Application status and prospect of residential heat pump heating technology in the Yangtze River Basin

Update time : 2021-07-01 17:42:00

Application status and prospect of residential heat pump heating technology in the Yangtze River Basin
By LI Zhenbo*, Wu Weldong, Wang Jing and LIu Fangron
【Abstract】: In recent years, the demand for heating of residential buildings in the Yangtze River Basin has been increasing, and the related heating technology has been paid more attention. Based on the climatic characteristics. building envelopes and heating characteristics of the Yangtze River Basin and focused on residential buildings, comprehensively analyses the common single heat source heat pump heating mode, the composite heat source heat pump heating mode and heating terminal forms. Considers that the household air-source heat pump heating is a shortcut to solve the residential heating problem in the Yangtze River Basin, the energy tower heat pump is an important way of heating in small region, and the multiple heat source compound heating is the developmer direction. In cities and towns, energy saving and efficient small district central heating should be the main heating way. supplemented by household heating. In rural areas. clean energy heating technology should be adopted in accordance with local conditions to make full use of the water and ground sources in the Yangtze River Basin. It is necessary to develop the composite heat source heating system to improve its energy saving. heating efficiency and economical efficiency. The radiant terminal has excellent comfort and energy saving, which is suitable and perfect heating terminal form. The building energy saving technology should be developed together with the heating technology.
【Keywords 】:the Yangtze River Basin heat pump heating. composite heat source. clean energy.
heating terminal
*University of Shanghai for Science and Technology. Shanghai, China
0 introduction
Worldwide energy shortage and environmental damage have become important problems hindering human development. According to the International Energy Agency (IEA) survey, due to the continuous growth of the global economy, increasing demand for heating and cooling, and increasing energy consumption, the global energy demand increased by 2.3% in 2018, the fastest growth rate in 10 years. With the rapid development of construction in China, the energy consumption is huge, in which the total energy consumption of buildings accounts for about 34% of the total energy consumption of the whole society, and the proportion is gradually increasing. However, in China's urban building energy consumption, air conditioning energy consumption accounts for about 50%~60% of the whole building energy consumption, especially in the Yangtze River Basin, where air conditioning energy consumption is huge in winter due to factors such as wet and cold climate and dense population in winter.
In the 1950s, China was facing a severe energy shortage problem, and relevant experts put forward that "Qinling-Huaihe River" line should be taken as the dividing line of central heating in the north and south of China to control the use of energy. However, in recent years, due to the influence of global climate change, extreme weather occurs frequently in winter in South China, cold air keeps going south, and rain and snow keep increasing. The indoor temperature is often around 0℃, and even lower than 0℃ for a long time, which leads to the worse indoor environment in winter in South China than in cold and cold areas in North China. With the continuous improvement of China's economic level, especially the great improvement of people's living standards in the Yangtze River Basin and the improvement of their quality of life, people's demand for heating in winter is increasingly strong. Therefore, it is of great significance to study the application of efficient and environmentally friendly winter heating system in residential buildings in the Yangtze River Basin.
How to use heat pump technology to solve the winter heating problem in the Yangtze River Basin has become a livelihood and scientific problem that government departments and related industries actively explore and strive to solve. The national and local governments have successively issued a number of policies and measures to encourage and support the development of new energy heating industry represented by air source and ground source heat pump, and promote heat pump heating in the Yangtze River Basin. It is suggested to apply various heat pump heating technologies according to local conditions. In 2016, the Ministry of Science and Technology and China Agenda 21 Management Center approved the establishment of the national key research and development plan project "Building Heating and Air Conditioning Solutions and Corresponding Systems in the Yangtze River Basin". Jiang Yi suggested adopting decentralized heating in hot summer and cold winter areas in south China in winter, and suggested using heat pumps for heating. Generally speaking, although the heating problem in the Yangtze River Basin has been widely concerned, the research on it is still in its infancy.
In this paper, from the perspective of climate and heating status in the Yangtze River Basin, focusing on residential buildings, the common single heat source heat pump heating, compound heat source heat pump heating and heat dissipation terminal equipment are comprehensively analyzed, and the technical direction and path that are more energy-saving and environmentally friendly and suitable for winter heating in the Yangtze River Basin are explored, providing reference for future related research and application.
1 .The status of heating in the Yangtze river basin
1.1 The climate characteristics and building envelope of the Yangtze river basin
According to the requirements of building thermal design in China, the whole country is divided into five thermal design climate zones: severe cold zone, cold zone, hot summer and cold winter zone, hot summer and warm winter zone and mild zone, with the average temperature of the hottest month and the average temperature of the years not higher than 5℃ and not lower than 25℃ as auxiliary indexes. The Yangtze River Basin is a typical hot summer and cold winter area, which is sultry and humid in summer, with an average temperature of about 25~30 in the hottest month, cold and humid in winter, with an average temperature of 0~10℃ in Leng Yue and an annual average relative humidity of 75%~80%. It needs cooling in summer and heating in winter, so it is a region with relatively poor climatic conditions at the same latitude in the world.
According to the relevant requirements of architectural design in China, buildings in hot summer and cold winter areas should meet the requirements of heat protection, shading and ventilation and cooling in summer, while cold protection should be considered in winter. At present, the wall thickness of residential buildings in hot summer and cold winter areas in China is usually 240mm, most of the external walls have no insulation structure, and the external windows generally use single-frame single-layer glass, and the window wall area is larger than that in cold areas, and the sealing performance of doors and windows is also poor, so the problems of running cold and running hot are serious.
Many scholars have studied the influence of envelope on air conditioning system. Jian Yiwen et al. studied the relationship between the annual total energy consumption of heating and air conditioning in Shanghai and area ratio of window to wall, and thought that the increase of north and east (west) to area ratio of window to wall would lead to the increase of the annual total energy consumption of heating and air conditioning. Li Xinyi and others explored the low-energy renovation strategy of residential buildings in Chongqing, and thought that improving the thermal performance of external windows could effectively reduce the energy consumption generated by heating and cooling of air-conditioning equipment throughout the year. After studying the air-conditioning system of passive residential buildings in the Yangtze River Basin, Kong Wenmao and others thought that enhancing the thermal insulation performance of residential buildings could effectively reduce the peak load of enclosure structure, and then simplify the air-conditioning system. Yu et al. analyzed the influence of envelope on energy saving of air conditioning system, and thought that the best strategy to reduce energy consumption of air conditioning system was to increase external insulation and shading.
To sum up, the temperature in the Yangtze River Basin is usually above 0C in winter, with cold and high humidity and hot and high humidity in summer. The thermal insulation performance of envelope is an important factor affecting indoor thermal environment and air conditioning energy consumption. However, the thermal insulation performance of envelope of residential buildings in the Yangtze River Basin is poor and the heat running is serious, which leads to the unsatisfactory heating effect in residential buildings. Therefore, more energy-saving building standards should be formulated, and building energy conservation should develop together with heating technology.
Many domestic scholars have studied and investigated the heating characteristics of the Yangtze River Basin, and think that the residential heating in the Yangtze River Basin has the following characteristics:
The heating time is about 3 months, and the demand starts from the beginning of December every year and basically stops in March of the following year;
The heating mode is mainly "part time, part space", and the residents start and stop the heating equipment according to the heating demand;
Most residents have the habit of opening windows and ventilating in winter;
The heating equipment is mainly split air conditioner, gas wall-hung boiler and local electric heating. Residents with better conditions use heating facilities such as radiant heating floor, and few communities use central heating;
At present, the utilization rate of renewable energy is very low. Although extensive attention has been paid to the use of water, air, ground and solar energy for heating, the application status is not good and the use ratio is very low.
From the above characteristics, it can be seen that the "full-time, full-space" central heating form in the north is not suitable for use in the Yangtze River Basin. The reasons are as follows: First, the heating time in the Yangtze River Basin is shorter than that in the north, and the construction of central heating equipment is costly and economical; Secondly, the heat supply in the form of "full-time, full-space" heating is far greater than the heat required by residents, which will not only cause huge energy pressure, but also lead to a lot of energy waste and environmental pollution; In addition, the Yangtze River Basin is a typical hot summer and cold winter area. From the perspective of the whole year, the system should not only meet the residents' winter heat load demand, but also meet the summer cooling load demand; Finally, the Yangtze River Basin, as one of the important economic and scientific research centers in China, should not use the traditional northern heating method, and should encourage new energy heating technology to accumulate experience for the future northern heating reform. Therefore, environmentally friendly, efficient and comfortable household heating and small area heating are the first choice in the Yangtze River Basin.
Compared with central heating, household heating is easier and faster to build in residential buildings, and the initial investment is relatively low. It is a quick way to solve the heating problem in the Yangtze River Basin, but there are also some problems such as the space occupied by heating equipment, poor heating effect in winter and energy waste caused by electric heating. Compared with central heating, small-area heating reduces the laying of pipelines, which can meet the residents' central heating demand in a small range and reduce the energy loss of household heating. It can meet the needs of heating and cooling in the same system, and its economy is higher than that of household heating. It is a better way of heating in the Yangtze River Basin in the future.
2 heating system analysis
Heating is one of the important factors affecting people's well-being. Many scholars have made comparative analysis on which heating system is more suitable for residential buildings in the Yangtze River Basin. In the aspect of heat source selection: Li Dan and others analyzed the current situation and related technologies of residential heating in hot summer and cold winter areas in South China, and thought that the use of renewable energy should be paid attention to in heating in South China; By analyzing the application of different single heat source heating systems in southern civil buildings, Yin Ping believes that air source heat pump is a shortcut to solve the heating problem of southern residential buildings; Bai Yunyun et al. analyzed the working principle and related applications of energy tower heat pump, and thought that energy tower heat pump technology can effectively solve the heating problem of residents in the south of the Yangtze River, and can also meet the cooling requirements in summer. Due to the shortcomings of single heat source in heating, composite heat source heating system has become a hot topic for scholars to discuss in recent years. After considering the shortcomings of single heat source, Xi Xiaozhen and others believe that the heat pump heating system coupled with solar energy and air heat energy is an ideal heating system in the Yangtze River Basin; Several common heating forms of military commanders in Sun Zhong are compared. According to the characteristics of the Yangtze River Delta region, it is proposed to adopt compound heat source heating, such as solar energy and heat pump compound heating, solar energy and biogas compound heating, etc. Wenjun Feng and others analyzed the climate characteristics of the Yangtze River Delta region, and also proposed compound heat source heating, and considered that centralized heating in small areas in residential quarters was the best scheme.
2.1 single heat source heat pump heating system
Electric heating system, gas wall-hung boiler system, air source heat pump system, energy tower heat pump system and ground source (soil source, surface water source, underground water source and reclaimed water source) heat system are common single heat source heating systems in the Yangtze River Basin.
Green, clean and efficient heating methods should be the first choice for future heating in the Yangtze River Basin, and renewable energy should be given priority as the heat source for heating. Because of its high energy consumption, electric heating is not suitable for large-scale heating. Wall-hung boiler heating has pollution and safety problems, which should be gradually replaced by cleaner, more environmentally friendly and safer heating methods.
2.1.1 air source heat pump
The principle of air-source heat pump is to extract the low-grade heat energy in the air for residential heating by consuming a small amount of electric energy, which has a high energy utilization rate, and can use the same system and energy (electric energy and air energy) as summer refrigeration. It is suitable for household heating and easy to build when used in the Yangtze River Basin. According to "Renewable Energy Law of the People's Republic of China" and relevant explanations, air heat energy conforms to the definition attribute of renewable energy, and in 2015, China brought air source heat pump into the category of renewable energy technology.
In terms of energy saving and emission reduction: Zhang et al. think that compared with traditional heating mode, air source heat pump heating is more environmentally friendly, which is an effective way to solve pollution problems and has a good application prospect in China: Sun Hongli et al. tested the winter thermal environment of some residential buildings in Chengdu (heating system including gas wall-hung boiler and air source heat pump), and the test results show that the energy consumption of heat pump system is lower, which is only 12.5% of that of gas wall-hung boiler system; Zhang Lu built an air source heat pump heating system in a residential building in Hefei. The results show that the average COP of the system is 2.67 on the premise of meeting the heating demand, and it has a good energy saving effect. Zhang Chuan et al. set up an air source heat pump system with small temperature difference heat transfer terminal in a residential building in Shanghai for testing. The results show that the COP of cold and heat of the system is above 3.0 whether it is heated by 35℃ hot water in typical winter conditions or cooled by 10℃ cold water in typical summer conditions. Zhao Kang et al. installed an air source heat pump in a residential building in Hefei and conducted experiments with radiation terminals. The research shows that the COP of the heat pump can reach 4.0 and the energy efficiency of the heat pump unit can reach 3.3 under the condition of satisfying indoor comfort. In terms of economy: Yu Tao and others have studied that the air source heat pump system is more economical than the traditional coal-fired heating system; Mikkel et al. analyzed the economy of common heating methods, and the results showed that air source heat pump had obvious advantages in economy. In terms of suitability, Yu Lixia et al. analyzed the climate suitability of air-source heat pump in 9 cities of Yangtze River Basin. The results showed that Chengdu and Chongqing had good suitability, Hangzhou, Changsha, Nanchang, Wuhan and Shanghai had better suitability, while Nanjing and Hefei had poor suitability.
To sum up, air source heat pump is a renewable energy heating system with remarkable economy, energy saving and environmental protection. It is widely used in the Yangtze River Basin and is the main way of household heating in residential buildings. However, there are the following problems in the heating of this kind of system in the Yangtze River Basin:
1) Insufficient cooling capacity in summer, high outdoor temperature in the Yangtze River Basin in summer, resulting in poor cooling performance of heat pumps, and air source heat pumps can not fully meet the requirements of indoor cooling load;
2) Frost exists in outdoor units in winter, which also leads to the fact that air-source heat pumps are not generally suitable in the Yangtze River Basin, because about 60% of the heating season in the Yangtze River Basin is in the heavy frost area, and frosting will seriously affect the unit performance, with the energy efficiency ratio of the unit decreasing by about 40% and the heating capacity decreasing by about 45%. It should be avoided that the performance of the unit will be lowered due to "frost is not removed".
In order to solve the above problems, some scholars have carried out research. Firstly, to solve the problem of insufficient cooling supply in summer, the technologies of supplementing air and increasing enthalpy, CO2 heat pump, two-stage heat pump and cascade heat pump are mainly adopted at present. Secondly, scholars at home and abroad have done a lot of research on defrosting and frost-free technology, which can be divided into three aspects: 1) increasing air volume and wind speed, improving the surface form and coating of heat exchangers; 2) Using heat storage water or energy storage materials to relieve frosting degree; 3) Using intelligent optimization defrosting control method, the heat pump system can better adapt to the change of outdoor environment. In addition, the energy tower technology is also one of the effective ways to solve the frosting problem. Based on the above technologies, the suitability of air source heat pump system in Yangtze River basin has been improved, but its initial investment has also increased correspondingly.
2.1.2 Energy Tower heat pump
Energy tower (also known as heat source tower) heat pump technology uses air as heat source through the heat exchange of energy tower and the function of heat pump unit, and realizes the functions of cooling, heating and hot water supply through the energy exchange between tower body and air. In winter, because the solidification temperature of the carrier medium used in the system is lower than 0℃, the heat energy in the air can be effectively extracted when the ambient temperature is lower than 0℃ to realize heating; In summer, the energy tower can be used as a cooling tower, which can efficiently discharge heat into the air and realize refrigeration. Energy tower heat pump system is a new type of single heat source heat pump system, which has been widely used in large buildings in southern China in recent years and is not limited by natural environment factors.
In terms of energy saving and emission reduction, the energy tower heat pump can be used as a cooling tower in summer and a heat absorption tower in winter, which can meet the annual cold and heat load demand and provide hot water. Especially in summer, the building waste heat can be recovered to heat domestic water, making full use of the waste heat and improving the efficiency of the unit. Song Yinggan and others think that the energy tower has obvious energy-saving effect and is suitable for the Yangtze River basin with outdoor wet bulb temperature above 9℃, mainly for central heating in small areas. Li et al. set up an energy tower heat pump heating test bench. The research results show that under the winter environment with dry bulb temperature of 1~5℃ and relative humidity of 71%~95%, the heating COP of the system is 2.58~4.34. The energy tower heat pump technology has broad application prospects in hot summer and cold winter areas. According to the analysis of Wang Wei and others, the COP of energy tower heat pump system in winter is about 3.2, and the frosting probability is reduced by 85%.
In terms of economy: Xu Zhengyu made an economic comparison among energy tower heat pump, air source heat pump and gas boiler. The results show that the operating cost of energy tower heat pump is 5,55% lower than that of air source heat pump and 10.73% lower than that of gas boiler; Ge Yulei compared the energy tower heat pump with the air source heat pump. The experimental results in Shanghai show that the performance coefficient of the former is 15% higher than the latter, and the operating cost is 5.9% lower than the latter. Chen Wei and others analyzed the heating system reconstruction project of a university in Zhejiang, and thought that the energy tower system has high equipment utilization rate, long life and obvious advantages in economy; Qi Fei et al. developed an energy tower heat pump system, and compared it economically with the commonly used VRF system. It is considered that replacing VRF system with energy tower heat pump in Yangtze River basin can save energy by 40%, and in addition, energy tower heat pump can provide hot water whole day.
It can be seen from the research of the above scholars that the energy tower heat pump has the advantages of wide working range, high energy efficiency ratio, high efficiency and environmental protection, long service life, relatively simple system and multi-purpose (cooling, heating and providing hot water), etc. It also solves the frosting problem of air source heat pump in winter operation, and is suitable for the Yangtze River Basin which is in the heavy frost area most of the time, and is a way to solve the residential heating problem in this area. However, the energy tower heat pump system also has some practical problems, such as relatively high initial investment, easy corrosion of plate heat exchangers caused by unfrozen liquid and immature related technologies. Energy tower heat pump technology has been widely used in large-scale buildings and residential quarters in the Yangtze River Basin, and has good economy and energy saving. It will be a breakthrough to solve the residential heating problem in the Yangtze River Basin, and it is a technology with development potential and application prospect, which is worthy of active promotion and in-depth study.
2.1.3 ground source heat pump
Ground-source heat pump technology is an efficient and environmentally-friendly air-conditioning technology, which uses shallow underground low-temperature geothermal sources, such as soil, groundwater or surface water, and transfers low-grade heat energy to high-grade heat energy by inputting a small amount of high-grade energy, thus cooling and heating buildings. Underground shallow rock and soil collected about 47% of solar energy, which is 500 times higher than the total annual energy consumption of human beings. It is almost infinite renewable energy, which is consistent with the trend of sustainable development in China. From the introduction of ground source heat pump technology into China in 1990s to 2013, China's ground source heat pump system has provided energy for about 400 million m2 of buildings, with a total energy supply area far exceeding that of the United States (about 199 million m2). Ground source heat pumps include surface water sources, soil sources, underground water sources and renewable water source heat pumps, among which surface water source heat pumps include lake water sources, river water sources and sea water source heat pumps, and renewable water source heat pumps include domestic sewage sources and industrial wastewater sources.
In terms of energy saving, the ground source heat pump has high energy efficiency, and the COP can reach 5.0 on some existing equipment. Based on the existing technology, the COP is improved to 6.0~8.0, and the theoretical maximum COP of this technology is 14.0; Wang Zejian and others take the ground source heat pump as an example to supply energy for a high-density residential building in Shanghai. The calculation and analysis show that the COP of the system is 4.35~5.23 for cooling in summer and COP3.43~3.82 for heating in winter; Taking a villa in Hangzhou as an example, Wang Lei built a ground source heat pump+floor/ceiling radiation+ventilation system. The test results show that the COP of the system is 3.0~4.0, which can meet the needs of cold and hot loads in winter and summer and ensure the annual hot water demand; Wang Qi et al. took Nantong underground water source pump as an example. Experimental analysis shows that the heating efficiency of underground water source heat pump is 108% higher than that of air source heat pump, and it is about 70%~80% lower than that of traditional electric heating operation, and there is less pollutant emission; Taking Xiangtan City's surface water source heat pump air conditioning project as an example, Liu Tingting compares the energy consumption of surface water source heat pump and air source heat pump. The results show that the former saves 31.5% more energy than the latter in hot summer and cold winter areas.
In terms of economy: Zhang Nan and others took a residential building in a hot summer and cold winter area as an example, and analyzed the economy of the ground source heat pump system. Although the initial investment of the ground source heat pump is higher than that of the conventional air conditioning system, its energy saving effect is remarkable, its operating cost is much lower than that of the air conditioning system, and its investment payback period is about half of its service life, which has good economy; Bolling et al. compared and analyzed four systems: ground source heat pump, high efficiency electric heating, absorption air conditioning, direct heating and heat driven heat pump, and thought that ground source heat pump had the best economy and the shortest payback period.
Although ground source heat pump has great advantages in energy saving, emission reduction and economy, there are also some problems in practical application. Surface water source heat pump system is limited in application location, and it needs to be close to water source or water storage device before it can be used. There are some problems in ground source heat pump system, such as heat imbalance, high initial investment and corrosion of soil to metal. Groundwater source heat pump system has some problems, such as blockage of recharge wells, imperfect evaluation system and limited exploitation of groundwater, among which the intractable recharge problem causes the groundwater resources in many areas to be damaged to varying degrees, and some areas even have ground collapse. The regenerative water source heat pump system has the problems of blockage, scaling and corrosion, and the sewage quality is one of the most critical factors affecting the system performance. Moreover, the actual utilization rate of reclaimed water in China is low, and the research on key technologies is not yet mature. Based on the above research, it is found that ground source heat pump has good energy saving and economy. For the Yangtze River Basin, it should be used according to local conditions, make full use of the advantages of water and ground source in the Yangtze River Basin, and maximize the advantages of ground source heat pump.
2.2 compound heat source heating system
It can be seen from the previous section that although the heating mode of single heat source can basically meet the heating demand of buildings in the Yangtze River Basin, there are still many problems to be solved and improved. In view of this situation, scholars at home and abroad put forward a series of heating forms with composite energy sources, which complement each other's advantages and further improve the energy saving and economy of the heating system.
2.2.1 the combination of solar energy and heat pump
Solar energy heating technology is a technology that converts dispersed solar energy into convenient hot water through a heat collector and delivers it to the heating end to heat the room. Its main advantage is that it has no pollution to the environment and is inexhaustible; Its deficiency lies in the insufficient light intensity in the Yangtze River basin. The technology is greatly affected by solar radiation and unstable, so it can only be used during the day, and its use time is severely limited. Therefore, at present, solar energy systems are mostly used to produce domestic water, and rarely used for heating alone. Solar energy and heat pump composite system combines solar heat utilization technology with heat pump technology, which makes up for the deficiency of single heat source and improves the working performance of heat pump and the heat collection efficiency of solar collector. Common forms include direct expansion solar heat pump, which combines solar energy with air source heat pump or ground source heat pump.
In terms of energy saving: Odeh et al. designed a solar energy and air source heat pump composite heating system, which can realize double heat sources to supply energy, and found that the performance of composite heat source heating is better than that of single heat source heating; Bellos et al. compared the heating performance of direct expansion solar heat pump system and ordinary air source heat pump system, and the results showed that the COP of the former was 60% higher than that of the latter; Huang Ziqi and others set up a solar energy and air source heat pump system test bench in Changzhou, Jiangsu Province. The experimental results show that the COP is above 5.0 when heating, and the average COP of the system that produces hot water while cooling rooms in summer is 6.0; Weng Wen and others took a villa in Nanjing as an example to analyze the composite system of solar energy and ground source heat pump. The results show that when the heat load is greater than the cold load, the comprehensive performance of the system is better than that of a single ground source heat pump system, and it can ensure the heat balance of the soil, which has certain feasibility; Zhi Chaoying and others verified through experimental research that the solar energy and ground source heat pump combined heating system can effectively solve the problem of heat imbalance, restore the soil temperature, improve the COP of heat pump units, and achieve long-term stable operation; Wu Pin et al. put forward an application system of solar power supply combined with air source heat pump, and simulated the operation of the system. The results show that the annual power generation of solar roof photovoltaic in multi-storey residential buildings in the Yangtze River Basin is greater than the annual power consumption of air source heat pump system, and the system has good energy saving.
In terms of economy: Hou Yaxiang and others compared the solar and air-source heat pump system, air-source heat pump system, electric heating boiler, coal-fired boiler and natural gas wall-hung boiler from five evaluation indexes, such as initial installation cost and operation cost. The results show that the comprehensive evaluation value of solar and air-source heat pump system heating is the highest (3.35) in hot summer and cold winter areas; Ma Hui and others believe that because the technology of direct expansion solar heat pump heating is still immature, the initial investment and operating cost are not economical, and the direct expansion solar heat pump heating replaces electric heating, the investment payback period is about 9 years, and the life is long; Liu Guangping and others believe that although the operating cost of the composite system of solar energy and ground source heat pump is low, the initial investment is relatively high and the system maintenance is complicated; Research by Dai Jianzhong and others shows that the initial investment of solar energy and ground source heat pump system is 20% higher than that of traditional ground source heat pump system, but the operating cost is about 18% lower than that of traditional ground source heat pump system.
From the above analysis of energy saving and economy, it can be concluded that the solar energy and heat pump combined heating system has excellent performance, high heating efficiency and low operating cost, and can realize multi-purpose of one machine. However, due to the immature technology, its economy is poor and its investment payback period is long. In the future development, it is necessary to further study related technologies, so as to improve the system performance and its economy. The composite heating system of solar energy and heat pump will have a wide application prospect in residential heating in the Yangtze River Basin.
2.2.2 combination of CCHP and heat pump
CCHP consists of two systems: power generation and waste heat recovery. Natural gas is used as the main fuel to drive power generation equipment such as micro-combustion engine, internal combustion engine or gas turbine to generate electric energy for users. The low-grade waste heat discharged from the system after power generation is supplied to users for heating and cooling through the waste heat recovery system, and the generated electric energy can be complemented by grid connection. CCHP has the advantages of energy saving, environmental protection, high comprehensive energy efficiency and good economic benefits. However, in practical application, there are some problems, such as the weakening of system energy cascade utilization advantage due to the dynamic change of actual load. CCHP and heat pump combination is to combine gas cooling, heating and power cogeneration technology with heat pump technology to realize multi-energy supply mode, which can improve the economy and flexibility of the system, give full play to the advantages of different heat sources, and further improve the utilization efficiency of primary energy. In terms of energy saving, Si Pengfei thinks that CCHP and water source heat pump composite system can give full play to the advantages of energy cascade utilization of CCHP; Yu Chunyao calculated and analyzed a regional energy project in Shanghai. The results showed that compared with CCHP alone, the comprehensive utilization rate of primary energy increased from 75.1% to 78.0%, and the cascade utilization ratio of natural gas increased from 57.6% to 100%. Shi Lijun and others made a comprehensive analysis of a CCHP and water source heat pump composite energy supply system in Chongqing. The experimental results show that the comprehensive utilization rate of primary energy is 82.3%, the cooling COP is 3.85, and the heating COP is 3.97. In terms of economy: Yu Chunyao believes that the energy cost of CCHP and water source heat pump composite system is reduced by 43% compared with single CCHP; Shine Chow compares the economy of CCHP and water source heat pump composite system with traditional water source heat pump system scheme, and thinks that the composite system can reduce the cost by about 43.1% every year; Through simulation calculation, Chen Bin et al. think that the annual operation cost of CCHP and ground source heat pump composite system is low, although the initial investment is high, the increased cost can be recovered within 7.9a years compared with the conventional system, and the system has good long-term benefits. From the above analysis, compared with traditional CCHP, CCHP and heat pump composite system has higher comprehensive utilization rate of primary energy, enhanced advantages of energy cascade utilization, improved refrigeration and heating efficiency, and can effectively solve the heat imbalance problem of ground source heat pump, which can guarantee the annual energy demand of users. In addition, the operating cost is reduced and the operating life is prolonged. However, the overall investment of the system is too large, and the payback period is slightly longer. The combined system of cooling, heating and power supply and heat pump is seldom used in China, and the immature related technologies lead to insufficient economic advantages, which is one of the important reasons hindering its development.
2.2.2 combination of CCHP and heat pump
CCHP consists of two systems: power generation and waste heat recovery. Natural gas is used as the main fuel to drive power generation equipment such as micro-combustion engine, internal combustion engine or gas turbine to generate electric energy for users. The low-grade waste heat discharged from the system after power generation is supplied to users for heating and cooling through the waste heat recovery system, and the generated electric energy can be complemented by grid connection.
CCHP has the advantages of energy saving, environmental protection, high comprehensive energy efficiency and good economic benefits. However, in practical application, there are some problems, such as the weakening of system energy cascade utilization advantage due to the dynamic change of actual load.
CCHP and heat pump combination is to combine gas cooling, heating and power cogeneration technology with heat pump technology to realize multi-energy supply mode, which can improve the economy and flexibility of the system, give full play to the advantages of different heat sources, and further improve the utilization efficiency of primary energy. In terms of energy saving, Si Pengfei thinks that CCHP and water source heat pump composite system can give full play to the advantages of energy cascade utilization of CCHP; Yu Chunyao calculated and analyzed a regional energy project in Shanghai. The results showed that compared with CCHP alone, the comprehensive utilization rate of primary energy increased from 75.1% to 78.0%, and the cascade utilization ratio of natural gas increased from 57.6% to 100%. Shi Lijun and others made a comprehensive analysis of a CCHP and water source heat pump composite energy supply system in Chongqing. The experimental results show that the comprehensive utilization rate of primary energy is 82.3%, the cooling COP is 3.85, and the heating COP is 3.97.
In terms of economy: Yu Chunyao believes that the energy cost of CCHP and water source heat pump composite system is reduced by 43% compared with single CCHP; Shine Chow compares the economy of CCHP and water source heat pump composite system with traditional water source heat pump system scheme, and thinks that the composite system can reduce the cost by about 43.1% every year; Through simulation calculation, Chen Bin et al. think that the annual operation cost of CCHP and ground source heat pump composite system is low, although the initial investment is high, the increased cost can be recovered within 7.9a years compared with the conventional system, and the system has good long-term benefits.
From the above analysis, compared with traditional CCHP, CCHP and heat pump composite system has higher comprehensive utilization rate of primary energy, enhanced advantages of energy cascade utilization, improved refrigeration and heating efficiency, and can effectively solve the heat imbalance problem of ground source heat pump, which can guarantee the annual energy demand of users. In addition, the operating cost is reduced and the operating life is prolonged. However, the overall investment of the system is too large, and the payback period is slightly longer. The combined system of cooling, heating and power supply and heat pump is seldom used in China, and the immature related technologies lead to insufficient economic advantages, which is one of the important reasons hindering its development.
3 heating terminal equipment
The terminal equipment of heating system is an important part of the system. It provides heat to the room to meet the heat load demand of the room and make the indoor temperature meet the requirements. At present, the heating terminal equipment mainly includes radiator, radiant heating equipment and hot air heating equipment. In this paper, the heat transfer modes, advantages and disadvantages and common systems of these three heating terminal equipments are summarized, as shown in Table 1.
Table 1 Comparison of Different Heating Terminal Equipment

	Heat transfer mode	Advantages	disadvantage	Common system
Radiator heating	Natural convection and radiation	Low cost, long service life, easy production, low thermal inertia and convenient transportation and installation	When the water quality is poor, the radiator is prone to oxidation corrosion and water leakage, with high heat loss and uneven indoor temperature	Boiler heating, gas wall-hung boiler, solar energy and triple supply of cold, heat and power, etc
Radiant heating	Radiation	Uniform room temperature, comfort, energy saving, low noise, saving indoor space, flexible and safe installation, long service life and improved hygiene	The system is hidden, with many projects, difficult maintenance and high initial investment. The floor coil is laid on the structural layer, which increases the load bearing of the floor. The floor is thermally inert and the indoor temperature rises slowly, which is not suitable for intermittent heating	Boiler heating, gas wall-hung boiler, heat pump heating and combined cooling, heating and power supply, etc.
Hot air heating	forced convection	Convenient installation and maintenance, simple structure, low initial investment and fast response	Strong sense of blowing reduces indoor comfort, and the indoor air environment in the whole space is poor and the indoor temperature is uneven	Heat pump heating and so on

In terms of energy saving, under the same comfort conditions, radiant heating saves 20%~30% energy than traditional convection radiator heating, and radiant heating system saves 28%~40% energy than conventional convection air conditioning heating system. In terms of comfort, the floor radiant heating room dissipates heat upward from the ground, the temperature gradually decreases, and the room is evenly heated, which can not only warm people's feet, but also avoid dust caused by air convection. The comfort is good, and the warmth of feet accords with the concept of keeping in good health of traditional Chinese medicine, which has a good health care effect. In terms of economy, the initial investment of radiant heating is slightly higher than that of radiator heating and hot air heating, but its service life is relatively long (about 50 years), which is more than twice that of other end forms. The economy of radiant heating is better than the other two forms. However, when cooling in summer, there are some problems such as easy condensation, poor comfort and reaction speed compared with convection heat transfer terminal. To sum up, radiant heating terminal is an energy-saving and comfortable terminal form with good economy, but it has certain limitations in summer cooling. When applied in the Yangtze River basin, radiation terminal and other terminal forms should be organically combined to meet the cold and hot demand of residential buildings. In the following research, we should fully study the condensation, unsuitability of intermittent heating and effective coupling with other terminals of the radiation terminal during cooling in the high humidity area of the Yangtze River Basin.
4 summary and prospect
1）In terms of heat source types: the heating period in the Yangtze River Basin is short and the heating temperature is low, so low-grade energy and renewable energy should be used as heat sources as much as possible, and coal-fired boilers and pure electric heating are not recommended; The Yangtze River basin is a hot summer and cold winter area, and the heat source should meet the requirements of residents for heating in winter and cooling in summer; In the future, heating in this area should not be limited to one heat source. It is suggested to use multiple heat sources for combined heating to avoid the deficiency of single heat source.
2) In terms of heating forms: central heating in the north is not suitable for the Yangtze River Basin. Green, clean, efficient, comfortable and sanitary household heating and small-area heating should be the first choice for the heating of residential buildings in the Yangtze River Basin. Due to the advantages of economy and energy saving, small-area heating is more suitable for the heating needs of residential buildings in the Yangtze River Basin in the future; Energy-saving and efficient central heating in small areas should be given priority to in towns, supplemented by household heating, and clean energy heating technology should be adopted according to local conditions in rural areas to make full use of the advantages of water and ground sources in the Yangtze River Basin.
3) In the heating system: air source heat pump technology is suitable for household heating, which is a quick way to solve the heating problem in the Yangtze River Basin; Energy tower heat pump technology is suitable for central heating in small areas and is an important way to solve the heating problem in the Yangtze river basin; Ground source heat pump needs to combine its own advantages with the characteristics of heating areas, apply it according to local conditions, and give full play to its technical advantages, which is a supplementary form of heating in the Yangtze River Basin; Composite heat source has the advantage of complementary energy, which can effectively improve the system performance, reduce the system operation cost, and effectively make up for the deficiency of single heat source. However, the related technology is not mature and the initial investment is too high, which is the development direction of heating in the Yangtze River Basin in the future and the focus of future research.
4) On the heating terminal: the radiant heat dissipation terminal is an ideal heating terminal form, which is more energy-saving and comfortable than other terminal forms, and is suitable for residential heating terminal form, but there are still some problems in cooling. The radiation terminal and other terminal forms should be organically combined to construct an efficient and comfortable terminal for both winter and summer.
5) In the envelope structure: heating technology and building energy-saving technology are complementary and indispensable, and the development of heating technology should also promote the progress of building energy-saving technology.
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Source: Journal of HV&AC Heating Ventilating & Air Conditioning

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