Energy supply

In this paragraph, the different energy supply systems considered for electricity and thermal energy are listed and the methods used to estimate the relevant specific costs of energy are described.

3.1.1 Electricity supply

Concerning the electricity supply, I considered two scenarios: grid supply and a combination of grid and photovoltaic (PV)-battery power system. I chose to investigate the PV technology rather than the wind technology, because it is more modular and easier to manage and because the power production has lower uncertainty. In fact, the lack of predictability of wind power production results in higher levelised cost of electricity (LCOE) due to the implementation of over-sized batteries.

3.1.1.1 Grid supply

For the case of grid supply, the specific cost of electricity and the CO2 grid intensity are relevant to the country where the industrial plant is located and depend on the mix of electricity carriers of the grid in the specific country. The data about the current mix of electricity carriers in the different countries are taken from [154]. The current cost of electricity for non-household consumers is given by [155]. Once the shares of total electricity output covered by the different carriers are known, it is possible to estimate the global CO2

emission factor by using the emission factors and the conversion efficiencies of the single carriers. The conversion efficiency is defined as the ratio between the output energy and the primary energy [156]. The CO2 emission factors referred to the primary energy demand are reported in Table 24 [157].

Table 24. CO2 emission factor for energy carriers [157].

Electricity carrier CO2 emission factor [kg/kWhprim]

Lignite 0.399

Hard Coal 0.335

Mineral Oil 0.270

Natural Gas 0.202

59 3.1.1.2 PV-battery power system

The PV-battery system is simulated by using a tool implemented in INSEL [158]. The tool estimates how much power is produced by the system in a specific location, with given number of PV modules and size of the battery. In this way, for different configurations, it is possible to estimate the share of the total required power that can be covered by the PV-battery system. To characterise the specific location in the tool, the relevant meteorological data of a typical year with hourly resolution are required. In particular, the inputs to provide are: the global horizontal irradiance (GHI), the diffuse horizontal irradiance (DHI) and the ambient temperature. The other inputs refer to the size of the system and, in particular, these are the installed PV power [MW] and the capacity of the battery [h]. With these inputs, the model calculates the power produced by the PV-battery system that corresponds to a share of the total electricity demand of the treatment chain. This share is defined as self-sufficiency [%] (equation 123) because it is self-generated by an ad-hoc built plant, whereas the remaining fraction is always supplied by the grid.

4/56 − 3266787/189 [%] ="/0.,$!!1*-,%*"+

"+13$'+ 100 (123)

Also, knowing the size of the system and the power produced, it is possible to calculate the capital and operating costs and the LCOE relevant to the PV-battery system. The capital costs are calculated as the sum of PV modules, battery and converter [159]. The operating costs are estimated as 1.5%/y and 2.5%/y of the investment cost for PV and battery, respectively [160].

The global LCOE is given by the combination of the cost of electricity from the grid and the LCOE calculated for the PV-battery system. The weights of the two terms in the combination correspond to the shares of the power demand covered by the two corresponding supply systems. Moreover, in the case of a combined grid-PV-battery supply, two sub-cases are included: one in which no taxation is imposed on the CO2 emissions and one in which a given tax is applied to the CO2 emissions generated by the fraction of power taken from the grid.

Finally, the CO2 emission factor of the combined electricity supply system is calculated by dividing the CO2 emissions due to the grid supply by the total power produced.

Overall, the INSEL tool can provide the global LCOE and CO2 emission factor of an electricity supply system given by a certain combination of grid and PV-battery. A wide range of combinations can be realised by varying installed PV power and capacity of the battery and the different systems correspond to different costs and CO2 emissions. As reported in the procedure in Figure 12, I performed parametric analyses by varying the ratio between the installed PV power and the power demand from 0.5 to 10 and by varying the full load hours of the battery from 0 to 17.5 h. These analyses gave rise to a scatter plot of LCOE values in function of the CO2 emission factor that is directly correlated to the self-sufficiency of the plant, as shown in Figure 13. I selected the configurations of the supply system corresponding to the points in the lower envelope of the scatter plot to get a characteristic curve of LCOE vs.

CO2 emission factor (in black in Figure 13). The cost of electricity and the corresponding CO2

emission factor are used as inputs in the simulation of the treatment chains to calculate the economic (LPCtot) and the environmental (CO2 emission per unit of product) global outputs.

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Figure 12. Procedure to estimate global LCOE and CO2 emission factors of mixed PV-battery-grid supply systems with the INSEL tool.

Figure 13. LCOE [$/kWh] of the PV-battery-grid supply as function of the CO2 emission factor [kg/kWh] obtained by varying the PPV,inst/Pdemand ratio from 0.5 to 10 (step of 0.5) and the full load hours of the battery from 0 to 17.5h (step of 2.5h). The CO2 emissions have a cost of 80€/tonCO2. The

asterisk symbol (*) indicates the grid supply point.

3.1.2 Heat supply

I investigated two possible sources of thermal energy: waste heat available in the industrial site and a combined heat and power cycle.

61 3.1.2.1 Waste heat available in the industrial site

The reference case used for the analyses presented in this doctoral thesis provides the availability of low-temperature waste heat in the industrial site. In fact, previous studies showed that most industrial sectors produce high amounts of waste heat, especially in the temperature range between 100°C and 200°C [161]. This thermal energy is suitable to the concentration technologies involved in the treatment chains, because these typically require low temperature heat. The MED unit works with steam at a temperature comprised between 70°C and 120°C and the MD units operate with a maximum temperature of the hot stream ranging from 50°C to 80°C.

Therefore, in most cases, thermal energy is supposed to be supplied from low-pressure steam lines at pressure of 1 bar (temperature of 100°C). I fix a specific cost and CO2 emission factor, mostly due to the electricity to be supplied for pumping and compressing the heat. The guidelines report that 0.09 GJ of electricity are required to recover 1 GJ of thermal energy [162]. Thus, I assumed a thermal energy cost of 0.01 $/kWhth, in agreement with other works [163, 164] and with the equivalent cost of the electricity required, when the specific electric energy cost is 0.1 $/kWhel [155]. The CO2 emissions are calculated on the basis of the electric consumption of the heat recovery operations.

3.1.2.2 Combined Heat and Power Cycle

An alternative thermal energy supply is a Combined Heat and Power (CHP) cycle with steam boiler and steam turbine. The system consists in a boiler fired by natural gas and a conventional steam turbine, which is used to provide thermal and electric energy simultaneously. To achieve this dual purpose, the steam is extracted at a given pressure. For desalination applications, the outlet steam pressure is typically between 0.9 and 2.5 bar.

The cost of the produced thermal energy depends on the outlet steam pressure and is estimated via the reference cycle method, proposed by [138]. This method calculates the price of steam extraction, by estimating the amount of electricity that could have been generated by further expanding the extracted steam in the low-pressure section of the turbine. The reference system is the turbine in which the steam is completely used for electricity generation.

Therefore, the thermal energy cost is calculated on the basis of the opportunity cost (missed revenues) that would have been given by the electricity selling. Figure 14 depicts the trends of electricity losses in kWhel per ton of steam and the corresponding costs of the thermal energy in function of the pressure of the extracted steam.

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Figure 14. Specific electricity losses [kWhel/ton] and heat costs [$/MWhth] as functions of the steam pressure [bar].