Category Principles, technologies, and impacts
The amount of carbon as CO2 in the atmosphere is currently around 800 Gt. For every Gt emitted into the atmosphere, approximately half is absorbed by the Earth, mainly by the oceans. The burning of fossil fuels has emitted some 225 Gt of carbon since 1955, so the amount of carbon in the atmosphere has increased by about 110 Gt during the last 50 years. This increase corresponded to a 63 ppm rise in CO2 and a rise in average global temperature of about 0.5°C. The amount of carbon currently emitted into the atmosphere each year (2005) is ^7 Gt, and this is predicted to rise to ~14 Gt by 2055 if there is no action to reduce carbon emissions. Assuming the same fraction is absorbed by the Earth, these emissions would add around 260 Gt of carbon to the atmosphere...Read More
The concentration of carbon dioxide in the Earth’s atmosphere has risen sharply over the
last 50 years. As explained in Chapter I, water vapour and carbon dioxide are the two main
greenhouse gases that trap the infrared radiation emitted by the Earth and thereby raise the temperature of the Earth’s surface. The current level of CO2 is 375 ppm (2005), while in 1955 it was 312 ppm and, in the pre-industrial era (before —1750), 280 ppm (Fig. 11.1(a)).
Analysis of ice cores shows that, over a much longer period, there have been fluctuations in the concentration of CO2 in the atmosphere (Fig. 11.1(b))...Read More
Fuel cells are receiving a lot of attention throughout the world. They provide carbon-free electricity with very low emissions and good efficiencies of ~50%. A fuel cell has no moving parts, so it is vibration-free, quiet, and reliable. There are many types of feedstock that can either provide hydrogen directly or indirectly as in the direct methanol fuel cell. Many of these supplies can be obtained using renewable sources such as hydro, wind, and solar power, or nuclear reactors. Hydrogen can be transported by pipeline or truck or can be reformed locally.
However, fuels cells are currently too expensive to compete without subsidies: the cost of a fuel cell power supply is ‘N-‘$450Q/kW compared with $1000/kW for a diesel generator...Read More
As yet, there is no way of storing hydrogen very compactly. It can be compressed up to 34 MPa (= 340 bar) in lightweight polycarbonate bottles, but even at that pressure there are only 31 g/litre of hydrogen. Liquefying H2 is expensive and only increases the density to 71 g/litre. Metal hydrides such as LaNisHg and NaAlH4 can hold 1.3 wt% and 3.7 wt% of hydrogen, respectively, which can be released at temperatures suitable for PEM fuel cells (^80°C).
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Hydrogen is introduced on one side of the cell and flows through a porous anode, where.:-. it dissociates into hydrogen ions and electrons on contact with a catalyst.. The. porous electrodes are often made of a porous carbon impregnated doth or paper 100-300 micron thick...Read More
A fuel cell is an electrochemical device that can be used to generate electricity or store energy in the form of hydrogen. Unlike a battery, a fuel cell is fed continuously by chemicals. The
Fig. 10.8 PEM fuel cell (schematic). The electrolyte is a thin plastic membrane and the anode and cathode are porous.
chemical feed consists of hydrogen and oxygen and the fuel cell acts as a means of combining them to make water, i. e. the opposite of electrolysis. There are two electrodes (the anode and cathode), where the chemical reactions take place, and a catalyst speeds up the reactions at the electrodes. There is virtually no pollution and the only byproduct is water...Read More
Batteries are electrochemical devices for storing energy in a form that can be readily converted into electrical energy. The chemicals are stored within the device from manufacture, unlike a fuel cell where the chemicals are renewed continuously. Batteries are either non-rechargeable (primary) or rechargeable (secondary), but only rechargeable batteries are of interest for large scale energy storage.
A battery is essentially a series of cells, each containing two electrodes immersed in an electrically conducting medium called the electrolyte. The lead-acid battery has two porous
Fig, 10.7 Lead-acid battery.
electrodes; a lead anode and a lead oxide cathode, immersed in an electrolyte of sulfuric acid (Fig. 10.7)...Read More
Superconducting magnetic energy storage (SMES) is the storage of energy in the magnetic field due to the flow of direct current in a superconducting material. The energy stored in the magnetic field is released by discharging the current in the coil. Since superconductors have no resistance, the current (and the associated magnetic field) does not decay with time once a direct current has been induced to flow in a superconducting coil.
Essentially, a SMES system consists of three components: a superconducting coil; a cooling system; and a power conditioning system (which converts AC to DC, and vice versa). The overall efficiency of an SMES is typically 95%, after allowing for losses in AC/DC conversion and cryogenic cooling of the superconducting material.
The advantages of SMES are that t...Read More
Another means of storing mechanical energy is in the form of rotational kinetic energy. The idea is not new.
® Grid controllers use the ‘spinning reserve’ of rotors to make minor adjustments to power supply and frequency.
® Flywheel-powered buses were used in Switzerland in the 1950s.
® The flywheel in a car provides kinetic energy to keep the engine turning between piston strokes.
In recent years, flywheels for energy storage have been developed for niche markets, e. g. providing power for testing switchgear equipment, which would otherwise cause large disturbances to the local distribution network due to sudden drops in current.
Conventional flywheels for energy storage are metallic with mechanical bearings and rotate up to around 4000 rpm...Read More