Solar PV-notes

8/10/2019 Solar PV-notes

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UNIT V

EMERGING SOLAR CELL TECHNOLOGIES

Third generation solar cells

The term third generation photovoltaics refers to all novel approaches that aim to overcome the Shockley-

Queisser (SQ) single band gap limit, preferably at a low cost. The efficiency limit is around 33% in the band

gap range from 1.0 eV up to 1.8 eV.

We are going to first look at the fundamental limitations of classical single-junction solar cells:

First, in single-junction solar cells only on band gap material is used. Hence, a large fraction of the

energy of the most energetic photons is lost as heat

Secondly, most solar cells concepts are based on an incident irradiance level of 1 sun. However, higher

irradiance means more current generation and also higher voltage levels, resulting in a higher overall

efficiency.

Thirdly, every photon only excites one electron in the conduction band creating only one electron-hole

pair. The energy of highly energetic photons could be utilized better if they would create more than one

excited electron in the conduction band.

Fourthly, the photons with energies below the band gap are not used. Hence, they do not result in charge

carrier excitation.

If these fundamental limitations could be solved, PV concepts with conversion efficiencies exceeding the

Shockley-Queisser limit could be developed. Some third generation concepts are Multi-junction solar cells

multi-exciton generation, intermediate band-gap solar cells and hot carrier solar cells. Besides multi-junction

and the concentrator approach, none of these concepts have resulted in high efficiency solar cells or even been

demonstrated yet. These other concepts are still in fundamental research phase and it is not clear whether they

will ever become a large scale PV technology

Multi-junction solar cells:In multi-junction cells, several cell materials with different band gaps are combined

in order to maximize the amount of the sun light that can be converted into electricity. To realize this, two or

more cells are stacked onto each other.

Multi-exciton generation: Another approach to enhance the charge carrier excitation by a single energetic

photon is called multiple exciton generation (MEG). Here, more than one electron-hole pair is generated from

high energy photons.

Intermediate band solar cells: The concept of intermediate band solar cells (IB) tries to tackle the problem

photons with energies below the bandgap cannot be utilised for current generation. In intermediate band cells

energylevels are created artificially in the bandgap of the absorber material. Photons with energies below the


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bandgap can excite an electron from the valence band in to the intermediate band. A second low-energy photon

is requiredto excite the electron from the intermediate band into the conduction band.

Hot carrier solar cells: The idea of hot carrier solar cells is to reduce the energy losses due to relaxation and

hence thermalizationthis should be achieved by collect electron-hole pairs of high energy photons just after light

excitation before they have a chance to relax back to the edges of the electronic bands.

ORGANIC SOLAR CELL

Organic Solar Cells are carbon-based materials possessing semiconductor characteristics. Organic materials

used presently in solar cells include for example conducting polymers, dyes, pigments, and liquid crystals. Light

absorption in organic materials almost always results in the production of a mobile excited state ratherthan in

free charge carriers. Therefore the usual product of light absorption in molecular materials is a tightly bound,

neutral electron/hole pair or exciton.

Working Principle:

In general, for a successful organic photovoltaic cell five important processes have to beoptimized to obtain a

high conversion efficiency of solar energy into electrical energy:

1. Absorption of light and generation of excitons

2. Diffusion of excitons to an active interface

3. Charge separation

4. Charge transport

5. Charge collection

To create a working photovoltaic cell, the two photoactive materials are sandwichedbetween two metallic

electrodes (of which one is transparent), to collect the photo generated charges. After the charge separation


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process, the charge carriers have to betransported to these electrodes without recombination. Finally, it is

important that the chargescan enter the external circuit at the electrodes without interface problems.

Schematic drawing of the working principle of an organic photovoltaic cell is shown below.

Illumination of a donor material (D) through a transparent electrode (TCO) results in the formation of an

exciton (1). Subsequently, the exciton is transported by diffusion (2) to the interface between the donor material

and an acceptor material (A). Electron is transferred to the acceptor material (- element), leaving a hole at the

donor material (+ element) (3).The photo generated charged carriers are then transported (4) to and collected at

opposite electrodes (5). A similar charge generation process can occur, when the acceptor is photoexcited

instead of the donor.


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DYE-SENSITIZED SOLAR CELL (DSSC)

DSSC is a concept presented in 1991. It can be considered as a thin film solar cell device. DSSC is a photo-

electro-chemical system. In its operation it involves a photon, an electron and a chemical reaction. The

operation of DSSC is considered similar to that of a photosynthesis process. For this reason, the operation of

DSSC is fundamentally different from that of a crystalline Si-based solar cell. In this cell, the functions of light

absorption and charge transport are done by two different materials unlike in other semiconductor-based cells

where both of these jobs are done by the same material. In DSSC, the light is absorbed by materials called dye

and the carriers are being transported by wide band gap semiconductors.

A DSSC basically is a thin layer solar cell formed by sandwiching two transparent conductive oxide (TCO)

electrodes. One of them has a TiO2 layer coated with photosensitizer (dye). The other electrode known as

counter electrode consists of platinum deposited on the other TCO.The inter-layer space is filled with an

organic electrolyte containing a redox mediator usually a mixture of iodine and iodide.

The main processes that occur in a DSSC:

1.Photoexcitation:The incident photon is absorbed by the

photosensitizers (dye) adsorbed on the TiO2surface.

S + h S

2. TiO2charge injection:The photosensitizers are excited

from the ground state (S) to the excited state (S). The

excited electrons are injected into the conduction band of

the TiO2electrode. This results in the oxidation of the

photosensitizer (S+).

S S++ e

(TiO2)

The injected electrons in the conduction band of TiO2are

transported between TiO2nanoparticles with diffusion

toward the back transparent contact. And the electrons

finally reach the counter electrode through the circuit.

4. Regeneration of S:The oxidized photosensitizer (S+) accepts electrons from the I

ion redox mediator

leading to regeneration of the ground state (S), and the Iis oxidized to the oxidized state, I3

.

2S++ 3I

-2S+I3

-

5. Regeneration of I-:The oxidized redox mediator, I3

, diffuses toward the counter electrode and then it is

reduced to Iions.

I3+ 2 e

3 I


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THERMOPHOTOVOLTAICS (TPV)

Thermo-photovoltaic (TPV) systems are direct-conversion heat engines that use photovoltaic cells to generate electricity

from the radiant energy emitted by a heated object.The TPV system consists of a heat source, a radiator, a filter, a PV cel

and a heat sink. The radiator is used to radiate a constant spectrum over the time and the filter is used to filter radiation

and pass only suitable radiation to the cell. A Heat sink is used to remove the heat from the cell as the high temperature o

the cell results in its performance degradation. The system is designed in a way that the unutilized radiation is reflected

back to the heat source to minimize losses. In typical TPV systems, the object is heated to temperatures of 13002000K.

Both Thermophotovoltaic and Photovoltaic cells work in basically the same way, the exception being that instead of the

light hitting the cells and creating an electric field, as in Photovoltaic, the Thermophotovoltaic cell uses a semiconductor

designed for longer wavelengths for non-visible light (infrared) emitted by any hot object. TPV cells can utilise the energy

generated, not only from direct sunlight, but from any other heat source like fossil fuels also.


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Floatovoltaics:

Floatovoltaics is a concept where solar PV panels are made to float over existing water surfaces like irrigation

ponds, lakes and large reservoirs. This system has the following advantages:

1. Improves Water Quality: As water bodies are exposed to the sun, photosynthesis promotes the growth

of organic matter including algae. The alga is typically not desirable, can clog pumping and filtration

systems and requires costly chemical treatment to control the problem. Installing SPG Solar

Floatovoltaics will shade the water and reduce photosynthesis. This in turn will reduce the formation of

algae and reduce your chemical and operational costs.

2. Cooler PV Panels: Solar PV panels perform better in cooler conditions. By installing Solar

Floatovoltaics over water, not only is the water cooled by the 100% shade but the panels will be

naturally cooled resulting in improved power production performance. The cooler environment also

reduces stress on the system, which reduces maintenance and increases the PV systems lifespan.

3. Evaporation Reduction: A substantial amount of water is lost to the atmosphere each year that could

be utilized for productive uses including crop production and industrial processes. By installing Solar

Floatovoltaics over water, evaporation reduces by 70%.


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Solar PV project execution

1. Site Selection:

Selecting a suitable site is a crucial part of developing a viable solar PV project. In selecting a site, the aim is

to maximize output and minimize cost. The main constraints that need to be assessed include:

Solar resource

Global Horizontal Irradiation, monthly, annual and inter-annual variation, impact of

shading. Actual measurement of solar irradiance at the specific site is also desired, in absence of

authenticdata by the providers of energy estimation websites, resources.

Local climate Dust accumulation on surface of modules, flooding, high winds, snow and extreme

temperatures.

Available area area required for different module technologies, required area for the evacuating

electrical substation ,its orientation with the outgoing transmission lines, access requirements, pitch

angle and minimizing inter-row shading, area optimization for modules mounted on solar trackers(single, multi axis). Land preparation to create a gradient to use gravity for rain water harvest and

recycling for modules washing

Land use this will impact land cost and environmental sensitivity. The impact of other land users on

the site should also be considered. Alternate thinking for Water body floating solar panels, as well as hill

top, hill incline mounts, which are free from real estate headaches .

Topography flat or slightly south facing slopes are preferable for projects in the northern hemisphere.

Geotechnical including consideration of groundwater, resistivity, load bearing properties, soil pH

levels and seismic risk.

Geopolitical sensitive military zones may be avoided.

Accessibility proximity to existing roads, extent of new roads required.

Grid connection cost, timescales, capacity, proximity and availability & technical feasibility by

Network analysis to determine the Smart Grid.

Module soilingincluding local weather, environmental, human and wildlife factors.

Water availabilitya reliable supply is required for module cleaning and use of Rain Water Harvest is

a must.

Financial incentives tariffs and other incentives vary between countries and regions within countries

Also to look forward and optimize in a situation when Incentive is Zero.


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2. Plant Design:

In designing the site layout, the following aspects are important:

Choosing row spacing to reduce inter-row shading and associated shading losses.

Choosing the layout to minimize cable runs and associated electrical losses.

Allowing sufficient distance between rows to allow access for maintenance purposes.

Choosing a tilt angle (to try for mechanical multistep arrangement in the support structure to reduce the

tracker costs) that optimizes the annual energy yield according to the latitude of the site and the annual

distribution of solar resource.

Orientating the modules to face a direction that yields the maximum annual revenue from power

production. In the northern hemisphere, this will usually be true south.

The electrical design of a PV project should adhere to Standards based to IEC,DIN , and Indian Standards if

made/available. It can be split into the DC and AC systems.

The DC system comprises the following:

Array(s) of PV modules.

Inverters.

String Monitors (Voltage & Currents in Real time )

DC cabling (module, string and main cable).

DC connectors (plugs and sockets).

Junction boxes/combiners.

Disconnects/switches.

Protection devices.

Earthing.

The AC system includes:

AC cabling.

Switchgear.

Transformers.

Substation.

Earthing and surge protection.

Metering, Net Metering is preferable.


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3. Permits and Licensing

Obtaining the relevant permits and licenses is essential to facilitate the timely completion of a project.

Clearances also help ensure that the development proceeds in harmony with the natural environment,existing land usage and other regulatory interests. The exact requirements vary from country to country but

the key permits, licenses and agreements typically required for renewable energy projects include:

Land lease contract.

Buildings permit/planning consent.

Grid connection contract.

Power purchase agreement.

The authorities, statutory bodies and stakeholders that should be consulted also vary from country to country

but usually include the following organization types:

Local and/or regional planning authority.

Environmental agencies/departments.

Archaeological agencies/departments.

Civil aviation authorities (if located near an airport).

Local communities.

Health and safety agencies/departments.

Electricity utilities.

Military authorities.

4. Economics and Financial Modeling

The development of solar PV projects can bring a range of economic costs and benefits at the local and

national levels.

Economic benefits can include:

Job creation.

Use of barren land.

Avoidance of carbon dioxide emissions.

Increased energy security and ability to increase capacity in a modular manner..

Reduction of dependence on imports.

Increased tax revenue by accurate metering & internet based energy management system, which

reduces manpower deployment which can be utilized in a better manner..

Grid Parity and less dependence on Fossil fuels.


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The financial benefits and drawbacks to the developer should be explored in detail through the development

of a full financial model. This model should include the following inputs:

Capital costs these should be broken down as far as possible. Initially, the CERC assumption can

be used but quoted prices should be included when possible.

Operations and maintenance costs in addition to the predicted O&M contract price, operational

expenditure will include comprehensive insurance, administration costs, salaries and labour wages.

Annual energy yieldas accurate an estimate as is available at the time.

Energy pricethis can be fixed or variable and will depend on the location of the project as well as

the tariff under which it has been developed.

Certified Emission Reductionsunder the Clean Development Mechanism, qualifying Indian solar

projects may generate certified Emission Reductions, which can then be sold. However, this revenue

is difficult to predict.

Financing assumptionsincluding proportion of debt and equity, interest rates and debt terms.

Sensitivity analysis sensitivity of the energy price to changes in the various input parameters

should be assessed.

Social Responsibility costs-when offering free energy to the underprivileged category in society.

LIFE CYCLE COSTING (LCC)


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5. Commissioning

Commissioning should prove three main criteria:

The power plant is structurally and electrically safe.

All pre commissioning tests of each sub system is in order.

The power plant is sufficiently robust (structurally and electrically) to operate for the specified

project lifetime.

The power plant operates as designed and performs as expected. Provisions shall be built in the

design & configuration for the guaranteed evacuation of energy, considering the irradiance variation

expected .

Commissioning tests are normally split into three groups:

Visual acceptance tests:These tests take place before any systems are energized and consist of a detailedvisual inspection of all significant aspects of the plant.

Pre-connection acceptance tests:These include an open circuit voltage test and short circuit current test.

These tests must take place before grid connection. Normally it is taken care in the design & specification of

equipments and type test reports of the manufacturer before factory dispatch.

Post-connection acceptance test: Once the plant is connected to the grid, a DC current test should be

carried out. Thereafter, the performance ratio of the plant is measured and compared with the value stated in

the contract. An availability test, usually over a period of 5 days, should also be carried out and may get

repeated depending on the magnitude of the irradiance measured at site.

6. Operations and Maintenance

Compared to most other power generating technologies, PV plants have low maintenance and servicing

requirements. However, suitable maintenance of a PV plant is essential to optimize energy yield and

maximize the life of the system. Maintenance consists of:

Scheduled or preventative maintenance

planned in advance and aimed to prevent faults from occurring

as well as to keep the plant operating at its optimum level. Scheduled maintenance typically includes:

Module cleaning.

Checking module connection integrity.

Checking junction / string combiner boxes.

Detection of faults.


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Inverter servicing.

Inspecting mechanical integrity of mounting structures.

Vegetation control.

Routine balance of plant servicing / inspection.

Unscheduled maintenance

carried out in response to failures. Common unscheduled maintenancerequirements include:

Tightening cable connections that have loosened.

Replacing blown fuses.

Repairing lightning damage.

Repairing equipment damaged by intruders or during module cleaning.

Rectifying supervisory control and data acquisition

(SCADA) faults.

Repairing mounting structure faults.

Rectifying tracking system faults.


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SOLAR PV APPLICATIONS

1.Solar vehicle

CAR WEIGHT & WEIGHT TO POWER RATIO: While weight is not the only or most important parameter that

controls car performance, it does have a significant effect. Every effort should be made to keep chassis weight to a

minimum. This not only improves acceleration and allows the car to reach full speed more quickly but also reduces

rolling resistance and loads on other components such as axles, wheels and guides.

BUILD QUALITY: It is important to manufacture your car with its critical components correctly aligned and with

the required clearances. Your car must be strong and stiff enough in critical areas to maintain these clearances

AERODYNAMICS: Good aerodynamics, by which is meant a car with low aerodynamic drag, is critical if your car

is to have the best performance possible. Aerodynamic drag varies with velocity squared

SOLAR PANEL: It is more important to use a good quality solar panel. Solar cells have both series and parallel

internal resistances in varying ratios and the ratio of these resistances (within the cells and externally when

assembled) can ultimately give a panel a ballasting advantage or disadvantage. Low quality panels are more l ikely to

have an undesirable ratio of resistances if power measured at 50% Sun. See the section on solar panels in the Design

Guide for details. Solar panel output varies with temperature. Panel power drops by nearly 0.5% per Deg C

temperature rise. Set the Voltage suitable for both the motor and electronics unit. In general, as a rule of thumb for

best results the panel voltage at maximum power output should be between 2 and 3 times the motors rated voltage.

ENERGY UTILISATION: It is important to use as much of the energy collected by your panel as possible to drive

the Car. Ensure you have selected the best gear ratio. By knowing where the energy is used you can take steps to use

it effectively. Energy is used in the following areas:

Overcoming air drag (shape and frontal area)

Giving the car Kinetic Energy (car mass and velocity)


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Electronics (unit efficiency and correct adjustment)

Motor (motor characteristics and operating point)

Rolling resistance (use of tyres, bearings fitment and lubrication, axle alignment

and use of steering)

Driving of car (tyre on drive wheel if required and gear reduction. Is the reduction ratio correct? Are the

gears correctly meshed and in alignment)

MOTOR: Voltage, power, torque constant and voltage constant must suit solar panel selected. Should be high

efficiency & preferably lightweight (Typically 85% and 80 grams). Not worn or damaged.

GEARS:

Good quality with properly formed teeth.

Adjusted for correct mesh.

Correct ratio chosen for the car.

BEARINGS:

Clean and undamaged

Correctly installed with no preload.

Lubricated with light oil

ELECTRONICS:

High efficiency at operating point

Correctly set to panel power point. Caution: the maximum power point voltage drops

Rapidly with increasing panel temperature.

WHEELS:

Must run freely and true especially radially.

Be in correct alignment particularly if steering is not used.


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2. Space based solar cell

Space-based solar power(SBSP) is the concept of collectingsolar powerinspace(using an "SPS", that is, a

"solar-power satellite" or a "satellite power system") for use onEarth. It has been in research since the early

1970s and it is still on the papers. Scientists are saying that to make this space based solar power they need lots

of money as it is very expensive. As economies of scale are achieved in the manufacture of solar collection

devices (both thermal and electric), and as petroleum prices gradually rise, solar energy will become more cost

competitive.

SBSP would differ from currentsolarcollection methods in that the means used to collectenergywould reside

on anorbitingsatelliteinstead of on Earth's surface. Some projected benefits of such a system are a higher

collection rate and a longer collection period due to the lack of a diffusingatmosphereand nighttimeinspace.

Part of thesolar energy(55-60%) is lost on its way through the atmosphere by the effects ofreflectionand

absorption. Space-based solar power systems convertsunlightto microwaves outside the atmosphere, avoiding

these losses, and the downtime due to theEarth's rotation.

Space-based solar power essentially consists of three elements:

a means of collecting solar power in space, for example via solar concentrators, solar cells or a heat engine

a means of transmitting power to earth, for example via microwave or laser

a means of receiving power on earth, for example via a microwave antenna (rectenna)

Advantages:

The SBSP concept is attractive because space has several major advantages over the Earth's surface for the

collection of solar power.

There is no air in space, so the collecting surfaces could receive much more intense sunlight, unobstructed

by the filtering effects of atmospheric gasses, cloud cover; there is no night, dust to be cleaned, clouds and
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other weather events. Consequently, the intensity in orbit is approximately 144% of the maximum attainable

intensity on Earth's surface

A satellite could be illuminated over 99% of the time, and be in Earth's shadow a maximum of only 72

minutes per night at the spring and fall equinoxes at local midnight.Orbiting satellites can be exposed to a

consistently high degree of solar radiation, generally for 24 hours per day, whereas the average earth surface

solar panels currently collect power for an average of 29% per day.

Power could be relatively quickly redirected directly to areas that need it most. A collecting satellite couldpossibly direct power on demand to different surface locations based on peak load power needs.

Elimination of plant and wildlife interference.

Disadvantages:

The SBSP concept also has a number of problems.

The large cost of launching a satellite into space

Inaccessibility: Maintenance of an earth-based solar panel is relatively simple, but construction and

maintenance on a solar panel in space would be difficult. In addition to cost, astronauts working in orbit are

exposed to unacceptably high radiation dangers.

After being decommissioned, parts of it may stay in orbit and become space debris. This space debris can

create trouble for other space satellites.

The space environment is hostile; panels suffer about 8 times the degradation they would on Earth.

The broadcast frequency of the microwave downlink (if used) would require isolating the SBSP systems

away from other satellites.

The large size and corresponding cost of the receiving station on the ground.

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Solar PV-notes