PHOTOVOLTAICS: Taking in the Sun

Governments are supporting renewable energy technologies like photovoltaics for several reasons. First, a growing proportion of voters is changing their attitudes about sustainability, moving from asking, “Is this a problem? to, “What can we do about it?”

As well, the economic gap between renewable energy technologies and “old” technologies is closing, while the supply of petroleum energy resources is becoming less secure.

Meanwhile, environmental policies are becoming global in scope. Reducing CO2 emissions is a major concern, as nations look for ways of meeting their international obligations to reduce greenhouse gases.

Photovoltaics currently account for less than half of one per cent of the world’s power generation. However, the global business volume for this solar technology has doubled over the last three years, while costs are dropping. The expansion of photovoltaics is concentrated in regions where energy is expensive, such as California, Europe and Japan.

In the U.S., more than 200,000 homes have photovoltaic power, mostly due to government incentives. The federal government offers grants that amount to about 40% of the photovoltaic investment. Canada has no national program of this scale, but various sources of funding are available for special types of projects.

What are photovoltaics?

Photovoltaic (PV) modules convert sunlight into direct current (DC) electricity. The basic technology has existed for over a century but has advanced to the point of being commercially viable over the last 40 years. PV modules are common in calculators, parking ticket dispensers, communication tower sites and remote cottages.

A PV module is an array of cells connected in series and encapsulated for protection from the environment. Each cell consists of two electrode layers on either side of a photoreactive semiconductor. One layer is an electron donor, and one is an electron acceptor. When light photons strike the electron donor side, electrons jump across the junction to the electron acceptor side. This action creates the voltage potential required to drive a current through the load — either a battery or an AC circuit — connected across the cell.

A PV system contains one or more modules and other components, termed “balance of system.” These can include:

inverters (to convert the DC supply to AC for grid-connected systems)

batteries (to store electricity produced in off-grid systems for later use)

controllers (to manage energy storage and deliver power to the load)

electrical connections and meters

a structural attachment (to mount PV modules and other components).

The power used to produce photovoltaic modules is recovered in about two years, leaving more than 20 years of net energy recovery with negligible cost. It is a technology that has great promise for addressing energy demands in buildings without emitting noxious substances or disrupting ecosystems.

Crystalline cells or thin film

Currently, commercial semiconductors are based on silicon semiconductor technology. The cells used in buildings are either crystalline silicon, or amorphous silicon (thin film).

Crystalline cells are assembled in flat panel arrays contained in a frame. The cells contain solid silicon wafers, laminated onto glass or plastic. The wafers are produced by growing large silicon crystals or by casting molten silicon. They are then treated to make them either an electron donor or acceptor wafer.

Crystalline cells are usually black, dark blue or purple. Custom colours are possible, but decrease efficiency. A typical crystalline PV module is 1.5 ft. by 4 ft. (6 sq. ft.), and generates about 12V of direct current.

In thin-film technology, photo-active materials are vacuum-deposited onto a substrate material, such as glass, steel, or plastic. The film can be applied to a variety of products and shapes, which gives it a range of architectural applications.

There are three types of thin-film technology currently available, based on the semiconductor material: amorphous silicon (a-Si), cadmium telluride (CdTe) and copper indium diselenide (ICS). Amorphous silicon has been available for many years; the others are just coming on the market.

These modules do not have mechanical connections between cells, as are required for crystalline modules. Most thin-film products are dark charcoal to black, and resemble black architectural glass. They are available as roof shingles, metal standing seam systems, skylights, sunshades and insulated glass units. Glass panels can be laser-etched to allow light transmission.

Research continues on both existing and new products to make PV more competitive with other technologies. In Canada, for example, ATS/Matrix Solar of Cambridge, Ontario is piloting “solar spheral” PVs. This technology is based on metallic beads embedded in a thin aluminum foil, creating very flexible cells that can be applied to a wide range of surfaces. They are expected to be less expensive to produce than silicon-based cells.

Integrating PV into the building envelope

PV power can be produced in large arrays and fed into a grid, or it can be generated in distributed arrays, located close to the demand. This article addresses distributed generation, which has benefits over central plant systems because it eliminates loss in the transmission line, does not require new transmission infrastructure, and uses the building structure to avoid the cost of a separate support system.

Over the last 10 years, there has been considerable activity in developing panels incorporated into the building envelope, called Building Integrated Photovoltaics (BIPVs). Systems can be added to an existing building envelope or integrated into the envelope as an element in the weather protection system. The U.S., for example, has a program in place to encourage up to a million homeowners to install PV systems in their roofs, and Germany and Japan have similar incentives.

There are three major techniques for mounting roof arrays.

Standoff systems require a separate membrane below the array and are installed on sloped roofs. They can be used on new or retrofit roofs.

Roof integrated systems act as part of the building envelope. Roof BIPV comes in architectural and structural standing seam, shingle, and panel formats.

Sloped arrays for flat roofs are fundamentally the same as standoff systems except frames raise the array farther away from the roof and provide slope for the panel.

Using thin film technology, PV can be added to insulating glass units, vertical or sloped glazing. The film can be laser-etched to allow light transmission. With crystalline systems, gaps can be left between individual modules to allow light transmission in proportion to the gap size.

Several PV installations have the modules mounted on sloped surfaces above windows to create sunshades. This strategy helps to reduce the building’s cooling load and control glare, thus adding to the benefits of the photovoltaic installation.

There are issues to overcome when installing PV on buildings. For example:

Fully developed standards do not exist, so building departments, utilities, trades and inspectors must be committed to the PV concept.

Inverters need to prevent a condition called “islanding;” this occurs when the system continues to produce energy during a power failure on the grid. Unless controlled, lines that would otherwise be dead are actually live and create a hazard for utility workers. Most modern inverters resolve the problem, but utilities generally look for an additional isolation switch.

Older systems can lose efficiency over time, but more current systems have proven to be reasonably stable.

Efficiency and costs

PV modules are rated based on electrical output, measured in “peak watts” under full sun in standard testing conditions. The potential amount of electricity produced by a PV panel depends on the building location and the panel’s efficiency and orientation.

Crystalline cells use a relatively large amount of silicon, so that while they are the most efficient at converting sunlight into electricity,
they are also the most expensive to install. By using less efficient amorphous silicon in thin layers, capital costs per square metre are less, but a larger array is required for the same output. According to various references, the following data (see table below) apply for installations in 2001.

Until there is a larger base of installed PV systems in Canada, full costing will be difficult to establish. Natural Resources Canada’s Energy Diversification Research Laboratory gives a cost of between 30 and 60 cents per kWh, but this would typically be for relatively small systems.

At present, the efficiency ratios of crystalline cells mean they have the lowest costs per watt, but extensive research is being done to increase the efficiency of amorphous silicon cells so that their lower capital cost can be converted into lower production costs as well. Laboratory efficiencies of about 18% can now be achieved.

Different geographic regions receive different amounts of solar radiation, depending on their latitude and cloud coverage. Urban centres in Canada are much farther south than most of Europe, where PV systems are in widespread use.

The maximum annual solar harvest on buildings north of the equator is achieved with south-facing modules set at a tilt from the horizontal equal to the site latitude less 10 to 15 degrees. The system designer will modify the orientation to maximize electrical output at particular times or seasons. The panels should be situated to obtain the most power when the owner is paying the most for electricity.

If one measures payback only in terms of electrical output, there is little basis for PV being widespread in grid-connected locations. For example, a standard 10W peak PV module installed on a vertical, south-facing wall in New York generates about 11 kWh per year. If one kWh costs 10 cents from the utility, and the BIPV system costs $10/W installed, it would take about 100 years of electrical output to compensate for the system’s initial cost. The system would not likely last this long, so the cost would never be recovered.

However, the analysis must consider other things:

For BIPVs, there are capital savings involved in reducing or eliminating the material and installation charges for the materials displaced by the PV modules. To the extent that PV can also make the building special, it may reduce the need for architectural features that are often added with no payback calculation.

Government or utility incentives often offset some of the costs or increase the revenue from energy produced.

BIPVs can be a highly visible public statement of environmental responsibility, with associated benefits to the owner’s image that transcend building construction cost calculations.

The installation cost for photovoltaic power is still several times more than the current cost for hydro-electric, fossil fuel or nuclear production. However, until lifecycle costing becomes a requirement for power projects, and until the true subsidy costs for fossil fuel power are worked out, it is difficult to compare the real costs of benign power with non-renewable, environmentally disruptive sources.

In the meantime, PV will be adopted by those building owners who want to be, and want to be seen as, a part of the solution process for global warming, environmentally sustainable development, and technological innovation.CCE

Peter J. Halsall, P.Eng., is a principal with Halsall Associates of Toronto.

Resources:

Neu, J. and Martel, S. Connecting MicroPower to the Grid. 2001. www.micropower-connect.org

World Photovoltaic Module Price & Cumulative Shipments 1980-1999

Source: Solar Design Associates, Cambridge, MA