Photosynthesis as a thermodynamic cycle

Abstract

Thermodynamics of photosynthesis has been a subject of interest to the scientific community; it is, therefore, addressed in this paper. This work reveals that traditional thermodynamic relationships may be used to calculate and project photosynthesis. Solar energy is required for the chemical reaction of green matter production. When the size of the green matter expands, less solar energy is received by the surroundings and more chemical energy is stored in plants and vegetation. If everything else is the same, the increase in the chemical energy produced is equal to the decrease in the heat of the biosphere and vice versa. Photosynthesis expansion is thus equivalent to heat transfer from the biosphere to the green matter. Plants surrounding air may be assumed as a heat reservoir at air dry bulb temperature, Tdb. The colder air enclosed by the space of the green matter may be assumed as a cold reservoir at air wet bulb temperature, Twb, and photosynthesis may be represented by a Carnot engine cycle. The thermal efficiency of the cycle is equal to 1-(Twb/Tdb)0.5. If everything else is the same, the difference, Tdb-Twb, is a limiting factor of terrestrial photosynthesis. Based on this understanding, equations to predict growth of the green matter and tree diameter are derived and validated based on observations. Other findings include photosynthesis global average thermal efficiency is between 0.61% and 0.72%, and seasonal greening is nearly 0.80%. Neglecting deforestation, surface greening trend with climate change is between 0.23% and 0.28% annually.