Energy and exergy analysis of the transient performance of a qanat-source heat pump using TRNSYS-MATLAB co-simulator

In this study, the transient performance of a qanat source heat pump is investigated using a TRNSYS-MATLAB co-simulator. The water/ethylene glycol-to-air compression heat pump and the helical coil heat exchanger, which is used to inject heat to or to extract heat from the qanat water, are mathematically modeled in matrix laboratory (MATLAB), and then, coupled to transient systems simulation (TRNSYS) model to evaluate the system transient performance and calculate the heating and cooling loads of the case study building. Comparison of the performance of the qanat source heat pump with an air source heat pump showed that the coefficient of performance of the qanat source heat pump is at least 5% and at most 34% higher than that of the air source heat pump. By increasing the flow rate of the working fluid in the helical coil heat exchanger from 2 L/min to 8 L/min, the coefficient of performance of the qanat source heat pump increases at least 12% and at most 34.1%. The maximum increase in energy efficiency ratio and free energy ratio of the system by the similar increase in the flow rate is 46.4% and 24.8%, respectively. The exergy analysis of the qanat source heat pump reveals that the minimum and maximum exergy efficiency of the system is 32% and 85.5%, respectively. The findings also indicate that the most exergy destruction occurs in the condenser in heating mode and in the evaporator in cooling mode.

The influence of 3-DOF film thickness disturbance on transient performance of typical spiral groove dry gas seals

Purpose The purpose of this paper is to investigate the influence rule and mechanism of three degrees of freedom film thickness disturbance on the transient performance of spiral groove, upstream pumping spiral groove dry gas seal (UP-SDGS) and double-row spiral groove dry gas seal (DR-SDGS). Design/methodology/approach The transient performance of spiral groove, UP-SDGS and DR-SDGS are obtained by solving the transient Reynolds equation under different axial and angular disturbance coefficients. The transient and steady performance of the above-mentioned DGSs are compared and analyzed. Findings The film thickness disturbance has a remarkable impact on the sealing performance of DGS with different structures and the calculation deviations of the leakage rate of the UP-DGS will increase significantly if the film thickness disturbance is ignored. The axial and angular disturbance jointly affect the film thickness distribution of DGS, but there is no significant interaction between them on the transient sealing performance. Originality/value The influence mechanism of axial disturbance and angular disturbance on the transient performance of typical SDGSs behavior has been explained by theory. Considering small and large disturbance, the interaction between axial disturbance and angular disturbance on the transient performance have been studied.

An improved 2-level MPPT scheme for photovoltaic systems using a novel high-frequency learning based adjustable gain-MRAC controller

Under rapidly changing environmental conditions, the model reference adaptive control (MRAC) based MPPT schemes need high adaptation gain to achieve fast convergence and guaranteed transient performance. The high adaptation gain causes high-frequency oscillations in the control signals resulting in numerical instability and inefficient operation. This paper proposes a novel high-frequency learning-based adjustable gain MRAC (HFLAG-MRAC) for a 2-level MPPT control architecture in photovoltaic (PV) systems to ensure maximum power delivery to the load under rapidly changing environmental conditions. In the proposed 2-level MPPT control architecture, the first level is the conventional ripple correlation control (RCC) that yields a steady-state ripple-free optimum duty cycle. The duty cycle obtained from the first level serves as an input to the proposed HFLAG-MRAC in the second level. In the proposed adaptive law, the adaptation gain varies as a function of the high-frequency ripple content of the tracking error. These high-frequency contents are the difference between the tracking error and its low-pass filtered version representing the fluctuations in output due to rapid changes in the environmental conditions. Thus, adjusting the adaptation gain by high-frequency content of the tracking error ensures fast convergence, guaranteed transient performance, and overall system stability without needing high adaptation gain. The adaptive law of the proposed HFLAG-MRAC is derived using the Lyapunov theory. Simulation studies, experimental analysis, and performance comparison with recent similar work validate the effectiveness of the proposed work.

Atypical start-up experiments on pump as turbine

At present, research on the characteristics of pumps as turbine (PATs) during the start-up process is still insufficient. To reveal the transient characteristics of a centrifugal PAT during the atypical start-up process, a test rig for the transient performance of the PAT was built; in addition, experiments on the transient hydraulic performance of three kinds of steady speed and three kinds of steady flow were conducted. Through these experiments, the evolution characteristics of the transient performance parameters of the PAT during the atypical start-up process were analyzed over time. Moreover, three dimensionless coefficients were employed to deeply reveal the transient characteristics of the PAT during atypical start-up. Results showed that the rise curves of flow rate and outlet static pressure exhibited shock phenomena. With the increase in the stable running speed after start-up, the impact phenomenon of the outlet static pressure presented a delayed trend. The dimensionless head and flow coefficients reached the maximum value at the initial stage of the atypical start-up process and then rapidly dropped to the minimum value before slowly rising to the final stable value. The dimensionless power coefficient had a maximum value at the initial stage of atypical start-up and then rapidly dropped to the final stable value.