The present invention concerns a regenerator, in particular to be applied in thermal engines.

Thermal engines with regenerators are known from the prior art. The regenerator heats up and cools down the working fluid of the thermal engine each time the working fluid passes through the regenerator. Known regenerators are thereby designed as porous structures which allow the fluid to flow through the regenerator and contain walls which perform the heat transfer between the regenerator material and the working fluid. However, the known regenerator designs are limited to relatively low temperature gradients, in particular because their length and the porousness are limited by the flow resistance that is acceptable for regenerators in thermal engines.

Object to the present invention is to provide a regenerator, which improves the temperature gradient achievable in the working fluid and thereby remains compact and has a low flow resistance of the regenerator.

This object is solved by a regenerator according to claim 1 . Further preferred embodiments of the invention are described in the claims.

According to the invention, a regenerator is provided, comprising at least two segments, wherein each segment comprises a permeable section with a circular and/or polygonal ring shape which extends around a flow axis, wherein two of said segments are spaced apart from each other by a spacer, wherein the spacers are made of other material than the permeable sections of said segments. The core feature of the regenerator according to the present invention is, that it is made of several, at least two, segments, which are insulated from each other by spacers. This feature allows for different material temperatures in the respective segments. Since the spacers prevent the temperatures of the segments to be conformed by inner conductive heat transfer between the segments, these different temperatures in the segments remain, while the thermal engine continues to another working cycle. When a hot working fluid flows through the regenerator according to the present invention, it transfers heat to the first segment it flows through, thereby lowers its own temperature and heats up the first segment. When reaching the second segment, the temperature gradient between the working fluid temperature and the material temperature of the second segment is lower than at the first segment. Consequently, the second segment is heated up about a lower temperature difference than the first segment. Due to the spacers, this temperature difference between the first and the second segment remains until the working fluid returns to the regenerator in the opposite direction with lower working fluid temperature for being heated up by the regenerator. When reaching the second segment (now the first segment with respect to the flow direction) with the lower temperature, the cold working fluid is heated up about a first temperature difference. The partly heated working fluid afterwards enters the first segment (now second segment with respect to the flow direction), which still has the higher material temperature from the previous contact with the working fluid. Accordingly, the temperature gradient between the partly heated working fluid and the first segment is still high and accordingly, a high amount of heat energy can be transferred from the first segment to the working fluid, which increases the working fluid temperature about a second temperature difference. The sum of the first and second temperature difference achieved by the first and the second segment of the regenerator is higher than the temperature gradient achievable by a one-piece regenerator of the same length as the two-segment regenerator according to the present invention. Thus, the regenerator according to the present invention combines a compact design with a higher working fluid temperature gradient, which increases the thermal efficiency of a thermal engine equipped with regenerator according to the present invention.

Preferably, at least one of said segments comprises a permeable section with a plurality of circular or polygonal flow channels that are separated from each other by walls, wherein the flow channels extend with substantially constant cross-sectional geometry parallel to the flow axis. In particular preferred, the permeable section of the respective segment comprises circular or polygonal flow channels that are separated from each other by walls. In a first preferred embodiment, the permeable section is made of a cellular tailor-made structure, e.g. a cross-sectional shape with constantly repeating patterns of for instance polygonal shapes. One preferred example of such cellular tailor-made structure is a honeycomb structure, which is easy to be manufactured and combines a high mechanical strength with a high flow rate due to relatively small wall thicknesses which result in low flow resistance of the honeycomb structure. However, in particular to increase the wall thickness and thereby the ability of the regenerator to retain a high amount of heat energy, the cross-sectional shape of the flow channels may preferably be circular, e.g. the permeable section comprises cylindrical flow channels. It is to be understood that a low wall thickness between the flow channels will reduce the flow resistance while at the same time the total heat capacity of the permeable section is reduced. Conversely, an increased wall thickness will allow the permeable section to retain and save more heat energy while the flow resistance also increases. In a preferred embodiment the average wall thickness of the permeable section is about 0.1 mm to 0.5 mm, in particular preferred about 0.1 mm to 0.2 mm.

In a particularly preferred embodiment the flow channels have a hexagonal shape which is easy to manufacture and allows for a high mechanical stability. Furthermore, the hexagonal shape allows for relatively constant wall thicknesses of the walls between the flow channels.

In another preferred embodiment at least one of said segments comprises a permeable section formed by a helically extending guide element, wherein at least one flow channel is formed between two sections of the guide element, wherein the at least one flow channel extends with substantially constant geometry parallel to the flow axis. The helically extending guide element extends around the flow axis while the sections positioned at the same annular position form a flow channel there in between. In a first embodiment it would be possible, that the helically extending guide element forms one helically extending flow channel. The embodiment of one guide element which is helically formed around the flow axis is very easy to produce and allows for low-flow resistance, since there are less wall sections compared to the circular, polygonal, or hexagonal flow channel geometry, which would cause flow resistance. Furthermore, since the guide element can be formed as one thin sheet metal which is then turned around the flow axis and held at one end, the distance between the sections of the guide element can be decreased or increased only by rotating one end of the guide element more or less. This allows for an easy optimization of the ratio between flow resistance and total heat capacity of the permeable section.

Furthermore preferred, the sections of the guide element are supported by a support element and positioned at a distance from each other in order to form the flow channels). To increase the stability of the permeable section of the segment, a support element holds the guide element in its helically deformed state. In a preferred embodiment the support element is made of the same material as the guide element and thereby contributes to the heat saving capacity of the segment.

Preferably the support element and the spacer are formed as one piece. The support element and the spacer can be one and the same element, which eases the production and assembling of the regenerator. Thereby the length of the support element parallel to the flow axis is larger than the guide element's extension parallel to the flow axis. It is to be understood, that in this case the support element is made of another material than the guide element, in particular material which is able to insulate the guide element of a first segment from the guide element of a neighbouring segment of the regenerator.

According to a preferred embodiment of the invention, at least one of the segments comprises an outer ring and/or an inner ring, wherein the permeable section is supported by at least one of the outer ring and the inner ring. The outer ring and the inner ring of the respective segment increase the stability of the segment and may furthermore improve the insulation of the segment's material at the radial inner and outer site of the regenerator. Preferably the outer ring and/or the inner ring comprise insulating material with low thermal conductivity. Furthermore alternatively, the outer ring and/or the inner ring are adapted to be fixed to insulating material and prevent the heat energy retained in the material of the segments from being transferred and thereby lost to the environment.

In another preferred embodiment, the permeable section is formed as one piece with at least one of the outer ring and the inner ring. This allows two ease the manufacturing process of the segment, since the rings and the permeable section can be formed at one and the same working step. Furthermore, the stability of the segment is increased when the outer ring and/or the inner ring are formed as one piece with the permeable section.

Furthermore preferred, the permeable sections are made of a material with a specific heat capacity of less than 1^-^, preferably less than 0,7^;, and in particular preferred less than The low specific heat capacity of the permeable section al lows for high temperature increases and or decreases of the material temperature of the permeable section. Consequently, the fluid streaming through the permeable section is confronted with high temperature gradients between the working fluid and the permeable section. The inventors found, that a specific heat capacity of less than 1^— is sufficient for providing an efficient regenerator, while the material costs can kc]'K be held low. In case the material costs are not the main restricting factor of manufacturing the regenerator according to the invention, it was found that a specific heat capacity of less than _can achieve very good results in terms of thermal effi- kc]'K ciency. In particular preferred, the heat capacity is lower than 0.5—^—, which allows kg "K for heat resisting materials to be used in the regenerator, that withstand fluid temperatures above 800°C.

Preferably, the at least two segments are held in position relative to each other by the at least one spacer. In this embodiment the spacer not only spaces the segments from each other and provides thermal insulation between the segments, but it also holds and positions the segments relative to each other. It is preferred that at least four or five segments are arranged along the flow axis and spaced and held relative to each other by a plurality of spacers. In a first embodiment the spacers can be formed as ring-shaped elements which contact the inner and/or the outer ring of the sections of the segments. Alternatively, the spacers can be formed as rectangular-shaped elements which extend substantially in radial direction from the flow axis. The spacers are preferably adhesively fit to the segments.

In a preferred embodiment, at least four segments, preferably at least five segments, are arranged substantially coaxially along the flow axis, wherein the flow channels are preferably arranged coaxially to each other along the flow axis. The inventors found that arranging at least four and in particular preferred five to six segments coaxially to each other along the flow axis, achieved high temperature gradients in the working fluid and still kept the regenerator design compact. Each of the segments is separated from the neighbouring segments by at least one spacer.

In a preferred embodiment the permeable section of at least one of the segments is made of stainless steel. Stainless steel has specific heat capacity between 0.5 to 0.7 kg-K . Furthermore, the material costs of stainless steel are rather low in view of the very good achievable thermal efficiency of the regenerator. Finally, stainless steel is resistant against corrosion and has a high structural stability.

Furthermore preferred at least one of the spacers is made after zirconium oxide.

This material has the very high mechanical strength and allows for good insulation of the segments of the regenerator. Alternatively preferred, the spacers can be made of ceramic material, which also has very good insulation stats and can be designed to have a high mechanical strength.

Further characteristics and advantages of preferred embodiments of the invention are described in the following, with regard to the attached drawings.

Figure 1 shows a first preferred embodiment of a regenerator 1 which preferably comprises five segments 2. The segments 2 are separated from each other by spacers (not shown). In this preferred embodiment the segments 2 comprises a permeable section 22 with a cellular tailor-made structure, preferably a honeycomb shaped structure. Furthermore preferred, the segments 2 comprise an outer ring 24 and an inner ring 26, wherein the permeable section 22 is held between the outer ring 24 and the inner ring 26. The regenerator 1 shown in figure 1 is adapted to be mounted on a Stirling engine with a circular outer contact surface.

Figure 2 shows a side view on the embodiment of figure 1 . Preferably, the spacers 4 are adapted to create a space between the segments 2. In a first preferred embodiment, the spacers 4 are formed as one piece with the inner ring 26 of the segments 2. In an alternative preferred embodiment, the spacers 4 are arranged between the segments 2 and particular preferred contact the inner rings 26 of two adjacent segments 2.

Figure 3 shows a more detailed view on the permeable section 22 of the segment 2 according to the embodiment of figures 1 and 2. The permeable section 22 comprises a structure with flow channels 22A which are separated from each other by walls 22B. The walls 22B preferably for a honeycomb structure such, that the flow channels 22A comprise hexagonal shape. It is preferred, not only for the honeycomb structure of the permeable section 22, but also for other cellular tailor-made structures, that the flow channels 22A have a constant inner geometry parallel to the flow axis A (figure 1 ). This allows for constant flow of the working fluid through the permeable section 22 with low flow resistance.

Figure 4 shows another preferred embodiment of a segment 2 which comprises a helically extending guide element 22C extending around the flow axis A. In this preferred embodiment there are 4 support elements 23 arranged at the 0°, 90°, 180° and 270° position of the segment 2 to hold the guide element 22C in position and stabilise the flow channels 22A formed between the respective sections of the guide element 22C. In a preferred alternative embodiment, only two support elements 23 according to fig. 4 would be sufficient to hold the guide element 22C in position. This could save weight and material costs wherein a reduced stability of the segment 2 would be the disadvantage. Furthermore preferred, the support elements 23 may also serve as spacers 4 and may be formed as one piece with the respective spacers described before. Figure 5 shows another preferred embodiment of the regenerator 1 with four segments 2, which comprise guide elements 22C according to the above described embodiment of figure 4. In this preferred embodiment, there are no support elements 23 necessary, but the guide element 22C is positioned only by the spacer 4 which may also be defined as inner ring 26. Of course, additionally or as an alternative, support elements 23 according to figure 4 may be used in the embodiment of figure 5, which would increase the flow resistance of the regenerator 1 and increase the stability.

Reference numerals:

1 - regenerator

2 - segment

4 - spacer

22 - permeable section

22A - flow channel

22B - wall

22C - guide element

23 - support element

24 - outer ring

26 - inner ring

A - flow axis