Environmental and economical analyses of transcritical CO2 heat pump combined with direct dedicated mechanical subcooling (DMS) for space heating in China
Introduction
Energy conversion and environmental protection are becoming the dominant factors for the sustainable development of modern society. However, the haze weather frequently appears during the heating season in recent years, especially for cold winter days in North China. Coal burning, the traditional heating method for space heating, is a main reason for the air pollution, which also operates with low energy efficiency. Thus, in order to replace the coal-fired boiler heating system resulting in serious pollution, the air source heat pump (ASHP) is used as a practical solution for clean space heating. It works with input power and carries the thermal energy from the ambient to the indoor air, and the coefficient of performance (COP) is greater than one. Therefore, governments give subsidy to the residents to widen the applications of air source heat pump.
Whereas, for the industries of refrigeration and air-conditioning, restricted by the Montreal Protocol [1] and that of Kigali Amendment [2], the widely used hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) is being or will be soon phased down. Most of the new produced air source heat pumps use HCFCs or HFCs (e. g. R22 or R410A) as working fluid, which are harmful to the ozone layer or can result in global warming. In contrast, natural refrigerants, such as CO2, NH3 and hydrocarbons (HCs) are harmless to the environment and with good thermodynamic and heat transfer properties [3]. Among all these working fluids, CO2 is considered as the most promising candidate because it is safe and environmentally-friendly [4], [5].
CO2 is favorable to produce sanitary hot water (SHW) due to its high temperature glide at the heat rejection process [6]. In Japan CO2 heat pump water heaters have been popularly used since 2001, which are well known as “Eco Cute” products [7]. The experimental results of Nekså et al. [8] show that the tap water can be heated from 9 to 60 °C with the heating COP of 4.3, and the hot water can reach up to 90 °C without operational difficulties. Tammaro et al. [9] modeled and compared heat pump systems to generate sanitary hot water with CO2 and R290 being used as refrigerant, separately. They concluded that the sizes of the CO2 compressor and heat exchanger are significantly smaller than those of R290 to generate the same heating capacity. Pitarch et al. [10] evaluated the heat pump systems employing natural refrigerants to produce sanitary hot water. They concluded that the COP can be improved by 11% when R290 is used to heat the water from 30 to 90 °C compared with the CO2 system.
Multiple uses (such as space heating, space cooling, sanitary hot water, air-conditioning, freeze and cold storage, etc.) of CO2 heat pump/refrigeration systems were developed, taking advantage of the CO2 large temperature glide on the same isobar. A hybrid space heating and hot water production CO2 heat pump system was put forward by Stene [11] for residential use. The water supply/return temperatures of floor heating are 33/28, 35/30 and 45/40 °C, and the set point temperatures of the domestic hot water are 60, 70 and 80 °C. The theoretical and experimental results indicate that seasonal performance factor (SPF) is not less than that of the most energy efficient brine-to-water heat pump. Jin et al. [12] introduced a CO2 ground-coupled heat pump system for space heating and cooling, and sanitary water heating. The simulation results show the combined COP ranges from 3.0 to 5.5 with the supply SHW of 65 °C, and the supply/return temperature of space heating fluid is 40/35 °C under the system boundary conditions. An integrated transcritical CO2 booster system for supermarket refrigeration and heating demands is also proposed [13], [14]. The heat recovery technology is adopted in order to provide space heating and meet the tap water heating demand. Karampour and Sawalha [13] concluded that the cost of the heating reclaimed from CO2 is about half of that purchasing from a district heating network, and 20% cheaper than that produced by air source heat pump. Zhang et al. [15] proposed a novel segmentation heating CO2 heat pump system based on the temperature glide matching method, which can be used to produce warm and hot water simultaneously. The warm water can meet the use of washing hands, vegetables etc, and the hot water can reach up to 95 °C using as drinking water or even 110 °C (pressurized water).
From the above summaries, we can conclude that CO2 shows great advantage in the applications of water heater with high water temperature lift (around 50 °C). Whereas, it should be noted that tap water is mostly used as the inlet fluid, and the temperature is relatively low (7–26 °C [12]). Thus the outlet temperature of CO2 fluid in the gas cooler could be cooled to a low degree, which can significantly reduce the irreversibility loss during the throttling process, and a high COP can be obtained. However, when it comes to the application of space heating, according to the recommended water supply/return temperature shown in Table 1, the water return temperature is apparently high, leading to a low energy efficiency. Hence, energy performance of CO2 heat pump system should be improved if used for space heating.
Dedicated mechanical subcooling (DMS) is a promising method to widen the application of transcritical CO2 heat pump system in the field of space heating. For CO2 system, DMS was firstly employed to improve the energy efficiency of refrigeration system, by cooling down the CO2 fluid at the outlet of gas cooler using a small vapor compression cycle [18]. Then, this concept was introduced by Yang et al. [19] to improve the COP of transcritical heat pump for space heating. Different from the traditional configuration of DMS (named direct DMS in this study) [20], the gas cooler outlet CO2 fluid is cooled by chilled water produced by a R134a refrigeration cycle rather than by the direct evaporating of DMS refrigerant. Then, experimental and theoretical studies were performed by Song et al. to extensively investigate the running characteristics of this new system, including the optimal discharge pressure [21], [22] and medium temperature [23], [24]. Meanwhile, the performances of combined R134a/CO2 heat pump and cascade R134a/CO2 heat pump were also compared and discussed [25], [26].
From the perspective of users and governments, the environmental and economical performances of a heat pump or refrigeration system are more concerned, which can be used to assess the emission behaviors as well as the initial and operating costs during the whole life cycle of the instruments. In this study, the environmental and economical performances of the CO2 heat pump system integrated with direct DMS (CO2 HPDMS) are discussed and compared with baseline CO2 heat pump system (CO2 HPBASE), and traditional heating solutions, including coal-fired boiler heating (CFB), wall hanging gas boiler heating (WGH), and direct electric heating (DEH). Additionally, the thermal performance deterioration due to frosting and defrosting is concerned, and the influences of building energy-saving potential, feed-in tariff, location of cities, and type of heating terminals are also analyzed and discussed.
Section snippets
System description
Fig. 1 shows the configuration of the CO2 heat pump system. The system illustrated in Fig. 1(a) is a typical traditional CO2 heat pump system (1-2-3-6-1), which includes four major components: compressor, gas cooler, throttling valve and evaporator. The return water is heated by the CO2 fluid in the gas cooler, and then the hot water flows to the uses to supply heat. Whereas, restricted by the high water return temperature, the CO2 temperature at the outlet of gas cooler is relatively large,
Analysis of heating load and primary energy consumption
The heating load per unit area of the building is calculated and the results are shown in Fig. 4. It can be found that the time for heating supply increases with the latitude of the city. Particularly, for Harbin located in the severe cold region, heating is also needed occasionally in the hottest month of July. When it comes to Shanghai, where no distinct heating was supplied to the buildings previously, the heating load is comparable to that of Harbin in some days during the winter. Thus, we
Discussion
In this study, the environmental and economical performances of CO2 heat pump system with direct dedicated mechanical subcooling are discussed. The energy efficiency of traditional CO2 heat pump is weakened significantly when the return temperature is high. The energetic performances can be alleviated by employing the direct subcooling method of DMS. Furthermore, a lower water supply/return temperature can lead to a better system overall thermal performance. Thus, under the premise of meeting
Conclusions
According to the above analyses and discussion about the environmental and economical performances of transcritical CO2 heat pump with direct dedicated mechanical subcooling (CO2 HPDMS) system and other heating methods for space heating, the following conclusions are achieved:
- (1)
Introducing DMS to traditional CO2 heat pump (CO2 HPBASE) system for space heating is an efficient method to reduce the primary energy consumption. Moreover, the energy consumption can be further decreased when small
Declaration of interest
The authors declared that there is no conflict of interest.
Acknowledgements
The authors gratefully acknowledge the financial supports from National Natural Science Foundation of China (No. 51806151 and 51776140), the Innovation Team of Cold Chain Units, Energy Saving and Cold Storage of College and University of Tianjin Municipality (TD13-5088), and Tianjin Application Foundation and Advanced Technology Research Project (15JCYBJC21600). The 6th author is also grateful for the financial support of the Japan Society for the Promotion of Science (JSPS).
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