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A scheme for the more efficient incorporation of MHD into an existing power generation network
Energy Conversion. Vol. 8, pp. 177-180. Pergamon Pre~, 1968. Printed in Great Britain
A Scl, ,e for the more Efficient Incor ration of MHO into an Existing Power Generation.Network R, V, HARROWELLt (Received23 May 1968)
1. Introduction The work described in this paper is one of a number of studies made at these laboratories to try to minimize the cost of electricity from prospective M H D power stations. These methods have aimed at the reduction of capital costs at the expense of thermal efficiency or vice versa, but none has given an advantage big enough to make open-cycle M H D power generation economically viable in the British context. However, the present method, which might provide gains up to 2 per cent points, could be worthwhile in the different economic circumstances of other countries. There have been several general studies [1, 2] of large power plant [,~ 1000 MW (T)] using an M H D generator as a thermodynamic topper to a conventional steam station. The incorporation of such plant into an existing network of generating stations would involve only electrical connection; no mechanical integration has been contemplated. The purpose of this contribution is to show that, if an MHD-steam station is allotted a typical existing conventional station as its twin and their water circulatory systems are linked, then the efficiency of these "Siamese twins" is significantly greater than that of the pair in mechanical isolation. In a typical tentative design by CERL of a 2000 MW (T) M H D station the heat would be produced in the cooling water from the combustion chamber and duct at a rate of about 200 MW at a temperature of about 250°C. Normally, the exhaust gas in modern steam plants is brought down to the stack temperature (,-~ 120°C) by the incorporation of a low-temperature (250°C) air heater. In the M H D plant, on the other hand, the air is heated over this temperature range in the compressor, and it has been proposed that the stack temperature be attained in the exhaust gases by the incorporation of a low-temperature economizer in the steam cycle. This would provide another 100 MW of low-grade heat, making a total of 300 MW to be accommodated by the displacement of all but one of the regenerative feed-beaters. There is a consequent drop in the steam cycle etticiency of 5 per cent points and it has been considered that this is about the maximum that can be tolerated.
The present idea involves the diversion of the lowgrade heat (N.B. not just surplus duct heat) from the steam section of the M H D plant, thus allowing the restoration of all the regenerative feed-water heating. This would raise the M H D steam cycle efficiency. The diverted low-grade heat is used for feed-water heating in the twin station, the greatly reduced direct fuelling of which now has to provide only the evaporative and super heat. The resultant changes in the numerator and denominator of the ratio of the new combined outputs to the new combined inputs leave this ratio (i.e. the overall efficiency) greater than when the units were not connetted.
2. Thermodynamics of Modem Medium-Power Steam Cycles The temperature-entropy diagram for a Rankine steam cycle with superheat is shown in Fig. 1. The sensible (low-grade) heat is represented by the shaded area, and the total heat input by the area abeedfg. In the diagram the conditions shown are for a 60-MW set working at 900 lb/in ~ with super-heating to 482°C (900°F). This type of plant, in which the low-grade heat is 34 per cent of the total heat input, was being installed in Great Britain up to about 1960. 500 482
d
400 -574 t,j
o 30C-I-.-" 27C
L
20C - l-
to( -
a
3o
/
e ~
Entropy
~f Central Electricity Research Laboratories, Leatherhead, Surrey, England.
Cleeve R o a d ,
S, joules
Fig. 1. T - S diagram for F~nklm~ Cycle ~ 177
*C" supeHmmting.
178
R . V . HARROWELL
Only a few of the more efficient 100-MW sets working at 15001b/in 2 and 566°C (1050°F) without reheat were installed (in the late 1950's); the percentage of low-grade heat in this cycle is about 38. Further increase of saturation temperature beyond 312°C (1500 lb/in 2) would have led to increased steam wetness; so the next move towards increasing efficiency was the re-introduction of the reheat cycle. The first sets of this type were of 100- and 120-MW size using steam at 1500 lb/in 2 superheated to 525°C (975°F) and 538°C (1000°F) respectively, with reheat to 510°C (950°F) and 538°C. The temperatureentropy for the 120-MW type is shown in Fig. 2. The low-grade heat in the feedwater as a percentage of the total heat input is 35.
M
T [
=
"q=(aT"M +C)-G L
I
D
~
"qzO • T
--
"q=
L,(yM-G + ~/,(¢tr- M+C))+Lz~" LjT+LzD/~f
'Twin' conventionol stotion
M
~/; (=T-M+C-D)-G
•
_
:- - i 'Siomese fwin' convenfionol :
?
"q =
,+o(,,,-,>,&
stotion
Fig. 3. MHD-stemn plant twinned with conventional plant (a) Me~=-ltally =¢eanected.
O
(b) "Siamese" twin arr~m~em~t.
i
Entropy S, joules g*C Fig. 2. T-S diagram for 120 MW reheat set.
If the low-grade heat in these cycles could be supplied from an external source, the fuel input, for the same output, would be reduced by about one-third, and the cycle efficiency would be increased by about 50 per cent. This is the principle on which the following argument largely rests. 3. The Alternative Arrangements
Figure 3 shows the alternative arrangements: (a) the assumed present arrangement, and (b) the proposed system linking the MHD station with an existing conventional station. The efficiency of (a) is given by LI[yM
=
--
G+
~l(o~T --
M + C)] + L2~72D/f
L z T + L2D/I~f
(1)
is the is the is the is the
*Tz is the efficiency of the MHD steam cycle with all but one of its regenerative feed-heaters displaced by M H D coolant and an L.T. economizer, ,~2 is the efficiency of the conventional steam cycle with its full regenerative heating, is the boiler efficiency of the MHD station (allowance for stack losses), /3 is the boiler efficiency of the conventional station, T is the thermal input to the MHD station, Lt is the load factor of the MHD station, L2 is the load factor of the conventional station, E is the low-grade heat from the MHD-steam economizer, F is the low-grade heat from the MHD duct and combustion chamber, D is the sum of E and F, f is the low-grade heat in the conventional station as a fraction of its total input. The load factor, L, is included to allow for the possibility of the existing conventional station having a position in the merit order at which its load factor might be less than that appropriate to the M H D station. In the first arrangement (a) the conventional station would be run at its economic load factor, but in the second (b) it would, of course, have to run at the M H D load factor. The efficiency of (b) is given by rl, = 7 M -
where M 7 G C
.l"~_[
G + 71~(o~T- M + C -
D) + ~ D / f
(2)
r + D(l/f-- 1)l~ MHD electrical output, inverter efficiency, combined compressor and auxiliary powers, compressor power,
where ~/{ is the efficiency of the MHD steam cycle with all its regenerative feed-heating restored,
A Scheme for the More Efficient Incorporation of M H D into an Existing Power Generation Network
nz is the efficiency of the conventional steam cycle with all its regenerative feed-heating displaced by low-grade heat from the MHD-steam station. Let the numerator of Equation (1) be x and let the denominator be y. Then
1.5
t~" I'Oi ~0
'5 X
~7 = Y
(3)
Since the change in '7 depends on changes in both x and y, 8,7 =
a'7
8x +
179
o.5i
=2 2g
a~
-Jc0.3
I
0.4
)
0.,5
I
[
0.6
0.7
I
O.a
[
0.9
1.0
LffL)
= l { s x - - X - SYy } , y
(4)
Fig. 4. Incremental etikiency of combiaed ~
and ¢~v~floml
stations as a function of the economic lead factor (L=) (normalL~ed) 8'7 > 0 when 8x < 0, 8y < 0, provided 8x
x
The changes involved in going from the arrangement of Fig. 3(a) to that of Fig. 3(b) will now be examined for a possible practical system. 3. A 2000-MWO') M I l D Station with 3 × 120MWOg) Conventional Station The system examined comprises a 2000-MW (T) MHD-steam cycle, chosen as typical from various CEGB studies, and a hypothetical conventional station similar to the type being installed up to about 1962, having 3 × 120-MW(E) sets working at 15001b/inz/538°C with reheat to 538°C. Under the arrangement (a)--Equation (1)--the six stages of regenerative feed-water heating are retained in the conventional plant; under (b)--Equation (2)--the regenerative heating is supplanted by low-grade heat from the M H D station. The following values have been used in Equations
of the conventional plant.
on the MHD-steam station, or 4.7 per cent points on the conventional station. When L2 < Lt the conventional station does not operate at its economic load factor in system b and thus the full advantage of this system is not realized. The effect of this displacement from the merit order is shown in Fig. 4 in terms of the gain in efficiency (~/) of b over a against the economic load factor of the conventional station (normalized to the load factor of the M H D station). 4. Discussion The reason for the increased efficiency of the rearranged system is contained in Equation (4), which gives the change in average efficiency due to changes in the numerator and denominator of (3). For the above values Equation (1) becomes 1247 - - 0.419, '7 -- 2972 and Equation (2) becomes
(1 a n d 2):
T= M = D = C= G= 'Tt = '71 =
2000 M W 500 M W 300 M W 160MW 166 M W 0.402 0-450
'72 = '7~ = ~= f= V= fl =
0.397 0.342 0.9 0.35 0.97 0.885
For L2/L1 --~ 1 these values give '7 = 0.419, '7' = 0.431, an improvement of 1.2 points in the average efficiency of the two stations. The efficiency of the MHD-steam station alone is 45.3 per cent. It would have to be raised to 46.9 per cent in order to give '7" = 0.431 when averaged with its twin, unconnected conventional station. The Siamese-twin arrangement, therefore, is equivalent to an increase of 1"65 per cent points in the efficiency of the MHD-steam station or, alternatively, to 3.4 per cent points in the conventional station. If the calculation is made for 60 M W (E) sets, of the kind being installed up to about 1960, the figures are slightly better: the improvement for the twinned stations is 1.6 per cent points, equivalent to 2.15 per cent points 15
1135 '7. . . . 2633
0.431.
In this ease $x = - - 112 M W has been more than offset by 8y = -- 339 MW, the criterion for ~'7 > 0,
8x/Sy < x/y = 0.42, being satisfied. The cause of this large reduction in y is to be found in the value o f f , the fraction of low-grade beat in the conventional steam cycle. This is equivalent to 339 M W of conventional fuel input, and is now supplied from the M H D system, where it was a much smaller fraction of the total. The best use for the proposed system would probably be with conventional plant using sets up to 120 M W at pressures not above 1500 lb/in~. It is to be expected that there would be some special uneasiness at the thought of running magnox nuclear plant in this way, although, in principle, there could be some gain here too. Because of the smaller proportion of total sensible heat that could be provided by M H D coolant of limited temperature in the most modern plant working at high
180
R.V. HARROWELL
temperatures, the gain would be less with this type of plant integrated into an MHD-steam station. However, calculations show that, even here, there would be some net gain. It is not unusual for fairly major plant alterations to be made during the life of any power station, and the above modifications, although needing close examination, would probably not be exceptional.
purpose of the new scheme then being to restore the efficiency to the datum level. As an example of this, a considerable capital saving could be achieved if the air pre-heater had to heat the combustion air to only 1000°C instead of 1200°C, as required at present. Such a reduction in temperature, however, would decrease the station efficiency by about 2 per cent points, an unacceptable amount for systems so far considered. But with the Siamese-twin system the 2 per cent points would be restored. Working at the lower air temperature would not only reduce the capital cost: the feasibility of the pre-heater, and thus of the whole MHD plant, would be very much increased too. The use of oxygen-enriched air heated to only 800°C, previously rejected on the grounds of reduced efficiency, might also be given further consideration.
4.1. Possible objections There are two principal objections that might be raised to the scheme: that the operation of the turbines in the conventional station will be seriously disturbed by the elimination of the bled steam circuit; and that the economic load factor of the existing conventional station is likely to be less than that of the newly-installed MHDsteam plant. 5. Conclusions Taking the second point first, it can be seen from Using reasonably realistic values for the calculation Fig. 4 that the load factor of the conventional station of the efficiencies, the proposed scheme increases the may be almost half that of the MHD-steam station before there is no gain from the Siamese twin arrange- overall efficiency of the Siamese-twin stations by about 1.5 per cent points above the efficiency obtainable when ment. The load factors in 20 yr time, of plant commissioned the two are paired but not mechanically connected; this in the 1960's cannot be forecast with great accuracy, but is equivalent to an increase of about 2 per cent points certain general predictions are in order. For instance, in the effective efficiency of the MHD-steam plant or since thermal efficiencies have steadily risen during about 4 per cent points in the conventional station. The optimum conventional station for this exercise recent decades, there is a wide spread in the efficiences of present power stations. With this rise in efficiencies appears to be one with 60 or 120 MW sets operating at now levelling off, it is reasonable to suppose that the about 1500 lb/in z, these having the biggest fraction of range will be narrower in, say, 1985 than at present. low-grade heat at temperatures attainable by MHD As the load factor is closely related to efficiency, it is coolant. The increase in the effective efficiency of the MHD likely, therefore, that L~/L1 will be nearer unity then plant can be exploited in two main ways: either the than now. As to the first objection, the flow conditions in the running (fuel) costs can be simply reduced, or the turbines can be left undisturbed if the steam is bled off capital cost of part of the plant might be reduced at the as at present but is then pumped back to the input expense of the intrinsic efficiency 0/1) of the MHD-steam pressure and temperature of the h.p. turbine instead of station (which is compensated by the increase in effective being passed through the feedheaters. The pumping efficiency). For instance, the cost of a pre-heater giving power for restoring pressure to the h.p. level would be air at 1000°C would be considerably less than that of a debited to the output from the turbines. This would 1200°C heat exchanger, although the intrinsic thermal leave them with roughly the same net output as would efficiency of the station would be reduced by about be obtained from turbines especially designed to use 2 per cent points. only the reduced steam flow resulting from the eliminaUnfortunately, the reduction in costs offered by this tion of the bled steam. scheme was not enough to make open-cycle MHD Suitable two-way valves would enable some or all of generation viable in Great Britain, although the scheme the bled steam either to circulate through the simple may be more attractive elsewhere. feedback loop just described or to be diverted through one or more of the regenerative feedheaters. This arrange- Acknowledgements--This study, carried out at the Central Electricity Research Laboratories, was part of the British MHD Collaborative ment would ensure reasonable flexibility, enabling the Research Programme. It is published by permission of the Central conventional plant to operate independently of the Electricity Generating Board. MHD station if necessary. References 4.2. Exploitation of increased efficiency [1] C. Carter, D. V. Freck, R. V. Harrowell and J. K. Wright, A surveyof results of the C.E.G.B. programme of research on An increase in efficiency of 2 per cent points, which open cycle MHD generation, in: Proc. lnt. Syrup. MHD is relatively large, can be used to give a simple reduction Electrical Power Generation, Paris, E.N.E.A. (O.E.C.D.), Vol. 3, p. 1299 (1964). in running costs, or it might be used to reduce capital [2] M. Rosner, The oil-fired MHD power plant, in: Proc. Syrup. costs in a situation where a cheaper item of plant could Electricity from MHD, Vol. 3, p. 771, Salzburg, I.A.E.A. and be introduced at the expense of decreased efficiency, the E.N.E.A. (1966) (and other papers in this volume).
A Scl, ,e for the more Efficient Incor ration of MHO into an Existing Power Generation.Network R, V, HARROWELLt (Received23 May 1968)
1. Introduction The work described in this paper is one of a number of studies made at these laboratories to try to minimize the cost of electricity from prospective M H D power stations. These methods have aimed at the reduction of capital costs at the expense of thermal efficiency or vice versa, but none has given an advantage big enough to make open-cycle M H D power generation economically viable in the British context. However, the present method, which might provide gains up to 2 per cent points, could be worthwhile in the different economic circumstances of other countries. There have been several general studies [1, 2] of large power plant [,~ 1000 MW (T)] using an M H D generator as a thermodynamic topper to a conventional steam station. The incorporation of such plant into an existing network of generating stations would involve only electrical connection; no mechanical integration has been contemplated. The purpose of this contribution is to show that, if an MHD-steam station is allotted a typical existing conventional station as its twin and their water circulatory systems are linked, then the efficiency of these "Siamese twins" is significantly greater than that of the pair in mechanical isolation. In a typical tentative design by CERL of a 2000 MW (T) M H D station the heat would be produced in the cooling water from the combustion chamber and duct at a rate of about 200 MW at a temperature of about 250°C. Normally, the exhaust gas in modern steam plants is brought down to the stack temperature (,-~ 120°C) by the incorporation of a low-temperature (250°C) air heater. In the M H D plant, on the other hand, the air is heated over this temperature range in the compressor, and it has been proposed that the stack temperature be attained in the exhaust gases by the incorporation of a low-temperature economizer in the steam cycle. This would provide another 100 MW of low-grade heat, making a total of 300 MW to be accommodated by the displacement of all but one of the regenerative feed-beaters. There is a consequent drop in the steam cycle etticiency of 5 per cent points and it has been considered that this is about the maximum that can be tolerated.
The present idea involves the diversion of the lowgrade heat (N.B. not just surplus duct heat) from the steam section of the M H D plant, thus allowing the restoration of all the regenerative feed-water heating. This would raise the M H D steam cycle efficiency. The diverted low-grade heat is used for feed-water heating in the twin station, the greatly reduced direct fuelling of which now has to provide only the evaporative and super heat. The resultant changes in the numerator and denominator of the ratio of the new combined outputs to the new combined inputs leave this ratio (i.e. the overall efficiency) greater than when the units were not connetted.
2. Thermodynamics of Modem Medium-Power Steam Cycles The temperature-entropy diagram for a Rankine steam cycle with superheat is shown in Fig. 1. The sensible (low-grade) heat is represented by the shaded area, and the total heat input by the area abeedfg. In the diagram the conditions shown are for a 60-MW set working at 900 lb/in ~ with super-heating to 482°C (900°F). This type of plant, in which the low-grade heat is 34 per cent of the total heat input, was being installed in Great Britain up to about 1960. 500 482
d
400 -574 t,j
o 30C-I-.-" 27C
L
20C - l-
to( -
a
3o
/
e ~
Entropy
~f Central Electricity Research Laboratories, Leatherhead, Surrey, England.
Cleeve R o a d ,
S, joules
Fig. 1. T - S diagram for F~nklm~ Cycle ~ 177
*C" supeHmmting.
178
R . V . HARROWELL
Only a few of the more efficient 100-MW sets working at 15001b/in 2 and 566°C (1050°F) without reheat were installed (in the late 1950's); the percentage of low-grade heat in this cycle is about 38. Further increase of saturation temperature beyond 312°C (1500 lb/in 2) would have led to increased steam wetness; so the next move towards increasing efficiency was the re-introduction of the reheat cycle. The first sets of this type were of 100- and 120-MW size using steam at 1500 lb/in 2 superheated to 525°C (975°F) and 538°C (1000°F) respectively, with reheat to 510°C (950°F) and 538°C. The temperatureentropy for the 120-MW type is shown in Fig. 2. The low-grade heat in the feedwater as a percentage of the total heat input is 35.
M
T [
=
"q=(aT"M +C)-G L
I
D
~
"qzO • T
--
"q=
L,(yM-G + ~/,(¢tr- M+C))+Lz~" LjT+LzD/~f
'Twin' conventionol stotion
M
~/; (=T-M+C-D)-G
•
_
:- - i 'Siomese fwin' convenfionol :
?
"q =
,+o(,,,-,>,&
stotion
Fig. 3. MHD-stemn plant twinned with conventional plant (a) Me~=-ltally =¢eanected.
O
(b) "Siamese" twin arr~m~em~t.
i
Entropy S, joules g*C Fig. 2. T-S diagram for 120 MW reheat set.
If the low-grade heat in these cycles could be supplied from an external source, the fuel input, for the same output, would be reduced by about one-third, and the cycle efficiency would be increased by about 50 per cent. This is the principle on which the following argument largely rests. 3. The Alternative Arrangements
Figure 3 shows the alternative arrangements: (a) the assumed present arrangement, and (b) the proposed system linking the MHD station with an existing conventional station. The efficiency of (a) is given by LI[yM
=
--
G+
~l(o~T --
M + C)] + L2~72D/f
L z T + L2D/I~f
(1)
is the is the is the is the
*Tz is the efficiency of the MHD steam cycle with all but one of its regenerative feed-heaters displaced by M H D coolant and an L.T. economizer, ,~2 is the efficiency of the conventional steam cycle with its full regenerative heating, is the boiler efficiency of the MHD station (allowance for stack losses), /3 is the boiler efficiency of the conventional station, T is the thermal input to the MHD station, Lt is the load factor of the MHD station, L2 is the load factor of the conventional station, E is the low-grade heat from the MHD-steam economizer, F is the low-grade heat from the MHD duct and combustion chamber, D is the sum of E and F, f is the low-grade heat in the conventional station as a fraction of its total input. The load factor, L, is included to allow for the possibility of the existing conventional station having a position in the merit order at which its load factor might be less than that appropriate to the M H D station. In the first arrangement (a) the conventional station would be run at its economic load factor, but in the second (b) it would, of course, have to run at the M H D load factor. The efficiency of (b) is given by rl, = 7 M -
where M 7 G C
.l"~_[
G + 71~(o~T- M + C -
D) + ~ D / f
(2)
r + D(l/f-- 1)l~ MHD electrical output, inverter efficiency, combined compressor and auxiliary powers, compressor power,
where ~/{ is the efficiency of the MHD steam cycle with all its regenerative feed-heating restored,
A Scheme for the More Efficient Incorporation of M H D into an Existing Power Generation Network
nz is the efficiency of the conventional steam cycle with all its regenerative feed-heating displaced by low-grade heat from the MHD-steam station. Let the numerator of Equation (1) be x and let the denominator be y. Then
1.5
t~" I'Oi ~0
'5 X
~7 = Y
(3)
Since the change in '7 depends on changes in both x and y, 8,7 =
a'7
8x +
179
o.5i
=2 2g
a~
-Jc0.3
I
0.4
)
0.,5
I
[
0.6
0.7
I
O.a
[
0.9
1.0
LffL)
= l { s x - - X - SYy } , y
(4)
Fig. 4. Incremental etikiency of combiaed ~
and ¢~v~floml
stations as a function of the economic lead factor (L=) (normalL~ed) 8'7 > 0 when 8x < 0, 8y < 0, provided 8x
x
The changes involved in going from the arrangement of Fig. 3(a) to that of Fig. 3(b) will now be examined for a possible practical system. 3. A 2000-MWO') M I l D Station with 3 × 120MWOg) Conventional Station The system examined comprises a 2000-MW (T) MHD-steam cycle, chosen as typical from various CEGB studies, and a hypothetical conventional station similar to the type being installed up to about 1962, having 3 × 120-MW(E) sets working at 15001b/inz/538°C with reheat to 538°C. Under the arrangement (a)--Equation (1)--the six stages of regenerative feed-water heating are retained in the conventional plant; under (b)--Equation (2)--the regenerative heating is supplanted by low-grade heat from the M H D station. The following values have been used in Equations
of the conventional plant.
on the MHD-steam station, or 4.7 per cent points on the conventional station. When L2 < Lt the conventional station does not operate at its economic load factor in system b and thus the full advantage of this system is not realized. The effect of this displacement from the merit order is shown in Fig. 4 in terms of the gain in efficiency (~/) of b over a against the economic load factor of the conventional station (normalized to the load factor of the M H D station). 4. Discussion The reason for the increased efficiency of the rearranged system is contained in Equation (4), which gives the change in average efficiency due to changes in the numerator and denominator of (3). For the above values Equation (1) becomes 1247 - - 0.419, '7 -- 2972 and Equation (2) becomes
(1 a n d 2):
T= M = D = C= G= 'Tt = '71 =
2000 M W 500 M W 300 M W 160MW 166 M W 0.402 0-450
'72 = '7~ = ~= f= V= fl =
0.397 0.342 0.9 0.35 0.97 0.885
For L2/L1 --~ 1 these values give '7 = 0.419, '7' = 0.431, an improvement of 1.2 points in the average efficiency of the two stations. The efficiency of the MHD-steam station alone is 45.3 per cent. It would have to be raised to 46.9 per cent in order to give '7" = 0.431 when averaged with its twin, unconnected conventional station. The Siamese-twin arrangement, therefore, is equivalent to an increase of 1"65 per cent points in the efficiency of the MHD-steam station or, alternatively, to 3.4 per cent points in the conventional station. If the calculation is made for 60 M W (E) sets, of the kind being installed up to about 1960, the figures are slightly better: the improvement for the twinned stations is 1.6 per cent points, equivalent to 2.15 per cent points 15
1135 '7. . . . 2633
0.431.
In this ease $x = - - 112 M W has been more than offset by 8y = -- 339 MW, the criterion for ~'7 > 0,
8x/Sy < x/y = 0.42, being satisfied. The cause of this large reduction in y is to be found in the value o f f , the fraction of low-grade beat in the conventional steam cycle. This is equivalent to 339 M W of conventional fuel input, and is now supplied from the M H D system, where it was a much smaller fraction of the total. The best use for the proposed system would probably be with conventional plant using sets up to 120 M W at pressures not above 1500 lb/in~. It is to be expected that there would be some special uneasiness at the thought of running magnox nuclear plant in this way, although, in principle, there could be some gain here too. Because of the smaller proportion of total sensible heat that could be provided by M H D coolant of limited temperature in the most modern plant working at high
180
R.V. HARROWELL
temperatures, the gain would be less with this type of plant integrated into an MHD-steam station. However, calculations show that, even here, there would be some net gain. It is not unusual for fairly major plant alterations to be made during the life of any power station, and the above modifications, although needing close examination, would probably not be exceptional.
purpose of the new scheme then being to restore the efficiency to the datum level. As an example of this, a considerable capital saving could be achieved if the air pre-heater had to heat the combustion air to only 1000°C instead of 1200°C, as required at present. Such a reduction in temperature, however, would decrease the station efficiency by about 2 per cent points, an unacceptable amount for systems so far considered. But with the Siamese-twin system the 2 per cent points would be restored. Working at the lower air temperature would not only reduce the capital cost: the feasibility of the pre-heater, and thus of the whole MHD plant, would be very much increased too. The use of oxygen-enriched air heated to only 800°C, previously rejected on the grounds of reduced efficiency, might also be given further consideration.
4.1. Possible objections There are two principal objections that might be raised to the scheme: that the operation of the turbines in the conventional station will be seriously disturbed by the elimination of the bled steam circuit; and that the economic load factor of the existing conventional station is likely to be less than that of the newly-installed MHDsteam plant. 5. Conclusions Taking the second point first, it can be seen from Using reasonably realistic values for the calculation Fig. 4 that the load factor of the conventional station of the efficiencies, the proposed scheme increases the may be almost half that of the MHD-steam station before there is no gain from the Siamese twin arrange- overall efficiency of the Siamese-twin stations by about 1.5 per cent points above the efficiency obtainable when ment. The load factors in 20 yr time, of plant commissioned the two are paired but not mechanically connected; this in the 1960's cannot be forecast with great accuracy, but is equivalent to an increase of about 2 per cent points certain general predictions are in order. For instance, in the effective efficiency of the MHD-steam plant or since thermal efficiencies have steadily risen during about 4 per cent points in the conventional station. The optimum conventional station for this exercise recent decades, there is a wide spread in the efficiences of present power stations. With this rise in efficiencies appears to be one with 60 or 120 MW sets operating at now levelling off, it is reasonable to suppose that the about 1500 lb/in z, these having the biggest fraction of range will be narrower in, say, 1985 than at present. low-grade heat at temperatures attainable by MHD As the load factor is closely related to efficiency, it is coolant. The increase in the effective efficiency of the MHD likely, therefore, that L~/L1 will be nearer unity then plant can be exploited in two main ways: either the than now. As to the first objection, the flow conditions in the running (fuel) costs can be simply reduced, or the turbines can be left undisturbed if the steam is bled off capital cost of part of the plant might be reduced at the as at present but is then pumped back to the input expense of the intrinsic efficiency 0/1) of the MHD-steam pressure and temperature of the h.p. turbine instead of station (which is compensated by the increase in effective being passed through the feedheaters. The pumping efficiency). For instance, the cost of a pre-heater giving power for restoring pressure to the h.p. level would be air at 1000°C would be considerably less than that of a debited to the output from the turbines. This would 1200°C heat exchanger, although the intrinsic thermal leave them with roughly the same net output as would efficiency of the station would be reduced by about be obtained from turbines especially designed to use 2 per cent points. only the reduced steam flow resulting from the eliminaUnfortunately, the reduction in costs offered by this tion of the bled steam. scheme was not enough to make open-cycle MHD Suitable two-way valves would enable some or all of generation viable in Great Britain, although the scheme the bled steam either to circulate through the simple may be more attractive elsewhere. feedback loop just described or to be diverted through one or more of the regenerative feedheaters. This arrange- Acknowledgements--This study, carried out at the Central Electricity Research Laboratories, was part of the British MHD Collaborative ment would ensure reasonable flexibility, enabling the Research Programme. It is published by permission of the Central conventional plant to operate independently of the Electricity Generating Board. MHD station if necessary. References 4.2. Exploitation of increased efficiency [1] C. Carter, D. V. Freck, R. V. Harrowell and J. K. Wright, A surveyof results of the C.E.G.B. programme of research on An increase in efficiency of 2 per cent points, which open cycle MHD generation, in: Proc. lnt. Syrup. MHD is relatively large, can be used to give a simple reduction Electrical Power Generation, Paris, E.N.E.A. (O.E.C.D.), Vol. 3, p. 1299 (1964). in running costs, or it might be used to reduce capital [2] M. Rosner, The oil-fired MHD power plant, in: Proc. Syrup. costs in a situation where a cheaper item of plant could Electricity from MHD, Vol. 3, p. 771, Salzburg, I.A.E.A. and be introduced at the expense of decreased efficiency, the E.N.E.A. (1966) (and other papers in this volume).