Why monocrystalline and bifacial panels perform differently in open-water UV conditions

By:sealite | August 27, 2026

Solar panels powering marine navigation equipment operate in one of the most demanding photovoltaic environments on the planet. Open water combines intense UV radiation, high reflectivity from the sea surface, salt-laden air, and extended periods of direct sun exposure with no shading relief. In these conditions, the differences between monocrystalline solar panels and bifacial solar panels are not marginal — they are operationally significant, particularly for Aids to Navigation (AtoN) systems that must perform reliably for years without maintenance intervention.

This article builds from the structural differences between both panel types through to the specific mechanisms of UV degradation in open-water marine environments, and concludes with practical guidance on what these differences mean when selecting solar power for AtoN applications. Each section builds on the one before it, so the final application guidance will make more sense if read in sequence.

What makes monocrystalline and bifacial solar panels structurally different

Understanding how these two panel types respond to UV exposure begins with understanding how they are constructed. The structural differences between monocrystalline and bifacial panels are not simply cosmetic — they reflect fundamentally different approaches to capturing solar energy, and those differences directly determine how each panel ages in high-UV marine environments.

A monocrystalline solar panel is built from a single continuous crystal of silicon, sliced into thin wafers and assembled into a cell array. The silicon cells are sandwiched between a front glass layer and a solid opaque backsheet, typically made from a polymer composite. The backsheet reflects no light back through the cells — its purpose is protection and insulation. Current is generated only from the front surface of the panel.

A bifacial solar panel replaces the opaque backsheet with a second glass layer or a transparent polymer rear surface. This allows light to enter from both the front and the rear of the panel. In practice, bifacial panels generate additional power from reflected or diffuse light reaching the rear surface — a characteristic referred to as bifacial gain. The key structural differences can be summarised as:

Monocrystalline panels use a single glass front layer and a solid polymer backsheet
Bifacial panels use either dual glass construction or a glass front with a transparent rear encapsulant
Monocrystalline panels generate power from one surface; bifacial panels generate power from two
Bifacial panels typically use a thinner encapsulant layer on the rear to allow light transmission

For example, a standard monocrystalline panel mounted on a navigation buoy captures direct solar irradiance from above. A bifacial panel in the same position can also capture light reflected upward from the sea surface — a meaningful additional source of energy in open-water deployments where surface reflectivity is consistently high.

How UV radiation behaves differently over open water

Before examining how each panel type degrades, it is important to understand why open-water UV conditions differ from terrestrial environments. UV radiation over open water is not simply more intense — it arrives from multiple directions simultaneously, which changes the cumulative UV load that a panel’s materials must absorb and resist over time.

On land, UV radiation reaches a solar panel primarily from above. The ground beneath the panel may reflect a small percentage of UV back upward, but most terrestrial surfaces — grass, concrete, soil — have relatively low UV reflectivity. Open water behaves differently. The sea surface acts as a dynamic reflector, with UV reflectivity that varies with wave angle, sun elevation, and surface conditions. In calm, high-sun conditions, UV reflectivity from water can be substantial, directing reflected UV radiation upward toward the underside of any panel mounted above the surface.

This creates a condition that solar panel manufacturers designing for rooftop or ground-mounted applications do not typically need to account for: bidirectional UV exposure. Both the front and rear surfaces of a panel deployed over open water receive meaningful UV irradiance. The practical implications of this are:

Front glass and front encapsulant receive direct solar UV as in any outdoor installation
Rear surfaces receive reflected UV from the water surface below
UV exposure is continuous across extended operational periods with no shading from structures or vegetation
Salt spray and humidity accelerate the chemical degradation processes that UV radiation initiates

For a monocrystalline panel with an opaque backsheet, the rear surface is not designed to transmit or even manage UV — it is designed for protection. For a bifacial panel with a transparent rear layer, that rear surface is intentionally exposed. This distinction becomes critical when examining how UV degrades the materials inside each panel type.

Why encapsulant and rear-surface materials drive UV degradation rates

The encapsulant is the transparent polymer layer that bonds the silicon cells to the glass layers and protects them from moisture, mechanical stress, and chemical attack. In both monocrystalline and bifacial panels, the encapsulant is the material most directly affected by UV radiation over time. However, the degradation dynamics differ significantly between the two panel types because of the structural differences established in the first section.

The most common encapsulant material is ethylene vinyl acetate (EVA). Under sustained UV exposure, EVA undergoes a photochemical process called yellowing or browning — the polymer chains break down, the material discolours, and its optical transmittance declines. As transmittance drops, less light reaches the silicon cells and power output falls. In a standard monocrystalline panel, this degradation process affects primarily the front encapsulant layer, because the rear backsheet blocks UV from reaching the rear encapsulant.

In a bifacial panel, both the front and rear encapsulant layers are exposed to UV radiation. In open-water deployments where reflected UV reaches the rear surface, the rear encapsulant degrades in parallel with the front. This means that bifacial panels in marine environments face a dual encapsulant degradation pathway that monocrystalline panels with opaque backsheets do not. The rear glass layer in a dual-glass bifacial panel does provide some UV filtering, but the degree of protection depends on the glass specification and any UV-blocking coatings applied during manufacture.

The rear surface material also matters. The options and their UV resistance characteristics differ in important ways:

Opaque polymer backsheet (monocrystalline): Provides a strong UV barrier for the rear encapsulant; does not transmit UV; degradation is limited to front-surface materials
Dual glass (bifacial): Rear glass provides partial UV filtering; glass is inherently more UV-stable than polymer backsheets, but the rear encapsulant still receives UV transmission through the glass
Transparent polymer rear layer (bifacial): Offers less UV protection than glass; more susceptible to yellowing and delamination under sustained UV and salt exposure

A useful analogy is to think of the encapsulant as a window frame sealant. On a building with windows on one side only, the sealant on the windowless side is protected from weathering. On a building with glass on both sides, both sealant lines are exposed to the elements. The glass itself provides some protection, but the sealant on the exposed side ages faster than the one that is sheltered. In a marine bifacial panel, both encapsulant layers are the exposed sealant — and in open-water UV conditions, that matters.

Comparing real-world power output decline between both panel types

Building on the encapsulant degradation mechanisms described above, it is possible to understand why monocrystalline and bifacial panels follow different power output decline curves in open-water UV conditions — even when their initial performance specifications appear similar.

All photovoltaic panels experience some degree of output degradation over time. This is a normal and expected characteristic of solar technology, and reputable manufacturers publish degradation rate specifications that describe the expected annual percentage decline in power output. In standard terrestrial conditions, high-quality monocrystalline panels typically degrade at rates that the industry considers well-characterised and predictable over a 25-year lifespan.

In open-water UV conditions, the degradation picture for bifacial panels becomes more complex. The bifacial gain — the additional power generated from rear-surface light capture — can offset some of the degradation loss in the early years of deployment, particularly in high-reflectivity marine environments. However, as the rear encapsulant yellows under sustained UV exposure, the bifacial gain diminishes progressively. The panel may therefore show an initial performance advantage over a monocrystalline equivalent, followed by an accelerated decline phase as rear encapsulant degradation compounds front-surface degradation.

Monocrystalline panels with opaque backsheets degrade more predictably in marine UV environments because their degradation pathway is essentially unidirectional — front encapsulant yellowing, front glass soiling, and silicon cell degradation are the primary mechanisms. There is no rear encapsulant UV degradation variable to account for. For AtoN system designers specifying solar power for multi-year deployments, this predictability has operational value: a power budget calculated at the time of installation is more likely to remain accurate over the service life of the equipment.

It is worth noting that dual-glass bifacial panels — which use tempered glass on both surfaces rather than a transparent polymer rear layer — perform better in UV conditions than bifacial panels with polymer rear encapsulants. The glass provides a more effective UV barrier for the rear encapsulant, and glass is inherently more resistant to UV-induced degradation than polymer alternatives. However, dual-glass bifacial panels carry a weight and cost premium that must be evaluated against the performance benefit in the specific deployment context.

What UV performance differences mean for AtoN system selection

The structural and degradation differences covered in the preceding sections have direct implications for anyone specifying solar power for marine navigation aids. The core question is not which panel type is technically superior in absolute terms — it is which panel type delivers the most reliable, predictable power output over the required service life in the specific deployment environment.

For AtoN applications, the consequences of power supply failure are not simply operational inconvenience. A marine lantern that fails to activate due to an undersized or degraded solar power supply creates a navigational hazard. This means that the power budget for an AtoN solar system must account for degradation over the full intended service life, not just initial rated output. The degradation predictability of monocrystalline panels with opaque backsheets makes them a conservative and well-understood choice for long-duration AtoN deployments in high-UV open-water environments.

Bifacial panels offer a genuine performance advantage in deployments where rear-surface light capture can be reliably maintained — for example, on elevated structures over highly reflective water surfaces where the rear encapsulant can be specified in dual-glass construction. However, the additional complexity of managing bifacial gain variability and rear encapsulant UV exposure requires careful engineering assessment. Key selection criteria for AtoN solar power in open-water UV conditions include:

Encapsulant specification: Confirm whether the panel uses EVA or a higher UV-stability encapsulant such as polyolefin (POE), which offers better resistance to yellowing under sustained UV exposure
Rear surface construction: For bifacial panels, dual-glass construction is preferable to transparent polymer rear layers in open-water marine deployments
Published degradation rate: Evaluate the manufacturer’s stated annual degradation rate and confirm whether it has been validated in marine or high-UV environments specifically
Service life power budget: Calculate expected power output at the end of the intended service life, not at initial installation, and verify that the lantern’s minimum operating power requirement is met throughout
UV stabilisation of supporting components: The panel housing, mounting hardware, and any enclosures should also be constructed from UV-stabilised materials — UV-stabilised polyethylene is a well-established choice for marine AtoN equipment enclosures

Sealite’s solar marine lanterns are engineered with these environmental realities in mind, incorporating premium-grade solar modules and UV-stabilised materials designed to maintain reliable performance across service lives of up to 15 years in demanding marine environments. For port authorities, coast guards, and waterway operators specifying AtoN solar power, understanding the UV performance characteristics of monocrystalline and bifacial panels at this level of detail is not academic — it is the foundation of a power system that will keep navigation aids operating reliably for the full duration of their intended deployment.