# Self-regulated dual-mode solar energy harvesting

## TL;DR

A new solar energy system can switch between producing heat or electricity based on the outside temperature, improving efficiency and usefulness.

## Contribution

A self-regulating dual-mode solar harvester that passively switches between heat and electricity based on environmental temperature.

## Key findings

- A fluidic optical switch using water as a phase-changing material enables passive toggling between heat and electricity.
- The system improves solar conversion efficiency by aligning energy output with ambient heating and cooling cycles.
- The design is scalable and applicable to vehicles, greenhouses, and buildings using common materials and methods.

## Abstract

Harvesting solar energy is of fundamental importance to the green economy. However, traditional technologies are single-mode, converting sunlight to either heat or electricity. We introduce dual-mode solar harvesters that leverage phase-changing fluidic optical switches to passively toggle between converting sunlight into either heat or electricity, depending on environmental temperature. By delivering heat when cold out and electricity when warm out, dual-mode harvesters can reduce temporal demand mismatch and improve useful solar conversion efficiency.

Traditional solar energy harvesters are single-mode—typically designed to convert available sunlight either into heat or electricity. However, neither energy form is continuously useful, and an optimal variant should instead be capable of autonomously toggling relative thermal and electrical yield based on need. Here, we introduce passive, dual-mode indoor solar energy harvesting by using a fluid layer trapped above a Fresnel lens as a phase-changing switch within a solar waveguide. When warm out and electricity is desired, the fluid is in its vapor phase, causing a refractive index difference with the microstructured lens beneath it to concentrate sunlight toward a solar cell. When cold out and heat is instead desired, the fluid condenses atop the Fresnel structure to reduce refractive index differences, causing sunlight to refract past the solar cell and convert to heat when absorbed indoors. Using water as a phase-changing switch, we built an exemplary system that self-regulates indoor temperature and solar cell light exposure over ambient heating and cooling cycles. Our approach is general across everyday materials and manufacturing methods and may be deployed on the surfaces of vehicles, greenhouses, and residential or commercial buildings.