1,721,152 research outputs found
Potenzialanalyse von Solarturmkraftwerken mit Flüssigmetallen als Wärmeträgermedium
No full textToday solar power plants with central receiver technology often use molten nitrate salts like Hitec or Solar Salt as heat transfer fluid and storage medium. Due to their high heat capacity and the low cost these are well suited for thermal energy storage. Nevertheless, these salts also inherit disadvantages, such as high melting points above 220°C demanding high energy for trace heating. The upper temperature limit of Solar Salt is at 565°C. Therefore, high temperature power conversion cycles with high efficiency are unfeasible. Additionally, its high density results in a high pressure drop in the riser, leading to additional parasitic losses. In plants operating with Solar Salt, freeze events and corrosion problems occur. Moreover, high pressure drops have to be accepted in the absorber tubes to achieve a reasonable heat transfer. In all the mentioned points liquid metals possess advantages compared to molten salts. Sodium is the most often used liquid metal in research and industry and was already tested at the solar test center PSA in Almería in Spain. The only disadvantage of sodium - its reactivity with water and oxygen - was demonstrated in a sodium fire and the eventual destruction of the test center in summer 1986. During the last 30 years measurement techniques and safety precautions were developed to avoid such accidents. The present work analyses the properties of liquid metals in detail and compares them with Solar Salt. The custom-built design and simulation tool for tubular receivers ASTRID makes a precise thermohydraulic calculation possible. The assessment of the liquid metal concepts is based on annual yields and LCOE calculations, which are compared to a reference system with Solar Salt. All concepts with liquid metals use electromagnetic pumps. After the solar heating in the receiver the heat is transferred from sodium to Solar Salt in a heat exchanger and then stored in a two-tank storage. Both the reference concept and the liquid metal concepts use the same power block and the same temperatures in storage and turbine. The results indicate a potential to reduction in LCOE with sodium of up to 16 % compared to the reference system with Solar Salt
Methanol Production via Solar Reforming of Methane
No full textIn der vorliegenden Arbeit wird ein solarer Reformierungsprozess zur Methanolherstellung untersucht. Der Prozess stellt eine Möglichkeit dar, die mit diesem Herstellungsprozess verbundenen Treibhausgasemissionen zeitnah bedeutend zu reduzieren. Hiermit wäre ein wesentlicher Schritt in der Entwicklung einer nachhaltigeren Chemieindustrie geleistet. Darüber hinaus lässt sich Methanol auch als Brennstoff einsetzen. So kann Methanol einen Beitrag zu einer klimaschonenderen Energieversorgung leisten, wenn er durch solare Reformierung produziert wird.
Zunächst wurde in der Arbeit ein Gesamtprozess auf Basis der indirekt beheizten solaren Reformierung entwickelt. Hierbei war ein Ziel die anfallenden Abwärmeströme zu nutzen. Infolgedessen wird ein großer Teil der Abwärme in einem Wasser-Dampf-Kreislauf zur Stromproduktion genutzt, da es hierfür keine sinnvolle Verwendung im Prozess gibt. Darüber hinaus wird der Off-Gas-Strom der Methanolsynthese teilweise zur Stromproduktion eingesetzt.
Der entwickelte Prozess nutzt Sonnenenergie und Erdgas und produziert hieraus Methanol und elektrischen Strom. Das Verhältnis der verschiedenen Ströme zueinander ist hierbei durch Parametervariation veränderbar. Eine Optimierung mit herkömmlichen Bewertungskriterien wie Energie- oder Exergiebilanzen ist daher nicht möglich. Folglich wurde ein Bewertungskriterium entwickelt, dass auf dem Ziel basiert den Verbrauch der Fossilen Rohstoffe und die damit verbundenen Treibhausgasemissionen zu reduzieren.
Auf Basis dieses Bewertungskriteriums wurde der Prozess mithilfe von Parametervariationen optimiert. Die Ergebnisse zeigen, dass der Prozess das Potential hat Sonnenenergie effektiver zu nutzen, als dies bei der reinen Stromproduktion der Fall ist. Eine anschließende Wirtschaftlichkeitsbetrachtung zeigte, dass der Prozess zur konventionellen Sonnenenergienutzung theoretisch konkurrenzfähig ist. In der Praxis müssten hierfür jedoch entsprechende Fördermechanismen, wie sie für die Stromproduktion existieren, für die Herstellung von Chemierohstoffen eingeführt werden
Forced convective heat loss from cavities of multi-megawatt scale solar receivers
No full textCavity receivers may increase the efficiency of concentrated solar thermal energy (CSTE) systems because they lose less heat via thermal radiation compared to external receivers. For large CSTE cavity receivers on the multi-megawatt scale, the understanding of forced and mixed convection has not advanced enough to predict the heat loss accurately. Hence, this doctoral thesis focuses on investigating (i) the heat transfer in the forced convection limit (Ri = 10^6); (ii) the heat transfer in the mixed convection regime (Ri ~ 1) for large cavity receivers (Gr >= 10^10, Re >= 10^6); and (iii) possible convective heat loss reduction measures. The forced convective heat loss from 5 geometrical configurations and 3 reduction measures was measured in a high-pressure wind tunnel. All models were scaled and included the relevant part of the tower. The experiment covered a Reynolds number range of between 1.5 x 10^6 and 6 x 10^6, based on the cavity diameter. For the measurements, novel ringlike hot-film sensors were designed and mounted on the inside of the cavity. These sensors were operated with a constant-temperature anemometry (CTA) system. In addition, a numerical model was validated with a selection of the wind tunnel measurement points. The numerical model was then adapted to the original scale for simulations of multi-megawatt cavity receivers in the mixed convection regime. The measurements showed that the forced convection from a cavity without reduction measures strongly varies with a factor of up to 6.1 depending on the wind speed and its direction. With a reduction measure a reduction of more than 50% of the forced convective heat loss may be achieved for specific wind directions. Further, it was observed that the forced convection in the cavity is governed by the external flow characteristics in direct vicinity of the aperture. The simulated cases revealed that the forced convective heat loss contributes substantially to the mixed convective heat loss. The mixed convective heat loss results to be of the same order of magnitude as the radiative heat loss. Finally, it was deduced that the optimization of the design of a multi-megawatt CSTE cavity receiver with respect to convective heat loss is possible when a specific site is given and its meteorological boundary conditions are well known
Design of Advanced Porous Geometry for Open Volumetric Solar Receivers Based on Numerical Predictions
No full textDue to the continuous global increase in energy demand, a possible source of renewable energy is certainly represented by the sun.Concentrated Solar Power (CSP) represents an excellent alternative, or add-on to existing systems for the production of energy on a large scale. In CSP systems, specular surfaces (heliostats) reflect the incoming sunlight, focusing it on a single or multiple focal points.In some of those 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. The core of such receivers is a porous structure called absorber. The latter, hit by the reflected solar radiation, transfers heat to the evolving fluid, which is generally air subject to forced convection. The proper design of this element is essential to achieve high thermal efficiencies, making such structures extremely beneficial for the overall performances in the energy production process.For this purpose, a proper combination of the structure parameters such as porosity, heat exchange surface and optical properties is needed. Thus, a deep preliminary study has been performed to evaluate the consequences of the variation of the effective parameters and properties on the thermal performance of the porous absorber.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 current in-house state-of-the-art technology till obtaining a brand new advanced geometry. The newly designed structure has been numerically tested, achieving good performance and the presence of the so-called volumetric effect, as the outlet fluid temperature is higher than the solid inlet temperature. Afterwards, a test sample has been produced for laboratory experiments at the DLR solar facility in Cologne, in the form of 3:1 scaled-up demonstrator due to technology limits. Additive manufacturing process technology and a titanium-aluminium alloy have been used for the production by the Fraunhofer IFAM Institute. The outcome of the experimental campaign has been useful for the validation of previously predicted numerical results and, in general, of the overall design procedure. This also confirmed that, as the manufacturing technology will progress and become cheaper in the near future, it will be possible to improve the overall performance of SPT using advanced micro-geometry in open volumetric receivers
Investigation of an improved acoustical method for determining airtightness of building envelopes
No full textUnintended air infiltration in buildings is responsible for 30 to 50 % of the buildingstock energy demand. The fan pressurization method, also known as the blower-doortest, is the most frequently used and standardized measurement method to evaluatea buildings’ airtightness and determine the airflow through a building or a buildingelement. While detection and quantification of individual leaks with smoke tracers orinfrared thermography are challenging, time-consuming, and depend on the respectiveoperator’s experience, acoustic methods have the potential to localize and quantifyleaks in building envelopes without the need for pressure or temperature differencebetween in and outside of the examined building. In this thesis, two acoustic methods, coherence measurements and beamforming, areintroduced to this field of application to estimate the leakage size and location ofindividual leaks in building elements. This work aims at finding if different leaksizes can be quantified and detected using these acoustic measurement methods. Foran estimation of leakage size, acoustic and airflow measurements are compared in alaboratory test apparatus. Test walls representing a single characteristic air leakagepath in the building envelope at a model scale and separate two chambers with speakerand microphones. Various types of wall structures with different slit geometry, wallthickness, and insulation materials are tested. The acoustic measurements are performed with a sound source placed in one chamber and ultrasonic microphones located in bothchambers. The results of these measurements are compared to the airflow through thetest wall measured using a flow nozzle. The results from laboratory measurements indicate a linear trend between acoustic coherence and leak size in the investigated range of several mm2. Although the acoustic measurement uncertainty is still significant(≈ ±50 %), the acoustic method shows the potential to give an order of magnitude of leak sizes. The findings are validated in a real building setup using various reproducible leaks constructed with cable ties wedged in a window gasket. The acoustic and airflow measurements in the building show results similar to the laboratory measurements. Further, the acoustic beamforming method (“acoustic camera”) using a microphone array to detect leak locations and visualize them shows promising results. The same constructed leaks in a window gasket are detected. With decreasing leak size and soundpressure, detection frequency increases. The thesis concludes that acoustic methods have the potential to detect and quantifythe leaks in building envelopes without test preparations as for common methods
Einflüsse von Klimavariabilität und -wandel auf Ausbau und Erzeugung im Europäischen Stromsystem
No full textThrough the combustion of fossil fuels, humankind contributes to an increase in the concentration of CO in the earth’ atmosphere and, thus, to increasing global warming. This is known as anthropogenic climate change. One instrument for mitigating the consequences of climate change is the expansion of low-emission technologies such as renewable energies. Less research has, however, been done into the opposite direction of effect, i.e. whether and how climate change affects the energy system. Closely linked to this topic of long-term-climate change is the so-called short-term-climate variability, which is reflected e.g. in warmer/colder or windier/calmer years. The aim of this thesis is the supply and demand-side analysis of climate variability and change effects on the expansion (installed capacity) and the generation (amount of energy) in the European electricity system. For this purpose, a generation expansion planning model encompassing 28 countries has been developed, which is capable of calculating scenario-based development paths. For comparison with different climate scenarios, a general reference scenario has been modelled, which is based on all other input data and assumptions. Climate variability is represented by an algorithm-based selection of historical weather years from a more-than-30-year period of wind and photovoltaic time series. Furthermore, the study investigate show climate change affects the feed-in of wind turbines and the demand for electricity under rising ambient temperatures in Europe. This is done using spatially and temporally high-resolution projection data from Earth system models under various greenhouse gas concentration scenarios. It is shown that although climate variability plausibly influences the model results in the direction of impact, the deviations from the reference scenario are in the low, single-digit range (<3%). An influence of climate change effects on wind power generation can also be demonstrated, but there is no clear spatial trend in the studied scenarios being found. However, the influence on electricity demand is spatially unambiguous, since higher outside temperatures induce lower heating loads in Northern Europe and increased air conditioning loads in Southern Europe, which, however, diverge seasonally. This insight should be taken into account in particular when dimensioning the expansion of the European interconnected grid as well as in legal requirements with regard to insulation standards for buildings
Konvektive Verluste an offenen volumetrischen Solarstrahlungsempfängern
No full textThe open volumetric air receiver is a promising technology for solar power towers. Researchers have investigated the behavior of the porous Absorber structures and the operational performance under fluctuating incident solar
radiation. All of the research work carried out is based on assumptions on the flow conditions at the receiver front, which so far were not investigated. The flow conditions at the front determine to what extent the receiver cycle is
driven as a closed loop, which is quantified by the air return ratio. The energy losses due to the partial exchange of hot cycled air by ambient air are referred to as convective losses.
In the present thesis, the influence of the air return ratio on the overall System efficiency was analyzed. More specifically, the interaction between receiver
cycle and power cycle was investigated to determine optimized design Parameters for future solar power towers. The simulations demonstrated the significant impact of the air return ratio on the overall system efficiency. This
result provided motivation for a thorough examination of the flow conditions at the receiver front. The approach was to develop a measurement setup which provides velocity fields in front of a modular section of the receiver, under real operating conditions. The experimental setup was assembled in the high flux solar simulator in Cologne, and using the installed Particle Image Velocimetry
measurement system, a quantitative examination of the flow conditions was feasible for the first time. The measurement results showed the turbulent
nature of the gas flow and highlighted the phenomena which affect the convective losses. A numerical model of the system was also developed using the open
source software OpenFOAM. Velocity fields and integral temperature values were compared with the experimental data in order to validate the simulations.
Good agreement between experiment and simulation allowed for an in depth analysis based on the numerical model. Hence, characteristic operating points
for the open volumetric air receiver were determined. A weak dependency of the air return ratio on the overall mass flow was found. Furthermore, an intentional
partially open loop for the receiver cycle was identified as an operating mode which can increase the performance of solar power towers in the future. Findings in this work lead to a better understanding of the convective losses
and the flow conditions at the receiver front. Further investigations are required in order to transfer this knowledge from the laboratory scale to the power
plant scale
Measurement methods for investigating the air return ratio of open volumetric receivers at solar power towers
No full textCost reduction plays a significant role in the field of concentrated solar thermal energy. It is therefore essential to quantify all factors that influence the energy conversion efficiency. The air return ratio is a key factor for the overall efficiency of the open volumetric receiver. It is the fraction of the blown out air which is sucked in again through the solar receiver. To achieve a high receiver efficiency it is therefore important to increase the air return ratio. Many variables such as wind speed and direction, geometry of the receiver design and operational mode influence the air flow in front of the receiver. This in turn influences the air return ratio. It is therefore of vital importance to be able to measure the air return ratio and furthermore visualize the air flow in front of the receiver. The air return value was prior to this work unknown on a large scale and under concentrated solar irradiation.The development of a measurement technique for the quantification of the air return ratio with maximum accuracy is the main objective of this thesis. The second objective lies in the visualization of the returned air. This improves the understanding of the occurring flow phenomena which govern the air return ratio. The measurement methods were developed at a lab scale, tested under operating conditions and successfully demonstrated at the solar tower Jülich. In order to measure the air return ratio, three variants of a novel circular tracer gas measurement technique have been developed. The tracer gas is injected either continuously or intermittently into the open air system. The tracer gas is diluted by the imperfect air return ratio. The mole fraction of the injected noble gas helium is measured with a mass spectrometer within the air system, from which the air return ratio is determined. A temporal resolution of 0.5 s has been achieved. A maximal air return ratio of (68.6 ± 0.7)% with 95% confidence interval has been measured during irradiation with concentrated sunlight at the solar tower power plant Jülich. This is higher than thepreviously assumed air return ratio of 60%. This difference corresponds to a 4 − 5% higher overall system efficiency. The return air in front of the receiver was visualized for the first time with the newly developed Induced Infrared Thermography. Hereby, carbon dioxide is added to the return air. This induces a larger amount of radiationbeing given off in the infrared region. This radiation from the return air is visualized using an infrared camera
Wärmerückgewinnung aus Partikeln mittels kugelförmiger Wärmeträgermedien in solaren thermochemischen Kreisprozessen
No full textIn the present work, a system to recover heat from a solar driven thermochemical cycle for hydrogen or syngas production was investigated. The recovery of the sensible heat from the used redox material can increase the process efficiency significantly. In the beginning, a concept was developed to recover heat from a particulate redox material. Therefore, a solid, spherical heat transfer medium is used, which transports the heat from the reduced to the oxidised redox material. Because of the direct contact between the redox material and the heat transfer medium in a mixture, heat can be transferred efficiently. In a combination with several co-current heat exchangers a quasi-counter-current principle is reached. An analysis shows the maximum achievable heat recovery rate of this concept. Afterwards, the heat transfer coefficient between the redox material and the heat transfer medium was investigated. Therefore, a well-known analytic model from the relevant literature was adapted to the characteristic properties of the binary mixture. Furthermore, a test rig was developed to examine the heat transfer experimentally. The method design of experiments was used to plan and analyse the experiments. It was found that heat losses are unavoidable and that they had to be identified by further tests to correct the measured values, which are necessary to determine the heat transfer coefficient. The result is an empirical model that describes the influence of the investigated factors and interactions on the heat transfer coefficient. Taking the heat transfer coefficient and the material properties into consideration, the heat recovery system was calculated in detail. Every single stage was examined separately. Assuming a geometry for the system, the heat losses were determined. The results show that the analysed concept has the potential to reach a satisfactory heat recovery rate and to increase the process efficiency significantly. Finally, it was examined how the heat recovery rate can be further increased. It was proposed to change the properties of the used material and the atmosphere in the system. Furthermore, two modified concepts, which can reach a higher maximum heat recovery rate than the investigated concept, were introduced. Based on the spherical heat transfer medium, a reactor concept, which can be combined with the analysed heat recovery system, was described
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