Fotovoltaik

(Omdirigeret fra Anti-solcelle)
Solpaneler i Marla, Spanien
World Photovoltaics Installed Capacity.svg
Fotovoltaisk 'træ' i Steiermark, Østrig

Fotovoltaik er omdannelsen af lys til elektrisk energi. Det sker ved den fotovoltaiske effekt via halvledere. Typiske materialer, der bruges til fotovoltaik, er monokrystalisk silicium, polykrystallisk silicium, amorf silicium, cadmiumtellurid og kobber indium selenid. Denne teknik benyttes gennem solceller også kaldet fotovoltaiske celler.

På grund af stor efterspørgsel efter vedvarende energi har forskningen og etableringen af solcelleanlæg gennem de senere år været i stor vækst.[1][2][3]

Fotovoltaisk produktion er øget med i gennemsnit 48% hvert år siden 2002, hvilket gør det til den hurtigst voksende energiteknologi i verden.[4]

Fotovoltaiske effektRediger

Den fotovoltaiske effekt er skabelsen af elektrisk spænding i et materiale der udsættes for lys. Til trods for at den fotovoltaiske effekt er direkte relateret til den fotoelektriske effekt er de to processer forskellige og bør være adskilte. I den fotoelektriske effekt bliver elektroner frastødt et materiales overflade, når det udsættes for bestråling med en tilstrækkelig mængde energi. Den fotovoltaiske effekt er forskellig på den måde, at de frigivne elektroner flyttes mellem forskellige bindinger (for eksempel fra valens til konduktions-bindinger) indenfor det samme materiale, dermed opbygges en spænding mellem to elektroder.[5]

Den fotovoltaiske effekt blev første gang observeret af den franske fysiker Alexandre Edmond Becquerel i 1839.[6][7]

SolcelleRediger

  Uddybende artikel: Solcelle

I de fleste fotovoltaiske enheder er bestrålingskilden sollys og derfor kendes disse som solceller. I tilfældet med en pn-overgang solcelle, der skaber belysning af materialet en elektrisk jævnspænding.[8]

Anti-solcelleRediger

  Hovedartikel: Termoelektrisk effekt.

Det er et faktum, at en pn-overgang med det rette båndgab, kan anvendes til at lave elektrisk energi af en temperaturforskel.[9]

Der er i flere artikler blevet fremsat den hypotese, at man lave fotovoltaik celler, som kan virke om natten - og om dagen hvis beskyttet mod sollys. Sådanne fotovoltaik celler kaldes en anti-solcelle (engelsk anti-solar cell), termisk udstrålings fotovoltaik celle (engelsk thermoradiative cell).[10][11] Både om natten og dagen skal himlen være mere (helst) eller mindre skyfri, så termisk infrarød stråling i bølgelængdeintervallet 8-13 um frit kan stråle gennem atmosfæren og ud i det lufttomme verdensrum. Ideelt vil en anti-solcelle kunne levere 54 W/m^2 og under typiske himmelforhold 10 W/m^2.[12][9] Da anti-solcellen køles ned på den himmelvendte side, skal der løbende tilføres varme på siden, der vender ned mod jorden, fx fra vinden, søvand, havvand, jorden - eller på anden vis.

Der er fundet flere materialer, som kan udstråle infrarød stråling i bølgelængdeintervallet 8-13 um. Sådanne materialer kan også anvendes til at køle objekter både dag og nat - fx huse og huses tage, mure mellem 5°C-10°C (resultat 2014-2019) i forhold til omgivelsestemperaturen, når der er klar himmel.[13][14]

Se ogsåRediger

ReferencerRediger

  1. ^ Tyska FV-marknaden
  2. ^ BP Solar to Expand Its Solar Cell Plants in Spain and India
  3. ^ Large-Scale, Cheap Solar Electricity
  4. ^ Earth Policy Institute (2007). Solar Cell Production Jumps 50 Percent in 2007
  5. ^ The Photovoltaic Effect – Introduction. Photovoltaics.sandia.gov (2001-02-01). Retrieved on 2010-12-12.
  6. ^ Photovoltaic Effect. Mrsolar.com. Retrieved on 2010-12-12.
  7. ^ The photovoltaic effect. Encyclobeamia.solarbotics.net. Retrieved on 2010-12-12.
  8. ^ The photovoltaic effect. Scienzagiovane.unibo.it (2006-12-01). Retrieved on 2010-12-12.
  9. ^ a b Open Access: 13 October 2016, nature.com: Entropic and Near-Field Improvements of Thermoradiative Cells Citat: "...Semiconductor p-n junctions, on the other hand, have been used to harvest photons from high temperature heat sources to generate electricity, such as photovoltaic (PV) and thermophotovoltaic devices9,10,11,12,13,14,15. In these devices, photons from external heat source enter the devices to generate electron-hole pairs and carry in entropy. Entropy of the incoming photons and entropy generated in the energy conversion process will be carried away by photons emitted during radiative recombination and via heat rejected to the environment...The idea of using photons to reject heat of a heat engine is recently explored by Byrnes et al.18. aiming at using outer space as the heat sink, and a thermoradiative cell can be a candidate for this approach...", backup
  10. ^ January 30, 2020, scitechdaily.com: Anti-Solar Cells: Thermoradiative Photovoltaic Cells Work at Night Citat: "...What if solar cells worked at night? That’s no joke, according to Jeremy Munday, professor in the Department of Electrical and Computer Engineering at UC Davis. In fact, a specially designed photovoltaic cell could generate up to 50 watts of power per square meter under ideal conditions at night, about a quarter of what a conventional solar panel can generate in daytime, according to a concept paper by Munday and graduate student Tristan Deppe. The article was published in, and featured on the cover of, the January 2020 issue of ACS Photonics...The device would work during the day as well, if you took steps to either block direct sunlight or pointed it away from the sun. Because this new type of solar cell could potentially operate around the clock, it is an intriguing option to balance the power grid over the day-night cycle..."
  11. ^ Feb 3, 2020, popularmechanics.com: How Reverse Solar Panels Could Generate Power at Night. It's called "optically coupling with deep space." Citat: "...In a thermal radiation cell, we reset the parameters so Earth is the new sun, and its even minimal accumulated heat dwarfs the cold, midnight black of outer space...", backup
  12. ^ Paywalled: ACS Photonics: Nighttime Photovoltaic Cells: Electrical Power Generation by Optically Coupling with Deep Space. Tristan Deppe Jeremy N. Munday* ACS Photonics 2020, 7, 1, 1-9 Citat: "...Explained in detail in the next section of this Perspective, a TR [Thermoradiative Cells] cell generates power because the emission of thermal radiation from the cell exceeds the absorption of irradiation from the surroundings during operation. The actual devices, a solar cell and a TR cell, are nearly identical; however, the operating currents and voltages have opposite signs because the radiative processes are reciprocal (Figure 1c)...The nighttime PV cell concept relies on the thermoradiative effect and uses the warmth of the earth, at about 300 K, as a heat source and the darkness of space, at 3 K, as a heat sink...an ideal cell could produce as much as 54 W/m2 under ideal conditions and potentially more than 10 W/m2 under typical sky conditions..."
  13. ^ Stanford School of Engineering. (2014, November 26). High-tech mirror beams heat away from buildings into space. ScienceDaily Citat: "..."As a result of professor Fan's work, we can now [use radiative cooling], not only at night but counter-intuitively in the daytime as well."...The first part of the coating's one-two punch radiates heat-bearing infrared light directly into space...This multilayered coating also acts as a highly efficient mirror, preventing 97 percent of sunlight from striking the building and heating it up...Together, the radiation and reflection make the photonic radiative cooler nearly 9 degrees Fahrenheit [5 °C] cooler than the surrounding air during the day...Its internal structure is tuned to radiate infrared rays at a frequency that lets them pass into space without warming the air near the building..."
  14. ^ May. 23, 2019, sciencemag.org: This engineered wood radiates heat into space, potentially slashing cooling costs Citat: "..."...If used on a building’s exterior, such as in siding and roofs, the material could drop a building’s temperature as much as 10°C and reduce cooling costs as much as 60%...But in the past 2 years, researchers have devised plastic films and paints that absorb heat and re-emit that energy at longer mid-IR wavelengths, which air doesn’t absorb. If emitted toward the sky, these photons pass unimpeded and dump their energy into deep space. But to use these materials in buildings, engineers need to laminate rooftop or siding materials with the plastics or apply the heat-emitting paints...The researchers hit upon a simple chemical procedure. They soaked basswood in a solution of hydrogen peroxide, which chops normally long lignin molecules into small fragments...it turns white, reflecting virtually all incoming light. The new composite also absorbs heat from its surroundings and reradiates it as mid-IR light. That allows the material to cool surfaces to which it is attached by up to 10°C..."
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