Nghiên cứu khả năng quá lạnh cho hệ thống điều hòa không khí co2 bằng địa nhiệt

BỘ GIÁO DỤC VÀ ĐÀO TẠO TRƯỜNG ĐẠI HỌC SƯ PHẠM KỸ THUẬT THÀNH PHỐ HỒ CHÍ MINH LUẬN VĂN THẠC SĨ NGUYỄN VĂN PHA NGHIÊN CỨU KHẢ NĂNG QUÁ LẠNH CHO HỆ THỐNG ĐIỀU HỊA KHƠNG KHÍ CO2 BẰNG ĐỊA NHIỆT NGÀNH: KỸ THUẬT NHIỆT Tp Hồ Chí Minh, tháng 11/2022 BỘ GIÁO DỤC VÀ ĐÀO TẠO TRƯỜNG ĐẠI HỌC SƯ PHẠM KỸ THUẬT THÀNH PHỐ HỒ CHÍ MINH LUẬN VĂN THẠC SĨ NGUYỄN VĂN PHA NGHIÊN CỨU KHẢ NĂNG Q LẠNH CHO HỆ THỐNG ĐIỀU HỊA KHƠNG KHÍ CO2 BẰNG ĐỊA NHIỆT NGÀNH: KỸ THUẬT NHIỆT - 8520115 Hướng dẫn khoa học: PGS.TS ĐẶNG THÀNH TRUNG TP.Hồ Chí Minh, Tháng 4/2022 LÝ LỊCH KHOA HỌC I LÝ LỊCH SƠ LƯỢC: Họ & tên: Nguyễn Văn Pha Giới tính: Nam Ngày, tháng, năm sinh: 20/10/1974 Nơi sinh: Quảng Ngãi Quê quán: Quảng Ngãi Dân tộc: Kinh Chỗ riêng địa liên lạc: 197/86 Thoại Ngọc Hầu, Phú Thạnh, Tân Phú, Thành phố Hồ Chí Minh Điện thoại quan: Điện thoại nhà riêng: Fax: E-mail: phanguyeniuh@gmail.com II QUÁ TRÌNH ĐÀO TẠO: Trung học chuyên nghiệp: Hệ đào tạo: Thời gian đào tạo từ …/… đến …/ … Nơi học (trường, thành phố): Ngành học: Đại học: Hệ đào tạo: Đại học quy Thời gian đào tạo: 1993-1998 Nơi học (trường, thành phố): Đại học Bách Khoa TP.Hồ Chí Minh Ngành học: Kỹ Thuật Nhiệt Tên đồ án, luận án môn thi tốt nghiệp: Thiết kế hệ thống điều hịa khơng khí trung tâm thương mại Bãi Sậy quận thành phố Hồ Chí Minh i Ngày & nơi bảo vệ đồ án, luận án thi tốt nghiệp: Tháng năm 1998 đại học Bách Khoa thành phố Hồ Chí Minh Người hướng dẫn: GS Lê Chí Hiệp III Q TRÌNH CƠNG TÁC CHUYÊN MÔN KỂ TỪ KHI TỐT NGHIỆP ĐẠI HỌC Thời gian Nơi công tác Công việc đảm nhiệm 1998-1999 Cơng ty Mía Đường II Chun gia 1999- Trường đại học Cơng Nghiệp TP Hồ Chí Minh Giáo viên ii LỜI CAM ĐOAN Tôi cam đoan luận văn thạc sĩ “Nghiên cứu khả lạnh cho hệ thống điều hịa khơng khí CO2 địa nhiệt” cơng trình nghiên cứu tơi Các số liệu, kết nêu luận văn trung thực chưa cơng bố cơng trình khác Tp Hồ Chí Minh, ngày … tháng … năm 2022 (Ký tên ghi rõ họ tên) iii LỜI CẢM ƠN Trước hết, tác giả xin gửi lời cảm ơn chân thành đến PGS.TS Đặng Thành Trung tận tình giúp đỡ, ln đưa dẫn phù hợp thực đề tài để hoàn thành tốt luận văn “Nghiên cứu khả lạnh cho hệ thống điều hịa khơng khí CO2 địa nhiệt” Đồng thời xin cảm ơn bạn kỹ sư, thạc sĩ cộng tác hỗ trợ suốt q trình thực cơng trình nghiên cứu Tác giả xin chân thành cảm ơn toàn thầy cô Bộ môn Công nghệ Nhiệt – Điện lạnh, Khoa Cơ Khí Động Lực, Trường Đại Học Sư phạm Kỹ Thuật TP Hồ Chí Minh Các thầy cô truyền đạt kiến thức quý báu tạo điều kiện tốt để tác giả nghiên cứu hồn thành luận văn Dù cố gắng để thực luận văn hạn chế trình độ, thời gian nguồn tài liệu tham khảo nên tác giả tránh khỏi thiếu sót Tác giả mong nhận đóng góp ý kiến từ q thầy để luận văn hồn thiện iv TĨM TẮT Đề tài tập trung đánh giá khả lạnh địa nhiệt hệ thống điều hịa khơng khí dùng môi chất CO2 vận hành tới hạn Trong nghiên cứu mối quan hệ đại lượng nhiệt độ lạnh, áp suất làm mát, lưu lượng khối lượng thay đổi để phân tích nghiên cứu Năng suất lạnh hệ số COP so sánh điều kiện vận hành hệ thống: không sử dụng địa nhiệt, sử dụng địa nhiệt với ống trơn, sử dụng địa nhiệt với ống mềm có bọc nước Kết nghiên cứu cho thấy rằng, sử dụng địa nhiệt với ống bọc nước đạt hệ số COP cao 5,69 sử dụng ống trơn 4,71 so với không sử dụng địa nhiệt 4,53 Khi giảm mức công suất hệ thống hệ số COP giảm đáng kể, xu hướng giảm COP tăng độ lạnh hệ thống trường hợp lạnh nghiên cứu Dữ liệu lưu lượng khối lượng phân tích nghiên cứu này, giảm lưu lượng khối lượng, độ lạnh có xu hướng giảm Tuy nhiên, trường hợp lạnh địa nhiệt ống trơn độ lạnh giảm mạnh so với ống có bọc nước Nghiên cứu dù việc lạnh địa nhiệt mang lại hiệu cao cần ý điều chỉnh giá trị áp suất làm mát phạm vi cho phép để mang lại hiệu lượng cao v ABSTRACT This study focused on assessing the geothermal subcooling potential for the air conditioning systems using CO2 that operates in the transcritical state In this study, the relationship between the parameters such as the subcooling temperature, the cooler pressure, and the mass flow rate was determined for the best operating conditions The cooling capacity and COP (Coefficient of Performance) are also compared under different conditions: Without geothermal, geothermal using the bare tube, and geothermal using the watercovered tube The study results show that, using geothermal with the watercovered tube achieved the highest COP of 5.69 while using the bare tube of 4.71 compared with 4.53 without using geothermal When reducing the capacity of the system, the COP coefficient also decreases significantly, the COP tendency decreases when increasing the subcooling temperature in both cases The mass flow rate data were also analyzed in this study When the mass flow rate is reduced, the subcooling temperature tends to decrease However, in the case of the geothermal subcooling by a bare tube, the subcooling temperature decreases more strongly than the water-covered tube The results show that although geothermal subcooling is highly effective, it is important to control the cooler pressure within the allowable range for the maximum energy efficiency vi MỤC LỤC LÝ LỊCH KHOA HỌC i LỜI CAM ĐOAN iii LỜI CẢM ƠN iv TÓM TẮT v ABSTRACT vi MỤC LỤC vii DANH SÁCH CÁC CHỮ VIẾT TẮT ix DANH MỤC HÌNH ẢNH x Chương TỔNG QUAN 1.1 Đặt vấn đề 1.2 Tình hình nghiên cứu ngồi nước 1.2.1 Tình hình nghiên cứu ngồi nước 1.2.2 Tình hình nghiên cứu nước 21 1.2.3 Tính cấp thiết đề tài 22 1.3 Mục tiêu đề tài 22 1.4 Đối tượng phạm vi nghiên cứu …………………………………… 1.5 Nội dung phương pháp nghiên cứu 23 Chương 24 CƠ SỞ LÝ THUYẾT 24 2.1 Tổng quan đặc tính mơi chất R744 (CO2) 24 2.2 Cơ sở truyền nhiệt 26 2.2.1 Dẫn nhiệt 26 2.2.2 Trao đổi nhiệt đối lưu 26 2.2.3 Trao đổi nhiệt xạ 27 2.3 Cơ sở điều hịa khơng khí 27 2.3.1 Nhiệt ẩn 28 2.3.2 Nhiệt 28 vii 2.4 Công thức liên quan 28 2.5 Tính tốn chu trình lạnh: 30 2.5.1 Trường hợp hệ thống khơng có q lạnh 30 2.5.2 Trường hợp hệ thống khơng có q lạnh sử dụng nguồn địa nhiệt 34 Chương 41 THIẾT LẬP THỰC NGHIỆM 41 3.1 Thiết lập mơ hình thực nghiệm 41 3.1.1 Mơ hình thực nghiệm 41 3.1.2 Sơ đồ hệ thống thực nghiệm: 42 3.2 Phương pháp thu thập liệu 44 3.2.1 Các thiết bị đo kiểm 44 3.2.2 Sai số thiết bị 47 Chương 48 KẾT QUẢ VÀ THẢO LUẬN 48 4.1 Ảnh hưởng nhiệt độ lạnh đến hệ số COP hệ thống 48 4.1.1 Trường hợp sử dụng lưu lượng môi chất mức cao 48 4.1.2 Trường hợp giảm lưu lượng tuần hoàn 51 4.2 Sự ảnh hưởng lưu lượng tuần hoàn đến hệ thống 55 Chương 60 KẾT LUẬN VÀ KIẾN NGHỊ 60 5.1 Kết luận 60 5.2 Kiến nghị 61 TÀI LIỆU THAM KHẢO 62 viii under heating and cooling applications, Renewable and Sustainable Energy Reviews, Vol 92, 2018, pp 658-675 [10] Y.T Ge, S.A Tassou, I Dewa Santosa, K Tsamos, Design optimisation of CO2 gas cooler/condenser in a refrigeration system, Applied Energy, Vol 160, 2015, pp 973-981 [11] Peihua Li, J.J.J Chen, Stuart Norris,Flow condensation heat transfer of CO2 in a horizontal tube at low temperatures, Applied Thermal Engineering, Vol 130, 2018, pp 561-570 [12] Mikolaj Mastrowski, Jacek Smolka, Armin Hafner, Michal Haida, Michal Palacz, Krzysztof Banasiak, 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Conference on System Science and Engineering (ICSSE) | 978-1-6654-4848-2/21/$31.00 ©2021 IEEE | DOI: 10.1109/ICSSE52999.2021.9538466 PROCEEDINGS OF 2021 INTERNATIONAL CONFERENCE ON SYSTEM SCIENCE AND ENGINEERING (ICSSE) 26-28 August 2021 Ho Chi Minh City, Vietnam Electronic ISBN 978-1-6654-4848-2 PROCEEDINGS OF 2021 INTERNATIONAL CONFERENCE ON SYSTEM SCIENCE AND ENGINEERING (ICSSE) Electronic ISBN: 978-1-6654-4848-2 Virtual Conference, 26-28 August 2021 Ho Chi Minh City, Vietnam Co-organized by: Sponsored by: An Experimental On Subcooling Potential By Geothermal In CO2 Air Conditioning System Thanhtrung Dang Department of Thermal Engineering HCMC University of Technology and Education, Ho Chi Minh city, Vietnam trungdang@hcmute.edu.vn Hoangtuan Nguyen Faculty of Refrigeration College of Technology II Ho Chi Minh city, Vietnam tuannguyenhoang@hvct.edu.vn Vanpha Nguyen Department of Thermal Engineering HCMC University of Technology and Education, Ho Chi Minh city, Vietnam phanguyeniuh@gmail.com Jau-Huai Lu Department of Mechanical Engineering National Chung Hsing University Taichung, Taiwan jhlu@dragon.nchu.edu.tw Abstract - This study focused on assessing the geothermal subcooling potential for the air conditioning system using CO that operates in the transcritical state In this study, the relationship between the parameters such as the subcooling quantity, the cooler pressure, and the mass flow rate was determined for the best operating conditions The cooling capacity and COP (Coefficient of Performance) are also compared under different conditions: Without geothermal subcooling, with geothermal subcooling using a bare tube, and with geothermal subcooling using a water-covered tube The results of study showed that, using geothermal subcooling with the water-covered tube may achieve the highest COP of 5.69 while using the bare tube of 4.71 compared to 4.53 without using geothermal subcooling When reducing the capacity of the system, the COP decreases significantly The COP tends to decrease when increasing the subcooling quantity in both cases The mass flow rates were also analyzed in this study When the mass flow rate is reduced, the subcooling quantity tends to decrease However, in the case of the geothermal subcooling by a bare tube, the subcooling quantity decreases more than that by a water-covered tube The results showed that although geothermal subcooling is highly effective, it is important to control the cooler pressure within the allowable range for the maximum energy efficiency Keywords – Air conditioning, geothermal, subcooling, experiment, COP, CO2 I INTRODUCTION Currently, the research and the applications of CO2 are becoming very important to meet the requirements of being friendly with the environment and reaching high energy efficiency The studies in the past few decades have mainly focused on clarifying the feasibility of using CO2, and the factors that affect the efficiency of the system It is noted that the subcooling potential of the refrigerant plays an important role in improving the refrigerant capacity and thereby increasing the efficiency of the system Taking advantage of Vietnam's hot and relatively low humidity climate and geothermal background, the uses of these geothermal basics are a relatively new study to fully exploit energy efficiency when using refrigerant CO2 Related to refrigeration systems using CO2 operated under transcritical condition, Son et al [1] investigated on the condensation heat transfer characteristics of CO2 at high saturation temperature in a horizontal and smooth microfin XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE Giahuy Dang Department of Thermal Engineering HCMC University of Technology and Education, Ho Chi Minh city, Vietnam Dghuy.0708@gmail.com tube The investigation was conducted by experiment with respect to condensation temperature and mass flux The test sections consist of 2400 mm length with a horizontal copper tube of 4.6 mm (smooth) and 4.95 mm (microfin) inner diameter The experiments were conducted at refrigerant mass flux of 400-800 kg/(m2s), and saturation temperature of 20 - 300C The main experimental results showed that annular flow almost dominated the majority of condensation flow in the horizontal smooth and microfin-tube The condensation heat transfer coefficients for the smooth and microfin-tube increase with the decreasing saturation temperature and increasing mass flux Ge et at [2] showed that CO2 has been widely applied in refrigeration systems where heat is conventionally released to the ambient through external airflow To achieve the study targets, two CO2 finned-tube gas coolers/condensers with different structural designs and controls have been purposely built, instrumented and connected with an existing test rig of a CO2 booster refrigeration system Consequently, the performance of the CO2 gas coolers/condensers with different structure designs, controls and system integration at different operating conditions can be thoroughly investigated through experimentation In the meantime, models of the finned-tube CO2 gas coolers/condensers have been developed using both the distributed (detailed model) and lumped (simple model) methods The former is employed to give a detailed prediction of the working fluid temperature profiles, localized heat transfer rates and effects of pipe circuitry arrangements, while the latter is suitable for the simulation and optimization of system integration with less computation time Tsamos et al [3] presented an experimental investigation into the performance of CO2 finned-tube gas coolers/condensers with different designs in a CO2 booster system The heat exchangers were mounted in a specially designed test facility that allowed the control of different test conditions and parameters, including air on temperatures and flow rates, approach temperatures and CO2 operation pressures The integrated refrigeration system can provide specified CO2 fluid parameters at the heat exchanger inlet, through which the system efficiency can be calculated Zhang et al [4] showed that finned-tube CO2 gas coolers play an important role to the system performance and thus need to be thoroughly investigated To achieve this, some effective parameters including the CO2 and air fluid velocity fields, temperature profiles and heat transfer characteristics at different operating conditions are predicted and analyzed by means of Computational Fluid Dynamics (CFD) modelling and simulation Idewa et al [5] investigated the overall heat transfer coefficient of two CO2 gas coolers through experiment and Computational Fluid Dynamics (CFD) The CFD modelling provided prediction accuracy for the overall heat transfer coefficient with a maximum error of 9% compared to the CFD predictions Comparing the two gas cooler designs, and from the experimental and modelling results it has been shown that the performance of the gas cooler can be improved by up to 20% through optimization of the circuit design of the gas cooler A horizontal slit between the 1st and 2nd row of tubes of the gas cooler can increase the overall heat transfer coefficient by 8% compared to a fin without the slit Wand et al [6] investigated the supercritical heat transfer characteristics of compact microchannel gas coolers applied in automobile CO2 heat pump systems A simulation model of automobile gas cooler was developed by using segment-by-segment method, and validated by experimental results Using this model, under the typical heat pump operation conditions, the temperature distributions along the CO2 refrigerant flow path were predicted, and the variation of heat transfer coefficient was analyzed Furthermore, the impacts of operation conditions such as refrigerant inlet pressure and temperature, as well as configuration parameters such as depth and pass number on the heating performance were discussed Results show that the established model has a good prediction accuracy for heating capacity of gas cooler It is found that there is unfavorable air temperature inhomogeneity on the airside outlet surface of gas cooler Li et al [7] investigated the performance of the integrated fin and micro-channel gas cooler Measured heat capacity for the gas cooler ranged from to kW Effects of various inlet air temperatures and velocities, refrigerant mass flow rates and operating pressures were studied to provide a better understanding on how these parameters affected the CO2 gas cooler performance A segment-by-segment model was developed to simulate the integrated fin and micro-channel gas cooler The model predicted the gas cooler heat capacity within 5% and refrigerant-side pressure drop within 8% deviated from the experimental data The validated model was used to analyze the impact of fin geometry and air-side maldistribution on the performance of the heat exchanger Bae et al [8] investigated the condensation heat transfer and the multi-phase pressure drop of CO2 near the critical point are occurring in a Printed Circuit Heat Exchanger (PCHE) for a supercritical CO2 power cycle application Homogeneous Equilibrium Model (HEM) approach is used to evaluate and develop appropriate heat transfer and pressure drop correlations The experiment was performed with the CO2 test facility called SCO2PE (Supercritical CO2 Pressurizing Experiment) in KAIST The heat transfer and pressure drop test was conducted with the PCHE in the facility Existing correlations were compared to the test data and a new set of correlations was necessary to be developed A new set of correlations is newly suggested in this paper which captures physical characteristics reasonably well Ehsan et al [9] presented a comprehensive review of heat transfer characteristics and correlations with supercritical CO2 employed under heating and cooling conditions in horizontal channels or tube An exhaustive review of implementation of supercritical CO2 used with horizontal and vertical orientation of tubes under turbulent flow condition and other operating parameters (inlet sCO2 pressure, mass flux, temperature, and heat flux) is also reported In the present work, possible reasons for heat transfer deterioration under heating of supercritical CO2 are discussed The characteristics of pressure drop, convective heat transfer behavior, effect of buoyancy, the wall temperature distribution and finally the comparison among different correlations are reviewed extensively for supercritical CO2 The study of these correlations with their range of applicability provides a good insight for efficient thermal design and optimization of heat exchanger especially in thermal power plants Related to the application of geothermal in the refrigeration systems, Kim et al [10] conducted a simulation study of a hybrid solar-geothermal heat pump system for residential applications using carbon dioxide was carried out under different operating conditions The system consists of a solar unit (concentric evacuated tube solar collector and heat storage tank) and a CO2 heat pump unit (three doublepipe heat exchangers, electric expansion valve, and compressor) As a result, the differential of pressure ratio between the inlet and the outlet of the compressor increases by 19.9%, and the compressor work increases from 4.5 to 5.3 kW when the operating temperature of the heat pump rises from 400C to 480C Besides, the pressure ratio of the compressor decreases from to 2.5 when the ground temperature increases from 110C to 190C The operating time of the heat pump is reduced by h as the daily solar radiation increases As the solar radiation increases from to 20 MJ/m2, the collector heat rises by 48% and the maximum collector heat becomes 47.8 kWh The heating load increases by 70% as the indoor design temperature increases from 180C to 260C However, the solar fraction is reduced from 11.4% to 5.8% because of the increases of the heating load Karampour et al [11] investigated the integration of geothermal storage into state of the CO2 transcritical booster systems to evaluate the impact of this integration on energy efficiency Three scenarios of integration are studied including stand-alone and integrated supermarket building systems The results show that for a stand-alone average size supermarket, heat recovery from the CO2 system should be prioritized over a separate ground source heat pump Extracting heat from the ground by an extra evaporator in the CO2 system has also little impact on this supermarket’s annual energy use However, in the case of supermarket integration with a neighboring building where the supermarket provides heat to the neighbor, geothermal storage integration can reduce the total annual running cost of the two non-integrated buildings by 20–30% with a payback time of less than 3.5 years The results also show there is no need for a separate ground source heat pump From the literature reviews above, most of the research exploited the heat transfer properties of CO2 refrigeration systems at low evaporation temperature, exploiting more in thermodynamic parameters such as mass flow rate, evaporation temperature and density, heat flux, focusing on COP assessment and cooling capacity of the system In the field of geothermal application in the cycle, they only focused on the heat pump The exploitation of the CO2 refrigeration system operating on the critical and geothermal application in air conditioning is a completely new research direction, very feasible in our country Therefore, it is very necessary to study the subcooling potential by geothermal in the CO2 air conditioning system II METHODOLOGY subcooled by geothermal, the refrigerant continues to go through the throttle valve, reducing the pressure to decrease the temperature and then goes into the minichannel evaporator to exchange heat with the air in the testing room CO2 finally returns to the compressor to complete a cycle The cycle continues to circulate and the room is cooled down gradually, as shown in Fig In this system, the compressor is a special type for CO2, manufactured by SANDEN company, model SRCACA A Calculation and Design The heat transfer rate was calculated as:   q  UA.ttm  The log-mean temperature difference was defined as: t  t (2) t lm  max t max ln t Pressure drop by friction was calculated as: p  f w Where A Dh f L U L L  f Re w Dh Dh (3) is heat transfer area (m2) is hydraulic diameter (m) is Fanning friction factor is channel/tube length (m) is overall heat transfer coefficient (W/m2 K) is the fluid velocity (m/s) is dynamic viscosity (N.s/m2) is density (kg/m3) is temperature (0C) w µ  t  Cooler capacity for G kg was calculated as: Qk  G (h2  h3 ) Fig Schematic of the test loop Before burying the copper tube, the soil temperatures in different conditions were surveyed in advance In hot days, the wet soil was in the temperature from 28.60C to 300C, while in cool and dry days, the wet soil was in the temperature ranging from 26.30C to 30.10C The temperature of the soil varies in each session of the day The location in survey (as shown in Fig 2) is beside a building with shade, and the sunlight intensity is normal In the geothermal subcooling area, the copper tube with the inner diameter of mm, the outer diameter of 6.4 mm, the total length of 10 m and was buried underground (4) Evaporator capacity was calculated as: Q0  G (h1  h4 ) (5) Work of adiabatic compression was determined by: N  G (h2  h1 ) (6) The COP of the cycle was quantified by: Q (7) COP  N Where COP is coefficient of performance h is enthalpy at state point (kJ/kg) is work of adiabatic compression (kW)  is mass flow rate (kg/s) G  B Experimental setup The experimental setup is shown in Fig.1 CO2 was used as the refrigerant in this system CO2 vapor enters the compressor and is compressed to high pressure and temperature The superheated vapor enters the cooler and then is cooled by the fan The vapor continues to flow into the copper tube that was buried underground After being Fig The location the copper tube was buried Parameters such as temperature, pressure at the nodes of the system shown in Fig were recorded during system operation The air velocity and the humidity of the cooling environment were also collected for calculating the cooling load The range and accuracy of the measuring apparatuses are given in Table Table Accuracies and ranges of testing apparatuses Accuracy Range Thermometer ±0.10 C -270 - 4000 C Anemometer % FS 0.3 - 45 m/s Humidity meter ±3 % FS 1.0 - 99.9% Pressure sensor 0.05 %FS - 100 bar Digital volumetric flow rate meter ±0.5%RS 400 to 5000 l/h Testing apparatus III RESULT AND DISCUSSION Three cases were investigated: without geothermal subcooling, with geothermal subcooling using the bare tube, and with geothermal subcooling using the water-covered tube Temperature, pressure and some typical parameters are shown in Table It should be noted that the state points of the cycle are indicated in Fig The temperature difference between points and is considered as the subcooling quantity of the system Parameters such as the evaporation pressure and the mass flow rate are changed; while, other parameters such as the cooling capacity, the power consumption and COP are calculated to comprehensively evaluate the efficiency of the system Table Typical parameters in the refrigeration cycle Case State Points Pressure (bar) Temperature(oC) Without G (kg/s) subcooling Q0 (kW) COP Pressure (bar) o Subcooling Temperature( C) with bare G (kg/s) tube Q0 (kW) COP Pressure (bar) Subcooling Temperature(oC) with a water G (kg/s) covered Q0 (kW) tube COP 1' 41.4 85.5 85.4 24.7 85.54 35.3 40.8 85.5 85.4 25.3 88.68 33.2 40.04 85.12 85.11 23.5 85.17 30.3 42.8 41.4 8.2 14.9 68.2 2.079 3.506 41.9 40.8 7.3 14.7 67.3 2.434 3.93 41.04 40.04 6.5 14.2 66.8 3.008 4.66 N/A N/A N/A N/A 85.5 35.6 85.5 33.6 Fig Effect of subcooling quantity to evaporation pressure Inside the cooler, the isothermal condensation did not occur like the condenser in the conventional refrigeration systems In the case of geothermal subcooling with a bare tube, the temperature at the outlet of the gas cooler would become lower Furthermore, if the tube was covered with water, the heat transfer rate was higher and relatively stable; the changes in the evaporative pressure did not significantly affect the subcooling quantity The soil can be considered as an infinite thermal reservoir; however, the heat transfer rate of the water-covered tube was restricted by the contact area of the tube and the soil, thus leading to the above experimental results 85.12 85.12 35.3 31.3 Figure shows the change in the subcooling quantity due to the influence of evaporation pressure In the case of geothermal subcooling with a bare tube, the evaporation pressure was in the range of 42 bar to 46 bar, and the highest subcooling quantity was 2.50C If a water-covered tube was used, the highest subcooling quantity was ranging from 4.50C to 50C In a CO2 air conditioning system operating on transcritical cycle, changes in vapor pressure can be carried out by varying the opening of the throttle valve as the system has no condensation in the cooler The amount of circulating refrigerant in the cooler was reduced when the throttle valve opening was decreased, leading to an increase in the cooler pressure and a decrease in the evaporator pressure Fig Effect of subcooling quantity to the power consumption Fig shows the power consumption in terms of the subcooling quantity In the case of geothermal subcooling with a bare tube, the power consumption tends to increase linearly with the subcooling quantity, from 563W to 620W The same trend can be observed for the case of geothermal subcooling with water-covered tube The power consumption of the system increases from 552W to 605W as the subcooling quantity increases from 4.50C to 5.00C Fig shows the variations of COP in terms of the temperature before the throttle valve for a two-compressor parallel system The performance curves were plotted together for three cases showing the effect of subcooling on the COP of the system Especially, without subcooling in the system, the highest COP of the system reaches 4.53 corresponding to the pre-throttle temperature at 340C For the case of geothermal with a bare tube, COP may reach 4.71 while the pre-throttle temperature is 31.60C Fig Experimental results of the mass flow rate with ranging from 76 to 85 bar Fig Effect of subcooling to COP In the third case, the system is subcooled by using a water-covered tube with 60mm diameter, the COP is the highest in all cases as high as 5.69 at the pre-throttle temperature of 28.70C It is noted that the water temperature at the inlet and the outlet sides of the flexible pipe did not change, ranging from 25 to 270C The system COP only increased by 0.18 using geothermal subcooling with the bare tube However, the increment was 1.16 using geothermal subcooling with the water-covered tube In all three cases, the COP increased and then decreased in terms of the temperature before the throttle valve, as shown in Fig They are due to the process of changing the cooler pressure corresponding to the increase and decrease of the compressor discharge temperature by adjusting the throttle valve At the cooler, the cycle did not occur the isothermal process like the condensation in the conventional refrigeration system So this state causes the increasing temperature at the end of the cooler, changes the subcooling quantity and changes the cooling capacity, leading to changing COP of the whole system It is noted that this rule is only obtained when changing the throttle area (corresponding to changes in suction and discharge pressures as well as flow rate through the compressor) The mass flow rates of CO2 at different cooler pressures are shown in Fig When the cooler pressure varies from 76 bars to 85 bars, the circulating mass flow rate drops from 71.2 to 68.2 kg/h for the case of none subcooling, and the mass flow rate drops from 70.7 to 67.3 kg/h for the case of geothermal subcooling with a bare tube As for the case of geothermal subcooling with a water-covered tube, the mass flow rate drops from 70.9 to 66.8 kg/h In general, when increasing the cooler pressure, the mass flow rate tends to decrease It is because the throttle valve opening should be closed gradually to increase the cooler pressure The refrigerant flow rate was reduced as a result of smaller valve opening It is noted that the same trend is valid as the compressor was operated at 60% and 40% of the load Fig shows the relationship between the mass flow rate and the cooling capacity Qo of the system using two parallel compressors operating in three cases The highest cooling capacity of the system reaches 2.49 kW at the mass flow rate of 69.6 kg/h without subcooling In case of geothermal subcooling with a bare tube, the cooling capacity reaches 2.7 kW at the mass flow rate of 69 kg/h While in the case of geothermal subcooling with a watercovered tube, the best result was obtained that the peak cooling capacity reaches 3.15 kW at the mass flow rate of 69.1 kg/h In all three cases, the cooling capacity increases with the mass flow rate and then decreases As mentioned above, the amount of circulating refrigerant decreases with the decreasing the cooling capacity gradually according to the Equation Fig Effect of mass flow rate to the cooling capacity The relationship between the mass flow rate and the subcooling quantity is shown in Fig In the case of subcooling with a bare tube, the mass flow rate ranges from 64.8 to 70.7 kg/h, and the maximum value of the subcooling quantity is 2.40C, while using a water-covered tube the highest subcooling quantity ranges from 4.60C to 50C, and the mass flow rate fluctuates from 66.8 kg/h to 70.9 kg/h It is noted that by narrowing the throttle valve opening, the circulating mass flow rate was reduced, and the cooling pressure was raised, leading to an increase in subcooling quantity This occurs in both cases of subcooling, but when the tube is covered with water, the heat transfer rate is higher and relatively stable, the mass flow rate does not change clearly the subcooling quantity The heat capacity of the geothermal stratum can be regarded as an infinitely huge thermal reservior; however, the heat transfer rate is restricted by the contact area as well as the heat conduction between the hose and soil, thus leading to the above experimental results Fig Effect of subcooling quantity to the mass flow rate IV C ONCLUSION This paper has evaluated the potential of geothermal subcooling on air conditioning systems using CO2 This study achieved with expected results: - When changing the evaporation pressure from 42 bar to 46 bar, in case of subcooling with the bare tube, the highest subcooling quantity is 2.50C, while using the water-covered tube, the highest subcooling quantity is between 4.50C and 50C - When the cooler pressure increases, the subcooling quantity tends to increase, but also increases the adiabatic compressive power, resulting in increased the energy consumption, which occurs both of full capacity operation and reduced power level at 40% and 60% - The COP system increases by 0.18 when using geothermal subcooling with the bare tube and 1.16 using geothermal subcooling with the water-covered tube respectively - When the cooler pressure changes from 76 - 85 bar, the circulating mass flow rate changes from 71.2 to 68.2 kg/h in case of none subcooling, from 70.7 - 67.3 kg/h corresponds to the case of subcooling with the bare tube, from 70.9 to 66.8 kg/h with the water-covered tube respectively In general when increasing the cooler pressure, the mass flow rate tends to decrease ACKNOWLEDGEMENTS The support of this work by the projects (No T202101TĐ sponsored by the specific research fields of HCMUTE and No B2020-SPK-04 sponsored by the Vietnam Ministry of Education and Training) are deeply appreciated REFERENCES [1] Chang-Hyo Son and Hoo-Kyu Oh, Condensation heat transfer characteristics of CO2 in a horizontal smooth- and microfintube at high saturation temperatures, Applied Thermal Engineering, Vol 36, April 2012, pp 51-62 [2] Y.T Ge , S.A Tassou, I Dewa Santosa, and K Tsamos, Design optimisation of CO gas cooler/condenser in a refrigeration system, Energy Procedia, Vol 61, 2014, pp 2311-2314 [3] K.M Tsamos, Y.T Ge, I.D.M.C Santosa, S.A Tassou, Experimental investigation of gas cooler/condenser designs and effects on a CO2 booster system, Apply Energy, Vol 186, 2017, pp 470-479 [4] XinyuZhang, YuntingGe, JiningSun, LiangLi, Savvas A.Tassou, CFD modelling of finned-tube CO2 gas cooler for refrigeration systems, Energy Procedia, Vol 119, 2019, pp 275-282 [5] Idewa, M.C.Santosa, Konstantinos, M.Tsamos, Baboo L.Gowreesunker, Savvas A.Tassou, Experimental and CFD investigation of overall 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supercritical CO2 under heating and cooling applications, Renewable and Sustainable Energy Reviews, Vol 92, 2018, pp 658-675 [10] Wonseok Kim, Jongmin Choi, Honghyun Cho, Performance analysis of hybrid solar-geothermal CO heat pump system for residential heating, Renewable Energy, Vol 50, 2013, pp 596604 [11] Mazyar Karampour, Samer Sawalha, Carlos Mateu-Royo, Jörgen Rogstam, Geothermal Storage Integration into Supermarket’s CO Refrigeration System, IGSHPA Research Track, 2018 S K L 0 ... hành tới hạn địa nhiệt ứng dụng vào điều hịa khơng khí hướng nghiên cứu hoàn toàn mới, khả thi nước ta Vì việc nghiên ứu Khả Năng Q Lạnh Cho Hệ Thống Điều Hịa Khơng Khí CO2 Bằng Địa Nhiệt cần thiết... nghiệm cho trình lạnh địa nhiệt hệ thống điều hịa khơng khí dùng mơi chất CO2 - Đánh giá khả lạnh địa nhiệt hệ thống điều hịa khơng khí dùng môi chất CO2 1.4 Đối tượng phạm vi nghiên cứu 1.4.1... tài ? ?Nghiên cứu khả lạnh cho hệ thống điều hịa khơng khí CO2 địa nhiệt? ?? với khả lạnh dự kiến từ 2-30C ứng với suất dự kiến 100W 1.2 Tình hình nghiên cứu ngồi nước 1.2.1 Tình hình nghiên cứu ngồi