Educational Resources

Frequently asked questions:

1.) Solar system

What is photovoltaics (solar electricity), or "PV"?

What do we mean by photovoltaics? The word itself helps to explain how photovoltaic (PV) or solar electric technologies work. First used in about 1890, the word has two parts: photo, a stem derived from the Greek phos, which means light, and volt, a measurement unit named for Alessandro Volta (1745-1827), a pioneer in the study of electricity. So, photovoltaics could literally be translated as light-electricity. And that's just what photovoltaic materials and devices do; they convert light energy to electricity, as Edmond Becquerel and others discovered in the 18th Century.

Back to top

How can we get electricity from the sun?

When certain semiconducting materials, such as certain kinds of silicon, are exposed to sunlight, they release small amounts of electricity. This process is known as the photoelectric effect. The photoelectric effect refers to the emission, or ejection, of electrons from the surface of a metal in response to light. It is the basic physical process in which a solar electric or photovoltaic (PV) cell converts sunlight to electricity.

Sunlight is made up of photons, or particles of solar energy. Photons contain various amounts of energy, corresponding to the different wavelengths of the solar spectrum. When photons strike a PV cell, they may be reflected or absorbed, or they may pass right through. Only the absorbed photons generate electricity. When this happens, the energy of the photon is transferred to an electron in an atom of the PV cell (which is actually a semiconductor).

With its newfound energy, the electron escapes from its normal position in an atom of the semiconductor material and becomes part of the current in an electrical circuit. By leaving its position, the electron causes a hole to form. Special electrical properties of the PV cell—a built-in electric field—provide the voltage needed to drive the current through an external load (such as a light bulb).

Back to top

What are the components of a photovoltaic (PV) system?

A PV system is made up of different components. These include PV modules (groups of PV cells), which are commonly called PV panels; one or more batteries; a charge regulator or controller for a stand-alone system; an inverter for a utility-grid-connected system and when alternating current (ac) rather than direct current (dc) is required; wiring; and mounting hardware or a framework.

Back to top

What's the difference between PV and other solar energy technologies?

There are four main types of solar energy technologies:

  1. Photovoltaic (PV) systems, which convert sunlight directly to electricity by means of PV cells made of semiconductor materials.
  2. Concentrating solar power (CSP) systems, which concentrate the sun's energy using reflective devices such as troughs or mirror panels to produce heat that is then used to generate electricity.
  3. Solar water heating systems, which contain a solar collector that faces the sun and either heats water directly or heats a "working fluid" that, in turn, is used to heat water.
  4. Transpired solar collectors, or "solar walls," which use solar energy to preheat ventilation air for a building.

Back to top

How does sunlight affect life on Earth?

All life on earth is supported by the sun, which produces an amazing amount of energy. Only a very small percentage of this energy strikes the earth but that is still enough to provide all our needs. A nearly constant 1.36 kilowatts per square meter (the solar constant) of solar radiant power impinges on the earth's outer atmosphere. Approximately 70% of this extraterrestrial radiation makes it through our atmosphere on a clear day. In the southwestern United States, the solar irradiance at ground level regularly exceeds 1,000 w/m2. In some mountain areas, readings over 1,200 w/m2 are often recorded. Average values are lower for most other areas, but maximum instantaneous values as high as 1,500 w/m2 can be received on days when puffy-clouds are present to focus the sunshine. These high levels seldom last more than a few minutes. The atmosphere is a powerful absorber and reduces the solar power reaching the earth at certain wavelengths. The part of the spectrum used by silicon PV modules is from 0.3 to 0.6 mirometers, approximately the same wavelengths to which the human eye is sensitive. These wavelengths encompass the highest energy region of the solar spectrum.

Talking about solar data requires some knowledge of terms because on any given day the solar radiation varies continuously from sunup to sundown and depends on cloud cover, sun position and content and turbidity of the atmosphere. The maximum irradiance is available at solar noon which is defined as the midpoint, in time, between sunrise and sunset. Irradiance is the amount of solar power striking a given area and is a measure of the intensity of the sunshine. PV engineers use units of watts (or kilowatts) per square meter (w/m2) for irradiance. Insolation (now commonly referred as irradation) differs from irradiance because of the inclusion of time. Insolation is the amount of solar energy received on a given area over time measured in kilowatt-hours per square meter (kwh/m2) - this value is equivalent to "peak sun hours". Peak sun hours is defined as the equivalent number of hours per day, with solar irradiance equaling 1,000 w/m2, that gives the same energy received from sunrise to sundown. In other words, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the sun shone for six hours with an irradiance of 1,000 w/m2. Therefore, peak sun hours corresponds directly to average daily insolation given in kwh/m2. Many tables of solar data are often presented as an average daily value of peak sun hours (kwh/m2) for each month. Insolation varies seasonally because of the changing relation of the earth to the sun. This change, both daily and annually, is the reason some systems use tracking arrays to keep the array pointed at the sun. For any location on earth the sun's elevation will change about 47° from winter solstice to summer solstice. Another way to picture the sun's movement is to understand the sun moves from 23.5° north of the equator on the summer solstice to 23.5° south of the equator on the winter solstice. On the equinoxes, March 21 and September 21, the sun circumnavigates the equator. For any location the sun angle, at solar noon, will change 47° from winter to summer.

The power output of a PV array is maximized by keeping the array pointed at the sun. Single-axis tracking of the array will increase the energy production in some locations by up to 50 percent for some months and by as much as 35 percent over the course of a year. The most benefit comes in the early morning and late afternoon when the tracking array will be pointing more nearly at the sun than a fixed array. Generally, tracking is more beneficial at sites between 30° latitude North and 30° latitude South. For higher latitudes the benefit is less because the sun drops low on the horizon during winter months.

For tracking (structures that follow the sun across the sky by various mechanisms, thereby increasing the energy captured from the sun) or fixed arrays, the annual energy production is maximum when the array is tilted at the latitude angle; i.e., at 40°N latitude, the array should be tilted 40° up from horizontal. If a wintertime load is the most critical, the array tilt angle should be set at the latitude angle plus 15° degrees. To maximize summertime production, fix the array tilt angle at latitude minus 15° degrees.

Using inaccurate solar data will cause design errors, so you should try to find accurate, long-term solar data for your system location. These data are becoming more available, even for tilted and tracking surfaces. Check local sources such as solar system installers, universities, airports, or government agencies to see if they are collecting such data or know where you might obtain these values. If measured values on a tilted surface are not available, you may use the modeled data here. Data for fixed and single-axis tracking surfaces at three tilt angles (latitude and latitude ±15°) are provided. Two-axis tracking data are given also, as well as a set of world maps that show seasonal values of total insolation at the three tilt angles. All data are in units of kilowatt-hours per square meter. This is equivalent to peak sun hours—the number of hours per day when the sun's intensity is one kilowatt per square meter.

Back to top

How long do PV systems last?

A well-designed and maintained PV system will operate for more than 20 years. The PV module, with no moving parts, has an expected lifetime exceeding 30 years. Experience shows most system problems occur because of poor or sloppy installation. Failed connections, insufficient wire size, components not rated for dc application, and so on, are the main culprits. The next most common cause of problems is the failure of electronic parts included in the Balance of Systems (BOS) - the controller, inverter, and protection components. Batteries will fail quickly if they are used outside their operating specification. In most applications, batteries are fully recharged shortly after use. In many PV systems the batteries are discharged AND recharged slowly, maybe over a period of days or weeks. Some batteries will fail quickly under these conditions. Be sure the batteries specified for your system are appropriate for the application.

Back to top

How much electricity does a photovoltaic (PV) system generate?

A 10% efficient PV system in most areas of the United States will generate about 180 kilowatt-hours per square meter. A PV system rated at 1 kilowatt will produce about 1800 kilowatt-hours a year. Most PV panels are warranted to last 20 years or more (perhaps as many as 30 years) and to degrade (lose efficiency) at a rate of less than 1% per year. Under these conditions, a PV system could generate close to 36,000 kilowatt-hours of electricity over 20 years and close to 54,000 kilowatt-hours over 30 years. This means that a PV system generates more than $10,000 worth of electricity over 30 years.

Back to top

What does energy conversion efficiency mean?

Energy conversion efficiency is an expression of the amount of energy produced in proportion to the amount of energy consumed, or available to a device. The sun produces a lot of energy in a wide light spectrum, but we have so far learned to capture only small portions of that spectrum and convert them to electricity using photovoltaics. So, today's commercial PV systems are about 7% to 17% efficient, which might seem low. And many PV systems degrade a little bit (lose efficiency) each year upon prolonged exposure to sunlight. For comparison, a typical fossil fuel generator has an efficiency of about 28%.

We're working on ways to convert more of the energy in sunlight to usable energy and increase the efficiency of PV systems, however. Some experimental PV cells now convert nearly 40% of the energy in light to electricity. In solar thermal systems (like solar water-heating roof panels), efficiency goes down as the solar heat is converted to a transfer medium such as water. Also, some of the heat radiates away from the system before it can be used.

Back to top

2.) Sprayfoam Insulation

When can Spray Foam be installed?

Spray Foam can be installed in both new and existing homes. For new homes, it is professionally installed at the same point in the construction cycle as other types of insulation. That is, it should be installed after the rough plumbing, electrical wiring, and heating and air conditioning ducts, but before the drywall has been installed. In existing homes, foam can be applied to the inside of roofs and under floors as well as inside walls with pour in place, low expansion foam or in wall cavaities when drywall or other interior wall sheathing is removed during major remodeling or renovation projects.

Back to top

What is Spray Foam made of?

Polyurethane which is thermoplastic polymer. Most manufacturers also offer products that include up to 20% renewable content in the form of vegetable oils like soy.

Back to top

How does Spray Foam work?

Polyurethane foam insulation consists of an A chemical (an isocynate) and a B chemcial (a resin) managed by a piece of equipment called a proportioner and then applied under high pressure and at high temperatures. As the two chemicals are combined at the tip of the spray gun, the foam expands, hardens and cures within a few minutes.

Back to top

Where can Spray Foam be placed?

Walls, ceilings, under floors and everywhere else you would expect insulation in your new or existing home.

Back to top

What does Spray Foam adhere to?

Spray Foam completely and permanently adhers to wood, masonry, metal studs and joists, basically to any clean, dry and non-oily surface.

Back to top

Why is Spray Foam better than Fiberglass or Polystyrene insulation?

Spray Foam insulation expands to fill up crevices, and it can be applied to places other types of insulation can’t reach. Spray Foam is also self-adhering, making it possible to spray it under a floor without taking up the floor first, and it is much easier to apply to ceilings than sheets of insulation that must be glued or stapled into place. Also, it lasts longer because it sticks to walls and does not fall off like fiberglass. Lastly, because it is an air barrier, Spray Foam insulation reduces air-infiltration in a seamless and monolithic fashion that is just not practical with conventional insulation products and methodologies; because of these air-barrier characteristics, Spray Foam dramatically reduces the convection transfer of energy and humidity that can lead to inefficient and expensive HVAC performance and condensation damage.

Back to top

How can Spray Foam increase air quality and reduce allergies?

Molds are part of the fungus family and are capable of growth without sunlight. Because [deletion of prepositional phrase here], molds can be found almost anywhere and can grow on almost anything as long as moisture and oxygen are present. [sentence deleted here] Because Spray Foam expands to completely fill any access to air movement through penetrations or seams in the wall or roof assembly, much more effective management of indoor air quality through the mechanical system is possible. Because hot air cannot readily mix with cold air through convection loops inside the wall or roof assmeblies, humidity cannot condense into bulk water . Mold growth is propagated by bulk water in the form of condensation, not humidity.

Back to top

Can Spray Foam be applied directly to electrical wiring?

Yes, Spray Foam can be applied directly to electrical wiring.

Back to top

How does Spray Foam work with recessed lighting?

Recessed lights or other fixtures may require a certain amount of air circulation around them for cooling purposes. In these cases, a box can be built around the fixture with gypsum wall board; then spray foam can be sprayed directly to the outside of the box. In the case of IC (insulation contact) approved cans, most Spray Foam manufacturers recommend that the can only be contacted with a conventional insulation like fiberglass and application of the foam to this fiberglass girdle. Because Spray Foam is most often applied to the under side of the roof deck in the creation of an unvented attic assembly, this condition is only present in cathredral ceiling assemblies where the interior drywall or other wall treatment is applied directly to the roof rafters.

Back to top

How long will Spray Foam last?

Spray Foam projects created 20 years ago show no signs of deterioration. Spray Foam is a product of plastics chemistry and is only broken down by ultra violet light. So long as it remains inside a wall cavity or in an unvented attic assembly and not in direct sunlight, it will remain as stable as the day it was installed.

Back to top

Does Spray Foam contain formaldehyde?

No. The products of modern Spray Foam Insulation chemistry have been tested to demonstrate none of the off gassing characteristics that bankrupted the industry in the 1970s and 80s. Modern Spray Foam chemistry has been successfully operational since the mid 1980s.

Back to top

Will Spray Foam insulation strengthen my house?

Closed cell Spray Foam will have a significant structural stiffining characteristic but that is not the case with open cell Spray Foam. With the application of closed cell foam, wall and roof assemblies will be more wind resistant. But because closed cell Spray Foam is a water barrier, bulk water can become trapped next to other organic building materials such as wood framing during a roof or plumbing leak and be difficult to dry or extract. All modern wall and roof assemblies employ some type of bulk water management methodology such as a residential house wrap but it is important to manage construction of the assembly so that bulk water does not become trapped between double moisture barriers. Closed cell Spray Foam is more expensive than open cell but can be a very effective insulation solution in metal buildings, commerical applications as well as in residential structures in wind storm conditions such as those found in coastal areas.

Back to top

Where is the best place to have Spray Foam in an existing home?

Penetrations in the sheetrock into your ceiling allow hot and cold air, dust, noise and insects into your home. The application of Spray Foam to the underside of the attic roof deck to create an unvented attic that places HVAC ducts and equipment inside the thermal envelope, is the first place to start in an existing home. If the HVAC equipment in the attic includes a gas or propane fueled furnace, one must always consider the management of combustion air that is being pulled from the inside of the conventional vented attic if the furnace has an efficiency lower than 90% AFUE. High efficiency gas furnaces have sealed combustion chambers and do not pull combustion air from the attic but rather from outside by means of a concentric flue that emits combustion gases through the center section of the flue and brings in combustion air through the outter section of the flue.

Back to top

3.) Rainwater Collection

Why Is Saving Water Important?

Nationally, population growth has put stress on available water supplies. Between 1950 and 2000, the U.S. population nearly doubled while the public demand for water more than tripled! Americans now use an average of 100 gallons of water each day—enough to fill 1,600 drinking glasses! This increased demand has put additional stress on water supplies and distribution systems, threatening both human health and the environment.

There's a reason that water has become a national priority. A recent government survey showed at least 36 states are anticipating local, regional, or statewide water shortages by 2013. But by using water more efficiently, we can help preserve water supplies for future generations, save money, and protect the environment.

Back to top

Why collect rainwater?

Rainwater is pure and unlike most well water, it is naturally soft water and has a hardness of zero without the use of a water softener. Soft water requires less soap on and calcium won't collect on your faucets, tiles, glassware and hair. And if you size your system correctly, your rainwater supply is as dependable a well. Unlike most public water supplies, which rely on chlorine for disinfection, rainwater can easily be made safe and potable without the use of any chemicals.

Back to top

How much rainwater can I collect?

Average collection rates are around 550 gallons for every 1000 square feet of collection surface for every one-inch of rain. To estimate how much you could collect in one year, take the square footage of your collection surface, divide by 1000, multiply by 550 and then multiply by the average annual rainfall for your area.

Back to top

How much rainwater do I need?

A reasonably conservative person will use between 25 and 50 gallons a day, excluding any landscaping needs. So, a family of four water-wise souls would need around 40,000 gallons per year. You'll want to store enough water to see you through the worst drought on record in your area. Here in the Texas Hill Country, that's around 75 days without rain, which means that family of four would need to store at least 7500.

Back to top

Is rainwater safe to drink?

Captured rainwater goes through six stages of filters before actually being used for everyday purposes during which no chemicals are added. The water is softer and purer than any ground water or public water system.

Back to top

What about Droughts?

Each system is custom designed for regional rainfall and homeowner demands. Once full, the storage tanks can store enough useable water to last as long as 18 months with no additional rain. The systems are designed to harvest more than enough water, based on the worst recorded drought conditions for that area.

Back to top

What are the Benefits?

No monthly water bill. No use restrictions during period of drought. No water softener is needed.
Less wear on appliances and plumbing fixtures.

Back to top

Learn more about building a better tomorrow: