From gas to geothermal energy

Abstract

The transition from fossil fuels to renewable energy sources brings along the reduction of the supply temperature of the distribution heating networks, because renewable energy sources usually provide heat at lower temperatures. This master thesis specifically focuses on the buildings because it is uncertain which adaptations are necessary to ensure thermal comfort when the supply temperature decreases. Especially in existing buildings, since they were originally designed for high operating temperatures. A case study was conducted by studying the performance of a non-residential building under different simulated scenarios to enable lower operating temperatures in the heating network. The objectives of the research are two: 1) to develop of a protocol for data collection and selection of promising adaptations that facilitates a case study on this topic and 2) to compare the observed effects of the different possible adaptations on the required operating temperatures in this case study with the results of the few other studies available. The most interesting results derived from the case study are that it is possible to lower the peak power, and thus the operating temperatures, in the current situation of the selected building, without any adaptation, from 78/70oC to 63/55oC. Assuming that the mass flow is decreased in order to achieve a bigger temperature difference between supply and return, operating temperatures can be 70/50oC. Mass flow control through radiators should then be implemented in order to obtain the lowest possible return temperature for a given supply temperature. During the implementation phase of the geothermal project, the implications of this measure need to be studied. When heat losses are small, smart settings in the building management system prove successful to lower the supply temperature further. In the controls and operation scenarios, adjusting the room temperature setpoints and the heating schedules enables even lower supply and return temperatures, 50/35oC. In old, poorly insulated buildings, minimal renovations in the buildings’ envelope are crucial (and often sufficient) to enable low temperature heating. When the building without renovations was simulated, it was not possible to lower the operating temperatures; instead, the required temperature increased to 90/70oC.
Changes in the energy system, such as the installation of radiators designed for lower temperature heating, also enable supply and return temperatures of 50/35oC. The case study building has a ventilation system with heat recovery and pre-heating of air to 19oC. A scenario is simulated with a system with only heat recovery. The ventilation air is introduced in the room at lower temperatures, and the heating system takes over the heat demand. For the same operating temperatures, 70/50oC, the heat recovery scenario suffers a decrease in the thermal comfort. Seeing the effect of the heating through ventilation air in lowering the supply and return temperatures, a central ventilation system with heat recovery and pre-heating that can operate at low temperatures can represent a good alternative to other decentralised adaptations in the energy system, such as changing radiators. The protocol for data collection and selection of promising adaptations created during this research facilitates the first stages of a case study, by providing an inventory of the main elements for the data collection and their implication in lowering the operating temperatures in the heating network. It also presents a list of possible adaptations to enable low temperature heating. Future case studies could focus on fixing thresholds for the properties of the envelope that limit the lowering of the supply and return temperature. Further research could also continue exploring the optimisation of schedules and room temperature settings to lower the temperatures in the heating network even more, considering the possibility to use the thermal mass of the building as passive storage. Generalisations from case study findings are necessary to start developing one common standardised procedure, with recommended tools and guidelines for simulation as well as practical tests. The standard procedure at building level ought to be included as part of a holistic district level and building level approach in order to facilitate the transition to more efficient and sustainable heating networks.