Background of the invention At present, all known design solutions of heat exchangers placed in vessels, mostly in thermal energy stores, require extensive heat exchange surfaces and relatively large volume vessels. This is most marked in cases where there is a high energy flux demand in relation to the accumulated energy in the vessel. The heat exchanger size needed to satisfy the required performance is governed by the low heat transfer coefficients associated with natural convection on the side of the heat exchanger immersed in the fluid contained in the vessel and, additionally, by the unwanted drop in temperature of the accumulated fluid due to disturbed stratification. A high energy draw-off results in convective currents being formed that disturb the stratification and result in the thermal homogeneity of the fluid content.

Present design solutions, due to the thermal homogeneity of the accumulated fluid, are limited in the amount of useful energy that can be drawn-off, defined as the amount of energy drawn-off before the flow temperature from the heat exchanger falls below the required value. For example, this is an important factor when low temperature heat sources are utilised. The cost of such designed heat exchangers and thermal energy stores are relatively high.

Additionally, large heat stores have greater heat losses, amplified by convective currents flowing on the inside surfaces of the stores that increase heat transfer.

Summary of the invention The above disadvantages are removed by an arrangement of the heat exchanger and the vessel, for example a thermal energy store, according to this invention. The substance of which is that it contains at least one dividing wall that with the wall of the thermal energy store form at least one discharge flow channel. The channel can be shaped, beginning in the top part and discharging in the bottom part of the vessel in which the heat exchanger is placed. The heat exchanger is preferably placed in the upper part of the channel and, for plain tube heat exchangers, should be arranged in at least two spirals so that the influence of the generated boundary layer on the surface of the heat exchanger is minimised, whilst with shaped tubes it is possible to use one row.

Preferably the heat exchangers are designed in such a way so that the resistance of individual spirals is the same.

It is preferable if at least one internal heat source, for example an electric heating element or a heat exchanger, is placed in the bottom part of the vessel.

Preferably the inlet from an external heat source, for example a boiler, is situated at the upper end of the discharge channel. It is also preferable if the return conduit to the charging circuit and the return conduit from the discharging circuit, for example a heating circuit, are situated in the bottom part of the vessel.

Preferably a device that lowers the entrance velocity is placed on the end of the conduit by which liquid enters the vessel so as to eliminate disturbance of the stratification.

It is advantageous to divide the volume of the vessel and the heat exchange surface area into two or more parts or vessels in such a way so that they are connected in series both the heat exchangers

or parts of a vessel or individual vessels.

The advantage of the invention is that it raises the critical coefficient of natural heat transfer of the heat exchanger on the side immersed in the liquid in the vessel by means of its arrangement which disturbs the developing boundary layer on the heat exchanger surface on the side immersed in the liquid and also by means of the discharge channel, with the heat exchanger situated in its upper part, which accelerates the gravity (buoyancy) flow of the liquid contained in the vessel over the heat exchanger. When designing the heat exchanger the temperature of the stored liquid, the temperature of the liquid entering and leaving the heat exchanger, the volume flow and the geometrical shape of the store have to be considered.

The heat exchanger in the discharge channel cools the liquid and fills the discharge channel with the cooled liquid which forms a column of cooled liquid and thereby generates a driving force which accelerates the flow across the heat exchanger and thereby raises the coefficient of heat transfer above the usual values associated with heat exchangers immersed in vessels.

The consequence of the use of the discharge channel is the separation of the flow of the cooled fluid from the stored fluid which enables a vertical piston movement of the stored liquid when the stored energy is being drawn off and thereby conserves stratification, i. e. the temperature gradient, and enhances the efficiency of the thermal energy store in the delivery of the heated liquid at the required parameters.

Thus designed heat exchangers and vessels are smaller than currently used and therefore cheaper.

The heat losses of thus designed thermal energy stores are lower,

partly because they are smaller but also because the outer case of the store is part of the discharge channel which is at a lower temperature than that of the stored liquid.

A further raising of the efficiency is possible by the division of the volume of the vessel and the heat exchange surface area into two or more parts or individual vessels in such a way so that they are connected in series whether they are heat exchangers or parts of a vessel or individual vessels. By these means, the storage efficiency is raised and it is possible to extract more useful energy from a given volume and, at the same time, it enables the utilisation of heat sources at differing energy levels and the supply of thermal energy at differing temperature levels according to demand while preserving all advantages of the invention.

Thus designed heat exchangers and stores are capable, if low temperature heat sources are utilised, of extracting from a given volume of stored liquid up to 30% more useful energy (defined as the amount of energy drawn-off before the flow temperature from the heat exchanger falls below the required value) than conventional solutions and, at the same time, will save 20% of the heat exchange surface area. Higher energy efficiency, lower heat losses and the possibility to save on capital cost is a significant contribution to the utilisation of ecological low temperature alternative heat sources and can decide whether utilisation is economically advantageous.

Brief description of drawings In order to explain the invention more clearly, examples of applications of the invention to thermal energy stores are described in detail with the help of the accompanying drawings.

Fig. 1 shows, in schematic vertical section, a thermal store with a two-row heat exchanger and a discharge flow channel. In fig. 2, two interconnected thermal stores are shown in schematic vertical

section. In fig. 3 a three-part store with several thermal energy sources and with a discharge of energy at varied thermal levels is shown in schematic vertical section.

Detailed description of the preferred embodiments In fig. 1 a store 1 comprising a cylindrical vessel is shown. In the upper part of the store 1 a two-row heat exchanger 2 is situated consisting of two separate spirals connected at the top and bottom of the heat exchanger 2. The position of the spirals and the distance between themselves and the dividing wall 3 and the wall of the store 1 and the spacing of the individual coils of the spiral are chosen to achieve the required heat transfer coefficient. The store 1 is provided either with a cylindrical shaped dividing wall 3 or a straight cylindrical dividing wall 3', which with the external wall of the store 1 form a discharge channel 5. The purpose of the dividing wall 3,3'is to guide the flow of the liquid flowing through the discharge channel 5, if need be, to lower its velocity before entering the inner space of the store 1.

The inlet 7 of the liquid to be heated is situated in the bottom part of the heat exchanger 2 and the outlet 8 of the heated liquid is situated in the top part of the heat exchanger 2. The inlet of the liquid from the thermal heat source 9 is situated in the upper part of the store 1. Baffle 10 prevents the disturbance of the thermal stratification and directs the incoming liquid into the discharge channel 5 thereby increasing the heat transfer coefficient. The return conduit of the charging circuit 11 is connected in the bottom part of the store 1. Alternatively or additionally the store can be provided with an internal heat source 15, for example an electric heating element, situated in the bottom part of the store 1. If required, the outlet to the discharge circuit 12 is situated in the top part of the store 1 and the return 13 in the bottom part of the store 1 on the end of

which is situated a device 14 to lower the entry velocity.

Alternatively or additionally an indirect discharge circuit can be used where energy is extracted by means of a heat exchanger.

Fig. 2 is a schematic drawing of two stores 1'and 1''connected in series. Store 1'is connected to store 11 1 by a connecting conduit 6. The stores 1'and 1''can be in close proximity, where appropriate they can form one vessel and the connecting conduit can be formed by an internal partition. The inlet 7 of the liquid to be heated is to the bottom part of the heat exchanger 2''of the lower store 1"The heated liquid then flows from the upper part of the heat exchanger 2''by means of the connecting conduit 4 into the bottom part of heat exchanger 2'of the upper store 1' and the heated liquid then exits through outlet 8. The inlet of the liquid from the heat source 9 is situated in the upper part of store 1'. The return of the charging circuit 11 is connected to the bottom part of the lower store 1''. The outlet to the discharge circuit 12 is situated in the top part of the upper store 1'and the return 13 in the bottom part of the lower store 1''.

In fig. 3, according to the invention, a three-part store 1 with several thermal energy sources and with a discharge of energy at varied thermal levels is shown. Store 1 is divided by internal partitions 22 and 23 into three spaces 19,20 and 21. The inlet 7 of the liquid to be heated is to the bottom part of heat exchanger 2'''of the bottom space 19 and the heated liquid then flows from the upper part of heat exchanger 2'''by means of a connecting conduit 4''into the bottom part of heat exchanger 2'/of the middle space 20 and from its top it then flows by means of connecting conduit 4'into the bottom part of heat exchanger 2'of the top space of the store 21 and the heated liquid then exits through outlet 8.

An internal heat source 15 having the lowest thermal energy level

is situated in the lower part of the bottom space 19. The inlet of the liquid from a higher level thermal energy source 9''is situated in the upper part of the middle space 20 and the return conduit 11''of this charging circuit is connected to the bottom part of the middle space 20. The inlet of the liquid from the highest level thermal energy source 9'is situated in the upper part of the top space 21 and the return conduit 11'in the bottom part of the top space 21.

The discharge circuits are connected to the relevant spaces of the store 1 according to the required thermal levels. The outlet to the discharge circuit requiring the highest thermal level 12'is situated in the upper part of top space 21 and the return conduit 13'in the bottom part of top space 21. The outlet to the discharge circuit requiring a lower thermal level 12"is situated in the upper part of the middle space 20 and the return conduit 13''in the bottom part of the middle space 20. If the temperature of the liquid returning from outlet 12''or 12'of the discharge circuit is lower than the thermal energy level in the bottom space 19, then the return conduit 13'''can be connected to the lower part of the bottom space 19.