5.8 Why renewables?
For several decades the finite nature of fossil fuel reserves has been appreciated. Although known reserves have increased, this is offset by their greater inaccessibility. But there is another imperative for a reduced dependency on coal, gas and oil. Over the past three decades there has been mounting evidence, first for the increasing level of pollutants and carbon dioxide in the atmosphere through the use of fossil fuels, and secondly that the increase in carbon dioxide is causing an increase in the temperature of the atmosphere through The Greenhouse Effect. It is almost universally accepted that this is having an effect on weather patterns and sea level. The debate is now over how large and catastrophic these effects will be, and whether they can be reversed or at least mitigated.
The Greenhouse Effect
The Greenhouse Effect refers to the way in which temperatures rise when heat loss is inhibited, originally used in connection with glass 'greenhouses' used in gardens to provide a warm environment for plant growth. Such a structure allows daylight to pass through the glass and fall upon objects within it. The warmed objects lose an equivalent amount of energy predominantly by emitting long wavelength infrared radiation. Because the glass is less transparent to this than it is to the shorter-wavelength light that comes in, the air and the objects inside the greenhouse continue to warm up until they are warm enough to lose energy as fast as they gain it.
This effect is not restricted to greenhouses. It also applies to the entire global system. Radiation incident from the Sun warms the whole of the planet. The average temperature of the Earth is the result of the amount of cooling by radiation emitted from the Earth's surface just balancing the amount of heating by radiation incident from the Sun. The atmosphere acts like the glass of the greenhouse. In particular, it is more difficult for radiation from the relatively cool surface of the Earth to penetrate the atmosphere than it is for radiation from the relatively hot surface of the Sun. In fact if this weren't the case and the Earth had no insulation provided by the atmosphere then life as we know it would not exist – the entire surface would be frozen. In truth the greenhouse effect – in moderation – is no bad thing.
The problem comes when, in effect, the Earth's insulation layer is supplemented by another layer or a more effective material. For the Earth, gases such as carbon dioxide and water vapour are responsible for ensuring that the atmosphere provides this insulation. So we have to be careful. Increased levels of carbon dioxide in the atmosphere resulting from the burning of fossil fuels causes an enhancing of the greenhouse effect.
Average global temperatures have certainly risen over the past century by about 0.5 °C and the level of carbon dioxide (a major greenhouse gas) has increased from 280 ppm (parts per million) in pre-industrial times to a current level of 392 ppm (at the time of writing, 2012). Predictions for the rise in global temperatures over the next 100 years vary from 0.5 °C to 3 °C. The lower end of this scale will almost certainly be survivable, but even the smallest temperature increase may lead to more frequent and more catastrophic weather events. The Intergovernmental Panel for Climate Change has predicted that several low lying Pacific islands will become swamped by increased sea levels as a result of Seawater expansion and that countries such as Bangladesh could lose as much as 17% of their land area – an area populated by around 20 million people. In addition to this, melting of the ice over Antarctica and Greenland will have an effect at least as great as that of seawater expansion upon sea levels.
Water does not expand much on heating: a mere 0.02% per degree Celsius. That's not much; surely it can't lead to significant deepening of warmed oceans? So what if the top few metres of water do become a little warmer?
Let's try a few sums. To keep things simple I'll presume that sideways expansion is restricted by the continental land masses so that any expansion of the water in the sea causes a simple increase in its depth, see Figure 95.
The US National Oceanic and Atmospheric Administration has examined seawater temperatures over 40 years towards the end of the twentieth century and reported the following:
- A temperature rise of about 0.3 °C on average over the first 300 m of depth has occurred.
- A temperature rise of about 0.06 °C on average over the range from 300 to 3000 m of depth has also occurred.
The depth of water expands by 0.02% per degree Celsius. That's 0.2 mm per metre of depth for each degree Celsius. For the first 300 m the expansion is (0.2 × 300) mm per degree Celsius, so a 0.3 °C rise would cause a depth increase of:
0.3 × (0.2 × 300) mm = 18 mm.
The next 2700 m will similarly account for an extra depth of over half as much again.
Activity 43 (self-assessment)
Show that expansion of 2700 m of deep sea on warming by a mere 0.06 °C will lead to a depth increase of about 30 mm.
The temperature rise × (the number of mm of expansion per metre of depth per degree × the depth in metres) = increase depth in mm.
- 0.06 × (0.2 × 2700) mm = 32 mm
It is clear that if the temperature of the oceans continues to rise over the next century then some lower lying lands could literally 'go under'.
There is a long record of data for the global average sea level, taken for the most part from tidal gauges, but latterly from more sophisticated tools such as satellite radar measurements. Figure 96 shows the long-term trend.
Hence, there are compelling reasons to rapidly expand energy generation technologies that are non-polluting. The facts are becoming recognised at an international level. The first climate change conference at Rio de Janeiro in 1992, followed by the conference at Kyoto in 1997, sought to set legally binding commitments on countries for reductions in carbon dioxide emissions so as to achieve the same levels of emission as existed in 1990. As a result of this the UK is now committed to a target of reducing carbon dioxide emissions by 80% relative to their 1990 level by the year 2050. Measures to achieve this include cleaning up factory and power station emissions, reducing energy consumption and increasing energy generation from renewable energy sources. This last has been translated into an increase in renewable energy generation to a level of 10% of UK energy production by 2020, from a level in 2000 of about 1%.
Unfortunately, the Kyoto Protocol does not go very far. If fully implemented it is likely to restrict the increase in global temperature by no more than a tenth of a degree. Predictions suggest that cuts in carbon dioxide emissions will have to be around 60% just to stabilise its level in the atmosphere at about 550 ppm (i.e. twice the pre-industrial level). This is a level that whilst still having some effect, is likely not to be catastrophic.
Against this background, renewable energy derived from sunlight appears promising. Approximately the same amount of energy falls on the Earth's surface in one hour as all human civilisation generates for its use in a whole year. Although most of this falls on the sea and relatively inaccessible land areas, and of course drives the weather and other global processes, there is still a vast store to be tapped, for which we are now developing the technology.
Activity 44 (self-assessment)
List the main problems associated with continued and increasing use of fossil fuels.
- Resource depletion: natural gas is forecast to run out first, oil will follow, coal reserves are good for a few centuries but will become increasingly costly to extract.
- Pollution: particulates from industry and transport, and CO 2 and other greenhouse gases.
You may also have mentioned various other socio-political problems associated with energy resources being concentrated in small areas of the globe.
In this course we are primarily interested in an engineering solution to reducing the emissions of greenhouse gases by implementation of renewable energy.