A Roof Integrated Solar Heating System Without Storage

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1 M. and W. Saman Sustainable Energy Centre University of South Australia Mawson Lakes Boulevard, Mawson Lakes, SA 5095, Australia Abstract A prototype of a roof integrated solar heating system has been installed in an occupied house. The system consists of a collector and an auxiliary gas heater, and was used throughout the winter of 2004 to investigate the impact of not having a dedicated storage facility. It was determined that the collector was able to make a meaningful contribution to the heating demand with a solar contribution of 26%. Furthermore, it was shown that heating with a collector only was an effective method of using solar energy to provide heating. Inconclusive evidence was found that the system was utilising the thermal mass of the building for storing energy. 1. INTRODUCTION In Australia, 36% of domestic energy consumption is estimated to be due to space heating (AGO, 1999). In winter the solar energy that an Australian home is exposed to is approximately 800 MJ/m 2. With winter heating loads of the order of 100 MJ/m 2, the available solar energy is 8 times the energy needed for heating. Despite this advantage, solar heating systems are virtually non existent in the residential market. This is mainly due to the cost of the system particularly the collector (Duffie and Beckman, 1991). Research at the Sustainable Energy Centre has been conducted into developing a roof integrated solar air heating system which is of low cost and acceptable to the domestic market ( et al, 2003). This system consists of a roof integrated solar air collector and a thermal storage unit with phase change materials. The collector is integrated into standard corrugated steel roofs, as described by ( et al, 2004). As shown in Figure 1, air is used to transfer heat from the corrugated surface into the system. The collector is glazed by a plastic glazing. The thermal storage unit, along with other components is located in the roof space. Solar Radiation Glazing Corrugated Roofing Iron Air Flow Moisture Barrier Figure 1. Roof integrated collector. In 2003 a prototype of the roof integrated solar heating system was installed in a newly built SA Housing Trust house in Adelaide (Figure 2). This house was also occupied in this year. In 2004 the system was modified such that the thermal storage unit was disengaged, leaving only the collector to provide solar heating. Without storage, the ability of a solar heating system to contribute to the heating demand is limited. This limitation is based on heating demand predominantly occurring during low and no sunshine periods. However, for a climate such as Adelaide, with high irradiance levels in winter, heating demand can occur during times of high irradiance levels. Furthermore, it may be

2 possible to make use of the building structure itself to provide some storage capability. Consequently it was decided to test the solar heating system in the home with solar heating being directly provided by the collector. Roof integrated collector Figure 2. Demonstration of the roof integrated solar heating system 2. SYSTEM DESCRIPTION The system was fully operational at the beginning of July Prior to this date the backup heater was the only source of heating. The system was fully monitored. Parameters measured include solar irradiance, gas flow and various temperatures. Figure 3 shows the major components of the solar heating system installed in the home. The roof integrated collector was connected to a fan and ducted with conventional air conditioning equipment. The system is ducted to the entire house. The system is controlled using a PC based controller specifically developed for this application. Backup heating is provided by a small wall gas heater installed in the living room with an efficiency of 81%. This heater is also controlled by the solar heating control system. The gas heater is only used to heat the living room and is the only heater in the house. The solar heating system also allows for 10% ventilation during operation. Generally, conventional heating systems provide for no ventilation. The ventilation provided by this system meets the recommended ventilation rates for residential buildings. The gas heater provides no ventilation. ANZSES

3 Fresh Air Roof Collector Air Flow Fan Insulation House Interior Outlet Figure 3. Roof integrated solar heating system The system operates in two modes, heating using the solar heating system and heating using the gas heater. The householder has the choice between each type of heating. When solar heating is chosen, once a suitable irradiance level is achieved the fan is engaged and air is pumped from within the home through the collector and back into the home. 10% of the flow entering into the collector is derived from outside. If gas heating is chosen, the fan is disengaged and the gas heater is turned on. However, if the gas heater is on and solar heat is available the system will automatically revert to this mode, maximizing the solar contribution. To further increase the solar contribution, the control system was designed to provide continuous whole of house heating when solar heat was available. The system would therefore tend to overheat the home until the householder became uncomfortable and would switch the system off. It was planned that some of this heat would be stored in the building structure delaying the need for conventional heating. 3. PERFORMANCE RESULTS 3.1. Energy Analysis The system was continuously monitored throughout From the monitored data the heating energy provided by the solar heater and the gas heater could be determined. The energy provided by the solar heater was determined based on the temperature difference between the air entering the house and the room temperature. Table 1 shows the amount of heating that the solar heater and the gas heater achieved over the period from July to September. Although the solar heater provided ventilation, the values presented do not include the energy needed for ventilation, enabling a direct comparison of the heating achieved by each system. The energy provided by the solar heater was determined for times only when the room temperature was below 22 C. Above this temperature it was assumed that there existing no heating demand and so any additional solar heat added to the building was ignored. Based on the heating energy only, the solar contribution to the total heating load was found to be 38%. Since the fan energy is derived from conventional means the solar contribution must take the associated primary energy into account. By reducing the solar heat by the primary energy of the fan, the solar contribution will therefore reflect the contribution made by the system to reducing greenhouse gas emissions. Consequently a contribution of 0 represents no savings in emissions, whereas a contribution of 100% means that all the energy used to heat the house is derived from solar. When considering the primary energy of the fan, the solar contribution reduces to 26%. ANZSES

4 Therefore, heating with only the collector can represent a substantial contribution to the overall heating provided to a building. This figure is conservative as it does not consider the primary energy of gas heater fan. Table 1. Heating energy in house (monitoring period July to September 2004). Heating provided by gas heater, MJ 5433 Heating provided by solar heater, MJ 3297 Solar contribution 38% Solar contribution considering fan energy 26% Table 2 shows a breakdown of energy provided by each heater over each month. The data highlights how the solar heater generally provided the same amount of energy for each month. As expected gas heating in the warmer month of September was reasonably lower than that of the winter months of July and August. This combination results in a higher solar contribution in September than in the winter months of July and August. During spring and autumn where heating is still required, it is to be expected that there are fewer periods when heating is in demand and solar heat is available. Therefore, the solar contribution should be less. However, the data in Table 2 reveal that this is not the case and that at least the same contribution levels can be expected. Therefore the solar contribution highlighted in Table 1 is representative of the entire heating season. Table 2. Monthly breakdown of heating energy in house. July August September Heating provided by solar heater, MJ Heating provided by gas heater, MJ Solar contribution 37% 34% 43% The extent to which heating can be provided by the solar heater during the day relates to how much solar energy is available to the heater during times when heating is required. The total amount of solar energy that was incident on the collector during times of operation was determined to be 130% of the total heating load over the July to September period. Therefore a significant amount of solar energy is available during times of heating demand. This available solar energy was found to represent 32% of the total irradiance on the collector over the same period. Therefore heating directly with the collector utilises a significant proportion of the total solar energy. Consequently, heating with only a solar collector is an effective method at meeting a major part of the heating demand Temperature Analysis It has been demonstrated that the solar heater did provide significant heating into the building. However, the data in Tables 1 and 2 only describe the energy delivered into the building, and does not indicate whether this heat has effectively provided or increased thermal comfort. Table 3 shows the average monthly ambient temperature and temperature within the house. The temperature within the house is represented by the difference between the actual room temperature and the ideal room temperature of 21 C. The average is taken over the entire month including night time periods. The months shown represent times when heating was conducted. The months prior to the system being commissioned represent times when only the gas heater was used. Comparing the months before and after the solar heater was commissioned, it is clear that the solar heater has not only provided the same temperature conditions as the gas heater but has increased the thermal comfort within the house. In the month of July, the house was at the same average temperature as in June, but July was a colder month. In August, the average room temperature was significantly closer to the ideal room temperature, but August had the same average ambient temperature as June. In September, the house was the warmest, again significantly warmer than either May or June, but this result will be affected by the higher ambient temperature. Overall, the solar heater achieved a high level of thermal comfort. ANZSES

5 Table 2. Average temperatures in house. Average of room Date temperature less 21 C, C Average ambient temperature, C May Jun System commissioned Jul Aug Sep The ability of the solar heater to contribute to the heating load relates not only to meeting demand which occurs during sunny periods, but also in how energy can be stored in the building structure. If solar energy is stored in the building, it will offset the need for conventional heating. Figure 4 shows temperature profiles of two consecutive days in the coldest month of July. Based on solar irradiance levels, the two days presented are a sunny day and a semi cloudy day respectively. On the first day, 6 hours of solar heating, until 4:00 pm, was able to a keep the house relatively comfortable until 7:00 pm, when the room temperature was 19 C. Although not confirmed, this amount of heating avoided the need for gas heating for that night. On the second day after 4 hours of solar heating, the room temperature was maintained at around 18 C which delayed the use of the gas heater by 2.5 hours. This suggests that during this period the room was comfortable. A preliminary interpretation indicates that the thermal mass and the insulation in the building are delaying the need for gas heating. This effect has yet to be fully confirmed, however represents a significant potential for the system. In dry climates such as Adelaide, many days exist where heating is needed in the evening and not during the day. If some solar energy can be stored in the building using the solar heating system, then this energy will maintain comfort temperatures within the home for part or all of the evening. Ultimately this will result in a significant reduction in conventional heating demand. Temperature, o C Outdoor temperature Indoor temperature Gas heater Solar heater Collector irradiance Gas heating 3.5 hrs Gas heating 6 hrs Solar heating 6 hrs Solar heating 4 hrs /07/04 0:00 13/07/04 6:00 13/07/04 12:00 13/07/04 18:00 14/07/04 0:00 14/07/04 6:00 14/07/04 12:00 14/07/04 18:00 15/07/04 0:00 15/07/04 6:00 Figure 4. Temperature profiles in the building. (Some radiation data for the first day is missing). ANZSES

6 4. CONCLUSIONS A roof integrated solar heating system was monitored within a home over a winter period. The system only heats using the collector during sunny periods. Auxiliary heating was provided using a room gas heater. It was demonstrated that the solar heater did provide a significant amount of heating and not only provided but improved thermal comfort within the house. Overall, the expected solar contribution over the heating season was found to be 26%. It was also shown that heating during the day is an effective use of the available solar energy. Temperature data from the house suggested that the some of the energy is being stored in the building structure which is delaying the need for gas heating. This process has a significant potential to further increase the solar contribution, particularly in dry climates. This result is only preliminary and requires further investigation. 5. REFERENCES Australian Greenhouse Office (AGO) (1999), Australian Residential Building Sector Greenhouse Gas Emissions , Federal Department of the Environment and Heritage, Canberra, Australia. M., Saman W. and Bruno F. (2004), Roof integrated solar heating system with glazed collector. Solar Energy, 76, M., Saman W. and Bruno F. (2003) Development of a low cost solar heating system, Preprints of the 2003 International Solar Energy Society, Göteburg, Sweden. Duffie J. A. and Beckman W. A. (1991), Solar Engineering of Thermal Processes, 2 nd edn, Wiley Interscience, New York. ANZSES