District Heating (DM) plays an important role in the infrastructure of all the Nordic countries, and the market share of the total heating market is significantly higher in these countries compared to others in the West. In Denmark about 50% of total heating is covered by DH.
With Denmark's leading position in DH technology considerable research and development has been initiated by Danish manufacturers as well as by technological institutes and universities.
In 1980 The Danish Ministry of Energy initiated a DH research programme and since then more than ISO research and development projects have been carried out in cooperation between research institutes, consulting engineers, DH companies, and manufacturers.
Much of the work to be described later has been financed by this programme, which has also played an important role as a basis for the work performed by the European standardisation organisation CEN.
In order to maintain and further develop much of the knowledge obtained in the Danish Energy Research Programme, the Danish Technological Institute (DTI) and the Technical University of Denmark (TUD) formed the Centre for District Heating Technology in 1991. The Departments of Energy Technology and Plastics Technology at DTI and the Laboratory of Heating and Air Conditioning at TUD form the backbone of the centre while experts from other departments also participate. Altogether 30 people at the centre are experienced in DH technology and more than 100 reports on DH have been published.
The centre has a number of test facilities for DH components (pipes, heat exchangers, etc) including an accredited flow rig for DH meters.
A DH programme within the Nordic research cooperation was started at the end of 1985 and has been based on the knowledge at the technical universities in the Nordic countries. A fundamental programme structure has been established by appointing research coordinators at the professorial level. supplemented by recipients of national scholarships. This organisation constitutes the backbone of the programme together with a number of reference DH networks that participate by opening their networks to various research projects.
The International Energy Agency's (IEA) Programme of Research, Development, and Demonstration on District Heating and Cooling should also be mentioned. It was initiated in 1982 and until now there have been three programme periods each with a number of specific research projects and analyses. The IEA Programme is directed by an executive committee, and ten countries participate at the moment.
In the following some trends in the development and research in DH technology will be mentioned briefly, divided into the topics: House Stations and Heat Meters, The DH Distribution Network, Production Plants, and Operational Optimisation of DH Systems.
The house stations in Denmark are often directly connected to the DH mains and the system configuration is relatively simple, i.e. in many systems the radiator circuit is not used for preheating the domestic hot water.
Much work has been done to standardise house stations for singlefamily houses and reduce the price of these units.
The demand for lower supply temperatures in the DH system has led to a need for more efficient heat exchangers in the units in order to maintain the hot water supply at the lower temperature level. During the last 10 years the return temperature from the DH units has been markedly reduced by introducing small plate heat exchangers.
In a project directed by Aarhus Municipal Works efficient house stations are being developed and tested in cooperation with the Centre for District Heating Technology and manufacturers of DH units and control valves. In particular, the performance at a lower supply temperature level is being investigated .
If the control valves do not operate fast and with precision, problems may arise in heat exchangers for domestic hot water, i.e. fouling of the heat exchanger may occur due to a high content of calcium carbonate (chalk). To obtain a fast response of the control valve, a special valve has been developed with a forward function. i.e. when hot water is being tapped the pressure in the cold water line is changed and this is sensed by the control valve, thus ensuring a fast response. The function of this special valve has been tested both experimentally and by mathematical modelling, and its marketing has been carried out successfully since 1988.
Work is in progress to develop tools for an online determination of the fouling of heat exchangers so that the cleaning and maintenance of large heat exchangers can be carried out only when necessary. In the longer perspective new composite heat exchanger materials as well as additives to the DH water may reduce the fouling problem.
Work on modelling the house stations is in progress in Denmark as well as within the IEA programme. In the near future complete stations can be modelled and the effect of heat exchanger areas and the type and size of control valves can be determined before construction actually begins. The overall performance and efficiency of different house station configurations can also be determined for different load situations.
The Centre for District Heating Technology is also developing a dynamic design tool for hot water tanks with internal heating coils. With information on the domestic hot water loads (tapping programmes) the temperature distribution inside the tank and the energy content can be calculated by the model and this will ensure a more optimal tank design.
The Danish accreditation scheme DANAK. was established in 1991. The scheme is a continuation of the accreditation scheme established in 1973 under the auspices of the former Danish National Testing Board. From 1992 all new heat meters smaller than 3 m3/h installed in Denmark have been tested at an accredited testing laboratory, and from July 1993 existing meters in operation will also be called for verification. Within a few years the performance of DH meters is consequently expected to be improved and a greater market share by static (nonmechanical) heat meters will be anticipated.
Work is in progress to document the influence from flow disturbances on meter performance, for instance from swirls created by elbows and pulsating flows generated by pumps or other devices. In the longer perspective it should be possible to compensate for these disturbances within the meter. It is also possible to design the meter so that it is less sensitive to disposals of magnetite in its interior.
Prototypes of "intelligent" meters have been constructed and tested, but are not in common use today. Usually the heat meter is connected to a control valve, and in this way the information contained inside the meter can be used to limit the DH flow or the consumption of heat or ensure the largest possible cooling of the DH water.
In the Danish Energy Research Programme much work has been done to improve the performance and lifetime of the DH pipes and develop methods that will reduce the overall costs of establishing a DH system. This includes methods for inexpensive laying of plastics pipes and laying pipes without preheating and making use of compensators. UNICHAL has recently initiated work to de termine the number of temperature cycles that different types of DH networks will experience in their lifetime in order to establish a reliable design methodology.
The substitution of CFC as a blowing agent when manufacturing preinsulated DH pipes has encouraged the making of a number of investigations and tests of the performance of nonCFC foams. Also the diffusion of oxygen and water vapour through plastic pipes has been analysed and resulted in an improved pipe design.
Many DH systems in Scandinavia as in the rest of Europe are in need of renovation, and this is planned for the future. For this reason, it is important to have at our disposal methods that can determine the heat loss and water leakage from the systems. Work has been done to test different methods for measuring the heat loss and thereby indirectly the heat conductivity of the insulation of the DH pipes as well.
The methods include (1) measuring the temperature field in the soil surrounding the pipes. (2) measuring the surface temperature above the pipes by infrared techniques (Thermovision), and (3) applying heat flux meters on the DH casing or concrete duct. Based on the developed methods the heat loss from a given pipe can be determined and a better plan for renovating the network can be worked out.
In 1948 it was discovered that adding chemicals or fibres to water could drastically reduce pipe friction. However, different problems inhibited any practical use of this until the 1980s when cationic surfactants proved to perform well in DH systems. While the development and testing of the drag reducing additives took place in Germany and USA, Homing Municipal Works in Jutland were the pioneers in testing the socalled smooth DH water in a transmission line in normal operation. The results have been very promising so far and pumping costs have been reduced by as much as 70%.
As has been described in some of the previous chapters, much work is being done to increase the use of natural gas and biomass in Danish DH systems. Many small CHP units with thermal storage have been built to meet the national requirements for efficient use of energy. However, the adding of many new production plants increases the complexity of the systems making them more difficult to operate. To cope with this challenge most new plants are equipped with modern SCADA systems which monitor a number of parameters in the plants as well as in the DH network. These systems have been found to be very useful for operational optimisation.
Optimum operation of a DH system means that operational costs, e.g. the pumping costs as well as production costs and heat losses are all minimised.
In order to optimise a DH system it is important to understand how its various parts interconnect. The problem is essentially dynamic. If, for instance, the supply temperature at the plant is changed, at some later time this will change not only the temperatures and heat accumulated in the pipes, but the flow and return temperature from the house stations as well.
An overview of optimisation methods applied for DH systems are given
below:
Quasisteady state optimisation
Instantaneous optimisation of heat production units
(Load dispatch)
- Online minimisation of the supply temperature
Quasidynamic optimisation of the complete DH system
An interesting approach has been to determine the time lag and temperature decrease in a distribution network by statistical methods (time series analysis) enabling the minimisation of supply temperature at the Vestkraft CHP plant in Esbjerg. Large savings have been reached annually by lowering the supply temperature by an average of 90C. However, in this approach the production and distributions costs were not considered explicitly, as the goal was mimmising the supply temperature.
A deterministic approach has also been adapted, i.e. to develop dynamic simulation models of the components in the DH system. combining the models and finally minimising a cost function of the operational costs. In each optimisation period (time step) the flow pattern in the network is considered to be known either from measurements or steadystate hydraulic calculations; for this reason the approach is called quasidynamic.
This is believed to be the first time an optimisation of a DH system has been carried out taking into account the dynamic behaviour of the complete system, namely, plants, network, and house stations.
Data from Ishoj DH system were used as a case study to test the method. The figure shows the Ishoj DH System consisting of 17 heat exchanger stations and a 7 km DH network made of preinsulated pipes with diameters ranging from 65 to 300 mm. In the top left corner of the figure the measured supply temperature at the DH plant and at substation No. 9 is shown as well as the simulated supply temperature at this substation obtained by a dynamic heatloss model. In the top righthand corner the transport times from the DH plant to the substation are shown resulting from two different strategies for the supply temperature at the DH plant.
The lower part of the figure shows two different supply temperature strategies: a constant supply temperature of 1050C, and a varying one which optimises the DH system when the heat is produced at a backpressure CHP plant.
So far a simple optimisation routine has been developed and implemented in the SCADA system in Ishoj. The final goal is an online optimisation of the system.
Work on artificial neural networks has already been done, for instance to forecast the heat loads at DH plants. In the future it should be possible to combine different neural networks to obtain a complete optimisation model of a DH system.
No doubt the use of different computer codes or intelligent SCADA systems will soon be at the disposal of the operators in the control room, and large savings are sure to be achieved by a more efficient operation of complex DH systems.