ELECTRONICS, PHOTONICS AND MICROSYSTEMS
Andrzej DZIEDZIC, Piotr MARKOWSKI
Autonomous Power Supplying Systems Topic 11. Photovoltaic and thermophotovoltaic microgenerators
1. Photovoltaics, photovoltaic effect – basic information 2. Parameters of solar cells and solar modules 3. Examples of photovoltaic microgenerators 4. Thermophotovoltaic conversion 5. Elements of thermophotovoltaic systems 6. Thermophotovoltaic (TPV) power microgenerators
The forecast of global use of energy sources Remaining renewable Annual energy consumption [EJ/a] Solar thermic
Solar energetic
Wind
Biomass
Water energy
Nuclear energy
Gas
Coal
Petroleum Source: solarwirtschaft.de Photovoltaics (PV) is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels comprising a number of cells containing a photovoltaic material. Materials presently used for photovoltaics include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride (CdTe), and copper indium gallium selenide (CuInxGa1-xSe – CIGS). 0.25
0.20
m] a) ultraviolet
0.15 2 b) visible radiation c) infrared
0.10 P [mW/cm P
0.05
0.00 Spectrum of solar radiation: 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 [m] (1) – radiation of excellent black-body for T = 6000 K (2) – solar radiation reaching high layer of earth atmosphere (3) – solar radiation at the sea level (with losses caused by presence in the
atmosphere of O2, O3, H2O and CO2) The photovoltaic effect is the creation of a voltage (or a corresponding electric current) in a material upon exposure to light - the generated electrons are transferred between different bands (i.e. from the valence to conduction bands) within the material, resulting in the buildup of a voltage between two electrodes. It refers to photons of light knocking electrons into a higher state of energy to create electricity. The term photovoltaic denotes the unbiased operating mode of a photodiode in which current through the device is entirely due to the transduced light energy. Virtually all photovoltaic devices are some type of photodiode. In most photovoltaic applications the radiation is sunlight and for this reason the devices are known as solar cells. In the case of a p-n junction solar cell, illumination of the material results in the creation of an electric current as excited electrons and the remaining holes are swept in different directions by the built-in electric field of the depletion region. Photovoltaic effect - simple explanation 1. Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon (except of this lower energy photons can pass straight through solar cell or the photon can reflect off surface of semiconducting material) 2. Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. Due to the special composition of solar cells, the electrons are only allowed to move in a single direction. 3. An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity. p-n homo- or heterojunction - charge carrier separation
1. drift of carriers, driven by an electrostatic field established across the device (dominant mode in thin film cells as a-Si or CIGS) 2. diffusion of carriers from zones of high carrier concentration to zones of low carrier concentration, following a gradient of electrochemical potential (dominant mode in mono- or multicrystalline silicon solar cells) The equation for total current I flowing in semiconductor is as follow: qU qU I I S1 exp 1 I S 2 exp 1 kT 2kT where IS1 i IS2 - diffusive and recombination part of diode dark saturation current, respectively:
Dn n p0 D p p n0 n I Sq I Sq i Wv N S1 L L S 2 2 th t n p
n p where W – width of active layer of junction:
2 Na Nd W VD q Na Nd
and ε – electrical permeability of semiconductor, Na, Nd – density of proper trap states (acceptor and donor ones), VD – diffusive voltage. I-V characteristics (black) and load characteristics (red) of typical illuminated solar cell with marked characteristic parameters of such device qV I [exp( OC ) 1] I 0 S AkT
Parameters of solar cells (1)
ISC - Short Circuit Current VOC - Open Circuit Voltage - can be calculated from Shockley equation (for I = 0)
AkT I VOC ln(1 ) q I S where I – photogenerated current (dependent on solar flux insolation, independent on voltage), V – voltage cross the output terminals, k – Boltzmann’s constant , T – junction temperature (in Kelvin), A – diode ideality factor. A is from the range 12 and shows the participation of recombination current and diffusion current in total photocurrent. Diode ideality factor can be calculated from the formula q V V A 1 2 kT 1 ln( x) (assuming that is voltage change for current change by an order of x), at T = 300 K = 0.026 V. Parameters of solar cells (2)
• (Vnom, Inom) – nominal power point Pnom • (Vm, Im) – maximal power point Pmax (Pmax = ImaxVmax) • Fill Factor, FF: I V FF max max I SC VOC
Efficiency, η – ratio of maximal power point Pmax to the power of solar irradiation (Pin): I U max max 100% Pin • Series resistance, RS • Shunt resistance, RSH All parameters of photovoltaic cell are strongly dependent on solar radiation and ambient temperature. qVD VD I I I S exp 1 AkT RSH
DC equivalent circuit of a solar cell – single diode equivalent (SEM) model
qV D 1 I I IS exp AkT V D RSH and
VD V I RS DC equivalent circuit of a solar cell – double diode equivalent (DEM) model
qVD I I I S1 exp 1 kT
qVD VD I S 2 exp 1 2kT RSH
and
VD V I RS Cell temperature Reverse saturation current
Effect of temperature on Effect of reverse the I-V characteristics of saturation current on the a solar cell I-V characteristics of a solar cell Series resistance Shunt resistance
Effect of series resistance Effect of shunt resistance on the I-V characteristics on the I-V characteristics of a solar cell of a solar cell Example - parameters of ST40 CIGS module - DEM model (1)
Arrhenius plot for IS1 – diffusive part Arrhenius plot for IS2 – of dark current of ST 40 CIGS module recombination part of dark current in a wide range of total insolation of ST 40 CIGS module in a wide Gipoa and module working range of insolation Gipoa and
temperature Tm module working temperature Tm Example - parameters of ST40 CIGS module - DEM model (2)
Temperature dependence of series Temperature dependence of shunt resistance of ST40 CIGS module in resistance of ST40 CIGS module in a wide range of total insolation Gipoa a wide range of total insolation Gipoa Example - parameters of ST40 CIGS module - DEM model (3)
Efficiency η of ST40 CIGS module in Fill factor FF of ST40 CIGS module a wide range of its working in a wide range of its working
temperature Tm temperature Tm Basic structure of a silicon based solar cell and its working Band diagram of a silicon solar cell mechanism Polycrystalline photovoltaic Solar cell made from a cells laminated to backing monocrystalline silicon wafer material in a module Sunlight
Metallization Antireflective layer Conductive layer Buffer layer Junction (n semiconductor) OVC layer Absorber layer (p semiconductor) Ohmic contact Substrate
Schema of cross-section of thin-film CIGS cell/module Cross-section through CIGS structure with marked places of module dividing onto many photovoltaic cells and roads of electrical current flow (SGL - …., Mo - …., CIGS - …., ZAO - …
Best laboratory efficiencies obtained for solar cells with various materials and technologies PV cell interconnection and module fabrication Solar cells are rarely used individually – cells with similar characteristics are connected and encapsulated to form modules (panels) which, in turn, are the basic building blocks of solar arrays. The module must be able to withstand such environmental conditions as dust, salt, sand, snow, humidity and hail, as well as maintaining performance under prolonged exposure to UV light. I-V characteristics of solar module (1) a) composed of identical cells (N cells in series × M cells in parallel) q(V /N) tot Itot MI MI 0[exp( )1] AkT b) composed of non-identical cells • mismatched cells connected in parallel: same voltage Cell 2 has lower output caused by: - manufacturing defects V - degradation - partial shading - higher temperature I-V characteristics of solar module (2)
An easy method of calculating The effect on current output the combined open circuit of mismatched cells voltage (VOC) of mismatched connected in parallel cells connected in parallel I-V characteristics of solar module (3)
2 1 An easy method of calculating I the combined short circuit Series connected current (I ) of mismatched mismatched cells and the SC cells connected in series effect on their voltage output PV modules – thermal considerations
It is desirable for modules to operate at as low temperature as possible because: • cell output is increased at lower temperatures, • thermal cycles and stress are reduced, • degradation rates approximately doubles for each 10oC increase in temperature.
The Nominal Operating Cell Temperature (NOCT) is defined as the temperature reached by open circuited cells in module under the following conditions: • irradiance on cell surface – 800 W/m2, • ait temperature – 20oC • wind velocity – 1 m/s • mounting – open back side
The following approximate expression for calculating the cell temperature is given
(NOCT 20) 2 TCELL TAIR S [ C] , where S – insolation (in mW/cm ) 80 Photovoltaic microgenerators - examples (1)
- 67 micro-solar cells - Single cell dimension: 550 x 300 μm - Matrix dimension: 5.9 x 7.9 mm2 - Lighting: laser 532 nm, 1W - Parameters: U = 101 V I = 88 μA P = 8.8 mW DS = 19 mW/cm2 Source: [1] Conversion coefficient: 0.88%
Photovoltaic microgenerators - examples (2)
MCM technique Lumped model of an assembled cell Photovoltaic microgenerators - examples (3)
Illuminated I-V characteristic and delivered power of an array (N = 15) lamp placed at different from the VOC scaling versus connected array is considered devices in series Photovoltaic microgenerators - examples (4)
2 Optical Source I [μA] V [V] DS [mW/cm ] conv. coef. [%] Sun light AM 1.5 26.5 7.25 0.50 0.5 (100 mW/cm2) 150 W IR lamp d = 15 cm 82.7 7.76 1.66 0.55 (300 mW/cm2) 150 W IR lamp d = 5 cm 287.0 8.24 6.22 0.62 (1 W/cm2) Nd:YAG laser λ=1064 nm 5.90 7.0 0.1 2.86 (3.5 mW/cm2) 15 microphotovoltaic cells; single cell area: 2 mm2; matrix area: 6,3 x 6,5 mm2 Photovoltaic microgenerators - examples (5)
Si a-Si:H
SiO2 ZnO
Cr ITO Thermophotovoltaic conversion (1)
Thermophotovoltaic (TPV) energy conversion is a direct conversion process from heat differentials to electricity via photons. A basic thermophotovoltaic system consists of a thermal emitter and a photovoltaic diode cell. The temperature of the thermal emitter varies between different systems from about 900°C to about 1300°C. The emitter can be a piece of solid material or a specially engineered structure (a conventional solar cell is effectively a TPV device in which the Sun serves as the emitter). Thermal emission is the spontaneous emission of photons due to thermal motion of charges in the material. For normal TPV temperatures, this radiation is mostly at near infrared and infrared frequencies. The photovoltaic diodes can absorb some of these radiated photons and convert them into free charge carriers, that is electricity. Thermophotovoltaic systems have few, if any, moving parts and are therefore very quiet and require low maintenance. Thermophotovoltaic conversion (2)
Schematic of TPV systems: (Left) - A blackbody emitter is heated (either by solar concentration or combustion) causing it emit radiation. The radiation passes through a filter that allows only optimal wavelengths for conversion and reflects others back to the emitter where they can absorbed again generating additional heat and photons. That light passed through the filter is absorbed by the PV converter to generate electricity. Any unabsorbed light passes to the reflector where it is sent back to the PV to be converter or to the emitter to be reabsorbed. (Right) - A similar system, where the blackbody emitter and narrow band filter are replaced with a selective emitter . Elements of thermophotovoltaic systems (1) Emitters - for the emitter, deviations from perfect absorbing and perfect blackbody behavior lead to light losses. For the case of selective emitters, any light emitted at wavelengths not matched to the bandgap energy of the PV may not be efficiently converted (for reasons discussed above) and leads to reduced efficiency. Efficiency, temperature resistance, and cost are the three major factors when choosing the radiator for TPVs: • Efficiency is determined by energy absorbed relative to total incoming radiation. • Ability to operate at high temperatures is a crucial factor because efficiency increases with operating temperature. As emitter temperature increases, the blackbody radiation shifts to shorter wavelengths, allowing for more efficient absorption by photovoltaic cells. • Cost is a major limitation in commercialization of TPVs. Elements of thermophotovoltaic systems (2)
Polycrystalline Silicon Carbide (SiC) is the most commonly used emitter for burner TPVs. SiC is thermally stable to about 1700°C
Tungsten - refractory metals are often used as selective emitters for burner TPVs. Tungsten is the most common choice. The emitter is usually in the shape of a cylinder with a sealed bottom, which can be considered a cavity. The emitter is attached to the back of a thermal absorber such as SiC and maintains the same temperature. Emission occurs in the visible and near IR range which can be readily converted by the PV to electrical energy. Elements of thermophotovoltaic systems (2)
Rare-earth Oxides such as ytterbium oxide (Yb2O3) and erbium oxide (Er2O3) are the most commonly used selective emitters for TPVs. These oxides emit a narrow band of wavelengths in the near infrared regions, allowing the tailoring of the emission spectra to better fit the absorbance characteristics of a particular PV cell.
Photonic Crystals - a class of novel periodic materials that allow the precise control of electromagnetic wave properties. Elements of thermophotovoltaic systems (3) Filters - for blackbody emitters or imperfect selective emitters, filters are needed to reflect nonideal wavelengths back to the emitter. In practice, these filters are rarely perfect. Any light that is absorbed or scattered and not redirected to the emitter or the converter is lost. Additionally, practical filters often reflect a small percentage of light in desired wavelength ranges or transmit light of non ideal wavelengths. Both can lead to inefficiencies. Converters - even for systems where only light of optimal wavelengths is passed to the converter, inefficiencies associated with non-radiative recombination and ohmic losses exist. Since these losses can depend on the intensity of light incident on the cell, real systems must consider the intensity produced by a given set of conditions (emitter material, filter, operating temperature). Elements of thermophotovoltaic systems (4) Blackbody Radiation - Planck's law states that a blackbody will emit light with a spectrum given by:
where I' is the flux of light of a specific wavelength, λ, given in units of 1/m3/s. Here, h is Planck’s constant, k is Boltzmann’s constant, c is the speed of light, and Temit is the temperature of the emitter. Thus, the flux of light with wavelengths in a specific range can be found by integrating over the range. Elements of thermophotovoltaic systems (4) The peak wavelength is determined by the temperature, Temit based on Wien’s displacement law:
, where b is Wien’s displacement constant.
For most materials, the maximum temperature an emitter can stably operate at is about 1800°C. This corresponds to an intensity which is peaked at λ ~ 1600 nm or an energy of about 0.75 eV. For more reasonable operation temperatures of 1200°C, this drops to about 0.5 eV. These energies dictate the range of band gaps that are needed for practical TPV converters (though the peak spectral power is slightly higher). Elements of thermophotovoltaic systems (5)
Photovoltaic Cells:
• Silicon - the relative wide band gap of Si (1.1 eV) is not ideal for use with a blackbody emitter at lower operating temperatures. • Germanium - Ge has a band gap of 0.66 eV. But Ge diodes have fast decaying “dark” current and therefore, a low open- circuit voltage. a) Gallium Antimonide - the gallium antimonide (GaSb) PV cells (band gap of 0.72 eV) are the basis of most PV cells in modern TPV systems. GaSb is a III-V semiconductor with a zinc blende crystal structure. The manufacturing process for the GaSb PV cell is quite simple. Czochralski Te-doped n-type GaSb wafers are commercially available. Vapor based Zn diffusion is then carried out at elevated temperatures ~450°C to allow for p-type doping. Lastly, front and back electrical contacts are patterned using traditional photolithography techniques and an anti- reflective coating is deposited. b) Indium Gallium Arsenide Antimonide (InGaAsSb) is a compound III-V semiconductor. The addition of GaAs allows for a narrower band gap (0.5 to 0.6 eV), and therefore better absorption of long wavelengths c) Indium Gallium Arsenide (InGaAs) is also a compound III-V semiconductor which has a band gap of 0.74 eV. Thermophotovoltaic (TPV) power microgenerators (1)
Sketch of microcylindrical TPV power generator Schematic of the TPV microsystem Thermophotovoltaic (TPV) power microgenerators (2) The overall efficiency of TPV microsystem is a product of the efficiencies of the radiator source (consisting of the burner and the emitter) - ηRS, the optimal filter - ηF, and the photovoltaic cells - ηPV
ηTPV= ηRS· ηF·ηPV (in TPV with selective emitters ηTPV = ηRS·ηPV)
where ηRS is the ratio of net radiated power (total emission in the whole spectrum minus the radiation returned and absorbed in the emitter) to the heat value of fuel multiplied by fuel flow rate, and
ηRS = Pmax/Prad Thermophotovoltaic (TPV) power microgenerators (3)
Power radiation spectrum as a function of photon energy for a greybody of emissivity 0.7 at 770oC and 1000oC Thermophotovoltaic (TPV) power microgenerators (4)
Schematic of the modular Schematic of the suspended- TPV microsystem tube micro-reactor (SμRE) in TPV microgenerator configuration Thermophotovoltaic (TPV) power microgenerators (5) Properties and efficiencies of four kinds of different TPV microsystems
Type 1 Type 2 Type 3 Type 4 Emitter SiC SiC Co/Ni-doped Co/Ni-doped MgO MgO PV cells GaSb GaInAsSb GaSb GaInAsSb 2 Pmax [W/cm ] 0.351 0.554 2.08 1.39 Output electrical power 0.74 1.17 4.40 2.93 Pel [W] ηRS 21.7% 21.7% 21.7% 21.7% ηPV 2.64% 4.18% 15.72% 10.50% ηTPV 0.57% 0.91% 3.48% 2.28%