Figure 1: Existing apartment buildings connected to the Viborg district heating network, Denmark
Normally, the total energy delivered for space heating during very low outdoor temperatures (below -5 °C ) only accounts for 3-5% of the total share. Hence, in line with 4GDH requirements, it will not be an issue for district heating operators to increase the supply temperatures in the networks only for a limited number of hours while maintaining a low supply temperature profile for most of the year. How does the digitalization of the demand side help our understanding? Historically, the absence of comprehensive data from substations and heating systems led to buildings being viewed as ‘black boxes’. This perception hindered the advancement of strategies for optimal control and operation of these systems, restricting the potential for achieving low-temperature operation. While the degree of digitalization on the demand side varies among countries, implementing the new European Energy Efficiency Directives (EED) in recent years has accelerated the adoption of remotely accessible digital devices. This development has led to innovative monitoring and control, facilitating low-temperature operations and enhancing end- user billing transparency. It is expected that all energy meters and submeters (heat cost allocators for space heating and hot water consumption) in large buildings with central heating or connected to district heating networks will soon be remotely readable. Lessons learned from heating system operations in existing apartment buildings in Viborg The basic knowledge is that to heat a flat, all available radiators must be in operation and controlled locally with thermostatic radiator valves to secure the expected indoor temperature according to the end user's preferences. However, analyzing data from energy meters, heat cost allocators mounted on each radiator ( see Figure 2), and a few temperature sensors allowed us to gain insight into the standard ways of controlling and operating the space heating systems in apartment buildings. It was found that, on average, 30 to 40% of the radiators are normally closed and not active.
Figure 2: Typical radiator in apartment buildings with thermostatic control valve and heat cost allocator
The main reasons can span from the end-users believing they can reduce energy consumption to having unoccupied flats in the buildings. The effects are the same independent of the cause. First, this leads to a non-uniform heating distribution among flats. Flats with fewer operating radiators and different indoor temperatures can enhance heat transfer from neighboring flats, “stealing” heat from each other. But, if the outdoor temperature drops and the heat transfer from adjacent flats is reduced or the supply temperature is controlled under the assumption that all radiators are in operation, increasing the number of active radiators is the only way to maintain the same comfort. This was evident from an ongoing test in one of the buildings with 156 radiators. It was found that during 16-18 December 2023, the average outdoor temperature in Viborg was around 8°C, and the number of radiators in operation was 94. Whereas, during 5-8 January 2024, the outdoor temperatures dropped to - 9°C, and the number of radiators in operation increased by almost 10%, as highlighted in Figure 3. Secondly, fewer radiators heating the entire flat inevitably increases the overall return temperature because the valve controlling the flow in the radiators will open more (or fully) to
16 HOTCOOL no.5 2024
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