03-10-2012, 02:55 PM
A Solar Dish-Engine
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INTRODUCTION
A Solar Dish-Engine System is an electric generator that “burns” sunlight instead of gas or coal to produce electricity. The major parts of a system are the solar concentrator and the power conversion unit.
The dish, which is more specifically referred to as a concentrator, is the primary solar component of the system. It collects the solar energy coming directly from the sun (the solar energy that causes you to cast a shadow) and concentrates or focuses it on a small area. The resultant solar beam has all of the power of the sunlight hitting the dish but is concentrated in a small area so that it can be more efficiently used. Glass mirrors reflect about 92% of the sunlight that hits them, are relatively inexpensive, can be cleaned, and last a long time in the outdoor environment, making them an excellent choice for the reflective surface of a solar concentrator. The dish structure must track the sun continuously to reflect the beam into 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 beam of solar energy, converts it 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, which is the heat transfer medium and also the working fluid for an engine. Alternate thermal receivers are heat pipes wherein the boiling and condensing of an intermediate fluid is used to transfer the heat to the engine.
SYSTEM DESCRIPTION
Dish/engine systems convert the thermal energy in solar radiation to mechanical energy and then to electrical energy in much the same way that conventional power plants convert thermal energy from combustion of a fossil fuel to electricity. As indicated in Figure 1, dish/engine systems use a mirror array to reflect and concentrate incoming direct normal isolation to a receiver, in order to achieve the temperatures required to efficiently convert heat to work. This requires that the dish track the sun in two axes. The concentrated solar radiation is absorbed by the receiver and transferred to an engine.
CONCENTRATORS
Dish/engine systems utilize concentrating solar collectors that track the sun in two axes. A reflective surface, metalized glass or plastic, reflects incident solar radiation to a small region called the focus. The size of the solar concentrator for dish/engine systems is determined by the engine. At a nominal maximum direct normal solar isolation of 1000 W/m2, a 25-Kw dish/Sterling system’s concentrator has a diameter of approximately 10 meters. The most durable reflective surfaces have been silver/glass mirrors, similar to decorative mirrors used in the home. Attempts to develop low-cost reflective polymer films have had limited success. Because dish concentrators have short focal lengths, relatively thin glass mirrors (thickness of approximately 1 mm) are required to accommodate the required curvatures. In addition, glass with low-iron content is desirable to improve reflectance.
RECEIVERS
The receiver absorbs energy reflected by the concentrator and transfers it to the engine’s working fluid. The absorbing surface is usually placed behind the focus of the concentrator to reduce the flux intensity incident on it. An aperture is placed at the focus to reduce radiation and convection heat losses. Each engine has its own interface issues. Stirling engine receivers must efficiently transfer concentrated solar energy to a high-pressure oscillating gas, usually helium or hydrogen. In Brayton receivers the flow is steady, but at relatively low pressures. There are two general types of Stirling receivers, direct-illumination receivers (DIR) and indirect receivers which use an intermediate heat-transfer fluid. Directly-illuminated Stirling receivers adapt the heater tubes of the Stirling engine to absorb the concentrated solar flux. Because of the high heat transfer capability of high-velocity, high-pressure helium or hydrogen; direct-illumination receivers are capable of absorbing high levels of solar flux (approximately 75 W/cm). However, balancing the temperatures and heat addition between the cylinders of a multiple cylinder Stirling engine is an integration issue. Liquid-metal, heat-pipe solar receivers help solve this issue.
ENGINES
The engine in a dish/engine system converts heat to mechanical power in a manner similar to conventional engines, that is by compressing a working fluid when it is cold, heating the compressed working fluid, and then expanding it through a turbine or with a piston to produce work. The mechanical power is converted to electrical power by an electric generator or alternator. A number of thermodynamic cycles and working fluids have been considered for dish/engine systems. These include Rankine cycles, using water or an organic working fluid; Brayton, both open and closed cycles; and Stirling cycles. Other, more exotic thermodynamic cycles and variations on the above cycles have also been considered. The heat engines that are generally favored use the Stirling and open Brayton (gas turbine) cycles. The use of conventional automotive Otto and Diesel engine cycles is not feasible because of the difficulties in integrating them with concentrated solar energy. Heat can also be supplied by a supplemental gas burner to allow operation during cloudy weather and at night. Electrical output in the current dish/engine prototypes is about 25 kWe for dish/Stirling systems and about 30 kW for the Brayton systems under consideration. Smaller 5 to 10KW dish/Stirling systems have also been demonstrated.
Brayton Cycle
The Brayton engine, also called the jet engine, combustion turbine, or gas turbine, is an internal Combustion engine which produces power by the controlled burning of fuel. In the Brayton engine, like in Otto and Diesel cycle engines, air is compressed, fuel is added, and the mixture is burned. In a dish/Brayton system, solar heat is used to replace (or supplement) the fuel. The resulting hot gas expands rapidly and is used to produce power. In the gas turbine, the continuous and the expanding gas is used to turn a turbine and alternator.
ANCILLARY EQUIPMENT
ALTERNATOR:
The mechanical-to-electrical conversion device used in dish/engine systems depends on the engine and application. Induction generators are used on kinematic Stirling engines tied to an electric-utility grid. Induction generators synchronize with the grid and can provide single or three-phase power of either 230 or 460 volts. Induction generators are off-the-shelf items and convert mechanical power to electricity with an efficiency of about 94%.
Alternators in which the output is conditioned by rectification (conversion to DC) and then inverted to produce AC power are sometimes employed to handle mismatches in speed between the engine output and the electrical grid. The high-speed output of a gas turbine, for example, is converted to very high frequency AC in a high-speed alternator, converted to DC by a rectifier, and then converted to 60 hertz single or three-phase power by an inverter. This approach can also have performance advantages for operation of the engine.
COOLING SYSTEM:
Heat engines need to transfer waste heat to the environment. Stirling engines use a radiator to exchange waste heat from the engine to the atmosphere. In open-cycle Brayton engines, most of the waste heat is rejected in the exhaust. Parasitic power required for operation of a Stirling cooling system fan and pump, concentrator drives, and controls is typically about 1 kW.
CONTROLS:
Autonomous operation is achieved by the use of microcomputer-based controls located on the dish to control dish tracking and engine operation. Some systems use a separate engine controller. For large installations, a central System Control and Data Acquisition (SCADA) computer is used to provide supervisory control, monitoring, and data acquisition.
CURRENT ACTIVITIES
Thermal Motors (STM) to develop a dish/Stirling system for utility-scale applications. The SAIC/STM team successfully demonstrated a 20-kW unit in Golden, Colorado, in Phase 1. In December 1996, Arizona Public Service.
Company (APS) partnered with SAIC and STM to build and demonstrate the next five prototype dish/engine systems.
The overall objective is to reduce costs while maintaining demonstrated performance levels. The economic potential of dish/engine systems continues to interest developers and investors Next-generation hybrid receiver technology based on sodium heat pipes is being developed by Sun Lab in collaboration with industrial partners. Although, heat-pipe receiver technology is promising and significant progress has been made, cost-effective designs capable of demonstrating the durability required of a commercial system still need to be proven. Sun Lab is also developing other solar specific technology in conjunction with industry.
[attachment=35142]
INTRODUCTION
A Solar Dish-Engine System is an electric generator that “burns” sunlight instead of gas or coal to produce electricity. The major parts of a system are the solar concentrator and the power conversion unit.
The dish, which is more specifically referred to as a concentrator, is the primary solar component of the system. It collects the solar energy coming directly from the sun (the solar energy that causes you to cast a shadow) and concentrates or focuses it on a small area. The resultant solar beam has all of the power of the sunlight hitting the dish but is concentrated in a small area so that it can be more efficiently used. Glass mirrors reflect about 92% of the sunlight that hits them, are relatively inexpensive, can be cleaned, and last a long time in the outdoor environment, making them an excellent choice for the reflective surface of a solar concentrator. The dish structure must track the sun continuously to reflect the beam into 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 beam of solar energy, converts it 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, which is the heat transfer medium and also the working fluid for an engine. Alternate thermal receivers are heat pipes wherein the boiling and condensing of an intermediate fluid is used to transfer the heat to the engine.
SYSTEM DESCRIPTION
Dish/engine systems convert the thermal energy in solar radiation to mechanical energy and then to electrical energy in much the same way that conventional power plants convert thermal energy from combustion of a fossil fuel to electricity. As indicated in Figure 1, dish/engine systems use a mirror array to reflect and concentrate incoming direct normal isolation to a receiver, in order to achieve the temperatures required to efficiently convert heat to work. This requires that the dish track the sun in two axes. The concentrated solar radiation is absorbed by the receiver and transferred to an engine.
CONCENTRATORS
Dish/engine systems utilize concentrating solar collectors that track the sun in two axes. A reflective surface, metalized glass or plastic, reflects incident solar radiation to a small region called the focus. The size of the solar concentrator for dish/engine systems is determined by the engine. At a nominal maximum direct normal solar isolation of 1000 W/m2, a 25-Kw dish/Sterling system’s concentrator has a diameter of approximately 10 meters. The most durable reflective surfaces have been silver/glass mirrors, similar to decorative mirrors used in the home. Attempts to develop low-cost reflective polymer films have had limited success. Because dish concentrators have short focal lengths, relatively thin glass mirrors (thickness of approximately 1 mm) are required to accommodate the required curvatures. In addition, glass with low-iron content is desirable to improve reflectance.
RECEIVERS
The receiver absorbs energy reflected by the concentrator and transfers it to the engine’s working fluid. The absorbing surface is usually placed behind the focus of the concentrator to reduce the flux intensity incident on it. An aperture is placed at the focus to reduce radiation and convection heat losses. Each engine has its own interface issues. Stirling engine receivers must efficiently transfer concentrated solar energy to a high-pressure oscillating gas, usually helium or hydrogen. In Brayton receivers the flow is steady, but at relatively low pressures. There are two general types of Stirling receivers, direct-illumination receivers (DIR) and indirect receivers which use an intermediate heat-transfer fluid. Directly-illuminated Stirling receivers adapt the heater tubes of the Stirling engine to absorb the concentrated solar flux. Because of the high heat transfer capability of high-velocity, high-pressure helium or hydrogen; direct-illumination receivers are capable of absorbing high levels of solar flux (approximately 75 W/cm). However, balancing the temperatures and heat addition between the cylinders of a multiple cylinder Stirling engine is an integration issue. Liquid-metal, heat-pipe solar receivers help solve this issue.
ENGINES
The engine in a dish/engine system converts heat to mechanical power in a manner similar to conventional engines, that is by compressing a working fluid when it is cold, heating the compressed working fluid, and then expanding it through a turbine or with a piston to produce work. The mechanical power is converted to electrical power by an electric generator or alternator. A number of thermodynamic cycles and working fluids have been considered for dish/engine systems. These include Rankine cycles, using water or an organic working fluid; Brayton, both open and closed cycles; and Stirling cycles. Other, more exotic thermodynamic cycles and variations on the above cycles have also been considered. The heat engines that are generally favored use the Stirling and open Brayton (gas turbine) cycles. The use of conventional automotive Otto and Diesel engine cycles is not feasible because of the difficulties in integrating them with concentrated solar energy. Heat can also be supplied by a supplemental gas burner to allow operation during cloudy weather and at night. Electrical output in the current dish/engine prototypes is about 25 kWe for dish/Stirling systems and about 30 kW for the Brayton systems under consideration. Smaller 5 to 10KW dish/Stirling systems have also been demonstrated.
Brayton Cycle
The Brayton engine, also called the jet engine, combustion turbine, or gas turbine, is an internal Combustion engine which produces power by the controlled burning of fuel. In the Brayton engine, like in Otto and Diesel cycle engines, air is compressed, fuel is added, and the mixture is burned. In a dish/Brayton system, solar heat is used to replace (or supplement) the fuel. The resulting hot gas expands rapidly and is used to produce power. In the gas turbine, the continuous and the expanding gas is used to turn a turbine and alternator.
ANCILLARY EQUIPMENT
ALTERNATOR:
The mechanical-to-electrical conversion device used in dish/engine systems depends on the engine and application. Induction generators are used on kinematic Stirling engines tied to an electric-utility grid. Induction generators synchronize with the grid and can provide single or three-phase power of either 230 or 460 volts. Induction generators are off-the-shelf items and convert mechanical power to electricity with an efficiency of about 94%.
Alternators in which the output is conditioned by rectification (conversion to DC) and then inverted to produce AC power are sometimes employed to handle mismatches in speed between the engine output and the electrical grid. The high-speed output of a gas turbine, for example, is converted to very high frequency AC in a high-speed alternator, converted to DC by a rectifier, and then converted to 60 hertz single or three-phase power by an inverter. This approach can also have performance advantages for operation of the engine.
COOLING SYSTEM:
Heat engines need to transfer waste heat to the environment. Stirling engines use a radiator to exchange waste heat from the engine to the atmosphere. In open-cycle Brayton engines, most of the waste heat is rejected in the exhaust. Parasitic power required for operation of a Stirling cooling system fan and pump, concentrator drives, and controls is typically about 1 kW.
CONTROLS:
Autonomous operation is achieved by the use of microcomputer-based controls located on the dish to control dish tracking and engine operation. Some systems use a separate engine controller. For large installations, a central System Control and Data Acquisition (SCADA) computer is used to provide supervisory control, monitoring, and data acquisition.
CURRENT ACTIVITIES
Thermal Motors (STM) to develop a dish/Stirling system for utility-scale applications. The SAIC/STM team successfully demonstrated a 20-kW unit in Golden, Colorado, in Phase 1. In December 1996, Arizona Public Service.
Company (APS) partnered with SAIC and STM to build and demonstrate the next five prototype dish/engine systems.
The overall objective is to reduce costs while maintaining demonstrated performance levels. The economic potential of dish/engine systems continues to interest developers and investors Next-generation hybrid receiver technology based on sodium heat pipes is being developed by Sun Lab in collaboration with industrial partners. Although, heat-pipe receiver technology is promising and significant progress has been made, cost-effective designs capable of demonstrating the durability required of a commercial system still need to be proven. Sun Lab is also developing other solar specific technology in conjunction with industry.