May 2018

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Evacuated Solar Tubes

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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.


The HS-VAC1.8 is the solar absorber. 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.


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 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

Structure All-Glass Double-tube co-axial structure
Glass Material High borosilicate 3.3 glass
External pipe diameter/thickness 58mm, +/- 0.6mm; T=1.6mm
Internal pipe diameter/thickness 58mm, +/- 0.6mm; T=1.6mm
Pipe length 1800mm
Coating Structure Cu/SS-ALN(H)/SS-ALN(L)/ALN
Sf*dime»nt method 3 Target magnetron sputtering plating
Specific absorption As=0.93~0.96(AM1.5)
Emission ratio eh=0.04~0.06(80°C+/-5)


 Vacuum tightness  P<=5.0*10~3Pa
 Idle sunning property parameters  Y=260-300m2 7KW
Solar Irradiation for preset water temperature H<=4.7MJ/m2(O58)
H<=3.7~4.2 MJ/m2
Solar irradiation for preset water temperature H<=3.7MJ/m2(047)
H<=2.9~3.2 MJ/m2
 Average Heat Loss coefficient  ULT= 0.4-0.6 w/( m2 °C)

Solar Hot Water Systems

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Solar Hot Water

Solar hot water heating systems use the free heat from the sun to provide your hot water requirements, be this for your home or your business. During the winter and at times in the springs, autumn and even summer, it is necessary to additionally heat the water with a conventional boiler or immersion heater. However, when solar energy is available the amount of fuel needed for this purpose is dramatically reduced or removed all together.

How do they work?

  • Solar water heating systems use panels, called Collectors that are fitted to a roof or mounted free-standing on flat roofs or even in flower beds. South facing, pitched 45° angle positions are best but Collectors can be east or west facing. Two distinct types of solar water heating panels are commercially available.
    • Flat Plate Collectors. These comprise an insulated frame with a glass cover in which sits an absorber plate and pipework. The Collectors can be fitted in the roof or on the roof, which is the easiest method for retrofit installations. More radiation is absorbed if the cover glass is made from iron-poor safety glass as this allows light in the short-wave spectrum to pass through. The shorter the wave length the more energy is in the light. Flat Plate Collectors are less efficient than Glass Evacuated Tube Collectors and do not work well in winter.
    • Glass Evacuated Tube Collectors. These are also called Solar Tubes and use the principle of the vacuum flask to efficiently capture the suns rays to heat water all the year round. Far more efficient than Flat Plate Collectors, each tube concentrates heat in its pure copper tip. The tip of each tube is inserted into a manifold where the heat is transferred to the water. Solar Tubes will heat water to over 60°c in strong sunshine when the outside temperature is -30°c. They also work better than simple Flat Panel Collectors in east-west orientations. Larger Glass Evacuated Tube Collector systems can se used to heat or supplement the heating of a home or business but mainly in summer and sunny days in spring and autumn.

When installing Collectors other factors such as the condition of the roof, shading, aesthetics and accessibility need to be considered when choosing where to locate a Collector.

  • An indirect circulation system is used to pump the heat transfer fluid (mixture of water and antifreeze) from the Collectors to the heat exchangers in the hot water cylinder or other heat dump zones. Some indirect protect themselves from over heating to stop the glycol fluid from becoming super-heated when demand is low and solar radiation is high.
    • Antifreeze. The indirect circulation system is pressurised and uses an expansion vessel. The heat transfer fluid comprises a non-toxic food grade glycol-water mixture with the concentration of glycol dependant on the likely minimum temperature. As the system is pressurised, the boiling point of the heat transfer fluid is much higher than that of water.
    • Drainback systems. These use pumps to push the water through the Collectors when there is solar radiation. When the pumps stop the water drains into a reservoir tank so there is no water in the Collect to freeze in cold weather. Fine in theory, but these systems are more difficult to install and get right so that the piping always slopes downward to completely drain from external area that can freeze.
  • Storage of hot water is the next consideration. Here, a Solar Cylinder needs to be installed to store the water heated by the sun. For most central heating systems this will mean replacing the existing gravity fed hot water cylinder. Modern combi-boilers have no hot water cylinder but flash heat mains pressure water on demand. This is a more difficult installation but be achieved by using valves to separate the two systems.
  • The important point here is to make space the largest Solar Cylinder that can be accommodated. The thermal efficiency of these means that water heated the day before yesterday can often be used today; or at worst act as pre-heated water that only needs to be topped up.

Why install solar hot water heating

  • Cut fuel bills. Once the initial installation cost is paid the hot water costs will be slashed.
  • Cut your carbon footprint. Solar hot water from sunlight is a green and will reduce carbon dioxide emissions.
  • The cost of gas, electricity, oil and coal is likely to rise faster than interest on savings and wages.

Cost and payback

  • The Energy Savings Trust says the typical cost of solar water heating system range from £3,000 to £5,000.
  • Savings are good, although the Energy Savings Trust just say “moderate – a solar water heating system can provide about a third of your hot water needs”. This depends on some extent to where you live in the UK and how much hot water you use. A guide would be 50-80 litres per person per day. From our experience a solar heating system provides nearer to 70% of hot water needs, which is double the modest claims of the Energy Savings Trust – and of course there’s the CO2 emission savings too.

Things to consider

  • The Energy Savings Trust suggest you will need a minimum of 5 m² of space for a solar hot water collector system, although two of our 2 m² glass evacuated tube Collector seem to be fine for most households.
  • Solar heating needs a sunny position for the Collectors. South facing is best but east to west will work with two panels, one facing east and one facing west. The installation of this configuration will be more expensive, but the parts costs from SolarSave are around the same.
  • Planning permission is not normally needed but it’s always best to consult your local planning authority if you live in a Listed Building, or a building in Conservation Area or World Heritage Site.
  • Collectors don’t always have to be mounted on a roof, they can be fixed to a frame on flat roofs or even in the garden. If you have a heated swimming pool then they are seriously worth considering – providing you have the space.