Mateja Trobec Lah*, Dr. Asist., Ales Krainer*, Dr. Prof.,

University of Ljubljana, Faculty of Civil and Geodetic Engineering, Chair for Buildings and Constructional Complexes

Jamova cesta 2, 1000Ljubljana, P. O. BOX 3422 Slovenia

Tel: +386 01 4768 609 fax: +386 01 4250 688 e-mail: MLah@fag. uni-li. si.

* ISES member

Abstract. The main aim of the paper is to present a progressive strategy in automatic control for a shading device — roller blind, based on the fuzzy logic, to obtain the desired inside illumination according to the available momentary solar radiation. To implement the suitable control system for the movable shading device, the combination of the fuzzy and conventional control algorithm was developed. For this purpose we studied the luminous efficacy of the solar radiant energy in all weather conditions. The developed control algorithm contains a cascade control with fuzzy controller as the main and conventional PID — proportional-integral — derivative controller as the auxiliary controller. The illumination fuzzy controller, which is essential for proper roller blind position, was designed progressively during the experimental procedure. It enables us to use the non-linear knowledge about the optical process and to transfer it to an appropriate control action (roller blind alternation) in a way that is close to human thinking. It was optimized iteratively. A well-designed illumination fuzzy controller means moderate continuous movement of the roller blind. It also assures the inside daylight illumination in the area, where the desired value deviates up to ± 50 lx. Automatic active response of the shading device to the outside weather conditions enables the optimization of internal visual performance together with efficient use of energy, also in the sense of the inside thermal performance.

1 Introduction

Using daylighting in buildings is an important and useful strategy in replacing the need for high-grade conventional energy for inside illumination. Energy flows through the building envelope are present all the time. The properties of the building envelope have significant influence on the interaction between the inner and the outer energy conditions in the sense of thermal and lighting flows. The available solar radiation conditioned the response of the building as naturally lit space. The positive aspect of the controlled luminous energy flow through the building envelope is enhanced with the development of the technology, i. e. with the possibility of the automatic active response of the shading elements to the outside conditions. In our case this is realized with a roller blind adaptable to the outside weather conditions. In daylighted spaces the psychological benefit is increased. Also, the lighting energy consumption and the heat gains associated with the electrical lighting are reduced. The main aim of the paper is to present the automatic adaptable roller blind, based on the fuzzy logic control system to obtain the desired inside illumination according to the momentary available solar radiation. A real model of a building, test chamber properly equipped, was built for the development of the fuzzy control system for changeable window geometry. The test chamber allows the investigation and experimentation of illumination control and enables us to study the influences of the movable shade on the luminous efficacy in the interior.

Control system design is a complex procedure usually based on modeling and simulations in the overall iterative approach [1]. In our case the design approach was based only on experimentation. The main focus of the study was on the design and development of the
fuzzy illumination controller. In order to develop an effective illumination control system, sets of preliminary measurements were done, where the daylighting response in the sense of the inside illumination dependent on different sizes of the transparent part of the window and the luminous efficacy was studied. The available daylight inside the building depends on the solar radiation and the building’s geometry. Weather conditions and the level of cloudiness determine the terrestrial total solar radiation and the ratio of diffuse/direct radiation. Actual daylight illuminance in a room is related to the luminance pattern of the sky and also to the window geometry with regard to the room’s dimensions.

Illumination and luminous efficacy correspond to the available solar radiant flux. Spectral distribution of the solar energy is roughly equal to black body spectrum at a temperature of T=5773 K. It is useful to know the impact of the total solar radiation on the optical effect in the interior space. The ratio of the luminous flux Фу [Watt] to the radiant flux Фе [lumen] is defined as luminous efficacy K [lm/W]. Luminous efficacy K of solar radiation is given with the ratio of the luminous flux of the visible light Фу (380 nm < X < 780 nm) and the total solar radiant flux Фу (0 nm <X~ 2500 nm). The maximal value for the solar "overall” luminous efficacy is 93 lm/W. The luminous flux is the part of the radiant power, which is perceived as light by human eye, and the value 683 lm/W is based upon the sensitivity of the eye at 555 nm, which is the peak efficiency of the photopic (daylight) vision curve [2].

In experiments the observed inside luminous efficacy of the solar radiation was the ratio between the measured external available solar radiation (global and reflected) and the measured internal illumination. The observed luminous efficacy K presented in the experiments is:

K [lm/W]: Luminous efficacy = (measured inside illumination)[lx]/(measured external

solar radiation) [W/m2]

Solar luminous efficacy is a quotient, which tells us the real efficacy of the daylight in the sense of internal visual performance taking into account changeable weather conditions. The ratio describes the relationship between the optical and the thermal effect of the available solar energy.

Other sets of experiments were intended to study the relationship between the outside available solar global and reflected radiation and the corresponding inside illumination.

The experiments were done throughout the year using different transparent area sizes to collect and examine the correlation between the solar radiation and the inside illumination. The non-linear mapping between the inside and the outside conditions was used as a basis for defining the fuzzy rules in the illumination fuzzy controllers. For fuzzy rules in the form of if-then statements only the input-output relations are important and not the information about the mechanism, which causes these relations.

The final aim was to create an illumination control algorithm, which enables harmonized variation of the window geometry, i. e. proper positioning of the roller blind in real time conditions. Well designed illumination control algorithm with precise setting of the parameters produces appropriate signals for moderate roller blind alternations according to the outdoor weather conditions and the desired indoor illumination. The result is efficient utilization of the solar energy for the inside harmonized lighting and thermal energy conditions.