Designing a solar power system

Photovoltaic Cells and Modules

The solar module (or photovoltaic component of the system) is the heart of the system. It transforms the sun's rays into useable electrical energy.

The solar module (or panel) is comprised of several individual photovoltaic cells connected in series or parallel with a metallic material. The energy produced by a solar module is influenced primarily by the number of cells within a module and how these cells are arranged within the module. When the cells are connected in series, the total voltage is the sum of the voltages from each individual cell. The output current in this configuration will remain the same as that produced from a single cell. When the cells are connected in parallel, the total current is the sum of the currents from the individual cells and the output voltage is the same as that produced from a single cell. Each cell in a module typically produces anywhere from 2 to 5 amperes and approximately 0.5 volts (about the same amount as produced from an ordinary flashlight battery). The cells can be arranged in a module to produce a specific voltage and a specific current to meet your electrical requirements. By multiplying the output voltage by the output current, one can calculate the total electricity produced (in watts). Typically, cells are arranged in a module to produce voltages in increments of 12. Hence, most modules in the marketplace are 12 volts, 24 volts, and even 36 volts. The trend is to higher voltage modules.

Like photovoltaic cells, solar modules can also be arranged to produce a specific current and voltage. By connecting solar panels in certain configurations (called a solar array), one can dictate the current and voltage of the array, thus dictating the electricity the system produces.

The size of your photovoltaic system will be dictated by the amount of daily energy required (loads) and the amount of energy available at your location. A professional supplier can assist you by performing a detailed analysis and preparing a quotation based on the analysis. Using energy efficiently will reduce the cost of your system.

Module Types

Modules are available in different power outputs, frame types, cell technology, life expectancy and efficiency. These factors will determine the best panel to suit your needs. BP Solar has a wide range of high efficiency solar modules to suit virtually every application.

For information on specific BP Solar modules, click here.

Module Colors

Modules can be produced in various colors. This is typically associated with the need to create a distinctive look either for security reasons or for architectural reasons. However, producing modules in a certain color is a custom production effort and requires a volume order in excess of 50,000+ cells for any given color. This also requires extensive lead times to complete the work and additional costs would be included for engineering development and manufacturing adjustments. The typical cost for adding color to cells is 2 to 3 times the price of normal cells (per cell). The color will also result in a degradation of performance over normal cells of about 20%.

How PV Modules Efficiencies are rated in the Factory

PV modules are rated at a well- defined set of conditions known as Standard Test Conditions (STC). These conditions include the temperature of the PV cells (25 C or 77 F.), the intensity of radiation (1 kW/square meter), and the spectral distribution of the light (air mass 1.5 or AM 1.5, which is the spectrum of sunlight that has been filtered by passing through 1.5 thicknesses of the earth's atmosphere). These conditions correspond to noon on a clear sunny day with the sun about 60 degrees above the horizon, the PV module directly facing the sun, and an air temperature of 0 C (32 F). In production, PV modules are tested in a chamber known as a flash simulator. This device contains a flash bulb and filter designed to mimic sunlight as closely as possible. It is accurate within about 3.1%. Because the flash takes place in only 50 milliseconds, the cells do not heat up appreciably. This allows the electrical characteristics of the module to be measured at a single temperature, the ambient temperature of the module/factory. Since this temperature is usually close to 25 C, a minor adjustment corrects output characteristics to the 25-degree standard temperature.

Approximate Energy Efficiency Ranges for Various Module Types

Does PV Work in the Cold?

Yes, very well in fact. Contrary to most people’s intuition, PVs actually generate more power at lower temperatures, other factors being equal. This is because PVs are really electronic devices and generate electricity from light, not heat. Like most electronic devices, PVs operate more efficiently at cooler temperature. In temperature climates, PVs will generate less energy in the winter than in the summer, but this is due to the shorter days, lower sun angles and greater cloud cover, not the cooler temperatures.

Does It Work In Cloudy Weather?

PVs do generate electricity in cloudy weather although their output is diminished. In general, the output varies linearly down to about 10% of the normal full sun intensity. Since flat plate PVs respond to a 180-degree window, they do not need direct sun and can even generate 50-70% of their rated output under a bright overcast. A dark overcast might correspond to only 5-10% of full sun intensity, so output could be diminished proportionately. Indoor light levels, even in a bright office are dramatically lower than outdoor light levels, typically by a factor of several hundred or more. PVs designed for outdoor use will generally not produce useful power at these light levels since they are optimized for much higher intensities. On the other hand, PVs designed for lower light levels like the cells found on calculators are optimized for those conditions and perform poorly in full sunlight.

Aside From PV Modules, What Else Do I Need In My PV System?

Although a PV system can be as simple as a module and a load (such as a direct driven fan), most PV systems are designed to supply power whenever it is needed and so must include batteries to store the energy generated by the PV array. Systems with batteries also need electronic devices to control their charging or limit the discharging of the batteries. Since PVs and batteries are inherently DC devices, large systems usually include DC/AC inverters to supply AC power in standard voltages and frequencies. This enables the use of standard appliances in the system. Otherwise special DC appliances (usually from the RV or marine industry) must be used. On the electrical side, protective devices such as diodes, fuses, circuit breakers, safety switches and grounds are required to meet electric code safety standards. In general, PV systems also require mounting hardware to support and elevate the PV modules and wiring to connect the PV modules and other components together.

Will Tracking Improve The Performance Of My System? How About Using Reflectors To Concentrate More Light On The Modules?

The effectiveness of tracking depends a lot on the climate and the application. Areas with a lot of haze or clouds won’t get much benefit from trackers because the light is scattered. Also, applications where the load is the same in every month will also derive little benefit because tracking doesn’t improve the performance of the system very much under worst case (usually winter) conditions. Under ideal conditions, trackers improve PV output per day up to 40% but they add to system complexity and expense and are not generally as strong as fixed mounting systems. Their use is generally limited to applications where the increased output matches increased demand (such as livestock watering) in drier areas (i.e., the US Southwest).

Reflectors can increase the output of PV arrays somewhat, although their effect is not linear because the increased light intensity causes the module to operate at higher temperatures, which reduces its efficiency. More importantly, the elevated module temperatures and light intensities can lead to premature failure of the module, and for this reason, the use of artificial ref lectors is not recommended and will in fact void the module’s warranty.

How Long Will My PV System Last? Do PV Modules Lose Power Over Time?

In general, the PV modules are the longest lived component of a PV system. Top quality modules such as BP Solar’s MEGA series are designed to last at least 30 years and carry a 20 year warranty. They are designed to withstand all of the rigors of the environment including arctic cold, desert heat, tropical humidity, winds in excess of 125 mph ((200kph),), and 1 inch (25 mm) hail at terminal velocity.

Batteries will at best last about 7 years (high quality industrial types). Smaller sealed units will typically last 3 to 5 years. Automotive batteries are poorly matched to the characteristics of PV systems and will generally only last 12 to 18 months in PV service.

Some types of PV modules (using thin film silicon) have a predictable fall-off in output in the first few months of operation which slows down and stops after some time. The modules’ output from then on is relatively stable. This is a comparatively small effect in current Solarex thin film modules which carry an 80% power warranty for 5 years. Polycrystalline modules such as BP Solar’s MEGA series do not experience this kind of degradation and in fact are warranted to produce 80% of their original minimum power rating for 20 years.

What About Breakage? Don’t Most Modules Contain Glass?

The most reliable, longest lived PV modules use a glass superstrate. For Solarex’s MEGA series this is low iron-tempered glass and is laminated with layers of plastics. This construction is very durable but given a strong enough impact, it will break. If the glass is shattered or punctured the module will eventually fail due to water getting into the solar cells and causing corrosion, It may take years for the module to completely fail (produce no power). On the other hand, if the module is damaged in such a way that the two electrical connections between any given pair of cells are both severed there will be no path for the current and the module will have no output.

BP Solar makes a series of products called Life modules which use an aluminum substrate rather than a glass superstrate. These modules are designed for light weight and ruggedness in applications such as camping and are shatterproof. In a permanent installation however, they will not last as long as equivalent glass front modules. This is because the plastic covering used is not as inert as glass and the aluminum is not as good a match (for thermal expansion) as glass is to the silicon solar cells.

In summary, given enough force anything will break. The most effective protection against vandalism, theft and other catastrophes is property/casualty insurance.

What Should I Look For When Purchasing A PV Module?

An informed buyer will look at a number of items when buying a PV module. First, ask the seller what external agencies have tested, qualified, or otherwise approved the module. In the US, look for a listing from Underwriters Laboratories (UL) and Factory Mutual Research (FM), organizations which certify the safety and performance of PV products. In Europe look for approval by the Commission of the European Communities (CEC). Ask if the module passes the tests established by the US Jet Propulsion Laboratory (JPL Block V) to verify long-term reliability. Find out if the manufacturer regularly qualifies production units (rather than laboratory samples) to international standards.

Next check out the module. Pick it up. Does it have a solid feel? Or does the frame easily twist. Look at the junction box. Is it solidly attached? Can it accommodate standard electrical fittings? Can it take heavy gauge wire? Can connections between modules be made in the box? Will it accommodate diodes and regulators if needed?

Look at the solar cells. Are they perilously close to the module frame (which can lead to electrical breakdown and premature failure)? Are the module bus bars open and well isolated or are they folded behind the cells where they can cause electrical shorts or delamination?

Study the label. Is the actual tested power of that individual module printed on the back, or is there only a generic label? If so, is it clear what the manufacturer’s tolerance is on power (how far below nominal can the power be and the module still be considered within specifications)? Ask the seller if it is not readily apparent. Does the module have enough voltage to charge batteries under all conditions (at least 16.5 volts at maximum power)?

Examine the warranty. Is it vague or does it guarantee a specific level of performance?

Finally, look at the manufacturer. How long have they been manufacturing photovoltaics? Are they an organization likely to still be in business in 10 years? What is their reputation? Have their products proven reliable in many years of operation? Do they have a trained sales force and authorized distributor team to back up their products in the field?

We at Southwest PV Systems invite you to put us to the test. We are confident that you will find our BP Solar photovoltaic modules and systems to be the best in the industry and we hope that the information in this catalog will help you to make PV part of your life.