Transcritical Carbon Dioxide Heat Pump Research Articles

Exergo-environmental cost optimization and thermodynamic analysis for a solar-driven combined heating and power system

Reasonable pricing of each product in a combined heating and power system is essential to improve the economic benefits of the system. However, the whole life cycle pollutant emissions cost of the system and energy level in the conventional exergo-economic method are ignored, leading to irrational pricing of each product. Hence, an exergo-environmental cost optimization methodology that considers the whole life cycle pollutant emissions cost and energy level is developed. The compressor discharge pressure of the transcritical carbon dioxide heat pump is employed as the optimization variable to obtain the lowest unit exergo-environmental cost of the system. The unit exergo-environmental cost is converted to the unit energy-environmental cost to determine the price of each product. Optimization results show that the lowest unit exergo-environmental cost of the system is 0.387 $/kWh when the compressor discharge pressure is 10.15 MPa, the unit exergo-environmental costs of the electricity and space heating water are 0.199 $/kWh and 0.989 $/kWh, respectively. The corresponding unit energy-environmental costs of the space heating water and electricity are 0.047 $/kWh and 0.199 $/kWh, respectively. Compared with the conventional exergo-economic method, the unit exergo-environmental costs of the electricity and space heating water are improved by 16.37 % and 11.37 %, respectively, Furthermore, under optimal configuration, the exergy, energy, and solar to electricity efficiencies of the system are 16.28 %, 88.09 %, and 13.79 %, respectively. The above results prove that the developed exergo-environmental cost analysis and optimization methodology can reasonably determine the price of each product of the building energy system.

Performance investigation and exergoeconomic optimization for a solar-driven direct heating and cooling system

Integrating solar energy with direct heating and cooling (DHC) systems can improve system efficiency and reduce polluting gas emissions. To assist buildings in better selecting a DHC system, an exergoeconomic analysis and optimization model combined with building loads and environmental parameters is proposed. The exergoeconomic optimization model is applied to a novel DHC system based on transcritical carbon dioxide heat pump (TCHP) and solar energy. The sold electricity ratio and operating time ratio are defined to transform the building loads and environmental parameters into specific operating conditions of the system. The compressor discharge pressure is optimized to obtain the lowest total unit exergy cost (UEC) of the DHC system. The exergy, energy, and exergoeconomic performance of the DHC system are analyzed under optimal conditions. The results indicate that when the compressor discharge pressure in the heating mode and cooling mode are 7.31 MPa and 8.93 MPa, respectively, the DHC system has the lowest UEC, 0.629 $/kWh. The total UEC is reduced by 43.56% compared to the reference system. Under optimal configuration, the exergy and energy efficiency of the DHC system are 1.81% and 40.30%, respectively. The sensitivity analysis shows that the exergy and energy efficiency, the total UEC, and the UEC in the heating and cooling modes are positively correlated with the sold electricity ratio. The above results demonstrate that the exergoeconomic optimization methods can provide better assistance in selecting the DHC system.

Thermodynamic analysis of a transcritical CO[formula omitted] heat pump integrating a vortex tube

A thermodynamic model is used to analyze a transcritical carbon dioxide heat pump cycle integrating a vortex tube. The objective is to prove that a vortex tube is beneficial in raising heat pump efficiency. The novelty of the current work lies in implementing a sophisticated thermodynamic model of the vortex tube operating with carbon dioxide at high pressures or in a two-phase state. However, for all other components of the heat pump, a commercial library (TIL) associated with the Dymola software is used to simulate the system. A transcritical carbon dioxide conventional heat pump model is validated to determine the feasibility of implementing the TIL library. Then, the vortex tube heat pump is analyzed to test the heat pump performance and the effect of different parameters under various operating conditions. Of the parameters discussed, the hot exit pressure of the vortex tube and the desuperheater glycol mass flow rate are the most effective parameters on the desuperheater heating capacity and the overall coefficient of performance. Meanwhile, the hot exit pressure significantly controls the desuperheater glycol exit temperature, yet, with limitation. On the other hand, the variation of the cold mass fraction shows insignificant changes in the desuperheater glycol exit temperature. The final conclusion is that the heating coefficient of performance of the vortex tube heat pump is improved by a maximum of 43.7% when compared to that of the conventional heat pump.

Transcritical carbon dioxide heat pump cycle: Validation and comparison of one-dimensional models of ejector with independent data sets

Two typical 1-D homogeneous equilibrium models from the literature (constant and variable pressure mixing models) were used to study the ejector expansion transcritical CO 2 heat pump cycle. A sensitivity analysis was performed to study the impact of the isentropic efficiency of the compressor and of each component of the ejector on the overall system performance. Two groups of independent experimental data from the literature were used to validate the two models. It was found that the cycle performance was more sensitive to the isentropic efficiency of the compressor than that of the individual components of the ejector. With appropriate input value of the compressor efficiency, both 1-D models accurately predicted the ejector outlet pressure and vapor quality. However, for the ejector entrainment ratio and cycle COP, the deviations were more than 50 %. These deviations can be significantly minimized to 10 % by eliminating the imbalance occurring in the system operation using a data post-processing method to correct the pseudo steady state in the real operation system. By comparing the simulation results of the two models, it has been shown that there is minor difference on the prediction accuracy. The research can serve as a practical reference for the operation of two-phase-flow ejector expansion transcritical heat pump cycle, and can also provide an insight and orientation for the subsequent optimization of 1-D ejector numerical models. • Two 1-D homogeneous equilibrium models of the ejector are experimental validated. • Neglecting the pressure variation during the mixing has no significant impact. • The liquid imbalance is a predominant source of the modeling deviation. • An innovative data post-processing method is proposed. • The system performance is more sensitive to the compressor isentropic efficiency.

Superhydrophobic heat exchangers delay frost formation and enhance efficency of electric vehicle heat pumps

The number of electric vehicles has rapidly expanded as high-performance automotive lithium-ion batteries have become more affordable. However, the range of electric vehicles decreases in cold climates partly because of the additional thermal loads associated with cabin heating. One way to improve the efficiency of cabin heating is to replace resistive heating elements with an air-source heat pump system. However, to gain the full benefit of heat pumping, frost formation on the outdoor heat exchanger must be minimized. In this work, we modified the surface wettability of aluminum louvered-fin automotive heat pump evaporators and tested them under realistic operating conditions in a transcritical carbon dioxide (CO2) heat pump. Each heat exchanger underwent several consecutive frosting and defrosting cycles to understand the cyclic performance of the system. The heat exchanger with a superhydrophobic outer surface was able to delay frost formation and maintain higher heat transfer rates when compared to heat exchangers with higher surface energies (hydrophilic). The delayed frost formation resulted in a system efficiency benefit in the first few frosting cycles but diminished in later cycles due to water retention and incomplete defrosting. However, for most automotive applications the superhydrophobic heat exchanger showed substantial benefits for normal driving trips. The scalable and optimized superhydrophobic heat exchangers developed here have the potential to increase the efficiency of automotive heat pumps and consequently increase the range and reduce energy consumption of electric vehicles.

Energy, economic and environmental assessment of transcritical carbon dioxide heat pump water heater in a typical Chinese city considering the defrosting

Considering the high demand for domestic hot water in China, the air source heat pump water heater is being popular due to higher energy efficiency. However, the heat pump is mainly studied from the energy perspective in the previous literature, and the influence of the frosting is also neglected. In this paper, with the presented domestic hot water load in a typical Chinese city, the carbon dioxide heat pump was tested at the varied ambient temperatures considering the different defrosting methods. With the proposed hot gas bypass defrosting method, a techno-economic analysis is required. To show the benefits of the proposed system, the unit is analyzed with the energy, economic and environmental analysis. The comparison with two traditional heating solutions is also presented. Results show that the seasonal performance factor is improved by 2.85%, and the annual fuel cost is decreased by 758.26 Chinese yuan in the proposed system compared with the baseline system. For the additional cost required by the proposed defrosting method, the proportion declines rapidly from 1.53% to 0.46% in the first 5 years. Compared with the direct electric heater, the life cycle cost is reduced by 2313.23 thousand Chinese yuan, and the carbon dioxide emission is declined by 266.46 tons in the proposed system. With the negligible direct emissions of 36.98 kg, the total equivalent warming impact is mainly determined by the indirect emissions. The primary energy consumption and CO2 pollutant emission is related with the domestic hot water load. The payback period is sensitive to the domestic hot water load, compressor cost and electricity price.

Experimental analysis of reverse cycle defrosting and control strategy optimization for transcritical carbon dioxide heat pump water heater

The heating performance of air source transcritical carbon dioxide heat pump will be dramatically reduced with frost formation when operating in cold regions. Although reverse cycle defrosting is widely used, no information is available in the transcritical carbon dioxide system. In this paper, the reverse cycle defrosting was experimentally investigated in a transcritical carbon dioxide heat pump with a microchannel evaporator. The effect of water flow rate, initial water tank temperature, compressor frequency and electronic expansion valve opening are respectively studied, and an optimized control strategy is proposed considering the defrosting time and stability. Results show that thermal energy is considerably affected by water side parameters, and stability is more sensitive to the water temperature and electronic expansion valve opening. With the optimized reverse cycle defrosting, defrosting time and total energy consumption are reduced by 95 s and 21.8% compared with the pre-set basic control strategy, and the total energy consumption is also saved by 38.6% compared with the other defrosting method. The optimized reverse cycle defrosting is verified to be preferable for the transcritical carbon dioxide system, and the presented discussion is beneficial for the application of reverse cycle defrosting in the air-to-water heat pump.

Energy, exergy, economic and environmental analysis of refrigerant charge in air source transcritical carbon dioxide heat pump water heater

Currently, the air source transcritical carbon dioxide heat pump is extensively used for water heating. The refrigerant charge plays a significant role in optimizing system performance. However, most previous studies only focus on energy analysis. In this paper, an experiment was conducted and the effect of refrigerant charge was evaluated with energy, exergy, economic and environmental analyses. Results showed that the optimal normalized refrigerant charge was 0.214 in the comprehensive analysis. In the most important energy analysis, the peak coefficient of performance was increased by up to 16.27% with optimal charge over other charge amounts, corresponding to the maximum heating capacity and superheat of 5.13 °C. The maximum exergy efficiency was 34.17%, and the largest exergy destruction was observed in the evaporator, accounting for 44.04–49.22% of the total. Compared with the direct electric heater, the annual operating cost was decreased by 184.13 thousand Chinese yuan at the optimum charge. Moreover, the payback period was lower than 1.0 years and most sensitive to operating time as shown by the sensitivity analysis. The reduction of carbon dioxide emissions reached the peak value of 378.01 tons at the optimal charge, which was profitable in reducing the greenhouse effect. Also, considering the various operating conditions and domestic hot water load, the energy, economic as well as environmental benefits and the applicability of the evaluated optimal charge were presented.

CFD modelling and exergy analysis of a heat pump cycle with Tesla turbine using CO2 as a working fluid

The transcritical carbon dioxide heat pump cycle has been drawing much research interest due to its environmental friendliness and the thermodynamic features of carbon dioxide. However, there is one concerning issue, which is the huge exergy loss associated with the isenthalpic process in the expansion valve. In the current study, a new transcritical carbon dioxide heat pump cycle is proposed, where a Tesla turbine replaces the expansion valve. The Tesla turbine is a bladeless turbine that works with any two-phase fluid, which is the case for the expansion of supercritical CO2. A 3D computational fluid dynamics model is first developed to simulate the flow of carbon dioxide within a Tesla turbine, and then the extracted results are used as data for subsequent thermodynamic modeling of the heat pump cycle. The Tesla turbine power production and exergy losses, as well as the proposed heat pump cycle coefficient of performance are investigated in terms of the turbine rotor angular velocity and the gas cooler and evaporator pressures. It is demonstrated that the coefficient of performance of the cycle where a Tesla Turbine is integrated is up to 16.3% higher than the traditional cycle with the expansion valve. In addition, at rotor angular velocity equals to 1000 rad/s, the turbine power is maximum and increasing the inlet pressure leads to the higher torque and consequently higher turbine power. At lower inlet pressure, the coefficient of performance of the heat pump cycle is higher. A thermodynamic trade-off is illustrated between the power production from the Tesla turbine and the vapor quality at the outlet of the Tesla turbine, as a function of rotor angular velocity. It is numerically proven that the optimum rotor angular velocity corresponds to the maximum exergy efficiency of the Tesla turbine, which in turn leads to the maximum coefficient of the performance of the whole cycle.

Application of the Peltier sub-cooled trans-critical carbon dioxide heat pump system for water heating – Modelling and performance analysis

Although the Peltier sub-cooled trans-critical CO2 cycle concept has been applied for refrigeration, which typically involves discharging the heat into ambient air, this system is rarely considered for heat pumping purposes. Therefore, this research aims to expand the scope of the Peltier sub-cooled trans-critical CO2 cycle into heat pump water heating where the generated heat is uniquely discharged into water at temperatures progressively higher than ambient. The heat flows between the CO2 and flowing water are modelled as Nusselt based convective heat transfers where a 1D model is imposed to the direct gas cooler to improve simulation accuracy. Moreover, important but often neglected characteristics such as Peltier device size and Peltier heating factor (PHF) will also be analyzed. Results indicate that the PHF has an extremely strong influence on the overall system’s coefficient of performance (COP). Specifically, an optimal PHF value exists as a trade-off between the benefit of sub-cooling and the losses due to reduced CO2 mass flow rate, the latter of which caused reductions in the convective heat transfer coefficient and the direct gas cooler’s heating capacity. In the meantime, although larger Peltier device sizes improves the system COP, the improvement will converge towards a specific maximum.

Dynamic Simulation Model of Trans-Critical Carbon Dioxide Heat Pump Application for Boosting Low Temperature Distribution Networks in Dwellings

This research investigates the role of new hybrid energy system applications for developing a new plant refurbishment strategy to deploy small scale smart energy systems. This work deals with a dynamic simulation of trans-critical carbon dioxide heat pump application for boosting low temperature distribution networks to share heat for dwellings. Heat pumps provide high temperature heat to use the traditional emission systems. The new plant layout consists of an air source heat pump, four trans-critical carbon dioxide heat pumps (CO2-HPs), photovoltaic arrays, and a combined heat and power (CHP) for both domestic hot water production and electricity to partially drive the heat pumps. Furthermore, electric storage devices adoption has been evaluated. That layout has been compared to the traditional one based on separated generation systems using several energy performance indicators. Additionally, a sensitivity analysis on the primary energy saving, primary fossil energy consumptions, renewable energy fraction and renewable heat, with changes in building power to heat ratios, has been carried out. Obtained results highlighted that using the hybrid system with storage device it is possible to get a saving of 50% approximately. Consequently, CO2-HPs and hybrid systems adoption could be a viable option to achieve Near Zero Energy Building (NZEB) qualification.

Turning industrial aerobic fermentation plants into thermal power stations

The industrial aerobic bioprocess of baker's yeast production requires large amounts of water to cool down large bubble column bioreactors. As result, an appreciable quantity of low-grade heat is generated. Because of the low temperature level of the water (~25°C) exiting the bioreactors cooling system, very little attention has been dedicated to heat recovery and conversion from this stream, which is usually released in rivers, streams, and canals. In this work, we simulated the generation of low-grade heat (up to 14.4 MW) from an industrial baker's yeast production plant consisting of seven 150-m3 bioreactors. Subsequently, a dedicated transcritical carbon dioxide heat pump system for the conversion of this low-grade heat into fourth generation district heat (~16.2 MW) was successfully designed. Fourth generation district heat employs low-temperature water (30-70°C) as heat carrier and is expected to play a major role in future sustainable energy system. Finally, an economic study confirmed the feasibility and the applicability of our approach and a concept for long-term energy storage including state-of-the-art phase change material (PCM)–units was discussed.

Preliminary investigation of a transcritical CO2heat pump driven by a solar-powered CO2Rankine cycle

SUMMARY In this paper, a transcritical carbon dioxide heat pump system driven by solar-owered CO2 Rankine cycle is proposed for simultaneous heating and cooling applications. Based on the first and second laws of thermodynamics, a theoretical analysis on the performance characteristic is carried out for this solar-powered heat pump cycle using CO2 as working fluid. Further, the effects of the governing parameters on the performance such as coefficient of performance (COP) and the system exergy destruction rate are investigated numerically. With the simulation results, it is found that, the cooling COP for the transcritical CO2 heat pump syatem is somewhat above 0.3 and the heating COP is above 0.9. It is also concluded that, the performance of the combined transcritical CO2 heat pump system can be significantly improved based on the optimized governing parameters, such as solar radiation, solar collector efficient area, the heat transfer area and the inlet water temperature of heat exchange components, and the CO2 flow rate of two sub-cycles. Where, the cooling capacity, heating capacity, and exergy destruction rate are found to increase with solar radiation, but the COPs of combined system are decreased with it. Furthermore, in terms of improvement in COPs and reduction in system exergy destruction at the same time, it is more effective to employ a large heat transfer area of heat exchange components in the combined heat pump system. Copyright © 2012 John Wiley & Sons, Ltd.

The Experimental Investigation and Theoretical Analysis on Transcritical Carbon Dioxide Heat Pump Cycle

The (H)CFC-phase out and the fear for future problems for other synthetic working fluids, because of their known and unknown impact on the environment, have introduced a rising interest in environmentally safe natural working fluids. CO2is one of the few non-toxic and non-flammable working fluids that do not contribute to ozone depletion or global warming, if leaked to the atmosphere. Because the critical temperature of CO2is only 31.1°C, the transcritical cycle can be used to improve the coefficient of performance of the system. The experimental investigation and theoretical analysis on transcritical carbon dioxide heat pump system are carried out in this paper. It points out that there is an optimum operational pressure on transcritical carbon dioxide heat pump cycle, when the outlet temperature of gas cooler is constant, the coefficient of performance increases with increasing evaporating temperature at the same conditions, and the operational efficiency increased with decrease of gas cooler exit temperature. So in order to obtain the optimum performance, the influence of evaporating temperature, gas cooler exit temperature, and the operational pressure should be considered during the designing and operating transcritical carbon dioxide heat pump system.

The Experimental Investigation and Theoretical Analysis on Transcritical Carbon Dioxide Heat Pump Cycle

The (H)CFC-phase out and the fear for future problems for other synthetic working fluids, because of their known and unknown impact on the environment, have introduced a rising interest in environmentally safe natural working fluids. CO2 is one of the few non-toxic and non-flammable working fluids that do not contribute to ozone depletion or global warming, if leaked to the atmosphere. Because the critical temperature of CO2 is only 31.1°C, the transcritical cycle can be used to improve the coefficient of performance of the system. The experimental investigation and theoretical analysis on transcritical carbon dioxide heat pump system are carried out in this paper. It points out that there is an optimum operational pressure on transcritical carbon dioxide heat pump cycle, when the outlet temperature of gas cooler is constant, the coefficient of performance increases with increasing evaporating temperature at the same conditions, and the operational efficiency increased with decrease of gas cooler exit temperature. So in order to obtain the optimum performance, the influence of evaporating temperature, gas cooler exit temperature, and the operational pressure should be considered during the designing and operating transcritical carbon dioxide heat pump system.

Exergy assessment of an optimized capillary tube-based transcritical CO2heat 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.

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 CO 2 refrigerant unlike NH 3 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.

Optimization of two-stage transcritical carbon dioxide heat pump cycles

Optimization studies of two-stage transcritical carbon dioxide heat pump cycles, incorporating options such as flash gas bypass, flash intercooling and compressor intercooling, are presented based on cycle simulation. Sub-critical and super-critical thermodynamic and transport properties of carbon dioxide coded and then integrated with the simulation code for further analyses. Results exhibit improvement in performance by adopting optimal operating conditions. The optimum interstage pressure, thus obtained, deviate from the classical estimate of geometric mean of gas cooler and evaporator pressure. It is observed that the flash gas bypass system yields the best performance among the three two stage cycles analyzed. Internal heat exchanger effectiveness and compressor isentropic efficiency shows marginal influence on the system performance. Internal heat exchanger effectiveness shows marginal influence on the system performance while compressor isentropic efficiency shows an about 10% variation in COP. However, optimum gas cooler pressure and optimum intermediate pressure are only marginally affected. Based on the cycle simulations, correlations of optimum gas cooler pressure and inter-stage pressure in terms of gas cooler temperature and evaporator temperature are obtained. This would be useful as a guideline in design of such systems.

Simulation of a transcritical CO 2 heat pump cycle for simultaneous cooling and heating applications

A steady state simulation model has been developed to evaluate the system performance of a transcritical carbon dioxide heat pump for simultaneous heating and cooling. The simulated results are found to be in reasonable agreement with experimental results reported in the literature. Such a system is suitable, for example, in dairy plants where simultaneous cooling at 4 °C and heating at 73 °C are required. The optimal COP was found to be a function of the compressor speed, the coolant inlet temperature to the evaporator and inlet temperature of the fluid to be heated in the gas cooler and compressor discharge pressure. An optimizing study for the best allocation of the fixed total heat exchanger inventory between the evaporator and the gas cooler based on the heat exchanger area has been carried out. Effect of heat transfer in the heat exchangers on system performance has been presented as well. Finally, a novel nomogram has been developed and it is expected to offer useful guidelines for system design and its optimisation.

Optimization of a transcritical CO 2 heat pump cycle for simultaneous cooling and heating applications

Energetic and exergetic analyses as well as optimization studies of a transcritical carbon dioxide heat pump system are presented in this article. A computer code has been developed to obtain sub-critical and super-critical thermodynamic and transport properties of carbon dioxide. Estimates for irreversibilities of individual components of the system lead to possible measures for performance improvement. The COP trends indicate that such a system is more suitable for high heating end temperatures and modest cold end temperatures. Expressions for optimum cycle parameters have been developed and these correlations offer useful guidelines for optimal system design and for selecting appropriate operating conditions.