An advanced lighting system combining solar and an artificial light source for constant illumination and energy saving in buildings
Graphical abstract
Constant illumination system mixing sunlight and artificial light.
Introduction
Globally, energy consumption in buildings has been steadily increasing over the years in both the residential and commercial sectors, constituting a considerable portion of gross energy consumption in developed countries. The high level of energy consumption in buildings is becoming a major concern in many countries. According to statistics released by the United Nations Environment Programme (UNEP) (2012), about 30%–40% of global energy is consumed in buildings [1], [2].
Among the various forms of global energy consumption, lighting is responsible for about 19% of electricity used worldwide. This situation, however, could be immensely improved by applying new lighting control technologies. In the residential sector, only 1% of buildings use energy-saving lighting controls that operate upon the detection of occupants or some value of daylight intensity. Energy savings from lighting controls will be even greater if applied to the nonresidential (commercial) sector, as this sector consumes about 60% of global electricity. Projections show that this could save about 68 billion dollars and result in 330 million tons of carbon dioxide (CO2) abatement [3].
In implementing an energy saving strategy, it is essential to deal with peak demand, especially in commercial or industrial buildings. Here, peak demand refers to the maximum power consumed by a building along the course of an entire year. This peak typically reaches its highest point during the hottest period in the summer in the case of commercial buildings, due to high occupancy and the cooling system running at maximum capacity. To size a mechanical cooling system, the peak cooling load should be considered, among other factors. Its reduction would result in system downsizing, and eventually allow for decreasing the initial cost.
During peak periods, building owners are often penalized by utility companies with significantly higher rates if they exceed a maximum load. Daylighting systems offer an attractive and effective option for reduction of peak demand because daylight is plentiful during summer peak periods. In some climates, however, the cooling load could increase due to solar heat gains from uncontrolled daylighting; thus an optimal balance between electric lighting use and cooling energy is necessary [4], [5]. Consideration of the admission of daylight indoors is more than just an energy issue; it is directly related to human comfort, promoting the visual environment and leading to health benefits. Controversies over buildings’ solar access are becoming more frequent, in line with the global trend of urbanization and concentration of high-rise buildings in limited urban areas [6].
Up till now, various passive and active systems have been developed and extensively used to utilize daylight as much as possible. Of these, active daylighting strategies usually provide a more efficient way to use sunlight for indoor illumination than passive strategies do. A fiber-optic daylighting system is one such system that can constantly track the sun for indoor illumination. Extensive studies have been conducted to improve the efficiency of fiber-optic daylighting and to promote the system's applicability under different operating conditions. New designs and various operating schemes have been proposed and tested based on theoretical investigations to enhance the energy effectiveness of the system, as well as its adaptability in solar energy utilization [7], [8], [9], [10], [11].
Energy efficiency of a daylighting system varies, depending on lighting requirements and application strategies. Many studies have evaluated the performance of daylighting designs to channel in natural light for indoor illumination to meet lighting needs. In general, daylighting in buildings can be largely divided into two distinct daylighting schemes; passive and active. Passive daylighting involves only nonmoving parts for the collection and transmission of sunlight, whereas active daylighting requires moving parts and parasitic power to track the sun for efficient collection of sunlight. Various strategies have been developed to practice active daylighting in conjunction with electric lighting, to achieve maximum energy efficiency and to deliver a quality visual environment. They generally involve daylight–responsive controls (lighting controls) to regulate the output of electric light (lamps) in accordance with the available daylight from an active daylighting system. Because two different luminaires (installed on a ceiling) are involved in providing an adequate amount of light to interior spaces, they are likely to fluctuate in brightness and illuminance [12], [13], [14], [15], [16].
There have been many studies performed to maintain a constant level of illumination at some reference point in a room through combining daylight and artificial light, where the latter is provided by an electric lighting system installed near the daylight exit (a distributing lens) of an active daylighting system. The electric lighting system is generally controlled by a single photo sensor (or by a number of photo sensors) installed on the ceiling (or wall). [17], [18], [19]. Unlike prior active daylighting practices with lighting controls, this work presents an active daylighting system where constant illumination is made possible by mixing sunlight and artificial light and maintaining a constant luminous power before the resulting light is discharged through a terminal device installed indoors.
A Fresnel lens mounted on a dual axis solar tracker was used with an LED lamp, and sunlight and artificial light were mixed together based on a feedback signal from a light control system developed in this work. The system's functional competitiveness and reliability were explored for practical applications.
Section snippets
Operational concept of the system
As mentioned, the present system differs from the existing active daylighting systems that work with lighting controls that regulate an electric light's output. That is, electric lighting systems (lamps) are activated upon the availability of sunlight due to active daylighting systems. Two different types of luminaires are required: one for electric light and the other for active solar daylighting. In the case of our system, however, only one type of luminaire is necessary, as the sunlight and
Performance assessment
To verify the photometric applicability of the daylighting system for constant illumination, a series of measurements were performed on clear-sky days, with a system installed on the roof of a test cell (Fig. 7). The cell was built with prefabricated sandwich panels, by sandwiching a 4.7 mm polystyrene foam insulation panel between metal skins 1.5 mm thick. Fig. 8 shows the plan view and dimensions of the test cell whose length, width and height were 3000 mm, 1500 mm and 1500 mm, respectively.
Results and discussions
Here, the experimental results are presented for those cases that demonstrate the effectiveness of the constant illumination system developed in this work. These cases were chosen from a number of cases measured with clear skies because they were deemed to be the most appropriate to test the reliability and validity of our developed system, especially in conjunction with the aforementioned artificial light control based on a standard curve applicable to resembling clear-sky conditions. As
Conclusions
This work was conducted to explore the possibility of applying an active daylighting system for constant illumination of interior spaces to improve energy efficiency and promote the indoor visual environment. An active daylighting system built with a Fresnel lens mounted on a dual axis solar tracker was used in conjunction with an LED lamp (120 W, CCT 5000 K) to generate a constant flux of light by adding sunlight and artificial light as directed by a lighting control regulating the output of
Declaration of competing interest
None.
Acknowledgments
The authors would like to acknowledge support from the National Research Foundation of Korea through the Ministry of Science, ICT & Future Planning (Grant number 2017R1A2A1A05001461, 2018R1A2B2008542, 2016R1D1A1B04934265).
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