Online Course: PV - Application
Online Course: PV - Application
Online Course: PV - Application
PV - application
Berlin, 2019-06-28
Table of content
1. Introduction ..................................................................................................................................... 3
1.1 Learning objectives of the course ........................................................................................... 3
1.2 Introduction to the course ...................................................................................................... 3
2. Application....................................................................................................................................... 5
2.1 PV system categories/application ........................................................................................... 5
2.2 Grid-connected configuration ................................................................................................. 6
2.3 Off-grid configuration .............................................................................................................. 8
3. Components of a PV system .......................................................................................................... 10
3.1 Overview of PV cell types ...................................................................................................... 10
3.2 PV modules ............................................................................................................................ 11
3.3 Introduction to inverters ....................................................................................................... 12
3.4 Introduction to mounting structures .................................................................................... 13
4. Physical aspects ............................................................................................................................. 15
4.1 PV cell power output ............................................................................................................. 15
4.2 Electrical characteristics and the I-V curve ........................................................................... 16
4.3 Factors affecting power output ............................................................................................. 16
4.3.1 Impact of irradiance ...................................................................................................... 17
4.3.2 Impact of temperature .................................................................................................. 18
4.3.3 Impact of load resistance .............................................................................................. 19
4.3.4 Standard test conditions ............................................................................................... 19
4.3.5 Temperature coefficient................................................................................................ 20
4.3.6 Calculation exercise ....................................................................................................... 20
4.3.7 Orientation and inclination (tilt) of PV modules ........................................................... 21
5. Economic aspects of PV ................................................................................................................. 23
5.1 Resource assessment ............................................................................................................ 23
5.2 Definition of Performance Ratio ........................................................................................... 24
5.3 Energy yield calculation and example ................................................................................... 25
5.4 Metering options ................................................................................................................... 26
5.5 Investment cost of a PV system ............................................................................................ 27
5.6 Price tendencies for modules and inverters ......................................................................... 28
5.7 Operating costs...................................................................................................................... 30
5.8 Endnotes ................................................................................................................................ 30
6. Summary........................................................................................................................................ 31
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6.1 Summary of the course ......................................................................................................... 31
6.2 References ............................................................................................................................. 31
7. Further reading.............................................................................................................................. 32
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1. Introduction
1.1 Learning objectives of the course
Upon completion of this course, participants will be able to
Photovoltaics (left) – sunlight is converted into electricity; solar thermal (right) – solar energy
generates heat, e.g. hot water (Source: RENAC)
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Residential grid-connected PV system (large array) and single solar thermal collector below PV array
on a roof in Germany (Source: RENAC)
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2. Application
2.1 PV system categories/application
Learning objective: Upon completion of this page, you should be able to
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Categories of PV systems (Source: RENAC)
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Central PV systems are also known as solar farms, solar parks or utility-scale PV systems. Large-scale
PV power generation is concentrated at one specific location. Such plants can take up large areas of
land and feed directly into the electricity grid. The configuration of such plants is more comparable to
conventional power plants, producing 3-phase power. They are connected to medium or high voltage
transmission networks.
In all grid-connected systems, the PV array (sometimes referred to as solar generator, see figure
“Typical residential PV system”) generates DC electricity and is connected via junction boxes to an
inverter, which converts the direct current (DC) into alternating current (AC). The inverter output is
connected to a meter which records the amount of energy being fed into the grid.
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2.3 Off-grid configuration
Learning objectives: Upon completion of this page, you should be able to
• name the different components used in off-grid PV systems and
• identify the different applications of off-grid PV systems.
Off-grid PV systems are not connected to the electricity grid. They are also called stand-alone PV
systems. The electricity produced is consumed very close to the location where it is generated. Off-
grid systems usually incorporate battery storage so that electricity can be supplied even when the sun
is not shining. Solar water pumping systems are a major exception to this. Off-grid systems are usually
found at locations where there is no electricity grid available.
The system components differ from grid-connected systems and the array size is typically much
smaller, especially for small off-grid applications like solar home systems.
The PV modules are connected to a charge controller which regulates the charging and discharging of
the batteries. Only DC power is supplied. Many of these systems also have inverters (connected directly
to the batteries) which supply AC power.
Micro- and mini-grids are basically small electricity grids providing electricity to, for example, a small
island or a remote building complex or village. Here we are classifying them as off-grid systems. They
usually have back-up power sources, most commonly a diesel generator. Some larger micro- and mini-
grid systems do not have battery storage.
Electricity is typically produced and stored at a central point from which it is distributed (at AC grid
voltages). A range of types and configurations are possible.
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Micro/mini-grid set-up (Source: RENAC)
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3. Components of a PV system
3.1 Overview of PV cell types
Learning objective: Upon completion of this page, you should be able to
Schematic view of the different PV cell types, in red are the main PV cell technologies. (Source:
RENAC)
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3.2 PV modules
Learning objectives: Upon completion of this page, you should be able to
• explain the function of PV modules and
• describe the design of PV modules.
PV modules are formed by connecting PV cells in series and/or in parallel and encasing them in a
protective material. PV modules can also be connected in series and/or in parallel in order to form a
PV array. The connections of cells and modules (series or parallel) are designed in such a way that they
produce the voltage and current required for the application (see figure “PV cells, modules and
arrays”). The number of modules also determines the maximum power generation capacity of a PV
plant.
The number of PV cells in crystalline modules varies typically between 36 and 72 cells. The PV cells are
electrically connected and sandwiched between two very thin transparent layers of vapour-proof
encapsulation material (commonly made of Ethylene Vinyl Acetate or EVA), then placed on top of a
reflective back sheet (commonly made from polyvinyl fluoride), and then sealed with a glass cover in
an aluminium frame (see figure “Cross section of a typical polycrystalline PV module”). The EVA layers
provide weather protection and ensure that vapour does not reach the PV cell, this being one of the
most common causes of module degradation. Electrical contacts are connected in a junction box at
the back of the module, and bypass diodes are soldered across the connections in order to reduce
power output losses due to shading.
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Cross-section of a typical polycrystalline PV module. (Source: RENAC)
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Principle of DC/AC conversion (Source: RENAC)
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The fundamental requirements of mounting structures are that they must support the weight of the
PV modules, they must distribute the load evenly onto the roof/ground below, and they must
withstand expected extra loading, e.g. from wind or snow. Since large array structures (including
modules) are heavy it is necessary to seek the advice of a structural engineer prior to roof mounting.
The structure should have optimum orientation and tilt angles to maximise power output and all
shading should be avoided. The structure will typically be made from aluminium or galvanised steel.
Painted steel and wood are also options though such structures will probably require more
maintenance over the 20+ year lifetime of the system.
Earthing/grounding as well as lightning protection may be required (see your local codes). Correct
earthing/grounding is a difficult issue and needs special attention by electrical experts.
Mounting system for sloping roof used for training purposes at RENAC (Source: RENAC)
Ground mounting structure, galvanised steel, front and rear view (Source: RENAC)
Tracking system at solar farm, inverter on the rear, Germany (Source: RENAC)
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4. Physical aspects
4.1 PV cell power output
Learning objectives: Upon completion of this page, you should be able to
The photoelectric effect is the phenomenon that converts electromagnetic radiation (such as from the
photons of solar irradiation) into electricity in certain materials. A PV cell is designed in such a way that
the energy from these photons is transferred to the electrons in the cell, causing them to become
mobile. When these electrons are all channelled together to run through conductive materials, such
as copper cables, they produce useful direct current (DC) electricity. (A more physical explanation is
provided in ‘PV Fundamentals II’.)
Thus, a PV cell produces current and a voltage is created across it. The electrical power P for DC
systems, measured in Watts [W], is the product of voltage V measured in Volts [V] and current I
measured in Amperes [A]:
𝑃 =𝑉 ×𝐼
Under normal sunlight conditions the PV cell’s voltage remains fairly constant. However, the cell’s
current is very sensitive to sunlight intensity. High solar irradiance will produce a high electrical
current; low solar irradiance will produce a low electrical current. The surface area of the PV cell also
affects the electric current output. A cell with a large surface area will produce more electric current
than a cell with a small surface area.
Thus, the two main factors that affect the output of a PV cell are the intensity of the sunlight falling
on it and the size of the cell. Other factors are also important and will be discussed in the following
sections.
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4.2 Electrical characteristics and the I-V curve
Learning objectives: Upon completion of this page, you should be able to
The I-V curve describes an important electrical characteristic of a PV cell. At any given time, a PV cell
is operating with a specific current and voltage which lies along its I-V curve (red curve in the figure).
This line shows the current I which is produced over a range of voltages. ISC represents the short circuit
(SC) current, i.e. the value at which the current is at a maximum and the voltage is equal to zero. VOC
represents the open circuit (OC) voltage, i.e. the value at which the voltage is at its maximum and the
current is equal to zero.
The power curve of the PV cell is shown by the blue line. This line shows the electric power produced
over a range of voltages. Under normal operating conditions the curves grow and contract along both
the current and voltage axis. The maximum power point (MPP), the point at which the cell produces
the maximum power, occurs at the ‘knee’ of the power curve where the product of voltage and current
are greatest. The maximum power is thus:
𝑃𝑀𝑃𝑃 = 𝑉 𝑀𝑃𝑃 × 𝐼𝑀𝑃𝑃
• learn how irradiance, shading, temperature, and module orientation and tilt angles influence
voltage, current and power output of a PV cell
• identify power output reductions on an I-V curve.
There are three critical factors which affect the instantaneous output of a PV cell or module. They are
the solar irradiance incident on the surface, the temperature of the PV cells, and the electrical load
resistance connected to the PV cell or module.
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4.3.1 Impact of irradiance
Learning objectives: Upon completion of this page, you should be able to
• identify the impact of irradiance on PV cell or module voltage, current and power and
• describe how the I-V curve changes with changing irradiance levels.
The output power of a PV cell or PV module directly depends on the solar irradiance incident on its
surface. As irradiance G increases, the current I increases due to an increase in the level of the
photoelectric effect. Voltage output V, on the other hand, varies only slightly with changing irradiance
(see figure). This means that as soon as the sun illuminates the surface of the cell/module, the voltage
rises to a value that is close to VOC. Regardless of a change in solar irradiance, such as shading from
passing clouds, the voltage will fluctuate only slightly below that VOC range. The current, however, will
increase in direct proportion to the irradiance, only reaching the module’s full current under strong
irradiance conditions, such as 1 kW/m2. For this reason, current produced by the photoelectric effect
in a PV cell is referred to as photocurrent. Passing clouds or people, which cast shadows on the PV
modules, will cause a drop in current output. Since current is directly proportional to power, shading
has a significant effect on power output.
To summarise, an increase/decrease in solar irradiance causes:
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4.3.2 Impact of temperature
Learning objectives: Upon completion of this page, you should be able to
• indicate the impact of temperature on PV cell or module voltage, current and power and
• describe how the I-V curve changes with changing temperature.
The performance of PV cells also varies with temperature. Since PV cells and modules cannot convert
100% of the absorbed light into usable electricity, some of this energy is lost in the form of heat, which
causes the temperature of the cell to increase. This is a normal part of the operation of a PV cell and
cannot be avoided. As the cell gets hot, its power output drops. This is because the open circuit
voltage VOC decreases significantly when the internal cell temperature rises (see figure). The short
circuit current ISC increases slightly with increasing temperature but not enough to compensate for the
large drop in VOC.
Under normal conditions, the PV cell will always operate at a temperature higher than the ambient
temperature. The ideal operating state for maximising power output is high irradiance with low
temperatures. Such conditions are rare, e.g. high up in the mountains in winter on a clear day.
To summarise, an increase in cell temperature causes:
• only a slight increase in current;
• a significant decrease in voltage;
• and therefore, a significant decrease in power output.
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4.3.3 Impact of load resistance
Learning objectives: Upon completion of this page, you should be able to
𝑉
𝐼=
𝑅
Note about batteries: if batteries are connected to the PV modules, as in an off-grid system, the
batteries will determine the voltage at which the PV modules operate. The module output current will
then correspond to the module’s I-V curve at that voltage.
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4.3.5 Temperature coefficient
Learning objectives: Upon completion of this page, you should be able to
• identify the temperature coefficients and how they can be used to determine PV cell voltage,
current and power at different temperatures.
The temperature coefficient shows how voltage, current or power output of a PV cell or module change
with changing temperature. Module datasheets give temperature coefficients (TC) for open circuit
voltage VOC under STC, short circuit current ISC under STC, and power at the maximum power point PMPP
under STC. The voltage temperature coefficient is the most common one used. Inverters (and other
devices, such as charge controllers) can be damaged by module/string voltages that exceed the
specified input voltages of these inverters (and other devices). Conversely, if the voltage is too low,
this can cause system underperformance.
Voltage temperature coefficients are given in the form of e.g. -0.156 V / °C or -156 mV / °C or as % /
⁰C (e.g. +0.45% / ⁰C).
Current temperature coefficients are given in the form of 0.0029 A / ⁰C or 2.9 mA / ⁰C or as % / ⁰C (e.g.
+ 0.45% / ⁰C).
Power temperature coefficients are given in the form of -0.42 W/ ⁰C or as %/ ⁰C (e.g -0.42% / ⁰C).
Assume the following data: The yearly daytime temperature at a location ranges from -10⁰C to +45⁰C.
A PV module is being installed which has:
- a VOC of 43.24 V at STC,
- a VMPP of 35.35 V at STC, and
- a temperature coefficient TC (VOC) of -0.168636 V / ⁰C.
Note: temperature coefficient Tc (VMPP) is slightly different than TC (VOC). In this case, for the
calculations, the difference will be neglected.
This means that for every C temperature drop below 25⁰C, the module voltage will rise by 0.168636
V. Similarly for every C temperature rise above 25⁰C, the module voltage will drop by 0.168636 V.
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What will be the maximum VMPP (voltage when the module is operating at its maximum power point,
i.e. in full sun) produced by the module?
𝑉
𝑉𝑀𝑃𝑃 (−10°𝐶) = 35.35 𝑉 + [(25°𝐶 − (−10°𝐶)) × 0.1686 ] = 41.25 𝑉
°𝐶
What will be the minimum VMPP (voltage when the module is operating at its maximum power point,
i.e. in full sun) produced by the module?
The minimum VMPP will be produced at the highest ambient temperature, 45⁰C. But the cell
temperature of roof-top or open field PV systems can as a rule of thumb be 25⁰C higher than the
ambient temperature, so this will be 70⁰C.
𝑉
𝑉𝑀𝑃𝑃 (+70°𝐶) = 35.35 𝑉 + [(25°𝐶 − 70°𝐶) × 0.1686 ] = 27.76 𝑉
°𝐶
To summarise: The highest voltage VOC will be at -10⁰C, i.e. 49.14 V. And the voltage range at the MPP
will be between 27.76 V and 41.25 V.
Similar calculations can be performed to estimate voltages at different temperatures for series strings
of PV modules.
• identify the impact of PV module orientation and inclination on energy yield and
• understand that the optimum orientation and inclination are dependent on latitude (i.e.
number of degrees north or south of the equator).
The PV module’s orientation and inclination (tilt) significantly affect the amount of irradiation that the
surface receives, and hence the amount of energy that the module produces. In the northern
hemisphere, the PV module should be facing south; in the southern hemisphere it should be facing
north. This guarantees the maximum irradiation level on the PV module throughout the year. In regions
close to the equator, the orientation is less important.
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The optimum tilt angle of the PV module depends strongly on the location. As a rule of thumb, the
module should be tilted to an angle equal to the latitude of the installation site. Roof mounted modules
are usually simply installed at the same angle as the roof since the extra cost of adjusting the tilt angle
exceeds the benefit of the extra energy that would be generated. In regions close to the equator the
most solar irradiation is captured if the PV module is flat. However, in practice a minimum tilt angle of
10-15 is recommended to allow for self-cleaning.
The figure shows a building located at a latitude of around 50N. It depicts the proportion of solar
radiation received at different orientations and inclinations compared to a reference PV module placed
at the optimum orientation (due south) and inclination (30) for this particular location.
Solar radiation received on PV modules at different orientations and inclinations (Source: RENAC)
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5. Economic aspects of PV
5.1 Resource assessment
Learning objectives: Upon completion of this page, you should be able to
• describe the area of use for colour-coded resource maps and their limitations and
• explain the need for resource studies conducted by independent site assessors.
In order to assess or calculate the energy yield (output) of a PV system, one has to know about the
available resource (input) to the PV system, meaning the solar irradiation. Colour-coded solar resource
maps show how irradiation is distributed in the target area. They provide a first indication of the
available solar energy production (AEP) potential in a certain region. The input data are usually
provided as an average annual sum in kWh/m² (the darker the colour, the better the resource
potential).
Solar irradiation maps are well-suited for relative valuation of different regions or siting areas. They
are, however, not well-suited for absolute determination of the energy yield potential of a specific
project site. The main reason is that the maps do not contain enough information regarding other on-
site conditions, such as roughness, shading and temperature at the micro site investigated.
Solar resource maps are not a substitute for detailed PV resource assessments prepared by certified
PV resource assessors. Rather, they serve to focus investigations and indicate where on-site
measurements would be merited. During a bank or investor’s due diligence, two independent PV
resource assessment studies are usually required, which take into account individual site
characteristics and the project-specific technology.
Global Horizontal Irradiation of Turkey (Data source: Data source: SolarGis and the World Bank
Group)
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5.2 Definition of Performance Ratio
Learning objectives: Upon completion of this page, you should be able to
• define the factors influencing energy yield and
• calculate the Performance Ratio and describe its use.
The economic feasibility of a PV plant critically depends on its electricity yield, which in turn depends
on both the meteorological conditions at the location and the performance of the plant itself.
As the electricity yield is heavily dependent on solar irradiation levels, it is essential that accurate and
long-term climate data are available for the given location. Solar irradiation at a site is often stated in
terms of the Peak Sun Hours (PSH) over a certain period.
Furthermore, component quality, plant design and layout as well as the maintenance plan will affect
the performance and consequently the energy yield of the plant.
The Performance Ratio (PR) is one of the most important parameters for assessing the performance
of a PV plant. It is the ratio of actual plant output to nominal plant output. It is determined by
monitoring the actual plant output over a defined period (e.g. one year) and dividing this by the
nominal plant output in that same period. The PR will change from period to period depending on
environmental and technical conditions at the plant.
A low PR is indicative of high system losses, for example due to high module temperature, reflection
of solar irradiation from the module, soiling of the front glass, shading, component failure, mismatched
module outputs, etc. Therefore, higher PR values are desirable.
In contrast, PR is also used with common reference values in the planning stage of a PV system. For
example, a PR of 0.75-0.80 is used for grid-connected PV plants and 0.65 for off-grid plants. This means
that the expected gross energy production potential would have to be multiplied by the performance
ratio value or reduced by (1-PR) to reflect the expected production after system losses.
Once the energy yield of a PV system has been estimated, the revenue from electricity sales can be
calculated in order to evaluate the profitability of a project.
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5.3 Energy yield calculation and example
Learning objective: Upon completion of this page, you should be able to
• calculate the energy yield for a given setting.
The following equation provides a quick and simple way to estimate the energy yield of a PV plant
over a particular period of time:
𝐸 = 𝑃𝑆𝐻 × 𝑃𝑃𝑒𝑎𝑘 × 𝑃𝑅
Where,
E: Energy yield of the PV plant over a period of time (e.g. over one year) [kWh/year]
PSH: Peak Sun Hours at the location of the PV plant over the same period of time [h/year]
PPeak: Peak nominal power of the PV plant [kWp]
PR: Performance Ratio of the PV plant, normally between 0.65 and 0.85
The Peak Sun Hour (PSH) is an imaginary case, where the sun shines at a constant irradiance of 1000
W/m² for one hour. We can visualise the PSH by looking at the area below the solar irradiance curve.
This area must be similar to the area of a rectangle with a “height” of 1000 W/m². The width of the
rectangle determines the PSH.
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𝑊ℎ 𝑘𝑊ℎ
𝐸𝑈𝑆𝐴 = 𝑃𝑆𝐻 × 𝑃𝑃𝑒𝑎𝑘 × 𝑃𝑅 = 1,900 ℎ × 1,000 𝑊𝑝 × 0.7 = 1,330,000 = 1,330
𝑎 𝑎
Although the system in the Southern USA has a lower estimated PR due to higher ambient
temperatures, the output is higher because more solar irradiation is available. Once the PV plants are
operational, the actual plant outputs can be monitored and thus the actual PRs calculated.
Technical set-up for metering option with feed-in tariff (Source: RENAC)
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Technical set-up for net metering option (Source: RENAC)
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Sample investment cost budget for a 5.0 MWp PV installation (Source: RENAC)
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Development of module prices 2017 – 2018 in € per Wp (Data source: PVMagazine 2018)
Inverter market details 2018 (Data source: Fraunhofer ISE, 2018, p. 37 based on various
sources)
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5.7 Operating costs
Learning objective: Upon completion of this page, you should be able to
• overview the operating cost structure of a PV project and
• identify the most important OPEX items.
Operating costs (OPEX) for PV systems are very low. However, in order to maximise energy yield and
hence revenues, OPEX will never be zero and should be looked at carefully. The OPEX for PV projects
predominantly include operations and maintenance (O&M) costs, land lease payments and insurance
costs.
O&M costs can consist of different sub-costs, such as fees payable under full service agreements for
the modules and/or inverters, costs for regular cleaning services for modules, and maintenance staff
(personnel in charge on-site). O&M costs are project-specific and have to be estimated in detail based
on existing contracts. In the initial evaluation phase, a rough first estimate of 2.0% of the total CAPEX
is often used.
Land lease contracts are also negotiated individually and their costs can vary from project to project.
A market-oriented estimate might, for example, be USD 1,000 per MWp. Insurance costs for PV projects
have meanwhile been standardised by most insurance companies.
Additionally, more general costs should be taken into consideration, such as costs for electricity
consumption by the production facility, grid connection and/or balancing energy and costs for
commercial administration and auditing of the project company. The OPEX budget should further
include a cost reserve for the dismantling of the project (and module recycling, if not provided by the
equipment manufacturer) once the operating period ends.
5.8 Endnotes
[1]: Fraunhofer ISE (2016), Photovoltaics Report, p.9
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6. Summary
6.1 Summary of the course
This first unit on PV systems provided an introduction to PV applications and components, explained
some physical characteristics of PV modules relating to power generation, and explained the basics of
yield estimation and economics of a PV plant.
The two general categories of PV systems were discussed, i.e. grid-connected PV systems and off-grid
PV systems. Grid-connected PV systems were further categorised into distributed and central PV
systems; off-grid PV systems into stand-alone systems (with or without batteries) and mini- or micro-
grids.
The heart of a PV system is the PV module which consists of many PV cells connected in series and/or
parallel. An inverter converts the direct current from the PV modules into alternating current which
can be injected into the grid. Off-grid PV systems with batteries require a charge controller to protect
the batteries against over or under charge.
The underlying physical principle of PV systems is the photoelectric effect whereby the energy of
photons (quants of light) causes electrons to move in a specific material. Thus, the intensity of solar
radiation and the size of the PV cells have an impact on the current produced. In addition, the
temperature has an impact on the voltage of the cells. Current and voltage determine the power
generation of the cell or module.
For investment decisions and revenue projections it is important to carry out an energy yield estimate
of a PV system prior to development. The energy yield can be estimated using Peak Sun Hours, peak
power capacity of the PV system and the estimated or known performance ratio.
6.2 References
Fraunhofer ISE (2016): Photovoltaics Report, Dr. Simon Philipps, Fraunhofer ISE und Werner
Warmuth, PSE AG; available online at
https://www.ise.fraunhofer.de/de/veroeffentlichungen/studien/photovoltaics-report.html (last
access 02.06.2017)
GHI Solar Map © 2014 GeoModel Solar, Wikimedia; available online at
https://commons.wikimedia.org/wiki/File:SolarGIS-Solar-map-Philippines-en.png (last access
02.06.2017)
Solarserver – Global Solar Industry Website: PVX Spot Market Price Index soler PV moules; available
at http://www.solarserver.com/service/pvx-spot-market-price-index-solar-pv-modules.html (last
access 02.06.2017)
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7. Further reading
James P. Dunlop; Photovoltaic systems; ISBN: 978-1-935941-05-7; American Technical publisher
Comprehensive guide to the fundamentals and principles involved in the planning, design and
installation of solar photovoltaic (PV) systems. It covers the key steps in project development and
specific requirements for installations.
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