**3. Methodology**

The energy consumption, GHG emission, and the unit product cost of the seawater desalination plants in China are assessed for the year of 2006–2016. The seawater desalination capacity data of China overall and selected provinces in 2016 are mainly derived from [14], and other data sources are explained where mentioned.

#### *3.1. Energy Consumption*

The energy consumption of seawater desalination plants in China can be calculated using mass balance equations along with the specific energy consumption (SEC) and the capacity of the plants. SEC in kWh/m<sup>3</sup> desalinated water, is one of the most critical factors characterizing the performance of the water supply [24].

$$EC\_u = \sum SE\_i \times C\_i \times \dots \times P\_a \tag{1}$$

where *ECa* is the annual energy consumption of the plant, kWh/y; *SECi* is the specific energy consumption of seawater desalination plants with process *i*, kWh/m3; the energy consumption of all desalination processes involved in this study (RO, MED, MSF, and ED) are converted to the form of electricity, kWh/m3; *Ci* is the capacity of the desalination plants with process *i*, m3/d. Pa is the availability of the plant, when specific data is not available, Pa is set as 90% based on the study of [25].

The specific energy consumption of various desalination processes is listed in Table 2.

**Table 2.** Specific energy consumption (SEC) of different processes (based on [4]).


#### *3.2. GHG Emissions*

Since there are no major direct GHG emissions in water desalination, the estimation of GHG emissions mainly considers the emissions from energy consumption. The estimation method is based on the calculation method of GHG emission from processing proposed by ISCC [26], which is shown as follows:

$$G\_{\varepsilon} = EM\_{\varepsilon} + EM\_{\text{in}} + EM\_{\text{unstable}} \tag{2}$$

where *Ge* is the annual GHG emissions of the desalination plants, t CO2eq/y; *EMe* is the emission of energy consumption, t CO2eq/y; *EMin* is the emission of material inputs, t CO2eq/y; and *EMwaste*, is the indirect emission from treating the waste generated from the desalination processes, t CO2eq/y. In this study, the GHG emission from energy consumption is estimated, and the emissions of material input (seawater) and waste (brine) are not considered due to the limit of data availability.

$$EM\_{\mathfrak{e}} = EC\_{\mathfrak{a}} \times E\_{f} \tag{3}$$

where *ECe* is the annual energy consumption of process *i*, kWh/m3; and Ef is the emission factors, t CO2eq/kWh. In this study, the Ef is set as 1.04 t CO2eq/kWh according to the reference [27].

#### *3.3. Unit Product Cost*

The cost of water desalination mainly includes capital cost and operating cost, with the later mainly consisting of energy cost for plant operation and the cost for maintenance. In this study, the unit product cost, which is the cost per m<sup>3</sup> desalted water, is calculated based on the method proposed by [25].

Estimation of the unit product cost is calculated as follows:

$$LUP = \frac{\frac{\text{CC}}{P\text{I}} + OP\_a}{\text{Ca} \times Pa} \tag{4}$$

where *UPC* is the unit product cost, USD/m3; *CC* is the capital cost of the plant over the lifespan, USD; *Pl* is the plant life, y; *OP*a is the annual operating cost; USD; *Ca* is the capacity of the plant, USD; *Pa* is the plant availability, %.

Capital Cost

> The capital cost is calculated according to the power law rule:

$$\frac{\text{CC}\_x}{\text{CC}\_{rf}} = \left[\frac{\text{Ca}\_x}{\text{Ca}\_{rf}}\right]^m \tag{5}$$

where CCx and Cax are the capacity (m3/d) and capital cost (MUSD) of the studied plant; CCrf and Carf are the capacity (m3/d) and capital cost (MUSD) of the reference plant, m is the power value.

Consequently, the capital cost of plant x can be calculated as:

$$\mathbb{C}\mathbb{C}\_{\mathcal{X}} = \varepsilon^{m \times \ln\left(\mathbb{C}a\_{\mathcal{X}}\right) - m \times \ln\left(\mathbb{C}a\_{\mathcal{I}}\right) + \ln\left(\mathbb{C}\mathbb{C}\_{\mathcal{I}f}\right)}\tag{6}$$

According to [25], m is set as 0.8 for seawater desalinate plants. A dataset of the year 2016 from the Carlsbad Desalination Plant, in San Diego County, USA, is selected as the reference plant to estimate the overall capital cost of the desalination plants (shown in Table 3).


**Table 3.** Basic data of the Carlsbad Desalination Plant, in San Diego County [25].

Operating Cost

The annual operating cost includes the energy consumption (electrical power), maintenance, labor, membrane replacement, as well as the cost for the chemicals. The operating cost is dependent on the operating process of the desalination plants, but in general, energy cost is the major component. Zhou et al. [28] studied the cost of thermal processes and found that energy cost is 87% of the total operating cost.

Wittholz et al. [25] investigated the cost of water desalination, and analyzed the breakdowns of the cost, including fixed cost (capital cost) and operating cost (maintenance, material and energy cost, etc). The contribution of the energy cost to operating cost is also estimated (Table 4).

**Table 4.** Average cost breakdowns of different desalination processes (based on [25]).


The operating cost can be calculated with energy cost and the energy/operating cost ratio as in Equation (7):

$$OP\_{\mathfrak{a}} = \text{Eco/EOP} \tag{7}$$

where *OPa* is the annual operating cost, MUSD, *Eco* is the energy cost, MUSD, and *EOP* is the ratio of energy cost and operating cost, %, which is shown in Table 4.

The cost of energy consumption can be calculated based on the specific energy consumption of different processes and the capacity as well as the price of the electric power, as shown in Equation (8):

$$Eco = SEC\_i \times C\_i \times P\_c \tag{8}$$

where SECi is the specific energy consumption, kWh/m3; the energy consumption of all desalination processes involved in this study (RO, MED, MSF, and ED) is converted to the form of electricity, kWh/m3; Pe is the price of the electricity supplied for desalination plant, USD, the price is estimated for the year of 2016.

The electricity price for water desalination plant is referred from reference [29]. The cost is estimated from water input to the gate of the plant; water conveyance and distribution, as well as the brine disposal are ye<sup>t</sup> considered.
