Young-Soo Lee

Numerical and experimental study on the design of a stratified thermal storage system

Abstract The purpose of this study is to figure out the thermal stratification mechanism of a storage tank and thereby to determine optimum design and operating conditions. To this end, a computer program is developed to investigate the fluid flow in a tank, using Patankars SIMPLE algorithm. The validation of program is made successfully by the comparison against experimental data measured with a bench scale facility. Further a systematic investigation has been made in terms of important design and operational parameters such as storage tank size (commercial-scale and bench-scale), loading time, shape of diffuser, turbulence model and inlet velocity or Fr No. Considering the thermal efficiency of storage tank is critically impaired by the effect of flow recirculation and mixing by turbulence, a novel model with minimum mixing between hot and cold water is proposed for the evaluation of the performance of storage system by the assumption of the uniform plug-type flow. This is made by solving only the following governing equation of temperature which has no convective mixing with constant axial velocity, that is, ∂ (ρT) ∂ t +u ∂ (ρT) ∂ x =∇· k c p ∇T Based on a series of parametric investigation, a number of useful results can be drawn. In general the large scale system shows better storage performance than the small system. As the increase of loading time, the degree of stratification lowered due to the increased effect of heat transfer by convection and diffusion via thermal stratification region. And the curved type diffuser shows better performance of thermal stratification compared to the diffuser of flat type. As for the case of large-scale tank, the effect of Fr No., or inlet velocity does not show any significant effect on the thermal stratification compared to the small size one. The calculated results show that the model of ideal type-plug flow can be used as a possible tool for the evaluation of performance of storage system by giving the reference condition of best thermal stratification or least mixing condition. But in the case of full scale system, the difference between ideal and actual cases not significant compared to the case of bench scale. It is considered possibly due to the relatively decreased heat transfer effect by the increased effect of water flow in the full scale system. Two turbulence models, say standard k – e and RNG k – e models, show no visible difference in this study.

Power Maximization of a Heat Engine Between the Heat Source and Sink with Finite Heat Capacity Rates

ABSTRACT:In this study, the theoretical maximum power of a heat engine was investigated by sequential Carnot cycle model, for a low-grade heat source of about 100℃. In contrast to conventional approaches, the pattern search algorithm was employed to optimize the two design variables to maximize power. Variations of the maximum power and the optimum values of design variables were investigated for a wide range of UA(overall heat transfer conductance) change. The results show that maximizing heat source utilization does not always maximize power.Keywords:Sequential carnot cycle(연속-카르노 사이클), Power maximization(출력 극대화) †Corresponding author Tel.: +82-42-860-3062; fax: +82-42-367-5067 E-mail address: minsungk@kier.re.kr 기 호 설 명  ：열용량,   [kW/K] ：질량유량 [kg/s] ：미소 카르노 사이클의 수 ：전열량 [kW] ：온도 [℃] ：열관류율 [kW/K] ：출력 [kW]  ：열교환기 내 대수평균 온도차 [℃]  ：열교환기 내 최소 온도차 [℃] 하첨자 C ：냉각수, 저온측H ：열원, 고온측h ：카르노 사이클의 고온부I ：입구L ：저온측l ：카르노 사이클의 저온부O ：출구 1. 서 론 에너지 및 환경 문제에 대처하기 위하여 우리 정부에서는 신에너지 및 재생에너지 개발․이용․보급 촉진법을 마련하여 정책적인 지원을 아끼지 않는 등 신에너지 및 재생에너지의 기술개발․이용․보급촉진과 관련 산업의 활성화를 위해 다각적인 노력을 경주하고 있다. 이와 관련하여, 최근에는 비화산지대의 지열, 태양열, 해양온도차 등 저온성 재생에너지원이 부각되면서 이들을 이용하여 전기에너지를 얻으려는 시도가 꾸준히 이루어지고 있다.

The effects of surface tension and wire diameter on the rise velocity of a bubble in a miniature two-phase closed thermosyphon

The rise of a large gas bubble in a miniature two-phase closed thermosyphon with a thin wire insert has been analyzed by the potential flow theory. The effect of the interfacial surface tension is explicitly accounted for by the application of the Kelvin-Laplace equation and solved for the bubble shape. The solution is expressed in terms of the Stokes stream function which consists of an infinite series of Bessel functions. The conditions of the bubble movement in a miniature two-phase closed thermosyphon were theoretically ascertained. The analytical results were compared with the experimental results.

Analysis of Performance of Heat Pump System with Flue Gas Heat Recovery through Field Test

Abstract A field test of a 70 ㎾ heat pump system with flue gas heat recovery was performed by an experiment at theKorea Institute of Energy Research. The flue gas is exhausted from a 320 RT absorption chiller-heater in the heating season.Using this flue gas, source water of the heat pump is heated by a condensed-type heat exchanger in the chimney. Theoperating characteristics of the heat recovery heat pump system were analyzed. Based on the results of the experiments, operating maps were obtained, and an optimum operating range is suggested, in which the return and heat source watertemperature are 51 ℃ and 31 ℃ , respectively. Additionally, economic analysis of this system was conducted and about 50%energy cost savings can be expected in the heating season. Key words Condensing heat recovery(응축열회수), Flue gas(배가스), Heat recovery system(열회수 시스템), Latent heat(잠열), Condensation(응축)†Corresponding author, E-mail: yslee@kier.re.kr 기호설명COP H ： 난방성능계수 Q cond ： 응축(난방)열량 [ ㎾ ] Q evap ： 증발열량 [ ㎾ ]

Potential Performance Enhancement of Dual Heat Pump Systems through Series Operation

In this study, the potential performance enhancement in a dual heat pump system through series operation was investigated by a comparison between the performance for parallel and series operation for a heating supply temperature of . To compare the performance of each configuration fairly, the heat transfer surface area of the heat exchangers was fixed. The inlet temperatures and the flow rates of the heat source and the load were also fixed. In addition, the heat transfer and pressure drop characteristics of the working fluids were considered to achieve a more realistic comparison. The results show that the heating coefficient of performance (COP) of the series configuration is approximately 5% higher than that of the parallel configuration under the simulation conditions considered in the present study.

Power Enhancement Potential of a Low-Temperature Heat-Source-Driven Rankine Power Cycle by Transcritical Operation

In this study, the power enhancement potential of a Rankine power cycle by transcritical operation was investigated by comparing the power of an HFC-134a subcritical cycle with that of an HFC-125 transcritical cycle, for a low-grade heat source with a temperature of about 100°C. For a fair comparison using different working fluids, each cycle was optimized by three design parameters from the viewpoint of power. In contrast to conventional approaches, the working fluids heat transfer and pressure drop characteristics were considered in the present approach, with the aim of ensuring a more realistic comparison. The results showed that the HFC-125 transcritical cycle yields 9.4% more power than does the HFC-134a subcritical cycle under the simulation conditions considered in the present study.

Power Optimization of Organic Rankine-cycle System with Low-Temperature Heat Source Using HFC-134a

: 엑서지 손실율 [W] f : 마찰계수 h : 2열전달계수 [W/mK] i : 엔탈피 [J/kg] k : 내측관의 열전도계수 [W/mK] L : 길이 [m] m& : 질량유량 [kg/s] P : 압력 [kPa] Q& : 전열량 [W] T : 온도 [℃] UA : 총열관류율 [W/K] W& : 동력 [W] x : 건도 하첨자 C : 응축기 CI : 냉각수 입구 CO : 냉각수 출구 CW : 냉각수 HI : 열원수 입구 Key Words : Organic Rankine Cycle(유기랭킨사이클), Low-Temperature Heat Source(저온 열원), HFC-134a 초록: 본 연구에서는 지열발전 등과 같은 저온 열원을 에너지원으로 하는 발전에 응용될 수 있는 HFC-134a 유기랭킨사이클의 출력 극대화를 수행하였다. 기존의 연구와는 달리, 본 연구에서는 열교환기 해석에 유한체적법을 적용함으로써 작동유체의 열전달 및 압력강하 특성을 고려하였다. 또한, 열원과 냉각수의 입구온도 및 유량, 그리고 사이클을 구성하는 열교환기들의 총 전열면적을 구속 조건으로 함으로써, 기존 연구들에 비해 보다 현실적인 결과를 얻을 수 있도록 하였다. 사이클의 출력은 3 개의 설계인자를 이용하여 최적화 하였다. 시뮬레이션 결과, 출력을 극대화 시킬 수 있는 설계인자들의 최적 조합이 존재함을 보였다. 또한, 출력 향상을 위해서는 증발과정의 개선이 우선적으로 필요함을 보였다. Abstract: In this study, an organic Rankine-cycle system using HFC-134a, which is a power cycle corresponding to a low-temperature heat source, such as that for geothermal power generation, was investigated from the view point of power optimization. In contrast to conventional approaches, the heat transfer and pressure drop characteristics of the working fluid within the heat exchangers were taken into account by using a discretized heat exchanger model. The inlet flow rates and temperatures of both the heat source and the heat sink were fixed. The total heat transfer area was fixed, whereas the heat-exchanger areas of the evaporator and the condenser were allocated to maximize the power output. The power was optimized on the basis of three design parameters. The optimal combination of parameters that can maximize power output was determined on the basis of the results of the study. The results also indicate that the evaporation process has to be optimized to increase the power output.

Distribution of Hot Tap Water Load for District Heating Substation with Hot Tap Water 2-Stage Heat Exchanger

According to the standards for district heating substation established by Korea District Heating Corporation, water heating supply systems at over 150 Mcal/h capacity must employ the 2-stage heat exchanger that improves the system efficiency by reusing the heat included in the return water of district heating system already used for space heating. In this paper, the operating characteristics of the system in accordance with the load distribution of two heat exchangers for pre-heating and re-heating cold city water are investigated. The results including mass flow rate, return temperature etc. help to manage district heating system economically.

A Simulation Study on the Annual Heating Performance of the Seawater-Source Screw Heat Pump

Inthisstudy,inordertoutilizetheseawaterasaheatsourceatGangneungcityneartheEastSeainKorea, anannualheatingperformanceofascrewheatpumpwassimulated.Forasimulation,themaximum heating capacityofheatpumpwasassumedat3.5MW.AnambienttemperatureatGangneungcitywascalculatedfrom theTMY2weatherdata,whiletheseawatertemperaturewascalculatedfromtheregressionequationbasedon themeasurementbytheNationalFisheriesResearchandDevelopmentInstituteofKorea.Theheatingloadwas assumedlinearlydependentontheambienttemperature,whilethemaximumheatingloadwasassumedtoappear whentheambienttemperatureisbelow-2.4℃,whichisthetemperatureofTAC2.5%forheatingatGangneung city.A heatpumpperformanceatfull-loadwascalculatedfrom theregressionequation,whichinvolves refrigerantsevaporatingandcondensingtemperatures,basedonacommercialscrewcompressorperformance map.Aheatingsupplytemperaturewhichdeterminesrefrigerantscondensingtemperaturewasassumedlinearly dependentontheheatingload.Aperformancedegradationduetothepart-loadoperationofheatpumpwasalso considered.Simulationresultsshowthatanannualheatingcoefficientofperformance(COPH)ofaseawater-source screwheatpumpisapproximately2.8andthatitisnecessarytoimprovepart-loadperformancetoincreasean annualperformanceoftheheatpump.

Simulation of an Absorption Power Cycle for Maximizing the Power Output of Low-Temperature Geothermal Power Generation

In this study, an absorption power cycle, which can be used for a low-temperature heat source driven power cycle such as geothermal power generation, was investigated and optimized in terms of power by the simulation method. A steady-state simulation model was adopted to analyze and optimize its performance. Simulations were carried out for the given heat source and sink inlet temperatures, and the given flow rates were based on the typical power plant thermal-capacitance-rate ratio. The cycle performance was evaluated for two independent variables: the ammonia fraction at the separator inlet and the maximum cycle pressure. Results showed that the absorption power cycle can generate electricity up to about 14 kW per 1 kg/s of heat source when the heat source temperature, heat sink temperature, and thermal-capacitance-rate ratio are 100°C, 20°C, and 5, respectively.