23 January 2013
Direct evaporation in the receiver tube
In the DUKE (Durchlaufkonzept – Entwicklung und Erprobung [Cycle Concept – Development and Testing]) test system, the steam needed to generate electricity is produced directly in the receiver tubes of the parabolic troughs. Intermediate steps employing a heat transfer medium are no longer needed and higher operating temperatures are possible.
DLR (CC-BY 3.0).
DUKE test facility for direct steam generation
In the DUKE (Durchlaufkonzept – Entwicklung und Erprobung; Once-through Concept – Development and Testing) test facility, at the Plataforma Solar in Almería (Spain), the steam that drives the turbine is generated directly in the receiver tube. In today's plants, synthetic thermal oil is used as the heat transfer medium.
At the Plataforma Solar de Almería in southern Spain, researchers from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have put a test facility for solar thermal power plants into operation. In the power plant, steam for generating power is produced directly in receiver tubes in the parabolic troughs. This avoids the need for intermediate stages using thermal transfer media and also allows for higher operating temperatures. The new technology enables parabolic trough power generators to produce power more efficiently and cost-effectively.
Everything in one tube
In the test facility in Almería, parabolic mirrors reflect the Sun's rays onto receiver tubes. These tubes absorb the solar radiation, convert it into thermal energy, and pass it on. What is special about this test facility is that the tubes contain water, instead of the more usual oil; the water is directly heated and turned into steam. The superheated steam generated in this way can be used to drive a turbine in a power generator. Researchers refer to this as a 'once-through concept'. "The main challenges with this type of direct steam generator are the high operating pressure – approximately 110 bar – in the receiver tubes and the control of the entire process. But the advantages outweigh these; with the 'once-through concept', there is no longer a need for heat exchangers and many other additional components such as oil treatment facilities," says project leader Fabian Feldhoff from the DLR Institute of Solar Research, describing the benefits of the new technology. “This enables a reduction in the cost of solar power generators. Furthermore, power generators using this technology can operate at higher temperatures, making the generation process more efficient.”
Using a 1000-metre-long collector array with a thermal output of three megawatts, researchers in the DUKE research project (Durchlaufkonzept – Entwicklung und Erprobung; Once-through Concept – Development and Testing) are attempting to demonstrate the 'once-through concept' on an industrial scale. The new test facility offers one-of-a-kind opportunities for research as well as for continuing to develop this technology.
Advantages of water as a thermal transfer medium
Parabolic trough power generators are currently the most proven solar thermal power generators. Almost every commercial facility constructed to date uses synthetic thermal oil in the receiver tubes of the mirror array. The disadvantage of such thermal oils is that they can only be heated to a maximum of 400 degrees Celsius, which gives rise to limitations in the level of efficiency that can be attained. The facility now being tested by DLR can operate at temperatures of up to 500 degrees Celsius using a new receiver design. Using water as the heat transfer medium has the additional advantage of being low-cost, and it is neither flammable nor harmful to the environment.
In facilities that use the 'once-through concept', the steam for the turbine is evaporated and superheated in one continuous process in the collector array. Previous commercially operated direct steam generation units work using the recirculation concept. With this method, the water flows through three areas in the solar array – the evaporation area where the steam is generated; the 'steam drum', where the liquid water and steam are separated; and the 'superheating area', where the steam is heated to even higher temperatures. Facilities of this type were preferred because they are easier to control. The 'once-through concept' now being developed and tested in Almería certainly presents greater challenges in terms of controlling the facility; however, the scientists believe that, overall, operation of the system will be more cost-effective while at the same time more efficient. Furthermore, facilities of this type are more easily scalable, as solar power generators can easily be expanded. This is particularly important for further cost reductions in the long term.
DUKE project description
The DUKE solar research project (Durchlaufkonzept – Entwicklung und Erprobung; Once-through Concept – Development and Testing) has been funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit; BMU) and is carried out together with industrial partners. The test facility is being operated at the Plataforma Solar de Almería (PSA) under an already-successful cooperation with the Spanish organisation CIEMAT (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas; Centre for Energy, Environment and Technology Research). The project is expected to run until April 2014.
The Plataforma Solar de Almería is a test centre for high-temperature concentrating solar technology. Since the very beginning, DLR has played a major role in its planning and construction, and has been making use of the facility with on-site scientific staff to conduct its solar technology testing and development work in close collaboration with its Spanish partner, CIEMAT, which owns and operates the facility.
Last modified:23/01/2013 10:02:42
23/01/2013 13:54 -
from Divyadarshan D.Purohit
Dear Friends at DLR,You are doing amazing job.Solar power plant is need of new generation,we have no alternate other than nature.I am sure a day will come when whole globe will join this movement.-- Divyadarshan D.PurohitGurudev Observatory,VadodaraIndia
24/01/2013 00:00 -
from Habib Torabi
This method would not be efficient at the larger industrial scale. Because of the lacking of the energy storage and without any doubt it has to be combined with the energy storage of molten salt at the higher temperatures and even combining with the new technology of liquid batteries. According to my opinion, direct steam generation can be used as a sub-part of a larger industrial design that would be combined with the salt molten energy storage and the liquid metal batteries of professor Sadoway that needs even no turbine and no generator for generating the electricity. One thing more, we have to focus on decreasing the steam temperature on behind of steam turbines for increasing the efficiency of the whole of system. For example, using of any kind of heat as a source of energy for producing cheaper drinking water from the sea water by a direct freezing-melting method but only as a sub-part of producing drinking water from the sea water!I am at your service by whole of my heart!
25/01/2013 13:18 -
from Fabian Feldhoff, DLR
Dear Habib:You are absolutely right that storage is crucial for this technology. The most suitable storage system for a direct steam generation (DSG) plant is a combination of a two-tank molten salt system (for super-heating of the steam) and a so-called phase change material storage system (for evaporation of the water). The two-tank molten salt system is applied in all parabolic trough plants with storage so far. It is a state-of-the-art technology. For the phase change material system, a prototype has been tested successfully by DLR in Spain. An industrial-scale prototype will be built in the current project DSG-Store very soon. This project is led by our DLR colleagues of the Institute of Technical Thermodynamics. For a 100 MW electric plant with storage, the efficiency of such a DSG system will be about 8 % higher than the one of a current plant with synthetic oil.The advantage of solar-thermal power plants is the heat storage with very low losses and low cost. The electricity is then generated from the heat when it is needed. This is a decisive difference to batteries which use electricity as input. Combining a solar-thermal plant with batteries is not very efficient. Batteries are better suited for fluctuating electricity generation, like from wind energy or photovoltaics.Best regards,Fabian Feldhoff
07/03/2013 11:12 -
from A Bittar
Hi ,good work that opens incremental gain options for parabolic trough technology, I am surprised that there is no mention of the "once through" implementation by the AREVA Solar Fresnel system, which showed that this was quite a straightforward method for producing superheated steam withour recirculation.how does the PTC DSG cost compare with the Fresnel cost, which presumably if of similar efficiency
14/03/2013 09:14 -
from Fabian Feldhoff
The mentioned Linear Fresnel system by AREVA uses a different approach than in parabolic troughs or in Novatec Solar's Fresnel system.AREVA uses many parallel pipes with smaller diameters instead of one receiver pipe with a larger diameter. Furthermore, AREVA does not foresee any injections to control the mass flow, but only the de-focusing (or moving) of mirrors. This is unfavorable for both control and efficiency.Regarding the cost, it is of course the secret of the project developers. The Fresnel system physically has a lower optical efficiency, so with the same collector surface we have less energy input. However, Fresnel manufacturers say that their collectors are much cheaper than parabolic troughs, such that you can build more collector surface for the same money. The latest studies suggest that the overall system costs (usually given in EUR/Megawatthour) are the same, if the Fresnel system surface only costs about 66% of the same parabolic trough system surface. However, this value strongly depends on the manufacturer and other influences.Best regards,Fabian Feldhoff