Hiki Hong

Effects of transverse convection and solid–liquid density difference on the steady close-contact melting

Abstract The steady close-contact melting phenomenon occurring between a phase change material and an isothermally heated flat surface in relative motion is investigated analytically, with the effects of transverse convection across the liquid film and solid–liquid density difference taken into account. Scale analysis is used to estimate the dependence of system variables on characteristic parameters. Also, an analytical solution to a set of simplified model equations is obtained to quantify the effects. Transverse convection can be characterized by a dimensionless interfacial temperature gradient which asymptotically approaches unity/zero with decreasing/increasing the Stefan number. The convection effect in the liquid film can be neglected approximately for the range of the Stefan number less than 0.1. It is found that the solid descending velocity depends linearly on the liquid-to-solid density ratio, and that the ratios of solid descending velocity, film thickness and friction coefficient to the conduction solution are proportional to 3/4, 1/4 and −1/4 powers of the interfacial temperature gradient, respectively.

An Energy Saving Technique Using Ondol Heating Schedule Control of Housing Units in Korea

This study was performed to determine the heating energy savings and carbon dioxide reduction that would result from turning off ondol heating in housing units in Korea. Lowering the room temperature and intermittent heating of a room are effective methods for reducing heating energy consumption. These techniques were proven in our previous studies by modelling simulations. To confirm the simulations schedule controllers, which automatically turned off the ondol heating in the living room and kitchen from 10 pm to 6 am, were installed at the hot water distributor in five households. The heating gas used in each of the housing units was measured over a period of 5 years, with and without the schedule controllers. We found that approximately 30% of gas consumption, mainly used for heating, can be saved compared to turning on the appliance during the night.

Method and Analysis of a Dynamic Simulation of Ondol Heating

Ondol heating, a kind of radiant floor heating, is the main form of heating being used in housing units in Korea. Although Ondol can provide a more comfortable and healthy indoor environment than any other form of heating systems, a more precise simulation is required to further improve the system to prevent over and under heating of the housing environment. However, due to complexity, building energy simulation has so far not been performed, for systems such as Ondol and other relevant facilities. For evaluating energy consumption and indoor temperature variation, a new method has been proposed. In this study, a dynamic simulation of Ondol heating was carried out by combining TRNSYS and EES. This new system of building simulation could contribute to determine over or under heating of the system in a housing unit. The simulation results of a typical housing unit in Korea showed a good trend in a viewpoint of actual behaviour of Ondol heating, maintaining a comfortable and healthy indoor environment with less energy consumption.

Enhancement of Stratification for Solar Water Storage Tank with Spiral Jacket and Coil(Part 1:Verification Experiment)

Large-scale active solar heat systems are generally using heat exchanger between collector and storage tank loops to prevent damage by freezing. It is difficult to maintain stratification in a storage tank in the system owing to greater flow rate enhancing heat transfer. In the previous study, we introduced a spiral-jacketed storage tank and obtained good results to keep system performance of general level without better stratification. We added a scroll-shaped heat exchanger coil on the upper part in the spiral-jacketed storage tank. As a result of the experiment, it was verified that degree of stratification of the new type storage tank is higher than that of the previous one with a possibility of better collector efficiency and solar fraction.

Microscopic energy conversion process in the ion drift region of electrohydrodynamic flow

We theoretically investigated the momentum transfer and energy conversion process of ion-neutral and ensuing neutral-neutral collisions in the ion drift region of electrohydrodynamic flow. Our results are presented in explicit equations with physical interpretations of the phenomena. The unit conversion process was estimated to sustain for 1.0 nano-second in a very tiny 0.5-μm-sized volume in the air. Also, the continuum-based equation formulations are presented according to the microscopic energy conversion phenomena. Numerical simulations reflecting those formulations are performed to verify the theoretical results and experimentally supported by an air corona discharge.

THE EFFECT OF UPPER-HEATING SYSTEM IN SOLAR WATER STORAGE TANK

Thermal stratification in the water storage tank of solar heating system is essential to increase the collection efficiency. We previously investigated the stratification performance of side-heating system, where the hot water returning from the collector heats up the side wall of storage tank. Subsequently, we studied an evolved heating system for further improvement, where we added an upper-heating to the side-heating. Here we thoroughly examine the stratification performance of the evolved heating system using TRNSYS-based simulation. As the essential result, contrary to expectation, evolved system does not show substantial improvement of collection efficiency compared to side-heating. However, we confirm that evolved system excels in the useful energy.

Development of an HVAC&R Systems Energy Evaluation Methodology and Simulation Program for Office Buildings

HVAC&R systems are the most energy consuming building services, representing approximately half of the final energy use in the building sector. Despite their significant energy use, there is a lack of a consistent and homogeneous framework to efficiently guide research. This research is about the energy consumptions of HVAC&R systems, with the aim of establishing a common idea for the analysis of building energy efficiency. Our approach deals with the concept of an HVAC&R set that is composed of subsystems. A matrix combination evaluation is examined, and a total of 960 sets can be implemented in a large scale office building. A method as a tool for energy evaluations of HVAC&R systems, and HEET as a energy simulation program, were developed.

Freeze Protection for Passive Solar Water Heating System

In the present work, a new freeze protection method has been proposed for a natural circulation system of solar water heater. Though electrothermal wire is popularly used for the purpose, there are freezing troubles by wire cut-off and shortage of excessive electric power consumption. In the experimental device, hot water in storage tank was used to heat the outlet pipe from the tank if the pipe surface temperature falls lower than a set point. The cold water pipe to the storage tank was installed to directly contact the hot water pipe surface temperature rose by transferred heat.

Performance Enhancement of Solar Thermal Storage Tank with Heat Exchange Coils (Part 1:Verification Experiment)

Abstract A thermal storage tank with internal heat exchange coils is commonly used in solar thermal systems with a collectorarea below 100 m 2 . The coils are installed in the lower part of the tank because the temperature of the upper part of thetank can drop if the outlet temperature of the collector becomes lower than the upper temperature of the tank, which is a kind of temperature reversal. As an alternative to the well-mixed storage tank with lower coil only, we have proposed a tank with lower and upper coils and have achieved superior thermal stratification in the tank, which results in increasedcollector efficiency and solar fraction. But, the phenomenon of temperature reversal was often observed in the tank when the load or solar radiation changed rapidly. In the present work, revised control was successfully applied, i.e., to heat only the lower coil using a three way valve if temperature reversal occurs and to operate the collector at a low flow rate whenthe quality of solar radiation is not good.

Enhancement of Stratification for Solar Water Storage Tank with Spiral Jacket and Coil(Part 2 Simulation)

Abstract We have performed experiments to enhance the stratification in a storage tank in order to raise the collector efficiencyand solar fraction in solar thermal systems. The storage tank with a spiral jacket in the side wall has a scroll-shaped heat exchanger coil added to the upper part. The performance was compared between only the side and upper-side heating partthrough simulation using TRNSYS under the same weather conditions and initial conditions. As a result, the upper-side heating has a 4.2% advantage in solar fraction, but almost no increase in collector efficiency. Key words Active solar heat system(설비형 태양열 시스템), Spiral-jacketed storage tank(나선재킷 축열조), Stratification(성층화), Collector efficiency(집열효율)†Corresponding author, E-mail: hhong@khu.ac.kr 기호설명 I t ：경사면 투사 일사량 [kW] Q u ：획득열량 [kW] Q aux ： 보조열원 열량 [kW] 그리스 문자  ： 집열 효율 1. 서 론 신재생에너지 분야에서도 변환형태가 열에너지인 태양열 시스템은 경제성 및 활용도가 높아 큰 경쟁력을 갖춘 분야로 평가된다. (1) 태양열 시스템 보급의 확대를 위하여 많은 연구가 이루어지고 있으며, 이를 바탕으로 적용분야가 점차적으로 넓어지고 있는 추세이다. 분야의 활성화를 위해서는 무엇보다도 가정용 및 소형건물, 복지시설용의 소형 및 중형시스템이 집중적으로 보급되어야 하며, 이를 위해서는 기존의 문제점이 해결되고 획기적으로 기술혁신이 이루어진 시스템의 도입이 전제되어야 한다. 이를 위해 지난 연구에서 나선재킷 축열조