Actuation Mechanisms

The Striling dish is shape like a satellite dish that reflects direct solar irradiation onto a receiver at the focal point of the dish.The receiver will absorbs the sun radiation and convert it into thermal energy. There is two way the thermal can be converted into electricity is either using an engine generator coupled directly to the receiver or the thermal is transported through pipes to a central power conversion system. These systems can excess the limit of the temperature to 1500 oC. Because the receivers are often called distributed-receiver systems.

This system use sunlight instead of crude oil or coal to produce electricity. The solar dish concentrator and the power conversion unit are the main parts of the system. Electricity are generates from a central power converter collect the absorbed sunlight from individual receivers and deliver it via a heat-transfer fluid to the power-conversion systems.

Energy at the focal point are employ with small generators, will produce a electricity rather than as heated fluid. The power conversion unit includes the thermal receiver and the heat engine. The heat is transfer to heat engine as the thermal receiver absorbs the concentrated beam of solar energy.

A thermal receiver can be a bank of tubes with a cooling fluid circulating through it. The heat transfer medium usually employed as the working fluid for an engine is hydrogen or helium. Alternate thermal receivers are heat pipes wherein the boiling and condensing of an intermediate fluid is used to transfer the heat to the engine. The heat engine system takes the heat from the thermal receiver and converts it to produce electricity.

Active Tracker

A large scale heliostat is powered by DC-motors connected to the gear system operates using sun tracking algorithm. This driver proves to be very effective. To track the sun there are a few of the actuator and mechanism needed to be involve.

Examples;

DC Motors

The tracking system would need to consist of two motors, which control the position of the array, and control circuit (either analog or digital) to direct these motors. The following sections discuss some possible types of motors that could be used for this type of application.

The second picture shows the inner working of a basic DC motor. The outside section of the motor is the stator (stationary part), while the inside section is the rotor (rotating part). The stator is comprised of two or more permanent magnet pole pairs, while the rotor is comprised of winding that are connected to a mechanical commuitator. The opposite polarities of the energized winding and the stator magnet attract each other. When this occurs the rotor will rotate until perfect alignment with the stator is achieved. When the rotor reaches alignment, the brushed move across the commutator contacts (middle section of rotor) and energise the next winding.

Second Example;

Light Sensing Element

In the propounded system LDR is used as light sensing element. Photo-diode or photo-transistors could also be used for light sensing. Photo-diodes are temperature sensitive and costly than LDRs. On the other hand, a photo-transistor is slower in response time. Thus LDR is preferred as the connection of LDR is simple and it is also bidirectional.

The LDRs are put into hollow tubes so that only perpendicular incident rays are sensed. Then these LDRs are connected to the controller circuit. When the sun moves from right to left, the incident light on the left LDR is more. Hence, the resistance of the left LDR decreases and vice versa. When the resistance of any particular LDR decreases; the controller circuit responses so that the panel is moved towards the position of that particular LDR. Therefore, dish is always kept at the position where the light intensity is maximum and making the dual axis solar tracker to be more efficient.

Passive Tracker

Operates on a thermo-hydraulic mechanism which is based on two cylindrical tubes containing a low boiling point gas fluid. This fluid is forced from one side or the other (by the solar radiation creating gas pressure) which causes the tracker to move from the resulting imbalance.

Example;

Stirling Engine

This device operates by cyclic compression and expansion of air or other gas (the working fluid) at different temperature. The Stirling is a closed cycle regenerative heat engine with a permanently gaseous working fluid. This Stirling engine has of two type which is Alpha and Beta.

Alpha type Stirling engine. There are two cylinders. The expansion cylinder (red) is maintained at a high temperature while the compression cylinder (blue) is cooled. The passage between the two cylinders contains the regenerator.

Beta type Stirling engine has only one cylinder. Hot at one end and cold at the other. A loose fitting displacer shunts the air between the hot and cold ends of the cylinder. A power piston at the end of the cylinder drives the flywheel.

The expansion cylinder (red) pushes the piston to the end point of the cylinder. This means the compression cylinder (blue) has small amount of gasses comparing to red cylinder.

More gases in blue cylinder mean the red cylinder has convert gas to blue cylinder. This shows that the gases were cooling down and the pressure is drop.

This cycle continue to empty the red cylinder which brings all the pressure at its low point.

The cycle will change hand after the blue cylinder reach to its maximum point of expansion.

The flywheel drives the power piston (top) and displacer piston (bottom) upward so that the hot gas move to heat exchanger.

The power piston and displacer piston is push to the top cylinder as the gases were heated

As the pressure is drop the flywheel drive the power piston and displacer piston downward

Geometical Arrangement

Solar Energy

The basic resource for all solar energy systems is the sun. Knowledge of the quantity and quality of solar energy available at a specific location is of prime importance for the design of any solar energy system. Although the solar radiation is relatively constant outside the earth's atmosphere, local climate influences can cause wide variations in available solar radiation on the earth’s surface from site to site. In addition, the relative motion of the sun with respect to the earth will allow surfaces with different orientations to intercept different amounts of solar energy.

The Picture below shows the regions of high solar radiation where solar energy conversion systems will produce the maximum amount of energy from a specific collector field size. However, solar energy is available over the entire globe, and only the size of the collector field needs to be increased to provide the same amount of heat or electricity as in the shaded areas.

Site Qualification

Solar technologies using concentrating systems for electrical production require sufficient beam radiation. Consequently, appropriate site locations are normally situated in arid to semi-arid regions. On a global scale, the solar resource in such regions are very high. More exactly, acceptable production costs of solar electricity typically occur where radiation levels exceed about 1,700 kWh/m²-yr, a radiation level found in many areas as illustrated in the picture above. Appropriate regions include the southwest United States, Northern Mexico, the North African desert, the Arabian Peninsula, major portions of India, central and Western Australia, the high plateaus of the Andean states, and north eastern Brazil. Promising site locations in Europe are found in southern Spain and several Mediterranean islands.

Solar electricity generation costs and feasibility of the project highly depend on the project site itself. A good site has to have a high annual beam solar radiation to obtain maximum solar electricity output. It must be reasonably flat to accommodate the solar field without prohibitive expensive earth works. It must also be close to the grid and a substation to avoid the need to build expensive electricity lines for evacuating the power. It needs sufficient water supply to cover the demand for cooling water of its steam cycle. A backup fuel must be available for granting firm power during the times when no solar energy is available.

Tracking

To track the sun the tracking algorithm were use. This algorithm uses the zenith and azimuth angle of the sun to position the dish facing directly into the sun. A control unit performs the tracking algorithm which controls the positioning systemthis system moves the dish to face directly at the sun throughout the day. [20]

A transmissioninclude all the parts that move the dish throughout the day, such as linear actuators, linear drives, swivel drives, hydraulic cylinders. A sensing devices such as LDR is use to detects the sun position using their own algorithm this allow a maximum accuracy to detect the sun. [20]

Economic & Environmental Considerations

The most important factor driving the solar energy system design process is whether the energy it produces is economical. Although there are factors other than economics that enter into a decision of when to use solar energy such as no pollution or no greenhouse effects.

Commercial applications from a few kilowatts to hundreds of megawatts are now feasible, and plants totalling 354 MW have been in operation in California since the 1980s. Plants can function in dispatchable, grid-connected markets or in distributed, stand-alone applications.

They are suitable for fossil-hybrid operation or can include cost-effective storage to meet dispatchability requirements. They can operate worldwide in regions having high beam-normal insolation, including large areas of the south western United States, and Central and South America, Africa, Australia, China, India, the Mediterranean region, and the Middle East.