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By a rearrangement of the traditional supply-converter-load system connection, partial-power-processing-based converters can be used to achieve a reduction in size and cost, increase in system efficiency and lower device power rating. The concept is promising for different applications such as photovoltaic arrays, electric vehicles and electrolysis. For photovoltaic applications, it can drive each cell in the array to its maximum power point with a relatively smaller converter; for electric-vehicle applications, both an onboard charger with reduced weight and improved efficiency as well as a fast charger station handling higher power can be considered. By showing different examples of partial-power-processing application for energy-conversion and storage units and systems, this paper discusses key limitations of partial-power-processing and related improvements from different perspectives to show the potential in future power electronic systems. Keywords: batteries; DC–DC converter; differential power processing; fuel cells; hydrogen production; partial- power processing; wide band-gap devices; fractional power processing intermittent, such as when the sun shines or the wind Introduction blows, an intermediate is needed to store the energy when Different sources for renewable-energy production are there is too much energy in the power system. currently in use and more are being researched in order A wide variety of storage systems are available for to bring them to the commercial level . Photovoltaic (PV) storing excess energy, for example batteries (many dif- panels, wind turbines and hydro power are a few examples ferent chemical compositions), pumped hydro, flywheel, of commercially available sources that are included in the compressed-air energy storage and some emerging tech- power system and more are added to the mix every day. nologies at various stages: hydrogen systems (electrolyser Other emerging renewable systems are enhanced geo- and fuel-cell), superconducting magnetic-energy storage thermal, artificial photosynthesis, concentrated solar and electrochemical capacitors . For the storage sys- and cellulosic ethanol . All of these sources can deliver tems, as was the case for the sources of renewable energy, electrical power but at different voltage levels, ranging the input and output voltages range from a few volts to from a few volts to kilo or mega volts, and either of the kilo or mega volts. Therefore, an interface between en- AC or DC type. Since some renewable-energy sources are ergy generation and storage is needed. Since the interface Received: 13 May 2019; Accepted: 1 September 2019 © The Author(s) 2019. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, 307 provided the original work is properly cited. For commercial re-use, please contact email@example.com Downloaded from https://academic.oup.com/ce/article-abstract/3/4/307/5608983 by DeepDyve user on 10 December 2019 308 | Clean Energy, 2019, Vol. 3, No. 4 needs to be used to both store and retrieve the power, it ESS needs to be highly efficient. ESS The conventional way of interconnecting an energy- storage system (ESS) to a renewable-energy source could ESS be through a DC bus, as shown in Fig. 1, where a positive bus terminal and a negative terminal of a bidirectional DC–DC dc converter are connected to the ESS while the other posi- V V 1 2 bus tive and negative terminals of the DC–DC converter are connected to the regulated DC bus. This means that the dc power (P) running into the DC–DC converter will also be the power supplied to the ESS (minus the losses of the DC– Fig. 2 Series-connected DC–DC converter system. The input of the con- DC converter), thus the DC–DC converter has to be rated at verter is at V, while the output is at V . 2 1 the same power level as the ESS and the supply. Instead of using the conventional way of interconnec- tion, the next-generation power electronic-converter appli- I I I in dc out cations are proposed to use the idea of connecting the ESS ++ dc 14.7A 4.7A 10A in series to the DC–DC converter and the DC bus, which is In Out boost shown in Fig. 2. The ESS is connected between the positive dc terminals of the DC–DC converter, while the DC bus is con- –– V V in out ESS nected to a positive terminal and the reference of the DC– 20V 28V out DC converter. In that manner and in accordance with Fig. 2, 10A the voltage V is set by the partial voltage between the DC bus and the ESS (V − V ), hence the DC–DC converter only 1 ESS processes the partial power between the DC bus and the ESS. The definition of V and V will be followed throughout 1 2 Fig. 3 Schematic overview of a Series Connected Boost Unit (SCBU) as the rest of the paper and their corresponding names in given in . V corresponds to V of Fig. 2 while V corresponds to V . in bus out ESS The power is flowing from V to V . other figures will be pointed out. Here, it should be noted in out that, in the literature, this concept is known under at least the following names: differential/fractional/partial-power for a DC–DC converter efficiency of 85%. This showcases processing [4–6] and, for the rest of the paper, it will be the higher-than-normal efficiency of the system view. The known as partial-power processing (PPP). increase in power density is also calculated by  for the Several different PPP schemes have already been de- system in Fig. 3 as scribed in the literature and here a couple of applications will be shown. The earliest configuration using PPP seems V 28V out Pd = · P = · 80W= 250 % .(2) SCBU dc to be  from 1996, where it is called a Series Connected V 8V boost Boost Unit (SCBU). The intended use was for PV arrays Another benefit of the SCBU according to the author is an supplying a battery, particularly in space use. The author inherent fault tolerance, which for space applications usu- specifies the idea as a ‘unique interconnect topology’ in- ally is covered by redundancy in the system. Reference  stead of a new DC–DC converter and suggests that many argues that the redundancy in the case of the SCBU can be isolated DC–DC converters can be used. The topology is a shorting of the output side, which would connect the PV shown in Fig. 3, which shows the configuration used to fur- array directly to the battery of this system. The usual re- ther boost the output voltage by stacking the voltage of the quirement for redundancy in spacecraft applications is to PV cells on top of the converter output. Based on the num- have the required number of converters plus a backup con- bers in Fig. 3, the efficiency of the system is calculated as verter, which translates to removing one converter from the spacecraft for both cost and weight savings. The last P V · I 28V · 10A out out out η = = = = 95.2%,(1) benefit worth mentioning is that an isolated DC–DC buck P V · I 20V · 14.7A in in in converter is used to achieve a boost converter, which is possible even with a lower-efficiency converter due to the ESS bus increase in efficiency from the SCBU topology. The limitations of the system given by  as a direct dc quote are V V ESS bus ESS ‘1. The SCBU is a boost con verter and requires that the dc output voltage be greater than or equal to the input voltage at all times, 2. Galv anic isolation must not be required between Fig. 1 Traditional DC–DC converter system. The input of the converter input and output circuits, and is at the left side, while the output is at the right side. Downloaded from https://academic.oup.com/ce/article-abstract/3/4/307/5608983 by DeepDyve user on 10 December 2019 Jørgensen et al. | 309 3. The input voltage source has a limited voltage full-bridge boost converter, which has a transformer to help range requiring only a small percentage (<50%) of boost the voltage between the input and the output. As in voltage boost.’ the other papers about PPP, the benefits are praised as lower initial cost and better efficiency than a full-rated converter, The last requirement has since been shown by others not but here it is taken a step further and calculates what the to be a limitation, whereas the first two requirements are improved efficiency means in kWh. A straightforward ap- still a hindrance to the PPP scheme. proach is used, where the loss over time is calculated as PPP has recently been gaining more attention than earlier, especially for PV arrays, where it is used for maximum (3) e = P (t)dt. loss loss power-point tracking for individual cells of the PV array. The full power processing is compared to the PPP approach A recent paper  organizes the different techniques used for the same charging profile, which shows almost a reduc- for PPP together with PV arrays into two main groups: series- tion of a factor of 10 in used kWh for their example. connected PV strings and parallel-connected PV strings, ex- The trend in power systems is a move towards distrib- amples shown in Fig. 4a and b , respectively. Series-connected uted energy-production and storage system , which PV strings are used to achieve a larger voltage than a single translates to a need for more converters placed close to the PV cell can supply on its own, while the parallel-connected production or storage. PPP is a promising concept in that PV strings are used if the required voltage is only slightly regard, since it is an effective way to minimize the phys- larger than the PV cells can supply on their own. The use of ical size, which is due to a lower power requirement. At the PPP here is to ensure that each cell is working at its max- same time, the initial and running costs of converters will imum power point by either supplying the cell with extra also be reduced, which is of great benefit when the instal- current from the bus or feeding excess current to the bus. lation of more converters is required. PPP has also been suggested in use for extreme fast- charging stations , 9 where the drawing has been redrawn in Fig. 5 to reflect our style of drawing. Fig. 5 highlights an- 1 PPP examples other feature of PPP: the excess energy can be delivered back to the source or the load, depending on what would be Different prototypes have been created at the Technical most beneficial to the actual application. The DC–DC con- University of Denmark and will here be reproduced as ex- verter chosen for this application is a current-fed resonant amples showing the application of PPP. I I A string load B load dc1 dc dc PV dc V V bus bus dc dc2 PV dc PV dc Fig. 4 Example of (a) a series-connected string of PV cells controlled by PPP and (b) a parallel string of PV cells controlled by PPP . The power is flowing from the PV panels to the bus; thus, with reference to Fig. 2, V is equal to V . bus ESS dc dc dc ESS ESS dc bus bus ESS ESS Fig. 5 PPP for use in an extreme fast-charging station. (a) Charging the ESS, (b) discharging the ESS. Downloaded from https://academic.oup.com/ce/article-abstract/3/4/307/5608983 by DeepDyve user on 10 December 2019 310 | Clean Energy, 2019, Vol. 3, No. 4 Before getting into the examples, a short explanation of system is 100% effective, which is expected, since, at the gain of PPP is given. When using PPP, there will be an k = 0, the converter is not processing any power from the efficiency (η) of the DC–DC converter and a different effi- system. This requires that V is 0 V, which requires in- ciency for the overall system. The efficiency of the system finite gain to get the required output voltage and is thus can be written as  hard to achieve. At the other extreme of = k 1, meaning the converter processes the full power, there is a small η = ,(4) system improvement if a 90% converter is used, but an improve- 1 + k · (1 − η − ) DC DC converter ment of 17% for a 50% efficient converter. k = 1 means where that V is equal to V and thus only half of the system ESS 2 P − V · I V − V DC DC converter 2 ESS 1 ESS power is being processed. This shows that the best way k = = ≈ ,(5) P V · I V ESS ESS ESS ESS to operate a PPP system is to have a small voltage dif- ference between the energy source and the ESS. One ap- V is the DC bus voltage, V is the converter input voltage 1 2 proach to achieving a high voltage at the input of the and V is the ESS voltage, thus V – V is the DC–DC con- ESS 1 ESS converter and a low voltage at the output is the so-called verter input voltage. This definition of voltages comes from input-series-output-parallel concept or, in reverse, the Fig. 2. input-parallel-output-series for a low-input voltage and Fig. 6 shows the system efficiency as a function of a high-output voltage. k and a function of converter efficiency. For k = 0, the η = 90% dc-dc η = 80% dc-dc η = 70% dc-dc η = 60% dc-dc η = 50% dc-dc 70 η = 40% dc-dc η = 30% dc-dc η = 20% dc-dc η = 10% dc-dc 0 0.2 0.4 0.6 0.81 Fig. 6 Normalized calculation of system efficiency for different efficiency DC–DC converters Vout = 50 Vout = 52 Vout = 54 Vout = 56 Vout = 58 0 500 1000 1500 2000 2500 3000 3500 P (W) EC Fig. 7 Power processed by the DC–DC converter versus power fed into the electrolyser cell for different output voltages of the DC–DC converter, where the output voltage is defined in Fig. 8c and corresponds to V of Fig. 2 [11, 12] P DC-DC converter (W) in η (%) system Downloaded from https://academic.oup.com/ce/article-abstract/3/4/307/5608983 by DeepDyve user on 10 December 2019 Jørgensen et al. | 311 1.1 Alkaline electrolyser cell converter The first prototype was made for an alkaline electrolyser cell and the system specifications are shown in Table 1, while the relationship between the power processed by the DC–DC converter versus the power fed into the electrolyser cell at different output voltages is shown in Fig. 7. There is no linear relationship between the two, which is due to the I-V characteristic of the electrolyser cell. The converter can be seen in Fig. 8a [11, 12], the system setup is shown in Fig. 8b and the system diagram is shown in Fig. 8c. An example of a positive load step is shown in Fig. 9a and a negative load step is shown in Fig. 9b. Both load steps occur with no overshoot and the negative load step is faster than the bus positive load step. The converter is made for a maximum operating power of 733 W, while the maximum system power is 3.5 kW. The DC-DC highest k factor is achieved when the input to the DC–DC converter converter is 2 V and the output is 48 V (corresponding to a V of 48 V), resulting in k = 0.04. This k factor occurs at the ESS EC same time as the efficiency of the DC–DC converter itself is low, as seen in Fig. 10a. Fig. 10a shows the efficiency of the DC–DC converter at different V levels, giving the worst-case DC–DC converter efficiency when V is the lowest. In Fig. 10b, the calculated system efficiency is shown. All the ef- ficiencies are within 0.5% and the highest efficiency is achieved when the converter efficiency is lowest, which is attributed to the fact that the k factor is lowest for this ESS ESS ESS dc Table 1 Alkaline electrolyser system specifications  V V V bus 1 2 V 35 ~ 48 V ESS dc I 0 ~ 72 A ESS V 58 ~ 50 V V = V – V 23 ~ 2 V Fig. 8 (a) DC–DC converter made for an alkaline electrolyser cell using 2 1 ESS the partial-power concept. (b) The DC–DC converter connected to an Maximum system power, P 3456 W system alkaline electrolyser system. (c) System diagram of the DC–DC con- Maximum dc converter power, P 733 W DC–DC verter and the ESS, which, in this case, is an electrolyser cell (EC) [11, k 0.21 12]. Reprinted from  with permission from IEEE. AB Vgs (10 V/div) Vgs (10 V/div) V2 (5 V/div) V2 (5 V/div) IESS (2 A/div) IESS (2 A/div) IL (20 A/div) IL (20 A/div) Fig. 9 Load-step response of the PPP system. (a) is a positive step and (b) a negative. The shown waveforms are identical between (a) and (b), and are as follows from top to bottom: V is the gate voltage (10 V/div), V is the converter input voltage (5 V/div), I is the ESS current (2 A/div) and I is the GS 2 ESS L inductor current (20 A/div), Rogowski coil (100 mV/A). Time scale is 6.4 ms/div [11, 12]. Reprinted from  with permission from IEEE. Downloaded from https://academic.oup.com/ce/article-abstract/3/4/307/5608983 by DeepDyve user on 10 December 2019 312 | Clean Energy, 2019, Vol. 3, No. 4 AB 99.5 98.5 88 98 0 500 1000 1500 2000 2500 3000 3500 V = 50 V = 52 V = 54 V = 56 82 1 V = 58 100 0 200 400 600 800 dc/dc converter output power (W) V = 50 V = 52 84 V = 54 84 V = 56 V = 58 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 3000 3500 System output power (W) System output power (W) Fig. 10 Alkaline electrolyser cell (a) DC–DC converter efficiency calculation, (b) system efficiency calculation and (c) system efficiency measurement [11, 12]. Reprinted from  with permission from IEEE. V . The measured system efficiency is shown in Fig. 10c efficiency of 90%, the system efficiency is still improved to and, again, the efficiency is flat at higher powers and the ~94%. Fig. 12 shows the converter and system efficiency, highest efficiency is obtained at the highest power level. while discharging (SOEC mode). The efficiency on the system This showcases one of the benefits of PPP, which is that level improves by around 2% and the system efficiency curve the usual worst-case operating mode for the DC–DC con- is relatively flat, which is usually hard to achieve since the verter (high voltage gain) is suddenly turned into a highly control reduces the efficiency at low power levels and con- efficient mode, since the k factor becomes low. duction losses reduces efficiency at high power levels. 1.2 Solid-oxide electrolyser/fuel-cell converter 1.3 Electric-vehicle converter The second prototype is of a solid-oxide electrolyser cell The last prototype is a DC–DC converter designed for char - (SOEC)/solid-oxide fuel-cell (SOFC) setup [1314 , ], which can ging a battery in an electric vehicle . The DC–DC con- be seen in Fig. 11a and the system diagram in Fig. 11b . The verter is shown in Fig. 13a, the setup is shown in Fig. 13b drawing of the system diagram is changed from the first and Table 3 lists the specifications of the system. The prototype, but the functionality of the two is equal. The system diagram is shown in Fig. 13c and it can be seen SOEC/SOFC system is in the background of Fig. 11a, while the that the configuration is the same as in the first prototype. converter is shown in the front. The I-V curves of the used Table 3 shows that the voltage difference between the SOEC/SOFC can be seen in Fig. 11c and the overall system ESS and the supply is smaller than the earlier prototypes, specifications can be seen in Table 2. The k factor for the which leads to a larger k factor and therefore a smaller im- charging is in the low end and, as shown in Fig. 6, the ex- provement in the efficiency. The converter and system ef- pected system efficiency is above 90% regardless of the effi- ficiency for k = 0.4 is shown in Fig. 14a and, for the lowest ciency of the DC–DC converter. For the discharging case, the converter efficiency, the system efficiency is improved by benefit is reduced due to the higher k but, with a converter 8%, while it is improved by around 4% for the rest of the System η (%) dc/dc converter η (%) System η (%) Downloaded from https://academic.oup.com/ce/article-abstract/3/4/307/5608983 by DeepDyve user on 10 December 2019 Jørgensen et al. | 313 Table 2 SOEC/SOFC system specifications  Charging Discharging SOEC/SOFC V 450 ~ 540 V 360 ~ 450 V ESS I 0 ~ 60 A 30 ~ 0 A ESS V 600 V 600 V V = V – V 150 ~ 60 V 240 ~ 150 V 2 1 ESS Maximum system power, P 32.4 kW 10.8 kW system Maximum dc converter power, P 3.6 kW 7.2 kW DC–DC k 0.11 0.67 DAB voltages and currents are from 2 to 600 V and from 0 to 72 A, B ESS respectively, but can in theory go both lower and higher than ESS that. The converter power level compared to the system power level also ranges by a factor of 10 (from 733 W to 7.2 kW), which can also vary more for the right applications. ESS I A benefit that has not been mentioned yet is that, due bus to a high-voltage/low-current side and a low-voltage/high- dc current side of the converter, the components used does not V V 1 2 bus need to be rated for both high voltage and high current as is usually the case for DC–DC converters converting voltages dc that are close between the source and the ESS. For example, take the system specifications in Table 1, which, for a trad- itional converter, would need devices rated at 48 V/72 A on one side and 58 V/~69 A on the other side. For the system Discharging mode Charging mode employing PPP, those ratings would change to 23 V/72 A and 48 V/~47 A, which is a ~50% reduction on the high-voltage side and 70% reduction on the low-voltage side. This might lead to more options of switching devices, lower-cost de- vices, less copper on the printed circuit board and reduced costs for the magnetic and capacitive components. So is this configuration the new way for all applica- tions? Certainly not. PPP has its limitations, as mentioned –60 –40 –20 0 20 40 earlier, which are: (i) best performance obtained for a I [A] ESS voltage difference between the source and load close to zero, (ii) source voltage greater than load voltage and (iii) Fig. 11 (a) SOEC/SOFC under partial-power test, (b) the system diagram, no galvanic isolation. One use of isolated DC–DC con- where V and V are the same as in Fig. 2 , and (c) I-V curve of the 1 2 verters is normally that they provide galvanic isolation, SOEC/SOFC setup which provides safety to both the source and the ESS, but also to any eventual human interaction with the devices. system specifications. The efficiencies for k = 0.9 is shown For the three cases here, galvanic isolation has either not in Fig. 14b, where the lowest converter efficiency is im- been a desired feature or it has been implemented in a proved by 2% at the system level and the rest of the ef- different manner. ficiencies are improved by around 1%. This comparison A difficulty that seems to occur in some DC–DC con- shows the benefit of having a big difference between the verters/some PPP configurations is that non-active power voltages of the ESS and the source, leaving only a small is circulating and erodes the benefits of the PPP [15, 16], part of the processed power to the DC–DC converter and the idea being that the reactive element of the DC–DC con- thus improving system efficiency. verter is storing non-active energy and losing a bit of that energy every switching cycle. 2 Results and discussion For the use cases presented in the previous section, a clear 3 Conclusions efficiency gain is obtained on a system level, even though The concept of PPP fits into the decentralization trend of the efficiency of the converter is not the highest that has society and thus of power electronics moving to distributed been reported in the literaturT eabl . e 4 summarizes the three systems, where smaller converter units are used close to the prototypes of the previous section and shows some of the source/load instead of bigger centralized units. There are variation that can occur in PPP systems. The range of the V [V] ESS Downloaded from https://academic.oup.com/ce/article-abstract/3/4/307/5608983 by DeepDyve user on 10 December 2019 12 cm 314 | Clean Energy, 2019, Vol. 3, No. 4 DC-DC converter efficiency System efficiency 100 100 98 98 96 96 94 94 92 92 0 12 34 2 46 810 P [kW] P [kW] ESS ESS Fig. 12 Converter and system efficiency. V = 300–450 V and P = 10 kW . ESS ESS Table 3 Electric-vehicle system specifications  V 288 ~ 403 V ESS I 10 A ESS V 489 ~ 566 V V = V – V 201 ~ 163 V 2 1 ESS Maximum system power, P 4 kW system Maximum dc converter power, P 2.8 kW DC–DC k 0.70 Charger efficiency Converter efficiency 0 500 1000 1500 2000 2500 3000 Battery charging power [W] C 98 ESS ESS ESS dc V V V bus 1 2 90 dc Charger efficiency Converter efficiency 0 500 1000 1500 2000 2500 Fig. 13 (a) DC–DC converter for charging of batteries in electric vehicles, Battery charging power [W] (b) DC–DC converter connected to batteries for testing and (c) system diagram of the proposed system. Reprinted from  with permission Fig. 14: Efficiency curves for the DC–DC converter of the electric-vehicle from IEEE. charging and the system efficiency. Operating conditions (a) V = 273 V, ESS V = 400 V, k = 0.4 and (b) V = 273 V, V = 540 V, k = 0.9. Reprinted from 1 ESS 1  with permission from IEEE. several benefits of using PPP, which include a lower initial comes for free and the tradeoff for PPP is the requirements price, higher efficiency (and thus lower operational cost), of no galvanic isolation and higher source voltage than load lower-rated devices (voltage, current and/or power) and voltage. For many applications, the voltage requirement is smaller size. As always in electrical engineering, nothing no problem by default or the system can be set up in such 23 cm η [%] dc-dc η [%] Efficiency [%] Efficiency [%] system 4 cm Downloaded from https://academic.oup.com/ce/article-abstract/3/4/307/5608983 by DeepDyve user on 10 December 2019 Jørgensen et al. | 315 Table 4 Summary of the three previous prototypes Solid-oxide electrolyser/fuel-cell converter Alkaline electrolyser Electric-vehicle cell converter Charging Discharging converter V 35 ~ 48 V 450 ~ 540 V 360 ~ 450 V 288 ~ 403 V ESS I 0 ~ 72 A 0 ~ 60 A 30 ~ 0 A 10 A ESS V 58 ~ 50 V 600 V 600 V 489 ~ 566 V V = V – V 23 ~ 2 V 150 ~ 60 V 240 ~ 150 V 201 ~ 163 V 2 1 ESS Maximum system power, P 3456 W 32.4k W 10.8k W 4k W system Maximum dc converter power, P 733 W 3.6k W 7.2k W 2.8k W DC–DC k 0.21 0.11 0.67 0.7  Levron Y, Clement DR, Choi B, et al. Control of submodule in- a way as to be no problem. 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Clean Energy – Oxford University Press
Published: Dec 6, 2019
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