Page 43 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

30 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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power cycles that are adjusted to the specific needs of solar power generation. Instead of providing
thermal energy via burning fossil fuels (which causes vast CO emissions and air pollution) or via
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nuclear reactions, the naturally and freely available solar irradiation is used to increase the working
fluid’s enthalpy, which finally drives a steam turbine. The turbine is connected to the shaft of an
electrical generator in order to generate electricity and the power produced is then fed into the grid.
Figure 1.11 shows such complete systems.
The main technologies that can be found on the market are steam based, engine based, and PV
based. In most cases, water/steam is used as working fluid (Rankine steam cycles), but there do exist
some concepts that are still under consideration in which air or CO is used as working fluid, which
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drives a gas turbine (Brayton gas turbine cycle). This last design enables the possibility to run an
attached bottoming steam cycle and by using heat recovery boilers results to the so-called solar ther-
mal combined cycle plant. In addition, typical Stirling engines are non-steam-based technologies,
and they are used to convert solar thermal energy into mechanical work.
CSP plants have the inherent energy storage capability in the form of heat, and with further
support from additional thermal storage systems or a hybrid system, CSP plants may continue to
generate electricity even when clouds block the sun rays. Although CSP has better performance for
grid integration than other renewable energy options, the relatively immature technology and the
high cost restrict its large-scale deployment. In addition, one disadvantage of CSP is that it requires
strong direct sunlight. Consequently, the highest share of CSP resources is mostly limited to semi-
arid, hot regions or deserts. However, similar to other thermal power generation plants, CSP requires
water for cooling, which constitutes a big challenge in exploiting CSP resources in arid regions.

2.4.1 Solar Thermal Power Concepts

Solar thermal power plant concepts can be divided into two basic categories, namely, line and point
focusing technologies. On one hand, line focusing systems concentrate the incident solar direct irra-
diation onto a focal line, in particular onto solar receiver tubes that are placed all around the focal
lines of the solar collectors. On the other hand, point focusing systems concentrate the incoming
solar irradiation onto a single focal point or better, onto a focal area that is very small compared to
the total size of the reflecting mirror surface. One single solar receiver is therefore placed at this
focal point or area [27–29]. Generally speaking, the concentration of the incoming solar irradiation
is the fundamental principle in order to reach high temperatures, thus providing high-quality thermal
power that enables the operation of conventional thermodynamic power cycles. In this sense, the
CSP is not based on the PV effect anymore.
Nowadays, the most used design is the parabolic trough collector technology (line focusing).
There, the power cycle’s working fluid is heated indirectly, using a heat transfer fluid that is heated
within the focal lines of the parabolic troughs [27]. Temperatures reached a range between 400 °C
and 550 °C, depending on the used heat transfer fluid (thermal oil or molten salts). Another concept
that has been tested and operated successfully is the solar power tower or central receiver concept
(point focusing) [28, 29]. In that case, the solar radiation is concentrated onto a central receiver area,
which is placed at the top of a tower, using many slightly curved mirror elements. At the receiver,
the concentrated solar energy is transferred to the working fluid or heat transfer fluid. Depending on
the concept, temperature levels can exceed 1000 °C [28]. Power towers promise high efficiency if
used with Brayton cycles [30], as they can be extended to solar-combined cycle plants. Additional
concepts are the linear Fresnel collector concept (line focusing) and the parabolic dish collector
concept (point focusing). The linear Fresnel concept is similar to the parabolic trough systems,
also line focusing. In this architecture, many flat mirror elements concentrate solar radiation onto a
horizontal receiver tube [29]. Parabolic dish collectors concentrate the sunlight onto the focal point,
where Stirling engines transform the thermal energy into electrical energy. They provide good effi-
ciency rates in small power classes, ideal for isolated and stand-alone applications [31]. The most
characteristic CSP designs are briefly described in the following: