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?