Page 31 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

18 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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2.1 INTRODUCTION
Conventional energy resources may run out in the future, along with the fast-growing economy
worldwide. Energy consumption and efficiency have become two issues of high concern in today’s
society. Through developing renewable energies, societal sustainability may be maintained, which
also prevents possible energy crisis. As an important source of renewable energies, solar energy
as a consequence is broadly utilized, in either a passive or an active manner [1]. At present, for
active solar power techniques, depending on how the solar energy is converted into electricity, the
following three technologies are commonly used: photovoltaic (PV), concentrated photovoltaic
(CPV), and concentrated solar power (CSP). The three solar techniques will thus be covered and
addressed in this chapter.
First, the PV technology will be presented. Basically, according to the PV effect [2], abundant
solar energy can be directly converted into direct current (DC) electricity with the help of semicon-
ducting materials (e.g., crystalline silicon), forming a PV cell. Unfortunately, the voltage level of
a single PV cell is relatively low compared to the voltage required for many loads (e.g., charging
a battery, powering a lamp, the grid). Hence, the PV cells are connected in series and in parallel to
form PV modules to increase the voltage level as well as the power rating, respectively. In practical
applications, the PV modules are also connected in the same way, which enables grid connection
and thereby powering heavy loads. Details of the grid-connection issues for PV systems will be
covered in Chapters 3 and 4.
Although a lot of applications using PV technology can be seen in our daily lives, the energy
conversion efficiency is still not satisfactory. Figure 2.1 shows the best research cell efficiencies of
different PV technologies, where there are mainly five groups of solar cell technologies. In general,
the multi-junction PV cells can achieve higher efficiency compared to the crystalline silicon cells.
By contrast, in terms of cost, thin-film solar cells are the cheapest among the five, but is still at the
early stage of massive utilization. Intensive research is ongoing to push forward the penetration of
the thin-film solar technology into the PV market [4, 5]. Additionally, the emerging PV technologies
have been developed for certain applications with attractive features like low cost and transparency.
However, most of the emerging technologies are still not commercialized, and the efficiencies are
the lowest as shown in Figure 2.1 [3].
Nevertheless, in order to improve the conversion efficiency, CPV (also referred to as concentra-
tor PV) technology was developed and employed, where sunlight is concentrated using lenses and/
or mirrors onto small solar cells (typically using multi-junction PV cells for high efficiency). As it
can be observed in Figure 2.1, the CPV structure is typically multi-layered using a wider spectrum
of irradiance. However, since the electricity generation is based on the PV effect, according to the
PV characteristics, cooling of the CPV cells is an important issue for maintaining high efficiency.
Nevertheless, although the number of CPV installations is quite low compared to that of the PV
technology, systems using CPV have the potential to become competitive in the near future [6].
Hence, the power electronics technology for CPV systems has been also touched in Chapter 1, and
the CPV technology is also detailed in this chapter.
The PV and CPV technologies mentioned earlier are based on the PV effect. Alternatively, the
solar energy can be utilized and converted using the CSP technology. In contrast to the CPV system,
the CSP technology electricity generation systems first concentrate a large area of sunlight to a
small area and then heat up the thermal-carrying materials (e.g., water or thermochemical materials).
Then, the heat is transferred to either drive a heat engine (e.g., a steam turbine) or power a thermo-
chemical reaction, and hereafter, electricity is generated. The role of power electronics here is thus
to convert and transmit the energy in a reliable and efficient way. However, the CSP systems can
provide large thermal energy that requires specific technology or systems to store. As CSP systems
are growing at a fast pace, this chapter will also conceptually introduce the CSP technology, where
its impact on the entire grid is discussed. It is worth mentioning that synchronous generators can
be used in the CSP systems, where the structure is similar to the classical power generation system.