Solar Storage

As a homeowner in rural Southern California, you might think I’m nearly an ideal case for solar power. Throw a few solar panels on my roof, attach an inverter, and cut the umbilical cord to SDG&E (our local power company).

You would be wrong.

In fact, solar power is not yet practical (in the economic sense) as a total energy replacement. The problem is energy storage (more on this in a moment). It is possible today, under certain fairly restrictive circumstances, to economically justify solar energy as a supplement to the power grid. For instance, I might be able to justify a supplemental solar system designed to run my air conditioner. The analysis would include the capital expense (presumably financed) for purchasing and installing the solar system, compared with the estimated future cost of energy. The problem I have with the justifications for supplemental solar systems is that they only make sense when one assumes quite high future energy costs. You place a large bet, in effect, by buying (say) a $75K supplemental solar array — you pay (bet) $300 or $400 a month (on the loan) for 20 or 30 years, and hope that you save at least that much on energy costs. For me, that sounds like a very risky bet — the real payoff is just a few hot months per year, and then only if our energy costs remain at historically high levels. Of course, I might just be a chump who doesn’t realize energy prices are on some kind of inevitable upward spiral, but who knows?

The economic justification would be on much more solid ground if the solar system could completely replace the power grid. Earlier I cited energy storage as the culprit, and it’s a doozy of a problem. The person who comes up with an answer for this is going to make a lot of money! Here’s the problem: your house uses some power all day and all night. The pattern of power usage varies by individual, geography, lifestyle, etc. — but darned few of us would be interested in a home with zero power at night. But solar panels only generate useful amounts of power when there is bright sunlight shining on them. Most solar panels are mounted in a fixed orientation (e.g., on a rooftop) — and in that case, the panels generate much less power in the morning and evening than they do during the middle of the day. Fog, clouds, and rain all reduce the power generated. Less power is generated in the winter both because the days are shorter and because the sun is lower in the sky. The only way to provide power for your house all day and all night is to store the power (for example, in car batteries) during the day when the sun is bright, and use it from storage at other times. This means that your solar array must generate much more power (typically 5 to 10 times) than you actually need during the day to fill that storage — only in this manner will you have sufficient power to run your house day and night. And you really need more than one day’s worth of storage — you need enough to get through whatever worst-case assumption about weather you want to make. Here in Southern California I’d probably get away with four or five days worth of storage; in other places you might want more.

The least-expensive storage technology available today is a fancy version of the good old-fashioned car battery: a lead-acid cell designed for day-in, day-out deep discharge cycling. These batteries cost something like $0.15 to $0.20 per watt/hour of storage. Sound cheap? Suppose your home uses 8 kilowatts on average (that’s modest) when the sun is not at its peak. Further suppose that in the winter you have 16 off-peak hours per day, and you want 5 days worth of storage. That means you need 640 kilowatt/hours of storage — and that will set you back about $100,000, for something like 500 batteries weighing 10 tons. Yikes! You see what I mean about that being a problem?

And it gets worse. Your battery investment is not a one-time affair. The typical lifetime of such a battery is 3 to 5 years under ideal conditions — something few of us can practicably provide. Some batteries (more expensive) are maintenance free; others need regular checking and maintenance. All batteries have various ways to fail; some of these involve violent explosions, acid leaks, and other delightful scenarios. Oh, and also poisonous gas. Not to mention the disposal problem of a few tons of lead each year. So it’s not really a $100,000 expense — it’s more like $20,000 a year (forever!) and a not-insignificant amount of hassle and risk.

So I’m still connected to the grid. But I’d sure like someone to solve the energy storage problem!

I was reminded of this entire issue by a very interesting article about a solar energy generation system that uses the ocean as a storage mechanism. This idea generates power through the temperature difference between the sun-warmed surface water and the cold deep water. The idea is not new, though making it practical is. In my mind, the primary benefit of this approach is that it completely solves the energy storage problem. The bad news is that this won’t work in my back yard <smile>!