AU2012359096A1

AU2012359096A1 - Cooling device and method for controlling a cooling device

Info

Publication number: AU2012359096A1
Application number: AU2012359096A
Authority: AU
Inventors: Mario LENTZ; Admilson Pinto; Uwe SCHRAMER
Original assignee: Dometic SARL
Current assignee: Dometic SARL
Priority date: 2011-12-20
Filing date: 2012-09-10
Publication date: 2014-07-10
Anticipated expiration: 2032-09-10
Also published as: WO2013091914A1; IN2014DN05650A; EP2795206B1; EP2795206A1; CN104024767B; KR20140105541A; KR20170043680A; CN104024767A; AU2012359096B2; DK2795206T3; US20150000318A1; HK1199922A1

Abstract

The invention relates to a cooling device, comprising at least one regeneratively operated primary cooling circuit, in particular a solar-powered cooling circuit, wherein the cooling circuit has at least one compressor, at least one condenser, at least one evaporator, at least one cooling space, at least one temperature sensor for measuring the cooling space temperature (T

Description

WO 2013/091914 PCT/EP2012/067607 Cooling Device and Method for Controlling a Cooling Device The present invention relates to a cooling device having at least one regeneratively operated primary cooling circuit, wherein the cooling circuit has at least one compressor, at least one condenser, at least one evaporator, at least one cooling space, at least one temperature sensor for measuring the cooling space temperature in the cooling space, and a controller, wherein a desired temperature value of the cooling space and a comparison temperature value can be stored in the controller. The present invention further relates to a method for controlling a cooling device. Typically, such cooling devices are employed in remote areas, in particular in developing countries where a stable and safe energy supply under normal circumstances cannot be ensured. Here, it has been found to be feasible to generate the power required for operation in a photovoltaic manner since the solar insolation in most developing countries is sufficiently high throughout the year. Thus, also delicate goods, such as for example medical products or food that need to be cooled can be stored safely, whereby the quality of life of the local people can be improved. For that, the World Health Organization (WHO) has made a catalogue with threshold criteria that has to be fulfilled by the used cooling device and the used cooling equipment for the transport and storage of medical products. Here, it is also a guideline that the cooling tem perature is substantially in the range between +21C and +81C, and this temperature range can be maintained over at least three days also independent from the power supply. However, this also directly indicates the greatest disadvantage of the photovoltaically oper ated cooling devices, namely that cooling during the solar insolation-free time (e.g. at night or with clouds) is not possible or can only be ensured with the help of an external power source, respectively. However, just the latter is generally not possible. Also the use of batter ies is not very practical, since these would significantly increase the first cost, the supply of spare parts and maintenance would turn out to be difficult, and an environmentally aware disposal of waste batteries is almost impossible to accomplish. Also, this results in a certain requirement on the controller of the actual cooling method, since the only limitedly available energy has to be used as intelligent as possible for cooling. 1 WO 2013/091914 2 PCT/EP2012/067607 Thus, in the solutions known from the prior art it is built on a high thermal mass of the cooling elements for storing the cold. Here, at solar insolation, if present, the compressor perma nently runs to maintain a sufficient refrigerating capacity. Then, the stored cold is often intro duced via a fan into the actual cooling space, in which the medical products are stored. To prevent that here the cooling space temperature is falling below a temperature of 01C at which the medical products storing in the cooling device would be damaged, such cooling devices have a heating that provides thermal energy as needed. This system has proved to be extremely practicable in longstanding field experiments. How ever, in these systems the storage capacity for the medical products is correspondingly lim ited by the required thermal mass for storing the cold. Moreover, a number of components is required, such as for example the heating device and the fan, the supply of spare parts and maintenance of which sometimes can involve problems. Moreover, it is also the fact that a uniform freezing of the thermal mass and the cooling elements used therefore, respectively, cannot arise or only after a very long operating period. Thus, it is the problem of the present invention to provide a cooling device in which the above-mentioned criteria of the WHO can be fulfilled, wherein a sufficient storage capacity of the cooling device with renunciation of an additional heating can be provided and a uniform and complete freezing of the used cooling elements is ensured. Moreover, it is the problem of the present invention to provide a method for controlling such a cooling device. The solution of the problem is accomplished with a cooling device according to claim 1 and a method for controlling a cooling device according to claim 7. Practical developments are de scribed in the dependent claims. The cooling device according to the invention in contrast to the cooling devices known from the prior art is characterized in that the cooling of the cooling space can be interrupted by the controller and the comparison temperature value can be changed by the controller depend ing on the time and/or the cooling space temperature. This is advantageous in that there is no permanent cooling capacity, but the cooling can be interrupted when a desired cooling space temperature has been reached. This is achieved by comparison of the comparison temperature value with the cooling space temperature. When after a certain time there is still energy available or the desired temperature has not yet been reached, respectively, the con troller can reduce the comparison temperature value such that a further cooling of the cool- WO 2013/091914 3 PCT/EP2012/067607 ing space or the used cooling elements takes place, respectively. Here, in order not to come into a critical range in which a damage of the cooled good must be expected an interruption of the cooling is also made possible as a function of the actual cooling space temperature. Thus, with an existing energy there is a cooling that is however interrupted as soon as a damage of the cooled good by too low temperatures is to be expected. Preferably, the primary cooling circuit has at least one extra cooling space and at least one extra evaporator assigned to the extra cooling space. The extra evaporator is connected to the primary cooling circuit via a valve, and the cooling of the cooling space can preferably be interrupted by switching the valve. Thus, with an existing energy despite already sufficiently low temperatures in the cooling space a further cooling can occur. Here, it is conceivable that the extra cooling space has a storage tank with which the cooling space can be cooled in the absence of energy. Alternatively, the extra cooling space may be a freezing compart ment for cooling ice bags or the like. Latter may be used to transport medical products for shorts routes. It may be advantageous if the cooling device has a second cooling circuit with a second compressor for cooling a second cooling space, wherein the second compressor can be supplied with energy via a switch. Thus, with a further excess of energy a second cooling circuit for cooling a further cooling space can be used. The second cooling space may have for example a storage tank or may be a freezing compartment. Preferably, the cooling device has an additional circuit, wherein the controller can be sup plied with energy via the additional circuit. So the energy supply of the controller is inde pendent from the energy supply of the cooling circuit. Here, it is advantageous if the additional circuit has a transformer. In this way, also with a drop in voltage in the input voltage a constant output voltage can be maintained. This pre vents a switching-off of the controller also in a drop of voltage that can be conditional on the turn-on transient of the compressor, for example. Preferably, the additional circuit has at least one condenser. In this way, the controller can be supplied with energy even if actually no energy is available any more, for example during a solar insolation-free time conditional on clouds.

WO 2013/091914 4 PCT/EP2012/067607 In terms of the method, the solution of the problem is accomplished in that the comparison temperature value in switching on the controller corresponds to the desired temperature val ue, and the cooling of the cooling space is interrupted if the actual cooling space tempera ture has reached the comparison temperature value, wherein the comparison temperature value is reduced after a predetermined time period by a stored correction value as long as the actual cooling space temperature has not reached the comparison temperature value within a predetermined time period. That is, if the desired cooling space temperature has not yet been reached after the predetermined time period the comparison temperature value is reduced. Background here is that a non-reaching of the desired temperature value within the predetermined time period also indicates a relatively high initial or outdoor temperature, re spectively, so that the cooling space preferably is cooled more and over a prolonged time period, so that the used cooling elements completely freeze. Here, it should be ensured that the correction value is not chosen so high that the temperature in the cooling space falls un der a critical range in which the cooled good can be damaged. Here, it can be suitable if the cooling of the cooling space is interrupted by switching off the compressor of the primary cooling circuit. It is particularly suitable if the compressor is switched off if the actual cooling space temperature falls below a comparison temperature value by a hysteresis value and the compressor is switched on if the actual cooling space temperature exceeds the comparison temperature value by a hysteresis value. In this way, an excessive cooling or an excessive heating, respectively, of the cooling space is prevent ed. In other words, the actual cooling space temperature oscillates by the comparison tem perature value, wherein the variations correspond to the hysteresis value. Therefore, also a delayed response of the cooling circuit or the compressor, respectively, can be taken into account. Preferably, the comparison temperature value is gradually increased by an amount of the stored correction value until the comparison temperature value corresponds to the desired temperature value with the comparison temperature value being increased in switching off the compressor by the next step. So it is ensured that the compressor runs as long as possi ble and cooling elements used for cooling the cooling space are completely frozen before the desired temperature value is reached. It is suitable if the cooling of the cooling space is interrupted by switching the valve. It is par ticularly suitable if the valve is switched if the actual cooling space temperature falls below WO 2013/091914 5 PCT/EP2012/067607 the comparison temperature value by a hysteresis value and the valve is switched if the ac tual cooling space temperature exceeds the comparison temperature value by the hysteresis value. That means, that by switching the valve the extra cooling space is cooled by the extra evaporator until the valve switches again and the evaporator again cools the cooling space. This is advantageous in that the compressor permanently runs and in phases wherein the cooling space must not be cooled the extra cooling space is cooled. Here, it is advantageous if the comparison temperature value is increased by an amount of the stored correction value until the comparison temperature value corresponds to the de sired temperature value, wherein in switching the valve the comparison temperature value is increased by the next step. This ensures that the compressor operating time is at the maxi mum and as much cold as possible can be stored. Moreover, it is considered to be advantageous that the second compressor is supplied with energy if the compressor of the primary cooling circuit is switched off. If there is sufficient energy available it is particularly suitable if the compressor of the primary cooling circuit and the second compressor are simultaneously supplied with energy. So, a maximum cold yield from the available energy can be achieved. Preferably, the output voltage of the additional circuit is maintained constant independent of the input voltage by the DC transformer. Thus, the controller does not switch off when the input voltage briefly drops for example by the turn-on transient of the compressor. This is in particular the case, if there is briefly not enough energy available, for example due to a cloud, and therefore the compressor switches off. This results in the advantage that the pos sibly already reduced or increased actually valid comparison temperature value stored in the controller is not lost. Moreover, this also ensures that the already elapsed time of the prede termined time period is maintained until the reduction of the comparison temperature value. Otherwise, this would cause that this period every time begins to run anew after switching off the controller. Thus, it can be prevented that the desired temperature value is reached be fore the cooling elements are completely frozen. It is advantageous if the switch-off of the controller by the at least one condenser of the addi tional circuit during a critical time period is prevented. Here, by a critical time period the time period can be understood at which no energy is available despite solar insolation. This is in particular the case due to clouds or animals or vehicles standing in front of the solar module.

WO 2013/091914 6 PCT/EP2012/067607 The capacity of the at least one condenser should be chosen such that a normally anticipat ed critical time period can be bridged. It can be considered to be advantageous if in switching on the controller the comparison temperature value is set to the desired temperature value. In the following, the invention is explained in detail by way of examples illustrated in the drawings. Here: Fig.1 schematically shows a principle sketch of the construction of the cooling de vice according to the invention; Fig. 2 schematically shows an operation image of a cooling device according to the invention with a primary cooling circuit and a second cooling circuit; Fig. 3 schematically shows a time chart for the cooling space temperature, the com parison temperature value, and the compressor operating time for a cooling device with a primary cooling circuit from putting into operation; Fig. 4 schematically shows a time chart for the cooling space temperature, the com parison temperature value, and the compressor operating time for a cooling device with a primary cooling circuit after putting into operation; and Fig. 5 schematically shows a time chart for the cooling space temperature, the com parison temperature value, the compressor operating time, and the valve posi tion for a cooling device with a primary cooling circuit with two evaporators. Fig. 1 shows a sketch of the principle construction of the cooling device 1 according to the invention. The illustrated cooling device 1 has a primary cooling circuit 2 (for that, see fig. 2), a controller 6, and an additional circuit 11. The controller 6 is connected to the various com ponents of the cooling circuit 2 via control lines 18. The represented cooling system 1 is re generatively supplied with energy by a solar module 16 converting the light of the sun into electric energy. Then, the energy is supplied via power lines 17, wherein the controller 6 is supplied with energy independently of the remaining cooling circuit 2 via the additional cir cuit 11. For that, the additional circuit 11 has a DC transformer (not illustrated) and a plurality of condensers (not illustrated) to maintain the output voltage for supplying the controller 6 also in case of a drop of the input voltage. Also, the condensers permit an energy supply of the controller 6 when the insolation of the sun does not provide sufficient energy for opera- WO 2013/091914 7 PCT/EP2012/067607 tion. In particular, with the condensers thus a prolonged cloud period or the like can be bridged. In fig. 2, an operation image of a cooling device according to the invention with a primary cooling circuit 2 and a second cooling circuit 9 is illustrated. The primary cooling circuit 2 consists of a compressor 3, a condenser 4, a filter drier 14 as well as two evaporators 5, 7 that are connected to the cooling circuit 2 via a valve 8. Between the valve 8 and the evapo rators 5, 7 each a throttle 15 arranged. Via the evaporator 5 the cooling space (not illustrat ed) is cooled and via the extra evaporator 7 the extra cooling space (not illustrated) is cooled. The latter may be for example a storage tank or a freezing compartment. The con troller 6 is connected to the compressor 3 via a first control line 19, so that the compressor 3 can be switched on and off by the controller. The valve 8, which in particular is a three-way solenoid valve, is connected to the controller 6a via a second control line 20, so that the con troller 6 can switch the cooling circuit either to the evaporator 5 or to the extra evaporator 7. In other words, by switching the valve 8 with the controller 6 it can be decided whether the cooling space or the extra cooling space is to be cooled. For that, the controller 6 detects the actual temperature in the cooling spaces via temperature sensors to obtain an optimum cool ing. Moreover, in fig. 2 a second cooling circuit 9 with a second compressor 10, a second con denser 13, a filter drier 14, and a second evaporator 12 is illustrated. A throttle 15 is ar ranged upstream of the second evaporator 12. This second cooling circuit 9 is used for cool ing a second cooling space (not shown) that may be for example a storage tank or a freezing compartment. The second compressor 10 is connected to the controller 6 via a third control line 21, so that the second compressor 10 can be switched on and off by the controller 6. That is, in the example illustrated in fig. 2 the controller 6 can control the cooling of three cooling spaces in total by switching on and off the compressors 3, 10 and/or by switching the valve 8. Fig. 3 shows a time chart for the cooling of a cooling device with a primary cooling circuit and a cooling space when putting the cooling device into operation. When mounting the cooling device the temperature in the cooling space Tair corresponds to the ambient temperature. As soon as the cooling device is put into operation the controller switches on the compressor (C on) and the cooling circuit begins to cool the cooling space whereby the cooling space temperature Tair decreases. For that, a desired temperature value SET, a comparison tem- WO 2013/091914 8 PCT/EP2012/067607 perature value TSET and a hysteresis value HW are stored in the controller. When putting into operation the comparison temperature value TSET corresponds to the desired temperature value SET. Further, the controller measures the temperature in the cooling space Tair via a temperature sensor and matches it with the comparison temperature value TSET. If the tem perature in the cooling space Tair after a predetermined time period to has not yet reached the comparison temperature value TSET, the controller reduces the comparison temperature value TSET by a stored correction value dSET. The stored correction value dSET is chosen such that there is no reduction of the comparison temperature value TSET below O'C. The cooling of the cooling space now is continued until the temperature in the cooling space Tair has reached the now reduced comparison temperature value TSET minus the hysteresis val ue HW. The controller switches off the compressor (C off). Next, the comparison tempera ture value TSET is gradually increased. Here, each increase of the comparison temperature value TSET in n steps corresponds to the n-th part of the correction value dSET. Since now there is no cooling the temperature in the cooling space Tair rises. As soon as the tempera ture in the cooling space has reached the comparison temperature value TSET now increased by dSET/n plus the hysteresis value HW the compressor is switched on again (C on) and the cooling space is cooled again. The temperature in the cooling space Tair drops. If this now has reached the comparison temperature value TSET minus the hysteresis value HW the compressor is switched off again (C off) and the comparison temperature value TSET is in creased by the next amount dSET/n. This process now repeats until the comparison tempera ture value TSET again corresponds to the desired temperature value SET. Since the controller has an energy supply independent of the compressor also in a brief switch-off of the com pressor by lacking solar insolation the cycle is not restarted every time, but continued from the point of interruption. If at the end of the solar cycle, that is at nightfall, not all n steps should have been completed, then the comparison temperature value TSET for the next solar cycle is set back to the desired temperature value SET. In this way, the compressor operat ing time is maximized and ensured that the cooling elements do completely and homogene ous freeze. In the following, the mode of operation is explained in detail by way of a numerical example. Here, WO 2013/091914 9 PCT/EP2012/067607 Cooling space temperature when putting into operation Tair: 301C Desired temperature value SET: 5CC Comparison temperature value when putting into operation TSET: 5cc Correction value dSET: 2cC Hysteresis value HW: 0.1C Number of steps n: 4 predetermined time period to: 6h When putting the cooling device into operation the compressor is switched on and the cool ing space temperature Tair of 30cG starts to drop. When after 6h the cooling space tempera ture Tair is e.g. 91C and thus is higher than the comparison temperature value TSET Of 50G, the comparison temperature value TSET is reduced by the correction value dSET to 3CG. As soon as the cooling space temperature Tair now has reached a value of 2.9C (T SET - HW = 3cC - 0.1cC) the compressor is switched off and the comparison temperature value is in creased by 0.5cC (d SET/n = 2cG/4) to 3.5cC. The cooling space temperature Tair starts to rise. As soon as the cooling space temperature Tair has reached a value of 3.6C (T SET + HW = 3.5cC + 0.1cC) the compressor is switched on again and the cooling space temperature Tair drops again. When the cooling space temperature has reached 3.4C (T SET - HW = 3.5C 0.1cc) the compressor is switched off again and the comparison temperature value is in creased to 4cC (TSET + dSET/n = 3.5C + 2cC/4). This procedure repeats in tota I n = 4 times until the comparison temperature value TSET again corresponds to the desired temperature value SET of 5cC. Then, the compressor is switched on at a temperature in the cooling space Tair of 5.10 (TSET + HW = 5cC + 0.1cC) and switched off at a cooling space tempera ture of 4.9cC (TSET - HW = 5cC - 0.1cC). In fig. 4, a time chart is illustrated that continues the time chart of fig. 3. In this illustration, the comparison temperature value TSET is already again identical to the desired temperature val ue SET and the compressor is switched on and off as stated above. The temperature in the cooling space Tair oscillates around the comparison temperature value TSET. If now there is a longer interruption of the energy supply, for example at night, the compressor is switched off (C off) and the temperature in the cooling space Tair rises. As soon as energy is available again the compressor is switched on (C on) and the cycle described in fig. 3 is restarted. The comparison temperature value TSET corresponds to the desired temperature value SET. The cooling space temperature Tair reaches the comparison temperature value TSET within the time t, wherein this time period t is less than the predetermined time period to. Thus, the WO 2013/091914 10 PCT/EP2012/067607 comparison temperature value TSET is not reduced by the correction value dSET, but remains identical to the desired temperature value SET. Fig. 5 in principal shows the same time chart as in fig. 3, wherein in this example additionally an extra evaporator and an extra cooling space are provided that can be controlled by a valve (V). The temperature of the extra cooling space Tbuffer when putting into operation cor responds to the ambient temperature and thus the cooling space temperature Tair. In contrast to the time chart illustrated in fig. 3 in this example the compressor is not switched off if the cooling space temperature Tair corresponds to the comparison temperature value TSET, but the valve is switched such that the extra cooling space (V on) is cooled and the temperature in the extra cooling space Tbuffer drops. When switching the valve also the comparison tem perature value TSET is gradually increased by the n-th part of the correction value dSET. As soon as the temperature in the cooling space Tair corresponds to the now increased compar ison temperature value TSET plus the hysteresis value HW the valve is switched off again (V off) and the cooling space is cooled, whereby the cooling space temperature Tair drops again. The valve is only switched again when the cooling space temperature Tair has reached the comparison temperature value TSET minus the hysteresis value HW. This pro cess is then repeated for n times in total until the comparison temperature value TSET again corresponds to the desired temperature value SET. As can well be seen in this example, the compressor is not switched off during the whole cycle, but remains permanently switched on (C on).

WO 2013/091914 11 PCT/EP2012/067607 List of Reference Numbers 1 cooling device 2 primary cooling circuit 3 compressor 4 condenser 5 evaporator 6 controller 7 extra evaporator 8 valve 9 second cooling circuit 10 compressor 11 additional circuit 12 second evaporator 13 condenser 14 filter drier 15 throttle 16 solar module 17 line 18 control line 19 first control line 20 second control line 21 third control line Tair cooling space temperature TSET comparison temperature value Tbuffer storage tank temperature SET desired temperature value dSET correction value to predetermined time period t time n steps C compressor switching V valve switching

Claims

1. A cooling device (1) having at least one regeneratively operated primary cooling circuit (2), in particular a solar-operated cooling circuit, wherein the cooling circuit (2) has at least one compressor (3), at least one condenser (4), at least one evaporator (5), at least one cooling space, at least one temperature sensor for measuring the cooling space temperature (Tair) in the cooling space, and a controller (6), wherein a desired temperature value (SET) of the cooling spaces and a comparison temperature value (TSET) can be stored in the control ler (6), characterized in that the cooling of the cooling space can be interrupted by the controller (6), and the comparison temperature value (TSET) can be changed by the controller (6) depend ing upon the time and/or the cooling space temperature (Tair).

2. The cooling device (1) according to claim 1, characterized in that the primary cooling circuit (2) has at least one extra cooling space and at least one extra evaporator (7) assigned to the extra cooling space, wherein the extra evaporator (7) is con nected to the primary cooling circuit (2) via a valve (8), and the cooling of the cooling space can be interrupted by switching the valve (8).

3. The cooling device (1) according to claim 1 or 2, characterized in that the cooling device (1) has a second cooling circuit (9) with a second compressor (10) for cooling a second cooling space, wherein the second compressor (10) can be supplied with energy via a switch.

4. The cooling device (1) according to anyone of the preceding claims, characterized in that the cooling device (1) has an additional circuit (11), wherein the controller (6) can be sup plied with energy via the additional circuit (11).

5. The cooling device (1) according to claim 4, characterized in that the additional circuit (11) has a DC transformer. WO 2013/091914 13 PCT/EP2012/067607

6. The cooling device (1) according to claim 4 or 5, characterized in that the additional circuit (11) has at least one condenser.

7. A method for controlling a cooling device according to anyone of the preceding claims 1 to 6, characterized in that when switching on the controller the comparison temperature value (TSET) corresponds to the desired temperature value (SET), and the cooling of the cooling space is interrupted when the actual cooling space temperature (Tair) has reached the comparison temperature val ue (TSET), wherein the comparison temperature value (TSET) is reduced after a predetermined time period (to) by a stored correction value (dSET), as long as the actual cooling space tem perature (Tair) has not reached the comparison temperature value (TSET) within the predeter mined time period (to).

8. The method according to claim 7, characterized in that the cooling of the cooling space is interrupted by switching off the compressor.

9. The method according to claim 8, characterized in that when the actual cooling space temperature (Tair) falls below the comparison temperature value (TSET) by a hysteresis value (HW) the compressor is switched off.

10. The method according to claim 9, characterized in that when the actual cooling space temperature (Tair) exceeds the comparison temperature val ues (TSET) by the hysteresis value (HW) the compressor is switched on.

11. The method according to claim 10, characterized in that the comparison temperature value (TSET) is gradually (n) increased by an amount of the stored correction value (dSET/n) until the comparison temperature value (TSET) corresponds to the desired temperature value (SET), wherein the comparison temperature value (TSET) is increased by the next step (dSET/n) when the compressor is switched off. WO 2013/091914 14 PCT/EP2012/067607

12. The method according to claim 7, characterized in that the cooling of the cooling space is interrupted by switching the valve.

13. The method according to claim 12, characterized in that when the actual cooling space temperature (Tair) falls below the comparison temperature value (TSET) by a hysteresis value (HW) the valve is switched.

14. The method according to claim 13, characterized in that when the actual cooling space temperature (Tair) exceeds the comparison temperature val ue (TSET) by the hysteresis value (HW) the valve is switched.

15. The method according to claim 14, characterized in that the comparison temperature value (TSET) is gradually (n) increased by an amount of the stored correction value (dSET/n) until the comparison temperature value (TSET) corresponds to the desired temperature value (SET), wherein the comparison temperature value (TSET) is increased by the next step (dSET/n) when the valve is switched.

16. The method according to anyone of the preceding claims 7 to 15, characterized in that the second compressor is supplied with energy via a switch.

17. The method according to claim 16, characterized in that the second compressor is supplied with energy when the compressor of the primary cooling circuit is switched off.

18. The method according to claim 16 or 17, characterized in that with sufficient energy the second compressor and the compressor of the primary cooling circuit are simultaneously supplied with energy. WO 2013/091914 15 PCT/EP2012/067607

19. The method according to anyone of the preceding claims 7 to 18, characterized in that the output voltage of the additional circuit is maintained constant by the DC transformer in dependent of the input voltage.

20. The method according to anyone of the preceding claims 7 to 19, characterized in that switching off the controller during a critical time period by the at least one condenser of the additional circuit is prevented.

21. The method according to anyone of the preceding claims 7 to 20, characterized in that when switching on the controller the comparison temperature value (TSET) is set to the de sired temperature value (SET).