Cloud physics

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Cloud_physics

Atmospheric sciences

Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of atmospheric clouds. These aerosols are found in the troposphere, stratosphere, and mesosphere, which collectively make up the greatest part of the homosphere. Clouds consist of microscopic droplets of liquid water (warm clouds), tiny crystals of ice (cold clouds), or both (mixed phase clouds). Cloud droplets initially form by the condensation of water vapor onto condensation nuclei when the supersaturation of air exceeds a critical value according to Köhler theory. Cloud condensation nuclei are necessary for cloud droplets formation because of the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface. At small radii, the amount of supersaturation needed for condensation to occur is so large, that it does not happen naturally. Raoult's law describes how the vapor pressure is dependent on the amount of solute in a solution. At high concentrations, when the cloud droplets are small, the supersaturation required is smaller than without the presence of a nucleus.

In warm clouds, larger cloud droplets fall at a higher terminal velocity; because at a given velocity, the drag force per unit of droplet weight on smaller droplets is larger than on large droplets. The large droplets can then collide with small droplets and combine to form even larger drops. When the drops become large enough that their downward velocity (relative to the surrounding air) is greater than the upward velocity (relative to the ground) of the surrounding air, the drops can fall as precipitation. The collision and coalescence is not as important in mixed phase clouds where the Bergeron process dominates. Other important processes that form precipitation are riming, when a supercooled liquid drop collides with a solid snowflake, and aggregation, when two solid snowflakes collide and combine. The precise mechanics of how a cloud forms and grows is not completely understood, but scientists have developed theories explaining the structure of clouds by studying the microphysics of individual droplets. Advances in weather radar and satellite technology have also allowed the precise study of clouds on a large scale.

History of cloud physics

The modern cloud physics began in the 19th century and was described in several publications. Otto von Guericke originated the idea that clouds were composed of water bubbles. In 1847 Augustus Waller used spider web to examine droplets under the microscope. These observations were confirmed by William Henry Dines in 1880 and Richard Assmann in 1884.

Cloud formation: how the air becomes saturated

Cooling air to its dew point

Late-summer rainstorm in Denmark. Nearly black color of base indicates main cloud in foreground probably cumulonimbus.

Adiabatic cooling: rising packets of moist air

See also: Adiabatic process

As water evaporates from an area of Earth's surface, the air over that area becomes moist. Moist air is lighter than the surrounding dry air, creating an unstable situation. When enough moist air has accumulated, all the moist air rises as a single packet, without mixing with the surrounding air. As more moist air forms along the surface, the process repeats, resulting in a series of discrete packets of moist air rising to form clouds.

This process occurs when one or more of three possible lifting agents—cyclonic/frontal, convective, or orographic—causes air containing invisible water vapor to rise and cool to its dew point, the temperature at which the air becomes saturated. The main mechanism behind this process is adiabatic cooling. Atmospheric pressure decreases with altitude, so the rising air expands in a process that expends energy and causes the air to cool, which makes water vapor condense into cloud. Water vapor in saturated air is normally attracted to condensation nuclei such as dust and salt particles that are small enough to be held aloft by normal circulation of the air. The water droplets in a cloud have a normal radius of about 0.002 mm (0.00008 in). The droplets may collide to form larger droplets, which remain aloft as long as the velocity of the rising air within the cloud is equal to or greater than the terminal velocity of the droplets.

For non-convective cloud, the altitude at which condensation begins to happen is called the lifted condensation level (LCL), which roughly determines the height of the cloud base. Free convective clouds generally form at the altitude of the convective condensation level (CCL). Water vapor in saturated air is normally attracted to condensation nuclei such as salt particles that are small enough to be held aloft by normal circulation of the air. If the condensation process occurs below the freezing level in the troposphere, the nuclei help transform the vapor into very small water droplets. Clouds that form just above the freezing level are composed mostly of supercooled liquid droplets, while those that condense out at higher altitudes where the air is much colder generally take the form of ice crystals. An absence of sufficient condensation particles at and above the condensation level causes the rising air to become supersaturated and the formation of cloud tends to be inhibited.

Frontal and cyclonic lift

See also: Extratropical cyclone, Warm front, Cold front, and Precipitation

Frontal and cyclonic lift occur in their purest manifestations when stable air, which has been subjected to little or no surface heating, is forced aloft at weather fronts and around centers of low pressure. Warm fronts associated with extratropical cyclones tend to generate mostly cirriform and stratiform clouds over a wide area unless the approaching warm airmass is unstable, in which case cumulus congestus or cumulonimbus clouds will usually be embedded in the main precipitating cloud layer. Cold fronts are usually faster moving and generate a narrower line of clouds which are mostly stratocumuliform, cumuliform, or cumulonimbiform depending on the stability of the warm air mass just ahead of the front.

Convective lift

See also: Atmospheric convection

Another agent is the buoyant convective upward motion caused by significant daytime solar heating at surface level, or by relatively high absolute humidity. Incoming short-wave radiation generated by the sun is re-emitted as long-wave radiation when it reaches Earth's surface. This process warms the air closest to ground and increases air mass instability by creating a steeper temperature gradient from warm or hot at surface level to cold aloft. This causes it to rise and cool until temperature equilibrium is achieved with the surrounding air aloft. Moderate instability allows for the formation of cumuliform clouds of moderate size that can produce light showers if the airmass is sufficiently moist. Typical convection upcurrents may allow the droplets to grow to a radius of about 0.015 millimetres (0.0006 in) before precipitating as showers. The equivalent diameter of these droplets is about 0.03 millimetres (0.001 in).

If air near the surface becomes extremely warm and unstable, its upward motion can become quite explosive, resulting in towering cumulonimbiform clouds that can cause severe weather. As tiny water particles that make up the cloud group together to form droplets of rain, they are pulled down to earth by the force of gravity. The droplets would normally evaporate below the condensation level, but strong updrafts buffer the falling droplets, and can keep them aloft much longer than they would otherwise. Violent updrafts can reach speeds of up to 180 miles per hour (290 km/h). The longer the rain droplets remain aloft, the more time they have to grow into larger droplets that eventually fall as heavy showers.

Rain droplets that are carried well above the freezing level become supercooled at first then freeze into small hail. A frozen ice nucleus can pick up 0.5 inches (1.3 cm) in size traveling through one of these updrafts and can cycle through several updrafts and downdrafts before finally becoming so heavy that it falls to the ground as large hail. Cutting a hailstone in half shows onion-like layers of ice, indicating distinct times when it passed through a layer of super-cooled water. Hailstones have been found with diameters of up to 7 inches (18 cm).

Convective lift can occur in an unstable air mass well away from any fronts. However, very warm unstable air can also be present around fronts and low-pressure centers, often producing cumuliform and cumulonimbiform clouds in heavier and more active concentrations because of the combined frontal and convective lifting agents. As with non-frontal convective lift, increasing instability promotes upward vertical cloud growth and raises the potential for severe weather. On comparatively rare occasions, convective lift can be powerful enough to penetrate the tropopause and push the cloud top into the stratosphere.

Orographic lift

Main article: Orographic lift

A third source of lift is wind circulation forcing air over a physical barrier such as a mountain (orographic lift). If the air is generally stable, nothing more than lenticular cap clouds will form. However, if the air becomes sufficiently moist and unstable, orographic showers or thunderstorms may appear.

Windy evening twilight enhanced by the Sun's angle, can visually mimic a tornado resulting from orographic lift

Non-adiabatic cooling

Along with adiabatic cooling that requires a lifting agent, there are three other main mechanisms for lowering the temperature of the air to its dew point, all of which occur near surface level and do not require any lifting of the air. Conductive, radiational, and evaporative cooling can cause condensation at surface level resulting in the formation of fog. Conductive cooling takes place when air from a relatively mild source area comes into contact with a colder surface, as when mild marine air moves across a colder land area. Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath. This type of cooling is common during the night when the sky is clear. Evaporative cooling happens when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature, or sometimes to the point of saturation.

Adding moisture to the air

There are five main ways water vapor can be added to the air. Increased vapor content can result from wind convergence over water or moist ground into areas of upward motion. Precipitation or virga falling from above also enhances moisture content. Daytime heating causes water to evaporate from the surface of oceans, water bodies or wet land. Transpiration from plants is another typical source of water vapor. Lastly, cool or dry air moving over warmer water will become more humid. As with daytime heating, the addition of moisture to the air increases its heat content and instability and helps set into motion those processes that lead to the formation of cloud or fog.

Supersaturation

The amount of water that can exist as vapor in a given volume increases with the temperature. When the amount of water vapor is in equilibrium above a flat surface of water the level of vapor pressure is called saturation and the relative humidity is 100%. At this equilibrium there are equal numbers of molecules evaporating from the water as there are condensing back into the water. If the relative humidity becomes greater than 100%, it is called supersaturated. Supersaturation occurs in the absence of condensation nuclei.

Since the saturation vapor pressure is proportional to temperature, cold air has a lower saturation point than warm air. The difference between these values is the basis for the formation of clouds. When saturated air cools, it can no longer contain the same amount of water vapor. If the conditions are right, the excess water will condense out of the air until the lower saturation point is reached. Another possibility is that the water stays in vapor form, even though it is beyond the saturation point, resulting in supersaturation.

Supersaturation of more than 1–2% relative to water is rarely seen in the atmosphere, since cloud condensation nuclei are usually present. Much higher degrees of supersaturation are possible in clean air, and are the basis of the cloud chamber.

There are no instruments to take measurements of supersaturation in clouds.

Supercooling

Water droplets commonly remain as liquid water and do not freeze, even well below 0 °C (32 °F). Ice nuclei that may be present in an atmospheric droplet become active for ice formation at specific temperatures in between 0 °C (32 °F) and −38 °C (−36 °F), depending on nucleus geometry and composition. Without ice nuclei, supercooled water droplets (as well as any extremely pure liquid water) can exist down to about −38 °C (−36 °F), at which point spontaneous freezing occurs.

Collision-coalescence

Main article: Coalescence (meteorology)

One theory explaining how the behavior of individual droplets in a cloud leads to the formation of precipitation is the collision-coalescence process. Droplets suspended in the air will interact with each other, either by colliding and bouncing off each other or by combining to form a larger droplet. Eventually, the droplets become large enough that they fall to the earth as precipitation. The collision-coalescence process does not make up a significant part of cloud formation, as water droplets have a relatively high surface tension. In addition, the occurrence of collision-coalescence is closely related to entrainment-mixing processes.

Bergeron process

Main article: Bergeron process

The primary mechanism for the formation of ice clouds was discovered by Tor Bergeron. The Bergeron process notes that the saturation vapor pressure of water, or how much water vapor a given volume can contain, depends on what the vapor is interacting with. Specifically, the saturation vapor pressure with respect to ice is lower than the saturation vapor pressure with respect to water. Water vapor interacting with a water droplet may be saturated, at 100% relative humidity, when interacting with a water droplet, but the same amount of water vapor would be supersaturated when interacting with an ice particle. The water vapor will attempt to return to equilibrium, so the extra water vapor will condense into ice on the surface of the particle. These ice particles end up as the nuclei of larger ice crystals. This process only happens at temperatures between 0 °C (32 °F) and −40 °C (−40 °F). Below −40 °C (−40 °F), liquid water will spontaneously nucleate, and freeze. The surface tension of the water allows the droplet to stay liquid well below its normal freezing point. When this happens, it is now supercooled liquid water. The Bergeron process relies on super cooled liquid water (SLW) interacting with ice nuclei to form larger particles. If there are few ice nuclei compared to the amount of SLW, droplets will be unable to form. A process whereby scientists seed a cloud with artificial ice nuclei to encourage precipitation is known as cloud seeding. This can help cause precipitation in clouds that otherwise may not rain. Cloud seeding adds excess artificial ice nuclei which shifts the balance so that there are many nuclei compared to the amount of super cooled liquid water. An over seeded cloud will form many particles, but each will be very small. This can be done as a preventative measure for areas that are at risk for hail storms.

Cloud classification

Main article: List of cloud types

Clouds in the troposphere, the atmospheric layer closest to Earth, are classified according to the height at which they are found, and their shape or appearance. There are five forms based on physical structure and process of formation. Cirriform clouds are high, thin and wispy, and are seen most extensively along the leading edges of organized weather disturbances. Stratiform clouds are non-convective and appear as extensive sheet-like layers, ranging from thin to very thick with considerable vertical development. They are mostly the product of large-scale lifting of stable air. Unstable free-convective cumuliform clouds are formed mostly into localized heaps. Stratocumuliform clouds of limited convection show a mix of cumuliform and stratiform characteristics which appear in the form of rolls or ripples. Highly convective cumulonimbiform clouds have complex structures often including cirriform tops and stratocumuliform accessory clouds.

These forms are cross-classified by altitude range or level into ten genus types which can be subdivided into species and lesser types. High-level clouds form at altitudes of 5 to 12 kilometers. All cirriform clouds are classified as high-level and therefore constitute a single cloud genus cirrus. Stratiform and stratocumuliform clouds in the high level of the troposphere have the prefix cirro- added to their names yielding the genera cirrostratus and cirrocumulus. Similar clouds found in the middle level (altitude range 2 to 7 kilometers) carry the prefix alto- resulting in the genus names altostratus and altocumulus.

Low level clouds have no height-related prefixes, so stratiform and stratocumuliform clouds based around 2 kilometres or lower are known simply as stratus and stratocumulus. Small cumulus clouds with little vertical development (species humilis) are also commonly classified as low level.

Cumuliform and cumulonimbiform heaps and deep stratiform layers often occupy at least two tropospheric levels, and the largest or deepest of these can occupy all three levels. They may be classified as low or mid-level, but are also commonly classified or characterized as vertical or multi-level. Nimbostratus clouds are stratiform layers with sufficient vertical extent to produce significant precipitation. Towering cumulus (species congestus), and cumulonimbus may form anywhere from near the surface to intermediate heights of around 3 kilometres. Of the vertically developed clouds, the cumulonimbus type is the tallest and can virtually span the entire troposphere from a few hundred metres above the ground up to the tropopause. It is the cloud responsible for thunderstorms.

Some clouds can form at very high to extreme levels above the troposphere, mostly above the polar regions of Earth. Polar stratospheric clouds are seen but rarely in winter at altitudes of 18 to 30 kilometers, while in summer, noctilucent clouds occasionally form at high latitudes at an altitude range of 76 to 85 kilometers. These polar clouds show some of the same forms as seen lower in the troposphere.

Homospheric types determined by cross-classification of forms and levels.

Forms and levels	Stratiform non-convective	Cirriform mostly non-convective	Stratocumuliform limited-convective	Cumuliform free-convective	Cumulonimbiform strong-convective
Extreme level	PMC: Noctilucent veils	Noctilucent billow or whirls	Noctilucent bands
Very high level	Nitric acid & water PSC	Cirriform nacreous PSC	Lenticular nacreous PSC
High-level	Cirrostratus	Cirrus	Cirrocumulus
Mid-level	Altostratus		Altocumulus
Low-level	Stratus		Stratocumulus	Cumulus humilis or fractus
Multi-level or moderate vertical	Nimbostratus			Cumulus mediocris
Towering vertical				Cumulus congestus	Cumulonimbus

Homospheric types include the ten tropospheric genera and several additional major types above the troposphere. The cumulus genus includes four species that indicate vertical size and structure.

Determination of properties

Satellites are used to gather data about cloud properties and other information such as Cloud Amount, height, IR emissivity, visible optical depth, icing, effective particle size for both liquid and ice, and cloud top temperature and pressure.

Detection

Data sets regarding cloud properties are gathered using satellites, such as MODIS, POLDER, CALIPSO or ATSR. The instruments measure the radiances of the clouds, from which the relevant parameters can be retrieved. This is usually done by using inverse theory.

The method of detection is based on the fact that the clouds tend to appear brighter and colder than the land surface. Because of this, difficulties rise in detecting clouds above bright (highly reflective) surfaces, such as oceans and ice.

Parameters

The value of a certain parameter is more reliable the more satellites are measuring the said parameter. This is because the range of errors and neglected details varies from instrument to instrument. Thus, if the analysed parameter has similar values for different instruments, it is accepted that the true value lies in the range given by the corresponding data sets.

The Global Energy and Water Cycle Experiment uses the following quantities in order to compare data quality from different satellites in order to establish a reliable quantification of the properties of the clouds:

• the cloud cover or cloud amount with values between 0 and 1
• the cloud temperature at cloud top ranging from 150 to 340 K
• the cloud pressure at top 1013 - 100 hPa
• the cloud height, measured above sea level, ranging from 0 to 20 km
• the cloud IR emissivity, with values between 0 and 1, with a global average around 0.7
• the effective cloud amount, the cloud amount weighted by the cloud IR emissivity, with a global average of 0.5
• the cloud (visible) optical depth varies within a range of 4 and 10.
• the cloud water path for the liquid and solid (ice) phases of the cloud particles
• the cloud effective particle size for both liquid and ice, ranging from 0 to 200 μm

Icing

Another vital property is the icing characteristic of various cloud genus types at various altitudes, which can have great impact on the safety of flying. The methodologies used to determine these characteristics include using CloudSat data for the analysis and retrieval of icing conditions, the location of clouds using cloud geometric and reflectivity data, the identification of cloud types using cloud classification data, and finding vertical temperature distribution along the CloudSat track (GFS).

The range of temperatures that can give rise to icing conditions is defined according to cloud types and altitude levels:

Low-level stratocumulus and stratus can cause icing at a temperature range of 0 to -10 °C.

For mid-level altocumulus and altostratus, the range is 0 to -20 °C.

Vertical or multi-level cumulus, cumulonimbus, and nimbostatus, create icing at a range of 0 to -25 °C.

High-level cirrus, cirrocumulus, and cirrostratus generally cause no icing because they are made mostly of ice crystals colder than -25 °C.

Cohesion and dissolution

There are forces throughout the homosphere (which includes the troposphere, stratosphere, and mesosphere) that can impact the structural integrity of a cloud. It has been speculated that as long as the air remains saturated, the natural force of cohesion that hold the molecules of a substance together may act to keep the cloud from breaking up. However, this speculation has a logical flaw in that the water droplets in the cloud are not in contact with each other and therefore not satisfying the condition required for the intermolecular forces of cohesion to act. Dissolution of the cloud can occur when the process of adiabatic cooling ceases and upward lift of the air is replaced by subsidence. This leads to at least some degree of adiabatic warming of the air which can result in the cloud droplets or crystals turning back into invisible water vapor. Stronger forces such as wind shear and downdrafts can impact a cloud, but these are largely confined to the troposphere where nearly all the Earth's weather takes place. A typical cumulus cloud weighs about 500 metric tons, or 1.1 million pounds, the weight of 100 elephants.

Models

There are two main model schemes that can represent cloud physics, the most common is bulk microphysics models that uses mean values to describe the cloud properties (e.g. rain water content, ice content), the properties can represent only the first order (concentration) or also the second order (mass). The second option is to use bin microphysics scheme that keep the moments (mass or concentration) in different for different size of particles. The bulk microphysics models are much faster than the bin models but are less accurate.

Oracle

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Oracle 

An oracle is a person or agency considered to provide wise and insightful counsel or prophetic predictions, most notably including precognition of the future, inspired by deities. As such, it is a form of divination.

Description

The word oracle comes from the Latin verb ōrāre, "to speak" and properly refers to the priest or priestess uttering the prediction. In extended use, oracle may also refer to the site of the oracle, and to the oracular utterances themselves, called khrēsmē 'tresme' (χρησμοί) in Greek.

Oracles were thought to be portals through which the gods spoke directly to people. In this sense, they were different from seers (manteis, μάντεις) who interpreted signs sent by the gods through bird signs, animal entrails, and other various methods.

The most important oracles of Greek antiquity were Pythia (priestess to Apollo at Delphi), and the oracle of Dione and Zeus at Dodona in Epirus. Other oracles of Apollo were located at Didyma and Mallus on the coast of Anatolia, at Corinth and Bassae in the Peloponnese, and at the islands of Delos and Aegina in the Aegean Sea.

The Sibylline Oracles are a collection of oracular utterances written in Greek hexameters ascribed to the Sibyls, prophetesses who uttered divine revelations in frenzied states.

Origins

Walter Burkert observes that "Frenzied women from whose lips the God speaks" are recorded in the Near East as in Mari in the second millennium BC and in Assyria in the first millennium BC. In Egypt, the goddess Wadjet (eye of the moon) was depicted as a snake-headed woman or a woman with two snake-heads. Her oracle was in the renowned temple in Per-Wadjet (Greek name Buto). The oracle of Wadjet may have been the source for the oracular tradition which spread from Egypt to Greece. Evans linked Wadjet with the "Minoan Snake Goddess".

At the oracle of Dodona she is called Diōnē (the feminine form of Diós, genitive of Zeus; or of dīos, "godly", literally "heavenly"), who represents the earth-fertile soil, probably the chief female goddess of the proto-Indo-European pantheon. Python, daughter (or son) of Gaia was the earth dragon of Delphi represented as a serpent and became the chthonic deity, enemy of Apollo, who slew her and possessed the oracle.

In classical antiquity

Pythia at Delphi

When the Prytanies' seat shines white in the island of Siphnos,
White-browed all the forum—need then of a true seer's wisdom—
Danger will threat from a wooden boat, and a herald in scarlet.

— The Pythoness, in The Histories, Herodotus.

The Pythia was the mouthpiece of the oracles of the god Apollo, and was also known as the Oracle of Delphi.

The Delphic Oracle exerted considerable influence throughout Hellenic culture. Distinctively, this woman was essentially the highest authority both civilly and religiously in male-dominated ancient Greece. She responded to the questions of citizens, foreigners, kings, and philosophers on issues of political impact, war, duty, crime, family, laws—even personal issues. The semi-Hellenic countries around the Greek world, such as Lydia, Caria, and even Egypt also respected her and came to Delphi as supplicants.

Croesus, king of Lydia beginning in 560 BC, tested the oracles of the world to discover which gave the most accurate prophecies. He sent out emissaries to seven sites who were all to ask the oracles on the same day what the king was doing at that very moment. Croesus proclaimed the oracle at Delphi to be the most accurate, who correctly reported that the king was making a lamb-and-tortoise stew, and so he graced her with a magnitude of precious gifts. He then consulted Delphi before attacking Persia, and according to Herodotus was advised: "If you cross the river, a great empire will be destroyed". Believing the response favourable, Croesus attacked, but it was his own empire that ultimately was destroyed by the Persians.

She allegedly also proclaimed that there was no man wiser than Socrates, to which Socrates said that, if so, this was because he alone was aware of his own ignorance. After this confrontation, Socrates dedicated his life to a search for knowledge that was one of the founding events of western philosophy. He claimed that she was "an essential guide to personal and state development." This oracle's last recorded response was given in 362 AD, to Julian the Apostate.

The oracle's powers were highly sought after and never doubted. Any inconsistencies between prophecies and events were dismissed as failure to correctly interpret the responses, not an error of the oracle. Very often prophecies were worded ambiguously, so as to cover all contingencies – especially so ex post facto. One famous such response to a query about participation in a military campaign was "You will go you will return never in war will you perish". This gives the recipient liberty to place a comma before or after the word "never", thus covering both possible outcomes. Another was the response to the Athenians when the vast army of king Xerxes I was approaching Athens with the intent of razing the city to the ground. "Only the wooden palisades may save you", answered the oracle, probably aware that there was sentiment for sailing to the safety of southern Italy and re-establishing Athens there. Some thought that it was a recommendation to fortify the Acropolis with a wooden fence and make a stand there. Others, Themistocles among them, said the oracle was clearly for fighting at sea, the metaphor intended to mean war ships. Others still insisted that their case was so hopeless that they should board every ship available and flee to Italy, where they would be safe beyond any doubt. In the event, variations of all three interpretations were attempted: some barricaded the Acropolis, the civilian population was evacuated over sea to nearby Salamis Island and to Troizen, and the war fleet fought victoriously at Salamis Bay. Should utter destruction have happened, it could always be claimed that the oracle had called for fleeing to Italy after all.

Sibyl at Cumae

Cumae was the first Greek colony on the mainland of Italy, near Naples, dating back to the 8th century BC. The sibylla or prophetess at Cumae became famous because of her proximity to Rome and the Sibylline Books acquired and consulted in emergencies by Rome wherein her prophecies were transcribed. The Cumaean Sibyl was called "Herophile" by Pausanias and Lactantius, "Deiphobe, daughter of Glaucus" by Virgil, as well as "Amaltheia", "Demophile", or "Taraxandra" by others. Sibyl's prophecies became popular with Christians as they were thought to predict the birth of Jesus Christ.

Sibyl at Erythrae

Erythrae near Ionia in Asia Minor was home to a prophetess.

Oracle at Didyma

The ruins of the Temple of Apollo at Didyma

Didyma near Ionia in Asia Minor in the domain of the famous city of Miletus.

Oracle at Dodona

Dodona in northwestern Greece was another oracle devoted to the Mother Goddess identified at other sites with Rhea or Gaia, but here called Dione. The shrine of Dodona, set in a grove of oak trees, was the oldest Hellenic oracle, according to the fifth-century historian Herodotus, and dated from pre-Hellenic times, perhaps as early as the second millennium BC, when the tradition may have spread from Egypt. By the time of Herodotus, Zeus had displaced the Mother Goddess, she had been assimilated to Aphrodite, and the worship of the deified hero Heracles had been added. Dodona became the second most important oracle in ancient Greece, after Delphi. At Dodona, Zeus was worshipped as Zeus Naios or Naos (god of springs Naiads, from a spring under the oaks), or as Zeus Bouleos (chancellor). Priestesses and priests interpreted the rustling of the leaves of the oak trees by the wind to determine the correct actions to be taken.

Oracle at Trophonius

Trophonius was an oracle at Lebadea of Boeotia devoted to the chthonian Zeus Trophonius. Trophonius is derived from the Greek word "trepho" (nourish) and he was a Greek hero, or demon or god. Demeter-Europa was his nurse. Europa (in Greek: broad-eyes) was a Phoenician princess whom Zeus, having transformed himself into a white bull, abducted and carried to Creta, and is equated with Astarte as a moon goddess by ancient sources. Some scholars connect Astarte with the Minoan snake goddess, whose cult as Aphrodite spread from Creta to Greece.

Oracle at Menestheus

Near the Menestheus's port or Menesthei Portus (Greek: Μενεσθέως λιμήν), modern El Puerto de Santa María, Spain, was the Oracle of Menestheus (Greek: Μαντεῖον τοῦ Μενεσθέως), to whom also the inhabitants of Gades offered sacrifices.

Oracle at the Ikaros island in the Persian Gulf

At the Ikaros island in the Persian Gulf (modern Failaka Island in Kuwait), there was an oracle of Artemis Tauropolus.

Oracle at Claros

At Claros, there was the oracle of Apollo Clarius.

Oracle at Ptoion

At Ptoion, there was an oracle of Ptoios and later of Apollo.

Oracle at Gryneium

At Gryneium, there was a sanctuary of Apollo with an ancient oracle.

Oracle of Zeus Ammon at Siwa and Aphytis

The oracle of Zeus Ammon at Siwa Oasis was so famous that Alexander the Great visited it when he conquered Egypt.

The oracle of Zeus Ammon at Aphytis in Chalkidiki.

In other cultures

Main article: Divination

The term "oracle" is also applied in modern English to parallel institutions of divination in other cultures. Specifically, it is used in the context of Christianity for the concept of divine revelation, and in the context of Judaism for the Urim and Thummim breastplate, and in general any utterance considered prophetic.

Celtic polytheism

In Celtic polytheism, divination was performed by the priestly caste, either the druids or the vates. This is reflected in the role of "seers" in Dark Age Wales (dryw) and Ireland (fáith).

China

Main articles: Oracle bone and I Ching

 

Oracle bone of the Shang dynasty, ancient China

In China, oracle bones were used for divination in the late Shang dynasty, (c. 1600–1046 BC). Diviners applied heat to these bones, usually ox scapulae or tortoise plastrons, and interpreted the resulting cracks.

A different divining method, using the stalks of the yarrow plant, was practiced in the subsequent Zhou dynasty (1046–256 BC). Around the late 9th century BC, the divination system was recorded in the I Ching, or "Book of Changes", a collection of linear signs used as oracles. In addition to its oracular power, the I Ching has had a major influence on the philosophy, literature and statecraft of China since the Zhou period.

Hawaii

In Hawaii, oracles were found at certain heiau, Hawaiian temples. These oracles were found in towers covered in white kapa cloth made from plant fibres. In here, priests received the will of gods. These towers were called 'Anu'u. An example of this can be found at Ahu'ena heiau in Kona.

India and Nepal

Main article: Divination

In ancient India, the oracle was known as akashawani or Ashareera vani (a voice without body or unseen) or asariri (Tamil), literally meaning "voice from the sky" and was related to the message of a god. Oracles played key roles in many of the major incidents of the epics Mahabharata and Ramayana. An example is that Kamsa (or Kansa), the evil uncle of Krishna, was informed by an oracle that the eighth son of his sister Devaki would kill him. The opening verse of the Tiruvalluva Maalai, a medieval Tamil anthology usually dated by modern scholars to between c. 7th and 10th centuries CE, is attributed to an asariri or oracle. However, there are no references in any Indian literature of the oracle being a specific person.

Contemporarily, Theyyam or "theiyam" in Malayalam - a south Indian language - the process by which a devotee invites a Hindu god or goddess to use his or her body as a medium or channel and answer other devotees' questions, still happens. The same is called "arulvaakku" or "arulvaak" in Tamil, another south Indian language - Adhiparasakthi Siddhar Peetam is famous for arulvakku in Tamil Nadu. The people in and around Mangalore in Karnataka call the same, Buta Kola, "paathri" or "darshin"; in other parts of Karnataka, it is known by various names such as, "prashnaavali", "vaagdaana", "asei", "aashirvachana" and so on. In Nepal it is known as, "Devta ka dhaamee" or "jhaakri".

In English, the closest translation for these is, "oracle."

Nigeria

The Igbo people of southeastern Nigeria in Africa have a long tradition of using oracles. In Igbo villages, oracles were usually female priestesses to a particular deity, usually dwelling in a cave or other secluded location away from urban areas, and, much as the oracles of ancient Greece, would deliver prophecies in an ecstatic state to visitors seeking advice. Two of their ancient oracles became especially famous during the pre-colonial period: the Agbala oracle at Awka and the Chukwu oracle at Arochukwu. Although the vast majority of Igbos today are Christian, many of them still use oracles.

Among the related Yoruba peoples of the same country, the Babalawos (and their female counterparts, the Iyanifas) serve collectively as the principal aspects of the tribe's world-famous Ifa divination system. Due to this, they customarily officiate at a great many of its traditional and religious ceremonies.

Norse mythology

In Norse mythology, Odin took the severed head of the god Mimir to Asgard for consultation as an oracle. The Havamal and other sources relate the sacrifice of Odin for the oracular runes whereby he lost an eye (external sight) and won wisdom (internal sight; insight).

Pre-Columbian Americas

In the migration myth of the Mexitin, i.e., the early Aztecs, a mummy-bundle (perhaps an effigy) carried by four priests directed the trek away from the cave of origins by giving oracles. An oracle led to the foundation of Mexico-Tenochtitlan. The Yucatec Mayas knew oracle priests or chilanes, literally 'mouthpieces' of the deity. Their written repositories of traditional knowledge, the Books of Chilam Balam, were all ascribed to one famous oracle priest who had correctly predicted the coming of the Spaniards and its associated disasters.

Tibet

In Tibet, oracles (Chinese: 护法) have played, and continue to play, an important part in religion and government. The word "oracle" is used by Tibetans to refer to the spirit that enters those men and women who act as media between the natural and the spiritual realms. The media are, therefore, known as kuten, which literally means, "the physical basis". In the 29-Article Ordinance for the More Effective Governing of Tibet (Chinese: 欽定藏內善後章程二十九條), an imperial decree published in 1793 by the Qianlong Emperor, article 1 states that the creation of Golden Urn is to ensure prosperity of Gelug, and to eliminate cheating and corruption in the selection process performed by oracles.

The Dalai Lama, who lives in exile in northern India, still consults an oracle known as the Nechung Oracle, which is considered the official state oracle of the government of Tibet. The Dalai Lama has according to centuries-old custom, consulted the Nechung Oracle during the new year festivities of Losar. Nechung and Gadhong are the primary oracles currently consulted; former oracles such as Karmashar and Darpoling are no longer active in exile. The Gadhong oracle has died leaving Nechung to be the only primary oracle. Another oracle the Dalai Lama consults is the Tenma Oracle, for which a young Tibetan woman by the name of Khandro La is the medium for the mountain goddesses Tseringma along with the other 11 goddesses. The Dalai Lama gives a complete description of the process of trance and spirit possession in his book Freedom in Exile. Dorje Shugden oracles were once consulted by the Dalai Lamas until the 14th Dalai Lama banned the practice, even though he consulted Dorje Shugden for advice to escape and was successful in it. Due to the ban, many of the abbots that were worshippers of Dorje Shugden have been forced to go against the Dalai Lama.

Cell damage

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Cell_damage 

Cell damage (also known as cell injury) is a variety of changes of stress that a cell suffers due to external as well as internal environmental changes. Amongst other causes, this can be due to physical, chemical, infectious, biological, nutritional or immunological factors. Cell damage can be reversible or irreversible. Depending on the extent of injury, the cellular response may be adaptive and where possible, homeostasis is restored. Cell death occurs when the severity of the injury exceeds the cell's ability to repair itself. Cell death is relative to both the length of exposure to a harmful stimulus and the severity of the damage caused. Cell death may occur by necrosis or apoptosis.

Causes

• Physical agents such as heat or radiation can damage a cell by literally cooking or coagulating their contents.
• Impaired nutrient supply, such as lack of oxygen or glucose, or impaired production of adenosine triphosphate (ATP) may deprive the cell of essential materials needed to survive.
• Metabolic: Hypoxia and Ischemia
• Chemical Agents
• Microbial Agents:-Virus & Bacteria
• Immunologic Agents: Allergy and autoimmune diseases such as Parkinson's and Alzheimer's disease.
• Genetic factors: Such as Down's syndrome and sickle cell anemia

Targets

The most notable components of the cell that are targets of cell damage are the DNA and the cell membrane.

• DNA damage: In human cells, both normal metabolic activities and environmental factors such as ultraviolet light and other radiations can cause DNA damage, resulting in as many as one million individual molecular lesions per cell per day.
• Membrane damage: Damage to the cell membrane disturbs the state of cell electrolytes, e.g. calcium, which when constantly increased, induces apoptosis.
• Mitochondrial damage: May occur due to ATP decrease or change in mitochondrial permeability.
• Ribosome damage: Damage to ribosomal and cellular proteins such as protein misfolding, leading to apoptotic enzyme activation.

Types of damage

Some cell damage can be reversed once the stress is removed or if compensatory cellular changes occur. Full function may return to cells but in some cases, a degree of injury will remain.

Reversible

Cellular swelling

Cellular swelling (or cloudy swelling) may occur due to cellular hypoxia, which damages the sodium-potassium membrane pump; it is reversible when the cause is eliminated. Cellular swelling is the first manifestation of almost all forms of injury to cells. When it affects many cells in an organ, it causes some pallor, increased turgor, and increase in weight of the organ. On microscopic examination, small clear vacuoles may be seen within the cytoplasm; these represent distended and pinched-off segments of the endoplasmic reticulum. This pattern of non-lethal injury is sometimes called hydropic change or vacuolar degeneration. Hydropic degeneration is a severe form of cloudy swelling. It occurs with hypokalemia due to vomiting or diarrhea.

The ultrastructural changes of reversible cell injury include:

• Blebbing
• Blunting
• distortion of microvilli
• loosening of intercellular attachments
• mitochondrial changes
• dilation of the endoplasmic reticulum

Fatty change

The cell has been damaged and is unable to adequately metabolize fat. Small vacuoles of fat accumulate and become dispersed within cytoplasm. Mild fatty change may have no effect on cell function; however, more severe fatty change can impair cellular function. In the liver, the enlargement of hepatocytes due to fatty change may compress adjacent bile canaliculi, leading to cholestasis. Depending on the cause and severity of the lipid accumulation, fatty change is generally reversible. Fatty Change is also known as fatty degeneration, fatty metamorphosis, or fatty steatosis.

Irreversible

Necrosis

Necrosis is characterised by cytoplasmic swelling, irreversible damage to the plasma membrane, and organelle breakdown leading to cell death. The stages of cellular necrosis include pyknosis; clumping of chromosomes and shrinking of the nucleus of the cell, karyorrhexis; fragmentation of the nucleus and break up of the chromatin into unstructured granules, and karyolysis; dissolution of the cell nucleus. Cytosolic components that leak through the damaged plasma membrane into the extracellular space can incur an inflammatory response.

There are six types of necrosis:

• Coagulative necrosis
• Liquefactive necrosis
• Caseous necrosis
• Fat necrosis
• Fibroid necrosis
• Gangrenous necrosis

Apoptosis

Apoptosis is the programmed cell death of superfluous or potentially harmful cells in the body. It is an energy-dependent process mediated by proteolytic enzymes called caspases, which trigger cell death through the cleaving of specific proteins in the cytoplasm and nucleus. The dying cells shrink and condense into apoptotic bodies. The cell surface is altered so as to display properties that lead to rapid phagocytosis by macrophages or neighbouring cells. Unlike necrotic cell death, Neighbouring cells are not damaged by apoptosis as cytosolic products are safely isolated by membranes prior to undergoing phagocytosis. It is considered an important component of various bioprocesses including cell turnover, hormone-dependent atrophy, proper development and functioning of the immune and embryonic system, it also helps in chemical-induced cell death which is genetically mediated. There is some evidence that certain symptoms of "apoptosis" such as endonuclease activation can be spuriously induced without engaging a genetic cascade. It is also becoming clear that mitosis and apoptosis are toggled or linked in some way and that the balance achieved depends on signals received from appropriate growth or survival factors. There are research being conducted to focus on the elucidation and analysis of the cell cycle machinery and signaling pathways that controls cell cycle arrest and apoptosis. In the average adult between 50 and 70 billion cells die each day due to apoptosis. Inhibition of apoptosis can result in a number of cancers, autoimmune diseases, inflammatory diseases, and viral infections. Hyperactive apoptosis can lead to neurodegenerative diseases, hematologic diseases, and tissue damage.

Repair

When a cell is damaged the body will try to repair or replace the cell to continue normal functions. If a cell dies the body will remove it and replace it with another functioning cell, or fill the gap with connective tissue to provide structural support for the remaining cells. The motto of the repair process is to fill a gap caused by the damaged cells to regain structural continuity. Normal cells try to regenerate the damaged cells but this cannot always happen. Asexual reproduction is what repairs cells

Regeneration

Regeneration of parenchyma cells, or the functional cells, of an organism. The body can make more cells to replace the damaged cells keeping the organ or tissue intact and fully functional.

Replacement

When a cell cannot be regenerated the body will replace it with stromal connective tissue to maintain tissue/organ function. Stromal cells are the cells that support the parenchymal cells in any organ. Fibroblasts, immune cells, pericytes, and inflammatory cells are the most common types of stromal cells.

Biochemical changes in cellular injury

ATP (adenosine triphosphate) depletion is a common biological alteration that occurs with cellular injury. This change can happen despite the inciting agent of the cell damage. A reduction in intracellular ATP can have a number of functional and morphologic consequences during cell injury. These effects include:

• Failure of the ATP dependent pumps (Na⁺
  /K⁺
  pump and Ca²⁺
  pump), resulting in a net influx of Na⁺
  and Ca²⁺
  ions and osmotic swelling.
• ATP-depleted cells begin to undertake anaerobic metabolism to derive energy from glycogen which is known as 'glycogenolysis'.
• A consequent decrease in the intracellular pH of the cell arises, which mediates harmful enzymatic processes.
• Early clumping of nuclear chromatin then occurs, known as 'pyknosis', and leads to eventual cell death.

DNA damage and repair

DNA damage

DNA damage (or RNA damage in the case of some virus genomes) appears to be a fundamental problem for life. As noted by Haynes, the subunits of DNA are not endowed with any peculiar kind of quantum mechanical stability, and thus DNA is vulnerable to all the "chemical horrors" that might befall any such molecule in a warm aqueous medium. These chemical horrors are DNA damages that include various types of modification of the DNA bases, single- and double-strand breaks, and inter-strand cross-links (see DNA damage (naturally occurring). DNA damages are distinct from mutations although both are errors in the DNA. Whereas DNA damages are abnormal chemical and structural alterations, mutations ordinarily involve the normal four bases in new arrangements. Mutations can be replicated, and thus inherited when the DNA replicates. In contrast, DNA damages are altered structures that cannot, themselves, be replicated.

Several different repair processes can remove DNA damages (see chart in DNA repair). However, those DNA damages that remain un-repaired can have detrimental consequences. DNA damages may block replication or gene transcription. These blockages can lead to cell death. In multicellular organisms, cell death in response to DNA damage may occur by a programmed process, apoptosis. Alternatively, when a DNA polymerase replicates a template strand containing a damaged site, it may inaccurately bypass the damage and, as a consequence, introduce an incorrect base leading to a mutation. Experimentally, mutation rates increase substantially in cells defective in DNA mismatch repair or in Homologous recombinational repair (HRR).

In both prokaryotes and eukaryotes, DNA genomes are vulnerable to attack by reactive chemicals naturally produced in the intracellular environment and by agents from external sources. An important internal source of DNA damage in both prokaryotes and eukaryotes is reactive oxygen species (ROS) formed as byproducts of normal aerobic metabolism. For eukaryotes, oxidative reactions are a major source of DNA damage (see DNA damage (naturally occurring) and Sedelnikova et al.). In humans, about 10,000 oxidative DNA damages occur per cell per day. In the rat, which has a higher metabolic rate than humans, about 100,000 oxidative DNA damages occur per cell per day. In aerobically growing bacteria, ROS appear to be a major source of DNA damage, as indicated by the observation that 89% of spontaneously occurring base substitution mutations are caused by introduction of ROS-induced single-strand damages followed by error-prone replication past these damages. Oxidative DNA damages usually involve only one of the DNA strands at any damaged site, but about 1–2% of damages involve both strands. The double-strand damages include double-strand breaks (DSBs) and inter-strand crosslinks. For humans, the estimated average number of endogenous DNA DSBs per cell occurring at each cell generation is about 50. This level of formation of DSBs likely reflects the natural level of damages caused, in large part, by ROS produced by active metabolism.

Repair of DNA damages

Five major pathways are employed in repairing different types of DNA damages. These five pathways are nucleotide excision repair, base excision repair, mismatch repair, non-homologous end-joining and homologous recombinational repair (HRR) (see chart in DNA repair) and reference. Only HRR can accurately repair double-strand damages, such as DSBs. The HRR pathway requires that a second homologous chromosome be available to allow recovery of the information lost by the first chromosome due to the double-strand damage.

DNA damage appears to play a key role in mammalian aging, and an adequate level of DNA repair promotes longevity (see DNA damage theory of aging and reference). In addition, an increased incidence of DNA damage and/or reduced DNA repair cause an increased risk of cancer (see Cancer, Carcinogenesis and Neoplasm) and reference). Furthermore, the ability of HRR to accurately and efficiently repair double-strand DNA damages likely played a key role in the evolution of sexual reproduction (see Evolution of sexual reproduction and reference). In extant eukaryotes, HRR during meiosis provides the major benefit of maintaining fertility.

Microgrid

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Microgrid 

A microgrid is a local electrical grid with defined electrical boundaries, acting as a single and controllable entity. It is able to operate in grid-connected and in island mode. A 'Stand-alone microgrid' or 'isolated microgrid' only operates off-the-grid and cannot be connected to a wider electric power system.

A grid-connected microgrid normally operates connected to and synchronous with the traditional wide area synchronous grid (macrogrid), but is able to disconnect from the interconnected grid and to function autonomously in "island mode" as technical or economic conditions dictate. In this way, they improve the security of supply within the microgrid cell, and can supply emergency power, changing between island and connected modes. This kind of grids are called 'islandable microgrids'.

A stand-alone microgrid has its own sources of electricity, supplemented with an energy storage system. They are used where power transmission and distribution from a major centralized energy source is too far and costly to operate. They offer an option for rural electrification in remote areas and on smaller geographical islands. A stand-alone microgrid can effectively integrate various sources of distributed generation (DG), especially renewable energy sources (RES).

Control and protection are difficulties to microgrids, as all ancillary services for system stabilization must be generated within the microgrid and low short-circuit levels can be challenging for selective operation of the protection systems. An important feature is also to provide multiple useful energy needs, such as heating and cooling besides electricity, since this allows energy carrier substitution and increased energy efficiency due to waste heat utilization for heating, domestic hot water, and cooling purposes (cross sectoral energy usage).

Definitions

The United States Department of Energy Microgrid Exchange Group defines a microgrid as a group of interconnected loads and distributed energy resources (DERs) within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both connected or island-mode.

The Berkeley Lab defines: "A microgrid consists of energy generation and energy storage that can power a building, campus, or community when not connected to the electric grid, e.g. in the event of a disaster." A microgrid that can be disconnected from the utility grid (at the 'point of common coupling' or PCC) is called an 'islandable microgrid'.

A EU research project describes a microgrid as comprising Low-Voltage (LV) distribution systems with distributed energy resources (DERs) (microturbines, fuel cells, photovoltaics (PV), etc.), storage devices (batteries, flywheels) energy storage system and flexible loads. Such systems can operate either connected or disconnected from the main grid. The operation of microsources in the network can provide benefits to the overall system performance, if managed and coordinated efficiently.

Electropedia defines a microgrid as a group of interconnected loads and distributed energy resources with defined electrical boundaries, which form a local electric power system at distribution voltage levels, meaning both low and medium voltage up to 35 kV. This cluster of associated consumer and producer nodes acts as a single controllable entity and is able to operate in either grid-connected or island mode.

A Stand-alone microgrid or isolated microgrid, sometimes called an 'island grid', only operates off-the-grid and cannot be connected to a wider electric power system. They are usually designed for geographical islands or for rural electrification.

Types of microgrids

A typical scheme of an electric based microgrid with renewable energy resources in grid-connected mode

Campus environment/institutional microgrids

The focus of campus microgrids is aggregating existing on-site generation to support multiple loads located in a tight geographical area where an owner can easily manage them.

Community microgrids

Community microgrids can serve thousands of customers and support the penetration of local energy (electricity, heating, and cooling). In a community microgrid, some houses may have some renewable sources that can supply their demand as well as that of their neighbors within the same community. The community microgrid may also have a centralized or several distributed energy storages. Such microgrids can be in the form of an ac and dc microgrid coupled together through a bi-directional power electronic converter.

Remote off-grid microgrids

These microgrids never connect to the macrogrid and instead operate in an island mode at all times because of economic issues or geographical position. Typically, an "off-grid" microgrid is built in areas that are far distant from any transmission and distribution infrastructure and, therefore, have no connection to the utility grid. Studies have demonstrated that operating a remote area or islands' off-grid microgrids, that are dominated by renewable sources, will reduce the levelized cost of electricity production over the life of such microgrid projects.

Large remote areas may be supplied by several independent microgrids, each with a different owner (operator). Although such microgrids are traditionally designed to be energy self-sufficient, intermittent renewable sources and their unexpected and sharp variations can cause unexpected power shortfall or excessive generation in those microgrids. This will immediately cause unacceptable voltage or frequency deviation in the microgrids. To remedy such situations, it is possible to interconnect such microgrids provisionally to a suitable neighboring microgrid to exchange power and improve the voltage and frequency deviations. This can be achieved through a power electronics-based switch after a proper synchronization or a back to back connection of two power electronic converters and after confirming the stability of the new system. The determination of a need to interconnect neighboring microgrids and finding the suitable microgrid to couple with can be achieved through optimization or decision making approaches.

Military base microgrids

These microgrids are being actively deployed with focus on both physical and cyber security for military facilities in order to assure reliable power without relying on the macrogrid.

Commercial and industrial (C&I) microgrids

These types of microgrids are maturing quickly in North America and eastern Asia; however, the lack of well-known standards for these types of microgrids limits them globally. Main reasons for the installation of an industrial microgrid are power supply security and its reliability. There are many manufacturing processes in which an interruption of the power supply may cause high revenue losses and long start-up time. Industrial microgrids can be designed to supply circular economy (near-)zero-emission industrial processes, and can integrate combined heat and power (CHP) generation, being fed by both renewable sources and waste processing; energy storage can be additionally used to optimize the operations of these sub-systems.

Topologies of microgrids

Architectures are needed to manage the flow of energy from different types of sources into the electrical grid. Thus, the microgrid can be classified into three topologies:

AC microgrid

Power sources with AC output are interfaced to AC bus through AC/AC converter which will transform the AC variable frequency and voltage to AC waveform with another frequency at another voltage. Whilst power sources with DC output use DC/AC converters for the connection to the AC bus.

DC microgrid

In DC microgrid topology, power sources with DC output are connected to DC bus directly or by DC/DC converters. On the other hand, power sources with AC output are connected to the DC bus through AC/DC converter.

Hybrid microgrid

The hybrid microgrid has topology for both power source AC and DC output. In addition, AC and DC buses are connected to each other through a bidirectional converter, allowing power to flow in both directions between the two buses.

Basic components in microgrids

The Solar Settlement, a sustainable housing community project in Freiburg, Germany.

Local generation

A microgrid presents various types of generation sources that feed electricity, heating, and cooling to the user. These sources are divided into two major groups – thermal energy sources (e.g,. natural gas or biogas generators or micro combined heat and power) and renewable generation sources (e.g. wind turbines and solar).

Consumption

In a microgrid, consumption simply refers to elements that consume electricity, heat, and cooling, which range from single devices to the lighting and heating systems of buildings, commercial centers, etc. In the case of controllable loads, electricity consumption can be modified according to the demands of the network.

Energy storage

In microgrid, energy storage is able to perform multiple functions, such as ensuring power quality, including frequency and voltage regulation, smoothing the output of renewable energy sources, providing backup power for the system and playing a crucial role in cost optimization. It includes all of chemical, electrical, pressure, gravitational, flywheel, and heat storage technologies. When multiple energy storages with various capacities are available in a microgrid, it is preferred to coordinate their charging and discharging such that a smaller energy storage does not discharge faster than those with larger capacities. Likewise, it is preferred a smaller one does not get fully charged before those with larger capacities. This can be achieved under a coordinated control of energy storages based on their state of charge. If multiple energy storage systems (possibly working on different technologies) are used and they are controlled by a unique supervising unit (an energy management system - EMS), a hierarchical control based on a master/slaves architecture can ensure best operations, particularly in the islanded mode.

Point of common coupling (PCC)

This is the point in the electric circuit where a microgrid is connected to a main grid. Microgrids that do not have a PCC are called isolated microgrids which are usually present in remote sites (e.g., remote communities or remote industrial sites) where an interconnection with the main grid is not feasible due to either technical or economic constraints.

Advantages and challenges of microgrids

Advantages

A microgrid is capable of operating in grid-connected and stand-alone modes and of handling the transition between the two. In the grid-connected mode, ancillary services can be provided by trading activity between the microgrid and the main grid. Other possible revenue streams exist. In the islanded mode, the real and reactive power generated within the microgrid, including that provided by the energy storage system, should be in balance with the demand of local loads. Microgrids offer an option to balance the need to reduce carbon emissions with continuing to provide reliable electric energy in periods of time when renewable sources of power are not available. Microgrids also offer the security of being hardened from severe weather and natural disasters by not having large assets and miles of above-ground wires and other electric infrastructure that need to be maintained or repaired following such events.

A microgrid may transition between these two modes because of scheduled maintenance, degraded power quality or a shortage in the host grid, faults in the local grid, or for economical reasons. By means of modifying energy flow through microgrid components, microgrids facilitate the integration of renewable energy, such as photovoltaic, wind and fuel cell generations, without requiring re-design of the national distribution system. Modern optimization methods can also be incorporated into the microgrid energy management system to improve efficiency, economics, and resiliency.

Challenges

Microgrids, and the integration of DER units in general, introduce a number of operational challenges that need to be addressed in the design of control and protection systems, in order to ensure that the present levels of reliability are not significantly affected, and the potential benefits of Distributed Generation (DG) units are fully harnessed. Some of these challenges arise from assumptions typically applied to conventional distribution systems that are no longer valid, while others are the result of stability issues formerly observed only at a transmission system level. The most relevant challenges in microgrid protection and control include:

• Bidirectional power flows: The presence of distributed generation (DG) units in the network at low voltage levels can cause reverse power flows that may lead to complications in protection coordination, undesirable power flow patterns, fault current distribution, and voltage control.
• Stability issues: Interactions between control system of DG units may create local oscillations, requiring a thorough small-disturbance stability analysis. Moreover, transition activities between the grid-connected and islanding (stand-alone) modes of operation in a microgrid can create transient instability. Recent studies have shown that direct-current (DC) microgrid interface can result in a significantly simpler control structure, more energy efficient distribution and higher current carrying capacity for the same line ratings.
• Modeling: Many characteristics of traditional schemes such as the prevalence of three-phase balanced conditions, primarily inductive transmission lines, and constant-power loads, do not necessarily hold true for microgrids, and consequently, models need to be revised.
• Low inertia: Microgrids exhibit a low-inertia characteristic that makes them different to bulk power systems, where a large number of synchronous generators ensures a relatively large inertia. This phenomenon is more evident if there is a significant proportion of power electronic-interfaced DG units in the microgrid. The low inertia in the system can lead to severe frequency deviations in island mode operation if a proper control mechanism is not implemented. Synchronous generators run at the same frequency as the grid, thus providing a natural damping effect on sudden frequency variations. Synchronverters are inverters which mimic synchronous generators to provide frequency control. Other options include controlling battery energy storage or a flywheel to balance the frequency.
• Uncertainty: The operation of microgrids involves addressing much uncertainty, which is something the economical and reliable operation of microgrids relies on. Load profile and weather are two uncertainties that make this coordination more challenging in isolated microgrids, where the critical demand-supply balance and typically higher component failure rates require solving a strongly coupled problem over an extended time horizon. This uncertainty is higher than those in bulk power systems, due to the reduced number of loads and highly correlated variations of available energy resources (the averaging effect is much more limited).

Modelling tools

To plan and install microgrids correctly, engineering modelling is needed. Multiple simulation tools and optimization tools exist to model the economic and electric effects of microgrids. A widely used economic optimization tool is the Distributed Energy Resources Customer Adoption Model (DER-CAM) from Lawrence Berkeley National Laboratory. Another is Homer Energy, originally designed by the National Renewable Energy Laboratory. There are also some power flow and electrical design tools guiding microgrid developers. The Pacific Northwest National Laboratory designed the publicly available GridLAB-D tool and the Electric Power Research Institute (EPRI) designed OpenDSS. A European tool that can be used for electrical, cooling, heating, and process heat demand simulation is EnergyPLAN from Aalborg University in Denmark. The open source grid planning tool OnSSET has been deployed to investigate microgrids using a three‑tier analysis beginning with settlement archetypes (case‑studied using Bolivia).

Microgrid control

Hierarchical Control

In regards to the architecture of microgrid control, or any control problem, there are two different approaches that can be identified: centralized and decentralized. A fully centralized control relies on a large amount of information transmittance between involving units before a decision is made at a single point. Implementation is difficult since interconnected power systems usually cover extended geographic locations and involve an enormous number of units. On the other hand, in a fully decentralized control, each unit is controlled by its local controller without knowing the situation of others. A compromise between those two extreme control schemes can be achieved by means of a hierarchical control scheme consisting of three control levels: primary, secondary, and tertiary.

Primary control

The primary control is designed to satisfy the following requirements:

• To stabilize the voltage and frequency
• To offer plug and play capability for DERs and properly share the active and reactive power among them, preferably, without any communication links
• To mitigate circulating currents that can cause over-current phenomenon in the power electronic devices

The primary control provides the setpoints for a lower controller which are the voltage and current control loops of DERs. These inner control loops are commonly referred to as zero-level control.

Secondary control

Secondary control has typically seconds to minutes sampling time (i.e. slower than the previous one) which justifies the decoupled dynamics of the primary and the secondary control loops and facilitates their individual designs. The setpoint of primary control is given by secondary control in which, as a centralized controller, it restores the microgrid voltage and frequency and compensates for the deviations caused by variations of loads or renewable sources. The secondary control can also be designed to satisfy the power quality requirements, e.g., voltage balancing at critical buses.

Tertiary control

Tertiary control is the last (and the slowest) control level, which considers economical concerns in the optimal operation of the microgrid (sampling time is from minutes to hours), and manages the power flow between microgrid and main grid. This level often involves the prediction of weather, grid tariff, and loads in the next hours or day to design a generator dispatch plan that achieves economic savings. More advanced techniques can also provide end to end control of a microgrid using machine learning techniques such as deep reinforcement learning.

In case of emergencies such as blackouts, tertiary control can manage a group of interconnected microgrids to form what is called "microgrid clustering", acting as a virtual power plant to continue supplying critical loads. During these situations the central controller should select one of the microgrids to be the slack (i.e. master) and the rest as PV and load buses according to a predefined algorithm and the existing conditions of the system (i.e. demand and generation). In this case, the control should be real time or at least at a high sampling rate.

IEEE 2030.7

A less utility-influenced controller framework is that from the Institute of Electrical and Electronics Engineers, the IEEE 2030.7. The concept relies on 4 blocks: a) Device level control (e.g. voltage and frequency control), b) Local area control (e.g. data communication), c) Supervisory (software) control (e.g. forward looking dispatch optimization of generation and load resources), and d) Grid layers (e.g. communication with utility).

Elementary control

A wide variety of complex control algorithms exist, making it difficult for small microgrids and residential distributed energy resource (DER) users to implement energy management and control systems. Communication upgrades and data information systems can be expensive. Some projects try to simplify and reduce the expense of control via off-the-shelf products (e.g. using a Raspberry Pi).

Examples

Hajjah and Lahj, Yemen

The UNDP project “Enhanced Rural Resilience in Yemen” (ERRY) uses community-owned solar microgrids. It cuts energy costs to just 2 cents per hour (whereas diesel-generated electricity costs 42 cents per hour). It won the Ashden Awards for Humanitarian Energy in 2020.

Île d'Yeu

A two year pilot program, called Harmon’Yeu, was initiated in the Spring of 2020 to interconnect 23 houses in the Ker Pissot neighborhood and surrounding areas with a microgrid that was automated as a smart grid with software from Engie. Sixty-four solar panels with a peak capacity of 23.7 kW were installed on five houses and a battery with a storage capacity of 15 kWh was installed on one house. Six houses store excess solar energy in their hot water heaters. A dynamic system apportions the energy provided by the solar panels and stored in the battery and hot water heaters to the system of 23 houses. The smart grid software dynamically updates energy supply and demand in 5 minute intervals, deciding whether to pull energy from the battery or from the panels and when to store it in the hot water heaters. This pilot program was the first such project in France.

Les Anglais, Haiti

A wirelessly managed microgrid is deployed in rural Les Anglais, Haiti. The system consists of a three-tiered architecture with a cloud-based monitoring and control service, a local embedded gateway infrastructure and a mesh network of wireless smart meters deployed at fifty-two buildings.

Non-technical loss (NTL) represents a major challenge when providing reliable electrical service in developing countries, where it often accounts for 11-15% of total generation capacity. An extensive data-driven simulation on seventy-two days of wireless meter data from a 430-home microgrid deployed in Les Anglais investigated how to distinguish NTL from the total power losses, aiding in energy theft detection.

Mpeketoni, Kenya

The Mpeketoni Electricity Project, a community-based diesel-powered micro-grid system, was set up in rural Kenya near Mpeketoni. Due to the installment of these microgrids, Mpeketoni has seen a large growth in its infrastructure. Such growth includes increased productivity per worker, at values of 100% to 200%, and an income level increase of 20–70% depending on the product.

Stone Edge Farm Winery

A micro-turbine, fuel-cell, multiple battery, hydrogen electrolyzer, and PV enabled winery in Sonoma, California.