atw 2018-04v6

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atw Vol. 63 (2018) | Issue 4 ı April

evaporator are elevated are that the

efficiency of power plant is increased.

When the temperature of main evaporator

and reheating steam increases

by 10 °C, the efficiency increases

by 0.5 %; and pressure increases by

10 kg/cm 2 , the efficiency increases by

about 0.2 %. Domestically, in 1990’s,

500 MW-grade standard coal thermal

power plant was designed and built,

and its operation condition was pressure

246 kg/cm 2 and temperature

538 °C.

In the case of Dangjin Thermal

Power No. 9, No. 10 and Samcheok

Thermal Power No. 1, No .2 that have

been being built, the pressure of

250 kg/cm 2 , temperature of 600 °C

were accomplished [4].

SCO 2 cycle is the power generation

technology of the Gas Brayton Cycle

method where pressurized carbon

dioxide is heated by the pressure

greater than critical condition to high

temperature and turbine is driven.

Presently, CO 2 power generation cycle

can be applied to most heat sources

used, and also it can be used for large

power plant, small scale distribution

power supply, or power supply for

marine plant.

Super critical condition means the

conditions for temperature and pressure

greater than critical point in the

general material state where liquid-gas

phase change occurs, and the

temperature and pressure at the lower

pressure part is greater than 32 °C, 74

atm, and all parts of cycle are maintained

over critical condition. While

operation is carried out at high

pressure, volumetric flow decreases,

so the size of overall heat conversion

cycle can be decreased; accordingly,

construction period and production

unit price can be lowered to secure

high economic feasibility.

Besides, compared to water vapor,

the compatibility with existing material

is excellent, so it can be supplied

to turbine at the temperature higher

than evaporator cycle. From this, the

increase of additional power generation

efficiency can be possible [5].

2.3 Heat Conversion Model

Design

IHX loop of VHTR that is studied in

the present study is the system where

the high temperature heat generated

in the reactor by connecting hydrogen

generation equipment and power

generation equipment in series can be

supplied in the same manner.

IHX loop nuclear reactor shown in

Figure 1 provides 350 MWt heat output,

and the heat generated from

| | Fig. 1.

IHX Loop Modelling.

nuclear fission is supplied to helium

fluid. For heat transfer to produce

hydrogen, heat exchanger, steam generator

for the power generation via

USC cycle, and in the power generation

via SCO 2 , one heat exchanger is

provided. In order to utilize the result

of the study regarding the existing

VHTR, the major principle and

variable if heat conversion model

were set as follows. Temperature and

pressure at No. 1, 2, 3, 4, 10 were

presumed by reference literature [8].

Temperature and pressure of ultrasuper

critical cycle No. 5, 6 and SCO 2

cycle, No. 8 were assumed by using

reference literature [9]. The model to

be explained below was defined as

reference model, and then the present

authors will plan to develop a model

that considers a variety of heat

efficiency improvement method. In

the present study, in the concept

similar to general Rankine cycle’s

reheating cycle, bypass mode was

proposed.

To begin with, the reference model

is as follows. After 910 °C helium fluid

discharging from VHTR carries out

heat exchange with heat exchanger 1,

hydrogen is produced by receiving

heat from high temperature helium

fluid in the heat exchanger 1. 846 °C

helium fluid passing heat exchanger 1

enters into steam generator 2 and go

through heat exchange. The fluid of

this steam generator is ultra-super

critical state water, and produces

power. The temperature of helium

fluid that passes through steam

generator 2 is 614.8 °C, this helium

fluid enters into heat exchanger 3

where heat exchange is carried out.

The fluid of this heat exchanger is

super critical-state carbon dioxide,

and it produces power by the heat

supplied. The temperature of helium

fluid coming out of heat exchanger 3

is 450 °C. The heat output that is produced

in heat exchanger 1 producing

hydrogen is 37.37 MWt. The mass flow

of helium from IHX is m 1 , and the

mass flow of water flowing in heat

exchanger 1 is m 2 , the mass flow of

water flowing in steam generator 2 is

m 3 , and the mass flow of CO 2 flowing

in heat exchanger 3 is m 4 . In this

study, the temperatures and pressures

from No.1 to No.10 in Figure 1 were

assumed, and m 1 and m 2 were calculated

by using the Equation (1), and

m 3 and m 4 were calculated by using

the Equation (2). Besides, considering

the characteristics of general longitudinal

temperature difference of heat

exchanger, the temperature at No. 6

and No. 9 was assumed to decrease by

10 °C compared to the temperature at

No. 4 and No. 7 of the steam generator

inlet.

Major equation or relationship for

heat equilibrium analysis is as follows:

• Equation used for calculating m 1

and m 2

: W = m∆h = m(h out – h in )... (1)

Here,W : Thermal power (MWt)

m : Mass flow (kg/hr)

h : Enthalpy (kJ/kg)

in : Entrance of the equipment

out : Outlet of equipment

• Equation used for calculating m 5

and m 8

∑m in h in = ∑m out h out ... (2)

In the case of hydrogen production, it

was assumed that all heat was

converted to work required, and in

OPERATION AND NEW BUILD 231

Operation and New Build

Heat Balance Analysis for Energy Conversion Systems of VHTR ı SangIL Lee, YeonJae Yoo, Deok Hoon Kye, Gyunyoung Heo, Eojin Jeon and Soyoung Park

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