intro to solar

A solar panel is composed of individual PV cells strung together
When light strikes the solar panel, it excites electrons between the P-type silicon and N-type silicon layers which creates DC power.

ELECTRICAL CURRENTS AND RATINGS

What is the difference between DC and AC current?

Direct Current (DC):

Electrical current flowing in one direction continuously. This is the power produced by solar before running through an inverter.

Alternating Current (AC):

Electrical current that cycles in alternating directions. This is the power used by residential homes and the product of running power from PV through an inverter.

How is current measured?

Voltage:

The force or "pressure" of electrical current

Amperage:

Measure of the amount of electricity in the electrical current

Wattage:

A watt is a unit of measure equal to Amperage x Voltage x Power Factor

Domestic circuits or loads are connected to the utility via breakers (or fuses) in an electrical enclosure referred to as a load center. The breakers attach to conductive busbars inside the load centers; single-pole breakers only get fed from one side of the bus, so they supply 120-Volt circuits [lighting loads, most small appliances, some dedicated appliances like microwaves]; and two-pole breakers get fed from both sides of the bus, so they supply 240-Volt circuits [dryers, ovens, air conditioners].

The rating on the breakers shows how much current (Amps or Amperage) is allowed to pass through them before the breaker trips, and so the circuits can be abbreviated by their Amperage and their one- or two-pole nature: An air conditioner might use a 30A/2P breaker, an electric water heater might use a 60A/2P breaker, and a microwave oven might use a 20A/1P breaker.

Other breaker types exist for condensing circuits into physically smaller spaces while still supplying the same Amperage and Voltage, and will be addressed later.

HOW PV GENERATED ELECTRICITY CAN BE USED IN RESIDENTIAL HOMES

So now we know that AC current is the type of current that residential electrical systems can use, but wait; how can we use the DC power produced by PV?

Direct current is inverted to AC power by an inverter for use by the existing electrical distribution system of the home.
Any excess energy created by the PV system that does not get used by the homeowner will be exported to the utility grid for redistribution.

Note that usage is recorded by utility using kWh (Kilowatt-hours) figures. That is a measure of power consumed over time. Likewise, the utility records production in kWh.

MODULE STRINGING:

Basic concepts of solar panel wiring (aka stringing)

To have a functional solar PV system, you need to wire the panels together to create an electrical circuit through which current will flow, and you also need to wire the panels to the inverter that will convert the DC power produced by the panels to AC power that can be used in your home and sent to the grid. In the solar industry. This is typically referred to as “stringing” and each series of panels connected together is referred to as a string.

String Types:

Series - Wiring solar panels in series involves connecting each panel to the next in a line (as illustrated in the diagram above). Just like a typical battery that you may be familiar with, solar panels have positive and negative terminals. When stringing in series, the wire from the positive terminal of one solar panel is connected to the negative terminal of the next panel and so on.

When stringing panels in series, each additional panel adds to the total voltage (V) of the string but the current (I) in the string remains the same. One drawback to stringing in series is that a shaded panel can reduce the current through the entire string. Because the current remains the same through the entire string, the current is reduced to that of the panel with the lowest current.

Parallel - Stringing solar panels in parallel (shown in the diagram) is a bit more complicated. Rather than connecting the positive terminal of one panel to the negative terminal of the next, when stringing in parallel, the positive terminals of all the panels on the string are connected to one wire and the negative terminals are all connected to another wire.

When stringing panels in parallel, each additional panel increases the current (amperage) of the circuit, however, the voltage of the circuit remains the same (equivalent to the voltage of each panel). Because of this, a benefit of stringing in parallel is that if one panel is heavily shaded, the rest of the panels can operate normally and the current of the entire string will not be reduced.

2 | SYSTEM PRODUCTION

FACTORS THAT AFFECT SOLAR PRODUCTION

What are the basic factors that affect solar production?

Azimuth
Tilt
Shading

The image to the right shows the overall direct irradiation for all of the continuous united states. The main takeaway being that production can also be heavily affected by location alone. That's why it is extra important to pay attention to the variables that we have reasonable control over when designing solar.

AZIMUTH

What is Azimuth?

Azimuth is the compass directional measurement in precise degrees.

Since we’re in the northern hemisphere:

A south facing orientation is preferred

for overall irradiance.

A north facing orientation

is the least preferred

Applying Azimuth to the layout:

Looking at the image below which roof mounting planes should we mount on?

South:
- Most of our mods should go here
East/West:
- Any additional mods go here
North:
- Rarely would we ever put any here

TILT

What is Tilt?

The horizontal tilt of your panels
Ideally, tilt should be adjusted based on Latitude to maximize production, but this is not always possible.
For flush mount applications, module tilt will match roof tilt.

Tilt: The measure of roof/module angle in relation to the horizon.

Which factor will have a more profound effect on production: Azimuth or Tilt?

As you can se from the example below, even a huge disparity in tilt such as 20 degrees vs 45 degrees only yields about 300kWh difference annually on a south facing roof in this location.

However, by simply moving all panels from the South facing roof to the North facing roof, production is decreased significantly. That's a 2,600kWh/year difference!

SHADING

Which roof areas should be avoided here?

What can cause shading?

As you can see, shading can make or break a design as well. On this one, we have a tree overhanging on a Southeast facing roof plane causing this roof to be less preferred than even the Northwest facing roof plane.

3 | NET METERING

WHAT IS IT?

Net Metering refers to the utility metering of consumption against customer generated production being exported to the grid:

Measured production is applied as a credit on the customer’s utility bill the following month.
This is measured either by a single bi-directional meter or by a secondary production meter specifically to measure solar production.

4 | PROJECT STAGES

As Design & Engineering, our goal should be to design an installable product that will pass through the project stages as smoothly as possible with as little rework as possible. As you can see, there are many points at which projects could become bottlenecked. The ultimate goal is to get to PTO (Permission to Operate) as soon as possible.

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