Exploiting volumetric effects in novel additively manufactured open solar receivers
Introduction
Solar receivers able to provide ever higher output temperatures at very high irradiance levels constitute one of the key elements to achieve high thermal conversion efficiencies in concentrating solar power plants. Three major solar receiver concepts can be found to date (Becker and Vant-Hull, 1991): tubular receivers, fluid/particle receivers, and volumetric receivers. All of them transform incident concentrated sunlight into thermal energy at the temperature required by the downstream mechanical, thermal or chemical conversion process. Regardless of the working fluid or thermodynamic cycle employed, design trends towards higher absorber output temperatures are general. Target operating conditions for their widespread industrial deployment are demanding (Mehos et al., 2016): working fluid temperatures at receiver exit in excess of 1000 K, thermal conversion efficiencies over 90%, minimum service life of 10 000 cycles and overall costs below 150 USD per kilowatt of thermal power delivered. Operating temperatures play a conflicting role in their performance because thermal losses become significant at the very high levels required for efficient downstream thermochemical or power cycles.
Volumetric receivers consist of radiative-convective heat exchangers in which porous interlocking structures are arranged to fill a volume. Concentrated solar radiation is gradually absorbed and conducted into their solid volume, and transferred to a working fluid by forced convection. The objective is that maximum receiver operating wall temperatures occur deep inside the heat exchanging matrix to reduce thermal emission losses from the aperture. When this occurs, it is also possible that the temperature of the working fluid leaving the receiver is higher than that found in its irradiated front face. The attainment of both of these conditions is widely referred to as the volumetric effect (Boehmer et al., 1991). The working fluid employed in open volumetric receivers is typically air because of its abundance, low environmental impact, and ability to reach very high temperatures without phase change or thermal degradation. Closed-loop pressurised volumetric receivers also exist, which employ external quartz windows to seal the flow field (Pozivil et al., 2015).
High (or selective) solar absorptivity, high (or directional) thermal conductivity, mechanical durability at severe operating conditions, and, where possible, low cost are desired features for volumetric receiver materials – especially if they are to operate under volumetric effect conditions. In addition, the geometry must be designed to allow for effective heat transfer processes, including an adequate absorption of incident radiation as a function of depth, high thermal conduction towards the interior, and high convective heat transfer coefficients. Minimising the pressure drop across the receiver is also desirable. The design of volumetric receivers is thus one of conjugate heat transfer processes and interconnected optical and thermal requirements where trade-offs are unavoidable.
There exists scarce experimental evidence of solar receivers achieving a significant volumetric effect. The exception is a double-layer selective receiver composed of an external silica square-channel monolithic honeycomb and an internal layer of silicon carbide particles (Menigault et al., 1991). A similar double-layer configuration where the internal particle layer was replaced by a ceramic silicon carbide monolith did not show the same behaviour (Pitz-Paal et al., 1992), however. Neither did, to name but a few studies, more conventional single-layer silicon carbide monolithic honeycombs (Téllez, 2003, Hoffschmidt et al., 2003), stainless steel wire grids (Ávila Marín et al., 2014), nor ceramic foams (Fend et al., 2004, Mey et al., 2016). Comprehensive reviews on solar receivers investigated since the 1980s have been given in Avila-Marín, 2011, Gómez-García et al., 2016, Ho, 2017. A detailed one-dimensional analysis has been employed to argue that receiver geometry and materials still lack adequate optimisation for maximum thermal conversion efficiencies (Kribus et al., 2014).
It is shown in this paper that variable geometry open volumetric air receivers can address the main problems still encountered in these components (where the incident radiative heat flux is almost completely absorbed in the front region), and potentially operate under volumetric effect conditions. Receivers described here have been built from the structured repetition of elementary cells, leading to hierarchically-layered configurations of decreasing porosity levels. This allows for an enhanced diffusion of incident sunlight and a shift of radiation absorption profiles towards the rear, which reduces thermal emission losses. An additional advantage is the augmentation of internal convective heat transfer in the receiver through a combination of aerothermal mechanisms: a gradually increasing wetted area surface, a reduction of flow cross-sectional areas (increasing flow velocities, Reynolds numbers, and heat transfer coefficients), and the generation and enhancement of turbulent flow structures within the intricate flow channels. Flow instabilities observed in monolithic configurations (Pitz-Paal et al., 1997) can also be avoided due to the flow field redistribution variable geometry receivers allow for.
Some so-called fractal-like solar receivers have been recently proposed, not based on variable porosity concepts, but on the repetition of elementary structures at larger scales: the SCRAP receiver (Lubkoll et al., 2016), a pin-shaped external micro-structure (Capuano et al., 2017), a bladed receiver (Wang et al., 2016), and staggered rearrangements of tubes along self-repeating patterns (Ortega et al., 2016). The SCRAP receiver is a pressurised air receiver where numerous outward radial spikes are internally cooled by recirculating flow channels. The pin-shaped micro-structure is similarly spiked, but intended as a frontal add-on to current open volumetric air receivers. An experimental prototype was manufactured by additive manufacturing (electron beam selective melting) in Titanium-Aluminium alloy (Ti6Al4V) and showed promising initial results. The last two have been designed as improvements for current molten salt tubular receivers. All are aimed at enhancing light-trapping performance and move the operating point of solar receivers closer towards volumetric effect conditions by minimising emission losses. This paper presents a fully-integrated experimental assessment of some new fractal-like receivers, which show potential as candidates for the replacement of current monolithic honeycombs and foams.
Section snippets
Design and manufacturing of hierarchically-layered receivers
The development of variable geometry receivers built from the fractal repetition of elementary cells has been made possible by recent advances in additive manufacturing techniques. These allow for the design of creative highly-customised components based on complex geometries with improved functionality. Selective laser melting (SLM), which utilises a laser beam to melt successive layers of powder into a finished part, has been the manufacturing technique employed here. This technique was
Receiver material optical properties
The influence of material reflectivity on radiation absorption profiles was numerically investigated in Alberti et al. (2016). Overall radiation absorption was shown to decrease when reflectivity increases, as expected, but the study also demonstrated that radiation absorption profiles could be aft-shifted with materials of moderate reflectivity. An optimum value of reflectivity then typically exists, as absorber materials of very high reflectivity not only cause large optical reflection losses
Experimental apparatus
Experimental measurements of thermal efficiency were conducted at the IMDEA Energy Institute. A schematic of the calorimetric facility employed is given in Fig. 9 (the location of air temperature measurement planes is discussed later on). Incident radiation is provided by a high flux solar simulator, composed of a single 7 kW Xenon-arc lamp and a truncated ellipsoidal reflector that acts as a concentrator. The calorimetric facility, in turn, consists of a radiation homogeniser, a calibrated air
Operating conditions
The radiative flux density distribution at the absorber aperture was measured with a Gardon gauge. The radiometer was installed on an automated motion control mechanism, which traversed a plane located 5 mm downstream from the homogeniser and coincident with the absorber aperture plane. Fig. 10 shows the irradiance distribution on the aperture of both absorber families. The black square in the figure indicates the relative size of the homogeniser outlet plane. The smaller hexagon refers to
Hydrodynamic measurements
Measurements of pressure drop, conducted during operation across the four absorbers, are shown in Fig. 12, plotted as a function of air velocity through each sample. They were acquired in experiments conducted at full power of the solar simulator, at a range of decreasing mass flow rates. A differential pressure transducer was utilised, connected to static pressure tappings in the ducts upstream and downstream of the sample holder module. Air velocities were calculated from the mass flow rate
Volumetric effects
Steady-state surface temperature measurements were conducted on the absorber lateral walls as a function of depth, and air temperature measurements in the absorber inlet and outlet planes, in order to assess the achievement of volumetric effects. Fig. 13 shows temperature distributions along one of the lateral walls of absorber sample 3, conducted for 9 values of incident power per unit mass flow rate (). These decrease from top to bottom as follows: 4770.8 kJ kg−1, 2385.4 kJ kg−1,
Thermal conversion efficiency
Absorbers solar-to-thermal conversion efficiency is calculated as the ratio between the power that is transferred to the working fluid by forced convection and the incident radiative power over the absorber aperture. Quantitatively, efficiency can be calculated as the rate of enthalpy rise of the working fluid divided by the total incident power. It thus gives an indication of the capability of solar absorbers to transform incident solar power into enthalpy of the working fluid through a series
Conclusions
The experimental aerothermal assessment of four novel hierarchically-layered fractal-like volumetric absorbers have been described in this paper. The absorbers were built by the lateral repetition of elementary cells, arranged into constituent layers which were, in turn, stacked up along the main longitudinal axis perpendicular to the irradiated face. Two absorber families with different porosity distributions as a function of depth were investigated. The hierarchical structures were employed
Conflict of interest
There are no potential conflicts of interest with this submission.
Acknowledgements
Research leading to these results has received funding from the regional government of Comunidad de Madrid through project ALCCONES (S2013/MAE-2985), and from the European Union Seventh Framework Programme (FP7/2007-2013) through grant agreements 609837 and REA 291803 (Marie Curie Actions). All are gratefully acknowledged.
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Present address: Cardiff School of Engineering, Cardiff University, The Parade, Cardiff CF24 3AA, UK.