PTC electric heater with ceramic insulation

Abstract

An electric PTC instantaneous water heater for heating and / or cooling systems with a heat pump, designed to heat a heat transfer fluid in a secondary circuit of a heat pump, comprising a container (10) with means for the inlet and outlet of the heat transfer fluid (6) and at least one PTC heating element (1) inserted into the container (10) to heat the heat transfer fluid (6), characterized in that the PTC heating element (1) comprises a ceramic PTC body (2), two electrodes (4, 5) made of electrically conductive material attached to the ceramic PTC body (2), an insulating ceramic filler (3) for insulating the electrodes (4, 5) and a metal housing (9) made of a highly thermally conductive material. The insulating ceramic filler (3) consists of a cured mixture containing 85-90 wt% powdered Al2O3 and 10-15 wt% adhesive.

Description

[0001] The technical solution involves an electric PTC heating of the heat transfer fluid in the secondary circuit of heating and / or cooling systems with a heat pump, in particular air-to-water heat pumps.

[0002] Modern heat pumps have cooling circuits that use novel, environmentally friendly refrigerants as working fluids.

[0003] An example of a newer refrigerant is R290, also known as propane. It is a natural hydrocarbon refrigerant with low environmental impact. This substance, with high thermodynamic performance comparable only to ammonia (R717) or difluoroethane (R152a), has low toxicity but is highly flammable.

[0004] Its use in Europe is currently regulated by regulations on refrigerant charge limits depending on the application and installation location. On the other hand, R290 refrigerant technology can be integrated into any system, regardless of whether it is used in commercial or industrial applications.

[0005] Unlike the previous generation of safety refrigerants, whose leakage did not pose any serious safety risks, the new refrigerants mentioned above, such as R290, are flammable and / or explosive.

[0006] The safety design of heating and / or cooling systems using such heat pumps is fraught with problems, as the refrigerants involved can cause hazardous events if released into the environment. Therefore, the system design must be based, at a minimum, on the principle that the system must be absolutely leak-proof during normal operation and that no refrigerant leaks may occur in either the condenser or the evaporator.

[0007] However, situations can arise where refrigerant leaks occur due to adverse circumstances. Therefore, when designing heating and / or cooling systems, measures for detecting refrigerant leaks and for disposing of refrigerant that has escaped from the refrigeration circuit are also taken into account.

[0008] The lower ignition limit of propane as a working fluid is approximately 1.7% by volume in air, which corresponds to 38 g / m³ in air. If the cooling process takes place in a surrounding, hermetically sealed, but otherwise air-filled space with propane as the working fluid, the problem arises of detecting a critical, explosive situation after failure, in which the working fluid escapes into this hermetically sealed space.

[0009] The security solutions are described, for example, in the documents EP3940314 A1, EP3805671 A1, EP3748257 A1.

[0010] In addition to leaks in the cooling circuit, any potential leakage of the working fluid (refrigerant) into the connected secondary heating and / or cooling circuit of the heat transfer fluid must also be comprehensively addressed. Examples of known solutions from the prior art include degassing devices for the safe removal of gaseous refrigerant from the heat transfer circuit, safety devices with leakage sensors, and similar devices.

[0011] Another device that must also be operationally safe in the event of a leak of flammable refrigerant is the electric auxiliary heater in the connected secondary heating and / or cooling circuit of the heat transfer fluid (back-up).

[0012] Electric auxiliary heating is used in heat pump systems primarily when the heat extracted from the outside environment (e.g., the air) is insufficient for comfortable heating and / or hot water production, especially during the winter months. At the same time, the electric heater also prevents the heat transfer fluid from freezing (during defrosting mode).

[0013] For heat pump heating systems, the maximum operating temperature of the heating medium is set to 75°C. A temperature difference of ΔT = 10°C is specified for heating the heat transfer fluid via the auxiliary heater; that is, if the heat transfer fluid has a temperature of, for example, 15°C at the inlet to the electric auxiliary heater, it should be heated by 10°C to 25°C.

[0014] In conventional electric auxiliary heaters, the heating elements are designed as resistance heating elements (rods, coils). Heating occurs via a flow-through process, with the resistance heating elements immersed in the heat transfer medium. The heating coil is heated to a temperature of up to 850°C, which can lead to overheating of adjacent parts if the temperature of the heating source is not properly regulated or monitored. To prevent thermal damage caused by the temperatures of these heating elements, which are significantly higher than the temperature of the medium being heated, a costly safety concept is required.

[0015] Electric resistance heaters are described, for example, in documents EP3910260 A1, EP3869118 A1 and EP3037741 A1.

[0016] Due to the high temperatures reached by the heating coils and for safety reasons, electric resistance heaters are not suitable for use in heating and / or cooling systems with heat pumps using flammable and / or explosive working media.

[0017] The auto-ignition temperature of the refrigerant R290, for example, is already 450 °C, which must be taken into account when designing an electric auxiliary heater if the adverse circumstances of refrigerant leakage into the circuit of the heated heat transfer fluid must be considered.

[0018] An alternative to electric heaters with resistance heating coils are the state-of-the-art electric heaters with PTC (Positive Temperature Coefficient) heating elements, whose use is particularly advantageous because the temperature of the heating element, depending on the design of the PTC element, does not exceed 300°C or even less. This temperature is not exceeded even in dry heat, as the PTC heating element is self-regulating.

[0019] The disadvantage of PTC heating cells is precisely the type of self-regulation through changes in electrical resistance depending on the temperature.

[0020] If sufficient heat transfer from the PTC heating cell to the environment and the heated medium is not ensured, a high inrush current occurs when the PTC heating cell is switched on, leading to a high initial power peak in the first few seconds after switching on, which is an undesirable phenomenon.

[0021] This is normal behavior for PTC heating elements, and the higher power typically lasts for up to 20 seconds, depending on the design of the PTC heating element.

[0022] Peak power increases directly in proportion to the device's power rating; for example, it is much lower for a 2 kW device with a peak power of 10% than for a 10 kW device with a peak power of more than 10%. Electric heating devices with a rated power of 12 kW to 25 kW are currently used in heat pump systems, with future models expected to reach up to 50 kW.

[0023] Peak performance also means higher power consumption.

[0024] Peak loads place a higher electrical load on network components (circuit breakers, cables) and all connected devices. In other words, the electrical grid must be designed for such a high load, requiring more powerful circuit breakers and thicker cables. For the customer, this means an undesirable increase in costs and often unacceptable modifications to the electrical installation, which can ultimately lead to damage to the connected devices.

[0025] Therefore, it is necessary to design PTC heating elements with the lowest possible inrush current.

[0026] WO2018141458A1 presents a solution for a PTC heating cell with reduced inrush current, the solution being aimed at improving the material, design and attachment of the electrodes to the PTC ceramic body.

[0027] The present technical solution describes a PTC electric heater for a heat transfer fluid in a secondary circuit of heating and / or cooling systems with a heat pump, with an improved performance characteristic compared to the known state of the art.

[0028] An electric PTC heating element according to the present embodiment comprises a container with an inlet for the heat transfer fluid, an outlet for the heat transfer fluid and at least one PTC heating element inserted into the container for the heat transfer fluid.

[0029] The heating takes place during the flow of the heat transfer fluid from the inlet to the outlet of the container; it is therefore a flow-through heater.

[0030] The PTC heating cell consists of a ceramic PTC core, two electrodes made of electrically conductive material attached to the ceramic PTC core, an insulating filling to protect the electrodes, and a metal casing that conducts heat well. The metal casing serves both as the housing for the PTC heating element and as the heat exchanger between the heated heat transfer fluid and the heat generated in the ceramic PTC core when the electrodes are connected to an electrical source.

[0031] The present solution provides an improved PTC heating cell design to reduce peak power and an innovative insulating filler to improve heat transfer from the PTC ceramic body. According to the prior art, insulating Kapton foil (or Kapton film), which is an electrical insulator with good thermal conductivity, is typically used to insulate the electrodes of the PTC heating cell.

[0032] This insulator has a specific thermal conductivity of approximately 0.5 W / (m·K) .

[0033] The thermal conductivity of the Kapton foil is insufficient for our purposes, and measurements have shown that the peak power when switching on the PTC heater with the Kapton foil exceeds 30%, while the acceptable peak power for an electric auxiliary heater is less than 10%.

[0034] Another disadvantage of Kapton foil is its low mechanical strength, which makes the production of the PTC heating cell more difficult.

[0035] In the present technical solution, the insulating filler is made of Al(2)O3, an electrical insulator with good thermal conductivity. Its specific thermal conductivity is approximately 25 W / (m·K).

[0036] The problem of joining two ceramic materials, i.e. a ceramic PTC body and a ceramic Al 2 O 3 material, is solved by forming the insulating filler with a cured mixture of powdered Al (2) O ( 3) and an adhesive.

[0037] Sodium silicate (Na₂SiO₃, or water glass) is preferably used as an adhesive.

[0038] Preferably, the insulating filler consists of a mixture of 85-90 wt.% powdered Al 2 O 3 and 10-15 wt.% aqueous sodium silicate Na 2 SiO 3 )

[0039] Preferably the diameter of the Al (2)O(3) dust particles used in the ceramic insulating filling is in the range of 0.05 to 1 mm.

[0040] In the production of the PTC heating cell, it is advantageous to add urea as an activator and curing accelerator to the insulating filler in a weight fraction of up to 1%.

[0041] Aqueous glass reacts with carbon dioxide from the air and hardens; therefore, CO2 is preferably blown in instead of air to accelerate the hardening process.

[0042] The process for manufacturing a PTC heating element is as follows: The uncured insulating filler (mixture) is poured into a metal housing and a ceramic PTC body with appropriately attached electrodes is inserted into the filler before curing.

[0043] The position and method for attaching the electrodes to the ceramic PTC body in order to minimize the inrush current in the PTC heating cell when it is connected to a power source are described in the prior art and do not need to be elaborated further.

[0044] The ceramic PTC body, the electrodes and the insulating filler are encapsulated in a metal housing after the insulating filler has hardened, so that at least part of the two electrodes remains free for the connection of the electrical conductors.

[0045] The metal housing can consist of various cross-sections - circular, rectangular, square, polygonal, since the housing also functions as a heat exchanger and therefore the overall size of the surfaces mediating heat exchange is important.

[0046] Such a design of the PTC heating element, and in particular the composition of the insulating filling, significantly limits the peak performance of the electric auxiliary heater.

[0047] Measurements under laboratory conditions revealed that the peak electrical power of the electric PTC auxiliary heater (with the described PTC heating element) is less than 3% of its rated power, which is the best value for known similar devices.

[0048] For comparison of the peak power of similar devices: If a commercially available Kapton foil is used as insulating material, the peak power of the electric auxiliary heater is more than 30% of its rated power.

[0049] The technical solution is explained using images, but is not limited to them.

[0050] The images show: Fig. 1 Known state of the art - PTC electric auxiliary heater with PTC heating elements with insulating Kapton foil, as described in document EP4290155 A1 Fig. 2: Cross-section through an exemplary arrangement of a PTC electric instantaneous water heater with embedded PTC-encapsulated heating element according to the described technical solution Fig. 3 Power-time diagram of an electric PTC auxiliary heater with a rated power of 6 kW, with PTC heating elements with insulating Kapton foil Fig. 4 : Power-time diagram of an electric PTC auxiliary heater with a nominal power of 5 kW, with PTC heating elements with insulating filling according to the presented solution

[0051] An electric PTC instantaneous water heater for heating and / or cooling systems with a heat pump, designed to heat a heat transfer fluid in a secondary circuit of a heat pump, comprises a container 10 for heating the heat transfer fluid, with means for the inlet and outlet of the heat transfer fluid 6 and at least one PTC heating element 1 inserted into the container 10.

[0052] The PTC heating cell 1 consists of a cuboid PTC ceramic body 2, electrodes 4, 5 made of electrically conductive material which are attached to the PTC ceramic body 2, an insulating filler 3 for insulating the electrodes 4, 5 and metal sleeves 9 made of a thermally conductive material.

[0053] The metal sleeve 9 has a circular cross-section and an essentially cylindrical shape and provides heat exchange between the heated heat transfer fluid 6 and the heat generated in the ceramic PTC body 2 after an electrical voltage has been applied to the electrodes 4, 5.

[0054] The metal housing 9 is filled with an insulating filler 3 in which a ceramic body 2 with attached electrodes 4, 5 is arranged, the electrodes 4, 5 protruding partially from the housing 9 for the connection of electrical conductors.

[0055] The PTC heating element 1 is partially immersed in the heat transfer fluid 6 during operation, and part of the electrodes 4, 5, which protrude from the housing 9, is not in contact with the heat transfer fluid 6.

[0056] The insulating ceramic filler 3 consists of a cured mixture containing 85-90 wt% powdered Al (2)O3 and 10-15 wt% of an adhesive.

[0057] Sodium silicate Na₂SiO₃ is preferably used as an adhesive.

[0058] The insulating filler 3 preferably contains urea in a weight fraction of up to 1%.

[0059] The preferred embodiment of the electric heater is a safe electric PTC auxiliary electric heater for heating and / or cooling systems with a heat pump, which serves to heat the heat transfer fluid 6 in the secondary circuit of the heat pump, wherein the heater is equipped with at least one PTC heating element 1 with a maximum operating temperature of 300°C and the heat pump has a refrigeration circuit with a flammable working fluid and the peak power of the electric heater is less than 3% of its rated power.

[0060] The power-over-time diagram of the electric PTC auxiliary heater with a nominal power of 5 kW, with PTC heating cells with insulating filling according to the presented solution, is in Figure 4 depicted.

[0061] For comparison, shows Figure 3also a diagram of the power over time of an electric PTC auxiliary heater with a nominal power of 6 kW and PTC heating cells with insulating Kapton foil.

[0062] The described electric PTC instantaneous water heater for heating and / or cooling systems with a heat pump serves to heat the heat transfer fluid in the secondary circuit of the heat pump and is also safe for systems with a cooling circuit containing a flammable working fluid (e.g., R290). The described electric PTC auxiliary heater is a device with a nominal output of 12 kW to 25 kW, expected to be up to 50 kW. Reference symbol list

[0063] 1 PTC heating element 2 PTC ceramic body 3 Insulating spatula 4 Electrode 5 Electrode 6 Heat transfer fluid 7 Graphical representation of power and time when using Kapton foil 8 Graph of power in relation to time when using a ceramic filling 9 Metal housing 10 Electric instantaneous water heater

Claims

1. Electric PTC instantaneous water heater for heating and / or cooling systems with heat pump, designed for the additional heating of the heat transfer fluid in the secondary circuit of the heat pump, comprising a container (10) with means for the inlet and outlet of the heat transfer fluid (6) and at least one PTC heating element (1) inserted into the container (10) to heat the heat transfer fluid (6). characterized by the fact that the PTC heating element (1) comprising a ceramic PTC body (2), two electrodes (4, 5) made of electrically conductive material attached to the PTC ceramic body (2), an insulating ceramic filler (3) for stripping the electrodes (4, 5), a metal housing (9) made of a thermally conductive material, wherein the insulating ceramic filler (3) is formed by a hardened mixture containing 85-90 wt% powdered Al2O3 and 10-15 wt% of an adhesive.

2. PTC electric instantaneous water heater according to claim 1, characterized by the fact thatthe metal housing (9) is filled with an insulating ceramic filling (3) in which a ceramic body (2) with attached electrodes (4, 5) is arranged, the electrodes (4, 5) partially protruding from the housing (9) for the connection of the electrical leads.

3. PTC instantaneous water heater according to claim 1 or 2, characterized by the fact that the diameter of the Al2O3 dust particles in the insulating ceramic filling (3) is 0.05 mm to 1 mm.

4. Electric PTC instantaneous water heater according to one of claims 1 to 3, characterized by the fact that The adhesive is an aqueous solution of Na2SiO3.

5. PTC instantaneous water heater according to one of the preceding claims, characterized by the fact that the insulating ceramic filler (3) contains up to 1 wt% urea.

6. Electric PTC instantaneous water heater according to one of the preceding claims, characterized by the fact that the value of its peak electrical power is less than 3% of its rated power.

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