Development of an open volumetric air receiver for a rock bed thermal storage system

Nel, Jacobus Coenraad (2017-12)

Thesis (MEng)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: Concentrating solar power technology combined with a thermal energy storage system offers a sustainable and dispatchable energy technology which can produce electricity on demand. However, the high costs and advanced technology requirements associated with these systems need to be addressed in order to make them more cost effective and viable for local implementation. In this study an open volumetric air receiver was developed as a suitable solution to overcome the mentioned limitations. This technology has an inherently simple design with the benefit of using air as HTF that is abundantly available and has no environmental impact. The ultimate goal of this work is to use the open volumetric air receiver to charge a rock bed thermal energy storage system. In this project the first steps were taken through developing an absorber concept for this use. The concept was aimed towards a simple and cost effective design, which satisfies local manufacturing capabilities and material availability. The charging requirements for the rock bed allowed for the use of a metallic absorber, since the maximum charging air temperature is 600 °C. A stack of stainless steel wire mesh screens with a gradually decreasing porosity through its depth was identified as a cost-effective and simple absorber concept. The stack of wire mesh screens was placed inside a modular stainless steel cup to form a scalable absorber module. A numerical model was developed to investigate the heat and radiation transfer inside the porous absorber. The model assumed local thermal non-equilibrium between the screen and the air temperatures and approximated the radiation as a volumetric heat source. The use of low porosity wire mesh screens in the front of the absorber, followed by a finer screen deeper in its volume reduced the thermal losses and allowed better penetration of the incident flux, as well as enhanced the heat transfer through the absorber. The air mass flow rate proved to have a significant influence on the outlet air temperature. A lower flow rate increased the outlet air temperature, but also caused an increase in thermal loss due to the higher front temperature and lower heat transfer coefficient, and hence lowered the thermal efficiency of the absorber. The absorber prototype and a small scale central receiver test tower was designed, manufactured and installed on an available 1 kWth medium flux concentrator to experimentally test and validate the concept. Different configurations of mesh screens were tested at different air mass flow rates. The best mesh configuration produced hot outlet air at 405 °C to 488 °C at thermal efficiencies of 87 % to 58 % in an average incident flux level of 55.3 kW/m2. The results produced by the model were in very good agreement with the experimental results at higher flow rates and adequately predicted its trend. At lower flow rates, however, the model overestimated the thermal performance. Finally this project proved that the wire mesh screen absorber concept with a gradually decreasing porosity is a suitable solution for the use in an open volumetric air receiver to charge a rock bed thermal energy storage system.

AFRIKAANSE OPSOMMING: Gekonsentreerde sonkragtegnologie gekombineer met 'n termiese-energie stoorstelsel bied 'n volhoubare en versendbare energietegnologie wat elektrisiteit op aanvraag kan lewer. Die hoë koste en gevorderde tegnologie-vereistes wat met hierdie stelsels verband hou, moet egter aangespreek word om hul meer kosteeffektief en lewensvatbaar te maak vir plaaslike implementering. In hierdie studie was 'n oop volumetriese lugontvanger ontwikkel as 'n geskikte oplossing om die genoemde beperkings te oorkom. Hierdie tegnologie het 'n inherente eenvoudige ontwerp, met die voordeel om lug te gebruik as hitte-oordragvloeistof wat oorvloedig beskikbaar is en geen omgewingsimpak het nie. Die uiteindelike doelwit van hierdie werk is om die oop volumetriese lugontvanger te gebruik om 'n rotsbed termiese energie stoorstelsel te laai. In hierdie projek was die eerste stappe geneem om 'n konsep vir ʼn absorbeerder te ontwikkel vir hierdie gebruik. Die konsep was gemik op 'n eenvoudige en kosteeffektiewe ontwerp wat voldoen aan plaaslike vervaardigingsvermoëns en materiaal beskikbaarheid. Die laaivereistes vir die rotsbed laat die gebruik van 'n metaalabsorbeerder toe, aangesien die maksimum lugtemperatuur 600 °C is. 'n Stapel vlekvrye staal gaas skerms met 'n geleidelik dalende porositeit deur die diepte was geïdentifiseer as 'n koste-effektiewe en eenvoudige absorbeerder konsep, en is geplaas in 'n modulêre vlekvrye staal koppie wat 'n skaalbare absorbeerder module vorm. 'n Numeriese model was ontwikkel om die hitte en bestraling oordrag binne die poreuse absorbeerder te ondersoek. Die model het lokale termiese nie-ewewig tussen die skerm en die lugtemperature aangeneem en die straling benader as 'n volumetriese hittebron. Die gebruik van lae porositeit gaas skerms in die voorkant van die absorbeerder, gevolg deur 'n fyner skerm dieper in die volume verminder die termiese verliese en laat beter penetrasie van die gekonsentreerde straling toe, sowel as die hitteoordrag deur die absorbeerder. Die lugmassa se snelheid het 'n beduidende invloed op die uitlaatlugtemperatuur. 'n Laer vloeitempo het die uitlaatlugtemperatuur verhoog, maar het ook 'n toename in termiese verlies as gevolg van die hoër voortemperatuur asook verswakte hitte oordrag koeffisiënt. Die prototipe van die absorbeerder, en 'n kleinskaalse sentrale ontvanger toetstoring, was ontwerp, vervaardig en geïnstalleer op 'n beskikbare 1 kW konsentrator om die konsep eksperimenteel te toets en te valideer. Verskillende konfigurasies van maasskerms is by verskillende vloeitempo's getoets. Die beste gaas konfigurasie het warm uitlaat lug by 405 °C tot 488 °C met ‘n termiese doeltreffendheid van 87 % tot 58 % gelewer, met 'n gemiddelde voorvalvloeivlak van 55.3 kW/m2. Die resultate wat deur die model geproduseer is, was in baie goeie ooreenstemming met die eksperimentele resultate by hoër vloeitempo's en het die tendens voldoende voorspel. Dit het egter die termiese doeltreffendheid oorskat by laer vloeitempo's. Uiteindelik het hierdie projek bewys dat die draad gaas skerms absorbeerder konsep met 'n geleidelik dalende porositeit 'n geskikte oplossing is vir die gebruik in 'n oop volumetriese lugontvanger om 'n rotsbed termiese energie stoorstelsel te laai.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102918
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