The Potential of Thermophotovoltaic Heat Recovery ...

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discusses TPV heat recovery in the glass industry. The work ..... likely to operate in an 8 to 24 hour cycle consisting of batch feeding, melting and the extraction of ...
The Potential of Thermophotovoltaic Heat Recovery for the Glass Industry T. Bauer, I. Forbes, R. Penlington and N. Pearsall Northumbria Photovoltaics Applications Centre (NPAC), School of Engineering University of Northumbria, Newcastle upon Tyne, NE1 8ST, UK, [email protected] Abstract. This paper aims to provide an overview of heat recovery from industrial hightemperature processes and uses the glass industry in the UK as an example. The work is part of a study of a range of potential industrial applications of thermophotovoltaic (TPV) in the UK being carried out by the Northumbria Photovoltaics Applications Centre. The paper partitions in the review of relevant facts about TPV technology and the glass industry, and afterwards discusses TPV heat recovery in the glass industry. The work identifies areas of use for TPV in the glass industry, which is assessed in terms of glass sector, furnaces type, process temperature, impact on the existing process, power scale and development effort of TPV. Knowledge of these factors should allow the design of an optimum TPV system. The paper estimates possible energy savings and reductions of CO2 emission using TPV in the glass industry.

INTRODUCTION AND MOTIVATION In modern industrial society the majority of energy is consumed in the sectors transportation, building and industry, and as primary source of this energy serve mostly fossil fuels. The use of fossil fuels has led to worldwide concerns about security of supply, increasing energy demand, limitation of resources and local and global environment impacts (e.g. acid rain, climate change 1). A consequence is increased interest in non-fossil fuel energy resources and the efficient use of fossil fuel. For fossil fuel combustion various methods can improve the efficient use of energy including methods like combined heat and power, energy management, new burner technology, improved insulation and heat recovery. Heat recovery is usually associated with heat upgrading systems such as heat exchanger or heat pumps. In practice the use of heat upgrading systems are often limited to applications where heat recovered and heat demand coincides since heat can be transported and stored only to a certain point. Heat recovery technologies, which generate electricity such as thermoelectrics or steam turbines combined with a generator offer a more flexible operation since electricity can be transmitted with lower losses compared to heat, the grid can act as a storage and electricity can be converted in other energy forms with high conversion efficiencies. This paper considers thermophotovoltaics as a heat

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It is thought that greenhouse gas emission causes global warming and climate change and an attempt is made to reduce this emission in a worldwide agreement (Kyoto Protocol). In the UK the Kyoto Protocol calls for the reduction of greenhouse gas emission to 12.5 % below 1990 levels by 2008 – 2012 [22].

recovery method to convert high-temperature heat into electricity to improve energy efficiency in the high-temperature industry. Classified by end-use, high-temperature processes account for the biggest part of energy consumption in the UK industry (Fig. 1) 2. The main high-temperature process uses of energy are coke ovens, blast furnaces and other furnaces, kilns, metal smelters and glass tanks [2]. 4.2% Mech. engineering

16.5% Space heating 34.8% High-temp. processes

1.4% Refrigeration 13.6% Motor and drivers

24.3% Low-temp. processes

1.4% Compressed air 7.8% Drying and separation

1.7% Potteries 4.2% Glass 10.8% Cement, lime and plaster 3.3% Bricks 6.7% Other minerals

60.0% Iron and Steel industry 9.2% Non-ferrous metals

FIGURE 1. Industrial energy consumption by end-use (left) and high-temperature processes by industry (right) in 1992 for the UK [2]

High temperature processes (Fig. 1, left) can be further classified by industry (Fig. 1, right). Most of these industries have energy-intensive processes operating at suitable for TPV operation with temperatures around or above 1000 °C [6,8]. Most high-temperature industries show a large diversity in terms of production volume, type of fuel, type of furnace, type of existing heat recovery method, nation specific processes and process temperature. For example the iron and steel industry uses various fuels 3, furnaces 4 [5] and existing heat recovery methods [8]. Although the glass industry is one of the smallest energy sectors (Fig 1, right), this industry was chosen as an example out of the high-temperature industries to examine TPV heat recovery systems. The glass industry was chosen because of its uniformity in terms of melting temperature (around 1500 °C), fuel 5 and type of furnaces. This paper consists of three parts, which review the relevant facts of both TPV (1) and the glass industry (2), and afterwards discusses TPV heat recovery in the glass industry (3).

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For this study worldwide data were not available, but it is thought that other industrial countries show similar distributions. In the UK, energy sources are electricity (40 %), coke (28 %), natural gas (28.5 %), oil (2%) and propane (1.5 %). 4 Usually melting is carried out in coke-fired cupolas, in electric arc or induction furnace. 5 Major fuel is natural gas with a share of 82 %. Other minor sources are oil 6 % and electricity 12 % [6]. In 1994 for the US a similar distribution is given: electricity (17.3 %), oil (1.6 %), natural gas (79.5 %) and others (0.8 %) [7]. 3

FACTS ABOUT THERMOPHOTOVOLTAICS Thermophotovoltaics (TPV) is the use of the photovoltaic effect to generate electricity from a high temperature thermal source [1]. Typically the temperature of a radiator (also sometimes named emitter) ranges from 1300 to 2000 K, which lead to a theoretical hemispherical total radiation per unit area of about 16 to 91 W/cm2 according to the Stefan-Boltzmann law (σ⋅T4). Photovoltaic cells can convert parts of the radiation below a certain wavelength into electricity and high wavelength radiation is usually reflected back to the heat source by any type of spectral control. The major photovoltaic cell materials in use for TPV are Gallium Antimonide (radiation below 1.9 µm), Silicon (1.1 µm) and Indium Gallium Arsenide (variable from 0.9 to 3.5 µm). Several combustion-based systems are in a prototype stage [1], which operate mostly in combined heat and power, or portable power applications, and only a few publications [1,3,4] suggest the use of TPV for high-temperature industrial heat recovery.

Characterisation Similar to combustion TPV systems, TPV heat recovery systems are also characterised by parameters like the total power (W), the power density (W/cm2), the efficiency and the costs (£/W). High power densities are required to improve the economics on the other hand high power densities are associated with high losses. Despite power densities as high as 2.5 W/cm2 are reported [18], in this work a value of 1 W/cm2 is seen as a reasonable compromise [1]. Unlike the case for combustion TPV systems, TPV heat recovery systems should be seen in the interaction of a given high-temperature process, rather than a stand-alone system. The attached TPV heat recovery system will extract energy of the given hightemperature process (Fig. 4). For each specific process the question can be asked, to what extent process changes occur, such as process temperature, product quality or fuel consumption, and whether they are acceptable?

Efficiency Usually, efficiency is defined as the useful output to the total input. Applied to TPV the useful output is commonly defined as the generated power 6 and the definition of the total input can be defined in different ways. For example the input for a combustion TPV system may be defined by the chemical fuel power entering the system, whereas the efficiency of the combustion can degrade the overall system efficiency. Other TPV systems exclude the heat source. For example, solar TPV systems are likely to be characterised by the total incident radiation onto the system (without the sun as the heat source). For TPV heat recovery systems, there are applications imaginable where even the radiator may not be defined as an inherent part of the system, since the process, such as a bulk of molten glass, may radiate with a

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For combined heat and power (CHP) the output may include additional the useful heat.

suitable infrared spectrum 7. The TPV heat recovery system efficiency hsys will exclude the heat source and possibly the radiator, which should lead to an inherently higher value of hsys compared to combustion TPV. The best current combustion based TPV systems under development are estimated to reach an overall efficiency (electricity output to chemical fuel power) in the order of 15 % [19,21]. TPV heat recovery will exclude the combustion efficiency (in the order of 80 %), therefore a value in the order of hsys = 20 % is seen as realistically achievable. A possible definition of hsys is given by the following equation, whereas Pel is the electrical output power and Pexheat the total amount of extracted heat from the process (Fig. 4). In practise, Pexheat is difficult to determine and can be replaced by the sum of all outputs of the TPV system (Fig. 4): Pel, the cooling power Pcool and the heat losses not ascribed to the cooling power Ploss (Equation 1). Usually the cooling power Pcool can be derived from the flow, the density, the heat capacity and the temperature change of the cooling fluid and for a proper designed system Ploss will be minimised and may be for some systems negligible. Pel Pel (1) = h sys = Pexheat Pel + Pcool + Ploss

FACTS ABOUT THE GLASS INDUSTRY Glass Industry Statistics TABLE 1. Overview of the glass industry in the UK in 1998 Furnace Sector [6] GJ per Furnace Typical Furnace Output [6] (share by Tonne Number Type and Melting (tonnes energy) [6] 8 [6] process per day) Container Tank, cross or end 14 150 – 300 9.0 (55%) fired, max. 1500 ºC Flat Tank, cross-fired, More than 3 9.2 (20%) max. 1600 ºC 300 Fibre Tank often with 8 20 – 300 9 15-25 (10%) electrical boosting Domestic Tank often with 3 150 – 300 20-30 (5.7%) electrical boosting Sc. & tech. 13 Day tank 1 – 3009 20-30 (8.3%) Giftware More Pot, day tanks, More Less than 1 (1%) than 30 900-1600 ºC than 50 7

Typical Heat Recovery Method

Typical Secondary Process

Regenerator

Forehearth

Regenerator

Float bath

Regenerator, recuperator9 Regenerator, recuperator Regenerator, recuperator Recuperator

Forehearth Forehearth Forehearth None

For the exclusion of heat source and radiator, the efficiency may be defined as the electricity output to the total incident radiation minors the backward reflected radiation to the heat source by the filter, whereas the efficiency becomes a function of the spectrum of the incident radiation. 8 The theoretical requirement to heat glass from room temperature to 1500 ºC is about 1.7 GJ per tonne. The theoretical energy required for the chemical reaction of the raw material is about 0.6 GJ per tonne (this energy is not required if exclusively cullet is used) [7]. It is noted that this values can differ for example due to different glass compositions [20]. For a modern furnace without cullet and an efficiency (heat transported to the glass to energy content of the fuel) of 40 % the values lead to an energy consumption of 5.75 GJ/tonne. It is noted that in literature other efficiency definitions are discussed [20]. 9 Value for the US market [7].

The major producers of glass are located in the US, Western European countries, Japan, China, Republic of Korea and the former USSR [17]. Glass production can be classified by the six sectors container, flat, fibre, domestic, scientific and technical, and giftware (Tab. 1) [6]. Flat and container are the two dominating sectors by energy use. In these two sectors glass furnaces tend to be large and efficient 10 (Tab. 1) and the industry is dominated by only a few often worldwide operating manufacturers. The giftware, fibre, scientific and technical and parts of domestic glass sector tend to show smaller production volume and work less efficient (Tab. 1) [6,7]. On a glass site the melting process in the furnace accounts for about 70 % of the total energy use [6,7]. The electricity demand on a glass site compared to the total site energy accounts for 10 - 30 % depending on the sector in the US and 14 % in mean for the UK [7].

Glass Production Process The upper part of Fig. 2 illustrates a generic type of a melting process. The batch consisting of the raw material, cullet (recycled glass) or a mixture of both is fed into the furnace. In the furnace the primary melting, the refining and conditioning phases take place sequently, before the molten glass product leaves the furnace. For large production volume tank furnace are applied (Fig. 3). In tank furnaces the batch is fed continuously on one side of the furnace (called doghouse) and the melting process takes place throughout the furnace and the molten glass is extracted on the other end, usually at the forehearth (Fig. 3). Tank furnaces show distinctive local process regions with constant temperature. For small production volume intermittently operating furnace are used, which are likely to operate in an 8 to 24 hour cycle consisting of batch feeding, melting and the extraction of the glass. The giftware sector often uses a refractory pot inside the furnace, which is externally heated and the temperature varies for melting (1300 – 1600 °C) and processing (900 – 1200 °C) of the glass. For large production volume and intermittent operation it is likely to use tanks (day tanks) instead of pot furnaces, which are similar in construction to tank furnaces [7]. Both intermittently and continuously operating furnaces operate usually 24 hours per day and sometimes electricity is used to boost the production volume 11. For electrical heating usually rods are introduced in the furnace and glass acts as an electrical conductor, which leads to an inherently high conversion efficiency from the electricity to the heat in the glass. The most common furnace type for large production volume is a regenerative natural gas-powered tank furnace. The largest among them are float glass furnaces, which have a production volume of up to 2000 tonnes glass per day and the area of 100 to 13 meters can contain up to 2500 tonnes of molten glass. An example of an end-fired regenerative furnace is shown in Fig. 3 and the estimated 12 energy distribution of a typical modern furnace is shown in Fig. 2 [7,9]. The regenerator requires an alternating cycle of the combustion air direction. The combustion air 10

Energy consumption per tonne of produced glass is a measure of the efficiency of a glass furnace. There are also furnaces with pure electrical heating in existence The percentage values given in the Fig. 2 may vary about ±10 % due to aspects like differences in optimisation and age of the furnace, the glass type, air temperature, the cullet to raw material ratio and the moisture content of the batch.

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passes the first regenerator and is heated up. Afterwards, natural gas together with the preheated combustion air is burned in an U-shape flame above the molten glass bulk and the flue gas leaves the furnace through the second regenerator (Fig. 3). After about 15 to 30 minutes the direction of firing is reversed and the second regenerator preheats the combustion air [14,11]. The energy content of the fuel (assumed to be 100 % in Fig. 4) is only partly transferred to the glass, other parts are lost in the structure (mainly through the furnace walls) and carried out by the flue gas. For this work, no data about the energy distribution of intermittently operating furnace were available, but it is thought, that a similar energy distribution exists as shown in Fig. 4. There are three areas where a TPV heat recovery system could be placed, which are secondary processes, the furnace or the regenerator (Fig. 4). These areas are separately reviewed in the next three sections. Cullet Furnace (Melting Process)

Batch

Molten Glass Secondary Processes

Heat Losses Pheat Electricity Pel

Raw Material

40 %

Furnace and Regenerator Structural Losses 30 %

Extracted Process Heat Pexheat

Fuel 100 %

TPV Heat Recovery System

Cooling Demand Pcool

Flue Gas Losses 30 % 150 %

80 %

Air 50 %

Regenerator

Air in 0%

FIGURE 2. Sankey diagram of typical natural gas-powered regenerative tank furnace8 and showing where TPV system could be sited for heat recovery12. Regenerator Port

Furnace Arch Tank Crown Furnace

Bridge Wall

Burner Air inlet and flue gas outlet

Regenerator Chamber with Checkers

Doghouse

Forehearth

FIGURE 3. End-fired 13 regenerative tank furnace with forehearth [10] 13

Beside end fired furnaces there are also cross-fired furnaces in existence, where on or more pair of regenerators are located on each side of the tank.

The Furnace The tank furnace is surrounded on all sites by refractory walls (Fig. 3) and comprises three distinctive semitransparent areas, which are the molten glass bulk 14 on the bottom, the luminous flame above and the surrounding hot gas 15. Heat transfer is dominated mainly by radiative heat exchange resulting in a net radiation heat transfer from the luminous flame, the hot gas and the furnace walls (mainly the crown) to the glass [13]. For both, the molten glass and the hot gas/flame region, the incident radiation heat flux at the inner furnace wall is high. Assuming grey-body radiation (ε⋅σ⋅T4) 16 for the molten glass bulk [12], the hemispherical total radiation flux is estimated as about 50 W/cm². In the crown area the incident radiation heat flux depends on the location and whether firing is carried out or not. For a flat glass furnace values between 32 and 71 W/cm² are reported [14], whereas the radiation is spectrally selective [13]. Most of the incident wall radiation is reflected or reradiated into the furnace and only a small fraction in the order of 0.1 to 1.8 W/cm² [11,15,16,20] is lost through the walls. The lower value in the order of 0.1 W/cm² can be achieved by means of an insulation layer on the outside. The problem associated with insulation is that the outer refractory temperature increases from a few hundred degrees to temperatures above 1000 °C [11,20] this higher refractory temperature will result in advanced refractory corrosion. Therefore, the insulation of a furnace is a compromise between energy losses through the wall (1), and the furnace lifetime limited by refractory corrosion and insulation costs (2) (a typical furnace lifetime may be given by 10 years [6]). Some areas of the furnace like the crown 17, the sidewall at the glass line or the throat 18 are especially susceptible to refractory corrosion and sometimes cooling is applied instead of insulation. Interruption of cooling due to power failure can be critical for furnace operation [20,11]. It is noted that for some furnace areas higher wall losses are deliberated to reduce glass temperature, an example is the conditioning zone of the float glass furnace [15,20]. Pot furnaces have typically one opening for batch feeding and glass extraction, which may be closed by a flap. Pot furnaces show some diversity in terms of the number of pots, and the arrangement and number of the burner.

The Regenerator And Recuperator The most common method to improve the energy efficiency of tank furnaces is the use of regenerators (Fig. 3), which can partly transfer heat from the flue gas to the combustion air. In the Sankey diagram (Fig. 2) typical values of reused (50 %) and wasted (30 %) energy are shown. It is estimated, that typically value of reused energy 14 The bulk of molten glass in practical furnace dimension can be regarded as optically thick (the product of physical layer thickness and absorption coefficient is bigger than 4) and a grey-body radiator with an emissivity of about 0.9 [12]. 15 The volume of gas in practical furnace dimension can be regarded as optically thin, where some spectral regions are nearly transparent and others are strongly radiating and absorbing. The selectivity is mainly caused by vibration and rotation bands of CO2 and H2O [13]. 16 Emissivity ε = 0.9 [12], Stefan-Boltzmann constant σ=5.671 ·10-8 W/m2K4, absolute temperature for soda-lime glass T=1773 K 17 In the crown silica bricks are used, which operate near their softening point at 1620 °C. Silica brick are used, because they avoid contamination of the molten glass. 18 Some furnaces are split into melting and working end, which are connected by the throat whose primary function is to accommodate the relatively large temperature difference between these two sections.

of 50 % can be further increased to a theoretical maximum limited of about 58 % 19. However, this maximum value is in practise not strived, because of increased costs for large regenerators and increased production of NOx due to high air preheating temperatures 20. The fact, that a part of the energy in the regenerator is usually wasted 21, is also reflected in the flue gas temperatures leaving the regenerator, which is still in the order of 300-650 °C [7,20]. Recuperators are often used for intermittently operating furnaces. In the glass industry, recuperator reach usually a lower efficiency of heat recovering than regenerator, because the recuperator materials in use limit the air preheating temperature. It is thought that in future new recuperator, for example from silicon carbide or silicon nitride, can be used, which withstand higher temperatures [9].

Secondary Processes In continuously operating furnaces the molten glass leaves the furnace through the forehearth (Fig. 3), for further processes such as blowing, pressing, casting or drawing. The only exception is the float glass process, which does not require a forehearth. In addition to the feeding of the glass for further processing, the forehearth has the function of thermal homogenisation of the glass (heating and forced cooling is carried out) to reach a high glass quality. The glass temperature at the forehearth outlet (around 1100 °C [7]) and forehearth wall losses (around 0.1 W/cm2 [16]) are low, if compared to the furnace. The average energy consumption of forehearths accounts for 5.6 % of the total energy consumption on a glass site in the UK [6]. Today, the float glass process produces nearly all of the flat glass, where the molten glass is poured continuously from the furnace onto a bath of molten tin. The molten tin is held on a temperature gradient with high-temperature at the entrance (1000 ºC) and low temperature at the exit (600 ºC) where the glass has reached dimensional stability. To prevent oxidation of the tin bath the process takes place in a chamber in an enriched hydrogen and nitrogen atmosphere [11]. The energy use in the float process for heating the tin bath and the chamber above the glass accounts for a mean value of 12 % of the total site energy in a float glass site in the US [7].

DISCUSSION OF A TPV SYSTEM IN THE GLASS INDUSTRY Furnace The furnace refractory walls offer large areas for TPV implementation, where there may be two possibilities to incorporate a TPV heat recovery system.

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The heat content of the preheated air to the heat content of the flue gas is given by a theoretical maximum of 72 % [9,20]. In the UK, stringent NOx emission limits lead partly to lower air-preheating, which increases the waste heat of the regenerator. There are a few glass sites, which are equipped with steam boilers for heat recovery. Sometimes the steam is used for electricity generation via a steam turbine, but the use is usually limited to multi-furnace sites in the UK [6]. 20 21

The first possibility may be, that the molten glass bulk 22 would act as a radiator and the TPV heat recovery system would ideally consist exclusively of a filter and a photovoltaic cell. It is thought that this kind of operation may allow the use of Silicon photovoltaic cells, if a suitable filter can be designed. For all other areas discussed in this work it is thought that Gallium Antimonide and Indium Gallium Arsenide are the more probable cell materials, because of the lower temperature range, which requires long wavelength (low bandgap) photovoltaic cells. For the implementation of TPV, the corrosive atmosphere, possible contamination of the glass due to TPV and the high radiation level inside the furnace as well as the structural importance of the refractories for the furnace (especially the arch crown) may cause difficulties. The second possibility may be to incorporate the TPV heat recovery system inside the wall. For an insulated wall, the temperature at the join between refractory and insulation is found still in the order above 1000 °C, which allow TPV operation. For both attempts the principal problem is seen in the small wall heat losses (commonly in the range of 0.1 to 0.3 W/cm² [15,16,20]) compared to the necessary heat flux to operate TPV (a reasonable value is seen as 5 W/cm²) 23. To overcome this disagreement it is possible to accept higher wall losses (resulting in higher fuel consumption), to accept lower power densities for TPV (resulting in higher TPV costs) or possibly to include any means to concentrate the heat flux locally for TPV operation and at the same time to keep the wall heat losses in mean ideally at the previous level (0.1 to 0.3 W/cm²). Areas in the furnace with increased losses (higher heat flux) but often limited area for TPV use are the sidewall at the glass line, the throat, parts of the conditioning zone of a float glass furnace or openings. Especially pot furnaces show a relatively large opening compared to their furnace size. Openings are usually easily accessible and can be applied for the test and the launch of TPV heat recovery systems. The redesign of burners with an including TPV device is another possibility of TPV use where the available area respectively maximum power output is limited.

The Regenerator and Recuperator The conditions for TPV operation differ markedly between the furnace area and the regenerator and recuperator area. In the furnace area the aim is to minimise wall losses, whereas in the regenerator and recuperator a part of the available heat is used for air preheating and the other part is usually wasted. Because of these conditions the efficiency of TPV may be in the regenerator and recuperator less critical than in the furnace. The right range of temperature for TPV operation is usually given in the region where the flue gas enters. TPV is likely to require a radiator, because of low and spectral selective radiation along with the contamination of the flue gas. Possible difficulties are seen in non-uniform temperature zones and the cycling of the temperature (15-30 minutes for the regenerative tank furnaces and daily for pot and day tank furnaces), and any type of storage for TPV operation may be required to bridge the thermal cycling. Additionally, the extraction of heat by the TPV heat 22 In the hot gas and flame region, it is difficult to estimate the incident radiation power within a spectral range, since the hot gas, the flame and the refractory bricks behave all spectrally selective [13]. 23 An efficiency hsys = 20 % and Pel per area 1 W/cm2 is assumed.

recovery system may result in a mean lower working temperature of the regenerator and recuperators, which could require a larger dimensioning of them.

Secondary Processes Forehearths are usually easily accessible and the heat arise from cooling is usually wasted and could be recovered by TPV. However, the extent and the arrangement of the cooling depend on the following forming process and may be subject of change within one furnace and for different furnaces. The available waste heat limits the use of TPV to small-scale application. It is thought that in terms of TPV use float glass chambers can be considered in a similar way as furnaces. Drawbacks for the use of TPV are the partly to low temperature, the limitation of the amount of available energy and the limitation to one glass sector (flat glass).

TPV Contribution to Energy Efficiency on a Glass Site A lot of possible TPV areas discussed so far are likely to generate a negligible portion of electricity compared to the overall energy consumption of the glass furnace. Such applications will only little improve the energy efficiency on a glass site, but they can be used to launch TPV technology and to offer a reliable grid-independent power supply on the glass site for power failures. In particular openings of small pot furnaces are thought to be ideal for testing and introduction of TPV technology. Technologies to improve energy efficiency of glass tank furnaces currently discussed and partly implemented are for example oxygen-fuel firing, batch preheating from flue gas, optimised combustion systems, increased furnace wall insulation and the use of cullet [6,9]. The potential to save melting energy of such methods was estimated without a rebuild of the furnace as 2 % and with a rebuild (long-term) as up to 20 % [9]. It should be noted, that the improvement of energy efficiency of these techniques is reached usually by reduced gas consumption or higher glass output. In contrast, electricity produced by TPV has a higher value 24 and can be used on-site without transmission and distribution losses. It is assumed that a TPV heat recovery system in long-term, which produces 4 % electricity of the melting energy, can compete with other heat recovery methods. Assuming a TPV power density of 1 W/cm2 [1], a electricity generation of 4 % of the melting energy and a furnace of 100 tonnes with the typical energy distribution as shown in Fig. 48, it is estimated that a TPV system would have a power output of about 270 kW and would require a hightemperature area of 27 m2. Such area is available at the furnace walls (and float glass chamber) or the regenerator respectively the recuperator. In the Sankey diagram the electricity generation of 4 % of the melting energy, would correspond to the use of 1/3 of 60 % waste heat (Fig. 2) assuming a TPV system conversion efficiency hsys = 20 %. Assuming furthermore, that 70 % of the energy on a glass site is used for melting, it can be concluded that TPV could provide about 3 % of the glass site energy in the form of electricity. Usually all electricity produced by TPV can be used on-site, because statistically at least 10 % of the energy 24

In the UK, cost of industrial electricity per unit energy is about 6 times higher than industrial natural gas [23].

on a glass site is required as electricity [7]. The total energy related 25 CO2 emission of the glass industry is estimated as 1.87 million tonnes of CO2 per year in 1998 [6]. If a conversion factor of 122.2 kgCO2/GJ for electricity generation and 3 % of the total annual energy consumption of the UK glass industry of about 30 PJ is assumed, the possible CO2 saving by using TPV accounts for 0.11 million tonnes of CO2 per year (approximately 6 % CO2 saving). The low values of energy (3 %) and CO2 (6 %) savings should be seen in the context of the potential of higher efficiencies of TPV (for example through combined heat and power operation), the potential of other hightemperature industries and current developments in the glass industry like oxygen-fuel firing 26, NOx limitations or carbon and energy taxes, which make the use of TPV more likely.

RESULT AND CONCLUSION The potential of thermophotovoltaic (TPV) to improve energy efficiency in the glass industry has been investigated. For this purpose relevant facts of the thermophotovoltaic technology and the glass industry were reviewed. Various areas with a suitable temperature for TPV were discussed, which can be split in small-scale or large scale applications. The small-scale application the sidewalls at the glass line, the throat, the float glass conditioning zone and float glass chamber, openings, the redesign of burner and the forehearth can be used to launch TPV technology and to offer a reliable grid-independent power supply on the glass site for power failures. The large-scale application furnace walls, regenerator and recuperator are possible areas to improve the energy efficiency on a glass site. Tab. 2 summarises all the areas. TABLE 2. Assessment of areas for TPV use Small or Area of TPV large Typical furnace use scale and process applic. Furnace walls

Large

All furnaces

High

Regenerator

Large

Tank furnace

Low

Recuperator

Large

Pot and day tank furnace

Low

Small

Float glass process

High

Small

Float glass furnace

High

Small

Tank furnace

High

Small

Tank furnace All without pot and float glass furnaces Most furnaces

High

Float glass process Conditioning zone wall Sidewalls at the glass line Throat

25 26

Impact on the existing process

Forehearth

Small

Openings

Small

High Low

Process temperature range and cycling Local const. temperature regions Varying temperature, 20-30 minutes cycle Varying temperature, daily melting cycle Local const. temperature regions Local const. temperature regions Local const. temperature regions Const. temperature Local const. temperature regions Const. temperature

Develop. Effort Long Medium Medium Long Medium Medium Medium Medium Short

About 0.56 million tonnes of CO2 emission comes from batch gas, which is not included here. Oxygen-fuel firing furnaces mostly do not use a regenerator or recuperator, which make the whole flue gas heat accessible.

Redesign of the burner

Small

All combustion furnaces

Low

Location and time varying temp.

Medium

In this paper the implementation of TPV in the glass industry were discussed. If the difficulties can be overcome, the large-scale use of TPV heat recovery on a glass site is realistically estimated to provide in mean 21 % of the electricity and reduces energy related CO2 emission by about 6 %. It is thought that other high temperature industries can be assessed with a similar methodology and show similar potential as in this work.

REFERENCES 1. Coutts, T. J., “A review of progress in thermophotovoltaic generation of electricity” in Renewable and Sustainable Energy Reviews 3, 1998, pp. 77-184 2. “Energy Consumption in the United Kingdom (Energy Paper 66)”, Department of Trade and Industry, UK, 1997, pp. 43-48 3. “Glass Project Fact Sheet - Thermophotovoltaic electric power generation using exhaust heat”, U.S. Department of Energy, Office of Industrial Technologies, Energy Efficiency and Renewable Energy, 2001 4. Fraas, L., Avery, J., Daniels, W., Huang, H. X., Malfa, E., Testi, G., “TPV Tube Generators for Apartment Building and Industrial Furnace Applications”, 17. European Photovoltaic Solar Energy Conference, 2001 5. “Energy Consumption in ferrous foundries (Second edition – Energy Consumption Guide 48)”, ETSU, Energy Efficiency Best Practice programme (EEBPP), 1999 6. “Technology Benchmarking in the UK Glass Sector (General Information Report 75)”, ETSU, Energy Efficiency Best Practice programme (EEBPP), 2000 7. “Glass Industry of the future, Energy and Environmental Profile of the U.S. Glass Industry”, Office of Industrial Technologies, Energy Efficiency and Renewable Energy, US Department of Energy, 2002 8. “Energy Efficient Technology in High Temperature Industries”, Centre for Renewable Energy Sources for the European Commission Directorate-General for Energy, 1998 9. Beerkens, R., “Application of Energy Saving Technologies for Glass Furnaces: A comparative study”, in Improved technologies for the rational use of energy in the glass industry, Fachinformationszentrum Karlsruhe, 1992, pp. 325-339 10. “Holloware Glass Furnaces”, Fused Cast Products Division, Product Catalogue, SEPR, France 11. Tooley, F. V., “The Handbook of Glass Manufacture”, Vol. 1 and 2, 3. Edition, Ashlee Publishing, 1984 12. Gardon, R., “A Review of Radiant Heat Transfer in Glass”, in Journal of The American Ceramic Society, Vol. 44, No. 7, 1961, pp. 305-312 13. Wieringa, J. A.; Elich, J. J. Ph.; Hoogendoorn, C. J., “Spectral effects of radiative heat-transfer in hightemperature furnaces burning natural gas”, Journal of the Institute of Energy, Vol LXIII No 456 Sep., 1990, pp. 101-108 14. Hayes, R.R., Brewster, S., Webb, B. W., McQuay, M. Q., Huber, A. M., “Crown incident radiant heat flux measurements in an industrial regenerative, gas-fired, flat-glass furnace” in Experimental Thermal and Fluid Science 24, 2001, pp. 35-46 15. “The Use of Refractories for Flat Glass Furnaces”, SEPR Group, 2000 16. “The Use of Refractories for Container Glass Furnaces”, SEPR Group, 2000 17. Darnay, A. J., “Manufacturing worldwide: industry analyses, statistics, products, and leading companies and countries”, Gale Research, 1999, pp. 452-453 18. Fraas, L.; Groeneveld, M.; Magendanz, G.; Custard, P., “A single TPV cell power density and efficiency measurement technique”, 4. NREL Conference on Thermophotovoltaic Generation of Electricity, 1999, pp. 312-316 19. Fraas, L. M., Avery, J. E., Han Xiang Huang, “Thermophotovoltaics: heat and electric power from low bandgap solar cells around gas fired radiant tube burners”, 29. IEEE Photovoltaic Specialists Conference, 2002 20. Trier, W., “Glass Furnaces, Design construction and operation”, translated by K. L. Loewenstein, Society of Glass Technology Sheffield, 1987 21. Horne, W. E., Morgan, M. D., Sundaram, V. S., Butcher, T., “A 500 Watt Diesel Fueled TPV Portable Power Supply”, 5. NREL Conference on Thermophotovoltaic Generation of Electricity, 2002 22. “Climate Change The UK Programme”, Department for Environment, Food & Rural Affairs, 2001 23. “Quarterly Energy Prices”, Department of Trade and Industry (DTI), National Statistics, UK, pp. 28-29, June 2001