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Solar cavity receiver design for a dish-Stirling system
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology. (Solar group)ORCID iD: 0000-0003-1792-0551
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The growing concern for the climate change has led to an increasing research effort in renewable energy technologies in order to achieve a more sustainable electricity production. Concentrating Solar Power (CSP) is identified as a promising technology to deal with part of the future electricity production. In CSP technologies, a solar receiver converts the concentrated sunlight into high temperature heat. The solar receiver is one of the most critical CSP components as it must provide high thermal power collection efficiencies while operating under very high temperatures and heat fluxes. Thereby, improving the solar receiver efficiency and endurance would benefit the technical and economic viability of CSP.

This PhD thesis aims at improving the efficiency and endurance of a typical solar cavity receiver for the dish-Stirling CSP technology. This research work includes new experimental and numerical analyses contributing to the state of the art of solar receiver design. The efficiency is improved through the analysis of the receiver cavity shape, geometry, operating conditions, and radiative properties, whereas the durability improvement is achieved through the study of advantageous receiver support structures using Finite Element Analysis (FEA). Moreover, a solar laboratory was developed and characterized to conduct representative experiments of the cavity receiver. Multiple parametric experiments were conducted in order to perform a comprehensive validation of the simulations.

During the development of the solar laboratory, it was observed that the commonly utilized flux mapping system (CMOS camera-Lambertian target) should not be used for the characterization of Fresnel lens-based solar simulators. Due to this, the lab characterization was approached combining measurements from a thermopile sensor (radiometer) and a self-designed flat plate calorimeter. Furthermore, a detailed Monte Carlo uncertainty analysis allowed an accurate evaluation of the uncertainty propagation. All the experiments were designed and conducted to increase the accuracy of the final results.

Regarding the cavity receiver design for a dish-Stirling system, the aperture diameter is the most important parameter towards improving the cavity receiver efficiency. The reverse-conical cavity shape provided higher efficiencies (up to 2%) than the cylindrical shape. Additionally, a potential efficiency increase of 0.6% could be achieved by using a cavity material/coating with optimal radiative properties(high emissivity/absorptivity ratio). Finally, the studies suggested that convection has a negligible influence on determining the optimum aperture diameter, whereas the Direct Normal Irradiance (DNI) has little influence. The simulations yielded a cavity receiver with a maximum total receiver efficiency of 91.5%.

Experimental measurements of the receiver displacements under thermal expansion allowed finding realistic mechanical boundary conditions of the receiver. Further structural simulations suggested that thermomechanical stresses can be reduced by setting the receiver supports to certain positions, which can be achieved with the application of external forces and torques. Moreover, the peak stresses can be moved to colder regions to improve the lifetime of the receiver. By shifting the support positions, the receiver simulations calculating creep lifetime under no relaxation showed a potential lifetime improvement of 57%.

Abstract [sv]

Den växande oron för den globala uppvärmningen har lett till en ökad forskningsinsats i förnybar energiteknik mot en hållbarare elproduktion. Koncentrerad solenergi (CSP) identifieras som en lovande teknik för att hantera en del av den framtida elproduktionen. I CSP-tekniken konverterar en solfångare det koncentrerade sollujset till högtemperaturvärme. Solfångaren är en av de kritiska CSPkomponenterna eftersom den arbetar under väldigt höga temperaturer och termiskflöde. Därigenom har solfångarens verkningsgraden och uthålligheten en direkt påverkan på CSP-tekniken och dess ekonomiska genomförbarhet.

Denna doktorsavhandling syftar att förbättra verkningsgraden och hålligheten hos en typisk solkavitetsfångare för dish-Stirling CSP-tekniken. Detta forskningsarbete innehåller nya experimentella och numeriska analyser som inriktar på att förbättra designen. Verkningsgraden förbättras genom analys av solfångarkavitets form, geometri och strålningsegenskaper. Hållfasthetsförbättringen uppnås genom studier av fördelaktiga stödstrukturer för solfångare. Dessutom har ett sollaboratorium utvecklats och karakteriserats för att genomföra representativa experiment. Multipla parametriska experiment andvändes för att validera de numeriska simuleringarna.

Under sollaboratoriets utveckling konstaterades att det allmänt använda CCD-kamera-Lambertian-mål systemet inte kunde användas för sollaboratorikarakteriseringen med Fresnel-linser. På grund av detta utfördes laboratoriekarakteriseringen med en termopil-sensor (mätning av termisk flöde) och en platt kalorimeter. Dessutom gjorde en detaljerad Monte Carlo-osäkerhetsanalys det möjligt att utvärdera osäkerhetskedjan. Experimenten utformades för att öka noggrannheten i de slutligaresultaten.

I den studerade kavitetsfångaren var öppningdiametern den viktigaste parametern för dess verkningrad. Den koniska kavitetsformen gav den högsta verksningsgraden medan verkningsgraden potentiellt kan ökats 0.6% genom idealiska kavitetstrålningsegenskaper (hög emissivitet/absorptivitet förhållande). Studierna antyder att konvektion har en försumbar inverka för att bestämma den optimala öppningsdiametern och DNI-värdet har liten påverkan. Slutligen gav simuleringarna en kavitetsfångare med en maximal total verkningsgrade av 91.5%.

Experimentella mätningar av solfongarens utböjning andändes för att hitta realistiska mekaniska randvillkor. Ytterligare strukturella simuleringar antydde att de termomekaniska spänningarna kan minskas genom justering av solfångarens stödpunkter. Detta kan uppnås med tillämpningen av krafter och vridmoment. Dessutom kan toppspänningarna flyttas till kallare regioner för att förlänga solfångarens livslängd. Med nya stödpositioner kan livslängden mot creep öka 57% för det studerade fallet.

Place, publisher, year, edition, pages
Stockholm, Sweden: KTH Royal Institute of Technology, 2020. , p. 80
Series
TRITA-ITM-AVL ; 2020:2
Keywords [en]
Concentrating Solar Power, Solar receiver, Solar simulator, Uncertainty analysis, Monte Carlo ray tracing, Experimental validation, Coatings, System modeling, Structural analysis, Creep damage, Thermal stress, Cavity receiver design
National Category
Engineering and Technology
Research subject
Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-266773ISBN: 978-91-7873-418-4 (print)OAI: oai:DiVA.org:kth-266773DiVA, id: diva2:1387204
Public defence
2020-02-18, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency, CSP-StirlingVinnova, 2016-02836Swedish Research Council Formas, 2016-02836Swedish Energy Agency, 2016-02836Available from: 2020-01-24 Created: 2020-01-20 Last updated: 2020-01-24Bibliographically approved
List of papers
1. Development of a Fresnel lens based high-flux solar simulator
Open this publication in new window or tab >>Development of a Fresnel lens based high-flux solar simulator
2017 (English)In: Solar Energy, ISSN 0038-092X, E-ISSN 1471-1257, Vol. 144, p. 436-444Article in journal (Refereed) Published
Abstract [en]

In this paper, a Fresnel lens based high flux solar simulator (HFSS) is developed for concentrating solar power research and high temperature material testing. In this design, each commercially available 7 kW(e) xenon-arc lamp is coupled with a silicone-on-glass Fresnel lenses as the optical concentrator, and 12 lamp-lens units are distributed in a circular array. In total, the power of the solar simulator can reach 84 kWe. A ray tracing model has been developed based on the real arc-emitter shape and the Fresnel lens optics for predicting the optical performance of the HFSS design. The testing result shows that the ray tracing model can predict the flux distribution on the focal plane accurately but a bit conservative in the center region. The flux distribution on the focal plane appears axisymmetric with a peak flux of 7.22 MW/m(2), and 19.7 kW of radiative power in total is delivered on a 280 mm diameter target. (C) 2017 Elsevier Ltd. All rights reserved.

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2017
Keywords
High flux solar simulator, Fresnel lens, Concentrating solar energy, Xenon lamp
National Category
Energy Systems
Identifiers
urn:nbn:se:kth:diva-205490 (URN)10.1016/j.solener.2017.01.050 (DOI)000397550500044 ()2-s2.0-85011317412 (Scopus ID)
Note

QC 20170524

Available from: 2017-05-24 Created: 2017-05-24 Last updated: 2020-01-20Bibliographically approved
2. Characterization of the KTH high-flux solar simulator combining three measurement methods
Open this publication in new window or tab >>Characterization of the KTH high-flux solar simulator combining three measurement methods
2017 (English)In: Energy, ISSN 0360-5442, E-ISSN 1873-6785, Vol. 141, p. 2091-2099Article in journal (Refereed) Published
Abstract [en]

This paper presents the characterization of the first Fresnel lens-based High-Flux Solar Simulator (HFSS) showing the evaluation of the total thermal radiative power dependent on the aperture radius at the focal plane. This result can be directly applied to calculate the thermal power input into any solar receiver tested in the KTH HFSS. Three measurement setups were implemented and their results combined to assess and verify the characterization of the solar simulator: a thermopile sensor measuring radiative flux, a CMOS camera coupled with a Lambertian target to obtain flux maps, and a calorimeter to measure the total thermal power within an area of 300×300 mm. Finally, a Monte Carlo analysis was performed to calculate the total uncertainties associated to each setup and to combine them to obtain the simulator characterization. The final result shows a peak flux of 6.8 ± 0.35 MW/m2 with a thermal power of 14.7 ± 0.75 kW within an aperture of 180 mm in diameter at the focal plane, and a thermal-electrical conversion efficiency of 25.8 ± 0.3%. It was found very good repeatability and a stable energy output from the lamps during the experiments.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Solar simulator, Characterization, Uncertainty analysis, Monte Carlo
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-219891 (URN)10.1016/j.energy.2017.11.067 (DOI)000423249200061 ()2-s2.0-85036450427 (Scopus ID)
Note

QC 20171215

Available from: 2017-12-15 Created: 2017-12-15 Last updated: 2020-01-20Bibliographically approved
3. Experimental and numerical performance analyses of a Dish-Stirling cavity receiver: Geometry and operating temperature studies
Open this publication in new window or tab >>Experimental and numerical performance analyses of a Dish-Stirling cavity receiver: Geometry and operating temperature studies
2018 (English)In: Solar Energy, ISSN 0038-092X, E-ISSN 1471-1257, Vol. 170, p. 913-923Article in journal (Refereed) Published
Abstract [en]

Higher performance cavity receivers are needed to increase the competitiveness of solar power plants. However, the design process needs to be improved with more relevant experimental and numerical analyses. Thereby, the performance of four different Dish-Stirling cavities is investigated experimentally analyzing the influence of the cavity aperture diameter and shape at various operating temperatures. Temperatures inside the cavity receiver were collected together with the electrical power produced by the engine-generator. Then, a thermal system simulation was modelled and a comprehensive multi-parameter and multi-operation validation was performed. To improve this validation, the temperature distribution across the receiver tubes was analyzed in order to relate temperatures on the irradiated region with the non-irradiated one, where thermocouples can measure. The simulations were later used to obtain cavity receiver efficiencies, temperatures and loss breakdowns. The results show that the cavity receiver must be studied in optimization processes in conjunction with the other system components. Moreover, the reverse-conical cavity was found to be more efficient than a nearly cylindrical shape. Regarding the cavity receiver thermal losses, radiation and natural convection present similar contributions in the system under study. Finally, it was found that thermocouples installed on a non-irradiated region can be used to obtain peak receiver temperatures if the measurements are rectified by a correction value proportional to the DNI.

Place, publisher, year, edition, pages
Pergamon Press, 2018
Keywords
Solar simulator, Experimental measurements, System modelling, Receiver design
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-234615 (URN)10.1016/j.solener.2018.06.031 (DOI)000442713900083 ()
Note

QC 20180914

Available from: 2018-09-14 Created: 2018-09-14 Last updated: 2020-01-20Bibliographically approved
4. Experimental and numerical performance analyses of Dish-Stirling cavity receivers: Radiative property study and design
Open this publication in new window or tab >>Experimental and numerical performance analyses of Dish-Stirling cavity receivers: Radiative property study and design
2019 (English)In: Energy, ISSN 0360-5442, E-ISSN 1873-6785, Vol. 169, p. 478-488Article in journal (Refereed) Published
Abstract [en]

The solar receiver performance has a direct impact on the CSP power plant performance and, thereby, its levelized cost of electricity. Improved receiver designs supported by new advanced numerical tools and experimental validation campaigns directly help to make CSP technology more competitive. This paper presents an experimental and numerical investigation of the influence of the cavity receiver radiative properties and the thermal power input on the Dish-Stirling performance. Three cavity coatings are experimentally investigated: the original cavity material (Fiberfrax 140), Pyromark 2500 and Pyro-paint 634-ZO. Moreover, simulations validated with the experimental measurements are utilized to define a higher performance cavity receiver for the Eurodish system. The results indicate that the absorptivity of the cavity should be as low as possible to increase the receiver efficiency whereas the optimum emissivity depends on the operating temperatures. If the cavity temperature is lower than the absorber temperature, low emissivities are recommended and vice-versa. All material/coatings analyzed for the cavity provide similar receiver efficiencies, being Fiberfrax 140 slightly more efficient. Finally, a total receiver efficiency of 91.5% is reached by the proposed Eurodish cavity receiver when operating under the most favorable external conditions. 

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2019
Keywords
Solar simulator, Experimental measurements, Coatings, System modelling, Receiver design
National Category
Energy Systems
Identifiers
urn:nbn:se:kth:diva-247838 (URN)10.1016/j.energy.2018.12.033 (DOI)000459528500038 ()2-s2.0-85058468339 (Scopus ID)
Note

QC 20190326

Available from: 2019-03-26 Created: 2019-03-26 Last updated: 2020-01-20Bibliographically approved

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Garrido Gálvez, Jorge

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