District Heating in Areas with Low Energy Houses: Detailed Analysis of District Heating Systems based on Low Temperature Operation and Use of Renewable Energy

This PhD thesis presents a summary of a three-year PhD project involving three case studies, each pertaining to a typical regional Danish energy planning scheme with regard to the extensive use of low-energy district heating systems, operating at temperatures as low as 55°C for supply and 25°C for return, and with the aim of intensive exploitation of renewable energy sources. The hypothesis is that a detailed analysis of energy performance and cost of construction and operation of low energy district heating systems can be used as a rational basis for planning use of district heating in areas with low energy houses. The first case study focus was concerned with developing a method for the designing of low-energy district heating systems for new settlements in which low-energy houses were to be built. The method involved primarily the development of a novel pipe dimensioning method based on optimization of the pipe diameters rather than use of rule-of-thumb methods, through consideration of a certain value of a maximum pressure gradient or a maximum velocity, or both. In addition, attention was directed at the assessment of (i) substation types considered for use in connection with the low-energy houses involved, together with the idea of utilizing booster pumps in the district heating network and (ii) use of network layouts of either a branched (tree-like) or a looped type. The methods developed were applied in a case study, the data of which was provided by the municipality of Roskilde in Denmark. The second case study was aimed at solving another regional energy planning scheme, one concerned with already existing houses, the heat requirements of which were currently being met by use of a natural gas grid or a conventional high-temperature district heating network. The idea considered for employing a low-energy district heating system here involved use of an operational control approach of boosting the supply temperature during the peak winter months due to their shorter durations when compared to a year period. This approach can be considered in two different respects: (i) in the municipal infrastructure, transforming the current heating systems into lowenergy district heating systems and (ii) in the operation of low-energy district heating systems. The building settlement in question, one located in the municipality of Gladsaxe, was chosen for the case study carried out, due to the existing houses there being considered for renovation to houses of a low-energy class, and due to the existing heat-supply energy infrastructure there being a natural gas grid. The third case study carried out aimed at developing energy conversion systems based on use of renewable energy sources that were available locally. This was carried out in an external stay at the University of Ontario Institute of Technology (UOIT) in Oshawa, ON, Canada under the supervision of Prof. Ibrahim Dincer. In this colleborative study, a novel method was developed to serve as the basis of a decision support tool in investigating the optimal use of renewable energy sources, particular consideration being given to the following: (i) the monthly satisfaction of energy requirements of various types: heating (including the demands of space heating and of domestic hot water production), electricity, and cooling, in order to study the improvement in efficiency achieved by use of multi-generation systems, (ii) various types of energy conversion systems, such as single-generation, cogeneration, and multi-generation systems, (iii) the long-term storage of heat energy to cope with the mismatch between the energy production from renewable energy sources and the heat energy requirements, both in terms of the variations involved, such through the excessive production of heat by means of solar based systems, heat that cannot be used immediately but can be stored in borehole storage systems, to be used then in the cold winter period, (iv) an extensive economic assessment of the technologies involved, taking several different parameters into account, each unique for the technology in question, such as the specific investmet costs based on an economy-of-scale, operation and maintenance costs, the lifetime of the technology, the capacity factor, and the salvage value of the energy conversion system at the end of its lifetime, (v) seasonal variation in the generation of energy, in line with the availability of the renewable source in question, (vi) on a limited scale, aimed at gaining as much insight as possible into the complexities of the questions involved, examining the environmental concerns possible to encounter during the operations of each conversion system, the security of supply being figured on the basis of the optimal solutions obtained. In summary, the methods developed in the case studies concern the technical framework for establishing an integrated energy supply scheme involving the use of renewable energy sources for meeting the energy needs of low-energy houses by means of a city-wide low-energy district heating system.

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