Wednesday, August 22, 2007

Energy-generation: Future sources.

Note

In this post, I consider energy sources that have not yet been widely-implemented. Some, such as solar and tidal, are in the beginning stages of widespread implementation. Others, such as biofuel (for power generation, not vehicle fuel), are not. So, caveat: some of the claims about suitability here are a little on the speculative side.

Biofuel

At least according to Wikipedia, the most reasonable use for biofuel for power generation is biogas. This makes a certain amount of sense, given that natural gas is the best overall energy source of the fossil fuels, and that biogas is chemically similar (mostly methane and CO2). The structure of a biogas power plant is, thus, identical to that of a natural gas plant.

Biogas is produced by treating organic material, including biodegradable waste materials such as paper, food, and sewage, and crops grown specifically for their biodegradable content, in an anaerobic digester. The digester is a tank sealed to prevent the introduction of oxygen and contains bacteria which, ultimately, break down the organic matter into methane, CO2 and water. Since the tank is sealed, none of these products are released into the atmosphere, but can be harvested for human use. The bacteria themselves produce a waste product, called digestate, which can be used for soil conditioning .

However, all these products must be treated before they can be used or, in the case of the wastewater, allowed to re-enter the surrounding ecosystem. Biogas may require treatment before it can be used as a fuel, given that trace levels of certain chemicals (e.g., hydrogen sulphide) must be kept within strict limits in some jurisdictions. Furthermore, as with all biofuels, a key issue in adopting biogas is the feedstock required for the digester. Different materials have different gas yields; and some materials can't be broken down by the bacteria. Although sewage and manure can be broken down, much energy content of the organic material has already been lost to the animal that produced the waste. If the feedstock is contaminated with materials the bacteria cannot digest (such as glass or metal), the feedstock must be treated first in order to remove these contaminants.

Overall, the ecological impact of biogas is not huge. Burning it does produce greenhouse gases, but these are gases that have been only recently stored in organic material. If biogas is created from fuel crops, or vegetable food waste, then replanting these crops would, it seems, make biogas power at least close to carbon-neutral. Since biogas is chemically similar to natural gas, I would expect the energy production to be comparable.

Solar

There are a number of ways to create electricity from solar energy. The most commonly-known is via photovoltaic panels. These panels consist of a series of photovolatic or solar cells which chemically convert sunlight into electricity. Perhaps the most obvious application of this technology for energy generation is not in the form of large power plants, but building (and even vehicle or road) installations. They can thus be added to other forms of energy production (e.g., on the roofs of power plants). Something I didn't know, until today, is that there are large-scale photovoltaic power plants in Europe, and otehrs planned in Australia. These consist, as one would expect, of large banks of solar panels, some of which are designed to follow the sun as it moves across the sky.

Heat from the sun can also be used to generate electricity. The process should be familiar at this point: water is heated by the heat from the sun, and used to drive a thermal power plant, which converts the hot water/steam's energy into electrical energy. There are several different designs for capturing the heat, ranging from large trough systems to flat-plate systems to parabolic dishes. One that I found particularly interesting is the "power tower", which uses an array of mirrors to focus the sun's rays on a central tower, which contains a substance capable of storing the heat energy for later use in boiling water for steam turbines. Since the material in the towers is capable of storing heat energy, it is at least in principle possible to use them to generate some electricity during the night. (Although they would, obviously, be unable to handle any sudden increases in demand.) Thermal systems have, however, fairly low conversion efficiencies: according to Wikipedia, this can range from 20% to 40%, at best.

There doesn't seem to be any necessary problem with the cost of these technologies; as demand increases, price will drop, thus reducing the capital cost (and, it should be noted, also associated costs, such as panel installation). The production of the panels, mirrors, etc. is, environmentally, a possible problem: the resources consumed are limited, and the production processes do produce waste. However, solar is in this respect clearly superior to fossil fuel plants; and, indeed, anything humans build is going to consume something. The biggest issue I see is one that parallels a problem with wind: you need some other sort of technology (either power storage or another sort of generating facility) to pick up the slack when the sun goes down, and the solar panels are incapable of generating. Power towers are a possible solution to this problem, but I remain dubious as to their ability to handle overnight shifts in demand.

Geothermal

Geothermal power is generated by boring holes into hard rock, then pumping water down one hole. The water would be heated, and emerge as steam from the second hole, which could be used to generate electricity by driving a good thermal power plant. Geothermal plants exist in a number of countries, but not on a particularly wide scale. Input costs are the obvious construction costs, plus the costs associated with developing and using the drills needed to penetrate the rock. Operating costs should be no different from any other power plant. Output costs would be minimal, if the geothermal energy is truly renewable. There is no fuel consumed and little waste, gaseous or otherwise, released; water/steam can be recycled and reused to drive the turbine(s), and waste gases are emitted in very low levels from plants which use underground hot water sources. It's not yet known whether locations used for geothermal energy would, eventually, cool down -- that is, if they can be depleted. This depletion is probably temporary, given the heat that exists in the Earth's mantle will replenish a given site, but it suggests this power source may not be as stable as we would like. There is also a (I would suspect small) chance that drilling for geothermal energy on a large scale would result in seismic instability.

Tidal

There are two types of tidal power installations. First are barrages, which convert potential energy of a higher pool of water into electrical energy, using turbines like those found in hydroelectric power plants. The higher pool or basin can be filled in one of two ways. Under ebb or outflow generation, water flows in until high tide, at which point a set of gates are closed until the sea level has fallen. The gates are then opened and the water flows through turbines. Under flood generation, the basin fills through the turbines, generating electricity at flood tide. Barrage systems have all the attendant problems of hydroelectric dams: environmental impacts and significant capital costs being the most prominent. However, like hydroelectric dams, barrage systems do not consume fuel, do not produce waste, and have very low operating costs.

Second are tidal stream generators, which are, in essence, underwater versions of a wind turbine. (See here for such a system about to open off the cost of Northern Ireland.) Evaluation of these is thus largely the same. Tides won't turn the turbines constantly, so the power generated must be stored somehow. However, tides are more predictable than the wind; and since the systems are underwater, the aesthetic considerations are nil. Furthermore, the tides are capable of producing more power, because they move water, which is hundreds of times denser than air. Like wind, tidal energy of this kind is essentially inexhaustible.

A related possible source of energy is wave power. Wave power can be harnessed to produce electricity in a variety of ways. For example, mechanical energy can be created by a line of buoys rising and falling with the waves, and this energy can be converted into electricity. Waves can also be directed into a reservoir and then used to drive hydroelectric generators. Most of these technologies appear to be in early development. In principle, though, they seem no better or worse than solar, wind, tidal or hydroelectric power generation: once the cost is expended to create the necessary infrastructure, the energy is renewable and emission-free.

Fuel Cell

Fuel cells produce electricity via a chemical reaction. A fuel and oxidant flow into an electrolyte, and the resulting chemical reaction produces electricity, which can be harvested for a variety of applications. There is a tremendously wide variety of fuel cell designs; I'm not even going to try to describe them in any depth. They use a variety of materials, operate at a range of temperatures, and have quite a range of energy efficiencies. Their application in power generation is, consequently, also quite varied. For example, according to Wikipedia, the Stuart Island Energy Intiative in Stuart Island, Washington uses solar panels to power an electrolyzer, which produces hydrogen (presumably from water). This hydrogen is then used to run a fuel cell which creates electricity. I mention them here only because, like small photovoltaic installations, fuel cells are a potential source of small-scale power generation. They could also be used to generate constant levels of electricity, serving as a back-up to power generating technologies (such as wind and solar) that are variable in their output.

Conclusion

Most of these technologies are not in widespread use, so evaluating them will get a little speculative. However, thus far, they all look fairly good. They consume fewer resources as input costs than conventional fossil fuel plants, or nuclear plants, for that matter. They also have low operating costs and output little more than electricity. Some are, however, limited. For example, solar technologies will only generate power during daylight hours. Wave farms and tidal power technologies are dependent on changes in water flow patterns; although these are predictable, they are nonetheless variable. Geothermal plants could temporarily exhaust underground thermal energy. However, biogas and fuel cell plants could, in principle, provide the needed backup to alleviate the loss of generation from one of these sources. No one is ideal, on its own, but a balanced and careful exploitation of a variety of energy generation sources may be, ultimately, the best solution.

The real question, though, is how long it would take to get these generating technologies working on a sufficient scale to meet both current and future energy needs. Except for fuel cells (depending on type), all the technologies discussed above require significant capital investment and time before they can generate significant amounts of electricity. And some, such as wave farms, are still largely in development.

Sources
http://en.wikipedia.org/wiki/Biofuel
http://en.wikipedia.org/wiki/Biogas
http://en.wikipedia.org/wiki/Biogas_powerplant
http://en.wikipedia.org/wiki/Digestate
http://en.wikipedia.org/wiki/Solar_power
http://en.wikipedia.org/wiki/Photovoltaics
http://en.wikipedia.org/wiki/Photovoltaic_power_stations
http://en.wikipedia.org/wiki/Solar_thermal_energy
http://en.wikipedia.org/wiki/Geothermal_power
http://en.wikipedia.org/wiki/Tidal_power
http://en.wikipedia.org/wiki/Wave_power
http://en.wikipedia.org/wiki/Wave_farm
http://en.wikipedia.org/wiki/Fuel_cell

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