THE EVOLUTION OF PV SOLAR POWER ARCHITECTURES: A QUANTITATIVE ANALYSIS OF MICRO- INVERTERS PERFORMANCE VS. CONVENTIONAL INVERTERS

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1 THE EVOLUTION OF PV SOLAR POWER ARCHITECTURES: A QUANTITATIVE ANALYSIS OF MICRO- INVERTERS PERFORMANCE VS. CONVENTIONAL INVERTERS Dr. Alaa Mohd Enecsys Europe GmbH, Louisenstr. 65, Bad Homburg, Germany addresses: Alaa.Mohd@enecsys.com ABSTRACT: The application of renewable energy in power generation is steadily increasing. This rapid growth is supported by the recognition of these systems as an effective generation source with positive environmental and economical effects. A key element at the grid side is the inverter. In conventional PV solar installations, modules are connected in series, like a daisy chain, to form a string. A central inverter converts the high voltage DC output of the string to AC for connection to the grid. The micro-inverter architecture allows each module to be an independent, standalone, solar AC grid connect system with its own optimized energy production output. The purpose of this work is to introduce a quantitative analysis of the yield gained of both Micro-inverters and Conventional inverters. The study will show data from different inverters and different locations. This data will be analysed and the advantages and disadvantages of both approaches will be discussed in detail. Keywords: Micro Inverter, Solar, Photovoltaic, Balance of System. 1 INTRODUCTION Traditional The electric energy is the backbone of our society. It continues to support the growth, welfare and progress of the human race since Thomas Edison began his work on the electric light in The electric energy demand of the world is continuously increasing, and the vast majority of it in most countries is generated by conventional sources of energy. However, the rapid growth of global climate change along with the fear of an energy supply shortage and limited fossil fuel is making the global energy situation tends to become more complex. The increasing demand for electric power than the offer, along with many developing countries lacking the resources to build power plants and distribution networks, and the industrialized countries that face insufficient power generation and greenhouse gas emission problem forces us to consider a better economical and environmental friendly alternative. Renewable energy sources (RESs) such as wind turbines, solar panels and fuel cells could be part of the solution. All of these sources require interfacing units to provide the necessary interface to the grid. The core of these interfacing units is power electronics technologies because they are fundamentally multifunctional and can provide not only their principle interfacing function but various utility functions as well. The evolution of the Solar Power Systems will be briefly discussed in the following section. Figure 1: Conventional string architecture in a solar PV system. Some companies now offer so-called optimizers so that more power can be extracted from each solar module. These are DC-DC converters that add about $200 to the cost of each module. The need for a large central inverter remains. In the micro inverter architecture, Figure 2, a small inverter is attached to the back of or integrated into the solar module. A 7kWp system might use 29 microinverters each rated at 240Wp. 2 SOLAR PV ARCHITECTURES COMPARED In conventional PV solar installations, shown in Figure 1, solar modules are wired in a series, creating a PV array, before connection to a string or central inverter. A string or central inverter converts the high voltage DC output from the PV array to AC for connection to the electricity grid. For residential and commercial installations, the inverter will typically be rated at 3kWp or 5kWp. In a typical 7kWp system, two string inverters may be used. Figure 2: A micro-inverter based PV system reduces the cost-per-harvested Watt by up to 20% over the lifetime of the system. At first sight, it might seem that the micro-inverter architecture is more complex and more expensive. Clearly, compared to conventional solar PV systems it has more components. Micro-inverters are also less efficient in power

2 conversion than central inverters. However, when considering the lifetime economics of solar PV systems, those based on micro inverters can be shown to offer a 20% advantage in LCOE terms. A detailed analysis of costs vs. total power harvested reveals the true picture. The following analysis compares a conventional string inverter system with one in which micro-inverters are integrated into the solar module to produce a so-called AC module. Today Micro inverters are connected as separate devices to DC solar PV modules but there is a growing trend to integrate them into the modules. For this discussion, we ll consider a system with a rating of 7kWp. All cost assumptions in this paper are based on research carried out during Costs are shown in US dollars and are based on wholesale prices. Figure 3: The Enecsys micro-inverter. Capital costs Whichever system architecture is selected, the basic cost of modules will be the same. Let s assume a solar module price of $2 per Wp. The module cost for our 7kW system is therefore $14,000. The string inverters will cost around $0.5 per Wp or $3,500. Micro inverters will be more expensive at perhaps $0.75 per Wp or $5,250. When the cost of inverters plus modules is added together, a micro-inverter based system ($19,250) costs around 10% more than the string inverter system ($17,500). When it comes to installation costs, the picture changes. You don t need DC wiring when using AC modules you simply connect the modules in parallel and feed the result into the grid. There are no high voltage DC switch boxes and protection circuits that need special skills and training to install. Solar modules within a DC string have to be closely matched to optimize system performance. You don t need to do this with AC modules. Furthermore, with AC modules you don t need to allocate an area within the building for the installation of the string inverters. The main costs of installation are design and planning, mounting structure kit, and labor costs for mounting, assembly, integration and commissioning. These costs will vary from installer to installer, but initial feedback suggests that a typical 7kWp AC module system will have installation costs of $12,500 while a string inverter system costs approximately $16,500. In the other words, AC modules offer around 24% savings in installation costs. Combining the total equipment plus installation and commissioning costs results in a compelling argument for considering ACmodules with built-in micro-inverters. A 7kWp system has a total initial set up cost of $31,750 when you use AC modules, versus $34,000 for a system with string inverters. That s a saving of nearly 7% in capital costs. Power harvesting It s in power harvesting that PV systems with micro inverters show the greatest advantage over string inverter systems. With DC strings, shading from any source such as tree branches or even something as small as an antenna or vent pipe can dramatically reduce the energy produced. Also, dirt and other debris that build up unevenly on solar module surfaces causes some modules or even a few cells of modules to see less solar energy. Because the modules are connected in series the whole system only performs at the level of the poorest performer in the string. A solar module in shadow limits the PV system s performance and the higher potential energy of the other modules that are not shadowed is wasted. Under these real world conditions the penalty to the potential energy harvest is severe with conventional string inverters systems. With AC modules, each module is an independent, standalone, solar AC grid connect system with its own optimized energy production output. Maximum Power Point Tracking (MPPT) is used to achieve the highest possible power yield. This is an electronic technique that varies the electrical operating point of the module in order to extract the maximum available power from it. Any degradation in the performance of a module, due to clouds, shadows, dirt or other obstructions, does not affect the performance of other modules and therefore has much less negative impact on the power harvested from the system as a whole when using micro-inverters or integrated AC modules. String inverters are typically around 96% efficient. Micro-inverters are a little less efficient, perhaps 94%. However, this marginal difference is more than negated when you consider the system level issues described above. In addition, string inverter systems typically lose about 2% of potential harvested power due the mismatch of solar modules, and a further 2% due to the use of DC wiring. Further, the inverter itself has been shown by experience to be the most unreliable part of the system and, as it is a single point of failure, a failed inverter means that no power is harvested until it is fixed or replaced. This is another factor that reduces energy harvesting, perhaps by a further 2% over the life of the system. When all of these factors are taken into account, systems that use micro inverters yield more power from every solar module, result in less downtime, and reduce the cost per Wp yielded from the system over its lifetime. The specific figures will depend upon external factors, such as the installation location and solar radiation available. Current best estimates show that the initial cost per yielded Watt (Wy) is around 12% lower for systems using a micro inverter integrated with a solar module (AC module). Maintenance Solar PV modules have been with us long enough that we can be confident that there will be only slight performance degradation over a period of 25 years or so. A 2003 study, presented at the 3rd World Conference on PV Energy Conversion in Osaka, Japan, showed only 4% total degradation of module output after 23 years outdoor exposure. Since the study, advances in solar

3 module technology are likely to have further improved this performance. This figure is important for estimating the lifetime yield of the system. PV solar systems with string inverters are relatively expensive to maintain. The first problem is that the installation can only be monitored at a system level, not at the level of each individual module. Finding a faulty solar module or one that has a shadow cast over it is a time-consuming and expensive task. By contrast, AC modules are monitored individually. Installers and maintenance companies can even monitor systems online, right down to the level of individual solar modules, from a web-enabled interface. This enables them to immediately identify the exact location of any faults and quickly fix it. String inverters rarely, if ever, last the lifetime of a PV solar system. This is reflected in relatively limited warranties of perhaps 5 or 10 years. Any LCOE calculation needs to take into account at least one replacement of this expensive component, plus the cost of labor involved. By the end of 2010, micro inverters with life expectancy of up to 25 years in realworld conditions (based on accelerated life test data to internationally recognized standards) will be widely available. This does not mean that there will be no failures. However, any failure will not bring down the whole system, it will just reduce the output by 1/29th or of one of the 29 micro inverters in our 7kWp system described above. Based on the currently available data, over a 20- year period the total estimated cost of maintenance and replacement for a conventional string inverter system is $5,192. For a system with high reliability AC modules this falls to just $739. When all these factors are taken into account, the total cost of maintenance as a percentage of initial PV system costs is 11% for a conventional solar system using string inverters but just 2% for an AC module system with integrated micro inverters. 3 MICRO INVERTER ECONOMICS IN SOLAR PV INSTALLATION Before investing in a grid-connected solar PV system, buyers want to know how long it s going to take to get a payback and how much the system is going to save them over its lifetime. Arriving at precise numbers for payback and savings is difficult because every installation is different. The numbers depend on various factors including the local cost of electricity from the grid, the incentives for clean solar power, such as feed-in tariffs (where the electricity supplier pays you for the solar electricity you generate), or capital cost subsidies, and unpredictable factors such as the amount of solar radiation that affects the amount of power generated. The utility scale PV companies talk in terms of levelized cost of electricity (LCOE) in order to enable comparisons between alternative energy technologies. A similar approach can be applied to residential and commercial PV systems. Here, LCOE can be used to compare the overall technical and economic performance of different PV system architectures. Comparing the lifetime system cost with the total power harvested over the same period helps determine the return on the investment. LCOE calculations need to take account of: 1. Capital costs. These include the solar modules, the cost of power conversion electronics (to convert the DC output of PV modules into AC suitable for grid connection), and installation. 2. Power harvesting. The total power harvested from the system, throughout its life, and the monetary value of each harvested kilowatt-hour (kw h), determines the financial return. The exact value of a kw h of electricity depends upon the comparative cost of electricity from the grid and how much your PV installation reduces power consumption from the grid. You also need to take account of feed-in tariffs and the other incentives provided by governments or utilities to install renewable energy systems. 3. Maintenance. Maintenance costs are directly related to the reliability of system components. Service calls are expensive. If it s difficult to locate the source of any problems, the time spent locating a fault drives up cost. Some system components have proven to be much less reliable than others. System reliability significantly impacts LCOE figures. Power ratings Before we go any further, it s worth clarifying some of the terminology used to describe power in solar PV systems. Watt-peak (Wp) is the most commonly used measure for PV modules. A related unit is kilowatt-peak (kwp). This defines the power output of a solar device under ideal conditions. There is an industry standard used to test PV modules that defines the watt-peak rating of a solar cell as its output power under exposure to 1000 Watts of light intensity per square meter, at 25 degrees C ambient temperature and with a light spectrum similar to sunlight that has passed through the atmosphere. For any PV system, the mean output will be lower but the Wp measurement provides a standard by which the relative performance of systems can be compared. In all solar PV installations some power is lost in the wiring and in the process of converting DC from the PV modules to AC in the inverter(s). Calculations based on true yielded Watt (Wy) take account of this and of the other factors that reduce the Wp figure. 4 A QUANTITATIVE ANALYSIS OF MICROINVERTERS PERFORMANCE VS. CONVENTIONAL INVERTERS A Comparison between a xyz and SMI Micro Inverter is done using 10 Suntech 185W panels for each system. Bins have been placed in front of 2 panels on each system to ensure there is some shading. Figure 4: The installed roof solar system.

4 Figure 5: The installed roof solar system. Figure 6: The kwh Generation. Figure 7: The kwh Generation in November. The Micro Inverters have generated kwh since the 28th of July. The xyz has generated kwh since the 28th of July. During November the Micro Inverters generated 53.9 kwh, whilst the xyz generated only 46.1 kwh 5 REDUCING SOLAR PV FIRE RISKS ON ROOFTOP INSTALLATIONS Firefighting operations in buildings with solar photovoltaic (PV) installations can present special problems. If a fire breaks out, firefighters need to take special safety precautions due to the presence of lethally high direct current (DC) voltages. A relatively new approach to eliminating the problem involves the use of a small inverters mounted on the rails behind the solar panels, or modules. These are connected together in such a way that high DC voltages are not present anywhere in the system. In conventional solar PV systems, high DC voltages are a source of danger during installation, operation and removal of equipment. Only specially trained electrical installation technicians can work on these systems and they need to have an understanding of firefighting procedures and requirements. By daylight, PV modules generate a low DC voltage. However, when the modules are connected in series the output voltage of the resulting string can reach up to 1,000V. This high DC voltage is converted into an AC (alternating current) voltage by means of an inverter and fed into the grid via a distribution box. Even if the inverter can be cut off via the main fuse or circuit breaker, the high DC voltage generated by the module continues to exist on the roof. This originates on the roof but is usually fed to a lower-level location for the inverter, perhaps in a garage or basement. Therefore, Fire risks due to short circuits and DC arcing exist throughout the building. Controlling AC voltage isn t so problematic as circuit breakers offer a disconnection function to help prevent a short circuit and the formation of an arc. With photovoltaic installations, however, a safe disconnection to stop electrical arcing is more difficult to implement, which increases the risk of fire. Many factors can cause a fire when using DC voltage such as ageing photovoltaic equipment or arcing triggered by defective or damaged insulation, particularly along the fuse panel of the generator junction box. According to VDE standards Making contact with DC voltage higher than 120V is life-threatening,. That s why fire-fighters need to locate the circuit breakers or isolating switches when they are called to a property with a solar PV installation and take great care when handling the terminal box. There are ways to help those tackling fires on roofs with solar modules, although many are still theoretical. For example, emergency-stop switches could be fitted directly to the modules, which could also be safely and centrally operated in the event of fire. However, it is not yet practical to develop such switches for mass production at acceptable prices. Another possibility would be to completely obscure the photovoltaic modules by means of a cover so that no further energy is generated and the system would automatically shut down. However, this concept could prove to be unreliable in cases of emergency. One other option would be to apply a thick layer of extinguisher foam onto the modules to reduce their output, but the original output levels might be reached again once the foam disappears, which could be after just a few minutes. Using micro inverters avoids this problem: The AC voltage conversion is carried out behind each module so that no dangerous high voltage DC is present. Enecsys micro inverters are mounted on the rails behind solar modules and convert DC to AC from one solar module or, in the case of the Duo, from two modules. This eliminates the need for the string inverters used in conventional solar PV architectures. In addition to improving safety, micro inverters provide effective solutions to many problems associated with conventional string or central inverters:

5 By operating each individual solar module at the point of maximum power (MPPT - Maximum Power Point Tracking) and power optimisation, micro inverters maximise the energy generated from each solar module and the overall PV system. Moreover, any reduced power output of individual modules due to shadowing by clouds or other obstacles (such as trees or chimneys, leaves or soiling) has no effect on the remaining modules and therefore only a minimal impact on the energy harvest from the overall installation. This provides between 5% and 20% improvement in energy generation over the lifetime of the system, depending on the site and system configuration. The performance and yield of each individual module can be monitored, which is not possible with string systems. Operator and installation engineers can obtain detailed information in real time and relay the performance of the solar installation during the whole operating period to end users. Faults with individual modules can be detected and rectified quickly and easily. String and central inverters are frequently the weakest link in the solar PV installation and usually need to be replaced at least once during the service life of the system. Enecsys micro inverters guarantee reliable operation over the entire anticipated service life of 25 years, which matches the life of solar modules. In order to achieve this high level of reliability, the patented design of the Enecsys micro inverter eliminates life-limiting components (electrolytic capacitors and opto-couplers). Micro inverters offer simplified PV array design and installation because solar modules do not need to be matched and can be installed in any orientation on any roof plane. Furthermore, there is no requirement for specialized high voltage DC isolators. by electronics designers is that every 10 degrees C rise in temperature will halve the mean time between failures (MTBF) of an electronic system. The MTBF figure is a guide to the predicted failure rate during the so-called useful life period of a product. It is a statistical calculation based on the predicted failure rates of the individual components within the product. However, the concept of MTBF is widely misunderstood, and often misrepresented in marketing material. The inference made by some suppliers in the solar industry is that a high MTBF supports an expectation of a long life for their products. Such statements are wrong. An MTBF of 600 years sounds great but 100% of the products may still fail in an unacceptably short time. This is because wear-out mechanisms determine the lifetime of products, and these failure mechanisms are not predicted by MTBF. A product with an MTBF of 300 years could actually live 40 years before the wear-out mechanisms lead all of them to break down. By contrast, a product with 600 years MTBF might have wear out mechanisms that limit its life expectancy to 15 years or less. If you know this, you would clearly choose the 40-year life expectancy product over the 15-year life expectancy product. For a solar micro inverter, this is very important. With a 15-year lifetime product you would have to replace all of them before the modules wear out, in about 25 years. With the 40-year lifetime product, you will only replace a few of them during the lifetime of the modules. Clearly, an overemphasis on MTBF as a measure of real-world reliability is dangerous. A company may claim an MTBF of 800 years for its products but only warrant them for 10 years. Where it s specified over a real-world ambient temperature range of -40 to +85 degrees C, the warranty is the better indicator of likely operating life. The safest approach is to select products that are backed by accelerated life test data derived from accepted test methodologies. For the solar industry, IEC is a recognized test methodology for solar modules and therefore an appropriate guideline for micro inverters. Two further important indicators of reliability are the temperature range over which the micro inverter is rated and the warranty offered by the micro inverter manufacturer. Solar modules are typically rated from -40 to +85 degrees C. Why would you then choose a micro inverter rated at a maximum temperature less than the maximum temperature for the solar module? Until now, the reliability of micro inverters has been limited by the need to use relatively unreliable components in their manufacture. These problems have now been overcome. 7 CONCLUSIONS Figure 8: a PV roof System. 6 THE RELIABILITY QUESTION Micro inverters are not a new idea. The challenge has been to design these products to be reliable in real-world conditions. Ambient temperatures can easily reach +85 degrees C behind a solar module and high temperatures are not good for electronic devices. A rule of thumb used A key element at the grid side of a PV power system is the inverter. For most rooftop PV solar installations in domestic and commercial premises, there is growing recognition that conventional DC string architectures are underperforming their potential for energy harvest. They are not delivering the efficiency required to enable more widespread adoption of solar-electric technology. The interim step of using DC-DC optimizers greatly improves energy harvesting, but at a cost that is unacceptable. The next few years will see micro inverter based systems become widely adopted and a growing number of PV

6 module manufacturers will build micro inverters into their products. The AC module will become an industry standard module and installation time, complexity and cost will be dramatically reduced. At the same time, users will harvest more electricity from their systems and their investments will be paid back over much shorter periods. 8 REFERENCES [1] P.C. Ghosh, B. Emonts, H. Janßen, J. Mergel, D. Stolten. Ten years of operational experience with a hydrogen-based renewable energy supply system, Solar Energy 75, 2003, pp [2] R.Teodorescu, F.Blaabjerg. Overview of Renewable energy systems, Aalborg University, Institute of Energy Technology. [3] A. Bilodeau, K. Agbossou. Control analysis of renewable energy system with hydrogen storage for residential applications, Journal of Power Sources, [4] Osama Omari, Conceptual Development of a General Supply Philosophy for Isolated Electrical Power Systems, PhD Thesis, South Westphalia University of Applied Sciences, Campus Soest, Germany, February [5] Three Dimensional Space Vector Modulation Strategy for Three-Leg Four-Wire Voltage Source Inverters. IET Power Electronics Research Journal. With Egon Ortjohann, Nedzad Hamsic, Max Lingemann, Andreas Schmelter, Worpong Sinsukthavorn, Danny Morton. [6] E. Ortjohann, A. Arias, D. Morton, A. Mohd, N. Hamsic, O. Omari. Grid-Forming Three-Phase Inverters for Unbalanced Loads in Hybrid Power Systems, IEEE 4th World Conference on Photovoltaic Energy Conversion, Waikoloa, Hawaii, May [7] O. Omari, E. Ortjohann, D. Morton S. Mekhilef. Active Integration of decentralised PV Systems in Conventional Electrical Grids, PV in Europe from PV Technology to Energy Solutions Conference and Exhibition, Mai 2005,Spain, Barcelona. [8] Supervisory Control and Energy Management of an Inverter-based Modular Smart Grid. IEEE PES Power Systems Conference & Exhibition (PSCE). Seattle, Washington, March With E. Ortjohann, W. Sinsukthavorn, M. Lingemann, N. Hamsic, D. Morton. [9] G. Seguier, F. Labrique., Power Electronic Converters, DC-AC Conversion, Springer-Verlag, Heidelberg, Germany, January [10] MTBF and reliability A misunderstood relationship in solar PV, Paul Engle, white paper [11] Micro-inverter Economics in Solar PV Installations, white paper, Enecsys [12] The Evolution of PV Solar Power Architectures, white paper, Enecsys [13] Review of control techniques for inverters parallel operation. Electric Power Systems Research Journal. With Egon Ortjohann, Nedzad Hamsic, Max Lingemann, Andreas Schmelter, Worpong Sinsukthavorn, Danny Morton, Osama Omari.