SOLAR ELECTRIC: PHOTOVOLTAICS
Say "solar" and most people will think of photovoltaics (PV), the dominant technology in the field. Photovoltaics turn sunlight into electricity, using either silicon (the same type used in semiconductors) or what's called thin-film technology. Typically, when you see a solar panel on a roof, it is a photovoltaic (PV) panel made out of a silicon material and covered with a tempered glass enclosure to protect it from the elements.
Photovoltaic solar panels, also called PV modules, offer a dizzying array of technology choices in terms of cells, panels and configurations. Even the cells themselves can use monocrystalline, polycrystalline, metallurgical grade or a hybrid mix of silicon materials. With so many different manufacturers and so many different panel types, much of the ultimate purchasing decision will be done with the guidance of a trusted professional installer.
Today, the price of the system and the associated cost per kilowatt hour (kWh) of electricity it generates is key. Solar photovoltaics is still an expensive means of generating electricity, especially when compared to coal, which is relatively cheap. Nationwide, retail residential electricity rates averaging $0.13/kWh. The amortized cost of electricity over the course of PV system's lifetime is typically around 18 cents/kWh. On a pure price basis, then, solar can't yet compete with conventional electricity. However, when additional factors are accounted for, a compelling case can be made for solar. Solar PV systems make the most financial sense when:
(1) They are accompanied by government-sponsored incentive programs, which include rebates, tax credits, low-interest loans and grants. For more information, see our U.S. map of solar incentives.
(2) They are deployed to help offset large monthly electricty bills. In many cases, your monthly financing payments will be lower than what you were previously shelling out to your electric utility every month. As a general rule of thumb, solar PV systems make the most sense for individuals and business that are paying a high, per-kWh rate for retail electricity.
(3) They are connected to the electricity grid, or "grid tied." With a net-metering agreement, the PV owner is in many states permitted to sell any excess power back to the utility. As more and more states create renewable portfolio standards (which require that a certain amount of power be generated by renewable resources), more and more utilities will likely be required to purchase renewable energy credits (RECs). By installing a solar PV system, you are becoming equipped to sell RECs to your utility so it can meet state-mandated RPS requirements. While not all states have REC programs in place, there's a good chance the adoption of the REC model will continue.
To get an idea of how much PV panels (also called modules) cost, see the monthly survey posted by SolarBuzz. Bear in mind that these prices are retail prices. In terms of installed costs, you should expect to pay between $3.15 and $4.00 per watt, though this figure will vary depending on your installer, location and project specifications.
For more information, feel free to browse our blog, where we post up-to-date news and commentary about solar PV technology and solar financing.
Thin Film Technologies
Amorphous silicon (a-silicon) cells are the oldest commercial thin-film technology. In contrast to c-silicon, which must be sawed into wafers at great loss of material, a-silicon is directly deposited on a substrate or superstrate. This process produces a continuous random network of bonds, as opposed to the crystalline lattice of c-silicon. This network is less efficient and prone to defects in the form of broken covalent bonds, or “dangling bonds.” These defects can be partially remedied by adding hydrogen to bond to the empty “slots” on the silicon, but the addition of hydrogen causes a premature degradation of the material when it is exposed to sunlight, a phenomenon known as the Staebler-Wronski effect. Thus a-silicon cells are not as efficient as crystalline cells, but are less expensive to produce.
Cadmium telluride (CdTe) cells take their name from the cadmium telluride gas that is used in their manufacture. The band gap (essentially the electromagnetic range at which the semiconductor is most efficient at absorbing energy) of the tellurium-based semiconductor is an excellent match for the solar spectrum. There is also a light-transmitting layer of cadmium sulfide in the cell which admits more solar energy. Those two advances combine the efficiency of crystalline silicon cells with the lower manufacturing costs of amorphous silicon processes. While these cells are still in the early stages of development, the technology is evolving rapidly and holds great promise for lower-cost, high-efficiency cells.
Copper-indium-diselenide (CuInSe2, or CIS) cells, like CdTe cells, are named for the semiconductor used to make the solar cell. They also have high efficiency but are at present too expensive for large-scale commercial production.
Solar Tiles are small photovoltaic cells that look like shingles. While less efficient per square unit of area than a solid photovoltaic cell would be, they are more likely to be permitted by local ordinances and neighborhood associations because of their lower aesthetic profile.
Solar thermal systems are a cousin of solar electric systems. Rather than converting sunlight into electricity, solar thermal systems capture sunlight to make use of its heat energy.
Solar Thermal systems may be broadly split into three categories: (1) Water heating, (2) swimming pool heating, and (3) space heating (or cooling). Each utilizes variations on one simple technology: heating a thermal storage medium with solar energy. This storage medium may be used directly, as is the case with open-loop water heating, or indirectly, as in a heat exchanger.
Solar-thermal systems offer a compelling economic rationale, with relatively low initial capital costs and relatively quick payback. As you'll learn below, there are two types of solar hot-water systems: Active Systems and Passive Systems.
Active Systems are the more complex of the two, using electric pumps and valves to move water around the system and transfer heat. This increased complexity generally is more expensive, but it also offers greater efficiency over the long run. Active systems may also provide overall better performance than passive systems, and are typically a better solution for retrofitting an existing home (active systems are modular and do not need to be installed in a single location).
Active Systems fall into two sub-categories: Closed Loop and Open Loop.
Closed Loop systems, as their name implies, use a closed system of fluid transfer to heat up hot water, while not actually running the potable water through the solar collector system. Instead, the solar collector heats up an anti-freeze mixture that is then pumped through a heat exchanger in a water tank to heat up potable water. These systems are designed for colder climates, where freezing temperatures and harsher conditions exist. They also have higher maintenance costs than some of the other systems.
Open Loop: Open Loop systems are simpler because they just route potable water through the solar collector to be heated and then into the storage tank for household use. Essentially, these systems eliminate an entire set of pipes and fluid transfer, lowering costs and complexity.
Passive Systems generally also come in two flavors: one is Integral Collectors Storage, or ICS; the other, called Thermosyphon, is more widely used. The beauty of the passive systems is their ease of use, low maintenance requirements, and ability to function without any electricity.
Thermosyphon Systems rely on natural convective circulation of water: hot water rises above the cold, and can then be collected in a tank. Cold water in the tank flows back down into the solar collector panel to be heated and circulated. Systems like this have been in use for many years, and can be found in many Mediterranean countries.
Finally, it's important to remember that solar thermal systems are one of the easiest and most affordable ways to incorporate renewable power into your home or business. The upfront costs associated with the purchase of a solar water heat system are considerably lower than those associated with a solar PV system.
Regardless of which technology interests you, one of our certified solar professionals in your area can help walk you through the numerous options.
Concentrated Solar uses reflectors to concentrate the sun’s rays onto a collector. The collector either stores the heat energy or uses a heat engine to convert it into mechanical energy.
The range of applications of this technology is enormous, and varies from the small solar cookers, which are becoming popular in developing countries, to vast fields of utility-scale mirrors, such as those employed by Solar Tres, a concentrated solar power (CSP) project slated for construction in Spain. This installation will focus hundreds of times the sun’s energy on a single collector tower, producing 5 megawatts of electricity using liquid sodium as a heat exchanger. Additionally, it will have the fringe benefit of being one of the very few renewable energy technologies to come out of a James Bond Movie. Take that, Biomass!
Types of Solar Power