• Transactions of the Korean hydrogen and new energy society
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• v.33 no.1
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• pp.95-104
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• 2022

Micro hydropower is a readily available renewable energy source that can be harvested utilizing hydrokinetic turbines from shallow water canals, irrigation and industrial channel flows, and run-off river stream flows. These sources generally have low head (<1 m) and low velocity which makes it difficult to harvest energy using conventional turbines. A horizontal-axis screw turbine was designed and numerically tested to extract power from such low-head water sources. The 3-bladed screw-type turbine is placed horizontally perpendicular to the incoming flow, partially submerged in a narrow water channel at no-head condition. The turbine hydraulic performances were studied using Computational Fluid Dynamics models. Turbine design parameters such as the shroud diameter, the hub-to-shroud ratios, and the submerged depths were obtained through a steady-state parametric study. The resulting turbine configuration was then tested by solving the unsteady multiphase free-surface equations mimicking an actual open channel flow scenario. The turbine performance in the shallow channel were studied for various Tip Speed Ratios (TSR). The highest power coefficient was obtained at a TSR of 0.3. The turbine was then scaled-up to test its performance on a real site condition at a head of 0.3 m. The highest power coefficient obtained was 0.18. Several losses were observed in the 3-bladed turbine design and to minimize losses, the number of blades were increased to five. The power coefficient improved by 236% for a 5-bladed screw turbine. The fluid losses were minimized by increasing the blade surface area submerged in water. The turbine performance was increased by 74.4% after dipping the turbine to a bottom wall clearance of 30 cm from 60 cm. The final output of the novel horizontal-axis screw turbine showed a 2.83 kW power output at a power coefficient of 0.63. The turbine is expected to produce 18,744 kWh/year of electricity. The design feasibility test of the turbine showed promising results to harvest energy from small hydropower sources.

• Journal of Korea Water Resources Association
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• v.51 no.3
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• pp.235-246
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• 2018

The main purpose of this study is to suggest a methodology for identifying vulnerable region in Choyang creek basin susceptible to soil losses based on runoff aggregation structure and energy expenditure pattern of natural river basin within the framework of power law distribution. To this end geomorphologic factors of every point in the basin of interest are extracted by using GIS, which define tractive force and stream power as well as drainage area, and then their complementary cumulative distributions are graphically analyzed through fitting them to power law distribution to identify the sensitive points within the basin susceptible to soil losses with respect to scaling regimes of tractive force and stream power. It is observed that the range of vulnerable region by scaling regime of tractive force is much narrower than by scaling regime of stream power. This result seems to be due to the tractive force is a kind of scale dependent factor which does not follow power law distribution and does not adequately reflect energy expenditure pattern of river basins. Therefore, stream power is preferred to be a more reasonable factor for the evaluation of soil losses. The methodology proposed in this study can be validated by visualizing the path of soil losses, which is generated from hill-slope process characterized by local slope, to the valley through fluvial process characterized by drainage area as well as local slope.

• The Transactions of the Korean Institute of Electrical Engineers A
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• v.52 no.7
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• pp.429-436
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• 2003

This paper presents a new approach for evaluating the transmission marginal loss factors (MLFs) considering the reactive power. Generally, MLFs are represented as the sensitivity of transmission losses, which is computed from the change of the generation at reference bus by the change of the load at the arbitrary bus-i. The conventional evaluation method for MLFs uses the only H matrix, which is a part of jacobian matrix. Therefore, the MLFs computed by the existing method, don't consider the effect of the reactive power, although the transmission losses are a function of the reactive power as well as the active power. To compensate the limits of the existing method for evaluating MLFs, the power factor at the bus-i is introduced for reflecting the effect of the reactive power in the evaluation method of the MLFs. Also, MLFs calculated by the developed method are applied to energy spot markets to reflect the impacts of reactive power. This method is tested with the sample system with 5-bus, and analyzed how much MLFs have an effect on the bidding/offer price, market clearing price(MCP), and settlement in the competitive energy spot market. This paper compared the results of MLFs calculated by the existing and proposed method for the IEEE 14-bus system, and the KEPCO system.

• Journal of the Korean Solar Energy Society
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• v.21 no.4
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• pp.63-71
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• 2001

This work has been carried out to find the ideal operating conditions for solar vacuum tube collectors which are widely used at present. Various types of solar collectors including a flat plate one were experimentally tested and examined to determine their thermal efficiencies and operating characteristics. Generally, solar vacuum tubes can be classified into two groups according to their design features. Of these, one is characterized by the insertion of a metallic device(such as a finned heat pipe) in an evacuated glass tube for the collection and transportation of solar energy. The other utilizes double glass tubes where the smaller one is contained inside the bigger one and soldered to each other after the small gap between them is evacuated. Both of these solar collectors are designed to minimize convection heat losses by removing the air which is in direct contact with the absorber surface. The performance of the former type can be readily analyzed by applying the relevant correlations developed for flat plate solar collectors. This has been demonstrated in the present study for the case of a solar collector where a heat pipe is inserted in an evacuated tube.

• Journal of Nutrition and Health
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• v.26 no.1
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• pp.76-88
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• 1993

Maternal body weight, skinfold thickness and circumference measurements were examined in 23 lactating women and 10 nonlactating women longitudinally from 1 week to 12 weeks postpartum. The dietary intakes of Korean lactating and nonlactating women were measured by 24-hour recall method and effects of maternal energy intake on body weight change were analyzed by Pearson correlation. The mean daily energy intake of breast-feeding(BF, n=12), combination of breast and formula-feeding (CF, n=11), and formula-feeding (FF, n=10) mothers were 1941kcal, 187kcal and 1727kcal, respectively. A significant decrease in weight was observed at all group. Weight losses at 12 weeks of postpartum in BF, CF and FF motheres were 3.18kg, 3.91kg and 5.15kg respectively. Weight losses increased as calorie intake decreased (${\gamma}$2=0.3803, p<0.05). The skinfold thickness and circumferences on all regions were decreased significantly except triceps and upper arm circumferences. However, there were no significant differences between lactating and nonlactating women. Anthropometric measurements decreased at the trunk but not at the limbs. Weight change was significantly correlated with changes of the scores for proximal circumference (${\gamma}$2=0.4999, p<0.05). There results suggest that lactation does not promete weight loss in well-nourished women and that the Korean RDA for energy in lactation may be too high.

• Advances in Energy Research
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• v.1 no.1
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• pp.79-95
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• 2013

The purpose of this paper is to investigate the surface temperatures of the shelter of three public buildings in the city of Xanthi, in northern Greece. The buildings were built in different time periods and consequently they have different technical characteristics. Respectively, we survey the three following buildings that have been built in 1880 (Municipality Hall of Xanthi), in 1970 (Municipality Amphitheatre) and in 2002 (Bank offices building). Data have been gathered by the use of thermal camera and the survey has been conducted from January up to July. The data gathered regard measurements of the surface temperature of the exterior walls of the shelters, both inside and outside. The study aims at the evaluation of the thermal behavior of the shelter of buildings, which built in different time and under different regulations. The gathered data of the surface temperatures compare the different thermal behavior of the shelter. The analysis of the results and diagrams show that the thick masonry of the traditional Municipality Hall offers an insulation that is adequate. The building of 1970, which was constructed with the previous buildings regulation, has thermal losses due to inadequate insulation. The new building of 2002 has low thermal losses.

• Journal of Energy Engineering
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• v.27 no.1
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• pp.65-75
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• 2018

The emission of greenhouse gases mainly carbon dioxide and methane is the result of rapid industrialization to meet the demands of ever-growing population. This has resulted in an increase of global temperature which in turn is responsible for severe environmental, social, ecological and economic losses, commonly known as to as 'climate change'. This study attempts to highlight the impacts of climate change mainly focussing on water contamination, sanitation and open defecation in India. The requirement for the instantaneous employment of environment friendly technologies along with improved sanitary system has been discussed. Various other issues which are also linked to climate change that need further management like managing water resources, deterioration in human health, economic losses, modification and successful implementation of policies have been pointed out. Furthermore, stress has been made for the urgent adaptation and rethinking for making strategies along with the involvement of women in order to cop up challenges offered by climate change.

• Journal of Power Electronics
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• v.16 no.3
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• pp.1152-1162
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• 2016

The unequal impedances of the interconnecting cables between paralleled inverters in the island mode of microgrids cause inaccurate reactive power sharing when the traditional droop control is used. Many studies in the literature adopt low speed communications between the inverters and the central control unit to overcome this problem. However, the losses of this communication link can be very detrimental to the performance of the controller. This paper proposes an improved reactive power-sharing control method. It employs infrequent measurements of the voltage at the point of common coupling (PCC) to estimate the output impedance between the inverters and the PCC and then readjust the voltage droop controller gains accordingly. The controller then reverts to being a traditional droop controller using the newly calculated gains. This increases the immunity of the controller against any losses in the communication links between the central control unit and the inverters. The capability of the proposed control method has been demonstrated by simulation and experimental results using a laboratory scale microgrid.

• Journal of the Korean Solar Energy Society
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• v.39 no.4
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• pp.25-39
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• 2019

Luminescent solar concentrator (LSC), consisting of luminophore included glass or substrate with edge-mounted photovoltaic cell, is semi-transparent, energy harvesting devices. The luminophore absorbs incident solar light and re-emit photons, while the waveguide plate allows re-emitted photons to reach edge or bottom mounted photovoltaic cells with reduced losses. If the area of LSC is much larger than that of photovoltaic cell, this system can effectively concentrate solar light. In order to improve the performance of LSC, new materials and optical structures have been suggested by many research groups. For decreasing re-abosprion losses, it is essential to minimize the overlap between absorption and photoluminescence solar spectrum of luminophoroe. Moreover, the combination of selective top reflector and reflective optical cavity structure significantly boosts the waveguide efficiency in the LSC. As a result of many efforts, commercially available LSCs have been demonstrated and verified in the outdoor. Also, it is expected to generate electricity in buildings by replacing conventional glass to LSCs.

• Nuclear Engineering and Technology
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• v.52 no.12
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• pp.2928-2938
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• 2020

A light water nuclear Reactor has been exergy analyzed, and the rate of irreversible exergy loss and exergy destruction is calculated for each of its components. The ratio of these losses compared to the total input exergy loss is calculated, which shows that most irreversible losses occur in the reactors, turbines, steam generators, respectively, as well as in the downstream operations. The main aim of this paper is to optimize the power plant using an innovative firefly algorithm and then to propose a novel strategy to improve the overall performance of the plant. As shown in the results, the exergy destruction rate of the plant decreased by 1.18% using the firefly method, and the exergy efficiency of the plant reached 29.3% comparing to the operational amount of 28.99%. Also, the results of the firefly optimization process compared to the Genetic algorithm and gravitational search algorithm to study the accuracy of the model for exergy analysis fitness problems in the power plants and the results of this comparison has shown that the results are nearly similar in the mentioned methods. However, the firefly is faster and more accurate in limited iterations.