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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 collector’s 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 50°C to 200°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 1500°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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𝑆 = 𝐼𝐵𝑅𝐵(𝜏𝛼)𝐵 + 𝐼𝐷(𝜏𝛼)𝐷 [1 + 𝑐𝑜𝑠𝛽

2] + 𝜌𝐺(𝐼𝐵 + 𝐼𝐷)(𝜏𝛼)𝐺 [

1 − 𝑐𝑜𝑠𝛽

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;

𝑄𝑙𝑜𝑠𝑠 = (𝑇𝑝 − 𝑇𝑎)/𝑅𝐿

Or can also be rewritten using the overall heat loss coefficient;

𝑄𝑙𝑜𝑠𝑠 = 𝑈𝐿𝐴𝐶(𝑇𝑝 − 𝑇𝑎)

𝑈𝐿 = ℎ𝑒𝑎𝑡 𝑙𝑜𝑠𝑠 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡

𝑇𝑝 = 𝑝𝑙𝑎𝑡𝑒 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒

As the thermal solar is based on the Rankine cycle, the thermal efficiency can be deduced using the

Carnot efficiency. [15]

𝜂 =𝑊𝑛𝑒𝑡

𝑄𝑖𝑛

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 “p” type and “n” 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];

𝐸 = 𝐴𝐶𝑟𝐻𝑃𝑟

𝑟 = 𝑠𝑜𝑙𝑎𝑟 𝑝𝑎𝑛𝑒𝑙 𝑦𝑖𝑒𝑙𝑑

𝐻 = 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑎𝑛𝑛𝑢𝑎𝑙 𝑠𝑜𝑙𝑎𝑟 𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑜𝑛 𝑜𝑛 𝑡𝑖𝑙𝑡𝑒𝑑 𝑝𝑎𝑛𝑒𝑙𝑠

𝑃𝑅 = 𝑃𝑒𝑟𝑓𝑜𝑟𝑚𝑎𝑛𝑐𝑒 𝑟𝑎𝑡𝑖𝑜

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;

𝜂 =𝑃𝑚𝑎𝑥

(𝐸𝑠𝑤 ∗ 𝐴𝐶)

The maximum power is the rated power as given by the solar manufacturer and the 𝐸𝑠𝑤 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

𝐸 = 𝐴𝐶𝑟𝐻𝑃𝑟

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;

𝜂 =𝑊𝑛𝑒𝑡

𝑄𝑖𝑛

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, 𝜃𝑒,𝐷 and the ground reflected radiation,

𝜃𝑒,𝐺. The effective incidence angle can be calculated from;

𝜃𝑒,𝐷 = 59.68 − 0.1388𝛽 + 0.001497𝛽2

𝜃𝑒,𝐺 . = 90 − 0.5788β + 0.002693𝛽2

β 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.