Preliminary Investigation into Solar Thermal Combi-system Performance

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Date
2014-09-29
Authors
Lee, Elizabeth
Keyword
combi system , solar dhw , combi-system , solar domestic hot water , solar space heating , solar thermal
Abstract
Solar thermal combi-systems use solar energy to provide thermal energy for space heating and domestic hot water. These systems come in many different designs and configurations, and their performance is dependent on many different factors such as location, solar collector type and orientation, and thermal storage size. In this study, two different combi-system configurations (a basic combi-system, and a single-tank combi-system) as well a stand-alone solar space heating system and a stand-alone solar domestic hot water system were investigated using TRNSYS models and simulations, and the thermal performances were compared using different combinations of solar collector and heat exchanger types, different locations, and a sensitivity analysis was performed using the basic solar combi-system. Using Toronto as the reference location, the performance of the solar stand-alone and combi-systems were compared using combinations of solar collector types (glazed vs. vacuum tube) and heat exchanger types (fixed effectiveness vs. natural convection). Each system combination was optimized for collector flow rate and tilt angle, and all systems were given the same solar collector area, thermal storage volume, and load profiles. This analysis revealed that the glazed type solar collector performed best when the heating load was continuous throughout the year, whereas the vacuum tube solar collector performed best with systems that had heating loads concentrated in the winter. Further comparisons of these systems were done using reference locations: Boulder, Colorado; Seattle, Washington; Winnipeg, Manitoba; and Toronto, Ontario. Results showed that the single-tank solar combi-system with internal stratifiers in the thermal storage tank outperformed the other solar thermal system configurations, although if examined solely based on space heating performance, the basic combi-system with preferential charging of the space heating thermal store had the best performance. Lastly, a sensitivity analysis of the basic combi-system was conducted, again using Toronto as the reference location. Several parameters were considered, including: the solar collector tilt angle and azimuth, the solar collector size, flow rate through the collector loop, thermal storage tank size, heat exchanger effectiveness, number of tank nodes, hydronic-floor supply temperature, and hydronic-floor R-value. Of these, the solar collector parameters and the hydronic-floor supply temperature had the greatest impact on the overall system performance.
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