GB2131527A

GB2131527A - Liquid heating apparatus and method of operating such apparatus

Info

Publication number: GB2131527A
Application number: GB08332178A
Authority: GB
Inventors: Carl Wilhelm Kolker; Maria Elisabeth Klossner
Original assignee: BAUCALOR AG
Current assignee: BAUCALOR AG
Priority date: 1982-12-04
Filing date: 1983-12-01
Publication date: 1984-06-20
Also published as: NL8304126A; BE898333A; LU85108A1; IT8349426A0; IT1169347B; DE3244908A1; FR2537250B1; GB2131527B; FR2537250A1; GB8332178D0

Abstract

Apparatus for heating water or other liquids comprises a heating vessel 1 having a cold liquid inlet duct 3 near its bottom, a heated liquid outlet duct 5 near its top, and a heating device 8, for example a heat exchanger through which heated fluid flows or an electric immersion heater, forming a heating zone 7 in the upper part of the vessel. A circulating duct 12 with a circulating pump 17 is provided to draw liquid from the lower part of the vessel and discharge it into the heating zone 7 and means 21- 24 is provided for adjustably limiting the amount of heat supplied by the heating device 8, whereby various different volumes of water in the vessel 1, or in the vessel 1 and in reservoirs 18 and 19, may be heated according to requirements. <IMAGE>

Description

SPECIFICATION Liquid heating apparatus and method of operating such apparatus This invention relates to apparatus for heating water or other liquids, the apparatus comprising a heating vessel having a cold liquid inlet duct, a hot liquid discharge duct, a heating device which is mounted in the upper part of the vessel and forms a heating zone in the upper part of the vessel and a liquid circulating duct having a circulating pump, the circulating duct leading from the lower part of the vessel and re-entering the vessel in the heating zone.

Such apparatus is used in general for heating and storing various liquids, but especially domestic hot water, and the apparatus therefore always fuifils a double function. With the apparatus it is possible slowly to heat up a quantity of water or other liquid, determined by the volume of the heating vessel to a desired final temperature and prepare it for discharge and use. The whole stored, heated volume of water or other liquid is usually discharged in a time which is relatively short compared with the heating time. Owing to the longer heating-up time of such apparatus compared with a directly heated boiler or a continuous flow heater, heat sources having a low heat energy output may be used.

Usually, hot water heating vessels have, at the bottom of the vessel, a cold water feed line and at the top a hot water discharge line; a heating device is situated in the lower part of the vessel, so that during heating the heated water ascends in the vessel and colder water flows to the heating device. This circulation ceases when the heating device is switched off, that is when the entire quantity of water in the vessel has reached the desired final temperature. If hot water is now drawn off, which usually does not happen until the required final temperature has been reached, cold water flows in at the bottom of the vessel.

This happens largely with the avoidance of turbulence, so that a stable temperature stratification is established in the water in the vessel, in which above a shallow mixed temperature intermediate zone the predetermined required hot water temperature is maintained and below this zone the feed water temperature is maintained. The intermediate zone at an intermediate temperature becomes gradually deeper in the course of time due to thermal conduction.

These heating vessels have the disadvantage that water having the desired final temperature cannot be drawn off until the entire volume of water in the vessel has been heated up, so that even where only small quantities are drawn off, the quantity of water not removed has previously to be heated to the desired temperature.

French Patent specification No. 7818882, discloses a heating device in the upper region of a heating vessel; a cold water feed line at the bottom of the vessel and a hot water discharge line at the top of the vessel. With this arrangement it is possible continually to provide a predetermined minimum volume of water at the final temperature. The heating device is capable within reasonable periods of time, however, of heating up only the water in the upper zone of the vessel, because natural convection with the cold water situated below does not take place, but the aforementioned stable temperature stratification becomes established. Therefore the remaining water in the lower part of the vessel must be specially heated. For this purpose a circulation line or duct is provided and leads from the bottom of the vessel.This line contains a circulating pump and a solar heating cell, and leads back into the heating vessel substantially below the heating zone of the heating device. Consequently, French Patent specification 781 8872 discloses the combination of a heating vessel having a heating device with a reservoir disposed below the true heating zone and further heating by means of an external heat source. The heating device is switched on and off in a thermostatically controlled manner under the control of a temperature sensor, so that a predetermined quantity of hot water at the desired temperature is continually available. This heating vessel is supplied, when hot water is drawn off, with cold water via the reservoir situated below the heating zone.This heating vessel/reservoir combination has the disadvantage that the quantity of water available at the desired final temperature cannot be varied when the heat supply through the solar cell is lacking. A similar hot water installation having the same disadvantages is also disclosed in French Patent specification 7605690.

The main aims of the present invention therefore are to provide an apparatus which makes possible the heating up of a limited quantity of water or other liquid to a required temperature, the apparatus being of the type initially described, and to provide an improved method of operating such an apparatus.

Preferably, the apparatus is constructed to ensure the heating up of the liquid with the smallest possible expenditure of energy and for this purpose it may utilize solar energy with considerable efficiency. The invention is based on the concept of pumping cold liquid from the bottom of the heating vessel into the heating zone situated in the upper part of the vessel for the purpose of heating the liquid and stopping the heating of the liquid after a specific quantity of heat has been transferred to the liquid from the heating device. In this manner the layer of liquid heated up in the heating zone is increased in the downward direction, in dependence upon the quantity of cold liquid circulated by pumping, to a depth which corresponds to the anticipated demand for the heated liquid.This inventive concept is put into effect by providing apparatus as initially described with means for adjustably limiting the quantity of heat supplied by the heating device.

The term "heating zone" is to be understood as meaning the convection zone in the upper part of the vessel which is influenced by the heating device when this is in operation.

The level in the vessel of the lower boundary of the heating zone is therefore determined mainly by the position of the lowest heating surface of the heating device. The heat conducting zone adjoining the convection zone below and constituting the transition to the cold zone is therefore basically not to be considered as part of the heating zone.

The feeding of cold liquid into the heating vessel from the circulating duct into the heating zone in the upper region of the vessel leads to a thorough mixing of already hot and still cold liquid. This cold liquid is then quite quickly heated up by the heating device almost on the through flow heater principle which is in effect realised with this arrangement. It will be understood that the feed of cold liquid into the heating zone must be dependent on the thermal output of the heating device in order for the liquid to reach the intended final temperature.

Various devices and systems may be used to form the means for limiting the quantity of heat supplied by the heating device, for example time switches, heat quantity meters having a switch contact or contact thermometers at specific levels in the heating vessel.

When heating only part of the liquid in the vessel, the heat quantity limiter switches off both the heating device and also the circulating pump, whereas when heating the entire content of the vessel, only the heating device is switched off after the supply of a set quantity of heat.

The invention ensures therefore the heating up of specific quantities of liquid in the heating vessel to a predetermined temperature by the use of a heating device having a relatively small output without requiring a notable expenditure on control equipment for this purpose.

The quantity of cold liquid circulated through the circulating duct can be adapted in an especially simple manner to the thermal output of the heating device by a temperature sensor in the heating zone which switches the circulating pump on and off. In this manner, even if heated liquid is intermittently drawn off from the vessel, a specific minimum temperature is always ensured in the heating zone. For this purpose, the temperature sensor advantageously switches off the circulating pump when, or at least does not switch it on, while the liquid temperature in the heating zone remains below a predetermined minimum value. Only after a predetermined final temperature has been reached is the circulating pump switched on.

The final heated temperature for a predetermined pertial quantity of liquid is advantageously ensured by the provision of at least one temperature sensor in the heating vessel which switches off the heating device when the temperature is reached. If switching off of the heating device is desired at the latest when the entire liquid content of the vessel has reached a maximum temperature, then the aforementioned temperature sensor is disposed at the bottom of the vessel. One or more temperature sensors, provided above the bottom of the vessel but below the heating zone in the vessel, make possible switching off of the heating device after the desired quantity of liquid has reached the final temperature.This quantity of liquid can be infinitely adjusted over a range from a minimum to a maximum by using a so-called band sensor, preferably in conjunction with at least one further temperature sensor in the heating zone.

The energy consumption of the heating device decreases, other things being unchanged, if the circulating duct is connected to an external energy source; a heat exchanger with its heat output side or circuit incorporated into the circulating duct is preferred for this purpose. Any thermal energy source may be used as the external energy source, for example, a heating boiler, waste heat, or a heat pump. Solar collectors are particularly suitable, since these are capable in some climates of heating up, for instance, domestic water to temperatures normal in domestic hot water systems. The disposition of the heating vessel within a solar collector is especially advantageous from the thermodynamic aspect, since insulation problems associated with the heating vessel then largely disappear.In this connection, the use is preferred of solar collectors having a, preferably cylindrical, parabolic reflector, around the focal axis of which the heating vessel, which is cylindrical, is centred. Such a parabolic reflector is advantageously pivoted about its focal axis to follow the path of the sun. In particular it is advantageous to arrange, as energy collectors, semiconductor elements concentrically around the focal point or focal axis of a parabolic reflector, preferably using the surface of the vessel as a support for the semiconductor elements. Such an apparatus achieves a considerable increase in the heat yield from solar energy and, if semiconductor elements are used, provides some electrical energy in addition to the sensible heat.

The parabolic reflectors can be more simply made without appreciable ioss of energy if they are only substantially parabolic and consist of at least three, and preferably five, plane reflecting surfaces, the arrangement of which is adapted as closely as possible to the parabolic form.

Other forms, for example hemispherical or spherical, of heat absorber or heating vessel serving as a heat absorber can be used within a solar collector which is preferably guided to follow the sun by means of one or more optical eyes, which preferably also provide a seasonal correction.

It will be understood that the above described solar collectors are also in general suitable for use in conjunction with any liquid reservoirs forming part of the apparatus instead of the heating vessel.

Further temperature sensors for switching on and off the circulating pump make possible, especially where an external energy source is used, a simple control of the apparatus to make the best possible utilisation of energy; they also enable overheating inside the entire apparatus to be prevented when they are used in conjunction with a safety valve in the top of the heating vessel.

A plurality of water reservoirs disposed in series and connected upstream of the heating vessel permit a considerable increase in the volume of liquid which can be heated and therefore made available without further heating devices being made necessary. Equally a minimum quantity of liquid at the draw-off temperature is available for intermittent consumption. The volumes of liquid present at the top of each reservoir which is connected to the bottom of the next succeeding reservoir and finally to the heating vessel are successively heated up. In this manner the quantities of hot water can be adapted over a very wide range to the particular consumption expected.In this case, the boundary layer between the hot and cold water shifts from the heating vessel to the next reservoir and beyond to the further reservoirs downstream so that with a single heater assembly, in practice, a predetermined quantity of liquid, as small or as large as desired, can be brought up to the desired temperature.

The present invention also consists, according to another of its aspects, in a method of operating an apparatus in accordance with the invention, the method being characterised in that while the liquid in the heating zone is being heated by the heating device the circulating pump is in operation and, after the emission of a predetermined quantity of heat from the heating device, the heating device is put out of operation.

By continuing to run the circulating pump after the heating device has been switched off, the entire contents of the heating vessel can be brought to a uniform intermediate temperature.

Such further continued running of the pump is, however, of advantage particularly when either only a part of the heating vessel content is to be heated by the heating device and the remaining content is only to be warmed up to the extent that an external energy source is available, or if the entire heating vessel content has already been heated and further heating to a higher temperature with the assistance of an external energy source is desired.

Examples of apparatus and of methods in accordance with the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a diagram of a first example of the apparatus; Figure 2 is a diagram of a second more complex example for the supply of hot water; Figure 3 is a diagrammatic cross-section of a solar collector with a cylindrical substantially parabolic reflector which may form part of the apparatus; and Figure 4 is a diagrammatic cross-section of another solar collector with a cylindrical parabolic reflector.

Referring to Figure 1, a heating vessel or calorifier 1, has at its bottom 2, a cold water feed line 3 and, at its top 4, a hot water discharge or draw-off line 5. In the upper part 6 of the vessel 1, a heating device 8, which defines a heating zone 7, and is preferably a through-flow fluid heater or an electrically operated heater, is disposed, with connections 9 and 10 leading to an energy source, not shown. The water convection which becomes established in the heating zone during heating moves downwards approximately as far as a boundary layer 11 shown by a dotted line. A circulating line 12 leads from the vessel bottom 2 and includes lines 13, 14, 15, 16 and 5 and also a circulating pump 1 7 and reservoirs 18 and 19.

The circulating line 1 2 can alternatively include the lines 13, 14, 14' and 5 and also the circulating pump 1 thus by-passing the reservoirs.

A temperature sensor 20, which disposed in the heating zone 7, and is in the form of a contact thermometer switches the circulating pump 17 on and off by means of electric circuitry not illustrated. Furthertemperature sensors 21,22, 23 constitute, together with a selector switch, not illustrated, a heat quantity limiter 24; they switch the heating device 8 off when the predetermined temperature is reached by electric circuitry which is not shown.

After the entire system has been filled with water, the heating device 8, for example in the form of a 2 kw electrical immersion heater, is switched on and heats up the water in the heating zone 7 as far as the boundary layer 11, that is approximately the upper third of the vessel content, to 600C. When this temperature is reached, the temperature sensor 20 switches on the circulating pump 1 7. Cold water is thereby introduced at the vessel top 4 into the heating zone 7 until the temperature at the temperature sensor 20 has fallen to 550C. The temperature sensor 20 then switches the circulating pump 17 off, and the procedure repeats itself as soon as a temperature of 600C is again reached at the temperature sensor 20. In this manner the boundary between hot and cold water in the calorifier 1 moves continually downwards.This procedure is terminated after a preselected period by switching off the heating device and the circulating pump 1 7. This switching off is preferably effected by one of the temperature sensors 21, 22 or 23, depending upon a preselection, when a temperature of 600C has been reached at the preselected temperature sensor. Instead, however, a temperature band sensor can be used for the infinitely variable preselection over a range of the quantity of hot water. In either case the heating device is switched off after a limited but selectable amount of heat has been supplied. If the contents of the reservoirs 18 and 1 9 are also to be heated entirely or partly to 600 C, further temperature sensors must be provided in an appropriate manner, singly or in the form of a temperature band sensor.In this case, however, the cold water feed line 3 is preferably connected to the bottom of the last reservoir 1 9 which in effect forms a part of the heating vessel.

After temination of heating, the entire preheated quantity of water can be supplied via the hot water discharge line 5 in a relatively short time via a line 25 to a consumer unit 26. Cold water automatically flows simultaneously in through the feed line 3. The consumer unit can, as with storage heating, constitute a closed system with the heating vessel.

Referring to Figure 2, a heat exchanger 27 is incorporated with its secondary or heat output side 28 into the circulating line 12. The primary or heat input side 29 of the heat exchanger 27 is part of a primary circuit 37, composed of lines 30 to 33, a circulating pump 34, an external energy source, in this example a solar collector 35, also optionally a multi-way valve 36. Alternatively, however, a so-called thermosyphon circulation in the primary circuit 37 is possible, if the energy source 35 is disposed lower than the heat exchanger 27.

This arrangement continually maintains the circuit liquid flowing to the solar collector at the lowest practicable temperature and therefore obtains energy from the collector even at a low level of incident solar radiation.

If there is no incident solar radiation, heating of the content of the vessel 1 is achieved entirely in the manner described in connection with Figure 1; i.e. the temperature sensor 20 either switches off the heating device 8 and also the circulating pump 17 at, for example, 600C, or switches off the circulating pump 17 between 55 and 600C and a temperature sensor 38 switches off the heating device 8 when 600C is reached. All the set-point values of the electronic control system can be adjusted upwards or downwards internally with respect to the value quoted above by way of example.

As soon as the energy source 35 supplies sufficient energy, this is used for maintaining the temperature in the vessel 1, since the pump 34 of the primary circuit 37 starts up when the discharge temperature of the energy source 35, measured at a temperature sensor 39, is higher than the secondary side discharge temperature of the heat exchanger 27, measured at a temperature sensor 40. The circulating pump 17 of the heating vessel or secondary circuit 41 is switched on only when the temperature at the sensor 40 of the heat exchanger is higher than that at the sensor 20 in the heating zone. The sensor 40 can also be disposed in the line downstream of the secondary side outlet from the heat exchanger 27.

If the liquid heated up in the heating zone by the heating device 8 without operation of the circulating pump 17 is sufficient as a minimum quantity, the remaining content of the heating vessel 1 can be heated up exclusively by the external energy source 35, in that the control electronic system, which is not illustrated in the drawing, switches off the heating device when the final required temperature at the temperature sensor 20 is reached, the circulating pump 34 then runs only when the temperature at the sensor 39 is higher than that at the sensor 40, and the circulating pump 1 7 is switched on only when the temperature at the sensor 40 is higher than that at the sensor 20.It will be understood that, in the same manner, the entire content of the vessel 1 can be heated up by means of the external energy source 35, without the heater assembly 8 being switched on, provided of course that the source 35 provides sufficient energy for this purpose.

A safety thermostat disposed in the heating zone 7 switches off the heating device 8 when a predetermined maximum temperature, e.g. 900C, is reached if the control electronic system should fail. As a further safety measure, an additionai melting fuse at the safety thermostat can switch off the heating device if this is electricaiiy operated.

One especially effective form of overheating protection for the apparatus or particular components thereof is assured by a preferably adjustable relief valve (safety valve) 42, disposed at the top of the vessel 1 in conjunction with a temperature sensor in the vicinity of the component to be specially protected. This safety valve opens when the predetermined maximum temperature at the aforementioned sensor is exceeded, so that hot water flows out from the boiler and cold water flows in until the temperature falls below a preset value at the relevant sensor. This temperature sensor may be one of the sensors already mentioned, especially the sensor 38 or 39.In case of such overheating, the circulating pumps and the other temperature sensors continue to operate in the otherwise usual manner, so that the incoming cold water is automatically supplied for cooling the overheated location of the hot water supply apparatus. Such protection, which is important particularly for solar collectors, can be amplified in that, for example, a pressure relief safety valve 43 may be disposed in the primary circuit 37. This valve opens when a predetermined pressure in the circuit of, for instance, 3 bar is exceeded. Such a pressure relief safety valve can also additionally indicate a sub-atmospheric pressure and cause automatic topping up of the water in the primary circuit 37. With a closed system, in the case of overheating, a consumption device connected downstream of the heating vessel may be set in operation instead of providing a discharge safety valve.

In the above-described apparatus therefore, two volumes of water of different magnitude can be heated up as desired solely with the heating device 8, namely either only the content of the heating zone 7 or the entire heating vessel content, the heat quantity limiter formed by the temperature sensors 20 and 38 cooperating with an electronic control system.

The apparatus described with reference to Figure 2 can be extended if, where there is adequate energy available, not only the entire content of the vessel 1 can be heated by the external energy source 35 to a predetermined required temperature and be maintained at this temperature, but furthermore sufficient energy is available for supplying another hot water producer, such as a swimming bath heater or the like. Such further plant, can, in the usual manner, consist for example of a heating boiler 44, a hot water reservoir 45 and a consuming device 46.

The coupling of such a plant to the apparatus of this invention is effected via a heat exchanger 47, which on the primary or heat input side is connected at the cold water inlet via a line 48 with the line 30 of the primary circuit 37 and on the hot water outlet via a line 49 with the multiway valve 36 of the line 32. Here, the multi-way valve 36 is connected to a priority circuit, not illustrated, by which when a predetermined maximum heating vessel temperature of, for instance, 650C at the temperature sensor 38 is reached, a comparison with temperature sensors 50, 51, 52 in a secondary or heat output circuit 54 of the heat exchanger 47 is carried out.The temperature sensor 50 is disposed at the secondary side outlet from the heat exchanger 47, whereas the temperature sensor 51 is disposed at the top and the temperature sensor 52 at the bottom.of the hot water reservoir 45. If the temperature at the sensor 50 is now higher than that at the sensor 51, after the multi-way valve 36 has already been switched over and the heat exchanger 47 is supplied from the external energy source 35, then a circulating pump 53 in the secondary circuit 54 of the heat exchanger 47 is switched on. In this manner, the content of the hot water reservoir 45 can be heated, possibly intermittently, until a predetermined temperature of, for example 900C is reached at the temperature sensor 52, whereupon the multi-way valve 36 can again be switched over to the heating vessel 1.This switching-over can also be carried out when the temperature sensors 20 or 38 at the vessel 1 indicate a drop below their setpoint temperatures.

In other respects, the hot water installation connected to the heating boiler 44 is controlled in a conventional manner. It is equipped with a priority valve 55 only for coupling it to the hot water supply apparatus of this invention. In principle, any desired number of consumer units can be connected in series and coupled with the heating vessel.

The solar collector shown in Figure 3 comprises an internally coated, five-flat surface substantially parabolic reflector 56 (three, seven or more surfaces and also a deeply concave instead of a shallow form of construction are also possible). The reflector 56 makes an angle 58 of 50 with a transparent covering surface 57 and an angle 60 of 141 with a rear surface 59, to achieve a good approach to a parabolic form. The width of the approximately rectangular, elongated cover 57 is approximately 1 m and its distance from the rear surface 59 is approximately 0.35 m.

The angle lines 61 and 62 are at a distance of approximately 0.08 m from the plane of the rear surface 59. A cylindrical vessel 63, which may form the heating vessel, having a diameter of approximately the width of the rear surface 59 extends within the cross-section constituted by the parabolic reflector 56 and the covering surfaces 57 concentrically with the focal axis 64 of the approximately parabolic form of the reflector and can be provided densely on its external surface with semiconductor elements 65 which emit heat under the action of incident light.

By this arrangement, an extraordinarily large and intensively utilized collector surface for solar energy is created. The external surface 66 of the reflector 56 can be thermally insulated, and the entire solar collector can be rotated about the focal axis 64 which forms a rotational axis to follow the path of the sun and, during night time or when there is no incident sunshine, can be protected from heat losses and harmful environmental influences by exposing the external surface 66 upwards to the atmosphere or by covering the surface 57 by a closable cover controlled in some other manner. This can be done, for example, by a further heat insulating semi-cylindrical shell 67, which is so disposed about its pivot axis 11 that it covers the surface 57 when there is no incident radiation.Such a solar collector achieves a higher energy output than a flat collector having an equal collector area. The vessel 63 can simultaneously, with particular advantage, undertake the function of the heating vessel of the apparatus of this invention and thereby ensure space-saving and thermodynamically especially efficient heating of water. The vessel then simultaneously fulfils the function of a heating vessel, a reservoir and a heat absorber. For this purpose the casing of the vessel may be of transparent material and may have radiation-absorbing material in the vessel. It is also possible to utilize the vessel solely as a hot water reservoir for the solar collector and to coupled the latter when required to the apparatus of this invention.In each case the facility is provided of pivoting the solar collector at a concentric, end inlet and outlet connection respectively of the vessel 63, for the purpose of following the course of the sun.

Spaces adjoining the vessel 63 can be evacuated in order to avoid heat losses.

A thermally insulated solar collector 35 as shown in Figure 4 is especially effective. It is again equipped with a reservoir or heating vessel in the form of a vessel 63 and a thermally insulated cover 67 of circular arc form. The focal axis of the parabolic reflector 56 coincides with the centre points 11 of the cylindrical reservoir and of the cover. The solar collector is guided with its outer edges 68, 69 along a circular arc 70 shown in a broken line to follow the course of the sun (the position for midday or the zenith is shown). Since the circular arc 70 has the same radius of curvature as the inner surface of the cover 67, the reservoir is completely thermally insulated or capable of being thermally insulated (the latter when the cover 67 is rotated about the axis 11), during the night and also when there is no incident sunlight.

Claims

1. Liquid heating apparatus comprising a heating vessel having a cold liquid inlet duct, a hot liquid discharge duct, a heating device which is mounted in the upper part of the vessel and forms a heating zone in the upper part of the vessel and a liquid circulating duct having a circulating pump, the circulating duct leading from the lower part of the vessel and re-entering the vessel in the heating zone, wherein means are provided for adjustably limiting the quantity of heat supplied by the heating device.

2. Apparatus according to claim 1, in which there is a temperature sensor in the heating zone, the sensor controlling the operation of the circulating pump.

3. Apparatus according to claim 1 or claim 2, in which there are one or more temperature sensors in the lower part of the vessel for controlling the heating device.

4. Apparatus according to any one of claims 1 to 3, in which there is a heat exchanger, the heat output side of which is incorporated in the circulating duct, and a heat input circuit of which is supplied with heat from an external heat source.

5. Apparatus according to any one of claims 1 to 4, in which there is a temperature sensor in the circulating duct or in the secondary heat output side of the heat exchanger.

6. Apparatus according to claim 4 or claim 5, in which the heat input circuit of the heat exchanger includes a further circulating pump.

7. Apparatus according to any one of claims 4 to 6, in which there is a temperature sensor in the heat input circuit of the heat exchanger.

8. Apparatus according to any one of claims 4 to 7, in which the external heat source is a solar collector.

9. Apparatus according to any one of claims 4 to 8, in which the heating vessel is disposed within the external energy source, for example the solar collector.

10. Apparatus according to claim 8 or claim 9, in which the solar collector comprises a reflector having a focal point, in the region of which at least one semi-conductor element is disposed.

11. Apparatus according to claim 10, in which there are a plurality of semi-conductor elements disposed concentrically around the focal point of the reflector which is substantially parabolic.

12. Apparatus according to claim 10 or claim 11, in which the reflector comprises at least three plane reflecting surfaces.

1 3. Apparatus according to any one of claims 9 to 12, in which the solar collector has a cover by which it can be covered in dependence upon the strength of the solar radiation incident upon it.

14. Apparatus according to any one of claims 9 to 13, in which the solar collector comprises a parabolic reflector of which the edges lie on a circular arc centred on the parabola focal point, a thermally insulating cover, the inner surface of which lies on the circular arc, and the heating vessel which is cylindrical and has its centre axis inside the parabola.

1 5. Apparatus according to any of the preceding claims, further comprising aciditional temperature sensors in the heating vessel and circulating duct for controlling the circulating pump.

1 6. Apparatus according to any one of the preceding claims, in which there is a safety relief valve connected to the top of the heating vessel.

1 7. Apparatus according to any one of the preceding claims, in which the circulating duct leads through a plurality of water reservoirs arranged in series with each other.

18. Apparatus according to any one of the preceding claims, in which there is a safety thermostat connected to the heating device to shut down the device when a predetermined maximum temperature is reached at some component of the apparatus.

1 9. Apparatus according to claim 4 or any one of claims 5 to 8 when dependent on claim 4, in which there is a further heat input circuit coupled by a priority circuit to the first heat input circuit of the heat exchanger.

20. Apparatus according to any one of the preceding claims, in which the or each temperature sensor is a contact thermometer which produces a switching signal at a temperature which is adjustable.

21. A method of operating an apparatus according to claim 1, characterised in that while the liquid in the heating zone is being heated by the heating device the circulating pump is in operation and, after the emission of a predetermined quantity of heat from the heating device, the heating device is put out of operation.

22. A method according to claim 21, in which a temperature sensor switches on the circulating pump when the temperature of the liquid in the heating zone rises to a predetermined value and switches off the pump when the said temperature falls below another lower predetermined value.

23. A method ccording to claim 21 or claim 22, characterised in that the heating device is put out of operation when a predetermined liquid temperature is reached at a specific level in the vessel below the heating device.

24. A method according to claim 23, characterised in that the circulating pump is kept in operation after the heating device has been put out of operation.

25. A method according to any one of claims 21 to 24, characterised in that a circulating pump in a heat input circuit of a heat exchanger, a heat output side of which is incorporated in the circulating duct, is switched on as soon as the temperature at the heat output side falls below the temperature in the heat input circuit.

26. A method according to claim 25, characterised in that a temperature sensor which switches on and off the circulating pump in the heat input circuit switches off the heating device when a predetermined temperature is exceeded.

27. A method according to claim 25 or claim 26, characterised in that a circulating pump in a circuit at the heat output side of the heat exchanger is switched on when the temperature at the heat output side of the heat exchanger is higher than the temperature In the heating zone.

28. A method according to any one or more of claims 21 to 27, characterised in that a safety relief valve connected to the top of the heating vessel opens when make-up cold liquid flows into the vessel if a temperature sensor situated adjacent a component of the apparatus exceeds a predetermined temperature.

29. A method according to any one of claims 21 to 28, characterised in that the quantity of liquid to be heated is infinitely regulated over a range by means of a temperature sensor in the heating zone and also a temperature band sensor in the heating zone.

30. Apparatus according to claim 1, substantially as described with reference to Figure 1, or Figure 2, or Figure 2 as modified with reference to Figure 3 or Figure 4, of the accompanying drawings.

31. A method according to claim 21, substantially as described with reference to Figure 1, or Figure 2, or Figure 2 as modified with refernce to Figure 3 or Figure 4, of the accompanying drawings.