# Economic Analysis of a Zero-Water Solar Power Plant for Energy Security
## 1 Introduction  
Thermal power plants withdraw and consume great volumes of water [1, 2, 3]. About 40% of water withdrawals in Europe and the United States are due to power generation, specifically due to thermal plants, while this percentage reaches 84% in China and 10% globally [4, 5, 6, 7]. This water is mainly used for cooling purposes, i.e., to condensate the steam at the outlet of the turbine in a Rankine cycle [8]. Cooling is a critical step in both fossil and renewable energy plants that must be close to a reliable water source when using wet cooling. The main two disadvantages of water use for cooling purposes are, first, the significant environment impact and, second, the vulnerability of the plant to water availability. In direct or indirect cooling, the temperature of both the water source and the ambient air, in the case of cooling towers, is increased [9, 10]. Part of the water used is usually returned to the water source at a higher temperature and an altered chemical composition, often including pollutants [11]. Avoiding water use in thermal plants calls for urgent solutions, considering expected water stress scenarios in various regions of the world during the next decades [12]. Water scarcity, greatly influenced by climate change, will cause water competition among different sectors with social and political conflicts [8]. Potable water and water for food production, residential use, power generation and other water-requiring processes will become scarce [13, 14, 15]. This kind of effect on power generation has already been seen during the last decades, when episodes of water scarcity occurred in countries such as France, the United States of America, India, and Australia [16]. This problem can be addressed by replacing water-based cooling systems with dry cooling systems that almost completely eliminate the use of water in thermal power plants. Dry cooling systems, as we know them today, are more expensive and less efficient than water-cooling systems because they depend on the usually higher temperature of the air (when compared to water) [17]. The higher air temperature leads to an increase of the steam pressure at the outlet of the turbines. This reduces the expansion in the turbine, that in turn reduces the efficiency of the Rankine cycle and the power output of the plant. The lower efficiency leads to higher environmental impacts and economic expenditures: more CO\({}_{2}\) is emitted to the atmosphere in the case of fossil-fuel plants, larger installations of heliostats are required in concentrating solar power (CSP) plants, and the local climate may be impacted by exhausted the cooling air at a higher temperature impacting. An earth-air heat exchanger (EAHE) is a technology used to cool and heat air mainly in residential and agricultural applications. These systems cool or heat the air through pipes buried at a certain depth, taking advantage of the relatively constant temperature of the ground [18]. EAHE systems can be open- or closed-loop and horizontal or vertical. This technology is also used with water or other heat transfer fluids, generically called ground-source heat pumps (GSHP) [19]. EAHE systems use fans to move the air through the pipes that may be made of different materials, filters to clean the air, and dehumidifiers or dryers to remove the moisture. In the case of systems using a liquid fluid instead of air, fans are replaced by hydraulic pumps [20; 21]. The economics of underground small-scale heat exchangers have been evaluated through methods such as the Net Present Value (NPV), the Internal Rate of Return (IRR), or the Pay-back Period (PBP). [PERSON] et al. (2012) studied the IRR for a 10-year period of an earth-air tunnel integrated into an evaporative system [22]. They estimated excavation costs of USD 10/h and USD 96 for a 0.15 m diameter and 24.4 m length PVC pipe. The cost of a standard blower was USD 200, while the operation and maintenance (O&M) costs of the whole system were estimated as 2% of the initial investment.