1

Magnetohydrodynamics For Advanced Power Generation System

First page Template Secretariat uses only. Do not type in this box.

HARADA, Nobuhiro Nagaoka University of Technology

1603-1 Kamitomioka, Nagaoka 940-2188, Japan

Abstract

Recently increase of electric power generation efficiency becomes much more important not only to save energy resources but also to reduce CO2 emission to decelerate global climate change. Conventional power generation systems are reviewed and suggestions are summarized as; increase working temperature, don’t use condenser of steam-turbine to reduce exhaust heat, and construct energy re-circulating type system in order to increase plant efficiency and also use nuclear power with high efficiency in order to reduce CO2 emission. Energy re-circulating type Nuclear/MHD single power system was proposed to achieve high efficiency of over 55% using high operating temperature and eliminating bottoming cycle. For reduction of CO2emission, CO2 recovery type generator system was proposed, which has special features of using coal synthesized gas burning with pure oxygen and with heat recovery systems of regenerative coal gasification process, fuel pre-heating, and steam de-composition. Plant efficiency can be expected over 50% including oxygen production.

Keywords: Magnetohydrodynamics, MHD power generation, high efficiency, high temperature gas cooled reactor, reduction of CO2 emission

1 INTRODUCTION

Magnetohydrodynamic (MHD) power generation has been studied as a novel commercial power plant due to its inherent advantage of high-efficiency with high-working temperatures. Let us review the principle of an MHD power generation shown in Figure 1. This process is based on Faraday’s electro-magnetic induction law. In an MHD generator, electrically conductive fluid is moving across magnetic field as an armature coil is rotating in magnetic field in the rotating generator. It is obvious that power generation is an energy conversion process. If electric power is extracted in an MHD generator, Lorentz force of output current and applied magnetic field acts to decelerate the working plasma, and therefore, the working plasma loses its

enthalpy which is converted into electric power in the MHD generator. Merits of MHD power generation are as follows; 1, simple structure, 2, working at high-temperatures, 3, high Carnot-cycle efficiency, and 4, easy to realize combined cycle with other systems. In the present study, main objectives are to point out issues of the currently used power generation systems and to propose near future high-efficiency power generation system based on MHD process.

Figure 2. Efficiency of various power generation systems.Figure 1. Principle of an MHD power generation.

The International Conference on Electrical Engineering 2008

No. O-043

2 ISSUES OF CURRENT POWER GENERATION SYSTEMS

It is recognized more important to increase efficiency not only for saving energy resources but also for reducing CO2emission recently. Electric power generation system is known as a heat cycle whose efficiency is restricted by the second law of thermodynamics. Actual efficiency is limited below the Carnot efficiency which is determined mainly by the highest cycle temperature because lower heat exhaust temperature is generally temperature of environment. Figure 2 shows efficiency of various power generation systems. Plant efficiency of advanced coal fired steam-turbine is slightly higher than 40% at the temperature of 500C. Efficiency of advanced BWR in Japan stays below 35% due to poor steam conditions for safety reasons. We understand that steam-turbine system can reach up to about 40% and we have to combine other system working higher temperatures if higher efficiency is required. Gas turbine system is already commercialized. Efficiency of combined cycle with gas-turbine/steam-turbine is up to 52% at gas temperature of 1500C. However, this level may be the upper limit because of complexity, material durability and delicate structures. On the other hand, for MHD generators, highest working temperature is 2400C for closed cycle MHD and up to 3300C for open cycle. Efficiency is expected in excess of 60%. For closed cycle system, working gas is inert gas of argon or helium. It needs high temperature heat exchanger in spite of any heat source can be used. Figure 3 shows schematic of conventional steam-turbine system. Efficiency is higher about 40% at relatively lower

temperature of 800C. However, we can see so much heat energy is exhausted to environment through condenser. It is necessary to recover steam to water. Although nuclear powered steam-turbine is one of the lowest CO2 emission system, efficiency is kept lower than 35% due to poor steam conditions. Figure 4 shows schematic of gas-turbine/steam-turbine combined cycle. Again, we can see that much heat is lost through condenser and as exhaust gas. And therefore, efficiency seems relatively low even if system temperature is increased from 800C of steam-turbine to 1500C of combined cycle.From the above review of the present power generation systems, it can be summarized as follows: In order to increase plant efficiency, 1, increase working temperature, 2, don’t use condenser of steam-turbine to reduce exhaust heat, and 3, construct energy re-circulating type system. Also, in order to reduce CO2 emission, use nuclear power with high efficiency. We have to construct nuclear powered energy re-circulating type system.

3 PROPOSAL OF NEAR FUTURE POWER GENERATION SYSTEM

3.1 Energy Re-Circulating LNG/MHD System

Figure 5 shows proposed energy re-circulating type MHD power generation system with LNG heat source which has been proposed by Prof. Y. Okuno at Tokyo Institute of Technology.[1] The system does not combined with any other system and is called closed cycle MHD single system. We can see that plant efficiency is expected over 60% even the enthalpy extraction ratio of the MHD generator is only 30%. Thermal input to the MHD generator is 200 and electric output is 60 in spite of only 100 input thermal energy to the system because 100 of heat is recovered by regenerator. Enthalpy extraction ratio of above 30% was achieved by experiments with shock tube facility.[2] So this estimation of efficiency is considered to be realistic in near future.

Figure 3. Conventional steam-turbine system.

Figure 4. Gas-turbine/steam-turbine combined cycle.

Figure 5. Energy Re-circulating type MHD single system with LNG as heat source.

2

July 6-10, 2008, OKINAWA, JAPAN

3.2 Energy Re-Circulating Nuclear/Gas Turbine System

It is pointed out that efficiency of power generation system with nuclear fission reactor must be increased in order to reduce CO2 emission. Energy re-circulating type gas-turbine single system with nuclear reactor is proposed. Schematic of this system is shown in Figure 6. Here, working gas is re-circulating helium and high temperature gas-cooled reactor (HTGR) is considered to be used in this system. We can expect high plant efficiency about 47% in contrast with 35% for the case of BWR/steam-turbine system. This remarkable increase in efficiency results in saving by over 25% of nuclear fuel consumption. Main issue may be development of increase operating temperature of HTGR.

3.3 Energy Re-Circulating Nuclear/MHD System

Because, in previous nuclear/gas-turbine system, highest working temperature is considered to be 850C owing to requirement from difficulty in developing HTGR, its efficiency is relatively low. A system using MHD generator to achieve higher efficiency is proposed as shown in Figure 7. This system is a special power generation system driven by HTGR directly connected with MHD single power generation system for space applications. Typical gasdynamic parameters of heat, Q in MW, temperature, T in K, and pressure, P in MPa are shown in this figure. Working medium of helium mixed with xenon is used so as to connect closed cycle MHD system directly to HTGR. We exclude alkali-metal seed from the system. Mixed inert gas (MIG) system has been studied to eliminate system complexity of seed injection, mixing and recovery.[3,4] Ionization potential of MIG working medium is much higher than that of inert gas seeded with alkali-metal, and therefore, ionization level, namely electrical conductivity, is not enough at the temperature of the reactor exit, ~1800K. So, it must be pre-ionized electrically. Disk shaped Hall-type MHD generator is used for simple geometry, fewer electrode connections and simple structure of superconducting magnet. Regenerator which is installed just downstream of the MHD generator can regenerate heat exhausted from the generator in order to minimize waste heat radiated from the radiation cooler and to improve plant efficiency. Other components are staged compressor with intercoolers and radiation cooler.

Figure 6. Energy Re-circulating type gas-turbine single system with nuclear reactor as heat source.

Input power from the HTGR is fixed as 5MWth. Thermal input to the MHD generator is about 13MW which is the sum of input thermal energy from HTGR of 5MW and recovered one from the regenerator of about 8MW. Enthalpy

extraction ratio, which is the ratio of output electrical power to thermal input, is assumed to be reasonable value of 35%. Generated electric power is 4.5MWe and net output is 2.76% because some of the generated power is used for compressor power. Finally, total plant efficiency reached to 55% with the 5MW input heat to this system and 2.76MWe net output electric power. Main reasons of high efficiency are high operating temperature and thermal energy recovery by regenerating heat exchanger. We have to note that we can reduce fuel consumption by 25% by the increase in efficiency from 40% to 50% and almost no CO2 emission. We recognized that this system will be an important candidate in near future.

Figure 7. Energy Re-circulating type MHD single system with nuclear reactor as heat source for space applications.

A) Number of Compressor Stages Figure 8 shows compressor power and total plant efficiency against number of compressor stages. It can be seen that plant efficiency increases with the increase of compressor stage number owing to increase number

3

The International Conference on Electrical Engineering 2008

of intercoolers which can reduce power worked by the working gas. We have to note, however, this effect is not

significant at the number in excess of 4. So, we should reasonably choose the number of compressor stages of 3. B) Regenerator Efficiency Figure 9 shows T-s (temperature-entropy) diagram of the present system for regenerator efficiencies of 1.0, 0.6 and 0.2. Here the conditions at the exit of HTGR are set as the reference point of entropy where both temperature and pressure are fixed. For the case of regenerator efficiency of 1.0, working gas gains thermal energy at the process 1 through 3 in the figure; heat from the regenerator at 1 to 2 and heat from the HTGR at 2 to 3. Point 3 and 4 correspond the exit of HTGR and of MHD generator, respectively. We can see that area enclosed T-s diagram becomes larger, namely increase in output power for higher regenerator efficiency. So, regenerator is necessary to achieve such high plant efficiency.

1 2 3 4 5 60.0

0.2

0.4

0.6

0.8

1.0

0.0

0.5

1.0

1.5

2.0

2.5

Plan

t Effi

cien

cy

Number of Compressor StagesC

ompr

esso

r Pow

er (M

We)

Comp. Power

Plant Eff.

Figure 8. Influence of compressor stage number on compressor power and plant efficiency.

0200400600800

100012001400160018002000

Reff =1.0Reff =0.6Reff =0.2

Tem

pera

ture

, T (K

)

Entropy, s

Influence of Regenerator EfficiencyReactor Exit

1

2

3

4

5

60

200400600800

100012001400160018002000

Reff =1.0Reff =0.6Reff =0.2

Tem

pera

ture

, T (K

)

Entropy, s

Influence of Regenerator EfficiencyReactor Exit

1

2

3

4

5

6

Figure 9. T-s (temperature-entropy) diagram of the present HTGR/MHD system for various regenerator efficiencies.

0.0 0.2 0.4 0.6 0.8 1.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0

2

4

6

8

10

12

14

16

18Influence of Regenerator Efficiency

Plan

t Effi

cien

cy

Regenerator Efficiency, Reff

Plant Efficiency

Qreg

Qre

g and

QM

HD (M

Wth

)

QMHD

Figure 10. Influence of regenerator efficiency on regenerated power, thermal input to MHD

generator and plant efficiency.

Figure 10 shows regenerated power Qreg by the regenerator (at the process 1-2 in Fig.9), thermal input to the MHD generator QMHD and total plant efficiency as a function of regenerator efficiency. Here, temperature difference between high- and low-temperature fluids is assumed as 50 K. When regenerator efficiency is decreased, i.e. regenerated heat is reduced; thermal input to the MHD generator is also decreased as the thermal output from the HTGR is kept the same level of 5 MW. Thus decreased output power from the MHD generator results in the decrease of total plant efficiency. If the regenerator is removed from the system, plant efficiency falls down to about 28%.

3.4 CO2 Recovery Type MHD Power System

To reduce CO2 emission is one of the urgent requirements to reduce global climate change based on green house effect. If we burn fossil fuel, CO2 must be exhausted. Therefore, we have to develop two directions; 1, increase plant efficiency which leads to reduce fuel consumption and 2, CO2 recovery type power generation system. We discuss how to increase plant efficiency using MHD generator previously. We would like to discuss how to design CO2 recovery type plant. At first, if we burn fossil fuel with air, exhaust gas contains so much N2 and we have to separate CO2 from N2 and H2O.This process requires so much energy and again increases CO2 production. If combustion exhaust contains only CO2and H2O, it is easy to separate CO2 from H2O. This can be achieved by burning fuel(s) with pure oxygen. Of course some amount of energy loss to produce oxygen takes place. However, temperature of combustion gas can be increased and if this increase in temperature can be effectively used to increase of plant efficiency, such penalty can be compensated. Basic ideas of CO2 recovery type MHD power generation system are as follows: Heat source is coal synthesized gas burning with pure oxygen. Coal must be considered as a heat source in near future with in 200years instead of LNG or oil. Nitrogen free with oxygen separate plant is included. Figure 11 shows typical CO2 recovery type MHD generator plant proposed by Prof. N. Kayukawa at Hokkaido University.[5] In this system, H2 and CO is burned with pure oxygen to drive MHD generator at the temperature around

4

July 6-10, 2008, OKINAWA, JAPAN

5

[3] N. Harada, L. C. Kien and T. Tashiro, “Closed Cycle MHD Generator using He/Xe Working Plasma,” AIAA Paper, 2002-2144, Proc. of the 14th International Conference on MHD Power Generation and High Temperature Technologies, pp.163-171, 2002.

2800C. Downstream part after MHD generator, heat is recovered by regenerative coal gasification process, fuel pre-heating, and steam de-composition. Energy penalty for oxygen production plant can be recovered due to operate at high temperature with high efficiency of the MHD generator. It is known that only MHD generators can be operated such high temperature regime. Total plant efficiency can be expected as over 50% with CO2 recovery.

[4] N. Harada and T, Tashiro, “Influence of Recombination Coefficient on Discharge Structure and Plasma Stability in Closed Cycle MHD Generator with He/Xe Working Gas,” AIAA Paper, 2003-3762, 2003.

[5] N. Kayukawa and Y. Wang, “Stand-Alone Scheme of Open-Cycle Magnetohydrodynamic Power Generation System,” Journal of Propulsion and Power, Vol. 19, No. 5, pp. 972-975, 2003.

Figure 11. CO2 Recovery type MHD generator plant.

4 CONCLUDING SUMMARY

We reviewed issues of current electrical power generation system. It can be summarized as follows: In order to increase plant efficiency, 1, increase working temperature, 2, don’t use condenser of steam-turbine to reduce exhaust heat, and 3, construct energy re-circulating type system. Also, in order to reduce CO2 emission, use nuclear power with high efficiency. We have to construct nuclear powered energy re-circulating type system. Also idea of CO2 recovery type power generation system must be developed. Energy re-circulating type Nuclear/MHD power system was proposed to achieve high efficiency using high operating temperature and eliminating bottoming cycle. For reduction of CO2 emission, CO2 recovery type generator system was proposed, which has special features of using coal synthesized gas burning with pure oxygen and heat recovery systems.

REFERENCES

[1] Y. Okuno, et al., “Proposal of a Highly Efficient CCMHD Single Power Generation System,” Trans. IEEJ, Vol. 118-B, No. 12, pp. 1457-1462.

[2] N. Harada, et al., “Improvement of Enthalpy Extraction over 30% using a Disk MHD Generator with Inlet Swirl,” Energy Conversion and Management, vol. 36, no. 5, pp. 355-364, 1995.

The International Conference on Electrical Engineering 2008