Evacuated Tube Solar Collector

Evacuated tube solar collectors are very efficient and can achieve very high temperatures. They are however, very expensive compared to flat plate collectors or batch solar collectors. They are well-suited to commercial 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 to eliminate heat loss through convection and radiation. There are two main types of evacuated tube collectors:

Direct-flow evacuated-tube collectors

A direct flow evacuated tube collector consist of a glass tube, with a flat or curved aluminum fin attached to a metal or glass pipe. The fin is covered with a selective coating that transfers heat to the fluid that is circulating through the pipes, one for inlet fluid and the other for outlet fluid.

Heat pipe evacuated-tube collectors

Heat pipe evacuated tube collectors contain a copper heat pipe, which is attached 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 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 when the fluid in the heat pipe condenses and flows back down the tube. The process then repeats.

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 its fluid.

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

The liquid within an Evacuated-tube collector can get much hotter than in flat-plate collectors. The high temperatures they produce, can exceed the boiling point of water and can cause significant problems in a domestic solar water heating or solar space heating system. It's crucial to make sure there is always an adequate load on the system to keep the temperatures below 100° (212°F) and to mix the hot water that will be used for human contact.

The glass tubes are fragile, as they are made of annealed glass, which is more delicate than tempered glass. Care must be taken when transporting and handling the glass tubes.

Evacuated-tube collectors, unlike flat-plate collectors, the surface of which is always warm, do not shed snow. Because evacuated tubes are good insulators, little heat escapes them and the snow that accumulates on the tubes can stick for a long time. Snow can pack between the tubes, rendering them ineffective. The fragility of the glass tubes makes it impossible to scrape the snow off.