Design and Operation Optimization of a Solar Collector System

This thesis introduces the concept of Concentrated Solar Power utilization for the production of Electrical Energy. As the world economy grows and the technology growth booms, the dependability on more robust, ingenious and cost effective ways of generating electricity becomes essential. Solar Energy is currently striving to become a major player in fulfilling the energy needs of the modern world with environmental aspects guaranteeing a safer future for our generations. There are two main ways through which solar energy from sun is employed to produce electricity i.e. Photo-voltaic cells and Concentrated Solar Power. Little work has been carried out in the field of Concentrated Solar Power (CSP) but there is still room for significant improvement. Concentrated Solar Power has a lot of potential in our country considering the abundance of solar radiation throughout the year particularly in the southern region. The proposed design will be scalable to industrial level. This research project is one of its kind considering not much work has been carried out previously in this regard in Pakistan. The following report contains detailed design strategies for making a Heliostat. We carried out work on Heliostat design in our institute and significantly improved the previous designs of any such tractable heliostat thus enhancing the efficiency of the system.

Heliostat Design

1        Heliostat Design

The Design of the heliostat is a very important task. A good design can enhance the efficiency of the system significantly. It is also necessary in order to effectively withstand various static & dynamic loads that the heliostat will face.

Following are some of the aspects of Heliostat design and other parameters associated with or related to the design of heliostat.

•      Field Selection for Heliostats/Polar Field

•      Selection of Frame Arrangement/Horizontally rectangular main frame

•      Shape and design of the Mirror/Slightly tilted mirrors

•      Wireless And autonomous control

•      Tracking of primary axis

•      Wind Protection

•      Hail protection /Rain protection

Design Features of A Heliostat

1        Heliostat Field Selection

From case studies of different mega projects involving SCSs (solar tower) we observed that the heliostats were arranged in one of the following configurations

1.      Surround Field

2.      Polar Field

1.1      Surround Field

In this configuration the solar tower is completely surrounded by heliostats on all sides. This configuration has an added advantage that the total insolation remains relatively more uniform throughout the day. When the sun moves from one side of the tower to the other side then the heliostats on the opposite side start to reflect the sun rays more effectively hence maintaining a uniform profile.

It has been experimentally observed that the efficiency of the heliostats nearest to the tower is maximum & it decreases as the distance increases. In surround field configuration more heliostats can be placed inside this high efficiency region.

This type of field is used in Ivanpah (392 MW).

1.2      Polar Field

In this type of field all the heliostats are placed on one side of the tower.

This characteristic can be utilized in medium to large sized solar tower systems(STS). If the same number of heliostats is arranged in polar field instead of surround field then due to their increased efficiency more energy will be generated with the same initial cost

Due to above mentioned reasons we are going to use “Polar Field”.

1.2.1    Mechanical Design

The mainframe of the heliostat is designed to be rectangular with its length greater than its height. This arrangement will give us following benefits

1. Due to smaller height, the heliostat will cast a smaller shadow on the mirrors behind it as opposed to a vertically erect rectangular heliostat.
2. The distance between consecutive rings can be decreased due to smaller shadow of front row heliostat which will result in increased system efficiency.
3.   The center of gravity will be lower to the ground which will increase stability of the mechanical system while facing wind.
4.      Smaller height will result in smaller moment arm so the assembly can bear high forces without being overturned.
5. The frame will be rotated along the horizontal axis while the mirror will rotate along the vertical axis.

Mirror Manufacturing for Solar Concentrator

Selection of Reflective Mirror

In almost all of the STSs plane mirrors are used. The reason is that the cost of manufacturing curved mirrors is enormous which results in an increased amount of capital required to install the plant.

Despite the higher cost, curved mirrors were selected for this project due to following reasons

1.      Through literature survey we came to know that in terms of “Levelized Cost of Electricity” (LCOE) reduction, a design improvement that results in 1% performance improvement is equivalent to a design improvement that reduces solar field cost by about 2.3% i.e. the leverage of performance vs. cost on LCOE is better by a factor of about 2.3x

The exact improvement in efficiency due to this decision is unknown at this stage & it is possible that this improvement might not be able to balance out the increase in capital cost. Any and all results will be reported at the end of the project including the impact of this decision.

2.      There is another factor that inspired us to use curved mirrors. As the distance from the solar tower increases, the effect of scattering from dust particles becomes more and more pronounced. Thus the advantage of using a converging beam of light become more evident as this kind of reflected beam will be able to transfer a considerably more amount energy to the solar tower even after scattering.

3.      As the beam is converging therefore the area of the “wave front” formed at the receiver will be of small size. This enables us to design a receiver of smaller size which will reduce cost.

4.     Higher concentration of light due to converging beam will result in increased temperature which will result in increased efficiency of the system.

5.     Very high precision tracking is usually not feasible. The decreased wave front area of the reflected beam gives us an increased margin of tolerable error in our tracking.

Heliostat Tracking Techniques and Control Mechanism

1.1      Solar Tracker

The solar tracker is a device and a mechanism for orienting the solar concentrator in such a way that it reflects maximum sunlight on the receiver directly from the sun and the deviation angle is minimum throughout the tracking time. A highly efficient Solar tracking mechanism drastically enhances the efficiency of the solar concentrator since it can collect more sun light as compared to a passive mechanism.

Solar trackers are implemented using Astronomical equations, light dependent resistors, sun pointing sensors etc. coupled with other devices such as Stepper motor, Dc Motors, Servo motors, gas filled piston etc.  

1.1.1    Dual Axis Trackers

The dual axis tracker has both horizontal and vertical axis and hence can track the position of the sun and its apparent motion anywhere in the sky throughout the day. Dual axis tracker track the sun through both East to West and North to South, which gives it an added benefit of capturing more sunlight directly from the sun, which increases the added power output and the theoretical efficiency by approximately 40%.

It can be implemented with the help of sun pointing sensors or light dependent resistors, astronomical data or predicted trajectory of sun throughout the day.

1.1.1.1  Disadvantage

Dual axis tracking comes with greater complexity for the system. More motors are to be used to dual track which increases the difficulty level. It is also argued that it can be less reliable considering greater number of devices used in this mechanism which increases the maintenance and down time.