Development of a dual-pressure air receiver system for the SUNDISC cycle

Heller, Lukas (2017-03)

Thesis (D.Phil)--Stellenbosch University, 2017.

Thesis

ENGLISH SUMMARY: Combined cycle CSP plants with solarized gas turbines have the potential to generate dispatchable electricity at a high efficiency and with the option to integrate cost-effective rock bed thermal energy storage. However, as only the energy downstream of the gas turbine can be dispatched in such a setup, solar-derived electricity generation is only possible for a few hours after sunset or at a low rating. The novel SUNDISC cycle is proposed to overcome this shortcoming by adding an additional, non-pressurized air receiver that directly charges the storage. A techno-economical model of a SUNDISC cycle plant was created based on efficiency estimates for the solar field and receivers, empirical correlations for the heat transfer in the rock bed storage, a part-load model for the Brayton cycle as well as a simplification of the steam cycle. A parametric study with this model suggests that electricity can be generated at an almost constant rating at a levelized cost of 0.143USD/kWe h (24% lower than the reference singlereceiver cycle) during more than 8200h per year. A learning-curve scenario predicts the levelized costs of such plants to drop to less than 0.075USD/kWe h by the year 2035. Heating of the non-pressurized air for charging the storage system and of the pressurized air in the Brayton cycle can be done in a dedicated receiver at each respective pressure level or in a hybrid receiver. Based on the previously proposed HPAR concept, the potential of a metallic tubular hybrid receiver to fulfill the requirements of a SUNDISC cycle plant was investigated. Models of the main heat transfer mechanisms (solar and thermal radiation as well as internal and external convection of the absorber tubes) were merged to estimate the optical and thermal efficiency of the receiver as well as the two air streams’ respective achievable outlet temperature. It was found that the desired temperature increase in the non-pressurized air streamcannotbeachievedforsensibleflowvelocitieswithabasicHPARreceiver design. Additionally, the thermal efficiency was calculated to be lower than in competing pressurized air receiver technologies and temperature gradients in the absorber tubes were high. The potential of three enhancements was investigated: (1) Externally finned absorber tubes can be designed to increase the convective heat transfer to reach the desired outlet temperatures but the energy input into the pressurized air stream is lowered and the assemblies’ durability is unproven. (2) Quartz glass elements can be used to minimize the effect of wind on the receiver and to increase mixing in the tube bundle. (3) A volumetric absorber behind the absorber tubes can help achieve the nominal outlet temperature of the flow. While the results suggest that these enhancementshavethepotentialtoimprovethereceiverperformance(combined optical and thermal efficiencies greater than 73% were calculated), the level of detail of the model has to be increased to validate the findings and their durability is critical. As further steps in the development of the SUNDISC cycle, improvements in models of the receivers, the steam generator and Rankine cycle as well as of component costs are recommended. Utilization of an HPAR concept receiver in the cycle depends on manufacturability, durability and cost of enhancements to the basic design.

AFRIKAANS OPSOMMING: Gekonsentreerde sonkragstasies met ’n gekombineerde gas- en stoomturbien kringloop het die potensiaal om elektrisiteit op aanvraag te lewer indien dit termiese energie in ’n lae koste rotsbed gestoor kan word. Die gekombineerde siklus maak ’n hoë termiese benuttingsgraad moontlik. In bestaande kringlope word warm uitlaatgas van die gasturbien in die rotsbed gestoor, wat opwekking in die stoomturbien beperk tot ’n paar ure na sononder. Hierdie werk beskryf die ontwikkeling van die SUNDISC kringloop. Dié kringloop maak gebruik van ’n addisionele laedruk ontvanger wat son energie direk in die rotsbed stoor. Sodoende kan die elektriese uitset van die stoomturbien, asook die duur van opwekking na sononder verleng word. ’n Model van die hele kragstasie, wat die heliostate, ontvanger, rots bed, en gas- en stoomturbiene insluit, is ontwikkel vanuit emperiesie vergelykings vir hierdie komponente. Die gemiddelde jaarlikse bedryfskoste, verwerk na ’n huidige waarde, is ook bereken. Hierdie model is in ’n parametriese studie gebruik, wat aandui dat elektrisiteit teen feitlik konstante tempo opgewek kan word vir meer as 8200 ure per jaar, teen ’n koste van 0.143USD/kWe h. Die koste is sowat 24% laer as wat met vergelykbare kringlope behaal kan word. Daar word voorspel dat ’n steil leerkurwe die koste van opwekking teen 2035 tot sowat 0.075USD/kWe h kan verlaag. Dit is moontlik om die lae- en hoëdruk ontvangers onafhanklik te bedryf. Deur hulle saam te voeg, hou egter beduidende voordele in. In ’n buis-tipe ontvanger, word die buitekant van die buise deur die laedruk lugstroom verkoel, wat lei tot laer termiese spannings, stralingsverliese en geen konveksieverliese vir die hoëdruk ontvanger. Hierdie sogenaamde HPAR konsep is verder ondersoek, om te sien of dit aan die vereistes van die SUNDISC kringloop kan voldoen. Son en termiese straling, sowel as konveksie warmteoordrag oor en deur die buise is in ’n enkele model saamgevat. Die model is gebruik om die vermoë van die onvanger om son energie in warmte om te skakel, sowel as die uitlaat temperature van beide lugstrome vooruit te skat. ’n Basiese twee-druk ontvanger ontwerp wat van gladde buise gebruik maak, kon nie die gewensde temperature teen sinvolle vloeitempos haal nie. Die benuttingsgraad was ook aansienlik laer as wat in bestaande ontvangers aangeteken is. Temperatuur gradiënte was hoog en kan lei tot faling as gevolg van termiese spannings. Drie aanpassings van die ontwerp is ondersoek, naamlik (1)diegebruikvanvinbuiseomkonveksiewarmteoordragaandielaedrukkantte verhoog, (2) kwarts vensters en balke om windeffekte te verlaag, en vermenging aan te help, en (3) ’n volumetriese onvanger aan die uitlaat kant van die laedruk ontvanger. Resultate dui daarop dat ’n kombinasie van die aanpassings die benuttingsgraad van die ontvanger tot 73% kan opstoot, en dat die gewensde temperature behaal kan word. Daar bestaan egter twyfel oor die langtermyn betroubaarheid van veral die vinbuise by hoë temperatuur. Die model was slegs daarop gemik om die potensiële voordele van die konsep uit te lig, en sal in die toekoms verfyn moet word om betroubare antwoorde te gee. Die studie het bevestig dat die SUNDISC kringloop tasbare voordel inhou, en stel verdere komponent ontwikkeling op alle vlakke voor om die koste te verlaag. Die gebruik van ’n HPAR konsep ontvanger sal daarvan afhang of probleme rondom die vervaardigbaarheid, betroubaarheid en koste daaraan verbonde suksesvol oorkom kan word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101315
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