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National Renewable Energy Laboratory

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National Renewable Energy Laboratory
Established 1977
Research Type Energy Efficiency & Renewable Energy
Budget 328 Million (2009)[1]
Director Dan E. Arvizu
Staff 1,230
Location Golden, CO
Operating Agency Midwest Research Institute and Battelle Memorial Institute
Website www.nrel.gov

NREL – Golden, Colorado

The National Renewable Energy Laboratory (NREL), located in Golden, Colorado, as part of the U.S. Department of Energy, is the United States‘ primary laboratory for renewable energy and energy efficiency research and development.

Contents

History

Established in 1974, NREL began operating in 1977 as the Solar Energy Research Institute.

Under the Carter administration, it was the recipient of a rather large budget and its activities went beyond research and development in solar energy as it tried to popularize knowledge about already existing technologies, like passive solar amongst the population.

In the Reagan years that followed the budget was cut by some 90%, many people ‘reduced in force’ and the activities reduced to R&D.

In later years renewed interest in the energy problem improved the institute’s position. But funding has fluctuated.

In 2006 its funding had dropped to the point it was forced to lay off 32 workers [2] .

It was designated a national laboratory of the U.S. Department of Energy (DOE) in September 1991 and its name changed to NREL. Since its inception it has been operated under contract by the Midwest Research Institute of Kansas City, Missouri.[3]


NREL is the principal research laboratory for the DOE Office of Energy Efficiency and Renewable Energy (EERE) which provides the majority of its funding.

Other funding comes from DOE’s Office of Science and Office of Electricity Transmission and Distribution.

NREL’s areas of research and development expertise are:

  • Renewable electricity
  • Renewable fuels
  • Integrated energy systems
  • Strategic energy analysis[4]

Funding in 2009

For 2009 funding is broken down between its major groups.

  • Wind $33.9 million
  • Biofuels $35.4 million
  • Solar $72.4 million[5]

NREL’s Technology Transfer Office supports the practical deployment of technologies developed, and this often involves collaborative research projects and licensed technologies with public and private partners.

NREL’s innovative technologies have been recognized with 39 “R&D 100” awards. The engineering and science behind these technology transfer successes and awards demonstrates NREL’s commitment to a sustainable energy future.[4]

Dr. Dan E. Arvizu became NREL’s eighth Laboratory Director in January 2005, and was previously an executive with CH2M HILL companies.

Solar cells

NREL PV R&D is performed under the National Center for Photovoltaics [6].

NREL tests and validates solar technologies.

The main research wind turbines at NREL

See also

References

  1. ^ Stimulus leaves NREL in cold
  2. ^ NREL to Lay Off 32 Staff, Budget Shortfall
  3. ^ MRI Quick Facts – mriresearch.org – Retrieved August 24, 2008
  4. ^ a b NREL Overview
  5. ^ Stimulus leaves NREL in cold
  6. ^ http://www.nrel.gov/pv/ncpv.html

External links

List of renewable energy organizations

From Wikipedia, the free encyclopedia

This is a list of notable renewable energy organizations:

Associations

Research institutions

See also

Second generation

Second generation materials have been developed to address energy requirements and production costs of solar cells. Alternative manufacturing techniques such as vapour deposition, electroplating, and use of Ultrasonic Nozzles are advantageous as they reduce high temperature processing significantly. It is commonly accepted that as manufacturing techniques evolve production costs will be dominated by constituent material requirements,[7] whether this be a silicon substrate, or glass cover.

The most successful second generation materials have been cadmium telluride (CdTe), copper indium gallium selenide, amorphous silicon and micromorphous silicon.[6] These materials are applied in a thin film to a supporting substrate such as glass or ceramics, reducing material mass and therefore costs. These technologies do hold promise of higher conversion efficiencies, particularly CIGS-CIS, DSC and CdTe offers significantly cheaper production costs.

Among major manufacturers there is certainly a trend toward second generation technologies, however commercialisation of these technologies has proven difficult.[9] In 2007 First Solar produced 200 MW of CdTe solar cells making it the fifth largest producer of solar cells in 2007 and the first ever to reach the top 10 from production of second generation technologies alone.[9] Wurth Solar commercialised its CIGS technology in 2007 producing 15 MW. Nanosolar commercialised its CIGS technology in 2007 with a production capacity of 430 MW for 2008 in the USA and Germany.[10]Honda, also began to commercialize their CIGS base solar panel in 2008.

In 2007, CdTe production represented 8.9% of total market share, thin-film silicon 5.2% and CIGS 0.5%.[9]

http://en.wikipedia.org/wiki/Solar_cell

**

My Note –

I started here because the thin film technology is more valuable to what is in my mind and then I thought it might be good to remember some of the other things found in the listing –

Third generation

Third generation technologies aim to enhance poor electrical performance of second generation (thin-film technologies) while maintaining very low production costs.

Current research is targeting conversion efficiencies of 30-60% while retaining low cost materials and manufacturing techniques.[7] They can exceed the theoretical solar conversion efficiency limit for a single energy threshold material, that was calculated in 1961 by Shockley and Queisser as 31% under 1 sun illumination and 40.8% under the maximal artificial concentration of sunlight (46,200 suns, which makes the latter limit more difficult to approach than the former).[11]

There are a few approaches to achieving these high efficiencies including the use of Multijunction photovoltaic cells, concentration of the incident spectrum, the use of thermal generation by UV light to enhance voltage or carrier collection, or the use of the infrared spectrum for night-time operation.

**

High efficiency cells

High efficiency solar cells are a class of solar cell that can generate more electricity per incident solar power unit (watt/watt). Much of the industry is focused on the most cost efficient technologies in terms of cost per generated power. The two main strategies to bring down the cost of photovoltaic electricity are increasing the efficiency (as many of the costs scale with the area occupied per unit of generated power), and decreasing the cost of the solar cells per generated unit of power. The latter approach might come at the expense of reduced efficiency, so the overall cost of the photovoltaic electricity does not necessarily decrease by decreasing the cost of the solar cells. The challenge of increasing the photovoltaic efficiency is thus of great interest, both from the academic and economic points of view.

Record efficiencies

Multiple junction solar cells

The record for multiple junction solar cells is disputed. Teams led by the University of Delaware, the Fraunhofer Institute, and NREL all claim the world record title at 42.8, 41.1, and 40.8 percent, respectively.[12][13][14] NREL claims that the other implementations have not been put under standardized tests and, in the case of the University of Delaware project, represents only hypothetical efficiencies of a panel that has not been fully assembled.[15] NREL claims it is one of only three laboratories in the world capable of conducting valid tests, although the Fraunhofer Institute is among those three facilities.

Thin film solar cells

In 2002, the highest reported efficiency for solar cells based on thin films of CdTe is 18%, which was achieved by research at Sheffield Hallam University, although this has not been confirmed by an external test laboratory[citation needed].

The US national renewable energy research facility NREL achieved an efficiency of 19.9% for the solar cells based on copper indium gallium selenide thin films, also known as CIGS. These CIGS films have been grown by physical vapour deposition in a three-stage co-evaporation process. In this process In, Ga and Se are evaporated in the first step; in the second step it is followed by Cu and Se co-evaporation and in the last step terminated by In, Ga and Se evaporation again

http://en.wikipedia.org/wiki/Solar_cell

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Effect of physical size

The values of I0, RS, and RSH are dependent upon the physical size of the solar cell. In comparing otherwise identical cells, a cell with twice the surface area of another will, in principle, have double the I0 because it has twice the junction area across which current can leak. It will also have half the RS and RSH because it has twice the cross-sectional area through which current can flow. For this reason, the characteristic equation is frequently written in terms of current density, or current produced per unit cell area:

J = J_{L} - J_{0} \left\{\exp\left[\frac{q(V + J r_{S})}{nkT}\right] - 1\right\} - \frac{V + J r_{S}}{r_{SH}}

where

  • J = current density (amperes/cm2)
  • JL = photogenerated current density (amperes/cm2)
  • Jo= reverse saturation current density (amperes/cm2)
  • rS = specific series resistance (Ω-cm2)
  • rSH = specific shunt resistance (Ω-cm2)

This formulation has several advantages. One is that since cell characteristics are referenced to a common cross-sectional area they may be compared for cells of different physical dimensions. While this is of limited benefit in a manufacturing setting, where all cells tend to be the same size, it is useful in research and in comparing cells between manufacturers. Another advantage is that the density equation naturally scales the parameter values to similar orders of magnitude, which can make numerical extraction of them simpler and more accurate even with naive solution methods.

A practical limitation of this formulation is that as cell sizes shrink, certain parasitic effects grow in importance and can affect the extracted parameter values. For example, recombination and contamination of the junction tend to be greatest at the perimeter of the cell, so very small cells may exhibit higher values of J0 or lower values of rSH than larger cells that are otherwise identical. In such cases, comparisons between cells must be made cautiously and with these effects in mind.

[edit] Cell temperature

Effect of temperature on the current-voltage characteristics of a solar cell

Temperature affects the characteristic equation in two ways: directly, via T in the exponential term, and indirectly via its effect on I0. (Strictly speaking, temperature affects all of the terms, but these two far more significantly than the others.) While increasing T reduces the magnitude of the exponent in the characteristic equation, the value of I0 increases in proportion to exp(T). The net effect is to reduce VOC (the open-circuit Voltage) linearly with increasing temperature. The magnitude of this reduction is inversely proportional to VOC; that is, cells with higher values of VOC suffer smaller reductions in voltage with increasing temperature. For most crystalline silicon solar cells the reduction is about 0.50%/°C, though the rate for the highest-efficiency crystalline silicon cells is around 0.35%/°C. By way of comparison, the rate for amorphous silicon solar cells is 0.20-0.30%/°C, depending on how the cell is made.

The amount of photogenerated current IL increases slightly with increasing temperature because of an increase in the number of thermally generated carriers in the cell. This effect is slight, however: about 0.065%/°C for crystalline silicon cells and 0.09% for amorphous silicon cells.

The overall effect of temperature on cell efficiency can be computed using these factors in combination with the characteristic equation. However, since the change in voltage is much stronger than the change in current, the overall effect on efficiency tends to be similar to that on voltage. Most crystalline silicon solar cells decline in efficiency by 0.50%/°C and most amorphous cells decline by 0.15-0.25%/°C. The figure to the right shows I-V curves that might typically be seen for a crystalline silicon solar cell at various temperatures.

[edit] Series resistance

Effect of series resistance on the current-voltage characteristics of a solar cell

As series resistance increases, the voltage drop between the junction voltage and the terminal voltage becomes greater for the same flow of current. The result is that the current-controlled portion of the I-V curve begins to sag toward the origin, producing a significant decrease in the terminal voltage V and a slight reduction in ISC, the short-circuit current. Very high values of RS will also produce a significant reduction in ISC; in these regimes, series resistance dominates and the behavior of the solar cell resembles that of a resistor. These effects are shown for crystalline silicon solar cells in the I-V curves displayed in the figure to the right.

[etc. ]

Most solar cells, which are quite large compared to conventional diodes, well approximate an infinite plane and will usually exhibit near-ideal behavior under Standard Test Condition (n \approx 1). Under certain operating conditions, however, device operation may be dominated by recombination in the space-charge region. This is characterized by a significant increase in I0 as well as an increase in ideality factor to n \approx 2. The latter tends to increase solar cell output voltage while the former acts to erode it. The net effect, therefore, is a combination of the increase in voltage shown for increasing n in the figure to the right and the decrease in voltage shown for increasing I0 in the figure above. Typically, I0 is the more significant factor and the result is a reduction in voltage.

[edit] Solar cell efficiency factors

[edit] Energy conversion efficiency

Dust often accumulates on the glass of solar panels seen here as black dots.

A solar cell’s energy conversion efficiency (η, “eta”), is the percentage of power converted (from absorbed light to electrical energy) and collected, when a solar cell is connected to an electrical circuit. This term is calculated using the ratio of the maximum power point, Pm, divided by the input light irradiance (E, in W/m2) under standard test conditions (STC) and the surface area of the solar cell (Ac in m2).

\eta = \frac{P_{m}}{E \times A_c}

STC specifies a temperature of 25°C and an irradiance of 1000 W/m2 with an air mass 1.5 (AM1.5) spectrum. These correspond to the irradiance and spectrum of sunlight incident on a clear day upon a sun-facing 37°-tilted surface with the sun at an angle of 41.81° above the horizon.[21][22] This condition approximately represents solar noon near the spring and autumn equinoxes in the continental United States with surface of the cell aimed directly at the sun. Thus, under these conditions a solar cell of 12% efficiency with a 100 cm2 (0.01 m2) surface area can be expected to produce approximately 1.2 watts of power.

The losses of a solar cell may be broken down into reflectance losses, thermodynamic efficiency, recombination losses and resistive electrical loss. The overall efficiency is the product of each of these individual losses.

Due to the difficulty in measuring these parameters directly, other parameters are measured instead: Thermodynamic Efficiency, Quantum Efficiency, VOC ratio, and Fill Factor. Reflectance losses are a portion of the Quantum Efficiency under “External Quantum Efficiency”. Recombination losses make up a portion of the Quantum Efficiency, VOC ratio, and Fill Factor. Resistive losses are predominantly categorized under Fill Factor, but also make up minor portions of the Quantum Efficiency, VOC ratio.

[23]

[edit] Thermodynamic Efficiency Limit

Solar cells operate as quantum energy conversion devices, and are therefore subject to the “Thermodynamic Efficiency Limit”. Photons with an energy below the band gap of the absorber material cannot generate a hole-electron pair, and so their energy is not converted to useful output and only generates heat if absorbed. For photons with an energy above the band gap energy, only a fraction of the energy above the band gap can be converted to useful output. When a photon of greater energy is absorbed, the excess energy above the band gap is converted to kinetic energy of the carrier combination. The excess kinetic energy is converted to heat through phonon interactions as the kinetic energy of the carriers slows to equilibrium velocity.

Solar cells with multiple band gap absorber materials are able to more efficiently convert the solar spectrum. By using multiple band gaps, the solar spectrum may be broken down into smaller bins where the thermodynamic efficiency limit is higher for each bin.[24]

[edit] Quantum efficiency

As described above, when a photon is absorbed by a solar cell it can produce a pair of free charge carriers, i.e. an electron-hole pair. One of the carriers (the minority carrier) may then be able to reach the p-n junction and contribute to the current produced by the solar cell; such a carrier is said to be collected. Alternatively, the carrier may give up its energy and once again become bound to an atom within the solar cell without being collected; this process is then called recombination since one electron and one hole recombine and thereby annihilate the associated free charge. The carriers that recombine do not contribute to the generation of electrical current.

Quantum efficiency refers to the percentage of photons that are converted to electric current (i.e., collected carriers) when the cell is operated under short circuit conditions. External quantum efficiency (EQE) is the fraction of incident photons that are converted to electrical current, while internal quantum efficiency (IQE) is the fraction of absorbed photons that are converted to electrical current. Mathematically, internal quantum efficiency is related to external quantum efficiency by the reflectance (R) and the transmittance (T) of the solar cell by IQE = EQE / (1 − RT). Please note that for a thick bulk Si solar cell T is approximately zero and is therefore in practical cases often neglected.

Quantum efficiency should not be confused with energy conversion efficiency, as it does not convey information about the fraction of power that is converted by the solar cell. Furthermore, quantum efficiency is most usefully expressed as a spectral measurement (that is, as a function of photon wavelength or energy). Since some wavelengths are absorbed more effectively than others in most semiconductors, spectral measurements of quantum efficiency can yield valuable information about the quality of the semiconductor bulk and surfaces.

[edit] Maximum-power point

A solar cell may operate over a wide range of voltages (V) and currents (I). By increasing the resistive load on an irradiated cell continuously from zero (a short circuit) to a very high value (an open circuit) one can determine the maximum-power point, the point that maximizes V×I; that is, the load for which the cell can deliver maximum electrical power at that level of irradiation. (The output power is zero in both the short circuit and open circuit extremes).

A high quality, monocrystalline silicon solar cell, at 25 °C cell temperature, may produce 0.60 volts open-circuit (Voc). The cell temperature in full sunlight, even with 25 °C air temperature, will probably be close to 45 °C, reducing the open-circuit voltage to 0.55 volts per cell. The voltage drops modestly, with this type of cell, until the short-circuit current is approached (Isc). Maximum power (with 45 °C cell temperature) is typically produced with 75% to 80% of the open-circuit voltage (0.43 volts in this case) and 90% of the short-circuit current. This output can be up to 70% of the Voc x Isc product. The short-circuit current (Isc) from a cell is nearly proportional to the illumination, while the open-circuit voltage (Voc) may drop only 10% with a 80% drop in illumination. Lower-quality cells have a more rapid drop in voltage with increasing current and could produce only 1/2 Voc at 1/2 Isc. The usable power output could thus drop from 70% of the Voc x Isc product to 50% or even as little as 25%. Vendors who rate their solar cell “power” only as Voc x Isc, without giving load curves, can be seriously distorting their actual performance.

The maximum power point of a photovoltaic varies with incident illumination. For systems large enough to justify the extra expense, a maximum power point tracker tracks the instantaneous power by continually measuring the voltage and current (and hence, power transfer), and uses this information to dynamically adjust the load so the maximum power is always transferred, regardless of the variation in lighting.

[edit] Fill factor

Another defining term in the overall behavior of a solar cell is the fill factor (FF). This is the ratio of the maximum power point divided by the open circuit voltage (Voc) and the short circuit current (Isc):

FF = \frac{P_{m}}{V_{oc} \times I_{sc}} = \frac{\eta \times A_c \times E}{V_{oc} \times I_{sc}}.

The fill factor is directly affected by the values of the cells series and shunt resistance. Increasing the shunt resistance (Rsh) and decreasing the series resistance (Rs) will lead to higher fill factor, thus resulting in greater efficiency, and pushing the cells output power closer towards its theoretical maximum[25]

[edit] Comparison of energy conversion efficiencies

At this point, discussion of the different ways to calculate efficiency for space cells and terrestrial cells is necessary to alleviate confusion. In space, where there is no atmosphere, the spectrum of the sun is relatively unfiltered. However, on earth, with air filtering the incoming light, the solar spectrum changes. To account for the spectral differences, a system was devised to calculate this filtering effect. Simply, the filtering effect ranges from Air Mass 0 (AM0) in space, to approximately Air Mass 1.5 on earth. Multiplying the spectral differences by the quantum efficiency of the solar cell in question will yield the efficiency of the device. For example, a Silicon solar cell in space might have an efficiency of 14% at AM0, but have an efficiency of 16% on earth at AM 1.5. Terrestrial efficiencies typically are greater than space efficiencies.

Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 40.7% with multiple-junction research lab cells and 42.8% with multiple dies assembled into a hybrid package.[26] Solar cell energy conversion efficiencies for commercially available multicrystalline Si solar cells are around 14-19%.[27] The highest efficiency cells have not always been the most economical — for example a 30% efficient multijunction cell based on exotic materials such as gallium arsenide or indium selenide and produced in low volume might well cost one hundred times as much as an 8% efficient amorphous silicon cell in mass production, while only delivering about four times the electrical power.

However, there is a way to “boost” solar power. By increasing the light intensity, typically photogenerated carriers are increased, resulting in increased efficiency by up to 15%. These so-called “concentrator systems” have only begun to become cost-competitive as a result of the development of high efficiency GaAs cells. The increase in intensity is typically accomplished by using concentrating optics. A typical concentrator system may use a light intensity 6-400 times the sun, and increase the efficiency of a one sun GaAs cell from 31% at AM 1.5 to 35%. See Solar_cell#Concentrating photovoltaics (CPV) below and Concentrating solar power (CSP).

A common method used to express economic costs of electricity-generating systems is to calculate a price per delivered kilowatt-hour (kWh). The solar cell efficiency in combination with the available irradiation has a major influence on the costs, but generally speaking the overall system efficiency is important. Using the commercially available solar cells (as of 2006) and system technology leads to system efficiencies between 5 and 19%. As of 2005, photovoltaic electricity generation costs ranged from ~0.60 US$/kWh (0.50 €/kWh) (central Europe) down to ~0.30 US$/kWh (0.25 €/kWh) in regions of high solar irradiation. This electricity is generally fed into the electrical grid on the customer’s side of the meter. The cost can be compared to prevailing retail electric pricing (as of 2005), which varied from between 0.04 and 0.50 US$/kWh worldwide. (Note: in addition to solar irradiance profiles, these costs/kwh calculations will vary depending on assumptions for years of useful life of a system. Most c-Si panels are warranted for 25 years and should see 35+ years of useful life.)

The chart at the right illustrates the various commercial large-area module energy conversion efficiencies and the best laboratory efficiencies obtained for various materials and technologies.

Reported timeline of solar cell energy conversion efficiencies (from National Renewable Energy Laboratory (USA)

Watts peak

Since solar cell output power depends on multiple factors, such as the sun‘s incidence angle, for comparison purposes between different cells and panels, the measure of watts peak (Wp) is used. It is the output power under these conditions known as STC. The standard test conditions imply an insolation (solar irradiance) of 1000 W/m2, a solar reference spectrum AM (airmass) of 1.5 and a cell temperature 25°C