Heat Flow in Building Components, Experiment and Analysis

Detta är en avhandling från Dep. of Building Science

Sammanfattning: Three building components exposed to natural climate were studied: a dynamic insulation in the ceiling of a house, an outer wall and one window in the same wall. The term dynamic insulation implies that part of the inlet or exhaust air passes through the insulation of a house. A house with dynamic insulation was continuously measured for approximately a year and a half. The performance of the dynamic insulation was estimated by using hourly values of the temperature distribution inside the insulation. The air flow through the insulation was calculated as the air flow that best matched the measured temperature distribution. For the calculations both the transient and steady state heat transfer equations were used. The dynamic U-value for the insulation was about 0.05 W/m2°C for the ceiling. This corresponds to a dynamic energy efficiency for the insulation of 35%. Taking into account that only 40% of the total supply air passed through the insulation, the total efficiency became 14%. A heat exchanger for ventilation air have an efficiency above 60%. The general conclusion from the measurements was that dynamic insulation requires a house constructed to much higher standards, as far as air leakage is concerned, in order to work properly. An outer ambient wall with a window were studied with both theoretical analyses and measurements performed under conditions with natural climate. The method used was to estimate the heat flow through wall and window from measured temperatures and solar radiation. The longwave radiation was calculated from surface temperatures. The convective heat transfer was calculated as the difference between the heat flow through the building element and the longwave radiation. With the one-dimensional dynamic heat transfer model for the window which included shortwave radiation it was possible to measure the continuous heat flow through a window from temperature sensors and solar radiation measurements. With the one-dimensional finite difference model for the heat transfer through the wall it was possible to calculate the heat flow through a wall from temperature sensors. It was possible to continuously measure the convective heat transfer coefficient on the inner surface of a wall or a window. The accuracy was not very good: at best ±15% for the window and ± 20% for the wall. Even with this low accuracy the effect of different heating and ventilation strategies on the inside could clearly be detected. The results showed that the importance of the ventilation design and the position of the radiator is crucial. Local convective heat transfer coefficients may be more than 10 times the expected, due to ventilation or position of the radiator.

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