Our Renewable Future

Chapter 1. Energy 101

It is impossible to overstate the importance of energy. Without it, we can do literally nothing. Further, the unfolding consequences of modern civilization’s energy use (including climate change), together with the inevitable energy transition from fossil fuels to renewables, will be the defining trends of the current century. How we address the climate–energy dilemma will make a life-or-death difference for current and future generations of humans, and for countless other species.

But we can’t participate usefully in discussions about energy without some basic knowledge of the subject. This chapter surveys a few simple energy concepts that everyone should be familiar with. These will include the definition of “energy” and an exploration of the forms it takes; the difference between energy and power; the Laws of Thermodynamics; the distinction between stocks and flows of energy; net energy (or energy returned on energy invested); lifecycle analysis (LCA) and lifecycle impacts; and the difference between operational energy and embodied energy.

What Is Energy? The Basics of the Basics

Energy is known by what it does: physicists define it as the capacity to do work. Energy exists in several forms—including thermal, radioactive decay, kinetic, mechanical, and electrical—and its form can change. For example, the energy stored in the molecular structures of coal can be released as heat through the process of combustion. That heat can be used to boil water, creating steam at high pressure, which can flow through turbines that spin magnets to produce an electric current, which is then passed through transformers and wires into homes and offices, where it is available to power computers, lights, and televisions.

Energy is measured in a variety of units, including joules, British thermal units (BTUs), and calories. When discussing electrical energy, the most common unit is the watt-hour (Wh). Sometimes the energy of fossil fuels is discussed in terms of barrels-of-oil-equivalent (boe). Where renewable energy is used in the form of electricity, we will discuss it in terms of watt-hours and megawatt-hours (MWh).

It is also important to understand the distinction between energy and power. While units of energy measure the total quantity of work done, they don’t tell us how fast that work is being accomplished. For example, you could lift a one-ton boulder up the side of a mountain using only a small electric motor and a system of pulleys, but it would take a long time. A more powerful electric motor could do the job faster, while a still more powerful rocket engine could rapidly propel a payload of identical weight to the top of the mountain in a matter of seconds. Power is therefore defined as the rate at which energy is produced or used. Think of it as energy per unit of time. The standard unit of electrical power is the watt (W). The amount of electrical energy a 10 W light bulb uses depends on how long it is lit: in one hour, it will use 10 Wh of energy. In the same amount of time, a hundred thousand such bulbs would use 1000 kilowatt-hours (kWh), which equals 1 MWh (1,000,000 watts = 1000 kilowatts = 1 megawatt).

Laws of Thermodynamics

Two important physical principles, known as the first and second laws of thermodynamics, describe important limits to the ways that energy works.

The first, known as the conservation law, states that energy cannot be created or destroyed, only transferred or transformed. In the example cited earlier (the use of coal to power household appliances), the total amount of energy is conserved at every stage. When energy chemically stored in the coal was transformed into heat, then electric current, and finally into the work of lighting our office or running our computer, some was “lost” at each stage. But that energy still exists; it is merely released to the environment, mostly as heat.

That’s where the second law, sometimes called the entropy law, comes in: it states that, whenever energy is converted from one form to another, at least some of it is dissipated (again, typically as heat). Though that dissipated energy still exists, it is now diffuse and scattered and thus not available to do work. Thus, in effect, usable energy is always being lost. The word entropy was coined by the German physicist Rudolf Clausius in 1868 as a measure of the amount of energy no longer practically capable of conversion into work. According to the second law, the entropy within an isolated system inevitably increases over time. Since it takes work to create and maintain order within a system, the entropy law tells us the depressing news that, in the battle between order and chaos, it is chaos that ultimately will win.

All of this means that it is technically incorrect to say that we “consume” (or “produce”) energy. We merely obtain it from places of higher concentration and get it to do work for us before it dissipates into less concentrated forms (ultimately, low-level dispersed heat). Hence energy density is an important criterion in assessing the likely value of potential energy resources (see the section “Energy Resource Criteria” later in the chapter). Only relatively concentrated energy is useful to us. We live in a universe teeming with energy; the trick is putting that energy to work. This is much easier when some of that energy happens to be temporarily concentrated in fossil fuels, wood, or uranium, or in a persistent flow pattern (e.g., a river, wind, or sunlight).

Areas of higher energy concentration can take either of two forms: stocks or flows. A stock is a store of energy—energy chemically stored in wood, oil, natural gas, or coal, or nuclear energy stored in stocks of uranium. Flows of energy include rivers, wind, and sunlight. Both stocks and flows present challenges: flows tend to be variable, whereas stocks can be depleted.

Some forms of energy are more versatile in their usefulness than others. For example, we can use electricity for a myriad of applications, whereas the heat from burning coal is currently used mostly for stationary applications like generating power (we formerly burned it in locomotives and ships, until oil proved its superiority for mobile applications). When we turn the heat from burning coal into electricity, a substantial amount of energy is lost due to the inefficiency of the process. But we are willing to accept that loss because coal is relatively cheap, and it would be difficult and inconvenient to use burning coal directly to power lights, computers, and refrigerators. In effect, we put a differing value on different forms of energy, with electricity at the top of the value ladder, liquid and gaseous fuels in the middle, and coal or firewood at the bottom. Solar and wind technologies have an advantage in that they produce high-value electricity directly.

Energy efficiency can be defined as minimizing the loss of energy in the process of obtaining work from an energy source. (How far can we get a gallon of gas to propel a two-ton automobile? That is the car’s fuel efficiency, which is often expressed in miles per gallon). Efficiency also applies to converting energy from one form to another. (How much electricity can a power plant generate from a ton of coal or a thousand cubic feet of natural gas? Power plant efficiency is usually expressed as a percentage, indicating how much of the initial energy is still available after conversion).

Net Energy

It takes energy to get energy: for example, energy is needed to drill an oil well or build a solar panel. Only net energy, what is left over after our energy investment is subtracted, is actually useful to us for end-use purposes. Sometimes the relationship between energy investments and yields is expressed as a ratio, energy returned on energy invested (EROEI, or sometimes just EROI). For example, an EROEI of 10:1 indicates ten units of energy returned for every unit invested.

The historic economic bonanza resulting from society’s use of fossil fuels partly ensued from the fact that, in the twentieth century, only trivial amounts of energy were required in drilling for oil or mining for coal as compared to the gush of energy yielded. High EROEI ratios (in the range of 100:1 or more[1]) for society’s energy-obtaining efforts meant that relatively small amounts of capital and labor were needed in order to supply all the energy that society could use. As a result, many people could be freed from basic energy-producing activities (like farming or forestry), their labor being substituted by fuel-fed machines. Channeled into manufacturing and managerial jobs, these people found ways to use abundant, cheap energy to produce ever more goods and services. The middle class mushroomed, as did cities and suburbs. In the process, we discovered an unintended consequence of having armies of cheap “energy slaves”: as manufacturing and other sectors of the economy became mechanized, many handcraft professions disappeared.

The EROEI ratios for fossil fuels are declining[2] as the best-quality resources are used up; meanwhile, the net energy figures of most renewable energy sources are relatively low compared to fossil fuels in their heyday (and this is especially true when buffering technologies—such as storage equipment and redundant capacity—are accounted for[3]). A practical result of declining overall societal EROEI is the need to devote proportionally more capital and labor to energy production processes. This would likely translate, for example, to the requirement for more farm labor, and to fewer opportunities in professions not centered on directly productive activities: we would need more people making or growing things, and fewer people marketing, advertising, financing, regulating, and litigating them.[4] For folks who think we have way too much marketing, advertising, financialization, regulation, and litigation in our current society, this might not seem like such a bad thing.

Net energy analysis (NEA) establishes a baseline for the economic usefulness of any energy resource. Decades of research suggest that, if an energy resource cannot yield at least three units of energy for every unit employed in energy production, it will not be economically useful in the long run[5] unless the utilization of the energy is highly productive. A low net energy resource (such as biofuel) could potentially be of value if it provides a large benefit (e.g., as fuel for aviation if petroleum were to become scarce), but a high-EROEI resource would then be needed to provide the energy for the production of that lower-EROEI resource.

Unfortunately, the net energy or EROEI literature is inconsistent because researchers have so far been unable to agree on a common set of system boundaries. Therefore two analysts may calculate very different EROEI ratios for the same energy source. This does not entirely undermine the usefulness of NEA; it merely requires us to use caution in comparing the findings of different studies.

Incorporating the dimension of time into EROEI analysis adds yet another layer of complexity, but doing so is essential if we are to realistically compare energy from flows (solar and wind) with energy from stocks (fossil fuels). The great majority of the energy investment into solar panels comes during their manufacture, while the energy return is delivered slowly over the decades of their projected usefulness. This front-loading of energy investment creates problems if we wish to push the energy transition very quickly (as is documented in a study by Dale and Benson at Stanford University, who found that all solar technology installed until about 2010 was a net energy sink, in the sense that it hadn’t yet paid for itself in energy terms[6]). If solar and wind build-out rates are very high, the net energy available from these sources will be smaller during the transition period (though wind’s higher EROEI should result in shorter delays in system-wide energy payback); see figure 1.1. During at least the early stages of the transition, the kinds of energy being invested in building and deploying renewable energy systems (mostly fossil fuels for high-heat industrial purposes and for transportation) will be different from the higher-quality electrical energy yielded from those systems.

WEB Figure 1-1 Energy input output and net power
Figure 1.1. Energy input, output, and net power during the build-out of new energy production infrastructure. GJ=gigajoules.
Source: Michael Carbajales-Dale, “Fueling the Energy Transition: The Net Energy Perspective,” presentation at Global Climate and Energy Project Workshop on Net Energy Analysis, Stanford University, Stanford, CA, April 1, 2015.

Life Cycle Impacts

Life cycle analysis (LCA) assesses the resource burden and potential environmental impacts associated with a product, process, or service by compiling an inventory of relevant energy and material inputs and environmental releases, and by evaluating the potential environmental impacts associated with identified inputs and releases. LCA is used to evaluate not just energy technologies but products and services of all kinds. However, since it typically tracks energy inputs (among other things), it is highly relevant for understanding our current energy systems and for planning the transition to renewables.

Virtually all energy processes entail environmental impacts, but some have greater impacts than others. These may occur during the acquisition of an energy resource (as in mining coal), or during the release of energy from the resource (as in burning wood, coal, oil, or natural gas), or in the conversion of the energy from one form to another (as in converting the kinetic energy of flowing water into electricity via a dam and hydro-turbines).

Some environmental impacts are indirect and occur in the manufacturing of the equipment used in energy harvesting or conversion. For example, the extraction and manipulation of resources used in manufacturing solar panels entail significantly more environmental damage than the operation of the panels themselves.

NEA and LCA are complementary: NEA ignores the environmental costs of energy production activities, whereas LCA identifies and quantifies these; on the other hand, LCA tells us nothing about the economic viability of an energy technology. LCA can be narrowed to encompass just materials use or energy use, or broadened to include greenhouse gas emissions and other environmental impacts. A shortcoming of LCA studies is that they represent a snapshot in time; for a process that hasn’t changed much in decades (e.g., cement making) this is not a big issue, whereas for communications technology it means that studies can quickly become outdated.

LCA also gives us useful information about our energy-demand activities—manufacturing, transportation, building operation and maintenance, food production, communication, health care, and so on.

Operational versus Embodied Energy

Another essential energy concept has to do with the difference between embodied and operational energy. When we estimate the energy required by an automobile, for example, we are likely to think at first only of the gasoline in its tank. However, a substantial amount of energy was expended in the car’s construction, in the mining of ores from which its metal components were made, in the manufacture of the mining equipment, and so on. Further, enormous amounts of energy were spent in building the infrastructure that enables us to use the car—including systems of roads and highways, and networks of service stations, refineries, pipelines, and oil wells. The gasoline in the car’s fuel tank supplies operational energy, but much more energy is embodied in the car itself and its support systems (fig. 1.2). This latter energy expenditure is easily overlooked.

WEB Figure 1-2 Life cycle embodied and operational
Figure 1.2. Life cycle embodied and operational energy of five vehicle types. Electricity use in electric vehicles (EV) and plug-in hybrid electric vehicles (PHEV) is based on the U.S. power supply mix.
Source: Nuri Cihat Onat, Murat Kucukvar, and Omer Tatari, “Conventional, Hybrid, Plug-in Hybrid or Electric Vehicles? State-Based Comparative Carbon and Energy Footprint Analysis in the United States,” Applied Energy 150 (2015): 36–49, doi:10.1016/j.apenergy.2015.04.001.

The energy glut of the twentieth century enabled us to embody energy in a mind-numbing array of buildings, roads, pipelines, machines, gadgets, and packaging. Middle-class families got used to buying and discarding enormous quantities of manufactured goods representing generous portions of previously expended energy. If we have less energy available to us in our renewable future, this may impact more than the operation of our machines and the lighting and heating of our buildings. It means those buildings, that infrastructure, and all those manufactured goods will be increasingly expensive to produce, which may translate to a shrinking flow of manufactured goods that embody past energy expenditure, and a reduced ability to construct high-energy-input structures. We might have to get used to consuming simpler foods again, rather than highly processed and excessively packaged ones. We might purchase, on average, fewer items of clothing and furniture and fewer electronic devices, and we might inhabit smaller spaces. We might use old goods longer, and reuse and repurpose whatever can be repaired. Exactly how far these trends might proceed is impossible to say. Nevertheless, under such conditions it is fair to assume that this overall shift might constitute the end of the network of economic arrangements collectively known as consumerism.[7] Here again, there are more than a few people who believe that advanced industrial nations consume excessively, and that some simplification of rich- and middle-class lifestyles would be a good thing.

The distinction between operational and embodied energy is important in the context of this report because it may well turn out to be much easier to operate machines and systems with renewable energy than to embody renewable energy into materials, machines, and infrastructure.

Energy Resource Criteria

In evaluating energy sources, NEA and LCA are essential, but the following criteria also need to be taken into account:

  1. Energy density
  2. Direct monetary cost
  3. Other resources needed
  4. Renewability
  5. Scalability
  6. Location
  7. Reliability
  8. Transportability

1. Energy density

Measures of energy density include mass density, volume density, and area density.

  1. Mass (or gravimetric) density. Mass density is the amount of energy contained per unit of mass of an energy resource. For example, if we use the megajoule (MJ) as a measure of energy and the kilogram (kg) as a measure of mass, coal has about 20 to 35 MJ/kg, natural gas about 55 MJ/kg, and oil around 42 MJ/kg (for comparison’s sake, the amount of food that a typical diet-conscious American eats throughout the day weighs a little over a kilogram (dry) and has an energy value of about 10 MJ, or 2,400 kilocalories[8]). However, an electric battery is typically able to store and deliver only about 0.1 to 0.5 MJ/kg, and this is why electric batteries are problematic in transport applications: they are very heavy in relation to their energy output. Thus electric cars tend to have limited driving ranges and electric aircraft (which are exceedingly rare) are able to carry only one or two people.
          Consumers and producers are sometimes willing to pay a premium for energy resources with a higher energy density by mass; therefore in some instances it might theoretically make economic sense to convert a lower-density fuel, such as coal, into a higher-density fuel, such as synthetic diesel, though the conversion process entails such high monetary and energy costs that most commercial efforts to do this have failed.
  2. Volume (or volumetric) density. Volume density is the amount of energy contained within a given volume unit of an energy resource (e.g., MJ per liter [L]). Obviously, gaseous fuels will tend to have lower volumetric energy density than solid or liquid fuels. Natural gas has about .035 MJ/L at sea level atmospheric pressure, and 6.2 MJ/L when pressurized to 200 atmospheres. Oil, though, contains about 35 MJ/L.[9]
          In most instances mass density is more important than volume density; however, for certain applications the latter can be decisive. For example, fueling airliners with hydrogen, which has high energy density by weight, would be problematic because it is a diffuse gas at common temperatures and surface atmospheric pressure; thus a hydrogen airliner would require very large tanks (themselves having large mass) even if the hydrogen were supercooled and highly pressurized.
          The greater ease of transporting a fuel of higher volume density is reflected in the fact that oil moved by tanker is traded globally in large quantities, while the global tanker trade in natural gas is relatively small. Consumers and producers will usually pay a premium for energy resources of higher volumetric density.
  3. Area density. The area density is the amount of energy that can be obtained from a given land area (e.g., an acre or a hectare) when the energy resource is in its original state. For example, the area energy density of wood as it grows in a forest is roughly 1 to 5 million MJ per acre.[10] The area density for oil is usually tens or hundreds of millions of MJ per acre where it occurs, though oilfields are much rarer than forests (except perhaps in Saudi Arabia).
          Area energy density matters because energy sources that are already highly concentrated in their original form generally require less investment and effort to be put to use. Thus energy producers often tend to prefer energy resources that have high area density, such as oil that can be refined into gasoline, over ones that are more widely dispersed, such as corn that is intended to make ethanol (fig 1.3).
WEB Figure 1-3 Volumetric and gravimetric density of fuels
Figure 1.3. Volumetric and gravimetric density of fuels and storage media.
Sources: Coal: Tadeusz Patzek and Gregory Croft, “A Global Coal Production Forecast with Multi-Hubbert Cycle Analysis,” Energy 35 (2010): 3111. Natural gas. Crude oil and wood. Batteries and additional batteries. All others: Charles Hall and Kent Klitgaard, Energy and the Wealth of Nations: Understanding the Biophysical Economy (New York: Springer, 2012).

2. Direct monetary cost

This is the criterion to which most attention is normally paid. Clearly, energy must be affordable and competitively priced if it is to be useful to society. However, the monetary cost of energy does not always reflect its true cost, as some energy resources may benefit from hidden subsidies or may have costs that are not currently directly paid for by the buyer (called external costs, such as health or environmental impacts). The monetary cost of energy resources is largely determined by the other criteria listed here, as well as supply and demand (table 1.1).

WEB Table 1-1 LCOE
Table 1.1. US average estimated levelized cost of electricity (LCOE) for new plants entering service in 2020 (2013 $/MWh). CCS, carbon control and sequestration; O&M, operations and maintenance; PV, photovoltaic.
Source: Energy Information Administration (EIA), Levelized Cost and Levelized Avoided Cost of New Generation Resources in the Annual Energy Outlook 2015, June 2015.

3. Other resources needed

Very few energy sources come in an immediately useable form. We can be warmed by the sunlight that falls on our shoulders on a spring day without exerting effort or employing any technology. But most energy resources, in order to be useful, require a method of gathering and/or converting the energy. This usually entails the use of some kind of apparatus, made of some kind of material (e.g., oil-drilling equipment is made from steel and the bits from diamonds). The extraction or conversion process generally also uses some kind of energy resource (e.g., the production of synthetic diesel fuel from tar sands requires water and heat; natural gas is often used). The availability or scarcity of the material or resource, and the complexity and cost of the apparatus, thus constitute limiting factors on energy production.

The requirements for ancillary resources in order to produce a given quantity of energy are usually reflected in the price paid for the energy. But this is not always or entirely the case. For example, some thin-film photovoltaic (PV) panels incorporate materials such as gallium and indium that are nonrenewable, rare, and depleting quickly.[11] While the price of thin-film PV panels reflects the current market price of these materials, it does not give an indication of future limits to the scaling up of thin-film PV due to these materials’ increasing scarcity.

4. Renewability

If we wish our society to continue using energy at industrial rates of flow not just for years or even decades but for centuries into the future, then we will require energy sources that can be sustained more or less indefinitely. Energy resources like oil, natural gas, and coal are clearly nonrenewable because the time required to form them through natural processes is measured in the tens of millions of years, whereas the stocks available will power society at best for only a few decades into the future at current rates of use. In contrast, solar PV and solar thermal energy sources rely on sunlight, which for practical purposes is not depleting and will presumably be available in similar quantities a thousand years hence.

Some energy resources are renewable yet are still capable of being depleted. For example, wood can be harvested from forests that regenerate themselves.However, the rate of harvest is crucial: if overharvested, the trees will be unable to regrow quickly enough and the forest will shrink and disappear. Even energy resources that are renewable and that do not suffer depletion are nevertheless limited by the size of the resource base (as will be discussed next).

5. Scalability

Estimating the potential contribution of an energy resource is obviously essential for macroplanning purposes, but such estimates are always subject to error—which can sometimes be significant. With fossil fuels, amounts that can be reasonably expected to be extracted and used on the basis of current extraction technologies and fuel prices are classified as reserves, which are always a fraction of resources (defined as the total amount of the substance estimated to be present in the ground). For example, the U.S. Geological Survey’s first estimate of national coal reserves, completed in 1907, identified 5000 years’ worth of supplies. In the decades since, most of those reserves have been reclassified as resources, so that today only 250 years’ worth of U.S. coal supplies are officially estimated to exist—a figure that is still probably much too optimistic (as the National Academy of Sciences concluded in its 2007 report, Coal: Research and Development to Support National Energy Policy[12]). Reserves are downgraded to resources when new limiting factors are taken into account, such as, in the case of coal, seam thickness and depth, chemical impurities, and location of the deposit.

On the other hand, reserves can sometimes grow as a result of the development of new extraction technologies, as has occurred in recent years with U.S. natural gas supplies: while the production of conventional American natural gas is declining, new horizontal drilling and underground fracturing technologies have enabled the recovery of “unconventional” gas from low-porosity rock (usually shale), significantly increasing the national natural gas production rate and expanding U.S. gas reserves.

Reserves estimation is especially difficult when dealing with energy resources that have little or no extraction history. This is the case, for example, with methane hydrates, with regard to which various experts have issued a very wide range of estimates of both total resources and extractable future supplies; it is also true of oil shale, also known as kerogen-rich marlstone (not to be confused with tight oil, which is sometimes confusingly called shale oil), and to a lesser degree tar sands, all of which have limited extraction histories.

Estimating potential supplies of renewable resources such as solar and wind power is likewise problematic, as many limiting factors are often initially overlooked. With regard to solar power, for example, a cursory examination of the ultimate potential is highly encouraging: the total amount of energy absorbed by Earth’s atmosphere, oceans, and land masses from sunlight annually is approximately 3,850,000 exajoules (EJ)—whereas the world’s human population currently uses just over 500 EJ of energy per year from all sources combined,[13] an insignificant fraction of the previous figure. However, the factors limiting the amount of sunlight that can potentially be put to work for humanity are numerous, starting with the materials and land use requirements for the building and siting of solar collectors. The technical potential for global solar power is calculated by various researchers at between 43 and 2,592 EJ; and for wind, between 72 and 700 EJ, according to figures collected by Moriarty and Honnery.[14]

Or consider the case of methane harvested from municipal landfills. In this instance, using the resource provides an environmental benefit: methane is a more powerful greenhouse gas than carbon dioxide, so harvesting and burning landfill gas (rather than letting it diffuse into the atmosphere) reduces climate impacts while also providing a local source of fuel. If landfill gas could power the U.S. electrical grid, then the nation could cease mining and burning coal. However, the potential size of the landfill gas resource is woefully insufficient to support this. Currently the nation derives about 16 billion kWh per year from landfill gas for commercial, industrial, and electric utility uses. This figure could probably be doubled if more landfills were tapped.[15] But U.S. electricity consumers use close to 200 times as much energy as that. Landfills deplete like oil wells. Further, more modern rubbish consists so much of paper and plastic that it probably won’t produce methane in quantities that are useful; the methane being harvested now is largely from trash buried in the 1970s when a greater proportion of rubbish was putrescible. There is still another wrinkle: if society were to become more environmentally prudent and energy efficient, the result would be that the amount of trash going into landfills would decline—and this would reduce the amount of energy that could be harvested from future landfills.

6. Location

The fossil fuel industry has long faced the problem of “stranded gas”—natural gas reservoirs that exist far from pipelines and that are too small to justify building pipelines to access them. Renewable resources often face similar hurdles.

The location of solar and wind installations is largely dictated by the availability of the primary energy resource; often, sun and wind are most abundant in sparsely populated areas. For example, in the United States there is large potential for the development of wind resources in Montana and North and South Dakota; however, these are three of the least-populous states in the nation. Therefore, to take full advantage of these resources it would be necessary to build high-capacity power lines from these states to more populated regions. There are also good wind resources offshore along the Atlantic and Pacific coasts, nearer to large urban centers, but taking advantage of these resources will entail overcoming challenges having to do with building and operating turbines in deep water and connecting them to the grid onshore. Similarly, the nation’s best solar resources are located in the Southwest, far from the population centers of the East Coast and Midwest.

Thus taking full advantage of these energy resources would require more than the construction of wind turbines and solar panels: much of the U.S. electricity grid would need to be reconfigured, and large-capacity, long-distance transmission lines would need to be constructed.

7. Reliability

Some energy resources are continuous: coal can be fed into a boiler at whatever rate the technology is able to accommodate, as long as the coal is available. But some energy sources, such as wind and solar, are subject to rapid and unpredictable fluctuations. Another way to say this is that our rate of using some resources is capacity constrained (e.g., by the size of the boiler and conveyor belts), while wind and solar are supply constrained, since the availability of the resource (sunshine or wind) dictates the rate of delivery. The wind often blows at greatest intensity at night, when electricity demand is lowest. The sun shines for the fewest hours per day during the winter—but power system operators are required to assure security of supply throughout the day and year (fig. 1.4).

WEB Figure 1-4 Monthly weekly daily electricity production
Figure 1.4. Monthly, weekly, and daily electricity production of solar and wind power in Germany, 2013.
Source: Bruno Burger, “Electricity Production from Solar and Wind in Germany in 2013” (Freiburg: Fraunhofer ISE, January 9, 2014).

As noted previously, intermittency of energy supply can be managed to a certain extent through storage systems, capacity redundancy, and grid upgrades. However, these imply extra infrastructure costs as well as energy losses.

8. Transportability

The transportability of energy is largely determined by the mass and volume density of the energy resource, as already discussed. But it is also affected by the state of the material (assuming that it is a substance)—whether it is a solid, liquid, or gas. In general, a solid fuel is less convenient to transport than a gaseous fuel because the latter can move by pipeline (pipes can move eight times the volume on a doubling of the size). Liquids are the most convenient of all because they can likewise move through hoses and pipes, take up less space than gases, and won’t dissipate into the air if not stored and distributed in perfectly airtight systems.

Energy resources that are fluxes or flows, like the energy from sunlight or wind, cannot be directly transported; they must first be converted into a form that can be—such as electricity. Electricity is highly transportable because it moves through wires, enabling it to be delivered not only to nearly every building in industrialized nations but to many locations within each building.

Transporting energy always entails costs—whether it is the cost of hauling coal (which may account for up to 70 percent of the delivered price of the fuel[16]), the cost of building and maintaining pipelines and pumping oil or gas, or the cost of building and maintaining an electricity grid. Using the grid entails costs too, since energy is lost in transmission (about 6 percent in the United States[17]). These costs can be expressed in monetary terms or in energy terms. The energy costs of transporting energy affect net energy.

Obviously, the evaluation of energy resources is a complicated process that entails the likelihood of estimation errors. The failure to take a single challenging criterion into account can lead to unwarranted optimism regarding the promise of an energy resource; on the other hand, the failure to foresee technological innovation can lead to too much pessimism. Some energy analysts advocate letting the market sort out energy winners and losers, but that’s not an adequate response to the challenges presented by climate change and fossil fuel depletion: if we wait for the market to force an energy transition, it will do so too slowly and too late to prevent both environmental and economic turmoil. Further, the market tends to look for short-term results; therefore, market-driven solutions may not be sustainable long-term.

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