Solar Panels and Battery Storage – Heaven or Hell?
Anybody who reads the green press on a regular basis has seen countless articles extolling the virtues of combining solar panels with storage batteries. While there’s no question that adding battery storage increases the flexibility, utility and value of solar panels, I have a hard time getting past the threshold question, “Is the game worth the candle?”
One of the biggest issues with any battery-based energy storage system is that batteries degrade over time. Part of the degradation arises from the imperfect nature of reversible electrochemical reactions. No matter how good the chemistry is, each charge-discharge cycle results in a slight loss of electrochemical potential. The balance of the degradation arises from age. It’s one of the main reasons that the adjective most commonly associated with the noun battery is “damned.” Our batteries never have the cycle-life we think they should have and they usually need replacing at the least opportune moment.
Performance degradation issues are well known in the battery industry, but they’re poorly understood outside the battery industry. In most cases, it’s difficult to find reliable data on what the expected degradation rates will be over the service life of an energy storage system. In mid-April, however, Greentech Media published an article that discussed Tesla Energy’s Powerwall warranty for customers the Australian market. With a little sleuthing, I was able to find the Powerball Manufacturer’s Limited Warranty Certificate (USA). Exhibit A to the warranty specifies the following minimum capacity retention levels:
1. The Product shall maintain >85% of its initial rated capacity until the earliest to occur of:
A.The lithium-ion battery cells in the Product have reached 4 MWh of aggregate discharge throughput (at the battery DC output); or
B. 2 years have expired from the Original Installation Date.
2. The Product shall maintain >72% of its initial rated capacity until the earliest to occur of:
A.The lithium-ion battery cells in the Product have reached 9 MWh of aggregate discharge throughput (at the battery DC output); or
B. 5 years have expired from the Original Installation Date.
3. The Product shall maintain >60% of its initial rated capacity until the earliest to occur of:
A. The lithium-ion battery cells in the Product have reached 18 MWh of aggregate discharge throughput (at the battery DC output); or
B. 10 years have expired from the Original Installation Date.
So Tesla Energy starts with a battery capacity of 7 kWh and rates the product at 6.4 kWh to leave room at the top and bottom of the state of charge range to avoid a 100% charge or discharge, both of which are very hard on batteries. It then builds in capacity retention factors to protect it from the inevitable battery degradation. In graphic form, the minimum capacity retention warranty looks like this.
If I needed 7 kWh of energy storage, I would not be terribly excited with a system that offered 6.4 kWh off the shelf and promised 4.6 kWh after five years and 3.8 kWh after ten years.
Traditionally, the battery industry has used a 20% capacity loss as the definitive end-of-life metric because most batteries lose capacity rapidly once they cross the 80% threshold. It’s my understanding that lithium-ion batteries do lose capacity more slowly than other chemistries, but I don’t have a deep enough knowledge of the technical aspects to judge whether the Tesla Energy’s capacity retention estimates are conservative or aggressive. In fact, I’m not sure that Tesla Energy knows the definitive answer in 2016 because current estimates are based on computer modeling rather than real world performance data over a 10-year service life. Since human beings are far more clever than computers when it comes to abusing their stuff, I’m not willing to assume that the current estimates will prove to be realistic over the long term.