#3 - USE RENEWABLE ENERGY - Photovoltaics - The Basics

Photovoltaics are increasingly becoming one of the more commonly accepted means of providing site sourced renewable energy to the project.

Photovoltaic science is the science of turning energy produced from the sun into electricity. Edmond Becquerel discovered the concept known as the photovoltaic effect in 1839. However, the first positive/negative (p/n) junction solar cell was not created until 1954 at Bell Labs.

PV Production Worldwide

WHAT IS THE SOLAR POTENTIAL OF YOUR SITE?

Although Carbon Neutral design strategies include the use of photovoltaic systems to provide electricity for systems and equipment, you need to find out the solar potential of your site. The links below can be used to determine relative irradiation values for sites in the United States and Canada.

Natural Resources Canada: Interactive Map for PV and Radiation
link to map

NREL Solar Maps:
http://www.nrel.gov/gis/solar.htm

How Do PV Cells Work?
Photovoltaics are solid-state semiconductor devices that convert light directly into electricity. They are usually made of silicon with traces of other elements and are first cousins to transistors, LEDs and other electronic devices. Production of PV has increased dramatically since the initial sustainable design conference in 1987. The efficiency has increased and the costs have decreased.

Solar cells are converters. They take the energy from sunlight and convert that energy into another form of energy, electricity. Solar cells convert sunlight to electricity without any moving parts, noise, pollution, radiation, or maintenance. The conversion of sunlight into electricity is made possible with the special properties of semi-conducting materials.

Sunlight Converted: At the atomic level, light is made of a stream of pure energy particles, called "photons." This pure energy flows from the sun and shines on the solar cell. The photons actually penetrate into the silicon and randomly strike silicon atoms. When a photon strikes a silicon atom, it ionizes the atom, giving all its energy to an outer electron and allowing the outer electron to break free of the atom. The photon disappears from the universe and all its energy is now in the form of electron movement energy. It is the movement of electrons with energy that we call "electric current.”

Sunlight to Electricity: A typical solar cell consists of a glass cover to seal the cell, an anti-reflective layer to maximize incoming sunlight, a front and back contact or electrode, and the semiconductor layers where the electrons begin and complete their voyages. The electric current stimulated by sunlight is collected on the front electrode and travels through a circuit back to the solar cell via the back electrode.

Solar cells are created from a semi-conducting material, usually silicon, that is treated or doped with a controlled amount of impurities such as Phosphorous (N type dopant) and Boron (P type dopant) to form a PN junction. Sunlight energy striking the face of the cell is absorbed by the semiconductor and excites electrons within the cell creating electron-hole pairs (negative and positive charges). These pairs are disassociated by the electric field generated by the PN junction: the electrons (-ve charges) drift towards the N region; the holes (+ve charges) drift towards the P region. The +ve and -ve charges are then collected at the top and bottom cell contacts and create a flow of electricity.

The surface of the solar cells is coated with an anti-reflective layer to provide higher solar absorption which gives them their typical blue or black color.

Solar cells alone cannot produce usable power. They need to be interconnected  with other system components that ultimately serve a specific electrical demand,  or ‘load’. PV systems can either be stand-alone, or grid-connected. The main  difference between these two basic types of systems is that in the latter case,  the PV system produces power in parallel with the electrical utility, and can  feed power back into the utility grid if the onsite load does not use all of  the PV system’s output. When the sun is shining, the direct current electricity (DC) from the PV modules  is converted to alternating current (AC) by the power of an electronic inverter,  and then fed directly into the building power distribution system where it  supplies electric power.

Each of the modules must be wired and connected to the next, to eventually transfer the electrical charge to the inverter. In most cases this is done via thin, flat wires that run through the cells. In the spheral solar application, the silicon balls are embedded into a metal mesh sheet, and this acts to carry the electrical charge. Inverters come in various sizes/types and result in some power loss via the process.

An individual solar cell can vary in size from 1cm to 15cm and produce between 1 and 2 watts. Main types on the market are crystalline and thin film. Cells are combined into modules, and modules into arrays. Arrays are ganged on a surface to provide the amount of power required.

Types of Silicon Cells:
Mono crystalline cells: are made from very pure mono-crystalline silicon. This type of silicon has a single and continuous crystal lattice structure with almost no defects. High efficiency (15%). Energy intensive manufacturing process. Expensive.

Poly- or multi-crystalline cells: are produced using numerous grains of mono-crystalline silicon and have a more irregular surface. In the manufacturing process the silicon is cast into ingots which are rectangular/square in shape. These are cut into very thin wafers and assembled into complete cells. They can also grow this on a substrate. Less efficient (12%). Less expensive.

Mono-crystalline cells tend to be flat black or deep blue in color. Polycrystalline cells have a mottled (like galvanized steel), cobalt blue appearance.

Thin film cells can be amorphous silicon, copper-indium-diselenide (CIS) and cadmium-telluride (CdTe) cells. They are omposed of silicon atoms arranged in a thin amorphous matrix rather than a crystalline structure. Amorphous silicon absorbs light more effectively than crystalline and the  product is much thinner. Cheaper to produce, but with efficiencies around 6%. These modules have a charcoal grey or bronze color and look like low-E coating or fretting when used on vision glass. Other colors are available but, the cost will be higher.

Relative Efficiencies of PV by Type

The Temperature Factor:
Photovoltaics produce heat as a by-product of the process by which sunlight is changed to electricity. They must be installed so that they are vented, as overheating will decrease their efficiency.

Photovoltaics actually work better in cold weather situations. This makes the Northern United States and Canada good climates for their use.

Contrary to most peoples' intuition, photovoltaics actually generate more power at lower temperatures with other factors being equal. This is because photovoltaics are electronic devices and generate electricity from light, not heat. Like most electronic devices, photovoltaics operate more efficiently at cooler temperature. In temperate climates, photovoltaics will generate less energy in the winter than in the summer, but this is due to the shorter days, lower sun angles and greater cloud cover, not the cooler temperatures.

Rain and Snow:
Rain will not adversely affect a PV array system since during periods of rainfall the solar irradiance is already low. Roof mounted PV arrays can become covered with snow in the winter. If the array is covered, it will not work. In snowy climates, sloped arrays are preferred to flat installations as theoretically the sun will penetrate the snow, heat the dark PV layer, melt the base of the snow and it will slide off of the panel. It is important to prevent such snow from piling up at the base of the array, or sliding uncontrolled onto passers by below the installation. Sometimes it is necessary to shovel the array. Care must be taken not to damage it. These sorts of logistics must be carefully worked into the design of the project, its installation and instructions for use and maintenance to the owners.

Dirt and Pollution:
Any factor that reduces light transmittance to the PV surface will reduce the output of the system. If dirt is allowed to accumulate (more likely in urban areas), the output can be reduced by 2% to 6%. The higher value occurs if the slope of the array is less than 30 degrees. The occasional heavy rainstorm is usually sufficient to clean the array. If PV is installed on a wall surface, rain can keep it clean if the array is exposed to such. Otherwise, the surface can be cleaned in the same way as window systems would be. If installed in a dirty environment, the building must be designed to allow reasonably easy access to the arrays for cleaning.

CMHC Healthy House, Toronto, Ontario, Canada
The solar panels that form the large south faciing overhang on this experimental housing is beyond the reach of any ordinary window cleaning equipment. The owners must hire a "cherry picker" to come and clean the arrays to remove environmental pollution.

Shading:
AVOID SHADING THE PANEL. The shaded area will not reduce the output proportionally to the area shaded -- loss is much higher. Within a chain of modules the output will be that of the weakest (shaded) module. If shade cannot be avoided at certain times, be sure to gang the affected modules together on the same circuit, leaving the sunny modules to fully function.

This goes for seasonal shading due to trees or vines, and even the shade from deciduous trees in the winter when they are bare. Watch for plant growth over time that can shade the panels. Locate the PV array away from the lengthening shadows of adjacent buildings or trees that will shade the panel in the early morning and late afternoon when the electrical generation is likely already being compromised by the lower sun angles and increased reflectance of the sun off of the panels.

Orientation:
It is essential to provide unobstructed access to sunlight to optimize efficiency.

Due south is ideal but deviations up to 45 degrees only result in a 10% loss of power. As a rule, BIPV installations are best when oriented south and tilted at an angle of 15 degrees higher than the site latitude; ie. The further north you go, the more vertical the panel as the sun angles are low in the sky and the system performs better when the rays strike at a right angle (less reflectance).

Effectiveness of PV at 52 degrees North Latitude

Optimum orientation must be worked for each latitude and can be derived from the climate data for the site, in particular the solar angles. PV Watts is an online performance calculator for grid connected systems.

PV VS. BIPV:
BIPV stands for “building integrated photovoltaic” systems.

These use PV, except attempt for a more “architectural integration” of the PV into the roof, wall, glazing and shading systems.

Integration aims to reverse the trend to think of PV as an “add-on” (and usually pretty ugly) system, and ensures that it works as part of the building envelope system.

This works with sustainable notions of having building elements “do” more than one thing. Roofs can easily accommodate another use -- by adding electrical production. The same with curtain walls, skylights, etc.

Lillis School of Business at the University of Oregon
Uses BIPV in its south facing front entrance wall to simultaneously capture the sun's energy and provide interior shading to the atrium.

LINKS TO SOME GREAT PV RESOURCES:

International Energy Agency:
Community Scale Solar Photovoltaics - Housing and Public Development Examples
http://www.iea-pvps.org/products/download/rep10_04.pdf

Urban BIPV in the New Residential Construction Industry
http://www.iea-pvps.org/products/download/IEA-PVPS_T10-03-2006__Urban%20BIPV%20in%20the%20New%20Residential%20Construction%20Industry.pdf

BIPV Educational Tool:
http://www.bipvtool.com/

CanMet Energy: Solar Photovoltaic Systems
http://canmetenergy-canmetenergie.nrcan-rncan.gc.ca/eng/renewables/
standalone_pv/publications/2006046.html

HELPFUL LINKS IN THE AIA 50to50 WIKI:

Photovoltaics