Elon Musk announcing the PowerWallElon Musk, in an event that may be for battery technology what the announcement of the iPhone was to cell phones, announced the Tesla PowerWall last month in a speech that spoke of a coming dawn of grid-tied photovoltaic arrays with battery storage.

The vision, according to Musk, is an era where power utilities become optional and the average suburban home can realistically be grid independent. Don’t like the power company? Fine, cut the cord.

The excitement and buzz surrounding Tesla’s announcement means many people are interested in understanding the battery storage markets and available technology.  In this article, we take a quick trip through battery history, so that we can discuss the PowerWall announcement in context and provide insight about the future of grid tied PV + battery storage.

Batteries and Solar Energy Systems

Solar for School in Haiti

Historically, battery-based systems only made sense in extremely remote areas, such as this system that ReVision Energy designed for a hospital in Milot, Haiti.

Though it seems like ancient history in the context of the fast moving solar energy industry, it wasn’t that long ago that pretty much ALL solar electricity systems included batteries (and in many parts of the world, they still do).

Before the popularization of the grid tied inverter in the late 1990s, all solar energy systems were grid independent, which meant that if you wanted to have power after the sun set, you had to have a battery of some kind to store the energy, usually a big bank of ‘golf Cart type’ lead acid batteries.

Fast forward twenty years and, at least in the US, the vast majority of solar energy systems no longer use batteries at all and instead take advantage of the utility grid and net metering to export excess electricity when the solar production exceeds the load, and to import electricity back from the grid when the loads exceed production.

Grid tied solar electric systems work great and are a real win-win for both society and the solar customer. The solar customer benefit by having the advantage of using the grid as storage, and the rest of the grid benefits because solar customers typically export excess electricity to the grid at periods of high wholesale prices and peak demand, and import power at periods of low wholesale prices and low grid demand. This is a market benefit above and beyond the obvious environmental benefits of solar.

The benefit that net metered solar systems provide to the grid are well documented in a variety of Cost/Benefit analysis of net metering including Maine’s own Value of Solar Study.

Grid-tied solar energy’s rapid growth has been fueled by staggering drops in equipment cost, roughly 75% over the past 10 years. And though batteries have not experienced the same meteoric progress in performance and cost, many industry observers think it is only a matter of time. Investment in battery R and D is growing worldwide, thanks largely to surging popularity of hybrid and plug in electric vehicles, and the distributed solar industry is well poised to benefit from those investments and economies of scale created by that market.

Tesla’s PowerWall announcement illustrates this perfectly as Musk’s expectation is that the home energy storage market can ride the coat-tails of his planned Gigafactory, designed primarily to produce batteries for next generation Tesla vehicles.

How Batteries Work

Pika Energy Solar Battery Backup Supply

Schematic of Pika Energy Island system – courtesy Pika Energy

Whether it’s the Tesla PowerWall, cell phone, cordless drill power-pack or golf cart battery, the basic concept is the same: a charged battery stores electrical energy in chemical form for later use.

In most renewable energy applications, a special class of battery, deep cycle batteries, are required, as the constant discharge and re-charge of the battery requires more durable construction (in contrast to your car’s battery, which is used for a short burst of energy before it is quickly re-charged by your alternator).

In addition to being able to survive the wear and tear of frequent use and deep cycles, a renewable energy battery will also have a number of other important performance characteristics that determines how well it works in a particular application. Among those are:

  • Energy Capacity: Maximum electric usable energy (kWh) stored in a battery.
  • Maximum Discharge and Charge rates: The peak Power (usually given as maximum current) the battery can either provide or accept without damage.
  • Depth of Discharge: What % of the battery’s capacity may be used before it needs to be recharged.
  • Cycle Life: How many recharge cycles a battery can undergo before it reaches end of life.
  • Calendar Life: How long, in time, a battery can be expected to last before it reaches end of life.
  • Energy Density: How much energy is stored per unit of volume.
  • Specific Energy: How much energy stored per unit of mass. Energy density and specific energy are related, but distinct. For example, a battery with the same energy capacity nay be large and heavy (low energy density and low specific energy), large and light (low energy density but high specific energy), small and heavy (high energy density and low specific energy), or small and light (high energy density and high specific energy).
  • Temperature Limitations: The acceptable operating temperature range of a battery. Some battery chemistries may not operate below freezing temperatures, or at very high temperatures, for example. Others generate significant heat either while charging or discharging and thus may need an active thermal management system to keep them from overheating.
  • Self Discharge Rates: The rate at which a battery loses charge while not producing energy.

There is no one single battery which optimizes all these characteristics, and in fact even within a particular battery chemistry (Lithium Ion or Lead Acid, for example), battery designers constantly have to make tradeoffs between different characteristics; for example sacrificing maximum discharge rate to increase overall capacity, or trading off cycle life against calendar life. Or trading any of the above against cost. Clearly there is no such thing as ‘the best battery’, there is only ‘the best battery for a particular application.’

So Why is the PowerWall a Big Deal?

Tesla PowerWall battery specsThe PowerWall is a lithium ion (LiOn) battery, the same high power, high specific energy and energy density rechargeable battery technology that runs your cell phone or laptop computer. Up until now, LiOn batteries were disproportionately expensive relative to their lead acid counterparts, necessarily limiting the applications where they made sense, such as in portable consumer electronics or in electric vehicles where power and energy density are extremely important.

Perhaps the most eye-grabbing headline of Musk’s product announcement was the PowerWall’s price: $3,500 for a 10kWh battery. Though a lot remains unknown about this battery, and, keeping in mind that the headline price is for just the battery, not the required power electronics, the stated price is still exceptional, making it competitive with maintenance-free heavy duty lead acid batteries. Further, the PowerWall has several properties (less weight, higher power, and possibly higher cycle life) that are superior.

While the PowerWall’s price-point is game-changing for a certain niche of the battery storage market, Tesla’s much bigger vision is to roll out battery storage on scale throughout the country, and, undoubtedly, the planet. Their in-progress Gigafactory is the cornerstone of this – the plan is to drop the price on battery storage not so much through technology breakthroughs, but through manufacturing efficiency and scale.

This is what has happened in the PV market. PV cells today are essentially the same technology in use since the 1970s, but benefiting from vastly improved manufacturing processes. Tesla is aware that even at their impressive announced price per kilowatt-hour for their battery, the pure economic case for adding energy storage to a grid-tied solar electric array is not yet compelling enough to make it a no-brainer for the average residential solar customer. And so, they will work tirelessly to reduce that price.

Like the iPhone was at its time of release, the Tesla PowerWall is a milestone, and only the beginning of a new era of battery storage. It is not a new technology, but a better packaging and experience of an existing technology; as has been proven before, a product that has pizzazz, consumer appeal, and strong engineering to back it up, can radically transform the world.

Further Reading: Applications of Energy Storage and Emerging Technology

Solar PV System Yarmouth MaineThis article is derived from a three-part battery series we ran in our blog. Read on to learn about the applications of battery storage and solar (Part 2), or to learn about the technologies (Tesla, and others) driving the current innovation race in solar energy storage.