• Energy Research

Abstract The dual receiver concept presented in this paper improves the adaptation of the central receiver to the steam cycle in a solar thermal power plant. By combination of an open volumetric air heater and a tubular evaporator the dual receiver concept profits from the advantages of these two concepts while their characteristic problems are avoided. The water is evaporated directly in the tubular steam generator; preheating and superheating are done in heat exchangers by using the hot air from the volumetric receiver. This paper presents a concept study that extends previous work on the 10MWel level (Buck et al., 2004, “Dual Receiver Concept for Solar Towers,” Proc. 12th Solar PACES Int. Symposium, Oct. 6–8, Oaxaca, Mexico) to a level of 100MWel, which is the expected power range of future plants. The results confirm the benefits of the new concept, resulting from higher thermal efficiency of the receiver and lower parasitic power consumption. The annual mean efficiency is increased from 13% to 16%. Advantageous are also the reduced thermal loads in the receiver components.

Abstract The Linear Fresnel Collector (LFC) technology is currently being commercialised by several companies for the application in solar thermal power plants. This study compares the electricity generation costs for LFC and Parabolic Trough Collector (PTC). PTC is the most commercial CSP technology to date and is therefore regarded as the benchmark. For reasons of comparability, direct steam generation is assumed for both LFC and PTC. For the LFC, cost data comparable to typical CSP plant sizes are hardly available. Therefore, the break even cost – referring to aperture-specific collector investment – is determined, where cost-parity of the electricity generation with a PTC reference plant is reached. This study varies the assumptions on collector performance and operation and maintenance costs to reflect different designs of LFC technologies. The calculations were carried out using cost and hourly simulation performance models. Depending on the assumptions, the costs for a linear Fresnel collector solar field should range between 78 and 216 €/m 2 to reach cost-parity at assumed reference solar field costs of 275 €/m 2 for the PTC. The LFC principle of arranging the mirrors horizontally leads to lower aperture-related optical efficiency which must be compensated by lower cost per m 2 of aperture compared to PTC. The LFC is a collector with significant cost reduction potential, mainly due to cheaper mirrors and structural advantages.The presented cost and performance targets shown in this study must be met by LFC technology developers to reach the PTC benchmark.

Abstract The DISS (DIrect Solar Steam) project is a complete R+TD program aimed at developing a new generation of solar thermal power plants with direct steam generation (DSG) in the absorber tubes of parabolic trough collectors. During the first phase of the project (1996–1998), a life-size test facility was implemented at the Plataforma Solar de Almería (PSA) to investigate under real solar conditions the basic DSG processes and evaluate the open technical questions concerning this new technology. This paper updates DISS project status and explains O&M-related experience (e.g. main problems faced and solutions applied) with the PSA DISS test facility since January 1999.

Parabolic trough power plants use concentrated direct insulation for steam generation in a Rankine cycle. The fundamentals for direct steam generation in parabolic troughs have been developed in recent years, the feasibility was successfully demonstrated at a solar powered test facility reaching steam temperatures of 400°C at a maximum pressure of 100 bar. Important components for the accelerated market penetration of solarthermal power plants are cost-effective local storage systems. Main benefits of storage systems are: extended utilisation of the power block, improved efficiency of components and facilitated integration into the electricity system. Within the R&D project WESPE, a test facility for solid sensible heat storage is developed and connected to a solar collector loop. A simulation tool for the analysis of the transient performance of the storage system integrated into the power plant was implemented. The results show a significant potential for economic optimisation by applying advanced operation strategies based on an improved adaptation of the storage unit to the characteristics of solar collectors and power cycle.

Abstract For sensible thermal energy storage (TES) in liquids in the temperature range from 250 °C to 550 °C, a mixture of 60 wt% sodium nitrate (NaNO 3 ) and 40 wt% potassium nitrate (KNO 3 ), known as Solar Salt, is commonly utilized. At the time of writing, TES technology for concentrating solar power is the major application. Although commercial systems have been demonstrated, there are still several material aspects to be investigated. In this paper we address thermophysical properties and metallic corrosion, as well as thermal decomposition processes. The paper reviews temperature dependent thermophysical properties of Solar Salt. Deviations among the authors of these properties were small for the density (±1.5%), medium for the heat capacity (±7%) and large for thermal diffusivity and thermal conductivity values (±15%). The paper gives an overview of the various aspects of steel corrosion in molten alkali nitrate salts. From literature data, four steel type categories mainly depending on the temperature range are defined. The paper presents thermal stability examinations of Solar Salt and NaNO 3 by isothermal lab-scale tests and thermal analysis measurements. Salt analysis in the isothermal test showed a steadily increasing oxide level at a constant nitrite to nitrate ratio. The result shows that there are kinetic differences in the first decomposition process with nitrite formation and the second decomposition process with oxide formation. The impact of the partial oxygen pressure on the decomposition temperature was examined by thermogravimetric measurements. Measurements show an improved stability limit for higher partial oxygen pressures.

For the control of the Direct Solar Steam Generation (DSG) Process, the state variable of the evaporator section as well as of the superheater section has to be measured. Furthermore, the knowledge of a simplified model of the system is of great interest. In this article, a simplified model for the dynamic behaviour of the superheater section of the DSG Process and a measurement technique for the determination of the steam quality are presented. In both cases, first, experimental results are presented.

Parabolic trough concentrating collectors play a major role in the energy efficiency and economics of concentrating solar power plants. Therefore, existing collector systems are constantly enhanced and new types were developed. Thermal performance testing is one step generally required in the course of their testing and qualification. For outdoor tests of prototypes, a heat transfer fluid loop (single collector or entire loop) needs to be equipped with measurement sensors for inlet, outlet, and ambient temperature as well as irradiance, wind speed, and mass or volumetric flow rate to evaluate the heat balance. Assessing the individual measurement uncertainties and their impact on the combined uncertainty of the desired measurement quantity one obtains the significance of the testing results. The method has been applied to a set of EuroTrough collector tests performed at Plataforma Solar de Almería, Spain. Test results include the uncertainty range of the resulting modeling function and exemplify the effects of sensors and their specifications on the parameters leading to an uncertainty of ±1.7% points for the optical collector efficiency. The measurement uncertainties of direct normal irradiance and mass flow rate are identified as determining uncertainty contributions and indicate room for improvement. Extended multiple sensor deployment and improved calibration procedures are the key to further reducing measurement uncertainty and hence increasing testing significance.

For future parabolic trough plants direct steam generation in the receiver pipes is a promising option for reducing the costs of solar thermal energy [1]. These new solar thermal power plants require innovative storage concepts, where the two phase heat transfer fluid poses a major challenge [2]. For the regions where the heat transfer fluid is in a single phase (water or steam), sensible heat storage using molten salt [3] or concrete [4] as storage material can be applied. However, efficient energy storage in the two-phase evaporation/condensation region requires heat storage operation within a narrow temperature range. For this two phase region, a high performance PCM storage technology was developed and demonstrated by DLR. A test module using 14 tons of PCM with 700 kWh capacity was built in 2009 and commissioned in 2010 in a direct steam test loop, set up at the power plant Litoral of Endesa in Carboneras, Spain [5]. The PCM-storage uses Sodium nitrate as phase change material with a melting temperature of 305 °C. Cycle testing has started end of 2010. Cycling tests have proven the expected discharge capacity of approx. 700 kWh for the PCM-storage module. System operation in constant pressure mode and sliding pressure mode has been conducted for the PCM-storage. While in the constant pressure mode a peak performance of the storage of more than 700 kW could be demonstrated, in the sliding pressure mode a constant power output over almost the whole charge and discharge period could be provided. The paper discusses the test results and evaluation for different operation modes for the phase change storage for discharge operation. Charging of the phase change storage is always in a once-through mode. However, for discharge, the steam can be generated either in forced or natural circulation mode or in once-through mode, leading to very different effects for the two-phase flow and filling level inside the heat exchanger pipes in the storage. The effects for forced and natural circulation discharge will be analysed and described in the paper.