Performance assessment of various BIPV concepts

Transcription

1 Performance assessment of various BIPV concepts Wiep Folkerts, Roland Valckenborg, Menno van den Donker, Corry de Keizer, Chris Tzikas, Minne de Jong and Kostas Sinapis SEAC Eindhoven, The Netherlands, Abstract Performance assessment of BIPV (building integrated photovoltaics) naturally focuses on those aspects where BIPV deviates from regular BAPV. The major aspects are: temperature, partial shading and geometry. This paper highlights measurements and analysis related to all three relevant factors. In addition the extension of BIPV to BIPVT is discussed. Keywords: BIPV, ventilation, partial shading, PVT, facades, energy performance 1. Introduction European regulations are aiming at (near) zero-energy buildings in the near future. This requires, besides other measures, the application of solar electricity and solar heat in buildings. These energy performance regulations are now becoming the main driver for the growing market in BIPV and BIPVT [1]. An essential aspect is the precise methodology in which the application of BIPV contributes in the calculation of energy performance of a building. Today in most calculation methodologies a discount factor has to be applied when BIPV is used instead of BAPV. In the calculation method for The Netherlands (EPC), the prescribed discount factor for non-ventilated BIPV is 87.5% and for partly ventilated BIPV 94%. The motivation behind this discount factor is that the built-in nature of BIPV will lead to an increase in temperature, and therefore to a reduced performance. This aspect is discussed in section 2 of this paper. As BIPV is usually applied because of aesthetical and architectural reasons, BIPV will, more than BAPV, suffer from partial shading. The reason is that a choice for application of BIPV will typically be applied to the entire architectural area under consideration, and not only to the shade-free areas. This makes the understanding of the effects of partial shading in BIPV essential. Moreover, mitigation of partial shading effects in BIPV, either by optimizing the design of the BIPV module or by optimizing the design of the electrical system, becomes very important. In section 3 we discuss some aspects of partial shading, relevant for BIPV. Another factor of consideration for the design and performance of BIPV products is geometry. In case of BAPV, geometry usually is made subordinate to electricity yield. A well-known example is the installation of a tilted BAPV system on a flat roof. However the application of BIPV involves by definition that more importance is attributed to architectural choices and as such that yield may become subordinate to geometry. In other words: areas that from a performance point of view are not optimal, may be assigned as areas for BIPV application. A clear example of this is application of BIPV on facades (section 4), an area of the building that usually does not have the optimal geometry from PV yield point of view, however in many buildings still is the most appropriate choice for meeting the energy performance requirements. Since a large share of the energy demand in the built environment in continental Europe consists of heat, the combination of both solar heat and solar electricity production in one roof is an interesting option. Combining photovoltaic panels and thermal collectors in a hybrid PVT collector may result in a higher roof energy yield and a more aesthetic unified appearance of the roof. We discuss experimental results on such systems in section 5. The performance of BIPV and BIPVT concepts and products, as discussed in this paper has been studied on SEAC s outdoor research location SolarBEAT [2]. Figure 1 shows an overview picture of the research facility. The picture shows in the left lower corner the meteorological measurement system. The following

2 parameters are monitored: global tilted irradiance (secondary standard pyranometer), pyrgeometer, in-plane ambient temperature, wind speed and wind direction. Figure 1. The SEAC outdoor research facility SolarBEAT for BIPV and BIPVT studies. The roof in the middle shows the three types of uncovered PVT systems as discussed in section 5. The meteorological measurement system is in the lower left corner of the picture. 2. Temperature and BIPV performance As explained above, the way that BIPV is valued in calculations of energy performance in buildings, requires a thorough understanding of these temperature effects and of ways to mitigate these temperature effects. Figure 2: Effect of a ventilation air gap behind the PV panels in a BIPV roof. ADVANCED BUILDING SKINS 1379

3 The built-in nature of BIPV gives often rise to a higher operating temperature compared to BAPV. This can be mitigated by applying a ventilation gap behind the BIPV construction. A better understanding and a better design of the ventilation in BIPV will avoid an unnecessary large reduction factor (discount factor) in the energy performance calculation as applied for buildings. Figure 2 shows experimental results for a BIPV roof in which we varied the ventilation air gap behind the PV panels from 4 cm to 8 cm. We measured the temperature difference between the PV panel and the ambient air as a function of irradiance G. We notice that at G = 800 W/m 2 the increase in ventilation gap reduces the panel temperature with 5 C. For a monocrystalline silicon PV panels that corresponds to a power difference of 2.4%. An interesting question is: how large should the ventilation gap be in order to minimize the additional heating effect of the built-in nature of BIPV? We addressed this question in figure 3. Figure 3: Calculated effect of the ventilation air gap in BIPV design on the temperature T of the PV cells. The irradiation is taken 1000 W/m 2 and the insulation value 0.26 K.m 2 /W. Figure 3 shows a simulation based on a thermal model validated by experimental results. The heat flow in the BIPV roof is modelled by a set of thermal resistances in an equivalent circuit model. Other parameters that can be varied in the model are wind speed and the speed of the air in the air gap. We find that for typical parameter settings the effect of the air gap width saturates at values between 10 and 16 cm. 3. Partial shading In the introduction we explained why BIPV may be more subject to partial shading compared to BAPV. This puts an additional challenge on the design of BIPV, both on the module design as on the design of the electrical system. The aspects of electrical system design in case of partial shading have been discussed in [3]. This paper section focuses on aspects related to the design of the BIPV component. We selected 10 commercial panels, of which 2 with crystalline Silicon technology, 6 with technology and 2 with CdTe technology. Table 1 gives an overview of the panel types as selected.

4 Table 1: Overview of PV modules selected for the partial shading study. Panel type Yingli YL265C-30b Sun Power SPR-X Avancis PowerMax 3.5 Solar Frontier 0549 SF170-S Solar Frontier 0699 SF170-S Solibro SL2-115 Stion 1286 STO-145 Stion 2024 STO-145 Calyxo CX3-77 First Solar FS265 Technology c-si c-si CdTe CdTe We applied partial shading in incremental steps of 5% in both landscape and portrait mode. After each step the situation was stabilized and the yield of the module was measured in a steady state solar simulator. For every step, we measured the total module dc yield. As expected, the experimental results mimic in the first place the configuration of the module from the individual PV cells and substrings with bypass diodes. Figure 4 shows the results for portrait shading. In the modules the applied shading is perpendicular to the length of the individual PV cells, giving rise to a linear decay of yield with shading. The cells in the Calyxo module are connected in two substrings, resulting in a two-step yield decay. The c-si modules as well as the First Solar module show an almost complete loss of power at 20% partial shading. For the c-si modules, fully shading of a single cell in a series connected string, eliminates the current flow in the complete string. Figure 4: Experimental power decay as function of partial shading applied in portrait mode for various PV modules. ADVANCED BUILDING SKINS 1381

5 Figure 5: Experimental power decay as function of partial shading applied in landscape mode for various PV modules. Figure 5 shows the results for landscape shading. Here, as expected the yield as function of shading shows the opposite result compared to the portrait shading. Now the CdTe modules show the linear decay. For an actual BIPV design for a specific building on a specific location at a specific geometrical orientation, the expected shapes of the partial shading can be calculated or simulated. This is especially true for frequently occurring shading objects like dormers, chimneys and neighbouring buildings. From the results presented here, straightforward design rules can be derived for applying specific BIPV components in the relevant situations in either portrait or landscape orientation. Or, if considered from the other end: to choose the suitable module technology for an architectural BIPV design that requires either portrait or landscape oriented modules. 4. BIPV for façades The most straightforward way of applying BIPV in facades is a vertical arrangement of (single size) PV panels in a curtain wall construction. The appearance of the façade is then dominated by the appearance of the PV modules of choice. This means that a straightforward high efficiency BIPV façade will show an opaque black or dark blue appearance.one of the options for achieving a different appearance, is the application of a so called zigzag façade [4]. The zigzag façade is composed of PV areas that face upwards and aesthetical areas that face downwards. The appearance of the façade is then dominated by the aesthetical downward facing areas. The PV performance is relatively independent from the choice for the aesthetical areas. We built a pilot façade composed of two parts. The lower part is a straightforward vertical BIPV façade consisting of 10 Solar Frontier panels of 1.23 m 2 and 160 Wp each. The total installed capacity is R vert = 1.60 kwp. The air gap behind the panels is 260 mm, large enough to avoid unnecessary loss of performance. The system is equipped with SolarEdge power optimizers and a SolarEdge 2.2 kw inverter. The upper part of the pilot embodies a zigzag façade. Figure 6 shows the actual design parameters for the zigzag façade. In this paper we discuss the performance of a single row of PV in this concept. The PV area of one row comprises 3 crystalline Silicon panels of 0.6 m 2 and 100 Wp each. The total installed capacity of a row is R ZZ = 0.30 kwp. The system is equipped with Heliox micro-inverters.

6 Factor topology: Figure 6: Design of the upper part of the BIPV façade pilot - the zigzag SCX: yield (year) facade specific yield GVI: PR (year) ZigZag: yield (year) specific yield GVI: boosting factor/pr specific yield facade G_POA_incassette PR (year) Figure 7: Photograph of the BIPV façade pilot, composed of two parts: the lower vertical BIPV curtain wall and the upper zigzag façade. Pyranometers are highlighted with a red circle. Figure 7 shows a photograph of the pilot set-up. Please note the pyranometers that have been positioned in the vertical plane as well as in the plane of the PV area of the zigzag façade. We have to realize that the application of the zigzag concept as a consequence reduces the installed PV capacity per façade area. In 2 2 our specific case (figure 6), the installed capacity per m façade is Czz = 74.0 Wp/m, compared to Cvert = Wp/m for the vertical façade.on the other hand the installed PV in the zigzag façade has a more favorable orientation towards the sun, resulting in a larger irradiation G on the plane of the PV and a larger specific yield in kwh/kwp. Table 2 summarizes the key parameters for the pilot. ADVANCED BUILDING SKINS 1383

7 Table 2: Overview of key parameters for the vertical BIPV façade and the zigzag BIPV facade. Symbol Unit Vertical BIPV facade Zigzag BIPV facade (one row) Installed PV capacity R kwp (nominal) Installed PV capacity C Wp/m per m 2 of facade Irradiation on the G kwh/m 2 / plane of the PV area (location Eindhoven) year Measured ac yield Y kwh for a full year Performance Ratio PR % 81% 78% (ac) Specific yield kwh/kwp Note that the value G ZZ = 1217 kwh/m 2 /year is the measured irradiation on the plane of the PV in the zigzag module. This value is built up from (1) direct radiation from the sun, (2) diffuse radiation from the open sky part of the hemisphere that the PV is exposed to and (3) reflection from the white façade panel above the PV. The first two terms can be estimated directly from the various pyranometer measurements on SolarBEAT to be 530 kwh/m 2 /year and 460 kwh/m 2 /year respectively. The contribution from reflection is then 227kWh/m 2 /year. This means that the irradiation on the plane of the PV is enhanced due to reflection on the white plane above by 23%. We measured the ac yield of the PV systems for a full year. The results Y in kwh are listed in the table. From these numbers we can directly derive the performance ratio PR for the full year as: PR = (Y/R) x (G STC /G) We notice that the PR for the zigzag facade is lower than the PR for the vertical facade. This is due partly to a higher efficiency of the dc-ac conversion for the vertical façade and partly to the fact that the contribution to the irradiation on the zigzag PV from the reflection is inhomogeneous over the area. In figure 8 we present the measured PR on a daily basis for the month of august The size of the measurement points mimics the irradiance and thus the weight in the overall PR calculation. In the left of the figure we notice some low PR values for the vertical BIPV. These are due to losses because of the specific configuration of power electronics chosen in this experiment. This effect is not representative for a larger well configured system. Furthermore we notice that for increasing irradiance the PR of the vertical façade becomes larger with respect to the PR of the zigzag façade. This has two reasons. First, the relative contribution of the reflection irradiance on the zigzag PV increases, which because of the inhomogeneous nature of this reflection leads to a lower PR. Secondly, the vertical PV façade () has a somewhat more favorable temperature coefficient compared to the zigzag PV (c-si). In summary, we see an interesting trade-off between the choice for a vertical BIPV façade and a zigzag BIPV façade. The zigzag façade offers interesting aesthetical opportunities to realize a distinguished architecture. At the same time the specific yield (that is: relative to the amount of PV installed) is clearly higher than for the vertical BIPV solution. That is mostly because of the more favorable orientation of the PV towards the sun. Another factor contributing to the higher specific yield of the zigzag façade is the reflection of incoming light via the aesthetical panel onto the PV. In the case of a white panel this diffuse reflection is estimated to give a 23% increase in irradiation. However, as this additional irradiation is inhomogeneous, the additional electricity yield is less (reduced PR). On the other hand, the vertical BIPV enables more PV to be installed at a given façade and thus the vertical BIPV offers a higher total electricity yield for a given façade.

8 Figure 8: Experimental daily PR values for the zigzag BIPV façade and for the vertical BIPV façade for the month of August Performance analysis of BIPVT concepts with heat transport by liquid and by air Combining photovoltaic panels and thermal collectors in a hybrid PVT collector may result in a higher roof energy yield and a more aesthetic unified appearance of the roof. Performance assessment of solar thermal collectors and PVT systems with liquid heat transport, requires integration of those systems in a controllable thermal loop. In our set-up on SolarBEAT, excess heat produced by the thermal systems is dumped in an aquifer. A combination of heaters, valves, chillers and control technology enables to set the liquid to a specified temperature. The input temperature for the systems can be set between 7 and 80 C. We use a 25 % glycol solution in the PVT loop. The thermal loop system and more details of the experimental set-up have been described in [5]. Another approach for the design of a PVT system is to use air as the heat transport medium. This may be a suitable approach if the hot air is used as an input for an air-based heat pump or to regenerate a ground source. In general, air-based PVT systems have a simpler design compared to liquid-based, however liquid based collectors are more efficient and offer more flexibility due to the option to include heat storage, a better conduction of the heat to the liquid and the use of closed loops. 5.1 Performance analysis of uncovered liquid based BIPVT concepts On the central roof in figure 1, our test setup for three different uncovered liquid-based PVT systems is shown. We denote the systems as A, B and C. The systems are built by different suppliers and differ with regards to the type of PV used, how the absorber is connected to the PV cells and the thermal insulation on the back of the PVT panel. These differences are summarized in Table 3. The individual collectors of each system are thermally connected in series. For each system, different flow rates are set in order to match the flows that are used in operational systems. However, the inlet temperature of the first collector of each system is the same. Each PVT panel was electrically connected to a SolarEdge power optimizer. The power optimizers are in series connected to the SolarEdge inverter. In this way, the electrical and thermal performance of the PVT modules is measured at maximum power point for each module ADVANCED BUILDING SKINS 1385

9 Table 3: Overview of the three liquid-based PVT systems. System Type of PV Type of absorber Thermal insulation between collector and ambient A Sheet and tube Back insulation aluminium absorber B c-si Add-on clamped No insulation polypropylene absorber C c-si Roll bond aluminium absorber Building integrated with back insulation PVT collectors produce both heat and power. The energy yield depends on different factors, of which the most important factors are: Irradiance, ambient temperature, average fluid temperature and wind speed. Figure 9 show the produced power (heat and electricity) in W/m 2 for the three collector types. We chose two sunny days with a low (7 C) and a high (35 C) inlet temperature in summer Irradiance G poa in the plane-of-array is shown in green. The thermal heat is depicted by continuous lines, while in the dotted lines the electrical power is added. System B (red) does not have any insulation at the back and therefore acts as a heat exchanger at night and produces heat, when the ambient temperature is higher than the collector temperature. While the PV yield is in a similar range on the two days, the thermal yield depends largely on the inlet temperature of the water. Please note, that the average collector temperature is very different for the different collectors. System C operates at higher temperatures and therefore, produces more useful heat. System A and C perform better at higher temperatures due to the insulation at the back. Figure 9: Thermal (continuous lines, Q th) and additional electrical power output (dashed lines, Q th+p PV) per m2 for a day with a fluid input temperature of 35 C (left) and 7 C (right). In-plane irradiance (G POA) is shown in green. The thermal efficiency is calculated based on measured data for a one year period from June 2015 to May The absorption factors of the different PVT panels were measured with an integrating sphere to be between 0.90 and 0.94 for the different panels.

10 The thermal collector efficiency curves for collector A, B and C are shown in Figure 10 for a wind speed of 0 and 3 m/s, with the PV in MPP. The PV efficiency (12-14%) should be added to calculate the total energy efficiency. Figure 10: Collector curve for PVT collector A, B and C with a wind speed of 0 (solid) and 3 m/s (dashed), based on measured data from June 2015 to May 2016, with PV operational and measured in MPP. There is a large difference between the measured thermal performance of the three different collectors. These differences can be explained by the differences in the PVT collector design (table 3). PVT collector C performs the best, since it has a good heat conduction and insulation at the back. Collectors A and B can be applied in systems that work with a low temperature heat input. As an example: systems that are connected to a ground-source heat pump, often operate below ambient temperature. The performance of these collectors can be improved by increasing the heat transfer, e.g. by applying heat conducting paste. However, this may not lead necessarily to an economically better solution. 5.2 Performance analysis of ventilated BIPVT roofs Air based PVT systems are easier from an installation point of view and are less costly. In general, a ventilated BIPVT system uses the input air from the ambient. The heated air can be used for instance for direct space heating or preheating. In the Netherlands, there is a large seasonable mismatch between supply potential of hot air and the demand for direct heating. Furthermore, the storage of hot air is not an attractive option. Therefore other applications of hot air from solar thermal systems are being studied, like drying processes or conversion to hot water for e.g. regeneration of a ground source. Any of these applications require that a forced air flow is applied. Figure 11: Measured efficiency of heat transfer from PV to air in a ventilated BIPVT system with forced air ventilation. We characterized the thermal performance of a ventilated BIPVT system, in which we measured the airflow in the cavity of a closed set up. Furthermore we measured the ambient or ingoing air temperature T amb, the outgoing air temperature T f3 and the irradiance G poa. From these data we calculate the efficiency η. ADVANCED BUILDING SKINS 1387

11 Figure 11 shows the efficiency η of heat transfer to air on the y-axis vs the reduced temperature for different ventilation speeds of the system. It shows that higher air flows lead to higher efficiencies (more energy transfer). This however comes together with lower temperatures and lower usability of the air output. The optimum setup depends on the specific application. 6. References Footnotes must be numbered and grouped together at the end of the text with the footnote number in square brackets. [1] F. Frontini, P. Bonomo, A. Chatzipanagi, G. Verberne, M. van den Donker, K. Sinapis and W. Folkerts, Building Integrated Photovoltaics Report 2015, SUPSI-SEAC, [2] R. Valckenborg, J.L.M. Hensen, W. Folkerts and A. de Vries, Characterization of BIPV(T) applications in research facility SOLARBEAT, Proceedings of the EU PVSEC 2015, 2015, pp [3] K. Sinapis, C. Tzikas, G. Litjens, M. van den Donker, W. Folkerts, W.G.J.H.M. van Sark and A. Smets, A comprehensive study on partial shading response of c-si modules and yield modeling of string inverter and module level power electronics, Solar Energy 135, 2016, pp [4] R. Valckenborg, R. Loonen, X. Xin, X., D. Verduijn, W. Folkerts and J.L.M. Hensen, Performance analysis of the ZigZagSolar BIPV façade system, Proceedings of Advanced Building Skins, [5] C. de Keizer, M. de Jong, M. Katiyar, W. Folkerts, C. Rindt and H. Zondag, The WenSDak project: analysis and development of aesthetic building integrated solar heat and power roofs, ISES Conference Proceedings of Eurosun 2016, 2016.