Introduction to Solar Energy Conversion - Nanophysics of Solar and Renewable Energy - page 114

Figure 5.1 Two-stage optical system taking
wide angular range emission from hot black
surface and focusing it into a small angular
range. In reverse, this will focus sun s rays on
black surface making use of a large angular
range. This system has a primary Fresnel lens

and a secondary imaging element made of glass
of high index of refraction n. The concentration
ratio in such a system can, in principle, reach n 2
(D es /R s ) 2 with 84 000 experimentally
attained [53].

focused. Focusing systems are almost useless on a cloudy day, while flat solar panels
may still operate at 30 - 40% of their normal capacity.
The direct heat absorption to heat water and to heat a home is actually important in
economic terms, perhaps larger than the electric conversion systems that are our
primary interest. These systems reduce the load on the electric power grid and should
be classi ed as renewable energy. Building of homes in several countries, notably
Israel, requires solar heating used for domestic hot water and to partially heat the home.

5.2
Heat Engines and Thermodynamics, Carnot Efficiency

Aheat engine works between a hot reservoir, T s and an ambient reservoir T a , and has a
limiting efficiency
g Carnot ¼ 1 T a =T s :

ð 5 : 3 Þ