Speed of Dark Blog

So, here’s a bit of technology that had, until today, evaded my alternative-energy gaze:  The Evacuated Tube Solar-Thermal Collector.  Composed of a group of double-walled vacuum sealed glass tubes, these devices act like the thermal heat pipe in your computer:  Heat energy is applied to the length of the tube, and a fluid evaporates and travels up the tube to the cooler end, where it condenses, transferring its heat to a Single channel of water.  That’s really the key to its efficiency. The system is under a vacuum, so there is no convection of conduction loss through the actual collector surface.  The water receiving the heat energy only travels across the very top, and is heavily insulated.

The system has very low heat loss when not in sun, and is effective clear through the winter due to the vacuum design:  Light passes through the vacuum between the tubes and strikes the inner tube, which is coated with a dark material that absorbs the majority of the light and converts it to heat.  That heat is not infra-red, and so does not travel back out of the system.  The evaporated fluid then does its job to move the heat to the top, where the water runs along well protected from the elements.  The round shape also keeps the tubes facing the sun at optimal angle, no matter what position it’s in.  I found a good report documenting the installation and monitoring of an industrial installation, to check it out by following this link.  With a decent amount of sunlight, the system will operate and save on energy bills clear through the heart of winter.  And the report I just linked to calculated over 30% efficiency for the entire system.  Far better than a photovoltaic system could ever dream.   And, did I mention, these are cheaper?  Quite a bit, actually.  A do-it-yourselfer could really make out a deal on a system made of these and see a return in little over a year.  Add some sort of reflector behind the tubes to reflect any waste light and your efficiency will be even higher.  And the report doesn’t even take into account that the heat it absorbs isn’t absorbed by your roof, possibly lowering your cooling bills as well.  Honestly, other than Nanosolar’s exciting new product, I don’t see anything that comes close to this in simplicity, cost effectiveness, and efficiency.  I’m not sure why this isn’t more commonly pointed out as an option to people looking to alternative energy sources.

Now for my obligatory crazy idea.  These units are efficient.  Really efficient.  More efficient than will ever be used during the summer, when the sun is the most intense and is out longer during the day.  There has been some work done on using R134A in an Organic Rankine Cycle Turbine to make use of low-temperature  (under 100 C) geothermal wells.  The technology is also used to generate electricity off of the waste heat at industrial sites.  These are big generators, of course, but the basic concept is, you run a refrigerator backwards like a turbine.  You can probably easily tell where I’m going.  The big point is, you have to have 100 degrees Fahrenheit difference between the sides of the generator for there to be enough of a pressure gradient to drive the turbine efficiently.  In a home application of the solar collectors above, summertime would be the highest yield time, while being the lowest utilization.  In the winter you could heat your home with the output during the day.  In the summer, the sun is out longer, and much more power would be produced.  A smaller turbine, for a 5 or 10 kW generator, could make use of the massive amount of energy the solar collectors would yield and convert it to electricity which would be used to power the home.  The big key would be to see if a 35 percent efficient solar heating system would be powerful enough to offset the costs of an R134A 10 kW turbine generator and make it worthwhile 6 months out of the year.  It would be a grid-tied unit, of course.