EP0029010B1 - Electric storage heater - Google Patents

Abstract

The accumulation block comprises an alternating series of accumulation elements (10) and transfer elements (11). The transfer elements are made of a thermo conductive material and are provided with combustion channels (14) delimiting between each other a transfer blade (14a) in contact with the adjacent accumulation elements. These transfer elements (10) also comprise grooves (12) wherein electric resistors (13) are nested. This arrangement contributes to increase the thermal transfer coefficient between the resistors (13) and the accumulation elements (10) and between the latter and the combustion channels (14). The arrangement may also provide heat directly to the combustion channels (14) without accumulation in the accumulation elements (10) for direct heating.

Description

Technical area

The present invention relates to an electric storage heater in which air circulates in a closed circuit passing through an air-water heat exchanger.

State of the art

Electric storage heaters allow heat to be stored during off-peak hours of power consumption, especially at night, and to restore it during the day when electricity consumption is higher, which allows the load to be better distributed. of the distribution network. In return, the user benefits from a reduced rate which represents significant savings.

Such devices have already been proposed, designed essentially for storing a sufficient quantity of heat during the eight hours of the night tariff to heat during the remaining sixteen day hours a given habitable volume and for a heat loss corresponding to the coldest days for a specific climatic region. The design of such devices is economically questionable given that we size these devices according to exceptional circumstances that occur only a few times a year. It follows that a large proportion of the storage volume is only necessary for a few days a year. The same applies to the additional insulation material intended to insulate this storage volume used exceptionally. Consequently, the corresponding investment for the storage material and the insulating material cannot be amortized by the price difference between off-peak hours and peak hours for this part of the storage volume used sporadically. In addition, the storage volume is a dead space in the home that should be reduced as much as possible.

Reducing the storage volume, however, faces another difficulty. If it is admitted that it is not necessary to size the accumulation block for the coldest days, it must nevertheless be possible to compensate for the heat loss of the habitat during these days. However, the refractory materials used for storage are at the same time poor heat conductors so that a significant time will elapse between the switching on of the resistors and the moment when the heat given off by them can be recovered. In addition, additional heat will be stored in the accumulation block at the full rate which increases consumption and unnecessarily overloads the network during daylight hours.

One could certainly consider replacing these refractory materials with materials that are good thermal conductors and with high storage capacity such as cast iron, however the price of cast iron is, with identical storage capacity at least twice as expensive as magnesia for example. However, the storage material alone constitutes a significant proportion of the price of the device, which cannot be compensated by the mere reduction of the insulation following the reduction in volume for equal storage capacity.

It has already been proposed to add to such a heater a boiler provided with an independent electrical resistance intended to directly heat the water supplying the central heating as soon as its temperature resulting from the heat exchange with the air flowing through the accumulation block drops below a certain threshold. The disadvantage of this solution lies essentially in the fact that it requires two independent heating circuits and two temperature control systems, which significantly increases the cost of the device.

Finally, all heaters of this type have a significant drawback resulting from the poor thermal conductivity of the storage material. The temperature gradient between the electric heating resistor and the storage block is therefore quite large, so that to heat the block to 650 ° C., the temperature of the resistors is appreciably higher and reaches 800 ° C. or even more. . We know that from 800 ° C, an armored resistance degrades quickly and that a breakdown can finally occur between the sheath and the resistance. It is therefore not uncommon in this kind of device to have to change an armored resistor whose price is high.

It has already been proposed, in patent GB-A-975,560, a thermal accumulation block formed of stacked refractory elements traversed by horizontal convection channels extending between two faces of the block adjacent to two spaces respectively connected to a distribution and to an air manifold. These channels are formed in two complementary portions formed on two adjacent refractory elements. A metal sheet is interposed between two adjacent blocks and extends through the convection channels to facilitate the removal of heat from the block. These metal sheets do not constitute a storage element but are only there as a thermal conductor. In addition, if these sheets allow the heat to be removed from the refractory blocks, they practically do not make it possible to facilitate the thermal loading of these blocks because they are not in direct contact with the heating resistors. This lack of contact makes Also direct heating during the day is less efficient. All the heat produced by the resistors cannot be removed as quickly as if a good contact existed between the resistors and the metal sheets, these being heated only by radiation. Furthermore, the arrangement of the convection channels in the refractory blocks has two drawbacks, firstly the refractory materials are brittle and do not allow the formation of a plurality of small channels increasing the surface of exchange with air for a section of given passage, then the walls of the convection channels are not thermally good conductors the exchange taking place essentially with the metal foil. This is a solution which is far from making the most of the presence of conductive elements in the block.

As for patent FR-A-1,542,246, it comprises a block formed of stacked refractory elements between which electrical resistances are embedded in a thermally good conductive material which also surrounds a column of water used for heat extraction accumulated in the block. The nature of the thermally conductive material is not specified, nor the mode of incorporation of the resistors in this material. Such a document apparently poses the problem without really giving a solution. In addition, it should be noted that the heating of the water is direct which practically does not allow good modulation of the heat transfer under penalty of working with very high vapor pressures.

DE-A-1 778 165 proposes to form an accumulation block of a core composed of elements with good thermal conductivity surrounded by an envelope composed of elements made of material with low conductivity. The elements of the core have grooves for receiving electrical heating resistors. Vertical convection channels are provided between the elements of the heart and the envelope. Such a structure necessarily creates a warmer area in the upper part of the core, since the elements with lower thermal conductivity are around, and therefore does not achieve the desired goal, so that the storage volume is not not optimally used since it will be necessary to stop the heating of the block when the top of the heart has reached the limit temperature supported by the resistors while the refractory elements will still be at a substantially lower temperature.

All these drawbacks explain the reason why this heating method is not very widespread despite the potential economic interest it presents and the regulatory effect it is likely to have on the electricity distribution which we seek to reduce. peak consumption which is very expensive in equipment and which therefore affects prices.

Statement of the invention

The object of the present invention is to remedy, at least in part, the above-mentioned drawbacks.

To this end, the subject of this invention is an electric storage heater comprising an accumulation block housed in a thermally insulating envelope and having an alternating stack of storage elements of refractory material and transfer elements of material. thermally conductive, this accumulation block being traversed by convection channels opening onto two of its opposite vertical faces and incorporating electrical heating resistors. This apparatus is characterized in that each of said transfer elements is formed by a parallelepipedal body made of cast iron extending from one to the other of said opposite vertical faces and that this body has on the one hand an alternating network of convection channels and conduction heat transfer fins formed on at least one of these faces adjacent to said accumulation elements and on the other hand at least one recess opening on one of the faces of said parallelepiped body and shaped to receive removably an electrical heating resistor.

Brief description of the figures

• La fig. 1 est une vue en élévation de l'ensemble de l'appareil.
• La fig. 2 est une vue en coupe agrandie selon la ligne II-II de la fig. 1.
• La fig. 3 est une vue en coupe agrandie selon la ligne III-III de la fig. 2.
• La fig. 4 est une vue en coupe d'une variante de la fig. 2.
• La fig. 5 est une vue en coupe d'une variante de la fig. 3.
• Les fig. 6 et 7 sont des vues en coupe de variantes de la fig. 2.
• La fig. 8 est une vue en coupe selon la ligne VIII-VIII de la fig. 7.

The accompanying drawing illustrates schematically and by way of example an embodiment and variants of the apparatus which is the subject of the present invention.

• Fig. 1 is an elevational view of the entire apparatus.
• Fig. 2 is an enlarged sectional view along line II-II of FIG. 1.
• Fig. 3 is an enlarged sectional view along line III-III of FIG. 2.
• Fig. 4 is a sectional view of a variant of FIG. 2.
• Fig. 5 is a sectional view of a variant of FIG. 3.
• Figs. 6 and 7 are sectional views of variants of FIG. 2.
• Fig. 8 is a sectional view along line VIII-VIII of FIG. 7.

Best way to realize the invention

The heater illustrated schematically in FIG. 1 is more particularly designed to heat the water of a central heating circuit. However, the invention is in no way limited to this application. In addition, only the main elements of this device are shown, the invention essentially relating to thermal storage.

This device comprises an insulating envelope 1 delimiting an enclosure divided into two superposed compartments, one 2 in which there is a tangential fan 3 and an air-water heat exchanger 4, the other 5 in which is a block of accumulation 6 which rests on the partition 7 separating the compartments 2 and 5. This partition 7 is pierced with two openings 7a and 7b which make the tangential fan 3 communicate with a distribution space 8 formed between one of the faces of the accumulation block 6 and the insulating envelope 1, and respectively the air-water heat exchanger 4 with a collecting space 9, formed between the face of the accumulation block 6 opposite the face adjacent to the distribution space 8 and the insulating envelope 1. The other lateral faces and the upper face of the accumulation block are in contact with the insulating casing 1 so that the air circulation between the tangential fan 3 and the air-water heat exchanger 4 can only take place 'through the accumulation block 6.

The structure of this storage block is illustrated in more detail in FIGS. 2 and 3. It is formed of an alternating succession of accumulation elements 10 made of a refractory material, such as magnesia, and transfer elements 11 made of a material which is a good thermal conductor such as cast iron. The accumulation elements 10 have a parallelepiped recess 10a in which is housed the transfer element 11.

This transfer element 11 has a groove 12 formed from parallel rectilinear segments connected by arcs of circles. This groove 12 is formed on one of the faces of the transfer element 11, adjacent to an accumulation element 10. It extends to a depth substantially equal to half the thickness of the transfer element 11 and serves as a housing for an armored resistor 13 fitted in this groove 12. A series of parallel convection channels 14 are provided on this same face of the transfer element as well as on the opposite face. Their section is small so as to pinch the flow and to favor its distribution over the entire height of the block 6. The cast iron separating the grooves from each other forms heat transfer fins 14a which are in contact with the accumulation elements 10 and provide thermal transfer by conduction between the transfer and accumulation elements 11 and 10. The depth of these convection channels 14 is chosen so that the armored resistance 13 placed in its groove 12 is located at a depth greater than the bottom convection channels 14.

In this embodiment with armored resistance 13 embedded in the transfer element 11, the presence of arcs of circles between the straight segments increases the thickness of the transfer element in cast iron, so that it is possible to reduce this thickness by removing the resistors from the transfer element at the end of each straight segment and replacing the shielded resistors 13 with ordinary resistors 13 '(fig. 4 and 5) housed in insulation tubes 15 made of alumina and connected at their ends by fittings 16.

This variant makes it possible to reduce the thickness of the transfer element 11 'by 1 cm by passing it by 25 mm in the embodiment of FIGS. 2 and 3 to 15 mm in the embodiment of FIGS. 4 and 5, which reduces the cast iron weight of the transfer elements by almost half for comparable performance. This reduction in thickness can obviously be compensated by a corresponding reduction in the depth of the parallelepiped recess 1 Oa.

Fig. 6 shows another variant in which the accumulation elements 10 ′ have in section the shape of an I capable of providing them with better mechanical resistance.

According to fig. 7, the accumulation elements 10 "have a rectangular cross-section and the transfer elements 11" then extend over all the surface of the accumulation elements 10 ". This variant makes it possible to improve the contact between the elements of transfer 11 "and accumulation 10".

In the embodiments of figs. 2 to 6, the advantage lies in the fact that the transfer elements 11, 11 ′ can be introduced into or removed from the accumulation block 6 like simple drawers. This notably facilitates the possible replacement of a resistor. In the case of the variant of FIG. 7, it is then possible to provide housings 12 "allowing the introduction and removal of the resistors 13" from one of the sides of the accumulation block 6 "so as not to have to dismantle it to change the resistors.

For this purpose, the transfer elements 11 "are formed in two parts 11" a and 11 "b to form between them housing 12". Each of the two parts 11 "a and 11" b has two flanges 17 which extend respectively along the two sides of the block 6 "adjacent to the spaces 8 and 9 formed between the envelope 1 and the block 6". These flanges 17 serve to retain the accumulation elements 10 ".

The two parts 11 "a, 11" b have recesses 18 (fig. 8) intended to receive an elastic member 19 intended to bear against the walls of the insulating envelope 1 so as to position the block 6 "in this envelope 1.

The convection channels not visible in Figs. 7 and 8. are formed on the external faces of the parts 11 "a and 11" b and extend transversely to the resistance 13 ". These convection channels pass through the flanges 17 so that these are crenellated.

Instead of using an armored resistor, the resistor 13 "can be a bare resistor, possessed on a ceramic insulating layer formed by a putty deposited in liquid form and then cured.

To prevent dust of magnesia from being deposited in the convection channels 14, a sheet 20 can be interposed between the elements. transfer elements 10, 10 ', 10 "and storage elements 11, 11', 11".

We will not describe here the control and adjustment systems of this device since they are outside the scope of the present invention. It should only be noted that an adjustment flap associated with a control device (not shown) is used to vary the air flow through the storage block 6.

The advantages between the accumulation block described according to any of the variants described and the solutions mentioned above all stem from the fact that the resistors 13, 13 ', 13 "and the convection channels 14 are in the transfer element 11 , 11 ', 11 "in cast iron and that this transfer element considerably increases the heat exchange surface by contact with the accumulation elements 10 made of refractory material.

This composite structure of the accumulation block 6 therefore improves the heating of the accumulation elements by reducing at the same time very strongly the temperature gradient between the resistors 13, 13 ′, 13 "and the accumulation block 6. This is as well as if the maximum temperature of the accumulation block 6 is 650 ° C., the temperature of the resistors 13, 13 ′, 13 "will be barely higher. In fact, when the resistors are in refractory material and heat it essentially by radiation, the maximum admissible power is between 1.5 and 2 W / cm2 with a high temperature gradient between the temperature of the accumulation block and that of resistance, whereas in the case of the present invention this gradient is greatly reduced for a power of 4 W / cm ² . It follows that the maximum temperature of the resistors 13, 13 ', 13 "when the accumulation block 6 is at 650 ° C, is very significantly below 800 ° to 900 ° C, which correspond to the critical temperatures for the life of the resistors.

In addition to this first advantage, it should also be mentioned that if the storage elements can be heated quickly without danger to the resistors, the restitution of this heat can also be rapid. In this example, we want to store 60 kWh to heat a home whose maximum possible loss is 15 kW, or a minimum autonomy of 4 hours. However, it is obviously not enough to be able to store 60 kWh, but it must be able, if necessary, to restore them quickly enough.

In the example described, the accumulation block 6 has a height of 1.2 m and a section of 0.4 × 0.5 m ² , the insulation being 5 cm thick. It has been calculated that, without the transfer elements 11, the restitution of the maximum 15 kW would require a number of convection channels such that the transfer elements 11 could not be produced in magnesia or in another refractory material which, because 'they are obtained by sintering, can not have a profile as finely cut as a block of cast iron. This amounts to saying that with an accumulation block 6 without the transfer elements 11 made of cast iron, it would be impossible to heat a dwelling whose maximum heat loss is 15 kW and that it would then be necessary to either an accumulation block of a larger volume, i.e. an auxiliary heater to provide this additional power if necessary.

Another important advantage of this device must still be noted. When the storage capacity is exhausted, it is essential to be able to ensure, without interruption or marked reduction in temperature, the heating of the water in the air-water exchanger 4. Since the resistors 13, 13 ', 13 "and the convection channels 14 are in the same transfer element; 11 in cast iron as soon as the resistors are again supplied, they heat the transfer element 11 and, like the tangential fan 3 circulates the air through the channels convection 14, the air instantly dissipates this heat without significantly heating the accumulation elements 10 so that the heat thus provided is immediately available, which is unthinkable with an accumulation block without transfer element. from this particularity that it is possible to realize an accumulation block whose capacity is chosen, for economic criteria, lower than the maximum loss of a habitat. It is indeed advantageous to be able to u use the same resistors and the same regulation system to store heat as needed and for direct heating. There is no need to double the resistors and the regulation system. In addition, the heating then being direct, the elements of accumulation do not accumulate heat while the apparatus uses electricity at a high rate, the storage being done only at the hours during which the electricity is distributed. at the reduced rate.

The apparatus described therefore offers, by its design, great flexibility of use and makes it possible to supply, if necessary, an extremely high quantity of heat reduced to the accumulation volume, hence an optimization of this volume and a reduction of the necessary investment and immobilized space, offering the possibility of placing this device in the living space itself, its dimensions on the ground can be reduced to that of an electrical household appliance such as a refrigerator , a stove or a washing machine, which allows to equip small dwellings without basement. Finally, the structure of the accumulation block allows it to be used either as an accumulation heater or as a direct heater using the same basic elements, which is also of great interest. In principle, the life of the resistors should be unlimited. mitée, however, the removable mounting of the transfer elements 11, 11 'or resistors 13 "allows to intervene without requiring the disassembly of the entire accumulation block 6 if necessary.

Claims (8)

1. Electrical heat storage apparatus comprising a storage block housed in a thermally insulated envelope and being composed of alternately stacked storage elements of refractory material and transfer elements of thermally conductive material, this storage block being traversed by convection ducts opening on two opposing vertical faces thereof and containing electric heating resistors, characterized in that each transfer element is formed of a cast iron parallelepipede shaped body extending from one to the other of said opposing faces, and in that said body includes on one hand a network of alternatively disposed convection ducts and heat transfer fins arranged on at least one of the faces adjacent said storage elements and on the other hand at least one cavity opening in one of the faces of said parallelepipede body and adapted for removably receiving on electric heating resistor.

2. Heating apparatus according to claim 1, characterized in that each storage element comprises on at least one of the two transversal faces relative to the storage block a parallelelepipede shaped cavity outspreading between the two faces of said storage block adjacent said portion of air circulation circuit, respectively, and adapted for receiving said transfer element.

3. Apparatus according to claim 1, characterized in that said electric resistor is a shielded electric resistor distributed over the surface of said transfer element and housed within- the body thereof at a depth exceeding that of said convection depth the value of which corresponds substantially to the diameter of said shielded resistor.

4. Apparatus according to claim 1, characterized in that a portion of said electric resistor is housed within a tubular insulator adapted in a groove between said ducts and parallel thereto and is connected externally to said transfer element to at least another portion of electric resistor housed in a second tubular insulator fitted in another groove parallel to the first one.

5. Apparatus according to claim 1, characterized in that said transfer element is divided into two parts between which there is managed a cavity for receiving said electric resistor, this cavity having an opening on one of the lateral faces of said block.

6. Apparatus according to claim 1, characterized in that two opposing sides of each transfer element is fitted with raised flanges for retaining between them said storage elements.

7. Apparatus according to claim 1, characterized in that resilient organs are disposed between said insulating envelope and the storage block said organs being retained by said transfer elements.

8. Apparatus according to claim 1, characterized in that an electrically insulating layer is interposed between said transfer element and said resistor, this layer being integral to said transfer element.

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