Cooling Coefficient Of Performance Research Articles

Experimental Study to Analyze Feasibility of a Novel Panelized Ground-Source Thermoelectric System for Building Space Heating and Cooling

A thermoelectric module is a device that converts electrical energy into thermal energy through a mechanism known as the Peltier effect. A Peltier device has hot and cold sides/substrates, and heat can be pumped from the cold side to the hot side under a given voltage. By applying it in buildings and attaching it to building envelope components, such as walls, as a heating and cooling device, the heating and cooling requirements can be met by reversing the voltage applied on these two sides/substrates. In this paper, we describe a novel, panelized, ground source, radiant system design for space heating and cooling in buildings by utilizing the Peltier effect. The system is equipped with water pipes that are attached to one side of the panel and connected with a ground loop to exchange heat between the cold/hot sides of the thermoelectric module and the underground region. The ground loop is inserted in boreholes, similar to those used for a vertical closed-loop Ground Source Heat Pump (GSHP) system, which could be more than a hundred meters deep. Experiments were conducted to evaluate the feasibility of the developed panel system applied in buildings. The results show that: (1) the average cooling Coefficients Of Performance (COP) of the system are low (0.6 or less) even though the ground is used as a heat sink, and thus additional studies are needed to improve it in the future, such as to arrange the thermoelectric modules in cascade and/or develop a new thermoelectric material that has a large Seebeck coefficient; and (2) the developed system using the underground region as the heat source has the potential of meeting heating loads of a building while maintaining at a higher system coefficient of performance (up to ~3.0) for space heating, compared to conventional heating devices, such as furnaces or boilers, especially in a region with mild winters and relatively warm ground.

Increasing the Performance of an Adsorption Chiller Operating in the Water Desalination Mode

The intensive development of the world economy and the expected population growth mean that demand for cooling and water will continue to rise. The use of conventional technologies to meet this demand is associated with an enormous expenditure of electricity, which still comes mainly from non-renewable sources. With the increasing demand for energy, the increasing scarcity of drinking water, and the negative impact of humankind on the environment due to global warming and ozone depletion, intensive research has been carried out to find modern desalination technologies Most of the technologies use electricity for the process of desalination, and over 6% of the world’s electricity is generated from non-renewable sources, thus increasing the emissions of harmful pollutants into the atmosphere. One possibility to reduce emissions is the use of adsorption chillers with desalination function, which allow the production of cooling simultaneously with the process of water desalination. These systems can be powered by low-temperature waste heat from industrial processes or from renewable sources (solar panels) and require little electricity to operate. This paper presents options to improve their performance and increase the production of condensate in the process of desalination of saline water. Moreover, also presented are the results of tests carried out on a two-bed adsorption chiller with desalination function. The aim of the study was to determine the effect of cycle time on the cooling coefficient of performance (COP) and on the production of condensate from water desalination. The obtained results confirmed that increasing the adsorption and desorption cycle time leads to an increase in the COP value of the adsorption chiller, but the efficiency of the desalination process and condensate production decreases with increasing cycle time.

Effect of Metal Additives in the Bed on the Performance Parameters of an Adsorption Chiller with Desalination Function

Adsorption chillers with desalination functionality, being devices characterised by very low electricity consumption, provide an alternative to conventional sources of cooling and water. The option of desalinating water means that the use of a single device enables obtaining two useful products. Adsorption chillers are not widely used at present. due to their low performance characteristics; these are, however, constantly being improved. This paper presents a verification of the possibility of increasing the cooling coefficient of performance (COP) and specific cooling power (SCP) of a laboratory adsorption chiller by optimising the length of cycle times and using a copper additive to silica gel with a mass fraction of 15% to increase heat transport in the bed. The choice of copper among other considered additives was determined by the conclusions from the research on the sorption kinetics of various mixtures, price and availability, and a high thermal conductivity. The device was operated in a two-bed mode aimed at producing cooling. The adsorbate was distilled water. The results were compared with those obtained under similar conditions when the beds were only filled with silica gel. As a result of the testing, it was found that the use of the copper additive with the sorbent increased both the COP and SCP. The tests were performed for different cycle times, of 100, 200, 300 and 600 s. With increasing cycle time COP also increased. In contrast, the specific cooling power increased only up to a certain point, whereafter its value decreased.

Flow thermohydraulic characterization of hierarchical-manifold microchannel heat sink with uniform flow distribution

• Flow uniformity of manifold microchannel (MMC) heat sink is analyzed. • Flow difference between adjacent microchannels is less than 7% in hierarchical-MMC. • Pressure drop of hierarchical-MMC is reduced by 28–32% compared to typical-MMC. • Heat dissipation of 450 W/cm 2 is achieved under heated area of 9 cm 2 in experiment. • Maximum value of cooling coefficient of performance (COP) is obtained to be 19,480. The manifold microchannel (MMC) heat sink has been used for thermal management of high heat flux electronics. Flow maldistribution among microchannels is one of the major hindrances that affect the proper functioning of the typical-MMC ( i.e. , a coplanar design of the manifolds and the inlet/outlet ports) cooling system, which is attributed to the Z-shaped flow of fluid between the microchannels and the manifolds. Herein, a novel hierarchical-MMC heat sink with stacked configuration is proposed, which mitigates flow maldistribution by optimizing the flow path. First, the fluid flow and heat transfer characteristics of the typical-MMC and the hierarchical-MMC are compared through single-phase conjugate simulation. Next, experimental tests are performed on the hierarchical-MMC heat sink fabricated by Selective Laser Melting (SLM) technology under single-phase and subcooled flow boiling conditions. The numerical results indicated that the hierarchical-MMC exhibits a uniform flow distribution, and the difference of percentage of volumetric flow rate in the section plane of the parallel microchannels is less than 5%. Compared with the typical-MMC, the total pressure drop and overall thermal resistance of the hierarchical-MMC are reduced by 28–32% and 7–13% at the volumetric flow rate of 0.12–1.17 L/min, respectively. Moreover, the experimental results verified the accuracy of the numerical simulation and the feasibility of fabricating the hierarchical-MMC through SLM technology. Under subcooled flow boiling condition, a heat dissipation of 450 W/cm 2 is achieved under a footprint area of 30 × 30 mm at the volumetric flow rate of 1 L/min. The maximum value of cooling coefficient of performance (COP) for hierarchical-MMC heat sink is obtained to be 19,480 at the volumetric flow rate of 0.6 L/min and heat flux of 260 W/cm 2 .

Energy, exergy, and environmental (3E) analysis of a compound ejector-heat pump with low GWP refrigerants for simultaneous data center cooling and district heating

This work presents an energy, exergy, and environmental evaluation of a novel compound PV/T (photovoltaic thermal) waste heat driven ejector-heat pump system for simultaneous data center cooling and waste heat recovery for district heating networks. The system uses PV/T waste heat with an evaporative-condenser as a driving force for an ejector while exploiting the generated electric power to operate the heat pump compressor and pumps. The vapor compression system assessed several environmentally friendly strategies. The study compares eleven lower global warming potential (GWP) refrigerants from different ASHRAE safety groups (R450A, R513A, R515A, R515B, R516A, R152a, R444A, R1234ze(E), R1234yf, R290, and R1243zf) with the hydrofluorocarbon (HFC) R134a. The results prove that the system presents a remarkable overall performance enhancement for all investigated refrigerants in both modes. Regarding the energy analysis, the cooling coefficient of performance (COPC) enhancement ranges from 15% to 54% compared with a traditional R134a heat pump. The most pronounced COPC enhancement is caused by R515B (a 54% COPC enhancement and 49% heating COP enhancement), followed by R515A and R1234ze(E). Concerning the exergy analysis, R515B shows the lowest exergy destruction, with the highest exergy efficiency than all investigated refrigerants.

Confirmation and prevention of vapor bypass in absorption heat pump with U-pipe pressure separation device caused by upward side two-phase flow

• Indication that two-phase flow in U-pipe causes vapor bypass in absorption heat pump. • Observation of vapor bypass in U-Pipe through experiment under vacuum condition. • Evaluation of effects of vapor bypass in AHP through field tests. • New design method to avoid vapor bypass and improve performance by over 6 %. A U-pipe is the key pressure separation device in a lithium bromide absorption heat pump (AHP) connecting the high-pressure condenser and the low-pressure evaporator. A recent study reported that two-phase flow occurs in the upward side of the U-pipe, which reduces the overall density of the fluid and impairs the pressure separation ability of the U-pipe. The liquid seal function of the U-pipe fails, and the vapor can flow directly from the high-pressure condenser to the low-pressure evaporator, causing a vapor bypass problem. This study experimentally investigates the U-pipe vapor bypass mass flow rate under vacuum conditions. The mass flow rate ratio of the bypassed vapor to the main liquid fluid in the U-Pipe can be as high as 10 %. The effect of this bypassed vapor on the AHP performance is investigated through simulations and field tests. The cooling coefficient of performance (COP) drops by 0.05 to 0.1 when vapor bypass occurs, providing evidence for the existence of U-pipe vapor bypass in AHPs. A practical method is suggested to solve this problem and to improve the cooling COP of the AHP by more than 6%.

Energy performance analysis and optimization of a coupled adsorption and absorption cascade refrigeration system

An energy cascade utilization system is an advanced technology that recoveries waste heat energy efficiently. However, research on cascade systems by utilizing waste hot water with 60–100 °C is still considerably lacking in the literature. Specifically, this paper investigates the waste heat recovery performance of a multistage coupled absorption chiller (ABC)-adsorption chiller (ADC) cascade system. The proposed ABC-ADC cascade system is capable of producing potable water and three streams of chilled water under different temperature settings. Firstly, the experimental analysis is judiciously carried out on a four-bed two-evaporator ADC subsystem prototype. Key results reveal that the ADC’s maximum specific daily water production is 10.5 m3/day/ton. Subsequently, the ABC-ADC cascade system’s performance is experimentally analysed and optimized. The achievable maximum cooling coefficient of performance (COPc) is obtained to be 0.55. Additionally, a general method is proposed to optimize the subsystem’s cooling capacity combination of the ABC-ADC cascade system in search of an optimal COPc. The results indicate that the optimized COPc can further be enhanced by around 20%. The performance efficiency of an optimized ABC-ADC cascade system incorporating a microturbine system is then investigated. Compared to the experimental ABC-ADC cascade system, the optimized system’ total coefficient of performance is demonstrated to improve by 18%. The primary energy saving ratio is also promoted from 9.8% to 18%. Moreover, 68.92% of the gas turbine’s dissipated energy is able to be recovered by the optimized ABC-ADC system. As far as application is concerned, this cascade system has been demonstrated to be superior to standalone heat-driven chillers with great commercial potential for implementation in industries where low-grade waste heat is readily available.

A hybrid vapor chamber heat sink incorporating a vapor chamber and liquid cooling channel with outstanding thermal performance and hydraulic characteristics

Facing higher and higher heat load and more rigorous limitations on energy consumption, conventional heat sinks and vapor chambers are falling short of new requirements. In this paper, a hybrid vapor chamber heat sink (HVCHS) of liquid cooling is developed, wherein the active liquid cooling channel is embedded in the vapor region of the vapor chamber so that two modes of heat transfer, phase change and single-phase heat transfer, are incorporated in a single device. The thermal performance, hydraulic characteristics, and cooling coefficient of performance (COP) of the HVCHS are investigated. The hybrid design provides a high cooling capacity of 1200 W using a 9 cm2 heater and maintains the maximum junction temperature of 95.25 °C simultaneously. The streamwise temperature rise is only 2.04 °C on the bottom surface under the maximum cooling capacity, which shows great temperature uniformity under high power. The minimum thermal resistance is as low as 0.04 °C/W and is compared with some previous studies. The minimum pressure drop is 4.83 kPa with the corresponding pump power of 80.5 mW. The cooling COP is above 12,500 for heat inputs of more than 850 W. The HVCHS may have great potential for the energy-saving cooling management of high-performance electronics.

Evaluating the Performance of Water Chillers Equipped with Constant- or Variable-Frequency Centrifugal Compressors

The cooling coefficient of performance (COPR) and energy efficiency ratio (EER) of refrigerant R-134a compressors (single- and double-compressors) with different refrigerant tonnage (200, 250, 300, 380, 500, and 700 RT) for centrifugal and Maglev centrifugal compressors change with different operating performance load percentages (10–100%), and constant-frequency and variable-frequency operation, resulting in performance differences. In particular, a water chiller can have a fixed cooling water inlet temperature of 32 °C and a variable cooling water inlet temperature between 18.33 °C and 32 °C. According to the actual test results, the commercial performance code program and parameter table of the water chiller were established. Based on the performance matching of different load chillers, the on-site load capacity was analyzed and the effective water chiller performance and model matching were determined as the best choice for the tonR number of the deicing machine and unit matching, providing a reference for a future large water chiller that cannot be used on site for a single unit tonR. To achieve energy-saving benefits, different types of compressors, different refrigeration tonR operation, constant-frequency unit and variable-frequency unit alternate operation, and different operating performance load percentage operation can be allocated. Finally, the results show that, when the cooling water inlet temperature is fixed, the Maglev variable-frequency centrifugal compressor water chiller is better than the constant-frequency centrifugal water chiller, and also better than the variable-frequency centrifugal water chiller. The larger the freezing tonR of the variable-frequency centrifugal water chiller, the smaller the difference between COPR and EER. When the cooling water inlet temperature changes, the Maglev variable-frequency centrifugal water chiller is better than the constant-frequency centrifugal water chiller, and it is also better than the variable-frequency centrifugal water chiller. The larger the freezing tonR of the variable-frequency centrifugal water chiller, the smaller the difference between COPR and EER. Moreover, the operating performance of the constant-frequency centrifugal water chiller is between 60% and 90%, which can maintain relatively high COPR and EER values. The operating performance of the variable-frequency centrifugal water chiller is between 40% and 70%, which can maintain relatively high COPR and EER values. Compared with the constant-frequency and variable-frequency, the Maglev variable-frequency centrifugal water chiller can maintain higher COPR and EER values when the operating performance is between 10% and 100%. When the operating performance is between 10% and 70%, it can maintain very high COPR and EER values. When the water chiller is selected in the field, the energy-saving of COPR and EER will be given priority. Therefore, the load capacity can be used to effectively manage the water chiller performance and model selection, so that the operation performance can reach the best percentage and energy saving can be achieved.

Energy and Exergy Analyses of a Refrigerant Pump Integrated Dual-Ejector Refrigeration (DER) System

The aims of the present study are to design and perform the analysis of a dual-ejector refrigeration system (DER). The system is constructed by adding a second ejector and a refrigeration pump to the classical single-ejector refrigeration system (SER). In the theoretical analysis, two different refrigerants are employed, R134a and R600, and the cooling coefficient of performance (COP), exergy destruction and exergy efficiency are selected as the performance indices. The performance indices of the DER system are investigated under the variation of the evaporation, and condensing temperatures and results are compared with the SER system operating at the same conditions. In the given conditions, the maximum cooling COP and exergy efficiency are achieved with the DER system by 7.52 and 38.8%, respectively. In the DER system, the minimum exergy destruction occurs with R134a by 9.3 kJ/kg at 10 °C evaporation and 40 °C condensing temperatures. Moreover, 5.3% increments in the cooling COP and exergy efficiency are achieved with R600 when the condenser temperature is 55 °C and the evaporator temperature is 5 °C. The results also showed that the improvements achieved in the cooling COP and exergy efficiency with the DER system are greater at high condensing and low evaporation temperatures.

Operation and performance of VRF systems: Mining a large-scale dataset

The energy consumption of air-conditioning systems has gained increasing attention as it contributes significantly to the global building energy use. The variable refrigerant flow (VRF) system is a common air-conditioning system applied widely in residential and office buildings in China. Understanding the actual operation and performance of VRF systems is fundamental for the energy-efficient design and operation of VRF systems. Previous research on VRF system operation used either limited field data covering certain building types and climate zones or used a questionnaire to obtain a larger dataset. However, they did not capture the wide applications of VRF systems quantitatively across all building types, climate zones, and operating conditions. To fill this gap, statistical and clustering analysis was conducted on the newly proposed key performance indicators of approximately 287,000 VRF systems for residential and commercial buildings in all five climate zones in China. The main findings are: (1) VRF systems are mainly used for cooling in all climate zones in China; (2) among all building types, the duration of use is lowest in residential buildings and highest in hotels and medical buildings; (3) the distribution of the ideal VRF cooling coefficient of performance (COP) is similar across all climate zones and building types; whereas the COPs of ideal VRF heating in the Severe Cold region and Cold regions are lower than those in other climate zones; and (4) partial load operations for VRF systems are common in residential buildings and office buildings due to the part-time-part-space operation mode. These findings can inform the actual application of VRF systems in China, supporting the design, operation, industry standard development, and performance optimization of VRF systems.

A novel solar cooling cycle – A ground coupled PV/T desiccant cooling (GPVTDC) system with low heat source temperatures

Abstract This study investigates the performance of a novel desiccant air dehumidification and cooling cycle that utilises ground source heat exchange and flat plate photovoltaic thermal (PV/T) collectors (the GPVTDC system) to provide conditioned air. The system was modelled in TRNSYS using verified components with a focus on three climates: (i) an oceanic climate (Cfb) (ii) a humid sub-tropical climate (Cfa), and (iii) a hot semi-arid climate (BSh). The GPVTDC system pre-cools the entry air by utilising a ground source heat exchange. This allows the desiccant regeneration process to operate at low temperatures ranging from 43 °C to 62 °C, which enables the use of glazed water type PV/T collectors as the main heat source. The collectors were sized to match both thermal and electrical energy requirements to drive the GPVTDC system. The system was simulated for a whole year in each climate to provide fresh air ventilation, dehumidification and cooling process for an office space. Evaluation of the annual cooling performance of the GPVTDC system shows that it can maintain an office space within the thermal comfort zone for up to 93% of the time, with an average system cooling coefficient of performance (COP) between 9.6 and 16.3 for the three analysed climates. In addition, electrical energy saving up to 87% is achieved in comparison to a conventional dew point air dehumidification and cooling process.

Simulation of the Performance of Solar Driven Thermoelectric Based Rotary Desiccant Wheel HVAC System

<div class="section abstract"><div class="htmlview paragraph">In the automotive applications, the main functionality of the HVAC system includes heating, ventilation, and cooling or air-conditioning of the vehicle to achieve the desired indoor thermal comfort. In the current scenario, the conventional vapor compression based HVAC system is widely used. The typical refrigerants used to operate this equipment include HFCs and HFOs which are susceptible to cause an environmental hazard. This article aims to assess the performance of a hypothetical solar-driven thermoelectric based rotary desiccant wheel HVAC system (D-HVAC) to be used for automotive applications. The D-HVAC system uses the desiccant wheel to remove the latent heat, energy wheel to remove the sensible heat, evaporating coolers to achieve further cooling, the regeneration of the desiccant wheel by hot air and water as the refrigerant. In the case of a solar-driven-DHVAC system, solar energy is utilized for the regeneration of the desiccant wheel in place of hot air. However, the intensity of incident solar energy varies throughout the day. To compensate for this energy fluctuation an additional thermoelectric system (Peltier device) can be added to achieve the optimum regeneration of the desiccant wheel. The Peltier device avails the advantage to simultaneously heat the regeneration air and cool the process air to achieve the highest cooling coefficient of performance (COP) of the solar-driven D-HVAC system. The usage of PV cells to produce the required electricity to operate the blowers (for process air and regeneration air) and motor of the rotary desiccant wheel and TEC. Overall, through numerical simulation, the increment in the COP of the solar-driven thermoelectric based rotary desiccant wheel HVAC system in comparison with the simple D-HVAC system and conventional vapor compression based HVAC system are investigated.</div></div>

Evaluating the performance of a coated tube adsorber for adsorption cooling

Abstract Adsorption cooling (AC) is an environmentally friendly alternative to conventional vapor compression cooling. In this paper, the performance of a coated tube adsorber suitable for AC systems is numerically evaluated. The developed adsorber uses silica gel-water as adsorbent-adsorbate working pair. A numerical model is used to analyze the influence of several governing parameters such as the evaporator, condenser and regeneration temperatures, cycle time, metal-adsorbent heat transfer coefficient, metal tube diameter, coating thickness and heat transfer fluid (HTF) velocity on the adsorber's performance. It is confirmed that increasing the evaporator temperature results on a performance increase whereas increasing the condenser temperature hindrances the system's performance. A regeneration temperature close to 70 oC results on the highest cooling coefficient of performance (COPc). The cycle time can be used as a system-controlling parameter to tune the COPc and the specific cooling power (SCP). The adsorber performs better when the regeneration time is 35% shorter than the adsorption time. The maximum COPc occurs when the metal-adsorbent heat transfer coefficient reaches values of 100 W.m-2.K-1. The SCP greatly increases until this coefficient reaches 350 W.m-2.K-1. The adsorbent coating thickness severely influences the performance of an AC system. It was found that the maximum COPc corresponds to a coating thickness of 1.75 mm and, the smaller the coating thickness the smaller the SCP. The metal tube's diameter and the HTF's velocity mainly influence the SCP since they have direct impact on the heat transfer rate exchanged between the HTF and the adsorbent material.

Near-Field Electroluminescent Refrigeration System Consisting of Two Graphene Schottky Diodes

Abstract It has been shown that the performance of a thermal radiative device, such as a thermophotovoltaic (TPV) and an electroluminescent (EL) refrigerator, can be significantly enhanced when the vacuum gap between a reservoir and a semiconductor diode becomes nanoscale. Recently, several studies have reported the integration of a TPV and a light emitting diode (LED) in one near-field thermal radiative device to improve the operation efficiency. However, surface polaritons were hardly exploited in previous research because bare semiconductor diodes were used. In this paper, we propose a TPV-LED integrated near-field EL refrigeration system consisting of two graphene-semiconductor Schottky diodes. A substantial refrigeration rate (101.9 kW/m2) is achieved owing to the coupling of surface plasmon-phonon polaritons excited by a symmetric configuration of graphene-polar materials. Moreover, the cooling coefficient of performance (COP) of the system can be enhanced up to 2.65 times by recycling the electrical power generated in the TPV cell. The cooling performance is further investigated in relation to design parameters, namely the doping concentration of Si and insulator thickness.

Investigation and Analysis of R463A as an Alternative Refrigerant to R404A with Lower Global Warming Potential

This research presents the development of R463A refrigerant, a nonflammable refrigerant that was retrofitted to replace R404A. R463A is primarily composed of hydrofluorocarbons/hydrocarbons/carbon dioxide (HFCs/HCs/CO2), and has global-warming potential (GWP) of 1494. It is a nonazeotropic mixture of R32 (36%), R125 (30%), R134a (14%), R1234yf (14%), and R744 (6%). R463A is composed of polyol ester oil (POE), and it is classified as a Class A1 incombustible and nontoxic refrigerant. R463A has a higher cooling capacity (Qe) than that of R404A, as it is composed of hydrofluorocarbons (HFCs) R32 and carbon dioxide (CO2) R744, and has lower GWP than that of R404A due to the use of hydrofluoroolefins (HFOs) from R1234yf. The results of this research showed that R463A can be retrofitted to replace R404A due to its composition of POE, Class A1 incombustibility, and lower toxicity. The properties of R463A and R404A, as analyzed using national institute of standards and technology (NIST) reference fluid thermodynamic and transport properties database (REFPROP) software and NIST vapor compression cycle model accounting for refrigerant thermodynamic and transport properties (CYCLE_D-HX) software, are in accordance with the CAN/ANSI/AHRI540 standards of the Air-Conditioning, Heating, and Refrigeration Institute (AHRI). The normal boiling point of R463A was found to be higher than that of R404A by 23%, with a higher cooling capacity and a 63% lower GWP value than that of R404A. The critical pressure and temperature of R463A were found to be higher than those of R404A; it can be used in a high-ambient-temperature environment, has higher refrigerant and heat-rejection effects, and has lower GWP than that of R404A by 52% due to the HFOs from the R1234yf component. The cooling coefficient of performance (COPc) of R463A was found to be higher than that of R404A by 10% under low-temperature applications. R463A is another refrigerant option that is composed of 7% carbon dioxide (CO2), and is consistent with the evolution of fourth-generation refrigerants that contain a mixture of HFCs, HFOs, HCs, and natural refrigerants, which are required to produce a low-GWP, zero-ozone-depletion-potential (ODP), high-capacity, low-operating-pressure, and nontoxic refrigerant.

A Integrated Dedicated Outdoor Air System to Optimize Energy Saving

Outdoor air supply is required to maintain good indoor air quality (IAQ). For tropical or subtropical regions, warm and humid outdoor air would cause excess air-conditioning energy use. This study has proposed an integrated dedicated outdoor air system (IDOAS), which integrates the enthalpy exchange and outdoor air cooling into a unitary system. IDOAS could operate independently of central air-conditioning systems thus saving tremendous piping cost and energy needed to deliver chilled water to outdoor air unit in a conventional centralized system. An experimental unit of IDOAS was built to prove this novel concept. Enthalpy exchange efficiency was tested to be about 44%. The test results show that about 44% of energy needed to condition the outdoor air can be saved. A reverse Rankine refrigeration cycle was integrated to cool the outdoor air. Due to this integrated configuration, the air passing through the condenser would be at a lower temperature. The consequent lower refrigerant condensing temperature would improve the cooling cycle efficiency. The cooling coefficient of performance (COP) was improved by about 46%. In addition, the outdoor air could be conditioned to a lower humidity before being supplied to space, which would improve the thermal comfort. The test results of this novel IDOAS show that it could provide good air quality at lower energy use.

Energy and exergy performance comparison of different HFC/R1234yf mixtures in vapor-compression cycles

To increase the COP and volumetric capacity of hydrofluoroolefins while reducing the global warming potential (GWP) of hydrofluorocarbons (HFCs), four HFC/R1234yf mixtures with various compositions are compared with verified thermodynamic models. Results show that a lower R1234yf mass fraction leads to a higher mixture latent heat; R32/R1234yf has the highest pressures, the lowest pressure ratios and the biggest temperature glides. As the R1234yf mass fraction increases from 0.0 to 1.0, the cooling coefficient of performance (COP) first increases from 5.25 to 5.52 and later decreases to 5.30 for R32/R1234yf, while it decreases from 5.46, 5.63 and 5.30 to 5.29 for R134a/R1234yf, R152a/R1234yf and R161/R1234yf. The heating COP first increases from 3.90 to 4.00 and later decreases to 3.79 for R32/R1234yf, while it decreases from 3.84, 3.95 and 4.02–3.79 for others. Caused by different volumetric capacities, R32/R1234yf requires a compressor enlarged by 2.8 times, R134a/R1234yf and R152a/R1234yf requires little change on compressor size, while R161/R1234yf requires a compressor enlarged by 1.5 times. R32/R1234yf yields the highest discharge temperature, while R134a/R1234yf yields the lowest. R32/R1234yf shows the highest exergy COPs (ECOPs) when the R1234yf mass fraction is above 60% in cooling mode and 46% in heating mode. Otherwise, R152a/R1234yf performs the best in cooling model and R161/R1234yf performs the best in heating mode. Considering both GWP and efficiency, the optimal composition is 20/80% for R32/R1234yf, 10/90% for R134a/R1234yf and 100/0% for both R152a/R1234yf and R161/R1234yf. This study provides suggestions for the determination of optimal compositions of different HFC/R1234yf mixture refrigerants.

Experimental study on CO2/R32 blends in a water-to-water heat pump system

The purpose of this study is to evaluate the cooling and heating performance as well as the compressor operating conditions of CO2/R32 blends in a water-to-water heat pump system. Experiments were conducted to reveal the effects of the refrigerant compositions, heat source temperatures, and internal heat exchanger (IHX) on the performance of the system. The results show that 23.3% and 65.2% maximum improvement in heating coefficient of performance (COPh) and cooling coefficient of performance (COPc) of the system were obtained when the mass fraction of CO2 was 0.6, respectively. Meanwhile, heating capacity, cooling capacity, and the compressor discharge pressure of CO2/R32 blends were decreased significantly. The thermal matching between the heat source and CO2/R32 blends significantly affected the overall performance of the system. In addition, IHX is recommended in the water-to-water heat pump to improve the system performance.

Effects of functionalization on the performance of metal-organic frameworks for adsorption-driven heat pumps by molecular simulations

Metal-organic frameworks (MOFs) are promising adsorbents for adsorption-driven heat pumps (AHPs) due to their outstanding adsorption performance. However, the role of functionality in tuning the AHP performance has not been elucidated. In this work, the ethanol adsorption behaviors and AHP performance of Zr-based MOFs in three topologies (i.e. csq, ftw and scu) functionalized by –NH2 and –CF3 exhibiting different affinities towards ethanol working fluid were investigated by grand canonical Monte Carlo simulations (GCMC). It was revealed that functionalization not only affected the adsorption capacity but also shifted the steps of isotherms to the low pressure upon functionalization by –NH2. scu-MOFs exhibited the higher coefficient of performance for cooling (COPC) than csq-MOFs and ftw-MOFs. Moreover, functionalization reduced the COPC of ftw-MOFs and scu-MOFs due to the lower working capacity resulting from the reduced pore volumes. On the contrary, the COPC of csq-MOFs can be improved upon functionalization by –NH2.