Cooling of plants: a constraint on growth?

Difficulties experienced with the cooling of the French nuclear plants in the heatwave of summer 2006 has been picked up by anti-nuclear forces as indicating a possible limitation on an expansive nuclear future. Plant operations had to be curtailed owing to the rise in the temperature of the cooling water at rivers and lakes.

Rather mischievously, some commentators have inferred that, far from being a solution to global warming, nuclear may itself get killed off by it, owing to a rise in global water temperatures. This is clearly rather a cute paradox, but essentially absurd. Rises of water temperature of such magnitude would have already destabilised the world to such an extent that civilisation as we know it would be threatened. But to what extent does the requirement for plant cooling pose any real potential difficulty for nuclear? This issue is addressed in a new World Nuclear Association (WNA) position statement, which is summarised hereafter.

Cooling is a necessity for all thermal power plants, consuming either fossil fuels – coal, natural gas, or oil – or uranium, where heat is produced to run the steam-driven turbines that generate most of the world’s electricity. The removal of surplus heat is essential in the steam cycle, and these thermal plants usually accomplish heat discharge using a local water source and various techniques of ‘wet’ cooling.

In the internal energy-transfer function, water is circulated continuously in a closed-loop cycle. The primary heat source turns water to steam to drive the turbine, and the water is then condensed and returned to the heat source. In this internal circuit, hardly any water is lost, so only a very small amount of make-up water is required. This function is much the same whether the power plant is nuclear, coal-fired, or conventionally gas-fired. Today most of the world’s non-hydro electricity is produced in this way.

Cooling is also needed to condense the low pressure steam in the internal circuit and recycle it. As the steam condenses back to water, the surplus heat must be discharged to the air or a body of water. Just how much heat must be discharged for any given electricity output is determined by a plant’s thermal efficiency.

Thermal efficiency depends on the temperature difference between the internal heat source and the external environment. Any steam cycle system loses about two thirds of the energy due to the physics of turning heat into mechanical energy – in this case the turning of a generator. But this loss can be reduced by expanding the temperature differential, and thereby increasing thermal efficiency. Hence the colder the cooling water, all other things being equal, the higher the efficiency.

Nuclear plants currently being built have about 34-36% thermal efficiency, while one of the new reactor designs boasts up to 39%. In comparison, a typical new coal plant runs at 36%, while some new coal-fired plants approach 40%. In determining the cooling requirement, these distinctions are not insignificant. For example, any power plant running at 33% thermal efficiency must discharge about 14% more heat than one at 36% efficiency. Coal plants have a slight edge over nuclear plants and a correspondingly reduced need for cooling water.

In both fossil and nuclear plants, a water circulation system is normally used to accomplish heat discharge through two types of wet cooling: ‘once through’ and ‘recirculating’. Where the power plant is next to a large water body, cooling is achieved simply by running a large amount of water through the condensers in a single pass and discharging it back a few degrees warmer in almost the same amount. In this simple method, the water may be salt or fresh. There is hardly any onsite water use in the sense of depletion, though some extra evaporation will occur offsite due to the water being slightly warmer.

Many nuclear power plants have once through cooling, since their location is not influenced by the source of the fuel, and depends first on where power is needed and secondly on water availability for cooling. A number of countries are able to use once through seawater cooling for all of their nuclear plants. Among these are the UK, Sweden, Finland, South Africa, Japan, Korea and China, while Canada uses once through cooling from the Great Lakes.

Any nuclear or coal-fired plant that is cooled by drawing water from a river or lake will have regulatory limits imposed either on the temperature of the returned water or on the temperature differential between inlet and discharge. In hot summer conditions, when even the inlet water from a river may approach the limit set for discharge, the plant will be unable to run at full power using once through cooling only. In these circumstances, recirculating wet cooling that capitalises on the physics of evaporation can be used to help.

Cooling towers with recirculating water are indeed a common visual feature of power plants, whether fossil or nuclear. Tall towers, usually hyperboloid in shape, employ a natural draft ‘chimney effect’. Shorter towers use a forced draft created by large fans. A big issue, however, is that recirculating cooling systems reduce the overall efficiency of a power plant by 3-5% compared with once through use of water from the sea, a lake or major river.

In these systems, water passes through the condenser and on to the top of the tower. From there it sprays downward to a collection basin while being cooled by an updraught that carries heat away, mainly by evaporation and with some direct heat transfer to the air. In temperate climates, an onsite pond can also be used to accomplish evaporation.

As the cooled water is returned to the condenser, the 3-5% of the flow that is lost to evaporation must be continuously replaced. The water loss equates to some 1.75-2.5 litres per kilowatt-hour. Moreover, because evaporation concentrates impurities in the water, some bleed of water (called ‘blowdown’) is required, raising the need for replacement water by another 50%.

Typical water consumption for a 1000MWe plant – providing electricity to perhaps 1 million people in an industrial country – might be 75 megalitres per day, or the

equivalent of 25 Olympic-sized swimming pools. In the USA, where inland siting causes more than half of coal and nuclear plants to use wet cooling towers, electric power generation reportedly accounts for 3% of all freshwater consumption. In France, where 58 nuclear reactors produce almost 80% of the nation’s electricity, 32 reactors use cooling towers, drawing replacement water from rivers and lakes, while 26 reactors use once through cooling from seawater and major rivers.

In addition to ‘wet’ cooling, some power plants (but no nuclear plants to date) use ‘dry’ cooling – ie cooling through heat transfer to the air without the physics of evaporation. This works like an automobile radiator, with a high-flow forced draft past a system of finned pipes through which the steam passes. Alternatively, there may still be a condenser cooling circuit with the enclosed water cooled by a flow of air.

Dry cooling involves much greater cost for the cooling setup and is less efficient than wet cooling towers that use the physics of evaporation. Large fans consume much power, and cooling solely by heat transfer is relatively inefficient. Because combined-cycle gas turbine (CCGT) plants release much of their heat to the air in the turbine exhaust, they require only about one third as much cooling as normal thermal plants, and this is commonly done through dry cooling. Hence they can achieve thermal efficiencies in the 60-70% range.

Cooling for coal-fired and nuclear plants is plainly not an issue where the availability of water is unlimited, as when the plant is sited by a large body of relatively cool water. The wet cooling problem can arise for plants sited on rivers and other locations where water availability is limited in quantity or by regulations on the temperature of returned water. But by capitalising on the great flexibility in site selection (not encumbered by fuel issues), nuclear energy planners can ensure the availability of reliable cooling as they optimise costs. Key factors in this calculation are the comparative extra expense of longer electricity transmission and of alternative and supplemental cooling technologies. The siting of coal plants, by contrast, is substantially motivated by another factor: fuel availability. Because coal-fired plants consume vast quantities, proximity to the fuel source or transport hubs is a key cost factor and a major constraint in their siting.

For any power plant, once through cooling systems using fresh water and seawater are less costly to build and more energy-efficient than systems using wet recirculation through cooling towers or ponds. Thus, the siting of coal and nuclear power plants on coastlines is usually preferable where other considerations allow. This siting preference – yielding simple and inexpensive cooling – must be balanced against the cost benefit of proximity to electricity load centres. In many instances, this balance is easily struck in favour of coastlines. In the UK, for example, all nuclear plants are on the coast and total electricity transmission losses in the system are only 1.5%.

Assuming that nuclear power enters a renewed strong growth phase, it is certain that many new plants in the USA, Russia and China and elsewhere will have to be built away from the sea. It’s inconceivable that we’ll have transmission lines over thousands of miles, given the potential losses. The costs of cooling towers, dry cooling and lower thermal efficiencies will not be insignificant and, in some cases, this may be sufficient to turn the economic balance against nuclear. Where the economic balance is fine, as in many parts of the USA today, the additional costs of an inland site could be significant.

If the world wants thousands of new nuclear plants this century, siting issues due solely to cooling needs are likely to become more pressing, especially if the current concerns about long-run water availability in many areas of the world are justified. Dry cooling may eventually be a possibility, but by then, reactor designs are likely to have moved on to such an extent (with Generation IV and beyond) that it’s hard to predict the outcome.

Thus, cooling is an essential but readily manageable aspect of prospective nuclear power operations and does not constitute a constraint on the future growth of nuclear power as a large-scale low-cost provider of clean energy with highly stable prices and strong security of supply.

Steve Kidd is Director of Strategy & Research at the World Nuclear Association, where he has worked since 1995 (when it was the Uranium Institute). Any views expressed are not necessarily those of the World Nuclear Association and/or its members

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