We have already talked about flat solar collectors and vacuum tube collectors.

Air collectors are also found in collectors without concentration category.

They are flat and their main characteristic is to have air as heat transfer fluid.

They do not have a maximum limit temperature (convective processes have less influence on the air) and work better in normal circulation conditions, but in contrast they have a low heat capacity and heat transfer process between plate and fluid is not good.

Its main application is heating.

Externally it is not possible to distinguish an air collector from a water collector. It is in the absorber where greatest differences are found. It has a rough shape and lacks the classic pipe of water collector ducts. The air circulates freely on absorber surface collecting the heat that it transforms.

Being a technology that has not been widely disseminated until now, there is no standardized solar collector model and each manufacturer makes its own model.

There are also conical or spherical thermal solar collectors.

Their main characteristic is that they simultaneously constitute collection and storage unit.

Its catchment surface is conical or spherical with a same geometry glass cover. With this form it is achieved that illuminated surface throughout the day, in shade absence, is constant.

Their installation is simple, but they present water stratification problems and useful catchment surface is small.

Its main application is sanitary hot water in single-family homes production and in very benign climates, since large storage surface, weather exposed, causes great energy losses.

Finally, in collectors without concentration category, we find outdoor swimming pools solar collectors.

They are made of rubber, polypropylene or polyethylene; and incorporate in their manufacturing process substances that protect them from plastics natural tendency to degrade under ultraviolet rays action.

They also carry other additives to protect them from chemical agents used in pool water purification. They have an acceptable night frosts resistance.

They are used mainly to heat pools water and thus be able to prolong its use for several more months.

These collectors do not have cover, neither with housing nor with insulating material. They are constituted by naked plate collector. This is because working temperature will not exceed 30ºC in any case and at this low temperature, radiation and conduction losses are very small, making it possible to dispense with covers and insulation.

It is not necessary to use any type of heat exchanger or accumulator, because pool water flows directly through collectors.

They need a frame because they are usually not rigid, but they can also be placed directly on a roof, or even on the ground. By being flexible, absorb surface irregularities on which they rest.

These equipment enjoy an approximate 10 years lifespan. They need little maintenance and there is little risk of corrosion, as they are synthetic.

The second major group is that of solar collectors with concentration.

Its most common use is not at domestic level but in thermoelectric plants and facilities that work at medium and high temperature.

These collectors concentrate the solar radiation received in a very small surface receiving element (a point, a line).

Being smallest receiver and concentrated radiation, it allows a better solar energy absorption.

They are capable of providing temperatures above 300 ° C with good yields.

Concentration collector plants generate high temperature steam for industrial processes and to produce electricity.

There are concentration collectors of various types (tower, cylindrical-parabolic, Stirling engine).

Nevertheless, all of them have in common that they demand to be equipped, to be efficient, with a tracking system that allows them to remain constantly located in best position to receive the Sun’s rays throughout the day.

One of the drawbacks of most concentration collectors (and especially the cylindrical-parabolic) is that they only take advantage of Sun direct radiation, that is, they only take advantage of solar rays that actually hit their surface. They are not able, on contrary, to capture diffuse solar radiation.

Therefore, they are not convenient in climatic zones that, although they receive an acceptable solar radiation amount, are relatively cloudy. They are only effective in authentically sunny áreas.

This content was extracted from the Solar Thermal Energy Technical-Commercial Manual and is part of Solar e-learning.

All you need is Sun. All you need is Sopelia.

Within group of solar collectors without concentration are the vacuum tubes collectors.

Currently they are the most used.

As was seen in flat collector’s analysis, conversion of radiant energy from the sun to thermal energy leads to radiation, conduction and convection losses that progressively decrease the yield as temperature difference between collector and environment increases.

Improvement provided by vacuum tube collectors is to avoid conduction and convection losses.

If less heat is lost, we will in most cases obtain more yield for same amount of Sun energy.

We will see that this is not always the case and depends on temperature of use.

Vacuum collectors find their main application in intermediate temperature systems (heating, air conditioning, industrial processes, etc.) and in cold places with high differences between collector temperature and the environment.

The vacuum technique used by fluorescent tubes manufacturers has been developed and is the one used by vacuum tube manifolds manufacturers.

Vacuum tube collector systems are based on evacuated tubes.

These are formed by two concentric tubes between which the air has been sucked up producing a vacuum. At one end, both tubes are joined by sealing the vacuum. Inside both tubes are located the different types of absorbers that determine the different systems.

Single evacuated tubes are evacuated tubes, assembled directly with accumulation tank or independently, which may contain only water or water plus antifreeze.

A dark colored layer of absorbent material is located on evacuated tube inner wall.

When solar radiation strikes the absorbent material layer it is transformed into heat and raises the temperature of the fluid that is in contact with it.

The fluid is heated by convection and begins to rise through the tube being replaced by cold fluid which in turn heats up and restarts the process.

This type of vacuum tube offers the advantage of having the aforementioned few heat losses and the disadvantages of being very sensitive to pressure.

U-Pipe vacuum manifolds are used both in individual collectors and in compact solar systems with integrated tank.

Absorber can be placed on tube wall as in evacuated tube case or on an absorbent material sheet.

In any case, absorber is run on its surface by a pipe (preferably copper) through which the fluid raises its temperature in contact with it flows.

U-Pipe tube manifolds have the advantage of being able to adopt both horizontal and vertical position without impairing their performance since tube can rotate on its axis by tilting absorber in the most appropriate way in case the absorber has sheet shape.

Finally, in vacuum tube technology we find heat pipe manifolds.

They employ a mechanism consisting of a closed tube into which a vaporizing fluid (alcohol mixture) of specific properties is introduced.

When sun hits absorber attached to the tube, fluid evaporates and absorbs heat (latent heat). As gas rises above liquid to top of the tube where the cold spot is located. There it liquefies (condenses) and yields its latent heat to the fluid we are interested in heating by falling back to tube bottom by capillarity or gravity.

This process (evaporation – condensation) is repeated for sun’s radiation duration or until collector has reached a very high temperature (around 130 ° C or more).

They have the advantage that each tube is independent being able to change in full system operation. It is highly frost resistant.

Since tubes can also rotate on their axis, it is possible to adopt vertical and horizontal positions as in the case of U-Pipe systems, although in this case generally a minimum tube inclination (between 15º and 20º according to manufacturer) to allow fluid, once liquefied, to fall by gravity.

There are 3 qualities of these collectors:

– Dry union: heat exchange occurs without direct contact between heat transfer fluid and tube, which makes them very suitable in areas with unfavorable water qualities.

– Diode function: heat transfer is always carried out only in one direction, from absorber to heat transfer fluid, and never other way round.

– Temperature limitation: evaporation – condensation cycle takes place as long as vaporizing fluid critical temperature is not reached, thus avoiding uncontrolled temperature rise inside the tubes risks.

This content was extracted from the Solar Thermal Energy Technical-Commercial Manual and is part of Solar e-learning.

All you need is Sun. All you need is Sopelia.

Within the solar collectors without concentration we find the flat plate.

They were the most used, but have lost ground in favor of vacuum tube.

In flat collectors, the collector is located in a rectangular box (housing), whose usual dimensions are between 80 and 120 cm wide, 150 and 200 cm high, and 5 and 10 cm thick (although there are larger models).

The face exposed to the sun is covered by a very fine glass, while the remaining five faces are opaque and are thermally insulated.

Inside the box, on the face that is exposed to the sun, is placed a metal plate (absorber).

This plate is attached or welded to a series of conduits through which a heat transfer agent (usually water, glycol, or a mixture of both) flows.

A selective surface treatment is applied to mentioned plate or is simply black painted, to increase its heat absorption.

Flat solar collectors work taking advantage of greenhouse effect (the same principle that can be experienced when entering a car parked in the sun in summer).

After passing through the glass (transparent for wavelengths between 0.3 μm and 3 μm) radiation reaches absorber surface, which is heated and emits radiation with a wavelength between 4.5 μm 7.2 μm, for which the glass is opaque.

Approximately half of this last radiation diffuses to the outside, being lost; but the other half returns inward and thus contributes to absorber surface further heating.

As it passes through the box, the heat transfer fluid heats up and increases its temperature at absorber expense, which temperature will decrease.

The heat transfer fluid then transports that heat energy to where it is desired.

The flat solar collector is formed by 4 main elements:

1) Transparent cover: it must possess the necessary qualities (suitable transmission and thermal conductivity coefficients) to provoke the greenhouse effect and to losses reduce; ensuring manifold water and air sealing, in conjunction with housing and joints; do not keep outer surface dirt adhering so that rain easily slips.

2) Absorber: receives the solar radiation and converts it into heat that is transmitted to the heat transfer fluid.

Shapes are diverse: metal plates separated by a few millimeters, a metal plate with welded or embedded tubes or two metal plates with a circuit inside.

Face exposed to sun must capture the largest radiation amount so it is usually black painted or endowed with a selective surface (very absorbent to radiation and with low emissivity).

Paints are cheaper than selective surfaces and have a better overall thermal behavior at near-ambient temperatures, but are marred by ultraviolet radiation continued action and temperature variations between day and night.

Selective surfaces generally have a better behavior and are obtained by several layers superposition (metal and metal compounds) or special surface treatments.

The most modern manufacturing technique is laser welding.

3) Insulation: it is used to reduce thermal losses in absorber rear part that must be of low thermal conductivity. Materials can be glass wool, rock wool, cork, polyethylene or polyurethane.

4) Housing: generally made of aluminum or stainless steel, it protects and supports manifold elements, also allowing manifold anchoring and holding to assembly structure. It must withstand temperature changes (dilatations) without tightness losing and must resist corrosion.

Solar energy wherever you are with Sopelia.

Solar Layout is the App for collectors and solar modules on site positioning.

This is the most intuitive Solar App of the market.

To use it on field is not necessary to have an Internet connection because it works from place latitude, obtained by GPS.

Today we will see solar thermal energy part.

To begin press left command shown in initial screen with the house, the solar collector and the user taking a hot shower.

If our Smartphone GPS is not enabled, the App will ask us to activate it to locate our position.

Intermittent earth planet image immediately appear with the legend “Localizing”.

When our device GPS have located our position, the following screen appears to confirm it.

By confirming our location the Solar Equipment Use Menu displays.

There are 3 applications in the Menu:

1 Hot water: represented by a shower image
2- Heating represented by a radiator image
3- Outdoor pool conditioning: represented by a pool ladder image

By selecting one of the 3 applications, Options Menu will display.

There are 3 variables in the Menu:

1- Inclination: represented by collector and angle image
2- Orientation: represented by collector and cardinal points image
3- Distance: represented by 3 collectors rows image

By pressing the Inclination option, we get recommended inclination value for location and solar application selected, accompanied by some Tips considering collector type used.

Pressing Orientation option, we obtain procedure to fix collectors orientation description and access to recommended compass App discharge, if we don´t have it.

Pressing Separation option, the Kind of Surface Menu is displayed for us to select the appropriate option (Horizontal / Non horizontal). If the surface on which the collectors will be placed is horizontal, we only must enter Collector Height in cm data.

If the surface on which the collectors will be placed is non horizontal, in addition to Collector Height in cm data, we must enter Surface Inclination Angle data. We will enter a positive value if it matches the collector inclination direction and a negative value if it is different.

In this way we obtain the Separation (distance) between collector’s rows in meters.

Pressing i button Tips related to shadows and singular locations (snow, desert and rain areas) are deployed.

Download Solar Layout and placed solar thermal collectors on site in the most intuitive way with Sopelia.

Este cronograma representa la dosificación recomendada de dedicación para una correcta asimilación de conocimientos durante el curso e-learning de Técnico – Comercial en Energía Solar Térmica impartido por Sopelia.
Puedes recibir esta formación íntegramente desde tu computadora, smartphone o dispositivo móvil.
Supone dedicar entre 1 y 2 horas diarias entre lunes y viernes de cada semana.
* Semana 1: Introducción a la Energía Solar
1.1) El futuro de la energía solar
1.2) El Sol
1.3) Nociones básicas de Física
* Semana 2: Introducción a la Energía Solar
1.4) Nociones básicas de Electricidad
1.5) Nociones básicas de Energía
1.6) Energía del sol
1.7) Tablas
– Resolución Test 1 y 2 y Ejercicio 1
* Semana 3: Energía Solar Térmica – Equipos
2.1.1) Colectores
2.1.2) Sujeción y anclaje
* Semana 4: Energía Solar Térmica – Equipos
2.1.3) Fluido caloportador
2.1.4) Protección de la instalación
* Semana 5: Energía Solar Térmica – Equipos
2.1.5) Tuberías
2.1.6) Tanques acumuladores
2.1.7) Intercambiadores
* Semana 6: Energía Solar Térmica – Equipos
2.1.8) Grupos de bombeo
2.1.9) Aislamiento
2.1.10) Otros componentes
– Resolución Test 3 y Ejercicio 2
* Semana 7: Energía Solar Térmica – Instalaciones
2.2.1) Principios básicos
2.2.2) Diseño
2.2.3) Regulación
* Semana 8: Energía Solar Térmica – Instalaciones
2.2.4) Proyecto de un sistema de ACS
2.2.5) Cálculo de la superficie colectora
2.2.6) Cálculo de los demás elementos de la instalación
* Semana 9: Energía Solar Térmica – Instalaciones
2.2.7) Presentación de un proyecto
2.2.8) Otras aplicaciones
2.2.9) Ejecución y mantenimiento de la instalación
* Semana 10: Energía Solar Térmica – Instalaciones
– Resolución Test 4 y 5 y Trabajo Práctico final
Se trata de la formación en Energía Solar con la mejor relación calidad-precio del mercado.
Puede recibirse donde quiera que estés.
Solamente se necesita una computadora, smartphone o dispositivo móvil y conexión a Internet.
Por tratarse de la 1era edición hay un 50% de descuento sobre el PVP.

Esta acción de formación brinda capacitación técnico – comercial en aplicaciones domésticas de energía solar con el objetivo de difundir la tecnología y desarrollar recursos humanos para su incorporación al mundo laboral y empresarial.
La edición 2016 comienza el día 19 de septiembre y finaliza el día 25 de noviembre.

El plazo de inscripción es hasta el día 16 de septiembre inclusive en www.energiasrenovables.lat
Ya no tienes excusas, energía solar donde quiera que estés con Sopelia.

On Internet we can find free tools for basic or low complexity solar systems dimensioning and for certain components or accessories estimation.

Sopelia research team has carried out an exhaustive search and testing from which a new corporate website section called Free Solar Tools has been created.

Selected tools were classified into 4 categories.

Today we will analyze the third of them: Solar Thermal.

In first category we have already analyzed tools to obtain data about solar resource and other variables to be considered in energy estimation solar system will provide in our location.

In the second category we have analyzed tools to calculate the “load”, ie the energy demand to be met.

Now we are going to analyze tools to solar thermal system dimensioning and others to estimate individual components of a system.

The order of the tools is not random. We have prioritized the most intuitive, the most universal and those that can be used online without download.

For this third category our selection is as follows:

1) Solar Thermal Calculator

Approximate calculation tool from which budget, production data and system performance study is automatically obtained.

A Navigation Guide and Manuals can be found at page bottom.

2) Simulation for Solar Thermal System Pre-design

Online application based on the TSOL software that allows solar energy system simulating to ACS and ACS + heating contribute.

Available in German, English, Spanish and French.

3) Solar Fraction Calculation

Free download program developed by IDAE (Institute for Energy Diversification and Saving) and ASIT (Solar Thermal Industry Association) that allows to define a wide variety of solar systems introducing a minimum of project parameters, associated to each system configuration; and in this way, obtain solar system coverage on ACS and pool conditioning energy demand.

4) Solar Expansion Vessel Calculation

Tool developed to calculate solar expansion vessel volume.

Volume values (total circuit, solar collectors, pipes), Maximum system temperature (ºC), Glycol concentration (%), Height between expansion vessel and system highest point (minimum value 1 Bar) and safety valve Pressure setting must be introduced.

5) Thickness Insulation Pipes Calculation

Calculator that allows to estimate minimum and more economical water pipes insulation thickness.

Pipe Grade and Size, Insulation Material, Humidity and Temperature (Internal and Ambient) must be entered.

Solar energy wherever you are with Sopelia.

Thermal solar collector is responsible for capturing solar radiation and converting its energy into heat energy.

A body exposed to the sun receives an energy flow Er and heats up.

Simultaneously, thermal losses occur due to radiation, convection and conduction, which grow as the body temperature increases.

There comes a time when thermal losses Ep equals the gains due to the incident energy flow, reaching the so-called equilibrium temperature:

 Er = Ep

The equilibrium temperature of the collectors is usually between 100º and 150º C under normal conditions of use and for irradiation values in the order of 1,000 W / m2.

If it is possible to continuously extract a part of the heat produced Ee to take advantage of it as usable energy, the equilibrium conditions change:

Er = Ep + Ee

Ep is now smaller because a part of the energy received Er is tapped Ee.

The body has become a solar thermal collector.

If we want to increase Ee we have two options: reduce thermal losses Ep or increase energy flow Er.

First option involves collector design and construction improving in order to reduce losses.

For the second option is used the concentration technique, which by some optical system concentrates the solar flux on a smaller surface so that as the area decreases, the intensity increases.

In a solar collector the energy is extracted through a fluid called heat carrier.

The greater the difference between operating temperature and ambient temperature, the greater the thermal losses and thus the lower energy amount that heat transfer fluid will be able to extract.

The collectors must be operated at the lowest possible temperature, provided that temperature is sufficient for the specific use in each case.

This is because collector efficiency decreases as the operating temperature increases.

Improved insulation helps thermal losses reduce.

Reflection losses are due to transparent cover that usually exists in almost all collectors.

It will be necessary to properly orient the collectors so that they receive the greatest radiation amount possible during the period of use.

The question: which is the best collector ?

A priori has no answer.

It will depend on system location and energy demand that is intended to be met.

There are many types of solar collectors, but there are two large groups: unconcentrated collectors and concentrated collectors.

Solar thermal collectors according to their working temperature:

1) Low temperatura

1.1) Flate: protected and not protected

1.2) Vacuum tubes: direct flow, heat pipe and solar concentrator CPC)

2) High temperatura

2.1) Parabolic Cylinder

2.2) Central receiver system

2.3) Parabolic disks

2.4) Solar chimney

3) Other collectors

3.1) Rubber

3.2) Spherical

3.3) Conical

In next posts we will analyze in detail each collector type.

This content was extracted from the Solar Thermal Energy Technical-Commercial Manual and is part of Solar e-learning.

Solar energy wherever you are with Sopelia.

Solar thermal energy systems for domestic applications will be increasingly present in the built landscape and will be promoted by regulations such as solar ordinances or future building technology standards.

The most basic system is the compact equipment called thermosiphon, which incorporates all subsystems and where the fluid circulates naturally (difference in densities).

Solar thermal systems use the sun’s rays to get hot water or air.

Special plates, called collectors, concentrate and accumulate Sun heat and transmit it to the fluid we want to heat.

This fluid can be home’s drinking water or home’s heating or cooling hydraulic system.

Generally a thermal solar energy system is constituted by several subsystems, which in turn can be considered as interdependent systems connected to each other.

However, sometimes the same physically independent element performs several functions within the solar system.

These different subsystems are:

a) Capture system: composed of solar collectors. They are responsible for receiving the solar radiation and transmit it to the fluid that circulates inside.

b) Accumulation system: composed of one or more deposits to accumulate the hot water generated up to the moment of its use.

c) Hydraulic system: composed of the pumps and pipes through which the working fluid circulates. A primary circuit transports the energy captured to the accumulator. The circulation of the fluid through the pipes is performed by a circulation pump or by natural circulation.

d) Exchange system: exists in case the fluid flowing through the solar collectors is not the same as the one used by the user; for example when there is frost risk or user fluid can damage the solar system. The exchanger can be part of the same accumulator or located outside.

e) Control system: in pumps forced circulation systems will be in charge to start and stop them. Different components system actuation (motorized valves, pumps, etc.) is done through control mechanisms.

f) Auxiliary energy system: generally solar system economic viability requires that total energy demand cannot be met with solar input at all times. The energy produced by solar system depends on climatic conditions and that is why an auxiliary energy production system is available. These support equipment complement the solar system in order to ensure at all times hot water service continuity.

Solar thermal systems have a great similarity with conventional thermal systems.

In fact, they share all their components (pipes, protection mechanisms, accumulation tanks, exchangers, pumping groups, insulation) except one: solar collectors.

This content was extracted from Solar Thermal Energy Technical & Commercial Manual and is part of Solar e-learning.

Solar energy wherever you are with Sopelia.

Solar Layout is the App for collectors and solar modules on site positioning.

This is the most intuitive Solar App of the market.

To use it on field is not necessary to have an Internet connection because it works from place latitude, obtained by GPS.

Today we will check solar thermal energy part.

To begin press left command shown in initial screen with the house, the solar collector and the user taking a hot shower.

If our Smartphone GPS is not enabled, the App will ask us to activate it to locate our position.

Intermittent earth planet image immediately appear with the legend “Localizing”.

When our device GPS have located our position, the following screen appears to confirm it.

By confirming our location the Solar Equipment Use Menu displays.

There are 3 applications in the Menu:

1- Hot water: represented by a shower image
2- Heating: represented by a radiator image
3- Outdoor pool conditioning: represented by a pool ladder image.

By selecting one of the 3 applications, Options Menu will display.

There are 3 variables in the Menu:

1- Inclination: represented by collector and angle image
2- Orientation: represented by collector and cardinal points image
3- Distance: represented by 3 collectors rows image.

By pressing the Inclination option, we get recommended inclination value for location and solar application selected, accompanied by some Tips considering collector type used.

Pressing Orientation option, we obtain description of procedure to fix collectors orientation and access to recommended compass App discharge, if we don´t have it.

Pressing Separation option, the Kind of Surface Menu is displayed for us to select the appropriate option (Horizontal / Non horizontal).

If the surface on which the collectors will be placed is horizontal, we only must enter Collector Height in cm data.

If the surface on which the collectors will be placed is non horizontal, in addition to Collector Height in cm data, we must enter Surface Inclination Angle data.

We will enter a positive value if it matches the collector inclination direction and a negative value if it is different.

In this way we obtain the Separation (distance) between collector’s rows in meters.

Pressing i button Tips related to shadows and singular locations (snow, desert and rain areas) are deployed.

Download Solar Layout and placed solar thermal collectors on site in the most intuitive way with Sopelia.

Hydraulics is the physics field that studies fluid mechanics and is divided into Hydrostatic (liquids at rest) and hydrodynamics (liquid in motion).

density of a body d is called the mass m and volume V ratio:

d = m / V

specific gravity pe is the weight (= m g.) and volume ratio:

pe = m. g / V

Fluids (liquids and gases) always exert a pressure pr in all directions.

The pressure is the quotient between a force f (the exerted by the fluid) and the surface area S acted upon by this force:

pr = f / S

The pressure unit in the SI is the Newton divided by m2 (N / m2) and is called pascal.

Pressure exerted by gravity and the forces tending to compress the fluid is called static pressure.

The pressure resulting from movement of a fluid is called dynamic pressure.

Knowing the density or specific gravity of a fluid we can find the static pressure due to gravity at any depth h from either of the following two formulas:

pr = d. g. h

pr = pe. h

The pressure difference is equal to the depths difference h between 2 points or vertical distance between them.

A typical static pressure, is the atmospheric pressure produced in all directions on the bodies placed on earth surface due to the large air column above them. The result of this all directions atmospheric pressure action produces no net force pushing the body to one side, tends to compress it.

In the case of a container, the atmospheric pressure acts inside and outside and therefore their actions cancel each other.

We are interested in knowing the excess pressure above atmospheric pressure that may be inside the container (tanks or pipes) through measuring devices (manometers).

If air can freely enter and leave a container through the edge of the lid, the liquid surface will only be subjected to atmospheric pressure. It is an open or non pressurized reservoir.

If we measured pressure at different heights in the tank with a manometer it will be equal to zero at the surface and maximum at the bottom.

If the container is now sealed and subjected to additional pressure p, transmitted through the pipes that communicate with distribution circuit; the measurement is equal to the previous one but increased in the value of p. Usually the small pressure difference caused by the height difference is negligible compared to the overall circuit pressure p.

Archimedes’s theorem allows us to know a body weight when it is immersed in a liquid.

This theorem can be applied to a same liquid portion.

Suppose that a liquid portion suffer a slight temperature increase relative to other liquid parts.

Bodies expand by its temperature increasing and when increasing its volume density decreases as mass remains unchanged.

If d1 is the new density of the portion considered (d1 < d):

Weight of the liquid portion: p = m. g = V1. d1. g

Thrust acting on the liquid portion: E = V1. d. g

Where V1 is the volume of the liquid portion

These are the called fluids natural convection currents, in which hot parts tend to rise. Natural circulation or thermosyphon systems are based on this phenomenon for supplying hot water by solar collectors.

This content is part of “Solar Energy Introduction” eBook and solar e-learning of Sopelia.