14 Applications of Solar Energy
Dr. Dhanya M.S M.S
- Introduction
The solar energy is provided by sun by the nuclear fusion reactions. The incident solar energy is not fully utilized for meeting the energy demands at the same time of insolation. The solar energy is generally transformed into thermal or electric energy using solar devices. The solar energy is, however, available during sunshine hours, and the demand of thermal or electric energy may also exist during non-sunshine hours. Also, the maximum availability of solar energy may not coincide exactly with the demand of thermal or electric energy. The availability of solar energy is sometime low for several days due to cloudy days, resulting in the substantial lowering of the output of thermal and electric energy from the solar radiation. Hence, it is essential to store energy output (thermal and electric) from solar devices during high insolation (in = incident, sol= solar and ation = radiation) from solar device during to meet the thermal and electric load demand during peak demand times. During low insolation times, solar energy storage system enables delivery of more power than what is generated by the solar electric or thermal plant, and so it enables to match the generation of energy with the load demand.
- Classification of solar energy storage system:
The solar energy storage systems can be classified as follows:
The thermal energy storage system
Chemical energy storage system
Electrical energy storage system
Hydrogen energy storage system
Electromagnetic energy storage system
Biological storage system
2.1 Thermal energy storage:
Thermal acid batteries are the most commonly used means in chemical energy storage system. The advantages are (i) good working efficiency (up to 80%), (ii) low cost, (iii) rapid change from charging to discharging mode and (iv) slow discharge rate. A storage battery takes electrical energy generated by solar radiation and stores it as chemical energy. It later supplies electric energy by converting this stored energy.
2.2 Electrical energy storage:
A capacitor is used to store electrical energy in electrostatic field when it is charged. The capacitor of large capacity is required to store a significant amount of energy.
2.3 Hydrogen energy storage:
The electrical energy is used to decompose water by the electrolysis reaction into hydrogen and oxygen. These substances can be recombined to release the stored energy when required.
2.4 Electromagnetic energy storage:
The electrical energy is used to store energy in a magnetic field. The resistance of the coil wire is made almost negligible so that the stored energy in the coil is not dissipated out and stored energy in the magnetic field can be maintained indefinitely. The electromagnetic energy storage requires the use of superconducting materials. These materials develop almost zero resistance to electricity flow when cooled below a critical or transition temperature. This method of storing electromagnetic energy is also called super conducting magnetic energy storage (SMES). The electric energy can be recovered when coil is discharged.
2.5 Biological storage: The solar energy is stored in plant by a process known as photosynthesis. Photosynthesis is the process in which organic compounds are formed in green plant using carbon from atmospheric carbon dioxide in the presence of sunlight. The plants on decaying from biomass which can be converted into various type of solid, liquid and gaseous fuels.
- Sensible heat storage:
Thermal energy is stored in this type of storage by virtue of heat capacity and temperature difference developed during charging and discharging. The temperature of the storage material rises when thermal energy is absorbed and temperature drops when thermal energy is taken out. In this storage, the charging and discharging can be performed reversibly for an unlimited number of time. The sensible heat storage can be liquid media storage and solid media storage.
3.1 Sensible heat storage by water
Water is considered as the most suitable media for storage below 100̊ C. liquid such as oils, liquid metals and molten salt are also used as liquid media storage.
The water thermal energy storage can be short term and long term. A short -term thermal energy storage system has a well-insulated storage tank as shown in figure (1). The storage in such tank is economical for few days only as heat losses over long duration make the storage uneconomical.
Long – term sensible heat storage by water is possible in underground reservoir having special insulation. In this system, water is heated in charging mode by passing it through a heat exchanger and then it is stored in an underground reservoir. In the discharge mode, the hot water is made to flow back through the heat exchanger, where release the stored energy as shown in figure 2 but with reverse circulation.
Fig 2 Long term sensible heat storage by water
The advantages of this storage system are
It is abundantly available.
It is inexpensive.
It has high specific heat which enables to store more heat per unit mass.
It has low viscosity requiring less energy to pump through the pipe system.
It can be used for both storage and working medium.
It is stable.
It has no harmful effect.
But water has the following disadvantages:
It has limited temperature range of 0-100̊ C .
Fig 2 Long term sensible heat storage by water
The advantages of this storage system are
It is abundantly available.
It is inexpensive.
It has high specific heat which enables to store more heat per unit mass.
It has low viscosity requiring less energy to pump through the pipe system.
It can be used for both storage and working medium.
It is stable.
It has no harmful effect.
But water has the following disadvantages:
It has limited temperature range of 0-100̊ C .
The charging or adding of heat is done by passing hot air through the bed in one direction and the removable of heat is done by passing the normal air through the bed in the opposite direction.
The advantages of solid media storage are as follows:
Stones or pebbles are abundantly available
Low cost
Non-combustible
Easy to handle
Possibility of high storage temperature
No freezing point during heat removal
No corrosion problem
No requirement of heat exchanger
The disadvantages are as follows
The size of the storage container should be large
Simultaneously charging and discharging of energy is impossible
Large pressure drop needs high capacity air blower
4. Latent heat storage
In latent heat storage, heat energy is stored by virtue of latent heat which is required to bring about phase change of storage medium. The heat required to bring about phase change of a material is much larger compared to sensible heat change of the same material. The phase change of material also involves absorption or release of large quantity of heat energy at constant temperature, which is impossible in the case of sensible heating and cooling. Therefore, latent heat storage system is more compact for a certain heat storage compared to sensible heat storage system. The phase change which can be used for storage system are solid-solid, solid- gas, solid – liquid and liquid- gas. Solid – gas and liquid –gas transformation involves large volume changes, thereby making such storage system impractical and complex. However, solid- solid transition involves transformation of the material from one crystalline form to another, thereby resulting in the transformation with small volume changes. Hence, such storage systems are practical and preferred in spite of small changes in latent heat possible during transformation.
For the phase-change storage media, salt hydrates called glauber’s salt (Na2SO4.10H2O) are preferred.These have solid – liquid transformation. Besides hydrates, paraffins (C18H36) and non-paraffins (ester, fatty acid, alcohols and glycols) are also suitable for such storage. The hydrate crystals have water of crystallization and these can be represented by X(Y)n.mH2O (one atom of X, n atoms of Y and m molecules of water in one crystal). When hydrate crystals are heated to transition temperature, these crystals release their water of crystallization and anhydrous salt (hydrates without water) get dissolved in the released water.
The problems faced with the use of salt hydrate for latent heat storage are as follows:
(i) The released water of crystallization is insufficient to dissolve all the solid salt produced on heating. The anhydrous salt settles down at the bottom of the container. The recrystallization becomes impossible on removal of heat. The process becomes irreversible and performance degradation takes place.
(ii) Mechanical means (vibration or stirring), the suspension media or thickening agents have to be used to make the system work in reversible manner without performance degradation. The problem can also be resolved by limiting vertical height of the container.
(iii) Heat of fusion is small (251 kJ/kg).
Applications of Solar Energy
- Solar Pond:
Principle: In ordinary pond, when water is heated up by the sun rays, the heated water rises to the top of the pond. The hot water loses heat to the atmosphere, and so the net temperature at the top of the pond remains nearly at atmospheric temperature. The solar pond technology ensures that heated brine water remains at the bottom of the pond due to more brine concentration and density in it.
The solar pond serves the dual purpose of a large flat collector and a thermal storage system. It consists of a large size brine pond (depth of about 1 m) which has salt concentration gradient in such a way that the most concentrated and dense part of the brine solution is at the bottom of the pond and brine concentration gradually reduce from bottom to top of the pond based on the variation of brine solution density. A solar pond has three zones, the top zone is surface zone which has the least salt content and its temperature is the atmospheric temperature. The bottom zone collects and stores the solar energy as heat energy. In between these two zones there is the gradient non- convective zone.
The hot brine solution from the bottom of solar pond is taken out without disturbing the brine gradient existing in the solar pond. This solution is taken to heat exchanger to remove used to run a turbine which is coupled to a generator to generate power. The refrigerant vapours existing from the outlet of the turbine are condensed to liquid state in a condenser and pumped to heat exchanger. Solar pond electric power plant is shown in figure 4.
5 Solar water heater:
A small capacity water heating system with natural circulation is as shown in figure 5. It is suitable to supply hot water for domestic purposes. It has two main component which include (i) flat plate collector to convert solar radiation in to heat energy and (ii) water storage tank to store hot water.
The tank is located above the level of collector. The natural circulation of water is established from the collector to water tank and then from water tank to the collector. The hot water for use is withdrawn from the top of tank, which is replaced by cold water entering at the bottom of the tank. Water heating system is also provided with an auxiliary heating system so that the system can also work during cloudy and rainy days when sufficient solar radiation is unavailable.
- Solar thermal pump:
Solar pumping utilizes the mechanical power generated by the solar radiation to run the water pump. Solar energy offers several beneficial features which make its utilization in irrigation pumping quite attractive. The features are as follows:
(i) The need for pumping arises most during the summer months when solar radiation is intense.
(ii) Pumping can be carried out intermittently without any problem.
(iii)Surplus pumped water can be stored in a reservoir or tank.
(iv) The requirement of water decreases during period of low radiation when solar pumping decrease. Evaporation losses reduce during cloudy days. Rainwater is also available during rainy days.
(v) There is relatively inexpensive running and maintenance cost.
The solar pump is similar to solar heat engine working in low- temperature range. The source of heat engine works in low- temperature range. The source of heat is a solar collector. The heat is transported to a heat exchanger where heat is transferred to a refrigerant of low boiling point. The refrigerant evaporates and high- pressure vapour is taken to a turbine to do useful mechanical work by running the solar pump as shown in the figure 6. The outlet refrigerant vapour from turbine is condensed and takes to heat exchanger using feed for reuse.
- Solar furnace:
Principle: Solar furnace is used to study the properties of the materials, such as physical, mechanical, chemical and electrical properties at high temperatures. The focusing type solar collectors can concentrate solar radiation over a small area in a furnace for heating of materials being tested. It is possible to obtain high temperatures which can be about 3500̊C .
The solar furnace has basically two main components; (i) a concentrator with arrangement to position testing materials at its focus and (ii) a system of a large number of small heliostats. The large number of heliostats are located and positioned in such a manner that they direct solar radiation onto a paraboloidal collector figure 7.
The solar radiation after reflection after reflection from the heliostats moves parallel to the optical axis of the concentrator. The heliostats are provided with the system of sun tracking. The concentrator focuses the incoming solar rays on the test material to heat up.
(i) It can be used during sunshine hours.
(ii) The cost of equipment is high.
(iii)Material with small area can be only heated and tested.
Passive Solar technology integrates building design with environmental factors that improves the harnessing of solar energy.
- Solar passive heating:
Solar energy can be used for passive heating of buildings to maintain comfortable temperature inside the buildings. Passive heating of buildings does not require any mechanical device.
This heating consists of natural processes such as convection, radiation and conduction design to ensure natural flow of heat in the space inside building. Such specially designed building is called solar house.
In the northern hemisphere, the sun rays come from south direction. Hence in order to achieve solar passive heating in cold regions, south facing wall is made thick using concrete or stone to store the maximum heat energy from the incident solar radiation. The entire south wall is further provided with a plastic or glass sheet covering with an air gap in between the wall and the sheet covering. The incident solar radiation after passing through the sheet covering is absorbed by the thermal storage wall. The warm air in air gap rises and enters into the space from the upper inlet vents and cold air is removed from the space from the lower outlet vents (Fig 8).
There is another method to provide solar passive space heating, which is shown in figure 9. In this method, a flat plate collector is provided to face south. The collector is provided with rock bed type storage system. During sunshine hours, the collector transfers and stores heat energy from incident solar radiation into the rock bed storage system. The available stored energy in the rock bed is used later at night when air is passed through the rock bed, and so warm air enters into the space to be heated.
- Solar passive space cooling:
The heat tends to enter a building in the following ways;
(i) Direct sunlight heat which can be reduced by using shading and providing venetian blinds to glass windows and doors.
(ii) Conduction of heat through walls, roof and floor. It can be reduced by providing insulation. Maximum heat is conducted through the exposed roof which has to be provided a false ceiling with a good insulating material to reduce the conduction of heat from it.(iii)Infiltration of outside hot air. It can be reduced by proper sealing of the space and reducing the openings of doors and windows.
The methods to reduce or prevent heating of the space are as follows.
(i) Shading of glass area and the walls
(ii) Providing air circulation or ventilation so that warm air is driven out cool air from outside is sucked into the space using chimney effect as shown in a figure (9).
(iii)Providing a pond on the roof to reduce radiation heating and achieve cooling below the pond.
(iv) Providing black plastic bags on a metallic roof which helps in radiating out the heat from the space during night-time
(v) Providing ground coupling or basement construction to maintain temperature of the space close to ground temperature.
The ground temperature is always lower than surrounding atmospheric temperature.
- Solar refrigeration and cooling system:
A simple solar operated absorption refrigeration system to cool a space is as shown in figure(10). The hot water transported from a flat plate collector is passed through a generator which is a heat exchanger.
The heat is transferred to a refrigerant and absorber solution. The refrigerant can be ammonia or water while absorber is water or lithium bromide which generates refrigerant vapours at high-pressure vapours are condensed into high- pressure liquid in the condenser. The high- pressure refrigerant liquid is throttled to low pressure and temperature by an expansion valve. The low pressure refrigerant takes heat from the evaporator and vapourises, thereby cooling air or water which can be used for cooling the space inside the building.
- Solar Cooker
A solar cooker consists of an
– insulated box of blackened aluminium in which utensils with food materials are kept and
– reflector mirror hinged to one side of the box so that angle of reflector is adjusted
– a glass layer consisting of two layers of clear window glass sheets
The box is kept in such a way that solar radiation falls directly on glass cover. The reflector mirror is adjusted in that way additional solar radiation after mirror reflection is also incident on glass cover. The glass cover traps heat owing to green house effect providing more heating effect. The air temperature inside the box ranges from 140-160oC which is enough for boiling and heating purposes
There are other types of solar cookers uses the solar collectors
– community solar cooker with arge reflectors concentrating solar radiation onto a black metal surface
– Dish type solar cookers with parabolic reflectors
- Solar Distillation
It is the process to convert saline water into pure water using solar radiation. This is done with the help of the solar device known as solar still. A solar still consists of a shallow blackened basin filled with saline water that is to be distilled. It is covered with sloping transparent roof. The solar rays pass through transparent roof and are absorbed by the blackened surface of basin increasing temperature of the water. The water in the basin gets evaporated due to heat and rises to roof. The water drops or condensed water slip down along the sloping roof. This water is collected by the condensate channel and drained out from solar still.
- Solar Dryer
This is used to dry the food items with the use of solar radiation. The drying can be achieved directly or by indirectly.
- Solar lighting:
The use of natural light to provide illumination that in turn reduce lighting load on HVAC. The architectural designs of building orientation, window orientation, skylights, etc. can control the entry of sunlight to the building passively. The lighting can also be through photovoltaic technology.
- Solar Electric Technology
This technology converts the solar radiation directly o electricity with the help of photovoltaic cells.
The applications of photovoltaics include
- – solar powered calculator
- – solar powered camping equipment
- – solar powered satellites
- – solar powered cars
Conclusion
The harnessing of solar energy is either by active or by passive technology. It has applications of heating, cooling, desalination, cooking and mainly the electricity generation.
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