EVALUATION OF A STRATIFIED MULTI-TANK THERMAL STORAGE FOR SOLAR HEATING APPLICATIONS by Cynthia Ann Cruickshank A thesis submitted to the Department of Mechanical and Materials Engineering In conformity with the requirements for the degree of Doctor of Philosophy Queen’s University Kingston, Ontario, Canada June, 2009 Copyright © Cynthia Ann Cruickshank, 2009 Abstract A novel multi-tank thermal energy storage (TES) was evaluated experimentally and numerically. The multi-tank storage is based on the interconnection of standard hot water storage tanks by a single charge flow loop. Each tank is charged through a thermosyphon loop and natural convection heat exchanger (NCHE). Both series- and parallel-connected configurations were investigated and results show that high degrees of stratification can occur. To predict the performance of the series- and parallel-connected multi-tank TES, a numerical model was developed and implemented in the TRNSYS simulation environment. Laboratory tests were also conducted to measure the unit’s performance under charge conditions representative of combinations of clear and overcast days. The effects of rising and falling charge loop temperatures and power levels on storage temperatures and heat transfer rates were studied and indicated that sequential stratification was achieved in the series-connected storage. Under certain conditions, reverse flow through the thermosyphon loops was identified, leading to destratification and carry-over of heat to the downstream storage tanks. Consequently, a new model was developed and showed to model reverse thermosyphon operation. A subsequent analysis showed that these effects could be minimized by careful system design. To quantify the relative benefits of the sequentially stratified TES, values of exergy stored versus time were determined and compared against fully stratified and fully mixed storages. Results show that the series configuration closely matches the exergy level attained by a perfectly stratified storage. i Finally, annual simulations conducted for a typical multi-family installation showed that the multi-tank storage performed at a level comparable to a single, fully stratified, storage. ii Acknowledgements I wish to express the utmost gratitude to my supervisor, Dr. Stephen J. Harrison of the Department of Mechanical and Materials Engineering at Queen’s University, for his support, guidance and friendship. His enthusiasm and expertise in this area have provided me with a wealth of knowledge and experiences that are invaluable. I will always remember his encouraging words, “If we knew the answer, we wouldn’t call it research.” It was truly a pleasure to work with him and learn from him. I would also like to express my appreciation to the Canadian Solar Buildings Research Network for funding this work. Without their support, this project would not have been possible. In addition, I want to acknowledge EnerWorks Inc. for providing the necessary equipment to conduct this study. To my colleagues at the Solar Calorimetry Lab, past and present, I would like to say thank you for your friendship, assistance and encouragement. I wish you good luck with your studies and your future projects. To my fiancé, thank you for your love, patience and understanding. Your encouraging words and ongoing support were always there at times when I needed them the most. Last but not least, I thank my parents, sister and brother for their unconditional love, kindness, support and encouragement. From the start until the accomplishment of this manuscript, they have been my source of strength and courage. To them, I dedicate this thesis. iii Statement of Originality I hereby certify that all of the work described within this thesis is the original work of the author. Any published (or unpublished) ideas and/or techniques from the work of others are fully acknowledged in accordance with the standard referencing practices. Cynthia Ann Cruickshank June, 2009 iv Table of Contents Abstract.............................................................................................................................................i Acknowledgements.........................................................................................................................iii Statement of Originality..................................................................................................................iv Table of Contents.............................................................................................................................v List of Figures.................................................................................................................................ix List of Tables ..............................................................................................................................xviii Nomenclature.................................................................................................................................xx Chapter 1 Introduction .....................................................................................................................1 1.1 Background............................................................................................................................1 1.2 Solar Thermal Energy Systems..............................................................................................3 1.2.1 Solar Domestic Hot Water Systems................................................................................4 1.3 Solar Thermal Energy Storage.............................................................................................10 1.3.1 Energy Storage Processes .............................................................................................12 1.3.2 Water-based Sensible Heat Thermal Storage................................................................15 1.3.3 Multi-tank Thermal Storage..........................................................................................18 1.4 Problem Definition...............................................................................................................20 1.4.1 Objectives .....................................................................................................................21 1.4.2 Contribution of Research ..............................................................................................22 1.5 Experimental and Computational Methodology ..................................................................22 1.5.1 Parameters Studied........................................................................................................23 1.6 Organization of Research and Thesis Document.................................................................24 Chapter 2 Literature Review..........................................................................................................27 2.1 Introduction..........................................................................................................................27 2.2 Single Tank Studies .............................................................................................................30 2.3 Natural Convection Heat Exchangers..................................................................................36 2.4 Multi-tank Studies................................................................................................................38 2.5 Performance Indices.............................................................................................................43 Chapter 3 Modeling of the Multi-tank Thermal Energy Storage...................................................44 3.1 Introduction..........................................................................................................................44 3.2 Numerical Modeling of the Multi-tank Thermal Energy Storage........................................46 3.2.1 Component Models.......................................................................................................47 v 3.2.2 Modeling of Stratified Thermal Energy Storage...........................................................50 3.2.3 Modeling of Natural Convection Heat Exchanger Performance ..................................57 3.3 Implementation of the Multi-tank Model in TRNSYS ........................................................61 3.3.1 Modeling of the Thermal Storage .................................................................................65 3.3.2 Modeling of Natural Convection Heat Exchanger (NCHE) .........................................68 3.4 Preliminary Comparison with Experimental Results...........................................................69 Chapter 4 Experimental Study.......................................................................................................72 4.1 Introduction..........................................................................................................................72 4.2 Description of the Storage System.......................................................................................73 4.3 Experimental Measurements................................................................................................77 4.3.1 Experimental Setup and Commissioning......................................................................80 4.4 Experimental Procedure.......................................................................................................83 4.4.1 Constant Temperature Charge Tests.............................................................................85