Earlier this year, NREL highlighted a trend which seems to be one factor driving increased interest in the energy storage sector. To put it simply, “as `net metering rates decline, the economics of using residential PV-plus-storage systems for self-consumption improves,” and if we can broadly store and distribute renewable energy, we can further approach the goal of a 100% renewable future. However, the fact remains: batteries are expensive, and their operation and maintenance is complex.
As far as our wallets are concerned, NREL estimates that on average, consumers pay about $560 per kWh of storage capacity, and that for the off-grid solar pioneer, a battery bank represents about 60% of total, upfront system costs.
The high upfront cost of a battery bank might not be so bad, if batteries could boast the same 25 to 30 year lifespan as other solar hardware, but they can’t. They require periodic replacement, and that replacement schedule drives up the levelized cost of electricity (LCOE) for the system owner. NREL has yet to publish on lifetime costs of solar + storage, but at Konza Solar, we have found that the owner of a stand-alone solar system will need to replace his or her battery bank about four times, over a 25 year system lifespan.
The number of times system owners will need to replace their battery banks depends on several factors. But the most common contributor to short battery life, for both lead-acid and lithium-ion batteries, is frequent and deep discharge. Even under ideal conditions and constant vigilance, battery systems degrade, but as a general rule, the deeper the per-cycle discharge, the shorter the lifespan of the battery.
And this is where solar tracking enters the scene. It is in this complicated middle ground, between an array of solar modules and a battery storage system that dual-axis solar tracking shines as a technology which benefits both sides of the expression solar + storage.
At Konza Solar, we have compared battery bank longevity in systems that use dual-axis tracking and fixed arrays, and we find that over the lifetime of a solar array, dual-axis tracking cuts total battery requirement by as much as half, while increasing net module output by about 1/3. The numbers are surprising. Yet the reason for this dramatic benefit is perfectly sensible.
Each replacement of a battery bank increases the lifetime system cost, or LCOE, reducing the cumulative payback of the system. The resulting loss of potential returns is substantial.
The Konza Solar Tracker, however, allow modules to produce peak energy as long as the sun is shining, so batteries are not solely responsible for supplying electricity outside the roughly 4.5 hours that a fixed array is producing. Additionally, south-facing fixed arrays do not produce energy during peak usage times - morning and evening.
A fixed array positioned for maximum harvest will briefly reach peak output at midday. However, morning and evening output is low or nill. This means that a battery bank has to work harder for longer, during peak usage times, relying entirely on energy harvested in the middle of the day.
This is a huge drain on batteries, and it means that the individual, business, or community with a fixed array + storage has two options.
One, they can oversize their battery bank (higher upfront and replacement costs), or, two, they can drain the batteries more deeply, reducing the lifespan of the batteries (more replacement cycles). Either way, those people can end up purchasing twice as many batteries as they would need with a tracker.
It is clear that as battery storage becomes more prevalent at every scale, from residential to community to utility, dual axis tracking should become more prevalent as well. The numbers are clear.
As we explore new ways to improve solar modules and storage, dual-axis solar tracking will assume an ever more important position between those two technologies. The simplest way to think about the dynamic between solar production and solar storage is to remember that the whole point is to provide energy when energy is needed.
As much as possible, energy storage should be used to supplement power produced by renewable sources and used at the time of production. Basically understanding the relationship between energy production and the longevity of battery systems, tells us that solar tracking just makes sense, at every scale.
And in order to increase renewable energy’s share of the energy landscape now, we must attempt to release as much or more energy from stored and real-time renewable sources as we release from fossil fuels by burning them. Dual-axis solar tracking optimizes both energy production and storage economics.