Thermal Conversion Efficiency Research Articles

Heat transfer measurement in a three-phase spray column direct contact heat exchanger for utilisation in energy recovery from low-grade sources

Energy recovery from low-grade energy resources requires an efficient thermal conversion system to be economically viable. The use of a liquid-liquid-vapour direct contact heat exchanger in such processes could be suitable due to their high thermal efficiency and low cost in comparison to a surface type heat exchanger.In this paper, the local volumetric heat transfer coefficient (Uv) and the active height (Hv) of a spray column three-phase direct contact heat exchanger (evaporator) have been investigated experimentally. The heat exchanger comprised a cylindrical Perspex tube of 100cm height and 10cm diameter. Liquid pentane at its saturation temperature and warm water at 45°C were used as the dispersed phase and the continuous phase respectively. Three different dispersed phase flow rates (10, 15 and 20 L/h) and four different continuous phase flow rates (10, 20, 30 and 40 L/h) were used throughout the experiments. In addition, three different sparger configurations (7, 19 and 36 nozzles) with two different nozzle diameters (1 and 1.25mm) were tested. The results showed that the local volumetric heat transfer coefficient (Uv) along the column decreases with height. An increase in both the continuous and dispersed phase flow rates had a positive effect on Uv, while an increase in the number of nozzles in the sparger caused Uv to decrease. The active height was significantly affected by the dispersed and continuous phase flow rates, the sparger configuration and the temperature driving force in terms of the Jacob number.

Directly absorbing Therminol-Al2O3 nano heat transfer fluid for linear solar concentrating collectors

This paper reports experimentally measured thermal and optical properties of Al2O3-Therminol®55 nano heat transfer fluid (nHTF) in conjunction with a specially developed line focusing Fresnel lens based solar thermal concentrator. Solar collector tests were conducted under real life outdoor ambient and solar radiation conditions. A range of volumetric proportions of Al2O3 (0.025–0.3vol%) have been covered. Highest thermal conductivity and refractive index were measured for nHTF with 0.1vol% concentration of Al2O3 with higher concentrations (>0.1vol%) concluded to be unfit for their use in the solar collector applications due to likely agglomeration and sedimentation in the hours of low or nil sunshine when fluid is not circulating. Thermal conductivity enhanced by 11.7% for 0.1vol% Al2O3 concentration nHTF. A highest temperature of 132.8°C was delivered by the solar collector using 0.1% concentration Al2O3 nHTF at a flow rate of 0.5lps. This was 44.7°C higher than that achieved by pure Therminol®55 (88.1°C) even though the DNI during 0.1% nHTF test was considerably lower than that for the later. The solar concentrator thermal efficiency increased with the proportion of NPs attaining a maximum of 52.2% for 0.1vol% Al2O3 nHTF. Directly absorbing nHTFs along with the solar collector developed in this study are predicted to be strong candidate to replace conventional metallic tube receiver using concentrators due to advantages of size compaction and higher thermal conversion efficiency.

Geometric optimization of thermoelectric elements for maximum efficiency and power output

The geometry of n- and p-type thermoelectric elements (TE) in terms of the cross-sectional area and length were optimized to yield either maximum thermal conversion efficiency ηth,max or maximum power output Po,max. The optimization process incorporated temperature-dependent material properties and independent thermal and electrical contact resistances that are a function of the contact area per TE leg. Additionally, ηth,max and Po,max were quantified by simultaneously optimizing the TE geometry and varying the hot-side fluid temperature, cold- and hot-side heat exchanger effective area and heat transfer coefficients using a complete one-dimensional thermal resistance network model. Optimized compared to non-optimized geometries, excluding contact resistances, achieve a maximum 10.4% increase in Po,max and 3.2% increase in ηth,max for conditions studied. Optimized compared to non-optimized geometries, taking into account independent thermal and electrical contact resistances, exhibit a 29% increase in volumetric power density and 12% increase in volumetric efficiency in comparison to non-optimized cases.

Thermodynamic Modeling and Validation of a 210-MW Capacity Coal-Fired Power Plant

This paper presents the thermodynamic modeling and validation of a 210-MW subcritical coal-fired power plant situated in North India. In the era of high computing facilities available, modeling and simulation have become a very pertinent tool for design and development. As such, the objective of the present work is not to enhance the thermal conversion efficiency, but to represent the thermal system in a simplistic yet functional manner. The thermal performance module consists of the following main sections such as steam boiler, economizer, superheater, reheater, steam turbine, condensate extraction pump, boiler feed pump, feedwater heaters and condenser. Thus, a semi-empirical module has been developed to predict thermal performance of coal-fired power plant. It includes the mass balance, energy balance, thermodynamic property relations for working substance followed by empirical correlations for steam bled pressure and thermodynamics of a steam power cycle. By using the MATLAB calculation tool, a process-based computer program of the coal-fired power plant has been successfully validated for thermal efficiency, steam flow rate and coal consumption rate at wide plant load range. The predictions were compared with operating data of 210-MW thermal power plant at different power output levels. The predicted results agree with plant operating data.

Study of a nTHGEM-based thermal neutron detector**Supported by National Natural Science Foundation of China (11127508, 11175199, 11205253, 11405191), Key Laboratory of Neutron Physics, CAEP (2013DB06, 2013BB04) and CAS (YZ201512)

With new generation neutron sources, traditional neutron detectors cannot satisfy the demands of the applications, especially under high flux. Furthermore, facing the global crisis in 3He gas supply, research on new types of neutron detector as an alternative to 3He is a research hotspot in the field of particle detection. GEM (Gaseous Electron Multiplier) neutron detectors have high counting rate, good spatial and time resolution, and could be one future direction of the development of neutron detectors. In this paper, the physical process of neutron detection is simulated with Geant4 code, studying the relations between thermal conversion efficiency, boron thickness and number of boron layers. Due to the special characteristics of neutron detection, we have developed a novel type of special ceramic nTHGEM (neutron THick GEM) for neutron detection. The performance of the nTHGEM working in different Ar/CO2 mixtures is presented, including measurements of the gain and the count rate plateau using a copper target X-ray source. A detector with a single nTHGEM has been tested for 2-D imaging using a 252Cf neutron source. The key parameters of the performance of the nTHGEM detector have been obtained, providing necessary experimental data as a reference for further research on this detector.

Review on recent approaches for hybrid PV/T solar technology

Summary This paper reviews the recent hybrid photovoltaic thermal (PV/T) structural/geometrical topologies to highlight the state of the art on this form of collector approaches and designs for both liquid and air-based systems. The review focuses on the development of the typical flat-plate collector – as an essential part in the PV/T system – in terms of new concept and novel configurations and specifically on the design of these collectors that use air or liquid as a heat transfer medium and their ability to extract useful heat from the back surface of the PV panel. Different mechanisms of fluid flow either natural or forced are considered. Many different design configurations for hybrid PV/T collectors have been cataloged and evaluated. It is shown that at least 30 distinct configurations have been introduced in the literature in the last 5 years. The paper concludes with identifying the major factors that affect the performance of typical PV/T systems and lead to effective enhancement of the heat removal mechanisms thus improving the electrical and thermal solar conversion efficiencies. This paper should serve as a significant form of reference for any future development in the design of the PV/T concept. Copyright © 2016 John Wiley & Sons, Ltd.

Thermal conversion of lignin to phenols: Relevance between chemical structure and pyrolysis behaviors

In this study, the Fourier transform infrared (FTIR) spectrometry and the 13C–1H correlation two-dimensional (2D) heteronuclear single-quantum coherence (HSQC) nuclear magnetic resonance (NMR) were introduced to determine the chemical structure of soda alkali lignin (SAL) and Alcell organosolv lignin (AOL), and the relevance between chemical structure and pyrolysis behaviors was evaluated by thermogravimetric analysis (TGA) and pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS). Results showed that the two lignin samples had similar functional groups and S/G ratio, excepting side-chain linkages. SAL was mainly cross-linked by β-β′ linkages, while the main linkage within AOL was β-O-4′. This difference proposed that AOL had the worse thermal stability and was easier to be pyrolyzed to phenols than SAL. Herein, the pyrolysis transformation of SAL was always promoted by the increased temperature, whereas the generated phenols from AOL would be redecomposed at high pyrolysis temperature (800°C). Moreover, the priority of demethoxylation occurring in SAL pyrolysis was higher than that in AOL pyrolysis. Further analysis on the types of phenols suggested that the formation of syringyl phenols benefitted from the increasing temperature. However, the formation of p-hydroxyphenyl phenols was inhibited as temperature increased, and the highest selectivity of guaiacyl phenols was obtained at 600°C. The reveal of the relevance between lignin chemical structure and pyrolysis behaviors is meaningful for the efficient thermal conversion of lignin to phenol compounds.

A roadmap for production of sustainable, consistent and reliable electric power from agricultural biomass- An Indian perspective

The utilization of agricultural biomass for production of electric power can help to reduce the environmental emissions while achieving energy security and sustainable development. This paper presents a methodology for estimating the power production potential of agricultural biomass in a country. Further, the methodology has been applied to develop a roadmap for producing reliable power in India. The present study reveals that about 650Mt/year of agricultural biomass is generated in India, while about one-third of this has been found to be surplus for energy applications. The cereal crops have major contribution (64.60%) in production of surplus biomass followed by sugarcane (24.60%) and cotton (10.68%). The energy potential of these resources is of the order of 3.72EJ, which represents a significant proportion of the primary energy consumption in the country. These biomass resources can produce electric power of 23–35GW depending upon the efficiency of thermal conversion. The delivery of biomass to the plants and selection of appropriate technology have been found as the major issues that need to be resolved carefully. In the end, the study summarizes various technological options for biomass collection and utilization that can be used for producing clean and consistent power supply.

Effect of Sodium Oxides in Ash Composition on Ash Fusibility

Ash fusibility is closely associated with ash slagging, which has great significance on efficiency and cleanliness of thermal conversion and coal utilization. Not being a dominant component of coal ash, Na2O is believed to be an inducement for slagging problems. In this study, for the realization of better understanding of the underlying mechanisms, the influence of Na2O on the ash fusibility was investigated from the perspectives of content variation and temperature rising. Experimental approaches, such as ash fusion temperature (AFT) test, X-ray diffraction (XRD), and scanning electronic microscope (SEM), were applied to quantify the ash fusibility and detect minerals’ transformation and surface morphology in ash melting. The ash melting process was simulated by a multicomponent system based on the FactSage database, which includes SiO2–Al2O3–Fe2O3–CaO–Na2O. This study also used Gibbs free energy theory to analyze the chemical reactions, mineral behaviors, and phase diagram and record the sodium migration in the melting process. In general, Na2O reduces the fusion temperatures of ash samples, which can be related to the performances of minerals albite, nepheline, and anorthite, as well as the eutectics they form.

Build and Test Research of a Coaxial Hybrid-power Gas Engine Heat Pump System Based on LiFePO4 Battery

Power batteries of hybrid-power gas engine heat pump (HPGHP) often encounter frequent charge and discharge operations, which have an adverse effect on nature gas conversion efficiency of system. In order to improve the dynamic efficiency and economic performance of HPGHP, this paper raises a coaxial HPGHP facility, which takes the LiFePO4 battery as the auxiliary power source to drive the compressor. Logic threshold control strategy is put forward to distribute power between the gas engine and battery pack. Test research was operated to verify the advantageous performance of LiFePO4 battery HPGHP. The results show that LiFePO4 battery HPGHP can achieve an obvious energy saving effect, especially in low-load condition and high-load condition. When compressor speed increases from700rpm to 2500rpm, COP decreases from 4.76 to 3.25, heating capacity increases from 14.52 to 33.13kW. The maximum difference of LiFePO4 battery HPGHP and lead-acid battery HPGHP is respectively 13.2%, 11.28% and 10.2% separately for engine thermal efficiency, gas-consumed rate, and energy conversion efficiency under the same load. In addition, reclaimed heat is tested and the result shows that LiFePO4 battery HPGHP is 11.82% lower than conventional nature gas heat pump (GHP).

Design of a High Performance Selective Solar Absorber with the Structure of SiO2-TiO2-TiNxOy-Cu

Using TiNxOy thin film as the absorbing layer, a spectrally selective solar absorber (SSA) with the structure of SiO2-TiO2-TiNxOy-Cu has been designed; and the SSA performance has been evaluated by both calculation and experiment. The TiNxOy layer is fabricated by RF magnetron sputtering of a pure TiN target with Ar and N2 gases. The introduction of oxygen in the TiNxOy is realized by using the residual oxygen in the deposition chamber. It is found that the N2 flow rate has a large effect on the complex refractive index of the TiNxOy thin film and thus the SSA performance can be optimized by controlling the N2 flow rate. With the TiNxOy absorbing layer deposited with the N2 flow rate of 2 sccm, the SSA achieves the highest solar thermal conversion efficiency for a solar radiation concentration factor of equal to or larger than 5. The SSA based on the TiNxOy layer deposited with the N2 flow rate of 2 sccm has a solar absorbance of 96.29% and a thermal emittance of 6.11% at the temperature of 400 °C.

Design, Fabrication, and Efficiency Study of a Novel Solar Thermal Water Heating System: Towards Sustainable Development

This paper investigated a novel loop-heat-pipe based solar thermal heat-pump system for small scale hot water production for household purposes. The effective use of solar energy is hindered by the intermittent nature of its availability, limiting its use and effectiveness in domestic and industrial applications especially in water heating. The easiest and the most used method is the conversion of solar energy into thermal energy. We developed a prototype solar water heating system for experimental test. We reported the investigation of solar thermal conversion efficiency in different seasons which is 29.24% in summer, 14.75% in winter, and 15.53% in rainy season. This paper also discusses the DC heater for backup system and the current by using thermoelectric generator which are 3.20 V in summer, 2.120 V in winter, and 1.843 V in rainy season. This solar water heating system is mostly suited for its ease of operation and simple maintenance. It is expected that such novel solar thermal technology would further contribute to the development of the renewable energy (solar) driven heating/hot water service and therefore lead to significant environmental benefits.

A Comparative Study on Thermal Performance of Solar Flat Plate and Evacuated Tube Collectors

Solar power harvesting is an important field of research around the globe due to the inadequacy of the natural resources like oil, gases and other fuels. Designing a solar water heater that could absorb more thermal energy with high efficiency is a challenging task of the researchers. It is important to consider the efficiency of the collector, and how well it transfers heat into working fluid. The overall performance of any solar collector and the amount of energy and the money spent is dependent on the performance of the collector right throughout the year. In this research work, an attempt has been made to investigate the thermal performance of two different water heaters, a flat plate collector and an evacuated tube collector. Experiments were conducted on the designed, two different solar water heaters such as flat plate collector and evacuated tube. The inlet and outlet temperatures of the water were measured for various experimental parameter such as mass flow rate of water and tilt angle. The solar irradiation was recorded through pyranometer, a solar radiation measuring device during experiment. Evacuated tubes made from borosilicate were used in this research work. From the analysis of the results it is found that the thermal conversion efficiency of evacuated tube is 41.3% than the flat plate collector tested in earlier work of the investigator. Numerical simulations were also conducted and the results obtained were found to be well matched with the experimental results.

Thermo-mechanical Analysis of Uncooled La0.67Sr0.33MnO3 Microbolometer Made on Circular SOI Membrane

We report on thermal analysis of uncooled antenna-coupled La0.67Sr0.33MnO3 microbolometer made on circular SOI membrane with no limitation in its active area (circular membrane with diameter up to 2 500 μm). A simple method how to investigate the thermal conversion efficiency (thermal resistance value) is introduced. Thermal analysis is supported by the ANSYS modelling and simulation. It is found that Rth of our LSMO bolometer (bolometer sensitivity) can be tuned by the SOI membrane thickness. Rth of the bolometer as high as 164°C/mW is estimated from the thermal simulation for SOI membrane with the thickness of 300nm.

Experimental research and performance study of a coaxial hybrid-power gas engine heat pump system based on LiFePO4 battery

Power batteries of hybrid-power gas engine heat pump (HPGHP) often encounter instantaneous power demand. Due to working modes switching caused by the frequent variation of external load, the batteries tend to perform frequent charge and discharge operations, which has an adverse effect on energy conversion efficiency of system. In order to improve the dynamic efficiency and economical performance of HPGHP, this paper raises a novel HPGHP system, which takes the LiFePO4 battery as the auxiliary power source to drive the compressor. Logic threshold control strategy is put forward to distribute power between the gas engine and battery pack. Test research on COP, heating capacity, engine thermal efficiency, fuel-consumed rate, energy conversion efficiency and reclaimed heat have been operated to verify the advantageous performance of LiFePO4 battery HPGHP. The results show that LiFePO4 battery HPGHP can achieve an obvious energy saving effect, especially in low-load condition and high-load condition. When compressor speed increases from700rpm to 2500rpm, COP decreases from 4.76 to 3.25, heating capacity increases from 14.52 to 33.13kW. Compared with lead-acid battery HPGHP under the same load, LiFePO4 battery HPGHP shows better performance. Engine thermal efficiency and energy conversion efficiency of LiFePO4 battery HPGHP are higher than those of lead-acid battery HPGHP, and fuel-consumed rate is lower. The maximum differences of engine thermal efficiency, energy conversion efficiency, fuel-consumed rate are 13.2%, 10.2% and 11.28%, respectively. In addition, reclaimed heat is also tested and the result shows that LiFePO4 battery HPGHP is 11.82% lower than conventional GHP.

Flexible thin-film black gold membranes with ultrabroadband plasmonic nanofocusing for efficient solar vapour generation.

Solar steam generation has been achieved by surface plasmon heating with metallic nanoshells or nanoparticles, which have inherently narrow absorption bandwidth. For efficient light-to-heat conversion from a wider solar spectrum, we employ adiabatic plasmonic nanofocusing to attain both polarization-independent ultrabroadband light absorption and high plasmon dissipation loss. Here we demonstrate large area, flexible thin-film black gold membranes, which have multiscale structures of varying metallic nanoscale gaps (0–200 nm) as well as microscale funnel structures. The adiabatic nanofocusing of self-aggregated metallic nanowire bundle arrays produces average absorption of 91% at 400–2,500 nm and the microscale funnel structures lead to average reflection of 7% at 2.5–17 μm. This membrane allows heat localization within the few micrometre-thick layer and continuous water provision through micropores. We efficiently generate water vapour with solar thermal conversion efficiency up to 57% at 20 kW m−2. This new structure has a variety of applications in solar energy harvesting, thermoplasmonics and related technologies.

Influence of leg sizing and spacing on power generation and thermal stresses of thermoelectric devices

The influence of leg dimensions and spacing on power-generation and thermo-mechanical performance of thermoelectric devices was investigated using numerical and statistical analyses tools. Bismuth-telluride based thermoelectric device models with rectangular-prism and cylindrical legs were simulated for a temperature range of 20–120°C and various leg heights between 1 and 5mm, widths/diameters between 1 and 2mm, and spacing between 0.5 and 1.5mm. Predicted power output, conversion efficiencies, and thermal stresses were validated with less than 8.9%, 1.2%, and 6.6% variations respectively. It is found that both leg width and height have a significant effect on power generation and thermal stresses: The relationship between power generation performance and thermal stress levels is inverse. Although leg spacing has effect on thermal stress and conversion efficiency, its effect on power output is negligible.

Thermo-Economic Assessment of a Olive Pomace Gasifier for Cogeneration Applications

Abstract A thermo-economic analysis of a combined heat and power (CHP) plant fed by syngas produced through the gasification of dry olive pomace is presented. The plant is composed by a 800 kWt downdraft gasifier, a gas clean-up system, a 200 kWe microturbine (MGT) and a heat recovery system to cogenerate hot water. Surplus heat is used to dry olive pomace from 50% to 17% wb moisture content. The plant is modeled in ASPEN Plus. Real data from experimental tests are used to calibrate the gasifier model, while the technical specification and performance of the CHP plant are collected from commercial plants in operation and data from manufacturers. Mass and energy balances are reported throughout the paper. The thermodynamic simulation of the biomass gasifier coupled to the MGT, the thermal and electrical conversion efficiency and temperature of cogenerated heat available are also presented. A thermo-economic assessment is then proposed, to investigate the economic profitability of this small scale CHP plant in the Italian energy policy scenario and considering the subsidies available for renewable electricity in the form of feed-in tariffs. For this purpose, the case study of base load CHP plant operation and heat supplied to different typologies of energy end user is assumed. The results allow quantifying the most influencing economic and technical factors that affect the performance and profitability of such investment and the bottlenecks that should be faced to facilitate a broader implementation of such CHP schemes for on site generation.

Thermodynamic and economic analysis for the pre-feasibility study of a binary geothermal power plant

Maximum power output and thermal conversion efficiency are the goals of ORC thermodynamic analysis. Co-optimization of energetic performance and system cost is needed for pre-feasibility design analysis. This paper presents a pre-feasibility design investigation for a binary geothermal power plant using a typical geothermal resource in New Zealand. Thermodynamic and economic analyses were conducted for key cycle design options, a range of working fluids and component selection parameters. The net electrical power output (Wnet) and the ratio of Wnet to total Purchased Equipment Cost (PEC) are used as the objective function to select the most thermo-economical designs. Three working fluids n-pentane, R245fa and R134a are investigated. The thermodynamic analysis shows that the net electrical power output (Wnet) of cycle design achieves a maximum level at a certain optimum turbine inlet pressure and mass flow rate of working fluid. The 2-stage designs produce higher Wnet and thermal and exergy efficiencies than the 1-stage designs. Economic comparison indicates that the type of working fluid and cycle configuration have a great effect on economic performance as measured by PEC. The profitability analysis was conducted for the top three options. The results indicate that a standard Rankine cycle with a 2-stage turbine using n-pentane is the most thermo-economical design for the particular brine resource and re-injection conditions.

Product distribution and sulfur behavior in sewage sludge pyrolysis: Synergistic effect of Fenton peroxidation and CaO conditioning

Iron- and calcium-based compounds play an important role in the devolatilization stage of solid fuels thermal disposal. This study aims to investigate the synergistic effects of Fenton’s reagent (Fe2+/H2O2) and conditioner CaO on product distribution and sulfur transformation in this process. Four samples were pyrolyzed by using a special drop-tube/fixed-bed furnace at 873–1273K with a high heating rate. Specific compositions of solid residue, tar and gas were also identified to further clarify the related mechanisms. According to the results, after Fenton peroxidation, both side chain and main chain of organic compounds were prone to crack, forming more gas and less tar with similar composition. When the oxidized sludge was further treated by CaO, altered carbon species evolution left additional COH, CO, C(O, N), OCO in char. Moreover, synergy between the two conditioners was likely to provide Fe2O3, CaO and H2O, further increasing the H2-rich gas yields. Besides, the observation also demonstrates that Fenton treatment favored the char-S production not only through catalyzing the cyclization reactions but also from producing relatively stable sulfonic acid-S/sulfone-S. When temperature increased, the peroxidation hindered H2S, CS2 and CH3SH releasing, while the decomposition of FeSO4 and Fe2(SO4)3 aggravated SO2 polluting. However, it is exciting that sulfur fixation rate eventually reached over 97.0%, since the residual Fe- and Ca-containing compounds cooperatively captured liberated sulfur. Thus reusing conditioners, which originally targeted for improving sludge dewaterability, is a promising strategy for achieving efficient thermal conversion of sewage sludge with low sulfur pollution.