Danish DH dates right back to the turn or the century, and at the same time use was first made of otherwise wasted heat from the expanding electricity production and from the incineration of burnable waste. DH is most suitable for exploiting surplus heat which would otherwise be wasted and permits firing with fuels which would be technically difficult for use in direct heating.
The transmission and distribution system comprised insulated steel pipelines laid in concrete ducts (see Fig. 1). It was not until the 1960s that preinsulated piping came into widespread use, mainly with inside steel pipes and outside plastic pipes. Large diameter pipes are manufactured also with outside steel pipes. During recent years preinsulated small-diameter pipe systems with two plastic tubes have been Introduced to the market.
From the beginning of the 60s the preinsulated pipe systems have been the main type used, except in the case of very large- dimension piping. Heat loss has thus been reduced considerably. In addition. experience has shown that the maintenance costs of preinsulated piping are very low compared with concrete ducts.
Consumers were in most cases directly connected. i.e. no heat exchangers were fitted between the DH water and the water in a radiator system. This was a simple and cheap solution, and at the same time the temperature decrease in the indirect, heat-exchanger type of plant was avoided. On the other hand, the pressure in the radiator system is forced to be higher, increasing the risk of comprehensive water damage should breaks occur (see Fig. 2).
Costs were allocated to consumers as a fixed tariff, calculated on the basis of building Moorage, plus a variable tariff based on the quantity of heat delivered, determined by measuring the water volume circulated through a consumer's heating installation. This form of measurement was cheap and motivated consumers to install large radiators which could cool the heating water to a low return temperature. This reduced the heat loss from the main network on the street and the water flow in the system
In the period 1955-1973, more than 200 new. independent DH networks were established all over Denmark. During World War II it had been difficult to get hold of any fuel at all, neither coal nor coke was readily available, instead fuel- wood and peat of a poor quality had to be used. From the mid-50's oil supplies stabilised. and the price of oil rose much less than inflation until the first oil crisis in 197.?. For 15 years just prior to the oil crisis the energy supply all over Europe was mainly oil-based. In housing, radiator systems were Installed together with hot water tanks providing kitchen and bathroom with hot water. In most houses in Denmark, individual oil-burning boilers were installed with their own chimneys and oil-storage tanks. This required a major investment, but it also improved the living quality of the occupants
However, in 200-250 of the small and middle-sized towns in Denmark DH schemes were introduced instead. Thousands of individual oil-burning boilers, oil tanks, and chimneys were replaced by a single, large supply plant with a DH network laid out underground for distribution to all consumers.
The aggregate investments in these two respective models were about the same, but with DH two major advantages were gained: (I) these large plants could utilise heavy fuel oil, which was relatively cheap, whereas the individual oil burners were limited to the more expensive gas oil, and (2) the large central plants were able to switch over relatively easily to other Fuels in case of failing or very costly oil supply.
During 1980-84. approx. 60 DH plants were converted to coal firing. Coal was so much cheaper than oil at the time that the payback period for this conversion was no more than 1-3 years. Subsequently, it was possible to reduce heating tariffs by 20-30%. There were also major environmental advantages in converting to coal, a factor which has seldom met sufficient acceptance. In the technical sphere, the misapprehension prevailed that oil was always cleaner to use than coal. Today, environmental loads gener ally are less in firing oil than coal, considering oil and coal qualities and modern burning techniques. However, considering the situation in the early 80s it has to be taken into account that the existing oil-fired plants were approx. 20 years old and used oil containing up to 1.5% sulphur, whereas coals usually contained less than 1% sulphur. When the new coal-fired plants were fitted out with very efficient bag filters there were. in practice, no problems from particles and dust, whereas at certain times there were serious problems with polluting fall-out of soot particles from old fuel oil-fired plants
In the years following the first oil crisis in 1973 it was decided to expand the DH supply on a large scale. This was based mainly on a greater utilisation of surplus heat from the large power stations. Originally, only buildings close to power stations were sup plied, but the DH networks were gradually extended to take in areas at a greater distance.
With the oil price increases in 1973 - and particularly the expectancy that energy prices would rise sharply in the following years - there was a basis for a very substantial expansion of DH produced by power stations. In many areas where heating was provided mainly by individual oil-burning boilers. DH schemes were laid out and large-scale transmission pipelines established. As Indicated above, factors of economy and security of supply were the principal considerations that spurred the strong expansion in utilising surplus heat. In recent years. however, improving the environment has gained evergrowing importance. The development of DH and utilisation of surplus heat from power production are shown in Fig. 3.
As a result of the 1973 oil crisis, the Danish government decided in June 1979 to commence exploiting the oil and natural gas resources in the Danish sector of the North Sea and introduce natural gas supply in Denmark. However, it quickly proved more difficult, and with a time horizon that was longer than originally anticipated, to establish a natural gas scheme which could take over a major part of the country's energy supply. In the 80s this conversion process was prepared by the municipal and regional planning following the 1979 Heat Supply Act. As part of a scheme to reduce the CO; emissions the Danish parliament decided in 1990 that all DH plants with a capacity of more than I MW were to be converted within the next 8 years either to combined heat and power production based on natural gas or to DH production on biomass.
In 1992 there are about 60 DH plants utilising straw, and some 40 utilising wood- chips. As one result of the present, substantial energy taxation on oil and coal, the "equivalent taxation" of natural gas I. and the non-taxing of straw and woodchips, the price of heating from the latter DH plant types has become extremely competitive. even in smaller plants. The distribution of production types in 1990 is shown in Fig. 4. An overview of capacities in Danish DH plants is given in Fig. 5.
As indicated in Fig. 4, in 1990 approx. 16% of DH supply was based on the direct burning of natural gas or coal (not counting the CHP production). Within the next 3-6 years most of these DH systems will be converted to CHP plants running on natural gas alone. This development can be seen as a continuation of the expansion of the DH networks coupled to the major coal-fired power stations, which progressed from the mid-1970's to the mid-1980s. In this ten-year period Denmark achieved a substantial. aggregate fuel saving via a wider utilisatlon of cogeneration of heat and power from large power stations. In the coming year!, this development trend will continue with cogeneration of heat and electricity in the many DH plants which today are limited to heat production. There are a number of reasons why this development has not started until recently:
Plants should be dimensioned so that a CHP unit can supply approx. 90% of the annual DH demand. The remaining 10% is usually produced by a gas-fired standby and peak- load boiler unit. Under Danish conditions this requirement corresponds to a cogeneration plant of capacity 60-70% of heating demand on the coldest hour of the year. The price of power delivered to the main grid depends on whether day or night delivery is made. In the morning and early evening hours, when electricity demand is at its maximum, the price is highest. At night, when demand Is small, it is lowest. This means that DH companies can gain the highest possible average price for electricity if a heat storage-facility is included as part of the plant. Thereby, the plant can always run at 100% load, to attain its highest efficiency level, and only at hours during which the price of electricity is highest. The storage facility or heat accumulator is a large-scale water tank in which hot water is stored. The accumulator normally can hold the accumulated heat production of the CHP unit that has been run at full load for approx. 10 hours. Investments in a complete, turnkey gas- fired cogeneration plant sized at 0.5-40 MW total approx. between DKK. 6.5 and 4.5 mill. per MW. Investments per MW are highest for the smaller plant and, of course, also depend on the location of the plant and its architectural quality. All the plant types mentioned will achieve an overall efficiency (power + heat) of approx. 90%. The smallest gas combustion engines or gas turbines will attain an electricity efficiency of 30-33%. and engines over 2 MW will reach 36-39%.. A combined-cycle plant under 50 MW will achieve an electricity efficiency in the range 40-44%.
Running costs for lubrication oils, maintenance. and personnel at a cogeneration plant total approx. DKK 0.02 per kWh electricity produced by either gas turbines or combined-cycle plants. In a gas combustion engine plant the corresponding costs are approx.. DKK 0.06 per kWh. At current Danish electricity prices, the higher electricity production of gas combustion engines more than outweighs their higher running costs. It is therefore expected that the majority of the many CHP facilities below 10 MWe to be established in the coming years will be gas combustion engine plants.
The relationship between extra fuel costs and revenue from the sale of electricity when a plant is converted from heat production alone to cogeneration is, of course, decisive for the economic feasibility of the particular project. Under Danish conditions the marginal fuel consumption would correspond to a an electricity efficiency of approx. 80%.. This means that for every extra GJ fuel purchased. 0.8 GJ electricity can be sold to the grid. With the current relationship between fuel and electricity prices this is more than sufficient to cover the extra capital and running costs involved. Because of this high efficiency, completely new DH networks have been established that are based even on natural gas-fired cogeneration. these serve in towns with only 300-500 people. It does not seem unrealistic that new DH networks in upwards of 50-100 towns with a total of 10.000-15.000 households will be established within the next 5 years. p<>With this trend it can be expected that be fore 2000, approx. 55% of the heating in Denmark will be district heating. Of this. about 70%, will be heat generated in the combined heat and power installations.
The District Healing Information Foundation was established by the Danish District Heating Association and suppliers to the DH sector, and has approx. ISO members today. The aim of the foundation is to sup port each DH company in its endeavour to obtain the greatest possible number of connections to DH by means of sales and in formation materials and participation in connection campaigns.
The Danish Board of District Healing consists of approx. 40 members: these are consulting engineering firms, manufacturers. contractors, and service industries as well as several municipal heat supply utilities and the two Copenhagen heat transmission companies. The object of the Danish Board of District Heating is to further the exportation of Danish know-how and Danish products for DH. This takes place primarily through information about the large fuel-savings and environmental advantages that Denmark has obtained through its extension of DH throughout the country
S.E. Perning. Danish Board of District Healing Jernbanevej 65. DK-5210 Odense NV