Preliminary investigation of a transcritical CO2 heat pump driven by a solar‐powered CO2 Rankine cycle

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.     

Exergy analysis and optimization of a hydrogen production process by a solar-liquefied natural gas hybrid driven transcritical CO2 power cycle

A solar transcritical CO₂ power cycle for hydrogen production is studied in this paper. Liquefied Natural Gas (LNG) is utilized to condense the CO₂. An exergy analysis of the whole process is performed to evaluate the effects of the key parameters, including the boiler inlet temperature, the turbine inlet temperature, the turbine inlet pressure and the condensation temperature, on the system power outputs and to guide the exergy efficiency improvement. In addition, parameter optimization is conducted via Particle Swarm Optimization to maximize the exergy efficiency of hydrogen production. The exergy analysis indicates that both the solar and LNG equally provide exergy to the CO₂ power system. The largest amount of exergy losses occurs in the solar collector and the condenser due to the great temperature differences during the heat transfer process. The exergy loss in condenser could be greatly reduced by increasing the LNG temperature at the inlet of the condenser. There exists an optimum turbine inlet pressure for achieving the maximum exergy efficiency. With the optimized turbine inlet pressure and other parameters, the system is able to provide 11.52 kW of cold exergy and 2.1 L/s of hydrogen. And the exergy efficiency of hydrogen production could reach 12.38%.     

Thermodynamic evaluation of solar integration into a natural gas combined cycle power plant

The term integrated solar combined-cycle (ISCC) has been used to define the combination of solar thermal energy into a natural gas combined-cycle (NGCC) power plant. Based on a detailed thermodynamic cycle model for a reference ISCC plant, the impact of solar addition is thoroughly evaluated for a wide range of input parameters such as solar thermal input and ambient temperature. It is shown that solar hybridization into an NGCC plant may give rise to a substantial benefit from a thermodynamic point of view. The work here also indicates that a significant solar contribution may be achieved in an ISCC plant, thus implying substantial fuel savings and environmental benefits.     

Thermodynamic analysis of the CO2‐based Rankine cycle powered by solar energy

Using carbon dioxide as working fluid receives increasing interest since the Kyoto Protocol. In this paper, thermodynamic analysis was conducted for proposed CO₂‐based Rankine cycle powered by solar energy. It can be used to provide power output, refrigeration and hot water. Carbon dioxide is used as working fluid with supercritical state in solar collector. Theoretical analysis was carried out to investigate performances of the CO₂‐based Rankine cycle. The interest was focused on comparison of the performance with that of solar cell and those when using other fluids as working fluids. In addition, the performance and characteristics of the thermodynamic cycle are studied for different seasons. The obtained results show that using CO₂ as working fluid in the Rankine cycle owns maximal thermal efficiency when the working temperature is lower than 250.0°C. The power generation efficiency is about 8%, which is comparable with that of solar cells. But in addition to power generation, the CO₂‐based solar utilization system can also supply thermal energy. Copyright © 2007 John Wiley & Sons, Ltd.     

A combined heat and power system for buildings driven by solar energy and gas

A novel hybrid solar/gas system intended to provide cooling/heating and electricity generation for buildings was developed. The system is based on the combination of an ejector heat pump cycle with a Rankine cycle. It is driven by solar energy and supplemented by a gas burner. The system also uses an environmentally friendly refrigerant to have minimal impact on the environment. Results of system computer modelling, prototype tests and economic analysis are reported. The system was judged to be viable and reliable. Technical improvements still have to be achieved to improve system economics.     

Parametric analysis and optimization of a Kalina cycle driven by solar energy

A solar-driven Kalina cycle is examined to utilize solar energy effectively due to using ammonia–water's varied temperature vaporizing characteristic. In order to ensure a continuous and stable operation for the system, a thermal storage system is introduced to store the collected solar energy and provide stable power when solar radiation is insufficient. A mathematical model is developed to simulate the solar-driven Kalina cycle under steady-state conditions, and a modified system efficiency is defined to evaluate the system performance over a period of time. A parametric analysis is conducted to examine the effects of some key thermodynamic parameters on the system performance. The solar-driven Kalina cycle is also optimized with the modified system efficiency as an objective function by means of genetic algorithm under the given conditions. Results indicate that there exists an optimal turbine inlet pressure under given conditions to maximize the net power output and the modified system efficiency. The net power output and the modified system efficiency are less sensitive to a change in the turbine inlet temperature. An optimal basic solution ammonia fraction can be identified that yields maximum net power output and modified system efficiency. The optimized modified system efficiency is 8.54% under the given conditions.     

太阳能辅助二氧化碳热泵热水系统实验研究

利用实验的方法,研究了太阳辐照度、外界气温和风速、初始水温、蒸发器出口温度和压力等对太阳能辅助二氧化碳热泵热水系统运行状况和COP的影响。实验结果表明,系统COP随初始水温的升高而增大;太阳辐照度、外界温度和风速对热泵系统性能的影响主要体现在对系统循环水温的影响;在一定范围内,蒸发压力和蒸发温度越高,热泵系统的COP越大。     

烟气余热驱动复叠式非共沸工质有机朗肯循环系统热力学分析

文章构建了复叠式非共沸工质有机朗肯循环系统模型,并利用该模型对复叠式非共沸工质有机朗肯循环系统的热力学性能进行分析,得到了高温级循环质量流量、低温级循环质量流量、冷却水质量流量、高温级循环净输出功率、低温级循环净输出功率、冷却水泵功耗和系统净输出功率等随工质摩尔组分的变化规律。分析结果表明,高温级循环蒸发泡点温度和高温级蒸发器夹点位置会影响复叠式非共沸工质有机朗肯循环各项性能参数随工质摩尔组分的变化趋势,当高温级循环混合物中环戊烷的摩尔组分为0.8,低温级循环混合物中异丁烷摩尔的组分为0.1时,复叠式非共沸工质有机朗肯循环系统的净输出功率达到最大值,为92.79 kW,比复叠式纯工质有机朗肯循环系统提高了3.83%。     

Second law analysis of two‐stage compression transcritical CO2 heat pump cycle

Because of the global warming impact of hydro fluorocarbons, the uses of natural refrigerants in automotive and HVAC industries have received worldwide attention. CO₂ is the most promising refrigerant in these industries, especially the transcritical CO₂ refrigeration cycle. The objective of this work is to identify the main factors that affect two‐stage compression transcritical CO₂ system efficiency. A second law of thermodynamic analysis on the entire two‐stage CO₂ cycle is conducted so that the exergy destruction of each system component can be deduced and ranked, allowing future efforts to focus on improving the components that have the highest potential for advancement. The inter‐stage pressure is used as a variable parameter in the analysis study. The second law efficiency, coefficient of cooling performance and total exergy destruction of the system variations with the inter‐stage pressure are presented graphically. It was concluded that there is an optimum inter‐stage pressure that maximizes both first law and second law efficiencies. Copyright © 2008 John Wiley & Sons, Ltd.     

The performance analysis of a two‐stage transcritical CO2 cooling cycle with various gas cooler pressures

A theoretical analysis of a two‐stage transcritical CO₂ cooling cycle is presented. The effect of a two‐stage cycle with intercooling process on the system coefficient of cooling performance is presented for various gas cooler pressures. However, the performance comparison between one‐stage and two‐stage cycles is presented for same operating conditions. Gas cooler pressure, compressor isentropic efficiency, gas cooler efficiency, intercooling quantity and refrigerant outlet temperature from the gas cooler are used as variable parameters in the analysis. It is concluded that the performance of the two‐stage transcritical CO₂ cycle is approximately 30% higher than that of the one‐stage transcritical CO₂ cycle. Hence, the two‐stage compression and intercooling processes can be assumed as valuable applications to improve the transcritical CO₂ cycle performance. Copyright © 2008 John Wiley & Sons, Ltd.     

Exergetic analysis of solar concentrator aided natural gas fired combined cycle power plant

This article deals with comparative energy and exergetic analysis for evaluation of natural gas fired combined cycle power plant and solar concentrator aided (feed water heating and low pressure steam generation options) natural gas fired combined cycle power plant. Heat Transfer analysis of Linear Fresnel reflecting solar concentrator (LFRSC) is used to predict the effect of focal distance and width of reflector upon the reflecting surface area. Performance analysis of LFRSC with energetic and exergetic methods and the effect, of concentration ratio and inlet temperature of the fluid is carried out to determine, overall heat loss coefficient of the circular evacuated tube absorber at different receiver temperatures. An instantaneous increase in power generation capacity of about 10% is observed by substituting solar thermal energy for feed water heater and low pressure steam generation. It is observed that the utilization of solar energy for feed water heating and low pressure steam generation is more effective based on exergetic analysis rather than energetic analysis. Furthermore, for a solar aided feed water heating and low pressure steam generation, it is found that the land area requirement is 7 ha/MW for large scale solar thermal storage system to run the plant for 24 h.     

Simulation of the optimal heat rejection pressure for transcritical CO2 expander cycle

In order to optimize and control transcritical CO₂ refrigeration cycle, a mathematical model was developed to simulate the system performance. The simulation results show that a maximum COP exists at the optimal heat rejection pressure not only for throttle valve cycle but also for expander cycle. Also, the optimal heat rejection pressures of the throttle valve cycle are greater than those of the expander cycle under the same condition. In order to further obtain correlation of the optimal heat rejection pressure for transcritical CO₂ expander cycle, it is necessary to analyze the impact degree of compressor efficiency, expander efficiency, gas cooler outlet temperature and evaporation temperature. Based on the simulation results, the values of the optimal heat rejection pressure for the expander cycle were regressed in terms of gas cooler outlet temperature and evaporation temperature at given compressor efficiency and expander efficiency. Finally, two types of polynomial correlations were obtained. One is cubic form, with an average deviation of less than 0.5% and the other is simplified form, with an average deviation of less than 1%. It is, therefore, convenient to use either correlation to simulate the performance of transcritical CO₂ expander cycle.     

Low temperature heat source for power generation: Exhaustive analysis of a carbon dioxide transcritical power cycle

The main results of a theoretical work on the use of a low temperature heat source for power generation through a carbon dioxide transcritical power cycle are reported in this paper. The procedure for analyzing the behaviour of the proposed cycle consisted in modifying the input pressure to the turbine from 66 bar, maintained constant each evaluated temperature (60 °C, 90 °C, 120 °C and 150 °C) until the net work was approximately zero. As a result, the maximum exergy efficiency was 50%, while the energy efficiencies obtained were 9.8%, 7.3%, 4.9% and 2.4% and the net specific work was 18.2 kJ/kg, 12.8 kJ/kg, 7.8 kJ/kg and 3.5 kJ/kg, respectively. Furthermore, the effect of the addition of an internal heat exchanger, which obviously supposed an increase in the efficiency, was analyzed. The analysis of the proposed system shows the viability of implementing this type of process as an energy alternative and/or strengthener of non-conventional energy sources in non-provided zones, or for increasing the energy efficiency in the industry.     

Development of a double acting free piston expander for power recovery in transcritical CO2 cycle

To replace the throttling valve with an expander is considered as an efficient method to improve the performance of the transcritical CO₂ refrigeration cycle. This paper presents the design and experimental validation of a double acting free piston expander, in which a slider-based inlet/outlet control scheme is used to realize a full expansion process for the expander. The power extracted from the expansion process is utilized by an auxiliary compressor, which is arranged in parallel with the main compressor. A design model is developed to determine the geometric parameters of the expander together with the auxiliary compressor. An expander prototype is manufactured and validated experimentally in the air test system, mainly by means of analyzing the dynamic pressures in the expander chamber. The experimental results show that the expander can work stably in a wide range of pressure differences/ratios at the frequency approximately linear with the pressure difference through the expander. The p–t diagrams in the expander indicate that the slider-based inlet/outlet control scheme enables the expander to have the proper suction, expansion and discharge processes. However, the prototype at high frequency doesn’t present isobaric suction process, which results in insufficient gas suction and therefore decrease in the expander efficiency. With the p–t diagrams at various frequencies compared, the optimal working frequency is found to range from 10 to 17 Hz in the air system. The isentropic efficiency of 62% is obtained from the p–V diagram analysis. Further validation of the expander in the CO₂ system will be conducted in the near future.     

Methodology for the energy analysis of an air cooled GAX absorption heat pump operated by natural gas and solar energy

In this work a methodological analysis and energy evaluation of an air cooled absorption system, with generator–absorber heat exchange (GAX), and operated by a hybrid natural gas–solar energy source is presented. Given the characteristic non-linearity of the resulting system of equations, the methodology proposed envisages a calculation sequence for the external currents and an iterative procedure for the internal currents.The system studied intents to be an alternative for space conditioning in the residential sector, the unit was designed with a capacity of 10.6 kW (3 Ton) of cooling and uses as working fluid ammonia–water. Giving priority to internal energy integration, an arrangement is proposed for the GAX cycle that allows 19% of solar contribution at full load, being greater at partial loads. In spite of using ambient air up to 40 °C with a relative humidity of 24% as cooling source, a COP value of 0.86 for cooling and 1.86 for heating was calculated, with an internal energy integration of 16.9 kW, 37% more than the energy that is supplied to the generator.     

Thermoeconomic analysis of a solar driven heat engine

Thermoeconomic optimization has been carried out for an endoreversible solar driven heat engine using finite-time/finite-size thermodynamic theory. In the considered heat engine model, heat transfer from the hot reservoir is assumed to be radiation mode and the heat transfer to the cold reservoir is assumed to be convection mode. The power output per unit total cost is taken as objective function and the optimum performance and design parameters have been investigated. The effects of the technical and economical parameters on the thermoeconomic performances have been also discussed.     

Performance optimization of transcritical CO2 cycle with parallel compression economization

Being a low critical temperature fluid, CO₂ transcritical system offers low COP for a given application. Parallel compression economization is one of the techniques to improve the COP for transcritical CO₂ cycle. An optimization study of transcritical CO₂ refrigeration cycle with parallel compression economization is presented in this paper. Further, performance comparisons of three different COP improvement techniques; parallel compression economization alone, parallel compression economization with recooler and multistage compression with flash gas bypass are also presented for chosen operating conditions. Results show that the parallel compression economization is more effective at lower evaporator temperature. The expression for optimum discharge pressure has been developed which offers useful guideline for optimal system design and operation. Study shows that the parallel compression with economizer is promising transcritical CO₂ cycle modifications over other studied cycle configurations. A maximum improvement of 47.3% in optimum COP is observed by employing parallel compression economization for the studied ranges.     

Performance optimization of transcritical CO2 refrigeration cycle with thermoelectric subcooler

Use of thermoelectric subcooler is one of the techniques to improve the performance of transcritical CO₂ cycle. Thermodynamic analyses and optimizations of transcritical CO₂ refrigeration cycle with thermoelectric subcooler are presented in this paper. Further, the effects of various operating parameters on cycle performances are studied. It is possible to optimize current supply, discharge pressure, and CO₂ subcooling simultaneously based on maximum cooling COP for thermoelectrically enhanced transcritical CO₂ refrigeration cycle to get best performance. Results show that thermoelectric current supply, COP improvement, and discharge pressure reduction increase with increase in cycle temperature lift, with maximum values of 11 A, 25.6%, and 15.4%, respectively, for studied ranges. Use of thermoelectric subcooler in CO₂ refrigeration system not only improves the cooling COP, also reduces the system high‐side pressure, compressor pressure ratio, and compressor discharge temperature, and enhances the volumetric cooling capacity. Component‐wise irreversibility distribution shows similar trend with basic CO₂ cycle, although values are lower leading to higher second law efficiency. Cooling capacity may be enhanced by increasing the current supply for the same thermoelectric configuration with penalty of COP. Study reveals that thermoelectrically enhanced CO₂ refrigeration cycle yields significant performance improvement especially for higher‐cycle temperature lift. Copyright © 2011 John Wiley & Sons, Ltd.