SOLAR ENERGY
1.1 INTRODUCTION
For a definition of energy we can keep it simple as we say energy is a physical unit. The physical unit of energy is Joule. But energy is a very complex phenomenon. To this very day no man can explain exactly the energy facts and figures. Energy has clearly a connection with the capability of doing work. If an object can do work, we say that the object possesses energy. For that reason energy is often said to be the ability to do work.
But the concept of energy is too complex to be described fully in such a brief statement.
There are many kinds of energy and one of them is Solar energy.
Here we could say Energy is the power from sources such as sun irradiance to make electricity or heat as another alternative energy source.
Remark that we don’t say that we make new energy. We convert energy from one form in another. An additional problem is de storage of the new converted energy, but that’s another story.
Energy has always exists and is hidden in all sorts of storage phenomenon. Sometimes in a, more or less, abstract shape. E.g. in case of placing an object, we have an amount of potential energy depending on level difference. Another example is de storage of fossil energy stored in earth layers. The energy is stored million years ago.
Energy can neither be created nor destroyed. When a loss occurs in one form of energy, an equal increase occurs in other forms of energy.
But in an energy conversion process not all conversion items are in the wanted energy form.
But the sum of the energy amount in all energy sources of a system before an after conversion remains constant. We call that the energy balance.
There are many forms of energy. Coal, oil, gasoline, and al other fuels possess energy because they can be burned (a chemical reaction) to do work. Examples of energy forms are:
- Chemical energy
- Electric energy
- Thermal energy
- Mechanical energy
- Nuclear energy
- Magnetic energy
- Irradiance energy
- Potential energy
- Gravity energy
- Kinetic energy
- Wind energy
- Etc.
When there is an energy- balanced situation in a system, the system feels “happy”. In this situation the system uses a minimum of energy. The system will remain in this steady state and will resist against disturbing actions. State changes always ask for energy and the system then enters the resistance. There is always a reaction force which is opposite to an action force trying to prevent the results of an action.
By now it’s clear the energy on earth we have to our disposal, always finds its origin in sun irradiance. Sun irradiance energy and its derivates in the past million years however are stored for indirect use. However the amount of fossil energy is limited.
Sooner or later the bottle is empty. A lot of the fossil energy is converted yet and only a minority of the original amount is left. Maybe it’s better to say that the far most “is spilled” for unimportant things. There is a much better use of fossil energy then heat production. E.g. Oil is the source for a lot of very useful materials with remarkable specifications. So it’s advisable to allocate fossil sources to the production for that stuff. In the near future there is a lack of fossil materials. Therefore we have to find other means satisfying our energy needs.
A very good solution will be found in the use of sunlight irradiance as a direct energy source.
Besides the exploration of renewable energy sources we have to look for a more efficient use of energy. Low consumption devices with great overall efficiency. No old fashioned electric glow lamp with a light output efficiency of 10 percent but highly efficient LED lights with a very low energy consumption. No energy swallowing CRT screen but a TFT flat screen. Etc.
The start of above-mentioned developments was not without problems. Not anyone was highly enthusiastic. However things are strongly improved. Now anybody has accepted the new arrivals as normal. Now various revolutionary new developments announce their selves. Not anyone is highly enthusiastic, but just in time things will strongly accepted. The most important new technology is based on the direct utilizing of sunlight as an infinitely energy source. It’s called a direct source because energy is directly utilized without intervention of long-term energy storage in e.g. plants or fossils.
In the following the direct utilizing of sun irradiance will be emphasized. There will be special attention for PV installations. We start with necessary essential background information
1.2 THE DIRECT USE OF SOLAR ENERGY
Using direct solar energy there are several elementary principles in gaining energy. Energy gaining principles are distinguished in:
1. concentrated solar power (CSP)
2. Solar thermal energy (passive solar techniques)
3. Photovoltaic solar energy (active solar techniques)
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Concentrating Solar Power (CSP) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated heat is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exists; the most developed are the parabolic trough, the concentrating linear fresnel reflector, the Stirling dish and the solar power tower. Various techniques are used to track the Sun and focus light. In all of these systems a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage. E.g. Water is heated to steam which is used to drive turbines. The turbines are coupled to electromechanical generators.
These generators produce electricity as in a conventional power station.
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These are solutions based on thermal solar collectors or PV solar collectors. A number of basic elements are grouped together in so called solar panels. Those panels are irradiated by sunlight in the energy conversion process resulting in the wanted form of energy. Solar modules produce heat or electricity.
What is the difference between thermal solar collectors and PV collectors with solar cells?
A thermal solar collector produces heat which can be used for the heating of a domestic hot water boiler or for a swimming pool. On other hand solar cells produce electricity as a direct result of the sun irradiance. Speaking about solar modules there can be a relation between both heat and electricity.
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1.2.1 Thermal Solar Collectors
There are several kinds of thermal solar collectors from simple to very sophisticated. The following types are most usual:
• Flat-plate collectors
• Evacuated flat-plate collectors
• Evacuated tube collectors
The flat- plate collectors and evacuated tube collectors always need a storage tank. The collector water volume itself is very low and water temperatures inside the collector can reach much more than 100°C if the water is not continuously replaced. The collector efficiency also decreases significantly at high operating temperatures.
Therefore, all solar collector systems for domestic water heating need a hot water storage tank to store larger quantities of absorbed collector heat and to provide a hot water supply during bad weather periods and darkness. In contrast, for solar swimming pool systems, the pool water itself is the storage.
1.2.2 Flat-plate collectors
Solar domestic water heating systems are used in many countries. The most common collectors for today are flat-plate collectors. These often consist of three components:
• The transparent cover
• The collector housing
• The absorber.
An absorber is mounted inside the flat-plate collector housing. This absorber converts sunlight to heat and transfers it to water in tubes, which passes through the system. The collector housing is highly insulated on the back and sides to keep heat losses to a minimum. But there are still losses. A glass pane covers the collector and avoids most of the convection losses. It reduces heat radiation from the absorber to the environment in the same way as a greenhouse. On the contrary, the glass also reflects a small part of the sunlight. That part can no longer reach the absorber.
In the figure opposite the kernel of a flat-plate collector is showed.
The figure below gives an impression of a complete advanced system
The foto photo below shows build in solar collectors on the roof of a swimming pool
1.2.3 Evacuated plate collectors
There is no big difference between the evacuated plate collector and the former plate collector. If the system is used in regions with a danger of frost, the heat losses cool both the collector and storage. Ultimately, they can freeze and be damaged. A way needs to be found for achieving significant reduction of the collector heat losses. Better insulation on the back is not a problem. The real problem is heat losses through the front cover. Sunlight must pass through the front with low absorption and reflection losses. The cover must therefore be transparent, and yet this leads to large heat losses through the cover. A vacuum can reduce the heat losses, but often not as much as is necessary. New so-called transparent insulation materials (TIM) brought a solution to these problems (Lien et al, 1997; Manz et al, 1997). Those materials have a slightly lower transmittance compared to low-iron solar safety glass. However, the heat transition coefficient is significantly lower so that the heat losses are reduced to acceptable levels. Extra measures can be taken in winter by a small frost protection heater in the collector.
1.2.4 Evacuated tube collectors
These are high efficiency thermal collectors using heat pipes which are mounted in double-walled glass tubes. There is a vacuum between the glass walls of the tube. The high vacuum inside the closed glass tube of the evacuated tube collector is easier to maintain over a long period of time than that in an evacuated flat- plate collector. Glass tubes can resist the ambient air pressure due to their shape so that no supports are necessary between the back and front sides. A metal absorber sheet with a heat pipe in the middle is embedded inside a closed glass tube with a diameter of a few centimetres. A temperature sensitive working medium such as methanol is used inside the heat pipe. The sun heats up and vaporizes this heat pipe fluid. The vapour rises to the condenser and heat exchanger at the end of the heat pipe. There, the vapour condenses and transfers the heat to the heat carrier of the solar cycle. The condensed heat pipe fluid flows back to the bottom of the heat pipe where the sun starts heating it again. The following figures give an impression.
The figures above show the kernel of a high vacuum tube solar collector and a detail of the top end of a heat pipe. Vacuum collectors exist with various dimensions. The length is usually about 2 meters. The width depends of the number of heat pipes which are mounted I a rack. The rack width starts at 10 pipes. The rack in our example contains 24 double-walled tubes of borosilicate glass. The glass tubes are comparable with a conventional double-walled thermos flask. The vacuum between the glass walls forms a highly thermo isolating space. There is hardly heat transport via the glass walls of the tubes. So the absorbed sun radiance energy passes through the copper heat pipes and not via the glass walls.
In order to improve the efficiency of the system the inside of the glass tubes is provided with a special absorbing coating. Finally the heat energy is collected in the top of a heat pipe.
In return those tops are embedded in a copper collector tube. This tube is called the header. The collector act as an heat exchanger. The gained heat energy is given to the water which flows through the header. The header on return is connected to the hot water storage. The figure below gives extra information.
The combination hot water storage and underfloor heating is ideal. This type of collector reacts quickly on temperature changes and is highly efficient. Also in winter those collectors are operational with minimum sunlight due to the high temperatures of the heat pipes.
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1.2.5 Photovoltaic solar energy
Light is a form of electromagnetic radiation. Electromagnetism is a manifestation of energy and as yet partially a mysterious matter. A manifestation of electromagnetic radiation is the existence of photons. Light consist of photons. A photon is created within an atom as an electron changes from a higher energy status to a lower energy status. During this process a photon is emitted. So a change of the energy state of an electron inside an atom creates a photon which generates light. The reverse process is also possible. The photons in light are capable to influence the energy state of an electron inside an atom during irradiation. A moving electron is an electric current. Moving electrons are responsible for moving electric charge which is electric current. The electric charge in a certain point exists as an electric potential. The potential difference between two terminals is a voltage.
The reverse process is also possible. A voltage can cause an electric current through a load connected to the voltage terminals.
A material or device that is capable of converting the energy contained in photons of light into an electrical voltage and current is said to be photovoltaic.
So it’s possible to create an electric voltage with the aid of light. This is a photovoltaic phenomenon and is essential for photovoltaic systems. The basic unit of a photovoltaic system is the photovoltaic cell.
1.2.6 PV cell
Photovoltaic cells or PV cells are electrical devices about 1/100th of an inch thick that convert sunlight into a direct current electricity through the photovoltaic effect.
They don’t consume fuel and have a life span of at least 25 years. In the near future PV cells produce a significant amount of our electric energy. The driving force to power photovoltaic systems comes from the sun. It‘s interesting to note that the surface of the earth receives something like 6000 times as much solar energy as our total energy demand.
That sounds very hopeful!
The physical background of a PV cell lays in the semiconductor technology and is rather complex. It’s not necessary to know all the facts and figures for the practical use of the PV cell. Normally one will work with simplified models.
Next a summary of a complex story:
At this moment a photovoltaic cell or PV cell is the most commonly used solar cell in practice. The kernel of a normal photovoltaic cell is a piece of semiconductor material with a barrier between a region with P-type and a region with N-type silicon. In fact it’s one big diode. Just as with a normal diode current can only flow in one direction. So a solar cell is polarity sensitive. When sunlight (electromagnetic radiation) irradiates the solar cell, electrons become free from their atoms. Those free electrons moves in the plus direction because there is a lack of negative charged electrons. The sum of all moving electrons is the electric current through the solar cel. The picture below shows a normal solar cell.
To be practical about the use of solar cells, solar panels are used. Solar cells are grouped together in a well arranged manner to a so called solar module. A module is an assembly of photovoltaic cells wired in series or series/parallel, to produce a desired voltage and current. A solar module delivers direct current which immediately can be used by connected DC devices. It’s also possible to store the energy in batteries for delayed use. When AC is wanted in stead of DC then an inverter is necessary. In the case of a surplus of gained solar energy grid delivery is possible via that inverter.
Energy storage in batteries is chiefly useful as no main grid exists. Then the solar system acts a stand alone or an autonomous system with a battery-storage. In that case the system can also be used by night for e.g. light beacons, street lights, etc.
1.2.7 PV Panel and PV array
PV cells are encapsulated within the module framework to protect them from weather and other environmental factors. Modules are available in a variety of sizes and shapes. Usually, they are flat panels that produce anywhere from 5 watts to around 300 watts. Most cells in a PV panel produce approximately one-half of a volt. Therefore a number of cells are connected in series in order to reach the higher desired voltage. So, a string of 36 cells delivers 18 volts under standard test conditions (STC) and a nominal voltage of 12 volts. The current output of the module is dictated by the amount of surface area and the cell efficiency of an individual cell in the module. Normally is one single module not sufficient to satisfy the energy need. Hence more modules are encapsulated within a panel frame. The terms “panel” and “module” are often used interchangeable. Our definition of a solar panel:
A PV panel is one or more modules wired together. A PV array is a group of modules or panels wired together to produce a desired voltage and current and fastened to a mounting structure. Solar energy can be used everywhere is the ultimate energy solution. Solar energy is also extremely suitable for space travel.
The picture below shows an example of a solar plant.
Further on in this explanation we’ll focus on the assembly of a solar panel, but first we will have a closer look to some technological aspects of a solar cell being the building brick of a solar panel.
