Patent Application
20200286724
This invention relates to gas-discharge lamps that produce electromagnetic radiation in the ultra-violet region of the electromagnetic spectrum. Such lamps may find use in various applications relating to disinfection, such as for the purification of water or treatment of food and beverages, in the manufacture of pharmaceuticals and also for curing and drying. More specifically, the invention relates to a mercury-free gas-discharge lamp and, in particular, a mercury-free radiation source for a gas-discharge lamp.
In a typical gas-discharge lamp, ultra-violet (UV) light is generated by passing an electrical discharge through an ionised gas (or “plasma”), as a consequence of the resulting transitions of electrons between energy states emitting photons of particular energies.
The use of ultra-violet (UV) electromagnetic radiation or light for disinfection and purification purposes is known. The most desirable wavelengths of UV radiation for disinfection purposes are generally understood to be in the 180 nm to 320 nm range, more preferably 200 nm to 300 nm (often referred to as UV-C), and optimally around 265 nm. UV radiation of such wavelengths has both a biological effect, inactivating (if only temporarily) micro-organisms primarily by genomic damage preventing replication, and a chemical effect, breaking chemical bonds (including those of micro-pollutants) by a process called photodissociation or photolysis.
UV electromagnetic radiation, typically of slightly higher wavelengths (up to approximately 400 nm), is also used for curing and drying.
Conventional UV gas-discharge lamps comprise an elongate tube of quartz or silica with electrodes at either end. The lamps are filled with a starting gas, typically a noble gas such as argon or xenon, and also a small quantity of radiating working material, typically mercury. At room temperature most of the mercury inside the lamp is in liquid form. The lamp is ignited by passing an electrical current across the electrodes of the lamp, which ionises the starting gas, the resulting atomic/electron collisions causing the mercury to evaporate. Once the lamp has reached operating condition, the mercury partial pressure is much higher than that of the starting gas, and mercury therefore dominates the electrical and radiating behaviour of the lamp.
There follows a short overview of high-pressure, low-pressure and metal-halide discharge lamps.
The development of sources of UV radiation is entwined with the development of sources of Electromagnetic Radiation (ER) in the visible spectrum i.e. visible lighting. These associations are not only in respect to the same fundamental principles of physics and design but practically as well. A key example being that of the low-pressure mercury (LP Hg) lamp which is essentially identical to that of a fluorescent lamp used commonly for residential lighting except for the addition of a phosphor coating which absorbs the UV atomic emission of mercury, and then subsequently emits in the visible region. As visible lighting consumes approximately 25% of the worlds produced electrical energy the goals for increased efficiency and extended lifetimes of visible ER sources are also aligned providing potential insight into an alternative method of UV ER generation. Sources of UV radiation are investigated and discussed below, however particular focus has been given to plasma sources, because of their current dominance of the market. Emerging sources and their impacts are also discussed.
Plasma lamps achieved commercial success in the 1930's following on from the incandescent lamp, where an incandescent lamp emits ER from a hot body e.g. a tungsten wire.
A plasma lamp (plasma being defined for example as “a gaseous mixture of positive ions and electrons”) provides several benefits over an incandescent lamp. Firstly, radiation is produced with increased energy efficiency (i.e. the ratio of energy output to energy input). Secondly, as plasma-derived photons are produced from direct atomic excitation, their wavelengths are determined by the atomic constituents of the plasma, thus enabling the production of UV radiation. A number of methods have been developed to use plasma to produce UV radiation. The historically most successful methods are summarised below (physical characteristics such as lamp size, electrode design etc can vary considerably depending on the plasma characteristics, however these are not discussed. Instead focus is given to the variation in plasma characteristics):
To produce suitable lamp plasma for use in UV disinfection an element or compound, the following characteristics must be achieved:
Mercury (Hg) meets these criteria and hence is the primary constituent of the majority of lamp plasmas for both visible lighting and UV disinfection. Although other elements can be and are used in limited quantities e.g. xenon, practical challenges include high internal lamp pressure creating problems when starting the lamp, and high running currents. Lamp pressures for compact Xenon lamps being in the region of 15 atm cold and up to 60 atm when running with the relevant temperature increase.
The Low Pressure (LP) Hg plasma discharge lamp is composed of a low internal Hg gas pressure (approximately 0.01 mbar) combined with a buffer gas that is usually argon. The low Hg pressure ensures that the majority of electron excitations are at two energy transitions producing 253.7 nm and 185.0 nm. The Hg pressure (and therefore the impedance and consequentially the lamp power) is determined by the running temperature of the lamp (increasing temperature meaning increasing pressure) and regulating the amount of Hg in the gas phase to that condensed on the cold spot, as shown in
With the optimal selection of lamp variables (i.e. lamp geometry, Hg content, temperature etc.) an energy efficiency of 60% at 253.7 nm can be achieved, however this is only at low power densities (<0.5 W/cm approx. 0.2-0.3 W/cm at 253.7 nm); increasing power densities by up to 400% with the use of an amalgam and increasing tube diameter (in the region of 26-33 mm) will reduce the lamp efficiency to the region of 36% at 253.7 nm. Even at the highest efficiency, 40% losses are incurred which can be attributed to: the production of other wavelengths (3%), losses at the electrodes (15%) and elastic collisions with the tube wall and argon (22%). With such a temperature sensitive design, a limitation can be the temperature of the surrounding water, which, if at 4° C., would reduce the radiant efficiency to approximately 20%.
The development trend in LP discharges is to increase power density whilst maintaining radiant efficiency. In addition to the adoption of amalgam as previously described, the selection of the lamp driver is critical and further efficiencies have been gained by the use of a high frequency driver with a square wave. During the mid-1970's the concept of active heat regulation of the lamp through heating of the cathode and external heating of the lamp was employed, enabling optimised lamp conditions and therefore increased power densities (in part due to a reduction of re-absorption through line broadening). This concept has recently been reapplied to provide an increased output for UV disinfection reiterating the desire for a high radiant efficiency and high power density UV source. Further developments in lamp driver electronics have seen the use of inductively coupled fluorescent lamps and are proposed as a future solution to enable further continuing improvement of the LP plasma lamp, by reducing net losses and extending lamp life by removing the need for electrodes.
The basic requirements for a High Pressure (HP)—a term which includes High Intensity Discharge (HID)—in terms of lamp fillings are the same as that of the LP discharge, and hence Hg is again the most commonly used filling. In contrast however, the amount of Hg (and hence consequentially the internal pressure) is significantly higher than that of a LP discharge and as a key distinction to that of the LP discharge, all the Hg is in the vapour phase. This is shown in
As in the LP discharge, an increase in Hg vapour pressure increases impedance, hence increasing voltage (V) and consequentially power density of the lamp. The pressure gradient is continuous between a LP and HP discharge however a clear distinction is made to that of a HP discharge when the temperature of the (Hg) ions and electrons reach an (approximate) equilibrium referred to as a Local Thermal Equilibrium (LTE) as shown in
Losses in elastic collision are proportional to the difference between a low energy electron to that of a high energy atom/ion (ie. LP discharge atom/ion temperature in the range of 300K to 700K and electron temperature above 10,000K. HP discharge has both atom/ion temperature generally between 4,000K and 11,000K depending on lamp conditions, meaning that when LTE is reached, elastic losses approach zero. Additionally, as power density increases so does the temperature of the lamp and in particular the arc which develops in the high pressure lamp, enabling thermal excitation and its subsequent emission. Although the lamp temperature increases, the thermal losses are not surprisingly low due to the low thermal conductivity of Hg. The implications being the LTE provides disproportionate radiant efficiency benefits to the HP discharge in comparison to that of the LP discharge. The arc develops because of a radial temperature gradient within the lamp; as temperature increases so does ionization (producing electrons referred to as current carriers) meaning that the current density is highest at the axis of the electrodes. This means that the LTE as a consequence has a significant increase in net radiant efficiency (
The second implication of increasing pressure and plasma temperature is that of changing spectral output. The LP discharge is dominated by atomic collision and spectral emission from excitation, hence the two narrow and dominant emission lines at 253.7 nm and 185 nm, this changes with increasing pressure, which is thought due to:
Therefore the HP discharge can be characterised by a high density high efficiency discharge with a spectral output form the UV to the Infra-Red (IR). Although the spectral output far exceeds that of LP discharge, the plasma efficiency enables the total radiant efficiency to be approximately ⅓ of that of a LP discharge. With similar advances in high frequency electronic drivers as for the LP discharge, the expected lamp life can be between 2,000 to 8,000 hours dependent on lamp design parameters. The practical implications means that compared to a LP discharge a far higher UVC density can be achieved in more efficient discharge in respect to radiant efficiency, however a compromise is made with a lower spectral efficiency.
The efficiency of the HP plasma cannot be optimised or improved by pressure control as discussed for the LP discharge because it already functions in the LTE. However in visible lighting a resourceful method has been employed to enable the use of elements with desirable excitation and ionization energies but with too high a boiling point or too low a vapour pressure. The use of a halogen in conjunction with a desirable element will in most cases result in the reduction of the boiling point, enabling it to be used as directly or as part of a HP plasma. Iodine is often the selected halogen over bromine and chlorine as it is less reactive with internal lamp components whilst also generally producing the highest vapour temperature compared to other halogen compounds. The halide (in addition to the halogen component) is usually metal and hence the term Metal Halide (MH) is/are added to a high pressure Hg discharge. The Hg then performs the role of a ‘buffer gas’ which provides majority of the required gas vapour and electrical properties, although in this case does also contribute to the spectral output. The spectral output is almost entirely determined by the additional metal content73 due to the fact the excitation potential of the metals used are comparatively much lower than Hg (
The lower vapour temperatures provided by the metals used in their halide form enables them to be in the vapour phase whilst at the operational temperatures of the lamp. As the temperatures increase towards the arc the halide dissociates and associates at lower temperatures at the lamp wall (
The MH lamp appears in many ways to be the ideal solution to the limitations of low power densities or low spectral efficiencies associated with the LP and HP discharges respectively. In fact, the potential for MH lamps to produce spectral efficiencies (visible region) of 34% and enhance colour rending facilitated its entry into the lighting market. The ability of MH lamps to be used for UV generation is limited. Experiments on iodide additives (iron (FeI2), cobalt (CoI2), manganese (MnI2), antimony (SbI2)) to assess their impacts on UV outputs, and although FeI2 and MnI2 enhanced the UVA output, none of the iodides improved the output in the UVC region. Presumably this limitation is associated with the need for a lower excitation potential required for effective MH operation.
Although the MH lamp provides highly desirable spectral and electrical characteristics, numerous practical problems were encountered and had to be overcome before commercial MH lamps were widely produced. One such limiting factor for the high intensity discharge (HID) is lamp life, which is closely associated with the high temperatures and small lamp geometry. One benefit of a lamp running at a temperature above 500° C. is that the absorption band at 215 nm which develops with time in quartz is removed. The absorption (thought to be due to loss of oxygen from the silica lattice) is removed by heating above 500° C. and thus a lamp with a quartz envelope running at or above this temperature is assumed to reverse such a formation. As a MH lamp is designed with much smaller geometries and higher pressures, a geometry and pressure similar to that of a MP lamp is likely to gain the benefits of a HP discharge without the geometry related issues of a visible HID lamp.
Low pressure (LP) and high pressure (HP) mercury (Hg) lamps dominate the UV disinfection market due to their relative operating simplicity and reasonable energy efficiency. Numerous improvements have been made in LP lamps, however their greatest limitation is internal losses caused by its low internal pressure. Improvements have also been made to HP lamps however ultimately their limitation in further efficiency improvements are related to the spectral output, determined by the lamp pressure.
To meet the needs of a high efficiency and high density lamp, the metal halide (MH) lamp has been proposed due to its success in visible lighting, and if the concept could be successfully applied to UV generation it would provide a desirable solution. The present work identifies one limitation of prior attempts as relating to the reliance upon Hg as the primary lamp filling which restricts the use of MH components with spectral lines of higher energy and therefore optimisation of spectral output in the UVC region.
Preferred performance objectives to enable widening of the upper energy density range of disinfection applications of the lamp include:
To warrant switching from a traditional Hg based HP lamp it would be preferable to offer a competitive advantage i.e. increased germicidal efficiency. An approximate figure of 12% germicidal efficiency is typical for a Hg HP lamp; however, efficiency will be related to lamp diameter i.e. the losses incurred from photon production at the lamp arc to that of emission of the lamp wall. Thus 12% can be used as a guideline figure but a direct efficiency comparison of any proposed lamp to a Hg lamp of equal diameter would need to be conducted.
Desirable performance objectives include:
These design characteristics are specifically of a narrow scope to enable a design concept and investigation to be undertaken. Additional performance data will relate to specific applications (including but not necessarily exclusive to water disinfection), comprising for example a detailed assessment including the effect on whole life costs (inclusive of lamp costs, lamp driver and combined efficiency) and specific application considerations such as the production of disinfection by-products (DBP).
To achieve the specified performance aim and objectives of the lamp the proposed concept is to produce a MH lamp with a dominant UVC output. This has been selected as a design concept as it is an adaptation of an existing approach used in visible lighting and is principally a high density discharge as required to meet the design objectives.
Potential reasons for selecting the concept of a UVC MH lamp may include the following:
Attempts to enhance the UVC spectral output of a Hg based MH lamp have not been successful to-date. One possible cause of this lack of success could be because of the previous selection of elements e.g. antimony which has preferential spectral lines that have a higher excitation energy than Hg and thus not favoured, as was seen for elements with lower excitation energies, e.g. iron. Therefore an alternative primary lamp filling is proposed which has similar physical characteristics to Hg whilst also having lower spectral lines (i.e. higher photon energies) than the lowest desired spectral region i.e. 200-230 nm. A suitable secondary lamp filling preferably has desirable excitation energies (spectral lines) and ionization energies, whilst providing functional vapour pressures both at lamp starting and running temperatures.
The minimum vapour pressure to produce useful radiation at 1000K (726.85° C.) is 133 Pa (1 torr) with possible elements to meet this condition being strontium, tellurium, magnesium, zinc, cadmium and caesium. Using an element in halide form in general increases vapour pressure, reduces the boiling temperature and metal iodides do not appreciably react with the fused silica such as magnesium and zinc.
The halide(s) and ideally iodide(s) preferably meet a number of criteria. The primary halide should ideally mimic the vapour pressure characteristics of Hg whilst having dominant spectral lines lower than 253.7 nm (i.e. a higher energy) enabling a secondary halide with a suitably high enough vapour temperature not to impact lamp characteristics, whilst having spectral lines of a desirable wavelengths 200-230 nm and/or 260-280 nm to be preferentially selected in excitation. The halide also preferably needs to be stable at lamp wall temperatures and dissociate at arc temperatures (4000-6000K). Consequentially a spectral and functional assessment of primary and secondary lamp fillings is required to enable a lamp concept to be developed.
According to a first aspect of the invention there is provided a mercury-free high-pressure metal-halide ultraviolet gas-discharge lamp comprising a primary filling of at least one of osmium, germanium and tellurium, and a secondary filling comprising at least one of tin, antimony, indium, tantalum and gold.
Preferably, the primary lamp filling is tellurium and the secondary lamp filling is antimony.
Preferably, the halogen of the metal-halide comprises iodine.
Preferably, the primary lamp filling is TeI2 and the secondary lamp filling is SbI3.
Preferably, the ratio of iodine to tellurium is non-stoichiometric, preferably with a reduced iodine content.
Preferably, the ratio of iodine to tellurium is no greater than 2:1, preferably no greater than 1.5, more preferably less than 1.0. The ratio may be by mass in gaseous form.
Preferably, the lamp output comprises electromagnetic radiation of wavelength in the range 200-300 nm.
Preferably, the primary lamp filling has similar physical characteristics, such as vapour pressure, to mercury whilst also having lower spectral lines (i.e. higher photon energies) than the lowest desired spectral region i.e. 200-230 nm, more preferably having dominant spectral lines lower than 253.7 nm.
Preferably, the secondary lamp filling has suitably high enough vapour temperature not to impact lamp characteristics, both at lamp starting and running temperatures, whilst having spectral lines of a desirable wavelengths 200-230 nm and/or 260-280 nm to be preferentially selected in excitation.
In some embodiments, alternative enclosure materials other than quartz may be used, such as (but not limited to) ceramic materials. This may reduce if not eliminate the effects of the lamp filling otherwise reacting with the lamp body material.
In some embodiments, the lamp may be driven without the use of electrodes, for example inductively or with the use of microwaves. This may limit the effects of material reactions which may arise, for example, when using tungsten based electrodes and/or iodine in the fillings.
Further features of the invention are characterised by the dependent claims.
Any apparatus feature as described herein may also be provided as a method feature, and vice versa.
The invention extends to methods and/or apparatus substantially as herein described with reference to the accompanying drawings.
Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination.
It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently.
These and other aspects of the present invention will become apparent from the following exemplary embodiments that are described with reference to the following figures in which:
An initial assessment of potential elemental candidates was undertaken by identifying dominant spectral lines (from a neutral atom) from elements of rows 4-6 of the transition metals, rows 4-6 of the metalloids and rows 3-4 of the post transition metals from the periodic table. A summary of this information is displayed below in Tables 1-3.
Spectral lines provided in tables are in order of relative values high to low. Where values are equal wavelengths they are stated in order of wavelength i.e. lowest wavelength first, except where there are more than 3 of equal value or more than 1 value in third wavelength where the values are stated
From the spectral information summary, eight possible elements appear to have desirable spectral characteristics, three for a primary filling (osmium, germanium and tellurium) and five for a secondary filling (tin, antimony, indium, tantalum and gold). To further assess these potential elements critical data of their known physical properties as elements and halides are presented in Table 4.
The three candidates for primary lamp fillings identified based on spectral criteria can be reduced to a single candidate, tellurium due to the insufficient data supporting the stability of osmium and germanium as an iodide in the gas phase.
Of the five candidates for secondary lamp fillings gold and indium were rejected as candidates as they would not produce suitable iodide, leaving tantalum, tin and antimony as possible candidates. Tantalum has a higher boiling point (BP) and although tin provides the lowest BP the spectral characteristics of antimony (two lines being approx. 260 nm) and its previously use in lamps makes it the preferred choice for the initial concept prototype. In addition, there are practical limitations are incurred with the use of tin.
A more detailed spectral assessment of tellurium and antimony was undertaken with the addition of Iodine due to common use as halogen for MH lamps. The dominant spectral lines from both neutral and singly ionized elements are displayed in tabular form and the complete spectral data has been displayed graphically. A summary of spectral data in the UVC regions of the three elements is also displayed. Data obtained for Sb contains spectral lines from neutral to −4 ionization whereas data for Te was only available for neutral and −1 ionization states.
Tellurium
Antimony
Iodine
The spectral data for tellurium exhibits predominant lines either below or in the lower region of the 200-230 nm target spectral range whilst maintaining 97.7% of the spectral range below 250 nm.
Seven of the eight most dominant spectral lines of antimony are ideally placed within the two target spectral areas. Although a secondary region of spectral emission occurs from antimony between 800 nm-1000 nm the overall lines produced appear favourable in respect of the desired spectral range meeting both the target areas of 200-230 nm and 260-280 nm.
The underlying question is that of the transition lines for both tellurium and antimony. The concept of a High Pressure discharge means that numerous transition lines will likely be produced under the lamp pressures from increased collision frequency as in the HP Hg discharge. Also with increased pressures will be spectral emission from other sources such as recombination and bremsstrahlung. Therefore the total spectral output and spectral radiant efficiency will only be determined when measured at the designed lamp pressures.
A spectral assessment of elements has been undertaken with tellurium and antimony highlighted as potentially suitable elements for use in a UV MH lamp. In addition to a desirable spectral output the fillings must also display functional characteristics.
Suitability of Halide Compounds for Lamp Plasma
There are a number of key physical characteristics which any potential halide must meet particularly in relation to its ionization energies, thermal and vapour characteristics and specific molecular interactions associated with the halide compound.
Ionization Energy
A necessary feature of a lamp filling is a relatively low ionization level, which aids the starting of a lamp. A lower ionization level means less energy is required to produce free elections which in turn produce more electrons and so on, in what is described as the avalanche effect. As displayed in Table 9 both antimony and tellurium have lower ionization levels compared to mercury and hence should be suitable to initiate a plasma discharge.
Arc Stability
A characteristic of the high pressure discharge is the arc contraction which if accounted for in design of a MP Hg lamp should produce a relatively stable straight arc; however, this is not guaranteed for a MH lamp. Previous work with Hg based MH lamps has identified significant impacts of MH additives on the lamp arc either constrictive or broadening even though the proportional amount of the MH additive is minimal to that of Hg within the lamp. Recorded examples in literature are thorium, scandium and other rare earth metals which constrict the arc and make it more susceptible to internal fluctuations, whereas addition of alkaline metals (i.e. caesium, sodium, potassium) have the opposite effect and broaden the lamp arc having a stabilising effect.
Arc stability is a critical factor in determining the functional suitability of the proposed plasma discharge concept, not simply because of undesirable anisotropic radiant characteristics due to the rising of the arc above the lamp axis when in the horizontal position (which can also cause condensation of MH on the underside of the arc), but in extreme cases the lamp wall can physically melt causing it to self-destruct. The reasoning for the instability of the HP arc can be identified when assessing its fundamental thermal characteristics. The HP pressure lamps used in UV disinfection are characterised by having a significantly longer arc length than lamp diameter of the lamp. The arc is central to the lamp which is in part due to the physical characteristics imposed upon the arc by the walls of the lamp and in this case is referred to as a ‘wall stabilised’ arc. This is a desirable feature of a well-designed MP lamp and is an aim for a high density, high efficiency MH lamp.
The wall stabilised arc is a feature of a positive radial profile temperature which displays a sharp decline in temperature towards the lamp wall from the arc. This means that movements in the arc are stabilised due to cooling/heating effects incurred from moving from the centre of the lamp. If the lamp has a temperature gradient that drops rapidly from the arc rather than at the lamp wall there is no stabilised effect. Such instability causes the arc to rise (when mounted horizontally) with resulting spectral problems but also causing the possibly of quartz softening or halide condensation under the arc. A critical design criterion to indicate a wall stabilised arc is the ratio of average excitation potential v to that of the ionization potential and vi being greater than 0.585 i.e.
Thermal Characteristics of Elements
The lamp arc as previously discussed has a temperature of approximately 3700-4700° C. however the temperature of the lamp envelope is expected to be lower than 800° C. This by implication means that the high degree of thermal insulation is required not only to provide protection for the quartz envelope but also to restrict thermal losses of the discharge to maximise efficiency of the discharge. A number of data points for thermal conductivity are provided in Table 10 for Te and Sb for comparison to Hg and Zn that also produces a relatively high vapour pressure in elemental form. Data for Te and Sb are similar although the key difference is that mercury exhibits a steady increasing trend with temperature whereas Te exhibits a decreasing trend. As the data for Sb is only a single point little can be interpreted however compared to the data on Zinc (Zn) showing considerable more thermal conduction it appears that approximately similar thermal characteristics to that of Hg may be provided by a Te based lamp at temperatures at the lamp wall.
Metal Halide Characteristics and Interactions
Critical to the stability of any halide lamp proposed is stability and interaction between the halide compounds filling used for lamp filling, particularly the primary fill compound. As spectral selection identified only Te as an appropriate primary filling an assessment of literature on Te as a metal iodide has been undertaken with key information being provided alongside information for Sb in Table 11.
Table 11 describes both Te and Sb as iodides TeI4 and SbI3 respectively with m.p. and b.p. data as previously stated in Table 4 with little additional information to note regarding SbI3. TeI4 presents additional complexity when in the gas phase as required for a HP lamp discharge. Core reactions between Te and I from the solid to the gas phase, are described in Equations 1-5 below:
Equation 1 Thermal decomposition of tellurium tetraiodide in the vapour phase
TeI4(g)TeI2(g)+I2(g)
(The proportion of TeI2 formed is temperature dependent and increases with temperature, at ≥500° C. this is near completely TeI2. There are also isolated (TeI4)4 tetramers.
Equation 2 Sublimation and deposition of tellurium dihalide
Te(s)+I2(g)TeI2(g)
Equation 3 Sublimation and deposition of tellurium tetraiodide
TeI4(g)Te(s)+2I2(g)
Equation 4 Thermal decomposition of iodine in the gas phase at temperatures above 600° C.
I2(g)+I2(g)2I2(g)
Equation 5 Thermal decomposition of tellurium iodide in the gas phase at temperatures above 600° C.
2TeI2(g)Te2(g)+2I2(g)
To have Te in the gas phase it must transition from TeI4 through various states and compounds however above 600° C. Te will be in the gas phase although interchangeably as an iodide or diatomic Te. It is unknown whether this will impact the stability of the arc however to ensure Te does not condense into the solid phase a wall temperature of 600° C. must be maintained with a minimum I to Te ratio of 2:1. Complex iodide vapours can form and this is a possibility between Te and Sb iodides, possibly adding to further spectral and functional complexity, however as Sb will be a secondary filling it will comprise only of a small proportion to lamp performance, and for design purposes only the Te iodide formations will be assessed.
The proportion of iodine to that of the element in question is critical. Two methods are used to ensure an adequate amount of iodine is present. Firstly, provide exact iodine to element ratios are added to form a complete number of halogen compounds; secondly, an excess of iodine can be added to that of the element reducing the likelihood of elemental condensation at the lamp wall. In the latter case the there are problems associated with free I2 which is a strong light absorber and can cause loss of metals over time can cause problems with lamp functionality. In a Hg based lamp this is resolved with the formation of HgI2 which is transparent and relatively unstable.
Pressure Characteristics of Selected Halides
A critical component of lamp plasmas described above is the ability for the lamp filling to provide sufficient internal lamp pressure. In contrast there are known issues relating to having too high a pressure from halides and the need to limit the amounts used. As the MH lamp is designed to function around the same principal design criteria as a Hg HP lamp it is prudent to assess pressure of the lamp fillings in relation to temperature compared to that of Hg. Pressure data for both TeI4 and SbI3 are limited however pressure curves Te2I2 are displayed alongside those of I2, TeBr4, and Hg in
The pressure curves displayed in
Spectrally Te provides suitable lines for use as a primary lamp filling with Sb as a secondary filling, with both elements providing evidence of suitable energy potentials to that required for ionization to indicate the production of a wall stabilised arc. Te appears to provide suitable thermal and pressure (as TeI4) characteristics to match Hg as the primary lamp filling. Te will provide a stable iodide at pre running conditions as TeI4 which will be converted to TeI2 in the gas phase. The only possible disadvantage identified in the assessment is that over 600° C. the Te iodide transitions back and forth to both Te and I (both of which are in the gas phase) and it is unknown whether this will cause any instability in the functioning of a lamp.
The functional assessment of selected halides Te and Sb provided a basis for a potential high efficiency UV MH lamp. The methods of UV generation development can be linked to visible lighting and as such could be the reason for the lack of such a MH development (i.e. the visible Hg MH lamp would not benefit from replacing Hg). There is still an underlying question as to the reason for a lack of such a MH development to date when considering the developments of LP UV sources. As such an assessment of related patents filed is listed in Table 12 with relevant associated data displayed in
Pat1 using a Sb halide produces a significant amount of UV radiation (
Pat4 shown in
Pat3 provides further spectral data on the use of Te as a lamp filling for UV production.
Pat2 is the closest representative of a HP lamp using Te. The data provided in Pat2 (
In particular, Pat2 appears to recite features such as:
Although the concept of a Te based MH lamp seems technically feasible from a functional assessment, no high efficiency UV HP MH has been published to date or a plasma with a visible output using of tellurium iodide in a stoichiometric ratio of Te:I of 1:2, and therefore practical verification of this technical proposal is required.
The use of a halogen is required for the benefits in increased vapour pressure however as described above the possibility of I2 formation is high (because there is no Hg to form HgI2) and therefore the dosing of tellurium to iodide proportions as described in the previous paragraph is not only novel but likely critical to producing a functional UV MH lamp.
A UV MH lamp was deemed to be feasible based on a primary lamp of filling of Te and iodide in the form of TeI2 and a secondary lamp filling of SbI3. In optimised quantities this combination of lamp fillings were expected to enable similar internal lamp pressures to that of an Hg HP lamp but with increased spectral efficiency due to the second filling with a lower excitation level and optimal spectral characteristics. The benefits of Te in conjunction with iodine is that relatively similar pressure characteristic to Hg should be achieved however at the temperature produced in a HP lamp (>600° C.) an interchangeable state is formed between the iodide compound in gas phase and its elemental constituent in the gas phase, it is unknown whether the elemental components particularly I2 with its high vapour pressure will affect the stability and functionality of the lamp. Excluding this, the suitability of both Te and Sb iodides to provide a functional alternative to Hg as a HD UV source looks technically promising however optimal quantities need to be practically assessed.
To achieve the proposed concept of a high efficiency MH HP lamp with Te Halide forming the primary constituent of lamp plasma and Sb iodide as a secondary filling maximising the spectral output in the UVC region a number of design stages had to be undertaken. These are described below:
Stage1—
Initial requirements are to establish the functionality and performance criteria of tellurium iodide as lamp plasma, particularly in respect to; arc stability, electrical characteristics during running, spectral output and spectral radiant efficiency.
This was achieved by using TeI4 and Te as the lamp fillings in a stoichiometric ratio of 2:1 (I:Te). Two initial lamp fillings with two lamp body geometries (15 mm Internal Diameter (ID) and 18 mm ID both with a 100 mm Arc Length AL) will be used to gain initial performance data. The 18 mm ID lamp geometry is more representative of a conventional MP lamp however the 15 mm ID geometry reduces the possibility of halide condensation particularly in maintaining the gas phase of tellurium iodide i.e. >400-600° C.
Stage 2—
Optimise the quantities of Te Iodide to provide optimal performance criteria using Hg MP lamp as a baseline. This will require balancing the spectral performance of the unit to power density whilst assessing arc stability. Assuming arc stability there may well be a balance between spectral optimisation depending on the two key areas i.e. 200-230 nm and 260-280 nm and pressure, and lamp pressure i.e. power density, hence this could lead to two separate designs to be optimised by Stage 3.
Stage 3—
Addition of Sb iodide to optimised Te iodide primary filling. Based on Hg based MH lamps only a small percentage will be required however this is not guaranteed and so a range of Sb iodide fillings should be used starting at 5% of the Te iodide value.
As initial guidance for stage 1 the following values were determined. Using total weight as a comparative value the lower values (those of half the quantity used in the prototype by Turner (1994)) in Table 13 with lamp geometries selected being in the region used for current HP Hg lamps (18 mm ID prototype lamps). Following the assessment of the results of these prototypes in respect of spectral output, spectral efficiency and visual verification of lamp performance (e.g. arc position and stability) optimisation of lamp fillings can proposed for Stage 2.
#1 Hg fill for concept lamp based on maximum loading whilst enabling a stable arc = 12 V cm−1. Hg dose for 15 mm prototype estimated voltage cm−1 = 25 mg and 18 mm prototype = 40 mg
All the prototype lamps were produced by Hanovia Ltd (Berkshire, UK). The Hg lamps were produced as per the standard manufacturing process to the author's specification (Table 14). All lamp bodies (lamps without fillings) produced for the metal halide prototypes were produced using the same production process as the Hg lamps until the point of inserting the lamp fillings, at which point the lamps were removed from the process whilst under vacuum using Swagelok (Hertfordshire, UK) vacuum fittings and were transferred into a Mbraun (Nottinghamshire, UK) Unilab Plus glovebox enabling a moisture and oxygen free environment (<0.5 ppm of measured H2O and O2). In these conditions the required lamp fillings were weighed using a VWR (Leicestershire, UK) precision balance with automatic calibration (SN: LPW-723i) sensitive to 1 mg. Fillings were added to the lamp bodies, re-sealed and returned to the standard lamp production process. All prototypes had platinum reflective paint to the rear of the electrodes to reflect infrared ER, preventing a cold spot forming behind the electrodes and the potential for condensation of lamp fillings from the lamp plasma.
The performance assessment was carried out in terms of three specific aspects; Physical characteristics (i.e. arc stability), Absolute spectral output and Electrical characteristics. All prototypes were driven with an Eta+ (Nuertingen, Germany) X series electronic ballast with a 4 kW power rating. If the prototype did not ignite it was cooled (this is stated in Table 16 in the comment section if cooling was required) using freezer spray (Artic Products, Leeds UK or Electrolube, Leicestershire, UK) to reduce the internal gas pressure and consequently the strike voltage. This was generally due to halide dissociation during manufacturing process e.g. the lamp temperature increasing due to the removal of the lamp stem (used to inset lamp fillings and gas).
The details of the lamp assessment are described below:
Physical Characteristics—
The first lamp of each prototype design was conducted in front of a viewing window (comprised of welding glass) to enable the viewing of the lamp when running the arc. Photographic images of the lamps running were taken through the viewing window using a Fujifilm (Fujifilm UK, Bedford, UK) s9600 bridge camera.
Spectral and Electrical Characteristics—
The lamps were operated horizontally in air in a dark room with the lamp radiation passing through a collimating tube (500 mm in length with internal baffles for collimation) with vertical entrance slit of 0.51 mm in width. When the lamp had stabilised, electrical characteristics were measured with a Voltech (Oxfordshire, UK) PM6000 3 phase universal power analyser. Germicidal efficiency was calculated from the spectral measurements accounting for the shaded slit width (0.53 mm), the measured distance from the lamp arc (0.5 m) and the Arc length (0.1 m) and correcting for germicidal weightings. Two action spectra (AS) were used to calculate germicidal weightings: Spectrum B representing a target pathogen with no sensitivity below 230 nm, and Spectrum A representing a target pathogen with a high sensitivity below 230 nm. The AS used were adapted so relative values equalled one at 253.7 nm.
During the production of the initial set of halide prototypes for design stage 1 an error was made during dosing of the lamps meaning that the amount of Te dosed was ten times higher than that desired in Table 13, with the final amounts for all subsequent design stages therefore provided in Table 15. In addition, two practical challenges emerged: the weighing of lamp fillings (determined to be due to a gas leak which as a consequence produced varying pressure during measurements, this was resolved for the second and third set of prototypes) and the process of de-stemming in the lamp production process (due to a marginal stem size increase to 6 mm for vacuum fittings, this was resolved through removal of the stem in stages to allow closure more gradually).
The practical problems described in the construction of stage 1 prototypes led to a significantly reduced number of functioning prototypes (Table 16) and thus the decision was made to use the increased proportional Te levels for design stage 2, due to the desirable lamp voltage (i.e. near 12V cm−1) produced by 18 mm Lamp I B and 15 mm Lamp II B. This meant that although slightly adjusted (due to the simplicity of not requiring balanced Te and I levels) the prototypes from stage 1 were re-built (Lamps III and IV) and tested with a third variant (Lamp V) with a reduced proportion of TeI4 to Te but with a combined high quantity of filling to attempt to produce a lamp with higher voltage. Following the completion of the second set of prototypes the lamp with the highest spectral output for both 18 mm (Lamp VI) and 15 mm (Lamp V) lamps was selected as the basis for stage 3 development. In addition, to identify the cause behind the similarities in lamp voltage produced a second set of lamp designs was produced with reduced fillings using one-fifth of the quantities of lamp fillings in stage 2. All lamp fillings are specified for stages 1, 2 and 3 prototypes and are displayed in Table 15.
The performance results from all 3 prototype stages are provided below in Tables 16 with related images to aid performance assessment being subsequently provided in
#2All lamps visually assessed based on a single set of Electrical measurements only.
#3Germ A % and Germ B % relates to the germicidal efficiency of the lamps when weighted with action displayed in FIG. 15.
#4The first set of electrical measurements from the second spectral scan was missing therefore 2nd set from first scan was used due to the short period of time between the scans
#5The first of four electrical measurements missing hence only one set of measurements was used for power calculation of the first spectral scan
Benchmark Hg Lamps—
The Hg based comparison lamps were made in a well-established process and were thus relatively simple to produce. The electrical performance of the lamps was extremely close and consistent (no greater then +/−3V) to that of the designed running voltage (120V). The lamps themselves ran well in respect of starting and stability with observed centralised arcs in both the 18 mm lamps (
The lamps both 15 mm and 18 mm provide a spectral output (
Stage 1—
The two initial prototypes illustrated that a lamp with a sustained plasma can be produced and run for a period of a least 20 min (the time limited by the need to carry out further scans rather than issues with the lamp), a voltage density of 9.57 V cm−1 can be produced (close to the comparative 12V cm−1 of the benchmark Hg lamps), and a non-stoichiometric Te and I lamp filling can be used to produce a functional plasma. The lamps that did not start could be visually identified as having halide dispersion near the stem removal which in conjunction with the fact that the lamps were unable to restart indicates the separation at least in part of the halogen into its elemental form.
Stage 2—
The functional yield of the second set of prototypes was increased to 75% largely due to improvements in lamp stem removal. This also enabled identification of halide residual in the lamp stem and lamp positioning post stem removal as the causes of the 25% of the failures. Lamps III, IV and also lamp V (containing a reduced percentage of TeI4 to Te) produced voltages in a narrow region between 85-95V. There was a marginal increase in voltage from lamp Ill to lamp IV for the 18 mm lamps however the difference was negligible between the lamps of 15 mm. The production of similar voltages rather than an expected change proportional to the amount of lamp filling used could indicate either a restriction of Te entering the gas phase cause by non-stoichiometric quantities of lamp fillings, or saturation of lamp filling in the gas phase, i.e. increasing the lamp filling will not result in further fillings entering the gas phase and a proportional increase in lamp voltage (hence the production of second lamp design with significantly reduced fillings in stage 3).
The germicidal efficiencies of the stage 2 prototypes were significantly lower than the design target, ranging from 0.4-0.9% (depending on lamp and germicidal weighting). This can in part be attributed to the spectral output produced for both 18 mm (
Stage 3—
The spectral outputs of all of the prototypes in stage 3 changed considerably, with numerous peaks developing throughout the previously established continuum in stage 2).
Both 15 mm and 18 mm lamps with design VI show a small but increased output below 220 nm however this is not the case with the 18 mm lamps. In fact in contrast to the proposed increase in lamp efficiency with Sb as a dopant the prototypes produced in stage 3 are lower than that of stage 2.
The lamp design VI for both 15 mm and 18 mm lamps was based on one-fifth of the lamp fillings for lamp VII however minimal change in voltage was measured especially for the 15 mm lamps. This indicates that the Te in the gas phase is saturated, however it appears I continues to enter the gas phase. This can be seen in the transition from the straight stable arc with a low visible output and no gas pockets to that of the final often turbulent lamp (as described in results stage 2). This was most clearly demonstrated in lamp 18 mm VII C shown in
When considering the overall results from design stages 1, 2 and 3 which at best has produced approximately one-tenth of the germicidal output compared to their Hg counterparts, and for the most part produced lamp arcs with erratic properties, particularly when close to the electrodes, it is clear that the design concept is far from being ready for production. However the research has enabled key theoretical design features of the lamp concept in its current state to have been verified. In addition the likely causes of the performance limitations of the prototypes were also identified and suggestions made as to how these can be addressed.
The prototypes lamps all produced a sustained high pressure plasma discharge produced an arc without the need for Hg as a filling. The lamps also produced a spectral continuum in the desired 200-300 nm spectral region and the lamp physical structure remained intact in all prototypes. These findings are not only novel but are critical characteristics of any future lamp to improve the performance of high UV density radiation sources. The challenge is how can the germicidal efficiency be increased and arc discharge stabilised, both of which have the same root cause.
The spectral output produced from the second set of prototypes displayed in
A key question was the halide stability above 600° C., specifically whether the reversible reaction between the formation and decomposition of TeI2 (2TeI2(g)⇄Te2(g)+2I2(g)) would either produce arc instability from 12 or condensation of Te. The inferred saturation of lamp filling suggests that within the plasma capacity for Te in the gas phase, condensation did not appear to be occurring, or if so, not to the detriment of the functionality of the lamp. In contrast I2 did appear to affect the stability of the arc and consequently the impedance of the plasma. This was supported by lamp design VI and VII which had lower outputs with Sb as a dopant and transitioned into a turbulent arc. This was due to the additional I from SbI3 which created increased turbulence near the electrodes and in the case of lamp 18 mm VII this extended the full underside of the arc. This would ordinarily be in itself a major design limitation due to the inability to maintain a halogen cycle however in this case two factors suggest that this is not the case. The first is the functional HP plasma even with the use of non-stoichiometric proportions of Te:I as lamp fillings. The second is the almost ideal characteristics of the lamp arcs during the majority of the lamp warm-up phases (Section 7.3.4). The capacity to be able to reduce the amount of I used to the point where it has little to no adverse effect on the arc stability and output above and beyond forming and maintaining the plasma will be critical on improving the performance of the lamp. The arc displayed during the warm-up phase, and intended to be reproduced permanently after lamp warm up with reduced ratios of I and reduced overall lamp fillings, displayed no visible turbulence and a minimal visible output (
The description of the two key current limitations and potential methods to address them for core functionality have been discussed, however to develop such a lamp for the commercial market will involve a number of additional development steps. This will likely include optimisation of the electrical lamp driver which may require optimisation of electrical frequency but will almost certainly require a configuration providing a higher strike voltage. The lamp strike voltage could be reduced with the use of a ‘penning gas’ replacing argon with a dual gas combination with differing ionization levels leading to a greater production of ions reducing the voltage required to strike the lamp. Conversely the use of an increased buffer gas pressure to increase the lamp impedance could be applied if the optimal pressure based purely on the Te lamp fillings is not high enough to achieve a suitable V cm−1 value (albeit with the consequence of increasing the strike voltage). Ultimately a number of subtle design iterations will be required following fundamental plasma improvements to produce a lamp to meet a design specification based upon market requirements.
The production of a lamp based upon a halide filling will require additional control in the manufacturing process due to their hygroscopic nature, however since they are currently used as additives to Hg based lamps with appropriate training and equipment this will easily mitigated. Te is industrially available for production (albeit for this investigation TeI4 was significantly harder to locate than elemental Te) however in a high purity form it is more expensive than Hg with example costs of Te being £3.78/g and Hg £1.26/g, (with Te costs based on 500 g 99.9999+% purity; Hg costs based on 250 g 99.999995% purity), which in the context of this study for a 18 mm ID Te lamp (Lamp 5 requiring 150 mg of Te) and 18 mm ID Hg lamp (lamp requiring 40 mg of Hg) would cost £0.57 and £0.05 respectively. This is based on the amount of fillings used during testing which as per the recommendations is likely to reduce with further development stages. Although the relative cost of Te is significantly higher than Hg, the cost per lamp is very low for both lamp fillings in respective of other lamp component costs e.g. the cost of quartz for both 18 mm diameter lamp examples being £13.00. The availability and cost of Te as a primary filling has practical promise.
The development stages presented in this work should enable what is currently a unique and novel plasma concept enabling a Hg-free production of UV radiation to that of high efficiency germicidal lamp for commercial applications. The upcoming ban (Minamata Convention on Mercury) on the use of Hg in a large number of products including visible lighting by 2020 including import and exports (excluding products where no alternative is available e.g. water disinfection) suggests a clear environmental motivation to reduce or remove the wide application of mercury. The potential increase in production costs caused by import/export restrictions could in conjunction with environmental factors drive the need for a Hg-free alternative and the development of LED's and DBD's in the low energy density range. Whether this is or will become a further driver re-emphasises the potential benefit of the proof-of-concept Te-based UV radiation source, which if further developed could have significant and far-reaching effects on the industry.
To address the suggestions from the conclusions from the initial research a further set of prototype lamps were produced (9 mm ID, 190 mm Arc Length and 2 mm Wall Thickness).
The filling details of each lamp are in the table below, spectral outputs of the lamps in the graphs following with key outcomes stated below:
In summary, a novel proof-of-concept Hg-free plasma enabling a high pressure UV discharge was produced. The germicidal efficiency of prototype designs were significantly lower than that of the Hg equivalents, however two fundamental limitations were identified as being the primary causes: excessively loaded lamps with plasma saturation and an iodine content greater than a stable UV-efficient discharge can contain. The study critically revealed that non-stoichiometric quantities of Te to I can be used whilst still producing functional lamp plasma that produces desirable electrical characteristics and a stable arc. With increasing environmental and economic drivers to produce a Hg-free, high efficiency and high power density lamp, the following recommendations are proposed to further develop the presented proof-of-concept into an applicable lamp for the water industry:
The results presented suggest a possible alternative to current Hg-based lamps as a source of UVC radiation. Although the results to-date provided approximately 1/10th the efficiency of current Hg lamp technology, this has been achieved without the use of Hg, with less toxic lamp fillings. It is anticipated that following additional rounds of improvements that the lamp efficiency will improve significantly and could increase beyond that of a conventional Hg lamp. If the efficiency of the lamp is increased above that of the conventional Hg lamp then a reduction in whole life costs and direct and indirect carbon costs will occur.
Excluding reactor design optimisation (e.g. hydraulic optimisation, lamp positioning etc.) the reactor efficiency will be determined by the efficiency of the lamp(s) in use. The findings provided are encouraging, offering the potential to not only provide the generation of UV radiation without Hg but also with a spectral output that is dominated by ER above 240 nm, ideal for the spectral application in all three validation protocols. Further development is required prior to production, however the unique selling point of the lamp being Hg-free and the potential of increased efficiency within currently applied technology (e.g. lamp geometries and lamp driver) makes a strong case for further investment.
Features of the present invention include:
It will be understood that the invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.
Reference numerals appearing in any claims are by way of illustration only and shall have no limiting effect on the scope of the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1609447.6 | May 2016 | GB | national |
This application is a Continuation of U.S. patent application Ser. No. 16/304,719 filed on Nov. 27, 2018, which is a National Phase of PCT Patent Application No. PCT/GB2017/051511 having International filing date of May 26, 2017, which claims the benefit of priority of United Kingdom Patent Application No. 1609447.6 filed on May 27, 2016. The contents of the above applications are all incorporated by reference as if fully set forth herein in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16304719 | Nov 2018 | US |
| Child | 16882780 | US |
