GB2493388A - Solid material thermal storage system and solar collector - Google Patents

Abstract

The thermal store 26 comprises a solid material such as sand, gravel, clay or clastic rock, that is supplied with sensible heat energy received by a solar collector 23. The store may be encased by vacuum insulation panels or an insulating material such as polyurethane hard foam, and include drainage to remove water. The store may be used to heat domestic hot water 21 or water for use in central heating 19,20 without build-up of legionella or calcification. Heat energy may be transferred to the store by copper pipe heat exchangers arranged in a bundle 39. A pump 24 may be actuated when the temperature 32 of the collector exceeds the temperature 33 of the bundle at the entry point into the store. The store may comprise a plurality of heat storage modules or segments, with one segment being kept cool so that it may be used to remove residual heat from the collector when needed. Other systems such as long distance heating and industrial processes may also be used to heat the store. The thermal store can be manufactured from mass produced parts that are easily assembled on site.

Description

I. Title Insulated solid material thermal storage systems, assembled on site to store and to supply sensible heat through direct and indirect heat exchanger

2. Background

This invention is about solid material thermal storage systems for sensible heat It focusses on the supply of domestic hot water and thermal heat through usage of solar energy although different sources of heat and other purposes for the application are possible.

Energy from a solar collector that is to supply domestic hot water or thermal heat or to support a heating system normally has to be stored because of the different points in time when demand and availability occur. In the private sector storage volumes between some hundreds to maximal 2500 litres are used to solve this problem.

The storage medium is water, because of its high thermal capacity and the possibility to use it directly.

If a solar thermal system in the private sector provides only domestic hot water, the storage has a volume not bigger than 500 litres. Such system works effectively from spring to autumn and can cover two thirds of the demand. However, a longer period of bad weather cannot be bridged and for these times and the period of winter an auxiliary heating system is needed.

Combination installations for domestic hot water and supporting the heating system have generally bigger storages (in the private sector up to 1000 litres and for blocks of flats several storages of this size are usual).

These storages are able to store heat some days and they work effectively in supporting the heating system in the transition period. However, they cannot store the whole amount of heat available in summer.

* That only a small proportion of the heat available in summer is used by both types of systems has various reasons. The main reason is that the maximal storage temperature is limited to 90°C to 95°C for water storages to avoid high temperature and pressure load. Once this temperature is reached, the supply of heat from the collector will be halted and the amount of stored heat cannot be boosted. The interruption of the heat supply leads to high thermal loads for the collector. Stagnation temperatures can go up to 200°C for flat collectors and up to 300°C for evacuated tube solar collectors, resulting in an accelerated aging of the collectors and the coolant.

Additionally parts which have been exposed to such high temperature have to be evacuated automatically. In some cases high temperatures are avoided by cooling the storage overnight misusing the collectors. Therefore the feature of evacuated tube solar collectors to create heat above 100°C effectively cannot be used.

The reason for not using storages with volumes capable of storing the whole amount of available energy are the high prices of those storages. One reason for the high prices is that the storages deliver drinking water and therefore high hygienic standards have to be met. Mother barrier is the limitation of the size arising from staircases, doors and windows at least for existing buildings. The latter allow normally only storages with small volumes. It is not common to assemble the storage on site from a number of parts to get greater volumes.

Because water is a fluid, the stratification of the heat within the storage is complicated and causes very often a considerable loss of temperature by the heat exchange process. To solve this problem a lot of different forms and designs of heat exchangers are on the market. Except for a small number of special forms, the use of water always leads to the highest temperatures near the storage surface (depending on the state of charge). Despite the fact that storages are generally well insulated this increases the unavoidable loss of heat further.

A general problem with all water storages delivering domestic hot water is a possible contamination with legionella which can be hazardous to health. To avoid a strong growth of legionella it is useful to store the water at temperatures above 60°C or to heat the storage up to this temperature at least once daily.

In areas with chalky water this warming up to 60°C or more can cause another problem, because these temperatures start calcination. Calcinations reduce the performance of the heat exchangers and can narrow or seal up the water pipes.

3. Statement of invention

To overcome the problems of high costs, of reduced system effectiveness because of small-sized storage volumes, of the stratification of the heat, and the related heat loss, of the possible contamination with legionella and the risk of calcination the present invention proposes insulated solid material thermal storage systems, assembled on site to store and to supply sensible heat by means of direct and indirect heat exchangers.

4. Advantages * All parts of a solid material thermal storage system are standardized components. For that reason they have reasonable prices. They are easily workable and can he adapted to the available space on site, allowing to build storages with an optimal volume for a high annual useful heat.

The design of the solid material storage systems makes it possible that alterations due to changing demands and extensions can easily be carried out.

The use of solid material (ground material) as storage material allows to store heat at high temperatures (above 100°C). Therefore it is not necessary to switch the collectors off, avoiding laster aging through stagnation and high temperature loads. Instead more useful heat can be collected and stored.

The possibility to place the heat exchangers for the supply and for the extraction spatially close together means a great advantage. The closeness (or a possible direct contact) of both types of heat exchangers results in almost identical temperatures in the heat exchanger and the surrounding area. In case of an empty storage (low storage temperature) shortly after a supply of heat has taken place useful heat is available. In case of a fully loaded storage (high storage temperature) the extraction of heat cools the area for the supply of heat down. Because the amount of heat a collector delivers increases with a lower storage temperature this leads to a higher collector output. When the solid material storage system supports a heating system under normal circumstances the collector delivers heat to the storage very often while the heating system uses heat. The closeness of both types of heal exchanger allows almost a direct use of the solar energy. Variations in the collector output and the heating demand will be compensated by the storage material.

The use of solid material to store the heat makes it easy to use only segments of the storage depending on the collector output and temperatures in these segments. Because there is no convection, only thermal conduction, the heat can be better stored in one or some specified area/s and used when required.

The modular concept of these systems makes it easy to limit the temperatures in certain parts of the storage. This can for example be necessary if the thinking water that is to be warmed up is chalky and temperatures above 60°C must be avoided to prevent calcinations. Another special use of a segment of storage is to store the residual heat in it. Residual heat is the heat left in the collector when the temperature in the normal storage is nearly the same as in the collector. Residual heat occurs at the end of a sunny day or after a break in the radiation, if there is always a segment in the storage with a low temperature, this segment can be used to store the residual heat. To cool this segment down again it can be used to preheat the domestic water or supply a floor heating system for

example.

Recause there is only a little quantity of drinking water in the storage system, the possibility of a contamination with legionella is minimal. When the heat supply takes place near the pipe(s) for the domestic hot water, temperatures above 60°C are common; meaning the risk of a contamination is very low because the legionella die off at these temperatures.

Such storage systems would practically have the form of a cubical or a cuboid. The plane outsides allow various * ways to construct highly efficient heat insulation. One possibility is the use of Vacui.un Insulation Panels (VIPs) in a sandwich construction. VIPs have an extremely low thermal conductivity which can minimize the heat loss and reduce the mom needed for the insulation. The sandwich construction is necessary because VIPs cannot sustain mechanical load and there is a limited maximal working temperature. Another possibility to use the advantage of a vacuum as a heat insulation is the construction of a gas-tight cover around the storage material.

The cover must have an appropriate distance to the storage material, which in this case has to be in a robust sheathing. The space between has to be emptied with a vacuum pump. The gas-tight cover could be copper foil, which is even impermeable for hydrogen atoms.

In addition to solar collectors other systems like long-distance heating, district heating or industrial processes can be used as a source of heat for the storage system. Because solar heat is not always available the combination of different heat sources is meaningful and causes no problem.

If the solid material storage system cannot longer be used, it can be dismantled without problems. Because all components can be recycled, there are no high costs for dumping and there is no pollution of the environment.

5. Introduction to drawings

The basic methods of operation of the inventions are described in the following and clarified by thirteen accompanying drawings, most of them in cross section: figure 1 shows a very simple construction of a solid material storage with a helical twin tube bundle for the heat exchange; * figure 2 shows different designs and forms of tube bundles, different in the number of tubes, their positioning and their distances * figure 3 shows a single tube bundle (for one way of the heat exchange) in undisturbed environment; * figure 4 shows a twin tube bundle in undisturbed environment; * figure 5 shows a twin tube bundle in a more compact construction with two contact areas; * figure 6 shows a twin tube bundle in a more compact construction with three contact areas; * figure 7 shows the scheme of a storage with two tube bundles for the heat exchange; * figure 8 shows a simple solid material storage with a small nozzle for the drainage; * figure 9 shows a system with three identical storages; * figure IC shows a three storages system with uniform heat insulation; * figure 11 shows the scheme of a segmented storage; * figure 12 shows a system with only one storage for the supply of domestic hot water; * figure 13 shows a system with two storages for supporting a heating system.

6. Detailed description

In order to tackle the problems with high costs, the low thermal efficiency caused by small storage volumes, the difficulties with the stratification of the heat and the resulting loss of heat, the possible contamination with legionella and the risk of calcination, the use of insulated solid material thermal storage systems, assembled on site to store and to supply sensible heat through direct and indirect heat exchangers makes sense.

A solid material storage system consists of one or more solid material storages. A solid material storage is a spatial construction, where the walls are well heat insulated and that contains (storage material I) solid material like sand or any other type of ground material. The heat exchange takes place through one or more copper tube bundles. These bundles can have different forms; a useful form for a bundle is a helix. Conceivable are also bundles of spirals, u-tubes and so forth. The choice depends on the task at hand and the available space.

Figure 1 shows a very simple construction of a solid material storage, with its heat insulated load-bearing walls 2, the solid material 1 as a storage material for the beat and a helical twin tube bundle 3 for the heat exchange.

Description of the different components of solid material storage systems: 6.1. Walls The construction of the walls must guarantee that they have a very good heat insulation and they can safely store the material. The inside of the walls has to be waterproof, too, in order to make sure that a possible rest of water does not penetrate the insulation.

These criteria can be met with high-temperature rigid foam like water repellent polyurethane hard foams. With * these materials it should be possible to build a complete closed covering. Polyurethane hard foam is produced in large quantities and can easily be cut in nearly every possible form. So, in principle all thinkable solids can be constructed, practically cuboids and cubes are the first choice. The thermal conductivity of these hard foams is very low, minimizing the heat loss. The chosen width of the walls is responsible for the amount of heat loss and must additionally make sure that the pressure caused by the storage material can be absorbed safely. When assembling on site the walls can be glued together, or, making the handling in the future easier, hold in position with straps from heavy load restraint systems. The used material must have a long term service temperature above 100°C. But this should be no problem, because these foams normally have an operating temperature of 120°C and can resist temperatures up to 250°C a short while. If the use of rigid material is not possible, it is necessary to build walls to contain the storage material. These can be done for example with sandwich-type plaster boards covered by a vapour barrier. This then can be insulated with various materials or surrounded with a gas-tight cover. The space between the storage material and the cover can be evacuated with a vacuum pump, leading to a highly effective heat insulation.

6.2. Storage material To store the heat, solid material, like all kinds of ground (such as sand, clay, pebble, gravel, clastic rock).

concrete or similar materials should be used, as long as these are not unduly corrosive. The disadvantage of these materials is a lower volumetric heat capacity compared to water. The advantage of these materials is, that there is virtually no limitation in the storage temperature and because the material is solid the heat can be stratified without losing too much temperature. The higher storage temperature leads to nearly equal or in some cases to higher values of stored heat compared to water storages. Most of the conceivable materials from above are normal construction materials and therefore inexpensive and available everywhere. And while they are a bulk, the transport to any place in a building can be done with minimal effort, providing an adequate floor loading.

When choosing the kind of storage material, a maximal heat capacity and low heat conductivity should be the selection criterion. Low heat conductivity makes sure, that the stored heat is as long as possible in the region of the tube bundles and can be used without great temperature loss. Thinkable are also combinations of different materials to use their special features. If for example a helical tube bundle with no space between the winding is used, it is possible to use material inside the bundle with higher conductivity than on the outside.

6.3. 1-leat exchanger The supply and the extraction of the heat should happen via one or more copper tube bundles. These bundles can have different forms, depending on the design of the storage and the available space. A usefbl form for a tube bundle is the form of a helix. Also possible are bundles of spirals, simple circles or u-tubes. These bundles should consist of one or a small number of parallel copper tubes. Industrially manufactured copper tubes have different pipe diameters and wall thicknesses (table 1). Normally they are supplied iii lengths of 25m or 50m.

The bending radius is minimal six to eight times the size of the tube radius. In order to use the storage systems for diverse fitnctions, it is necessary to use parallel naming tubes. Conceivable designs for tube bundles with up to four tubes are shown in figure 2. The sensible positioning of the tubes in quad bundles 7, triple bundles 6, * twin bundles S and a single bundle 4 is displayed; all tubes have the same radius. Also thinkable are bundles of tubes with a different radius and bundles with more than four tubes. There can be a distance between the windings of the bundles or the windings can have contact the whole way around. Because there are a lot of companies manufacturing copper components the assumption is that these types of heat exchangers are inexpensive.

The description above shows that a lot of designs of heat exchangers are possible. The two basic conditions for the surrounding of possible heat exchangers are shown in figure 3 and in figure 4. Figure 3 shows one copper tube 8 through which the coolant 9 flows and which is exclusively used either for the transfer of heat into the storage or exclusively used for the heat extraction out of the storage. In its proximity no more tube or tubes are placed and the process of heat exchange 10 happens independently from other processe& The process of heat exchange happens only between the coolant and solid material. Surely there must be another tube in (a greater) distance from this tube to supply or extract the heat.

Usually the fluid in the solar collector circuit is a special antifreeze coolant (expensive), while the heating system uses ordinary water (inexpensive). This approach makes separate tubes necessary in order not to mix the fluids.

But if the fluids in both systems are of the same kind, it is possible, that the supply and the extraction of the heat takes place through one tube only. The same is true if for example another heat exchanger is used.

Figure 4 shows a twin bundle consisting of one tube for the supply of heat II and one tube for the extraction of heat 12. The tubes are positioned in a way that they have a contact area 13 and an interference zone 14, in which a special form of heat exchange takes place. This form of heat exchange is called indirect heat exchange and is the process where the heat is not only transferred from one fluid to another fluid but also from a fluid to solid material or from a solid material to a fluid. The twin bundle shown in figure 4 exchanges the heat 10 not steadily like a single tube, but there is also an influence of the tube working in the opposite way and the solid material around. The interference zone between both tubes leads to the following positive effects: -because copper is one of the best conductors of heat (hundreds of times better than solid dry ground) the second tube can support the heat exchange of the first tube by an increase of surface for the heat exchange -the heat exchange between both tubes is better than between a tube and the solid material of the storage, resulting in a fast temperature increase in the tube for the heat extraction when heat is supplied in the other tube, also when the temperature in the ("empty") storage is low -when heat is extracted by one tube, the temperature in both tubes drops, leading to better efficiency of the collector (the amount of useful heat ijicreases with a decreasing coolant temperature) Figure 5 and figure 6 show further designs of heat exchangers with indirect heat exchange. If the heat exchangers are built in a compact way that is without a space between the windings, for twin bundles this leads to two contact areas 13 for each tube (figure 5) or to three contact areas 13 for each tube (figure 6) between the two different types of functions (supply of heat Il, extraction of heat 12). The design from figure 6 leads to an * even change of temperature in the region of the heat exchangers, while the design from figureS leads first to a change in temperature lateral ("left" or "right"). Which design is used depends on the actual conditions and the requirements.

The main functions of a single tube from a tube bundle are: -the supply of heat from a solar collector -other possible sources are water cooled oven or fire places, near or long distance heating or heat from other (industrial) processes -the supply of heat from a heating system to boost the storage temperature -the extraction of heat for the supply of domestic hot water -the extraction of heat for the preheating of domestic hot water -the extraction of heat for heating -the extraction of heat to preheat the return flow of the heating system -the extraction of heat for technical processes -or a combination of the functions described above

S

It is also possible to use one or more tubes of a lube bundle for the same huiction. For example, if there is a quad tube bundle in the storage, two tubes can be used for the supply and two tubes can be used for the extraction of heat.

Depending on the task of the storage system more than one tube bundle can be used. For a storage with a basis of a square meter two helices 16,17 are an option. This shows figure 7. Also possible are four tube bundles with a smaller radius. Another alternative is the segmentation of the bundles (splitting into shorter sections). In order to reduce for example the pressure loss, a tube with a length of 50m can be split into two tubes with the length of 25m each and be operated parallel. The optimal number of tube bundles and their design must be chosen very carefully in order to meet the demand of a planned storage system.

The surface of a copper tube with a length of 50m and a diameter of 22mm, amounts to more than three square meter, making sure that the heat transfer can happen without great loss of temperature (table 1).

6.4. Drainage Possible residual moisture increases the heat conductivity in a way unwanted and must be removed from the storage material. Therefore there must be drainage 15 at bottom level to make sure that the residual water can flow out of the system (figure 8).

6.5. Combination options/ construction The daily quantity of heat coming from a solar collector varies strongly over the course of a year, making the segmentation of a bigger system into a number of smaller storages advisable. The advantage of this approach is * that for the amount of heat gained an adequate insulated storage volume is available. Figure 9 shows for instance a system of three identical storages placed next to each other. Each having a twin bundle 18 for the heat exchange. When using only the mid-position storage, this has the advantage of very good heat insulation on two sides. Depending on the amount of heat to store the left or right or both of the lateral storages can be used in addition.

Figure 10 shows a possible construction in case the space for the storage system is limited, or the system operates in a way, where all three storages are often used at the same time. Here all walls have the same thickness, reducing the volume needed for the system or if the volume is identical increasing the volume for the storage material and therefore the storage capacity.

To avoid possible calcination (caused by chalky water and temperatures above 60°C) in one part of the storage the temperature can be limited to 60°C. Heat with higher temperature must then be stored in the remaining storage and reloaded when needed.

The approach to build a system out of smaller solid material storages makes changes and extensions easy and inexpensive.

If there is room for a bigger storage (>10m3) and an appropriate solar collector a design shown in flgw-e 11 is a good possibility. This cuboidal storage can fill a complete room in the cellar or can be buried in the ground outside the building or beneath it. Again the penetration of water must be avoided and there must be a drainage for the possible residual moisture on the bottom level. For the heat exchange u-tubes twin bundles are a good choice. All entries and outlets furm the u-tubes for the supply of heat 11 and the extraction of heat 12 should be practically on one side of the storage, where they can be connected in such a way, that they form segments 38.

Those segments 38 can have different sizes and can be operated separately or in combinations to optimize the amount of useful heat gained from the storage.

6.6. System description

The above description for possible designs of storages and useful designs of heat exchangers allows the building of appropriate systems for all kinds of different tasks. Below two useful systems are described briefly.

Figure 12 shows a single storage system 26 exclusively for the supply of domestic hot water. The storage is fitted with two tube bundles. The outer one is a single tube bundle 29 and the inner one is a Iriple tube bundle 39.

The heat from the solar collector 23 is supplied to the storage material oniy by one tube of the Iriple tube bundle 39. If the temperature 32 at the collector outlet is a certain amount AT (normally 5 to 10K) higher than the temperature 33 at the lower entry of the triple tube bundle the pump 24 starts the solar circulation. The pump stops when the temperature 35 at the upper outlet of the bundle is higher than the collector temperature 32 reduced by AT. This way an equal temperature distribution from bottom to top is ensured. The heat spreads first around the tube bundle. l'hat ensures that the inner heat exchanger for the heating up of the domestic water is * always near the highest temperature in the storage. More heat supply leads then to an increase in the temperature also in the centre and the outer region of the storage. The inlet for the cold water always is through the single tube bundle, because this region of the storage has after a while of operation at least room temperature which is higher than the temperature the cold water has when entering the building (normally around 10°C). So the cold water is heated up and heat is extracted from this part of the storage. Is the storage temperature high enough the cold water can be heated up only in the single bundle. The upper outlet temperature 34 decides if the heated up cold water is warm enough or needs further heating up. If the temperature 34 is too low, the three way valve 27 must be opened in such a way that the water flows through the triple bundle as well. A high storage temperature leads to hot water, which might be too hot for normal usage. In order to cool the water down a mixing valve 28 can be used. The mixing valve 28 cools the hot water down to an adjustable useful temperature by mixing cold water to the hot water. If the temperature 35 at the outlet from the triple bundle is too low, a backup heater must provide the missing heat. In standard solar systems this is done via a separate heat exchanger in the storage supplied by the heating system. fri this case the third tube in the triple bundle must be connected with the flow and the backflow of the existing heating system. Alternatively a continuous flow heater can be used to increase the temperature of the water to a useful level.

Figure 13 shows a two storage system 30,31 for heating or supporting a heating system exclusively. Both storages are built identically and equipped with two twin tube bundles (an inner and outer) for the heat exchange.

So four tubes exist for each direction of the heat exchange. These four can be used singly, in groups or all at the same time. For both directions of the possible heat exchange there are 15 different ways of using the tubes. To run, monitor and optimize the operation of such a two storage system a control system (not included in Figure 13) is necessary.

7. Summary

The above description shows that solid material storage systems can be used in a lot of different ways. These systems are adaptable to all conceivable structural and thermo technical circumstances and they are easy to extend and alter. The use of components from mass productions assures comparatively low prices.

The direct closeness of the heat exchangers for the supply and extraction makes sure that, if solar heat (or heat from a different source) is available, this heat can be quickly used and without a great loss of temperature, because of the very high heat conductivity of the copper tubes (compared to the solid material), resulting in warming up the tubes with their contents first. A certain amount of warm water is always present and can be used directly because of the relatively great volume of the tube bundles (table I).

Another advantage of the direct closeness of both heat exchanger types is that the extraction of heat cools down the area for the heat supply first. This increases the collector output when supplying heat, because the efficiency of a collector drops with higher fluid temperatures.

Compared with a standard warm water storage, there is only a small reservoir of warm water ready to use in a * solid material storage and this water is permanently heated up by high temperatures of the heat supply, minimizing the risk of contamination with legionella, because high temperatures kill this bacterium.

Because the highest temperatures are always in the inner part of the storage and not near the storage surface (inner surface of the insulation), the loss of heat is limited. If the walls of the storage are flat, it is possible to use an insulation based on a vacuum. Either by using vacuum insulation panels (VJPs) or by building a gas-tight cover around the storage material with space between, which must be evacuated with a vacuum pump. Both ways lead to ahighly effective heat insulation because of the extreme low heat conductivity of vacuum.

If evacuated tube solar collectors are the supply source of the heat, their capability of producing heat in a temperature range above 100°C with high efficiency can be used. With the right design of the storage system ai inten,iption of the heat supply is not required and stagnation can be avoided. The use of evacuated tube solar collectors leads to a heat contents comparable to or higher than that of a water storage. 1!

tube diameter outside tube surface volume volume (wall thickness = 1mm (wall thickness = 0,8mm lengths0m) Iength= SOm) [mm] [m2] [I] [1] 12 1,88 3,93 4,25 14 2,20 5,65 6,04 2,36 6,64 7,05 18 2,83 10,05 10,56 22 3,46 15,71 16,34 28 4,40 26,55 27,37 5,50 42,76 43,81 Table I The measurements of copper tubes for drinking water and heating systems

Claims (1)

1. <claim-text>8. Claims 1 Insulated solid material storage systems, which are assembled on site to store and supply sensible heat through direct and indirect heat exchangers.</claim-text> <claim-text>2 A storage system according to claim 1, with three main parts: the storage material, the heat exchangers and the surrounding walls, are all mass produced, which are easy to process and to adapt, and by that allowing the construction systems from small to very large volumes (may consist of a number of modules), using the conditions on site optimally and to which more modules can be added easily.</claim-text> <claim-text>3 A storage system according to claim 1, which storage material allows to store the heat in a range above 100°C, and by that increasing the heat capacity and allowing the use of evacuated tube solar collectors without stagnation, making an automatic working system for the evacuation of the collectors redundant.</claim-text> <claim-text>4 A storage system according to claim I, which storage material is solid, allowing an exact spatial storing of the heat, resulting in a temperature of the stored heat only a little bit lower than the temperature of the heat source.</claim-text> <claim-text>A storage system according to claim I, which heat exchangers consist of one or a number of tube bundles, having different forms like helix, spiral, circle, an U, etc. with the same or different dimensions or diameters, with spaces between the windings if necessary, fUnctioning equally or differently and can be segmented.</claim-text> <claim-text>6 A storage system according to claim 1, with heat exchangers according to claim 5, containing only a small quantity of heated up drinking water, because most heat is stored in solid material and because of this and because the drinking water always is it' the area of the highest storage temperature, the risk of contamination with legionella is distinctly reduced compared to normal water storages.</claim-text> <claim-text>7 A storage system according to claim 1, which construction allows one or more areas with limited storage temperature, to avoid for example the calcination of fresh water.</claim-text> <claim-text>8 A storage system according to claim I, which construction has out of practical reasons flat sidewalls, allowing the use of a vacuum based heat insulation with an extreme low heat conductivity.</claim-text> <claim-text>9 A storage system according to claim I, which can consist of a nunther of storage modules, each usable alone or in combination depending on the current input of heat, what makes the system very energy efficient.A storage system according to claim I, which construction allows the use of different materials with different properties in one storage.II A storage system according to claim I, which is able to use the residual heat left in the collector.</claim-text>