The ever growing population and industrialized world of 21st century is facing severe problems such as climate change and ozone layer depletion. The 20th century saw the industrial revolution of mankind, which majorly increased fossil fuel consumption to many folds. During such period, the business pragmatism was of utter importance and environmental impacts were thrown aside. It was not until the beginning of the 21st century when mankind raised concerns about climate change due to the increasing levels of CO2 emissions, notwithstanding the ever swelling energy demand. In fact, at present CO2 emissions have already exceeded the upper safety limit of 350 ppm , and the energy demand is also expected to reach the 550 quadrillion BTU  by the end of year 2013. It is even of higher concern that the continuous population growth and increasing use of modern heavy-energy technologies (which could lead to increasing CO2 emissions) will be worsening the situation. Hence, the search of sustainable means of power generation should be considered.
In such a jeopardy, renewable energy sources are proving to be a feasible solution, mankind to rely on. Indeed, nowadays the world receives approximately 17-18% of its energy from renewables, including about 9% from ‘traditional biomass’ and about 8% from other renewable sources . These renewable shares are ideally expected to grow in the energy outlook, to bring down carbon emissions along with providing energy.
Solar energy is one of these renewable sources which has tremendous unexploited potential, especially in the country like India which are geographically placed in the Earth’s Sun-belt. Solar energy could be harnessed in two major ways. Solar photovoltaic panels are made of number of photovoltaic cells, which work on the principle of energy absorption by an electron-hole pair. These cells absorb the incident solar radiation and convert into the electricity, which can be stored in batteries. The other way to utilize the immense capacity of solar energy is through solar thermal technologies. Solar water heaters which are commonly used for domestic and industrial purposes is also one of these technologies. Concentrating Solar Power (CSP) came into light in the late 20th and early 21st century. CSPs are majorly employed for the high capacity power generation based using steam cycle (or Rankine power cycle) for operation. In CSP technology, the incident solar radiations are reflected onto the receiver placed at the focal point (or along the focal line, in case of line-focusing CSP) to increase the temperature of the surface up to even 1400 0C. These technologies are based on a typical property of a parabola geometry, which says that any ray perpendicular to the plan of the parabola will be reflected back to the focus of the parabola, where the receiver is mounted. A working fluid (or the heat transfer fluid) is passed through these receivers to get heated. The gained heat energy is then transformed into the usable form of energy such as electricity, using turbines and generators.
Historically, CSP was first introduced by Archimedes to repel the invading army , but it was not until the late 19th century when the first parabolic trough technology  using steam for power generation was demonstrated. Today CSP represents a reliable technology for electricity generation with a global installed capacity that exceeds the 2GWe. Further, based on the number of projects that are being planned or currently under construction the International Energy Agency has estimated that, even in the case of a conservative scenario, CSP installed capacity will exceed the 10 GWe by 2020 . In such regard, it is worth highlighting the construction of Ivanpah Solar power plant  located in the Mojave Desert in California, which will have a nominal capacity of approximately 320MWe, being the largest CSP project ever deployed. It is expected to go online in September 2013 in United States of America (USA) at Primm city of Nevada province  .
Introduction to CSP Technologies
The geographical location of the power plants based on CSP technologies is instrumental due to the fact that CSP deals with the assimilation of incident solar irradiation, normally denoted as DNI (Direct Normal Irradiance) and measured in terms of solar energy incident per unit area (W/m2). Modern day CSP technology has evolved many folds from the initial attempts and put into use for many different applications such as power production and process steam generation etc. The choice of technology to be implemented therefore, depends upon the end usage and the required highest temperature. These technologies can be distinguished on their focusing paradigms such as line focusing and point focussing. The present chapter deals with the different CSP technologies with an example of the existing power plants based on the respective technology as well as the highlighting differences between these technologies.
1. Line-focusing CSP
In line-focusing CSP technologies, the incident solar energy is reflected onto the receiver placed along the focal point of the reflector. The line-focusing technologies are generally employed to reach temperature up to 400 0C  for which molten salts, oils or water inclusively can be used as heat transfer fluid. Currently, there are two main types of line-focusing CSP technologies, namely parabolic trough and linear Fresnel. These are briefly described in the subsequent sections of the chapter.
a. Parabolic Trough Concentrator
In Parabolic Trough Concentrator (PTC), parabolic geometry is the working principle, which says the incident rays perpendicular to the plane of parabola are reflected and concentrated at the focus. The working fluid is passed through the receiver, which is made up of a metal pipe enveloped inside a vacuum tube to minimize mainly the convective losses. For power generation, many PTCs are connected in series to reach up to 400 0C  needed as per the end use. The PTC have tracking systems which allow them to track the Sun in the search of maintaining the perpendicularity of the incident rays . A general schematic of such power plant is shown in Figure 1. The PTC plants represent around 80% of the total CSP installed capacity worldwide , being worth to mention the ANDASOL  complex in southern Spain . It consists of three 50MWe CSP plants that commenced in 2006 and where the use of storage using a molten salts system was first demonstrated at large scale, thus boosting the development of CSP and encouraging new research fields. The Figure 2 shows parabolic trough in ANDASOL 1 power plant.b. Linear Fresnel Reflector
In Linear Fresnel Reflector (LFR) technology, flat mirror reflectors reflect and concentrate onto the receiver through which working fluid is pumped . A typical LFR is shown in Figure 3. Compared to PTCs, LFRs are less expensive and also allow for larger reflective areas . Furthermore, recent developments in LFR demonstrate that arrangements accounting for two receivers can yield a better overall performance. Such arrangement is known as Compact Linear Fresnel Reflector (CLFR)  as shown in Figure 4. Indeed when compared against PTCs, although cheaper, LFRs have other issues such as more optical losses and building complex tracking systems. Given that LFRs are flat mirror reflectors, these are easier to manufacture and install (that is why they are cheaper) and this is why they are typically used in hybridization modes (together with coal). The Puerto Errado 2  power plant is a pure linear Fresnel plant. However, the largest LFR power plant with 100 MWe gross capacity is being constructed in India in Dhursar district of Rajasthan . The Figure 5 shows one of such LFR power plant of 12 MWe gross capacity located at Ghisonaccia (Corsica Island), France .
2. Point-focusing CSP
In Point-Focusing CSPs, the receiver is placed at the focal point of the reflector field. These technologies are usually employed to achieve very high temperatures, hence are used in power production. The temperature range, these technologies can achieve is up to 1500 0C  because of the very high concentration ratios .
a. Dish Stirling
This system consists of stand-alone parabolic concentrator with a Stirling engine mounted at the focal point onto which the rays are concentrated. Because of its construction, this kind of concentrator can track Sun’s movement along both the axes, i.e. Sun’s position with respect to equator as seasonal tracking and Sun’s movement throughout the day as daily tracking. Dish Stirling has the highest Solar-to-electric energy efficiency because of high concentration ratios and two-axial tracking . The Figure 6 shows a typical Dish Stirling System.The complexity of construction and costs in manufacturing, installation and maintenance have limited this technology from penetrating CSP market. In turn, commercial power plants using this technology are very few  . One such power plant exists in California shown in Figure 7, USA with 300 MWe gross capacity .
b. Solar Central Tower System
The Solar Central Tower systems (SCTS) consists of a centrally located receiver mounted at the top of a tower surrounded by a heliostats field. This heliostats field consists of large number of flat mirrors attached to the metallic frame and supported by stands on the ground. These heliostats track the Sun though out the year with seasonal and daily tracking. The maximum temperature that can be achieved with SCTS is approximately 1200 0C .
Figure 8 illustrates the Solar Central Tower system. After PTC, Solar Tower has been the most successful technology used for CSP plants . In case of molten salts, the heat is transferred to water in a heat exchanger to convert to steam however, in case of water as a working fluid, steam is directly produced out of the receiver hence usually referred as direct steam generation (DSG). In Spain, Gemasolar Power plant  uses molten salt as working fluid and has 19.9 MWe  of gross capacity. The Figure 9 shows a filed photograph of Gemasolar power plant.Moreover, Spain also hosts a DSG power plant located in Sevila known as PS-10 and PS-20 with 11 MW and 20 MW of gross capacities respectively. The most recent solar thermal power plant with DSG technology is being constructed in USA, which is known as Ivanpah Solar thermal power plant.
The Ivanpah Solar Thermal power plant is installed with the gross capacity of 392 MW and comprises of three tower and heliostat field systems. The working pressure in the power cycle is 160 bar with the receiver outlet temperature of steam is 580 0C .
The CSP from the days of being used to evade enemy invasions to the massive power generating source has evolved many folds to capture the eyes of environmentalists and renewable energy enthusiasts. As the end consumer is generally unaware of the technology being employed to generate the electricity, these technologies do not hold any special importance. However, as this is the very consumer who is going to suffer the most with the hazards from climate change. If at all we want to mitigate the effects of climate change, green energy generation seems only possible solution and CSP can share major part of it. Therefore, researchers and policy makers need to work hand-in-hand to make the most out of such a magnificent technologies. As a human being, we owe a lot to this mother earth and its eco-system, hence it is our responsibility to protect the very existence of this natural home. Hence, I believe and we all should believe that the future in indeed green!
This article reflects the views of the author and not necessarily those of collaborative policy consultants.
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