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AC/DC – Edison vs. Westinghouse

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

Thomas EdisonWhy 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 Transmission Lines 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 InvertersHere 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.


Santa Rosa: A Solar America City

When I went to Santa Rosa City Hall today to pick up my “Solar PV Rooftop Installation” permit, a plaque placed at the entrance caught my eye.  It seems that Santa Rosa is one of 25 cities in the US that’s participating in the Department of Energy’s Solar America Communities project!

The DoE is coordinating with leaders at a local level to encourage the adoption of renewable solar energy across the country.   Many of the 25 cities are much larger (most have NFL or NBA teams) but Santa Rosa and Sonoma County are doing their part to lead the way.  According to the website, the city was placed at #5 in the nation’s top ten green cities in National Geographic’s Green Guide 2006.

To help encourage photovoltaic installations, the city has streamlined the permitting process and conducted special training for building inspectors to familiarize them with the technologies and applicable building codes.  They’ve also established a Clean Energy Advocate to help homeowners with conservation measures including Solar PV and Solar hot water systems.  The County of Sonoma has set up a special development fund to help homeowners make their homes more energy efficient.

Hopefully this program will spread to more cities across the country!


Video: Soldier’s experience guides solar decision

Many homeowners make the decision to go solar based on financial issues or environmental beliefs.  Veteran Patrick Padilla’s experiences serving in Iraq convinced him that installing solar panels on his house would help the country’s move toward energy independence.

See the video on CNN/Money.com


Article: Solar costs continue to fall

Solar costs are continuing to fall, as reported on Bloomberg News.  Costs of industrial arrays are projected to continue falling to $1.45 a watt in the next ten years, prompting further growth in the industry.  Manufacturing output continues to grow, which will only mean lower prices and more choices for consumers.

Read the article here.


Hire a contractor or D.I.Y.?

Today I went down to the city planning office to submit my application for a permit to install the solar system on our roof.  I was more than a little surprised when they commented “Gee, you’re the first person to get an owner/builder (D.I.Y. without a licensed contractor) permit for a solar installation.”   Really?  I live in a pretty large city (150,000 residents here in Santa Rosa) and I see plenty of solar panels on neighboring rooftops.  It seems like there are one or two houses on every block with a PV installation.  Since you can’t install a solar power system without a permit (if you hook it up to the grid it has to be inspected by both the city and the utility) that means the hundreds of nearby installations were all done by contractors.

Is it really that difficult?

Well, yes and no.  There are a number of skills required to install a rooftop solar system but we’re not talking about rocket science here.  The rewards can be high if you do the job on your own.  Besides the pride in doing the task yourself you’ll also save yourself quite a bit of money in the process.  Here are some factors that you need to consider:

Home repair skills – If you’ve done other improvement projects around the house, regularly watch the D.I.Y. channel or HGTV and if you know you way around a couple of basic power tools, you probably have the skills to do most of the work yourself.  While some of the wiring is best left to a licensed electrician, the bulk of the work requires nothing more sophisticated than a power drill and a ladder.

Project management skills - This job is going to require planning and paperwork.  Lots and lots (and lots) of paperwork.  Many cities are working to encourage renewable energy installations by cutting fees and permit requirements but you’re still going to need rudimentary drafting skills and you’re going to need to submit plenty of documentation about the installation to both the city and the utility.

Research skills – Unless you’re already a solar installer there’s a lot to learn about putting a bunch of panels up on your roof and plugging them into your power meter.  From product selection to building codes you’ll end up doing a lot of homework before you ever climb a ladder.  Fortunately there are plenty of helpful resources and I’ll be showing you where to find them.

Safety issues – More than likely you’ll be putting the solar panels on your roof.  How steeply sloped is the ‘pitch’ of your roof?  How high off the ground is the roof?  If you’re working on a one story building with a gentle slope then you’re in luck.  If you’re working on a multi-story building with a steeply sloped roof, unless you’re a mountain goat you may want to think about  leaving the job to professionals.

Financing the project – Many contractors can also arrange financing for the project.   If you can’t pay for the system out of pocket and if you don’t want to set up the financing yourself, hiring a contractor could still be an option.  Believe it or not, there are now some companies that will actually lease your rooftop and install a solar system for free  – the savings are much lower but there’s zero up front cost.

I’d describe a do it yourself solar setup as “straightforward but complicated.”  When you break things down into individual parts, each step of the project will be pretty easy.  When you put it all together there sure are a lot of individual pieces to figure out and keep track of.   Knowing what the steps are and learning how to do them correctly is the name of the game here.

If you’ve made it this far through the article you’re probably still thinking of doing the job by yourself – good for you!  This may be the most rewarding home improvement project you ever get involved in.   You’ll be feeling good when you see how you’re helping the environment by installing a renewable source of energy and you’ll really feel good when you open up that electric bill and the utility company says “Hey!  We owe you money this month!”