# Reducing the Operational Cost of Cloud Data Centers through Renewable Energy
## 5 Results
### Proper Placement of Photovoltaic Panels  
RE generators reduce operational costs, but they require an initial investment. Given the total investment, i.e., given the total size of the PV panels and battery capacity that the company is willing to invest in, the distribution of the generators among the DCs can have a different impact on the overall cost, which depends on a combination of the variables that are involved in the proposed approach: energy price, irradiation, values of PUE of the sites. Figure 10 reports cost-saving (in dark blue) and the amount of energy that was bought from the power grid (in light blue) in two scenarios, both corresponding to a total of 200-kWp PV panels. In the first scenario, the PV panels were installed in the various DCs in proportion to their value of PUE; while the opposite was done in Scenario 2, in  
Figure 8: Number of VMs per DC versus time. Figure 9: Cost-saving versus PV panel size for different migration policies; battery capacity equal to 0 kWh, 200 kWh and 400 kWh. which panels were installed preferentially in those DCs with high efficiency and low PUE. The details of the scenarios are reported in Table 2.  
The total amount of energy purchased from the power grid was the same. However, Scenario 1 allowed achieving 25% higher cost-saving, suggesting that it is better to invest in the power supply of those DCs that consume more, so as to get the highest benefit from the green energy that is produced. However, price also should be taken into account. To further investigate the PV panel distribution strategies, Figure 11 reports the savings that can be achieved when the PV panels are installed in one DC only, according to what is reported in the \(x\)-axis. Observe that cost-saving does not only depend on PUE, but it depends also on the price of electricity, which is, on average, 0.1, 0.09, 0.14 and 0.16 $/kWh per each of the DCs. The lowest saving of about 7% was obtained by placing the RE generator where the average energy price was the lowest, i.e., 0.09 $/kWh, with an intermediate value of PUE. Slightly higher cost-savings were obtained by placing the PV panel in DC 1, having a similar average energy price and a slightly lower PUE with respect to DC 1. By placing the PV panels in DC 4, showing the highest average energy price (77% higher than for DC 1) and value of PUE, cost-saving results more than doubled. This behavior is partly due to the variability of energy prices over time and the location-dependent RE production levels. Indeed, the average daily RE production varied depending on the location, being 4.2, 3.7, 3.2 and 3 kWh per kWp of PV panel capacity in each of the different DC locations, respectively. \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & **DC 1** & **DC 2** & **DC 3** & **DC 4** \\ \hline Scenario 1 & 20 kWp & 40 kWp & 60 kWp & 80 kWp \\ \hline Scenario 2 & 80 kWp & 60 kWp & 40 kWp & 20 kWp \\ \hline Scenario A & 20 kWp & 20 kWp & 20 kWp & 20 kWp \\ \hline Scenario B & 40 kWp & 0 kWp & 40 kWp & 0 kWp \\ \hline Scenario C & 0 kWp & 0 kWp & 80 kWp & 0 kWp \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Scenarios for the distribution of PV panels. Figure 10: Cost-saving and total grid energy demand for the scenarios 1 and 2 reported in Table 2.  
Finally, the effects of PV panel capacity consolidation is analyzed in Figure 12, which shows the cost-saving obtained in three different scenarios having the same total PV panel capacity of 80 kWp. In Scenario A, each DC was equipped with a 20-kWp PV panel; in Scenario B, two PV panels with a capacity of 40 kWp were installed in two DCs; in Scenario C, an 80-kWp PV panel was installed in a single DC. The different scenario configurations are reported in Table 2. For Scenarios B and C, the DCs equipped with PV panels were selected considering the configuration providing the highest cost-saving. Consolidating the RE generation capacity in 50% of the DCs allowed achieving the same cost-saving of almost 15%, which was obtained by distributing the RE generation capacity among all DCs.