# A Review of the Water Desalination Technologies
## 3 Statistics on Desalination  
As introduced in the previous section, several technologies have been proposed for desalination. The research is currently investigating new solutions to reduce energy consumption and improve environmental sustainability, considering renewables as an energy source to supply the process. Many technologies described above are under investigation, so limited data are available. Considering the commercial technologies for desalination, the trends of the installed capacity and number of plants are reported in Figure 34[12]. As introduced above, thermally driven desalination plants (MSF and MED) dominated this sector until the 1990s [22; 36]. As the freshwater demand is still growing, new plants are being installed. MSF continues to grow linearly, while the installed capacity of RO is increasing very quickly [85]. Figure 34: The trend of installed capacity and operative desalination plants. Reproduced with Elsevier License permission n. 4965830049121 [12]. Figure 33: HDH unit using a heat pump with an air refrigerated condenser. In [12], [PERSON] et al. reported some statistics on the current big desalination plants installed around the world, using data from the Global Water Intelligence (GWI) [86]. In detail, GWI is a database containing information on the installed desalination plants, including their status, operational years, installed capacity, geographic position, technology, and water sources. Considering the desalination technology, the global installed capacity (95.37 \(\times\) 10\({}^{6}\) m\({}^{3}\)/day) is composed of RO (68.7%), MSF (17.6%) MED (6.9%) NF (3.4%) ED (2.4%), and other (1.0%) [12]. Figure 35 reports graphically the worldwide distribution of desalination plants, considering size, technology and the water source used to produce freshwater [12]. Most of desalination plants are installed along the coastline to adopt the sea as a water source. Indeed, 60.8% of world's installed capacity uses the sea as a water source, whereas brackish water is utilized in 20.6% of cases [12]. The Middle East and North Africa are currently equipped with 4826 plants, with an installed capacity equal to 45.32 \(\times\) 10\({}^{6}\) m\({}^{3}\)/day (47.5% of the world's installed capacity). The remaining desalination plants are distributed in the following way: 3505 plants in East Asia and the Pacific (18.4% of worldwide installed capacity), 2341 in North America (11.9%), 2337 in Western Europe (9.2%), 1373 in Latin America and the Caribbean (5.7%), 655 in Southern Asia (3.1%), 566 in Eastern Europe and Central Asia (2.4), and 303 in Sub-Saharan Africa (1.9%) [12]. The main sectors are industry (7757 plants with a capacity of 28.8 \(\times\) 10\({}^{6}\) m\({}^{3}\)/day) and municipality (6055 plants, 59.39 \(\times\) 10\({}^{6}\) m\({}^{3}\)/day). Table 1 shows statistics on the state of the art of operative desalination plants [5; 12; 13; 14; 31; 87]. According to these data, RO is the most adaptable solution to exploit sea and brackish water, with an average capacity ranging from 1000 (or lower) to 320,000 m\({}^{3}\)/day. As for the energy supply, RO exclusively requires electricity (or mechanical energy) to run the pumps along the water circuits. This feature simplifies the potential coupling with renewable energy sources, as demonstrated by several studies [19; 36; 87; 88]. The water cost is lower than other desalination technologies, explaining its quick diffusion around the world. Figure 35: A map of desalination plants around the world, by size and technology. Reproduced with Elsevier License permission n. 4965830049121 [12]. To improve the energy efficiency of desalination technologies, exergy analysis has been recently proposed in several researches [89; 90; 91; 92; 93; 94]. Introducing the Second Thermodynamic Law, exergy represents the maximal available energy related to a system, obtainable if reversible processes are only used to achieve equilibrium conditions within the environment (dead state).