What is a Dish or Stirling System?

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A stirling system is basically a heat engine that converts heat into some sort of mechanical work by a series of cyclical expansiona snd contractions of air or gases. Dish-Stirling Systems converts solar radiation into electrical energy. In the past 20 years, the Stirling energy system dish has held the world record in converting solar energy into electricity. In the year 2008, it achieved a new record with a 31.25% efficiency rate. This is all due to their modular design which gives them an edge in power output in both the small and large scale projects. They have thousands of dishes erected in a solar park. Two solar parks using the Stirling Energy Systems or SES systems, are currently being developed in US with a capacituy of  1.4GW. 

What is the Histroy of Stirling Engine?

Stirling engine was discovered by the Scottish Rev. Robert Stirling. But his Stirling Cycle was not understood properly until the scientist Sandi Carnot developed the Carnot Cycle.

What are the Components of Dish-stirling Systems?

The system consists of the following components:

• Parabolic solar concentrator: There is a concentrator that delivers  fuel to the Stirling engine. Waht it does is basically concnetrate as its name states, the suns rays or radiation into a focal point. The concentrator has about 12 single segments, each made of glass fiber resin which are arranged in a parabolic shell. For the operation of the Stirling engine, a very high temperatureshould be obtained. Therefore a quite large point-focus concentrator is used for this system
• Tracking system: There is a tracking suystem to make sure the concentrator always be perfectly oriented towards the sun. Hence it is mounted on a two-axial tracking system. A simple movable steel construction erected  on six wheels has been developed. The horizontal and the vertical orientation of the concentrator aremanaged bythe servomotor. The orientation of the concentrator towards the sun is done by either the sun tracking sensor, or a computer program which actually predicts the position of the sun and orients is accordingly.

• Solar heat exchanger (Receiver): The receiver is the link between the concentrator and the Stirling engine. Generally, there are two types of receivers. One type is a solar-only receiver which can only be operated during sunshine. The other type is a hybrid receiver which is additionally equipped with a gas burner and can be operated the whole day. Receivers have essentially two duties:
  • To absorb as much of the solar radiation reflected onto it from the concentrator as possible.
  • To pass on this absorbed energy to the Stirling engine in form of heat with the least possible losses.

• Stirling engine with generator: The Stirling cycle is the most efficient thermodynamic cycle to transform heat into mechanical or electrical energy. The Stirling engine has some amazing properties:
  • The Stirling engine depends only on external heat supply, with no preference on how the heat is generated.
  • In the Stirling engine a constant amount of working gas (helium or hydrogen) is constantly heated and cooled.
  • Since the efficiency of the Stirling engine increases with increasing upper process temperature, this engine is the ideal combination to produce energy with a solar collector.
  • As there is no internal combustion, this engine produces almost no noise.
  • The impending life-cycle of a Stirling engine is amazingly high since there is no internal installed burner, the required heat can also be generated with fossil fuels (Bio-gas etc.).Thus the system is also online during cloudy periods and during night-time.

How does a Stirling Engine operate?

In the simplest version, a Stirling engine consists of a sealed system with two cylinders (expansion and compression cylinder) filled with a working gas (helium). The pistons of these cylinders are connected to one crankshaft (The shaft for converting the up-and down piston motion into rotary). If the working gas in the expansion cylinder is heated (by the sun) it will expand due to the increasing temperature; pushes the piston down and thus induces power. Part of this power is now used to push the hot working gas from the expansion cylinder into the compression cylinder. On its way, the working gas passes through a re-generator where a major part of its heat is stored and also through a water cooled gas cooler, where it will be cooled further down. Once completely cooled in the compression cylinder, this piston will return due to the inertia of the crankshaft, and the working gas is compressed at low temperature. By reabsorbing the heat stored in the re generator the gas is pushed back into the working cylinder. On the whole, the expansion of the hot gas in the working cylinder delivers more energy than is needed for the compression of the cold gas in the compression cylinder. This surplus of energy can be used to operate an electric generator which is directly hooked to the crankshaft of the engine.

The parabolic concentrator reflects the incoming solar radiation onto a cavity receiver which is located at the concentrator ’s focal point. The solar radiation is absorbed by the heat exchanger (receiver) and thus heats the working gas (helium) of the Stirling engine to temperatures of about 650° C. This heat is converted into mechanical energy by the Stirling engine. An electrical generator, directly connected to the crankshaft of the engine, converts the mechanical energy into electricity (AC). To constantly keep the reflected radiation at the focal point during the day, a sun tracking system rotates the solar concentrator continuously about two axes to follow the daily path of the sun. The electrical output of the system is proportional to the size of the reflector, its optical losses and the efficiencies of the Stirling engine and the generator.

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