On the smart grid roadmap team, we would declare “Every end node is a microgrid!” When pressed, we would say “Remember, microgrids are recursive.” Sometimes an observer would ask how we communicated with these microgrids. “Information only flows through the ESI!” (Energy Services Interface). We would then stop, often leaving the listener more mystified than before we spoke. This month, I am going to address this mystery, to describe what we meant by that.

The typical traveler, crossing the country and stopping in airports, is the model for our microgrid. He travels with his laptop, which has an internal DC power distribution system  and power storage in a battery. The system manages its internal affairs to provide computing service. It monitors the battery life and makes operating decisions accordingly. It may sense what parts of the system are being used, dimming and brightening the screen according to use. The businessman defines policies, opting for higher performance or for longer battery life.

What makes the laptop part of smart energy, though, is how it adapts to changing power availability. An experienced traveler is always alert for changes of energy service provider. In the Charlotte, NC airport, power is available at the gate only when sitting on the floor using the housekeeping receptacle on the pillars. Travelers staying longer can use the power plugs by the rocking chairs in the main terminal. In San Jose, plugs are available from seats by the windows at the gates. Travelers on Southwest  can vie for the limited row of big seats with plugs, or opt to belly up to the device bar, a long, high shelf with plugs and bar stools. 

Every seating decision is a power management decision. Should I arrive earlier, and get a power supply in comfort before the gate area fills up? Should I choose pants in which I don’t mind sitting on the floor? Does this flight have power on-board? Can I use bright settings, consume power faster, because I know I will have power soon? Do I plan just to sleep through this flight?  All the power management technology avails me little, unless I know the context of power use, and have ready access to situation awareness on current and predicted power availability, and awareness of how I want to use it.

Defining the Microgrid

A microgrid is a system of systems that optimizes its own ability to serve some purpose or purposes in accord with policies established by its owners, operators, or occupants. The purposes of a microgrid are met, in accord with those local policies, by managing energy over time. The support its purpose, a microgrid balances over time, energy use, generation, storage, recycling, conversion (from one form of energy to another), and market operations to make up any deficits or dispose of any surpluses.

A microgrid takes responsibility for itself. Each microgrid might encompass an entirely different set of technologies. Each microgrid has its own purposes, and the policies that govern them.

For our efforts to create smart energy to be successful, we will need new technology—a lot of new technology. We will need faster adoption of each new technology—a lot faster. We will see more diversity in the end nodes—a lot more diversity, as a system with a 20 year life spans 10 generations of accelerated technology. The partners of and suppliers to the microgrid must not care what is inside each end node.

For a microgrid to participate in a smart grid, the only thing it needs is an Energy Services Interface, a locus where it can talk to the marketplace of the next larger grid.

Different microgrid end-nodes will take advantage of different functions. Many will choose to acquire their energy, i.e., electrical power and gas, from an external supplier, as they do today. Others will use energy storage to buy power when it meets their policy objectives. Note that the objective could be least price, or it could be buying exclusively from renewable sources. There is no need to buy “blended” power, if one can choose to buy only from a named wind farm, with the market clearing supply and demand in real time.

Other microgrids will choose to add generation to the mix. I have indicated a diesel generator, but a home that chooses to have wind or solar generation is in the same category.

Almost always, the best option is if an end node can consume its surplus generation within the microgrid. You would not bring the summer bounty from your garden to the local supermarket, only to buy less good produce that evening. If you did, you would not expect to profit from it. A microgrid that consumes what it produces does not impose additional costs on the larger grid, costs of balancing power and of protecting the other nodes.

There are off-grid vacation homes today that are designed for intermittent use. The renewable sources they use are not adequate to support the lifestyle of their owners. A home may slowly charge batteries every day from the sun; it may slowly pump water from the well each day with wind; it may conserve and prepare energy bound services for the weekend when it is in use.

Building with the Energy Services Interface (ESI)

The OASIS Energy Interoperation (EI) committee formed to meet the needs for high-level interactions between a grid and its end nodes. The OpenADR Alliance is developing profiles of the EI specification and developing test suites for interoperability. The EI specification defines the Energy Services Interface (ESI) as the locus that exchanges EI-based messages.

EI names the market participants, the nodes that are either side of an ESI as Virtual Top Nodes (VTN) and Virtual End Nodes (VEN). The sketch below, taken from the specification, shows a cascade of interactions, with each VEN receiving information from a VTN, and then itself acting as a VTN to send messages to another level of VENs.

But this is almost too abstract to find the microgrids. Let’s say that I, J, K, and L are each building-microgrids. Let’s say as well they have empowered G to act as their buying agent. In this case, G, I, J, K, and L together make up a larger microgrid—and the ESI for that microgrid is on the outside of G.

Let’s take this scenario and instantiate it as something real. Let’s put this microgrid on a college campus.

In this picture, some of the buildings have become microgrids. (If only the ones on the map really were!) Others are still as they have always been. Together they are all participants the microgrid that is the campus.

[an error occurred while processing this directive] Because of the definitions in the ESI, we can begin to move toward a model based on autonomous buildings, each with their own purpose, and each subject to its own policies, interacting with the local microgrid.

There is no need to limit this model to campuses, or to military bases. It can work for an office park or for a new neighborhood. It can scale up, as these microgrids themselves participate in larger interactions. (This approach is described in the Galvin Perfect Power model, described at www.galvinpower.org.)

The microgrid model also scales down.  A wing or a floor of a building could act as a microgrid within the building microgrid. With a smaller scale, the purpose becomes purer, and the policies easier to implement. Equipment, even appliances, could act as microgrids—even down to the laptop that we started with.

Then there are the mobile microgrids, the electric vehicles. They move from microgrid to microgrid, and negotiate with each. Cars, too, demand more room for innovation. Cars need controls based on safety, and performance. Cars, like other microgrids, should not expose their inner working, merely an ESI.

Every node is a microgrid. Start planning how you will control inside your building, to better meet the purposes of the building occupants, while negotiating with the supplying grid as to when to buy, and when not to buy. And think about hiding what’s inside, to simplify the smart grid, and to allow yourself some room for innovation: you will want to sell additional innovations to the same buildings next year.

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