Assessing the energy use and IAQ of various HVAC systems during the early design stage

The early design stage of a building is decisive for describing the concept of the HVAC system. Designers and practitioners can adjust and optimize the design during this stage as it provides them with enough resilience to adapt new changes. In practice, a well-defined optimization process is essentially required in order to achieve the project’s goals within a reasonable time span. These goals vary from one project to another, and sometimes they require a comprehensive study to identify the factual and stochastic parameters and their impact on the design. This paper presents a workflow process to design three Heating, Ventilation, and Air-Conditioning (HVAC) systems for a modern Swedish office building focusing on operational energy use and Indoor Air Quality (IAQ) indices. The workflow follows a bottom-up approach starting from analysing the building’s requirements, then defining multiple concepts for the HVAC system, and eventually, quantifying the systems’ performance by the means of comparative study. The three HVAC concepts included in this study are; airborne constant air volume (ON/OFF), airborne Variable Controlled Ventilation (VAV), and hydronic system integrating Active Chilled Beams (ACB). The first two systems rely solely on air as heating and cooling medium, while the ACB system requires water circuit and primary fresh air to provide the necessary induction. All three systems are designed to maintain a certain limit of operative temperature that varies depending on season and occupancy schedule. Furthermore, the design criteria states that all systems must achieve a good indoor climate with low sound levels and air speeds in the occupied zone. The study shows that all three systems have an operational energy use under the limits prescribed by the Swedish building regulations. However, a significant reduction in the size of air ducts and operational energy use can be achieved when implementing an ACB system when compared against ON/OFF and VAV systems. This reduction in energy use comes in the expense of relatively higher operative temperatures during the occupied summer periods, as lesser amount of air is supplied to the zone. This increase in temperatures is correlated with higher performance loss in office tasks performed by the building occupants. An optimization of the ACB system is suggested in order to compensate the performance loss by lowering the thermostat control point of the ACB’s. This scenario would increase the cooling energy use compared to the base case, but, at the same time, ensures an overall reduction in comparison with ON/OFF and VAV systems.