• Energy Research

Abstract This paper presents an experimental and simulation study for quantifying the daylighting performance of bottom-up roller shades installed in office spaces. The bottom-up shade is a motorized roller shade that opens from top to bottom operating in the opposite direction of a conventional roller shade, so as to cover the bottom part of the window, while allowing daylight to enter from the top part of the window, reaching deeper into the room. A daylighting simulation model, validated with full-scale experiments, was developed in order to establish correlations between the shade position, outdoor illuminance and work plane illuminance for different outdoor conditions. Then, a shading control algorithm was developed for application in any location and orientation. The validated model was employed for a sensitivity analysis of the impact of shade optical properties and control on the potential energy savings due to the use of daylighting. The results showed that Daylight Autonomy for the bottom-up shade is 8–58% higher compared to a conventional roller shade, with a difference of 46% further away from the facade, where the use of electric lighting is needed most of the time. The potential reduction in energy consumption for lighting is 21–41%.

Abstract In this paper we present a methodology to map individual occupants' thermal preference votes and indoor environmental variables into personalized preference models. Our modeling approach includes a new Bayesian classification and inference algorithm that incorporates hidden parameters and informative priors to account for the uncertainty associated with variables that are noisy or difficult to measure (unobserved) in real buildings (for example, the metabolic rate, air speed and occupants’ clothing level). To demonstrate our approach, we conducted an experimental study in private offices by considering thermal comfort delivery conditions that are representative of typical office buildings. Personalized preference models were developed with the training dataset and the developed algorithms were used in a detailed validation process. The proposed model showed better prediction performance compared to previous methods. Towards realization of preference-based control systems, this study also addresses practical limitations associated with controlling model complexity and data efficiency as well as using effective model evaluation metrics to train reliable personalized preference models in the real world.

Abstract This paper presents a method to incorporate personalized visual preferences in real-time optimal daylighting control without using general discomfort-based assumptions. A personalized shading control framework is developed to maximize occupant satisfaction while minimizing lighting energy use in daylit offices with automated shading systems. Personalized visual satisfaction utility functions were used along with model-predicted lighting energy use to form an optimization framework using two approaches. In the multi-objective optimization scheme, the satisfaction utility and predicted lighting energy consumption are used as parallel objectives to provide a set of Pareto solutions at each time step. In the single-objective optimization scheme, the satisfaction utility is converted into a constraint when minimizing lighting energy use. A simulation study with two distinct visual satisfaction models, inferred from experimental data, was conducted to evaluate the implementation feasibility and optimization effectiveness. Daily and annual simulation results are presented to demonstrate the different patterns of optimal points depending on preference profiles, occupant sensitivity to utility function, and exterior conditions. Finally, we present a new way to apply the multi-objective optimization without assigning arbitrary weights to objectives: allowing occupants to be the final decision makers in real-time balancing between their personalized visual satisfaction and energy use considerations, within dynamic hidden optimal bounds. A slider is introduced as a dynamic user interface with mapped and sorted optimal solutions.

Abstract This paper presents an experimental and simulation study to evaluate daylight glare probability (DGP) in office spaces with roller shades. Roller shades can be controlled in various ways and have an openness, transmitting direct and diffuse light even when fully closed. Since DGP combines the overall brightness of the visual field and the perceived contrast of the scene in one metric, the development of glare protection guidelines is complex in this case. Full-scale experiments with an HDR camera in test offices were combined with a validated, integrated daylighting and glare model. Correlations between DGP and design parameters (work plane or vertical illuminance) were developed and the applicability of DGP and DGPs for closed and controlled shades is discussed. The results show that DGPs is not an accurate metric when the sun is within the field of view – even for low openness fabrics – while the DGP equation might need a correction for such cases, due to extreme values of the solar corona's luminance influencing the luminance term. DGP and work plane illuminance are not well correlated, except for the case of perfectly diffuse fabrics. However, for all instances when the sun is not visible by the occupant, DGPs can be used to approximate daylight glare, including cases with sunlight on various surfaces in the space, for any fabric openness and control type. This enables the development of model-based, real-time glare control shading operation, with vertical illuminance being the basic parameter. Simple sunlight protection strategies cannot prevent glare, despite maximizing daylight utilization.

While it is well-known that cool roofs can efficiently reduce cooling demand in buildings, their overall energy performance in mixed and cold climates has been a topic of debate. This paper presents a comprehensive simulation study to evaluate the combined impact of roof reflectivity, insulation level, and construction type (adhered vs attached) on annual energy demand and energy costs in the United States, for different buildings and climate zones. EnergyPlus was used to model three building types (retail, office, and school buildings) for the 16 most climate-representative locations in the US using typical reflectivity and insulation values. The results show that (i) roof reflectivity is equally important to roof insulation in warm climates; (ii) for low-rise offices and schools, the benefits of reflective roofs vs dark-colored roofs are clear for all US climatic zones, with higher savings in warm climates; (iii) for big-box-retail buildings, reflective roofs perform better except for cold climate zones 7–8; (iv) dark-colored, mechanically attached roofs achieve slightly better performance than reflective roofs in mixed and cold climates. Decision makers should consider building type, climatic conditions, roof insulation levels, and durability performance, along with roof reflectivity, when assessing the overall potential benefits of cool roofs.

Abstract This paper presents a hybrid ray-tracing and radiosity method for processing luminous flux in spaces equipped with horizontal venetian blinds. The method considers both diffuse and specular characteristics of the blinds and aims to establish a balance between computational speed and accuracy. Direct components are treated with ray-tracing techniques employing a shining factor for the blinds to split between specularly and diffusely reflected components. The specular components are traced inside the blind cavity and inside the room while the direct-diffuse components inside the blind cavity are processed in a two-dimensional radiosity calculation until the final diffuse flux departing the cavity is determined. Diffuse-to-diffuse transmission is considered using a traditional radiosity method. Each room surface is divided into sub-surfaces and given an initial luminous exitance, after accounting for directly traced portions. Then a three-dimensional radiosity method is employed for the entire room to compute illuminance distributions on each sub-surface and on the work plane. Comparison between the current model and results obtained with full radiosity showed that significant errors can be introduced by improper modeling of blind specular components for almost all profile angles and slat angles. An in-depth analysis of the transmission process with different profile angles and shining factors showed the potential of blinds with diffuse characteristics to transmit more light than blinds with highly specular surfaces for certain angles. Further analysis for profile angle close to 45°, showed that for certain commonly used slat angles, a second reflection occurs at the bottom side of the upper slat, redirecting direct light towards the work plane, with serious potential glare consequences. The model contributes to rapid and accurate assessment of illuminance/solar radiation distribution in spaces with venetian blinds and related potential lighting energy savings when electric lighting controls are utilized. Analysis of the number of inter-reflections as a function of profile and slat angles with simultaneous consideration of different reflectances and shining factors is particularly important for providing guidelines and recommendations for venetian blinds optimized design and control.

Shading control strategies are often required to optimize the balance between solar gains, daylight availability, glare protection, and view to the outside. Automated shading operation, when proper...

Abstract This paper presents the development of new venetian blind control strategies considering daylight provision, lighting energy use and visual comfort. A hybrid ray-tracing and radiosity method is used to calculate transmission through the window-blind system and interior illuminance distributions taking into account specular and diffuse material properties. Work plane illuminances are used to extract daylighting metrics, while directional light distribution and respective luminance values are used to calculate glare potential. Four types of control strategies were evaluated – a cut-off angle control, a daylight-redirecting control, and two glare protection control modes. Using the common “cut-off” angle may result in a strong second reflection that might significantly affect visual discomfort depending on view direction and profile angle. Proper rotation of the tilt angle and consideration of specular characteristics can minimize this effect and redistribute daylight deeper into the space, although glare problems cannot be eliminated in some cases. Moreover, two additional control strategies were developed based on glare probability and sky conditions: the first is a model-based control derived from real time glare simulation results. The second is a “simplified” control that uses pre-calculated correlations between glare probability and transmitted illuminance to establish binary set points that can be used in model-predictive control algorithms. A detailed daylighting analysis was performed in order to evaluate set points for blind control actions, annual daylighting metrics, lighting energy use and daylight glare probability and duration. Different room sizes, glazing properties, blind material characteristics and orientations were studied. The results showed that diffuse blinds do not reduce the risk of glare when light redirection is desired; double-sided slats with a diffuse top and a specular bottom surface are better in this regard. Fixed angles at 60° or higher provide satisfactory results for most cases, although there is a penalty in daylighting efficiency. The two glare protection control algorithms proved to be efficient in providing high daylight autonomy values without the risk of glare for all studied cases.