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[en] For a thermoelectric device driven by the temperature difference between soil and ambient air, previous analytical work has been performed wherein a sinusoidal surface boundary condition was imposed; the results suggested that optimal placement of the lower terminal of the device should be at a nondimensional depth of x* = 2.28, resulting in a 7% increase in power over placement at a much greater depth. Analysis of temperature data taken in conjunction with operation of a thermoelectric device has shown that the optimal depth for the lower thermal reservoir is much shallower than originally thought (x* ∼ 1), with increases in performance approaching 70% over that experienced at greater depths. Representative data are presented and interpreted, along with recommendations for future work.
[en] Highlights: • A new method for quick calculation of TEG length and cross-section is proposed. • There exist different optimal lengths and areas with different thermal boundaries. • Different combinations of lengths and areas can lead to same maximum power. • This method is approved to be accurate and time-efficient. - Abstract: A comprehensive mathematical model is proposed to calculate the optimal leg length and cross-section area of TEG unit to maximize the peak output power. The model shows that for a TEG unit, there exists an optimal ratio of leg length and leg cross-section area corresponding to the maximum peak output power with a convective thermal boundary condition, and the optimal leg length and cross-section area can be further calculated based on the optimal ratio. The mathematical model is also validated in this paper, and the corresponding error is within a reasonable range. Moreover, the effects of the leg length and leg area on the peak output power, the peak output power density and the efficiency of TEG unit with different thermal boundary conditions are also discussed. This study will provide guidance for the structure design optimization of TEG unit.
[en] Highlights: • A PV system can be used to increase the autonomy of a mobile system. • The goal is to develop the models of a PVS concerning all its components. • Measured, forecast and calculated values of radiation and temperature can be considered. • The implementation of the MPPT charge regulator is also included. • The model can be considered a useful tool to estimate the performance of a PVS. - Abstract: High autonomy is an important goal of most mobile systems, it can be achieved in three ways: reducing energy consumption, increasing the capacity of batteries and replenishing battery energy over time. Another possibility comes from using an on-board generating system: in this context an embedded photovoltaic system (PVS) is a feasible solution. To model the whole system and to evaluate its energy performance, a Simulink model in Matlab environment has been developed. Meteorological variables (radiance and ambient temperature) are essential inputs to estimate the power production of a PVS. In this simulator, measured, forecast as well as calculated values of radiation and temperature can be considered. Once that the path followed by a mobile PVS (MPVS) is known, the Simulink tool calculates both the energy produced by PVS and the energy consumption. Different scenarios concerning the optimization of the PV production, considering the path followed by the MPVS, its consumption and the available solar radiation, have been considered and simulated. This model can become a very helpful tool to estimate the power production of PVS applied to any mobile system in such a way not only to optimize the energy performances but also to refine the sizing of the PVS. A mobile robot is used as test-bed
[en] Highlights: • We propose rotating thermoelectric (TE) device operating in periodical steady state. • We describe analytical method to investigate the behavior of the rotating TE device. • The solution of optimization problem for TE device parameters is found. • Cooling regime in sinusoidal mode is found to demonstrate better performance. - Abstract: We propose a novel rotating thermoelectric device operating in periodic steady state, whether it is possible to achieve better energy conversion performance for rotating device comparing to conventional stationary steady state. The rotating thermoelectric (TE) cooling device consisting of the single TE conductor is described. It operates in two periodical steady state modes: the switching periodical mode (P-mode) when hot and cold ends of the TE conductor are periodically instantly reversed and the continuous sinusoidal mode (S-mode) when the temperature of TE conductor edges varies continuously according to a sine wave. Cooling and power generation regimes of rotating (TE) device in the periodic steady state was studied analytically. The efficiency and the cooling temperature of rotating TE device was found to depend not only on the dimensionless TE figure of merit, but also upon an additional dimensionless parameter comprising of the rotation period, the size and the thermal diffusivity of the TE conductor. The proposed analytical method can be generalized to even more complex rotating cooling modes and allows us to solve the optimization problem for cooling TE device parameters. The S-mode was shown to demonstrate deeper cooling at certain times
[en] The applications of photovoltaic (PV) systems have become more widespread in both developed and developing countries. The most critical exercise in designing a PV system is the determination of the optimum size of the solar panel and the battery capacity to meet the load demand with acceptable reliability. The amount of power generated by a PV system strongly depends on the availability of solar insolation at the required location. Also, the efficiency of a PV system is influenced considerably by a number of climatic variables, such as solar irradiance availability and ambient temperature. The technical information is provided for standard test conditions that may never occur in practice. Reliable knowledge of the performance of PV systems under actual operating conditions is essential for correct product selection and accurate performance prediction. This paper investigates the operational performance and efficiency characteristic of a small PV system installed at the City University of Hong Kong. The solar data and the power generated by the PV system are systematically recorded and analysed. The findings provide technical data in different months for designers and engineers to assess and size PV systems
[en] The resultant Seebeck coefficient αR(Tz) of a thermoelectric (TE) element was derived analytically from the temperature dependence of the intrinsic Seebeck coefficient αI(Tz) by taking into account the Thomson effect, where Tz is a temperature at z along a TE element. The analysis was performed by expanding αI(Tz) in a power series in (Tz - T), where T is a mean temperature. As a result, when αI(Tz) has a convex curve exhibiting a local maximum at Tz = T, αR(Tz) is increased at the interfaces of a TE element, while when it has a concave curve giving a local minimum at Tz = T, αR(Tz) deteriorates there. If the p-type (Bi0.4Sb0.6)2Te3 with a local maximum of αI(Tz) at T = 390 K is employed for a TE element, αR(Tz)/αI(T) at both interfaces is increased up to 1.53 under the condition of T = 390 K and ΔT = 200 K. A similar enhancement in αR(Tz)/αI(T) appeared even in the n-type (Zr-Hf)NiSn half-Heusler. When αI(Tz) varies nonlinearly with changes in Tz, therefore, the TE figure of merit ZR(Tz)Tz is found to be affected dramatically at the interfaces. The average resultants ZAR(T) estimated for the p-type Bi-Te and n-type half-Heusler compound reach great values of 1.46 and 1.26 times as large as their intrinsic Z(T), respectively. The experimental method to confirm such a phenomenon is also proposed here. The performance of a TE element is thus expected to be enhanced significantly not only by improving the intrinsic Z(Tz)Tz but also by optimizing the Tz-dependence of αI(Tz).
[en] Highlights: • The paper presents a new control method of a photovoltaic grid connected system. • The control strategy is simple, no need for regulators or modulation techniques. • The control method uses a cost function to be minimized for each voltage vector. • Once the strategy is approved, a multilevel inverter is used to improve the results. - Abstract: The paper presents an improved predictive power control for a photovoltaic conversion chain connected to a grid based on finite states space model of the converter. The proposed control algorithm selects the switching state of the inverter that minimizes the error between active and reactive power predictions to their computed values for all different voltage vectors. The optimal voltage vector that minimizes a cost function is then applied to the output of the power converter. Once the proposed predictive strategy is validated, a multilevel converter is then used to improve and highlight the obtained results. The proposed predictive control strategy uses only one sample time prediction and it is very intuitive since it is very simple and provides best performances compared to other modulation techniques
[en] The availability of more comprehensive solar irradiance data is invaluable for the reduction of cooling load in buildings and for the evaluation of the performance of photovoltaic plants. In many parts of the world, however, the basic solar irradiance data are not always readily available. This paper presents an approach to calculate the solar irradiance on sloped planes by integrating the sky radiance distribution. Sky radiance data recorded from January 1999 to December 2001 in Hong Kong were used to estimate the solar irradiance for the horizontal and four principal vertical surfaces (N, E, S and W). The performance of this approach was assessed against data measured in the same period. Statistical results showed that using sky radiance distributions to predict solar irradiance can give reasonably good agreement with measured data for both horizontal and vertical planes. The prediction approach was also employed to compute the solar irradiance of a 22.3 deg. (latitude angle of Hong Kong) inclined south oriented surface. The findings indicated that the method can provide an accurate alternative to determine the amount of solar irradiance on inclined surfaces facing various orientations when sky radiance data are available
[en] Highlights: • A new method for design of segmented TEG is proposed. • Optimal length ratios for output power and efficiency are different. • Material properties, geometry and heat transfer all need to be considered. • This method is approved to be accurate and time-efficient. - Abstract: A comprehensive method for indicating the length ratio of segmented thermoelectric generator (TEG) is proposed to increase the output power and thermoelectric conversion efficiency. It is found that for a segmented TEG, there is an optimal length ratio corresponding to the highest maximum output power or thermoelectric conversion efficiency, which is not only dependent on the material properties but also the heat transfer conditions and geometry structure. The optimal length ratios corresponding to the output power and thermoelectric conversion efficiency are different. This method is also validated, and the error is within a reasonable range, indicating that this method can be used accurately and time-efficiently for the design of segmented TEGs.
[en] Highlights: • A novel PV-TE system was presented. • Mathematical model of the system was built. • Performance under different ambient conditions was analyzed. • New PV-TE and the conventional PV were compared. • Preliminary economic analysis was demonstrated. - Abstract: Photovoltaic-thermoelectric (PV-TE) hybrid system is one typical electrical production based on the solar wide-band spectral absorption. However the PV-TE system appears to be economically unfeasible owing to the significantly higher cost and lower power output. In order to overcome this disadvantage, a novel PV-TE system based on the flat plate micro-channel heat pipe was proposed in this paper. The mathematic model was built and the performance under different ambient conditions was analyzed. In addition, the annual performance and the preliminary economic analysis of the new PV-TE system was also made to compare to the conventional PV system. The results showed that the new PV-TE has a higher electrical output and economic performance.