Sustainability: Humanity Perspective - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 773

The radiation energy from the sun powers the ecological systems on earth. Fig. 15.15
shows the distribution of thermal radiation energy on earth. The thermal radiation can be
quantified by

E ¼ ε sA S T 4

(15.22)

where ε is the emissivity of the body, s is Stefan e Boltzmann constant or

$ K 4 , A S is the surface area of the body, and T is the surface temperature
of the emitting body. For thermal radiation absorption, A S is the surface area of the body that
receiving the radiation energy. The thermal radiation absorptivity has the same value as the
emissivity.
As shown in Fig. 15.15 , 174 PWor 174

10 8 W $ m 2

5.6704

10 15 Wof solar radiation reaches the earth's atmo-
sphere, only about half of which (89 PW) is absorbed via earth surface (land and oceans). Not
surprisingly, nearly half of the radiation reached to earth surface (40 PW) results in the evap-
oration of water, powering the water cycle on earth ( Fig. 15.5 ). In total, there are 122 PW of
solar radiation actively powering the ecosystems on earth, before the energy is radiated
into the space. Partially intercepting the solar radiation beamed to earth surface to power
directly the human society can be beneficial to the ecosystems as a whole on earth.
Solar energy has been harnessed by humans since ancient times using a range of ever-
evolving technologies. Solar radiation, along with secondary solar-powered resources such
as wind and wave power, hydroelectricity and biomass, accounts for most of the available
renewable energy on earth. Only a minuscule fraction of the available solar energy is used.
Solar-powered electrical generation relies on heat engines (indirect conversion) and PVs
(direct conversion). Solar energy's uses are limited only by human ingenuity. Indirect conver-
sion of solar energy to meet the needs of humans is easier to implement. A partial list of solar
applications includes space heating and cooling through solar architecture, potable water via
distillation and disinfection, daylighting, solar hot water, solar cooking, and high

Radiated to
Space from
atmosphere
111 PW

Radiated to
Space from
surface
11 PW

Reflected
by clouds
35 PW

Reflected by
atmosphere
10 PW

Reflected
by surface
7 PW

Incoming solar:
174 PW

Radiation
absorbed
by
atmosphere
26 PW

Conduction
and air
convection
12 PW

Water
Vapor
40 PW

Absorbed by
atmosphere
33 PW

89 PW absorbed by surface
(land and oceans)

FIGURE 15.15 A schematic of major pathways of energy flow to around earth.