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Topic Last Updated on 10-07-2024
Warming the Air
Post on topic: The power of Energy.
The losing party was represented by another famous inventor and entrepreneur, Thomas Edison, who advocated the advantages of direct current. The century-old dispute was finally resolved by a strong argument — economics. The main disadvantage of direct current, when transmitted over long distances, can be summed up as follows: longer wires lead to greater resistance. The power produced by a generator depends on the voltage and current:
Current = Power / Voltage
Notice that at a low voltage, we get a higher current. Based on Joule’s law, the power of heating generated by a conductor is proportional to the product of its resistance and the square of the current:
Heat = Current² × Resistance × Time
A large portion of this heat is wasted on heating the conductor. In the practice of direct current transmission technology, which Edison so vehemently defended, low voltage (below 200 V) was used, which allowed transmitting the current across 0.9–1.2 mi. Essentially, electrical grids could not possibly be centralized on a regional — much less a national — level with regards to direct current.
An Acceptable Loss
There are only two ways to minimize losses. The first is to reduce the resistance of a wire (R) by using materials with a low specific resistance. Following this criterion, copper (its specific resistance ρ equals 16–18 Ohm*mm²/m) can only be replaced by silver (ρ=15–16 Ohm*mm²/m). The payoff, as we can see, is only 5–6%, while the cost is increased twofold — silver is much more expensive than copper.
The other method is far more promising — reducing the electrical current (I) that is transported through wires (remember the quadratic dependence of power loss in wires). At the end of the 19th century, there wasn’t a device that could be used to regulate the flow of current and voltage in direct current power lines. That’s why Edison couldn’t use this option.
Diagram of electric power transmission from a generating site to the consumer
Westinghouse had more luck: he had an electric transformer. Even at the time, it was able to transform alternating current power with an energy conversion efficiency of 98 %! By maintaining the overall power (we should note that this is equal to the product of the voltage and current), a transformer increased input voltage by hundreds of times while decreasing the current by the same amount; this radically decreased parasitic heat losses during transmission. Afterward, a reverse action must be performed on the ground, near electricity consumers.
A Pyrrhic Victory
So, that’s all. The alternating current won, right? That’s the impression we get when we look at the contemporary large-scale energy infrastructure. Meanwhile, oddly enough, gigantic energy infrastructure is experiencing its own difficulties.
The ultra-high-voltage electricity transmission of 750–800 kV, achieved in the last 40 years, has been virtually unsurpassed: building higher-voltage Megavolt lines is considered impractical. The limits of voltage that can be transmitted through these lines reach up to 2–3 GW; a further increase, though much needed for industry and demanding consumers, runs into serious difficulties.
The main such difficulty is reactance. Any conductor, even a simple piece of copper wire, has capacitance and inductance; that is, it acts as an inductor and capacitor at the same time. Even if it isn’t as effective as specialized elements, it still somewhat contributes to unwanted inductance and increases capacitance.
Schematic diagram of an inductor
The situation gets worse if we consider energy distribution as a complex system that’s rich in intertwined wires, consumers with a high inductance (transformers, motors, converters, etc.), switching nodes, and transitions through different environments (soil-water-air). Then the waste reactive power can reach a considerable amount — up to 40–50 %!
Energy | skin effect
There is another nuance that is common for alternating current transmission lines — the skin effect. This phenomenon occurs when conduction electrons are pushed onto the surface of the conductor. Basically, it means that the effective cross-sectional area of a wire decreases; that is, its resistance rises, therefore, more heat gets generated. On an 800kV transmission line with a length of 1243 mi, losses associated with the skin effect can reach 300 MW (10 % of the 3 GW in transmitted power), and that occurs at lower voltages (50–60 Hz) when the effect is not as pronounced! For high-voltage electronic equipment, this harmful effect presents a major problem.
In addition to the above, difficulties with the synchronization of networks thousands of miles in length make it obvious that creating large electricity grids on a national scale requires immense resources and investments. Meanwhile, creating a dependable and economical system is almost impossible — the losses are inevitable and very high.
Energy | Not Exactly a Wire
In modern power cables, a current flows through internal copper or aluminum conductors with a cross-sectional area under 2 in2 (it can be visually represented as a square with a side of 1.4 in — its area is close to the number above), that are made of dozens of intertwined thick wires. In cables with a voltage under 420 kV, a cross-linked polyethylene (PEX, XPE, or XLPE)— a polymer foam with a modified molecular structure — acts as insulating material. The point of “cross-linking” is this: under the influence of high temperatures, oxygen saturation and electron-beam irradiation, the hydrogen atoms in the molecules of ethylene are substituted by additional intermolecular bonds. The tensile strength of the resulting material is increased by 100 times; it has low thermal conductivity and low water absorption.
Current conductors are “soldered” into an entire insulating sheath made of cross-linked polyethylene, on top of which sits actual “armor” — woven wire mesh that protects the cable from mechanical impacts. Of course, this isn’t the only possible design of power cables. In the product lines of leading world manufacturers, such as Prysmian Group (Italy), Nexans (France), Hengtong (China), and Sumitomo (Japan), apart from traditional options with polymer insulation, you can find oil- and gas-filled cables for high and ultra-high voltages. The combined sales of power and signal cables of listed companies recently reached $30 billion. Still, the majority of orders were direct current cables.
Energy | The Sand and the Sun
Perhaps the strongest stimulus that pushed electrical engineers to think about other concepts for building networks was the “green” project Desertec.
This pet project from a consortium of European electrical equipment manufacturers was announced back in 2003, and its goal was to convert part of the Sahara Desert into an energy “oasis.” Engineers proposed the construction of gigantic solar and wind energy plants in North Africa to lower Europe’s evening peaks of energy consumption and supply 15–25 % of its total energy needs. (The solar power plants were planned to be of the collector type — a device that uses mirrors to heat water while driving the produced vapor to the generator’s turbines.)
Transmitting this energy would require thousands of miles of cables, part of which would travel across the Mediterranean seafloor. That’s where the difficulties arose. Research showed that during transmission by high voltage alternating current power lines, losses could reach 60 % at a voltage of 750 kV, and 50 % at 1150 kV. The reason for this lies in the reactance of the underwater cable with a large cross-sectional area with a great length.
