PUE evaluation
According to the statistics on average DC spends about 40% of its operating expenses for payment of electricity bills.
Figure 1. Budget distribution in DC (APC source 2005)
In its turn electricity is consumed for:
- Power supply of IT-equipment;
- Power supply of cooling equipment;
- Power supply of supporting systems (lightning, ACS, fire alarm etc.);
- Losses in transmission lines and equipment;
However in most cases costs for energy supply of cooling systems and losses account for up to 80% of all electricity consumption.
PUE (Power Usage Effectiveness) index is commonly used for evaluation of energy efficiency.
PUE measures how effectively a DC uses electricity, specifically how much electricity is consumed by IT-equipment versus full electricity used by DC that includes IT-load, cooling, lightning and other consumers. Ideal PUE = 1, which means that all capacity, supplied to DC, is only used by IT-load. PUE could be calculated according to the formula:
The calculations show how important to reduce electricity consumption of system elements that do not refer to IT-equipment. For most of DC’s this is the only way to increase their economic efficiency.
Let us give an example (Business Case) of how energy efficiency impacts on the economy of a DC:
| Racks installed | 80 |
| Average consumption capacity for 1 rack | 5 KW |
| Capacity consumed by IT | 400 KW |
| Capacity consumed by cooling | 550 KW |
| Losses in power lines and equipment | 50 KW |
| Total capacity of DC1 | 1000 KW |
Calculating PUE we shall obtain: PUE = 1000/400 = 2,5
If 1 KWh equals 3.81 rub, total consumption equals 2 743 200 per month or 32 918 400 rub per year. Costs for cooling equal 1 508 760 rub per month or 18 105 120 rub per year and it accounts for 55% of all electricity costs.
The best practices in the field of DC construction allow to reach PUE=1.15-1.2 during one year evaluation.
Let us recalculate our case with PUE=1.2 and we shall obtain:
| Racks installed | 80 |
| Average consumption capacity for 1 rack | 5 KW |
| Capacity consumed by IT | 400 KW |
| Capacity consumed by cooling | 60 KW |
| Losses in power lines and equipment | 20 KW |
| Total capacity of DC2 | 480 KW |
Total consumption equals 1 316 736 rub per month or 15 800 832 rub per year. Costs for cooling equal 164 592 rub per month or 1 975 104 rub per year and it accounts for only 12.5% of all electricity costs.
It is worth noticing that leaving the level of IT-load at 400 KW we obtain the sum of economic effect that equals 17 117 568 rub. given annual operating costs.
It is difficult to reach the same level of efficiency at an operating DC without deep upgrading. Though if to conduct an expert study and analysis of current process solutions, in most cases it is possible to reach PUE=1.5 ÷ 1.7 with moderate investments involved (with payback period less than or equal to one year). It may give an economic effect up to 12 million rub (compared to our DC 1 in Business Case) that is also a very good result, which allows to increase DC’s economic efficiency and obtain additional competitive advantage in terms of greater safety factor in service cost value.