As with almost every other part of the data center, uninterruptible power supplies (UPSs) are being scrutinized in the quest for the lowest possible power usage effectiveness (PUE) rating. There is no doubt that cooling continues to be the main source of power consumption but, when it comes to the mechanical and electrical functions within the data center, the energy efficiency of the UPS can have a negative impact on PUE.

There have been numerous innovations in recent years that have made the UPS more efficient – for example, the inverter has been continually optimized to achieve optimum waveform. However, it is modular UPS systems that so far appear to be the most popular choice for data center operators.

Total installation
The traditional approach to UPS is to install a system with a total load in one go. If an owner aims to have a 500kW data center it could be tempting for him or her to opt for all-or-nothing as a more cost-effective approach. But managing partner at Spanish consulting and engineering company PQC Garcerán Rojas said in the time it takes for a data center to build up to a 500kW operation the UPS may be only partially used, which equates to losses in efficiency.

“Therefore opting for modularity could be a suitable alternative in this case,” Rojas said.

Rojas’ team knows about data centers. It was responsible for Telefonica’s massive Tier IV facility in Madrid. Rojas heavily promotes the use of modular UPS systems. He said modular systems can cope with data center expansion, and they tend to have a higher performance rate than traditional models because power demand and usage are closely linked. This results in increased energy efficiency.

Modular UPSs, however, come with their drawbacks. There are generally far more components involved in a modular system than in conventional models and, for this reason, they can be more complex and can cost about 30% more than traditional models, according to Óscar Pons, business development and product specialist at Schneider Electric.

Because of their scalability and because they take less time to repair compared with traditional versions, modular UPSs are, despite this, growing in popularity.

Another benefit of the modular system is that battery use can be more energy efficient, ABB head of power protection Miguel Angel Jimeno said.

“UPS-related batteries wear down and age whether they are used or not. This means if you install a UPS with a battery of 100kVA, after six years you will not have reached the total load and the battery will be as worn out as if you had. If I install a cabinet intended for 100kVA but the charge is only 40kVA, I can use a 40kVA battery and, six years later, buy another one,” Jimeno said.

Taking into account all the potential benefits of a modular UPS, it could be a good alternative to the more traditional approach. But does modularity have its limitations? Rojas believes it does.

“In the example of a 500kW data center above, you could install a 500kW machine, or two 250kW machines, or five at 100kW, in stages. However, the big question for data center owners is: ‘At what rate should I set my modularity?’ because the more you modulate, the greater the number of components,” Rojas said.

Risk vs reliability
Modularity usually involves the installation of elements in parallel. The greater the number of items, the greater the risk of failure which could, in turn, have a considerable impact on reliability.

According to Rojas the trick is “finding the point where investment and quality meet”.

“There is always a point where I am no longer interested in improving modularity further because I am going to risk failures,” Rojas said.

The focus of this question changes for ABB. It is not so much a question of assessing the increasing possibility of failure when adding more components but more about the impact on the business if the UPS fails, Jimeno said.

“If I have four batteries working in parallel, the possibility of malfunction in one of them is greater than if there is only one battery,” Jimeno said.

“However, if I have four batteries and one fails, then what happens? I have to isolate that equipment and the other three batteries continue to power the load. If I only have one battery and it dies, then I’m left with nothing.”

The risk of failure can be reduced by installing redundant devices, such as another UPS as a back up.

“You must have a 100% redundancy because if a module fails, the other module has to support the load,” said Raphael García, director Emerson Network Power’s data center business for Latin America.

Many companies, especially in a tough economic climate, tend to think they will not expand, so they do not plan for future investment in infrastructure. However, when opting for a modular design, the potential for expansion should be a major consideration from the outset.

This article first appeared in DCD Focus 35. To read the digital edition click here.