Promising emerging technologies for supporting DC power distribution

Different techniques can be used for on-site power generation in data centers, which can eliminate dependence on grid power or be used for backup power in situations where the grid power is expensive or unreliable. On-site power production is most commonly achieved via diesel generators, as illustrated in both Figure 2 and Figure 3 [5], [8], but alternative sustainable technologies, delivering significant advantages in terms of reduced emissions, reduced noise levels and reduced fuel dependency are already available and could be of interest to data centers as well.

Modern distributed generation technologies provide various topologies of small size generators, in the order of tens of kW, that generate – from sustainable energy sources – DC directly (photovoltaic arrays and fuel cells) or require a DC intermediate conversion stage for transforming the high frequency electrical energy into suitable AC energy (micro-turbines, Stirling engines, micro-hydro turbines, variable speed wind generators) [10],[14]. Figure 7 shows some examples.

Figure 7 Generation technologies from sustainable energy sources that could provide on-site power support for data centers, (a) static fuel cells, (b) micro-turbines and (c) photovoltaic cells

Most of the electrical storage systems, such as osmosis batteries, super capacitors and superconducting magnetic energy storage (SMES), supply DC electrical energy. Others, like flywheels, generate high frequency currents that must be converted into DC, or implement a DC intermediate conversion stage for AC connection [14], [17]. Figure 8 shows some examples.

Figure 8 Energy storage technologies that could provide on-site power support for data centers, (a) super capacitors, (b) high speed flywheel and (c) osmosis battery stack

Including all the above mentioned on-site power generation and power storage equipment into the AC distribution architecture shown in Figure 2 requires six power conversion stages to connect the DC and high frequency AC sources to the AC bus, as shown in Figure 9. The DC-AC inverters then simultaneously perform the synchronization between the on-site generated power frequency (> 50/60 Hz) and the AC distribution power frequency (typically 50 or 60 Hz) [17].

Figure 9 AC distribution architecture with DC support (storage & generation, dotted components are optional)

Similarly, connecting all these sources and storage components to the DC distribution architecture shown in Figure 3, requires only two power conversion stages, namely for the high frequency AC sources, as shown in Figure 10. Optionally, to benefit from advanced efficiencies and power control of the DC sources and storage elements, these could be connected to the DC bus using a DC-DC converter, as shown.

The connection of these sources and storage elements to a DC distribution bus does not require any synchronization between the DC power sources.

Figure 10 DC distribution architecture with DC support (storage & generation, dotted components are optional)

It can be seen that the reliability and efficiency of the overall data center power distribution will be higher in the DC distribution architecture due to the considerably lower number of power conversions, as discussed in previous sections of this paper.

As the technology for the on-site power generation and power storage improves and more capacity is installed within a data center, a point will be reached where the on-site generation matches, or exceeds, the energy demand of the data center. When this occurs the data center could become independent of the public supply grid and be controlled as a so-called microgrid, with DC being the obvious distribution architecture. In this case the public grid will then only serve as backup to the on-site power generation and on-site power quality can be controlled locally and very specifically.

Microgrids can be designed to meet the exact needs of a data center, such as enhancing local reliability, reducing losses, supporting local voltages, providing increased efficiency, implementing voltage sag correction and providing uninterruptible power supply functions, amongst others, according to Lasseter [18]. This is possible as the microgrid spans only the data center and all of its parameters can be controlled by the data center operator/owner. Kakigano et al. [19] reports that a DC microgrid enables superior high quality power supply, easy isolation of system faults, higher conversion efficiencies due to downsized, transformerless converters, amongst others. Although the technology needs to mature a bit further, self-sustaining microgrids could be providing power to data centers in the near future.