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Energy Out of the Blue: Generating Electric power from the Clash of River and Sea Water Two pilot projects are testing the potential of "salt power," a renewable energy that relies on the differing salinities at river mouths to make watts


Freshwater, Saltwater, Salt power 1.


In the hunt for alternatives to polluting and climate-warming fossil fuels, attention has turned to where rivers meet the sea. Here, freshwater and saltwater naturally settle their salinity difference, a phenomenon that two pioneering projects in Europe will try to harness to generate clean energy. This concept of "salt power"—also known as osmotic, or salinity-gradient, power—has been kicked around for decades, and now, proponents hope, technology has advanced enough to make it economically competitive. On November 24, the world's first large-scale prototype facility for developing a form of salt power called pressure-retarded osmosis is expected to begin fully operating in Norway. "The big reason to build this thing is to answer important questions [about osmotic power], and while we've done a lot of theoretical studies, we need live experience," says Stein Erik Skilhagen, vice president of osmotic power at Statkraft, Norway's state-owned power utility that built the plant. The prototype will have no customers, although the very small amount of electricity it generates will technically be directed into the power grid. Statkraft's approximately $5-million prototype plant is a converted paper mill in the seaside village of Tofte, about 60 kilometers south of Oslo. The plant's pressure-retarded osmosis setup will place freshwater and brine on either side of a semipermeable membrane that prevents the passage of salt particles but allows water through. Water from the fresh side naturally flows into the salty side, generating pressure equivalent to a column of water 120 meters high. This pressurized water can be used to turn a turbine to make electricity. Statkraft's goal is to yield five watts per square meter of membrane, although current capacity is about three watts. If successful, the utility hopes to build a commercial salt power plant for paying customers around 2015 with a targeted cost ranging from seven to 14 cents per kilowatt-hour (pdf) (at current euro–dollar conversion rates), which at the low end would be competitive with coal and natural gas prices. To the south in the Netherlands, a Dutch research firm called Wetsus has fired up its own salt power experiment to evaluate what is essentially a saltwater–freshwater battery. Wetsus, with the collaboration of a spin-off company called Redstack, is pursuing a version of salt power dubbed "blue energy". A pilot-scale installation that is about two times the size of a big American refrigerator is up and running in Harlingen, by the Wadden Sea, says Gert Jan Euverink, Wetsus's deputy scientific director. The technology relies on reverse electrodialysis, wherein a series of fresh and saltwater streams are diverted via underground pipes to opposite sides of two kinds of membranes. These let sodium or chlorine ions—the constituent elements of salt—dissolved in the water to pass into separated freshwater streams. This builds an electrical potential across the membranes, like a battery, and this charge reacts with iron to form an electric current. Joost Veerman, a researcher at Wetsus, says the company aims to get five watts per square meter of membrane, the same result as Statkraft's process. Read Comments (9) | Post a comment 1 2 Next >


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* [generic_50x50 ] Facing the Freshwater Crisis * [generic_50x50 ] First U.S. "power Tower" Lights Up California power (physics)


In physics, power is the rate at which work is performed or energy is converted. It is an energy per unit of time. As a rate of change of work done or the energy of a subsystem, power is


where P is power, W is work and t is time. The average power (often simply called "power" when the context makes it clear) is the average amount of work done or energy converted per unit time. The instantaneous power is then the limiting value of the average power as the time interval Δt approaches zero.


* 1 Units * 2 Mechanical power * 3 Electrical power + 3.1 Instantaneous electrical power + 3.2 Average electrical power for sinusoidal voltages + 3.3 Average electrical power for AC + 3.4 Peak power and duty cycle * 4 power in optics * 5 See also


The dimension of power is energy divided by time. The SI unit of power is the watt (W), which is equal to one joule per second. Non-SI units of power include ergs per second (erg/s), horsepower (hp), metric horsepower (Pferdestärke (PS) or cheval vapeur, CV), and foot-pounds per minute. One horsepower is equivalent to 33,000 foot-pounds per minute, or the power required to lift 550 pounds by one foot in one second, and is equivalent to about 746 watts. Other units include dBm, a relative logarithmic measure with 1 milliwatt as reference; (food) calories per hour (often referred to as kilocalories per hour); Btu per hour (Btu/h); and tons of refrigeration (12,000 Btu/h). [edit] Mechanical power


Differentiating by time gives that the instantaneous power is equal to the force times the object's velocity v(t):


The average power is then


This formula is important in characterizing engines—the power output of an engine is equal to the force it exerts multiplied by its velocity. In rotational systems, power is related to the torque (τ) and angular velocity (ω):


The average power is therefore


In systems with fluid flow, power is related to pressure, p and volumetric flow rate, Q:


[edit] Electrical power Main article: Electric power [edit] Instantaneous electrical power The instantaneous electrical power P delivered to a component is given by


P(t) is the instantaneous power, measured in watts (joules over second) V(t) is the potential difference (or voltage drop) across the component, measured in volts


If the component is reactive (e.g. a capacitor or an inductor), then the instantaneous power is negative when the component is giving stored energy back to its environment, i.e., when the current and voltage are of opposite signs. [edit] Average electrical power for sinusoidal voltages The average power consumed by a sinusoidally-driven linear two-terminal electrical device is a function of the root mean square (rms) values of the voltage across the terminals and the current through the device, and of the phase angle between the voltage and current sinusoids. That is,


P is the average power, measured in watts I is the root mean square value of the sinusoidal alternating current (AC), measured in amperes


This figure can also be called the effective power, as compared to the larger apparent power which is expressed in volt-amperes (VA) and does not include the cos φ term due to the current and voltage being out of phase. For simple domestic appliances or a purely resistive network, the cos φ term (called the power factor) can often be assumed to be unity, and can therefore be omitted from the equation. In this case, the effective and apparent power are assumed to be equal. [edit] Average electrical power for AC


For purely resistive devices, the average power is equal to the product of the rms voltage and rms current, even if the waveforms are not sinusoidal. The formula works for any waveform, periodic or otherwise, that has a mean square; that is why the rms formulation is so useful. For devices more complex than a resistor, the average effective power can still be expressed in general as a power factor times the product of rms voltage and rms current, but the power factor is no longer as simple as the cosine of a phase angle if the drive is non-sinusoidal or the device is not linear. [edit] Peak power and duty cycle In a train of identical pulses, the instantaneous power is a periodic function of time. The ratio of the pulse duration to the period is equal to the ratio of the average power to the peak power. It is also called the duty cycle (see text for definitions). In the case of a periodic signal s(t) of period T, like a train of identical pulses, the instantaneous power p(t) = | s(t) | ^2 is also a periodic function of period T. The peak power is simply defined by:


The peak power is not always readily measurable, however, and the measurement of the average power P[avg] is more commonly performed by an instrument. If one defines the energy per pulse as:


then the average power is:


[edit] power in optics Main article: Optical power In optics, or radiometry, the term power sometimes refers to radiant flux, the average rate of energy transport by electromagnetic radiation, measured in watts. The term "power" is also, however, used to express the ability of a lens or other optical device to focus light. It is measured in dioptres (inverse metres), and equals the inverse of the focal length of the optical device.


* Motive power * Orders of magnitude (power) * Pulsed power * Intensity — in the radiative sense, power per area * power gain — for linear, two-port networks. The Minimum-power Effect of a Magnetized Plasma


Abstract. An experimental and theoretical analysis is performed of a fully ionized, magnetized, quasi-steady plasma being separated from a surrounding neutral gas blanket by partially ionized boundary regions: (i) For rotating as well as high-frequency heated plasmas there exists a sharply defined minimum heating power P[m] and temperature T[0m] below which a steady fully ionized state cannot be maintained. At laboratory dimensions and ion densities of about 10^21 m^-3, these parameters become P[m] simeq 0.5 MW and T[0m] simeq 2 × 10^4 K. This "minimum-P effect" is important both to the processes of creating and maintaining laboratory plasmas in fusion research and to the studies of certain cosmical plasmas. (ii) The theoretically predicted behaviour of the plasma is roughly in agreement with the experiments in the power range close to the minimum value. [spacer ] Publications & Services> Books> Book Series> The World as Active power: Studies in the History of European Reason The World as Active power: Studies in the History of European Reason Edited by Juhani Pietarinen and Valtteri Viljanen


Introduction, Juhani Pietarinen and Valtteri Viljanen 1. Platoâs power Dualism, Juhani Pietarinen 2. The Active Principle in Stoic Philosophy, HÃ¥vard Løkke 3. Plotinus on Act and power, Eyjólfur Kjalar Emilsson 4. power and Activity in Early Medieval Thought, Tomas Ekenberg 5. power and Possibility in Thomas Aquinas, Andreas Schmid 6. Causal power in Descartesâs Mind-Body Union, Juhani Pietarinen 7. De novo creat: Descartes on Action, Interaction, and Continuous Creation, Timo Kajamies 8. Motion and Reason: Hobbesâs Difficulties with the Idea of Active power, Juhani Pietarinen 9. Spinozaâs Actualist Model of power, Valtteri Viljanen 10. Leibniz on Force, Activity, and Passivity, Arto Repo and Valtteri Viljanen


Juhani Pietarinen, Ph.D. (1972) in Philosophy, University of Helsinki, is Professor Emeritus of Practical Philosophy at the University of Turku. He is the coeditor of Perspectives on Human Conduct (1988), Genes and Morality (1999) and Philosophy and Biodiversity (2004), and the author of various articles on ethics and history of philosophy. Valtteri Viljanen, Ph.D. (2007) in Philosophy, University of Turku, is Academy of Finland Postdoctoral Fellow at the University of Turku. He is the author of Spinoza's Dynamics of Being: The Concept of power and its Role in Spinoza's Metaphysics (2007) and of various articles on history of philosophy, especially on Spinoza. What is the ultimate explanatory factor for the existence of the world, for all its changing phenomena and the enduring order found in it? In the history of Western thought, we can find a longstanding philosophical tendency to answer this question in terms of power: the universe is understood as an ordered whole produced by a rational power, that is, by the power of reason. That power is thought to be active in the sense of being capable of existing and acting âin itselfâ as an infinite, eternal, and unchangeable cause of the world. The essays in this collection discuss the idea of active power in the world-explanations of Plato, the Stoics, Neoplatonism, early and late medieval scholasticism, Descartes, Hobbes, Spinoza, Leibniz, Kant, Hegel, and Schopenhauer. [icon_print ] Print Version


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