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.
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 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.
With systems configured as in Figures 2, 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. 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 PV generated electricity is individually measured
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 Canadian Solar Industries Association (CanSIA) offers a PV Technician certificate program, and graduates have good knowledge of the design, installation and operation of home-sized PV systems. In addition, a number of community colleges across Canada have started to offer programs that cover PV system installations.
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 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 charts 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||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
|Regina (Saskatchewan)||1361||Cairo, Egypt||1635|
|Calgary (Alberta)||1292||Capetown, South Africa||1538|
|Winnipeg (Manitoba)||1277||New Delhi, India||1523|
|Edmonton (Alberta)||1245||Los Angeles, U.S.A.||1485|
|Ottawa (Ontario)||1198||Mexico City, Mexico||1425|
|Montréal (Quebec)||1185||Regina, Canada||1361|
|Toronto (Ontario)||1161||Sydney, Australia||1343|
|Fredericton (New Brunswick)||1145||Rome, Italy||1283|
|Québec (Quebec)||1134||Rio de Janeiro, Brazil||1253|
|Charlottetown (Prince Edward Island)||1095||Beijing, China||1148|
|Yellowknife (Northwest Territories)||1094||Washington, D.C., U.S.A.||1133|
|Victoria (British Columbia)||1091||Paris, France||838|
|Halifax (Nova Scotia)||1074||St. John’s, Canada||933|
|Iqaluit (Nunavut)||1059||Tokyo, Japan||885|
|Vancouver (British Columbia)||1009||Berlin, Germany||848|
|Whitehorse (Yukon)||960||Moscow, Russia||803|
|St. John’s (Newfoundland and Labrador)||933||London, England||728|
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.
(excerpt from CMHC http://www.cmhc-schl.gc.ca)
Canadian Solar Industries Association (February 2010)
Ontario Power Authority Renewable Energy Feed-in Tariff Program (February 2010)
Solar Energy Society of Canada Inc. (February 2010)
Photovoltaic potential near Ottawa