Evacuated tube collectors
Figure 10-4 shows the construction of an evacuated tube solar collector. A
row of glass envelopes (tubes) have had the air removed, which creates a
vacuum and makes for excellent insulation. Copper rods inside the tubes are
physically connected to a massive copper tube inside the enclosed header,
through which water (or other liquid) flows. The copper rods get very hot on
a sunny day, and the heat moves directly into the copper mass in the header,
and from there into the fluid. When reflectors are located behind or around
the tubes, these collectors are even more effective.
These collectors are impervious to freezing conditions and work very well,
even when the air temperatures go below zero. They are more expensive,
however. That being said, prices are coming down because so many are
being built that economies of scale are kicking into high gear.
People can also make these systems with fluid-filled tubes inside, instead of
rods. Then you have a direct, batch-type evacuated collector (see the preced-
ing section). Look for more of these in the future.
Evacuated tube collectors work well in windy and wet conditions that would
normally cool down other types of collectors. This type of system doesn’t
freeze and is extremely well insulated, yielding effective heat storage. In fact,
the rods can reach temperatures exceeding 250°F while the glass envelopes
are cool to the touch. But they don’t shed snow, and they’re way too fragile
to try to brush the snow away or scrape ice off.
These collectors perform better than other types of collectors on cloudy
days, and on sunny days, they work for more of the daylight hours because
the tubes are always perpendicular to the sun.
Evacuated tube collectors are fragile; if the seal breaks, performance is poor
because the vacuum is compromised, and insulation is poor. On the positive
side, it’s very easy to change a single element if it breaks. With most collectors,
breakage means removing and changing the entire collector.Producing energy can be dirty work. Carbon emissions, coal slurry,
nuclear waste, and other pollutants can wreak havoc on the environ-
ment, cause health problems, and make people hopping mad. And many
energy sources are in limited supply, particularly fossil fuel sources that have
traditionally dominated the world’s energy usage. Not only does that drive
prices up, but it also leads to political conflicts when people decide they’re
not willing to share. You’re probably not ready to go completely unplugged,
but you do want to play your humble part to save the environment, help the
country become less dependent on foreign energy sources, and save money.
Tall order? Maybe not. Above all the energy sources in use today, solar
shows the most long-term promise for solving the world’s energy problems.
Solar power works well on both large and small scales, and it is possible
to start using solar power right away. You can start small, and work your
way up.
On any given day, 35,000 times the total amount of energy that humans use
falls onto the face of the Earth from the sun. If people could just tap into a
tiny fraction of what the sun is providing each day, society would be set. Of
course, some problems do crop up, but they’re solvable, and going solar can
be well worth the effort.
To understand the role solar energy can play in your home, you need to
have a good understanding of where your own energy comes from, where it’s
used, and how much pollution each of your energy sources generates. In this
chapter, I explain how solar fits into your day-to-day life — and why it’s such
a good energy option.Erasing your carbon footprint
Most energy resources are burned in order to create the useable energy we
so take for granted. The worst offender, in terms of pollution, is coal, and the
United States gets around 50 percent of its electrical power from coal-fired
power plants. Put simply, there’s no clean way to burn coal, and that situa-
tion is not likely to change in the next decade, or even longer. While you hear
a lot of talk these days about “clean burning coal” plants, it’s all relative. A
clean-burning coal fired power plant is simply cleaner than those that exist
now; in the grand scheme of power generating systems, coal is one of the
worst offenders in terms of greenhouse gas emissions.
Your carbon dioxide footprint is a measure of how much carbon dioxide
you’re releasing into the environment by virtue of your energy-consuming
habits. A typical American carbon dioxide footprint is around 36,000 pounds
(18 tons) per year. That’s a lot!
Solar, however, has no carbon footprint, other than the energy is takes to
manufacture a solar panel (referred to as grey or invested energy). For each
kilowatt-hour (kWh) of energy-generating capacity you install with solar, you’ll
save that much from other sources, most likely the electrical power grid. No
other alternative energy resource can offer this impact except wind power
and hydro, but solar is far more versatile and widely available. Wind power
is practical only where there’s a lot of wind (which doesn’t include all the hot
air blowing around in Washington, DC). Hydropower is only available where
suitable water resources are abundant, and hydropower disrupts the environ-
ment in ways that have resulted in the environmental movement frowning
upon further hydropower development. (Plus all significant hydro sources in
the U.S. have been tapped, anyway.)
Your carbon footprint is valuable for calculating cost versus gain for install-
ing solar systems because — face it — even though pollution isn’t costing
you directly in your wallet, you need to factor it into your thinking.
When you generate solar electricity, you don’t need transmission lines and all
the associated inefficiency. Solar is right there, where you use it. When you
install a 3kWh active solar system, you’re offsetting the need for that much
power from your utility company. But you’re saving about 9kWh of total
power consumption because of inefficiencies in the power grid. Therefore,