a high pressure window for volumetric solar receivers

A HIGH PRESSURE WINDOW FOR VOLUMETRIC SOLAR RECEIVERS

J. Karni, A. Kribus Environmental Sciences and Energy Research Dept. The Weizmann Institute of Science Rehovot 76100, ISRAEL B. Ostraich and E. Kochavi Rotem Industries Ltd. P.O.B. 9046, Beer Sheva 84190 , ISRAEL

Submitted to: J. Solar Energy Engineering June 18, 1997 Revised, February 1998

Abstract The absorbing matrix of a Volumetric (Directly Irradiated) solar receiver must be exposed t o the concentrated incoming sunlight. Most applications require that the receiver operates at an elevated pressure and in many cases the working fluid is not air. These requirements can be met only if the receiver is equipped with a transparent window. A novel Frustum-Like High-Pressure (FLHiP) window, made of fused silica, is presented. Optical, mechanical and thermal analyses, over a thousand hours of accelerated life-time tests, and several hundred hours of tests in a solar receiver, show that this window satisfies the required criteria for operation in a volumetric solar receiver, whose operating pressure and peak absorber temperature reach 30 bar and 1700°C, respectively.

1 Introduction In a solar-thermal system the conversion of sunlight to a ‘useful’ form of energy takes place in a Solar Receiver. This is a key component of the system where concentrated light, provided via an optical concentrator, is absorbed and its energy is transferred to a working medium (gas, liquid or solid particles), either in a thermal or a thermochemical process. The receiver design depends on the operating ranges of temperature, pressure and radiation flux. When the energy conversion process requires high temperatures, the concentration of the light must also be high, to minimize reradiation losses. In collecting and concentrating systems such as a parabolic dish or a central receiver, the concentration ratio is typically high, 500–10,000 times that of normal incident sunlight reaching the earth. The receiver working temperatures are also quite high: ranging from 500 to 1,300°C for various applications. Various receivers have been proposed and tested over the last three decades. Overviews of this technology are provided by Winter et al., (1991), De Laquil et al., (1992) and Becker et al., (1995). It is generally agreed that a Volumetric receiver design is the preferred choice for high-

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flux, high-temperature applications. A Volumetric (Directly Irradiated) receiver is a type of receiver where the working fluid either absorbs the radiation directly, or is in contact with a solid surface which absorbs the radiation. In the latter case, the absorbing surface may be a stationary matrix, or moving particles. An overview of volumetric receivers is given by Karni et al. (1998). The main advantage of these receivers is the ability to absorb high solar fluxes and operate at high temperatures, while using a relatively simple and compact design. In some directly irradiated receivers the absorbing matrix is exposed to the ambient. In most applications, however, the receiver operates under elevated pressure (or vacuum), and/or utilizes other working gases besides air. In these cases the receiver must be equipped with a transparent window, which enables the irradiated sunlight to enter the receiver cavity, while preventing the working fluid and much of the reradiation emitted in the cavity from escaping to the ambiance. Clearly, without a window the use of volumetric receivers is very limited. Many attempts at making a receiver window were reported. An early windowed receiver, at

ambient pressure, was built and tested by Hunt (1979). A concept for a pressurized windowed receiver/reactor was first proposed by Flamant and Olalde (1983). Tests of windowed receivers and receiver-reactors are described by Buck (1990), Pritzkow (1991), Anikeev et al. (1992), Posnansky and Pylkkänen (1991), Posnansky and Pylkkänen (1992), Buck et al. (1996) and Abele et al. (1996). These studies demonstrated that the window poses a very difficult design problem because it must be highly transparent, yet strong and durable at high temperatures. The attempts to develop a workable windowed receiver have achieved lower operating pressure (only up to 3 bar) and temperature (up to 1000°C) than originally planned. Consequently, the lack of a suitable window, capable of withstanding higher pressure and temperature, was considered the main drawback of the Directly Irradiated (Volumetric) receivers (De Laquil et al., 1992). Recently, Karni et al. (1997) tested a volumetric, windowed receiver (the DIAPR) that operates at aperture flux of up to 10 MW/m2 , a pressure of 10–30 bar, and air exit temperature of up to 1300°C. A Frustum-Like High-Pressure (FLHiP) window, made of fused silica was used in 3

the DIAPR. Extensive analysis and testing have shown that this window is highly durable and can sustain the most severe working conditions demanded in solar driven power cycles. The development of the FLHiP window is described in the present article.

2 Window Design 2.1 Design Requirements The purpose of the window for a pressurized, directly irradiated receiver is to separate the receiver cavity from the ambient air and allow operation at high pressure, while minimizing reflection, reradiation and convection losses. An effective window must satisfy all of the following criteria over a long period of operation, i.e., thousands of heating-cooling cycles and hundreds of pressurization-depressurization cycles: •

Good optical properties: minimize reflection and absorption of incoming light.

•

Mechanical strength: ability to endure stresses caused by the receiver operating pressure and temperature conditions.

•

High, variable working temperatures: operate at a continuous window temperature of up to 800°C, peak temperature of 1000°C and thermal gradients of up to 25°C/mm.

•

Reliable, stress-free installation and sealing: prevent placement–induced stresses and leaks of the pressurized working gas.

•

Cooling capability: inner receiver temperatures could reach 1500–1700°C, i.e., a few hundred degrees higher than the maximum allowable window temperature; therefore, the window must be cooled.

•

Prevent dust accumulation: settling of dust could reduce the window optical performance and cause overheating.

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•

Simple, low cost production: the window should be made in a relatively simple method, using inexpensive material.

Because of its optical, mechanical and thermal properties, fused silica (fused quartz), a vitreous material which is readily forged and used ordinarily in the furnace industry, has been the material of choice in many studies. Our design of a fused silica, Frustum-Like High-Pressure (FLHiP) window for a directly irradiated receiver, is presented in Figure 1. This design satisfies all of the above criteria for the expected working environment. As we will show in the following sections, other window configurations may meet some of the above criteria, but not all of them.

2.2 Optics of the FLHiP window The optical performance of the window is quantified based on its optical efficiency, defined as the fraction of the irradiation energy reaching the receiver aperture, which is transmitted through the window into the receiver cavity. Optical performance analyses of various window configurations were conducted by Heller (1991) and Kribus (1994). Since the absorption coefficient of fused silica for solar radiation is very low, and a practical window thickness would not exceed 10–20 mm, the only significant irradiation losses associated with the window are due to reflection. A flat window over the receiver aperture would lose about 8% of the irradiation by reflection. As shown in both of the above studies, concave shapes such as a section of a sphere, paraboloid, straight cylindrical tube or a cone, which provide for multiple ray impingement on the window, significantly improve its optical efficiency. Kribus (1994) calculated the transmitted fraction for various conical window configurations using ray tracing; the receiver–window geometry he implemented is shown schematically in Figure 2. He found that a window transmission of over 99% is possible in a fairly broad range of geometric parameters. In this range, only up to about 3% of the incoming radiation reaches the Back Plane Reflector, therefore it has only a minor effect on the overall performance, even at a moderate reflectivity. This good performance across a wide range of cone dimensions implies

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that the input-side optical considerations (ignoring re-radiation) are not a limiting factor in the design of a conical window. Furthermore, the optical (transmission) efficiency can be fitted to a single parameter γ (Figure 2b) representing the window’s geometry. This simplifies considerably the design process for a conical window, since it is possible to vary the cone dimensions (e.g. t o satisfy mechanical design requirements), while keeping the parameter γ unchanged, thus keeping the window’s optical performance constant. Note that a straight tube is the limiting case of the frustum-like shaped window; to keep γ constant, a straight tube window must be relatively long. Like many other transparent materials, fused silica has high transmittance to sunlight (in the range of 0.3 < λ < 3 µm), but its transmittance drops sharply as the wavelength λ increases beyond 3 µm, making it completely opaque for radiation at wavelengths larger than about 5 µm. Therefore a fused silica window absorbs a part of the reradiation emitted from the hot receiver components (i.e., the absorber). If some of this absorbed energy is transferred back to the working flow (see section 2.6), then the window becomes a partial radiation shield, thus significantly reducing reradiation losses from the receiver cavity.

2.3 Mechanical Strength Since all the candidate transparent materials are brittle in the operating temperature range, the first and foremost design problem is to avoid stress-induced failure. The theory of brittle materials failure was introduced by Inglis (1913), expanded by Griffith (1920, 1924), and extensively studied since then (Tipper, 1962; Gordon, 1976; Lawn, 1993). This theory and supporting experiments indicate that a typical brittle solid contains tiny slit-shaped flaws (‘microcracks’) and/or other centers of heterogeneity. Due to these microcracks, the opening mode of failure, which corresponds to separation of crack walls under the action of a tensile load normal to the crack plane, is by far the most germane to crack propagation in brittle materials. This mode of failure cannot occur under compressive load.

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Ample observations confirm that materials that are brittle in tension usually exhibit ductile behavior in compression; see for example Hockey (1983), Stavrinidis and Holloway (1983), Evans and Faber (1984) and Stevens (1987). The result, as can be seen in common Physical Properties Tables, is that brittle materials are at least one order of magnitude stronger in compression than in tension. For example, according to technical brochures of Corning Glass 9

Works and GE Quartz Products, the design compressive strength of fused silica (over 1.1x10 Pa) 7

is more than 20 times higher than its design tensile strength (~5x10 Pa), and about 2.5 times higher than the design strength of carbon steel. The window should therefore be designed such that it is entirely under compression, during any possible operating conditions. The frustum shape of the FLHiP window and the method used to mount it ensure that the combined effect of the receiver pressure and thermal load generates only compressive stresses. This was verified in calculations (section 3) and supported by tests at a pressure of up to 55 bar (section 4). Other concave windows, e.g., a spherical or a paraboloid section, and a straight tube, could also be designed for a solely compressive load when the receiver cavity is pressurized. Other failure modes such as creep were not analyzed in detail, but the long-term cycling tests (section 4.3) did not indicate susceptibility to this mechanism.

2.4 Thermal and Longevity Requirement The allowable operating temperature range and thermal gradients depend on the window material. An important reason for choosing fused silica as the window material is its working temperature, which is higher than that of other vitreous substances: it may sustain peak temperatures of up to 1050°C and can be used continuously up to a temperature of 850°C. At these conditions it has a very low devitrification rate. The FLHiP window durability and longevity was studied experimentally. This included pressure loading (section 4.1); combined pressure and thermal load under solar input (section 4.2); and accelerated lifetime thermal and pressure cycling (section 4.3). 7

2.5 Mounting and Sealing The window mounting and sealing technique is shown in Figure 1. The seals between the window and the metal housing are made of a commercial high-temperature gasket material: Klingerit Royal CAF-C, made by Rich. KLINGER GmbH in Germany. This material is rated at maximum temperature of 550°C and maximum pressure of 200 bar. The seals are pressed tight t o the window’s end rims as the receiver cavity is pressurized. The seals were tested successfully with the window at pressures of up to 55 bars. Static leak rate of the entire vessel was up to 3% pressure loss (1.0 bar when initially pressurized to 31 bar) during a 30 minute period. This leak rate is insignificant for air heating, but may require additional optimization of the seal design for other applications involving toxic or inflammable gases. This installation and sealing method has several important aspects: •

The bellows eliminates the possibility of stresses development in the window due to either misalignment during installation, or different thermal expansion of the stainless steel receiver housing and the window when the receiver is heated.

•

The bellows diameter is smaller then the narrowest window diameter, which is attached to it. When the receiver cavity is pressurized, the pressure is applied on the window and the bellows from the outside. Hence, the seal of the narrow window end is pressed axially onto the window. This load is also transmitted to the large diameter seal, which is pressed additionally by the axial component of the pressure load on the conical section of the window. The receiver pressure is therefore used for sealing the window on both sides. This method of mounting also creates compressive stress components in the window, along its axial and radial directions.

•

The short, straight cylindrical sections near the window ends assure that the force on the seals is parallel to the longitudinal load applied on the window, thus preventing window failure due to a shear stress or a bending moment near its ends.

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•

The centering O-rings, which are made of soft high-temperature alumina-silica fibers, are not used to provide sealing. Together with the bellows, which is slightly compressed during assembly, they hold the window in a center position, preventing contact with metal parts, during assembly and when the receiver is depressurized. This feature is not possible with spherical, parabolic or other single-rim windows. Consequently, such windows are commonly furnished with a flange at their rim. The stress pattern near a flange is complex, and tensile stresses can not be completely avoided over the entire desirable range of operating conditions.

•

The window installation technique and overall receiver design allow for a very simple and easy replacement of a window; in the present 50 kW DIAPR model it takes two people about 30 minutes. We estimate that in a multi-megawatt commercial system, window replacement may take a team of 2–3 people about 1–2 hours.

2.6 Cooling and Prevention of Dust Accumulation As explained in section 2.2, the window absorbs part of the reradiation emitted from the receiver. Moreover, the hottest regions in the receiver can reach up to a 1000°C higher than the window’s upper continuous working temperature (800°C). This temperature gradient implies that heat is transferred to the window by radiation and convection. Cooling of the window is therefore necessary to protect it. The most efficient way to cool the window is to utilize the cold working gas entering the receiver cavity; in this way, the energy removed from the window is not lost. T o assure an effective convective cooling, the inlet gas must flow over the entire inner surface of the window and cool it before extracting heat from the absorber, while avoiding mixing with the hot-side flow. Various inlet flow configurations were investigated experimentally and numerically; see Kribus et. al. (1994) and Karni et. al. (1997). Figure 1 shows schematically the selected flow pattern in the receiver cavity, which was employed in DIAPR tests. Using this method, it was possible to maintain the window temperature at about 600°C, while the absorber temperature reached about 1700°C (Karni et. al. 1997). Note that the frustum shape provides for a 9

significantly better convective cooling over the entire window inner surface than that possible with any single-rim window geometry. The prevention of dust accumulation is an added feature, which is uniquely possible with the FLHiP window. As shown in Figure 1, a low pressure external air stream can be used to flush the outer window surface. In some applications the frustum shape, having two open-ended baseplanes, allows some or all of the dust-removing air to flow through the bellows and be utilized for preheating of the working fluid, or another low-temperature heat requirement.

2.7 Simple, Low Cost Production The FLHiP window is relatively simple to produce from standard fused silica tubes. Since the required window thickness is only 1–3% of its average diameter (depending on the operating pressure range), the amount of material required and the corresponding cost are relatively low. For example, in an electricity producing solar Central Receiver plant, where the average sunlight concentration at the receiver aperture is 5,000 kW/m2 , the specific cost of the window would be about $10/kW of electricity output. We have received several quotations for manufacturing of FLHiP windows of various sizes and configurations. Windows of up to 500 mm large-diameter aperture can be fabricated using existing manufacturing facilities. According to manufacturers, equipment allowing production of up to a 1000 mm large-diameter aperture window is expected to become available in the near future. The window aperture area, i.e., the area of the frustum base-plane with the larger diameter, is proportional to the receiver power. A 1000 mm diameter window would correspond to a receiver power of 5–6 MWt. Based on the price of purchased windows and various supplier quotations, the window cost per unit aperture area (i.e., per kW) decreases somewhat as the window diameter is increased.

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3 Stress Analysis Stress and strain analysis was conducted using an axisymmetric finite-element model of the window. In a real window some asymmetry will always be present; additional work is underway t o evaluate these effects and update the conclusions regarding the window performance envelope. A 3-D model of buckling failure mode was also analyzed. Initial calculations were performed on simplified models and compared to analytical solutions. Detailed description of the stress analysis procedure is provided by Ostraich et. al. (1996). The calculation assumptions and main variables were: • Material properties based on GE Type 214 fused quartz. • Receiver pressure loading of 5–55 bars. • Various window surface and seals temperature distributions, including temperature gradients that are up to ten times larger than those anticipated during solar heating. • Window cone half angle of β=10° and 17°. • Scaling the window between 0.12 and 1.0 m large-aperture diameter. According to these calculations, no tensile stresses would develop anywhere in the window, under any foreseen operating conditions; the window is therefore always subjected to compressive stresses only. Figure 3 shows typical distributions of minimum principal stresses, S3, along the window at various pressure loading conditions; the negative values represent compression. The longitudinal axis, z, is measured from the window’s front aperture (the large-diameter aperture) towards the back, narrow end. The temperature profile specified was based on ray-tracing calculation of the radiation absorbed in the window under maximum solar load. Maximum window temperature was 1000°C, corresponding to the maximum permitted for fused quartz for short intervals. The maximum gradient across the window thickness was 50°C over 2.25 mm. Even at the highest

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pressure loading of 55 bar, the largest compressive stress was less than 0.15 of the design 9

compressive strength of over 1.1x10 Pa. Since the pressure-induced compressive stresses are dominant in all cases, the direction of the principal stress S3 is nearly parallel to the radial coordinate. The distribution of S3 along the window is then roughly proportional to the ratio between local cone diameter D and the wall thickness t. The peak compressive stress is therefore located near the large-diameter end of the window. The local variation of the stress near the window ends is due to the transition from conical to cylindrical shape. The calculations show that scaling up of the receiver without changing its relative proportions does not effect the stress distribution, as expected. Changing the window cone angle from 10° to 17° was done in the calculations by increasing its larger diameter, while keeping the diameter of the smaller frustum plane and the window length unchanged. This variation produced an increase of the peak compression (near the larger window diameter) by a factor of about 1.5, and a steeper decline of the stress towards the narrower window end, where the stress distribution was similar in both cases (Figure 4). No tensile (positive) stresses exist anywhere in the window at either geometry. The effect of the boundaries at the seal locations was investigated assuming first a completely fixed boundary, and then a free movement in the radial direction. The results show that the boundaries affect the stresses only in a small region adjacent to them. The boundary effect was more prominent along the S1 direction, where the stresses are one to two orders of magnitude lower than along the S3 direction. This sensitivity check is related to the effects of the stiffness of the seal material, and does not deal with external effects such as thermal expansion of the seal holder. In the comparison presented in Figure 5 the temperature gradients across the window wall were increased, at a fixed typical pressure loading (22 bar), until positive tensile stresses begun t o develop in the direction of maximum principal stresses, S1. As can be seen, a temperature 12

gradient of 500°C across the 2.25 mm thickness of the window (222°C/mm) was required t o offset the compressive stresses created in the window by the pressure load. This temperature gradient is one order-of-magnitude higher than the maximum gradient anticipated in the window at the most severe working conditions.

4 Experimental Results 4.1 Cold Tests Pyrex and fused silica models of several candidate windows were tested in pressure vessels at room temperature. Among the configurations tested was a hemispherical window with a flange, a straight tube installed in several ways and a double-pane frustum-like window. Various sealing and mounting techniques were also tested. The chosen design of the FLHiP window was pressuretested, using Pyrex specimens, which were installed in a cylindrical brass container equipped with seals and mounting similar to those used later in the receiver (Figure 1). The vessel was filled with water and immersed in a large (350 liter) water barrel. Controlled pressurization was provided using a compressed air tank and a needle valve. The tests indicated that the Pyrex FLHiP window model can withstand up to 55± 1 bar of internal receiver pressure. The test was terminated at 55 bar due to the design limitations of the pressure vessel. No significant leaks were observed during the tests. Note, that the compressive strength of fused silica, which is used for the actual receiver window, is 1.77 times higher than that of Pyrex, which was used in the cold tests.

4.2 High-Temperature Solar Tests Solar tests of the FLHiP window were conducted in the high-temperature, high-pressure receiver, DIAPR; they are reported in detail by Karni et. al., 1996 and Karni et al., 1997. About 250 test hours and 120 heating-cooling cycles were performed. Table 1 summarizes the range of operating conditions of these tests.

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Table 1. Range of solar tests operating conditions in which a DIAPR receiver with a FLHiP window was used

Power level [kW]

10

40

Absorber temperature [°C]

400–1100

950–1600

Gas exit temperature [°C]

600–800

850–1200

15–25

17–25

Window temperature [°C]

not measured

400-600

Total flowrate [kg/s]

0.010–0.017

0.022–0.035

0–0.014

0.022–0.028

Working pressure [bar]

Window cooling flowrate [kg/s]

The early 10 kW level tests with the DIAPR, conducted at the Weizmann Institute Solar Furnace, were performed mainly to determine the performance of the window and other key receiver components under high pressure, large temperature gradients, and frequent heatingcooling cycles. This test series included approximately 60 hours of on-sun experiments, with about 25 heating–cooling cycles, under a pressure of 15–25 bar. The receiver was positioned horizontally, i.e., the window axis was perpendicular to the gravitational force, promoting strong natural convection currents. The portion of the inlet flow designated initially for window cooling was varied in these tests between 25 and 100% of the total mass flow rate of the working fluid. The inlet velocity of the window cooling flow was relatively low and in most cases it protected the window only over a fraction of its length. As a result of the above conditions, the temperature and its gradients varied considerably over the window surface and between tests. Four different quartz windows were used and none of them broke during a test; no transmittance degradation was observed. The first window was used in solar tests for about 10 hours (about 5 heating–cooling cycles) and then broke when the receiver was idle, cold and at ambient pressure. This window was manufactured about 10 mm shorter than the specified design length. According to the design, when the receiver is pressurized the window is pushed against the seals, which are compressed plastically (see Section 2.5). The bellows is designed to remain slightly compressed 14

when the pressure is removed, to hold the window in place even after some shrinkage of the seals. In the case of the short window, the bellows extended to its neutral position as the system was depressurized, and thus could not support the window. The window slipped and turned sideways until it touched the metal seal holder and broke. The crack pattern seen in the window supports this scenario of failure. The working gas in these tests was CO2 , circulated in a closed loop. In the first test run the working gas unintentionally contained about 10% of methane, which was accidentally left in one of the storage tanks. Consequently, carbon particles, which were formed as the methane decomposed in the hot receiver, settled on the inner surface of the window, creating a black, completely opaque layer over most of its area. Only 10–15% of the window surface area remained transparent; this was a crescent-shaped region, located at the upper-frontal part of the window, where its temperature was highest (900–1000°C) and the carbon particles reacted with the CO2 to form CO. The picture shown in Figure 6 was taken through the receiver aperture after that test. The duration of the solar test was about 3.5 hours, at an internal pressure of 20 bar. We estimate that the condition shown in Figure 6 existed at least during the last hour of the test. The window was therefore exposed to temperature gradients which greatly exceeded those anticipated for normal operating conditions, especially near the boundary between the transparent region and that coated with carbon. Despite the extreme loading conditions, the window did not break. On the following day, the window was cleaned by circulating air (instead of CO2 ) at 20 bar and heating the receiver with sunlight until nearly all the carbon was burned away. Subsequently, tests were resumed according to plan. This accidental situation is indicative of the window’s ability t o withstand severe off-design conditions. Most of the receiver tests were conducted at the Weizmann Institute Solar Tower (Karni et. al., 1996). Here, the nominal power level was 40 kW and the receiver was inclined downwards 25° from the horizon, alleviating significantly the effect of natural convection, which dominated the window cooling flow in previous tests. In these experiments the absorber temperature could reach 1700°C, yet the quartz window had to be kept at its recommended working temperature of 15

600-800°C. Therefore, 70–100% of the working fluid (air) entered the receiver cavity along the window (Figure 1). The working gas inlet was modified such that the air entered the receiver’s cavity through a 0.5 mm wide circular slot, located near the back (smaller) end of the window. The narrow inlet slot forced the cold incoming gas to flow at a relatively high velocity. Hence, its momentum was sufficient to overcome the downwards pull of natural convection. Consequently, most of the gas flowed along the window’s inner wall, towards the front of the receiver, thereby cooling all of the window before turning back and flowing across the hot absorber elements. The cool incoming stream also prevented the hotter gas from coming in direct contact with the window. In latter runs the window’s temperature was measured with an infrared camera; it was maintained at around 600°C, while the absorber temperatures reached 1500–1600°C. The working pressure in this test series was also 15–25 bar. The window failed during a solar test only once. The cause of the failure was a flow malfunction, which deprived the window from its convective cooling. A significant part of the inlet working fluid stream, designated for window cooling, leaked through internal cracks, which apparently formed in ceramic components at the back of the receiver; hence, the flow created a ‘short circuit,’ bypassing the receiver cavity and flowing directly into the outlet tube. This diminished the window cooling and produced a rapid increase of the absorber temperature, which exceeded 1700°C. The lack of sufficient convective cooling, combined with a relatively high radiative heating from the absorber, caused local overheating of the window, which softened and failed at its hottest spot. A 3–4 cm2 oval-shape hole with its edges bent outwards, in the direction forced by the escaping high-pressure air, was observed after the system was shut down. It is interesting to note that only very few small cracks were observed around the hole and the rest of the window remained intact. The failure remained localized since following the formation of the hole, the receiver pressure declined slowly over about 15 minutes, while the system cooled down. The pressure-induced compressive stresses were therefore present and maintained the integrity of the undamaged parts of the window. The flow leakage problem was subsequently corrected and tests were resumed. 16

With the aforementioned exception, there where no mechanical failure or transmittance degradation in any of the tests. A failure-time distribution analysis, based on 150 hours of trouble-free operation of the window, yields an estimate of 3,000 hours MTTF (Mean Time T o Failure) at a confidence interval of 90%.

4.3 Accelerated Temperature and Pressure Cycling Long term durability of the window, working under thermal and pressure cycles typical to a solar receiver, is crucial for extended operation of the receiver. Figure 7 shows the temperature and pressure cycling apparatus used for accelerated longevity tests of the window. The window was installed in the device in the same way that it was mounted in the receiver. Heating was provided by a cylindrical electrical element, inserted through the larger window aperture. Compressed nitrogen was used to pressurize the test vessel and to cool it down. The window temperature distribution was calculated from thermocouple readings over the heating element and the radiation shield surrounding the window. Each thermal cycle included heating of the window to an average temperature of about 800°C and then cooling it to below 200°C. Each thermal cycle lasted approximately 15 minutes. The system was pressurized to 20 bar and depressurized once or twice a day, i.e., every 20–50 thermal cycles. This ratio of thermal to pressure cycles is similar to the operating conditions expected in a solar plant. About 1200 temperature cycles and 30 pressure cycles were performed. In the early tests the window temperature was very non-uniform, varying over 400°C from top to bottom. The local peak temperatures were considerably higher than planned, reaching about 1100°C. Consequently, after about 200 thermal cycles, white devitrification dots, typical for fused silica overheating, begun to form on the window; nevertheless, they did not lead to mechanical failure. In the following cycles the maximum window temperature was kept below 1000°C and neither devitrification nor mechanical degradation were observed.

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5 Conclusions The purpose of the volumetric receiver window is to separate the receiver cavity from the ambient air and allow operation with various working gases at high pressure. This should be accomplished while minimizing losses due to reflection of solar radiation entering the receiver, reradiation emitted from the hot inner receiver components, and natural convection. A satisfactory window solution must satisfy all the required performance criteria over thousands heating-cooling cycles and hundreds of pressurization-depressurization cycles. Optical, thermo-mechanical and flow analyses, cold pressure tests, hundreds of hours of hightemperature solar experiments, and over a thousand cycles of accelerated lifetime tests were performed with the Frustum-Like High-Pressure (FLHiP) window. The window successfully sustained its design working conditions: receiver pressure of up to 30 bar, continuous window working temperature of up to 800°C, peak window temperature of 1000°C, and surrounding absorber temperature of up to 1700°C. These analyses and experiments show that the fused silica FLHiP window goes a long way towards achieving all the optical, mechanical, thermal, longevity, installation, cooling, and manufacturability goals that were set for a successful high-performance receiver window. The successful development of the FLHiP window opens up numerous possibilities for volumetric receiver applications. One of them, the heating of pressurized gas to temperature levels compatible with high-temperature gas turbines, has been demonstrated (Karni et al. 1997) and is proposed for application in high-performance solar power plants (Kribus et al., 1997). Other possible applications are gas-phase solar chemistry (e.g., reforming of methane), metal oxides reduction and gas-dynamic laser.

Acknowledgments Support for this work was provided by the Israel Ministry of Energy and Infrastructure. Additional support was provided by a generous grant from the late professor Albert Sabin. The 18

authors thank R. Rubin, P. Doron, D. Sagie, L. Velonsky, M. Danino, Y. Mimon, E. Taragan and I. Anteby for valuable contributions to this work.

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Nomenclature D 1 , D2

Large and small aperture diameters of the window

L

Length of window

S1, S3

Maximum and minimum principal stresses

t

Window thickness

z

Axial coordinate

Greek β

Cone half-angle

γ

Characteristic angle of cone-frustum window

λ

Radiation wavelength

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References Abele, M., Bauer, H., Buck, R., Tamme, R. and Wörner, A., 1996, Design and Test Results of a Receiver-Reactor for Solar Methane Reforming. Proc. ASME Solar Engineering 1996, pp. 339-346. Anikeev, V. I., Bobrin, A. S. and Kirillov, V. A., 1992, New Conception of Catalytic Volumetric Reactor-Receiver. Proc. 6 International Symp. Solar Thermal Concentrating Technologies, Vol. 1, pp. 387-394. Becker, M., Gupta, B., Meinecke, W. and Bohn, M., 1995, Solar Energy Concentrating Systems–Applications and Technologies. C. F. Müller Verlag, Heidelberg. Buck, R., 1990, Test and Calculations for a Volumetric Ceramic Receiver. Solar Thermal Technology–Research Development and Applications, Proc. 4 International Symp., B. P . Gupta, W. H. Traugott ed., Hemisphere Publishing Corp., New York, pp. 279-286. Buck, R., Heller, P. and Koch, H., 1996, Receiver Development for a Dish-Brayton System. Proc. ASME Solar Engineering 1996, pp. 91-96. De Laquil, P., Kearney, D., Geyer, M. and Diver, R., 1992, Solar-Thermal Electric Technology. In: Renewable Energy, Sources for Fuel and Electricity, Island Press, Washington, D.C., Chapter 5. Evans, A. G. and Faber, K. T., 1984, Crack Growth Resistance of Microcracking Brittle Materials. J. Amer. Ceram. Soc., Vol. 67, p. 255. Flamant, G. and Olalde, G., 1983, High Temperature Solar Gas Heating Comparison Between Packed and Fluidized Bed Receivers–I. Solar Energy, Vol. 31, No. 5, pp 463-471. Gordon, J. E., 1976, The New Science of Strong Materials. Penguin, Harmondsworth. Griffith, A. A., 1920, The Phenomena of Rupture and Flow in Solids. Phil. Trans. Roy. Soc. Lond. Vol. A221, p 163. Griffith, A. A., 1924, The Theory of Rupture. In: Proc. First Internat. Congr. Appl. Mech. (ed. C.B. Biezeno & J. M. Burgers), J. Waltman Jr., Delft, p 55. Heller, P., 1991, Optimization of Windows for Closed Receivers and Receiver-Reactors: Enhancement of Optical Performance. Solar Energy Materials, Vol. 24, pp. 720-724. Hockey, B. J., 1983, Crack Healing in Brittle Materials. Fracture Mechanics of Ceramics, R. C. Bradt, A. G. Evans, D. P. H. Hasselman and F. F. Lange ed., Plenum, New York, Vol. 6, P . 637.

Hunt, A. J., “A New Solar Receiver Utilizing a Small Particle Heat exchanger,” Proceeding of the 14th International Society of Energy Conversion Engineering Conference, Vol. 1, No. 14, pp. 159-163, 1979 Inglis, G. E., 1913, Stresses in a Plate Due to the Presence of Cracks and Sharp Corners. Trans. Inst. Naval Archit. Vol. 55, p 219. 21

Lawn, B., 1993, Fracture of Brittle Solids. Cambridge University Press, New York. Karni, J., Rubin, R., Kribus, A., Doron, P. and Sagie, D., 1996, Test Results with the Directly Irradiated Annular Pressurized Receiver. 8 International Symp. Solar Thermal Concentrating Technologies, Köln, Germany, October 1996 Karni, J., Kribus, A., Rubin, R., Doron, P., Fiterman, A. and Sagie, D., 1997, The DIAPR: A High-Pressure, High-Temperature Solar Receiver. J. Solar Energy Engineering, Vol. 119, pp. 74-78. Karni J., Kribus A., Rubin R. and Doron P., 1998, The Porcupine: A Novel High-Flux Absorber For Volumetric Solar Receivers. Accepted to J. Solar Energy Engineering. Kribus, A., 1994, Optical performance of conical windows for concentrated solar radiation. J. Solar Energy Engineering, Vol. 116, pp. 47-52. Kribus, A., Fiterman, A., Doron, P. Karni, J., and Agranat, V., 1994, Energy Transport in a DIAPR-Type Receiver. Proc. 7 International Symp. Solar Thermal Concentrating Technologies, Vol. 4, pp. 864-872, Moscow, Russia, September, 1994. Kribus, A., Zaibel, R., Carey, D., Segal, A., and Karni, J., 1997, A solar-driven combined cycle power plant. Accepted for publication in: Solar Energy. Ostraich B., Kochavi E., Taragan E., Anteby I. and Mimon Y., 1996, Solar Receiver’s Quartz Window—Thermomechanical Analysis. 26 Israel Conf. Mechanical Engineering, Haifa, Israel, May 1996. Posnansky, M. and Pylkkänen, T., 1991, Development and Testing of a Volumetric Gas Receiver for High-Temperature Application. Solar energy Materials, Vol. 24, pp. 204-209. Posnansky, M. and Pylkkänen, T., 1992, High Temperature Volumetric Gas Receiver—Results of the Development and Testing of the Atlantis Ceramic Receiver. Proc. 6 International Symp. Solar Thermal Concentrating Technologies, Vol. 1, pp. 291-298. Pritzkow, W. E. C., 1991, Pressure Loaded Volumetric Ceramic Receiver. Solar Energy Materials, Vol. 24, pp. 498-507. Stavrinidis, B. and Holloway, D. G., 1983, Crack Healing in Glass. Phys. and Chem. Glasses, Vol. 24, p. 19. Stevens, K. K., 1987, Statics and Strength of Materials. Prentice-Hall, Englewood Cliffs, New Jersey. Tipper, C. F. E., 1962, The Brittle Fracture Story. Cambridge University Press, New York. Winter, C.-J., Sizmann, R. L. and Vant-Hull, L. L., 1991, Solar Power Plants. Springer-Verlag, Berlin, Heidelberg.

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Figures Figure 1. The Frustum-Like High-Pressure (FLHiP) window. For the 50 kWth receiver that was tested (Karni et. al., 1997), L=210 mm, D 1 =120 mm, D 2 =60 mm, β=10°, and t=2.25 mm. Figure 2. Geometry of the window. (a) cutaway view of an annular solar receiver with a FLHiP window (b) receiver cross-section and the angle parameter γ (After Kribus, 1994). Figure 3. Distribution of minimum principal stresses, S3, along the window, β=10°. Negative values indicate compression. Figure 4. Distribution of minimum principal stresses S3 (maximum compression) and maximum principal stresses S1 (maximum tension) along the window, for β=10° and 17°, under a pressure of 22 bar. Figure 5. Comparison of the maximum principal stresses, S1, at the largest expected (ΔT=50°C) and an exaggerated (ΔT=500°C) temperature difference across the 2.25 mm thick window wall. Pressure is 22 bar; β=10°. Figure 6. A view through the aperture into the receiver, after the deposition of carbon on the window during the first solar test. Figure 7. Window accelerated lifetime testing apparatus.

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Figure 1. The Frustum-Like High-Pressure (FLHiP) window. For the 50 kWth receiver that was tested (Karni et. al., 1997), L=210 mm, D1 =120 mm, D2 =60 mm, β=10°, and t=2.25 mm. 24

Window

(a)

Back Plane Reflector Absorber Face Aperture Plane

Absor

Concentrated Sunlight

ber

Windo

w

γ

Back Plane Reflector

(b)

Aperture

Figure 2. Geometry of the window. (a) cutaway view of an annular solar receiver with a FLHiP window (b) receiver cross-section and the angle parameter γ (After Kribus, 1994).

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Figure 3. Distribution o f minimum principal stresses, S3, along the window, β=10°. Negative values indicate compression.

26

Figure 4. Distribution of minimum principal stresses S3 (maximum compression) and maximum principal stresses S1 (maximum tension) along the window, for β=10° and 17°, under a pressure of 22 bar.

27

Figure 5. Comparison of the maximum principal stresses, S1, at the largest expected (ΔT=50°C) and an exaggerated (ΔT=500°C) temperature difference across the 2.25 mm thick window wall. Pressure is 22 bar; β=10°.

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Boundary of carbon covered region

Transparent region (exposed Porcupine) Carbon covered region

Back Reflector

Figure 6. A view through the aperture into the receiver, after the deposition of carbon on the window during the first solar test.

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COOLING WATER OUT

COOLING WATER IN

N2 OUT

INSULATION N2 OUT

THERMOCOUPLE PORTS

ELECTRIC HEATER WINDOW

RADIATION SHIELD N2 IN

COOLING WATER IN

Figure 7. Window accelerated lifetime testing apparatus.

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