Maximum Efficiency Ηmax Research Articles

Optimization of runner and vane blade angle of an oscillating water column based on genetic algorithm and neural network

Oscillating water column (OWC) converts ocean wave energy into electrical energy based on a reversible vane-runner-vane device. The blade profile is crucial and requires optimization design for a better performance. In this study, computational fluid dynamics (CFD) and genetic algorithm (GA) are used for optimization design. A weighted objective function is constructed by using multi-condition maximum efficiency ηmax and high efficiency range Rwid. Flow rate of maximum efficiency point φEmax is also monitored. GA helps to find the optimal 01 solution after 10 generation's iteration. Considering the time-consuming of GA in the local optimization, artificial neural network (ANN) is trained to assist the optimization. An optimal 02 solution with better performance was found by ANN and validated by CFD. The angle of runner and guide vane of optimal 02 is changed from 60° to 30°–55.739° and 35.082° with a flatter profile. ηmax is maintained at a high level over 43% and Rwid increases from 1.5 to 1.89. Compared with initial solution, the energy loss of optimal 02 is greatly reduced especially at high flow points by reducing the vortex scale. Widening of the high efficiency range enhances the ability of the unit to adapt to wave energy utilization.

International Round Robin Test of Thermoelectric Generator Modules.

The status of metrology for the characterization of thermoelectric generator modules (TEM) is investigated in this work by an international round robin (RR) test including twelve laboratories from nine countries on three continents. Measurements have been performed with three samples of a Bi2Te3-based commercial TEM type, which has prevailed over three competing types during previous tests on the short- and long-term stability. A comparison of temperature-dependent results is provided up to 200 °C hot side temperature for the maximum power output Pmax, the incident heat flow (at maximum efficiency conditions), and the maximum efficiency ηmax. Data evaluation from all RR participants reveals maximum standard deviations for these measurands of 27.2% (Pmax), 59.2% (), and 25.9% (ηmax). A comparison between RR data sets and reference data from manufacturer specifications shows high deviations of up to 46%, too. These deviations reflect the absence of measurement guidelines and reference samples and confirm the need for improvements in the standardization of TEM metrology. Accordingly, the results of the RR are presented against the background of our own investigations on the uncertainty budgets for the determination of the abovementioned TEM properties using inhouse-developed characterization facilities, which comprise reference and absolute measurement techniques for the determination of heat flow.

Numerical heat transfer analysis & predicting thermal performance of fins for a novel heat exchanger using machine learning

In the present case study, the thermal performance of fins for a novel axial finned-tube heat exchanger is investigated and predicted using machine learning regression technique. The effects of variation in the fin spacing, fin thickness, material, and the convective heat transfer coefficient on the overall efficiency and total effectiveness have been analyzed and commented upon. The k-Nearest Neighbor (k-NN), a machine learning algorithm, is used for regression analysis to predict the thermal performance outputs and the results showed high prediction accuracies. The k-NN algorithm is robust and precise which can be used by thermal system design engineers for predicting output variables. The temperature profiles of various geometries have been depicted and compared in the results. It was concluded that the efficiency is increasing with fin thickness & decreasing with fin spacing and the maximum efficiency ηmax=0.99975 is achieved at δ∗=0.1&t∗=0.0133 having h=5W/m2.K for copper material. The effectiveness is increasing with fin spacing & fin thickness and the maximum effectiveness εmax=122.766 is for δ∗=8&t∗=0.4 having h=5W/m2.K.

Parametric study on the thermoelectric conversion performance of a concentrated solar-driven thermionic-thermoelectric hybrid generator

Summary A concentrated solar-driven thermionic-thermoelectric hybrid generator composed of solar heat collector, thermionic generator (TIG), thermoelectric generator (TEG), and radiator is introduced in this paper. A theoretical model of thermoelectric conversion performance for the hybrid generator is built up based on the heat source of the concentrated solar radiation rather than isothermal heat source. Based on the model, the impacts of related parameters on the internal temperature distributions, output power, and efficiency have been discussed. Moreover, the optimal operating conditions of the TIG-TEG hybrid device at its maximum output power and efficiency have been determined. Results show that when cascading the TEG with the TIG, there is very little change of the TIG cathode temperature in most conditions, namely, TC ≈ TC′. Meanwhile, the anode temperature becomes higher, and the TEG cold end temperature T2 is close to the anode temperature TA′ for the single TIG system, ie, TA > TA′ ≈ T2. In theory, the optimal concentrated solar radiation I0 for the maximum output power Pmax and the maximum efficiency ηmax differs, which are I0,P = 2.5 × 106 W/m2 and I0,η = 2 × 106 W/m2, respectively, whereas the output power and efficiency of the TIG-TEG hybrid system simultaneously reach their maximum values when the optimal TIG anode temperature TA,opt = 1025 K, the optimal TIG output voltage Vopt = 2 V, and the optimal ratio of load resistance to internal resistance (R2/R)opt = 2. However, in practice, the parameter values of I0, ΦA, and TA should be strictly controlled under 1.8 × 106 W/m2, 1.4 eV, and 660 K, respectively. Generally, the maximum output power and efficiency of the hybrid TIG-TEG system are, respectively, 35% and 4% higher than that of the single TIG.

Enhancement of thermoelectric properties in liquid-phase sintered Te-excess bismuth antimony tellurides prepared by hot-press sintering

Various approaches have been applied to improve the thermoelectric performance of bismuth antimony telluride based materials. Here, we prepare the bulk pellets of nominal Te-excess p-type Bi0.4Sb1.6Te3+x (x = 0, 0.2, 0.4, 0.6, 0.8, 1) samples by a high-temperature solid state reaction followed by a hot-press sintering, and performe structural investigations including X-ray diffraction measurements and a high-resolution transmission electron microscopy analysis as well as a thermoelectric property characterization including electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall effect. We demonstrate that a liquid-phase sintering of Te-excess samples can provide a preferential orientation of a bulk pellet along a measured direction, an improvement in hole carrier concentration, and sub-micron sized Sb precipitates acting as a phonon scattering center, resulting in an enhancement in power factor and a reduction in lattice thermal conductivity compared to the pristine x = 0 sample. Consequently, the hot-pressed bulk pellet of the nominal Bi0.4Sb1.6Te3.4 composition exhibits an impressively high ZT of 1.41 at 417 K along the vertical direction, which is about 31 % enhancement in ZT compared to the pristine Bi0.4Sb1.6Te3 sample. The significantly improved values of practical thermoelectric properties such as engineering (ZT)eng, maximum efficiency ηmax, and output power density Pd indicates a practical applicability in thermoelectric energy harvesting technologies.

The relationship between the thermoelectric generator efficiency and the device engineering figure of merit Zd,eng. The maximum efficiency 𝜼max

Thermoelectric materials are used today in thermoelectric devices for heat to electricity(thermoelectric generators-TEG) or electricity to heat(heat pumps) conversion in a large range of applications. In the case of TEGs the final measure of their performance is given by a quantity named the maximum efficiency which shows how much from the heat input is converted into electrical power. Therefore it is of great interest to know correctly how much is the efficiency of a device to can make commercial assessments. The concept of engineering figure of merit, Zeng, and engineering power factor, Peng, were already introduced in the field to quantify the efficiency of a single material under temperature dependent thermoelectric properties, with the mention that the formulas derivation was limited to one leg of the thermoelectric generator. In this paper we propose to extend the concept of engineering figure of merit to a thermoelectric generator by introducing a more general concept of device engineering thermoelectric figure of merit, Zd,eng, which depends on the both TEG materials properties and which shall be the right quantity to be used when we are interested in the evaluation of the efficiency. Also, this work takes into account the electrical contact resistance between the electrodes and thermoelement legs in an attempt to quantify its influence upon the performance of a TEG. Finally, a new formula is proposed for the maximum efficiency of a TEG.

High absorption coefficients of the CuSb(Se,Te)2 and CuBi(S,Se)2 alloys enable high-efficient 100 nm thin-film photovoltaics

We demonstrate that the band-gap energies Eg of CuSb(Se,Te)2 and CuBi(S,Se)2 can be optimized for high energy conversion in very thin photovoltaic devices, and that the alloys then exhibit excellent optical properties, especially for tellurium rich CuSb(Se1−xTex)2. This is explained by multi-valley band structure with flat energy dispersions, mainly due to the localized character of the Sb/Bi p-like conduction band states. Still the effective electron mass is reasonable small: mc ≈ 0.25m0 for CuSbTe2. The absorption coefficient α(ω) for CuSb(Se1−xTex)2 is at ħω = Eg + 1 eV as much as 5–7 times larger than α(ω) for traditional thin-film absorber materials. Auger recombination does limit the efficiency if the carrier concentration becomes too high, and this effect needs to be suppressed. However with high absorptivity, the alloys can be utilized for extremely thin inorganic solar cells with the maximum efficiency ηmax ≈ 25% even for film thicknesses d ≈ 50 − 150 nm, and the efficiency increases to ∼30% if the Auger effect is diminished.

Enhancement of thermoelectric properties by effective K-doping and nano precipitation in quaternary compounds of (Pb1−xKxTe)0.70(PbSe)0.25(PbS)0.05

Temperature-dependent ZT values for series compounds of (Pb1−xKxTe)0.7(PbSe)0.25(PbS)0.05(x= 0.01, 0.015, 0.02, 0.025, and 0.03).

Gamma ray driven photovoltaic cells: an interface between nuclear and semiconductor physics

We explore the possibility of utilizing low energy γ-photon radiation from single or multiple radioactive sources, for the production of electron–hole pairs in the semiconductors of photovoltaic cells. Our aim is to produce and store electric energy using this mechanism. The radioactive sources in the GRPVC (gamma ray photovoltaic cell) can be either naturally occurring or manufactured nuclear isomers. The γ-photon intensity can be controlled, hence the output and efficiency can also be controlled. A spherical GRPVC of 40 cm diameter, with the nuclear isomer Th*-229 (Eγ = 3.5 eV, T1/2 = 45 h) can produce a maximum open circuit voltage Vocmax = 2.626 V, at maximum efficiency ηmax = 52.2% initially, which occurs when the semiconductor energy gap, Eg, equals the γ-photon energy, Eγ. This reduces to the lower value Voc = 0.5 V at η = 12% after 188 days. The ultimate efficiency for the GRPVC studied here is about 70%, which occurs for Eg = Eγ ∼ 10 eV, and is above 40% for Eg = Eγ (with Eγ ⩾ 3 eV). It is also shown that nuclear isomers with long half-life can keep the GRPVC operating at high Voc and high efficiency levels for many decades, and even for centuries.

Theory of the Nernst-Ettingshausen generator

The theory is developed for using the Nernst-Ettingshausen effect in non-degenerate semiconductors as a generator of power. A figure of merit is determined, and it is shown that the maximum efficiency ηmax depends upon this together with the operating temperatures. The efficiency is determined for the semiconductor itself, disregarding the power required to produce the magnetic field. For an extrinsic semiconductor ηmax passes through a maximum as magnetic field rises, but for the temperatures considered (TH = 1000° C, TC = 0° C) this is only 0.75%, even when the semiconductor parameters are particularly favourable. For an intrinsic semiconductor, however, much better performance is possible, but only at very high magnetic fields and with favourable materials. Electrons and holes must both have high mobilities at the operating temperatures, and the lattice thermal conductivity must be very low. The energy gap must also be within an optimum range. Magnetic fields well above 104 Oe will be necessary. The use of superconducting magnets would, however, make this possible.