Design of Active Chilled Beam systems: Detailed economic analyses for different design temperatures and airflows

Abstract

The predominant space-cooling system prevailing in Swedish commercial buildings today is the variable air volume (VAV) system. The system efficiently modulates the supplied air volume to meet changing cooling demands. An alternative cooling system is active chilled beams (ACB). ACB is a ceiling-mounted convection unit that decouples ventilation from thermal conditioning facilitating a potential for energy and cost savings. This thesis aims to compare the energy and life-cycle costs (LCC) between the VAV and ACB systems, highlighting the impact of the cooling system choice on building energy demand and energy source design. The study grounds its investigations on the energy simulations of a case building located in Gothenburg, Sweden. The building is a six-story lightweight commercial building constituted primarily of open office landscapes. The energy is sourced from a combination of district energy and borehole heat-exchanger (BHE). The study concludes that ACB systems can be designed at costs commensurate to comparable conventional VAV systems due to the savings attained from a smaller air-handling unit and ductwork, attributed to reduced airflows, exceeding the investment cost of ACB units and accessory equipment. Although the study can confirm energy savings achieved through reduced airflows, it could not validate other sources of energy savings. It was observed that VAV consumes less total energy but requires higher total power compared to ACB. Translated into annual costs, the higher power consumption of the VAV pertains to larger power costs which, combined with larger fan energy, results in higher operational expenses for the VAV system. Maintenance costs cause a further divergence between the systems, in favor of the ACB. Investigating the different design parameters found that increasing the airflow of ACB systems generally leads to more balanced building loads. Contrarily, increasing supply air temperature cause an opposite effect on the specific component energies and the balance of building loads. Modulation of air temperature should conclusively be accompanied by proportional alterations to airflow rate to reduce the impact on investment costs; prioritizing a lower number of units over minimum airflow.