SOLAR ENERGY COLLECTION
SOLAR THERMAL ENERGY SYSTEMS
Solar thermal energy systems are broadly characterized as follows, depending on the way they capture, convert and distribute solar energy.
(i) Passive solar thermal energy systems
(ii) Active solar thermal energy systems.
Passive Solar Thermal Energy Systems
in passive solar energy systems, the heating is carried out without any special device for energy conversion, Heat is directly received and used for heating. Heating the houses, water, cooking. Drying, etc. Are the examples of this application. These are the low temperature applications of solar energy. These systems are a simple and cost-effective way to take the advantage of the sun's free, renewable energy.
A passive solar system does not involve mechanical devices or the use of conventional energy sources beyond the limit of needing to regulate dampers and other controls, if any, examples of basic passive solar structures are greenhouses, sunroofs. As the Sun rays pass through the glass windows, the interior will absorb and retain the heat.
In passive solar energy systems, the energy collected through the solar system is distributed according to a law of thermodynamics. It states that the heat moves from warm to cool areas and surfaces. The simple way of transferring heat from passive solar collectors is by convection mode.
Ancient people used passive solar energy systems. They build their houses out of stone or clay which absorbed the sun's heat during the day and stayed warm after dark, providing heat throughout the night. Hence, the properties of building materials and building design both play a role in the energy balance of the system. However, it requires a careful design and is often difficult to implement in an older and present buildings, those did not have the design feature when manufactured. Buildings today use similar methods for building new buildings for passively capturing solar energy.
Passive solar energy systems include orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties, and designing spaces that naturally circulate air. Strategic planning of building location, orientation and materials provide a great control over the inside temperature. Installing large south facing windows, planting shade trees and using Tromped walls which are made of absorptive materials that store heat during the day and slowly release it at night are measures that can be adopted for new and old buildings alike. For example, Sun spaces or glass rooms built on the south side of a building can provide up to 60 % of a home's winter heating Also, builders construct houses with large double- or triple panel windows that get direct sunlight to capture and magnify the sun's warmth. The inside air becomes much warmer than the air outside because the windows let in the sun's energy and trap it, gradually raising the temperature.
Active Solar Thermal Energy Systems
In an active solar thermal energy system, a solar collector holding a heat-transfer medium such as air or liquid captures the solar radiation, which is then distributed through the building via electric fans or pumps. For example, a solar collector positioned on the roofs of buildings heats the fluid and then pumps it through a system of pipes to heat the whole building
The technology is simple and used in many possible applications of low temperature heat use systems. The most common application of these systems is the production of domestic water heater known as solar water heaters.
The typical basic components of an active solar heating system include the following
(i) Solar collector
(ii) Storage unit
(iii) Load and
(iv) Auxiliary source.
Active solar energy systems are the most cost-effective in cold climates with good solar resources when they are displacing more expensive heating fuels such as electricity, propane, and oil.
The disadvantage of active solar systems is that the use of this device external power sources can fail, which needs more controls and maintenance.
The following active solar thermal energy systems are commonly used:
(i) Solar water heaters
(ii) Photovoltaic (PV) cells or solar cells and
(iii) Concentrated solar power (CSP).
(i) Solar water heaters:
Solar water heaters (active) produce thermal energy to heat water for households, commercial entities and swimming pools. These heaters are one of the most commonly implemented renewable energy technology because of their cost-effectiveness and relatively simple installation. With the proper model installed, they heat efficiently regardless of outside temperature. Solar water heaters typically need a backup conventional gas or electric water heater to account for cloudy days or unusually high water demand.
Solar water heaters consist of two parts, such as solar collector and storage tank. In warm climates, collectors heat water directly, but in cold climates, a denser fluid is heated and then transported to a water tank where it heats the water indirectly. The heater can be built to use an active or passive system for circulating warm fluid, depending on climate and the time of day when water demand is the highest. The maximum heating temperature varies with collector model, but water temperature can exceed 90 °C, suitable for commercial purposes.
Solar water heaters can reduce conventional energy consumption for heating water by 60% in commercial applications and up to 75% in homes. Although initial home installation costs at least double that of conventional heaters, the reduction in gas or electric bills realized over their 15-20 year lifespan allows solar water heaters to equal or better the long term cost of other water heater
(ii) Photovoltaic (PV) cells or solar cells:
Photovoltaic (PV) cells or solar cells are an active system in which small panels are applied with semiconducting material. PV panels convert the sun's rays into electricity, which can power a variety of individual items from personal computers and streetlights to water pumps. This material, usually made of silicon but potentially other poly crystalline thin films, generates a direct current when sunlight hits the panel. PV cells are effective in all regions of the country. In a solar PV array, it does not matter how hot the sun is by the time its light and warmth reach the home, as long as some of the sun's rays are hitting the solar panels.
Commercially available PV panels are up to 22.5% efficient at converting sunlight into electricity in optimal conditions but even in partly cloudy weather they can operate at 80% of their maximum output. PV cells can be installed on Windows and roof tiles. PV systems can be tailored to meet a building's energy needs by adding concentrating or sun-tracking devices, DC-AC converters and/or battery storage.
PV systems may or may not be connected to the electric transmission grid. When the number of PV panels are arranged as an array, and it is connected to the grid, the excess power can be sold to the electric company.
(iii) Concentrated solar power (CSP):
Concentrated Solar Power (CSP) is an active system distinguished from other sol energy systems by its ability to function as a utility-scale power plant. CSP uses the fields of mirrors to concentrate solar energy into channels holding heat-responsive fluid. The u temperatures excite the fluid to a point where it powers a turbine or engine, which in turn runs an electric generator. Without storage facilities, CSP systems can generate electricity about eleven hours on a sunny summer day. However, CSP systems do have the potential to provide base load power for utilities. A CSP system that uses oil or molten salt as a medium in the heat-transfer process can retain the thermal energy in thermos-like tanks for the use when sunlight is not available.
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