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PV Systems Installed in Marine Vessels: Technologies and Specifications

PV Systems Installed in Marine Vessels: Technologies and Specifications Hindawi Publishing Corporation Advances in Power Electronics Volume 2013, Article ID 831560, 8 pages http://dx.doi.org/10.1155/2013/831560 Research Article PV Systems Installed in Marine Vessels: Technologies and Specifications 1 2 1 Ioannis Kobougias, Emmanuel Tatakis, and John Prousalidis Division of Marine Engineering, School of Naval Architecture and Marine Engineering, National Technical University of Athens, 9 Heroon Politechniou Street, 15773 Zografou, Greece Laboratory of Electromechanical Energy Conversion, Power Systems Division, Department of Electrical and Computer Engineering, University of Patras,UniversityCampusofPatras, 26504Rio,Greece Correspondence should be addressed to Ioannis Kobougias; ikob@central.ntua.gr Received 25 October 2012; Accepted 22 January 2013 Academic Editor: George Antonopoulos Copyright © 2013 Ioannis Kobougias et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Considerations are held about the specificationin which the PV plants have to fulfill so that they can be installed on marine vessels. Initially, a brief description of the typical electrical grid of ships is presented, distinguishing the main parts, reporting the typical electrical magnitudes, and choosing the most preferable installation areas. The technical specifications,in which the PV plants have to be compatible with, are fully described. eTh y are determined by the special marine environmental conditions, taking into consideration parameters like wind, humidity, shading, corrosion, and limited installation area. The work is carried out with the presentation of the most popular trends in the field of solar cell types and PV system technologies and their ability to keep up with the aforementioned specifications. 1. Introduction limited, mainly working as suppliers to small lighthouses, buoys, and chargers for the batteries of small sailing yachts Without doubt the last decade was the golden age of the [8, 9]. The rising transport expenses due to the fuel prices, the photovoltaic systems. The large number of technological increasing restrictions of CO and nitric oxides NO emis- 2 𝑥 breakthroughs on the research areas of power electronics, sion due to new ecological policies, and generally the need for photovoltaic (PV) panels, and microgrids made the use of PV more eco-friendly transportation were the reasons that forced panels feasible to numerous applications of modern life. the marine companies to reexamine the systematic use of PV The PV systems of today generate electric power that systems on large vessels [10–12]. ranges from W to MW. Small solar chargers for portable The photovoltaic technology can indeed be a really cost- devices such as laptops, cell phones, and calculators are very eecti ff ve solution for ships. PV systems can act as ideal popular. Single or arrays of PV panels produce electric subsidiary power sources, independent from the vessel elec- power for street lights, advertising signs, isolated agricultural tromechanical settlement because they [13–15] electric pumps, even small houses not connected to the utility (i) produce electric power without the need of trans- grid. In addition, PV systems, wind power systems, batteries, ferred gas or liquid fuel, fuel cell generators, and other renewable energy systems work together and organize reliable microgrids [1–4]. But the most (ii) have no by-productssuchasgas emissionsornoise, common PV applications are the grid-tied ones, where single (iii) have low maintenance cost, PV panels or large scale PV plants apply auxiliary electric (iv) have limited or no use of mechanical moving parts, power to the grid [4–7]. Despite their extended use at mainland applications, the (v) consist of few parts, with easy installation and fast PV systems presence in modern marine technology remains replacement in case of aging or defectiveness, 2 Advances in Power Electronics Main 3 kV–13.8 kV , generator 50–60 Hz,3𝜑 Load Possible M G power AC3𝜑 conversion 400 V , 50 Hz TM Or Load Main Possible generator Power power AC1𝜑 Main motor converter conversion 240 V , 50 Hz and/or Gear box Load Auxiliary Possible power generator number 1 DC conversion 400 V Diesel DM G motor number 1 Load Diesel Possible DM power DC motor number 2 G conversion 24 V Auxiliary generator number 2 Figure 1: Diagram of a typical electrical system for a ship. (vi) have satisfactory life time with a warranted PV panel Besides the main generator, at least two additional aux- output power by the manufacturers, which usually iliary (diesel powered) electrical generators are connected cannot be less than the 80% of the nominal one aeft r to the electrical grid providing with electric energy. Addi- 25 years of operation, tionally, in high load or breakdown cases (malfunction of main motor and/or main generator) they have to sustain the (vii) can be placed in small surfaces with no practical use minimum operational standards of the vessel [18, 19]. such as roofs, walls, funnels, and superstructure, According to the bibliography, the output voltages of In this paper, aresearchisheldtogiveanswers abouthow themainand thedieselgeneratorsvary. Typicalvaluesof the most popular technologies of the mainland PV systems their magnitude and frequency are 3 kV, 3.3 kV, 4.16 kV, 6 kV, can be applied, and what specifications must be fulfilled in 6.6 kV, and 13.8 kV and 50 to 60 Hz [16, 20, 21]. With the help order to be appropriate for partially or fully electrical marine of power converters and transformers, the generated power is vessels. supplied to an internal bus that transfers the power in every load over the ship. The bus can be either AC or DC. eTh DC 2. The Electrical System of a Marine Vessel buses are more popular to modern naval ships while the AC ones to cargo and passenger ships. However, the research in Four separate areas can be distinguished in the electric system progress investigates the most protfi able solution for the large of a typical ship: the main propulsion engine, the generators, civil vessels. Typical values for a three-phase AC bus are the themaindistributionbus,and theloads.In Figure 1,a 400V,50or60Hzand foraDC busthe 400V. diagram of a typical electrical system for a ship is presented. The last part of the ship electrical system is the loads. A thermal motor consuming diesel or heavy fuel oil is Different types of loads are connected, but the most common used as a prime mover. Its rotational movement can be used are the DC 24V and 400V, the single-phase AC 230V, 50Hz, either for both propulsion and electric generation, or exclu- andthe three-phaseAC400V, 50Hz loads. sively for electric power generation. Among the aforementioned parts of the vessel’s electric In thefirstcase, thegenerator is coupledtothe shaft system, the potential spots for the PV systems to be connected throughastep-upgear. It is knowntothe worldwidebibli- to the distribution buses along with the loads. ography as shaft generator; and according to its position, the type of coupling and the control equipment can be catego- rizedintofourteentypes [16, 17]. It aims not only to produce 3. Technical Specifications for PV Systems in electric power for the ship but also, in certain cases, to operate as a propulsion motor and assist the main motor engine. Marine Vessels In the second case, the mechanical power of the thermal engine is dedicated only to drive the main generator. eTh gen- Even though the installation of PV plants to mainland is com- erator simultaneously supplies the ship with electric power mon and thoroughly examined, extra considerations must and drives an electric motor attached to the propeller. eTh be done when they have to be installed on ships. eTh most propulsion system is known as integrated full electric propul- important distinction between the mainland and marine sion (IFEP) configuration, and its main characteristic is that PV applications is the environmental conditions, forcing PV the shaft system is minimized, if not completely eliminated, systems to be more tolerant to extreme winds, high humidity, that is, in the IFEP-pod case. and salt. Bus (DC 400 V or AC 3𝜑 , 400 V, 50/60 Hz) Advances in Power Electronics 3 The winds on a ship are characterized by their great muta- installation areas. On the contrary, in marine applications the bility of direction and speed, a fact that has great impact to open spaces are limited, and the shades are more difficult the orientations of the applied PV panels. eTh use of xfi ed tilt to be predicted due to the continuous changes of the vessel oer ff s easy and strong embracement to the ship. eTh orienta- orientation. For these reasons, the use of bypass diodes and tion is preferable to concur with the keel so as to help the ves- smaller-scale installed PV systems are preferable. sel aerodynamic. But the xfi ed tilt results an important draw- eTh re are two additional specica fi tions that the smaller- back. PV panels cannot fully exploit the sun radiation due to scale installations meet. Firstly, PV plants on ships should be the variety of the ship routes and constantly changing latitude. connected closely to crucial loads so as to minimize the dis- PV plants with embedded tracking control system may oeff r tribution losses; and secondly, they have to produce dispersed more efficiency but have many mechanical moving parts electric power, to apply the necessary reserve for PV power which are vulnerable to ocean storms and are more expensive generation. and need extra maintenance costs even higher than the Finally, the operational specifications of a PV system are ones of the corresponding on land systems. eTh golden the same for both the mainland and marine applications. mean is the use of PV panels placed tangentially to the ship Namely, the output characteristics ought to be compatible surfaces. eTh air resistance of the PV panels to strong winds with the electric magnitudes on the installing points of the becomes negligible, and its incident solar radiation is hardly grid; high efficiency, and power factor are essential; maxi- aeff cted by the sailing routes. However, this type of orienta- mum power point tracking (MPPT) and anti-islanding con- tion maximizes the necessary installation surface and stint trol are necessary. the air flow behind the panels reducing the possibility of cooling. 4. Solar Cell Types eTh marine environment can be also harmful for both According to their structure, the solar cell types can be the electronics and the panels of a PV system. The high classified into two basic groups, the crystalline cells and thin levels of humidity and salt can cause short-circuits and induce films [ 24]. At crystalline cells, the raw material is silicon corrosion to the mechanical parts of the converters. eTh European Committee for Electro Technical Standardization (Si), whose molecules are organized in crystalline grids. The orientation of the grid defines whether the cell is monocrys- (CENELEC) has developed an ingression protection rating talline or polycrystalline [24, 25]. (IP Code) that scales the electronic circuit protection levels from solidobjects,materials,and liquids[22, 23]. According The monocrystalline solar cells (Mono c-Si) owe their name to their almost perfect single-crystal silicon structure. to this, the protection class of the converters embedded to the eTh yare constructedbythe Czochralskiprocess [ 24], a spe- marine PV plants must be at least IP54 or IP54 W (especially cial method according to which the polysilicon starting mate- the ones installed outside the ship’s shell), making the ventila- rial is melted, specially cooled to monocrystal semiround or tion weaker and the total cost higher. square bars and cut into wafers with wire saws. The use of To encounter the corrosion problems, the PV panel almost pure silicon (an expensive raw material), the spe- metallic frames must be specially constructed. Every metal cialized production process, and the large amount of silicon surfacemustbegalvanizedorcovered by specialantirust coatings. High-quality metal must be used especially at xin fi g losses during the construction of wafers rise to the top of the production cost. However, the monocrystalline solar cells points (e.g., aluminium or V2A stainless steel). In addition, havethelongestestimatedlifetime(over30years),thehighest since any moisture penetration would result in cell degrada- tion (especially in CIS cells) extra consideration must be done efficiency (15–18%), and power density (W/m )among the for the encapsulation materials (i.e., additional glass sheet as commercial cells [24, 25]. the front weatherproong fi heat-strengthened, or -toughened On the other hand, the polycrystalline cells (Poly c-Si) safety glass) [24]. consist of crystal grids with different orientations. The aim eTh installation of PV plants on a marine vessel is also of this dieff renceisthe massiveproductionand thelesscon- under area restrictions. eTh systems must neither impede the trolled cooling method of silicon into cuboid form, which sig- cargo and human transfer nor cover places with n fi ancial nificantly diminish the production cost. As before, the wafers impact such as deck, storage halls, and tanks. eTh y also should are formed by cutting the extracted silicon cubes or ingots be kept out of reach in order to prevent electrical shocks, as with wire saws. The resulting square form of the wafers oer ff s well as damage of PV panels and converters, keeping at the higher back plane cover than the square monocrystaline cells. same time their maintenance work easy for the specialized The higher internal resistance at the vicinity of the multicrys- staff. u Th s, appropriate installation areas can be the unused talline structures decreases the overall efficiency of poly- roof and the facades of the superstructure, the funnel, the crystalline cells to 13–15% while their estimation life time port, and the starboard, even the glazing and glass fronts. exceeds the 25 years [24, 25]. Shading is another problem the PV systems are facing. In contrast to the aforementioned types of cells, thin eTh full or partial block of sun radiation in a cell or even a films have no crystallinestructure.They areconstructed by whole panel in a string grid not only decreases the efficiency applying thin layers of photoactive semiconductors on low- but can also be harmful to the system. eTh phenomenon is cost substrate (in most cases, glass). eTh most common raw knownashot spot andoccurswhenthe shaded cellsorpanels materials are the amorphous silicon (a-Si), the copper indium stop acting as generators and become electric loads [24]. diselenide (CIS), and cadmium telluride (CdTe), with the In land applications, shading is avoided by choosing open a-Si to be the most popular because of the low cost and 4 Advances in Power Electronics 52.3% ··· ··· . . 4.7% . . . . 1.8% 2.9% ··· 38.3% ··· Ribbon/sheet c-Si Poly c-Si CdTe, CIS, and others Mono c-Si a-Si ··· Figure 2: World trends of the most popular commercial PV cells at 2009 [26]. DC DC Or the lack of heavy metals. eTh cheap and small amount of raw AC DC material, the simple and low energy-cost production process, and n fi ally the easy installation make thin films an attractive Or choice formassproduction. Becauseoftheir cell form (long Ship’s bus narrow strips) and their interconnection, thin film cells are (AC or DC) less sensitive to shading and are able to construct cheaper Centralized technology and more transparent modules than the aforementioned cells. Counting on their ability to be installed in curved, even Figure 3: Centralized technology. flexible, surfaces, thin films are perfect for PV plants installed in building facades and glazing and glass fronts. eTh main drawbacks are the low efficiency (a-Si: 5–7%, CIT: 9–11%, power. As it is shown in Figure 3, its main characteristic is the CdTe: 5–8.5%) [24, 25] and the least lifetime (20 years) which use of a single converter. The power is generated by parallel aimtothelackofcrystallinestructure.Thoughthinfilmsoeff r connected strings of panels. Each string ensures the necessary better utilization of diffuse and low light and more favorable high DC voltage that drives the converter, while their parallel temperature coefficient, properties that the new technology connection generates a high current. Extra characteristics of hybrid PV modules take advantage of. of thecentralized technology arethe useof(a) asingle In marine PV applications, the choice of the type of cells maximum power point tracking (MPPT) control system and is relatedtothe totalcostand thetypeofinstallationsurface. (b) diodes at the end of every string that block the reverse Crystalline cells are more appropriate for flat surfaces. eTh y currents because of the shadings or in grid temperature have low cost, high efficiency, and high power density that differences. help them to fully exploit small installing areas. eTh selection eTh pros andthe cons of thecentralized technology at of monocrystalline or polycrystalline is determined by the marine applications can be summarized as follows. total budget and the available area. eTh thin film modules aremorepreferablefortransparentsurfaceslikewindowsand Advantages glass facades, for curves, and shadowy places. (i) Large amounts of electric power can be generated. Finally, in Figure 2 the world trends of the most popular Due to space limitations on vessels the power produc- commercial PV cells at 2009 are presented [26]. tion cannot be equal to respective mainland applica- tions (10 kW to 400 kW) but definitely overcame the 5. PV System Technologies 10 kW [7, 27]. (ii) The string formation of panels oeff rs satisfactory high eTh power of a PV plant is directly related to the number of DC voltageatthe inputofthe converter, making the installed panels while their parallel and in series connec- needless extra voltage amplification (by the converter tion defines the output current and voltage. There are four or transformer). eTh high DC is essential for the main technologies for the interconnection of the PV panels connection to the ship electric bus. and converters, which are presented in the following para- (iii) The converter can be away of the PV grid, protected graphs, adjusted for marine applications. from humidity and dust. The first technology is referred in the worldwide bibliog- raphy as centralized technology [7, 26–29]. It is one of the (iv) It can be applied both to single- and three-phase AC oldestones, appliedtoPVplantswithlarge amount of output bus. Advances in Power Electronics 5 Disadvantages (i) Large installation areas are necessary. (ii) The PV panels are more likely to be shaded and stroke by hotspot phenomena. . . . . . . . . . (iii) The single converter lacks of reserve, raising the risk of total system breakdown in case of a malfunction. (iv) Only the same type of PV panels can be installed. (v) eTh re is high voltage at the connection point between PV grid and converter that results to (a) higher risk of electroshock, (b) higher cost of the wiring (special specifications DC DC for insulation and high power transfer), AC DC (c) higher installation cost due to special protection and ground systems [26]. DC DC Or AC DC -If necessary- (vi) The single MPPT control system cannot help each panel to operate at its maximum power, leading to Or Or reduced overall efficiency. Ship’s bus (AC or DC) (vii) The upgrade of the total installed power capacity is String not feasible. technology (viii) High total cost. Figure 4: String technology. (ix) Lower efficiency in relation to other technologies. Possible replacer of the aforementioned technology is the Disadvantages string technology [7, 26–29], which is popular in countries, pioneers of PV technology, such as Germany. Unlike central- (i) eTh problems of high DC voltage to the connection ized technology, the PV grid consists only of a single string point between PV grid and converter and hotspot that is attached to a converter, a characteristic that decreases phenomena still remain. the installed power capacity but gives many advantages to (ii) Only the same type of PV panels can be installed. the new technology. A diagram of the string technology is presented in Figure 4. (iii) Depending on the installed number of PV panels, the generated power from every string ranges between 0.7 kW and 3 kW. Advantages (iv) The multistring technology is an evolution of the (i) Because of theinseriesconnectionofthe panels, string technology whichlatelygains ground on the there is not always necessary extra amplification of the world market. eTh power management that is offered converter’s input voltage. is similar to the centralized technology without the disadvantagesofthe second.Asitispresented at (ii) Less installation area needs but still considerable. Figure 5 in the design requires multiple strings of (iii) Lower total cost than before. panels with their respective converters, parallel con- (iv) More effective MPPT control system since it is applied nected to a single central converter. to smallernumberofpanels. (v) er Th e is no need for blocking diodes. Advantages (vi) The upgrade of the installed power capacity is suc- (i) Similar power generation to centralized technology. ceeded by installing more strings and converters (ii) More effective MPPT control system. to the bus without any further restriction by the precedent system design. (iii) Different types and numbers of panels can be installed at every string. (vii) Higher efficiency compared to the centralized tech- nology. (iv) The blocking diodes are not necessary. (viii) The converter can be away from the PV grid, pro- (v) eTh converter can be away from the PV grid, pro- tected from humidity and dust. tected from humidity and dust. (ix) It can be applied both to single- and three-phase AC (vi) It can be applied both to single- and three-phase AC bus. bus. 6 Advances in Power Electronics Module-integrated converter (MIC) technology DC DC DC Or AC AC DC DC DC ··· DC DC ··· Ship’s bus DC DC Or (AC or DC) DC AC Or Ship’s bus Figure 6: MIC technology. (AC or DC) Multistring technology Advantages Figure 5: Multistring technology. (i) eTh one to one correspondence between panel and inverter oer ff s the optimal MPPT control. (ii) The voltage at the connection point between panel Disadvantages and converter is low. (i) eTh additional converter in every string increases the (iii) Low total cost. total cost (iv) The installation does not require special staff. (ii) The problems of high DC voltage to the connection (v) eTh y demand the minimum installation area, making point between PV grid and converters and hotspot them ideal for marine applications, especially in PV phenomena still remain. plants integrated in windows and glass facades. (iii) Even if the system is tolerant during a string’s con- (vi) Easily installed closely to crucial loads. verter breakdown, it cannot remain operational if the (vii) The upgrade of the installed power capacity is suc- main converter is damage. ceeded by installing more modules to the bus with- out any further restriction by the precedent system (iv) Largeinstallationareas arenecessary. design. (v) eTh last technology is called AC-PV module (in AC (viii) A PV plant based on numerous MIC can generate applications) or module-integrated converter (MIC) power even when one or more converters fail to [7, 26–29] and is the newest one in the eld fi of operate. Residential PV applications. eTh y are low power PV (ix) The panels are free of hotspot risk. devices that are consisted of a single PV panel and a converter directly connected to the bus. The lack of PV arrays and the more eeff ctive control of every Disadvantages device (panel and converter) lead to higher total (i) eTh MIC in mainland applications is dedicated for efficiency. eTh converter can be attached either to single-phase applications and the output voltage the support mechanism beside the panel or directly to its back side. The diagram of MIC technology is hardly overcomes the 300 V (without the use of trans- former). However, in marine applications, they may presented in Figure 6. have to be not only able of supplying three-phase In comparison with the aforementioned technologies loads but also reaching output voltages equal to 400 V MIC have the following characteristics. (DC or AC). Advances in Power Electronics 7 (ii) The maximum generated power is determined by the [5] Large-Scale Photovoltaic Power Plants, http://www.sunener- gysite.eu/download/AnnualReview FreeEdition.pdf. PV panel’s nominal power. 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PV Systems Installed in Marine Vessels: Technologies and Specifications

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Hindawi Publishing Corporation
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Copyright © 2013 Ioannis Kobougias et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Hindawi Publishing Corporation Advances in Power Electronics Volume 2013, Article ID 831560, 8 pages http://dx.doi.org/10.1155/2013/831560 Research Article PV Systems Installed in Marine Vessels: Technologies and Specifications 1 2 1 Ioannis Kobougias, Emmanuel Tatakis, and John Prousalidis Division of Marine Engineering, School of Naval Architecture and Marine Engineering, National Technical University of Athens, 9 Heroon Politechniou Street, 15773 Zografou, Greece Laboratory of Electromechanical Energy Conversion, Power Systems Division, Department of Electrical and Computer Engineering, University of Patras,UniversityCampusofPatras, 26504Rio,Greece Correspondence should be addressed to Ioannis Kobougias; ikob@central.ntua.gr Received 25 October 2012; Accepted 22 January 2013 Academic Editor: George Antonopoulos Copyright © 2013 Ioannis Kobougias et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Considerations are held about the specificationin which the PV plants have to fulfill so that they can be installed on marine vessels. Initially, a brief description of the typical electrical grid of ships is presented, distinguishing the main parts, reporting the typical electrical magnitudes, and choosing the most preferable installation areas. The technical specifications,in which the PV plants have to be compatible with, are fully described. eTh y are determined by the special marine environmental conditions, taking into consideration parameters like wind, humidity, shading, corrosion, and limited installation area. The work is carried out with the presentation of the most popular trends in the field of solar cell types and PV system technologies and their ability to keep up with the aforementioned specifications. 1. Introduction limited, mainly working as suppliers to small lighthouses, buoys, and chargers for the batteries of small sailing yachts Without doubt the last decade was the golden age of the [8, 9]. The rising transport expenses due to the fuel prices, the photovoltaic systems. The large number of technological increasing restrictions of CO and nitric oxides NO emis- 2 𝑥 breakthroughs on the research areas of power electronics, sion due to new ecological policies, and generally the need for photovoltaic (PV) panels, and microgrids made the use of PV more eco-friendly transportation were the reasons that forced panels feasible to numerous applications of modern life. the marine companies to reexamine the systematic use of PV The PV systems of today generate electric power that systems on large vessels [10–12]. ranges from W to MW. Small solar chargers for portable The photovoltaic technology can indeed be a really cost- devices such as laptops, cell phones, and calculators are very eecti ff ve solution for ships. PV systems can act as ideal popular. Single or arrays of PV panels produce electric subsidiary power sources, independent from the vessel elec- power for street lights, advertising signs, isolated agricultural tromechanical settlement because they [13–15] electric pumps, even small houses not connected to the utility (i) produce electric power without the need of trans- grid. In addition, PV systems, wind power systems, batteries, ferred gas or liquid fuel, fuel cell generators, and other renewable energy systems work together and organize reliable microgrids [1–4]. But the most (ii) have no by-productssuchasgas emissionsornoise, common PV applications are the grid-tied ones, where single (iii) have low maintenance cost, PV panels or large scale PV plants apply auxiliary electric (iv) have limited or no use of mechanical moving parts, power to the grid [4–7]. Despite their extended use at mainland applications, the (v) consist of few parts, with easy installation and fast PV systems presence in modern marine technology remains replacement in case of aging or defectiveness, 2 Advances in Power Electronics Main 3 kV–13.8 kV , generator 50–60 Hz,3𝜑 Load Possible M G power AC3𝜑 conversion 400 V , 50 Hz TM Or Load Main Possible generator Power power AC1𝜑 Main motor converter conversion 240 V , 50 Hz and/or Gear box Load Auxiliary Possible power generator number 1 DC conversion 400 V Diesel DM G motor number 1 Load Diesel Possible DM power DC motor number 2 G conversion 24 V Auxiliary generator number 2 Figure 1: Diagram of a typical electrical system for a ship. (vi) have satisfactory life time with a warranted PV panel Besides the main generator, at least two additional aux- output power by the manufacturers, which usually iliary (diesel powered) electrical generators are connected cannot be less than the 80% of the nominal one aeft r to the electrical grid providing with electric energy. Addi- 25 years of operation, tionally, in high load or breakdown cases (malfunction of main motor and/or main generator) they have to sustain the (vii) can be placed in small surfaces with no practical use minimum operational standards of the vessel [18, 19]. such as roofs, walls, funnels, and superstructure, According to the bibliography, the output voltages of In this paper, aresearchisheldtogiveanswers abouthow themainand thedieselgeneratorsvary. Typicalvaluesof the most popular technologies of the mainland PV systems their magnitude and frequency are 3 kV, 3.3 kV, 4.16 kV, 6 kV, can be applied, and what specifications must be fulfilled in 6.6 kV, and 13.8 kV and 50 to 60 Hz [16, 20, 21]. With the help order to be appropriate for partially or fully electrical marine of power converters and transformers, the generated power is vessels. supplied to an internal bus that transfers the power in every load over the ship. The bus can be either AC or DC. eTh DC 2. The Electrical System of a Marine Vessel buses are more popular to modern naval ships while the AC ones to cargo and passenger ships. However, the research in Four separate areas can be distinguished in the electric system progress investigates the most protfi able solution for the large of a typical ship: the main propulsion engine, the generators, civil vessels. Typical values for a three-phase AC bus are the themaindistributionbus,and theloads.In Figure 1,a 400V,50or60Hzand foraDC busthe 400V. diagram of a typical electrical system for a ship is presented. The last part of the ship electrical system is the loads. A thermal motor consuming diesel or heavy fuel oil is Different types of loads are connected, but the most common used as a prime mover. Its rotational movement can be used are the DC 24V and 400V, the single-phase AC 230V, 50Hz, either for both propulsion and electric generation, or exclu- andthe three-phaseAC400V, 50Hz loads. sively for electric power generation. Among the aforementioned parts of the vessel’s electric In thefirstcase, thegenerator is coupledtothe shaft system, the potential spots for the PV systems to be connected throughastep-upgear. It is knowntothe worldwidebibli- to the distribution buses along with the loads. ography as shaft generator; and according to its position, the type of coupling and the control equipment can be catego- rizedintofourteentypes [16, 17]. It aims not only to produce 3. Technical Specifications for PV Systems in electric power for the ship but also, in certain cases, to operate as a propulsion motor and assist the main motor engine. Marine Vessels In the second case, the mechanical power of the thermal engine is dedicated only to drive the main generator. eTh gen- Even though the installation of PV plants to mainland is com- erator simultaneously supplies the ship with electric power mon and thoroughly examined, extra considerations must and drives an electric motor attached to the propeller. eTh be done when they have to be installed on ships. eTh most propulsion system is known as integrated full electric propul- important distinction between the mainland and marine sion (IFEP) configuration, and its main characteristic is that PV applications is the environmental conditions, forcing PV the shaft system is minimized, if not completely eliminated, systems to be more tolerant to extreme winds, high humidity, that is, in the IFEP-pod case. and salt. Bus (DC 400 V or AC 3𝜑 , 400 V, 50/60 Hz) Advances in Power Electronics 3 The winds on a ship are characterized by their great muta- installation areas. On the contrary, in marine applications the bility of direction and speed, a fact that has great impact to open spaces are limited, and the shades are more difficult the orientations of the applied PV panels. eTh use of xfi ed tilt to be predicted due to the continuous changes of the vessel oer ff s easy and strong embracement to the ship. eTh orienta- orientation. For these reasons, the use of bypass diodes and tion is preferable to concur with the keel so as to help the ves- smaller-scale installed PV systems are preferable. sel aerodynamic. But the xfi ed tilt results an important draw- eTh re are two additional specica fi tions that the smaller- back. PV panels cannot fully exploit the sun radiation due to scale installations meet. Firstly, PV plants on ships should be the variety of the ship routes and constantly changing latitude. connected closely to crucial loads so as to minimize the dis- PV plants with embedded tracking control system may oeff r tribution losses; and secondly, they have to produce dispersed more efficiency but have many mechanical moving parts electric power, to apply the necessary reserve for PV power which are vulnerable to ocean storms and are more expensive generation. and need extra maintenance costs even higher than the Finally, the operational specifications of a PV system are ones of the corresponding on land systems. eTh golden the same for both the mainland and marine applications. mean is the use of PV panels placed tangentially to the ship Namely, the output characteristics ought to be compatible surfaces. eTh air resistance of the PV panels to strong winds with the electric magnitudes on the installing points of the becomes negligible, and its incident solar radiation is hardly grid; high efficiency, and power factor are essential; maxi- aeff cted by the sailing routes. However, this type of orienta- mum power point tracking (MPPT) and anti-islanding con- tion maximizes the necessary installation surface and stint trol are necessary. the air flow behind the panels reducing the possibility of cooling. 4. Solar Cell Types eTh marine environment can be also harmful for both According to their structure, the solar cell types can be the electronics and the panels of a PV system. The high classified into two basic groups, the crystalline cells and thin levels of humidity and salt can cause short-circuits and induce films [ 24]. At crystalline cells, the raw material is silicon corrosion to the mechanical parts of the converters. eTh European Committee for Electro Technical Standardization (Si), whose molecules are organized in crystalline grids. The orientation of the grid defines whether the cell is monocrys- (CENELEC) has developed an ingression protection rating talline or polycrystalline [24, 25]. (IP Code) that scales the electronic circuit protection levels from solidobjects,materials,and liquids[22, 23]. According The monocrystalline solar cells (Mono c-Si) owe their name to their almost perfect single-crystal silicon structure. to this, the protection class of the converters embedded to the eTh yare constructedbythe Czochralskiprocess [ 24], a spe- marine PV plants must be at least IP54 or IP54 W (especially cial method according to which the polysilicon starting mate- the ones installed outside the ship’s shell), making the ventila- rial is melted, specially cooled to monocrystal semiround or tion weaker and the total cost higher. square bars and cut into wafers with wire saws. The use of To encounter the corrosion problems, the PV panel almost pure silicon (an expensive raw material), the spe- metallic frames must be specially constructed. Every metal cialized production process, and the large amount of silicon surfacemustbegalvanizedorcovered by specialantirust coatings. High-quality metal must be used especially at xin fi g losses during the construction of wafers rise to the top of the production cost. However, the monocrystalline solar cells points (e.g., aluminium or V2A stainless steel). In addition, havethelongestestimatedlifetime(over30years),thehighest since any moisture penetration would result in cell degrada- tion (especially in CIS cells) extra consideration must be done efficiency (15–18%), and power density (W/m )among the for the encapsulation materials (i.e., additional glass sheet as commercial cells [24, 25]. the front weatherproong fi heat-strengthened, or -toughened On the other hand, the polycrystalline cells (Poly c-Si) safety glass) [24]. consist of crystal grids with different orientations. The aim eTh installation of PV plants on a marine vessel is also of this dieff renceisthe massiveproductionand thelesscon- under area restrictions. eTh systems must neither impede the trolled cooling method of silicon into cuboid form, which sig- cargo and human transfer nor cover places with n fi ancial nificantly diminish the production cost. As before, the wafers impact such as deck, storage halls, and tanks. eTh y also should are formed by cutting the extracted silicon cubes or ingots be kept out of reach in order to prevent electrical shocks, as with wire saws. The resulting square form of the wafers oer ff s well as damage of PV panels and converters, keeping at the higher back plane cover than the square monocrystaline cells. same time their maintenance work easy for the specialized The higher internal resistance at the vicinity of the multicrys- staff. u Th s, appropriate installation areas can be the unused talline structures decreases the overall efficiency of poly- roof and the facades of the superstructure, the funnel, the crystalline cells to 13–15% while their estimation life time port, and the starboard, even the glazing and glass fronts. exceeds the 25 years [24, 25]. Shading is another problem the PV systems are facing. In contrast to the aforementioned types of cells, thin eTh full or partial block of sun radiation in a cell or even a films have no crystallinestructure.They areconstructed by whole panel in a string grid not only decreases the efficiency applying thin layers of photoactive semiconductors on low- but can also be harmful to the system. eTh phenomenon is cost substrate (in most cases, glass). eTh most common raw knownashot spot andoccurswhenthe shaded cellsorpanels materials are the amorphous silicon (a-Si), the copper indium stop acting as generators and become electric loads [24]. diselenide (CIS), and cadmium telluride (CdTe), with the In land applications, shading is avoided by choosing open a-Si to be the most popular because of the low cost and 4 Advances in Power Electronics 52.3% ··· ··· . . 4.7% . . . . 1.8% 2.9% ··· 38.3% ··· Ribbon/sheet c-Si Poly c-Si CdTe, CIS, and others Mono c-Si a-Si ··· Figure 2: World trends of the most popular commercial PV cells at 2009 [26]. DC DC Or the lack of heavy metals. eTh cheap and small amount of raw AC DC material, the simple and low energy-cost production process, and n fi ally the easy installation make thin films an attractive Or choice formassproduction. Becauseoftheir cell form (long Ship’s bus narrow strips) and their interconnection, thin film cells are (AC or DC) less sensitive to shading and are able to construct cheaper Centralized technology and more transparent modules than the aforementioned cells. Counting on their ability to be installed in curved, even Figure 3: Centralized technology. flexible, surfaces, thin films are perfect for PV plants installed in building facades and glazing and glass fronts. eTh main drawbacks are the low efficiency (a-Si: 5–7%, CIT: 9–11%, power. As it is shown in Figure 3, its main characteristic is the CdTe: 5–8.5%) [24, 25] and the least lifetime (20 years) which use of a single converter. The power is generated by parallel aimtothelackofcrystallinestructure.Thoughthinfilmsoeff r connected strings of panels. Each string ensures the necessary better utilization of diffuse and low light and more favorable high DC voltage that drives the converter, while their parallel temperature coefficient, properties that the new technology connection generates a high current. Extra characteristics of hybrid PV modules take advantage of. of thecentralized technology arethe useof(a) asingle In marine PV applications, the choice of the type of cells maximum power point tracking (MPPT) control system and is relatedtothe totalcostand thetypeofinstallationsurface. (b) diodes at the end of every string that block the reverse Crystalline cells are more appropriate for flat surfaces. eTh y currents because of the shadings or in grid temperature have low cost, high efficiency, and high power density that differences. help them to fully exploit small installing areas. eTh selection eTh pros andthe cons of thecentralized technology at of monocrystalline or polycrystalline is determined by the marine applications can be summarized as follows. total budget and the available area. eTh thin film modules aremorepreferablefortransparentsurfaceslikewindowsand Advantages glass facades, for curves, and shadowy places. (i) Large amounts of electric power can be generated. Finally, in Figure 2 the world trends of the most popular Due to space limitations on vessels the power produc- commercial PV cells at 2009 are presented [26]. tion cannot be equal to respective mainland applica- tions (10 kW to 400 kW) but definitely overcame the 5. PV System Technologies 10 kW [7, 27]. (ii) The string formation of panels oeff rs satisfactory high eTh power of a PV plant is directly related to the number of DC voltageatthe inputofthe converter, making the installed panels while their parallel and in series connec- needless extra voltage amplification (by the converter tion defines the output current and voltage. There are four or transformer). eTh high DC is essential for the main technologies for the interconnection of the PV panels connection to the ship electric bus. and converters, which are presented in the following para- (iii) The converter can be away of the PV grid, protected graphs, adjusted for marine applications. from humidity and dust. The first technology is referred in the worldwide bibliog- raphy as centralized technology [7, 26–29]. It is one of the (iv) It can be applied both to single- and three-phase AC oldestones, appliedtoPVplantswithlarge amount of output bus. Advances in Power Electronics 5 Disadvantages (i) Large installation areas are necessary. (ii) The PV panels are more likely to be shaded and stroke by hotspot phenomena. . . . . . . . . . (iii) The single converter lacks of reserve, raising the risk of total system breakdown in case of a malfunction. (iv) Only the same type of PV panels can be installed. (v) eTh re is high voltage at the connection point between PV grid and converter that results to (a) higher risk of electroshock, (b) higher cost of the wiring (special specifications DC DC for insulation and high power transfer), AC DC (c) higher installation cost due to special protection and ground systems [26]. DC DC Or AC DC -If necessary- (vi) The single MPPT control system cannot help each panel to operate at its maximum power, leading to Or Or reduced overall efficiency. Ship’s bus (AC or DC) (vii) The upgrade of the total installed power capacity is String not feasible. technology (viii) High total cost. Figure 4: String technology. (ix) Lower efficiency in relation to other technologies. Possible replacer of the aforementioned technology is the Disadvantages string technology [7, 26–29], which is popular in countries, pioneers of PV technology, such as Germany. Unlike central- (i) eTh problems of high DC voltage to the connection ized technology, the PV grid consists only of a single string point between PV grid and converter and hotspot that is attached to a converter, a characteristic that decreases phenomena still remain. the installed power capacity but gives many advantages to (ii) Only the same type of PV panels can be installed. the new technology. A diagram of the string technology is presented in Figure 4. (iii) Depending on the installed number of PV panels, the generated power from every string ranges between 0.7 kW and 3 kW. Advantages (iv) The multistring technology is an evolution of the (i) Because of theinseriesconnectionofthe panels, string technology whichlatelygains ground on the there is not always necessary extra amplification of the world market. eTh power management that is offered converter’s input voltage. is similar to the centralized technology without the disadvantagesofthe second.Asitispresented at (ii) Less installation area needs but still considerable. Figure 5 in the design requires multiple strings of (iii) Lower total cost than before. panels with their respective converters, parallel con- (iv) More effective MPPT control system since it is applied nected to a single central converter. to smallernumberofpanels. (v) er Th e is no need for blocking diodes. Advantages (vi) The upgrade of the installed power capacity is suc- (i) Similar power generation to centralized technology. ceeded by installing more strings and converters (ii) More effective MPPT control system. to the bus without any further restriction by the precedent system design. (iii) Different types and numbers of panels can be installed at every string. (vii) Higher efficiency compared to the centralized tech- nology. (iv) The blocking diodes are not necessary. (viii) The converter can be away from the PV grid, pro- (v) eTh converter can be away from the PV grid, pro- tected from humidity and dust. tected from humidity and dust. (ix) It can be applied both to single- and three-phase AC (vi) It can be applied both to single- and three-phase AC bus. bus. 6 Advances in Power Electronics Module-integrated converter (MIC) technology DC DC DC Or AC AC DC DC DC ··· DC DC ··· Ship’s bus DC DC Or (AC or DC) DC AC Or Ship’s bus Figure 6: MIC technology. (AC or DC) Multistring technology Advantages Figure 5: Multistring technology. (i) eTh one to one correspondence between panel and inverter oer ff s the optimal MPPT control. (ii) The voltage at the connection point between panel Disadvantages and converter is low. (i) eTh additional converter in every string increases the (iii) Low total cost. total cost (iv) The installation does not require special staff. (ii) The problems of high DC voltage to the connection (v) eTh y demand the minimum installation area, making point between PV grid and converters and hotspot them ideal for marine applications, especially in PV phenomena still remain. plants integrated in windows and glass facades. (iii) Even if the system is tolerant during a string’s con- (vi) Easily installed closely to crucial loads. verter breakdown, it cannot remain operational if the (vii) The upgrade of the installed power capacity is suc- main converter is damage. ceeded by installing more modules to the bus with- out any further restriction by the precedent system (iv) Largeinstallationareas arenecessary. design. (v) eTh last technology is called AC-PV module (in AC (viii) A PV plant based on numerous MIC can generate applications) or module-integrated converter (MIC) power even when one or more converters fail to [7, 26–29] and is the newest one in the eld fi of operate. Residential PV applications. eTh y are low power PV (ix) The panels are free of hotspot risk. devices that are consisted of a single PV panel and a converter directly connected to the bus. The lack of PV arrays and the more eeff ctive control of every Disadvantages device (panel and converter) lead to higher total (i) eTh MIC in mainland applications is dedicated for efficiency. eTh converter can be attached either to single-phase applications and the output voltage the support mechanism beside the panel or directly to its back side. The diagram of MIC technology is hardly overcomes the 300 V (without the use of trans- former). However, in marine applications, they may presented in Figure 6. have to be not only able of supplying three-phase In comparison with the aforementioned technologies loads but also reaching output voltages equal to 400 V MIC have the following characteristics. (DC or AC). Advances in Power Electronics 7 (ii) The maximum generated power is determined by the [5] Large-Scale Photovoltaic Power Plants, http://www.sunener- gysite.eu/download/AnnualReview FreeEdition.pdf. PV panel’s nominal power. 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