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Ammonia Diffusion Absorption Heating and Cooling Cycle The evaluation of a solar-driven aqua- ammonia diffusion absorption heating and cooling cycle MC Potgieter 20268890 Dissertation submitted in partial fulfillment of the requirements for the degree Magister in Mechanical Engineering at the Potchefstroom Campus of the North-West University Supervisor: Prof CP Storm September 2013 The evaluation of a solar-driven aqua-ammonia diffusion absorption heating and cooling cycle Marthinus Christiaan Potgieter Bachelor of Engineering (Mechanical Engineering) Dissertation submitted in partial fulfilment of the requirements for the degree Master of Engineering in Mechanical Engineering North-West University Potchefstroom Campus Student number: 20268890 Supervisor: Prof. C.P. Storm Potchefstroom Augustus 2013 (Rev3) Acknowledgements i ACKNOWLEDGEMENTS One cannot journey through life and not marvel at the beauty and complexity of all things. Be it from the most miniscule sub-atomic particle theorised to the greatest supernova and beyond, man has ever strived to gain knowledge of all that he beholds. Through his exploitations and attempted mastery over nature, man must, however, always realise with the appropriate humility that he still stands to answer to One. It is to the Great Architect of the Universe I give thanks for the means and opportunity to reconnoitre in his cosmic garden – spread at our feet. Tread carefully, I shall... To my parents, my continued appreciation for all their love, guidance and support – without ever expecting anything in return and knowing of what I am capable even before my realising it. To Alma, for all her unending love, understanding and encouragement for which I am truly blessed. You have my eternal love and gratitude – for you to believe in my success is my success. To my colleague Stefan, for sharing his insights and helping guide this study through the research being done in parallel on specific system components. A word of thanks is also due to Prof. Pieter Rousseau for his insights during both of his post- graduate Thermal-fluid Systems Modelling courses, and for lending a fresh perspective on the simulation section of this study. Also to the personnel of the NWU Potchefstroom Campus natural sciences branch library for their diligent assistance and uncompromising service which continually supports research efforts even in a digital era. Finally I would like to extend my sincere gratitude to my supervisor Prof. Chris Storm for his special counsel, attention to detail and most of all vision, without which this study would be non- existant. His passion and enduring strive towards a “more efficient” approach is always inspiring, be it for a thermodynamic cycle, working environment, standing policies or the making of yet another cup of coffee (many of which were needed in order to successfully complete this research). SOLI DEO GLORIA The evaluation of a solar-driven aqua-ammonia diffusion absorption heating and cooling cycle Abstract ii ABSTRACT Title: The evaluation of a solar-driven aqua-ammonia diffusion absorption heating and cooling cycle Author: Mr M.C. Potgieter Supervisor: Prof. C.P. Storm School: School of Mechanical and Nuclear Engineering Degree: Master of Engineering An ever-increasing demand for energy with virtually no decline in our dependence on fossil fuels puts the world’s non-renewable resources under great pressure. This problem necessitates careful consideration of alternative methods of energy conversion and usage by investigation of novel and more sustainable methods or new applications for old ideas. Using solar thermal energy to drive an absorption cycle is one such new use for technology which has been around for almost a century. Low values for coefficient of performance (COP) and high capital cost are large drawbacks of such systems. However due to negligible operating costs the investment can be regained back quickly. There are several possible mixtures to use as working fluid of which aqua-ammonia is selected for diffusion absorption systems as well as solar-driven variants. This is due to the low temperatures that can be achieved in the evaporator for refrigeration purposes. Another factor for its use in a simulation is the availability of accurate thermo-physical property correlations. The simulation of such a cycle is required in order to prove its feasibility in a domestic setting within South Africa. After thorough investigation of cycle configurations and alternative working fluids, the study focusses on simulating a zero-order model of a solar-powered diffusion absorption cycle for refrigeration purposes. Climate data of Potchefstroom over a 22-month period is used in order to establish boundary values for temperatures and along with certain system requirements, parameters are set for the overall cycle. Varying performance and efficiency is also investigated. By following a systems-CFD approach the cycle is represented as a thermal-fluid system network of components connected by nodes to closely resemble reality. Where previous studies have neglected zero-order requirements or adequately accounted for irreversibilities, here the conservation equations have been applied with the most terms used and the least amount of assumptions made in order to achieve the greatest accuracy. The evaluation of a solar-driven aqua-ammonia diffusion absorption heating and cooling cycle Abstract iii Several steps are followed in order to evaluate the cycle as the title suggests. The diffusion absorption refrigerator (DAR) cycle performance is evaluated when using helium or hydrogen as auxiliary gas. A slight increase in COP is found when using helium, but it is not sufficient to justify the cost. A secondary simulation of an alternate dual-pressure cycle using a pump is done as feasibility comparison with the same parameters as the diffusion cycle. It was found that the second cycle is not acceptable due to high evaporator temperatures needed to ensure liquid enters the pump instead of partially evaporated solution. This would greatly increase the work input required for what essentially becomes a compressor. Optimisation of the DAR is evaluated by simulating the use of a rectification column and the effects of different design points on overall performance. Meteorological data for Potchefstroom, South Africa is used to perform a yearly analysis on the simulated cycle and to specify a suitable design point. The use of a radiative cooling system as heat sink for the system is then investigated and incorporated into the system model. Finally, the performance characteristics of the simulated DAR cycle are discussed, verified and compared with available data from similar research. It is shown that a 40% solution aqua- ammonia-hydrogen cycle driven by 526 kW of solar thermal energy at 130°C and a system pressure of 1.5 MPa can easily achieve a COP over 0.4 with an air-cooled absorber at 40°C and a water- cooled condenser at 35°C. A 231 kW refrigeration capacity at an average evaporator temperature of –20°C is achieved, satisfying the requirements for a domestic refrigeration system. Keywords: Solar, aqua-ammonia, diffusion, absorption cycle, DAR, radiative cooling system, solar collectors, generator, absorber, condenser, evaporator, renewable energy. The evaluation of a solar-driven aqua-ammonia diffusion absorption heating and cooling cycle Uittreksel iv UITTREKSEL Titel: Die evaluasie van ‘n son-gedrewe akwa-ammonia diffusie absorpsie verhittings- en verkoelingsiklus Outeurr: Mnr M.C. Potgieter Studieleier: Prof. C.P. Storm Skool: Skool vir Meganiese en Kern Ingenieurswese Graad: Magister in Ingenieurswese ‘n Toenemende aanvraag na energiebronne terwyl die mensdom steeds nie minder staat maak op fossielbrandstowwe nie, plaas die wereld se nie-hernubare hulpbronne onder toenemende druk. Hierdie probleem verg noukerige oorweging van alternatiewe metodes van energie omsetting en gebruik deur die ondersoek van innoverende en meer volhoubare metodes of nuwe toepassings van reeds gevestigde idees. Die gebruik van termiese son-energie vir die aandrywing van ‘n absorpsie siklus is een so ‘n toepassing van tegnologie wat reeds ‘n eeu gevestig is. Lae vertoningskoëffisiënte (VK) en hoë kapitaal uitgawes is sterk teenvoeters van bogenoemde stelsels, maar met lae bedryfskostes in ag genome, kan die belegging vinnig teruggewin word. Daar is verskeie moontlikhede vir werksvloeiers van diffusie absorpsie siklusse sowel as hul son- aangedrewe variante, waarvan aqua-ammoniak die gekose een sal wees. Dit is weens die lae temperature wat in die verdamper bereik kan word, gepas vir verkoelings doeleindes. Nog ‘n bydraende faktor vir aqua-ammoniak se gebruik in ‘n simulasie is die beskikbaarheid van akkurate termo-fisiese eienskap korrelasies. Die simulasie van hierdie siklusse is baie gesogd, aangesien dit die lewensvatbaarheid in ‘n huishoudelike opset in Suid-Afrika kan bewys. Na ‘n deeglike ondersoek van verskeie siklus- opstellings en alternatiewe werksvloeiers, het die studie gefokus op die simulasie van ‘n nulde-orde model son-aangedrewe diffusie absorpsie siklus vir verkoelings doeleindes. Metereologiese data oor ‘n duur van 22 maande van Potchefstroom is gebruik om die grenswaardes vir die temperature te bepaal. Tesame met voorafbepaalde stelselvereisdes kon parameters vasgestel word vir die algehele siklus. Deur gebruik te maak van ‘n stelsel-BVM benadering word die siklus as ‘n termo-vloeier stelsel netwerk van komponente geheg met nodepunte voorgestel om sodoende die werklikheid so na as The evaluation of a solar-driven aqua-ammonia diffusion absorption
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