While the Australian rock band AC/DC are often described as “powerful” and “electric” they aren’t the topic of discussion today – instead we’re talking about the two common forms of electrical delivery and why they’re important to your solar photovoltaic installation. The band’s name does have an electrical origin though, they came up with the idea when they saw a label on the back of a sewing machine that said “AC/DC” meaning that it could operate from either alternating current or direct current.
So what is the difference between alternating current (AC) and direct current (DC)?
DC is the simpler form of electricity where there is a constant electrical potential (voltage) that moves electrons through a circuit in a linear fashion – much like water flowing through a pipe. DC voltage is found in batteries which commonly range from 1.5 volts (AA batteries) to 12 volts (automobile batteries). DC voltage can be generated in a battery through a chemical process, by a generator (like the alternator in your car) and by a photovoltaic cell. In a PV cell, photons knock electrons from one layer of a silicon semiconductor down to another, “pushing” them forward through the circuit.
AC works more like a wave, with electrons moving back and forth in the wire depending on the potential of that moment. The voltage driving the electrons continuously changes from positive to negative and back again; the frequency of the change is commonly measured in cycles per second (Hz). In the United States, the common line voltage is 120 volts cycled at 60 hertz (120V AC, 60 Hz) though some high power equipment is run at 240V AC. AC voltage is most often created using a rotating generator. Power plants (coal or nuclear) boil water to make steam that turns a turbine, hydroelectric and wind power also come from turning a generator using mechanical energy.
AC and DC power are not interchangeable. If you connect a device designed for AC usage to a DC power source (or the other way around) you will most likely have a front row seat to a display of smoke, sparks and melted components. If you’re lucky a fuse or circuit breaker will attempt to protect the device.
Most of the small devices in our modern life use DC power (cell phones, computers, flat screen TV’s) but the power that comes out of the wall socket is AC power. The solution is a power adapter – those bulky “wall warts” you plug into the wall convert the electricity from one form to another. In some instances like computers or televisions the adapter is built into the device and is usually referred to as a “power supply”. All of the parts inside your computer (PC or laptop) from CPU to memory to hard drive use DC power and need a power supply to translate the wall socket’s AC power into DC power. Many of the larger, “old school” devices in your home (toaster, air conditioner, washing machine) require more power and are designed to use the household AC directly.
Edison vs. Westinghouse
Why don’t we use DC power so we don’t need adapters and power supplies? Well, we almost did. The earliest form of DC electricity known is the Baghdad Battery, a simple chemical battery dating back to roughly 250 BC. In more (relatively) modern times, Thomas Edison built a network of DC electrical generating stations in the 1880s. The problem was that the voltage dropped off rather quickly as the current moved through the wires. The power plants needed to be less than a mile from the customer. Raising the voltage would solve the problem but it’s difficult to convert high DC voltages to low DC voltages – even more so in the nineteenth century! The answer to the transmission losses turned out to be high voltage AC power. Losing ten volts per mile isn’t a problem when you start out with a hundred thousand volts but it’s a deal breaker when you’re using 250 volts. Edison’s problem was that he owned the patents on DC power generation and distribution and stood to make a lot of money if the country adopted DC as the standard. George Westinghouse, his chief business rival, had bought the patents for AC power generation from Nikola Tesla and was lobbying for AC power. Edison fought against the AC system but eventually lost out to the more efficient way to transmit power from the generation stations to the home.
AC power can be converted from one voltage to another by using a simple transformer. Electricity is generated at the power plant, stepped up using a transformer to hundreds of thousands of volts AC for transmission across the country (via “the grid”), stepped back down again to low thousands of volts to distribute around neighborhoods and stepped down again using transformers on utility poles to 120 volts AC for delivery to your home. System wide the power loss is quite minimal, around 6%, even though you may live hundreds of miles from the nearest hydroelectric dam or coal-fired power plant.
What does this have to do with solar power? It all goes back to something I mentioned earlier – solar photovoltaic cells generate DC voltage. That doesn’t work so well when our house wiring uses AC voltage. That means if we want to use PV panels to run our homes, we need to change the power from DC to AC. Fortunately with modern electronics and circuitry we can efficiently convert from DC to AC using a device called an inverter. An inverter does what a cell phone’s power adapter does but in reverse, it takes the steady DC current coming from your solar panel and changes it to the regular alternating current used in your house. In general inverters are pretty efficient, losing only 1% to 5% of the power during the conversion.
Types of inverters
There are a number of different types of inverters depending on the intended application. The major types found in home installations are:
Off Grid Inverters – As the name suggests, off-grid systems aren’t attached to the local utility company; these are usually vacation homes or buildings in remote locations. Solar panels are used to charge up a bank of batteries and the inverter draws power out of the batteries to generate AC electricity. The battery storage system can come in pretty handy if the off-grid system is expected to be used at night.
Grid Tied Inverters – Here there are no batteries involved. The DC power from the array of solar panels is combined together and passed to the inverter. The output of the inverter is connected to the house wiring and, through the meter, the utility company. If the panels generate more power than the home is currently consuming the balance is sent to the grid; power flows “backwards” from the home to the utility where it is used to power other homes nearby.
Grid Tied Battery Backup Inverters – This is a hybrid of the two previous systems. The inverter is connected to the utility company but there are also batteries being charged by the solar cells. If the power company experiences a blackout the inverter will continue to provide power for the home until the main power is restored (or until the batteries run out)
Microinverters - A relatively new product, a microinverter is as the name suggests – a small inverter. While older solar installations had a single, large inverter (sometimes weighing hundreds of pounds) some newer setups use a small microinverter at each solar panel. The electricity is converted from DC to AC up on the roof and integrated into the household power using standard wiring. Generally microinverters are more efficient and easier to install than single, large inverters.
There will be more articles discussing inverters in the future. After the solar panels, the inverter is next largest piece of a solar installation and often represents 20% of the system cost (compared to 60% for the PV panels) Choosing the right inverter for your project is possibly the most important decision you’ll make during the design of the system.