Solar Energy – A renewable and emission-free power source (Part II)
By: Marina Thurau
In the first part of this article we had a closer look at the sun and the processes creating the different types of electromagnetic radiation and how scientists found ways and materials of using this renewable and emission-free solar power.
In part two we want to have a closer look at – solar-thermal energy (STE) and its applications.
STE generates electricity indirectly, by heating a fluid (water, oil, alcohol etc.) through special panels. The heated fluid (e.g. water) can be used directly in households, office buildings or remote places, where power supply for heating water would be too expensive, or it can be transformed into steam powering a generator or turbine that produces electricity in a second step.
Solar Thermal Applications can be divided into three different temperature ranges: low temperature (> 30°C) for swimming pool heating; medium temperature (30°C – 100°C) for domestic water and space heating, commercial cafeterias, laundries, hotels and high temperature (> 100°C) for industrial process heating and electricity generation.
For each temperature range special collector types (Figure 1) are recommended because of their high efficiency in this particular range: low temperature – unglazed mats for water heating; mid-temperature – glazed and insulated collectors and high-temperature – evacuated tubes or focusing collectors.
Figure 1: Types of collectors for the different temperature ranges (source: NREL)
a) Mid-Temperature Example: Water Heating System
Solar water heating systems are using natural energy from the sun. Solar energy heats up large panels called thermal/ solar collectors (Figure 2 and 3). The energy is transferred through a fluid (often water, but it can also be a heat-transfer fluid, such as a water-glycol antifreeze mixture) to a reservoir tank for storage and subsequent use. It is then used to heat water for commercial or domestic use and also as an energy input for heating and cooling devices and for industrial ‘process heat’ applications.
Solar water heaters can either be passive or active. A passive system operates without a pump, while an active system uses an electronic pump to circulate the heat-transfer fluid.
The system can also be distinguished as a direct or indirect system. The direct system circulates the water between the collector and the tank itself, while the indirect system uses a heat transfer fluid (mentioned above) and a heat exchanger, so that the water in the tank never reaches the collector.
There are various factors influencing the amount of hot water a system produces: type and size of the system, amount of sun available, proper installation and the tilt angle and orientation of the collector at the site.
Figure 2: Schematic design of a typical solar water heating system (Source: Home energy magazine online, 1995)
For sizing a system experts use worksheets and special computer programmes to assist with the determination of the total collector area and the storage volume required for the provision of 90 to 100 % of a household´s hot water needed during the summer.
The rule of thumb concerning the tank size is normally roughly 50 litres capacity for each person in a household. So if for example you are a family of two, use the small 100-litre system, if you are four, look at the 200 litre tank measurement results. Add an additional 50 litre, if you may use a bit more water than average.
Another aspect is the size of the collector area; allow about 1.5 square meters of collector area for each of the first two family members. For any additional family member add about 0.6 square meters of collector area. This works in areas like Accra, Kumasi or Tamale with a daily average radiation of 5 kWh/m².
Advantages of a solar water heating system are: 1. very safe, because of minimal risk of electric shock; 2. environmentally friendly; 3. system will pay for itself in about 4 to 5 years and then save money for the rest of its life-expectancy (usually 15 to 30 years depending on the set up, model and conditions); 4. Storage capacity of hot water (tank) is larger than a standard water heater; 5. independence of the electric power provision and future increase of fossil fuel prices; 6. increases the market value of your house; 7. For warmer climates like in Ghana, anti-freezing units and controllers are not necessarily required.
Disadvantages: 1. the initial investment; 2. Maintenance of the panels or tank may be needed; 3. a electric powered backup heater may be necessary for further heating the water and kill any bacteria, or at night time and cloudy days when the system doesn´t receive enough sun energy for producing enough heat. This will involve extra costs.
Figure 3: Solar water heating system on a roof top in Cape Town (Source: www.solarquotes.co.za, 2011)
b) High-Temperature Example: Indirect Energy
Here, solar energy is concentrated on the absorber tube located in the focal line of solar mirrors (see Figure 1) lined up in large fields of single-axis tracking parabolic trough solar collectors (Figure 4). This process heats a heat transfer fluid in the pipe to about 400 °C and, via heat exchanger, generates high-pressure superheated steam to drive a turbine, which will produce electricity. The spent steam from the turbine is condensed in a standard condenser and returned to the heat exchanger via condensate and feedwater pumps to be transformed back into steam. The process is similar to conventional power plants, just that there are no fossil fuels necessary to run the turbines. The advantage is that solar thermal power clean, easy to predict and that it allows economical storage. Unlike other technologies, the energy can be stored as heat in large tanks during sunshine hours and released during cloudy periods or at night. Therefore solar thermal power plants can provide constant, reliable power. There are several solar thermal power plants operating in the U.S., Europe (Spain), Saudi Arabia and North Africa. More and larger plants are planned to be build in future in the deserts of North Africa and Asia.
Figure 4: Solar Thermal Electric Power Plant using Parabolic Trough Mirrors (source: http://www.solarmillennium.de/deutsch/archiv/technologie/parabolrinnen-kraftwerke/index.html)
In part three of this article we will have a closer look at the Photovoltaic (PV) which is applied for generating solar power directly.
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