Photovoltaic (PV) Systems
Photovoltaic System Overview
Photovoltaic (PV) systems are used to convert sunlight into electricity. They are a safe, reliable, low-maintenance source of solar electricity that produces no on-site pollution or emissions. PV systems incur few operating costs and are easy to install on most Canadian homes. PV systems fall into two main categories — off-grid and grid-connected. The “grid” refers to the local electric utility’s infrastructure that supplies electricity to homes and businesses. Off-grid systems are installed in remote locations where there is no utility grid available.
PV systems have been used effectively in Canada to provide power in remote locations for transport route signalling, navigational aids, remote homes, telecommunication, and remote sensing and monitoring. Internationally, utility grid-connected PV systems represent the majority of installations, growing at a rate of over 30% annually. In Canada, as of 2009, 90% of the capacity is in off-grid applications; however, the number of grid-connected systems continues to grow because many of the barriers to interconnection have been addressed through the adoption of harmonized standards and codes. In addition, provincial policies supporting grid interconnection of PV power have encouraged a number of building-integrated PV applications throughout Canada.
With rising electricity costs, concerns with respect to the reliability of continuous service delivery and increased environmental awareness of homeowners, the demand for residential PV systems is increasing. This About Your House aims to inform homeowners of what they need to consider before purchasing a system. The information presented will focus on grid-connected PV systems. To learn more about off-grid applications, consult CMHC’s Research Highlight fact sheet Energy Use Patterns in Off-Grid Houses.
PV System Components
The most critical component of any PV system is the PV module, which is composed of a number of interconnected solar cells. PV modules are connected together into panels and arrays to meet various energy needs, as shown in Figure 1. The solar array is connected to an inverter that converts the Direct Current (DC) generated by the PV array into Alternating Current (AC) compatible with the electricity supplied from the grid. AC output from the inverter is connected to the home’s electrical panel or utility meter, depending on the configuration. Various AC and DC disconnects are installed to ensure safety when working on the systems.
Figure 1 — Components of a PV array
There are two different types of metering arrangements that can be used, depending on the local utility. The first is net metering, depicted in Figure 2. In this configuration, the utility charges you for your net consumption of electricity. When you are producing more electricity than you are consuming, your meter will essentially run backwards providing you with a credit. If you have a large system and produce a net surplus of electricity over the course of a year, utilities generally do not currently pay you for the surplus. Instead, accounts are generally reset to zero after a given period, often on a given day every year.
The second metering arrangement is depicted in Figure 3, where the electricity generated by the PV system is measured by a separate utility meter. This metering configuration is used when the utility pays homeowners a different rate for electricity that is generated than what is taken from the grid. For example, in 2009, the Ontario provincial government started offering 20-year fixed price contracts paying homeowners $0.802 for every kilowatt-hour produced from rooftop systems of less than 10 kW1. These types of contracts, known as feed-in tariffs, are used to accelerate the adoption of renewable energy technologies and are discussed in more detail later.
1 Visit http://fit.powerauthority.on.ca for more information.
With systems configured as in Figures 2 and 3, the system shuts down during power outages. In such a case, inverters are designed to sense the outage and automatically disconnect all power going to the utility meter as a safety requirement to protect utility service employees that may be working on the power lines. So even though you have a PV system, it would not be available during power outages. In order to have backup power, you need to add a battery bank. The whole domestic electrical load is too large to be entirely powered, but some inverters have the capability to continue powering an emergency sub-panel that can be used to provide power to critical loads (e.g. refrigerator, security systems, etc.) in the case of a power outage, as depicted in Figure 4. In addition to a battery bank, this configuration requires a charge controller that is able to effectively manage the batteries charging from the PV system, to ensure their optimal performance and extend their life expectancy. This system is more costly and loses some of the efficiency advantages of a battery-less system.
Figure 2 — Net-metering PV system configuration
Figure 3 — PV generated electricity is individually measured
Figure 4 — Net-metering PV system configuration with emergency backup
System Design Issues
Evaluating Solar Electricity Generation Potential
It is wise to consult a PV professional at the design stage, as most dealers offer design and consultation services. Ensure that the dealer has proven experience in designing and installing the type of system you want.
The first step in evaluating the potential of solar electricity for your home is a site assessment. PV modules are extremely sensitive to shading. Cells within a PV module and PV modules within an array are often connected in series. Think of these cells as forming a long chain, and the amount of current flowing through the chain is limited by the weakest link, i.e. the shaded cell or module. The shaded cell or module will act as a resistor. For example, if one PV module in an array of 20 modules is completely shaded, it can reduce the output power of the entire array by 100%. In addition, given that the module will be acting as a resistor stopping the current flow, it will heat up to the point where it can become damaged.
Therefore, when evaluating different locations to mount a PV array, a shading analysis needs to be performed that will identify when and where shading will occur taking into consideration that during the winter months the sun is lower in the sky and tall objects, such as trees and buildings, cast longer shadows. In most cases, the ideal location for a solar array is on the roof of the house. This alleviates most shading concerns, and its large, flat surface makes mounting relatively easy. However, chimneys and other rooftop projections need to be considered in the shading analysis. Also, the future mature height of nearby trees should be used in the evaluation instead of current tree heights.
Properly aiming modules due south with an appropriate tilt will maximize the solar energy that the PV array collects; however, small variations of up to 15° in orientation or tilt will not significantly affect performance. As a general rule, a tilt angle equal to the latitude of the site will maximize yearly performance. Reducing the tilt by 15° does not affect performance significantly (see Table 1); however, a lower tilt will result in more snow accumulation in the winter. At higher angles, snow generally melts off on its own. At lower angles, snow can accumulate, reducing the power produced in the winter. However, given that most of the yearly output is produced outside winter, snow accumulation will not drastically reduce the annual performance of the system.
In order to assist in assessing the PV generation potential across Canada, Natural Resources Canada developed Photovoltaic potential and solar resource maps of Canada that give an estimated PV electricity production for over 3500 Canadian municipalities. The maps and tables provided present monthly and annual electricity generation per kilowatt of installed PV. As shown in Table 2, Canadian cities have a good solar potential, compared to many cities worldwide. One of our least sunny locations, St. John’s, has more solar potential than cities in Germany and Japan, which are the world leading countries in solar electricity generation.
Table 1 — Yearly PV potential (kWh/kW) at varying tilts
|All south facing
||Yearly PV potential (kWh/kW)
|Latitude tilt -15°
||Latitude tilt +15°
||Vertical, 90° tilt
PV System Sizing
In off-grid PV system applications, the PV array and associated battery banks must be carefully sized to be able to meet the load demands through periods with the lowest solar availability. In grid-connected applications, the presence of the grid eliminates the need to closely match the system size with the year-round electrical loads. For net-metered systems where the utility does not pay for excess electricity generation, the estimated annual solar electricity generation should be less than or equal to the annual electricity consumption as there is no financial benefit to generating more electricity than you need. For systems with a battery bank serving an emergency sub-panel, the battery bank must be sized factoring in the size of the emergency electrical loads, the PV system size, and how long emergency backup power is needed (see CMHC’s About Your House: Backup Power for Your House).
Sizing of grid-connected PV systems can be approached in a number of ways depending on your objectives which could include:
- To maximize PV generation for a given budget;
- To offset your yearly purchased electricity;
- To offset a portion of your family’s carbon footprint;
- To completely take advantage of available unshaded south-facing roof area;
- To reshingle a south-facing roof with PV roofing tiles;
- To improve aesthetics; and/or
- To take advantage of a government or utility incentive.
Table 2 — Yearly PV potential of major Canadian cities and major cities worldwide
|Major Canadian cities and capitals
||Yearly PV potential
|Major cities worldwide
||Yearly PV potential
||Capetown, South Africa
||New Delhi, India
||Los Angeles, U.S.A.
||Mexico City, Mexico
|Fredericton (New Brunswick)
||Rio de Janeiro, Brazil
|Charlottetown (Prince Edward Island)
|Yellowknife (Northwest Territories)
||Washington, D.C., U.S.A.
|Victoria (British Columbia)
|Halifax (Nova Scotia)
||St. John's, Canada
|Vancouver (British Columbia)
|St. John's (Newfoundland and Labrador)
|Source: Natural Resources Canada. (2007). Photovoltaic potential and solar resources maps of Canada.
The three most common types of solar cells are distinguished by the type of silicon used in them: monocrystalline, polycrystalline and amorphous. Monocrystalline cells produce the most electricity per unit area and amorphous cells the least. If you want to maximize solar electricity generation for a given area, then you should select the most efficient monocrystalline PV panels you can afford. If, on the other hand, your goal is to cover a given area at the lowest cost, then you may wish to buy amorphous panels. If you are concerned with maximizing your solar electricity generation for the lowest cost, then it is best to look at the cost-effectiveness of a panel regardless of its technology by examining its cost per rated production:
For example, you want to compare the cost-effectiveness of a 160-watt PV panel from manufacturer A selling at $800, to a 60-watt PV panel from manufacturer B selling for $350. In this case, the more expensive panel from manufacturer A is more cost-effective at $5/watt compared to $5.83/watt for the other panel. Other factors should also be considered, such as the quality of the product. Good quality PV panels have 20- to 25-year warranties, have gone through testing evaluations and bear the appropriate certification labels. Also, some PV panels might be more expensive, but may also be more easily installed and thus less expensive overall. As discussed in the next section, some PV panels are designed to act as roofing tiles or shingles. Although they might be more expensive on a $/watt basis, you also need to factor in the avoided cost of shingles or other roofing material.
Once the PV array is sized, the size of the inverter is determined to maximize the performance of the system. If you plan to expand your PV system in the future, you may wish to oversize the inverter in order to be able to meet the additional demands of the larger system. Adequate wall space to mount the inverter and other associated components is also required in the utility room or next to your electrical panel. Small systems may only require a 0.6 m x 0.9 m (2 ft. x 3 ft.) wall area, while larger systems may require a 1.2 m x 1.2 m (4 ft. x 4 ft.) space. Some inverters are designed to withstand harsh conditions and can be mounted on an exterior wall, therefore not requiring any interior wall space. Alternatively, each PV module can be fitted with its own micro-inverter eliminating the need for one large inverter and minimizing the impacts of shading on the performance of the overall PV array.
If the system has batteries, then a battery enclosure that is vented and protected against freezing will be necessary. Car batteries are not optimal for PV systems as they are designed to deliver a high current for a short period, whereas backup batteries for household applications need to deliver a relatively continuous current over extended periods. Special deep-discharge batteries are best suited. Certain types of deep-discharge batteries release small quantities of hydrogen when being charged and should be kept in a ventilated enclosure, well away from open flames or sparks. Consult your PV or battery dealer to determine the size of battery bank you need, and the installation and venting requirements for your chosen battery system.
PV System Installation
When it comes to installing PV panels on your house, there are a number of mounting options available.
EQuilibrium™ housing strives to achieve a balance between our housing needs and those of our natural environment. Click here to learn more about the CMHC EQuilibrium™ initiative and demonstration homes across Canada.
A number of building-integrated PV (BIPV) products are available, where the PV system essentially becomes an integral part of the building envelope. PV roofing tiles are available and were used on an EQuilibrium™ demonstration home, as shown in Figures 5 and 6.
Another EQuilibrium™ home used a different option where a flexible, thin, amorphous PV panel is applied to a standing-seam metal roof, as shown in Figures 7 and 8. With that system, it is very difficult to distinguish the PV array from the metal roof.
|Figures 5 and 6 — Avalon Discovery 3, an EQuilibrium™ demonstration home in Red Deer, Alberta uses PV roofing tiles
|Figures 7 and 8 — ÉcoTerra™, an EQuilibrium™ demonstration home in Eastman, Quebec uses amorphous PV panels stuck directly on its metal roof
Standard PV Panels Installed on Racking System
Standard PV panels can be mounted together on racking systems that fit on a typical roof (see Figure 9). PV systems convert 5% to 20% of the incident solar energy into electricity, a small portion is reflected, and the rest gets converted into heat. Without dissipating this heat, PV panels heat up and their efficiencies start to decrease. To address this, a small air space is typically left between the PV panels and the roof to allow for air circulation to help cool the PV panels.
Figure 9 — The Now House®, an EQuilibrium™ demonstration home in Toronto, Ontario has standard PV panels mounted on its roof. Now House® is a registered trademark of the Now House Project Inc. used under license.
If you do not have sufficient roof space, a PV racking system can extend beyond your roof, like that on the EQuilibrium™ home shown in Figures 10 and 11. This configuration will experience greater wind loads, which should be considered when the system is designed.
|Figures 10 and 11 — The Riverdale NetZero Project, an EQuilibrium™ demonstration duplex in Edmonton, Alberta has a PV racking system that extends beyond the roof
There are a number of factors that need to be taken into consideration when designing and installing a racking system. You need to ensure that the panels are safely secured to the rack, and that the rack is safely secured to the roof. You may need to get your system certified by a structural engineer. Consult with your installer or your municipality to see what requirements exist in your area.
It is best to select a racking system designed for roofs and to follow the manufacturer’s installation specifications. All roof penetrations for both the mounting hardware and electrical equipment need to be carefully sealed to avoid any water penetration in the future. PV systems can also be mounted vertically on a wall, but will produce less electricity, as shown in Table 1. If you do not have sufficient south-facing roof space but have a large yard, there are a number of pole-mounting options available.
If you are installing a PV system on an existing roof, you may wish to replace the existing shingles, if they have only a few years of life remaining. You do not want to have to take off the PV system shortly after its installation in order to replace the underlying roof. If you are installing a PV system on a new roof that is covered under warranty, you should ensure that adding a racking system with roof penetrations will not void your warranty. Adding a PV system on top of an existing roof can help extend its life, as the PV system will shelter the roof from the elements.
While safe installations of electrical systems are covered under the Canadian Electrical Code, the Canadian Standards Association (CSA) governs product safety. CSA has standards for all electrical components, including solar equipment and all electrical equipment must carry an approval label. Products that are purchased outside Canada may not have undergone the testing process that the same product goes through when brought in by a solar product distributor. It is possible to find good quality PV modules that meet testing standards such as IEC 61215 crystalline silicon design qualification test performance (or the IEC 61646 for thin film modules) and the IEC 61730 (or the equivalent UL 1703) safety test. In addition, inverters have to meet the CSA C22.2 standard no. 107.1-01 to allow their interconnection to the grid. Discuss this with your solar dealer and electrical inspector before proceeding to install these products — often a “special inspection” or extra safety measures will satisfy electrical code requirements.
It is important to remember that PV systems are modular, and can be expanded as energy needs grow or as budgets allow. It is wise to anticipate future needs by purchasing larger or oversized wires, switching gears and controls, so that these components will not have to be replaced to accommodate a larger PV system.
PV components have no moving parts — which keeps maintenance requirements to a minimum. Good quality PV modules are typically warranted for 20 to 25 years, and have life expectancies exceeding 40 years. PV panel efficiency can degrade over the years and warranties