Objective
The low water temperature in the district heating systems confers a number of important advantages: reduction of heat loss in pipes; significant reductions in investments (the specifications for components are less stringent, as expansion and pressure are reduced, leading to savings); consumers do not require heat exchangers when connected to the network;
possibility of using surplus heat (industry) or natural heat (geothermal and solar). At the same time, the practice of heat-power cogeneration allows optimum energy utilisation! with the benefit of economic and environmental advantages.
The municipality of Aarhus had already worked out that for each degree the temperature was lowered, about 1.3% could be saved in primary energy consumption. The new study's objective was to determine if, in networks that already have low temperatures, like Denmark, it was still possible to reduce the temperature by a few degrees, without affecting the service provided to the inhabitants.
The overall economic calculations show that for a village the size of Sabro, for a 15?C (from 80 to 653C) reduction in temperature of heating water, the saving on the global cost can be estimated at more or less two per cent. That is equivalent to a potential saving in the district heating budget of about DKK 40,000/year. The additional costs for the increased flow are insignificant. By reducing
demand is satisfied. Nevertheless, there are exceptions resulting from limitations due to the capacity of the heat installations of the consumers concerned .
To lower the temperature to 70?C, without any noticeable deterioration in the quality of service offered, it is not necessary to make any substantial investments. However, in each network. a relatively small number of installations determine the lower temperature limit, below which it is risky to go. The strategy to follow therefore consists of going ahead with a planned temperature lowering programme compatible with these "critical"installations . Beyond that, it is necessary to establish a balance between the estimated costs of proceeding with a temperature lowering programme (changing faulty installations) and the savings such a programme can bring. There is clearly no point, from an economical viewpoint, in saving on energy consumption when the transformations made by the consumer cost more than the savings.
In any case, if the experience gained in Sabro is to be transferred to the whole city of Aarhus, a 15?C lowering of the water heating temperature would mean a yearly saving of DKK 8.5 million (or 1.9% of the total budget, in 1990, of DKK 455 M), and of 5,600 tonnes of coal/year. If the theoretical transformations intended to achieve a better cogeneration are carried out at the CHP stations, savings will amount to DKK 20 M, equal to 25,000 tonnes/year.
The municipality of Aarhus has already begun to apply the lessons of this study. Initially, the water temperature has been reduced from 80? to 75?C.But there is agreement that it will not be possible to drop below 65?C, without running the risk of failing to offer a maximum quality service. On the other hand, according to the specificities of each district and their equipment, it is impossible to say at present whether the temperature of 70?C, which has been adopted for Sabro, will be tried elsewhere. This will be the subject for future studies, which will determine temperatures on an individual district basis.
More generally, the experience of Aarhus can be considered promising for the rest of Europe. In the Danish area, the temperature of 80?c was the starting point for experiments. But, in the rest of Europe, district heating plants use water at over 100?C, often above 140?C. In all likelihood, these plants will never be able to bring temperatures down to the level known in Aarhus, unless large investments are made. Yet, the potential for savings there is certainly much greater than in Aarhus.
In Denmark, the use of district heating is very important, presently accounting for 45% of the total Danish heat requirement. Over the next fifteen years, the political authorities plan to increase this public service to cover 60% or more of demand. As a result, in Denmark, heating is increasingly becoming as much a service delivered as water or electricity. A city like Aarhus, for example, supplies 84% of its 260,000 inhabitants with heat, which represents approximately 8,100 TJ/year.
Historically, district heating used to use steam as the heat-carrying vector, making its first appearance in the USA a century ago. In Europe, hot water systems are more common. However, the temperatures and pressures vary greatly from one region to another. Most plants have opted for temperatures between 100 and 180 deg. C, requiring high pressures to operate.
In Denmark, the temperatures used are lower: 80% of plants carry water of 82 deg. C or less and a maximum pressure of 6 bars. The municipality of Aarhus collaborated with its counterpart in Kiel, Germany, to do this study. To measure the feasibility and efficiency of such an operation, from both economical and environmental viewpoints, a wide-ranging experiment was tried out on the small district heating system in Sabro, a small village in the Aarhus region.
From October 1990 to July 1991, this site (650 households, a school, an institution, six industries) was used as a testing ground for tests to lower the inlet water temperature as a heat-carrying medium. For the test, all consumers' heating systems were fitted with energy meters.
system was gradual (one degree centigrade per week)and, a teach stage of the tests, all necessary measurements (water circulation, temperature, pressure) were recorded. This was done at consumer installations and also at the main station and in selected sources. The test was spread over three distinct periods:
- In Winter, the temperature was gradually decreased from 80 deg. C at first to65 deg. C.
- In Spring, the operational cycle was divided into two periods. During the first, the temperature was progressively lowered from 66 deg. C to 61 deg. C. During the second, it was kept at a constant 65 deg. C.
- In Summer, the temperature was reduced from 67 deg. C to 61 deg. C, taking into account the fact that there was no heat requirement -merely a need for domestic hot water.
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DEM - Demonstration contractsCoordinator
8210 ARHUS
Denmark