Monday, July 7, 2008

Green Snake Oil?

In the news today: Toyota is planning on putting a solar panel on their Prius.

Sounds good, doesn't it? An electric hybrid that's able to be partially powered by the sun.


How are we sure that its not essentially just a clever sales gimmick?

Here's one take on the "Reality Check" of physics.

A conventional solar panel (Mitsubishi; Sharp) provides 185W of output, and is 65" long x 33" wide: call it roughly 5ft x 3ft. Let's assume no sunroof, so you can install two of them on the roof of a car like a Prius.

Those two panels result in roughly 370W of power under ideal conditions. Let's assume that there's another 10% of roof real estate available and round up the number to a conveniently easy to work with 400W of output.

The article (see link) reports that the A/C system needs up to 5kW of power. Standardizing units, that's 5,000W.

Thus, the panel can provide (400W/5,000W) = 8% of the A/C system's peak demand.

Thus, you can run your A/C "totally on solar" if you only run it at ~8% of its capacity. That doesn't sound like all that much cooling capacity to me.

Okay, let's look at things differently: let's assume that the solar panels can charge the battery pack, even though this isn't mentioned in the article. While we're also at it, let's be an irrational optimist and assume that this storage will be 100% efficient and has no other trade-off factors (such as needing a bigger battery).

So how much power can we store? Classically, solar systems are discussed and designed around the time of day which results in roughly 80% of the total daily collection. This is commonly defined as 10AM-3PM, which is a period of 5 hours. True, there is solar gain before 10AM and after 3PM, but generally it isn't planned for too much, as the sun angle, strength and shading factors are some of the factors for why the panels run at below their ratings and all of these other hours of daylight only collect around ~20%. This is classical Pareto Principle.

So in those 5 hours of good gain, the panels will run at their rating, so this sytem will generate 5hr * 400W =2,000W-Hrs.

Continuing to assume that idealized 100% efficient storage battery, this means that the system can provide 0.4 hours (24 minutes) worth of full A/C per day. For a to/from work commute, that's 12 minutes each way...assuming that you park your car in the sun at work and don't go out to lunch.

Okay, it is better than nothing, but do keep in mind that those panels aren't free. Their retail price is around $2,000.

And more importantly, they aren't weighless. In standard trim, they're just over 35lbs each. Figure that with packaging economies, they can be reduced from 70lbs down to around a 50lb increase in vehicle curb weights. Now ask yourself: an increase of 50lbs in the vehicle's weight hurt the car's fuel economy by how much?

And ditto for that idealized 100% efficient battery. To do 2000 W-hr worth of power storage, you're roughly going to need a battery roughly the size/capacity of a standard automotive Lead-Acid battery. In NiMH, I'd SWAG it at another $500 and 25-30lbs increase in curb weight.

This is just a quick "back of envelope" system analysis, but it looks like at least a 50% salesmanship snake oil for a 'Green' product to me at this point. I'd personally like to see a more comprehensive one done with real values, to specifically include how much the vehicle's cost goes up, weight goes up and MPG goes down (because of that higher weight).

Don't hold your breath.


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