Sunday, July 18, 2010

Heat engines

Driving over the Mount Harris "pass" from Dixmont to Jackson yesterday I had an all-too brief flash of cosmic understanding.

This saddle between hills is 900 feet above sea level, while Jackson spreads out several hundred feet lower. On a good day you can see all the way to Acadia National Park, where the First Family is currently on vacation.

But what I saw yesterday wasn't the President and family, or even Air Force One which must have flown overhead at some time or other, but photosynthesis, on a grand scale. The Maine woods are in full leaf, and Jackson, indeed all of northern Waldo County, is of course just one big broad leaf forest, with a light scattering of dwellings and hayfields here and there.

A hazy, green, abundant, massively productive forest countryside. A New World Sherwood.

Yeah, Robin.

Under the shade of the trees, this time of year, it's much cooler than it is in the sunlight. Buggy, but cooler.

So, an obvious science geek question is, where does all this abundant solar energy go to if it's not reaching the ground under the trees? At this time of year more than a kilowatt of energy is hitting every square meter every hour the sun shines. That's like having a two-bar electric toaster oven in each square meter.

This of course makes for a lot of heat anywhere the sun's light hits the ground.

Sunshine energy is primarily ultraviolet light: invisible moving photons. Some of this light energy is reflected back to space as visible light in that part of the spectrum that humans experience as green.

Very green, in this case. The whole of Maine seems green, this time of year.

Some of the energy heats the air, which moves the energy around a bit. The more humid the air, the more energy the air can hold, the more energy the air moves around. This is why shade doesn't work as well, and why thunderstorms are more frequent, on humid days.

But an awful lot of energy is being absorbed by the leaves of the trees and used to combine carbon dioxide with water to make sugars and cellulose. I'm not sure how much, but enough to help make it feel much cooler under the trees.

A square meter of solar panels properly positioned on a day like this will capture about 15% of this energy in the form of electricity.

But I wouldn't cut down trees to put up solar panels. That would be a waste. Trees are probably much more efficient at collecting sunlight and reducing carbon levels than solar panels are. That doesn't mean to say I wouldn't cut down trees, though.

Here's our Womerlippi Farm woodpile, about four cords, 70% white or gray ash. At 24 million BTUs per cord, and 3.4 million BTUs per MWH, that's 7 MWH per cord, or 28 MWH total.

These four cords were harvested from an area less than 20 meters square. That's only 400 square meters. And, although the cutting was heavy, big trees were still standing on that 400 square meters when I was done cutting. No tree cut was older than 15 years, and most were around 10 years old.

28 MHW divided by 400 square meters is 700 kilowatts per square meter for the whole time period the trees were growing.

700 kilowatts per meter divided by ten years is 70 kilowatts per square meter per year.

Our square meter of solar panels, by comparison, will collect 200 watts per hour, average 4.5 hours per day, 365 days per year, or 328 kilowatts per meter per year.

So solar panels are more efficient, right?

But that doesn't count the energy taken to make the panels, while the trees grow without this help.

Nor does it count the energy in leaves, which in this case were cycled through sheep, although in the fall the remaining energy in the leaves of the trees still standing will cycle through the soil.

So this is all very philosophical, isn't it. What do I think about all this?

I think I'm glad to have my firewood cut, split, covered and drying for winter.

And I'm glad to live in a beautiful Maine forest.


CoolClay said...

Good stuff, and very interesting comparison Mick, obviously completely theoretical, but I like it!

Mick said...

Theoretical? Not when the cold weather comes! Then we'd better have enough wattage stacked outside, right?