Cost Estimate

Saturated steam turbines are more expensive than superheated steam turbines because their components must fulfil higher technical specifications. The blades must withstand harder working conditions, which demands the use of expensive raw material. The design and assembly costs are also higher and an additional separator is required between the high pressure and low pressure stages of the saturated steam turbine.

For small sizes (less than 15 MWe), the information given by turbine manufacturers show that the price of the power block set (i. e., steam turbine, generator, condenser, lubrication and control systems) is between 25% and 40% (depending on the manufacturer) more expensive for saturated steam when the same manufacturer is considered for both options. If the price of a saturated steam power block is compared with the price of a superheated steam power block delivered by a different manufacturer, the difference can be even of 90%. So, for instance, offers issued by different manufacturers for a 10 MWe superheated steam power block are in the range from 2,5 million € and 3,6 million €, which means a 44% difference, though the offers are related to the same steam option and nominal power.

For the small size considered it is more realistic to consider a price difference of 40% between both options because the number of manufacturers for small saturated steam turbines is limited. Taking into consideration offers available, the price considered for a superheated steam power block is 1,5 Mio. €, while the price considered for the saturated steam option is 2,1 Mio. €. The cost of the other power block components (deaerator, auxiliary pumps, piping and fittings) is very similar for both options and, therefore, the cost difference between both options related to the power block is 0,6 million €.

As far as the solar field is concerned, the superheated steam option requires some items that are not needed in the saturated steam solar field. These items are:

— the water/steam separator and associated equipment (solenoid valve, non-return valve, piping and fittings) between the evaporating and superheating section.

— the water injector and associated equipment (piping, fittings, control valve and temperature sensor) used to control the steam temperature

The number of parabolic trough collectors required for the saturated steam solar field of the DSG power plant considered in this study is 72, while only 70 collectors are needed for the superheated steam option. This bigger solar field compensates the lower design efficiency of the saturated steam option. The differences between the solar fields of the two options considered are listed in Table 4, together with the number of units and the cost associated to every item. Costs are marked with “+” when the relevant item only exists in the superheated steam option, while the sign “-“ is used for the items required in the saturated steam option only.

Table 4. Economic comparison of saturated and superheated steam DSG solar fields

Item

Units

Unit cost (€)

Total cost (€)

Middle water/steam separator and associated equipment

7

10 500

+ 73 500

Water injector and associated equipment

7

5 000

+ 35 000

548 m2 parabolic trough solar collectors (extra)

2

109 600

— 219 200

Total:

— 110700

Taking into account the cost difference of both the power block and the solar field, it is concluded that the initial investment required by the saturated steam option is 710 700 € higher than that of the superheated steam option. This means a 5% higher investment.

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