vacustruct® is a low vacuum insulation glass building system for greenhouses and daylight buildings.

Abstract

Modules (up to 6m x 3m x 0,3m) with a framework made out of ultra high performance concrete with a patented double sealing, integrated shadow and heating elements and an u-value of 0,2W/(m² x K) will meet the demands of price-sensitive markets with huge volumes.

Introduction

100 million m² of single glass, screens as additional thermal insulation and an average annual energy requirement of approx. 350 kWh/m² - which sounds like energy-related building mistakes from the past - are current figures from Dutch horticulture. In Germany, the figures are smaller - but not because of improved energy efficiency, but because of the widespread phase-out of the production of crops in need of heat.
Greenhouses with high light transmission have thus far been clearly inferior to other types of buildings in terms of thermal insulation performance. Both the ZINEG project (the future initiative for low-energy greenhouses) and the Dutch project "Kas als Energiebron" (greenhouse as an energy source) have produced solutions that promise significant improvements over the (often obsolete) greenhouse stock. Through double glazing, glass-foil combinations or multi-layer foil roofing in combination with several energy screens, greenhouse roofs can theoretically achieve U-values of approx. 1.0 W (m² K)-1. This value is already achieved today with triple-glazed standing walls in garden centre buildings, but this is accompanied by low light transmission. Vacuum is indispensable for achieving significantly better insulation properties with high light transmission.

R&D project GIFpro

GIFpro - Prefabricated low vacuum insulating glass construction system: prototypical implementation and evaluation of a roofing system for greenhouses (and other buildings with daylight utilization) In the project, vacuum-insulated pre-fabricated greenhouse elements with solar glass and a frame made of a new, vacuum-sealed special concrete (ultra-high strength concrete, UHPC) are developed to practical maturity, implemented as a prototype and scientifically evaluated.
The sealing of the vacuum is decisive for the long-term performance of a vacuum insulation. The modules have a double seal with protective vacuum (1mbar to 10mbar) in the frame. In the event of a pressure increase, this can be evacuated by means of a variable or permanently connected vacuum pump. This type of sealing is patented and allows the use of elastic seals that can compensate for different material expansions. Gas-tight glass-lot or glass-metal connections are not able to do this, or only to a limited extent, and represent the weak point of vacuum insulation glass.

A challenge with vacuum insulation glazing is the atmospheric pressure, which affects the panes and the frame with 10to/m². Spacers between the panes can be integrated almost invisibly with a narrow gap (high vacuum), but this is not possible with low vacuum insulation with large distances between the panes. Unlike windows, greenhouses are all about light transmission and not about an unobstructed view, so the less complex low vacuum insulation can be used.

A large space between the panes also offers the possibility of integrating movable sun protection elements that act as external shading to protect against heat. The material used for this purpose is inexpensive, tinned sheet metal ("can sheet"), which has the optimum optical properties and also retains these thanks to the "vacuum protection" (corrosion and soiling are prevented).
The frame of the modules is made of an ultra-high strength concrete (UHPC), which has no cavities due to aggregates in the nanometer range. The advantages of the UHPC frames are the possibilities offered by the material. Essentially, these are - in comparison to aluminium or steel - low material costs, the processing as jointless casting (no welding seams or similar), the almost variable formation of the crosssections including the insertion of vacuum bodies, as well as the impermeability for water and gas. The total width of the double-walled frame with cavity will be approx. 6cm, the two walls will be only 0.8cm to 1.2cm thick. The distance between the panes of the modules is 30 cm, pre-calculations indicate an Uvalue of 0.2 W (m² K)-1.
The GIFpro project will not only differ from other greenhouses in terms of its outstanding thermal insulation properties, but also in terms of its modular prefabricated element construction with further integrated functions. In addition to the sun protection, heat exchanger tubes are integrated in a further step, with which the entire glass surface can be thermally activated. Both the defrosting of snow, the heating of the interior as well as the cooling can be carried out with small temperature differences over the envelope surface. The high stability of the elements makes it possible to largely dispense with additional substructures and reduces investment costs. The modules can be dismantled and reused elsewhere. This opens up new opportunities, especially in the area of project financing.

Conclusions and outlook

The combination of concrete and glass with vacuum is completely new and requires interdisciplinary cooperation, because hitherto unknown questions arise (e. g.: does concrete crawl if it is permanently exposed to a pressure difference of 1 bar). The consortium with its different competencies and fields of activity would probably not have come together without a funded research project. In the opinion of all those involved, the greenhouse is very well suited to the new technology, since building regulations do not act as an obstacle to innovation to the extent that they do in other areas of the building industry. Scale effects can be achieved very quickly, as greenhouse systems are often built over several hectares. The future areas of application will not be limited to greenhouse construction. This construction method is also suitable for facades and roofs and will allow a much better use of natural daylight. With regard to thermal insulation and heat protection, no more compromises have to be made.

Acknowledgements

This project (HA-Project No.: 493/16-05) is funded within the framework of HessenModellProjects from funds of LOEWE - national offensive on the development of scientific and economic Excellence, funding line 3: SME joint project

Contact

Stefan Lück An den Weilerwiesen D-61169 Friedberg
www.vacustruct.com
Tel.: +49 173 2841925 Fax: +49 6031 62999 E-Mail: stefan.lueck@email.de

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