Our comfortable lifestyles are completely dependent on energy consumption. Today, our energy comes from many sources: some more efficient at delivering that energy than others, some producing more carbon dioxide into the atmosphere than others. OurWorldinData.org has a great deal of energy data depicted as graphs to help us visualize where energy is produced and used. See examples below.
Background Material
The main sources from which energy is produced today are the non-renewables coal, oil and natural gas. Traditional biomass, which is essentially considered carbon-neutral, is also still a major contributor to our global energy production. Growing in usage are some renewable energy sources including hydropower, wind, and solar. And, we must also keep nuclear energy in mind. This assignment connects the following learning objectives from our course: Explain general principles of environmental geology including: sources and fate of water and air pollution; how we get energy, water, and food; and demonstrate basic data interpretation skills including formulating hypotheses, detecting patterns in environmental data, testing hypotheses and drawing conclusions.
POWER GENERATION THROUGH FOSSIL FUELS
Coal-Fueled Thermal Power Stations
Coal is the world’s premier fuel source still, however the USA has been replacing coal with natural gas. Coal is pulverised and fed into a furnace where it burns, heating a water boiler to produce steam. The hot flue gasses rise up a tall chimney without assistance because they are far lighter than (cool) air. These rising gases produce the draft which brings a blast of air into the furnace.
The furnace operates continuously with lightweight “fly ash” rising with the flue gases, while heavy ash falls down below the flame and is removed. This simple furnace is probably the most common type found world-wide. It operates at a thermal efficiency of approximately 30%. That is, around 70% of the energy in the fuel is discharged into the environment (aka wasted).
The steam that is generated in the boiler (heated by the furnace) enters a turbine at its centre where the blades are small. The steam expands through these blades, spinning them. There are several sets of blades that the steam passes through as it expands and cools, each set of spinning blades turning the turbine to produce energy. Typical coal plants are about 600MW.
Natural Gas-Fueled Thermal Power Stations
There are three main options for natural gas-fueled power stations. First, it is possible to simply burn natural gas instead of coal in a furnace to clean up the process. However, the (former coal, now natural gas) power plant’s efficiency still remains at ~30%. The gas lends itself to a completely different type of generator. Instead of generating steam to spin a turbine, the gas is burned directly in the turbine (so it is a gas turbine, not a steam turbine).
Second, the modern gas turbine and generator combination, designed for use with natural gas rather than converted, yields electricity at an efficiency of around 45%. This can be improved if the heat of the exhaust is also used, which is the third type. The exhaust loses much of its heat expanding through the turbine, however it is still hot. This heat can be used to operate a steam turbine. However, often a ‘reheat’ stage is added, burning more gas to optimise boiler temperature for a steam turbine. This combined cycle generation reaches efficiencies of around 55%. Typical natural gas plants are about 500 MW.
Oil-Fueled Thermal Power Stations
Petroleum, usually as residual material from a refinery, is sometimes burned to raise steam for a turbine generator. This residue is solid at ambient temperature and its alternative use is as binder for road paving (asphalt) or roofing. Petroleum-rich countries in the Middle-East frequently burn it in power stations, sometimes using the waste heat to distill sea water (desalinization). However, in general, petroleum is too valuable as a refinery feedstock to be burned in a power plant, especially considering their efficiency for producing electricity is only around 40%. These power stations are much smaller than coal or gas, some only being 2MW! Many are in the 10MW size range, and some as big as 50MW. It is worth keeping in mind that one gallon of gasoline is equivalent to about 100 tons of buried plant material, buried under miles of overburden, “cooked” at the right temperature, and protected from escaping by impermeable caprock for millions of years.
Traditional Biomass
Finally, wood has been used as a source of heat energy for one million years, and it’s renewable, up to a point. The use of wood as a fuel has caused deforestation in several regions of high population density. Examples are much of India and China, and parts of Africa. Where fuel wood is no longer available, people have been collecting and drying animal dung (largely cow) as fuel. Wood is considered “carbon neutral” fuels because carbon dioxide was absorbed from the atmosphere when the trees grew. A similar argument can be made for animal dung which started out as plant matter (eg: pasture grass) which animals ate and processed. Biomass is being explored now also for power generation through thermal stations like those for fossil fuels described above. The new technology is showing high efficiency, over 70% and up to 90%, but power plants are small: typically, less than 25MW.
NUCLEAR POWER
Nuclear powered generating plants are usually touted as having no carbon emissions. Of course, this is not entirely true. Immense amounts of emitted carbon are “embedded” in the nuclear plant’s structure – principally as ferroconcrete in the huge slab poured under the installation and in containment domes that are built to contain radioactive gaseous emissions in case of an explosion. The reactor itself will have a steel shell around one metre thick in pressurised water reactors (PWR) and advanced gas-cooled reactors (AGR).
The ore from which the uranium fuel is produced usually contains actual uranium in tiny quantities – always less than 5% metal. Mining this involves digging and moving vast quantities of rock and then extracting the valuable portion, creating more emissions (as well as huge amounts of waste rock). Finally, all nuclear reactors (except the Canadian CANDU type) use uranium in which the fissile 235U isotope has been enriched from the naturally occurring 0.7% to around 3% – generating more carbon emissions. At the end of it there is a substantial quantity of depleted uranium, 238U with little if any of the fissile 235U isotope.
Efficiency generally runs around the 30% mark for the PWR. Any advantage gained by operating the primary water circuit at high temperature is balanced off by the need for a secondary water circuit. The AGR has the potential of running at considerably higher temperature that a PWR. As such, it can potentially post a higher efficiency. On average nuclear power plants are 1GW or 1000MW.
POWER GENERATION THROUGH RENEWABLE SOURCES
Hydropower
Modern water turbines can yield huge amounts of power, such as Niagara Falls, which is one of the largest hydroelectricity generators in North America. These installations have a nameplate output of around 4500MW, and are capable of operating 24/7. Hydropower is extremely size flexible – from a single kW to Niagara Falls and everywhere in between. The power output is dependent on the flow of the river the hydro system is installed upon. Hydro is extremely efficient, operating around 90% efficiency. It’s important to remember that although the operation of hydropower is CO2-free, CO2 emissions are created during construction and installation.
Wind Turbines
Land-based wind turbines are being built with ratings of 1.5MW to 5MW, while turbines to be installed in relatively shallow seas are rated at up to 14MW, with 100m long blades! Of course, wind turbines are only installed in areas with a decent wind regime allowing them to generate at their rated capacity for at least 30% of the time, but in windy areas (like the North Sea around Scotland) they may operate up to 70% of the time.
Wind power is touted as zero-carbon emission, but one has to remember that the steel or concrete tower and composite blades of a wind turbine both represent one-time carbon emissions related to: metal smelting for steel, limestone roasting for cement, melting silica sand for glass fibres, and a hydrocarbon resin which is made to harden around the turbine blades. This is not to mention transportation of materials, and the installation of copper or aluminum wire to bring the electrical power to the transmission grid.
Photovoltaic Solar Power
The first practical photovoltaic device, a silicon crystal, was invented in 1939 and patented in 1941. Initially costly (the silicon must be of very high purity) and of low efficiency, they were impractical until very recently. Efficiency has improved over time (from below 10% to 40% for the best – 22% is typical for commercial panels today). Most photovoltaic power generation plants are small, on the order of 5-10MW, however there are several 200MW plants around the world, and even some at the GW scale!
Once again, the CO2 emissions related to producing, transporting and installing the photovoltaics are a one-time thing; operation produces no carbon emissions. The main concern against solar photovoltaics lies in the toxic pollution created during manufacture.
Solar Thermal Power Generation
These are the large-scale mirror installations that work to concentrate solar energy onto a receiver pipe containing a fluid (typically oil, but sometimes water, or even molten nitrate salt). The super-heated fluid is used to generate steam and turn a turbine, much like the power plants described for fossil fuels, and have a similar efficiency of 35%. These power plants vary in size, most are around 250 MW.
Quantitative Assignment 3: Energy Comparison Calculation (50 pts)
Part 1: Power Generation (20 pts: 10 pts each)
- Choose a non-renewable and a renewable energy source to compare with nuclear. For each of these three calculate how much energy they can produce in a year. For example; if your source can produce 200MW of energy and it runs about 40% of the time over the span of a year (24hrs/day, 365 days/year), this would look like: 200 x 24 x 365 x 0.40 = 700,800 megawatt-hours/year. You should have three calculations (one for each source) and show your work.
- Perhaps you are comparing a nuclear power plant with a single wind turbine or dam and a natural gas power plant. The scale of these power generation facilities differ greatly. Set them to equal power generation and compare again (e.g. how many wind turbines equals one coal fired plant?). Show your work.
Part 2: Electricity Consumption (20 pts: 5 pts each)
- A home modem/router system uses about 25 watts. To deliver our internet connectivity that we so depend upon they run 24 hrs a day year-round. Using the same formula described above, calculate how much energy a typical modem uses in a year (in megawatt-hours). Show your work.
- How many of each of your chosen power generation sources does it take to power a typical modem for a year?
- Several years ago there was a massive push to replace 100-watt incandescent lightbulbs with fluorescents, and now, LEDs. Assuming a lightbulb might be on 10 hours a day, calculate how much energy the old incandescent bulbs consumed in a year. Now compare this with a modern LED bulb that uses about 10 watts to emit the same amount of light. Show your work.
- How many of each of your chosen power generation sources does it take to power a lightbulb for a year?
Part 3: Whaddaya Think? (10 points)
Comment (reflect) on your comparisons. What do these calculations mean to you?
