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Types of Solar Power Systems
This second blog post is an Introduction to Solar Electric Systems. The first blog post was Introduction to Electricity Basics. This is about five (5) of the different types of solar electric systems:
Grid tied with battery backup
We will look at the pros and cons of each type. We will briefly cover the basic equipment needed for each type. Ready?
PV DIRECT SYSTEM
PV Direct systems are the most basic type. The most common one is solar water pumping. This is for remote irrigation, watering livestock, or pumping from a well or stream to a cistern. An example in the video is a community garden in Massachusetts that was near a river. They were able to pump water out of the river into tanks to allow people to water their garden plots. A PV direct system can consist of as few as 2 components, the solar panel and the powered device, like a pump or a fan. There is often a controller or a linear current booster between the two. There are also some fuses and breakers, but not much more than that. It’s a very basic system where the solar panel makes the power, and the pump uses what it can get. When the sun is bright and the panel is making more power. The pump will pump faster than when it is overcast or early morning or late afternoon. The key here is to use the power when you have it, and not need it when the sun isn’t available. Generally, in the example of a pump, you’ll be pumping to a tank or cistern during the day. This allows you to use the water all day and night without having to use batteries. It’s easier to store water than power. The obvious advantage of a PV direct system is it’s simplicity. When the sun shines, the device runs. When it’s not shining, the device doesn’t run. It’s generally easy to install, and you don’t have the expense and hassle of batteries. When you are choosing a PV direct system, you need to determine what DC voltage the device runs on. It MUST be a DC device, you can’t plug an AC device into a solar panel instead of into a wall socket. You need to know how much power your device needs. The specs will generally give you this information. Remember, if it only gives you volts and amps, you can calculate the watts by multiplying volts and amps. You also need to consider how and where you want the panel mounted. Will you be mounting it on a shed roof, or installing it on the side of a pole?
A common set-up for residential systems are grid-tied solar powered systems. These provide some of your power. You sell any extra back to the electric company during the day, and buy it back at night as needed. Even if you don’t have extra to sell throughout the day, your power meter will be spinning slower. This is because you buy less from the grid as you are using your own power that you generated. In a grid tied system, the solar panels are wired through fuses (or circuit breakers) to a grid-tied inverter. The inverter is wired to a breaker in your Main AC panel. The electric company may require you to switch your power meter to a bi-directional meter. This can spin backwards when selling power, and forward when buying. The nice thing about a grid tied system is that it will reduce your electric bill by making some of the power you use. You don’t need to make all of your power, although you can if you want to and have the space and budget to do so. But most people get a big enough system to reduce their bill, but not eliminate it. The down side to a straight grid-tied system is that if the grid goes out, so will power to your house. Even if it’s a beautiful sunny day, your system is required by law to turn off. A grid-tied inverter should have this feature built-in. This is because the inverter is connected to the grid. If the grid is out, the linemen could be out there working on restoring the power. They are NOT expecting any power on the lines. If your system was still up, you run the risk of electrocuting them. When deciding on a grid-tied system, there are several decisions. First find out how much power you currently use by reading your electric bill. How many KWH do you currently use each month? You need to decide what percentage of your use you want to offset. That can be determined by how much money you want to spend, or how much space you have available for the panels. That ties in to the next question, where do you want the panels? If you are putting them on your roof, what kind of roofing does it have, and are they in good shape? If you need a new roof within the next 10 years, get it replaced before installing the panels. Remember that they don’t have to be on your roof if you have space in the yard or field for a ground or pole mounted system.
An off-grid system is a way to power a remote location that doesn’t have access to the power grid. Since you don’t have the grid available for backup if needed, you need to carefully plan your system. Off grid systems consist of the solar panels, the solar charge controller (which manages charging the batteries), the battery bank, and if AC devices are used, an inverter to convert the DC power from the batteries to AC like you have available in the wall sockets. Many off-grid systems have a backup power source like a generator, for times when the sun is not proving enough power for your needs. Unlike a grid tied system where you can just base the system on how much power you want to offset, an off-grid system needs to provide all of your power. With an off-grid system, you need to take a good look at what you are powering, and how long you need to power it. Going off-grid often takes changes to your lifestyle. It’s not as simple as just saying, I’m tired of paying the electric company, I’m taking my whole house off grid. You have to figure worst case scenarios; winter time, with little to no sun for days. Unless you are in a climate that has long sunny days all year round, most people who are doing a year round off-grid system will have a generator available to pitch in in the winter.
The first step to determining the size of your off-grid system is to do a loads list. You can’t just say it is an “average” size system. There is no such thing as an average off-grid system. Without knowing your exact loads, you will end up either buying a system that’s too small, or paying too much for a system that’s too big. A load assessment is a list that tells the exact amount of devices you will be running, how much power they draw, how long a day they will run, and if they have a high surge that needs to be taken into consideration. Additionally, we need to know which devices may potentially be on at the same time. While you can generally control when you turn on a light, you have little control over when a well pump may kick on, or when your refrigerator compressor turns on. Once you have an accurate loads list, you can now figure out what size solar system you need (6 Steps to Design a DIY Off Grid Solar Power System). When calculating the size of the system, one of the questions asked is how many days do you want to run off your battery bank without sun or generator. Remember that every day added to this increases the most expensive part of your system, the batteries. So plan on enough storage, but not too much. As you can see, sizing an off-grid system is a delicate balance.
GRID TIED WITH BATTERY BACK-UP
Grid-tied battery backup systems are becoming popular. This is because of the increase of power outages due to an aging power infrastructure. Another is because of the increase in severity and frequency of storms. A grid tied battery backup system is the best of both worlds. You don’t have to generate all your power. You can offset some of your power needs with the solar, and buy the rest from the grid. When the grid does go out, your system will switch over to powering the devices connected to the Critical Loads Panel. The rest of your house will go out. When designing this system, you decide what you need to have running when the grid goes out. Examples are refrigerators, freezers, well pump, furnace fan, outlets for lights and phone chargers, etc. In a grid tied battery backup system, you have your solar panels charging you battery bank. The inverter is connected to the Main breaker panel. This in turn is connected to the grid, as well as the Critical Loads Panel, which is isolated from the grid. When the grid is up, the solar keeps the batteries topped off. The solar also provides some power to the house, both to items connected to the Critical Loads Panel, and the Main Breaker box. The grid provides any extra power needed. If you are generating more power than you need, the inverter sells the extra power back to the grid, spinning your meter backwards. But when the grid goes out, the inverter immediately disconnects itself from the grid, and will provide power only to the Critical Loads Panel. During the day, the solar will continue to generate electricity to recharge your batteries. But at night, you’ll be running solely on the batteries. These systems also have the option of also having a generator to charge the batteries when needed. There is an advantage of this over a generator system without a battery bank. You can briefly turn on the generator to charge the batteries, and then turn it off. This saves you fuel, and allows you to silently run the critical loads at night. When selecting a grid tied battery backup system, you have to ask your self all of the same questions as with both the grid tied and the off grid systems. Remember that you have the added expense of the batteries now, so it will be a more expensive system than a straight grid-tied. You’ll also have to do a loads list on your critical loads, to determine the correct battery bank and minimum size solar array to recharge the batteries during a power outage.
In a hybrid system you have another power source, such as a wind turbine or microhydro from a river. These systems have a general advantage. When the sun power is at its lowest, that’s usually when the wind is blowing hard or the river is flowing fast. An example is a combination of both solar and wind to provide power for the battery bank. Most turbines will have their own controller to manage charging the battery bank. You would wire it in parallel with the solar charge controller. Be sure to get both solar and turbine to support the same size battery bank. Solar charge controllers prevent overcharging by disconnecting the solar panels when the battery bank is full. You can’t use the same controller to also control the turbine. Unlike solar, a turbine can spin itself out of control to the point of self destruction. It needs a place to put the power it is generating. Be sure to use a controller designed for the turbine. A hybrid system can be very useful in locations that don’t have good sun in the winter. The two technologies can compliment each other.