DC Coupled Energy Storage

Given common inverter loading ratios of 1.25:1 up to 1.5:1 on utility-scale PV (PVDC rating : PVAC rating), there is opportunity for the recapture of clipped energy through the addition of energy storage. Using a simplified system for illustrative purposes, consider a 14MWDC PV array behind a total inverter capacity of 10MWAC. Depending on your location and type of racking, the total clipped energy can be over 1,000,000 kWh per year. Without energy storage these kWhs are lost and revenues stunted. With storage attached to the array, the batteries can be charged with excess PV output when the PV inverter hits its peak rating and would otherwise begin clipping. This stored energy can then be fed into the grid at the appropriate time. Note that this ability to capture clipped DC output is only possible using a DC-coupled storage system.

Clipping is a phenomenon where the DC-AC PV inverter has hit its peak AC output and therefore must drive the PV DC array voltage off of the maximum power point in order to effectively curtail the PV array. It is not possible to move or shunt this power to an AC-coupled battery system because doing so would force the PV inverter to exceed its rating to pass any excess PV energy onto the common AC bus.

Using a DC-coupled storage configuration, the DC-DC converter charges the batteries directly from the DC bus with the excess energy that the PV inverter cannot use. In the simple example of Figure 2 where there is a 1MW AC inverter with a 1.4MW DC array, during times when the PV array is producing greater than 1MW DC, excess energy can be used by the DC-DC converter to charge the batteries, then discharged later when the PV output is low or when there is a peak demand on the grid for kWh production.