COP Of System Research Articles

Theoretical investigation of a novel high-temperature heat pump exploiting low-temperature waste heat and using two-phase machines with zero electric consumption

A novel high-temperature heat pump (HTHP) exploiting low-temperature waste heat is presented, which is enabled by two-phase compressors and expanders without the need for any requirement of external electric power. The investigation involves a sensitivity analysis at the design point, considering the concurrent variation of five independent variables. The aim is to identify the best-performing conditions for a rubber manufacturer and a canning industry which have different temperatures of waste heat and required heating power. The COP of the overall HTHP system, which is the ratio between the output heating power and the input thermal power (coming from the heat source) is evaluated. The maximum COP values are 0.258 for the rubber manufacturer and 0.452 for the canning industry. The primary energy saving is assessed in terms of equivalent fuel consumption. For the rubber manufacturer, the resulting yearly cost saving is in the range of 129 ÷ 646 €/kWth of HTHP installed capacity, and 140 €/kWth for the canning industry. The economic viability of the HTHP is contingent upon achieving isentropic efficiencies of the expanders and compressors higher than 0.59 and 0.55, respectively, for the rubber manufacturer, and both higher than 0.45 for the canning industry.

The influence of NH3-H2O-LiBr ternary working fluid on the performance and solution circulation in ammonia absorption refrigeration system

The ammonia absorption refrigeration system(AARS) has been widely applied in the recovery of industrial waste heat. Using NH3-H2O-LiBr ternary working fluid is an effective way to promote the performance of AARS. While previous studies mainly focused on the vapor–liquid equilibrium of the NH3-H2O-LiBr solution and the performance variation of the ternary system under specific working conditions, this paper aims to experimentally investigate the influence of LiBr on performance and solution circulation under various operation conditions. Experiment results indicated that 15%(w/w) was the optimum LiBr concentration across all working conditions. The COP of the ternary system was 7.55% higher than the binary system at most and the heat source temperature of the ternary system can be reduced by 25 °C to reach the same COP of the binary system. The COP was promoted because the rectifier cooling duty decreased by 53% at most, resulting in a reduction of its proportion in the heating capacity from 28.6% to 17.0%. Although the circulation ratio increased by 52% when the LiBr concentration increased from 5% to 15%, the absorber cooling duty experienced an average increase of 6.19%. The effectiveness of the solution heat exchanger was improved from 66.8% to 76.3% when the LiBr concentration increased from 5% to 20%. As the rise of the circulation ratio leads to higher pump power, the viscosities of the NH3-H2O, H2O-LiBr, and NH3-H2O-LiBr solutions were measured. The results demonstrated that solution temperature and NH3 concentration had more influence on solution viscosity than LiBr concentration. The viscosity increased by 6.2%, 17.1%, and 7.1% when the LiBr concentration increased by 11.26%, NH3 concentration increased by 9.5%, and solution temperature dropped by 2 °C, respectively.

CO2 system integrated with ejector and mechanical subcooling: A comprehensive assessment

To achieve the goal of clean annual space heating and cooling for residential buildings, the concept of annual space heating and cooling transcritical CO2 system combined with ejector and mechanical subcooling is proposed. The problem of large throttling loss and performance degradation when CO2 system is applied for residential space heating and cooling can be solved. A theoretical model of the new proposed system is developed, and then optimized by the method of genetic algorithm. Moreover, taking an ordinary residential building located in 70 cities belonging to different climate zones as the application scenario, the seasonal, annual, environmental, and economic performance of eight heating and cooling solutions are compared and discussed. The results indicate that the COP and exergy efficiency of the new proposed system is significantly improved in contrast to conventional mechanical subcooling system, which increases by 11.48% and 21.64% in heating mode, and 8.99% and 8.69% in cooling mode, respectively. The annual performance coefficient of the new proposed system is 7.21%∼8.21% higher than conventional mechanical subcooling system. Additionally, the life cycle carbon emissions of the new proposed system can be significantly reduced by 12.23%∼89.58% compared with the combined solution of coal-fired boiler and R32. With the advancement of CO2 technology, it is possible to improve the economic performance of CO2 systems and the price of the new proposed system is lower than all solutions when the price of compressor is reduced by 60%. This study can provide a theoretical reference for the development and optimization of CO2 combined heating and cooling system, and provide a feasible solution for residential annual space heating and cooling.

4E analysis and multi-objective optimization of compression/ejection transcritical CO2 heat pump with latent thermal heat storage

To promote the goal of peak carbon dioxide emissions and carbon neutrality, low-energy consumption buildings require innovative technologies and efficient energy management. In this paper, the multi-objective optimization and the energy, exergy, economic and environmental (4E) analyses of the compression/ejection transcritical CO2 heat pump with latent thermal storage (TPE-LTES) system are conducted, and the influences of crucial parameters on the system performance are evaluated. The results show that the optimal comprehensive COP and annual hot water production (mDHW) of the TPE-LTES system are 3.59 and 3110 tons, respectively. Then, the comprehensive exergy efficiency of the TPE-LTES system owns 48.93 % higher than that of the heat pump unit. Furthermore, the optimal life-cycle cost (LCC) and life-cycle CO2 emissions (LCCE) of the TPE-LTES system are 708,073 CNY and 575,512.79 kg, respectively. Moreover, with constant discharge pressure, the evaporative superheat can be preferentially increased to reduce the system LCC and LCCE. Finally, the comprehensive COP and exergy efficiency of the system can be improved by increasing the inlet water temperature, while the increasing inlet water flow rate is the least desirable. This work is helpful to promote the research and application of latent heat storage unit integrated into air source heat pump.

Performance analysis of interactive cascade ventilation combined with solar energy for the epidemic prevention and control

Increasing low-grade and renewable energy sources in the HVAC systems has become necessary to reduce heating energy consumption. Hence, seeking a new air conditioning system that can be combined with renewable energy is significant to maintain a healthy and comfortable indoor environment with a low cost. This paper proposes a novel air source heat pumps assisted solar hot water with interactive cascade ventilation system (shortened as SWAP-ICV). Taking the traditional air source heat pumps with mixing ventilation system (AP-MV) as a reference, the virus control performance and energy efficiency of SWAP-ICV is investigated through CFD and TRNSYS simulations. The results demonstrate that interactive cascade ventilation under full-fresh air operation mode and normal-epidemic linkage mode can make the annual infection risk reduce by 41.16% and 9.33% compared with mixing ventilation under full-fresh air operation mode. Meanwhile, the proposed system can reduce energy consumption by 59.92% and 69.8% during the normal period and epidemic period by introducing solar energy. The average system COP during the heating season indicates that SWAP-ICV can improve the system efficiency by 1.9–2.58 times than AP-MV system. The conclusions obtained from this research can provide new ideas for the design and operation of air conditioning systems in the post-epidemic era.

Performance analysis and optimization of a coupled open and close liquid desiccant sorption system using LiCl solution

The efficient utilization of low-grade heat for air conditioning has been a topic of world concern, especially for low-grade heat below 80℃. In this study, a novel coupled open and close liquid desiccant sorption system driven by 70 ∼ 80℃ low-grade heat is proposed. This system employs a generator to provide strong solution for both the dehumidifier and absorber, achieving the air temperature and humidity independent control to reduce driving temperature and enhance system performance. A mathematical model is developed to simulate the proposed new system, and a comparative analysis is conducted with a traditional combined absorption refrigeration with liquid desiccant dehumidification system using LiCl solution. It is found that the COP of the new system is 0.76, which is higher than 0.65 of the traditional system when the hot water temperature is 75℃, the reason is that the new system avoids the sensible heat consumption of regenerated air which accounts for 17% of the total heat consumption in the traditional system. Furthermore, it is observed that the outlet concentration difference between the absorber and dehumidifier exists in the new system. The cycle is optimized to improve the concentration difference, leading to a 5.5% increase in the COP. After that, economic performance analysis shows a payback period of 2.65 years for a 150 kW cooling capacity system, with a reduction of 86.6 tons of CO2 emissions per year due to the decrease in power consumption.

Performance improvement of air-source heat pump via optimum control based on artificial neural network

The use of air source heat pump (ASHP) systems is an economically viable strategy for building decarbonization. However, the ASHP energy performance deteriorates under severe conditions around a building. Researchers have conducted studies for optimal compressor control to overcome these limitations. Despite these efforts, users can seldom perform the optimal regulation of heat pump systems because manufacturers are reluctant to relinquish control to them. Therefore, we developed an artificial neural network (ANN)-based optimum control logic (OCL) system such that users can directly control and optimize heat pump systems without a compressor. The developed ANN-based OCL controls the secondary-side working fluid of the heat pump considering general building conditions, and its practical applicability has been verified using dynamic simulation. The improvement in the energy performance of the optimum models with respect to the conventional models was: 1.52% and 3.58% for the cooling and heating system COP, respectively, and 0.76% and 0.81% for the heat pump COP. These results demonstrate the potential for reduction in the carbon footprint of a building while maintaining comfort by enabling the energy-efficient operation and stable load response of the heating and cooling system.

열교환기 최적 설계를 통한 친환경 냉동탑차용 냉동시스템의 성능향상 연구

An analytical study on the optimization of heat exchanger design parameters of a refrigeration system for a one-ton eco-friendly refrigerator truck was conducted to improve its performance. The components of the refrigeration system were modeled analytically. The developed model was validated by comparing the predicted values with the measured data. The effects of the major design parameters on system performance were analyzed using the developed model. The heat exchangers were optimized according to the geometric parameter to improve system efficiency. The COP of the optimized refrigeration system was an average of 12.5% higher than that of the conventional system. Additionally, a drop-in simulation for the optimized refrigeration system using R455A was conducted to determine an alternative for R404A. The COP of the refrigeration system with R455A (COPR455A) was almost the same as that of the refrigeration system with R404A (COPR404A) because the increase in the heat transfer rate was the same as that of the compressor work. However, the COPR455A was 8.7% higher than the COPR404A at the same heat transfer rate.

Dynamic characteristics of a mechanically coupled organic Rankine-vapor compression system for heat-driven cooling

There are large quantities of compression heat wasted and cooling capacity required in cryogenic air separation units (ASUs). Mechanically coupled organic Rankine-vapor compression (ORVC) system is an attractive technology to realize heat-driven refrigeration for compression heat recovery and compression power saving in cryogenic ASUs. As the states of the compression heat inevitably change with the ambient environment, off-design operation of the ORVC system is worth investigating. A mechanically coupled ORVC test bench is designed and built in this paper. Off-design experiments are carried out to figure out the dynamic response characteristics and system performance variations. Results show that, the ORC working fluid flowrate and the opening of the throttle valve are the key operation parameters affecting the system dynamic response. The experimental system responds quickly with delay time less than 10 s and response time less than 1 min. The opening of the throttle valve has the greatest impact on the cooling capacity and system COP which reaches up to 0.61. This research may provide reference for control scheme design and actual operation for the compression heat recovery system in cryogenic ASUs.

Study on effects of multi-climatic parameters on performance of ground source heat pump through coaxial borehole heat exchanger

ABSTRACT The accurate calculation for heat flux of borehole heat exchanger (BHE) is of great importance for design of ground source heat pump (GSHP); the climatic conditions influence GSHP performance through GHE. In this study, a 2-D heat transfer model for coaxial BHE was built to investigate the effects of multi-climatic parameters on BHE heat transfer and GSHP performance. The outlet temperature, ground temperature profiles and COP of GSHP system for the cooling period were concerned. The effects of climatic conditions under different borehole depths, ground thermal conductivities and flow rates were studied. The studies showed that the outlet temperature from BHE increased about 10% and system COP declined about 5% in cooling mode with considering the effect of climatic conditions. The comparison indicates that the effects of climatic conditions on coaxial BHE are stronger than that on U-tube BHE. Generally, the climatic conditions influence BHE heat transfer mainly through the shallow ground and the influences strengthen with time. The effects can be more significant with a larger ground thermal conductivity, but it weaken with increase of borehole depth. Also, the effects of climatic conditions on BHE and GSHP seem to be independent of flow rate.

Experiment and performance evaluation of an integrated low-temperature proton exchange membrane fuel cell system with an absorption chiller

When the typical operation temperature of the Proton Exchange Membrane Fuel Cell (PEMFC) reaches 333.15–363.15 K, the ratio of electrical energy to waste heat generated by the PEMFC is about 1. In this paper, an absorption chiller with LiBr/[BMIM]Cl/CH3OH as the working pair is proposed to convert the waste heat into cooling capacity. The PEMFC and absorption chiller (AC) systems have been studied from the energy and efficiency point of view. The COP of the integrated system can reach a maximum of 0.85, which is remarkably 0.35 higher than that of the individual PEMFCs. The electrochemical process of the PEMFC and the thermodynamic process of the AC systems are calculated using MATLAB. The results show that the PEMFC operation temperature of 353.15–358.15K is sufficient to drive the AC and meet the requirement of 273.15–283.15K evaporation temperature and 308.15–313.15K condensation temperature. Meanwhile, when the electric power output from the PEMFC is 100 kW, the AC can output approximately 60 kW cooling capacity, which is sufficient to meet the cooling demand of the commercial bus.

Comprehensive performance of pv/t-gchps under heating conditions

In HSCW (Hot Summer and Cold Winter) climate zone of China, the building heat load is usually less than the cooling load which caused the problem of thermal accumulation of underground soil to damaged the COP of the heat pump system. PV/T-GCHPs (Photovoltaic/photothermal - Ground-coupled heat pump system) is a system that realizes the complementary utilization of geothermal energy and solar energy to solve the problem. The experimental platform of the system was designed and built, and the effect of the temperature of the outlet water from the ground buried pipes on the photoelectric efficiency of the PV panel was analyzed through experiments and simulations under the heating conditions of the PV/T-GCHPs system, and the heating performance coefficients of PV/T-GCHPs and GCHPs were compared. The results show that the PV plate temperature reaches the lowest when the heat pump load is 75% and the circulating water flow is 0.28 kg/s, when the outlet temperature of buried pipe is 8℃. The maximum photoelectric efficiency increased by 6.4% under above conditions, the average heating performance coefficient of PV/T-GCHPs is increased by 2.2% compared with GCHPs. The daily average comprehensive efficiency is 63.32%, the maximum comprehensive efficiency of the system is 74.67%. The maximum heat collection efficiency of PV/T collector is 48.33%. This paper hopes to provide a theoretical basis for the promotion of PV/T-GCHPs in HSCW climate zone of China.

Experimental investigation on the cooling performance of direct and secondary loop CO2 air conditioning systems for electric vehicles

As a natural refrigerant, CO2 is considered one of next generation refrigerants of electric vehicle (EV) applications for excellent environmental and heating performance. The integration and safety performance of CO2 systems could be enhanced by secondary loop configuration, which is highly valuable for practical applications. This paper investigates the cooling performance characteristics of direct (DAC) and secondary loop (SLAC) transcritical CO2 air conditioning systems based on liquid cooled gas coolers (LCGC). A series of steady-state experiments were carried out for both systems to study the effects of compressor speed, EXV opening, outdoor air velocity, and indoor air flow rate on the cooling performances. Furthermore, the exergy destruction of each component and the exergy efficiency of both systems were analyzed based on the experimental results, showing that the evaporator needs to be better designed to balance heat transfer performance with pressure loss performance. The results showed that the cooling capacity and COP of SLAC system were reduced by up to 9.1% and 10.8% under the same compressor speed, compared with the DAC system. This study presents experimental references for further research and practical extension of the CO2 mobile air conditioner.

Exploring the feasibility of next-generation CO2-based adsorption cooling systems using different adsorbent reactor configurations

The present investigation expounds on the effects of the reactor geometry, cycle time and heating/cooling fluid temperatures on the performance of a 4-bed CO2 adsorption cooling system. Cylindrical adsorbent reactors of varying aspect ratios (AR) with an optimum number of internal longitudinal fins and non-finned reactors are compared. The study confirms the substantial effect of reactor configuration on the system COP, specific cooling effect (SCE) and power, energy interactions during various processes and the total cycle time (TCT), which the equilibrium or lumped system analysis fails to predict. An increment of 98 % and 189 % in system COP and SCE is attained by varying the aspect ratio of the reactor with fins under subcritical operations. In continuation, a reduction of 41 % in TCT is observed. The study confirms the optimum adsorption/desorption time for which the COP and SCE are maximum, especially for higher AR reactors. It is noteworthy that proper selection of reactor aspect ratio results in a better system COP, even at lower heating fluid temperatures. During the transcritical operations, an increment of 23 % in COP and 6 % in SCE is achieved for a gas cooler exit temperature of 35 °C. Moreover, the reactor configuration also affects the optimum gas cooler pressure for the transcritical cycle, like the gas cooler exit temperature. One of the crucial observations is that, for a given combination of reactor configuration, heating/cooling fluid temperatures, and operating pressures, there exists a minimum cycle time below which a cyclic steady-state condition is impossible to achieve. During the subcritical and transcritical operations, a maximum COP of 0.1 and 0.052 is achieved for the proposed adsorption chiller configuration, respectively.

Experimental Investigation for TiO2 nanoparticles as a Lubricant-Additive for a Compressor ofWindow Type Air-Conditioner

The coefficient of performance of a window type Air-Conditioner system can be improved if a reduction in the work of compressor can be achieved by a suitable technique. The present study investigates the effect of dispersing a low concentration of TiO2 nanoparticles in the mineral oil based lubricant, as well as on the overall performance of a window type Air-Conditioner system using R22 as the working fluid. An enhancement in the COP of the refrigeration system has been observed and the existence of an optimum volume fraction noticed, with low concentrations of nanoparticles suspended in the mineral oil. Results showed that the average compressor work reduced by 13.3%, which ultimately resulted in an increase of 11.99% in the COP due to the addition of nanoparticles in the lubricating oil.

A novel indirect evaporative cooler with porous media under dual spraying modes: A comparative analysis from energy, exergy, and environmental perspectives

Indirect evaporative cooling (IEC), which cools the air through the physical process, is regarded as one of the high-efficient air conditioning (AC) technology. The reliable performance of traditional IEC is maintained by consistent water spraying, consuming much energy and hindering further performance improvement. In this regard, we proposed a novel indirect evaporative cooler with porous media on the secondary air channel (PIEC) to mitigate the dependence on water spraying. For the PIEC, the water storage capacity of porous media enables periodic spraying. However, the performance comparison between periodic spraying and traditional consistent spraying is rarely carried out although the two modes have been achieved, which promotes this research. In this study, the PIEC model with consistent spraying and periodic spraying was briefly described based on our previous results. According to the results from the established model, the PIEC performance was analyzed and compared under dual spraying modes from the energy, exergy, and environmental (3E) perspectives. It is demonstrated that periodic spraying can significantly improve the system COP by 58.7% at the cost of tiny temperature fluctuations. Additionally, the periodic mode can increase the exergy efficiency and lessen the exergy loss ratio in the studied cases. Eventually, the PIEC system using periodic spraying can annually reduce up to 24.2% of CO2 emissions compared with the traditional consistent mode, indicating better environmental benefits.

A novel R744 multi-temperature cycle for refrigerated transport applications with low-temperature ejector: Experimental ejector characterization and thermodynamic cycle assessment

A novel vapor-compression system concept employing carbon dioxide as the refrigerant is proposed to serve the needs of a typical medium-size refrigerated truck used for multi-temperature (MT and LT) goods delivery. The system design is based on the implementation of an ejector as the only component increasing the refrigerant pressure from the LT evaporation pressure to the MT evaporation pressure, thus providing cooling effect at two different temperature levels with only one stage of compression. The ejector was experimentally tested and its ability to effectively entrain mass flow rate from low pressure suction conditions (corresponding to a LT evaporation temperature of −25 °C) was assessed. Lower external ambient temperatures and consequent lower expansion energy available at the ejector motive nozzle leads to a reduction of the maximum achievable pressure lift. Moreover, a significant degradation of the ejector performance towards the highest pressure lifts is experienced. Based on the ejector experimental data, a numerical evaluation of the proposed cooling unit performance has been performed, highlighting that in design conditions (LT evaporation at −25 °C) the cooling unit provides a LT freezing power ranging between 1.1 kW and 2.3 kW and a corresponding minimum MT cooling power ranging between 5.1 kW and 3.8 kW, depending on the chosen ejector lift. The MT cooling power can be further increased by increasing the compressor mass flow rate. The system COP is maximized at the maximum available lift provided by the ejector.

Design and optimization of absorption cooling system operating under low solar radiation for residential use

In this study, the single-stage ammonia–water (NH3–H2O) absorption cooling system driven by low solar radiation was designed for the residential use. The proposed system was investigated for the different collector type namely flat plate (FPC), evacuated tube (ETC), compound parabolic (CPC), and parabolic trough (PTC). The absorption cooling system was equipped with two thermal energy storage units to obtain a whole day operating. The dynamic evaluation on the basis of hourly data was conducted to evaluate the performance of the system throughout the energy and exergy methods. The system was also evaluated from the economic point of view using net present value method. As conclusion, the system with evacuated tube collector was determined as the most investable one whereas the system with parabolic trough collector was determined as the most efficient one. As a result of NPV calculations, it was seen that the most profitable system was the system using 6726.13 US$ ETC. The highest system exergy efficiency to be obtained with the system using ETC was calculated as 2.6% and the highest system COP was calculated as 0.25.

Effect of flow channel structures and refrigerants on the motor cooling process of compressor with gas bearings

Energy-saving and environment-friendly water chiller is the development trend. The efficient compressors and new refrigerants are hot spots. The gas bearing can significantly improve the performance of centrifugal refrigeration compressors, and effective motor cooling is the guarantee of performance and reliability. The influence of channel structure and refrigerant on the performance of the motor cooling process and refrigeration system is analyzed. The results show that adding the bearing gas supply circuit decreases the motor temperature and the maximum variation of permanent magnet temperature is about 60 °C. The cooling channel structure in the interlayer of the motor shell will affect the cooling process. The spiral channel can reduce the stator temperature and improve the uniformity of temperature distribution. But the temperature of the motor changes slightly when the length of the channel is more than 6000 mm. The motor temperature is the highest with refrigerant R1234ze(E), followed by it R1234yf and R134a. The maximum temperature of the permanent magnet for R1234ze(E) is 20–50 °C higher than it for R1234yf and 50–60 °C higher than it for R134a under different inlet diameters. The maximum temperature of the permanent magnet for R1234ze(E) is 30 °C higher than it for R1234yf and 50 °C higher than it for R134a under different inlet temperatures. The system with a motor cooling circuit has controlled the motor temperature within the safe operation range, while the COP of systems with R134a, R1234yf, and R1234ze(E) has only decreased by 4.63 - 5.36%, 4.22 - 4.6%, and 3.78 - 4.15% respectively.

Evaluation of high-speed aircraft thermal management system based on spray cooling technology: Energy analysis, global cooling, and multi-objective optimization

With the increase in aircraft speed, the aerodynamic heating on the aircraft wall has become one of the key factors restricting flight safety. Although passive thermal protection can solve the thermal protection problem of the aircraft body, there will always be some heat transferred into the cabin. And the temperature of the cabin interior environment and cabin wall increases, which reduces safety. The spray cooling system with R134a as a coolant can reduce the cabin wall temperature to 275 K, and the temperature drop rate can reach 1.05 K/s, so that the cabin cooling needs can be realized. The refrigeration system is used to connect the spray system with the fuel loop to form the closed-air spray cooling heat management system of the aircraft. The refrigeration system and the tandem radiator are optimized according to the genetic algorithm. After repeated iterations with different design parameters, the exergy efficiency and COP of the refrigeration system can reach 80 % and 2.36, and the pressure loss and the bottom plate temperature of the microchannel heat exchanger are 0.98 Mpa and 313 K, respectively. According to the analysis of the efficiency of the heat management system, the ejector efficiency, the energy efficiency ratio (EER) of the total system, and the required primary fluid flow rate, the required primary flow rate is at least 0.0027 kg/s to drive the thermal management system, EER of high-speed aircraft thermal management system can reach 15.62 %, exergy efficiency of the refrigeration system and the ejector efficiency can reach 44 % and 1 4% respectively.