Method and apparatus for reducing the initial start-up and subsequent stabilization period losses, for increasing the usable power and for improving the controllability of a thermal power plant

In a method for reducing the start-up and stabilization period losses, for increasing the usable power and to improve the controllability of a thermal power plant, there are integrated into the power plant's steam cycle pressurized heat storage reservoirs which are charged by feeding them with excess heat produced in the said power plant as, for example, during the start-up and load stabilizing periods or during periods of reduced electrical power production and, when there is an increased demand for heat, the said heat storage reservoirs are discharged by the release of stored heat into the water-steam cycle. Control deviations in the electrical power while the power plant is in full operation are counterbalanced by changes in the charging and discharging streams of the pressurized heat storage reservoirs. In the apparatus for carrying out this method, pressurized heat storage reservoirs are connected, on the water-side, to the condensate system and, on the steam side, to the medium pressure of intermediate superheater network of the steam cycle or also to the power plant's medium pressure or low pressure turbine extraction points.

The invention relates to a method and apparatus for reducing the losses 
which occur initially on start-up and which subsequently occur during the 
following stabilization period prior to normal running conditions; the 
invention also relates to a method and apparatus for increasing the usable 
power and for improving the controllability of a thermal power plant. 
During the initial start-up period and the immediately following 
stabilization period in conventionally operated thermal power plants, 
large quantities of steam must bypass the turbine complex through the 
condenser. During this process, enormous quantities of heat remain unused 
and are given off via the condenser cooling water and the cooling tower 
into the atmosphere. 
In particular, with large power plant systems, the initial start-up period 
and the subsequent stabilizing period can take up to an hour or more, 
depending on the condition of the installation. In addition to this, many 
conventional power plant systems must be shut down regularly at weekends 
and at night. The quantities of heat which are produced during these 
start-up and stabilizing periods, and which constitute a considerable 
fraction of the total amount of converted thermal energy, are released 
unutilized. 
It has also been found, unfortunately, that a power plant system cannot be 
controllably operated when working at its upper load limit, especially at 
the limit of its rated pyrometric power, since, in order to attain 
power-controlled operation, it is necessary to retain a certain 
controllable load reserve in order to compensate for fluctuations in the 
demand. 
Compensation of the control deviations in the electrical power of a power 
plant system from the demand power can only be effected via the 
time-behavior of the steam production and the storage capacity of the 
steam generator which decisively determines the controllability of the 
power plant system. 
The object of the invention is to improve the economy of operation of a 
power plant by reducing the initial start-up and following stabilizing 
period losses and thereby increasing the plant's utilizable power. A 
further objective of the invention is to improve the controllability of a 
power plant. 
In accordance with the invention, this object is accomplished by arranging 
for one or more pressurized heat storage reservoirs to be integrated into 
the power plant's water-steam cycle, the said heat storage reservoirs 
being charged by supplying them with excess heat produced in the power 
plant and, in the event of an increased demand for steam, discharging the 
said heat storage reservoirs by releasing stored heat back again into the 
water-steam cycle. 
The pressurized heat storage reservoirs are charged during the start-up and 
stabilizing processes with start-up steam or stabilization period steam. 
The pressurized heat storage reservoirs return their charged energy to the 
power plant's water-steam cycle during periods of high load or periods 
when there is an increased demand for power to produce electrical energy. 
Thus, it is possible, with the method according to the invention, to store 
an appreciable fraction of the energy, hitherto given-off, unused, into 
the atmosphere during the start-up and following stabilizing phases of a 
power plant system, and to use this energy during periods of increased 
power demand. 
In order to effect a further increase in the upper load limit, it is 
additionally advantageous, during low or partial load periods, to charge 
the pressurized heat storage reservoirs with hot condensate via medium 
pressure/low pressure preheaters using steam bled from the medium pressure 
steam system and/or suitable extractions from the medium pressure and/or 
low pressure turbines. 
In an additional embodiment of the method according to the invention, 
control deviations in the electric power from the demand power of a power 
plant system are at least partially compensated by changes made in the 
pressurized heat storage reservoirs' charging or discharging flow. 
By this means, it is additionally possible to reduce a power plant system's 
available demand load reserve, which must necessarily be retained, by the 
normal output of the pressurized heat storage reservoirs, and by 
correspondingly increasing the rated load of the power plant system.

Further detail concerning the invention may be derived from the 
constructional example illustrated schematically in the appended figure. 
In the power plant system shown by way of example in the figure, steam 
flows in succession through a high pressure turbine 31, an intermediate 
super-heater 34, a medium pressure turbine 32 and also through a double 
channel low pressure turbine 33. The condensate formed in a condenser 1 is 
fed into a feed-water tank 6 via the condensate pumps 2 and the low 
pressure-medium pressure preheaters 4a to 4n, and passes again from the 
tank 6 into the steam generator via a feed-water pump 7. A sink-tank for 
storing condensate is denoted by 3. 
On the water side, a pressurized heat storage reservoir 21 is connected in 
shunt with the condensate system via pipe-lines 23 and 26 and a pump 22. 
In the example illustrated, a pressurized pipe-line following the 
discharge pump 22 discharges into a condensate pipe-line 30 at a point 
between the last medium pressure-low pressure preheater 4n and the 
feed-water tank 6. The pressurized pipe-line can, however, also feed 
directly into the feed-water tank 6. 
On the steam side, the pressurized heat storage reservoir 21 is connected 
via a pipe-line 27 to the medium pressure or intermediate super-heater 
network of the power plant unit and/or to other commercially suitable 
steam networks and steam systems having a steam pressure higher than that 
present in the pressurized heat storage reservoir 21 as, for example, to 
an extraction pipe 28 which also supplies the feed-water tank 6 with 
steam. In order to charge the pressurized heat storage reservoir 21 during 
a start-up or a stabilization period, steam is fed from the medium 
pressure intermediate heater network via the pipe-line 27, if need be, via 
the intermediary of a pressure reducing regulator, into the pressurized 
heat storage reservoir 21 which is prefilled with cold condensate, whereby 
the condensate contained therein is heated. 
In another start-up and stabilization period control arrangement, the steam 
used during the start-up or stabilization period heats a stream of 
condensate through pipeline 29, either directly or via a steam pressure 
reducing regulator and in a controlled or uncontrolled manner, to produce 
a boiling water or hot water stream with which the pressurized heat 
storage reservoir 21 is charged. 
During power operation, the pressurized heat storage reservoir 21 is 
charged in low or partial load periods with hot condensate via the low 
pressure-medium pressure preheaters 4a to 4n, whereby the hot stream of 
condensate from the same extraction pipe-line 28, which also supplies the 
feed-water tank 6 with steam, is further heated in a preheating mixing and 
degassing stage (not shown in the figure) located directly ahead of the 
pressurized heat storage reservoir 21. 
Along with the discharge, hot condensate from the pressurized heat storage 
reservoir 21 is admixed, via pipe-line 26, the depressurizer 24 and the 
discharging pump 22, with the condensate flowing in the pipe-line 30 to 
the feed-water tank 6. 
If, from time to time, the pressurized heat storage reservoir 21 is 
operated at a pressure which is elevated with respect to the feed-water 
tank 6, the hot storage discharge stream from the reservoir can be 
depressurized in the depressurizer 24 to the pressure present in the 
feed-water tank 6, and fed into the condensate pipe-line 30. 
The stream of depressurized steam is fed via a pipe-line 35 directly into 
the feed-water tank 6 or alternatively into a steam line 25 which leads to 
the feed-water tank 6. 
By this means, the discharge stream and the contents of the feed-water tank 
attain the same thermodynamic state. 
In a simplified circuit, the depressurizer 24 and the pipe-line 35 can be 
dispensed with and the discharge stream, with the enthalpy of the contents 
of the pressurized heat storage reservoir, can be fed directly into the 
condensate pipe-line 30. Of course, associated with this arrangement is a 
limitation to the discharge flow in the lower load periods when the 
pressure in the pressurized heat storage reservoir 21 is greater than the 
pressure in the feed-water tank 6. Consequently, in this simplified 
thermal cycle, it is necessary to provide a safety-control cycle which 
prevents evaporation from occurring in the condensate pipe-line 30 and at 
the inlet to the feed-water tank. 
By using the charging and discharging streams of the pressurized heat 
storage reservoir 21 as regulating quantities in load regulation, it is 
possible, when the plant is in operation, to control, simply and rapidly, 
any departures of the electrical power from its demand value within the 
desired power control range.