There’s finally been some good news for a change: According to a recent analysis performed by market research company IHS, the market for photovoltaic storage systems is about to boom. In Germany alone, the total capacity is set to increase from eight to 4,900 megawatts (MW) between now and 2017. The success of storage systems is dependent on one condition, however: They must be able to pay for themselves within the estimated 20-year lifetime of a solar plant, as well as operate reliably in basements throughout the year.
What could make more sense than drawing on proven lead-acid batteries or the more modern lead-acid gel batteries? They are less expensive than the new lithium-ion batteries and boast a decisive advantage in terms of conforming to safety standards, having been used for many years to supply power without interruption in buildings such as hospitals. Unlike lithium-ion batteries, they are backed by well-founded empirical data. Figures on the lifespan of lithium-based technologies, their potential cycle lives and residual capacities after a particular amount of time are based on relatively short aging tests under extreme conditions, the results of which are then extrapolated to 20 years. “There is currently no alternative to lead-acid batteries,” states André Haubrock, the manager responsible for business development at the battery manufacturing company Hoppecke, which is headquartered in Brilon, Germany.
However, more and more suppliers of photovoltaic storage systems are using lithium-ion batteries thanks to their technical potential being significantly greater than that of lead-acid batteries. They are capable of storing more solar power in a more compact space, as well as promising a longer lifespan and higher efficiency. According to data from the battery expert Eric Maiser at the German Engineering Federation VDMA, lead-acid batteries start to lose some of their capacity after around 3,000 complete charge and discharge cycles, while lithium-ion batteries optimized for stationary applications are capable of completing 7,000 full cycles before experiencing such losses. “Lithium-ion batteries grapple less with capacity losses caused by wear and tear,” explains Maiser. Both anode and cathode are each wrapped in metal foil, with the cathode foil containing porous graphite. During charging, lithium migrates through an electrolyte into the graphite pores, where it absorbs electrons that it then releases during discharge. The advantage of this is that the electrolyte only serves as a means of transport and does not take part in the chemical reaction.
In lead-acid batteries, on the other hand, the electrolyte is slowly depleted of its dilute sulfuric acid. During discharge, this acid reacts with the two lead electrodes to form lead sulfate. When the chemical process is reversed during recharging, small quantities of the lead sulfate do not redissolve, progressively weakening the battery. The greater the depth of discharge, the more the battery is weakened. In light of this, manufacturers advise against discharging batteries completely. The problem caused by sulfation is less pronounced in the more modern lead-acid gel batteries, in which the sulfuric acid is fixed in the form of a gel. However, these batteries also show signs of the type of electrode corrosion typically seen in lead-based batteries and must generally be exchanged after 10 to 15 years.
As a result, the operators of lead-based systems incur additional costs at a later date that are not expected with the more robust lithium-based technology which, according to manufacturers, has a lifespan of at least 20 years thanks to higher cycling stability. “This compensates for the initial higher purchase price of lithium-ion batteries,” says Maiser. What’s more, the technology may actually become significantly less expensive in the future. In contrast to lead-acid batteries, only very few standardized manufacturing processes have to date been introduced for lithium-ion batteries. According to a joint study entitled Energy Storage Revolution conducted by the consultancy Roland Berger and the VDMA, using standard modular batteries lowers manufacturing costs by 8%. Savings stem above all from the fact that using standard components reduces the amount of materials required and increases cycle times.
The industry is currently also developing more robust and higher-performance electrode materials. Today’s lithium-ion batteries use graphite for the anode and lithium metal for the cathode, which serves as a chemical reactant for the graphite. Manufacturers want to use new anodes made from lithium titanate in the future, which charge faster and can withstand more charge cycles than graphite. According to Maiser, technical progress and economies of scale arising from larger quantities being produced may result in the overall costs of lithium-ion batteries falling from the current amount of roughly €800 per kilowatt hour (kWh) to €250 per kWh over the years to come, leading to the technology reaching a price level equal to that of lead-acid batteries. In contrast, the latter’s development potential has almost been exhausted. Does the classic battery stand a chance against lithium-ion technology in the future?
This article was originally published on Solar Energy Storage and was republished with permission.
Lead image: Starting line via Shutterstock