Technical questions and answers

Making solar hot water is intrinsically simple. However, there are some subtle details. Here are some technical questions posed to us by customers about Solartwin's unique alternative energy system. Thanks for the opportunity to reply:

Q: 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: Solartwin collects solar energy and operates independent of temperature. The box in which our panel is housed is double glazed and very well insulated indeed. The answer is nevertheless a very remote yes. It might cool water by 5 degrees C if you put it in the 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 is usually above our competitors. According to our research at Napier University, payback is further boosted by having no parasitic electricity costs (for 2 motorised valves, a solar controller which is on 24 hours a day, and a pump). Whether this remote yes outweighs the fact that conventional solar uses mains electricity 24 hours a day depends on you.

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Q: What is solar energy? (not too long please!)

A: Solar energy is the radiation which comes from the sun. This comprises around 55% visible radiation, which we can see. Most of the remaining solar radiation is infrared radiation. A tiny amount is ultraviolet radation. Solar water heating panels 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 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. May I stop now?!

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Q: 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 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 pump runs only on solar energy in the form of electricity. It has no temperature sensor, only a high pressure bypass in case the panel or its pipes are frozen. In response to your question: First - 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 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. Fifth - if they really want to put cold water in under these circumstances they can connect a second cylinder behind the first and draw water off it! This will also allow for more summer hot storage and is a neat solution for people with AGAs and Rayburns.

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

A: It draws water from the bottom and delivers it hot to the top, to be pedantic. This brings huge benefits. The 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 hot water on days of half sun. Most others deliver a full tank of tepid water. You choose.

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Q: What is the maximum heated water temperature that the solar panel can achieve?

A: Water and steam at 100C is theoretically possible if you were to switch the pump off on a hot sunny day. But please don't do this. With the pump running we have only ever achieved 85C so far at Napier! But 160C internally for the panel if left dry in the sun, is possible.

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Q: My family is hoping to self-build in the next few months and would be interested in installing a DIY Solartwin panel 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 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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Q: What is the life expectancy of the pump and does it come with a guarantee?

A: Intended design life of the whole system is 20 year minimum. The performance warranty is 5 years, although some products, such as the G.E.C. Lexan glazing actually come to us with 10 years warranty. I would expect the 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 a brushless DC motor. Using a brushless, rather than a brushed motor reduces wear and increases life.

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Q: What wattage does the photovoltaic panel produce?

A: 5 watts, which we then reduce to 4W using a shunt resistor, so the pump's output is approx 0.7 litres a minute peak.

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Q: I am slightly concerned about the head available from the 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?

A: Please don't be concerned. The 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 motor but the pump head itself has been de-rated by fitting an internal pressure bypass which opens at about 1 bar if any pipe is frozen. Hydraulically, our system is fine for a total pipe length of 30m of our own 6mm microbore flexible pipe. If you want to go over this, then you need to increase the pipe diameter on the side which supplies cooler water to the 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 losses. First its surface area is about half that of normal pipes. Second, its volume at 6mm diameter is about a fifth of the volume of 15mm pipes, so 'dead leg' losses are minimised. Another way of looking at this benefit is that you can site the panel much further away from the cylinder with our system compared to conventional solar, if you want.

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Q: I'm concerned that if I relied on the PV only there might be a tendency for the system to 'leak' 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!) would be infinitesimal.

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Q: The web info suggests the unit 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 are a specially formulated silicone rubber. They can expand over 100% by volume. Water expands 4% on freezing.

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Q: I have a bungalow with a flat roof ground floor extension. The upper, sloping roof could be used to support a 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 it will not sit in any puddles. You will need an angled A-frame for support. We can make these at any angle if you want. 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 it sitting tight and to do away with roof penetrations.

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Q: You provide lots of lovely information but as regards payback time you just say that it is shorter. Do you have some estimates of the payback time?

A: The issue of 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
• the microclimate
• the orientation of your panel
• the thermal efficiency of your boiler - if you have one
• what proportion of the delivered solar hot water you actually use
• the annual fuel 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 you consider to be a capital value boost to the value of your home and a few others besides.

All this makes for gigantic spreadsheets with assumptions compounding upon each other and a huge opportunity for financial trickery. For example one unfortunate solar salesman I invited home (as part of a competitor analysis) applied a crazy 13% fuel price inflator to the fuel displaced and pretended that the system also saved on central heating, thus hastening the payback by a factor of about three! 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 a third of electricity. We estimate that for mains gas (not more expensive bottled gas) you are unlikely to exceed £60 on fuel savings and perhaps save another £10 or so per annum on boiler life extension. For electricity (peak rate) the savings can be well over £100 P.A. The environmental benefits which do not directly accrue to you can be calculated / estimated using various methodologies. Some are optimistic and some pessimistic. By using midrange figures we reckon these can be around £25 P.A. (gas displaced) and around £50 for electricity. I hope this gives you a flavour of why I cannot pluck a 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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Q. What about underfloor heating?

Not a very good application 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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Q: Have you got a solar panel on your home?

A: Yes, 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 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 lots of hot water for our hands and for the paddling pool in summer...

Q - 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 could heat a 150 litre cylinder, not a standard household 120 litre cylinder. We were not in the test, originally. At the very last minute one company dropped out and we were asked to supply a system. But 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 and faced lots of flak, mostly based on simple misrepresentation of the paper. In terms of reliability, of the 8 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 which did not break down, perhaps indicating that simplicity is a reliability bonus.

Even though it was undersized for this “beauty contest”, Solartwin 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 plates Solartwin came consistently close to the top in terms of carbon savings per square.

In terms of carbon clawback, and the question of why invest mains electricity in order to save heating fuels such as gas? Solartwin also eliminated the carbon footprint associated with conventional solar which, using the report’s calculation methodology, averaged 17% for flat plates and an astonishing 23% for evacuated tubes.

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Q - Is Solartwin’s PV pumping a gimmick?

No, for the above reason. PV pumping makes Solartwin is around 20% more sustainable than old solar based on this side by side test report.

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Q - Should I replace my cylinder when installing a Solartwin simply because it is poorly insulated?

Usually not. You money is probably better spent on buying one or two slip on insulation jackets at £10 each. You may then have some spare cash left over from the cylinder budget for other energy efficiency measures. Of course there may be additional reasons to replace a cylinder.

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Q - Must I only use my existing cylinder with Solartwin?

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

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Q - 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, 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. 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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Q - Is continuous circulation 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 uses a controller to switch off the pump when the top of the hot water cylinder reaches a certain temperature, usually somewhere in the range of 65-85C. But by not removing the heat the panel gets hotter and hotter, typically 150 plus C in full sun. This exposes the panel to “solar stagnation” which brings very high temperatures and pressures, engineering and stress considerations which inevitably constrain the design of its absorber 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. We chose continuous pumping as low temperature and low pressure, safe design. Particularly if the system uses a smallish cylinder, then to prevent boiling or overheating in the summer the system deliberately recirculates water to the panel, mainly in the afternoon, to cool it so that if you are on a summer holiday, say, the 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” panels). For balance it would be worth saying that conventional solar will switch off the panel, which is another way of not collecting all the available heat. Either way, the heat ends up not being stored, which is a good thing from a safety perspective.

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Q - Does 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 in life, but what is the big thrill about roof slates? Solar customers who pay extra for a 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 saving initiatives.

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Q - Will solar thermal panels pay back their initial cost?

Who knows? Maybe not, unless the carbon savings are valued 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 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 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...

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Q - Don’t older designs of solar thermal system install more parts?

True. We are in the sustainability business, of saving carbon for your money, 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 locally, all our pipes, pumps and 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 renewables business, to offer what we regard as a genuinely more sustainable way of working. Energy savers in the British Isles have 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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Q - Why can’t they be sold for half the price?

Because of the above plus, unfortunately, highway robbery: the enormous cost of market entry 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 may colleges in UK. Some colleges have recognised its limitations and some have not. Based largely on an outdated manual from 10 years ago which takes an “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 course based on it at CAT 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 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 hope this helps to move the debate forward.

Says Barry Johnston, Managing Director.

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