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Solar system design basics

In the following chapter I’ll explain the basics of solar system design and why Optimizers increase the safety and production of your system. First, we’ll refresh some basic concepts of electricity and then apply these to solar power. As this reading is quite technical, do not hesitate to reach out to us for guidance. GRØNBID team has experience from dozens of solar installations and will provide assistance with all your questions.

The structure of this post is first to refresh the basic electrical concepts, then the ones specific to solar systems and finally compare different types of inverters.

Voltage Current Power

Basic Terms:

 

Voltage: According to the common water-analogy, Voltage (marked with V for Volt) is for electricity, what height difference is for water. Water flows faster in river down a mountain than in the lowlands of Skåne. Water from a fire hydrant will spray far because of the high pressure caused by the water being stored high in a water tower. If there is no height difference, no water fill flow; if there is no voltage, no electric current will flow. Direct current or DC produced by Solar panels acts just like water, where the flow is always in the same direction, from higher to lower potential.

Higher voltage means the electricity has more potential to find paths. If you hold the positive and the negative poles of a battery in your hands, the voltage is not enough to travel through your skin. But if you put two metal spikes into a socket, the 230V is enough to penetrate your skin and the electricity will travel between the 2 poles of the socket through your body. In extreme cases the electricity can jump a small distance causing an arc. You might have seen this plugging in an appliance to an old socket. Because the arcs cause high temperature, they can easily lead to fires if in a place with flammable material.

Current: According to the same water analogy, current (marked with A for Ampere) is the amount of water flowing in a pipe or river. More current requires larger pipes, or in case of electricity: conductors. Electric current always needs a closed circuit to flow, as well as voltage difference. If a switch is turned OFF, the conductors are not terminated and no current will flow. The potential difference still exists and current starts flowing as soon as it finds a path to close the circuit. If there is no voltage, current will not flow even if there is a path, just like water will not flow in a lake.

Current is the millions of charged particles that travel in a conductor and voltage is what puts them to motion.

Power: Like water spinning a wheel in a river, electrical power is the product of voltage and current. The more water there is, the more work you can get out. Alternatively, if the water is rushing down a high mountain at a fast speed, it has more energy to turn the wheel. If there’s a malfunction that limits the voltage or restricts the current, power is reduced.

Voltage arc analogy

Energy: Energy is simply put Power x Time. For electric Energy the unit of power is 1 kW or kilo-Watt and unit for time is 1 hour. This is why your electrical consumption in the electric bill is shown as kWh. In grid-tied solar system design the objective is usually to maximize the amount of produced energy, especially if the rates for used and sold electricity don’t change per time of day. This is the situation in Sweden, but in California for example it does make sense to time the maximum output to the peak-price hours of the afternoon.

Conductor: Anything that conducts electricity, usually a metal. Wires and cables are conductors, but any metal object or solution can act as a conductor. The opposite of conductor is insulator, that doesn’t conduct electricity at all. Cable is a conductor wrapped in an insulator, that’s why it’s safe to use.

Solar Specific terms

Series connection: Like most of us remember from elementary school, when connecting two batteries in series their voltages are summed up. The same current flows through each battery, with each increasing the voltage. Solar panels are always connected in series, each panel increasing the voltage at a constant current. A typical panel produces 30V, so 15 panels in series, or a string produce 450V. This is higher than the voltage coming from our electrical sockets, which is only 230V.

Parallel connection: The opposite of series connection would be to connect all the panels parallel. In this case the current is summed up. This would lead to very high currents and large conductors, so this is never used for solar panels.

Solar panels: Through Photovoltaic reaction solar panels produce Direct Current or DC. Panels have typically maximum voltage of 30V and maximum current of 9A. This gives the output of 270W under maximal conditions. This DC electric power is converter to AC that our grid uses in an inverter. Inverters are explained shortly in this article.

Solar panels produce electricity always when they’re exposed to sunlight. In case the circuit is open and current doesn’t flow, there exists a potential between the positive and the negative conductor. That potential difference is looking for a path to close the circuit and anything conductive will serve that purpose be it a steel storm drain or firefighter’s spray.

Since the same electrons flow through each solar panel, it becomes of paramount importance that the flow is not restricted by any single panel in the string. When a panel is shaded it stops producing power and actually starts restricting the flow of current through it. One partially shaded panel in a string limits the output of every single panel in the string. Because this is quite problematic for a solar system, Optimizers were developed to resolve the issue.

Inverter

Because the grid works with Alternative Current or AC, the output of the solar panels needs to be changed from DC to AC. That is the main task of the inverter, but it also performs all the protective functions required by the Electrical Code. Solar system design is very much dependent on which inverter type is used.

There are 3 types of inverters for residential rooftop installations:

  • String inverters
  • Inverters with optimizers
  • Microinverters

 

String micro Solar Edge inverter

Photo Source: LetsGoSolar.com

 

String Inverters

String inverter is the classic model that has been on the market for decades, while inverters with optimizers and microinverters have become more common in the last 10 years. The main difference between these is that with string inverters, all the solar panels are connected in series and controlled as one long chain. Therefore, the monitoring can’t be done on panel-level, but instead per string. Also, because the inverter has no way of controlling the panels, even if you turn off the inverter, the string of panels is still energized meaning that the voltage between the positive and negative conductor is 450V. This is not ideal for safety, because high voltage in a cut wire can energize objects it touches.

In terms of production, as explained previously, string inverters are sensitive to partial shading. This problem of “weakest link” means that a partially shaded module restricts the flow of current in the whole string, reducing production. Also, discovering problems in production is significantly more complex, because there is no way to know which panel of the string has the low production.

Laying out the panels on your roof also becomes more restricted. If your panels are at different orientation or tilts, they will perform differently depending on the time of the day, but the string inverter will set the current per the lowest module, so that instead of the other panels producing better in the afternoon and other panels better in the morning, all panels perform poorly all the time. Most string inverters have 2 or more string inputs, but for a small system this still limits the design as all the panels in the string must face the same exact direction.

 

Inverters with Optimizers

Optimizers are small boxes connected directly to each solar panel acting as a gatekeeper for the panel to the string. The optimizers are connected to series and under normal operation they adjust the voltage and current of all the panels in the string so that there are no losses. Importantly, the optimizers communicate with the inverter and can sense when the inverter is turned off or a conductor is broken. Acting as the gatekeeper, the optimizer will sense a problem and stop feeding power, thus isolating the panel. When you turn off the power at the inverter you will have no voltage in the conductors coming in. The only voltage is the 30V between the panel and the optimizer in the 90 cm of wire. This makes the inverters with optimizers very safe.

Another feature of optimizers is the panel-level monitoring. Optimizers communicate with the inverter and the internet so that you can see the production of every single panel on your roof from your laptop or phone. This way you can detect and locate problems immediately, even if it was nothing more than partial shading from a bunch of leaves fallen down on a panel in the fall.

Third benefit of the optimizers is that they do not limit the production of the string in case one module is shaded. By adjusting the voltage and current output of the panel, they find the combination that only adds to the string, never restricts it. This allows every module to put out its maximal power under all conditions thereby increasing overall production. You can also use the roof space more efficiently, because partial shade doesn’t impact the whole string.

Inverters with partial shade

Photo source: Laminar

Microinverters

Instead of having one electrical component to transform the DC to AC, microinverters are tiny inverters connected directly to a panel. They offer all the benefits of Optimizers but are more expensive because the DC to AC conversion is done separately for every single panel. Microinverters are justified for very small systems, but don’t justify the added price for a larger system. From one point of view microinverters are more resilient to failure because there is no central unit whose malfunction would cause the whole system to shut down. On the other hand, microinverters have downsides that neither string or optimizer inverters have:

  • Many complex electric components need to be installed on the roof exposed to heat, cold and other extreme conditions, increasing the risk of failure.
  • Efficiency is lower because of the small size and poor heat dissipation.
  • Only work with Wi-fi connection i.e. no hard-wired internet connection

Because Optimizers do everything that microinverters without the downsides, they have replaced them to a large degree for all but the smallest installations.

 

Summary

This overview was to give GRØNBID’s customers insight to why we are so strongly in favor of Solar Edge inverters with Optimizers for a less than 15 kW system. Like with all technology, we keep our eyes open and constantly research the market to find the best and most reliable available solutions. Unlike the Contractors, we don’t have any affiliations to Equipment Manufacturers, which makes us independent and objective in providing our customers the best projects.

If you have already received a quote from a contractor, we are happy to evaluate it for you in terms of technology and sizing. Do not hesitate to contacts us at info@gronbid.com or at 070 189 1006.

 


About the Author : Tuukka


3 Comments
  1. […] the next article I will explain in detail why Solar Edge Optimizers are such a good solution that we don’t even […]

  2. Gustav A. September 21, 2018 at 6:27 pm - Reply

    Thank you for this article, it explains the different terms pretty well to me.

  3. Faruk October 13, 2018 at 10:53 am - Reply

    I read your post very helpful to me because i am a Engineer.

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