Md. I. A. AnsariDepartment of Agricultural Engineering(e-mail: irfan26200@yahoo.com)

Refrigeration System

Mechanical vapour compression systemSolar vapour absorption System

• Food materials are refrigerated to slow down the reactions that cause spoilage.

• On an average, reaction rates are reduced by half when the temperature is reduced by 10 oC.

• Refrigeration is the removal of heat from a material or space, so that it’s temperature is lower than that of it’s surroundings.

Vapour Compression System

Vapour Compression System

• Evaporation : (Liquid + Vapor) to Vapor• Compression : Vapor to Vapor• Condensation : Vapor to (Liquid• Expansion : Liquid to (Liquid + Vapor)

• A refrigerant with a low boiling point refrigerant is compressed.

• The compressed gas, now with a much higher boiling point, moves to the condenser.

• • The boiling point of the refrigerant is now higher

than the temperature of the surrounding air.

• Therefore, the gas will give up heat and condense (the latent heat of vaporization is given up to the surroundings).

• The high-pressure liquid refrigerant then flows to the expansion valve or capillary tube), which produces a substantial pressure drop.

• This lowers the boiling point of the liquid to a very low value—well below the temperature around the evaporator coils.

• Heat gained by the evaporator coils causes the liquid to boil.

• The low pressure vapor then flows to the compressor where it is compressed to continue the cycle.

• This vapor compression refrigeration cycle is commonly used in the refrigerators, freezers, and air conditioners used in the home and in many commercial applications.

Coefficient of PerformanceThe coefficient of performance (C.O.P.) is defined as a ratio between the heat absorbed by the refrigerant as it flows through the evaporator to the heat equivalence of the energy supplied to the compressor.

Unit of Refrigeration• Refrigeration capacity / refrigeration effect

of cooling systems is sometimes given in “tons.”

• Now unit of refrigeration used in S.I. system is kW.

• 1 ton of refrigeration = mass of ice x latent heat of ice / 24 hrs

• A ton of refrigeration is defined as the quantity of latent heat required to be removed from one ton of water of 0° C temp. to convert it into ice of 0° C temp within 24 hours .

Ton in metric unit- (1000 kg X 80Kcal/kg)/(24X60) = 55.4 Kcal/min=3.5 kW

This is approximated to 50 Kcal/min and it is called One Ton of Refrigeration .

Solar Absorption Refrigeration System

• Some liquids like water have great affinity for absorbing large quantities of vapours (NH3) and reduce the total volume greatly.

• The absorption refrigeration system differs fundamentally from vapor compression system only in the method of compressing the refrigerant.

• An absorber, generator and pump in the absorption refrigerating system replace the compressor of a vapor compression system.

• Figure shows the schematic diagram of a vapor absorption system.

• Ammonia vapor is produced in the generator at high pressure from the strong solution of NH3 by an external heating source.

• The water vapor carried with ammonia is removed in the rectifier and only the dehydrated ammonia gas enters into the condenser.

• High pressure NH3 vapor is condensed in the condenser.

• The cooled NH3 solution is passed through a throttle valve and the pressure and temperature of the refrigerant are reduced below the temperature to be maintained in the evaporator.

• The low temperature refrigerant enters the evaporator and absorbs the required heat from the evaporator and leaves the evaporator as saturated vapor.

• Slightly superheated, low pressure NH3 vapor is absorbed by the weak solution of NH3 which is sprayed in the absorber as shown in Fig.

• Weak NH3 solution (aqua–ammonia) entering the absorber becomes strong solution after absorbing NH3 vapor and then it is pumped to the generator through the heat exchanger.

• The pump increases the pressure of the strong solution to generator pressure.

• The strong NH3 solution coming from the absorber absorbs heat form high temperature weak NH3 solution in the heat exchanger.

• The solution in the generator becomes weak as NH3 vapor comes out of it.

• The weak high temperature ammonia solution from the generator is passed to the heat exchanger through the throttle valve.

• The pressure of the liquid is reduced to the absorber pressure by the throttle valve.

The water heated in a flat plate collector array, is passed through a heat exchanger called the generator where transfer of heat takes place to a solution (absorbent + Refrigerant), which is rich in refrigerant.

Refrigerant vapour boiled off at a high pressure and goes to the condenser where it is condensed into a high pressure liquid.

• The high pressure liquid is throttled to a low pressure and temperature in an expansion value and passes through an evaporator coil.

• Hence, the refrigerant vapour absorbs heat and cooling is obtained in the space surrounding this coil.

• The refrigerant vapour is now absorbed back into a solution mixture withdrawn from the generator.

The refrigerant concentration is weak in this solution and pumped back into the generator, there by completing the cycle.

The common refrigerant absorbent liquids are ammonia-water, water-lithium bromide. The later is used in air conditioning.

Conversion of Solar Energy into Electricity

Photoelectric Effect• The photoelectric effect relies on the

principle that whenever light strikes the surface of certain metals electrons are released. These free electrons then travel into a circuit built into solar cell to form electric current.

• Solar photovoltaic (SPV) is the process of converting solar radiation into electricity using a device called solar cell.

• A solar cell is a semi-conducting device made of silicon or other materials which when exposed to sunlight, generates electricity.

• Solar cells require little maintenance, do not pollute and they operate silently, making photovoltaic energy the cleanest and safest method of power generation.

• PV cell is a semiconductor made from a crystalline silicon (Si) material.

• Semiconductor materials have a conductivity in-between conductors and insulators.

• A pure silicon crystal or germanium crystal is known as an intrinsic semiconductor.

• There are not enough free electrons and holes in an intrinsic semi-conductor to produce a usable current.

• The electrical action of these can be modified by doping means adding impurity atoms to a crystal to increase either the number of free holes or no of free electrons. The conductivity of a contaminated silicon crystal can be increased by a factor of 106.

• When a crystal has been doped, it is called a extrinsic semi-conductor.

• Types of semiconductor:• n-type semiconductor • p-type semiconductor

Doping of Semiconductors • The addition of a small percentage of foreign

atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties, producing n-type and p-type semiconductors.

• Pentavalent impurities Impurity atoms with 5 valence electrons produce n-type semiconductors by contributing extra electrons.

• A silicon atom has 4 relatively weakly bound (valence) electrons, which bond to adjacent atoms.

• Replacing a silicon atom with an atom that has either 3 or 5 valence electrons therefore produce either a space with no electron (a hole) or one spare electron that can move more freely than the others, this is the basis of doping.

• P-type doping, the creation of excess holes, is achieved by the incorporation into the silicon of atoms with 3 valence electrons, most often boron and

• • N-type doping, the creation of extra

electrons is achieved by incorporating an atom with 5 valence electrons, most often phosphorus.

• N-type doped silicon is a much better conductor than pure silicon.

PV Cell Construction

• The most common type of semiconductor currently in use is made of silicon crystal.

• Silicon crystals are laminated into n-type and p-type layers, stacked on top of each other.

PV Cell Construction

1) Low Iron Glass2) Polymer layer (EVA)3) Anti-reflective coating4) PV Cell consisting of

a) Contact for electricityb) Emitter (n-type Si)c) Base (p-type Si)d) Contact

5) Back polymer layer

1

23a

n-type Siliconb

p-type Silicon

c

d

5

- - - - - - - - - - - -

+ + + + + + + + + + + +

Antireflective coating • Since, silicon is a shiny gray material and

can act as a mirror, reflecting more than 30% of the light that shines on it.

• Two techniques are commonly used to reduce reflection. The first technique is to coat the top surface with a thin layer of silicon monoxide (SiO).

• A single layer reduces surface reflection to about 10%, and a second layer can lower the reflection to less than 4%.

• A second technique is to texture the top surface. Chemical etching creates a pattern of cones and pyramids, which capture light rays that might otherwise to reflect away from the cell.

• Reflected light is redirected down into the cell, where it has another chance to be absorbed.

• Solar cells are made up of extremely thin silicon wafers (about 300 micron m) to protect cells from damage, cells are hermetically sealed between a layer of toughened glass and layers of ethyl vinyl acetate..

• Connected by an external circuit electrons flow from the n-side to create electricity and end up in the p-side.

• The amount of power available from a pv device is determined by: type and area of material, intensity of sunlight.

• The electricity produced is called direct current (DC) and can be used immediately or stored in a battery.

• An inverter is used to convert into alternating current (AC), the standard power used in residential homes.

Basic types of silicon solar cells are: 1. Mono crystalline silicon solar cells2. poly crystalline silicon solar cells3. Thin film or Amorphous silicon solar cells.

• Mono crystalline silicon solar cells

• The conversion efficiency for single-silicon commercial modules ranges between 15-25%.

• Costlier

• Poly crystalline silicon solar cells: The energy conversion efficiency for a commercial module made of polycrystalline silicon ranges between 10 to 20%.

• Less expensive to produce due to simpler manufacturing process

Thin-film solar cells:

• The thin film cells are produced from amorphous silicon.

• The efficiency is 5 to 8%.

• These are very cheap to manufacture.

• The magnitude of the electric current generated depends on the intensity of the solar radiation, exposed area of the solar cell, the type of the materials used in fabricating the solar cell and ambient temperature.

• Solar cells are connected in series and parallel combinations to form modules that provide the required power.

• The power output of a solar cell can be increased by using tracking mechanism to keep the PV device directly facing the sun or by concentrating the sunlight using lenses or mirror.

• Batteries are often used in PV systems for the purpose of storing energy produced by the PV array during the day.

• All of these components allow the system to power a water pump, appliances, commercial sites, or even a whole community.

• Photovoltaic cells come in many sizes, but most are 10 cm by 10 cm and generate about 0.5v and 1 W.

• PV cells are bundled together in modules or panels to produce higher voltages and increased power.

• Sun intensity at surface ~1000 watt / square meterPV cells about 15% efficient = ~150 watt / square meter

The wattage output of a PV module is rated in peak watt (Wp).

The peak watt output power from a module is defined as the maximum power output that the module could deliver under standard test conditions (1000 W/m2 solar radiation intensity at 25 oC).

Inverter: DC power to AC power

Flat-Plate PV Systems The most common array design uses flat-plate PV

modules or panels. These panels can either be fixed in place or

allowed to track the movement of the sun. They respond to sunlight that is either direct or

diffuse. Even in clear skies, the diffuse component of

sunlight accounts for between 10% and 20% of the total solar radiation on a horizontal surface.

On partly sunny days, up to 50% of that radiation is diffuse.

And on cloudy days, 100% of the radiation is diffuse.

A PV system normally consists of:A PV panel arrayA control panel to regulate power A power storage system (batteries)An inverter for converting the DC to AC

power Backup power supplies (generators)Charge controller needed for large systems

to prevent overcharging

http://www.solarenergy.gen.in/wp-content/uploads/2011/10/solar-energy-working.gif

SPV systems

Stand alone system SPV pump system

• Cell efficiency is defined as the ratio of electric power output of the cell, module, or array to the power content of sunlight over its total exposed area.

• The maximum theoretical efficiency of solar cells is around 47 percent.

• Generally, the performances rating of photovoltaic are expressed in terms of peak watt.

• Conversion efficiency:

• P = E•I = E2/R = I2•R,where P is power, E is electromotive force, I is intensity or current, and R is resistance

• Energy = P•t, where t is the time that power flows

• V = I•R• One unit of electricity=1kWh

Advantages Of Photovoltaic Solar Energy Conversion

Absence of moving parts. Direct conversion of light to electricity at

room temperature. Can function unattended for long time. Low maintenance cost. No environmental pollution. Very long life. Highly reliable. Solar energy is free and no fuel required.

Limitations Of Photovoltaic Solar Energy Conversion

Manufacture of silicon crystals is labour and energy intensive.

The principle limitation is high cost. The insolation is unreliable and therefore

storage batteries are needed. Solar power plants require very large land

areas. Electrical generation cost is very high. The energy spent in the manufacture of solar

cells is very high. The initial cost of the plant is very high

Solar Lantern

A Solar lantern is a simple application of solar photovoltaic technology, which has found good acceptance in rural regions where the power supply is irregular and scarce.

Solar Lantern is made of three main components: Solar PV panel, the storage battery and the lamp.

The measure of a battery capacity is ampere hour.

A single charge can operate the lamp for about 4-5 hours.

Applications and uses:• Emergency and/or house lighting, table

lamp, Hawker / Vendor Stalls, non-electrified remote places for education

• Easy and convenient alternative to kerosene / petromax / gas.

• It is easy to install, no electrical connection is required and no electricity charges.

Solar Lanterns

8W, 10W, 12W: 8W, 10W, 12W: PanelPanel

12 V, 7 AH-battery12 V, 7 AH-battery

CFL: 5 or 7WCFL: 5 or 7W

Green LED light Green LED light indicates the indicates the charging of the charging of the batterybattery

Cost: Approx. Rs. Cost: Approx. Rs. 30003000

Solar Charging Station (SCS)• Solar charging stations

– At any central location• Direct charging during the day

– Batteries inside the lanterns

D(PBD) inaugurating SCS, Sathla KSK Rooftop solar panels, Sathla KSK

• Customers pay rent for lantern & a fee for charging lantern on a daily/monthly basis– Every evening, Customers bring the discharged

lantern and take a charged lantern with them– Alternatively, the charging station owner

arranges for delivery & collection of lanterns

IndianOil’s Solar Lantern Charging Rack

Solar Lantern in Use

Solar Street Light

This system is designed for outdoor application in un-electrified remote rural areas.

This system is an ideal application for campus and village street lighting.

The system is provided with battery storage backup sufficient to operate the light for 10-11 hours daily.

The system is provided with automatic ON/OFF time switch for dusk to down operation and overcharge /deep discharge prevention cut-off with LED indicators.

• The solar street light system comprise of :a) 74 Wp Solar PV Module b) 12 V, 75 Ah Tubular plate battery with battery box c) Charge Controller cum inverterd) 11 Watt CFL Lamp with fixtures ON/OFF time switch for dusk to down operationOvercharge/deep discharge prevention cut off with LED

indicatorse) 4 m mild steel lamp post above ground level with

weather proof paint and mounting hardware. • Life: 15-20 yrs

Solar Fencing System • The solar fence is scientific fence and works

on solar energy with backup facility to run uninterruptedly during the nights as well as cloudy days.

• Working principle • The solar module generates the DC energy and

charges the battery. • The output of the battery is connected to

energizer or controller or charger.

The basic building blocks of a power fence are: 1. Energizer 2. Earthing (Grounding System) and 3. Fence system

Energizer: The heart of the power fence is the Energizer. The

energizer is selected depending on the animals to be controls, length of the fence and number of strands.

Main function of the energizer is to produce short and sharp pulses of about 8000 volts at regular intervals On:1/10th of a second and OFF:4/5th of a second.

Max current should not be more than 0.008 amp. The solar module generates DC and charges battery. The power input is from the DC energy from battery. The output of the battery is connected to energizer or

controller. The energizer should be protected from children,

should be enclosed, free from mechanical damage and away from inflammable.

• Fencer: The energizer produces a short, high voltage pulse at regular rate of one pulse per second. The live wire of the energizer is connected to the fence wire and the earth terminal to the Earth system.

• Animal / Intruder touching the live wire creates a path for the current through its body to the ground and back to the energizer via the earth system and completes the circuit.

• Thus the intruder will receive a shock, the greater the shock the intruder receives the more lasting the memory will be avoided in future.

Solar water pumping system• The solar water pumping system is a

stand-alone system operating on power generated using solar PV (photovoltaic) system.

• The power generated by solar cells is used for operating DC surface centrifugal mono-block pump set for lifting water from bore / open well or water reservoir for minor irrigation and drinking water purpose.

• The system is provided with 1800 W solar PV panel (24 nos. X 75 Wp) and 2 HP centrifugal DC mono-block / AC submersible with inverter.

• The average water delivery of 2 HP solar pump will be around 1.38 to 1.40 lakh litre per day

• The size of suction & delivery lines is 2.5 inches (62.5 mm).

Advantages of solar pump sets

No fuel cost-uses abundantly available free sun light

No conventional grid electricity required Long operating life Highly reliable and durable- free performance Easy to operate and maintain Eco-friendly Saving of conventional diesel fuel

Other PV usesOther appropriate uses for PV systems on farms

include: Power for feed or product grinding Electric-powered egg collection and handling equipment Product refrigeration Electric fencing to contain livestock Rural electrification Water disinfection in water treatment systems (Aqua

Guard) Desalination of brackish water by the use of PV powered

RO systems Battery charging Evaporative cooling Running home appliances

Solar Park

Solar Park

Solar Cell ProducersGermany

Japan

China

Taiwan

Australia

Future of Solar Energy

• PV is cost-effective for providing electricity in remote areas and in niche applications.

• As the costs of fossil fuels and electricity increase, PV is becoming more cost-effective compared to electricity from conventional sources.

• The market for photovoltaic cells is presently growing at about 30 % per year and cost is declining continuously due to advancement in technologies and mass production.