Exergy analysis of solar energy systems - Solar Energy Sciences and Engineering Applications - page 97

Exergy analysis of solar energy systems
Ibrahim Dincer &Tahir Abdul Hussain Ratlamwala
Faculty of Engineering and Applied Science, University of Ontario Institute of
Technology, Oshawa, Ontario, Canada
3.1 INTRODUCTION
Extensive use of fossil fuels in past decades has led us to the era of global warming
and depletion of the stratospheric ozone layer. Fossil fuels such as gasoline, diesel,
natural gas etc. when used emit harmful greenhouse gases such as CO, CO 2 ,SO 2 ,
NO x etc. Since the realization of the harmful effect of using fossil fuels, researchers
have started looking for alternative sources of energy which are renewable and envi-
ronmentally friendly. One of the very promising alternative and renewable sources of
energy is solar energy. Solar energy converts solar flux entering the Earth's surface
into electricity or heat. The electricity generated by the solar energy system is in direct
current format which can be used as it is or can be converted to alternative current
based on end user requirements. Major benefits of using a solar energy system include
(a) environmentally benign operation, (b) no moving parts, (c) no wearing of parts
if the system is carefully protected from the environment, (d) energy output can vary
from watts to megawatts based on the size of the system, (e) can be used to power
phones or to power a community, and (f) module by module construction so that the
size of the system can be altered based on the requirements.
Solar energy systems are characterized as passive solar energy systems or active
solar energy systems. Solar energy systems are assigned to these categories based on the
way they capture, convert and distribute solar energy. Passive solar techniques include
(a) designing a building in a way that it uses solar energy for day lighting, (b) selecting
materials with favorable thermal mass or light dispersing properties, and (c) designing
spaces such that they naturally circulate air.
Active solar energy systems are divided in to three categories which are (a)
photovoltaic systems which generate electricity, (b) thermal systems which generate
heat, and (c) photovoltaic/thermal (PV/T) systems which generate both electricity
and heat.
There are two types of photovoltaic panels available in the market, which are
rigid photovoltaic panels and flexible photovoltaic panels. Rigid photovoltaic pan-
els have higher power to area density and flexible photovoltaic panels have low
power to area density. Solar thermal systems are sub-divided into following cate-
gories: (a) low temperature such as flat plate collectors, (b) medium temperature such
as concentrated collectors or solar dishes, and (c) high temperature such as heliostat
fields.