Power plants – Utilizing natural heat – Solar
Reexamination Certificate
2001-07-31
2002-12-03
Nguyen, Hoang (Department: 3748)
Power plants
Utilizing natural heat
Solar
C060S641110, C060S641150
Reexamination Certificate
active
06487859
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to hybrid solar fossil fuel receivers and, in particular to hybrid sodium heat pipe receivers for dish/stirling systems in follow up to and based upon Provisional Application Ser. No. 60/222,875, filed Aug. 3, 2000, and claims the benefit of the priority filing date of said Provisional Application under 35 U.S.C. Section 119(e).
2. Description of the Related Art
Solar dish/sterling systems continue to receive strong interest in concentrating solar research programs, because of their demonstrated high efficiency for conversion of sunlight to electricity. Potential end users have indicated that to satisfy their requirements for continuous, reliable, and economical electricity, these systems will need to be hybridized. Hybridization adds a combustor and two heat exchangers to the existing concentrator, receiver, engine, and electrical system. This addition should cost less than $300/kW to compete with its diesel alternative. In addition to this economic challenge, there is the technical challenge of efficiently firing an engine at 700° C. or more. This requires a well-designed primary heat exchanger as well as a carefully-integrated combustor and recuperator.
Over the past decade or so, a number of programs have addressed various aspects of these challenges. Most have used alkali-metal reflux receivers as the starting point. These receivers are popular because of their isothermal behavior. Their primary benefit is higher system efficiency, enabled by uniform temperature at the Stirling-engine heater heads. For hybrid systems, reflux receivers have a further benefit: they allow separate solar and fired heat-transfer surfaces, and therefore independent optimizations. Conceived nearly 20 years ago, Osborn, D. B., et al., “Solar Power Converter with Pool Boiling Receiver and Integral Heat Exchanger,” U.S. Pat. No. 4,335,578, Jun. 22, 1982, alkali-metal reflux receivers have been under intensive development since about 1987. Andraka, C. E., et al., “Reflux Heat-Pipe Receivers for Dish Electric Systems,”
Proceedings of the
22
nd Intersociety Energy Conversion Engineering Conference
, Philadelphia, Pa., 1987; Diver, R. B., et al., “Solar Test of an Integrated Sodium Reflux Heat Pipe Receiver/Reactor for Thermochemical Energy Transport,” Journal of Solar Energy, 1990; Andraka, C. E., et al., “Testing of Stirling Engine Solar Reflux Receivers,”
Proceedings of the
28
th Intersociety Energy Conversion Engineering Conference
, Atlanta, Ga., 1993; and Adkins, D. R., et al., “Heat Pipe Solar Receiver Development Activities at Sandia National Laboratories,”
Proceedings of the Renewable and Advanced Energy Conference
, Maui, Hi., 1999.
In 1991, the Institute for Physics and Power Engineering (IPPE, Obninsk, Russia) reported on several sodium and NaK heat-pipe designs used to transmit power to Stirling engines. Gonnov, I. V., et al., “Design and Testing of Heat Exchangers with Liquid Metal Heat Pipes for Stirling Engines,”
Proceedings of the
26
h Intersociety Energy Conversion Engineering Conference
, Boston, Mass., 1991. The IPPE designs included gas-fired and solar-heated versions, all with screen wicks. The gas-fired surfaces were elaborate high-parts-count assemblies. Nominally-isothermal operation was demonstrated with metal-vapor temperatures up to 750° C. and electrical output up to 4 kW
e
. The issues of simultaneous gas and solar (hybrid) operation were not addressed.
Also in 1991, The German Aerospace Research Establishment (DLR) Institute for Technical Thermodynamics (Stuttgart, Germany) reported on their development of a sodium heat pipe receiver with screen wicks, demonstrating transport of 32 kWt at 780° C. Laing, Doerte, et al., “Sodium Heat Pipe Solar Receiver for a SPS V-160 Stirling Engine: Development, Laboratory and On-Sun Test results,”
Proceedings of the
26
th Intersociety Energy Conversion Engineering Conference
, Boston, Mass., 1991. Since then, the DLR has continued the development of its design. Laing, D., et al., “Second Generation Sodium Heat Pipe Receiver for a USAB V-160 Stirling Engine: Evaluation of On-Sun Test Results Using the Proposed IEA Guidelines and Analysis of Heat Pipe Damage,”
Journal of Solar Energy Engineering
, November, 1997, and most recently, reported on first- and second-generation hybrid designs. Laing, D., et al., “Design and Test Results of First and Second Generation Hybrid Sodium Heat Pipe Receivers for Dish/Stirling Systems,”
Proceedings of the ASME International Solar Energy Conference
, Albuquerque, N.Mex., 1998. The DLR hybrids are completely-integrated systems, including a Stirling engine, screen-wick heat-pipe receiver with separate solar and gas-fired surfaces, a natural-gas combustor, a brazed-fin primary heat exchanger, and a recuperator. The first system used a diffusion gas-swirl burner. It was operated for more than 60 hours, with “very acceptable” behavior. The DLR has presented results showing burner operation between about 8 and 22.8 kW
t
, sodium vapor temperatures up to 790° C., system efficiencies up to 20% (gas only, with the aperture plugged) and combustor efficiencies up to 90%. The second DLR hybrid represents a significant re-design. It uses a lean pre-mix combustion system, chosen to reduce exhaust emissions. The engine heater tubes are relocated to simplify manufacturing.
In 1994, Thermacore reported on its first hybrid heat-pipe receiver, developed for the Cummins Power Generation 7.5 kW
e
dish/Stirling system. Hartenstine, J. R., et al., “Development of a Solar and Gas-Fired Heat Pipe Receiver for the Cummins Power Generation 7.5 kWe Dish/Stirling System,”
Proceedings of the
29
th Intersociety Energy Conversion Engineering Conference
, Washington, D.C., 1994. Thermacore's first system included a sodium heat-pipe receiver, separate solar and gas-fired surfaces, a natural-gas combustor, and an integrated recuperator. It featured nickel-powder wicks, fins milled from the heat-pipe wall, and nozzle-mixing burners. Test results (not reported in the literature) led to a second design that uses pre-mixed metal-matrix burners and circular-finned secondary heat pipes to supply heat to the primary heat-pipe solar receiver. It is believed that this system was tested successfully, although, once again, the test results are not reported in the literature.
In 1995, Stirling Technology Company (STC) reported on its development of a hybrid 10 kWt NaK pool-boiler receiver. Noble, J. E., et al., “Test Results from a 10 kWt Solar/Natural Gas Hybrid Pool Boiler Receiver,”
Proceedings of the
4
th
ASME/JSME Solar Engineering Joint Conference
., Maui, Hi., 1995. The system comprises a NaK pool boiler, separate solar and gas-fired surfaces, a natural-gas combustor, and a stand alone recuperator. The burner was a pre-mixed metal matrix type, delivering heat radiatively and convectively to the pool-boiler wall. The system was thermally loaded with a water-cooled gas-gap calorimeter. Tests were carried out with lamp heating at STC, and later with solar heating at the High Flux Solar Furnace at National Renewable Energy Laboratory (NREL). Full hybrid operation at nominally 700° C. was demonstrated during simulated natural cloud transients, with burner power varying by 2:1.
In 1995, our nascent hybrid receiver efforts were combined to develop a 75-kWt hybrid reflux receiver, with emphasis on manufacturability, cost, and lifetime. Using a ⅙th-scale gas-fired sodium heat pipe, the initial step was to select a candidate burner type and candidate gas fired surface configuration. In 1997, we reported on our study of the applicability of premixed metal-matrix radiant burner technology to hybrid systems. Bohn, M. S., “Application of Radiant Burner Technology to Hybrid Dish/Stirling Systems,”
ASME International Solar Energy Conference
, Washington, D.C., 1997.
However none of the foregoing art enables a fully-integrated system, including a burner, pin-fin primary heat exchanger, recuperator, solar absorber, and sodium heat pipe, which is chara
Andraka Charles E.
Anselmo Kenneth M.
Bohn Mark S.
Corey John
Mehos Mark S.
Midwest Research Institute
Nguyen Hoang
White Paul J.
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