News: A Different Kind Of System

Recent Article describes how solar power has recently been utilized in a new and different kind of way.  Rather than photovoltaic cells generating electricity, the panels heat up water which runs a turbine.  It also heats up tanks of salt.
Read about it.
Quiz Question:  describe energy, in its relation to electricity and heat.  

Ideal Conduit Runs: Attic

In every roof mount installation, one endeavor is always the same:  the panels are on top of the house, and the electrical service equipment is in the basement.  NEC 690 states that you need to be inside of EMT conduit if you are running DC above 60v indoors.  It's not really possible to snake conduit through walls without completely dismantling the sheetrock to fasten the conduit properly.  Some newer houses are pre-equipped with chaseways, which are often in PVC.  Some inspectors have allowed PV installers to run either EMT or flexible metal conduit down large PVC chaseways, often at the request of homeowners who don't want external PV runs on their house.  I can't blame em.

This is a good example of the ideal situation, regarding the entry to the house of the DC wires from the array, and the run from the combiner box to the exterior wall.

System Design - Microinverters

So basically there are two different ways to wire a residential PV system.  You can either have one inverter (macroinverter system), which typically resides in the basement of your home next to your service panel.  Or you can have many inverters (microinverter system), on the same sized system, with the same number of panels.  It's just a different approach.

Picture the system in the graphic above.  These are two identical systems with the same STC rating (16 panels, let's call 'em Solarian 250's), meaning that the system is a 4kw, effectively.

The Macroinverter system [A] is a single inverter.  If any panel or the inverter itself fails, the single device will register a fault code or if something becomes unplugged up there on the roof, you'll lose the string.  The system is also wired as such that the DC power from the panels continues through the house to the inverter.  That requires EMT conduit if the wires are indoors, per NEC 690.

The Microinverter system [B] are many inverters.  They're not in the basement, they're on your roof underneath the array.  Each panel has its own inverter.  They're probably just as pricey in number as it were to just get one single unit.

Pro's and Con's
Salespeople were pushing the microinverter systems hard when they first became popular, back in 2010.  I was a bit skeptical of their popularity.  Not just because I was an old dog who only wanted to install systems a certain way.  Admittedly I had more experience with what I'm describing as "macroinverter" systems, a name they were never given, but makes sense to describe them as, since microinverters have their own definitive terminological distinction.

Here's why I was skeptical.

Microinverters are more subject to fail, if not solely for the reason that they're contantly exposed to the elements.  Temperature cycles are known to damage electronic equipment, and daytime/nighttime temperature differentials, during certain times of the year, can exceed 30 degrees a cycle.  Also, inverters are more efficient in cool, dry places.  It just seems to make more sense to install a single-inverter system, where it can be installed in the basement next to the service panel.

Aside from their exposure to the elements, multiple inverters also multiply the chance for system failure.  Without an advanced monitoring system (which could also potentially be subject to fail), there's no way to know if a single inverter gets "knocked out," or loses its ability to produce power.  Meaning that you'd have no idea knowing just how many of your panels are actually producing power.  The panels, remember, aren't wired in series.  They're each wired individually.

Classroom Exercises
Fill out a MicroInverter Question Sheet and a Wiring Diagram.

Site Survey: Montessi VW

Today I got my oil changed on my '05 TDI Jetta Wagon at Montessi in North Haven.  It takes about an hour and I thought, why not climb up on the roof and see what was going on up there.  So I found a hatch and got looking around with my tape measure.

Site surveys are the first step towards determining how much solar can fit on a roof.  But how do you wrap your head around it when there are obstructions on a roof everywhere?

Here are a few simple pointers.  The first?
Think Zones.

Here is an original drawing of what my original site survey appeared like.  With solar panels, they are usually installed in blocks.  By thinking in terms of these blocks, you can map out how much space is viable for solar on a complicated commercial roof.  Yes, zones.

  1. Get the entire perimeter.  Subtract 2' from the edges. 
  2. Measure out the "zones," which are areas of space that are square which do not have any obstructions.  
  3. Verify with distances to the air vents. 
I then label the zones by the roof surface.  In the illustration to your right, you see that this is "Roof B" and thus there is an area marked "B-1, B-2," and so on.  Solar panels, on flat roofs, are often counter-ballasted and require space between rows.  Typical amount of space is 2' and the panels are mounted with a 10° to 15° angle

Air Handler
Then I take these drawings home and digitize them on a simple graphics program, such as "Flash" or "Photoshop."  One thing that helps?  The original drawings are to scale, where each square represents 50 square inches (with solar, measure in inches, not feet).

For a smaller roof, the scale may be 10" per square.  That would also be fine.  In this instance, though, I'm using 50" per square.
So here we go.  

Satellite Photo
This is an actual look at the roof.  As you can see, there are many vents and other complicated parts about it.  By examining a roof with an aerial satellite photo, you can't really determine the accuracy of space available, with the same level that you can if you're actually standing on the roof. 
The actual roof

This drawing, to the left, was made using the data that I created from actually measuring the roof with a 25' measuring tape.  
What is the angle of the roof?

This is now with the areas marked out which are suitable for solar.  Imagine walking around the roof after the panels are installed.  What if you need to get to one of the air handlers?  That's the reason why it's important to be conservative about the amount of space that you give for your array in a situation like this, with a large complicated commercial roof.  

The zones are also listed by their area.  Note that they're all rectangular (no rhombus!  no parallelograms!).  

So here's your final design.  This is how many panels will reasonably fit on this roof surface.  Note the distance between the rows, for the inter-row shading.  That's important.  

The next question though is, how much power will that make them?  What about the rest of the math?

How many panels is that?

We're about to get into some of that math really soon in just a matter of moments.  

The Math Behind The Scenes
Ok.  So you know how many panels can fit.  

I devised the above layout for Sunpower 310w panels, which are 41.5 by 61.5 inches in dimension.   I don't know how expensive they are at the moment, but this is just a hypothetical site survey.   Here is the electrical data:

What can you gather from that information?  Well, here is some other information to consider.  You have an open circuit voltage (60.3v) limits the number of modules you can put in series.  If you match that with the temperature derating factor in the NEC, you must over-rate your voltage by a factor of 120%.  Therefore, 8 in series is 482v (x1.2) = 578.88 which is as close to the maximum 600v allowed by law!

In other words, you can group the array.  5 in series is too low, because the rated voltage is 50.1 and the inverter kicks in at 250v (see inverter operating voltage).  In this case, I'd aim for 7 in series for the most optimal operating voltage for the inverters.  

Areas  A:  42
A - 1 :  500 x 80   (10)
A - 2 :  200 x 200 (6)
A - 3 :  200 x 200 (6)
A - 4 :  500 x 100 (20)

Areas  B:  108
B - 1 :  200 x 225 (4)
B - 2 :  100 x 300 (5)
B - 3 :  200 x 200 (4)
B - 4 :  300 x 400 (16)
B - 5 :  300 x 550 (25)
B - 6 :  150 x 350 (9)

B - 7 :  200 x 350 (15)
B - 8 :  200 x 250 (12)
B - 9 :  200 x 250 (18)

Areas  C, D, E:  46
C :  550 x 250   (20)
D :  750 x 250 (20)
E :  100 x 300 (6)

That's 196 panels.  Divisible by 7?  Yes.  28 strings of 7, which will fit if each of the 4 strings go onto an inverter.  That's 7 inverters.  Here are some more awesome questions you can ask.

How Much Power is This System?  
It's technically a 60kw.  But that's if the panels were in a laboratory at exactly 1kw/m².  In reality, it's not going to reach that number ever.  What it's more likely to perform at is based on the angle, orientation, and shading, along with wire transmission loss and inverter loss.  A more accurate number?

60kw x .97 (angle/orientation) x .8 (shading) x .75 (other losses) = 34kw.  Yes, life isn't a laboratory, and the .8 rating for shading is because of all of the tall air handlers which will block the sunlight from panels at certain parts of the day.  It might be smart to eliminate some of those panels, in order to streamline the system and increase its functionality.  A rule?  Count 3x the height of the obstruction if the panels are directly north of the object, and 2x the height to the west or east of the shading object.

In the Northeast United States, you get 4.4 hours of sun on average per day, according to NREL.  That means this system will average about 150 kWh per day.  That's 54,750 kWh per year, or 54 mWh.  At current rates, this system is worth $12,000 in savings per year.  In 10 years, that's $120,000.

Do you think this system will cost more than $120,000?

Photos From Installations

These are photos of houses in Connecticut that I installed PV grid-tied systems on.


I have developed a board game, and the purpose of it is to display how to design solar electrical systems on a variety of hypothetical sites, estimating the size of the system, selecting the components, and evaluating the annual production of that system.  You can check your answers to ensure that your calculations are correct.

After establishing that your methods of calculation are accurate (and by familiarizing yourself with all the parts of a system), you should be able to accurately assess your home for its own solar electrical system.  That is the purpose and meaning behind the board game.