Small solar organic rankine cycle for electricity generation

Lombard, Hans Jurgens (2015-12)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: The global the demand for energy has risen dramatically in recent years and the cost of electricity has risen considerably. Local electricity sales prices followed the global trend, increasing by an average of 25% per annum from 2008 to 2012. This upward trend has opened the market for new technologies, such as renewable energy alternatives, better use of waste streams and low temperature power generation. The Organic Rankine Cycle is an example of a low temperature power generation cycle that can utilize a renewably energy source. The Organic Rankine Cycle is in essence a Rankine Cycle that employs a different working fluid than water. The primary objective of this thesis is to build a functioning ORC. The cycle is to utilize a low temperature (< 120°C) and pressure heat source to generate up to 1 kW of electricity. Each component has to be sized and bought or designed and manufactured. The performance of each component is to be predicted using theoretical mathematical models. This model will be validated through comparison with the experimental results obtained. The condenser has to be driven by natural convection in order to minimize the energy consumption of the system. Finally the feasibility of such a system will be investigated. The experimental Organic Rankine Cycle was run for several different scenarios and measurements were taken to characterize and evaluate the performance of this system. The performance of the system components was satisfactory in comparison with the design specifications with the exception of the boiler and scroll expander electric motor. The boiler capacity was experimentally determined to be 8 kW, much lower than the design capacity of 20 kW. The lowered boiler capacity inhibited the system performance by as much as 25%. The conversion efficiency of the scroll expander electric generator was low at 7%, but the scroll expander demonstrated an isentropic efficiency up to 47%. A theoretical model was developed to predict the thermodynamic performance of the system components. The theoretical model predicted the performance of the system components within 10%, except for the pump performance which deviated due to the bypass valve adjustment. A study was conducted to determine the feasibility of an Organic Rankine Cycle system. The study takes into account the initial capital cost, discount rate, operations and maintenance costs and system life span. The study found that the ORC is currently a viable option for remote, off-grid electricity generation with an levelized cost of electricity of R 2,55 compared to the Eskom tariff of R 3,90. The additional cost of line rental increases the Eskom tariff considerably. For urban generation the levelized cost of electricity for renewable sources is still higher (R 2,55) than the Eskom tariff (R 1,00), but the increasing Eskom tariff and decreasing cost of renewable energy solutions should see renewable energy technologies become a financially feasible solution by the year 2020.

AFRIKAANSE OPSOMMING: 'n Drastiese toename in die wêreldwye aanvraag na energie die afgelope paar het die koste van elektrisiteit aansienlik laat toeneem. In Suid Afrika het die prys van elektrisiteit met 'n gemiddeld van 25% jaarliks, gedurende die tydperk van 2008 tot 2012, gestyg. Hierdie opwaartse neiging in die mark het dit makliker gemaak vir hernubare energie bronne, die beter benutting van energie strome en lae temperatuur kragopwekking om die markte betree. 'n Voorbeeld van 'n lae temperatuur kragopwekkingsiklus wat hernubare bronne kan benut, is die organiese Rankine siklus. 'n Rankine siklus wat 'n organiese vloeier gebruik in plaas van water, staan bekend as 'n organiese Rankine siklus. Die primêre doel van hierdie tesis is om 'n werkende organiese Rankine siklus te bou. Die siklus moet gebruik maak van 'n lae druk en lae temperatuur hitte bron van minder as 120C. Die siklus moet in staat wees om 1 kW op te wek. Komponente moet ontwerp en vervaardig word waar nodig of gekoopte komponente se grootte moet bepaal word. 'n Teoretiese model wat in staat is om die werksverrigting van elke komponent voorspel moet opgestel word. Die teoretiese model moet geverifieer word deur gebruik te maak word van die eksperimentele resultate. Die kondensor moet gedryf word deur natuurlike konveksie om die energie verbruik van die stelsel te verminder. Laastens word ondersoek ingestel om die finansiële lewensvatbaarheid van so 'n stelsel te bepaal. Die eksperimentele organiese Rankine siklus was getoets onder verskillende omstandighede en metings is geneem om die eienskappe van die stelsel, asook die werksverrigting van die onderskeie komponente, te bepaal. Die werksverrigting van die onderskeie komponente was bevredigend in vergelyking met die ontwerp werksverrigting, met die uitsondering van die stoomketel en die scroll turbine generator. Die stoomketel kapasiteit was eksperimenteel bepaal as 8 kW, heelwat minder as die ontwerp kapasiteit van 20 kW. Die omskakelings effektiwiteit van die generator was slegs 7% terwyl die isentropiese effektiwiteit van die scroll turbine bereken was as 47%. Dit word aanbeveel dat die generator vervang word met 'n GS generator of 'n WS generator met 'n aparte terminale vir die stator windings om die omskakelings effektiwiteit te verbeter. 'n Teoretiese model was ontwikkel om die termodinamiese werksverrigting van die stelsel komponente te voorspel. Die teoretiese model voorspel die werksverrigting van die komponente binne 10%, met die uitsondering van die stoomketel toevoerpomp, omdat die hersirkulasie klep verstel word. 'n Studie om die lewensvatbaarheid van 'n organiese Rankine siklus te bepaal, was ook gedoen. Die studie neem in ag die aanvanklike kapitaal inset koste, afslag koers, bedryfskoste en instandhoudings koste asook die lewensduur van die stelsel. Die studie het bevind dat die organiese Rankine siklus tans lewensvatbaar is in landelike gebiede met 'n koste van R 2,55/kWh teenoor die R3,90/kWh tarief van Eskom. Die addisionele koste van die lyn huur dra 'n aansienlike deel by tot die Eskom tarief. Die stedelike Eskom tarief van R1,00/kWh is steeds minder as die koste van 'n hernubare stelsel (R2,55/kWh), maar dit word verwag dat die stelsels teen 2020 lewensvatbaar sal wees.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97980
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