Globally, the size and scale of data centers are an unknown quantity. Data centers are a 24/7 operation, forming a critical part of the global infrastructure and varying greatly in terms of physical size, capacity, ownership and resilience.

Our consumption of data and use of web-based services has been growing rapidly at typically above 50 percent a year for over 20 years (over 4 percent a month). This is a level of consistent growth not seen in many other market sectors.

On a global basis, the ‘best guess’ power consumption has been growing at 10 percent CAGR. With issues such as proximity to end-user business, availability of power, and connectivity being key location drivers, the larger data center builds tend to be focused in key locations, including Slough, West London, and Hemel Hempstead in the UK, and Dublin in Ireland.

Dealing with the application of a generating set to any requirement can be complex. This is particularly true of data centers. This WBPS article aims to cover some of the key elements that need to be considered.

Why are standby generators required?

The primary reason standby generators are included within a data center is to provide backup power in the event of a prolonged power utility failure.

This utility failure can be anything from just a few seconds to hours or possibly days. Most operators will set a minimum operational period before refueling the installation is required.

The data center electrical load off, when viewed very simplistically, can be split into two areas:

1. UPS (Feeding the technical IT loads)
2. Cooling load (mechanical plant)

The UPS package will typically offer a support time of five to 10 minutes which is more than sufficient to deal with any short-term “brownout” or the 10 to 15 seconds required to bring the standby generating sets online.

The mechanical plant, and particularly the cooling plant, usually has more flexibility and resilience built-in by way of cold-water buffer vessels.

Power Rating Categories - BS ISO 8528 – 1; 2018 Section 14.3

For commercial reasons, generating set manufacturers use the same engine (with model variations) across a range of power nodes. In addition, a single model of generating set can carry a number of different capacity ratings i.e. 1000kVA ESP, 1000kVA PRP, and 700kVA COP; the set rating is dependent on the operational duty/types of loading applied to the generating set, rather than the maximum engine capacity.

Section 14.3 of the standard covers the five different ways in which a generating set can be “rated”. These are COP, PRP, ESP, LTP and DCP. Some of these ratings, for example, PRP (Prime) rated sets, have their power ratings set against the ability to deliver into a varying load. They deliver an average load level over a 24-hour period whilst still being able to provide a 10 percent overload, one hour in twelve.

In the case of the ESP rating, there is no overload capability but there is a limitation on the number of running hours it can be operated over a twelve-month period. It is important to note that the way in which these ratings are allocated will vary from manufacturer to manufacturer, based on engine performance, connected rating of alternator performance, and maximum operating ambient temperature.

First step load acceptance

We identified earlier that the BS ISO standard does not require the generating set to achieve any specific level of first-step load acceptance, as this is determined by the BMEP that the engine is able to deliver. Hence there is a wide variation between manufacturers and the engines used at various power nodes.

60 percent first step load acceptance for “any rating” of generating set has for a long time been considered an “industry norm” and is often written into many “standard” consultant specifications. However specific consideration is not given to the actual operating requirements of the infrastructure or being qualified with the essential giving performance classification (G1-G4).

The design of many of the larger engines (1500kVA +) used on the larger generating sets in the healthcare, water treatment, or hyperscale data centers, in particular, has been around for a very long time. Whilst they have been enhanced and improved over time (more electronic engine management, improvements in the combustion process, high-pressure fuel injection, improved materials, etc) there are few truly new engines. Those that are new typically offer lower fuel consumption, hence lower emissions, and have a more compact footprint i.e. higher power density.

Those improvements can come at a cost, and this can be the first step of the load acceptance capability of the generating set. Whilst these advancements are accepted within the standard, the “industry norms” often do not keep pace with, nor understand, the changes or embrace the advantages that they bring. This is often the case when we look at the data center environment.

Fuels and emissions

This is a subject that weighs heavily on the minds of data center designers and users due to extensive regulation and licensing issues. For some time, there has been a drive to reduce CO2 emissions. Many of the larger engine/generating set manufacturers are certifying their engines to use hydrotreated vegetable oil (HVO) which offers a reduction in CO2 of up to 90 percent (over the life cycle of the fuel) over conventional BS EN 590 B7 fuel. As with conventional diesel fuel, HVO needs to be looked after during its life cycle with polishing and cleaning systems. There are likely to be some early availability issues, but its adoption is likely to provide a measure of an interim solution.

Some designers and operators have looked to gas as an alternative as it is cleaner in emissions and particulates than diesel. However, on-site storage is a space and safety problem, with having a local source of sufficient capacity and reliability another.

Gas, having a lower calorific value than diesel, means the engine swept volume must increase in order to deliver the same power, meaning higher initial capital costs. Starting a gas engine can take much longer and the set is much slower at accepting site loads. Those issues, combined with higher maintenance costs, have ensured gas generation is very rarely adopted.

Many of the larger manufacturers are currently working on a hydrogen fuel solution, running test programs to assess long-term reliability issues. This option looks to offer a promising future but there is some way to go with work still to be done in order to provide a truly “green” source of hydrogen fuel, workable local safe storage and distribution methods.

In relation to NOx emissions specifically, it is important to study the information provided by each individual manufacturer. The results provided (as with all of the other elements that are present in the gas stream) are measured under specific conditions, including engine load and operating temperature, ambient temperature, and distance from engine gas flow. All these parameters will vary when at the site, hence it is important that each installation is treated and assessed on its own relative merits. The MCPD indicates a requirement to achieve a NOX level better than 190mm/m3 at 100 percent load and after a max of 20 mins of operation.

It is also important to ensure that the units of measurement and remedy are the same. NOx levels are usually given in mg/Nm3 at five percent O2 at 100 percent PRP. The established method of NOx reduction is the introduction of a selective catalytic reduction unit (SCR) into the exhaust gas flow. NOx reduction/elimination is achieved by injecting ammonia into the gas flow prior to it passing over the catalyst. As the NOx produced in the engine cylinders is directly linked to engine pressure and temperature it should be noted that as the exhaust gases expand and cool then the NOx levels will fall on their own.

There are of course other elements present in the exhaust gases emitted from a diesel engine. These include HC, PM, and soot. The amount discharged depends on the rating of the set, engine type, fuel consumption, quality of fuel used, and engine load. Each of these elements can be reduced by fitting a specific reduction unit. There is however a limit to what components can be added whilst staying within the engine’s operating parameters.

Some additional considerations

Enhancing generator starting reliability is key to ensuring the performance of any data center. Fitting dual starting batteries, dual battery chargers and dual starter motors to each set is a quick and cost-effective way of achieving that objective.

Selecting the right generating set control system is key to the overall operation of the system. Firstly, ensure that the selected control system can provide all the necessary communications required for connection to any BMS/ EMS system and site-wide protocols. Getting the generator package (multisets) up to speed and on load as quickly as possible follows. The use of dead bus synchronizing can see this achieved in around 10 seconds from the start signal regardless of the number of sets to be synchronized.

As discussed earlier in this article, different manufacturers have their own way of arriving at a DCP rating for a given size of set. It is important to understand this in the context of each operational environment. At one end you can have a situation where the DCP rating is the ESP rating whilst at the other end it could be a COP rating. There is an over 40 percent difference in power output/capacity between the two extremes.

Right-sizing the generator package in a scalable way can mitigate this problem but a good design strategy will ensure that a permanent load bank, both resistive and inductive, is connected to achieve a full load test on each machine and raise the load to >25-30 percent at near to unity power factor regardless of the actual load across all machines.

For further information, please email Robb Shingles at [email protected].