Photovoltaic efficiency keeps improving. SunPower recently achieved 24 percent efficiency with its panels, delivering 70% more electricity than conventional solar panels from the same space over 25 years.
you are thinking of building a new home, and you want it to be as energy
efficient as possible. You’ve heard that the cost of solar panels has dropped
significantly. But you aren’t sure of the next steps. First
the good news -- you heard right! The cost of photovoltaics has dropped
precipitously in recent years, thanks to a rush of imports, improved solar
efficiencies, and bigger economies of manufacturing scale. The installed cost
of a residential system has fallen to a tempting $2.70 a watt, making it very
competitive with purchasing power from the utility.
led to a big increase in residential installations – they nearly doubled from
2014 to 2015 alone. “Things are really looking good for solar,” said Ron
Fergle, of Solart Inc., speaking to a packed room of architects at this year’s
American Institute of Architects conference. It’s
reached the point where producing solar electricity is cheaper in the long run
than buying it from a utility, as long as you are willing to make an upfront
investment. “Photovoltaics are cost competitive with fossil fuels,” says Jason
Jewhurst, a senior associate with Bruner/Cott Architects & Planners, which
specializes in urban housing projects.
much would it cost to buy a system for your new home? Installers say the size
of the typical photovoltaic installation is about 5 kilowatts (kW). At $3 per
watt, that means the system only costs $15,000, half of what it cost five years
ago. A super energy efficient new home, on the other hand, may only need a 2kW
system. That would run about $6000. And that’s before tax breaks. The first is
a federal solar tax credit of 30 percent. States may offer tax
economics of photovoltaics are so compelling that some people design net-positive
homes that produce more power than they consume. In more than 35 states, the
excess power can be sold to the local utility at full retail rather than
wholesale rates. And you can still buy electricity from the utility when you
might need it.
are typically only one part – arguably the most important one – of a system
that powers your home. Most photovoltaic panels produce direct-current (DC) power
that must be converted to alternating current (AC) with a stand-alone inverter.
Some newer panels now come with built-in inverters.
missing piece of this equation has been a reliable battery for storing
electricity for use on a cloudy day. Inefficient lead-acid batteries used to be
the only choice. In the last two years a new generation of deep-charging
batteries – some work with lithium-ion technology, others with salt water –
have hit the market that
can hold four times the electricity and last three
times longer. Acquion, whose battery modules are shown above, employs environmentally benign salt-water technology to produce a deep-charging battery that’s compatible with the new generation of lithium-ion batteries that have hit the market.
solar systems also come with software that tracks your energy consumption and
use. You can come home in the evening, find out how much electricity your home
produced during the day, and adjust your nighttime use accordingly. Real-time
data may remind you to close doors and windows so that conditioned air doesn’t
escape. Some systems tell you how much electricity different home appliances
If you think you are located in a part of the country
where photovoltaics won’t work well due to clouds or industrial pollution,
think again. One of the toughest places to run a solar system in the United
States is Seattle, where storm clouds cover rooftops for much of the year,
Fergle said. But Germany has roughly the same solar insolation levels (or solar exposure) as
Seattle, and it was the world leader in electricity produced by PVs until it
was recently passed by China. The United States is in a way better position to use
photovoltaics than Germany. “The United States has huge untapped potential,”
Where Should I Put the Panels?
make optimal use of PVs, your home must have a clear view of the sun
for most or all of the day — unobstructed by trees, roof gables,
chimneys, and buildings. The best location for a PV system is usually on a
south-facing roof, since in the U.S. the sun is always in the southern half of
the sky. That said, a roof that faces east or west may also be acceptable. If
you can’t use the roof, you could
mount the panels on a pole or a trellis in
the yard, as here at a Deltec demonstration house in Asheville, SC.
with composition shingles are the easiest to work with; slate the most
difficult. PV panels can often be integrated into the roof itself. Some modules
are actually designed as three-tab shingles or metal-roof sections. Using PV panels as an awning can provide both electricity and shade.
What Type of Solar Should I Use?
There are two basic kinds of photovoltaics to
consider. The first employ Crystalline Silicon cells. They are the most
efficient and command 80 percent of the market. If your home will have good
solar exposure – a big roof facing south in a sunny climate -- they are often
the best choice.
may also live longer. They commonly come with a 25-year warranty. The actual
efficiency of the panels ranges from 15% to as high as 24%, depending on the
manufacturer. SunPower achieved a record 24.1 percent efficiency with its
are two types of crystalline silicon cells -- monocrystalline and
polycrystalline. Monocrystalline are more expensive but yield better results.
Thin film photovoltaics, on the other hand, may perform better at lower light
levels, and at high temperatures. Thin film may be a better choice with low
light levels — where the lot is heavily treed, or if the home isn’t oriented to
have much south-facing roof space.
film photovoltaics are made of thin layers of PV material deposited by gas on
backing material. The film is basically rolled on roofs, louvers, or walls.
(Here thin film is part of the roof in an early version of the Future Farmstead project in Georgia.) The
film is flexible, lending itself to application on a wide range of surfaces –
not just the roof. Thin film also may make more sense where space is not
an issue – like on a farm or in a rural setting. Homogenous
in appearance, thin film isn’t as efficient as crystalline silicon at
converting sunlight to electricity, with efficiency ranging between 7 and 13
percent. It also degrades faster and comes with a shorter warranty.
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