Skynative – brings daylight into the depths of buildings

There are various ways of directing daylight into the interior of a building or a room. In cooperation with BASF, Bartenbach has developed a prototype of a horizontal daylight duct integrated into the suspended ceiling that efficiently transports daylight from the outside into the underexposed “core” of a building. “Skynative” is the name of the system, named after BASF spin-off Skynative® UG, which is working on market integration as well as continuous improvement of the system in an open research collaboration with Bartenbach. In this paper, the system concept development, simulation-based application studies as well as the setup of a real-scale mock-up, including results from a long-term performance monitoring at the premises at Bartenbach, are presented. First potential application projects are shown, and the system efficiency is discussed.


Introduction
People spend more than 90% of their time indoors, often without direct access to daylight from windows [1].The problem increases in urban areas due to deeper building floorplans and urban compaction.Furthermore, the EU is driving a drastic increase in the refurbishment rate of existing building stock to help achieve its climate goals of reducing greenhouse gases by 55% by 2035 and reaching climate neutrality by 2050 [2].In Germany, 64% of the existing building stock was built before 1978 [3].These buildings usually have a significant lack of quality and quantity of daylight.Revitalization of unused buildings (industry halls, retail centers...) is a vastly growing market, representing 22% of the overall office construction market.And finally, the COVID pandemic has had a massive impact on working environments towards creating inclusive workplaces (e.g.modular seating solutions, desk sharing, less density) and changed therefore the usage concepts for new and existing buildings -also leading to a new focus and requirements on revitalization [4].
At the same time, a growing body of research shows that daylight plays a critical role in human health, wellbeing and productivity [5]- [8].Lack of daylight negatively influences people.It verifiably causes short-term issues like low concentration, productivity and a disrupted circadian rhythm [9].Lack of daylight also causes significant long-term health problems such as seasonal affective disorder, fatigue, and stress as well as slow recovery, especially at intensive care units [10].EU regulatory authorities and independent green building certification agencies are therefore pushing higher daylighting standards (e.g.EN-17037) and supports building certification schemes (e.g.DGNB, WELL, BREEAM, LEED) to promote human health effects through daylight in recent years.1320 (2024) 012019 IOP Publishing doi:10.1088/1755-1315/1320/1/012019 2 So how to bring daylight effectively and efficiently deep into the interior of a building so that these areas also receive daylight and can manage without electric light during the day?Although the electric lighting industry seeks to position the LED as a daylight-equivalent light source, research reveals that electric light does not provide the same health and productivity benefits as natural sunlight [11].
Consequently, what the market lacks is a daylighting system that enhances daylight levels in the building core, independent of the location and in an optimal balance with electric light via a human-centered control approach.
Especially the so-called "central zones" of a building, which on average account for around one third of the net floor area, are often poorly supplied with daylight and correspondingly dark.Due to normative requirements, hardly any workplaces are found here.Valuable and often cost-intensive areas are thus lost to companies and economic use of the space is hardly possible.Especially in times of the "new work" movement, this is a major aspect, as interior, open spaces in the core area of an office floor plan are increasingly being used as exchange and networking zones across professional and organizational boundaries [12].
The desire for optimal use of space from the occupant's and company's point of view and dealing with a growing urban development density requires a rethinking of daylighting strategies.Systems that direct daylight from the exterior façade into the interior offer an important approach to solving this problem -and therefore the author team developed and tested a daylighting system prototype, referred to under the term Skynative.
A horizontal daylight duct efficiently transports daylight from the façade "invisible" through the suspended ceiling into the interior of buildings.Thus, the Skynative prototype makes it possible to improve the visual indoor quality, promote human health by using daylight, and make numerous rooms usable through daylighting.
Besides new built structures, Skynative can bring benefits especially for the retrofit and revitalization of old buildings, which must be increased drastically within the next years to achieve the climate protection targets.In contrast to most vertical, roof-installed light tubes available on the market (ref. chapter 1.1), Skynative utilizes a horizontal duct system across the façade, which is easier to apply as a retrofit solution.Especially in old industry halls, which are increasingly being revitalized into office buildings, there are often large room heights, which makes a subsequent integration of Skynative very advantageous.However, Skynative not only offers potential benefits in terms of occupant comfort and satisfaction but can also contribute to the building owner's financial interests and help achieve environmental goals.

State of the art and scientific literature review
On the market, so called "light tubes" (also known as sun tubes) have been established as one variant of daylighting systems for many years and are offered by well-known companies such as Velux, Solatube, Suntek and SunPipe, to name a few.Kingspan Light + Air offers a system of a combined daylighting and air tube solution to enhance natural light and fresh air in buildings within one system.This solution attempts to optimally solve the problem of simultaneous integration of both technologies in the very limited space of a suspended ceiling, which has also proven to be one of the biggest challenges in the design of horizontal light ducts.
But also in research, several works have been published presenting new concepts on daylighting duct concepts.In the following review, a special focus is given to horizontal duct systems.Just recently, Mesloub et al. [13] published a feasibility study on the potential for visual comfort, overall energy consumption and the economic feasibility of tubular daylighting systems in deep plan offices in the MENA countries.This work also includes a keyword-based bibliometric analysis to extract relevant publications and research done on tubular daylighting in the period of 2010 -April 2022.Thus, most of the research work was focussing on the performance of light pipe design, followed by the energy efficiency and their impact on energy savings achieved.
Numerous works on lighting duct concepts have been done by Obradovic et al. [14]- [16].In an initial paper, Obradovic et al. invested in a literature study on daylight transport systems spanning from the 80's to 2019 and evaluates their most appropriate application at high latitudes [15].A clear distinction is made to system applications at equatorial latitudes, as most studies are published from this region.As a conclusion, it is highlighted that the local daylighting conditions (clear vs. overcast conditions, sun angle, façade orientation…) are most essential for deciding about the type of collector, while building structure, room geometry and functionality are decisive factors for choosing the right transport element.
In Obradovic et al. [14], horizontal light pipes in specific are analysed in terms of their daylight efficiency, especially for high sun angles.By attaching light-deflecting panels on the duct entry element, the potential for daylighting performance improvement was analyzed by reducing the number of interreflections in the lighting duct.The result showed that depending on the length of the pipe, an improvement in daylight autonomy (DA300) of an indoor space between 16% and 19% is possible.
As a continuation of the previous mentioned studies, Obradovic et al. investigated the horizontal light pipe concept studied in their previous works in a full-scale test office to analyse illumination levels and lighting energy use [16].The chosen room, within an office building located in southern Norway, included southwest-oriented windows for natural daylighting via the façade and a horizontal light pipe with daylight entrance facing south.The focus in the case study was to evaluate the improvement of daylight levels.As a result, the reduction of the electric light demand especially in the rear part of the office could be shown because of the daylight provision by the daylight duct.The study included a monitoring of the outdoor and indoor situation and could be used for further theoretical studies.The presented study is indeed a good supplement to the monitoring results gained from the Skynative mockup in this paper.
Roshan and Barau presented a study on applying an anidolic daylighting system under tropical climate conditions [17].Mayhoub et al. addressed in their work one of the major challenges to integrate a horizontal lighting duct with other building service ducts like ventilation or air conditioning [18].They compare the differences and similarities of the different ducting systems and proposes a schematic design on an innovative dual ducting system, which allows the avoidance of conflicts between the building systems and simplifies the installation process.Lu et al. took a more general look and explored multi-dimensional evaluation methods for suitability of applying light duct systems [19].

Skynative -Design and function
There are various ways of transporting daylight into the interior of a building or room: In cooperation with BASF, Bartenbach has developed from 2017 to 2019 the concept of a horizontal daylight duct that efficiently guides daylight from the outside into the interior of a building.
In 2019, BASF outsourced the development and founded the start-up Skynative, which has since been marketing and further developing the system together with Bartenbach."Skynative" means as much as "natural sky".The daylightdirecting system can be used where conventional daylighting via the facade reaches its limits.2.1."Skynative" -Functionality and setup The light guiding system consists of three components: incoupling element, light duct, and decoupling technology, shown in Figure 1.The daylight entry surface is equipped with a daylight-redirecting film from 3M and is positioned directly above the transparent window of the conventional building façade.
The redirecting film enables that for a broad variety of sun positions, direct sunlight can be transported as effectively as possible through the duct with a reduced number of interreflections.By this, it is possible to transport daylight up to 20 meters into the depth of the building.Depending on the height of the duct, even longer distances could be achieved, but at least 30 centimeters of installation height are required for a proper functionality of the Skynative daylighting system.

Mock-up installation and monitoring setup
To test, experience and demonstrate the technology, a real-scale mock-up as system prototype has been installed at the premises of Bartenbach in Aldrans, Austria in 2017 (Figure 2).In addition to providing a real demonstration for future clients, from a research point of view it was the goal to obtain realistic observation data from the mock-up.Therefore, an office room was recreated with a typical furniture and room design.
In Figure 3 a panoramic view of this office room is shown.The external reference ("view outside") is established as a virtual window with an HD screen that mimics the real environment (seen on the right of the image: showing a live video of the external situation in the same viewing direction).Thus, the available daylighting in the room is provided exclusively by the integrated Skynative light ducts in the ceiling.For research purpose, the two ducts installed were equipped with different reflective materials: • Duct 1: 3M Specular Film DF2000MA • Duct 2: Alanod PRIME SILVER 27 DL The product "PRIME SILVER 27 DL" from Alanod is an aluminium sheet with special coatings to provide high light reflectivity (>98%) and UV-stability.In contrast, the 3M Specular Film DF2000MA is a polymeric film providing specular reflection with high light reflectivity, that is metal free and noncorroding.The purpose of applying two different materials was to test to different application methods (coated aluminium vs. polymeric film) and to obtain comparisons in their optical efficiency, a possible spectral shift and long-term stability.While the 3M-film resulted in a Melanopic Daylight Efficacy Ratio MDER close to 0,9, the Alanod aluminium sheet showed a spectral shift of the reflected light, resulting in a MDER about 0,7.The two ducts were sealed on the outside with insulating glazing (Optiwhite) and covered on the inside with a 3M Daylight Redirecting Film.To evaluate the system behaviour in the long term, illuminance levels have been measured indoors and outdoors since October 2018.In addition, luminance cameras were put into operation in January / February 2019.The results of the monitoring at Bartenbach presented in this paper lasted until the end of December 2019.

Monitoring setup
As part of the monitoring process, illuminance levels are measured at various positions (Figure 4).In addition, fisheye photos are taken, from which luminance images are subsequently created to obtain information about the luminance distributions.The measurement of the illuminances as well as the creation of the luminance images is carried out in the exterior area as well as in the interior area.
The illuminance measurements are implemented with an Ahlborn Almemo809 PC interface and corresponding sensors.Two FLA 613 VLM sensors are used outdoors and four FLA 623 VL sensors indoors.The measured values are stored every minute.The positions of the sensors are shown in Figure 4.The sensors S1 and S2 correspond to those in the outdoor area and are positioned horizontally and vertically.Sensor S3 is placed at the center of the ceiling to create an "integrating space" situation and to estimate the luminous flux.Sensors S4 and S5 are mounted on the standing table (110 cm height) and working table (75 cm height) respectively.In addition, a sensor S6 is mounted on the worktable vertically at eye level.On the standing table and the work table, the sensors are positioned directly under a duct opening: on the standing table under the 2nd opening of the 3M duct and on the work table under the first opening of the Alanod duct.
The positioning and orientation of the sensors is based on following questions, to get a benchmark for the system performance (1-3), possible health effects (4-5), and information to quantify the visual appearance in the room (6-7): (1) What illuminance levels are achieved at the workplace?(Measurement at the workplace) (2) Can 500 lx be reached at the workplace?(Measurement at the workplace) (3) How much light in total enters the room through the system?(4) What daily light dose can be expected?(Measurement at eye level -vertical) (5) Occurring luminance in the room?(Luminance camera with fisheye lens) (6) Sunspots and structures in space?(Luminance camera with fisheye lens) (7) Dynamics of the light situation and influence of the outside situation?(webcam / L-cameras) Outdoor measurements are mainly investigated in order to quantify the monitoring boundary conditions as well to serve input data for the simulation models.

Monitoring results
By monitoring the "Skynative" prototype, its mode of operation and performance can be measured and evaluated.The challenge is to record and map the dynamic processes of the daylighting system, as it is a key aspect, along with the photometric data, that adds value to the solution.

Daylight availability
For the 2019 measurement period, average continuous daylight autonomy (cDA) at the standing desk was 58.5% based on 300 lx and 46.8% based on 500 lx.While 300 lx is the target minimum for circulation areas in the building, 500 lx is the target minimum for desk lighting.In Figure 5, the monthly values for the achieved continuous daylight autonomy on the standing desk (S4) are shown.

5: Monthly results for the continuous daylight autonomy at the standing table (S4)
Comparing the collected annual results at the standing desk with the theoretical calculations for different climatic data shows a wide range of variation for the individual months.However, when viewed over the entire course of the year, there is good agreement with the measured values in the monitoring.

Illuminance levels
In Figure 6, the measured illuminance values on working desk level are shown in one-minute intervals.It can be seen that in the sunny periods with low sun angles (February, October, November) the highest illuminance levels can be achieved.
The higher daylight availability is due both to the generally higher radiation gains in these months on south-facing facades, and because transport efficiency increases due to lower internal reflections in the light channel.The illuminance levels achieved are lowest in the summer months.Nevertheless, the overall level matches the criteria with > 500lx during almost all working hours.3. Glare probability and melanopic equivalent daylight illuminance While illuminance levels reach peaks especially in months with low sun angles, also highest glare probabilities are measured in this period.Nevertheless, the glare ratings stay within acceptable levels throughout the year (Figure 7).The DGP values determined from the luminance images with values for the DGPmax below 0.25 are very moderate.As a comparison, the EN 17037 standard "Daylight in buildings" recommends for high glare protection that a DPG value of 0.35 should not exceed 5% of the usage time.It should also be noted that the vertical illuminances are often below 380 lx and are thus actually too low for an application of the DPG (compared to the right chart in Figure 7).Also, according to Evalglare, DGP values below 0.2 can underestimate glare.In general, it must also be mentioned at this point that the use of DGP in this context may have some limitations, as this method was originally developed for side windows.
Considering a Melanopic Daylight Efficacy Ratio MDER of 0,889 for the 3M duct according to Table 1, which means comparable to natural sunlight (DER = 0,9) at clear sky conditions, a melanopic equivalent daylight illuminance value (MEDI) of 250 lx vertically at eye level can be achieved for at least four hours at most of the days, especially in the more important winter period, when people working in offices are less exposed to natural sunlight due to the late sunrise and early sunset.

Daylight dose
As a benchmark for evaluating the potential for non-visual light effects and circadian entrainment of the Skynative daylight system, the daily daylight dose for the working hours of 08:00 to 18:00 was evaluated based on vertical illuminance measured on eye level (S6).A threshold of 5000 lxh is based on recommendations for light therapy [20], [21], where a vertical illuminance of 5000 lx for one hour has a noticeable effect, according to proband studies.As seen for the horizontal illuminances on work desk level, the highest daylight dose values were reached in the sunny winter period, while the minimum is reached in the summer month due to less radiation gains at the façade as well as higher internal reflections in the light duct (Figure 8).In terms of circadian entrainment, this is an advantageous behavior, as especially in short-day winter months, additional daylight will be beneficial for the daylight supply of office workers, while in long-day summer months, sufficient daylight dose might be reached even outdoors in times before and after working time.By further applying the simulation model in performing calculations based on theoretical EnergyPlus Weather Datasets [22], an extended comparison of the simulation results with the gained measurements was possible.
Analysis was performed using an Excel-based design tool, based on pre-calculated simulation data from the simulation model and verified with measurement data from the real mock-up.It is based on a standard room size of 5,4 x 5,4m (double size of the real mock-up) and 4 installed light ducts.The tool estimates, depending on the duct geometry (height and width) and the number of outlets, the maximal possible duct length to achieve a minimum continuous daylight autonomy of 50% on average for the defined room setting.In Figure 9 the outcome is represented in form of a performance matrix, showing the maximal duct length depending on duct width (x-axis) and height (y-axis).The tool allows a fast and easy estimation of the required number of light ducts and the required duct geometry; in addition, the possible length of the light transport can be evaluated.Nevertheless, it cannot replace a detailed engineering of the light duct in a later design stage.

Simulation case study on an open-floor plan
To evaluate the potential and the application of the Skynative daylighting system, especially in combination with a sidelight office setting, a simulation study for a representative floor plan with different room heights (2,75m -3,75m) was done (Figure 10).
For the simulation study.the sidelight windows have been equipped with a shading screen and a daylight-depending control, which activates the shading for an incident solar radiation on the façade of 120W/m2.The façade control was evaluated in two scenarios: an identical operation of the façade in all 4 cardinal directions, and an individual operation.
For the daylight calculation of the floor plan including the sidelight window and the sun shading control, Ladybug tools have been used.The daylight supplementation via the Skynative daylighting ducts have been added on top using the verified simulation model described under chapter 5.The results for the continuous daylight autonomy in the case of the floor plan daylight simulation without Skynative installation results in an achieved cDA > 50% for 78,1% of the floor area.Especially the core part was clearly below the threshold as seen in the top chart of Figure 11.
By including the supplied additional daylight via the Skynative daylighting ducts, a continuous daylight autonomy of > 60% in a wide range of the core part could be achieved.Thus, a utilization for additional workplaces seems reasonable according to the available daylight levels.The criteria of "view outside" can be still guaranteed by the existing sidelight windows at all four façade directions.

Conclusion
This paper shows the concept, the development, and the real-field application of the prototype light duct system Skynative, developed by Bartenbach and BASF and continued by the outsourced start-up with the same name (Skynative).An annual monitoring of a real-scale mock-up has revealed many positive aspects: 'Skynative' reproduces a light spectrum that closely resembles the dynamics of daylight and therefore likely has a similarly activating aspect on people indoors.The fact that daylight dose levels of up to 4,000 lxh per day have been achieved, especially during the winter period, can be evaluated positively.Considering a MDER of 0,9 for the 3M duct, a MEDI-value of 250 lx vertically at eye level can be achieved for at least four hours at most of the days, especially during the winter period.The monitoring results prove that with the use of highly reflective materials with a reflectance > 98.5 % in the light duct, a continuous daylight autonomy of more than 50 % related to 500 lx is possible even in windowless rooms.Orientation and external obstruction play a significant role in the system efficiency, as a maximum number of direct sunlight hours on the duct inlet is required to achieve high performance values as shown in the monitoring as well as the performance matrix in this work.

Figure 1 :
Figure 1: Schematic functionality and components of the Skynative ® daylighting system

Figure 4 :
Figure 4: Schematic arrangement of lux sensors (S1 -S6), L-camera and skycam in outdoor and indoor areas

Figure
Figure 5: Monthly results for the continuous daylight autonomy at the standing table (S4)

Figure 8 :
Figure 8: Daily daylight dose levels reached throughout the year of monitoring

Figure 9 :
Figure 9: Performance matrix based on the calculation of a reference setting

Figure 10 :
Figure 10: Representative office floor plan for the revitalisation simulation study

Figure 11 :
Figure 11: Simulation results in continuous Daylight autonomy: without (top) and with Skynative installation (bottom).

Table 1 :
Reached MDER values for the different duct materials