SYSTEM

Technical field The present invention relates to a method of fabricating a component of a solar energy system, and relates

particularly, though not exclusively to a method of fabricating an ejector of a solar energy system, such as a solar cooling system. Background

Cooling systems such as air conditioning and refrigeration units require considerable amounts of electrical energy that is often generated from fossil fuels associated with emission of pollutants and greenhouse gases. Photovoltaic solar panels may be used to convert sunlight into electricity that subsequently powers a compressor of a cooling system. This may reduce the consumption of fossil fuels but the efficiency is relatively low and the capital cost is relatively high. Steam-driven ejector heat pump cooling systems have been used for air conditioning of very large spaces within buildings that are equipped with fossil fuel powered steam boilers. The application of ejector heat pump cooling systems outside of large-scale niche applications, however, has not been a commercial success, at least in part because efficient and inexpensive large scale production of suitable ejectors has proved to be a challenge . Summary of the Invention

In accordance with a first aspect of the present invention there is provided a method of fabricating a component of a solar energy system, the method comprising the steps of: providing a tube, the tube comprising a material that deforms when at least a length of the tube is exposed to a suitable difference in pressure between an interior portion of the length of the tube and an exterior portion of the length of the tube; providing a die having a cavity arranged to receive the length of the tube, the cavity defining a shape that is related to that of the component of the solar energy system; positioning the length of the tube in the cavity of the die; and

increasing a relative pressure of a fluid within the interior portion of the length of the tube relative to a pressure within the cavity and outside the interior portion of the length of the tube such that at least a portion of the length of the tube expands to a shape that is related to that of the component of the solar energy system.

The component of the solar energy system may be an ejector and may be arranged for pumping a fluid. The ejector may be a refrigeration ejector suitable for solar cooling applications .

Embodiments of the present invention facilitate

fabrication of ejectors at a relatively fast rate and typically with reduced energy consumption. Material wastage may also be reduced compared with known methods.

In accordance with a second aspect of the present

invention there is provided a method of fabricating at an ejector of a solar energy system, the method comprising the steps of:

providing a tube, the tube comprising a material that deforms when at least a length of the tube is exposed to a suitable difference in pressure between an interior portion of the length of the tube and an exterior portion of the length of the tube; providing a die having a cavity arranged to receive the length of the tube, the cavity defining a shape that is related to that of the ejector; positioning the length of the tube in the cavity of the die; and

increasing a relative pressure of a fluid within the interior portion of the length of the tube relative to a pressure within the cavity and outside the interior portion of the length of the tube such that at least a portion of the length of the tube expands to a shape that is related to that of the ejector.

The ejector typically is arranged for pumping a fluid and may be a refrigeration ejector suitable for solar cooling applications. The following introduces features that embodiments of the first and second aspects of the present invention may have .

In one embodiment the step of increasing the relative pressure is conducted such that at least the portion of the length of the tube expands until that portion of the length of the tube is in contact with the cavity of the die .

The step of increasing the relative pressure of the fluid within the interior portion the tube relative to a pressure within the cavity and outside the interior portion of the tube may comprise increasing the pressure within the interior portion of the tube. Alternatively, the step of increasing the relative pressure of the fluid within the interior portion the tube relative to a pressure within the cavity and outside the interior portion of the tube may comprise reducing the pressure of a fluid within the cavity and outside the interior portion of the tube. In one specific embodiment the step of providing the tube comprises shaping the tube. For example, the tube may initially have an exterior diameter that larger than that of a portion of the ejector, such as a throat portion of the ejector. The method may comprise locally reducing the exterior diameter of the tube such that the tube has a non-uniform exterior diameter that may be profiled. For example, the method may comprise locally reducing the exterior diameter of the tube at a throat portion of the ejector. Reducing the external diameter of the tube may comprise any suitable process, such as rotary swaging. The exterior diameter of the tube typically is selected and reducing the external diameter of the tube typically is conducted such that the step of increasing the relative pressure within an interior portion of the tube results in less expansion compared to the use of a tube having a uniform and smaller exterior diameter. Consequently, likelihood of overstretching and tearing of tube material as a consequence of the expansion is reduced.

Further, the step of providing the tube may comprise pre- forming or pre-machining tube material such that an additional amount of the tube material is located at a region of the length of the tube that is subjected to more expansion than another region of the length of the tube. Consequently, likelihood of overstretching and tearing of tube material as a consequence of the expansion is further reduced .

The method typically also comprises the step of heating the length of the tube prior or during the step of increasing the relative pressure. Heating of the length of the tube may relieve work hardening of the material. In one example the tube comprises a metallic material and at least the length of the tube may be heated to a

temperature above a transition temperature at which the material changes from a brittle state to a ductile state. The tube material may also be provided in the form of an annealed material and the method may comprise heat treating the tube material after formation of the ejector to improve material properties.

In an embodiment, the step of increasing the relative pressure is conducted such that a hoop stress is induced in at least the portion of the length of the tube and the hoop stress is greater than a yield strength of at least the portion of the length of the tube.

The tube may not necessarily comprise a metallic material, but may alternatively comprise another suitable material. For example, the tube may comprise a polymeric material, a ceramic or glass. Examples of suitable metallic materials include steel, copper, aluminium, brass, carbon steel, an alloy, and high elongation steel that may have a

relatively low carbon content.

In an embodiment, the method comprises the step of exposing the length of the tube to an axial compression during expansion of the length of the tube. Exposing the length of the tube to an axial compression during

expansion may reduce the risk of tearing of the tube material. The method may further comprise the step of disposing a lubricant between the length of the tube and the die. The lubricant may reduce friction between the tube and the die. The lubricant may comprise molybdenum disulphide, although any suitable lubricant may be used as appropriate. Suitable alternative lubricants may comprise graphite, boron nitride, chalk, calcium fluoride, cerium fluoride and tungsten disulphide.

The shape that is related to that of the ejector may comprise the shape of a compressor portion of the ejector. The shape that is related to that of the ejector may also comprise a nozzle housing of the ejector such that the compressor portion and the nozzle housing are formed integrally . The fluid within the interior portion of the length of the tube typically is a liquid and the method may comprise the step of charging an interior portion of the length of the tube with the fluid. The fluid typically is selected such that the fluid can be heated to a temperature that is appropriate for heat treatment of the tube material without suffering any substantial deleterious effects. The fluid typically is selected such that the fluid will not vapourise when the fluid is pressurised and/or when the tube is released from the die. The fluid typically is not flammable. For example, the fluid may be a silicone oil.

In one embodiment of the present invention the die is arranged and the tube material is selected such that the tube slightly contracts ("springs back") in diameter when the relative pressure within the interior portion of the length of the tube is reduced. This may facilitate separation of the length of the tube from the die . It will be appreciated that this contraction in diameter is dependent on the tube material and not all materials show such a contraction.

The method may also comprise the step of locally or globally controlling the temperature of the fluid, which may provide a number of advantages. For example, moderate heating temperatures may be used to assist in reducing or preventing local stresses in the tube material. In a further example, controlling the temperature of the fluid may also comprise exposing the tube material to rapid cooling after expansion of the length of the tube. In an embodiment the method comprises the step of

determining a temporal pressure profile of the fluid, which may comprise determining a rate of relative pressure increase of the interior portion of the length of the tube and a rate of subsequent relative pressure decrease of the interior portion of the length of the tube. The step of increasing the relative pressure may be defined at least in part by the determined temporal pressure profile.

The invention will be more fully understood from the following description of specific embodiments of the invention. The description is provided with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a flow chart illustrating a method of forming an ejector in accordance with an embodiment of the present invention ;

Figure 2 is a schematic cross-sectional representation of an ejector fabricated in accordance with an embodiment of the present invention;

Figure 3 is a perspective view of a schematic (wire frame) representation of a compressor portion of an ejector fabricated in accordance with an embodiment of the present invention ;

Figure 4 is a perspective view of a solid representation of the compressor portion of Figure 2; Figure 5 is a side view of the compressor portion of Figure 2;

Figure 6 is a cross-sectional view of two portions of a die used to fabricate the compressor portion of Figure 2 in accordance with an embodiment of the present invention;

Figure 7 is the die of Figure 6 positioned around a tube from which the compressor portion of Figure 2 is

fabricated in accordance with an embodiment of the present invention; and Figure 8 is a schematic diagram of one embodiment of an ejector cooling system circuit in accordance with an embodiment of the present invention.

Detailed Description of Specific Embodiments Embodiments of the present invention relate a method of fabricating a component of a solar energy system, such as an ejector.

Referring initially to Figure 1, a method 10 in accordance with an embodiment of the present invention is now described. Further details of the method will be described further below with reference to Figures 2 to 8.

The method 10 comprises the initial step 12 of providing a tube, such as tube composed of copper or another suitable material. The tube material is selected such that the tube material deforms when the tube is exposed to a suitable difference in pressure between an interior portion of the tube and an exterior portion of the tube. As will be described further below in more detail, the tube may comprise a portion that is processed such that a diameter of the tube is locally reduced. Step 14 provides a die that has a cavity arranged to receive the length of the tube. The cavity defines a shape that is related to that of the component of the solar energy system.

Step 16 positions the length of the tube in the cavity of the die. Step 18 increases a relative pressure of a fluid within the interior portion of the length of the tube relative to a pressure within the cavity and outside the interior portion of the length of the tube such that at least a portion of the length of the tube expands to a shape that is related to that of the component of the solar energy system.

Referring now to Figure 2 an embodiment of the present invention is described in further detail. Figure 2 shows an ejector 20 that was formed using a method in accordance with the present invention. The ejector 20 may be operated to drive a heat pump refrigeration cycle, in which case, the ejector may be used in place of an electrically driven compressor. The ejector 20 has no moving parts and is suitable for widespread commercial and domestic use. The ejector 20 uses thermal energy rather than electrical energy to generate a compression effect. Figure 8 shows an example of a solar cooling system 200 comprising a solar panel 204 that supplies thermal energy to the ejector 20.

In the example illustrated in Figure 2 the ejector 20 comprises a hollow body 22 that has a partially closed end 25 and an open end 29. The ejector 20 is often

cylindrical, and in this embodiment is substantially symmetric around a central axis 36. The hollow body 22 has a nozzle housing 42 attached to a compressor portion 34. A nozzle 30 penetrates the end 25 of the hollow body 22. The nozzle 30 has an inlet 38 external of the hollow body 22 and an outlet 40 interior of the hollow body 22. The nozzle 30 has a constriction 31 intermediate the inlet 38 and the outlet 40.

It will be appreciated that other designs are envisaged. For example, both ends 25, 29 may be open and the ejector 20 may be arranged such that evaporator flow is in an axial direction through open end 25 and a nozzle 30 may enter the hollow body 22 of the ejector 20 through a side portion of the nozzle housing 42. It will also be appreciated that designs comprising an annular shaped nozzle and/or multiple nozzles are envisaged.

The nozzle housing 42 defines an entry chamber 24. A wall 32 of the entry chamber 24 has an entrained flow inlet formed therein. The compressor portion 44 defines a mixing chamber 26 in communication with the entry chamber 24. The compressor portion 44 also defines a diffusing chamber 28, and an intermediate chamber 27 in

communication with and intermediate of the mixing and diffusing chambers. The intermediate chamber is

restricted relative to the mixing and diffusing chamber. Figures 3 to 5 show views of the compression portion 44 of the ej ector 20.

An embodiment of a method of manufacturing the compressor portion 44 will now be described with reference to the Figures 6 and 7. Figure 6 shows a cross-sectional view of die 100. The die 100 is typically cylindrical and comprises a die portions 101 and 102 that are initially spaced apart from each other. The die 100 is formed when the portions 101 and 102 are brought together. The die 100 is configured with an internal space that has a shape that is complementary to that of a portion of an ejector, for example the ejector 20 illustrated in Figure 2.

Prior to inserting a tube 104 for forming an ejector portion into the die 100, the tube 104 is sometimes deformed or machined. In this example the tube 104 has initially an exterior diameter that larger than that of a narrow throat portion of the ejector 20. The method may comprise locally reducing the exterior diameter at the throat portion of the tube 104 such that the tube 104 has a non-uniform exterior diameter (not shown in Figures 6 and 7) . In this example the exterior diameter of the tube 104 is locally reduced using rotary swaging. The exterior diameter of the tube 104 is selected and reducing of the external diameter of the tube 104 is conducted such that an expansion required for formation of the ejector portion is reduced. Consequently, likelihood of tearing of material of the tube 104 as a consequence of the expansion is also reduced.

Further, tube 104 may also be pre-machined such that an additional amount of the tube material is located at a region of the tube that is subjected to more expansion than another region of the tube.

The tube 104 is then inserted between the spaced apart die portions 101, 102. A fluid 110 is then introduced into a first end 112 the tube. The fluid may be a silicone oil, and preferably a fluid that is able to be heated to temperatures that are appropriate for annealing the tube without the fluid suffering any deleterious effects. It is also be preferable for the fluid to not vapourise when the fluid is heated or de-pressurised, and particularly when the tube is released from the die. It is also preferable that the fluid not be flammable. The second end 114 of the tube may be pinched closed or capped, for example. The pressure of the fluid 110 in the tube is then increased using a suitable pump. In this embodiment, the pump is a piston type pump, however other embodiments may use any suitable pump, examples of which include but are not limited to a rotary type positive displacement pump, a reciprocating type positive displacement pump (such as a piston or diaphragm pump) , and a linear type positive displacement pump (such as a rope pump or chain pump) .

The increased fluid pressure within the tube induces a hoop stress of the tube which is greater than the hoop strength of the tube to plastically deform the tube 104 into contact with the internal walls 106, 108 of the die portions 101, 102, respectively.

Generally, but necessarily, during the increase of the fluid pressure the die portions 106,108 may be held in a mechanical press, for example a clamp or vice. The tube 104 may be composed of any suitable material. In the examples of Figures 4 to 7, the tube 104 is a copper or stainless steel tube. Further examples of the tube material include a high elongation steel with a low carbon content. In some examples, the tube may comprise a non metallic material such as a polymeric material, glass or ceramic .

An axial compression is applied to the tube 104 when the fluid pressure is being increased to compensate for thinning of a wall of the tube that may occur as the wall is urged outwardly by the pressure of the fluid. For example, the tube 104 may be grasped at two points of either side of the die 100 by jaws which are then urged to move together by a hydraulic piston, rack and pinion, or other suitable compression means. Generally, the axial tension that is applied to the tube 104 is determined prior to its application. This may be determined using computational finite element analysis of the process.

During the fabrication the tube 104 is expanded into the die 100 which may result in localised thinning of the material. This may possibly lead to rupture of the tube. Applying an axial tension may relieve this unwanted side effect .

A lubricant is disposed between the tube 104 and the die portions 106, 108. The lubricant may be, for example, molybdenum disulphide although any suitable lubricant may be used. Lubrication is favourable during the application of an axial compression to the tube.

The tube 104 is heated before expansion above a material brittle-to-ductile transition temperature. The method should also be carried out below the melting temperature of the tube material, around 1085°C for a copper tube. It will be appreciated that the values of the transition and melting temperatures vary from material to material. The - Im ^¬

material of the tube 104 may also be provided in annealed form .

The fluid pressure is then increased until the tube 104 expands within the die 104 such that an exterior surface of the tube 104 is in contact with the entire die surfaces 106 and 108. The pressure is then realised and the die 100 is configured such that at least for some suitable tube materials) the tube 104 contracts ("springs back") when the pressure of the fluid is reduced. The formed portion of the ejector 20 is then machined and processed using known techniques to form the ejector 20.

The temperature of the fluid may be controlled to improve the method. The step of controlling the temperature of the fluid may provide a number of advantages. For example, moderate heating temperatures may be used to assist in preventing local stresses in the material that might otherwise lead to rupturing of the tube. In some cases, the tube material may be exposed to rapid and this may be achieved by admitting cool fluid into the tube. Further, admitting a cool fluid after hydroforming may cause the tube to shrink sufficiently to facilitate its removal from the die.

The temporal pressure profile may be determined prior to placing the tube in the die. This may be determined by, for example, computational finite element analysis of the method. The pressure of the fluid may be increased and/or decreased as defined by the output of the analysis.

After the tube has been deformed by the increased pressure therein to form the compressor portion, the vice or press is released and the die portions 106 and 108 are

separated. The fluid is drained from the formed

compression portion and the tube is subsequently removed from the die and cleaned. The compressor portion 44 may then be machined or trimmed if required, and then may be attached to the nozzle housing 42 by any suitable means including but not limited to brazing, wielding or by use of an adhesive. In one embodiment, complementary threads are formed on the compressor portion 44 and the nozzle housing 42 which are then engaged to attach the nozzle housing 42 to the compressor portion 44.

Alternatively, the die may be configured so as to

facilitate formation of the nozzle housing 42 such that the compressor portion 34 and the nozzle housing 42 are formed integrally.

The operation of the ejector 20 may be generally

understood with reference to Figures 3 and 8. A source of vapour is coupled to the exterior end 38 of the nozzle 30. The vapour passes through the nozzle 30 and leaves the nozzle through the interior end 40. The passage of the vapour through the ejector 20 causes a reduction in pressure at the entrained flow inlet 34. Entrained flow inlet 34 is in communication with a vessel having a fluid in the form of a refrigerant, examples of which include but are not limited to hydrofluorocarbons , hydrocarbons, alcohols and water. In the embodiment of Figure 8, the vessel is contained in an evaporator 208. The relatively low pressure at the entrained flow inlet 34 causes evaporation of the refrigerant which in turn cools the remaining refrigerant in the vessel. The cooled refrigerant may then be used for subsequent cooling applications such as air conditioning.

The heat pump refrigeration cycle may consist of high 210 and low 212 temperature sub cycles. In the high

temperature sub cycle, heat that is transferred to the ejector from the heat source (such as a solar collector 204) through a vapour generator causing vaporisation of the ejector cycle working fluid in the generator at a temperature slightly above the saturation temperature of the refrigerant. Vapour then flows to the ejector where it is accelerated through the nozzle 30.

A pump 201 may be required to generate a pressure

difference for the ejector 20 to operate, but since liquid is being compressed, the electricity required may be relatively small. All other components in the heat pump circuit 202 may be, but may not be, conventional.

Since much of the vapour enthalpy may be converted to kinetic energy, conservation of energy suggests that the vapour temperature and pressure within the inlet housing 22 may be very low. The low pressure within the inlet housing may act to draw vapour flow from the evaporator.

The generator and evaporator flows may then mix in the ejector and the combined flow may undergo a compression shock. Thus thermal compression may replace the electrical compressor in a conventional heat pump. Further

compression may take place in the diffusing chamber such that a subsonic stream emerging from the ejector then flows into the condenser 206. At the condenser 206, heat is rejected from the working fluid (refrigerant) to the surroundings, resulting in a condensed refrigerant liquid at the condenser exit. The ejector 20 needs to provide sufficient exit pressure such that the saturation temperature of the refrigerant at this point is greater than the condenser cooling medium, otherwise heat cannot be rejected and the cycle ceases to operate. This is the malfunction mode of the ejector, caused by excessive condensing backpressure. Malfunction can be overcome by suppling greater generator pressure and temperature, for example from a generator 214.

Liquid refrigerant leaving the condenser is then divided into two streams; one enters the evaporator 208 after a pressure reduction through the expansion valve, the other is routed back into the generator after undergoing a pressure increase through the refrigerant pump 201. The refrigerant fluid is evaporated in the evaporator, absorbing heat from the environment that is being cooled, and then it is entrained back into the ejector 20

completing the cycle.

The ejector heat pump cycle may benefit from sub cooling prior to evaporation and from minimising superheating through compression.

The ejector mechanism may offer freedom of choice of refrigerant and is not complicated by the need for compressor lubricant compatibility. Also, the ejector is tolerant of liquid slugging since both generator and evaporator ports are essentially open tubes. There are a number of means to model the performance of an ejector. Modelling may be based on thermodynamic

compressible flow theory with minor corrections for non- ideal behaviour, or numerically derived using

computational fluid dynamics and/or finite element analysis. Modelling may be aided with reference to:

• Eames, IW, Aphornratana, S & Haider, H 1995, ^Λ Α

theoretical and experimental study of a small-scale steam jet refrigerator' , International Journal of Refrigeration, vol.18, no.6, pp. 378-86.

• Huang B., Petrenko V., Chang J, Lin C, Hu S . , ^Λ Α combined cycle refrigeration system using ejector cooling cycle as bottoming cycle', International Journal of Refrigeration 24 (2001) 391-399. · Zhu C, Wen L., Shock Circle method for ejector

performance evaluation, Energy Conversion and

Management, Vol 48, pp 2533-2541, 2007.

• Eames I., ^Λ Α new prescription for the design of

supersonic jet pumps: the constant rate of momentum change method' , Applied Thermal Engineering, Vol 22, ppl21-131, 2002.

Computational Fluid Dynamics (CFD) has matured over the last decade with the advance in hardware computational capability. This is allowing researchers to investigate the ejector processes in greater detail including

supersonic shock effects, real gas behaviour, metastable refrigerant states, boundary layer flow, flow separation and the like. Due to the complexity of highly turbulent supersonic compressible flow involving a real gas model, only highly developed CFD packages maybe suitable for ejector modelling. Ejector modellers may use Fluent or ANSYS CFD, or any other suitable softonne. The selection of a turbulence model is required for CFD modelling. The standard κ-ε turbulence model may not be adequate. In particular, the hybrid κ-ω-sst model seems to offer good result, as described by Bartosiewicz Y., Aidoun Z., Desevaux P.,Mercadier Y., CFD experiments integration in the evaluation of six turbulence models for supersonic ejector modelling, Proceedings of Integrating CFD and Experiments, Glasgow, 2003.

Insights into real ejector flows may be provided by advanced visualisation techniques involving transparent ejectors.

It will be appreciated that numerous variations and/or modifications may be made to the disclosed embodiments. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. For example, the component of the ejector may be formed by reducing a pressure at a region that is exterior to the tube 104 resulting in an increase in relative pressure within an interior region of the tube 104.

Reference that is made to prior publication is not an admission that the prior publication are part of the common general knowledge of a skilled person in Australia or any other country.