CASE STUDY ON HYBRIDIZATION OF MAGNETO HYDRO DYNAMIC GENERATOR

MHD Generator:

MHD is a direct heat-to-electricity conversion technic based on Faraday law that when an electric conductor moves across the magnetic field,a voltage is induced in it which produces an electric current.MHD power generation has great potential for power production in excess of 1000MW.The overall energy utilization can be improved by employing combined cycle power plant consisting of MHD Generator as a topping plant & a gas or steam turbine as a bottoming plant.The overall efficiency of about 60% can be achieved in the combined cycle.In the present work, a case study has been made on hybridization of MAGNETO HYDRO DYNAMIC GENERATOR. The performance of MHD generator without hybridization is compared with the hybridized unit. It is observed that efficiency of MHD generator can be almost increased by three times by hybridizing it with gas and steam turbines.

INTRODUCTION

Magneto hydrodynamic generator is a direct heat to electricity conversion technic based on faraday’s law. The field of magneto hydro dynamic was initiated by Hannes Alfrel for which he received Noble prize in 1970.Magneto means``magnetic field”.

Hydro means ``fluid”and dynamic means``movement”.

FARADAY’S LAW:

When an electric conductor moves across a magnetic field a voltage is induced in it which produces an electric current.Here the conductor is an ionized gas which is passed at a high velocity through a powerful magnetic field,current is generated and can be extracted by placing electrodes in suitable position in the stream.It produces D.C.power directly.

WORKING OF OPEN CYCLE MHD:

In. open cycle, the fuel is burned with preheated oxygen at 1100° c.The hot pressurized working fluid at 2300° c to 2700° c is seeded with potassium carbonate to ionize the gas.This is passed through convergent-divergent nozzle which is used to increase the velocity to 1000m/sec to get desired mass motion energy.After this,these gases passes through MHD duct where magnetic field acts perpendicular to the direction of gas motion.The electrode pair is connected in different ways to reduce lossea. The D.C.power generated is converted into A.C. power by inverter. In preheater, the oxygen or oxygen enriched air is heated to a temperature of 1100° c.In seed recovery unit we seed the working fluid in the combustion chamber inorder to uncrease the conductivity of fluid.

WORKING OF CLOSED CYCLE MHD:

In a closed system, Helium or Argon is used as working fluid,which is heated in a heat exchanger.Due to this higher temperature and better thermal efficiency are possible.Here a liquid metal is mixed with a inert gas to form working fluid.The liquid metal provides conductivity.The gas is heated by the combustion of fluid gas to 1900°c and seeded by Cesium injection.It is passed through MHD at high speeds and produced D.C.power is converted to A.C. by inverter.The working fluid is slowed down in diffuser and precooled.The precooled gas is compressed for heating.

HYBRIDISATION OF MHD GENERATOR:

The overall energy utilization can be improved by employing combined cycle power plant consisting of MHD generator as a topping plant and a gas or steam turbine as a bottoming.The overall efficiency of about 60% can be achieved in the combined cycle.If the gas entering the MHD duct at about 3000°c could be expanded to the ambient temperature oe 30°c.The carnot efficiency would have reached 90%.Unfortunately the MHD power is restricted because by the time the gas temperature falls to 200° c,the electrical conductivity becomes very low with the electrons combuning with ions to form nutral atoms,and the generator then ceases to operate satisfactorly.Therefore the MHD exhaust at about 200° c is utilized in rising steam to drive turbine and generate electricity in a conventitional steam power plant.MHD lopped steam plants can operate either in an open or closed cycle.A gas turbine plant can also be used as a bottoming unit.Since the combined cycle plant operates over a large rate temperature difference the efficiency will obviously be higher.

PERFORMANCE ANALYSYS OF MHD GENERATOR

The air enters the low pressure compressor and gets compressed but it’s compression ratio is less.Then it passes through intercooler and fed to high pressure compressor. As it’s compression ratio is high, the air coming out of HP compressor has high pressure.It is preheated in regenerator and supplied to combustor.As it is preheated ,heat required to be supplied in combustor is less.After this it is expanded in the nozzle and comes out with high velocity.This high velocity gas enters into MHDgenerator.The outlet power from MHD is used to run the compressors and for regeneration purpose.

Assuming the following specifications as per INDIAN experience:

P1=Pressure at inlet of low pressure compressor=1bar

T1=Temperature at inlet of low pressure compressor=300k

T6=Temperature after combustion process=2900k

Pressure ratio=4

P4=Pressure after combustion process=4bar

MHD duct length=1.2m

MHD inlet crosssection=8*10cm2

MHD outlet crosssection=8*25cm2

Magnetic flux density=3.5wb/m2

Gas conductivity=10mhos/m

Efficiency of compressor=80%

Efficiency of nozzle=90%

Effectiveness of regenerator

CALCULATIONS:

As it is perfect inter cooling

P2=sq root of(P1*P4)

P2=2bar

1-2`: Isentropic compression in LP compressor

(T2`/T1)=(P2/P1)^(g-1/g)

g=1.4 for air

T2`=365.7k

1-2:Actual expansion in LPC:

Eff of compressor=(T2`-T1)/(T2-T1)

T2=382.13k

As it is a perfect inter cooling ,T3=T1=300k

T2`=T4`=365.7k

T2=T4=382.13k

Cp=Sp. Heat at constant pressure of gas=1.0465kj/kg-k

Cv=Sp. Heat at constant volume of gas=.7744kj/kg-k

g=(Cp/Cv)

g=1.3513

6-7`:Isentropic expansion in nozzle

T7`/T6=(P7`/P6)^(g-1/g)

T7`=2022.314k

Eff of nozzle =(Actual expansion/Isentropic expansion)

=(T6-T7)/(T6-T7`)

T7=1924.79k

Cp(T6-T7)=( C7^2-C6^2)/2*g

C7=1428.674m/sec

Considering MHD crosssection

Inlet area=8*10cm^2

Outlet area=8*25cm^2

Average plate area=.42m2

PV=RT

V=5.2373m3/kg

By mass balance at inlet section

M=A7*C7./v

M=2.1823kg/sec

By mass balance at outlet section

M=A8*C8/v

C8=571.468m/sec

Avg velocity of flow=U=(C8+C7)/2

U=1000m/sec

Open circuit voltage,E0=B*U*d

Where,

B=Magnetic flux density

U=Average velocity

D=distance between plates

E0=280v

Generator resistance,Rg=(distsnce between electrodes/gas conductivity*average area)

=(d/s*A)

=0.019W

maximum power,Wmax=(Eo^2/4Rg)

=1.029MW

Part of this generated power is used to run the compressor and it is equal to=2mCp(T2-T1)

=0.375MW

Total power output=MHD output-compressor input

=0.654MW

Thermal input:

Regenarator effectiveness E=(T5-T4)/(T8-T4)

Pv=RT8

T8=1924.78k

T5=1539k

Thermal input=mCp(T6-T5)

=3.108MW

MHD Efficiency,h=21%

PERFORMANCE ANALYSIS OF MHD GENERATOR WITH

GAS TURBINE

The output from MHD generator is passed through gas turbine,as the gases from MHD generator have high velocity.And this is used in generating power in gas turbine. Therefore overall efficiency of MHD increases.

Consider 50% reaction turbine for which Cf1=Cf2,

a1=b2

a2=b1

And blade velocity ratio=0.4

a1=20°

Ca1=571.468m/sec

Cb=228.5872m/sec

From velocity diagram,

Cf1=200m/sec,

Cw1=535m/sce,

b1=35°

Ca2=380m/sec,

Cw2=290m/sec

Power output from gas turbine=(Cw1+Cw2)*Cb*m/gc

=0.41153MW

Efficiency =(MHD output+Gas turbine output)/Thermal input

=(0.654+0.41153)/3.108

h =34.28%

PERFORMANCE ANALYSIS OF MHD GENERATOR WITH GAS AND STEAM TURBINE :

The output from the gas turbine is used to heat the water in the boiler . The heated steam is fed to the steam turbine and their the steam is expanded upto the condenser pressure . By using alternator at the turbine we get power output.

Assuming , the condenser pressure =40 º c

Degree of superheat = 50º c

Saturated pressure at 40º c=20 bar

Temperature at a =362.4º c =635.4 k

Calculations:

At 40º c , Pressure = 0.07375 bar

Specific enthalpy = 167.5 KJ/KG

At 20 bar , Saturated Temperature = 212.4 º c

Superheated Temperature = 262.4º c

Specific enthalpy =2932.80 KJ/KG

Temperature out of gas turbine =1924.78 k

Heat utilized for heating in the boiler = Q =m Cp (T8- Ta)

=2.9446 MW

Heat increase of the water in the boiler = Q =mw (h at 20 bar - h at 0.07375 bar )

By Heat Balance,

mCp (T8-Ta)=mw (h at 20 bar – h at 0.07375 bar)

=> mw = 1.06495 Kg/Sec

Calculations of the Steam Turbine ,

At 20 bar, sp.entropy = 7.00216 KJ/Kg-Sec

1-2 is isentropic process,

s1=s2

s1= sf+x(sfg)

=> x2 =0.8366

h2=hf+x2 hfg = 2181.128 KJ/Kg

h1= 2932.80 KJ/Kg

h1-h2 = (c1^2 - c2^2)/ 2g

=> c1 = 1226.108 m/Sec

Cw1+Cw2 = 2Ca1 Cos α1 –Cb

= 1813.886 m/Sec

Output Power = (Cw1+Cw2)Cb(m/g)

= 0.9465 MW

Overall Efficiency=(Total Power / Thermal Input )

= 64.73 %

CONCLUSION:

In the present work a case study has been made on the hybridization of MHD generator. The thermal efficiency of MHD generator is estimated with and without hybridization. The following performance values are obtained:

Thermal efficiency of MHD Generator without hybridization = 21%

Thermal efficiency of MHD Generator combined with Gas Turbine Plant = 34%

Thermal efficiency of MHD Generator combined with Gas & Steam Turbine Plants = 64%

From the above case study results, it is understood that the performance of the MHD Generator can be drastically improved by hybridization. This case study will be useful in ascertaining the improvement in the thermal efficiency of hybridized MHD Generators.

REFERENCES:

1. Non - Conventional Energy Resources - Dr . R.K. Singal

2 . Power Plant Engineering - G.D.Rai.

3 . Power Plant Engineering - Arora & Domkunduwar