Energy density and the prospects for renewably-powered societies

In the post prior to last week’s, I looked in some detail at the energy densities associated with each of the conventional fossil fuels that together account for over 80 percent of global primary energy supply. As I pointed out, the highly concentrated nature of these energy sources is a fundamental enabling factor in relation to the forms of social and economic organisation that have evolved over the course of the industrial age. The norms, expectations habits and tendencies with which we live together today—and that for most of us, most of the time, remain largely below our thresholds of awareness—are intertwined in various ways with the characteristics of our energy sources. Different energy sources necessarily entail differences in these characteristics. In transitioning between energy source regimes, if key characteristics associated with an emerging regime differ sufficiently from those with which our major techno-economic infrastructure and socio-cultural institutions have developed, then at some point the infrastructure and institutions will themselves need to change for the process of transition to proceed. When such transition points are reached, the connections between energy resources and cultural expectations can no longer remain submerged from view: we’re required to confront the changing situation, and in many cases, we too must undergo our own transformations, individually and collectively.

We can see just such a process of transformation in relation to an area that I pointed out specifically last time—the concept of base-load electricity supply, that’s itself an artifact of centralised electricity infrastructure with limited capacity for matching fluctuating supply and demand. We often hear that stable base-load supply is essential for electricity grids to function without major and even catastrophic breakdown; this is frequently cited as a key impediment to providing large proportions of our electricity supply from intermittent renewable generators such as wind turbines and solar photovoltaics. It might seem reasonable to assume, then, that this is somehow essential to electricity’s use as a utility-scale energy source. This, however, may only be contingently so.

We require stable base-load supply now because historically the ways in which electricity has been supplied have provided us with precisely this. The great majority of all electricity—both now, but more importantly for this story, starting from the time when grid-based electricity supply infrastructure was first being rolled out on a large scale in the early decades of the twentieth century—is thermally generated in steam power plants. That is, most electricity generation plants use a heat source—such as combustion of coal, oil or gas, or nuclear fission of uranium fuels—to heat water in a boiler in order to form steam, which then drives a turbine connected to an alternating current generator. The basic technology involved has limited turn-down capacity—the minimum capacity at which a plant can operate continuously, as a proportion of its rated design capacity, is relatively high compared with other technologies. For instance, hydraulic turbines (as used in hydro-electric generation plants) and even open-cycle gas turbines (operating on a similar principle to the engines for commercial jet aircraft) generally allow much larger turn-down ratios. In addition to this, steam power plants require a lead time ranging from many hours to days in order to start up from cold. This is exacerbated where the plant is coal-fired, as the coal supply and combustion system involves extensive trains of complicated mechanical equipment that must be properly synchronised in order to work at all.

If a plant’s output does need to be reduced below the minimum level, then it must step down from that level to zero—but if any of its capacity is required on short notice (in response to spikes in electricity demand), then it must be maintained on hot stand-by. The boilers are fired continuously, and the steam circuit is maintained close to working conditions. In order to provide very short-term capacity for response, the turbines and generators must be kept spinning without load. A consequence of this is that it’s very expensive to have a plant operate below its minimum electricity output—most of the operating costs (e.g. fuel, operating and maintenance staff) continue to accrue, but without providing any revenue opportunity. This in turn raises the overall cost of electricity to customers. We’ve therefore developed electricity systems for which there are strong structural incentives ‘built in’ to the techno-economic foundations favouring minimum continuous supply levels. In turn, we’ve developed electricity demand patterns that match this preference for minimum continuous supply. Once these demand patterns are established, supply patterns—regardless of the technology by which electricity might later be generated—need to match the demand expectations of end-users. These demand expectations are very often built into the electrically-powered equipment itself, and particularly in industrial situations, into the production processes and ways of organising work with which they are associated. To borrow by analogy a concept from Humberto Maturan and Francisco Varela, introduced at the end of a post a few weeks back, electricity systems have evolved through a process of structural coupling between supply and demand infrastructures and institutions. The base-load construct is an artifact of this evolutionary process.

In an important sense, the extent to which we’re now relatively locked in to the need for stable, continuous base-load supply is a function not only of technical considerations, but of socio-cultural structures and expectations that have formed in the context of electricity supply systems that have these same base-load supply characteristics built-in by default. In principle, it is technically possible to build electricity supply-and-use systems that do not require continuous base-load supply—though to date, we haven’t done so because we haven’t needed to. This situation is now changing, as higher and higher proportions of electricity supply from sources that are not base-load friendly are mandated in political jurisdictions around the world. In a sense, the technical naivety of such mandated targets is driving changes in what we understand to be technically feasible. There seems to be little doubt that it’s far more challenging to build, operate and maintain electricity systems with intermittent supply. But we’re also only now learning what might in fact be possible in this respect (and what might not) by being required to do so via policy prescriptions for which the policy makers might justifiably be viewed as ignorant of the technical consequences. In the process though, once significant levels of intermittent supply are brought online, we will need to figure out together what electricity systems of this nature will look like, and how they will work. They may not perform at all as we’ve come to expect on the basis of conventional supply arrangements—and as a consequence, we may, individually and collectively, be required to transform our expectations of what it means to live with electricity.

With this excursion into base-load electricity supply, we’re dealing with the implications of a particular characteristic of fossil fuels: their great convenience, resulting in particular from their reliability and versatility. In important respects, this convenience is a double-edged sword. By enabling such vast expansion in the range and extent of economic and social activity, and in becoming dependent on—or at least, significantly attached to—the benefits that this provides, contemporary societies are highly vulnerable to disruptions in the supply of key energy sources, whether due to short-term break down or to longer-term resource depletion. Were the energy sources upon which we’ve grown our industrial economies and their attendant cultural expectations less reliable and versatile, then our present economic activity might well be less spectacular in its diversity and extent, but it’s also quite possible that our societies might have greater resilience—i.e. that we’d be better positioned for responding and adapting to disruptive circumstances.

Returning to the signature characteristic of fossil fuels with which I started the discussion today—their high energy densities—it’s apparent that this has similar implications. By virtue of their having evolved in the context of energy sources with unusually high energy densities, our present highly complex socio-cultural and techno-economic arrangements have built into them a vulnerability associated with their dependence on continued availability of energy sources with similar characteristics.

On the basis of this, it seems that there might be some merit in comparing the energy densities associated with fossil fuels with those associated with alternative energy sources carrying out a similar supply task. This in turn might give some sense of the prospects for those alternatives to replace or displace fossil sources on a large scale. Qualitative comparisons of this nature are frequently made—for instance, in considering the prospects for wind turbines or photovoltaic panels to substitute for coal- or gas-fired electricity generation. I often do this myself, as a way to quickly illustrate differences between energy sources. As a communication device, the language of energy concentration is often both useful and effective.

On close inspection, however, the concept of energy concentration turns out to be fraught with hidden traps for the casual user; avoiding inadvertent distortions requires considerable care in the concept’s application. This becomes much more obvious when we move from general, qualitative overview, to context-specific, quantitative analysis. Once again—and by now the ubiquitous influence of this theme is likely to be quite apparent to regular readers—the basis for this is to be found in the systemic nature of the energy concept. There’s little question that comparing fuels on the basis of energy density is both useful and relatively uncontroversial. Many fuels are—at least in principle—substitutable for a given energy supply task. The standard approach to determining heating values discussed previously ensures that energy densities are readily quantified, and that what is meant by such a measure is relatively free from ambiguity.

But the utility of the energy density concept—which I’m treating as synonymous with ‘energy concentration’—arises specifically because of the cognitive sleight-of-hand by which a fuel’s heating value becomes metaphorically characterised as ‘energy content’ i.e. we proceed as if the energy associated with a fuel is some sort of entity-like thing physically contained within the fuel material. And when we’re dealing with fuels, this works quite well—for most practical purposes, even where this sleight-of-hand remains hidden from view or is forgotten altogether, we can get by adequately on this basis. We can organise our practical social and economic affairs with sufficient success to meet our needs, and so there’s little reason to question it. In fact, a strong argument could be made that it is precisely because this conceptual simplification works so well in the case of fuels that we’ve been so successful in using the energy concept to organise our economic affairs—i.e. it’s made the energy concept more widely accessible, and hence easier to apply practically than it might otherwise be.

A quantity of fuel, viewed as a store of chemical potential energy, provides us with a handy proxy for the wider system via which its associated energy becomes available to us. For instance, the oxygen required for the fuel’s combustion is drawn directly from the atmosphere; its literal invisibility renders it insubstantial to us—and hence we tend to treat it as relatively inconsequential in the provision of energy. In principle, there’s no reason that we couldn’t propose an alternative convention for conceptualising fuels, turning the tables and speaking of the heating value of a quantity of air—requiring only the introduction of a suitable quantity of combustible material with appropriate chemical characteristics, under appropriate conditions, in order to make the heat content of that air available to us. At first glance it may appear ridiculous to suggest this—it seems obvious that the standing convention centred on the fuel as the critical component in the provision of thermal energy, rather than the air, is the natural approach. With a little reflection though, it’s apparent that it would be equally ridiculous to treat a fuel as an ‘energy store’ in the absence of availability of oxygen, or any of the other conditions required to make the energy that we metaphorically regard as stored in that fuel available to us. Why, then, would we base the quantification of energy density on the volume of fuel involved, rather than some other characteristic spatial volume associated with the overall system by which the energy associated with the fuel is made available to us? There’s no absolute basis for this—it’s simply a matter of convention and convenience that we go about this in the established way. This works quite well, provided the overall systems involved are indeed of a sufficiently commensurate nature. If we’re comparing, say, black coal and brown coal as fuels for a given electricity generation task, then the energy density of the fuels is indeed a useful high-level measure for roughly gauging the relative utility of those fuels, and the infrastructure implications for the particular task.

The adequacy of the insight that an appraisal based on energy density of fuels provides us with is, however, very much dependent on how we draw the boundary of the system that we have in mind. This becomes more apparent when we start to distinguish between fuels and primary energy sources more carefully. To date, except where context expressly required finer distinction, I’ve treated the term fossil fuel as more-or-less synonymous with both natural resources prior to production (the materials as they exists ‘in the ground’), and the materials provided to consumers by the production of those resources. The table of heating values and energy densities for major fuels that I provided previously related specifically to this latter category i.e. the produced fuel materials, rather than raw materials in their natural state. If we consider both the raw materials in their natural state, and the produced fuels, on a fine enough scale, then their properties will be the same. If, on the other hand, we take into account the broader geological context in which the resources occur, we might start to see energy density somewhat differently. The difference between conventional natural gas and coal seam or shale gas provides an interesting case in point. Large-scale production of coal seam gas in Australia involves recovery of relatively small quantities of gas from many wells distributed over large areas, as this photo depicts. At the scale of an individual well, the energy density of gas ‘in the ground’ may be equivalent to that for conventional wells, such as those in the offshore fields of Bass Strait from which most of the natural gas in the State of Victoria where I live is produced; certainly, at the well-heads themselves, give or take a little to allow for composition differences, the energy density will be similar. Ascertaining resource-wide values for such a measure is a far more involved matter though when taking into account distribution across many wells and large geographic areas. Issues such as the depth from which the gas is recovered also must come into consideration. Similar circumstances prevail in relation to coal and oil: the energy density of the fuels produced does not necessarily reflect well the energy density of the overall resource prior to production, and this is subject to change over time as the easiest-to-exploit resources are produced first, with later resources generally being associated with lower ‘in the ground’ overall energy density. We can also extend this to thinking about bio-fuels—in fact, the main point is perhaps more obvious in this case: compare a dense rainforest, with an open woodland, for example, in terms of the energy density associated with the resource prior to production. This is also reflected in differences in crop yields per unit of cultivated area for farmed bio-fuel feed-stocks, but it applies equally to our energy sources, the food we eat.

The difference though between energy density of refined fuels and the primary resources from which they’re manufactured perhaps reaches its zenith with uranium fuels for nuclear power plants. The uranium fuel loaded into the reactors has a heating value that’s typically in the range 500 to 4000 GJ/kg, depending on the type of reactor [1]. With a density of around 11,000 kg/m3 for uranium dioxide, UO2, this range of heating values corresponds with energy densities in the range 5.5 x 106 to 44  x 106 GJ/m3. Black coal, in comparison, has a specific energy around 24 MJ/kg and energy density of 20,000 MJ/m3. That’s a factor of between 20,000 and 170,000 difference between uranium fuel and black coal on a specific energy basis, and a factor of between 275,000 and 2.2 million on an energy density basis. We find a very different picture emerging though on shifting focus from refined fuels to primary resources. While black coal has essentially the same characteristics in situ as post-production (in fact, it’s energy density is effectively higher when in the ground; in that condition, as a solid material it is free of the voids which give it a reduced bulk density once produced in lump form), production of uranium fuel involves extensive, multi-step processing starting with the uranium oxide-containing ore. The great majority of the world’s known uranium resources comprise ores with grades in the range 0.02 percent to 0.5 percent triuranium octoxide , U3O8 [2], the principal component in yellowcake. As a consequence of these low ore grades, the equivalent specific energy of uranium ores is far lower than for the refined fuel, typically by factors very roughly in the range of 200 to 5000. For a factor of 2000, this brings the in situ heating value equivalent for uranium ore back to a factor of between 10 and 85 greater than coal, and the in situ energy density equivalent back to a factor of around 140 to 1,100 greater. These are still very substantial differences, but the effect of expanding the system boundary, for the purpose of comparing coal and uranium, from one that takes into account only the refined fuels, to one that sweeps in the primary resources, clearly has a dramatic impact on the picture that emerges.

It’s apparent from this that energy density is not on its own a sufficient measure by which to compare the utility of different fuels for a given energy supply task. Even so, provided we keep its limitations in mind, energy density does provide us with a handy guide for thinking about the relative merits of different fuels. The waters become far murkier though when we shift from a focus on the energy density of fuels—materials that can usefully be characterised, if only metaphorically, as stores or stocks of energy—to considering the energy density of flow-based sources, such as wind and direct solar radiation; we can also include electricity here, given that it cannot be stored and must be generated ‘on demand’. Whereas with fuels we can readily agree on an appropriate characteristic volume for quantifying energy density—i.e. we use the spatial volume occupied by the fuel itself—for a source best characterised in terms of the size of the energy flow rate associated with it, rather than the size of the energy stock that it provides, there’s no obvious characteristic volume that can be pinned down. This makes it particularly contentious to compare the energy density of stock-based sources with flow-based sources: there is no immediately obvious basis for drawing an equivalence in this respect between, say, a cubic metre of diesel fuel stored in a tank on the one hand, and a cubic metre of atmospheric air flowing past the blades of a wind turbine. Even though we can quickly arrive at a measure of the energy associated with each of these physical phenomena, and even though it is immediately apparent that the energy associated with the moving air is far more diffuse than that associated with the diesel, we’re really not comparing apples with apples by considering the air and the diesel in isolation, as the broader system contexts in each situation are so vastly different. For instance, consider a 2 MW generator powered by a diesel engine, along side a 2 MW wind turbine. Leaving aside issues of intermittency for the time being, while the physical scale of the wind turbine equipment will dwarf the diesel generator set by an order of magnitude or so, the support infrastructure that each requires is fundamentally different. The diesel generator set is tied to a fuel production and supply infrastructure that is globe-spanning in its extent, while the wind turbine’s primary energy source is exploited at point-of-use without the need for any further human infrastructure. Moreover, comparing the energy density associated with renewable energy flows—to the extent that sensible measures for this can be established—first with diesel fuel, and then with petroleum ‘in the ground’ across an entire oil field, the picture that emerges is likely to be significantly different. In the latter case, wind or solar radiation are likely to look somewhat less marginal than they would in the former.  This makes it very difficult to move from broad qualitative observations about comparative energy concentration, to sensible quantitative analysis that can guide assessment of the prospects for renewable energy to support an industrial civilization.

Just to be clear, my intention here is not to refute statements to the effect that the diffuse nature of renewable energy sources is likely to preclude them from powering the industrial societies necessary for providing the ways of life that citizens of the rich world are accustomed to at present. For a range of reasons that I’ll be looking at in more detail as we proceed, at this stage I think it’s quite likely that such a view will turn out to be pretty much on the money. Rather, what I’m attempting to point out is that making such a future-oriented appraisal in the present requires more than a qualitative assessment of energy density—and that even carrying out a quantitative analysis that focuses more narrowly on energy density will not really illuminate the situation much further.

Such caveats aside though, there is a sense in which quantitative energy density comparisons, even between flow- and stock-based sources, can be both valid and useful, provided the limitations are kept in mind. This relates to the tricky business of visualising our energy systems. For most of us—including those who are trained in energy-related fields of science and engineering or who work with energy infrastructure in a hands-on capacity—getting a concrete feel for what the abstract numbers that we’re dealing with in relation to global and even national energy use mean in practice can pose a significant challenge. Even when we’re confronted in person by the infrastructure involved in energy supply—as I have been on numerous occasions, for instance while accompanying students on tours of the electricity generation plants and coal mines of the Latrobe Valley east of Melbourne—it can be very difficult to get a sense of the difference in magnitude and scale involved in meeting the same supply task from alternative sources, and how this might relate to the prospects for large scale transitions in energy sources. By translating figures for energy use into spatial parameters, energy densities do provide us with a way of envisaging the infrastructure scales involved, and hence getting a rough sense of what such transitions might imply in terms of overall economic activity. In taking this approach, we’re still dealing with abstractions—in seeking an appreciation of what might and might not be possible in transitioning from fossil to renewable energy sources, the complexities associated with investigating systems-oriented questions with society- and economy-spanning scope mean that there’s no ultimate substitute for the action-inquiry of building actual infrastructure and learning from the experience. Nonetheless, even the very practical business of creating concrete things starts within present imaginations. With this as background, in the next post I’ll take up the task of very roughly comparing fossil and renewable energy sources quantitatively, using energy densities as the starting point, with a view to seeing if we can get a sense of the respective infrastructure scales that might be involved in powering our present industrial economies with renewables.




1 thought on “Energy density and the prospects for renewably-powered societies

  1. Pingback: Sustainable energy - the many dimensions of energy quality

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