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Researchers have developed a once-through concept that directly heats water in the receiver tube and can be used to drive power plant turbines. The parabolic trough test facility is located in southern Spain at the Plataforma Solar de Almería.
© DLR (QFly); PSA
Research projects revisited
02.10.2015

The illustration shows a comparison between the previous recirculation concept (above) with a central steam separator and the currently developed once-through concept with two injectors in each collector string (below).
© DLR

The principle of direct evaporation is similar to the one used in conventional power plants with the exception that here the steam is generated directly in the solar array.
© DLR

Greater efficiency: Generating steam in receivers

On the Plataforma Solar de Almería in southern Spain, scientists from the German Aerospace Center (DLR) have developed a new way of generating solar steam. In the installation, the steam used for producing electricity is generated directly in the receiver tubes in the parabolic troughs. This therefore eliminates intermediate steps using heat transfer media and also enables higher operating temperatures of up to 550 degrees Celsius. This should therefore enable parabolic trough power plants to produce electricity more efficiently and cheaply.

In the Spanish installation, parabolic mirrors 1,000 metres in length focus the sun‘s rays onto receiver tubes. The special feature of this collector string is that the tubes contain water instead of oil as the heat transfer medium. This effectively makes it one of the largest solar water heaters in the world – and the largest of its kind. Scientists from the German Aerospace Centre (DLR) have developed and tested the so-called “once-through” concept as part of the DUKE solar research project. Here the water first of all evaporates within the collector string and is then further heated to a higher temperature. With a temperature of up to 550 °C and a pressure of about 110 bar, the superheated steam produced in this manner can then be used in principle to drive turbines in power plants. This direct steam generation in the solar array is known as direct evaporation.

Previous direct evaporation systems have worked according to the so-called recirculation concept (Figure top left). Here the water in the solar array flows through three zones: the evaporating area for generating the steam, the steam drum where liquid water and steam are separated, and the last section of the solar array where the temperature of the steam is further increased. This concept features a simple process management.
“With our further enhanced approach with the forced once-through concept, the steam for the turbine is vaporised in a continuous string and superheated,” explains Jan Fabian Feldhoff, project manager of the DUKE project at DLR, and adds: “This concept also increases, however, the demands made on the installation’s control system.” The steam drum in the old approach was able to successfully dampen the influence of clouds. In the new once-through concept, however, passing clouds have an almost immediate effect, so it is necessary to react very quickly. The particular difficulty with this is that it is difficult to know precisely when and where a cloud will appear. Solar radiation measurements are expensive and reliable forecasts have only just begun to be developed – but the controller needs, of course, to work already. Amongst other things, the measurement of the steam temperature is therefore used to identify the current situation in order to accordingly adapt the controller automatically.

Once-through concept more flexible than recirculation operation

The development of direct evaporation is benefiting from the knowledge gained from conventional technologies used for generating steam, for example in coal-fired or nuclear power plants. The desired increase in efficiency leads, however, to a rise in the pressure and the steam temperature. Instead of using recirculation operation, modern power plants therefore also operate forced flow systems that correspond to the once-through concept. Especially at high pressures, the once-through concept provides significantly more flexibility during operation.
A particular advantage of the once-through approach relative to recirculation is the ease with which the collector string can be scaled up. Each string has the same structure and provides the same performance. In the case of the test facility, that amounts to a thermal output of around three megawatts when there is good direct sunlight. “To create a large power plant, you simply have to construct as many strings in parallel as required until the desired performance is achieved. It is precisely this simple design that offers considerable potential for savings,” sums up the project manager.

Water as a heat transfer medium

Parabolic trough power plants are currently the most tried and tested solar power plants. Almost all commercial plants built to date use a synthetic heat transfer oil in the receiver tubes situated in the mirror array. The disadvantage is that the tubes can only be heated to temperatures below 400 degrees Celsius, which limits their maximum efficiency. The plant now being tested can work with operating temperatures of up to 500 degrees Celsius in the new type of receivers. Having water as the heat transfer medium also has the advantages that it is cheap, always available and is neither flammable nor harmful to the environment.

Controlling the temperature of the once-through concept

The higher the temperature, the greater the efficiency with which the power plant can be operated. “In actual operation it is sufficient to keep the temperature in a small range around the desired target temperature,” says Feldhoff. The problem with this: “This is not so easy with strongly fluctuating radiation, for example, with passing clouds.” The energy for producing and superheating the steam can break down within a few seconds. The temperature must remain constant, since otherwise the steam turbine would automatically switch off for safety reasons. The amount of water in the collector string must therefore always be adapted to the current irradiance conditions.
If water is added at the inlet of the collector string, there is a considerable lag in terms of the temperature’s response at the outlet. If the amount of water is reduced by one per cent, the temperature does not respond for five to ten minutes and then rapidly increases by about eight degrees. The situation worsens if the cloud has drawn away in the meantime. This is why water is added at one or two points within the collector string. This is injected into the steam. This enables the temperature to be controlled considerably more quickly, which represents a crucial step in the commercial implementation of the once-through concept.

Follow-up project to bring technology to market maturity

The results enabled the researchers to eliminate previously lingering doubts about the concept and establish the findings on a sound footing. However, long-term experience is still required in order to convince all doubters – especially in order to reduce the risk premiums when financing such power stations. “We are planning a follow-up project to bring the technology to market quickly,” says Jan Fabian Feldhoff.

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