Spatio-Temporal Visualisation of Reflections from Building Integrated Photovoltaics
2.1. Current Legislation
2.2. Current Technical Guidelines and Assessment Criteria
2.3. PV Reflection Simulation
3. Case Study
4. Simulation Methodology
4.2. Workflow Overview
4.3. General Workflow
4.3.1. Sky Model Generation
4.3.2. Simulation Model Generation
Geometric Model of Church and Surroundings
Candidate PV Material Models
4.3.3. Cumulative PV Reflection Simulation
Photon Map Generation
Absolute Annual Irradiance
Relative Annual Irradiance
4.4. Spatio-Temporal Workflow
4.4.3. Feature Detection (Sustained Glare)
5. Results and Discussion
5.1. Annual Reflected Irradiance
5.2. Application of Recommended Criteria, Glare Duration
5.3. Practical Applications and Impact
- Assessing proposed PV installations for compliance with regulations by building authorities and urban planners.
- Developing low-glare PV module surfaces by PV manufacturers.
- Defining PV glare assessment criteria, thresholds, and technical guidelines by trade associations.
- Establishing urban regulations concerning PV installations by government agencies.
- While using irradiance as physical metric is convenient to indicate potential glare, it is an integral quantity; as such, it may underrepresent isolated incidents of high radiance/luminance from reflectors subtending very small solid angles within an observer’s field of view. While the resulting glare can even be disabling, such events tend to be of short duration (corresponding to glint), and may not appreciably impact the sustained glare duration.
- The background luminance (and therefore contrast) is not considered.
- The simulation depends on the accuracy and timeliness of the digital terrain model; in particular, vegetation is assumed to be a static, effectively opaque barrier irrespective of season in the case study.
- The PV surface models are derived from new production samples, and do not take weathering or manufacturing tolerances into account.
- While a sunny sky assesses the worst-case scenario, a more realistic assessment would be obtained using a climate-based sky model that accounts for typical cloud cover on site.
- The temporal resolution is currently 15 min, which may be insufficient in detecting high-frequency glare. While the timestep increments can be finer grained, this increases simulation and postprocessing times.
- Timestamp increments are fixed, thus the simulation also expends considerable computational effort on timestamps that contribute negligible irradiance, rather than sampling adaptively.
- The current workflow is static and non-interactive, since the simulation must complete before the results can be assessed.
6. Conclusions and Future Work
- A general workflow to assess cumulative annual irradiance from PV reflection, both in absolute terms and in relation to a reference material in order to compare different candidate PV surface materials.
- A novel, image-based spatio-temporal workflow that applies recommended assessment criteria and thresholds to quantify the cumulative annual glare duration, and the maximum sustained glare duration for any day of the year.
- A PV reflection simulation using data-driven material models of measured BSDFs.
- A digital terrain model of the site providing the urban context of the simulation.
- Photon mapping to efficiently precompute only the reflected component from the PV on the surrounding buildings.
- A falsecolour visualisation of reflected irradiance in the context of the built environment suitable for practitioners, municipal planning authorities and clients of PV installations. Including a background image of the built environment enables identification of potentially affected buildings. Such a visualisation is readily accessible to non-experts as well as practitioners.
- While the satinated PV module exhibits a homogeneous reflection distribution and no potential for glare at all, the standard PV module concentrates its reflections towards the west of the site during summer mornings (see Figure 11).
- The proposed spatio-temporal workflow identifies glare in the west when applying the conservative irradiance threshold of 10 W/m, which translates into an annual glare duration of ca. 100 h (see Figure 14) and a maximum sustained glare duration of 1h for any day of the year (see Figure 15). These exceed the recommended limits of = 50 h and = 30 min, respectively.
- In consideration of the noncritical nature of the identified glare with the standard PV module, both candidates are viable replacements for the current roof. Because the site is heritage protected however, the satinated PV is more suitable in preserving the current appearance of the roof (see Figure 9).
- A comparison with predictions made in an earlier proposal for a PV roof retrofit confirmed that the proposed method indeed complements and supports the expertise of a PV planner (see Figure 13).
- Optimisation of the scripts comprising the spatio-temporal workflow, particularly the memory footprint of the sustained glare extraction, which currently limits it to iteratively processing daily timesteps as the matrices tend to be very large (several gigabytes).
- Improvements to the daylight simulation engine. Progressive photon mapping techniques with visual feedback, for example, would introduce a degree of interaction to the simulations by allowing a planner to identify and react to hotspots early, while the results are still being refined. Currently the simulation runs to completion before the results can be reviewed.
- On a more fundamental level, the light transport algorithm at the heart of the simulation engine should be modified to more efficiently sample adaptively in both the spatial andtemporal domains. The raytracing algorithms currently employed in Radiance do not account for the latter, and annual simulations always use large numbers of fixed timesteps, with few actually contributing significantly to the results (i.e., as detectable glare in the analysis).
- The proposed method should be applied to a more comprehensive suite of extant case studies and recommended criteria, with the goal of refining the latter in terms of how the predicted glare is actually perceived on site.
Conflicts of Interest
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|BAFU||Swiss federal environmental agency (Bundesamt für Umwelt)|
|BIPV||Building Integrated Photovoltaics|
|BSDF||Bidirectional Scattering Distribution Function|
|HDR||High Dynamic Range|
|LAI||German federal/state association for pollution control|
(Bund/Länder-Arbeitsgemeinschaft für Immissionsschutz)
|RPG||Swiss federal spatial planning law (Bundesgesetz über die Raumplanung)|
|RPV||Swiss federal spatial planning ordinance (Raumplanungsverordnung)|
|falsecolor||Applies linear or logarithmic falsecolour scale to HDR image|
|genbsdf||Generates BSDF from measured data as XML file|
|gensky||Generates sky model for given date/time/location (solar position + optional sky dome)|
|mkpmap||Forward raytracer, generates photon map|
|oconv||Generates simulation model as octree (sky + geometry + materials)|
|pcomb||Modifies/filters HDR image pixels|
|pcompos||Composites multiple HDR images|
|pcond||Tone maps or adjusts exposure of HDR image|
|rcontrib||Source/timestamp contribution raytracer, computes time-series irradiance from photons|
|rtrace||General backward raytracer, computes cumulative irradiance from photons|
|vwrays||Generates primary rays for a given viewpoint to pass into raytracer|
|DoY (ΔDoY)||Day of Year (resp. increment) [day]|
|HoD (ΔHoD)||Hour of Day (resp. increment) [h]|
|Maximum area of PV [m]|
|Maximum distance to PV [m]|
|Maximum reflected luminance from PV [cd/m]|
|Maximum irradiance on receiver from PV [W/m]|
|Maximum sustained glare duration on any day of the year [h]|
|Maximum cumulative glare duration per year [h]|
|Parameter, Critical Condition||Description||Threshold Value||Units|
|<||Distance to PV||50 (commercial) |
|>||Area of PV||100 (commercial) |
|>||Angle subtended by PV at receiver||7.5||°(degrees)|
|>||Reflected luminance from PV||30 (MIT) 50 (Swissolar)||kcd/m|
|>||Irradiance from PV at receiver||10 (Sandia Labs) ≈16.8 (MIT) 30 (Swissolar)||W/m|
|>||Maximum sustained glare duration on any day of the year||30||min|
|>||Cumulative glare duration per year||50||hours|
|DoY||Day of year||[0, 181]|
|ΔDoY||Day of year increment||7|
|HoD||Hour of day||[4.5, 20.5]|
|ΔHoD||Hour of day increment||0.25|
|Number of timesteps (sun positions)||1280|
|Lat, long||Site latitude, longitude||47.038° N, 8.312° E|
|Merid||Timezone meridian||15° E (CET)|
|Number of photons||250 M|
|Photon lookup bandwidth||400 (rtrace), 4000 (rcontrib)|
© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Schregle, R.; Renken, C.; Wittkopf, S. Spatio-Temporal Visualisation of Reflections from Building Integrated Photovoltaics. Buildings 2018, 8, 101. https://doi.org/10.3390/buildings8080101
Schregle R, Renken C, Wittkopf S. Spatio-Temporal Visualisation of Reflections from Building Integrated Photovoltaics. Buildings. 2018; 8(8):101. https://doi.org/10.3390/buildings8080101Chicago/Turabian Style
Schregle, Roland, Christian Renken, and Stephen Wittkopf. 2018. "Spatio-Temporal Visualisation of Reflections from Building Integrated Photovoltaics" Buildings 8, no. 8: 101. https://doi.org/10.3390/buildings8080101