Life Cycle Assessment of District Heat Distribution in Suburban Areas using PEX Pipes Insulated with Expanded Polystyrene

Artikel i vetenskaplig tidskrift, 2006

Goal, Scope and Background. Combined heat and power (CHP) is a strategy aimed at reducing the impact of the energy sector on the climate by more efficient use of the energy content of the fuel. The implementation of CHP requires the utilisation of the heat produced. Space heating by means of district heating is one possible use for such heat. In countries such as Sweden, where district heating is already extensively used, many multiapartment buildings are connected to district heating. For increased use, the distribution systems will have to expand into suburbs with single family homes. However, the environmental impact and cost of the district heat distribution system increase when the pipe networks are extended into such areas. This is due to the production and installation of longer pipe networks and increased heat losses from the system. Attempts have been made to find new types of pipe constructions in order to lower the costs of connecting single family homes to district heating. These should be evaluated from an environmental perspective. The EPSPEX system is a distribution system intended for suburban areas. This system consists of cross-linked polyethylene (PEX) pipes in insulating blocks of expanded polystyrene (EPS). This paper presents a life cycle assessment of the EPSPEX district heat distribution system. In a second scenario, sub-stations were added. The results indicate areas that require improvement and provide a basis for comparison with other types of district heat distribution systems. Methods. Production, network construction and use of the district heat system were studied by means of life cycle methodology, employing specific data for the EPSPEX system and generic data for upstream impacts of the materials used. The system constructed in Vråen, Värnamo, Sweden, in 2002 was studied. The district heating used in Vråen is mainly based on biofuels. The functional unit was the use of one metre of an EPSPEX district heating system over a period of one year. The expected system life was 30 years. The results were characterised as global warming potential, acidification potential, eutrophication potential and the use of finite resources, as well as weighted by EPS 2000, ExternE and EcoIndicator 99. No external review was performed, but a reference group of district heating experts familiar with the practice has reviewed the study. Results. Heat losses are clearly the main environmental impact in all characterisations and weightings (71-92% of the total impact), despite the fact that the heat production studied was mainly based on biomass combustion, generally perceived to be environmentally friendly. Of the system components, the production of EPS insulation blocks had the largest environmental impact. Discussion. This impact, however, is compensated for by the fact that the need to produce less heat leads to a lower level of emissions. Several characterisation methods revealed that the production and combustion of diesel for excavating the pipe trench has a significant environmental impact. The jointing brass swaged coupling used for the PEX fluid pipes has a surprisingly high impact in terms of acidification and EPS 2000, considering the small amount of brass in the system. Conclusions. The life cycle environmental impact is dominated by the heat production needed to compensate for heat losses from the system, despite the fact that the EPSPEX system is relatively well insulated compared to a conventional district heating system. It is possible to shut down the heating circuit and only use the hot tap water circuit during the summer months; this reduces the heat losses and is an advantageous feature of the system. The second largest environmental impact of the EPSPEX system arises from the production of the EPS insulation blocks. A decrease in nitrogen oxide emissions, especially those caused by the excavation and filling of pipe trenches, would be beneficial. A rough comparison has been made with available literature data for conventional DN25 twin pipes. The results indicate that the environmental impact of the EPSPEX system is probably lower. However, the pipes are not identical, as the water delivery capacity of the conventional pipe is slightly lower. Recommendations and Perspectives. In Sweden, new types of pipes are being developed for district heating in suburban areas, and there is a need for an environmental comparison between such new alternatives and previous results for conventional polyurethane insulated steel pipes. This study reveals that biofuels, although perceived to be environmentally friendly, must be used with caution in order to ensure a satisfactory environmental performance. Heat loss from district heating should be minimized also when biofuels are used. The most immediate way to reduce such environmental impact is to increase the insulation. The environmental trade-off between lower heat losses achieved by the use of more insulation and the production of greater amounts of insulation material should be further studied. © 2007 ecomed publishers (Verlagsgruppe Hüthig Jehle Rehm GmbH).