What is photovoltaics (PV)?
Photovoltaics is the process of conversion of light into electricity. Photovoltaic cell is a device that converts light into electricity. Photovoltaic cells were first developed in 1954 by Bell Labs but cost of producing electricity from the device was prohibitive. The space industry is credited for developing the technology and making it usable. Today photovoltaic panels are used extensively in consumer products & building industry.
Sunlight is free and abundant. PV is one of the sustainable means to supplement our growing energy needs and reduce carbon emissions and nuclear waste/hazard from fossil fuel based and nuclear power plants.
There are several types but for the purpose of this article we'll look at the following:
Crystalline Silicon: This is the most common and the type you will most likely get for roof top installation. Typical efficiency of a crystalline silicon PV module is about 15%- which means that a PV module will convert about 15% of incident solar power into electricity.
Thin film: Most visible application of thin film photovoltaic cells are electronic gadgets - solar powered calculators or watches. Its efficiency is less than that of crystalline type but its cheaper to produce and hence suited for low power electronics. Thin film cells have also gained popularity in building integrated photovoltaics as it can be 'printed' on glass or roof membrane or fabric and convert an otherwise dormant building component into an electricity generating system.
Hybrid: Though the term hybrid could be used for many systems but I'm pointing to the thin film-crystalline hybrid. its a double layer system with both thin film and crystalline cells that increases the efficiency of the PV modules to a much higher level. If the installation area is limited, designers could look at this option to get higher power generation.
System design & application
PV panels: Photovoltaic panel or module is a system of PC cells connected together and packaged for durability and ease of use. In domestic or commercial installations, PV modules are connected in an array usually on the rooftop or an empty lot. Modules could be fixed or mounted on a tracking system that moves with sun altitude for maximum exposure. Normally efficiency gained by using a movable tracking system is not enough to justify the expnce, hence fixed systems with the PV module mounted at the optimum tilt angle work best. Optimum tilt angle depends on the location and the time of year when you want to maximize output. For a system designed for maximum efficiency thought the year the best angle is your latitude, times 0.76, plus 3.1 degrees.
PV modules produce electricity in the form of direct current (DC) which could be used to charge batteries connected to an inverter as power backup/supplement or feed the building electrical system directly using an inverter. The latter could be considered for office buildings where daytime power usage is the primary concern. 'Grid tie' PV systems where the system is wired to the supply electricity to the utility grid if it's not being used is also feasible in some cities.
Budget, required power and available area are the factors we normally use for concept level planning. If you use a PV system that generates peak 15W per hour per square foot then you know that a 100sq foot installation will give you 1.5KW per hour.
Building integrated photovoltaics (BIPV): PV cells built into roof tiles or glazing are available and could be considered during the design stage to build power generation right into the building envelope. This remains a niche and expensive design option for high profile commercial or institutional buildings. In the case of high-rise buildings the available roof area is insufficient but the large south facing facade surfaces offer a good location to consider glazing integrated PV.
Cost remains a big disadvantage but with government subsidies or long term lease contracts with utility companies, solar power becomes a viable option.
A good PV system is of no use if sunlight is not there. Areas with less sunny days may not be able to justify the expense of installing a PV system.
Photovoltaic technology is here to stay and will remain a viable and sustainable means of producing on-site electricity. PV is one of the most important systems to be considered for net zero buildings.