Patent Description:
One application for thermoacoustic heat engines is powering satellites, particularly in deep space. While Earth-orbiting satellites typically use solar cells as an energy source, this is not available as satellites move farther from the Sun. Consequently, these satellites are powered by a radio-isotope heat source such as plutonium whose heat is converted into electricity via the thermoelectric effect. However, this system for generating electricity is not very efficient.

Thermoacoustic heat engines are a more efficient way to generate power from a heat source. In essence, a thermoacoustic heat engine consists of a tube filled with a gas. Applying heat at one end of the tube creates a heat differential along the length of the tube and induces sound waves which can be used to convert the heat into mechanical energy. Thermoacoustic heat engines can generally be classified as either resonant or traveling wave types.

A schematic of a representative traveling wave thermoacoustic heat engine is shown in <FIG>. A sealed system filled with, for example, pressurized helium gas, includes a torus <NUM>, a resonator <NUM> and a variable acoustic load <NUM>. Torus <NUM> includes cold heat exchanger <NUM>, regenerator <NUM> and hot heat exchanger <NUM>. A sound wave is induced in the helium gas by creating a temperature difference across regenerator <NUM> by using cold heat exchanger <NUM> and hot heat exchanger <NUM>. Thermal buffer tube <NUM> provides a thermal buffer between hot heat exchanger <NUM> and the cold side (<NUM> and <NUM>) by providing space for the heated helium gas to oscillate without reaching the cold side. A flow straightener and heat exchanger <NUM> suppress prevent certain types of gas flow and reduces heat loss thereby improving the thermal efficiency of the heat engine.

Torus <NUM> also includes a feedback inertance <NUM> which provides a path for the helium gas to flow through to compliance <NUM>, jet pump <NUM> and finally back to cold heat exchanger <NUM>. The configuration and volume of inertance <NUM> and compliance <NUM> are selected to control the phase of the traveling wave induced in the helium gas. Jet pump <NUM> is used to reduce gas streaming and thereby improve thermal efficiency.

<FIG> depicts a single stage thermoacoustic heat engine. The amount of acoustic power output per acoustic power input (the gain) per stage of a thermoacoustic heat engine is limited by the temperature ratio between the hot and cold ends of the regenerator. To increase the overall gain of the engine, single stage thermoacoustic heat engines are sometimes connected in series to form a multi-stage heat engine. However, prior art multi-stage engines are thermally and mechanically cumbersome, volumetrically inefficient, do not scale down well and are subject to high thermal stresses. This is due to the geometries used to expose the gaseous working fluid's acoustic power path multiple times to a common set of thermal interfaces. A folded loop topology has been used to provide a common set of thermal interface points, but it is very large and only works for a subclass of traveling wave heat engines. <CIT> discloses a multi stage travelling wave thermoacoustic heat engine. A plurality of heat engine stages are formed as a toroidal spiral cascade of N stages inside a pressure vessel. Each stage feeds into the next stage such that all of the thermoacoustic power cycles past a common set of thermal interfaces multiple times with a single domed pressure vessel.

Thus, a need exists for a multi-stage thermoacoustic engine with improved volumetric and thermal efficiency, better scalability and greater resistance to high thermal stresses.

The invention in one implementation encompasses a multistage traveling wave thermoacoustic engine with a topological folding of the acoustic power path to re-access the same thermal interface multiple times within a single domed pressure vessel. The inventive thermoacoustic engine is simpler and cheaper to manufacture and more reliable due to the minimization of hot joints.

In an embodiment, the invention encompasses a multi-stage traveling wave thermoacoustic engine according to claim <NUM>.

In a further embodiment, the second thermal buffer tube stage includes an opening for receiving the gas flow from the first regenerator stage.

In another embodiment, the thermal buffer tube stages are operatively coupled such that the gas flows past the flow turner heat exchanger into a second regenerator stage and is directed by the one or more regenerator partitions into a third thermal buffer tube stage adjacent to the second thermal buffer tube stage.

In a further embodiment, the third thermal buffer tube stage includes an opening for receiving the gas flow from the second regenerator stage.

The yet another embodiment, the multi-stage traveling wave thermoacoustic engine includes a compliance; an inertance coupled to said compliance; and a linear alternator operatively coupled to the first thermal buffer tube stage; wherein the gas flow exits a regenerator stage and enters the compliance.

In another embodiment, the linear alternator includes a jet pump.

In another embodiment, the multi-stage traveling wave thermoacoustic engine includes two or more pistons operatively coupled to the linear alternator, said pistons actuated by acoustic energy in a traveling wave of the flow of gas through the linear alternator; and a motor operatively coupled to each piston for generating an electric current.

In an embodiment, the multi-stage traveling wave thermoacoustic engine includes four thermal buffer tube stages and four regenerator stages.

In an embodiment, the multi-stage traveling wave thermoacoustic engine includes three thermal buffer tube stages and three regenerator stages.

In an embodiment, the multi-stage traveling wave thermoacoustic engine includes five thermal buffer tube stages and five regenerator stages.

In another embodiment the invention encompasses a satellite including any of the above multi-stage traveling wave thermoacoustic engines.

In another embodiment, the invention encompasses a torus for a multi-stage traveling wave thermoacoustic heat engine including a domed cylindrical shell; a slotted heat exchanger and flow turner heat exchanger in the domed end of the shell; a thermal buffer tube inside and coaxial to said shell below the slotted heat exchanger and flow turner heat exchanger; an annular regenerator between said shell and said thermal buffer tube; a first plurality of partitions inside said thermal buffer tube, said plurality of partitions dividing said thermal buffer tube into a plurality of thermal buffer tube stages; and a second plurality of partitions equal to the first plurality of partitions dividing said annular regenerator into a plurality of regenerator stages corresponding to the thermal buffer tube stages.

In a further embodiment, the torus has four thermal buffer tube stages and four regenerator stages.

In a further embodiment, the torus has three thermal buffer tube stages and three regenerator stages.

In a further embodiment, the torus has five thermal buffer tube stages and five regenerator stages.

Features of example implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which:.

Reference will now be made in detail to one or more embodiments of the invention. While the invention will be described with respect to these embodiments, it should be understood that the invention is not limited to any particular embodiment. On the contrary, the invention includes alternatives, modifications, and equivalents as may come within the scope of the appended claims. Furthermore, in the following description, numerous specific details are set forth to provide a thorough understanding of the invention. The invention may be practiced without some or all of these specific details. In other instances, well-known structures and principles of operation have not been described in detail to avoid obscuring the invention.

<FIG> depicts an overview of a multi-stage traveling wave thermoacoustic engine <NUM> according to the present invention. In an embodiment, thermoacoustic engine <NUM> is a sealed system filled with high pressure helium gas, although any suitable gas could be used. A heat source <NUM> is attached to one end of shell <NUM> by a heat spreader <NUM>. In an embodiment, heat source <NUM> is a general purpose heat source, for example, a radio-isotope substance such as plutonium. Heat spreader <NUM> couples energy from heat source <NUM> to a heat exchanger inside shell <NUM> which functions similarly to hot heat exchanger <NUM> of <FIG>. Further details of the structures inside shell <NUM> will be discussed in connection with subsequent Figures. Thermal mechanical interface (TMI) <NUM> is located at the opposite end of shell <NUM>. TMI <NUM> coordinates with another heat exchanger inside shell <NUM> which functions similarly to cold heat exchanger <NUM> of <FIG>. Heat source <NUM> and TMI <NUM> create a temperature difference between the ends of shell <NUM> which causes sufficient acoustic gain to sustain an oscillation in the helium gas inside shell <NUM>.

The oscillating gas forms a traveling wave in shell <NUM>, enters a compliance (shown in <FIG>), then passes through inertance <NUM> to linear alternator <NUM>, where it is fed back to shell <NUM>. Cylinder body <NUM> encloses a piston and motor assembly that is used to convert the acoustic power in the traveling wave of oscillating helium gas into electrical energy as explained further below.

<FIG> is a section view of the thermoacoustic engine of <FIG>. Common elements are denoted with common reference numbers. Regenerator shell <NUM> encloses thermal buffer tube <NUM>. A set of partitions as represented by partition <NUM> divides thermal buffer tube into stages as will be discussed in more detail in connection with <FIG>. Regenerator <NUM> is a cylinder located in the space between shell <NUM> and tube <NUM>. Regenerator <NUM> is also divided into stages by partitions that are not shown so that other features of the thermoacoustic engine can be depicted clearly. In general, a flow turner heat exchanger <NUM> supports the transfer of heat from heat source <NUM> (<FIG>) to the traveling wave of gas, and helps direct the wave from thermal buffer tube <NUM> into regenerator <NUM>. From regenerator <NUM> the gas passes through slotted heat exchanger <NUM> which facilitates heat transfer between the gas and TMI <NUM>. The cooled gas enters annular compliance <NUM> and is fed back to linear alternator <NUM> through inertance <NUM>. Jet pump <NUM> assists in maintaining an even flow of gas through the system.

The acoustic energy of the traveling wave of gas is translated into mechanical and then electrical energy through linear alternator <NUM>, which is coupled to piston <NUM> oscillating inside cylinder <NUM> and coupled to motor <NUM>.

<FIG> is a perspective, partial cutaway view of the thermoacoustic engine of <FIG>. In particular, <FIG> depicts the feedback path from inertance <NUM> to linear alternator <NUM> through passage <NUM>. Inertance <NUM> is used to control the phase of gas oscillations through the thermoacoustic engine.

As explained above, the power gain provided by a single stage engine is limited by the ratio between the temperatures at either end of regenerator. Typically, a temperature ratio of <NUM> is the maximum difference allowed by conventional material between the hot end and a cold end near ambient temperature. Many potential applications are enabled by temperature ratio of <NUM> and below. Therefore, gain is increased by cascading several single stage engines in series, as depicted conceptually in <FIG>. In particular, <FIG> illustrates the flow of acoustic power <NUM> through the cascaded stages <NUM> and <NUM>. As a practical matter, the stages could be interconnected in a variety of orientations. The hot end of stage <NUM> is connected to the cold end of the stage <NUM> by a thermal buffer tube <NUM>. Although two stages are shown in <FIG>, a multi-stage thermoacoustic engine may include any number of stages, depending on performance criteria.

Physically separating thermoacoustic engine stages has several disadvantages. Each stage experiences more heat loss, is less efficient and the ratio of surface area to volume is poor. In addition, constraints on physical orientation of several stages require tradeoffs in the location of the heat source as well as internal losses associated with moving acoustic power from one stage to another. The present invention encompasses a multi-stage thermoacoustic heat engine that minimizes these disadvantages.

<FIG> depict a coaxial toroidal spiral cascaded multi-stage traveling wave thermoacoustic engine according to the present invention. <FIG> depicts a side view of the thermoacoustic engine. Thermal buffer tube <NUM> and heat exchanger <NUM> are enclosed by regenerator shell <NUM>, similarly to <FIG>. An annular regenerator is also included but not shown in this view. The space inside shell <NUM> is divided into stages. Thermal buffer tube <NUM> is divided into stages <NUM>, <NUM>, <NUM> and <NUM> by partitions <NUM>, <NUM>, <NUM> and <NUM> as shown in a top view in <FIG>. Although partitions <NUM>, <NUM>, <NUM> and <NUM> are shown as individual elements, they may also be formed as a single piece.

Similarly, regenerator <NUM> is also divided into four stages <NUM>, <NUM>, <NUM> and <NUM> by partitions <NUM>, <NUM>, <NUM> and <NUM> as shown in <FIG>. This configuration creates a four stage thermoacoustic engine within regenerator shell <NUM>. Three stages, <NUM>, <NUM> and <NUM>, of thermal buffer tube <NUM> are blocked by a lower plate <NUM> located between thermal buffer tube <NUM> and linear alternator <NUM>. Although four stages are depicted in <FIG>, the invention is not limited to a particular number of stages. In an alternative, the embodiment of <FIG> could also be configured with, for example, two, three, five or other numbers of stages, depending on desired performance characteristics. In addition, stages or other fractional components may not be of uniform size, in order to further fine tune performance characteristics.

The flow of acoustic power through the gas in the multistage thermoacoustic engine will now be described as depicted by line <NUM> as depicted in both <FIG>. Acoustic power (oscillating pressure in the gas) enters stage <NUM> of thermal buffer tube <NUM> from linear alternator <NUM>. It travels up stage <NUM> between partitions <NUM> and <NUM>, over the top of thermal buffer tube <NUM> through heat exchanger <NUM> and back down regenerator stage <NUM>. Curved portions <NUM> and <NUM> of partitions <NUM> and <NUM>, respectively (shown in more detail in <FIG>) direct the flow of the oscillating gas into slot <NUM> at a lower end of adjacent stage <NUM> in thermal buffer tube <NUM>. Similarly, the acoustic power in the gas travels up stage <NUM>, over the top through heat exchanger <NUM>, back down regenerator stage <NUM>, around and into a slot (not shown) in the lower end of adjacent stage <NUM> of thermal buffer tube <NUM>. From there acoustic power travels up stage <NUM>, over the top through heat exchanger <NUM>, back down regenerator stage <NUM>, around and into a slot (not shown) in the lower end of adjacent stage <NUM> of thermal buffer tube <NUM>. Finally, the acoustic power in the gas travels up stage <NUM> of thermal buffer tube <NUM> and down regenerator stage <NUM> to compliance <NUM> and then to inertance <NUM> for the feedback path to linear alternator <NUM> as described above.

The above described acoustic power flow path is similar the path of electrical current in a toroidal inductor. The toroidal multi-stage thermoacoustic heat engine combines a series of traveling wave thermoacoustic heat engine stages into a toroidal spiral inside a single domed pressure vessel such that all of the thermoacoustic power cycles past a common set of thermal interfaces multiple times within the common pressure vessel and common coaxial buffer tube space. This enables an N-stage engine (where N can be <NUM>, <NUM>, <NUM>, etc) within the compact and low stress geometry of a single domed pressure vessel.

<FIG> depict perspective views of a portion of the interior of the multi-stage thermoacoustic engine of <FIG>. Common elements are denoted with common reference numbers. Thermal buffer tube <NUM> includes partitions <NUM>, <NUM>, <NUM> and <NUM>. Regenerator partitions <NUM>, <NUM>, <NUM> and <NUM> are also shown. Line <NUM> in <FIG> shows a portion of the flow of acoustic power through the thermal buffer tube and regenerator stages as explained above.

<FIG> is a schematic diagram illustrating another view of the flow of acoustic power in the multi-stage thermoacoustic engine of <FIG>. In effect, <FIG> shows the regenerator stages of <FIG> in an unrolled view so as to clarify the flow of acoustic power through the stages. Thermal buffer tube <NUM> is unrolled and effectively lying flat against the page. Stages in the regenerator are shown by partitions <NUM>, <NUM>, <NUM> and <NUM>. The acoustic power flow <NUM> enters a first stage of thermal buffer tube <NUM> from linear alternator <NUM> (<FIG>) at <NUM>. It exits the top of thermal buffer tube <NUM> into the regenerator stage between partitions <NUM> and <NUM>, past curved portions <NUM> and <NUM> into slot <NUM> of the adjacent thermal buffer tube stage. Similarly the flow of acoustic power <NUM> moves through the next regenerator stage between partitions <NUM> and <NUM> into slot <NUM>, through thermal buffer tube <NUM> again, then down through the regenerator stage between partitions <NUM> and <NUM>. After traveling through the last thermal buffer tube and regenerator stage, acoustic power flow <NUM> wraps around to the right end of thermal buffer tube <NUM> and exits to compliance <NUM> at <NUM>.

There are several viable construction approaches to achieving the requisite pressure seal between segments; a) brazing or diffusion bonding partitions between coaxial cylindrical elements b) inserting formed segments into the coaxial cylindrical elements, c) tight fit interference seals or c) <NUM>-D printing. This is not an exhaustive list of construction methods, nor does any particular method need to be used throughout the engine (e.g. the segmenting of the annular regenerator could utilize one technique, while the segmenting of the thermal buffer tube could use another technique). The segments in the thermal buffer tube should be straight in order to preserve the planar traveling wavefront needed to prevent thermal mixing. Any of the other components (regenerator, flow turner heat exchanger, slotted heat exchanger, or plenums) can be used to transition. The optimum transition between segments is plenum following each cold heat exchanger.

In an embodiment, a thermoacoustic engine according to the present invention has overall dimensions of approximately <NUM> by approximately <NUM> by approximately <NUM>. However, these dimensions are not limiting and principles of the invention may be applied to a thermoacoustic engine of any size. A variety of materials may be used to construct the inventive thermoacoustic engine, including metals and ceramics.

If used and unless otherwise stated, the terms "upper," "lower," "front," "back," "over," "under," and similar such terms are not to be construed as limiting the invention to a particular orientation. Instead, these terms are used only on a relative basis.

An illustrative description of operation of the apparatus <NUM> is presented, for explanatory purposes.

Claim 1:
A multi-stage traveling wave thermoacoustic heat engine (<NUM>), characterised by:
a domed cylindrical shell (<NUM>);
a thermal buffer tube (<NUM>, <NUM>) coaxially located inside said shell (<NUM>), said thermal buffer tube (<NUM>, <NUM>) further comprising one or more tube partitions (<NUM>) dividing the thermal buffer tube (<NUM>, <NUM>) into a plurality of thermal buffer tube stages (<NUM>, <NUM>, <NUM>, <NUM>);
an annular regenerator (<NUM>) located between the shell (<NUM>) and the thermal buffer tube (<NUM>, <NUM>), said regenerator (<NUM>) further comprising one or more regenerator partitions (<NUM>, <NUM>, <NUM>, <NUM>) dividing the regenerator into a plurality of sealed regenerator stages (<NUM>, <NUM>, <NUM>, <NUM>) corresponding to the stages in the thermal buffer tube;
a flow turner heat exchanger (<NUM>) at a first end of said shell; and
a slotted heat exchanger (<NUM>) at a second end of said shell opposite the first end;
wherein said thermal buffer tube stages (<NUM>, <NUM>, <NUM>, <NUM>) and said regenerator stages (<NUM>, <NUM>, <NUM>, <NUM>) are operatively coupled such that a gas flow enters a lower end of a first thermal buffer tube stage (<NUM>), flows past the flow turner heat exchanger (<NUM>) into a first regenerator stage (<NUM>) and is directed by the one or more regenerator partitions (<NUM>, <NUM>, <NUM>, <NUM>) into a second thermal buffer tube stage (<NUM>) adjacent to the first thermal buffer tube stage (<NUM>).