News
Reaching a consensus on
indoor photovoltaics testing
Photo
Credits: Jonne Renvall, Tampere University.
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Standardising indoor
photovoltaics measurement and reporting
Indoor photovoltaics has
exploded as a research field over the past five years, driven by the
urgent need for sustainable energy supply for the Internet of Things, as
well as the influx of new materials with potential for efficient
performance. This has put a spotlight on the lack of standard testing
conditions. A group of 28 researchers, including many involved in the
MENTOR and MASAUTO doctoral training networks[1],
have come together to discuss this problem, and the direction the
community needs to take as it matures. This discussion is captured as a
commentary, just published in Joule.
|
Standardising indoor
photovoltaics measurement and reporting
Indoor photovoltaics has
exploded as a research field over the past five years, driven by the
urgent need for sustainable energy supply for the Internet of Things, as
well as the influx of new materials with potential for efficient
performance. This has put a spotlight on the lack of standard testing
conditions. A group of 28 researchers, including many involved in the
MENTOR and MASAUTO doctoral training networks[1],
have come together to discuss this problem, and the direction the
community needs to take as it matures. This discussion is captured as a
commentary, just published in Joule.
Wild west of indoor
photovoltaics
Photovoltaics operating indoors
used to be a niche field, with few materials. This changed over the past
five years. A key driver is the rapidly growing importance of the
Internet of Things (IoT). The IoT is an ecosystem of smart devices
connected via the cloud, and is a central pillar of the fourth
industrial revolution. There are now tens of billions of devices part of
the IoT, potentially rising to a trillion in the near-future. Powering
these small, autonomous devices solely with batteries will result in
significant waste, as well as practicality challenges that limit how
fast the IoT ecosystem could grow. Harvesting energy freely available
from the environment is an ideal solution, and indoor photovoltaics
(IPVs) are particularly appealing because of the ubiquitous nature of
ambient lighting. Simultaneously to this growing need for IPV, there has
been a rapid increase in the classes of materials with potential to be
efficient at indoor light harvesting. Together, these factors have
fuelled a rapid rise in prominence of the IPV field.
But the IPV field right now is
still a wild west. Groups measure under a wide range of different
lighting conditions, with inconsistent reporting. This means that
improvements in efficiency could be achieved simply by finding a better
light source for a particular device, rather than necessarily improving
the technology. This lack of comparability in results between groups
hinders the quantification of progress. Yet, a key conceptual barrier
towards standardisation is that there is no standard indoor light
source. Indeed, lighting conditions will vary over the course of a day
as the mix of indoor and outdoor lighting changes, and will depend on
reflections within the room. There are many in the community who would
argue that we should not have standards in order to avoid biasing the
optimisation of IPV materials towards an arbitrarily-chosen light
source. But maintaining the status quo will prevent a fair comparison of
technological progress between different groups.
Standards
A group of 28 researchers
working on IPV, mostly from across Europe, debated this issue, and
reached an agreement. In order for the field to mature, it is critical
that we all agree on standard testing conditions (STCs). At the
individual device level, measuring under STCs will ensure comparability,
and that breakthroughs in efficiency can be certified reliably. Standard
indoor lighting conditions were recently defined in the first
international standard published for IPVs (IEC TS 62607-7-2:2023).
However, this is yet to gain widespread adoption, and, given that
international standards take a long time to develop, the proposed
lighting conditions (5000 K white light emitting diodes and fluorescent
light tubes) are arguably not the most representative average across
conditions IPV would typically experience. Debating on what the standard
light source should be as a community is critical and will feed into
future updates of these standards.
Outlook for the community
In the future, we can build upon
these STCs to develop energy rating standards to quantify a standard set
of lighting conditions reflective of real-world conditions. This allows
the power density and operating voltage of IPVs to be quantified across
different settings. Combining energy rating standards with STCs for
individual devices reconciles the debate within the community about the
diversity of indoor lighting conditions.
However, we can only reach this
point if we first agree on STCs. Developing energy rating standards will
take significant inter-laboratory comparisons, along with careful
measurements of real-world spectra and irradiance across the diverse
range of conditions IPVs are used in. In the meantime, researchers can
measure the resilience of their IPVs across different testing
conditions.
Above all, it is critical that
the community across different sectors (academic, industrial, national
metrology labs, IoT developers and equipment manufacturers) continue
discussing together. Fora, like that offered by the annual Indoor
Photovoltaics Conference (IPVC), are valuable opportunities, as well as
the two doctoral training networks that bring together key groups active
in the field.
The consensus statement is
published in Joule, and the consensus reached by the authors in IPV
measurement and reporting is summarised in Table 1. Joule, 2025, DOI:
10.1016/j.joule.2025.102127
[1]
MASAUTO and MENTOR have
received funding from the European Union’s Horizon Europe
Research and Innovation Programme under the Marie
Skłodowska-Curie Doctoral Networks grant agreements no.
101168161 (MASAUTO) and 101169056 (MENTOR).
Contacts:
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Dr. José
Silva
Scientific Project
Coordinator
josesilva@fisica.uminho.pt |
Prof. Luís
Marques
Network Coordinator
lsam@fisica.uminho.pt |
University of Minho
Campus de Gualtar
PT-4710 - 057 Braga
Portugal |