The considerable extent of district heating (DH) in Denmark has resulted in a distribution system with a total pipe length of over 17,000 km (double pipes). The number of connections/house stations is about 500,000, which supplies more than 1 million dwellings.
It is characteristic that both city centres as well as the surroundings more sparsely populated, residential developments are supplied with district heating.
An essential basis for the wide distribution of DH in Denmark is the use of comparatively low temperatures and pressures. Thus more than 90% of the systems are usually operated at 700C to 900C in the supply pipe, and W'C to 450C in the return pipe, and at pressures below 6 bar.
A considerable number of existing systems are connected with transmission pipes from cogeneration and refuse incineration plants. These systems operate at comparatively low temperatures, too, which means about 900C to 1250C in the supply pipe.
Only in areas where the heat is used for industrial purposes are high water temperatures used, viz. from 1200C to 1700C. A few other systems have been supplied by steam for many years.
The existing Danish distribution network for DH has been established mainly since 1955. In the beginning of this period, steel pipes in concrete ducts with an insulation of mineral wool or cellular concrete were predominant.
In 1963, Denmark introduced a prefabricated and preinsulated DH pipe, which could be buried directly in the ground. This marked a new era in the distribution of district heating.
The carrier pipe of steel was surrounded by insulating polyurethane (PUR) foam. Because of positive experiences with pipes of plastics for other applications, it was an obvious idea to use an outside protective casing/jacket pipe of this material. Today, PEHplastics are used for the casings.
The experience with preinsulated pipes has been very positive. Today, practically all pipelines are made this way, and it is estimated that no less than twothirds of the existing pipelines are of this type.
A major problem in propagating DH throughout Denmark is the rather large in vestment required, mainly in transmission and distribution networks. The use of preinsulated lowers the total investment cost considerably, and this has had a considerable influence on the progress of DH in Denmark.
It is quite important, too, that the distribution system is durable and requires little or no need of maintenance. In this connection, also, preinsulated pipes are in front.
Practically all preinsulated pipes in the Danish DH network are produced in Denmark. There are four manufacturers which export these pipes worldwide.
Preinsulated pipes are normally for use up to 1300C. It is characteristic for the Danish preinsulated pipe systems that they are 'complete systems', in that they include straight pipes as well as pipe tees, bends, compensators, and anchors. Furthermore, they are produced in every carrier pipe dimension up to 1016 mm, in lengths of 6 to 24 m, and in both straight and curved forms. Only the joints are completed in the field. The main pipes are often provided with an electronic surveillance or alarm system.
The first prefabricated pipe system, which was the sliding system, reacted to high temperatures quite like those in the steel pipes in ducts. The carrier pipe was covered with a sliding agent in order to make it movable in proportion to its insulation and casing. For expansions, builtin compensators or bends (e.g. loop expansions) were used. In the latter case, the carrier pipe and the insulation were separated by a hollow space or a soft insulation which allowed transverse movements when used.
In the bonded pipe system, which now predominates, the steel pipe is bonded to the insulation and casing. Under the influence of variable temperatures the pipe, insulation and casing move as one.
The axial movements are, however, rather restricted due to the friction between casing and filling/soil materials. The friction forces of the moving pipe length result in comparatively high axial stresses in the carrier pipe.
At very low supply temperatures (60 700C) this is no problem, but at higher temperatures it is necessary to
use:
expansion bends or perhaps axial compensators, operating on both the medium pipe and casing
axial compensators activated only once, i.e. during the first heating, and afterwards welded together
prestressing or preheating of the pipeline before covering or to:
design the pipeline so that it can be documented that stresses that occur will not result in unacceptable strains.
In the current improvement of the preinsulated pipe systems and its documentation, the last method has lately gained ground be cause it results in cheaper pipelines.
Finally, it should be mentioned that for DH networks with very low supply temperatures (70750C), preinsulated pipes with a plastic carrier pipe are also being used today.
Contrary to the carrier pipes, which are nearly always welded, a number of various jointing methods have been used regarding the casings. The main types that are being used
are:
Flanged joints or muffs
Shrinked joints
Welded joints.
At the flange joints, two pipe halves, equipped with flanges are to be pressed together around the casings by means of bolts or wedges.
At the shrink joint, a plastic muff, sleeve or plate shell with builtin shrinkage effect (when heated by a gasburner) is to be mounted.
In the field, welded joints are based primarily on electricayl heated conductors fastened to the inside of the muff or a band. Several different types of these joints are used.
Insulation in the joints is provided by means of precasted half shells or by onsite pressureinjected foam.
It should also be mentioned that service and branch pipes are often made of flexible pipes (cable pipes). The flexibility is gained by using softer materials for carrier pipe, e.g. soft steel, copper or plastic materials (especially PEX), and insulations as well as corrugated casings in some cases. These pipes are all laid out in a wavy pattern to avoid including expansion joints. New methods are being developed for pipe laying. The latest method is a special "plowingin" technique which makes it possible, in one working process, to excavate, lay the pipes, and recover them. Substantial time and construction savings are thus achieved.
As it is of crucial importance that the DH network operate almost flawlessly over many years, there has been considerable activity in documenting, laboratory testing, and improving of the preinsulated pipes. Of course, these activities are instigated by the pipe manufacturers, but several projects concerning DH networks and preinsulated pipes are being carried out by consulting firms and by the Danish Technological Institute. These projects have been supported by the Danish Ministry of Energy (EFPProjects) and to some extent by the Ministry of Industry.
The projects have been dealing with, e.g.:
-Damage statistics of district heating pipes
-Renewal of older district heating pipes in concrete ducts
-Expansion and stresses in preinsulated pipes
-Mechanical and chemical testing of the foam, casing, and joints of preinsulated pipes, including estimates of reliability and durability
-Using of PURfoams based on nonCFC materials
-Diffusion of oxygen and water in preinsulated pipes with media pipes of plastic materials
The technological development of the Danish distribution system for DH has thus been an integrated interaction among different partners, an interaction that has resulted in a wellfunctioning Danish distribution network.
Underlining the efforts of obtaining a satisfactory distribution system, it should be mentioned that the very first initiatives to completing technical requirements for district heating pipes were taken in Denmark.
This resulted in 1982 in a code of practice for DH networks (Danish Standard DS 448) as wen as some standards on preinsulated pipes (DS 217882).
In 1982, the Danish initiative concerning standards resulted in the setting up of a standardization workgroup under CEN (European Committee for Standardization). Here Danish DH plants, consulting companies, manufacturers of pipes, as well as the Danish Technological Institute are represented. Up till now, it has resulted in 4 European Standards and draft standards (EN 253, 448, 488, and 489).
Also the code of practice for DH networks has been revised and a new edition of Danish standard 448 was approved in 1992.
The space heating Installation at the consumers is typically a central heating twostring system, or in some cases a onestring system. Because of the considerable distribution in Denmark of DH at low temperatures and pressures, the consumers' space heating installations can be connected either indirectly by a heat exchanger or directly to the DH.
Both systems are used in small singlefamily houses as well as in large multistorey buildings, but the indirect system is predominant in large cities. The requirements of the DH plants and the needs of the consumers determine the choice.
Domestic hot water is heated in a storage waterheater tank or instantaneously by a separate heat exchanger. e.g. a flow water heater, directly connected to the district heating.
The connections/house stations and consumer installations are supplied with various automatic controls in order to provide as high an energy efficiency and comfort as possible. Nearly all houses are supplied with radiator thermostats. Many are supplied with a central water flow control and a weather compensator as well, by means of an outdoor sensor and a room sensor, if necessary, these control the flow temperature to the heating installation so that the heat supply is centrally controlled and follows consumption. In this way unnecessary heat losses from the installations in the building are reduced and energy is saved.
By using a weather compensator, a lot of other energysaving functions can be realized. e.g. effective and economic night set back, pump control, and temperature limiting.
In a directly connected house station the DH water circulates directly through the consumer installation, i.e. the radiators of the consumers are directly heated by DH water. The control of the radiator system can be either in the form of a quantity control. where the water flow is adapted to the heating requirement, or temperature control, where instead the flow temperature is adapted.
With quantity control the temperature to the radiator system is the same as that in the distribution system.
A central differential pressure control ensures a constant differential pressure. In that way, the decentralised control of the flow through the individual radiators or thermostatic return valves can be carried out quite freely.
Directly connected house stations with quantity control are simple, inexpensive, easy to operate and result in an efficient cooling of the DH water.
With temperature control, the adaption of the flow temperature to the heat demand is accomplished, often by means of a weather compensator. This is implemented by means of bypass systems with a pump in the flow or the return pipe of the heating installation and a twoport motor valve in the return pipe.
Quantitycontrolled as well as temperaturecontrolled systems are equipped with a differential pressure control. In this way, a noiseless and wellfunctioning installation is achieved.
In indirectly connected house stations the heat is transferred from the DH system (the primary side) to the central heating system (the secondary side) via a heat exchanger. i.e. indirect connections do not prescribe any demands on the DH temperatures and water pressure, provided there is a heat exchanger corresponding to the actual operating conditions. Indirectly connected house stations win usually have a temperature control based on weather compensation. Typically, it is a thermostatic valve or motor valve at the primary return pipe of the heat exchanger which is regulated in accordance with the outdoor temperature.
Furthermore, a differential pressure controller is recommended in order to ensure the best operating conditions for the control valve.
Hot water tanks are usually equipped with internal spiral heat exchangers and protected against corrosion. The temperature may be controlled by means of a thermostatic valve with a sensor placed in the tank or by a return valve installed in the DH return pipe.
Flow water heaters can also be controlled by means of a thermostatic valve with a sensor placed in the domestic hot water flow on the secondary side.
Alternatives are to (1) use a pressurecontrolled valve which regulates the flow of district heating water depending on the consumption of hot service water, or (2) mix it with a builtin thermostatic valve. The thermostat keeps the hot water temperature constant although the flow temperature and differential pressure in the supply system vary.
Besides, the thermostatic valve ensures that the comfort temperature is maintained in the heat exchanger outside the comfort period. In addition, water is saved as the hot water is already in the heat exchanger when ready for use.
Water heaters for multistorey houses are normally designed after the same principles but according to the individual needs.
Another possibility for multistorey houses is to use a flow water heater combined with a tank. With this system. It is possible to heat the tank during a prolonged period and thus obtain an improved cooling of the DH water.
Very often, especially concerning the use of an indirect connection, the space heating system and hot water system are connected together in a prefabricated unit possibly with an insulated casing.
The advantage of prefabricated units is that it is more convenient to mount heat exchangers. security equipment, pumps, and automatics at a factory, thereby lowering costs.
Also, there is an ongoing Improvement of the consumer connection systems In other ways. e.g. various computer programs are being used for designing and equipping the heat exchangers, valves, etc, and of connections as a whole. The manufacturers them selves are taking the lead in this process in cooperation with the DH plants, the consultancy firms, the Centre of District Heating and others. Several more general R&D projects and tests have also been carried out.
The main principles of settling the accounts of district heating are that each consumer is to pay his or her proportionate share of the expenditures necessary for operating the district heating plant and yielding a return on investments. Practical and financial considerations, however, require different ways of following this principle.
Normally, the following elements are part of the settling account:
Connection fee
Fixed, annual fee
Variable fee depending on the heat consumption.
The fixed annual fee is normally chosen to be comparatively low and the variable fee comparatively high in order to stimulate the consumers to reduce the consumption of energy.
Measuring the consumption for determining the variable fee can take place
by:
Volume measuring (m3)
-(kWh) or a combination.
A m3measurement settles the account from the consumption of hot water (independently of the cooling).
A kWhmeasurement settles the account from the consumed heat energy, which means a dependency on both the water consumption and cooling.
In Denmark. the number of DH plants using energy measuring is increasing considerably partly because this method is fairer to the consumers (the registered consumption does not depend on the flow temperature from the plant but on the real consumption), and partly because it allows the DH plant to lower the flow temperature which reduces heat losses and improves the possibility of using alternative energy sources for heating the water.
Corresponding to the measurement principles decided upon either a hot water meter or a heat meter can be used.
The heat meter consists n principle of:
-a flow meter part
-a temperature meter part
-a calculation unit
which in new meters often are integrated in a compact unit.
For small hot water and heat meters one of the following flow meter principles are usually applied :
-Impellers:
Rotating impellers actuated by one or more water streams. The actuatlons of the impeller are transferred to the meter via a counting device or by a magnetic device.
Turbine (Woltman):
Rotating impellers, but in contrast to the impeller meters, the axis of rotation is placed in the middle of the water stream and in parallel with it
-Fluidistor:
The instability of the fluid beam when an obstacle is passing is utilised by a ball actuated at the measuring point. The actuations of the ball, being proportional to the water speed, will be transferred magnetically.
MagneticInductive:
Measuring the voltage induced by actuating a conductor in a magnetic field. As water is electrically conducting, a voltage is induced corresponding to the flow speed .
Ultrasonic:
Ultrasound Is emitted and. e.g.. the time delay following the speed of the water Is measured.
The first three flow meters more or less operate mechanically. They can however be equipped with a device to magnetically transfer the signals. The magnetic inductive and the ultrasonic meters are fully electronic. Furthermore, it should be mentioned that a recent production of an improved fluidistor meter has been started up. Here a pressure sensor rather than the ball is actuated.
The heat meter calculations are today always fully electronic.
Because of the need to manufacture heat meters of high reliability and long durability, the trend in general is to design these units with no or very simple mechanical elements.
In Denmark. heat meters for DH are type tested and approved and installed meters are regularly calibrated. This is in accordance with a departmental directive and technical directives dating from 1989.
Finally, it should be mentioned that in multistorey buildings with separate consumers or other residential properties with several apartments. DFI meters for the complete buildings are often supplemented with meters used for apportioning individual consumption. Such devices are either evaporation types or relatively simple electronic meters.