Concentrated Solar Power and Desalination technologies

Concentrated Solar Power and Desalination technologies

Concentrated solar power technology generates thermal power by using sunlight energy as single or multi reflectors supply the radiation to a receiver where the flux concentrated and the temperature rises to a useful level. Commonly this heat is used to generate electricity by using an electrical generator engaged to a steam or gas turbine powered by the heat of the targeted receiver, it also can be used as a power source for chemical production plants [1]. A newly growing use of CSP systems is for water desalination, it is used directly as a source of heat for the desalination process for the thermal desalination technologies or used undirectly as it provides the power for an individual facility that consumes part or total of its power for water desalination.

Historically CSP technologies were used by ancient Greek and Chinese since they used mirrors or glasses to make fire. In the 20 th century during the oil crises in the 1970s, the interest in solar energy began to increase to replace fossil fuel resources [1]. There are four main technologies for harvesting the concentrated solar power: parabolic trough, linear fresnel reflector (IFR), central receiver, and parabolic dish [2], see fig 1. All types of CSP works with the same principle ( reflector and receiver) depending on the solar radiation:

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1. Parabolic trough (fig 2): this type of CSP uses longitudinal sheets of reflectors with a parabolic cross to focus maximum radiation on the receiver which commonly made as a tube in such CSP types. Parabolic trough reflectors were invented by the Swedish engineer John Ericsson in 1870 [4]

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Fig 2: parabolic trough reflector [5]

2. Linear Fresnel Reflector (LFR): this type of reflectors consists of multi rectangular plane reflectors that concentrate the flux on a single or double receiver, see fig 3. What makes this type special is its simplicity and low capital cost for design and construction in comparison with other types [6]

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Fig 3: Linear Fresnel reflector [6]

3. Parabolic Dish (shown in fig. 4): consist of parabolic circular reflector reflects the solar radiation to a receiver placed in the center of the parabola, this type has a single reflector and single receiver only [7]. It is the most efficient technology in CSP systems as it always supplies all the radiation to the receiver directly without the cosine loss effect which happens in other technologies. Cosine loss effect is an important loss occurs in planed reflectors because they cannot be always aligned normally to sun flux direction [8]

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Fig 4: Parabolic Dish Receiver [5]

4. Central Receiver: this is the most commonly used type of CSP technologies in last years, it consists of a big number of mirrors or reflector directed to a single receiver, the reflectors placed on the ground and the receiver placed in high position on a column in which very concentrated sunlight radiated on it causing a very high temperature able to be used to generate steam or molten salt for power generation [2] as shown in fig. 5. It is called also Heliostats which means large mirrors with two-axis tracking [8]

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Fig. 5: central receiver [8]

As a conclusion comparison, the parabolic dish and the central receiver technologies have an advantage of thermodynamic efficiency over the other two types but they are more expensive, see table 2.

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Table 2: Comparison of CSP technologies [8]

In January 2016 CSP technologies industry has a global total capacity of 7638 MWe which 4801 MWe of it is operational and 2837 MWe of it is during construction and also about 8472 MWe is under development which gives a good overview of how this industry is growing in the near future. Spain took the lead in CSP technology with a production of 2305MWe, the USA comes secondly with a production of 1893MWe then other countries are like South Africa, Chile, India, China, and few countries in the middle east show interest in developing their power generation grids with CSP plants [3].

CSP technologies are one of the most developing technologies in the world, the most characteristic that CSP technology developers and researchers are working on is the capital cost of the construction of a plant since no fuel cost which make it very competitive with fossil fuel plant unless it has construction low price to be also competitive with other renewable power plants. Fig 6 shows the actual growth rate from 1984 until 2011 with an estimated growth rate based on 19% compound average growth per year since 1984 and another extrapolation growth rate depending on the compound average growth rate per year of 40% since 2005. Fig 8 shows the same data of fig. 7 but with expansion on the vertical axis and shows also a comparison of historical installed capacity data between CSP technology with photovoltaic and wind power plants [8].

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Fig. 6: Global installed capacity of CSP plants, both actual and possible future compound growth rates [8].

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Fig. 7: Global installed capacity of CSP plants, both actual and possible future compound growth rates together with historical data [8].

The desalination process focuses on separating the seawater or any brackish water into two streams, the first one is the distilled water which is the useful product and the brine which rejected. There are many types of water desalination differ in there used technologies and capabilities from method to another, table 3 below shows a brief classification of water desalination technologies, mainly they can be divided into two major types: thermal processes which consumes considerable amount of thermal energy because it is an endothermic process and the thermal capacity of water is high comparatively, and the other process is membrane processes which also consumes energy in high rate, water desalination also can be classified according to the used source of energy and also by the physical process of separation [9].

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Table 3: water desalination processes classification [9]

1. Multi-Stage Flash (MSF): this process based on evaporation of brine as it enters the chamber of low pressure where the brine start to evaporate then the brine enters another chamber or stage with lower pressure which will cause more amount of water evaporated and the brine becomes more concentrated withs alt then it enters another stage until the last stage, see fig. 8.

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Fig 8: MSF desalination process [9]

2. Multi-effect distillation (MED): the single stage of this process consists of an evaporator and condenser (or preacher) where the seawater or brackish water get heat before entering the evaporator and get evaporated then the vapor enters the condenser again to be condensed and heating the coming seawater. Seawater fed more than the needed value to be condensed, this extra seawater acts as a cooler in the condenser, see fig. 9.

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Fig 9: MED process [9]

3. Reverse Osmosis (RO): is a type of mechanical desalination that uses the filtration process to produce clean water. It is used commonly due to its simplicity, reliability, and availability.

Every desalination process has its characteristics which make it suitable for specific applications or conditions, table 4 below a brief comparison between the main types of the desalination process.

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Table 4: techno-economic data of the most common desalination process [9]

CSP systems can be used as a source of heat energy for the water desalination process, or for power generation, or both as a combined plant, see figure 10. The planet earth receives a tremendous amount of heat every year, for example, each square of the in the middle east and north Africa (MENA) receives an amount every year that is equal to 1.5 million barrels of oil [10], see figure 11, this energy is not being used sufficiently because of the difficulties of the generation and storage of the power. Harvesting a considerable amount of this energy for water desalination will greatly enhance the agriculture and industries that are depending on water as well as providing water for drinking. In most CSP plant as shown in figure 10, solar power used to heat a thermal storage tank firstly, this is important because solar power is not availbe every time. Then this thermal storage tank will feed both the electricity generation system and desalination plant which has boiler to be heated to a specific temperature to contribute in the evaporation of the seawater

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Fig. 10: annual direct normal irradiance in MENA and southern Europe in kWh/mA2/year.[12]

Computational Techniques Softwares became a necessary tool for designing and optimization of concentrated solar power systems as they provide good preparation information about the plant before construction and installation to estimate the performance of the target (receiver) and the heliostat (reflector) and the other parts. Approved by the experimental and applied data as a measure for the fidelity of the simulations and optimization methods. Many software tools and codes have been developed for decades and they also need a lot of work to adapt to the different features and projects in different conditions, therefore, no one can be a standard software tool to fit with all cases. Due to that software tools must be screened and the suitable one selected.

Regarding concentrated solar power plants two major issues have to be concerned, “the first one is an optimization problem: what is the best heliostat field layout to maximize the collected solar energy or to minimize the cost of that energy?”[13]. The second issue is the performance:” What is the power reflected by the field and arriving on the receiver aperture?”[13]. Answering these questions is the key criteria for selecting suitable software since efficiency matrixes are required to be calculated by the flux maps.

The most common computational techniques will be shown below with there features and the comparison between them:

1. SPRAY (MIRVAL): This software was written for evaluation of the performance of the reflectors, it was developed by SANIA labs and one of Monte Carlo ray tracing programs. The core of the FORTRAN based code SPRAY was developed by DLR since the original form of MIRVAL was not available. Both field efficiencies and flux maps can be determined using this software for individual or fixed reflectors, because of the long history of development it has a variety of features and geometries available for the user which gives it a considerable advantage over other software tools. One limitation of it is its difficulty to use because it has no user interface and it just run by ASCII files [14], ,[16],[17], [18].
2. TieSOL: this software was developed by the University of Houston field codes UHC or RCELL. This software is not available commercially or as open source because it just used at the University of Houston on an outdated computer with special conversion codes. It was designed and developed for the design and optimization of solar power reflectors and receivers [16]. A software company called Tietronix was responsible for its development into a commercial package as TieSOL [19],[14].
3. HFLCAL: Michael Kiera in the 80's started developing this software for two aims; layout and optimization of solar reflector and receivers plant and the other aim is to measure the output of the plant annually at certain configuration. This software employed Monte Carlo methods for field optimization sharing this with just little codes and it has many other features like automatic multiaiming, secondary concentrator optics, tower reflectors systems, various receiver models and the ability of least-cost optimization [20], as well as the interaction between reflective surfaces can be reproduced by ray tracing techniques, this software is available commercially by DLR[14].
4. CRS4-2: if a FORTRAN based software used for simulation of concentrated solar power systems to measure their performance it is capable to calculate “cosine, shading and blocking effect through tessellation”[14]. This software is not available at the present but it could be in the future [21].
5. TieSol: it is a very fast software of Monte Carlo ray-tracing because it uses the parallel processing technique it has many abilities including; design analysis and optimization of concentrated solar power systems as well as receiver flux map and annual performance calculations. An advanced visualization tool has been developed by Tietronix capable of tracking reflectors in real-time mode. This software is commercially available from Tietronix [22], [23].
6. Tonatuih: it is an open-source Monte Carlo ray-tracing program used for the simulation and analysis of the optical solar power plant and systems, it has a variety of features and different geometries just like SolTrace as well as it can model many stages, Python or Mathematica programming languages have to be used externally for calculating flux maps. Its source code can be freely accessed with General Public License which gives it the ability to the user to change it to fit any needed conditions and projects [24],[14], it can be downloaded for free from its website [25] .
7. SolTrace: this program was completed in 2011 as free software use for big sizes of solar energy plants, it is written in C++ programming language using parallel processing methods, it has several geometries of reflectors and receivers available and can measure the output and the flux maps, it uses Google SketchUp plug-in for visualization. SolTrace can be downloaded for free from the NREL website [26].
8.
9. STRAL: it is very efficient software used to precisely configure the reflectors in a way where every one supply solar irradiation to the targeted receiver with many details and geometries available, it is very modern ray-tracing software, it can be downloaded commercially by DLR [20],[27].
10. ISOS: is a software-based on MATLAB program using numerical techniques to find the isosurfaces (homogenous flux regions with 3D flux maps that let SASEC 2012 4 to measure the flux in all levels on the heliostats. this software is suitable for academic use [28], [29].
11. Biomimetic: a software uses biomimetic pattern for the calculation of the CSP systems performance and is also capable of the calculations of the annual irradiation of the location for cosine losing, shading, and blocking, aberration as well as atmospheric attenuation. The software is unavailable right now [30].
12. HFLD: Heliostat Field Layout Design, is MATLAB based program, its considerable features are low calculation time for design and optimization of reflectors and the measurement of the annual solar irradiation totally on the lant of the concentrated solar power system plant, it is available commercially [31].

There are other codes for this technology that differs in their features and suitable applications for use, in summary, table 5 shows a brief comparison between the mentioned software. Table 6 shows the capabilities of each software. Table 7 shows a brief comparison of the most commonly used codes in this field.

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Table 5: summary of the main features of the codes and their availability [14]

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Table 6: summary of software capability [14]

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Table 7: the main features of the five most-used codes [13]

References

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