U.S. patent application number 13/805311 was filed with the patent office on 2013-08-22 for heat transfer device.
The applicant listed for this patent is John Philip Roger Hammerbeck, Keith Robert Pullen, Matthew Gordon Read. Invention is credited to John Philip Roger Hammerbeck, Keith Robert Pullen, Matthew Gordon Read.
Application Number | 20130213613 13/805311 |
Document ID | / |
Family ID | 42582669 |
Filed Date | 2013-08-22 |
United States Patent
Application |
20130213613 |
Kind Code |
A1 |
Hammerbeck; John Philip Roger ;
et al. |
August 22, 2013 |
HEAT TRANSFER DEVICE
Abstract
The invention relates to a device for transferring heat and a
method of controlling such a device, the device comprising a stator
chamber containing: a liquid; an input heat exchange surface; an
output heat exchanger; and a rotor arranged to be rotated by vapour
bubbles.
Inventors: |
Hammerbeck; John Philip Roger;
(London, GB) ; Pullen; Keith Robert; (London,
GB) ; Read; Matthew Gordon; (London, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hammerbeck; John Philip Roger
Pullen; Keith Robert
Read; Matthew Gordon |
London
London
London |
|
GB
GB
GB |
|
|
Family ID: |
42582669 |
Appl. No.: |
13/805311 |
Filed: |
June 20, 2011 |
PCT Filed: |
June 20, 2011 |
PCT NO: |
PCT/GB11/00928 |
371 Date: |
March 26, 2013 |
Current U.S.
Class: |
165/104.27 ;
416/223R |
Current CPC
Class: |
Y02B 10/50 20130101;
F28D 15/02 20130101; F03B 13/00 20130101; F03B 17/005 20130101;
F28F 2250/08 20130101; F28D 15/00 20130101; F28D 15/06 20130101;
F05B 2240/243 20130101; F28F 13/125 20130101; F28D 2015/0291
20130101; F28F 27/00 20130101; F28F 2245/00 20130101; F05B 2220/602
20130101 |
Class at
Publication: |
165/104.27 ;
416/223.R |
International
Class: |
F28D 15/00 20060101
F28D015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 18, 2010 |
GB |
1010308.3 |
Sep 15, 2010 |
GB |
1015435.9 |
Claims
1. A heat transfer device having a housing, the housing containing
an evaporator and a rotor, the rotor rotationally drivable by
bubbles produced when a liquid is heated by the evaporator, the
rotor further arranged proximate to a surface of the evaporator so
as to aid removal of bubbles from the surface.
2. A device according to claim 1 wherein the removal of bubbles is
by scraping, brushing or turbulence.
3. A heat transfer device having a housing, the housing containing
an evaporator and a rotor, the rotor rotationally drivable by
bubbles produced when a liquid is heated by the evaporator, wherein
the rotor is further arranged to have successive compartments for
containing bubbles and each successive compartment in the direction
of bubble travel has increased volume so as to accommodate bubble
growth.
4. A device according to claim 3, wherein the rotor has multiple
rotor sections which are coupled together, each successive rotor
section having larger compartments than the preceding rotor
section.
5. A device according to claim 3, wherein the rotor is arranged to
provide a mechanical drive output and/or an electrical power
output.
6. A device according to any preceding claim 3, wherein the rotor
has a plurality of at least partially radial vanes around its
circumference, so forming cells.
7. A device according to claim 6 wherein the rotor comprises a
one-way valve arranged to allow fluid displaced by bubble growth to
flow out of a rotor cell in a direction contrary to the direction
of rotor rotation.
8. A device according to claim 7, wherein the rotor rotational axis
is substantially horizontal and the rotor vanes are curved so as to
trap bubbles in the cells for an extended range of rotor rotational
angles in operation.
9. A device according to claim 8 wherein the rotor axis is inclined
from about 0 degrees to about 45 degrees from horizontal.
10. A device according to claim 8, wherein the housing comprises a
downward duct to deliver condensate near the descending side of the
rotor.
11. A device according to claim 6, wherein the rotor rotational
axis is substantially vertical.
12. A device according to claim 11 wherein the rotor axis is
inclined from about 0 degrees to about 45 degrees from
vertical.
13. A device according to claim 11, wherein the housing
incorporates shrouding around the rotor, providing first and second
ports for bubble release and condensate return, respectively.
14. A device according to claim 13 wherein the ports communicate
with upward and downward ducts for conveying bubbles from the
evaporator to the condenser, and from the condenser to the
evaporator, respectively.
15. A device according to claims 1, wherein the rotor comprises a
screw having a vertical component to its axis of rotation, and the
compartments are formed between screw vanes.
16. A device according to claim 15 wherein the rotor screw
comprises downwardly facing projections for capturing bubbles.
17. A device according to claim 15, wherein the rotor is an
Archimedes screw.
18. A device according to claim 17 wherein the screw rotor has an
axial skew, such that the screw vanes extend from the axis of
rotation in a direction, which when viewed side-on in cross-section
through a diameter of the screw, the direction is inclined from
perpendicular.
19. A device according to claim 17, wherein the screw rotor has a
radial skew, such that the screw vanes extend from the axis of
rotation in a direction which when viewed end-on in cross-section
is inclined from a radius.
20. A device according to claim 17, wherein the screw rotor further
comprises a plurality of blocking members radiating from the axis,
the outer edge of each member extending from a radially outer end
of a first vane of a pair of adjacent vanes, towards a radially
inner position and towards a second vane of the pair, the outer
edge of each blocking member arranged so as to close with a
liquid/gas surface, when the blocking member is oriented maximally
down/up in use, so as to isolate a gas/liquid portion between
adjacent vanes in use.
21. A device according to claim 20 wherein the number of blocking
members per screw rotation is between about 6 and 12.
22. A device according to claim 15, wherein the rotor screw has an
expanding cross-sectional area in the direction of bubble
travel.
23. A device according to claim 22 wherein the expanding
cross-sectional area is achieved by increasing the screw pitch,
increasing the screw outer diameter, decreasing the screw inner
core volume, and/or decreasing the volume of a projection located
between the screw vanes.
24. A device according to claim 15, wherein the rotor screw has a
co-rotating tube around it.
25. A device according to claim 24 wherein a rotating seal is
arranged between the co-rotating tube and the housing of the
screw.
26. A device according to claim 1, wherein the rotor comprises an
internal duct.
27. A device according to claim 1, wherein the rotor has a
bubble-repellant coating.
28. A device according to claim 1, wherein the housing comprises an
upward duct for conveying bubbles from the evaporator to a
condenser.
29. A device according to claim 1, wherein the housing comprises a
downward duct to carry condensate from the condenser to the
evaporator.
30. A device according to claim 29 wherein the downward duct is
positioned and angled so as to direct condensate onto the rotor in
the direction of rotation.
31. A device according to claim 1, wherein the housing is arranged
to shroud at least one rotor cell at a position close to the
evaporator.
32. A heat transfer device having a housing, the housing containing
an evaporator and a rotor, the rotor rotationally drivable by
bubbles produced when a liquid is heated by the evaporator, wherein
the rotor has a plurality of radially arranged cells, the housing
and rotor are arranged to close at least one cell when the cell is
rotated adjacent to the evaporator, and the cell has a one-way
valve arranged to allow fluid displaced by bubble growth to flow
out of the cell in a direction contrary to the direction of rotor
rotation.
33. A heat transfer device having a housing, the housing containing
an evaporator and a rotor, the rotor rotationally drivable by
bubbles produced when a liquid is heated by the evaporator, wherein
the rotor has a plurality of radially arranged cells, the housing
and rotor are arranged to close at least one cell when the cell is
rotated adjacent to the evaporator, and the housing has a duct
positioned adjacent to the evaporator to allow fluid displaced by
bubble growth to flow out of the cell.
34. A device according to claim 33 wherein the duct incorporates a
one-way valve.
35. A heat transfer device according to claim 32 when used for
cooling exhaust gas from an internal combustion engine.
36. A device according to claim 35 wherein multiple devices are
arranged in series, and optionally wherein successive devices are
arranged to operate at lower temperatures and/or exhaust gas
pressures.
37. A device according to claim 35, wherein the device incorporates
a screw rotor having increasing cross-sectional areas between screw
vanes in the direction of gas travel.
38. A heat transfer device according to claim 33, wherein the rotor
is arranged to drive a fan for increasing air flow over a heat
source.
39. A device according to claim 38 wherein the heat source is a
domestic hot water radiator, refrigeration condenser, refrigeration
compressor, LED, photo-voltaic cell, solar collector, heat
exchanger or any suitable source of heat having a higher
temperature than the housing.
40. A heat transfer device according to claim 33, wherein the rotor
provides mechanical power and electrical power using electrical
generating means.
41. A heat transfer device according to claim 33 further comprising
a pressure regulator device, the regulating device comprising a
spring and piston, or a bellows arrangement, and optionally wherein
the spring is bi-metallic.
42. A heat transfer device according to claim 33 further comprising
a vacuum pump.
43. A heat transfer device according to claim 33 further comprising
a rotor shroud arranged direct bubbles into the rotor.
44. An Archimedes screw having vanes formed from a spiral which is
arranged around a rotational axis within a cylinder, the screw
further comprising a plurality of blocking members radiating from
the axis, the outer edge of each member extending from a radially
outer end of a first vane of a pair of adjacent vanes, towards a
radially inner position and towards a second vane of the pair, the
outer edge of each blocking member arranged so as to close with a
liquid/gas surface, when the blocking member is oriented maximally
down/up in use, so as to isolate a gas/liquid portion between
adjacent vanes in use.
45. An Archimedes screw having vanes formed from a spiral which is
arranged around a rotational axis within a cylinder, the screw
further comprising a plurality of blocking members radiating from
the axis, the outer edge of each member extending from a radially
outer end of a first vane of a pair of adjacent vanes, towards a
radially inner position and towards a second vane of the pair.
46. An Archimedes screw according to claim 45 wherein the first and
second vanes are respectively upper and lower in use, such that the
outer edge of the member closes with a gas/liquid surface to
isolate a gas portion between adjacent vanes when the member is
oriented maximally downwards in use.
47. An Archimedes screw according to claim 45 wherein the first and
second vanes being respectively lower and upper in use, such that
the outer edge of the member closes with a gas/liquid surface to
isolate a liquid portion between adjacent vanes when the member is
oriented maximally upwards in use.
48. An Archimedes screw according to claim 45, wherein the number
of blocking members per screw rotation is between about 6 and
12.
49. An Archimedes screw having vanes, wherein the vanes have a
radial skew, such that the vanes extend from the axis of rotation
in a direction which when viewed end-on in cross-section is
inclined from a radius.
50. An Archimedes screw having vanes, wherein the vanes have an
axial skew, such that the vanes extend from the axis of rotation in
a direction, which when viewed side-on in cross-section through a
diameter of the screw, the direction is inclined from
perpendicular.
51. (canceled)
52. A method of controlling the transfer of heat through a heat
transfer device, the heat transfer device having a housing
containing a condenser and an evaporator arranged to boil a liquid
by heat input, the method comprising controlling the rate of
transfer of heat by carrying out one or more of the following
steps: a) controlling the level of heat input to the system; b)
changing the level of vacuum or pressure in the housing; c)
changing the level of heat transfer out of the system; d) changing
the size of the system by closing off parts of the condenser or
removing some liquid from circulation; e) restricting circulation
between parts of the system, in particular by reducing the flow of
liquid from the condenser to the heat input (evaporator) 170; f)
actively introducing hot water from the evaporator to higher parts
of the rotor, thereby increasing vapour generated within the rotor;
and g) reducing or increasing air flow over the condenser.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a device and method for
transferring heat.
BACKGROUND
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
SUMMARY OF THE INVENTION
[0007] The invention is defined in the appended claims.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
DESCRIPTION OF FIGURES
[0014] FIG. 1a shows a heat transfer device according to an
embodiment of the invention.
[0015] FIG. 1b shows a rotor for a heat exchanger, the rotor having
one-way liquid jet valves.
[0016] FIG. 1c shows an improved rotor having curved/spiral
vanes.
[0017] FIG. 1d shows a disc rotor arranged to rotate close to a
side wall of a stator chamber.
[0018] FIG. 2 is a detail view of a rotor and stator housing for a
heat exchanger, incorporating ducts for bubble-propulsion of
liquid.
[0019] FIG. 3 shows a heat transfer device arranged to cool a 3D
semiconductor chip and having two horizontal-axis rotors.
[0020] FIG. 4a shows a heat transfer device having a vertical axis
rotor propelled by liquid jets.
[0021] FIG. 4b is a top-down view of the heat transfer device of
FIG. 4a.
[0022] FIG. 5a shows a heat transfer device employing a vertical
axis screw-type rotor
[0023] FIG. 5b shows a heat transfer device employing an inclined
Archimedes screw as a rotor.
[0024] FIG. 6a shows a vertical-axis rotor having a central
passageway for the passage of fluids.
[0025] FIG. 6b shows a cross-section view through the rotor and
housing of FIG. 6a.
[0026] FIG. 7 shows a screw-type rotor device having a tapered
rotor.
[0027] FIG. 8a shows a pressure regulating bellows-type device for
regulating the pressure within a heat transfer device
[0028] FIG. 8b shows an alternative pressure regulating device
having a spring and piston.
[0029] FIG. 9 shows an improved Archimedes screw having blocking
members so as to allow a reduction in the size of the central core
of the screw.
[0030] FIG. 10 shows a conventional Archimedes screw vane,
[0031] FIG. 11 shows the vanes of an Archimedes screw, the vanes
having a radial skew.
[0032] FIG. 12 shows the vanes of an Archimedes screw, the vanes
having an axial skew.
[0033] FIGS. 13 to 16 show an Archimedes screw having radially
skewed vanes.
[0034] FIGS. 17 to 20 show an Archimedes screw having axially
skewed vanes.
DETAILED DESCRIPTION
[0035] 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.
[0036] As shown in FIG. 1a, a first horizontal rotational axis
embodiment of a heat transfer device 100 that is optimised for
transferring heat from hot spots in electronic devices comprises a
stator chamber 110 containing a closely fitting rotor 120 and a
suitable charge of liquid. A stator duct 130 is provided on the
upwardly moving (in use) side of the rotor and is fluidly coupled
to a heat exchanger 140 which may be remote from the rotor. A
second duct 150 fluidly couples (or connects) the heat exchanger
140 to the downwardly moving (in use) side of the rotor. The rotor
periphery is provided with cells 160 (also termed "compartments" or
"pockets"), which, for example, may be created by radial vanes
165.
[0037] 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.
[0038] Simple horizontal axis versions of the invention (where the
rotor axis is horizontal, or substantially horizontal, as shown in
FIG. 1a) comprise a rotor 120, of any suitable length, with
projecting vanes 165 that create cells 160 which are arranged
around the circumference of the rotor 120. As a cell 160 moves over
a heat input surface (or input heat exchange surface) 170, the vane
165 dislodges (or "scrapes") bubbles from the heat input surface
170 and the cell 160 is gradually filled with bubbles, imparting a
lifting force, due to buoyancy, on the rotor 120. As the rotor
turns further, such that the rotor vane 165 approaches the
horizontal orientation, the leverage of the bubbles exerts an
increasing rotational force on the rotor. Rotor work surplus to the
scraping effect is generated and can be applied to driving a
connected pump, enabling the condenser 140 to be placed below the
evaporator (or heat exchange surface 170), or applied to generating
electrical power or any other device or process requiring rotary
power. Thus a thermosyphon according to embodiments of the
invention can be used in applications which were previously the
preserve of heat pipes where the condenser 140 is below the
evaporator 170. The scraping (or sweeping, or brushing) of bubbles
from the surface of the input heat exchange surface 170 assists
with removal of the bubbles from the heat exchange surface 170,
thereby avoiding the possibility of a blanket of insulating vapour
bubbles forming. Thus, such a 2-phase cooling device can be
operated over a wider range of heat input rates ("extending the
curve"), and undesirable effects such as "kettling" are avoided or
reduced.
[0039] In other embodiments, as shown in FIG. 1c, the rotor 120 can
incorporate curved or spiral vanes 1065, which curve such that the
outer tip 1066 of each vane 1065 is rotationally retarded with
respect to the radially inner part 1067 of the vane 1065. This
arrangement allows bubbles 181 to be trapped within the cells 160
for longer (in terms of rotational procession of the rotor), being
released only after or near the point where the vane outer tip 1066
passes the rotor 120 centreline 185 (on the non-driving side), such
that the bubbles impart greater work to the rotor 120. FIG. 1d
shows an embodiment where a rotor 120 is arranged to rotate close
to a side wall of the stator chamber 110, with cells 160 passing
over a heat exchange surface 170 (also termed an evaporator), and
the stator chamber wall optimally arranged at an angle of about 20
degrees to horizontal.
[0040] 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.
[0041] 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).
[0042] 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").
[0043] 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 FIG. 1a, the liquid
displaced by the pressure of growing bubbles 181 escapes the cell
160 in all directions through the gaps between the rotor 120 and
casing 110. In more highly engineered and tight fitting devices
this liquid can be channelled through non-return valves 180 as
shown in Figure 1b, or flow biasing ducts 230 (as shown in FIG. 2)
as jets of liquid 185, as described above.
[0044] 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 FIG. 1b), or flow biasing ducts (diodic fluid valves); and
second, by providing one or more ducts leading from a lower portion
of a stator wall adjacent to the input heat source 175.
[0045] As shown in FIG. 1b, first, by connecting each cell 160 on a
rotor to the following cell through non return valves 180, or flow
biasing ducts (diodic fluid valves). A cell non-return valve 180
may be created by having a vane that is radially increasingly
compliant in the trailing direction, or comprises a rigid vane of
reduced radial length or width that supports a trailing compliant
member so that the compliant member can only bend backwards. As the
vapour bubbles accumulate in a cell 160 they cause the pressure in
the cell to increase (at least some cells are shrouded, e.g. by the
housing, also termed stator chamber, thus preventing pressure from
leaking away without being directed through the valves). This
pressure forces liquid through the valve 180 or duct into the
following cells, creating a reaction force on the rotor and also
displacing the rotor vis-a-vis the fluid mass surrounding it.
Pressure can be released from an un-shrouded cell, away from the
evaporator, thus avoiding circulation and equalisation of pressure
around the rotor. This process allows the device to consistently
start rotating in the designed direction and with heat input from
directly below the rotor. A device relying on bubble lift alone may
rely on heat input and boiling past the bottom dead centre of the
rotor (i.e. to one side of the rotor centreline 185), or if (as
shown in FIG. 1c) a rotor with curved vanes is employed, heat input
can be provided directly below the rotor.
[0046] 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
FIG. 2) leading from a lower portion of a stator wall adjacent to
the heat input 170 (also referred to as the evaporator, or boiler).
Each duct is below the level of liquid in the cell, for the time
which the cell 160 is open to the duct, and as pressure in the cell
grows, liquid is forced into the duct and may be used for cooling
or other purposes elsewhere. In particular, the pumped flow may be
used in cooling computer chips with 3D architecture, where it is
advantageous to pump fluid through micro channels within the chip
to achieve the required heat removal.
[0047] In further embodiments, as shown in FIG. 2, ducts 230 in the
stator wall, below the liquid level in a cell 160 (i.e. below the
level at which bubbles collect in a cell 160), may be used to
harness the pressure fluctuation as each cell 160 passes the ducts,
so as to pump liquid through micro-channels in the target cooling
area (element to be cooled 175). In embodiments, a plurality of
smaller micro-channels lead away from each duct, such that the duct
acts as a header `tank` for the micro-channels. Because the fluid
is liquid and pumping force fluctuates with the passing of each
cell, the diameter of the duct 230 from the stator wall to the
header entry to micro channels can be relatively large (compared
with the micro-channels). The science of minimising pressure drop
from the header to the micro-channels, and even (equal) flow
through each micro-channel is non-trivial, and yet is important if
even cooling of the device to be cooled is to be achieved. If
header losses (pressure loss in the transition from the duct to the
micro-channels) prove relatively high, and/or uneven distribution
of flow between micro-channels proves problematic, multiple micro
ducts may instead be provided (in some embodiments), so as to
directly connect the cell 160 to the ducts and/or micro-channels
within the element to be cooled (e.g. a chip) 175.
[0048] To maximise the efficiency of the pressure pumping feature,
in an embodiment a layout (as shown in FIG. 3, which may be used,
for example, for 3D chip cooling) has a vertically oriented
multi-layer chip (or any other layered heat source which is to be
cooled) 310 sandwiched between the heat input areas of two heat
transfer devices 100 similar to that illustrated in FIG. 1a.
Devices of this type take heat from the outer layers of the chip,
but also pump liquid via a duct 330 under the chip 310 and up
vertical micro channels 340 within the chip 310 to cool the inner
layers. Such micro channels 340 may be of varying section and
extended with a channel extension 350 to vent vapour and liquid at
or near the top of the condensing heat exchanger 140. This venting
of vapour can provide an additional pumping force as any bubbles
expand. If heat is inputted from the chip 310 to one vertically
divided side of each of the heat transfer devices 100 (e.g. to the
right of the centreline 185 shown in FIG. 1a), it is advantageous
to close the other vertically divided side of each of the cells 160
with a shroud or shrouds, and thereby reduce leakage of pressure
that might otherwise be usefully directed to driving liquid through
the micro channels 340. Other methods of utilising the pressure
fluctuation include driving diaphragm pumps via the duct 230.
[0049] 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 FIG. 4. This means
that the axis of rotation has at least a vertical component, as
well as possibly also a horizontal component. In such embodiments,
the top of the rotor 410 is closed such that cells 160 are shrouded
on their upper side, and after rotating through a heat input zone
170 each cell 160 releases its bubbles to an upward duct 430
through a first port (or cut-out) 440 in the stator wall.
Similarly, a second port (or cut-out) 450 in the stator wall
connects the cell 160 to a liquid return downward duct 460. This
type of embodiment can also, for example, be applied to a conveyor
type rotor (not shown) that rotates round two or more pulleys and
can move heat horizontally.
[0050] 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.
[0051] Vertical axis devices as shown in FIG. 4 have partially
shrouded cells and the rotor is rotated by pressure rise causing
jetting through valves into the following cell. Other vertical axis
embodiments of the invention as shown in FIG. 5a (where the rotor
axis is vertical, or substantially vertical) have a spiral rising
duct and the rotor is rotated by the pressure of bubbles rising up
the spiral.
[0052] In the embodiment shown in FIG. 5a, there is provided a
substantially vertical stator chamber 510 containing a close
fitting cylindrical rotor 520 or conical rotor and filled with a
suitable fluid. Such embodiments are advantageous for use in
situations where heat input is effected via relatively large
surface areas, since heat can be inputted around the whole
circumference of the rotor 520. The rotor 520 has one or more
spiral strips (also termed "projections" 530 projecting from the
rotor surface and extending from the bottom of the rotor to the top
of the rotor. The strips 530 may be provided with turned down outer
edges or seals 540 to retain bubbles under the spiral strip 530.
The strips 530 may also be provided with one or more downwardly
projecting fins 550 to catch and slow bubbles, thereby increasing
the torque passed to the rotor 520, and also helping to prevent
formation of long sausage-like bubbles through which vapour may
rise too rapidly. A conical rotor can be arranged such that it has
a larger diameter towards the top of the rotor, so 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. Thus, additional work can be extracted from the bubble
expansion. Alternatively, instead of a conical screw, a parallel
sided screw can be used but with a member of decreasing
cross-section (in the direction of bubble rise) can be inserted
into the space between the vanes, thus effectively providing
increasing volume between the vanes in the direction of bubble
rise, so as to provide for bubble expansion. Alternatively,
multiple screws of increasing sizes can be stacked to allow for
expansion, and these can be mechanically coupled.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] Another embodiment shown in FIG. 5b employs an Archimedes
(Archimedean) screw as the rotor 520, inclined at an approximately
45 degree angle (although other angles between 0 degrees and 90
degrees could be used). This embodiment offers improved resistance
to bubbles travelling unimpeded up the spiral, since in use the
bubbles become trapped in compartments between the rotor vanes 530.
The screw housing or "cylinder" 505 extends downwards so as to
cover the area close to the heat source and thereby capture the
majority of the vapour bubbles. The bubbles impart a force on the
rotor, causing the rotor to turn, and allowing the bubbles to rise.
The vapour bubbles then travel to the condenser 140, where they
give up heat and condense. The condensate returns to the reservoir
510 via return pipe 560, the system being sealed such that the
screw remains filled with liquid apart from the vapour bubbles
rising through it.
[0059] 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.
[0060] 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.
[0061] In a yet further embodiment shown in FIG. 6, employing a
vertical rotation axis rotor with a spiral, there is provided a
substantially vertical stator housing 610 comprising an outer
stator cylinder 615 and an inner stator cylinder 616 with end
closures to define a hollow cylindrical chamber 620 in which a
close fitting rotor 630 is rotate-ably mounted. The inner cylinder
616 of the stator housing 610 defines an inner cylindrical duct 640
for passage of a fluid to be cooled. Such an embodiment is thus
optimised for cooling a flow of fluid internal to the rotor. The
rotor 630 is provided with one or more spiral projections 650 to
define spiral passages up the rotor 630. A duct 660 is provided to
join, via a heat exchanger 670 (which can, for example, be
external), a fluid inlet 680 at the lower end of the stator housing
610 with a fluid outlet 690 at the top end.
[0062] 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.
[0063] Turning to FIG. 7, an embodiment adapted for operation in
low gravity condition comprises a stator chamber 710 containing a
close fitting rotor 720 and a quantity of fluid. The rotor
comprises a first axial end section 730 with radially separated
cells 740 at the periphery (around the circumference of the rotor
first axial end 730). The cells 740 have one-way valves or flow
biasing openings (as described above with reference to FIGS. 1 and
2) connecting each cell 740 to the next. A second axial end 750 of
the rotor 720 (which, in embodiments, optionally has a tapering
section) terminates in a heat exchange volume of the stator chamber
710, the heat exchange volume and stator chamber forming together a
heat exchanger 760. The rotor 720 has an interior duct 770
connecting the second axial end 750 to the first axial end 730. The
stator wall at the first axial end 730 has a cut-out or passageway
780, at least partially opposite the interior of one of the cells
740, to provide a connecting duct 790 between: the interior duct
770 of the rotor; and the cells 740 of the rotor (as rotation of
the rotor 720 brings the cells 740 into position adjacent to the
cut-out 780, either one or two cells will be fluidly coupled by the
cut-out 780 to the rotor interior duct 770, depending on the exact
orientation of the rotor 720). The second rotor section 750 has one
or more spiral projections 705 on its surface that conform closely
to the inner surface of the stator chamber 710. The projections 705
extend beyond the second axial end 750 to become scrapers (for
scraping bubbles off the inner surface of the stator housing 710)
in the heat exchanger 760. On the rotor 720, the spiral projections
705 define spiral ducts 7100 that lead from the stator heat
exchanger 760 volume to a stator wall projection 7110 that
sealingly separates the first section 730 of the rotor from the
second section 750 of the rotor. The projection 7110 is provided
with an opening 7120 further round in the direction of rotation
than the cut-out 780 so that it will connect the spiral ducts 7100
with a cell 740 after the cell 740 has ceased to be in
communication with the cut-out 780.
[0064] 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.
[0065] 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.
[0066] 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.
[0067] 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.
[0068] 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 FIG. 8a.
[0069] 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.
[0070] 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.
[0071] Another pressure regulating device 805, attachable to the
stator chamber 110, for regulating the internal pressure inside the
stator chamber is shown in FIG. 8b. The regulator device 805 has a
body 809, within which is a spring 808 and piston 811, the spring
acting against the piston 811. A diaphragm seals the piston against
the body 809 (although a sliding seal could be used instead of a
diaphragm). In use, the spring acts against the piston to increase
the space inside the body, thereby lowering the pressure. The
spring is optionally bi-metallic so as to provide a
temperature-dependent pressure. An opening 812 connects the
interior of the body to the interior of the stator chamber 110,
thereby communicating lowered pressure with the inside of the
stator chamber 110. The spring pressure and body volume can be
adjusted so as to provide the appropriate pressure inside the
stator chamber 110.
[0072] 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).
[0073] 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.
[0074] 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.
[0075] The following two factors have been considered for
improvements to conventional Archimedes screws:
[0076] 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.
[0077] 2) increasing the lifting force of the screw by increasing
the ratio of gas to liquid within the Archimedes screw.
[0078] 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.
[0079] 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.
[0080] In the light of the above analysis, an improved Archimedes
screw has been produced, as follows:
[0081] 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 FIG. 9. Extending from the central
core 920 there are provided blocking members 930, regularly spaced
both radially around the core 920 and axially along the length of
the core 920. These blocking members extend between each pair of
vanes, reaching axially downwards towards the lower end of the
screw 900 from the trailing side of each upper vane 910. Each
blocking member 930 extends substantially radially outwards from
the core 920, with the outer edge 940 of each blocking member 930
oriented at an angle that results in the outer edge 940 of that
member 930 being substantially horizontal when that member 930
points radially downwards (the outer edge 940 of each blocking
member 930 extends from near the tip 955 of the upper 950 of the
two flights of vanes 950,960 between which it lies, towards the
core 920 and towards a radially inward part 965 of the lower vane
960). In other words, the outer edge of each blocking member
extends from a radially outer end 955 of the upper vane 950 of a
pair of adjacent vanes, towards a radially inner position and
towards the lower vane 960 of the pair, such that the radially
outer edge 940 of the blocking member closes with a liquid surface
941 to isolate a gas portion 980 between adjacent vanes (when the
blocking member is oriented maximally downwards in use). In use,
the screw is inclined typically at 30 to 60 degrees from the
horizontal.
[0082] 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.
[0083] 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).
[0084] 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.
[0085] 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 FIG. 9,
a seal (which is, for example a circular washer) 907 is arranged at
the top of the cylinder between the co-rotating tube 906 and
cylinder 905, so as to trap gas between the co-rotating tube 906
and the cylinder 905, and thereby to reduce drag. For example, the
seal 907 in this embodiment is fixed to the screw cylinder 905 and
the inner portion of the seal 907 rests on top of the co-rotating
tube 906. At the start of operation, the interior of the screw 900
is flooded with liquid. Vapour from an evaporator 170 is conducted
into the device via duct 911, or by virtue of the cylinder 905
extending over the evaporator 170. In the present embodiment, the
vapour is not ducted into the co-rotating tube, but advantageously
rises to the upper wall of the cylinder 905 and up the gap between
the container 905 and co-rotating tube 906. The vapour is prevented
from escaping at the top of the cylinder 905 by the seal 907, and
as the vapour accumulates it pushes the liquid surface down the
cylinder 905 until the vapour begins to escape into the screw
(inside the co-rotating tube 906), which it fills and causes the
screw to rotate, as described elsewhere. Because the pressure of
liquid on the top side of the seal 907 is similar to the vapour
pressure underneath, the seal can be preloaded to form an effective
rotating seal that is lubricated by the operating fluid. As the
liquid at the top of the device is not in communication with the
liquid at the bottom (they are isolated by the seal), a by-pass
duct 560 is provided to connect the two ends of the device
(although in embodiments with a co-rotating tube 906 and without
seal 907, liquid can return to the bottom of the screw via the
clearance between the co-rotating tube 906 and the cylinder 905,
therefore no separate duct 560 is required in those embodiments).
The seal 907 thus reduces the drag, the heat loss through the outer
container and the charge of fluid required in the device, which
reduces warm-up time. Clearly the seal 907 can be implemented in a
variety of ways and can be fixed to the cylinder 905 or the top of
the co-rotating tube 906, or even not be fixed to either. The
relative rotational speed of the seal and sealing surface contacted
by the seal can be reduced by narrowing the neck of the co-rotating
tube 906 in the vicinity of the seal 907, thus reducing frictional
losses.
[0086] 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.times. 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.
[0087] A further improvement to Archimedes screws will now be
described.
[0088] 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 (FIG. 10). The maximum volumetric flow rate
and the associated geometric parameters of this type of screw have
previously been characterised ("The turn of the screw--optimal
design of an Archimedes screw", Rorres, 2000).
[0089] 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.
[0090] It is clear that when the axial and radial skew angles are
both zero, the conventional screw geometry is obtained as shown in
FIG. 10. Using a non-zero value for one or both of the axial and
radial skew angles can increase the volumetric flow rate of the
screw compared to the maximum value possible with a conventional
screw of the same outer radius. This is achieved because of an
increase in the volume of fluid held between each pair of screw
vanes. Although the total volume (i.e. the volume of liquid plus
the volume of gas) between each pair of screw vanes is
substantially unchanged, the volume of liquid, or fluid, able to be
held in each compartment, between the screw casing and the
"tip-over" level (the "tip-over" level is the fill level in a
compartment at which fluid begins to overflow the central section,
or core 1340, of the screw--from one compartment into the next), is
increased or reduced (the ratio of gas to liquid is changed).
Volumetric efficiency and/or torque are thereby improved. These
novel screw geometries therefore represent an improvement over the
conventional design.
[0091] FIG. 11 shows an embodiment, having what is referred to as
`radial skew`, where the vanes 1320 are radially inclined, i.e. a
cross-sectional view of the vanes, taken across the end of the
cylinder, shows that each vane section 1330 is inclined with
respect to a radius from the axis of rotation 1310. Experiment has
shown that improved volumetric efficiency is achieved when the vane
sections 1330 are aligned at an angle (shown in the cross-section)
tending towards a tangent to the circumference of the core
1340.
[0092] FIG. 12 shows an embodiment, having what is referred to as
`axial skew`, where the vanes 1320 are axially inclined (i.e. a
cross-sectional view of the vanes, taken along the length of the
cylinder and across a diameter of the cylinder, would show each
vane section inclined with respect to a normal to the cylinder
surface). The result of this vane orientation, when viewed as a
cross section across the end of the cylinder, as shown in FIG. 14,
is that the vane sections 1330 appear curved (viewed end-on to the
cylinder).
[0093] 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.
[0094] FIGS. 13 to 16 (radially skewed screw 1500) and 17 to 20
(axially skewed screw 1700) show further views of the two improved
Archimedes screw designs, including end-on and side-on views and
two 3D views for each. In both cases (radial skew and axial skew)
the illustrated ratio of inner radius to outer radius is 0.5, the
screw pitch equals the outer radius, and a skew angle (radial or
axial respectively) of 45 degrees has been used to illustrate the
general case, although the optimum geometry is almost certainly
different and will depend on the application (including depending
on the angle of inclination of the screw).
[0095] Further, one or more heat exchange devices can be nested
inside another heat exchange device.
[0096] 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:
[0097] a) controlling the level of heat input to the system. This
gradually affects the whole system.
[0098] 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.
[0099] c) changing the level of heat transfer out of the system, by
changing the level of insulation at various points around the
system.
[0100] d) by changing the size of the system, for example closing
off parts of the condenser 140 or removing some liquid from
circulation.
[0101] 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.
[0102] 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.
[0103] g) by reducing or increasing air flow over the condenser
140.
[0104] 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.
[0105] The features of the various aspects and embodiments can
optionally be combined in any combination.
[0106] 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).
[0107] 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.
[0108] 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.
[0109] 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.
[0110] 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.
[0111] 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.
[0112] 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.
[0113] 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.
[0114] 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.
[0115] 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.
[0116] 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.
[0117] 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.
[0118] Some further examples of domestic applications follow:
[0119] 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.
[0120] 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.
[0121] 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.
[0122] 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.
[0123] 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.
[0124] Some further examples of small industrial applications
follow:
[0125] 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.
[0126] b) Buoyancy engines can recover exhaust energy from
distributed power generation units such as Diesel generators.
[0127] c) Buoyancy engines can recover energy from warm exhaust
from air-conditioned buildings systems.
[0128] d) Buoyancy engines can recover energy from compressors and
refrigerators.
[0129] Some further examples of large Industrial applications
follow:
[0130] a) Vertical versions of buoyancy engines can be built into
industrial chimneys so as to recover power, in mechanical and/or
electrical form.
[0131] 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.
[0132] 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.
[0133] 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.degree. 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.degree. C., to the BEs to stir and distribute sewage.
[0134] 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.
* * * * *