Renewable energy refers to energy from sources that we can’t run out of. We have at least seven sources of renewable energy that can eventually replace fossil fuels.

When we burn coal or oil, it’s gone. It took eons for it to form. We can’t make any more of it. And we’re beginning to understand that the supply is not inexhaustible.

Renewable energy, on the other hand, is inexhaustible. As long as the sun shines, the wind blows, water moves in rivers and oceans, and underground temperatures differ from surface temperatures, they can produce power.

And as long as plants die, we excrete wastes from our bodies, and continue to generate garbage, all this waste is likewise inexhaustible. All these renewable energy sources replenish themselves on a human timescale.

Renewable energy is also clean energy. It doesn’t depend on burning anything, so it emits no greenhouse gases or particulate matter.

Humanity has burned stuff to make energy since time immemorial. We have explored renewable energy sources only in recent decades. We have put some of them to good use already. Others await more scientific research and technological innovation before they become practical.

I will present an overview of the sources of renewable energy, but first, we need to look at how and why we started using fossil fuels to begin with.

Why people started using fossil fuels in the first place

From prehistoric times, everyone has had to burn something to heat their homes and cook their food. Certain trades, such as metal work, also required fire.  Coal has been known and used for centuries, but the fuel of choice was firewood wherever it was available until fairly recently.

When most people lived in the countryside, they couldn’t possibly use up wood for building and burning faster than forests could replenish. Firewood was a renewable energy source. As more and more people started living in urban areas, however, the situation changed.

Colonial Philadelphia, for example, was established near dense forests. It didn’t take half a century, though, before firewood became scarce. Benjamin Franklin introduced a cast-iron stove in 1744 to burn wood more efficiently than standard fireplaces could. It had limited success. Philadelphians started using coal to heat their homes in the 1810s. It became overwhelmingly the main fuel by the 1850s.

The need for cheap, abundant fuel expanded during the Industrial Revolution to operate factories. Coal must have seemed like an attractive alternative to deforestation. Oil and natural gas joined the energy mix by the end of the 19th century.

The world has run on fossil fuels now for only a little more than two centuries. At first, they seemed inexhaustible. Unfortunately, in such a short time, we have used up the most easily accessible fossil fuels. We have also found the environmental cost of using them unacceptable.

Now, we are moving away from fossil fuels. Here are the main sources of renewable energy:

Solar power

All the energy used by people all over the world doesn’t equal the energy in one hour’s worth of sunlight.

Solar energy has a long history, dating back to magnifying glasses to start fires by the 7th century B.C. Edmond Becquerel discovered the photovoltaic effect in 1839. August Mouchet developed what eventually became parabolic dish collectors in the 1860s.

Discoveries by other scientists eventually led to William J. Bailley’s invention of the solar panel in 1908.  The first solar cell that could convert enough solar power to run electrical equipment made its appearance in 1954. Since then, solar panels have become progressively more efficient and less expensive.

Solar energy is scalable. Utility-scale solar projects contribute electricity to the grid. Homesteaders who want to live off-grid can use solar power for their own personal needs.

Solar installations cost a lot of money, but once installed, the energy is free. It emits no greenhouse gases and causes no pollution.

But solar power has one obvious disadvantage: the sun produces no energy where it is not shining. The recent invention of various battery storage technologies has largely solved the problem of intermittency.

Solar power also has a few less obvious problems:

First, we have no good mechanism for recycling or repurposing panels that have outlived their usefulness.

Second, utility-scale solar farms must be sited far from end users of the electricity, requiring expensive distribution lines.

Third, they require a lot of land. Some proposed projects have run into opposition because they require clearcutting forests.

Wind power

Windmills have provided energy to pump water and grind grain since the days of the ancient Persians.

Dependable technology for generating electricity from the wind came only in the 1990s. Yet wind power is set to overtake hydroelectric power as the largest source of renewable energy.

Wind and solar power share some of the same advantages and disadvantages.

• They require expensive installations to harvest free energy.
• They do not contribute to pollution of air, water, or ground.
• Both potentially provide more energy than the world can use.
• Both require large plots of land far distant from major population centers.
• Large scale solar and wind farms can take land away from other uses, although wind turbines are spaced far enough apart that they can more easily coexist with farming.
• Problems of disposal of broken or worn equipment have not yet been solved.
• Just as the sun provides no power when it’s not shining, wind provides no power when it’s not blowing.

Wind power has some of its own problems. Turbines have grown so large that they have become noisy eyesores. They routinely kill high-flying birds such as eagles and condors. Development of offshore windfarms can solve at least some of these problems.

Wind turbines for residential use lag behind solar panels. That’s at least in part because most of them rely on the same vertical axis turbine technology as utility-scale turbines, which requires huge size for their efficiency. Research into new designs shows promise of eventually making wind energy affordable and practical for homes and office buildings.

Some houses already have both solar panels and wind turbines, combining two renewable energy sources to power one home.

Meritless criticisms of wind power

I’m not sure why wind energy seems to get more criticism than other sources of renewable energy.

The recent cold snap that shut down the grid in Texas brought out predictable criticism from opponents of wind power. Texas is the leader in the adoption of utility-scale wind power.

The same criticisms faced wind power after a catastrophic grid failure in Australia in 2016. I examined them carefully and found them without merit. Administrative incompetence adequately explains both crises.

In 2017, Hurricane Harvey caused massive damage in Texas. The wind farms came back online quickly. In fact, one continued to operate throughout the storm.

Geothermal energy

Temperature varies greatly at the Earth’s surface. A few feet below ground, it is fairly constant regardless of weather at the surface. But a mile or so underground, the temperature is much hotter than the surface.

Heat ultimately comes from the earth’s core, which is about as hot as the sun. In areas with lots of hot springs or volcanoes, high heat is closer to the surface than at most places on earth.

Utility-scale geothermal plants operate by drilling wells a mile or two deep to find hot water. A pump brings the water to the surface at high pressure. When the pressure drops there, the water turns to steam. This steam operates a turbine to generate electricity.

From there, the steam goes to a cooling tower to return to liquid form, which is pumped back underground. The system, therefore, is a closed loop with the most important portions of it below ground. It produces few emissions.

And conditions underground remain constant. Where sources of renewable energy such as solar or wind produce intermittently, geothermal plants can operate around the clock.

Geothermal energy plants cost a lot of money to construct. They are unsuitable for areas that experience a lot of earthquakes. Indeed, sometimes they can increase earthquake activity.

Heat pumps provide residential geothermal energy. Horizontal loop systems, with pipes four to six feet below the surface, require a lot of land with little rock. Vertical loop systems take less area but must extend to a depth of several hundred feet. The pipes contain a refrigerant and operate on the same principles as a refrigerator or air conditioner.

Since heat pumps can move heat into a building or out of a building, they function as both furnace and air conditioner. They, too, are expensive to build.

Hydroelectric power

Hydroelectric power, the largest renewable energy source in the US, comes from using falling water to power turbines. It requires the building of dams to create a fast enough flow. As long as water runs in the rivers, it is an inexhaustible power supply. It does not depend on weather or the time of day.

It does, however, require a dammable river. The larger the dam, the farther it must be from major population centers that will use the power.

The dams have their own environmental problems. The largest hydroelectric projects require huge dams that restrict the natural flow of the water. Environmentalists have fought them from the beginning and cheer when one is demolished.

Smaller projects, with an installed capacity of less than about 40 megawatts, divert less water and cause less environmental damage.

Tidal energy

Along every seacoast, tides roll in and out in a highly predictable rhythm. Tidal energy seeks to harness the power of all that moving water. It is a much more difficult feat than damming a river. For one thing, rivers flow in only one direction.

Although astronomers understand the movements of the sun and moon that cause tides, no one yet understands what actually happens in the water. And so no one knows the environmental impact of submerging machinery in the water to generate electricity. It would include impact on marine life, sedimentation patterns, magnetic fields, etc.

Tidal power is in its infancy. Where wind power has, for now, settled on large vertical axis wind turbines, tidal power has no standard design. Machinery can operate at the surface, near the seafloor, or somewhere between.

Most installations are experimental. Some haven’t worked at all.  Some run out of money before they can demonstrate whether they could work or not.

Right now, tidal power costs much more per kilowatt-hour than any other existing technology. More experience and more research will bring costs down. It didn’t take wind and solar power long to become less expensive than fossil fuels. And while building a tidal power plant is expensive, it doesn’t cost a lot to operate.

In theory, tidal power has a tremendous potential upside. Not only does it produce no emissions, but it also requires much less space than either solar or wind farms. Turbines can be much smaller and closer together. By mid-century, it might be possible to install 1,000 gigawatts of tidal power, which is about half of the current capacity of coal-fired plants.

Do you want to know something even more theoretical? It’s possible to generate electricity from falling rainwater! Not much yet, but maybe someday.

Biomass energy

Burning firewood is biomass. It was renewable as long as trees could replace themselves at least as quickly as they were cut down.

Nowadays, organic material can still be burned to produce heat, but biomass generally means one of two technologies.

Landfills, manure lagoons, and wastewater treatment plants all generate methane. It is a potent greenhouse gas. It is also the main component of natural gas. So it just makes sense to capture it and refine it for renewable natural gas.

Even the maximum feasible production of renewable natural gas cannot replace all the fossil fuels we burn. But replacing some of it with waste methane instead of releasing it into the atmosphere has a significant environmental upside.

Biomass can also make alcohol. Making ethanol from corn has become a big business. Unfortunately, it takes more energy to grow and transport the corn than the ethanol produces. It would be more environmentally friendly to make ethanol from the cellulose in agricultural waste or non-food crops such as miscanthus.

The slow pace of scientific progress in creating cellulosic ethanol

Unfortunately, it is easy to extract ethanol from the simple sugars in corn but not from cellulose. Cellulose is a natural polymer, a string of more than ten simple sugars. The technology of breaking it into simple sugars has not yet developed to where making ethanol from it is financially practical.

Research on marine pests called gribbles shows how scientists have started to discover processes that will eventually work. Gribbles eat wood in the oceans, including wooden ships and piers.

In 2009, a British research team announced that it was beginning to study how gribbles digest the wood in hope of mimicking it to make simple sugars from cellulose.

In 2013, the team announced that it had analyzed how the gribbles’ digestive system works. Unlike other cellulose-eating animals, it does not rely on gut bacteria. Instead, it secretes a very strong enzyme. The scientists also discovered how to get microbes to generate the same enzyme in large quantities.

Most recently, in 2018, the team announced discovering a protein that begins to break down cellulose before the enzyme starts to work on it. It works more efficiently than industrial pretreatment. If they can replicate the gribbles’ digestive system on industrial scale, cellulosic ethanol will become economically feasible.

Other research teams are working with other natural processes of breaking down cellulose. One or more of them may or may not lead to discovery of some kind of useful biomimicry before the research on gribbles does. The discoveries in nine years of research on gribbles may be very rapid scientifically. But it shows how long we’ll have to wait for cellulosic ethanol to join the ranks of practical sources of renewable energy.

Hydrogen fuel cells

Hydrogen fuel cells work like batteries with one important exception: They do not run down or need to be recharged. Instead, they depend on a constant supply of hydrogen. Like a battery, they have two electrodes separated by an electrolyte. Hydrogen flows to the negative electrode (anode), where a catalyst separates each atom into protons and electrons.

The electrons make an electrical circuit that ends at the positive terminal (cathode). The protons move though the electrolyte to the cathode. There, the electrodes and protons recombine to make more hydrogen. It, in turn, combines with oxygen from the air at the cathode to produce water, which the cell emits into the atmosphere.

Fuel cells have a wide range of applications, including providing emergency backup power and operating vehicles. They operate more efficiently and more quietly than combustion engines. Because they emit only water, they do not contribute to air pollution.

Where does the hydrogen come from? It can come either from natural gas or water. Hydrogen from natural gas is not a renewable fuel. What’s more, the carbon emissions from producing it offset the environmental advantages of using it.

Hydrogen from water requires an electric current to cause a process called hydrolysis to separate the hydrogen from the oxygen. It produces hydrogen as a source of renewable energy only if the required electricity comes from some other renewable energy source. And so far, making hydrogen from water is considerably more expensive than making it from natural gas.

None of these seven sources of renewable energy is perfect. None can supply all the world’s demand for energy. As technology improves and becomes less costly, however, some combination of them can ultimately replace fossil fuels entirely. If they don’t completely eliminate emissions of greenhouse gases, they can reduce them to where they will do little environmental harm.