PV system fundamentals

Photovoltaic (PV) systems are one of the best options for on-site power generation. Commercial buildings must generate power as we move forward with the AIA 2030 challenge, net zero energy, and other energy efficiency programs


Learning objectives:

  • Outline the basic information about solar photovoltaic (PV) systems.
  • Analyze the best application, location, and use for PV systems.
  • Calculate PV panel size and electrical requirements.

The latest solar photovoltaic (PV) market report from Greentech Media and the Solar Energy Industries Association (SEIA) forecasts another record year for the U.S. PV market in 2015, with an estimated 27% growth over 2014. Moreover, growth should accelerate beyond 2015 as the 30% federal Solar Investment Tax Credit (ITC) is set to expire Dec. 31, 2016. It’s anticipated the expiration of the ITC will send the market into a frenzy as consumers rush to take advantage of the benefit. As costs continue to decrease and consumers continue to yearn for PV, the demand will only increase. The engineering community must quickly get up to speed on how to design and specify PV systems.

The sun produces essentially an infinite amount of energy. The Earth receives approximately 170 million GW of power from the sun, which is millions of times greater than global power demand. Solar irradiance is the power of solar radiation per unit of area, measured in Watts per square meter (W/m2). The Earth’s solar irradiance constant is approximately 1,366 W/m2 (nominally 1,000 W/m2). Solar irradiation, or solar energy, is the solar power accumulated over time. The unit of measure is Watt-hours per square meter (Wh/m2). The greater the solar irradiance for a given location, the more energy is generated.

The preceding information is necessary for interpreting insolation, or solar irradiation, maps. Insolation maps are used to rate the solar energy resources of a location. The National Renewable Energy Laboratory (NREL) produces insolation maps for reference.

Figure 1: This showcases the 21.6-kW PV system array at the Oakland University Engineering Center in Rochester, Mich. All graphics courtesy: SmithGroupJJRIt’s important to also understand the sun path or solar window for a given location, which depends on geographical location and time of year. The two angles most important to PV designers are the azimuth angle (relationship to due south) and altitude angle (the sun elevation angle), as they directly impact the design and energy yield. A PV module (i.e., panel) receives the maximum amount of energy when the direct component of solar radiation is exactly perpendicular to the module surface. Most locations in the U.S. will have a steeper tilt in the winter months to align with the lower winter sun and a flatter tilt in the summer for the high summer sun. The University of Oregon has an excellent program for generating sun path charts to analyze sun angles throughout the year for various locations. Because PV modules are generally fixed, the designer is required to make a compromise between summer and winter energy production. The angle is typically chosen to maximize annual energy generation with an angle within 15 deg of the latitude resulting in the best yield.

PV cells and modules

Photovoltaics is a solar technology that uses the electrical properties of various types of semiconductor materials to directly convert sunlight into electric power. The resulting electricity is direct current (non-sinusoidal). The basic physical process is known as the PV effect. When high-energy photons of sunlight strike a semiconductor material and are absorbed, the energy is transferred to the electrons in the semiconductor material. The extra energy excites the electrons to the point where they break free from their associated atom. The free electrons are directed through an external circuit to a load where the flow of electrons (i.e., current) can do useful work. This process occurs continuously as the semiconductor material is exposed to daylight.

Figure 2: A grid-tied (utility-interactive) PV system configuration is shown.

Photovoltaic cells are the basic building blocks of a PV module and are typically made of crystalline silicon material. Many other materials are available for PV cell production, but crystalline silicon dominates the power generation market. Crystalline silicon cells are manufactured through a variety of techniques. The basic steps in the manufacturing process are silicon purification, crystal ingot formation, wafer production, doping, application of electrical contacts, and cell encapsulation.

Monocrystalline silicon cells are made from a single silicon crystal resulting in high efficiency (16% to 21%). Polycrystalline silicon cells are composed of many silicon crystals and have a lower efficiency (11% to 15%). The individual PV cells are combined in series and parallel connections to form modules, often referred to as panels. Series connections are made to build voltage. Parallel connections are made to build current. The power output of the module is voltage multiplied by the current (P = V x I). Standard modules are either 60 or 72 cells and current modules are rated at 260 W to 300 W. Similar to PV cells, modules are connected in series and parallel to increase voltage and current. A group of modules is called an array (see Figure 1).

Figure 3: An off-grid (stand-alone) PV system configuration uses batteries.A PV system is defined as all the components required to allow the system to produce useful power. This includes modules, wiring, conduit, combiner boxes, overcurrent protective devices (e.g., fuses and circuit breakers), disconnect switches, inverter(s), meters, mounting hardware, etc. PV systems can be configured in various ways such as grid-tied (utility interactive) or off grid (stand-alone) (see Figures 2 and 3).

Grid-tied is the most common PV system configuration because it’s the easiest to design and less expensive when compared with a stand-alone system, which relies on batteries. The application of the system will determine the system configuration. Applications for PV systems include space exploration, vehicles, electronics, remote lighting, signage, telecommunications, water pumping, and baseload power. With respect to size classification, residential systems are rated less than 20 kW, commercial systems are 20 kW to 1 MW, and utility size systems are larger than 1 MW.

Figure 4: A current-voltage (I-V) graph shows the effect of temperature on photovoltaic module voltage and current output.Site survey and planning

The first stages of a PV project involve researching the location and surveying the site. It is crucial to know the climatic conditions, most notably the high and low temperatures and solar insolation. The Solar America Board of Codes and Standards has a reliable reference map for obtaining temperature data. Temperature is perhaps the most important factor when sizing the array and balance of system (BOS) components. This is due to the fact that the voltage and current produced by the cells are dependent on the ambient temperature and insolation at the site. That is, the voltage of the module is higher in colder temperatures and the current increases with higher insolation values (see Figure 4).

It’s also important to consider factors such as wind speed, corrosion, severe weather, lightning strikes, seismic activity, and the potential for vandalism and theft. Local building codes and city ordinances, permitting, inspection, interconnection requirements, and financial incentives must also be investigated prior to design. These factors may influence how the system is designed and installed.

The location should be surveyed prior to design to evaluate shading concerns and existing roof conditions. Shading is a huge concern. Even a small amount of shade during the day could have a detrimental effect on the system performance. The age and condition of the roof may warrant a total roof replacement, as the life of the PV system is 30 yrs or more.

Following the initial site survey, array sizing is the next task to consider. At this point, the first question to ask is, “What am I designing for?” The answer will influence how the system is designed. For example, are you trying to maximize annual energy yield? Maximize the kilowatt rating? Are you working within a limited footprint? Are you working with a fixed budget? You won’t be able to design an array layout until you know the objectives and constraints.

Most PV modules are manufactured to a standard dimension around 66x38 in. By using software such as Autodesk AutoCAD or Revit, you can quickly create different layouts using portrait or landscape module orientation, different tilt angles, etc. This is a trial-and-error exercise where the goal is to determine the most efficient layout while satisfying your constraints and objectives. There will be several iterations before you arrive at a preferred layout. You must also calculate the row-spacing distance to avoid inter-row shading. You can use trigonometry to hand-calculate the spacing, or use an online calculator. The approximate rule of thumb for spacing between rows should be 2½ to 3 times the height of the tilted module.

Your final PV array layout provides the total quantity of modules, which will give the dc power rating for the system. The rule of thumb for array size based on area is 8 to 10 W/sq ft. A useful tool for estimating annual energy production is the PVWatts calculator from NREL. The PVWatts calculator allows you to quickly estimate your production based on location, tilt angle, and other factors. More sophisticated designers can use a program called PVsyst for PV-system modeling.

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