Page 14 - A Comprehensive Guide to Solar Energy Systems

6 A CoMPREHENSIVE GUIdE To SoLAR ENERGy SySTEMS

1.3 Types of Technology That Can Harness Solar Energy

There are two main types of solar power:solar thermal and solar PV.
Solar thermal includes domestic hot water systems (Chapter 6), cooking [13], solar-
disinfecting water [14], energy storage—molten salts [4], solar power transport [15], fuel
production [16], and CSP (Chapter 7). The latter involves focusing and tracking the sun’s
rays using mirrors (usually parabolic troughs or dishes) onto a working fluid, which vapor-
izes and expands and is used to drive a turbine. The temperature of the working fluid can
o
reach 800 C. The great advantage of CSP is that the sun’s energy is converted into heat,
which can be readily stored. This is not true for PV systems, because electricity is more dif-
ficult to store, although battery technology is rapidly improving.
It has been estimated that solar energy could be used to supply up to 70% of household
hot water in the United Kingdom and in sunnier climates, providing almost all domes-
tic hot water. Today worldwide solar water heaters are responsible for 435 GW th [17]. CSP
supplies 5.01 GW electricity globally, this being less than 2% of all electricity supplied by
solar energy; Spain is the CSP world leader with 2.5 GW investment followed by the United
States (1.9 GW) [17].
Solar PV panels (Chapters 8 to 12) produce electricity directly and can be effective in
both, direct, or diffuse cloudy solar radiation, although the systems are obviously more
efficient in direct sunlight. Electricity is produced as a result of the sun’s energy strik-
ing a solar panel (at present usually pure silicon), which causes electrons to be released;
these in turn then travel through wires (Chapter 8). Until recently, the only solar panels
(wafers) available were made of pure silicon (99.9999 purity), which is both costly and
energy-intensive to manufacture (Chapters 9 and 21). Recent research into wafer technol-
ogy has produced a range of new solar wafers, which include materials, such as cadmium
telluride (Chapter 10) interesting alloys of copper indium and gallium (Chapter 21) and
more recently perovskites (Chapter 11). Some of these involve elements, which are in short
supply; and some involve elements, which are toxic, for example, cadmium (Chapter 21).
Silicon wafers have improved significantly over the past 2 decades and the efficiency is
of the order of 20%. Furthermore, with mass production, the price of silicon wafers has
decreased enormously.
A recent report by Fraunhofer stated that in Germany, in 1990, the price for a typical
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rooftop system of 10–100 kW p PV, was around 14 € (kW p ) . At the end of 2016, such sys-
tems cost about 1.3 € (kW p ) . This is a net-price regression of about 90% over a period of
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26 years [18]. Solar panels suitable for use on roofs are now manufactured in such quanti-
ties that the electricity generated in several favorable locations, according to the World
Economic Forum (WEF), has reached grid parity; that is the point where the direct, un-
subsidized, cost of PV generated electricity is equal to that of fossil fuel generated power
[19]. The growth in PV manufacturing has been driven by government incentives where,
for example, in countries, such as the UK, Germany, Spain, and Australia the cost of elec-
tricity and technological innovation is subsidized. Under such schemes a premium tariff