Making Photovoltaics Work

To work its best, a complete photovoltaic system requires both proper installation and a way to transfer the electricity to an end use. This requires mounting structures that help arrays gain the best tilt towards the sun and intermediary technologies that condition the electricity for consumption. These elements are called balance of system components because they help in matching a photovoltaic array to its installation site and use.

Installing an Array to Maximize Efficiency

A primary consideration in installing a photovoltaic array on a building is the availability of solar energy in the space where the system will be mounted. As solar cells are connected within panels and as panels are connected to each other in the array, any shade from a tree, building or other structure that falls on a cell or panel can significantly reduce the efficiency of the entire system. For this reason the majority of arrays are installed on roofs where they can receive unimpeded solar energy throughout the day.

A second consideration for installations is the angle at which the array is mounted. As explained in Using the Sun, solar energy does not reach the Earth at the same angle throughout the day or in different parts of the world. In the Northern Hemisphere, the summer sun is almost directly overhead, but, as the Earth tilts away from the sun in the winter, the sun follows a path lower in the sky and towards the south, causing solar energy to reach the Earth's surface at a much more acute angle.

While the sun's angle changes throughout the year, our need for electricity does not change very much. To allow for the breadth of angles of solar energy, photovoltaic systems are typically mounted at an angle that accommodates both the high summer sun and the low winter sun, maximizing its efficiency at all times of year.

As a rule of thumb, photovoltaic panels that best accommodate the range of solar angles in a particular location are tilted at an angle equal to the latitude of the location. In Massachusetts, this latitude is around 42 degrees north, so photovoltaic systems in the state should ideally be mounted at an angle of 42 degrees and face due south.

While a photovoltaic system can operate without directly facing the path of solar energy, the closer it comes to meeting this path, the more efficiently it works. However, this efficiency is often traded off with the additional cost of certain mounting structures.

Flat Mounting

Flat mounting is the simplest way to install photovoltaics on a roof. In this situation, photovoltaic panels are simply arranged in an array and mounted to the roof using direct attachments or a weighted framework to make the system resistant to the wind.

While efficiency is diminished, the system is still relatively effective and can be an attractive choice for commercial or office buildings that want to install large arrays at minimal cost.

Flat mounted systems can also be installed on slanted roofs, more typical on residential buildings, which keeps installation costs down while gaining a tilt closer to the region's ideal angle.

Rack Structures

Rack mounting systems allow more control over the array's angle. These systems rely on a simple metal frame that supports the array at the desired angle toward the south. Rack systems are best used on buildings with flat roofs or on the ground, as even a slightly tilted roof can sometimes make installation difficult.

Pole Mounting

Pole mounting is used similarly to rack mounting but supports the photovoltaic array on a pole mounted in the ground. These systems are most often used in rural locations or locations where the best sunlight is not near a building.

Tracking Structures

Tracking structures literally track the sun's angle as it changes throughout the day and year. Two types of tracking structures are available: one-axis and two-axis. One-axis trackers follow the sun from east to west as it passes through the sky and still need to be mounted at a 42 degree angle facing the south. Two-axis trackers can track both the sun's daily course and its changing path throughout the year.

While these systems are the most effective in capturing direct sunlight as its angle changes, they also require more expensive, high-maintenance components than other mounting structures. They are typically reserved for technologies like photovoltaic concentrator systems which depend solely on direct sunlight to function.

Connecting an Array to a Load

Because photovoltaic technologies rely on the sun, their energy production changes with the availability of solar energy. To ensure that a photovoltaic system can provide electricity when it is needed, additional components are needed to either temporarily store electricity for later use, or to connect the array to a building that has an alternate power source, like the local utility, available when electricity from the array is not.

Another factor complicating connection of an array to a building is that buildings use electricity in a different form than the electricity provided by a photovoltaic array. The electricity from photovoltaic arrays travels in a direct current (DC current) while buildings are structured to rely on alternating current (AC current). To make photovoltaic electricity usable, it needs to be transformed from direct current to alternating current and its flow needs to be controlled as it joins the currents used in different buildings.

There are several different ways to structure a photovoltaic array in relation to its load. The most straightforward is a direct connection, or direct-coupled system which connects the direct current to an end use. These systems are useful for small scale daytime applications like water pumps and ventilation fans, but because of the complicating factors mentioned above, most applications require several additional components.

Utility Connected Systems

Increasingly, the most practical way to use photovoltaics is to connect it to a building that is also served by its local utility. In this arrangement, the photovoltaic system provides a certain amount of the building's electricity and the rest is provided through the utility. Other terms for this arrangement are grid-connected or utility-interactive systems.

At night, when the photovoltaic system is not in operation, all electricity comes from the utility. During the day, particularly in the early afternoon, the photovoltaic system can provide most or all of the electricity needed. In some cases, the photovoltaic system will produce more than the building needs. When this happens, electricity can actually be fed back to the utility, gaining a credit on the building's electric bill and ultimately earning money on the extra electricity produced. This approach is called net metering.

Utility connected systems do not require many additional parts, though a device that can translate DC current into AC current is needed. This device, called an inverter, receives the DC current from the array and translates it into AC current which is then fed to a distribution panel. This panel combines the electricity from the array with the electricity provided by the utility and distributes it to the load. In cases where net metering is also used, a special meter needs to be connected to this system.

Battery Storage Systems

Battery storage systems can be used in places where utility connected systems are not an option. In this arrangement, all the electricity produced by the photovoltaic system is fed through a battery, which transfers electricity on when it is needed and stores it when it is not. Battery systems can also store electricity well after the sun has gone down, allowing the system to continue powering an end use during the night.

Battery systems require an additional component called a charge controller to regulate the quality of electricity flowing from the photovoltaic array to the battery. This charge controller can serve a dual purpose in channeling electricity to both the battery and a separate DC electric load.

Hybrid Systems

Hybrid systems are a less common but equally functional method for ensuring continuous electricity. A typical system combines photovoltaics with wind or gas power and can also connect to the utility for any remaining electricity needed.

This arrangement uses the same structure as a battery system but introduces a rectifier, which works the opposite way from an inverter. This rectifier translates the electricity from a utility or other AC power source into DC current to be fed into the battery. The battery and other components then perform the same functions as in a battery storage system. As additional backup, utility power can still be provided directly to the end use.

Photovoltaic Installation Guide

We provide detailed information on solar installations for homeowners and homebuilders in our catalog of How To Guides. For information on current residential installations in Massachusetts, see our section on Solar Energy in Massachusetts.

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