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In solar tower power plants, aim point optimization is suitable to find aim point distributions resulting in intercept powers close to the theoretical maximum. However, the application in real time operation often faces the problem of long optimization duration. To counteract this issue, the convergence of an existing strategy, the ant colony optimization meta-heuristic, is enhanced. The raytracing is already replaced by pre-calculated flux maps of the individual heliostats in previous works to increase the optimization speed. In this work, the optimization is merged with a grouping strategy and implemented on a GPU to achieve further time reductions. Here, a k-means clustering algorithm performs the heliostats grouping. The use of groups reduces the solution space for the optimizer and additionally the amount of pre-calculated flux maps, so that the data fits in the global memory of the GPU. Over 100 billion flux values can be evaluated per second using this adapted approach. In this way, the algorithm finds suitable aim point distributions within a few seconds up to a minute. The achieved intercepts are 1–4% higher then those found by a single factor aiming strategy for the evaluated central receiver reference power plant. Moreover, the approach has proved its applicability in clouded environments that lead to spatially fluctuating solar radiation. There, a spillage reduction compared to the single factor aiming of 35% is reached.

15. ASME Energy Sustainability Conference, ASME ES 2021, online, 16 Jun 2021 - 18 Jun 2021; Journal of solar energy engineering 144(3), 030903 (2022). doi:10.1115/1.4053464, Published by ASME, New York, NY

Author(s): Kolsch, B; Walker, IS; Schiricke, B; Delp, WW; Hoffschmidt, B | Abstract: Today, the fan pressurization method is the most frequently used method to evaluate a building's airtightness. However, the localization and quantification of leaks remain difficult. In this paper, an acoustic method is introduced to estimate the leakage size of single leaks. Acoustic and airflow measurements were conducted and compared in laboratory tests within the same boundary conditions. This work aims to investigate if various leak sizes can be predicted using acoustic measurement methods. The test apparatus consists of two chambers, separated by a test wall. This wall represents a single characteristic air leakage path. Various types of wall structures with different slit geometries, wall thicknesses, and insulation materials were investigated. The acoustic measurements were performed with a sound source placed in one chamber and ultrasonic microphones located in both chambers. These results were compared to measured airflows through the test wall to provide estimates of uncertainty in the acoustic approach, which indicate a linear trend. Finally, these laboratory measurements were compared to the same measurements at a real office building. Although the acoustic measurement uncertainty is still significant (greater than +/- 50%), the acoustic method has the potential to give an order of magnitude of single leak sizes.

Existing buildings often have low energy efficiency standards. For the preparation of retrofits, reliable high-quality data about built-in materials is required. Contactless measuring technologies, especially microwave radar, have the potential to enable an easy-to-apply and automatable way to analyse the structures and thermal properties of existing building walls, but the relationship between materials, their thermal properties, and their electromagnetic properties, such as the permittivity, is needed for its application. This article presents an analysis of the relationship between electromagnetic and thermal properties for a variety of building materials. Systematic measurements were performed for samples (burnt clay bricks, calcium silicate bricks, autoclaved aerated concrete and lightweight concrete) mainly originating from demolished buildings. The thermal conductivity, thermal capacitance, and dielectric permittivity were measured and the hypothesis of a correlation between permittivity and thermal parameters was partly confirmed. This information is a prerequisite for using microwave radar sensing technology to determine heat transfer coefficients of existing building walls. The next research step is the development of a corresponding measurement and evaluation method.

Building stocks account for a large share of energy consumption and harbour great potential for reducing greenhouse gas emissions. The field of urban building energy modelling (UBEM) offers a range of approaches to inform climate protection policies, producing output of different granularity and quality. We compare two typology-based (archetype) approaches to urban heat demand calculation in a mixed-use area in Berlin, Germany. The goal is to show challenges and pitfalls and how remote sensing can improve the modelling. The first approach uses 2D cadastral data and specific heat demand values from a typology. For the second approach, we derive a 3D building model from aerial imagery, use parameters from the same typology, and calculate the heat balance for each building. We compare the differences in several geometric parameters, U-values and the heat demand. Additionally, we analyse if window detection on aerial image textures and surface temperatures from aerial infrared thermography can improve the estimated window-wall ratios and U-values. The two heat demand approaches lead to different results for individual buildings. Averaging effects reduce the differences at an aggregated level. Remote sensing can be used to improve some geometric parameters needed for modelling, but still requires additional research regarding U-value estimation.

Heating energy represents a significant proportion of the total energy consumption of theFederal Republic of Germany. Thus, a reduction of heating energy demand in buildings is one of the central points of German energy saving and climate goals. Considering that the current rate of new building construction is approximately 1 %, the most significant energy saving potential lies in existing buildings. The analysis of existing buildings will thus become more important in the years to come. As a result, the development of new measurement systems to assess the need for renovation is required. In this context, detailed Information about heat insulation and airtightness is essential. The current approaches to detect air leakage in building envelopes are mostly time consuming and expensive. Therefore, the development of an advanced and more inexpensive method is a vital step. In this paper, we2 present a study about the feasibility and suitability of ultrasound technology to evaluate the size of air leakages in building envelopes. The advantage of ultrasound technology is that the corresponding wavelengths are small enough to penetrate small openings and that disturbing background noise is usually low. Moreover, the frequencies are above the human threshold of hearing; hence, working and living inside the building would be affected less by the measurement than by a blower door test or leakage detection with audible sound. To examine the suitability of this technology, we use an experimental setup of ultrasound microphones and a speaker in a test chamber. Within this setup, we measured different hole sizes and assessed whether they could be detected using ultrasonic frequency swept signals.

The air return ratio is a key factor for the overall efficiency of the open volumetric receiver concept. Although first measurements of the air return ratio exist for smaller setups of the open volumetric receiver concept, so far no measurement of the air return ratio has been presented for the Solar Thermal Test and Demonstration Power Plant Julich ( ≈ 1.5 MW, ≈10 (kg air)/s; ≈700 °C). This paper describes the application of a tracer gas method at the solar tower Julich to determine this substantial ARR. As tracer gas the environmentally friendly helium has been chosen. The helium is injected dynamically into the circular air flow of the system and the helium mole fraction is measured using a mass spectrometer. The dynamic concentration response of the system is used to determine the air return ratio. This dynamic method only requires one location of measurement. First measurements with this dynamic method were conducted at the solar tower Julich. The ARR of STJ was measured with and without irradiation of the main receiver with high accuracy. Under low-wind conditions and without irradiation of the main receiver the air return ratio was measured to be ( 67.7 ± 0.5 ) % for an air mass flow of ( 9.96 ± 0.04 ) kg / s . A slightly higher air return ratio of ( 68.6 ± 0.7 ) % was measured under irradiation with an air mass flow of ( 9.94 ± 0.04 ) kg / s . The air return ratio was sensitive to the air mass flow, showing significantly lower rates when moving further away from the 10 kg / s design air mass flow to 5 kg / s .

The transition to renewable energies has mainly happened in the field of electric energy. In industrial processes only a negligible share of the energy consumption is supplied by renewable energies. To increase this share, several processes were proposed in the past. However, for a meaningful evaluation and optimization of such processes, no appropriate criterion exists. In particular, renewable energy is still the limited resource. To identify the best use of renewable energy, a new evaluation procedure for renewable industrial processes is proposed. The novel procedure is applied to the co-production of electricity and methanol from solar energy and natural gas. The SOLME process for solar methanol production is proposed, optimized and evaluated with the proposed criterion. The results indicate that the proposed SOLME process can make more efficient use of the solar energy than conventional solar power plants. Hence, the solar methanol production is promising to initiate a further penetration of solar energy into the chemical industry.

Flux density measurement is precious for operation as well as for performance testing of solar power towers. The radiation reflected off the absorber can be used for this task. The disturbance of the plant operation is close to zero. Besides a digital camera, a computer and optionally a radiometer, almost no further hardware is required. That’s why it is suitable for industrial-scale receivers. This paper presents several enhancements for this measuring process. Especially the so-called “scan method” appears promising, because it leads to significantly smoother flux density results compared to formerly presented methods. The article also deals with the determination of the irradiation’s directional composition as well as with calibration routines. The proposed innovations were demonstrated at the Juelich Solar Tower with its open volumetric receiver and are likely to be suitable for external tube receivers, too. A plausibility check using CFD simulations was carried out. In summary, the discussed techniques show promise for future application at large-scale solar towers.

In solar reforming, the heating value of natural gas is increased by utilization of concentrated solar radiation. Hence, it is a process for storing solar energy in a stable and transportable form that also permits further conversion into liquid fuels likemethanol. This process has the potential to significantly decrease the natural gas consumption and the associated CO2-emissions of methanol production with only few open questions to be addressed prior to commercialization. In the medium and long term, it has the potential to generate methanol as an environmentally friendly fuel for both transport aswell as flexible electricity production in combined cycle gas turbines, when biogas is used as reactant. In a previous study the high potential of indirectly heated solar reforming with solar air receiverswas shown; however, the efficiency is limited when using state of the art open volumetric receivers. Therefore, different types of air receivers are implemented into an indirectly heated solar reforming process and the overall efficiency potential is assessed in the present study. The implemented receivers are an open volumetric cavity receiver, a closed volumetric cavity receiver and a tubular cavity receiver. The open volumetric cavity receiver and tubular cavity receiver achieve the best results due to their capability of operating efficiently at temperatures well above 700 °C. For these receivers peak efficiencies up to 29% and 27% respectively are predicted. As the utilization of an open volumetric cavity receiver constitutes an open heat transfer loop, the air return ratio together with air return temperature is identified as a major impact factor for overall efficiency. With an improved air return ratio, annual process efficiency of 18.4% is feasible. Considering its relative technical simplicity, this makes indirectly heated solar reforming a promising technology to efficiently provide renewable energy for methanol production and thereby diversify solar energy utilization.

A high share of heating energy consumption by buildings is due to poorly insulated building envelopes. Identification of poorly insulated building envelopes is still a challenge. Nowadays, remote sensing equipment has become increasingly inexpensive and accessible. This paper evaluates the potential of using unmanned aerial vehicles (UAVs) carried equipment for quantitative analysis of the thermal properties of building envelopes. A comprehensive uncertainty analysis of the U-value measurement is conducted. In the uncertainty budget, an error in temperature measurement with IR camera contributes the largest share of uncertainty. Additionally, IRT measurements are compared with the heat flux meter (HFM) method.

We investigated the forced convective heat loss from a model of a multi-megawatt cavity receiver of a concentrated solar power (CSP) tower system in a high-pressure wind tunnel. Measurements of 5 geometrical configurations of this model as well as measurement uncertainties are reported in this contribution. The experiment covered a Reynolds number range of between 2 · 10 6 and 8 · 10 6 , based on the external dimensions and flow field. In general, the measured values are highly sensitive to the geometrical configuration, the wind velocity, and the wind direction. The results show that the maximum forced convective heat loss for all configurations occurs when the wind blows from frontal directions of between 60 ° and 80 ° relative to the tower symmetry plane. We found that the peak location does not vary for different inclinations, but does vary for different aperture openings. Also, the results show that the direction of the wind causes the forced convective heat loss to vary with a factor of up to 6.1, but at least with a factor of 2.6. Last but not least, our power-law correlation of the dependency of the forced convective heat loss on the Reynolds number matches literature values.

We investigated the forced convective heat loss from a model of a multi-MW cavity receiver of a concentrated solar power (CSP) tower system in a high-pressure wind tunnel. Measurements of 5 geometrical configurations of this model are reported in this contribution. The experiment covered a Reynolds number range of between 2 ⋅ 106 and 8 ⋅ 106, based on the external flow field. The results show that the maximum forced convective heat loss for all configurations occurs when the wind blows from frontal directions of between 60° and 80° relative to the tower symmetry plane. We found that the peak location does not vary for different inclinations, but does vary for different apertures. Also, the results show that the direction of the wind causes the forced convective heat loss to vary with a factor of up to 6.3. Last but not least, we augmented the interpretation of our heat loss measurements with images produced with a Background-oriented Schlieren Imaging (BOS) setup.

As of now the worldwide solar research community has a unique new tool: Synlight is a high-flux solar simulator of a new performance category. It bridges the gap between laboratory scale tests and demonstration, and offers large-scale testing opportunities for up to three independently operating test campaigns. Following the commissioning and opening in March, 2017, the facility has been validated recently. The gained results met or exceeded the expectations from simulation and prototype. In particular, solar radiation powers of up to 310kW with a simultaneous peak flux of 12.5MW/m² could be measured.

In solar reforming, the heating value of natural gas is increased by utilization of concentrated solar radiation. Hence, it is a process for storing solar energy in a stable and transportable form that also permits further conversion into liquid fuels like methanol. The feasibility of solar reforming is proved. However, its overall process efficiency potential has not been studied systematically. In this work, an indirectly heated solar reforming process with air as heat transfer fluid is designed and modelled to produce syngas suitable for subsequent methanol synthesis. For provision of solar high temperature heat, an open volumetric receiver is implemented into the process model and the overall performance is investigated. Results show the paramount significance of the air return ratio of the receiver and its ability to achieve high efficiencies at temperatures above 850 °C. For realistic air return ratios, design point process efficiencies of 19% can be achieved, for an increased air return ratio, values up to 23% are feasible. The determined corresponding annual efficiencies are 12% and 14% respectively. Considering its relative technical simplicity this makes indirectly heated solar reforming a promising technology to overcome the current limitations of solar energy in the medium term.

The influence of wind on the convective losses of cavity receivers for solar power towers was analyzed experimentally in a cryogenic wind tunnel. At an ambient temperature of −173 °C a Grashof number of Gr = 3.9·1010 can be reached. Six different wind directions ranging from head-on to rearward flow, five wind speeds up to Re = 5.2·105 and four inclination angles of the cavity in the range of ϕ = 0°…90° were analyzed. With a similarity approach the results can be transferred to a receiver in normal ambient conditions with an inner diameter of 2.4 m and a wall temperature of approximately 730 °C. The methodology and its restrictions are discussed in detail. The experiments show that the influence of wind on large horizontal receivers is small. However, with increasing inclination angle the receiver becomes more susceptible to wind, although the convective losses never exceed those of the horizontal cavity. In some cases a reduction of the convective losses under the level of natural convection was observed if wind is present. Additionally, local information about the heat losses of different heater sections are presented, which are used to analyze the heat loss mechanisms inside the cavity. A shrinking of the stagnant zone is found to be the main reason for the increasing losses.

Concentrated Solar Power plants (CSP) provide several advantages to other energy conversion concepts. However, they are still not competitive in price, compared to conventional power plants. But there is still potential to reduce the cost. The higher heat conductivity of liquid metals leads to very high heat transfer coefficients - ten times higher as with molten salts. This big advantage could increase the efficiency and lower the cost of CSP plants. In order to optimize such a power plant regarding efficiency and cost, detailed information of each component of the system is necessary. This paper shows several system concepts in which liquid metals can be used as heat transfer fluid. In addition, several liquid metal pump types are investigated as well as thermal energy storage concepts. With the assumptions and boundary conditions taken in this model, direct thermal energy storage with liquid metal is prohibitive. But some innovative indirect storage systems with liquid metals show costs in the same range as with a 2-Tank molten salt storage. (C) 2015 The Authors. Published by Elsevier Ltd.

Current measurement techniques do not allow the visualization of the return air flow of open volumetric receivers in solar tower power plants. The reason is that the region of interest is irradiated by concentrated solar radiation and is located on top of a tower. Therefore, a novel method of measurement, the Induced Infrared Thermography (IIT) is introduced within this paper. With this method the return air can easily be observed with an infrared camera. As air has a very low emissivity in the infrared region the activity has to be induced by the addition of an infrared-active component, here carbon dioxide. The temperature of the infrared-active component has the greatest influence on the signal strength but the mole fraction of the component and the distance to the infrared camera are also important. Due to temperature restrictions, the measured signal to noise ratio is low and therefore several post-processing steps have to be conducted to visualize the return air. The most important step for the visualization is subtracting a background image. Furthermore, a video filter is employed in order to remove noise. A time series of ITT images can be used to obtain information on the velocity fields of the flow. The process, called Infrared Image Velocimetry (IRIV) here, is similar to Particle Image Velocimetry (PIV) and is applied in this paper to external fluid flows of the open volumetric receiver. As IRIV is in an early stage of development the depicted results are treated as qualitative vector fields.

In several studies, the efficiency of solarized industrial processes was assessed and showed promising results. However, the economic competitiveness of such processes is a mainly unanswered question. In this work, the economic performance of a solar aided methanol production process is determined. The assessment yields promising results for the considered co-production process. The presented results also show that for a penetration of renewable energies beyond electricity production, the common support model of fixed feed-in-tariffs is not suitable. A reasonable cost for CO2 emissions or reward for their reduction will allow finding the most economic application for renewable energies independent of the end product of the process. When reasonable rewards of 75 $/tCO2 for the reduction of CO2 emissions compared to fossil reference processes are considered, the process can produce methanol and electricity at competitive prices.

In the last decade, different absorber geometries, such as foams and honeycombs, have been tested at laboratory or industrial scale in order to achieve high performance in the conversion of the solar radiation into usable heat, with the current state-of-the-art , the HiTRec-II monolithic honeycomb, characterized by a square-channel section and made out of siliconized silicon carbide (SiSiC). Such geometry has been so far the best compromise for large-scale application thanks to the low production costs, easy manufacturability through extrusion procedure and overall acceptable performance. However, it does present some drawbacks, since the geometry is not able to contain the radiative heat losses, especially from the front surface. An optimized absorber geometry, capable to reduce overall thermal losses, is presented in this work, being able to increase the final thermal efficiency of more than 12% compared to the current state-of-the-art and showing the presence of the so-called volumetric effect, since the outlet fluid temperature is higher than the solid inlet temperature. A test sample has been produced for laboratory-scale experiments, in the form of a 3:1 scaled prototype through additive manufacturing procedure, using a titanium-aluminium alloy (Ti6Al4V) and the experimental results were in good agreement with the numerical calculation, with a deviation of 3%, computed considering a 3:1 Ti6Al4V scaled-up sample. As the manufacturing technology will progress and become cheaper in the near future, it will be possible to improve the overall Solar Power Tower (SPT) plants performance using advanced micro-geometry for open volumetric receivers.

The design of central receivers in solar thermal power plants is crucial for efficiency and operating behavior of the plant. The local heat flux distribution on the absorber tubes varies, depending on time, weather conditions and aim point strategy. A proper receiver design needs accurate thermodynamic models to detect local temperature distribution, especially hot spots on the absorber tubes. Moreover, due to highly transient boundary conditions, the dynamic behavior of the model is important. Starting with a detailed CFD model of a single receiver tube several simplified FEM models were investigated. The influence of an inhomogeneous heat flux distribution on the absorber and the dynamic behavior after a sudden change of heat flux (e.g. due to passage of clouds) were analyzed. In order to consider radiation exchange between surfaces, simulations with a whole receiver panel were also conducted. The FEM model with one-dimensional fluid elements and constant heat transfer coefficients shows a very good agreement with the detailed CFD model. Further simplifications like the presented model, where the tubes are discretized as projected surfaces are computationally very efficient and can be used for relative comparison between receiver configurations. However, this simplification has deviations in the prediction of tube temperatures and radiation losses. Finally, the receiver simulation of the Solar Two power plant validates the FEM model with the measured data for solar salt. The investigation of liquid metals considers a single tube with an inhomogeneous heat flux on its surface. The detailed analysis shows, that the Nusselt number correlation plays an important role for the tube wall temperature. If the Nusselt number is overestimated in the region of the peak heat flux, the simplified model results in a lower tube wall temperature.

German Aerospace Center (DLR) is currently constructing a new high-flux solar simulator synlight which shall be commissioned in late 2016. The new facility will provide three separately operated experimental spaces with expected radiant powers of about 300kW / 240kW / 240kW respectively.synlight was presented to the public for the first time at SolarPACES 2015 [1]. Its engineering and erection is running according to plan. The current presentation reports about the engineering and the ongoing erection of the novel facility, and gives an outlook on its new level of possibilities for solar testing and qualification.

This article presents expressions for the Sherwood number as a function of the Schmidt and Grashof numbers for the evaporation–condensation thermal processes in water at temperatures ranging from 70 °C to 99 °C. The aim is to predict the condensation rate in these processes using temperature data. To collect experimental data to develop the expressions, a one-stage thermal desalination unit was built with controlled temperatures. In the measurements, water evaporation temperature ranged from 20 °C to near boiling, at atmospheric pressure. By using a well thermally insulated unit, all losses were minimized and only drop back losses, which represent drops that do not reach the collection flow channel, were presented. These losses were reduced by the condensation surface tilt angle, which facilitated the flow of condensate. The measured temperatures and condensation rates allowed the calculation of the mass transfer coefficient and the Sherwood number, considering the average distance between the plates as the characteristic length. To estimate the constants in the expression for the Sherwood number, a non-linear regression model was implemented for 698 data points. The condensation surface tilt angles were: 10°, 14°, and 16.8°. The results show that the expressions are more adequate for a temperature range of 70 °C–95 °C, where there is a lack of more information in the literature.

Qualitative measurements of leaks in air flows are frequently conducted in engineering. However, an environmentally friendly leak rate measurement technique which yields quantitative results and is applicable to hot, large-scale circular air flows (≈10 kg/s; ≈ 700 °C) with high leak rates has not been presented yet. This paper describes the development, test and validation of a stationary and a dynamic, helium based tracer gas method on a lab-scale model of a solar air receiver with partial air recirculation. Helium is chosen as tracer gas since it is environmentally friendly, stable under high temperatures and is also cheap for large-scale air flows. The tracer gas is injected either continuously or intermittently into the model system, its concentration is measured using a mass spectrometer and the static or dynamic concentration response of the system is used to determine the leak rate. The stationary method needs two measurement points upstream and downstream the leak, the dynamic method only one measurement point if applied to a circular air flow system. Since the dynamic method is time dependent the transfer function of the measurement setup was determined and the dynamic measurement error was considered. An extensive uncertainty analysis is presented for both the stationary and the dynamic method. Exemplary measurements were conducted at the model system with very good results. Both dynamic and static measurements yield the same result within their confidence intervals. The leak rate of the solar receiver model with a mass flow rate of m in = ( 0.247 ± 0.008 ) / kg s was measured to be l stat = ( 36.1 ± 2.3 ) % and l dyn = ( 34.5 ± 3.6 ) % with the static and the dynamic method respectively.

Convective losses reduce the efficiency of cavity receivers used in solar power towers especially under windy conditions. Therefore, measures should be taken to reduce these losses. In this paper two different measures are analyzed: an air curtain and a partial window which covers one third of the aperture opening. The cavity without modifications and the usage of a partial window were analyzed in a cryogenic wind tunnel at −173°C. The cryogenic environment allows transforming the results from the small model cavity to a large scale receiver with Gr≈3.9·1010. The cavity with the two modifications in the wind tunnel environment was analyzed with a CFD model as well. By comparing the numerical and experimental results the model was validated. Both modifications are capable of reducing the convection losses. In the best case a reduction of about 50 % was achieved.

Nowadays, due to the pressing global energy demand, a possible source of renewable energy is represented by solar energy. Concentrated solar power (CSP) represents an excellent alternative, or add-on to existing systems for large-scale energy production. In those systems and, particularly, in solar tower plants (STP), mirroring surfaces reflect sunlight, focusing it on the core-component defined as volumetric receiver. Here, a porous structure (absorber) is responsible for converting the solar radiation into heat with the use of evolving air under natural convection. The proper design of this element is essential in order to achieve high efficiencies that can make such facilities extremely advantageous. In this paper, an overview of the different approaches for the numerical modeling developed at the German Aerospace Center (DLR), are presented, based both on detailed and homogeneous representation (discrete and continuum approaches, respectively) of the absorber. The corresponding numerical models were run again and compared where possible, using as reference case pre-exiting experimental evaluation, pointing out shortcomings and differences. Temperature profiles for solid and fluid phase, as well as efficiency value, are used as key parameters in the comparison, showing a substantial gap between numerical and experimental results. This inconsistency can be addressed both to inherent difficulties in the experiments butalso on the lack of a proper characterization of the absorber radiative behavior that directly affects the heat transfer process. This gap is even more pronounced when the use of a continuum approach is considered, due to implicit approximation introduced by the homogeneous representation of the porous volume. For those reasons, a better representation of the radiative heat transfer must be introduced in future conjugate numerical models and the use of a continuum approach can be addressed to a preliminary design procedure, given the possibility to quickly manipulate geometric and thermodynamic parameters of porous structures.

This paper gives a general overview of the concept of a high temperature gas-to-particle heat exchanger, the corresponding test facilities and the results of laboratory tests. A description of the optimal bulk material and separator properties and their influences on the operating conditions is also given. The three phenomena pinning, blistering and blocking could be observed during the tests and were analysed in more detail using simulation software.

Due to the continuous global increase in energy demand, Concentrated Solar Power (CSP) represents an excellent alternative, or add-on to existing systems for the production of energy on a large scale. In some of these systems, the Solar Power Tower plants (SPT), the conversion of solar radiation into heat occurs in certain components defined as solar receivers, placed in correspondence of the focus of the reflected sunlight. In a particular type of solar receivers, defined as volumetric, the use of porous materials is foreseen. These receivers are characterized by a porous structure called absorber. The latter, hit by the reflected solar radiation, transfers the heat to the evolving fluid, generally air subject to natural convection. The proper design of these elements is essential in order to achieve high efficiencies, making such structures extremely beneficial for the overall performances of the energy production process. In the following study, a parametric analysis and an optimized characterization of the structure have been performed with the use of self-developed numerical models. The knowledge and results gained through this study have been used to define an optimization path in order to improve the absorber microstructure, starting from the current in-house state-of-the-art technology until obtaining a new advanced geometry.

Since the 80s power production with solar thermal power plants has been a way to substitute fossil fuels. By concentrating direct solar radiation from heliostats very high temperatures of a thermal fluid can be reached. The resulting heat is converted to mechanical energy in a steam cycle which generates electricity. High efficiencies and fast start-up are reached by using air as a heat medium, as well as using porous ceramic materials as solar receiver of the concentrated sunlight. In Germany the construction of a 1.5 MWe solar tower power plant began in 2008. It is operational since December 2008 and started production of electricity in the spring of 2009. In Greece and Cyprus, countries with high solar potential, the development of this competitive solar thermal technology is imperative, since it has already been implemented in other Mediterranean countries.

A new solar thermal desalination system has been developed and tested in field conditions. The system has two components: a desalination tower with multiple stages and one or more solar collectors. Sea and ground water can be used to feed the desalination tower, which produces desalinated and decontaminated water in its 5–7 horizontal stages. The solar collectors are used to absorb solar energy and a fluid, the desalinated water itself, transports the heat energy to the tower. Four different systems have been tested in Germany, Spain, India, and Brazil. The field tests show some important results: the system produces about 15–18 L/m 2 /day, which represents a factor of 5–6 times higher than the production of a still type distiller; it is modular and each unit produces approximately 35 L/day that allows the installation of larger systems (1000–2000 L/day).

This article reports on two novel porous materials, which have been foreseen as volumetric receivers for concentrated solar radiation: a double-layer silicon carbide foam and a screen-printed porous silicon carbide material. Volumetric receivers are used in the solar tower technology. In this technology ambient air flows through the porous solid, which is heated by concentrated solar radiation. A heat exchanger then transfers the energy to a conventional steam turbine process. The general thermophysical and permeability properties of materials required for this application are reviewed. Experimental set-up and results of pressure loss and laboratory scale tests in concentrated solar radiation are reported. For the foam, efficiency data could be determined from the test results. Finally, a comparison is presented between the efficiency properties of the foam and those of materials used for the same application until now. This comparison shows, that the efficiency of the double-layer foam material is significantly higher. Up to now, porous materials consisting of a parallel channel geometry with thin walls showed disadvantageous permeability properties. By applying a new manufacturing process and modifying the channel geometry, the permeability properties of the printed material could be significantly changed, so that it now meets the requirements for an application as a volumetric receiver.

Porous solids like extruded monoliths with parallel channels and thin walls made from various oxide and non-oxide ceramics, ceramic foams and metal structures have been tested in the past with the objective of applying them as open volumetric receivers in concentrated solar radiation. In this application, ambient air flows through the solid, which is heated by concentrated solar radiation. A heat exchanger then transfers the energy to a conventional steam turbine process. In all cases, to obtain high efficiencies, high absorptivity in the visible and near infrared range has to be combined with a high porosity to create large surfaces for convective heat transfer from the solid absorber to the fluid. However, it can be shown that especially high performance absorbers tend to be sensitive to inhomogeneous flux distributions, which may cause local overheating of the material. In various tests with specific kinds of materials, flow instabilities occurred, which partly leads to hot spots and a sudden destruction of the receiver. To achieve both high efficiencies and reliable operation, an optimised combination of geometrical as well as thermal conductivity and heat transfer parameters has to be selected. A precise knowledge of these quantities for a number of various materials is necessary to estimate the limits for stable flow conditions on the basis of complex numerical simulation programs. Finally, efficiency and performance tests with candidate materials have been carried out. In this paper, the experimental work on a variety of porous materials is reported. The paper will report on methodology and results of thermal conductivity, convective heat transfer coefficient and efficiency measurements of these monolithic materials. It will also present an experimental set-up designed to investigate how the properties of the porous materials affect flow stability. Based on these results, a recommendation for the design of volumetric absorbers will be given.

This paper reports results from long-term durability tests of reflector materials to be used for solar concentrating systems. The studies have been conducted under the auspices of an IEA–SolarPACES collaboration between the National Renewable Energy Laboratory (NREL, USA), the Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT, Spain) and Deutsches Zentrum fur Luft- und Raumfahrt (DLR, Germany). In this co-operative effort, accelerated ageing tests as well as outdoor exposures at a number of test sites having various climatic conditions have been carried out since 1995. In addition to materials already in use at solar power stations, newer materials offering the chance of a significant cost reduction in solar electricity and process heat generation are being investigated. Comparative optical tests are carried out to assess the efficiency as a function of exposure/service time in a solar concentrator. Among the materials showing promise for long-term outdoor applications are various silvered glass mirrors, a silvered polymer film, and an anodized sheet aluminium having an additional protective polymer coating. In addition to durability tests of reflector material samples, practical results are also reported for experiences with field applications of silvered thin glass and anodized sheet aluminium mirrors.

The High Temperature Receiver (HitTRec) consists of a modular ceramic absorber, a supporting structure and an air-return system. It has been designed to prevent possible flow instability at 700-800°C average outlet air temperature with atmospheric pressure. The HiTRec-II prototype was developed to solve the structural problems of the first prototype (HiTRec-I). Testing in the Plataforma Solar de Almerı´a (PSA) test bed lasted from November 2000 through May 2001, accumulating 150 test hours under concentrated sun. Results demonstrated the durability of the modified stainless-steel structure. Inlet aperture flux was up to 900kW/m2 and average outlet air temperatures of up to 840°C with peak outlet air temperatures of up to 950°C. Thermal efficiency under steady-state conditions was 76±7% at 700°C, nominal conditions for a PHOEBUS-type volumetric receiver. Other performance characteristics were also evaluated (e.g., Air Return Ratio of 46% and characteristic receiver response time of 70 s).

Numerical results found in the literature predict the capability of volumetric receivers to produce gas outlet temperatures of more than 1000°C. Other approaches found the tendency for an inherent flow instability for volumetric absorbers at high outlet temperatures. Both aspects, which are based on one-dimensional approaches, do not fit the experimental experience available even for semi-commercial volumetric receivers in the case of non-homogeneous flux distribution. Since this is the general case in solar power plants, a new analytical approach which takes into consideration a three-dimensional irradiance distribution and its influence on fluid flow and radial heat transfer is presented in this article. A comparison with experimental data for four totally different volumetric absorber types coincides well with the measured overall thermal efficiency as well as with the local temperature distribution. The model helps to explain how thermal efficiencies and temperature distributions are strongly influenced by non-homogeneous irradiation. Moreover, it demonstrates how flow instabilities are partly prevented by radial heat transfer. Finally, it is shown that a volumetric receiver consisting of sufficiently small absorber modules which are equipped with an additional orifice plate at the rear side will always run under stable flow conditions.

This paper describes the modeling of a high-temperature storage system for an existing solar tower power plant with open volumetric receiver technology, which uses air as heat transfer medium (HTF). The storage system model has been developed in the simulation environment Matlab/Simulink ® . The storage type under investigation is a packed bed thermal energy storage system which has the characteristics of a regenerator. Thermal energy can be stored and discharged as required via the HTF air. The air mass flow distribution is controlled by valves, and the mass flow by two blowers. The thermal storage operation strategy has a direct and significant impact on the energetic and economic efficiency of the solar tower power plants.

This article begins with a short introduction and continues with a presentation of solar tower power plants around the world. The focus is set on the developments of the last five years and in the near future of the most important components of a central receiver system (CRS). For each of the components, a description of each technology, its applicability, and the advantages and disadvantages is presented. The chapter also includes a state of the art review for almost each system and points out the Research and Development (R&D) needs. A further issue is the choice of the heat-transfer fluid (HTF) as well as the improvement of new solar tower power plant cycle systems. The paper introduces recent R&D activities, with the majority occurring in Europe and the United States, as has been presented in international journals and conferences by scientific personnel. It also reflects opinions of experts who specialized in the planning and erection of solar power plants and carrying out CRS research projects. The future effort of R&D in these fields should improve efficiencies, increase durability and reliability, and reduce investment and Operation & Maintenance (O&M) costs. Nowadays, and in this decade, the most advances in CRS occur in the United States, in Europe, and in China. Furthermore, a great interest, especially for add-ons to existing research platforms as well as the construction of new ones, can be observed for the last years for countries in the Middle East & North Africa region, such as Morocco as well as India, Australia, and South Africa. WIREs Energy Environ 2017, 6:e217. doi: 10.1002/wene.217 For further resources related to this article, please visit the WIREs website.

Solar thermal concentrated power is an emerging technology that provides clean electricity for the growing energy market. To the solar thermal concentrated power plant systems belong the parabolic trough, the Fresnel collector, the solar dish, and the central receiver system. For high-concentration solar collector systems, optical and thermal analysis is essential. There exist a number of measurement techniques and systems for the optical and thermal characterization of the efficiency of solar thermal concentrated systems. For each system, structure, components, and specific characteristics types are described. The chapter presents additionally an outline for the calculation of system performance and operation and maintenance topics. One main focus is set to the models of components and their construction details as well as different types on the market. In the later part of this chapter, different criteria for the choice of technology are analyzed in detail.

The potential of renewable energies varies significantly from North to South Europe. Southern Europe has a high solar potential and is ideal for the implementation of solar concentrated power plants. To this group of solar thermal power systems belong the solar tower, parabolic trough, solar dish and linear Fresnel systems. North European countries, especially the Scandinavian countries, have a high biomass and hydropower potential. This paper focuses on calculation of the power production for hybrid systems of solar tower with gas turbine in Southern Europe and biogas-only operation in Northern Europe. The solar tower system consists of a heliostat field, which concentrates direct solar irradiation on an open volumetric central receiver. The receiver heats up ambient air to temperatures of around 700°C. The hot air's heat energy is transferred to a steam Rankine cycle in a heat recovery steam generator (HRSG). The steam drives a steam turbine, which in turn drives a generator for producing electricity. In order to increase the operational hours of a solar tower power plant, a heat storage system and/ or hybridization may be considered. The advantage of solar-fossil hybrid power plants, compared to solar-only systems, lies in low additional investment costs due to an adaptable solar share and reduced technical and economical risks. On sunny days the hybrid system operates in a solar-only mode with the central receiver and on cloudy days and at night with the gas turbine only. As an alternative to methane gas, environmentally neutral biogas can be used for operating the gas turbine. Hence, the hybrid system is operated to 100% from renewable energy sources. An advanced software tool library has been developed for modelling such solar hybrid power plants. This library includes the components of the solar-heated hot gas cycle and the steam cycle. Moreover, a choice of different gas turbine and duct burner components is given. When developing a simulation model for the calculation of a small hybrid power plant, components from the library are inserted into the model. The software tool features the possibility of either calculating the energy output of individual operating points or of time intervals in the range of days up to an entire year. With this simulation tool, hybrid solar tower systems are calculated for various locations with high solar potential within Europe. In addition, locations in North Scandinavian countries with high biomass potential are investigated and power plants with biogas as fuel without solar input are calculated.

In view of rising energy prices and an increasing share of power generated by renewable energy sources, the importance of energy storage is growing. In the framework of this project, a thermal energy storage concept for solar power towers is being developed, in which quartz sand serves as a storage medium. Sand is suitable due to its properties such as high thermal stability, specific heat capacity, and low-cost availability. Compared with storages based on ceramic bodies, the use of sand promises to reduce costs of energy storage and thus to reduce the costs of electricity generation. In addition, the storage concept could be applicable in the steel industry. The central element of the storage concept is an air-sand heat exchanger, which is presently under development. This paper describes simulation results and measurements of the heat exchanger prototype. It includes sand flow behavior and experience with different porous walls as well as up-scaling options.