Hydronic design of cogeneration in collective residential heating systems: state-of-the-art, comparison and improvements

Highlights: • Hydronic designs of an ICE-CHP with auxiliary boiler and storage tank are examined. • A morphological chart is developed to structure existing and new proposed designs. • A CHP with variable flow has the highest energy savings and stability of operation. • A parallel boiler should be equipped with a modulating control valve. • The proposed methodology can guide engineers towards an hydronic design. -- Abstract: Cogeneration (CHP) has great potential to save primary energy in collective residential buildings. However, these savings and their associated financial benefits are influenced by the design of the complete heat production system, which typically includes, besides the CHP itself, an auxiliary boiler and a storage tank. Both scientific literature and design guides focus little on the hydronic aspects of that design, although the performance of a heat production system is extremely prone to it. Therefore, this paper evaluates different hydronic designs of a central heat production system with CHP. First, an overview is given of the state-of-the-art hydronic design concepts that are used in the private sector. Two new concepts are proposed as improvements to the existing ones: one to integrate the CHP and one to integrate the boiler. A morphological chart is developed to classify the features of both the conventional and novel designs. Second, the performance of all the design concepts (and their 54 combinations) are evaluated based on a case study of an apartment block with 24 apartments. This evaluation is made by means of dynamic building system simulations. The results show that maximal primary energy can be saved if the CHP is integrated according to the novel CHP design, which allows a variable flow rate through the CHP. This concept should be preferred in a design process. Multiple hydronic configurations of the boiler, of which one is the novel hydronic boiler concept, resulted in a similar performance. Therefore, designers are advised to make a case-specific comparison to decide which one to take. The morphological chart and methodology elaborated in this paper provide a basis to make that decision.