DIMINISHING RETURNS IN SOLAR PANEL AREA AND EFFICIENCY

You might think that the energy collected from a solar thermal panel was:

Light energy x panel efficiency x panel area

Seems reasonable enough, but it's not true. The reason is that there is a limited storage capacity, and a limited daily demand.

On a day of high light levels, once the hot water cylinder reaches its maximum safe temperature, the solar panel has to switch off - there's nowhere to put the energy. A lower efficiency panel might heat your hot water cylinder up to the maximum temperature a little more slowly than a more efficient panel of the same area, but they'll both harvest the same amount of energy on that day.

The same argument applies to a larger area of solar panel compared to a smaller one.

So, you might ask, why not put in a bigger hot water cylinder? Yes, for those sunny days, you'll collect more of the available energy, and you might carry some of it over for the next day - if the heat losses from the cylinder don't get it first. The problem with using a very large storage volume comes on less bright days. The solar panel will then only achieve a large volume of luke-warm water rather than a smaller volume of water at a useful temperature, and the boiler will needlessly fire to heat it up.

The fact is that for a solar water heating installation, the relationship between area and useful energy collected follows a curve of diminishing returns. The same is true for efficiency.

Here's a graph to illustrate the point:

Click on the image for more detail

It shows how the energy collected from a solar panel varies against the installed panel area. The household demand for this average-sized house is around 2,200 kWh per year, and is shown in blue. The green line is a typical evacuated tube solar panel, the orange is a flat plate panel.

Both show a characteristic curve of diminishing return as you increase the installed area, for the reasons already discussed. The other feature of note is the small difference in performance between the high efficiency evacuated tube, and the flat plate panel, a difference that is normally overcome in practice by the fact that flat plate solar panel installations tend to have a larger area.

For real-life data on the energy performance of solar panels, have a look at the Viridian automated solar test house.

Next: Integration with heating systems