• Energy Research

Abstract A great amount of researches on thermodynamic cycles have been active in recent years, such as ORC (organic Rankine cycle), Kalina cycle, et al. However, the ultimate aim of such researches, which could even be traced back to more than one century ago, has not changed with a tireless pursuing to Carnot cycle. In exiting researches, the working fluid, as a medium for energy conversion, is commonly considered to play an important role in the thermodynamic cycle: (1) relative to ideal cycle, most of actual power cycles in the engineering field cannot operate without working fluid; (2) energy efficiency, considering the analysis of second-law efficiency, of actual cycle has a significant decrease due to the introduction of working fluid. Thus, working fluid is a hot spot in the research of thermodynamic cycle in recent years. Zeotropic mixture, which commonly consists of two or more pure working fluids, has flexibility in thermos-physical properties with a possible potential to enhance the cycle performance. The effects of thermos-physical properties of zeotropic mixture should be considered when determining the cycle structure and the design of components. This paper presents a novel construction method of thermodynamic cycle based on the zeotropic mixture. By adding the thermodynamic coordinate of working fluid, a 3D cycle diagram based on the traditional temperature and entropy cycle diagram is applied for performance analysis of cycle. According the proposed construction method, a baseline cycle, composed by ORC sub-system and compositions regulating sub-system, is put forward and available compositions regulating techniques for such cycle are discussed as well. Finally, a representative case is described briefly and the features are summarized. This work provides a new methodology view to guide researchers in energy-efficient design of thermodynamic cycles.

Abstract Solar heating, as an alternative technology to coal-fired heating, could significantly alleviate the air pollution in China. The value of design radiation should be carefully determined when designing a solar system, as it would commonly affect the solar collector area, and then the economics and performance of the system. However, in the existing publications, most studies ignore the influence of the daily and seasonal variation of radiation, and commonly choose the fixed value of design radiation in some typical days, leading to a non-optimal performance of the system with unreasonable setting parameters. To solve this problem in the design stage, a method is proposed to determine the design radiation of a parabolic trough collector (PTC) heating system. In this method, the concept of non-guaranteed days is introduced to quickly and conveniently determine the system rated capacity. Additionally, the cost of unit heating supply is employed in this method as an optimized object of the heating system. Based on the proposed method, a case study for four typical solar radiation districts in China is conducted. Firstly, the design radiation values of each district are decided through the proposed method. Then, the relationship between non-guaranteed days and the initial investment of the system under the optimal design radiation is obtained. Finally, the economic analysis of the PTC system in Mentougou (suburbs of Beijing) is performed. This work could be a practical guide to select the design radiation for a PTC heating system without the auxiliary boiler. Moreover, the method could also be applied to more types of solar systems and regions.

Abstract The application of T-junction has already been extended from the petroleum engineering to the emerging advanced thermodynamic cycles. How a T-junction, as a separator in an emerging thermodynamic cycle, affects the cycle efficiency is discussed in this paper. Firstly, based on the Eulerian method, parametric studies on the local pressure drop coefficients are conducted and two new local pressure drop (LPD) coefficients are predicted in this paper. Then, the effects of pressure drop on a composition adjustable organic Rankine cycle efficiency is analyzed. In the process, a composition adjustable organic Rankine cycle with zeotropic mixtures R245fa/R123 is chosen to calculate the effects of pressure drop on thermodynamic cycle efficiency. The quality of the outlet of the evaporator is assumed as 0.7, and the composition ratio as 0.6/0.4. The results show that the pressure variation of T-junction affects the adjustable organic Rankine cycle efficiency, and reasonable phase separation ratio can improve the thermodynamic cycle efficiency. The pressure drop could cause composition diffusion. In addition, two new local pressure drop coefficients are obtained. And the results demonstrate that K12J declines slowly with the increases of density ratio (ρ2/ρ1) and decreases with the increases of mass fraction of taken off (F). K13J shows an obvious trend with diameter ratio (d3/d1) and F. This work could push the research of T-junction as a separator in the adjustable organic Rankine cycle.

Abstract As one of the most promising methods to convert medium- and low-temperature heat into power, organic Rankine cycle (ORC) has been widely studied. Working fluid, which plays the most important role in ORC, is the root of the huge gap on energy-efficiency between the actual cycle and ideal cycle. This paper presents the limiting thermal efficiency and limiting thermodynamics perfection of simple organic Rankine cycle (S-ORC) and regenerative organic Rankine cycle (R-ORC) in subcritical region to quantitatively describe the role of the pure working fluids. The expressions of limiting thermal efficiency and limiting thermodynamics perfection of S-ORC and R-ORC are derived respectively. 20 working fluids are employed in S-ORC and 10 working fluids are employed in R-ORC to demonstrate the effects of working fluids and operating conditions on limiting thermal efficiency and limiting thermodynamics perfection. The limiting thermal efficiency of S-ORC increases with the increase of the slope of working fluid saturated liquid line and latent heat of vaporization. The limiting thermal efficiency of R-ORC increases with the increase of the slope of working fluid saturated liquid line and latent heat of vaporization and the decrease of the slope of working fluid saturated gas line and specific heat capacity of superheat gas at constant pressure. According to the results of limiting thermal efficiency, the maps for S-ORC and R-ORC which might guide the selection of working fluids for different operating temperature are provided as well.

Abstract This paper studied two different types of Organic Rankine cycle (ORC) driven by waste heat from diesel engine and mainly concentrates on the performances under different working conditions. The transient responses of ORC when alternating the condition of engine is also investigated. Compared to basic ORC, it shows the alternation of engine poses smaller impact on ORC system with auxiliary oil cycle (OS/ORC) because of the big thermal inertia of this system, but the time it takes to return to steady state gets longer. On the other hand, the response of ORC system is quick and the large variation amplitude of system parameters can cause the system to an undesirable working condition, damaging the key components. ORC system can generates more power which is 22.23kW than the OS/ORC which is 18.89 kW, but the ORC system gets affected more by the impact of disturbance, as the output power is decreases by 48.27% when engine condition changed while that for the OS/ORC is just decreases by 16.62%.

Abstract Organic Rankine cycle (ORC) system has been widely used in waste heat recovery (WHR) from exhaust gas of internal combustion engine (ICE) to improve efficiency. Due to the real road condition, the heat source of ORC system is always fluctuant and volatile for WHR in automotive internal combustion engine (A-ICE). Thus, two types of ORC system are analyzed to explore the dynamic performance in this paper based on the dynamic models programmed in Dymola. The dynamic behavior is analyzed when ICE operating condition changes and when the changed operating condition gets back to the original state after a period of time. The results show that with fluctuant heat source, the Oil storage/Organic Rankine Cycle (OS/ORC) system has a stronger resistance to the change of operating conditions as well as a better heat recovery performance, and the rate of decline of system power output is lower; but it's apt to occur low temperature corrosion. In addition, with consideration of PID control strategy, evaporation pressure drops 1.43 bar and 2.57 bar for the ORC and OS/ORC system rather than 8.69 bar and 11.54 bar, and the degree of superheat can be prevented from declining to zero, ensuring a safety operation of system.

Abstract Solar radiation data is critical to the design and operation of solar energy utilization systems, so a large number of models have been proposed and developed to estimate solar radiation in the past ten years. However, the performances of such models are controversial in different studies, and there is a lack of systematic comparison among them. In addition, few studies pay attention to the time scales and practicability of the models. This paper focuses on solving these questions through a critical literature review and the authors believe it can benefit researchers to perform further investigations about solar radiation. This paper reviews and compares the models from the points of view of time scale and estimation type for the first time. Furthermore, a large amount of data about the evaluation metrics (root mean square error and mean absolute percentage error) from different studies is summarized to clarify the performances of proposed models. The questions arising from the processing of source data are also carefully examined. This paper has presented a novel method to compare the estimation models and has provided a detailed analysis on available models. The results indicate that the sunshine duration fraction models and artificial neural networks have similar performances when used to estimate monthly average daily global radiation and daily global radiation, while more work is needed to study the estimation method on smaller time intervals and the mechanisms of atmospheric attenuation for solar radiation.

Abstract An obvious knowledge gap exists in the design of ORC-based solar-CCHP, due to an abuse and indiscreet application of the existing research methodology specifically developed for solar-ORC. Such extended research methodology, which emphasizes on the promotion of power generation performance, does not provide a reasonable design result, because of a significant application difference between the ORC-based solar-CCHP and solar-ORC. Although the two systems have some similar technologies elements, a bifurcation point on design has not been clarified which lead to the lack of clear methodology framework or profile in this research field. To fill the gap and clarify the bifurcation point, this paper focuses on the ORC-based solar-CCHP system with the energy cascade utilization principle. Considered the medium-temperature working fluids, including Benzene, Toluene, Decane, D4, MM, the whole system model is established which consists of a parabolic trough collector, ORC, heat exchanger for heating system, and a single-effect absorption chiller. An assessment framework for the system energy efficiency is proposed. And then a comparison study between the solar-ORC and ORC-based solar-CCHP is conducted. The results show that the configuration characteristics have an important effect on the system energy efficiency for the CCHP system, which should be taken into account prior to the performance of the solar-ORC system. Specifically, for the solar-ORC system, the system energy efficiency increases as the increasing of the evaporating temperature. However, for the ORC-based solar-CCHP system, the behaviors of the system thermal performance, such as the overall efficiency of solar energy are totally changed compared with the solar-ORC system. Consequently, the optimal configuration parameters should be a tradeoff between the thermal performance, size and initial investment cost. Finally, the optimum configuration design for the ORC-based solar-CCHP system with MM is achieved, where the overall efficiency of solar energy is 40.95%, with the cooling-to-power ratio of 4.95 and heat-to-power ratio of 4.2.

Abstract Organic Rankine cycle (ORC) is one of the most promising methods for converting medium-low temperature thermal energy into power. Many studies have been performed to optimize the operating conditions, select proper working fluids, design efficient expansion machines, etc. However, as the major power-consuming component in ORC system, the circulating pump is rarely studied. Most of theoretical studies only assume a constant value of isentropic efficiency for the compression process, but this value varies with different studies. A small-scale ORC system is built in the present work to test the performance of a diaphragm pump, and the working fluids of R245fa, R123, R152a and R600a are tested under various conditions. The isentropic efficiencies of diaphragm pump for compressing those working fluids are between 57.22% and 93.51%. The results show that volume flow rate and pressure difference have great influence on isentropic efficiency, and the isentropic efficiency increases with the volume flow rate and pressure difference. Under the condition of same volume flow rate and pressure difference, the isentropic efficiency conforms to the order of η R245fa > η R123 > η R600a > η R152a . The analyses about the variation trend of isentropic efficiency are made from the perspectives of physical properties of working fluid and the driving mode of diaphragm pump, which prove that the hitting velocity of diaphragm to the liquid is the main cause for irreversible enthalpy increment. A novel parameter α V / ρc p is proposed to identify the influence of physical properties of working fluid on the performance of pump. The experimental results show that under same volume flow rate and pressure difference, the isentropic efficiency of pump decreases with the increment of α V / ρc p of different working fluids.