A study on community heating system using woody biomass for high performance passive ventilation houses

Abstract

Introduction: In Hokkaido, passive ventilation system has been developed as the distinctive technology in cold region. It is essential to reduce the use of fossil fuel for space heating, therefore the planning of space heating system in high performance houses is required significant improvement of insulation as well as airtight. Over the past decades, the use of woody biomass resources for space heating and hot water supply system has been increased mainly in the mountainous areas in Japan. In some of these areas, there are "community heating system" as a small district heating system with small biomass boiler, which are consisted of some residential houses, city halls, and other public facilities. This is because woody biomass resources which can be obtained in the neighborhood mountain can contribute to reduce the use of fossil fuel and electricity. In this study from these backgrounds, we focused on how to design high-efficiency community heating system using woody biomass which supplies thermal energy to a group of high performance passive ventilation houses in a town of Hokkaido. Analytical model of supply and demand sides in community heating system: In the past studies regarding the district heating system, a size of the heating demand has been given as the numerical values of the basic unit about the energy use according to the building types. In this study, annual heating demands of communities were assumed based on the insulation performance of houses and the living activities of households. Three types of community: new-region, mixed-region, and old-region were assumed from the combinations of three different insulation levels of houses. New-region is composed of only high insulated houses, mixed-region is mixed the low insulated with the high insulated houses, and old-region is mainly composed of low insulated houses. Annual heating demands per a household (a ratio of load levelling) were calculated 11 MWh/year (37%) in new-region, 16 MWh/year (30%) in mixed-region, and 20 MWh/year (25%) in old-region. On the other hand, a size of the supply side system as a biomass boiler, pumps and pipes was assumed to satisfy the peak of heating demands of the communities. The energy efficiency of a biomass boiler of 85%, the insulation thickness of the polyethylene pipe lines of 30mm, and the flow velocity of hot water of 1.0 to 2.0 m/s were assumed. Energy and exergy analysis of community heating system: From the results of analysis, it was found that SCOP of community heating system tends to decrease gradually by the number of households increases despite the difference in new-region, mixed-region, and old-region. The reason is due to the fact that pump load gets larger with increasing speed. In the case of 20 to 100 households, SCOP is highest in newregion, followed in order by mixed-region and old-region. SCOP in all cases of community heating system is higher than that of individual heating system. In addition, the community heating system composed of the residential houses is required a combination with some public buildings such as office school, hospital, and day-service center for the elderly which have large heating demand in the daytime. From the exergy analysis in case of 40 households, total input exergy for the community heating system were 1.1 to 1.4 times larger than that for individual heating system. However, 72% of this input exergy of the community heating system was equivalent to the chemical exergy within woody biomass which can be obtained in the neighborhood.