In the most recent posts last year, I looked in some detail at what the energy costs of energy supply imply for global-scale transition from fossil fuels to (mostly) renewable energy (RE) sources. The modelling presented there highlighted the importance of taking a dynamic view of transition – rather than just looking at the start and end states. If we’re serious about identifying feasible transition pathways, this type of approach has an important role to play. It’s reassuring to see that more significant effort is starting to be made in this area.
One reason this has been slow to gain traction is the idea that renewable energy sources are so abundant as to be without practical limits. It’s a popular and compelling story, but unfortunately, also one that obscures as much as it reveals. Here, I’ll explain why, and set out the detailed case for why we are much better served by thinking in terms of the practically realisable potential for renewable energy, rather than the raw physical flows. At the heart of this is a basic insight, expressed in a simple aphorism: ‘each joule of energy is not equal’.
1. Global RE resource potential
Energy from renewable sources that is available for conversion to forms useful for human purposes is ultimately both dependent on and constrained by the Earth’s natural energy flows. These are the streams of energy associated with naturally occurring physical phenomena that are ever-present in our physical surrounds and readily amenable to harnessing via conversion techniques of human origin.
As summarised in Table 1, the largest of these flows, from direct solar radiation and wind, are respectively almost four and two orders of magnitude greater than the current global total primary energy supply of around 550 EJ per year (EJ = 1018 J), or 17 TW (TW = 1012 W).
|Earth annual energy flow (EJ/year)||3,900,000||28,400||700–3,350||130–315||1,900–3,000||1,325|
|Earth energy flow rate (TW)||123,600||900||22–106||5–10||60–95||42|
Table 1: Earth energy flows. Sources: ,. Where reported figures differ significantly, lowest and highest values are included as a range. Ocean includes tidal, wave, ocean current and ocean thermal energy. The figure for geothermal relates to flows potentially suitable for electricity generation due to the heat source media being at a sufficiently high temperature. This excludes the far larger flows potentially suitable for low-temperature heating.
Set against the total primary energy supply figures, the earth flows are indeed staggeringly large. Their scale can, quite naturally, inspire awe. But when we compare these figures, we would also do well to hold in awareness that qualitatively we’re dealing with two very different things. We might express their magnitudes in the same units, but this doesn’t imply a common identity. Consider an analogy with water. We can compare the volume of the Earth’s oceans with the volume of water used for human purposes, without implying that the former is available for the latter.
Human exploitation of RE from the sources in Table 1 will always lie within the bounds established by Earth flow rates. But how far within these bounds? This is the question that matters for envisaging human societies that are entirely renewably powered, and for navigating pathways towards them. The very large difference between the total of the figures in Table 1 and current supply from existing sources is often invoked as grounds for asserting that there is, in effect, no practical limit to human energy use below this level. Many will now have seen Al Gore’s February 2016 TED talk in which he proclaims that ‘enough energy from the sun comes to the Earth every hour to supply the full world’s energy needs for an entire year – it’s actually a little bit less than an hour.’
In Al Gore’s case, this is primarily a rhetorical device to stir emotions, rather than sober scientific reportage. Elsewhere though, similar outlooks form the basis for serious technical analysis. One high-profile example from a few years back is found in Teske & Vincent’s  Greenpeace report ‘Energy (r)evolution: A sustainable Australia energy outlook’. The authors make their assessment of the future prospects for RE development in relation to theoretical potential, which they define as the ‘physical upper limit of the energy available from a certain source’ (emphasis added) (p.19), ostensibly equivalent to the quantity for which we’ve adopted the term Earth energy flow. The use of the term available here is important, as it implies a view that this physical upper limit could, in principle at least, be exploited for human purposes.Note 1
The approach to understanding RE ‘theoretical potential’ that I’ll outline here is different to one in which physical upper limits of natural energy flows are assumed to be available for human exploitation.Note 2 The upper bounds represented by Earth energy flows are perhaps more usefully conceptualised as ‘stocks’ or ‘pools’ from which the far smaller energy flows potentially available for human use can be drawn, rather than as practically-realisable limits on which this use could converge, given sufficient technological capability and economic scope. The size of the overall pool acts as a key driver for the practically-realisable rate of human use, but as we’ll see in the next section, this is governed by a host of considerations in addition to the rate of Earth energy flow.Note 3
The central contention here is that ‘theoretical potential’ is, under all circumstances, an inadequate metric upon which to base an argument for the straightforward nature of transitioning current energy use to RE sources. There are myriad factors that attenuate the proportion of such potential that can be practically realised. Investigating these matters adequately demands far more nuanced and painstaking analysis than some relatively superficial appraisals might imply at face value.
2. Energy for society: technical and economic potential
The case outlined in the last section is based on a very simple but often neglected premise: a joule of energy in one situation is not necessarily of equal value, in terms of the utility it can provide, to a joule in another. In order to make a quantity of energy in a desired form available for human use, resource costs will always be incurred.
Exploiting different sources—for example, wind in one geographic location, and wind in another—incurs different conversion and supply costs. Higher quality sources—locations with higher average wind speed and closer proximity to electricity users—incur relatively lower costs for meeting a given supply task. As such, to understand the prospects for an energy source to meet a given demand expectation, it is necessary to look beyond its gross size, measured by the number of joules associated with it, differentiating the source on the basis of the relative availability—or more colloquially, the relative quality—of one joule compared with another.
The distinction between economically recoverable reserves, technically recoverable resources and ultimately recoverable resources for fossil fuels reflects a similar understanding that the financial cost and technical challenge involved in exploiting a resource formation varies over its production life. In RE analysis, it is common to employ the analogous concepts of technical and economic resource potential. It is these measures that provide the appropriate reference points for assessing RE’s prospects to satisfy human demand, rather than raw Earth energy flows.
While there is no universal standard for defining technical and economic potential, here I’ll adopt the convention described by de Vries et al.  and Hoogwijk et al. , that, according to Hoogwijk et al., was introduced for use specifically in relation to wind energy by van Wijk et al.
- Theoretical potential: equivalent to Earth energy flow;
- Geographical potential: the theoretical potential for a specified geographic territory (e.g. “all non-glaciated land with wind speeds of class 3 and above”);
- Technical potential: energy extractable from geographical potential with a given configuration of converters (e.g. wind turbine power rating and spacing), minus losses due to energy conversion from primary energy flux to final energy carrier (electricity or fuels);
- Economic potential: the technical potential that can be realised at a given financial cost level, usually determined via market mechanisms in which various supply alternatives compete with one another.
Technical potential is the indicator of specific interest in estimating the upper bound for future energy supply, in the form of electricity and fuels, that might be realised in practice from RE sources. Estimates of RE technical potential are shown in Table 2, based on a survey conducted by Moriarty & Honnery , and incorporating additional data from de Castro et al. , and Smil .
As a comparison of the figures in Tables 1 and 2 makes clear, the relationship between Earth energy flows and technical potential for various sources is highly non-linear. That is, there is no general rule of proportionality between the size of the pool, and the amount of energy that can be appropriated for human use, as the popular view—typified by the report from Teske & Vincent  discussed in the previous section—tends to suggest.
It is important to note that published estimates are available that place technical potential for RE beyond the high ends of the ranges reported in Table 2. A prominent example is the study by Jacobson & Delucchi , the findings of which are summarised in Table 3, for locations classified by the authors as “high-energy”. From this it is apparent that estimates of technical potential are subject to considerable variation depending on the methodology employed.
The figures for wind reported by Jacobson & Delucchi provide a valuable illustration. The low figure they give (72 TW) is based on an earlier study by Archer & Jacobson . Jacobson & Archer  have more recently published an updated estimate of global technical potential for wind energy, using a different methodology, in which the equivalent figure, adjusted for the same geographical potential, is almost an order of magnitude lower, at approximately 9 TW.
|Solar||Wind||Ocean||Hydro||Biomass||Geothermal (electricity only)|
|Studies range (EJ/year)||60–2,592||32–600||1.8–33||32–95||27–1,500||1.1–22|
|Studies range (TW)||1.9–82||1.0–19||0.06–1.1||1.0–3.0||0.9–48||0.03–0.7|
Table 2: Estimates of technical potential for RE, for studies reviewed by Moriarty & Honnery , and incorporating recent additional sources that report values outside the ranges in their review. For solar and wind, the low range figures are from de Castro et al. ,; and for hydro, the low range figure is from Smil . All figures are for electricity output, except biomass which is based on heating value of fuels.
|Solar||Wind||Ocean||Hydro||Biomass||Geothermal (electricity only)|
|High-energy locations (EJ/year)||70,000||2,272–5,365
|High-energy locations (TW)||2,218||72-170
Table 3: Estimates of technical potential for RE from Jacobson & Delucchi . Figures in parentheses under wind are based on a more recent estimate from Jacobson & Archer  using a revised methodology; the figure presented is based on the reported finding from Jacobson & Archer for 100 m above all land outside Antarctica, using the land fraction for the wind regime on which the low figure reported by Jacobson & Delucchi is based.
A clear implication here is that any estimate of technical potential should be evaluated in relation to the adequacy of the methodology with which it is derived. This is an issue that, specifically in relation to wind, has received close attention recently from researchers who point out that estimates must take into account both a) the capacity of the Earth system to replenish atmospheric circulation, and b) the impact of human extraction on this circulation ,,,.
Until recently, technical potential estimates for wind have commonly been conducted on a bottom-up basis. This involves calculating the output that would be achieved from an array of wind turbines with known performance characteristics, assuming that air flow in the atmosphere is unaffected by the number of turbines added. In practice though, the energy extracted by one turbine reduces the energy available in the atmospheric air flow for another turbine to extract. The significance of this effect increases with the scale of wind development.
Miller et al.  instead take a top-down approach, in which energy available for extraction is fixed by the atmospheric solar heating replenishment rate. This gives estimates for wind power potential of between 18 and 68 TW (using three methodologies of varying sophistication).
Taking a different approach to addressing this issue, Adams & Keith  estimate that for large wind farms greater than 100 square kilometres in area, electricity output is limited to approximately 1 W/m2. On this basis, the global technical potential for wind power is 510 TW, assuming maximum exploitation over the entire surface area of the planet, including oceans and glaciated regions. For land area only, this equates to approximately 125 TW. This provides a physical upper bound only, rather than an estimate of practically realisable electricity supply from wind. The portion of this that could be developed in practice is a small fraction of the total, once considerations such as land use competition and economic viability of local wind regime are taken into account.
Even so, it is not uncommon to encounter claims that there is no barrier to meeting current global energy demand of 550 EJ/year (17 TW) from RE sources – and even projected future energy demand significantly greater than this – on the basis that an estimate for technical potential exceeds such demand. Indeed, this is the view expressed by Jacobson & Archer , who conclude that their estimated technical potential for wind of 80 TW over land and coastal area outside Antarctica implies that there is no ‘fundamental barrier’ to sourcing 5.75 TW electricity from wind in 2030. The legitimacy of such a claim rests, however, on the relevance of the comparison between estimated gross technical potential (omitting large and fundamental costs), and demand.
3. From technical to practically-realisable potential
Here the qualitative distinction discussed earlier between one unit of potentially exploitable energy and another must be taken into account. In assessing the technical potential for a given RE source, power generated in different locations has different value, even if all conversion systems are assumed to be of identical design. This follows from the geographic variability of Earth energy flows, and variability of the geographic relationship between generators and end use sites. For example, in the case of wind, electricity generated in a location with higher average wind speed and with end users located nearby will be of higher nominal value than electricity generated in a location with lower wind speeds and more distant end users. This is because life cycle supply costs will tend to be lower in the location with higher winds and close end-users than the location with lower winds and end-users further away.
In this respect, the development of RE resources follows a pattern typical of all resources: decreasing marginal return on effort, as exploitation of the overall resource potential expands. In very general terms, this implies an inverse relationship between the cost of development, and the ‘head room’ between the current level of resource development and the overall potential. As more potential is exploited, this head room decreases and effort required for further development increases. While early development may be relatively easy, as development expands further, each increment is more costly than the last, although this may be mitigated to some degree by concurrent technology improvement. In short, subject to political incentives and constraints, the ‘best’ locations tend to be exploited first, and as production expands lower quality locations are necessarily taken up.
Typically, well before aggregate development converges on the estimated technical potential, the increase in marginal development cost will constrain the level of resource exploitation. For any given resource, where this ceiling lies in relation to estimated technical potential is subject to significant uncertainty. Nonetheless, the general principle means that the fact that an estimate of technical potential exceeds anticipated demand cannot be taken to imply that there is no basic barrier to meeting such a demand level.
Following from this, adequately assessing the prospects for RE requires a nuanced and comprehensive understanding of development costs, and of how these costs vary as each RE source is developed. Here it becomes necessary to differentiate between financial costs, which are typically taken into consideration in assessing the economic potential for an RE source, and physical costs, which are directly relevant to estimating net, as distinct from gross, technical potential, but that in practice are rarely accounted for.
As I’ve discussed here at length in previous posts, for any energy resource development the physical costs that are of principal concern are energy costs: the energy that must be invested, over the life cycle of a facility, to make a quantity of energy available in the desired form – energy return on investment (EROI).
The specific relevance of energy investment for assessing technical – rather than economic – potential is its direct impact on the technical viability of an energy supply facility, in relation to the facility’s nominal function. If a facility has EROI < 1, then it is a net energy user and cannot contribute to the net supply that enables other economic activity. There may still be economic grounds for operating the facility, but it cannot make a positive contribution to aggregate technical potential. On the other hand, a facility may have EROI > 1, while providing a negative return on financial investment. In this case, it would still contribute to technical potential, but development of that potential would then only be possible with sufficient financial subsidy. Whether or not to develop such potential is a political-economic question to be determined on the basis of collective social values, rather than a technical question to be addressed on empirical grounds.
None of this implies that EROI analysis is free from uncertainty, or that its findings are not contestable. As with any analysis, its findings are dependent on the methodology employed, and the adequacy of its inputs. However, it can be made completely transparent, so that all methodological and data input assumptions are available to anyone wishing to assess the validity of the analysis. This allows for open dialogue on validity, and the possibility of convergence on a consensus view of technical potential. This is the approach adopted for the energy transition modelling initiative discussed in the previous posts. If, however, energy costs are omitted from consideration, arriving at a consensus view of RE technical potential appears less likely.
The broad implication of taking energy investment costs into account is that the fundamental physical limit for RE development, regardless of the uncertainties involved in estimating this in practice, is lower than most technical potential estimates imply. Moreover, the lower the EROI of a renewable energy source in general, the more significant this reduction in the physical limit will be. As reported in an earlier post, Moriarty & Honnery  show that non-linear increase in the energy costs of energy supply as total RE output increases leads to an optimum level of RE supply beyond which further development will decrease net energy return. Furthermore, they estimate that, when energy costs associated with managing ecological impacts of RE development are also included, this optimum level for all sources is likely to lie within the range of most mainstream estimates of future energy demand. Indeed, when practical environmental, economic, social and political factors, plus issues associated with transition timing, are taken into account, Moriarty & Honnery argue that this optimum level may well be less than current demand.
As discussed previously, in order to investigate the significance of the increase in marginal energy costs as total RE output expands, Moriarty & Honnery consider the case of global technical potential for wind power. They take as their reference case a scenario in which 2 MW wind turbines are distributed over the entire Earth land area at a density of one unit per square kilometre. This gives a gross electrical energy output of 516 EJ per year, equating to around 0.13 W/m2. As a check on the plausibility of this figure, it can be compared with the maximum extraction limit of 1 W/m2 estimated by Adams & Keith  and the equivalent maximum from the top-down analysis of Miller et al.  of 0.14 – 0.53 W/m2.
From the initial unconstrained gross technical potential of 516 EJ/year (16 TW), and accounting only for a series of realistic constraints associated with land use and supply reliability (i.e. ignoring other plausible constraints), the net energy available may be reduced by as much as 80 percent, to a little over 3 TW. This stands in strong contrast to Jacobson & Archer’s claim that their estimated technical potential of 80 TW makes achieving 5.75 TW electricity generation from wind a straightforward matter.
Note 1: The values Teske & Vincent report for theoretical potential exceed, in all cases except solar, the broadly accepted values for Earth energy flow reported in Table 1. In the case of wind, their figure appears to be based on the rate at which kinetic energy in the global atmospheric circulation is replenished via solar heating. But only a fraction of this (~25%) is dissipated in the region of the atmosphere adjacent to the Earth’s surface (and hence forming the Earth energy flow from which human exploitation can occur). For direct solar radiation on the other hand, they report a figure that is 36 percent of the Earth flow. They give the ‘potential’ (their exact terminology, presumably theoretical potential, as they understand it) of all RE sources as ‘3078 times the current global energy needs’ – around 41 percent of the total of all Earth energy flows in Table 1. This suggests that their use of the term theoretical potential is inconsistent. At times they seem to mean by this what we have termed here Earth energy flow, and at other times they seem to mean some version of what is typically termed geographical potential. It’s difficult to be sure though. Consider, for instance, how Teske & Vincent’s solar figure contrasts with Smil’s  estimated geographical potential for solar (he uses the term ‘potentially usable flux’) of 13 percent of Earth flow.
Note 2: On this point, a colleague asked if it was fair to say that ‘in principle’ earth energy flow is the physical upper limit for human exploitation, drawing an analogy with water harvesting where ‘in principle, all of the rainwater falling in a watershed could be captured if cost was no barrier.’ It seems on the surface like a neat analogy. It’s certainly consistent with the way that energy as a physical phenomenon is understood by most people. Energy conversion, however, is very different in kind to water harvesting. Removing energy from any flow changes the physics of that flow. Remove ‘all’ the energy, and the physical phenomenon from which energy is being exploited ceases altogether. So you can only take energy up to some limit well below this. This relates to one of Beyond this Brief Anomaly’s key conceptual themes: energy is not a ‘thing’ or a ‘substance’, in the way that it is useful to think of water as a ‘thing’ or a ‘substance’. It’s a metaphorical construct that we use to make sense of observed regularities in the way that change unfolds in physical systems. We treat it colloquially as an ‘object-like thing’, because that’s a convenient way for us to grasp such an abstract concept in concrete terms. And along the way, most people, even specialists in energy related fields, ‘forget’ this, and take energy to be an ‘object-like thing’. The origins of much confusion around energy issues can be found here.
Note 3: Nonetheless, as a comparison of the figures presented in Table 1 and Table 2 (see following section) indicates, the relative sizes of the various Earth flows do provide a very rough proportional indication of scope for each source to contribute to overall human energy use.
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