Patent Application
20130213613
The invention relates to a device and method for transferring heat.
A thermosyphon is a heat transfer device in which heat is inputted at the bottom of a pipe or loop, and heat is extracted at the top, so as to drive circulation of a liquid which transfers heat from the bottom to the top. Variations include two phase thermosyphons (in which the liquid boils at the heat input end and the rising bubbles of vapour help drive the liquid convection), and actively pumped thermosyphons (which have a pump in the circuit). The latter is familiar as a central heating system. A passive thermosyphon requires gravity to operate, and unless it has pumped assistance the output heat exchanger must be above the input.
A heat pipe is often described as a two phase thermosyphon where liquid in a tube boils, thereby removing local heat, and the vapour condenses some distance away, giving up its heat. The liquid condensate flows back to the input end by gravity and or capillary means through very narrow passages. Capillary flow frees the heat pipe from requiring gravity to operate, so that a heat pipe can operate with the input higher then the output, and a heat pipe will operate in zero gravity in space. However, heat pipes are limited in size and can dry out (leading to a loss of capillary action) if they get too hot.
Thermosyphons and heat pipes are generally designed to operate at low internal pressures because this reduces the boiling point of the liquid employed. However thermosyphons can be operated as systems open to the atmosphere and with many different combinations of liquids and pressures according to the application and boiling point required.
These technologies are of great interest because heat density in computer chips, photovoltaics, IGBTs and LEDs is rapidly increasing, posing heat removal problems. Two phase, or boiling, heat transfer is up to 1000 times faster than heat transfer through copper, which is the best performing easily available material. However, in two phase transfer, as the rate of heat input rises, Critical Heat Flux is reached when the production of bubbles becomes so general over the input surface that the bubbles coalesce to form an insulating blanket on the surface and heat transfer drops dramatically. This is a problem that limits the maximum performance of all boiling heat transfer devices including heat pipes and two phase thermosyphons. The situation can be improved by pumping liquid across the heat input surface or by liquid jets impinging on the heat input surface. These techniques force liquid on to the surface and dislodge the bubbles. However, pumped solutions add cost, complexity and noise as well as requiring external power.
The present invention improves boiling heat transfer in heat pipes and thermosyphons. Thus, thermosyphons can be used in applications where previously only heat pipes were used.
The invention is defined in the appended claims.
By way of introduction, devices according to embodiments of the invention are self-powered by the lifting force of bubbles from boiling. The liquid in which a bubble is immersed exerts a force on the bubble equal to the weight of liquid displaced. Because the gas in a submerged bubble is at equal pressure to the surrounding liquid it will exert the same force on a surface that prevents it rising as the surrounding liquid exerts on it. This enables work to be extracted from bubbles formed by boiling a liquid. The energy to boil the liquid may come from any suitable heat source such as fluids that are required to be cooled, solar and other radiation, waste heat, and chemical combination or disassociation.
In operation, the rotor, which may be of any suitable shapes and dimensions including conveyors rotating round two or more axes, is powered by the buoyancy force of the bubbles themselves, and, optionally, by the power that can be extracted from bubble growth. Devices can be optimised for transferring heat or generating power or a combination, depending on the requirement of the application.
A further advantage of the embodiments, described in more detail below, is that more power is produced than is required to operate the device (i.e. more than is required to cause rotation of the rotor). Such excess power can be extracted mechanically from the rotor and used to drive mechanical devices such as fans (for example, a fan can be driven by the rotor and used to force air flow through the condenser), or can be used to drive a generator so as to produce electrical energy. This enables the device to fulfil requirements in the field of recycling waste heat, harnessing low grade heat, solar powered electrical generation and improving the efficiency of engines.
As will be described by reference to the Figures, embodiments of the invention firstly provide a means to increase thermal transfer in thermosyphons by using a self-powered rotor to increase turbulence and to scrape or sweep, without surface contact, the vapour bubbles created by boiling heat transfer off the input surface as the bubbles form.
Secondly, there is provided a means of power generation by harnessing the buoyancy of bubbles and/or pressure increase from the formation of bubbles. Mechanical power take-off can also be used to drive a fan for the forced circulation of air through the fins of the condenser heat-exchanger and/or a heat source.
Thirdly, there is provided an improved method of extracting work from vapour bubble buoyancy in a liquid, using an Archimedes screw. Embodiments include improvements to Archimedes screws, including radially and axially tilted vanes which increase volumetric capacity, and blocking members which enable a reduction in the size of the central core thereby increasing volumetric capacity.
a shows a heat transfer device according to an embodiment of the invention.
b shows a rotor for a heat exchanger, the rotor having one-way liquid jet valves.
c shows an improved rotor having curved/spiral vanes.
d shows a disc rotor arranged to rotate close to a side wall of a stator chamber.
a shows a heat transfer device having a vertical axis rotor propelled by liquid jets.
b is a top-down view of the heat transfer device of
a shows a heat transfer device employing a vertical axis screw-type rotor
b shows a heat transfer device employing an inclined Archimedes screw as a rotor.
a shows a vertical-axis rotor having a central passageway for the passage of fluids.
b shows a cross-section view through the rotor and housing of
a shows a pressure regulating bellows-type device for regulating the pressure within a heat transfer device
b shows an alternative pressure regulating device having a spring and piston.
There follow, by way of illustration, various embodiments and features of the invention. These are not intended to define the invention, which is instead defined in the appended claims. It will be clear, however, to persons skilled in the art, that numerous combinations and variations can be applied in particular fields without departing from the claimed invention.
As shown in
In operation, an input heat exchange surface 170 at or near the bottom of the device is bonded to an element or heat source 175 (for example, a computer chip) which is required to be cooled. A suitable pressure and fluid charge is established in the device 100, and heat is transferred from the element to be cooled, via the input heat exchange surface 170, to the liquid in the device 100. The input heat from the element to be cooled causes the liquid inside the stator chamber 110 to boil, creating bubbles, which are trapped in the rotor cells 160, which rise due to gravity, and whose upward buoyancy rotates the rotor 120. The bubbles then escape from the rotor cells 160 after their surrounding rotor cell 160 has rotated to an upwardly facing position, and the bubbles then rise up the upward duct 130 to the heat exchanger 140 where heat carried in the vapour bubbles is transferred to the heat exchanger 140 and out of the stator chamber 110, and the bubbles condense into liquid. Condensed liquid descends from the heat exchanger 140 under the action of gravity, is carried by the downward duct 150, and continuously charges the rotor cells 160 with cooled liquid which is denser than the heated liquid and vapour bubbles in the upward duct 130. The downward thrust adds further impetus to the rotor.
Simple horizontal axis versions of the invention (where the rotor axis is horizontal, or substantially horizontal, as shown in
In other embodiments, as shown in
Suitable bearings on which the rotor 120 rotates are plastic, stainless steel or glass ball or roller bearings. The rotor and stator can be made from metals such as aluminium, or alternatively from plastics or ceramics or any suitable material having similar characteristics. The evaporator is preferably made from a thermally conductive material such as copper or aluminium. The condenser is arranged with fins made of a thermally conductive material such as copper or aluminium.
In a prototype of an embodiment it has been found that at high rotor speeds there is insufficient time for all the bubbles to leave a cell as it approaches top dead centre and that a large upward duct opening and a suitable cell depth are required to give time for all the bubbles to leave. A large size of the duct opening is preferable for assisting bubbles to leave the cells 160, but duct opening size may be balanced against the desirability of avoiding the kinetic energy of down-flowing liquid being dissipated by leakage across to the upward duct 130 (it has been found that this can be substantially avoided by keeping at all times at least two full cell widths of stator wall between the upward duct 130 and the downward duct 150).
Additionally, in embodiments, it has been observed that some bubbles stick to the rotor. This is advantageous when bubbles stick to the axial ends of the rotor, as it reduces liquid drag between the ends of the rotor and stator wall. However, it is not advantageous when bubbles stick within a cell, and are carried past the centreline 185 of the rotor, such that they are carried over into the next cycle (into the downwardly moving half of the rotor). To maximise efficiency, therefore, the rotor may have a bubble-repellant surface or coating within the cells 160, and a vapour or bubble attracting surface or coating at the axial ends of the rotor vanes 165. The ends may also be provided with spiral grooves to assist in maintaining a layer of gas between the rotor 120 and the end wall of the stator chamber 110 (also termed “housing”).
In other embodiments, additional features described below such as non-return valves 180 may be added to increase torque. In simple horizontal devices, as shown in
Additional power, beyond the buoyancy force, that can be extracted from bubble growth in devices according to the invention, arises when a discrete cell on a rotor passes a heat input surface. When the fluid within the cell is heated, vapour forms and pressure within the cell rises (since the rotor is arranged to fit closely within the stator housing such that fluid flow from each cell is controlled and optimally kept to a minimum). This pressure rise can optionally be harnessed in two ways: first, by connecting each cell on a rotor to the following cell through non return valves 180 (as shown in
As shown in
The second method by which the pressure rise in the cell can be harnessed is by providing one or more ducts 230 (as shown in
In further embodiments, as shown in
To maximise the efficiency of the pressure pumping feature, in an embodiment a layout (as shown in
Embodiments having rotors 120 with cells 160 and non-return valves (otherwise known as jet valves) 180 can also have a vertical axis of rotation, as in the embodiment shown in
Additional work can be extracted from vertical axis celled rotors by stacking more rotors on top of the first to extract further work from the rising bubbles. In this case the rotors may be mechanically linked.
Vertical axis devices as shown in
In the embodiment shown in
There is further provided heat exchange means (e.g. a heat exchanger 140) in the stator chamber 510 wall, above the rotor, at or near the top of the rotor, by which means heat can be transferred outside of the heat transfer device 100. There is further provided one or more ducts 560 external to or within the stator chamber 510, or internal to the rotor 520, through which cooled fluid can return from the heat exchanger 140 to the bottom of the rotor 520.
In operation, the internal pressure in the device (100) is lowered by external pressure reducing means (such as an external vacuum pump) such that, when placed in contact with a heat source (e.g. a surface or fluid flow that is required to be cooled), the fluid in the device 100 boils at the heat interface (i.e. at the portion of the stator chamber 510 which is in contact with the heat source). The use of a vacuum lowers the temperature at which boiling takes place.
The heat interface is preferably as close to the base of the rotor as possible, but the device 100 can operate, albeit with reduced efficiency, if the heat interface extends partially up the rotor. The bubbles of vapour, which are produced by boiling of the fluid in the device, rise and are deflected by the spiral (or spirals) 530, imparting a turning force on the rotor 520. The bubbles rise up each spiral 530 until they encounter a downward projection 550 and are restrained from rising until following bubbles merge with them to create a bubble large enough for a portion to escape from under the projection 550.
A downwardly pointing conical rotor can be employed and has the advantage that any bubbles escaping between a spiral 530 and the stator wall 510 are trapped again by the spiral above, rather than moving vertically up the wall and so potentially avoiding being trapped by the spiral above. Such a conical rotor has a larger diameter towards the top of the rotor. A further advantage of such a conical rotor is that as bubbles rise through the rotor, their expansion is accommodated in the progressively larger volume of the rotor as its diameter increases with height. Furthermore, as diameter increases, so does torque exerted on the rotor by the bubbles.
Thus, additional work can be extracted from the bubble expansion. In other embodiments, multiple vertical-axis rotors are stacked on top of each other. In such embodiments, each successive rotor optionally has a larger diameter than the one below.
Another embodiment shown in
Devices according to embodiments can be chained, with the heat exchanger (condenser) of a first device acting to provide heat to the heat input source (evaporator) of the next device in the chain. Each device can be arranged with appropriate liquid and internal pressure so as to maximise efficiency of each device, each device operating at a different temperature range. Thus, devices can be “compounded”. Multiple devices according to embodiments can be deployed in exhaust systems and boilers etc., in which embodiments it is advantageous if they share a heat exchanger 140.
All devices according to this invention can produce work (mechanical energy) which may be used within the device (for example for driving a pump to provide pressurised flow for hydraulic actuation) or transferred outside the device by mechanical or magnetic means. Also, for the highest heat transfer rates it may be necessary to provide additional power to the rotor to drive it at speeds that bubble growth alone cannot provide. In those embodiments it should be noticed that as long as there is gas in the cells on the upwardly moving side of the rotor, there will be a torque supplied by the liquid on the downward side, reducing the input work to the rotor. Furthermore, the greater the height of the liquid column the greater the torque supplied. The rotor can be used to power an electrical generator for generating electrical power, or a mechanical fan for forcing air flow through fins of the condenser and/or a heat source, among other uses. Cooling the condenser 140 can help to improve device efficiency.
In a yet further embodiment shown in
In operation, the stator housing 610 is filled with liquid at a suitable pressure, which may be externally controlled. Hot fluid moves through the inner duct 640 and heat is transferred from the hot fluid through the stator wall 610 to liquid in the stator chamber 620, causing it to boil and produce vapour bubbles. The bubbles rise and produce a lifting and turning force on the rotor 630, which rotates. Because the rotor is arranged to be close to the heat exchange surface, as the rotor 630 rotates, the spirals 650 brush, scrape or sweep the bubbles off the surface of the stator housing 610, to prevent insulation of the heat exchange surface (between the hot liquid in the inner duct 640 and the liquid in the stator chamber 620) by the bubbles. Rotation of the rotor causes pumping of fluid in a circuit via fluid outlet 690, the duct 660, and the heat exchanger 670 where the fluid is cooled and then returns to the stator liquid inlet 680. The spirals optionally have turned down edges and vapour restraining projections as described above. Further optional variations also have external spirals 655 to the rotor so that fluid flow past the exterior surface of the stator may also be cooled and thus provide a more compact cooling device.
Turning to
In operation, the internal pressure of the device is first set to a level suitable for the fluid employed and the temperatures of the source and sink. Heat transfer through the stator chamber wall 710 at a heat input area heats and boils liquid in the rotating cell 740 for the time being over the heat input area. The heat input area is in the region of the larger end of the rotor. The boiled off vapour is forced to the radially inner part of the cell 740 by pressure from the liquid which is centrifuged outward by the rotation of the cell. Rising vapour pressure from increased boil off in the cell forces a liquid jet through one way valves between the cells 740, or through flow biasing passages on the trailing edge of each cell 740. This produces a reaction thrust on the rotor 720. The passage of fluid into each following cell will increase the pressure in each following cell with similar but diminishing reaction effect. The reaction thrust on the rotor causes it to rotate. Rotation brings the cell under the cut out 780 and the still slightly pressurized vapour leaves the cell 740 and moves up the rotor interior duct 770 to the stator heat exchanger 760. Here the vapour is discharged against a stator vane 7130 which is set at an angle to assist in establishing a vortex of the same direction of rotation as the rotor 720 in the heat exchanger 760 volume, and the vapour is cooled and condenses.
One or more spiral scrapers 706 attached to the rotor 720 are used to gather liquid condensate into the rotor spiral ducts 7100. The scrapers 706 also have the effect of further encouraging a vortex in the heat exchanger 760 volume. The vortex moves liquid to the scraped inner surface of the stator housing 710 by centrifugal force.
If the heat exchange surface is tapered down in the axial direction away from the celled end of the rotor, the rotating vortex of vapour will act on the condensate on the tapering surface to drive it towards the rotor. This may be sufficient to allow the rotor spirals to be dispensed with unless it is desired to have the heat exchanger at some distance from the rotor. In this case surplus rotor work may be used to operate a fan to improve the vortex or be used to power a pump as described above.
Having entered a rotor spiral 7100 the liquid is moved to the other end of the spiral, which in some embodiments is shrouded, and the liquid is injected through the projection opening 7120 into the most recently vapour discharged cell 740 which has by now rotated under the opening 7120.
All the above devices may be operated at any suitable internal pressure to suit the target temperature and fluid selected, but will usually be designed for low internal pressure. A method of facilitating manufacture and providing both pressure control and increased surface for transferring heat out of the above-described type of thermosyphon is to use a bellows device as shown in
In such a bellows device 800, adapted to the present purpose of regulating pressure within the stator chamber 110, a rigid flat plate 810 is bonded to a thin sheet 820 that has concentric or other suitable ribbing to make the sheet flexible. A compression spring 830 (biasing member), which is bimetallic so as to change rate with temperature, and which is arranged between the plate and the sheet, in operation pushes the sheet 820 away from the plate 810, increasing the internal volume. The internal volume is connected to the internal volume of the stator chamber 110, so that the bellows device 800 can regulate the internal pressure of the stator chamber 110. The bellows device 800 may have a second ribbed sheet instead of the plate. In other embodiments, a second bi-metallic spring 840 external to the device is also used, to adjust the internal volume and thereby the pressure, according to ambient temperature. In other embodiments, other means such as an external screw 850, to compress the internal spring 830 or allow it to move out, can be employed for controlling the internal pressure.
For assembly, the internal spring is compressed by external force, reducing the internal volume, and air evacuated through a suitable duct. A valve in the duct allows retention of the vacuum until the device is attached to a heat transfer device (e.g. a thermosyphon) such as that described above. The heat transfer device is charged with liquid before use, and the bellows device is attached, inducing a vacuum in the heat transfer device, and sealing both devices. The spring 830 is released and the plate 810 and sheet 820 are forced apart by the spring, lowering the internal pressure. When used on a heat transfer device, such as a thermosyphon, this device produces the required vacuum in the thermosyphon, acts as a pressure controller and also as a heat exchanger with the ribs acting as fins. Thus the device can provide a self regulating fin type heat exchange surface of improved capacity.
Another pressure regulating device 805, attachable to the stator chamber 110, for regulating the internal pressure inside the stator chamber is shown in
The above-described pressure regulator devices 800,805 additionally accommodate the change in volume required within the stator chamber 110 when vapour bubbles grow (since vapour occupies greater volume than liquid).
As described, certain embodiments employ an Archimedes screw as the rotor. Archimedes screws have been used for lifting water and for power generation from low head water sources. In these devices force is produced by the lifting force of displacement of liquid and this is combined with the rotational speed to produce power. Such devices have a power to volume ratio of the same order as a large wind turbine tower, however it would be desirable for this ratio to be increased. It is a further purpose of this invention to increase the useful displacement of an Archimedes screw, to increase the lifting force.
Traditional water power Archimedes screws have been found by experiment and analysis (Chris Rorres-Journal of Hydraulic Engineering January 2000 pages 72-80) to have a maximum fill ratio of 60% of available volume when operating to lift water. This is less than half the total volume of the screw because the volume of the central cylindrical core is 25% of the total volume.
The following two factors have been considered for improvements to conventional Archimedes screws:
1) increasing the screw volume available to be filled with fluid (liquid and gas). This is restricted in optimised conventional Archimedes screws by the central cylinder to which the vanes are attached and which takes up about 25% of the cross-sectional area of the screw.
2) increasing the lifting force of the screw by increasing the ratio of gas to liquid within the Archimedes screw.
Considering the above factors, the volume available for fluid can be increased by decreasing the central core cross-sectional area. However, the central core surface acts to separate pockets of fluid or particulate solids, and enables them to be moved up the screw. Furthermore the reason the conventional inner cylindrical core is so large is that its diameter determines the level at which fluid overflows into the next compartment. Any reduction in cross-sectional area of the central core must be achieved without decreasing the ability to separate liquid and gas. It has been further realised that more of the total volume could be filled with fluid if the point at which the fluid escapes into the next compartment could be: lowered if operating on gas flowing upwards; or raised if operating on water flowing downwards. By way of illustration, in an Archimedes screw operating on gas flowing upwards, if a vertical barrier for preventing gas escaping from each compartment to the next is extended downwards, i.e. lowered, then the core size can be reduced without allowing gas to escape from one compartment to the next (which would otherwise result in a loss of torque). Similarly, in an Archimedes screw operating on water flowing downwards, if a vertical barrier for preventing water escaping from each compartment to the next is extended upwards, i.e. raised, then the core size can be reduced without allowing water to escape.
In order to solve this problem with reference to an Archimedes screw operating on ascending gas or vapour, it has been further realised that the liquid in each compartment is acting as a rolling valve between the gas packages as they rise up the screw. The size of this water valve is limited on the upside by spillage downwards as in a water screw pump, however the limit on it being small is firstly that it must prevent unimpeded gas flow upwards and that there must be room through the valve gap for the gas to circulate upwards as the screw turns. Gas can move rapidly and with low drag through a small gap, compared to fluid. Reducing the operational size of the water valve will also reduce any work required to lift the water valve to the top of the screw.
In the light of the above analysis, an improved Archimedes screw has been produced, as follows:
An embodiment of a screw architecture that fulfils the requirement of minimizing the water valve (central core) size comprises a tilted screw 900 with one or more flights of vanes 910 (of optionally narrow pitch), mounted on and around a central core 920 (which can be relatively small compared to conventional Archimedes screws), as shown in
In operation the screw 900 rotates and at any time at least one blocking member 930 outer edge 940 is extended into the surface of the liquid valve 970 (the liquid which is trapped between the two flights of vanes 950,960) so as to provide a seal, isolating a portion of gas 980, and thereby preventing gas escaping from one compartment (also termed “pocket” or “cell”) into the next compartment. The vane tips and/or blocking member outer edges (or tips) 940 are optionally angled radially or axially or otherwise shaped to reduce agitation as each member 930 enters the liquid 970. Clearly it is important (although not essential) to have the minimum number of vanes 910 that allows a desirably small liquid valve 970, without the blocking members 930 agitating the liquid surface so much that gas 980 can pass. An additional factor is that, because of the curvature of the cylinder 905, at the bottom of the cylinder in each compartment, a decrease in liquid depth brings a proportionally smaller decrease in surface over which the liquid rolls (and a corresponding decrease in drag). This means that for ever smaller decreases in drag, more blocking members 930 are required for sealing. A number of members between 6 and 12 per revolution has been found to provide the optimum for most liquids, although a greater or smaller number of members can be used, albeit with increased complexity or drag. In other embodiments, the vanes 910 are supported only by the blocking members 930 which are arranged radially around the rotational axis of the screw 900 and connect between each vane 910 such that a central core 920 is not required.
The above description concerns separation of gas portions in compartments between vanes. However, by reversing certain aspects, the design can be adapted in other embodiments to separate portions of liquid instead. For example, if instead the outer edge of each blocking member extends from a radially outer end of an upper vane of a pair of adjacent vanes, towards a radially inner position and towards the lower vane of the pair, when in use the blocking member is oriented maximally upwards, the outer edge of the blocking member is able to isolate a liquid portion between adjacent vanes (and by analogy the blocking member closes with a gas surface). Depending on the screw inclination when in use (such a screw can be mounted at various inclination angles in use), the radially outer edge of the blocking members are optimally shaped so as to match (i.e. close with) the liquid/gas surface boundary (which remains substantially horizontal, regardless of screw inclination), when the screw is suitably rotated in use (to either the most upwardly position for liquid separation, or the most downwardly position for gas separation).
The top and bottom turns (at the top and bottom of the screw 900) may have truncated and/or tapering blocking members, since for the first and last turns, the blocking members 930 otherwise agitate the liquid and add drag without an increase in lift. The screw can optionally be shrouded by a co-rotating tube 906 within the screw cylinder 905. The cylinder 905 optimally extends over the evaporator such that the majority of bubbles are captured by the end of the screw, and/or optionally a duct 911 connects the evaporator 170 to the cylinder 905.
Further enhancement to screws enclosed in co-rotating tubes 906 can be made by reducing drag from the liquid between the co-rotating tube 906 and the (outer) screw cylinder 905 (also termed “housing”). In this aspect of the invention shown in
In all embodiments having a spiral screw rotor, including those using vertical screw rotors and/or Archimedes screw rotors, the pitch of the screw can optionally be arranged to increase in the direction of bubble travel, such that the expansion of bubbles is accommodated. For example, it has been found that with a 3 metre high screw, the bubbles expand to approximately 5× their volume at the top compared with at the bottom of the screw (although rotor can optionally be increased in the direction of bubble travel, such that the rotor is cone-shaped. Alternatively, a combination of these features can be applied in embodiments. Alternatively, the number of rotor vanes can be increased or decreased by changing the spiral from a single spiral to a double or triple spiral at various points along the length of the spiral screw rotor, thereby changing the volume between each rotor vane. By accommodating the expansion of bubbles, further work is imparted to the rotor, thus increasing efficiency.
A further improvement to Archimedes screws will now be described.
In its conventional form, an Archimedes screw consists of one or more vanes (sometimes also referred to as ‘blades’) twisting around a central shaft. This is enclosed within an outer cylinder. A cross-section through the screw perpendicular to the axis of rotation shows that this conventional design is characterised by a radial blade profile (
The volumetric flow rate is an important factor as it influences the cost and size of a device for a particular application. Two design modifications are presented here which result in increased volumetric flow rate compared to the conventional screw. For clarity, the two design improvements have been illustrated schematically for a single blade. They can also be applied to screws with multiple blades.
It is clear that when the axial and radial skew angles are both zero, the conventional screw geometry is obtained as shown in
Both radial skew and axial skew features result in an improvement in volumetric capacity (the amount of fluid conveyed for a given rotor size and for a single rotation). The increase in fluid volumetric capacity increases torque produced by liquid travelling down the screw and/or by vapour bubble travelling up the screw. For a particular application, the design of a screw can thus be optimised so as to maximise the volume of liquid conveyed per revolution of the screw. Volumetric flow rate is the product of the volume per revolution and the rotational speed of the screw. Thus, for the same size and rotational speed, these improvements allow a higher volumetric flow rate. Both radial skew and axial skew can be used alone or combined, and can also be combined with the above-described blocking member embodiments so as to further improve performance. It has also been found that a greater number of vanes per unit length increases the volumetric efficiency, although a limit is anticipated where additional drag from additional vanes cancels out the increase in torque resulting from increased volumetric efficiency.
Further, one or more heat exchange devices can be nested inside another heat exchange device.
In operation, various methods of control may be used to control the rate of transfer of heat through a heat transfer device according to the above-described embodiments. The rate of heat transfer can be controlled by one or more of the following methods:
a) controlling the level of heat input to the system. This gradually affects the whole system.
b) changing the level of vacuum or pressure, which immediately affects the whole system by changing the boiling and the saturation point of the liquid and vapour.
c) changing the level of heat transfer out of the system, by changing the level of insulation at various points around the system.
d) by changing the size of the system, for example closing off parts of the condenser 140 or removing some liquid from circulation.
e) by restricting circulation between parts of the system, in particular by reducing the flow of liquid from the condenser 140 to the heat input (evaporator) 170.
f) by actively introducing hot water from the heat input 170 to higher parts of the screw 910, which results in a large increase in vapour generated within the screw 910. This can be done via a hollow central tube 640.
g) by reducing or increasing air flow over the condenser 140.
h) by using mechanical output from the rotor to rotate a fan for forced air flow through the condenser, and by controlling the fan speed or the fan's proximity to the condenser.
The features of the various aspects and embodiments can optionally be combined in any combination.
Here follow, by way of example, some more complex applications of the invention as applied to cooling and power generation (this list is non-exhaustive).
Whereas the advantages of recovering energy from low grade heat such as engine exhausts and warm air exhausted by air conditioning systems are well known, there has been little attention paid to the advantages of more rapid cooling of exhaust streams. Conventional internal combustion engines exploit the difference between the internal pressure generated and exhaust back pressure. Lowering back pressure increases power output. However, achieving this by shortening the exhaust tends to work against correct timing of pressure wave reflections within the exhaust, which improve power by scavenging exhaust gas from the cylinder at an optimal time during the cycle. Exhaust system lengths are optimised for a particular engine speed band. Outside that band, reflection timing may seriously degrade engine performance. The timing of pressure wave reflections is a product of exhaust length and the speed of sound in the exhaust. A method of rapidly reducing the temperature of the exhaust gases and so reducing the back pressure would be beneficial if it was sufficiently controllable to also control the speed of sound in the exhaust. The current device removes heat rapidly and controllably and could fulfil the specified need, perhaps by using the engine's existing liquid cooling system as a first heat sink for high temperatures and ambient air as a second heat sink for second stage cooling. Generating electrical power from the cooling process would be an added benefit.
Similarly in the field of gas compression, gas is discharged at high temperatures and cooled in heat exchangers, incurring a pressure drop. More rapid heat exchange would enable a smaller pressure drop in the exchanger by shortening the required heat exchange passage length and lower the back pressure. This would lower the pressure ratio across the compressor and reduce the compressor work consumed to achieve the required compression. Thus, parasitic losses are reduced.
Reducing pressure drop would also be advantageous in refrigeration systems, air conditioning systems, desalination and other industrial processes. In a typical domestic fridge the condenser is 12 to 18 metres of narrow bore finned tubing with many bends. This imposes a pressure drop which consumes about 10% of the system's energy input. As pressure drop is proportional to length, cooling the flow over a length of 1.2 metres instead of 12 metres would reduce the system's energy consumption by 9% from this effect alone. This would result in a reduction in power consumed by the compressor, and a reduction in motor heat generated. In hermetic domestic systems this introduces a virtuous circle of a reduction in motor heat, thus reducing the inlet temperature rise incurred from cooling the motor with inlet gas, thus leading to lower compressor work requirement and so on. Another advantage of reducing the condenser length is that it significantly reduces the refrigerant charge, lowering cost and increasing system response. Again, work can be recovered from the transfer of heat by a rapid cooling device according to the present invention.
Further, in refrigeration systems, a fan can be driven from a rotor 120 of a heat transfer device 100 as claimed, with the heat source being the compressor heat. A fan has been found to provide a useful effect wherever there is approximately a 15 degree temperature difference between the evaporator 170 and the condenser 140. The fan can be used to drive air flow through the condenser of the refrigeration system, in use, thereby improving the heat transfer from the refrigeration system condenser. Heat can also be removed from the hermetic compressor container. Both of these effects can lead to improved refrigeration efficiency.
In industrial electric motors a power failure, trip or shut down, can lead to temperature excursions because the air cooling fan is part of the motor (and therefore rotates with it). A quick restart with high current draw causes high temperature damage to the winding varnish. Lengthy cooling is therefore mandated before a motor is restarted, and this can result in considerable production loss on complex process lines. Self powered devices according to the invention continue to provide cooling as long as there is heat flow above the designed temperature, thus enabling processes to be re-started more quickly. Parasitic losses (due to electrical resistance which generally increases with temperature) are thereby also reduced.
A further application for such a heat transfer device is for cooling concentrator-type photo-voltaic cells, for example in domestic installations, where the efficiency of the cells can be increased by cooling them, and at the same time hot water can be produced by exchanging heat from the heat exchanger (condenser) 140 to water contained in a domestic hot water tank. Similarly, such a heat transfer device is useful for cooling high power LEDs.
An application of the invention in the field of mechanical power that fulfils a domestic heating requirement. Domestic radiators are large because of the heat surface area required to transfer heat to air by purely convectional means. Such radiators have to be even larger to transfer heat from the low temperature water produced by heat pumps. The rate of heat transfer slows as the room temperature and water temperature converge. Slow response and warm-up may result from the use of standard radiators. This problem is being overcome by the addition of electrically driven fans to increase flow of air. However this requires electrical cabling, supply and controls to turn the fan on and off.
Aspects of the present invention, called a fan system in this application, can be applied as a solution by making part of the radiator into the container for an evaporator for a two phase vacuum driven mechanical and electrical power producing system.
In a favoured embodiment the fan system is closed and filled with water under vacuum. The evaporator is in thermal contact with the hottest part of the radiator, which is where the hot water enters the radiator. The system is provided with a fan driven by magnetic means by preferably an Archimedes type screw according to various aspects of the invention. Optionally the system is also provided with a generator to convert a fraction of the power output into electricity.
In operation, hot water reaches the radiator soon after the heating system starts circulating water. Heat transfers to the fan system and heats up its charge of water. When the water boils the rising vapour causes the rotor to turn, which in turn causes the fan to turn via a coupling (preferably a coupling having no rotating seals, for example a magnetic coupling). The fan turns, causing an air flow through the radiator and its ducting, and thus produces improved heat transfer. Some of the power transmitted by the coupling may be used to generate electricity and power temperature sensors for wireless transmission with a central controller, and thereby allow control of the radiator valves to adjust the room temperature. Because the condenser of the fan system also acts as a radiator there is additional improvement in heat transfer to the room. The fan system optionally has a separate air path from that of the main body of the radiator, although in some embodiments the condenser 140 of the fan system could be the sole heat exchanger with the room, such that maximum energy is available for rotating the fan.
Thus the benefits of a fan driven radiator can be obtained without use of externally supplied electrical power or cabling and controls. As such radiators are inherently safe because of their low temperature and low pressure design, they are eminently suitable for installation in bathrooms, where an electric fan might be a hazard. Furthermore the addition of a self-powered fan would also improve the performance of electric oil-filled radiators. The addition of a fan would also be helpful in cooling PCs. Industrial applications where cooling must be guaranteed in the event of power failure are also possible.
Some further examples of domestic applications follow:
a) The heat transfer device (also termed a buoyancy engine in the examples below) can operate on solar power to produce both electricity and hot water during the day. Given a reasonable amount of insulated hot water storage, generation can continue after sundown by generating from stored hot water. When this is exhausted, burning wood or fossil fuels can keep the engine supplied with heat.
b) In far northern winters the engine can operate on a low boiling point liquid and generate electricity by using the below freezing heat from permafrost or the sea etc to generate, by using the even lower night air temperature as a sink. The electricity can be used to power a heat pump to provide hot water.
c) The buoyancy engine can operate on the exhaust heat from a domestic boiler to provide electricity to run the boiler controls and fan, thus freeing the boiler from grid dependence. The same engine can switch to operating on solar during the day when the boiler is off.
d) A small device can operate off waste fridge heat (from the heat exchanger of a refrigerator unit) to charge a battery, which in turn is used for mobile phone charging.
Further, control can be exercised over pressure in the system to enable mechanical power to be extracted after the heat source has been removed. Since there is a large amount of energy stored in the hot liquid (e.g. water) after the heat (solar or other source) is no longer available, if the pressure is gradually reduced then power generation can continue due to continued boiling of the liquid at lower temperatures under the lower pressure. Although the embodiments described above have been described as operating under a lowered pressure or vacuum, by the selection of different liquids having different boiling temperatures, it is possible for such a system to operate under raised pressure, for example during the day when input heat is highest (when the sun is shining on a solar collector acting as heat source). Then, at night, when only a lower temperature heat source is available (such as a tank of hot water which was heated during the day when there was an excess of heat provided by the solar collector), the system pressure can be reduced so as to allow the liquid to boil at a lower temperature. It is also possible to enhance this effect by deliberately increasing the volume of water in the system, or within an auxiliary (or “side”) tank (used for storing excess heat collected during the day), should there be additional heat available which is not being used for generation at the time or should the value of stored energy be high. An example is where the engine is sized to produce say 100 W during the way and this requires 1000 W of heat for an engine of 10% efficiency. However, additional heat is available particularly at the peak of the day and this is used to store hot water. When the sun goes down, this second, auxiliary, tank of hot water is circulated and used as a heat source to allow generation to continue. Pressure inside the engine can be reduced as the temperature of the heat source decreases, thus lowering the boiling point and allowing further power generation. This can eliminate the need for a battery in stand-alone off-grid systems, which is a major cost and source of complication for off-grid power systems.
Some further examples of small industrial applications follow:
a) Wireless sensors can be powered or re-charged by very small buoyancy engines. These would be particularly effective in or on industrial or central heating ducts where solar is not practicable and where the system may remain cold for months. Buoyancy engines would start generating as soon as heat flowed, overcoming the problem of discharged batteries.
b) Buoyancy engines can recover exhaust energy from distributed power generation units such as Diesel generators.
c) Buoyancy engines can recover energy from warm exhaust from air-conditioned buildings systems.
d) Buoyancy engines can recover energy from compressors and refrigerators.
Some further examples of large Industrial applications follow:
a) Vertical versions of buoyancy engines can be built into industrial chimneys so as to recover power, in mechanical and/or electrical form.
b) Concentrator solar systems in deserts require large amounts of cooling water. The hot water can be stored to produce more valuable night-time electricity. As the desert night air cools, the engine can continue to run on the progressively lower heat levels in the stored water of the plant.
c) Heat removal in data centres is becoming a major problem. Buoyancy enginess can utilize the heat from chillers and refrigeration plants (e.g. from their heat exchangers) to produce recycled energy.
d) There are many uses in water treatment where Archimedes screws are widely used for lifting water. Sewers and septic tanks are at a temperature of approx 15-20° C. all year round, from the warm water released and bio-decay. This heat can be applied to generation. In sewage farms Buoyancy Engines (BEs) can be used to directly drive lifting Archimedes screws, driving distributors and agitators. In this application, cooling water from electric motors and biogas generators can be piped at, say 60° C., to the BEs to stir and distribute sewage.
e) Mine cooling. Like the London Underground, mines can have uncomfortably high air temperatures. Like heat pipes, BEs can transfer heat from evaporator to condenser at supersonic speed over long distances. Thus they have the capacity to generate power at the workface, while simultaneously cooling the working environment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1010308.3 | Jun 2010 | GB | national |
| 1015435.9 | Sep 2010 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB11/00928 | 6/20/2011 | WO | 00 | 3/26/2013 |
