Wet to dry conversion

7 August 2020

A cost effective way to deal with water stress.

A significant proportion of thermal power plants* with steam cycle operating around the world today rely on evaporative wet cooling towers for condensing the exhaust steam of their steam turbines. The water consumed in such a wet cooling tower may be up to 3 m3 for each MWh of electricity generated by the turbine.

Climate variations, recurring droughts and growing demand from other consumers can restrict the water available to such power plants, and their generating capacities therefore fall victim to their thirsty cooling systems. In severe cases the annual electricity output is seriously reduced, because the plant needs to shut down for weeks or months, as has been seen in several plants in Europe, USA, India, Mexico, Brazil and elsewhere recently.

Dry cooling by air, as an alternative to wet cooling, is increasingly used for new-build thermal power plants with steam cycle.

It is commonly argued that dry cooling reduces generation efficiency, but the annual electricity output of a wet cooled power plant that has experienced curtailed generation due to water shortage will be less than its dry cooled counterpart, given that the dependence on water of the latter is nil.

For water saving at existing thermal power plants with steam cycle, Belgium based cooling system manufacturer SPG (formerly SPX) Dry Cooling, offers conversion of wet cooling systems to wet+dry hybrid or all-dry cooling, with maximised NPV (net present value) and minimal payback time.

A simulation is run modelling the year-round power output and water consumption of the plant with its existing wet cooling tower and looking at the effect on power output and water consumption of implementing various cooling system conversion schemes.

Several case studies have proven that wet-to-hybrid or wet-to-dry conversions not only help improve drought resilience and availability, but also enable annual capacity factors to be increased, resulting in increased revenues.

A recent example is an analysis carried out by SPG Dry Cooling that examined dry cooling options for a US thermal power plant. This demonstrated that conversion to semi-dry or all-dry cooling would achieve a very short investment payback period, and accumulate nearly 100 million USD in additional revenue by the end of the plant’s commercial life, offering a retrofit option with unparalleled economics for the plant’s owner/operator.

The flow diagrams (above) show the current wet cooling configuration and options for the converted scheme.

A year round simulation of turbine output and water consumption for the existing cooling scheme and future conversion options was carried out, making use of actual ambient air dry and wet bulb temperature duration curves and steam turbine output curves (see graphs below).

Then the NPV of the extra electricity generation revenues, netted with the investment cost of the conversion, were calculated for the remaining commercial life of the plant for all conversion options.

The NPV of the extra electricity revenues becomes positive and steadily rising after the investment for the conversion has been paid back. Any of the input variables can be adjusted to check NPV and payback sensitivities.

The two charts above show example plots of NPV versus remaining commercial operation years for two different assumptions for gross profit (USD) per MWh (8 $/MWh and 12 $/MWh).

In summary, customised cooling system conversions reduce power plant vulnerability to water scarcity, and ensure increased generation revenue through the remaining plant life, providing a reliable and sustainable answer to water stress problems in wet- cooled thermal power plants.

* Includes, as well as fossil-fuelled units, concentrated solar, nuclear, waste-to-energy and biomass plants

NPV of conversion options relative to base case (wet cooling), as a function of remaining commercial life, for two different assumptions about gross profit per MWh, 8 $/MWh (left) and 12 $/MWh (right)

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