Issue
Renew. Energy Environ. Sustain.
Volume 2, 2017
Sustainable energy systems for the future
Article Number 29
Number of page(s) 7
DOI https://doi.org/10.1051/rees/2017044
Published online 08 September 2017

© H.F.O. Mueller, published by EDP Sciences, 2017

Licence Creative Commons
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction, state of the art

“Micro-optical structures” are generally defined as surface structures with lateral and depth dimensions in the order of 1–1000 μm. They are applied in transparent building elements for advanced daylighting as well artificial lighting systems in order to improve quality as well as energy and cost efficiency of lighting.

Figure 1 shows the principle solution of sunlight redirection to the depth of the room and a double-sided microstructure for this application [1]. The specific shape of the structures redirects incident light from a solar altitude range between 15° and 65° to emergent angles larger than 0° (to the ceiling) without solar tracking.

Figure 2 shows the integration of LED lighting in windows. The angle-selective light emission by the cylindrical microstructure allows to look through the lightguide and the window unless you do not look directly into the emitted light.

The initial development of suitable micro-optics for the described applications in Figures 1 and 2 by simulation, manufacturing of prototypes and measurements is proceeding now in a joint development project with a run-time from 2016 to 2019. Scientists, manufacturers of microstructures and building components as well as architects are co-operating in the integrated approach of the TALED project (see “Acknowledgements” section).

thumbnail Fig. 1

Left: Window with sunlight redirecting glass in upper window area and shading in lower window area. Right: Double-sided microstructure with lenses and prisms for redirection of sunlight.

thumbnail Fig. 2

Left: Window with LED lighting in upper window area by transparent lightguide. Right: Edge-lit lightguide with cylindrical microstructure for one-sided, selective light emission [2,3].

1.1 Objectives

The main objectives and work packages of the project are defined as follows. The present state microstructures have to be optimized as to optical performance and largescale manufacturing. The technical integration of micro-structured daylighting and LED systems in windows is developed in a systematic approach of modular systems. Prototypes of lighting systems in realistic dimensions and constructions are manufactured and their performance is measured in laboratories and test rooms. Last but not least, the options of architectural integration are demonstrated in exemplary designs, complemented by simulations of lighting performance and energy efficiency.

2 Methodology and results

2.1 Optimization of structures and manufacturing technology

The optimization of micro-optical structures concerns system performance and largescale manufacturing technologies. Experienced project partners are involved for the two the different technologies: hot embossing (Fig. 3) and UV embossing (Fig. 4).

Hot embossing is applied for one-sided as well as for two-sided polymer structures in one step. Structure dimensions are 200–500 μm, produced in maximum element sizes of 1500 mm × 600 mm. The master is a flat mould cut in steel or brass (for prototypes) by flywheel or fad. Manufacturing is carried out in a plate to plate process in PMMA and PDMS. The de-moulding of the hot embossing process required an adaption of structure geometries.

UV embossing is applied for one-sided as well as for double-sided polymer structures. Structure dimensions are 20–500 μm, produced in a maximum element width of 1000 mm. The master is produced by photolithographic technology on a cylindric metal mould or on a film. Manufacturing is carried out in a roll to plate or roll to roll process in varnish which is hardened by UV light. The substrate can be glass or PMMA.

For the LED lightguides two options of light emission were examined: even or uneven light emission depending on the distribution of structural elements in relation to the light source. Figure 5 shows that the reference system with regular distribution leads to a high emission of the light flux on the first 100 mm of the light guide. By a specific stochastic distribution of the structure a uniform light emission over the length of the lightguide is achieved.

Simulations of the illuminance of an office room have shown, that the uniformity of light emission from the lightguide has no significant influence on the illuminance distribution in the room.

The optimization of LED lighting included the examination of shape and thickness of lightguides. Findings are: the thinner the light guide, the higher the emitted light flux. A wedge-shaped lightguide increases the emitted light flux. In consideration of construction and available material 2 mm thick.

PMMA panes with parallel surfaces were chosen as light guides. Actual prototypes of the LED lightguides show a higher transparency for the samples produced in UV embossing (Fig. 6).

thumbnail Fig. 3

Hot embossing.

thumbnail Fig. 4

UV embossing.

thumbnail Fig. 5

Optimized light emission over the length of lightguide by distribution function of structure. Red line: Reference system with even distribution of structure. Black line: System with distribution function ([4], RIF).

thumbnail Fig. 6

Prototypes of LED light guides in hot embossing (left) and UV embossing technology (right).

2.2 Building integration

The microstructures are protected against physical impact and atmospheric deposits by positioning them in the void between two glass panes. The variation of glass integration in Figure 7 shows solutions for double and triple glazing. The two-sided structure for sunlight redirection can be integrated either on a third glass pane or as a composite film on one of the void surfaces of a double glass unit. The one-sided structure of LED lightguides can be positioned on surface 2 of a double glass unit, if surface 1 is kept clean. A third pane has to be installed, if the risk of light emission by dust on surface 1 has to be eliminated. The combination of microstructures for both, sunlight redirection and LED light guides, can only be realized in a triple glass unit, using a composite film for sunlight redirection.

For the LED lighting 2 mm lightguides were integrated in a double glass unit. Fitting LED, mounted on strips, are fixed on the edge of the lightguide. There is a variety of LED on the market, offering the necessary light flux of approximately 3000 lm/m (Fig. 8).

Integrating LED's in window frames means to control the heat emitted. Simulations and tests have shown, that LED with 20 W/m (3000 lm/m) can create temperatures >100 °C in a well insulated window frame (Fig. 9). By special means of heat conduction from the LED to the outer shell of the frame temperatures can be reduced to acceptable values <60 °C. Tests with aluminium and timber frames have bee carried out (Fig. 10).

The construction details of integrating LED's in windows are solved in co-operation with the involved manufacturers of glass, luminaires and facades. A special challenge is the accurate positioning of LED's on the edge of the light guide in the glazing unit, which has to work under conditions of initial mounting as well as maintenance. A sample is shown in Figure 10.

thumbnail Fig. 7

Integration of microstructures for sunlight redirection and lightguides in double and triple glass units.

thumbnail Fig. 8

Triple glazing with light guide in the middle (left) and fitting LED strips with differing LED dimensions (right).

thumbnail Fig. 9

Temperature in window frame with LED (20 W/m) mounted on edge of lightguide. Left: Well insulated frame without heat conduction. Right: Heat conduction by metal connection from LED's to the room facing surface of the frame ([4], ai3).

thumbnail Fig. 10

LED lighting integrated in a timber window frame. Laboratory mock-up.

2.3 Architecture and building types

The application of daylighting and LED systems with microstructures is strongly influenced by architecture and building type. The position and size of sunlight redirecting elements in facades are influenced by the depth and height of rooms to be illuminated naturally. Specific solutions are offered for atria, courtyards and lightshafts to be illuminated from skylight by redirected sunlight (Figs. 11 and 12).

For the integration of LED lightguides in transparent buildings components like windows, partitions and roof lights typical rooms and buildings are analysed.

Actual designs for different kinds of buildings are used in co-operation with architects to check the applicability of the lighting systems with microstructures and to adjust them to the specific situation. Based on systematic siumulations and measurements in test rooms, thumb rules for dimensioning are developed for both, sunligthing and LED lighting systems.

Figure 13 shows the example of a classroom illuminated by LED lightguides in facade a partitition. Room dimensions (length × width × height) are 8 m × 7 m × 3 m. The technical lighting data are:

Two lightguides (window and partition) with 2 LED-strips each 3000 lm/m; room depth 7 m.

Overall light flux 2 × 2 × 3000 lm/m/7 m = 1714 lm/m2.

Mean illumination 500 lx.

Room utilization factor (overall efficiency) 500 lm/m2/1714 lm/m2 = 0.29.

Installed wattage 1714 lm/m2/150 lm/W = 11 W/m2.

The LED lighting offers a variation of illuminance and colour temperature [5]. Thus it allows to adjust the illumination to the biorhythm and activity level of the occupants, as Figure 14 shows. Lightguides with LED at the upper and lower edges are positioned in the clear-storeys of windows and partitions. In addition desk lights with transparent lightguides are distributd in the room.

thumbnail Fig. 11

Architectural integration of sunlight directing elements.

thumbnail Fig. 12

Example for sunlight redirection in a shopping center. Daylit floor areas red.

thumbnail Fig. 13

Example of a classroom illuminated by LED lightguides in facade and partition.

thumbnail Fig. 14

Office room with LED lightguides in upper areas of windows and partitions and in task lights. Left: Cool white, high illuminance (activity support, e.g. in early morning). Right: Warm white, low illuminance (activity reduction, e.g. in late afternoon).

3 Conclusions

Micro-optical structures are used for advanced daylighting and LED systems, which can be integrated in transparent building elements of facades and partitions as well as in roof lights. Large scale manufacturing of the structures is applied and the first prototypes have been integrated in glass- and window constructions. The combination of both systems will increase the lighting comfort and the energy efficiency in non-residential buildings. The next steps towards marketable products will be to apply and monitor the systems in test rooms and demonstration buildings.

Table 1

Participants of the joint research project TALED with matter of collaboration, name and logo of participant.

Acknowledgments

The participants of the joint research project TALED, duration from 2016 to 2019, are shown in Table 1.

References

  1. S. Klammt, H.F.O. Müller, A. Neyer, Microoptics for efficient redirection of sunlight, Appl. Opt. 51, 2051 (2012) [CrossRef] (In the text)
  2. F.-C. Tengler, M. Jakubowski, A. Neyer, High Transparent Light Guiding Plate for Single-Sided Light Emission, Poster Abstract, Micro and Nano Engineering (MNE), London, 2013 (In the text)
  3. H.F.O. Mueller, F. Sasso, Energy-efficient lighting by LED, in Proceedings WREC XIII, 03–08 August (University of Kingston, UK, 2014) (In the text)
  4. Research Reports of Project TALED, Energy- and cost efficient, façade integrated Day- and LED lighting based on mico-optical components (2016–2019) (unpublished), Co-ordination: Fraunhofer Institute Building Physics, Stuttgart, Germany (2016–2019), Partners: RIF e.V., Karl Jungbecker GmbH, Temicon GmbH, Durlum GmbH, Saint-Gobain Securit GmbH, Green Building R&D GmbH, ai3, SSP AG, Funding: German Federal Ministry for Economic Affairs and Energy (In the text)
  5. H.F.O. Muller, Spectral variation of energy-efficient room lighting, in Proceedings Med Green Forum & Renewable Energy, edited by A. Sayigh (Springer, Switzerland, 2017), pp. 755–765, ISBN 978-3-319-30745-9 (In the text)

Cite this article as: Helmut F.O. Mueller, Micro-optical structures for daylighting and led systems, Renew. Energy Environ. Sustain. 2, 29 (2017)

All Tables

Table 1

Participants of the joint research project TALED with matter of collaboration, name and logo of participant.

All Figures

thumbnail Fig. 1

Left: Window with sunlight redirecting glass in upper window area and shading in lower window area. Right: Double-sided microstructure with lenses and prisms for redirection of sunlight.

In the text
thumbnail Fig. 2

Left: Window with LED lighting in upper window area by transparent lightguide. Right: Edge-lit lightguide with cylindrical microstructure for one-sided, selective light emission [2,3].

In the text
thumbnail Fig. 3

Hot embossing.

In the text
thumbnail Fig. 4

UV embossing.

In the text
thumbnail Fig. 5

Optimized light emission over the length of lightguide by distribution function of structure. Red line: Reference system with even distribution of structure. Black line: System with distribution function ([4], RIF).

In the text
thumbnail Fig. 6

Prototypes of LED light guides in hot embossing (left) and UV embossing technology (right).

In the text
thumbnail Fig. 7

Integration of microstructures for sunlight redirection and lightguides in double and triple glass units.

In the text
thumbnail Fig. 8

Triple glazing with light guide in the middle (left) and fitting LED strips with differing LED dimensions (right).

In the text
thumbnail Fig. 9

Temperature in window frame with LED (20 W/m) mounted on edge of lightguide. Left: Well insulated frame without heat conduction. Right: Heat conduction by metal connection from LED's to the room facing surface of the frame ([4], ai3).

In the text
thumbnail Fig. 10

LED lighting integrated in a timber window frame. Laboratory mock-up.

In the text
thumbnail Fig. 11

Architectural integration of sunlight directing elements.

In the text
thumbnail Fig. 12

Example for sunlight redirection in a shopping center. Daylit floor areas red.

In the text
thumbnail Fig. 13

Example of a classroom illuminated by LED lightguides in facade and partition.

In the text
thumbnail Fig. 14

Office room with LED lightguides in upper areas of windows and partitions and in task lights. Left: Cool white, high illuminance (activity support, e.g. in early morning). Right: Warm white, low illuminance (activity reduction, e.g. in late afternoon).

In the text

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