy in a useful way (winter-peaking utilities will typically see demand during the day and at night).
In Georgia, thermal storage cost savings for cooling typically don’t justify the higher upfront costs (and other issues). There are several reasons for this:
- When a chiller is run at low temperatures (or used to make ice),the chiller derates. This means that it takes MORE energy to generate the same amount of cooling capability (as measured in ton-hours). The additional energy requirement in terms of kilowatt-hours offsets some of the demand savings.
- Depending on the planned storage temperature, you may need to use a chiller with a different refrigerant (glycol, for example). This can add cost upfront and in maintenance.
- Many designers suggest that thermal storage can be accomplished with a smaller chiller (since it runs more hours at night than it would during the day, it can achieve the same quantity of ton-hours); this is rarely so in Georgia, since you have a large number of cooling hours relative to setback times, and chillers derate at low temperatures.
- Thermal storage takes up a lot of room.
However, thermal storage of hot water can make sense when:
- There is a large domestic/process hot water load that peaks during the day (laundry or healthcare settings)
- Upfront cost is less important than operating cost
There are some advantages to thermal storage:
- Utility bills are typically reduced somewhat
- When chilled water storage is used, availability of an added fire-protection water source.
- Better dehumidification (because of lower coil temperatures); this advantage could also be achieved by running the chiller at a lower supply temperature during the day.
- Can be used to augment chiller capacity (run the chiller constantly, and use the stored cooling capacity to meet peak loads); this can reduce the size of the chiller and provide some redundancy in the event of a chiller outage.
- Smaller piping, ducts, and supporting pumps can offset some of the capital cost considerations. Since the water is typically supplied at a lower temperature, the duct and piping requirements are reduced. This advantage could also be achieved by operating the chiller at a lower supply temperature during the day (which would increase operating costs for the chiller).
- Increases first cost of HVAC system.
- More complicated system design.
- Requires well-trained maintenance crew.
- Requires a large space for the tank (can be buried, which requires excavation).
- 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.
Other system types (such as encapsulated ice, iceballs, eutectic salt storage) are variations being developed and commercialized, but are not covered here.
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. This is because the water is typically supplied at lower temperatures, resulting in better humidity control.
The best cool storage applications are in any building:
- Being charged on a time-of-use electric rate schedule with a high differential between on-peak and off-peak
- Having high on-peak demand charges, with relatively low or no off-peak demand charge
- With peak cooling loads during utility on-peak hours
- Having 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 (but with the understanding that thermal storage requires large tanks; small ducts can be used in supply temperatures are lowered on the chiller)
- Facilities where the cold storage can be tied into existing ammonia or other refrigeration systems
Applications to Avoid
One sour project can wipe out the benefit of a hundred "atta-boy" installations in a service territory. There are some applications which may seem to be cool storage candidates, but should be avoided if these conditions exist:
- No benefit from off-peak operation
- Speculative designs built on low-first cost premises
- Likelihood of little or no attention to providing and keeping trained operators
- Seasonal projects where the staff changes from season to season
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 requires more space (½ to 1 gal per sq ft of conditioned space) than ice storage (1/16 to 1/8 gal).