Patent Publication Number: US-9423151-B2

Title: Electric heating system, a control head and a heating liquid

Description:
CROSS REFERENCE TO A RELATED APPLICATION 
     This application is a National Stage Application of International Application Number PCT/EP2012/004282, filed Oct. 12, 2012; which claims priority to European Application No. 11 008 313.6, filed Oct. 14, 2011; and claims the benefit of U.S. Provisional Application Ser. No. 61/547,163, filed Oct. 14, 2011; all of which are incorporated herein by reference in their entirety. 
    
    
     The present invention relates to an electric heating system, a control head and a liquid to be used in an electric heating system and, in particular, to an electric heating system, wherein electric energy is converted into thermal energy by directing an electric current through the heating liquid, thereby heating the heating liquid. 
     BACKGROUND OF THE INVENTION 
     Electric energy is considered to be a clean energy, which does not produce any pollution when consuming the electric energy. In addition, electric energy can easily be managed and controlled to meet particular demands and, moreover, is widely available. Therefore, the use of electric energy also for heating purposes gains increased importance. 
     Conventional heating systems rely mostly on chemical energy stored, e.g., in oil, gas or coal which cause a significant amount of pollution when producing heat by burning these fossil recourses to heat, for example, water for buildings. The corresponding emissions lower the air quality, in particular, in densely populated areas and, in addition, transport still a significant amount of heat in the environment, because chemical energy can not be used to 100% for the desired purpose. 
     Also conventional electric heaters are disadvantageous in that they rely on electric current applied to heat wires, thereby heating the wires and subsequently the water or air surrounding the heat wires. However, conventional electric heaters need always some isolation, thereby slowing down the heating process. In addition, the heat wires are subject to significant wear and tear and thus become less efficient with time. 
     Therefore, there is a need for providing heating systems, which work efficiently, provide thermal energy in short time, and are easy to manage and to control while working at zero emission. 
     SUMMARY OF THE INVENTION 
     The aforesaid problems are solved by an electric heating system according to claim  1 , a control head according to claim  14  and a heating liquid according to claim  15 . Claims  2 - 13  refer to specifically advantageous realizations of the subject matter of claim  1 . 
     The present invention solves the aforesaid problems in that a heating system to heat a main heating circulation comprises an electric heater, a control head, a heat exchanger, a pump, and a plurality of tubes. The electric heater is adapted to heat a primary heating liquid by applying an electric current directly to the primary heating liquid. The control head is adapted to determine a temperature and a pressure of the primary heating liquid. The heat exchanger comprises a first liquid passage for the primary heating liquid and a second liquid passage for a secondary heating liquid in the main heating circulation. The second liquid passage is in thermal contact with the first liquid passage to heat the secondary heating liquid while cooling the primary heating liquid. The tubes connect the electric heater, the control head, the heat exchanger and the pump to define a circulation for the primary heating liquid. The pump is adapted to pump the primary heating liquid such that heat is transferred from the heater via the heat exchanger into the main heating circulation (secondary circulation system). 
     In further embodiments, the heat exchanger provides a galvanic separation of the primary circulation systems from the secondary circulation system. For example, dielectric materials may be arranged between the primary liquid and the secondary liquid so that no electric current can flow between the primary liquid and the secondary liquid. The blocked electric current can either relate to a DC current (no transport of charge carriers between both heating liquids), but may also refer to an AC current (for example, in that the complex impedance of the heat exchanger is infinite). The galvanic separation provides thus an improved safety. 
     In further embodiments the safety is further improved in that the heat exchanger and the plurality of tubes (or system of tubes) are configured to prevent a user from getting into electric contact with the primary liquid. For this reason, also the tubes may, for example, comprise dielectric materials. In addition, the tubes may comprise optional metal fittings at the respective ends of the tubes and the metallic fittings can be grounded such that the primary liquid is in electric contact to a ground potential (e.g. zero potential). As consequence, a liquid flowing through the different components is in electric contact with the ground potential so that if the primarily liquid still contains some net electric charges (i.e. it is charged relative to the ground potential), these net charges will be transferred to the ground potential and can not cause any harm for a user which touches one of these components. 
     Further embodiments relate to a heating system, wherein said electric heater is connectable to a power supply comprising at least two power lines and said primary heating liquid flows through said electric heater along a flow path. The electric heater further comprises a central electrode connected to a central electrode terminal and a cylindrical outer electrode connected to an outer electrode terminal. The central electrode and the cylindrical outer electrode are separated by the flow path in a coaxial arrangement such that an electric current flows between the central electrode and the cylindrical outer electrode when the at least two power lines are connected to the central electrode terminal and to the outer electrode terminal. An advantage of such coaxial arrangements is that they do not need much space and are easy to manufacture. In addition, dependent on the particular demand they can be combined. For example, three of them can be arranged in parallel and connected to different phases of a power supply. 
     In further embodiments, the electric heater comprises a liquid inlet and a liquid outlet for the primary liquid, and, in addition, may comprise ground electrodes being arranged at the liquid inlet and/or at the liquid outlet. The ground electrodes may be arranged perpendicular to the liquid flow such that the liquid passes the ground electrodes (e.g., in that they comprise different openings for the flow of the primary liquid). As result the primary liquid outside the electric heater is on the ground potential. In further embodiments, the electric heater comprises an electrode assembly with a plurality of electrodes, which are arranged such that each of the electrodes is connected to the power source and the electrodes are formed such that the area, which is exposed to the primary liquid, is equal for the different electric power lines. For example, the electric power source can be a three-phase power supply so that, for example, five terminals are available, three of them for the three different phases of the power line, a further terminal for the neutral or null signal and a ground or earth terminal (which shall comprise a lower impedance than the neutral terminal). The ground terminal can be connected to one of the ground electrodes provided at the fluid inlet and fluid outlet, whereas the three phase electrodes are connected to the electrode assembly such that two adjacent electrodes are connected to different lines of the three-phase supply and the primary liquid flows between these adjacent electrodes. As a result, the electric current generated in the primary fluid is homogeneous throughout the electric heater, thereby providing an efficient mechanism for transforming the electric energy into heat energy. 
     In further embodiments, the control head comprises a working thermostat sensor and a safety thermostat sensor, wherein the working thermostat sensor is used to determine the temperature of the primary liquid. The safety thermostat sensor provides, for example, a signal when a temperature threshold signal is reached, thereby providing a security measure such that a maximum temperature can be set and monitored. For example, when the maximum threshold temperature is exceeded, the heating is automatically interrupted, e.g. in that the electric current through the primary liquid is interrupted. The working thermostat sensor may, e.g., be used to define two limits, an upper limit and a lower limit so that when the temperature reaches the lower limit, the heating starts and when the temperature of the primary liquid reaches the upper limit, the heating is interrupted. This defines a working range of the heating system. 
     Further embodiments comprise an optional control unit, which is configured to obtain the temperature and pressure from the control head and, based on the measured quantities, to operate the electric heater accordingly. For example, the control unit may be configured to use the measured temperature and pressure to control the electric heater in that the value of the current applied to the electric heater is modified. In addition, the electric current may be applied as pulses to the primary liquid (pulsed mode) and the control unit may be configured to modify a frequency of the pulses such that the temperature and/or the pressure is adjusted to be within acceptable operational limits. Optionally, the control unit and/or the control head may comprise a display for a user to show the current temperature and pressure and to show the operational limits. For example, an optional thermal-manometer may be arranged at the control head to display the current temperature and pressure in the heating system, which can thus be monitored by the user. 
     Optionally, the control head comprises an air vent which is configured to release air from the plurality of tubes to optimize the circulation of the primary heating liquid. In further embodiments the heating system comprises an expansion unit which is adapted to provide a constant (predetermined) pressure of said primary heating liquid in that a varying volume is provided for the primary heating liquid. Therefore, volume modifications due to heating and cooling of the system are compensated. The expansion unit may, e.g., comprise a bellow or similar devices which are able to expand the volume in case the pressure increases and shrink the volume when the pressure decreases. 
     In further embodiments the control head comprises access ports providing contact to the primary heating liquid and being configured to couple one or more devices selected from the group consisting of: the working thermostat sensor, the safety thermostat sensor, the pressure sensor, the expansion unit, and the air vent. Therefore, the control head may comprise seven inlets and/or outlets so that, in addition to the access ports a heating liquid inlet, which may be connected to the electric heater (via a tube or directly), and an outlet is provided. The control head may be provided as integral component. 
     The control unit can be configured to control the electric heater to operate in the pulsed mode, because the liquid is heated very quickly. In the pulsed mode no continuous electric current is applied to the primary liquid, but pulsed electric signals in an operational frequency are applied to the primary liquid. By changing the operational frequency of the pulsed signals, the temperature of the primary liquid can be controlled to be in predetermined ranges. In addition, the pulsed operational mode may ensure that no electrolytic gas is generated at the different electrodes (as e.g. hydrogen), because any generated gas ions can recombine in the periods between the pulses. In addition, the control unit may be configured to apply an alternating current to the electrodes of the electric heater so that also the frequency of the applied alternating current may ensure that no electrolytic gases can be generated by the current flowing through the primary liquid. Thus, the use of alternating current also suppresses the aggregation of gas at particular electrodes. The pulsed mode can, e.g., be set up in that the power of the power supply is periodically supplied to the primary liquid so that the current flowing through the primary liquid will sharply increase and drop rapidly after the power is disconnected from the electrodes. 
     In further embodiments the primary heating liquid may be any kind of fluid (or medium) suitable to generate thermal energy when electric current is applied thereto and which is suitable to transport the generated thermal energy to the heat exchanger. 
     Further embodiments relate to a specific liquid as primary heating liquid for use in a heating system as described before. For example, the primary liquid may comprise compounds such that the electric conductivity (or electric resistance) is within a predetermined range of 40-380 μS (micro Siemens). 
     This can, e.g., be achieved in that the following mixture of materials: 1. Distilled water (30-80%), 2. Sodium tetraborate (Borax—Na 2 B 4 O 7 ×10H 2 O—0.40-0.10%), 3. Propylene glycol (C 3 H 8 O 2  or HO—CH 2 —CHOH—CH 3 —20-65%), 4. Waterglass (sodium or potassium silicate—Na 2 SiO 3 —0.002-0.025%), 5. Ammonium molybdate ((NH 4 ) 2 MoO 4 —0.01-0.15%) and 6. Acetic acid (CH 3 COOH—1-3%). The ratio of the various components of the heating liquid is important to obtain a desired output power of the heating device (here and in the following all %-values may refer to volume-%). 
     In further embodiments the secondary liquid may be a mixture of (distilled) water, alcohol and/or glycol (e.g. 50% distilled water and 50% alcohol), or any other liquid. 
     Embodiments of the invention relate also to a control head for use in a heating system as described before. The control head comprises a plurality of access ports providing contact to the primary heating liquid and being configured: to couple to the working thermostat sensor for providing the temperature of the primary heating liquid, to couple to the safety thermostat sensor to provide a temperature threshold signal, to couple to said pressure sensor for providing the pressure of the primary heating liquid, and to couple to the expansion unit for compensating a volume expansion of the primary heating liquid. The control head is integrally formed. 
     Embodiments of the present invention have a number of advantages over the prior art. For example, by using an electric heater which applies electric current directly to water the water heats up very quickly. As consequence, a pulsed mode can be used to heat the water directly, which in turn can easily be controlled. This efficient operation mode is not possible in conventional systems, because of the heating delay of those systems. In addition, the control head can combine all needed monitoring devices (manometer, thermostat, thermometer, etc.) within a single piece, which can be connected directly or close to the electric heater. If, for example, the electric heater is in downstream direction from the control head, the temperature and pressure of the heated water can be monitored directly and immediately after the heater without much time delay. 
     Moreover, compared to conventional heating systems, which use, e.g., gas or oil and need a burning chamber, the heating system according to the present invention is very small. It is very simple in operation and, because a liquid is heated directly by the electric current, there is practically no possibility of damages or heater burning out. Due to the direct heating of the primary liquid, the heating is also very efficient and inexpensive. In case of any leakage of fluid, the heating system will stop immediately (because the pressure and/or temperature will exceed the operation limits) which prevents damages or even fire. The room temperature may, e.g., be automatically regulated by thermostats, which may control the heating system and turn it on and off as soon as the temperature has reached a predetermined limit. 
     Thus, the heating system provides a high measure of security and a high degree of protection, because the heat system would immediately cease to work upon depletion of water even without using the thermostat, auto fuse or an auto clutch. 
     The secondary circulation system provides the possibility to distribute the heated water also over different floors. Unlike other heating systems based on boilers burning fossil recourses, the electric heating system according to the present invention does not create any source of toxic fumes, ashes or any other hazardous materials for the health of the users and the environment. Finally, the heating system is completely silent at work. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and numerous advantages of the invention will be described hereafter in further detail with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates the heating system according to an embodiment of the present invention; 
         FIG. 2  illustrates the different components of the heating system; 
         FIG. 3  depicts different electrode assemblies and its connection within the electric heater; 
         FIG. 4  depicts two graphs illustrating the controlling of the heating system; 
         FIG. 5  depicts different sides of the control head according to embodiments; 
         FIG. 6  depicts the liquid inlet and outlet of the control head according to embodiment; and 
         FIG. 7  depicts two cross sectional views of the control head. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a heating system  100  comprising an electric heater  110 , a control head  120 , a heat exchanger  130 , a pump  140  and a plurality of tubes  105  connecting the electric heater  110 , the control head  120 , the heat exchanger  130  and the pump  140 . The electric heater is adapted to heat a primary (heating) liquid flowing through the tubes  105  by applying an electric current directly to the primary liquid. For example, the electric current (or electric voltage) may be applied on the primary heating liquid along a fluid passage  115  inside the electric heater  110 . The control head  120  is adapted to determine the operational parameter, as, e.g., a temperature and a pressure of the primary liquid. Optionally, the control head  120  may be configured to control that the operational parameters are in operational limits (e.g. to lower the pressure when an upper limit is exceeded). The heat exchanger  130  comprises a first liquid passage  131  for the primary heating liquid and a second liquid passage  132  for a secondary heating liquid in a main heating circulation, wherein the second liquid passage  132  is in thermal contact with the first liquid passage  131  to heat the secondary heating liquid while cooling the primary heating liquid. The pump  140  is adapted to pump the primary liquid through the system of tubes  105  within the primary circulation system (circulation for the primary liquid). 
       FIG. 2  shows in detail a preferred embodiment with different components of the heating system  100 . On top of the heating system  100  an optional control unit or control panel  160  is arranged and below the control panel  160  the heating system as shown in  FIG. 1  is accommodated within a case or housing  300 . 
     The electric heater  110  is connected with the control head  120 , either directly or via one of the plurality of tubes  105 . The control head  120  comprises a working thermostat sensor  230 , a safety thermostat sensor  240 , an air vent  250  and a connection  127  for an expansion unit  270 , which is connected to the control head  120  (directly or) via a first tube  105   a . The control head  120  comprises, moreover, a thermo-manometer  260 , which is adapted to show the temperature and/or the pressure of the primary liquid flowing in the system of tubes  105  (as indicated by the arrows). The control head  120  is connected with the heat exchanger  130  with a second tube  105   b.    
     Between the heat exchanger  130  and the pump  140  an optional connector  280  is arranged. The optional connector  280  comprises a further inlet  210  for the primary liquid (to fill the primary liquid in the tubes, e.g., via filling valve). In addition, the connector  280  comprises a pressure safety valve  211 , which is configured to open in case the pressure within the system of tubes  105  exceeds a safety threshold, to thereby prevent damages of the heating system. The heat exchanger  130  is connected with the optional connector  280  via a third tube  105   c.    
     The pump  140  may, e.g., be connected directly to the optional connector  280  (or via a further tube) and is configured to pump the primary liquid circulating within the system of tubes  105  such that the primary liquid flows from the pump  140  towards the electric heater  110 . Therefore, the pump  140  may be arranged upstream from the electric heater  110 , wherein the pump  140  may be directly connected to the electric heater  110  or may be connected via a fourth tube  105   d.    
     The system of tubes  105  can optionally be grounded by a plurality of fittings  205 , which are arranged at some or each end of the tubes  105 . The tubes  105  may, e.g., be formed by an insulating (electrically and/or thermally) material and the optional fittings  205  at the ends of the tubes  105  may comprise electrically conducting material (e.g. metal) such that the primary liquid is in electric contact with the electrical conducting fittings  205 . By connecting the fittings  205  or some of the fittings  205  to a ground potential GND, the primary liquid can be discharged so that the flow of the primary liquid does not cause an electric flow via the system of tubes  105 . 
     The heat exchanger  130  is configured to provide a heat flow from the primary liquid to a secondary liquid in the tubes of the main (secondary) circulation system  135 . Preferably, the heat exchanger  130  comprises dielectric material such that no electric connection is provided between the primary liquid and the secondary liquid. In addition, the heat exchanger  130  may preferably comprise a material with high thermal conductivity such that an efficient heat transport between the primary liquid and the secondary liquid can be achieved. Optionally, the heat exchanger  130  may also be electrically connected to the ground potential GND. The heat exchanger  130  ( FIG. 2 ) may also be made of metal materials and may be electrically conductive. However, the material of the tubes  135  ( FIG. 2 ) should be made of dielectric material such as plastic or alike. 
     For security, a galvanic separation as indicated by the line  134  between the primary circulation system and the secondary circulation system is therefore be provided so that no electric current can leave the heating system via the tubes of the main circulation system  135 . 
     The monitoring and controlling of the system as shown in  FIG. 2  may be provided by the control panel  160 , which can, e.g., be arranged on top of the heating system within the same housing  300 . The control panel  160  may, e.g., comprise a working thermostat  161  and a safety thermostat  162 , which are configured to adjust or show the temperature as set for the safety (e.g., 95° C.) and to define a working range (as e.g. within 50-70° C. or 30-90° C.). These temperatures depend on the particular composition of the primary heating liquid and may, for example, be at least 5% below the boiling temperature of the primary liquid. 
     The temperature and pressure is measured and displayed on the pressure sensor  260  ( FIG. 2 ). The working sensor  230  ( FIG. 2 ) on the control head  120  ( FIG. 2 ) is connected to the working thermostat  161  ( FIG. 2 ) on the control unit  160  ( FIG. 2 ); the safety sensor  240  ( FIG. 2 ) is connected to the safety thermostat  162  ( FIG. 2 ) on the control unit ( FIG. 2, 160 ). 
     The control panel  160  may, moreover, comprise one or more fuses  163  which may interrupt the operation in case the applied current to the primary liquid exceeds a predetermined upper threshold (e.g. 30 A or of 40 A) and/or in case the pressure or temperature within the system exceeds further thresholds to prevent damages. In addition, the control panel  160  may comprise a switch  164  to turn on/off the system, an Ampere-meter  165  to show the value of the electric current applied to the primary liquid. Finally, the control panel  160  may optionally comprise an LED light indicator  166  to show that the system is currently working or is turned off. The fuse  163  ( FIG. 2 ) may be of 1.6 A and may protect the control panel only. The fuse of the building in which the heating device is installed might be of 30 A so that the heating system should not exceed 20-25 A. 
     The control panel  160  may together with the heating system be grounded by connecting the housing  300  to the ground potential GND. The heating system  100  is connectable to an AC current supply  310  as, e.g., the usual 220 V power supply or a 3×380 V (three phase) power supply. 
       FIG. 3A-B  depict different electrode assemblies for the heater  110  and  FIGS. 3C ,D depict a possible connection of the electrodes to the power supply. 
       FIG. 3A  shows a first embodiment for the heating cell inside the electric heater  110  with a plurality of electrodes arranged inside the heating cell along the fluid passage  115 . This embodiment uses a coaxial electrode arrangement with a central electrode  118  connected to a terminal  318  and an outer electrode  117  connected to a terminal  317 , which are arranged in a cylindrical configuration between a liquid inlet  110   a  and a liquid outlet  110   b  of the electric heater  110 . In addition, at the liquid inlet  110   a  a ground electrode connected to the ground potential GND is provided with an opening  412  to provide a passage for the primary liquid. Downstream of the ground electrode (with respect to the primary liquid) a neutral electrode  119   a  connected to a terminal  319   a  is provided, which is again arranged perpendicular to the flow path of the primary liquid and which also comprises an opening  419   a  for the primary liquid to pass after entering the heating cell from the liquid inlet  110   a . After passing the opening  419   a  the primary liquid enters the fluid passage  115  which is arranged between the central electrode  118  and the cylindrical outer electrode  117 . After leaving the fluid passage  115  the primary liquid passes a further opening  419   b  of a further neutral electrode  119   b  before the primary liquid passes the opening of a further ground electrode provided at the liquid outlet  110   b  of the electric heating cell  110 . The further neutral electrode  119   b  is connected to a terminal  319   b  and the ground electrode is connected to the ground potential. Therefore, each of the electrodes  117 ,  118 ,  119  and the ground electrode are provided with separate terminal  317 ,  318 ,  319  to be contacted with a power supply, which may, for example, either be a three-phase, a two-phase or a mono-phase power signal. 
       FIG. 3B  shows a cross-sectional view of the embodiment of  FIG. 3A  perpendicular to the fluid passage  115  crossing the central electrode  118  and the cylindrical outer electrode  117 . In this embodiment the electric heater  110  (or more particular, the electric heating cell) comprises a circular shape as shown in  FIG. 3B , wherein a cylindrical outer electrode  117  is arranged around the central electrode  118  in a coaxial shape. The central electrode  118  is supported, e.g., by four support elements  410   a ,  410   b ,  410   c  and  410   d.    
     To initiate an electric current between the central electrode  118  and the outer circular electrode  117  an electric voltage is applied, for example, by connecting both electrodes to different phases of the provided power supply. If, e.g., a three-phase power supply is used a first phase of the three phases can be connected to the outer cylindrical electrode  117  and a second phase of the three phases can be connected to the central electrode  118 . The third of the three phases may in this configuration not be used. The electrode  119  at the liquid inlet  110   a  and/or at the liquid outlet  110   b  may be connected to the neutral (null) potential of the three-phase power supply or may optionally be connected to third phase of the 3-phase power supply. Finally, the ground electrode is connectable to the ground potential GND. For these connections the terminals  317 ,  318  and  319  can be used, wherein these terminals can be arranged at different positions of the heating cell. 
     The support elements  410  comprise, e.g., a dielectric material which can withstand the temperature of the electric heater  110 . Alternatively, the support elements  410  can also be used for the electric connection to the central electrode  118 , in which case, the support elements  410  are provided along the axial direction such that they do not contact the outer cylindrical electrode  117 . 
       FIG. 3C  shows an embodiment for the connection of the electric heater  110 , which comprises three heating cells  110   a ,  110   b ,  110   c  arranged in parallel along the flow path of the primary heating liquid. The liquid inlet  110   a  and the liquid outlet  110   b  are provided with fittings  205 , which are both connected to the ground potential GND. The terminals  317 ,  318 ,  319  of the electrodes  117 ,  118 ,  119  are connected either to the neutral potential (O) or to one of the three phases R, S, T of a three-phase power supply for the electric heater  110 . 
     In the embodiment as shown in  FIG. 3C  each of the heating cells  110   a ,  110   b ,  110   c  comprises a central electrode  118   a,b,c  and a cylindrical electrode  117   a,b,c  so that in a first cell  110   a  a central electrode  118   a  is connected via the terminal  318   a  to the R-phase of the power supply and the cylindrical electrode  117   a  is connected via the terminal  317   a  to the neutral potential O. The second heating cell  110   b  has a central electrode  118   b  connected via the terminal  318   b  to the S-phase of the power supply and a cylindrical electrode  117   b  connected via a terminal  317   b  to the neutral terminal O. The third heating cell  110   c  has also a central electrode  118   c  connected via a terminal  318   c  to the T-phase of the power supply and the cylindrical  117   c  is connected via a terminal  317   c  to the neutral terminal O. 
     In addition, at the fluid inlet  110   a  a neutral electrode  119   a  is provided, which is downstream from the fitting  205  and is also connected via a terminal  319   a  to the neutral terminal O. Similarly, at the fluid outlet  110   b , a further neutral electrode  119   b  is provided which is upstream from the further fitting  205   b  and which is also connected via the further terminal  319   b  to the neutral terminal O. Therefore, the connection as shown in  FIG. 3C  comprises three heating cells as shown in  FIGS. 3A , B, which are electrically connected to different phases of the power supply. 
       FIG. 3D  shows a further embodiment for a different connection of the heating cells as described in  FIGS. 3A , B. Again, three heating cells  110   a ,  110   b ,  110   c  are arranged in parallel along the heating flow between the fluid inlet  110   a  and the fluid outlet  110   b . This embodiment differs from the embodiment as shown in  FIG. 3C  in that the circular electrodes  117   a ,  117   b ,  117   c  are now connected to different phases (instead of being connected to the neutral terminal O as in  FIG. 3C ). In detail, the first heating cell  110   a  has a cylindrical electrode  117   a  connected via the terminal  317   a  to the T-phase of the power supply, the middle heating cell  110   b  as the cylindrical electrode  117   b  connected via the terminal  317   b  to the R-phase, and the third heating cell  110   c  has a cylindrical electrode  117   c  connected via the terminal  317   c  to the S-phase of the power supply. The central electrodes  118   a ,  118   b ,  118   c  are connected via the terminal  318   a ,  318   b ,  318   c  in the same way to different phase as shown in  FIG. 3C . By this connection also between the central electrodes  118  and the cylindrical electrodes  117  of each of the three heating cells  110   a ,  110   b ,  110   c  potentials of different phases are applied so that a current is flowing between the central electrode  118  and the cylindrical electrode  117  in each of these three parallel heating cells when a connection to the power supply is established. 
       FIGS. 4 a  and 4 b    illustrate the pulsed operational mode for the electric heater. Because the electric heater  110  is operating by applying an electric current directly to the liquid, the conversion of the electric energy into heat of the primarily liquid is very efficient and the liquid is heated immediately if a current is applied to the primary liquid. This is the reason why the heating system  110  of the present invention can be operated in a pulse mode, wherein the electric current is not continuously applied to the primary heating liquid but as pulses with a certain pulse frequency. 
     In the embodiment shown in  FIG. 4  a first pulse is generated at a time t 1  for a time period Δt 1 , a second pulse is generated at the time t 2  for a second time period Δt 2  and the third pulse is generated at the time t 3  for a third time period Δt 3 . The difference between the time t 2  and t 1  is given by a first delay T 1 . The difference between the time t 3  and t 2  is given by a second time delay T 2 . In further embodiments the time delays T 1  and T 2  can be selected equally or can differ (e.g. T 1 &gt;T 2 ). The pulse frequency is, e.g., defined as 1/T, wherein T=T 1 −T 2 . 
     Therefore, the voltage can be applied at the times t 1 , t 2  and t 3 , wherein the voltage is applied over time periods Δt 1  to Δt 3 . Between theses time periods the voltage is turned off until the next on-time (e.g. t 2 ), where again for a time period Δt 2  the voltage is applied to the electrodes. The electric current (see dashed line in  FIG. 4 a   ) will (almost) immediately rise when the voltage is applied and will fall rapidly after the voltage is turned off. Therefore, when the voltage is applied as pulses (as shown in  FIG. 4 a   ) the current will rapidly increase at the times t 1 , t 2  and t 3  until it also reaches a maximum value. After turning off the voltage (e.g. after the predetermined time Δt), the current will drop rapidly to a zero value. 
     As consequence, the primary liquid is not constantly subject to an electric current, but only during short periods of time the current is flowing through the liquid. The predetermined time period Δt can be adjusted in such a way that a gas generation by electrolyze in the primary liquid is suppressed. Moreover, the frequency of the pulses (or the times t 1 , t 2 , t 3 , . . . ) are controlled by the control panel  160  to adjust the operational temperature of the primary liquid accordingly. The time periods can also be adjusted differently so that, for example, the time period Δt 1 &gt;Δt 2 &gt;Δt 3  or, alternatively, the time period Δt is at first smaller and increases with the time t. 
       FIG. 4 b    shows the temperature as function of time, wherein at an initial time t 4  the temperature reaches a lower limit Temp 2  indicating that the electric heater shall start to operate. At this time, the pulse mode is turned on a pulsed electric current as shown in  FIG. 4 a    flows through the primary liquid so that the heater starts heating until the temperature of primary liquid reaches at time t 5  an upper limit Temp 1 . At this time, the heater stops operating until the time t 6 , where the primary liquid again reaches the lower temperature threshold Temp 2 . At this time, the heater again starts to operate until the temperature reaches (or exceeds) the upper temperature Temp 1 , where the electric heater again ceases to apply current to the primary liquid. In case the temperature rapidly increases to exceed a maximal temperature Temp max  at the time t 9 , where a safety thermostat sensor generates an emergency signal, the whole system is turned-off to prevent damages from the system. 
     As for the operation, different modes can be envisioned. For example, in case the temperature reaches the lower limit Temp 2 , a first pulse mode is initiated (e.g., with a pulse frequency of 17 or 10 or 20 Hz) and is maintained until the temperature of the primary liquid reaches the upper limit Temp 1 . At this time, the pulse mode is turned off, so that no current is applied to the primary liquid until the temperature of the primary liquid reaches the lower limit Temp  2 . In a different operational mode, the frequency of the applied current or voltage to the primary liquid is modified such that when the temperature reaches the lower limit Temp 2 , the pulse frequency of the pulses is increased until the temperature reaches the upper temperature limit Temp 1 , where the pulse frequency of the applied electric current is again lowered, to thereby lower also the temperature until the primary liquid again reaches the lower temperature Temp 2 . In a different operational mode also the time duration Δt can be modified such that the pulse length (see  FIG. 4 a   ) of the voltage signal is modified to thereby apply more energy to the primary liquid and to increase the temperature of the liquid. For example, the pulse length Δt can be increased, when the primary liquid reaches the lower temperature Temp 2  until the primary liquid again reaches the upper temperature Temp 1 , where the pulse length Δt of the applied voltage signals to the primary liquid can be decreased. In this latter operational mode, the pulse frequency of the pulsed signals can remain constant, whereas in the first operational mode the pulse length can remain constant, whereas the frequency of the applied pulse signals is modified. 
     The frequency of the pulsed signal (1/T) may, e.g., be modified in a range between 5 and 1000 Hz or between 10 Hz and 50 Hz or preferably be more than 17 Hz. The pulse length Δt may, e.g., selected to be more than 1 ms or more than 10 ms or between 50 ms and 100 ms. 
       FIGS. 5 to 7  show embodiments for the control head  120  with various access ports  123 ,  124 ,  125 ,  126 ,  127 ,  128  to provide access to the primary heating liquid for different components.  FIG. 5  shows views from the front, top and back side of the control head  120 ,  FIG. 6  shows side views and  FIG. 7  show two cross-sectional views of the control head  120 . 
       FIG. 5A  depicts the side, on which the access port  126  for the optional thermo-manometer  260  is formed (see  FIG. 2 ). Below the opening  126  for the thermo-nanometer  260  the access port (opening)  127  for the connection to the expansion unit  270  is shown on the left hand side of the control head  120 .  FIG. 5B  depicts the side, where the access port  125  for the air vent  250  is formed. Finally,  FIG. 5C  depicts the side, where the two openings  123 ,  124  for the working thermostat sensor  230  and for the safety thermostat sensor  240  are formed. 
     The dimensions are, e.g., as follows: the length L 1  (distance of opening  127  from the left side): L 1 =29 mm (or between 15 . . . 40 mm), the length L 2  (distance between openings  127  and  126 ): L 2 =32 mm (or between 25 . . . 40 mm), the length L 3  (distance of opening  126  from the right side): L 3 =39 mm (or between 30 . . . 50 mm), the length L 4  (width of a connecting portion  128  on the right hand side): L 4 =15.5 mm (or between 10 . . . 20 mm), the angle A 1  (slope angle of left flange): A 1 =15° (or between 10 . . . 20°), the length L 5  (diameter of opening  127 ): L 5 =13 mm (or ½ inch or between 10 . . . 20 mm), the length L 6  (diameter of opening  126 ): L 6 =13 mm (or ½ inch or between 10 . . . 20 mm) and the angle A 2  (slope angle of right flange): A 2 =15° (or between 10 . . . 20°), the length L 7  (width of recess in portion  128 ): L 7 =4 mm (or between 2 . . . 6 mm) and the length L 8  (width of a connecting portion  128  on the left hand side): L 8 =11 mm (or between 6 . . . 20 mm). The length L 9  (distance between openings  123  and  124 ): L 9 =63.11 mm (or between 50 . . . 70 mm), the length L 10  (overall length of control head  120 ): L 0 =100 mm (or between 50 . . . 150 mm). 
       FIG. 6A , B show side views of the control head  120 , i.e. views perpendicular to the flow direction of the primary liquid when flowing from the electric heater  110  to the control head  120 . The metal fittings  205  of the tubes  105  are connectable to portions  128  surrounding the flow path of the primary liquid. 
       FIGS. 7A and 7B  show cross-sectional views along the cross-sectional line A-A and B-B as shown in  FIG. 5C .  FIG. 7A  shows the cross-sectional view along the cross-section A-A, wherein the opening  124  for the safety thermostat sensor  240  is shown together with the opening  126  for the thermo-manometer  260 .  FIG. 7B  shows a cross-sectional view along the cross-section B-B, with the opening  125  for the air vent  250 , the opening  123  for the thermostat sensor  230  and the opening  127  for the expansion unit  270 . 
     Possible operational parameters of the heating system  100  may comprise the following values. The heating system  100  can be used to heat a space of up to 900 m 3  (or for spaces between 100-500 m 3 ). The volume of the primary liquid in the primary circle may, e.g., within the range of 1 to 5 L or, preferably, between 2.3-2.5 L. The voltage used for heating can be within the range of 90 V to 600 V (single phase or three phases or combination thereof at the same time; e.g. 220 V or 3×380 V). The frequency used for the pulsed mode may be modified from 0 to 1000 Hz or be more than 17 Hz (or between 10 . . . 40 Hz). The electric current supplied to the primary liquid may, e.g., be within the range of 1-25 A (or vary from 0 to 40 A). The applied power may be in the range between 1 and 24 kW (or 1 to 50 kW). The working pressure of the primary liquid within the system of tubes may be within the range of 1-2.2 bars (or between 1 and 4 bars). The maximum temperature Temp max  limited by the safety thermostat may be up to 95° C. (or 10% below the boiling temperature of the primary liquid). The operational temperature can be varied continuously up to the maximal temperature, wherein a higher operating temperature of the primary circle may be set dependent on the used primary liquid. 
     The tubes  105  may comprise dielectric material and have a diameter of ¾ inch (or between 10 mm to 30 mm). The control head may comprise a cylindrical shape with a diameter of, e.g., 80.5 mm (or between 50 and 200 mm). The system of tubes  105  can be covered by a metal cladding or a metal shell, which improves the galvanic separation in that the whole system can be easily connected to the ground potential. 
     The primary liquid may contain ions or particular salts and can, in particular to be adapted to ensure that no sedimentation occurs during operation. 
     The embodiments described above an the accompanying drawing merely serve to illustrate the subject matter of the present invention and the beneficial effects associated therewith, and should not be understood to imply any limitation. The features of the invention, which are disclosed in the description, claims and drawings, may be relevant to the realization of the invention, both individually and in any combination.