Control of Thermal Energy Storage in Commercial Buildings for California Utility Tariffs and Demand Response:

Thermal Energy Storage (TES) is an established technology that shifts heating or cooling energy use from an on-peak period when demand and rates are highest to an off-peak period, when rates are lower. This study evaluates the two main types of TES systems: Full storage TES systems designed to shift the entire cooling system load to the off-peak period, and partial storage TES systems, which are designed to shift only a portion of the cooling load off-peak. The cooling load profile on the peak day is selected for the TES system design, which ensures that a full storage TES system is sufficient to meet the cooling load requirement. For a utility tariff that has a monthly demand charge and on-peak demand charge as well, a full storage system can provide bill savings by reducing both peak demand and energy use. For a partial storage system, the cooling system supplements the TES system during peak hours, which can reduce a portion of peak demand with reduced cooling plant capacity. TES systems shift electricity use from on-peak periods to off-peak periods on a recurring basis, which is characterized as permanent load shifting (PLS). For TES’ participations in DR, partial storage TES systems are better suited than full storage systems for participating in demand response (DR) programs because full storage systems create peak period baselines with little to no room for shedding cooling related loads. For DR events called on peak demand days, the integration of partial TES systems with typical DR control strategies (e.g. global temperature adjustment (GTA)) can also provide one-hour or 20-minute load shed resources by aggregating the cooling load reduction during the GTA deployment period. Buildings with partial TES systems can be good resources for participating in DR programs requiring faster response times and shorter response durations. TES demand shifting and economic payback is greatly influenced by the following factors: (1) utility rate structures; (2) building load characteristics (e.g. load pattern, ratio of on-peak and off-peak cooling load); (3) climate; and (4) available physical space for retrofit installations. In this study, a matrix of various TES use cases was simulated to evaluate the impact of building load, climate and California utility tariffs. Simulations show that typical TES installations will have enough excess capacity to provide cooling demand shifting on most days. TES is fully discharged on less than 5% of the total number of weekdays during the year because the TES storage capacity is designed based on the total cooling load on the peak day. With current retail DR programs that have a relatively small number of “event” days, typically on the hottest days—the amount of excess capacity is minimal, and, so is the benefit to customers of participating in DR with only TES. Because the cooling load is lower on non-peak days, partial TES systems have excess capacity that can be used during DR event hours, which will enable customers to participate in DR by turning off chiller(s). For older office buildings in PG&E territory, bill reduction is greatest with a full 9-h TES, but payback is faster with a full 6-h TES. Similarly, for old and new office buildings in SDG&E territory, a full 9-h TES provides the lowest annual utility costs, but payback is faster with a partial 9-h. Utilities currently look to TES to provide maximum peak period reduction. In most cases studied here, the TES configuration that provided the greatest economic benefit to the customer also provided the greatest peak period load reduction to achieve the demand charge savings. However, small-to-medium retail customers will have the lowest utility costs with a partial storage system, which only provides a fraction, typically half, of peak period demand reduction compared to that of a full storage system. Older less efficient buildings have higher peak period loads and present greater potential demand reductions that can be achieved with TES. Incentives structured as dollar per kW of TES installed will achieve greater peak period reductions per dollar of incentive if targeted at new buildings, but, all other things being equal, the peak period load reduction provided by TES will be lower with a newer building charge savings. However, small-to-medium retail customers will have the lowest utility costs with a partial storage system, which only provides a fraction, typically half, of peak period demand reduction compared to that of a full storage system. Older less efficient buildings have higher peak period loads and present greater potential demand reductions that can be achieved with TES. Incentives structured as dollar per kW of TES installed will achieve greater peak period reductions per dollar of incentive if targeted at new buildings, but, all other things being equal, the peak period load reduction provided by TES will be lower with a newer buildin