ABSTRACT
Heat radiation from the sun is the main primary energy source for life on earth (another source is geothermal heat, mainly due to the decay of natural radioactive nuclei, like Uranium 235, and the cooling of the hot earth’s core). There is an average heat flux of about 1.4 kW/m2 from the sun arriving at the outer atmosphere of our planet; the more exact value of 1.367 kW/m2
refers to the solar constant. Absorption and reflection from the atmosphere reduce the received power flux on the ground by about 20−30%. There are two main technological pathways to harvest this energy directly, namely, with solar thermal and with photovoltaic energy conversion. It turns out that the endoreversible thermodynamics of Chapter 5 provides a convenient framework for a treatment of both of them. We will see, that the two endoreversible engines for solar energy harvesting are a thermal engine for solar thermal, and an electrochemical engine for photovoltaic power. The upper heat and particle reservoir is the sun, which radiates photons from its surface at a temperature of approximately T1 ≈ 5800K. The lower bath, in a terrestrial application, is assumed to be at the ambient temperature, T2 ≈ 300 K. The Carnot efficiency of an associated heat engine is then η0 = 1−T2/T1 = 0.95. Real efficiencies of solar thermal and of photovoltaic devices are, however, significantly smaller. The following subsections review a number of the concepts for discussing such solar energy efficiencies. For more details see, e.g., Refs. [Bej88], [DV00], [DV08], and [Mac15].
