Solar energy is cost and carbon free. However, the intensity of solar radiation depends on weather, climate and season. It is ironic that solar biosolids dryers were first developed in Germany, a rather cool country with little sunshine, at least during fall and winter. Required footprint and the costs of greenhouses depend on solar energy input, and both can be very large where the climatic conditions are less than ideal.

Another common problem is odor generation. This happens where pasty and non-porous sludge cake is fed, becomes anaerobic and develops volatile and odorous fatty acids.
Operator access should be kept to an absolute minimum due to high infection risks. Biosolids are usually not disinfected and the greenhouse air contains many pathogens. 

Solar dryers should be:

-effective and efficient, independent of climate and weather,
-virtually odor-free, i.e. anaerobic conditions must be prevented,
-fully automated and very reliable, including their feeding, moving and discharge equipment

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