How Does It Work?
Sunlight strikes a semiconductor material and bumps electrons off in a continuous stream called direct current (DC). Home appliances and lights use alternating current (AC), so it is the job of the inverter to change the DC to AC. If your home is using electric energy (i.e. refrigerator, lights) solar-produced energy is consumed. If your solar system is making more energy than your house is using, the surplus gets sent back to the grid, banking the power you produce for later use. [See battery based grid tied & off grid for alternate system designs]
- Panels and inverters must be matched according to the amount of energy your solar panels can generate.
- The inverter needs to be close to the panels and connected to your main house electric panel.
- For grid-tied solar systems, when the grid is down your system will be automatically shut off until the grid comes back online. This occurs primarily for safety reasons.
Semiconductor material used in solar electric systems falls into two broad categories: crystalline (hard/stiff) and amorphous (soft/flexible). Crystalline technology can be monocrystalline grown in a large crucible or polycrystalline grown and cast into a large ingot. Both of these types are sliced into wafers and printed with electrical circuitry to become photovoltaic (PV) cells. The cells are then electrically connected in a long series and assembled into panels. Panel sizes vary; generally speaking, a 200 watt panel would be about 15 to 20 square feet and weigh 35 - 40 lbs. Conversion efficiency, the ratio of light energy converted to electrical energy, is in the range of 15 - 18%.
Amorphous photovoltaic material generally comes in sheets. Some amorphous material is transparent and can be applied to windows and other vertical surfaces. A more common application is a narrow sheet that fits between the seams of a metal roof. Amorphous PV is less efficient than crystalline panels, meaning that for the same area amorphous panels will convert less energy from the same amount of sunlight. However, much research is being done to improve conversion efficiency.
- Amorphous systems require about twice as much space as crystalline panels, however it may integrate better with your roof.
- Amorphous PV material shows up in many portable applications where flexibility and light weight are advantageous.
Inverters take direct current (DC) energy from solar panels and change it to alternating current (AC) that is used in buildings. For systems that are tied to the electric grid they must also match the frequency (hertz) within a very tight tolerance. All grid tied inverters are tested to Underwriter Laboratory's test standard UL 1741.
The inverter also contains safety and utility-interface circuitry, which enables a solar photovoltaic system to integrate and interconnect seamlessly and safely with the utility grid system and all the building’s electrical loads. Most modern inverters also contain data monitoring and web communication circuits so the owner and system stakeholders can keep an eye on performance.
Inverters generally work best when the panels of the solar array are oriented the same way with the same tilt. However, multiple inverters can be connected to your electric panel. For instance, if you have a south facing solar array and west facing array, separate inverters can be used on the same house.
The advent and development of inverter technology is what made grid tied PV systems possible.Inverter technology continues to improve and warranties are getting longer. Just a few years ago a typical warranty was 5 years, but now you can expect an inverter warranty of 10-20 years.
- Inverters must be sized according to the output of the PV Panels
- Inverters should be located near the PV panels
- Inverters have a low hum when they are working so they should be located in an area where this daytime noise is not an issue.
The advent of the new generation micro inverter has opened an exciting era in small PV systems. It is now possible to install single panel grid-tie PV systems in apartments, condos, and small homes. For the cost of a little as a single PV panel (Nominally $500 for 200 watts) and a single micro inverter (Nominally $200), a PV system can be added to a home.
Micro inverters mounted on each PV panel provide solutions for sites with shading issues or complex roofs with arrays mounted in more than one orientation. For example, in difficult installations where a shadow passes across the array during the day, a string inverter reduces production from all panels on the string.
Meters & Monitoring
Having a meter that tracks your solar electric generation can be a useful tool.
Net metering allows you to use the electric grid, and the company that otherwise supplies you with electricity, as if it were a big battery. There will be times when your electricity needs are less than the amount of electricity your generating system is providing at the moment. Your PV system puts the excess electricity you do not need back into the electric grid to be used by others and allows you to take this same amount of electricity back out of the electric grid at a later date. Net metering permits you to "bank" your excess electricity and then withdraw it from the grid for your use.
For example, on a sunny summer day when no one is home, a photovoltaic system might produce more electricity than needed at the time. Conversely, in the evening, when everyone is home, electricity needs would exceed the output of the system.
Most electric meters measure electricity moving both into and out of your home or business. Generally, we are taking electricity from the electric grid for our needs. The meter runs "forward" as it counts up the kilowatt hours we have consumed. But if you generate electricity with a photovoltaic or wind generating system and you make more electricity than you need, net metering legally allows this excess electricity to run the other way through the meter and back into the electric grid. Just like running your car in reverse, the meter now turns backwards.
Net metering, thus, might result in your meter turning backwards at mid-day when the sun is the strongest and running forwards at night when a solar system stops operating. If you put 10 kilowatt-hours (kWh) of excess electricity into the electric grid during the day, net metering allows you to take 10 kWh of electricity out of the grid later and pay nothing extra for them. In effect, you are allowed to "bank" these 10 kWh and use them later to offset your need to buy 10 kWh. Thus, you can get full retail value for the electricity you generate. Source: RNP
Your inverter probably has a display showing cumulative generation, but if for any reasons you change inverters, you would lose the count to that point. So the best way to monitor the output of your solar electric system is a dedicated meter at the inverter.
Once they are installed, PV systems are safe (with the usual caveats of any electrical system) and require little maintenance.
During the installation process there are many important safety precautions. Among the considerations is the fact that work is being done on a roof, usually a sloped one, and that solar panels have the potential for generating energy whenever light strikes them. Installers use harnesses in order to maintain a safe working environment and many put a tarp or sheet of cardboard over the panels while the wiring is being connected to reduce the voltage.
All solar modules and inverters meet stringent Underwriters Laboratories (UL), Institute of Electrical and Electronics Engineers (IEEE), and American Society for the Testing of Materials (ASTM) standards, in addition to extensive European and global standards for safety and integrity. Safe installation is covered by the National Electrical Code (NEC), in which Chapter 690 – Photovoltaic Systems, has been in place for nearly 20 years. Locally enforced structural Codes govern the safe integration into buildings.
Much of the training for solar professionals focuses on safety issues; solar PV installation is not a DIY weekend project.
As with windows, more light gets through when the glass is clean. Those of us who live in areas with high pollen count or other atmospheric pollution might want to wash the panels down once or twice a year. Those with panels at a steep angle and a clean environment might never have to think about it. There are probably more systems that don't get washed than do.
Inverters, disconnect switches and meters for grid tied systems are zero maintenance. Battery based grid tied and off grid systems use batteries for energy storage, and these absolutely must be maintained according to manufacturer’s instructions.
Battery Based Grid Tied and Off Grid
There are three basic types of PV systems:
- A Grid-tied system operates only when the utility grid is working
- A Stand-alone off-grid is not connected to the grid at all
- A Battery-based on-grid operates in grid-tied or stand alone mode
If your site is on-grid already, a grid-tied system is the least expensive to install and the simplest to operate and maintain. There is no need for owner maintenance or technical ability, only the pleasure of feeding renewable electricity into the power grid through your meter!
Depending on grid reliability and the length of power outages in a particular service area, some people opt for the simplicity of a grid-tied system with a portable generator for emergency back-up power. Others may opt for the reliable backup of a battery-based on-grid system, which will pick up loads seamlessly when utility power cuts out. The price of a battery-based on-grid system is higher installation cost, and more owner participation in maintaining the batteries. Modern battery-based on-grid systems are nearly (but not quite) as efficient as grid-tied.
If your site is remote and grid electricity is not available, then an off-grid system is the only choice. An off-grid system will be a good choice for you, and save you money over extending the power line, if the following are all
- Your residence is over 1/4 mile from existing power lines
- You are willing to accept that you may not have unlimited power on-demand
- You are willing to be the manager of your own power system
If some of these are not true, the cost of owning and operating a stand-alone off-grid system will increase, and may be more than the cost of extending the power line.
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