Petri Nets, Energy and Data Center Architecture

The data center IT infrastructure is basically a value stack. A supply chain of stack elements who act as a service component (People, Process and IT that adds up to an IT service). Although IT infrastructure delivers no direct business value, much of the business value is created in business processes that depends upon a solid and stable IT infrastructure. With an IT infrastructure in place, you can run applications, but they can’t deliver any value without a physical IT infrastructure of server, storage, network and power and cooling components. These stack elements are all interdependent, and, of course, any one of them can be further decomposed (see the proposed Stack Framework of Datacenterpulse.org) .

For each element in the stack the IT organization has to assure quality as agreed on. In essence these quality attributes were performance, availability, confidentiality and integrity. One of the most big challenges for the IT organization was and is to coherently manage these quality attributes for the complete service stack or supply chain. Sustainability as quality attribute is a new kid on the block. The sustainability attribute is composed of the power, cooling and floor space sub attributes.

As insurance against the outage of IT services, most data centers have some level of redundancy of system components. The costs associated with additional extra energy usage as a result of redundancy will be taken for granted. The key question is how to ensure that the redundancy is achieved in the most energy efficient way possible. So how to architecture and design sustainability for a data center? The Greenmetrics Conference 2009 in Seattle produced a paper named “Quantifying the Sustainability Impact of Data Center Availability” which tries to answer the question how to estimate the sustainability impact for different availability architectures of a data center power infrastructure.

For a high-level approximation of the environmental impacts the thermodynamic metric of exergy (also called usable available energy), which represents the part of a  system that can be converted into useful work, is used in a life-cycle exergy analysis (LCEA). Stochastic Petri Net (SPN) models are used to estimate availability for different power infrastructures.

In the presented case study power infrastructure architectures with increasing redundancy, such that each successive architecture has an additional component duplicated, are compared. For each architecture availability (using a SPN model) and lifetime exergy is estimated. Sounds complicated? Read the article it is pretty straightforward although it looks like that it is the first time such a approach is taken.

Hopefully this article gets more airplay and gets a follow-up where other cases will be shared.

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Data Center Emergency? No Emergy!

If we see energy efficiency as a topic that has something to do about sustainability than sooner or later you are confronted with words like exergy and emergy. Energy is a word we all know and it means the ability to cause work were work is defined as any useful energy transformation. In all kinds of work, one type of energy is transformed into another where some of the energy going into a used form that no longer has potential for further work.

Not all forms of energy are equivalent. While they can all be converted to heat one cannot say that joules of one form of energy are equal to joules of another form in their ability to cause work. This concept of quality required a new concept of energy. Exergy quantifies the useful work that may be done by a certain quantity of energy. But energy has also another aspect, it has a history. Embodied energy or Emergy is defined as the available energy that was used in the work of making a product. Embodied energy is an accounting methodology which aims to find the sum total of the energy necessary for an entire product lifecycle: raw material extraction, transport, manufacture, assembly, installation, disassembly, deconstruction and/or decomposition.

In this energy  transformation chain at each transformation step some energy is degraded and some is passed to the next step in the chain. The emergy of one type required to make a unit of energy of another type is called transformity. These Energy Transformation chains can be described with the help of the Energy Systems Language (ESL) of Howard Thomas Odum. See Emergystems.org. A simple example of  data center energy chain presented as an ESL diagram is given below.

An ESL example

Emergy aims to provide an ecocentric value of industrial products and processes instead of the economic value, which is anthropocentric. Emergy analysis claims to provide a more holistic alternative to many existing methods for environmentally conscious decision-making. All though there is still discussion about the practical usefulness of emergy in an engineering environment if you want to make some effort on sustainability it is worthwhile to spend some time on it. There is a very nice EMERGY SIMULATOR available. This Java application is written by Raphael Valyi and you can find and download it on Sourceforge

So the new question can be what is your data center emergy. Enjoy your new sustainability project!

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