Solar Energy Materials 24 (1991) 222-230 North-Holland

Solar Energy Materials

Test loop for research on direct steam generation in parabolic trough power plants

M a r t i n Miil ler

z s w - Centre of Solar Energy and Hydrogen Research, Section Solar Thermal Engineering Hessbriihlstrasse 61, D-7000 Stuttgart 80, Germany

In future generations of parabolic trough power plants high-pressure live steam is to be generated directly in the tubes of the reflector field. The unusual combination of geometric dimensions and values of thermodynamic parameters will be the cause for thermodynamic peculiarities. Preliminary predictions of the two-phase flow patte~n,.~ of the fluid and the tempera- ture distribution in the tube wall are presented. Experiments fot the validation of the computa- tions and for further research are described.

1. Introduction

With this presentation of the activities of the Centre of Solar Energy and Hydrogen Research in the field of direct steam generation power plants, I do not want to present results today, but I want to describe some points of interest connected with this new technology and I want to show the concepts for experi- ments we are going to set up in order to provide tools for the examination of the peculiarities of direct steam generation power plants.

First of all, I want to compare the basic concept of solar farm power plants using today's technology, that is circulating thermal oil in the primary loop, with future concepts of solar farm power plants using direct steam generation (fig. 1).

Today, all solar farm power plants being in operation use a concept with two loops. The primary loop contains thermal oil as working fluid. The oil circulates through the absorber tubes of the parabolic troughs and is heated by the con- centrated radiation of the sun to a maximum temperature of about 390 ° C. A heat exchanger is used to transfer the heat of the oil to the secondary loop, which contains water and steam as working fluid. The secondary loop is a conventional loop for a thermal power plant with a single reheat system. Live steam conditions are 100 bar, 370°C. One major advantage of the system is, that only a moderate pressure of about 16 bar is required in the pr imary loop. Therefore, flex hoses for the connection of the pipes are available.

On the other hand, the thermal oil is responsible for a big part of the operational costs, since the oil will age due to thermal cracking and therefore has to be exchanged from time to time. In order to reduce costs and increase the efficiency of

0165-1633/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

M. MiiUer / Steam generation in parabolic trough power plants 223

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Fig. 1. Simplified

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heat flow diagrams of solar farm power plants: (a) with thermal oil in tb. • primary loop: (b) with direct steam generation.

the power plants, the oil has to be replaced by a heat transfer fluid with the following conditions: - it has to be cheap: a power plant of the 30 MW class contains about 350 m 3 of

oil; - it should be t~tsed in the heat collecting elements as well as in the turbine in order

to eliminate '~he second loop; - it should be non-critical for the environment in case of leakages. Water and steam at high pressure meet these conditions excellently.

2. Experimental concepts

By using water as the only fluid in a solar power plant, the concept becomes much simpler since the pump for the oil loop arid the heat exchangers are no longer necessary. In addition to that, this concept allows to use higher live steam tempera- tures since there is no restriction from the thermal stability of the working fluid.

With this concept, commonly referred to as Direct Steam Generation (DSG) because the steam is generated directly in the absorber tubes of the solar field, the design of the parabolic troughs and of the absorber tubes has to be modified, mainly since the absorber tube has to withstand a live steam pressure of at least 100 bar.

224 M. Miiller / Steam generation in parabolic trough power plants

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Fig. 2. DSG-Ioop, artist's view.

Flex hoses or other types of rotatable fittings are expensive and very !ikeiy to become trouble spots. Therefore, the pipe has to be fixed relative to the ground and the parabolic mirror has to rotate around the pipe. A first impression of the design of the new collector can be seen from fig. 2.

Parabolic trough collectors are usually tracked by only one axis which is the centre line of the absorber tube in the case of direct steam generation power plants. Therefore, they always suffer cosine losses. By lining-up the mirrors Ln the *"--"- I,~ U I t l l -

South axis and thus turning them in the course of the day and by mounting them with an inclination depending on the degree of latitude, the cosine losses can be minimized. This target leads to a saw-tooth-like shape of the lines of the solar field, which can also be seen from fig. 2.

Due to the fact that the concentration ratio of the mirrors cannot exceed a value of about 80.. . 100, the heat absorber tubes for troughs with an aperture of about 11 m will have an inner diameter of about 100 ram. In order to achieve a mass flow density of at least 500 k g / m a s, the length will have to be more than 1 kin.

The described improvements in the design of solar farm power plants show several peculiarities which are subject to our research activities. The main peculiari- ties are: - the length of the evaporator tubes, - the unusually large inner diameter of the tubes,

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34. Mi~ller / Steam generation in parabolic trough power plants 225

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- the small inclination, - the saw-tooth-like downsteps within the absorber lines, - the non-uniform heating of the tubes and the fact, that the heated zone moves

around the tube during the course of the day. A first point to be determined is the sequence of flow patterns of the fluid along

its path through the absorber. Vaporization does not take place at a certain spot of the evaporator tube, but

within a tube section of a given length. In this evaporation section, a two-phase flow of water and steam occurs in the tube. With respect to the steam contents and the speed of the phases of the flow, several flow patterns may he observed. The flow patterns themselves will affect the conditions of heat transfer from the tube wall to the fluid and therefore are of greatest interest.

For one realistic layout case, we have calculated a flow pattern map according to Steiner, University of Karlsruhe (fig. 3). By using dimensionless numbers, the map defines regions for the characteristic flow patterns occuring in the two-phase flow. The regions for the flow patterns are plotted versus the Martinelli parameter, which is a dimensionless number depending on the steam content of the two-phase mixture. By connecting the points calculated for different values of steam content. the sequence of flow patterns occuring in the evaporator tube can be determined. It turns out that the region of stratified wavy flow is to be expected especially for low

226 M. Miiller / Steam generation.in parabolic trough power plants

wail thickness not in correct scale

Fig. 4. Two-dimensional temperature field of a pipe wall heated from one side.

steam contents. At higher steam contents, slug and plug flow may occur and later there will be dispersed drops in the steam flow.

It has to be noted, that this map is only valid for a non-heated flow which is completely developed. To achieve a completely developed flow, a length of about 200 diameters (about 20 m in a real-scale DSG loop) is necessary.

Therefore, it is not possible at present, to give a reliable I-~:diction of the flow patterns that will occur. The main reason for this is the long part of the pipe, where the flow is not completely developed after the downsteps. On the other hand, the stability of the flow and the heat transfer to the fluid depend highly on the actual flow pattern.

In order to show the influence of the flow pattern and of the circumferentially non-uniformly heated zone of the pipe, we made a preliminary prediction of the wall temperatures of the pipe and the distribution of the heat flow.

By using a finite-difference program, the two-dimensional temperature field of a cross-section of the wall can be calculated. The temperature field in fig. 4 is computed by subdividing the pipe wall into 6 radial layers and 36 circumferential sections. The tube is u~Jf'.~rmly heated from only one side, Inside the tube, the steam-water flow is assumed to be stratified. From the plot of the isothermals, the importance of azimuthal heat conduction through the pipe wall can be clearly seen.

M. Miiiler / Steam generation in parabolic trough power plants 227

Of course, the highest values of temperature occur in the heated, non-wetted region. For the data used here, the temperature is about 80 K higher than the lowest temperature in the wall. The width of this range of temperatures is very sensitive to the coefficients of heat transfer from the tu~: e wall to water as well as from the tube wall to steam. Therefore, the ;emperatures have to be measured by experiments and by well adapted calculation methods of the heat transfer coefficients. It is obvious, that thermal stress will occur in the tube wall and that the tube will be bent due to these temperature differences.

The purpose of showing the flow pattern map and the temperature field of the tube wall was to give a brief insight into some of the physical effects which are to be expected in the pipes of direct steam generation power plants.

The tools for profound investigation on the DSG process and its pecularities would therefore be full-scale and reduced-scale experiments. Of course, the full-scale experiment under solar conditions gives the most reliable results, but experiments with down-scaled geometric dimensions and with other fluids than water and steam at high pressure provide a tool to carry out the experiments under easily controlled and stable conditions.

The Centre of Solar Energy and Hydrogen Research is installing two different experimental set-ups for the examination of the two-phase flow.

A preliminary and simple experiment consists in observing the flow in a pressure- less tube, using water and air at ambient temperatures. The observation of the flow is very easy, since a glass or plexiglass tube can be used. On the other hand, only phenomena of the non-boiling two-phase flow can be determined, because no heat is transferred to the fluid. We will use this experiment especially in order to get a first understanding of the effects due to the downsteps between the mirrors.

From the flow diagram of the experimental set-up, the general concept can be seen (fig. 5). Two plexiglass pipes with an inner diameter of 40 mm are connected with a transp~tzent plastic hose. The inclinatic, n of the pipes as well as the flow rates

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g/rain max., mass fraction 0.1 kg air/kg H:~O max.

228 M. Miiller / Steam generation in parabofic trough power plants

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of water and air can be adjusted. The water is circulated by the use of a pump, whereas the air is blown into the water flow from a compre,.,sor by using a special mixing device. At the end of the test section, the air is released from the flow by separating water and air in an open vessel.

The main experiment will use the original fluid at the real thermodynamic parameters. Unfortunately, it is not possible to set up a full-scale experiment on the controlled conditions of a laboratory, since even one single line of the evaporator field would require an energy input of about 10 MW and a pipe length of more than 1 km. The energy demand can be reduced by using smaller pipe diameters, but a true modeling of the whole loop at reduced scale is not possible as could be shown by evaluation of the relevant similarh, y conditions, and the pipe would still be very long. Yhe energy required for the experiments as well as the pipe length can be reduced by modeling individual sections of the evaporator tube. In a similar way as described for the experiment with water and air, the two phases of the fluid (water and steam at high pressure) have to be mixed together at the inlet of the test section. By mixing water and steam to any required steam contents, the whole evaporator tube may be examined by simulating it section by section.

Until now, we are considering both, to build up the experiment in the laboratory as well as to couple the test section to the existing ~Jat~,r and steam in~,'allations of a power plant. The most probable concept for the laboratory loop is shown iii fig. 6,, It consists of a high-pressure water-steam loop. Preferably, the high pressure and temperature levels of the fluid should not be lost at the end of the test section. The heat supplies at the evaporator preceding the mixing device and the heat sink at the

M. Miiller / ,gteam generation in parabolic trough power plants 229

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230 114. Miiller / Steam generation in parabolic trough t~ower plants

condenser are realized by the use of ~: thermal oii~ This concept has the advantage that, due to small mean temperature difference~ al the evaporator and the con- denser, the control of the steam conditions is exact.

The general design data for the test loop are the following: - Maximum live steam pressure: 160 bar - Maximum live steam temperature: 450 ° C - Steam contents at the inlet of the test section: 0 . . . 1 - Maximum mass flow: 350 k g / h

The observation of the flow inside the high-pressure tube is, of course, not as easy as in the water /a i r experiment. A very flexible technique for flow observation is available by using a gamma-ray densitometer, since the tube wall needs no modification (fig. 7). With the gamma densitometer, the location for the measure- ments does not need to be fixed in advance, since the device may be moved to any part of the test loop where the evaporator tube is accessible from catside. In addition to that, we will set up the device as well as the evaluation equipement in a portable manner, so that it can also be transferred to the absorber field of a solar farm power plant.

The principle of operation of the densi tometer is that the beam of a gamma source is absorbed when going through water and steam, but much more by water than by steam. Therefore, the sigr,.a! received from a detector on the other side of the tube indicates the medium density of the fluid along the beam. By combini:~g the signals of several beams, conclusions on the flow pat tern inside the tube can be drawn.

With the high-pressure loop and the results obtained from the densi tometer measurements, it is possible to validate the computed flow pat tern and to achieve additional information about the phenomena of the developing flow. Thus, prob- lems of the boiling process in solar farm power plants can be located and improve- ments can be suggested.