The solar power industry has given plenty of attention to battery systems lately, and there is a very strong reason for this. Utility-scale solar farms can now produce electricity at a lower cost than conventional power plants fired by fossil fuels, but photovoltaic panels still have a key disadvantage: inability to deliver power at night, and limited generation capacity during cloudy days. This limitation disappears if solar power is complemented with batteries, but they are still held back by their high upfront cost.
In the case of commercial solar power, batteries allow companies to rely even less on electricity retailers. They can simply size the solar photovoltaic array larger, and accumulate surplus generation for hours when sunlight is limited or unavailable.
Although there are many types of batteries, two technologies dominate the market: lead-acid batteries have been used for decades in the automotive industry, and have also been deployed to complement solar photovoltaic systems in remote locations. On the other hand, lithium-ion batteries have been used mostly in electronic devices, and the technology is now being adapted for the energy industry – Tesla batteries are perhaps the best-known example. This article will compare the main characteristics of both types of batteries.
As you might expect, lead acid batteries are the most affordable, in great part because they have been in the market longer and the technology has had more time to evolve. You can expect an upfront cost of around AU$250 per kWh of storage capacity for lead acid batteries, but this is increased to AU$500/kWh for lithium-ion. Although price changes depending on the manufacturer and economies of scale, you can expect the upfront investment to be 2-3 times higher for lithium-ion batteries.
However, price is the only advantage of lead acid batteries. They are outclassed by lithium-ion batteries in practically all performance aspects. In addition, if you deploy a commercial solar array through a Power Purchase Agreement and include lithium-ion batteries in the contract, you can avoid the upfront expense.
Despite their lower upfront cost, lead acid batteries can tolerate much less discharge cycles than lithium-ion batteries. Although the actual number of cycles varies depending on the manufacturer and the demands of the specific application, most lead acid batteries last for less than 1,000 cycles. On the other hand, commercial lithium-ion batteries typically have a rated service life in the range of 3,000 to 5,000 cycles.
Although the upfront cost of lithium-ion batteries is higher, their life cycle cost is actually lower. For example, a lead acid battery with a price of $250/kWh and rated for 1,000 cycles has an operating cost of $0.25 per kWh per cycle. On the other hand, a lithium-ion battery priced at $500/kWh and rated for 4,000 cycles achieves an operating cost of $0.125 per kWh per cycle, which is 50% less!
Lead acid batteries must be fully charged between cycles, or they start to experience a chemical reaction called sulfation, which reduces their charge-holding capacity. This is not a problem with lithium-ion batteries, making them a better option to complement commercial solar power systems. Photovoltaic arrays don’t always produce enough surplus energy to fully charge the batteries.
When dealing with batteries we tend to focus on the storage capacity, but the power output is equally important because it determines how much load the battery system can handle at once. Keep in mind that a higher power output results in charge being depleted faster. Lead acid batteries are very sensitive to discharge speed, and their effective capacity is much less when they are discharged fast. On the other hand, lithium-ion batteries can tolerate faster discharge rates without a drastic reduction of their effective capacity. This makes lithium-ion batteries better suited for demanding applications where a high power output is required in a short time.
The round-trip efficiency of both battery types is also an important aspect to consider. Lead-acid batteries typically have an efficiency of around 80%, which means you get 8 kWh back if you store 10 kWh. On the other hand, most lithium-ion batteries have a round-trip efficiency of around 95%.
Lithium-ion can also tolerate an increased depth of discharge (DoD). Most lead acid batteries are rated for a DoD of only 50%, and suffer a drastic reduction of their service life beyond than point. On the other hand, most lithium-ion batteries can deliver their rated service life at 70-80% DoD.
Another key difference between both battery types is how voltage behaves as charge is depleted. Lithium-ion batteries are capable of sustaining a stable voltage output throughout their entire discharge cycle, only experiencing a reduction at very low levels. On the other hand, the voltage output of lead acid batteries decreases gradually in proportion to how much charge is left.
So far we have compared lithium-ion batteries and lead acid batteries in terms of electrical performance. However, there are also important differences in size and weight. For a given energy storage capacity, a lithium-ion battery system only has one-half of the volume and one-third of the weight of a lead acid battery array.
The reduced volume and weight of lithium-ion batteries is a significant advantage in transportation, since vehicles consume less energy to carry the weight of batteries, and more compact vehicles are possible if the space requirement of batteries is reduced by 50%. In fixed applications, such as when lithium-ion batteries complement commercial solar power, the weight advantage is less important but they still save plenty of space.
Lead acid batteries offer an advantage in terms of upfront cost, but lithium-ion batteries have a lower lifetime cost while offering superior performance. Consider that lithium-ion batteries also have more flexibility during operation, allowing them to handle the variable energy generation of commercial solar power systems. Large lithium-ion batteries are already being deployed throughout Australia by energy retailers and large industrial users: the Hornsdale Power Reserve was the largest battery in the world at the start of 2018, delivered by Tesla and rated at 100 MW and 129 MWh.