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Your Restaurant in Brooklyn: Designing the PV system

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.

(My apologies to those who were looking for this article yesterday; the reason for delay will become clear in a few weeks.)

It’s now week 5.  You’ve decided that it’s a good idea to install a micro-grid to power your restaurant.  So, how do you go about it?  Micro-grids are built of several components – power sources, storage (batteries), and the logic to control all the components.   We’ll start be looking at the power sources first.  Over the next couple weeks, we’ll look at the storage, and how to size that appropriately.  And, we’ll look at finding the right equipment that holds the logic to control all of these components.

So, what are your power sources?  Already, you have the electrical grid and the diesel generator.  Why do you want to add other power sources?  Well, in this case, the grid is unstable, so you added the generator to have a consistent power source.  But, the diesel generator has its own downsides.  By adding another source of electricity, you hope to fix the problems you’re having with your current sources of electricity.

From an engineering point of view, more sources of electricity leads to having a more stable electricity source.  If each source of electricity (electrical grid, diesel generator, and solar panels) is available 30% of the time, at least one of them will be available 97.3% of the time [100% - (30% x 30% x 30%)].  (Refer to your probability textbook for more detail about how this works.)   From a business point of view, each additional electricity source has an additional cost, so you have to look at the benefit of the additional electricity source as opposed to the cost.  As long as the benefit exceeds the cost, you should proceed.  We’ll discuss more about this cost / benefit tradeoff in a future article.

You’ve thought through the other sources of electricity you could install – bio-diesel, micro-hydro, etc. – and have determined that solar power is best fit for you situation.  (Refer to week 3 for the details around this decision.)  So where do you go from here?  How do you design a PV system?

(At this point, I feel it’s appropriate to provide you a little more information about me, so you understand how I’m qualified to design a PV system.  I’m an engineer by training, and have spent the last 5 years working with commercial scale PV systems.  “Commercial” in the PV industry just means, not for residential and not for utility.  In that time I’ve designed, installed, commissioned, and maintained PV systems from 3.7 kW (16 solar panels) to 1,124 kW (4,784 solar panels).  In that time, I’ve hit a few records, including the largest commercial PV systems in Illinois, Michigan, and Minnesota.  And, I was involved in the largest commercial PV project in Ohio which installed 2,020 kW on 53 buildings in 6 months.  I’m also certified by as a PV system installer by North American Board of Certified Energy Practitioners (NABCEP – www.nabcep.org).

The first thing you need to do is to determine how many solar panels can fit on the roof of your restaurant.  (You can also install solar panels on the ground, but in this case we’re going to install them on the roof.)   You start by looking at your biggest PV system you can fit, because in my experience you typically can’t produce all the electricity you use by a PV system mounted on the roof of a building.  (The only exception to this rule of thumb is a large warehouse that has a very large roof, and uses very little electricity per square foot of building space.)  So, here are the rough plans for your restaurant:

In this case, you have a pitched roof that looks like this:

We need to determine is how much space on the roof is available for installing solar panels.  Honestly, you could do the trigonometry to figure out how many square feet there are on the roof.  You’re welcome to do the math, but I find it easier to just draw the building in 3D.  (I use and recommend Sketchup – www.sketchup.com.  After years of using AutoCAD, the simple user interface for Sketchup was a welcome change.  Plus, the basic version is free.)  So, the drawings above turn into this 3D model:



The next step is to determine where you can install solar panels.  Design guidelines from the California Fire Marshall that are being adopted by the latest building codes require a 3 foot wide clear area along the roof peak and along the edges of the roof.  These requirements have not been adopted everywhere, so you may not be required to follow this.  From experience, I’d recommend following them.  It makes installation and maintenance much easier to undertaken, and at the end of the day, these design guidelines were put in place to allow firefighters to more easily and safely fight fires on a building with a PV system.

The next step is to layout the solar panels.  The company that makes the largest solar panels is SolarWorld (www.solarworld-usa.com).  (I’m talking 60-cell versus 72-cell here solar panels here.  The 60-cell solar panels are more prevalent in residential and small commercial.)  These solar panels are 65.94 in long by 39.41 in wide by 1.22 in thick.  (SolarWorld solar panels are also the thinnest solar panels around.  But, this won’t have any impact on how you design the PV system.)  I always size with SolarWorld solar panels because I can go with a physically smaller solar panels at a later point if I so choose.  (If I size with another solar panel, I may not be able to install the same number of SolarWorld solar panels at a later point if I so choose.)  Plus, the size difference is only up to 2 in in any given direction, so it won’t make much of a difference on a small PV system.

I built a 3D model of the SolarWorld solar panels in Sketchup that I use for the layout process.  It makes short work of laying out the solar panels because I only need to copy and paste them.  (I’m happy to share the model with anyone who would like it.  Just get in touch with me.)  Also, remember to layout the solar panels in both portrait and landscape orientation.  You’ll typically be able to layout more solar panels in one orientation versus another depending on the dimensions of the roof.  For the roof of your restaurant, landscape orientation made more sense.



With this layout, you’ll be able to fit 88 solar panels on the roof of your restaurant – 44 on the south face of the roof, and 44 on the north facing side of the roof.  You might notice a couple things I did “wrong” with this design.  First, I didn’t follow my own guidelines from above regarding the 3 foot clear space along the peak of the roof.  This was a deliberate choice on my part to increase the size of the PV system.  In the area where I’m designing this, there is no requirement for this space.  (There aren’t many requirements of any kind.  But, I’ll get into that at a later date.)  It’s one of those trade-offs that you’ll run across during the design process.  Second, you’ll notice that I installed solar panels on the north facing side of the roof.  This is normally a bad idea.  But, I’m designing this PV system for a building in Lagos, Nigeria.  Lagos is located 5 degrees north of the equator.  The loss in production from the solar panels facing north on this roof is minimal, so it made sense to install them on that side of the roof as well.  Typically, it doesn’t make sense to do this in northern latitudes.  (This is where my experience influences the design a little bit, because I know what types of values I’m going to get from the next few steps.)

So, I now know the number of solar panels I can fit on the roof, but that’s not really what I can about.  What I want to know is how much electricity (kWh) will this PV system generate?  How do I figure that number out?

The good news, there’s an app for that.  Well, not quite, but the National Renewable Energy Laboratory (NREL – www.nrel.gov) has been kind enough to provide us with an on-line tool to ball-park the production from a PV system in a given location.  (NREL also has some additional resources related to solar and other renewable energies.  I highly recommend checking them out.)  The tool is called PV Watts.  There are now a few different versions of this tool.  I’d recommend using the PV Watts Viewer (http://gisatnrel.nrel.gov/PVWatts_Viewer/index.html) for any PV systems located in the United States.  (For the PV system I’m designing in Nigeria, I used the PV Watts v1, because only this version allowed me to pull solar data for Nigeria.)

PV Watts requires a few inputs to make it work.  The first is to determine the location of the PV system.  You can input the zip code, the address, or the longitude and latitude.  For Brooklyn, the zip code I used was 11226.  Next, click on “Send to PV Watts”.  This next page is where all the heavy lifting happens.  You have required inputs for “DC Rating”, “DC to AC Derate Factor”, “Array Tilt”, and “Array Azimuth”.  I’ll walk you through what to enter for all these inputs.  First, enter “1.0” for “DC Rating”.  This allows for a “kWh/kW” factor that you can use as the PV system size changes without having to come back to PV Watts several times in the future.  Second, enter “0.80” for “DC to AC Derate Factor”.  This is completely based on my experience with PV system production as compared to expected.  The “Array Tilt” for this PV system is about 5 degrees.  (Since the solar panels will be installed flat on the roof, the tilt of the PV system is the tilt of the roof.  The information above shows a rise of 1” for every f 12” of run.  Going back to trigonometry, we have Tan(x) = 1/12.  X = 4.77 degrees)  “Array Azumith” the orientation of the PV system as compared to due south.  In the case of PV Watts, due south is set to 180 degrees.  The output from PV Watts is the expected production of a PV system with these design specs on a monthly basis.  With these details, the PV system will produce 1074 kWh / kW.

In the case of the PV system in Nigeria, you actually get two “kWh / kW” values, one for the part of the PV system facing south (1368 kWh / kW), and one for the part of the PV system facing north (1354 kWh / kW).

This is all fine and dandy, but how do I get to the total kWh produced by the PV system?  A little more math and we’re there.  First you need to determine the number of kW the PV system represents.  You currently know that you can fit 88 solar panels on the roof.  And, I know from experience that solar panels that produce 235 W are widely available.  I also know that it is possible to get solar panels that produce 250 W.  So, if you install the 235 W panels, you’ll have a 20.68 kW system [(88 x 235 W) * ( 1 kW / 1000 W)].  And if you install the 250 W panels, you’ll have a 22.00 kW system [(88 x 250 W) * ( 1 kW / 1000 W)].

All else being equal, you’d always want to install the 250 W solar panels, but the 250 W solar panels tend to be more expensive than the 235 W solar panels on a $ / W basis.  So, this is another one of those cost versus benefit trade-offs that you’re going to have to make.  Unfortunately, you won’t have all the information to make a good decision about this until later, so we’ll just use both values for now.  The 235 W to 250 W values provide a good low end and high end for the available sizes of solar panels.

Congratulations!  You’ve finished your initial PV system design.  You’ve determined the high level metrics of what your PV system will do.  Here are the highlights:

Number of solar panels:                                  88

PV system size:

235 W solar panels:        20.68 kW

250 W solar panels:        22.00 kW

PV system production

235 W solar panels:        28,145 kWh

250 W solar panels:        29,942 kWh


You’re not quite to the level of detail you’ll need to be able to install the PV system, but this is enough information to move forward with understanding the other parts of the electricity system.  Tune in over the next few weeks as we look at the two other major components of this electricity system – storage (batteries) and the controls.

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