Comparison of natural convection heat exchangers for solar water heating systems
Thermosyphon heat exchangers are used in indirect solar water heating systems to avoid using a pump to circulate water from the storage tank to the heat exchanger. In this study, the authors consider the effect of heat exchanger design on system performance. They also compare performance of a system with thermosyphon flow to the same system with a 40W pump in the water loop. In the first part of the study, the authors consider the impact of heat exchanger design on the thermal performance of both one- and two-collector solar water heaters. The comparison is based on Solar Rating and Certification Corporation (SRCC) OG300 simulations. The thermosyphon heat exchangers considered are (1) a one-pass, double wall, 0.22 m{sup 2}, four tube-in-shell heat exchanger manufactured by AAA Service and Supply, Inc., (the Quad-Rod); (2) a two-pass, double wall, 0.2 m{sup 2}, tube-in-shell made by Heliodyne, Inc., but not intended for commercial development; (3) a one-pass, single wall, 0.28 m{sup 2}, 31 tube-in-shell heat exchanger from Young Radiator Company, and (4) a one-pass single-wall, 0.61 m{sup 2}, four coil-in-shell heat exchanger made by ThermoDynamics Ltd. The authors compare performance of the systems with thermosyphon heat exchangers to a system with a 40 W pump used with the Quad-Rod heat exchanger. In the second part of the study, the effects of reducing frictional losses through the heat exchanger and/or the pipes connecting the heat exchanger to the storage tank, and increasing heat transfer area are evaluated in terms of OG300 ratings.
Retrofitting Conventional Electric Domestic Hot Water Heaters to Solar Water Heating Systems in Single-Family Houses—Model Validation and Optimization
System cost reductions and development of standardised plug-and-function systems are some of the most important goals for solar heating technology development. Retrofitting hot water boilers in single-family houses when installing solar collectors has the potential to significantly reduce both material and installation costs. In this study, the TRNSYS simulation models of the retrofitting solar thermal system were validated against measurements. Results show that the validated models are in good agreement with measurements. On an annual basis a deviation of 2.5% out of 1099 kWh was obtained between the auxiliary energy from results and from the simulation model for a complete system. Using the validated model a system optimization was carried out with respect to control strategies for auxiliary heating, heat losses and volume of auxiliary storage. A sensitivity analysis was carried out regarding different volumes of retrofitted hot water boiler, DHW profiles and climates. It was estimated that, with adequate improvements, extended annual solar fractions of 60%, 78% and 81% can be achieved for Lund (Sweden), Lisbon (Portugal) and Lusaka (Zambia), respectively. The correspondent collector area was 6, 4 and 3 m 2 , respectively. The studied retrofitted system achieves a comparable performance with conventional solar thermal systems with the potential to reduce the investment cost.
Predictors of health-promoting behaviors among freshman dental students at Istanbul University.
The aim of this cross-sectional study was to determine the predictors of health-promoting behaviors among freshman dental students at Istanbul University. Data were collected from a convenience sample of 111 students using questionnaires including a sociodemographic section, the Health-Promoting Lifestyle Profile II, the Health Value Scale, the Generalized Self-Efficacy Scale, the Perceived Stress Scale, and the Multidimensional Scale of Perceived Social Support. Data were analyzed using descriptive statistics, t-test, Pearson's correlation, and stepwise multiple linear regression. The overall health-promoting lifestyle behaviors among these students were at a moderate level, indicating that they engaged in health-promoting behaviors sometimes to often. They scored highest on the spiritual growth but lower on the health responsibility subscales. Females reported having engaged more in health responsibility behaviors and lower in physical activity than did males. Students from high-income families reported more physical activity. Significant correlations were found between overall health-promoting lifestyle behaviors, self-efficacy, health value, stress, and social support. Among all the related factors, self-efficacy was the most strongly predictive factor: it explained 61 percent of the variance in overall health-promoting behaviors. Thus, self-efficacy as a cognitive-perceptual factor should be considered by dental educators when planning educational interventions to promote students' healthy lifestyle.