By Jan Eric Thorsen, Director, Danfoss Heating Segment Application Centre; Dr. Marek Brand Application Specialist, Danfoss Heating Segment; Dr. Oddgeir Gudmundsson Director, Projects, Danfoss Heating Segment
The substation is a parallel indirect system, as shown in figure 1. The heating system was weather compensated, where the heating flow temperature, T22, is a linear function of the ambient temperature, Tamb. Details on data measuring devices are not shown on figure 1.
The case example presented, focusing on a building substation, reveals the positive and needed effects on applying heat exchangers with increased thermal length going towards the next generation of DH systems, for increasing the energy efficiency and reducing the temperatures in the district heating network. The concept of district heating (DH) has a bright future; the DH concept is an enabler for the demands of a flexible and renewable based energy infrastructure of the future. However, to maximize its potential the DH industry must have a vision for improvements and develop the technology. For instance, there are several challenges that have been identified when going from the 3rd generation DH to the 4th generation DH, including the focus on minimizing bypasses, effect of individual metering, lack of systematic monitoring and problem correcting, thermal length of radiators and heat exchangers, effect of automatic balancing on building level and holdup time and volume of domestic hot water (DHW) related to risk of bacteria growth. One of these identified challenges is addressed in this article: the heat exchangers applied in the building substation. By applying a longer thermal length of the heat exchanger, a better cooling of the supply can be achieved, both for the heat exchanger for space heating and DHW preparation.
Figure 1: Scheme of DH substation used in field case.
In figure 2, the installation is shown, including the heating part and the DHW part. The lower pictures show the heat exchangers applied for the two systems. The heat exchangers had the same main dimensions, which is appropriate for one-to-one replacement, but a different thermal length due to pattern design. The heat exchanger platformdimensions are 320x95mm, which is common for this kind of application.
THE HEAT EXCHANGERS IN THE ONE-FAMILY HOUSE SUBSTATION:
The focus was on the space heating and instantaneous DHW heat exchangers placed in the DH substation of a one-family house, located in the southern part of Denmark. The aim was to verify the reduced DH return temperature as a result of increased thermal length. Two measuring series were made. The first one in the winter where measurements were made directly at the heating circuit heat exchanger. Hereby the influence from the DHW system was eliminated. The second series was made during the non-heating season and focused on the DHW part. The building for the field test installation was built in 1979, with an area of 224m^2 and a total heating consumption of 13.3 MWh/y, a DHW consumption of 2.9 MWh/y, resulting in total DH demand of 16.2 MWh/y. Inhabitants were 2 adults and 3 teenagers. The house had to a large extent two-plate radiators with internal convection fins. Energy renovations had been made previously on the building envelope, including increased insulation and replaced windows.
Figure 2: Pictures of the DH substation installation and heat exchangers, to the left heating, to the right DHW
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