Posts tagged ‘dessert’

Renewable Energy- Solar (A Brief Discussion)


Renewable energy is energy which comes from natural resources that are essentially inexhaustible. such as sunlight, wind, rain, tides, and geothermal heat, which are renewable (naturally replenished). The most important feature of renewable energy is that it can be harnessed without the release of harmful pollutants. Renewable energy is derived from natural processes that are replenished constantly. In its various forms, it derives directly from the sun, or from heat generated deep within the earth. Included in the definition is electricity and heat generated from solar, wind, ocean, hydropower, biomass, geothermal resources, and biofuels and hydrogen derived from renewable resources. Renewable energy replaces conventional fuels in four distinct areas: power generation, hot water/ space heating, transport fuels, and rural (off-grid) energy services.

Solar energy

Solar energy is the energy derived from the sun through the form of solar radiation. Solar powered electrical generation relies on photovoltaics and heat engines. A partial list of other solar applications includes space heating and cooling through solar architecture, daylighting, solar hot water, solar cooking, and high temperature process heat for industrial purposes.

Solar technologies are broadly characterized as either passive solar or active solar depending on the way they capture, convert and distribute solar energy. Active solar techniques include the use of photovoltaic panels and solar thermal collectors to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties, and designing spaces that naturally circulate air.

Solar power

Solar power is the conversion of sunlight into electricity, either directly using photovoltaics (PV), or indirectly using concentrated solar power (CSP). CSP systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. PV converts light into electric current using the photoelectric effect.

Solar power is the conversion of sunlight into electricity. Sunlight can be converted directly into electricity using photovoltaics (PV), or indirectly with concentrated solar power (CSP), which normally focuses the sun’s energy to boil water which is then used to provide power, and other technologies, such as the sterling engine dishes which use a sterling cycle engine to power a generator. Photovoltaics were initially used to power small and medium-sized applications, from the calculator powered by a single solar cell to off-grid homes powered by a photovoltaic array.

The only significant problem with solar power is installation cost, although cost has been decreasing due to the learning curve. Developing countries in particular may not have the funds to build solar power plants, although small solar applications are now replacing other sources in the developing world.

Structure of the Sun & Spectrum

Energy production: By Fusion : H atoms combine to form Helium

Effective Black Body Temperature (as seen from earth) = 5,777 K

48% of the total radiation- Visible, 6% – UV and 46%-  IR

Annual average radiation 1,366 W/m2 in space (Called the Solar Constant)

Spectral Irradiance

The amount of electromagnetic energy incident on a surface per unit time per unit area is called Irradiance. In the past this quantity has often been referred to as “flux”. The unit is W/m2.

A light source is characterized by its spectral irradiance

F(λ) = power density at a given λ

= irradiance per unit wavelength interval at a given wavelength

Unit is Wm-3 or commonly Wm-2 nm-1

Radiant power density = total power density emitted by a source

Terrestrial Radiation

u Beam radiation (Direct): Solar radiation propagating along the line joining the sun and the receiving surface

u Diffused radiation: radiation scattered by dust, aerosols, molecules etc. No preferred direction

u Total radiation =Beam + Diffused

u Irradiance (W/m2): The rate at which the radiant energy is incident on a  unit area of the surface ( incident radiant flux). Denoted by G.

u Emissive power: The rate at which the energy leaves a surface through emission

u Albedo: The reflection from the earth~30%

Solar radiation Components

The solar radiation arriving at the earth’s surface has two components

  1. Direct: can be focused
  2. Diffused >10%: cannot be focused

(Direct / diffused) Ratio:   0.9 Cloudless, clear day

0.0 Completely overcast day

The total irradiance at any surface is the sum of the two components

Gt = Gbeam +Gdiffused



Used to measure global solar radiation

Must respond to both the direct solar beam and to diffuse sky radiation from the whole hemisphere

Sensing element is a flat horizontal surface



Measures the direct solar beam

Must be kept normal to the solar beam (pointed at the sun) Thermopile (multiple thermocouples) is used to measure the temperature difference between the source and the reference cavity

Photovoltaic detectors

Uses a silicon detector whose response typically extends from 400 to 1100 nm, with a non-uniform response over this range

Must be calibrated against a high-quality thermopile-type sensor (pyranometer)

Much less expensive than a thermopile-type sensor

Gives accurate direct solar measurement because it avoids the effects of dome heating due to absorption of solar radiation.

Fundamentals of Energy & Electricity

  • Electricity is defined as the directional flow of electric charge
  • Electrical Energy: One form of energy which made available by the flow of electric charge through a conductor
  • Electric power is defined as the rate at which electrical energy is transferred to a load by an electric circuit. The SI unit of power is the watt (W). 1 watt = 1 joule/second  P=E/t
  • Direct Current: Direct current does not change directions- the current flow is always from the positive pole to the negative pole
  • DC power:      P = VI or P = I2R or P=V2/R
  • Alternating Current: AC changes directions. For a 50 Hz AC, current changes direction 50 times per second.
  • AC power:      P = V I cosφ where, cosφ is power factor and φ is the phase angle between current and voltage.
  • The power factor = (active true or real power) / (apparent power).
  • Active (Real or True) Power is measured in watts (W) and is the power drawn by the electrical resistance of a system doing useful work.
  • Reactive Power is measured in volt-amperes reactive (VAR). Reactive Power is power stored in and discharged by inductive motors, transformers and solenoids
  • Anything that uses electricity (either AC or DC) is referred to as an “electrical load”.
  • AC load: Any module or instrument that uses AC energy. e.g. Household light, fan, TV, refrigerator etc.
  • DC load: Any module or instrument that uses DC energy. e.g. DC fan, light, battery powered instruments etc.
  • RESISTIVE LOAD (heaters and incandescent lights)
  • The voltage and current peaks coincide and are therefore in phase
  • Power factor is in unity.
  • INDUCTIVE LOAD (Motors and transformers)
  • With an inductive load the current waveform is lagging behind the voltage waveform, the amount of phase delay is given by the cosine of the angel (Cos) between the vectors representing voltage and current.
  • Capacitive Load: (Capacitors, wiring, cable)
  • The capacitive load has a current waveform which is leading the voltage waveform, therefore the voltage peaks and current peaks are not in phase. The amount of phase delay is given by the cosine of the angle between the vectors representing voltage and current.
  • Kilowatt-hour (kWh):  Energy of 1,000 watts working for one hour. The amount of power a power plant generates or a customer uses over a period of time is measured in kilowatt-hours (kWh). Kilowatt-hours are determined by multiplying the number of kW required by the number of hours of use. For example, if we use a 40-watt light bulb 5 hours a day, we have used 0.2 kilowatt-hours of electrical energy/day.

Photovoltaic System

Solar cell or photovoltaic cell (PV), is a device that converts light into electric current using the photoelectric effect. The first solar cell was constructed by Charles Fritts in the 1880s. In 1931 a German engineer, Dr Bruno Lange, developed a photo cell using silver selenide in place of copper oxide. Although the prototype selenium cells converted less than 1% of incident light into electricity, both Ernst Werner von Siemens and James Clerk Maxwell recognized the importance of this discovery. Following the work of Russell Ohl in the 1940s, researchers Gerald Pearson, Calvin Fuller and Daryl Chapin created the silicon solar cell in 1954. These early solar cells cost 286 USD/watt and reached efficiencies of 4.5–6%.

There are many competing technologies, including fourteen types of photovoltaic cells, such as thin film, mono crystalline silicon, polycrystalline silicon, and amorphous cells, as well as multiple types of concentrating solar power. It is too early to know which technology will become dominant.  The earliest significant application of solar cells was as a back-up power source to the Vanguard I satellite in 1958, which allowed it to continue transmitting for over a year after its chemical battery was exhausted.  The successful operation of solar cells on this mission was duplicated in many other Soviet and American satellites, and by the late 1960s, PV had become the established source of power for them.  Photovoltaic went on to play an essential part in the success of early commercial satellites such as Telstar, and they remain vital to the telecommunications infrastructure today.

The high cost of solar cells limited terrestrial uses throughout the 1960s. This changed in the early 1970s when prices reached levels that made PV generation competitive in remote areas without grid access. Early terrestrial uses included powering telecommunication stations, off-shore oil rigs, navigational buoys and railroad crossings. These off-grid applications accounted for over half of worldwide installed capacity until 2004.

Silicon Wafer-Based Solar Cell

¡  The most commonly known solar cell is configured as a large-area p-n junction made from silicon. A p-n junction of silicon solar cell is made by diffusing an n-type dopant into one side of a p-type wafer (or vice versa).

¡  Despite the numerous attempts at making better solar cells by using new and exotic materials, the reality is that the photovoltaic’s market is still dominated by silicon wafer-based solar cells (first-generation solar cells). This means that most solar cell manufacturers are currently equipped to produce this type of solar cells.

¡  Consequently, a large body of research is being done all over the world to manufacture silicon wafer-based solar cells at lower cost and to increase the conversion efficiencies without an exorbitant increase in production cost.

¡  The ultimate goal for both wafer-based and alternative photovoltaic concepts is to produce solar electricity at a cost comparable to currently market-dominant coal, natural gas, and nuclear power in order to make it the leading primary energy source.

¡  Since a major part of the final cost of a traditional bulk silicon module is related to the high cost of solar grade polysilicon feedstock (about US$ 0.4/Watt peak) there exists substantial drive to make Si solar cells thinner (material savings) or to make solar cells from cheaper upgraded metallurgical silicon (so called “dirty Si”).

¡  IBM has a semiconductor wafer reclamation process that uses a specialized pattern removal technique to repurpose scrap semiconductor wafers to a form used to manufacture silicon-based solar panels.

Crystalline Silicon Solar Cell

¡  The highest efficiencies on silicon have been achieved on mono crystalline cells. The highest commercial efficiency (22%) is produced by Sun Power, which uses expensive, high-quality silicon wafers. The University of New South Wales has achieved 25% efficiency on mono crystalline silicon in the lab[3]

¡  The most prevalent bulk material for solar cells is crystalline silicon (c-Si), also known as “solar grade silicon”. Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon, or wafer.

¡  Mono crystalline silicon (c-Si): often made using the Czochralski process. Single-crystal wafer cells tend to be expensive, and because they are cut from cylindrical ingots, do not completely cover a square solar cell module without a substantial waste of refined silicon. Hence most c-Si panels have uncovered gaps at the four corners of the cells.

¡  Multi crystalline silicon (mc-Si): made from cast square ingots, large blocks of molten silicon carefully cooled and solidified. Poly-Si cells are less expensive to produce than single crystal silicon cells, but are less efficient. US DOE data shows that there were a higher number of multi crystalline sales than mono crystalline silicon sales.

¡  Ribbon silicon   : is a type of multi crystalline silicon, it is formed by drawing flat thin films from molten silicon and results in a multi crystalline structure. These cells have lower efficiencies than poly-Si, but save on production costs due to a great reduction in silicon waste, as this approach does not require sawing from ingots.

Amorphous Silicon Solar Cell

¡  Single-crystal silicon solar cells tend to be expensive and are limited to approximately six inches diameter.

¡  A system powered by solar cells requires, in general, a very large area solar cell array to generate the power.  Amorphous silicon solar cells provide the possibility of fabricating large area and relatively inexpensive solar cell system.

¡  When silicon is deposited by CVD techniques at temperature below 873K, an amorphous film is formed regardless of the type of substrate.

¡  In amorphous silicon, there is only very short range of order, and no crystalline regions are observed.

¡  Hydrogen may be incorporated in the silicon to reduce the number of dangling bonds, creating a material called hydrogenated amorphous silicon.

¡  Amorphous silicon has a very high optical absorption coefficient, so most sun-light is absorbed within approximately 1μm of the surface. Consequently, only a very thin layer of amorphous silicon is required for a solar cell.

¡  A typical amorphous silicon solar cell is a PIN device

¡  The amorphous silicon is deposited on an optically transparent indium tin oxide-coated glass substrate. If aluminum is used as the back contact it will reflect any transmitted photons back through the PIN device.

¡  Amorphous silicon solar cells approximately 40cm wide and many meters long have been fabricated[4].

¡  Conversion efficiencies are smaller than in single-crystal silicon, but the reduce  cost makes this technology attractive.

Thin Film Solar Cell

¡  The various thin-film technologies currently being developed reduce the amount (or mass) of light absorbing material required in creating a solar cell.

¡  This can lead to reduced processing costs from that of bulk materials but also tends to reduce energy conversion efficiency, although many multi-layer thin films have efficiencies above those of bulk silicon wafers.

¡  They have become popular compared to wafer silicon due to lower costs and advantages including flexibility, lighter weights, and ease of integration.

¡  CdTe (cadmium telluride) solar cell is a solar cell based on cadmium telluride, an efficient light-absorbing material for thin-film cells. Compared to other thin-film materials, CdTe is easier to deposit and more suitable for large-scale production.

¡  CIGS(Copper Indium/Gallium Diselenide) a solid mixture of the semiconductors CuInSe2 and CuGaSe2. Unlike the conventional silicon based solar cell, which can be modeled as a simple p-n junction, these cells are best described by a more complex hetero   junction model.

¡  The best efficiency of a thin-film solar cell as of March 2008 was 19.9% with CIGS absorber layer.[5]

¡  The use of gallium increases the optical band gap of the CIGS layer as compared to pure CIS, thus increasing the open-circuit voltage, but decreasing the short circuit current.

¡  Organic/polymer solar cells: are built from thin films (typically 100 nm) of organic semiconductors such as polymers and small-molecule compounds like poly phenylene vinylene, copper phthalocyanine (a blue or green organic pigment) and carbon fullerenes and fullerene derivatives such as PCBM.

¡  These devices differ from inorganic semiconductor solar cells in that they do not rely on the large built-in electric field of a p-n junction to separate the electrons and holes created when photons are absorbed.

¡  Energy conversion efficiencies achieved to date using conductive polymers are low compared to inorganic materials.

¡  However, it improved quickly in the last few years and the highest NREL (National Renewable Energy Laboratory) certified efficiency has reached 6.77%[6] .


Silicon Thin-Film Cell

¡  Silicon thin-film cells: are mainly deposited by chemical vapor deposition(typically plasma-enhanced (PE-CVD)) from silane gas and hydrogen gas. Depending on the deposition parameters, this can yield:

I.  Amorphous silicon (a-Si)

II. Proto crystalline silicon or

III. Nano crystalline silicon (nc-Si), also called microcrystalline silicon.

.    Amorphous silicon: has a higher band gap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it absorbs the visible part of the solar spectrum more strongly than the infrared portion of the spectrum.

.    Proto crystalline silicon: present dangling and twisted bonds, which results in deep defects (energy levels in the band gap) as well as deformation of the valence and conduction bands (band tails). The solar cells made from these materials tend to have lower energy conversion efficiency than bulk silicon, but are also less expensive to produce. The quantum efficiency of thin film solar cells is also lower due to reduced number of collected charge carriers per incident photon.

¡  Nano crystalline solar cell: These structures make use of some of the same thin-film light absorbing materials but are overlain as an extremely thin absorber on a supporting matrix of conductive polymer or mesoporous metal oxide having a very high surface area to increase internal reflections.

¡  Using Nano crystals allows one to design architectures on the length scale of nanometers. In particular, single-Nano crystal devices, an array of single p-n junctions between the electrodes and separated by a period of about a diffusion length, represent a new architecture for solar cells and potentially high efficiency.

¡  A silicon thin film technology is being developed for building integrated photovoltaic’s (BIPV) in the form of semi-transparent solar cells which can be applied as window glazing. These cells function as window tinting while generating electricity.

Currents & Voltages in Multi-junction Solar Cells

¡  Since all the sub cells are connected in series, the current flowing through all of them is the same. The voltages generated by individual cells add.

¡  The current is controlled by the cell which provides the minimum current. Optimization of the design of individual cells and testing of their performance (individual cells) is by itself an important problem.

¡  The two important performance parameters of multi-junction solar cell are short circuit current and open circuit voltage.

¡  Reverse saturation current is also important parameter for this cell.

¡  The absorption co-efficient is also another performance parameter, if absorption co-efficient is high, then light is entered deep to the cell.

¡  Each sub-cell of multi-junction solar cell would be either lattice matched or mismatched.

Efficiency of Multi-Junction Solar Cell

¡  The efficiency of multi-junction solar cell depends on the band gap, concentration of the incident spectrum and temperature.

¡  If concentration is increased then efficiency also increased. The same thing is also true for temperature.

¡  The short-circuit current density can calculate directly from the irradiance data.

¡  The open circuit voltage, reverse saturation current, maximum current, maximum voltage, fill-factor and finally efficiency can calculate from spectral p-n junction model.

Losses of Solar Cell

1) Losses for the Top Solar Cell

a) Optical Losses:

(i)surface reflection, anti-reflection-coating

(ii)2.6 percent

(iii)grid shading 4 .O percent

(iv)absorption 2.0 percent

(v)recombination 6.2 percent

(vi)series resistan2ce.l0o s ses percent

b) Electrical Losses           series resistance losses 2.0 percent

2) Losses for the Bottom Solar Cell

a) Optical Losses:

(i) interface reflection 2.0 percent

(ii) surface reflection

(iii)anti-reflection-coating 2.0 percent

(iv)absorption 9.0 percent

(v)recombination 6.2 percent

b) Electrical Losses     series resistance losses 2.0 percent

Hot Carrier Cell

¡  The concept of using multiple energy levels to increase the efficiency of a solar cell has shown real improvements over standard solar cells. But it could be possible to increase solar cell efficiencies for a single junction by utilizing hot carriers.

¡  In all solar cells, an incoming photon with energy in excess of the band gap produces an EHP where the total energy is greater than the band gap. The electrons and holes will first interact with other electrons and holes through carrier-carrier interactions to form carrier populations that can be described by a Boltzmann distribution. At this point, the temperature defining the carrier distribution is above the lattice temperature and hence the carriers are referred to as hot carriers.

¡  Typically the additional energy associated with the elevated temperature is contained by the electron due to it lower effective mass. In a typical solar cell, the hot electrons will give off their excess energy to the lattice by producing optical phonons. These optical phonons then interact with other phonons and the energy in excess of the band gap is lost. In bulk semiconductors, all of this happens in less than 0.5picoseconds.

¡  The theoretical treatments of the hot carrier extraction have shown that efficiencies exceeding 80% are possible under fully concentrated sunlight. However, it is not easy to separate hot electrons and holes to different contact.

¡  The entire concept behind maintaining hot carrier populations is a minimization of electron-phonon interactions. In the presence of metal contact, it would be very easy for the hot carriers to cool to the lattice temperature through the large number of available electronic states.

¡  The important issue that will need to be addressed before hot carrier solar cells are produced is the geometry of the cell.

¡  Even with the improved hot carrier lifetimes in quantum systems, the distant the hot carrier can travel before cooling is likely to be short. Therefore it may be necessary to design the cell in such a way that all the EHPs are generated very close to the energy selective contacts to ensure the carriers do not cool before being collected.

Charge Controllers in PV Systems

The primary function of a charge controller in a stand-alone PV system is to maintain the battery at highest possible state of charge while protecting it from overcharge by the array and from over-discharge by the loads.

Without charge control, the current from the array will flow into a battery proportional to the irradiance, whether the battery needs charging or not. If the battery is fully charged, unregulated charging will cause the battery voltage to reach exceedingly high levels, causing severe gassing, electrolyte loss, internal heating and accelerated grid corrosion. In most cases if a battery is not protected from overcharge in PV system, premature failure of the battery and loss of load are likely to occur.

When a battery is over-discharged, the reaction in the battery occurs close to the grids, and weakens the bond between the active materials and the grids. When a battery is repeatedly over-discharged, a loss of battery capacity and life will eventually occur.

Important functions of a charge controller are

Prevent Battery Over-charge: to limit the energy supplied to the battery by the PV array when the battery becomes fully charged.

Prevent Battery Over-discharge: to disconnect the battery from electrical loads when the battery reaches low state of charge.

Provide Load Control Functions: to automatically connect and disconnect an electrical load at a specified time, for example operating a lighting load from sunset to sunrise.

In order to obtain a long battery life time, over-charging and over-discharging must be avoided. In the case of lead-acid battery the voltage is a measure of its state-of-charge. Therefore, by measuring this voltage, one can determine whether the battery is working outside its normal regime.

Charge controllers set points

The battery voltage levels at which a charge controller performs switching functions is called the charge controllers set points.

There are four basic switching set points which are defined for most charge controllers that have battery over-charge and over-discharge protection features.

These are:

  •  Voltage Regulation (VR) set point
  •  Array Reconnect Voltage (ARV) set point
  •  Low voltage Load Disconnect (LVD) set point
  •  Load Reconnect Voltage (LRV) set point

 Selection of Proper Charge Controller

A charge regulator should be selected to pass the expected continuous current from the array into the battery and should be able to withstand temporary peak currents due bright conditions.

Normally it is known that a PV module generates a maximum power current of Imp amps. But this current level is measured under standard irradiance conditions of 1000W/m2.

It is sometimes possible that environmental conditions produce momentary irradiance levels quite higher than this value of 1000W/m2. Because of light reflection and the edge of cloud effect, sporadically increased current levels are not uncommon.

Therefore charge regulators should be selected to control up to 125% of standard module current for short periods.

The regulator is selected by multiplying the total standard current from an array times this safety factor. The total current from an array is given by the number of modules in parallel multiplied times the module current.

To be very conservative, the short circuit current ISC should be used instead of Imp. Regulator size can be determined by the following relation:

Charge controller size = (Number of modules in parallel) × Isc × 1.25


Maximum Power Point Tracker (MPPT):

Maximum Power Point Tracking, frequently referred to as MPPT, is an electronic system that operates the Photovoltaic (PV) modules in a manner that allows the modules to produce all the power they are capable of.

The current-voltage characteristics of photovoltaic module are such that it can be considered to be a constant current source up to a certain voltage range. This voltage is close to the open circuit voltage of the PV module.

Principle of Maximum power point tracking:

In a photovoltaic (PV) system it is desirable to extract the maximum amount of energy output from the PV array. This is possible if the array is operated at maximum power point (MPP) every instant, because the maximum power point is fluctuating due to change in insolation and temperature. With reference to Fig.1 (c), when the load resistance RL reaches RL2, the load line corresponding to RL2 intersects the maximum power point and maximum power will be dissipated through RL; we can then say that impedance matching between the PV system and the load is achieved.

Shifting of maximum power point with the variation of insolation and the setting of optimum voltage

A photovoltaic array can deliver its maximum power only when the load impedance matches its dynamic impedance which however varies with insolation and temperature. Thus for maximum power transfer and minimization of mismatch losses, an impedance transformer is needed which will continuously match the dynamic impedance of the PV-array to the fixed impedance of the load for all insolation levels and temperature. Such an impedance matching device is called a maximum power point tracker (MPPT).

In Fig.2 it is seen that if the insolation is increased to I1 the MPP is shifted to P1 increasing the voltage V1 at maximum power point  and when the insolation is decreased to I4 the MPP will be shifted to P4 decreasing  the voltage V4 . Now it seems that if the PV-array is operated at the average value V0 of V1 and V4 it is quite possible that almost the maximum power P0 from the PV-array will always be supplied to the load.

When this principle is adopted in designing an MPPT circuit and if the efficiency of the circuit is very high about 85-90% then approximately almost all the power can be delivered to the load. This type of MPPT is adequate for many practical purposes.

Maximum power point tracking can also be conveniently achieved by a pulse width modulated DC to DC converter having a variable duty cycle which is a function of the light intensity. The duty cycle can be varied by automatic control circuit. Automatic duty cycle adjustment can be achieved by using simple circuit or by a microprocessor based control circuit. The microprocessor based circuit hunts for the maximum power point more accurately under all circumstances (insolation and temperature). On the other hand simple control circuit tracks the maximum power point approximately rather than accurately.

Designing a PV System

Input Data Required For PV System Sizing:

  • The daily or hourly load requirements during a typical year
  • The required security of supply, taking into account the back-up source, if any.
  • The mean daily irradiation in the plane of the array at the chosen site for every month of a typical year.
  • The maximum number of consecutive sunless days likely to be experienced.
  • The mean daily ambient temperature for every month of a typical year.
  • The estimated cell temperature rise above ambient of the modules in the array.
  • Typical current-voltage characteristics of the module at various irradiances.
  • The selected DC bus voltage.
  • The maximum allowable depth of discharge of the battery.
  • The estimated percentage energy losses in the battery, power conditioning equipment and control system.
  • The estimated losses in the array from module mismatch, cables and voltage drop across blocking diodes.
  • The estimated losses from dust and shading.

In practice, designing a PV system depends if it is off-grid or grid-tied.

Off-grid systems require a rigorous design, often with several iterations to optimize the number of modules, batteries, and stand-by generators, if necessary, to minimize system costs.  Loads must be carefully calculated.

Grid-tied systems generally are sized by one of two methods:

a.  How big of a system is possible with the available budget, i.e. budget constrained.

b.  How big of a system is possible given a limited area, i.e. area constrained.


Grid-Connected PV Systems

Grid-connected PV systems, although small compared with other power generation sources are becoming very popular all over the world. Grid-Connected PV Systems, also called utility interactive PV systems, always have a connection to the public electricity grid via a suitable inverter, because a PV module delivers only a dc power. Normally there is almost no effect of the PV system on the grid affecting power quality, load on lines, and transformers etc. However, for a larger share of PV in low-voltage grids, as in solar installations, these aspects need to be taken into account. From a technical point of view, there will be no difficulty in integrating as much PV into low voltage grids as the peak load of the respective segment.

Decentralized Grid-connected PV systems

Decentralized Grid-connected PV systems [Fig.1 (a)] have mostly a small power range and are installed on the roof-top of buildings (roof-top or flat-roof installation) or integrated into building facades.

Energy storage is not necessary in this case. On sunny days, the solar generator provides power, e.g., for the electrical appliances in the house. Excess energy is supplied to the national grid. During the night and overcast days, the house draws its power from the grid. In this way, the electricity grid can be regarded as a large “storage unit”. In the case of a favorable rate-based tariff for PV electricity, as in force in some countries, it is more advantageous to feed all solar electricity into the grid.

For example, in Germany around 80% of the more than 50,000 existing grid-connected PV systems are installed either on the roof-top of a building or integrated into a building façade. The benefit of the installation of a PV system into or onto a building is that no separate area for the solar generator is needed.

Central Grid-connected PV systems

Central Grid-connected PV systems [Fig.1 (b)] have an installed power up to MW range. With such central photovoltaic power stations it is possible to feed directly into the medium or high voltage grid. Mostly central photovoltaic power stations are set on unused land, but in some cases an installation on buildings, mostly on the flat roof of greater buildings, is also possible.



As world-wide energy demand increases, conventional energy resources, such as fossil fuels, will be exhausted in the not-too-distant future. Therefore we must develop and use alternative energy resources, especially our only long-term natural resource, the sun. The solar cell is considered a major candidate for obtaining energy from the sun, since it can convert sunlight directly to electricity with high conversion efficiency, by the photovoltaic effect, occurring no chemical reaction, can provide permanent power at low operating cost, and virtually free from pollution.

The photovoltaic effect was first recognized in 1839 by French physicist A. E. Becquerel. The solar cell was first developed by Chapin, Fuller, and Pearson in 1954 using a diffused silicon  p-n  junction.

Recently, research and development of low, flat-panel solar cells, thin-film devices, concentrator systems, and many innovative concepts have increased. In the near future, the costs of small solar-power modular units and solar-power plants will be economically feasible for large-scale use of solar energy.


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Cinnamon Roll Pie Y2, Week Five: Summer Was So Yesterday Pie Spring in southern Wisconsin is maybe best described as bipolar. It isn’t so much a period of pleasant, mild weather as it is a violent lurching between winter and summer until one of them (hopefully summer) sticks. Just a couple of weeks ago we had a 95° day immediately followed by a day with lows cold enough to bring on frost warnings. Only the foolish and uninitiated put away their winter clothes, and … Read More

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