A comprehensive review of greenhouse shapes and its applications

Greenhouse technology is a practical option for the production and drying of agricultural products in controlled environment. For the successful design of a greenhouse, the selection of a suitable shape and orientation is of great importance. Of various shapes of greenhouses, the even-span roof and the Quonset shape greenhouses are the most commonly used for crop cultivation and drying. The orientation of greenhouses is kept east–west for maximum utilization of solar radiations. Hybrid and modified greenhouse dryers have been proposed for drying of products. The agricultural products dried in greenhouses are found to be better in quality as compared to open sun drying because they are protected from dust, rain, insects, birds and animals. Moreover, various greenhouses shapes along with their applications have been reviewed.     

A comprehensive review on power-to-gas with hydrogen options for cleaner applications

This review presents the power-to-gas concept, particularly with hydrogen, from renewable energy sources to end-use applications in various sectors, ranging from transportation to natural gas distribution networks. The paper includes an overview of the leading related studies for comparative evaluation. Due to the intermittent/fluctuating phenomena of most renewables, power-to-hydrogen appears to be a promising option to offset any mismatch between demand and supply. It is a novel concept to increase the renewability of fuels and reach a sustainable energy system for future transportation, power and thermal process sectors. Comparisons of different hydrogen production methods fed by several energy sources are made regarding environmental impact, cost and efficiency. The present results show that hydrogen production (with power-to-hydrogen concept) via polymer electrolyte membrane electrolyser has lower environmental effects than other traditional methods, such as coal gasification and reforming and steam methane reforming. The geothermal energy-based system has the lowest levelized cost of electricity during hydrogen production, while natural gas has the highest value. The best option for the plant efficiency is found for high-temperature steam electrolysis fed from biogas, while the lowest efficiency value belongs to polymer electrolyte membrane electrolyser driven by solar photovoltaics, respectively.     

Deep learning methods and applications for electrical power systems: A comprehensive review

Over the past decades, electric power systems (EPSs) have undergone an evolution from an ordinary bulk structure to intelligent flexible systems by way of advanced electronics and control technologies. Moreover, EPS has become a more complex, unstable and nonlinear structure with the integration of distributed energy resources in comparison with traditional power grids. Unlike classical approaches, physical methods, statistical approaches and computer calculation techniques are commonly used to solve EPS problems. Artificial intelligent (AI) techniques have especially been used recently in many fields. Deep neural networks have become increasingly attractive as an AI approach due to their robustness and flexibility in handling nonlinear complex relationships on large scale data sets. Major deep learning concepts addressing some problems in EPS have been reviewed in the present study by a comprehensive literature survey. The practices of deep learning and its combinations are well organized with up-to-date references in various fields such as load forecasting, wind and solar power forecasting, power quality disturbances detection and classifications, fault detection power system equipment, energy security, energy management and energy optimization. Furthermore, the difficulties encountered in implementation and the future trends of this method in EPS are discussed subject to the findings of current studies. It concludes that deep learning has a huge application potential on EPS, due to smart technologies integration that will increase considerably in the future.     

A comprehensive review on assembly design strategies on proton exchange membrane applications

Proton exchange membranes (PEMs) are widely used in fuel cells, electrolyzers, and electrolytic dehumidifiers. Despite significant improvements in material preparation, operational performance and durability for these applications remain impractical, mainly due to poor multilayer structures and component design. This paper reviews recent ideas on improving the assembly design for three types of PEM applications, focusing on aspects of configuration optimization of the membrane electrode assembly and microstructured anode/cathode materials. Well-designed interfaces in catalyst layers (CLs) and diffusion layers (DLs), and between multilayers, are critical for reducing contact resistance and enhancing water management. Modification of catalyst/ionomer interfaces, such as order-structured CLs and patterned PEM/CLs, can improve catalyst utilization and notably reduce loading. Combining multiple CLs and DLs with different porosity or hydrophobicity, or using metal-based DLs (metal fiber or foam) is also feasible. Another approach receiving much attentions is to improve the common plate-and-frame structure to a plate-less one. Eliminating bipolar plates with ridge grooves or integrating the flow field with DLs can improve the reaction, and reduce mass transfer and interface ohmic resistances. Stack compactness can also be enhanced. From the microstructure of materials to the macrostructure of assembly configurations, this review pointed out systematical guidance for further research to achieve well-designed PEM assemblies.     

A review of hydrogen-air cloud explosions: The fundamentals,overpressure prediction methods,and influencing factors

Hydrogen is one of the most promising renewable energies that has been observing rapid development over the past years. Recent accidental explosion incidents and the associated damages have demonstrated the importance of hydrogen safety against potential explosions. This article presents a systematic review on hydrogen explosions. Potential explosion scenarios including the existence of impurities and rich-oxygen environment in the production, storage with extreme-high pressure and ultra-low temperature, transportation, and consumption processes are reviewed. Different types of hydrogen-air cloud explosion include expansion and deflagration, detonation, and deflagration-to-detonation transition (DDT). Existing studies on hydrogen explosion covering laboratory and field blasting test, numerical simulation utilizing various computational approaches, and theoretical derivation are reviewed and summarized. CFD modeling is currently one of the main research methods because of its cost effectiveness, though challenges existing in simulation hydrogen-air cloud detonation comparing with testing results. Apart from the properties of hydrogen-air cloud such as concentration, size and heterogeneity, environmental factors such as ignition, ventilation and obstacle are found to strongly influence the loading characteristics of hydrogen-air cloud explosion. Existing prediction approaches for estimating blast loading from hydrogen-air cloud explosion including the TNT equivalent method (TNT-EM), TNO multi-energy method (TNO MEM), and Baker-Strehlow-Tang method (BST) are primarily empirical based. Because of the inherited difference of hydrogen-air cloud from solid explosives and conventional flammable gases, the accuracies of these approaches are still doubtable, which requires further study.     

A review on microcombustion: Fundamentals,devices and applications

Microcombustion research has flourished over the past decade. Yet, most of the commercial potential of microcombustion is still to come. Aside from portable electronics, emerging drivers stem from the energy problem of declining fossil fuel reserves and their large environmental footprint upon combustion. The need to capitalize on underutilized energy sources and renewables further stimulate energy research in microsystems. In this review paper, technological drivers, applications, devices, and fabrication protocols of microburners are presented. Then, a review of homogeneous, catalytic, homogeneous-heterogeneous and heat recirculating microburners is given. Results are presented that interpret literature findings. An outlook of microcombustion research is finally outlined.     

A comprehensive review and analysis of solar forecasting techniques

In the last two decades, renewable energy has been paid immeasurable attention to toward the attainment of electricity requirements for domestic, industrial, and agriculture sectors. Solar forecasting plays a vital role in smooth operation, scheduling, and balancing of electricity production by standalone PV plants as well as grid interconnected solar PV plants. Numerous models and techniques have been developed in short, mid and long-term solar forecasting. This paper analyzes some of the potential solar forecasting models based on various methodologies discussed in literature, by mainly focusing on investigating the influence of meteorological variables, time horizon, climatic zone, pre-processing techniques, air pollution, and sample size on the complexity and accuracy of the model. To make the paper reader-friendly, it presents all-important parameters and findings of the models revealed from different studies in a tabular mode having the year of publication, time resolution, input parameters, forecasted parameters, error metrics, and performance. The literature studied showed that ANN-based models outperform the others due to their nonlinear complex problem-solving capabilities. Their accuracy can be further improved by hybridization of the two models or by performing pre-processing on the input data. Besides, it also discusses the diverse key constituents that affect the accuracy of a model. It has been observed that the proper selection of training and testing period along with the correlated dependent variables also enhances the accuracy of the model.     

A review of nanofluid preparation,stability, and thermo‐physical properties

This paper represents a comprehensive review on the preparation and stability of nanofluid, the convective heat transfer coefficient and different thermo‐physical properties such as thermal conductivity, specific heat capacity, viscosity, and so on. Here, for each thermo‐physical property, measurement methods, enhancement mechanisms, and criticisms of different studies are also presented. However, based on the available literature, it is concluded that a nanofluid has, in general, better thermo‐physical properties even at a very low particle concentration (typically 1% or less) than conventional heat transfer fluids. The only drawback is high viscosity which leads to a higher pressure drop. At a very low particle concentration, this drawback can be minimized. Three tables are provided for three thermo‐physical properties namely thermal conductivity, specific heat capacity, and viscosity, which can be used as a ready reference for calculating the nanofluid properties.     

Ground heat exchangers—A review of systems,models and applications

The temperature at a certain depth in the ground remains nearly constant throughout the year and the ground capacitance is regarded as a passive means of heating and cooling of buildings. To exploit effectively the heat capacity of the ground, a heat-exchanger system has to be constructed. This is usually an array of buried pipes running along the length of a building, a nearby field or buried vertically into the ground. A circulating medium (water or air) is used in summer to extract heat from the hot environment of the building and dump it to the ground and vice versa in winter. A heat pump may also be coupled to the ground heat exchanger to increase its efficiency. In the literature, several calculation models are found for ground heat exchangers. The main input data are the geometrical characteristics of the system, the thermal characteristics of the ground, the thermal characteristics of the pipe and the undisturbed ground temperature during the operation of the system. During the first stages of the geothermal systems study, one-dimensional models were devised which were replaced by two-dimensional models during the 1990s and three-dimensional systems during recent years. The present models are further refined and can accommodate for any type of grid geometry that may give greater detail of the temperature variation around the pipes and in the ground. Monitoring systems have been set up to test various prototype constructions with satisfactory results.     

A review of free-piston engine history and applications

This document reviews the history of free-piston internal combustion engines, from the air compressors and gas generators used in the mid-20th century through to recent free-piston hydraulic engines and linear electric generators. Unique features of the free-piston engine are presented and their effects on engine operation are discussed, along with potential advantages and disadvantages compared to conventional engines. The paper focuses mainly on developed engines where operational data has been reported. Finally, the potential of the free-piston engine is evaluated and the most promising designs identified.     

A comprehensive review on PEM water electrolysis

Hydrogen is often considered the best means by which to store energy coming from renewable and intermittent power sources. With the growing capacity of localized renewable energy sources surpassing the gigawatt range, a storage system of equal magnitude is required. PEM electrolysis provides a sustainable solution for the production of hydrogen, and is well suited to couple with energy sources such as wind and solar. However, due to low demand in electrolytic hydrogen in the last century, little research has been done on PEM electrolysis with many challenges still unexplored. The ever increasing desire for green energy has rekindled the interest on PEM electrolysis, thus the compilation and recovery of past research and developments is important and necessary. In this review, PEM water electrolysis is comprehensively highlighted and discussed. The challenges new and old related to electrocatalysts, solid electrolyte, current collectors, separator plates and modeling efforts will also be addressed. The main message is to clearly set the state-of-the-art for the PEM electrolysis technology, be insightful of the research that is already done and the challenges that still exist. This information will provide several future research directions and a road map in order to aid scientists in establishing PEM electrolysis as a commercially viable hydrogen production solution.     

Underground hydrogen storage: A comprehensive review

Increased emissions of greenhouse gasses into the atmosphere has adversely been contributing to global warming as a result of burning fossil fuels. Therefore, the energy sectors have been looking into renewable sources such as wind, solar, and hydro energy to make electricity. However, the strongly fluctuating nature of electricity from such energy sources requires a bulk energy storage system to store the excess energy as a buffer and to fulfill the demand constantly. Underground storage is a proven way to store a huge amount of energy (electricity) after converting it into hydrogen as it has higher energy content per unit mass than other gases such as methane and natural gas. This paper reviews the technical aspects and feasibility of the underground storage of hydrogen into depleted hydrocarbon reservoirs, aquifers, and manmade underground cavity (caverns). Mechanisms of underground hydrogen storage (UHS) followed by numerous phenomena such as hydrodynamics, geochemical, physiochemical, bio-chemical, or microbial reactions have been deliberated. Modeling studies have also been incorporated in the literature to assess the feasibility of the process that are also reviewed in this paper. Worldwide ongoing lab study, field study together with potential storage sites have been reported as well. Technical challenges along with proper remedial techniques and economic viability have been briefly discussed. Finally, this paper delivers some feasible strategies for the underground hydrogen storage process, which would be helpful for future research and development of UHS.     

A comprehensive review on biological hydrogen production

Clean fuels are the critical requirement for industrialized world to combat emission of greenhouse gas. Hydrogen is one of the cleanest fuels that generates water as a result of combustion. Production of hydrogen from renewable and nonpolluting resources is an imperative task for sustainable clean fuel production. Biological processes provide an opportunity to produce hydrogen from renewable and economical bio-resources like biomass and solar energy through various processes such as direct/indirect photolysis, photo-fermentation, dark-fermentation, and CO gas-fermentation. This paper provides a comprehensive review on biological hydrogen production including organisms, type of substrates and their concentrations, role of chemical addition, operation conditions such as temperature, pH, and agitation, as well as illumination systems in case of light dependent processes. Further discussions in this work comprise various configuration of integrated biological processes of photolysis, dark, and photo-fermentation such as two component and three-component systems.     

A comprehensive review of direct borohydride fuel cells

A direct borohydride fuel cell (DBFC) is a device that converts chemical energy stored in borohydride ion (BH₄⁻) and an oxidant directly into electricity by redox processes. Usually, a DBFC employs an alkaline solution of sodium borohydride (NaBH₄) as fuel and oxygen or hydrogen peroxide as oxidant. DBFC has some attractive features such as high open circuit potential, ease of electro-oxidation of BH₄⁻ on non-precious metals such as nickel, low operational temperature and high power density. The DBFC is a promising power system for portable applications. This article discusses prominent features of DBFC, reviews recent developments in DBFC research, and points out future directions in DBFC research.     

A comprehensive review of solar thermoelectric cooling systems

The negative environmental impacts of burning fossil fuels have forced the energy research community seriously to consider renewable sources, such as naturally available solar energy. This paper provides an overview of solar thermoelectric (TE) cooling systems. Thus, this review presents the details referring to TE cooling parameters and formulations of the performance indicators and focuses on the development of TE cooling systems in recent decade with particular attention on advances in materials and modeling and design approaches. Additionally, the TE cooling applications have been also reviewed in aspects of electronic cooling, domestic refrigeration, air conditioning, and power generation. Finally, the possibility of solar TE cooling technologies application in “nearly zero” energy buildings is briefly discussed, and some future research directions are included. This research shows that TE cooling systems have advantages over conventional cooling devices, including compact in size, light in weight, high reliability, no mechanical moving parts, no working fluid, being powered by direct current, and easily switching between cooling and heating modes.     

A comprehensive techno-economical review of indirect solar desalination

Solar powered desalination has been the focus of great interest recently worldwide. In the past, majority of the experimental investigations focused on solar coupled thermally driven conventional desalination technologies such as Multi-Stage Flash (MSF) and Multi-Effect Distillation (MED). With the advancement in membrane technology and its advantages such as high Recovery Ratios (RR) and low specific energy requirements Reverse Osmosis (RO) desalination has gained popularity. Currently, 52% of the indirect solar desalination plants are RO based with MED and MSF having a 13% and 9% share respectively. Membrane Distillation (MD) based plants represent 16% of the total and have been a focus of recent research efforts. This paper aims to provide a comprehensive review of all the indirect solar desalination technologies along with plant specific technical details. Efforts assessing the economic feasibility and cost affecting parameters for each desalination technology are also reviewed.     

Hotspots,frontiers, and emerging trends of tandem solar cell research: A comprehensive review

A tandem solar cell (TSC) is a kind of special photovoltaic (PV) device with two or further sub-cells stacked in it. On the basis of Web of Science database and CiteSpace software, the literature about TSC research from 2000 to 2019 is reviewed. The top 10 hotspots are deduced (efficiency, performance, film, silicon, design, open circuit voltage, polymer, morphology, oxide, and growth), yielding prominence of the primary roles of devices and materials in PV research. The top 10 research clusters are analyzed (organic compounds, polymer solar cells, perovskite, non-fullerene acceptors, silicon, high frequency-glow discharge, solution process, light trapping, liquid phase epitaxy, and water splitting), revealing the development orientation of TSC research. High co-citation, strong burst citation, and representative frontier literature are highlighted. Five evolution trends/clusters are examined. Organic solar cells are the mainstream of TSC research and are gradually replaced by the emerging trend of non-fullerenes. Perovskite solar cells are a typical emerging trend, which rejuvenates the traditional silicon solar cells. This review provides a visual panorama of TSC research over the past two decades.     

Energy harvesting from pavements and roadways: A comprehensive review of technologies,materials, and challenges

Energy harvesting from pavements has been a topic of extensive research in the recent past. This domain has attracted not only the research community but also the industry and governmental authorities. The various sources exploited for energy harvesting from pavements and roadways are solar radiation, mechanical energy dissipated due to moving vehicles and pedestrians, geothermal energy, rainwater, and wind. This article presents an exhaustive and updated review of all potential means of energy harvesting from these sources. Following the introductory section, the article sequentially covers the energy harvesting methods and their research progress, materials, development of practical systems, commercial status, comparison of technologies, challenges, and concluding remarks. This study reveals that there is wide scope for further research and feasibility studies, which could lead to a wide‐spread implementation of the various technologies for energy harvesting from pavements and roads.