Solar photovoltaic (PV) is installed as an integrated component of roofing system. PV cell converts light to electricity. But how is solar energy produced & purchased and what are the best solar panel products? These are some of the questions addressed in the article.
The solar cells can be made from a number of different substrates. The most common is silicon. Simply speaking, when light strikes, the surface of a solar cell some of it is absorbed into the silicon. This light energy bumps the electrons loose and causes energy to flow.
The percent of the sunlight absorbed into the solar cell that is converted to usable energy is referred to as its efficiency. For example, if a solar cell is rated at 10% efficiency that means that 10% of the captured energy from the sun is being converted into electricity. If solar PV module ‘A’ is rated at 10% and module ‘B’ is 20% then you would need to purchase twice as many ‘A’ modules to equal the capacity of module ‘B’.
Solar Irradiation is the total amount of solar energy accumulated on an area over time, (i.e. one day) and is often expressed as “peak sun hours per day”. For example, in a given city, the sun may shine 15 hours per day in the summertime. That sun energy equates to 5.63 hours of full sun or sun shining at the “optimal angle” which is considered to be the angle of the sun around high noon. The government authorities have calculated data available of full sun hours by region within each state. This metric is a key factor in determining the economics of solar power in any given area. It is also an essential metric for solar installers who must adequately measure and verify that the output of an installed PV system is consistent with expectations.
Solar panels come in at least three popular forms: rigid panels, flexible “thin” films, and tubular “fluorescent tube” forms. There are basically four different types of technology:
• Crystalline Silicon (c-Si)
• Amorphous Silicon (a-Si)
• Copper Indium Gallium Diselenide (CIGS)
• Cadmium Telluride (CdTe)
They all have advantages and disadvantages, but interestingly, most have a similar installed cost range per watt capacity.
Crystalline Silicon (c-Si) – Typically referred to as “rigid panels” because that is the predominant manufactured form. Rigid panels encased in glass are by far the most common. These panels are typically placed at an angle towards the sun to optimize efficiency. Thinner panels and “building integrated” forms that would affix directly to the roof are now just starting to be introduced at increased cost.
Efficiency: Efficiency is defined as the amount of sunlight that is converted into energy. It can vary greatly among manufacturers. Typical rates are between 10% and 20%. Some (< 5%) of its efficiency output is typically lost during its life, with most manufacturers providing efficiency and power output warranties over the life of the panel.
Advantage: This is the most proven solar PV technology, having been introduced over thirty years ago. It has the highest efficiency vs. the other commonly used products, which basically translates into less real estate required to generate the same capacity of watt power.
Disadvantage: A disadvantage of c-Si vs. the thin film products, can be their weight. A structural engineer inspection of the building is mandatory to ensure that the structure can handle an additional 4-5lbs/sqft of roof space.
Shading used to be a major disadvantage of c-Si, whereby if you had eight panels strung together in a solar array and panel number two became shaded, the entire string would have a major decline in solar output. However, that has changed and c-Si and other products are affected by shade in a minor way.
Amorphous Silicon Thin Film (a-Si) – a-Si is available in rigid panels, encased in glass and in a flexible form with either stainless steel or a plastic as the base layer. In the rigid panel form, they can be placed at an angle just like c-Si. Most thin film, however, lays flat on the roof.
Efficiency: Efficiency tends to be in the 6-9% range. Twenty percent of its efficiency is typically lost after the first few years and then it levels off.
Advantage: a-Si is the oldest thin film technology, introduced approximately 10 years ago, with many long-term, real world tests and installation data, but less than c-Si.
Disadvantage: Lower efficiency vs. c-Si. a-Si doesn’t have the track record of c-Si.
Copper Indium Gallium Diselenide (CIGS) – As with a-Si thin film, CIGS can be supplied in a rigid panel, encased in glass and in a flexible form with either stainless steel or a plastic as the base layer. In addition, CIGS is also supplied in a glass “fluorescent looking” tube.
Efficiency: The thin film form’s efficiency is similar to a-Si, in the 6-9% range. In the tube form the efficiency is slightly higher due to the cylindrical surface having more “optimal angle” with the sun during the day and from the reflectivity generated if using a white membrane. Ten percent of its efficiency is lost after the first 10 years.
Advantage (tubes): Given that a reflective roofing membrane may be considered an integral part of the PV system due to its reflectivity benefit, you may be able to depreciate the roofing membrane, which is normally depreciated over 39 years, to that of five years like the solar PV.
Disadvantage: CIGS has not been proven yet, as it is just being introduced to the market.
Efficiency: Efficiency range is 11-14%.
Advantage: CdTe is more proven than CIGS and the cost is very low
Disadvantage: Safety concerns of cadmium have slowed the product’s growth.
Purchasing Solar energy
Solar energy is purchased based on the watt capacity (or peak production) of the system, also known as the “nameplate” or “system capacity”. Costs can vary widely. As an example, a 30,000 square feet manufacturing facility typically has usable solar space of 15,000-20,000 square feet. For space to be considered usable, it must be free of shading from other buildings, mechanical apparatus on the rooftop, or other obstructions. Generally, if 15,000 square feet are available for solar panels, a system with a capacity of 150kW (kilowatts) or greater can be installed. Based on hypothesized energy use of a 30,000 square feet manufacturing facility, the 150kW system may generate as much as 50% of the energy needs of that building. The “avoided electricity cost” that no longer has to be paid because the solar system is generating the electricity, form the basic economics of the financial return of a PV system.
Members of the distribution system include:
• Solar panel manufacturers
• Accessory parts manufacturers
• Solar panel installers
• Power Purchase Agreement Providers
The Integrators are the people who can assess the current situation and make a recommendation regarding the type of solar panel to purchase, how much watt capacity can be installed and how much energy can be generated based on the solar irradiation of that system. The Integrator, typically, will purchase the solar panels and accessories and resell them to client. Most Integrators specialize only in “Solar Integration.” Other companies, including roofing contractors and roofing system manufacturers, are starting to offer integration services.