Direct Steam Generation Solar Plants Research Articles

Direct steam generation solar systems with screw expanders and parabolic trough collectors: Energetic assessment at part-load operating conditions

This paper explains a numerical optimization of a novel screw expander-based solar thermal electricity plant to evaluate the energetic benefits in specific case studies. In the proposed solar electricity generation system, which is based on the steam Rankine cycle, water is used as working fluid and storage, parabolic trough collectors as a thermal source and screw expander as power machine. Such solar system offers major advantages over conventional power plants adopting steam turbines: low operating pressures, good exploitation of low temperature heat sources, acceptable efficiency in energy conversion with steam-liquid mixtures and reduced size. Since screw expanders can operate at off-design working conditions in several situations when installed in direct steam generation solar plants, the chief purpose of the present study is to develop a thermodynamic model to analyse the energy performance of the planned solar power system when off-design operating conditions befall. To assess maximum efficiency of the whole power plant at part-load operating conditions, numerical optimization is then performed in a specific range of fluctuating evaporation temperatures under fixed condensation pressures.

Exergetic Analysis of a New Direct Steam Generation Solar Plant Using Screw Expanders

Screw expanders are volumetric machines particularly suitable in energy conversion with steam–liquid mixtures and for the exploitation of low temperature heat sources. This study explored the main criteria to evaluate the thermodynamic advantages and exergetic assessment of an innovative solar electricity generation system: Screw expanders are utilized as power machines and parabolic trough collectors as a thermal source. Such a direct steam generation solar system, which is based on the Rankine cycle, offers benefits in comparison with usual power plants with dynamic expanders: The best exploitation of low temperature heat sources, satisfactory thermal efficiency with steam–liquid mixtures, lower evaporation pressures, and reduced size. Under real working conditions, screw expanders can work at part-load operating conditions; thus, the chief purpose of the present paper was to analyze the exergetic advantages of the planned solar power system when solar radiation and off-design working conditions fluctuate. Initially, the polytropic expansion phase with a specific numerical model is described to evaluate the energy losses that affect the thermodynamic performance of screw expanders when installed in the planned renewable energy power plant. Subsequently, to explore the exergy harnessing in the exhausted steam at off-design working conditions and then to appraise the maximum exergetic efficiency of the proposed screw expander-based solar thermal electricity plant, numerical optimization was performed in a broad range of evaporation and condensation temperatures.

Optimization of medium temperature direct steam generation solar plant

The object of this study is the plan of original power systems of reduced size, investigating on the energetic benefits of a solar plant with parabolic trough collectors and steam screw expander. Usually, medium temperature solar systems need to be coupled with organic fluids cycles which allow high specific expansion work at low temperature and pressure, so involving a heat exchange between two fluids with the consequent exergetic degradation of the available thermal energy. This paper focuses on the steam screw expander-based solar thermal electricity system with water as heat transfer and storage fluid, clarifying in which operating conditions this simple plant layout could be convenient in terms of energy saving. A mathematical model on the system part-load behaviour is analysed and the optimum working conditions of this solar system were determined, revealing variations of maximum solar thermal power efficiency with operating pressure ratio and beam solar radiation.

Thermal energy storage evaluation in direct steam generation solar plants

Existing commercial parabolic trough power plants use thermal oil as a heat transfer fluid, with working temperatures in the region of 400 °C. In order to achieve more efficient generating systems, a second generation of parabolic troughs that operate at temperatures higher than 400 °C is being developed. One possibility Abengoa Solar is assessing is the use of direct steam generation (DSG) inside parabolic troughs in order to achieve higher temperatures; in a first stage heating up to 450 °C and in a second stage heating up to 550 °C. For the future market potential of parabolic trough power plants with DSG, it is beneficial to integrate thermal energy storage (TES) systems. Different TES options based on the most known technologies, steam accumulators, molten salts (MS), and phase change materials (PCM), are presented and compared in this paper. This comparison shows as main conclusion of the study that a combined system based on PCM-MS has a clear advantage in the ratio with 6 or more equivalent hours of storage, while with lower than 6 h, steam accumulators are considered the best option.

Exergy and exergoeconomic analysis of sustainable direct steam generation solar power plants

Solar direct steam generation is considered as a promising technology for steam production in thermal power generation due to high temperature levels that can be achieved compared to other technologies that use indirect steam generation. This paper demonstrates exergy and exergoeconomic analysis of commercial-size direct steam generation parabolic trough solar thermal power plant. For steam power cycles, reheating might be necessary to avoid great wetness of steam which shortens the lifetime of the turbines. Therefore, two configurations have been considered in this study; the non-reheating configuration as well as reheating by steam–steam heat exchanger. For each component, exergy and exergy-costing balance equations have been formulated based on a proper definition of fuel–product–loss. Exergy results show that particular attention should be paid to solar field, condenser, low pressure turbine and high pressure turbine (in a descendant order) as they constitute the major sources of exergy destruction. Results from exergoeconomic analysis, however, show that the condenser should be the fourth component in the order of importance after the solar field and low/high pressure turbines. Increasing the temperature at the inlet of the low pressure turbine by 100K using steam–steam reheating is shown to result in 9.1% increase in the vapor fraction at the exit of turbine. This increase in steam quality, however, would be achieved by drop less than 1.5% in thermal and exergetic efficiencies, and about 2% increase in cost of electricity. Moreover, the effect of degree of reheating on exergetic exergoeconomic parameters has been investigated. The results revealed that there is a specific value of degree of reheating for which the exergetic efficiency would be on it’s lowest value. This point would be of importance during optimization procedure of reheating direct steam generation solar plants.

Performance and cost assessment of Integrated Solar Combined Cycle Systems (ISCCSs) using CO2 as heat transfer fluid

In this paper, a performance and cost assessment of Integrated Solar Combined Cycle Systems (ISCCSs) based on parabolic troughs using CO2 as heat transfer fluid is reported on. The use of CO2 instead of the more conventional thermal oil as heat transfer fluid allows an increase in the temperature of the heat transfer fluid and thus in solar energy conversion efficiency. In particular, the ISCCS plant considered here was developed on the basis of a triple-pressure, reheated combined cycle power plant rated about 250MW. Two different solutions for the solar steam generator are considered and compared.The results of the performance assessment show that the solar energy conversion efficiency ranges from 23% to 25% for a CO2 maximum temperature of 550°C. For a CO2 temperature of 450°C, solar efficiency decreases by about 1.5–2.0% points. The use of a solar steam generator including only the evaporation section instead of the preheating, evaporation and superheating sections allows the achievement of slightly better conversion efficiencies. However, the adoption of this solution leads to a maximum value of the solar share of around 10% on the ISCCS power output. The solar conversion efficiencies of the ISCCS systems considered here are slightly greater than those of the more conventional Concentrating Solar Power (CSP) systems based on steam cycles (20–23%) and are very similar to the predicted conversion efficiencies of the more advanced direct steam generation solar plants (22–27%).The results of a preliminary cost analysis show that due to the installation of the solar field, the electrical energy production cost for ISCCS power plants increases in comparison to the natural gas combined cycle (NGCC). In particular, the specific cost of electrical energy produced from solar energy is much greater (about two-fold) than that of electrical energy produced from natural gas.