Renewable Energy Powered Water Treatment Technologies

The most common renewable energy technologies for powering water treatment systems in the past have been PV, solar thermal energy, and wind energy. This is shown in Fig. 6 for the following desalination technologies: reverse osmosis (RO) including nanofiltration, multieffect distillation (MED), electrodialysis (ED), multistage flash (MSF), and mechanical vapor compression (MVC). Although, no examples of renewable energy powered water recycling schemes exist at the present time, there are plans to develop such schemes. An Australian scheme is described in a section below.

Before considering the energy consumption ofvarious technologies, it is important to understand the implications of the chosen desalination technology. Desalination can be accomplished via phase change (including MED, MSF, MVC) or membrane separation (including RO, ED, NF) processes. The former all involve a phase change of the feedwater (either to vapour or solid), whereas technologies like RO or ED rely on the filtration properties of polymeric membranes. The figure of merit for clean water production is the specific energy consumption (SEC), defined as how many

Solar MED _ 5% Hybrid

\ Other

Solar MED _ 5% Hybrid

Figure 6 Breakdown of renewable energy powered desalination system technologies implemented worldwide Ref. [10].

units of clean drinking water can be produced for each unit of energy consumed (units: kWh/m3). The SEC of a phase change process is proportional to the amount of water produced, whereas the energy requirements for a membrane separation process are proportional to the salinity of the feedwater.

Table 1 compares all existing renewable energy powered desalination technologies, highlighting the energy consumption and disadvantages of each technology. For RO and NF systems, the major energy requirement is for pressurising the feedwater, with brackish water systems typically operating at pressures of about 5-15 bar, while seawater desalination ranges from about 40 to 60 bar. It should be noted that many technologies are still going through a learning curve: overall, the SEC of Spanish seawater desalination plants has decreased from 22 kWh/m3 in 1970 to 8 kWh/m3 by 1990, and is presently at 4 kWh/m3 [9].

Given that the capital cost of installing a renewable energy system is high, it is naturally desirable to couple this with the desalination technology with the lowest SEC. However, this is not the only consideration. For example, if significant amounts of low-grade heat are available, then perhaps one of the phase change processes could offer a lower lifecycle water cost if operation and maintenance costs are less than for RO systems.

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