In our proposed design we have assumed a Green Data Center (GDC) consists of different homogeneous servers. Figure 1 simply demonstrates the hybrid power supply system for the proposed GDC. Here IT equipment (servers of GDC) is considered as DC load and the other equipment such as lights and cooling device are defined as AC load. To provide optimum electricity to the green data center for various load, GDC are equipped with different types of renewable and non-renewable hybrid energy sources through conductor. To perform AC-DC conversion of our proposed design a converter (dual mode) is used and vice versa. An energy management unit (EMU) is stated to connect all renewable energy sources and storage devices together. EMU also protects the devices and ensures the better quality of battery bank for charging and non-charging state.

Green data center (Figure 2) generally receives a large number of service requests and a workload distribution server is used to allocate all of the incoming requests to the available and accessible data center. As we know more power consumption is occurred due to the increase of incoming service requests. In our proposed design we have considered a data center with several hundred to thousand of servers with workload distribution server with T (Working period of time). We can find out total number of incoming server requests by the following equations [8],

N r [ t ] = ∑ k = 1 D N r [ k , t ] (1)

N r [ t ] denotes the number of request to the data center k at time slot t. As we know each data center has some constraints to process certain number of requests and we can find it out by the following [9],

0 ≤ n r [ k , t ] ≤ n s [ k , t ] × q r [ k , t ] (2)

0 ≤ n s [ k , t ] ≤ N s [ k , t ] (3)

here, n s [ k , t ] defines the total number of active server in k^th data center, q r [ k , t ] denotes the request processing ability per second and N s [ k , t ] simply defines the maximum number of server in every k^th data center.

The power is consumed in GDC due to the different kinds of IT equipment’s such as server and the facility equipment’s (cooling and lighting). In GDC the entire server consumes a fixed amount of power in the idle state and it increases with the increase of workload. We can find out the power consumption of a server by the following [10],

Q s = Q I + U f × ( Q P − Q I ) (4)

U f [ k , t ] = n r [ k , t ] n s [ k , t ] × q r [ k , t ] (5)

here, Q_I is the power consumption in idle state, Q_p denotes the peak power consumption and U_f represents the utilization factor.

As we know (Figure 4) that cooling and lightning devices consume power that depends on the power usage effectiveness (PUE). And the optimum value for PUE is considered as 2.0 in several industries. Many researchers use the value of PUE as 1.3, 1.5, and 1.7 and also by the following we can calculate the power consumption of GDC [11],

Q D C [ k , t ] = n s [ k , t ] × [ Q I + U f [ k , t ] × ( Q P − Q I ) ] × σ PUE (6)

σ PUE = T o t a l   p o w e r   c o m s u m p t i o n   b y   G D C T o t a l   p o w e r   c o m s u m p t i o n   b y   s e r v e r s (7)

where, σ_PUE denotes as the power usage effectiveness (PUE).

As we know (Figure 3) the fact that the nature is unpredictable and the power generation by the solar and wind can varies due to the nature so that the supply of electricity at the GDC sometimes can fluctuate without any prior notification. A diesel generator is used in GDC when the power generations by the renewable sources are insufficient for the GDC. Let consider, P P V [ k , t ] is power generation from solar source, P W T [ k , t ] is the power generation from wind turbine and P D G [ k , t ] denotes the generation from the DG and therefore we can now derive a equation as follows [12],

P D G [ k , t ] = Q D C [ k , t ] − P r e n w [ k , t ] (8)

P r e n w [ k , t ] = P P V [ k , t ] + P W T [ k , t ] (9)

From above equation it is clear that when P r e n w [ k , t ] < Q D C [ k , t ] then P D G [ k , t ] is positive and it shows the power consumption from DG and for P r e n w [ k , t ] = Q D C [ k , t ] the value of P D G [ k , t ] is zero which simply defines that there is no power consumption from DG.

Solar PV Array: As we know that we can get electrical energy from the solar irradiation and it depends on the solar radiation of a particular geographical area. The energy production from the renewable energy source PV can be calculated as following [13],

E P v = C P V × γ r × d P V × μ × 365   days/yr (10)

Here, C_PV is denoted as the capacity of the PV array in kW, γ r is the daily average solar radiation in kWh/m²/day, d_PV is denoted as the derating factor and μ is the dual axis tracking factor [14],

Wind Turbine (WT): As we know that wind turbine uses wind to produce energy by the use of turbines mechanical rotation. It transfers the energy to the generator and harvest electrical energy. The production of energy using WT also depends on the geographical area as like PV. The approximate power from the wind is expressed as follows [15],

P W T = 1 2 ρ v 3 C w t (11)

Here C_wt is the coefficient of the Betz limit and can obtain almost 59% maximum value for all turbines, v defines as the wind speed, ρ is denoted the monthly air density (kg/m³).

Energy Storage System (ESS): The series and parallel combination of several batteries is denoted as the Energy Storage System. And ESS is used as a backup energy at the time of impairment of the renewable and non-renewable energy sources. We can calculate the battery autonomy by the following expression [16],

B a u t o = N b a t × V n o × A n o × B D O D × ( 24   h/day ) L G D C (12)

Here, N_bat denotes the total number of batteries, V_no is denoted as the nominal voltage (V), A_no is denoted as the single battery capacity, L_GDC is the daily average load and B_DOD is defined as the depth of discharge.

Diesel Generator (DG): We can find out the production of energy from DG by the following expression [17],

E D G = P D G × δ D G × t r   kWh (13)

Here P_DG, δ_DG, t_r is defines as the rated power output, efficiency of DG and running duration of DG. We also can calculate the diesel consumption by the following expression where F_sp is denoted as the specific fuel consumption (L/kWh) [18],

F c = E D G × F s p (14)

Here, EDG denotes the specific fuel consumption (L/kWh).

In our proposed design we have used various numbers of homogeneous servers such as 500, 100, 1500, 2000, 2500, 3000, 3500, and 4000 for our green data center. The power consumption by the each computer server is assumed as 120 W whereas 60% power is allocated for the server at its idle state [22]. We also vary utilization factor (U_f) from 0 to 1.0 and σ_PUE = 1.3, 1.5, 1.7, 2.0. The lifespan of our project is considered as 10 years and annual rate of interest is considered as 6.75% [23]. The simulation setup of PV/WT/DG powered GDC is presented in Table 1.

We have discussed the energy analysis, optimum size and cost evaluation in this section. HOMER software calculates the energy contribution for various servers for our proposed study in Table 2. Figures 5-8 show the power consumption by the GDC under various system configurations. Here from Figure 5 to Figure 8 we can find the power consumption for our green data center with different servers varying utilization factor and power usage effectiveness. We have calculated different power consumption under the various PUE. All the figures that we have included in this study shows the power consumption by the GDC escalates linearly with the increase of number of servers (N_s), utilization factor (U_f), number of server requests (N_r) and power usage effectiveness (σ_PUE). As we know that larger PUE denotes the high power consumption by cooling, lighting equipment and workload of the server rises with the increment of utilization factor.

Optimal System Design: We have used the HOMER software for utilizing the design for optimal system configuration which is shown in Table 1. From this table we can see that PV size increases with the increase of large number of servers especially when the number of servers is larger than 1000. We also can see from the table that The DG size does not significantly change up to 1500 servers but for 3000 to 4000 server’s size of the diesel generator changes dramatically and the converter size also changes in the same way.

Energy Yield Analysis: Solar panel for our suggested sharp ND-250QC polycrystalline module with a power of 250W. Nominal voltage and current for our model is 29.50V and 8.50A. The calculated early electricity providing by the PV source has been estimated by the help of HOMER software. HOMER calculates for energy contribution for N_s = 1500, V_f = 1 and σ_PUE = 1.3 which is shown in Table 3.