Leaf-inspired photovoltaic cell is efficient and provides “free” water, too

Photovoltaic (PV) cells—often referred to as “solar cells”—have a hard life. They are exposed to the weather and the sun’s energy that they capture to generate electricity which also causes them to heat up. According to some tests, for every 10°C increase in operating temperature, the efficiency of Si-based PV panels typically decreases by 4.0 to 6.5% and their ageing rate doubles. Cell temperatures up to 65°C are common in sunny and hot settings.

The solution to “too hot” is well known to engineers: use some sort of cooling arrangement. This can be done using an active approach using forced air or water, but the required heat exchangers, pumps, and plumbing adds cost, complexity, and power consumption. Alternatively, a less complicated and less costly, but also less efficient, passive approach can be used. In addition to well-known convection cooling, some passive designs use optical techniques of sub-bandgap reflection or selectively emissive coatings in order to emit heat via radiation to the cold outer space, but these can reduce heat by only about 4°C.

As a result, there’s a need to explore new approaches, and that’s what university researchers are doing. A team based at Imperial College London has devised a “bio-inspired” PV-leaf technology which uses low-cost materials and relatively simple construction to overcome the thermal problem and actually provide new benefits. This design eliminates the need for pumps, fans, control units and expensive porous materials. It automatically adapts to ambient-temperature and solar-condition variations and can even provide additional clean water. According to their various tests, the PV-leaf can generate over 10% more electricity compared to conventional solar panels.

The design takes its inspiration from plant leaves—nature’s own solar-energy capture process—and mimics the transpiration process which allows water to move, be distributed, and evaporate. Natural fibers mimic leaf vein bundles while hydrogels simulate sponge cells, so a PV-leaf can effectively and affordably remove heat from solar PV cells.

[In case you’ve forgotten: Transpiration is the process of water movement through a plant and its subsequent evaporation (some call it “exhalation”) from leaves, stems, and flowers. It is a passive process that requires no energy expense by the plant. Transpiration cools plants, changes osmotic pressure of cells, and enables mass flow of mineral nutrients.]

In this design, a biomimetic transpiration (BT) layer is attached to the back of a solar PV cell in order to remove the heat generated in the cell (Figure 1). The bamboo-fiber bundles mimic the vascular bundles in transporting and distributing liquid water over the cell’s surface, while hydrogel cells with a large specific surface area and excellent water-absorption performance are used to mimic the sponge cells in providing effective evaporation.

Figure 1 Schematic illustration of the PV cell and transpiration structure arrangement within the bio-inspired PV-leaf: a) Typical internal structure of a real leaf. The vascular bundles uniformly distribute liquid water throughout the whole surface of the leaf. Effective transpiration cooling protects the photosynthetic process. b) Internal structure of the bio-inspired transpiration structure. Hydrophilic fiber bundles and hydrogel cells are used to mimic the vascular bundles and sponge cells. c) Exploded view of the transpiration structure. The BT layer is constructed of bamboo fiber bundles and packed hydrogel cells. The root of the fiber bundles is soaked in bulk water. d) Diagram and working principle of the PV-leaf transpiration structure. Water flows from the root to the hydrogel cells driven by capillary and osmotic processes. The water molecules in the molecular mesh then evaporate, removing PV heat. e) Photograph of the single PV-leaf prototype. Source: Imperial College London

The configuration of the PV-leaf transpiration structure comprises a BT layer (~1 mm thick) and a supporting mesh (0.5 mm thick) connected to the underside of a PV cell layer (~150 μm thick) over an effective area of 10 × 10 cm2. In the BT layer, around 30 branches of the bamboo fiber bundles are homogeneously embedded into the potassium polyacrylate (PAAK) superabsorbent polymer (SAP) hydrogel cells, distributing water over the entire area covered by the BT layer. The ends of the fiber branches are gathered together and soaked in water.

Such a structure is interesting, but it needs to be tested, of course. The research team tested indoors under controlled lighting as well as outdoors, and the results were impressive and in line with the design simulation: reduced heat, increased power output, and water, Figure 2. For increased efficiency, which is a primary metric here, the number seems to be about a 10% improvement, which is a significant number.

Figure 2 Synergistic generation of electricity, heat, and clean vapor of the hybrid multi-generation PV-leaf. a) Schematic of the PV-leaf. A chamber was attached below the BT layer to collect clean vapor. b) Temperature profile (temperatures of the solar cell Tcell, the vapor on the interface Tvap, and the outlet of the chamber Tout) and transpiration rate of the PV-leaf when the ventilation fan power is Pf = 60 mW. c) Comparison of outputs of the PV-leaf and the standalone PV cell when Pf = 60 mW. d) Electrical efficiency, thermal efficiency, and transpiration rate of the PV-leaf as a function of Tvap, which was adjusted by changing the ventilation fan power. Tvap increases as the ventilation power decreases. e) Different condensation technologies and corresponding condensation rate (CR) limits. A separate condenser is needed to condense the vapor. The limit of the water-based condenser (hc = 50 W/m2/K) is assumed to be five times that of the air-based condenser. f) The salinity of the input saline and output freshwater tested by a refractometer. A separate flat-plate water-based condenser was used to condense the vapor generated by the PV-leaf. The saline was dyed blue to help visualize the cleaning effect. Source: Imperial College London

If you are interested in all the details, check out the 10-page paper “High-efficiency bio-inspired hybrid multi-generation photovoltaic leaf” published in Nature Communications, along with a 16-page Supplementary Information file which provides extensive additional insight into the design and test as well as the impact of variations in test conditions.

While they are justifiably pleased with their idea, its execution, and the results, I was also impressed that that they didn’t label it as a “breakthrough” or “revolutionary”. We already have way too much hype, especially when a development is related to clean or renewables, and the eternal quest for research grants.

The harsh reality is that for most small-scale projects, and especially those related to energy and power issues, these developments often don’t scale well. As you go from benchtop to pilot run and then full size, all sorts of things happen. Production, fabrication, and installation issues interfere; non-linearities in the materials become more prominent; and inescapable factors such as the changing relationship between surface area and volume affect mechanical, chemical, and thermal performance.

If you don’t think this is the case, look at the many battery-related advances created in the lab touted as “breakthroughs”, but which never made it much further. The laws of physics and chemistry, as well as economic realities, dictate what you can and cannot achieve at various scales.

What’s your view on the practicality of this novel approach? Is it viable on a larger scale, or is it another innovation which will prove to be not-quite feasible in the larger real world, as has been the case with so many innovations?

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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Leaf-inspired photovoltaic cell is efficient and provides “free” water, too

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