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Energy saving in buildings is an important measure for mitigation of climate change. There exists a large potential for energy saving in buildings by improving the thermal performance of windows. For decisions on energy saving window retrofits, accurate estimation of the energy saved and its uncertainty is of importance. The ISO 15099 standard, which is normative for thermal modelling of windows within the building sector, does not give uncertainty estimates. The main novelty of this study was to provide uncertainty analysis for model prediction of the thermal transmittance of windows, in the perspective of decisions on window retrofits. For this purpose, we proposed a new simplified model, which facilitated uncertainty analysis, and still was similar to the ISO 15099 window model. The model was validated by application of a benchmark validation procedure to a set of previously performed validation experiments. Main conclusions were: (i) The model was accurate within a prediction uncertainty equal to 0.20 Wm−2K−1; (ii) The domain where the model is valid was described using existing well-documented validation experiments. This domain was restricted to windows with glazing thermal transmittance corresponding to 2-layer glazing, and to windows where the frame area is a minor part of the total window area. (iii) The prediction uncertainty was mainly determined by the measurement uncertainty in the validation experiments; (iv) If a window retrofit is based on reduction of window thermal transmittance, then this reduction has to be larger than 0.56 Wm−2K−1 in order to yield energy savings above the uncertainty limit.

The practice of validation and uncertainty analysis (UA) of building energy models (BEM) is critically reviewed. The background for this review is the recognized need for improvement of the accuracy in prediction of building energy performance, e.g. as part of an efficient mitigation response to climate change. The review was performed using benchmark comparison to verification and validation (V&V) frameworks obtained from the field of scientific computing. First, the current practice of V&V of BEM was reviewed, with a special focus on UA, and on the existing validation experiments (VE), used to provide the measurement data required for validation of BEM. Second, the review included a case study on the V&V of the European and International standard BEMs, CEN ISO 13790 and 52016–1, for calculation of the hourly energy use for space heating and cooling. From the perspective of the benchmark V&V frameworks, the conclusion was that these standard models cannot be considered as validated. Based on the present review, suggestions are given on how to strengthen the Building Information Modelling (BIM) initiative in its support for the development of accurate BEM. Finally, scientific challenges in terms of V&V of BEM are identified, where the most important is to increase the degree of consensus among scientists on the procedure for V&V as a condition for creating scientifically based standard BEMs.

Heat flow measurement with a heat flow meter is a standardized method (ISO 9869-1) to estimate thermal transmittance (U-value) of a building element. The heat flow meter is a thin plate mounted on top of the surface of the element, and measures the heat flux q through the plate. The measured q is the product of the difference in temperatures between exterior and interior environment, and the U-value. The heat transferred from the element is based on the radiant and the convective heat transfer.

ISO 9869-1 specifies that the environment temperature T_e “is a notional temperature" and it "cannot be measured directly” (section A.3.1). The air temperature T_a is proposed as a reasonable approximation for the indoor environment, while overcast conditions and absence of significant solar radiation are specified conditions for replacing T_e with T_a for the exterior environment.

The sol-air thermometer (SAT) measures the sol-air temperature T_sa, i.e. the equivalent temperature of the convective and the radiative environment. In the absence of solar radiation, T_e = T_sa. SAT is a sensor consisting of a thin flat solid plate, of high thermal conductivity. The front side of the sensor is exposed to the environment, whose T_sa is to be measured, and the backside is thermally insulated. The temperature of the SAT-plate equals T_sa.

In this work we propose introduction of the measured T_e in the existing standard (ISO 9869-1). The method for measurement of T_sa, using the SAT, has been demonstrated experimentally for different periods, without solar radiation present and under stable climatic conditions.

There exists a building energy performance gap between theoretical simulations and the actual energy usage as measured. One potential reason for this gap might be a mismatch between predicted and measured values of the heat flux q through the building envelope. There is therefore a need to develop accurate and more cost-efficient methods for measurement of q. The standard ISO 9869-1 states that, at the outdoor surface, q = ho(Ts − Tenv), where ho is the overall heat transfer coefficient, including both convective and radiative components, Tenv is the environmental temperature, and Ts is the temperature of the building surface. It has previously been shown that the sol-air thermometer (SAT) could be used for convenient measurement of Tenv under dark conditions. In the present work, two SAT units, one heated and the other unheated, were employed for accurate outdoor measurements of ho in cold winter climate. Validation was performed by comparison of results from the new method against measurements, where previously established methodology was used. With current operating conditions, the measurement uncertainty was estimated to be 3.0 and 4.4%, for ho equal to 13 and 29 Wm−2K−1, respectively. The new SAT steady-state method is more cost-effective compared to previous methodology, in that the former involves fewer input quantities (surface emissivity and infrared radiation temperature are unnecessary) to be measured, while giving the same ho results, without any sacrifice in accuracy. SAT methodology thus enables measurement of both Tenv and ho, which characterizes the building thermal environment, and supports estimation of q.

Models predicting ecosystem carbon dioxide (CO2) exchange under future climate change rely on relatively few real-world tests of their assumptions and outputs. Here, we demonstrate a rapid and cost-effective method to estimateCO2exchange from intact vegetation patches under varying atmospheric CO2concentrations.We find that net ecosys-tem CO2uptake (NEE) in a boreal forest rose linearly by 4.7  0.2% of the current ambient rate for every 10 ppmCO2increase, with no detectable influence of foliar biomass, season, or nitrogen (N) fertilization. The lack of any clearshort-term NEE response to fertilization in such an N-limited system is inconsistent with the instantaneous downreg-ulation of photosynthesis formalized in many global models. Incorporating an alternative mechanism with consider-able empirical support – diversion of excess carbon to storage compounds – into an existing earth system modelbrings the model output into closer agreement with our field measurements. A global simulation incorporating thismodified model reduces a long-standing mismatch between the modeled and observed seasonal amplitude of atmo-spheric CO2. Wider application of this chamber approach would provide critical data needed to further improvemodeled projections of biosphere–atmosphere CO2exchange in a changing climate.

Heat flow measurement with a heat flow meter is a standardized method (ISO 9869-1) to estimate thermal transmittance (U-value) of a building element. The heat flow meter is a thin plate mounted on top of the surface of the element, and measures the heat flux q through the plate. The measured q is the product of the difference in temperatures between exterior and interior environment, and the U-value. The heat transferred from the element is based on the radiant and the convective heat transfer.

ISO 9869-1 specifies that the environment temperature T_e “is a notional temperature" and it "cannot be measured directly” (section A.3.1). The air temperature T_a is proposed as a reasonable approximation for the indoor environment, while overcast conditions and absence of significant solar radiation are specified conditions for replacing T_e with T_a for the exterior environment.

The sol-air thermometer (SAT) measures the sol-air temperature T_sa, i.e. the equivalent temperature of the convective and the radiative environment. In the absence of solar radiation, T_e = T_sa. SAT is a sensor consisting of a thin flat solid plate, of high thermal conductivity. The front side of the sensor is exposed to the environment, whose T_sa is to be measured, and the backside is thermally insulated. The temperature of the SAT-plate equals T_sa.

In this work we propose introduction of the measured T_e in the existing standard (ISO 9869-1). The method for measurement of T_sa, using the SAT, has been demonstrated experimentally for different periods, without solar radiation present and under stable climatic conditions.

Carbon dioxide (CO2) has often been used as tracer gas for measurement of the air change rate l (h1 ) in buildings. In such measurements, a correction is required for the presence of indoor CO2, which commonly consists of atmospheric CO2 mixed with human respired CO2. Here, 13C isotope-labelled CO2 was employed as tracer gas, and cavity ring-down spectroscopy (CRDS) was used for simultaneous measurement of the two isotope analogues 12CO2 and 13CO2. This enabled the simultaneous measurement of the 13CO2 tracer gas, with correction for background 13CO2, and the concentration of indoor CO2, allowing for presence of occupants. The background correction procedure assumes that the isotope delta of the background indoor CO2 equals dB ¼ 19‰, based on the prior information that the carbon isotope ratio RB ¼ 13C/12C of all carbon in the bio-geosphere of earth is in the interval 0.010900 < RB < 0.011237. Evidence supported that l could be accurately measured, using the new 13CO2 tracer method, even when the background 13CO2 concentration varied during the measurement time interval, or when the actual dB value differed from the assumed value. The measurement uncertainty for l was estimated at 3%. Uncertainty in l due to uncertainty in RB, uRB(l), was estimated to increase with a decreasing amount of 13CO2 tracer. This indicated that at least 4 ppm tracer must be used, in order to obtain uRB(l)/l < 2%. The temporal resolution of the l measurement was 1.25/l h.

Accurate measurement of the convective heat transfer coefficient h_c at external surfaces, e.g. at building facades and roofs, is of fundamental importance for heat transfer studies of the built environment. There are two basic methods for measurement of h_c, the Loveday and Ito methods, which use one and two heated sensor units, respectively. To guide in selection of method and operating conditions, and in design of the sensor, we performed an error analysis. This included estimation of systematic errors, comparison between methods, and to established Nusselt number correlations, sensitivity analysis, and an evaluation of the measurement uncertainty. The main conclusion was that both methods, at forced convection, yielded measurement uncertainties at the 4 % level, provided that the Ito method was operated under the new condition, where one of its sensors remained unheated. However, at natural convection conditions, the Ito method cannot be operated with one of its sensors unheated, since h_c is then zero at that sensor surface, which violates the method assumption that h_c is the same at both sensors. Sensitivity analysis showed that systematic errors will be reduced by decreasing the sensor surface emissivity. The major source of measurement uncertainty was the conductive heat flux estimate.