Already in the beginning of the 1960´es, Denmark had more than 50 years of experience in using district heating and the development was now strengthened by the fact that some Danish suppliers launched the idea of town development with district heating based on a low supply temperature (up to some 90°C). Hereby the district heating stations and the distribution network would become cheaper. Also on the operational side this led to savings since the demands on personnel etc. did not have to live up to the regulations for hot-water plants or power stations.
The pipes of the first piping networks were made of iron and insulated with cellular concrete, but this turned out to cause corrosion of the distribution pipes, especially in cases of the nets being placed in poorly drained soil. In order to fight this problem, the pipes were insulated with mineral wool and hung up in concrete ducts, so that they were secured against humidity. However, this solution was expensive, which made it suitable only for heat supply to major heat customers such as hospitals and industries with considerable heat consumption. Therefore the development of new district heating systems in housing areas therefore partially came to a halt for a period.
The breakthrough for the heat supply of the housing areas came when a number of Danish companies developed the pre-insulated district heating pipes. Here the iron pipes from the factory were covered with a heat insulating layer of polyurethane finished by a dense non-corrodible plastic jacket. The problem with this solution was how to obtain a completely tight assembling of the pipes; but with a special technique where the joints were assembled with muffs that had been welded or screwed on, it proved a success as well. In addition, electrodes were built into the insulation layer of the pipes, which made it possible to discover any intrusion of water and identify a leak so that it could be repaired relatively easy.
Initially the plastic jacket allowed only low temperature, with flow temperatures preferably not exceeding about 90°C. Soon the material was improved and the allowed temperature was raised to 120°C. The low operation temperature made it possible to utilise the surplus heat from industrial enterprises, and not least from waste incineration plants for solid waste, which in Denmark at that time was most often based on heat disposal by means of hot-water boilers. The combination of these two facts caused a boom in the establishment of district heating networks and waste incineration plants. Thus the first post-war waste incineration plants were built at the beginning of the 1960´es.
The energy crises boosted the development in Denmark
It is important to note that district heating in Denmark developed in the same manner as in other countries before the energy crisis hit the western countries in 1973/74. At that time, energy consumption per inhabitant was extremely high. The outcome of the crisis was, however, that it became necessary to save energy in Denmark, including energy for space heating. One reason for this was that the heating budgets were all of a sudden multiplied, and another was that Denmark at the time imported almost 100% of all the fossil fuels required to generate heat.
Therefore, the Danish government was forced to devise methods for saving fuels in order to safeguard the interests of society and to reduce the consumers´ heat bills. A number of initiatives were launched, including:
- Systematic planning of the heat supply in all areas
- Greatest possible CHP generation on the basis of such planning
- Insulation of buildings
- Development of highly effective district heating pipe systems, pre-insulated pipes with high insulation properties and low installation costs
- Reduction of operating temperatures in district heating systems and a variable flow in the pipelines to secure the most economical operation in the overall production and distribution system, while at the same time allowing the possibility to apply pre-insulated pipes
Due to a firm energy policy and cooperation between central and local authorities, heat supplying companies and private companies, considerable results have been achieved in Denmark during the period since the energy crisis. This has, among other things, meant that here at the beginning of the 21st century, the energy requirements per inhabitant for space heating have dropped a little more than 50% of the requirement in 1973.
In the fuel area, a number of initiatives have been launched. In the large cities, former small areas with their own heat supply have been consolidated into major overall areas, in which heat can be distributed everywhere through integrated networks from CHP units, waste-to-energy systems, industrial enterprises, etc. The existing heat plants are mostly maintained as peak and reserve load units which will start to operate in critical supply situations.
Vestforbrænding, the largest waste-to-energy plant in Denmark handling more than 700,000 tons of waste anually
In rural areas with as little as 250-500 inhabitants, so-called decentralised CHP plants have been established. They are designed on the basis of the heat demand and connected to the overall power supply network. This means that in principle, these systems are only in operation when heat demand makes it necessary, and since almost all of them are equipped with heat accumulators, the overall power and heat generation can be controlled and optimised. The plants are fired by means of natural gas, straw, wood waste, biogas and other indigenous fuels, which only affect CO2 emission to a limited extent.
The wood-fired CHP-plant in Assens supplies approximately 5,400 inhabitants of the community
Zoning - regulating the supply of gas and district heat
In 1979, a new heat supply act was implemented which started a heat planning process in the municipalities with the aim of introducing a complete new infrastructure for domestic natural gas to cover 15% of the heat marked (by direct supply) and with the aim of raising the district heating market share up to around 60% in year 2000.
The central authorities issued guidelines, supervised the planning and approved the plans, and the municipalities did the planning in collaboration with energy utilities and consultants.
The main result of the heat planning was a least cost zoning of natural gas networks and district heating networks to substitute individual oil boilers. Another result was the zoning of new integrated district heating transmission systems to supply local systems from large CHP plants and waste incinerators.
Example of a least-cost zoned area between gas network to one-family houses (green) and district heating supply to large buildings (red)
As district heating networks are much more capital intensive than gas networks, districts with sufficient heat density could have a district heating monopoly network for heating and a gas network for cooking.
A typical objection to zoning is that it eliminates market forces. However, this need not be true. On the contrary zoning of the modern district heating system, which is regulated by a flexible national least cost planning, will make even better use of the market forces. About zoning in general it can be concluded:
- A natural gas network in a city district will link the district to one strong monopolist, namely one supplier of gas, which delivers gas in accordance with the heat load curve. Even when the gas market opens, one may question the real competition as there will be only few dominating suppliers, often integrated by cross ownership and other measures. This will, in fact, reduce competition.
- A district heating network in a city district will open for real competition among all suppliers of all kinds of fuels and surplus heat sources, summer, winter, day and night. As a special case the district heating network with heat accumulators will form an efficient link between the power and the gas market, but only if there is a stable and fair competition in both networks.
In the municipal heat planning, the zoning can be regarded as a result of a co-ordinated competition between the district heating company and the gas company. The company, able to offer the alternative that best meets the objectives of the national energy strategy, such as lowest long term costs, high security of supply, low environmental impact, use of local resources etc., will win the competition. That will give the basis for monopoly of heat supply in the district in question in at least a period equal to the economic lifetime of the investment. That could, e.g., be 25 years for district heating and 10 years for natural gas. The alternative would be wild competition, duplication of investments, bankruptcy and loss of assets.
Today, the objectives of the planning are fulfilled. Denmark is a net exporter of oil and gas, and the consumers can easily afford to pay the real costs of heat.
The example from Copenhagen
Copenhagen, the capital of Denmark, had to enter entirely new ways of thinking when the planning started back in the beginning of the 1980s. This meant that five municipalities, which differ widely in sizes as well as in political views, agreed to join forces in order to comply with the requests of the government. This was achieved even though their experiences with district heating were also very heterogeneous; thus two of the participating municipalities did not have any consumer networks established at all on beforehand.
From the outset, some district heating systems were already established in the central part of the Greater Copenhagen area, which were supplied by a variety of small and medium sized plants each connected to its own pipeline network. At the same time three large power blocks were located on the island of Amager, but due to the specific geographical conditions it was not possible to utilise the full capacity on the heat side, i.e. to extend the field of activity beyond the borders of the island. A link was thus missing which could connect the different district heating networks and which could also utilise the full capacity of the large power blocks. The link that was found, and which has afterwards been established, was a so-called inter-municipal or regional transmission network.
Installation of transmission pipelines in Central Copenhagen
Fundamentally, a transmission network is a connecting link between the local district heating networks and the production plants. The design concept of a transmission system thus implies that production, transmission and distribution are separated into three different areas of operation. This means that the transmission company buys heat from the production units, transports it through the network and sells it to the partner municipalities, which on their side take care of the further transportation to the individual consumers.
One of the major advantages of this design concept is that the transmission company can choose freely among the various production plants due to the integrated structure of the system. The choice is among others based on which production plant is the cheapest to take into operation at the moment, which one is the most suitable to cope with the environmental directives prescribed by the government and which one should merely be operated occasionally and be given status as peak load and reserve unit.
What has been experienced in the Copenhagen area is that district heating is an incredibly flexible system both with regard to choice of production plant and with regard to the fuels used at the production plants.
In this connection, it has also been possible to kill two birds with one stone. As in most metropolitan areas all over the world we are faced with the problems of the increasing amounts of waste despite the best intentions of reversing this development. Recently, the capacities at the two large waste incineration plants in the area have therefore been extended. The heat produced during this process is sent into the transmission network. An increasing part of the waste incineration is producing combined heat and power. Looking at the figures for year 2003, waste incineration was thus able to cover more than 25 per cent of the total heat demand in the Copenhagen area.
Today, the Copenhagen district heating system is one of the world´s largest, supplying heat for a floor area of around 50 million square metres. The transmission system connects four CHP plants, four waste incinerators and more than 50 peak load boiler plants to more than 20 distribution companies in a one pool-operated system with a total heat production of around 30,000 TJ.
The transmission network in Greater Copenhagen
Technological development
Concurrently with the development and expansion of the district heating systems in Denmark, the industry supplying components and systems to the sector also carried out intense research and development of their individual products. The objective was that each component in the systems should provide more efficient operation and contribute to the overall energy saving targets. In fact, very early the industry became aware that if their products did not meet these objectives in a cost efficient way, they would not be able to compete as suppliers to the sector because the district heating companies would evaluate operational cost and efficiency equally with the price of the components.
During the past 25 years, the industry has provided results that have made it possible for the Danish district heating sector to achieve results appreciated world-wide. In short, few examples out of many can be highlighted just to illustrate the magnitude of the work carried out.
Pre-insulated pipes
The concept of pre-insulating steel pipes and covering the insulation with a water-resistant casing was invented in Denmark in 1960. The concept became a major success and abundant number of kilometres of pipes has been installed in Denmark and abroad creating the basis for modern cost-effective district heating systems.
Intense research and development activities in Denmark have, over time, provided better quality and even more cost effective systems. Today, pre-insulated pipes are buried directly in the ground, carried under seawater and fitted without the use of compensators or other stress releasing methods. The service life is calculated to minimum 30 years, but we will se pipes in service many years after this theoretical limit. Furthermore, it is interesting to know that many of the pre-insulated pipes installed in the 60s and 70s still operate satisfactorily.
Directly buried pre-insulated pipes under installation
Heat exchangers
Plate heat exchangers are widely used in Denmark to separate circulating water in different networks. It is an example of compromising investment and operating costs, with the objective of minimising pressure loss and simultaneously maximising the heat transfer coefficient.
Plate heat exchangers have the additional advantages of demanding less space than other types and being built in modular form. The capacity of a plate heat exchanger can thus be increased by adding more plates - an operation that can proceed in parallel with the expansion of the network. In this way, investments in heat exchanger capacity can be scheduled to follow demand.
Variable flow and speed-controlled pumps
To deliver heat in abundant quantities 24 hours a day is a simple task, but to deliver the exact amount of heat needed at any given time to all consumers requires modern equipment and operational skills. It is, nevertheless, the way Danish district heating systems are operated today.
The primary reason for using this concept is the large potential for saving power for pumping. Often the reduction of pump energy can be reduced to one third over the year, and the pay-back time on the installation is often less than two years. Another reason for using this concept is the reduction of heat loss from the network due to reduced temperatures outside peak hours. Furthermore, wear and tear of the pump system is reduced, resulting in prolongation of the service life.
Pumping station at the Avedøre Power Station in Copenhagen
Even in small house installations and apartment buildings, speed-controlled circulating pumps are used providing energy savings without losing comfort. In such installations, the pay-back time is short, and the reason is that the cost of small speed-controlled circulation pumps are low because they are produced in large numbers and due to the technological development of the electronic control unit.
Heat metering
In Denmark, it is mandatory to pay for heat and other utilities like water and electricity according to consumption. Therefore, all district heating installations are equipped with energy meters and heat allocators.
The installation of heat meters does not, in itself, bring along energy savings; it is the consumer´s awareness of his own consumption that motivates the consumer to consider how energy can be saved. Often simple measures are taken by the consumer to reduce heat consumption such as closing the radiator valve instead of opening the window, or avoiding excessive use of hot water, but it is important to remember that the energy savings are initiated by the consumer´s own awareness about his consumption.
One way of measuring heat consumption of a multi-storage building is to install a heat meter at the point of heat supply to the building, and place heat allocaters at the radiators in the flats
Modern energy meters are provided with facilities for remote reading. This is widely used by the district heating company to conduct better planning of the operation and to give alarm signals when pre-set values are transgressed, e.g. through leaking. Remote reading also provides facilities for the consumer who can follow his own consumption via the internet.
