Development and testing of an externally finned tube cavity receiver for Brayton cycle preheating purposes

Basson, E. J. J. (2019-04)

Thesis (MA)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: The low cost energy storage of concentrated solar power is the biggest advantage of this technology over other renewable energy sources. This storage enables such a power plant to dispatch the energy on demand. A combined cycle air driven CSP plant called the SUNDISC, consisting of an upstream solarised Brayton cycle with an intermediate rock bed energy storage and a downstream steam Rankine cycle, was conceived at Stellenbosch University. The use of air as working fluid enables potential co-firing with natural gas in the solarised Brayton cycle. This increases the annual capacity factor and ensures nominal operation during periods of intermittent irradiation. The bottoming Rankine cycle further increases the annual capacity factor of the plant and with the aid of the rock bed, the bottoming cycle can deliver power on demand after the sun has set. The Brayton cycle gas turbine, however, limits the mass throughput and thereby the charging quantity to the storage and bottoming cycle during periods of high solar resource. In past studies of the SUNDISC cycle a receiver system capable of bypassing the Brayton cycle and directly charging the rock bed was conceived and investigated numerically. The Hybrid Pressurised Air Receiver (HPAR) intends to supply compressed air to the Brayton cycle and atmospheric air to the rock bed simultaneously. Simulation suggested that the desired thermal output to the rock bed could not be achieved. Several suggestions to improve the heat transfer to the non-pressurised side were made, among which was the addition of external fins to the tubes. The objective of this study is to experimentally determine the ray trapping and heat transfer enhancement effect of external fins. In this project, a test receiver was designed based on the suggestions from previous studies. The receiver was instrumented and installed at the receiver test facility of STERG at Stellenbosch University where several tests were conducted to determine the behaviour of the modified HPAR. The experimental investigation yielded on-sun data with which future investigations can be validated. The circumferential temperature distribution on selected tubes, the volumetric cavity temperature, the heat removal relationship between the inner and outer fluid and the ray penetration depth was captured. Using the test data a sensitivity analysis of the on-sun tests were conducted which highlights the sensitivities of the receiver to the environmental effects. During the experimental investigation, the receiver demonstrated the ability to modulate the circumferential temperature gradient on the tubes which can potentially reduce thermal fatigue of the components. The receiver further demonstrated the ability to control the different energy absorption quantities to the two streams, enabling better thermal control of the receiver and cycle. Lastly, the receiver was found to be fairly insensitive to wind and demonstrated the ability to capture and repurpose the convective losses, thereby further increasing its efficiency.

AFRIKAANSE OPSOMMING: Die lae koste energie berging van gekonsentreerde sonkrag is die grootste voordeel bo ander hernubare energiebronne. Hierdie lae koste berging stel so 'n kragsentrale instaat om energie op aanvraag te voorsien. 'n Gekombineerde siklus, lug gedrewe, CSP-aanleg, met 'n primêre Brayton-siklus, 'n intermediêre rotsbed energie-stoor en 'n sekondêre Rankine-siklus, bekend as die SUNDISC siklus is deur die Universiteit Stellenbosch ontwikkel. Die gebruik van lug as werksvloeier maak die verbranding van gasse moontlik in die Brayton-siklus. Hierdie verbranding van gasse verhoog die jaarlikse kapasiteitsfaktor en verseker nominale uitsette van die aanleg gedurende die nag en bewolkte toestande. Die sekondêre Rankine-siklus verhoog die jaarlikse kapasiteitsfaktor van die plant verder. Met behulp van die rotsbed energie berging kan die sekondêre siklus op aanvraag energie lewer nadat die son gesak het. Die Brayton-siklus gasturbine beperk egter die lug se massavloei en sodoende die laaikapasitiet van die energie-stoor en sekondêre siklus gedurende periodes van hoë straling op die heliostaat veld. In vorige studies was 'n ontvangerstelsel wat die Brayton-siklus kan omseil en die rotsbed direk laai opgeteken en ondersoek. Die Hybrid Pressurised Air Receiver (HPAR) beoog om gelyktydig saamgeperste lug na die Brayton-siklus en atmosferiese lug na die energie-stoor te voorsien. Simulasies het bevind dat die termiese doelwitte van die rotsbed nie bereik kan word deur die ontvanger nie. Verskeie voorstelle om die warmteoordrag na die atmosferiese kant van die ontvanger te verbeter is gemaak, mees prominent die byvoeging van eksterne vinne aan die buise. Die uiteindelike doel van hierdie studie is om eksperimenteel die stralings-vasvang- en warmoordrag-effek van hierdie toevoeging te bepaal. In hierdie projek is 'n toetsontvanger ontwerp op grond van die voorstelle van vorige studies. Die ontvanger is by die ontvanger toetsfasiliteit van STERG aan die Universiteit Stellenbosch geïnstalleer. Verskeie toetse was uitgevoer in ‘n poging om die gedrag van die gewysigde HPAR te bepaal. Die eksperimentele ondersoek het data gelewer waarmee toekomstige ondersoeke gevalideer kan word. Die omtrek-temperatuur verspreiding op geselekteerde buise, die volumetriese holte temperatuur, die hitte verwydering verhouding tussen die binneste en buitenste vloeier en die penetrasie diepte is vasgelê. Deur gebruik te maak van die toetsdata is 'n sensitiwiteitsanalise uitgevoer. Die sensitiwiteit van die ontvanger ten opsigte van die omgewings-effekte was beklemtoon. Tydens die eksperimentele ondersoek het die ontvanger die vermoë getoon om die omtrek-temperatuurgradiënt op die buise te beheer. Hierdie beheer kan die termiese uitputtingsvermoë van die komponente moontlik verbeter. Die ontvanger het ook die vermoë om die verskillende energie-absorpsie kwantiteite van die twee strome te beheer bewys, wat beter beheer oor die ontvanger en siklus instaat stel. Laastens was die ontvanger redelik onsensitief vir eksterne winde tydens die toetse. Die ontvanger het die vermoë getoon om die konveksie-verliese vas te vang en oor te dra na die interne vloeier en sodoende die doeltreffendheid verder te verhoog.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/106106
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