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 article is to provide a summary of relevant theory regarding main aspects of concentrated solar powered energy production. The main idea behind the topics covered is to give the reader a better understanding of the path the sun follows, a background of concentrated solar power, different solar tracking methods used in industry and the ways CSP is implemented to generate energy.
Table of Contents
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 being the Sun. The azimuth is the angle between a celestial body and the true geographical North measured in the clockwise direction 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 the coordinates are used in various industries such as engineering, navigation, astronomy, mapping, mining and artillery. (Rouse, 2006)
1.2 Degrees of freedom
The amount of degrees of freedom a system has defines the capability of a system to move in more than one direction. As explained in section 2.1 Azimuth and Elevation the sun moves about two paths. For accurate solar tracking a tracking device should be a two degrees of freedom system. A two Degrees of freedom system would be able to track the sun along the azimuth and elevation axis. A one degree of freedom 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
The solar tracking error is the physical error experienced between the position of the sun at a specific moment in time and the exact direction in which the solar tracking system is focused upon at that specific moment. This error would typically be measured in radians or degrees as the error is an angular error. (Forristal, 2003)
1.4 Solar receiver
A solar receiver can be described as a device that is used in high temperature solar concentrators. The receiver’s main function is to convert solar radiation to heat in a concentrated solar power system. The receiver’s purpose is to absorb as much solar radiation as possible and thermodynamically transfer the absorbed energy to the heat transfer fluid. The receiver is thus 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 will suit the application best. Receivers can be manufactured from various materials including aluminium, steel and copper. In some cases the receiver can be manufactured from fibres, this is normally done for air based systems. The amount of radiation absorbed by the receiver is dependent 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 efficiency of the system. The main heat loss mechanisms one can expect from a receiver are conduction, convection and radiation. These are all aspects that should be considered when planning to design a solar receiver. (Govender, 2013)
2 Solar tracking systems
2.1 Static system
A static system consists of no moving parts. Static solar system 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 in 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 degree error even once a day. (Kalogirou, 2004) (Richard G. Budynas, 2011)
2.2 One degree of freedom system
A one degree of freedom system consists of some mechanical parts that is only able to move in one direction. This direction normally being along the elevation axis. A one degree of freedom tracking system is limited in its energy generation capabilities but is 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 reduce the solar tracking error somewhat. A one degree of freedom system can follow the sun’s path to such an extent that make concentrated solar power methods such as the Fresnel reflector design possible. (Kalogirou, 2004) (Richard G. Budynas, 2011)
2.3 Two degrees of freedom system
A two degree of freedom system has many moving mechanical parts to allow the system to follow the suns path in both the azimuth and elevation directions. A two degree of freedom system has very little limitation to its method of energy generation. This statement can be made because of the low solar tracking error the system possesses. As previously explained the suns path changes constantly along the azimuth and elevation axis and with a two degree of freedom system being able to follow both these paths systems with high efficiencies such as a 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 a 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 dependant on the type of results the system has to provide. The solar dish mounted still manual tracking system used for this research project would have to be adjusted every few minutes in order 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 still stay relatively high. In general there will be some sort of 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 the solar tracking error as there is a human error factor coming into play. The tracking error depends on the time increments between repositioning. This is not a very cost effective system and would not normally be implemented in industry. (Shah, 2011) (Zip, 2013)
3.2 Passive tracker
A passive tracker is a system that consists purely of mechanical parts with no electricity to power a motor in order 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 that is driven to one side or the other making use of the solar heat. This process creates gas pressure which moves the centre of gravity of the entire system to the one side. This system is very difficult to be calibrated 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 sends the needed electricity to the motor and gears which in turn tries to align the system with the sun. An active tracker is able to work in one direction with the system then consisting of one motor, one sensor and a set of gears. A drawback of this one degree of freedom system is that one would see a large tracking error as the sun’s path is along two directions as previously mentioned in the report. An active tracking system can also be implemented as a two degree of freedom system with the system then consisting of at least two motors, several sensors and an appropriate set of gears. The system would then show a small tracking error as the system would be able to track the sun along both the azimuth and elevation directions. (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 suns position is determined via accurate mathematical calculations. The chronological tracking method is seen as one of the best solar tracking methods. (Shah, 2011) (Zip, 2013)
4 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. Two main types of single-axis tracking systems exist namely 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 above-mentioned systems will be looked at 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 fluids-temperature to up to 400oC. 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 that is bent into a parabolic shape, a black metal tube that is enclosed inside a glass tube in order to reduce heat losses is then mounted along the focal line of the receiver. Once the parabolic through is constructed the system is pointed towards the sun the system can then start operating as the reflective material reflects the sun’s rays onto the receiver. (Kalogirou, 2004)
Figure 1 Parabolic through solar collector
The parabolic through collector can be placed in an East-West direction which then allows the system to track the sun from North to South. The main advantage of orientating the collector in this specific manner is to allow the full aperture to face the sun for most of the day and in turn receiving 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 through solar collector’s tracking mechanism can be classified as a mechanical or an electric system. A video representation of an entire plant consisting of parabolic trough collectors can be seen when following the link below. (Kalogirou, 2004)
4.1.2 Linear Fresnel reflector
Linear Fresnel technology relies on an array of linear mirror strips which concentrates 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 the difference, however, is that the linear Fresnel reflector does not have to be of parabolic shape. Large receiver/absorbers can be constructed and the absorbers does not have to move. A great advantage of this type of solar collector is the fact that is 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 being the Khi Solar One CSP plant near Upington. More information on the solar power plant can be read by following the first link below, the second link is a short fly-by video of what the solar power plant looks like. (Kalogirou, 2004)
Figure 2 Linear Fresnel reflector field (Kalogirou, 2004)
4.1.3 Heated receiver thermodynamic system
Parabolic dish reflector systems 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. In order 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 making use of an engine generator coupled directly to the receiver or the energy 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, these temperatures can be in excess of 1500oC. 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 is used to concentrate 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 has the ability to absorb many photons and convert them into heat. The receiver will then act like a boiler in a typical thermodynamic system which can generate energy for various applications. This system has the ability to produce higher efficiencies than most of the other solar tracking systems. As with many other applications, high efficiencies mean high setup costs and this is a common drawback of a complex heated receiver thermodynamic system. (Kalogirou, 2004)
4.2 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 making use of this method solar energy can be turned into electrical energy. The solar cells are normally set up next to each other in a flat plate, this 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 23o from the horizontal.
As with any method of harnessing solar energy there are a few advantages and disadvantages that accompany the photovoltaic method. An advantage is that a photovoltaic system setup is relatively cheap when comparing it to other methods such as a heated receiver thermodynamic system that is implemented on a large scale. Unlike with solar tracking dishes the solar tracking error does not have a large effect on the efficiency of a photovoltaic system. Photovoltaic solar cells used in industry has an efficiency of between 15% and 19%, this can be seen as 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 a desired end result. A simple thermodynamic closed cycle typically consists of the following components; a flclosed-cyclerculated by a pump/compressor, a boiler and a condenser/heat exchanger. The boiler in this case being 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 industry. The Brayton cycle 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 (Combustion chamber), and a turbine. When applying this system in concentrated solar energy applications the combustion chamber is replaced by a receiver that is heated up by the concentrated power of the sun. (Roux, Bello-Ochende, & Meyer, Thermodynamic optimisation of the integrated design of a small scale solar thermal Brayton cycle, 2012)
Figure 4 Typical Closed Brayton Cycle Process
A closed cycle consists of a compressor, combustion chamber, turbine and a 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. 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 in a way similar 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. An appropriate shape or size must be chosen for the receiver in order to increase the amount of energy that can be captured. The working fluid of a Rankine cycle is usually water as the Rankine cycle is a model that is used to predict 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 section 6.1 and 6.2. (Claus Borgnakke, 2013)
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