Applications of Solar Energy
1- Solar Distillation
Fresh water is a problem in many areas of the world, not because water is not available, but because the solid content of the available water is too high to make it drinkable or useable for irrigation.
One method of producing fresh water from either salt water, contaminated water, or even liquid wastes from houses and communities is by means of a solar still.
A solar still has a basin which contains the salt water or contaminated water or waste water. This basin is covered by inclined sheet of glass with a collecting trough at its bottom. The sun shining through this glass will heat the water and vaporize some of it, then the steam coming in contact with the glass will condense on the glass and run down and collect as distilled water in the collecting container. The glass cover can be either sloping south only or it can be a roof-type construction.
The stills are usually oriented along the east west line with the glass inclined with the horizontal at 30 to 45 degrees. Depending on whether the water is shallow or three to four feet deep classifies these stills as shallow basin or deep basin designs.
Plastics can be used instead of the glass but usually will form drop wise condensation which reflects solar energy on the glass and, therefore, the yield from these stills is not as satisfactory. One can expect from ½ to one pound of fresh water from each square foot of pan area and this is the equivalent of up two inches of rainfall per week.
Fresh water is a problem in many areas of the world, not because water is not available, but because the solid content of the available water is too high to make it drinkable or useable for irrigation.
One method of producing fresh water from either salt water, contaminated water, or even liquid wastes from houses and communities is by means of a solar still.
A solar still has a basin which contains the salt water or contaminated water or waste water. This basin is covered by inclined sheet of glass with a collecting trough at its bottom. The sun shining through this glass will heat the water and vaporize some of it, then the steam coming in contact with the glass will condense on the glass and run down and collect as distilled water in the collecting container. The glass cover can be either sloping south only or it can be a roof-type construction.
The stills are usually oriented along the east west line with the glass inclined with the horizontal at 30 to 45 degrees. Depending on whether the water is shallow or three to four feet deep classifies these stills as shallow basin or deep basin designs.
Plastics can be used instead of the glass but usually will form drop wise condensation which reflects solar energy on the glass and, therefore, the yield from these stills is not as satisfactory. One can expect from ½ to one pound of fresh water from each square foot of pan area and this is the equivalent of up two inches of rainfall per week.
2- Heating Water
Solar water heaters for domestic use. Generally, two main parts in the system: (a) The collector, and (b) A storage tank (larger than the usual ones). The designer should plan for heating 20 gallons of water/day/person (in an upper class due to dish-& clothes-washers).
Many solar hot water systems have supplemental heaters (provide ~ 30% of the hot water during inclement weather) controlled by a thermostat and which use an electric element to heat the water. Or an alternative is (1) to use a large storage tank of high capacity or (2) to change one’s life style a little in order to dispense with the supplemental heater. A gas heater is cheaper than an electric one.
There are two main types of solar hot water heating systems:
(a) The closed box type.
(b) The recirculation type.
a) The closed box
It is very simple. It consists of a metal box insulated on the sides and bottom which contains several large diameter block pipes. The top of the box is covered by a transparent sheet of plastic. The tops of the large pipes are exposed to the sum and serve as the heat collectors, the hot water is also stored in the large pipe. Hot water is drawn off the top ends of the pipes via a manifold and replacement water is introduced at the bottom ends.
b) The recirculating type
It consists of a flat plate collector and a separate insulated storage tank. The collector consists of a copper sheet which is painted flat block and has water tubes embedded in the surface. Heat is trapped (behind the glass sheet covering) and is carried off by water flowing through the tubes. Since the flat plate collector is located lower than the storage tank, a convection current is set up automatically (due to density differences), the warm water in collector displaces the cooler water in the storage tank. At night, the convection current ceases since the water in the solar collector becomes cooler than the water in the tank, this prevents the loss of heat to the night time air.
Advantages of closed box type
– Danger of freezing is minimized due to the large amount of heat stored in the water in the tubes.( the supply pipes must be wrapped in insulation to prevent them freezing)
Disadvantages of closed box type
1- May lose a large amount of heat at night through the uninsulated plastic cover.
2- Does not produce enough hot water.
Advantages of recirculating type
1- Does not lose much heat at night due to the tank is insulated.
2- More hot water is produced.
Disadvantages of recirculating type
– It is more susceptible to freezing in its flat plate collector, Double glazing provides extra insulation at night &prevents freezing.
The solar heating system sketched in Figure (1.4) uses solar energy to heat a liquid coolant such as water or anti-freeze. The heat exchanger uses heat from the liquid coolant in the primary circulation system to heat water in the secondary circulation system. The control valve in the lower right of the figure allows water to be added to the secondary circulation system. An auxiliary heater in the upper right of the figure is included in the system to supplement the supply of heat from the solar collector. It is a reminder that solar energy collection is not a continuous process. A supplemental energy supply or a solar energy storage system must be included in the design of the heating system to assure continuous availability of heat from the solar heating system.
Solar water heaters for domestic use. Generally, two main parts in the system: (a) The collector, and (b) A storage tank (larger than the usual ones). The designer should plan for heating 20 gallons of water/day/person (in an upper class due to dish-& clothes-washers).
Many solar hot water systems have supplemental heaters (provide ~ 30% of the hot water during inclement weather) controlled by a thermostat and which use an electric element to heat the water. Or an alternative is (1) to use a large storage tank of high capacity or (2) to change one’s life style a little in order to dispense with the supplemental heater. A gas heater is cheaper than an electric one.
There are two main types of solar hot water heating systems:
(a) The closed box type.
(b) The recirculation type.
a) The closed box
It is very simple. It consists of a metal box insulated on the sides and bottom which contains several large diameter block pipes. The top of the box is covered by a transparent sheet of plastic. The tops of the large pipes are exposed to the sum and serve as the heat collectors, the hot water is also stored in the large pipe. Hot water is drawn off the top ends of the pipes via a manifold and replacement water is introduced at the bottom ends.
b) The recirculating type
It consists of a flat plate collector and a separate insulated storage tank. The collector consists of a copper sheet which is painted flat block and has water tubes embedded in the surface. Heat is trapped (behind the glass sheet covering) and is carried off by water flowing through the tubes. Since the flat plate collector is located lower than the storage tank, a convection current is set up automatically (due to density differences), the warm water in collector displaces the cooler water in the storage tank. At night, the convection current ceases since the water in the solar collector becomes cooler than the water in the tank, this prevents the loss of heat to the night time air.
Advantages of closed box type
– Danger of freezing is minimized due to the large amount of heat stored in the water in the tubes.( the supply pipes must be wrapped in insulation to prevent them freezing)
Disadvantages of closed box type
1- May lose a large amount of heat at night through the uninsulated plastic cover.
2- Does not produce enough hot water.
Advantages of recirculating type
1- Does not lose much heat at night due to the tank is insulated.
2- More hot water is produced.
Disadvantages of recirculating type
– It is more susceptible to freezing in its flat plate collector, Double glazing provides extra insulation at night &prevents freezing.
The solar heating system sketched in Figure (1.4) uses solar energy to heat a liquid coolant such as water or anti-freeze. The heat exchanger uses heat from the liquid coolant in the primary circulation system to heat water in the secondary circulation system. The control valve in the lower right of the figure allows water to be added to the secondary circulation system. An auxiliary heater in the upper right of the figure is included in the system to supplement the supply of heat from the solar collector. It is a reminder that solar energy collection is not a continuous process. A supplemental energy supply or a solar energy storage system must be included in the design of the heating system to assure continuous availability of heat from the solar heating system.
3- Electricity from Solar Cells
Photovoltaic energy is the conversion of sunlight into electricity through a photovoltaic (PVs) cell, commonly called a solar cell. A photovoltaic cell is a non mechanical device usually made from silicon alloys.
Sunlight is composed of photons, or particles of solar energy. These photons contain various amounts of energy corresponding to the different wavelengths of the solar spectrum. When photons strike a photovoltaic cell, they may be reflected, pass right through, or be absorbed. Only the absorbed photons provide energy to generate electricity. When enough sunlight (energy) is absorbed by the material (a semiconductor), electrons are dislodged from the material’s atoms. Special treatment of the material surface during manufacturing makes the front surface of the cell more receptive to free electrons, so the electrons naturally migrate to the surface.
Photovoltaic energy is the conversion of sunlight into electricity through a photovoltaic (PVs) cell, commonly called a solar cell. A photovoltaic cell is a non mechanical device usually made from silicon alloys.
Sunlight is composed of photons, or particles of solar energy. These photons contain various amounts of energy corresponding to the different wavelengths of the solar spectrum. When photons strike a photovoltaic cell, they may be reflected, pass right through, or be absorbed. Only the absorbed photons provide energy to generate electricity. When enough sunlight (energy) is absorbed by the material (a semiconductor), electrons are dislodged from the material’s atoms. Special treatment of the material surface during manufacturing makes the front surface of the cell more receptive to free electrons, so the electrons naturally migrate to the surface.
When the electrons leave their position, holes are formed. When many electrons, each carrying a negative charge, travel toward the front surface of the cell, the resulting imbalance of charge between the cell’s front and back surfaces creates a voltage potential like the negative and positive terminals of a battery. When the two surfaces are connected through an external load, electricity flows.
The photovoltaic cell is the basic building block of a PV system. Individual cells can vary in size from about 1 cm (1/2 inch) to about 10 cm (4 inches) across. However, one cell only produces 1 or 2 watts, which isn’t enough power for most applications. To increase power output, cells are electrically connected into a packaged weather-tight module. Modules can be further connected to form an array. The term array refers to the entire generating plant, whether it is made up of one or several thousand modules. As many modules as needed can be connected to form the array size (power output) needed.
The performance of a photovoltaic array is dependent upon sunlight. Climate conditions (e.g., clouds, fog) have a significant effect on the amount of solar energy received by a PV array and, in turn, its performance. Most current technology photovoltaic modules are about 10 percent efficient in converting sunlight with further research being conducted to raise this efficiency to 20 percent.
The PV cell was discovered in 1954 by Bell Telephone researchers examining the sensitivity of a properly prepared silicon wafer to sunlight. Beginning in the late 1950s, PVs were used to power U.S. space satellites. The success of PVs in space generated commercial applications for PV technology. The simplest photovoltaic systems power many of the small calculators and wrist watches used everyday. More complicated systems provide electricity to pump water, power communications equipment, and even provide electricity to our homes.
Photovoltaic conversion is useful for several reasons. Conversion from sunlight to electricity is direct, so that bulky mechanical generator systems are unnecessary. The modular characteristic of photovoltaic energy allows arrays to be installed quickly and in any size required or allowed.
Also, the environmental impact of a photovoltaic system is minimal, requiring no water for system cooling and generating no by-products. Photovoltaic cells, like batteries, generate direct current (DC) which is generally used for small loads (electronic equipment). When DC from photovoltaic cells is used for commercial applications or sold to electric utilities using the electric grid, it must be converted to alternating current (AC) using inverters, solid state devices that convert DC power to AC. Historically, PVs have been used at remote sites to provide electricity.
Disadvantages:
1. High cost (~40 times that of electricity by conventional sources).
2. Decreased performance at elevated temperatures.
3. Gradual diminution of power over several years.
4. Difficulty of storing electricity.
5. A terrestrial solar cell system provides electricity for a limited number of hours and not on all days.
To overcome (4) and (5) two approaches:
1. Use the electricity to electrolyze water and store 2h as a fuel.
2. to place the solar cells on large satellite arrays in a synchronous orbit. Where the satellite remains above a fixed point on the earth. The solar generated electricity is converted on the satellite to microwaves which are beamed to a receiving station on the earth below. The energy is reconverted into conventional electricity and conveyed by power lines.
The photovoltaic cell is the basic building block of a PV system. Individual cells can vary in size from about 1 cm (1/2 inch) to about 10 cm (4 inches) across. However, one cell only produces 1 or 2 watts, which isn’t enough power for most applications. To increase power output, cells are electrically connected into a packaged weather-tight module. Modules can be further connected to form an array. The term array refers to the entire generating plant, whether it is made up of one or several thousand modules. As many modules as needed can be connected to form the array size (power output) needed.
The performance of a photovoltaic array is dependent upon sunlight. Climate conditions (e.g., clouds, fog) have a significant effect on the amount of solar energy received by a PV array and, in turn, its performance. Most current technology photovoltaic modules are about 10 percent efficient in converting sunlight with further research being conducted to raise this efficiency to 20 percent.
The PV cell was discovered in 1954 by Bell Telephone researchers examining the sensitivity of a properly prepared silicon wafer to sunlight. Beginning in the late 1950s, PVs were used to power U.S. space satellites. The success of PVs in space generated commercial applications for PV technology. The simplest photovoltaic systems power many of the small calculators and wrist watches used everyday. More complicated systems provide electricity to pump water, power communications equipment, and even provide electricity to our homes.
Photovoltaic conversion is useful for several reasons. Conversion from sunlight to electricity is direct, so that bulky mechanical generator systems are unnecessary. The modular characteristic of photovoltaic energy allows arrays to be installed quickly and in any size required or allowed.
Also, the environmental impact of a photovoltaic system is minimal, requiring no water for system cooling and generating no by-products. Photovoltaic cells, like batteries, generate direct current (DC) which is generally used for small loads (electronic equipment). When DC from photovoltaic cells is used for commercial applications or sold to electric utilities using the electric grid, it must be converted to alternating current (AC) using inverters, solid state devices that convert DC power to AC. Historically, PVs have been used at remote sites to provide electricity.
Disadvantages:
1. High cost (~40 times that of electricity by conventional sources).
2. Decreased performance at elevated temperatures.
3. Gradual diminution of power over several years.
4. Difficulty of storing electricity.
5. A terrestrial solar cell system provides electricity for a limited number of hours and not on all days.
To overcome (4) and (5) two approaches:
1. Use the electricity to electrolyze water and store 2h as a fuel.
2. to place the solar cells on large satellite arrays in a synchronous orbit. Where the satellite remains above a fixed point on the earth. The solar generated electricity is converted on the satellite to microwaves which are beamed to a receiving station on the earth below. The energy is reconverted into conventional electricity and conveyed by power lines.
4- Space heating and cooling
• Absorption cooling cycle:
Uses two working fluids: (1) A refrigerant (2) An absorbent. The evaporated refrigerant is absorbed from the cooling coils into the secondary fluid. The resulting solution is transferred by a low-power pump to a regenerator, where energy in the form of heat causes the refrigerant to be distilled out of the absorbent fluid. The refrigerant (now liquid) goes back to the evaporating coils (evaporator), vaporizing and cooling the inside of the system. The absorbent is transferred back to the absorber where the vaporized and heated refrigerant can be absorbed and carried away again.
The absorption cooling process uses the vaporization of liquid refrigerant to draw heat out of the air or water to be cooled.
Practical current fluid pairs
1- NH3- water 2- water- Lithium bromide
• Absorption cooling cycle:
Uses two working fluids: (1) A refrigerant (2) An absorbent. The evaporated refrigerant is absorbed from the cooling coils into the secondary fluid. The resulting solution is transferred by a low-power pump to a regenerator, where energy in the form of heat causes the refrigerant to be distilled out of the absorbent fluid. The refrigerant (now liquid) goes back to the evaporating coils (evaporator), vaporizing and cooling the inside of the system. The absorbent is transferred back to the absorber where the vaporized and heated refrigerant can be absorbed and carried away again.
The absorption cooling process uses the vaporization of liquid refrigerant to draw heat out of the air or water to be cooled.
Practical current fluid pairs
1- NH3- water 2- water- Lithium bromide
5- Solar Cooking
A solar cooker is basically a tool used to gather solar energy for cooking. It gathers solar energy and reflects it onto the container, which is not strictly a component of a solar cooker. The container may be in the form of a pan, pot or otherwise.
Types of solar cooker
Box Cooker Panel Cooker Parabolic Cooker
A solar cooker is basically a tool used to gather solar energy for cooking. It gathers solar energy and reflects it onto the container, which is not strictly a component of a solar cooker. The container may be in the form of a pan, pot or otherwise.
Types of solar cooker
Box Cooker Panel Cooker Parabolic Cooker
Physics of solar Cooking
There are 3 main principles incorporated into solar cooking which are fundamental to any cooker. These principles are:
• Reflection of the greatest possible amount of sunlight to the food.
• Converting these light waves to heat.
• Effectively retaining the heat by insulation.
There are 3 main principles incorporated into solar cooking which are fundamental to any cooker. These principles are:
• Reflection of the greatest possible amount of sunlight to the food.
• Converting these light waves to heat.
• Effectively retaining the heat by insulation.
Reflection
The aim is to make sure your cooker can adjust to the sun’s varying positions to capture enough light to cook. The reflector directs the sun’s rays onto the pot.
The shape of the parabolic model is made such that it reflects solely on a single point where the food is to be placed. Hence, the cooker can only provide one point for cooking.
The panel model has walls that reflect towards the middle of the cooker instead of at one point. Thus this model is more effective than the first one because it is less affected by the direction of the sun’s rays.
The box model has an adjustable reflector lid and also sides that are covered with reflective material. Thus, this model has the most effective response to the variations of the sun’s directions, making it the most effective.
The aim is to make sure your cooker can adjust to the sun’s varying positions to capture enough light to cook. The reflector directs the sun’s rays onto the pot.
The shape of the parabolic model is made such that it reflects solely on a single point where the food is to be placed. Hence, the cooker can only provide one point for cooking.
The panel model has walls that reflect towards the middle of the cooker instead of at one point. Thus this model is more effective than the first one because it is less affected by the direction of the sun’s rays.
The box model has an adjustable reflector lid and also sides that are covered with reflective material. Thus, this model has the most effective response to the variations of the sun’s directions, making it the most effective.
Convection
The aim is to make sure that your model can effectively absorb and convert the sun’s rays into heat. Using pots made of dark material can do this. Dark-coloured materials absorb more heat while light-coloured ones reflect the sun’s rays.
If a shiny pot is used, the rays are reflected out of the cooker allowing no light energy to be converted to heat. On the contrary, the black pot absorbs the rays and converts them to heat.
This principle is used in all models. To further increase the intake of heat, a black bottom should be placed in the cooker. The presence of this bottom will produce more heat energy, making the model more effective.
The aim is to make sure that your model can effectively absorb and convert the sun’s rays into heat. Using pots made of dark material can do this. Dark-coloured materials absorb more heat while light-coloured ones reflect the sun’s rays.
If a shiny pot is used, the rays are reflected out of the cooker allowing no light energy to be converted to heat. On the contrary, the black pot absorbs the rays and converts them to heat.
This principle is used in all models. To further increase the intake of heat, a black bottom should be placed in the cooker. The presence of this bottom will produce more heat energy, making the model more effective.
Retaining Heat
Heat retainment allows heat to remain inside the cooker; insulation is the heat barrier the heat capacity and efficiency of the cooker: using insulation reduces cooking time. Various insulation materials are used in solar cookers–glass, plastic, wood, cardboard, still air barriers (for box models), or plastic bags.
If little insulation is used, models do not achieve a high temperature. Models that achieve high temperatures make good use of insulation, making them efficient models.
Another method to increase the efficiency of your cooker is placing adobe or bricks in the cooking volume. These materials of high heat capacity retain heat for a long time. Also, the thicker the pot used, the greater amount of heat they will absorb. These methods will allow food to cook long after the sun has set.
Heat retainment allows heat to remain inside the cooker; insulation is the heat barrier the heat capacity and efficiency of the cooker: using insulation reduces cooking time. Various insulation materials are used in solar cookers–glass, plastic, wood, cardboard, still air barriers (for box models), or plastic bags.
If little insulation is used, models do not achieve a high temperature. Models that achieve high temperatures make good use of insulation, making them efficient models.
Another method to increase the efficiency of your cooker is placing adobe or bricks in the cooking volume. These materials of high heat capacity retain heat for a long time. Also, the thicker the pot used, the greater amount of heat they will absorb. These methods will allow food to cook long after the sun has set.
6- Agricultural and Industrial Drying
Hay and other crops have been solar dried in open fields since the beginning of agriculture.
Solar drying equipment include indirect heating by hot air stream of low relative humidity. The material is placed in deep layers covering a smaller area. Large volumes of air are required since Cp of air is low. The air is forced through by electrically driven fans or by thermo-siphoning (by solar heating of tall thermal chimneys or collectors).
Solar drying may also be accomplished by exposing the solid material to sun’s radiation with or without a transparent cover.
Hay and other crops have been solar dried in open fields since the beginning of agriculture.
Solar drying equipment include indirect heating by hot air stream of low relative humidity. The material is placed in deep layers covering a smaller area. Large volumes of air are required since Cp of air is low. The air is forced through by electrically driven fans or by thermo-siphoning (by solar heating of tall thermal chimneys or collectors).
Solar drying may also be accomplished by exposing the solid material to sun’s radiation with or without a transparent cover.
7- Solar Furnaces
They are capable of giving higher temperatures than can be obtained in fuel-operated furnaces and most electric ones. Large parabolic mirrors, ~ 3 m in diameter give temperatures up to 3500°C in a small area without contamination from combustion products.
They are capable of giving higher temperatures than can be obtained in fuel-operated furnaces and most electric ones. Large parabolic mirrors, ~ 3 m in diameter give temperatures up to 3500°C in a small area without contamination from combustion products.
Solar Energy Storage
In order that a solar-operated thermal system be continuous, heat or energy storage must be used.
There are four main systems for storing heat in a reasonable quantities:
1. Sensible heat storage.
2. Latent heat storage.
3. Salt ponds.
4. Chemical storage.
There are four main systems for storing heat in a reasonable quantities:
1. Sensible heat storage.
2. Latent heat storage.
3. Salt ponds.
4. Chemical storage.
No comments:
Post a Comment