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A simple T-junction has been tested as a phase separator to separate liquid from gas. Tests have been performed with a 5cm inner diameter T-junction that is oriented with the main arm horizontal and the side arm directed vertically upwards. The flow media were liquid water and air at pressures between 1 and 2 bar. Separation efficiency and pressure loss were evaluated. As a result it is observed that the separation efficiency can be significantly improved if the whole T-junction is slightly inclined downwards. Separation of more than 90% of the liquid is feasible if the flow at the inlet of the T-junction is stratified to wavy.
Parabolic trough collectors are the most mature technology for large scale solar electricity production as demonstrated by the 354MWe operating since a decade in California. Those systems use thermal oil to transfer heat from the collectors via suitable heat exchangers to a steam turbine power plant.
Significant cost reductions, performance improvements and environmental benefits are expected from direct steam generation (DSG) in the absorber tubes, avoiding the intermediate heat exchange, eliminating temperature limitations imposed by the thermal stability of the oil, and reducing the pumping power required for the solar field. During the projects DISS and INDITEP, technical feasibility of the DSG process has been proven and the basic tools and components to design and implement a first DSG demonstration plant have been developed. However, there is still room for significant improvement of components and O&M processes related to DSG plants. Development of new components and/or O&M process require the availability of a life-size DSG test facility where they can be tested under real solar conditions. Such a DSG test facility is now available at the Plataforma Solar de Almería, which is owned by the Spanish CIEMAT, and it is the result of seven years of R+D work. This is the so-called PSA DISS Test Facility.
Companies and/or research centers performing activities related to the DSG process could use the test facility offered here to test their developments or products in order to qualify them under real solar conditions.
A numerical tool is set up based on the object-oriented Modelica language to simulate the transient behaviour of parabolic trough collector rows. Detailed analysis on the response of the system to fluctuations in solar irradiation are carried out. The tool allows for implementation of a plant control system. Due to the object-oriented structure of the code the components can be arranged in various configurations. Also adaption to other pipe flow applications can be simulated after minor adaptions to the models.
Integration of DSG solar field and power block require specific equipment, the definition of which is closely related to start-up and shut-down procedures for the power plant. Non-conventional equipment and systems have been specified, according to the necessities of the different operation procedures to be implemented in this power plant. Mainly buffer tanks, water-steam separators and recirculation pumps are required, but other systems have also been completed or re-designed in order to comply with the operational requirements.
Lay-out and specific isometric drawings were generated in order to clearly define the integration of both parts. A global lay-out of the whole power plant was also produced.
A phase separator for the use in parabolic trough collector fields with direct steam generation was designed, constructed and tested under steam conditions. Baffles arranged inside the pressure vessel are responsible for separating droplets from the gas stream. Separated water is collected at the bottom of the vessel and can be extracted from the device through two pipes. Design rules are set up to dimensionally define the separator. In water-steam tests a very good phase separation of more than 95% was achieved over a wide range of operating conditions.
Electrical engineering of the whole power plant has been carried out. Apart from conventional power cycle electrical engineering design, a specific task was carried out for the design of electrical power supply systems in the solar field. A specific design was derived for feeding electrical motors at the drive pylons, as well as for earthing parabolic trough structures and surge arrestors in the solar field.
Economy assessment of different DSG power plant configurations.
For a number of sites, and different power plant sizes (namely 5, 12 and 50MWe), a complete economic and technical study was carried out were conceptual designs were defined and estimates were given for investment and O&M costs as well as for yearly electricity production.
A satellite model was used in order to derive annual series of hourly values DNI (Direct Normal Irradiances) for each site. Radiation data were used as input in complete models were solar field plus power cycle were integrated, based in the software tool IpsePro. The output values of yearly electricity production, and consumption of different sources (such as fuel, water, electricity) were introduced in an excel spreadsheet, which also integrated estimates for O&M values in order to derive LEC, IRR and NPV values for each option.
Control architecture for the whole 5MWe DSG power plant were defined. Control room at the main building, control units for the main systems and communications infrastructure.
Regarding the solar field, Local Control Units, which communicate, with the DCS were fully integrated in the control engineering design of the power plant. Such LOC's, which have already been designed, tested and validated at the PSA, operate in open loop arrangement. They calculate the position of the sun via astronomic algorithm, check the current position of the parabolic trough collector via lineal encoder mounted in the axis of the structure and command the hydraulic drive units located at every drive pylon.
The metal support structures being previously defined as Eurotrough model, mechanical engineering design of the solar field was required in order to define the interconnection of all the mechanical components within the solar field.
Piping, valves, auxiliaries, pressure vessels and pumping stations were designed according to international standards and a complete set of drawings and specifications was produced.
Solar field civil engineering design.
Parabolic trough foundations have been sized and designed according to the loads trasmitted by the metal structure. Metallic structures and foundations for the rest of the equipment in the solar field (piping, vessels, tanks, pumps, etc) have also been designed. Inner and roads and drainage and rainwater collecting system for the platform of the solar field have been designed and quantified. A flood prevention system has also been conceived.
Conceptual development of the solar field for a DSG plant of 5MW. A DSG solar field was designed to be coupled to a power cycle delivering 5MWe net electrical power.
Parabolic trough collector loops were sized and the process values defined according to the requirements of the steam turbine. An iterative process was carried out in order to maximize solar-to-electric efficiency of the power plant. Special attention was paid at this preliminary stage to power cycle - solar field integration issues, defining specific equipment in order to properly link the requirements of water and steam from both sides.
CIEMAT has developed a new selective coating for stainless steel pipes. This coating is stable in air at 500ºC. Samples of this new coating have been tested in an open oven at 500ºC for six months and no degradation was detected because the absorptance and emittance remained unchanged after this period. Selective coatings commercially available at present for working temperatures of up to 400ºC are not stable in hot air and a rapid degradation occurs when the glass envelope is broken and the vacuum conditions between the steel pipe and the glass cover is lost.
The coating developed by CIEMAT is manufactured using the inexpensive technique of Sol-gel, using platinum as both infrared reflector and metallic component for the cermet. The amount of platinum is very small and therefore the price of this noble metal is not a barrier to be cost-effective.
The structure of the complete absorber is composed of several layers. First of all, nickel-tungsten and silica layers are used to protect stainless steel substrate against thermal degradation and the diffusion barrier is employed to avoid platinum diffusion in the former layers due to high temperature. Platinum reflective layer is used to assure low thermal emittance of the absorber and two platinum-alumina cermets, with different metal content, are the absorber of solar radiation. Finally a silica antireflective layer is deposited over the absorber to reduce front surface reflection in the absorber and to optimise solar absorptance.
Solar absorptance (AM1.5) of the coating developed is 0.95 and thermal emittance is 0.15 at 500ºC.
Improved reflectors were developed and tested. This included reinforced reflectors using 5mm glass, a new fixation technique using ball joints and "click in" mounting concepts.
Prototype reflectors were fabricated and tested at PSA using high precision photogrammetric methods, where the intercept factor could be measured. This was performed in a laboratory test setup and in the Inditep Collector. From the measurements synthetic yearly intercept values were produced, with the aid of a statistical ray tracing code and insolation data of the Guadix site for 2003 and 2004.
Lists of instruments have been generated for the whole power plant, according to the requirements determined by process & control engineering. Hook-up typicl drawings have been generated and instruments have also been implemented in isometric drawings of the solar field.
Procedures of start-up and shut down have been established so much in cold as in warm, both in winter and in summer. These procedures will be implemented when a DSG plant may be operated.
A model library for the commercial heat balance calculating program IPSEpro has been developed. This library contains all components necessary for the calculation of solar thermal power plants with direct steam generation, such as i.e. collectors and separators. This program library can be used for the basic engineering of parabolic trough collector fields with direct steam generation as well as for the investigation of their part load behaviour and the electricity production of these solar thermal power plants at every site where weather data are available.
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