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Your Restaurant in Brooklyn: Into the depths of the Micro-grid

We’re back again to your restaurant in Brooklyn once again. Let’s review what has occurred so far.  In week 1, you opened the restaurant only to discover that it had no electricity.  In week 2, you corrected the problem by installing a diesel generator.  In week 3, you starting seeing the downsides of having a generator – the smell, the black smoke, the sound, and the potential to run out of fuel.  Week 4 was a slight detour from the story to look at the impact of Hurricane Sandy, and see what would have happened if your restaurant was operating on a micro-grid at the time the storm hit. In week 5, we started getting into the design of the micro-grid, and determine the size of the PV system that could fit on the roof of your restaurant.

And, here we are in week 7 (already!).  It’s time to get a little deeper into the other components of a micro-grid. At a high level, a micro-grid includes some sort of generation, it usually includes storage of some kind, and a piece of hardware that makes the generators, storage, and electrical loads talk nicely to each other. This piece of hardware, or pieces of hardware, make the up the brain of the micro-grid.  The brain dispatches and controls the generation sources, dispatches and maintains the storage, and controls the electrical loads as necessary. This intelligence is all geared around making sure the generation of electricity matches the demand of electricity, and maintaining the generation, storage, and electrical loads for a long time to come. In our case, the diesel generator, the PV system, and the electrical grid will fill the “generator” role (when it works) and batteries will fill the storage role. I’ll get into the details of spec’ing batteries in a future article, but today we’re going talk through the “brain” of the system.

With everything that the “brain” is expected to handle, you might start to think that this is going to be a very complex system, with multiple pieces of hardware that need to be configured to talk to one another. In reality, there is one piece of hardware available on the market that is designed and built to be the “brains” of micro-grids. It comes from SMA, and it is called the Sunny Island:

It doesn’t seem like much, but this piece of hardware can handle everything necessary in a micro-grid and then some. A company called SMA (www.sma-america.com) makes this handy piece of hardware. SMA is one of the big boys when it comes to solar and micro-grids. They’ve been around since 1981, and are the market leader when it comes to solar PV inverters. Plus, the Sunny Islands have been installed in a ton of micro-grid applications across the world, including the 3 largest micro-grids to date (- http://www.sma-australia.com.au/en_AU/news-information/current-news/news/news/1596.html), each over 1 MW. You can get some more specs on the Sunny Island here – http://www.sma-america.com/en_US/products/off-grid-and-back-up-solutions/sunny-island-5048-5048-us.html – and get some training from SMA on their use here – http://www.sma-america.com/en_US/smasolaracademy/overview.html.  (I’ve attended a few of their training sessions myself, and the instructors are entertaining and informative.)

So, how exactly do you spec out an SMA Sunny Island for your micro-grid? Well, first let’s look at the electrical loads that you’re trying to serve with this micro-grid. It’s important to find out exactly how much electricity you need and where it’s going. By looking at your electrical bill, you’ll typically get a total amount of electricity you pulled from the electrical grid over a period of time. This is measured in kWh. This is important, but you’ll also want to know what is the most amount of power you’ll draw at any given time.  If you have a demand meter or a more advanced meter, this is indicated as kW on your electrical bill. (If you have an AMI meter, you can get power measurements for every 15-minutes.) Typically, you’ll want more detailed information than this if you taking a hard look at a micro-grid, but this is a good start.

I’ll take a moment here to clarify the difference between power (kW) and energy (kWh). For those who are observant, you’ll notice that kW stands for kiloWatts, or thousands of Watts, and kWh stands for kiloWatts times hours, or thousands of Watts over hours. This might be a little abstract, so let’s make it a little more concrete, and hopefully improve your understanding of how this works. Think of water flowing through a pipe. Power (kW) is a measure of the size of the pipe, or the amount of water than flows out of the pipe at a given time. Energy (kWh) is the total amount of water that flows out of the pipe over a period of time.  If the entire pipe was filled with water and it flowed out of the pipe like that for a full hour the value for power (kW) would be equal to the value energy (kWh) because the time (h) is equal to one.

So what do you do if you don’t have this data? The good news is that you can measure it. You need to get a CT (current transducer) or amp clamp and place it on the wires that connect a particular load to the electrical panel. These devices will measure the amount of current that flows across any of these wires. Once you have the current, you’ll be able to determine how much power a particular load draws at a given point in time.  Here’s the math:

P = I x V

Power (P) = Current (I) x Voltage (V)

If I = 30 amps (A), and V = 230 volts (V)

Power = 30 x 230 = 6,900 Watts (W)

                  Here’s what a CT looks like:

And, an amp clamp:

You can also look at the power tags on the equipment to determine what the max power draw by each piece of equipment would be. Here’s what the power tag looks like:

Ultimately, you’ll want to collect all this data over a period of time so you can get a better understanding of your load profile. For the initial design of the SMA Sunny Islands, you’re only going to need to know your max power draw. You’ll want to know your full energy consumption over time as we spend more time doing a load analysis. This will be important for determining the size of your battery bank and for understanding from where your electricity will be coming. We’ll spend more time on this next week.

For getting an idea of how many SMA Sunny Islands you need you need to know how many phases (typically, 1-phase or 3-phase) you have, and what is the most amount of power you draw at any one given time. For your restaurant, you have a 3-phase system that draws at most 35 kW (35,000 Watts) at any given time. So, because you have a 3-phase system, you’ll need to have a number of SMA Sunny Islands that is evenly divisible by 3. You need one Sunny Island for each phase, and you need to have at least three for the phases to be balanced. Since you’re dealing with 3 phases, each group of Sunny Islands on a phase will have to be able to handle about 12 kW (12,000 Watts). SMA Sunny Islands come in different sizes based on the amount of Watts that will come through the Sunny Island; they’re sized from 2000 W to 6000 W. But, if you’re going use more than three Sunny Islands, you’re going to need to use a multi-cluster box. Multi-cluster boxes are geared to work with 5000 W Sunny Islands (model 5048), so for your restaurant, we’re going to spec six 5000 W Sunny Islands.

But, if you’re doing the math at home, you realize that we’re not going to provide the full amount of power you’re going to need (6 x 5000 W = 30000 W < 35000 W). So, here’s where my design experience kicks in. First, the Sunny Islands are slightly over-engineered and can handle higher power throughput on a short-term basis. Per the specs, the 5000 W Sunny Island can handle up to 6500 W for 30 minutes, which means 6 x 6500 W = 39,000 W for 30 minutes. I’m familiar with the load profile for your restaurant, so this is going to work out for most of the time.

What about the few times when you draw more than 30000 W for more than 30 min? What happens then? Well, the Sunny Island is built with a number of relays that can be controlled by the Sunny Island when certain parameters are met. We’re going to set one of these up so that it trips off a set of loads that we define when this situation occurs. That way, we can guarantee that we’re tripping off non-critical loads for the short period of time when the power draw for you store exceeds 30000 W for more than 30 minutes. For your restaurant, it’s much better to trip of the refrigerator for a short period of time than the oven. Your refrigerator will stay cold for a relatively long time, even during the day, but you can’t really continue to cook meals for you customers if your oven doesn’t work.

So, we’re going to have a 22 kW PV system, 6 Sunny Islands 5048, and your current diesel generator. We’re well on our way to getting your micro-grid fully designed – we already have two of the 3 major components sized!

Next week, we’ll take a look at a more details load analysis of your electricity usage, and start to think about how the generators (PV system, diesel generator, and electrical grid) are going to interact with each other. We’ll also start thinking about the amount of storage (batteries) we’ll want to incorporate into the micro-grid.

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