Si-Doek Oh

Exergoeconomic analysis of thermal systems

A combination of exergetic and economic analysis for complex energy systems is proposed. We derive a general cost-balance equation which can be applied to any component of a thermal system. In this study, the exergy of a material stream is decomposed into thermal, mechanical and chemical exergy flows and an entropy-production flow. A unit exergy cost is assigned to each disaggregated exergy in the streams at any state. This methodology permits us to obtain a set of equations for the unit costs of various exergies by applying the cost-balance equation to each component of the system and to each junction. The monetary evaluations of various exergy (thermal, mechanical, etc.) costs, as well as the production cost of electricity of the thermal system, are obtained by solving the set of equations. The lost costs of each component of the system can also be obtained by this method. The proposed exergy-costing method has been applied to a 1000-kW gas turbine cogeneration system. Our exergy costing method provides information on decisions about the design and operation of the cogeneration system.

Bubble dynamics on the evolving bubble formed from the droplet at the superheat limit

Abstract The violent oscillation of the bubble formed from the evaporated droplet at the superheat limit has been investigated analytically and numerically. In this study, we have formulated a general bubble dynamics model, which is suitable for the oscillating bubble in an incompressible liquid medium. One of distinct features of this model is that the velocity and temperature distribution of the gas inside the bubble are obtained by solving continuity and energy equations for the gas analytically. With uniform density and temperature distribution approximation, the calculated values of the far field pressure signal from the evolving bubble formed from the fully evaporated droplet are in good agreement with experimental results.

A Study of Bubble Behavior and Boiling Heat Transfer Enhancement under Electric Field

The effect of a D.C. electric field on nucleate boiling heat transfer for refrigerants, R11, R113, and FC72, was investigated experimentally in a single-tube shell/tube heat exchanger by using the temperature control method of wall superheat. Also the behavior of bubble under nonuniform electric field produced by wire electrodes was studied by numerical calculation. For R11, the electrohydrodynamic (EHD) enhancement for boiling heat transfer was observed for all ranges of wall superheat tested. However, the enhancement in boiling heat transfer disappeared if the wall superheat exceeded 13°C for R113, and no electric field effect on the boiling heat transfer was observed for FC72. An application of approximately 5 kV was enough to eliminate the boiling hysteresis for R11 and R113. Numerical study of the electric field in a single medium has hinted that the bubbles are forced away from the heating surface and toward the electrostatic stagnation point by the dielectrophoretic force. Such modified bubble motion turns out to promote the boiling heat transfer if one uses proper electrode configuration.

Thermoeconomic installation limit of PV/WT hybrid energy systems for Off-grid islands

ABSTRACT An ideal off-grid island can become 100% energy-sufficient if one installs renewable energy systems such as solar photovoltaic (PV) and wind turbine (WT) systems. However, the intermittent and uncertain nature of the power supply from renewable energy systems hinders a 100% autonomy level (AL) without an infinite energy storage capacity. The thermoeconomic installation limit (TEIL) of a PV/WT hybrid energy system was studied using hourly weather data and the energy demand profile for off-grid islands. An appropriate battery size for the TEIL was also determined. Given the current installation cost of the hybrid energy system and the battery unit, the AL for a PV/WT hybrid energy system at the TEIL is calculated to be approximately 70%. Above the limit, the size of the energy storage unit and, correspondingly, the total annual cost of the PV/WT hybrid energy system increase sharply.

Electrohydrodynamic (EHD) enhancement of boiling heat transfer of R 113+WT4% ethanol

Nucleate boiling heat transfer for refrigerants, R113, and R113+ wt4% ethanol mixture, an azeotropic mixture under electric field was investigated experimentally in a single-tube shell/ tube heat exchanger. A special electrode configuration which provides a more uniform electric field that produces more higher voltage limit against the dielectric breakdown was used in this study. Experimental study has revealed that the electrical charge relaxation time is an important parameter for the boiling heat transfer enhancement under electric field. Up to 1210% enhancement of boiling heat transfer was obtained for R113+wt4% ethanol mixture which has the electrical charge relaxation time of 0.0053 sec whereas only 280% enhancement obtained for R113 which has relaxation time of 0.97 sec. With artificially machined boiling surface, more enhancement in the heat transfer coefficient in the azeotropic mixture was obtained.

Economic Optimization of a Cogeneration System for Apartments in Korea

A cogeneration system which can be used as a distributed generation source produces electricity and heat energy simultaneously from a single source of fuel. For industrial and domestic applications, where both kinds of energy are required, the cogeneration system can return fossil fuel energy savings up to 30%, and can reduce CO2 emissions correspondingly as compared with a conventional system. In this study, eight apartments with residential areas in the range of 7200 m2 to 18200 m2 were chosen to study how much energy savings can be achieved by adoption of the cogeneration system in those apartments. Based on the energy demand data for heat and electricity, an optimum configuration of the cogeneration system for each apartment was determined by a developed computer program. The economic gain achieved by introducing the cogeneration system in those apartments was estimated and the monitored values compared with the estimated ones. By adoption of the cogeneration system, the fossil fuel saved was more than 30% and an average economic gain of

Exergoeconomic analysis of gas turbine cogeneration systems

3.6/m2 per year was obtained.Copyright