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1、Proceedings of the ASME 2013 International Mechanical Engineering Congress and ExpositionIMECE2013November 15-21, 2013, San Diego, California, USAIMECE2013-65433NON-CONCENTRATED SOLAR COLLECTOR FOR SOLARTHERMAL CHEMICAL REACTIONSNico HotzDuke UniversityDepartment of Mechanical Engineering and Materi

2、als Science 303 Hudson Hall, Box 90300Durham. North Carolina, 27708-0300, USA Phone: 919 660 5118: ABSTRACTsurfaces, the methanol-water mixture can be effectively heated to 240-250°C and converted to hydrogen-rich gas mixture. For liquid methanol-water inlet flow rates up to 1

3、ml/min per m2 of solar collector area can be converted to hydrogen with a methanol conversion rate above 90%.This study will present the design and fabrication of the solar collector-reactor, its testing and optimization, and its integration into an entire hydrogen-fed Polymer Electrolyte Membrane f

4、uel cell system.The purpose of this study is the proof that non- concentrating solar-thermal collectors can supply the thermal energy needed to power endothermic chemicalreactions such as steam reforof alcoholic (bio-)fuels. Traditional steam reformers require the combustionof up to 50% of the prima

5、ry fuel to enable theendothermic reforreaction. Our goal is to use aselective solar absorber coating on top of a collector-reactor surrounded by vacuum insulation. For methanol refor , a reaction temperature of 220-250°C is required for effective methanol-to-hydrogen conversion. A multilayer ab

6、sorber coating (TiNOX) is used, as well as a turbomolecular pump to reach ultra-high. TheINTRODUCTIONThe main objective addressed in this study is to prove that a non-concentrating solar-thermal collector can supply the energy needed to power the steam reformation of alcoholic bio-fuels, such as met

7、hanol and bio-ethanol. Integrating such a collector into a low-temperature PEM fuel cell system can provide a cost-effective hybrid solar energy system for stationary electric power generation; useful for single residential and non-residential buildings. This proof-of-concept is conducted in the for

8、m of experiments, assisted by analytical and numerical analysis and optimization of the system.collector-reactor isof copper tubes and plates and aCu/ZnO/Al2O3 catalyst is integrated in a porous ceramicstructure towards the end of the reactor tube. The device is tested under 1000 W/m2 solar irradiat

9、ion (using an ABB class solar simulator, air mass 1.5).Numerical and experimental results show that convective and conductive heat losses are eliminated at vacuum pressures of <10-4 Torr. By reducing radiative losses through chemical polishing of the non-absorbing1Copyright © 2013 by ASMEFig

10、ure 1: Schematic of the proposed hybrid systemThe combination of these technologies (low- temperature fuel cells and solar power, Fig. 1) will be advantageous in terms of cost and energetic efficiency compared to systems based on a single energy source and energy conversion technology 1. Direct sola

11、r-to-electric energy conversion, such as photovoltaics, is currently not economically competitive with traditional electric power generation. Fuel cell technology using alcoholic fuel possibly generated from biomass (e.g. methanol and ethanol) is, at present, not competitive in terms of costs either

12、. However, the system proposed for this project consists of relatively cheap, commercially available hardware components (an intermediate-temperature solar collector, pressurized gas tank, and hydrogen-fed Proton Exchange Membrane (PEM) fuel cell) and benefits in terms of energetic efficiency from t

13、he cost- supply of solar heat. The storage of liquid methanol as primary fuelthe power demand is high, but inco aly low.solar power isThe major scientific objectives of this proposed project are:(1)Development of a selective solar absorbercoating. The selective solar absorber coating mustbalance hig

14、h solar absorptance with low thermal emittance at its operating temperature 2. These two properties are equal for a given wavelength in traditional materials. It is necessary then to selectively absorb visible light, while emitting little in the infrared. This way energy from the visible light of th

15、e sun is absorbed as thermal energy, while infrared radiation from the absorber isprevented from esca. The solar spectrum on a brightday consists predominantly of wavelengths below 3 mm. Figure 2 shows both absorptance or emmitance, and irradiance, as a function of wavelength 2. The dark grey, jagge

16、d line is the irradiance of solar light on a bright day. The light grey line is the emission spectra of a blackbody at 450°C. The dashed line is the absorptance or emittance qualities of an ideal solar absorber at 450°C.in simple and saferequires almost 1700 times lessspace than traditiona

17、l hydrogen gas canisters.Additionally, this hybrid system, with hydrogen as intermediate product, benefits from the efficient short- term storage of a limited amount of hydrogen in a pressurized tank. This allows for the temporary storage of energy in the system avoiding the need for expensive and i

18、nefficient electric energy storage, e.g. batteries. For electric power costumers and electric grid companies this leads to the significant advantage of a more controllable and responsive power supply that will be better equipped to meet the quickly fluctuating demands of the modern energy user throu

19、gh the near instantaneous conversion of stored hydrogen to electric power via the fuel cell. This system configuration also solves the problem of the unbalanced supply of sunlight during the day-night cycle, an intrinsic disadvantage of solar power. The proposed system can be operated without relyin

20、g on power d by the electric grid during peak hours in theearly evening, as is typical for residential buildings, when(2)Development of a solar-thermal collector. Thesolar-thermal collector has to provide enough solar power to the reactor and the evaporator to achieve the requiredtemperaturesforalco

21、holsteamrefor.Thesetemperatures are typically above 200C, e.g. 220-250Cfor methanol. The design presents two challenges. First the construction of a highly efficient vacuum-basedinsulationaroundthereactor;andsecond,theminimization of irradiative losses through non-collectingsurfaces. Numerical analy

22、sis has shown that the necessary temperatures can be obtained without concentration of sunlight, enabling a simple and compact design.2Copyright © 2013 by ASMEFigure 2: Absorbance and emittance of perfect selective absorber, emission of blackbody at 450°C, and direct terrestrial solar irra

23、dianceIn “Review of Mid- to High- Temperature Solar Selective Absorber Materials”, NREL describes six types of coatings and surface treatments for selective absorption of light 2. These include: 1) Intrinsic absorbers, which use the intrinsic properties of the materials, 2) semiconductor-metal tande

24、ms, which use semiconductors to absorb short-wavelength radiation while the underlying metal provides low emittance, 3) multilayer absorbers, which utilize the selective effect caused by the multiple reflectance passes through the bottom dielectric layer, 4) metal-dielectric composites, which utiliz

25、e either fine metal particles in a dielectric or ceramic matrix; or, a porous oxide impregnated with metal, 5) surface texturing, which optically traps solar energy, and 6) solar- transmitting coatings/blackbody-like absorbers, which use doped semiconductors on an absorber.porous metal dielectric co

26、atings are planned, which are currently less established. In particular, porous metal- dielectric composites are very promising. Other solar absorbers create a quasi-2D interface through which heat is transferred to the methanol.Figure 3: Schematic of the hybrid reformer with integrated catalystA no

27、vel method will be used to fabricate a porousEXPERIMENTmetal-dielectriccompositeimpregnatedwithCu/ZnO/Al2O3 catalytic nanoparticles. This material would create a catalytically active area inside the reactorThe challenge of this system component is to design and fabricate a solar collector using avai

28、lable technologymaterial with a surface-to-volume ratio in the order ofto reach aum temperature of 220 - 250°C. This108 m-1 4. Such a coating would allow the reforofcan be achieved through the use of vacuum insulated fluid channels coated with an efficient solar-thermal absorberfuel vapor in-si

29、tu within the porous 3D oxide structure, thereby decreasing heat losses. This 3D structure will(Fig. 3). Intrinsic, multilayer, and blackbody absorbers are al y well-established technologies. Multilayer absorbers will be used to establish an initial working system because this technology offers the

30、best performance at a modest price premium. Computer simulations have shown that flat-plate solar-thermal collectors using selective multilayer coatings have the potential to heat methanol to 225°C at flow rates of 0.15 ml/h/cm2 3. In future work, experiments withcreate a bulk thermal heterojun

31、ction if you will. Such a coating is, to the best of the authors knowledge, a completely novel concept.By using solar power to fulfill the heating requirement instead of combustion of precious fuel, this project will prove that we can automatically increase thefuel-to-electricity efficiency realisti

32、cally by at least 50%by at least 25%, more if heat losses of practicalsemiconductor-metaltandem,opticaltrap,and3Copyright © 2013 by ASMEapplications are included. This has been shown analytically and numerically in a previous study 1. Furthermore, this coating would not need to be limited to us

33、e with non-concentrating collectors. By using this coating with a concentrating system, alternative fuels such as biodiesel could be utilized with all of the benefits remaining in the new system.(HiPace80,PfeifferVacuum),reachingvacuum pressures below <10-5 Torr.To introduce catalytic nanoparticl

34、es into the copper tubes of the solar collector, a sol-gelation method was used. The CuO/ZnO/Al2O3 nanoparticles were prepared in a one-step process by flame spray synthesis as described in previous studies 5. To a dry mixture containing silica sand (size: 5070 mesh) and sodium metasilicate pentahyd

35、rate or Al2Si2O7 as ceramic binder, triammonium citrate aqueous solution (0.225g/ml, 0.9wt% dry triammonium citrate) as gelation agent and water were added. The mixture was placed in an ultrasonic bath for 5 min to accelerate the dissolving of the binder and form a homogeneous solution, into which24

36、.1 wt% catalyst was then added. The mixture was then manually stirred and placed in ultrasonic bath again for 5 mins respectively to ensure the homogeneity and a gel- like foam precursor was obtained. The gel-like precursor was then fed into the copper tube of the solar collector using plastic pipet

37、tes to scoop and then push it through the orifice. Two plastic rubber tubes were connected at both ends of the reactor after the gel was inserted. The gel could be moved in the copper tube by small amount of air generated by squeezing either side of the rubber tube. The reactor with rubber tubes con

38、nected was immersed into a water bath with the ends of rubber tubes above water to let the evaporated vapor out. After 24 hours in the water bath at 60°C, the reactor was disconnected from the rubber tubes and then placed into oven. The reactor was thermal treated at 120°C for 2 hours with

39、 temperature ramp at 2°C/min from room temperature.A solar collector system based on previous numerical simulations and experiments has been designed and built. This system currently consists of a TiNOX multilayer absorptive coating on copper substrate. Methanol is fed through copper tubing joi

40、ned to the reverse side of the copper substrate. The solar collector is shown in Fig. 4, placed inside a vacuum chamber.Figure 4: Photograph of solar collector inside of vacuum chamberUsing this method, the relatively large silica sand was used as buffer material to avoid hot spots due to its therma

41、l conductivity, to increase the average pore size, and therefore, to decrease the pressure drop caused by gas flow through the foam-like porous ceramic. The gelationAll experiments use a simulated solar light source or solar simulator to ensure an intensity and spectrum oflightto natural sunlight. T

42、he performance of thesolar simulator fulfills the requirements and specificationsof the Class ABB Standard by the American Society for Testing and Materials (ASTM), defined as a spatial uniformity and temporal stability within 5% error and a spectral match within 0.7 and 1.25 (ASTM E927). The light

43、source illuminates an area of 10 x 10 cm2 for the small-scale demonstrator.agent causes a gelation or foaprocess that leads to asignificantly higher porosity at dried state than a comparable packed bed of loose particles. The ceramic binder helps all catalytic and SiO2 particles adheretogether and t

44、o the reactor wall.The solar collector is placed inside a vacuum chamber to reduce heat losses due to conduction and convection from the collector to the ambient. The vacuum is achieved by a roughening pump (Rotary Vane Pump D4B, Oerlikon Leybold) and a turbomolecular pumpRESULTSIn Fig. 5, the resul

45、ting methanol conversion is shown for different reactor temperatures (from 200 to 250C) and various mass flow rates of methanol (from 0.0126 to0.126 mg/s) using one single tubular reactor. This reactor4Copyright © 2013 by ASMEcomprises of a reactor volume of 17.7 mm3 and contains15 mg of Cu/ZnO

46、/Al2O3 catalyst micro-particles. The reactor tube is in this case externally heated by an electric heater.drops; however, even for 10 ml/min/m2, the collector temperature is at a high level, still reaching 224ºC.Figure 6: Reaction temperature inside the solar collector for different liquid inle

47、t flow rates per collector areaFigure 5: Methanol conversion for a single tubular reactor containing 15 mg of catalyst, for different reaction temperatures and mass flow rates of methanol.Previously, it has been shown through simulation that temperatures of 240 or 250C can be achieved by solarirradi

48、ationwithoutconcentrationofsunlightforreasonable flow rates 1, 3. These new experimental results (Fig. 6) confirm these temperatures.As Fig. 5 shows, for a realistic flow rate of methanol fuel, a temperature of more than 230C is required to achieve between 85 and 90% of methanol conversion. For 240C

49、, about 0.02 mg/s methanol input are possible and almost 0.04 mg/s for 250C. This leads to the result that for the bench test demonstrator generating 10 W of electric power, 20 to 40 small reactor tubes are required. This corresponds to a reactor volume of between 0.35 to0.70 cm3 and 0.3 to 0.6 g of

50、 catalyst for the experiments. Using these results as well to estimate the necessary size of the final steam reformer powering an entire singleFigure 7 shows the catalytic performance of foam reactor and corresponding packed bed reactor (loose particles, no sol-gelation) in terms of methanol convers

51、ion. The packed bed reactor contains 45 mg catalyst while the foam reactor has a slightly smaller size with 42 mg catalyst because due to transport transport losses during the fabrication of reactor inside the copper tube. From Fig. 7, it can be seen that at low temperature and low input low rate, t

52、he packed bed reactor has a relatively higher methanol conversion due to slightly larger size. However, when the temperature increased up to 255C, the foam reactor surpassed the packed bed reactor and showed higher methanol conversion at the entire input flow rate range. It is noteworthy that the di

53、fference of methanol conversion between foam reactor and packed bed reactor is getting larger with increased reactor temperature and input flow rate. It is believed that the better performance of the foam reactor at higher temperature and input flow rate is due to its advantage in terms of robustnes

54、s and mechanical stability at those conditions. Compared with foam reactor, the packed bed reactor is relatively fragile and exhibits less efficient heat transfer within the reactor, which might be less obvious atfamily household, the followings are drawn:Theum methanol inlet mass flow rate is up to

55、0.28 g/s, requiring a reactor volume between 120 and 250 cm3 and 105 210 g of catalyst, for reactor temperaturesof 240 - 250C.Figure 6 shows first results from the solar-thermal collector. These results show that a stagnation temperature slightly above 250ºC can be achieved. For flow rates belo

56、w 2 ml/min/m2 (total liquid inlet flow rate per solar collector area), reaction temperatures above 240ºC are possible. For higher flow rates, which is beyond the goal for this system, the collector temperature5Copyright © 2013 by ASMElow temperature and low flow rate. However, when it come

57、s to higher flow rate and higher temperature, the packed bed reactor, which is not as durable as foam reactor, is not able to resist the force of flow and it isunique foam structure and the addition of SiO2 sand as buffer pressed by the performance.flow, leadingto lower catalyticFigure 8

58、: CO concentration of foam reactor (F) and packed bed reactor (PB)SFigure 7: Methanol conversion of sol-gelation foamreactor (F) and packed bed reactor reactor temperatures and flow rates.(PB) at differentThe present study analyzes the possibility to generate the reactor temperature required for methanol steamreforby sunlight. A novel, non-concentrating solarA low concentration of CO is poisoning of the subsequently usedcrucial to avoid fuel cell. Sincecollector design was fabricated and