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Published on October 16th, 2007 | by Stephanie Evans


Solar Energy and Photovoltaics: A Primer

Among the most promising technologies that we have in the effort to fight global warming is photovoltaics, or PV.  PV technology employs silicon to produce a voltage when encountering light.  Solar electric power systems have gained widespread popularity over the past quarter century because of their many applications. 

Let’s review some of these applications, as well as information and tools that can help you to determine if PV is an effective alternative energy choice for your home.

Ground-mounted Solar Array

The photo-electric effect of light on silicon was first described theoretically by Albert Einstein in 1905 and was finally demonstrated by Bell Laboratories in 1954.  During the 1960’s, NASA drew from these early developments to field the technology for powering their spacecraft.  Subsequently, PV technology has continued to improve in efficiency.  Although PV’s uses were initially limited to off-grid applications, within the past decade PV has become cost-effective and competitive in the grid-connected energy market.

 Unlike solar water heaters, photovoltaic systems do not utilize heat from the sun, but rather the light photons that cause heat.  Most PV modules are designed with a series of silicone wafers that are soldered together in rows.  Imagine an ingot of silicone that is thinly sliced like cheese.  These slices are treated with doping chemicals and layered with other materials in such a way that when sunlight strikes them, electrons become free to do work.

These newly-freed electrons roll off the silicon wafers, called cells, and are collected and channeled onto positive and negative direct current (DC) wires.  Cells are configured to achieve a desired operating voltage and usable current, usually 12, 24, or 48 volts DC.  Many solar cells comprise a solar module.  Power from solar modules can be used to charge batteries, directly supply electrical loads, or for connection to a utility interactive inverter.

Off-Grid PV

It did not take long for people to realize that electricity produced by the photovoltaic effect had many potential uses.  Most of these involved charging storage batteries so that a load could be serviced whenever needed.  In fact, this was the exact application that NASA envisioned at the outset of the space program, when it decided to power all of its space vehicles and satellites with PV instead of nuclear power.

In the off-grid residential and telecommunications power markets, PV is similarly used to charge batteries which in turn supply power to DC loads or to an AC inverter.  The inverter takes DC battery voltage and produces 120 or 240 volts AC, just like the grid voltage found in most homes.  The AC output from the inverter can supply a standard electrical service panel, and in so doing, becomes a totally independent utility company!

Another of PV’s traditional applications has been remote water pumping.  Homesteaders and small villages in rural locations are able to power DC water pumps directly from PV modules.  These systems may have no batteries or inverters.  Operation is simple: The water pump comes on when sunlight strikes the array.  Systems like this have been donated by charitable organizations to the residents of rural African villages, commonly replacing costly diesel-powered water pumps, and dramatically increasing their quality of life.

The most basic analysis reveals that the money saved in electricity each month by utilizing a PV system would be greater than the monthly payment for a loan taken to install a PV system.

Photovoltaic Cell  

Grid-Tied Applications

Off-grid and rural customers were the backbone of the PV industry for many years, until the introduction of grid-tied inverters in the mid-1990’s.  These inverters do not require batteries, but simply wire into a customer’s main service panel through a regular circuit breaker.  Coupled with net-metering laws, the new inverter technology enabled PV to finally go mainstream.

PV has also found new advocates in the business and commercial sectors.  There are significant incentives for businesses seeking to reduce their monthly electricity costs by utilizing solar technology.  Corporate PV customers are able to enjoy numerous incentives that homeowners can only dream of, such as accelerated depreciation, business tax write-offs, and other tax benefits.  Many megawatts of PV presently are, or are in the process of being installed on commercial and municipal establishments, representing a huge sector of the solar industry.

At present the incentives for homeowners are not quite as attractive as they are for government or businesses.  In California, rebate levels for residential customers have steadily declined over the past few years, even as PV module prices increased.  This trend has effectively precluded a national tidal wave of interest in solar energy, though interest is increasing nonetheless.

Placement of Array

PV systems are generally either roof-mounted or ground-mounted.  Arrays are oriented as close to due south as possible and are angled at or near a 30 degree pitch (in North America or similar latitudes).  For a roof, this is essentially a 6:12 pitch.  Photovoltaic arrays are most efficiently installed on south or southwest rooftops of structures.  Metal racks are secured to the roof framing, with the PV modules fastened to the racks.  This is an efficient placement because the array utilizes dead roof space rather than unused ground space.  The roof’s framing is already in place and provides a secure support for the array.An additional benefit to rooftop installations is that when installed on a sunny roof, the building’s interior is naturally kept cooler by the array, reducing radiant heat gain and helping to maintain more comfortable temperatures during the summer.  This can have the added benefit of reducing the cooling load of an air conditioner, thus saving electricity even as energy is produced.

Sometimes rooftops are not the best option, however.  If there is insufficient south-facing roof space available on a structure, the effects of shading, concerns about roofing materials, or the added weight of an array can make ground-mounted installations a desirable option.  Ground arrays can be oriented to true south and located where there is minimal shading.  These arrays can also be designed more efficiently if they serve a dual purpose, such as providing a shade overhang for a yard (sometimes referred to as a California patio), or functioning as a parking lot shade.  Generally speaking, the installation of ground-mounted arrays is more expensive than that of roof-mounted arrays, due to the necessity of providing concrete footings, a code-compliant structure, and any necessary trenching.

Should I Invest in Solar?

Residential candidates for a PV system are those with large homes and large utility bills.  Screening out potential candidates for home solar systems requires that a financial analysis be conducted to determine what the return on investment would be for the purchase of a PV system.  There are a number of moderately complex factors to consider when conducting such an analysis.

Your electric bill is the logical starting point in making any determination of feasibility.  Look for the monthly kilowatt hours (kWh) and take note.  Contact your utility company to request a copy of your past 12 months of usage.  Ideally a one-page data sheet will provide all relevant usage and billing information for the past year.  Adding together the totals from each month and dividing by 12 will give you your monthly average.

Notice where your utility-assigned "baseline" usage ends, usually around 400 kWh.  After passing your baseline threshold, you enter a new rate tier and pay a higher price per kilowatt hour.  For many customers, Rate Tier 2 gives way to Tier 3, then Tier 4.  Finally there is Rate Tier 5, in which customers are typically paying upwards of $.40/kWh.  For some, there will be a substantial number of kilowatt hours used in this expensive rate tier alone.  These are customers who should think seriously about investing in solar as a matter of financial prudence, and as a way of reducing their environmental footprint.

When a customer’s electric bill reaches $400/month or more, the economic benefits of investing in solar PV become much more clear.  The most basic analysis reveals that the money saved in electricity each month by utilizing a PV system would be greater than the monthly payment for a loan taken to install a PV system.  Playing with some more numbers, one can forecast the return on investment in years until the break-even point.

From the break-even point until the modules no longer work, the power system will produce free electricity.  Modules carry a manufacturer’s warranty for a period of 25 years, but will not degrade significantly for 30 to 40 or more years.  For a PV system with a 10-year payback period, this means that modules will provide anywhere from 20 to 30 years of free electricity after they have paid for themselves.  That is an impressive investment.  

On the other hand, for homeowners whose electric bills are less than $100 per month, a PV system with a net cost of $15,000 may take over 18 years to pay for itself.  For these potential customers, efforts to go green would probably be better focused elsewhere: a solar or tankless water heater, energy star appliances, or other energy efficient measures would be among the most practical investments.

Deciding to "generate" power by harnessing the sun’s rays should ultimately come down to pragmatic realities and considerations.  In the absence of highly attractive government incentives, any decision to go solar must weigh-in various factors.  This article has outlined the primary considerations involved in deciding whether a PV system constitutes a good investment for you.  The final decision to purchase PV for your home should ultimately be informed by: your price per kWh; over-baseline kWh; available rate schedules (time of use or flat rate); viable roof space; budget; available rebates and tax incentives.  Clearly the decision is not a simple one, and the variables amount to a complex energy equation.   But when this equation is correctly solved, a properly sized PV system can result in benefits both for your pocketbook and the environment.

Article Contributors: Kevin Byrne

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