Reliability and Maintenance of Solar Renewable Systems - Case Study Example

?Reliability and Maintenance of Solar renewable systems Reliability being the ability of an item to perform its intended function under pre-defined conditions for a stated period of time, it has an inherent inverse relationship with ‘failures’; lesser the failures, higher the reliability and vice versa; higher the failures, higher the maintenance being the logical corollary. The renowned “Bath Tub Curve” concept highlighting the failure zones which precede and succeed the useful lifetime of any product, when depicted on a time scale as shown below, can be deemed to be a measure of its performance, and is more relevant for rotating equipments. Every failure has a cost attached to it. The principal reliability-linked factors of cost through the useful life of an item being Design & Development, Production and Maintenance & Repair, evaluated in the backdrop of installation and environmental parameters like temperature, humidity, vibration, corrosion by chemical attack etc. and the degradation caused by the combination of a few or all of them. A higher reliability invariably envisages higher production, design and development costs, whereas, appropriately factored-in maintenance and repair costs can lead to higher levels of reliability. In the context of non-sustainability of the current energy practices, dawning of global awareness that renewable energy sources are the only way out, has led to a sensible emphasis on solar energy as one of the (wind energy being the other) most abundant and attractive renewable energy source, easily available for harnessing. In the scenario of rapidly depleting natural energy sources like coal, oil and natural gas, the solar option has presented itself as an alternative which is being vigorously pursued for viability for commercial exploitation in the longer run. A system that uses solar cells and solar panels in a mechanical system that collects, transforms DC current into AC current and stores the electricity for later use is often called Active Solar Energy. The deeper “Bath Tub” assumes the shape of a “Shallow Pan” in the case of static equipments predominantly deployed for renewable energy applications, as far as reliability is concerned; the failure zones in this case are therefore not pronounced. Listed below are a few indicative amongst the many, though not exhaustive, but popular applications: Solar air conditioning Solar ovens for heating of foodstuffs Agricultural irrigation Hydrogen generation by the use of photo-electro-chemical cells The following applications however stand out amongst the most popular commercial applications: 1. Generation of electricity in Solar updraft towers Solar towers use an array of reflectors termed ‘heliostats’, approximately 120 sq. meters in size. Located on top of 150 meters tall towers within receptacles, these are directed towards the sun for direct absorption of the heat of the concentrated solar radiation. Operating temperatures close to 1000oC can be attained depending upon the choice of the heat transfer fluid (HTF) which in turn also decides the design of the receptacle, lined with the very best quality insulation material for optimum performance. High reliability is foreseen at the design stage itself. Likely failure points are identified in advance using the FMECA (Failure Mode Criticality and Effect Analysis) technique and appropriate specification de-rating vis-a-vis the liberal vendor rating is resorted to, for ensuring realistic unhindered performance. Finite element thermal simulation and sensitivity analysis to comply with the thermal specifications also forms part of the internal product review. External third party assessment provides inputs for MTBF (Mean Time between Failures) calculation for useful lifetime assessment, maintainability and manufacturing as part of an iterative design process for consistent improvement of product designs and individual components. Selection of the most appropriate materials of construction of all the components of solar renewable energy systems forms the last and the most important lap of the exercise to afford reliability and longevity to them to render them maintenance-free. 2. Generation of electricity using photovoltaic (PV) solar cells Abundance of free and seemingly infinite quantity of sunlight available to the earth is a resource of energy, which has remained relatively untapped. Harnessing of this energy for conversion to electricity by photovoltaic technology is of recent origin and offers considerable potential for its commercial exploitation in the years to come, amidst dwindling resources of fossilized fuels. It has emerged as a major power source, because of its proven reliability and several economic and environmental benefits. The factors of reliability which support its cause are listed below: It is FREE, clean and permits storage for use, when needed. Its biggest advantage is that it is virtually maintenance free and absolutely reliable, because of the absence of moving parts; it is designed for ease of assembly, use and maintenance. Its yet another significant benefit is that, it offers an option for nations to rid themselves of avoidable dependence on oil for fulfilling their gross energy needs. It is a nascent technology possessing the long term potential to create additional jobs for strengthening of economies. It does not generate hazardous waste or cause atmospheric pollution by release of gases like carbon dioxide, sulphur dioxide, nitrous oxide or mercury like many traditional forms of electrical generation do. It does not require liquid or gaseous fuel to be transported or combusted. It does not contribute to global warming, acid rain or smog. Its far reaching benefits outweigh the high initial cost of the systems Innovations in product design in recent times have led to the feasibility of execution of maintenance of roof mounted PV systems from ground level, without taking recourse to body harnesses, ladders and cranes for accessing the roofs in compliance with the safety requirements. In colder countries where snow accumulation on roofs could pose a maintenance problem, safety and liability issues associated with jobs involving working at heights with slippery work surfaces have virtually been dispensed with. Contrarily, in hotter tropical countries, where accumulations of dust and foreign matter over the PV solar panels pose a severe maintenance problem, it has been intelligently resolved by providing them with dust-repellant coatings and by mechanisms for auto-blowing of the dust periodically. High wind velocities are known to cause problems of stability of solar arrays. Gyro-controlled hydro-electronic devices engineered with dual axis tracking systems to align the arrays in a horizontal position to allow the flow of air above and below them in cyclonic weather, has resulted in protecting them against undesirable stresses for enhanced reliability. Research and advanced technology have also made it possible for the solar arrays to align themselves with the trajectory of the Sun’s path so that the Sun’s rays are always incident upon them, leading to enhanced performance of the PV systems with the advent of similar control mechanisms. Functional and operational needs have led to the broad categorization of PV systems into two types, viz; Stand Alone and Grid-Connected. As the name itself suggests, the former type of PV systems are ideally suited for DC and/or AC electrical loads and have established their reliability for daylight hour applications like water pumps, ventilation fans, small circulation pumps for solar thermal water heating systems and ventilation blowers etc, which are designed to operate independent of the electric utility grid; however, batteries could be used for energy storage, if deemed necessary, for specific end applications. The reliability of consistent AC voltage and power quality required by any utility grid is assured by the DC power generating PV arrays, which receive the energy feed from the core component of the latter type of Grid-Connected PV systems called the Inverter. Depending on whether the on-site electrical loads are required to be fed or to reverse feed the grid when the PV system output exceeds the on-site load demand, a bi-directional distribution interface on the downstream side of the inverter, performs the regulation function. Bulk of the equipments being static without any rotating parts, the bare minimum maintenance is limited to only the storage batteries. Established technologies are getting outperformed by the emerging energy storage technologies encompassing the usage of Li-ion, NiMH, flow batteries, ultra-capacitors or flywheels that are expected to provide reliable and safe energy storage at competitive cost. 3. Large scale electricity generation using Concentrated Solar Power (CSP) The method by which thermal energy of the Sun is collected during the daylight hours and stored in specially designed storage systems for eventual generation of electrical power is called the CSP technique of solar energy power generation. A schematic of the “Parabolic Trough” system, in which an array of solar energy collectors, each of which can swivel on the N-S axis to trace the trajectory of the path of the Sun, and harness its energy appears as shown below. Each parabola shaped linear collector is a receptacle of the direct heat radiation from the Sun, which transmits the energy absorbed to the focus of the parabolic trough, where the temperature is of the order of 4000 C. The stored thermal energy redeployed for running steam turbines, which convert the thermal energy to electricity is the most common method in use. The swiveling action of the parabolic reflector troughs is achieved by ball and socket joint assemblies, which apart from interconnecting the receptacles arranged in parallel rows, also serve as a conduit for the HTF to flow to the distribution pipes. Insulation of these ball joint assemblies to allow free and flexible movement, poses the toughest challenge for ensuring system reliability. The other important factors of reliability with maintenance orientation which merit attention among others include: Thermal insulation quality which can bring down heat losses by 30-40 % Lowering of installation cost by resorting to one layer insulation system Zero degradation to achieve the declared lifetime of the solar plant Non combustible to render it fire accident free Environment friendly with total recyclability Free from health hazards 4. Passive Solar Heating of buildings Landscaping, insulation, use of overhangs and thermal windows are becoming the modern norms for construction of homes with passive solar energy initiatives. Designed to make a house more energy efficient and to trap the heat of the sun to help retain internal heat during colder months, passive solar heating can be used to heat parts of new homes and reduce the overall heat loss in a house to reduce energy consumption. This concept is finding extensive acceptance in the sunlight starved countries, for which hardly any maintenance is foreseen. The single determining factor, apart from those listed above, however is the criteria for selection of the location of installation and orientation of these PV systems. While sunlight is the all pervading nature’s free bounty, but countries in the proximity to the equator, lying between the tropic of Cancer and the tropic of Capricorn called tropical countries are the ones endowed with the brightest sunshine virtually all round the year. They obviously possess the brightest potential for harnessing of the abundant solar energy by virtue of the geographical advantage they enjoy, more particularly in the high noon conditions of the summer months. Solar panels facing southward with a tilt facilitating sun rays to fall at right angles to the earth assure maximal output. It is interesting to note that the world’s largest solar plant spread over 1000 acres, and which accounts for 90% of the global commercial production, is located in the Mojave Desert of North America and has a good record of reliability; solar power produced in the desert is also for obvious reasons, estimated to cost three times lesser than in the plains or elsewhere. Other countries proactively engaged in their solar energy pursuits are North America, Germany, Italy, Spain, Brazil, Peru, Middle East countries in UAE, Israel, Saharan Africa, Korea and Japan to name a few, not to mention of China and India who have also joined this league. North America, and Germany have over the years, acquired a distinct edge over the others in respect of their focused approach at developing both captive solar installations and Japan for grid-connected, maintenance-free, special synthetic resin- coated or polymer foil lined photo voltaic panels for residential applications, in particular for illumination and street lighting. The creation of solar panels typically involves cutting Crystalline Silicon into tiny disks less than a centimeter thick. These thin, wafer-like disks are then carefully polished and treated to repair and gloss any damage from the slicing process. After polishing, dopants (materials added to alter an electrical charge in a semiconductor or photovoltaic solar cell) and metal conductors are spread across each disk. The conductors are aligned in a thin, grid-like matrix on the top of the solar panel, and are spread in a flat, thin sheet on the side facing the earth. To protect the solar panels after processing, a thin layer of cover glass is then bonded to the top of the photovoltaic cell. After the bonding of protective glass, the nearly-finished panel is attached to a substrate by an effective, thermally conductive layer of cement. The thermally conductive properties of the cement keep the solar panels from becoming overheated and obviate the need for maintenance. Reliability improvement is ensured by the making the provision for air to blow above and below them to dissipate the leftover energy that the solar panel is unable to convert to electricity and would otherwise overheat the unit and impair the efficiency of the solar cells. Amorphous Silicon (A-Si) solar panels, thinner than their traditional crystalline counterparts, are a powerful, emerging line of photovoltaics, which differ in output, structure, and manufacture. A-Si solar cells are developed in a continuous roll-to-roll process by vapor-depositing silicon alloys in multiple layers, with each extremely thin layer specializing in the absorption of different parts of the solar spectrum. Reduced materials cost apart, this option is efficient, reliable and virtually failure-free. Some Amorphous Solar Panels also come with shade-resistant technology or multiple circuits within the cells, so that if an entire row of cells is subject to complete shading, the circuit continuity would not be fully impaired and some output can still be extracted. The element of maneuverability built into the development process of Amorphous Silicon solar panels also renders them much less susceptible to breakage during transport or installation, thereby reducing the risk of damaging the significant investments made in photovoltaic systems. By virtue of the materials chosen for manufacture, both these options possess the anti-weathering properties needed for toughness against nature’s fury like extremes of heat and cold (deserts and snowy environs), thunder storms accompanied by lightening, excessive wind velocity-induced stresses from cyclones/hurricanes/tornadoes and sandstorms, corrosion linked to salinity associated with coastal locations, gales and hailstorms etc. which have a direct bearing on their reliability with barely any need of maintenance. With the element of portability built into the design of these systems, modern versions provide ease of accessibility for maintenance, particularly against traffic induced dust, leaves during spring fall and bird droppings. The benefits accruing on account of barely any moving parts in the case of solar equipments, current research is focused at practically doing away with the “Running-in” period of the “Bath Tub” curve, and thereby extending their “Effective working life”. Unlike the post utility phase of rotating equipment which is marked by wear related deterioration in reliable performance and increased maintenance costs, highlighted by the steeper gradient of the “Bath Tub” curve, this degenerative phase is virtually non-existent for components of renewable energy equipments and can be deemed to be an unmixed blessing. Amongst the many innovative solutions developed in the recent years after decades of research include pressure sensitive tapes, industrial cleaners and wipers, epoxy grouting and anchoring systems, gaskets and seam sealants, anti-slip, protective and safety coatings, Butyl mastic tapes, sealants, industrial lubricants and fluids, electrical potting, encapsulants and equipment chocking systems, have contributed to the reliable working of the solar renewable energy systems. It has been predicted that 50% of the world's energy will come from renewable sources by 2040. Eminent global industrial players and stakeholders who are likely to occupy the stage of renewable energy are bigwigs like British Petroleum, Total, Occidental Petroleum Corp, General Electric, Rolls Royce, Fiat and Mitsubishi, to cite a few. Their energies would be ostensibly channelized at pumping in funds for research in increasing the reliability levels of the solar energy equipments by seeking cost effective solutions, so that not only do they last longer but also call for virtually zero maintenance. They would explore the possibility of making the solar panels deliver more than the current average rates of 0.45 – 1.35 KWH/m2/day and the ways and means to enhance the peak solar power availability levels in excess of 1020 Watts/m2. The methodology shall lie in devising technologies to cut down on the absorption losses caused by clouds, rain, snow, fog, haze and smog before the solar energy reaches the earth. An exclusive ministry or department dedicated entirely to the research and development of reliable renewable energy resources in virtually all the countries across the world, with specific allocations of budgets in furtherance of the cause and their incremental increase year after year in the recent decades, is adequate proof of the kind of priority assigned to this domain by the respective governments. The sun will never set on solar power. The more crucial impediment however appears to be the need to change attitudes towards the environment, one another and at increased positive human interaction for seeding a sustainable world. ? ? ? ? ? Bibliography: 1) Reliability and MTBF Overview, Pages 2 – 4/10, (http://www.vicr.com/documents/quality/Rel_MTBF.pdf) 2) High Temperature Insulation in Concentrated Solar Power, (http://www.microthermgroup.com/high/EXEN/site/concentrated-solar-power.aspx) 3) Solar Electricity Basics, (http://homepower.com/basics/solar/) 4) Technical Note – SolarEdge Reliability Methodology – 2/2011, Pages 3 & 5/8, (http://www.solaredge.com/files/pdfs/solaredge-reliability-methodology.pdf) 5) Complete Photovoltaic Systems, July 15, 2011 (http://www.solardirect.com/pv/systems/systems.htm) 6) 30 Facts about Solar Energy, July 17, 2011 (http://www.alternate-energy-sources.com/facts-about-solar-energy.html) 7) How are Solar Panels Made?, July 19, 2011, (http://www.solarpanelinfo.com/solar-panels/how-are-solar-panels-made.php) 8) Residential Solar Energy; Recent Developments and Most Common Usage, July 19, 2011 (http://www.solar-for-energy.com/residential-solar-energy.html) 9) Industrial Solar Power Solutions, July 19, 2011 (http://www.sunwize.com/products/sunwize-solar-power-station.php) 10) Making the case for Renewable Energy, Page 44, June 2011, (http://www.solartoday-digital.org/solartoday/201106) 11) Systems-Driven Approach for Solar Applications of Energy Storage, U. S. Department of Energy, November 5-6, 2003, (http://www1.eere.energy.gov/solar/pdfs/sda_consolidated_minutes.pdf) Read More