Commercially available since the 1970s, photovoltaic (PV) technology converts energy from solar radiation directly into electricity using semiconductor materials. It has no mechanical moving parts, so it lasts for decades and requires only minimal maintenance. Photovoltaic projects range from small-scale projects for lighting and pumping to large-scale projects for whole buildings and even utility-scale photovoltaic “farms.”
In general, solar electricity is more expensive per kilowatt (kW) than many other sources of electricity, but it has a number of advantages. Because a photovoltaic system can be located at the user site, it can often offset the full retail electricity rate of the facility, rather than the wholesale power price. In addition, photovoltaic electricity often matches peak demand very well, especially in warmer climates, and can offset peak electricity rates. It is modular and can be installed in any size necessary, with the only limitation being the availability of a sunny roof or ground space. Additionally, photovoltaic technology often qualifies for more incentives than other renewable energy technologies. For example, in states that have a “solar set-aside” in their renewable energy requirements, the renewable energy certificates can be much higher than those provided for other technologies.
A photovoltaic system is made up of several major components including:
- Mounting racks
- Electric panel
- Battery bank (optional).
First is the photovoltaic panel or module. This is a panel of photovoltaic cells manufactured into a discreet system with a power rating of some level, such as 200 watts. Photovoltaic modules come in a wide variety of types and sizes, so a photovoltaic system can be designed to best match the space and load of the project. A photovoltaic module creates relatively low-voltage direct current (DC) electricity.
Modules vary by the type of photovoltaic technology being used and the efficiency of the conversion. All modules are given a power rating based on tested performance at the industry’s standard test conditions. So, if a panel is rated at 200 watts, it means that it produced that amount of power at those particular test conditions. This provides consumers with the knowledge that each manufacturer’s system is rated the same. Most photovoltaic modules have warranties both for defects and for system performance. A typical performance warranty guarantees that the module will produce 80% of its rated power for up to 20 years.
Photovoltaic modules are connected to create an array. The various modules are connected in series and in parallel as needed to reach the specific voltage and current requirements for the array. Combiner boxes are used in solar installations to combine the inputs from multiple strings of modules into one output circuit.
The array has to be secured and oriented in some way. The structures holding the modules are referred to as the mounting racks. The mounting racks can be directly attached to a roof, the ground, or another structure. Roof-top systems can also utilize racks that use weights (ballast) to hold the system in place without requiring any roof penetration. Mounting racks, like all building structures, must withstand wind loads in the 90 to 110 mph range for most areas or as high as 150+ mph for areas with hurricane potential.
The next key component is the inverter. An inverter converts the DC electricity from the photovoltaic array into an alternating current (AC) that can connect seamlessly to the electricity grid. Most facilities are wired for AC, so the inverter plays a critical role. Efficiency of the modern inverter can be as high as 98%. The inverter also senses the utility power frequency and synchronizes the photovoltaic-produced power to that frequency. When utility power is not present, the inverter will stop producing AC power to prevent islanding, or putting power into the grid while utility workers try to fix what they assume is a de-energized distribution system. This safety feature is built into all grid-connected inverters on the market.
There are two primary types of inverters for grid-connected systems: string and micro inverters. Each type has strengths and weakness that help determine the most appropriate fit for different types of installations.
String inverters are most common and typically range in size from 1.5 kW to 500 kW. Benefits of these inverters are that they tend to be cheaper per watt of capacity. A selection includes a large range of output voltages. On the downside, if the inverter goes down, a significant part of production could be lost during the outage. For larger photovoltaic systems, these inverters can be combined in parallel to produce a single point of interconnection with the grid. Warranties are available for all sizes and typically run between 5 to 10 years, with 10 years being the current industry standard. On larger inverters, extended warranties of up to 20 years may be available for additional cost. It is expected that the inverter will be replaced one time during the life of the photovoltaic system.
Micro inverters are dedicated to the inversion of a single photovoltaic module’s power output. The AC output from each module is connected in parallel to create the array. Present micro inverters range in size from 175 watts to 380 watts and can be the most expensive option per watt of capacity. Warranties range from 10 to 20 years. Small projects with irregular modules and shading issues typically benefit from micro inverters.
The next part of a photovoltaic system is the electrical panel and related equipment. All types of photovoltaic systems require switch gear and protections as required by electrical code (e.g. NEC 690) and good system design. This gear may be as simple as a few disconnect switches, fuses, or breakers, with transformers required for higher voltage interconnections.
In some instances, the photovoltaic system may also include a battery bank. The power from the photovoltaic array keeps the batteries charged, and, as needed, provides the building with power from the batteries when there is no connection to an electricity grid or if the grid is unavailable. Batteries can add significant cost to a system and typically have a shorter life than the photovoltaic modules. Photovoltaic systems with battery storage include a charge controller that is either integrated into the inverter or a separate component. The charge controller controls both the DC voltage that is coming off the photovoltaic array and the voltage going into the batteries. To help extend their longevity, batteries require the specific stages of charging produced by charge controllers.
How Does It Work?
When light energy, or photons, strikes a photovoltaic cell, electrons are “knocked” loose from a layer in the cell designed to give up electrons easily. The charge difference that is built into the cell pulls the loose electrons to another cell layer before they can recombine in their originating layer. This migration of electrons creates a charge between layers in the photovoltaic cell. Electrically connecting the positively and negatively charged layers of a photovoltaic cell through a load (e.g. a light bulb) will produce electricity as the electrons flow through the circuit, thus, lighting the light bulb as they are attracted back to the positive layer of the cell.
Photovoltaic cells integrated into a system, or photovoltaic module, create electricity. This energy is then converted through the inverter to be used by electric machines, appliances, lights, and so on.
Types and Costs of Technology
Photovoltaic material and technologies have been growing and improving considerably. The price of photovoltaics has decreased tremendously in the last 20 years, due to increasing manufacturing scale and technology advances.
Crystalline Silicon Modules
Traditional photovoltaic cells are made from silicon crystals. These cells are combined and wired together into a module that is a flat-plate panel which can be used alone or with others. Materials presently used include mono-crystalline and poly-crystalline silicon. Crystalline-type modules are generally the most common, most efficient, and the most expensive of the commercially available technologies. Silicon is quite abundant and non toxic. Crystalline-type photovoltaics have a strong industry base and successful track record of over 45 years in the field. Although photovoltaic modules degrade over time, crystalline-type modules are typically guaranteed through warranties to produce at least 80% of their original power after 20 to 25 years. Crystalline -type modules have a lifespan in the 25 to 30 year range but can keep producing energy beyond this range. Typical overall efficiency of crystalline solar panels is between 12% and 15% (the percentage of the solar energy they convert to electricity).
These types of photovoltaic modules are useful in most applications, and are especially useful where space is limited. In general, the system cost is considered high, and depends on the capacity and other factors such as geographic location and mounting. Potential of further cost reduction is expected. Typical installed cost of a federal roof-mounted system is $6.5 to $8 per watt (as indicated by the General Services Administration (GSA) Schedule, effective April 2010 through November 2011).
Second-generation solar cells are known as thin-film solar cells. They are made from amorphous silicon or non-silicon materials such as cadmium telluride. Thin film solar cells use layers of semiconductor materials only a few micrometers thick. They are flexible and frequently used in building-integrated photovoltaic (BIPV) applications such as roof shingles, tiles, building facades, or the glazing for skylights. BIPV can be well blended into building architecture, providing an additional aesthetic option for designers. These new materials allow for cheaper resources and production, so the cost of the system—as a function of its output—is reduced.
The efficiency of thin-film solar cells is generally lower than crystalline cells—typically in the range of 6% to 8%. As such, thin-film technologies require more space to produce the same amount of electricity as a crystalline-type module. Where space is not an issue, these types of modules can be a more cost-effective option. Typical installed cost of a large-scale ground-mounted system is $3 to $4 per watt.
Concentrating Photovoltaic Modules
Concentrating photovoltaic modules use plastic lenses or mirrors to concentrate sunlight onto a very small piece of high-efficiency photovoltaic material. As a result, very little of the costly photovoltaic cells are used and can reach cell efficiency much higher than flat-plate systems. Concentrating photovoltaic systems are becoming a more cost-effective option for utilities and industry. Furthermore, system efficiency can be as high as 30%. However, because the system must use direct sun beams only, the modules must be moved so that they are always pointed directly at the sun. This also limits the use of concentrating collectors to the sunniest parts of the country. Concentrating photovoltaic systems are not as common in the commercial market as flat-plate technologies such as crystalline-type and thin-film modules. Additional applications are undergoing research, development, and demonstration, and are expected in the near future.
Third-generation solar cells are being made from a variety of new materials besides silicon, including solar inks, conventional printing press technologies, solar dyes, and conductive plastics. These options are technically attractive because they are made of low-cost materials. Manufacturing could also be significantly less expensive than older solid-state cell designs. However, the efficiency of these technologies is expected to be a lot lower than the typical thin film.
When making a decision about whether photovoltaics are a good match for a particular construction project, several factors must be considered. The most important is that a site will not be shaded most of the day (typically from 9 a.m. to 3 p.m. on the winter solstice). Excellent photovoltaic conditions are large open roofs or ground areas with few obstructions. In roof applications, it is important to consider if the roof can handle the added weight of a photovoltaic system (typically 2 to 8 lbs/ft²) and roof orientation to the south if it is not flat. For ground applications, it is best to use previously disturbed land to reduce National Environmental Policy Act (NEPA) compliance requirements.
There are three common types of photovoltaic system design:
- The system is connected to the utility grid and has no batteries to store extra electricity
- The system is connected to the utility grid and has battery storage to provide emergency power back-up
- The system is not connected to the utility and uses battery storage to provide the power needs of the location.
Grid-connected photovoltaic systems without battery storage are the most common type of application. If the system is connected to the grid and has critical loads that need emergency backup power, a grid-connected with emergency battery backup system is recommended. If the site does not have grid power and runs on generators an off-grid system with battery storage can reduce or eliminate generator run time and save on fuel and operation and maintenance costs.
The Federal Energy Management Program (FEMP)’s Integrating Renewable Energy into Federal Construction Guide has more information on assessing renewable energy options, and the following sections describe on-site and use characteristics, economics, potential power production, and operations and maintenance (O&M) requirements.
Photovoltaics will produce energy anywhere there is sun and will produce more where there is a lot of sun. However, economic viability depends not only on the solar resource, but on economic factors pertaining to the site. Economic incentives for photovoltaics, such as tax incentives or state renewable electricity requirements with specific solar targets, increase the value of solar-produced electricity. To maximize the value of the photovoltaic electricity, net metering must be available from the local utility on a grid-connected system. Net metering allows the user to offset the retail rate of their electricity with the production from the photovoltaic system; in effect, running the meter backwards.
Other key economic factors that can improve a photovoltaic system’s viability are high electricity rates or time of use electricity rates that are high during the sunny parts of the day, solar feed-in tariffs, and any other solar incentive.
Assessing Resource Availability
The National Renewable Energy Laboratory (NREL) has developed detailed information on the solar radiation and other factors that contribute to the energy production from photovoltaic systems throughout the United States and in much of the world. The following map shows the United States’ solar photovoltaics resource potential for the more common “flat-plate” technologies such as crystalline-type and thin-film modules. Warmer colors indicate better solar resource.
Typical operating conditions for a photovoltaic module are different from the standard test conditions under which it is rated. In the 1990s, the PVUSA Test Conditions (PTC) were developed to determine how much power a module would produce under more “normal” conditions. Information about the PTC energy production of most modules is available from Go Solar California.
NREL’s PVWatts™ solar calculator determines the energy production and cost savings of grid-connected photovoltaic energy systems throughout the world. It allows homeowners, installers, manufacturers, and researchers to easily develop estimates of the performance of hypothetical photovoltaic installations. Note that PVWatts is designed for flat-plate systems, not concentrating photovoltaic systems.
The PVWatts calculator works by creating hour-by-hour performance simulations that provide estimated monthly and annual energy production in kilowatts and energy value. Users can select a location and choose to use default values or their own system parameters for size, electric cost, array type and efficiency, tilt angle, and azimuth angle. The azimuth angle is the compass bearing toward which the modules are pointed. A system facing true north has an azimuth of 0°, due east 90°, south 180°, and west 270°. In addition, the PVWatts calculator can provide hourly performance data for the selected location.
Photovoltaic systems should ideally be designed and installed with an azimuth within 45° of true south (for the Northern Hemisphere) to maximize electricity production. Panels typically produce the most energy if tilted at an angle equal to the latitude of the location but system design economics may dictate a more cost optimal orientation. In addition, avoid any shade on photovoltaic modules since shade on any single module can negatively affect the output of the entire array.
Using typical meteorological year weather data for the selected location, the PVWatts calculator determines hourly performance data for the system and adjusts for losses—both in production of energy and the conversion from DC to alternating current AC power. Hourly values of AC energy are then summed to calculate monthly and annual AC energy production.
This output is particularly useful in matching seasonal loads to output of the photovoltaic system. Running PVWatts with different scenarios is also helpful in understanding the variations in output from design changes to the system such as size, angle, and orientation.
Concentrating Solar Resource
The next map shows the targeted solar resource potential used for concentrating photovoltaics and other concentrating solar technologies. The map shows only the direct radiation from the sun, as concentrating systems cannot make use of any reflective solar energy.
Concentrating photovoltaics typically require at least 6.0 kWh/m²/day of direct normal solar radiation, which is indicated by the orange color in the resource maps. In general, if the site is in the southwest United States, concentrating solar technologies are feasible and should be given further consideration.
When integrating photovoltaics into construction projects, it is important to consider the following design considerations:
- Provide south-facing roof area (60 to 160 ft²/ kW, depending on efficiency)
- Provide blocking between rafters or other structural support as required
- Minimize roof penetrations of solar installation. Provide roofing materials that accommodate connections on sloped roofs.
- Provide pathway for electrical conduit into electrical room
- Provide footprint or wall space in electrical room for power conditioning equipment and accessories (consider outdoor locations for these)
- Upgrade the panel that will receive solar power to accommodate sum of utility power plus solar power, as per code requirement.
- For example, designers may need to specify a 400-amp panel where the design without solar might have only needed a 225-amp panel. This allows the busbar to accommodate the maximum expected power.
- Integrate measurement points from photovoltaic system into overall building control system
- For example, AC power output.
Once a good solar project site has been identified, there are some general procurement points to consider regarding the financing/contracting mechanism selected for the project. General considerations include interaction with the utility to resolve interconnection issues, compliance with the NEPA, review of the site master plan to confirm that the selected site will not interfere with planned future use, discussion of historic building issues, and ability to connect the system to building automated controls.
Photovoltaic projects often make use of renewable energy project funding to take advantage of tax incentives or other incentives. This can be complex for Federal agencies and will need to be carefully handled in procurement. Procuring Solar Energy: A Guide for Federal Facility Decision Makers provides more information and links to case studies.
Design and installation of a photovoltaic system should be limited to those organizations and individuals with significant photovoltaics experience in projects that are of a similar size and type to the one being proposed. Industry certification of experienced solar providers is managed by the North America Board of Certified Energy Practitioners. Providers with this certification in photovoltaics have undergone rigorous testing and have demonstrated significant experience.
As opposed to typical construction projects where a detailed design is created and then contractors bid on building the design (i.e. design-bid-build), it is recommended that a performance specification be used in a design-build scenario for photovoltaic projects. For this reason, solar developers can use their competitive advantages to create the most cost-effective designs that deliver the required performance.
Operation and Maintenance
Operations and maintenance (O&M) is an important procedure to ensure that a system operates optimally and safely and extends the life of the system components. Typical O&M procedures can be applied to a photovoltaic system. For example, major system components should be scheduled for a regular inspection and maintenance to ensure mounting racks and connectors are tight and there is no sign of corrosion. Facility personnel should fully understand the system operation and safety whether the O&M is performed by the facility personnel or a third-party contractor.
O&M for photovoltaic systems can be done by facility personnel or a third-party contractor as part of a service contract that may come with the system installation package. O&M costs can range between 0.02 to 0.1 cents/kWh. The higher reported costs included maintenance costs for generators in remote hybrid PV systems. Some studies report that O&M costs are well correlated to the system size, so 1% of total O&M costs is expected. For more information, visit PVResources.com.
FEMP’s Integrating Renewable Energy into Federal Construction Guide has more information on O&M procedures.
Photovoltaic modules normally come with an excellent manufacturer warranty for any manufactured defects. A regular visual inspection is a simple way to check for any damage that may have been caused by severe weather. Also, since dust build up on the photovoltaic module surface can dramatically decrease system performance, it is recommended that a module cleaning be performed one to two times per year or as needed.
Besides the photovoltaic modules, the inverter is the second most expensive component of a grid-connected system. It requires regular inspection and maintenance, with inverter maintenance costs likely being the most expensive of all components. It is expected to have to replace the inverter during the life of the photovoltaic system. Regular maintenance can identify signs that the inverter is about to fail, such as excessive vibration and noise, and limit down times from a failed inverter.
Special considerations for photovoltaics system design and installation include interconnection, environmental review and permitting, security issues, and relevant codes and standards.
A photovoltaic system that will be connected to the utility grid must meet interconnection requirements of the local utility. Many states or localities have guidelines that require interconnection of many customer-owned power projects. Some guidelines limit the size of a project that can be interconnected, or place a grid-wide limit on the amount of capacity a utility must interconnect. Photovoltaic projects sometimes have different interconnection standards than other technologies, with the best resource being the local utility for the site. A federal agency should confirm early in the discussion with the utility if it can sign the utility interconnection agreement as there have been some cases where utility indemnification clauses prevented an agency from legally signing the agreement.
Rules also vary for net metering of interconnected systems, including availability of net metering, terms of the tariff, and the size of eligible systems. In many cases, the rules for photovoltaic systems vary from other energy sources. FEMP’s Integrating Renewable Energy into Federal Construction Guide has more information on interconnection requirements and net metering.
Financing of photovoltaic systems often makes use of on-site power purchase agreements, where a third-party owns the photovoltaic system and sells the power to the facility. This is a newer model for utilities, however, and is not yet allowed by each state’s public utility commission.
The Database of State Incentives for Renewables & Efficiency (DSIRE) is a comprehensive source of information on state, local, utility, and federal incentives and policies that promote renewable energy and energy efficiency.
Environmental Review / Permitting
If the project is located on Federal land or uses Federal funding (besides a tax credit), the project must comply with NEPA. The effort involved to comply with NEPA greatly depends on where the project is located on the site and on the project’s scale. The impact of a rooftop system, for example, is typically less than that of a ground-mounted system which may be located in an environmentally sensitive area. Photovoltaic systems mounted on existing facilities typically qualify for a categorical exemption. Photovoltaics added to new construction should rarely add any additional burden to the NEPA process. Consulting with an agency or environmental expert on procedures for implementing NEPA is recommended.
There are various local and national codes and standards for distributed electric generator integration and interconnection with utilities. The following list identifies the existing codes and standards applicable to common photovoltaic installations.
- National Electrical Code Article 690 (Solar photovoltaic systems installation)
- NFPA 70 Uniform Solar Energy Code (Standard for electrical safety)
- ICC, ASCE 7 (Minimum building design loads)
- DOE’s EERE Solar Photovoltaics Technology Basics gives a brief description of how the photovoltaic materials convert sunlight into electrical energy.
- Database of State Incentives for Renewables & Efficiency (DSIRE) provides a comprehensive list of federal, state, and local incentives that promote renewable energy and energy efficiency.
- Go Solar California has information about the PVUSA Test Conditions energy production of most modules.
- North American Board of Certified Energy Practitioners (NABCEP) provides an industry certification of experienced photovoltaic installers. NABCEP was designed to raise industry standards and promote consumer confidence in photovoltaic and solar thermal system installations.
- National Center for Photovoltaics (NCPV) focuses on innovations in photovoltaic technology that drive industry growth in photovoltaic manufacturing nationwide. Formed by the U.S. Department of Energy (DOE) and based at NREL, the NCPV focuses on research and development and increasing U.S. competitiveness.
- NREL’s PVWatts calculator determines the energy production and cost savings of grid-connected photovoltaic energy systems throughout the world. Version 1 allows you to select a location from a map or text list of pre-determined locations. Version 2 allows you to select any location in the United States.
- Procuring Solar Energy: A Guide for Federal Facility Decision Makers provides an overview of federal facility managers and their procurement terms on the process of installing solar electric and solar thermal systems.
- Sandia National Laboratories teams with the U.S. Department of Energy, industry, and academia to improve the performance and reliability of photovoltaic technologies and grid integration.