I want to focus solar energy on a solar cell with the use of mirrors (that I already have) and maybe some aluminum foil.

Should I not go overboard on the focusing? How can I detect if the focused solar is too much?

Background information: I live at 45° latitude, the sun is not that strong here.

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    Are you sure focussing is helpful/economical? Focussing will only help when the Sun is at one particular angle, whereas unfocussed arrays work for an entire hemisphere. You'd need moving parts, and I doubt you want to go there. – gerrit Jan 13 '15 at 16:44

Yes, if the photovoltaics aren't designed for operating as concentrated PV, it can shorten the life of the cell: the additional thermal stress can be huge; the cell will operate at a higher temperature, as it can't dissipate heat that quickly. This can crack the encapsulant, leading to moisture ingress and the corroding of the cell and/or its contacts.

Higher operating temperatures also reduce the efficiency of the cell during the period of raised temperatures. The panel's datasheet will normally have a coefficient for this: the rate at which efficiency or peak power drops off per degree change in temperature.

You'd have to find positions for your reflectors that would adequately bounce sun off onto the panel, without shading the panel - that might not be easy, particularly at your latitude, where the position of the sun in the sky can vary a lot over the year. And the maintenance and wind-loading on the reflectors should be taken into consideration.

Solar panels have become so cheap now, that concentrating the sun is rarely worth it.

Even at 45° latitude, there will be periods of hours of intense sunlight. If I were to try the reflectors, I'd put a temperature sensor on it (it may already have one), and get an idea of normal cell rear-surface temperatures at 3 or 4 continuous hours of peak summer sun without the reflectors; then I'd keep an eye on that temperature when operating with reflectors over the same sort of exposure period.

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