Energy Storage System

PlantPredict offers modeling of AC-coupled storage systems; DC-coupled systems are currently not supported in PlantPredict. The section below describes the inputs and basic functionality of modeling these systems in PlantPredict. In many cases, the modeling requires calculating energy at a number of “nodes” within the storage system and so the diagram below may be a helpful reference when thinking about these systems and when referencing system nodal data.

Figure 1. Nodal reference diagram for PV+ Storage system

 

To add an energy storage system to your power plant, click the Add button corresponding to the Energy Storage System section under Power Plant Specifications. The system can be later deleted by clicking the Delete link, or updated by clicking the Update button.

An Energy Storage System is defined on the Energy Capacity tab by:

1. Defining two of the three parameters:

a) Energy Capacity Nameplate

b) Energy Capacity Factor

c) Energy Capacity Usable

2. Providing degradation rates (calendar and cycling-dependent) of energy capacity and DC Roundtrip Efficiency

3. Providing efficiencies (DC Roundtrip Efficiency and Inverter) and losses (MV Transformer and HVAC)

4. Defining power ratings (Inverter Real Power and MV Transformer Power Rating)

 

An Energy Storage System Dispatch algorithm is defined on the Dispatch Algorithm tab by:

1. Choosing an available Dispatch Algorithm, either predefined or the Custom option.

a. If the Custom option is selected:

i. the template Custom dispatch file can be downloaded

ii. charging and discharging times must be identified

iii. the fraction of inverter rated capacity (at the input to the MV transformer) desired to be charged or discharged (dispatch will be limited to the available capacity of the system) must be defined, for each index corresponding to the time steps in the prediction weather file

iv. the Custom dispatch file must be uploaded

v. skip Step 2

2. Identifying the desired dispatch hours in the Target Period table. The selected hours will direct the algorithm to target output at the interconnect capacity during those hours.

The ESS is always assumed to be at a full State of Charge during the first time interval of the prediction.

The system input values should be carefully considered, as they are dependent on the storage technology being modeled.

 

Key Terms

 The total storage nameplate DC energy capacity.

The percent of the nameplate DC energy capacity that is usable energy capacity.

 

The initial total storage usable DC energy capacity. This is the energy capacity within the usable State of Charge window.

 

The percent of the initial usable DC energy capacity by which  the usable DC energy capacity decreases over time, linearly.

 

The percent of the initial usable DC energy capacity that the usable DC energy capacity decreases as a function of cumulative storage cycles.

 

Initial ratio of stored energy to input energy for the storage system.

 

The percent of the initial DC Roundtrip Efficiency by which  the DC Roundtrip Efficiency decreases over time, linearly.

 

The percent of the initial DC Roundtrip Efficiency by which  the DC Roundtrip Efficiency decreases as a function of cumulative storage cycles.

 

The total active power capacity setpoint of the storage inverters.

 

Ratio of output power to input power of the storage inverters.

 

The total kVA rating of the storage MV transformers. Often assumed to be the same as the Inverter Real Power rating, for simplicity.

 

The energy consumption of storage MV transformer equipment as a percent of MV transformer rating, defined for the no-load operation of the transformers.

 

The energy consumption of storage MV transformer equipment as a percent of MV transformer rating, defined for the full load operation of the transformers.

 

The energy consumption due to HVAC per MWh of storage nameplate DC energy capacity.

 

The energy consumption due to HVAC per MW of DC power input or output at the ESS.

 

The Interconnect Excess charging algorithm charges all energy generated by the PV that exceeds the Power Output Limit (set under the System menu item), except during target period hours.

 

The Energy Available charging algorithm charges all energy generated by the PV system until the storage is at full usable State of Charge.

 

The Custom dispatch algorithm allows the user to define a charge and discharge target profile. The dispatch will be limited to the available capacity of the system. The file must contain indexes that correspond to the number of time steps in the prediction weather file.

 

File format for the Custom dispatch:

• First row contains column headers. Each following row contains the consecutive value.
• Column A header: Index

– Incremental index corresponding to each timestep in the weather file, beginning with 1.

• Column B header: Command

– Field can be blank, Discharge, or Charge, as desired.

• Column C header: Inverter Capacity Fraction
  • The proportion of total ESS inverter capacity to be transferred to or from the storage, at the Low Voltage AC (node 3) point on the Energy Storage System, in standard number format.

 

Note that a command to discharge energy that results in the total discharged and PV generated power exceeding the interconnection capacity is followed but causes the excess energy to be clipped.

 

Upload the file containing the Custom dispatch algorithm.

Boxes checked in the Dispatch Table are target period hours, and those that are not checked are not. The storage system will discharge only during checked hours. It will discharge at the maximum amount up to the stored energy, the power allowed by the inverter power capacity, and the power allowed to meet the interconnect capacity that exceeds the PV capacity.