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Journal ArticleDOI

Performance characteristics of the Spiky Central Receiver Air Pre-heater (SCRAP)

01 May 2020-Solar Energy (Pergamon)-Vol. 201, pp 773-786
TL;DR: In this paper, a spiky central receiver air pre-heater (SCRAP) technology is proposed to overcome barriers experienced to date, which can transfer heat from concentrated solar irradiation to a pressurized air stream in a gas turbine.
About: This article is published in Solar Energy.The article was published on 2020-05-01 and is currently open access. It has received 14 citations till now. The article focuses on the topics: Thermal efficiency & Compressed air.

Summary (5 min read)

List of Figures

  • 1 Heat transfer coefficient htip for different nozzle diameter dnozzle . .
  • The deviation of the measured pressure drop slope error, ∆p′, over the comparison between fD,sim and fD,exp, computed from experimental data.
  • Photograph of the 114 mm section after EDM machining and the same section from the other side with inner tube inserted . . . . . .


  • CSP plants are currently considered to provide LCOEs at about 0.12 USD/kWh, whereas new built coal in South Africa is estimated at about 0.07 USD/kWh (DoE, 2016) and solar PV has recently reduced to 0.04 USD/kWh (Bischof-Niemz and Fourie, 2016).
  • As with the PTC, also with CR, the HTF limitations dictated the operating temperature of the receiver outlet temperature.
  • This trend is explained by the higher plant performance and thereby reduced LCOE is expected from these plants.


  • This current generation of CSP CR plants operates with molten salt as HTF, as well as sensible thermal storage medium (using a two tank system).
  • Furthermore, cost reduction can be achieved by an increased large scale roll out of CSP, as well as further innovation into increasing plant performance.
  • The SUNSPOT cycle is a manifestation of a possible next generation CSP CR plant utilizing pressurized air as HTF and an asynchronous combined cycle (CC) for efficient and dispatchable power generation.
  • The turbine outlet air, as in conventional CC plants is used to drive a steam generator for the bottoming Rankine cycle.
  • Not shown are dish projects due to marginal capacities as well as solar park projects.


  • A central receiver technology, heating a pressurized air stream inbetween the compressor and the turbine stage, should be able to heat the air to outlet temperatures of above 1000 ◦C, while at the same time, operating at a low pressure drop to minimize the performance drop in the Brayton cycle.
  • The proposed SCRAP receiver is an attempt to overcome weaknesses of the current technologies.
  • 2 Review of pressurized air receiver technology A common goal for developing a central receiver is maximized central receiver system solar-thermal efficiency.
  • This review describes initially the relevant optical and thermal losses influencing that efficiency.
  • Numerous criteria influence the design of a central receiver system.

1.2.1 Receiver efficiency

  • The total losses affecting receiver system efficiency are the optical losses, thermal losses and pumping losses (Stine and Geyer, 2001).
  • The optical loss of solar re-reflection from the receiver is typically represented as part of the thermal efficiency, while the remaining optical losses only appear in a total efficiency analysis.
  • One attempt is the application of a selective coating to the absorber surface, improving the absorptive capabilities of the absorber directly, while providing low emittance values at the temperature range of the material.
  • Typically, spillage losses are not accounted for in the receiver efficiency, ηthermal, but are represented in a total systems analysis.
  • Heliostats at different distances to the tower cast images of different sizes and shapes onto the absorber.


  • A homogeneous flux distribution over the absorber surface.
  • While spillage losses are usually small (Stine and Geyer, 2001) and apply to a certain degree to any receiver design, CPC losses can be significant (Ávila-Marín, 2011; Hischier et al., 2012).
  • These are losses of minor nature and are often neglected (Solgate Report, 2005).
  • Radiative heat losses occur from hot receiver surfaces towards the environment.
  • Heat transfer fluid pumping losses can be of relevance in terms of parasitic losses or, as in the case of a Brayton cycle, directly reducing the pressure ratio on the turbine side.

1.2.2 Overview of generic receiver concepts

  • The central receiver concepts of interest for a pressurized air system are reviewed.
  • The overarching concepts of external and cavity receivers are first discussed, before elaborating on proposed concepts.

External receiver

  • The conventional external receiver is the simplest and cheapest receiver design, where the absorber system, usually numerous vertical tubes, is externally mounted to the receiver tower.
  • For a surrounding heliostat field multiple similar panels can be mounted to cover larger areas, e.g. reproducing a rectangular or cylindrical shape system).
  • The external receiver concept shows high exposure of the absorber tubes to ambient.
  • This results in high heat losses by means of convection and radiation.
  • Presently, external receivers are widely used for molten salt and DSG systems (both operating below 600 ◦C).


  • The external receiver, as shown in Figure 1.3, can utilize a surrounding heliostat field.
  • An advantage that a surrounding heliostat field shows over a polar3 field is a stable solar field optical efficiency over the course of the day, while a polar heliostat field has a higher noon performance (Vant-Hull, 2012).
  • Hence, employing a surrounding receiver system can allow for an increased optical efficiency of the heliostat field.
  • For large scale plants, the surrounding field becomes inevitable, as the distance of the farthest heliostats grows too large for efficient operation.
  • A conventional external receiver is not suited for low heat transfer coefficient fluids, such as air, as the required large exposed absorber surfaces would lead to significant heat losses.

Cavity receiver

  • A cavity receiver is a receiver system where the absorber system is encased inside a space with an opening towards the heliostat field.
  • Having the absorber system encased, can improve the thermal efficiency by means of reducing the convective heat loss (which can further be enhanced by a window in the opening), as well as by trapping reflected light and radiated heat.
  • At a more sophisticated level, high temperature systems such as pressurized air receivers have been conceived as cavity receivers (see chapter 1.2.4).

1.2.3 Overview of generic absorber concepts

  • It is employed to effectively transfer the energy of the concentrated sunlight into the heat transfer fluid.
  • The two absorber types commonly employed are then introduced.
  • These are the tubular and the volumetric absorber principles.

Tubular absorber

  • Current central receiver technologies for molten salt and direct steam generating systems use a number of tubes to form the absorber surface.
  • The heat transfer fluid is pumped through the tubes and is heated up in the process.
  • In order to minimize absorber surface area and temperature, fluids with high thermal conductivity are preferred, as they maintain an efficient system.
  • As shown in chapter 1.2.4, innovations in pipe manufacturing, heat transfer enhancements and intelligent receiver design can make tubular receivers a viable technology for pressurized air systems.

Volumetric absorber

  • Increasing attention has recently been given to volumetric receiver/absorber systems.
  • A porous surface, allowing the radiation to penetrate into the depth of the absorber, reduces reflection losses.
  • The heat transfer fluid, which is forced through the porous absorber from the irradiated side, provides the highest cooling effect at the exposed surface.
  • The volumetric effect can be described by the peak surface temperature occurring deep in the structure.
  • It furthermore does not expose the highest temperature parts of the system, reducing the exposed surface temperature and with that heat losses.

1.2.4 Review of existing pressurized air receiver concepts

  • The research area of air receiver systems for elevated temperatures, capable of supplying a Brayton cycle, is relatively young.
  • To date, mainly the German Aerospace Center (DLR) and the Weizmann Institute of Science (WIS) have driven development, leading to demonstration scale systems with published findings.
  • Both research institutions have developed cavity receiver systems with volumetric absorber technology, capable of reaching mean outlet temperatures of above 1000 ◦C.
  • This review covers the progress to date and highlights important problems encountered.
  • The gap between the lower air temperature after a solar receiver and the turbine requirement is usually overcome by introducing a fuel combustor.

Attempts by DLR

  • The DLR began tests on volumetric pressurized air receiver systems in 1989 with the PLVCR5 receiver, which employed a dome-shaped quartz glass window (Pritzkow, 1991).
  • The receiver concept has since been continuously improved, and the current version is known as the REFOS receiver, shown in Figure 1.6.
  • The volumetric absorber material used in the receiver can be either a silicon-carbide (Si-C) ceramic mesh for high temperatures or a metal wire mesh for pre-heating purposes.
  • In the SOLGATE project, a hexagonal compound parabolic concentrator.


  • (CPC) and two pre-heating sections (with identical CPCs) were included into the setup.
  • The receiver cluster consisted of a multi-tube coil pre-heater , a medium temperature REFOS pre-heater and a high temperature REFOS receiver.
  • The aforementioned tubular pre-heating section was not satisfactory.
  • More importantly, the cross-sectional temperature gradient between the radiated and unradiated pipe surface was calculated at up to 220 K, resulting in high thermal stresses, thereby reducing the receiver lifetime (Heller, 2010).
  • The new SOLUGAS receiver was meant to replace the tubular cavity receiver pre-heater and the REFOS receiver pre-heating stage of the SOLGATE project.


  • The target of the SOLTREC project was to achieve a mean air temperature of 1000 ◦C.
  • The SOLUGAS receiver was successfully operated at air outlet temperatures of 800 ◦C (Korzynietz et al., 2016).
  • The solar-thermal efficiency of the SOLUGAS receiver reached just below 80 % during testing which uses the flux on aperture as reference.
  • During the same test series the receiver system pressure drop was measured at around 200 mbar (Korzynietz et al., 2016).
  • Buck et al. (2002) stated an efficiency goal for the REFOS receiver (without a pre-heater system) of 80 % at 800 ◦C, including the optical losses of the CPC but did not report test results.


  • The quartz glass of the dome-shaped window needs to withstand high temperatures and pressures in order to separate the hot pressurized air flow (up to 20 bar) from ambient.
  • With the mentioned 500 h of test time, representing about 60 days of normal operation, deterioration was already visible within a fraction of a plant’s lifetime (Hofmann et al., 2009).
  • Concerns are that besides deterioration of the optical quality, cracking of the window occurs under pressure and high solar flux density (Grange et al., 2011).
  • For unpressurized systems higher diameters are possible.

The DIAPR receiver

  • In parallel to the DLR, the WIS (Weizmann Institute of Science) developed a pressurized air volumetric cavity receiver, the Directly Irradiated Annular Pressurized Receiver .
  • The DIAPR is based on the porcupine model, where concentrated solar radiation impinges on high temperature resistant alumina-silica pins (Karni et al., 1998).
  • The pressurized air stream is guided past the pins and is heated up in the process.
  • In an attempt to increase the system efficiency, a multistage DIAPR was developed that employed, similarly to the DLR approach, a coiled tubular cavity pre-heater (Kribus et al., 1999).


  • Efficiency information on the cluster or the pre-heater is not available.
  • These values were provided for a flux density of 5 MW/m2 on the receiver aperture.
  • Even though the DIAPR also utilizes a pressurized fused quartz glass, no concerns with regards to its durability were mentioned, and even up-scaling of the receiver was discussed.
  • While further work has been proposed, no new information on development progress with regards to the DIAPR system has been published in recent years.
  • The DIAPR technology was recently implemented in the 100 kWe AORA Solar micro-turbine central receiver system (Ávila-Marín, 2011).the authors.the authors.


  • With increasing attention to the development of pressurized air receivers, new concepts were recently proposed.
  • Further results, limitations and capabilities of the system are not provided.
  • The Australian Commonwealth Scientific and Industrial Research Organisation is developing a solar Brayton cycle system in cooperation with the Japanese Mitsubishi Heavy Industries (MHI).
  • In the PEGASE project, the development of a pressurized air receiver, based on compact heat exchanger technology, is pursued by the French CNRS/PROMES.
  • The experiments reported on in Bellard et al. (2012) highlight problems with unexpected high pressure drop, but no information on thermal efficiency of the system has been provided.


  • Hischier et al. (2012) proposed a cavity receiver design with a similar absorber system approach to the PEGASE receiver.
  • The absorbed radiation in a cylindrical cavity receiver is conducted into an annular reticulate porous ceramic (RPC) foam material (SiC), through which the pressurized air is sucked and in the process heated up .
  • The ceramic foam is shielded from the cavity by the absorber surface.
  • Small scale laboratory testing has been conducted with a 3 kWt receiver prototype.

1.2.5 Conclusion on review of current systems

  • The field of pressurized air receivers is relatively young, with only two research institutions driving development to prototype and demonstration scale.
  • Generally, limited information is available on the proposed systems.
  • The tubular pre-heating sections are equally poorly discussed in literature.
  • It can furthermore be concluded that the volumetric receivers and tubular pre-heaters proposed by DLR and WIS (and in fact also the new approaches introduced in Section 1.2.4) are cavity receivers, mostly equipped with secondary concentrators.

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Frequently Asked Questions (1)
Q1. What have the authors contributed in "Performance characteristics of the spiky central receiver air pre-heater (scrap)" ?

The novel spiky central receiver air pre-heater ( SCRAP ) technology is proposed to provide such a receiver and overcome barriers experienced by developments to date. The SCRAP receiver is a novel metallic receiver technology aimed at preheating an air stream to about 800 ◦C, either prior to a combustion chamber or alternatively a cascaded secondary non-metallic receiver system, capable of achieving elevated temperatures.