Decentralized storage in combined heat distribution circuits: how to control?

Abstract

In apartment buildings, collective heating networks have great energetic advantages. One form gaining more attention for the last decades is a combined heat distribution circuit (CHDC), in Belgium called “combilus”. It is a two-pipe system for the distribution of both space heating (SH) and domestic hot water (DHW). The supply temperature is set to the highest needed temperature, which is around 65°C for DHW (considering a temperature difference for enabling heat transfer). However, if decentralised DHW storage tanks in combination with low-temperature emitters for SH are used, the supply temperature could be optimised. In this research, two innovative control strategies were studied for such a CHDC to lower the distribution temperature (to the required temperature for SH) as much as possible by grouping the charging periods of those storage tanks. One control strategy is time-based, with pre-defined charging schemes, while the other is based on two sensors in the storage tanks. In order to test the control strategies, a simulation environment was set up in Matlab that represents the thermal dynamic behaviour of CHDC. However, to fully focus on the evaluation of the control strategies, an idealised central boiler room was assumed, which immediately delivers the desired temperatures. Besides the evaluation of the control strategies, the design of the storage tanks is also optimised by performing sensitivity analyses on the volume, hysteresis and charging flow rates. The results show that larger storage tanks provide better DHW comfort combined with less PE use (for the proposed controls) and that the charging flow rate can significantly reduce the central peak power, while DHW comfort and PE use remain the same. With the time-based control, the charging cycles and volume have a high impact on the performance and comfort. The two-sensor control is always able to reduce PE use and deliver the same or even better comfort than the reference control.