Attaining High Energy Efficiency with Less Materials Using
Smaller-Diameter, Inner-Grooved Copper Tubes
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HEAT PUMPS GAIN POPULARITY IN CHINA | |
Integrated Dual “Cooling and Heating” Systems in China Boast Energy Conservation and Superior Comfort | |
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China and the United States are a lot alike in terms of the development of new home heating technologies. Clean energy policies in both countries are driving the rapid adoption of heat pumps. Government policies in both countries are accelerating the transition from fossil fuels to heat pumps for residential heating. Additionally, the climate zones of China and the USA are similar.
A white paper published in the Appliance and HVAC Report describes five trends in heat pump design for residential heating [1].
The present article can be considered an extension of that earlier article from the point of view of heat pump technology trends in China. This article is based on a longer Chinese language report to the International Copper Association. The complete article translated in English is available online [2].
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Feet Warm, Heads Cool
A particular focus of the present article is on an integrated dual supply system. By “integrated” is meant that the heating and cooling functions are built into one system. By “dual supply” is meant that the system provides both heating and cooling, including room cooling via a supply of condensed AC refrigerant and room heating via the circulation of hot water. This integrated dual supply heat pump system has become popular in many regions of China. The system is depicted in Figure 1.
A single air source heat pump (ASHP) outdoor unit distributes refrigerant through copper pipes to wall-hanging indoor units in the summer and circulates hot water through radiators for heating in the winter. In many cases, the hot water is circulated beneath the floor to provide radiant floor heating.
Wall hanging split units for cooling and radiant heating through floors is harmonious with traditional Chinese medicine. One Chinese saying is “keep feet warm and head cool.” In other words, Chinese medicine advocates that people should keep their feet warm and their heads relatively cool.
Radiant heating through the floor emits heat from bottom up and the heat gradually decreases from the ground to the roof. The air conditioner cooling through wall hanging split units releases cool air overhead and the temperature gradually increases from the ceiling to the floor. That’s in good agreement with the health preservation principle of traditional Chinese medicine. The integrated dual supply heat pump system described here delivers warmer air in the lower part and cooler air in the top part, complying with the concept of this health safeguard. It’s no surprise then that this system is immensely popular in Chinese homes.
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1. Dual “Cooling and Heating” Systems
2. Stainless steel bellows
3. Defrosting water drain pipe
4. Drainage at the lowest point
5. Pressure gauge
6. System circulation pump
7. Filter ball valve
8. Ball valve
9. Pressure exhaust relief valve
10. Buffer tank
11. Blow-down valve
12. Expansion tank and safety components
13. Automatic water supply valve
14. Differential pressure bypass valve
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15. Prefilter
16. Multi-function filter ball valve
17. Floor heating water collector
18. Vent valve
19. Drain valve
20. Electrothermal actuator
21. Geothermal coil
22. Automatic exhaust valve
23. Filter ball valve
24. Electric two-way valve
25. Stainless steel bellows
26. Fan coil unit
27. Fan condensate pipe
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Figure 1: Schematic of dual-supply heat pump system. | |
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Split or Integral
Figure 2 shows a schematic diagram of the outdoor unit. The outdoor unit drives the inverter compressor with a small amount of electricity to absorb a large amount of low-temperature heat from the outdoor air and convert the heat into high-temperature heat through the reverse Carnot cycle.
The refrigerant-to-water heat exchanger produces hot water near 55 °C (131 °F) at the highest. The hot water is then driven by a circulating water pump to flow through the pipes of the floor heating (radiator) end equipment and warms up the indoor air through heat conduction through the ground and radiative heating of the room.
In cold regions, an intermediate economizer or a flash evaporator as well as other technical solutions may be added to the basic system to inject refrigerant of medium temperature and medium pressure to the enhanced vapor injection compressor to ensure enough heat.
The dual-supply heat pump system currently comes in two types, namely the split type and the integral type. The split type is composed of an outdoor main unit, an indoor unit, and a water module, while the integral type is composed of an outdoor main unit with a built-in water module and an indoor unit.
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Figure 2: Outdoor unit of the dual-supply heat pump. | |
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Innovative Enhancements
Multiple provinces in northern China successively launched coal-to-clean energy policies in early 2016. By 2018, the dual-supply heat pump system had been developed, integrating a refrigerant distribution system for cooling and a water circulation system for heating. [The Chinese name for this product can be literally translated as the "Air Fluorine and Ground Water" system, apparently with reference to the HFC refrigerants commonly used for cooling.]
Integrating the advantages of stability, energy savings, easy usage, and intelligence among others, the system became remarkably popular among high-end users from the beginning and has continued to grow in popularity.
With the continuous boom of the heat pump dual-supply (cooling and heating) market, the "Air Fluorine and Ground Water" system further integrated fresh air and heat recovery systems in 2021 to support air conditioning, floor heating, hot water, air purification, and dehumidification functions at the same time.
These new developments add new meaning to the term “integrated” system.
Design for Intelligence
From the point of view of product development, intelligent solutions adopt a DC variable-frequency heat pump system to precisely control the heating and cooling effects according to real-time loads and the demand, enabling highly efficient and stable operations in both heating and cooling scenarios.
Indoor relative humidity is a key indicator of comfort. There are two main ways to maintain its stability, namely dehumidification and humidification.
Dehumidification is where the system shines.
In the "dehumidification + floor heating" mode, the indoor unit plays the main dehumidifying role. The indoor heat as well as the waste heat generated by the outdoor unit are transported to the indoor room through the water pump to dehumidify the air without cooling it.
In recent years, people have set a higher indoor temperature in the winter for a higher level of comfort. That reduces the relative humidity and creates a need for humidification, especially for dry northern China regions. Nowadays, the system can draw fresh air, humidify the fresh air with a total heat exchanger and a central humidifier, and humidify the indoor air through internal circulation.
Newer systems are also equipped with a fresh air ventilator, a total heat exchanger, a built-in coarse filter, a PM2 high-efficiency filter, and an activated carbon filter to remove harmful gases such as sulfur dioxide (SO2) and nitrogen dioxide (NO2). Such components and innovations help create a healthy and clean breathing environment.
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Figure 3: Regional market share and growth rate of air-source heat pump dual-supply cooling and heating system in 2021. | |
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Climate Zones in China
As mentioned above, the climate zones in China in many ways mirror the climate zones in the USA. They include the Cold Zone B, which has hot-summers with cold winters; and Zone A, which has hot-summers with warm-winters. This approximately correspond to ASHRAE climate zones [3].
The dual-supply heat pump system is widely applicable to both zones and especially applicable in the Yangtze River Basin, including Jiangsu, Zhejiang, Shanghai, Anhui, and Hubei regions. Generally, homes in the Yangtze River Basin do not have central heating systems. Nonetheless, living standards are improving; as the consumption power and consumer awareness increase, the demand for heating in the winter also will increase. The inherent advantages dual-supply heat pump systems ensure their adoption.
Floor heating and other heating methods can be expected to grow in popularity as the socioeconomic status of residents in these regions increases and they demand comfort and heating performance. Gas water heating was the first to enter the market. As the gas cost increases, the "air conditioner + floor heating" dual-supply heating and cooling product will enjoy greater recognition in the marketplace, unlocking new opportunities for heat pumps in this market.
The market share of air-source heat pump (ASHP) dual-supply cooling and heating systems in East China reached nearly as high as fifty percent in 2021, which was up 27 percent compared with 2020. Many manufacturers are investing resources into East China. In addition, the overall market share of the dual-supply system market in North China, Central China, and Southwest China was higher than 10 percent. Due to constraints such as climate and consumption levels in the Northwest, the Northeast, and South China, the overall market share was still small but growing at an increasing rate.
The heat pump dual-supply cooling and heating system is highly dependent upon the outdoor temperature. Market growth in the northwest and northeast regions of China, where the temperature can be as low as – 30 °C during the heating season has been greatly limited. However, in South China, where the temperature in the heating season is usually above 10 °C, the demand for heating also has been low. See Figure 3 for details.
Multipurpose Home Comfort System
The main technical characteristic of the dual-supply heat pump system is its dual purposes. In summer, refrigerant from the outdoor unit is used as a cooling source for air conditioners, and in winter, the water system is used as a heating source for end equipment such as ground radiators, highlighting a perfect combination of the refrigerant and water systems.
In summer, the air conditioner serves as the refrigerant end equipment. In the winter, the floor radiator (or other heat sink in a small number of renovation projects) serves as the water end equipment. During the transitional season or intermittent heating periods, the refrigerant end is used in combination with the purification system to become a multi-purpose home comfort system for air conditioning, heating, and dehumidification, and air purification. (Figure 5)
Compared with other heating types such as electric heating and gas heating, the system is playing an increasingly prominent role in empowering a green environment and realization of the "dual-carbon" goals. (China’s “dual carbon” targets, sometimes known as the 2030/2060 goals, are to reach its carbon emissions peak in 2030 and become carbon neutral before 2060.) See the original report for detailed comparisons between these various systems [2].
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Figure 4: Chinese Advertisement for dual-supply heat pump. | |
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Conclusions
The dual supply heat pump system is derived from two systems: 1) the air-source multi-connect air conditioning unit and 2) the household water unit. A such, it enjoys the advantages of both systems.
Indoor units enjoy multiple connections with refrigerants transferred via seamless copper tubes to avoid water leaks and water-soaked roofs. It is based on a mature technology with a diversity of models available, free and flexible control, and efficient operation of partial loads.
The heating end usually connects with a ground radiator for radiant floor heating. The human body temperature is higher at the head and lower at the feet. If fan coils supply air via top convection, the hot air is relatively light and tends to gather in the upper part of the room, resulting in the unbalanced heat distribution, namely "warm at head and cold at feet." Ground radiator heating disperses heat from bottom to top, making people "cold at head and warm at feet," which is more comfortable.
The market share of the dual-supply heat pump systems skyrocketed in China. End users are increasingly installing this integrated heating and cooling solution, which boasts high energy conservation and superior comfort. With the upgrading of products and the accumulation of experience, it is expected that the market will further expand in the future and more designers, manufacturers and installers will participate in this booming sector.
The dual-supply heat pump system not only meets the needs of ordinary houses and villas but also can be extended to ultra-low energy consuming (near-zero energy) buildings, hotels, penthouses and other projects due to its larger capacity of ceiling-vent air products.
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USST Conducts Research on Heat Exchangers for Refrigerated Display Cabinets | |
Small-Diameter Copper Tubes are Key to Performance Improvements | |
Figure 1: Admin Building on the University of Shanghai for Science and Technology Campus. (Courtesy of USST. Wikimedia Creative Commons) | |
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The University of Shanghai for Science and Technology (USST) is just a few miles downstream from the historic Bund on the Huangpu River, the gateway to China’s largest city. The USST is one of the first sights to greet passengers on ships travelling up the Huangpu. (Figure 1)
The Shanghai Baptist College and Seminary was founded by Northern and Southern Baptists of the United States in 1906. Over the course of the twentieth century, the college merged with other learning institutions and evolved into a “key university” of Shanghai, specializing in applied research and engineering.
Currently, USST is home to the School of Energy and Power Engineering (SEPE) https://en.usst.edu.cn/info/1023/2378.htm, offering doctoral programs in multidisciplinary fields of science and engineering. In particular, the Institute for Refrigeration Technology offers advanced degrees in refrigeration and cryogenic engineering.
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The International Copper Association collaborated with USST on research project relating to refrigerated display cabinets with special emphasis on the design of heat exchangers for such equipment. The result was four research reports. [1-4]
- Investigation Report on the Development of Refrigerated Display Cabinet Industry (43 pages) PDF
- Research Report on Heat Transfer and Pressure Drop Performance of Small-diameter Heat Exchanger of Refrigerated cabinet Unit for Refrigerated Truck and Refrigerated Cabinet (24 pages) PDF
- Research Report on Flow Path Design of Small Diameter Heat Exchanger for Refrigerated Cabinet (32 pages) PDF
- Research Report on the Testing of Small Diameter Heat Exchanger for Refrigerated Cabinet (36 pages) PDF
These reports are now available in English for sharing with a global audience. They are available for downloading from the technical literature webpage of microgroove.net, that is, www.microgroove.net/technical-literature. They are briefly reviewed here “In the Spotlight.”
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First Report: The Display Cabinet Industry
The first report reviews cold chain logistics in China as well as the display cabinet industry in China. It also examines the existing refrigeration technology for display cabinets and the key role played by evaporators and condensers. The report explains the rationale for using 5 mm small diameter copper tubes in these heat exchangers and the feasibility of practical research.
Second Report: General Performance of Prototypical Design
The second report describes a theoretical analysis of the effects of small-diameter copper tubes on the heat transfer coefficients and pressure drops of the heat exchanger as well as the performance of the whole system. A replacement prototype is designed from theoretical principles and installed in the refrigerated cabinet. Thermal and economic advantages are determined by comparing the heat exchanger with smaller diameter copper tubes with the original (traditional) heat exchanger. This second report mainly focuses on the immediate changes in heat transfer coefficients and pressure drops, which occur when larger diameter tubes are replaced by smaller diameter tubes in the heat exchangers.
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Figure 2: Flow Paths (top) and 3D Renditions (bottom) in HXSim for three candidate designs (C1, C2 and C3). | |
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Third Report: Design Principles and Simulations
The third report describes a detailed scheme for the design of condensers. The structural parameters of the original condenser first are extracted from the drawings of the manufacturer. Next, a model for numerical calculations was established and its reliability verified using a modified correlation formula for heat transfer and pressure drop on the air side and the refrigerant side.
The design process proper begins with the application of basic principles in the development of candidate designs, which can be further analyzed, modified and compared using theoretical methods as well as simulations. Relevant design principles were identified after a careful rev iew of an extensive body of literature. For example, one basic design principle is that countercurrent heat transfer between the condenser refrigerant and the outside air improves the logarithmic average temperature difference and enhances heat transfer.
Another principle of design is that the inlet of the condenser circuit should be located higher than the outlet to avoid the adverse effects of gravity on the heat exchanger. Heat transfer efficiency losses from reheating can be avoided by placing entrances close to each other; and placing exits close to each other but as far away as possible from entrances. The total length of tubes within a single path should be kept the same for all paths, to ensure uniform heat transfer of different channels. In the second half of the condenser circuit, merging tubes from parallel paths could improve the overall heat transfer uniformity and help meet comprehensive performance requirements of heat transfer and pressure drop.
Considering these design principles as well as various other factors, candidate condensers were designed with 450 mm tube lengths (including condenser designs with 56 tubes and 60 tubes) as well as condenser designs with 420 mm tube lengths. Further theoretical analysis and simulations using HXSim heat exchanger simulation software helped to choose a few condensers to be built and tested in the laboratory [5]. It was determined that if the single tube length was appropriately reduced, then the utilization rate of the heat exchanger area could be improved by increasing the number of tubes. The single tube length of the A Series and B Series was the same as the original design (450 mm); however, for the C Series, the single tube length was reduced from 450 mm to 420 mm (including condenser designs with 60 tubes and 64 tubes).
Figure 2 shows flow paths for the C Series of candidate designs and Table 1 lists the corresponding simulation results. The C1 condenser contains fifteen single tube lengths per row, while C2 and C3 condensers contain sixteen single tube lengths per row. The heat transfer of C1, C2 and C3 are all greatly improved compared with the original design. The heat transfer is slightly less for C1 compared to C2 or C3; however, the C1 condenser has the obvious advantage of a lower internal tube volume and a smaller pressure drop among the three designs, as indicated in Table 1.
The 2022 Purdue Conferences Paper 2331 titled “Design of 5 mm Copper Tube Heat Exchangers for Display Cabinets with R404A” describes the design principles and simulation results for several series of condensers, including A and B as well as C. [6]
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Table 1: Simulation results for C-Series of condenser prototypes. | |
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Fourth Report: Testing Results
The fourth report describe a laboratory study of the performance of the newly designed condensing heat exchangers in refrigerated display cabinets. Experimental results using different heat exchanger designs were compared and analyzed. The results show that the cooling effect and thermal efficiency could be significantly improved compared with the prototype unit. The newly designed condensers with the C1 and C3 produced the best effect. Surface temperatures of the counters of the display cabinets were lowest for the systems using C1 and C3 condensers while the thermal efficiency was improved.
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Figure 3: Photographs of prototype C1 | |
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Collaborative Research
This USST research was commissioned by the International Copper Association (ICA). Professor Liu Jianhua from USST supervised the research in collaboration with Yu Xiaoxiao, Wang Haoyu, He Kuan and Feng Guangdong and other postgraduates of the refrigeration specialty at Shanghai Polytechnic University.
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"In the Spotlight" References
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ICA. (2019a). Investigation Report on the Development of Refrigerated Display Cabinet Industry. University of Shanghai for Science and Technology, Institute of Refrigeration Technology (Report 1). Translated from Chinese: 43 pages.
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ICA. (2019b). Research Report on Heat Transfer and Pressure Drop Performance of Small-diameter Heat Exchanger of Refrigerated cabinet Unit for Refrigerated Cabinet. University of Shanghai for Science and Technology, Institute of refrigeration Technology (Report 2). Translated from Chinese: 24 pages.
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ICA. (2019c). Research Report on Flow Path Design of Small Diameter Heat Exchanger for Refrigerated Cabinet. University of Shanghai for Science and Technology, Institute of Refrigeration Technology (Report 3). Translated from Chinese. 32 pages.
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ICA. (2019d). Research Report on the Test of Small Diameter Heat Exchanger for Refrigerated Cabinet. University of Shanghai for Science and Technology, Institute of Refrigeration Technology (Report 4). Translated from Chinese. 36 pages.
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Ding, G. (2019). User guide for heat exchanger simulation for refrigeration. International Copper Association and Shanghai Jiao Tong University, Institute of Refrigeration & Cryogenics, Shanghai, China. Download from microgroove.net/hxsim.
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Kerry Song, Frank Gao, Yoram Shabtay "Design of 5 mm Copper Tube Heat Exchangers for Display Cabinets with R404A" Proceedings of 19th International Refrigeration and Air Conditioning Conference. Purdue University, West Lafayette, Indiana, Paper 331.
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New White Paper
Frank Gao. Kerry Song, Yoram Shabtay and Harry Schmitz, "Heat Pumps Deliver High Efficiency Heating" Five Trends in Heat Pump Design for Residential Heating," Appliance and HVAC Report. October 2022.
Digital Issue Download PDF All White Papers
New Technical Publications
Kerry Song, Frank Gao, Yoram Shabtay "Design of 5 mm Copper Tube Heat Exchangers for Display Cabinets with R404A" Proceedings of 19th International Refrigeration and Air Conditioning Conference. Purdue University, West Lafayette, Indiana, Paper 331.
Yoram Shabtay, Frank Gao, Kerry Song, "Simulation of the effects of copper tube diameter on refrigerant charge reduction in split AC systems and refrigerated cabinets" TPTPR, 2021 6th IIR Thermophysical properties and Transfer Processes of Refrigerants Conference (Virtual) Sept., 2021, Paper 1969.
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