Hi Henriëtte - if you want to use air you can look at this set up - just put your air pump in the top nook between the window and backdraft pannel. the build-it-solar site has lots of intressting goodies - most with scientific validations. Maybe play arround with different humidity factors in your solar heater? Also some degradation in air pump efficiency at higher air temperature?
Liquids (water) would be beter heat carriers - but there is always the freeze fear.