Solar Cell & Panel Basics

The sun produces an unbelievable amount of energy that reaches the earth. The amount of energy that is absorbed by the Earth in one hour is more energy than mankind uses in one year. The total amount of solar energy reaching the earth in one year is huge - twice as much energy as ever existed from all sources of coal, oil, natural gas, and uranium combined. The sun strikes the surface of the earth at different angles ranging from 0 degrees (no sun) at the poles to 90 degrees at the equator during spring and fall. At the equator during noon time, the earth's surface gets the maximum amount of energy. As one moves away from the equator, the sun's rays have to travel longer through the atmosphere. Along the way, some rays are reflected into space or scattered by clouds causing a loss of energy as they go. On average, about 50% of the sun's energy makes it through the atmosphere and strikes the earth. The tilt in the earth's axis of rotation also causes variations in the amount of sunshine received. The North Pole receives little sunshine in the winter months and likewise, the South Pole receives little in the summer months. So the amount of solar energy that reaches any given location varies by latitude, time of year, time of day, and local weather. The theoretical basis of solar cells and solar panels is based on the photoelectric effect, that is, when sunlight shines on a specific semiconductor material, the electrons in the material will be excited and generate an electric current. This effect is the basis for converting solar energy into electrical energy.

Solar Cell Basics
A large majority of solar cells are made from silicon. In this section, we will discuss only crystalline silicon cells (see the section in Solar Basics for thin film types of cells). When silicon absorbs sunlight, the energy from the sun excites some of the cell's electrons into a mobile state where they are free to move around the entire cell. However, in a piece of plain silicon, there is no reason for it to go in one direction rather than another (electricity is the movement of electrons in one direction). However, in solar cells, there is a separator called a junction, where two slightly different types of silicon meet. The two types of silicon are pretty much the same, except each one is "doped" - has a tiny percentage of other materials mixed in. The two types of doping (called n-type and p-type) determine its electrical properties. When a random electron reaches the junction, it is accelerated across it (think about a waterfall ... the water can only go one way - down). So this flow establishes an electron direction in the system. If a wire is attached to each side of the junction, and sunlight is absorbed by the silicon, the free electrons flow from one side of the junction to the other. This flow of electrons induces a similar flow through the external circuit. This is "electricity" - electrons flowing in a single direction through conductors. This particular flow is called a DC current.

Electricity can be thought of as the flow of electrons (current) through a copper wire under electrical pressure (voltage) and is analogous to the flow of water through a pipe. If we think of the copper wire in an electrical circuit as the pipe, then the voltage is equivalent to the water pressure (pounds per square inch) and the current is equivalent to the water flow rate (gallons per minute). Power is measured in watts and is the product of voltage multiplied by current. Electrical energy is power (watts) consumed over time and is expressed as kilowatt-hours (kw/h). A kilowatt hour is 1000 watt hours. If a 100-watt light bulb is on for 10 hours, it uses 1 kilowatt-hour of power (100 watts times 10 hours = 1000 watt hours or 1 kilowatt-hour). Your electric bill is expressed in terms of how many kilowatt-hours are used each month. 

Solar Modules (Panels)
An individual silicon solar cell is quite small, typically about 6 inches square producing only about 1 or 2 watts of power. To boost the total power output from solar cells, they are connected together to form larger units called solar modules (panels). These modules are usually encased in glass or plastic to provide protection from the weather. Solar modules, in turn, can be connected to form larger units called solar arrays. These arrays can be interconnected to produce even more power. In this way, solar systems can be built to meet almost any electric power requirement, small or large. The reliability of solar arrays is an important factor in the cost of systems and in consumers accepting this technology. The solar cell itself is a "solid-state" device with no moving parts, and therefore, it is highly reliable and long-lived. The electricity produced by solar cells and solar modules is direct current (DC). Nearly all home appliances are alternating current (AC). Solar arrays are connected to an "inverter" which converts the DC current into AC. Inverters also synchronize the solar current and voltage to match that of the grid solar system attached.

Houses Off the Grid (Stand Alone)
Residential solar systems, called photovoltaic (PV) systems, are said to be stand-alone if they are not connected to the local electrical grid provided by the local power company. The illustration at the left shows the elements needed to convert power created by a stand-alone solar system into a usable form for a house. Most households use only alternating current (AC). Solar panels produce only DC current. An inverter converts DC current into AC current. A stand-alone solar system uses batteries for storage to provide electricity on cloudy days and at night. However, energy stored in a battery has losses. Only about 80% of the energy that enters a battery can be retrieved. Also, batteries are kind of messy and they need to be replaced from time to time. However, if you have a cabin in a remote area, solar power may be your only choice. Likewise, the space station and spaceships depend on the conversion of solar energy to make living in space bearable.

The reliability of solar systems focuses not cells, but on solar modules and other system components. Failures in solar systems usually originate from interconnection failures, glass breakage, or electrical insulation breakdowns. Solar systems normally have fault-tolerant circuit designs that utilize redundant features in the circuitry to allow continuous operation in spite of a partial failure somewhere in the system. 

Houses on the Grid
A house is said to be "on the grid" if, in addition to having solar panels installed, it is also still connected to the local electricity "grid" from the local power company. Homes on the grid do not normally have battery storage, instead, they draw power from the grid during evenings and very cloudy days, When they are drawing power from the grid their meters operate normally and they pay the normal price for their electrical service. If during daylight hours their solar system is generating more electricity than they are using, their meter runs backwards and they pump electricity back into the grid for others to use. When their meter runs backward, they sell electricity back to their local power company at the same price that they buy it. Residential solar systems have become very popular in Arizona. Currently (Q3 2011) demand exceeds supply and the incentive budget of the State is forecast to be overrun. This has caused the government to reduce the per-residence incentive in order to stretch the budget. 

Utility Systems
Utility use of solar power differs completely from residential use. As the picture on the right of Alamosa, CO by SunEdison indicates, a large amount of land is required to "house" the incredible amount of panel arrays necessary to generate electricity on a scale to compete with other sources of energy such as coal, gas, and nuclear reactors. On the other hand, solar electrical systems are relatively easy to build compared to alternatives. It takes a year or so to construct a "solar field" as opposed to several years to build a coal or gas-fired plant and ten years to bring a nuclear facility online. Although decreasing, subsidies from government agencies are still necessary to make utility solar systems competitive.

Why would government agencies subsidize solar power? Because solar power (like wind) is carbon-free and is a very "green" source of power. Although nuclear power is also carbon-free, disposing of the waste uranium is a major problem. If solar electricity can be generated on massive scales (like other energy sources), costs will come down and become competitive without subsidies. Solar power is forecast to be competitive with coal without subsidies by the year 2014. Solar power is very popular with conservationists and most government agencies. 

US Solar Radiation
Not every area of the United States receives enough sunshine to generate solar power on a utility scale. The map at the left shows the average daily solar radiation across the US. The areas in red, orange, and yellow are the primary targets for utility solar generation. There is more than enough energy from the sun in the southwest to supply the electricity needs of the rest of the US. Moving electricity from the southwest to the northeast along high-speed transmission lines (ala US freeways) incurs a loss of only about 11%. So transmission of long-distance solar power is technically feasible. The main challenges for solar electricity to become a major player are cost and storage facilities. In spite of the fact that current costs of solar electricity are higher than other fuel sources, solar costs are coming down rapidly. Very large storage facilities are necessary to supply electricity when the sun is not shining, i.e. at night and on cloudy days. Storage facilities of this magnitude are years into the future, so for now solar will be used in conjunction with other sources of energy such as wind, natural gas, and nuclear. There are no new coal facilities being built, but quite a few existing plants are being upgraded.

Sun "Tracker" Arrays
Shown at the right are single-axis "tracker" modules at Nellis Air Force Base in Nevada, USA. The "tilted" arrays were designed and built by SunPower Corporation of San Jose California. The arrays are tilted 20 degrees back from the vertical position. Tracker tilt angles reduce the wind profile and decrease the elevated end’s height off the ground. Tilted single-axis trackers typically have the face of the module oriented parallel to the axis of rotation. As a module tracks the sun, it sweeps a cylinder that is rotationally symmetric around the axis of rotation. Notice the shading in the photograph. Field layouts must consider shading from one panel to the one behind it and to the ones adjacent to it. The spacing has to be optimized considering the amount and cost of land versus any shading losses. The figure to the left below shows a typical sample of power generated over the course of a day.

The red line shows the power curve generated by fixed panels. The yellow is for trackers. The dotted black line is the typical power load during the summertime in California on a scale to the right. The green line shows the peak summer load at 5:00 in the afternoon with air conditioners still running at the office and also at home. On average a tracker system will generate 30% more power than a fixed system. However, at the peak time of day when utilities are taxed the most, trackers generate 40% more power and at this particular time of day, power is worth much more in the marketplace than the average daily price.

From actual test data, SunPower maintains that the benefits of the increased harvest of electric power far exceed the extra costs of the motor, mounting items, and control systems of the tracker.