I have a system in my house drawing air from the top ridge area of the loft space, through a large (vehicle) air filter (effectively outside air and dust in loft) and down into the house using a continuously rated extractor fan (Melrose). I first used a differential temperature switch unit thinking it would be clever, but found the simple solution of a manual wall switch was better, if the air feels warm I leave it running, if not I switch it off.
I had measured the temperature in the loft over time and it is very surprising how little sunshine it needs to raise the temperature by at least 5degC over ambient, and from March until into October I can get effective warm air for nearly free (fan takes 23W max, I estimate heat delivery can be up to 200W equivalent when the sun shines). The roof does face due south one side. It can be 10C outside and 20C in the loft, useful pleasantly warm air.
It will however take some years to repay the cost, even doing it myself, it just gives me a warm feeling in more ways than one. I would recommend others spend the money on the fuel bills instead.
(Edit - checked just now and it's 14C outside and 21C coming from the loft, it hasn't been particularly sunny today).