Author’s note 6-7-12: I wrote this in an attempt to compile my knowledge of all available energy options and to assess the relative potentials of these options to replace fossil fuels as our primary source of energy. I am a graduate student in biological engineering with a focus on photosynthetic hydrogen production, and I am by no means an expert on all that is covered here. This is an evolving document, and I am open to critiques and discussions.
If you are interested in reading more about the potentials and limitations of various renewable energy sources in greater depth, I recommend Tom Murphy’s blog Do the Math. Tom is a physics professor at UC San Diego who has been applying his math-savvy to separate the hope from the hype.
For a comprehensive, honest treatment of all renewable energy options (albeit a bit light on discussion of storage), I recommend the free online book Sustainable Energy – Without the Hot Air by David MacKay.
As a renewable energy researcher, I find myself always trying to find the best way forward, the technology or suite of technologies that will allow humans to maintain something like our current standard of living in the next millennium – a millennium in which the abundant supplies of fossil fuels will be gone and we will of necessity have replaced our ethic of dominion over nature with one of stewardship and cooperation.
We live in an age where energy is so abundant as to be taken for granted completely. A human laborer, working hard, has an output of around 100 watts. A horse can generate 750 watts. Let’s compare that to some modern amenities. A Toyota Camry engine, at a modest four cylinders, puts out 150,000 watts at full power. A clothes dryer uses 5000 watts. A kilowatt-hour, the energy equivalent of ten hours of human labor, costs an average of ten cents.
In 2010, the United States burned through 98 quadrillion Btu (“quads”) of energy (global demand was about 450 quads). This is equivalent to each man, woman, and child working 24/7 for 109 years. 83% of that energy came from fossil fuels – coal, oil, and natural gas – with an additional 8.6% from nuclear energy and only 8.2% from renewable sources. With fossil fuels running out and leading to climate change, clearly in the future we either need to use 83% less energy – an impossible proposition if we wish to continue living in cold climates and moving ourselves long distances, or we need to vastly increase our utilization of renewable energy, or (more likely) we need to reduce our demand to a point that can be satisfied with renewable sources.
My goal, in writing this, is to provide both a sense of optimism – that our energy challenge is not fundamentally intractable – and a sense of direction. Too often, as will be highlighted below, energy policy promotes the wrong decisions – decisions that are labeled as “green” but provide little or no benefit in terms of reducing our demand for fossil fuels.
To understand our energy system, it is necessary to consider energy forms (light, kinetic, potential, chemical, electricity, and heat) and energy streams (solid, liquid, gas, electricity, steam). Our differing abilities to store various energy forms and the energy losses associated with conversions between forms will constrain our choices as we build a renewable energy economy. With those constraints in mind, it is then necessary to consider energy sources – where the energy comes from and whether that energy can be sustained indefinitely.
In our world, we encounter meaningful amounts of energy in six main forms: light, kinetic, potential, chemical, electricity, and heat.
All energy from the sun arrives in the form of light, or more technically, electromagnetic radiation: massless packets of energy called photons traveling at 186,000 miles per second, arriving on Earth eight minutes after leaving the sun. We can also create light from chemical reactions (fire) or from electricity (light bulbs, LEDs, and antennas). We use low-energy forms of light (microwaves and radio waves) to heat food and transmit information. Light can be converted to chemical energy (photosynthesis and artificial variants), electricity (photovoltaics), and heat. The former two processes are thermodynamically constrained to operate at low efficiency (0.1-20% are common real-world values), while conversion to heat can be nearly 100% efficient (80-95% real-world). Importantly, energy cannot be stored as light.
Kinetic energy is energy of motion, and it is one of the major forms of useful energy – in transportation, washing machines, industry, food processing, and thousands of other applications. Engines convert heat and/or chemical energy to kinetic energy at relatively low efficiency (around 15-50%). Electric motors convert electrical energy to kinetic energy at high efficiency (80-95%). Kinetic energy can be stored in flywheels, but this is not an economical way to store useful amounts of energy. Two energy sources, wind and waves, are in the form of kinetic energy, which can be most easily converted to electrical energy.
Lifting objects against gravity requires energy, and energy is released as these objects fall. Just ask Newton about his apple. In reality, the only material moving from high to low elevation in quantity is water. The potential energy of elevated water is transformed into kinetic energy as the water falls, and this kinetic energy can be used to turn a waterwheel (as in the old grist mills) or a generator turbine (as in modern hydroelectricity). Gravitational potential energy can be stored in reservoirs, and is thus available on demand within the constraints of reservoir capacity. Gas at high pressure also has potential energy, in that in can generate kinetic energy if released through a turbine. This makes possible bulk energy storage in the form of compressed air in tanks and caverns.
Chemical energy is technically a form of potential energy – the energy stored in bonds between atoms is released when the atoms rearrange to form stronger bonds at a lower energy state. During combustion of gasoline, hydrogen and carbon atoms transition from being bonded to each other to being bonded to oxygen in the form of water and carbon dioxide. In combustion, chemical energy is converted almost entirely to heat at nearly 100% efficiency, and that heat can then be converted to kinetic energy at lower efficiency. In some cases, namely batteries and fuel cells, chemical energy can be converted directly to electrical energy, with efficiencies in the range of 50% (fuel cells) to 95% (some batteries).
Chemical energy can be stored in large quantities and, in gaseous and liquid forms, easily transported in pipelines. This feature has enabled the “anywhere, anytime” mentality of the fossil fuel era, which will be a challenge to maintain in a future where none of the energy sources are in chemical form.
Our newest energy form, arriving on the scene a mere 150 years ago, electricity has come to dominate modern life. Electrons move freely through metal, and given a voltage – a difference in potential – the movement of these electrons carries a great deal of energy. Electricity can be converted into kinetic and heat energy at high efficiency and light at moderate efficiency. It can be used to encode and process information – the foundation of the computer era. It is transmitted over wires that are easily plugged in or unplugged, and produces no exhaust gases when used. These are the manifold advantages of an electric world that have brought us to where we are today.
None of our energy sources, existing or future, are in the form of electricity. Moreover, electrical energy cannot be stored, with the exception of capacitors that store very small amounts useful only on the timescale of seconds to minutes. Electricity must be generated at the same rate that it is used, or at least that has been the rule in a world of “anywhere, anytime” energy. Alternatively, electricity must be used at the same rate that it is generated, and in a future of intermittent energy supplies and smart grids that will likely become the new guiding rule. We will increasingly need to adapt our demand to match supply on a real-time basis, cutting back usage when generation is low.
Electricity can be “stored” as chemical or potential energy, accepting some energy losses for each conversion step. In practice this means pumping water into reservoirs when the wind is blowing, or compressing air in underground caverns when the sun is shining. Electricity can also be used to charge batteries or to split water into hydrogen and oxygen. All of these processes can be reversed at a later time to regenerate electricity when it is needed. Note, however, that all so-called electricity “storage” is actually a two-step energy conversion requiring added infrastructure. That means that it makes sense to first modulate demand to match supply, and to use storage only to maintain essential power and as an emergency buffer.
Heat is the lowest “grade” of energy. All other forms of energy can be converted to heat at nearly 100% efficiency, but conversion of heat to other forms entails an efficiency hit, usually on the order of 50% or more. The lost energy remains as heat. The tendency of all energy to become heat is part of the Second Law of Thermodynamics. Entropy, or disorder, always increases, and heat is nothing more than an increase in the disordered motions of atoms and molecules. The reason why engines and power plants are only 20-40% efficient is that combustion creates only heat. In a confined chamber, that heat then creates pressure which can be used to drive a turbine or piston. However, the exhaust or steam leaving the engine is still hot, and more heat is lost into the engine itself, thus necessitating a radiator for cooling.
The nature of heat leads to some logical conclusions, all of which have been ignored in an era of abundant, cheap fossil energy.
1) High grade forms of energy should not be used to generate heat. If you burn natural gas or wood to heat your home, 80-90% of the chemical energy in the fuel becomes heat in your home. If you have electric heat, and that electricity was produced from natural gas, only about 25-35% of the energy in the gas ends up as heat in your home, and the rest is wasted.
2) Heat-wasting energy conversions should be located where heat is needed. This is the principle of municipal utility plants that burn coal or gas to provide both electricity and steam heat to community buildings. In the future, it may entail fuel cells in each home that provide both heat and electricity. Unfortunately what we have now is massive power plants far from population centers that pump half of the energy in the fuel into the environment as waste heat.
3) Efficient conversion of solar light into heat should be leveraged. Nearly all single-family homes contain enough roof area to economically provide both home heating and hot water, if solar energy is captured as heat and stored in hot water tanks. At present solar heating is vastly underappreciated and underutilized.
Heat can be stored in water, though only on the scale of a few days. Heat can be transported in steam or hot water pipes, though only for a few miles before losses become a problem. Heat is thus a home- or community-scale form of energy.
All energy not produced on-site must be imported, and for all of our complexity we have developed only five major streams of energy: 1) solid fuels (coal, wood), 2) liquid fuels (gasoline, diesel, kerosene, heating oil), 3) gas (LP, propane, natural gas), 4) electricity, and 5) local steam heating. To avoid costly investment in new infrastructure, future energy sources should ideally feed into these streams. Solid fuels move in trucks and railcars. Liquid fuels move in pipelines and trucks. Gases move in pressurized pipelines. Electricity moves in a complex grid of transmission lines. Steam moves through local networks of insulated, pressurized pipes. One challenge of some alternative fuels (e.g. hydrogen, ethanol) is that they do not fit in well with existing energy streams and infrastructure.
EROEI – A Brief Aside
Energy Return on Energy Invested, or EROEI, is a simple ratio of the energy produced by a technology divided by the energy required to produce, transport, install, and maintain that technology. The denominator is devilishly difficult to measure precisely, and thus you will see vastly different estimates for the same technology depending on which inputs are included. An analysis will probably include the energy to mine the nickel, platinum, etc. Do you then include the energy required to build the mining machines? Do you include the energy to provide food and shelter for the miners? Including all of the inputs without extending so far as to include the whole world is more art than science.
Although difficult to measure exactly, EROEI provides an important assessment tool. With an EROEI of 2:1, half of the energy produced is required to build the technology, and only half can be used. Substituting a technology with an EROEI of 3:1 over one with an EROEI of 2:1 will require 25% less total energy. The returns diminish from there, but it is clear that in general higher is better, and any technology with an EROEI of less than 2:1 is borderline useless, especially as we will need to use our limited remaining fossil energy to build our renewable energy infrastructure.
With energy forms, streams, and EROEI in mind, it is time to look at where our energy comes from. All the energy that we use, or can use, comes from one of two main sources. It is either taken from the Earth or provided by our Sun. Nuclear, geothermal, tidal, and fossil fuel energy are Earth energy. Biofuels, wind, waves, hydropower, and all variations of solar are Sun energy. It is worth examining all of these sources to see which, if any, could supply 100 quadrillion Btu indefinitely and at an acceptable cost to humanity and our natural environment.
Nuclear – fission and fusion
Einstein’s famous equation, e = mc2, has a certain appeal. Our entire 100-quad demand would require a little over 1,000 metric tons – about 30 truckloads – of uranium. Creating the same amount of energy through fusion (combining hydrogen atoms to produce helium, the reaction that powers the Sun) would require only about 200 tons of hydrogen.
Fission would be renewable but for two caveats: 1) uranium is a very rare element on Earth, and 2) fission generates radioactive waste with extremely long half-lives, on the order of thousands of years. If we were to transition to a nuclear energy economy we would soon be facing twin crises of uranium shortages and nuclear waste disposal. It is surprisingly difficult to find geologic sites guaranteed to remain stable, with no cracking or leaching, for over 10,000 years. Thorium, another fissile element, is much more abundant than uranium, and thorium reactors produce at least 10 times less long-lived waste. Thorium fission is an emerging, largely untested technology at present, but it has the potential for larger-scale deployment. On the scale of hundreds of years, however, thorium reserves would dwindle, so it is not ultimately a sustainable technology. Overall, despite multiple drawbacks and safety concerns, nuclear fission is climate-neutral and not presently resource-constrained, so it may buy us some time and comfort as we transition to truly renewable sources of energy.
Fusion has long been the promise of the future, and indeed if it were to succeed it could well power our society indefinitely. One tenth of one percent of the water on Earth would supply enough hydrogen to support a 500-quad world energy demand for over 100 billion years. The problem with fusion is that no one has yet found a way to achieve the necessary temperatures and pressures without putting more energy in than comes out. The Sun is large enough that gravity alone supplies the temperature and pressure deep in the core. It may be possible to replicate this on Earth using advanced physics and intense magnetic fields, but to date EROEI has been much less than 1. Importantly, no one has been able to prove that high-EROEI controlled fusion is fundamentally possible on Earth. Barring some major breakthrough, it seems likely that EROEI will remain less than 2 for at least the next 50 years and possibly indefinitely, and thus we should not count on fusion to solve our energy crisis.
Our planet is extremely warm inside – the core may be as hot as 10,000ºF. The problem is that in most places the heat is too far down for us to make use of it without running into an EROEI crisis. Five-mile-deep wells are too expensive, both on financial and energetic grounds, to justify the energy they could produce. Where the heat is closer to the surface, geothermal can be a good option, and we will probably see more of it in the future. Unfortunately it can play havoc with fragile geology like hot springs and geysers, so geothermal resources must be developed with care.
That said, the Earth provides an excellent and underutilized thermal buffer. Despite massive fluctuations in air temperature, ground temperature 6-10 feet down remains constant year-round. Thus we can heat and cool our living and working spaces by shuttling excess heat into the ground in the summer and removing heat from the ground in the winter. Using energy to move heat rather than generate heat produces around four times more heat per unit of energy in, opening the door to immense (and much-needed) energy savings by installing heat pumps.
Tidal energy is unique in that it ultimately derives from the rotational kinetic energy of the Earth and the orbital kinetic energy of the Earth-Moon-Sun system. There is no risk of depleting that energy if we make use of it. Even so, the difference between high and low tide is only about 10 feet, limiting the amount of gravitational potential energy available as water flows in and out of estuaries. Furthermore, constructing the necessary dams across estuaries is environmentally damaging and disrupts fragile ecosystems. In all likelihood tidal energy will never be a major source, though it may be developed in some niche applications.
Fossil fuels are Sun energy harvested from the Earth. During the Carboniferous period, 300 to 400 million years ago, plants evolved the ability to produce lignin, a structural polymer that is very difficult to break down. For about 100 million years, until some fungi evolved the ability to degrade lignin, woody material accumulated. As sedimentation buried this lignin and associated cellulose, it was cooked by the heat and pressure within the Earth until only the carbon remained. Today we call it coal. Oil formed from aquatic organisms deposited in anaerobic seafloors where decomposition could not keep up with accumulation. At high temperatures and pressures, various organic molecules were pyrolyzed to form oil. The geologic processes that create oil and coal also produce natural gas, and thus gas can be found in oilfields and coal beds.
The industrial revolution was driven by the discovery of fossil fuels and the invention of machines that could use them. Compared to the wood fires and waterwheels that preceded them, fossil fuels seemed infinite. They could be supplied in any amount to anywhere in the world. No longer was the mill limited by the size of the river. Whole cities were built around factories and steel mills, powered by coal. Oil allowed highways, suburbs, and airplanes. Electricity from coal and natural gas powered the information age. Industrial fertilizers, produced from natural gas, fueled the Green Revolution and increased the carrying capacity of the planet.
We are now, only 300 years after the invention of the steam engine, approaching two hard limits: 1) fossil fuel supplies are finite and rapidly dwindling, and 2) atmospheric carbon dioxide has risen to levels that threaten to dramatically change Earth’s climate. The huge and often unspoken question is whether we can power an industrial society and feed 7-9 billion humans in the absence of fossil fuels. I believe that we can, but we will need to make the right energy choices at the right times if we are to avoid crises.
From the previous section, it should be clear that aside from the possibility of nuclear fusion, we cannot continue to meet our energy needs with Earth energy. That means we must turn to the Sun, the nuclear furnace 93 million miles away at the center of the Solar System. The good news is that energy from the Sun arrives on the Earth’s surface at a rate of 129 petawatts, or 129 quadrillion watts. This is 7600 times larger than the 17 terawatts (17 trillion watts) used by humankind. If we could convert just 0.013% of that energy, or one hour’s worth, into useful forms, we could power all of our inefficient homes, cars, airplanes, factories, and cities for a year. If we look just at the United States, the most energy-hungry nation, we would need to convert 0.21% of incident sunlight on the lower 48 into useful forms to meet current energy demands. That is not an unreasonable number, and as it is much smaller than natural annual variations in incoming sunlight, capturing and using that much is unlikely to disrupt natural processes to any great degree.
The challenge, then, is not one of possibility but of technical and economic feasibility. There are many ways we can directly and indirectly utilize solar energy, and our future comfort (and for many people, survival) will depend on our ability to select and implement the technologies with the greatest potential and fewest drawbacks. I will outline the options in three major sections for direct, biological, and indirect sources, with a fourth section focusing on energy storage and conversion that will be necessary in a solar energy economy.
Nineteen-year-old Edmund Becquerel discovered the photoelectric effect in 1839 while shining light on an electrolytic cell. In semiconducting materials such as silicon, selenium, and germanium, absorption of a photon can elevate an electron into a “conduction band”, leaving behind a “hole.” In so doing, light energy has been effectively converted into an electrical potential. Each electron activation requires a precise amount of energy. As sunlight contains photons with a wide range of energies, some will have too little energy and will generate no electricity, while some will have too much and the excess energy will be wasted. The theoretical maximum efficiency of a single-layer photovoltaic cell is 33.5%, with present real-world efficiencies on the order of 14-22%. Higher efficiencies can be achieved by layering cells optimized to absorb different wavelengths of light.
Current-generation photovoltaic cells are made from crystalline silicon in a very energy-intensive manufacturing process; thus EROEI is relatively low. I have seen EROEI estimates ranging from 1:1 up to 30:1, with a most likely range between 3:1 and 10:1. Assuming the lowest estimates are incorrect, this is high enough to be a viable technology, but it means that we should build plenty of photovoltaics while fossil energy is still cheap, to avoid finding ourselves constrained by an shortage of energy to manufacture energy-generating devices. As photovoltaic installations scale up from square feet to acres to square miles, we may also confront materials shortages. The good news is that solar cells can now be produced from a variety of materials, hopefully allowing for substitution if one or more is in short supply. Newer innovations, such as thin-film cells, require far less energy to produce and promise to raise EROEI and shorten energy payback times.
By concentrating sunlight from a wide area onto one location, using parabolic troughs or mirrors, it is possible to achieve very high temperatures, enough to allow electricity generation using conventional steam turbines. This process is limited by the same heat-to-electricity constraint as fossil-fuel-powered plants, and real-world efficiencies are in the range of 20-30%. Concentrated solar works only in direct sunlight, so it is well suited to desert regions. EROEI appears to be around 10:1. An added benefit is that the heat can be stored into the night, partially decoupling energy production from solar irradiance to balance electricity output. It is likely that as coal plants are decommissioned, concentrated solar plants will sprawl across the desert Southwest. Environmentalists will cry foul, but if one thing is clear it is that we cannot transition to a solar energy economy without devoting large areas of land to solar energy conversion. For instance, if we wanted to provide 100 quads of energy using 25% efficient concentrated solar, we would need to cover 0.8% of contiguous US land area – the equivalent of the state of Maine.
Solar Hot Water, Solar Heating, and Passive Solar Design
Noon solar irradiance to a south-tilted surface is approximately 1000 watts per square meter. While converting that energy to electricity entails a major energy loss, converting it to heat is subject to no such restrictions. Solar collectors can be as simple as black painted aluminum sheets surrounding copper water pipes, and they can capture around 90% of incoming solar energy as heat. The hot water can then be used directly or circulated through radiators or in-floor pipes to heat the home. Costs are relatively high, but only because solar heating is a niche market at the moment. There are no rare metals or expensive components required. With home heating and water heating using around a third of domestic electricity consumption, this is a huge opportunity for improvement. Virtually every home has sufficient sun-exposed area to install enough solar collectors to offset 70-80% of home heating and water heating electricity. Passive solar design is architecture aimed to maximize solar heat gain in winter while keeping the house cool in summer. In milder climates, passive solar design can completely eliminate the need for home heating.
MIT is developing an “artificial leaf,” a device that couples a photovoltaic cell with catalysts to produce hydrogen and oxygen from water. It is too early at this point to know if this technology will be viable on a large scale, but if it is it could allow for the production of storable chemical energy (hydrogen) directly from sunlight. Similar processes, also in development, use concentrated solar thermal energy to produce hydrogen or other chemical fuels. Our best course, in my opinion, is to continue to fund cutting-edge research, knowing that while most ideas will fall short a few may become the oilfields of the future.
Over two billion years ago, cyanobacteria evolved the ability to harvest energy from sunlight and to use that energy to split water and build organic carbon compounds from carbon dioxide. Cyanobacteria are the reason we have oxygen in our atmosphere, and they are the predecessors of all modern plants.
Photosynthesis is not particularly efficient, with maximum sunlight-to-biomass efficiencies estimated at around 6%. Taking into account the facts that plants grow only for a few months of the year and don’t absorb all incident sunlight, and that much of the biomass energy is used by plants to build roots, transport water, replicate DNA, construct cells, etc., real-life efficiencies are in the range of 0.2-0.5%.
These low efficiencies are balanced, however, by the miracle of self-construction and self-replication. Covering five acres of land with photovoltaic panels costs $5-10 million. A farmer can plant and harvest a crop on the same land for around $3000. Biomass can be burned directly to produce heat; wood has heated human dwellings for thousands of years and will probably continue to do so on a smaller scale far into the future. In the larger picture, however, biomass is mainly targeted for production of liquid fuels.
Ethanol and Biodiesel
Most people are aware of corn ethanol, a product that now makes up 10% of the gasoline supply across most of the nation. Unfortunately, corn ethanol accomplishes nothing in terms of weaning us from fossil fuels. In brief, it is the wrong solution to the wrong problem. Corn-based ethanol has an EROEI of 0.9-1.5, meaning that for every unit of ethanol energy produced, approximately one unit of fossil energy is consumed. This works out economically because corn production is subsidized and because most of the energy inputs are in the form of coal and natural gas, which are cheaper than gasoline. In the mid-1990s, Midwestern corn farmers were faced with a demand shortage. Rather than bank fallow land to rebuild fertility for future food production (the right solution), they elected to create demand by building ethanol plants. The plan worked for farmers, as corn prices doubled in a few years, but it failed dramatically in its aim to replace fossil fuels with “green” energy.
Corn is an energy-intensive crop to grow, requiring large inputs of nitrogen fertilizer. The kernels, the only part harvested, make up only about half of the standing biomass. Within the kernel, only the starch can be converted into ethanol. Yeast consume some of the energy in fermentation. Finally, purifying the ethanol requires distillation, an enormous energy input that alone amounts to 30-40% of the energy output. The end result is ethanol yields of around 1800 liters per hectare, which is 0.06% of the annual solar energy input. Of every 10,000 units of solar energy striking a field, only 6 end up in the ethanol. If the EROEI is 1.2:1, then this is effectively reduced to 1 in 10,000.
Even with an EROEI of 10:1 or higher, we would still need 3.6 times the contiguous US land area to produce 100 quads of energy at 0.06% solar conversion efficiency. Soybean-based biodiesel, while achieving a slightly higher EROEI, has an efficiency of only 0.03%. Clearly we need to think differently.
In Brazil, the ethanol picture looks better because it is produced from sugarcane in which the whole plant is harvested and residual plant debris is burned to provide energy for distillation. Similar results could be achieved by using corn stover (debris), but an even greater improvement can be attained by moving away from corn. Before white settlers arrived, Midwestern prairies grew tall grasses – big bluestem, Indian grass, and switchgrass. These crops require minimal fertilizer and grow thicker than corn. Using gasification, in which biomass is partially combusted at high temperatures, sunlight-to-fuel efficiencies can reach 0.28%. That is still not high enough (100 quads would require 77% of the contiguous US land area), but it is potentially sufficient to make a dent in our liquid fuel demands.
Algae and cyanobacteria offer two key advantages: 1) with their simple, one-celled structure, they require less energy for growth and maintenance and so offer higher solar conversion efficiencies, and 2) they can be grown in tanks anywhere there is sunlight, avoiding the food vs. fuel competition inherent in energy crops. Attempts to extract diesel-like oils from algae run into an EROEI crisis; it takes a lot of energy to separate a gallon of oil from thousands of gallons of algae slurry. More promising are engineered microbes that produce fuels continuously. Joule Unlimited has patented cyanobacteria that produce hydrocarbons from sunlight and secrete them. The insoluble hydrocarbons accumulate at the top of the tank where they can be skimmed off and burned in any diesel engine. Other researchers are developing algae and cyanobacteria that produce hydrogen.
As with any emerging technologies, there is at this point no guarantee that efficiencies will improve or that EROEI will rise to an acceptable level, but if successful these technologies will allow for direct production of chemical energy from solar energy. Even if efficiencies are only around 5%, biofuels could supply 25 quads of energy (current US transportation demand) using 1% of contiguous US land area.
Indirect Solar Energy
Wind results from uneven solar heating of the Earth. Warm air rises near the equator, then moves poleward and sinks. As the equator is moving faster than the mid-latitudes due to simple geometry, when the air sinks it is moving faster than the planet, and voila, we have wind. Fronts, topography, and weather patterns conspire to make winds violent, variable, and unpredictable.
Wind turbines convert linear kinetic energy of moving air into rotational kinetic energy of turning blades and finally electrical energy as the shaft turns inside a generator. Constrained by laws of physics, a turbine can convert no more than 59.3% of the kinetic energy in wind into useful form. Real-world efficiencies for turbines are in the 30-45% range.
Wind is the largest and fastest growing renewable energy technology today. EROEI is relatively high, above 10, and payback times are relatively short. One US Dept. of Energy analysis suggested that wind potential in the US is around 22 quads, more electrical energy than the 13 quads currently consumed. Even more than photovoltaics, wind suffers from erratic production, making it imperative to be able to store some of the energy, and indeed wind energy development is already constrained by a lack of storage capacity. I will discuss storage possibilities below.
Waves are driven by wind over the ocean and carry a large amount of energy distributed over a very large area. Anchored generators can be constructed that bob up and down as waves pass over, converting kinetic energy into electricity. Estimated potential is around 2 quads in US coastal waters, with very uncertain cost and EROEI at this point as no major installations have been completed.
Roughly 25% of incoming solar energy drives evaporation of water, and when this water returns as rain at high elevations it has potential energy. Hydropower supplies 2.5 quads of energy, mostly electricity, at present, and it is not projected to grow much in the future. Most large rivers have been dammed, and few people want to see dams on the remaining wild flows. There is fair potential for small-scale hydropower on tributaries. While this will enable many homesteaders to be “off the grid,” it will probably total less than a quad.
The largest renewable energy sources yield electricity or heat, but aside from biofuels none yield storable chemical energy. While a future “smart grid” will likely incorporate variable demand, such as charging electric cars during periods of excess supply, it will still be necessary to store vast amounts of energy in chemical or potential form. There are presently four options for bulk energy storage: batteries, pumped hydro, compressed air, and hydrogen.
Batteries are a mature technology, but they have very low energy densities and require metals that become expensive in large volumes. Lead-acid batteries store 41 Wh/kg. Storing a GWh, the amount a medium-sized city uses in an hour, would require 24,000 tons of batteries, roughly two fully loaded 100-car freight trains. Lithium-ion batteries have three times higher energy density but far higher costs. Flow batteries, in which liquid electrolyte is stored in tanks and flows through an electrolytic cell during charging and discharging, offer better economies of scale, but in general batteries are only economical for smaller installations and short discharge times.
Pumping water uphill converts electrical energy into gravitational potential energy; allowing it to return through a generator turbine yields electricity again. This process has a round-trip efficiency of 70-80%. Application is limited mainly by available sites. In mountainous regions, it may be possible to build reservoirs near large lakes and rivers to expand energy storage capabilities. Where reservoirs already exist, pumped hydro is by far the most economical energy storage option.
Many parts of the US have salt domes and other underground caverns that could be pumped full of compressed air to store energy. The idea sounds far-fetched, but the technology is proven and large facilities (150-300 MW) are currently in development. Excess electricity powers a compressor, and compressed air then powers a turbine generator when electricity is needed. The expected round-trip efficiency is 50-70%, with losses occurring as hot compressed air cools naturally in storage.
Hydrogen got plenty of media hype a few years ago but has recently gone out of vogue. There is no hydrogen to be mined on Earth; it is essentially an energy storage medium. At present no large electrolysis facilities exist in the US; 95% of hydrogen is produced from natural gas. Hydrogen is challenging to store when size is a constraint, such as in vehicle applications. For bulk energy storage that is less of a problem. Compared to compressed air storage, hydrogen has a large space advantage but a cost and efficiency disadvantage. At the same pressure, hydrogen contains 71 times more energy than compressed air. Hydrogen contains mostly chemical energy, while energy in compressed air is entirely potential. Catalysts for electrolysis (to generate the hydrogen from electricity) and fuel cells (to reverse the process) have traditionally contained platinum and palladium, keeping costs high. More recently, catalysts have been developed using nickel and other cheaper metals, lowering costs dramatically. Electrolysis is about 70% efficient and fuel cells are 50-60% efficient, yielding only 40% round-trip efficiency. This efficiency rises substantially, to around 70%, if fuel cells are located where the waste heat is needed, such as in homes or business districts.
I am uncertain at this point whether hydrogen will play a central role in our post-fossil energy future or whether it will serve primarily as an energy storage medium alongside compressed air and pumped hydro. Hydrogen systems can be affordably produced at small scales, so it seems likely that in the future we will see hydrogen tanks and fuel cells replacing diesel generators as emergency backup power. If hydrogen becomes the dominant form of energy storage, then we may see the natural gas pipeline network converted to hydrogen, with hydrogen supplied to each home for heating, cooking, and electricity (via fuel cells) when wind and solar supplies are low.
Summary – Where do we go from here?
Compared to our food systems, our current energy systems are far less transparent and more mysterious to most people in the population. Electricity and natural gas are simply there in our houses, with no intuitive sense of how much is consumed or how much is available. Perhaps it is not surprising, therefore, that people draw vastly different conclusions from the same data when it comes to our energy future. The majority, still, expect business as usual, that electricity and gas will always be there, and that prices will continue to rise slowly. People have lived through enough oil price spikes to be less certain about gasoline, but are still not worried enough to buy electric cars en masse. Those who understand that the fossil fuel bubble will soon burst are split between two camps. The optimists, epitomized by Amory Lovins, believe that as fossil fuel prices rise we will naturally develop alternatives and otherwise continue business as usual. The pessimists, led perhaps by Richard Heinberg, believe that alternatives cannot replace fossil fuels, or at least that they cannot expand rapidly enough to match the rate of fossil fuel decline, and that we will thus face an uncomfortable “energy descent.” I tend to side with the pessimists in believing that we will need to, for a time at least, get by using significantly less energy than we are using today and spending a larger portion of our income on energy. However, I don’t believe that this transition necessarily entails a collapse of society, and I remain optimistic that we will over the longer term succeed in developing technologies to make renewable energy abundant and affordable.
Part of the problem is that we don’t yet have a model. There is no portion of the developed world that has created a functioning renewable energy economy. We can argue back and forth about EROEI and what factors should or shouldn’t be included, but until we have a solar panel factory operating on solar power, using metals smelted with wind-produced hydrogen and carried by biofuel-powered trains – until we know that we can close all the loops entirely with renewable energy and still keep everything running – then some level of uncertainty will remain.
The good news, from a transition standpoint in the United States, is that our fossil fuels will not all run out at once. Oil is peaking now and will begin to decline. Natural gas might peak in 20 years. We have abundant coal reserves, enough for at least 50 years at current rates of use and probably more. We can, if need be, transition our energy streams one at a time, using what is left of our coal energy to build the solar and wind generation that will serve us ad infinitum. Furthermore, in many cases we can fulfill the same needs with less energy. A 50-mpg vehicle with four occupants is tenfold more efficient than a 20-mpg vehicle with one occupant. Thus we may find that advances in conservation and efficiency will offset fossil energy declines and make a transition to renewable energy more feasible.
The other good news, over the longer term, is that we live on an energy-rich planet. The amount of energy arriving from the sun dwarfs humanity’s energy demand by a factor of 7600, and if atomic fusion can ever be harnessed safely and economically the available energy will be effectively limitless. Our crisis is not one of energy availability but of energy conversion. It seems likely that should our species and our technological capability survive another millennium, human ingenuity will find a way to tap these sources to create a world in which energy is as abundant as it is in today’s age of fossil fuels.
The question mark, then, is our ability to collectively act rationally – to select appropriate technologies with large energy gains and high EROEI, and to do so while fossil energy is still relatively abundant. There is a risk that we will squander more resources on non-solutions like corn ethanol that provide financial benefits to a group with political influence while doing nothing to ease fossil fuel dependence. And there is a risk that fossil energy scarcity or climate change could exacerbate social inequality to the point that finite fossil resources are wasted in conflict rather than used for transition.
Finally, there is a risk that we will continue to multiply to the point that our planet simply cannot support us. Energy is not the only element in the equation; food is perhaps more important, and as food consumption edges closer to production capacity the system will become vulnerable to natural disruptions – events such droughts or volcanic eruptions that occur with some regularity on our planet. If there were fivefold fewer people on the planet we would not now be facing climate change and fossil fuel depletion, even with the same per capita energy use. If there were two- or three-fold more people on the planet we would face vast famines and food shortages. Population is thus a wild card, a factor that is seldom considered in energy futures analyses but that is critical to how we will experience the end of the fossil fuel age.
In the end, I am hopeful. Somewhere between the status quo and the doomsayers lies reality – a reality marked by increasing fossil fuel costs driving exponential growth in wind and solar, something we are already seeing. A reality marked by communities quietly building local food and energy networks while politicians posture loudly but fail to act. A reality marked by less discretionary travel and a greater sense of place. A reality marked by rediscovery of barter, exchange, and local currency as the larger economic system implodes and restructures. A reality, in short, that I am glad to be part of and to assist in creating.
Mark’s projected US Energy Portfolio, post-fossil fuels (c. 2100)
Total energy: 50 quads (1/2 of present)
- Solar photovoltaic 10 quads
- Concentrated solar 10 quads
- Wind 10 quads
- Biofuels 10 quads
- Hydropower 3 quads
- Geothermal 1 quad
- Ocean (waves, tides) 1 quad
- Nuclear (mainly thorium) 5 quads
Storage capacity (compressed air, hydrogen, pumped hydro): 2 quads
Mark’s top 10 renewable energy priorities (that we should be spending more time and energy on)
- Solar heating, hot water, and passive solar design
- Bulk electricity storage (pumped hydro, compressed air, hydrogen)
- Resilient local energy systems, including local energy storage
- Heat pumps for heating and cooling, coupled with weatherization
- Fuel economy (can and should be doubled/tripled using current technology)
- Photovoltaic and concentrated solar installations
- Public transportation
- Next-generation biofuels
- Wind (a high priority, but we are on top of this one)
- Nuclear fusion (a game-changer if it works)
Mark’s bottom 5 “renewable” energy priorities (that we are currently wasting time and energy on)
- Corn-based ethanol
- Soybean biodiesel
- Hydrogen from natural gas
- Cellulosic ethanol (EROEI still too low)
- Carbon trading (no net gain guaranteed, more money to speculators)