DER VET User Guide/Technologies/Energy Storage

The battery model employed by DER-VET uses three variables to characterize the state of the system – charge power (ch), discharge power (dis), and state of energy (ene). Two binary variables are also employed when the binary input is on to ensure that the storage system does not concurrently charge and discharge and to handle minimum power requirements. These binary variables indicate whether the storage system is charging (on_c), discharging (on_d), or neither. The binary variables need to be disabled to perform size optimization, done by setting binary model parameter to 0.

The state of energy of the storage system at a given time is defined as the amount of AC energy you would get out if you were to discharge the storage system completely at full power. This definition means that charging power and discharging power will modify the state of energy of the storage system differently via the following equation.

∆SOE=[η*ch-dis]*dt

Where η is the roundtrip efficiency of the storage system and dt is the time resolution of the data. Other things can modify the state of energy of the storage system, though. These include self-discharge (∆SOE=-SOE*SDR/100*dt where SDR is the self-discharge rate of the battery [%/hr]) and power use from intra-period cycling such as for regulation.

Inputs

Charging Power Capacity

Tag	Key	Description
Battery	ch_max_rated	This input sets the power capacity of the storage system in the charging direction. Set this to 0 to have DER-VET optimally size this parameter.

Discharging Power Capacity

Tag	Key	Description
Battery	dis_max_rated	This input sets the power capacity of the storage system in the discharging direction. Set this to 0 to have DER-VET optimally size this parameter.

Minimum Charging Power

Tag	Key	Description
Battery	ch_min_rated	This input sets the minimum charging power of the storage system. This parameter only applies when the binary input is 1. This parameter is not compatible with size optimization.

Minimum Discharging Power

Tag	Key	Description
Battery	dis_min_rated	This input sets the minimum discharging power of the storage system. This parameter only applies when the binary input is 1. This parameter is not compatible with size optimization.

Energy Capacity

Tag	Key	Description
Battery	ene_max_rated	This input sets the energy capacity of the storage system. (Duration = ene_max_rated/dis_max_rated) Set this to 0 to have DER-VET optimally size this parameter.

Maximum Duration

Tag	Key	Description
Battery	duration_max	When optimally sizing the energy capacity of a storage system, this input can limit the duration the size optimization will produce to keep the results feasible. Set this to 0 to ignore.

Upper Limit on SOC

Tag	Key	Description
Battery	ulsoc	Restrict the storage system from using some energy capacity by setting a maximum SOC. The usable energy capacity = ene_max_rated * (ulsoc - llsoc).

Lower Limit on SOC

Tag	Key	Description
Battery	llsoc	Restrict the storage system from using some energy capacity by setting a minimum SOC. The usable energy capacity = ene_max_rated * (ulsoc - llsoc).

Roundtrip Efficiency

Tag	Key	Description
Battery	rte	What is the roundtrip efficiency of the storage system? For one full depth of discharge cycle (0% -> 100% -> 0% SOC), the roundtrip efficiency is the energy discharged/energy charged. The roundtrip efficiency is constant in DER-VET.

Self Discharge Rate

Tag	Key	Description
Battery	sdr	If the storage system is not operated, how quickly (in %/hr) does the energy stored disappear?

SOC Target

Tag	Key	Description
Battery	soc_target	What SOC should the storage system return to at the beginning/end of each optimization window?

Calendar Degradation

Tag	Key	Description
Battery	yearly_degrade	This input will degrade the energy capacity of the storage system in %/yr. The total degradation will be the calendar degradation combined with cycling degradation.

Cycle Degradation

Tag	Key	Description
Battery	incl_cycle_degrade	This binary input (0 or 1) determines whether or not degradation due to cycling will be calculated and applied.

Cycle Life Filename

Tag	Key	Description
Battery	cycle_life_filename	This input points to a file containing the cycle life curve of the storage system.

Charging Startup Cost

Tag	Key	Description
Battery	p_start_ch	When startup costs are turned on, this is the cost of starting to charge.

Discharging Startup Cost

Tag	Key	Description
Battery	p_start_dis	When startup costs are turned on, this is the cost of starting to discharge.

Cycle Limit

Tag	Key	Description
Battery	daily_cycle_limit	In any given 24-hr period, this input limits the number of cycles a storage system can perform, perhaps to comply with a warranty or performance guarantee.

Auxiliary Load

Tag	Key	Description
Battery	hp	Auxiliary load or "housekeeping power" is a constant AC power draw that does not discharge the storage system but does incur an energy cost. This is meant to represent HVAC computers, fans, lights, inverter power, etc.

Replacement Condition in Cycle Life File

Tag	Key	Description
Battery	state_of_health	What SOH was used to trigger a replacement by the people who made the cycle life file? A larger number indicates that replacement happens sooner, so the batteries would have a lower cycle life value when doing testing. A larger number with the same cycle life file will, in DER-VET, result in a longer modeled life for storage systems with degradation turned on.