Direct numerical simulation of interphase heat and mass transfer in multicomponent vapour–liquid flows

A Volume-of-Fluid methodology for direct numerical simulation of interface dynamics and simultaneous interphase heat and mass transfer in systems with multiple chemical species is presented. This approach is broadly applicable to many industrially important applications, where coupled interphase heat and mass transfer occurs, including distillation. Volume-of-Fluid interface tracking allows investigation of systems with arbitrarily complex interface dynamics. Further, the present method incorporates the full interface species and energy jump conditions for vapour–liquid interphase heat and mass transfer, thus, making it applicable to systems with multiple phase changing species. The model was validated using the ethanol–water system for the cases of wetted-wall vapour–liquid contacting and vapour flow over a smooth, stationary liquid. Good agreement was observed between empirical correlations, experimental data and numerical predictions for vapour and liquid phase mass transfer coefficients. Direct numerical simulation of interphase heat and mass transfer offers the clear advantage of providing detailed information about local heat and mass transfer rates. This local information can be used to develop accurate heat and mass transfer models that may be integrated into large scale process simulation tools and used for equipment design and optimization.     

Energy efficiency analysis and impact evaluation of the application of thermoelectric power cycle to today’s CHP systems

High efficiency thermoelectric generators (TEG) can recover waste heat from both industrial and private sectors. Thus, the development and deployment of TEG may represent one of the main drives for technological change and fuel substitution. This paper will present an analysis of system efficiency related to the integration of TEG into thermal energy systems, especially Combined Heat and Power production (CHP). Representative implementations of installing TEG in CHP plants to utilize waste heat, wherein electricity can be generated in situ as a by-product, will be described to show advantageous configurations for combustion systems. The feasible deployment of TEG in various CHP plants will be examined in terms of heat source temperature range, influences on CHP power specification and thermal environment, as well as potential benefits. The overall conversion efficiency improvements and economic benefits, together with the environmental impact of this deployment, will then be estimated. By using the Danish thermal energy system as a paradigm, this paper will consider the TEG application to district heating systems and power plants through the EnergyPLAN model, which has been created to design suitable energy strategies for the integration of electricity production into the overall energy system.     

Techno-economic assessment of green hydrogen production via two-step thermochemical water splitting using microwave

This study evaluates a two-step thermochemical water-splitting method for green hydrogen production and considers the economic feasibility of technically available designs under harsh hydrogen production conditions. As layouts of hydrogen production, two thermochemical water–splitting systems are evaluated in this study. One system is the process via high temperature from solar concentration power systems. The other system uses microwaves for thermochemical water splitting under low temperatures from advanced nuclear power plants. As part of the hydrogen production system, possible solid–solid and fluid–fluid heat recuperators of printed circuit heat exchanger (PCHE) are proposed and evaluated through the effectiveness-number of transfer units (ε-NTU) method and logarithmic mean temperature difference (LMTD) method. The required heat transfer area and volume are calculated according to the operating conditions and considered in the economic assessment of the hydrogen production system. Optimum geometries of the PCHE are proposed considering the cost analysis. The Levelized cost of hydrogen (LCOH) and system efficiency are calculated for the conventional system with solar power and system-using microwave with HTGR. The importance of heat recuperation systems is confirmed in that they account for approximately 10–20% of the cost for both system layouts. To evaluate the technology development level to achieve the ultimate target, LCOH according to various cost factors is evaluated and further research areas essential for commercialization are represented.     

Secondary reactions of turbulent reacting flows over a film-cooled surface

Demand for greater engine efficiency and thrust-to-weight ratio has driven the production of aircraft engines with higher core temperatures and pressures. Such engines operate at higher fuel–air ratios, resulting in the potential for significant heat release and chemical reactions on a film-cooled surface. Currently, there is little basis for understanding the effects on aero-performance and durability due to such secondary reactions. In this paper, the chemically reactive turbulent film cooling over a surface for three cases of an inclined single and two parallel and compound-angle coolant holes is investigated by a Reynolds-averaged Navier–Stokes approach with the shear-stress transport (SST) turbulence model using OpenFoam. To take into account the secondary combustion resulting from the unburned fuels in the crossflow, a two-step reaction scheme was used for the combustion of propane. The relative increase in surface heat flux due to near wall reactions was investigated for film cooling with N₂ and air injections. An eddy dissipation concept fast chemistry approach was used to account for the turbulence–chemistry interaction. Results demonstrate that reactions in the turbine cooling film can result in increased heat transferred to the surface. Failure to design for this effect could result in augmented heat transfer caused by the cooling scheme, and turbine life could be degraded substantially. The analysis suggests that high fuel–air ratio designs may have to consider changes to cooling strategies to accommodate secondary combustion.     

Three-dimensional simulation of saturated film boiling on a horizontal cylinder

Three-dimensional simulations of film boiling on a horizontal cylinder have been performed. A finite difference method is used to solve the equations governing the conservation of mass, momentum and energy in vapor and liquid phases. A level set formulation for tracking the liquid–vapor interface is modified to include the effect of phase change at the liquid–vapor interface and to treat the no-slip condition at the fluid–solid interface. From the numerical simulations, the effects of cylinder diameter and gravity on the interfacial motion and heat transfer in film boiling are quantified. The heat transfer coefficients obtained from numerical analysis are found to compare well with those predicted from empirical correlations reported in the literature.     

Estimating the value of energy saving in industry by different cost allocation methods

In complicated systems, such as a highly integrated industrial plant with its own energy production, estimating the value of energy conservation is not so straightforward. Often, heat is priced using different kinds of methods for allocating the fuel cost to heat and electricity. However, there is no consistent way to valuate the process steam in industry, and not just one useful method for allocating costs to heat and power. In this paper, the energy method, exergy method, benefit distribution method and market‐based method are evaluated and compared from different decision‐making perspectives. The results of this study indicate that the allocation methods may overestimate by up to 200–300% the benefits from the mill perspective compared to the benefits from the mill site perspective. So, the most suitable method may vary, depending on the selected system boundary, i.e. the decision‐making perspective, the type of CHP plant and energy prices. Based on the results of this study, the exergy method fits well with the CCGT plant with a condensing unit and constant fuel input. On the other hand, the market‐based method is the most correct way to estimate the value of heat when heat conservation reduces the production of CHP electricity. Copyright © 2010 John Wiley & Sons, Ltd.     

A review of principle and sun-tracking methods for maximizing solar systems output

Finding energy sources to satisfy the world's growing demand is one of society's foremost challenges for the next half-century. The challenge in converting sunlight to electricity via photovoltaic solar cells is dramatically reducing $/watt of delivered solar electricity. In this context the sun trackers are such devices for efficiency improvement.The diurnal and seasonal movement of earth affects the radiation intensity on the solar systems. Sun-trackers move the solar systems to compensate for these motions, keeping the best orientation relative to the sun. Although using sun-tracker is not essential, its use can boost the collected energy 10–100% in different periods of time and geographical conditions. However, it is not recommended to use tracking system for small solar panels because of high energy losses in the driving systems. It is found that the power consumption by tracking device is 2–3% of the increased energy.In this paper different types of sun-tracking systems are reviewed and their cons and pros are discussed. The most efficient and popular sun-tracking device was found to be in the form of polar-axis and azimuth/elevation types.     

Performance analyses of Cu–Cl hydrogen production integrated solar parabolic trough collector system under Algerian climate

The potential of hydrogen production by thermochemical cycle in Algeria using solar radiation as heat sources is estimated under the climate conditions of the country. The study analyzes an integrated copper–chlorine (Cu–Cl) thermochemical cycle with solar parabolic trough system for hydrogen production. In order to determine the most promising solar sites available for deploying the integrated system, the direct normal solar irradiance (DNI) for horizontal tracking system oriented in North-South has been estimated and compared for different locations. Heat gain from parabolic trough collector model is evaluated under Algerian conditions. To describe the different steps of the Cu–Cl cycle for hydrogen production, we perform a thermodynamic analysis accounting for relevant chemical reactions and including the determination of the energy necessary to the cycle. A parametric study is conducted to investigate the effect of heat gain from the parabolic trough collector (PTC) on the hydrogen production rate. Furthermore, the rate production of hydrogen by the Cu–Cl cycle is analyzed and compared for performance improvement of the system for different climatic regions in Algeria. Simulation results reveal great opportunities of hydrogen production using Cu–Cl cycle combined with solar PTC in the south of Algeria with annual hydrogen production exceeds 84 Tons H₂/year (around 0,30 kg/m²/day).     

Operation Characteristics of Heat Pump Systems With Ground Heat Exchangers

This article examines the merits of heating systems with a vapor compressor heat pump unit for small residential homes. In this case, the ground is assumed to serve as the low heat source, and the ground heat exchanger may be horizontal or vertical in form. A mathematical model for all vapor compressor heat pump (VCHP)–ground heat exchanger (GHE) systems is briefly presented. The model consists of two elements. The first contains the calculation model of the ground heat exchanger with the adjoining area of the ground. The equations for the elements of the VCHP, supplemented by the state equations for the working medium, form the second element of the general model. This model was used to perform thermal calculations for typical ground and VCHP parameters. The cumulative energy and ecological results of the application of VCHP–GHE systems were also estimated.     

A performance comparison between a horizontal source and a vertical source heat pump systems for a greenhouse heating in the mild climate Elaziğ, Turkey

The investigation presented in this article is aimed at demonstrating the technical and design feasibility of using ground-source heat pump systems in the mild climate applications for greenhouse heating, where heating requirements are dominant. An experimental comparison between a horizontal ground-source heat pump system and a vertical ground-source heat pump system was shown by focusing on the heating performance. For this purpose, an experimental set-up was constructed. The heating system mainly consists of two different ground heat exchangers, a heat pump, measuring units and a heating space of a model-sized glass greenhouse with 30 m² located in the greenhouse location.The heating coefficient of performances of the two ground-source heat pumps (COP_HP,H–V) and the overall system (COP_sys,H–V) were obtained to be in the range of 3.1–3.6 for HGSHP and 3.2–3.8 for VGSHP and 2.7–3.3 for HGSHP and 2.9–3.5 for VGSHP, respectively. Although significant savings are possible with these heating systems, a substantial investment in equipment and facilities may be required. The experimental results were obtained from November to April in heating seasons of 2007–2008. The results showed that the utilization of the ground-source heat pump is suitable for greenhouse heating in this district.     

Use of waste for heat,electricity and transport—Challenges when performing energy system analysis

This paper presents a comparative energy system analysis of different technologies utilising organic waste for heat and power production as well as fuel for transport. Technologies included in the analysis are second-generation biofuel production, gasification, fermentation (biogas production) and improved incineration. It is argued that energy technologies should be assessed together with the energy systems of which they form part and influence. The energy system analysis is performed by use of the EnergyPLAN model, which simulates the Danish energy system hour by hour. The analysis shows that most fossil fuel is saved by gasifying the organic waste and using the syngas for combined heat and power production. On the other hand, least greenhouse gases are emitted if biogas is produced from organic waste and used for combined heat and power production; assuming that the use of organic waste for biogas production facilitates the use of manure for biogas production. The technology which provides the cheapest CO₂ reduction is gasification of waste with the subsequent conversion of gas into transport fuel.     

Prospective application of farm cattle manure for bioenergy production in Portugal

Biogas is a promising renewable fuel, which can be produced from a variety of organic raw materials and used for various energetic purposes, such as heat, combined heat and power or as a vehicle fuel. Biogas systems implementation are, therefore, subjected to several support measures but also to several constraints, related with policy measures on energy, waste treatment and agriculture. In this work, different policies and policy instruments, as well as other factors, which influence a potential expansion of Portuguese biogas systems are identified and evaluated. The result of this analysis shows that the use of the cattle manure for biogas production is still far from its potential. The main reason is the reduced dimension of the Portuguese farms, which makes biogas production unfeasible. Various options are suggested to increase or improve biogas production such as co-digestion, centralized plants and modular plants. Horizontal digesters are the most suitable for the typical Portuguese plant size and have the advantage of being also suitable for co-digestion due to the very good mixing conditions. Mesophilic anaerobic digestion due to a more robustness, stability and lower energy consumption should be the choice. The recent increase in the feed-in tariffs for the electricity production based on anaerobic digestion biogas is seen as a political push to this sector.     

Economic analysis and comparison of chemical heat pump systems

Over the last years great interest has been shown in chemical heat pump systems. Chemical heat pumps represent a new technology with great potential to reduce the energy consumption in very different sectors. They can provide the ability to capture the rejected low-grade heat and to reuse it at increased temperature levels in various industrial processes. Heat can be removed from a heat source at low-temperature by an endothermic reaction and can be boosted to a heat sink at high-temperature by an exothermic reaction.Since chemical heat pumps can operate without compression, with less electrical power and at higher temperature levels compared to conventional heat pumps, they can afford high performance advantages. As an additional advantage, energy storage can also be accomplished so that intermittent energy sources can be utilized in a chemical heat pump system.The objective of this work was to study methanol–formaldehyde–hydrogen, ethanol–acetaldehyde–hydrogen, i-propanol–acetone–hydrogen and n-butanol–butyraldehyde–hydrogen chemical heat pump systems based on catalytic dehydrogenation of alcohols at low-temperature and hydrogenation of aldehydes and a ketone at high-temperature. On the base of economic analysis, the quantity of waste-heat that must be supplied to produce the benefits of the process heat and also the improvement in the net gain reached were determined and compared.     

Experimental characterisation of the operation and comparative study of two semi-indirect evaporative systems

The study described in this paper aims to present the fundamentals in which the operation of two different evaporative cooling systems is based, as well as the experimental results developed to characterise their behaviour in different conditions of outside air. These results will permit to define, according to the ideas of the systems’ operation, appropriate parameters to characterise the heat and mass exchange processes that take place as well as to compare them, like cooling capacity, thermal or energetic effectiveness; and afterwards developing this comparative analysis. The first system consists of a bank of ceramic pipes arranged vertically and staggered acting as a heat exchanger (SIERCP). In the second case an evaporative cooler has been manufactured with hollow bricks filled with still water (SIECHB). Both systems are called “semi-indirect” because they are designed to act as either direct or indirect evaporative systems depending on the relative humidity of the outdoor and return air streams. Results show that parameters related to the air humidity should be considered; and that the second system behaves generally as a direct evaporative cooler and provides a better performance.     

System performance of a deep borehole heat exchanger

Deep borehole heat exchanger (BHE) systems, installed in abandoned boreholes, have been operative in Switzerland for several years now. The operational conditions of the 2302 m deep BHE plant at Weggis have been monitored continuously since 1994. In the first operational phase, lasting from October 1994 to May 1996, the plant was severely underused, as shown by the high production temperatures (40 °C). This behaviour was investigated by a numerical model accounting for the heat transport in the rock matrix and along the different tubing systems, with special emphasis on the heat transfer in a multi-layer insulated central pipe. Lacking detailed logging data or undisturbed temperature profiles, an axis-symmetrical model had to be used, assuming uniform rock parameters. Sensitivity studies highlighted the effect of varying flow rate or operation/recovery cycle lengths and helped to develop a strategy that allowed us to make an accurate calculation of the long-term Weggis production history. The initial model assumptions, based on this detailed treatment of the tubing system, could explain the operational data. By means of slight model variations that account only for the minor effects of metallic sleeves, the long-term production temperature history of the Weggis plant could be accurately fitted. These findings were confirmed by a detailed analysis of the May 1996 data. Due to the low degree of utilization, only numerical sensitivity analyses were able to highlight the potential of the deep BHE plant at Weggis. The results indicate that the low utilisation of 40 kW during the first operational phase could be increased to over 200 kW. The specific yield of deep systems is much higher than in conventional shallow BHE systems. Our simulation procedure proves that the heat transfer in a deep BHE system is well understood.     

Influences of installation and tracking errors on the optical performance of a solar parabolic trough collector

The parabolic trough collector is an important component of parabolic trough solar thermal power generation systems. Coordinate transformations and the Monte Carlo Ray Trace (MCRT) method were combined to simulate the circumferential flux distribution on absorber tubes. The simulation model includes the optics cone with non-parallel rays, geometric concentration ratios (GCs), the glass tube transmissivity, the absorber tube absorptance and the collector surface reflectivity. The mode is used to analyze the effects of absorber tube installation errors and reflector tracking errors. The results are compared with reference data to verify the model accuracy. Influences of installation and tracking errors on the flux distribution are analyzed for different errors, incident angles and GCs. For a GC of 20 and 90° rim angle, X direction installation errors are −0.2%∼0.2%, Y direction installation errors are −1.0%–0.5%, and the tracking error should be less than 4 mrad. As the incident angle increases, the errors become larger, but the errors become smaller as concentration ratios are increased. The results provide foundations for heat transfer analysis of the absorber tube, for parabolic trough plant to ensure the safe intensity, and for economic analysis of the installation process and control system.     

Environmental assessment of grid connected photovoltaic plants with 2-axis tracking versus fixed modules systems

The use of two axes tracking systems has been widely implemented because of the higher rates in energy production that these systems can achieve. However, the reduction of the PV modules cost makes the economic advantage of these tracking systems not so evident and this has aroused the interest of analysing them from other points of view such as efficiency or energy performance and environmental impact.Most of the existing LCA studies related to Photovoltaic systems are focused in the comparison of the different technologies used for cell production; some reports include also the module assembly, but there is little information regarding the environmental impact caused by the complete solar photovoltaic plant.In this paper, a Life cycle analysis of two types of installations (with and without solar tracking) in different geographic locations is presented. The methodology, based on recognized international standards, provides the best framework for assessing the most relevant factors causing the environmental impacts and gives relevant information for further improvements. The results also allow the comparison of different solutions and the calculation of the Energy and Environmental Payback time of both configurations.     

Distributed multi-generation: A comprehensive view

The recent development of efficient thermal prime movers for distributed generation is changing the focus of the production of electricity from large centralized power plants to local generation units scattered over the territory. The scientific community is addressing the analysis and planning of distributed energy resources with widespread approaches, taking into account technical, environmental, economic and social issues. The coupling of cogeneration systems to absorption/electric chillers or heat pumps, as well as the interactions with renewable sources, allow for setting up multi-generation systems for combined local production of different energy vectors such as electricity, heat (at different enthalpy levels), cooling power, hydrogen, various chemical substances, and so forth. Adoption of composite multi-generation systems may lead to significant benefits in terms of higher energy efficiency, reduced CO₂ emissions, and enhanced economy. In this light, a key direction for improving the characteristics of the local energy production concerns the integration of the concepts of distributed energy resources and combined production of different energy vectors into a comprehensive distributed multi-generation (DMG) framework that entails various approaches to energy planning currently available in the literature. This paper outlines the main aspects of the DMG framework, illustrating its characteristics and summarizing the relevant DMG structures. The presentation is backed by an extended review of the most recent journal publications and reports.     

Analysis of the yield and production cost of large-scale electrolytic hydrogen from different solar technologies and under several Moroccan climate zones

The objective of this paper is to conduct an analysis of the green hydrogen production by the mean of water electrolysis from different solar energy systems and under different climate conditions in Morocco. To this end, simulation of four solar power plants configurations -with a nominal capacity of 100 MW_e from different technologies (fixed PV, 1 axis tracking PV, 2 axis tracking PV, and Stirling Dish) coupled with a PEM electrolyzer has been done. For the sake of precision, 3 years average of high quality meteorological data measured in-situ and at 5 different locations were used as simulation inputs. To have an idea about the potential of Morocco in the green hydrogen production market, we benchmarked the simulation results against the ones from Almeria, Spain and Stellenbosch, South Africa. Results show that for almost all sites, the 1 axis tracking PV system is the optimal technology -from techno-economic aspect-for green Hydrogen production in Morocco, even though the 2 axis tracking PV systems can generate the highest amounts of hydrogen (~4500 Tons/year), the fixed PV has the lowest LCOH₂ (5.8 $/Kg) and the Stirling Dish is the most efficient one (~12%). Besides, Morocco can be considered as a very competitive country for green hydrogen production (especially for PV technology) with an LCO_H2 of 5.57 $/Kg, against 5,96$/Kg in Southern Spain and 6,51$/Kg for south Africa.