Energy efficiency in a renovated modern office with activity-based work style
Abstract: During renovation Ljusåret 2 was converted to a modern office with an activity based work style (ABW) with a Demand Controlled Volume (DCV) ventilation system connected to a closed-loop duct. Cooling is provided through air handling units and active water based beams, the underfloor heating system was kept. Written instruction and specification have been studied for the two different control systems Schneider EcoStructure and Lindinspect. Both control systems have been analyzed according to time schedule, set-point and process value by using different functions in software. To be able to perform a energy audit and look at indoor climate for Ljusåret 2 there have been studies according to underfloor heating, constructions of ventilation system, diversity factor for DCV, closed-loop-ducts, heat losses from ducts, cooling demand and energy certification. According to this audit, energy performance is calculated to 89.1 kWh/m2 according to building energy, activity energy is not audited or calculated. During design phase, an energy calculation was made by an energy consultant with the result of 81.3 kWh/m2. The estimated performance is a 9.6 % increase. This building is designed for Miljöbyggnad certification of level silver and should be ≤ 109 kWh/m2,year. According to audit and calculation for energy performance this level is possible to keep. The estimated energy performance have been calculated with only 4 month of statistics from January until April 2019 because Ljusåret 2 have just been renovated. District heating has been estimated through the energy signature by data from energy meter. Electrical components for the building have been measured and energy usage calculated. Energy produced by compression chiller have been estimated with calculated performance from design phase and adding heat transfer between rooms and supply ducts. Energy between rooms and supply ducts were not included in energy calculation during the design phase. According to the control system for the DCV system there have been some issues with high temperature in supply ducts even when they are supplied with 15 ºC from air- handling unit. There have been measurement to the ventilation system 5701-5704 that is connected to a close-loop duct with a result of temperatures between 15.2 ºC up to 21.4 ºC and the velocity has varied between 0.05-2.1 m/s in different measurement spots. This is an increase of 6.4 ºC. A heat transfer calculation have been made in Paroc Calculus to estimate heat transfer between room and supply ducts. The results of this calculation indicates the same level of temperature increases as when the system was measured. With no thermal insulation cooling capacity is lost to half after less than 5 m with a velocity of 0.2 m/s, after 15 m with a velocity of 1 m/s and 30 m with a velocity of 2 m/s . This should be compared with supply duct with 20 mm of thermal insulation that has lost its cooling capacity after less than 13 m with a velocity of 0.2 m/s, after 63 m with a velocity of 1 m/s and is increase with 4 ºC after 100 m with a velocity of 2 m/s. Using closed-loop ducts with velocity below 2.0 m/s and without thermal insulation combined with under tempered supply air is not a good combination. Even short length with low velocity and lack of thermal insulation is devastating because of heat transfer according to logarithmical temperature difference between room and supply ducts. A closed-loop duct is often designed as a pressure chamber and recommended when using DCV and/or VAV ventilation to avoid problems with noise and to be able to reduce the need of dampers. Problems with temperature increasing according to velocity in ducts must be taken in consideration. For Ljusåret 2 this will affect district heating usage where ducts are placed because underfloor heating must compensate heat transfer. Chilled water must be provided an extra time for rooms with both DCV and chilled beams and rooms with only DCV is less comfortable which they could been with a correct installation.
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