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 i t through a turbine or with a piston to produce work. The mechanical power is converted to electrical power by a n electric generator or alternator. A number of thermodynamic cycles and working fluids have been considered fo r dish/engin e systems. These include Rankine cycles, using water or an organic working fluid; Brayton, both open an d closed cycles; and Stirling cycles. Other, more exotic thermodynamic cycles and variations on the above cycles hav e 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 dfficultie s in integrating them with concentrated solar energy. Heat can also be supplied by a supplemental gas burner to allo w operation during cloudy weather and at night. Electrical output in the current dish/engine prototypes is about 25 kW e for dish/Stirling systems and about 30 kWe for the Brayton systems under consideration. Smaller 5 to 10 kWe dish/Stirling systems have also been demonstrated.

Stirling Cycle: Stirling cycle engines used in solar dish/Stirling systems are high-temperature, high-pressure externally heated engines that use a hydrogen or helium working gas. Working gas temperatures of over 700 oC (1292°F) and as high as 20 MPa are used in modern high-performance Stirling engines. In the Stirling cycle, the working gas i s alternately heated and cooled by constant-temperature and constant-volume processes. Stirling engines usuall y incorporate an efficiency-enhancing regenerator that captures heat during constant-volume cooling and replaces it when the gas is heated at constant volume. Figure 4 shows the four basic processes of a Stirling cycle engine. There are a number of mechanical configurations that implement these constant-temperature and constant-volume processes. Most involve the use of pistons and cylinders. Some use a displacer (a piston that displaces the working gas withou t changing its volume) to shuttle the working gas back and forth from the hot region to the cold region of the engine . For most engine designs, power is extracted kinematically by a rotating crankshaft. An exception is the free-pisto n configuration, where the pistons are not constrained by crankshafts or other mechanisms. They bounce back and forth on springs and the power is extracted from the power piston by a linear alternator or pump. A number of excellen t references are available that describe the principles of Stirling machines. The best of the Stirling engines achieve

Solar Linear Stirling Engines
Figure 4. Schematic showing the principle of operation of a Stirling engine.

thermal-to-electric conversion efficiencies of about 40% [6-8]. Stirling engines are a leading candidate for dish/engine systems because their external heating makes them adaptable to concentrated solar flux and because of their hig h efficiency.

Currently, the contending Stirling engines for dish/engine systems include the SOLO 161 11-kW e kinematic Stirling engine, the Kockums (previously United Stirling) 4-95 25-kWe kinematic Stirling engine, and the Stirling Therma l Motors STM 4-120 25-kWe kinematic Stirling engine. (At present, no free-piston Stirling engines are being developed for dish/engine applications.) All of the kinematic Stirling engines under consideration for solar applications are being built for other applications. Successful commercialization of any of these engines will eliminate a major barrier to the introduction of dish/engine technology. The primary application of the SOLO 161 is for cogeneration in Germany ; Kockums is developing a larger version of the 4-95 for submarine propulsion for the Swedish navy; and the STM4-120 is being developed with General Motors for the DOE Partnership for the Next Generation (Hybrid) Vehicle Program .

Brayton Cycle: The Brayton engine, also called the jet engine, combustion turbine, or gas turbine, is an interna l combustion engine which produces power by the controlled burning of fuel. In the Brayton engine, like in Otto an d Diesel cycle engines, air is compressed, fuel is added, and the mixture is burned. In a dish/Brayton system, solar hea t is used to replace (or supplement) the fuel. The resulting hot gas expands rapidly and is used to produce power. I n the gas turbine, the burning is continuous and the expanding gas is used to turn a turbine and alternator. As in th e



Experimen Stirling Engine
Figure 5. Schematic of a Dish/Brayton system.

Stirling engine, recuperation of waste heat is a key to achieving high efficiency. Therefore, waste heat exhausted from the turbine is used to preheat air from the compressor. A schematic of a single-shaft, solarized, recuperated Brayto n engine is shown in Figure 5. The recuperated gas turbine engines that are candidates for solarization have pressur e ratios of approximately 2.5, and turbine inlet temperatures of about 850 oC (1,562°F). Predicted thermal-to-electric efficiencies of Brayton engines for dish/Brayton applications are over 30% [9,10].

The commercialization of similar turbo-machinery for various applications by Allied Signal, Williams International , Capstone Turbines Corp., Northern Research and Engineering Company (NREC), and others may create an opportunity for dish/Brayton system developers.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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