Energy performance of ventilation, heating and cooling systems integrated in sandwich panel of high performance concrete

Abstract

Spaces with a high density of occupants have high internal heat gains and need relatively high air change rates to be able to deliver the required amount of fresh air to the space. Classrooms often have elevated concentrations of CO2 mainly as a result of limited air change rates. Using traditional mechanical ventilation diffusers, it is a challenge to supply large amounts of fresh air to the space without creating local discomfort for occupants. This often leads to spaces with poor indoor air quality and problems with overheating, which have a negative influence on the comfort, performance, and health of occupants. One solution to this problem is to use a diffuse ceiling inlet that supplies fresh air in the room through a large area of perforated suspended ceiling, so that the air supplied has a low velocity. However, such ventilation systems have limited cooling ability, because the cooling capacity of outdoor air is considerably decreased during the summer. A promising solution to this problem is to use radiant cooling systems integrated into the inner structures of building elements. The ventilation system can supply fresh air and remove latent heat gains, while the radiant cooling system can remove large amounts of sensible heat gain. The large areas of internal surface available for radiant systems can give an increase in cooling capacity without compromising the comfort of occupants. The aim of the research for this thesis was to design, optimize and contribute to the development of new concepts of cooling, heating, and ventilation systems integrated into the sandwich wall elements made of high performance concrete. The goal was to find solutions that would work well with respect to energy efficiency and the indoor environment, and that would minimise the cost of components in a new building system made of high performance concrete. This thesis reports on the behaviour of wall elements made of high performance concrete with an integrated water-based radiant cooling and heating system which has been developed over the course of the PhD study and implemented in a full scale test building. The designed system of radiant cooling and heating is based on plastic capillary tubes cast in the inner layer of wall elements made of high performance concrete. The plastic capillary tubes represent a way of implementing a radiant cooling and heating system in a thin building structure. The temperature distribution around the integrated plastic capillary tubes was studied using numerical calculations. Measurements were made to evaluate the dynamic behaviour of the room equipped with a wall radiant cooling system combined with a diffuse ceiling inlet for ventilation. The proposed solution for ventilation is based on a diffuse ceiling inlet for mechanical ventilation made of perforated gypsum board with airtight connections utilizing the full potential of a diffuse layer without undesirable crack flow. Methods applied in this work included measurements and numerical simulations. Measurements were carried out in the full scale test building. The test room represented a classroom with a high density of occupants. Theoretical investigations were carried out with a CFD model of the test room. The aim of the development of the CFD model was to allow for a deeper understanding of the diffuse ceiling inlet and wall radiant system and to facilitate efficient and economical optimization of the design taking into account various parameters. The results of the investigations presented show that a diffuse ceiling inlet can successfully ventilate and cool the room with a high density of occupants using supply air at an average temperature of 21 °C. The resulting cooling power was 23 W/m2 at a flow rate of 5.8 l/s·m2 of floor area. The average air temperature in the test room was 24.5 °C. The cooling power of 32 W/m2 was available at a flow rate of 8.0 l/s·m2 of floor area, which resulted in an average air temperature in the test room of 24 °C. This creates a comfortable indoor environment without draughts. Sufficient mixing was obtained mainly as a result of the interaction of incoming air and heat sources situated in the test room. The diffuse ceiling inlet can therefore be considered a well-performing alternative to the traditional means of mechanical ventilation in spaces with a high density of occupants. The results also show that plastic capillary tubes integrated in a layer of high performance concrete can provide the energy needed for cooling between 29 W/m2 and 59 W/m2 of floor area with cooling water temperatures between 22 °C and 18 °C. This resulted in indoor air temperatures of 24.5 °C and 22 °C, respectively, and a draught-free indoor environment. The relatively high reaction speed of the designed system of radiant cooling was achieved as a result of the slim construction of high performance concrete. Measured values were used to validate a developed CFD model, with the aim of achieving a precise CFD model which can be used to evaluate indoor comfort numerically. The results show that transient calculations using Large Eddy Simulation turbulent models can give a good prediction of temperatures and air flow velocity magnitude in a room ventilated using a diffuse ceiling inlet. However, steady-state turbulent models needed to be applied to obtain adequate predictions in the rooms equipped with a wall radiant cooling system.