Life cycle assessment of the district heat distribution system -: Part 2:: Network construction

Goal, Scope and Background. In a district heating network, hot water is transported from a central heat generation plant to buildings where the heat is utilised for space heating and domestic hot water generation. This paper presents a life cycle assessment of the construction of district heating pipe networks, based on a gate-to-gate life cycle inventory commissioned by the Swedish District Heating Association. In the literature, environmental studies oil district heating mainly consider emissions from heat generation; environmental impacts from construction of the distribution system are seldom discussed. The purpose of the study is to identify environmentally significant parts in the construction of district heat distribution networks and to provide information for a larger study including more parts of the life cycle of such district heat distribution. No external review has been performed, but a reference group of district heating experts familiar with the practice was involved in the choice of systems to be studied as well as in reviewing parts of the study. Methods. The Study covers construction Of the Main Pipe system according to the guidelines from the Swedish District Heating Association. Construction of the pipe system was assumed to take place in Sweden by Swedish entrepreneurs during the time period 1999-2000. Transport of the district heating pipes from the factory to the excavation site is included in this Study, but not the production of the pipes. The functional unit used in the study is 100 metres of pipe system (flow and return pipe). The studied systems are: twin pipe of the dimension DN25 and single pipes of the dimensions DN25, DN100 and DN500. Two different surroundings were Studied: urban environment, characterised by the need to break open and to restore asphalt cover and to remove excavated material from the site, and green areas, Without my asphalt and where some of the excavated material might be left at the site and reused. Results and Discussion. A short description of the inventory, some inventory results and life cycle impact assessments are presented. Characterisations according to GWP, AP, POCP and resource depletion are given as well as two weightings: Eco-Indicator99 and Ecoscarcity. Emissions from production and use of the diesel needed for excavation of the pipe trench gives rise to a dominating part of the environmental impact. Recommendations and Perspective. To minimise the need for excavation is the most important feature in order to reduce the environmental impact from construction of the district heating pipe network. A twin pipe uses a narrower pipe trench than the equivalent two single pipes, and is an already available option. CO-utilising the trenches with cables for electricity, for instance, will not make the environmental impact from the trench any smaller, but will decrease the total need for excavation in society. It is important to make Sure that environmental improvements from changes ill the network construction phase are not off-set by other effects in the total life cycle of district heat distribution.