carbon limited

As part of the design of a UK secondary school, we were looking at using solar thermal panels. Normally, solar thermal doesn’t work well in schools because in the UK almost half of your energy is harvested in the summer when the building isn’t in use.

As a solution, we talked about trying to bias the output of solar thermal panels towards winter months by increasing their inclination. By putting them at a steeper pitch, you expose less collector area to direct sun in summer and, providing the spacing is sufficient, more area to low angle winter sun. Rowland our boiler man said he was doing this with a large array on his own house, though for different reasons. In his case, he was using the output from solar thermal to supplement his space heating. While the theory sounds good, I’d never looked closely at it.

Using weather files for Kew, I took the monthly solar exposure for a 1m x 1m panel at a number of inclinations. Here’s how it came out:

Not so good. The 40° inclination outperforms the steeper panels in every month of the year. There’s no benefit in winter to having more steeply inclined panels so you might as well just angle them correctly and have fewer of them. Looking at the weather file, it made sense: there’s not much direct winter sun. So generally in overcast conditions the panel that can see more of the sky is going to get more radiation. But if you do the same thing using Rome weather, you get a different result:

Now the steeper panels have an advantage in the winter. Compared to the lower inclination you get almost 30% more output in January and 30% less in July. In terms of total output for the year, it’s only about 10% less. These are just two particular weather files but at least it shows that the concept works where you’ve got winter sun.

It doesn’t look like it works well enough to do what Rowland wants to do though. Even with steeper panels the peak output is still in summer. So if you size your array to meet 100% of your summertime hot water demand (more than this and the system will overheat), you’ll meet less than 100% of the hot water demand in non summer months. In the heating season you’ll still be using all of the output from solar thermal for hot water, not for heating. So it looks to me like you either have to shade your panels in the summer or have a big old heat store.