Concentrated Solar Energy
As solar energy is diluted, the direct solar flux received on Earth surface can hardly exceed 1000 W.m-2. To capture this energy and transform it into heat, specific solar devices should be used. There are two main solar technologies that valorize the solar radiant energy into heat: flat-plate technologies (Pandey and Chaurasiya, 2017) and concentrators (Helman, 2017). The first kind is often used for domestic heating/air conditioning and in some cases, in industry for process heat generation. Due to excessive heat losses, these systems barely achieve 100-200°C. Solar concentrators enable to deliver energy at high temperature, which makes them crucial for many industrial and energy applications. Such technologies are based on the use of a receiver whose surface aperture is minimized (to reduce the thermal radiative losses) and optical systems, which collect and concentrate sunlight towards the receiver. There are two main areas of interest in concentrated solar energy: chemicals and power generation. Currently, power generation by concentrated solar energy (commonly referred to Concentrated Solar Power or CSP) is the most mature. CSP plants use the sunlight to heat a Heat Transfer Fluid (HTF), the latter exchanges energy with water, which becomes superheated and drives a conventional steam turbine-generator. Very often, to smooth out the energy delivery on cloudy and nocturnal hours, CSP plants are also equipped with thermal energy storage (TES) units making use of energy storage medium such as molten salt mixtures to store for several hours (e.g. 6-10 hours for a majority of active CSP projects) midday surplus sensible energy (Achkari and El Fadar, 2020).
CSP in the world
CSP is commercially available since 1984 (REN21, 2014) and it reached a world electrical production capacity of 5 500 MW in 2018 (REN21, 2019).Spain and USA are today the world leading countries in CSP, holding around 66% of the total power production capacity. Besides, more than third of major CSP projects are either under development or under construction especially in China, Chile, Australia, South Africa and MENA region, which denotes a growing interest in CSP worldwide (SolarPACES, 2020). The commercial viability of the plants depends on a large number of criteria, including the location of the power plants. In fact, solar concentration concerns exclusively the direct component of sunlight, which needs to be as high as possible. The best locations for CSP plants are therefore limited areas on Earth, which should guarantee a high level of DNI (Direct Normal Irradiance) all year round. The DLR (Deutsches Zentrum fur Luft-under Raumfahrt) drawn up a map of potential sites, which are auspicious for CSP (Figure I.1).
These sites are characterized by a DNI higher than 2000 kWh.m-2.year-1, a large open area (as flat as possible) with no property nearby, the presence of a power distribution network and a great supply of water. Note that under these conditions, only 1% of the Sahara desert surface is able to satisfy today’s world electricity demand (Letcher, 2020).
Concentrating solar systems
Four CSP technologies are generally accepted and commercially available for the production of electricity (Figure I.2). Each technology has its own concentration level, which is expressed by the concentration ratio (C). From an energetic point of view, this ratio can be defined as the flux entering the collector over solar radiation hitting onto the receiver. However, the flux on the receiver is neither uniformly distributed, nor easily measurable. Besides, it varies during the day. For this reason, the geometrical concentration was introduced. This parameter is more convenient for practical engineering applications as it depends on the geometry of the technology as manufactured. It is expressed as the ratio between the concentrator aperture surface and the surface of the receiver aperture.
The maximum temperature level delivered by a CSP technology depends above all on this parameter. Parabolic Trough (PT) and Linear Fresnel (LF) reflectors (Figure I.2-a,b) are one-axis tracking and line-focusing technologies. Their operating range is 250-400°C with concentration ratios of about 50-100. Central receivers (i.e. tower configuration, Figure I.2-c) and parabolic dish concentrators (Figure I.2-d) are two-axis tracking and point-focusing technologies, and their concentration ratios can easily reach 1000 at the expense of a greater cost and complexity. This allows reaching higher temperatures at the receiver, which improves the solar-to-electric (STE) efficiency and reduces the energy costs (Fernández-García et al., 2014; IRENA, 2012; Weinstein et al., 2015). In the four configurations, the concentration ratio (and thus the receiver temperature) can be increased by adding further optical components such as 2D and 3D CPCs (Compounds Parabolic Collectors) (Tian et al., 2018).
Even if PT technologies dominated the market during the last two decades, tower plants show today an increasing interest. In fact, the total power capacity of tower plants under construction or development is twice that of PTs (Gauché et al., 2017). LF reflectors and dish collectors utilization remains marginal, except in some small projects worldwide.
The solar tower configuration gains attractiveness and shows a high potential for coupling with high temperature thermochemical processes at large scale (Weinstein et al., 2015; Yadav and Banerjee, 2016). This configuration presents various possible arrangements depending on the heliostats (i.e. tracking mirrors) layout and on the central receiver design/location (Figure I.3).
Figure I.3-a shows an external receiver with an aperture receiving radiation all around the circumference. This concept is used in a number of large-capacity tower plants such as Noor in Morocco and Ivanpah in California. Figure I.3-b depicts a cavity receiver mounted on the top of a tower with a polar heliostat field. The receiver is an insulated enclosure with an aperture irradiated from one side of the tower only. PS10 plant near Seville in Spain is the first ever built commercial solar tower plant and it is based on this principle.
The third possible configuration shown in Figure I.3-c makes use of the Cassegrain optical arrangement borrowed from telescopes. The cavity receiver is set on the ground and a secondary reflector at the top of the tower redirects the impinging light towards the receiver. This significantly reduces the weight and the cost of the tower, which thus supports only the secondary reflecting component (Yogev et al., 1998). A commercial plant taking use of beam-down/reflector technology was recently built (for the first time) in China (http://www.xinchen-csp.com/).
Energy losses in CSP plants are basically of two types: optical and thermodynamic. The optical losses are due to different factors, which vary with the mirrors properties, the plant location and the period of the year. These losses are generally classified as follows: (i) the reflection losses are due to the non-perfect specular reflection on the mirrors surface, (ii) the cosine losses are due to the angle between the incident solar rays and the mirrors normal vector, (iii) the shading losses are caused by shading induced by some solar components relative to others (e.g. solar tower/heliostat field), (iv) the blockage losses occur when some reflectors block part of the reflected solar rays especially at low sun elevation angles, (v) the spillage losses are due to a proportion of irradiation that misses the receiver aperture because of tracking inaccuracies and mirrors shape defects, (vi) the attenuation losses are due to atmospheric scattering/absorption of radiation especially in large tower plants where the collector/receiver distance is high. With this in mind, the plant optical efficiency (ηopt) can be written as the product of each factor efficiency, as depicted in Eq.I.1. = . . ℎ . . . I.1
Accordingly, the optical efficiency of a solar plant is neither a fixed nor a predetermined value. It varies and decreases significantly during the day and throughout the years due to aggressive and repetitive stress factors such as temperature, humidity, saline mist, wind and sand storms (Pescheux et al., 2019). The maintenance of mirrors as well as their frequent washing are essential to guarantee a good conversion efficiency in the long-term. Globally, the lifetime of the solar mirrors is about 25–30 years (IRENA, 2017).
The overall CSP plants efficiency results from the combination of the optical efficiency, the receiver absorption efficiency and the thermodynamic efficiency of the cycle. Assuming that the solar receiver behaves like a grey body and that it only exchanges heat by radiation with outside, the efficiency of a solar receiver brought to the Trec temperature can be written as a function of the geometric concentration C=Amirrors/Arec, the optical efficiency ηopt, the DNI and the receiver radiative properties (α and ε are the absorptivity and the emissivity of the receiver) (Eq.I.2).
Where Qabs and Qrec represent the absorbed solar power and the one hitting onto the receiver, Arec is the receiver surface (receiving the radiation) and DNI (W.m-2) is the Direct Normal Irradiance, a measured solar data characterizing the flux received at Earth surface, which is perpendicular to sunlight. σrad is the Stefan-Boltzmann constant (~5.670 374.10-8 W.m-2.K-4).
The theoretical (maximum) thermal-to-electrical efficiency is given by the Carnot efficiency (Eq.I.3). Accordingly, the overall ideal plant efficiency is given by Eq.I.4, where Tamb and Trec are the lower and upper temperature limits of the Carnot engine.
For thermochemical applications, the primary goal is not to produce electricity, so the coupling with a thermodynamic cycle is generally not considered. The energy efficiency of the solar system is therefore limited only by the receiver efficiency.
The exergy efficiency of thermochemical processes is another interesting parameter to quantify because it gives an indication on how well solar energy is converted into chemical energy. It is defined as the ratio of the maximum work that may be extracted from output products to the solar power input that drives the process. By applying the second principle of thermodynamics, the exergy efficiency of a solar thermochemical process can be expressed by Eq.I.4, which is similar to Eq.I.4 (Steinfeld and Meier, 2004).
Figure I. 4 shows the absorption, the ideal CSP and the Carnot efficiencies of a black body receiver operating at different temperatures and concentration ratios. It can be seen that higher concentration ratios increase the absorption efficiency. Moreover, for a given concentration ratio, the absorption efficiency decreases drastically with temperature. Indeed, the thermal radiative losses evolve with temperature to the power 4. Furthermore, the exergy efficiency curves show that there is an optimum temperature at each concentration ratio that maximizes the conversion efficiency. Above this temperature, re-radiation losses become higher.
Figure I.4 Absorption, ideal CSP and Carnot efficiencies of a blackbody receiver (assuming ηopt=1.0) at different temperatures and concentration ratios (Fletcher and Moen, 1977)
Various innovative concepts were developed to take advantage of solar energy and convert it into added-value chemical products. Powered either by solar electricity, solar photons or solar heat, these concepts were classified in three groups (Steinfeld and Meier, 2004): the electrochemical group converts electric power into chemicals using an electrolytic process; the photochemical and photobiological group uses the solar photons for photochemical and biological processes; the thermochemical group uses high temperature solar heat to drive endothermic chemical reactions. The latter is of particular interest as it offers thermodynamic advantages. In this regard, a number of pioneering processes using either water, carbon dioxide or carbonaceous resources or any combination of the three as a primary feedstock for hydrogen and syngas (i.e. mixture of CO and H2) generation were investigated (Yadav and Banerjee, 2016). Within this scope, the gasification process has shown great promise and has been the subject of several research studies across the globe (Loutzenhiser and Muroyama, 2017; Puig-Arnavat et al., 2013).
Gasification is an endothermic process, which converts biomass into a spectrum of added value and marketable products. In line with the use of solar energy, solar gasification is considered to ensure the complete thermochemical conversion of biomass into syngas. In that respect, solar energy can be stored in the form of gaseous products composed mainly of hydrogen and carbon monoxide. The overall theoretical gasification process either with steam or CO2 can be written as follow:
The actual process is more complex and involves three major steps. First, the pyrolysis consists of a thermal decomposition of wood at high temperature (300 to 1000°C) mainly producing incondensable gases, tars and chars. Then the remaining char is gasified (second step) with the help of an oxidizing agent. Gas phase reactions, such as the reforming reaction or the Boudouard reaction, occur in a third step. In conventional autothermal gasification reactors, the required heat is provided by burning at least 30% of the initial feedstock. The use of concentrated solar energy to drive the endothermal reactions results in saving biomass resources and producing more syngas with a higher gas quality.
Experimental research at laboratory scale gave preference to cavity-type solar reactors where the solar cavity is at the same time the receiver and the reactor. This configuration ensures the highest temperatures, avoids the use of a heat transfer fluid (flowing between the solar receiver and the chemical reactor) and limits the heat losses. Cavity-type solar reactors can generally be classified depending on the method of heating the reactants i.e. directly or indirectly (Figure I.5).
Directly irradiated solar gasifiers generally make use of a transparent window that allows the concentrated sunlight to enter directly the reaction chamber. In such configuration, efficient solar absorption is achieved and heat losses are lowered, which enables to reach and maintain high temperatures (1000–1500°C). However, the transparent window mechanical resistance and cleanness are restraining factors due to inherent limitations regarding pressure and particles/smoke soiling/deposition. Alternatively, indirectly irradiated solar reactors use an opaque heat transfer wall to capture the highly concentrated solar flux and then transfer it to the reaction zone, thus avoiding the need for a transparent window. The type of material selected for the emissive wall should be constrained by its resistance to thermal shocks and maximum operating temperature, while it should offer chemical inertness, radiative absorbance, high thermal conductivity, and suitability for transient operation.
Different performance metrics were used in the literature to assess the performance of the solar conversion. The main performance indicators are recalled here: (i) the Carbon Conversion Efficiency (CCE, Eq.I.7) (ii) the Cold Gas Efficiency (CGE, Eq.I.8) and (iii) the Solar-to-Fuel Efficiency (SFE, I.9). The CCE quantifies the extent of biomass conversion inside the reactor. The CGE (also called upgrade factor) is the ratio between the calorific value of syngas and that of the initial feedstock. The SFE represents the ratio between the calorific value of syngas over the total thermal energy that enters the reactor including solar and biomass.
LHV of syngas/feedstock is the species Low Heating Value (J.kg-1), mi (kg) is the converted/produced feedstock/syngas mass. Qsolar (J) is the solar energy received by the reactor. Some authors expressed the SFE by another formulation, which is shown in Eq.I.10 (Kodama et al., 2002, 2010, 2010; Taylor et al., 1983). With ΔHreaction the energy absorbed by the reaction.
The first solar gasifier (Gregg et al., 1980) was a directly irradiated L-shaped continuous fixed bed reactor (Table I.1-a). It was successful to convert different carbonaceous materials into syngas with more than 20% of the incoming sunlight effectively stored as fuel value in the product gas. Taylor et al., (Taylor et al., 1983a) studied few years later a directly irradiated fixed bed reactor for charcoal, wood and paper gasification (Table I.1-b, left). This reactor was irradiated from the top and the charge was pushed upwards the focal point using a piston as the gasification progressed. The obtained performance was compared to that of a directly irradiated fluidized bed reactor for CO2 charcoal gasification. Compared to its fixed bed equivalent (Table I.1-b, right), which has a SFE (Eq.I.10) of 40%, the fluidized bed reactor reached only 10% efficiency at 950°C because of the more pronounced radiation losses on the upper part of the reactor and additional sensible losses in the exit gas. Piatkowski et al. (Piatkowski et al., 2009) studied another fixed bed reactor with batch mode operation (Table I.1-c). This reactor was composed of two cavities to minimize the heat losses; the upper one receives the solar radiation and heats the emissive plate, the lower one gets heated by the hot emitter to drive the thermochemical gasification reactions. The reactor was used to convert a wide variety of carbonaceous waste feedstocks such as charcoal, scrap tire powder, industrial sludge and sewage sludge. Its main drawbacks were the very low conductive heat transfer rate throughout the bed that entailed significant temperature gradients and build-up that leads to slagging and sintering inside the reactor. CGE and SFE (Eq.I.9) maximum values were 1.30 and 0.29 respectively and were achieved with beech charcoal feedsotck. Kodama et al. (Kodama et al., 2002) gasified bituminous coal with CO2 in a small-scale fluidized bed reactor directly irradiated by a Xe lamp from the side (Table I.1-d, left). The fluidized coal particles were in direct contact with the quartz window, which was detrimental to its cleanliness and mechanical integrity. Therefore, in another study, the concept was improved by developing a new solar fluidized bed reactor irradiated from the top to insure a gap between the window area and the reacting particles (Kodama et al., 2010) (Table I.1-d, middle) a maximum SFE value of 14% (expressed by Eq.I.11 and not considering the CO2 term) was achieved at optimal conditions. This fluidized bed reactor was further improved (Gokon et al., 2012) by adding a draft tube in the center of the reactor, which allowed homogenizing the temperature throughout the bed (Table I.1-d, right) and enhance the stirring. CCE values of up to 73% were achieved. Entrained and vortex flow reactors were also solarized, and tested under both direct and indirect heating modes. Z’ Graggen et al., (Z’Graggen et al., 2006) studied directly irradiated petroleum coke steam-gasification (Table I.1-e). The vortex flow configuration allowed achieving a CCE of up to 87% in a single pass of 1s residence time and a SFE (Eq.I.10) ranging between 5-9%. In the same vein, Müller et al. (Müller et al., 2017) gasified charcoal-water slurry in a windowless indirectly irradiated reactor at elevated pressure ranging from 1 to 6 bar (Table I.1-f). The radiations first heated a U-shaped SiC cavity, around which the gas-particle stream flows in the form of a vortex. A CCE of more than 94% was obtained in less than 5s with a CGE of 1.35 and a SFE (Eq.I.9) of 0.29. Other reactor designs were investigated such as two-zone drop tube/packed bed reactors (Bellouard et al., 2017b; Kruesi et al., 2013) where a porous material was placed in the hot region of the reactor to increase the particles residence time in the reaction zone, and molten salt reactors (Hathaway et al., 2014) in which molten salt was used as both heat transfer medium and catalyst for the reaction.
In short, the main studied reactor designs were packed-bed, fluidized bed, entrained and vortex flow reactors. Packed-bed reactors were operated in batch and continuous mode. These reactors accepted a wide variety of feedstocks with different shapes and sizes thanks to long residence times. However, they faced inherent issues related to high tars content, unreacted products and temperature gradients within the bed due to poor heat and mass transfer rates. Fluidized-bed solar reactors were developed and tested to achieve homogeneous temperature distribution. They required excess steam or inert carrier gas to achieve effective mixing, which reduced their net energetic performance. Entrained and vortex flow solar reactors offered excellent heat and mass transfer rates. Their drawback is the short residence time, and the need for finely ground particle (<1 mm) feed along with the gasifying agent.
Management of intermittency
The significant interest and benefits of solar gasification raise the need to address the issue of solar energy intermittency to tackle varying input power and to ensure continuous operation. The continuous duty of solar gasification was pointed out very early. Bruckner (Bruckner, 1985) proposed a novel high temperature approach that increases the throughput, decreases the cost of solar driven gasification and deals with solar energy fluctuations. It consisted primarily of separating the reactor from the receiver. The receiver, placed at the top of a solar tower was filled with molten slag and was heated to temperatures up to 1800K. A thermal storage refractory vessel of molten slag was designed to insure the supply of energy to the gasifier during off-sun periods for up to 16 hours. The schematic of the process is shown in Figure I.6.
Since then, other innovative concepts were published to overcome the discontinuous solar flux. The work that comes closest to Bruckner’s study considering concentrated solar energy and gasification distinctly is that of Xiao et al. (Xiao et al., 2013), Wu et al. (Wu et al., 2020) and Guo et al. (Guo et al., 2015). Xiao et al., 2013 studied experimentally a supercritical water gasification process heated by molten salt in a drop-tube helical heat exchanger/reactor (Table I.2-a). The reactor consisted of two concentric tubes where the molten salt and the supercritical water/biomass mixture flow separately. The molten salt was heated in a solar receiver represented by an electrical heater and flowed in closed circuit between the storage tank, the solar receiver, the reactor and the preheater (of biomass and water). In line with this, Wu et al., 2020 used parabolic dish collectors to generate high temperature steam, which was fed directly to a conventional gasification reactor (Table I.2-b). The high temperature steam served at the same time as the heat transfer fluid and the reactant of steam-pyrogasification. When the solar intensity was lower than the design point, air was injected in the gasifier to supply the reactor with the deficient process heat thanks to partial feedstock air-combustion. Guo et al., 2015 simulated a dual fluidized bed gasifier where the bed particles serve both as the heat storage media of high temperature solar heat and as bed materials for fluidization (Table I.2-c). In fact, the thermal energy required by steam-gasification was provided by the hot bed materials, which were heated in a solar receiver. The bed materials described a complete loop between the warm tank, the solar receiver, the hot tank, the gasifier (which cools down the inert particles due to the endothermic nature of the reactions) and the combustor. The flow rate of the bed materials sent to the solar receiver, the hot materials storage tank level, and air injection in the combustor were controlled to deliver constant gas production rate and quality despite solar energy variability.
Besides, several studies concerning the management of solar intermittency were carried out on cavity-type reactors. This alleviates the complex interaction and control between the different systems components (including the HTF, piping, solar receiver, chemical reactor, storage units, heat exchangers, etc.) in addition to the significant energy and material savings that the configuration provides. Hathaway and Davidson (Hathaway and Davidson, 2017) proposed a novel solar reactor concept that makes use of molten salts acting as both reaction and heat storage media (Table I.3-a). Beyond the thermal benefits that molten salts provide concerning the stabilization of process temperature, molten salts were found to provide an effective catalytic effect on the gasification process, thereby improving the gas quality. Alternatively, a number of system-level simulation studies on biomass and coal continuous steam solar gasification considered in-situ injection of pure O2 inside the solar cavity to overcome solar energy transients and elevate the reactor temperature (Kaniyal et al., 2013; Li et al., 2018; Sudiro and Bertucco, 2007). Muroyama et al. (Muroyama et al., 2018) were the first to demonstrate experimentally on an indirectly irradiated fluidized bed reactor the effectiveness of hybridization (i.e. combined solar gasification and oxy-combustion) to increase the reactor temperature (Table I.3-b).
Table of contents :
I. Chapter 1 Background on concentrated solar energy, gasification and modelling
I.2 Concentrated Solar Energy
I.2.2 CSP in the world
I.2.3 Concentrating solar systems
I.2.4 Performance metrics
I.3 Solar gasification
I.3.1 Reactor designs
I.3.2 Management of intermittency
I.3.3 Scale up
I.4 Spouted bed reactors
I.4.3 Pyrogasification reactors
I.5 Pyrogasification: modelling and simulation
I.6 Conclusion and methodology
II. Chapter 2 Numerical and experimental study of a novel solar reactor for biomass gasification
II.2 Allothermal operation
II.2.1 Principle and objectives
II.2.2 Experimental test bench
II.2.3 Time scales analysis
II.2.4 Model development
II.2.5 Experimental tests
II.2.6 Numerical study
II.3 Hybrid operation
II.3.1 Principle and objectives
II.3.2 Numerical study
II.3.3 Experimental study
II.4 Waste solar gasification
II.4.1 Principle and objectives
II.4.2 Experimental study
III. Chapter 3 Experimental and numerical investigation of inert bed materials effects in a high-temperature conical cavity-type reactor for continuous solar-driven steam gasification of biomass
III.2 Bed materials
III.3.1 Geometry, mesh and boundary conditions
III.3.2 Mathematical formulation
III.3.3 Numerical procedure
III.4 Results and discussion
III.4.1 Numerical study
III.4.2 Experimental study
IV. Chapter 4 Dynamic simulation and scale up study of a hybrid solar gasifier for biomass steam gasification
IV.2 Model development
IV.2.1 General principle
IV.2.2 Model parameters
IV.2.3 Mathematical model formulation
IV.3 Results and discussion
IV.3.1 Model validation at 1.5 kWthermal scale
IV.3.2 Large-scale reactor simulation
IV.3.3 Annual simulation
V. Chapter 5 Large-scale hydrogen production from solar-driven steam gasification of biomass: a technoeconomic study
V.2 Solar hydrogen cost model
V.2.1 General principle
V.2.2 Model assumptions
V.2.3 Design parameters
V.3 Results and discussion
V.3.1 Cost assessment
V.3.2 Comparison with other hydrogen production methods
CONCLUSION & PERSPECTIVES
VI. ANNEX 1
VI.1 Comparison between REACSOL design and literature recommendations
VI.2 Time scale characteristics
VI.2.1 Fluid dynamics
VI.3 Patent applications
VI.3.1 Heat exchange intensification
VI.3.2 Melting ash continuous evacuation
VII. ANNEX 2
VII.1 Bed materials hydrodynamic simulations
VII.2 Chemical equilibrium model for 0D simulations
VII.2.1 Optimization problem formulation