Micro Stirling engine

Abstract

A micro-heat engine that performs highly efficient heat conversion using a temperature difference may include a housing having a high-temperature side and a low-temperature side, a plurality of pistons disposed within the housing and each extending from the high-temperature side to the low-temperature side, and a plurality of regenerators, each configured to be positioned between two adjacent pistons and configured to store heat generated when the working gas moves between the adjacent pistons.

Description

Technical Field

[0001] (Cross - reference to related applications) This application claims priority to U.S. Provisional Application Serial No. 63 / 495,359, filed on April 11, 2023, the disclosure of which is hereby incorporated by reference in its entirety.

[0002] Aspects of the present disclosure generally relate to systems and methods for a micro - Stirling engine.

Brief Description of the Drawings

[0003] Embodiments of the present disclosure are particularly set forth in the appended claims. However, other features of the various embodiments will become more apparent and best understood by reference to the following detailed description and the accompanying drawings.

[0004] [Figure 1] A cross - sectional view of an exemplary thermoelectric converter is shown. [Figure 2] A perspective view of an exemplary double - acting Stirling engine is shown. [Figure 3] A top view of the exemplary double - acting Stirling engine of FIG. 2 is shown. [Figure 4] A side view of the exemplary double - acting Stirling engine of FIG. 2 is shown. [Figure 5] Another cross - sectional view of the exemplary double - acting Stirling engine of FIG. 2, depicted with exemplary dimensions, is shown. [Figure 6] A cross - sectional view of the exemplary double - acting Stirling engine of FIG. 2, showing the sinusoidal motion of pistons arranged in series with each other. [Figure 7] A chart showing an exemplary relationship between glass housing area and efficiency is shown. [Figure 8] A chart showing an exemplary relationship between heat flow and efficiency is shown. [Figure 9] A chart showing an exemplary relationship between attenuation and efficiency is shown. [Figure 10]A chart illustrating the exemplary relationship between damping and thermal energy is shown. [Figure 11] A chart illustrating the exemplary relationship between spring constant and efficiency is shown. [Figure 12] A chart illustrating an exemplary relationship between the spring constant and thermal energy is shown. [Figure 13] A chart illustrating the exemplary relationship between gas pressure and efficiency is shown. [Figure 14] The chart shows an illustrative relationship between gas pressure and thermal energy. [Figure 15] This chart shows the movement of a piston over time at gas pressures of 8.0 atmospheres or higher. [Figure 16A] This chart shows the piston movement over time, relating to piston deviation. [Figure 16B] This chart shows the movement of the piston over time, relating to piston stability. [Figure 17] This shows an exemplary two-piston model of a Stirling engine. [Figure 18] The following shows exemplary values ​​for various housing characteristics. [Figure 19] An illustrative chart illustrating the relationship between efficiency and high-temperature side is shown. [Figure 20] This chart illustrates the efficiency based on the relationship between damping and spring constant. [Figure 21] This chart illustrates the efficiency based on the relationship between damping and piston mass. [Figure 22] This chart shows another illustrative example of efficiency based on the relationship between damping and spring constant. [Figure 23] This chart shows another exemplary efficiency based on the relationship between damping and piston mass. [Modes for carrying out the invention]

[0005] Detailed embodiments of the present invention are disclosed herein as necessary. However, it should be understood that the disclosed embodiments are merely examples of the present invention, which may be embodied in various alternative forms. The drawings are not necessarily to scale. Some features may be exaggerated or reduced in order to illustrate the details of certain components. Accordingly, certain structural and functional details disclosed herein should not be constrained, but rather understood merely as representative grounds for those skilled in the art to apply the invention in various ways.

[0006] As technology continues to advance, there is a growing demand for increased available energy. In many cases, self-sufficient power systems are required, but energy demands cannot be met by batteries alone. Specifically, there is a need for small, high-power energy sources. For example, space probes, portable power supplies, and other highly reliable systems require power supply in remote, harsh environments, necessitating the development of special materials to meet increasing power demands and thermal management requirements while maintaining a small form factor. Currently, no micro-power system exists that meets all the mechanical, thermal, and system requirements for the power needs of space probes and other small form factor devices, while maintaining an efficiency of approximately 2% or more. Furthermore, this technology can also be applied to cooling microelectronics, waste heat utilization, and other applications.

[0007] Thermoelectric power sources such as thermoelectric generators (TEGs) and solar power are commonly used as energy sources in the space industry. However, as the size of these components decreases, the power they can supply also decreases. Currently, the operating efficiency of small-scale (mW-class) TEGs is 2%.

[0008] Other thermoelectric technologies, such as Stirling engines, are known to achieve high efficiency (30 - 40%). A Stirling engine is a heat engine that operates by the periodic compression and expansion of air or other gases between different temperatures, resulting in heat conversion. The Stirling engine utilizes input heat in a closed-cycle regenerative heat engine to convert thermal energy into mechanical work. When the power output is large, the Stirling engine is highly efficient compared to thermoelectric generators, but there may still be associated losses in such small-scale applications.

[0009] This specification discloses a Stirling engine manufactured from a group of MEMS materials that can be miniaturized for small (in mm units) high-efficiency power generation, reducing losses based on optimized parameters. As an example, this engine is a 2W Stirling engine, but other examples, specifically those in the range of 1Wth to 200Wth, are achievable. This engine has the ability to operate with a low-quality or relatively low-temperature heat source, enabling waste heat recovery where other power generation devices are inapplicable or inoperable. Therefore, this engine can be used as a portable power source and can utilize various fuels such as butane. It can also be operated for applications such as heat production and cooling. Furthermore, the miniaturization of the engine enables the use of additional power generation devices. For example, power generation includes piezoelectric generators, capacitive coupling, and pairs of embedded magnets and coils.

[0010] Figure 1 shows a cross-sectional view of an exemplary thermoelectric converter 100.

[0011] Figure 2 shows a perspective view of an exemplary double-acting Stirling engine 200. Figure 3 shows a top view of the exemplary double-acting Stirling engine 200 of Figure 2, and Figure 4 shows a side view of the engine of Figure 2.

[0012] Referring to FIGS. 2 - 4, the double - acting Stirling engine 200 includes four pistons 202 and a regenerator 206. In particular, the engine 200 can accept any even number of pistons. The pistons 202 are generally block - shaped and are equally spaced from each other in a 2×2 arrangement to form an overall cubic shape. The pistons are connected to each other via rods or tubes and may include various valves, seals, etc. The pistons 202 and the regenerator 206 are disposed within a housing 208. The housing 208 is sealed and encloses a fluid, usually a gas, inside. The housing 208 includes a high - temperature plate 210 at one end of the pistons 202 and a low - temperature plate 212 at the opposite end. Since the Stirling engine obtains efficiency based on the temperature difference, the greater the distance between the high - temperature side and the low - temperature side, the higher the efficiency. The high - temperature side, or the high - temperature plate 212, starts at a temperature of 500K. The temperature of the low - temperature side is about 300K. These temperatures are examples, and other temperatures are also envisioned.

[0013] The piston 202 has a hollow structure, maintains a vacuum inside, and has a low - emissivity surface to reduce heat loss. The piston 202 is housed within a cubic housing and has a piston rod connected to a spring. Although not shown, in another example, the piston may be cylindrical. The spring has a desired spring constant and is configured to resist the pressure generated by the gas within the cylinder.

[0014] The piston 202 may be a combination of a physically hard-stop piston and a spring piston. The use of a double-acting free piston simplifies the engine 200's mechanism and extends its lifespan due to the absence of at least partially mechanical linkage mechanisms. Furthermore, this allows for a completely closed system, facilitating heat containment. Using four pistons 202 in particular facilitates engine phase adjustment and balancing, and allows for piston stabilization. As an example, the dimensions of the piston 202 are approximately 15mm x 22mm x 22mm, but other dimensions are also conceivable. In some cases, one or more of the pistons 202 may be approximately 8mm x 2.5mm x 2.5mm. The cross-sectional area of ​​the housing 208 is, in this regard, approximately 2.204e-6m² as an example. 2 It is possible to do so.

[0015] The pistons 202 form a cavity 204 between them, which is configured to optimize space utilization and minimize parasitic conduction losses through the frame. The cross-sectional shape and material composition may also be formed to reduce flow losses and maximize heat transfer. Regenerators are attached to each piston and are described in more detail herein. As shown in the figures, the housing 208 has a square cross-section, creating a square space that houses the pistons 202 and forms a cavity 206. This reduces parasitic conduction losses through the housing 208. The pistons 202 are arranged in a 2x2 arrangement, forming four symmetrical quadrants of the engine 208.

[0016] A regenerator 206 is positioned between each piston 202. The regenerator 206 stores heat as the working gas moves from one side of each piston 202 to the cooler side. Thus, these regenerators 206 store at least some of the heat that could otherwise be lost through the cavity 206. The cross-section of the regenerator 206 can be optimized to reduce flow losses and maximize heat transfer between the pistons 202. As an example, the regenerator 206 may contain a phase-change material and be positioned horizontally rather than vertically within the housing 208.

[0017] The regenerator 206 can be, for example, an internal heat exchanger made of copper and a temporary heat storage location between the high-temperature and low-temperature sides. This allows the working fluid to pass through in one direction first and then in the reverse direction, thereby removing heat from the fluid in one direction and returning heat in the reverse direction. For example, the regenerator 206 may be resized to optimize heat transfer and efficiency between the high-temperature and low-temperature sides of a micro-Stirling engine.

[0018] The regenerator 206 consists of one or more materials capable of accumulating excess heat as the working fluid moves through the regenerator towards the next cycle (from the low-temperature side to the high-temperature side), and the accumulated heat is reintroduced into the system to substantially preheat the working fluid. This reduces heat loss during the engine cycle, improving engine efficiency and power output. As an example, the regenerator 206 may be made of copper. Copper acts as an efficient heat exchanger and is readily available.

[0019] [Table 1]

[0020] The efficiency of the example engine 200 is higher than the conventional efficiency of 2%. The larger temperature difference results in a more efficient cycle. Since most of the energy loss is thought to occur through the surface area of ​​the housing, using a small housing such as housing 208 can result in a more efficient engine than existing engines. The heat flow is calculated as Q = ΔT × A / L, where ΔT is the temperature difference, A is the housing area, and L is the housing height. Higher temperatures improve the efficiency of the Stirling cycle, but may also increase parasitic losses through housing 208. To reduce losses, it has a minimum size of four pistons 202 and four regenerators 202, and the temperature may be set according to the specific application based on the available heat source and local environment. At least one of the pistons 202 receives transient input during startup. As an example, conventional magnets and piezoelectric films can be replaced with electrostatic alternatives for power generation and startup operation. This reduces conduction losses through the piston magnets and eliminates films that have losses on the flexure.

[0021] In addition to the above, the housing 208 can be made of a rigid material that can maintain a stable operating temperature, has low thermal conductivity to avoid parasitic losses through the structure, and can have sufficient strength to withstand environmental loads and internal pressures at the operating temperature. As an example, the housing 208 is formed of glass at least partially. Plastics, elastomers, metals, alloys, ceramics, foams, composite materials, silicon, Kapton membranes, air, or any combination thereof can be added to the glass. Glass provides low thermal conductivity. In addition to low thermal conductivity, the reduction in surface area due to the small housing can improve overall mechanical efficiency. JPEG2026516627000003.jpg11169

[0022] Other materials or combinations of materials, including polyimide and titanium, may be considered for the housing. Furthermore, to optimize the structural and thermal requirements of the housing 208, the housing 208 may be composed of multiple different materials in different regions or parts of the housing. The housing 208 is sealed to enclose the internal gas. Therefore, the materials forming the housing 208 are in contact with the gas, and heat may be transferred from the gas to the housing material during operation. Again, this is usually not a problem in large systems. However, such losses can be a problem in small Stirling engines. Therefore, as mentioned above, glass may be chosen as the housing material due to its low thermal conductivity (e.g., 1.2 W / mm-K) and availability.

[0023] The housing 208 may be sealed with a cap 220, as shown in Figures 2 and 5. The cap 220 further seals the housing 208, ensuring complete airtightness. The cap 220 is positioned between the hot and cold plates of the housing material and acts as an insulator, minimizing heat transfer between the housing 208 and the cap 220. As an example, the cap 220 may be formed of silicon.

[0024] In addition to the materials discussed here, materials other than copper, glass, and silicon may also be considered. For example, advanced polymers and nanocomposites may offer additional or alternative advantages to the aforementioned materials. These advantages include lighter engines and reduced overall mass and volume due to improved flexibility. Other advantages include improved thermal properties or property tuning to suit specific environmental conditions.

[0025] Figure 5 is another cross-sectional view of the exemplary engine 200, depicted with exemplary dimensions. In this example, each piston is 15 mm high and 2.5 mm wide, and the overall engine width is 10 mm. These dimensions are examples, and other dimensions and proportions may be considered.

[0026] Figure 6 is a side view of an exemplary engine, showing pistons 202 performing sinusoidal motion in series with each other. As previously mentioned, the pistons 202 are heated at the top and cooled at the bottom. In this example, the working gas can be moved between adjacent pistons 202 via a connecting rod. The force exerted by the expanding gas on one side of piston 202 is amplified by the contracting gas on the adjacent piston, increasing the pressure difference between the pistons 202. The relative motion of the pistons relative to the adjacent pistons can be performed in a predetermined phase. This sinusoidal motion is generated by the reciprocating motion of the adjacent pistons. The relative phase can be forced by electronic force feedback.

[0027] This process and feedback provide thermodynamic stability, maintaining aligned piston phases. The engine may include a power extraction device to recover power from the structure. A typical method is a magnet / coil pair, but it is also possible to convert mechanical motion into electrical output using electrets or other capacitive couplings. The advantage of capacitive coupling is that it eliminates heavy magnets from the structure and allows the system to operate in higher temperature environments. Additional methods that become available through miniaturization, such as piezoelectric generators and coupled capacitors, should also be considered.

[0028] Mechanical structures aim to minimize thermal conductivity while maximizing the mechanical strength necessary to support the structure and internal pressures. This involves material selection (high-strength, low-thermal-conductivity materials), hybrid structures (e.g., variable-thickness piston sleeves using materials different from the main housing), and selective material removal to reduce thermal conductivity in areas where material strength is not critical. For example, topology optimization can be used to remove material from areas where strain due to loads (pressure, environment, etc.) is minimized. Specific examples include hollowing out the inside of pistons and removing material from the central part of the housing located between all pistons, particularly the upper and lower sections where bending stress is low.

[0029] The example engine mitigates some of the parasitic heat losses that existing solutions cannot address at such a small scale, thereby enabling the use of a Stirling engine.

[0030] Figure 7 shows a chart illustrating an exemplary relationship between glass housing area and efficiency. As shown in the figure, efficiency decreases as the area increases.

[0031] Figure 8 shows a chart illustrating an exemplary relationship between heat flow and efficiency. As shown in the illustration, in this example, high efficiency (e.g., 8%, as shown in Figure 8) is achievable with a heat flow of approximately 3W. The following parameters are achieved in these figures.

[0032] [Table 2]

[0033] Reducing the housing area can be expected to improve overall mechanical efficiency from 4% to 7%.

[0034] In addition to the housing size, various parameters such as damping, spring constant, gas pressure, piston mass, regenerator size, and gas type (He, N) can be set to achieve efficient assembly.

[0035] Figure 9 shows a chart illustrating an exemplary relationship between damping and efficiency.

[0036] Figure 10 shows a chart illustrating an exemplary relationship between damping and thermal energy. As an example, a damping of 0.0213 is achievable to balance efficiency and thermal energy (W).

[0037] Figure 11 shows a chart illustrating an exemplary relationship between spring constant and efficiency.

[0038] Figure 12 shows a chart illustrating an exemplary relationship between spring constant and thermal energy. Under constant damping and constant GHA, the exemplary spring constant is in the range of 50 to 110k. Specifically, the spring constant can be set to 100k.

[0039] Figure 13 shows a chart illustrating an exemplary relationship between gas pressure and efficiency. Figure 14 shows a chart illustrating an exemplary relationship between gas pressure and thermal energy.

[0040] Figure 15 shows a chart illustrating the piston movement over time at gas pressures above 8.0 atmospheres. Beyond 7.4 atmospheres, efficiency begins to decrease while the high-temperature and low-temperature sides remain at constant temperatures, but heat continues to increase. The piston movement can become unstable when the threshold gas pressure is exceeded, such as above 8 atmospheres.

[0041] In addition to the parameters mentioned above, slightly increasing the piston mass may not only improve efficiency, but also slightly increase the total heat output to compensate for the increase in mass. For example, 2.34e -4 A piston mass of kg is desirable.

[0042] Regarding the type of gas, helium gas can achieve higher efficiency, which corresponds to an increase in the total heat in the system. However, nitrogen gas is easier to contain and can provide an efficiency of approximately 6.5% while reducing the total heat.

[0043] Figures 16A and 16B show charts illustrating the movement of the piston over time, comparing piston deviation and piston stability.

[0044] Various manufacturing methods are possible. Load resistance can be evaluated using a mechanical model of the MEMS Stirling system. The basic design can be a four-piston system. The MEMS is modeled as a single component of borosilicate glass. In this example, the following characteristics are observed:

[0045] Elastic modulus: 63 GPa

[0046] Poisson's ratio: 0.2

[0047] Shear modulus: 27 GPa

[0048] Fracture toughness: 0.770 MPa - m^0.5

[0049] Thermal expansion coefficient: 4μm / m-℃

[0050] Thermal conductivity: 1.2W / m-K

[0051] Figure 17 shows an exemplary two-piston model of the Stirling engine 300. In this example, it is possible to create a model using a two-piston MEMS Stirling engine 302 with multiple devices arranged around the heat source (integration into solar probes, portable power supplies, micro-cooling applications, and other waste heat utilization applications). This allows the engine to utilize the shape of the potential heat source, enabling longer pistons and greater separation between the high-temperature and low-temperature sides. Furthermore, structural stability can be improved by positioning the MEMS Stirling engine externally.

[0052] When selecting housing materials, it is desirable to maximize thermal resistance while also possessing sufficient mechanical strength to withstand the expected load. The figure below shows the evaluation index for yield strength / thermal conductivity.

[0053] [Table 3]

[0054] [Table 4]

[0055] Based on the exemplified housing size, the mechanical properties of the glass, specifically SCHOTT BOROFLOAT glass, are shown below. In this case, the solid glass area corresponds to the square area, the hollow glass area corresponds to the volume inside the piston, and the regenerator is excluded.

[0056] [Table 5]

[0057] The selection of materials is based on compressive, tensile, and bending loads.

[0058] Figure 18 shows exemplary values ​​for various glass properties. Figure 18 also shows the change in load on a micro-Stirling engine due to pressurized gas.

[0059] Figure 19 shows an exemplary chart illustrating the relationship between efficiency and high-temperature side, with the low-temperature side assumed to be 15°C, and the efficiency of the micro-Stirling engine disclosed herein may lie between the two curves. JPEG2026516627000008.jpg14170

[0060] Figure 19 further shows the boundary between the lowest and highest possible efficiencies. Other example parameters can also be used. The table below lists additional parameters.

[0061] [Table 6]

[0062] Each parameter may be related to others and can affect the overall efficiency. Various relationships between parameters (e.g., damping, spring constant, efficiency, piston mass, etc.) are interrelated and define interdependencies between three or more parameters. Examples of various parameter interdependencies can be seen in Figure 20-23.

[0063] Figure 20 shows an exemplary efficiency chart based on the relationship between damping and spring constant. This chart shows a multivariable design with constant heat flow, input power of 2Wth, and initial high-temperature side temperature of 500K, with all other parameters being constant.

[0064] Figure 21 shows an exemplary efficiency chart based on the relationship between damping and piston mass. This chart shows a multivariable design with constant heat flow, input power of 2Wth, and initial high-temperature side temperature of 500K, with all other parameters being constant.

[0065] Figure 22 shows an exemplary efficiency chart based on the relationship between damping and spring constant. This chart shows a multivariable design with constant heat flow, input power of 2Wth, and an initial high-temperature side temperature of 500K, with all other parameters being constant. In this example, the high and low temperatures are kept constant, and the total heat flow entering the system is not controlled. By keeping other parameters constant, it is possible to observe not only other heat flows but also how changing parameters affect efficiency. In this example, the efficiency of engine 200 generally improves as the spring constant decreases or as the damping increases.

[0066] Figure 23 shows an exemplary efficiency chart based on the relationship between damping and piston mass. This chart shows a multivariable design with constant heat flow, input power of 2Wth, and an initial high-temperature side temperature of 500K, with all other parameters being constant. In this example, the high and low temperatures are kept constant, and the total heat flow entering the system is not controlled. By keeping other parameters constant, we can not only observe other heat flows, but also see how changing parameters affect efficiency. In this example, generally, the efficiency of engine 200 improves as the spring constant decreases and the damping increases.

[0067] Overall, Engine 200 enables reduced material usage, miniaturization, improved reliability, and increased efficiency compared to existing solutions. For example, Engine 200 can achieve up to three times the efficiency at the same heat.

[0068] Further applications of Engine 200 are also conceivable. For example, since more energy is lost at low temperatures, waste heat could be recovered. Low-cost conversion technology that can recover even small amounts of lost energy could have a significant impact. Furthermore, a miniaturized cooling system could be used for high-density cooling of microelectronics, in which case Engine 200 could operate in reverse cooling mode.

[0069] The computing devices disclosed herein generally include computer executable instructions, which are executable by one or more of the computing devices listed above. Computer executable instructions are compiled or interpreted from computer programs written using a variety of programming languages ​​and / or technologies, including but not limited to Java, C, C++, C#, Visual Basic, JavaScript, and Perl. Generally, a processor (e.g., a microprocessor) receives instructions from memory, computer-readable media, etc., and executes these instructions to run one or more processes, including one or more processes described herein. Such instructions and other data are stored and transmitted using a variety of computer-readable media.

[0070] While exemplary embodiments have been described above, they are not intended to describe all forms of the invention. Rather, the terminology used in this specification is for illustrative purposes only, not limitation, and it should be understood that various modifications are possible without departing from the spirit and scope of the invention. Furthermore, it is possible to combine features of various embodiments to form further embodiments of the invention.

Claims

1. A heat engine that performs highly efficient heat conversion by utilizing a temperature difference, A housing having a high-temperature side and a low-temperature side, A plurality of pistons are arranged within the housing, each extending from the high-temperature side to the low-temperature side, and configured to operate in response to the working gas heated internally by the high-temperature side of the housing, A heat engine comprising a plurality of regenerators, each configured to be positioned between two adjacent pistons and configured to store heat generated as the working gas moves between the pistons.

2. The heat engine according to claim 1, wherein at least one material of the housing is selected based on thermal conductivity and the size of the housing.

3. The heat engine according to claim 2, wherein at least a portion of the housing is made of glass.

4. The heat engine according to claim 1, wherein the plurality of pistons include an even number of pistons.

5. The heat engine according to claim 4, wherein the pistons are arranged in a 2x2 arrangement within the housing.

6. The heat engine according to claim 4, further comprising a piezoelectric generator for recovering power from the operation of the piston.

7. The heat engine according to claim 1, wherein the regenerator is located in a cavity formed between adjacent pistons.

8. The thermal engine according to claim 1, wherein the piston is at least one of a double-acting free piston, a physically hard-stop piston, or a spring piston.

9. The heat engine according to claim 1, wherein the piston operates in a predetermined phase that is forcibly given by electronic force feedback.

10. The heat engine according to claim 1, wherein the piston is a hollow piston with a vacuum inside.

11. The heat engine according to claim 1, further comprising a coupling capacitor for recovering power from the operation of the piston.

12. The heat engine according to claim 1, further comprising at least one pair of magnets and coils for recovering power from the operation of the piston.

13. The heat engine according to claim 1, wherein the housing includes at least one end cap made of silicon, and the housing is sealed.

14. The heat engine according to claim 1, wherein the piston has characteristics that affect the efficiency of the heat engine, and such characteristics include at least a spring constant and damping.

15. The heat engine according to claim 14, wherein the efficiency of the heat engine increases as the damping increases and the spring constant decreases.

16. The heat engine according to claim 1, wherein the piston has a piston mass of 2 to 4 kg.

17. A heat engine that performs highly efficient heat conversion by utilizing a temperature difference, A housing having a high-temperature side and a low-temperature side, A plurality of pistons are arranged within the housing, each extending from the high-temperature side to the low-temperature side, and configured to operate in response to the working gas heated internally by the high-temperature side of the housing, A heat engine comprising at least one regenerator positioned between two adjacent pistons and configured to store heat generated as the working gas moves between the pistons.

18. The heat engine according to claim 17, wherein the housing is sealed to retain internal gases, and the housing is formed of a material that maintains a stable operating temperature.

19. The heat engine according to claim 17, wherein the housing is formed in at least part of it from at least one of glass and silicon.

20. The heat engine according to claim 17, wherein the housing is made of glass and sealed with a silicone cap.

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