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Swithenbanks solar online FAQ's 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. |
| What weight are the collectors? |
| The 20 tube 58mm panel is 80kg, the others are more or less proportional to this weight. |
| Are The Tubes Fragile? |
| Are they tough enough to survive
impact from falling hailstones and
animals? 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. Impact from falling animals is not normally considered in the design of the tubes! |
| What size solar cylinder should I fit? |
| We recommend that the Swithenbanks 20 tube panel is used with cylinders up to 220 litres, and the Swithenbanks 30 tube panel is used with cylinders from 200 - 350 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. |
| What is the output of the panel in terms of kWh? |
| This varies according to the time of year - in summer we get almost 10 times the amount of solar energy that we do in the winter. In the summer, you can expect to get 21kWh of heat per day from a single 20 tube 58mm panel, whereas in December you can expect only 1.72kWh per day on average. Even in the winter, it is possible to preheat the water to 40°C or more if the sun comes out. In fact, if you fit multiple panels, it is possible to heat your hot water to 100°C in January (we have seen this in Cornwall). |
| What is the difference between the 58mm and 70mm tubes? |
| The standard tubes are 58mm diameter,1.8m long, and are of twin wall construction. These 58mm tubes are longer and wider than the cheaper much less powerful 47mm tubes. 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 70mm solar tubes are single-walled construction. This is more expensive to manufacture, but provides better performance in extremely cold conditions. |
| How hot will the water get? |
| 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 these 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 give 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 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 our location in the UK, this is 53?. This is 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 15 and 90° angle of inclination. This is actually academic, 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 80kg - 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. |
| Are spares available? |
| 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. |
| Are the panels guaranteed? |
| Yes, we offer a 5 year guarantee on the panels, although, with no moving parts the panels are extremely long-lived, probably in excess of 25 years. 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! |
| Can I fit this system DIY? |
| 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 through Eppoge, for those who wish to cover the subject in detail. 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? |
| Yes, well you can but you will need a bypass valve as most boilers will not accept pre-heated water. At present these valves are very expensive around £250! |
| Can I come on a course to learn how to install solar water heating systems? |
| Yes you can! We run training courses every month through Eppoge, which are designed to run through everything you need to know, whether you are planning to DIY, or become a professional installer. |
| 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 1-2 bar, 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? |
| The solar panel is 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 (use our non-toxic solar antifreeze) or you can use a controller which has 'freeze protection' - these controllers monitor the temperature of the collector - and if it falls below 4°C, it will automatically turn on the pump, allowing water to circulate and heat the manifold. You should turn this function off if you use 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 (these can be expensive). Please note that you will have to have a pressure vessel certificate to install these. Alternatively, you can fit a 'solar store' cylinder (see below). |
| What is a Thermal Store? |
| 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 pressue hot water to the household taps. |
| Can I get a grant for these systems? |
| Grants are only available for professionally installed systems (NOT DIY). Swithenbanks panels are MCS approved. |
| Do I need planning permission? |
| In most cases, no. Evacuated tube solar panels are considered in the same way as roof windows. Unless you live in a listed building, it is unlikely that it will require planning permission. Even if you do live in a listed building, you can usually fit the panel to the rear of the building, or at ground level without requiring planning permission. |
| 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, 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 as system. |
| 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 heatpipe 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 heatpipe 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 2-3 minutes) - so it is slightly more efficient in marginal conditions. However, it also cools down quicker, whereas the standard tube will continue to heat for 10-15 minutes after the sun goes in. |
| 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. A young couple I spoke to estimated 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, thus 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 9.7 - 12.75 litres of hot water cylinder (i.e. a 20 tube panel can supply hot water for a 180-255 litre cylinder, and a 30 tube panel can supply 270 - 382 litres). This assumes you have an unshaded south-facing roof slope to mount the panel. |
| Can I heat my house with evacuated solar tubes? |
| There is 10 times more sun energy in the summer than in the winter. (this is the reason for the different seasons, after all). 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, in 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, but you will 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 - ie 0.2MJ). If we divide this by 3.6M, we arrive at the number of kWh storage ie 14.3kWh. |
| What about overheating? |
| 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 mid summer (i.e. no water drawn off for a week or two). There are various ways to do it, using the BS3 controller: 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 pipework to cool the cylinder. When the cylinder drops below the set temperature, it ceases to circulate. 2. 'After cooling' 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 - eg 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 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. |
| Do I need planning permission for solar panels? |
| You should refer this question to your local planning department for a definitive answer, but the answer is normally 'No, you do not require planning permission'. 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, 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 usually possible to mount the panels at ground level to avoid the need for planning permission, if necessary! |
| How many panels do I need to heat my pool? |
| It depends what area of pool you need to heat, how well insulated it is, whether it has a pool cover, if it is above/below ground, how many weeks of the year you need to use it etc. However, as a rule of thumb we recommend that you start with solar panels equivalent to 25% of the pool area, but you can increase this up to 35% if more heat is required. As the panels are modular, it is easy to upgrade the panels at a later date if required. We strongly recommend the use of a pool cover when the pool is not in use, as it dramatically reduces heat loss through evaporation. |
| Can I use a Swithenbanks 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 Navitron
solar antifreeze or the frost-protection
feature of the Navitron controllers.
However, if you already have a
drain-down system, it may be possible to
replace your existing panel with a
Navitron panel, although it could cause
air locks.
Posted
on
Wednesday
29
August
2007
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