Solar Energy
| Author | William Webster |
| Pages | 205-226 |
Chapter 9
Solar Energy
PHOTOVOLTAIC SYSTEM
9.1 Solar power is the conversion of energy from sunlight into electricity. The energy from sunlight is typically converted into electricity by means of photovoltaics (PV), which comprises an arrangement of several components including solar panels to absorb and convert the sunlight into electricity, a solar inverter to convert the output from direct to alternating current, as well as mounting, cabling and other electrical accessories to set up a working system. A solar array (a loosely defined term referring to a group of PV solar panels or cells arranged and linked in such a way as to operate as a single unit) encompasses only the solar panels (which is the visible part of the system) and does not include all the other components. PV systems should not be confused with other technologies such as concentrated solar power or solar thermal power which is used for heating and cooling.
SOLAR CELLS
9.2 The essence of a PV system is its solar cells. PV cells are packed together in a frame known as a solar panel. Solar cells are made of semi-conductors such as silicon, which absorb the sunlight and convert it into electricity. A solar array consists of one or many solar panels although the solar array does not encompass the entire system.
9.3 A typical 150 watt solar panel is about a square metre in size and has a power output of around 265 watts, although it can range anywhere from as little as 225 watts to more than 350 watts. The higher the wattage of a solar panel, the more electricity it can produce under the same conditions. The life of a solar panel is typically 25 years. The payback period for an investment in a PV solar installation varies, but is likely to be in the region of 10–20 years.
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TYPES OF PHOTOVOLTAIC SYSTEMS
9.4 PV systems range from small, rooftop-mounted or building-integrated systems with capacities from a few to several tens of kilowatts, to large utility-scale power stations capable of generating hundreds of megawatts. Most PV systems are connected to the grid while those off the grid account for only a small portion of the market. PV systems rarely use battery storage although this will change as improvements in technology make it economically viable for smaller systems. Operating silently and without any moving parts or environmental emissions, PV systems have developed into a mature technology used for mainstream electricity generation. Solar energy is obviously important as the sun is a sustainable source of energy, whereas fossil fuels are finite and add to greenhouse gases.
9.5 As indicated above, PV systems are usually divided into three distinct categories:
(a) Residential rooftop: a solar array of a typical residential PV system is rack-mounted on the roof rather than integrated into the roof or facade of the building whose function is to supplement power within the dwelling. A typical small rooftop array mounted on a sloped roof may produce up to 10kW producing around 95% of clean net renewable energy over a 30-year service lifetime. Although rooftop-mounted systems are small and have a higher cost per watt than large commercial-scale installations, they account for the largest share of the market. There will usually be a connection to the grid so that any surplus energy produced by the PV system can be sold to the grid provider.
(b) Rooftop, mobile and portable: a small PV system is capable of providing enough electricity to power a single home or isolated devices such as street lights, construction and traffic signs, powering electric cars or solar-powered tents. Portable and mobile PV systems are also commonly used on recreational vehicles and boats.
(c) Building integrated: in urban areas, PV arrays are often used on rooftops to supplement power use and the building will usually have a connection to the grid, in which case the energy produced by the PV array can be sold back to the provider in some sort of net metering agreement. Some providers use the rooftops of commercial customers and telephone poles to support their use of PV panels.
PERFORMANCE
9.6 Key performance factors are set out in the following list:
(a) Weather fluctuations and changes in the Earth’s position towards the sun throughout the year. If sunlight is not constant irradiance will vary throughout the day. The angle of the sun, passing clouds, hazy weather and air pollution can all affect irradiance levels. Temperature also affects the efficiency of solar cells and the warmer solar cells get, the less efficient they are. Snow also blocks production of solar energy. It is perhaps worthy of note that although one imagines that a sunny climate is essential, Germany leads the world in installed solar capacity.
(b) The efficiency of the solar panels. The more efficient the panel the more electricity it can create from sunlight. Solar panel efficiency is measured by how much sunlight a solar panel can convert into usable electricity. The majority of residential solar panels typically have an efficiency of 15–18% although premium models can reach over 21%. The most efficient solar panel on the market is currently rated at 23.8%.
(c) Losses due to shading and dirt, dust, grit and other debris settling on the surface of the solar panels will block sunlight from reaching the solar cells and will reduce performance. In areas with frequent rain soiling is not usually significant, whereas areas that experience long periods of dry weather will give rise to more soiling during the summer. The same applies in the case of systems located near to construction sites and other places that produce dust. Solar panels therefore need to be regularly maintained and cleaned.
(d) Defects in panels: the most common problems with solar panels are hot spots on the panels; micro-cracks; snail trail contamination; degradation in the crystalline PV modules; internal corrosion/delamination.
PHOTOVOLTAIC MOUNTING
9.7 PV modules (and a solar panel consists of a string of connected modules which are assemblies of packaged PV cells) are assembled into arrays on some kind of mounting system which may be classified as ground mount, roof mount or pole mount. For solar farms (also known as solar parks or solar fields) large racks are mounted on the ground and the modules are mounted on the racks. For buildings, many different racks have been devised for pitched roofs. For flat roofs, racks, bins and building integrated solutions are used. Solar panel racks mounted on top of poles can be stationary or moving. Side-of-pole mounts are suitable for situations where a pole has something else mounted at its top, such as a light fixture or an antenna. Pole mounting raises what would otherwise be a ground-mounted array above weed shadows and livestock and may satisfy regulatory requirements regarding inaccessibility of exposed wiring. Pole mounted panels are open to more cooling air on their underside which increases performance. A
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multiplicity of pole top racks can be formed into a parking carport or other shaded structure.
CABLING
9.8 Owing to outdoor usage, solar cables are designed to be resistant against ultra violet radiation and extremely high temperature fluctuations and are generally unaffected by the weather. Standards specifying the usage of electrical wiring in PV systems include the British Standard BS 7671, which incorporates regulations relating to microgeneration and PV systems.
SOLAR TRACKER
9.9 A solar tracking system tilts a solar panel throughout the day. Depending on the type of tracking system, the panel is either aimed directly at the sun or the brightest area of a partly clouded sky. Trackers greatly enhance early morning and late afternoon performance increasing the total amount of power produced by a system by about 20–25% for a single axis tracker and about 30% or more for a dual axis tracker, depending on latitude. Trackers are effective in regions that receive a large portion of sunlight directly. In diffuse light (under cloud or fog) tracking has little or no value. Because most concentrated PV systems are very sensitive to the sunlight's angle, tracking systems allow them to produce useful power for more than a brief period each day. Tracking systems improve performance for two main reasons. First, when a solar panel is perpendicular to the sunlight, it receives more light on its surface than if it were angled. Second, direct light is used more efficiently than angled light.
9.10 Trackers and sensors to optimise performance are often seen as optional, but they can increase output by up to 45%. Arrays that approach or exceed 1MW often use solar trackers. For large systems, the energy gained by using tracking systems can outweigh the added complexity. For very large systems, the added maintenance of tracking is a substantial detriment. Tracking is not required for flat panel and low-concentration PV systems, but for high-concentration PV systems, dual axis tracking is considered to be a necessity.
BATTERY
9.11 PV systems increasingly use rechargeable batteries to store a surplus to be later used at night. Batteries used for grid storage also stabilise the electrical grid by levelling out peak loads and play an important role in a smart grid as they can
charge during periods of low demand and feed their stored energy into the grid when demand is high. PV systems with an integrated battery solution also need a charge controller as the varying voltage and current from the solar array requires constant adjustment to prevent damage from overcharging. Batteries for one’s home or business are heavy and large.
SELLING SOLAR ELECTRICITY BACK TO THE GRID (DOMESTIC INSTALLATIONS)
9.12 In 2009, it was possible to install solar panels and start breaking into profit at some point in the system’s lifetime. The first income stream involved the immediate savings on domestic energy bills. The second income...
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