10 Future outlook
10.1 Unit size and rating
The selection of unit size for a particular application depends on:
- The economies of scale; i.e., the generally lower capital cost per unit output given by larger output plant.
- The steam cycle and conditions associated with the selected steam generation system.
- The availability of plant designs for the steam generator and the turbine-generator.
- The size of the grid system and the individual generating units on it.
- The size of the organisation owning the plant, and its economic environment.
- The available construction time.
- The available site area.
In the late 1970s and early 1980s, there was a general period of consolidation worldwide, with relatively few new stations being ordered. Those that were ordered were generally established design, or part of a deliberate series ordering of plants. In the larger industrial countries, tandem-compound turbine sizes have stabilised in the range 600-800 MW for superheat plant, and 900-1300 MW for saturated steam plant. There have, however, been a few individual exceptions, with superheat plant including a 1200 MW tandem-compound unit in the USSR, and 1300 MW cross-compound units in the USA. Saturated steam plant has included a 1500 MW tandem-compound design in France in association with a PWR.
With the adoption of modular design principles, a turbine manufacturer can offer a range of outputs in his product range of large machines, typically from 200 MW upwards. There are many smaller or less-developed countries where the size of the grid system will preclude the larger plants, and the 250-350 MW range is often favoured.
Looking to the future, it appears that the selection of the unit size will be increasingly determined by the system size, the plant ownership and the rating of nuclear steam supply systems. In the USA, where there are a number of relatively small utilities, the preferred unit sizes for fossil-fired plant are likely to be around 350 MW and 700 MW. If new nuclear plant is ordered, it is expected that this would be at most only a modest development of the 1300 MW reactors. In France, with a large utility (EOF), the plan is likely to be a series of orders for 1500 MW PWRs, with little or no fossil-fired plant. In Germany, with a number of utilities, the American pattern is more likely. In Japan, there is a committed plan of development towards fossil-fired units rated at up to 1000 MW, and nuclear plant up to about 1200 MW.
The United Kingdom has also gone through a period of consolidation in the 1970s and 1980s. After a relatively rapid increase in unit size from 30 MW in 1945 to 500 MW in 1970, this has only increased to 660 MW subsequently. The earliest 660 MW unit was ordered in 1966 and was in service by 1974. This modest rate of development has produced plant of excellent thermal efficiency and reliability, and UK manufacturers' designs have been progressively developed and refined to take advantage of new technology, design methods, and manufacturing methods in the intervening years.
In the late 1980s, it has been judged that the time is right for a further increase in unit size for superheat plant to 900 MW, in order to meet both domestic and overseas requirements. For the domestic UK needs, this offers substantial benefits from the economies of scale — typified by the physical dimensions of the turbine-generator being only marginally increased from the 660 MW units. The initial steam conditions have been advanced from 160 bar, 565°C to 176 bar, 565°C, thus providing an increase in thermal efficiency without sacrificing operational flexibility.
For nuclear plant, the choice will be determined mainly by the reactor type and size. For advanced gas-cooled reactor plant (AGR), the preferred reactor size is in the 660-750 MW range. Being a superheat cycle, this requires a steam turbine almost identical to that for a fossil-fired unit of similar output. Any turbines required can therefore be derived from the existing families of modules covering plant up to 900 MW or more.
For water-cooled reactors, the reactor sizes are up to 1200-1300 MW, although some enhancement of this may be expected. For the first UK PWR at Sizewell B, the decision was made in 1979 to employ two 630 MW turbine-generators with the 1260 MW(e) reactor, as these made substantial use of modules developed and proven for the 660 MW fossil-fired plant, in particular the LP turbines and the generator. Those elements specific to the saturated steam cycle, such as the HP turbine, moisture separator, and steam/steam reheater (see Chapter 2), had been developed and proven in plant supplied to overseas by UK manufacturers at ratings up to 110 MW. It is expected that, if there is a series of PWR stations in the UK, the decision will be made at some stage to select a single turbine-generator per reactor, thus taking advantage of recognised savings in capital cost, construction time and operational costs. In line with developments elsewhere, it is not expected that reactor size, and hence turbine-generator size, will advance beyond 1500 MW in the foreseeable future.
There is therefore no perceived need for unit sizes to advance substantially in the next ten or twenty years, beyond about 1000 MW for superheat plant or 1500 MW for saturated steam plant.