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Increasing the energy supplement from renewable energy systems is one of the major issues for preparing the climate change challenges. In order to expanding the energy supplement from the renewable energy, the surplus energy should be generated in the community. This study presents the feasibility of configuration of net plus energy community by real-scale experimental research and measurements with an eco-friendly energy town constructed in Jincheon, stated in the middle of the South Korea. The 72,000 m2 of town contains six public buildings: a high school, library, youth center, childcare center, public health center, and management center, and the facilities incorporated a hybrid renewable energy system (HRES). This paper demonstrates the field tests and measurement results of annual and monthly source and site energy consumption and generation. The annual average daily final yield of the PV system installed in the town achieved 3.5 kWh/kWp/d, and the monthly performance ratio ranged from 65% to 79%. It was also found that the HRES contributed to generate over 24% more surplus energy compared with conventional heat pump system. Consequently, the annual source energy balance of the town showed that the proposed town achieved 134.5% net plus energy town in South Korea.

District cooling and heating methods that can utilize highly efficient heat pumps and various unused new and renewable types of energy are required to achieve low carbon emissions and zero energy usage in buildings and community units. The technical requirements for the implementation of decentralized thermal networks and heat trading are increasing, both for thermal networks in new buildings and for those remodeled based on existing centralized thermal networks. In this study, a conventional centralized thermal network was implemented as a decentralized thermal network and the possibility of heat prosumers feeding thermal networks was demonstrated experimentally. A real-scale plant was constructed by employing unused thermal energy facilities as prosumers in a school and childcare center based on the existing small-scale block heating and cooling thermal network. The decentralized thermal network and heat prosumer concepts were proven through operation experiments performed on the constructed system in summer and winter. An economic benefit can be achieved by increasing the peak power cost. The experimental results also showed that the proposed bi-directional heat trading reduced carbon emissions by 12.7% compared with conventional centralized thermal systems.

The overall heat transfer coefficient, pressure drop, and the fouling thermal resistance were compared between a clean and a seawater-exposed, fouled plate-frame heat exchanger. The fouled heat exchanger was utilized for heat recovery from the seawater-effluent for 3 months at a powerplant site. The performance tests were conducted at temperatures of 20 °C and 30 °C on the fouled-side, while the corresponding temperatures on the clean-side were 15 °C and 25 °C, respectively. Flow rate ranged from 20 to 40 l·min−1 on the fouled-side and the clean-side of the heat exchanger. In the absence of fouling, the average U value was higher than that under the fouling condition by 21.3 %. The average thermal resistance ranged from 0.135 m2·K·kW−1 to 0.157 m2·K·kW−1. The pressure drop was less than 2 kPa with the change of temperature, but the pressure drop of the fouled-side increased by 400 % compared with that of the clean-side.

This study investigated the thermal performance of an independent dedicated outdoor air system integrated with thermal energy storage (TES) used in a childcare center located in the eco-friendly energy town of Jincheon. During the cooling season, chilled water from the TES served buildings based on the use of a thermal network. To thermally charge the TES, a ground source heat pump was operated during off-peak periods (i.e., overnight). Based on the monitoring data for the childcare center and building leakage test results, the required sensible and latent cooling loads were estimated through simulations. The potential for operational energy savings in the proposed system was investigated and compared with the existing heat pump system, which was integrated with the TES and a dehumidifier. It was found that the proposed system could save up to 38% of the operating energy consumption compared with existing systems.

In this study, a plate heat exchanger was developed for applying to a heat pump system using the heated effluent from a thermal power plant. Since seawater is used as a heat source and a heat sink in this heat pump system, Titanium was chosen as the heat plate material for its high corrosion resistance. One of the purposes of the study was to get an optimized distribution area in the heat plate in order that better thermal performances could be achieved through equal flow rates along the heat transfer area. It was revealed from the performance tests that the mean heat transfer rate and the overall heat transfer coefficient were enhanced by 2.9% and 7.7%, respectively, compared to those measured before the design optimization.

This study investigated the applicability and thermal performance of heat prosumers in a bidirectional thermal network through experimental analysis. To realize the bidirectional thermal network and heat prosumers, the existing small-scale centralized district cooling and heating system was retrofitted to form a small-scale heat network with three thermal energy prosumers. The existing small-scale centralized district cooling and heating network consists of seven buildings: high schools, management center, childcare center, baby care center, youth center, library, and public health center. Among these, high school and childcare center were retrofitted from heat consumers to heat prosumers. Control logic for the proposed heat prosumer was also demonstrated. For the case study, the operation of the specific day was selected. The amount of heat supplied from the STES in the management center was 798.7 kWh that contained 17% of the total load. The heat supply from the TES connected to the heat pump in the management center was 2,270.5 kWh, the absorption heat pump for high school. The amount of heat supply for public buildings and schools was 824.0 kWh and 513.8 kWh, respectively, and the heat supply from the childcare center through geothermal heat pump was 295.2 kWh.

The demand for district heating and cooling systems in block units with a heat pump that utilizes various unused energy sources for energy supply has been increasing. This study investigated experimentally the ground source heat pump (GSHP) and sewage water source heat pump (SWSHP) facilities used in block cooling and heating networks. Then, a heat pump performance prediction model was derived for utilization in future designs. Operational data for heating and cooling energy supply from an experimental site were investigated for the period between 2018 and 2020. During the cooling season, the coefficient of performance (COP) of the GSHP was approximately 4.1, and that of the SWSHP was approximately 2.9. The cooling performance of the SWSHP gradually decreased because of the fouling. The COP of the GSHP and SWSHP during the heating season was approximately 3.6 and 3.4, respectively. The results also demonstrated that, if fouling in the SWSHP can be prevented or reduced, the acquired COP can be similar to that of the GSHP. The derived prediction model serves as a good reference for engineers who require information on the performance of field operations.

This study compares the performance of estimation methods for site-specific solar collector yield using two simplified collector efficiency equations-based models for training and testing of the model. Two different collector efficiency equations are used for comparative evaluation: One employs a first order collector efficiency equation defined in terms of collector inlet temperature, solar irradiance, and outdoor temperature, termed first-order equation model with constant T ci . The other equation model is a modified collector efficiency equation expressed with the terms of heating load, solar irradiance, and outdoor temperature, termed modified equation model with heat load input. In the approach with the first-order equation model, the collector inlet temperature term is treated as another parameter to be estimated as an effective collector inlet temperature. To evaluate the equation-based models, the Polysun simulation program is used for generating a data set of solar water heating system. For evaluation of daily solar collector yield estimation, two data sets were generated: training data set and testing data set. For evaluation of hourly solar collector yield estimation, training and testing performances were evaluated respectively. By using hourly simulated data, the hourly collector estimation performance was evaluated in terms of daily root-mean-squared error for the process of model training to compare the performance of fitness of the model for hourly estimation. The daily performance of two models showed quite similar estimation results. The comparison of the hourly performance of the two models resulted in a better performance of the modified equation model with heat load input.

The borehole thermal energy system, KINTEX, located in Ilsan, South Korea was simulated using TRNSYS. The effect of the depth and spacing of the boreholes in this Borehole Thermal Energy Storage (BTES) system on the average stored energy and ground temperature were investigated over three years of its operation. Borehole thermal energy storage was simulated using the Duct Ground Heat Storage Model built in TRNSYS. The BTES system is composed of solar collectors, a heat exchanger, borehole thermal energy storage, and a thermal storage tank. The effects on the internal energy of the system were investigated by varying borehole depth and spacing while the volume and number of boreholes were kept constant. The total number of boreholes is 90 and all boreholes are connected in series. The borehole depths varied from 40 to 47.5[Formula: see text]m with steps of 2.5[Formula: see text]m, and the borehole spacing varied from 2.5 to 4.0[Formula: see text]m while the total volume of the BTES was maintained at 21,[Formula: see text]236 [Formula: see text]m3. The internal energy stored ranged from 1528.91 GJ to 1618.69 GJ over three years of calculations. With an increase of borehole depth and associated decrease of borehole spacing, the internal energy increased and ground temperatures approached 35∘C over a simulation period of three years.

A prediction model of CO 2 condensation heat transfer coefficients was developed. The flow patterns in the present model were predicted by the Soliman model. The existing Thome et al. model was applied to predict heat transfer coefficients for the wavy-intermittent and annular flow. The modified Dittus–Boelter equation was utilized for the mist flow. Heat transfer coefficients in the annular-mist flow were calculated from the combination of models for the annular and the mist flow with the ratio of We number. The mean deviation of the present model with the experimental data of 589 points from seven different sources was 44.8%. The present model showed improved predictability compared to the existing models under high mass flux and high condensation temperature conditions, and with a small diameter tube.

The objective of this study is to investigate the performance characteristics of an injection CO 2 (carbon dioxide) HPWH (heat pump water heater) at various operating conditions. In the standard and standby heat loss test conditions, the performance of a two-stage CO 2 HPWH adopting a SCVI (sub-cooler vapor injection HPWH) cycle was measured and analyzed by varying the compressor frequency, the water flow rate, and the injection ratio at various outdoor temperatures. In the standard tests at the outdoor temperature of −15 °C, the COP (coefficient of performance) of the optimized SCVI HPWH was 7.6% higher than that of the non-injection HPWH. During the standby loss tests at the water flow rate of 200 kg h −1 , the COP of the optimized SCVI HPWH was 7.1% higher than that of the non-injection HPWH.

To investigate the behavior of the round-trip efficiency of transcritical-CO 2 -cycle-based TEES (thermo-electric energy storage) according to the changes in the temperature of the low-temperature hot storage tank, the charging and discharging processes were optimized at various temperature conditions of the tank. In contrast to previous studies, to achieve a more realistic simulation, heat exchangers of identical shape and size were shared by charging and discharging processes, and the characteristics of the working fluid's heat transfer and pressure drop were considered. Based on this study, it was confirmed that an optimal temperature exists in the low-temperature hot storage tank for maximizing the round-trip efficiency. The reason for this existence was discussed. Furthermore, it was found that a common design approach—using all the stored heat as a heat source during discharging—does not always yield optimal results from the viewpoint of round-trip efficiency.

The objective of this study is to investigate the effects of vapor injection techniques on the heating performance of a CO2 heat pump. The performances of the flash tank vapor injection (FTVI), sub-cooler vapor injection (SCVI) and FTVI with a suction line heat exchanger (FTSX) cycles were measured and analyzed with variations of the outdoor temperature, compressor frequency, and injection mass flow rate. At the outdoor temperature of −15 °C and compressor frequency of 55 Hz, the heating capacity and COP of the optimized SCVI cycle were 12.1% and 12.7% higher than those of the optimized FTVI cycle, respectively, because the total mass flow rate in the SCVI cycle was higher than that in the FTVI cycle by the large temperature and pressure differences in the sub-cooler of the SCVI cycle. In addition, the optimum injection flow rate ratios in the vapor injection CO2 cycles yielding the maximum COP were determined at various compressor frequencies.

The cooling mode performance of a residential split heat pump operating under nine different faults was described using normalized performance parameters determined from a ratio of the fault value to the no-fault value obtained from heat pump tests in environmental chambers. The normalized parameters were the coefficient of performance (COP), total capacity, refrigerant-side capacity, sensible heat ratio, outdoor unit power, and total power. A correlation for the normalized performance parameters was developed to produce a continuous representation of the performance characteristics given the fault level and indoor and outdoor temperature conditions.

The injection mass flow rate in a vapor injection CO2 cycle is an important design factor to improve the heating performance of a heat pump at low ambient temperatures. The intermediate pressure in a twin rotary compressor strongly affects the injection mass flow rate, and can be adjusted by the cylinder volume ratio of the second-stage to the first-stage. In this study, the heating performances of the vapor injection CO2 cycles including the flash tank vapor injection (FTVI) and sub-cooler vapor injection (SCVI) cycles were measured by varying the cylinder volume ratio of the twin rotary compressor at the ambient temperature of −15 °C. From the results, the optimum injection ratio yielding the maximum COP was determined for each cylinder volume ratio. In addition, in the FTVI and SCVI cycles, the optimum cylinder volume ratio was determined as 0.7 in order to achieve the maximum COP with the designed heating capacity.

This paper explores the thermodynamic performance limits of the vapor compression cycle. We have applied evolutionary algorithms to explore the performance of hypothetical refrigerants defined by the thermodynamic parameters used by the extended corresponding states model for fluid properties. We identified optimal values of these parameters required to reach the performance limits. The study confirmed the fundamental trade-off between the coefficient of performance (COP) and volumetric capacity, and indicated refrigerant critical temperature as the dominant parameter influencing the tradeoff. Thermodynamic performance limits depend on the operating conditions and the cycle design. These limits are represented by Pareto fronts developed for the objective functions COP and volumetric capacity. As expected, the performance of current refrigerants falls below the Pareto front limits. We demonstrate that for practical cycles the developed methodology and resulting Pareto fronts are more realistic benchmarks for the performance potentials of refrigerants than is COP alone, which is an efficiency only metric.

A techno-economic analysis of a hybrid renewable energy system, consisting of a solar thermal system, seasonal thermal energy storage (STES), heat pump systems, and district heating network for a net zero energy community has been conducted. Thermal and electric energy performance of the proposed systems were evaluated using detailed simulation models and experimental results of the Jincheon eco-friendly energy town in South Korea. Comparative environmental and economic analyses on two conventional systems that use gas-fired boilers (Case 1) and a centralized heat pump system (Case 2) were also performed. Assessment criteria in the environmental analysis include equivalent CO2 emissions and primary energy savings. The levelized cost of heat, a benefit-cost method and a payback period calculation were conducted to evaluate the economic benefit. The analyses were also conducted with different sizes for system components. Results showed that the increase in solar fraction of the proposed system reduces CO2 emission by up to 61% compared with Case 2 and enhances primary energy savings by up to 73% compared with Case 1. The levelized cost of heat was lower for the proposed system than the conventional systems. The proposed system showed a 6 year payback period and a benefit-cost ratio of 1.7.

The in-tube condensation heat transfer characteristics of CO2 in three different microchannels were investigated. The channels were rectangular, and the numbers of ports were 7, 19, and 23. The hydraulic diameters were 1.5, 0.78, and 0.68 mm for the 7, 23, and 19 ports, respectively. The test temperature ranged from −5 to 5 °C, and the range of the mass flux was from 400 to 800 kg/m2 s. The condensation heat transfer coefficient increased with the decrease in hydraulic diameter, which is dominant at condensation temperatures of 0 and −5 °C. Increasing and decreasing the heat transfer coefficient at critical vapor quality was observed for the microchannel of 23 ports. The highest pressure drop was found in the microchannel of 23 ports too. The model by Thome et al. showed the smallest mean deviation of heat transfer coefficient, and the Mishima and Hibiki model had a mean deviation of ±30.1% for the frictional pressure drop.

The condensation heat transfer coefficient and pressure drop of CO 2 in a multiport microchannel with a hydraulic diameter of 1.5 mm was investigated with variation of the mass flux from 400 to 1000 kgm −2 s −1 and of the condensation temperature from −5 to 5 °C. The heat transfer coefficient and pressure drop increased with the decrease of condensation temperature and the increase of mass flux. However, the rate of increase of the heat transfer coefficient was retarded by these changes. The gradient of the pressure drop with respect to vapor quality is significant with the increase of mass flux. The existing models for heat transfer coefficient overpredicted the experimental data, and the deviation increased at high vapor quality and at high heat transfer coefficient. The smallest mean deviation of ±51.8% was found by the Thome et al. model. For the pressure drop, the Mishima and Hibiki model showed mean deviation of 29.1%.

The objective of this study is to investigate the control methods of the gas-cooler pressure in a CO2 heat pump. The cooling performance of the CO2 heat pump was measured by varying the refrigerant charge amount, EEV opening, compressor frequency, and outdoor fan speed at various outdoor temperatures. The effects of the EEV opening and the outdoor fan speed on the gas-cooler pressure and the COP were analyzed by using the experimental data. In the standard cooling condition at the compressor frequency of 45 Hz, the optimum gas-cooler pressure and the maximum COP were 9200 kPa and 3.04, respectively, at the optimum EEV opening of 41% and outdoor fan speed of 500 rpm. As the compressor frequency increased from 45 Hz to 55 Hz at the standard cooling condition, the optimum outdoor fan speed increased from 500 rpm to 700 rpm.

In residential air-source heat pump systems, the refrigerant injection technique has rapidly developed in recent years due to its outstanding performance at low ambient temperatures. A numerical simulation model of a flash tank vapor-injection heat pump with a twin rotary compressor is newly presented in this paper. The performance of the twin rotary compressor with refrigerant injection was simulated using a loss and efficiency model. The model was validated by comparing the predictions with measured data. In addition, the model was used to analyze the performance characteristics of the vapor-injection heat pump according to the cylinder volume ratio of the twin rotary compressor. Based on the simulation results, optimum cylinder volume ratios were suggested for various design conditions, including for the fixed first cylinder volume, the fixed total cylinder volume, and the fixed heating capacity.

The condensation heat transfer characteristics of CO 2 flowing in a horizontal smooth tube were investigated. The tube diameter was 5.15 mm. The condensation temperature ranged from −10 to 5 °C, and the mass flux was from 600 to 1000 kg m −2 s −1 . When the temperature changed from 0 to −10 °C, the increase rate of the heat transfer coefficient was from 9.4 to 14.6%, and the pressure drop increased from 6.2 to 52.9%. When the mass flux increased from 600 to 1000 kg m −2 s −1 , the heat transfer coefficient increased from 6 to 35%, and the pressure drop increased from 60 to 165%, which were dependent on the condensation temperature. The increases come from the change of vapor velocity and thermophysical properties with condensation temperature. Considering large variation of mass flux from 200 to 1200 kg m −2 s −1 by including the existing studies, the effect of mass flux on condensation heat transfer coefficient was minor.

In this study, a simulation model for a CO 2 heat pump using vapor injection was developed and validated. It was used to predict the improvement of the heating performance of the CO 2 heat pump at various operating conditions. The simulation results showed consistent results with the measured data. The heating performances of the vapor injection and non-injection heat pumps were compared by varying the outdoor temperature and compressor frequency. The heating capacity of the vapor injection heat pump was 40% higher than that of the non-injection heat pump at the outdoor temperature of -8℃. The performance of the vapor injection heat pump was consistently higher than that of the non-injection heat pump even when the compressor frequency was reduced to 35 Hz at the outdoor temperature of -3℃.Keywords: CO 2 heat pump(이산화탄소 열펌프), Vapor injection(가스인젝션), Simulation(시뮬레이션) †Corresponding author Tel.: +82-2-3290-3366; fax: +82-2-921-5439 E-mail address: yongckim@korea.ac.kr 기 호 설 명

A cooling system is adopted to control the thermal load from avionic equipment in a reconnaissance aircraft such as an unmanned aerial vehicle that undergoes large and rapid environmental changes according to the flight altitude and velocity. In this study, an avionic cooling system using R236fa was manufactured by adopting a vapor compression cycle with a closed-loop air-circulation system. Before the normal tests, the cooling system was optimized by varying the refrigerant charge amount and the expansion valve opening. The performance characteristics of the cooling system were measured by varying the compressor rotating speed, thermal load, evaporator and condenser air flow rates, and condenser inlet air temperature. The performance of the cooling system was analyzed according to these operating parameters, and possible control methods for stable operation of the cooling system were suggested.

Vapor injection technique has been applied to prevent performance degrdation of a heat pump at low ambient temperatures. In this study, the heating performance of a heat pump with a variable speed injection scroll compressor using R-410A was investigated by applying sub-cooler vapor injection(SCVI) and flash tank vapor injection(FTVI). The heating performance of the heat pump was measured by varying compressor frequency and outdoor temperature. The heating capacity of the FTVI system was 8~10% higher than that of the SCVI system at all operating conditions. On the other hand, the heating performance improvement with the increase in the compressor frequency was more prominent in the SCVI system than in the FTVI system.

The refrigerant injection technique has rapidly developed in recent years due to its outstanding performance at low ambient temperatures, and various control methods for a heat pump with injection have been presented. However, most studies on cycle control have been theoretical, and practical control methods for cycle optimization based on experimental results hardly have been presented. In this study, an optimum cycle control method was proposed for a refrigerant injection heat pump with a double expansion sub-cooler based on the intermediate pressure and the injection ratio. The optimum sub-cooler pressure ratio was proposed from 0.4 to 0.7 in view of the heating capacity, and from 0.7 to 0.8 in view of the COP. The optimum injection ratio increased from 0.1 to 0.3 with an increase in the compression ratio.

Aluminum plate-fin heat exchangers (ALPHEs) are widely used in petroleum and gas-treated processes. The performance of the ALPHE is strongly dependent on the distribution characteristics of the header and the distributors of each fluid. In this paper, we define the aspect ratio in the side-entry type distributor and analyze the distribution characteristics using CFD-based numerical methods. The phenomena of velocity deviation and distribution with the aspect ratio and the inlet Reynolds number were analyzed by applying relative and absolute maldistribution parameters, and an optimum aspect ratio with the inlet Reynolds number was presented.

In this study, the steady state performance of a heat pump water heater was measured with a variation of operating conditions such as refrigerant charge amount, compressor frequency, EEV opening, and water mass flow rate. Transient state performance tests were also conducted to investigate major system effects associated with the interaction between the heat pump water heater and the water tank. Optimum refrigerant charge amount for the system was 1600 g. At compressor frequencies of 50 Hz and 60 Hz, water mass flow rates of 95 kg/h and 105 kg/h, and EEV opening of 8% and 16%, the water heating temperatures were and and COPs were 3.0 and 2.8, respectively. In the transient condition, the instantaneous COP decreased with an increase in the inlet water temperature.

The performance degradation of air-source heat pumps cannot be avoided when they operate at both very low and high ambient temperatures. The refrigerant injection technique has rapidly developed in recent years due to its outstanding performance at low ambient temperatures. This study measured the heating performance of air-source heat pumps in which novel vapor injection techniques of a combined flash tank and sub-cooler (FTSC) cycle and a double expansion sub-cooler (DESC) cycle were applied. The performance of these cycles was compared with that of a flash tank (FT) and a sub-cooler (SC) cycle. The average heating capacities of the FT, FTSC, and DESC cycles were higher by 14.4%, 6.0%, and 3.8%, respectively, relative to that of the SC cycle, but the average COPs for the respective cycle options were very similar.