Gas-electric hybrid wall-mounted boiler

. In common terminology, a hybrid (dual-fuel) heat supply system combines a gas boiler and a heat pump. These systems are considered the most energy-efficient of the currently existing autonomous household heat supply systems. Analysis of the efficiency and carbon footprint of household heat supply systems shows that at low atmospheric temperatures and the use of radiator-type heating devices, the question of the advantage of heat pumps is not unambiguous. A new solution for a thermal generator (Patent for invention RU 2782081 C1) is proposed - a hybrid gas-electric boiler that has a number of advantages when operating systems at atmospheric temperatures from 0 ° C to + 10 ° C. The technical characteristics of the hybrid boiler, the purpose and the field of effective application are determined. A comparison of the carbon footprint of a hybrid boiler with heat pumps and a typical gas boiler is presented. The area of effective use of the invention is a household with a heated area of up to 100 m2, apartment heat supply. The main advantage of a hybrid gas-electric boiler is the exclusion of cyclical operation of the heat generator at a low thermal load, characteristic of space heating at atmospheric temperatures from 0 ° C to 10 ° C.


Introduction
In commonly accepted terminology, a hybrid (dual-fuel) heating system combines a gas boiler with a heat pump.These systems are considered the most energy-efficient among autonomous household heating systems currently available.Hybrid systems automatically switch to the most efficient fuel source, minimizing the carbon footprint during heat generation.
It is believed that using heat pumps for heating reduces humanity's reliance on hydrocarbon fuels, and hybrid setups with traditional boilers are a necessary measure for operation in low ambient temperatures when the heat pump's efficiency decreases.The range of atmospheric air temperature within which air-to-water heat pumps are energyefficient is a critical factor in choosing the type of heat generator for autonomous household heating systems.
When assessing the energy efficiency of heating systems, it's essential to consider the regional heating device preferences and climate factors.For instance, in the UK and Russia, up to 97% of heating devices are radiators and convectors with an effective heating temperature of 50-65°C.Climate conditions, specifically the duration of the heating period below 0°C, also play a significant role.Several studies [1][2][3][4][5] and many other authors have explored the Seasonal Coefficient of Performance (SCOP) of heat pumps under different climate conditions.Generalized SCOP versus air temperature graphs are presented in works [6,7].The average SCOP values for different types of heat pumps can be described linearly, ranging from 1.4 at -10°C to 3.5 at +15°C.An analysis of the SCOP for air-source heat pumps (ASHP) as a function of the temperature difference between ambient air and radiator temperatures is presented in work [8].Of particular interest is an SCOP value of 2.0-2.5, at which the carbon footprint from using electrical energy conversion equals that of burning hydrocarbon fuels.According to the graphs in the referenced study, this SCOP coefficient corresponds to a temperature difference of 60°C, which, with radiator temperatures at 65°C, equates to an ambient temperature of +5°C.In conclusion, using air-to-water heat pumps (ASHP) with radiator-type heating devices compared to traditional gas boilers does not provide an advantage in terms of carbon footprint, especially in regions with ambient temperatures below 0°C.When natural gas is burned into the atmosphere, it releases 197 grams of CO2 per kilowatt of thermal power.Condensing wall-mounted gas boilers, in nominal operation, release 215 grams of CO2 per kWh, while conventional boilers release 235 grams per kWh [9,10].
According to data presented in work [11], in 2021, the total annual electricity generation in the USA by municipal-scale power plants resulted in CO2 emissions of 388 grams per kWh.Consequently, in the USA, the emissions equality coefficient for heating using gas or electricity is 1.96.This coefficient varies from one country to another and depends on the type of fuel used for electricity generation.According to data [12,13], it is 3.35 in China, 2.2 in Germany, and averages 2.64 worldwide.In developed countries, it typically falls in the range of 2-2.5.
Hydronic heating for households can be achieved with low-temperature devices like "underfloor heating," which, in some cases, proves cost-effective and allows for the use of air-to-water heat pumps at ambient temperatures down to -10°C [14]."Underfloor heating" exhibits high thermal inertia, and standard room air thermostatic heating control systems are ineffective with such heating devices.Additionally, "underfloor heating" as the sole heating device in a space is not suitable when ambient temperatures drop below -15-20°C, as it would require a floor temperature exceeding 45°C to compensate for heat loss, which poses comfort and technical challenges.Numerous authors have compared the effectiveness of heating with radiators and "underfloor heating" [15][16][17][18][19][20], but they haven't shown a clear advantage of one over the other.This work focuses on heating residential spaces with radiator-type heating devices.
The energy efficiency of hydrocarbon fuel boilers is not constant within the range of changing thermal loads.When using radiators for heating, the efficiency coefficient of wall-mounted convective boilers is 80% (GCV) and corresponds to the nominal thermal load.For heating small households (up to 100 m2), particularly apartments in multi-story buildings, heat losses for heating range from 1-4 kWh.The operation of convective gas boilers under such thermal loads shifts to a cyclical mode, where the boiler's efficiency can drop to 50% (GCV).
Wall-mounted gas convection boilers with atmospheric burners have a modulation coefficient of no more than 2.5 and cannot operate smoothly at thermal loads below 40% of their nominal capacity [21][22][23].Condensing wall-mounted gas boilers, when used with radiator heating devices under nominal loads, have efficiencies of 84-86% (GCV) [24][25][26][27][28][29].The proportional modulation range of these boilers, with full preliminary gas mixture valve pneumatic type control, is 5-6.However, at thermal loads below 3 kWh, they also shift to a cyclic mode with low efficiency.The Seasonal Coefficient of Performance (SCOP) for airto-water heat pumps at an atmospheric temperature of 0°C averages around 1.7-1.8,and the coefficient for electricity generation exceeds 2. Consequently, when the temperature drops below 0°C and radiator-type heating devices are used, the carbon footprint of air-to-water heat pumps is higher than that of gas boilers.At temperatures below -10°C, heat generation for heat pumps occurs through electrical heaters, with a carbon footprint twice that of gas boilers, or by switching hybrid heat pumps to hydrocarbon fuels.Given the above, there is a question about the feasibility of installing heat pumps for heating systems in cold climates with an average heating season temperature below -5°C.This equipment can be expensive, and maintaining indoor temperatures during the winter season without hydrocarbon fuel boilers, using only heat pumps, may lead to a higher carbon footprint.The issue of supplying residential spaces with thermal energy can be resolved without heat pumps using an alternative method.To avoid the operation of a thermal generator in a cyclic mode at low thermal loads, a device with a smooth modulation range of around 50 is required.This thermal generator can be a hybrid gas-electric wall-mounted boiler with a nominal capacity of up to 30-40 kWh, capable of providing thermal energy for an autonomous household heating system with an area of up to 300 m2 and eliminating the operation of gas boilers in a cyclic mode with low efficiency at temperatures above 0°C [30].
The aim of this work is to describe the operation and technical characteristics of the gaselectric hybrid wall-mounted boiler, assess its carbon footprint in comparison to air-towater heat pumps and gas boilers operating in a cyclic mode.

Materials & Methods
Research Object: The research focuses on the "ARDERIA" hybrid gas-electric boiler with a nominal capacity of 24 kWh.In Figure 1, there is a representation of the indirect electrical heating of the heat transfer medium using resistor heating elements.The heat transfer medium flows through a compressed spiral pipeline encased in an aluminum block, with resistor heating elements positioned at its core.The indirect heating of the heat transfer medium is achieved by heating the entire surface of the pipe within the block.The block is heated to the desired temperature through the operation of the boiler's electrical heating elements, following a specific algorithm.The maximum temperature limit of the heat transfer medium is controlled by a thermostat situated on the heating block's casing, with safety mechanisms in place to prevent overheating.The electrical heating block is integrated into the thermal energy generation circuit with the primary heat exchanger of the gas boiler.The hybrid boiler's control system selects the type of thermal energy generation based on the outdoor temperature at the location of the installation and the heat losses from the heated premises' enclosure.The combined boiler serves both heating and domestic hot water (DHW) purposes, with a thermal power range of 15-25 kWh when operating in DHW mode.In the "heating" mode, the boiler operates with a smooth modulation range of thermal power, ranging from 9.4-24 kWh when using gas as a fuel.The boiler features a closed chamber, forced flue gas exhaust, and an atmospheric multi-fuel burner.In the "heating" mode when utilizing electrical heating, the thermal power range is 0.5-9 kWh.The cumulative modulation coefficient of the thermal unit is 48.Power management for the boiler is handled differently based on the energy source in use.When using gas, power is controlled by a proportional current-driven gas valve, whereas, in electric mode, power control relies on thyristor-based management of the thermal heating elements.Comparing the carbon footprint of heat pumps and condensing boilers involved the application of numerical modeling methods.This comparison was based on averaged technical characteristics derived from manufacturers' data for heat pumps and condensing gas wall-mounted boilers.

Results & Discussion
The results of changes in the coefficient of energy efficiency based on the proportion of thermal power are depicted in Figure 2. Zone I correspond to the use of electrical energy, while Zone II represents the energy derived from burning gaseous fuel.The boiler's efficiency is determined based on the Gross Calorific Value (GCV).The efficiency values within the range of electrical energy use are assessed without considering the method of its generation.The efficiency (GCV) of a non-condensing boiler in the range of 40% to 100% of its nominal load gradually increases from 78.5% to 82.2%.In contrast, the efficiency of an electric boiler in the range of 0% to 40% of its nominal load remains constant at 97.3%. Figure 2 also depicts a hybrid boiler with a condensing design when used with radiator-type heating devices.In Zone II, the efficiency varies from 88% at 40% load to 86% (GCV) at full load.These values are based on averaged characteristics obtained from boiler manufacturers' data.
The cost of a hybrid gas-electric wall-mounted boiler differs from standard wallmounted gas boilers by only 25%, which is significantly lower than the cost of heat pumps.The operational expenses of hybrid boilers are nearly identical to those of standard gas boilers.Using electric heating of the heat transfer medium in a household heating system at low thermal loads allows for the almost complete elimination of the cyclic operation mode of the gas boiler.This is crucial as cyclic operation negatively impacts the service life and maintenance frequency of gas boilers.
A qualitative analysis of the carbon footprint of heating systems is presented in Figure 3.The carbon footprint assessment for heating systems uses the comparison unit of CO2 emissions during the combustion of gaseous fuel, equivalent to 0.197 kg of CO2 per kWh of generated energy.It's essential to note that losses associated with the transportation of electrical energy from production to usage locations were not considered in this study, as it falls outside the scope of this research.In the perspective of the author, there should be no unequivocal claims that heat pumps always have the smallest carbon footprint compared to heating systems based on hydrocarbon-fueled thermal generators.This especially holds true when using heat pumps in regions with an average winter temperature below -5°C.In the coming 15-20 years, a significant portion of electricity production will continue to rely on the combustion of hydrocarbon fuels.Therefore, in terms of CO2 emissions generated for the production of electrical energy and its subsequent use for space heating in cold-climate countries, gas boilers are considered one of the cleanest heating methods when compared to heat pumps.It's crucial to note that it is erroneous to spread information about stable characteristics of CO2 emissions across the entire operational range of gas boilers.Using gas boilers for household heating systems during periods of low thermal load leads to their E3S Web of Conferences 458, 01032 (2023) EMMFT-2023 https://doi.org/10.1051/e3sconf/202345801032cyclic operation, and the energy efficiency coefficients in these modes can decrease by more than 2 times.The use of hybrid gas-electric boilers is economically and environmentally reasonable for households located in climate zones with an average winter temperature ranging from 0 to +5°C.This is particularly applicable to multi-story buildings with apartment-based heating systems, where thermal loads almost never exceed 3-4 kWh, and gas wall-mounted boilers almost always operate in cyclic mode.
In Figure 3, there is a qualitative chart of specific CO2 emissions in kg of CO2 per kWh for autonomous household heating systems.The data is presented for a statistically averaged air-to-water heat pump (ASHP) in a radiator heating system, a wall-mounted gas boiler of the convection type, and a hybrid gas-electric boiler.The charts are presented for the case of a space with heat losses from its envelope of 15 kWh at -25°C and 0.8 kWh at +15°C.The conversion coefficient of hydrocarbon fuel to electricity is assumed to be 2.5.The emission coefficient KCO2 shows the ratio of CO2 emissions from the thermal generator to CO2 emissions from the combustion of gaseous hydrocarbon fuel.
When using a heat pump as a heat generator, the results are as follows: CO2 emissions increase from 0.2 kg/kW-hr at +10°C to 0.28 kg/kW-hr at 0°C.As the atmospheric temperature further decreases, the heat pump struggles to meet the space heating demands, necessitating additional heat generation.On the other hand, when electric heating elements are used, there is a sharper rise in CO2 emissions, reaching 0.48 kg/kW-hr at an atmospheric temperature of -5°C.At even lower temperatures, employing air-to-water heat pumps with radiator-type heating devices becomes impractical.Using a non-condensing gas wall boiler as a heat generator, the specific emissions of the gas heat generator range from 0.25 to 0.28 kg/kW-hr over temperatures from -25°C to -7°C.Above -5°C, the boiler transitions from smoothly modulating its power output to cycling operation, leading to reduced efficiency and increased CO2 emissions, reaching 0.5 kg/kW-hr at an atmospheric temperature of +13°C and continuing to rise with higher temperatures.With a hybrid gaselectric boiler, at temperatures ranging from -25°C to +4°C, the boiler operates similarly to a gas boiler with the aforementioned CO2 emissions.When the temperature exceeds +4°C, the hybrid boiler switches to electric mode, eliminating the need for the gas boiler to operate cyclically with low efficiency and high CO2 emissions, especially at atmospheric temperatures above +13°C.The graphs clearly illustrate that air-to-water heat pumps have the lowest carbon footprint for space heating when the average atmospheric temperatures during the heating season are above +5°C.At lower outdoor temperatures, gas boilers perform better in terms of carbon emissions.When the average atmospheric temperatures during the heating season drop below -3°C, gas boilers emit only half as much CO2 as heat pumps.
In terms of the complete life cycle cost assessment of autonomous heating systems, the hybrid gas-electric boiler offers advantages, particularly for households with heating areas of less than 150 m² or apartments in multi-story buildings with independent heating systems.This is due to the elimination of the boiler's cyclic operation under low heating loads.For apartments smaller than 60 square meters in the southern regions of Russia, hybrid boilers outperform both in economic and carbon footprint aspects when compared to heat pumps and gas boilers.

Conclusions
1.When assessing the carbon footprint of autonomous residential heating systems, it's essential to consider the average atmospheric temperature during the heating season and the type of heating devices in use.In countries with cold climates and a tradition of using radiator-type heating devices, gas thermal generators have the smallest carbon footprint.Air-to-water heat pumps, when the temperature falls below -5 to -8°C, exhibit a carbon footprint that is twice as high as that of natural gas-based heat generators.
2. Hybrid gas-electric boilers eliminate the operation of gas thermal generators under low heating loads in a cyclic operation mode with low energy efficiency.This results in an increased lifespan of the boilers and reduced carbon footprint when operating at atmospheric temperatures between +5°C and +15°C.
3. Hybrid gas-electric boilers offer the best overall life cycle cost and carbon footprint performance for autonomous residential heating systems in multi-story buildings located in the central and southern regions of the Russian Federation when compared to heat pumps and traditional wall-mounted gas boilers.

Fig. 1 .
Fig. 1.Block of indirect heating of the heat carrier with the use of resistor heating elements.1. Aluminum body of the heating block 2. Electric heating element 3. Heat carrier tube

Fig. 2 .
Fig. 2. Efficiency of a hybrid boiler.(GCV).Zone I -the use of electrical energy.Zone II -use of natural gas combustion energy.1. Non-condensing gas wall hung boiler (GCV) option. 2 -option of a condensing gas boiler with a radiator version of heating devices (GCV).