Patent Application
20130206356
The present invention relates to a heat storage device according to the preamble of claim 1 and a heat storage device according to claim 16.
Heat storage devices are used, inter alia, in power plants, in particular in solar power plants. Heat storage devices inserted in the ground are known for the storage of smaller quantities of heat (in contrast to storage on an industrial scale, see below), see Hahne, E: “The ITW solar heating system: an oldtimer fully in action”, SOLAR ENERGY, PERGAMON PRESS, OXFORD, GB, Vol. 69, No. 6, 1 Jan. 2000 (200-01-01), Pages 469-493, XPOO4221026, ISSN: 0038-092X(00) 00115-8. The heat storage device disclosed by Hahne has a water-gravel filling and is operated in a low-temperature range of 0° C. to about 35° C. together with a heat pump (see Hahne, loc. cit.,
DE 3 101 537 also discloses a heat storage device inserted in the ground in particular for supplying a house with heat, which is to be heated via electric current, where the operating temperature is suitable for operating the heating system of the house (
At the lower operating temperatures of this heat storage device lying below 100° C. in conjunction with the small dimensions, the thermal expansion of the heat-storing material is insignificant.
Smaller or larger cylindrical steel tanks which rest on supports and have a dry filling of gravel or ceramic have become known for the storage of heat at higher temperatures, in particular above 100° C. Steel tanks are used to absorb the operating forces in the storage device, which occur at operating temperatures above 100° C. and can then have very high values.
A dry filling of gravel or, as mentioned, also ceramic or another suitable bulk material (where reference is made hereinafter only to gravel instead of the various possible materials for the sake of simplicity) has a comparatively large heat capacity and can be perfused by a fluid, for example, a gas, since the intermediate spaces between the stones are sufficiently connected and allow a uniform perfusion over the entire cross-section of the gravel filling. The gravel filling is heated by a hot fluid which can then for its part deliver heat subsequently to a cooler fluid. Overall, heat storage devices having a heat-storing bulk material are suitable for storing heat produced by solar energy.
In particular, in the generation of heat by solar energy, however, the amount of heat actually produced depends on weather conditions (and naturally on the time of day), where the fluctuations due to the weather can be very large over the day. Frequent weather fluctuations or periods of bad weather bring with it losses of heat production which can be so serious that, due to the weather, the production of solar heat is not considered at sites which otherwise need not be eliminated in advance from the solar irradiation point of view.
In solar thermal power plants the sun's radiation is reflected by collectors with the aid of a concentrator and is specifically focussed onto a location at which high temperatures are thereby produced. The concentrated heat can be removed and used for operating thermal machines such as turbines which in turn drive power-generating generators.
Three basic forms of solar thermal power plants are in use today: dish Stirling systems, solar tower power plant systems and parabolic trough systems.
Dish Stirling systems are fitted with parabolic mirrors which concentrate the sunlight onto a focal point, where a heat receiver is located. The mirrors are mounted so that they can be rotated biaxially so that they can track the current position of the sun and have a diameter from a few metres up to 10 m and more whereby powers of up to 50 kW per module can then be achieved. A Stirling engine installed at the heat receiver converts the thermal energy directly into mechanical work, through which power is generated in turn.
At this point reference is made to the embodiments presented in U.S. Pat. No. 4,543,945 and the EU Distal and Eurodish installed systems in Spain.
Solar tower power plant systems have a central absorber mounted at an elevated point (on the “tower) for the sunlight reflected to it by the hundreds to thousands of individual mirrors, whereby the radiation energy of the sun is concentrated by means of the many mirrors or concentrators in the absorber and temperatures up to 1300° C. can be achieved, which is favourable for the efficiency of downstream thermal machines (usually a steam or fluid turbine power plant for power generation). The “Solar two” plant in California has a power of several MW.
Parabolic trough systems have a large number of collectors which have long concentrators having small transverse dimensions and therefore do not have a focal point but a focal line. These linear concentrators today have a length of 20 m to 150 m. An absorber pipe for the concentrated heat (up to around 500° C.) runs in the focal line, which transports this to the power plant. Thermal oil or superheated water vapour for example can be considered as transport medium.
The 9 SEGS parabolic trough power plants in Southern California together produce a power of about 350 MW. The “Nevada Solar One” power plant which went onto the grid in 2007 has trough collectors with 182,400 curved mirrors which are arranged over an area of 140 hectares and produces 65 MW.
Another example of a parabolic trough power plant is Andasol 1 in Andalusia having a concentrator area of 510,000 m2 and 50 MW power, where the temperature in the absorber pipes should reach about 400° C. The costs amounted to three hundred million Euro. Likewise, Andasol 2 which started test operation in mid 2009 and Andasol 3 which is to start operating in mid 2011.
It is found that there is a very broad requirement for the storage of heat:
On the one hand for the most diverse amounts of heat from single small systems such as, for example, dish Stirling systems as far as heat produced on an industrial scale by power plants such as Andasol, where heat storage devices having a capacity not achieved to date (and therefore a size not achieved) are required.
On the other hand, the storage should only take place short-term (shading, maintenance works, wind etc.), then medium-term for use at night or longer-term during a period of bad weather.
Finally, and particularly importantly, heat should be stored at the highest possible temperature since high temperatures are required when converting to another energy form (for example, current) for a high efficiency and as mentioned above, can increasingly be provided by parabolic trough power plants, in particular by solar tower systems and also by small parabolic mirrors. The losses during heat storage are therefore not only determined by the insulation during the storage duration and thereby by the temperature drop but also by the temperature of the heat extracted from the storage device. As mentioned above, the heat generated in the concentrator of a parabolic trough power plant can reach above 100° C., for example, 200° C., 400° C. or 500° C. or more, with even higher temperatures being strived for by further development.
The gravel filling (or dry filling, since a water filling at the said temperatures no longer makes sense as a result of the steam formation and the associated operating pressure) of a heat storage device for storage of heat on an industrial scale now has an appreciable weight so that the container must have a correspondingly solid steel wall in order to withstand the pressure of this gravel. For small heat storage devices the construction of a steel wall is at first glance less demanding, for large heat storage devices it is certainly no longer trivial.
The pressure at rest of the gravel filling (active pressure) is less problematical here since as a result of the empty spaces between the stones, an active pressure of about 30% of a water column must be expected. Naturally the active pressure depends on the density of the bulk material and can have higher values. Usually (but in no way necessarily) however, the desired perfusibility of the bulk material will have the result that a bulk material having a not very high density will be used.
During the operation of the heat storage device, by the very nature of the matter the gravel filling must be heated and cooled. During heating to the aforesaid temperatures of above 100° C., in a range of up to 400° C., 500° C., or even higher, the gravel filling expands, which produces an expansion pressure which the steel wall must withstand, so that a correspondingly solid structure is required. If an expansion of the gravel filling is not possible due to the rigid wall, the expansion of the individual stones will be compensated by a displacement of their arrangement with respect to one another so that the empty space between the stones is reduced by the volume consumed by the thermal expansion of the stones. The expansion pressure therefore corresponds in order of magnitude to that pressure which must be applied from outside to push together a gravel filling by a vertical wall (passive pressure).
The passive (naturally like the active) pressure is dependent on the type of bulk material and in the case of loosely poured gravel, is approximately three times the pressure of a water column, or more. In other words, it is the case that the determining operating pressure in the conventional heat storage device corresponds to the passive pressure which is substantially higher than the active pressure of the gravel filling.
Here it should be borne in mind that steel is heated more rapidly than gravel, with the consequence that during heating of the gravel the steel wall of the gravel container is in advance of this in the temperature and therefore also in the thermal expansion. During the heating process, the volume of the container therefore increases more rapidly than that of the gravel filling, which therefore collapses into itself to some extent (i.e. occupies a somewhat broader but less high volume). When the container steel wall has reached the temperature of the heating fluid (which will be somewhat above the storage device temperature), the gravel still trails behind in its temperature and begins to build up expansion pressure due to a corresponding increase in volume, which reaches its maximum at the storage temperature. This can have the result that the yield strength of the (hot, see above, and therefore less stress-resistant) steel wall is exceeded and the container becomes deformed, and is ultimately destroyed.
Overall the container must really be quite armour-plated in order to be able to withstand the pressures occurring during operation, which is constructively complex, and complicated and expensive to manufacture.
Accordingly, it is the object of the present invention to provide a simple heat storage device which allows the storage of small and also large amounts of heat as far as the industrial scale.
This object is achieved by a heat storage device having the features of claim 1 or claim 19 as well as by an operating method according to claim 27.
Since the side wall is supported from outside, it is substantially only compressively stressed without needing to withstand a circumferential expansion as a result of a pressure-dependent increase in diameter, which results in a considerably lower stressing of the side wall and allows a correspondingly simple construction. Since the support is provided by bulk material, a sufficiently robust support can surprisingly be achieved in the simplest manner since the support has an active pressure in advance which lies in the order of magnitude of the operating pressure (i.e. of the active pressure) of the heat-storing bulk material.
The side wall used in the prior art at operating temperatures of the bulk material of above 100° C., which is configured in a constructively complex manner in order to be able to absorb forces caused by the thermal expansion of the bulk material, is omitted.
In a preferred embodiment, the side wall (or at least areas of the side wall) are inclined at an angle of inclination which has the result that the heat-storing bulk material is displaced upwards to some extent during the expansion which takes place during the storage of heat, so that the volume of the bulk material filling is increased and therefore the operating pressure that really occurs is lower than the expansion pressure in the case of a vertically disposed side wall.
The pressure loading of the side wall is therefore reduced once again with the consequence that not only steel can be used for the side wall but according to another embodiment, this can also consist of concrete.
Further preferred embodiments exhibit the features of the dependent claims.
The invention is described in somewhat greater detail hereinafter with reference to the figures.
In the figures:
a to c show schematically further embodiments in which the supporting bulk material is heaped up above the ground, and
a to 5b show various configurations for a heat storage device according to the invention and
The figure does not show the further components by which means the power house 6 is connected to a heat source such as a solar power plant. The components to which the stored heat extracted from the heat storage device 1 is guided from the power house 6 are also not shown.
The figure shows a perfusion with fluid from bottom to top. The heat-storing bulk material 4 can thus be heated to above 100° C. by hot fluid coming from the heat source. As required, cold fluid can be passed subsequently through the heat-storing bulk material 4, which is accordingly heated and can then be used for conversion of energy, possibly for steam generation, where the steam for its part can then drive a turbine for the production of power.
Preferably a gas such as air is used as fluid. Other fluids, including liquid, are also feasible. Preferably a filling of gravel is used as heat-storing bulk material 4. Other bulk materials are also feasible. Finally, various solutions are possible for the guidance of the fluid through the heat-storing bulk material 4, for example, in such a manner that a temperature stratification is obtained in the heat-storing bulk material 4 and the perfusion is accomplished in concurrent flow or in countercurrent flow, see the description to
The person skilled in the art can design or modify the afore-mentioned elements in order to achieve an optimal heat storage according to the circumstances on site. In particular, the heat storage device 1 can be designed for the storage of the heat of a small unit or for the storage of heat on an industrial scale and can be configured accordingly.
The ground 2 shown in the figure has a different structure. On the one hand a solid structure such as, for example, rock 13 and on the other hand, a bulk material-like structure 14 such as, for example, earth or gravel.
During operation, the side wall 5 is loaded by the heating, heat-storing bulk material by outwardly directed, in the case of a cylindrical container 3, by radial operating pressure, where the side wall 5 for its part is supported towards the outside on the supporting bulk material 14. As a result, the side wall 5 is substantially only compressively stressed. If the side wall 5 consists of steel, the diameter of the container 3 becomes greater due to the thermal expansion with the result that the supporting bulk material builds up counterpressure corresponding to its active pressure so that during the subsequent increase in the diameter of the filling of heat-storing bulk material the side wall 5 is already stably supported by the supporting bulk material and a further increase in diameter of the side walls is largely or completely suppressed: according to the invention it remains basically as only compressive stressing of the side wall, which allows a correspondingly easy and simple design. Here it should be added that a denser material (compared with the heat-storing material) is preferably used for the supporting bulk material 14 since this need not be perfused by a fluid. This will usually be the case when using spoil from the pit in which the heat storage device 1 according to the invention is laid. The passive pressure of the supporting bulk material 14 is therefore greater than that of the heat-storing bulk material 4 from the outset, with the corresponding advantage of a stable support for the side wall 5.
In a preferred embodiment the supporting bulk material 14 is compacted compared with the looser filling state, which can be advantageous in particular with certain bulk materials but is hardly necessary with coarse bulk materials consisting of hard individual parts such as coarse gravel. Here it should be noted that when selecting the supporting bulk material, the person skilled in the art will in particular take into account its passive pressure but is fundamentally free in the determination of the material of the bulk material and this can be matched to the needs on site. The bulk material 14 can then be different from the material of the ground.
As a result of the inclination of the side wall 21, the heat-storing bulk material 4 under expansion pressure can be displaced upwards to some extent since the reaction of the wall to its individual stones has an upwardly directed component thanks to the oblique position.
A surface unit of the side wall 21 exerts a reaction force in accordance with the vector 25 of the reaction force, where this vector 25 has a horizontally inwardly directed component 26 as well as the afore-mentioned vertically upwardly directed component 27, which in turn results in a movement of the individual stones of the heat-storing bulk material 4 towards the top.
If stones are now displaced to some extent towards the top, a pressure relief is accomplished due to the simultaneous increase in volume. This effect occurs over the entire height of the container 20. This effect certainly occurs only to a restricted extent as a result of the friction of the individual stones amongst one another and with the side wall 21 but nevertheless to such an extent that the real operating pressure is noticeably reduced compared with the expansion pressure corresponding to the active pressure in the case of a vertical side wall. This in turn results in pressure relief of the side wall since specifically the operating pressure acting on said wall is smaller than that in the case of a vertically aligned side wall.
In one embodiment of the present invention, during operation of the heat-storage device the heat-storing bulk material has a passive pressure which is higher than the desired operating compressive strength (without safety margin) of the side wall of the container containing the heat-storing bulk material 4. This is possible because the angle of inclination of the side wall 21 with respect to the horizontal has a value such that the operating pressure of the heat-storing bulk material 4 is smaller than the operating compressive strength of the side wall 21.
The inclination of the side wall 21 not only gives the advantageous effect on the heat-storing bulk material 4 described above but at the same time the advantageous effect described by means of the force vector 40 on the supporting bulk material: a surface unit of the side wall 21 is loaded by the operating pressure and exerts a force according to the vector 40 on the corresponding region of the supporting bulk material 23 which has an outwardly directed horizontal component 41 and a downwardly directed vertical component 42.
Due to the horizontal component 42 alone, the supporting bulk material 23 at the surface 43 could be partially pressed away upwards, as is indicated by the outline of a heap 44. As a result, the effect of the supporting bulk material would be reduced in the area of the surface 43 with the risk that this effect continues downwards and the supporting bulk material 34 loosens in the area of the side wall 23 with the result that the side wall 21 must be designed for a correspondingly higher stressing.
Due to the vertically downwardly directed component 42, however, the risk of a heap 44 is avoided since the pressurised supporting bulk material 23 is not only pressed away horizontally outwards but is also pressed simultaneously downwards.
In summary, on the one hand the side wall can be supported by a supporting bulk material which overall allows larger heat storage devices (i.e. heat storage devices whose filling has a large volume) since without such support, the side wall would hardly be economical to manufacture as a result of the forces to be intercepted. In addition, the operating pressure can be reduced appreciably due to the described inclination of the side wall, which facilitates the construction of a conventional side wall. In combination, these two construction principles enable the stressing of the side wall to be reduced in such a manner that even for large heat storage devices, materials other than steel such as, for example, concrete can be used, which in turn brings with it the further appreciable advantages according to the invention, as is described in connection with
Since the specific optimal angle of inclination of the side wall depends on which bulk materials are used, it should be added at this point that an angle of inclination between 50 and 85 degrees enables the effect according to the invention to be achieved in almost all possible combinations of bulk materials. An angle of inclination between 60 and 80 degrees is suitable for most common bulk materials (gravel or ceramic in combination with material from the ground) whereas an angle of inclination of 70 degrees may serve as an average if uncertainties regarding the angle of friction of the materials are present or non-homogeneous materials are used.
For example, when selecting a heat-storing bulk material such as rounded gravel having a grain size of 28 to 32 mm, a specific weight of 15 kN/m3 and an angle of friction of 40 degrees in combination with a supporting bulk material such as looser, non-cohesive “standard soil” having a specific weight of 22 kN/m3 and an angle of friction of 30 degrees with an angle of inclination of the side wall of 80 degrees with respect to the horizontal, a passive pressure of the gravel of around 269 kN/m2 is obtained whereas the passive pressure of the supporting “standard soil” is 420 kN/m2.
a to 3c show various possible configurations in the structure of the supporting bulk material.
a shows a container 30 of a heat storage device partially sunk on the ground, where the appurtenant fluid lines and further components are omitted to unburden the figure. A bulk material 31 surrounds the side wall 32 over its entire height, on the one hand fills the pit 33 and on the other hand forms a mound 34. The flank of the mound 34 is flatter than the angle of friction of the supporting bulk material 31 with the result that the mound remains stable and can absorb the appreciable forces of the operating pressure. The stability of the flank of the mound 34 is supported by the inclination of the side wall 32 which additionally compacts the mound 34 in a stabilising manner under operating pressure by means of the downwardly directed force component (see
b shows a container 35 partially sunk in the ground, where the ground produces sufficient active pressure to support the side wall 36. The mound 37 of supporting bulk material is for its part supported on the outside by an outer end wall 38. This results in a reduction in the diameter of the arrangement since the flank of the supporting bulk material shown in
c shows a container 40 standing on the ground (or even above the ground, for example, on a frame) where the supporting bulk material 42 is heaped around the side wall 41 so that its side wall 41 is protected by a mound 42 of supporting bulk material, where an outer end wall 43 surrounds and delimits the mound 42.
The arrangements in
The longitudinal edges 52, 53 of the concrete elements 50 have a step 54 so that a broader surface 55 and a narrower surface 56 is formed. The concrete elements 50 are now positioned adjacent to one another, with the broader surface 55 and then the narrower surface 56 directed outward, with the result that the stepped longitudinal edges 52, 53 are supported on one another so that the side wall 51 is closed.
The person skilled in the art can also provide a different geometry of the edges 52, 53 so that the concrete elements 50 suitably intermesh. As a result of this intermeshing, a mutually defined position of the respectively adjoining concrete elements is obtained. At the same time, due to the step shape, a slight relative movement of the adjoining elements 50 in the built-in state is possible as before, whereby the flanks of the steps slide on one another to some extent so that remaining extremely small displacements of the elements are possible as a result of the operation of the heat storage device.
In detail, a concrete element 50 is formed as an elongate, flat panel whose two straight longitudinal edges 52, 53 are formed as a step 54 which can be brought into engagement with the step 54 of an adjacent concrete element 50. The width of a concrete element 50 at the lower edge 62 is smaller than the width at the upper end 63. In addition, the lower width 62, the upper width 63 and the ratio of the widths 62, 63 are formed in such a manner that a number of concrete elements 50 with engaged longitudinal edges can be joined together to form a closed jacket of a truncated cone, as shown in the figure.
The body of a concrete element 50 can preferably be curved in the direction of its width (this is however over its entire length), where the radius of curvature becomes smaller from the broader end towards the narrower end and is configured in such a manner that it substantially corresponds to the radius of curvature of the truncated cone.
The supports 57 shown by the dashed lines are further provided in the figure, which allow the concrete elements 50 to be arranged and aligned with respect to one another in the prepared pit during construction of the heat storage device so that the pit can then be filled with the supporting bulk material. During operation of the heat storage device according to the invention, however, the supporting effect of these supports 57 is of subordinate importance.
A base 65 of the container 51 can also be seen.
The thermal conductivity of concrete is massively lower than that of steel which allows the longer storage of high-temperature heat without greater additional expenditure on insulation. In order to prevent any cooling convection of air, according to the embodiment of
Such containers can, for example, have a diameter of 5 m to 25 m and a height of 4 m to 9.5 m.
a shows another configuration according to the invention for, a heat storage device according to the invention. The side wall 70 as a whole is divided into different segments 71, 72. In this case, only the segment 71 is inclined with respect to the horizontal and the segment 72 is arranged vertically, which however is sufficient to reduce the operating pressure of the heat-storing bulk material provided therein (and omitted to unburden the figure) in such a manner that the side wall can be formed of non-metal materials such as the concrete elements 50 described above. The person skilled in the art will, for example, select such a configuration when the available space on the one side of the pit provided for the heat storage device is small. At this point it should be noted that, for example, at the location of the vertical segment 72 rocky ground could be present so that the person skilled in the art need only support the inclined segment 72 by a supporting bulk material whilst the vertical segment 71 would be directly supported by the rock. Such mixed forms are certainly within the range of the present invention but are rare and are usually motivated by special soil information. Usually the entire side wall 70 is configured to be round as a truncated cone and surrounded by supporting bulk material.
b shows an additional configuration according to the invention for a heat storage device according to the invention. The side wall 80 is divided as a whole into different segments 81 to 84. One of the segments 81 is vertically aligned, the other segments 82 to 84 are aligned with respect to the horizontal according to the invention, which is sufficient to reduce the operating pressure of the heat-storing bulk material present therein (and omitted to unburden the figure) in such a manner that the side wall can be formed of non-metal materials such as the concrete elements 50 described above. In addition, the segment 84 is configured to be flat.
The person skilled in the art will determine such geometrically mixed forms according to the conditions on site, but will provide the inclination and the surface fraction of the inclined segments in such a manner that the effect of the inclined side wall according to the invention is effective.
A diagram is shown where curve 90 gives the temperature distribution in ° C. in a heat-storing bulk material of a heat storage device according to
A temperature stratification is found with an uppermost layer or zone A of the bulk material having the highest temperature (section 91 of curve 90) and a lowermost layer C having the lowest temperature (section 92 of curve 90). In this case, the highest temperature reaches and exceeds 500° C. whilst the lowest temperature lies below 100° C.
A middle zone or zone B of the bulk material (section 93 of curve 90) shows an approximately uniform temperature drop over its height.
In cases of longer-lasting or shorter-lasting supply of heat to the storage device, the curve 90 is shifted to the right or to the left, where the flank thereof, i.e. the section 93 thereof, remains substantially unchanged. The height of the layer or zone B therefore remains substantially the same.
Accordingly, zone A can extend over a height of a few cm at the beginning of the heat storage as far as about 2 m, where the temperature of the bulk material at the bottom in the layer C which is still present as before but now only just 0.5 m thick, remains at around 50° C.
This means that the supporting of the side wall according to the invention preferably extends over the entire height of the heat storage device even when the temperature of the bulk material in layer C is kept below 100° C. This applies likewise for the synergetic slope of the side wall for supporting according to the invention.
In an operating method according to the present invention only so much heat is supplied to the heat storage device that a lower layer C having a temperature below 100° C. is always present (which is not compulsory) so that the outlet temperature of the fluid supplying the heat to the storage device is also 100° C. (the temperature of the fluid is always somewhat higher than the bulk material heated by it) or is preferably lower at around 50° C. In this operating mode the fluid then delivers a maximum amount of heat to the storage device, which is advantageous for the efficiency of the heat storage in the power plant.
The advantages according to the invention are achieved as soon as the inlet temperature of the fluid exceeds 100° C., i.e. the heat storage takes place by means of a dry filling at temperatures relevant for thermal expansion thereof. Depending on the design of the power plant on site or the installations provided, in which a heat storage device according to the invention is integrated, the inlet temperature can be or exceed 200° C., 300° C., 400° C. or 500° C.
Accordingly, an operating method for a heat storage device according to the invention having a heat-storing dry filling of bulk material, which is surrounded by a side wall (5) enclosing said material, is characterised in that
Overall, the present invention provides a heat storage device which is suitable for small systems and for the storage of large amounts of heat such as are produced in large solar power plants. For large amounts of heat a single large, or several suitably connected smaller, heat storage devices are provided since the heat storage device according to the invention, in particular in a design having a side wall made of concrete elements, can be produced in series cost-effectively and on site. As a result of the insulating effect of the supporting bulk material and a side wall consisting of concrete (naturally the person skilled in the art can also provide the base and the cover of the container made of a material such as concrete), a long-term storage time is possible even for heat having a high temperature which is 500 degrees C. or higher, e.g. 650 degrees C.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1396/10 | Aug 2010 | CH | national |
| 1414/10 | Sep 2010 | CH | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CH2011/000198 | 8/30/2011 | WO | 00 | 4/9/2013 |
