Why small-scale energy storage sucks. (but there is hope)

2015 has full of developments on solar energy helpings it competitiveness against other renewable’s and non-renewable’s combined. Ever since the price of photovoltic’s, mainly silicon based ones have dropped below 1$/Watt , suddenly the expensive world of solar supported by innovative financing schemes became accessible to anyone with a half decent credit score. With everything lined up, Tesla released the Power Wall a Lithium Polymer LiPo battery bank, that will allow you to store your excess solar production for use during the night. Why did Tesla do this? Well, the energy storage department has been a mess for a long time and is ripe for disruption. I’ll try to explain why below.

For those familiar with general solar systems this concept of using batteries to store energy for nighttime use has been there for decades. Off-grid solar systems can’t function without batteries and often enough the batteries are the critical component which fails first. The biggest difference between Tesla’s Power Wall and the status-quo today is basic chemistry and packaging. Currently the most common form of battery chemistry used is Lead Acid batteries. These might seem similar to what you’ll find in your car, but actually they are a bit different. For the most part Lead Acid can mean a lot of things, generally a broad range of battery constructions are classified under this term; everything from AGM, GEL , VRLA to tubular traction batteries. What differentiates these from your car battery is the internal structure, arrangement and what the electrolyte is sitting in. Car batteries are designed to give a lot of current quickly and then get charged by the alternator once the engine is on, hence they are called shallow cycle, meaning you don’t drain them too much. Shallow cycle batteries generally have the electrolyte fluid free flowing around the electrodes so they need to be held upright orientation.

In most solar systems deep cycle batteries are required, this where the later generations of AGM, Gel, VRLA batteries come in (https://en.wikipedia.org/wiki/VRLA_battery). These types of batteris are designed for deeper discharges and longer discharges cycles and require almost or no maintenance due. When I say designed this simply means that they can withstand it better than others. Though, still very far from perfect as they are extremely heavy and some pretty ugly characteristics , which is exactly why the Tesla power wall is here.

You see, anyone with a cellphone knows that batteries lose their capacity with time. Slowly with every charge and discharge something in the battery changes preventing it from keeping it’s charge as it did. Anyone who’s had a smartphone will know how their battery slowly holds less and less charge through use. This is called the cycle life of a battery. Each battery is born with a number of cycles that’s it’s rated to go through. With each cycle some of the chemicals go through a change that is irreversible (forming crystals, lattices, etc) Though the problem is that cycle life of the battery depends on a few things, most importantly the depth of discharge or DoD. A simple graph to illustrate how this works is below for a Lead Acid VRLA is below.

Keep in mind the scale is logarithmic. As you can see if you only drain your battery to 75% (meaning you only use 25% of your capacity) you get tremendously more cycle’s, then say using 50%. So when systems are designed for energy storage you generally want your batteries to last a long time (since they’re expensive) so you want the maximum amount of cycles for them, which means that you should only discharge them at most 20%. As you can see this is getting quite frustrating , as now simply to power you’re off-grid system at night you need to purchase 80% more battery capacity than you actually need. You are only ever tapping in to the tip of the capacity present. Even more scary, is that if you ever over discharge, meaning you hit 0%, the batteries will likely be permanently damaged at least one of the batteries in your bank. Just like the weakest link in a chain this can cause the whole battery bank to short and/or kill all the rest of your batteries with it.

Deeply discharged VRLA batteries in a solar health clinic in Rwanda

So we know now that with our batteries we need to be careful. So far, we’ve seen that if we want them to last we need to use very little of them. Though we want to keep the lights on at night as well, so there begs the question. How do you know how much batteries you need? This is a bit more difficult than it sounds. Lead acid (LA) batteries have an interesting property which makes calculating how much batteries you need a bit more difficult. Batteries are generally rated with a capacity of amp hours or watt hours (these 2 are interchangeable and Wh generally gives you a better picture since it already takes the voltage into consideration). The first thing that comes to mind is simply calculating the average watt hours used every evening and say getting batteries that will supply 80% more of that. Well, that’s a start. Lead Acid batteries have an interesting and unfortunate property that makes this a bit more complicated. The Peukert Law (https://en.wikipedia.org/wiki/Peukert%27s_law) dictates that if you drain the battery faster, the capacity of the battery actually decreases. A simple analogy is that if your battery is your cup of water, imagine the cup of water getting smaller if the straw you put in gets bigger.

So now you find that if you draw all your power suddenly, say because you’re heating some water, or washing some dishes, you need even more batteries as you’ve lost a significant amount of your capacity due to the high current draw. Suddenly you have all these batteries and you’re only using 10%. That seems oddly inefficient. The reality is generally a lot of cycles are fore gone and systems are designed for 50% DoD. Though even with 50% the lifetime is reduced by 30%. Though the sad truth is that generally these batteries will be long dead even before their full life cycle lifetime. Simply due to an extreme over discharge situation or improper use and care. LA batteries require certain charge characteristics and maintenance events like equalization in order from them to live through their designated cycles. Alas anyone that has a car knows that if you leave it in the garage for a few extra weeks and you might have some problems getting it stared again. This is similar to a few cloudy days that can drain a off grid solar system deeper, causing crystals to form on the plates preventing recharge.

Given the dire situation with LA batteries finally we got the Lithium’s that show up on the market. These are the basis of what makes smart phones and consumer electronics so powerful. This clearly didn’t happen overnight. It requires decades of R&D for the this culmination to happen. Lithium batteries provide more energy density per KG than LA batteries they are also much more resilient to deep cycles and offer a much better DoD cycle curve

Lithium's still are much more expensive to other battery chemistries. The specific chemistry I’ll be looking at is LiFePo4 which is a newer safer version of Lithium ions (https://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery)

If we do a simple Wh/$ comparison we can quickly see that the difference is stark.

Lithium Calb $/Wh = $0.40

VRLA $/Wh = $0.19

Though this is not a fair comparison, when the battery chemistry changes, it effects more than just power density. Lithium offer several advantages over LA batteries. Firstly, the Peukert component is close to 0, meaning whether you charge them or discharge them very fast their capacity does not change. Secondly, they have a much better lifetime and can be cycled deeper then LA. This is the feature, that is the most disruptive. Deeper cycles mean you need less of them to get the same amount of usable power. So when we do a true Wh/$ comparison this isn’t actually fair, we need to normalize this around $/Wh around how actually how many batteries are required.

To illustrate this clearly we can go over an example. Let’s say we need a system that will last 10 years and provide us 20KWh of usable stored electricity.

10 years roughly means 3500 cycles, looking at the graphs above for VRLA we would need to use around 20% DoD daily and for Lithium we could do 70% DoD. So given we need 20 KWh in total, the system size for VRLA would need to provide 20 KWh with %20 percent of it’s capacity so the bank capacity we need would be 100 KWh and subsequently for lithium we would need a 28.5 KWh bank. So if we normalize around the factor of the system size needed, we get:

Lithium Normalized = 0.41 $/Wh

VRLA Normalized = 0.61 $/ Wh

When we compare the normalized cost we see that this actually is much closer than it was before and lithium’s actually come out cheaper. This is incredibly exciting, as now we can do all sorts of interesting things like debt financing and actually leasing of battery systems.

Granted, lithium's also have their downsides, like sensitivity to over charging and discharging and needing more complicated charging equipment like battery management systems, balancer’s, etc. Also VRLA batteries are one of the most recycled products on earth ,while lithium’s are a different story altogether. The downstream recycle methodology is still to be figured out and finalized.

With the Panasonic, Tesla partnership we can predict that this will ramp up the production lithium cell’s drastically. It’s not too optimistic to bet seeing a similar collapse in Lithium cell prices similar to the one we saw with silicon solar cells. Batteries are essential to normalizing power distribution and solving one of the grids major problems. It’s through batteries we will get electric cars, motorcycles and somewhere in the future planes ( we already have drones). This is an exciting time for being in the renewable space as systems now can be built much more reliably and last for significant duration's allowing us to light up more and more of the energy starved parts of the world with renewable s.