In the future, a higher focus will be on utilizing low grade renewable and waste heat sources to a larger extend. A common denominator for renewable low grade heat sources is that they tend to be difficult to access on a building level, either due to location, required investments or in general the economy of scale factor. To overcome the issues with both utilizing the low temperature renewable heat sources and the reduced heat demand of buildings, the DH networks can take advantage of the low-energy buildings and operate at even lower supply temperatures than commonly applied for 3rd generation DH systems. The reduced supply and return temperature not only increases the efficiency of the system but also increases the flexibility of the DH system towards potential new heat sources and towards the whole energy system.
The key characteristic of the 3rd generation DH system is the material and labor lean components applied combined with lower temperatures. The components consist of pre-insulated pipes buried directly into the ground and fixed without expansion loops, prefabricated compact substations, the use of compact brazed stainless steel plate heat exchangers and the use of material lean components such as combination valves. The 3rd generation DH is also referred to as the “Scandinavian” DH technology, since it was mainly driven by suppliers based in Scandinavia. Pressurized water is used as heat carrier, typically operated at temperatures below 100 °C. The 3rd generation DH was introduced in the 1970s and has been applied in almost all new schemes and renovation projects from approximately 1980. The motivation for this generation was the increased focus on lower construction cost and energy efficiency, e.g. triggered by the oil crises in the 1970s. Related factors were the security of supply aspect, where alternatives to oil found their way into the energy system. This included e.g. biomass and waste incineration. As a direct consequence of the oil crisis, CHP in Denmark became mandatory and was applied in many other countries as well. This boosted the deployment of DH heavily. The fuel mix has been altered by feeding in greener and renewable sources. The 3rd generation has made a huge contribution, and still has enormous potential, towards fulfilling the political energy goals. Currently, most DH schemes are 3rd generation systems starting a transition to the 4th generation to enable a future non-fossil and renewable based energy system. To meet these demands, existing DH schemes will develop towards focusing more on the integrated energy system, including buildings and integration of low quality fluctuating renewable and surplus energy sources. DH has to be seen as an integrated part of the future smart energy system, including district cooling, electricity and gas grids as well as buildings HVAC systems. A graphical comparison between the four generations of DH concepts / technology is shown in figure 1.
Low temperature DH cases in Denmark – example of 4th generation DH
A number of pilot projects have been made supplying existing and new low-energy buildings with low-temperature DH. One example is the DH system in Lystrup/Denmark. 41 row house flats are successfully supplied with 50°C DH without decentralized boosting of the temperature. The concept is to apply instantaneous domestic hot water heat exchangers with sufficient thermal length to be able to produce 47°C domestic hot water at a supply temperature of 50°C while they still have a low primary return temperature. For the heating circuit radiators were installed with sufficient area for operating at 50°C supply temperature at design load. The target was to operate the heating system at a primary temperature set of 50/25°C.
Figure 2: Low-temperature DH supply for an area in Lystrup/Denmark.
The outcome of the project was very positive with a thermal distribution loss of 17%, which is to be considered low, given the
design energy consumption of 6 MWh/year/flat. A contributing factor was the design of the DH network, which was designed for media speeds up to 2 m/s and the pressure head of 6 bars for the 766m trench length. In case normal design rules and temperature levels were applied, the thermal distribution loss would have been around 40%.
Figure 1: Illustration of the concept of 4th Generation DH in comparison to the previous three generations
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