Building energy storage system with multiple demand charge cost optimization
A building energy system includes a controller configured to obtain representative loads and rates for a plurality of scenarios and generate a cost function comprising a risk attribute and multiple demand charges. Each of the demand charges corresponds to a demand charge period and defines a cost based on a maximum amount of at least one of the energy resources purchased within the corresponding demand charge period. The controller is configured to determine, for each of the multiple demand charges, a peak demand target for the corresponding demand charge period by performing a first optimization of the risk attribute over the plurality of the scenarios, allocate an amount of the one or more energy resources to be consumed, produced, stored, or discharged by the building equipment by performing a second optimization subject to one or more constraints based on the peak demand target for each of the multiple demand charges.
1. A building energy system configured to serve energy loads of a building or campus, the system comprising:
equipment configured to consume, produce, store, or discharge one or more energy resources, at least one of the energy resources purchased from a utility supplier;
a controller configured to:
obtain representative loads and rates for the building or campus for each of a plurality of scenarios;
generate a cost function comprising a risk attribute and multiple demand charges, each of the demand charges corresponding to a demand charge period and defining a cost based on a maximum amount of at least one of the energy resources purchased from the utility supplier during any time step within the corresponding demand charge period;
determine, for each of the multiple demand charges, a peak demand target for the corresponding demand charge period by performing a first optimization of the risk attribute over the plurality of the scenarios;
allocate, to each of a plurality of time steps within an optimization period, an amount of the one or more energy resources to be consumed, produced, stored, or discharged by the equipment by performing a second optimization of the cost function over the optimization period subject to one or more constraints based on the peak demand target for each of the multiple demand charges; and
operate the equipment to consume, produce, store, or discharge the one or more energy resources at each of the plurality of time steps in accordance with a result of the second optimization.
2. The building energy system of claim 1 , wherein the controller is configured to modify the cost function by applying a demand charge mask to each of the multiple demand charges, wherein demand charge masks cause the controller to disregard a resource purchased from the utility supplier during any time steps that occur outside the corresponding demand charge period when calculating a value for a demand charge.
3. The building energy system of claim 1 , wherein the risk attribute of the cost function comprises at least one of a conditional value at risk, a value at risk, or an expected cost.
4. The building energy system of claim 1 , wherein performing the second optimization comprises using each peak demand target to implement a peak demand constraint that limits a maximum purchase of an energy resource subject to a demand charge during a corresponding demand period.
5. The building energy system of claim 1 , wherein the cost function comprises a revenue term that accounts for revenue generated by operating the equipment to participate in an incentive-based demand response program.
6. The building energy system of claim 1 , wherein the controller is configured to obtain the representative loads and rates by:
receiving user input defining the loads and rates for several scenarios; and
at least one of:
sampling the representative loads and rates from the user input defining the loads and rates for the several scenarios; or
generating an estimated distribution based on the user input and sampling the representative loads and rates from the estimated distribution.
7. The building energy system of claim 1 , wherein the controller is configured to obtain the representative loads and rates by:
receiving input defining loads and rates for several scenarios, each of the scenarios corresponding to a different time period used by a planning tool; and
sampling the representative loads and rates for each scenario from the loads and rates for a corresponding time period used by the planning tool.
8. The building energy system of claim 1 , wherein the controller is configured to obtain the representative loads and rates by:
storing a history of past scenarios comprising actual values for historical loads and rates; and
at least one of:
sampling the representative loads and rates from the history of past scenarios; or
generating an estimated distribution based on the history of past scenarios and sampling the representative loads and rates from the estimated distribution.
9. The building energy system of claim 8 , wherein:
each of the historical loads and rates corresponds to different time period; and
the controller is configured to sample the representative loads and rates for each scenario from the historical loads and rates corresponding to a time period having similar characteristics as the scenario.
10. The building energy system of claim 1 , wherein the cost function comprises a nonlinear maximum value function for each of the multiple demand charges and the controller is configured to linearize the cost function by replacing each nonlinear maximum value function with an auxiliary demand charge variable.
11. The building energy system of claim 1 , wherein the controller is configured to apply a weighting factor to each of the multiple demand charges in the cost function, each weighting factor scaling a corresponding demand charge to the optimization period.
12. The building energy system of claim 11 , wherein the controller is configured to calculate each weighting factor by:
determining a first number of time steps that occur within both the optimization period and the corresponding demand charge period;
determining a second number of time steps that occur within the corresponding demand charge period but not within the optimization period; and
calculating a ratio of the first number of time steps to the second number of time steps.
13. A method for managing a building energy system, comprising:
operating equipment to consume, store, or discharge one or more energy resources purchased from a utility supplier;
determining an allocation of the energy resources across the equipment over an optimization period by:
obtaining representative loads and rates for a building or campus for each of a plurality of scenarios;
generating a cost function comprising a risk attribute and multiple demand charges, each of the demand charges corresponding to a demand charge period and defining a cost based on a maximum amount of the at least one energy resource purchased from the utility supplier during any time step within the corresponding demand charge period;
determining, for each of the multiple demand charges, a peak demand target for the corresponding demand charge period by performing a first optimization of the risk attribute over the plurality of scenarios; and
allocating, to each of a plurality of time steps within the optimization period, an amount of the one or more energy resources to be consumed, produced, stored, or discharged by the equipment by performing a second optimization of the cost function over the optimization period subject to one or more constraints based on the peak demand target for each of the multiple demand charges; and
controlling the equipment to store or discharge the amount of the one or more energy resources allocated for a current time step of the plurality of time steps.
14. The method of claim 13 , comprising modifying the cost function by applying a demand charge mask to each of the multiple demand charges, wherein demand charge masks cause a controller to disregard a resource purchased from the utility supplier during any time steps that occur outside the corresponding demand charge period when calculating a value for the demand charges.
15. The method of claim 13 , wherein the risk attribute of the cost function comprises at least one of a conditional value at risk, a value at risk, or an expected cost.
16. The method of claim 13 , wherein optimizing the cost function comprises using each peak demand target to implement a peak demand constraint that limits a maximum purchase of an energy resource subject to a demand charge during a corresponding demand period.
17. The method of claim 13 , wherein obtaining the representative loads and rates comprises:
receiving user input defining the loads and rates for several scenarios; and
at least one of:
sampling the representative loads and rates from the user input defining the loads and rates for the several scenarios; or
generating an estimated distribution based on the user input and sampling the representative loads and rates from the estimated distribution.
18. The method of claim 13 , wherein obtaining the representative loads and rates comprises:
receiving input defining loads and rates for several scenarios, each of the scenarios corresponding to a different time period used by a planning tool; and
sampling the representative loads and rates for each scenario from the loads and rates for a corresponding time period used by the planning tool.
19. The method of claim 13 , wherein obtaining the representative loads and rates comprises:
storing a history of past scenarios comprising actual values for historical loads and rates; and
at least one of:
sampling the representative loads and rates from the history of past scenarios; or
generating an estimated distribution based on the history of past scenarios and sampling the representative loads and rates from the estimated distribution.
20. The method of claim 19 , wherein:
each of the historical loads and rates corresponds to different time period; and
the method includes sampling the representative loads and rates for each scenario from the historical loads and rates corresponding to a time period having similar characteristics as the scenario.