How cellular gases influence insulation properties of district heating pipes and the competitiveness of district energy
Description of the Project
The objective of this project is to clarify how the cellular gases replacing the CFCs influence the long-term insulation property. Included is a calculation of the cost of the heat loss costs when using the R11 or cyclopentane cellular gases in pre-insulated pipes. Finally, the project is completed with a review of whether the use of vacuum as cellular gas is within the scope of possibilities. This technology can reduce the heat loss by up to 75%.
The heat loss of piping systems has gained increased focus as the energy price has increased and as energy to a greater extent has become a taxation objective. The heat loss of the piping system amounts to 5 - 40% depending on the type of cellular gas, thickness of insulation, casing pipe dimension, etc. Therefore the heat loss in certain systems is a significant economic factor.
As the cellular gas R11 of environmental reasons has been phased out and replaced by cyclopentane, the heat conductivity of cellular plastic containing cyclopentane increased by app. 10%. That is due to the fact that cyclopentane has greater heat conductivity than R11. It has subsequently been seen that heat conductivity in cellular plastic containing cyclopentane increases additionally by up to 15% in the course of the lifetime of the pipe. Measurements have demonstrated that this is due to the diffusion of atmospheric air in the foam. The increase is greatest for the small dimensions.
This alone means that new piping systems must have thicker insulation so the heat loss does not increase, as that would make costs increase.
In some piping systems, carbon dioxide is used as insulation gas. This cellu-lar gas diffuses quickly and is replaced by atmospheric air; that makes the heat loss of the piping system increase sharply. Calculation models show an increase of app. 50%. In spite of this, carbon dioxide is still used as cellular gas in certain piping systems giving rise to increased costs for the entire life-time of the piping system.
Outside Europe other cellular gases of HCFC type such as 141b are used and new gases are on their way, e.g. 245fa and 365 mfc, which are of the HFC type. The long-term insulation properties of these cellular gases should be investigated before the gases become available in the market, so the users can calculate the expected heat loss of the piping system in comparison with the traditional pre-insulated piping systems.
Experiments are carried out with better types of polyurethane for preinsulated pipes for district energy, but optimisation of raw materials and processes are not estimated to reduce the heat loss of the piping system by more than 5%. A significant reduction in the heat loss of the piping system can therefore not take place by using this technology.
For these reasons, a study should be carried out of how the heat loss of the piping system will develop in the future, depending on the pipe dimension, insulation thickness, type of cellular gas etc.
In other industries, experiments are carried out to replace cellular gases with vacuum. In addition, PUR foam systems intended for vacuum are marketed. This implies that the traditional production technology of pre-insulated pipes can be used. When using vacuum and applicable foam systems, the calculation show that the heat loss can be reduced by up to 75% compared to existing products. This makes it possible to use smaller pipe dimensions and to further develop district energy in new areas.
However, a number of conditions have not been investigated, e.g. the mechanical properties and the lifetime. Especially these two properties separate the foam systems used for pipes in district heating and cooling from other applications. In addition, it should be clarified how the vacuum has to be maintained as it currently is not believed to be possible to keep a constant vacuum in the insulation without periodic maintenance. That requires a closer study of perspectives, possibilities and restrictions.
Summary of the final report of the project
The report deals with a number of different methods to determine the composition of cellular gas in the foam according to the results of the experiments which have been made over time.
A program for calculating heat loss has been developed and it is able to calculate the value of heat loss in a CFC pre-pipe system produced in 1985. The calculations are based on data from 2005 to 2035.
A questionnaire survey has been conducted in the countries associated with Euroheat & Power and answers received from 25 plants in six countries.
The section on vacuum insulation deals with the basic properties of energy transmission in materials, where the properties of a series of panels in the market are checked. This is followed by a section on technology used in order to acquire the vacuum necessary to obtain the insulating property. The section ends with general experiences from the use of vacuum.
Conclusions Still after 20 years in use, CFC-blown pipes will have app. 5% lower heat loss than pre-insulated pipes with traditional blown PUR-foam. This will be the case also for the coming 30 years period, meaning that the phasing out of CFC has reduced the competitiveness and probably the spread of district heating to new areas. Also the environmental impact has contributed to a reduction in competitiveness of district heating caused by the phasing out of CFC.
The new microcellular PUR-foam that is foreseen to penetrate the market within a few years will change this situation and bring the competitiveness and environmental impact of district heating back to the level before CFC was phased out. This is very positive, especially if carbon dioxide quota, increasing energy prices, new duties and increasing VAT will bring new burdens on the district heating area.
The answers in the questionnaire show that the users prioritise cyclopentane and that the long-term insulation properties are important, but comparison results between heat loss from CFC-blown pipes and cyclopentane pipes are seldom.
The use of vacuum insulation technology based on PUR-foam is useful for shells or rings with a thickness up to 4 cm . Due to the high temperature in the district heating systems, the diffusion rate thru the barrier material is accelerated and the lifetime as to the low thermal conductivity of the product is reduced.
Juha-Pekka Lemponen, Lappeenranta University of Technology, Finland
Micheal Kraaz, Ingenierbüro Kraaz, Germany
Henning D. Smidt, Danish Technology Institute, Denmark (project leader)
Danish Technological Institute, Denmark
Lappeenranta University Technology, Department of Energy Technology, Finland
Ing.-Büro M. Kraaz, Germany