2.4.1. Water Footprint

The total water footprint *Wf* was calculated as the sum of the water consumption within systems (direct water use, *Wd*) and the water footprint of energy consumed (indirect water use, *Wind*) according to Equation (1):

$$\mathcal{W}\_f = \mathcal{W}\_d + \mathcal{W}\_{ind} = k \cdot \mathcal{W}\_{\text{cv}} + \mathcal{C}\_{\mathcal{W}, \text{cl}} E\_{\text{cl}}.\tag{1}$$

In the present evaluation, we did not account for water pollution impacts (so-called grey water), but only for blue water footprint, which measures the consumptive use of surface and ground water.

Direct water consumption only occurs in CT configurations due to evaporation loss, drift and makeup-water requirements. Evaporated quantities were calculated with TRNSYS [33] using *type51b*. Additional water losses due to bleed o ff and drift were quantified as in [9] using a multiplicative coe fficient *k* on the evaporated water *Wev*, taking *k* = 2 as a reasonable estimate [47]. The footprint calculation approach and the data sources reported in [9] were used to derive the indirect water consumption rate *CW,el* for each reference city based on the national electricity production mix reported in Table 2, elaborated from the WorldBank database [48].

The total electricity demand *Eel* was determined as the sum of the energy required for each component simulated in TRNSYS. The chiller performance was considered in the energy consumption calculation by using the corresponding TRNSYS types. In-built TRNSYS performance data files were used to evaluate the EER and consequently the energy consumption, which is related to the cooling water temperature returning from the heat rejection device (DC or CT) as well as the temperature of chilled outlet water. For the absorption chiller, the inlet hot water temperature was also introduced as parameter to determine the EER. As a result, the yearly average EER values obtained from simulations in the climate regions of concern ranged between 0.52 and 0.55 for absorption cooling systems, and between 5.24 and 9.61 for compression cooling systems.

#### 2.4.2. Carbon Footprint and Primary Energy Demand Calculation

Carbon footprint has been defined as "the quantity of GHGs expressed in terms of CO2 equivalent mass emitted into the atmosphere by an individual, organization, process, product or event from within a specified boundary" [49].

As in the case of water footprint, di fferences in the carbon footprint of the configurations examined are exclusively bound to electricity consumption, since none of the air conditioning alternatives examined implies any direct fuel consumption. Carbon footprint was thus calculated according to Equation (2).

$$\text{CO2}\_f = \text{CO2}\_{\text{ind}} = \text{C}\_{\text{CO2}, \text{el}} \text{E}\_{\text{el}}.\tag{2}$$

On the other hand, based on the data sources used in this study (see [9]), carbon footprint coe fficients for electricity consumption *CCO2,el* were estimated with a life cycle approach (i.e., all *CO2eq* emissions consumption from extraction to plant construction were considered).

In a similar manner to [50], in this study it was assumed that the changes in direct carbon equivalent emissions from refrigerant leaks induced by switching from vapour compression units to absorption cooling systems were negligible compared to the emissions of greenhouse gases embodied in purchased electricity.

The primary energy consumption associated with purchased electricity was calculated according to Equation (3):

$$PED = C\_{PED,el} E\_{el} \,. \tag{3}$$

Site-to-source energy conversion factors *CPED*,*el* reported in Table 2 were obtained with the methodology and data sources discussed in [9,51] based on national energy mix data reported in Table 2.



#### *2.5. Basis for Economic Assessment*

The life cycle cost was used as a basis for economic assessment. According to the scheme proposed in [46], life cycle costs of buildings under an energy efficiency assessment framework may include initial, operation, repair, spare, downtime, loss, maintenance (corrective, preventive and predictive) and disposal costs. Based on available data, only initial (capital) costs of installations and operational costs of electricity and of water were considered.

For each configuration, the life cycle cost (LCC) for an interest rate of 10% and a lifetime of 10 years was calculated using the standard formula:

$$LCC = C\_{op} \left(\frac{q^n - 1}{q^n \cdot i}\right) + C\_{cap} \tag{4}$$

where:


$$- \qquad q = 1 + i.$$

A payback analysis was also later introduced to compare the economic feasibility between ABS and FC configurations. The payback period (PB) was evaluated using the following formula:

$$PB = \frac{\mathbb{C}\_{p,ABS} - \mathbb{C}\_{p,FC}}{\mathbb{C}\_{o,FC} - \mathbb{C}\_{o,ABS}},$$

where:


One should bear in mind that the simple payback time calculated according to Equation (5) allows a direct comparison between ABS and FC alternatives, but does not account for interest rates, which may lead to slightly different results with respect to LCC-based comparisons.

Capital cost estimates for absorption cooling systems, mechanical vapour compression cooling systems, dry coolers and cooling towers were obtained from the cost functions reported in [9], summarized in Table 3.

**Table 3.** Equipment cost functions used in this work.


The operational expenses for different configurations were mainly determined by electricity consumption and—for CT configurations—by water consumption. It was not possible to retrieve electricity and industrial water prices all over the world. To obtain an approximate estimate for reasonable ranges, Western European values were derived from [9] as guide values, and are reported in Table 4.


**Table 4.** Prices of electricity and water used in this work.

## **3. Results and Discussion**

#### *3.1. Average Total Cabin Cooling Load*

Figure 5 shows the total cooling load, averaged over operating hours of one year, for the reference electric cabin in the climate zones of interest. On a yearly basis, heat transfer through the cabin envelope led to lower cooling loads, particularly in colder climates. However, the difference between the average cooling load in climate 7 (cold) and in climate 1A (hot) Was less than 3% of the total cooling load.

**Figure 5.** Simulated average cabin load over 16 climate zones.
