District Heating for Residential Areas with Single-Family Housing -with Special Emphasis on Domestic Hot Water Comfort
Abstract: The concept of district heating (DH) involves centralised heat production where heat is distributed to consumers via a piping network. It is essential that benefits achieved from concentrated, large-scale production instead of local production in each building exceed the costs associated with heat distribution. Since the distribution cost per delivered kWh increases as the heat density of an area decreases, the possible competitive advantage of DH will be greater in areas with multi-family housing than in residential districts dominated by single-family dwellings. Nevertheless, in the last few years there has been a growing interest among Swedish DH companies in expansion into low heat density areas. This is a consequence of an already large market share in high heat density areas, increasing economic incentives to invest in combined heat and power production, and an understanding that DH can play an important role in fulfilling political ambitions for Sweden to comply with the obligations according to the Kyoto Protocol. This thesis focuses on DH in residential areas with single-family housing with particular emphasis on domestic hot water (DHW) comfort, which can sometimes be problematic in the non-heating season. The investigations are mainly based on theoretical findings achieved by performing dynamic simulations and finite element calculations. Laboratory measurements, field experimentation and questionnaires posed to operating companies were used to provide in part empirical verification, as well as a framework for practical engineering interpretation. Four recurrent problems in DH systems in single-family housing areas are: low primary water cooling, high specific heat losses, unsatisfactory DHW comfort during summertime, and high specific installation costs. All these topics are examined in this work, either directly or indirectly. State-of-the-art 2-pipe systems with indirect connection of space heating systems and instantaneous hot water heaters were assumed when modelling conventional systems. For the following topics numerical results were derived: thermostatic valves for securing an acceptable DH supply temperature during summertime, design and functioning of DHW controllers, and sizing of DH pipelines. Desirable characteristics of thermostatic valves and their influence on the temperature level in a DHW system were identified. Based on a numerical model of positive or negative bacterial growth, predictions were made of extent of growth in faulty DHW systems. Heat losses were calculated for a novel type of DH piping arrangement that has the potential to reduce investment costs. The system studied is a 4-pipe system with carrier pipes made of PEX (cross-linked polyethylene), arranged with EPS (expanded polystyrene) blocks that provide both heat insulation and mechanical protection. Calculations and field measurements show that the heat losses from the EPS-PEX culvert studied are approximately half those of a corresponding 4-pipe system consisting of two conventional twin pipes. For 4-pipe system networks, temperature problems similar to the ones experienced when employing DHW re-circulation in residential buildings were studied. In areas of low heat density, it is imperative that a high degree of market penetration is achieved to avoid inordinately high specific investment costs and heat losses. With this objective in mind, a survey was carried out among a large number of DH companies to identify current marketing strategies. The main result was that companies generally posses too crude a knowledge of income, age and other sociological factors about their potential customers. Such information could provide a basis for making reasonable hypotheses about consumer preferences, to form a basis for making future marketing more effective.
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