transfer calculations for each site individually. This enables the application of the AATTENUATION model for virtually all resource assessments for towerplants and in an operational mode in real time within plant monitoring systems around the world. The LUT also facilitates the generation of solar attenuation maps on the basis of long-term meteorological data sets which can be considered during resource assessment for CSP tower plant projects. The LUT s are provided together with this manuscript as supplementary files. The LUT for the AATTENUATION model was developed for a solar zenith angle ( SZA ) grid of 1 ◦ , an altitude grid of 100 m, 7 different standard aerosol types and the standard AFGL atmospheres for mid-latitudes and the tropics. The LUT was tested against the original version of the AATTENUATION model at 4 sites in Morocco and Spain, and it was found that the additional uncertainty introduced by the application of the LUT is negligible. With the information of latitude, longitude, altitude above mean sea level, DNI , relative
both corrected data sets lies within this estimated uncertainty limit. For yearly averages several error influences are expected to average out and absolute uncertainties of 0.02 and 0.054 can be expected for the FS11 and the LPV4, respectively. A longer path way for the LPV4 measurement will result in lower uncertainties for the measurement. In general, a noticeable decrease from about -0.0281 to 0.01 in the overall difference of FS11 and LPV4 measurement can be achieved by applying the ABC procedure. There- fore, it is recommended to apply the ABC method for atmospheric extinction measure- ments instead of using only raw measurements of the FS11 or LPV4 for solar resource assessment. The developed correction for the LPV4 transmissometer is rather small compared to the correction applied to the scatterometer of Vaisala (FS11). Correcting the transmissometer only with a mean diurnal correction curve might be therefore also an option to correct the measurement sufficiently. The FS11 scatterometer can be recom- mended to derive atmospheric extinction in solar towerplants. For other scatterometers, this potential has to be investigated.
A model developed in [ 21 ] based on DNI measurements to derive the atmospheric transmittance in solar towerplants has been validated at three sites in Spain and Morocco. The transmittance model (TM) can be applied to the desired site during solar resource assessment as only data sets of usually available meteorological parameters like DNI, temperature, relative humidity and barometric pressure are necessary. Three different aerosol profiles have been tested to be applied as key assumption for the transmittance model development. In one approach it is assumed that all aerosol particles are homogeneously distributed within the first kilometer above ground. In a second approach, the aerosol profiles of the LIVAS database ([ 28 ]) are assumed to describe the vertical aerosol distribution. The third transmittance model uses the boundary layer height (BLH) of the ECMWF ERA-interim dataset ([ 29 ]) to scale the height above ground in which all aerosol particles are assumed to be distributed homogeneously.
One promising point-focusing solar-thermal technology is the solar tower plant technology. In contrary to linear- focusing systems, e.g. parabolic trough plants, a concentrator (in the case of towerplants called heliostat) is focusing the reflected solar irradiance onto a receiver on the top of a tower. A heliostat field consisting of many heliostats can therefore achieve high temperatures at the surface of the receiver due to the high resulting concentration factor. The direct normal irradiance (DNI), which is the important parameter for con- centrated solar power (CSP) plants, is one part of the incom- ing solar radiation reaching the Earth’s surface. It is known that the “sun belt” region is displaying high potential for CSP plant technologies due to the high DNI resources available. Large parts of this area are in semi-arid or arid conditions, like the largest desert of the world, the African Sahara. It is the primary source for mineral dust aerosol particles (Wash- ington et al., 2003) and next to sea salt particles, mineral dust has a considerable contribution to atmospheric aerosol (D’Almeida and Schütz, 1983). Atmospheric aerosol extinc- tion can lower the DNI reaching the surface by up to 30 % (Gueymard, 2003). Especially in the lowest hundreds of me- ters of the Earth’s atmosphere higher loads of aerosol parti- cles like mineral dust or e.g. sea salt can be expected. This might be a crucial factor for solar towerplants. The solar ra- diation which is reflected by the heliostats to the receiver at the top of the tower has to travel a second time through the lowest atmospheric layers and this distance might be up to a few kilometers (depending on the solar field size). There- fore, the extinction has to be accounted for in solar resource assessment as well as plant optimization.
Atmospheric extinction between the heliostat field and receiver in solar towerplants is known to cause significant losses of reflected Direct Normal Irradiance. This phenomenon brings a limitation on the size of the heliostat field and is included in some raytracing and plant optimization tools. Usually, no detailed information about the local meteorological conditions is available for many sites that are now of interest for tower plant projects. Therefore, only standard atmospheric conditions are commonly used to describe the attenuation and also the height profiles of relative humidity and aerosol concentration. First of all the existing models are presented. The use of the Pitman and Vant-Hull model with real measurement data represents an improvement with respect to site independent calculations. Thus different commercially available instruments that can provide the input for the state of the art models are described, tested and intercompared. Also the limitations of the state of the art are discussed and methods to overcome these limitations are shown. The choice of the tested instrumentation and the evaluation of the different instruments have been performed with regard to necessary enhancements. Several months of MOR (Meteorological Optical Range) measurements from the Plataforma Solar de Almería (PSA) are presented. These data provide a base for further evaluation of the investigated instruments. The FS11 scatter meters display satisfying accuracies on transmittance measurements and their robust composition and low sensitivity to soiling facilitate application at remote sites. The Degreane TR 30AC transmissometer is rather suitable for smaller slant ranges than those required for many solar towerplants as uncertainties for clear atmospheric conditions are high. The Optec LPV-4 transmissometer obtains high accuracies for clear conditions if large working path distances are used to exploit the preciseness of the instrument. The presented measurement methods enable improvements in tower plant design and yield analysis, but still enhancements of the existing models are required. The discussed instruments, additional sensors and modeling approaches can be used to develop such methods. Keywords: beam attenuation, atmospheric extinction, solar towerplants, transmission, visibility, solar resource
These results suggest that all three TES concepts are well justifiable. They can be regarded as development steps towards a further improved cost-effectiveness of the technology: TES concept #3 is an advancement of the storage technology used in the Jülich tower, is ready for demonstration-scale solar deployment or for use in other industrial applications. Packed bed TES as outlined in concepts #1 and #2 is an interesting alternative with excellent prospects, both cost-wise and with respect to installation costs. Though substantial progress could be made in the course the HOTSPOT project, it still needs further development and further testing in pilot-scale to address remaining open questions on the thermo-mechanical implications and on the material questions. For concept #3, the industrial realization has been looked at in more detail: a blueprint planning including project time schedule and further manufacturing aspects has been worked out.
is exact enough for PSA, but might not be true for other locations and could increase the uncertainty there. Here, only the uncertainty for the case of an homogeneous atmo- spheric layer is investigated.
The layer thickness for the second RT calculation is defined to be 1 km. Several tests with different layer thicknesses have been conducted to test the sensitivity of the procedure to this parameter. It could be seen that the simulated spectral DNI after passing through the homogeneous layer differed strongly dependent on layer thickness. For high thicknesses (e.g. 10 km), radiation in wavelength bands with low contribution to the complete spectrum vanish completely. Derived transmittances for different slant ranges from the transmitted broadband radiation differ therefore for different layer thicknesses. Hence, the ABC algorithm was optimized for a slant range of 1 km as this is a realistic distance between a heliostat and the receiver in a solar tower plant (see also Sect. 3.1.1.B). Instrumentation of measurement data contributing to the ABC algorithm are located at PSA at different locations (FS11 scatterometer in the north-east of PSA, LPV4 trans- missometer transmitter in the south, receiver in the south-west of PSA, meteorological station in the south of PSA, sun photometer in the south-west of PSA). Local distur- bances which affect one instrument but not the others are possible as different construc- tion works were performed at PSA throughout the test year. The assumption that PSA is one homogeneous location, or all instruments measure at the exact same location and under the same atmospheric conditions therefore will introduce uncertainties.
Each configuration is defined by a specific receiver inlet and outlet temperature. For each temperature set, the solar subsystem is optimized for minimal LcoE. The following parameters are varied during the optimization: receiver aperture area, receiver tilt angle, tower height and field layout. A radially staggered field layout is assumed. The simulation tool HFLCAL  was used to determine the optimal parameters for each temperature set.
Abstract. New heat transfer and storage media offer for solar tower systems a much broader temperature range. Higher temperatures allow the integration of steam power cycles with increased efficiency. The present study evaluates modular solar towerplants using solid particles as heat transfer medium (HTM), allowing temperatures up to 1000°C. In a parameter study the influence of upper and lower HTM temperature on levelized cost of electricity (LCOE) is evaluated. The results show a significant impact of the HTM temperature selection, mainly governed by the HTM temperature difference. A high temperature difference results in reduced LCOE. The most important factors for this reduction are the cost decrease of particle inventory, storage containment, and particle steam generator. This decrease is partially offset by an increase in heliostat field and tower cost. The results indicate that the use of solid particles for high efficiency steam power cycles offers unique advantages due to the wide temperature range of the particles.
It is obvious that the temperature generally decreases from start (time of shut-down of the plant) to end of the measurement, as already described above. It becomes clear that a high amount of thermal energy losses take place at the uppermost part of the storage. In the upper mid section (level 5 to 7), the temperature remains almost constant, a clear deviation is visible between level 3 and 5. In the lower levels, only very little change of temperature happens during standstill. To easily locate the areas of significant temperature change, the local temperature differences after 12 h in all chambers are plotted as contours in Fig. 7 and Fig. 8 for both cases. It becomes obvious that the highest thermal losses take place at level 8 and also the area around level 3. This indicates that a significant amount of heat from the storage gets lost to the dome and the piping connection to the storage. A drawing of the thermal energy story of the Jülich Power Tower is shown in Fig. 9. It is assumed that heat conduction through the storage and piping material causes the thermal energy loss, since the temperature gradient of the piping, which leads to the receiver, induces heat flow through the material. This in turn leads to a reallocation of isothermals inside the storage since hot air moves upwards to the top of the storage, which is why the temperature in the core of the storage chambers also decreases in some regions as the cooled region expands. The most significant peak of local temperature change is observed in chamber 2 for both cases. The effects seem to be almost independent from the damper being open or closed, which is owed to heat conduction through the piping material, although the temperature decrease is somewhat lower for the closed damper case. Due to the closed damper, the heat loss is restricted, which suggests that the total heat loss is only partially conduction but also convection. With the damper at the TES closed, more thermal energy remains in the storage.
The outcome of unambiguous paternity analyses targeting dispersal kernel fitting comprises the locations of all potential pollen donors within the study area, the locations of selected mother plants, and the paternal origin of seed samples harvested from the mothers. Such data sets can be interpreted as complex marked point patterns (Stoyan & Stoyan 1994), and related spatial analysis methods allow estimation of empirical pollen dispersal kernels and should be able to reveal whether estimated dispersal probabilities at increasing distances are statistically informative, especially with regard to potentially non-random patterns of individual plants. Broadly, randomizing individual identities over all potential donors, while keeping the spatial attributes of the remaining paternity data, provides a null random mating model against which the empirical pollen dispersal kernel can be confronted. Then, simulation envelopes for the null model, constructed via Monte Carlo procedures, allow for formal testing whether the empirical pollen dispersal kernel significantly differs from that expected under random mating, and therefore how informative paternity data is at different distances for kernel fit purposes.
With the storage model implemented into the overall Jülich power tower model total annual yield calculations can be carried out. Furthermore, hybridized power tower systems with open-volumetric air receiver technology can be simulated. The simulation model has been optimized such that charging and discharging modes can be simulated for the implemented control strategies. In a further step annual energy yield simulations shall be carried out for scaled-up power tower systems of the Jülich type for different locations within the Earth’s Sun Belt regions.
In a power system characterised by increasing shares of renewable power generation, the flexibil- ity requirements placed on existing conventional capacities rise significantly. The main cause of an increased need for flexibility is the variable nature of power generation from wind power and photo- voltaics (PV). Both technologies depend on weather conditions, daily and seasonal changes, and therefore cannot generate “on demand” like conventional power plants. Furthermore, renewables have almost no mar- ginal costs. This means that they produce “for free” whenever the primary resource (i.e. wind or sun) is available. These factors entail a fundamental trans- formation of power systems, because of the need to respond flexibly to variation in renewables feed-in. Several options currently exist to provide more system flexibility for the integration of renewables. Encouraging demand-side flexibility (e.g. more flex- ible manufacturing processes) is one option. Another is to promote grid development, so that power can be transported with greater ease between regions and
Abstract— In the field of aviation, “Remote Tower” is a current and fast-growing concept offering cost-efficient Air Traffic Services (ATS) for aerodromes. In its basics it relies on optical camera sensor, whose video images are relayed from the aerodrome to an ATS facility situated anywhere, to be displayed on a video panorama to provide ATS independent on the out-of-the-tower-window view. Bandwidth, often limited and costly, plays a crucial role in such a cost-efficient system. Reducing the Frame Rate (FR, expressed in fps) of the relayed video stream is one parameter to save bandwidth, but at the cost of video quality. Therefore, the present article evaluates how much FR can be reduced without compromising operational performance and human factor issues. In our study, seven Air Traffic Control Officers watched real air traffic videos, recorded by the Remote Tower field test platform at the German Aerospace Center (DLR e.V.) at Braunschweig-Wolfsburg Airport (BWE). In a passive shadow mode, they executed ATS relevant tasks in four different FR conditions (2 fps, 5 fps, 10 fps & 15 fps) to objectively measure their visual detection performance and subjectively assess their current physiological state and their perceived video quality and system operability. Study results have shown that by reducing the FR, neither the visual detection performance nor physiological state is impaired. Only the perceived video quality and the perceived system operability drop by reducing FR to 2 fps. The findings of this study will help to better adjust video parameters in bandwidth limited applications in general, and in particular to alleviate large scale deployment of Remote Towers in a safe and cost-efficient way.