Reference:
    1- Simplified Methodology for Designing Parabolic Trough Solar Power
        by Ricardo Vasquez Padilla
        University of South Florida, rsvasque@mail.usf.edu
        2011
    2- Parabolic Trough Collector Overview
        by Dr. David W. Kearney
        National Renewable Energy Laboratory, Golden CO
        Workshop 2007

 A Parabolic Trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The receiver is a tube positioned directly above the middle of the parabolic mirror and filled with a working fluid. The reflector follows the sun during the daylight hours by tracking along a single axis. A working fluid (e.g. molten salt) is heated to 150–350 °C (423–623 K (302–662 °F)) as it flows through the receiver and is then used as a heat source for a power generation system. Trough systems are the most developed CSP technology.

The trough is usually aligned on a north-south axis, and rotated to track the sun as it moves across the sky each day. Alternatively, the trough can be aligned on an east-west axis; this reduces the overall efficiency of the collector due to cosine loss but only requires the trough to be aligned with the change in seasons, avoiding the need for tracking motors. This tracking method approaches theoretical efficiencies at the spring and fall equinoxes with less accurate focusing of the light at other times during the year. The daily motion of the sun across the sky also introduces errors, greatest at the sunrise and sunset and smallest at solar noon. Due to these sources of error, seasonally adjusted parabolic troughs are generally designed with a lower concentration acceptance product.

Parabolic trough concentrators have a simple geometry, but their concentration is about 1/3 of the theoretical maximum for the same acceptance angle, that is, for the same overall tolerances of the system to all kinds of errors, including those referenced above. The theoretical maximum is better achieved with more elaborate concentrators based on primary-secondary designs using nonimaging optics which may nearly double the concentration of conventional parabolic troughs and are used to improve practical designs such as those with fixed receivers.

 Reference, Technical Paper:
    1- Progress Towards Cost-Competitive Solar Power Tower Plants
        by K.L. Santelmann, D.T. Wasyluk, and B. Sakadjian
        Babcock & Wilcox Power Generation Group, Inc., Barberton, Ohio, U.S.A.
        Presented to:  Power-Gen Middle East, 2014, Abu Dhabi, United Arab Emirates

 This  type of solar tower is the open volumetric receiver concept (2-2a). A blower transports ambient air through the receiver, which is heated up by the reflected sunlight. The receiver consists of wire mesh or ceramic or metallic materials in a honeycomb structure, and air is drawn through this and heated up to temperatures between 650°C and 850°C. On the front side, cold, incoming air cools down the receiver surface. Therefore, the volumetric structure produces the highest temperatures inside the receiver material, reducing the heat radiation losses on the receiver surface. Next, the air reaches the heat boiler, where steam is produced. A duct burner and thermal storage can also guarantee capacity with this type of solar thermal power plant.

 The volumetric pressurized air receiver concept (2.2b) offers totally new opportunities for solar thermal tower plants. A compressor pressurizes air to about 15 bar; a transparent glass dome covers the receiver and separates the absorber from the environment. Inside the pressurized receiver, the air is heated to temperatures of up to 1100°C, and the hot air drives a gas turbine. This turbine is connected to the compressor and a generator that produces electricity. The waste heat of the gas turbine goes to a heat boiler and in addition to this drives a steam-cycle process. The combined gas and steam turbine process can reach efficiencies of over 50%, whereas the efficiency of a simple steam turbine cycle is only 35%. Therefore, solar system efficiencies of over 20% are possible.

   Compared to other technologies, the investment costs per square meter of collector field using linear Fresnel technology tend to be lower because of the simpler solar field construction, and the use of direct steam generation promises relatively high conversion efficiency and a simpler thermal cycle design. The Fresnel design uses less expensive reflector materials and absorber components. It has lower optical performance and thermal output but this is offset by lower investment and operation and maintenance costs.
 

This solar dish-engine system collects the sun’s energy and concentrates it on a small receiver. The thermal receiver absorbs the concentrated beam of solar energy, converts it to heat, and transfers the heat to the engine/generator. 150 kW of grid-ready electricity. this dish unit consists of 82 mirrors.

 3- Schematic Diagram

Dish/engine systems use parabolic dishes of mirrors to direct and concentrate sunlight onto a central engine that produces electricity. The dish/engine system produces relatively small amounts of electricity compared to other CSP technologies-typically in the range of 3 to 25 kilowatts.
The Solar Concentrator, or dish, gathers the solar energy coming directly from the sun. The resulting beam of concentrated sunlight is reflected onto a thermal receiver that collects the solar heat. The dish is mounted on a structure that tracks the sun continuously throughout the day to reflect the highest percentage of sunlight possible onto the thermal receiver.
The Power Conversion Unit includes the thermal receiver and the engine/generator. The thermal receiver is the interface between the dish and the engine/generator. It absorbs the concentrated beams of solar energy, converts them to heat, and transfers the heat to the engine/generator. A thermal receiver can be a bank of tubes with a cooling fluid—usually hydrogen or helium—that typically is the heat-transfer medium and also the working fluid for an engine. Alternate thermal receivers are heat pipes, where the boiling and condensing of an intermediate fluid transfers the heat to the engine.
The engine/generator system is the subsystem that takes the heat from the thermal receiver and uses it to produce electricity. The most common type of heat engine used in dish/engine systems is the Stirling engine. A Stirling engine uses the heated fluid to move pistons and create mechanical power. The mechanical work, in the form of the rotation of the engine's crankshaft, drives a generator and produces electrical power.

• Parabolic solar concentrator
• Tracking system
• Solar heat exchanger (Receiver)
• Stirling engine with generator

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 oC. 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 suntracking 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.

 Technical Data:

• The enclosed trough is a variant of the technology parabolic trough
• The design encapsulates the solar thermal system within a greenhouse-like glasshouse.
• The glasshouse creates a protected environment to withstand the elements that can negatively impact reliability and efficiency of the solar thermal system.
• Lightweight curved solar-reflecting mirrors are suspended from the ceiling of the glasshouse by wires. A single-axis tracking system positions the mirrors to retrieve the optimal amount of sunlight.
• The mirrors concentrate the sunlight and focus it on a network of stationary steel pipes, also suspended from the glasshouse structure. Water is carried throughout the length of the pipe, which is boiled to generate steam when intense sun radiation is applied. Sheltering the mirrors from the wind allows them to achieve higher temperature rates and prevents dust from building up on the mirrors.

The Solar Steam Generation Pilot (SSGP) plant has a solar field footprint of 17,280 m2, with a peak output of over 7 MW thermal. The Enclosed trough system is protected by a glass structure, which is similar to an agricultural greenhouse. Lightweight parabolic mirrors (4 of Fig 1) are hung within the glasshouse (2 of Fig 1). The glasshouse provides structural support and isolates the solar collectors from wind and moisture.
Roof washer. The enclosed trough glasshouse structure is fitted with an automated roof washing system (1 of Fig 1) capable of cleaning the entire roof surface each night. The majority of wash water is returned in the gutter system and can be recovered for re-use. Dust infiltration is minimized by positive pressure from an air-handling unit (6 of Fig 1), which provides filtered, dried air at slight overpressure within the structure in all conditions.
Reflector. Due to the absence of wind forces acting on the collectors, a lightweight parabolic reflector can deliver consistently high optical accuracy. Total weight of the mirror and frame is only 4.2 kg/m.2 The lightweight reflector enables a simple cable drive aiming system.  The fully automatic control system delivers less than 0.5 mrad pointing error at hundreds of points within the glasshouse.
Receiver. The low system weight allows the entire mirror system to be suspended from the fixed receiver system. This fixed receiver eliminates all moving parts from the high-pressure direct steam receiver system. Tubular glass shields minimize heat losses from convection (3 of Fig 1). The receivers are suspended from the structure by steel rods (5 of Fig 1).
Boiler. This system is specifically designed to mimic a conventional oilfield OTSG boiler process, and accommodates feed water, with total dissolved solids as high as 30,000 ppm, and produces 80% quality steam at 100 bar.
Water tank. An 80-m3 upright, insulated water tank was added to the system during the year to improve overall performance.

• Step One - There are no solar panels in a GlassPoint system. instead, large curved mirrors are suspended inside an agricultural glasshouse.
• Step Two - The mirrors automatically track the sun throughout the day, focusing sunlight  on a stationary boiler tube containing water
• Setp Three -  The concentrated sunlight heats the water to efficiently produce high-pressure steam