Reducing water consumption for cooling of thermal generation plants

The most energy-efficient way of cooling thermal plants is using the once-through system, whereby “water is withdrawn from nearby bodies of water, diverted through a condenser where it absorbs heat from the steam, and then discharged back to its original source at higher temperatures. Because once-through cooling systems do not recycle the cooling water, this leads to very high volumes of daily water withdrawals. The water intake structures at power plants with once-through cooling can kill several millions of fish annually, and the thermal discharge downstream can also harm aquatic organisms, affecting the whole aquatic ecosystems. In addition, the large volume of water required to operate once-through cooling systems makes power plants especially vulnerable in times of drought and extreme heat” (NDRC 2014).

Recirculating tower cooling and dry cooling are alternative cooling options that considerably reduce water use compared to once-trough cooling systems.

Recirculating tower cooling still foresees an intake of water from external sources, but the amount withdrawn is 95% lower than in once-trough cooling systems, with a comparable reduction of negative impacts on ecosystems. Water is kept circulating in the system, absorbing the heat from the steam used to generate power through a condenser, and releasing it through evaporation within a cooling tower. However, since cooling takes place through evaporation of a fraction of the water withdrawn, recirculating wet cooling can still be problematic in conditions of severe water scarcity.

Dry cooling relies on air as the medium of heat transfer, rather than evaporation from the condenser circuit. As a result, water losses are minimal. There are two basic types of dry cooling techniques available. Direct dry cooling uses an air-cooled condenser pretty much as in an automobile radiator. It employs high-flow forced air through a system of finned tubes in the condenser within which the steam circulates. It thus transfers the steam’s heat to the ambient air directly. Cooling a power plant in this way requires less than 10% of the water used in an equivalent wet-cooled plant. Around 1-1.5% of the power station's output is consumed to propel the large fans. An alternative design includes a condenser cooling circuit as in wet recirculating cooling, but the water being used is enclosed and cooled by a flow of air through finned tubes in a cooling tower. Heat is thus transferred to air by means of a process less efficient than wet cooling, but improving on direct dry cooling, as energy use is only 0.5% of output. According to EIA, there were 719 once-through systems in place, 819 recirculating systems, and only 61 dry cooling and hybrid systems installed in the USA in 2012. In the absence of analogous information for the EU and assuming that roughly the same technology maturity levels apply to the electricity sector across developed countries, it is possible to assume that dry/hybrid cooling counts for less than 4% of all cooling systems installed in thermal plants in the EU.

NDRC, taking as reference a conventional coal-fired power plant, quantifies the water use of alternative cooling options in two ways: water withdrawals, that is, how much water is taken from the water basin and then, possibly and partially, returned to it; and water consumption, that is, how much of the water withdrawn is transformed into vapour and hence not directly returned to the water basin after cooling. For dry cooling systems, they both amount to 0 l/MWh. Water withdrawal requirements for once-through cooling and closed-cycle cooling systems are, respectively, about 75,710 - 189,270 litres per megawatt-hour (l/MWh) and 1,890 – 4,540 l/MWh. Water consumption, on the other hand, results in about 380 – 1,200 l/MWh for once-through and 1,820 – 4,169 l/MWh for closed-cycle cooling. Thus once-through systems withdraw more water from the water basin, but also return more water to it than closed-cycle systems. However, it is the withdrawal process that brings about more serious negative effects on the environment, by directly killing river fauna and by returning water at a temperature above the ecologically desirable ranges.

IPCC categories

Structural and physical: Engineering and built environment options, Structural and physical: Technological options

Stakeholder participation

Stakeholder involvement is an important part of the authorising process for electricity generation plants, but it is difficult to extrapolate the implications for a specific component of the plant. Cooling towers, which can be over 50 m high, are arguably one of the most visible components of a plant, and hence there may well be local opposition to the negative aesthetical impact of an imposing tower on a landscape. However, mitigation and compensation measures can be put in place, for instance by designing and siting the plant in order to minimise visibility of its most prominent infrastructures from nearby inhabited areas, or by screening it by planting trees around the plant and /or by building artificial hills (soil berms) that blend into the natural landscape and block the view of the plant. Local communities can be directly financially compensated for the welfare loss caused by the aesthetical impacts suffered, or other compensatory actions can be undertaken, such as building socially useful infrastructure such as parks, schools, etc.

Since these options reduce water withdrawals from a basin, they are expected to be seen favourably by stakeholders relying on the same water resources as the power plants implementing these measures. The resulting changes in water use rights should be discussed among all stakeholders and agreed upon with them and with the water basin authorities accordingly.

Success and Limiting Factors

Recirculating tower cooling is about 40% more expensive (US DOE, 2009) than once-through wet cooling, and can be applied where water availability is limited or the impact of entrainment and impingement and thermal discharges needs to be reduced.

Both dry cooling options provide much greater flexibility in the location of new power plants, as it becomes independent from the availability of a major body of water. The major drawback of this option lies in its economic costs. With both types of dry cooling, heat transfer is significantly less efficient than with “wet” cooling options, and hence it requires very large and mechanically complex cooling plants. This results in higher costs. The operation of a dry cooling system requires in fact 1-1.5% of the power generated by the plant, compared to 0.5 % of a recirculating system and virtually zero for once-through. The physics of evaporation applied in wet cooling towers allows in fact a more efficient transfer of heat than the one from steam or water to air via metal fins, and hence increases the whole technical and economic efficiency of the plant. Note that thermal efficiency and therefore economic conditions of operation vary with the climatic conditions of the location of the plants, and can be considerably different across Europe.

This points to a second, technical limitation of dry cooling: in a hot climate, ambient air with temperatures above 40° C substantially reduces the cooling potential of a dry cooling system, compared with a “wet” system, which bases its potential on much lower wet bulb temperatures.

A possible way out could be a hybrid dry/recirculating system. Dry cooling could be used in condition of water scarcity and could be coupled with a limited use of a recirculating cooling tower system when temperatures peak. The recirculating tower cooling system can also be used during periods in which there is an abundance of water.

Costs and Benefits

Cost figures obviously vary with the specific conditions of each plant. However, in general US DOE (2009) reports that wet recirculating cooling systems are 40% more expensive than pass-through systems, while dry cooling systems are three to four times more expensive than a recirculating wet cooling system. At the moment, wet recirculating systems are considered the Best Available Technology for thermal plant cooling by the US Environmental Protection Agency (EPA), because they minimize the impact on water ecosystems while keeping the increase in costs affordable.

On the plus side, both recirculating and dry systems have virtually no intake of water and no impact on water ecosystems, which can at least partially compensate for the extra capital and operation costs, in particular in conditions of water scarcity brought about by climate change.

Legal Aspects

The choice of the cooling system is an important part of a power plant’s design. It is subject to the authorization processes applied to grant the permission to build and operate power plants, which varies from country to country. Since dry cooling systems are less energy efficient than other cooling systems, at the moment they rank last in the order of EU Best Available Technologies for cooling, and are outranked by recirculating tower cooling. While the use of dry cooling is not excluded, it is limited to locations with very restricted water resources or with particular environmental concerns related to water use.

For large units, safety implications concerning the removal of decay heat after an emergency shutdown with loss of power should also be considered.

Modifications to water use agreements resulting from the reduced water needs of plants implementing these options should be formally agreed upon with water basin authorities, based on consultations with all affected stakeholders.

Implementation Time

For new plants, the implementation time is the same as of the plants they belong to. For retrofits, it varies with the technologies. To replace a pass-through system, a study on retrofitting Californian coastal power plants (Tetra Tech, 2008) indicates a downtime of the plant (to allow installing and connecting the new cooling system) of six weeks as a conservative estimate for fossil plants whereas retrofitting the cooling system of nuclear power plants could require up to 12 months due to their technical complexity.

Life Time

The lifetime is the same as the electricity generation plant to which the specific measure belongs. Life-span of thermal plants varies with the technology: nuclear plants, although their design lifetime is typically 40 years, can keep functioning up to 70 years (Scientific American, 2009), while fossil fuel plants vary between 25 and 50 years (natural gas and coal plants, respectively).

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