An In-Depth Discussion on Concentrated Solar Power

Concentrated Solar Power (CSP) has long been proposed as a source for energy generation applications. The purpose of this EPCM article is to summarise relevant theories regarding the main aspects of concentrated solar-powered energy production. The main objective of the topics covered is to provide the reader with a better understanding of the sun’s path, an overview of CSP, various solar tracking methods used in the industry, and the implementation of CSP for energy generation.

1. Concentrated Solar Power: Theory

1.1 Azimuth and Elevation

Azimuth and elevation are the two coordinates that define the position of a celestial object at any given time in the sky as it is viewed from a certain location. An example of a celestial object is the Sun. The azimuth is the angle between a celestial body and the true geographical North measured clockwise around the observer’s horizon. The elevation is the vertical angular distance between a celestial body and the observer’s horizon or local plane. For an observer on the Earth, the sun’s elevation is the angle between the direction of the geometric centre of the sun’s apparent circular shape and the observer’s local horizon. The concept of coordinates is used in various industries, such as engineering, navigation, astronomy, mapping, mining and artillery (Rouse, 2006).

1.2 Concentrated Solar Power: Degrees of Freedom

The degree of freedom a system has defines the capability of a system to move in more than one direction. As explained in section 1.1: Azimuth and Elevation, the sun follows two paths. For accurate solar tracking, a tracking device should have a two-degree-of-freedom system. A two-degree freedom system could track the sun along the azimuth and elevation axis. A one-degree-of-freedom system can be used, but a larger tracking error would then be experienced. A static system can also be used; this specific system has zero degrees of freedom (Richard G. Budynas, 2011).

1.3 Solar Tracking Error

Solar tracking error refers to the deviation between the sun’s actual position at a given moment and the direction in which the solar tracking system is aimed at that same moment. This error is an angular error and is typically measured in radians or degrees (Forristal, 2003).

1.4 Concentrated Solar Power: Solar Receiver

A solar receiver is a device used in high-temperature solar concentrators. Its main function is to convert solar radiation to heat in a concentrated solar power system. The receiver aims to absorb as much solar radiation as possible and transfer the absorbed energy to the heat transfer fluid thermodynamically. Thus, the receiver is a vital component in any solar concentrated solar power system.

In the specific case of a parabolic dish reflector system, the receiver may be designed in various ways that best suit the application. Receivers can be manufactured from various materials, including aluminium, steel and copper. The receiver can sometimes be manufactured from fibres, normally done for air-based systems. The amount of radiation absorbed by the receiver depends on a few variables, namely the geometry of the receiver, the use of a selective absorber and the use of glass with low iron content. Being such a vital component in a concentrated solar-powered system, the heat loss from the receiver can greatly influence the system’s efficiency. One can expect that a receiver’s main heat loss mechanisms are conduction, convection, and radiation. These are all aspects that should be considered when designing a solar receiver (Govender, 2013).

2. Concentrated Solar Power: Solar Tracking Systems

2.1 Static System

A static system consists of no moving parts. Static solar systems used in industry are normally photovoltaic panels that are static mounts orientated in a specific direction. A static tracking system has a large solar tracking error as the system can’t follow the sun’s path. A high solar tracking error limits the methods by which the system can generate energy. The reason for this is the sun’s path changes every day of every year, so the static mount won’t even be able to have zero degrees error even once a day (Kalogirou, 2004; Richard G. Budynas, 2011).

2.2 One Degree of Freedom System

One degree of freedom system consists of mechanical parts that can only move in one direction. This direction normally is along the elevation axis. One degree of the freedom tracking system is limited in energy generation capabilities but less limited than a static system. The one-degree-of-freedom mounted tracking system cannot follow the sun’s exact path, but it can get close enough to somewhat reduce the solar tracking error. One degree of freedom system can follow the sun’s path to an extent that makes concentrated solar power methods such as the Fresnel reflector design possible (Kalogirou, 2004; Richard G. Budynas, 2011).

2.3 Two Degrees of Freedom System: Azimuth and Elevation

A two-degree-of-freedom system has many moving mechanical parts to allow the system to follow the sun’s path in both the azimuth and elevation directions. A two-degree freedom system has very few limitations in its method of energy generation. This statement can be made because of the system’s low solar tracking error. As previously explained, the sun’s path changes constantly along the azimuth and elevation axis, and with a two-degree-freedom system able to follow both these paths, systems with high efficiencies, such as parabolic or dish design methods, can be implemented with great effect. A two-degree-of-freedom system requires high initial costs to manufacture because of the complex mechanical moving system it possesses. These high costs are, however, rewarded with high energy efficiency (Kalogirou, 2004; Richard G. Budynas, 2011).

3. Solar Tracking Methods

A solar tracker is defined as a device that can orientate a payload towards the sun with the payload being any form of solar energy harnessing devices; ranging from solar panels, parabolic troughs, Fresnel reflectors and parabolic dish reflectors (Kalogirou, 2004).

3.1 Manual Tracker

A manual tracker has to be repositioned by an operator throughout the day. This will normally have to be done in small increments of time, depending on the type of results the system has to provide. The solar dish mounted on a still manual tracking system used for this research project would have to be adjusted every few minutes to obtain accurate results. If photovoltaic panels were to be used, the operator would only have to adjust the system a few times a day, and the efficiency would remain relatively high. In general, there will be some shadow aligning the object to allow the operator to align the system with some accuracy. This is a very basic type of solar tracking method. It would be difficult to establish solar tracking errors as human error factors come into play. The tracking error depends on the time increments between repositioning. This system is not cost-effective and would not normally be implemented in industry (Shah, 2011; Zip, 2013).

3.2 Passive Tracker

A passive tracker is a system consisting purely of mechanical parts with no electricity to power a motor to get the gears moving and, in turn, move the solar mount. A passive tracker normally consists of a housing filled with a compressed gas fluid with a low boiling point driven to one side or the other using solar heat. This process creates gas pressure, which moves the centre of gravity of the entire system to one side. This system is very difficult to calibrate correctly and will, more often than not, mean a large tracking error (Shah, 2011; Zip, 2013).

3.3 Active Tracker

An active tracker consists of a mechanical system using several sensors that send the needed electricity to the motor and gears, which try to align the system with the sun. An activity tracker can operate in one direction, consisting of a single motor, sensor, and a set of gears. However, a significant drawback of this one-degree-of-freedom system is that it experiences large tracking errors, as the sun’s path spans two directions, as mentioned earlier in the report. In contrast, an active tracking system can be implemented as a two-degree-of-freedom system requiring at least two motors, multiple sensors, and an appropriate set of gears. This configuration reduces tracking errors, enabling the system to accurately track the sun along the azimuth and elevation axes (Shah, 2011; Zip, 2013).

3.4 Chronological Tracker

A chronological tracker is based on the theory that if the exact position of an object is known (the exact longitudinal and latitudinal coordinates of the object) and if the precise time and date are known, certain calculations can be done to determine the exact position of the sun from the objects specific position. As one can imagine, the chronological tracker produces a very small solar tracking error. This is because the sun’s position is determined via accurate mathematical calculations. The chronological tracking method is considered one of the best solar tracking methods (Shah, 2011; Zip, 2013).

4. Concentrated Solar Power: Methods of Harnessing Solar Energy

4.1 Concentrated Solar Power

Concentrated solar power is a method of harnessing solar energy by collecting the rays from the sun and concentrating them on a single point or axis. There are two main types of single-axis tracking systems: parabolic trough collectors and Fresnel reflector collectors. A two-degree-of-freedom system, namely a heated receiver thermodynamic system, is used for many different applications in industry. The systems mentioned above will be examined in more detail below (Washburn, 2012).

4.1.1 Parabolic Trough Collectors

Parabolic trough collectors can effectively supply heat to a working fluid, increasing the fluid’s temperature to up to 400^oC. As one can expect, this wide range of heat supplied makes this system applicable to many applications. A schematic representation of a parabolic trough collector can be seen in Figure 1 (Kalogirou, 2004). The setup consists of a sheet of reflective material bent into a parabolic shape, a black metal tube enclosed inside a glass tube to reduce heat losses. It is then mounted along the focal line of the receiver. Once the parabolic trough is constructed, the system is pointed towards the sun. The system can start operating as the reflective material reflects the sun’s rays onto the receiver (Kalogirou, 2004).

Figure 1: Parabolic Trough Solar Collector

The parabolic trough collector can be placed in an East-West direction, allowing the system to track the sun from North to South. The main advantage of orienting the collector in this specific manner is allowing the full aperture to face the sun for most of the day and, in turn, receive less sun exposure in the early mornings and late afternoons. Another orientation one can consider is to place the parabolic through in a North-South direction, which will allow the system to track the sun from East to West; this has the exact opposite effect of the East-West orientation as the system will have greater losses at noon and be more effective in the mornings and late afternoons. A parabolic trough solar collector’s tracking mechanism can be classified as a mechanical or electric system (Kalogirou, 2004).

4.1.2 Concentrated Solar Power: Linear Fresnel Reflector

Linear Fresnel technology relies on an array of linear mirror strips which concentrate the sun’s rays onto a fixed receiver that is normally mounted on a linear tower. The linear Fresnel collector field is similar to a broken-up parabolic trough collector; however, the difference is that the linear Fresnel reflector does not have to be of a parabolic shape. Large receivers/absorbers can be constructed, and the absorbers do not have to move. A great advantage of this type of solar collector is the fact that it uses flat or elastically curved reflectors, which are a lot cheaper when compared to parabolic glass reflectors. Another advantage of a linear Fresnel reflector field setup is that the reflectors are mounted close to the ground, which minimizes structural requirements. A typical linear Fresnel reflector field can be seen in Figure 2. South Africa’s most notable Fresnel reflector field is the Khi Solar One CSP plant near Upington. More information on the solar power plant can be found by following the links below. 

http://www.energy.org.za/news/khi-solar-one-near-upington-achieves-a-technological-milestone

Figure 2: Linear Fresnel Reflector Field (Kalogirou, 2004)

4.1.3 Heated Receiver Thermodynamic System

A parabolic dish reflector system is a point-of-focus collector that normally can track the sun along both the sun’s paths, concentrating as much solar energy as possible onto a receiver located at the focal point of the dish. To obtain higher efficiencies from the system, the dish structure must be able to fully track the sun to reflect as much of the sun’s photons as possible onto the thermal receiver. The receiver absorbs the radiant solar energy and converts it into thermal energy in a circulating fluid. The thermal energy absorbed can then be converted into electricity by using an engine generator coupled directly to the receiver of the energy, which can be transported through other materials such as pipes or elements to a central power-conversion system. Parabolic dish systems can achieve very high temperatures, sometimes exceeding 1500^oC. Parabolic dish reflectors are the most efficient of all collector systems. A schematic representation of a parabolic dish reflector can be seen in Figure 3 (Kalogirou, 2004).

Figure 3: Schematic of a Parabolic Dish Collector (Kalogirou, 2004)

A concentrated solar power method, such as a parabolic or dish design, concentrates as much of the sun’s photons as possible on a single spot or line in the system. The single spot or line previously mentioned is commonly known as a receiver. This receiver can absorb many photons and convert them into heat. The receiver will then act as a boiler in a typical thermodynamic system, which can generate energy for various applications. This system can produce higher efficiencies than most of the other solar tracking systems. As with many other applications, high efficiencies mean high setup costs, a common drawback of a complex heated receiver thermodynamic system (Kalogirou, 2004).

4.2 Concentrated Solar Power: Photovoltaic System

Photovoltaic is a method in which solar cells are used to absorb photons of light to excite electrons to a higher state of energy; this allows the electrons to act as carriers for electricity. By using this method, solar energy can be turned into electrical energy. The solar cells are normally set up next to each other on a flat plate, which allows the maximum surface area of all the solar cells aligned next to each other to be exposed to the sun. A common angle to which this solar cell plate is tilted towards the sun is 23^o from the horizontal.

As with any method of harnessing solar energy, a few advantages and disadvantages accompany the photovoltaic method. An advantage is that a photovoltaic system setup is relatively cheap when compared to other methods, such as a heated receiver thermodynamic system that is implemented on a large scale. Unlike solar tracking dishes, the solar tracking error does not greatly affect the efficiency of a photovoltaic system. Photovoltaic solar cells used in the industry typically have an efficiency range of 15% to 19%, which can be considered a disadvantage (Jayakumar, 2009).

5. Common Thermodynamic Systems Used in Solar Energy Applications

In many solar energy applications, a thermodynamic process is followed to achieve the desired result. A simple thermodynamic closed-cycle typically consists of the following components: a closed-cycle curated by a pump/compressor, a boiler and a condenser/heat exchanger. The boiler, in this case, is the solar receiver that is heated by the concentrated solar power of the sun. An example of a closed thermodynamic cycle used to produce steam to power a steam turbine, which in turn generates electricity, can be seen in the visual representation by following the video link below.

5.1 Brayton Cycle

The Brayton cycle is probably one of the most popular thermodynamic systems used in the industry. It mainly uses air as the working fluid. A thermodynamic cycle can be classified as an open or closed cycle. In an open cycle, the working fluid is only used once through the system, while in a closed system, the working fluid can be used several times.

An open-cycle system consists of a compressor, boiler (or combustion chamber), and a turbine. In concentrated solar energy applications, the combustion chamber is replaced by a receiver heated by the concentrated power of the sun (Roux, Bello-Ochende, & Meyer, 2012).

Figure 4: Typical Closed Brayton Cycle Process

A closed cycle consists of a compressor, combustion chamber, turbine and heat exchanger. In Figure 4, the Heat exchanger after phase 2 will act as the combustion chamber, and the Heat exchanger after phase 4 will act as a condensing unit. As mentioned above, the same principle will be implemented for concentrated solar applications, as the combustion chamber will be replaced by a receiver that is to be heated by the concentrated power of the sun.

The Brayton cycle is known for the high-efficiency output the system can produce. A visual representation of how the Brayton cycle is implemented in the CSP industry can be seen by following the link below (Roux, Solar Tracking for a Parabolic Dish Used in a Solar Thermal Brayton Cycle).

5.2 Rankine Cycle

The Rankine cycle is a thermodynamic cycle that usually operates as a closed cycle. A Rankine cycle can be implemented in solar applications similarly to the Brayton cycle. The combustion chamber in a closed cycle is replaced by a receiver that is then heated by the sun’s concentrated power. To maximize energy capture, an appropriate shape and size must be selected for the receiver. In the Rankine cycle, the working fluid is typically water, as the cycle is commonly used to model the performance of steam turbine systems (Claus Borgnakke, 2013).

5.3 Kalina Cycle

The Kalina cycle is similar to the Rankine cycle as this system also operates as a closed thermodynamic cycle. The Kalina Cycle can create mechanical energy by converting thermal energy through a thermodynamic process. These two cycles use different working fluids in the respective closed cycles. Unlike the Rankine cycle, the Kalina cycle uses a mixture of water and ammonia as the working fluid. Water and ammonia have different boiling points, this solution can thus boil over a larger temperature range which in turn means that more heat can be extracted from a reservoir containing this fluid. This working fluid enables the Kalina cycle to produce higher efficiencies. The Kalina cycle can be effectively implemented in a concentrated solar power system using the same principles as explained in sections 6.1 and 6.2 (Claus Borgnakke, 2013).

 

6. References

Ghajar, A. J. and Cengel, Y. A., 2011. Heat and Mass Transfer: Fundamentals and Applications. 4th ed. New York: McGraw-Hill.

Budynas, R. G. and Richard, J. K., 2011. Shigley’s Mechanical Engineering Design. 9th ed. New York: McGraw-Hill.

Le Roux, W. G., Bello-Ochende, T. and Meyer, J. P., 2012. Thermodynamic optimisation of the integrated design of a small-scale solar thermal Brayton cycle. Department of Mechanical and Aeronautical Engineering, University of Pretoria, Pretoria.

Le Roux, W. G. et al., 2012. Solar tracking for a parabolic dish used in a solar thermal Brayton cycle. Department of Mechanical and Aeronautical Engineering, University of Pretoria.

Rouse, M., 2006. “Azimuth and Elevation.” Whatis.com, 1 January. Available at: http://whatis.techtarget.com/definition/azimuth-and-elevation [Accessed 1 March 2016].

Govender, P., 2013. Construction and analysis of the receiver for a solar thermal cooker system. University of KwaZulu-Natal, Durban.

Jayakumar, P., 2009. “Solar Energy.” Renewable Energy Cooperation-Network for the Asia Pacific, India.

Shah, A., 2011. “Solar trackers guide.” Green World Investor, 6 July. Available at: http://www.greenworldinvestor.com/2011/07/06/solar-trackers-guide-types-passivesingle-axisdual-axis-2-axisprice-and-uses/ [Accessed 6 March 2016].

Washburn, K. K., 2012. “Concentrating solar power technologies.” Solar Energy Development Programmatic EIS, 6 December. Available at: http://solareis.anl.gov/guide/solar/csp/ [Accessed 2 April 2016].

Sonntag, R. E. and Borgnakke, C., 2013. Fundamentals of Thermodynamics. 8th ed. Michigan: Wiley.

Kalogirou, S. A., 2004. “Solar collectors and applications.” Progress in Energy and Combustion Science, Cyprus.

Zip, K., 2013. “How does a solar tracker work.” Solar Power World, 4 April. Available at: http://www.solarpowerworldonline.com/2013/04/how-does-a-solar-tracker-work/ [Accessed 6 March 2016].

Sustainable.co.za, 2002. “Renewable energy systems.” 1 January. Available at: http://www.sustainable.co.za/energy-efficiency/solar-cookers/parabolic-solar-cookers.html [Accessed 1 July 2016].