The invention relates to vacuum thermal insulation suitable for the transmission of thrust forces and more particularly for heat storage means in motor vehicles, comprising an evacuated insulating zone and a load-bearing brace member of pore-forming insulating material.
Owing to the exacting requirements as regards costs, weight, volume and efficiency which have to be met by heat storage means, which are intended for use in motor vehicles, the invention will be described with reference to such heat storage means without the application thereof being limited to this purpose.
These exacting requirements lead to a series of problems. Owing to the requirement for high efficiency, in the present case with respect to the tolerable heat dissipation, it is only possible to employ vacuum insulation. Owing to the limited amount of space available in modern motor vehicles, which is due, among other reasons, to the higher expectations as regards mileage and exhaust gas emission, such heat storage means are to be as small as possible and light in weight and the external configuration is to be able to be modified by the designer so that the heat storage means may be accommodated in small spaces.
Conventional insulating techniques provide two types of vacuum insulation, that is to say high vacuum insulation on the basis of the Dewar vessel, which has been known for approximately 100 years, and microporous insulation. In this case the thermal flux is interrupted by three main paths--gas convection, thermal radiation and thermal conduction--in various different ways, although the external structure remains the same: the heat storage means is enclosed within a double-walled insulating vessel constituted by an inner and an outer container, such vessel being hermetically sealed and being able to maintain a vacuum for a long period of time.
The term high vacuum insulation is used to denote a type of thermal insulation, in the case of which the gas pressure in the insulated zone is under 10.sup.-3 mbar and in the entire insulating zone devices are arranged for the reflexion of the thermal radiation.
In the case of high vacuum insulation thermal conduction and gas convection are prevented because the space is evacuated to be substantially free of gas and measures are taken to ensure that such vacuum is maintained for prolonged periods of time of for the instance 10 years, for which purpose getters may be used.
In the case of high vacuum insulation the pressure varies between 10.sup.-7 and 10.sup.-3 mbar, the free paths of the gas molecules amounting to between 10 m and 1 mm. Therefore below a pressure of 10.sup.-3 mbar it is possible for the path length to be under its characteristic value in the insulated space of 1 mm, which is at the limit of what may be manufactured, for the prevention of thermal conduction of the gases. The gas convection is prevented by the low mass density.
The thermal radiation is for instance prevented by coating the walls of the insulated space with reflecting materials such as films of aluminum, copper or silver. These measures are slow to perform and are cost-intensive. The result is a highly intensive thermal insulation with a minimum space requirement, because the insulating effect is not dependent on the thickness of the gap, that is to say the wall distance between the inner and the outer containers. It only has to exclude the possibility of contact between the walls. The weight of the insulation only results from the weight of the outer container.
In the case of insulating vessels with a curved encompassing surface running between two end surfaces, and more particularly of circularly cylindrical insulating vessels, the forces resulting from the pressure difference between the vacuum space and the surroundings as compressive forces are taken up in the outer container, the wall needing some means to prevent buckling. This preventative means may be for instance in the form of circularly cylindrical corrugations following the curvature so that there is a dimensionally stable and lightweight outer container.
In the case of other configurations of vessel, more particularly with planar surfaces, high vacuum insulation is seldom utilized owing to the costs, the space requirement and the weight of the brace means to resist the vacuum pressure.
The microporous vacuum insulation consists of solid materials with a low thermal conductivity. The spatial distribution of the material is such that it is pervaded by a system of small pores (micropores), which may be evacuated. It is generally a question of fibers, powders and foams. Owing to the widespread use of glass fiber insulation this material will be taken as a basis for further explanations.
The pore size is defined as the diameter of the fibers and their distance apart. The fiber distance may be readily ascertained indirectly by weighing and bulk density, referred to as density for short, measurement. In lieu of the pore size, which may be hardly measured directly, calculations are therefore based on bulk density and fiber diameter.
The commonest fiber diameter is approximately 5 microns. In the case of a density of 200 to 300 g/l the minimum thermal conductivity is attained at 1 mbar. If at this pressure the density is increased, the thermal conductivity of the insulation will increase as well, because the conduction through the fibers increases. If at this pressure the density is reduced, the thermal conductivity of the insulation will also increase, because conduction in the gases also increases. Therefore it is possible to conclude that in the case of these values for the fiber diameter and the density the characteristic pore size is just less than the free path of the gas molecules, which at a pressure of 1 mbar is equal to approximately 1 micron.
In order to reduce the flow of heat by colliding gas molecules, the condition "pore size&lt;free path" has to be fulfilled. Furthermore, convection due to gas molecules has to be prevented, something that is also effected by the glass fiber.
If on the other hand the density is maintained constant at for instance 250 g/l and if the pressure is reduced to under 1 mb, there will be no further reduction in the thermal transfer at the latest on reaching the high vacuum range (below 10.sup.-3) and it will maintain itself at a level which is high in comparison with high vacuum insulation. This amount corresponds to the conduction in solids, that is to say in a glass fiber and its value decreases with the thickness of the insulating layer.
Therefore it is necessary for the thickness of the insulating layer to have a value corresponding to the desired thermal insulating effect and owing to the high density, which is necessary to make the pores sufficiently small, micropore insulating structures have a large bulk and weight. Furthermore the costs of the insulating material are dependent on this.
In the case of micropore glass fiber insulation the thermal radiation is checked by the fibers. This effect may be increased by the use of suitably colored fibers.
As regards its efficiency, volume and weight high vacuum is clearly superior to micropore insulation. With respect to the comparative costs experts are not however unanimous in their views. As regards the shaping of the structures to be insulated micropore insulation has a clear lead since it is suited to all different configurations of the insulating vessel, even those with flat container walls, because it can transmit thrust forces.
Steps taken towards a combination of the advantages of both forms of insulation are for instance described in the German patent publication 3,725,167 A. In this case the thermal insulation is divided up into brace members with micropore thermal insulation and interstices with a high vacuum insulation so that the insulating zone is subdivided up into alternating zones, in which respectively the one or the other of the above mentioned conventional thermal insulating structures are utilized, namely high vacuum insulation and micropore insulation.
It would be possible to attempt to adapt the quantity of the supporting insulating material employed to the load. However, the complexity of production and assembly of mutually separated supporting brace means renders it impossible to attain sufficiently small supporting distances in order, without reinforcement of the walls of the insulating zone, to take full advantage of the mechanical load limits of the insulating material. Therefore the thermal conduction, the volume and the weight are higher in practical applications than would appear to be theoretically possible. Practical testing has therefore not fulfilled expectations.