Danish District Heating - Lessons Learned
In Denmark district heating (DH) has become a natural
part of the infrastructure of almost every city. Most of the 5
million Danes consider a district heating connection to be as
commonplace as the connection to telephone, electricity, water,
and sewage systems.
As in many other European countries. district heating was introduced in our larger cities and towns in the mid-1920s. By then waste heat was recovered from local electricity works and distributed to town halls, rail way stations and other buildings In the city centres.
In 1950 the first DH plants based on heat- only boilers were put into operation. Till then. DH systems were initiated and operated by municipal organisations. In the 1960s a large number of similar plants throughout the country were initiated, however, by groups of individuals. They saw the benefits of district heating as opposed to individual central heating.
Following the oil-crisis, a national policy was formulated and gradually put into force to reduce the dependency on imported oil. Increased use of district heating based on combined heat and power production plants and conversion of existing and new power plants from oil to coal were some of the important means. In these years DH plants are converted to combined heat and power plants based on indigenous natural gas and biofuels.
In 1979 the parliament passed the so-called Heat Supply Act, which required every municipality to analyse its energy supply in detail. So-called heating plans, which were very similar to what we know as master plans today, were carried out. A number of alternatives were set up and compared with the results expected by a further development of the previous energy supply structure. The consumption of various types of energy as well as economic and environmental effects of the alternatives were thoroughly investigated. A planning period of 20 years was considered.
District heating had already played an important role by 1973, but it was almost entirely based on oil. Typically, further extensions of district heating were selected as the most feasible options in the cities and larger towns, and wherever possible, a combined production of heat and power (CUP) was introduced to derive the maximum amount of useful energy from the burned fuel. Also surplus heat from industries and waste incineration were tied into many systems, and in re cent years many different kinds of renewable energy supplies have been included.
Although for many years it had been quite normal to register the consumption and charge the use of district heating by means of water volume meters, there still existed systems where the settlement of consumption was made according to the total area being heated. Most people found this method to be acceptable as long as energy was cheap, but when prices soared, the demand for individual metering arose. It is a well-known general rule that total consumption goes down markedly when each consumer must pay for his or her actual consumption.
The present trend is that a number of the previous simple and inexpensive volume meters are being exchanged with the more accurate but costly heat-energy meters.
During the past 20 years, we have gained very much from the extensive use of DH in Denmark. We have improved our balance of payment by importing far less fuel and have improved our environment by burning less fuel to meet our heating and power needs. And we have learned a lot! This can be summarized by 6 key items:
Heating private homes, offices, and Institutions to approx. 21°C and providing hot water at about 55°C does not require high temperatures In the DH networks. The great majority of Danish systems operate at temperatures of around 80°C, and the trend for the future is to bring this temperature closer to the 55°C level actually needed. Both energy resources and money can be saved by doing so.
Low-temperature DH favours the use of any source of surplus heat. As noted earlier. in combined heat and power plants far more total energy (heat and electricity) can be extracted per kilogram of fuel than from separate plants for heat and power. If the temperature requirement were lowered, a greater proportion of electricity could also be generated.
There are many reasons why it is possible to operate district heating at low temperatures In Denmark. Although some of them cannot be transferred to other countries. It is always worthwhile to aim at operating a DH system at the lowest possible flow and return temperatures.
Figure 1 shows the result of a test operation in
which the forward temperature was reduced. Obviously, in order
to provide the required amount of heat, the flow at lower temperature
had to be stepped up. This required more energy consumption by
the pumps, but the reduced heat losses through the pipes more
than made up for this increase in pumping energy.
In many countries, the output of district heating systems is controlled by varying the temperature in the supply pipes. As the demand increases, the temperature is raised while the flow is held constant.
In Denmark, however, the temperature in the supply pipes is held fairly constant, while the flow is automatically kept in step with the demand at any time.
Figure 2 illustrates the mechanism. When radiators
are opened either manually or automatically by means of thermostatic
valves the differential pressure in the network will change. This
is detected by automatic devices in the network, and a message
is sent to the pump to increase the number of revolutions. The
adjustment to the increased load takes place immediately.
The traditional use of relatively low temperatures facilitated the development of preinsulated pipe systems. They were Invented in the late 1960s. and during the 1970s were developed to the very high standard we know to day. Provided they are assembled according to their specifications, which can be learned by any skilled Otter, such pipes represent a high degree of reliability and cost efficiency.
This matter of cost efficiency Is Important In Denmark. Unlike other countries', many of Denmark's district heating networks are established in low-heat density' areas - typically in districts limited to one-family houses. As the networks represent by far the largest share of the total Investment, the costs of procurement, installation, and maintenance of pipelines must be kept as low as possible. This has been an important consideration in developing preinsulated pipes.
Avoiding high temperatures and pressures in the networks also reduces the physical impact arising, for instance, from thermal expansion of the steel. Add to this that temperatures are kept fairly constant throughout the operational year, thus sharply limiting the risk of damage to the network. Both of these facts are important in holding construction and maintenance costs to an acceptable level.
Pipe leaks cannot be avoided entirely, and consequently it Is important that operators are trained to deal with such events. Interruptions In the heat supply should not exceed 6 to 8 hours, and in most utilities this commitment is adhered to.
The general acceptance and popularity of district heating in Denmark also testify to the high degree of reliability which has been achieved.
60% of the heat production for DH in Denmark is now derived from CUP plants. As noted earlier, a number of existing power plants have been converted to CUP since the mid-1970s. Almost all new power plants in the past 15 years have been CUP units, and a large number of such smaller, so-called decentralised units are being planned for the future. The share of heat from combined plants is going to increase further. Another 20% of DH production is made up by local, non-fossil Fuels mentioned below.
Biofuels like surplus straw from farming and wood chips from forestry are gaining ground as replacements of fossil fuels. More than 50 DH systems are benefiting from these energy sources, which, unlike coal, oil and gas, are neutral to the emission of CO2, as only the amount absorbed during the life time of the vegetation is released. This amount will have been released to the environment in any case whether digested in nature or combusted in a boiler.
The utilisation of waste heat from industry is another example of rational use of local resources in DH networks. As is the case with CUP, the fuel is used efficiently, and consequently the pollution levels will be lowered. Wherever possible, energy is "recycled" in Denmark to the economic benefit of both industry and the DH consumers and to the nation at large in the form of a less polluted environment.
An important local fuel supply is the waste which cannot be recycled and therefore is available for incineration.
The last 20% of DH production is made up by burning oil, gas, and coal in conventional combustors. In the years to come coal will be substituted by gas CUP and biomass DH.
Comparing a CHP plant producing the same amount of
electricity as a condensing power plant, additional fuel has to
be used at the CHP plant. In terms of energy the additional fuel
roughly equals half the energy needed to produce the hot district
heating water. The other half is extracted from the cooling water
otherwise wasted. Therefore, as a rule-of-thumb it can be said
that the primary energy for producing one heat unit in a CHP plant
is only half of that in heat- only boilers. It can be calculated
that out of the total production of district heating in Denmark
- in 1990 approx. 89.000 TJ - only about half was generated by
means of direct burning of fossil fuels. The rest originated from
"recycled" energy or local non-fossil fuels. Figure
3 shows the "energy balance" of the Danish district
heating sector in 1990.
in 1980 and 1990.
Going ten years back, more that 50% of DH production in Denmark was based on oil. An essential goal of Danish energy policy since the 70s has been to reduce oil consumption. The district heating sector has been very successful in this respect: Today only 2.5% is oil-based. Increased use of mainly combined heat and power production but also waste, biofuels and natural gas, has played a role in reaching this objective. Figure 4 shows this successful development, achieved in only a single decade.
And no doubt this development will continue. Presently,
a large number of new combined heat and power schemes are being
introduced. A large investment has been made in the infrastructure
to establish distribution systems all over the country. Only the
production units will have to be adjusted to
new energy supplies, and an additional number of
current consumers can still be connect ed at limited cost. Figure
5 shows how the share of DH in providing space and water heating
has grown from 1986 to 1990.
Twenty years ago, approx. 685 MJ per m2 and year were used to supply the heating of homes in Denmark. In 1990 this figure has been reduced to 361. There are many reasons for this, but it is also a result of the desire to reduce the dependency on foreign fuels and improve the environment.
Increased use of district heating supplied by CUP has been one of the important measures ures taken.
The trends of the previous 40 years in the district heating supply of the city of Herning shows the development quite dramatically (see figure 6). Until 1973 the number of consumers and total consumption grew steadily.
When the prices of oil rose sharply in 1973 and 1979, the consumers reacted immediately and reduced their consumption. On a longer term various means (involving both information, incentives, and investments) introduced and supported by the government, have enforced and consolidated the development. The number of consumers has grown steadily over the years, and the coverage of DH within the supply areas of Morning, as in many other cities of Denmark, is almost 100%.
The figures for Herning prove that DM, especially when CUP is used, helps provide a less- polluted environment. Figures 7, 8, and 9 show the development during the past ten years and indicate the expected trend over the next ten years.
In the future, a major effort will be made to increase the use of local biofuels.
During the early 1990s plants exploiting straw and landfill gas were introduced. Also a large-scale biogas plant utilising manure and organic wastes from both industry and households began operating and is still being developed, and a small-scale CHP plant based on gasification of wood chips has been scheduled to begin producing power and heat in 1993.
Had all of our houses been equipped with individual boilers for oil or natural gas, it would not have been possible to change the pattern of consumption as rapidly as has been the case in Denmark during the past decade.
The established infrastructure of district heating has proved to be very flexible and enabled this "revolution" to take place to the benefit of conserving energy resources and the environment. And it will go on!