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Solar energy can be harvested via several methods. It can mainly be broken down to 2 main methods
of harvesting energy
Photovoltaic solar panels
Thermal energy via solar collectors
Both this method will be reviewed extensively.
Thermal energy via solar collectors
Figure 1: Rankine Cycle
Solar thermal energy basically applies the Rankine cycle principle. Heat, represented as Qwe in the
diagram is added into the boiler to increase the temperature of the liquid/vapour. As water in the
boiler increases in temperature with the heat in, steam is produced. Steam then travels to the
turbine in order to generate work. The heat, Qwe in this system is supplied via thermal energy from
the solar panels. For smaller application, such as central water heating for residential homes, the
water in the boiler is directly used as hot water.
The heat is captured via solar collectors that are usually placed on rooftop of buildings for small scale
power generation or in an open area of a solar farm. Solar energy collectors are a type of heat
exchangers that converts solar radiation energy and converts it into heat. The heat is then
transferred to a flowing fluid through the collector. The heat in the fluid is then sent to either a
thermal energy storage tank, space conditioning equipment or used directly as hot water. The 3
main types of solar collectors in the market currently are:
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Flat-plate collectors (FPC)
Stationary compound parabolic collectors (CPC)
Evacuated tube collectors (ETC)
Flat Plate Collectors
Figure 2: Flat Plate Collectors
When solar radiation passes through the transparent cover, the absorber plate which has high
absorptivity absorbs a large portion of the energy and transfers the heat to the fluids in the flow
pipe. The underside of the absorber plate is insulated to reduce heat loss via conduction. The header
tube is a larger tube connected at the end of the liquid flow tubes. Flat plate collectors are cheap
and easy to manufacture. The components of a flat plate collector are:
Covers consist of one or more sheets of glass that transmits radiation
Fluid tubes that conduct and allows the fluids to flow from the inlet to the outlet
Absorber plate is where the tubes, fins and passages are attached. The plate is usually
coated with high-absorbing and low emitting layer.
Headers and manifolds where pipes discharges the fluids
Insulation is important to reduce the heat loss via conduction
Figure 3: Flat Plate collector
Compound Parabolic collectors (CPC)
CPC is capable of reflecting all the incident radiation to the absorber in a very wide angle. A trough
with 2 section of parabola facing each other would reduce the need to change the orientation to
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accommodate the angle of incidence from the solar rays. Multiple internal reflections can also be
used to enable any radiation entering the aperture within the collectorβs acceptance angle to be
directed to the absorber surface. The absorber is commonly a tube that contains flowing fluid.
Evacuated Tube collectors (ETC)
These types of collectors are designed in such a way that it is still usable during cold, cloudy and
windy days. They consist of heat pipes inside a vacuum-sealed tube. Several tubes are then
connected to the same manifold.
ETC shows that a combination of selective surface and effective convection suppressor can produce
good performance at high temperature. The vacuum envelope acts as an insulator and reduces the
heat loss via convection and conduction, subsequently increasing the usable temperature. Their
efficiency is also higher at low incidence angles, which means it can be used for a daylong operation.
The tube collectors in the ETC uses liquid-vapour phase change materials to perform heat transfer.
The sealed copper heat pipes are attached to a black copper fin that covers the absorber plate. The
ETC operates on a simple process cycle:
The fluid in the heat pipes (Usually methanol) undergoes an evaporating-condensing cycle.
Solar energy then heats this evaporated liquid and the vapour travels to the heat sink,
where it condenses and releases latent heat.
Finally, the condensed fluid then returns to the solar collector and the process is repeated.
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Figure 4: ETC collector
Sun Tracking Concentrating Collectors
The temperature of the collector can be increased if the area is reduced. By concentrating the solar
radiation on a small area, a higher temperature can be achieved. This further increases the thermal
efficiency of the system as the heat loss is reduced. There are several types of concentrating
collectors.
Parabolic Trough Collectors
One of the most efficient solar systems, the PTC can effectively produce heat from 50Β°C to 200Β°C. A
reflective sheet of material is bent into a parabolic shape to make up the collector. Then, a black
metal tube covered with glass is placed along the focal line of the receiver. Glass tube is used to
minimize the heat loss. When the parabola is pointed towards the sun, the solar rays are reflected
onto the receiver tube. The tracking system ensures that the collectors are always pointed in the
best direction of the sun.
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Figure 5: Parabolic Trough Collectors
[12]
Fresnel Collectors
There are two types of Fresnel collectors, The Fresnel lens collector (FLC) and the linear Fresnel
reflector (LFR). The Fresnel lens collector is made from plastic and shaped to focus the solar rays to a
single point on the receiver. The linear Fresnel collector uses an array of linear mirror strips and
concentrates the light onto the receiver. This is best explained using the diagram.
Figure 6: Fresnel Collectors
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Figure 7: Fresnel Reflector
Parabolic Dish Reflectors (PDR)
A PDR is a point focus collector that tracks the sun and concentrates the solar energy onto a receiver
that is located at the focal point of the dish. The dish fully tracks the sun to reflect the beam into the
thermal receiver. The tracking mechanism in this type of reflector is very advanced. The receiver
absorbs the solar radiation and converts it into thermal energy. The parabolic reflectors can achieve
temperatures of up to 1500Β°C. The fact that they are always pointed to the direction of the sun
makes it the most efficient system.
Figure 8: Parabolic Dish Reflectors
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Small scale solar generation
Solar power is also used in small scale, such as heating of water heater or space heating for
residential usage. A solar water heater uses concept of heat pipes that runs to the boilers. The heat
from the sun is collected via the solar collectors, and it is used to heat up the water in the pipes. Hot
water is then stored in the tank, as shown in the diagram. [13]
Figure 9: Solar generation for residential usage
Thermodynamic analysis and physics of solar collectors
Considering the radiation transfer between the glass cover and the absorber plate of a flat panel
collector, the collector is capable of capturing both incident solar flux that consists of direct radiation
and diffuse radiation. The concentrating collectors however can only utilize direct radiation if the
concentration ratio is greater than 10.
From the figure above, we can see the radiation being transferred from the glass cover to the
absorber plate. By using the isotropic model, the equation for the absorbed radiation, S, is as
follows:
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π = πΌπ΅π π΅(ππΌ)π΅ + πΌπ·(ππΌ)π· [1 + πππ π½
2] + ππΊ(πΌπ΅ + πΌπ·)(ππΌ)πΊ [
1 β πππ π½
2]
The energy that is not absorbed by the flowing fluid is known as the energy lost. This is unavoidable
as heat loss to the environment happens in any thermal system. Thermal loss can happen from
conduction, convection or radiation. The equation for the energy loss is;
ππππ π = (ππ β ππ)/π πΏ
Or can also be rewritten using the overall heat loss coefficient;
ππππ π = ππΏπ΄πΆ(ππ β ππ)
ππΏ = βπππ‘ πππ π πππππππππππ‘
ππ = ππππ‘π π‘πππππππ‘π’ππ
As the thermal solar is based on the Rankine cycle, the thermal efficiency can be deduced using the
Carnot efficiency. [15]
π =ππππ‘
πππ
Photovoltaic Solar Panels
There are certain materials that show the property known as photoelectric effect. This effect causes
them to absorb photons from light and release electrons. As the electrons flow freely and are
captured, electric current will be produced. This is how PV converts light energy into electricity. [16]
When a photon hits a PV cell, they will either be absorbed, reflected or pass through. The photons
that are absorbed by the cells will generate electricity. The energy of the photon is transferred to an
electron in an atom of the cell. The electron is now full of energy and is able to escape from its
original position to produce electrical current.
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Figure 10: Inside a PV cell
In order to induce an electrical field, the βpβ type and βnβ type semiconductors are place together.
The n-type semiconductors have excess electrons and the p-type semiconductors have excess holes.
But placing them together, an electric field is created. The electric field will allow the n-type material
to flow to the holes in the p-type material. The flow of electron enables the two semiconductors to
act as a battery, creating an electric field at the junction of two semiconductors. [17]
Figure 11: P and N layer of a PV cell
In order to allow the photons to be absorbed in the p-layer, it is imperative to tune the layer to the
properties of the photons. This will increase the absorption quantity. It is also possible that the
electrons might collide with one another and combining before they escape the cell. Hence,
materials are designed so the electrons escape and travel as close as possible to the junction. This
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can improve the overall efficiency of the PV cell. By maximizing absorption and minimizing reflection
and recombination, the efficiency can be maximized as well.
Photovoltaic efficiency
The electricity generated by a PV solar panel can be calculated from the formula [18];
πΈ = π΄πΆππ»ππ
π = π ππππ πππππ π¦ππππ
π» = ππ£πππππ ππππ’ππ π ππππ ππππππ‘πππ ππ π‘πππ‘ππ ππππππ
ππ = πππππππππππ πππ‘ππ
The efficiency is measured by the ability of the panel to absorb radiation and convert it to electricity.
The maximum efficiency equation is defined as;
π =ππππ₯
(πΈπ π€ β π΄πΆ)
The maximum power is the rated power as given by the solar manufacturer and the πΈπ π€ is the
incident radiation flux.
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State the major factors that enhances the efficiency of a Solar Energy System for the purposes of generating electrical energy Type of collector
The type of collector greatly influences the efficiency of the system. This can be highly associated
with cost as well. Collectors such as the parabolic dish reflectors are very efficient. This is because
the light energy is focused to a single spot. The two axis tracking mechanism also ensures that the
collector maximizes the solar radiation availability by always facing the direction of the sun.
However, this would result in a very high cost as many parts will be required. Cheaper collectors,
such as the flat panel collectors have lower efficiency, but very much lower in cost as well.
Size of collector
Based on the equation of energy below
πΈ = π΄πΆππ»ππ
It can be observed that the area of the collector influences the energy in the system. A large
collector size will provide more energy. However, if the collector size is too large, the heat loss can
also be high, resulting in lower efficiency. The key is to calculate the appropriate size of collector for
the application.
Heat loss
The efficiency of the system can be improved by reducing the heat loss in the system. Based on the
Carnot efficiency formula;
π =ππππ‘
πππ
The efficiency can be improved by reducing the heat loss in the system. This can be done by
concentrating insulation on the collectors. In a flat panel collector, the underside of the absorber
plate and both its side is well insulated with insulation material. This is to prevent heat loss due to
conduction. Next, in order to reduce heat loss via convection, a transparent cover is used.
Otherwise, heat loss via convection will take place through the stagnant air layer between the
absorber plate and the glass. Next, the transparent glass is used to reduce the heat loss via radiation.
By minimizing the 3 heat loss potential, the overall efficiency of the system can be increased.
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Solar irradiation level
The basis of solar energy is first and foremost, the availability of the energy itself. In some areas, the
solar energy availability is higher compared to others. In order to build a solar farm, a suitable
geographical location needs to be considered. Meteonorm database of Meteotest has the data
required for global solar irradiation level from 1981 to 2000. These values can be used to determine
the most appropriate location for a solar farm.
Incidence angle
The effective incidence angle will provide maximum amount of radiation on the collectors. This will
determine the tilt and the slope of the solar collector. There are two types of incidence angle.
Effective incidence angle for diffuse radiation from the sky, ππ,π· and the ground reflected radiation,
ππ,πΊ. The effective incidence angle can be calculated from;
ππ,π· = 59.68 β 0.1388π½ + 0.001497π½2
ππ,πΊ . = 90 β 0.5788Ξ² + 0.002693π½2
Ξ² is the collector slope angle in degrees. Maximizing this value will give the best incidence angles.
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Multijunction cells
The limitation of the PV cell is that only photons whose energy is equal or greater than the band gap
has the ability to free an electron from the circuit. However, this can be overcome by using
multijunction. Multijunction cells are where two or more difference cells with different band gap are
used to produce voltage via multiple junctions. This method is more efficient as it can convert more
energy spectrum of light to electricity. Most multijunction cells are focusing on gallium arsenide. The
diagram below shows how a multijunction PV cell is used.
Figure 12: Multijunction cell
Figure below shows how light energy in 3 different spectrums is absorbed separately by the multiple
junctions.
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