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The energy provide by sun is very large but just a little bit can be used by us. The technology of converting solar energy into electricity can be mainly divided into two parts, the solar thermal and solar photovoltaic.
Since this technology is relatively new, some social changes or problems will be experienced. At the beginning, the settlement in some locations may be needed before starting installation of the PV systems. As the land used is very large, we need to consider and communicate to the residents who may be affected.
Meanwhile, the availability of power supply will give a change in the populations of remote areas. The existence of electrical power supply motivates the population to use more appliances that are normally in use in grid-connected areas and bring them a modern daily life. Someone may start to move back to these remote areas resulting in overload of the PV supply systems. In addition, some countries may provide subsidy to the users of PV system for promoting solar power.
The society and technology provider are affected each other and communicate is needed. (I. M. Saleh Ibrahim Al-Jadi)
Nowadays, solar thermal power plants are economically with annual irradiations of more than 1300 kWh/m2. Below this level, the electricity cost of PV systems is lower and PV systems show some slight cost advantages in some regions with above 1600 kWh/m2 due to achievable cost reductions.
Solar PV becomes more and more economical.
In remote areas, connection to the electric grid and building of power line are expensive or inconvenient. Solar power is an economically alternative. Off-grid, Standalone PV system can be used as it can generate electricity without the grid and batteries are used to store electricity for nighttime or emergency use. Sometimes, PV Direct systems may be used for some devices which run and use electricity from PV panels directly when the sun is available. It is the simplest and cheapest system, and quite economic. This method is often used to light a roadside billboard at night, for an emergency roadside cellular telephone, even Satellites in outer space. Usually, water is pumped up to a watering trough when the sun shines. There is no electric energy storage, but potential energy in water. Then, the energy generated will not be wasted.
Moreover, PV array works in parallel with the grid to meet the electricity demand in Grid-connected PV systems. Net metering is an economic way that the electric meter operates backwards for sending back the excess power to the grid if the PVs supply is more than immediate demand. The grid loading reduced but the supply will not be affected and the electric bill can be reduced by selling the energy back to the utility.
Although the installation is quite high, the systems can last for a long time and running cost is low. During long-run period, only the little maintenance is required. Also, the sun is free, unlimited and highly reliable. The benefits of solar power is much significant than the initial cost the systems.
There are some benefits of solar power. Solar power is very clean. As electricity generated from light and no combustion, PV produces less pollution than fossil fuels, no greenhouse gases or hazardous waste. Also, it doesn't require transportation of liquid or gaseous fuel. For example, a 4 kWp PV system prevent environmental effect from reducing 2.4 tons of coal, over 3,600 gallons of water from being burned, 6.2 tons of CO2, 34 pounds of NOx and SO2, and 1.8 pounds of particulates. (A. Shahed, 2010)
Moreover, the limitations of environment are important factors considered for using solar power. For example, large area for installation, the sun does not shine at night and intensity is affected by the weather. So, different systems will be designed in order to minimize the impact or adapt the environment. For example, the size of array can be adjusted, a wind/PV hybrid system will be used if there is wind energy, and PV arrays are installed at different angle to obtain maximum electricity over a year. In Hong Kong, PV arrays are installed as facing south-west with angle of 40°. (EMSD, 2007)
Solar energy is converted into thermal energy (heat) directly. The solar thermal technology can be divided into three types: Firstly, low temp (<100â„ƒ) which use solar flat plate collectors, solar chimney and solar pond. Secondly, medium Temp (up to 400â„ƒ) which use line focusing parabolic. The light is focused onto a "line". Thirdly, high Temp (>400â„ƒ) which use parabolic dish or central tower. The light is focused onto a point.
Different CSP systems for electricity are showed in the figure below.
Normally, the higher the temperature, the higher the efficiency. Line focus has lower efficiency but less expensive and technically less difficult.
The following table shows the comparison of main CSP technologies.
Basically, there are several steps for converting solar energy into heat: light absorption by heat collector, heat transfer by fluid, thermal insulation and heat storage. Sometimes, vapor is used to drive a turbine and produce electricity, and then condensed back into a liquid. There are two basic solar hot water systems. A Thermosiphon system contains solar collector and hot-water tank. The cold water flow from tank to collector while hot water return to tank for use. A doble-cycle system with forced circulation with a conventional boiler for back-up heating. There are a temperature sensor and control unit to contol the pump for pumping water. Sometimes, passive solar heating is used at buildings based on the convection principle of air. The heat is absorbed during the day and is given off at night. ( Concentrating Solar Power Global Outlook 2009)
A solar cell converts the energy contained in photons of light into an electrical voltage and current. When solar cell is stroked by sunlight, electrons absorb enough energy from photon and flow through the external circuit from n-type silicon, and back to the p-type silicon to induce electric current. This photoelectric phenomenon is called Photovoltaic effect.
Solar cells can be grouped together in a PV module, and further become array by wired the modules in series to increase voltage, and in parallel to increase current in order to increase power.
The performance of a solar cell can be evaluated by the efficiency. Nowadays, the efficiency increase from a few percents to almost 20%. Moreover, maximum current increase at increasing irradiance level and maximum voltage decreases at increasing temperature. Therefore, PVs may perform better on cold, clear days. Besides, the output of a PV module can be reduced dramatically even if a small portion of PV module is shaded. To solve that, external diodes can help maintain the performance. (EMSD, 2007)
Solar PV systems
PV panels can be applied on different devices. A PV system can be divided into a PV array and the balance-of-system (BOS) which may include inverters for converting DC power from solar panel to AC power, batteries for storing electricity, charge controller for prevent batteries from being over charged, a meter for measuring electricity. Firstly, PV Direct systems used electricity from PV panels directly. Secondly, Standalone PV systems can work without electric grid. In addition, a backup generator may use to top up the batteries when solar is insufficient. The third one is Grid-connected PV systems. The PV system works in parallel with the grid to meet the electricity demand. The bidirectional electric meter can transfer energy from grid to load or from PVs to grid according to immediate demand. Finally, PV arrays can be built as an integral part of the building to be Building-integrated photovoltaic system (BIPV) to afford a portion of the power demand of building. In Hong Kong, there is a 350 kW (PV) installed on the roof of the EMSD Headquarters in Kowloon Bay which comprises a solar array made up of 2,300 PV modules. (EMSD, 2007)
For improving the solar power, there are different generation of solar cells, and technologies. The solar cell evolved from crystalline PV cells to Thin-Film Solar Cells, and further to multi-junction solar cell which can process more wavelength of sun light with maximum efficiency of almost 40%. Some technologies are developed to save the expensive PV materials that. High-concentration PV technology use mirrors or lenses (Fresnel lens) to focus sunlight onto a small PV cell. Also, sliver technology use long thin solar cells and reflector to reduce half of the surface area of module cells occupied. (EMSD, 2007)
The development is keep going to achieve the goal that high performance solar cells with low cost.
In most locations, the user need to obtain various permits from city or county building department before installation of a PV system, such as building permit, electrical permit. Also, the user may need to obtain approval from an architectural committee or association to comply with the "Covenants, Codes and Restrictions". Therefore, the users need to seek agreement from neighbors and submit system plans to homeowner committee. Code requirements for PV systems are mostly based on the National Electrical Code which defines the requirements for designing and installing safe, reliable of PV systems. The inspector will determine if the PV system is designed and installed properly based on Article 690. (CaliforniaEnergyCommission, 2003)
Sometimes, social movement needs a legislation to take up the initiatives. For example, Spain is on verge of making solar installation legally binding for all new building. When the laws promoting solar energy in Denmark were allowed to decline in 2000, many active groups immediately disappeared from the scene. A social movement needs visible successes to keep going. Moreover, Federal Tax Credit is promoted by Energy Policy Act of 2005. (Scheer, 2007) Homeowners with solar energy systems will receive a tax credit worth 30% of the system cost, limited at $2,000. Businesses that purchase solar equipment will receive a credit worth 30% of the overall system cost with no caps. In addition, these tax credits are authorized in 2008. The benefits of solar powered products can be maximized by adding numerous products to other conventional system with the addition of environmental and economic benefits. (SolarDirect, 2011)