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Thursday, November 4, 2010

Vacuum Tube Water Heater


Solar evacuated tubes AKA solar vacuum tubes are pyrex (borosilicate) glass that does not crack or break when exposed to radical temperature changes. This glass is crystal clear, very strong and have been used for years in China. Evacuated tubes offer an inexpensive method for efficiently transferring the sun's power to heat water. These tubes have a clear outer layer with a dark inner absorber layer (selective coating) containing a vacuum between the glass walls.

Just like the internal double glass walls of a traditional glass thermos, the vacuum (lacking air molecules) in the solar tubes prevents heat from escaping into the atmosphere. Unlike z glass thermos, solar tubes are clear and not mirrored allowing sunlight can penetrate the first layer with a 90%+ efficiency as sunlight is absorbed on the dark inner layer.

The entire unit requires the tubes to be exposed to pressurized water in a traditional setup. Because the glass cannot withstand pressure exceeding 20 PSI, these heaters incorporate a copper spiraled core that is placed inside the length of the tube connected to a nipple (heat pipe condenser). Only this copper nipple, which gets very hot, is exposed to water pressure. These copper nipples can withstand several hundred PSI. This setup of heat transfer reduces the units overall efficiency.

For the backyard scientist, the copper core is not needed. By filling these tubes with water and setting them out in the sun, water boils with no additional Reflectitve Film, Fresnel Lens, or Parabolic Mirror in about an hour. Adding any of the above increases boiling times dramatically.

Our video with a Parabolic trough "Solar Water Heater Steam Parabolic Mirror Trough"I used a black steel pipe as the absorber. Light refracted can be seen throughout the video due to heat loss into the atmosphere dramatically reducing efficiency. Touching this pipe with your hand would be an instant mistake! By replacing the black pipe with a solar vacuum tube in the same design, the outer glass tube would be warm to the touch "Solar Water Heater Steam Parabolic Mirror Trough".

Solar tubes come in a variety of lengths and diameters. They are a great tools for the solar backyard scientist and are used in many professional jumbo scale installations. The concept can boost efficiency of your project by 50% saving you time, money and unit footprint dimensions. These tubes are also a great teaching tool for science teachers.

One thing to remember; NEVER leave these tubes in the path of a Fresnel Lens, Parabolic Reflector or in the sun EMPTY. The tubes can build extreme heat exceeding the tolerance of the black coating and adding water to a 300c tube will shock even the best borosilicate glass.

Dan Rojas

UNDER $200 PV Solar Panel Installation


by Dan Rojas

Written for

A popular question is regarding "DIY make your own solar panels". Many people see websites offering this information and assume it is a "from scratch process". The question is actually a bit deceiving and similar to "building your own computer". Making "solar cells" is a complex process using a form of silicon and can best be equated to "making an Intel Processor", not a likely DIY in your garage or backyard task. On the other hand, making a "solar panel" is more like "building a computer" where a pre-manufactured processor is combined with a custom ready to install hard drive etc.

To "build your own solar panels", you can buy pre-manufactured solar cells off the internet, buy the ribbon to hook them together, add a blocking diode, seal them behind glass and save 50¢ to $2.00 a watt.

The "Harbor Freight Model", a 45 watt system with everything you need to get started is about $200 and well worth the expense, if you are getting your feet wet for the first time. This kit is designed to charge 12 volt batteries and comes with a charging controller, two 12 volt compact florescent bulbs and a stand for your roof or yard. All the basic wiring is included. I like this kit because it is durable and produces enough power to charge batteries at a reasonable rate and direct power medium sized DC motors for demonstrations. These kits are also expandable.

So how do you get the power to your house? Your house runs on Alternating Current or AC power so an inverter that converts DC to AC is required. The process is pretty simple. Solar panels hook to the controller and the controller hooks to a deep cycle marine battery. You now have 12 volt DC ready to go. Next, you take the inverter and hook it to your battery. For simple installations you use an extension cord no more than 50 feet in length. This setup is usually good for applications requiring 200-500 watts or less. There are 12 volt inverters that can produce over 3000 watts but these at a full load will drain a single battery in less than an hour. The 45 watt Harbor Freight model charges a drained mid-range deep cycle battery to near full capacity in about 12-24 hours of daytime light. So for more power, you need more panels, and for a longer charge, you need more batteries.

Installing solar directly into your house's electrical circuit breaker requires advanced professional electrical advice and installation. I have consulted a few electricians on the basics and they used their expertise to complete the project offering a solar only single outlet, by combining two of the 45 watt Harbor Freight Models. Remember, this type of installation requires a skilled electrician.

Even if you are using a simple extension cord for a small power application, keep in mind you are dealing with electricity and the risk of getting shocked or a fire hazard exists if done improperly. Read all the instructions that come with the kit you purchase and seek professional advice if you are in doubt. This system offers a low cost entry level solar alternative that produces usable power without draining your wallet.


Harbor Freight Model introduction 45Watt Solar Panels
Assembly HF 45 Watt
Wiring Solar and DC information P2 Solar Series
Wiring Part 3
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Dan Rojas
Heat a Swimming Pool with a Large Fresnel Lens.

I receive viewer mail weekly asking, "With all that heat produced by a Fresnel Lens, can I point a lens at my swimming pool a few hours a day and heat it?" The simple answer is no. Water is clear and any light collected from the Fresnel lens will simply pass the focal point and continue into a wide pattern exiting as reflective light bouncing off the bottom of the pool. The big misconception regarding any magnifying glass or 
Fresnel lens it the belief that they "magnify or amplify the sunlight" when all they really do is concentrate it.

"What if I focused the lens on a dark object just below the water surface?"

While that would be much better, you would transfer more heat to your pool if you simply got a black object the size of your lens and sunk it to the bottom, no lens needed. Square footage or square meters are what counts when it comes to sunlight. A one meter by one meter (39" x39") Fresnel lens concentrates sunlight to produce roughly 1000 watts of heat energy to a very small point. A sheet of heavy black plastic absorbs the same amount of solar energy but over a large area. Since plastic or almost any material transfers heat effectively when submerged in water, the 1M x 1M black plastic sheet would add 1000 watts of solar heating. The sheet is heating water at the bottom of the pool so you are taking advantage of the thermal layer above. Several black sheets can add a lot of heat assuming sunlight reaches the bottom of your pool. Many pools do not get direct sunlight to the bottom especially in the winter months due to the angle of the Sun.

I know of a person who incorporated 300 Fresnel page magnifiers into a solar blanket for a pool and tested it against a normal clear solar blanket. He was thinking of applying for a patent but was confused with the negative results he was generating. The non-Fresnel blanket was better insulated so it kept the heat in. The Fresnel Lenses basically did nothing other than provide an escape route for heat. I am not sure what came of his project.

If you are set on using a Fresnel Lens, the best way is to create a small copper loop and circulate the pool water through it with either a small solar pump or tapping the pool's pump system. The advantage is that the lens is not blocking light that falls on the pool so you are adding 1 square meter off ADDITIONAL LIGHT to the side at a high transfer efficiency. You will still need to track the Sun with a heliostat.

This is true for large water tanks also. Unless the Fresnel lens is larger than the water tank, focusing light does nothing assuming the entire tank is painted black. Mirrors are a different story. Because mirrors redirect sunlight, they do not impede the direct light falling on the tank so each additional mirror adds sunlight based on whatever size it is. Again, a Fresnel lens would be best suited away from the tank circulating fluid, in this case, as a passive system.

The easiest way to heat your pool is described in the three videos below:
Heating a Swimming Pool Solar Hot Water Part 1
Solar Hot Water Part 2

Check out our other videos at
Dan Rojas


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