Power Density: A Key to Understanding Energy Sources and Uses

  • Vaclav Smil
MIT Press: 2015. 9780262029148 | ISBN: 978-0-2620-2914-8

Vaclav Smil is a prolific, sometimes controversial, but invariably thought-provoking author. His latest book centres on a simple but important concept. Power density — defined in the book as “energy's rate of flow [transfer] per unit of surface area” of land or water — matters because these densities differ vastly for different methods of energy generation and use. That difference needs to be reconciled: massive, costly energy infrastructures such as long-range transport and storage are involved.

Wind turbines are low-density sources of power. Credit: Tom Paiva Photography/Getty

Megacities, with their concentrations of high-rise buildings, demand enormous quantities of energy in a comparatively small amount of space at any one time — and so have a high density of power demand. But diffuse renewable energy sources such as sunlight or biomass have low energy yields per hectare and intermittent availability, and so are low-density power supplies. Hence, in any transition towards renewables, cities will require vast renewable-energy hinterlands along with extensive cropland for food.

Smil excels when discussing the historical context of how our economy, cities and industry rose by harnessing high-power-density fossil fuels. He is also good at explaining the fundamentals that underlie the low power densities of renewables, such as the low efficiency by which plants convert the radiant energy of sunlight into chemical energy in the form of biomass (0.5%, Smil shows). Photos illustrate the energy options well.

Cities will require vast renewable-energy hinterlands.

However, Smil's technical discussion of options from renewables such as solar and wind to fossil fuels such as coal and gas, with its many site-specific examples, is necessarily repetitive and thus a tedious read. Moreover, he fails to clarify some fundamental concepts or to differentiate appropriately between different qualitative aspects of power and area.

Most fundamentally, he does not explain the concepts of power and energy per se; he seems to presume that readers are familiar with them. And it is unclear why he focuses on power density rather than energy density, or does not combine the two (given that energy = power × time). Imagine a stream of water running at 1 litre per second. The total amount of water (86,400 litres a day) is the energy; the flow at any one moment is the power (1 litre). If you need more than the available flow, your power demand exceeds the power supply and cannot be met. If you dam and store the flow, then suddenly release it, you vastly increase the power; but you cannot increase the total energy.

In my view, the primary constraint is the total usable energy flow. The rate at which energy can be transferred (power) acts as a secondary constraint, particularly for renewables, because of their low energy density and intermittent availability. Conversely, a high power density does not necessarily translate into a high energy density. To illustrate: lightning has an electric power equivalent to the output of ten large nuclear power stations. But because it lasts for just a fraction of a second, it only provides enough energy to power an electric car for less than 1 kilometre.

The book also creates confusion because many of Smil's examples of power density are actually energy densities. He is aware that power density as a measure should not focus on brief bursts of extreme energy availability, such as the solar electricity generated at noon in southern Arizona; it should be averaged over a larger territory and longer periods. And he appropriately calculates average, representative power densities, expressed as watts per square metre (W m−2). The standard unit for power, the watt, is defined as joules per second. But Smil's power densities are averages calculated over an entire year, and so become energy densities (in watt-years per year), an equivalence he fails to explain. However, he is not alone in this confusing use of power units that actually denote energy flows. Climate scientists also use W m−2 to describe the radiative balance of the planet and its alteration by increasing concentrations of greenhouse gases.

There are also important qualitative aspects that need to be considered. It matters whether power refers to electricity or biomass. And it matters whether land is used exclusively or only partly for energy. Smil raises this issue but does not address it systemically. He could have used the concept of exergy (energy's ability to perform useful work) to differentiate between high-quality energy (such as electricity, which is versatile) and low-quality energy (such as straw, which demands costly conversions to become usable beyond burning). However, his comparisons look only at power densities, irrespective of quality. Smil sketches out a valuable taxonomy of energy-related land uses along two dimensions. The first is exclusivity: the site of a power plant, for instance, is unusable for agriculture; right-of-way land underneath transmission lines is not. The second is longevity of use: a nuclear-waste repository will be in place for centuries, whereas an annual crop such as maize (corn) for conversion to ethanol can be grown in rotation. Yet Smil does not use this taxonomy, and as a result sometimes compares 'apples and oranges'.

Unlike in some of Smil's other books, the production quality of Power Density is regrettably low. I prefer readable graphics accompanied consistently by source and data referencing. In this book, many graph labels are hardly legible, and figures with references are the exception. Sometimes sources are mentioned in the text but not in the caption; at others, they are not even in the text. Graphs plot data on population and energy use, but statistical data sources are not specified or referenced.

Power Density's detailed examination of the spatial constraints of energy options adds to Smil's earlier, pioneering treatment of the subject, making it useful for energy specialists interested in exploring a massive ramp-up of renewables. But its technical nature and language make it rather inaccessible to a wider audience. And its failure to explain fundamental concepts such as the difference between power and energy, and to provide adequate data and source referencing, make it unsuitable as a textbook.