Evacuated Solar Tubes

Solar Collectors using glass evacuated tubes, called Solar Tubes, are the most efficient Collectors available and are particularly suitable for the UK and other northern European countries. Their efficiency is such that they heat water all year round working even on cloudy days.

SolarSave standardises on its 1.8 metre, 58mm diameter Solar Tube (part code HS-VAC1.8). The following text is the product specification for the HS-VAC1.8.

General

The HS-VAC1.8 is the solar absorber within the SolarSave 15 Solar Tube Collector. It absorbs solar energy converting it into heat and is a "twin-glass tube". This type of tube performs reliably and is a component part of tube type solar water heating panel. Fifteen HS-VAC1.8 tubes are mounted on a vertical incline into a heat exchanger to form a solar hot water Collector.

Construction

Each HS-VAC1.8 consists of two glass tubes made from extremely strong borosilicate glass. The outer tube is transparent allowing light rays to pass through with minimal reflection. The inner tube is coated with a special selective coating which absorbs the solar radiation with minimum reflection whilst a second coating reflects the infra red heat inwards. The top of the two tubes are fused together and the air within the space between the two layers evacuated. This "evacuation" process forms a vacuum, which is an important factor in the performance of the evacuated tubes.

The HS-VAC1.8 has similar properties to a glass lined vacuum/thermos flask. The vacuum is an excellent thermal insulator. This is important because once the evacuated tube absorbs the radiation from the sun and converts it to heat, the heal loss from the tube needs to be minimised. The insulation properties are so good that while the inside of the tube may be 150°C, the outer tube is cold to touch.

To maintain the vacuum between the two glass layers, a barium getter is used (the same as in television tubes). During manufacture of the HS-VAC1.8 this getter is exposed to high temperature which causes the bottom of the evacuated tube to be coated with a layer of barium metal. The barium absorbs any CO, CO2, N2, O2, H2O and H2 produced from the HS-VAC1.8 during storage and operation, thus helping to maintaining the vacuum. The barium layer also provides a clear visual indicator of the vacuum status. The silver coloured barium layer will turn white if the vacuum is ever lost, making it easy to determine if a tube needs to be replaced.

How it works

The process by which solar radiation is converted into heat is as follows.

The HS-VAC1.8 has an excellent selective absorption coating which produces an efficient method of heat generation. This heats the inside skin of the glass tube where the heat is transferred through conduction and radiation to the centreing metal heat fins and the central copper tube. This tube contains pure water with anti corrosion additives. The inside of the tube is at a reduced pressure so that the water boils at around 30°c. The hot water vapour rises to top of heat pipe where the heat is transferred to the circulating water, the vapour condenses and drains back down the tube.

As the HS-VAC1.8 has less parts to absorb heat before any meaningful heat transfer can take place, it starts producing heat more quickly than flat solar panels, even on cloudy days. It can even produce heat on sunny days at -30°c. It also is more resistant to frost damage and scaling. Each HS-VAC1.8 works independently of other tubes in the same racking system. This means that if a tube is damaged, the other tubes are unaffected and the system still produces solar energy.

The HS-VAC1.8 performs better than flat plate solar collectors in cold weather because it minimises heat transfer losses to the water.

Solar water heaters using the HS-VAC1.8 are aligned in parallel; the angle of mounting depends upon the latitude of the particular location. In a North South orientation the tubes can passively track heat from the sun all day. In an East West orientation they can track the sun all year round. In general, the efficiency of a HS-VAC1.8 system depends upon a number of factors, one important one being the level of solar in the region.

Basic Specifications of the HS-VAC1.8