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ABSTRACT
Based on heat balance and momentum balance among different components, dynamic models to predict the thermal performance of a domestic thermosyphon solar water heating system (TSWHS) were established. To validate these models, an outdoor experimental platform was built and tests were conducted on three typical summer days in Zhuhai city, China. Results indicate that the temperature rise of water in a tank predicted by these models agrees very well with experimental data. Therefore, these models were verified. Then, these models were used to investigate the effect of design parameters on the thermal performance of a TSWHS. For the TSWHS, results indicate that its air gap thickness prefers to be set at 25 mm because the heat loss through its glass cover stops decreasing above this value; both the energy loss and the final temperature of water in the tank decrease with increasing water tank volume. Therefore, considering the balance between photo-thermal efficiency and water temperature requirement, the tank volume can be determined among 130–150 L in this work. Because the improvement of energy gain from an optimal installation slope of the TSWHS cannot compensate for the operation and maintenance costs, the installation slope can be fixed at a value which is equal to the local latitude.
Nomenclature
| A | = | area, m2 |
| C | = | specific heat capacity, J/(kg°C) |
| D | = | diameter, m |
| d | = | hydraulic diameter, m |
| f | = | friction factor |
| G | = | solar radiation, W/m2 |
| g | = | gravitational constant = 9.8 m/s2 |
| GGS | = | gas geyser system |
| H | = | height, m |
| h | = | heat transfer coefficient, W/(km2) |
| k | = | thermal conductivity, W/(km) |
| L | = | length, m |
| l | = | cell length, m |
| M | = | mass, kg |
| = | mass flow rate, kg/s | |
| N | = | number of riser pipes |
| n | = | cell number |
| Nu | = | Nusselt number |
| P | = | pressure, Pa |
| R | = | thermal resistance, (m2 K)/W |
| Re | = | Reynolds number |
| SWHS | = | solar water heating system |
| T | = | temperature, °C |
| t | = | time, s |
| TRNSYS | = | transient system simulation |
| TSWHS | = | thermosyphon solar water heating system |
| V | = | velocity, m/s |
| W | = | width, m |
| y | = | travelling distance, m |
| Greek symbols | ||
| α | = | absorptivity |
| β | = | solar collector slope angle,° |
| ϵ | = | emissivity |
| ν | = | kinematic viscosity, m2/s |
| ρ | = | density, kg/m3 |
| ζ | = | coefficient of local pressure loss |
| σ | = | Stefan-Boltzmann's constant = 5.67×10-8, W/(m2 K4) |
| τ | = | transmissivity |
| δ | = | thickness, mm |
| Subscripts | ||
| a | = | ambient condition |
| air | = | air gap |
| c | = | collector |
| f | = | water flow |
| g | = | glass cover |
| in | = | water inlet |
| out | = | water outlet |
| ph | = | head pipe |
| pi | = | water inlet pipe |
| po | = | water outlet pipe |
| pr | = | riser pipe |
| r | = | radiation |
| t | = | water tank |
| wind | = | wind induced |
| a-g | = | heat transfer between ambient condition and glass cover |
| g-c | = | heat transfer between glass cover and absorber |
| c-f | = | heat transfer between absorber and water flow |
| a-t | = | heat transfer between ambient condition and tank |
Acknowledgments
This work is financially supported by the National Nature Science Foundation of China (No. U1401249), and the Natural Science Foundation of Guangdong Province, China (No. 2014A030312017). The authors also thank the Pearl River S&T Nova Program of Guangzhou and the Fundamental Research Funds for the Central Universities.
Additional information
Notes on contributors
Longsheng Lu
Longsheng Lu received the B.S. and Ph.D. degrees in Manufacturing Engineering from South China University of Technology, Guangzhou, China, in 2004 and 2009, respectively. Then he joined the same University to be a faculty in the Department of Manufacturing Engineering. Currently, he is an Associate Professor. He has authored or coauthored more than 50 journal papers. His research interests include advanced manufacture and design technology and its applications, e.g., heat sinks, PVT collector, carbon fiber reinforced composites.
Xiaowu Wang
Xiaowu Wang is a Professor at South China University of Technology. She received her Ph.D. from South China University of Technology in 2006. Her research interests include microscale heat transfer enhancement, energetic materials, thermodynamics model of thermal phenomenon, and renewable energy technology. She has co-authored 70 papers in journals and conference proceedings.
Shuai Wang
Shuai Wang received the M.S. degree in Mechanical design from South China University of Technology, Guangzhou, China in 2012. He is currently working as a Mechanical Engineer at Yutong Bus Company, Zhenzhou, China. His research interest focuses on the numerical analysis of heat and mass transfer in the field of solar energy.
Xiaokang Liu
Xiaokang Liu received the B.S. degree in Mechanical Engineering in 1986 from Jiangsu University, Zhenjiang, China. She is currently a Professor in the Department of Mechanical Design of South China University of Technology, Guangzhou, China. Her current research interests include modern design theory and application of new energy devices, precious manufacture technology, and the fabrication of carbon fiber reinforced composites.