Choosing the SPS pool size based on application requirements

Choosing the SPS pool size based on application requirements

The SPS pool size should be determined on the basis of assessing the typical daily cycle energy requirement of the system, and the extent to which this daily energy demand varies over different days.

If the daily discharge energy requirement is the same every day, and the discharge duration is relatively short, then there is little merit in using SPS mode. Just ‘right size’ the array of RUN mode batteries and be done with it.

A scenario where SPS mode offers significant benefits is for sites where there is a variation in the amount of energy discharged each day.

There may be a typical/common ‘lower’ daily total discharge need on most days, but with some days (perhaps unpredictable in advance) where a larger amount of energy has to be delivered before the array is recharged.

(As an example, consider an off-grid site that spends some of its time with no occupation, and some of its time with people present, operating energy demands related to their presence).

The optimal SPS pool size is generally determined in this ‘variable demand’ case by leaving enough ‘RUN’ mode batteries to handle the minimum daily demand, with the rest in SPS mode.

Once the system has charged/discharged through enough maintenance cycles to establish the SPS pool and for the SPS batteries to hibernate, the minimum daily demand can be handled by the RUN mode batteries and the SPS batteries will all be ‘asleep’.

On days when the energy demand turns out to be higher than the energy stored in the RUN mode batteries, the BMS can dynamically dispatch SPS batteries to deliver more energy as required. The BMS can be configured to define the point at which that dispatch of those batteries occurs, and to determine how many SPS batteries are woken up at each dispatch event.

(Note: The history of these dispatch events over time can be viewed in the BMS Event Log)

In some sites, there can be merit in seasonal adjustment of the SPS pool size based on observing the real world results.

The core advantage of using SPS mode in a hybrid pool scenario is that the SPS mode batteries, once hibernated, have no self-discharge rate and present no energy demand to keep their own internal pumps running. On a large array, significant ‘parasitic load’ reduction can be achieved in this manner.

One other application for SPS operation of relevance is ‘long duration storage’ applications, where a large array is required to deliver a (relatively) smaller output rate over a (relatively) longer timeframe In this scenario, most batteries can charged and hibernated. The BMS can then sequentially dispatch SPS batteries to generate an extended, yet high efficiency, discharge sequence over a long timeframe.

Another scenario is ‘100% SPS’. If configured, this turns the system into a ‘virtual standby genset’. The entire array is charged and hibernated and sits in standby. If the primary energy source fails, the entire ZBM array can fire up to deliver its energy to bring the site back in to operation until the primary energy source gets re-established.

The ‘100% SPS’ scenario works right down to the edge case of a single ZBM in SPS Mode on a small site (such as a cellphone tower site) as a short duration ‘grid outage’ backup energy source.