Energy and Meteorology Portal

Non-Renewable Alternatives

Although nuclear and thermal power plants (either fuel with coal or gas) do not depend on renewable sources to generate electricity, they do need water or air for their cooling systems. The amount of cooling required by any steam-cycle power plant (of a given size) is determined by its thermal efficiency. It has essentially nothing to do with whether it is fuelled by coal, gas or uranium (NWA, 2020).

These plants function by transferring the heat, produced by the nuclear reaction or fuel combustion, into water transforming it into steam to drive the turbines which in turn are used to generate electricity.  The water is circulated continuously in a closed loop steam cycle: after the steam has impulsed the turbine, it is then cooled to condense it back to liquid and return it to the heat source in a closed system.

Cooling down the steam to condense it back into water, requires also more water or air but this time in an open system. This water is used to capture the residual heat from the low-pressure steam, and this heat is then discharged either to the air or to a body of water. For this, the temperature difference between the steam and the water for cooling is key, as the bigger the difference between the coolant and the internal heat source, the more energy the water in the closed loop can absorb and the more efficient the overall system is, therefore resulting in a larger electrical output.

After the cooling water captures the residual heat, this is then directed to a cooling tower (Figure 1), to evaporate the residual heat or alternatively abundant water can be used to cool down the steam but without rising its temperature too much. Once the water reaches legal temperature levels it is discharged into the environment.

Figure 2: Schematic process of how the Cooling Tower interacts with the other systems in nuclear power plant (Source: Wikimedia Commons)

Availability of cooling water therefore becomes a major consideration in siting power plants, because they have to be located near an abundant source of water usually a river, estuary or the ocean, to accommodate hefty water usage. The amount of water can range between about 4000 l/MWh to 170 000 l/MWh depending on the cooling technology (Macknick et al. 2012). During the cooling process, some of the water is lost through evaporation in cooling towers, otherwise the warmer water is pumped back into lakes, rivers, or bays, which can raise the ambient temperature, potentially endangering fish and other aquatic organisms and creating toxic algae blooms (Hayat, 2014).

Some cooling towers use air flow instead of water, to cool down the closed system, where the hot moist air raises up the tower, which can also be facilitated by using fans at the bottom or top of the tower to speed up the process.

Recent innovations have the potential to drastically reduce the environmental impact of cooling systems. This can be achieved for example through the use of metal fins with a geometric design that favourably alters the air flow over them. This solution provides far better heat convection to cool the steam in the air-cooled condenser (Miller 2019). Another example is the electrified mesh that efficiently collects water on its surface, where the water vapor itself is also electrically charged using an ion beam, increasing the amount of water vapor attracted to the mesh. The water then drips down to a tank and the purity of this water allows it to be used in a plant’s boiler system as well, and also is a good alternative to conventional desalination plants, effectively reducing the water needed from a local water system (Chandler, 2021).

Increasing temperatures can result in reduced nuclear reactor efficiency by directly impacting nuclear equipment or warming the plant’s source of cooling water and also affect their safety as the operational safety within the core and spent fuel storage can not be ensured. Previous quantitative research suggests that for every 1˚C increase in outdoor temperature, electrical output decreases by between 0.37 – 0.72%. This poses a risk for all thermal power plants, not just nuclear units. But, nuclear power is uniquely vulnerable to increasing temperatures because of its reliance on cooling water for safe operations.

Nuclear and thermal power plants and W&CS

Forecasting water availability and water temperature at the correct spatial and temporal resolution is important for a correct water management for cooling systems, as heat waves and droughts can have a strong impact on their efficiency and they could even halt power generation altogether. For this reason, sub-seasonal to seasonal climate forecasting can provide an important input in the management of the power plant. Climate projections would be useful for planning the development of new nuclear and thermal power plants, or the decommissioning of old ones, as they would provide an estimate of how water temperature could change under different climate scenarios. For example, for three different scenarios above 2°C, it is expected that 12% (RCP 4.5), 26 % (RCP 6.0) and 75% (RCP 8.5) of nuclear power plants will be affected by 2040 (Figure 1; PRIS 2019).

W&CSs can also assist in climate risk assessments of especially water cooled power plants by estimating projected sea level rise, droughts and more generally water availability and temperature.