Traducción lingüística

Pressurized air receiver designs with pressurized windows allow for the direct cooling of the absorber surfaces with pressurized air, similar to an open volu- metric air receiver. This has two main advantages: Firstly, there is no thermal resistance caused by conduction through the absorber material. The pressur- ized air is, therefore, in direct contact with the hottest surface of the receiver. Secondly, the absorber does not have to sustain the pressure difference between the air flow and ambient because the window creates a pressurized chamber. This allows, for example, for the use of porous, high temperature-resistant ceramics as the (volumetric) absorber material. To the author’s knowledge, only two designs of pressurized air receivers with pressurized windows have been developed to a pilot scale. In this section, firstly the general characteristics of pressurized windows for pressurized air receivers are described, followed by a presentation of these two designs and findings on them.

Quartz glass windows

Fused silica (commonly called quartz glass) is well suited as the material of pressurized air receiver windows as it has a high transmissivity for radiation in the solar spectrum, a low thermal expansion coefficient and a relatively high tensile strength at temperatures up to 800◦_{C (Röger et al., 2009). However, it}

has a high absorptivity for radiation in the infrared/thermal spectrum, which leads to elevated window temperatures. Furthermore, analyses of quartz glass windows showed defects, namely devitrification spots (crystal formation) and burnt-in contaminations, already after short-term experiments in pressurized air receivers (Hofmann et al., 2009).

To increase the reflectance of thermal radiation, Röger et al. (2009) applied coatings to the pressurized side of a quartz glass window. They found tempera- ture decreases of the window up to 78 K compared to the uncoated window, leading to higher achievable thermal efficiencies and air outlet temperatures within the tolerated operating temperature range of the quartz glass.

Hertel et al. (2016) experimentally determined the failure stress of quartz glass samples with detrifications and after treating their surfaces, respectively, at ambient temperature and at 800◦_{C. While they measured a lowered trans-}

6.3. PRESSURIZED AIR RECEIVERS 59

after heat treatment of the samples. They also found all samples to have survival probabilities of more than 99.99 % at the design stress level for a specific pressurized air receiver.

In conclusion, quartz glass is a potentially suitable material of pressurized windows in volumetric air receivers. However, besides such windows’ high costs, their long-term performance under solar conditions has yet to be proven.

DIAPR

The Directly Irradiated Annular Pressurized Receiver (DIAPR) was developed in the 1990s by the Israeli Weizmann Institute of Science. The concept is build around a volumetric ceramic absorber structure called the Porcupine which is depicted in Figure 6.5(a). The ceramic quills of the absorber allow for the penetration of concentrated solar radiation into the depth of the structure and form a large surface area for the heat transfer to the pressurized air stream passing through. This stream is separated from ambient by use of a Frustum- Like High-Pressure (FLHiP) window, manufactured from quartz glass. Due to the load on the window, its poorer high-temperature material strength and its exposed position, it is designed to be cooled by the coldest available air directly from the flow inlet as depicted in Figure 6.5(b).

A prototype of the DIAPR technology was successfully tested at air outlet temperatures of up to 1300◦_{Cand an absolute pressure of up to 30 bar at solar}

fluxes up to 10 MWt/m2 (Karni et al., 1997). The thermal efficiency of a later

manifestation of the receiver was calculated at 70 % to 90 %, however, direct measurement of the solar energy input was not conducted (Kribus et al., 2001). While absorber temperatures of up to 1600◦_{C were measured, the window}

temperature could be kept below 600◦_C.

In further experiments, Kribus et al. (1999) tested a multistage receiver system consisting of a high-temperature DIAPR receiver and several metallic tubular pre-heaters as depicted in Figure 6.5(c). Due to deteriorated optical equipment during tests, namely the heliostat field and the CPC, the thermal input into the receiver was lower than the design value. Its thermal output (30 kWt to 60 kWt), outlet temperature (1000◦C) and efficiency, therefore, also

did not reach the previously achieved values.

The very high flux required to operate the DIAPR at temperatures above 1000◦_{C and at a high thermal efficiency, approximately 5 MW}

t/m2

to 10 MWt/m2, necessitates secondary or even tertiary (beam-down) concen-

tration devices with high concentration ratios. These systems have a negative impact on the optical performance of the solar field and receiver system besides imposing high costs.

(a)

(b) (c)

Figure 6.5: (a) Photo of the Porcupine ceramic absorber (Kribus et al., 2001), (b)

half-section drawing of the DIAPR assembly (Kribus et al., 1999) and (c) drawing of the multi-stage DIAPR receiver cluster (Kribus et al., 1999)

6.3. PRESSURIZED AIR RECEIVERS 61

REFOS

The design of the REFOS receiver of the German Aerospace Center (DLR) is similar to that of the DIAPR in many ways: It utilizes a ceramic volumetric absorber (although there were also manifestations with an Inconel wire absorber) behind a pressurized quartz glass window and features a CPC for further concentration as well as to enable clusters of receiver modules with minimized spillage (see Section 6.1.2). Some differences between the two concepts can be seen in the drawing in Figure 6.6: The volumetric absorber is a mesh or porous medium instead of a defined structure and the window is dome- instead of frustum-shaped.

REFOS receivers with an Inconel wire absorber were tested for more than a hundred hours reaching a maximum air outlet temperature of 815◦_{C and}

absolute pressures up to 15 bar (Buck, 2003, p. 3). Higher temperatures and efficiencies could be achieved by replacing the metallic absorber with silicone carbide (SiC) foams. These are available with high porosities, generating a micro-cavity effect, and have higher allowable operating temperatures.

In the SOLGATE and HST projects, a cluster consisting of a metallic tubular pre-heater (presented in Section 6.3.2) and two REFOS receivers was connected to a gas turbine. The air outlet temperature of the high-temperature REFOS module with ceramic absorber could be increased to 1030◦_{C (Buck,}

2005, p. 55). Problems with the mounting of a quartz glass window prevented reaching the originally declared goal of an air outlet temperature above 1100◦_C.

The thermal efficiency of the cluster was calculated at approximately 80 % to 85 % at air outlet temperatures between 650◦_{C and 1000}◦_{C, however, there}

was doubt regarding the validity of these values (see Section 2.1.3). Conclusions

Air receivers with pressurized glass windows have shown remarkable performance in terms of air outlet temperatures and thermal efficiencies. However, the two tested concepts both suffer from similar problems, namely their complexity and high cost. Both of these issues are directly related to the quartz glass window, which enables the technology but has undesirable side-effects. Additionally, the optical performance is greatly lowered by the necessary CPCs, which also increase the cost further. To replace these complex receiver systems, or at least supplement them, more robust options are sought after.