The Demand Limiting strategy limits peak energy usage by monitoring the energy rate from a meter, comparing it to a user configured tariff target, then shedding loads as needed to prevent energy consumption from exceeding the tariff target. For Demand Limiting, a tariff target is the maximum permitted average energy for the current demand interval. The current demand interval is the duration in minutes over which energy consumption is averaged for billing purposes.
Examples of utilities that can be controlled include electricity, natural gas, and steam. For example, during a hot summer day, electricity usage can be reduced by lowering the speed of a variable speed fan. Chilled water supply temperature or a discharge air temperature setpoint can be temporarily adjusted during the few hours of the day that are identified as peak times.
Demand Limiting calculates projected demand, which is the anticipated future energy usage that DLLR expects to control. DLLR supports two mutually exclusive methods for calculating this projected demand:
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Sliding Window - algorithm used if the utility calculates average demand over any possible x number of minutes, where x is equal to the time period over which average demand is measured. The DLLR feature approximates this calculation by assuming that a new billing interval starts each minute, and includes all of the data from the previous interval except for the first minute of the interval.
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Fixed Window - algorithm used if the energy utility billing interval starts at the end of the previous interval with no overlap of data from the previous interval. This algorithm typically uses an end of interval (EOI) pulse, supplied by the utility company. The EOI pulse is used to synchronize the DLLR demand interval to the utility demand interval.
For details on these algorithms, see DLLR Equations.