EP0429875A1

EP0429875A1 - Storage heater

Info

Publication number: EP0429875A1
Application number: EP90120844A
Authority: EP
Inventors: Keith Jarvis; Arthur Samuel Morrison; Derek Herrell
Original assignee: Norton Co
Current assignee: Saint Gobain Abrasives Inc
Priority date: 1989-11-01
Filing date: 1990-10-30
Publication date: 1991-06-05
Also published as: GB2237628A; GB8924591D0

Abstract

A laminar heat storage block (57) for use with a storage heater, which block includes a relatively thick layer (12′) of an electrically conductive material which has a high volumetric heat capacity and a relatively thin layer (14′) of an electrically insulating material having a volumetric heat capacity which is relatively high but lower than the volumetric heat capacity of the thick layer (12′). In one form of the invention the block (10) does not have an electrical heating element for use in a heater which has an electrical resistance heater. In a still further form of the invention, a layer (58) of an electrically conductive resistance heating material is fixed to the outer face surface of the thin layer (14′). The invention also consists of a method of making all forms of the storage block described above. The method includes forming a green brick in a die cavity from mixtures of clay and particulate materials having the desired thermal and electrical conduction and sintering the green brick, the heating elements applied to the electrically insulating face of the sintered brick.

Description

The present invention is directed generally to a storage heater and specifically to a heat storage block and a method of making same which storage block is heated by an electrical heating element for a first specified period of time, the stored heat being given out during a second specified period of time.
The storage block component of a storage heater is heated by an electrical heating element during off-peak periods when electric rates are low. At the end of the off-peak period, electricity to the electrical heating element is shut off and the heat which is stored in the block is given up to the surroundings. The unit sometimes incorporates a system which enables electricity to be used outside of the off peak period to boost the amount of stored heat.
Many types of synthetic and naturally occurring materials have been used for the storage block of storage heaters. Some of the general criteria for selection of a particular material are:

1. high volumetric thermal capacity
2. chemical strength
3. high electrical resistivity
4. resistance to thermal shock
5. resistance to corrosion

There are synthetic materials which have very high volumetric heat capacity but which are too expensive for use in storage heaters. concentrated natural ores are much cheaper and have been used extensively. Olivine, a mixture of fosterite and fayalite, is a naturally occurring mineral which has been used extensively for the heat storage block in storage heaters. Olivine meets most of the criteria for a heat storage block, including high specific heat and volumetric heat capacity and is electrically insulating. The insulating quality of olivine enables an unsheathed electrical heating element to be used for heating the block. The unsheathed heating element is less expensive and transfers heat to the block more efficiently. Olivine has been supplanted as the mineral of choice for storage heaters by certain iron ores such as magnetite and fayalite which have a greater volumetric heat capacity than olivine. However, the iron ores readily conduct electricity so that a sheathed heating element must be used. The greater volumetric heat capacity of the iron ore enables a smaller heat storage block to be used with the heater. This reduces the overall size of the heater for a given capacity. The cost and space saving advantages of the iron ore heater blocks are offset by the disadvantages of using a sheathed heating element. The sheathed heating element is more costly than the unsheathed heating element, and is less efficient in transferring heat to the heater block and has a limited life. The constant high temperature of the heater block has an adverse effect on the insulating sheath of the heater element. These and other difficulties experienced with the prior art heat storage blocks have been obviated by the present invention.
It is, therefore, the principal object of the invention to provide a heat storage block which is composed of a material having a high volumetric heat capacity and which utilizes an unsheathed heating element and a method of making same. For solving these objects, the invention provides a heat storage block according to independent claim 1 and a method of making a heat storage block according to independent claims 13 and 14. Further advantageous features of the block and the method are evident from the dependent claims, the description and the drawings. The claims are intended to be a first non-limiting approach of defining the invention in general terms.
The invention provides a heat storage block which consists primarily of an electrically conductive material which has a high volumetric heat capacity and which utilizes an unsheathed electrical heating element.
A further aspects of the present invention is the provision of a heat storage block which utilizes natural occurring and inexpensive materials having high volumetric heat capacity and a relatively inexpensive unsheathed electrical heating element.
It is another aspect of the present invention to provide a method of making a heat storage block which is a composite of an electrically conductive material having a high volumetric heat capacity and a material which is electrically insulating and which has a relatively lower volumetric heat capacity and which is capable of utilizing an unsheathed electrical heating element.
A still further aspect of the invention is the provision of a heat storage block which is composed of a material having a volumetric heat capacity and which has at least one electrically insulating face for use in a storage heater having an electrical resistance heater.
With these and other aspects in view, as will be apparent to those skilled in the art, the invention resides in the combination parts set forth in the specification and covered by the claims appended hereto.
In general, the invention consists of a composite heat storage block for use with storage heaters for storage and dissipation of heat. The heater block comprises a first layer of a solid crystalline compound which is electrically conductive and which has a substantially high volumetric heat capacity and a second layer of a solid crystalline compound which is electrically insulating and which has a volumetric heat capacity which is substantially high but relatively lower than the volumetric heat capacity of the first layer. The second layer of material is bound to the first layer and defines a heating face surface close to which is mounted a open wire electrical heating element for transmitting heat to the heater block. More specifically, the first layer consists of an iron ore, preferably magnetite, and the second layer consists of olivine. The heater block is formed by filling a die cavity having the shape of the block which is to be formed to a first level with a first mixture of water, clay and particles of a first electrically conductive mineral such as magnetite. A second mixture of water, clay and particles of a second electrically insulating mineral, such as olivine, is added to the die cavity on top of the first layer to a second level. The first and second mixtures are pressed within the die cavity to form a compacted green brick which is removed from the die cavity and allowed to dry. The dried brick is heated within a kiln to a predetermined temperature to affect sintering of the brick. An open wire or unsheathed heating element is then mounted close to the face side of the brick which has the electrically insulating material.
The character of the invention, however, may be best understood by reference to one of its structural forms, as illustrated by the accompanying drawings and which:

FIG. 1 is a perspective view of a heater block embodying the principals of the present invention,
FIG. 2 is a diagrammatic view of a heat storage mechanism utilizing the heater block of the present invention,
FIGS. 3-7 illustrate the steps of forming the heater block of the present invention,
FIG. 8 is a flow diagram of the steps of forming the heater block of FIG. 1,
FIG. 9 is a perspective view of a first modified heater block,
FIG. 10 is a flow diagram of the steps of forming the heater block of FIG. 9, and
FIG. 11 is a perspective view of a second modified heater block.

Referring first to FIGS. 1 and 2 which best show the general features of the invention, the heat storage block of the present invention is generally indicated by the reference numeral 10 and comprises a first relatively thick layer 12 and a relatively thin layer 14. The face 18 is placed adjacent to the heating element and heat is transmitted through this face into the body of the block. When the element has been turned off, air flows over this face and is heated from the brick by convection.
Referring particularly to FIG. 2, the heat storage blocks 10 are shown as part of a storage heater, generally indicated by the reference numeral 24 During the heating cycle, heat is transferred by radiation and convection from the heating element 20 to the heater blocks 10. The heat is transmitted through the block by conduction. The numeral 16 refers to the rear surface of the heater block 10. Number 22 is a source of electricity and 21 identifies the lead wires to the block 10. The blower portion of the complete storage heater assembly is indicated by number 24. The insulation (25) covers all surfaces of the heater block 10 except the rear surface 16 from which the heat extracted by the air blown by the blower portion 24. The thermal insulation 25 protects the heater block 10 from becoming too hot.
After the heating cycle has been completed air at room temperature enters the bottom of the heater and by a process of natural convection rises through the heater around the heating elements. This air draws heat from the insulated face of the heater block and vents out through the top of the heater.
* 1 Cal = 4.19 J
** cc = cubic centimeter (cm³)
In the preferred embodiment of the invention, the layer 12 is from 7 to 15 times the thickness of the layer 14. For example, a block having a heat discharge or a heating face dimension of 230 mm by 220 mm would have a layer 14 thickness of between 3 mm and 6 mm and a layer 12 thickness of between 42 mm and 45 mm. Preferably, the layer 12 is an aggregate of iron ore particles bonded by clay i.e. fine particles of hydrous aluminum silicates. The volumetric heat capacity of the layer 12 is substantially .87 Cal*/cc°**C. Preferably, the layer 14 is an aggregate of olivine particles bonded by clay and fused to the layer 12. The preferred iron ore of the present invention is magnetite Fe₃O₄ which has a high volumetric heat capacity but readily conducts electricity. The olivine layer 14 is a complex silicate of magnesium and iron, i.e. fosterite and fayalite. The olivine layer 14 has a volumetric heat storage capacity which is substantially .70 Cal/cc°C. However, the olivine layer 14 is electrically insulating. Since the block consists principally of magnetite, the block has a higher volumetric heat storage capacity than an olivine. block of the same size. Although the olivine layer 14 has a lower per unit heat storage capacity than the layer 12, its volumetric heat storage capacity is significant so that the overall heat capacity of the block 10 is only slightly reduced from that of an all magnetite block of the same size. However, the insulating capacity of the olivine layer 14 enables an unsheathed electrical heating element 20 to be sited on the heating face 18 in the same manner as the all olivine heat storage block of the prior art. The combination of magnetite and olivine in the example given above is preferred, since these two minerals are naturally occurring and inexpensive, and are thermally compatible. This thermal compatibility insures that the olivine layer will not separate from the magnetite layer after repeated thermal cycling. However, it is contemplated that the concept of the present invention can be applied to any high density electrically conductive material which can be combined with a thin layer of lower density electrically insulating material. Although it is essential that the thin layer of the heat storage block is electrically insulating it is also important that the thin layer has a relatively high volumetric heat capacity and a relatively high heat conductivity. The relatively high heat capacity of the thin layer does not significantly reduce the overall heat storage capacity of the block and the relatively high heat conductivity of the thin layer maintains the efficiency of heat transfer from the heating element to the thin layer and from the thin layer to the thick layer of the block.
The method of making the heater block 10 of the present invention is illustrated in FIG. 8, using by way of example the mineral olivine for the thin layer 14 and the mineral magnetite for the thick layer 12. The first step of the method as shown in block 40 consists of preparing a first mixture of magnetite particles, clay and water. This first mixture is introduced into a die cavity such as the cavity 28 of a die 30, as depicted in block 42 and illustrated in FIG. 3. The die cavity 28 is filled with the first mixture, indicated by the reference numeral 26, to a first level 27. The third step of the process, indicated by block 44, consists of preparing a second mixture of olivine particles, clay and water. The second mixture, indicated by the reference numeral 32, is introduced into the die cavity 28 on top of the first mixture 26 to a second level 34, as indicated by block 46 in FIG. 8 and illustrated in FIG. 4. The first and second mixtures 32 and 26, respectively, are then compressed to form a compacted and green or unsintered brick, as indicated by block 48 in FIG. 8 and illustrated in FIG. 5. The compacting of the layers 32 and 26 can be accomplished in any known manner, as for example by the use of a ram 36 as shown in FIG. 5. The green brick, generally indicated by the reference numeral 38, is then removed from the die 30 as indicated by block 50 in the diagram of FIG. 8 and illustrated in FIG. 6. The green brick 38 is dried, as indicated in block 52, by means of a conventional drying method. The green brick 38 is then sintered, as indicated in block 54 and illustrated in FIG. 7. The sintering of the brick 38 is accomplished in a conventional kiln 39 which includes a heating element 41. The sintering temperature in the kiln is between 1050°C and 1200°C. This temperature effectively sinters the clay within each of the layers 26 and 32 for bonding the magnetite and olivine particles.
1 inch = 2.54 cm
Referring to FIG. 9, there is shown a first modified heat storage block, generally indicated by the reference numeral 57. The block 57 comprises a first relatively thick layer 12′ and a second relatively thin layer 14′ which are identical to the layers 12 and 14, respectively, of the heat storage block 10. The layer 14′ has an outer heating face 18′ to which is fixed a layer 58 of an electrically conductive material. Electrical leads 59 are connected to the layer 58 for operatively connecting the layer 58 to a source of electrical power. Although the layer 58 is electrically conductive it has sufficient resistance to electrical flow to enable the layer 58 to function as a resistance heater when an electrical current is applied to the leads 59. The layer 58 covers at least 60% of the surface area of the face 18′ (70% coverage being ideal) and is spaced from the peripheral edge of the face 18′ a minimum distance of 10mm. This keeps the edges of the block clear of conducting material and avoids shorting out when the block 57 is installed as part of an electric storage heater. The thickness of the layer 56 is preferably less than .20 inches*.
Referring to FIG. 10, the method of making the heat storage block 57 is similar to that for making the block 10. The steps for forming a green brick for the block 57 are the same as for forming the green brick for block 10. The steps which are depicted in FIG. 10 by diagrammatic blocks 40′, 42′, 44′, 46′, 48′ and 50′ identical to the steps depicted by diagrammatic block 40, 42, 44, 46, 48, 50, 52 and 54, respectively, of FIG. 8. The block 57 is completed by depositing an electrically conductive material onto the face 18′ of the green brick after the brick has been sintered as depicted in diagrammatic block 60 of FIG. 10.
Referring to FIG. 11, there is shown a second modified heat storage block, generally indicated by the reference numeral 70. The block 70 comprises a first relatively thick layer 12˝ and a second relatively thin layer 14˝ which are identical to the layers 12 and 14, respectively, of the heat storage block 10. The layer 14˝ has an outer heating face 18˝.
The heating face 18˝ of the heat storage block 70 does not have an electrical heating element applied thereto. The block 70 is adapted to be used in a conventional storage heater which has an open wire electrical resister element for providing heat to the surface 18˝ of the heat storage block 70.
A conventional electrical resister storage heater includes a metal case which has a layer of insulating material such as ceramic fibre immediately adjacent the case. An electrical resistance heater is located in the center of the case and a heater block is placed with the case so that the olivine face of the heat storage block 70 abuts the resistance heater. Depending on the size and design of the storage heater, a second heat storage block is placed on the opposite side of the resistance heater element. The bottom wall of the case has slots to allow cool air to enter the case. The top wall of the case also has slots to allow the heated air to pass from the heater into the room.
The method of making the heat storage block 70 is identical to that of making the heat storage block 10 except for the last step of fixing an open wire electrical heating element to the heating face 18˝
Clearly, minor changes may be made in the form and construction of this invention and in the embodiments of the process without departing from the material spirit of either. Therefore, it is not desired to confine the invention to the exact forms shown herein and described but it is desired to include all subject matter that properly comes within the scope claimed.

Claims

1. A block (10) for storage and dissipation of heat, especially for use in a storage heater having an electrical resistance heater for providing heat to at least one of the storage blocks and for dissipating heat from said storage blocks, comprising :

(a) a first layer (12) of a solid compound which is electrically conductive and which has a substantially high volumetric heat capacity, said first layer (12) forming a first heat discharge face surface at one side of the block, and

(b) a second layer (14) of a solid compound which is electrically insulating and which has a volumetric heat capacity which is substantially high but relatively lower than the volumetric heat capacity of said first layer, said second layer (14) being bound to said first layer (12) and defining a second heating face surface (18) at the opposite side of said block from said first heating face surface.

2. A block as recited in claim 1, wherein the thickness of said first layer (12) is from seven to fifteen times that of said second layer (14).

3. A block as recited in claims 1 or 2 wherein the volumetric heat capacity of said second layer is substantially .70Cal/cm³°C and the volumetric heat capacity of said first layer is substantially .87 Cal*/cm³ °C.

4. A block as recited in one of the preceding claims wherein the composition of said second layer (14) comprises olivine and a mineral bonding agent and the composition of said first layer (12) comprises magnetite and a mineral bonding agent.

5. A block as recited in claim 4, wherein the olivine of said second layer is a mixture of fosterite and fayalite.

6. A block as recited in claim 4, wherein said olivine is (Mg, Fe)₂ SiO₄.

7. A block as recited in one of the preceding claims wherein the bonding agent for each of said first and second layers is clay.

8. A heat storage block as recited in one of the preceding claims comprising a third layer of an electrically conductive resistance heating material on said second face surface, the peripheral edge of said third layer being preferably spaced from the peripheral edge of said second layer.

9. A heat storage block as recited in claim 8, wherein the peripheral edge of said third layer is spaced from the peripheral edge of said second face a minimum distance of substantially 10mm.

10. A heat storage block as recited in claim 8 or 9, wherein said third layer covers at least 60% of the surface area of said second face.

11. A heat storage block as recited in one of claims 8 to 10, wherein said third layer is an alloy of primarily iron, aluminum and chromium.

12. A heat storage block as recited in claim 11, wherein said alloy also includes a minor amount of yttrium.

13. A method of forming a block for storage and dissipation of heat especially for use in a storage heater having an electrical resistance heater for providing heat to at least one of the storage blocks and for dissipating heat from said storage blocks comprising the following steps:

a. Forming a first mixture of water, clay and particles of a first mineral which is electrically conductive and which has a substantially high volumetric heat capacity,

b. forming a second mixture of water, clay and particles of a second mineral which is electrically insulating and which has a volumetric heat capacity which is substantially high but relatively lower than the volumetric heat capacity of said first mineral,

c. introducing said first mixture into a die cavity having the shape of the block to be formed until the cavity is filled to a predetermined level,

d. introducing said second mixture into said cavity on said first mixture until said cavity is filled to a second predetermined level,

e. pressing said first and second mixture under a predetermined pressure to form a compacted uncured brick having two distinct layers of material, a first layer of said first mixture which has a first face surface at one end of the brick and a second layer of said second mixture which has a second face surface at the opposite end of the brick,

f. drying said green brick, and

g. heating said brick to a predetermined temperature to affect sintering of the brick.

14. A method of forming a block for storage and dissipation of heat especially according to claim 13, comprising the following steps:

a. introducing a first wet mixture of magnetite and clay particles into a die cavity having the shape of the block to be formed until the cavity is filled to a first predetermined level,

b. introducing a second wet mixture of olivine and clay particles into said die cavity on top of said first magnetite and clay mixture until said cavity is filled to a second predetermined level,

c. pressing said first and second mixture preferably under a predetermined pressure to form a compacted green brick having two distinct layers of material, a first layer of said first mixture which has a first face surface at one end of the brick and a second layer of said second mixture which has a second face surface at the opposite end of the brick,

d. drying said green brick ,and

e. heating said dried green brick to a predetermined temperature to affect sintering of the brick.

15. A method as recited in claims 13 or 14 in which the pressing is effected in said die cavity and in which the compacted green brick is removed after this from said die cavity and/or wherein the thickness of said first layer is from seven to fifteen times that of said second layer.

16. A method of forming a block as recited in one of claims 13 to 15 wherein said olivine is (Mg, Fe)₂SiO₄ and is a mixture of fosterite and fayalite.

17. A method of forming a block as recited in one of claims 13 to 16 wherein said dried green brick is heated in the sintering step to a temperature between 1050°C and 1200°C.

18. A method as recited in one of the claims 13 to 17 comprising the further step of depositing an electrically conductive resisitance heating material on said second face surface.

19. A method of forming a block as recited in claim 18 wherein said electrically conductive material is spaced from the edges of said second face a minimum distance of substantially 10mm.

20. A method of forming a block as recited in one of claims 18 or 19, wherein said electrically conductive material covers at least 60% of the surface area of said second face.

21. A method of forming a block as recited in one of claims 18 to 20, wherein said resistance heating material is an alloy of primarily iron aluminum and chromium and secondarily of yttrium.