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The Light is Green




The Story of Solar & Thermal Systems

Long ago, back in '86, when a new state-of-the-art silicon cell topped the efficiency scale at 20%, I started thinking about methods of applying this new material to engineering plastics by vapor deposition.  The idea was to cover roofs with inexpensive sheets of cell material.  My associate Joe Mancinelli had experience with engineering plastics and solar degradation, and our research pretty much concluded that it just wasn't going to pay with the materials available at the time.  Oh well, it was an interesting and enlightening (pun?) experience.  I think electricity cost less than a nickel per kW-Hr.

I had yet to buy my first PC, but had installed them for others.  I had heard of electronic-mail, but the internet only really existed in the computer science departments of colleges and universities, a few big companies, and some military installations.  Little did I know the PC was about dominate my career for the rest of the millennium ...


Fifteen years later, early 2001, One day at lunch...

Thinking again about how much energy was being wasted, simply heating up the pavement and the roof, (yes, this is the kind of stuff I think about at lunch) the question of how much $$$ the sunlight falling on the ground was worth, and could that free flux be harnessed as electricity came to mind.

I was recalling the work of years before and a recent electric bill.  Having seen solar panels popping up more and more, I had reason to expect that the 20% efficiency threshold had become the norm rather than the academic state-of-the-art.  Recalling the often recited value of insolation 1kW·m-2 (that's 1000 watts per square meter), and the $0.10 to $0.12 per kWHr, I did a quick back of the envelope calculation arriving at the figure of $0.25·m-2·day-1.  

I stood up, stretched my arms (I could do that then), and told my lunch buddies that out of that Quarter, I bet I could pickup a Nickel.  Some chuckles ensued, the notion of 5˘ a day inspired humor in the group.  Out comes the trusty tape measure.  Back inside, the numbers feel too high; they don't seem right, but how can they be wrong?  This is one of those moments when you realize that the population in general is "mathematically challenged".  Remember, these are folks who understand calculus, state machines, and picosecond propagation effects.  

But the scope of my statement wasn't immediately understood.


The Story Behind the STS

So on average each square meter is intercepting about 3 kWHr (~980 BTU per sq ft) per day.  It's spread out, peaking at about 900W at noon, and somewhat less in each hour on opposite sides of lunchtime, and falling off rapidly as we depart from local solar noontime.  That's not the way I use energy (or is it?).  What to do?

There are two basic types of energy consumers residential & commercial.  Homes don't use a lot during the day, businesses do.  Homes use less power during the day than they could generate, businesses use far more.  There's got to be a way to get surplus power from homes to businesses.

Technically speaking, if we hookup to the power lines, the meter will turn in the direction of energy flow.  The reality is tainted by the business office at the power company.  Lately there has been news of utility co.'s and homeowners in court about their right to generate their own electricity.  A set of laws with the friendly name of "net-metering" are becoming prominent.  The utilities cite safety, but the suspicion is that $$$ is the reason for the monopoly reticence to accept energy from private sources at government mandated prices.  Very few folks had even heard of Enron at this time.

So the technical solution of dumping the power onto the grid, eliminates the need to try to store the energy in batteries, or hot water, or any other local device.  We can push as much energy as we're able back out the mains.

All that's needed is a technology to convert sunlight into electricity...


The Story Behind the STS

Today if you Google "Solar Energy" you'll get 43 million hits, back in 2001 the result was a far more modest.  If you were to repeat the exercise, in a short time you'll learn that there is a short list of viable solar energy technologies, broken down into two broad categories: Passive & Active

  • Passive technologies include things like south facing windows, skylights, agriculture/aquaculture, plantings (trees, bushes), rain gutters, etc.
  • Active technologies include photovoltaic (solar cells or PV), and solar-thermal.  These also come in two basic varieties, straight and concentrating.

After some extensive browsing, I realize that PV still doesn't have the conversion efficiency, is way too expensive.  In fact I encounter many articles that point out that the energy needed to process the silicon, melt it, saw it, heat treat it, solder, encapsulate, etc. can be more than the energy that the cell could produce in a typical 20 year lifetime.  That's called an "energy sink".  I want an energy source.

More research yields a technology developed in the western desert, it's called concentrating solar-thermal, and it uses a huge parabolic mirror to concentrate the sun and make steam.  Anyone whose ever picked up a magnifying glass knows you can start a fire by focusing the suns rays on a tiny spot.  So they focus the sun onto a special boiler, and hook it up to a conventional steam turbine.  Acres and acres of tracking mirrors, and a huge concrete tower, fine in the desert, not so easy in southwestern Connecticut.

But wait, it gets better.  They also work on another technology, it's called the parabolic trough.  Not only does it work, it works big!  Megawatts of power, thousands of homes supplied.  Instead of a tower and an army of free standing tracking mirrors, they curve huge glass/ceramic panels into highly polished and silvered mirrors that focus the suns rays onto a slender tube with a special non-radiating coating.  These troughs are long, about 150 feet, and they are very heavy too, the mirrors are 1/2 inch thick glass, but they aren't a hundred feet in the air. Inside the tube they pass a special oil.  The oil gets very hot almost 500 degrees.  That hot oil is then pumped to a boiler where is again makes steam that powers a conventional turbine.

Now, that's a lot of hardware, and even worse, even very hot water can freeze during a cold New England winter night.  There's got to be a better way!



The Story Behind the STS

OK what do I know?

  • The total peak flux of sunlight on my roof at home is about 50kW
  • Radiation loss is proportional to T4 .
  • Carnot efficiency is proportional to delta-T
  • Compounds with hydrogen bonds have high specific heats
  • NH3 won't freeze even at the poles
  • NH3 is compatible with iron and aluminum, but not copper.
  • Ammonia is cheap, can be (and is) poured on the ground
  • Chlorofluorocarbons are bad for the ozone layer
  • The speed of sound in CFCs is very low
  • Expansion ratios in steam turbines is restricted because wet steam eats blades
  • Pistons can be used to pump fluids
  • I know I can harness 10% of that energy with black paper, and a bicycle pump
  • I know I can do better with a purposely designed system

What else did I need to learn?  Quite a bit!





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Updated: 09/11/10 19:34