The low-down on Cooling Towers

As I understand it, if the site is confirmed, the power station will be either:

(a) Two reactors each with output equivalent to 1600 megawatt

electrical, that is a total output of 3200 mw(e).

(b)  Three reactors each with output equivalent to 1000

megawatt electrical, that is a total output 3000 mw(e).

Note: mw(e) is megawatt electrical.

mw(th) is megawatt thermal.

As you well know I am a supporter of nuclear power, so I have no problem with the nuclear side of the proposed nuclear power station at Oldbury. However, there maybe a choice in its design to do with the cooling system associated with the exhaust steam from the turbines.

I am aware of four ways that exhaust steam can be cooled in nuclear or conventional power stations. In all four ways the turbine exhaust steam and the cooling water (CW) are kept completely separate from each other. However in three of the ways (2, 3 and 4) the cooling water (CW) has itself to be cooled after it has cooled the exhaust steam back to water.

WAY 1. (Direct River/Sea Cooling)

This is the one in use at the existing Oldbury P.S.  where water is extracted from the river Severn passed through a condenser to cool the turbine exhaust steam and then the river water returned straight back to the river Severn: warmer than when it was extracted from the river.

This way of cooling has the minimum visual impact, particularly from a ‘distance’ point of view. However, there are statutory constraints on how the water is extracted from the river; then also how warm it is and its condition, before it is returned back to the Severn: these legal constraints are there to protect human health, the fauna, fish and wildlife. This currently limits the heat discharge to the Severn from a designed station output of about 600mw(e). (With a present thermal efficiency of about 25% this is about 1800mwth).

NOTE:

This method cannot be used by a new power station of the proposed size because the amount of heat returned to the  Severn in the warmed water ((in generating over 3000mw(e)) would be far too great to comply with the legal constraints associated with the Severn. (With a thermal efficiency of a new power station approaching 40% a heat rejection to the river Severn would be about 4800mwth., approaching three times the present licence limit of the existing P.S.  

WAY 2. (Concrete Tower-Natural Convection Wet Cooling)

A second way is the use of natural draught concrete cooling towers. The CW that cools the exhaust steam in the condenser is in a ‘closed system’ the same water going around and around: not unlike the water in a household central heating system. But instead of having a boiler to put heat into the water, as in a central heating system, you have a cooling tower to remove heat from the water that has been put into it from cooling the exhaust steam of the turbines: to turn it back to water.

It does this by taking the warmed water part way up the concrete cooling tower and then dispersing it in a designed manner, so that it falls like water from a bathroom shower head. As the water falls in billions of small droplets it meets a powerful draught of air coming up from the bottom of the cooling tower which cools the water droplets which eventually fall into a pond at the bottom of the cooling tower. These towers can either be cylindrical or of a hyperbolic shape: for technical and cost reasons they are normally hyperbolic in the UK.

These cooling towers can currently be up to 200 metres(m) high depending on the amount of energy generated by the power station, its thermal efficiency, planning constraints, cost and construction considerations: for example two smaller cooling towers could or might have to be used instead of one larger one. When one of these cooling towers is in use it emits a significant visual white water vapour plume from its top. This water vapour plume also means that some water is being lost from the ‘closed’ system so that some water has to be continually added to the system to keep the amount of water constant: this is generally called ‘make up water’. Water vapour is considered to be a ‘greenhouse gas’.

WAY 3. (-Forced Draught Wet Cooling)

The third way is again a cooling tower. In principle the same mechanism for cooling the exhaust steam as is the use of concrete cooling towers, but forced air draught is used to do the cooling instead of natural draught. Together with another design feature they stop/reduce the visual vapour plume: water vapour is still emitted but the part that is visible is insignificant compared to that from natural draught concrete cooling towers in WAY 2. These forced draught towers which require electrical fans to drive the cooling air through them to do the water cooling are referred to as a ‘hybrid cooling towers’.

Because fans are needed some of the power station electrical output is needed to drive these fans, so station output is reduced slightly (3%?). However, the height of these towers is significantly reduced to about one third the height of conventional concrete cooling towers in WAY 2.

WAY 4. (Dry Concrete Cooling Towers)

The fourth way is again a cooling tower. This design of cooling tower emits no water vapour, since the warmed water to be cooled and the cooling air do not come into contact with each other. My knowledge of these is limited. But from that limited knowledge these towers are of a considerably larger size than those of wet concrete cooling towers for a similar power station rating. None seem to have been constructed for a power station approaching anywhere near the size of the proposed Oldbury power station. Hence like WAY 1 not an option for the proposed power station at Oldbury.

CONCLUSION:

The choice is either WAY 2 or WAY 3. My choice is WAY 3: Hybrid Cooling Towers.

FOOTNOTE:

Colin,

In addition to the information I have given in ‘WAY 1’ above, a few people have asked me for the technical information about why the river Severn cannot be used for cooling the new station, in the same way as the present station, My explanation below may also be of interest to you.

Oldbury’s current output is around 440mw (electrical). To make the arithmetic easier (let’s say) 450mw (e) with a thermal efficiency (let’s say) of 25%. (which is determined by design and thermodynamic laws). The two reactors produce together heat totalling 1800mw (t)hermal, 25% of that heat makes the 450mw (e) and the rest, which is 1350mw of heat goes into the cooling water (CW), that is, the river Severn. The Severn environment can cope with that.

‘Guess’ figures for the new station: 3200mw (e) and thermal efficiency 40%( let’s say, latest design figure?) means  that the two reactors produce together about 8000mw(t), 40% of that heat makes the 3200mw(e) and the rest, which is 4800mw of heat, (over three times that of the current power station): has to go somewhere. It could not be dispersed into the River Severn because this is well outside what could be allowed.

As I understand it, if the site is confirmed, the power station will be either:

(a) Two reactors each with output equivalent to 1600 megawatt

electrical, that is a total output of 3200 mw(e).

(b) Three reactors each with output equivalent to 1000

megawatt electrical, that is a total output 3000 mw(e).

Note: mw(e) is megawatt electrical.

mw(th) is megawatt thermal.

As you well know I am a supporter of nuclear power, so I have no problem with the nuclear side of the proposed nuclear power station at Oldbury. However, there maybe a choice in its design to do with the cooling system associated with the exhaust steam from the turbines.

I am aware of four ways that exhaust steam can be cooled in nuclear or conventional power stations. In all four ways the turbine exhaust steam and the cooling water (CW) are kept completely separate from each other. However in three of the ways (2, 3 and 4) the cooling water (CW) has itself to be cooled after it has cooled the exhaust steam back to water.

WAY 1. (Direct River/Sea Cooling)

This is the one in use at the existing Oldbury P.S. where water is extracted from the river Severn passed through a condenser to cool the turbine exhaust steam and then the river water returned straight back to the river Severn: warmer than when it was extracted from the river.

This way of cooling has the minimum visual impact, particularly from a ‘distance’ point of view. However, there are statutory constraints on how the water is extracted from the river; then also how warm it is and its condition, before it is returned back to the Severn: these legal constraints are there to protect human health, the fauna, fish and wildlife. This currently limits the heat discharge to the Severn from a designed station output of about 600mw(e). (With a present thermal efficiency of about 25% this is about 1800mwth).

NOTE:

This method cannot be used by a new power station of the proposed size because the amount of heat returned to the Severn in the warmed water ((in generating over 3000mw(e)) would be far too great to comply with the legal constraints associated with the Severn. (With a thermal efficiency of a new power station approaching 40% a heat rejection to the river Severn would be about 4800mwth., approaching three times the present licence limit of the existing P.S.

WAY 2. (Concrete Tower-Natural Convection Wet Cooling)

A second way is the use of natural draught concrete cooling towers. The CW that cools the exhaust steam in the condenser is in a ‘closed system’ the same water going around and around: not unlike the water in a household central heating system. But instead of having a boiler to put heat into the water, as in a central heating system, you have a cooling tower to remove heat from the water that has been put into it from cooling the exhaust steam of the turbines: to turn it back to water.

It does this by taking the warmed water part way up the concrete cooling tower and then dispersing it in a designed manner, so that it falls like water from a bathroom shower head. As the water falls in billions of small droplets it meets a powerful draught of air coming up from the bottom of the cooling tower which cools the water droplets which eventually fall into a pond at the bottom of the cooling tower. These towers can either be cylindrical or of a hyperbolic shape: for technical and cost reasons they are normally hyperbolic in the UK.

These cooling towers can currently be up to 200 metres(m) high depending on the amount of energy generated by the power station, its thermal efficiency, planning constraints, cost and construction considerations: for example two smaller cooling towers could or might have to be used instead of one larger one. When one of these cooling towers is in use it emits a significant visual white water vapour plume from its top. This water vapour plume also means that some water is being lost from the ‘closed’ system so that some water has to be continually added to the system to keep the amount of water constant: this is generally called ‘make up water’. Water vapour is considered to be a ‘greenhouse gas’.

WAY 3. (-Forced Draught Wet Cooling)

The third way is again a cooling tower. In principle the same mechanism for cooling the exhaust steam as is the use of concrete cooling towers, but forced air draught is used to do the cooling instead of natural draught. Together with another design feature they stop/reduce the visual vapour plume: water vapour is still emitted but the part that is visible is insignificant compared to that from natural draught concrete cooling towers in WAY 2. These forced draught towers which require electrical fans to drive the cooling air through them to do the water cooling are referred to as a ‘hybrid cooling towers’.

Because fans are needed some of the power station electrical output is needed to drive these fans, so station output is reduced slightly (3%?). However, the height of these towers is significantly reduced to about one third the height of conventional concrete cooling towers in WAY 2.

WAY 4. (Dry Concrete Cooling Towers)

The fourth way is again a cooling tower. This design of cooling tower emits no water vapour, since the warmed water to be cooled and the cooling air do not come into contact with each other. My knowledge of these is limited. But from that limited knowledge these towers are of a considerably larger size than those of wet concrete cooling towers for a similar power station rating. None seem to have been constructed for a power station approaching anywhere near the size of the proposed Oldbury power station. Hence like WAY 1 not an option for the proposed power station at Oldbury.

CONCLUSION:

The choice is either WAY 2 or WAY 3. My choice is WAY 3: Hybrid Cooling Towers.

FOOTNOTE:

Colin,

In addition to the information I have given in ‘WAY 1’ above, a few people have asked me for the technical information about why the river Severn cannot be used for cooling the new station, in the same way as the present station, My explanation below may also be of interest to you.

Oldbury’s current output is around 440mw (electrical). To make the arithmetic easier (let’s say) 450mw (e) with a thermal efficiency (let’s say) of 25%. (which is determined by design and thermodynamic laws). The two reactors produce together heat totalling 1800mw (t)hermal, 25% of that heat makes the 450mw (e) and the rest, which is 1350mw of heat goes into the cooling water (CW), that is, the river Severn. The Severn environment can cope with that.

‘Guess’ figures for the new station: 3200mw (e) and thermal efficiency 40%( let’s say, latest design figure?) means that the two reactors produce together about 8000mw(t), 40% of that heat makes the 3200mw(e) and the rest, which is 4800mw of heat, (over three times that of the current power station): has to go somewhere. It could not be dispersed into the River Severn because this is well outside what could be allowed.