Sunday, 10 February 2013

Tilting at the Sun - Vertical Solar Panels

Do Vertical Solar Panels get More Energy in Winter?

Can you install solar heating panels to collect more energy in winter?  If you tip them up vertically and over-size them, is it possible to get nearly constant heat output throughout the year?

Works in Theory?

In wintertime the sun follows a lower path across the sky, in summer the sun rises higher. The concept is straightforward – by installing solar panels at a vertical angle they face more directly towards the winter sun and should collect more energy at that time of year.  Of course, vertically installed panels also collect less heat in the summer, but since solar thermal systems often produce more energy than can be used in the building at that time of year, perhaps you could get more coverage of your hot water with vertical panels.

We set up a model in a solar thermal modeling software (T*Sol), and simulated a year of hot water generation with solar panels set up at a range of different tilt angles.  We did this for a “right size” system, 6m2 square metres of flat plate solar thermal panels serving a hot water use of 200 litres per day at 50C, and an “over size” system of 18m2 serving the same demand.

The table below shows the prediction of the simulation software for this range of systems.  The orange colour is the weekly energy in hot water used, and the yellow colour is the energy supplied by the solar thermal system.

Weekly Solar Energy (yellow) for Cambridge, UK 200litres/day water use (orange)

(Thanks to Ian Barclay at Viridian Solar for the modelling)

To make things a little clearer, here is a graph contrasting the solar energy delivered by panels installed at 35° and panels installed at 75° for the 6m2 system.  The more vertical panel produces much less energy in summer and only a little more in winter.  The loss in summer is not compensated by the gain in winter.

The reduction of summer energy is not compensated by increase in winter

Perhaps if we oversize the system to produce a massive excess in summer, can we bring up the winter energy and get a more constant output through the year?  The graph below does the same comparison for the 18m2 system. 

Even for oversized system, winter gains are less than summer loss

The larger system produces near 100% hot water coverage for much more of the year than the smaller system, but the effect of tilting towards vertical is the same as for the smaller system – the extra energy in winter does not compensate for that lost in summer.  Note also the effect of diminishing returns in solar thermal design – we have tripled the solar panel area for a 50% increase in solar energy (read more on this effect here).

Perhaps we should blame the UK climate?  What if we were to consider a more continental climate.  Cold, but sunny winters instead of the mild, cloudy, gloomy UK weather?  We repeated the analysis with climate data from Dresden, a location with a continental climate having a latitude similar to Cambridge. 

For Dresden, Germany the results are similar to the UK

The results show a similar pattern to the UK simulation.

Our modelling suggests that the main effect of tilting solar panels towards vertical is to reduce the summer energy collection.  Although the sun is lower in winter, it’s also weaker and more often obscured by clouds, so the winter benefits are small.

So much for theoretical modeling, does anyone have any real-word experience to share?


  1. Stuart,

    I can't say I have personal experience, but I know of one European that recommends 60 degree frames for heating applications, presumably for this reason.

    They also have a vertical mounting option, which has been used in facade installations, so obviously there are some situations where people think it worthwhile, I suspect for architectural style.... I believe these installations were done in conjunction with interseasonal storage to reduce stagnation and make more economic sense

    I can put you in touch with someone if you would like to find out more?


  2. Thanks for sharing such an ideal post regarding vertical solar panel. One of the profoundly promoted applications of vertical solar panels is their use in building retrofits. I will stay in touch for more facts.
    Solar Panels and applications

  3. Nicely explained, never knew about this before.

  4. Yeah, but you don't really get snow in the UK. The benefit to vertical tilting for panels in winter is self-clearance under snow, which should make a large difference in winter production in the snowbelt. That's the analysis I was hoping to see...

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  6. Firstly I am located in a not so sunny Co Antrim and much of my story flys in the face of convention but that would be normal for me maybe. I find theories and research a bit of a bore as whats the use of all such MCS type numbers if reality is seldom going to come up this level of efficiencies. I have an avid interest in all things energy and engineering. I am not a tree hugger per say but I believe that if we can make it ourselves then why not. I'd much rather have this than a Breitling, real or imitation as most folk dont care what watch one has let alone admire it. More likely despise it. The overall effect of the increase in wind and now particular solar PV worldwide has been partly I feel responsible for the lower oil price just now. I wouldnt think there are many outside of the oil barrons and oil workers who would not like this and I feel they have ruled our lives for too long. I am happy that I can have a lower oil price and drive to the shops and motorhome at a much reduced rate. We can easily add renewables to stationary buildings but not so easily to vehicles. Around 17yrs back I bought an un-used Mitsubishi 2m flat plate collector. I installed it temporally which ended up being 3 years. It was mounted at 45 degree's facing SE and fed direct to the hot cylinder. I drained the system down end of Oct each year. For the 3 years in question it saved us about 900ltrs of heating oil in summer. The water was just about warm enough for a showering even on not so bright days. On brighter days it had the tank hot and we soon learned that of the sun shone use all you can becasue this thing will soon heat it up again. After adding another structure and shifting the hot cylinder the panel had to be moved and permanently as I did not want to be without this great toy as I saw it. So the only obvious place was a vertical siting as I fancied thermosyphon direct and the site suited thermo to our tank. Very conveniently we had several windows 2m high x 1m wide glass facing SE again. I swore to wifey that this was not planned!!!! I mounted the same panel inside the double glazing and plumbed direct thermo to the cylinder which unusually was in the roof space. It worked marvellously for 2 years and being inside the double glazing it didnt need drained in winter so I discovered that it could actually do better than take the chill off a cold tank in winter just as long as the sun came out which is actually more often in winter than we tend to realise. With the knowledge I had gained I added another 2m flat plate beside the existing and can safely say I am happy with the results.

  7. And a little more as these things have a limitation which doesnt suit a rainy day and plenty of time to write
    My conclusions have led me to an assumption that my hot tank didnt need all the sun can deliver to an angular panel in summer even from one 2m panel and in summer sun the vertical single panel behind double glazing facing SE was doing admirable. I added the second panel in the hope that on the many summer days we have with little sun that there would be a benefit. There are now few days in summer that the tank needs help from an immersion and we have always been a family of 4 and earlier 5. My wife and I are now moving to a cabin and giving the old house over to our son. I have mounted 2 x 2m landscape shaped panels this time vertically on the south wall of our cabin as low as is practical to allow thermosyphon again simply because its simple. The panels this time are fed to the tank via a coil. Given that the cabin is obviously a small dwelling and that being a new system I had the choice as to where everything went probably made it easier. The panel medium is a glycol water mix and header system is provided by a truck header tank rather than what seems to be deemed the norm a pressure vessel. I opted for this as I do not like any form of pressurised system no matter how many PRVs and safety's are used. A header tank if properly mounted fills the system and doesnt get very hot as no circulation go through the header. This type of tank can take all kinds of heating and cooling and even boiling for years sitting outdoors in the suns rays so there should be no problems in the dark. The expansion blow off is directed to a "safe" place. The domestic end of the system is a traditional vented un-pressurised system with shower pump for the shower as there is only one floor and the header tank for the cylinder has to be beneath the 8'6" roof level. Again with an uncontrolled heatsource such a solar a pressurised system would require many devices including some method, usually pumped to disperse excess heat whether the event will ever happen or not. The vent pipe from the top of the cylinder instead of being directed back the the cold header tank is directed up and over the roof in case of a bubble of two. This was the way olden day solid fuel systems were vented here as often the ladies in the process of cooking etc would boil the water system and its better to vent the steam and today not return hot water to our plastic headers. Better vented outside and to safety. I now have many and varied heat and energy sources today but this new solar thermal system has no other form of heating connected as yet. I drain the water (140l tank) out every eve and each eve the tank varies between warm to hot (to the bottom) I dont know how much worse this will be than my direct system in the house but it had the disadvantage of being behind more glass than usual. This is now Aug 2015 and the last 3 months have seen behaviour similar to the house system with 140l of warm to hot water being made from a cold start every morning. Mostly systems dont get totally drained down to cold every eve so I think I am being fairly harsh on the system. I am assuming like the house that if one were using hot water during the day the sun will make quite a bit up on that again as solar seems to only reach whatever temp the panels have available and then stop at that. That would be par for the existing house system anyhow. So use all you can early in the day and the panels will make it up before eve again is our

  8. And yet again I had too many words etc
    theory. I dont go for a huge thermal store as 1 I dont have the space and 2 I gave up a long time back on trying to maximise on every available watt of every hour every available day. For the most part my solar thermal, solar PV and Wind power operates up to around 50% only a limited time is there above 50% available so why try and store so much for so small an input window. Much better to add more input then less storage will do. I hope this may help even a few readers and I also hope I dont stir a load of theoretical "it wont work" nonsense. It does work and I'm happy with it. Cheers and many sunny days to you all D