Solar Energy in Winter

Solar PV is the most accessible of the sustainable technologies.  Unlike wind turbines which are relatively expensive and require carefully selected locations to be effective, solar panels are relatively cheap and can be located in backyards, fields and roofs.  The downside of solar PV is that it does not work well in an English winter and not at all at night.

Under a clear sky, the ideal mounting for a PV panel is on a tracker device which keeps it pointed at the sun, however, this type of device is expensive and a common solution is to place them on a south facing frame tilted according the latitude of the installation.  This approach will maximise the yield over the period of a year allowing the panel to take advantage of the clear or sparsely clouded skies of summer.

Treat the comments below with caution as they are based on two experiments using  basic measuring devices and I have not yet hat the opportunity to test the ideas with a PV panel.  The hypothesis is that the yield of a PV panel may by higher in winter if it is mounted horizontally.  I'm contemplating some further work for next winter with a project which attempts to maximise the use of wind and solar power, but at the time of writing, no decision has been made.

The first experiment was conducted in 2010.  The equipment was crude, it consisted of a light dependent resistor (LDR) mounted at one end of a short length of waste pipe, this was mounted on some woodwork which allowed it to take readings around the hemisphere of the sky.  The calibration of the LDR was in Lux and whilst this was not ideal, the objective was to observe relative intensities, so units were not too important.

On occasional trips to the Science Museum, I'm inspired by the well crafted instruments and the neatly written notebooks of observations.  These are things I aspire to, this particular device was used as firewood after it had produced a few graphs.  The link below gives a slightly more detailed description of the exercise, but the graphics below show the extremes.

The first graph shows measurements made under a clear sky.

Obviously, the maximum irradiance is mainly direct and at a maximum in the direction of the sun.  Under an overcast sky, this is not the case.

In this case, the irradiance is diffuse and more or less evenly distributed around the hemisphere, albeit at a much lower intensity than under a clear sky.

The second experiment took place during the winter of 2010/11.  The equipment used was the first attempt at a solar radiometer.  Whilst the device did produce some informative data, it also taught the lesson that simplicity, reliability and repeatability should be the design objectives.  The procedure was stand in the back yard with the improvised radiometer around noon and take two observations, the first with the instrument in the horizontal position, then with it inclined to the same angle as the pitch of our roof.  The absolute magnitude of the irradiance was probably dubious, but the presented as ratios is informative.  The graph shows the distribution of the ratio of the irradiance of the horizontal surface to that of the sloped one broken down by cloud cover.

The results show two peaks.  the left one in the colours of a clear or moderately clouded sky shows that the irradiance of the horizontal surface is less than the sloped one.  More interestingly, this is reversed under an overcast sky in which case the irradiance of the horizontal surface is greater, albeit at a much lower level.

One of the challenges of sustainable energy is managing seasonal variation.  In the case of solar energy, the irradiance is determined by sun-earth geometry with the clear sky irradiance in January being less than 20% of that in July and the increased frequency of overcast skies in winter reduces this still further.  Horizontal mounting might increase the winter yield, but by how much is not clear.  I am currently working on a cloud sky computer model which might allow the concept to be explored.