Evacuated Tube Solar Collector

Evacuated tube solar collectors are very efficient and can achieve very high temperatures. They can be very expensive compared to flat plate collectors or batch solar collectors, though some of the Integrated Tank Collectors are the cheapest to purchase and operate as they don't need most of the Balance-Of-System components. Evacuated tube solar collectors are well-suited to commercial (laundromat) and industrial heating applications and can be an effective alternative to flat-plate collectors for domestic space heating, especially in areas where it is often cloudy.

An evacuated-tube collector contains several rows of glass tubes connected to a header pipe. Each tube has the air removed from it (evacuated) to eliminate heat loss through convection and conduction. Inside the glass tube, a flat or curved aluminum or copper fin is attached to a metal pipe. The fin is covered with a selective coating that transfers heat to the fluid that is circulating through the pipe. There are two main types of evacuated tube collectors:

Direct-flow evacuated-tube collectors

A direct-flow evacuated tube collector has two pipes that run down and back, inside the tube. One pipe is for inlet fluid and the other for outlet fluid. Since the fluid flows into and out of each tube, the tubes are not easily replaced. Also, should a tube break, it's possible that all of the fluid could be pumped out of the system - if a closed loop is used, or your water will flow out as in a broken pipe, if an open loop is used.

Heat pipe evacuated-tube collectors

Heat pipe evacuated tube collectors contain a copper heat pipe, which is attached to an absorber plate, inside a vacuum sealed solar tube. The heat pipe is hollow and the space inside is also evacuated. Inside the heat pipe is a small quantity of liquid, such as alcohol or purified water plus special additives. The vacuum enables the liquid to boil at lower temperatures than it would at normal atmospheric pressure. When sunlight falls the surface of the absorber, the liquid in the heat tube quickly turns to hot vapor and rises to the top of the pipe. Water or glycol, flows through a manifold and picks up the heat. The fluid in the heat pipe condenses and flows back down the tube. This process continues, as long as the sun shines.

Since there is a "dry" connection between the absorber and the header, installation is much easier than with direct flow collectors. Individual tubes can also be exchanged without emptying the entire system of it's fluid and should one tube break, there is little impact on the complete system.

Heat pipe collectors must be mounted with a minimum tilt angle of around 25° in order for the internal fluid of the heat pipe to return to the hot absorber.

Integrated Tank Solar Collectors

Where temperatures are not likely to drop into the freezing zone, many evacuated tube solar collectors are made with an integrated storage tank at the top of the collector. This design has many advantages over a system that uses a separate standalone heat-exchanger tank. With the tank separate, you are required to operate solar controllers, water pumps, expansion tanks, etc. All this extra equipment can greatly increase the cost of the system. The separate heat exchanger tank can also be the single most expensive component in your system.

With the tank included in the design, water flow is controlled via your standard household water pressure. The reduction in electronics not only reduces the cost but also reduces failure points and operational complexity.

Globe Solar Energy has recently introduced a model to the Canadian market (yes that's in an area with freezing temperatures), which has governmental approval to be included in their ecoAction program. This unit comes with a built in heating wire for the outside piping and a controller to turn the heater on if needed. The cost of this simple system is half of the cost of a closed loop glycol system. This product could greatly increase the acceptance of solar water heaters in cooler regions of the globe.

Other considerations with evacuated solar collectors

Evacuated-tube collectors can get very hot, exceeding the boiling point of water and can cause significant issues in an existing domestic solar water system. You need to use your hot water every day to ensure the temperature doesn't overheat in the tanks. Mixers are easily installed just after the last hot water tank and mix your regular (cool) water supply with the hot water, to ensure the temperature never exceeds a set limit.

Notice on the picture above, the black hose leading away from the pressure relief valve on the top of the tank. After only a single day without use, this tank could exceed the temperature and pressure levels, blowing hot water onto the lawn and killing the grass. The black pipe was added to safely take the hot water away to the drainage system. On a ground mounted panel, you can also cover the unit before leaving the house for an extended period. Be careful of the tubes when covering them, as they are made of annealed glass, which is more delicate than tempered glass. You need to consider overhangs (slide ice and snow loads), falling branches, etc., when selecting a location for your panels.

Evacuated-tube collectors don't heat up like flat-plate collectors. Thus in the winter, they won't melt large quantities of snow that fall on them at one time. It can be very difficult to clear the snow from the glass tubes without breakage, be careful. Of course, heat-pipe evacuated tubes will not impact the system if they break - you just have one less tube to heat the water. If a direct-flow tube breaks, you have a problem!