Coupling

What it is

The point where the battery connects to the system. An AC-coupled battery has its own inverter and connects on the AC side, next to the grid connection. A DC-coupled battery connects through a DC/DC converter to the DC bus ahead of the PV inverters, so it can store DC power the inverters would clip and deliver it later through the same inverters. The surplus exists in timesteps where the inverter sits at its capability: its output at the AC ceiling, or its input current at its maximum DC input current. With the current limit binding, the battery holds the array nearer its maximum-power point, which lowers the bus voltage and with it the power the inverter takes at that current, so the capture gives up some of that hour's output. The response reports that part of the captured energy as e_inverter_input_given_up_dc_kwh: captured less given up is the extra array energy the battery harvests.

Typical values and how to get it

AC coupling for a battery with its own inverter, added to an existing system or sized independently of the PV inverters. DC coupling for a battery sharing the PV inverters, typically on an array oversized against them (a DC/AC ratio of 1.3 to 1.5) to recover the energy the inverters clip at their AC ceiling or their input-current limit. The system's single-line diagram shows which one applies.

Values

AC-coupled = ac (default): the battery charges from the system's grid injection and delivers to it · DC-coupled = dc: the battery charges first from the DC power above the inverter's capability, delivers DC power into the inverter's spare capability, and the inverter efficiency and the AC losses apply to the combined input

How it is applied

$$x_{dc,i} = \max\!\left(0,\ P_{arr,i}(U_i) - P_{cap,i}\right), \qquad P_{cap,i} = \min\!\left(P_{in,lim,i},\ \frac{U_i\,I_{max}}{1000}\right), \qquad P_{ac}\!\left(P_{in,lim,i},\ U_i\right) = P_{lim,i}$$

where

Symbol	Meaning	Unit
$x_{dc,i}$	the DC surplus a DC-coupled battery can take at timestep $i$	kW
$P_{arr,i}(U_i)$	the array's DC power at the bus voltage	kW
$U_i$	the DC bus voltage of timestep $i$: the array's maximum-power voltage held inside the MPPT window	V
$P_{cap,i}$	the inverter's DC capability: the most DC input it takes at the bus voltage	kW
$P_{in,lim,i}$	the DC input at which the inverter output reaches its ceiling	kW
$I_{max}$	the inverter's maximum DC input current	A
$P_{ac}$	the inverter's AC output as a function of the DC input and the voltage	kW
$P_{lim,i}$	the inverter's AC ceiling at timestep $i$	kW

References

1. Denholm, Eichman & Margolis 2017, "Evaluating the Technical and Economic Performance of PV Plus Storage Power Plants", NREL/TP-6A20-68737
2. DiOrio, Dobos & Janzou 2015, "Technoeconomic Modeling of Battery Energy Storage in SAM", NREL/TP-6A20-64641
3. DiOrio, Denholm & Hobbs 2020, "A model for evaluating the configuration and dispatch of PV plus battery power plants", Applied Energy 262 114465

API

storage.coupling

Symbols follow the symbol registry.