Energy diverted to storage

What this step does

With AC coupling, the AC energy taken from the system's grid injection to charge the battery, metered before the charge loss. It acts on the grid energy. With DC coupling the row is replaced by Change at the POI from storage.

The math

1. The row on the running total
2. Clip capture
3. Daily full cycle, outside the scheduled hours or with the battery at its floor
4. Timed discharge, outside the window

$$E_{after} = E_{before} - \frac{E_{chg}}{P_{DC}},\qquad E_{chg} = \sum_i c_i\,\Delta t \tag{1}$$ $$c_i = \min\!\left(p_i - P_{tgt},\ H_i,\ \frac{P_{bat}}{\eta_c}\right) \quad \text{when } p_i > P_{tgt} \tag{2}$$ $$c_i = \begin{cases} \min\!\left(x_i,\ H_i,\ \frac{P_{bat}}{\eta_c}\right) & x_i + F_i \ge H_i \\ \min\!\left(p_i,\ H_i - F_i,\ \frac{P_{bat}}{\eta_c}\right) & \text{otherwise} \end{cases} \tag{3}$$ $$c_i = \min\!\left(p_i,\ H_i,\ \frac{P_{bat}}{\eta_c}\right) \quad \text{when } p_i > 0 \tag{4}$$

where

Symbol	Meaning	Unit
$E_{after}$	the running total after this row	kWh/kWp
$E_{before}$	the running total before this row	kWh/kWp
$E_{chg}$	the energy taken to charge	kWh
$P_{DC}$	the system DC nameplate	kWp
$c_i$	the power taken to charge the battery at timestep $i$	kW
$\Delta t$	the length of one timestep	h
$p_i$	the system's grid injection before the battery at timestep $i$	kW
$P_{tgt}$	the interconnect target (storage.interconnect_limit_kw): the grid injection above which clip capture charges and up to which the battery discharges	kW
$H_i$	the charge power that fills the battery within timestep $i$: the room left below the state-of-charge ceiling divided by $\eta_c\,\Delta t$	kW
$P_{bat}$	the battery's power rating: it caps the power delivered on discharge and the power stored in the cells on charge	kW
$\eta_c$	the charge efficiency, one minus half of the round-trip loss	—
$x_i$	the surplus above the target, $\max(0,\ p_i - P_{tgt})$	kW
$F_i$	the surplus energy expected over the lookahead window after timestep $i$, divided by $\Delta t$	kW

Reading the number

A loss, negative. With clip capture it is the production above the target that the battery can hold: a few percent of the solar total for a system that often exports at its connection limit, under 1 % where the injection rises little above the target. It cannot exceed the injection above the target in a run without the battery (series p_grid_w with include_series), and a capacity above the largest daily total of that surplus is never filled. With daily full cycle or timed discharge it is close to one usable capacity a day. A battery of half the AC nameplate for 4 hours then takes about 730 h times the AC nameplate a year: a third to a half of the solar total for a utility plant, the larger share where the system yields less energy per kW of AC nameplate, and about half for a rooftop system with a low DC-to-AC ratio. A small battery, a high target or a battery kept full make it small.

References

1. DiOrio, Dobos & Janzou 2015, "Technoeconomic Modeling of Battery Energy Storage in SAM", NREL/TP-6A20-64641

API

storage.charged

Symbols follow the symbol registry.