*2.2. Cooling Systems Configurations*

Three alternative cabin air conditioning configurations are modelled and compared in this study: the baseline mechanical vapour compression chiller (MVC) described in Section 2.2.1; an energy-saving mechanical vapour compression configuration based on airside free cooling with outside air (FC), presented in Section 2.2.2; and a waste heat recovery absorption-cooling-based configuration (ABS), as specified in Section 2.2.3. In particular, as in [5] and [9], it is proposed to recover waste heat from the hot gas line cooling system of conventional electric arc furnaces based on the plant layout and temperature profiles reported in [9]. In fact, in conventional EAFs, o ff-gases leaving the furnace and the following dropout box are cooled down to at least 600 ◦C, as required for the operation of subsequent plant components, by flowing through a modular gas-tight water-cooled duct [5,35], known in the industry as a WCD. In conventional configurations, the water used as refrigerant in the WCD needs to be cooled down in heat rejection units (i.e., either DC or CT). Total removed heat loads vary over time due to process intermittence, and depending on steelworks capacity, reaching values ranging between 10 and 20 MW for a 130 t nominal tap weight furnace [36]. For the heat recovery system of concern, we considered the opportunity to derive a water flow from a module of the cooling water circuit corresponding to an average heat flow of about 3100 kW. To obtain a simple and homogenous assessment of the impact of heat rejection units depending on climate, it was assumed that the same

technology (i.e., either DC or CT) was used both for heat rejection at the WCD and as condenser for cabin refrigeration cycles.

## 2.2.1. Water-Cooled MVC Chiller

Mechanical vapour compression chillers are the most common refrigerators for air-conditioning purposes. In this study a water-cooled magnetic centrifugal chiller was selected as baseline refrigeration system for electric cabin air conditioning. The nominal capacity installed was 1300 kW and the performance was taken from a York catalogue for chillers [37]. The EER was 6.4, evaluated at an entering/leaving chilled water temperature of 12/7 ◦C and entering/leaving condenser water temperature of 30/35 ◦C.

Figure 1 shows the scheme of this configuration, which depicts both the cabin air conditioning system and the module of the WCD cooling circuit selected for heat recovery in configuration 2.2.3.

**Figure 1.** Mechanical vapour compression chiller schematic diagram.

#### 2.2.2. Free Cooling and MVC chiller

The FC configuration analysed in this paper, represented in Figure 2, consists of an MVC air conditioning configuration coupled with an external air ventilation system which draws air from outside and, after filtering, directly introduces it into the cabin, thereby reducing the cooling load for the conventional MVC chiller. In order to reduce the computational load without losing the significance in comparison, a fixed value of external air temperature was chosen to control the operation of the free cooling system. A value of 18 ◦C was assumed as the switch-off temperature, allowing the capacity of the free cooling ventilation system to be comparable with the internal fans' capacity. Thus, when the external air temperature is higher than 18 ◦C, the standard MVC chiller operates to cool the internal cabin air. Otherwise, the system operates in FC mode. Also in this case, no heat recovery from the WCD occurs and its full load is dissipated at heat rejection units.

**Figure 2.** Mechanical vapour compression chiller with free cooling system schematic diagram.

#### 2.2.3. Air-Cooler and Water-Cooled ABS Chiller

The waste-heat-recovery-based cooling system represented in Figure 3 relies on a hot-water-fed single effect absorption chiller. As underlined in [5], in conventional WCDs at EAFs, due to no further utilization purposes of the emitted thermal energy, the cooling water outlet temperature is usually in the range of 50 ◦C [38]. If thermal energy recovery is considered, the design temperature of the cooling system has to be increased.

**Figure 3.** Absorption chiller schematic diagram.

While it is also feasible to increase it to 200 ◦C, as demonstrated in [5], for this absorption cooling application the choice was made to increase it only to the average value of 90 ◦C. In this way, the system was designed to operate with hot water, in order to avoid introducing additional complexities from steam operation, such as additional maintenance and safety requirements related to higher temperature, pressure and phase change, which would be an additional burden in EAF plants without or with minimal steam networks. With hot water, single e ffect absorption chillers are used, whose reference EER is in the order of 0.7, in accordance with manufacturers' catalogues [39,40] and the literature [21]. A commercial absorption cooling system with a nominal capacity of 1319 kW was assumed to be installed, based on the LG Absorption Chiller catalogue [40]. At EAF steelmaking sites where steam networks exist, an integrated development of heat-recovery-based steam generation as in [5] and of absorption-based cooling could be considered in order to exploit more e fficient double effect absorption cycles [18,26]. However, this is beyond the scope of the present paper. Given the intermittence of the EAF melting process, based on the aforementioned tap-to-tap cycle, variations in flue gas temperatures correspond to oscillations in cooling water temperature at the heat recovery outlet. Thus, as in [5] and [9] a water storage tank is used as a hot water reservoir to compensate for power-o ff phases by limiting the temperature variability, which for single e ffect absorption cooling purposes is deemed acceptable in the range of 85 to 95 ◦C. The hot storage size was also designed to meet safety design criteria for cabin air conditioning systems, which imply that the cooling load to be removed from electric cabins was assumed to be constantly present during steelworks operations and to persist, during maintenance stops, for a period of three hours after the steelworks stop.

### *2.3. TRNSYS Simulation Model Development*

TRNSYS [33] was used in this work to perform a dynamic simulation of the behaviour of the elements used in the various configurations analysed. TRNSYS libraries consist of components such as heating, ventilation and air conditioning (HVAC), electronics, controls, hydronics, etc. The elements are called types, and can be linked to others to simulate entire systems.

Dynamic system simulation is possible by including performance data and simulation parameters for individual elements. The configurations defined in Section 2 were modelled in TRNSYS based on the schematic diagrams shown in Figures 1–3, obtaining TRNSYS input files (usually referred to as decks). As an example, the TRNSYS deck for the ABS configuration is represented in Figure 4.

The mass flowrates of chilled water and condenser water required by chillers were taken from manufacturers' catalogues. MVC and ABS systems were simulated using technical data (reference chilled, cooling, and hot water flow rates) from the manufacturers' catalogues [37] and [40], respectively, and the TRNSYS inbuilt performance data file, which allows EER simulation as a function of cooling, chilled, and hot water temperatures. The hot water tank was a stratified, five-layer adiabatic liquid storage tank simulated using TRNSYS *type60*. Cooling water from CTs or DCs served as input for the chiller condenser while the water leaving the chiller condenser was used as the input in CTs or DCs depending on the configuration studied. For the simulation of heat rejection units, technical data required as TRNSYS input were taken from LU-VE catalogue [41] and YWCT catalogue [42] for DCs and CTs, respectively.

A weather data file derived from the EnergyPlus weather database [43] was provided as input to CT, DC, and the cabin building to capture the e ffect of external environment conditions. Climate zones were selected according to ASHRAE [34]. Table 1 shows the cities selected here to represent each climate zone and their climate characteristics. The three cooling configurations described in the previous section were simulated in 16 out of 17 of the selected cities. Climate zone number 8 was not considered for simulations since cooling towers are inoperable in this zone [32] due to the extremely low temperatures (see Table 1). Therefore, a total of 96 simulations were performed.

**Figure 4.** Absorption cooling (ABS) configuration modelled in TRNSYS. Dashed lines are used as control indicators.


**Table 1.** Climate zones defined by ASHRAE and relative representative city. Zone number 8 (in italics) was not considered in this work.

#### *2.4. Calculation of Water–Energy–Greenhouse Gas (GHG) Nexus Indicators*

In accordance with [9,44], the total blue water footprint, carbon footprint and primary energy demand were selected as water–energy–carbon nexus indicators in this analysis. They meet most requirements reported by [45,46] for sustainability indicators; in particular, they are easy to interpret, able to show trends over time and sensitive to changes in the systems analysed here (i.e., different configurations of cabin refrigeration systems).
