*2.4. Natural Gas Thermodynamic Parameters*

Several parameters have to be evaluated to assess the Italian hydrogen blending threshold. Table 2 shows the natural gas composition that is conveyed by the Italian natural gas networks [43]. A natural gas density equal to 0.904 kg/m3 (= 0.7 <sup>×</sup> 1.292) at 0 ◦C and 101,325 Pa is conservatively assumed. In fact, to assume the highest possible density for the natural gas density signifies to evaluate the minimum BT in accordance to Equation (1). In addition, applying the ideal gas law a molar mass of 20.3 g/mol results in the case study. For hydrogen a density equal to 0.0899 kg/m3 is assumed in the same thermodynamic conditions.


**Table 2.** Mean natural gas composition conveyed by the Italian natural gas networks [43].

(\*) Values are limited by the respect of gas mixture's Wobbe Index.

The values reported in Table 3 are conservatively assumed for the hydrogen blending threshold evaluation.


**Table 3.** Values assumed for the evaluation of hydrogen blending threshold.

(\*) Since gas natural mixture composition varies, an average value was considered.

#### *2.5. Considerations about Reliability of Available Data on Natural Gas Flowrates*

Concerning the safety factor SF, the value was defined by the Authors in accordance to the available data about MNG. In particular, Snam provides the data of hourly gas imports, storages and exports that enter the national transmission system. In a preliminary evaluation, it can be assumed that the sum of the natural gas imports, the national production and eventually of the natural gas from the storage fields correspond to the flowrate that is going to be conveyed through the transportation system. In Figure 4 the average daily natural gas imports per months for the period 2017–2019 is reported, and also the maximum and the minimum daily gas imports are shown by the error bars that show the maximum positive and negative deviations of the daily average values from the average. As expected,

higher energy is delivered in the winter season. On the other hand, despite of the winter months, natural gas flow rate varies a little during the summer months regardless of the year. Minimum values equal to 877.3 kWh/day, 840.4 kWh/day and 850.1 kWh/day were calculated in August respectively for the year 2017, 2018 and 2019.

**Figure 4.** Average daily gas energy in the transportation system during 2017, 2018 and 2019. Data elaborated from [47].

Since instantaneous natural gas flowrate could change during the day, the hourly flowrates of fifty days randomly selected were analysed for a statistical evaluation: 17 days were selected in both 2019 and 2018, 16 days in 2017.

### *2.6. Identification of P2H Plants Size and Location in the Italian Territory*

Based on the hydrogen blending threshold calculated in the previous section, the P2H plants' total capacity (PP2H) in (kW) can be calculated from Equation (18):

$$\rm{P\_{P2H}} = \rm{BT}\_{\rm corr} \times \rm{LHV\_{H2}} \tag{18}$$

Furthermore, electrical needs have to be calculated. Three main electrical equipment are considered in the following analysis: (i) the electrolyzers, (ii) the compressor units and (iii) other auxiliaries. Since compressed physical storage is considered as the preferred option for its maturity level, no additional energy consumption due to storage section is considered. Particularly, the P2H total electric power capacity in (kW), PEL,P2H2, is calculated in accordance to Equations (19) as the sum of the electric capacity of the components that are implemented in the plant.

$$P\_{\rm El, P2H} = P\_{\rm El, El, E, CTROLYSER} + P\_{\rm El, COPRESOR} + P\_{\rm El, AUXILARIES} \tag{19}$$

Water electrolyzer electric capacity is calculated in Equation (20) as the ratio between the P2H plants' total capacity and electrolyzer efficiency (ηELECTROLYSER) in [%]. It should be noted that electrolyzer' efficiency is calculated as the ratio between the hydrogen energy production (based on LHVH2) and electrical power consumption.

$$P\_{\text{EL,ELECTROLYER}} = \frac{P\_{\text{P2H}}}{\eta\_{\text{ELECTROLYER}}} \tag{20}$$

Hydrogen compressors' electric capacity is calculated in Equation (21) as the ratio between the isentropic compression power and compressors' total efficiency. It should be noted that Lis,COMPRESSOR is the isentropic work of compression [kJ/kg]. An isentropic (ηis,COMPRESSOR) and electric (ηel,COMPRESSOR) efficiencies are also introduced [%]:

$$P\_{\rm EL,COMPRESOR} = \frac{\rm BT\_{corr} \times L\_{is,\ COMPRESOR} \times \rho\_{\rm H2}}{\eta\_{is,\ COMPRESOR} \times \eta\_{\rm el,\ COMPRESOR}}\tag{21}$$

Auxiliaries' electric capacity are calculated as in Equation (22). For the purpose a safety factor (SF') is introduced:

$$P\_{\text{EL},\text{AU}\text{XILARIES}} = \text{SF}' \times \left(P\_{\text{EL},\text{ELECTROLYER}} + P\_{\text{EL},\text{COMPRESOR}}\right) \tag{22}$$

Concerning the electrolysis section, due to the higher maturity level and the lower Capital Expenditures respect to alternative solutions, alkaline electrolyzers are assumed as the preferred option for P2H Italian plants. Based on state of the art [4], an average efficiency between [62%, 82%] is recognized for alkaline electrolyzers. A conservative efficiency of 65% is considered in the paper. Concerning the compression section, a downstream pressure up to 70 bar is considered as appropriate for hydrogen blending into the transmission system. Based on available review in the literature [48], reciprocating, linear and diaphragm compressors can be considered for the purpose. Particularly, diaphragm compressors are considered for the following analysis. In this case efficiencies in the range [80%, 85%] can be considered. For a conservative approach an efficiency value of 80% is considered. Therefore, based on Equation (21) and data reported in Table 4, the compression isentropic work between 1 bar and 70 bar is calculated equal to 9940 kJ/kg, i.e., a real work of 12.4 MW. Considering the total hydrogen flowrate (2326 kg/h), a total installed electrical power supply up to 8 MW is required to operate hydrogen compressors.


**Table 4.** Data used for the calculation of the isentropic compression work.

Once total P2H plants' capacity is calculated, the localization of each plant should be performed. However, in this preliminary estimation, it was assumed that P2H plants are located at the eight Italian transportation system import points. Based on this conceptualization, a distribution of P2H plants in Italy would be possible. P2H plants' capacity localization in the national territory is affected by both current renewable power plants distribution and concentration as well as renewable power potential. Since the paper aim is to firstly assess the compatibility of the proposed P2H power plants distribution and the existing renewable power plants, only a qualitative comparison will be performed between available and needed power on a regional basis.

#### *2.7. Estimation of the Economic Investment Required*

Based on the quantitative evaluation of P2H Italian potential, a preliminary assessment of the capital expenditure (CAPEX) is performed. The realization of new renewable power plants is not considered. For the purpose, referenced data available in the literature were used. Particularly, CAPEX is the investment required for P2H plant design, realization and tests. As shown in Table 5, the voice regarding the hydrogen storage volume is considered. For the purpose, a storage volume able to store up to 1 h of the nominal hydrogen capacity is considered. Concerning hydrogen compression, as reported in the available literature, estimates for compressors' investment vary widely from 144 €/kW to 18,500 €/kW [49]. Therefore, an average value equal to 10,000 €/kW was assumed as reported in Table 5. Since several assumptions were made, a safety factor for the purpose was defined. Particularly, since P2H plant's design strictly depends on the specific boundary conditions, a value equal to 25% was selected to take into account all the expenditures that were not included in electrolyzers, compressors and storage tanks, such as for example, engineering activities, ATEX certification, the purchase of interconnecting and bulk materials, the purchase of the land, etc.


**Table 5.** Capital expenditure (CAPEX) parameters.

Based on the specific costs reported in Table 1, the following CAPEXs (€) are calculated in accordance to Equations (23) and (24):

$$\text{CAPEX}\_{\text{ELECROLYER}} = \text{c}\_{\text{electrolyzer}} \times \text{P}\_{\text{EL}, \text{ELECROLYER}} \tag{23}$$

$$\text{CAPEX}\_{\text{COMPRESOR}} = \text{c}\_{\text{compressor}} \times \text{P}\_{\text{EL,COMPRESOR}} \tag{24}$$

$$\text{CAPEX}\_{\text{STORAGE}} = \text{c}\_{\text{storage}} \times \text{V}\_{\text{storage}} \tag{25}$$

$$\text{CAPEX}\_{\text{OTHER}} = \left( \text{CAPEX}\_{\text{ELECTORS}} + \text{CAPEX}\_{\text{CMMPRESOR}} + \text{CAPEX}\_{\text{STOCKAGE}} \right) \times \text{SF}^{\prime} \tag{26}$$

where celectrolyser, ccompressor and cstorage are the specific cost of water electrolyzers and compressors in [€/kW] as reported in Table 5. Vstorage is the storage volume [m3]. SF is the safety factor to take into account other costs that are necessary to design, install and operate a P2H plant.
