To Örebro University

Goal, Scope and Background. The idea of district heating is to transport centrally produced heat to buildings where it is used for space heating and for domestic hot water generation. Water is used as a heat carrier. Many different heat sources are used to supply district heating networks with hot water. In literature, environmental studies on district heating mainly consider emissions from the heat generation; environmental impacts from the distribution system are seldom discussed. This paper is the third in an article series on the environmental impacts from the district heat distribution system. The paper presents an evaluation of the use phase of district heat distribution, focusing on long-term thermal performance of different district heating pipes. An overall discussion, in which environmental impacts from the different life cycle phases of district heat distribution are compared, is also presented.

Methods for the Use Phase Study. Environmental impacts from use of district heat distribution systems were evaluated based on heat losses from the networks, which depend on the long-term thermal performance of the district heating pipes. The heat losses cause environmental impacts from extra heat generation needed to cover the losses.

The long-term thermal performance of preinsulated bonded district heating pipes with steel tube, polyurethane foam insulation and polyethylene casing, depends on the thickness and quality of both the insulation and the casing. One important attribute of the foam is the blowing agent used. The blowing agent influences both the initial insulating capacity of the foam and the ageing characteristics, due to differences in migration rates of different substances through the materials.

Heat losses were calculated for different district heating pipe dimensions (DN25 twin pipe, and DN25, DN100 and DN500 Series 2 single pipes). Pipes with two different foam blowing agents (cyclopentane and carbon dioxide) were studied, taking into account the differences in long-term thermal performance of the foams. Concerning emissions from heat generation, two heat sources were considered: heat generation according to the average district heating fuel mix used in Sweden in the year of 2000, and heat generation using natural gas heat only boilers. The functional unit used is 100 m of district heat distribution network during 30 years of use.

Results and Discussion on the Use Phase. A short description of the inventory, some inventory results and a life cycle impact assessment are presented. Characterizations according to GWP, AP, POCP and resource depletion are given as well as two weightings: EcoIndicator99 and Ecoscarcity. The DN25 twin pipe network has about 25% lower environmental impacts from use than the DN25 Series 2 single pipe network. The district heating pipes insulated with cyclopentane blown polyurethane foam have a better environmental performance during use compared to those insulated with carbon dioxide blown foam (6-13%). This is partly dependent on a higher initial insulating capacity of the cyclopentane blown foam, but also due to a slower deterioration of the insulating capacity over time. For the two heat sources considered, different impact assessments give different indications to which option that is environmentally preferable.

Overall Results and Discussion on Pipe Production, Network Construction and Network Use Phases. A comparison of the three life cycle phases studied in this article series was made concerning four emissions, the four characterizations and the two weightings. The use phase represents over half of the total environmental impact for most, but not all, environmental parameters studied. It is important to keep the heat losses from the network down and to strive for heat sources with low environmental impacts. The larger the pipe, the larger is the relative impact from pipe production. The network construction phase has a relatively small contribution to the total environmental impact in most systems studied. However, the emissions during network construction often occur in residential areas and may therefore not be neglected when immediate nuisances and health aspects are considered. A very small change of the material flows in the production phase, the change between two different blowing agents (cyclopentane and carbon dioxide), can give dramatic results for the total environmental outcome for the district heating network because of a large change in influence on environmental impacts during use. The DN25 twin pipe network proves to be environmentally advantageous compared to the DN25 Series 2 single pipe network during all of the studied life cycle phases. Recommendations and Perspectives. It is important to make sure that improvements in the production and construction phases do not lower the insulating capacity of the district heating system. A good initial insulating capacity, maintained over time, is important for the environmental performance of a district heat distribution network. Using DN25 twin pipes instead of DN25 Series 2 single pipes is a better choice, when possible, regarding all studied life cycle phases. The environmental impact from use of the district heat distribution system depends heavily on the type of energy source that is utilized to supply the network with heat.