Much of the world's energy supply comes from natural gas and nonrenewable fossil fuels, such as petroleum, coal, methane and propane, which are being rapidly depleted. The burning of natural gas, fossil fuels and other hydrocarbon fuels is the largest source of CO2 emissions worldwide, and produces other pollutants such as nitrogen oxides, sulfur dioxide, VOCs (volatile organic compounds), and heavy metals. Many of the major fossil fuel sources are also found in foreign countries, which may take advantage of another country's dependence on them for oil and other fossil fuels. Over time, a lack of energy independence can lead to wars, monopolization, inflation, manipulation and oppression, both between and within nations. There are alternative energy sources however, which unlike fossil fuels, are renewable, sustainable, affordable, and globally available. Some of the most well known green energy sources include thermoelectric, solar, thermal, hydrogen, hydroelectric, wind, and biomass energy. Other, less common green energy sources still being developed include hygroelectric, piezoelectric and ambient RF energy, as well as a number of other emerging small-scale energy harvesting technologies.

Thermoelectric Energy

Thermoelectric devices, such as thermocouples and thermopiles, thermoelectric generators, heat pumps, heaters, coolers, refrigerators and freezers, can convert temperature differences directly into electricity, or electricity into hot and cold temperatures. Thermoelectric devices are generally eco-friendly (depending on materials and production methods used), silent, long lasting, have no circulating fluids or moving parts, and relatively easy to find. The energy efficiency of such devices however, is only typically about 0.3-4%.

Solar Energy

The earth recieves more solar energy from sunlight every minute than is used in fossil fuels worldwide every year. Solar energy can be eco-friendly, sustainable, energy efficient and affordable, depending on the materials and methods used. Solar energy can be used in a wide variety of applications, such as solar HVAC (space heating, ventilation and air conditioning), drying, cooking, lighting, PV (photovoltaic) and thermal electricity production, CHP (combined heat and power) systems, water heating, and water treatment systems, such as solar saltwater desalination, solar distillation and UV disinfection. Though solar energy is not available at night and less available in shady or overcast weather conditions, the electricity it can be used to produce can be stored for later use.

Thermal Energy

Thermal heat energy may be used directly for such purposes as HVAC, drying, cooking, CHP systems, water heating, distillation and saltwater desalination, or indirectly for producing electricity or mechanical power via heat engines (such as steam or stirling engines) or thermoelectric devices. Though heat can be provided by burning something, such as wood, biomass or fuels, solar thermal energy (i.e., heat energy from the sun), heat storage or waste heat may be used as eco-friendly, renewable, affordable, energy efficient heat source alternatives. Solar heat collectors and solar concentrators may be used to collect solar heat energy, while heat storage mediums (such as hot water, sand, soil and/or PCMs) can be used to collect heat from any heat source, and store heat for later and extended use.

Hydrogen Energy

Hydrogen energy is generally obtained from hydrogen fuel, which may be used in fuel cells to produce electricity, or burned directly for space and water heating, cooking, lighting, distillation, motive power (for motors and vehicles), or producing electricity from heat. Production of hydrogen fuel via the electrolysis of water is eco-friendly and sustainable, but not very energy efficient or affordable, as it generally takes more energy to produce hydrogen fuel than the energy it yields. The conversion of hydrogen fuel to electricity using fuel cells can be eco-friendly, sustainable and energy efficient, but fuel cells available to the general public typically use toxic chemicals as electrolytes and fuels other than hydrogen for power, which contribute to environmental pollution and may pose significant health risks. Burning hydrogen fuel directly produces water, but also nitrogen oxides, which contribute to pollution.

Hydroelectric Energy

Hydroelectricity accounts for about 20% of all electricity produced and consumed worldwide. Hydroelectric systems can be on or off-grid and as small or as large as needed, but the water source is most often as close to the point of use as possible. Electricity is produced using dammed water or a running water source such as a river, creek, ocean tides or water falls to drive a water turbine and generator. Hydroelectricity can be green, energy efficient, affordable, and power an entire household, but is generally most suitable for land owners with sufficient space and water to drive the system.

Wind Energy

Wind energy can be environmentally friendly, renewable, energy efficient and affordable. Wind energy can be converted to mechanical power and used directly for such purposes as pumping water, grinding grains, or running a small saw mill. Wind power can also be used to drive a wind turbine and electrical generator to produce electricity. Mechanical or electrical output from wind energy is generally unpredictable due to the unpredictable nature of wind, but the electricity it can be used to produce can be stored for later use.

Biomass Energy

Biomass includes organic matter such as trees, wood, plants, leaves, grasses, agricultural crops, animals, human and animal waste. Biomass can be burned directly for space and water heating, cooking, lighting, producing electricity or mechanical power via heat engines (such as steam or stirling engines) or thermoelectric devices. Biomass can also be used for the production of biofuels, such as methanol, ethanol, methane, biodiesel and syngas, which can be burned for similar purposes as burning biomass directly, as well as for motive power in motors and vehicles. Methanol, ethanol and methane are produced from the anaerobic decay of biomass, while biodiesel is produced from vegetable oils, and syngas is a mixture of carbon monoxide, hydrogen and other hydrocarbons produced by the partial combustion of biomass.

Biomass energy is renewable and earth friendly, and the burning of biomass and biofuels can be energy efficient and carbon neutral. When biomass is burned however, previously stored carbon dioxide is released, along with carbon monoxide, sulfur, hydrocarbons and particulates, which contribute to pollution and pose significant health risks. Production of biofuels via the anaerobic decay of biomass releases methane and other environmental pollutants, depending on the biomass and methods used. The burning of biofuels also releases previously stored CO2, as well as additional CO2, carbon monoxide, methane, sulfur oxides, hydrocarbons, particulate matter and/or other emissions that contribute to pollution and likewise pose significant health risks. Use of biomass energy and the production of biofuels from biomass also requires a significant amount of land, water, work and energy (typically more energy than it produces). Likely the best, most environmentally friendly use of biomass would be as compost, which is produced from the aerobic decay of biomass.

Combining Green Energy Sources

Nearly all green energy sources work best under certain conditions, such as temperature, light, heat, water, space, general weather conditions, and so on. Solar energy for example, is best collected during the day and summer, while hydroelectric energy is most abundant during the spring, and wind energy is most abundant during the fall and winter. Multiple green energy sources can be combined and electricity produced can be stored for later use however, so as to provide a constant, reliable source of green energy under most or all conditions.

Green Energy Storage and Conversion

While combining green energy sources can help to provide a continuous, reliable source of electrical power under most conditions, any energy system can still exceed or fail to meet energy requirements, depending on a number of additional factors. Such factors include energy sources, energy storage or conversion methods if any, energy efficiency, peak and off-peak hours, minimum and maximum energy requirements. Therefore, most green energy systems use some form of energy storage and conversion method to store excess energy when available, reduce additional electricity production requirements during peak hours, convert stored energy to another usable form (such as hot or cold temperatures, mechanical or electrical energy, DC or AC power) if necessary, regulate energy use and/or electrical output, continuously and reliably distribute stored energy, in desired forms and amounts, under all or nearly all normal operating conditions.

Energy is usually stored in the form of thermal heat energy, biofuels, chemical energy, in flywheels, pumped water, compressed air, electric or magnetic fields, or fuel cells.

• Thermal heat energy can be collected from eco-friendly heat sources as solar energy, heat storage or waste heat, then stored using heat storage mediums such as evacuated space, still air, wood, cork paper, fiberglass, cotton, wool, soil, sand, gravel, stone, glass, rice hulls, masonry and/or PCMs (i.e., phase change materials, such as water, fats, oils, waxes and calcium chloride salts). Natural, eco-friendly insulation, such as evacuated space, still air, wood, cork, cardboard, fiberglass, or natural fiber insulation materials such as cotton, hemp, straw or wool, can greatly increase energy efficiency and storage time. Stored heat energy can then be used directly when needed, or to heat water and create steam to generate electricity by means of steam turbines and generators, or a heat engine such as a steam or stirling engine. Such systems are therefore often used in CHP (combined heat and power) applications.
• Biofuels can be used as a heat source in addition to or instead of solar, heat storage or waste heat sources, but the burning of any fuel is a fire hazard, all fuels burned must continually be replaced, and all fuels except hydrogen (depending on materials, methods and energy sources used to produce the hydrogen) contribute to pollution, and may have adverse health effects. Using biofuels as an energy source or for energy storage also merely shifts consumption of nonrenewable fuels to food crops, which requires a lot of land, water and energy to produce. While the production and burning of hydrogen may be done in a sustainable manner, hydrogen is explosive, and difficult to store safely, especially under pressure.
• The most common method of storing energy is in the form of chemical energy such as batteries or a bank of batteries. Most batteries however, contain nonrenewable rare earth materials, toxic chemicals and/or heavy metals, and though batteries can be rechargeable, they cannot be recharged indefinately. EEStor ceramic batteries are claimed to have 10 times the energy density of lead-acid batteries at 1/10 the weight and volume, but they are produced by a process of chemical fabrication that uses calcined composition-modified barium titanate powder and barium-calcium-zirconium-titanate (CMBT). Barium is a toxic heavy metal, which is explosive when combined with water, can accumulate in the body and cause barium poisoning. In the end, all dead batteries eventually end up in landfills, are replaced with new batteries, and the cycle continues.
• Flywheels are an eco-friendly, affordable, energy efficient method of storing kinetic energy. Flywheels tend to have a very long lifespan, but they cannot store energy for very long, must be heavy and large to store the amount of electricity needed by most buildings or vehicles, and can be quite dangerous due to the potential shattering of the massive wheel from overload.
• Water can be pumped from a lower level to a higher level, and is often used as a green, affordable, practical energy storage method in hydroelectric systems.
• CAES (compressed air energy storage) can be eco-friendly, but pressure (and thus electrical output) can vary greatly, operation range is limited, CAES systems are generally inefficient, expensive, impractical, and require a lot of space due to their very poor energy density. CAES systems also require a compressor (which can be loud), and compressed air can be difficult to store safely. It is possible however, to use the thermal energy produced by a CAES system to increase energy efficiency and reduce space requirements, but large-scale compressed air energy storage for household energy or motive energy (as in motors and vehicles) is not yet commonly practical, affordable, or available to the general public.
• Energy can also be stored in capacitors, inductors and/or oscillators. Capacitors store energy in an electric field, while inductors store energy in a magnetic field. A capacitor and inductor can also be combined to form an oscillator. Conventional alkaline batteries however, have an energy density of about 234 to 1639 times that of capacitors, ultracapacitors or supercapacitors. Thus, it would take 234 to 1639 times the amount of space and capacitors to store the same amount of energy as alkaline batteries. Many capacitors are polarized in a certain way, and if voltage is applied across the terminals, the capacitor itself can pop. Many capacitors also contain chlorine gas, which raises capacitance, but is highly toxic. In addition, capacitors remain charged until a load is placed across its terminals, even if a device is turned off or has not been in use for a long time, so they can be dangerous, even deadly. Likewise, if current flowing through an inductor is interrupted, a dangerous, high voltage pulse can result. The magnetic field of an inductor can also interfere with, damage or destroy nearby digital circuits in pacemakers, computers and other electronic equipment. Furthermore, capacitors, inductors and oscillators are typically used for storing and distributing short bursts of energy at a time, rather than storing and providing regular, continuous, long-term energy distribution.
• Fuel cells can convert chemical energy from a fuel (such as hydrogen, hydrocarbons such as natural gas, or alcohols such as ethanol or methanol) to electricity. Fuel cells are about 40-85% energy efficient and can be eco-friendly and sustainable, depending on the fuel used to power them, and methods for obtaining the fuel used. Unlike batteries however, fuel cells require a constant source of fuel and oxygen to run. The fuel is therefore used as a form of chemical energy source and storage, while the fuel cell itself is used for energy conversion and electricity production. Fuel cells available to the public however, typically use toxic chemicals as electrolytes and non-renewable fuels for power, which contribute to environmental pollution and may pose significant health risks.

Environmental and Financial Benefits

Collecting, storing, converting and distributing your own energy reduces your contribution to pollution, saves money by reducing or eliminating your energy costs, and increases self-sufficiency by reducing or eliminating the need to pay for others to provide energy for you. Excess renewable energy can be stored for later use, or can be sold to electric companies through the power grid. In addition, there are state and federal tax incentives (such as credits and deductions on income taxes) available for purchasing and using earth friendly, clean energy systems, renewable energy products and equipment.

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