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Abstract: Physical or virtual models are commonly used to visualize the conceptual ideas of architects, lighting designers and researchers; they are also employed to assess the daylighting performance of buildings, particularly in cases where Complex Fenestration Systems (CFS) are considered. Recent studies have however revealed a general tendency of physical models to over-estimate this performance, compared to those of real buildings; these discrepancies can be attributed to several reasons. In order to identify the main error sources, a series of comparisons in-between a real building (a single office room within a test module) and the corresponding physical and virtual models was undertaken. The physical model was placed in outdoor conditions, which were strictly identical to those of the real building, as well as underneath a scanning sky simulator. The virtual model simulations were carried out by way of the Radiance program using the GenSky function; an alternative evaluation method, named Partial Daylight Factor method (PDF method), was also employed with the physical model together with sky luminance distributions acquired by a digital sky scanner during the monitoring of the real building. The overall daylighting performance of physical and virtual models were assessed and compared. The causes of discrepancies between the daylighting performance of the real building and the models were analysed. The main identified sources of errors are the reproduction of building details, the CFS modelling and the mocking-up of the geometrical and photometrical properties. To study the impact of these errors on daylighting performance assessment, computer simulation models created using the Radiance program were also used to carry out a sensitivity analysis of modelling errors. The study of the models showed that large discrepancies can occur in daylighting performance assessment. In case of improper mocking-up of the glazing for instance, relative divergences of 25–40% can be found in different room locations, suggesting that more light is entering than actually monitored in the real building. All these discrepancies can however be reduced by making an effort to carefully mock up the geometry and photometry of the real building. A synthesis is presented in this article which can be used as guidelines for daylighting designers to avoid or estimate errors during CFS daylighting performance assessment. [Copyright &y& Elsevier]

• A comparative study between photovoltaic and low-e insulated glass units were conducted experimentally. • The net energy saving potential of the BIPV IGU was identified based on the power, thermal and daylighting performance. • BIPV IGU is better than Low-E IGU in reducing discomfort glare. • BIPV IGC can reduce the cooling energy consumption significantly. • About 16.8% electricity can be saved by replacing Low-E IGU with BIPV IGU. A novel semi-transparent building integrated photovoltaic (BIPV) laminate was developed and introduced in this paper. It was produced by cutting standard mono-crystalline silicon solar cells into small strips and then making electrical connections between each strip before laminating the cells between two layers of glass. The overall energy performance and energy saving potential of the BIPV insulated glass unit (IGU) under real world conditions were identified through a side by side comparative study. Compared to the reference IGU, the BIPV IGU had lower solar heat gain coefficient (SHGC) but much higher U-factor. The average HVAC electricity saving of the BIPV IGU was about 10% relative to the reference IGU. Daylighting measurement and analysis were carried out to evaluate the trade-offs associated with the BIPV IGU between daylight, glare, and lighting energy use. The results indicated that the BIPV IGU is better than the reference IGU in reducing discomfort glare. However, if the most conservative viewpoint near the window is used for the assessment, a lower transmittance BIPV IGU is required to bring the overall discomfort levels below the perceptible level. Lastly, the net energy saving potential associated with the novel BIPV IGU was identified based on the power, thermal and daylighting performance. On average, the BIPV IGU saved 16.8% of the total electricity use of the room. Further studies and improvement on the energy conversion efficiency of solar cells, the optimal transmittance as well as the thermal properties would make this technology more energy-efficient and affordable. [ABSTRACT FROM AUTHOR]

• A novel c-Si based semi-transparent BIPV laminate was introduced. • Energy performance of the BIPV insulated glass unit (IGU) was investigated. • The BIPV laminate has high energy conversion efficiency and visible transmittance. • The BIPV laminate could reduce discomfort glare compared to clear glass windows. • The shade effect was influenced by the solar cell configurations and shading patterns. This paper introduces a novel c-Si based building integrated photovoltaic (BIPV) laminate. It was produced by cutting standard crystalline silicon solar cells into narrow strips and then automatically welding and connecting the strips into continuous strings for laminating between two layers of glass. The overall energy performance of the BIPV insulated glass unit (IGU) including power, thermal and daylighting performance was investigated experimentally. The daily energy conversion efficiency of the active solar cell area was about 15% on sunny days, but it was less than 12% on cloudy days and overcast days. Due to the combined effect of both the semi-transparent PV laminate and the insulated glass unit, the solar heat gain coefficient (SHGC) of the BIPV IGU was as low as 0.25, which was much lower than commonly used glazing windows. Daylight analysis by means of high dynamic range (HDR) cameras and daylight glare probability (DGP) indicated that the BIPV IGU could reduce discomfort glare to some extent compared to clear glass windows. The net energy production of the BIPV IGU was estimated without considering the differences in HVAC energy use in this study. The artificial lighting energy consumption was about 431 W h/day while the average BIPV electricity production for the same period was 1940 W h/day. The net power generation was therefore 1509 W h/day for this BIPV IGU in Berkeley, California. Shading tests for the BIPV IGU were also conducted and the results revealed that the vertical configuration of solar cell wiring in the BIPV laminates was much more sensitive to horizontal shading patterns than vertical shading models. Thus, if shading was unavoidable in some cases, a reasonable arrangement of PV strings should be considered to bring down the energy loss as much as possible. Also, the impacts of environmental factors on the energy conversion efficiency of BIPV IGU were analyzed. Specifically, the power output declined by 0.42% of the peak power for each Celsius degree temperature rise. Thus, if more attention was paid to the heat dissipation issue of BIPV IGU, the overall energy conversion efficiency would be improved. [ABSTRACT FROM AUTHOR]

Exterior shades are the most effective way to control solar load in buildings. Twelve different coplanar shades with different geometry, material properties, and cut-off angles were investigated for two California climates: the moderate San Francisco Bay Area climate and a hot and dry Southern California climate. The presented results distinguish themselves from other simulation studies by a newly developed method that combines three research-grade software programs (Radiance, EnergyPlus, and Window 7) to calculate heat transfer, daylight, and glare resulting from optically-complex fenestration systems more accurately. Simulations were run for a case with constant electric lighting and a case with daylighting controls for a prototypical, internal load dominated office building. In the case of daylighting controls, the choice of slat angle and solar cut-off angle of a fixed exterior slat shading system is non trivial. An optimum slat angle was identified for the considered cases. Material properties (e.g., solar and visible reflectance) did not affect energy use if constant electric lighting was assumed, but they did have a significant influence on energy use intensity (EUI) when daylighting controls were assumed. Energy use increased substantially when an additional interior shade was used for glare control. [ABSTRACT FROM AUTHOR]

Abstract: Large-area polymer thermochromic (TC) laminated windows were evaluated in a full-scale testbed office. The TC interlayer film exhibited thermochromism through a ligand exchange process, producing a change in solar absorption primarily in the visible range while maintaining transparent, undistorted views through the material. The film had a broad switching temperature range and when combined to make an insulating window unit had center-of-glass properties of Tsol=0.12–0.03, Tvis=0.28–0.03 for a glass temperature range of 24–75°C. Field test measurements enabled characterization of switching as a function of incident solar irradiance and outdoor air temperature, illustrating how radiation influences glass temperature and thus effectively lowers the critical switching temperature of TC devices. This was further supported by EnergyPlus building energy simulations. Both empirical and simulation data were used to illustrate how the ideal critical switching temperature or temperature range for TC devices should be based on zone heat balance, not ambient air temperature. Annual energy use data are given to illustrate the energy savings potential of this type of thermochromic. Based on observations in the field, a broad switching temperature range was found to be useful in ensuring a uniform appearance when incident irradiance is non-uniform across the facade. As indicated in prior research, a high visible transmittance in both the switched and unswitched state is also desirable to enable reduction of lighting energy use and enhance indoor environmental quality. [Copyright &y& Elsevier]