Experimental And Numerical Study On A Building-Integrated Dual-Function Solar Collector

Energy consumed in buildings accounts for approximately 30-40% of the global energy consumption. Some major consumptions(around 40%) are in lighting, ventilating, space heating/cooling, and water heating. To fight against the worldwide deteriorating energy wastage and pollution problems, much effort in the building sector has been on promoting solar energy utilization. To this, the applications of solar thermal technology-with passive space heating and water heating as the two major approaches-have been highly successful.For passive space heating in buildings, many distinct design features have been put forward, such as the Trombe wall system, composite Trombe-Michel wall, and PV-Trombe wall. Also introduced in the Barra-Costantini system is the thin metal-plate absorber. While these named systems have been well developed, hurdles remain such as the low annual utilization rate in places with hot and lengthy summer. Summer overheating is also found to be problematic. Although in both the Trombe wall system and Barra-Costantini system the glazing is provided with two top-and-bottom vents for summer heat dissipation, this provision is often not adequate to remove the absorbed solar heat at the building fa(?)ade. Besides, the design also introduces other operational inconvenience, such as the difficulty in getting access to the two vents at the outside.In order to overcome the above barriers, a hybrid solar collector system-here named as the "building-integrated dual-function solar collector"-is introduced. The system involves a novel solar collector, which is modified from the conventional flat-plate collector by broadening the air gap between the absorber plate and the backboard, on which two vents are added. This system not only performs passive space heating in cold winter, but also as a fa(?)ade water heating collector when space heating is no longer required. Such integrated design makes it more cost-effective than those conventional systems solely for solar water heating or passive space heating. On the other hand, the elimination of the vent openings at the front surface makes the application more convenient and practical.A testing system of building-integrated dual-function solar collector was designed and tested in a hot-box. For the two modes of the building-integrated dual-function solar collector:thermosiphon water heating mode and space heating mode, we separately experimentally examined the thermal performance. The experimental results show that both of the novel system's operating modes worked well during the correlative periods. The novel system also represented some special thermal characteristic when working in thermosiphon water heating mode or space heating mode, i.e. when in water heating mode, the temperature of the heated water was almost same in the summer/spring and the solar heat gain of the water was smaller in the summer than in the spring, and when in space heating mode, there was a linear relationship between the temperature stratification and the intensity of the incident solar radiation on the solar collector.For the two modes of the building-integrated dual-function solar collector: thermosiphon water heating mode and space heating mode, two dynamic numerical models based on finite-difference method were respectively developed. At each operating mode, full experimental data was provided for model validation. At the thermosiphon water heating mode, it is proved by comparison of the experimental and simulated results that during the experimental periods, the prediction of the daily thermal efficiencies only had a maximal relative absolute deviation of 2.6%, and the RMSD of the simulated result of the indoor temperature was around 0.4℃. For the system in the space heating mode, the numerical model can also give accurate predictions but with higher deviation compared with the model for the system in the thermosiphon water heating mode. Therefore, the models for the building-integrated dual-function solar collector were entirely validated.From the comparison between the instantaneous indoor temperature of the experimental system and that of the system using the black paint, it can be seen that the selective absorbing coating has much advantage in improving the thermal performance of the novel system working in the space heating mode. The numerical results indicate the effect of the optical property of all kinds of nickel-alumina coatings on the thermal performance of the novel system in space heating:under the same absorptivity of the nickel-alumina coatings, there existed a linear relationship between the mean indoor temperature and the emissivity of the nickel-alumina coatings; and under the same emissivity of the nickel-alumina coatings, there also existed a linear relationship between the mean indoor temperature and the absorptivity of the nickel-alumina coatings.Our proposed collector system works with natural-circulation for water heating. The thermal behavior is different from that of an active system. A numerical study is made to show its thermal effect on the building thermal environment respectively in summer and autumn, by comparing with the cooling loads of the rooms with/without the novel collector. The results show that the daily cooling load of the room with the novel collector has a 2.05% reduction compared with that of the room without the collector in summer. It indicates that when in summer, the dual-function solar collector integrated with building can serve an available hot water in utilizing solar energy, and it does not have the overheating problem which most of the traditional passive space heating systems have in summer, and even the building's thermal environment can be improved slightly. While in autumn, the novel collector in water heating mode with natural circulation can result in an increase of space cooling load but a reduction of space heating load; this is sustainable since the cooling load can be readily handled by free air cooling.For the novel system in water heating mode with natural circulation, the numerical simulation is carried out to show influence of the thermal insulation of the backboard and the operating water volume on the thermal performance. The results show that the appropriate thickness of the thermal insulation of backboard is around 0.05m and that the appropriate operating water volume is around 40-50kg/m2 in summer and 50-60kg/m2 in autumn.