Chapter 4 ̶ Solar Energy

Chapter 4 ̶ Solar Energy

Solar energy is the largest available energy on earth. The life on the earth is due to Sun, directly or indirectly. All the plants make their food through photo-synthesis in presence of Sunlight. In turn this food is eaten by humans and animals. For harnessing solar energy effectively and efficiently various methods have been proposed. In the following these methods will be discussed. Solar energy may be harnessed in terms of heat, warm water, electricity etc. About half of the energy in the world is utilized as heat. Heat is required in industry for chemical processes and in residential sector for heating the space and as warm water.

4.1 Solar Ponds

Solar pond contains water or any other liquid, which absorbs solar radiation to heat the water/liquid present in the pond. The thermal energy stored in the pond in this manner is exchanged with heat exchanger to transfer heat energy to other place for specific application. This thermal energy can be used for heating the space and building, in industries for specific reaction or for electricity production. The pond may be natural or artificial. The size of the pond ranges from few hundred sq metre to few thousand sq metre, depending on the requirement and application. The temperature of water/liquid rises in the pond after absorbing solar radiation. Due to convection currents and conduction process this heat reaches at the surface of pond. From there heat losses through evaporation by the surface. To prevent the thermal energy and thus to store heat inside the solar pond special mechanism is adopted. On this basis it can be broadly divided into two category; Convective solar ponds and non-convective solar ponds.

4.1.1 Convective Solar Ponds

This is a normal pond containing water/liquid of homogeneous density. The surface is covered with transparent sheet to prevent the loss of heat through evaporation. The depth of such a pond is not large. It is a shallow pond with depth of 4 to 15 cm.

4.1.2 Non-Convective Solar Ponds

These are further divided into two category; Salt-gradient solar ponds and Viscosity stabilised solar ponds.

4.1.2.1 Salt-Gradient Solar Ponds

            In this pond, salt is dissolved in water, such that it divides the layers of ponds in three different zones. The density of salt water increases with depth. The upper zone is the layer of fresh water. Middle zone maintains the salt-gradient. The lowest layers form storage zone, which has saturated salt concentration. When solar radiation is absorbed, the temperature of water increases. As the density of water layer with high temperature is less, it rises up due to convection currents.  In absence of salt gradient, this heat goes to the surface, and is lost because of evaporation. In the salt̶ gradient solar pond, the density of hot water is decreased, but it is still higher than the layer above it due to presence of salt. In storage zone the molecules are so heavy because of saturated solution that they cannot move in upward direction. Hence the heat is preserved in the storage zone. The temperature may rise up to 90° C to 100°C in the storage zone. As one moves towards the surface the temperature is decreased.  If the surface is covered with transparent sheet, it will further prevent the heat loss and maintain the high temperature in solar pond. However, some of the heat losses due to conduction and salt&gradient is disturbed in the pond. To maintain the salt&gradient, salt is added and fresh water is replaced at the upper layer. Common salts which are utilised in the solar pond is sodium chloride (NaCl) and  magnesium chloride (MgCl₂). Additional alternatives are potassium chloride (KCl), Calcium chloride (CaCl₂), Ammonium nitrate (NH₄NO₃), Potassium nitrate (KNO₃), Borax (Na₂B₄O₇) and Sodium sulphate (Na₂SO₄), which are widely available from the waste product of flue gases produced from coal-fired power plants.       

4.1.2.2 Viscosity&stabilised Solar Ponds

            In this method instead of salt, gel is mixed with water in an amount which can prevent precipitation. It is not much common.

4.1.3 Factors affecting the performance of solar pond

·       Wind free condition: Climate must be wind free to prevent the heat loss from evaporation.

·       Dust free atmosphere: Dust dissolved in water will stop the solar radiation to penetrate into  the depth of pond.

·       Water clarity: Water of the pond must be free from external particles and algae.

·       Ground water level: Water level must be deep, so that heat cannot loss from the ground of storage zone.

·       High solar radiation: The intensity of solar radiation must be high.

·       Replacement of fresh water: There must be another nearby pond where the water can be replaced to maintain the salt&gradient and fresh water can be added to the top layer.

·       Application Site: The site of application should be close to solar pond.

4.1.4 Removal of heat from the solar pond

            This is done by heat exchanger. Heat may be exchanged through submerged heat exchanger and the brine withdrawal method.

4.1.4.1 Submerged Heat Exchanger

In this method a pipe containing liquid (internal heat exchanger) is inserted inside the storage zone. The liquid in the pipe becomes hot due to high temperature of storage zone. Then this hot liquid is transferred to another place of application, where it is again submerged in the water or liquid (external heat exchanger) which has to be heated.

4.1.4.2 Brine Withdrawal Method

In this method hot brine (salt water) is   pumped directly from the storage zone by using an extraction diffuser. The heat from the brine is extracted and the brine is then returned to the bottom of the pond by using a return diffuser. This method of heat exchange is considered more effective than the submerged heat exchanger. For better efficiency and to maintain salt&gradient the suction diffuser is placed just below the salt&gradient zone. The return stream is injected on opposite side of extraction diffuser and at the bottom of storage zone.

4.2 Solar Water Heater

In solar water heater, there is a collector of black painted metal, on which metal pipes containing fluid is attached. Solar radiation is absorbed by collector and metal pipes. Heat is also transferred from collector to the metal pipe. The whole assembly is covered with glass to minimize the heat loss from the system. The fluid present in the pipes get heated. This heat is exchanged in heat exchanger, where the pipe containing the hot fluid is submerged in tank containing another liquid at low temperature. In this way heat is exchanged from solar collector to the application point. In the solar water heater, there are three parts; collector, heat exchanger and storage tank of hot water/liquid. Amount of hot water supply depends on the size of collector array, the fluid used in the pipe and intensity of solar radiation. The temperature may be reached up to 100°C.

4.2.1 Type of solar water heater

When the fluid used in the collector is same as the fluid for utility, it is known as direct or open loop. If the fluid in application is different from the fluid used in collector and heat exchanger is used, it is called indirect or close loop.

The solar water heater may also be categorized in a different way. If no pump is used for transportation of heat, it is called passive system. Heat is transferred through natural convection method. When pump is used for circulation of hot fluid, it is known as active method. In passive method flow rate is low and in active method flow rate can be maintained at better rate.

4.2.2 Types of collectors

Collector may be of three varieties; covered, uncovered and vacuum. If the temperature difference between collector and surrounding is more, the transparent cover is necessary to stop heat loss to the surrounding. However, transparent covered collector receives less radiation in comparison to uncovered collector due to reflection loss. Vacuum covering stops the heat loss but is expensive. It depends on the application that which type of collector is required. Vacuum collector may provide high temperature.

Collectors may be flat plate type or concentrating. In flat plate collector solar radiation is absorbed directly from the sun. In concentrating system, an optical device is placed in between the sun and collector, which concentrate the solar radiation on the collector. In this method size of collector is small, so heat loss is also less. High temperature is achieved in comparison to flat plate collector. Sun tracking system is necessary in concentrating collector, which increases the cost of the system.

4.2.3 Array of collectors

Collectors may further be attached with each other in an array. This array may be parallel or series. In parallel array collector, the input and output temperature of each collector is same. In series array, the output of first collector is the input for second. Therefore in series array high temperature may be achieved. The combination of both parallel and series may also be used. The choice of array depends on the application and the requirement of temperature.

4.3 Solar Distillation

Solar distillation is a system to get fresh distilled water from salty water (brine). For this a shallow basin is constructed with black inner surface and covered with sloping air-tight plastic or glass sheet. The basin is filled with brine. The solar radiation heats the water to evaporate and the vapours condense on the slope and get collected in a storage tank.

4.4 Solar Cooker

Solar cookers are the devices, in which foods are cooked by the heat of solar radiation. In solar cookers foods are kept in pots and pots receive sunlight, such that the temperature of the food increases and it is cooked without the supply of any extra heat. On the basis of the construction of solar cooker it is categorized in three varieties.

4.4.1 Solar Box Cookers

These are the simplest type of solar cookers. There are two boxes, one of diminished size from another. Smaller box is placed inside and the gap between the two is filled with insulating material. Outer box may be of insulating material like wood, so that heat can be retained inside the box. Black paint may be applied on the inner side to absorb more heat. The box is covered with transparent glass or plastic. It will insure the minimum loss of heat from the cooker. A reflector of aluminium may be attached to it to concentrate the solar radiation on the pot containing the food. In box type solar cooker, temperature may rise up to 140°C.

       

4.4.2 Panel Type of Solar Cookers

In this type of solar cookers, four panels are used and the black painted pot containing the food is placed at the center. The food pot is kept inside heat resistant plastic bag or glass bowl. The panels are covered with reflecting sheets like aluminum. Thus panels reflect the sun rays on the food and increase the temperature of the food. This is not a very good system as it is open and may not work during wind blowing season.

4.4.3 Parabolic solar Cookers

In this system a reflector of parabolic shape is used and food vessel is kept at the focal point. The shape of the reflector must be very precise so that it can concentrate the solar radiation at the food. The larger the panel, the more heat it will absorb. In this type of solar cooker temperature of 200°C to 300°C may be reached. It is the fastest cooking solar cooker. It is suitable for baking, roasting and grilling.

 

4.5 Solar Greenhouse

Generally greenhouses are built to maintain the climate inside the greenhouse to grow vegetables. The atmosphere outside the greenhouse may not be suitable for a particular type of vegetables. Therefore an artificial climate is produced inside the greenhouse for growing a specific vegetable or any other plant. In an ordinary or conventional greenhouse a frame is created on the ground which is covered with plastic sheet. It traps the solar heat and does not allow it to escape.  There is arrangement of air ventilation inside the greenhouse. Temperature is maintained artificially inside the greenhouse.

Although conventional greenhouses are inexpensive, they are not safe in extreme weather condition like, snow, wind, intense sun and frost zone. Hence another design is required to overcome the difficult weather conditions. The best option is specifically designed solar greenhouses. Solar Greenhouse is based on following principles:

1.               Orientation of greenhouse should be in such a direction that it can collect most of the solar heat.

2.               The incident solar radiation might be stored as heat inside the greenhouse.

3.               Insulation should be done from all other areas.

4.               There should be minimum heat loss through leakage.

5.               There should be mechanism of maximizing the natural ventilation.

4.5.1 Orintation

The direction of greenhouse is such that, the axis of roof is in east-west direction. Both side of roof, north and south face solar radiation. South roof receives many times more radiation than north roof. Roof and covering sheet may be of any material given below:

·                 Glass (regular or tempered)

·                 Polyethylene sheeting (single or double layer)

·                 Acrylic

·                 Polycarbonate (single, double, triple wall)

·                 Fiberglass

4.5.2 Heat Storage

For heat storage inside the greenhouse, dark non-reflective water tanks and soil is used. Heat is stored in shallow soil depth and in deep soil depth. Thus the heat is stored when sun is shining and it is emitted when sun is not shining in the night. For active solar heating systems, solar liquid collectors, fan convectors and solar air collectors may be used.

4.5.3 Insulation

Walls, floor and foundation must be insulated. North, east and west walls should be well insulated. Insulating curtains may also be used. Greenhouse is sealed to prevent air infiltration.

                  

  4.6  Solar Air-conditioning and Refrigeration

Solar energy can also be used for cooling space inside the building i.e., air-conditioning and for refrigeration purposes. There are three modes of solar cooling.

4.6.1       Evaporative Cooling

It is a passive cooling system and used in hot and dry climate. Its principle is that when hot air is used to evaporate water, the air itself becomes cool. Thus it cools the space inside the building.

4.6.2       Absorption cooling system

This system has four parts:

i.                 Generator- The generator contains a solution mixture of absorbent and refrigerant. Suitable mixtures are (i) NH₃-H₂O, where NH₃ is the working fluid and (ii) LiBr-H₂O, where H₂O is working fluid. This mixture in the generator is heated with solar energy derived from flat plate collector. Because of this heat, refrigerant vapour is boiled-off at a high pressure and flows into condenser.

ii.               Condenser- In condenser the refrigerant vapours condense rejecting heat and becomes liquid at high pressure. After this, refrigerant liquid passes through expansion valve and goes into evaporator.

iii.             Evaporator- This refrigerant liquid evaporates in evaporator and cools the space.

iv.             Absorber- This low pressure vapour of refrigerant goes into absorber where it is absorbed in the solution mixture taken from generator, in which refrigerant concentration is low. This solution rich in refrigerant concentration is pumped back into generator.

4.6.3 Passive desiccant cooling

This method is used in warm and humid climate. The moisture is removed from room air using absorbent or adsorbent. After this, evaporation technique is used to cool the space.

4.7 Solar Cell (Solar Photovoltaic System)

In active solar system, solar energy can be utilized by three methods;

·       PV system- When solar radiation is converted directly into electrical energy.

·       Thermal solar system- In this method solar radiation is first converted into heat. This heat can be directly used for heating purposes or thermal energy can be further changed into electricity.

·       Solar fuels- In this system solar energy are converted into chemical energy.

The term photovoltaic consists of the greek word phos, which means light, and -volt, which refers to electricity and is a reverence to the Italian physicist Alessandro Volta (1745-1827) who invented the battery. Solar cells use  p-n junction semiconductor materials to produce photovoltaic effect. Typical efficiencies of the most commercial solar modules are in the range of 15-20%.

4.7.1 The Working Principle of a Solar Cell

Generation of potential difference at the junction of two different materials in response to absorption of electromagnetic radiation (solar radiation/ photon) is known as photovoltaic effect. This effect is closely related to the photo electric effect, in which electrons are emitted by a surface after absorbing photons. The energy of such a photon is given by E = hn, where h is Planck’s constant and n is the frequency of the photon. For photoelectric effect, Einstein received the Nobel Prize in Physics in 1921.

The photovoltaic effect consists of three basic processes:

1. Creation of charge carriers due to the absorption of photons in the materials that form a junction.

When photon of energy higher than band gap is incident on the semiconductor p-n junction, electron from valence band goes into conduction band, leaving behind a hole. Thus an electron hole pair of opposite charges is created in the material. If the maximum of valence band and minimum of conduction band is at the same k-value (wave vector), the excitation is done without changing crystal momentum (ђk). This semiconductor is called direct band gap material. If there is change in crystal momentum during excitation, it is known as in-direct semiconductor. The absorption coefficient in a direct band gap material is much higher than in an indirect band gap material, thus the absorber can be much thinner.

In this way the radiative energy of the photon is converted to the chemical energy of the electron-hole pair. The maximal conversion efficiency from radiative energy to chemical energy is limited by thermodynamics. This thermodynamic limit lies in between 67% for non-concentrated sunlight and 86% for fully concentrated sunlight.

2. Subsequent separation of charge carriers created due to absorption of photons at the junction.

After production of electron-hole pair, they usually recombine i.e. electron goes to its initial energy level and energy is emitted in the form of electromagnetic radiation (photon) or   transferred to other electrons or holes or to the lattice as non-radiative emission of energy. Hence it is required to separate electrons and holes before they could recombine with each other. This is done by creating junction of n and p type semiconductors.

3. Collection of the electrons and holes at the terminals of the junction.

When electrons and holes reach at their respective terminals, they are extracted from the solar cell into external circuit to perform electrical work. Electrons flow in external circuit and on reaching at another terminal recombines with holes. Thus, finally the chemical energy of electron-hole pair is converted in electrical energy.  

4.7.2 Solar Cell Parameters

Performance of solar cells are depicted by parameters like, the peak power Pmax, the short-circuit current density Jsc, the open-circuit voltage Voc, and the fill factor FF. These parameters are determined from the illuminated J-V characteristic. The conversion efficiency η can be determined from these parameters.

 The short-circuit current Isc is the current that flows through the external circuit when the electrodes of the solar cell are short circuited. The short-circuit current of a solar cell depends on the photon flux density incident on the solar cell, which is determined by the spectrum of the incident light The Isc depends on the area of the solar cell. In order to remove the dependence of the solar cell area on Isc, often the short-circuit current density is used to describe the maximum current delivered by a solar cell. In laboratory c-Si solar cells the measured Jsc is above 42 mA/cm², while commercial solar cell has a Jsc exceeding 35 mA/cm².

When no current flows through the external circuit, the voltage is measured as Open-circuit voltage. It is the maximum voltage that a solar cell can deliver.  Voc is a measure of the amount of recombination in the device. Laboratory crystalline silicon solar cells have a Voc of up to 720 mV, while commercial solar cells typically have Voc exceeding 600 mV.

The fill factor  (FF) is the ratio between the maximum power (Pmax = JmpVmp) generated by a solar cell and the product of Voc with Jsc. FF =(JmpVmp/JscVoc). For a silicon laboratory device and a typical commercial solar cell FF is about 0.85 and 0.83, respectively. However, the variation in maximum FF can be significant for solar cells made from different materials. For example, a GaAs solar cell may have a FF approaching 0.89.

The conversion efficiency is calculated as the ratio between the maximal generated power and the incident power.

η = Pmax/Pin = Jmp Vmp/Pin = Jsc Voc FF/Pin

Typical external parameters of a crystalline silicon solar cell are; Jsc » 35 mA/cm², Voc up to 0.65 V and FF in the range 0.75 to 0.80. The conversion efficiency lies in the range of 17 to 18%.

4.7.3 The equivalent circuit of Solar Cell

The J-V characteristic of an illuminated solar cell that

behaves as the ideal diode is given by Eq. (8.27),

J (V_a) = J_rec (V_a) &J_gen (V_a)& J_ph

= J₀ [exp(eV_a/kT)&1]&J_ph

Where,

V_a = applied voltage

J_rec = recombination current density

J_gen  = thermal generated current density

J_ph = photo generated current density

This behaviour can be described by a simple equivalent

circuit, in which a diode and a current source are connected in parallel. The diode is formed by a p-n junction.

Equivalent circuit of an (a) ideal solar cell and a (b) solar cell with a series resistance Rs and a shunt resistance Rp.

4.7.4 Loss mechanism in solar cell

Loss in the efficiency of solar cell is due to following reasons:

1. Loss due to non-absorption of long wavelengths,

2. Loss due to thermalisation of the excess energy of photons,

3. Loss due to the total reflection,

4. Loss by incomplete absorption due to the finite

thickness,

5. Loss due to recombination,

6. Loss by metal electrode coverage, shading losses,

7. Loss due to voltage factor,

8. Loss due to fill factor.

4.7.5 Design Rules for Solar Cells

For better efficiency three factors may be taken into account.

1. Utilisation of the band gap energy,

2. Spectral utilisation,

3. Light trapping.

4.7.6 Classifications of Solar Cell

Solar cells can be classified on the basis of :

       i.          Cell size

     ii.          Thickness of active material

   iii.          Type of junction structure

   iv.          Type of active material

4.7.7 Components of a Solar PV System

A single solar cell can produce only 0.1 to 3 watt of power and 0.5 to 0.55 volt of voltage. Actually for practical purposes more power and more voltage are needed. Therefore many solar cells are connected in series or parallel to give required output for the specific applications. In such a manner solar PV module is prepared. Generally 36 solar cells are connected in series to give 12 volts. PV modules prepared in this way are further arranged in series or parallel to form an array to give more output. When modules are connected in series, voltage is increased and if connected in parallel, current is increased. This is done according to the requirement for specific application.

Solar panels are the main part of a PV system, but few other components are also required according to the applications.

·       Mounting structure- A mounting structure is needed to fix the panel and to direct it towards solar radiation.

·       Energy storage- Battery is required to store the solar energy to assure that it can deliver the energy in night and bad weather condition also, when no solar radiation is present.

·       DC Converters- It is used to convert the panel output to a fixed voltage, as the actual voltage may have fluctuations due to weather condition.

·       DC-AC converters (Inverters)- It is required to convert DC electricity produced by solar PV system to AC electricity to feed the output voltage of solar panel  to the electricity grid.

·       Cables- Cables are used to connect different components of the system.

·       Load- Loads are not the direct part of the system, but they affect very much the design of the whole system.

4.7.8 Types of PV systems

PV systems may be very simple or may contain many components, depending on the required applications. It can be divided into three categories:

4.7.8.1 Stand alone system- This system gain power only by solar PV system itself. Hence the whole system contains only solar panel and load. It may also contain a battery for energy storage. Charge regulators are used to prevent batteries from over-charging and over-discharging.

4.7.8.2 Grid-connected systems- In this system the whole DC electricity produced by solar panels are converted to AC electricity and it is fed to electricity grid to supply electricity in houses. In this system storage batteries are not required as the whole power is transferred to the grid. For this purpose, solar parks are established, where a large number of solar panels are mounted to produce large amount of electricity.

4.7.8.3 Hybrid system- This system contains solar panels for producing electricity and a complementary method of electricity generation, like diesel, gas or wind generator.

4.8 Sun Tracking Systems

Solar panels are used in remote areas to get electricity. For best efficiency, solar panels must get maximum solar radiation at all the time. For this purpose, special sensors are attached with solar panels, which track the direction of sun and accordingly rotate the panel in the direction of sun. In a typical sun tracking system, following components are attached.

·       Computer hardware and software

·       Sun Tracking Sensors (STS)

·        Night Time Fault Detector (NTFD)

·        Day Time Fault Detector (DTFD)

·        Night and Cloud Detection