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Dynamic modeling and analysis of stocks and flows in building systems
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The building stock is a major consumer of material and energy resources, and also an important source for waste generation and emissions. The building stock represents a major capital stock for society, and also a large anthropogenic stock of materials. There has been a rapid growth of stocks in many countries due to rising population, changes in household structures, and improvements in standard of living in the latter half of the previous century, and as the building stock ages, buildings will reach the end of their lifetime and become obsolete. The materials in these buildings will then become available as potential secondary materials. Anthropogenic material stocks are becoming increasingly important as secondary resources due to scarcities of primary resources and adverse effects from mining and production. Society demands a building stock with a certain size, standard and composition, and this demand exerts a pull on material and energy resources, and as a consequence also pushes materials into the waste management system. Understanding the drivers and mechanisms in building systems is important for understanding how stocks and activities of construction, renovation and demolition develop over time. Buildings have a long lifetime and long-term studies of building systems are therefore required for understanding the stock dynamics and for anticipating changes in the associated flows of floor area and materials. The work in this thesis explores the metabolism of building stocks, and the long-term dynamic mechanisms inherent in building systems. Methodological challenges relate to how the dynamic behavior of stocks and flows can be modeled and simulated in a fully dynamic and consistent manner, and what parameters are relevant to represent the metabolic behavior of the system. Modeling building stocks and flows of floor area also provides the foundation for estimating the associated material stocks and flows. Material stocks in buildings are diverse with regard to composition and characteristics. Some building materials represent a potential secondary resource, while other materials or substances are merely waste management problems with potentially negative effects. Wood and concrete are studied as examples of the former and are relevant because of their abundant use in buildings. Polychlorinated biphenyls (PCBs) are studied as an example of toxic or hazardous contaminants to see how unwanted substances affect the stocks and flows over time. The results show that good knowledge about the past is critical for understanding the future in systems with a long product lifetime like buildings. Low demolition activity for residential buildings in Norway has been combined with an unprecedented growth in demand for floor area in the last decades. This has led to increasing stocks, and as a result, future demolition activity can be expected to increase as the building stock ages and more buildings reach the end of their lifetime. This will also increase construction demand to replace the obsolete buildings. The cyclic behavior with peaks in activity levels that are displaced according to lifetime is also reflected in material stocks and flows. Increased waste generation and demand for materials and resources have important implications for future waste management and resource consumption. Modeling dynamic systems with long-lifetime products will involve uncertainties related to the selection of modeling parameters, quality of historical data, and not least, assumptions and scenarios about future developments. Buildings in general, and residential buildings in particular, have long lifetimes. Results from the case studies demonstrate the importance of lifetime as a modeling parameter, with regard to both timing and magnitude of flows. The long lifetime also means that scenarios have to be developed for the future, and historical developments show how much stocks and flows can change over a few decades, meaning that long-term scenarios are inherently uncertain. The modeling parameters in the case studies are aggregated variables that are best suited to inform about long-term trends. Comparison with historical statistical data confirms that the modeling approach is better suited to simulate long-term trends than short-term fluctuations. A study of Chinese residential housing shows a strong growth in construction activity and stocks in the last decades. China is currently experiencing a very strong urbanization trend that has significant implications also for building stocks and resource consumption. At the same time, UN population projections predict a decline in total population. This calls for examining the development in rural and urban population stocks and the potential impacts on rural and urban building systems. Stocks and flows of population and residential housing were modeled for both rural and urban areas. Results show that projected trends in urbanization and population will have a strong influence on stocks and construction activity. Model simulations suggest that rural construction activity has already peaked and will experience a strong decline in the decades to come. While urbanization maintains increasing construction activity in the short-term in urban areas, a peak is expected here also, possibly already within the next decade, before construction activity starts declining substantially, as projected. The decline is dramatic for both rural and urban areas, and while this can be positive from a resource conservation point of view, the situation might be severe with regard to investments and employment levels in the economically important construction sector. The impacts on the construction sector will be more severe in urban areas where construction firms are responsible for most of the construction, as opposed to the more common practice in the rural areas of building your own house. For resource conservation and consumption, the impacts are also more pronounced related to urban stocks due to the absolute stock size and the higher quality of the materials used. While construction has traditionally been the dominating building activity, the ageing building stocks and the more-frequent upgrading of existing buildings signal a shift towards renovation becoming as important as, or even more important than, construction. The future increase in waste generation from demolition poses challenges for everyone involved in waste management, both for the industry and for legislation. While historically being regarded merely as a problem, building waste can also be a source for secondary materials, and with appropriate technology and legislation, these resources can be utilized in a more sustainable way, while at the same time being economically competitive. Some materials are however not suitable for reuse and recycling and constitute a waste problem. This is the case for toxic and other hazardous materials that are contaminating building waste. A case study of PCB in Norwegian buildings concludes that although PCBs were banned several decades ago, considerable amounts still remain in in-use stocks and are expected to do so for many more decades, with associated challenges for identification and safe disposal.