‘Vidulka’ Energy Exhibition conducted by Sustainable Energy authority in collaboration with the Ministry of Power and Renewable Energy will be held for the 6th consecutive year from the 17th- 19th August 2018 at the BMICH. Co-sponsored by JLanka Technologies, in line with their mission of 100% self-sufficient energy in Sri Lanka.
We have already discussed several types of concentrating solar power technologies. In this article, we are going to discuss a crucial technique used to improve the heat-to-electricity conversion efficiency in almost all the concentrating solar power technologies. It is solar tracking technique!
Why solar tracking is indispensable?
We have nothing to worry about the position of the Sun or its relative motion if it rises in a certain direction and stays at rest for several hours until it sets. Simply, if the Sun was a star at rest.
Just assume that our Sun is not in a relative motion with respect to the Earth. Now take a look at the diagram below.
A light ray coming from the Sun hits the surface and get reflected. As we can see it, the angle (θ) at which the light ray hits the surface (a flat reflector) is the same as the angle (θ) at which the light ray is get reflected by the surface (Law of reflection). The point onto which the reflector focus incoming rays depends on both the shape of the reflector and angle of incidence.
In concentrated solar power, the reflectors or mirrors must always be directed towards the Sun in order to harness as much as solar energy possible from solar radiation.
But…
As we know it, our planet is not something at rest. It is rotating about its own axis while revolving around the Sun. As a result, we observe a continuous change in the relative position of the Sun. Of cause, the Sun is not moving around the Earth. But that is how it looks like. A revolving Sun around the Earth!
It is an apparent motion!
So….
It is our Sun’s daily routine that it rises in the East and sets in the West (Due to the relative motion of the Sun)! As a result of the continuous change in the relative position of the Sun, the incident angle of the Sun’s rays is also continuously changing. This continuous change in the angle of incidence of Sun’s rays is what matters to CSP.
Since the relative position of the Sun is continuously changing with time the orientation of the reflectors or mirrors must be changed to follow the Sun’s path.
How solar tracking is achieved?
In order to optimize the energy yield from solar radiation, the orientation of solar concentrating systems is changed with the help of solar tracking devices. Solar tracking devices are devices which always monitor the real-time position of the Sun and keep the solar concentrating systems orientated towards the Sun accordingly.
Solar trackers consume some energy for their operation. However, they can significantly boost the overall system efficiency by about 10%- 100% depending on the geological conditions, and period of time whilst a typical tracking system consumes only about 2-3% of improved energy yield [1]. So, the solar tracking systems play a vital role in CSP systems by improving the energy yield per system and shrinking the payback time of a given CSP system.
How solar tracking systems work
Solar tracking systems are classified into two groups depending on their working principle. They are “active trackers “and “passive trackers”
Usually, active solar trackers employ sensors like LDRs (Light Dependent Resistors) to monitor the Sun’s path and use electric motors to orient the concentrating system accordingly.
Unlike, active tracking systems, passive tracking systems do not employ motors or electronic sensors. Instead, they employ a liquid or compressed gas to track the Sun’s path. These systems do not need electricity as they work with the heat of solar radiation. Upon exposure to sunlight, the gas or liquid expand which rotates the reflector or mirror until it is perpendicular to Sun’s rays.
Currently, two types of tracking systems are available: Dual axis and Single axis systems.
Single axis trackers have only one moving axis. Typically, they are aligned in North-South direction which enables the reflectors or mirrors to follow the Sun’s position as Sun rises in the east and sets in the west. But these systems are unaware of the seasonal variation in the Sun’s path and therefore, they cannot orientate the reflectors or mirrors from the north to the east.
Dual axis systems have two moving axes and thus they can orientate the reflectors or mirrors to follow the Sun very precisely. So, dual axis systems help harness as much as energy as possible.
Single axis vs. dual-axis tracking system
Single axis systems have fewer moving components than dual axis systems. So, single axis systems are less expensive, straightforward, and more reliable compared to their dual-axis counterpart. Further, single axis systems offer a greater life expectancy. However, dual axis systems offer a greater system efficiency than their single axis counterpart.
Which is the best tracking system?
One cannot just determine that the dual axis tracking system is the best. Both systems have specific advantages and disadvantages compared to the other. We should evaluate various factors such as the initial cost of the system, efficiency, location of the site, system efficiency, maintenance required, and system reliability before we decide which tracking system is the best fit for us.
References
[1] Mousazadeh, H., Keyhani, A., Javadi, A., Mobli, H., Abrinia, K., & Sharifi, A. (2009). A review of principle and sun-tracking methods for maximizing solar systems output. Renewable and sustainable energy reviews, 13 (8), 1800-1818.
In the previous article, we discussed two concentrating solar power technologies. In this article, we are going to discuss some other concentrating solar power systems, in detail.
Just like parabolic trough systems, linear Fresnel reflectors create a linear focus above them along which is a fixed linear heat absorbing receiver. Fresnel reflectors are flat or marginally curved mirrors equipped with single-axis solar trackers. Just like in other concentrating solar power systems absorbed heat is used to produce steam which turns a turbine and generate electricity.
Figure 01: A diagram of a linear Fresnel solar thermal power system with alternatively inclined mirrors
As shown in the above figure, some systems use alternatively inclined mirrors with two linear heat receivers. This configuration helps reduce the space required and also helps avoid any blocking between the mirrors.
Figure 02: A linear heat receiver of a linear Fresnel solar thermal power system
As shown in above figure 02, these systems have a smart configuration that their heat receiver is always fixed, pointing down, and the exterior of the receiver is well-insulated. This design avoids convective heat loss, minimize radiative loss and improves the system energy conversion efficiency.
The fundamental advantage of linear Fresnel reflectors is their simple geometry. Simply, they are easy and cheap to be fabricated since they are flat or slightly bent mirrors. These systems are more efficient than power tower systems but not competent than dish Stirling systems or parabolic trough systems. The demonstrated annual efficiency is in the range of 9- 11%. Anyway linear Fresnel systems are believed to be a good alternative to parabolic trough systems in terms of the cost, simplicity and land-use efficiency.
Figure 03: A power tower system
A typical concentrated solar thermal power system employs hundreds of heliostats to concentrate solar radiation onto a receiver installed on a tall tower. Heliostats are large mirrors with two-axis sun-tracking systems which help follow the Sun’s path precisely and thereby significantly improve the overall system efficiency. Inside the receiver is a heat transfer liquid which absorbs focused heat and transfers to a working fluid to produce steam. Just like in coal-powered or diesel-powered plants, the next part of the system is a steam turbine connected to a generator.
These systems usually reach a temperature of 800 ˚C- 1000 ˚C or even higher by concentrating solar radiation 600- 1000 times. The key advantage of these systems is that they archive higher energy concentration ratios and higher temperatures at a single receiving station which allows generating electricity using a single steam turbine, and an electric generator. However, the demonstrated annual efficiency of these systems is in the range of 8- 10%. That means the annual conversion efficiency of power tower systems is almost half that of dish Stirling systems. Furthermore, power tower systems require a large open area to receive and concentrate sunlight. Therefore, this model is more suitable for deserts and barren or infertile locations.
Concentrated photovoltaics where Fresnel lenses are used with solar PVs to concentrate sunlight onto solar PVs is also considered to be a concentrated solar power technology. Unlike other concentrating solar power technologies, this concept is not a solar thermal technology as it does not employ heat receivers, steam generators, turbines and electric generators to generate electricity. Instead, they use solar photovoltaics onto which sunlight is focused using Fresnel lenses.
Let us discuss concentrated photovoltaics technology in a separate article in detail.
References
[1] Müller-Steinhagen, H., & Trieb, F. (2004). Concentrating solar power. A review of the technology. Ingenia Inform QR Acad Eng, 18, 43-50.
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Theoretically, concentrated solar power is one of the simplest renewable energy technologies. The term “concentrated solar power” generally refers to both concentrated solar thermal power (CSTP) and concentrated photovoltaics. To begin with, let us start our discussion from concentrated solar thermal power technology.
Concentrated solar thermal power plants convert solar energy into heat of a working fluid and the heat is then converted into electricity. A typical concentrated solar thermal power plant employs mirrors or reflectors to focus solar radiation onto a small surface area where a working fluid has been enclosed. The fluid absorbs heat from focused solar radiation and channeled into a steam generator which turns a steam turbine and generates electricity.
There are several types of concentrated solar thermal power plants in operation. The key difference among them is that they employ different types of reflectors. But they all (except concentrated photovoltaics) share almost the same concept to convert heat into electricity: It is nothing but a steam-powered electric generator!
As we discussed in the previous article, several configurations of concentrated solar thermal power plants have been developed and some of them have been employed in commercial-scale.
Among them, followings are the most common configurations.
In addition to above, several other configurations are available to concentrate solar radiation. However, most of them are not economically/ technically viable and thus have not yet been used to generate electricity in commercial-scale. So, let us concentrate our discussion on potential concentrated solar thermal power concepts only.
Figure 01: Parabolic dish concentrators with Stirling engines
These systems use parabolic dishes to concentrate solar radiation onto its focal point. The paraboloid is the three-dimensional counterpart of a two-dimensional parabola. A typical parabolic dish system employs a heat receiver (At its focal point) coupled with a Stirling engine to collect heat. Collected heat is then channeled into the Stirling engine which exploits the heat to generate electricity. Their dual-axis solar tracking systems always track the Sun’s path and guide the dish so that the full aperture of the dish is always exposed directly to sunlight. The unique and intrinsic configuration of these systems allows to achieve even over 1000 ˚C and offers the highest conversion efficiency over other concentrating solar power systems. An annual efficiency of 16-18% has been already demonstrated with parabolic dish systems [1]. However, these systems are still too expensive for commercial electricity generation.
Figure 02: A parabolic trough reflector
The parabolic trough reflectors look like a parabola in the X-Y plane as shown in figure 03. Their three-dimensional design produces a linear focus along the Z-axis along which a heat absorbing tube runs. The absorbed heat is used to produce steam which turns a turbine connected to an electric generator. The demonstrated annual efficiency for this technology is 10- 15% [1].
Figure 03: Diagram of a parabolic trough system
This technology is technically feasible and economically cost-effective compared to other concentrating solar power technologies and therefore, parabolic trough technology is today’s most popular concentrating solar thermal technology.
In the next article, let’s discuss some other concentrating solar power technologies.
References
[1] Müller-Steinhagen, H., & Trieb, F. (2004). Concentrating solar power. A review of the technology. Ingenia Inform QR Acad Eng, 18, 43-50.
Image credits
Ctd
As we discussed in a previous article, the intensity of sunlight at the Sun’s surface is about 64100 kW m-2(64.1 MW m-2) which diminish down with the distance and is about 1000 W m-2 at the Earth’s surface when the Sun is at the zenith.
Figure 01: Beam divergence of an EM (sunlight) beam with distance
This reduction in the intensity is due to the divergence of Sun’s rays as they travel a long distance from the Sun (Our Sun would look like a point source if you take a look at the long journey of Sun’s rays travelling from the Sun to Earth).
On average, the amount of energy received by a solar energy harvesting device with an area of 1 m2 at the Earth’s surface is about 1000 J per second (Since the intensity of solar radiation is about 1000 W m-2). But….What if we can increase this figure?
Yes!
It would create a flood of energy and will offer a great opportunity to harness energy from a small area leading to a more space-efficient and more cost-effective energy harvesting technology.
It is obvious that the energy yield (energy harvest) increases when the intensity of solar radiation increases. Further, high energy yield (energy harvest) obviously curtails the payback time.
The higher the intensity of solar radiation the shorter the payback time!
That means, the energy yield by a given solar energy harvesting device would,
So, it is crystal clear that the energy yield per unit area can be improved by increasing the intensity of solar radiation.
But, how can we increase the intensity?
That is the idea of concentrated solar technology.
For ease of understanding, let’s take a look at a simple analogy.
An analogy
It would take minutes to fill your cup with rainwater if you wish to collect rainwater by exposing the cup to rain. It would take, however, much less time to fill the same cup with rainwater if you are collecting rainwater from a gutter outlet.
Why?
The gutter has, of course, a small area but collects rainwater falling onto a large surface area (Roof)! So the gutter outlet would fill your cup in an instant.
Simply, you can collect a large amount of water from a small gutter outlet within a short period of time while a large surface area is needed to collect the same amount of rainwater within the same period of time if you are collecting water from rain.
This is analogous to the concentrated solar energy concept. Incoming sunlight received by reflectors (analogous to a roof) is focused onto a small area (analogous to a gutter) by using mirrors or reflectors and intensified solar radiation is then collected and converted into useful energy. Several configurations have been tested and implemented successfully to concentrate solar radiation. The payback time, initial investment, and system efficiency largely depend on the configuration used to concentrate solar radiation. Each configuration has unique and specific pros and cons. In the next article, let us discuss them one by one.
CTD
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