Section 19. 'Flux' and Renewable Energy | |
Slide A, 'Flux' and Renewable EnergyRenewable energy is not like our conventional 'fuel-based' systems of energy use. Renewable energy is not 'fuel', it's a flux (or flow) of energy running through the natural environment. This means that as an energy source it's less dense than a fuel-based system, and it's also more difficult to gather this flux of energy because of the area of collecting equipment/infrastructure required. As the energy flow is lower, it also means that the energy return on energy invested ratio for renewable energy is lower than for the higher quality fuels such as oil and gas, and so we have to produce a greater quantity of energy than the equivalent value of fossil fuels that it replaces. The way we capture renewable energy sources influences how we develop the capacity to capture energy: for solar energy, it's directly proportional to the area of panels/cells optimally aligned to the incoming sunlight; for hydro-power it's how much water we can make vertically fall the greatest distance possible; for wind power and tidal streams it's the area of the rotating blades (not the height of the structure) in the flow of wind or sea water; and for biomass sources it's the area of land upon which we can grow a crop to produce energy. The scale of resource available depends upon the amount of energy flux in the media we are capturing. Wind is very light (about 1.2 kilos per cubic metre of air) and so doesn't produce much compared to a similar sized tidal stream turbine where the higher density (about 1 tonnes per cubic metre) means that about 900 times more power is produced. Even for the same source it also depends upon technology. For example, for each £1 spent, solar thermal (hot water) systems can produce nearly forty times more usable energy than solar photovoltaic (solar power) systems, because heat is easier to capture than the small part of the sunlight that is converted to electricity. The details of how we can capture and use renewable energy is a long presentation in its own right, and so in the Energy Beyond Oil presentation we'll look at two related sources: biomass and biofuels. Slide B, 'Flux' and Renewable EnergyIf we look at energy in terms of the Laws of Thermodynamics, the amount of energy we can get from any environmental flux of energy must be less than the total flux within that part of the biosphere. Given the distribution of energy flows across the Earth's surface and the efficiency of collection, the amount of usable energy may ultimately be quite low. The diagram above is a model of how energy flows through the Earth's biosphere. The values are given in peta-Watts (the instantaneous flow of energy measured in PW, 1015 or a million billion Watts) and in exa-Joules (the annual amount of energy measured in EJ, 1018 or a thousand million billion Joules). Most of the energy is solar energy, but there is a small input from the moon as gravitational energy (the tides) and from geothermal energy. The arrows show where energy can be extracted from the system. The limitation is that our collection of certain types of energy, such as solar power or wind power, can only trap a very small quantity of the total flow because we only have practical access to a small part of the Earth's atmosphere and the Earth's surface (e.g., why do we call this planet 'Earth' when 70% of it's surface is made of water?). For example, solar energy is highest at the equator, but the wind does not blow with any strength at the equator and there is little available water for hydro power outside of rainforests. Conversely, in temperate zones the wind and water options are better, but the solar resource (whilst still useful) could be a tenth of that at the equator. In the polar region there is very little of anything as much of the water is frozen for a large part of the year, there's very little biomass available, it's dark for 6 months of the year so there's negligible solar potential, and so the only viable option would be wind or geothermal power. If we take biomass, the energy flow through the 'terrestrial biomass' bubble is about 4,200EJ/year. The most efficient collection and processing system to produce electricity are perhaps 30% efficient, and so if we could collect all the biomass on the planet we might yield 1,260EJ of energy (unfortunately, that includes all our food too!). Slide C, 'Flux' and Renewable EnergyBiomass is one of our longest-used sources of renewable energy. Historically our food is our most important source of biomass-based renewable energy, after which it was firewood. Today biomass energy has taken-on a whole new meaning as it's been developed into a source of electricity, raw materials such as plastics, and fuel oils. The energy issue related to biomass is that plants are not necessarily the most effective way of capturing solar energy. A one-hectare (2½ acre) field in southern England has around 36,000GJ (about 400 houses worth) of energy shone onto it each year. However as the plants don't grow all year, and the leaves don't precisely track the sun, the amount absorbed by the plant is only 5% of this. The plant has to live and so only a small part of the absorbed energy is stored as usable biomass, so the energy available for use is about 0.6% of the energy input to the field. The most efficient form of energy conversion to electricity, gasification, is 40% efficient, and some of this will be lost in the electricity grid, and so the amount of solar energy utilised is around 0.2% of the amount that fell on the field. Conversely a solar thermal panel will capture 8% to 10% of the solar energy that shines on it. Therefore it's 40 to 50 times more efficient to heat water with solar energy directly rather than grow biomass and heat the water electrically – even if you used a combined heat and power system to use some of the waste heat from the process. The point related to heating water shows that there is a hierarchy of renewable energy sources. However, rather than a hierarchy related to the source of energy it's a hierarchy related to the use of the energy. For this reason the government doesn't consider the efficiency issue because a hierarchy based upon use (the demand-side of the energy system) is far too difficult and complex to be considered within our supply-side (the source of energy) centred national energy policy. For example, Britain could never get its electricity from biomass. If we take the net (the value once we take production and collection into account) value of short rotation coppice biomass, we'd have to cover 3.3 times the UK's total land area in coppice just to get our electricity supply (which is just less than a fifth of our total energy use at the present time!). Slide D, 'Flux' and Renewable EnergyBiofuels are very similar to biomass, with one important difference; as we only want a small part of the plant, and because we have to process the biomass to get the part we want, the energy return on the solar energy is even smaller. The solar energy flow shown above is very similar to biomass, but because we're only harvesting the plant oil the return is only about 1% of the solar energy. This would power the average diesel-powered car (this analysis is based upon oilseed used to produce biodiesel, which is by far the most efficient option) about 10,500 miles per year. There are about 30,000,000 cars and small vans registered on the UK's roads, each one running an average 9,000 miles per year. To produce enough biofuel for in the UK car fleet would require that we cover the entire UK in oilseed. Obviously, this isn't going to happen, and arguably an electric car would be slightly more efficient if operated from a more dense source of power, such as wind or wave. The problem is that producing that much electricity would also entail a massive development of wind turbines and wave power devices, and so in reality it would be far easier to use less cars for transport than try and convert or replace the current car fleet. There is of course a more efficient transport device than the private car – it's called a bus. And even more efficient than the bus, a bicycle! Background InformationBooks:
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 The Energy Beyond
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Energy Systems and Sustainability
Energy Beyond Oil
Renewable Energy
Evaluation of the Comparative Energy, global Warming,
Socio-Economic Costs and Benefits of Biodiesel
The Annotated 2-hour EBO Presentation Slides
