Development and validation of static simulation model for CO2 heat pump

A simulation model for the CO₂ heat pump water heater was developed and validated in this study. Component models of the gas cooler, evaporator, compressor, and expansion valve were constructed with careful consideration for the heat transfer performances. To validate the simulation model, experiments were carried out using an actual CO₂ heat pump water heater (water heating capacity: 22.3 kW; hot-water temperature: 90 °C). In simulations and experiments, the effects of the inlet water temperature and outside air temperature on the system characteristics were discussed. As a result, the average difference in COP between the simulation results and experimental results is 1.5%.     

Theoretical analysis and experimental research on transcritical CO2 two stage compression cycle with two gas coolers (TSCC + TG) and the cycle with intercooler (TSCC + IC)

As one of the natural refrigerants, CO₂ is a potential substitute for synthesized refrigerants with favorable environmental properties. In order to improve the performance of the CO₂ transcritical compression cycle, the performance of the two stage compression cycle with two gas coolers (TSCC + TG) and the two stage compression cycle with intercooler (TSCC + IC) were analyzed, respectively. Under the given calculation condition, the optimum intermediate pressure of the cycle TSCC + TG and the TSCC + IC are 7.09 MPa and 5.89 MPa, and the maximal COP are 2.77 and 3.08, respectively. Range of the given evaporating temperature and outlet temperature of gas cooler, the experimental testing shows that the performance of cycle TSCC + IC are 11.88% and 10.87% better than that of the cycle TSCC + TG, respectively. Range of the given inlet temperature and cooling water volume flow of gas cooler, the refrigeration COP (COP_c) and heat COP (COP_h) of the cycle TSCC + IC are average 10.97% and 4.39% higher than that of the cycle TSCC + TG. Range of the given inlet temperature and chilled water volume flow of evaporator, the refrigeration COP (COP_c) and heat COP (COP_h) of the cycle TSCC + IC are average 10.71% and 3.67% higher than that of the cycle TSCC + TG, respectively. The error between theoretical calculation and experimental testing is not exceeds 20%.     

Performance assessment of a direct expansion air conditioner working with R407C as an R22 alternative

This paper presents an experimental investigation of a direct expansion air conditioner working with R407C as an R22 alternative. Experiments are conducted on a vapor compression refrigeration system using air as a secondary fluid through both the evaporator and the condenser. The influences of the evaporator air inlet temperature (20–32 °C), the evaporator air flow rate (250–700 m³/h) and the evaporator air humidity ratio (9 and 14.5 g_wv/kg_a) at the condenser air temperature and volume flow rate of 35 °C and 850 m³/h, respectively on the system performance are investigated. Experimental results revealed that the evaporator air inlet temperature has pronounced effects on the air exit temperatures, pressures of the evaporator and the condenser, cooling capacity, condenser heat load, compressor pressure ratio and the COP of both refrigerants at humidity ratios of 9 and 14.5 g_wv/kg_a. Significant effects of the evaporator air flow rate are also gathered on the preceding parameters at the same values of mentioned-humidity ratios. The best performance, in terms of operating parameters as well as COP, can be accomplished using R22 compared to R407C. The inlet humidity ratio affects dramatically the performance of vapor compression system using R22 and R407C. The raising up humidity ratio from 9 to 14.5 g_wv/kg_a leads to an augmentation in the average cooling capacity by 29.4% and 38.5% and an enhancement in the average COP by 30% and 24.1% for R22 and R407C, respectively.     

Design and performance of a solar-powered heating and cooling system using silica gel/water adsorption chiller

In this paper, a solar-powered compound system for heating and cooling was designed and constructed in a golf course in Taiwan. An integrated, two-bed, closed-type adsorption chiller was developed in the Industrial Technology Research Institute in Taiwan. Plate fin and tube heat exchangers were adopted as an adsorber and evaporator/condenser. Some test runs have been conducted in the laboratory. Under the test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 9 kW and a COP (coefficient of performance for cooling) of 0.37 can be achieved. It has provided a SCP (specific cooling power) of about 72 W/(kg adsorbent). Some field tests have been performed from July to October 2006 for providing air-conditioning and hot water. The efficiency of the collector field lies in 18.5–32.4%, with an average value of 27.3%. The daily average COP of the adsorption chiller lies in 33.8–49.7%, with an average COP of 40.3% and an average cooling power of 7.79 kW. A typical daily operation shows that the efficiency of the solar heating system, the adsorption cooling and the entirely solar cooling system is 28.4%, 45.2%, and 12.8%, respectively.     

Exergetic analysis of transcritical CO2 residential air-conditioning system based on experimental data

The experimental system for the transcritical CO₂ residential air-conditioning with an internal heat exchanger was built. The effects of working conditions on system performance were experimentally studied. Based on the experimental dada, the second law analysis on the transcritical CO₂ system was performed. The effects of working conditions on the total exergetic efficiency of the system were investigated. The results show that in the studied parameter ranges, the exergetic efficiency of the system increases with the increases of gas cooler side air inlet temperature, gas cooler side air inlet velocity and evaporating temperature. And it will decrease with the increases of evaporator side air inlet temperature and velocity. Then, a complete exergetic analysis was performed for the entire CO₂ transcritical cycle including compressor, gas cooler, expansion valve, evaporator and internal heat exchanger under different working conditions. The average exergy loss in gas cooler is the highest one under all working conditions which is about 30.7% of the total exergy loss in the system. The second is the average exergy loss in expansion valve which is about 24.9% of the total exergy loss, followed by the exergy losses in evaporator and compressor, which account for 21.9% and 19.5%, respectively. The exergy loss in internal heat exchanger is the lowest one which is only about 3.0%. So in the optimization design of the transcritical CO₂ residential air-conditioning system more attentions should be paid to the gas cooler and expansion valve.     

Modeling and simulating the transcritical CO2 heat pump system

In this paper, a mathematical model for steady-state simulation of transcritical CO₂ water-to-water heat pump system with an expander has been developed. It is used to simulate the performance of transcritical CO₂ system with CO₂ expander prototype. Simulated results are compared with experimental data to verify the accuracy of the simulation model. The comparison results show the average deviation of about 15% for COP_c(cooling coefficient of performance) and COP_h(heating coefficient of performance), about 17% for cooling and heating capacity at experimental high pressure ranges. With this model, which has been validated in a limited high pressure range, the influence of water mass flow rate and water inlet temperature of both evaporator and gas cooler on the performance of transcritical CO₂ expander system is analyzed. The results show that decreasing inlet temperature and increasing mass flow rate of cooling water cannot only increase the system performance but also reduce the optimal heat rejection pressure, at which the maximum COP (coefficient of performance) can be obtained. For chilling water, increasing its inlet temperature and mass flow rate is favorable for increasing the system performance, while the optimal heat rejection pressure does not vary very much.     

Experimental study of a solid adsorption cooling system using flat-tube heat exchangers as adsorption bed

In this paper, a solid adsorption cooling system with silica gel as the adsorbent and water as the adsorbate was experimentally studied. To reduce the manufacturing costs and simplify the construction of the adsorption chiller, a vacuum tank was designed to contain the adsorption bed and evaporator/condenser. Flat-tube type heat exchangers were used for adsorption beds in order to increase the heat transfer area and improve the heat transfer ability between the adsorbent and heat exchanger fins. Under the standard test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 4.3 kW and a coefficient of performance (COP) for cooling of 0.45 can be achieved. It has provided a specific cooling power (SCP) of about 176 W/(kg adsorbent). With lower hot water flow rates, a higher COP of 0.53 can be achieved.     

Theoretical performance analysis of heat pump water heaters using carbon dioxide as refrigerant

The aim of this paper is to simulate the performance of an air source heat pump water heater using carbon dioxide (CO₂) as a working fluid. The heat pump water heating system consists of a compressor, a gas cooler, an expansion device and an evaporator. The computer simulation model has been developed by using the heat transfer data and the thermodynamic properties of CO₂. The effects on the heat pump performance by the operating parameters such as the compressor rotational speed, the inlet water temperature at the gas cooler, the inlet air temperature at the evaporator and the mass flow rate ratio of water to refrigerant were presented. For rated capacities of a 4 kW compressor with a 10 kW gas cooler and a 6 kW evaporator, the coefficient of performance is found to be between 2.0 and 3.0. The mass flow rate ratio of water and CO₂ between 1.2 and 2.2 is the most suitable value for generating hot water temperature above 60°C at 15–25°C ambient air temperature. Copyright © 2007 John Wiley & Sons, Ltd.     

Performance characteristics of mobile heat pump for a large passenger electric vehicle

In this study, the performance of a mobile heat pump for an electric bus, which uses the wasted heat of electric devices for a heating and air source for a cooling, was evaluated. Both cooling and heating performances of the mobile heat pump were tested under various experimental conditions, and then optimized by varying the refrigerant charge and the compressor frequency. The cooling capacity at all compressor frequencies was over 23.0 kW, which is sufficient for the cooling loads of an electric bus. The heating COP decreased but the heating capacity increased with the rise of outdoor temperature and the compressor frequency. The heating COP was 2.4 at an outdoor temperature of 10.0 °C. The observed heating and cooling performance characteristics of the mobile heat pump means it could be used for cabin heating and air conditioning of an electric vehicle with a short driving range.     

Modelling of an adsorption system driven by engine waste heat for truck cabin A/C. Performance estimation for a standard driving cycle

This paper presents the main characteristics of an innovative cooling system for the air conditioning of a truck cabin, as well as a first estimation of its performance during a standard driving cycle, obtained with a specifically developed vehicle-engine-cooling system overall model. The innovative cooling system consists of a water–zeolite adsorption–desorption system, which employs the waste heat from the engine to produce the cooling of the vehicle cabin. The developed global model is completely dynamic and is able to: reproduce the operation of the engine through a standard driving cycle, evaluate the waste heat available at the engine hydraulic loop; calculate the sequential operation of an adsorption–desorption system, calculate the condensed water per cycle, the cooling effect produced at the evaporator, and finally, the temperature and humidity evolution of the air inside the cabin. The model was validated by experimental data. The experimental tests were performed in a lab-scale adsorption chiller prototype specifically designed and realized to be driven by the low grade waste heat (80–90 °C) from the engine coolant loop of a truck. The experimental activity carried out showed that the chiller is able to generate up to 5 kW of peak cooling power at 10 °C (35 °C of condensation temperature) with a COP of 0.6. The obtained results show that the system could be able to provide a significant amount of the required cooling.     

Performance analysis of a waste heat driven activated carbon based composite adsorbent – Water adsorption chiller using simulation model

This study aims at improving the performance of a waste heat driven adsorption chiller by applying a novel composite adsorbent which is synthesized from activated carbon impregnated by soaking in sodium silicate solution and then in calcium chloride solution. Modeling is performed to analyze the influence of the hot water inlet temperature, cooling water inlet temperature, chilled water inlet temperatures, and adsorption/desorption cycle time on the specific cooling power (SCP) and coefficient of performance (COP) of the chiller system with the composite adsorbent. The simulation calculation indicates a COP value of 0.65 with a driving source temperature of 85 °C in combination with coolant inlet and chilled water inlet temperature of 30 °C and 14 °C, respectively. The most optimum adsorption–desorption cycle time is approximately 360 s based on the performance from COP and SCP. The delivered chilled water temperature is about 9 °C under these operating conditions, achieving a SCP of 380 W/kg.     

Cycle parameter optimization of vortex tube expansion transcritical CO2 system

Optimization studies along with optimum parameter correlations are presented in this article for a vortex tube expansion transcritical CO₂ refrigeration cycle with two cycle layouts based on the Maurer model (1999) and the Keller model (1997). A simple thermodynamic model is proposed and used for vortex tube analysis. Finally, the COP improvement and effect on optimum discharge pressure by using vortex tube in transcritical CO₂ cycle instead of expansion valve are presented. The results show that the effect of cold mass fraction and inlet water temperature to desuperheater (used to cool hot gas from vortex tube) on the cycle optimization is negligible. The Maurer model is better than the Keller model in terms of moderately more COP improvement and lower cost due to less components. The use of a vortex tube is more effective for higher gas cooler exit temperature for both models. Results show that the vortex tube expansion transcritical CO₂ cycle for the Maurer model can give higher COP improvement for lower cooling temperature applications; however the trend is reverse for the Keller model.     

Steam jet ejector cooling powered by waste or solar heat

A small scale steam jet ejector experimental setup was designed and manufactured. This ejector setup consists of an open loop configuration and the boiler operate in the temperature range of T_b = 85–140 °C. The typical evaporator liquid temperatures range from T_e = 5 °C to 10 °C while the typical water-cooled condenser pressure ranges from P_c = 1.70 kPa to 5.63 kPa (T_c = 15–35 °C). The boiler is powered by two 4 kW electric elements while a 3 kW electric element simulates the cooling load in the evaporator. The electric elements are controlled by means of variacs.Primary nozzles with throat diameters of 2.5 mm, 3.0 mm and 3.5 mm are tested while the secondary ejector throat diameter remains unchanged at 18 mm. These primary nozzles allow the boiler to operate in the temperature range of T_b = 85–110 °C. When the nozzle throat diameter is increased, the minimum boiler temperature decreases. A primary nozzle with a 3.5 mm throat diameter was tested at a boiler temperature of T_b = 95 °C, an evaporator temperature of T_e = 10 °C and a critical condenser pressure of P_crit = 2.67 kPa (22.6 °C). The system's COP is 0.253.In a case study the experimental data of a solar powered steam jet ejector air conditioner is investigated. Solar powered steam ejector air conditioning systems are technical and economical viable when compared to conventional vapour compression air conditioners. Such a system can either utilise flat plate or evacuated tube solar thermal collectors depending on the type of solar energy available.     

Experimental evaluation of a low-power direct air-cooled double-effect LiBr–H2O absorption prototype

This paper describes a new small air-cooled double-effect LiBr–H₂O absorption prototype directly powered by fuel and discusses the experimental findings for some tests carried out in Madrid in 2007, with natural gas as energy source. The prototype, which has been designed to supply 7 kW of cooling power, was able to chill water up to 7–18 °C under extreme outdoor temperatures. A new flat-sheet adiabatic absorber was used allowing it to operate at outdoor temperatures about 45 °C without any sign of crystallization. A mean daily coefficient of performance (COP) of about 1.05 was obtained. Since this absorption machine does not need cooling tower, there is neither water consumption nor Legionella pollution. Moreover, it is a quite compact unit. The ratio of cooling power over volume is about 6.0 kW/m³, while for the only air-cooled absorption chiller, Rotartica 045v, in the marked until 2009 this ratio is 4 kW/m³. When comparing with electric chillers presently on the market, this prototype was found to have a cooling cost approximately 15.9% higher and an environmental impact 16.7% lower. The absorption prototype is a more environmentally friendly solution as it does not emit fluorinated refrigerants.     

Performance analysis and optimization of a new two-stage ejector-expansion transcritical CO2 refrigeration cycle

In this paper, a new two-stage configuration of ejector-expansion transcritical CO₂ (TRCC) refrigeration cycle is presented, which uses an internal heat exchanger and intercooler to enhance the performance of the new cycle. The theoretical analysis on the performance characteristics was carried out for the new cycle based on the first and second laws of thermodynamics. Based on the simulation results, it is found that, compared with the conventional two-stage transcritical CO₂ cycle, the COP and second law efficiency of the new two-stage cycle are about 12.5–21% higher than that of conventional two-stage cycle. It is also concluded that, the performance of the new two-stage transcritical CO₂ refrigeration can be significantly improved based on the presented new two-stage cycle. Hence the new two-stage refrigeration cycle is a promising refrigeration cycle from the thermodynamically and technical point of views. A regression analysis in terms of evaporator and gas cooler exit temperatures has been used, in order to develop mathematical expressions for maximum COP, optimum discharge and inter-stage pressures and entrainment ratio.     

Influence of the superheat associated to a semihermetic compressor of a transcritical CO2 refrigeration plant

This work evaluates, from an energetic point of view, the effects of the superheat caused in the refrigerant by the electric motor cooling (SH_SC) in a semihermetic compressor installed in an experimental refrigerating plant, which operates with CO₂ as the working fluid in transcritical conditions. The analysis is based on 84 experimental tests which cover a wide range of operating conditions of the plant: three evaporating levels (0, ?10, ?17 °C) at four compressor speeds (1150, 1300, 1450, 1600 rpm) over a range of discharge pressures from 74.2 to 104.9 bar.The paper presents the empirical model of the compressor and its validation with the experimental measurements, which allows the behaviour of the facility to be calculated with prediction errors below 5%. With the model, the effect of the SH_SC on the energy efficiency of the plant is evaluated by comparing the actual performance of the facility with the simulated behaviour of the plant without considering the SH_SC produced by the semihermetic compressor. The maximum estimated reductions, due to the SH_SC of the compressor, are a 20% in cooling capacity and a 23% in COP.     

Particular characteristics of transcritical CO2 refrigeration cycle with an ejector

The present study describes a theoretical analysis of a transcritical CO₂ ejector expansion refrigeration cycle (EERC) which uses an ejector as the main expansion device instead of an expansion valve. The system performance is strongly coupled to the ejector entrainment ratio which must produce the proper CO₂ quality at the ejector exit. If the exit quality is not correct, either the liquid will enter the compressor or the evaporator will be filled with vapor. Thus, the ejector entrainment ratio significantly influences the refrigeration effect with an optimum ratio giving the ideal system performance. For the working conditions studied in this paper, the ejector expansion system maximum cooling COP is up to 18.6% better than the internal heat exchanger cycle (IHEC) cooling COP and 22.0% better than the conventional vapor compression refrigeration cycle (VCRC) cooling COP. At the conditions for the maximum cooling COP, the ejector expansion cycle refrigeration output is 8.2% better than the internal heat exchanger cycle refrigeration output and 11.5% better than the conventional cycle refrigeration output. An exergy analysis showed that the ejector expansion cycle greatly reduces the throttling losses. The analysis was also used to study the variations of the ejector expansion cycle cooling COP for various heat rejection pressures, refrigerant temperatures at the gas cooler exit, nozzle efficiencies and diffuser efficiencies.     

Exergy assessment of an optimized capillary tube‐based transcritical CO2 heat pump system

A capillary tube‐based CO₂ heat pump is unique because of the transcritical nature of the system. The transcritical cycle has two independent parameters, pressure and temperature, unlike the subcritical cycle. A comparative study for various operating conditions, based on system COP and exergetic efficiency, of a capillary tube and a controllable expansion valve‐based transcritical carbon dioxide heat pump systems for simultaneous heating and cooling at 73 and 4°C, respectively, is presented here. Two optimized capillary tubes having diameter of 1.5 and 1.6 mm are compared with an equivalent controllable throttle valve. Heat transfer and fluid flow effects are included in the gas cooler and evaporator model and capillary tube employs the homogeneous flow model to simulate two‐phase flow. Subcritical and supercritical thermodynamic and transport properties of CO₂ are calculated employing a precision in‐house property code. Optimization of effective distribution of total heat exchanger area ratio between gas cooler and evaporator is investigated. The exergetic efficiency is better in case of the capillary tube than that of a controllable throttle valve‐based system. Capillary tube‐based system is shown to be quite flexible regarding changes in ambient temperature, almost behaving to offer an optimal pressure control just like the controllable expansion valve yielding both, maximum system COP and maximum exergetic efficiency. Relatively at a smaller diameter, the capillary tube exhibits better exergetic efficiency. Capillary tube length is the critical parameter that influences system optimum conditions. The exergy flow diagram exhibits that compressor, gas cooler and capillary tube contribute a larger share, in that order, to system irreversibility. It is fairly established in this study that a capillary tube can be a good engineering option for small capacity systems in lieu of an expansion valve, which has been thought of as the only possible solution to attain the pressure optimization, an important feature of all transcritical CO₂ systems. Copyright © 2009 John Wiley & Sons, Ltd.     

Study on a wire-type Joule Thomson microcooler with a concentric heat exchanger

This study examines the performance of a wire-type Joule Thomson microcooler utilizing a flexible concentric counterflow heat exchanger. Three gases: C₂H₄, CO₂ and N₂ were used separately for trials conducted at inlet pressures ranging from 0.5 MPa to 5 MPa with C₂H₄ having the best performance. During unloaded tests at an inlet pressure of 2.0 MPa, C₂H₄ obtained a minimum temperature of 225 K while CO₂ obtained a minimum temperature of 232 K. Using CO₂ the microcooler was able to maintain a temperature of 273 K at 100 mW heat input and 2 MPa inlet pressure. An inlet pressure of 3 MPa allowed a 550 mW heat input at 273 K. Theoretical performance calculations were conducted and compared to experimental results revealing considerable reduction of microcooler performance due to the presence of heat in-leak. Results have displayed that the JT coefficient of the coolant gas is a more dominant factor than heat transfer properties in determining the performance of the coolant. Due to the microscale of the device, relevant scaling effects were evaluated, particularly entrance effects, surface roughness and axial conduction.     

Studies on a two-stage transcritical carbon dioxide heat pump cycle with flash intercooling

Simulation studies on a two-stage flash intercooling transcritical carbon dioxide heat pump cycle are presented. Sub-critical and super-critical thermodynamic and transport properties of carbon dioxide are calculated employing an exclusive precision property code based on recently published correlations. Results exhibit that flash intercooling technique is not economical with CO₂ refrigerant unlike NH₃ as the refrigerant. COP is considerably lower than that of the single cycle for a given gas cooler and evaporator temperature. There is no optimum inter-stage pressure as well. However, a marginal increase in COP occurs as inter-stage pressure decreases from the classical estimate of geometric mean of gas cooler and evaporator pressure. It is observed that incorporation of desuperheating of vapour in the intercooler almost doubles the mass flow rate in the second stage which can be attributed to the large flashing that occurs in the intercooler; this increase depends on the discharge temperature from the first stage and mass flow rate of refrigerant flow in the evaporator. Compressor isentropic efficiency shows marginal influence on system performance.