GB2280746A - Electric storage heater - Google Patents

Abstract

An electric storage heater uses one or more containers 7 for a phase change heat storage medium which changes its state at a relatively low temperature, say less than 70 DEG C. Air drawn in towards the lower end of the heater 5 emerges from the upper end 6 after having passed over the surface of the containers containing the phase change material. The relatively low temperature of operation permits the output flow to be controlled by a damper (not shown) which provides a variable restriction to the outlet 6. <IMAGE>

Description

ELECTRIC STORAGE HEATERS This invention relates to electric storage heaters.

Such heaters are intended to be charged during periods when electricity is available at a reduced tariff. In the past, materials such as fire clay or the higher capacity iron oxide have been formed into bricks and heated with electric heating elements. The bricks reach relatively high temperatures (hundreds of degrees Centigrade) and it is therefore necessary to provide thermal insulating material between the bricks and the outer casing of the heater. As well as increasing the bulk of the heater, this also adds to the cost.

For this reason, it has been proposed to use as the heat storage medium a phase change material in which thermal energy can be stored as latent heat absorbed as the material undergoes a change of state: because this is a more efficient way to store heat, sufficient heat can be stored in a suitably sized volume of phase change material without it having to be heated to a high level e.g. a maximum temperature of well under 100"C is possible. A typical suitable phase change material is a hydrate which can exist as a solid, but which can release water of crystallisation when heated and dissolve in it to form a liquid. When heat is no longer applied, the material gradually reverts to solid form, returning the latent heat.

The invention provides a storage heater comprising electric heating means, phase change heat storage material which undergoes a change of state from solid to liquid when heated by the electric heating means, an outer casing having an inlet in its lower region and an outlet in its upper region, and a damper for restricting the outlet to control the flow of heated air leaving the outlet Such an arrangement makes it possible to control the heat output from a relatively large surface area with one damper.

Advantageously, the phase change heat storage material is contained in a pair of containers, opposed walls of each of which extend in substantially vertical planes and which are spaced apart from each other in a direction normal to those planes.

However, more than two such containers may be used, or just a single container.

The damper may be a pivotable flap.

Electric storage heaters constructed in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a perspective view of the heater; Figure 2 is a side view of the heater mounted on a wall; Figure 3 is an enlarged view of a part of the heater; Figure 4 is a partly cut-away view of an individual heating module of the storage heater; Figure 5 is a perspective view of a panel supporting an electric heating element; Figure 6 is a side view of the panel; Figure 7 shows an alternative form for the phase change material container of a heating module (on an enlarged scale); and Figure 8 shows an altemative form of individual heating module.

Referring to Figure 1, the electric storage heater consists of two heating units indicated generally by the reference numeral 1, arranged side by side. Each unit may be supported on a pair of U-shaped supports 2 and/or is secured to a wall by means of brackets 3. The outer casing 4 of each unit has an inlet 5 for air at its lower end and an outlet 6, in the form of a grille, at its upper end.

Each unit consists of a pair of identical heating modules 7, which are separated by a central upward channel 8, through which rises ambient air drawn in at the bottom of each unit, the air thus being heated before emerging through the outlet grille 6.

Referring to Figure 4, each heating module utilises a phase change material as the heat storage medium. A typical material is a hydrate e.g. sodium acetate trihydrate (NaC2 H3O2. 3H2O). When the hydrate is heated, it changes its state from solid to liquid, by virtue of dissolving in its own water of crystallisation. The temperature of the phase change material rises as it is heated from ambient, but then remains at a substantially fixed value while it is in the process of absorbing latent heat and changing to a liquid. In the case of the example of the suitable hydrate given, the fixed value is around 58"C. When this process is complete, the temperature would rise beyond this level if further heating were applied, but an over-temperature sensor is included in the module to prevent this.When the temperature of the liquid in the module exceeds a temperature a little above the temperature at which the phase change takes place, the sensor interrupts the flow of electricity to the electric heating means.

The latent heat absorbed to liquefy the phase change material is given out when the material cools. Thus, the heating can take place during a period when electricity is at a lower tariff (off-peak) e.g. overnight, and can give out its heat gradually over another period when electricity costs are higher.

The phase change material in each module is housed in a sealed container 9, which also houses the respective electrical heating means in the form of a heating element 10 distributed over a flat panel 11. Each container is provided with two filling holes (not shown) at the upper end to enable the container to be filled at the outset.

The holes are then sealed. The panel 11 is made of two sheets of plastic film which are secured together e.g. by heat bonding with the element sandwiched between. The element may be made of foil e.g. aluminium pierced to form a continuous circuit consisting of linked rows of undulations. The interior of the container is provided with integral grooves 12 to locate the panel. The upper end of each casing is sealed with an air space above the phase change material to allow for the resulting expansion of the phase change material.

The walls of the container are parallel to the panel, and are arranged in a substantially vertical plane in use, and only a relatively thin layer of phase change material therefore lies on each side of the element. This enables a large proportion of the latent heat stored in the dissolved phase change material to be transmitted to the room in which the heater is housed. If the container were thicker, the phase change material would solidify first at the walls of the container and, because such solidified material is a poor conductor, the inner layers adjacent the panel would not be able to transfer efficiently their heat to the surroundings. They would rather tend to solidify more slowly than the outer layers.

The phase change material contracts when it cools and expands when heated.

Clearly, when it expands, it stresses the walls of the container, and since this alternate heating/cooling cycle is something that happens on a regular basis, there is a risk of long term damage to the container. It is to overcome this problem that the heating capacity of the element in its upper region is greater than over lower regions. This then ensures that the phase change material dissolves from the top downwards thereby substantially obviating stresses on the walls of the container. This feature forms the subject of our co-pending patent application no. P/9340/CRE

The grading of the heating is achieved by arranging that the pitch of the undulations is least in the top row and progressively increases for the lower rows i.e.

the undulations are closest spaced in the top row and progressively less closely spaced for each succeeding lower row. In addition, the upper row of undulations extends above the surface of the phase change material in the container.

Referring to Figure 2, heat is transferred from the containers by virtue of convection currents of air A, B, C passing, respectively, between the rear container and the wall against which the heater is mounted, between the pair of containers, and over the outer surface of the casing. The intake for the air flows A, B is the air inlet 5 at the base of the heater.

Because the maximum temperature reached by the phase change material is around 58"C, it is not necessary to surround the containers with insulation (as it is with existing storage heaters employing iron oxide bricks as the heat storage medium, since the bricks can reach temperatures of several hundred degrees Centigrade). The outer casing is desirably in conductive contact with the front wall of the front container, and such a surface presents relatively little risk to the occupants of the room if touched. It is possible to position the modules close to each other since there is no need for insulation at the sides of each unit. It follows that further units may be placed side by side with each other to increase the capacity of the heater, although one unit could be used on its own if desired.The only major item which is individual to each heater made in this way is the outer casing.

It is not easy to control the amount of heat stored in accordance with the room's heat required, as dictated by the prevailing weather and it is thus important to effect a control upon the amount of heat actually discharged from the appliance. In accordance with the invention, the heat output of the storage heater is controllable by means of a damper in the form of a flap valve 13, which is pivotable about its lower edge to open and close the opening formed by the grille 6. The flap valve 13 is controlled by a bi-metallic component 14 which is secured at 15 to the roof of the outer casing. When the heater has been fully charged, in the sense that sufficient heat has been applied for all the heat storage material to have been liquified, the right hand end of the bi-metallic component is deflected to one extreme position in which the flap valve closes the opening in the grille.As the heater cools, the temperature in the upper part of the casing likewise cools, and the right hand end of the bi-metallic component gradually retracts in a left hand direction, pulling the flap valve 13 open: this boosts the convection flow through the heater and hence the amount of heat transferred to the room. It is safe to occlude any or all of the associated ventilation channels because in normal duty the temperature inside cannot exceed the phase change temperature, say 60"C and any abnormal excursion above this would cause the safety cut-out to operate at a level say but 100C above the norm.Because of this fact a damper can be positioned at the exit air grille 6 and the internal ducting arranged to collect all the air from the ventilation channels, most importantly that obtaining between the rear panels of the modules and the wall on which the heater is mounted. These two facts make this damper arrangement different from any damper hitherto fitted to a heater. Thus, such an arrangement would not be possible with a storage heater of the iron oxide brick type, since a damper closing the outlet at the upper end of the heater would result in the temperature in the casing being raised to dangerous levels, of hundreds of degrees Centigrade, greatly increasing the risk of fire if a fabric came into contact with the heater.

Of course variations may be made from the above embodiment without departing from the scope of the invention. For example, it is not necessary for the pitch of the undulations on the panel to increase progressively from one row to the next. So long as they are closest spaced in the upper row, the other rows could have a constant, greater spacing. Also, if desired there could be more than one element per panel.

It is not essential for the panel to be supported by internally extending grooves.

The container could be shorter and grooves could be formed by pinching in the ends of the container. Alternatively, the panel itself could be provided with projections to locate it in the container, or, as shown in Figure 8, opposed walls of the container may be provided with depressions 16 arranged to engage regions of the panel free of element, in order to hold the panel in position. As shown in Figure 8, the upper region of the container could be cut away so that a tab 17 projected from each panel, to facilitate electrical connections to the element. Filling holes 18, 19 are shown in Figure 8.

If desired, the motive power for moving the flap valve 13 could be provided by a hydraulic system. Thus, a bulb containing a suitable fluid could be positioned below the main body of the heater e.g. mounted on the supports 2, the bulb being in communication with bellows, such that movement of the bellows consequent on rise or fall of the vapour pressure of the fluid (as a result of temperature changes of the bulb), would be translated into pivoting of the flap valve 13.

There is no need for the electrical connections to be at the top of each container, if desired, they could be at the lower end. The walls of the container could be provided with ribs for stiffening purposes (Figure 7). The containers could be made by blow moulding an extruded tube of plastics material between two dies. It is not essential for the outer casing to extend over the front surface of each module. The front surface of the front module could be finished in such a way as to form a suitable front surface for the heater. It would only then be necessary for a housing to be secured on top of the modules, and for a tray to be provided to secure the modules at the lower end for mounting on the supports 2. The elements need not be pierced metal foil, but could be printed if desired. A typical size for a module is in the region of 500 mm by 350 mm by 30 mm. A suitable intermodule air gap would be 20 mm and a suitable gap between the rear module and the wall would be around 15 mm.

While two modules per heating unit have been illustrated, only one module could be used, or three or more modules spaced in the direction of the normal to the surface, could be used.

Claims (8)

1. Astorage heater comprising electric heating means, phase change heat storage material which undergoes a change of state from solid to liquid when heated by the electric heating means, an outer casing having an inlet in its lower region and an outlet in its upper region, and a damper for restricting the outlet to control the flow of heated air leaving the outlet.

2. A storage heater as claimed in claim 1, in which the phase change heat storage material is contained in a pair of containers, opposed walls of each of which extend in substantially vertical planes and which are spaced apart from each other in a direction normal to those planes.

3. A storage heater as claimed in claim 2, in which the heater is provided with a spacer to space apart the rear wall of the rear container from an adjacent wall of a room.

4. A storage heater as claimed in any one of claims 1 to 3, in which the damper is a pivotable flap.

5. A storage heater as claimed in any one of claims 1 to 4 in which the damper is controlled by means responsive to the temperature within the outer casing.

6. A storage heater as claimed in claim 5, in which the means are contained in an upper region of the casing.

7. A storage heater as claimed in claim 6, in which the means include a bi-metallic component.

8. An electric storage heater substantially as herein described with reference to the accompanying drawings.