Category: Science

New discoveries, the scientific method, and all matters relating to the scientific study of the physical world

Carnot efficiency

Twist 1.5, Major's Hill Park, Ottawa

For a bit of light entertainment, I have been reading Tom Rogers’ book Insultingly Stupid Movie Physics, which basically covers the same terrain as his entertaining website, though at greater length and with more detail. Of course, one can never entirely escape climate change related information, and the book includes a discussion of Carnot efficiency: the maximum theoretical efficiency with which heat engines can convert thermal energy into useful power.

The efficiency depends on two factors: the high temperature produced using combustion, solar energy, geothermal energy, etc, and the cold temperature where the heat is expended into the surrounding environment:

Efficiency = ( 1 – Cold temperature / Hot temperature ) * 100

This has implications for technologies like the co-generation of heat and power. If the heat source for a power plant is 375°C (648°K) and it is dumping waste heat into 10°C (283°K) outdoor weather, the Carnot efficiency is about 56.3% (the actual efficiency is lower, for various reasons). If, instead, it is dumping the heat into buildings at 25°C (198°K), the Carnot efficiency falls to 54.0%. In a case where the heat source is just 200°C (473°K), the difference between a 10°C cold area and a 25°C cold area cuts the Carnot efficiency from 40.2% to 37.0%. In many cases, cogeneration is still worthwhile, despite the loss of useful electrical or kinetic energy, but it should be appreciated that the redirection is not without cost.

Carnot efficiency also helps explain why waste heat is not always worth capturing. If the temperature difference between the source and an available destination for the thermal energy is not large, there isn’t much useful power that can be produced.

[Update: 4:47pm] Remember to express the temperatures in Degrees Kelvin, by adding 273.15 to the figure in Degrees Celsius.

Balancing the environment and economy

Two mechanical diggers

When dealing with climate change, politicians often talk about the need to ‘balance the economy and the environment.’ I think this is a misleading categorization for two reasons.

Firstly, the balance has always been tilted virtually 100% towards the economy, in Canada at least. When the government talks about the need to scale back climate mitigation programs for economic reasons, they are talking about scaling back a handful of ineffectual programs that are not proving effective at reducing greenhouse gas emissions. The ‘balance’ dial between environment and economy is already twisted sharply towards the latter.

Secondly, even if we completely ignore the natural environment, the need to mitigate emissions remains. The Canadian economy could not survive the consequences of unrestrained emissions and climate change, with a temperature increase of 5.5°C to 7.1°C by 2100. If we care at all about the state of the economy 20, 50, and 80 years out, we need to avoid catastrophic climate change.

The economic analyses of mitigation that have been undertaken in the UK, Australia, and elsewhere have painted the same broad picture: it is possible to reduce greenhouse gas emissions significantly at a modest cost, provided you start early. The costs associated with inaction are much higher than those associated with this mitigation programme. To succeed, the whole economy needs to be pushed in the direction of decarbonization – a fact that remains true regardless of what balance you care to strike between economic health across the long term and environmental protection.

Limits of aquaculture

Common Grackle (Quiscalus quiscula), near Mud Lake, Ottawa

Seen from a simplistic and very selfish human perspective, ecosystems are devices for converting sunlight into human food. Sometimes, this happens fairly directly: sun hits soybean leaves, soybeans grow, and people eat them. In the case of the fish we eat, it is generally much less direct: sun hits phytoplankton, zooplankton eats that, they get eaten by fish that can eaten by successively larger fish, finally the largest fish get caught and eaten by us. In at least one important sense, this pyramid of energy use is quite different from the terrestrial one. In terrestrial agriculture, we manage the initial sun collection and can increase its amount in various ways. We are not, and perhaps never can be, farmers of plankton at the scale necessary to sustain the global marine food web. The effort involved in boosting the global plankton supply significantly would presumably be very large, given the immense biomass involved. Also, since energy is lost in each conversion, the amount of additional high-level species that would result from any increase would be smaller than the amount of additional plankton generated.

We are seriously overfishing the stocks that depend on the energy from existing phytoplankton stocks. If we start growing tuna and salmon in farms, feeding them fish from progressively lower in the marine ecosystem, we will eventually hit the bottom (if we keep having enough fuel for all those fishing boats). It is a fallacy to think that fish farms are like livestock farming on land. In the latter case, we are responsible for providing the inputs. In the former, we are still gathering from natural ecosystems, and doing so at an unsustainable rate.

Two partial solutions seem to exist. Firstly, we can get more fish per person by eating more plentiful species with lower trophic levels (closer to being creatures that eat plankton). That means anchovies for dinner, rather than tuna. Secondly, we could conceivably feed fish in farms using food from the land. That allows us to increase the basic solar energy being collected, and sustain a larger amount of tasty fish as a result. Of course, extending land-based agriculture entails other financial and environmental costs. Not least among these are the marine dead areas produced by pollution and fertilizer runoff.

The sensible way to run global fisheries is to avoid activities that cause disproportionate harm (dynamite fishing, catching juvenile fish) and then eat the sustainable portion of the output from different trophic levels. This means basically accepting a total level of sustainable human fish consumption for different species, then resisting political and financial pressures to exceed that limit. Of course, the record of human societies on doing this is dismal. We basically only fish sustainably when we are physically incapable of fishing more. Partly as a result of that, the general outlook for the world’s marine fisheries is dire.

Increasing renewable capacity is much harder than increasing energy consumption

David MacKay’s book (described here) makes an excellent point about the asymmetry between energy supply and demand, in terms of the difficulty or ease of increasing either:

It’s so simple for me to consume an extra 30 [kilowatt-hours] (kWh) per day. But squeezing an extra 30 kWh per day per person from renewables requires an industrialization of the environment so large it is hard to imagine.

For instance, buying a car and traveling 50 km per day in it means adding 40 kilowatt-hours per day (kWh/d) to your energy consumption. By contrast, surrounding all of the United Kingdom with wind turbines – with 15 per km of coastline, extending 4 km out to sea – would produce 16 kWh/d for every UK resident, if the wind was blowing all the time, and probably about 1/3 of that in actuality.

Statistics like that deepen my suspicion that a world without fossil fuel consumption will be one where there is much less energy consumption going on, overall. While increased efficiency can offset part of that, it also seems extremely likely that some very energy intensive activities will need to cease.

Sloppy reporting in The New York Times

In an article on refrigerants and climate change, New York Times reporter Irwin Arieff uses some rather misleading language to describe the warming effect associated with HFCs:

Environmentalists, meanwhile, say the shift to HFC-410A is only a halfway measure because the new refrigerant, while good for the ozone, still throws off heat, contributing to global warming.

As explained here before, greenhouse gasses (GHGs) do not cause the planet to warm because they themselves are warm or ‘throw off heat.’ Rather, they are opaque to the wavelengths of infrared light the planet radiates, and thus prevent some of that energy from escaping into space.

That said, it’s good to see that refrigerants are getting some attention as a category of GHGs, given how powerful they are relative to carbon dioxide and the special challenges involved in incorporating their management into an overall mitigation strategy. (See: Problems with carbon markets)

David MacKay’s sustainable energy calculations

For all the readers on this site interesting in climate change, policy, and technology, David MacKay’s book Sustainable Energy – without the hot air is a text that could be very profitably incorporated into our discussions. It seeks to evaluate whether (and how) society could operate without fossil fuels. It does so systematically, with all work shown, allowing you to question the methods and perform your own calculations for different circumstances. Another nice feature is that it is available online for free, though you may find it worthwhile to buy a professionally printed and bound copy.

The book is all about what is physically possible, rather than what is economical. As such, it sets a kind of base standard for sustainability. It evaluates whether something can be done at any cost, a pre-requisite to it being possible at a reasonable one.

To begin with, here is the methodology (p. 22 -28). It explains the exercise being undertaken and explains the key units to be used. The main unit of power selected is the somewhat unusual kilowatt-hour per day (kWh/d) per person (/p). While watts are more conventional, this unit does have some virtues in making things easily comparable and comprehensible. After all, if a kilowatt-hour (kWh) of electricity costs me about five cents, it is easy to start thinking about the economics of an activity that requires 30 or 40 kWh/d.

Here are a few chapters that touch directly on debates that have occurred (sometimes raged) on this site:

All the other chapters are relevant, as well, but these seem especially likely to inject some new information and thinking into long-running discussions.

The United Kingdom seems to be spoiled with people who are willing to perform comprehensive analyses of how their whole societal energy system could be rendered comparable with a stable climate (George Monbiot’s book is another example). It almost seems worth going through this entire text and re-performing the calculations with Canadian figures as inputs.

Somewhat short of that, would anyone be interested in going through the book chapter by chapter?

Bicycle physics

For those with an interest in both cycling and physics, the Wikipedia article “Bicycle and motorcycle dynamics” is well worth reading. It is interesting to note that lateral movements of bicycles (basically, those involving turning) are so mathematically complex that they require “two coupled, second-order differential equations… to capture the principle motions” and that these equations cannot produce exact solutions.

That contrasts in an interesting way with the experience of making turns at speed on a bicycle, and the appreciation one gains for the relationship between body movements, bicycle movements, and the condition of the ground.

Photos of Ontario and Quebec birds

Here is a list of the birds I have photographed so far as part of my open-ended project. The links go back to the posts in which the photos originally appeared. Eventually, I might sub-divide this list according to type or location.

Presently unidentified birds: none.

Climate sensitivity roulette

Big Bird in a cage

As discussed several times previously, two of the key uncertainties relating to climate change is (a) how much temperature would increase in response to any particular change in the atmospheric concentration of greenhouse gasses and (b) what humanity will actually emit between now and the achievement of global carbon neutrality. One way to express those uncertainties colourfully is with the Roulette wheels the MIT Joint Program on the Science and Policy of Global Change has created.

The wheels are based on results from the MIT Integrated Global Systems Model and have shaded areas proportional in size to different possible levels of temperature increase. The projections were recently updated, and the new ones contain significantly higher estimates of the risks of high levels of warming:

The new projections, published this month in the American Meteorological Society’s Journal of Climate, indicate a median probability of surface warming of 5.2 degrees Celsius by 2100, with a 90% probability range of 3.5 to 7.4 degrees. This can be compared to a median projected increase in the 2003 study of just 2.4 degrees. The difference is caused by several factors rather than any single big change. Among these are improved economic modeling and newer economic data showing less chance of low emissions than had been projected in the earlier scenarios. Other changes include accounting for the past masking of underlying warming by the cooling induced by 20th century volcanoes, and for emissions of soot, which can add to the warming effect. In addition, measurements of deep ocean temperature rises, which enable estimates of how fast heat and carbon dioxide are removed from the atmosphere and transferred to the ocean depths, imply lower transfer rates than previously estimated.

Full article

The ‘policy’ wheel assumes aggressive mitigation action, while the ‘no policy’ wheel assumes a business-as-usual course. It is notable that the chances of keeping warming below 2°C are infinitesimal, on that wheel. Even with aggressive action, our changes of keeping below 2°C of increase are looking increasingly distant, with effects that may be severe for both human and natural systems.

In addition to being a good visual image, I like the conceptual linkage between climate change and gambling. We are certainly taking a chance, whatever we do, but science can help us to assess the odds we face and make choices that reduce the risks of unacceptable outcomes.