IMPROVEMENTS IN SYSTEMS FOR HEATING WATER

Abstract

A system for heating water in a hot water storage tank, comprising a heater; and means for drawing water onto the heater to be heated from beneath the heater.

Claims

1. A system for heating water in a hot water storage tank, comprising: a heater; and means for drawing water onto the heater to be heated from beneath the heater.

2. A system according to claim 1, wherein the means for drawing water is arranged to prolong the dwell time of water proximate to the heater.

3. A system according to claim 1 or 2, wherein the means for drawing water is arranged to channel the flow of water at least partway along the length of the heater.

4. A system according to any of claims 1 to 3, wherein the means for drawing water comprises a cover arranged to shroud at least part of the heater.

5. A system according to claim 4, wherein the means for drawing water is arranged to feed drawn water into the cover.

6. A system according to claim 5, wherein the means for drawing water is arranged to generate swirl in the drawn water as it is fed into the cover.

7. A system according to any of claims 4 to 6, wherein the cover is arranged to be secured to a wall of the storage tank with the heater substantially enclosed within.

8. A system according to any of claims 4 to 7, wherein the cover is provided with one or more openings through which drawn water can discharge.

9. A system according to claim 8, wherein the one or more openings are spaced from the region of the cover where drawn water is fed into the cover, such that drawn water fed into the cover must pass at least part of the heater before it can discharge through the one or more openings.

10. A system according to any of claim 8 or 9, wherein the one or more openings are configured to control the flow direction of heated water as it discharges out of the cover.

11. A system according to any of claims 8 to 10, wherein the one or more openings are configured to control the flow rate of the heated water in different directions as it discharges out of the cover.

12. A system according to any of claims 8 to 11, wherein the one or more openings are of non-uniform size, shape and/or distribution about the cover, such that a flow of heated water can be directed in a particular direction.

13. A system according to any of claims 4 to 12, wherein the cover is arranged to enclose substantially said at least part of the heater, preferably wherein the cover is arranged to enclose substantially the entire heater, for example wherein the cover is arranged as a canister.

14. A system according to any preceding claim, wherein the means for drawing water is arranged to draw water from the base of the storage tank.

15. A system according to any preceding claim, wherein the means for drawing water comprises a fluid conduit that is external of the storage tank.

16. A system according to claim 15, wherein the fluid conduit extends between a fluid outlet and a fluid inlet on the storage tank, and wherein the fluid outlet is lower than the fluid inlet.

17. A system according to claim 15 or 16, further comprising a pump configured to draw fluid along the fluid conduit.

18. A system according to claim 17, wherein the pump is configured to provide a flow rate of between 0.2 litres/min and 15 litres/min.

19. A system according to claim 17 or 18, wherein the pump is controllable such that the flow rate of the drawn water can be controlled.

20. A system according to any of claims 17 to 19, wherein the pump is arranged in-line with the fluid conduit.

21. A system according to any of claims 15 to 20, wherein the means for drawing water further comprises a non-return valve.

22. A system according to claim 21, wherein the non-return valve is configured to open at a threshold pressure of about 0.1 MPa.

23. A system according to claim 21 or 22, wherein the non-return valve is arranged in-line in the fluid conduit.

24. A system according to claim 23, wherein the non-return valve is arranged downstream of the pump.

25. A system according to any preceding claim, wherein the heater comprises an immersion heater.

26. A system according to claim 25, wherein the immersion heater is arranged to be mounted in an immersion heater port on a wall of the storage tank.

27. A system according to claim 25 or 26, wherein the immersion heater comprises at least one heating element having a power rating of between about 2 kW and about 4 kW, and preferably about 3 kW.

28. A system according to any preceding claim, wherein the heater a is a heat exchanger disposed inside the storage tank, the heat exchanger preferably being arranged to receive a heated fluid from an external heat source.

29. A system according to any preceding claim, further arranged such that the water is heated external to the tank.

30. A system according to any preceding claim, further comprising at least one heat exchanger arranged external to the tank, wherein the heat exchanger is arranged to heat the water before it reaches the heater.

31. A system according to claim 30, wherein the at least one heat exchanger is a plate heat exchanger, for example supplied with heated water from a boiler or heat pump.

32. A system according to any of claims 29 to 31, further comprising a second heat exchanger, wherein at least one of the heat exchangers is supplied with waste heat harvested from a computing application, such as a microprocessor or a central processing unit (CPU).

33. A system according to claim 32, further comprising an electric heater arranged in series with the two heat exchangers, for example wherein the electric heater is an immersion heater disposed inside the tank, preferably wherein at least part of the immersion heater is shrouded.

34. A system according to any of claims 14 to 32, further comprising a diffuser element disposed inside the tank proximate to a fluid inlet of the tank, wherein the diffuser element is arranged to diffuse water as it enters the tank via said inlet.

35. A system according to claim 34, wherein the fluid inlet is arranged to inlet fluid towards the top of the tank, for example via an immersion port.

36. A system according to any preceding claim, further comprising means for sensing the temperature of the heated water.

37. A system according to claim 36, further comprising means for controlling the heater to heat drawn water to a desired temperature.

38. A system according to claim 36 or 37, further comprising means for controlling the flow rate of water being drawn onto the heater.

39. A system according to any preceding claim, comprising a thermocline sensor arranged to determine the position of a thermocline in the water stored in the storage tank.

40. A system according to any preceding claim, further comprising a user interface arranged to receive a user input indicating that a particular draw event is required, determine a time period until a predetermined volume of heated water associated with the indicated draw event is available; and display the determined time period to the user.

41. A method of heating water in a hot water storage tank, comprising the steps of: providing a heater; and drawing water onto the heater to be heated from beneath the heater.

42. A method according to claim 41, further comprising: sensing the temperature of the drawn water heated by the heater; and controlling the heater to heat the drawn water to a desired temperature.

43. A method according to claim 41 or 42, further comprising: sensing the temperature of the drawn water heated by the heater; and controlling the flow rate of the drawn water onto the heater such that the drawn water is heated to a desired temperature by the heater.

44. A method according to any of claims 41 to 43, further comprising monitoring the position of the thermocline in the storage tank to determine a volume of heated water that is available at the desired temperature.

45. A method according to claim 44, further comprising controlling the supply of energy to the heater based on the determined volume of available heated water.

46. A method according to claim 45, wherein the supply of heat is controlled based on a difference between the determined volume of available heated water and an expected volume of available heated water.

47. A method according to any of claims 41 to 46, wherein the heater is a heat exchanger.

48. A method according to any of claims 41 to 47, further comprising providing a (further) heat exchanger in series with said heater, wherein the heat exchanger is configured to pre-heat water before it reaches said heater.

49. A method according to any of claims 41 to 48, further comprising: receiving a user input indicating that a draw event is required from the hot water storage tank; determining a time delay required to ensure that a predetermined volume of water associated with the draw event is heated to a predetermined temperature; and informing the user of the determined time delay.

50. A method according to any of claims 41 to 49, further comprising: determining an average volume of heated water taken from the storage tank over a predetermined period; and heating water drawn from beneath the heater until a volume of heated water roughly equal to the determined average volume of heated water used is available.

51. A method according to any of claims 41 to 50, further comprising positioning a diffuser element inside the tank, wherein the diffuser element is arranged to intercept the heated water as it enters the tank so as to diffuse the flow of water.

52. A method of supplying energy to a heater arranged to heat water for storing in a hot water storage tank, comprising: monitoring the position of the thermocline in the storage tank; determining the volume of heated water that is available at a desired temperature based on the position of the thermocline; and supplying energy to the heater, for example based on a difference between the determined volume of available heated water and an expected volume of available heated water.

53. A heater assembly for heating water in a hot water storage tank, comprising an immersion heater having a heating element; and means for shrouding at least part of the heating element.

54. A heater assembly according to claim 53, wherein the heater assembly comprises a fluid inlet arranged to provide a fluid path into the water storage tank via the immersion heater.

55. A heater assembly according to claim 54, wherein the fluid inlet is arranged to extend at least partway along the length of the heating element.

56. A heater assembly according to any of claims 53 to 55, wherein the means for shrouding is arranged to have one or more openings through which water contained inside the cover can discharge.

57. A heater assembly according to claim 56, wherein the one or more openings in the means for shrouding are spaced from the end of the fluid inlet.

58. A heater assembly according to any of claims 53 to 57, wherein the means for shrouding is a cover having a length of between about 100 mm and about 300 mm, and preferably about 200 mm.

59. A heater assembly according to any of claims 53 to 58, wherein the means for shrouding is a generally cylindrical cover, preferably having a diameter of about of about 40 mm to about 150 mm, and preferably about 100 mm, for example wherein the means for shrouding is arranged as a canister.

60. A heater assembly according to claims 53 to 59, and configured to be mounted in an immersion port on a water storage tank.

61. A heater assembly according to any of claims 53 to 60, wherein the means for shrouding is arranged to be inserted through an immersion port of a water tank.

62. A heater assembly according to claim 61, wherein the means for shrouding is configured to expand once inserted through the immersion port.

63. A heater assembly according to any of claims 53 to 62, further comprising a temperature sensor arranged to detect the temperature of the drawn water.

64. An immersion heater for a water storage tank, comprising a fluid inlet arranged to provide a fluid pathway into the water storage tank when mounted therein.

65. An immersion heater for a hot water storage tank, comprising: a plug arranged to be mounted to the hot water storage tank; and a heating element arranged to extend away from the plug into the hot water storage tank; wherein the plug is further arranged to provide a fluid inlet to the hot water storage tank.

66. An immersion heater according to claim 65, further comprising a tube that extends at least partially through the fluid inlet in the plug.

67. A water storage tank, incorporating a system according to any of claims 1 to 40.

68. A water storage tank comprising a heater assembly or immersion heater according to any of claims 53 to 66.

69. A kit of parts, comprising: a heater for a hot water storage tank; and a cover arranged to be placed over the heater, for example when inside the hot water storage tank, such that the cover at least partially shrouds the heater.

70. A kit of parts according to claim 69, wherein the cover is provided with one or more openings through which water can discharge.

71. A kit of parts according to claim 69 or 70, wherein the cover is arranged to enclose substantially at least part of the heater, for example wherein the openings are provided towards an end of the cover and a substantially opposing end of the cover into which the heater is received is substantially closed.

72. A kit of parts according to claims 69 to 71, wherein the cover is arranged to be secured over the heater to a wall of a hot water storage tank, preferably an internal wall.

73. A kit of parts according to claim 72, wherein the cover is provided with one or more fixing points such that it can be secured to a wall of a water storage tank, preferably using a welding process.

74. A water storage tank substantially as described herein and illustrated in the accompanying drawings.

75. A method of heating water substantially as described herein and illustrated in the accompanying drawings.

76. A system for heating water substantially as described herein and illustrated in the accompanying drawings.

77. An immersion heater substantially as described herein and illustrated in the accompanying drawings.

78. A heater assembly substantially as described herein and illustrated in the accompanying drawings.

Description

[0079] An example of the present invention will now be described with reference to the accompanying figures, in which similar features may be labelled with corresponding reference numerals, and in which:

[0080] FIG. 1A shows a first embodiment of a system for heating water in a hot water storage tank;

[0081] FIG. 1B shows a close up of a heater used in the system shown in FIG. 1A;

[0082] FIG. 2 shows an example of a heater shrouded by a cover;

[0083] FIG. 3 shows another example of a heater shrouded by a cover;

[0084] FIG. 4 shows a first example of a cover secured to a relief dome of a tank;

[0085] FIG. 5 shows a second example of a cover having a plurality of castellated openings;

[0086] FIG. 6 shows a third example cover having a plurality of holes;

[0087] FIGS. 7A to 7C show an integral heater assembly;

[0088] FIGS. 8A to 8C show a second embodiment of a system for heating water in a hot water storage tank;

[0089] FIG. 9 shows another embodiment of a system for heating water;

[0090] FIG. 10 shows yet another embodiment of a system for heating water; and

[0091] FIG. 11 shows exemplary control architecture for the system.

[0092] FIG. 1A shows a system 100 for heating water according to a first exemplary embodiment of the invention, wherein the system 100 is incorporated into a hot water storage tank 102.

[0093] The tank 102 is generally cylindrical with a dome-shaped base 104 and a dome-shaped top 106, which may be referred to as relief domes. A main cold inlet 108 is arranged in the tank base 104 for filling and/or replenishing the tank 102 with water, preferably from a mains water supply, and a main hot outlet 110 arranged in the tank top 106, through which heated water may be drawn for use. A heat exchanger 112 is disposed in the tank base 104 for heating the water, the heat exchanger 112 in this example being arranged as a helical coil tapered to contour with the domed shape of the base 104, and having an inlet/outlet 114 for connection to a heat pump, or similar heat source (not shown).

[0094] A heater 116 is disposed inside the tank 102. As can be seen in the enlarged view of FIG. 1B, the heater 116 is mounted in a heater port 118 (sometimes referred to as an immersion port) provided in the tank 102. The heater 116 is, preferably, arranged to be screwed into the port 118, which be provided with a threaded boss for that purpose.

[0095] A fluid connection 134 extends between the tank base 104 and the heater 116, the fluid connection being arranged to draw water onto the heater 116 to be heated from below the heater 116. The fluid connection 134 in this example is external of the tank 102. The fluid connection 134 may be referred to from herein as a circulation tube, and incorporates a pump 136 that is arranged to circulate water through the circulation tube 134. The pump 136 is, ideally, suitable for use with potable water.

[0096] In this example, a non-return valve 138 is also provided to prevent cold water at the tank base 104 being drawn up the external circulation tube 134 during a draw event, where there might otherwise be a tendency for cold water to be drawn into the tank top 106. Preferably, the non-return valve 138 is configured to open at a threshold pressure, which can be achieved by the circulating pump 136, whereas a draw event is typically limited by the pressure drop of about 0.1 MPa (1 bar) at the inlet of a pump (not shown) upstream of the tank 102. If the pressure threshold to open the non-return valve 138 is above 0.1 MPa (1 bar), the risk of cold water being drawn through the circulation tube during a draw event is eliminated. Of course, the requirement for a non-return valve 138 depends on the pressure drop in the tank 102 during a draw event.

[0097] The circulation tube 134, pump 136 and non-return valve 138 each form part of a water circulation loop that draws water from towards the base of the tank 102, where the water is cold, and feeds the drawn water back into the tank 102 higher up, preferably towards the top 106 of the tank 102. The water is drawn onto the heater 116 as it re-enters the tank 102. The circulation tube 134 may be connected to the cold inlet 108 just as it enters the tank base 104 via a T-piece connector (not shown). Alternatively, the circulation tube 134 may be brazed to a flared hole (not shown) provided in the tank base 104, for example.

[0098] The circulation tube 134 is fluidly connected to a dip tube 130 that provides a fluid path into the tank 102 via the heater 116, and hence via the heater port 118.

[0099] It is desirable to ensure that water drawn onto the heater 116 passes over as much of the heater 116 surface area as possible, so that the water can be optimally heated before it is discharged into the tank 102. A cover 126 is placed over the heater 116, substantially shrouding the heater 116. As drawn water is displaced through the cover 126 by the pump 136, it is heated by the heater 116 until it emerges hot through one or more openings 140 provided in the cover. Ideally, as the heated water discharges from the cover 126 it should cause minimal disturbance to the stratified water in the tank 102 into which it discharges.

[0100] The one or more openings 140 are ideally arranged in a region, preferably towards a distal end, of the cover 126 that is spaced (or remote) from the point at which water is fed into the cover 126. This helps to ensure optimal exposure of the drawn water to the heater 116 as it is displaced through the cover 126 before it discharges out of the cover 126. The dip tube 130 extends alongside the heater 116 and feeds water into the distal end of the cover 126 with the one or more openings 140 arranged at a proximal end of the cover, as shown in FIG. 1B.

[0101] The dwell time of drawn water on, or near, the heater 116 can be dictated by the flow rate of the water through the cover 126. The flow rate can be adjusted by controlling the speed of the pump 136, and hence the system may be controlled to attain a desired discharge temperature of heated water corresponding to the dwell time.

[0102] It may also be desirable to provide one or more features inside the cover 126, such as a helical protrusion (not shown), that can induce swirl in the water as it flows through the cover 126, to improve the heat transfer coefficient.

[0103] The cover 126 preferably has a length of between about 100 mm and about 300 mm, and preferably has a diameter of about 40 mm to about 150 mm. The cover 126 may be secured to the wall of the tank 102, preferably either spot welded or TIG welded.

[0104] FIG. 2 shows an exemplary arrangement of a heater 116 and a cover 126 in a tank 102. The heater 116 in this example is an immersion heater, which includes a heating element 120 having terminals 122 for connecting the heating element 120 to an electrical power supply (not shown). The terminals 122 are housed in a plug (or base) 124, which is arranged to be secured into the immersion heater port 118 provided in the tank 102, preferably via a screw-thread engagement.

[0105] When the plug 124 is mounted in the heater port 118, the terminals 122 are exposed outside of the tank 102, and the heating element 120 extends into the tank 102, as shown in FIG. 1B. The immersion heating element 116 may further comprise a flange (not shown) that is arranged to rest on the exterior surface of the tank 102 to prevent the terminal housing 124 from falling into the tank 102.

[0106] Each heating element 120 may have an output of about 2.75 kW to 3 kW, and multiple heating elements 120 may be provided inside a cover 136. For example, four such 3 kW heating elements 120 may be desired in a cover 136 to heat sufficient hot water to deliver a substantially instantaneous shower.

[0107] The circulation tube 134 shown includes a pump 136 and a non-return valve 138, as described above. For a 3 kW heating element 120, a flow rate of up to about 3 litres/min might be desired. For multiple heating elements 120 inside the same cover 126, a flow rate of up to about 15 litres/min may be desired.

[0108] The rating of the heating element 120 is not limited to about 3 kW, and may be higher or lower depending on the requirements of a system. For example, a heating element of up to 9 kW, or beyond, may also be used, preferably in conjunction with a pump configured to feed it water at a higher flow rate, for example 15 litres/min, in a system from which large quantities of heater water might be demanded quickly. For example, a system may be required to deliver heated water at the rate of an electric shower at a time of day when energy prices are at a premium.

[0109] The dip tube 130 extends alongside the heating element 120 inside the cover 134. The dip tube 130 is connected to the circulation tube 134 via the terminal housing 124 of the heater 116, through which the dip tube 130 passes. The dip tube 130 has an outlet 132 arranged to feed water into the cover 126 at a distal, closed end of the cover 126, relative to the electrical terminals 122. A plurality of openings 140 are provided around the mouth of the cover 126, at a proximal end of the cover 126, such that the openings 140 are remote from the feed outlet 132.

[0110] A pocket 142, arranged to accommodate a mechanical thermostat probe and/or PT100 (resistance thermometer), thermocouple or thermistor based sensor, may also be provided inside the cover 136 as part of the system 100.

[0111] A series of tabs 144 are spaced around the open end of the cover 126 for securing it to a wall of a tank 102, preferably using a spot welding technique, as previously mentioned.

[0112] FIG. 3 shows another embodiment of a heater 316 and cover 326 arrangement, similar to that shown in FIG. 2, with the additional feature of an angled outlet 332 provided at the end of the dip tube 330. By having the angled outlet 332 of the dip tube 330 arranged to feed drawn water into the cover 326 at an angle tangential to the circumference of the cover 326, swirl can be induced in the water flow to enhance the heat transfer coefficient between the surface of the heating element 320 and the water within the cover 326.

[0113] FIGS. 4 to 6 show three different exemplary embodiments of a heater 416, 516, 616 surrounded by a cover 426, 526, 626 arranged to discharge heated water into a hot water storage tank.

[0114] In FIG. 4, a first embodiment of a cover 426 is shown having a straight edge at its open end, with no other openings in the cover 426. The cover 426 is secured to the underside of the relief dome 406 at the top of a tank. In this embodiment, water can discharge from the open end of the cover 426 via the clearances created between tabs 444 provided for securing the cover 426 to the tank, due to the curvature of the relief dome at the top 406 of the tank. This approach is simple in that minimal machining of the cover 426 is required, though less control of the water flow can be achieved through design. Furthermore, the clearances that are created between the tabs 444 in this embodiment may also be of a size that is prone to blockage from scale and other sources of debris that may be carried by the water.

[0115] In FIG. 5 a second embodiment of a cover 526 is shown having castellated openings 540 spaced around an open end of the cover 526, similar to the exemplary covers 126, 326 shown in FIGS. 1 to 3. The castellated openings 540 may be quite sinuous and possibly non-uniform to reduce mixing as heated water is discharged from the cover 526. The openings 540 are preferably between about 5 mm to about 20 mm in depth, with ideally between about 2 to about 8 periods about the circumference of the cover 526.

[0116] In FIG. 6, a third embodiment of a cover 626 is provided having a plurality of holes 640 around an open end of the cover 626. The size of the holes 640 may be varied to tune the flow rate. For example, it may be that slightly more flow is desired towards the side of the cover 626 facing the main outlet 108 of a tank because less water will be diverted downwards which could otherwise result in undesired mixing of water in the tank, which could disturb or otherwise affect the thermocline. The holes 640 may be between about 5 mm and 10 mm, for example and may not necessarily be of uniform size to minimise mixing of water and avoid blockage due to scale. Tabs 644 are also provided.

[0117] The covers may be fabricated from rolled stainless steel or copper, in which it is easier/quicker to laser/water/plasma jet cut a desired profile than to form a series of holes, as are provided in the cover 626 of FIG. 6. The covers may also be formed of certain grades of polypropylene plastic that are approved for contact with potable water.

[0118] A heater 724 and a cover 726 may be provided as a single heater assembly 700, which can be screwed into the heater port 118 to simplify installation and allow the heater unit 700 to be retrofitted into existing hot water tanks. The cover 726 must be of a diameter that allows it to fit through the heater port 118, though the cover 726 could be arranged to expand once it has passed through the heater port 118 to provide a cover 726 having a larger diameter.

[0119] An example of a heater assembly 700, comprising an immersion heater 716 and a cover 726, is shown in FIGS. 7A to 7C. As can be seen in FIG. 7A, the heating element 720 of the immersion heater 716 is shrouded by the cover 726, which is integral with the immersion heater 716. In other words, the heating element 720 is substantially contained by the cover 726. A dip tube 730 extends alongside the heating element 720 towards the distal end of the cover 726.

[0120] The dip tube 730 extends out through a plug 724 of the immersion heater 716, thereby providing a fluid pathway into the cover 726, as shown in FIG. 7B. In use, the dip tube 730 may be connected to a fluid conduit, such as a circulation tube (not shown), to draw water from beneath onto the heater assembly 700 to be heated, as described earlier.

[0121] The plug 724 of the immersion heater 716 assembly 700 may be provided with an external screw thread that corresponds to a screw thread on an immersion boss (not shown) installed in the heater port 118 of the tank 102. One or more openings 740 are provided at a proximal end of the cover 726 that allow drawn water to discharge from the cover 726 once it has been heated, as can be seen in FIG. 7C. The openings 740 here are, ideally, spaced from the end of the dip tube 730 to optimise the duration of time that the drawn water is heated before it discharges (into the tank) through the openings 740.

[0122] FIGS. 8A to 8C show a second exemplary hot water system 800, comprising a hot water tank 802 having a similar construction to the hot water tank 102 in FIG. 1A, described above.

[0123] In this alternative system 800, a heat exchanger 812 for heating the water in the tank 802 is provided towards the top 806 of the tank 802. The heat exchanger 812 has an inlet 814A for receiving a heated fluid from a gas boiler, or another separate heat source, and an outlet 814B for returning the fluid to the heat source. In this regard, the heat exchanger 812 is similar in operation to the heat exchanger 112 in FIG. 1A.

[0124] The heat exchanger 812 is enclosed by a cover 826 having a dip tube 830 that extends into the cover 826 to feed drawn water into the bottom of the cover 826. A fluid connection (not shown) is arranged to draw water onto the heat exchanger 812 to be heated from lower down in the tank 802, as described earlier. One or more openings 840 are provided at the top of the cover 826, to allow heated water to discharge from the cover 826 once it has risen through the cover and been heated by the heat exchanger 812.

[0125] An additional heater 816 may also be provided as an auxiliary heat source to heat the drawn water in the tank 802. As illustrated in FIG. 8B, the additional heater 816 is, ideally, an immersion heater having a heating element 820 that extends inside the cover 826 inside the tank 802. In practice, however, the immersion heater 816 may need to be electrically connected outside of the tank 802, though this will depend on the size of the heat exchanger 812.

[0126] FIG. 8C shows an external view of the cover 826, enclosing the heat exchanger 812 and immersion heater 816, positioned inside a tank 802. The heat exchanger 812 has an inlet 814A and an outlet 814B for the heated fluid to flow to and from the external heat source (not shown).

[0127] In another embodiment (not shown) the heat exchanger 812 may be provided towards the top 806 of a tank 802 for heating water, as described above, but without the additional heater.

[0128] A heat exchanger might also be included in the tank base 104 if a heat pump or biomass boiler is connected to the system 800. A heat pump heated tank 802 may have both a gas coil and heat pump coil. By positioning a heat exchanger at the top of the tank, gas fired hot water storage tanks may be controlled to provide the same benefits around heat losses and speed of hot water production as the immersion heater embodiment of the invention described earlier.

[0129] FIG. 9 shows another embodiment of a system 900 for heating water. In this embodiment, a heat exchanger 950 is arranged in series with the heating element 920 that is disposed in the tank 902, which is shown here within a cover 926 shrouding the heating element 920. The heat exchanger 950 is arranged external to the tank 902, so that fluid taken from the base 904 of the tank 902 to be introduced towards the top 906 of the tank 902 via the fluid conduit 934 first passes the heat exchanger 950 before being introduced (back) into the tank 902, via the heating element 920. The heat exchanger 950 may therefore pre-heat the fluid.

[0130] The heat exchanger 950 may be arranged to transfer heat from a low grade renewable energy source (such as solar, thermal or heat pump) via a standard plate heat exchanger (not shown). Alternatively, the heat exchanger may be arranged to absorb waste heat from a microprocessor (not shown), for example.

[0131] FIG. 10 shows another exemplary embodiment of a water storage tank 1002, comprising two heat exchangers 1050, 1052. Similar to the example shown in FIG. 9, the two heat exchangers 1050, 1052 are arranged to pre-heat water (in this example, which is extracted from the base of the tank 1002) before it is (re)introduced into the tank 1002.

[0132] The first heat exchanger 1050 is a plate heat exchanger taking input, for example, from a heat pump water glycol circuit (the arrows show the direction of flow of the hot water / glycol mixture across the heat exchanger 1050). The second heat exchanger 1052 uses a secondary source of heat, for example (waste) heat emitted from a CPU/core processor (for example a CPU heat-harvesting arrangement).

[0133] Other types of heat exchanger may of course be utilised in the present invention.

[0134] In the example of a tank 1002 shown in FIG. 10, a diffuser element 1060 is disposed inside the tank 1002. The diffuser element 1060 is positioned such that as pre-heated water is (re)introduced into the tank 1002 it immediately encounters the diffuser element 1060. This arrangement of a top diffuser element 1060 helps to minimise the disturbance of the water already stored within the tank 1002 by the water being (re)introduced at the top of the tank 1002, for example to minimise disturbance to a thermocline that has been (or is being) established in the tank 1002.

[0135] The diffuser element 1060 is, ideally, disposed inside the tank 1002 at an end of the fluid conduit feed pipe having a diameter of between around 6 mm to 22 mm, for example. The diffuser element 1060 may emerge from the feed pipe The diffuser element 1060 is, preferably, circular and may comprise 316 stainless steel or a WRAS approved plastic material such as polyethylene, for example. The diffuser element 1060 itself, preferably, has a diameter of between around 35 mm and 60 mm in diameter. The diffuser element 1060, ideally, has a plurality of radially-spaced holes, and preferably circular holes of around 5 mm to 15 mm diameter.

[0136] Also in this example, a pump 1054 is shown arranged to pump fluid up through the fluid conduit 1034 and past the heat exchangers 1050, 1052 before being reintroduced into the tank 1002. The pump 1054 may have a variable control flow rate, which in the example shown may be controlled to attain a correct (or desired) temperature at the diffuser element by increasing or decreasing the flow of water into the tank 1002, as necessary.

[0137] The arrangement of two heat exchangers could of course also be used with a tank having an internal heater assembly, similar to the arrangement shown in FIG. 9, or indeed with a heater assembly disposed externally of the tank (not shown), for example a heating element contained in a housing, such as a canister, substantially as described herein.

[0138] In another exemplary arrangement (not shown), a heater assembly comprising a heating element (such as an immersion heating element) substantially enclosed or shrouded (at least partially) within a housing (or casing) having a fluid input and a fluid output could alternatively/additionally be provided external to the tank. In this example, the heater assembly may be arranged such that water entering the housing is caused to flow past at least part of the heating element, such that the water is thereby heated before exiting the housing.

[0139] This externally disposed heater assembly may be arranged in-line with a fluid conduit that is arranged to extract water from the base of a water storage tank and reintroduce the water into the tank towards the top of the tank, such that the water is pre-heated by the heater assembly external to the tank before being reintroduced into the tank.

[0140] Such an external arrangement may incorporate a diffuser element inside the tank, which is arranged to diffuse the pre-heated water as it is reintroduced into the tank, similar to the diffuser element 1060 described above with reference to FIG. 10, for example.

[0141] Such an external arrangement may also be used with one or more heat exchangers, similar to those described above with reference to FIGS. 9 and 10, for example.

[0142] A further (third) heating element (not shown), for example an electric heating element, may also be arranged in series with two heat exchangers further to pre-heat the water and/or add additional heating power, if and when required.

[0143] FIG. 11 shows exemplary control architecture for controlling a variety of functions of the invention.

[0144] In a simple embodiment, a continuous proportional-integral-derivative (PID) controller can provide closed loop control of the speed of the pump 136 to ensure that there is a sufficient volume of water at a desired temperature available in the tank 102 to be drawn from the main outlet 110. The desired temperature would normally be about 60 degrees Celsius, which could be monitored using a temperature sensor arranged at the top 106 of the tank 102.

[0145] A higher level control logic can be used to control the heater 116 to increase the water temperature (up to 80 degrees Celsius, for example) in the event that there is surplus energy to store, for example when the bulk of the water is heated using renewable energy such as wind or photovoltaics), or to reduce the temperature to a lower temperature to reduce standing heat losses (down to 50 degrees Celsius, for example).

[0146] A temperature sensor may be provided at the inlet 118 where drawn water re-enters the tank 102 to inform the control logic when water in the tank 102 has attained an operating temperature. The temperature at the inlet may be used as an additional control parameter to modulate the speed of the water pump 136 more effectively.

[0147] A tank 102 state of charge sensor can be used to monitor the position of the thermocline in the tank 102 and hence determine a volume of heated water available for use. The state of charge sensor may be used to enable the system to determine the usage of the tank 102 and then the control logic can be used to move the thermocline to the correct position accordingly.

[0148] The control logic can be used to move the thermocline to a desired position in the tank 102, according to the average usage of the tank 102, by heating more or less drawn water. For example, if it is determined that the tank 102 is never more than half discharged, the thermocline could be moved down halfway towards the bottom of the tank 102, thereby ensuring that there is always sufficient hot water available for use.

[0149] Even if only part of the tank 102 is used, because of a single occupant in a large dwelling for example, it would still be desirable for the system to run a periodic sterilising cycle on the tank, ideally at least once a week, to ensure that bugs do not flourish. A sterilising cycle as referred to herein preferably connotes a process during which the entire volume of the tank 102, including the circulation loop 134, is heated to at least 50 degrees Celsius (more preferably at least 60 degrees Celsius) to sterilise the system.

[0150] A user interface could inform a user when a specific amount of hot water has been heated or how long the user is required to wait for an activity such as a bath or shower.

[0151] The water pump 136 would, ideally, be suitable for use with potable water (typically a brass pump impellor and housing would be specified). The water pump 136 could be driven by a DC brushed or AC brushless motor (i.e. switched reluctance, induction, permanent magnet, etc.). The drive circuitry might be a simple FET turning on and off at a particular frequency (e.g. at a constant frequency but with a variable duty cycle) in the case of a brushed motor. Alternatively, the drive circuitry might incorporate a variable frequency inverter for a brushless system.

[0152] With reference to the embodiment of FIG. 9, for example, a control architecture may be adapted to modify the flow rate of water through the fluid connection 934, which feeds the internal heater arrangement 916, to ensure that a consistent temperature at the outlet 910 is maintained from the tank 902 by way of the internal heater 916 arrangement regardless of a varying thermal input from the additional heat exchanger 950. Similar control architecture may be used in the embodiment shown in FIG. 10.

[0153] The power input to the immersion heating element 920 of the internal heater arrangement 916 may also be controlled to assist in maintaining a consistent temperature at the outlet 910. The supply of power to the heating element 920 may be controlled via on-off duty control using a Thyristor, solid state relay (SSR), insulated-gate bipolar transistor (IGBT) or other suitable switching device, ideally at the zero-crossings in the mains power supply, to ensure minimal harmonic distortion and electromagnetic emissions during operation.

[0154] Alternatively, the supply of power to the immersion heating element 920 might be controlled via high frequency pulse width modulation (PWM) of the current using an IGBT switching device, for example.

[0155] The heat (or power) supplied to the heat exchanger 950 may be controlled or scheduled based on a measurement of the state of charge of the tank 902.

[0156] For example, the supply of heat to the heat exchanger 950 may simply be controlled to be On or Off. Optionally, the flow rate of fluid being pumped through or past the heat exchanger 950 may be controlled to compensate for a fluctuating input of heat to the heat exchanger 950 alongside the heating element to achieve the desired outlet temperature into the top of tank 902.

[0157] With reference to FIG. 10, for example, the energy being provided to the heat exchanger 1050 may be a constant output from a boiler or heat pump, for example, whereas the energy being provided to the further heat exchanger 1052 may be a variable output, which can be dispatched depending on the cost of energy source and the state of charge of the tank, for example.

[0158] In the figures, similar features of different embodiments are labelled using similar, corresponding reference numerals.

[0159] It will be understood that the present invention has been described herein purely by way of example, and modifications of detail can be made within the spirit and the scope of the invention.

[0160] Furthermore, it will be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently.