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In the endeavor to accomplish a fully de-carbonized globe, sparkling interest is growing towards using natural gas (NG) having as vastly major component methane (CH4). This has the lowest carbon/hydrogen atom ratio compared to other conventional fossil fuels used in engines and power-plants hence mitigating carbon dioxide (CO2) emissions. Given that using neat hydrogen (H2) containing nil carbon still possesses several issues, blending CH4 with H2 constitutes a stepping-stone towards the ultimate goal of zero producing CO2. In this context, the current work investigates the exergy terms development in high-speed spark-ignition engine (SI) fueled with various hydrogen/methane blends from neat CH4 to 50% vol. fraction H2, at equivalence ratios (EQR) from stoichiometric into the lean region. Experimental data available for that engine were used for validation from the first-law (energy) perspective plus emissions and cycle-by-cycle variations (CCV), using in-house, comprehensive, two-zone (unburned and burned), quasi-dimensional turbulent combustion model tracking tightly the flame-front pathway, developed and reported recently by authors. The latter is expanded to comprise exergy terms accompanying the energy outcomes, affording extra valuable information on judicious energy usage. The development in each zone, over the engine cycle, of various exergy terms accounting too for the reactive and diffusion components making up the chemical exergy is calculated and assessed. The correct calculation of species and temperature histories inside the burned zone subsequent to entrainment of fresh mixture from the unburned zone contributes to more exact computation, especially considering the H2 percentage in the fuel blend modifying temperature-levels, which is key factor when the irreversibility is calculated from a balance comprising all rest exergy terms. Illustrative diagrams of the exergy terms in every zone and whole charge reveal the influence of H2 and EQR values on exergy terms, furnishing thorough information. Concerning the joint content of both zones normalized exergy values over the engine cycle, the heat loss transfer exergy curves acquire higher values the higher the H2 or EQR, the work transfer exergy curves acquire slightly higher values the higher the H2 and slightly higher values the lower the EQR, and the irreversibility curves acquire lower values the higher the H2 or EQR. This exergy approach can offer new reflection for the prospective research to advancing engines performance along judicious use of fully friendly ecological fuel as H2. This extended and in-depth exergy analysis on the use of hydrogen in engines has not appeared in the literature. It can lead to undertaking corrective actions for the irreversibility, exergy losses, and chemical exergy, eventually increasing the knowledge of the SI engines science and technology for building smarter control devices when fueling the IC engines with H2 fuel, which can prove to be game changer to attaining a clean energy environment transition. [ABSTRACT FROM AUTHOR]

An integrated numerical model that describes the operation of a renewable-energy-based system for a building's heating, cooling, and domestic hot water needs is described in this study. The examined energy system includes a vapor compression multi-source heat pump, PVT collectors, borehole thermal energy storage, and water tanks. Energy balance equations for the collectors and the tanks are coupled with correlations for the heat pump and the piping losses within a thermal network approach. The non-linear system of equations that arises is solved by employing in-house software developed in Python v. 3.7.3. The performance of the numerical tool is validated against measurements collected during the pilot operation of such a system installed in Athens (Greece) for two 5-day periods (summer and winter). It is shown that the proposed model can predict, both qualitatively and quantitatively, the building's energy system performance, whereas limited deviations from the experimental findings are mostly observed when highly transient phenomena occur. The numerical tool is designed with flexibility in mind and can be easily adapted to accommodate additional energy-system configurations and operational modes. Thus, it can be utilized as a supporting decision tool for new energy systems' designs and the optimization of existing ones. [ABSTRACT FROM AUTHOR]

A major drawback of Reverse Osmosis (RO) desalination units is their poor performance at off-design conditions. On the other hand, they can operate with low specific energy consumption and produce high-quality fresh water when they are supplied with almost constant power, according to their design and nominal operating conditions. This aspect brings additional effort when there is the need to desalinate seawater or even brackish water using unsteady power sources, such as renewable energy systems powered by, for example, solar or wind energy. In order to overcome this issue, an alternative small-scale RO unit is investigated here, whose major advantage is that it can operate with almost constant specific energy consumption and produce high-quality fresh water with low Total Dissolved Solids (TDS) for a wide range of power input. Such unit is suitable to be combined with small-scale energy systems, which can have a very high variety on their power output, such as PV, wind turbines, or even solar Organic Rankine Cycle, for power production. The developed RO unit includes three identical sub-units, each placed on its own skid and with a fresh water capacity of 0.7 m3/h. An energy recovery unit of axial piston motor type is equipped at each sub-unit for decreasing its power consumption. These sub-units are switched on/off, according to the power availability, in order to operate them within a range, thus with high efficiency and acceptable fresh water quality. Such small-scale RO unit, capable of producing 2.1 m3/h of fresh water in total, is examined here, focusing on its operation, performance, and fresh water production. The main parameter adjusted is the power availability, ranging from almost zero up to the maximum value of 12 kW. The analysis first focuses on a single sub-unit, exploring its performance for variable power input, while afterwards all three sub-units are synthesized, in order to form the complete RO unit, concluding to the dependence of the fresh water production and specific energy consumption from the power supply. For the single RO sub-unit at low power input, the TDS level is very high, whereas the specific energy consumption is low, since the membrane pressure is much lower than the designed one and slightly above the osmotic pressure. For available power, more than around 1 kW, the TDS level holds acceptable values lower than 400–500 mg/l, which is achieved with the high pressurizing of the feed seawater, with specific energy consumption up to 4–5 kWh/m3. For power input higher than 3 kW, the second and third sub-units are successively switched on. In this case, the specific energy consumption is almost constant and equal to 4.5 kWh/m3, including the required power of the feed pump, while the produced fresh water TDS has small variations and is approximately equal to 200–250 mg/l. [ABSTRACT FROM AUTHOR]

Photovoltaics are methods used to generate electricity by using solar cells, which convert natural energy from the sun. This generation makes use of unlimited natural energy. However, this generation is irregular because they depend on weather occurrences. For this reason, there is a need to improve their economic efficiency through accurate predictions and reducing their uncertainty. Most researches were conducted to predict photovoltaic generation with various machine learning and deep learning methods that have complicated structures and over-fitted performances. As improving the performance, this paper explores the probabilistic approach to improve the prediction of the photovoltaic rate of power output per hour. This research conducted a variable correlation analysis with output values and a specific EM algorithm (expectation and maximization) made from 6054 observations. A comparison was made between the performance of the EM algorithm with five different machine learning algorithms. The EM algorithm exhibited the best performance compared to other algorithms with an average of 0.75 accuracies. Notably, there is the benefit of performance, stability, the goodness of fit, lightness, and avoiding overfitting issues using the EM algorithm. According to the results, the EM algorithm improves photovoltaic power output prediction with simple weather forecasting services. [ABSTRACT FROM AUTHOR]

A power system can be defined as flexible if it can within economic and technological boundaries respond quickly and adequately to variations in supply and demand. The ongoing penetration of variable and intermittent renewable energy sources (RES) like wind and solar imposes additional and more critical requirement on power system flexibility. In this paper we propose a method to quantify these requirements based on the comparison of seven demand side parameters describing the statistical properties of the net load and the residual load of the referred power system. Each one of these parameters reflects a separate requirement on the available conventional generation in hourly and daily time scales—ramp up and ramp-down capabilities, technological minimum of generation, daily variation of generation. The proposed approach can be used to predict the requirements for generation flexibility according to the expected scenario of RES penetration in the future development of energy power system. It has been applied and integrated from the Bulgarian Transmission System Operator (TSO) which name is the Bulgarian Electricity System Operator (ESO). [ABSTRACT FROM AUTHOR]

The purpose of this paper is to examine several real house cases as renewable energy resources are installed. It is an empirical study, based on first principles applied to measured data. In the first case presented, a PV solar system has been installed and a hybrid vehicle purchased. Battery storage is being considered. Smart Meter data (provided in Victoria, Australia) measures the electrical energy flowing to and from the grid in each half hour. Missing is the story about what the house is generating and what its energy requirements are through each half hour interval. We apply actual (on site) solar PV data to this study, resolving the unknown energy flows. Analysing energy flow has revealed that there are five fundamental quantities which determine performance, namely energy load, energy import, energy harvesting, energy export and energy storage. As a function of PV size these quantities depend on four parameters, easily derivable from the Smart Meter data, namely the house load, the night-time house load (no PV generation), the rating of the solar PV system and the tariffs charged. This reveals most of the information for providing advice on PV array size and whether to install a battery. An important discovery is that a battery, no matter what size, needs a PV system large enough to charge it during the winter months. The analysis is extended to two more houses located within 5 km for which detailed solar data is unavailable. [ABSTRACT FROM AUTHOR]

Photovoltaic water pumping systems (PVWPS) are a promising solution to improve domestic water access in low-income rural areas. It is challenging, however, to make them more affordable for the local communities. We develop here a comparative methodology to assess relevant features of both widely employed PVWPS architecture with water tank storage, and hardly used PVWPS architecture with a battery bank instead of tank storage. The quantitative comparison is carried out through techno-economic optimization, with the goal of minimizing the life cycle cost of PVWPS with constraints on the satisfaction of the water demand of local inhabitants and on the groundwater resource sustainability. It is aimed to support decision-makers in selecting most appropriate storage for domestic water supply projects. We applied the methodology in the rural village of Gogma, Burkina Faso. Results indicate that the life-cycle cost of an optimized PVWPS with batteries is $24.1k while it is $31.1k if a tank is used instead. Moreover, reduced impact on groundwater resources and greater modularity to adapt to evolving water demand is noted if using batteries. However, as batteries must be replaced regularly and recycled adequately, PVWPS' financial accessibility could increase only if sustainable and efficient operation, maintenance, and recycling facilities for batteries were present or developed locally. [ABSTRACT FROM AUTHOR]

A twisted-fin array as an innovative structure for intensifying the charging response of a phase-change material (PCM) within a shell-and-tube storage system is introduced in this work. A three-dimensional model describing the thermal management with charging phase change process in PCM was developed and numerically analyzed by the enthalpy-porosity method using commercial CFD software. Efficacy of the proposed structure of fins for performing better heat communication between the active heating surface and the adjacent layers of PCM was verified via comparing with conventional longitudinal fins within the same design limitations of fin material and volume usage. Optimization of the fin geometric parameters including the pitch, number, thickness, and the height of the twisted fins for superior performance of the proposed fin structure, was also introduced via the Taguchi method. The results show that a faster charging rate, higher storage rate, and better uniformity in temperature distribution could be achieved in the PCMs with Twisted fins. Based on the design of twisted fins, it was found that the energy charging time could be reduced by up to 42%, and the energy storage rate could be enhanced up to 63% compared to the reference case of straight longitudinal fins within the same PCM mass limitations. [ABSTRACT FROM AUTHOR]

Using numerical methods, studies have been carried out to determine the effect of the introduction of the technology of two-stage combustion of high-ash Karaganda coal on the main characteristics of heat and mass transfer processes in the furnace of the BKZ-75 boiler at Shakhtinskaya TPP (Kazakhstan). Various regimes of supplying additional air into the combustion space, the volume of which varied from 0% (traditional basic version) to 30% of the total volume of air required for fuel combustion, have been investigated using 3D computer modeling methods. The performed computational experiments made it possible to obtain the distributions of the total velocity vector, temperature fields, concentration fields of carbon monoxide CO and nitrogen dioxide NO2 over the entire volume of the furnace and at the outlet from it. The introduction of the two-stage combustion technology made it possible to optimize the combustion of high-ash coal, since in this case there is an increase in the temperature in the torch core and a decrease in it at the outlet from the furnace, which has a significant effect on the chemical processes of the formation of combustion products. Based on the results obtained, it can be concluded that an increase in the percentage of air supplied through additional injectors to 18% leads to a decrease in the concentrations of carbon monoxide CO by about 36%, and nitrogen dioxide NO2 by 25% compared to the base case. A further increase in the volume of additional air leads to a deterioration in these indicators. The results obtained will make it possible to optimize the combustion of low-grade fuel in the furnace of the BKZ-75 boiler, increase the efficiency of fuel burnout, reduce harmful emissions into the atmosphere, and introduce a two-stage combustion technology at other coal-fired TPPs. [ABSTRACT FROM AUTHOR]

Because of the unpredictable activity of solar energy sources, photovoltaic (PV) maximum power point tracking (MPPT) is essential to guarantee the continuous operation of electrical energy generation at optimal power levels. Several works have extensively examined the generation of the maximum power from the PV systems under normal and shading conditions. The fuzzy logic control (FLC) method is one of the effective MPPT techniques, but it needs to be adapted to work in partial shading conditions. The current paper presents the FLC-based on dynamic safety margin (DSM) as an MPPT technique for a PV system to overcome the limitations of FLC in shading conditions. The DSM is a performance index that measures the system state deviation from the normal situation. As a performance index, DSM is used to adapt the FLC controller output to rapidly reach the global maxima of the PV system. The ability of the proposed algorithm and its performance are evaluated using simulation and practical implementation results for single phase grid-connected PV system under normal and partial shading operating conditions. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to the article "Uremic pruritus," by S.R. Keithi-Reddy and colleagues, in a 2007 issue.