Ambient and river water temperature

As cooling systems use either air or water as the heat sink, ambient and water temperatures can influence the cooling system's cooling efficiency. The lower the ambient or intake water temperature is, the higher the cooling efficiency gets. In 1824 Nicolas Léonard Sadi Carnot discovered that the maximum energy conversion efficiency of a heat engine is decided by the temperature difference between its hot heat source and cold heat sink, as seen in Figure 2.6. A heat engine refers to a system that converts heat or thermal energy and chemical energy to mechanical energy.

In Figure 2.6, W refers to the work/output of the engine, QH0T is the heat input at a high temperature, QC0LD is the heat rejected at a low temperahire. W equals to QH0T minus OCOLD. According to Carnot, the ratio between QH0T and Qcold equals the ratio between the high-temperature TH0T and low-temperature TC0ID. Jiang and Ramaswami (2015) highlighted the seasonal variations of power plants’ water intensities based on field data of 19

A heat engine operating between two temperaturescoal-fired power plants in Shandong, China

Figure 2.6 A heat engine operating between two temperaturescoal-fired power plants in Shandong, China. Power plants' water intensities are much higher in July when both ambient and river water temperatures are high, and therefore cooling efficiencies are low.

As a result, air-cooling systems' efficiency decreases as ambient air temperature increases, whereas open-loop cooling systems are mostly vulnerable to increased river water temperature, especially when the maximum discharged water temperature is regulated (Forster and Lilliestam, 2009). France and southern California had to curtail their electricity productions in 2003 and 2007, respectively, due to the compounded effects of low flows and increased river water temperatures. Moreover, the warm water discharge from thermoelectric power plants with open-loop cooling systems can potentially cause harmful impacts to river and marine ecosystems by interfering with fish breeding and species range (Raptis and Pfister, 2016; Raptis, Van Vliet and Pfister, 2016; Raptis. Boucher and Pfister, 2017).

Alternative energy sources for electricity

Apart from coal power plants, electricity generated from other primary energy also requires water inputs at different stages. According to Macknick et al. (2012), apart from cooling technologies, fuel types also have significant impacts on power production’s water uses, mostly due to the differences in thermal efficiencies. To generate a certain amount of electricity, gas-fired power requires the least water while nuclear power needs the most. Gas-fired power generation requires the least water because allnatural gas plants use a gas turbine where natural gas is mixed with air and combusts and expands to cause a generator to produce electricity. No water is required for generating or cooling the steam in a gas turbine cycle. However, gas turbines can be used in combination with a steam turbine in a combined cycle power plant with very high efficiency. Nuclear technologies require the largest water withdrawals among the thermoelectric generating technologies (EIA. 2019). In order to avoid damages to the finely engineered nuclear fuel assemblies, nuclear power plants operate at a lower temperature than conventional coal or gas power plants and therefore have lower thermal efficiency. Moreover, nuclear power plants do not lose heat through combustion gases; therefore, they withdraw and consume more water per unit of electricity produced (World Nuclear Association, 2013).

In terms of hydropower, although Mekonnen and Hoekstra (2012) have highlighted that hydropower is a significant water consumer because of water evaporation in the dammed reservoirs, there exist many methodological disputes, especially on how to attribute the water consumption to different uses (e.g. hydropower, agriculture, navigation, flood control) in multi-purpose reservoirs (Bakken et al., 2013, 2016). Furthermore, evaporation rates from open water exhibit substantial geographical differences affected by local climatic conditions, such as humidity, wind, temperature and so forth (Scherer and Pfister, 2016; Hogeboom, Knook and Hoekstra, 2018). As a result, the literature estimates exhibit huge uncertainties (Zhang and Anadon. 2013; Bakken, Killingtveit and Alfredsen, 2017). Zhao and Liu (2015) proposed to allocate the water consumption to reservoirs' different uses according to their ecosystem services. Based on this allocation method, Liu et al. (2015) calculated the water consumption intensities for electricity production of China's 209 hydropower plants. The following sections illustrate how water is used for hydropower, gas-fired power and concentrated solar power.

Hydropower

There are four main types of hydropower technologies: (1) Storage Hydropower Stations (Figure 2.7): Water stored in a dammed reservoir can be released to transform its potential energy to kinetic energy to drive a water turbine and generate electricity.

  • (2) Run-of-the-River Hydropower Stations (Figure 2.8): Some hydropower stations have small or no reservoir capacity so that only the water coming from upstream can be used for generation at that moment. The kinetic energy in running water is converted to mechanical energy at a turbine and spins the generator to produce electric energy.
  • (3) Pumped storage hydropower stations (Figure 2.9) use excess electricity when electricity demand is lower than production to pump water to higher reservoirs acting as a type of energy storage and release water when electricity demand is high. When electricity demand is low, excessive electricity is used to pump water in the lower water body to the upper reservoir. Excessive electric power is converted to potential energy stored
Schematic of a hydroelectric power station

Figure 2.7 Schematic of a hydroelectric power station

A typical run-of-river hydropower station

Figure 2.8 A typical run-of-river hydropower station

Schematic of a pumped storage hydropower station

Figure 2.9 Schematic of a pumped storage hydropower station

in the upper reservoir. During the high-electricity demand period, water is released from the upper reservoir to drive the turbine and generator to produce electricity.

In addition, there are various offshore renewable energy technologies, but they are not considered here as they do not consume freshwater.

Engineering background 23

The capacity of hydropower plants can be calculated according to the following equation:

P is power (watts); m refers to the water flow (kg s-1); g is the gravitational constant (i.e. 9.81m s*2); Hnet is the net head and q is the conversion efficiency of hydroelectric power plants, which is the product of efficiencies of all different components, including the turbine, drive system and generator. Modem hydropower stations have an energy conversion efficiency of about 90%, while smaller plants may have efficiencies between 60%-80% (Nazari-Heris and Mohammadi-Ivatloo, 2017). Net head is the gross head minus head losses. Gross head refers to the vertical distance between the water intake and the turbine. which can be measured on site. However, there are head losses due to pipeline friction, and net head is calculated as the gross head that can be measured at the site, deducting any head losses. A properly designed pipeline normally yields a net head of about 85%-90% of the gross head (Canyon Hydro, 2013).

 
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