Direct Steam Generation Process Research Articles

Thermo-hydraulic analysis of direct steam generation in a 500 m long row of parabolic trough solar collector using Eulerian two-fluid modeling approach

Direct steam generation (DSG) in the parabolic trough solar collectors is a potential alternative for economic and performance improvement of the solar thermal system. This process comprises the preheating, evaporation, and superheating of water/steam in the solar collectors. The modeling and simulation of the evaporation section are challenging because of the complex physics associated with the boiling phase change process. This work describes the Eulerian two-fluid modeling of the once-through mode of the DSG process in solar collectors. The interfacial interactions between phases have been modeled using the appropriate empirical correlations. The experimental data from the Spanish DISS test facility is used to validate the numerical model. Further, the 3-D thermal–hydraulic investigation of DSG in a 500 m long solar collectors' row has been performed for operating pressures of 60 bar and 100 bar; inlet mass flow rates of 0.4 kg/s and 0.6 kg/s; DNI of 750 W/m2 for the solar time at 12:00 h and 13:00 h. It has been observed that the maximum absorber temperature is reduced by 27 % at 60 bar and 20 % at 100 bar operating pressure when the inlet mass flow rate increases from 0.4 kg/s to 0.6 kg/s. Similarly, the maximum circumferential temperature difference reduces by 12 % and 14 %, respectively. Moreover, the fluid temperature, steam quality, absorber temperature distributions, fluid velocity, and pressure loss under the subjected boundary conditions have been summarized.

Numerical investigation on the effect of Al2O3-water nanofluid on direct steam generation in parabolic trough collectors

The utilization of solar energy through direct steam generation (DSG) in parabolic trough collectors is a novel approach. In addition, due to higher thermophysical properties, the effect of using nanofluid as the coolant has been the subject of some of the recent studies but the applicability of the nanofluids in DSG process has not been studied before. In this study, the flow boiling of Al2O3 nanofluid at volume fractions 1%, 1.5% and 2%, under uniform and non-uniform heat fluxes has been studied numerically. The Eulerian model was used and non-equilibrium RPI was applied as the boiling model. The validation was done for flow boiling of pure water and Al2O3 nanofluid separately, with the available data in the literature. The results showed that the vapor volume fraction at the outlet is higher under non-uniform heat flux and adding nanoparticles would increase the vapor generation in the tube. Adding nanoparticles to the pure water increased the vapor volume fraction at the outlet up to about 8.64% in respect to pure water under uniform and non-uniform heat fluxes. The average Nu was up to 544% higher under non-uniform heat flux compared to uniform heat flux. Due to the presence of stratified flow, increasing nanofluid concentration reduced the average Nu up to 21.24% and 10.24% under uniform and non-uniform heat flux, respectively.

Investigation of stress and deflection in absorber of parabolic trough solar collector for direct steam generation

The tubular absorber of a parabolic trough solar collector (PTSC) experiences deflection and thermal stress during the operation due to the adverse circumferential and longitudinal thermal gradient in the direct steam generation (DSG) process. This study presents the coupled three-dimensional thermal–structural analysis of the absorber tube by combining heat transfer and static structural analyses using ANSYS Fluent 2021R1 software. The receiver of the LS-3 solar collector unit with a length of 4.06 m is considered as a computational domain with mass flow rates of 0.4 kg/s and 0.6 kg/s, an operating pressure of 100 bar, and the direct normal irradiance (DNI) of 750 W/m2. The deformation and stress analyses have been performed for preheating, evaporation, and superheating stages of the DSG process at solar noon. Under the subjected boundary conditions, the circumferential thermal gradient at the middle of the absorber in preheating, evaporation, and superheating sections are 27 K, 13 K, and 31 K, respectively. The maximum and minimum deflections observed in the absorber tube are 17.6 mm and 7.6 mm, respectively. The maximum and minimum normal stress along the axial direction on top of the absorber are 26.0 MPa and 12.6 MPa, respectively. It is observed that normal stresses along the axial direction is maximum at the mid of the absorber. The maximum thermal strains observed in preheating, evaporation, and superheating sections are 4.6 mm/m, 5.2 mm/m, and 5.6 mm/m, respectively.

Effect of position of heat flux profile on the absorber surface of parabolic trough solar collector for direct steam generation

The overall performance of parabolic trough solar collector (PTSC) based power plants could be improved by introducing the Direct steam generation (DSG) in the receiver of the solar collector. However, the thermal-hydraulic instability induced in the DSG process is a severe issue for the commercial application of the technology. The concentrated solar flux falling on the dry portion of the absorber before or after solar noon generates a high circumferential thermal gradient in the stratified flow region. In this work, numerical analysis of thermo-hydrodynamics of DSG has been performed to study the effect of position of solar flux profile using CFD solver ANSYS Fluent 2020R1. The TPF in the solar collectors is modeled through two-fluid modeling approach. The inlet mass flow rate and operating pressure for PTSC are considered as 0.6 kg/s, and 100 bar, respectively. The solar beam radiations are considered as 750 W/m2 and 1000 W/m2. The obtained results revealed that temperature distribution at the absorber outer surface varies in the range of 585 K to 643 K. The maximum circumferential temperature difference is observed as 55.5 K. The volume fraction of vapor at the absorber outlet are found as 0.31 and 0.37 respectively for DNI 750 W/m2 and 1000 W/m2. The corresponding pressure losses are 316 Pa and 350 Pa, respectively. The obtained results could be employed to characterize the thermal behavior of the DSG solar collectors. The model is useful to configure the solar field operation for optimum performance.

Effect of transient concentrated solar flux profile on the absorber surface for direct steam generation in the parabolic trough solar collector

The parabolic trough solar collector with Direct Steam Generation (DSG) has the potential to bring down the electricity cost. The viability of DSG technology is proved using various experimental demonstrations (e.g., DISS test facility) and running commercial-scale power plants (Thai Solar One). However, the DSG technology is in the nascent stage and needs improvement in the solar collector's components and control mechanism. The thermo-hydrodynamic instability induced in the absorber tube due to the non-uniform concentrated solar flux and flow boiling. This study investigates the effects of transient solar flux profiles on the thermal-hydraulics of flow boiling in the DSG process using CFD software ANSYS Fluent. The Eulerian two-fluid model is developed using conservation equations, turbulence model, and phase change model. The concentrated solar flux profiles are considered corresponding to various times of the day from 12:00 h (solar noon) to 17:00 h at an interval of 1 h. The three dimensional (3-D) numerical investigations are performed for DNI of 750 W/m2 and 1000 W/m2; operating pressures 60 bar and 100 bar; mass flow rates 0.4 kg/s and 0.6 kg/s. The flow regimes under the considered boundary conditions are observed as stratified flow. The vapor volume fractions at the absorber outlet exist between 0.30 and 0.45 for DNI of 750 W/m2 and 0.35 to 0.50 for DNI of 1000 W/m2. The absorber surface temperature has been calculated for operation at various times of the day, and it stands between 550 K and 656 K. Further, the circumferential temperature distributions and temperature contours have been plotted for various operating conditions. The adopted multiphase model shows very good performance and is useful for designing and optimizing solar collectors for DSG.

Thermo-hydrodynamic modeling of flow boiling through the horizontal tube using Eulerian two-fluid modeling approach

The flow boiling process is common in many industrial applications as well as direct steam generation (DSG) in the receiver of parabolic trough solar collector (PTSC) and linear Fresnel reflector (LFR) systems. The objective of the present work is to investigate the flow boiling heat transfer through the horizontal tube numerically for the various potential applications in the DSG process in the solar collectors. The thermo-hydrodynamic study of flow boiling through the horizontal tube is presented through the two-fluid model (TFM) approach using computational fluid dynamics (CFD) software, ANSYS Fluent 19.0. Three dimensional (3-D), steady-state numerical simulations are performed by applying the Eulerian multiphase critical heat flux (CHF) boiling model. The flow conditions considered are similar as in the DSG in the solar collectors. The simulations are performed for 12 m length of the horizontal stainless-steel tube having inner and outer diameter 70 mm and 50 mm respectively with the mass flow rates ranging from 0.3 kg/s to 0.6 kg/s, operating pressures 30 to 100 bar, and wall heat flux 17.74 kW/m2. The variations in the pressure drop, vapor volume fraction, circumferential wall temperature, contours of vapor volume fraction, velocities have been predicted under the considered operating conditions. The pressure drop in the tube for the various operating conditions considered is in the range of 115 Pa to 426 Pa and the pressure drop is observed higher at the lower operating pressure. The pressure drops are observed as 222.4 Pa, 145 Pa, and 115.2 Pa respectively for the operating pressure of 30 bar, 60 bar, and 100 bar at the mass flow rate (MFR) of 0.3 kg/s. The large thermal gradient in the tube wall is observed in the circumferential direction at the liquid-vapor interface. The minimum and maximum value of ∆T (∆T = Tmax – Tmin) around the circumference are observed as 28.3 K and 77.1 K respectively for the given operating conditions. The circumferential temperature difference decreases with an increase in the operating pressure. The value of ∆T around the circumference is observed for the MFR of 0.3 kg/s as 77.1 K, 58.1 K, and 45.7 K respectively for operating pressure of 30 bar, 60 bar, and 100 bar. The contours of tube wall temperature at various axial positions have been plotted. The vapor volume fractions for the MFR of 0.3 kg/s are 0.6, 0.51, and 0.45 respectively for operating pressures of 30 bar, 60 bar, and 100 bar. The vapor volume fraction (VVF) decreases at the outlet of the tube as the MFR or operating pressure increases. The mixture velocity and the relative velocity between the phases have been predicted. The present study concluded that higher operating pressure is more suitable in terms of pressure loss and the thermal gradient. The presented numerical model is useful for the thermal performance analysis of DSG in the PTSC as well as the LFR system and other industrial applications having flow boiling through the horizontal tubes.

Comparison of Two-Phase Flow Correlations for Thermo-Hydraulic Modeling of Direct Steam Generation in a Solar Parabolic Trough Collector System

Direct steam generation (DSG) in parabolic trough collector (PTC) is an efficient and feasible option for solar thermal power generation as well as for industrial process heat supply. The two-phase flow inside the absorber tube complicates the thermo-hydraulic modeling of the DSG process. In the present work, a thermo-hydraulic model is developed for the DSG process in the receiver of a solar PTC. The two-phase flow in the evaporating section is analyzed using two empirical correlations of heat transfer and pressure drop, and a flow map integrated heat transfer and pressure drop model. The results of the thermo-hydraulic simulation using different two-phase heat transfer and pressure drop correlations were compared with experimental data from the direct solar steam (DISS) test facility at Plataforma Solar de Almeria (PSA), Spain. The test facility has collectors with aperture width of 5.76 m, focal length of 1.71 m, and absorber tube with inner and outer diameters of 50 mm and 70 mm, respectively. The simulation results using the aforementioned two-phase models were found to be satisfactory and consistent within the experimental uncertainty. The flow map based heat transfer model predicted the mean fluid temperature with root-mean-square error (RMSE) of 0.45% and 1.40%, for the cases considered in the present study. Whereas the flow pattern map based pressure drop model predicts the variation of pressure along the length of the collector with RMSE of 0.5% and 0.14%. Moreover, the flow pattern map based model predicts the different flow regimes paving a better understanding of the two-phase flow and helps in identifying the critical sections along the collector length.

Thermo-hydraulic analysis and numerical simulation of a parabolic trough solar collector for direct steam generation

Direct Steam Generation (DSG) is one of the most promising alternatives for parabolic trough solar plants to replace the synthetic oil and reduce the electricity cost. The focus of this work is to develop a comprehensive optical and thermo-hydraulic model for the performance prediction of DSG process under real operating conditions. Pressure drop and heat transfer characteristics are determined considering the effect of the non-uniform heat flux distribution due to the concentration of the sunlight. A numerical-geometrical method based on ray trace and finite volume method techniques is used to determine the solar flux distribution around the absorber tube with high accuracy. A heat transfer model based on energy balance is applied to predict the thermal performances of the different flow regimes in the DSG loop. The thermo-hydraulic behavior of the different DSG sections i.e. preheating, evaporation and superheating is investigated under different operating conditions. The validity of the model has been tested by being compared with experimental data from DISS test facility and other available models in the literature. The study also presents a comparative study of the effect of different parameters on the thermal gradient around the absorber tube. The analysis shows that the highest thermal gradient is occurring in the superheating section with a high risk of thermal bending and a potential damage risk. The model is also capable to evaluate the efficiency of a DSG loop for different conditions and help to take the appropriate control strategies to avoid flow instabilities in the DSG rows.

Thermo-hydraulic modelling for Direct Steam Generation

Parabolic trough solar collectors (PTCs) are one of the most mature and prominent available solar technologies for electricity generation and heat process applications. Most of the commercial PTC plants are currently based on synthetic oil as heat transfer fluid. In order to reduce the electricity cost and improve the overall efficiency, Direct Steam generation (DSG) emerged as promising option for PTC technology with lower environmental impacts. However, the use of water/steam as heat transfer fluid may lead to a more complex thermo-hydraulic process. Moreover, the realistic non uniform solar flux applied on the absorber tubes may provoke a thermal stress on these tubes and cause the damage of the heat collector element. The focus of this work is to develop a comprehensive optical and thermo-hydraulic model to determine the temperature and pressure drop in the DSG process taking in consideration the effect of the non-uniform heat flux distribution due to the concentration of the sunlight. A numerical-geometrical method is used to determine the solar flux distribution around the absorber tube with high accuracy. A heat transfer model is developed to predict the heat transfer characteristics of the different flow regimes in the DSG loop. The numerical model is validated with experimental data from DISS test facility and other available models in the literature.

On thermo-hydraulic modeling of direct steam generation

The advancement in parabolic trough technologies has led to the development of direct steam generation (DSG); an efficient and feasible solution that utilizes the sun’s energy for thermal power generation as well as for heat supply in industrial processes. The complexity of the two-phase flow in the absorber tube has always remained a challenge in the development of reliable simulation tools of DSG process. The focus of this work is to improve the thermo-hydraulic modeling of DSG process by introducing a new modeling approach. A more recent flow pattern map is utilized in the prediction of flow patterns in the evaporation section. In addition, a recent development of a flow pattern oriented heat transfer model is integrated in the two-phase modeling to cover all flow regimes. Moreover, the two-phase pressure drop is modeled by following a phenomenological approach that takes the flow structure of different patterns into consideration. The validity of the model has been tested by comparing simulation results with real experimental data from DISS test facility and the results were found to be very consistent. The flow pattern analysis shows that low inlet pressures will be favorable to increase the occupation length of annular flow. On the other hand, the filling percentage of annular flow will significantly diminish in the evaporation section for increased values of inlet temperature.

Theoretical Study of Direct Steam Generation in Two Parallel Pipes

In this work it was simulated the direct steam generation (DSG) in a system of two horizontal parallel pipes by using the commercial simulator RELAP. The pipes were 500 m long, internal diameter 5cm, and wall thickness of 10mm. Pipes were fed with sub-cooled water and the output can be: 1) hot water, 2) a steam-water mixture, 3) overheated steam. The flow was at steady state and pipes were undergoing to uniform and concentrated solar heating. Moreover, the effect of symmetric and asymmetric heating, on the DSG process, was studied.RELAP code allowed predicts flow distribution and inlet pressure. Predictions of inlet pressure and flow distribution were compared against predictions from literature. The pressure drop calculated by RELAP presented good fitting with field data from the Plataforma Solar de Almería (PSA).The study of flow distribution showed that RELAP is able to get the most stable solutions. It was found that for symmetric solar heating, asymmetric flow distribution solutions may be present in the system. Moreover, for asymmetric heating the mass flow rate is larger in the pipe with smaller heating. This is an unfavorable condition under a practical point of view. The concentrated solar heating does not affect in a considerable way the flow distribution in relation to the uniform solar heating; however, it could affect the flow pattern transition along the pipe (s).

Thermal Storage Concept for Solar Thermal Power Plants with Direct Steam Generation

One possibility to increase the efficiency and thus economic viability of solar thermal power plants is to increase their operating temperature. This approach demands the substitution of the state-of-the-art heat transfer fluid (HTF) that limits the operating temperature to roughly 400°C. Promising heat transfer fluids for future applications are molten salts or water/steam. If water/steam is used as HTF, the feed-water from the power block is fed to the solar field (SF) and directly evaporated and superheated. This process is called direct steam generation (DSG). A recent study [1] has pointed out that the economic potential of the DSG process is utilized only, if the SF design is simplified and a competitive thermal storage is available. Thus, an R&D project was launched in Germany to develop a complete storage system covering the energy of the evaporation as well as of the pre- and superheating section. It consists of a phase change material (PCM) storage for evaporation and a molten salt storage for pre- and superheating. One specific feature of superheated steam is its changing specific heat capacity with temperature. Using molten salt as storage medium with a nearly constant specific heat and the application of an obvious simple heat exchange would lead to an inefficient process. A significantly reduced live steam temperature and thus power block efficiency during discharge would be the results. Furthermore, the specific storage density of the molten salt system would be reduced too. In this paper this effect will be discussed in more detail. The consequences for the storage system will be discussed and solutions of the developed processes for the integration of such storage into a DSG power plant will be presented that reduce or overcome the mentioned restrictions.

Thermal analysis of solar receiver pipes with superheated steam

Direct steam generation (DSG) process in the absorber tubes of parabolic-trough solar collectors is experiencing an important development in the last decade. This concentrated solar technology can be used in solar fields for electricity production or industrial-process heat supply. The modeling and simulation of the DSG process in the parabolic-troughs are essential for the detailed design of the collector field. One of the critical aspects that should be identified is which process conditions may cause an overheating of the absorber tubes.A Computational Fluid Dynamics (CFD) model has been developed to calculate the temperature profile in the wall of absorber tubes of parabolic-trough solar collectors using steam as heat-transfer fluid. Steam presents lower thermal conductivity than water or other typical liquid heat-transfer fluids used in parabolic troughs. Therefore, for similar process conditions, the thermal stress in the absorber tubes is higher when the fluid is steam. The Finite Volume Method (FVM) package FLUENT is used to implement the model and to perform the investigation. This article summarizes the model used and a comparison of simulation results and measurements taken at a DSG solar test facility located at the Plataforma Solar de Almería, Spain.

Economic Potential of Solar Thermal Power Plants With Direct Steam Generation Compared With HTF Plants

The direct steam generation (DSG) in parabolic trough collectors is a promising option to improve the mature parabolic trough solar thermal power plant technology of the solar energy generating systems (SEGS) in California. According to previous studies [Langenkamp, 1998, “Revised LEC Projections and Discussion of Different DSG Benefits,” Technical Report No. DISS-SC-QA-02, Almeria, Spain; Price, et al., 2002, “Advances in Parabolic Trough Solar Power Technology,” ASME J. Sol. Energy Eng., 124(2), pp. 109–125; Zarza, E., 2002, “DISS Phase II Final Report,” Technical Report EU Contract No. JOR3-CT98-0277, Almeria, Spain], the cost reduction in the DSG process compared with the SEGS technology is expected to be 8–25%. All these studies were more or less preliminary since they lacked detailed information on the design of collector fields, absorber tubes required for steam temperatures higher than 400°C, and power blocks adapted to the specific needs of the direct steam generation. Power blocks and collector fields were designed for four different capacities (5 MWel, 10 MWel, 50 MWel, and 100 MWel) and different live steam parameters. The live steam temperature was varied between saturation temperature and 500°C and live steam pressures of 40 bars, 64 bars, and 100 bars were investigated. To assess the different cases, detailed yield analyses of the overall system were performed using hourly data for the direct normal irradiation and the ambient temperature for typical years. Based on these results, the levelized costs of electricity were determined for all cases and compared with a reference system using synthetic oil as heat transfer fluid. This paper focuses on two main project findings. First, the 50 MWel DSG system parameter comparisons are presented. Second, the detailed comparison between a DSG and a SEGS-like 100 MWel system is given. The main result of the investigation is that the benefit of the DSG process depends on the project site and can reach an 11% reduction in the levelized electricity cost.

Field Test of Water-Steam Separators for Direct Steam Generation in Parabolic Troughs

Direct steam generation (DSG) represents a promising option to improve today’s parabolic trough technology for solar thermal power generation. The European DISS and INDITEP projects have proven the feasibility of the DSG process under real solar conditions at the DISS test facility at the Plataforma Solar de Almería (PSA) (Zarza, E., Valenzuela, L., Léon, J., Hennecke, K., Eck, M., Weyers, H.-D., Eickhoff, M., 2004, “Direct Steam Generation in Parabolic Troughs Final Results and Conslusions of the DISS Project,” Energy, 29, pp. 635–644). These projects have also shown that the recirculation mode is the preferred operation mode for DSG collector fields. This concept requires water-steam separators at the end of the evaporation section of the collector loop. Both compact water-steam separators for every single row or huge separation drums for the whole collector field are considered. Small compact water-steam separators show a lower inertia, reducing the time for start-up. Within INDITEP and the German R&D project SOLDI compact water-steam separators have been developed, manufactured, and tested by DLR and Siemens, with its subcontractor Framatome ANP. Prototypes of a cyclone and a baffle separator have been implemented into the DISS test facility. More than 200 tests have been performed to investigate the separation efficiency, the pressure loss, and the performance under transient conditions. This paper focuses on the steady-state tests.

INDITEP: The first pre-commercial DSG solar power plant

This paper presents the conceptual design of the first solar power plant using Direct Steam Generation (DSG) in a parabolic-trough solar field. Experience and know-how in the DSG process acquired during the DISS project were applied in designing the solar field of this plant. The 5-MWe plant is composed of a DSG parabolic-trough solar field connected to a superheated steam Rankine power cycle. The solar field produces 410 °C/70-bar superheated steam. Detail engineering of this plant is currently underway within the framework of the INDITEP project, which is promoted by a German-Spanish consortium with the financial support of the European Commission (Contract No. ENK5-CT-2001-00540). The main design objective is to assure high operational flexibility and reliability. This is the reason why a robust superheated steam turbine has been selected, though the efficiency of its power block is modest.

Modelling and Design of Direct Solar Steam Generating Collector Fields

The direct steam generation (DSG) is an attractive option regarding the economic improvement of parabolic trough technology for solar thermal electricity generation in the multi megawatt range. According to Price, H., Lu¨pfert, E., Kearney, D., Zarza, E., Cohen, G., Gee, R. Mahoney, R., 2002, “Advances in Parabolic Trough Solar Power Technology,” J. Sol. Energy Eng., 124 and Zarza, E., 2002, DISS Phase II-Final Project Report, EU Project No. JOR3-CT 980277 a 10% reduction of the LEC is expected compared to conventional SEGS like parabolic trough power plants. The European DISS project has proven the feasibility of the DSG process under real solar conditions at pressures up to 100 bar and temperatures up to 400°C in more than 4000 operation hours (Eck, M., Zarza, E., Eickhoff, M., Rheinla¨nder, J., Valenzuela, L., 2003, “Applied Research Concerning the Direct Steam Generation in Parabolic Troughs,” Solar Energy 74, pp. 341–351). In a next step the detailed engineering for a precommercial DSG solar thermal power plant will be performed. This detailed engineering of the collector field requires the consideration of the occurring thermohydraulic phenomena and their influence on the stability of the absorber tubes.

Direct steam generation in parabolic troughs: Final results and conclusions of the DISS project

This paper presents a summary of the main results and conclusions achieved in the DISS (Direct Solar Steam) project. The test facility implemented at the Plataforma Solar de Almería (PSA) in 1997–8, the so-called PSA DISS test facility, was operated for more than 3000 h in 1999–2000 and 2001 to investigate the Direct Steam Generation (DSG) process under real solar conditions. The feasibility of the DSG process in horizontal parabolic trough collectors has been proven and an important know how has been acquired by the project partners regarding the thermo-hydraulic parameters of the water/steam flow in DSG solar fields.

Maximum temperature difference in horizontal and tilted absorber pipes with direct steam generation

The direct steam generation (DSG) in parabolic trough collectors is a promising option for the im provement of the reliable parabolic trough technology for solar thermal electricity generation. The DISS (DIrect Solar Steam) test facility at the Plataforma Solar de Almería is a full-scale solar steam generator consisting of parabolic trough collectors, used to investigate the DSG process under real operating conditions. According to theory and results from lab-scale experiments it was expected that the tilt of the absorber tubes can improve the thermohydraulic behavior of the DSG process significantly. Within the DISS project a test campaign has been performed to investigate the influence of tilt. The evaluation of the test data showed, that the horizontal absorber tube guarantees sufficient cooling confirmed by low temperature differences even at very high steam qualities.

Applied research concerning the direct steam generation in parabolic troughs

With levelized electricity costs (LEC) of 10–12 USCts/kWh the well-known SEGS (Solar Electric Generating Systems) plants in California are presently the most successful solar technology for electricity generation [Price and Cable (2001) Proc. ASME Int. Solar Energy Conf. Forum 2001]. The SEGS plants apply a two-circuit system, consisting of the collector circuit and the Rankine cycle of the power block. These two-circuits are connected via a heat exchanger. In the case of the Direct Steam Generation (DSG) in the collector field [Zarza et al. (2001) Proc. Solar Forum 2001, Washington], the two-circuit system turns into a single-circuit system, where the collector field is directly coupled to the power block. This renders a lower investment and higher process temperatures resulting in a higher system efficiency. Due to the lower investment and the higher efficiency a reduction of the LEC of 10% is expected when the DSG process is combined with improved components of the solar collectors [Zarza (2002) DISS Phase II Final Report, EU Contract No. JOR3-CT98-0277]. Within the European DISS (Direct Solar Steam) project the feasibility of the direct steam generation has been proven in more than 3700 operation hours. Steam conditions of 100 bar and 400 °C have been demonstrated. This paper presents the main scientific results of the DISS project that aims at the investigation and demonstration of the DSG process in parabolic troughs under real solar conditions.