Space between: Swiss scientist uses vacuum as insulation medium
With the soaring cost of energy to heat and cool homes, energy conscious glass and window manufacturers search for technologies to reduce energy loss through window glass panes and frames. At present, state-of-the-art insulation glass technologies achieve, at best, U-values of 0.2 BTU/hrft2F0. One BTU, or British Thermal Unit, is equal to the amount of heat required to raise the temperature of one pound of liquid water by 1 degree Fahrenheit at its maximum density per hour per square foot of glass surface, expressed in Fahrenheit. To put this insulation value in perspective, this type of glass reduces temperature loss by a factor of 5 compared to a single, uncoated glass pane as used in older windows. However, the most modern glass insulation still is about five times worse than the insulation in modern house walls or roofs. Insulating glass researchers strive to achieve insulation value equal to building walls and roofs. Scientific studies suggest that might be achieved by using vacuum as the insulation media.
For example, a single-family home with 230 square feet of window surface area of high efficiency insulating glass with a U-value of 0.2 suffers an energy loss of 49,720 BTUs per day. In comparison, the same home with window surface with a U-value of 0.04 would decrease the energy loss to 110 watts per day, a saving of 40,710 BTUs per day.
Emil Bächli, a Swiss engineer, has developed an innovative vacuum insulating glass along with an automated manufacturing process for a contiguously sealed double-pane window. The window achieves, as validated in tests conducted by the organizations listed on p. 83, an insulation equivalent to a U-value of 0.035 BTU/ft2hrF0. During the last 18 years, Bächli and his team have conducted intensive research and development efforts resulting in a patented proprietary insulating vacuum glass—Bächli-Glass—and a manufacturing process for the production of windows. This effort is not the only research contributing to the development of vacuum IG units worldwide. Similar projects exist in Russia, Japan and Australia. However, at present, none have resulted in a vacuum glass with sufficient longevity and insulation capacity compared to conventional high efficiency insulating glasses available in the market place.
The finished Bächli vacuum glass unit, ready for insertion into a window frame, exemplifies an insulation performance as outlined in the paragraph above based on a total thickness of the glass unit of 0.325 inches. The space between the two glass panes is about 1⁄100 inch with a vacuum within this space of 10-4 millibars. This insulation capacity is about equal to the insulation values achieved in modern house walls and roofs. As a result of the geometry of the design, the Bächli-Glass obtains, in addition, a reduction in sound transmission of 38 decibel at 3150 hertz, a notable advantage for buildings in areas exposed to excessive noise. The Bächli-Glass product and manufacturing process have been patent protected in Europe, the United States and Japan.
An insulation media
Vacuum, as insulation media, is not yet well understood. Heat transfer occurs when warmer air molecules come in contact with colder air molecules, and the process starts an internal convection inside the space separating the two window panes. That results in warm air radiating out and cool air radiating in through the window, or the reverse in warm climates. The laws of vacuum physics say that vacuum separates the air molecules by a distance greater than the mean free path of an air molecule; consequently, these molecules are prevented from contacting each other, and, as a result, temperature transfer does not occur. The physical space containing a vacuum must be smaller than the mean free path between air molecules.
In a vacuum IG unit, at a vacuum of 10-3 mB, air molecules are prevented from contacting each other if the space between the two glass panes is smaller than the mean free path of air molecules at that vacuum. As an example, the mean free pathway of air molecules at a vacuum of 10-3 mB is 23.6 inches, compared to the space between the two glass panes of 1⁄100 inch. Theoretically, vacuum inhibits temperature transfer completely. The reason for residue temperature transfer in vacuum insulate windows is that the two glass panes must be separated from each other through spacers, which in turn allow some temperature transfer. Window frames made of wood or aluminum also allow temperature transfer.
Vacuum spaces up to 0.4 inches are safe from risk hazards of explosion and implosion in case of accidental breaking of the vacuum glass. The extremely small space between the glass panes prevents ambient air from invading the vacuum space at sufficient force to create an explosion of the panes following the implosion created by the breakage.
Design and process
The Bächli-Glass design consists of two floating glass panes, either tempered or regular flat glass, with a thickness of 0.155 inches each, spaced 1⁄100 inch apart. The panes are separated by proprietary, transparent polyester pillars with a diameter of 0.4 inch and height of 0.01 inch with minimal temperature transfer coefficients. These pillars also are extremely resistant to pressure, but not hard enough to scratch the inside surface of the glass panes during independent expansion-contraction of the panes due to temperature differential between the outside and inside of the glass pane. The adhesion force of a vacuum is enormous, equal to a pressure of about 2,000 pounds per square foot. Despite their ability to absorb high pressure loads, the pillars are discreet and do not cast shadows even when exposed to direct sunlight. The panes are sealed together with an innovative and proprietary edge-capping system, in which the metal alloy and the glass bond together on a molecular level.
Worldwide, researchers have been working at designing Bächli-Glass for several years. They have arrived at various ways of keeping the panes separate. Other means include glass pillars to serve the dual purpose of resisting the force of the vacuum and keeping the panes separate. However, glass has high thermal conductivity, and therefore, using spacers made of glass decreases the insulation efficiency significantly of such a vacuum glass. Furthermore, in cases of extreme temperature differentials between the outside and the inside surfaces of a window, the panes expand and contract independently, resulting in small movements of the pillars inside the vacuum space. With the hardness of glass, these pillars tend to scratch the low emissivity coating on the inside surface of the panes, thereby creating a “star-light” effect over time when exposed to bright sunshine.
The edge-capping frame ensures vacuum seal integrity and creates a bond stronger than the material properties of the glass pane itself. This is achieved through a proprietary cold-sputtering cathode deposition process utilizing argon gas which deposits a 0.4-inch band of three extremely thin layers of chrome, nickel and tin around the four sides of the glass pane on the outside. This glass-metal compound that fuses the edges of the glass panes to the noble steel band results in a durable, high integrity and totally gas-tight bond. It ensures total vacuum seal integrity and allows the two glass panes to independently expand and contract during mechanical loads and climate influences.
Prototypes of this vacuum glass have been tested and the results have been confirmed utilizing gas spectrometry and electron microscopy by the Frauenhofer Institute for Solar Energy Systems, in Germany, the Paul Scherrer Institute for nuclear research, in Switzerland, and the Swiss Federal Laboratories for Materials Testing and Research, in Switzerland. The results are not public, but are on file as hard copies and available to interested parties. The tests performed demonstrated that the glass breaks before the edge capping sealing system disintegrates, and the vacuum is preserved in excess of 30 years. The schematic above represents the components and buildup of Bächli vacuum glass.
From the onset, Bächli, with his lifelong experience in vacuum technology and its applications, was aware that vacuum glass with a guaranteed longevity of 30 years must be produced within a vacuum. The reason for his scientifically well-founded thesis is that moisture within the vacuum space, even at a molecular level, will destroy the vacuum. It is impossible to remove molecular residues of moisture from the inside surface of glass panes if they are manufactured, assembled and then sealed under atmospheric conditions. The vacuum between the glass panes is created at the end of the process. Consequently, Bächli, together with his process engineers, designed and specified each machinery and equipment involved in the manufacturing process to function under vacuum conditions.
The Bächli manufacturing process is integrated into a vacuum tunnel and operates in one continuous process stream, in separate and sequential process steps. Within the vacuum tunnel, encapsulated by air-locks and loading stations at both ends, the line with its five manufacturing stations operates autonomously in a continuous flow, and utilizes proven robotic technology to manipulate and manufacture the vacuum glass.
The vacuum tunnel layout determines the maximum dimensions of the vacuum glass panes. The current pilot plant engineering design can accommodate rectangular glass up to 7 feet by 10 feet with a capacity to produce up to 2,000,000 square feet of vacuum glass per year. Smaller sizes can be manufactured without modifications to the machinery. The process technology could be applied to bent or curved glass but would require some engineering modifications.
The aerospace industry over the last 20 years has developed amazing technologies related to vacuum conditions that are available today for applications within private industry. Numerous companies, notably in the electronic industry, use these technologies in manufacturing processes.
Status and economics
Bächli and his team have manufactured and tested every part of the equipment needed for the process, but have not yet realized continuous manufacturing under vacuum conditions. Having financed the very substantial development, Bächli is looking for investors to support him for the realization of the first production unit.
Upon completion of this first manufacturing unit, there will be a strong demand for licenses to exploit the technology worldwide. The detailed engineering of this first unit anticipates that vacuum glass can be produced at a cost of approximately $2 per square foot based on an annual output of 2 million square feet of vacuum glass. These costs include more than 10 years for amortization of the plant and related engineering and startup costs. The investment is estimated at $12 million.
Taking into consideration the savings in energy costs by using vacuum insulation in windows, market research in Europe has confirmed the strong interest by architects and homeowners for such novel breakthrough technology. However, as with any novel technology, to advance it to a commercial reality, there must be entrepreneurs with the vision and the will to make it happen.