Solar Panels - Frequently Asked Questions
The following information provides answers to many of the questions you may have. If however you cannot find the
information you require, please do not hesitate to e-mail or call and we will get back to you as soon as possible.
You will also find this information extremely helpful in making a decision about product suitability.
Thank you.
Introduction to Solar Hot Water Heating Systems.
What size is the collector?
How do vacuum tube solar panels compare with flat plates?
What weight are the collectors?
Are the tubes fragile?
What is the difference between the single-walled and double-walled solar panels?
What size solar cylinder should I fit?
How many panels/tubes do I require to meet my household needs?
What is the difference between the 47mm, 58mm and 70mm tubes?
How hot will the water get?
Where do the panels need placing and at what angle?
Does the roof need to be strengthened at all?
Are spares available?
Are the panels guaranteed?
Can I fit this system DIY?
What if I have a combi boiler?
Will the system need to be inspected by a plumber or be installed by a plumber?
How much maintenance do the systems require?
What about freezing in cold weather?
Can I use Solar Power with a mains pressure hot water cylinder?
What is a Thermal Store?
Can I get a grant for these systems?
Do I need planning permission?
Can I power the pump with solar energy?
Can I heat my house with evacuated solar tubes?
How much heat energy can I store in my cylinder?
What about overheating?
How many panels do I need to heat my pool?
Can I use a WindPowerUK vacuum tube solar panel with a drain-down system?
Introduction to Solar Hot Water Heating Systems.
Some types of renewable energy are only available to people living in suitable locations, however free solar hot water heating is potentially available to almost every house in the UK! With high efficiency, they are much more efficient and cost-effective than electric photovoltaic solar panels. If we compare them on a like-for-like basis, the price of a Solar hot Water Heater is around 30p/watt compared to £6+/watt for solar photovoltaics, and once installed, your solar hot water heating panel will produce virtually free heat from solar power for decades to come.
An average household (3 people) would expect to produce most of its hot water from the end of March to the end of September from a single 20 tube 47mm hot water heating solar panel, although it will depend on water usage. Even outside this period, solar will produce a worthwhile contribution towards your hot water requirements, resulting in significant reduction of fuel usage and carbon dioxide emission. In fact, fitting a hot water heating solar system to your house is probably one of the most positive ways you can reduce your carbon footprint, without making major lifestyle changes.
For larger households (4-5people), it may be necessary to fit a larger solar hot water heating kit with 30-40 tubes to get the same percentage of hot water requirements, but even if a smaller system is fitted, every kWh produced will save 1 kWh of fossil fuels, and 0.5kg of CO2 emission. If you decide to fit an undersized system now, it is easy to increase the capacity of the system at a later stage, as the panels are modular - if you would like advice on sizing solar hot water heating panels for your requirements, please feel free to give us a call or send an email.
The 10 tube double-walled tube collector (47mm tubes) is 1760x 760 x130mm (LxWxH)
The 20 tube double-walled tube collector (47mm tubes) is 1760x1500x130mm (LxWxH)
The 30 tube double-walled tube collector (47mm tubes) is 1760x2170x130mm (LxWxH)
The 20 tube double-walled tube collector (58mm tubes) is 1900x1660x130mm (LxWxH)
The 30 tube double-walled tube collector (58mm tubes) is 1900x2406x130mm (LxWxH)
The 10 tube single-walled tube collector (70mm tubes) is 1760x1000x130mm (LxWxH)
The 20 tube single-walled tube collector (70mm tubes) is 1760x1950x130mm (LxWxH)
How do vacuum tube solar panels compare with flat plates?
On a hot, sunny day, flat plate panels will produce the same amount of heat, more or less, as for vacuum tube panels of the same area. Under ALL other conditions, the vacuum tube panel will outperform flat plate panels - usually by a very significant margin. Flat plate panels perform badly during windy weather, in cold conditions, in overcast conditions etc. Some of the very best flat plate collectors can compensate for this with more advanced design, but even then, they still do not perform as well as vacuum tube panels. This is why the vacuum tube panels are better suited to our temperate UK climate.
What weight are the collectors?
The 20tube 47mm panel is 48kg, the others are approximately proportional to this weight.
The tubes are made from borosilicate glass, which is very tough (also known as Pyrex). The tubes are designed to withstand hailstones up to 25mm - so they are unlikely to be broken, unless dropped onto a solid floor prior to installation
What is the difference between the single-walled and double-walled solar panels?
The difference relates to the design of the solar tubes. The standard tubes are double-walled, with a vacuum between the two walls of the glass. The centre is filled with air and the heat pipe runs up through the centre. The single walled tube is entirely filled with vacuum, and the vacuum is sealed by a glass-metal weld sealing the heat pipe to the glass. This is technically much more difficult to do, hence the cost is much higher. The single walled tube has a marginal advantage over the double-walled tube in that it reacts much quicker to sunlight (e.g. in winter it starts to heat water within 5-10 minutes rather than 10-15 minutes, and in summer the single walled tubes will start heating in just 30-45 seconds whereas the double-walled tubes will take 2-3minutes) - so it is slightly more efficient in marginal conditions. However, it also cools down quicker, whereas the standard tube will continue to heat for 10-15minutes after the sun goes in. A 10 tube single walled panel (70mm diameter tubes) produces about 10% less heat than a standard 20 tube double-walled panel (47mm diameter tubes).
What size solar cylinder should I fit?
We recommend that the WindPowerUK 20 tube panel is used with cylinders up to 175 litres, and the WindPowerUK 30 tube panel is used with cylinders from 175 - 260 litres. Larger cylinders can be heated with 2 or more panels, and it is possible to add an extra panel to benefit from higher temperatures in the winter. In fact, if you fit a smaller cylinder, you will have hotter water, but of course less of it. We would recommend fitting a tank of 135 litres if you prefer a smaller amount of hotter water.
How many panels/tubes do I require to meet my household needs?
This is a difficult question to answer as it depends on your water usage. For example, one person that contacted us lives in a motor home, and uses only 70 litres of hot water over a two week period and on the other end of the scale a young couple I spoke to estimate their hot water consumption to be at least 400 litres per day! First you should calculate how large a hot water cylinder you require to meet your needs. Remember that solar cylinders are normally designed to store water for use over 2 days, rather than one, allowing you hot water even if the following day is cloudy. Once you have calculated your cylinder size, you can estimate your panel size on the basis of one solar tube per 6.5 - 8.5 litres of hot water cylinder (e.g. a 20 tube panel can supply hot water for a 120-170 litre cylinder, and a 30 tube panel can supply 180 - 255 litres). This assumes you have an unshaded south-facing roof slope to mount the panel.
What is the difference between the 47mm, 58mm and 70mm tubes?
The standard tubes are 47mm diameter, 1.5m long, and are of twin wall construction. The 58mm tubes are also twin-walled, but longer (1.8m) and wider. This means that for a given number of tubes the 58mm tubes will produce 50% more heat than the same number of 47mm tubes. The 47mm tubes are easier to transport, and are less fragile in handling due to their shorter length. The 70mm solar tubes are single-walled construction. This is more expensive to manufacture, but provides better performance in extremely cold conditions. A 10 tube 70mm panel should produce around 90% of the output of a 20 tube 47mm panel.
This depends on the size of the cylinder you chose, and the amount of water you use. A smaller cylinder will be heated to higher temperatures, whereas a larger cylinder will heat more water, but not to the same extent. Systems are generally designed to heat water to 65°C or so, although during the winter, it may be necessary to 'top-up' the heat on less sunny days. Large arrays, running high temperature solar antifreeze at high pressures can be designed to achieve temperatures of 170°C or more!
Where do the panels need placing and at what angle?
Panels should be sited on a south facing wall or roof, in fact, anywhere between south-west and south-east will produce good results. If you are limited to an east-west facing system, then you will need two panels to provide the same amount of hot water as a single panel on a south facing roof slope. The most frequent solution to this problem is normally to mount one panel on the east slope and a second panel on the west slope. A special controller is available for east/west facing installations. The panels should be mounted at the angle of your latitude. For example, at Gloucester UK, this is 51°. This is the AVERAGE optimum angle. In fact, in winter the optimum angle is around 10° steeper, whereas in the summer it is 10° shallower. The panels will function anywhere between a 15° and 90° angle of inclination. However there is no need to get out your protractors, a variation of 10° will make very little difference to the output, so most people simply settle for whatever angle their roof slope is. The extra cost of trying to stand panels off the roof to achieve better efficiency would probably be better spent on purchasing a second panel, or energy-saving measures such as extra loft insulation!
Does the roof need to be strengthened at all?
The solar panel in operation weighs around 50kg - this is really not much weight for a roof, when you consider the weight of the tiles or slates. Generally there is no requirement to strengthen a house roof prior to mounting the solar panel, except if you are fitting a panel to a very old roof, which has sagging and decaying timbers.
Yes, spares are always available, should you need them. With no moving parts, it is very unlikely that you will need to replace anything, but occasionally customers break tubes during the installation process, in which case you can purchase a modestly-priced replacement. However, it is not possible to post a replacement tube, so you will need to collect it. The tubes are made of borosilicate glass (aka 'Pyrex') so they are actually very tough.
Yes, we offer a 24 month guarantee on the panels, although, with no moving parts the panels are extremely hard wearing and have a life probably in excess of 25years. In fact, the manufacturers offer a much longer guarantee, but in the UK we do not offer this extended period due to the legal requirements - if we were to offer a 25 year guarantee, we would need to take out an 'indemnity' policy on the product. The insurance company bases its risk on an assumption that if it priced the policy on the basis of replacing the unit once within 25 years, then they will ensure profitability regardless of claims - thus they would cost this in at probably £300 or more per item sold. The cost is out of line with our policy of keeping prices as low as possible, and we decided that most customers probably would not want to pay this additional fee!
Yes you can. These systems are very easy to fit, and anyone with basic plumbing and electric skills can carry out a DIY installation. We also run training courses, for those who wish to cover the subject in detail. We run training courses every month, which are designed to run through everything you need to know, whether you are planning to DIY, or become a professional installer. Mounting the panel on the roof is sometimes daunting, although it is actually quite simple - but we can offer this service, if required.
What if I have a combi boiler?
You can fit a Combi Solar Divert Valve. This is a hot water divert valve for use with combination boilers or instantaneous water heaters. This means that a solar system can now be linked with a combi boiler as back up. Cross-flow contamination is eliminated by the deployment of solar check valves integrated within the valve. This valve can ONLY be used with a combi that accepts preheated hot water.
Will the system need to be inspected by a plumber or be installed by a plumber?
No, you can do this yourself. However, you can call in a plumber to carry out the work if you feel you are not able to tackle to plumbing yourself. We normally recommend that you fit a pressurised solar loop. This is not essential, but it allows you to place the solar panel on the roof without worrying about whether it is possible to place the header tank high enough to function correctly. With a pressurised system, the plumbing is simpler, and by running the system at 1-2bar, it is possible to increase the boiling point of the water to 120°C or higher - allowing greater safety margins, and lower chance of fluid loss.
How much maintenance do the systems require?
Very little maintenance is required for solar water heating systems. You should occasionally check the system pressure, to make sure there has been no water loss, and to check for any air in the system. The only other requirement is to ensure that there is an electricity supply connected at all times, otherwise without pump circulation, in strong sunshine, the panel could overheat, and start to boil off water.
What about freezing in cold weather?
Our solar panels are very well insulated - the manifold is surrounded by 2"of rockwool insulation. This is better than your outdoor water pipes, so it is unlikely to freeze except in exceptionally cold weather. However, it is recommended that you take precautions to prevent the possibility of pipes freezing, by either adding antifreeze to the system (see other products for our non-toxic solar antifreeze
Can I use Solar Power with a mains pressure hot water cylinder? Yes. There are two ways to achieve this. You can either purchase a mains pressure unvented water cylinder, which can be expensive and you will have to have a pressure vessel certificate to install these, alternatively, you can fit a 'solar store' cylinder (see next question). A thermal store is a tank which has an additional large surface area high efficiency coil fitted. The mains cold water is fed into this coil, thus heating the water on its way through. The mains water exits the tank as hot as the hottest water in the tank, but without losing any pressure - thus providing mains pressure hot water to the household taps. Can I get a grant for these systems? Grants are only available for professionally installed systems (NOT DIY). WindPowerUK panels are approved under the Clearskies grant scheme, also for the Republic of Ireland and Scottish Grant schemes. The new BRE scheme is not considered viable. In order to gain approval, it is a necessary for us, as a distributor, to spend tens of thousands of pounds to register for the BRE's 'grant' scheme - and installers also have to spend a couple of thousand pounds to register. The scheme is supposed to be self-funding - in other words, the money we spend on registering for the scheme will then be used to provide the grants (after approximately 50% deduction for administration costs). If we do this, the price of the wind turbines and solar panels would have to be increased substantially to cover our costs. Rather than subsidise the cost of this 'scheme', we feel it more beneficial to offer all customers good prices. Hopefully the government will re-think its grant scheme! Do I need planning permission? You should refer this question to your local planning department for a definitive answer, but the answer is normally no. This is because solar panels satisfy the 'permitted development' requirements that they 'do not significantly project above the roof plane'. If you live in a listed building, an AONB or Conservation Area etc, then you may be subject to different planning rules. In these circumstances, it is usually possible to site solar panels without the need for planning permission (do a search on our forum pages for more information), but it varies from case to case. It is also generally possible to mount the panels at ground level to avoid the need for planning permission, if necessary! Can I power the pump with solar energy? Yes, this is possible. For flat plate collectors, manufacturers will specify a 5W photovoltaic panel, and a 5-10W low voltage pump. This is because flat plate collectors do not work efficiently in low sun conditions, so the pump only needs to function in bright sunlight. This is insufficient for evacuated tubes, which due to their high efficiency will require pump circulation even in overcast conditions. For this reason, the pump should be rated at least 10W, and the panel 20W to provide sufficient circulation. At the moment, there is no cost-effective low voltage pump on the market suitable for pumping hot water. The most popular 12v solar pump, the 'ivan' retails at around £150-£200, making this an expensive option. It is possible to use a solar photovoltaic panel to drive a mains inverter, powering a standard 220v circulation pump, but most customers do not want the complexity of such a system. Can I heat my house with evacuated solar tubes? There is 10 times more sun energy in the summer than in the winter. Therefore you immediately have the problem that the bulk of the energy is at the wrong time of year. The other problem with using solar to heat your house is that it is not present at the times when it is really cold - at night, on very overcast days, and on winter evenings etc. In practice you can provide a significant amount of supplementary heat in the spring and autumn (and some people have implemented such systems), but the contribution in the winter will be minimal, restricted to sunny days and you would need to fit many more panels, as heating a house is a lot larger task than heating an insulated cylinder of water. Solar water heating on the other hand, can be effective even in the winter, as the amount of heat required is considerably less than that required to heat a house. How much heat energy can I store in my cylinder? Let's use this example - a 260 litre cylinder of water, heated to 65°C. Assuming that room temperature in the house is 18°C, then the difference between the cylinder water and ambient air temperature (dT) is 47°C. The heat energy stored can be calculated by multiplying together the volume of water in the tank (in litres) by the dT by the specific heat capacity of water (4200kJ/kg K) - i.e. 260 x 47 x 4200 = 51.3MJ. (If we are really fussy, we can add the heat capacity of the copper cylinder itself - 0.2MJ). If we divide this by 3.6M, we arrive at the number of kWh storage - 14.3kWh. Many people worry about this, although on systems with long pipe runs the maximum temperature will be controlled by the insulation on the pipe-runs, as the pipes get hotter, heat losses will increase. If you correctly size a system, then overheating is not that likely, but if you oversize the cylinders - for example if you are a heavy water user and you want maximum gain, then you will need to consider cooling issues- particularly if you plan to go away on holiday during midsummer so no water would be drawn off for a week or two. There are various ways to do it, using the BS3 controller: How many panels do I need to heat my pool? Swimming pools are an ideal application for solar hot water heating, as the temperature required is quite low - less than 30°C. Most installers will recommend solar hot water flat plates to keep the price down because the amount of solar hot water panels required is much larger than for heating domestic hot water, however, if you can buy a high efficiency evacuated tube solar panel for the same money you might as well enjoy solar heated pool water for a greater proportion of the year - instead of just in July and August! Can I use a WindPowerUK vacuum tube solar panel with a drain-down system? Drain-down systems are a fairly primitive method for frost-protection. The components are large, expensive, and can easily be avoided using WindPowerUK solar antifreeze or the frost-protection feature of the WindPowerUK controllers. However, if you already have a drain-down system, it may be possible to replace your existing panel with a WindPowerUK panel, although it could cause air locks.
1. Overheating protection can be achieved by setting the controller to continue to circulate the water after the sun goes down (OREC) - this effectively utilises the pipe work to cool the cylinder. When the cylinder drops below the set temperature, it ceases to circulate.
2. 'Aftercooling' function - when enabled, this function will divert the solar heated water through a separate loop to dump the excess heat - this could be:
a.) A towel radiator in a bathroom.
b.) A radiator installed simply for dumping heat - e.g. on the outside of the house (assuming it is weatherproof) or in the roof space).
c.) If the existing boiler system connected to the top coil in the solar cylinder is pumped, you can use the BS3 solar controller relay to drive the existing central heating pump directly, which will effectively extract heat from the coil inside the cylinder by pumping water around the boiler and house radiators. You should ensure that the boiler is turned off or use a relay between the pump and the boiler controller, to prevent the boiler switching on (normally if the pump is powered-up, as it is connected to the 'call' function of the boiler controller, it will also fire up the boiler). This method involves NO PLUMBING so it can be more attractive for that reason.
3. Even if you have not implemented any of the above, when the panel gets too hot, and steam is produced, the extra volume of the system will be absorbed by the expansion vessel. When the system re-cools, the steam will condense again and the system will start to work again, without the need to refill or top-up. You should not rely on this method routinely, but it will protect the system without causing damage.
If you used solar hot water flat plates, a rule of thumb for calculating what you need is to fit the equivalent of 50% of the pool's surface area. For solar hot water vacuum tube panels, this is reduced to 25%-35% (depending on type of pool, level of insulation, pool covers, indoor/outdoor, above/below ground level etc)- and of course, they will carry on working in the autumn, winter and spring.
The solar hot water panel we sell is 2.25m² so (working on the 25% coverage figure) each panel will heat 11m² of pool area. A 40m² pool would only need 4 solar hot water heating panels. If you compare this to the cost of heating a pool for a year, large pool installations can pay for themselves in less than 18 months!
Installation is very simple. If you use chlorine you will need to fit a stainless steel heat exchanger (this is because chlorine causes copper to corrode). These are available from pool suppliers (they are needed for conventional heating methods for the same reason).
If you do not use chlorine, you can simply put the solar hot water heating panel in-line with the pool filter, so that the pump directs water through the panel before returning it to the pool.
As the panel will NOT radiate heat on cold days, heat losses will be confined to your pipe runs only - so you do not necessarily need a controller, although one can be fitted, to control a 3-way solenoid valve to divert the water through the panel when the panel is hotter than the pool water.
