AU5393301A - Solar thermal power production - Google Patents

Description

P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT S. j, *S Invention Title: "SOLAR THERMAL POWER PRODUCTION" *r S S. S

*SS*S

The following statement is a full description of this invention, including the best method of performing it known to us: I b 2

TITLE

"SOLAR THERMAL POWER PRODUCTION" FIELD OF THE INVENTION THIS INVENTION relates to the field of power production and, in particular, solar thermal power production.

BACKGROUND ART Many attempts have been made to harness the sun's energy for producing or enhancing power production and, in particular, for contributing to the production of electricity. The drive to develop an efficient method of solar power harvesting has been potentiated by growing environmental concerns in relation to traditional coal-fired generation plants.

Several methods are currently available for generating electricity by concentrating radiation from the sun. A system of concentrating parabolic 15 troughs is known as the LUZ system. This system is based on specially shaped glass reflectors which are used to focus sunlight onto evacuated ":glass tubes and onto a selective surface of an oil-filled tube. The LUZ system uses low pressure oil which is returned to a central area for heat ooooo S- transfer from the oil to generation of super-heated steam. This steam is then passed through a turbine of conventional Rankine Cycle design.

One of the problems in this system is that oil is necessary because water changes state at 100'C and has a massive increase in volume and a consequent reduction in heat transfer capability. If water is heated in a horizontal tube, then the process is very difficult, if not impossible to 3 control. The use of oil allows temperatures up to 4000C to be achieved in this circuit. The use of oil increases the complexity of the process as well as presenting a fire hazard and risk of environmental contamination.

The LUZ system has the focal point of the sunlight mounted directly onto the parabolic reflector. As the sun moves across the sky, both daily and according to the season, a flexible joint is required to bring the oil lines attached to the ground to the oil passageway in front of the parabolic trough. This joint is both expensive and also leads to a loss of heat due to poor insulation around a moving part.

An alternative system is known as the Power Tower system which uses a single large tower to collect heat from various flat plate reflectors located around the tower. Computer control of the plates focuses the sunlight to a single point at the top of the tower where heat is absorbed by oil which is subsequently returned to ground level. The heat is then transferred to water to produce steam and thereby drive a conventional turbine.

This type of system has problems in being at substantial heights off the ground resulting in very expensive structures. Accuracy in reflecting S, sunlight to a single point is also a difficult technological task. Once again, oil must be used in the collector which both increases costs and presents S* some element of environmental risk in the event of a leak from the operative pipeline.

A system called a Linear Freznel Mirror System is described in International Application No PCT/AU97/00864 to The University of am 4 Sydney. This system relies on mirrors positioned to avoid shadow overlap between adjacent structures while being aimed towards one of two collecting targets located at either side of an array of the mirrors. The system contemplates the use of horizontal collectors, some of which will be up to 15m above the ground. This system is still currently under development.

A system based on floating flat freznel mirrors has also been developed in which the problem of focussing is addressed by very slow rotation of a large collector relative to the sun. This system, however, has the problem of requiring a large volume and surface area of water and still requires some sort of moving joint to connect steam back to an engine or turbine to be driven.

The Australian National University has conducted research into solar dish technology providing a system capable of generating steam at oo very high temperatures of up to 6000C to 700 0 C. This system, however, •requires a dish of very precise structural features. It is necessary to have ".very pure water in the boiler tubes and this requirement renders the system unsuitable for most commercial water available. In addition, the dish has movements in two planes and thus requires flexible and S 20 leakproof joints on two axis. Problems then arise in controlling the steam temperature effectively in a once through system and also in the requirement for moving joints. While conventional "once through" boilers •have control of the heat in their algorithm, this capacity is not available because the sun is uncontrollable as a fuel input. It would be of advantage to provide a once through boiler system for harnessing the sun's energy which solves or at least decreases one or more of the above outlined problems.

SUMMARY OF THE INVENTION In one aspect, although it need not be the only or indeed the broadest aspect, the invention lies in a solar heating unit for producing steam comprising: a thermal transfer conduit having a fluid outlet elevated relative to a fluid inlet; a fluid vessel normally having a liquid in a lower region of said fluid vessel and a gas space above a level of said liquid in said vessel; a fluid return conduit communicating at one end with said lower region of said vessel and at an opposite end to said fluid inlet associated with said thermal transfer conduit, said fluid outlet of said thermal transfer 15 conduit being in fluid communication with said fluid vessel intermediate -said level of liquid and a base of said vessel; and, a gas outlet associated with said gas space in an upper region of said fluid vessel, said apparatus further including a solar concentrator °oo .,*means to concentrate, in use, solar energy onto said thermal transfer 20 conduit whereby a liquid contained in said thermal transfer conduit is •heated at least to a boiling point of said liquid whereby gas bubbles are formed therein to induce a flow of fluid in said thermal transfer conduit from said fluid inlet towards said fluid outlet by thermal and/or gaseous convection.

Suitably the liquid employed in the unit comprises water or an aqueous solution.

If required the water may contain dissolved inorganic or organic materials to selectively alter the boiling point of the liquid.

At least portion of the thermal transfer conduit may be inclined upwardly towards said fluid outlet.

If required, the thermal transfer conduit may be inclined upwardly from said fluid inlet towards said fluid outlet.

Suitably the transfer conduit is inclined at an angle of 50 to 450 relative to a horizontal datum.

The solar concentrator means may comprise one or more concavely dished reflectors and/or one or more convexly curved lenses.

Preferably the solar reflector means comprises trough one or more like reflectors.

15 If required tracking means may be employed to cause relative movement between said reflector means and said thermal transfer ~conduit as the earth rotates to maximise collection of solar energy.

Preferably said tracking means includes means to selectively move *the solar reflector means relative to said thermal transfer conduit.

Most preferably said tracking means comprises means to ~selectively rotate said reflector means about a rotational axis substantially coincident with a longitudinal axis of said thermal transfer conduit.

The solar heating unit may further comprise a convection shield for use with the thermal transfer conduit. If required the thermal transfer conduit may comprise one or more boiler pipes. The convection shield may be in the form of a shaped cavity locatable in operative position with the boiler pipe so that heat loss from the boiler pipe is reduced. The convection shield may be an inverted U in cross-section or any other suitable shape. The convection shield may include an inner layer and an outer layer. Preferably, the convection shield has insulation between the inner layer and outer layer. The convection shield may include a transparent plate to enclose an air space around the boiler pipe.

Preferably, the inner layer is reflective.

In a further aspect, the invention may reside in a steam generating array for generating saturated steam comprising a plurality of solar heating units as described above, each solar heating unit connected in series or parallel with at least one other solar heating unit. The steam generating array may further include a boiler drum in fluid connection with the solar array. The steam generating array may heat water to o approximately 300 0

C.

'°According to another aspect of the invention, there is provided a method of converting solar energy to a usable form of thermal energy, 0 said method comprising the steps of:concentrating solar energy onto a thermal transfer conduit containing water or an aqueous solution, said thermal transfer conduit having a fluid outlet elevated relative to a fluid inlet, said thermal transfer .conduit being fluidically coupled to a steam stripping vessel having a volume of liquid normally occupying a lower region thereof and, in use, a 8 gas space above a level of said liquid in said vessel, said fluid inlet being coupled to a fluid conduit in fluid communication with a lower region of said vessel and said fluid outlet being coupled fluidically to said vessel intermediate said liquid level and said lower region of said vessel whereby in use, sufficient thermal energy is absorbed by said water or aqueous solution to raise the water or aqueous solution to raise the water or aqueous solution to boiling point whereupon steam bubbles are formed and the combined effects of thermal and gaseous convection causes a cyclic flow of fluid from a lower region of said vessel via said thermal transfer conduit to an intermediate region of said vessel and saturated steam is collected from said gas space as a working fluid.

Suitably said thermal transfer conduit is oriented in a north/south axis.

If required, the thermal transfer conduit, when utilised in the southern hemisphere, is oriented with a southerly end thereof elevated relative to a northerly end thereof.

Alternatively, the thermal transfer conduit, when utilised in the northern hemisphere, is oriented with a northerly end thereof elevated relative to a southern end thereof.

BRIEF DESCRIPTION OF THE DRAWINGS FIG 1 shows a schematic side elevation of a solar collection unit for producing steam according to the invention.

FIG. 2 is a side schematic view of a solar trough of the present invention.

9 FIG. 3 is a front schematic view of a further embodiment of a solar trough of the invention.

FIG. 4 is a schematic view of an array of solar troughs located on a slope.

FIG. 5 is a top view of the array of FIG. 3.

FIG. 6 is a sectional view of a convection shield.

DETAILED DESCRIPTION OF THE DRAWINGS In this specification, like numbers indicate the same or similar features.

In FIG 1 the solar collection unit comprises a fluid vessel 1 having a return fluid conduit 2 communicating with a lower region la of vessel 1.

Conduit 2 is in fluid communication with an inlet end 3 of thermal transfer conduit 4 and an outlet end 4a of conduit 4 is in communication with vessel 1 below the level 5 of liquid contained therein.

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A gas space 6 is located in the vessel 1 above the liquid level and is selectively drawn off via conduit 7 for use as a working fluid in an ":energy extraction device such as a steam turbine (not shown) or the like.

The inclined thermal transfer conduit or boiler pipe 4 has located

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o therebeneath a solar energy collector/concentrator 8 such as a curved mirror to focus on a selected region 9 of thermal transfer conduit or boiler pipe 4.

0 In use, water (or an aqueous solution) in region 9 is heated by concentrated solar energy to the boiling point of the liquid in boiler pipe 4.

As the water boils, small gas bubbles are formed as a consequence of phase change whereby the density of the gas/liquid mixture in boiler pipe 4 above region 9 is less than that below region 9, due in part to the concentration of small gas bubbles in the fluid. Typically, the system operates with a boiler pipe pressure range of from 2000kPa to 10,000 kPa.

The lesser density fluid located in the inclined boiler pipe 4 above region 9 is caused to flow towards vessel 1 by a thermal/gaseous convection current which is replenished by cooler liquid via conduit 2. As the upper end or outlet of boiler pipe 4 is located below the level 5 of liquid in vessel 1, saturated steam is effectively stripped from the liquid in vessel 1 at the gas/liquid interface 5. Makeup water is introduced via conduit 10 to maintain fluid level 5 within predetermined limits.

If readily will be apparent to a skilled addressee that the unit according to the invention represents a simple, relatively inexpensive and maintenance free system for generating a working fluid from solar energy when compared with prior art units.

~Various aspects of preferred embodiments are described with reference to FIGS 2 to 6 and it should be understood that many forms of o:.ooi solar concentrator/collector may be employed with the invention to greater or lesser advantage.

Referring to FIG. 2, there is shown a single solar heating unit in the S• form of a concave parabolic trough 11 formed by a curved mirror 12 with a reflective mirror surface 13. The solar trough 11 has a support frame 14 and support trestles 15 located on either side of the support frame 14 and 11 curved mirror 12. Only one support trestle 15 is apparent in this view. A boiler pipe 16 is situated at the focal point of the curved mirror 12. The curved mirror 12 and its support frame 14 are mounted to the support trestles 15 so that they can rotate about an axis coincident with the longitudinally extending focus region of mirror 12. The reflective mirror surface 13, in cross-section, forms a parabolic curve which is focussed on the boiler pipe 16. The reflective mirror surface 13 may be formed from glass applied over a steel base and may be flexible. The reflective mirror surface 13 may in turn be formed by a mosaic of smaller individual mirrors formed of glass and a metallic backing which, in combination, form a single reflector for a heating unit. Boiler pipe 16 may therefore be kept stationary while the mirror rotates around it maintaining focussed sunlight on the boiler pipe 16. Suitably frame 14 includes a counterweight 14a.

FIG. 3 shows an embodiment of a second solar trough 17 which is 15 identical to that of FIG. 2 other than for the fact that a first support trestle •18 supports the curved mirror 12 and support frame 14 at a point higher :•':above the ground surface 20 than the point of support of a second support trestle 19 so that the reflective surface is inclined to the surrounding ground. This results in the curved mirror 12 being orientated 20 at an angle to the ground surface 20. The advantage of this angulation means the curved mirror 12 may be oriented to more effectively track the S•azimuthal path of the sun. That is, in the southern hemisphere, the curved mirror 12 may be oriented towards the North so that more incident sunlight is gathered as the sun's position in the sky moves to the North 12 during winter. Conversely, in the northern hemisphere, the curved mirror 12 may be oriented to the South. It is clear to a skilled addressee that a number of variations on this arrangement may be practicable. For example, the first unit may be sited in a trench so that the height of second and successive units may be minimised if the boiler pipe is run in a single inclined span through successive units.

The boiler pipe 16 is located at the focal point of the curved mirror 12 and runs between the two points of support 21, 22 of the curved mirror 12 and support frame 14 on the first support trestle 18 and second support trestle 19, respectively. The advantage of this location includes the fact that the boiler pipe 16 is held stationary while the curved mirror 12 and support frame 14 rotate around the points of support 21, 22. In the absence of movement of the boiler pipe 16, there is no requirement for flexible joints with the inherent problems and weaknesses associated with 15 such couplings. Rather, the boiler pipe 16 may be permanently statically *connected to both a delivery pipe 23 and discharge pipe 24.

*O o 9 As shown on the present single heating unit, the discharge pipe 24 is connected to a boiler drum 25 which is supported on access platform 26 26.

The ability to have the boiler tube stationary allows the system to :'operate at any desirable pressure with a minimised risk of leakage at moving joints and consequently, substantially reduced maintenance.

0.S*o The reflective mirror surface 13 may be conveniently formed from a laminated glass attached to steel to reflect the sunlight. This system 13 allows use of conventional steel backing and a simple structure to be built to hold the laminate. In addition, the laminate also permits a mosaic to be used to build a single large trough from multiple sections of laminate. The parabolic trough may include a patchwork of mirrors.

The applicant has also developed a system of control with active feedback to keep the sun's rays concentrated on the boiler tube for maximum performance.

Rotation of the curved mirror 12 and support frame 14 is for the purpose of tracking the sun during daylight. Rotation of the mirror may be by any suitable mechanical means, including manual operation of a winding device or any automated system known to those skilled in the art, including a motor source such as an electric motor.

The focal length of the curved mirror 12 may conveniently be in the order of 3m. This results in the boiler pipe 16 being similarly located at 15 slightly over 3m above the ground allowing easy access for construction and maintenance purposes. Individual mirror mosaics may be in the 9 order of 6m wide by 6m long giving a single trough a reflective surface of approximately 36m 2 *9999* o* The applicant has found that one or more heating units may be sited on a slope to increase exposure of the reflective surface to sunlight.

~In this situation, the boiler tube is located substantially on a North/South line and the solar trough is tilted in the same direction to increase the direct incidence of sunlight. As the slope tilts the solar trough, it is not necessary to have a disparity of support heights between trestles on the

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14 same heating unit. They may be paired. However, if desired, uneven support heights may be used to enhance a natural slope to provide a preferred angle of tilt.

In FIG. 4, solar troughs 27 are shown located on a slope 28 which is a natural slope and which, preferably, is a northern slope in the southern hemisphere or a southern slope in the northern hemisphere.

The preferred angle of the slope is in the range of 50 to 400, although other angles my also be utilised. In this view, the lower tier 29 of solar troughs forms the boiler section of the device. Water is delivered to the boiler system by feedpipe 30 at the bottom 31 of the boiler section 29.

Water is fed into the first solar trough 32 and then the second solar trough 33 and so on through successive solar troughs until returned to a steam separation vessel 34 as a mixture of liquid and steam. If required, the steam is then delivered to a superheater 35 for passage through further 0 solar troughs 36 for superheating and from there to a turbine (not shown).

O* 0o Otherwise the steam could be extracted from vessel 34 as saturated :.:':steam.

There are considerable advantages in this arrangement in that the 000000 ~natural convection flow of water and steam allows for passage through the boiler system as opposed to a system which is horizontal and thereby ~requires pumping of positive control of water and steam flow to allow for its functioning. The mere fact that a single solar trough may be located on ~an angle to the ground because of variations in the height of support trestles does not practically allow for successive solar troughs to be situated higher than each preceding device. Location on a slope, either natural or artificial, provides for this outcome and thereby provides a mechanism for producing superheated steam.

In FIG. 5 the boiler section 29 is formed by rows of series connected troughs 27. A feed pump 37 provides water to the bottom of each column via manifold 38. Water is provided to the lowest row 39 of solar troughs and then passes through each successive row (as exemplified at 40) being heated to greater temperatures with passage through each solar trough until substantially converted to steam. The boiler section columns are each connected to a steam 41 which, in turn, provides steam to superheater 35 wherein the steam may be superheated up to temperatures in the range of 450 The superheated steam is then delivered via pipeline 42 to a turbine 43 which, in turn, generates electricity in a conventional method. Exhaust steam is directed to a cooling tower 44 for condensation into reservoir 45 and subsequent

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recycling by feed pump 37.

The solar trough may include a convection shield for location in operative relationship to the boiler pipe to reduce heat loss due to loss to 0 Sothe air and, in particular, due to wind loss. A suitable shield is shown in FIG. 6. A boiler pipe 46 is shown in cross sectional view. Convection shield 47 includes an outer layer 48 and inner layer 49. The inner layer 49 is reflective to direct any incident sunlight onto the boiler pipe 46. The inner layer 49 may be formed from stainless steel and its natural finish may be adequate for reflection purposes. Insulating material 50 such as 1) 16 synthetic mineral fibre is located between the outer layer 48 and inner layer 49 to minimise heat loss and increase the efficiency of protection.

An enclosed space 51 is created by application of a transparent plate 52 held in place by retaining bolts 53. The convection shield 47 is sited so that transparent plate 52 is directed towards the reflective trough of a heating unit. Reflected sunlight may penetrate transparent plate 52 and heat boiler pipe 46. Convection losses will be greatly reduced. As the enclosed space heats up, loss to the general environment will be impeded by the convection shield 47.

Throughout the specification, the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features. Various changes and modifications may be made to the embodiments described and illustrated without departing from the present invention.

S* 0.

Claims (15)

1. 2. A solar heating unit as claimed in claim 1, wherein the liquid 0 employed in the unit comprises water or an aqueous solution.
2. 3. A solar heating unit as claimed in claim 2 wherein the water contains dissolved inorganic or organic materials to selectively alter he boiling point of the liquid. 18
3. 4. A solar heating unit as claimed in any preceding claim wherein at least portion of the thermal transfer conduit is inclined upwardly towards said fluid outlet. A solar heating unit as claimed in any preceding claim wherein the thermal transfer conduit is inclined upwardly from said fluid inlet towards said fluid outlet.
4. 6. A solar heating unit as claimed in any preceding claim wherein the transfer conduit is inclined at an angle of 150 to 400 relative to a horizontal datum.
5. 7. A solar heating unit as claimed in any preceding claim wherein the solar concentrator means comprises one or more concavely dished reflectors and/or one or more convexly curved lenses.
6. 8. A solar heating unit as claimed in claim 7 wherein the solar reflector means comprises trough one or more like reflectors. 15 9. A solar heating unit as claimed in any preceding claim wherein tracking means is employed to cause relative movement between said concentrator means and said thermal transfer conduit as the earth rotates to maximise collection of solar energy. oo.ooi A solar heating unit as claimed in claim 9 wherein said tracking means includes means to selectively move the solar concentration means relative to said thermal transfer conduit.
7. 11. A solar heating unit as claimed in claim 10 wherein said tracking means comprises means to selectively rotate said concentrator means about a rotational axis substantially coincident with a longitudinal axis of 19 said thermal transfer conduit.
8. 12. A solar heating unit as claimed in any preceding claim including a convection shield for use with the thermal transfer conduit.
9. 13. A solar heating unit as claimed in any preceding claim wherein the thermal transfer conduit comprises one or more boiler pipes.
10. 14. A steam generating array for solar generated steam comprising a plurality of solar heating units as claimed in any one of claims 1 to 13 wherein each solar heating unit is connected in series or parallel with at least one other solar heating unit.
11. 15. A method of converting solar energy to a usable form of thermal energy, said method comprising the steps of:- concentrating solar energy onto a thermal transfer conduit containing water or an aqueous solution, said thermal transfer conduit having a fluid outlet elevated relative to a fluid inlet, said thermal transfer conduit being fluidically coupled to a steam stripping apparatus comprising a fluid vessel having a volume of liquid normally occupying a lower region thereof and, in use, a gas space above a level of said liquid in said vessel, said fluid inlet being coupled to a fluid conduit in fluid communication with a lower region of said vessel and said fluid outlet *g being coupled fluidically to said vessel intermediate said liquid level and said lower region of said vessel whereby in use, sufficient thermal energy is absorbed by said water or aqueous solution to raise the water or aqueous solution to boiling point whereupon steam bubbles are formed and the combined effects of thermal and gaseous convection causes a 1 d' cyclic flow of fluid from a lower region of said vessel via said thermal transfer conduit to an intermediate region of said vessel and saturated steam is collected from said gas space as a working fluid.
12. 16. A method according to claim 15 wherein said thermal transfer conduit is oriented in a north/south axis.
13. 17. A method as claimed in claim 16 wherein the thermal transfer conduit, when utilised in the southern hemisphere, is oriented with a southerly end thereof elevated relative to a northerly end thereof.
14. 18. A method as claimed in claim 16 wherein the thermal transfer conduit, when utilised in he northern hemisphere, is oriented with a northerly end thereof elevated relative to a southern end thereof.
15. 19. A solar heating unit substantially as hereinbefore described with reference to the accompanying drawings. A method of converting solar energy to a usable form of thermal 15 energy substantially as hereinbefore described with reference to the accompanying drawings. DATED this Nineteenth day of June 2001. STANWELL CORPORATION LIMITED .oo.•i By its Patent Attorneys FISHER ADAMS KELLY o..oo