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The Science of Heat Transfer: What Is Conduction? - bop13mo - 20.04.2022
The Science of Heat Transfer: What Is Conduction?
Heat is an interesting form of energy. Not only does it sustain life, make us comfortable and help us prepare our food, but understanding its properties is key to many fields of scientific research. For example, knowing how heat is transferred and the degree to which different materials can exchange thermal energy governs everything from building heaters and understanding seasonal change to sending ships into space.
Heat can only be transferred through three means: conduction, convection and radiation. Of these, conduction is perhaps the most common, and occurs regularly in nature. In short, it is the transfer of heat through physical contact. It occurs when you press your hand onto a window pane, when you place a pot of water on an active element, and when you place an iron in the fire.
This transfer occurs at the molecular level — from one body to another — when heat energy is absorbed by a surface and causes the molecules of that surface to move more quickly. In the process, they bump into their neighbors and transfer the energy to them, a process which continues as long as heat is still being added.
The process of heat conduction depends on four basic factors: the temperature gradient, the cross section of the materials involved, their path length, and the properties of those materials.
A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes in a specific location. Temperature always flows from the hottest to coldest source, due to the fact that cold is nothing but the absence of heat energy. This transfer between bodies continues until the temperature difference decays, and a state known as thermal equilibrium occurs.
Cross-section and path length are also important factors. The greater the size of the material involved in the transfer, the more heat is needed to warm it. Also, the more surface area that is exposed to open air, the greater likelihood for heat loss. So shorter objects with a smaller cross-section are the best means of minimizing the loss of heat energy.
Last, but certainly not least, is the physical properties of the materials involved. Basically, when it comes to conducting heat, not all substances are created equal. Metals and stone are considered good conductors since they can speedily transfer heat, whereas materials like wood, paper, air, and cloth are poor conductors of heat.
These conductive properties are rated based on a “coefficient” which is measured relative to silver. In this respect, silver has a coefficient of heat conduction of 100, whereas other materials are ranked lower. These include copper (92), iron (11), water (0.12), and wood (0.03). At the opposite end of the spectrum is a perfect vacuum, which is incapable of conducting heat, and is therefore ranked at zero.
Materials that are poor conductors of heat are called insulators. Air, which has a conduction coefficient of .006, is an exceptional insulator because it is capable of being contained within an enclosed space. This is why artificial insulators make use of air compartments, such as double-pane glass windows which are used for cutting heating bills. Basically, they act as buffers against heat loss.
Feather, fur, and natural fibers are all examples of natural insulators. These are materials that allows birds, mammals and human beings to stay warm. Sea otters, for example, live in ocean waters that are often very cold and their luxuriously thick fur keeps them warm. Other sea mammals like sea lions, whales and penguins rely on thick layers of fat (aka. blubber) – a very poor conductor – to prevent heat loss through their skin.
LV & MV Aerial Bundle Cable (ABC)
Aerial Bundle Cables, often referred to as Aerial Bundled Conductors or simply ABC, are cables for overhead line power, so called for combining multiple single core cables together. With applications including temporary power distribution to street lighting and secondary pole-to-pole service cables, they are lightweight stranded aluminium conductors, both single core and multi-cores. Whilst Aerial Bundle Cables are used in rural power distribution in some countries, they are more commonly used in temporary power installations such as on construction sites. As insulated cables they are often preferred to bare conductors which are installed and separated by air gaps but where sparks and shorts in the event of high winds may cause resulting bushfires in dry climates or risk nearby property. The XLPE insulation material, and where relevant the sheathing material, allows the ABC to be tightly bundled together – additional steel wire supports similar to those in ACSR can also be incorporated as catenary wires .
LOW VOLTAGE ABC
Our Low Voltage ABC are manufactured in accordance with a range of national standards - British standard BS7870, French standard NF C33 209, Australasian standard AS/NZS 3560 Part 1, and IEC standards IEC 60502-1, TNB Specification, and HD 626 S1. They have a voltage rating of 0.6/1kV. With the insulation it also meets Class II according to IEC 61140 in protecting against electric shock. Aerial Bundle Cables have an operating temperature range of -40oC to +80oC and can be installed in temperatures as low as -20oC. These ABC are unscreened and without an additional outer sheath.
The LV ABC have both phase conductors and a neutral conductor - both Class 2 stranded Aluminium - with core identification being provided by ribs on the insulation: Phases by longitudinal ribs (I, II, III), Neutral core by longitudinal ribs (≤ 50 mm2 min.12 ribs; ≥ 50 mm2 min.16 ribs).
MEDIUM VOLTAGE ABC
We also supply Medium Voltage Aerial Bundle Cables, in voltages of 6.35/11kV, 12.7/22kV and 19/33kV. Manufactured in accordance with IEC 60502-2, we also have variants made specifically to Australian and New Zealand standards AS/NZS 3599 Part 1. In addition to the XLPE insulation, these higher voltage cables are sheathed with High Density Polyethylene (HDPE). They are available with and without additional screening, in light and heavy duty copper wire or tape, depending on the installation parameters and requirements.
What Types of Wire Are Used on Solar Farms?
Solar power installations in the US have grown 35 fold since 2008, while the average cost of photovoltaic panels has dropped 50% since 2014. Over 242,000 people work in the solar power industry in the US, which is more than double the number in 2012.
With this undeniable level of growth there's a lot of interest in solar panel wire, or photovoltaic (PV) wire (UL 4703). What makes solar farm wire unique comes down to its material, form, and insulation.
Wire Materials used in Solar Farms
The two main materials used to make solar farm wiring are copper and aluminum. Copper is more conductive than aluminum, which means a copper wire carries more current than an aluminum wire of the same size. Aluminum wiring is also more vulnerable to bending and flexing during installation, which can weaken it faster than copper wire. Another challenge with aluminum wire is the higher maintenance costs. Aluminum is more susceptible to temperature extremes. The expansion and contraction of the metal will require a technician to periodically tighten the terminals where aluminum is used. The main benefit to using aluminum is that the up-front cost is cheaper than copper.
Solid and Stranded Forms of Wire
Solar farm wire mainly comes in two forms, solid or stranded. Solid wire is one single conductor, which makes it more compact while still providing the same current as stranded wire. It also makes it less flexible, so it is best used in static applications. This can be a particular concern when building solar farms where wire can be exposed to wind and other vibration interference.
Stranded wire is made of multiple conductors put together. This configuration makes stranded wire more flexible than solid wire, and more resistant to vibrations.
Solar Farm Wire Insulation and Durability
Photovoltaic wire insulation has to be tough. It needs to resist UV radiation, weather, and abrasion caused by chemicals and salt water. Most electrical contractors use cross-linked polyethylene insulation, also known as PEX, XPE, or XLPE. This insulation is resistant to moisture, oil, and gasoline, can withstand temperatures up to 90 °C, and uses a black coating to resist UV radiation.
USE-2 Wire vs. PV Wire
Most solar installations are outdoors in harsher environments. Therefore the wiring has to meet standards for heat, moisture, and UV resistance.
There are two types of wire commonly used in solar farms: PV wire and USE-2 wire. They can both be used in grounded arrays, but only PV wire can be used in ungrounded ones.
PV wire is used for interconnecting PV modules, and was developed to be able to handle 90°C in wet conditions and 105°C in dry conditions. Characteristics of PV wire are thicker insulation and stranded wire construction for resilience and flexibility. It also requires a more stringent vertical flame test.
USE-2 wire is used for connecting terminals of service equipment. It is mostly found underground and for 90°C in wet conditions and 105°C in dry conditions. Like PV wire, USE-2 wire must pass a flame test, but it requires a less stringent horizontal flame test. Additionally, it needs to pass physical testing outlined in the UL standard.
Three Major Types Of Network Cables Used In Communication Systems
Fiber optic cable, twisted pair cable, and coaxial cable are the three main types of network cables used in communication systems. Each of them is different and suitable for various applications.
Fiber Optic Cable
Fiber optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves.
Fiber Optic cable has a complicated design and structure. This type of cable has an outer optical casing that surrounds the light and traps it within a central core. The inside of the cable (the core) must configured in two different ways – Single-mode and multi-mode; although the difference may seem small, it makes a tremendous difference to the performance and the usage of fiber optic cables.
Twisted Pair Cable
Twisted pair cable is a type of ordinary wiring which connects home and many business computers to the telephone company. It is made by putting two separate insulated wires together in a twisted pattern and running them parallel to each other, which helps to reduce crosstalk or electromagnetic induction between pairs of wires. Twisted pair cable is suitable for transferring balanced differential signals. The method of transmitting signals dates back to the early days of the telegraph and radio. The advantages of improved signal-to-noise ratio, crosstalk, and ground bounce that balanced signal transmission brings are particularly valuable in wide bandwidth and high fidelity systems.
According to whether the cable has a shielding layer, there are two common types of twisted pair cables—shielded twisted pair (STP) cable and unshielded twisted pair (UTP) cable. STP cable is available for Token Ring networks, while the UTP cable is more suitable for Ethernet networks. The most common UTP cable types applied in Ethernet network are cat5e, cat6a, and cat7 cables, etc. The following image shows the different structures of UTP and STP cables.
Aluminum processing
aluminum processing, preparation of the ore for use in various products.
Aluminum, or aluminium (Al), is a silvery white metal with a melting point of 660 °C (1,220 °F) and a density of 2.7 grams per cubic cm. The most abundant metallic element, it constitutes 8.1 percent of Earth’s crust. In nature it occurs chemically combined with oxygen and other elements. In the pure state it is soft and ductile, but it can be alloyed with many other elements to increase strength and provide a number of useful properties. Alloys of aluminum are light, strong, and formable by almost all known metalworking processes. They can be cast, joined by many techniques, and machined easily, and they accept a wide variety of finishes.
In addition to its low density, many of the applications of aluminum and its alloys are based on its high electrical and thermal conductivity, high reflectivity, and resistance to corrosion. It owes its corrosion resistance to a continuous film of aluminum oxide that grows rapidly on a nascent aluminum surface exposed to air.