Photovoltaic Cells, Solar Power, and LEDs

Most of the world’s energy can go back to our sun. Every day we are heated by its electromagnetic rays, and plants use the sun’s energy to make sugars and ultimately proteins and other good things to eat. Fossil fuels were also once made from these plant and other organisms that relied on the sun’s energy millions of years ago. Today, humans can convert the sun’s energy directly into electricity, through solar thermal and solar photovoltaic systems.


Solar panels, also called solar thermal, convert sunlight to heat and then heat to electricity. Photovoltaic cells, or solar cells, convert sunlight directly into electric current by way of carefully-engineered semiconductor materials.

Though solar photovoltaics are more efficient converters of sunlight, they are also more expensive.

As of May 2011, the world’s largest solar power plant is a concentrating solar thermal power plant in the Mohave desert in California. Solar Energy Generating Systems has a capacity of 310 megawatts and uses parabola-shaped reflective troughs to concentrate electromagnetic radiation.

The world’s largest solar photovoltaic plant is probably the Sarnia Solar Project in Ontario, Canada. It has a capacity of roughly 80 megawatts.


Sunlight heats a design element (water, air, chemical fluids), and that thermal energy is transmitted for other applications, such as heating water, heating space, or generating electricity. In solar thermal power plants, sunlight heats a specialized fluid, which in turn heats water into steam, which can run turbines and produce electricity.

Solar thermal power plants use concentrators that bounce the sunlight off elliptical mirrors to a central tube, in which the specialized fluid lies.


Photovoltaic cells are made of specialized diodes. Electrons (natural components of atoms) in the photovoltaic cells absorb light, which excites them to a state where they can be conducted as electrical current. This difference in energy, between the valence band (the state of a normal electron staying around its home atom) to the conduction band (electron free to move between atoms) is called the band gap.

Solar photovoltaic farm in Indonesia. Photo by Chandra Marsono.

Well-engineered photovoltaics have a band gap that coincides with the energies of as broad a spectrum of light as possible, to convert the maximum amount of the sunlight into electricity.

As sunlight energy pops electrons into the conduction band and away from their home atoms, an electric field is produced. The negatively-charged electrons separate from the positively-charged “holes” they leave behind, so that when electrons are freed into the conduction band, they move as electric current in the electric field, electricity.


An ever-expanding variety of semiconductor materials can be used to make solar cells; universities and companies worldwide are researching these options, from special bio-plastics to semiconductor nanocrystals. Nonetheless, the photovoltaic cells available today require precise manufacturing conditions and are therefore far more expensive to produce than solar panels.

Silicon has to be processed under clean room conditions — carefully regulated atmospheres — to remove impurities and prevent introducing contaminants, both of which can change the band gap. Thin film-based photovoltaics require special production methods, like chemical vapor deposition. Semiconductor processing also uses strong acids and often dangerous chemicals for etching.

Today, commercially-sold cells are made from purified silicon or other crystalline semiconductors like cadmium telluride or copper indium gallium selenide.


Silicon is plentiful in the Earth’s crust. Cadmium is a readily available but highly toxic heavy metal, as is arsenic, another chemical used in some cells. As tellurium demand is only recently rising in response to solar demand, it’s unknown what the global supply is for this unusual element but it may be quite abundant. Photovoltaics are a lively area of research, and the future production and environmental costs of starter materials, production, and pollution are difficult to predict.

California, Massachusetts, Ohio, and Michigan produced the most photovoltaics in 2009. However, that year, 58 percent of photovoltaics were imports, primarily from Asian countries like China, Japan, and the Philippines.


Photovoltaic cells work in the opposite direction of light-emitting diodes, or LEDs. LEDs are used interchangeably with other lighting, like light bulbs. However, LED’s work in a completely different manner, far closer to the way photovoltaics work.

Click here see a bar chart comparing how much energy is used by various light sources.

LEDs absorb energy in the form of electricity, exciting electrons into the conduction band. When the electrons in the semiconductor material drop back into the valence band from the conduction band, they emit energy in the form of photons, or electromagnetic radiation.

It’s a highly efficient process because energy isn’t wasted on producing heat, which happens with standard tungsten filament bulbs. LEDs also last a much longer time as they do not have filaments to burn out, and because they are very small and several units are used to replace one large traditional lamp, they do not all burn out at once. That makes LEDs a good choice for stoplights or other safety critical applications.


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Energy Efficiency, Principles of Consumption, and Conservation

A blower-door test.

Transportation efficiency
Calculating home energy
Lighting efficiency
Heating and Cooling



When trying to lower your energy use, a good place to start is getting a picture of the many ways you use energy now.




An average American uses more than four times as much energy per year than the global average, 308 million British thermal units (Btu) annually, compared to 73 million Btu per person per year globally,according to recent U.S. government estimates. That guess doesn’t account for foreigners’ use of gathered fuels like wood or manure. However, it also doesn’t include the foreign energy used to source, assemble, and ship an endless profusion of products to the United States from other countries, like China.

The most straightforward uses that you can measure and control are probably in the home and through transportation. Every year, the average car in the United States is driven 12,300 miles and consumes about 67.8 million Btu worth of fuel. On average, Americans use more energy in homes than for transport.  The average household uses less (around 41 million Btu worth of electricity). However, to use electricity at home, we have to generate an additional 90 million Btu of primary energy at the power plant, according to the U.S. Energy Information Administration. What is a Btu?



Untangling the individual’s footprint comes with unrelenting complexities. Perhaps you live in an apartment in a big city and commute to work on the train, plug in your phone and computer at work, eat out every day, shower at a gym, and only come home to sleep. Maybe you travel for work, and your employer pays the expenses. You may pay almost nothing for energy directly. Yet, you are participating in energy use through your work, transportation, food, clothes, water, air travel, and electronic devices.

It’s also difficult to calculate how much energy is used up in buying new things. If you replace your car every two years, or you have a large home that you’re constantly remodeling, chances are your true energy footprint is much larger than you will be able to calculate.

The good news is you can calculate some aspects of your energy use and reduce it. And even if you plug in at work, it’s quite possible to make a decent ballpark estimate of how much energy that takes, too.



As a driving culture with access to cheap fuels — relative to our incomes — Americans use a lot of energy getting around. Transportation of goods and people accounted for almost a third of greenhouse gas emissions in 2009, according to the U.S. Energy Information Administration.

Reducing energy use in transportation is guaranteed by replacing car, truck, or motorcycle trips with biking or walking. For a normal healthy adult, walking a mile or two daily should be well within reach. Biking is a faster option, but it’s often considered a child’s transportation method in the United States. In countries like the Netherlands, it’s ordinary to see anyone on a bike, from babies in handlebar seats to well-groomed professionals.

Nonetheless, social customs, transportation infrastructure, suburban development, weather, and promotion of driving over other forms of transportation make it inconvenient and sometimes impossible to change Americans’ driving habits, at least without changing jobs or moving to a new city. A 2005 ABC News/Time magazine/Washington Post poll found that only 4 percent of 1,203 Americans used public transportation to get to work.

Even if driving is a must, driving efficiency can be improved. More efficient vehicles are available, like hybrids and some electric vehicles. Fuel economy can be improved by better car design and better driving. There’s also car-sharing and carpooling.

Analyzing, grouping, and prioritizing destinations can cut down on unnecessary trips. Yes, getting to work is mandatory perhaps, but a whopping 85 percent of car trips are for shopping, errands, and social or recreational reasons, according to a 2001-2002 government survey.

Other alternatives include public transit, ridesharing, and smaller transportation modes like skateboards, scooters, Segways and even electric bikes.

In China, the low-speed electric bicycle is extremely popular and far more efficient than driving or even taking the bus. It’s a regular pedal bike with a rechargeable battery that boosts the pedaler’s power but doesn’t travel faster than about 12.4 miles per hour. Somewhat heavier than standard bikes, electric bikes can still be pedaled without power on the flat or downhill, and the battery can help the rider stay sweat-free and comfortable on the uphill climb.



Estimating home energy use is getting easier now that utilities have installed smart meters that display electricity demand moment-to-moment. Depending on the utility that supplies your power, if you have a smart meter, you may already be able to log in online and track your hour-by-hour power use on any particular day, compare weekdays to weekends, or see if the house-sitter blasted the air conditioning. You can see how much electricity your home draws right now, and you can turn on and off appliances to see how each one contributes.

If you don’t have a smart meter, to calculate the energy that individual items in your home use, you need to look up how many watts each device — televisions, refrigerators, computers, routers, lights, electric air and water heaters — uses. That nameplate wattage is usually printed on the device.

Some sample nameplate wattages (watts):

Clock radio: 10
Coffeemaker: 10
Dishwasher: 1200-2400
Ceiling fan: 65-175
Space heater: 750-1500
Computer: 200-300 (awake), 20-60 asleep
Laptop: 50
Refrigerator: 725

Weekly energy per device = wattage x hours it’s “ON” per week

For devices that cycle on and off, like refrigerators and air conditioners, you’ll divide the resulting number by three.

You’ll also want to examine how much natural gas, propane, or other fuels you use for heating and cooling space, heating water, and cooking. While electric devices tend to be more efficient than gas-powered devices in your home, electric devices actually tend to use more energy overall because of loss of efficiency when the electricity was generated and transmitted to your home.

If you’re in the market for replacing you refrigerator or other appliance, and want to find out more about efficient options, a good resource for information is the Energy Star program.

Another detailed resource for tracking your energy-related emissions of greenhouse gas is the Home Energy Saver, built by the U.S. Department of Energy and Lawrence Berkeley Laboratory.

Know that devices don’t precisely use what their nameplate wattage says. Various factors affect how much energy something uses. For example, using the maximum brightness setting on a laptop computer will require more energy. Air conditioners will require much more energy to operate in very hot weather not only because it’s hotter outside but because the refrigerant becomes less efficient as it gets warmer, particularly if the refrigerant gets into the high nineties Fahrenheit. See below for more about heating and cooling.



You can improve your efficiency by replacing appliances and redoing construction, but you can also conserve energy by using less demanding settings, adjusting the thermostat, and turning items like computers and televisions off when they’re unused.



Unlike the days of candles and whale oil lamps, today we have many electrical lighting options. Our most popular, the standard 100 watt bulb, is being phased out, in part due to Clean Energy Act signed into law by President George W. Bush in 2007.  The maximum wattage incandescent bulb allowed will be 29 watts by 2014, down 70 percent from pre-2011 levels.

Instead, that type of bulb will be replaced by lower wattage incandescent bulbs, as well as compact fluorescent bulbs and even light-emitting diodes.

We can save lighting energy by

1. Turning off unused lights

2. Changing the type of light bulbs we use (see chart)

3. Changing the lighting plan, including adding natural light in the form of windows and skylights and solar tubes.

For more information about design, see the Energy Savers website.

Light can be measured in lumens. A 100 watt incandescent light bulb gives off around 1750 lumens.

The standard light bulb has a tungsten filament that exhibits incandescence when electric current travels through it. The filament burns out over time. The bulb keeps the filament in a special gas atmosphere like argon, instead of being exposed to regular air. Tungsten halogen bulbs operate somewhat similarly, with an incandescent filament, but the bulb contains halogen gas, which helps keep the filament from burning out as quickly.

Compact fluorescent bulbs, the sometimes spiral-looking bulbs, fluoresce instead of incandesce. Electric current travels through argon gas and a small amount of mercury vapor, which emit ultraviolet light. That light, in turn, excites a phosphor (fluorescent) coating on the inside of the bulb, which then emits visible light. So called CFLs are far more efficient and have much longer lifetimes. They do, however, contain a small amount of toxic mercury vapor and shouldn’t be thrown into the trash.

LEDs are also much more efficient than incandescent bulbs and don’t emit mercury if they’re broken. This technology is  sometimes called Solid State — even though the type of physics that the name is based upon has now changed to Condensed Matter. Extremely long-lived and very energy efficient, LED’s use around 20 percent of the energy of an incandescent for the same amount of light. However, they are far more expensive than similar fluorescent or incandescent options. For more about how LEDs work, go here.



Heating and cooling take a lot of energy. Replacing heaters, refrigerators, and single-paned windows costs money. Ripping out walls to add insulation is scary and can become a huge project.

However, today, a wide array of tools and professionals are available to assess the efficiency of heating and cooling and put it into perspective with cost. Home efficiency experts can use infrared detectors to track where heat is lost, and they can use blower door tests to check how quickly air is being exchanged with the outdoors through holes and leaky ducts.

Blower door tests change the air pressure inside a building relative to the outside to measure how quickly the air pressure returns to normal. If you walks through a pressurized house during the test, you can also track where air is leaking.

Even without a professional, you can reassess your home energy use. For tips on do-it-yourself home energy assessment, try the U.S. Department of Energy’s Energy Savers website.



1. Repairing leaky ducts, an often neglected source of heat loss! Ducts are much easier to access than replacing insulation, and they often have holes and cracks, making them a major  source of cold air infiltration, and also indoor air pollution.  Leaks suck in cold, dirty crawl space air including asbestos, dirt, and volatile chemicals (paint thinners, pesticides) that we stow or spray under the house. For more about indoor air quality see the Environmental Protection Agency’s website here.

2. Improve insulation and weather stripping, and seal up cracks. Use curtains or blinds to trap heat in during the winter and block sun out during the summer.

3. Replace air conditioners and heaters with more efficient models.

4. If you live in a dry climate, open windows to vent your home in the evenings, keep windows closed and A/C on during the morning before its the hottest hour of the day. Resist cranking the A/C up during the hottest hours of the day when the coolant fluid is the least efficient.

5. Replace windows and doors with better rated ones. For more about how windows are rated see the National Fenestration Rating Council.



The invention of new electricity-dependent devices outstrips the speed that we are making our homes more efficient. Today, heating, refrigerators, and air conditioners are using less energy, but televisions, computers, and an ever-expanding selection of other electronics are demanding more. For more about electricity in the home see the Basics of Electricity and how energy moves through the home.



A British thermal unit – almost always written Btu or BTU – is a measurement of thermal energy.  The scientific community usually uses the more manageable unit of the joule, which is a metric measurement of energy.  (A Btu is roughly 1,000 joules) A Btu is the English unit.

Fuels are often measured in Btu to show how much potential they have to heat water into steam or provide energy in other ways, like to engines. Steam turbines produce most of the electricity in the United States.


For more about the Smart Grid go to the Power Grid Technology section.

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