Design and commissioning of an energy efficient air distribution system in a university hospital.

Energy efficiency was a significant by-condition in the design work of the New Block (23, 700 m²; 255, 100 ft²) of the University Hospital in the city of Linkoping in Sweden. The block is made up of several interconnecting buildings of between two to four floors, and contains the Heart Centre, the Ear Centre and the Clinic for Infectious Diseases. The principal aim of the design work was to decrease the electrical energy end-uses for air distribution, cooling and lighting (electronic ballasts, occupancy sensors). Over 1,300 windows with very low U-values contributed to the decrease in the heat energy use. The building was occupied and commissioned during the spring and summer of 1995. A pre-study was made of approximately half the air distribution system. i.e. air handling units as well as the duct system. The air flow rate of this half was assumed to be 30 m³/s (63,500 cfm); constant round the clock. The marginal profitability from the base case through three more energy-efficient designs was calculated, assuming a depreciation time of 15 years, an electrical energy price of SEK 0.30 (US¢ 4-5) per kWh, and 15% minimum allowed marginal internal rate of return. The fan energy use was expressed through an energy performance ratio, i.e. the Specific Fan Power SFP [kW/(m³/s); 0.472 W/cfm]. The gradual steps which decreased SFP from 4.0 kW/(m³/s) to 3.0, 2.0 and 1.5 were all found to be profitable, leading to a more detailed design with the goal of obtaining SFP= 1.5 kW/(m³/s). During the commissioning in late summer 1995, the SFP was measured for the actual air distribution system. For the two largest air handling units (custom built), the measured SFP was 1.86 kW/(m³/s) and 1.63 kW/(m³/s), respectively. The reasons for this increase compared with the pre-study were mainly due to more efficient filters being installed on the return air side of all air handling units. Furthermore, the filter class had been increased one step on the supply air side of the air handling unit with the highest measured SFP and the dual-duct mixing boxes needed higher static pressures than stated in the manufacturer's data.