The present invention concerns from a first aspect a water tank of the kind defined in the descriptive part of claim 1, from a second aspect a tank lid of the kind defined in the descriptive part of claim 7 and to a method of the kind des- cribed in claim 10.

The water tank according to the invention is primarily intended for storage of heat such as solar heat and is not pressurised. Such accumulating tanks with water can be used for heat storage in single family houses, for multi-family houses or for groups of houses, and occur in many varying sizes. The volume of the tanks may vary from some tens of cubic meters up to some tens of thousand cubic me- ters, and with a height from a few meters up to a few tens of meters. Prize consid- erations usually do not permit choice of stainless steel for such a tank, and they are thus usually made from cheaper steel grades. Consequently, one usually has to master corrosion problems.

With the purpose of reducing the corrosion problems, one tries to restrict the contact between water and air, to avoid or reduce diffusion of oxygen into the water. It is common for this purpose to put a thin layer of paraffin oil on the surface of the water to avoid oxygen dissolution in the water. Alternatively, the tank can be provided with a lid floating on the water surface.

It is previously known from EP 0 067 428 to combine these two methods of reducing contact of the water with the air. According to that publication, the tank is thus provided with a floating lid, and provided with a layer of paraffin oil on the an- nular water surface between the lid and the cylindrical tank walls.

The known designs with a covering layer of paraffin oil, a floating lid or a combination of these can, however, only partially solve the corrosion problem. Cor- rosion has been observed to appear on the part of the tank wall close to the water surface, and may sometimes be severe. This is caused by water level variations due to the expansion of water, starting from a cold discharged tank to a hot fully heated tank. This variation may reach around 4 % of the effective height of the tank. When the water level sinks, a narrow cylindrical surface is gradually exposed to the air. This wet surface is then subject to corrosion. The water tank shown in

said EP-document has a lid with such a relatively low edge that the amount of par- affin oil which can be stored in the annular region can not be thick enough to avoid exposure of wet tank wall to the air.

Since the paraffin oil layer is thin, the diffusion distance for oxygen through the layer is so short that some oxygenation of the water can not be avoided.

The purpose of the present invention is to eliminate also the last mention- ed reasons for corrosion of the tank walls, and to isolate the water even better from the air.

From a first aspect, this has been achieved by providing a water tank de- scribed in the descriptive part of claim 1 with the special characteristics of the cha- racterizing part of this claim. From a second aspect by providing a lid described in the descriptive part of claim 7 with the special characteristics of the characterizing part of this claim, and from a third aspect by using a method described in the des- criptive part of claim 10 with the special features specified in the characterizing part of this claim.

Since the lid edge has a height equal to at least 4 % of the effective depth of the tank, a narrow channel will be created between the lid edge and the tank wall with this height. This allows providing a paraffin oil layer with sufficient height to correspond to the water level variations due to changes in temperature. Due to the design of the invention, the diffusion distance through the paraffin oil layer will also be considerably increased and the oxygenation of the water much reduced.

The term"effective depth"in the present application refers to the height of the wa- ter filled volume of the tank when used as intended.

According to a preferred embodiment of the invention, the lid of the tank comprises a saucer-shaped disc and ballast means provided on the lid. The weight of the ballast can be adjusted and chosen to make the lid enter into the water far enough that almost its whole vertical peripheral surface will be located below the free liquid surface, which is the upper surface of the paraffin oil layer.

According to another preferred embodiment of a tank according to the in- vention, the lid is made with an underside which slopes upward from its central re- gion toward the periphery. For a cylindrical tank, this means that the underside may be slightly conical or saucer-shaped.

This shape of the lid also has the advantage that gases dissolve in the water but released during heating are permitted to leave the water freely without getting captured at the underside of the lid.

According to another preferred embodiment of the tank according to the in- vention, the tank may be provided with a detector for the liquid level to measure and indicate the level of the paraffin oil or water surface. This will allow supervision of the actual liquid levels. Indirectly, this provides information about the heat ener- gy content of the water.

The embodiments mentioned above and others preferred are mentioned in claims dependent upon claim 1.

The lid described by claim 7 carries advantages corresponding to what has been claimed above for the entire tank. By means of the lid according to the inven- tion it is thus possible to avoid the corrosion problems on the inside wall of the tank in the vicinity of the water surface in a tank where the lid is intended to be applied.

A preferred embodiment of the lid has a peripheral lid edge with a height equal to at least 4 % of the horizontal measure of the lid, which for a circular lid is its diameter. Since water tanks of the relevant kind are usually made with a dia- meter in the interval from 1 to 2 times the depth of the water, said lid edge height will be enough to compensate the water level variations.

The method described in claim 10 is an advantageous and practical me- thod to ascertain that the water tank is efficiently protected against corrosion.

The invention will be more closely explained in the following description of a preferred embodiment, with reference to the accompanying figures.

Figure 1 shows a vertical section through a water tank according to the inven- tion.

Figure 2 is a figure showing in principle the function of the invention at a varia- tion of the water level in the tank.

Figure 3 is a figure corresponding to figure 2, but related to prior art.

Figure 1 shows a section through a tank according to the invention. The tank is to be used as a heat accumulator for solar heat. When the water is as cold- est and the heat energy fully discharged, the temperature is around 4-5 degrees.

Fully heated and loaded with energy it has a temperature around 90-95 degrees.

The tank is shown as cylindrical, but naturally square, rectangular or any arbitrary cross-section is possible. The tank comprises a container part 1 with vertical walls and a lid part 2 floating on the water 4. The lid part 2 consists of a saucer-shaped lid 6,10 on which a ballast of bricks is loaded. Of course, sand, concrete or corre- sponding materials can also be used. The lid has a cylindrical edge forming a pe- ripheral surface 10 and the lid bottom 6 is slightly conical rising towards the peri- pheral surface 10. The peripheral surface has a height corresponding to at least 4 % of the effective depth of the water tank. In the channel between the peripheral surface 10 and the surrounding wall of the container part 1 a layer of paraffin oil 5 is provided. The weight of the ballast 3 is adjusted so that the lid part 2 is immers- ed so far in the water that the height of the liquid in the channel is at least 4 % of the effective height of the tank. Since the channel is filled with paraffin oil, the thickness of the oil layer will have this value.

The layer of paraffin oil 5 will thus have a height corresponding to the maximum variation of the water level due to the temperature variation. As the wa- ter volume decreases when heat is discharged, the water level and thus also the paraffin oil level will sink to expose part of the wall of the container part 1. This ex- posed wall part has only been in contact with paraffin oil, and will thus not be sub- ject to corrosion. The great thickness of the paraffin oil layer will also constitute a long diffusion distance, and very small amounts of oxygen will dissolve in the wa- ter.

Gas bubbles created in the water 1 when it is heated will rise toward the underside of the lid, and due to its conical shape further to the channel.

To supervise the actual water levels, a first tube 7 is connected to the tank at a height slightly above the water level which is normal when the water is as coldest. A second tube 8 is connected at a height representative of the hottest water. The distance between the tubes 7,8 is thus approximately 4 % of the tank depth. The upper tube is provided with a vertical transparent branch 9 with a tem- perature scale. Since the level of paraffin oil in the branch tube 9 is linearly depen- dent upon the average water temperature, it can be used as information about the available heat energy in the tank. The tank can also be provided with a number of

temperature indicators at different levels in the water to supply information about the temperature at different levels and the temperature layering.

The space 11 above the lid part 2 is connected to the free air through a ventilation tube 12 with a lower located opening 13.

When the tank is to be filled with water, the lid part 2 has a height of the peripheral surface 10 and a ballast weight 3 predetermined to match the largest expected level variation for the water surface. The tank is then filled with the desir- ed volume of water. If correctly calculated, the water will extend into the channel to a height around 4 % of the effective depth of the tank. Finally the paraffin oil 5 is added in the channel, and will press the water down as the oil forms a layer on top of the water. Enough paraffin oil is added until basically the whole channel is filled.

Figure 2 explains what happens when the water level recedes from its highest level B to its lowest level A. In the figure, solid lines refer to lowest situation and broken lines to the highest situation. In the highest situation the paraffin oil surface is at level C and the water surface at level B. When the water is cooled down and recedes to its lowest level A, the paraffin oil level will recede from its highest level C to its lowest level D (which is approximately the same as level B).

The tank wall 1 between C and D will then gradually be exposed to the air. Since this portion of the wall has only been in contact with the paraffin oil, there will be no corrosion.

Figure 3 shows in a corresponding manner what happens with prior art technology with a lower peripheral flange 110 and a thinner paraffin oil layer 105.

When the water level recedes from level B'to level A', the paraffin oil surface re- cedes from level C'to level D'. The tank wall 101 will then be exposed to air in the region between levels C'and D'. Of this region, the portion between B'and D'has been in contact with water and will be subject to corrosion.