Sustainability: Humanity Perspective - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 775

Solar energy can be harnessed in different levels around the world. Depending on
a geographical location the closer to the equator the more “potential” solar energy is avail-
able. Plant biomass coverage on the ground is a key factor is whether or how the solar energy
should be captured.
Solar technologies are broadly characterized as either passive or active depending on the
way they capture, convert, and distribute sunlight. Active solar techniques use PV panels,
pumps, and fans to convert sunlight into useful outputs. Passive solar techniques include
selecting materials with favorable thermal properties, designing spaces that naturally circu-
late air, and referencing the position of a building to the Sun. Active solar technologies
increase the supply of energy and are considered supply-side technologies, while passive
solar technologies reduce the need for alternate resources and are generally considered
demand-side technologies.
Solar power is the conversion of sunlight into electricity, either directly using PVs or indi-
rectly using concentrated solar power (CSP). CSP systems use heliostats (lenses or mirrors)
and tracking systems to focus a large area (via hundreds of movable mirrors or heliostats)
of sunlight into a small beam. At the central location (focal point), heat engine is employed
to convert the radiant heat into electricity. PV converts light into electric current using the
photoelectric effect.
Commercial CSP plants were first developed in the 1980s, and the 354 MW SEGS CSP
installation is the largest solar power plant in the world and is located in the Mojave Desert
of California. Other large CSP plants include the Solnova Solar Power Station (150 MW) and
the Andasol solar power station (100 MW), both in Spain. The 80 MW Sarnia Photovoltaic
Power Plant in Canada is the world's largest PV plant.
Commercial solar water heaters began appearing in the United States in the 1890s.
These systems saw increasing use until the 1920s but were gradually replaced by cheaper
and more reliable heating fuels. As with PVs, solar water heating attracted renewed
attention as a result of the oil crises in the 1970s but interest subsided in the 1980s
due to falling petroleum prices. Development in the solar water heating sector pro-
gressed steadily throughout the 1990s and growth rates have averaged 20% per year
since 1999. Although generally underestimated, solar water heating and cooling is by
far the most widely deployed solar technology with an estimated capacity of 154 GW
as of 2007.
15.7. GEOTHERMAL ENERGY
The Earth's core is very hot. Soil and rock have very low thermal conductivity. As the earth
surface cooled for millions of years, it is the low thermal conductivity of rocks and soil that
kept the earth's core hot. The sustainability of earth relies on the slow heat removal process to
keep the earth warm. Therefore, deliberate drawing down the thermal energy stored deep
underneath the earth is not a responsible action to the ecosystem.
Geothermal energy is thermal energy generated and stored in the Earth. Earth's
geothermal energy originates from the original formation of the planet, from radioactive
decay of minerals, from volcanic activity, and from solar energy absorbed at the surface.
The geothermal gradient, which is the difference in temperature between the core of the