Considerations Regarding the Use of Dish/Stirling Systems in Heliotechnics

Transcription

1 Considerations Regarding the Use of Dish/Stirling Systems in Heliotechnics N. Sorea and D. Cernomazu Abstract -- The paper presents the considerations regarding the use of the Dish/Stirling systems in heliotechnics and their advantages in converting the solar heat to electricity. This technology makes possible for large power facilities to provide clean, reliable, cost-effective and truly sustainable solar-electric power to communities worldwide, and tackle global climate change by actively reducing our planet s carbon emissions. Index Terms -- Solar power generation, solar energy, heat engines, energy conversion. I. INTRODUCTION he Stirling's air engine (as it is referred to in early text T books) was invented by reverend Dr. Robert Stirling and patented by him in 1816 (fig. 1). It followed earlier attempts at making an air engine and it was probably the first to be put to practical use when in 1818 an engine built by Stirling was employed pumping water in a quarry. When the name became simplified to Stirling engine is not known, but may be as recently as the mid twentieth century when the Philips company began to experiment with working fluids other than still refers to it as an 'air engine'[1]. Fig. 1. The Stirling engine patented in 1816 [11] Stirling engines using solar radiation have the greatest future. They seem to be the most convenient for the following reasons: Stirling engine is the only piston engine which is able to transform the heat of solar radiation directly to mechanical and subsequently electrical energy; The price of a Stirling engine is lower than the one for combustion applications; Depending on the geographic latitude, the engine is able to run for hours per year as a maximum; The realistically achievable servicing interval is approximately 2 years; Solar power plants with a power output of 5 50 MW can be installed in one site (easy servicing of engines); In comparison with solar electro-voltaic panels as well as other solar technologies, Stirling engines achieve the highest efficiency of transformation of thermal energy into electricity. II. STIRLING ENERGY SYSTEMS The solar dish Stirling technology is well beyond the research and development phase, with more than 20 years of recorded operating history. The equipment is well characterized with over 50,000 hours of on-sun time. Since 1984, the Solar Dish Stirling equipment has held the world's efficiency record for converting solar energy into grid-quality electricity. SES coordinated with the U.S. Department of Energy and Sun-Labs (National Renewable Energy Laboratory and Sandia National Laboratories) to conduct an endurance test of the solar dish Stirling system and to bring the technology to market. The SunCatcher is a 25-kilowatt-electrical (kwe) solar dish Stirling system which consists of a unique radial solar concentrator dish structure that supports an array of curved glass mirror facets, designed to automatically track the sun, collect and focus, that is, concentrate, its solar energy onto a patented Power Conversion Unit (PCU). The PCU is coupled with, and powered by, a completely re-engineered SES Stirling engine that generates power grid-quality electricity. (fig. 2) Nicolae SOREA, Dorel CERNOMAZU - Ştefan cel Mare University, Suceava, Romania, nsorea@eed.usv.ro, dorelc@eed.usv.ro 349

2 Fig. 2. The SunCatcher Dish/Stirling system [5] The PCU converts the focused solar thermal energy into grid-quality electricity. The conversion process in the PCU involves a closed-cycle, high-efficiency four-cylinder, reciprocating Solar Stirling Engine utilizing an internal working fluid that is recycled through the engine. The Solar Stirling Engine operates with heat input from the sun that is focused by the SunCatcher s dish assembly mirrors onto the PCU's solar receiver tubes which contain hydrogen gas. The PCU solar receiver is an external heat exchanger that absorbs the incoming solar thermal energy. This heats and pressurizes the gas in the heat exchanger tubing, and this gas in turn powers the Solar Stirling Engine. [5] The SunCatcher uses an innovative radial design for its concentrating mirrors (fig. 3). At sunrise, each SunCatcher automatically rotates to face The Sun, and with sophisticated automation software, tracks, collects, and focuses the sun s energy onto a single point, the Power Conversion Unit. innovation in CSP engineering design combining the best of three technologies that result in the world s most efficient solar electric power generation. The SunCatcher is a mirrored concentrator dish powered by a proven high-performance Stirling engine, specifically redesigned for high conversion efficiencies of reflected, focused sunlight to grid-quality electricity at utility-scale. The SunCatcher offers advanced dual axis rotational tracking capability, which means the parabolic dish moves continuously maintaining the optimal angle at 90 degrees with the sun s most direct solar radiation pattern; which optimizes collection and concentration of the highest quality of sunlight 100% of the time. Each SunCatcher is powered by its own Stirling dish engine, which produces 25kW of grid-quality solar-electric power. The Stirling engine has earned its reputation as the preeminent CSP-to-grid quality electricity generation technology after nearly 200 years of research, development and testing, by leading engineering scientists across several continents. The Stirling engine is the consummate PCU/heat engine, with proven and unequalled performance efficiency and reliability for given operating temperatures for electrical generation; and has powered industrial strength agricultural, manufacturing, Naval, commercial and automotive applications. The SunCatcher s three specialized computer controllers optimize the two-axis concentrated parabolic Dish technology on-sun tracking performance, and the PCU/heat engine performance efficiency for given operating temperatures for electrical generation, and facility Control & Data Acquisition designed to maximize the overall efficiency of solar-to-grid quality electricity generation and cost of plant operations and management. A generator is connected to the Solar Stirling Engine; and produces the grid-quality electrical output of the SunCatcher (fig. 4). Waste heat from the engine is transferred to the ambient air via a radiator system similar to those used in automobiles. The gas is cooled by a radiator system and is continually recycled within the engine during the power cycle. The conversion process does not consume water, as is required by most thermal-powered generating systems. Fig. 3. The SunCatcher Dish/Stirling systems [5] The SunCatcher solar dish Stirling system technology enables power plants to build capacity with modular building-block simplicity, and fundamentally shift the way we look at our 21st century utility energy infrastructures. The SunCatcher solar dish Stirling system is the latest 350

3 Fig. 4. Stirling engine used in SunCatcher [4] These systems have the following advantages: Lowest Cost - The SunCatcher represents over a decade of reengineering and redesign of the most advanced Concentrating SolarThermal Power (CSP) generation equipment technology offering highest efficiency, reliability and performance at the lowest competitive costs. Flexibility - The SunCatcher unique modularity allows the units to be installed on sloping land with up to a 5% grade. This significantly reduces the requirements for grading of sites and minimizes the ground disturbance. The most advanced Concentrating SolarThermal Power (CSP) generation equipment technology available, offering the highest efficiency, proven reliability and performance, at the lowest competitive costs. Scalable - Each SunCatcher generates 25kW of gridquality electricity immediately, as they re connected to the power grid. Now 21st century solar power plants will be able to scale daily, from 25MW to over MW. And, should a subsystem need replacing, the SunCatcher offers a modular plug & play design that simply replaces a subsystem when needed on site, with minimal impact on operations. There are no wait times for parts or engines which optimizes ongrid availability. The SunCatcher sets a new standard for cost effective modularity and scalability. Highest Conversion Efficiency - The SunCatcher consists of an innovative concentrating mirrored radial design that automatically rotates to track the sun, focuses and concentrates solar energy onto a single point, the Power Conversion Unit, which generate the world s highest solar energytogrid quality electricity peak conversion efficiency, at 31.25%. (fig. 5) Lowest Water Use - The SunCatcher does not need water for electricity generation or cooling cycles. Only a fraction of the amount of water required by competitive CSP technologies is used for washing its mirrors. Significant cost savings are gained, in addition to the permitting, and environmental advantages. Fig. 5. The SunCatcher efficiency vs. peer technologies [5] Other CSP technology designs require certain turbine generator power capacities as energy demands change. For example, a 10MW facility would require a 10MW turbine. However, as energy consumption increases, let s say to 50MW, the solar facility would then require a new 50MW turbine generator. This process continues as demand dictates, whether it s a 100MW, 1GW, or anywhere in between. Changing out large turbines to match capacity is costly, and, impacts the solar power plant on-line availability at risk. And, should turbine generators require service or need replacement, long lead times take the power plant off line, an extremely costly scenario to both power facilities and their customer communities. The power generation sector is the largest contributor to Greenhouse Gas emissions (GHGs), over 60%, worldwide. Projections indicate global electricity demand will increase 80% by However, SES has secured two of the world s largest solar plant build projects, and is preparing for fullscale production of the SunCatcher ; enabling power facilities to provide the next generation of clean, reliable, cost competitive and truly sustainable solar electricity. SunCatchers produce grid-quality solar power, adding new solar power to the grid daily and are based on the uniquely modular SunCatcher plug & play design. Working with San Diego Gas & Electric and Southern California Edison, Imperial Valley and Calico projects will produce up to a combined 1.750MW of clean, renewable energy. Both projects include up to SunCatcher units and over the next several years, other projects will follow, further validating the commercial appeal of this modular and scalable technology. Solar energy at a scale the world has never seen. SES Manufacturing has the experience and resources to deliver flexible, dependable, and cost competitive grid quality solar power. The SunCatcher is designed to meet the growing global need for renewable energy. The complete product cycle, engineering design to power generation is managed by a team of SES, bringing together world-class experience to meet customer needs across the globe. This renewable energy solution meets a wide range of customer needs, from 1MW to 1.000MW, with a modular, scalable design. Site installation is swiftly achieved with minimum environmental disturbance. Each SunCatcher generates 25KW of grid-quality electricity immediately, as they re connected to the power grid. The 21st century solar power plants are able to scale daily from 25MW to over 1.000MW. And should a component or subsystem need replacing, there are no wait times for parts or engines. As leaders in Concentrating Solar-Thermal Power, they are committed to the ability to execute on time at all levels to achieve high quality with large production capacity at the lowest competitive cost. An innovative and collaborative model designed to ensure dynamic mode visibility, on-time responsiveness, and high quality-performance metrics. The Quality Management System (QMS) at SES focuses 351

4 and closely monitors on every Supply Chain process with every partner-customer. The SES SunCatcher modularity is designed to allow for high volume and low cost automotive-style mass production. This type of mass production also provides the opportunity for high precision and highly consistent quality of manufacturing, which translates into higher reliability of system performance. The factory environments are designed and operated based on lean manufacturing principles. Cost optimization is an imperative. This involves that this products design enhanced for efficient manufacturing and leveraging scale, through long-term technology roadmap commitments that optimize global Supply Chain and logistics costs: Increase competitive advantage through manufacturing excellence Implementing lean manufacturing principles Innovation-to-Production processes and technologies. Stirling Energy Systems, Inc. (SES), the exclusive manufacturer of the SunCatcher, the latest innovation in grid quality, solar electric power generation technology for new energy infrastructures, is committed to a global quality system. III. EURODISH PROJECT The EuroDish is an integrated system developed in the EnviroDish project which combines a parabolic concentrator with a solar powered Solo Stirling engine to produce 10 kw electricity (fig. 6). This complete system is under investigation at PROMES-CNRS for further analysis than the initial project: efficiency improvement, operating cost demonstration, hybridation, cogeneration, etc. The parabolic concentrator tracks the sun by moving according to two axis, placing the dish always in the direction of the Sun. an industrial scale, as this concentrator technology is commercially available. The modifications mainly consist of adding the specially developed items: Observation cameras: focus, general overview; Network, power, water and pressurized air for the experiments; Z-axis high precision platform to adjust the experiment position up to 400 kg load; Water-cooled shutter to adjust thermal power from to 0 to 100 %, suitable for process regulation; 600 kg crane to easily install and remove the Solo Stirling engine and the platforms supporting the experiments, allowing mounting and tuning in the nearby workshop instead of outdoor. A pyrheliometer is installed and monitored at the EuroDish facility allowing precise DNI measurements even in cloudy conditions for correct efficiency evaluation of the hosted process. Dish-Stirling Systems transfer concentrated solar radiation with high efficiencies into electrical energy. Essentially the system consists of the following components: Parabolic solar concentrator; Tracking system; Solar heat exchanger (Receiver); Stirling engine with generator. (fig. 7) Fig. 7. Dish/Stirling system with SOLO 161 Stirling engine [8] Fig. 6. EnviroDish project [7] The EuroDish system allows the experiment processes at The parabolic concentrator reflects the incoming solar radiation onto a cavity receiver which is located at the concentrator s focal point [9]. 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 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. (fig. 8) 352

5 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. The concentrator has the following parameters: Diameter: 8.5 m; Projected area: 56.7 m²; Focal Length: 4.5 m; Average concentration factor: 2500; Reflectivity: 94 % ; Suspension : azimuth; Stow position: face down; Max. allowable wind velocity: 65 km/h; Survival wind velocity in stow position: 160 km/h; Drive: servo motor; Drive velocity: 60 /min; Control system: PC, micro controller; Data transfer: InterBus-S; Remote control: Telephone / WW; Burner performance, min-max: kw; Gas line pressure: 50+15/ -5 mbar; Exhaust back pressure, partial-full load: max. 2 mbar; Exhaust gas temperature: 85 0 C; Volume of exhaust gas flow: kg/h; System: flameless oxidation; Flame control system start/operation: ionization/temperature; Emission of nitrogen monoxide: mg/m 3 ; Emission of carbon monoxide: mg/ m 3 ; Cooling system: Volume of cooling fluid, internal: 4,12 l; Plate heat exchanger: stainless steel, copper, soldered; Cooling water flow via external pump: 0,5-2 m3/h; Cooling water pressure: 3 bar; Mains network connection: Voltage: 400 V, 50 Hz, 3 phase: Stating current: 25 A (operating current: 15,5 A); Capital investment and maintenance costs: Capital investment Cost of unit: approx ; Specific cost of unit ( /kwe): ; Service intervals: operating hours. Fig. 8. SOLO 161 Stirling engine [7] The Stirling engine has the following parameters: External dimensions: Length: 1280 mm; Depth: 700 mm; Height: 980 mm; Weight: 460 kg; General performance data: Maximum exit temperature outer circuit: 65 0 C; Performance temperature at heating inlet: 50 0 C; Electrical output capacity: 2-9,5 kw; Thermal output capacity: 8-26 kw; Electrical efficiency: 22-24,5 %; Thermal efficiency: %; Total efficiency: % ; Engine data: Type: V 2- Stirling engine; Cylinder capacity: 160 ccm; Operating gas: helium; Max. medium operating pressure: 150 bar; Nominal engine speed: 1500 rpm; Burner and combustion chamber: IV. INFINIA S SOLAR SYSTEM Infinia s Solar System relies on a Free-Piston Stirling Engine (FPSE) to convert concentrated solar heat to electricity. Infinia FPSEs convert thermal energy from external energy sources to linear motion which drives an integral linear alternator, thus generating clean, reliable electricity. This system is characterized by nearly 24% efficiency, low maintenance and operating autonomy. The main difference compared to other thermal technologies is that it does not need water or gas to produce electricity. Moreover, the engines are quiet, do not require lubrication and can be connected to the network in modular phases. A 50 MW plant using this technology can be built in less than a year. Fig. 9. PowerDish [6] The facility operates during sunlight hours and remains out of service during night hours. That is why the department of Renovalia together with a team of international consultants 353

6 and Infinia are working to develop a energy storage system to complement this technology. Simplicity means achieving excellent results with minimum hassles. PowerDish is a simple solution an it is a selfcontained system which is easy to install, operates itself, and generates clean, AC power (no plumbing, no complex balance-of-systems needs, no new transmission lines, just proven technologies, great product design and all the benefits of mass production). The result is megawatts installed in weeks. A single 3kW Stirling Generator will produce over 8MW per year. Each megawatt generated removes 1,867 tons of carbon dioxide, 7 tons of nitrogen oxides and 11 tons of sulfur dioxides from the atmosphere annually. Carbon dioxide avoidance alone is the equivalent of burning over 190,000 gallons of gasoline. The conversion efficiency of 24% enables it to generate up to twice as many kilowatt hours of electricity as Photo Voltaic (PV) systems of similar size. A Stirling Solar system will produce 1.5 MW of power on a 3 acre site versus the 13.5 acres required for generation of that same 1.5 MW using PV based systems. Groups of units can be installed in phases to produce increased power. Performance means turning the smallest investment into the most kwh at a site, over time. Versus PV, PowerDish does this by capitalizing on its much higher conversion efficiency and lower installed cost. Versus troughs and towers, PowerDish does this by capitalizing on its speedier and more flexible deployment, higher conversion efficiency and lower operating cost. Excellent technology, simple installation and very low operating costs by combining these in a single product, PowerDish is one of the preferred solar asset development solution for ground mount applications. Fig. 10. Infinia free piston Stirling engine and linear generator [13] V. TEDOM STIRLING ENGINE In 2001, TEDOM s.r.o. decided to commence development of a cogeneration unit based on a Stirling engine. It was a rather courageous decision to start from zero. Nevertheless, the work on the development was commenced at the beginning of In 2002 and 2003, the development was carried out partially with support of the Ministry of Industry and Trade within the framework of the project entitled: Research and development of the equipment for combined production of electrical energy and heat in the capacity class of micro-generation on the basis of a Stirling engine with a possibility of biomass combustion. Fig. 11. The functional specimen of TEDOM s Stirling engine [10] At present, the development is already underway with the use of the company s own financial sources, and the overall costs of the project exceeded CZK 40 million quite significantly. In the course of the work, the original purpose of use the engines have been reviewed several times. In 2008 it was decided that the first commercial version of the engine will be formed by a solar unit with the power output of 10 kwel. The target parameters have been determined as follows: engine output of 11 kw at 1500 rpm; electrical efficiency min. 25% at temperatures of 650 C / 60 C (hot tube temperature/input temperature into the engine); operation medium: helium; maximum mean pressure of the operation gas: 14 MPa; time between schedule maintenance: 2 years; minimum service life to general overhaul hours; low price of the engine; The development of the engine was commenced with a demanding phase consisting in the selection of suitable concepts. After long evaluations, it was decided to use a single-acting modification with the engine displacement of 183 cm 3. The main advantage of this modification is its simplicity and therefore also the presupposed low production costs which were preferred over top parameters. [2] Only a prototype is in operation at present, and this prototype serves for all the development work, i.e. verification of partial solutions and characteristics of periphery parts during tests in the extent of hundreds of hours. The following results have been ensured so far: 354

7 engine output: 7.4 kw at 1500 rpm; engine efficiency: 24.13% at 670 C/50 C temperatures; mean helium pressure: 10 MPa; the longest test: 700 hours; total number of test hours 1 200; fuel: natural gas; engine conditions after the test: satisfactory. Fig. 12. Typical demonstration of the course of TEDOM s Stirling engine output and efficiency [10] The engine development continues and all the knowledge obtained is transferred, on an ongoing basis, into the design of individual parts which are verified again. The entire concepts were motivated by the efforts of achievement of the smallest possible dimensions. On the other hand, the task of the robust design of the crankcase and cylinders is to attenuate the mechanical noise of connecting rods and crossheads as much as possible. The engine weight does not exceed 110 kg. An absolute majority of parts are castings of conventional materials, such as ductile and grey cast iron, possibly steel, of course, with an exception of the hot side of the. At present already more than 80% of all the castings presupposed are used on the engine. [3] For the transfer of power output from pistons the engine uses a complete crank mechanism which is a certain complication, without any doubt, but is necessary for the creation of a pressure-free crankcase. The crossheads transfer lateral forces and the pistons are then loaded on an axial basis only, which is necessary for their correct guiding, without the presence of lubricants. Only the lower part of the crank mechanism is lubricated in a hydrodynamic manner. For this purpose, a gear pump, pressure reducing valve and oil filter are installed in the front cover of the engine. [10] The oil filling is only 2 liters of engine or compressor oil. On the piston rod, a special packing box is used to seal the entire operation pressure of the engine, but this also prevents oil from penetrating into the working chamber of the engine, which is a problem with much higher importance. The packing box assembly is without any doubt the most demanding core of the entire engine. Its function decides whether the engine is capable of operating or not. For its correct function it was necessary to change the design of some parts several times, and mainly it was necessary to increase their quality in a principal manner. A full innovation in comparison with the original functioning sample is the use of a compressor for helium pressure reduction while regulating the power output, and the group of solutions for limitation of the influence of thermal expansion of the hot tube in order to increase its cycle lifetime. The engine development will continue with a focus on properties of periphery equipment, such as sensors, electromagnetic valves, control system, etc. On the basis of the existing experience it is possible to state that the development of the Stirling engine is formed, in a larger part by solutions of production and technological issues which are to be transferred, in the case of the series production implementation, to the way of how to obtain a high level of repeatability. On July 2008, the prototype of the TEDOM Stirling engine successfully passed the basic lifetime test in the range of 700 hours with an average power output of 6.1 kw. The maximum engine power output during the test was 7.6 kw at a pressure of 10 MPa and a temperature mode of 620/40 C (the hot tube temperature / cooling fluid temperature on the entry into the engine). The engine was fully functioning at the time of the test completion. The disassembly carried out subsequently was performed in order to ascertain the condition of key parts. The engine has proven a good basic functionality, even though it could be seen that in some areas it is still desirable to achieve some improvements. Nevertheless, the condition of the engine makes it possible, after small interventions in the design, to commence production of another ten prototypes for long-term tests. Fig. 13. The functional specimen of TEDOM s Stirling engine [10] VI. CONCLUSIONS The paper presents the main Dish/Stirling systems instaled all over the world and their advantages in producing clean 355

8 energy; The shape of the concentrator must be adjusted at the beginning in order to maximize Dish/Stirling performances; These systems are silent in use, low pollution, less lubricate and larger functioning time; The acting mechanisms are simple and there is no danger of explosion; The energy costs produced by a Dish/Stirling in climatic condition of Romania are estimated at 0,6 Euro/kWh and for EURODISH at 0.4 to 0.3 Euro/kWh. [12] VII. REFERENCES [1] SOREA, N. Stadiul actual al soluţiilor în domeniul motoarelor solare bazate pe conversia termomecanică - Referat I în cadrul stagiului de pregătire pentru doctorat. Conducător ştiinţific: prof. univ. dr. ing. Dorel Cernomazu, Universitatea Ştefan cel Mare, Facultatea de Inginerie Electrică şi Ştiinţa Calculatoarelor, Suceava, [2] SOREA, N. Contribuţii teoretice şi experimentale preliminarii în domeniul motoarelor solare bazate pe conversia termomecanică - Referat II în cadrul stagiului de pregătire pentru doctorat. Conducător ştiinţific: prof. univ. dr. ing. Dorel Cernomazu, Universitatea Ştefan cel Mare, Facultatea de Inginerie Electrică şi Ştiinţa Calculatoarelor, Suceava, [3] SOREA, N. Contribuţii privind realizarea unor noi motoare solare bazate pe conversia helio-termo-mecanică - Lucrare de disertaţie, Conducător ştiinţific: prof. univ. dr. ing. Dorel Cernomazu, Universitatea Ştefan cel Mare, Facultatea de Inginerie Electrică şi Ştiinţa Calculatoarelor, Suceava, [4] SOREA, N.; CERNOMAZU, D. Perspectives on using Stirling engines in heliotehnics In: 7th International Conference of Electromechanical and Power Systems SIELMEN 2009, Proceedings, ISBN , Iaşi, 8th - 9th of October 2009, p [5] *** Stirling Energy Systems, Inc. (SES) :: A Global Leader in the utility scale solar electric market, [6] *** Focus your energy, [7] *** CESI Eurodish solar generator, dispower_static/documents/highlight008.pdf [8] *** Eurodish, eng-eurodish.htm [9] *** Stirling Sun Power International SSPi GmbH, [10] *** TEDOM Stirling engine, [11] SOREA, N.; OLARIU E. D.; CREŢU N. et al Contribution concerning the extend of the Stirling engines in electrotechnics In: National Conference New and Renewable Energy Sources CNSNRE 2009, Edition X, Bucharest, 6 th -10 th of October, 2009 [12] PASCULETE, E; STANOIU, L; MACAVESCU, M. Experimental results at PSA facilities, estimation of implementing Dish/Stirling system in Romania, papers_w02/pascul_w02.pdf [13] SOREA, N. Stadiul actual al soluţiilor în domeniul motoarelor Stirling In: DOCT-US, Nr. 1, ISSN , Editura Universităţii Ştefan cel Mare, Suceava, 2009, p