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» LINEAR FOCUSING FACILITIES

The PSA has several linear-focusing solar collector facilities, both parabolic-trough and Fresnel. Many of these experimental installations, such as the innovative fluids test loop or the DISS plant, are the only one of their kind, and place the PSA in a privileged worldwide position for research and development of new parabolic-trough collector applications. The main characteristics of these facilities are briefly explained below.

» THE HTF TEST LOOP

The HTF test loop was erected in 1997 and it is an ideal facility for evaluating parabolic-trough collector components under real solar energy operating conditions.

The facility consists of a closed thermal-oil circuit connected to several solar collectors of 75-m long connected in parallel (up to three collectors can be installed in parallel), being able to operate only one at a time. The east-west rotating axis of the solar collectors increases the number of hours per year in which the angle of incidence of the solar radiation is less than 5º. The thermal oil used in this facility (Syltherm 800) has a maximum working temperature of 420ºC and a freezing point of -40ºC.

The facility’s oil circuit, which has a maximum working pressure of 18 bar, is made up of the following elements:

  • 1-m3-capacity oil expansion tank, with automatic nitrogen inertization
  • Oil circuit sump tank.
  • Mechanical-draft oil cooler, with air speed control and 400-kW maximum cooling
  • Centrifugal oil pump, with a flow rate of up to 8.3 liters per second
  • Two 40-kW electric oil heaters

Diagram of the PSA "HTF test Loop"

The first EUROtrough collector prototype developed by an European consortium with the financial aid of the European Commission was installed and evaluated under real working conditions at this facility in 1998 The facility is appropriately instrumented for qualifying and monitoring of the follow-ing components:

  • New designs of parabolic-trough collectors (up to 75 m long)
  • Parabolic-trough collector mirrors
  • Parabolic-trough collector absorber tubes
  • New designs of ball-joints or flexholes for connecting parabolic-trough collectors in the solar fields.
  • Solar tracking systems.

Main activities are currently related to study the optical and thermal performance of complete para-bolic-trough collectors (optical efficiency, IAM coefficient, and global efficiency/heat losses) and receiver tubes.

» TEST LOOP FOR PRESSURIZED GAS IN PARABOLIC-TROUGH COLLECTORS

The purpose of this experimental facility is to study the use pressurized gases as working fluids in parabolic-trough collectors, which has not been done to date, evaluating their behavior under a diversity of real operating conditions.

The experimental test loop is located north of the DISS loop control building, which houses the equipment necessary for its control and data acquisition.


View of the pressurized gas test loop connected to a molten salt thermal storage system.

The facility was originally designed to work at pressures and temperatures of up to 100 bar and 400ºC, and consists of the following components:

  • Two East-West-oriented EUROtrough parabolic-trough collectors, each 50 m long with a 274.2-m2 collector surface. The collectors are connected in series.
  • An 400-kW air-cooler able to dissipate the thermal energy in the fluid delivered by the collectors. It has two 4-kW motorized fans
  • A blower driven by a 15-kW motor which supplies the gas flow necessary to cool the collectors adequately
  • A data acquisition and control system that allows the temperature, flow rate, pressure, irradiation and humidity in the loop to be completely monitored.
  • Automatic control valves that allow precise, safe variation in the collector fluid feed flow rate.
  • A secondary loop for filling the test loop with gas.

Since testing at 400ºC was successfully completed at the end of 2009, this facility was then up-graded to achieve temperatures of up to 515ºC and it was connected to a two-tank molten-salt thermal storage system to test their joint capacity for collecting and storing solar thermal energy with a view to making use of them in dispatchable high-performance thermal cycles. This increase in test loop design conditions to 100 bar and 515ºC made the implementation of different improve-ments necessary (conventional absorber tubes in Collector 2 were replaced with advanced high-temperature tubes, stainless steal pipes were installed for the high temperature zone and changes were made in the control system).

 


Simplified system diagram of the innovative fluieds test loop connected to a molten-salt thermal storage system

The molten salt thermal storage system which forms part of a larger experimental salt loop, basically consists of:

  • Two 39-ton salt tanks, hot and cold, able to provide about six hours of thermal storage
  • An 344-kW air cooler to cool the salt with ambient air
  • A 344-kW gas/salt exchanger providing the salt circuit with the solar energy collected in the innovate fluids test loop

The thermal storage system is also connected to a small 344-kWt thermal oil loop, with VP-1 oil, allowing the thermal storage system to be charged and discharged by using this thermal oil system, with a salt/oil heat exchanger. This oil circuit consists of: expansion tank, drainage tank, oil heater, salt/oil heat exchanger and oil cooler.

» THE DISS EXPERIMENTAL PLANT

This test facility was erected and put into operation in 1998 for experimenting with direct generation of high-pressure-high temperature (100 bar/400ºC) steam in parabolic-trough collector absorber tubes. It was the first facility built in the world where two-phase-flow water/steam processes in parabolic-trough collectors could be studied under real solar conditions.

The facility consists of two subsystems, the solar field of parabolic-trough collectors and the balance of plant (BOP). In the solar field, feed water is preheated, evaporated and converted into superheated steam at a maximum pressure of 100 bar and maximum temperature of 400ºC as it circulates through the absorber tubes of a 700-m-long row of parabolic-trough collectors with a total solar collecting surface of 3,838 m2. The system can produce a nominal superheated steam flow rate of 1 kg/s. In the balance of plant, this superheated steam is condensed, processed and reused as feed water for the solar field (closed loop operation).

In 2012, within the Project DUKE, three additional parabolic-trough collectors were installed in the solar field and all the absorber tubes were replaced by new ones, to increase up to 500ºC the temperature of the superheated steam produced, enabling to generate direct steam at 100bar and 500ºC.


Simplified flow diagram of the PSA DISS loop

Facility operation is highly flexible and can work from very low pressures up to 100 bar. It is also equipped with a complete set of valves allowing the solar field to be configured for Recirculation (perfectly differentiated evaporation and superheating zones), for Once-Through (the intermediate water-steam separator and the recirculation pump located in the solar field are not used in this operatin mode) and in Injection mode (feed water is injected in different points along the collector row).

The facility is provided with a wide range of instrumentation for full system monitoring (flow rates and fluid temperatures in the various zones of the solar field, pressure drops in collectors and piping, temperature and thermal gradients in the cross sections of the absorber tubes, etc.) and a data acquisition and process control system which has a database where 5-s process data are recorded 24 hours a day.


View of the DISS plant solar field in operation

Among the capacities associated with this facility are the following:

  • Component testing for parabolic-trough collector solar fields with direct steam generation in their receiver tubes (receivers, ball joints or flexholes, water-steam separators, specific instrumentation, etc.).
  • Study and development of control schemes for solar fields with direct steam generation
  • Study and optimization of the operating procedures that must be implemented in this type of solar field
  • Thermo-hydraulic study of two-phase of water/steam in horizontal tubes with non-homogeneous heat flux.
» CAPSOL FACILITY

CAPSOL is a concentrating solar thermal energy test facility designed and built at the PSA for testing of small-sized, high-precision parabolic-trough solar collectors under real environmental conditions.

The facility is designed to operate with pressurized water under a wide range of operating conditions: fluid temperatures from ambient to 230ºC, flow rates from 0.3 to 2.0 m3/h and pressures up to 25 bar. It also allows testing of different collector orientations and sizes (apertures up to 3 m). High-precision instrumentation has been installed for measuring all of the parameters required for adequate evaluation of parabolic-trough collectors. In particular, the facility has a mass flowmeter (Coriolis-type, with a ±0,1% measurement accuracy), a pyrheliometer (Eppley, with 8 μV/(Wm-2) sensitivity) and two types of temperature sensors at the inlet and outlet of the solar field (4-wire PT-100 with an accuracy of ±0.3ºC in a 100 to 200ºC range). In addition to these instruments, the facility has sensors for measuring other parameters, such as fluid temperature at various points in the circuit, pressure, tank level, ambient temperature, wind speed and direction, etc.

Figure below shows a Photo of the CAPSOL test facility with two prototypes of small-size parabolic-trough collectors installed.


CAPSOL solar thermal test facility for small-size parabolic-trough collectors.

This test facility makes it possible to find the efficiency parameters required for characterizing parabolic-trough collectors: peak optical-geometric efficiency, incident angle modifier, overall efficiency and thermal losses when collectors are out of focus. The stationary state conditions needed for performing these tests is reached thanks to the inertia of the expansion tank and auxiliary heating and cooling systems. The data acquisition and control system facilitates monitoring and recording of the parameters measured as well as system operation from the control room.

Both complete small-sized parabolic-trough collectors and their components, such as absorber tubes, reflectors or tracking systems, can be tested in this facility. Furthermore, the facility also allows analysis of technical aspects of the collectors, such as materials durability, structural resistance, component assembly, etc. under real operating conditions.

» THE FRESDEMO LOOP

The FRESDEMO loop is a “Linear Fresnel  concentrator” technology pilot demonstration plant. This 100m-long, 21-m-wide module has a primary mirror surface of 1,433 m2, distributed among 1,200 facets mounted in 25 parallel rows spanning the length of the loop. This collector loop is designed for direct steam generation at a maximum pressure of 100 bar and maximum temperature of 450º.

This pilot facility is presently connected to the piping system of the PSA DISS plant from where it is supplied with solar steam at different pressures and temperatures for testing in the three working modes: preheating, evaporation and superheating.


Figure 1.- View of the linear Fresnel concentrator erected at the PSA.

» KONTAS

A rotary test bench for parabolic trough collector components, KONTAS,  was erected at Plataforma Solar de Almería in 2009. The concept was developed by DLR and within the framework of the Spanish-German agreement between CIEMAT and DLR, this test facility is now jointly used by both institutes.  

The test bench allows the qualification of all collector components and complete modules of a length of up to 20 m, i.e. structures, reflectors, receivers and flexible joints. It enables for a tracking at any desired angle of incidence of the solar radiation. It is equipped with high precision instrumentation and controls for precise, quick and automated measurements.

The test bench rests on rails directly mounted on top of the foundation. These rails form an inner and an outer ring. The collector itself is mounted on a steel platform with six steel wheels. The rotation of the platform on the rails around the central bearing is performed by motors driving four of these wheels.

The collector module is connected to a heating and cooling unit, which is also situated on the platform. A pump circulates Syltherm 800®* thermal oil as heat transfer fluid (HTF) with a mass flow similar to that of commercial plants. Mass flow is measured directly using the Coriolis measuring principle avoiding uncertainties of the density. The heating and cooling unit dissipates the energy the hot HTF collected on the way through the module and ensures a constant HTF temperature (±1K) at the inlet of the collector. Sensors for measurement of inlet and outlet temperatures are highly precise and may be calibrated on site. A high precision meteorological station delivers accurate radiation and wind data.

kontas
Side view of Kontas test bench and the heating cooling unit.