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.

1.3      Semi-Autonomous

If each and every heliostat is to be wired to the mainframe computer then it will require a considerable length of communication line/optical fibre resulting in huge additional cost. Therefore we have designed our heliostats to be equipped with microcontrollers that will perform the calculations on spot & send it to the motors.

An idea was proposed of using Arduino only for data acquisition which will then be transferred to the mainframe. At this moment we believe that the calculations required for tolerable error will be well within the capabilities of the controller & there is no need to implement the above mentioned scheme. However if any such communication between the controller & mainframe is established then it will be done wireless.

In order to make the heliostats completely autonomous PVs & batteries are needed which will increase the capital cost immensely. Such an increase in cost is neither economically feasible nor justified.

1.4      Leveling screws

The surface on which the heliostat is to be placed might not necessarily be even. For such cases the designed heliostat is equipped with leveling screws along with a spirit level at its base.

An idea was proposed of using an electronic “spirit level” sensor enabling the heliostat to auto level itself. This will require usage of additional motors along with burdening the microprocessor which cannot be justified.

1.5      Wind Protection

Usually there are strong storms or wind in particular direction in deserts or places where collector towers can be placed, so measures should be taken to make the system wind protected.

Protection against high speed wind was kept in mind while designing the frame of the heliostats. Pressure sensors will be present on our heliostats. When the wind exceeds a certain threshold the heliostat will go into a stall condition. The frame & mirror will align themselves & then turn in such a way that their normal is perpendicular to the direction of wind flow. This configuration will greatly enhance their capability of enduring high speed wind.

1.6      Hail protection /Rain protection

Rain and hail are a natural phenomenon and usually they can be disastrous for the working of Solar concentrators so for smooth working of a solar concentrator, it must be rain and hail protected.

For this purpose there will be a damper at the back side of the main frame. When there is rain we will send signal to motor using RFID and motor will rotate the frame upside down.

Damping can be done using thermo pore sheet at back side of the main frame.