A Returned Energy Architecture for Improved Photovoltaic Systems Efficiency Yigal Nimni, Doron Shmilovitz School of Electrical Engineering Tel Aviv University Tel-Aviv, Israel. e-mail:
[email protected] Abstract—A major loss mechanism in photovoltaic (PV) power systems is known as mismatch loss, which may result in up to 30% power reduction, depending on a PV system’s installation configuration and atmospheric conditions. To overcome excess losses due to mismatch losses, a new photovoltaic architecture is proposed which applies an internal power feedback (RECCs – returned energy current converters) to restore the symmetry among the PV modules i-v curves. This results in recovery of most of the internally trapped energy. The RECC units are realized by means of power electronics.
I.
II.
PARTIAL SHADING PROBLEMS IN PV SYSTEMS
Building Integrated PV systems (BIPV) are often subjected to shading. As a consequence, these systems may produce less energy than optimally positioned systems. This is due to the loss of available irradiation caused by shading and also to the so-called mismatch loss caused by partial shading. A widely accepted model of PV cells is shown in Figure 1.
INTRODUCTION
Mismatch losses in photovoltaic systems may be caused due to a number of reasons such as manufacturing tolerances of cell characteristics, environmental stresses, and unbalanced atmospheric conditions such as partial shading situations [1– 3]. Regardless of the mismatch cause, this situation manifests as an asymmetry in the current-voltage (i-v) characteristics of PV modules. Thus, since some of the PV modules within a PV installation exhibit different i-v characteristics, it is impossible to find an operation point at which all the modules within the installation operate at their maximum power point (MPP). To some extent, this problem is addressed in terms of bypass diodes that prevent a situation in which a module that generates low current (due to shading or malfunctioning) blocks the current of a whole string. However this is a suboptimal solution since the partial power that a bypassed module could generate is totally lost when it is bypassed. Furthermore, the action of the bypass diodes result in a step like characteristic in the i-v plane which results in a power graph with multiple maxima whose global maximum is difficult to track. In attempting to address this problem, there are numerous recent papers suggesting the concept of 'distributed power conditioning' where MPP tracking is performed at the module level by means of module integrated converters and the converters' outputs are connected in series to form the string [4, 5]. The module integrated converter seems to be the state-of-the-art in this regard, enabling operation of each module at the MPP. We propose injection of a current in parallel with the shaded modules in order to restore their symmetry. The energy for injecting this current is taken from the PV array's general dc output.
978-1-4244-5309-2/10/$26.00 ©2010 IEEE
Figure 1. PV cell electrical model.
Since a single PV cell's output voltage is quite low (around 0.55 V as a result of the cell’s internal forward biased diode) and since PV systems are used to generate substantial power, a long string consisting of many cells connected in series is needed in order to attain a sufficiently high voltage, which can be processed efficiently. This string is particularly long in grid-tied applications where the PV generated power is eventually fed into the ac power grid. For example, an ac inverter which generates 220VAC at its output is typically supplied with a dc voltage of at least 300VDC at its input, implying about 600 individual PV cells in series. Furthermore, the number of PV strings connected in parallel is determined by the required power. Actually, these long PV string are split into smaller strings which are implemented as standard solar modules. To demonstrate the effect of shading, consider a PV module consisting of 16 PV cells connected in series, as depicted in Figure 2. The shading is a continuous phenomenon which means it may acquire values between 0% and 100%. Furthermore, partial shading implies that some areas within the module are shaded while others are not. Therefore one may consider two independent parameters regarding partial shading: the percentage of shaded PV module area, and the shading irradiance percentage over each shaded area. Figure 2 shows a shading situation example where 50% of the module area is shaded by two shaded spots: the upper one is 30% shaded (i.e. the irradiance is reduced by
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30%) and the lower one is 70% shaded. Consequently the loss of irradiance is: 30%·25%+70%·25%= 25% and the raw power that may be generated by the module is 75% of the nominal power. We assume the following characteristics for the module: 16 cells of 0.5 V, 1 A each (nominal values for 1 Sun irradiance), thus 6 W is the upper bound of the output electrical power (75%·16·0.5V·1A = 6 W). Furthermore, we employ a simplified PV cell model consisting only of the irradiance current Iph and the forward biased diode, since this model is sufficient for demonstration of the principal effect. From Figure 2 it may be concluded that, without the bypass diodes, the 70% shaded PV cells dictate the overall current Iload which would be 0.3 A. The maximum power in this situation is around 2.4 W (~16·0.5·0.3). This is equivalent to a 70% loss of power (compared to nominal conditions), even though the loss of irradiance is only 25%. To some extent, this problem is addressed by means of bypass diodes (one across every 4 cells in this example). +
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Id 0.7A
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module is larger than 4.2 W (which is the maximum output power that can be achieved even with bypass diodes). ( 8cells ⋅ 1A + 4cells ⋅ 0.7A + 4cells ⋅ 0.3A ) ⋅ 0.5V = 6Watt The explanation for this mismatch is that a partially shaded group of cells still has some power to be output, but if it is bypassed (by the bypass diodes), then this energy cannot be used. It remains trapped within the PV cell group while the bypass diode clamps the voltage of this group to only 0.6 V, reducing the string's voltage and accordingly the string's output power. There is another drawback associated with the employment of bypass diodes; the string's i-v curve acquires a step-like shape rather than the monotonic concave curve of PV sources (with no bypass diodes), see Figure. 3. Consequently, the output power curve p(v) exhibits multiple maxima, which imposes difficulties for tracking the maximum power point. This becomes even more problematic when connecting a number of such PV sources together. In this case it is impossible to obtain the total potential maximum power. The next paragraph shows a concept that solves these problems and allows all the potential energy to be extracted from the partial shaded modules, with a better output power curve.
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Figure 3. PV module Simulation results (Iload & Pload versus Vload).
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Figure 2. 16 cell PV module with 50% area shading in 2 spots (30% & 70%).
These diodes provide a bypass path for the total current, so it is not blocked by PV cells that generate reduced current (due to shading or malfunctioning).Using bypass diodes, the string can supply 1 A up to the voltage of the 8 (50%) fully irradiated cells (4 V). Under these conditions, the two bypass diodes conduct and the MPP up to 4 V is 3.7 W (~8·0.5V·1A). Over the next highest voltage range (4–6 V), the four 30% shaded PV cells must contribute to the voltage buildup (the 30% bypass diode is not conducting). Thus, in this range of the i-v curve, the load current drops to 0.7 A. The maximum power for Vload between 4 V and 6 V is equal to 4.2 W (~12·0.5·0.7). The highest voltage range (6–8 V) is possible only if all PV cells contribute to the total voltage. In this case, no bypass diodes conduct. Thus, the higher voltage is attained at the cost of a reduction in load current to 0.3A, resulting in a maximum power of approximately 2.4W. The iv and p-v characteristics of the module with the four bypass diodes are shown in Figure 3 (obtained by means of a PSPICE simulation of the schematic of Figure 2). As mentioned above (see Figure 2), the potential power of the
THE RECC CONCEPT
Regardless of the source of mismatch (which may result either from differences in atmospheric conditions or module characteristics), it is always manifested in a lack of symmetry in the i-v curves among modules composing a PV system. Thus, the mismatch losses can be eliminated if the symmetry among a PV system's components is restored, i.e. by making all i-v curves within the PV system identical. Typically, in solar systems a relatively high voltage is obtained by connecting multiple modules in series to form a string of modules. Thus, symmetry restoration implies regulating all cells/module currents to the same current. The photongenerated current, Iph in Figure 1 is roughly proportional to the irradiation intensity, thus Iph is lower in shaded cells. We propose to replace the current that is missing due to shading (or any other mismatch) with a shunted current source. The power associated with the injection of the shunt current is taken from the overall dc output of the string (from the dclink capacitor). This restores symmetry among all string components and therefore allows extraction of the entire PV system's potential energy towards the load. Figure 4 shows the general form of a PV system shunted by RECC units (n=9 shading conditions of Figure 5). These shunted current sources are implemented as Returned Energy Current Converter (RECC) units. The RECC units are controlled so that the correct magnitude of current is injected by each
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RECC unit (for example by a CPU which scans the solar array rows periodically and tunes the current converters to achieve maximum output power from the overall string).
The voltage source 'UniDir Vs' connected to the string output is swept from 0 to 180 V in order to obtain the system i-v curve. The shaded modules are optionally bypassed by diodes (by setting the values of the resistors Rd1-Rd4) and the resistors Rp1-Rp4 serve to enable or disable the RECC units. All other resistors are used for current measurements. Simulation results for the array operating with bypass diodes and with RECC are shown in Figure 6. The advantage of the RECC concept over the bypass diodes is evident; extraction of the otherwise trapped potential energy of the shaded cells into the load yields in this example nearly 42% increase of output power. Furthermore the Pout(v) and Iout(v) curves are seen to become continuous and smooth (see Figure 6) which is particularly advantageous when combining a number of PV power sources together regarding tuning the operation to the Maximum Power Point (MPP) of the entire PV system.
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Figure 4. A string within a PV system with RECC units.
IV.
Figure 6. Pout & Iout for RECC and bypass diodes concepts according to simulation results.
SIMULATION EXAMPLE
Provided small modifications are made to adopt the RECC converters to operate in the PV system – RECC architecture (such as preventing dc current loops), almost any isolated dcdc converter topology can be used to implement the RECC unit. As shown in Figure 7, we have used a synchronous flyback topology with two additional MOSFETs to open a possible dc current loop. T2 is operated as the conventional fly-back pulse width modulated (PWM) switch, in effect setting the transfer ratio value k, while MOSFET T3 replaces the usual fly-back diode (otherwise a short-circuit would be formed across the RECC through a bypass path consisting of the transformer secondary and diode). In our implementation, since the RECC behaves like a transformer and since the PV module voltage is quite insensitive to irradiance (as opposed to current), all RECC units receive the same PWM signal. However, they are enabled one at a time through Enable inputs, switches T1 and T4.
Consider a PV system of 9 modules in series, each module having a nominal 20V open circuit voltage and 1 A short circuit current at 1 Sun irradiance (in total Voc = 180 V and Isc = 1 A for the array). Four modules out of nine are partially shaded: 50% of each module area and the shading is also 50% in terms of sun irradiance (22% of the array's area receives 50% irradiance). All other modules receive full sun. Assume 4 RECC units (UD_ERCC) shunt the shaded PV modules, as in Figure 5.
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Note: F9 F8 ==== PP+ PP+ Jsc = Short Circuit Density Current (ma/cm*cm) A = Cell Area (cm*cm) Rsn = Cell Serial Peculiar Resistance 0%A_0%S Rshn = Cell Parallel Peculiar Resistance F1 0%A_0%S P-
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Figure 5. PV system simulation diagram at 50% irradiance over 50% area of 4 modules, partial shading situation.
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Enable_m PWM
Figure 7. RECC Block implementation using a fly-back topology.
Figure 8 shows the internal signals of the Fly_RECC unit when k = 9 and each PV module generates about 16V. The shunted RECC contributes about 0.5 A since its parallel PV module suffers from 50% partial shading (in this example) and the PV module current is reduced accordingly from 1 A at full sun to ~ 0.5A due to shading. Moreover, the RECC output current Iout is seen to be 0.5A while its average input current is 55mA (according to the transfer ratio k=9). Figure 10. Sensitivity of total power to RECC transfer ratio k.
to the optimal k, and that the sensitivity does not vary much with irradiance conditions. VI. Figure 8. Flyback RECC typical signals.
V.
SENSITIVITY OF THE RECC TRANSFER RATIO K
The current injected by the RECC unit is determined by its transfer ratio k (which for a PWM converter based RECC is attained in practice through the converter duty ratio). On the other hand, the injected current (all injected currents within a PV system) sets the operating point of the system and determines how well the system is tuned to operate at the MPP. Thus, it is interesting to examine the sensitivity of the overall string output power to variations of transfer ratio k. Consider a circuit test set-up, a string consisting of 7 PV modules (m=7) connected in series. Each PV module produces a peak power of 20 V, 1 A at full irradiation (1 Sun). The sensitivity is tested under two irradiance profile conditions: LP1 light-moderate partial shading (requiring a light current feedback); and LP2 a more severe partial shading situation requiring a more intense current feedback, LP1 LP2 see Figure 9. The set-up is tested under 100% 100% these two irradiance conditions, while 5 RECC units compensate the partially 90% 50% shaded modules. The simulation model of the PV modules is based on the widely 75% 35% accepted basic PV cell model, such as the one shown in 50% 10% Figure 1, with the following parameters: 29 cm² area, short circuit current density, Jsc=34.3 75% 35% mA/cm², resistance losses: Rsh=170kΩ, Rs=60mΩ. Figure 10 shows the total PV string 90% 50% output power versus RECC transfer ratio k (Vin/Vout or Iout/Iin) variation k/koptimal. It 100% 100% can be seen that the sensitivity to transfer ratio is quite low, Figure 9. PV set-up reasonably close lighting conditions
CONCLUSIONS
Contrary to the module integrated scheme, in this new approach, just a small portion of the overall power (the portion which is necessary for symmetry restoration) is processed by the dc-dc converters, while most of the power is transferred directly to the dc bus. This results in three major advantages compared to module integrated converters: lower installed dc-dc converter rating (about 50%-70% rating reduction), lower losses, and smooth monotonic i-v and power curves for the overall PV array. RECC modules may be implemented either as gyrators or transformers. It would be interesting to investigate the impact of the type of RECC module on system performance and to determine which of the two is more suitable. REFERENCES [1]
M. C. Alonso-Gracia, J. M. Ruiz, and F. Chenlo, “Experimental study of mismatch and shading effects in the I-V characteristic of a photovoltaic module,” Solar Energy Mater. Solar Cells, vol. 90, no. 3, pp. 329–340, Feb. 2006. [2] H. Patel, V. Agarwal “MATLAB-Based Modeling to Study the Effects of Partial Shading on PV Array Characteristics” IEEE Transactions on Energy Conversion, vol. 23, NO. 1, March 2008. [3] N.D. Kaushikaa,_, Anil K. Rai, "An investigation of mismatch losses in solar photovoltaic cell networks", Elsevier Energy 32, 2007, pp. 755– 759. [4] Femia, N.; Lisi, G.; Petrone, G.; Spagnuolo, G.; Vitelli, M., "Distributed Maximum Power Point Tracking of Photovoltaic Arrays: Novel Approach and System Analysis", IEEE Transactions on Industrial Electronics, Volume 55, Issue 7, July 2008, pp.2610 – 2621. [5] Linares, L.; Erickson, R.W.; MacAlpine, S.; Brandemuehl, M., "Improved Energy Capture in Series String Photovoltaics via Smart Distributed Power Electronics", IEEE APEC 2009. 15-19 Feb. 2009 pp.904 – 910. [6] H. Kawamura, K. Naka, N. Yonekura, S. Yamanaka, H. Kawamura, H. Ohno, and K. Naito, “Simulation of I–V characteristics of a PV module with shaded PV cells,” Solar Energy Mater. Solar Cells, vol. 75, no. 3/4, pp. 613–621, Feb. 2003. [7] V.Quaschning and R.Hanitsch, “Numerical simulation of current– voltage characteristics of photovoltaic systems with shaded solar cells,” Solar Energy, vol. 56, no. 6, pp. 513–520, Feb. 1996. [8] G. R. Walker and J. C. Pierce, "Photovoltaic DC-DC Module Integrated Converter for Novel Cascaded and Bypass Grid Connection Topologies - Design and Optimization," IEEE Power Electronics Specialists Conference, 2006 Record, June 2006. [9] G. R. Walker and P. C. Sernia, “Cascaded DC–DC converter connection of photovoltaic modules,” IEEE Trans. Power Electron., vol. 19, no. 4, pp. 1130–1139, Jul. 2004. [10] E. Roman, R. Alonso, P. Ibanez, S. Elorduizapatarietxe, and D. Goitia, “Intelligent PV module for grid-connected PV systems,” IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1066–1073, Aug. 2006.
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