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Mercury Loss in Fluorescent Lamps - Experiment and Modelling
von Daniel LenkDespite the recent trend towards substitution of fluorescent lamps by LEDs, they remain an important part of global illumination due to their high efficiency, long lifetime and low initial cost. The fundamental working principle of a fluorescent lamp is that the electric discharge of a gas emits ultraviolet (UV) light, which is absorbed by a phosphor layer deposited on the inner surface of a soda-lime glass tube, which in turn reemits visible light. The gas, which is most efficient to convert an electric current into UV light by electric discharge, is mercury at low pressure. Thus, mercury is indespensible in fluorescent lamps.
Over the course of the lifetime of the lamp, the mercury is bound at the inner glass surface, resulting in graying of the glass and, consequently, in a decrease of luminous efficiency. To reduce the mercury loss by transport into the glass, an additional coating layer is deposited between the phosphor layer and the glass tube. However, mercury has also been observed to be bound in the coating layer and in the phosphor.
If not compensated, these mercury loss processes would reduce the amount of mercury available for the discharge and the lifetime of the lamp would be severely limited. However, since mercury is toxic and, thus, complicates the recycling of the lamps, a reduction of the dose of mercury in fluorescent lamps is desired for ecological reasons and also enforced by increasingly severe limitations by public authorities. However, the processes underlying the mercury loss were still widely not understood. While several approaches existed to investigate and/or simulate the mercury loss processes for simplified systems, containing either a phosphor or a coating layer, yielding evidence for a diffusive transport process and an electrostatic reaction process, there was still no systematic study and a consistent model of the process in a realistic phosphor/coating double-layer system available yet.
The goal of the presented study was to find a consistent description of the mercury loss process in such a double-layer setup and, subsequently apply it to the results of a systematic experimental lamp aging study to verify its applicability. This yields the possibility to predict the mercury loss properties of a specific layer design and material combination to reduce the mercury consumption in fluorescent lamps and, thus, reduce the amount of mercury used in such lamps.
This book provides an extensive study of the aforementioned mercury loss process by investigating the mercury content in the coatings and the luminous flux of hundreds of prototype lamps over the course of thousands of hours. The obtained results are discussed with respect to the electrostatic and structural properties of the materials used for the coatings of the lamps. Empirical models to describe the mercury loss process are developed and a consistent description of the mercury loss process is provided. The presented results isolate the UV-load on the glass as distinct key parameter to reduce the mercury loss in fluorescent lamps.
Over the course of the lifetime of the lamp, the mercury is bound at the inner glass surface, resulting in graying of the glass and, consequently, in a decrease of luminous efficiency. To reduce the mercury loss by transport into the glass, an additional coating layer is deposited between the phosphor layer and the glass tube. However, mercury has also been observed to be bound in the coating layer and in the phosphor.
If not compensated, these mercury loss processes would reduce the amount of mercury available for the discharge and the lifetime of the lamp would be severely limited. However, since mercury is toxic and, thus, complicates the recycling of the lamps, a reduction of the dose of mercury in fluorescent lamps is desired for ecological reasons and also enforced by increasingly severe limitations by public authorities. However, the processes underlying the mercury loss were still widely not understood. While several approaches existed to investigate and/or simulate the mercury loss processes for simplified systems, containing either a phosphor or a coating layer, yielding evidence for a diffusive transport process and an electrostatic reaction process, there was still no systematic study and a consistent model of the process in a realistic phosphor/coating double-layer system available yet.
The goal of the presented study was to find a consistent description of the mercury loss process in such a double-layer setup and, subsequently apply it to the results of a systematic experimental lamp aging study to verify its applicability. This yields the possibility to predict the mercury loss properties of a specific layer design and material combination to reduce the mercury consumption in fluorescent lamps and, thus, reduce the amount of mercury used in such lamps.
This book provides an extensive study of the aforementioned mercury loss process by investigating the mercury content in the coatings and the luminous flux of hundreds of prototype lamps over the course of thousands of hours. The obtained results are discussed with respect to the electrostatic and structural properties of the materials used for the coatings of the lamps. Empirical models to describe the mercury loss process are developed and a consistent description of the mercury loss process is provided. The presented results isolate the UV-load on the glass as distinct key parameter to reduce the mercury loss in fluorescent lamps.