Monthly Archives: February 2011

General

Fourth choice: Lowest Solar Panel Operating Temperature

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The fourth choice will actually determine what type of solar panel plans will actually work. As it turns out, the popular solar panels are polycrystalline. And this cell technology has a negative output voltage vs. temperature coefficient. Another way to state that is that the voltage gets higher as the temperature gets lower.

Believe it or not this is a very critical design parameter. Systems that are designed for use in Florida can self destruct when they are installed in Michigan because the Florida design would probably assume the lowest operating temperature of around 25 degrees. Here in Michigan sometimes get sunny days with below zero temperatures. This difference can raise the output voltage enough that the rest of the system is at risk of failing.

So in our case we needed to do some research to find the historic low temperatures and make some assumptions about low temperatures when sunlight would be hitting the panels. The record low temperature going back eighteen years waas -22 degrees F. And that would have been at night when the solar panels wouldn’t be generating any voltage. We chose to design our system based on a lowest possible temperature of -14 degrees F.

This design choice is particularly interesting when you realize that two of the system quotes we received were actually improperly designed for Michigan installations. I spoke to another engineer-homeowner who installed solar panels in 2010 and he also received proposals that had this same design error. So, be careful with this!

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Second Choice Grid Tie Solar Panels or Battery Charging Solar Panels

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So we have chosen to do solar electric on our Ann Arbor home, the next big questions is which type of topology. That is: Solar panels connected to inverter connected to the utility grid or solar panels connected to a charge controller connected to batteries, connected to an inverter connected to the home electric system.

In the grid tie arrangement the utility company acts as the power storage eliminating theu don’t have  need for batteries.

The downside to a grid tied arrangement is that if the utility company power actually goes out, the solar panel/inverter system shuts down. So you don’t have any kind of back-up power in a standard grid tie system. (The practical solution for this concern is to just have a back-up generator. )

The upside to a grid tied system is that you don’t have batteries to pay for or maintain. Having enough batteries to power a household for a couple days in case of cloudy weather can cost several thousand dollars. And those same batteries may need replacement in four to six years.

In fact, the cost of batteries and battery maintenance alone makes battery charging systems really only suitable for locations without grid power available.

Also, utility rebates and tax incentives are only available with grid-tie systems.

So the second choice ends up being fairly easy. A grid tie system it will be!

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Third Choice. String Tie or Microinverter

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So we’ve decided to install a grid tied system to take advantage of the utility and government subsidies and not have to deal with batteries.

The next choice is microinverters or strings.

String systems have a number of solar panels connected in series much like a flashlight might have two or three 1.5V batteries connected in series. In series connections the voltages add. So a flashlight with 3 1.5V batteries has a total of 4.5V to the bulb. In a solar installation a set of 13 solar panels might be connected in series. If they each put out about 30 Volts then the sum would be 390V.  Many solar panels have about 200W of power output under full sun so that might mean that the panels produce a current of about 7 Amps. (Assuming 210 Watt panels.)

If you need more power than 390 V at 7 Amps, (2,730 Watts), then you need to add additional strings. You can’t practically mix string voltages so this makes it a little difficult to design and build. In our example you have a choice of a 2,730 Watt system or a 5,460 Watt system or a 8,190 Watt system. Trust me, it starts to get a little complicated.

Also with a string system the whole string can be negatively impacted if one panel has a problem. For example if a shadow from a tree is over one panel that will limit the output from the whole string, even if the other panels aren’t shaded.

The alternative that has just been practical for the last few years is having an inverter connected directly to each panel. One inverter per panel. The output of the inverter is the 240VAC that can feed back into your breaker panel. These inverters that mount on the back of the solar panels are called microinverters. The most popular one is from Enphase.

Up to 15 of these can be grouped together on one circuit, and there is basically no limit to how many groups you can have.

This is a very flexible alternative because with microinverters you can start with just one panel and one inverter and build from there.

Again since I have a background in power supply design I found these systems fascinating. There were just two problems:

1. I am concerned about the long term reliability of that many power converters in that extreme environment. Roof temperatures in the summer can get quite high and obviously during the winters they can get quite low. And that temperature swing will be happening every 24 hours for the next twenty/thirty/forty years. (Solar panels can easily last 40 years, however their output will drop some over time.)

2. I wanted the option of doing something else with the power from the panels at some point in the future.  (This is a geeky engineer thing most people probably won’t care about.)

The microinverters do cost a bit more on a per Watt basis, but the installation is easier so they end up fairly close in total price.

In the end we decided on a string tie system with three strings of 14 panels.

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