I think that I cannot use pyroelectric materials in the same way I could use Seebeck effect materials for thermal energy harvesting. Pyroelectric materials do not need opposites sources - heat and cold , to generate voltage. Instead they need an either hot or cold temperature change. Once the temperature becomes constant, the voltage drops. In that case, day and night temperature changes is beneficial and it depends on what type of pyroelectric material I am using. The question is what is the rate of change (in temperature) for pyroelectric material that is available. If the material has a steep rate of change then a day/night temperature changes are not useful. If I have a pyroelectric material with a rate comparable to rate of change in day/night temperature changes then I will need only one component/material (pyroelectric). For Seebeck effect system to work, for a hot day I need a cold source to make Seebeck system to work. For a cold night, I need a hot source as well. And this complicates the design. I think that pyroelectric system simplifies a design of harvesting system provided that I have a material that operates in day/night temperature rates. Therefore, Pyroelectric system is useful where temperature changes occurs often and are not constant. Seebeck system is useful where you have source of hot/cold temperature and it is constant.