The Analysis of an organic rankine cycle for smaller concentrated solar powered systems.

Karsten, Louis (2018-12)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Small-scale energy consumers (500 kWe – 5 MWe), that are situated in off-grid areas are often left reliant on fossil fuels (like diesel generators) to meet their energy needs. The International Energy Association (IEA) however attempts to decarbonise the entire energy system and has set certain goals for the renewable energy sector. Most renewable energy sources offer only intermitted power solutions for small-scale off-grid applications and storing electrical energy for this scale becomes expensive. Concentrated solar power (CSP) can offer a non-intermitted solution by having the added benefit of thermal energy storage (TES). However, the critical barrier to the widespread usage of small-scale CSP is the lack of optimised and economically competitive technology. Steam Rankine cycles are typically integrated with CSP in large-scale applications but the thermodynamic properties of steam however prohibit water being used as suitable working fluid in lower temperature and lower power output applications. The aim of this thesis is therefore to determine whether the integration of an organic Rankine cycle (ORC), with a smaller CSP system can result in a feasible energy solution for off-grid applications. The first step taken was to determine the technical feasibility of solar integrated organic Rankine cycle (SORC) technology. It was proven feasible by the operation of existing small-scale SORC plants. ORC’s and CSP as separate types of technology are being used in various industries. The integration of CSP with an ORC is however considered an immature technology. A small-scale application in the higher DNI region of South Africa was identified to use as case study. Black Mountain mine near the town Aggeneys was selected by using a multi criteria decision analysis tool. Even though Black Mountain mine is grid-connected, it is still a viable case study to analyse whether an SORC can be a feasible solution for a mine. In proving so, the proposition then exists to use a SORC for future mining developments in off-grid areas. A SORC was theoretically analysed by looking at the solar field, TES and power block separately. The theory developed was then combined to create a model for a SORC. The simulation was conducted on MatLab and the design point was resultantly determined at a plant efficiency of 12.8 % and a turbine inlet pressure and evaporating temperature of 2858.8 kPa and 186 °C respectively. The simulation was then expanded to run over a period of one year and a maximum capacity factor of 83 % was recorded. The lowest achievable levelised cost of electricity (LCOE) was 16.7 $c/kWh at a solar multiple (SM) of 1.7 and TES size of 6 hours. The conclusion was reached that SORC’s are economically competitive with diesel generators but cannot solely meet the required energy demand and must therefore utilise auxiliary energy sources.

AFRIKAANSE OPSOMMING: Kleinskaalse energie verbruikers (500 kWe – 5 MWe), wat buite elektrisiteitsnetwerkgebiede geleë is, is dikwels afhanklik van fossiel brandstowwe (soos diesel kragopwekkers) om hul energie behoeftes te bevredig. Die Internasionale Energievereniging (IEA) poog egter om die hele energiesisteem te koolstof vry te maak en het sekere doelwitte vir die hernubare energie sektor gestel. Die meeste hernubare energiebronne bied slegs onderbroke kragoplossings vir kleinskaalse buite elektrisiteitsnetwerk toepassings. Die stoor van elektriese energie op hierdie skaal word buitendien ook duur. Gekonsentreerde sonkrag (CSP) kan 'n ononderbroke oplossing bied deur die bykomende voordeel van warmte-energie berging (TES) te hê. Die kritieke beperking vir die wydverspreide gebruik van kleinskaalse CSP is egter die gebrek aan optimale en ekonomies mededingende tegnologie. Stoom Rankine siklusse word tipies geïntegreer met CSP in grootskaalse toepassings, maar die termodinamiese eienskappe van stoom verbied egter dat water as geskikte werkvloeistof in laer temperature en laer kraguitsettoepassings gebruik word. Die doel van hierdie tesis is dus om vas te stel of die integrasie van 'n organiese Rankine-siklus (ORC) met 'n kleiner CSP-stelsel tot 'n haalbare energie-oplossing vir buite elektrisiteitsnetwerk toepassings kan lei. Die eerste stap is uitgevoer deur die tegniese uitvoerbaarheid van sonkrag-geïntegreerde organiese Rankine-siklus (SORC) tegnologie te bepaal. Die uitvoerbaarheid is bewys deur die bedryf van bestaande kleinskaalse SORC-aanlegte. ORC's en CSP as afsonderlike tipes tegnologie word in verskeie nywerhede gebruik. Die integrasie van CSP met 'n ORC word egter beskou as 'n onvolwasse tegnologie. 'n Kleinskaalse toepassing in die hoër DNI-streek van Suid-Afrika is geïdentifiseer as gevallestudie. Black Mountain myn naby die dorp Aggeneys is gekies met behulp van 'n multi-kriteria besluitanalise instrument. Selfs al is die Black Mountain myn aan die elektrisiteitsnetwerk verbind, is dit steeds 'n lewensvatbare gevallestudie om te ontleed of 'n SORC 'n haalbare oplossing vir 'n myn kan wees. Sodoende, bestaan die voorstel om 'n SORC te gebruik vir toekomstige ontginningsontwikkelings wat nie aan die elektrisiteitsnetwerk verbind is nie. 'n SORC is teoreties ontleed deur afsonderlik na die sonveld, TES en kragblok te kyk. Die teorie wat ontwikkel is, is dan gekombineer om 'n model vir 'n SORC te skep. Die simulasie is op MatLab uitgevoer en die ontwerppunt is gevolglik bepaal by 'n aanlegsbenuttingsgraad van 12.8 % en 'n turbine inlaatdruk en verdampingstemperatuur van onderskeidelik 2858,8 kPa en 186 °C. Die simulasie is dan uitgebrei om oor 'n tydperk van een jaar te hardloop en 'n maksimum kapasiteitsfaktor van 83 % is aangeteken. Die laagste haalbare gerealiseerde koste van elektrisiteit (LCOE) was 16.7 $c/kWh teen 'n sonveelvoud (SM) van 1.7 en TES grootte van 6 uur. Die gevolgtrekking is gemaak dat SORC's ekonomies mededingend is met dieselgenerators, maar nie alleen in die energie vraag kan voorsien nie en dus bykommende energie hulpbronne moet gebruik.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105075
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