Operating efficiency is now a generic ‘tick-box’ specification when comparing uninterruptible power supplies. Within the data centre market, the ability to easily expand a UPS system is becoming a more important consideration, fuelled by virtualisation and the launch of new modular UPS systems. Robin Koffler, director of the Thamesgate Group, explains
Today, standalone and modular uninterruptible power supplies supplied by most UPS manufacturers offer almost identical operating efficiencies. Most on-line double conversion UPS will achieve an operating efficiency of around 96-97%. When run in their Eco mode format (a line interactive or standby operation) this efficiency can rise to around 99%.
These benchmarks are impressive and a sign of the commitment of UPS manufacturers to respond to demand for more efficient systems. However, the move to 97% is only two or more percentage points from the operating efficiency of legacy UPS systems which would operate around 94-95% or less. The resultant saving in electricity costs may only really impact the operating budgets of larger data centres (at least 1MVA or several hundred kVA). Running the UPS in Eco mode (99%) would increase this saving further but again this may only be marginal.
So, how can the modular decision be used to select between UPS systems? For data centre operators looking for flexibility and upgrade paths, the modular UPS allows ‘right-sizing’ on day one with options to expand existing infrastructures later. Consider a 40kW load installation, noting that some UPS manufacturers now rate their UPS at Unity Power Factor in kW rather than kVA. If the day one load is 40kW, a 50kW UPS module could be installed on day one. The module is housed within a UPS frame that can be expanded to 150kW total or house 100kW N+1 or 50kW N+1.
This approach gives more expansion options as the cabinet is supplied sized to meet a future maximum size. An added benefit is it is designed for an in-row installation.
Whilst the same power configuration can be achieved with a standalone system, the floor space taken up will be greater as the system has to expand horizontally rather than vertically. In addition, further interconnecting cables will have to be installed between the standalone UPS, either under raised floors or from overhead containment. These include electrical supply and load cables back to the distribution board and communication cables for the UPS parallel cards.
Today, the downside of a modular UPS system is one of initial cost. Sometimes this can be by a factor of 1.5 to 2 compared to a standalone UPS system. This is due to scale economies and a more complex design. However, for the small-to-medium sized data centre looking for an in-row system that can be easily expanded at a later date, the modular approach may in the long run prove less costly over the planned lifetime of the installation.