Evacuated Tube Solar Panels

The advantage this design has over the flat-plate type is that the constant profile of the round tube means that the collector is always perpendicular to the sun's rays and therefore the energy absorbed is approximately constant over the course of a day. Ideally the panel wants to face south, so it can catch light all day, however it will still function well if it is at South East or South West. When this is not possible, it is recommended to have two panels, one facing East and the other facing West.

Although they work best in direct sunlight evacuated tubes do not rely on it to function. They work much like a thermos flask. In a double walled tube there are two layers of glass which has a vacuum between them. This is shown on the diagram below with the light grey area.

Cross section of an Evacuated Solar Tube

cross section of an evacuated solar tube

 

The inner tube is treated with a heat absorbing coating. The quality of the coating here is important. Check what compound is used by the manufacturer. Aluminium is one of the best. The vacuum prevents heat from escaping. The glass on a solar tube that is functioning properly will feel completely cold as it should be transferring all the heat captured to the heat pipe and up to the manifold or collector. Within the copper pipe will be a liquid to assist heat transfer, as shown above.

In most cases metalic fins run down the side of the copper pipe to transfer heat from the inner glass tube to the copper. Some sort of conductor here is crucial for efficiency. Like the copper pipe, they should run the full length of tubes. Anything shorter will result in a less efficient tube. 

The copper tip of a good quality solar tube can reach temperatures of up to 220 degrees centigrade if it is left in the sun. However, a correctly functioning solar panel system will never allow this to happen. 

The image below shows the layers of coating applied to the inner tube. In most cases the darker the tube is, the better.

evacuated tube coating


 

A complete evacuated tube solar panel - with the manifold cut-away

20 x evacuated solar tubes mounted into a manifold (also known as header or collector). The manifold has been cut away to show the internal copper pipe work (which the tubes push into) and the insulation around it to prevent heat loss. This is typically the hottest part of a solar panel.

complete panel with manifold cut away

 

1. Sunlight hits the panel, is absorbed and trapped by the vacuum tubes. 

2. Heat travels up the inside of the tubes to the copper pipe within the manifold.

3. Cool water from the return of the hot water cylinder enters the panel.

4. This water is heated as it travels along the panel, leaves and enters the hot water tank's coil where it acts as an element to heat the water within.

inside a manifold

A manifold with the outer casing and top layer of insulation removed to reveal an evacuated solar tube plugged into the copper header pipe within. From this you can see that at no point does the water in the panel come into contact with the tubes. This is why if a tube is broken or removed, the panel won't leak.

Things to check when buying an evacuated tube panel:

- The copper heat pipe runs the entire length of the tube. This will allow for maximum efficiency as the heat absorbed by the glass will be conducted all the way along. Some companies cut costs and corners by having shorter copper heat pipes within the glass tube. Remove the tube and bung from the top to check it. Beware of any tube that has the copper heat pipe sealed in! If you cannot remove it to check it, it is best to steer clear. A removable copper pipe also means that if a tube is broken, the inner copper and reflectors can be re-used on a replacement, which saves time, money and resources.

good/bad copper heat pipes

The tip of the heat pipe. This works best if it is tapered as all the heat is condensed to one area for better transfer to the manifold header. It also helps resist freezing which can split the copper. Although this is not essential, it is recommended and should be something to look out for.

Thickness of the copper pipe. The thicker the better. Lower grade tubes only use around 0.4mm thick copper. For best results look for 0.7mm or higher. 

Welding. Although this may be hard to check yourself, the manufacturer or supplier should be able to tell you what method is used for welding the copper pipe. Silver is ideal, although it is more expensive, it is far less prone to damage from either freezing or overheating. Check the weld used, then check it on the periodic table to see its melting and freezing points.

Glass. The glass on a solar tube should be checked, by the manufacturers, under a polarised lens for stresses and strains. These can then be repaired by applying heat. The glass should also be tested against hails stones.

Broken Tubes: 

What happens if I break a tube? - One of the benefits of an evacuated tube solar panel is that if a tube breaks, the panel will still function, although at a reduced efficiency. With flat plates, if it gets broken it is rendered useless and is a difficult and expensive job to replace. A solar tube is quick, easy and cheap to replace. The water in a solar panel system should never come in to direct contact with the solar tube and so breaking or replacing one will not cause a leak. Be very wary of any evacuated tube solar panel on which individual tubes cannot be removed or replaced!

Sometimes a breakage in a tube will be very obvious. However, any compromise to the tubes vacuum will reduce its efficiency significantly and this might not be easy to spot by eye or touch. Good quality tubes have an indicator as to whether the vacuum seal has been compromised. Silver is good and undamaged. Cloudy or White and the vacuum has been compromised.

undamaged/damaged evacuated tube

These tubes have a patented gas in the vacuum which turns the tip of the tube silver. If there is a leak in the vacuum, this gas escapes and turns the ends a cloudy white colour. Check this on all tubes when recieving a delivery. Transportation is the number one cause of damage to solar tubes.

manifold cross section

This cross section image shows the basic materials which make up a solar collector

The internal header pipe will nearly always be copper. However the insulation type and outer casing may vary.

Rock or glass wool are a particularly good material as a small amount can keep a lot of heat in. In the early days of solar panels there were restrictions in terms of planning permission as to how high a panel could pertrude from a roof. The insulation in the header often determined how big the panel was, so it had to be kept as small as possible but at the same time prevent heat loss. Check what insulation is used in a manifold before you buy. For further information on the rules and regulations on solar panels, see our Planning Permission section. 

Where do Solar Tubes come from? 

China have been producing Solar Panels for many years and are probably world leaders in design and manufacturing of evacuated solar tubes. However Germany, Greece, India, Italy, Turkey, America and the Czech Republic also have a fairly large Solar Panel industry. It really doesn't matter where your tubes come from. What is most important is the standard to which they are made. People who argue that tubes from China or a long way overseas are not eco-friendly due to the transportation costs, are quite frankly incorrect. Inferior tubes from any overseas manufacturer are a waste of money and resources. There is no point shipping a product halfway around the world that is not going to perform well or have a long lifetime. Good quality solar products should last a long time (25 years+), work efficiently when the sun is out and be fairly maintenance free. If this is the case they will soon pay for their own transport cost by reducing the need for gas and electric heating.

Beware of any company that claim to manufacture their panels in the UK. More often than not they only manufacture a very small percentage in the UK so they can make this claim. The rest tend to come from China or one of the other big solar countries. That being said, some companies do go the extra mile and manufacture 100% of their stock in the UK.

If the country of origin is important to you, then ask your supplier where they are from. But in most cases you should look at quality first.

Any respectable Solar supplier should have data available on the cost in terms of Product Emissions of the manufacture and transport of their equipment.

Sizes and types of Solar Tube 

Solar tubes come in all shapes and sizes. Most of them are differentiated by their diameter or length. The image below shows some of the variation in sizes. The right hand tube is a single walled evacuated tube and is built slightly differently to the twin walled tubes described earlier. The metalic top and heat pipe cannot be removed, unlike with the double walled tube as they preserve the vacuum. This means if it is broken anywhere, the entire tube must be replaced. This type of tube is considered more efficient than the other two, as the finned absorber inside is larger. Single walled tubes are often used in colder weather as they have a higher resistance to cooler temperatures, they do, however, need direct sunlight and also cost more. Most UK installers and suppliers predominently deal with the twin walled style.

The 58mm tube, being longer and wider should produce approximately 50% more energy than the 47mm. The 70mm tube, being even bigger, should produce around 90% more energy than the 47mm.

47mm, 58mm and 70mm solar tubes

solar tube legnths

 

What size tubes should I use and how many do I need? 

Twin walled evacuated tubes usually range up to about 58mm in diameter. After that they become single walled, which are better in colder conditions. See our section on the different types of tube for more information. 

As long as quality is assured, then it is a general rule of thumb to say that the larger the tube, the more efficient it is. It is also fair to say that the larger the tube the more expensive it is.

It is difficult to say how many tubes someone would need without first knowing some important details. Firstly it depends on the available sun light and secondly how much water you are trying to heat. There are so many variables to be considered when setting up a solar water heating system, by being aware of them and taking measures, where possible, to eliminate them, you can have a well functioning solar system. As a completely generalised rule of thumb, a correctly sized and installed system in the UK should provide roughly 50% of your annual hot water needs and contribute strongly toward them in the colder winter months.

If the solar tubes conform to the points outlined in the 'things to check when buying evacuated tubes' section, are placed on a south facing roof and you have read, understood and catered for the variables mentioned below, then the following table will be a good rough guide for how many tubes you require to heat domestic hot water in a twin coil cylinder in the UK. Please, however, do not use it as a hard and fast rule.

 

Cylinder's capacity (litres)
No. and diameter size of evacuated solar tubes
up to 80L
10 x 47mm
90L - 150L
20 x 47mm
160L - 240L
30 x 47mm OR 20 x 58mm
250L - 320L
40 x 47mm OR 30 x 58mm
330L +
Use the multiplications to calculate

 

Variables:

This section aims to identify the potential variables involed with installing a solar panel system and give a better understanding on what needs to be done to tackle them and get the best you can from the sun's energy.

New Cylinder or Retro-Fit?

See our sections on new cylinders and retro-fitting for more information. However be aware that although it is usually more expensive, a new cylinder with a coil designed for solar will be more efficient than retro-fitting a solar panel to an existing hot water system.

Pipe run

The length of the pipe run from the solar panel to the water store can have a huge effect on its performance. The longer the pipe run, the larger the potential for heat loss. Also the bigger the pipe the more water is held within the system. It is recommended to use small diameter pipe in as short a run as possible. Where pipe runs are likely to be very long it is worth increasing to compensate. 10mm or 15mm copper pipe is readily available from most plumbers merchants and solar suppliers and is usually ideal for most installations. 

Any pipe work used must be well insulated. This is somewhere where you should not cut costs or corners. High Temperature pipe insulation is easy to come by. It is not cheap but is well worth having. Low grade insulation may easily melt on hot solar pipe work, rendering it useless and difficult to remove. High temperature insulation will prevent heat loss from your pipe work and will tolerate the kind of temperatures a solar system can potentially reach. It is most important to insulate any pipe work which is situated outside and close to the panel as this is where the system will be hottest and so can lose the most heat. 

Cloud cover and shading

As obvious as it sounds, any shading on the panel will reduce it's efficiency. Pick a spot for it where it can catch the sun from dawn until dusk. Please also bear in mind that the UK often is an overcast and cloudy place. Although vacuum tubes function much better than flat plates in cloudy conditions, they cannot perform miracles.

thermal image of a solar panel

The above thermal image demonstrates that absolutely no heat is being lost from the solar panel and it is functioning well, which is why it is very dark. Thermal imaging is a great way of identifying heat loss on a building. In this instance it is clear that most of the house's heat is escaping from the chimney.

What is the optimum angle for a panel to sit at? 

In the UK anything from 15degrees to 90degrees is ok for a panel to sit at. They cannot sit flat as this prevents the absorbed heat from travelling up the internal copper pipe. Also panels on a flat vertical wall will not be at their best as they will not gain 100% of the available energy from the sun. Normally the angle of your roof will be ideal for your solar panel, if not most companies will sell a standing frame so the solar panel can be placed on a flat surface. These are not a specialised piece of equipment. Please do not be convinced otherwise and pay an extortionate amount for one.

solar panel angle calculator

 

Should I 'over spec' my system so I can get more out of it in winter?

People often look at buying more tubes or panels than is recommended for their cylinder's capacity so that they can get more hot water during the winter months and rely less on domestic boilers etc. This is called 'over specing' and is a perfectly reasonable choice, but bear in mind that it works both ways and will probably require a heat dispensing system normally in the form of a by-pass valve and radiator.

If you're going to over spec your system it is wise to consider adding some form of heat dump (or a way of getting rid of hot water other than from the taps). A heated towel rail is a common option or a single radiator. Most good quality solar controllers can be set to automatically dump excess heat if you have the equipment to do it.

Can I also run my central heating from solar? 

In theory, yes you can. However, the time of year you most need central heating, i.e. the winter, is when the solar panel will be gaining the least amount of energy from the sun.

To run most central heating systems you will need to 'over spec' the system by adding more tubes or panels. This can bring about the excess heating problems outlined in the above section.

Rather than over specing, consider an additional eco-friendly heat source, such as a wood burning stove or ground source heat pump, which will help heat a room and your domestic hot water, and can be used when needed.

How much should I spend on evacuated tube solar panels?

Hopefully this website has helped you understand that solar hot water panels are not an overly complicated piece of equipment. There should be no myths surrounding them. Any company that is evasive as to how solar panels work should be avoided.

The market has already been overrun by companies charging extortionate prices for their system based on totally false promises. And when your system breaks they're either no where to be found or charge a fortune to get it up and running again. 

Installing a solar panel yourself will always be cheaper than having an installer do it. Although it may seem like a very daunting task, all the information on how to 'do it yourself' should be readily available. If, however, your not competant with basic plumbing, electrics or heights then by all means have the system installed by a professional.

The main costs in a solar panel system is the panel itself and a new cylinder. If your retro-fitting a system then you should be paying around £1000 for the kit. But always check that the products are of good quality and carry some kind of guarantee.

A replacement cylinder will cost more money. The larger the cylinder you choose the larger panel you need and so the greater the cost will be. Its all a relative equation.

An evacuated tube solar panel kit with a cylinder should be in the region of £1500.

An installer may charge considerably more to fit the system. Please be aware that whilst there are some people out there who are quite literally ripping people off, not all solar installers are cowboys. Many are self employed, well trained professionals with a good work ethic. They have overheads and costs that may not seem immediately apparent, such as insurance, equipment hire/costs, staff, tools, transport etc which they must cover in their pricing.

A installation service from anything up to around £4000 is reasonable. Anything above and you should ask exactly what 'special' service you are getting. Nearly all installers offer a free survey and quote. See our case studies section for some interesting solar projects, many of them will tell you who installed them, how much they cost and a response from the customer. SolarHelp also has a large network of solar installers who offer good prices and practices, for more information see our Installations section, where we can help put you in touch with one.

How quickly will I get my money back and start saving?

This is the most common question and unfortunately its the most difficult to answer. Obviously the bigger solar panel you have the more energy you gain and so the quicker you save money but the panel will cost more in the first place. This is why it is imperative that the panel is of good quality.

Your money back is ultimately determined by how much you currently pay your gas or electricity supplier. If your on a flat rate of payment then the solar panel will bring down your monthly power usage and so your gas/electricity company will cut their rate based on these lower averages. Make sure you shop around or get on a good deal with your utilities company before installing a solar panel so that you can see a noticeable reduction in your bills and gain the maximum benefit from the investment. According to a recent article from the BBC , gas and electricity prices are only likely to go up in the forseeable future. 

It is also worth considering, given the current financial climate and low interest rates, that an investment in solar may beat an investment with a bank or building society.

To find out your exact savings use the following formula - Please bear in mind to be completely precise it will get very complicated!

equation

 

E = Insolation (energy available in the sun) (kWh/m2/yr)*

A = Aperture area (m2) - see our section on differences between aperture and absorber areas

n0 = Zero loss collector efficiency (%) - percentage of E available after efficiency losses - this will be given on your panel's performance report.** Change the percentage to a decimal for this equation. e.g. 30% = 0.3

Cost = Energy cost (pence/kWh) - from your gas/electric company. Divide this by 100 to get pounds

£ = Amount saved per year.

* - the average Insolation energy for the UK is 1100kWh, if your panel is at a 40degree angle. You can use this amount to give you a fairly accurate calculation of your annual savings. However, if you want to use an exact figure for section E, visit the PVGIS Solar Irradiation Data website to first find out the best panel angle for your area. Then, using your lat/long co-ordinates and that angle you can get the average insolation level for your area per year. This data, however, is displayed in Wh per day and must be changed to the total kWh per year in order to get the correct value for E. 

** - for an average amount here use 0.65. All solar companies should offer a performance report for their solar panels.

This calculation does not account for the variables mentioned in the variables section above.

Should I re-inforce my roof for an evacuated tube panel? 

Evacuated tube solar panels are typically lighter than flat plates. Unless your roof is damaged or the supports are very old rotting, you should not need to re-inforce the roof.

If you are using an evacuated tube panel with an integral tank, you may want to have a look at its total weight as there will be a significant amount of water on the roof. 

What about freezing?

Most good solar controllers will have a frost protection setting. However many people put anti-freeze in their system to prevent freezing. This is a perfectly good solution. Try if possible to use a non-toxic substance and dilute it with water, rather than use a concentrate. A good antifreeze will prevent damage to the system from temperatures upto around -30degrees.

heat pipe temperature

The copper tip of a single walled 70mm evacuated solar tube left in the sun on an october day in the UK, shown reaching temperatures of 140+ degrees. had this tube been in a fitted solar panel this heat would have been transferred to the cylinder and the controller would not allow it to reach such a high temperature.

Other necessary equipment includes:

A Solar Controller, Twin Coil Cylinder or Heat Exchanger, High Temperature Insulation, Pump or Pumpstation, Expansion Vessel, Anti-syphon Valve.

 
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