RESEARCH ON PHOTOVOLTAICS: REVIEW, TRENDS AND PERSPECTIVES Moacyr A. G. de Brito, Leonardo P. Sampaio, Luigi G. Junior, Carlos A. Canesin* *São Paulo State University – UNESP - Power Electronics Laboratory – Electrical Engineering Department Av. Prof. José Carlos Rossi, 1370, 15385-000, Ilha Solteira, SP -
[email protected] Abstract - This paper presents a briefly review, some trends and perspectives in the field of Photovoltaic energy conversion, which is considered to be the most important renewable energy source in few years, in the coming decades. The power electronics plays a fundamental role in this process, developing systems each times more competitive, efficient, reliable, and also reducing costs and reducing the payback time. Some trends are visible, which are the use of Silicon Carbide devices in PV inverters, the use of integrated inverter structures, the integration of power converters into the PV module or the use of few PV series connection, the development of thinner and more efficient solar cells. Moreover, the discussion about the necessity of MPPT and anti-island schemes are presented, mainly considering the expected growth of grid-tied applications. Keywords – Photovoltaic Energy, Renewable Energy, Integrated Converters, Solar Inverters, Trends and Concepts on PV, Power Electronics. I. INTRODUCTION In the last decades the widespread use of renewable energy sources in distribution networks throughout the world is visible. This scenario is the result of the continuous increase in energy demand coupled with the possibility of reduced supply of conventional fuels, evidenced by petroleum crisis, along with growing concerns about environmental conservation [1]. Among the alternative energy sources the electrical energy from photovoltaic panels (PV) is currently regarded as the natural energy source more useful, since it is free, abundant, produces no greenhouse gases during power generation, it is distributed over the earth, and participates as a primary factor of all other processes of energy production on Earth [2,3], and many governments and companies consider PV as the future of energy production. According to expertise the energy obtained from PVs will become the most important alternative renewable energy source until 2040 [4]. Nowadays it is mandatory that production, distribution and use of electrical energy are done as efficient as possible. In the Distribute Generation systems all the drawbacks of energy distribution is overcome, and for the end-user the Power Electronics plays a fundamental role in developing power converters more efficient and reliable. Thus, the development of photovoltaic energy conversion system is notable and progressive. Every year the prices of solar cells are decreasing and the installed PV systems are increasing at rates of more than 40% per year [5], although the total system costs is higher when compared to other conventional fuels. In this context, in order to reach the grid-connection
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market, the first goal for PV systems is to improve the efficiency and to reduce installation costs. The efficiency of PV system is improved using maximum power point tracking (MPPT) algorithms, high efficiency solar cells and high efficiency power electronic converters. Mechanical solar trackers are also desirable or mandatory for concentrator photovoltaic. The installation cost is composed by solar cells cost and inverter costs. Cheaper and more efficient solar cells are continuously being developed by respected research centers worldwide and they are close to reach the market expectations. Power converters also must reduce their cost, improve efficiency and increase lifetime. It should be pointed out that the solar panels are almost reaching the same cost/watt ratio as the power converters. Therefore, the cost reduction of power converters is meaningful in PV systems. Its efficiency has always been far better than the solar cells, but it cannot be neglected since it is directly related with the payback time. The lifetime is the drawback of the converters compared to the solar cells and they need to be improved to avoid maintenance costs along their lifetime, i.e., the converters lifetime must increase to 20-25 years [6]. In the field of solar cells, the PV market is dominated by wafer based Crystalline Silicon Cells with efficiency about 13-17%, but thinner wafers are being developed in order to reduce costs and increase efficiency and the nanotechnology will be responsible to overcome the expectations. Furthermore, there are other proposals as ultra-thin, multijunction and cross-fertilization solar cells. Another trend is in the field of power converters, where the use of Silicon Carbide semiconductors will permit higher switching frequencies without significant increase of losses, and reliable operation with higher junction temperatures. That means reduction of reactive elements and heatsinks, in order to manufacture converters with low weight, volume and costs, increasing structure power density [7, 8]. Furthermore, the integration of power electronics into the PV module offers new advances but also represents motivating challenges. In this new approach it is necessary to develop converters which will be capable to boost the low voltage of the PV panel with high currents without significant losses. Another point will be the series connection of few modules that can make the system more reliable reducing the payback time. Finally, the overall grid connected system efficiency and reliability can be improved with digital control concepts, which can create even more efficient algorithms. II. TRENDS IN PV INVERTER CONCEPTS A. Structures for PV systems Historically, the first implementations of converters for PVs were based on large series-parallel associations of
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photovoltaic modules attached in a single converter (central inverter), as shown in Fig. 1. This approach has low efficiency because of the obligatory series diodes inserted in each parallel branch of the structure in order to avoid power flow among branches, and also because of the unequal power distribution. Moreover, due to centralized MPPT algorithm, each branch might not operate at MPP, presenting loss of power. There are others limitations such as use of high voltage DC cables between PV modules and inverter and the nonflexible design which does not permit mass production. D1
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the use of limited and few series-connection of PV panels for a single inverter. In this context, the high-quality concepts of micro-inverter could be used with low production costs, on a novel Mid-solar-inverter technology. Thus, with only few series PV panels, shading will not a trouble anymore. Of course, installers have to put panels as close as possible and with the same sun inclination. In this new scenario, the system maybe not exposed to weather, increasing life-time, reliability and reducing maintenance costs and presents less payback time when compared to micro-inverters or central inverters for high power.
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The choice for topologies directly related to series connection, Figs. 2(a) and 2(b), allows increase in efficiency and better use of PV panels working at optimum power point of the association. In this approach, there are no power losses due to the absence of series diodes and dedicated MPPT algorithms can be applied at each branch. Thus, increasing the overall efficiency and also enables the cost reduction due to possible mass production. Increase power capacity is easily obtained through simple insertion of new PVs branches in the existing platform. A current trend nowadays is the use of the so-called microinverters (AC modules) that are dedicated to a single panel, and therefore specifically for low power applications, typically in the range of 200 to 300W, as shown in Fig. 2(c) [9]. Using a single inverter structure, each PV panel, regardless of shade, will operate at the maximum power point whenever possible. In great amount of series connection shading phenomena is a drawback. Therefore, if one PV panel suffers from shading the overall MPP efficiency will be limited by the shaded PV. Microinverters have the advantage of having low voltage DC link, easy replacement in case of failure, easy interconnection with the grid and easy parallelism. The microinverter is attached back in the panel and because of that, it has to support all weather conditions. This implies in a non-repairable converter due to use of resins that only can be removed with chemical processes. Additionally, for these systems have a long useful life, its components, semiconductors, filters and mainly the control and communication systems should be capable to operate at elevated temperatures, increasing the manufacture and sale costs. Therefore, it increases the payback time and maybe difficults its widespread use. Another trend could be
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Fig. 2. Tendencies in the use of PV Inverters - Series Association (a, b) and AC Module (c).
B. Classification of PV Inverters There are wide varieties of inverter topologies used for PV systems [10-18], which are usually classified by the following features: presence of galvanic isolation, number of stages, stand-alone or grid-connection operation and the place of the decoupling capacitor. Galvanic isolation is compulsory in several countries due to safety reasons. The galvanic isolation allows the system to be doubly grounded, but there are some topologies which allow PV panel grounding without galvanic isolation. Transformers also help to adjust voltage levels according to the transformer ratio, but switching-mode power electronic converters are more efficient than transformers. Low frequency (LF) transformers are heavy and inefficient. For this reason, several topologies have been proposed using high frequency (HF) transformers, in order to overcome transformers drawbacks, but it increases the complexity of the circuitry. Furthermore, safety is a design parameter that may be obtained without the use of transformers. The use of differential breaker and grounded package are examples of good safety designs without galvanic isolation. In brief, as described in [10], technically transformers are not adequate elements in PV systems, unless it is determined by law. Thus, eliminating transformers is a good option to develop even more efficient converters. Usually, the most common topologies have one or two stages. The number of stages is important to give an idea of complexity and cost. The two stage topology is based typically on Boost Converter cascaded with Full Bridge inverter. This approach presents a DC link power decoupling that permits to the converters to work in function of voltage
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DC link fluctuations. So, the DC-DC converter is responsible for boost the voltage and for the MPPT, the inverter is responsible for grid synchronization, anti-island and injection of power into the grid. It is clearly noticeable that reducing components reduces weight, volume and maybe costs. So, the one stage approach is the desirable solution, which is performed by integrated structures. However, some drawbacks may appear and there are no more DC links and the power converter itself has to perform all control functions. Thus, the designer must find the best compromise among the control algorithms. The mode of operation implies first on the output type required: current source (grid-connected) or voltage source (stand-alone). These differences change basically the output passive components of the inverter. Voltage Source Inverters (VSI) are the most common, where a voltage in a DC link is modulated in the output. Current Source Inverter (CSI) modulates the current, requiring an AC capacitor as the first passive component after the inverter. The grid-tied mode (grid-connected mode) eliminates the need for storage elements which increases the system efficiency, life-time and decrease significantly the overal cost [19]. The power decoupling is a requirement to operate effectively in the panel maximum power point, once the AC power is naturally pulsed at twice of the grid frequency and PV power must be constant. If pulsed power is supplied by PV panel, the voltage ripple will be very high, the operating point will oscillate around the MPP and the MPP tracking will not be effective. The power decoupling is achieved with an energy storage element, such the DC link capacitor. For single stage, the decoupling capacitor must be placed in parallel with the PV. The energy in DC link capacitor can be controlled providing a more effective decoupling with a smaller component, when compared with the second case, where the capacitance is usually around 0.5 mF/kW [10]. C. New Integrated Inverters Concepts The main challenge nowadays in relation to PV widespread is the reduction costs. Moreover, the lifetime is the drawback of the converters compared to the solar cells and they need to be improved to avoid maintenance costs along their lifetime. Thus, the reduction costs can be achieved using integrated structures, where the number of passive and active components is reduced. It indirectly means cost, efficiency and lifetime improvements. Typically the PV cells do not attend the specificities of the electrical equipments in alternated current (AC) as they present low DC voltage. To overcome this problem, the most used technique is the cascaded association between a boost DC-DC converter and a Voltage Source Inverter (VSI) [14, 15]. The great use of VSI is due to simplicity of design and implementation, since this converter is inherently stable. However, the cascaded association presents less efficiency than each individual structure, due to multiplication of individual efficiencies, in addition of increasing weight, volume and cost [20, 21]. Several works have proposed boost
inverter topologies as a combination of duplicated DC-DC converters, as shown in Fig. 3, one for positive and other for negative output, and this approach normally leads to complex circuits with hard control [22]. An alternative would be the use of an integrated structure which allows boost and inverter principles, besides offering a higher efficiency, uses fewer components, reducing costs and increasing the structure power density [20, 21], examples are shown in Fig. 4. In both structures two switches works in AC grid low frequency, one modulates during the positive and other during the negative cycle, while the others modulate at high frequency reducing the switching losses. Furthermore, these structures present less EMI and more reliability. Lac
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Therefore, the integration shown in Fig. 4 aims to unite the inverter and the boost stages into a single stage capable to boost and invert [23]. In this context, Fig. 5(I) shows the integration steps of the full-bridge and the boost converter, where the red components are removed. The first simplification (Fig. 5(a)) is to make the input boost current be injected directly in the output capacitor. Then the DC link capacitor and the inverter output inductor can be excluded. The next simplification is to remove the boost diode considering that the switches do not have anti-parallel diodes. And, finally, the boost switch may be eliminated once it is in parallel with the full-bridge switches and they can make the same function in the circuit. The final topology obtained in Fig. 5(I-c) is the known boost current source inverter (CSI). The operation of this inverter is not as usual as the standard voltage source inverter (VSI). In fact there is a condition where the output voltage is not controlled and it is when the output voltage is smaller than the input voltage, because the buck property disappeared during this integration. It means that this converter cannot theoretically provide voltage gain smaller than one. This is an undesired feature for AC output and may lead to deformations of the waveform close to the zero crossing. A possibility to solve this problem is the integration of the buck-boost with the full-bridge inverter, as shown in Fig. 5(II). The integration also starts making the output DC-DC converter current be sent directly to the output capacitor. But
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in this case, only the diode can be removed, because the switch is not in parallel with the inverter switches. The buckboost inverter is the result of this integration and it can better modulate the sinusoidal output current or voltage waveforms. This integration is valid for stand-alone or grid-connected operation. In case of grid-connection, a coupling inductor needs to be added. Furthermore, this procedure can also be easily extended for three-phase inverters, and it is interesting to note that this integration procedure may be done for any DC-DC converters. Thus, applying this technique to the other DC-DC converters, a novel family of grid-tied integrated inverters is found as shown in Fig. 6. In this context, Cuk integration is interesting for gridconnected PV systems, because its input and output have current source characteristics, correspondent to the DC-DC converter. It helps to reduce the decoupling capacitor in parallel with PV and avoid the use of an extra coupling inductor to the grid. It is interesting to point out that some integrated inverters like Cuk and Zeta maintain the VSI output characteristic, facilitating the prototyping and the control of the injected current. The Buck-Boost, Zeta and Sepic also present the isolation feature in HF, if necessary. Vdc
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It is important to remember that CSI needs the use of special switches with no anti-parallel diodes to permit reverse voltage blocking. Series diodes with the switches are common solutions where the anti-parallel diode is neglected and the reverse blocking is achieved. However, it increases the conduction losses. To improve the efficiency of CSI there are two basic options: the use of series schottky diodes with proper reverse voltage capability, or, reversing blocking IGBTs (RB-IGBT), which are made to support high reverse voltages and have lower losses compared with the sum of IGBT plus Diode [24]. This paper has not the scope to demonstrate experimental results, but it is interesting to show the feasibilty of the integration procedure presented in this topic. Thus, some integrate inverters were built and the main experimental results are shown in Fig. 7, considering the stand-alone operation. The tri-state modulation can be applied in these integrated inverters in order to reduce the order of the control system, increasing performance and the static voltage gain [25, 26]. The tri-state converters were useful originally to improve the dynamic performance reducing the right half-plane zero effect, typical in conventional boost converters in voltage mode control [27, 28].
Fig. 7. The Prototypes and the Main Waveforms for the Integrated Inverters.
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Considering the results showed in Fig. 7, the output voltage total harmonic distortion for the Boost integrated inverter was 6.5%, while the tri-state Boost integrated presented 5% and the tri-state Buck-Boost integrated presented 4%. III. CONTROL ALGORITHMS A. Maximum Power Point Tracking Control It should be noted that there is only one point of maximum power (MPP - Maximum Power Point) in photovoltaic panels, and this varies according to climatic conditions. The photovoltaic power characteristics is nonlinear, which vary with the level of solar irradiation and temperature, which make the extraction of maximum power a complex task, considering load variations. To overcome this problem, it is necessary to implement a maximum power point tracking (MPPT) algorithm. The most common methods are based on the idea of continuously perturb the PV operation point, like P&O, Hill Climbing and IC methods [29]. However there are others with great advantages as Beta, Correlation, and Temperature [30]. The simulation results of the Tracking Factor (TF: % of the energy harvested in a period of time) of the most interesting MPPT algorithms can be observed in Fig. 8 [31]. Tracking Factor was obtained via Matlab/Simulink PV and inverter models, applying the same temperature and irradiation steps for each algorithm. All algorithms were specified to provide its best results.
Fig. 8. Percent of Energy Extracted from the PV Panel - TF.
Furthermore, the methods can be compared to the accuracy of tracking the optimal steady state, dynamic response, ease and cost of implementation. Usually the slowest methods have tracking speed sufficient for most applications. The cost is basically related to the number of sensors, but the large number of sensors tends to improve the algorithm efficiency and compensate the extra cost. Interesting references about MPPT comparisons can be found in [29-32]. B. Synchronism and Anti-Island Control In order to make the connection of the PV system to the distribution network in AC low voltage it is necessary to take certain precautions, such as synchronization and prediction/ management of islanding. In this context, in order to implement the grid connection it is mandatory that the frequency and phase angle are the same. These are usually identified by Phase-Locked Loop (PLL) or zero crossing detection. After synchronization and
the grid attachment, the injection of power is initiated. The grid frequency varies slightly throughout the day, requesting that it be monitored constantly by the converter. Furthermore, small differences in frequency can lead to phase detachment, varying the power flow to the grid. The phase angle control between two AC power sources is the most common method among large generators. But, in small converters, impedance connection has to be very large in order to prevent that small angles do not transfer major powers. Besides this factor, the control becomes more susceptible to instability decreasing the security and reliability of the power converter. Another interesting alternative is to make the connection through a current source. The injected current can be modulated directly in the form of grid voltage ensuring only the active power transfer. Thus, in order to avoid propagation of distortions in voltage to injected current, the current reference can also be obtained by means of a PLL. In normal operation, the RMS current is increased until reach PV maximum power transfer. The island phenomena for a distributed generation system is defined when it continues feeding local loads in the absence of the supply network [33, 34]. That means that the converter feeds the loads specified to it and others loads connected to the grid, resulting in operational problems due to inability of local generation to detection the island mode, among others even more serious grid problems. Thus, standards that deals with the interconnection of photovoltaic systems to the network requires effective methods to detect island such as IEEE 929-2000, IEEE 1547 and UL1741 [34]. During islanding security, power quality and reliability problems may occur. The power grid can no longer control the voltage and frequency during islanding, creating the possibility of consumers equipment damage in a situation where the grid has no longer the control. This effect may create a situation of great danger to workers of the distribution lines or to ordinary people as a part of the distribution network remains energized even disconnected from the main network. Further, the reconnection of the network during an island can damage equipments and even the DG system, because of an out of phase connection, and also, islanding can interfere with the restoration of service over the network. The island detection methods are divided into passive, active and remote [33-37]. The passive ones are based on detection of abnormalities in amplitude, frequency or phase voltage in the point of common coupling (PCC) between the inverter and the grid when in islanding. Passive methods consists of relays as usual protections, i.e., under/overvoltage and under/over frequency protections. The active methods are based on direct converters actuation in the grid and in the maximum compensation of frequency and voltage. These methods introduce disturbances and monitor the response to determine if the grid, with its stable frequency, voltage and impedance, is still connected. If small perturbations are able to affect the parameters of the voltage at PCC, the active circuit turns down the inverter. Several active methods are presented in literature, but the most usual are reactive injection, phase-shift and impedance
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measurements. Among them, the most usual are the ones based on phase-shift method. Two of these methods must receive attention, because the others are based on it. They are Active Frequency Drift (AFD) and Slip Mode Frequency Shift (SMS). In AFD method the injected current has a frequency slight higher than the grid voltage, synchronized with the zero crossing of the sinusoidal network. In case of failure, the frequency rises continuously. In SMS, the nominal operating frequency for the inverter is unstable, which forces the frequency to lead or lag to stabilize at a frequency higher or lower than line frequency. Therefore, the frequency remains the same only if the grid is connected. Figure 9 shows the frequency change when the grid is disconnected from an inverter feeding power to the grid. The remote methods have gained attention recently by advances in communications technology, such as power line carrier (PLC). These are very effective, but are not yet economically viable nowadays, mainly for low power DG applications.
Fig. 9. Examples of SMS and AFD anti-island methods.
IV. TRENDS IN SOLAR CELLS Electrical bypass diodes for PV modules are available as integrated electronics and ensures that a shaded cell does not lead to reduce the total power of the PV. The simplest bypass circuit is the common bypass diode. Since all cell in the same module are connected in series they presents the same current. In case of shade, the bypass diode presents a path for the current and the PV module does not suffer from limiting the system current, permitting the unshaded cells to conduct the full current. In theory, one bypass diode per cell would be the best solution, but it is almost impossible in practice. Usually, a string composed by 24 cells is bypassed by one diode and up to three strings are series connected in a PV module. The current in a series connection of PV cells is of course related to the irradiation, but also to the surface area of one cell. Nowadays, for state-of-the art 6 inch solar cells, the bypass diodes must conduct currents up to 9A. The next generation cells will produce currents up to 16A and even more, as the cells surface are increasing [7]. These higher currents lead to higher power dissipation in the bypass diodes and it may become a serious issue in future. So, in order to overcome this problem it is necessary to adopt Active Bypass Diodes. These are electronic circuits which act as diodes, but have a very limited voltage drop, reducing dissipation issues and preventing hot spots. Indeed, the control of this circuit is more complex but its performance is far better than of a simple diode. Nowadays more than 90% of PV is used as flat panel modules, designed for 1kW/m2 (1 sun) operation. In some
parts of the world with more than 2500 hours of sunshine per year there is an increasing interest in using 1-axis or 2-axis concentrators based on mirrors or lenses onto the cell in order to maximize solar absorption. The wafer thickness is typically 0.2mm and the tendency is to produce cells in the order of 0.08mm thick, increasing efficiency up to 20%. Another future trend is to produce cells with help of nanotechnology, which are less than 0.02mm thick. Another expectation is in the reduction of silicon per watt peak, going from the actual 10 grams to 2 grams close to 2030. In order to reinforce the PV growth, it is expected by 2030 that PV industry will produce 1000 GWp globally [5, 38]. V. GRID PARITY The recent growth of PV market is dependent of subsidies. As examples one can verify the greatly growth in residential PV system installations in Japan during 1994 to 2005 because of govern efforts. More recently, Europe, especially Germany, has showed their strong in the PV market. The Germany PV installed capacity has grown up to more than 4GW due to the introduction of a special feed in electrical tariffs, where the electric power companies were obligated to buy PV electricity with higher rates than the market rate [38]. One shortcoming of subsidies is the increase for the consumer electricity payments, but the most modern cities can afford for this payment until the PV electricity can be equal the conventional one. This is called grid parity. In order to reach this stage, it is necessary to achieve cost reduction of PV power generation to grid electricity rates, security for the PV resources and ensure infrastructure maintenance for PV power quality. The stage for grid parity is coming and must be achieve soon in Europe by the years of 2015-2020. In case of Brazil, the high price of PV energy is expected to decrease and become a competitor of hydroelectric power, and the grid parity will be reached by the years 2020-2030 in almost all regions, if incentive programs for the application of this source can be applied, as was done in Germany [39]. VII. CONCLUSIONS It is clear that the PV energy will become the most important alternative renewable energy in the future, but some important steps must occur before, such as increase worldwide government subsidies, use of thinner and more efficient solar cells, development of more reliable, efficient power electronics converters, based on Silicon Carbide devices and/or integrated structures, and use of more efficient digital control algorithms. ACKNOWLEDGEMENT The authors would like to thank FAPESP, CAPES and CNPq for supporting this work. REFERENCES [1] H. Cha, S. Lee. "Design and Implementation of Photovoltaic Power Conditioning System using a Current based Maximum Power Point Tracking", in Proc. of IAS’08, vol.43, 2008, pp.1-5.
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