Solar Heating - Some Main Questions

Q: Hello All, These questions apply, generally, and are not just for DIY. There seem to be 2 main types - Flat panel and Tubular. 1. What are the Pros and Cons for the 2 types ? 2. Has either an advantage in terms of a. Effective power for a given roof area ? b. Effective power for a given active area ? c. Cost of panel for a given power. I am thinking about the fact that some tube systems have considerable gaps between tubes. Evidently, this is to reduce likelihood of lift off in high winds, but obviously useful roof area is lost. 3. It seems that there are 3 different methods of mounting. a. In roof - mainly for flat panels. b. On a roof mount, fixed to rafters, and possibly standing off the roof. c. Slung from straps, passing under slates or tiles and fixed to rafters. Panels resting on roof. It would appear that the Pros and Cons for these 3 are :- Cost Danger of Wind lift Variable Angle 3.a. Highest Least Fixed 3.b Intermediate Perhaps highest Variable but more secure than 3.b 3.c Lowest Perhaps intermediate Fixed If any of you have experience, or know some good websites where I can obtain such information, I should be grateful.

A: It may be an advantage, but is not the reason for doing it. Anyway, the space between the tubes is often taken up with reflectors, so the wind loading is similar to a flat panel. One of the advantages of vacuum tubes is that to some extent they track the sun as it moves around the sky. Closing the gaps would reduce this. You left out mounting on the wall, which can be either done vertically or at an angle using some sort of strut to make the bottom of the panel stand out further from the wall. And before anybody chimes in to say 'waste of time' - we had a solar tube system at the last house and it covered the majority of our dhw needs from May to September.... sadly we didn't stay there long enough to get the payback - but it's great to have a shower or bath in solar-heated waterWhat really matters (like wind power), isn't the panel's ultimate performance in ideal conditions but rather how badly it falls off in non-ideal conditions and thus how well it performs over an average year. Remember that the "Spanish laundry" is the easy problem, we're looking at how to save money through a UK winter heating period and that's about coping with poor sun. A second question, nearly but not quite as important, is the outlet temperature. This is why heat pipe tubes are now doing so well. IMHO, the best UK solar system is evacuated heatpipe tubes running a low-temperature underfloor heating system, with some degree of heat storage. This isn't the best performing system, but it's the one with the best payback against capital investment, and also reasonable performance. Space-heating with radiators works too, but underfloor is a better fit to available temperature and availability. There are many factors, its not simple. If I were building a HW system today I'd opt for heavily featured panels. Add reflectors to give some concentration, and add either 2ndary glazing, maybe in plastic film, or an external bubble that collects warm air as well as acting as 2ndary insulation. These features change a low cost flat panel's characteristics to be closer to those of tubes by raising the stagnation temp. You can hybridise the 2 too. A flat panel type outer casing, but inside it runs black pipes with shallow trough reflectors, somewhat like a 2'x2' office ceiling light. The plus of these is mostly tube- like performance at much lower cost. And lower cost means better ROI. It also means you can have more active area for less cost, giving more energy return too. But at the end of the day, if youre looking at winter space heat I'd generally use heated air panels, not massively increase the cost by using a wet system.

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