Low-pressure full DC gas water heater technology insight report

In recent years, with the intensification of the global energy crisis and the improvement of environmental awareness, energy conservation and emission reduction have become the focus of scientific and technological research in various countries. Statistics show that global greenhouse gas emissions are on the rise every year, and the main source of this is carbon emissions from traditional energy consumption. In order to deal with this problem, the international community has paid extensive attention to and promoted the research and development and application of new energy technologies. In terms of electricity consumption in the construction field, the energy consumption of household appliances accounts for a relatively large proportion, so improving the energy efficiency of household appliances and reducing energy consumption has become a common concern of governments and scientific research institutions.

In this context, the low-voltage full DC technology will change the power supply mode of household appliances from the traditional alternating current to direct current, and by directly providing a stable low-voltage full DC power supply, it avoids the energy conversion of multiple links such as frequency conversion and rectification in the traditional AC power supply, so as to improve the efficiency of energy utilization, and the mainland has also developed high-efficiency household appliances such as DC air conditioners and DC refrigerators. DC technology is widely regarded as one of the important development directions of household appliances in the future. And by adopting low-voltage full DC technology, it can not only improve energy efficiency and reduce carbon emissions, but also improve the safety performance of electrical appliances, especially the dual coupling effect of electrical performance and gas appliances, which can significantly reduce the risk of accidents. The development and application of low-voltage full DC technology is of great significance for improving the sustainable development of the home appliance industry and promoting the popularization of green and low-carbon lifestyles, but the low-voltage full DC technology for the gas appliance industry is still rarely involved.

From 1985 to the present, Wanjiale has been committed to providing high-quality hot water heating solutions. In recent years, with the research and development of low-voltage full DC technology and DC microgrid, full DC gas water heater is likely to become one of the future technology upgrade targets.

Under the guidance of the China Household Appliances Research Institute, the National Household Appliances Industry Information Center and Guangdong Wanjiale Gas Appliance Co., Ltd. jointly compiled the "2024 Low-Voltage Full DC Gas Water Heater Technology Insight Report". It is hoped that this report will provide some new ideas for the development of the gas appliance industry.

1. Relevant policies and development trends of the home appliance industry

In recent years, with the intensification of the global energy crisis and the improvement of environmental awareness, energy conservation and emission reduction have become the focus of scientific and technological research in various countries. In terms of building electricity, the energy consumption of household appliances accounts for a relatively large proportion, so improving the energy efficiency of household appliances and reducing energy consumption has become a common concern of governments and scientific research institutions. The mainland government has successively issued a number of national policies to encourage the development of green and smart home appliances, and drive the industry to upgrade and transform in the direction of low energy consumption and high energy efficiency.

1.1 Sorting out policies related to household appliances

1.1.1 Energy conservation and environmental protection policies

As the global climate change problem becomes increasingly severe, many countries have put forward emission reduction commitments. As the world's largest developing country, China has clearly put forward the "dual carbon" goal of striving to achieve "carbon peak" by 2030 and "carbon neutrality" by 2060.

Since the "Eleventh Five-Year Plan", the State Council has formulated a comprehensive work plan for energy conservation and emission reduction in each five-year planning period, and has made overall arrangements for the national energy conservation and emission reduction work, which has played an important role in accelerating the green transformation of the economy and society. In 2021, the Central Committee of the Communist Party of China and the State Council issued the "Opinions on Completely, Accurately and Comprehensively Implementing the New Development Concept and Doing a Good Job in Carbon Peak and Carbon Neutrality" and the "Action Plan for Carbon Peaking Before 2030", taking energy conservation and emission reduction as a major measure to promote carbon peak and carbon neutrality. The notice of the State Council on printing and distributing the action plan for carbon peaking before 2030 (Guo Fa [2021] No. 23) proposes to improve the level of electrification of building terminals and build "photovoltaic power generation, energy storage, DC power distribution, and flexible electricity consumption" buildings. "Light" and "storage" refer to distributed photovoltaic and distributed energy storage, respectively, as an important part of the building power distribution system; "Straight" refers to the change in the form of the building distribution grid, from the traditional AC distribution network to the use of low-voltage DC distribution network; "Flexible" means that the building electrical equipment should have the ability to be interrupted and adjusted, so that the building electricity demand changes from rigid to flexible. Compared with conventional photovoltaic buildings, photovoltaic DC buildings have the advantages of high power utilization rate (6-8% increase), obvious energy-saving advantages, less equipment investment, and short payback period. In addition, the application of DC power distribution in buildings can obtain the advantages of significantly improving system performance, significantly improving safety, and improving power quality.

In 2022, the State Council issued the "14th Five-Year Plan for Energy Conservation and Emission Reduction", which clarified the overall requirements, main objectives, key projects, policy mechanisms and safeguard measures for promoting energy conservation and emission reduction during the 14th Five-Year Plan, providing important work support for completing the goals and tasks of energy conservation and emission reduction during the 14th Five-Year Plan and helping to achieve carbon peak and carbon neutrality.

1.1.2 Consumption upgrade policy

Consumption is the primary driving force for the mainland's economic growth, and the consumption of household appliances is an important part of residents' consumption, and is one of the "four King Kongs" of traditional consumption (automobiles, home appliances, furniture, and catering). In recent years, with the increase in per capita home appliance ownership, the growth rate of traditional home appliance sales has slowed down, and the home appliance industry has entered the stock from the incremental market.

In 2020, it has been nearly ten years since the end of the last round of trade-in policy, and a large number of household appliances will enter the replacement period, and the following risks may exist in the expired use of old household appliances: (1) There are increased potential safety hazards, and household appliances that exceed the safe service life may have problems such as aging parts and circuits, which increases the possibility of potential safety hazards such as leakage, short circuit, and fire; (2) Energy waste, aging household appliances usually have high energy consumption and low energy utilization, and a lot of electricity will be wasted in the process of use, increasing energy consumption and electricity expenses; (3) Environmental pollution, aging household appliances may have problems such as the release of harmful substances, causing pollution and negative impact on the environment, which is not conducive to environmental protection and sustainable development.

At the beginning of 2021, the Ministry of Commerce and 12 other departments jointly issued the "Notice on Several Measures to Boost Bulk Consumption, Key Consumption, and Promote the Release of Rural Consumption Potential", proposing to activate the furniture and home appliance market, encourage qualified areas to eliminate old furniture and old household appliances, and provide subsidies for the purchase of green smart home appliances and environmentally friendly furniture. On June 9, 2023, the General Office of the Ministry of Commerce, the General Office of the National Development and Reform Commission, the General Office of the Ministry of Industry and Information Technology, and the General Office of the State Administration for Market Regulation jointly issued the "Notice on Promoting the Consumption of Green Smart Home Appliances in 2023", proposing to carry out in-depth trade-in of household appliances to promote the consumption of green and smart home appliances. On March 7, 2024, the State Council issued a notice on the "Action Plan for Promoting Large-scale Equipment Renewal and Trade-in of Consumer Goods" to implement the trade-in action of consumer goods and support the trade-in of durable consumer goods in areas such as home appliances.

1.1.3 Innovation and development policies

At present, the consumption of household appliances in the mainland is surging, on the one hand, the stock characteristics are obvious, and on the other hand, new household appliances have gradually become new favorites. Actively expanding market demand, grasping the trend of consumption upgrading, and helping users build a harmonious and beautiful "future home" are becoming the core driving force for the upgrading of the home appliance industry and moving towards high-quality development.

In February 2023, the Central Committee of the Communist Party of China and the State Council specially issued the "Outline for the Construction of a Quality Power", and household appliances were included in the key consumer goods. In July 2023, the Ministry of Commerce, together with the National Development and Reform Commission and 12 other departments, issued the "Several Measures to Promote Household Consumption", proposing to take green, intelligent, and age-appropriate as the starting point to improve the quality of supply, innovate consumption scenarios, improve consumption conditions, optimize the consumption environment, unblock points, solve difficulties, and treat pain points, so as to promote the recovery and upgrading of household consumption. In August 2023, the Ministry of Industry and Information Technology, the National Development and Reform Commission, and the Ministry of Commerce jointly issued the Work Plan for the Steady Growth of Light Industry (2023-2024), which puts forward practical measures for optimizing the structure of key industries, expanding domestic demand, promoting consumption, stabilizing exports, promoting the coordinated development of industrial ecology, and improving the modernization level of the industrial chain.

Figure 1. Sorting out relevant policies for the home appliance industry

1.2 Analysis of electricity consumption trend of household appliances

1.2.1 Analysis of power supply types of household appliances

Direct current and alternating current are the two most common types of electricity in power engineering and electronics, and they are fundamentally different in many ways.

Direct current (DC) refers to an electric current that does not change in magnitude or direction with time. The generation of direct current is usually done through devices such as batteries, solar panels, or DC generators. The current of direct current flows from the positive electrode to the negative electrode in a consistent direction. The advantages of direct current include high stability, low transmission loss, no electromagnetic radiation, easy control, etc., while the disadvantages of direct current include limited transmission distance and high conversion cost.

Alternating current (AC) refers to the current direction that periodically changes the direction of the current over time. The generation of alternating current is usually done by the alternators of power plants, which produce currents that periodically change from positive to negative to positive, like a sine wave. Alternating current has the advantages of convenient long-distance transmission, easy generation, simple motor structure, low manufacturing cost, and convenient maintenance, but there are also some disadvantages, such as electromagnetic radiation and voltage fluctuations in the transmission process.

Since 1882, when Edison lit up the Morgan Building with a DC power supply system, bringing light and warmth to the night, DC power has been dominant. In 1895, Tesla invented the alternating current power equipment to transmit electricity to 35 kilometers away, solving the problem of long-distance power transmission, ending the "power war", AC power supply technology developed rapidly and fully replaced DC power supply technology, all kinds of electrical equipment are basically driven by AC electricity, in history only due to the immaturity of electronic technology at that time, so that AC transmission replaced DC transmission.

The battle between direct current and alternating current has been over for more than a century, but they have not gone away. In view of the problems existing in AC transmission, the mainland is also gradually using ultra-high voltage DC transmission, which is especially suitable for transnational and cross-regional power transmission and grid interconnection due to its low loss and high stability in long-distance transmission, which helps to realize the optimal allocation of global energy resources. It can effectively concentrate renewable energy such as wind power and solar energy into the grid, supporting the global energy transition and carbon emission reduction goals; Many countries and regions around the world have taken the development of HVDC transmission technology as a national strategy, and the support of relevant policies and capital investment will further promote its commercialization process and technological innovation. To sum up, HVDC transmission technology, as an important part of the modern power grid, has a promising development prospect and will play an increasingly important role in the future power system.

Figure 2. AC and DC disputes

1.2.2 The power revolution in the context of green and low-carbon

In 2021, the State Council issued the Notice of the State Council on Printing and Distributing the Action Plan for Carbon Peaking Before 2030, which clearly requires that the level of electrification of building terminals be improved, and the construction of "photovoltaic storage, direct and flexible" buildings integrating photovoltaic power generation, energy storage, DC power distribution, and flexible electricity consumption will provide clear direction and support for the development of "photovoltaic storage, direct and flexible" for the first time at the policy level. "Optical storage, direct and flexible" is the main development direction of DC transmission technology, and DC electrical appliances will become an important support for the development of this technology.

At present, the power supply system of mainland residents is mainly based on alternating current, that is, what we see in daily life is "220V 50Hz" sinusoidal alternating current. On the power generation side, with the development of new energy sources such as photovoltaics and the rise of microgrids, low-voltage DC power supply systems are attracting widespread attention in the industry due to their high-efficiency and low-carbon characteristics. On the demand side of electricity, residents end up using a lot of direct current. From LED lighting, routers, to computers, it may extend to more categories of household appliances in the future, such as fans, air conditioners, refrigerators, washing machines, water heaters, etc. In order to provide direct current to these electrical appliances, a power module will be equipped on the outside or inside of the electrical equipment, which will rectify and step down the 220V alternating current provided by the grid and convert it into low-voltage direct current before it can be used by the electrical appliances. However, this conversion will bring a series of problems, such as high energy consumption, high cost, and complex equipment. If all the power generation is changed to direct current to form a low-voltage DC power supply system, it can break through the limitations of energy acquisition, energy transmission and distribution, and energy use of traditional electrical appliances, realize the direct current power generation, distribution, and power consumption, and improve system efficiency; Simplify the structure of related equipment and reduce costs; Let distributed photovoltaic and other power sources be connected to the grid nearby, so as to create a low-carbon, environmentally friendly, comfortable, intelligent and safe home environment.

The internal circuit of household appliances uses DC power supply, which can reduce equipment cost investment, reduce energy conversion loss, improve energy efficiency, improve home safety, convenience, comfort and artistry, and realize an environmentally friendly and energy-saving intelligent experience environment. At present, household appliances, such as refrigerators, washing machines, air conditioners, etc., use DC motors to achieve partial DC technology, and some enterprises have also done corresponding technical reserve research for full DC technology.

Figure 3. Schematic diagram of the application of optical storage, direct and flexible

1.2.3 Relevant technical reserves of the enterprise

1) Full DC technical reserves of other enterprises

In 2021, Gree launched a photovoltaic storage direct flexible air conditioning system; In 2022, Gree and Guochuang Lianneng, together with Xiaoshan Power Supply Company and NARI Group, will build the first multi-station integrated low-voltage DC distribution network demonstration project in Hangzhou, Zhejiang. In February 2023, Haier participated in the "14th Five-Year Plan" National Key R&D Program "Development and Demonstration of DC Products for Building Mechanical and Electrical Equipment".

2.) Wanjiale full DC technical reserves

Founded in 1985, Wanjiale has been focusing on improving China's household hot water and kitchen life with the brand vision of "creating a beautiful kitchen and bathroom life for hundreds of millions of families around the world".

Wanjiale has always been committed to providing high-quality hot water heating solutions, and has successfully created four gas water heater product series: zero cold water, small volume, healthy water and precise control constant temperature. From the first gas water heater with flame-out protection to the first small-volume gas water heater, from the first zero cold water technology to the innovative non-electric washing technology, Wanjiale has won many industry firsts.

In the context of the continuous rise in demand for high-efficiency and energy-saving products, Wanjiale conducted an in-depth analysis of the internal energy consumption mode of gas water heaters, and at the same time, after understanding that the relevant performance requirements (such as: poor anti-interference ability, low reliability, difficult maintenance, and relatively large harmful substance emissions) lagged behind the national green development needs, it actively responded to the national "double carbon" call and vigorously carried out product innovation and technology research and development. As a national key high-tech enterprise and a nationally recognized enterprise technology center, Wanjiale has a postdoctoral research workstation and a nationally recognized laboratory, and has developed innovative technologies that subvert the industry, such as small-volume integration technology, zero cold water, no electric washing, household central hot water, and kitchen automatic cooking. Wanjiale has led dozens of pioneering major technological innovations in the industry, led or participated in the formulation of more than 100 national and industry standards, and won nearly 3,000 authoritative industry awards at home and abroad. In order to develop a new, efficient and safe gas water heater platform technology, improve the quality of family life, and promote the technological progress of the home appliance industry, the company convened more than ten engineers, invested a lot of research and development funds, and overcame technical problems after repeated trials, optimization and improvement, which lasted 3 years, and finally successfully developed the full DC 24V gas water heater platform technology, and promoted the revision of the national standard for full DC gas water heaters, and contributed to the low-voltage full DC application of gas water heaters with cutting-edge technological innovation and research and development achievements.

1.2.4 Current situation and development prospects of flexible direct transmission

In recent years, China's electricity demand has grown rapidly. According to the data of the National Energy Administration, from 2016 to 2023, the electricity consumption of the mainland society increased year by year, and the electricity consumption of the whole society in 2023 was 9,224.1 billion kilowatt hours, a year-on-year increase of 6.7%, of which the domestic electricity consumption of urban and rural residents was 1,352.4 billion kilowatt hours, and the electricity consumption of residents accounted for 14.7% of the total electricity consumption of the society. In the context of the continuous increase in power demand and the shortage of electricity in some areas, it is not only necessary to call on the whole society to save electricity, use electricity scientifically, and use green and energy-saving household appliances and equipment, but also optimize the power grid structure, accelerate the construction of new power systems, and improve the power supply capacity of the whole network.

Figure 4. 2016-2023 China's social electricity consumption and growth

Flexible DC transmission, using a new generation of DC transmission technology, can improve the transmission capacity and safety level of the power grid, and is an important way to solve the problem of new energy consumption, long-distance transmission and grid security. The development of Continental Flexible Straightening Technology began in 2006 with the State Grid Intelligence Research Institute, and then developed rapidly. As of October 2023, a total of 11 flexible and direct transmission projects have been put into operation in China, and the transmission capacity and voltage level have reached the international leading level.

According to Rao Hong's paper "Engineering Application and Development Prospects of Flexible DC Transmission Technology", it is expected that there will be more than 50 new flexible direct transmission projects abroad in 2033, with an additional capacity of more than 60GW. Domestically, flexible DC transmission technology has been fully developed and applied in scenarios such as wind power transmission, grid interconnection, passive network power supply and long-distance large-capacity transmission, and its transmission capacity has reached the UHV level. It can be seen that flexible DC transmission is an important way for new energy grid-connected consumption, grid interconnection and long-distance power transmission, and the application scenarios include onshore flexible straight, sea breeze flexible and back-to-back grid flexible and straight interconnection, etc., with broad development prospects.

2. Analysis of the current situation and prospect of the gas water heater market

2.1 Gas Water Heater Market Size and Growth

Water heaters are household appliances that we often use in our daily lives. According to the National Bureau of Statistics, by the end of 2022, the average number of water heaters per 100 households in mainland China was 89.9 units. In 2023, with the recovery of the macro economy and the gradual release of consumption potential, the water heater market will pick up, according to the data of the National Household Appliance Industry Information Center, the retail scale of domestic water heaters in 2023 will be 51.35 billion yuan, and the retail sales will increase by 1.9% year-on-year. Among them, the sales scale of gas water heaters was 26.3 billion yuan, a year-on-year increase of 3.5%, and from the data, the market performance of gas water heaters was better than the whole. In recent years, with the accelerated coverage of natural gas pipelines, gas water heaters have become the first choice for many consumers when decorating new houses or replacing water heaters due to their advantages of small size, high energy efficiency, fast heating speed, and ready-to-use.

Figure 5. The size and growth of the domestic water heater market

2.2 Advantages of gas water heater products

As the main sales category of the water heater market, gas water heaters have gained a star-like halo by virtue of double-digit growth for many years, the government's "coal-to-gas" policy and the continuous promotion of natural gas pipeline laying across the country. In recent years, with the pursuit of consumers' pursuit of quality of life and the improvement of environmental awareness, the gas water heater market has shown a steady growth trend. In addition, the government's promotion and support of clean energy also provides favorable conditions for the development of gas water heaters. Gas water heaters have the following advantages:

1. Fast heating speed: Gas water heaters use gas as a heat source, which has a fast heating speed and can provide a large amount of hot water in a short time to meet the needs of users for immediate use.
2. Energy saving and environmental protection: gas water heaters produce less exhaust gas during the heating process, and the energy utilization rate is higher, which is more energy-saving and environmentally friendly. In 2005, the condensing gas water heater was born with high efficiency, energy saving and environmental protection, and the full premixed combustion technology vigorously developed by the gas appliance industry in recent years, which greatly improved the combustion heat exchange efficiency and reduced gas consumption, so as to achieve the purpose of energy saving.
3. High safety: Gas water heaters are usually equipped with safety devices, such as flame-out protection, overheating protection, etc., which can ensure safety during use under normal circumstances.
4. Wide range of application: Gas water heaters are suitable for various household types and water use scenarios, and can meet the needs of different users.

In recent years, consumers in the urban market who pay more and more attention to home design and user experience have given more consideration to gas water heaters with the advantages of energy saving, comfort and small size when replacing water heaters. According to the "2021 Household Water Heater Consumption Upgrade Trend Survey", gas water heaters have become the primary choice for most families, accounting for 53.8%.

Figure 6. Survey on the type of household water heater for users

Driven by factors such as manufacturers' vigorous publicity, continuous product upgrading and improvement, and improvement of gas pipeline network facilities, gas water heater products are rapidly becoming the leading category in the primary to tertiary markets.

2.3 Development trend of gas water heater products

In the process of continuous expansion of the market scale, the gas water heater industry continues to usher in new forces, especially in online channels, and the influx of a large number of brands has greatly increased the number of brands in the gas water heater market. According to the data of the National Household Appliance Industry Information Center, in 2023, the number of online brands of gas water heaters in mainland China will be 149, an increase of 26 from the previous year, and the number of brands on sale in offline channels will be 94, a decrease of 9 from the previous year. In the online market, the top three brands accounted for more than half of the country, with a retail sales share of 58.0% in 2023, an increase of 5.6 percentage points from the previous year, and the 4-5 brands accounted for 19.3% of the market share, a decrease of 2.4 percentage points from last year, and the performance of brands in the offline market was relatively stable, with the top three brands accounting for 36.7% of the market share, accounting for a slight decrease of 0.2 percentage points from last year, and the 4-5 brands gaining 22.1% The market share increased by 1.1 percentage points over last year.

Figure 7. Changes in the number and concentration of gas water heater brands in 2023

With the intensification of market competition, the determination of the brand to accelerate the transformation has become firmer, and the structural adjustment, transformation and upgrading of the gas water heater industry has become an inevitable choice. At the same time, under the general situation of consumption upgrading, consumers' requirements for household appliances are changing from "usable" to "easy to use", and the pursuit of quality life and buying good products has become a trend. Insufficient water supply, cold water waiting, water temperature fluctuations, and bathing health are the main pain points encountered by consumers in the process of using gas water heaters.

(1) Constant temperature. In view of the problem of "water temperature fluctuation" caused by external interference in the process of water use, the introduction and application of constant temperature technology has ensured the constant temperature of bathing water to a large extent. According to the data, the online and offline retail sales of gas water heaters with water servos increased by 72.1% and 68.9% respectively over the same period last year.

(2) Security. Gas water heaters generally use a closed combustion chamber and do not produce harmful gases. Modern gas water heaters are also equipped with a variety of safety protection devices, such as flame-out protection, overheating protection, anti-freezing protection, anti-backflow and gas leakage alarm protection system to ensure the safety of users; However, in actual use, there are still safety problems such as the ground wire being electrified due to the complex power grid environment of the home or the potential difference between the ground wire and the gas pipeline, which causes the pipeline to accelerate aging and heat burn-through.

(3) Health. Health has become the topic that users are most concerned about, and it has become the most publicized selling point of various companies. In terms of health experience, gas water heater products mainly achieve health effects through functions such as softening water, filtration, sterilization, and sparkling water. According to the data, the online and offline retail sales of gas water heaters with antibacterial functions will account for 46.6% and 19.4% respectively in 2023, an increase of 9.9% and 6.9% year-on-year respectively. Water health will become a breakthrough in the high-end upgrading of water heater products.

(4) Mute. In recent years, "mute" has become a high-frequency propaganda word in the home appliance industry, and many home appliances use "mute" as a selling point. In the process of operation, the gas water heater still cannot avoid the generation of noise due to the combustion and heating of the fuel and the continuous air supply of the fan, which causes trouble to the user's life. The data shows that in 2023, the retail volume of gas water heaters with mute function in the online and offline markets will account for 3.1% and 7.1% respectively, an increase of 15.6% and 29.3% year-on-year respectively, indicating that consumers have a certain willingness to buy silent water heaters.

Figure 8. Changes in the proportion of main specifications of gas water heaters in 2023

With the concept of "green and low-carbon, energy conservation and environmental protection" deeply rooted in the hearts of the people, consumers' demand for high-efficiency, low-carbon emission products continues to release, and at the same time puts forward higher requirements for bathing experience. The launch and application of these innovative technologies not only meet the current concept of green energy conservation and respond to the national "double carbon" policy, but also drive the industry through several rounds of upgrading, which not only meets the needs of consumers, but also brings new growth opportunities for the development of the industry.

3. Application of DC technology in gas water heaters

3.1 Technical analysis of electricity consumption of gas water heaters

There are two main power supply modes of gas water heater: plug-in power supply and battery power supply, in which plug-in power supply is connected to the power supply by plug-in mode, the advantage of this method is that the power supply is stable and the line is relatively simple; The battery power supply is mainly to supply power to the gas water heater through the battery, and its advantage is that the power supply of the gas water heater no longer depends on the power grid, and there is no need to lay power lines and plug-in boards and other facilities. According to the analysis of sales data fed back from the current market, the strong exhaust gas water heater with plug-in power supply is the absolute main force in the current market, and the retail share of the strong exhaust gas water heater in the offline market has been as high as 97%.

The role of power supply in gas water heater is mainly to support the work of ignition system, control system, and protection system. The spark generator of the gas water heater uses electricity to ignite the gas, and the electronic controllers, sensors and actuators in the control system rely on the current to control and regulate the flow, temperature, pressure and other parameters of the gas, and the temperature sensors, pressure switches, and leakage protectors in the protection system also need to be driven by current, monitor and control the temperature, pressure, and leakage of the water heater, and take corresponding measures in case of abnormalities to ensure the safe operation of the water heater. There are many circuit components in the gas water heater, the circuit is complex, and the power supply voltage in the access line needs to be converted to adapt to the different rated voltages, so the electricity safety of the product and the energy loss in the voltage conversion have become a common concern of enterprises and consumers. Based on the starting point of improving the safety and energy-saving type of gas water heaters, many gas water heater manufacturers are committed to promoting the innovation and development of gas water heater power technology, and improving their safety and energy consumption by adjusting the power supply type and underlying electrical architecture connected to gas water heaters.

3.1.1 Traditional AC gas water heaters

AC gas water heater refers to a gas water heater with AC power supply for the main components such as fans, pumps, and antifreeze heating elements inside the water heater. Among them, AC fans usually have a fixed speed and wind output, and lack sufficient adjustment capacity. Therefore, its wind pressure resistance is low, only about 100Pa, when the external pressure increases, the AC fan may not be able to provide enough air flow to meet the combustion needs of the water heater, which will lead to the performance of the water heater to deteriorate; Or the combustion is insufficient to produce harmful gases, which endangers the user's health, and can not meet the needs of different users for different environments in their homes. In addition, it is generally difficult to regulate the speed of the AC fan, which cannot be accurately adjusted according to the actual operation of the water heater, and if the combustion demand changes, the AC fan may not be able to provide the right amount of air in time, resulting in a decrease in combustion efficiency. In most of the combustion load sections, the AC gas water heater will take away too much heat due to the high wind speed, which will reduce the thermal efficiency of the water heater. Compared with DC fans, AC fans will also cause noise problems due to high speed, especially at night, which will also interfere with the rest and sleep of family members. In addition, AC gas water heaters generally use air pressure switches to judge the changes in flue and external wind pressure, while air pressure switches are a wearing part, with short life and high failure rate, which will seriously affect the user's experience.

In addition, the AC water pump has the problems of high electric power, large volume and low efficiency, and the weak performance under the same volume will seriously affect the user's experience.

3.1.2 DC inverter gas water heater

The traditional AC gas water heater mainly uses an AC motor, once the motor is started, it will work at the rated current and constant speed, in this working mode, regardless of the water flow, the gas water heater fan will run at a fixed power. The DC inverter gas water heater adopts DC inverter technology, which controls the speed of the motor through the inverter, so as to control the speed of the water heater fan, and then realizes the precise control of the heating effect. However, in the current stage of AC and DC coexistence, there are still problems such as electrical safety, low electrical efficiency, and insufficient interference performance. For electrical safety, the main problem is that the grounding wire of the home power grid is live or there is a potential difference between the gas pipeline, and it is easy to appear on the water heater as a hidden danger of "so-called electrical safety"; The low electrical efficiency is mainly due to the internal voltages such as AC220, DC40/36, 24/12/5V, etc., in addition to the power loss of AC/DC, there is also a large loss in the conversion between other DC voltages; The lack of interference performance is mainly due to the interference of switching power supply and servo devices, as well as poor adaptability to the complex external environment. Based on the design concept of efficient use of energy and ultimate safety exploration, we continue to explore the ultimate safety technology of high-efficiency DC, keep up with the road of green and low-carbon development, and the ultra-low voltage full DC technology leads the industry into the full DC era, as shown in Figure 9.

Figure 9. The development history of full DC gas water heater

3.1.3 Full DC gas water heater

Full DC technology is an advanced electric drive technology, which has the advantages of high efficiency, energy saving and environmental protection. The full DC gas water heater is a kind of water heater that combines the full DC technology and gas heating function, whether it is the external power supply end of the whole machine body, or the power supply mode of the electronic control components and other components inside the product, the power supply mode is DC power supply, and there is no need for AC and DC conversion inside, thus replacing the traditional AC power supply and realizing the direct control of the motor. It incorporates DC inverter technology to achieve an efficient and stable hot water output by controlling the gas supply and combustion process. In gas water heaters, full DC technology is mainly used in the fan air supply and exhaust system, the pump hydraulic system and the control system of various sensors, etc., so as to improve the operation efficiency of the whole machine and the stability of the product. In recent years, with the research and development of low-voltage full DC technology and DC microgrid, full DC gas water heater is likely to become one of the future technology upgrade goals.

At the same time, it also has greater advantages in the following aspects: first, the internal DC voltage of the full DC gas water heater is low-voltage DC, which will be more safe and reliable to use than AC 220V; The second is that the full DC gas water heater can achieve full-load accurate speed regulation and matching air volume, so that the combustion is more sufficient, the carbon monoxide emission is low and more environmentally friendly, especially when the user's flue resistance is different and larger, or the outdoor wind disturbance, the DC water heater can still work normally and safely, and the combustion conditions are normal, and the flue gas indicators are controlled within a reasonable range, energy conservation and emission reduction are in line with the pursuit of green environmental protection concept in today's society; Third, the full DC gas water heater also has the characteristics of intelligence, it not only has an intelligent constant temperature system, but also has an intelligent fault diagnosis and early warning function, at the same time, it uses a stepless inverter fan and pump, equipped with the whole electromechanical power self-distribution technology, the noise will be smaller when in use, providing users with a better hot water experience.

3.2 The status quo and advantages of using DC technology in gas water heaters

Technological innovation is the general trend of the development of gas water heaters, with the progress of science and technology, DC gas water heaters have been significantly improved in terms of appearance design, combustion technology, safety performance, etc. At the same time, the direct flow technology enables the gas water heater to control the gas supply and combustion process more accurately, thereby improving the energy efficiency of the water heater. Compared with traditional water heaters, DC gas water heaters can achieve higher thermal efficiency, quickly supply hot water, and meet people's high-quality bathing experience.

3.2.1 Technical difficulties in the design of direct flow of gas water heaters

At present, the power supply mode of gas water heater in the circulation field still adopts the AC220V power supply mode, and the power cord usually uses a three-wire pin plug, in which the ground wire will be directly connected to the metal shell of the water heater to ensure that the water heater can be safely grounded and prevent the occurrence of electric shock accidents. In addition, the zero and live wires of the water heater power supply will also be directly connected to the internal power supply system through the terminals, and the input alternating current will be converted into the direct current required by the internal circuit of the water heater through the transformer and rectifier circuit, which is used to drive the voltage required for the operation of different components in the water heater, such as: exhaust fan, igniter and circulating pump, etc., all need to obtain electric energy through this system. At the same time, it also needs to supply power to the electronic components that control the operation of these components, such as temperature sensors, pressure switches, etc., to ensure the normal operation of the water heater.

However, such a structural design cannot well meet the needs of direct access to new energy and energy storage technology in the "DC" stage of household appliances. In order to make up for these deficiencies, Wanjiale, a national enterprise of kitchen and bathroom appliances, draws on the design experience of DC electrical appliances in other industries at home and abroad, conducts a systematic analysis of the electrical structure of the existing gas water heater (as shown in Figure 10) from design to production, use and maintenance, and conducts in-depth research on the shortcomings of the existing gas water heater partial DC, and finds that there will be several technical difficulties in the DC design process of the gas water heater.

Figure 10. Electrical diagram of the interior of an existing gas water heater

3.2.1.1 Low energy efficiency

Most of the gas water heater products with DC fans or DC water pumps in the industry adopt the integrated design of controller and power supply, and there are many kinds of internal voltages, such as: AC220V, DC40V, 36V, 24V, 12V, 5V, etc. From the energy point of view, in addition to the power loss in the switching power supply in the AC-DC conversion process, the step-down conversion circuit between the other levels of DC voltage (as shown in Figure 11) will also have a large power loss. According to the analysis of the internal test experience of Wanjiale Enterprise Laboratory, the overall energy utilization rate of such products is reduced by about 10% when applied in AC power grid, which deviates from the original design concept of DC high efficiency.

Figure 11. Gas water heater voltage conversion circuit

3.2.1.2 Electromagnetic compatibility interference

Electromagnetic compatibility (EMC) is an issue that must be concerned in the integrated design of power supply and controller. The integrated design of strong and weak current of the control system will have strong interference, which will affect the normal operation of the power supply and the controller. This interference can be high-frequency switching noise from the power supply, electromagnetic radiation, or coupling interference from the signal and power lines inside the controller. Even after the treatment of common mode inductance, the interference phenomenon still exists, which will have a certain impact on the stable operation of the system. In addition, there is a certain amount of interference when the DC servo device is started, and the two are coupled to each other, and even cause some interference to the power grid, as shown in Figure 12. Therefore, it is necessary to use many anti-interference isolation devices or protection devices to optimize this during the design phase.

Figure 12. Schematic diagram of EMC interference in gas water heater

3.2.1.3 Safety hazards of external leakage

Generally, the electrical parts inside household appliances have relevant standard requirements in terms of voltage resistance and insulation, so they will be safer in the process of use, but it is not excluded that there will be leakage problems caused by factors such as aging in the process of long-term use over time, or damage to the insulation layer caused by other reasons, and changes in the user's complex ground wire environment, resulting in the metal shell of the appliance being electrified. When the ground wire fails, the human body will form a circuit with the ground after contacting the live parts, which will cause an electric shock accident.

When there is leakage inside other electrical appliances in the home, the metal shell of the water heater will also be energized through the ground wire transmission, and a loop will be formed through the gas pipeline and the earth, and the size of the loop current depends on the size of the gas pipeline's resistance to the ground, as shown in Figure 13 (the yellow line represents the direction of the leakage current, and the normal ground wire is not electrified, which is indicated in green). Therefore, electricity safety is also one of the problems to be solved in the process of DC water heater.

Figure 13. Schematic diagram of gas water heater electrification caused by leakage of other electrical appliances

3.2.2 Advantages of DC design of gas water heater

The advantages of the DC design of water heaters are mainly manifested in energy efficiency, safety, intelligence, user comfort and adaptability, etc., which can meet the diversified needs of modern families for gas water heaters.

3.2.2.1 Security design advantages

In terms of safety, the full DC technology can provide a stable DC power supply for the control system of the gas water heater, without the need for rectifiers to carry out AC-DC voltage conversion, and can optimize the internal structure of the gas water heater. In addition, fans, pumps and other components can use DC power supply, and full DC technology can be used to accurately control their voltage and current, so as to realize the functions of intelligent ignition and precise temperature control of the water heater, and improve the heating speed of the water heater and the stability of the outlet temperature.

3.2.2.2 Advantages of high-efficiency and energy-saving design

Components such as fans and pumps in gas water heaters are usually driven by motors, and the quality of the motors will directly affect the performance of the whole machine. Traditional AC motors generate large energy losses and noise during start-up and operation. The gas water heater using DC inverter motor will use DC current to control the heating power, which can realize the adjustment of the heating effect. Compared with the traditional AC inverter technology, the DC inverter technology can automatically adjust the gas supply and precise control of the combustion state according to the actual demand, and convert the electrical energy into mechanical energy more efficiently, so that the energy efficiency of the whole water heater can be significantly improved and the household electricity cost can be reduced. In addition, the brushless structural design of the DC inverter motor will make it produce relatively small noise when working; On the contrary, when the traditional AC motor is working, it will cause large fluctuations in its torque due to single-phase power supply, and the motor will not run smoothly enough, and its working noise will be much larger than the noise of DC frequency conversion.

3.2.2.3 Advantages of intelligent design

There is a mutually reinforcing relationship between the intelligence of gas water heaters and DC technology. First of all, DC technology provides a stable and efficient power supply for the intelligence of water heaters. In addition, the DC power supply has the advantages of good voltage stability and low power loss, so that the gas water heater can obtain continuous and stable power supply during the working process, and then ensure the normal and stable operation of the intelligent system (such as adjusting water temperature, water volume and other parameters). Therefore, the direct flow technology is the basic guarantee for the intelligence of gas water heaters. Secondly, the perfect combination of DC technology and intelligence also enables gas water heaters to adapt to different use scenarios and meet the needs of users. Let the water heater intelligently learn the user's bathing habits, and automatically adjust the water temperature, water volume and other parameters based on these data, providing users with a more comfortable and energy-saving bathing experience. In addition, users can also remotely control the water heater through mobile apps or other smart devices to achieve accurate water temperature setting and timing functions. This intelligent design greatly improves the convenience of daily use and meets the needs of modern families for smart life.

3.3 Summary

With the enhancement of consumers' awareness of energy conservation and environmental protection, high-efficiency and energy-saving gas water heaters have gradually become the mainstream products in the market. The DC design of the gas water heater can optimize its internal structure and working principle; At the same time, it also promotes the intelligent development of gas water heaters, making water heaters more safe and reliable, reducing energy consumption, and diversified appearance design and intelligent development also meet consumers' yearning for quality life.

Fourth, the technical interpretation and advantages of DC gas water heater

4.1 Technical interpretation of low-pressure full DC gas water heater

The low-pressure full-DC gas water heater is to apply the low-voltage DC technology to the gas water heater control system to achieve accurate control of the working state of the water heater. Direct current is easier to detect and control than alternating current, so gas water heaters using low-voltage full DC technology will perform better in terms of electrical safety. At the same time, low-voltage DC technology can also reduce power loss and improve energy utilization, so as to achieve the effect of energy saving. In terms of functionality, low-pressure full DC gas water heater technology can also be combined with other advanced technologies to further improve the performance of water heaters.

4.1.1 The overall design idea of low-pressure full DC gas water heater

The overall design idea of low-pressure full DC gas water heater mainly focuses on high efficiency, safety, stability and intelligence. As a leading enterprise in the gas water heater industry, Wanjiale has conducted painstaking research on the technical difficulties in the design of low-pressure full-DC gas water heaters and successfully developed low-pressure full-DC gas water heater products to conform to the general trend of future development of DC home appliance technology.

4.1.2 "Simplify" the underlying electrical architecture

The internal electrical circuits of gas water heaters are complex, a variety of control components are staggered with each other, and different components need to be loaded with different DC voltages, so the electrical architecture is the foundation and skeleton of the entire circuit. Only the most reasonable and smooth design of the infrastructure can ensure the stability and safety of the components. To this end, Wanjiale's engineers have innovatively designed and integrated the underlying electrical architecture of the gas water heater, and all servo devices have been unified into a high-efficiency low-voltage DC drive. On the basis of unified voltage, let the components work stably, not only can save energy consumption but also reduce the cost of electrical parts, but also improve the reliability of the product, so the electrical system of the whole machine is mainly divided into low-voltage power supply system, low-voltage main control system, low-voltage gas control, wind control, water control and other main modules.

Figure 14: Schematic diagram of the electrical underlying architecture of a gas water heater

4.1.3 Design of full DC control system

In practical applications, the switching power supply will be in a working state of high voltage and high current when working, and the waveform of its working voltage and current will also contain a large number of higher harmonic components. These higher harmonic components can generate electromagnetic fields inside the product, resulting in unstable operation and reducing the performance of the product. Secondly, the main components in the switching power supply are: transformer, power switch, capacitor, etc. In the process of high-frequency switching, they may generate harmonic oscillations that are transmitted to internal circuits through distributed capacitors, or they may radiate into space through radiators and transformers, causing interference to surrounding electronics. The server trim in the water heater contains complex circuits and electrical components, such as motors, drives, sensors, etc. These components also generate electromagnetic fields when they are working, and due to the influence of electromagnetic coupling between components, electromagnetic interference may occur, resulting in more ripple in the DC system and interfering with the normal operation of the control system, as shown in Figure 15.

Figure 15: Schematic diagram of the spatial propagation of electromagnetic interference

In order to solve this problem, Wanjiale's engineers optimized the design of the full DC control system, and adopted physical isolation and software control to reduce the impact of electromagnetic interference on the product.

4.1.4 Low-voltage power supply design

For DC home appliances, there is currently no unified DC voltage classification standard at home and abroad. At the earliest, according to different production environments and workplaces, the safety voltage was divided into five levels of 42V, 36V, 24V, 12V and 6V in the GB 3805-1983 "Safety Voltage" standard to prevent personal injury caused by electric shock in the labor and production process. The standard stipulates that when the voltage used in electrical equipment exceeds 24V, protective measures must be taken to prevent direct contact with live bodies. With the deepening of research, this standard has been updated to GB/T 3805-2008 "Extra-low voltage (ELV) limits", and is graded by environmental conditions and fault types, while the definition of safety voltage in other standards is also based on GB/T 3805-2008, as shown in Table 1.

Under different conditions of use, the requirements for safety voltage in the standard are not the same, so the use of "safety voltage" is not necessarily absolutely safe, and the current factor needs to be fully considered.

The damage caused by an electric shock to the human body depends mainly on the strength and duration of the current passing through the body. GB/T 13870.1-2022 "Effect of electric current on humans and domestic animals – Part 1: General requirements" stipulates that when exposed to DC voltage, the magnitude of the current flowing through the human body depends on the voltage and the magnitude of the resistance of the human body. The size of the human body resistance is related to the contact area, voltage, skin condition, current path and other factors, and the current flowing through the human body cannot exceed 25mA, as shown in Figure 16 and Table 2 below.

Figure 16.Regional division of the effects of DC current and duration on humans

Table 2 Physiological effects by region

Since October 1, 2019, household gas appliances have been officially included in the management scope of CCC certification, and in order to ensure product quality, gas water heater products must implement the two standards of GB 6932 and GB 4706.1. In both standards, the safety extra low voltage should not exceed 42 V, and when the safe extra low voltage is obtained from the grid, it should be passed through a safety isolation transformer or a converter with separate windings. Obviously, the grid specified in the standard is an alternating current grid. Therefore, the selection of DC safety voltage can also refer to GB/T156-2017 "Standard Voltage", which stipulates the preferred value and alternative table of DC rated voltage, as shown in Table 3. Since the DC voltage of 30V has fewer options for electrical parts during the implementation process, the engineers of Wanjiale finally chose the preferred value that is close to it - 24V as the power supply value for the rated voltage to be used in wet areas to provide the rated voltage required for the operation of the low-voltage full DC gas water heater.

4.1.5 AC and DC grid compatibility access design

At present, all provinces and cities across the country are actively promoting demonstration buildings such as smart buildings, zero-carbon buildings, green industrial parks, and photovoltaic storage, direct and flexible buildings, which also accelerate the implementation of DC power supply technology. Nevertheless, the current existing power supply mode will still be based on AC power supply, and will also exist in the state of coexistence of AC and DC power grids in a long transition time frame in the future. Therefore, at this stage, the low-voltage full DC gas water heater still needs a detachable external power adapter to convert the alternating current in the grid into direct current to supply power to the water heater. As shown in Figure 17.

Figure 17: Schematic diagram of the power supply interface of the low-voltage full DC gas water heater at this stage

4.2 The improvement of the performance of gas water heaters by low-pressure full DC technology

4.2.1 Energy consumption

First of all, from the perspective of the whole machine, the DC inverter fan used inside the low-pressure full DC gas water heater can accurately adjust the speed and match the air volume according to the user's use needs, so that the water heater can be fully burned, and will not take away too much heat due to excessive wind speed, which can provide higher combustion efficiency than the traditional AC gas water heater.

Secondly, when the water heater is working in the low-load combustion stage, the DC fan can reduce the wind speed and reduce the power of the motor; In the high-load combustion phase, the wind speed can be increased. Under the same conditions, it is difficult to achieve full combustion by using different sizes of air consumption and air volume according to load changes, especially at the usual load point, the half-open thermal efficiency is increased by about 4.2%Note 1.

In summary, the low-pressure full DC gas water heater has significant advantages in terms of energy consumption, and its efficient combustion technology, intelligent adjustment function, and DC frequency conversion technology all enable the gas water heater to minimize energy consumption during operation, reducing the power of the whole machine by 40% compared with the traditional full AC gas water heater Note 1, and achieving multi-faceted energy-saving improvement.

Note 1: The above test data comes from third-party testing institutions and Wanjiale Enterprise Laboratory.

4.2.2 Security aspects

The power supply of traditional household appliances is almost AC220V power supply, and the voltage will be relatively high when used, so you need to pay attention to the safety of electricity. If the ground wire is electrified, it may cause other home appliances to become electrified, resulting in the risk of electric shock. If you contact the live parts for a long time in a dry environment, a large area, and hand-to-hand, it will cause a strong muscle response to the muscles, and the DC voltage threshold for its contact is DC43V, as shown in Table 4 below.

In view of the current power consumption environment is still dominated by AC power grid, DC water heaters need to be used with power adapters at this stage. The power adapter uses a safety isolation transformer inside, the AC power is completely isolated outside the gas water heater, and the power adapter uses a plastic shell, which can be used as a reinforced insulation to wrap the live parts to prevent accidental leakage of the power adapter, which can improve the safety of the gas water heater from the root.

For the full DC low-pressure gas water heater, the internal electrical components are all supplied with a safe ultra-low voltage, and the voltage between the components will not be higher than the safe ultra-low voltage. Then, according to clause 8.1.4 in GB 4706.1, it is considered to be inelectrified, and even if the internal low-voltage full direct current is aged, its leakage current will not cause electric shock to the human body. Therefore, the low-voltage all-DC gas water heater has no hidden danger of leakage, and gives full play to the safety advantages of low-voltage DC power supply. In addition, because the water heater does not need to be intervened by the ground wire, it solves the series of problems caused by the leakage of the ground wire and avoids all the hidden dangers caused by the ground wire being electrified.

4.2.3 Comfort

The comfort of using gas hot water is generally reflected in the aspects of constant temperature, silence and zero cold water and large amount of water. First of all, in terms of the constant temperature of the water heater, the low-pressure full DC gas water heater can combine the DC frequency conversion technology with the constant temperature technology, and control and adjust the heating power of the gas water heater through the DC frequency conversion technology, so as to achieve accurate control of the outlet water temperature.

Secondly, in terms of noise, the noise of gas water heaters mainly includes combustion noise, vaporization noise, fan noise and mechanical noise. When the gas water heater is in different combustion stages, the burner will open different segments of the combustion fire under the control of the solenoid valve, so as to provide the combustion load required by the whole machine at this stage. Moreover, the DC fan does not have a traditional capacitive noise reduction circuit in the process of electronic speed regulation, which also makes the full DC gas water heater perform better in terms of noise.

In terms of zero cold water and large amount of water, the "pressurization" and "zero cold water" functions of gas water heaters are generally achieved through built-in water pumps. The pumps of traditional gas water heaters are usually directly driven by AC power supply, which has problems such as low efficiency and high noise. On the contrary, if the DC water pump of the same specification is used, it can achieve wide voltage operation, and under the same energy consumption, a larger amount of water can be generated, so that even users with poor water supply capacity can enjoy a comfortable bathing experience with a large amount of water, and at the same time, it also reduces energy loss, realizes the effect of energy saving, and further improves the adaptability and flexibility of the product.

In addition, the low-pressure full DC gas water heater also has the function of intelligent control, which can automatically adjust the heating power and outlet flow according to the user's water habits and actual needs, which helps to reduce operating costs and improve product adaptability, make full use of the power of the power supply, achieve the ultimate experience of product comfort, and save energy.

4.3 Summary

The low-pressure full DC gas water heater has significant advanced and technical advantages in terms of high efficiency and energy saving, zero cold water technology, large water supply, constant temperature performance, intelligent operation, safety and reliability. These technical advantages will make the full DC gas water heater more competitive in the market, and at the same time, it can also provide users with a more comfortable, convenient and safe hot water experience. Wanjiale's low-voltage full DC technology will also become the leader of the water heater industry in the future.

V. Conclusion

As the power end of the user-side power grid, the full DC gas water heater supports the development and final application of "photovoltaic storage, direct and flexible", and has become one of the important paths to achieve carbon neutrality in the field of civil buildings.

As a leading enterprise in the gas water heater industry, Wanjiale has conducted painstaking research on the technical difficulties in the design of low-pressure full-DC gas water heaters and put forward the overall design scheme of low-pressure full-DC gas water heaters, so that the water heater is more efficient and energy-saving in the process of use, and also ensures the safe and stable operation of products in a complex household electricity environment.

The full DC of gas water heaters is the inevitable direction of industrial and consumption upgrading, which is of great significance for the implementation of the "double carbon" policy and the promotion and implementation of the concept of green environmental protection. In general, the application of low-voltage full DC technology in household appliances can not only improve the quality of family life, save energy and protect the environment, but also contribute to the realization of green homes and sustainable development goals. Therefore, the application of low-voltage full DC technology in household appliances has important practical significance and development prospects.