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Miscellaneous topics:

1. Light-Emitting Diode (LED)

2. Heat Pump + Coefficient of Performance (COP)

3. Ocean Thermal Energy Conversion (OTEC)

Benoit Cushman-Roisin23 February 2012

1. Light-emitting diode (LED) versus other types of light bulbs

Let us begin with what light is.

Light is a form of electromagnetic radiation, that portion that falls in a certain range of wavelengths: 400 nm < < 700 nm.

Our eyes do not see radiation of shorter and longer wavelengths. Shorter wavelengths (ultraviolet down to X-rays) can be harmful to us, while longer wavelengths (infrared) are felt as heat.

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From the quantum physics perspective, electromagnetic radiation, including light, can be seen as a beam of photon particles.

Each photon possesses a bit of energy proportional to its frequency:

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The only way known to make light is to have electrons jump energy levels around the atomic nuclei that they orbit.

For the photon to be perceived as light, the energy jump must be in the correct range:

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In the traditional light bulb (Thomas Edison’s invention), an electrical current is passed through a very thin filament of tungsten or similar material. The electricity energizes some electrons by making them jump up onto outer orbits, from which they promptly drop down to their initial level of equilibrium.

tungstenfilament

The glass bulb that encloses the filament serves to protect the weak filament from physical destruction. Most

The major problem with the incandescent light bulb is that most of the electrons’ jumps down are insufficiently energetic and produce radiation in the infrared range rather than the visible range. Thus, the

importantly, it encloses a vacuum or gas with no oxygen to prevent destruction of the filament by combustion.

typical incandescent light bulb produces mostly heat with only a little bit of light.

A (perverted) advantage of the light bulb is that it needs a lot of electricity and works well with the 110 volt wiring of the building (220 volts in Europe). Alternative current (AC) is not an issue; it only makes the light flicker at a rate faster than our eyes can perceive.

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The fluorescent lamp is a gas-filled tube in which the gas (mostly mercury vapor) is made to glow by hitting its atoms with energized electrons.

Electricity is passed through a ballast to increase its voltage to the point where electrons start being ejected. These “boiling” electrons are then flying through the gas in the tube and, by collisions, excite the orbiting electrons of these atoms. When the electrons in the atoms drop back down to lower excitation states, photons are emitted. For most tubes, these are primarily UV photons, which are not visible. The phosphor coating absorbs these UV photons and de-excite them through a cascade process to the point where several visible light photons are prod ced from each UV photon

The compact fluorescent light (CFL) is no more than a coiled tube with the ballast squeezed into the socket.

light photons are produced from each UV photon.

Spectrum of light emitted by a fluorescent bulb compared to other bulbs

Whereas the incandescent bulb produces about 15 lumens per watt of input power, the fluorescent bulb produces less heat and can produce between 50 and 100 lumens per watt. This makes fluorescent bulbs four to six times more efficient than incandescent bulbs. That is why you can buy a 15-watt fluorescent bulb that produces the same amount of light as a 60-watt incandescent bulb.

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The Light-emitting diode (LED)

The core of an LED is a diode, consisting of a p-n junction.

p-doped silicon has doping atoms (such as boron or gallium) that have one less electron, each creating a missing lattice link, called a “hole”, acting like a positive charge.

n doped silicon has doping atoms (such as phosphorus) that have one extra electronn-doped silicon has doping atoms (such as phosphorus) that have one extra electron,each creating an unattached, floating electron, a negative charge.

A p-n junction is the juxtaposition of a p-zone with an n-zone. This called a diode.

Holes and electrons diffuse by means of random thermal agitation.

Unbalanced charges generate a potentialUnbalanced charges generate a potential, which creates a barrier to further diffusion.

Voltage has reached equilibrium level, and the junction is in a stable state.

From p-n junction to emission of light

When the p-n junction is subjected to an external opposing voltage, with the + on the p-side and the – on the n-side, electrons begin to flow through the n-type then drop into holes when they can to the p-side. Their “drop” is a lowering of their energy.

Energy is constantly fed by the power source that applies the opposing voltage. It needs to be a DC (= direct current = continuous current) source, of about 2 volts

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be a DC ( direct current continuous current) source, of about 2 volts

E = energy released by electron of charge qe dropping its voltage by V.This E bit of energy goes into a photon of frequency , corresponding to light of wavelength .qe = 1.602 x 10-19 C = electron chargeh = 6.626 x 10-34 kg.m2/s = Planck’s const.c = 2.998 x 108 m/s = speed of light.

Note: never connect an LED directly to a battery or power supply! It will be destroyed almost instantly because too much current will pass through and burn it out. LEDs must have a resistor in series to limit the current to a small, safe value. Currents of 2 mA, 10 mA and 20 mA are common.

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LED diode assembly into a bulb

For insertion in a 110V-AC socket, the 9V-DC LED needs a transformer.It is also fitted with a reflector to direct the light.

Advantages of LEDs over Compact-Fluorescent Lights (CFLs) and conventional incandescent light bulbs

LED CFL Incandescent

800 lumens 800 lumens 800 lumens

LEDs last up to 10 times as long as CFL bulbs and far longer than incandescent bulbs.

LEDs use an average of 3-5% of the power of an incandescent lamp.

LED CFL incandescent

7 W 14 W 60 W

2,100 days 330 days 50 days

$33/year $77/year $329/year

(http://www.designrecycleinc.com/led%20comp%20chart.html)

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Unlike CFLs, LEDs do not contain mercury and do not need to be recycled with care. They can be disposed of with regular domestic trash.

LEDs can be made in very small sizes and turn on extremely fast, much faster than CFLs.

Early LEDs could only emit red light. Advances in semiconductors presently allow LEDS to emit light according to a spectrum that comes close to the solar spectrum (daylight).

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2. Heat Pumps

Heat spontaneously flows from warm to cold, but heat flow in reverse, from cold to warm, is nonetheless possible. The catch is that some energy needs to be spent to achieve this.

3-ton heat pump$1,369.99Warm reservoir (ex. indoor air or water for shower)

Heat delivery

cold reservoir (ex. outdoor air or water in the ground)Free Energy!

Spontaneous heat flow due to leaks

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Heat delivery

Work ($$)The question is:What should go in the ?

Heat pump in reverse

Air-conditioners, refrigerators and freezers are heat pumps working in reverse.

They use the very same technology. The hot and cold sides are simply flipped.

Warm reservoir (ex. outdoor air)What we get for free.

cold reservoir (ex. indoor air)What we want to have6!

Spontaneous heat flow due to leaks

?

Heat removal

Work ($$)

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Heat pump – How it worksA refrigerant(*) (1) is first taken in the vapor phase through a compressor which makes its temperature and pressure rise significantly; its temperature is now higher than the temperature of the hot side;(2) it is then brought into a heat exchange, in which it loses some of its heat to the (relatively colder) hot side; in the process of doing so, it condenses into a liquid, still at a high pressure;(3) The liquid is then passed through an expansion valve, which drops its pressure and temperature significantly; it is then a semi-evaporated/semi-condensed phase; the temperature is then lower than that of the cold side;(4) finally, the refrigerant is made to complete its evaporation by taking heat p p y gfrom the cold side, which is warmer than itself.

(*) A refrigerant is a substance that condenses and evaporates at temperatures in the vicinity of the low and high temperatures of the system.

Switching between heating and cooling by the simple action of valve

Cooling mode(air-conditioning)

Heating mode

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When the heat sink/source is no longer outdoor air but geothermal

Cooling mode Heating modeg(air-conditioning)

Heating mode

Coefficient of Performance (COP)

cold

warm

W

Q1

Q2 = Q1 + W

The COP is defined as the ratio of the heat delivered to the energy spent in performing the heat transfer (usually electricity to power the compressor).

For a heat pump used to deliver heat:

W

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W

QCOP

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CompressorbyConsumedEnergyElectrical

EndWarmtoBroughtHeat

COP = 1 is no better than a plain resistor (electricity degenerating into heat)!

For a heat pump used for refrigeration:

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QCOP 1

CompressorbyConsumedEnergyElectrical

EndColdfromRemovedHeat

Note:COPcooling = COPheating – 1

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Examples of COP values and curves

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Note values that exceed unity and that can reach as high as 5.

For COP = 5, one gets 5 joules ps.co.uk/gra

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of heat transferred from cold to warm for every joule of electricity spent on the compressor. This amounts to 17,060 BTUs of heat transferred from every kWh of electricity.(1 kWh = 3412 BTUs)

Remember, however, thatCOP COP 1

http://www.ecoairpump.co.uk/mcs-air-source-heat-pump-pv-solar-thermal-products/

COPcooling = COPheating – 1

3. Ocean Thermal Energy Conversion (OTEC)

Any temperature difference can be used to produce mechanical work, and OTEC applies this principle to the existing ocean temperature gradient. The ocean temperature weakly in the horizontal but quite rapidly in the vertical, especially in the tropics (warm surface water under high sun overlaying cold water from polar origin rising from mid-depth through process called equatorial upwelling).

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The system works exactly as a heat pump used in reversepump used in reverse, with work produced instead of spent, and heat cascading from warm to cold instead of moving up from cold to warm.

In 1980, the U.S. Department of Energy built OTEC-1, a test site for closed-cycle OTEC heat exchangers installed on board a converted U.S. Navy tanker. Test results demonstrated that OTEC systems can operate from slowly moving ships with little effect on the marine

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A coast-based test facility was established (circa 1980) as a joint project of the State of Hawaii and DOE, and desalinated water was produced by using the open-cycle process.

environment. A new design for suspended cold-water pipes was validated at that test site.

In 1981, Japan demonstrated a shore-based, 100-kWe closed-cycle plant in the Republic of Nauru in the Pacific Ocean. This plant employed cold-water pipe laid on the sea bed to a depth of 580 meters. Freon was the working fluid, and a titanium shell-and-tube heat exchanger was used. The plant surpassed engineering expectations by producing 31.5 kWe of net power during continuous operating tests.

http://www.nrel.gov/otec/achievements.html

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Advantages of OTEC technology

1. The source of energy is perfectly renewable, and it also exists on a large scale.

2. The generation of energy is continuous in time, unlike solar and wind energy.

3 The system once installed is relatively maintenance free

Limitations of OTEC technology

1 Working temperature differences will always remain modest leading to very low

3. The system, once installed, is relatively maintenance free.

4. The technology is ideally combined with water desalination because places by the sea and in need of freshwater (like Arabia) are sunny locations with warm ocean surface water.

1. Working temperature differences will always remain modest, leading to very low conversion efficiencies. This means a relatively large capital investment for low return.

2. Working with seawater is challenging because of the corrosion by salt.

3. The technology can only be applied where there is a pronounced vertical temperature gradient right below the surface, and the best spots on earth are in the middle of the ocean, from where energy could hardly be transported. The next best places are next to relatively unpopulated islands where energy demand isn’t very large.