Compressed air energy storage (CAES) system is an “electricity to electricity” device. To reveal the energy conversion process and understand the energy loss principle are critical to improve the ...energy conversion efficiency. In this paper, the charge/discharge process analysis of an axial turbine based CAES with constant volume is executed in constant turbine inlet pressure mode and variable turbine inlet pressure mode. Firstly, the energy efficiency analysis of a whole round trip cycle of CAES system under design condition is carried out. The second law efficiencies of the operation mode 1 and the operation mode 2 are 44.56% and 45.16%, respectively. It also can be found that the throttling loss in operation mode 1 can be offset by the benefits of higher isentropic efficiency of turbines. Then, the off-design analysis of these two operation modes is implemented by considering the different power levels and speed levels. The mass flow rates of working fluids across the expansion train both increase with the increased load rate or the decreased speed. The load rate or speed both have the positive effect on the exergy efficiency and the energy generated per unit volume of storage, but have the negative effect on the heat rate.
•The modeling of diabatic CAES with axial turbine is proposed.•The energy efficiency analysis of CAES under design condition is carried out.•The off-design analysis of CAES during charging is implemented.•The off-design analysis of CAES in two discharge modes is laid out.
Electricity generated from renewable wind sources is highly erratic due to the intermittent nature of wind. This uncertainty of wind power can lead to challenges regarding power system operation and ...dispatch. Energy storage system in conjunction with wind energy system can offset these effects, making the wind power controllable. Moreover, the power spectrum of wind power exhibits that the fluctuations of wind power include various components with different frequencies and amplitudes. Thus, the hybrid energy storage system is more suitable for smoothing out the wind power fluctuations effectively rather than the independent energy storage system. A hybrid energy storage system consisting of adiabatic compressed air energy storage (A-CAES) system and flywheel energy storage system (FESS) is proposed for wind energy application. The design of the proposed system is laid out firstly. The A-CAES system operates in variable cavern pressure, constant turbine inlet pressure mode, whereas the FESS is controlled by constant power strategy. Then, the off-design analysis of the proposed system is carried out. Meanwhile, a parametric analysis is also performed to investigate the effects of several parameters on the system performance, including the ambient conditions, inlet temperature of compressor, storage cavern temperature, maximum and minimum pressures of storage cavern.
•A wind-hybrid energy storage system composed of A-CAES and FESS is proposed.•The design of the proposed hybrid energy storage system is laid out.•The off-design analysis of the proposed system is carried out.•A parametric analysis is conducted to examine the system performance.
Organic Rankine cycles for low grade waste heat recovery are described with different working fluids. The effects of the thermodynamic parameters on the ORC performance are examined, and the ...thermodynamic parameters of the ORC for each working fluid are optimized with exergy efficiency as an objective function by means of the genetic algorithm. The optimum performance of cycles with different working fluids was compared and analyzed under the same waste heat condition. The results show that the cycles with organic working fluids are much better than the cycle with water in converting low grade waste heat to useful work. The cycle with R236EA has the highest exergy efficiency, and adding an internal heat exchanger into the ORC system could not improve the performance under the given waste heat condition. In addition, for the working fluids with non-positive saturation vapor curve slope, the cycle has the best performance property with saturated vapor at the turbine inlet.
This paper investigates a transcritical CO2 cycle using geothermal resources to generate electricity. Liquefied natural gas (LNG) is employed as heat sink to drop the CO2 turbine back pressure ...sharply. The mathematical model of the transcritical CO2 geothermal power generation system is established for system simulations under steady-state conditions. A parametric analysis is conducted to evaluate the effect of several key thermodynamic parameters on system performance. Additionally, a multi-objective optimization using NSGA-II method is carried out to find the optimum performance of system from both thermodynamic and economic aspects. The results show that there is an optimal CO2 turbine inlet pressure that yields the maximum exergy efficiency. A higher CO2 turbine inlet temperature or a lower CO2 turbine back pressure brings about a higher exergy efficiency. In addition, an optimal CO2 turbine inlet pressure obtains the minimum required heat exchange area per net power output. The lower the CO2 turbine inlet temperature or the CO2 turbine back pressure is, the smaller the required heat exchange area per net power output is. By the multi-objective optimization, a Pareto optimal solution is obtained, which shows that an increase in exergy efficiency would increase the required heat exchange area per net power output.
•LNG is employed as heat sink of a transcritical CO2 power cycle.•The effects of several parameters on system performance are examined.•A multi-objective parametric optimization is conducted by NSGA-II.
•Solar-powered organic Rankine cycle with CPC and thermal storage unit is studied.•Off-design performances encountering the changes of key parameters are examined.•Off-design performance is analyzed ...over a whole day and in different months.
Performance evaluation of a thermodynamic system under off-design conditions is very important for reliable and cost-effective operation. In this study, an off-design model of an organic Rankine cycle driven by solar energy is established with compound parabolic collector (CPC) to collect the solar radiation and thermal storage unit to achieve the continuous operation of the overall system. The system off-design behavior is examined under the change in environment temperature, as well as thermal oil mass flow rates of vapor generator and CPC. In addition, the off-design performance of the system is analyzed over a whole day and in different months. The results indicate that a decrease in environment temperature, or the increases in thermal oil mass flow rates of vapor generator and CPC could improve the off-design performance. The system obtains the maximum average exergy efficiency in December and the maximum net power output in June or in September. Both the net power output and the average exergy efficiency reach minimum values in August.
Plant functional traits are a representation of plant resource utilization strategies. Plants with higher specific leaf area (SLA) and lower leaf dry matter content (LDMC) exhibit faster ...investment-return resource utilization strategies. However, the distribution patterns and driving factors of plant resource utilization strategies at the macroscale are rarely studied. We investigated the relative importance of climatic and soil factors in shaping plant resource utilization strategies at different life forms in forests using data collected from 926 plots across 163 forests in China. SLA and LDMC of plants at different life forms (i.e., trees, shrubs, and herbs) differ significantly. Resource utilization strategies show significant geographical differences, with vegetation in the western arid regions adopting a slower investment-return survival strategy and vegetation in warmer and wetter areas adopting a faster investment-return survival strategy. SLA decreases significantly with increased temperature and reduced rainfall, and vegetation growing in these conditions exhibits conservative resource utilization. Mean annual precipitation (MAP) is a key climatic factor that controls the resource utilization strategies of plants at the macroscale. Plants use resources more conservatively as soil pH increases. The influence of climate and soil factors is coupled to determine the resource utilization strategies of plants occupying different life forms at the macroscale, but the relative contribution of each varies across life forms. Our findings provide a theoretical framework for understanding the potential impact of increasing global temperatures on plant resource utilization.
This paper presents a regenerative organic Rankine cycle (ORC) to utilize the solar energy over a low temperature range. Flat-plate solar collectors are used to collect the solar radiation for their ...low costs. A thermal storage system is employed to store the collected solar energy and provide continuous power output when solar radiation is insufficient. A daily average efficiency is defined to evaluate the system performance exactly instead of instantaneous efficiency. By establishing mathematical models to simulate the system under steady-state conditions, parametric analysis is conducted to examine the effects of some thermodynamic parameters on the system performance using different working fluids. The system is also optimized with the daily average efficiency as its objective function by means of genetic algorithm under the given conditions. The results indicate that under the actual constraints, increasing turbine inlet pressure and temperature or lowering the turbine back pressure could improve the system performance. The parametric optimization also implies that a higher turbine inlet temperature with saturated vapor state could obtain the better system performance. Compared with other working fluids, R245fa and R123 are the most suitable working fluids for the system due to their high system performance and low operation pressure.
► A solar-driven regenerative ORC using flat-plate solar collectors is simulated. ► Sensitivity analysis of parameters and parameter optimization are conducted. ► The daily average efficiency is defined to evaluate the system performance.
This paper proposes a transcritical CO₂ power cycle driven by solar energy while utilizing the cold heat rejection to an liquified natural gas (LNG) evaporation system. 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 to provide stable power output when solar radiation is insufficient. A mathematical model is developed to simulate the solar-driven transcritical CO₂ power cycle under steady-state conditions, and a modified system efficiency is defined to better evaluate the cycle performance over a period of time. The thermodynamic analysis focuses on the effects of some key parameters, including the turbine inlet pressure, the turbine inlet temperature and the condensation temperature, on the system performance. Results indicate that the net power output mainly depends on the solar radiation over a day, yet the system is still capable of generating electricity long after sunset by virtue of the thermal storage tank. An optimum turbine inlet pressure exists under given conditions where the net power output and the system efficiency both reach maximum values. The net power output and the system efficiency are less sensitive to the change in the turbine inlet temperature, but the condensation temperature exerts a significant influence on the system performance. The surface area of heat exchangers increases with the rise in the turbine inlet temperature, while changes in the turbine inlet pressure have no significant impact on the heat exchanging area under the given conditions.
Using fuel cell systems for distributed generation (DG) applications represents a meaningful candidate to conventional plants due to their high power density and the heat recovery potential during ...the electrochemical reaction. A hybrid power system consisting of a proton exchange membrane (PEM) fuel cell stack and an organic Rankine cycle (ORC) is proposed to utilize the waste heat generated from PEM fuel cell. The system performance is evaluated by the steady-state mathematical models and thermodynamic laws. Meanwhile, a parametric analysis is also carried out to investigate the effects of some key parameters on the system performance, including the fuel flow rate, PEM fuel cell operating pressure, turbine inlet pressure and turbine backpressure. Results show that the electrical efficiency of the hybrid system combined by PEM fuel cell stack and ORC can be improved by about 5% compared to that of the single PEM fuel cell stack without ORC, and it is also indicated that the high fuel flow rate can reduce the PEM fuel cell electrical efficiency and overall electrical efficiency. Moreover, with an increased fuel cell operating pressure, both PEM fuel cell electrical efficiency and overall electrical efficiency firstly increase, and then decrease. Turbine inlet pressure and backpressure also have effects on the performance of the hybrid power system.
► A hybrid power system composed of a PEMFC stack and an ORC is proposed. ► A parametric analysis is conducted to examine the hybrid system performance. ► The results show such a hybrid power system increases the maximum power output effectively.
To recover the waste heat from solid oxide fuel cell (SOFC) and improve the overall electrical efficiency, a new integrated power system driven by SOFC is proposed to achieve the cascade energy ...utilization. This system integrates an SOFC–GT system with an organic Rankine cycle (ORC) using liquefied natural gas (LNG) as heat sink to recover the cryogenic energy of LNG. Based on the mathematical model, a parametric analysis is conducted to examine the effects of some key thermodynamic parameters on the system performance. The results indicate that the overall electrical efficiency of 67% can be easily achieved for the current system, which can be further improved with parametric optimization. An increase in fuel flow rate of SOFC can raise the net power output, but it has a negative effect on SOFC and overall electrical efficiency. The compressor pressure ratio contributes to an increase in SOFC and overall electrical efficiency, which are contrary to the effects of air flow rate and steam-to-carbon ratio. Under the given conditions, compared with the Kalina sub-system, the ORC sub-system produces 12.6% more power output by utilizing the cryogenic energy of LNG with simple configuration.
► An SOFC–GT–ORC integrated power system with LNG as heat sink is proposed. ► LNG is used as fuel for the SOFC and its cryogenic energy is recovered. ► Recovering the waste heat of SOFC achieves the cascade energy utilization.
