Indoor Solar Panels: End Use of Sun and Batteries Forever

Indoor photovoltaics (IPV), also known as indoor solar panels, is a technology that uses manufactured light sources to control indoor solar cells. Common manufactured lighting sources include LEDs, CFLs, and halogen lights. An indoor light test system must test IPVs to mirror indoor light sources. These panels are especially useful for small gadgets. They kill the requirement for batteries, making them a more maintainable alternative. IPV-powered devices are often classified as smart or a part of the Internet of Things (IoT), which is fundamental in various industrial sectors such as wearable fitness monitors, home security sensors, and autonomous vehicles. IPVs have a wide run of applications and can reduce battery dependence.

How Do Indoor Solar Panels Work?

The technology behind indoor photovoltaics (IPV) consists of a routine photovoltaic (PV) system. PVs contain a semiconducting safeguard layer with a bandgap generally between 1.1 and 2.0 eV. During light, electrons absorb incoming photons and can become excited. This causes the electrons to jump from the valance band to the conduction band. By extracting these electrons and creating holes, we can operate an external circuit.

Despite the success and growth of photovoltaics, traditional solar cells involve huge losses under manufactured light due to the difference in light range. Photovoltaics used outside are chosen to fit the sun-powered range. However, inside the incident photons are from a manufactured light source, with a diverse range. Therefore, outdoor photovoltaics are not suitable for indoor applications.

Indoor PV Spectrum

This change in light source and range has a negative effect on the performance of traditional outdoor solar panels when used indoors. You are required to test outdoor solar cells under light that mirrors solar irradiance. This is usually the AM1.5 standard range. This is a standard range that represents the light that has traveled through the discussed mass at a zenith angle of 41.8°. This is generally 1000 W m2 for wavelengths in the range of 290 to 3000 nm.

Spectrum of Indoor Light Sources compared to AM1.5G Spectrum (dark blue). The two examples given here are a cool LED and a typical fluorescent light.

The figure above shows the AM1.5 range near spectrums of manufactured light sources such as LEDs. A few key differences between the AM1.5 range and manufactured light sources include:

• Artificial light sources tend to work exclusively in the unmistakable locale, without the infrared component found in sunlight. Therefore, their spectrums are much smaller than that of the AM1.5 reference.
• Pecunia (2021) noted that artificial lighting tends to have much lower power. Up to three orders of magnitude weaker than the earthly incident light.

To efficiently collect light, the bandgap of your solar cell material must coordinate the range of light’s greatest point. The ideal bandgap of a photovoltaic working beneath earthbound sunlight is 1.1-1.4 eV. While the smaller range of indoor lighting relates to an ideal bandgap of 1.9 – 2.0 eV (Pecunia, 2021).

This difference in optimal bandgap for indoor PV vs. open-air PV explains why traditional PV materials, such as crystalline silicon (c-Si), are not perfect for use under manufactured light sources. To target the ideal bandgap for indoor applications, we must use wide bandgap materials. So also, existing PV materials with tuneable bandgaps can be adjusted for indoor applications.

Defining Photovoltaics Standards

The standards for making and testing outdoor solar cells are very well-defined. The solar range is both well-defined and generally reliable (the AM1.5G range). Therefore, there are standards specifically for classifying the light source for testing outdoor devices (ASTM E927-19, JIS C 8904-9, IEC 60904-9:2020).

Testing indoor solar panels is harder due to the variety of indoor lights used in different areas. In 2023, the IEC introduced new details assessing photovoltaics beneath indoor light. These standards include details about the testing handle and light source calibration. The indoor testing detail is device-dependent, as well as light source-dependent.

These new standards include two examples of calibrated light sources: one for a standard fluorescent light and one for a standard cool LED. In general, these light sources should also be compared to a color temperature of 5000 K and under the illuminance of standard indoor light sources (e.g. 1000 lx, 200 lx, 50 lx. Light sources are classified by three factors: spectral coincidence, non-uniformity, and temporal stability. Please remember that testing indoor photovoltaics compared to outdoor photovoltaics involves defining them by light rather than irradiance. You should measure the efficiency of indoor photovoltaics under a calibrated indoor light simulator. This is very similar to a solar test system. The spectral intensity is greatly reduced and the range is modified to better represent indoor light sources.

Indoor Solar Cells Have Higher Maximum Efficiency

The theoretical Shockley-Queisser limit of 33% for single junction photovoltaics under the AM1.5 range is not applicable to indoor applications due to its different measurement ranges. Instep calculates a theoretical maximum of 52.5% for indoor photovoltaics, based on a 1.87 eV bandgap safeguard under 1000 lux cool white LEDs. The solar range emits in UV and IR districts, with photovoltaics aiming for the top of this range, which results in thermalization losses from UV components and insufficient energy from IR components. By tuning the bandgap to target this crest. Indoor photovoltaics have a higher theoretical maximum efficiency compared to outdoor photovoltaics.

Indoor PV materials

There is a range of wide bandgap materials suitable for indoor photovoltaic applications. Each has its benefits as well as disadvantages such as stability and cost. The table below shows a summary of common photovoltaic materials used for indoor applications and compares their record efficiencies at a research level.

Benefits of Indoor Photovoltaics

• Abundant light sources

By 2030, LEDs are expected to make up 84% of all lighting deals, making indoor lighting an inevitable and readily available option. Indoor photovoltaics (PV) is more controllable and reliable than solar light, making it a vast and abundant light source for indoor devices.

• Sustainability

Successful selection of indoor photovoltaics reduces the requirement for disposable batteries. This promotes a more feasible approach to personal electronic devices. Buildings now have the opportunity to use a two-tiered feasible energy approach. Where both the mains and wireless devices are worked more sustainably.

• Ease of integration

The solution-processability of materials such as perovskites allows indoor photovoltaics to be flexible. Therefore, they are more space-efficient than batteries. And can be easily integrated into existing gadget structures.

• Lifetime 

Indoor photovoltaics (IPVs) have the potential to outperform outdoor c-Si photovoltaics, which have a lifespan of 20-30 years. Perovskite, a novel material, has a record lifetime of over 10,000 hours in a lab environment, promising improved longevity and commercialization. This could reduce the running and maintenance costs of indoor PV-powered devices, outperforming battery lifetimes for IoT applications.

• Autonomy

Without the requirement to replace batteries, indoor PV-fueled gadgets are very low support. By combining indoor PV with manufactured insights there is an opportunity for devices to be self-sufficient and completely autonomous.

Challenges for Indoor Photovoltaics

• New material development

The unearthly differences between indoor and outdoor photovoltaic use require wide bandgap materials to be created. These new materials must outperform the efficiency of battery choices and work successfully at significantly lower power than the AM1.5 spectrum.

• Cost

Material improvement will be the largest supporter of the cost of upcoming photovoltaics. C-Si cells for outdoor applications have an estimated manufacturing cost per cm2 of $0.006 (Mathews, 2019). Early approaches to IPVs such as undefined silicon were significantly higher in cost at $0.2 / cm2. The manufacturing cost of III-V devices remains tall, at $0.1 / cm2, due to rare components. However, progressive technologies such as perovskite and organic PVs are falling to $0.004 /cm2 and $0.003 / cm2 respectively.

• Power management

Like photovoltaics under solar light, indoor photovoltaics will not experience reliable light. Make sure the device has adequate capacity to ensure operation in any light conditions.

The Future of Indoor Photovoltaics

The future of indoor photovoltaics looks bright, with continuous research and improvement balanced to overcome current challenges and grow the possibilities of this technology. As efficiency improves and costs decrease, we can expect to see indoor solar panels become a standard feature in homes, offices, and other indoor spaces.

Conclusion:

Indoor photovoltaics is a promising alternative to traditional solar technology, using manufactured light to produce and expend energy. As developments develop, indoor solar panels may play a significant part in the future of energy, giving a reliable, proficient, and eco-friendly control source for different applications. Embracing this technology today can lead to a more sustainable future where we integrate renewable energy into our daily lives. 

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