• April 9, 2014 - Domestic Renewable Heat Incentive Released - Solar Thermal has got the great government funding through the Domestic RHI which it deserves! Solartwin Solar Thermal panels have just become not only a cost cutting and very green option to heat your water but now you will receive additional... More →
  • November 6, 2013 - Notice to Solartwin customers - Letter's from Stenfield & Co - A number of Solar Twin Ltd customers have been in touch this week to query a letter they have received from legal firm Stenfield & Co. - The content relates to Solar Twin Ltd's liquidation, and claims that customers "undoubtedly paid more than... More →
  • July 12, 2013 - Renewable Heat Incentive Updated - Solar Thermal to have Heavy Support - The Government have today released the Renewable Heat Incentive: The first step to transforming the way we heat our homes. If you are looking to buy a SolarTwin System contact Genfit. "the scheme will be open to anyone in these groups who installed... More →
  • May 27, 2013 - DECC announce they are doubling the RHPP grant for solar thermal from £300 to £600 - Solar Thermal Panels Receive Huge Renewable Heat Premium Payment - Solar Thermal has received a huge boost, it's a great time to buy a Solartwin Product. - Greg Barker tweeted about the scheme - - Big uplift for #RHPP Vouchers to put a shot in the... More →
  • February 14, 2013 - Solartwin DIY reviews - SolarTwin Reviews from Reviewcentre - "Review Centre is a community of real people, just like you, sharing their product and service experiences. - Our community of reviewers are a passionate bunch who want to help you discover what's right for... More →
  • January 17, 2013 - Crackdown on aggresive solar selling - YouGen, a leading consumer body serving the renewable and energy efficiency sectors, is calling for a change in the law after a review by the Office of Fair Trading revealed unscrupulous selling tactics by solar and other energy efficiency... More →

Solar panels + solar energy. Solar thermal+ Photovoltaic solar power & PV pumped solar hot water heating systems.

Solar panels for solar water heating: Q&A – FAQ 

Using solar panels to collect solar energy to make eco-efficient solar hot water systems is intrinsically simple. However, there are some subtle details about solar water heating systems. The real challenge is to make 100% solar powered solar hot water systems which are reliable, green and easy to fit.

Here are some technical solar questions posed to us by customers about Solartwin’s unique alternative energy system, and how do solar panels work more generally. Many thanks for the opportunity to reply to these solar questions:

FAQ: I want a DIY solar eco-home, but many of the solar hot water heating systems that I’ve looked at have temperature sensors which compare the temperature in the cylinder with that in the solar panel and switch on the pump when there is a gain. In your system, if say it’s sunny in the morning and the cylinder heats up and then it goes cloudy and the pump slows down but is still active, could I actually cool down my water?

A: In the past, Solartwin used to collect renewable solar energy used to operate independent of temperature. The box in which our solar panel is housed is double glazed and very well insulated indeed. The answer used to be, nevertheless a remote yes. It might have cooled water by 5 degrees C if you put it in the solar panel at a starting temperature of 65C if the weather was cold, below -5C outside, but it would normally add another 5-20C in nearly all circumstances. But generally, even a tank whose thermostat has cut out the boiler at 55C by sensing halfway up the cylinder will have water 5-10C cooler at the bottom, from where we draw off. Even if this remote event were to happen the total system efficiency was usually above our competitors. According to our research at Napier University, payback is further boosted by having no parasitic mains electricity costs (for 2 motorised valves, a solar controller which is on 24 hours a day, and a pump).

Since 2008 all Solartwin solar heating systems now come with a specially designed PV (solar electric) pumped solar pump controller. We expect this to further boost solar panel performance, perhaps adding 1-10% more energy a day compared to before. However we have not yet had independent tests of our solar heating systems to confirm this expectation! Interestingly, some conventional solar panels could also be fitted with this particular solar controller too (plus a PV panel sand solar suitable pump), but few are actually doing this yet.

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FAQ: What is solar energy? How does solar work? (not too long please!)

A: Solar energy is the radiation which comes from the sun. Solar energy comprises around 55% visible solar radiation, which we can see. Most of the remaining solar radiation is infrared radiation. A tiny amount is ultraviolet radation. Solar water heating panels (thermal solar collectors) work on visible and infrared radiation, so they use most of the sun’s spectral output. When this radiation hits the dark inside of a solar panel it heats it up. This then heats your water. Another use of solar radiation is to make solar electricity. Solartwin also uses this, but only in small amounts, to power the solar pump. However, per unit of energy created, solar electricity is about four times more expensive to generate than solar hot water. Plus you need about four times more roof area to make the same amount of solar electricity. So making solar hot water directly as Solartwin does, rather than via electricity makes a great deal of sense. There are some great videos on youtube to take a look at. May I stop now?!

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FAQ: I’m planning an eco-build and I want to buy a solar panel for it. During the winter months when most of the hot water will be generated by the boiler, what happens if there is enough sun to set the solar heating systems’ pump in action. At which time there is not enough thermal energy to heat up the water sufficiently so that when it is returned to the hot water tank it reduces the temperature there?

A: The solar pump runs only on solar energy in the form of photovoltaic electricity. So will your solar controller. Besides the pump being switched on and off by the solar controller only when the panel is hotter than the bottom of the hot water cylinder, the solar pump also conatains a high pressure bypass in case the solar panel or its pipes are frozen. In response to your question: Nevertheless – it is best not to have the hot water system on all day since this is wasteful anyway and does not allow for optimum solar performance. Most boilers have separate timers for this, but not all. Ideally time the boiler to add heat to the domestic hot water after 4pm. Second, even in winter some hot water is made by Solartwin, not all by the boiler as you say. Third – the solar thermal panel is well insulated and so will still raise the temperature of water going into it since it collects heat from the sun and not the air. Fourth – at 100% sun and a water input temperature of 50C and air temperature of freezing our mathematical model (based on extensive tests at Napier University) suggests that the water will still leave the panel at least 10C hotter than when it went in.

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FAQ: Your solar hotwater system heats the water at the top of the cylinder rather than at the bottom. Does this have any benefit?

A: The pump from our solar water heating systyem draws water from the bottom of the hot water cylinder, passes it through the solar panel and delivers it, solar heated to the top of the cylinder, to be pedantic. This type of solar circulation brings huge benefits. The solar hot water floats there, and stays hot instead of (with heat exchange delivery to the bottom of the panel) mixing the whole cylinder to become tepid albeit with the water still a bit warmer at the top. When you open your hot tap you draw off from the top so this is where you need hot water to be. We deliver half a cylinder of solar hot water on days of half sun. Most others deliver a full tank of tepid solar water. You choose.

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FAQ: What is the maximum solar heated water temperature that your solarpanels can achieve?

A: Water and steam at 100C is theoretically possible if you were to switch the solar pump off on a hot sunny day. But please don’t do this. With the pump running we have only ever achieved 87C so far In independent tests on our complete solar water heating system in UK. But 130C internally for the thermal solar panel if left dry in the sun, is possible but does not hapen in normal operation of the solar panels since they are always kept pumped at temperatures.

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FAQ: My family is hoping to self-build a zerocarbonhouse in the next few months and would be interested in installing a DIY Solartwin solar pannels system. However, I would prefer to have a rising main water system and avoid cold water storage tanks altogether. After some unpleasant past experiences of water tanks – leakage and dead wildlife among them – my prejudice on the matter is strong enough to rule out solar heating on this grounds alone! Is it possible to fit your system without a storage tank?

A: The principle of using solar hot water, of any solar technology, is to usually store water as and when the sun shines and then use it subsequently, so a hot water cylinder, as you know, is always needed but we too would like to do away with the cold water tank (even though it no longer functions as a pigeon/bat/rat/Santa trap when kept covered). High pressure Solartwin solar water heating solutions now include: using it to heat a low pressure vented heat store through which a high pressure heat exchanger at the top which then pre-feeds, via a thermostatic blender valve to the high pressure cylinder or a solar-ready combi boiler.

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FAQ: What is the life expectancy of the solar pump and does it come with a guarantee?

A: Intended design life of the whole solar water heating system is 20 year minimum. The performance warranty is 5 years, although some products, such as the polycarbonate glazing actually come to us with 10 years warranty. I would expect the solar pump to need replacing before 20 years, however, since it is the only moving part and moving parts are the most subject to wear. We are using an relatively expensive but deliberately reliable pump. The motor in it is a brushless DC motor. Using a brushless, rather than a brushed motor reduces wear and increases life several fold.

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FAQ: What wattage does the solar photovoltaic panel produce?

A: 5 watts maxmimum, usually less, is the PV’s output. The solar pump’s peak output is approx 0.7 litres a minute peak. Again it it usually less than this except in full perpendicular sun.

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FAQ: I am slightly concerned about the head available from the solar PV pump. Do you have engineering data that I could use to determine whether I’m likely to have a problem with the length and type of pipe I might use for my solar panels installation?

A: Please don’t be concerned. The photovoltaic solar pump is actually too beefy for the job. This is a deliberate part of our design as a way of getting a very long life from it. Not only has it a long life brushless DC motor but the solar pump head itself has been dramatically de-rated by fitting an internal pressure bypass which opens at about 1 bar if any pipe is frozen. Without this, the solar pump could deliver about 5 bar, which is way beyong th emaximum pressure allowed for the panel.

Hydraulically, this pressure limitation is fine for a total pipe length of 30m of our own 6mm microbore flexible pipe. If you want to go longer, then you need to increase the pipe diameter on the side which supplies cooler water to the solar heating panel. If you use 15mm diameter for all this, then the whole return pipe in microbore can be up to 30m long!

The great thing about our microbore pipe is that it is far better than conventional pipe at reducing distribution heat losses. First its surface area is about half that of normal pipe so it loses less solar energy. Second, its volume at 6mm diameter is about a fifth of the volume of 15mm pipes, so ‘dead leg’ losses (from solar hot water sitting uselessly in the pipes betwen panel and store) are minimised. Another way of looking at this benefit is that you can site the solar heating panel much further away from the cylinder with our system compared to conventional solar installations, if you want.

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FAQ: I do like the idea of 100% renewable solar energy from your solar panels but I’m concerned that if I relied on the solar PV only there might be a tendency for the system to ‘leak’ solar energy slowly through convection and/or PV pumping slowly when the achieved water temperature rise after losses in the transfer mean it is not economical to run through the store. Is that completely unfounded?

A: No problem. Convection through 6mm microbore pipes (even without our non-return valves in the pump stopping this from happening!) would be infinitesimal.

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FAQ: The web info suggests the solar panel isn’t damaged by freezing but there’s the extra water from the pipes too – does it have a pressure valve?

A: No problem, rubber expands when it freezes. Our pipes (in both the thermal solar panels and used for distribution to and from it) are a specially formulated silicone rubber. They can expand over 100% by volume. Water expands only 4% on freezing. So freezing is easily accommodated – with plenty of strech left to go. So there is no need for a pressure relief valve in the pipes or the solar panel.

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FAQ: I have a bungalow with a flat roof ground floor extension. I want a thermal solar panel for it, because I am trying to turn my low carbon home into a zero carbon home, or at least one which is carbon neutral. The upper, sloping roof could be used to support a solar panel but it does not receive the sun all of the day. The flat roof receives full sun from dawn to dusk. Installation of the pipes would also be easier from the flat roof as they could be taken up under the tiles to avoid drilling. Is it possible to have the solar unit free-standing on the flat roof? Would its own weight, plus water, be enough to hold it if the mounting brackets were fixed to a wooden frame?

A: Flat roofs are fine for solar water heating panels but you need to make sure it will not take off airborne and that the solar panel will not sit in any puddles. You will need an angled A-frame for support. We can make these at almost any angle if you want. Typically 30 degrees off the horizontal will maximise total perfprmance – but may have a summer excess. Generally people choose 45 degrees or even 60 degrees in order to sacrifuce a little of the possible summer solar energy excess for a smaller bit of useful winter gain, when the sun is at a lower angle. The weight itself is not enough to prevent flight! On some flat roof plus A-frame installations we use 2-3 tonne stainless steel straining wires (from any ships chandlers) to keep the solar panel sitting tight and to do away with roof penetrations.

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FAQ: You provide lots of lovely information but as regards solar payback time, you just say that it may be shorter. Do you have some estimates of the solar savings and payback time for your solar panels in particular?

A: The issue of solar energy payback is a complex one and has many different variables, not least the type and cost of fuel that the system would be displacing. Others are:

  • the latitude at which you live and you install the solar panel
  • the microclimate
  • the orientation and pitch of your solar panel
  • the thermal efficiency of your boiler – if you have one
  • what proportion of the delivered solar hot water you actually use
  • the price of the energy that you displace today by having a solar panel
  • the future annual energy price inflator you apply each year
  • the cost of the finance or lost interest on the capital spent on the installation
  • how much of the cost of the solar panel you consider to be a capital value boost to the value of your home
  • and a few others besides.

Is a solar costs-benefits calculation easy? No way. All this makes for gigantic spreadsheets with assumptions compounding upon each other and a huge opportunity for financial trickery. For example one unfortunate solar panel salesman I invited home (as part of a solar energy competitor analysis in 1999) applied a 13% fuel price inflator to the fuel displaced and pretended that his solar panels also saved on central heating, thus hastening the solar payback by a factor of about three! At the time, a 13% fuel price inflator seemed unreasonably high but not now.

We have recently commissioned some preliminary research that has resulted in outline potential savings of Solartwin, in terms of environmental and cost benefits. Maximum costs saving in the UK can be overestimated, not least since VAT on domestic fuel is only 5%! Mains gas costs about a third of electricity, even with the recent price rises. We estimate that for mains gas (not more expensive bottled gas, where savings may be higher) you are unlikely to exceed £100 on fuel savings. Perhaps you may save another £10 or so per annum on what we call the boiler life extension, since it may last longer if it is used less. For electricity (peak rate) the savings can be around £150 P.A. These are very rough figures only.

The environmental benefits (eg CO2 savings) of using solar panels to heat water, those which do not directly accrue to you as money saving but as CO2 savings, can be calculated / estimated as money too, by using various methodologies. Some are optimistic and some pessimistic. By using midrange figures we reckon these can in addition be around £50 P.A. (gas displaced) and maybe £100 for electricity displaced by solar energy. Remember, these are environmental valuations – which come on top of any money savings from solar. I hope this gives you a flavour of why I cannot pluck a solar payback figure out of thin air on the basis of a few lines of email and I hope you accept that I’m not just trying to be evasive. Do let me know your response – or calculations. For a step by step guide to payback calculation click here.

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FAQ. What about solar panels for underfloor heating?

Not usually a very good application for solar heating for reasons of practicality. We recommend that solar themal is used mainly for water heating for washing and bathing since the time of year that underfloor heating is needed usually coincides with the time when there is least sun – in winter and at night. This is not a good load/demand match.

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FAQ: Have you got a solar panel on your home?

A: Yes, I have a solar panel, but not on the main house roof. This is because Chester city centre, where I live, is in a Conservation Area. As a result, we are probably one of the few homes in UK to have an outside toilet with 100% solar hot water. The solar panel is poorly positioned, but it still does its job OK. It is due west facing, but its main illuminance problem is from shading. A huge ugly council tower block (the environmental strategy unit used to be based there!) overshadows it until noon for 7 months of the year. Trees also shade it in spring and autumn, when they are in leaf. Nevertheless we still have solar hot water heated up to 70C for our hands and for the paddling pool in summer.

FAQ – How did the Solartwin system do in the side by side tests of 8 solar water heating systems? Remarkably well. The tests were set up to see how well different solar water heating systems (not just thermal solar panels alone) could heat a 150 litre cylinder, not a standard household 120 litre cylinder. Our solar panels were not in the test, originally. At the very last minute one solar heating supplier dropped out and we were asked to supply a system, including one Solartwin solar panel. But we faced a quandry… Our standard Solartwin system is designed to heat a standard 120 litre cylinder so what were we to do? The options were (1) not to participate, (2) to make a special panel 25% bigger, (3) to install 2 panels since there was no limit on panel size or (4) to supply one standard sized panel and to take the flak.

We chose option 4 (install one solar panel which was a bit undersized according to the rules we were to play by) and faced lots of flak, mostly based on simple misrepresentation of the paper. In terms of reliability, of the 8 solar water heating systems which were installed as part of these tests, within 6 months, five of them had broken down, some more than once. Solartwin was one of the three solar thermal systems which did not break down, perhaps indicating that simplicity is a reliability bonus. Even though it was undersized for this “beauty contest”, the Solartwin solar water heating system came above midway of all 8 panels in terms of annual carbon savings.

Moving to a carbon per square metre footprint measure, the report also shows that of the flat plate solar panels, Solartwin came consistently close to the top (within 2%) in terms of carbon savings per square.

In terms of carbon clawback, there is the the question of why should anyone have to buy extra mains electricity in order to save heating fuels such as gas because they bought a solar panel? We think that the carbon clawback issue need sto be addressed at a strategic level. in Europe. Solartwin also eliminated the carbon footprint associated with conventional solar panels which, using the report’s calculation methodology, averaged 17% for flat plate solar panes and an astonishing 23% for evacuated tubes (vacuum tube solar panels).

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FAQ – Is Solartwin’s PV (photovoltaic) pumping a gimmick? No, for the above reason. Solar PV pumping makes Solartwin is around 20% more sustainable than old solar technology based on this side by side test report. Solartwin is a Zero Carbon solar panel water heating system. Most others are merely low carbon technology in that your electricity bill tends to go up with a solar panel which is used to replace gas or oil as a fuel with a consequent rise in global warming gases being produced. Hence why Solartwin is about 20% more sustainable.

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FAQ – What about Solar Trade Association membership? Why do some STA members slag off Solartwin?

It is interesting that the previous questioner used the term “gimmick” because recently a long standing member of the Solar Trade Association (of which we are no longer a member, more about this in a moment) attempted to stop us from gaining a significant solar heating contract for a school buildiing by making a number of strange and misleading claims including one claim that Solartwin was gimmicky.

We robustly contested this view and all of his claims were promptly and unreservedly withdrawn. But is Solartwin gimmicky?

We believe that Solartwin is an innovative solar water heating system which is proven over 10 years as a clever and effective zero carbon technology. Independent tests validate its operational carbon clawback as being zero instead of around 20%. It has also won awards from DTI for innovation and again for achievement.

The compact Oxford Dictionary defines a gimmick as “a trick or device intended to attract attention rather than fulfill a useful purpose”.

Solartwin is not a trick, but a cute device which is deliberately designed to perform more sustainably than conventional mains powered solar can ever do. As for attracting attention, well, we are entitled to state our technology’s benefits but for doing so we were once unlawfully fined by STA for making entirely valid carbon clawback claims. The fine was rescinded.

Solartwin appeared to have attracted attention, albeit hostile, from a competitor who appeared to be unwilling or perhaps unable to deliver our sustainability criteria and who seemed concerned that we were seeking to operate, perhaps unwelcome, in “his patch”. Well, tough. We will compete – hard but fairly. As for the useful purpose issue in the gimmick definition above, it is our view that Solartwin fulfils a very useful purpose in combating global warming by offering the public / private sector zero carbon solar water heating instead of low carbon solar.

This discussion raises the unfortunate issue of the overt hostility of certain members of the Solar Trade Association. If any readers have any comments or evidence of “spoiling” or misleading behavoiur by STA members we are interested in adding your experiences to our files – please do let us know!.

We have not been members of the STA since 2007. As for its influence, although we are unable to verify this figure, we we believe that under half of all UK’s domestic solar thermal panel installations are performed by STA members and we do not regard membership of the STA as being particularly valuable except that the UK Government see it is as the place to go for accurate facts about the solar thermal industry. We hope that readers do not regard our non-membership of the STA as being problematic: there has been a history of what can sometimes be almost Kafkaesque behaviour by STA regarding our business model and our technology.

Here are three instances of this.

  1. Apparent double standards regarding access to membership. Last year this Association, of which we were members for several years, refused to renew our membership, on the grounds that our solar heating panel must pass one particular test in a European solar panel Standard called EN 12975, a standard which contains more than ten tests for panels. (This standard is widely viewed as being out of date and due for review by CEN, who are Europe’s main standard setting body.) But the objectives of this particular panel test is not relevant to the particular way that our technology is used, a fact which was independently confirmed by a test house in Austria called Arsenal Research. So rather than abandon this solar panel test, Arsenal devised with us, a test which was as close to the original as possible but which was, in fact, relevant, and which was passed by our solar panel. So we pass all relevant tests in EN 12975. It is worth pointing out that regarding the irrelevant test, rather than simply abandon it, at the time of our exclusion, our solar panel passed pass a redesigned but relevant test. Nevertheless the STA insisted on our exclusion. This is funny because the deeper we look at this topic the more inconsistent the STA’s approach to us becomes. For example, our research into non-relevant tests (and exemptions to them) regarding other solar technologies reveals that, for years, evacuated tube type solar panels have been exempted, again on the grounds or not being relevant, from a certain, although different, test in EN 12975. It is noteworthy, and seemingly inconsistent, that evacuated tubes can apparently be certified to EN 12975 even though they are not required to pass this also irrelevant test and it is interesting that they are not required to pass even a relevant remodelled test (as our solar panel does). It is probably coincidental that several board member companies of the STA distribute evacuated tubes. It is probably a further coincidence that one major evacuated tube supplier which was on the STA board for years, sponsored Solar Keymark, the European solar panel super-standard. This is based on EN 12975 plus independent factory inspections. It is interesting to note that only last autumn Solar Keymark retrospectively rubber stamped this evacuated tube exemption, even though it had been taking place for years. So it seems to be the case that STA member companies can be exempted from non-relevant tests under EN 12975, unless they are Solartwin, in which case they are excluded from membership. The fact that, before we were offered the Solartwin patent, two STA board member companies tried to develop it but did not commercialise it, apparently “burying” it instead, is of interest, but presumably also of no relevance whatever to this discussion.
  2. £86K of state funds was allocated towards developing a solar panel installer training course (called the BPEC solar course). Channelled via the STA, its authorship was subcontracted to a previous STA Board Chairman’s company called Filsol. Early versions of this excluded our technology from fair coverage, on the rather debatable grounds that it was not a proven technology thereby forcing us to develop our own material and run our own seminars, at our own expense, in order to prevent innovative technology from being misunderstood and mis-installed.
  3. Attempted censorship. A few years ago the STA fined us £1000 for not editing our promotional literature as they wanted us to and for re-publishing independent DTI funded research showing that conventional solar technology had an operational carbon clawback which averaged 17% for flat plate collectors, and 23% for evacuated tubes in domestic sized installations. We had named the seven solar thermal companies involved, most of which were on the STA board at the time. Although it is our policy to compete fairly and openly on the carbon budget question, since we see this as an important issue of sustainability, and one which the consumer should know about, we now pull our punches – deliberately. We no longer name any companies involved in these tests because, being a small fish in a big pond, just standing our ground on such issues is not worth the hassle: it costs us too much. The £1000 fine was eventually rescinded as having been unlawfully levied, but resisting the fine, of course cost us far more than this.

Although no longer in the STA, we actively support the concept of Trade Associations. Besides being members of the Micropower Council, a highly focussed body which works on national policy issues affecting microgeneration, we are also active members of the Renewable Energy Association. The REA are, by far, UK’s largest renewable energy trade association representing a range of solar thermal technologies, including solar panels such as Solartwin. So we are members of a larger, and reputable UK trade association. The STA boycott is shocking – but fully understandable in the commercial context.

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FAQ – Should I replace my hot water cylinder with a special solar cylinder when installing a Solartwin solar panels simply because it is poorly insulated?

Usually not. When you install a Solartwin solar panel installation, your money might be better spent on buying one or two slip on insulation jackets at £10 each. You may then have cash left over from the cylinder budget for other energy efficiency measures. Of course there may be good, additional reasons to replace a cylinder instead in some circumstances.

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FAQ – Must I only use my existing cylinder with a Solartwin solar panel installation?

No. You have a choice. To clear up any misunderstanding: you are quite free to use a larger cylinder if you want with a Solartwin: you are not constrained to using your existing cylinder. You can choose to have a bigger cylinder at the time of your solar panels installation, or later on.

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FAQ – If I am changing cylinders then what choices do I have with Solartwin?

Several. The choice depends on individual circumstances but here are some general issues. The first choice is size. It is usually a good idea to go larger whne you install solar panels, since this will allow you to store more hot water. Then it can be a direct cylinder or an indirect one (ie with a heat exchanger, also called a coil). You may also use a thermal store with solar panels. We have just had clearance from DCLG (the building regs people) to use Solartwin to heat a heat exchanger in a high pressure G3 cylinder, but will will not be launching this just yet.

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FAQ – Is continuous circulation via solar panels, instead of on-off circulation (off when things get too hot) a valid design?

Yes. To control potential overheating, besides using “downstream” thermostatic blender valves say at the top of a cylinder in appropriate places such as at showers or at the top of the cylinder, solar thermal design niches can use several more “upstream” approaches. Old solar pansl usually use a solar controller to switch off the solar pump when the top of the solar hot water cylinder reaches a certain temperature, usually somewhere in the range of 65-85C.

But, by not removing the heat the solar panel gets hotter and hotter, typically 130 plus C in full sun. This exposes all closed circuit solar panels to “solar stagnation” which brings very high temperatures and pressures, engineering and stress considerations which inevitably constrain the design of its solar absorber surface to being made of metals. High pressure stagnation also constrains the plumbing design to a system, which if it were ever to burst would release superheated steam. Another older option is to remove the pressure challenge and to drain down the panel at high temperatures with an open vented low pressure plumbing design. This reduces the pressures but not the high temperature challenges.

So we chose continuous pumping as low temperature and low pressure, safe design. Particularly if the solar heating system uses a smallish cylinder, then to prevent boiling or overheating in the summer the system deliberately recirculates water to the solar panel, mainly in the afternoon, to cool it so that if you are on a summer holiday, say, the solar panels system will not boil. This heat export approach only works in places where the sun does not shine continuously (such as on Earth) and where it varies its intensity at the panel (so we do not recommend “tracked” solar panels).

For balance, it might be worth saying that conventional solar pump controllers will switch off the pump supplying the solar panel, which is another way of not collecting all the available heat. Either way, the solar heat ends up not being stored to the point where the cylinder might boil, which is a good thing from any safety perspective.

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FAQ – Does solar collector efficiency per square matter a lot?

Enormously if you live in a shed with a roof that is smaller than 2-6 sqm, which is a typical size range of a solar thermal installation. In terms of design, we have relegated efficiency per square metre to below sustainability which is at the top. Make the panel a bit bigger is our particular approach. I may be missing something important in life, but what is the big thrill about seeing a few extra roof slates? Solar panel customers who pay a shedload extra for a solar panels water heating system which is more efficient per square metre may end up paying £1000 more to see an extra square metre of slates on a roof. I’d rather spend £1000 to see an extra lovely painting on my living room wall, or perhaps save it to invest in further carbon footpring reduction initiatives.

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FAQ – Will solar thermal panels pay back their initial cost?

Who knows? Maybe not, unless the carbon and energy savings from solar are valued in cash terms, as well as the money saving. The financial calculation is a nightmare… First, capital outlay. Look at price paid, minus any grant, minus any possible resale value if you think that the installation might add value to your home. I hope that the final sum here will be less than say £4000, preferably a good deal less. Call this total your net capital expense. Secondly savings. Look at the savings which relate to the actually used solar energy which is actually delivered to the taps or showers divided by the hot water delivery efficiency (what’s that? a percentage usually in the range of 15% to 70%) of your gas boiler or other water heating system. There follows a calculation of converting energy into money, the outcome of which is usually unlikely to exceed £100. It could be far less. Maybe add a “boiler life extension” in money terms based on your boiler probably lasting a few percent longer before it packs up because less fuel will be put though it every year. Add on any carbon saving solar bonus cheque which the government gives you: this is currently non-existent. The total here will be your gross savings. Thirdly, expenditure or lost income. Add up any costs of owning the system such as maintenance in parts and labour. Add the extra electricity for the pump and controller, if they use mains electricity. Will your house insurance go up slightly because of higher rebuild costs? If so, then add in this increase. Will having a solar panel just slip you up into a higher council tax band? Let’s hope not. If you have bought the panel using funds which were in a bank account then you need to look at the lost interest, less any tax that you have saved. The total here is your gross expenses. Fourthly, subtract your gross expenses from your gross savings. What is left, in money terms is your net revenue gain. Finally, divide your net capital expense by your net revenue gain. This gives the solar panels payback in years assuming that fuel prices and all the other assumptions you have made do not change over time. If they do…aaarggghhh. (When did you last do a payback calculation on a car, computer, diamond ring anyway?) gasp…

Of course besides the money, there is the issue of energy payback, not just money. Making a solar panel uses energy, as does making the rest of the solar components. (And some solar panels but not Solartwin, use additional energy to operate them as well). So how long does it take to pay back the energy that is used to manufacture a Solartwin solar panel and its related components?

We asked Bath University this question and the answer is encouraging. In just a year and a half you will be breaking into energy credit, according to their calculations. The equivalent energy payback figure for solar electric panels (photovoltaics) is 4.2 years. So in energy payback terms Solartwin is better by a considerable margin. We do not know the exact position regarding conventional solar panel installations, but given that they generally require more, and heavier components it is probably reasonable to assume that their energy payback would be somwehat longer that Solartwin’s surprisingly short year and a half solar energy payback time.

Regarding the financial payback issues of our solar water heating system, Bath University’s calculations are that 30% of the upfront cost on a solar thermnal system is paid for during its life. So the balance (70%) would therefore need to be funded by either grants or an increase in value of your home, ot a combination of both for payback to break even. Naturally, there were lots of assumptions underpinning Bath University’s calculation, including that the Solartwin solar panel installation cost £3,523, that it delivered 815kWh of solar energy per year, that it displaced more that this, and that electricity cost 12p per kWh, and that gas cost 3.5p/kWh.

The Bath University reference for these figures is a paper called: INTEGRATED APPRAISAL OF MICRO-GENERATORS:METHODS AND APPLICATIONS, S.R. Allen, G.P. Hammond, H. Harajli, C.I. Jones, M.C. McManus and A.B. Winnett, University of Bath, Bath. BA2 7AY. Published 2008.

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FAQ – Don’t older designs of solar thermal system install more parts?

True. We are in the sustainability business, of saving carbon for your money, and of delivering solar energy with minimum carbon clawback, not selling you clobber. Trying to minimise the environmental impact of renewable energy technology this inevitably means fewer parts. As for thinking locally and buying solar panels locally, all our pipes, pumps and solar panels are made in England and we pay patent royalties to Scotland. And zero carbon solar inevitably costs extra: we supply you with an expensively and specially designed solar pump and a PV, not an imported £20 bronze pump and a length of cable for you to connect to the national grid. That is our particular philosophy of being in the solar energy business, to offer what we regard as a genuinely more sustainable way of working. Energy saving businesses in the British Isles ofer you lots of very different business models in this industry to choose from. As long as more people choose to reduce their carbon footprints effectively that’s great for us all.

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FAQ – Why can’t the solarpanels be sold for half the price? We want a low cost DIY solar solution.

Because of the above plus, unfortunately, highway robbery: the enormous cost of market entry with new solar water heating products in an industry where, in a few places a gold rush mentality pervades. For example there is a £86K state funded solar water heating training manual which is currently being used in many many plumbing colleges in UK. Some colleges have recognised its limitations, but some have not. Based largely on an outdated manual from 10 years ago which takes a mistaken “old solar is gold solar” approach, it contains numerous inaccuracies and recycles many of the myths about solar being only able to operate in a very limited technological niche. I went on a solar installers training course based on it at CAT (the Centre for Alternative Technology) where the trainers were well intentioned but the course did not cover our technology in a balanced way.

Unfortunately this manual has now been used to train and even accredit for grants, 1000 plumbers, some of whom may now mis-specify or mis-install our technology on the basis of this manual. We need to offer retraining to 1000 plumbers! How are we to fund this? We are a small business and it takes enormous time and effort (including missed family holidays) to keep rebutting crummy documents like these. The good news is that there is some light on the horizon: the latest building regulations on solar thermal panels have now been recognised as haing been drawn too narrowly, and other reviews are taking place including that of the self-referencing of incorrect government documents in order to create a false consensus that Solartwin is too marginal to be bothered with. Until UK and Europe develop a transparent approach to regulation and innovation, sorting such shenaigans will require a lot of luck and money. The money can can only come from early adopting users of new technology. I’m sorry about this. We can get a bit frustrated too. I hope this helps to move the debate forward.

Says Barry Johnston, Managing Director. Solar Twin Ltd.

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