Wall-mounted electric boilers on semiconductor thermistor PTC heating elements

. The improvement of energy efficiency in HVAC equipment makes the greatest contribution to reducing the environmental impact of household technical equipment on the natural environment. Currently, electric water boilers with resistive heating elements have gained the widest popularity for autonomous household heating systems. Thermoresistive semiconductor heating elements (РТС) represent a new direction in the production of electric water heat generators. An experimental wall-mounted electric boiler with РТС thermoresistive heating elements, with a nominal thermal power of 8 kW-h, was investigated. The characteristic of the РТС heating element within the boiler's heat exchange unit was determined as a function of temperature. The energy efficiency in the power modulation range of 0.65-8.0 kW-h ranged from 0.983 to 0.988. The specific maximum power of the РТС element was 20 watts/cm2, and the hydraulic resistance of the boiler at a flow rate of 1.0 m3/h was 0.38 m.w.c. Electric indirect heating unit, installed in the design of the serial-produced electric boiler, passed the full test cycle and showed higher energy performance than when using a standard indirect electric heating unit with resistive ETH. Considering that РТС element s have significantly higher reliability and service life, it is recommended to release a trial batch of electric wall-mounted boilers for autonomous household heating systems and small industrial facilities.


Introduction
Improving the energy efficiency of HVAC equipment makes the greatest contribution to reducing the environmental impact of household technical equipment on the natural environment.This is because the share of total energy consumption for heating and hot water supply is 78% in Russia [1], 79% in the European Union [2,3], and 60% in the United States [4].To reduce greenhouse gas emissions, advanced countries focus on increasing the energy efficiency of buildings and heating systems [5][6][7].Power-to-Heat (PtH or P2H) technologies belong to equipment where electrical energy generates heat.P2H offers numerous advantages for managing the energy transition.Water-based P2H systems utilizing surplus variable renewable energy allow for energy optimization and reduce the use of hydrocarbon fuels.
Electric boiler is one of the popular solutions for individual heating systems (P2N) and is widely used in households for heating water-based systems with radiators and lowtemperature heating devices like "warm floors."This technology is advantageous due to the comfortable, silent, and safe operation of the devices, low initial cost, ease of maintenance, energy storage capability, and integration with renewable energy generation devices.The classification of electric heat generation devices in residential and industrial buildings is presented in various studies [8][9][10].In the vast majority of small-sized households and commercial premises, electric boilers are used, where the conversion of electrical energy into heat is based on resistive heating [11][12][13][14][15][16].The main disadvantages of standard tubular heating elements used in electric boilers for water-based autonomous heating systems are: 1. Limited service life of heating elements due to manufacturing materials, design, and intensity of operation.
2. High temperature of the surface of the ETH shell in the area of contact with liquid coolants (in most cases water), leading to boiling of the surface layer of the coolant and intensifying the process of scaling.
3. Formation on the surface of the heating element deposits of hardness salts, which significantly reduce the efficiency of heating the coolant and change the mode of heat transfer of the pair "ETH-heat-carrier" during the operation of the boiler.
4. High thermal inertia of heating elements, leading to difficulty in controlling the heat load.
5. Possible destruction of ETHs in case of irregularities in heat exchange modes of the pair "ETH-heat-carrier" in the course of boiler operation.
The mentioned disadvantages of resistive heating elements for P2N technologies are largely eliminated or reduced if thermistors (thermoresistors) are used as resistors for indirect heating of the heat carrier in the heating system.Thermistors are semiconductor devices whose electrical resistance varies with temperature.It is advisable to use thermoresistors with variable resistance characteristics for autonomous heating systems with electric heat generation.In the temperature range of 0-80 °C, which is the active operating range for electric boilers, the electrical resistance of thermistors should change insignificantly and have a minimum in the range of 40-70 °C.The Curie point of the resistance characteristic (the resistance value is twice the minimum) should correspond to 80-90 °C, and at temperatures above 95 °C, the resistance of the thermistor should change significantly.PTC thermistors are made of doped polycrystalline ceramics based on barium titanate.The low resistance is achieved by doping the ceramic with materials of higher valence than the crystal lattice.Some barium and titanate ions in the crystal lattice are replaced by ions of higher valence to create a certain number of free electrons, which make the ceramic conductive [17].The fundamental difference between resistive and thermoresistive indirect heating lies in the following.The temperature coefficient of resistance for a resistor is a relatively stable value.The heat energy generated by a resistive heating element increases with increasing current.
The designs of resistive heating elements have a maximum power dissipation intensity threshold, beyond which no transformations occur in the heating element components.With such properties of heating elements, it is important to prevent exceeding the thermal load thresholds to avoid the destruction of the heating element.In the case of a thermoresistive heating element, when a certain temperature is reached, determined by the chemical composition and physical properties of the element, as well as external thermal pressure, there is a sudden increase in its electrical resistance, resulting in a sudden decrease in the generated thermal power.The ability to release thermal energy with increasing temperature is eliminated by the physical properties of the thermoresistive heating element.It is necessary to consider the dynamic characteristics of heating elements when using resistors and thermistors, or the dynamic characteristic of heating.When using resistors with practically constant resistance characteristics, the heating intensity depends on the magnitude of the current.With a constant current, the released thermal power is proportional to time, and the power remains constant.In the case of a thermoresistive heating element, the initial resistance of the element is small, and a large current flows through the element, resulting in self-heating of the positive temperature coefficient (PTC), which further increases the current by reducing resistance.The power of the thermoresistive heating element at the beginning of heating and until it reaches the temperature with the minimum resistance value is greater than that of a resistive heating element with the same nominal values as the thermoresistor.Thus, it is possible to declare a higher heating power for the thermoresistive heating element compared to the resistive one with the same starting power.There is a significant number of applications for PTC thermistors in HVAC equipment [18][19][20][21][22][23].Experimental studies on heating system equipment are known within the scope of heating systems [24][25][26].

Purpose of work
Purpose of work: to investigate the functioning of an experimental wall-mounted electric boiler with thermogeneration using PTC thermistor heating elements with a nominal power of 8 kW-h.Determine the indicators of energy efficiency, hydraulic resistance, and the feasibility of application in autonomous heating systems.
The research object is the mass-produced "ARDERIA E9" wall-mounted electric boiler with a nominal heat power of 9 kW-h, thyristor power control, circulation pump, expansion tank, and hydraulic safety group, manufactured at the Russian "ARDERIA" factory.The standard electric energy conversion unit into heat has been replaced with an experimental unit featuring PTC thermistor heating elements with a nominal power of 8 kW-h.The indirect "dry" heat transfer block with PTC thermistor elements consists of a central channel for the flow of the heat transfer medium with 1-inch connecting fittings, an aluminum housing with six channels of rectangular shape in which packs composed of PTC thermistors, stainless steel electrode plates with power supply cables, heat-conductive and current-insulating sleeves, are pressed.The power supply voltage for the PTC elements is 220 volts at 50 hertz.The total nominal power of the six blocks-elements is 8 kW-h.The construction of the block is housed in an aluminum housing, with the space between the block and the housing filled with thermal insulation material.The boiler's operating algorithm remains unchanged and is identical to the mass-produced "ARDERIA E9" boiler, with the maximum nominal power set at 8 kW-h.The tests were conducted at the research stand of the Russian "ARDERIA" factory [27].The measurement of electrical resistance was performed using the OWON XDM1241 digital multimeter.

Research results
Figure 1 presents the characteristic of the electrical resistance of the PTC thermistor as a function of temperature without external thermal influence.The resistance scale is logarithmic due to the wide range of values.The graph also shows the resistance variation of traditional resistor heating elements with the same nominal resistance as the PTC thermistor.
From the graphs, it can be observed that the resistance of the resistor heating elements remains almost unchanged within the operating temperature range of the electric boiler [28][29][30].-resistance heating elements (RHE) -thermistor PTC heating elements The resistance of the nichrome (80%-20%) wire, commonly used in traditional tubular heating elements with a copper shell, changes by 8% over temperatures ranging from 20°C to 650°C [31].The graph also displays the dynamic heating component of the PTC thermistor and resistor heating elements without external thermal load.Since the resistance of the resistor element is minimally affected by temperature and thermal load, its heating power is assumed to be constant.In PTC thermistor heating elements, the point of minimum electrical resistance is within the range of 40-50°C (Ohms), and at the start-up temperature under normal conditions of 15-25°C, it is 30% higher (Ohms).The temperature corresponding to twice the minimum electrical resistance is 75°C (Curie point).At a temperature of 100°C, the electrical resistance of the PTC elements is 95 times greater than at the Curie point.The change in the resistance graphs of the PTC elements compared to traditional resistor heating elements indicates that PTC elements have greater heating dynamics in the temperature range of 30-60°C.The heating power of the heat transfer medium in this operating temperature range is 20-30% higher.PTC heating elements, when applied to heating systems, exhibit selfregulation.As the temperature increases above the Curie point, the heating of the PTC elements practically ceases.In the working temperature zone of the heating system, the resistance characteristics of the PTC elements during self-heating and external thermal load are the same [32].Measurements of the electrical resistance of the PTC elements were taken at negative temperatures down to -17°C.The resistance characteristics remain stable and do not differ from the values at 10°C.The resistance characteristics of individual PTC element plates have a variation of up to 10-15%.However, since the nominal power of one element is approximately 60 watts and more than 120 PTC elements are required for an 8 kW-h boiler, the variation in characteristics does not significantly affect the heating properties of the heat transfer medium as a whole.The levelling of the variation in PTC element characteristics also occurs due to the indirect heating of the heat transfer medium through the body of the block, made of an aluminum alloy.The hydraulic resistance of the block with PTC heating elements is insignificant, and the total hydraulic resistance of the wall-mounted gas boiler is determined by the internal elements controlling the movement of the heat transfer medium.The total hydraulic resistance of the experimental wall-mounted boiler with PTC heating elements is 0.38 meters of water column at a flow rate of 1.0 m3/h, which is significantly lower than the hydraulic resistance of wall-mounted gas boilers of similar power [33] The experimental electric wall-mounted boiler with PTC thermistor heating elements, based on the design solutions of the standard, mass-produced electric boiler "ARDERIA-E9," has successfully passed all the bench tests specified in the factory's product testing program and is recommended for manufacturing a pilot batch.Electric boilers with PTC heating elements can be used in systems with prioritized control of allocated limited electrical power for households [34].

1.
PTC thermistor heating elements can be used in autonomous heating and hot water preparation systems in indirect heat transfer units.The installation of a heating unit with PTC elements does not require the presence of an emergency bimetallic thermostat (overheat sensor) since the properties of PTC heating elements eliminate the possibility of overheating.The destruction of the heat exchanger structure due to exceeding the thermal load threshold is impossible with PTC heating elements.
2. PTC thermistors have a service life that significantly (multiple times) exceeds the service life of resistive tubular heating elements.The specific power of a PTC heating element at a coolant temperature of 50°C is 20 watts/cm².
3. The characteristics of electrical resistance of PTC thermistor heating elements can be customized according to the customer's requirements, allowing the creation of highly energyefficient heating systems.The coefficient of energy efficiency for a wall-mounted electric boiler with PTC thermistor heating elements ranges from 0.983 to 0.988 within the thermal power range of 0.650 to 8.0 kW-h.

E3SFig 1 .
Fig 1. Graphs of changes in resistance and current depending on temperature.