Note: Descriptions are shown in the official language in which they were submitted.
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Pipeline System and Method for Draining a Pipeline System
Description
The invention relates to a pipeline system for conveying a salt melt,
comprising at least
one pipeline through which the salt melt flows, at least one inlet and at
least one outlet.
The invention furthermore relates to a method for draining a pipeline system
for conveying
a salt melt.
Pipeline systems for conveying a salt melt are used for example in solar power
plants,
particularly in parabolic trough solar power plants or Fresnel power plants.
The pipeline
systems are generally configured in the form of networks, which are used to
collect solar
energy in the solar power plant. In such a solar power plant, the radiation
energy of the
sun is concentrated by means of parabolic mirrors onto receivers. The
parabolic mirror and
receiver combination is referred to as a collector. A row of collectors is
connected in series
to form so-called solar loops. To this end, the receivers are respectively
connected to the
pipeline system or constitute a part of the pipeline system. A heat transfer
liquid, to which
the radiation energy collected by the receivers is transferred, flows through
the pipeline
system.
At present, a biphenyl/diphenyl ether mixture in particular is used as the
heat transfer
liquid, although the maximum operating temperature of this is limited by its
decomposition
temperature of about 400 C. In order to achieve higher operating temperatures,
which
allow greater efficiency, other heat transfer liquids are necessary. To this
end salt melts,
for example so-called solar salt which is a mixture of sodium nitrate and
potassium nitrate
in a ratio of 60:40, are used in particular.
A disadvantage of salt melts is, however, that they have high melting points.
A sodium
nitrate/potassium nitrate mixture melts, for example at the eutectic, that is
to say with a
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mixing ratio of 44:56, at a temperature of 218 C. In long pipeline systems,
such as are
encountered in solar power plants, salt melts with high melting points are
difficult to work
with reliably. Freezing of the salt melt can cause great economic damage in
pipeline
systems. One reason for the damage is, for example, the large volume expansion
of salts
when they melt. There is a risk that valves and pipelines will be placed under
pressure and
greatly damaged.
When the salt melt freezes, which may essentially happen outside the operating
times of
the solar power plant, that is to say outside the radiation times of the sun
or when the solar
radiation is interrupted owing to the weather, a volume contraction takes
place which can
lead to a different solidification state depending on the pipeline system and
operating
state. It is to be expected that, in general when unvented, evacuated bubbles
will be
created in the pipeline and merge to form more or less sizeable units. When
remelting
takes place, owing to a possibly large spatial distance between the melting
sites with
volume expansion and the evacuated regions, there may be insufficient volume
compensation to relieve pressures building up.
In order to prevent freezing of the salt melt in the pipeline system, it is
customary to drain
the pipeline system during prolonged offline times. In the case of current
pipeline systems
having a storage container for the salt melt, however, the drainage takes a
long time and
cannot be ensured reliably in particular for sudden outages, for example in
the event of an
electricity failure, so that damage to the pipelines can occur especially in
such cases.
For the drainage, a drainage container is currently provided which is
installed in a pit and
is protected against spillage by a container trough. The individual solar
loops, which are
formed by the pipeline system, have a slight gradient of about 0.3% so that
during
drainage the liquid contained in the pipelines is driven in the direction of
the drainage
container because of the gradient.
In modern systems with only one drainage container, the slight gradient used
is generally
insufficient for sufficiently rapid and complete drainage of, in particular,
pipeline systems
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with long pipelines such as are used in parabolic trough solar power plants or
Fresnel
power plants and which may often have a total pipeline length of 100
kilometers. On the
other hand, it is customary to use valves and cocks which do not have a safety
position.
Thus, in the event of a power failure, the valves may not for example lead the
solar loop
into a safe drained state. In this case, freezing of the salt used as a heat
transfer medium
is certainly likely. The solution of backing up the power supply by a
substitute source is not
sufficiently secure against all functional problems in the system. Lastly,
drainage into a
central drainage container entails long flow paths and flow times, with the
risk that the heat
transfer salt will solidify during the flow. Furthermore, a problem in one
solar loop can lead
to all the other solar loops being taken off line.
Furthermore, in currently used pipeline systems, collector banks are generally
connected
to the distributors for the heat transfer medium through flexible hoses or
ball-joint
connections. These, however, are not configured with a continuous gradient.
During
drainage, therefore, there is a risk that salt residues will remain in the
flexible connections
and solidify there.
Currently, salt with a low melting point is generally used in order to
minimize the problems
occurring in the pipelines due to the salt melt. Such salt melts, however,
have considerable
disadvantages. Examples of known heat transfer salts with a low melting
temperature are
mixtures of nitrates and nitrites of sodium and potassium, and of potassium
nitrate, sodium
nitrate and calcium nitrate.
Such mixtures, however, have a lower thermal stability than the solar salt
conventionally
used, consisting of potassium nitrate and sodium nitrate, so that the working
range is
limited to a temperature of less than 500 C. The effect of this is that a
lower efficiency of
the power plant has to be accepted. The salts furthermore have to be kept in
closed
systems, which leads to additional outlay in the area of the solar field since
inerting
systems, gas purifying systems or gas balance systems have to be installed in
the solar
field. The inerting is necessary because, on the one hand in the case of salts
containing
nitrite, atmospheric oxygen can oxidize the nitrite into nitrate and the
melting point of the
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salt can therefore rise uncontrolledly, and, in the case of systems containing
calcium,
carbon dioxide reacts with calcium ions to form insoluble calcium carbonate.
Other alternative salts contain significant amounts of expensive and not
readily available
elements, which restrict economic use to systems with low hold-up. Examples of
expensive components in these salts are lithium, rubidium and cesium.
Heat transfer systems other than salts generally have a high vapor pressure or
entail
considerable outlay for the corrosion protection of long pipeline systems.
Systems for heating salt bath reactors are known from the chemical industry,
at the lowest
point in which there is a drainage tank covered with nitrogen. All control
devices in the
system are in a safety position, so that in the event of an unintended
operating state the
molten heat transfer salt, generally a binary mixture of sodium nitrite and
potassium
nitrate, flows into the drainage container. To this end, all the pipelines are
arranged with a
gradient in the direction of the drainage container. The pipelines have such a
large
diameter that the lines are emptied even if no further venting is provided.
Regions
incapable of flow, for example above control devices and downpipes, have their
own
drainage lines via which they can be drained even in the event of valve
blockage. The
molten heat transfer salt is transported from the drainage containers with the
aid of
immersion pumps into the chemical systems.
These typical solutions of salt bath reactors, however, are not applicable and
not sufficient
in a solar field owing to its large size. For example, it is not suitable to
use one drainage
container for a solar power plant since the drainage process would take much
too long to
reliably prevent freezing. Furthermore, salt bath reactors are generally
operated
continuously, that is to say the system runs continuously after start-up of
the reactor until
the next revision. Until then, the system is constantly hot and there is flow
through all the
parts of the system. By means of this, an attempt is made to avoid
obstructions occurring
because of solidification of the salt, which could be removed only with great
difficulty - if at
all. Solar power plants, however, are subjected to a continual on-off cycle.
For example,
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the solar field is not supplied with radiation energy at night. Continuous hot
operation of all
the parts of the system would lead to excessive radiation losses in the solar
field. In order
to avoid the high radiation losses, it is therefore expedient to operate the
solar power plant
discontinuously, particularly in order to keep overnight energy losses low.
5
Furthermore, salt bath reactors and pipeline systems in solar power plants
differ in their
size. For instance, solar bath reactors conventionally have pipeline lengths
of at most a
few hundred meters, while the length of the pipelines in parabolic trough
solar power
plants can exceed 100 kilometers. This also entails an amount of salt greater
by a factor of
about 1000. Merely owing to their size, therefore, these pipeline systems in
solar power
plants cannot be operated in a similar way to pipeline systems for example in
salt bath
reactors.
It is therefore an object of the present invention to provide a pipeline
system for conveying
a salt melt and a method for draining a pipeline system for conveying a salt
melt, which
can be used in solar power plants and do not have the disadvantages of the
prior art.
The object is achieved by a pipeline system for conveying a salt melt,
comprising at least
one pipeline through which the salt melt flows, at least one inlet and at
least one outlet,
wherein the pipeline through which the salt melt flows has at least one
gradient inclined
with respect to the horizontal and is respectively connected at the lowest
positions via a
drainage valve to a drainage line and at the highest positions to a venting
valve.
The object is furthermore achieved by a method for draining a pipeline system
for
conveying a salt melt, in which the drainage valves and the venting valve are
opened for
drainage so that the salt melt can flow out of the pipeline through the
drainage line.
The advantage of providing the venting valve is that gas can flow back into
the pipeline
system during drainage and the drainage can thereby be accelerated in
comparison with
drainage without gas flowing back in. Furthermore, the diameter of the
pipelines can be
kept smaller without salt melt becoming blocked during drainage from inside
the pipelines.
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For example, in a closed system without a corresponding venting valve, the
flow of the salt
out would be hindered by air flowing against it. Particularly in thin
pipelines and in the case
of a very small gradient, the salt would not be able to flow away at all.
A gas suitable for being supplied to the pipeline system through the venting
valves is, for
example, air when using a salt which does not oxidize in the presence of
oxygen. Thus,
venting with air is possible in particular when using a solar salt, that is to
say a mixture of
potassium nitrate and sodium nitrate, preferably in a ratio of 40:60, wherein
the air can be
freed from water vapor and/or carbon dioxide.
When using a salt in the pipeline system which reacts chemically in the
presence of
atmospheric oxygen, for example a salt which contains calcium ions or nitrite,
a gas which
is inert with respect to the salt being used, for example nitrogen, will be
supplied through
the venting valve.
In order to permit full drainage of the pipeline system when required, it is
preferable for all
components of the pipeline system to be formed with a gradient. For example,
receiver
banks in parabolic trough solar power plants are arranged in a mobile fashion
so that the
parabolic mirrors can always ideally capture the radiation energy of the sun.
In order to be
able to move the receiver banks, the pipelines extending through the receiver
banks are
configured in a mobile fashion and, for example, connected by flexible lines
to statically
installed connections such as manifolds, distributors and drainage lines. The
flexible lines,
to which the individual receiver banks are connected, are also to be installed
with a
continuous gradient from the venting valve to the drainage valve in order to
be able to
permit reliable drainage. Movement arcs such as are currently used according
to the prior
art, and which extend upward, are to be avoided in this case. If movable
receiver banks
are used, at least one position has to be provided which allows the salt melt
to drain off.
This position must be fail-safe, that even in case of power failure the
receiver banks move
in a position which allows the salt melt to drain off. This can be achieved
for example
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driven by spring or pressurized air. If the position which allows the salt
melt to drain off
shall be achieved spring driven, it is advantageous to use pressurized air
storage units.
In a preferred embodiment, each drainage valve and each venting valve in the
pipeline
system is a valve with a failsafe function, which opens when a situation
requiring drainage
occurs. Such situations which require drainage are, for example, the
occurrence of an
elevated temperature or a reduced temperature in the solar loop, the
occurrence of an
elevated pressure or a reduced pressure in the solar loop, a deviation of the
quantity
flowing through the solar loop or an electricity failure. Furthermore, the
drainage may also
be instigated for example by automatic control, for example overnight drainage
in
continuous operation or drainage when the solar irradiation is not sufficient
for the solar
power plant to be operable safely. Furthermore, drainage should also be
possible for
manual intervention.
The occurrence of an elevated or reduced temperature in the solar loop or an
insulation
problem may, for example, be localized rapidly by an infrared optical scanning
system over
the entire solar field. Such a scanning system may also, for example, trigger
drainage of
the pipeline system when values deviating from the standard are measured.
The drainage valves and venting valves used as valves with a failsafe function
are closed
during normal operation of the solar power plant. When drainage takes place,
the valves
are automatically opened. In the case of the venting valve, this means opening
the valve,
and in the case of the drainage valve this means opening the pipeline into the
drainage
line so that the salt melt can flow out of the pipeline system into a drainage
container.
In a solar power plant, the individual pipelines of the pipeline system are
conventionally
configured as a U-shaped loop, the inlet and outlet respectively being
arranged at the
branch ends of the U-shaped loop. The branch ends are in general respectively
connected
to a manifold line, the salt melt being delivered to the pipeline via one
manifold line in
continuous operation and the heated salt melt being removed from the pipeline
via the
other manifold line and fed into an evaporator. In the evaporator, water is
evaporated and
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superheated by the salt melt and an electricity generation turbine is driven
using the steam
produced in this way. The salt melt is cooled in the evaporator and fed back
via the
manifold line into the pipelines of the pipeline system, where the salt melt
is heated again
in the receivers.
In a preferred embodiment of the invention, the drainage valves provided in
the pipeline
system are arranged so that both the pipeline and the inlet and respectively
the outlet are
drained into the manifold lines when they are opened. In order to permit rapid
drainage of
the pipeline system, it is in this case preferable that each individual solar
loop can be
drained via drainage valves into the drainage line.
In order to minimize the respective distances which the salt melt has to
travel for drainage,
it is furthermore preferable to position the venting valve centrally between
the drainage
valves of the U-shaped pipeline. This ensures that the maximum distance from
the venting
valve to the drainage valve in the respective pipeline is always of the same
length.
In order to be able to further accelerate the drainage of the pipeline, it is
furthermore
preferable for the venting valve to be connected to a pressurized gas line.
Depending on
the salt used, compressed air may for example be used as the pressurized gas
if the salt
melt does not contain any components which react chemically with constituents
of air. As
an alternative, it is for example also possible to use an inert gas as the
pressurized gas,
for example nitrogen, or alternatively synthetic air or CO2-scrubbed air. By
using a
pressurized gas, when the venting valve is opened gas is introduced under
pressure into
the pipeline and the salt melt is thus expelled from the pipeline. This leads
to accelerated
drainage. In order to obtain a failsafe pressurized gas supply, it is
particularly preferable
for the pressurized gas to be provided in pressurized gas storage units, which
are
connected to the venting valve via the pressurized gas line. The pressurized
gas storage
units can be set up decentralized.
In one embodiment of the invention, the pipeline system comprises at least two
pipelines
preferably configured in the shape of a U, which respectively have a gradient
inclined with
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respect to the horizontal and are respectively connected at the lowest
positions via a
drainage valve to a drainage line and at the highest positions to a venting
valve. The use
of at least two, and preferably more than two pipelines makes it possible to
reduce the
total length of the individual pipelines. Connecting the respective pipeline
to a drainage
valve furthermore serves the purpose that each individual pipeline can be
drained
separately and it is not necessary to drain all the pipelines via the common
manifold line.
This also allows more rapid drainage than drainage via the manifold lines into
a common
drainage container.
In order to collect the salt melt taken from the pipelines, it is preferable
for the drainage
lines respectively to be connected to a drainage container. In this case, it
is furthermore
advantageous for the drainage containers to be positioned close to the
respective pipeline
in order to avoid long distances from the pipeline into the pipeline container
and therefore
long drainage lines.
In order to be able to remove the salt fully from the pipelines, it is
furthermore
advantageous for the drainage containers to have a volume which corresponds at
least to
the volume of all the pipelines opening via the respective drainage lines into
the drainage
containers.
In order to reduce the number of drainage containers, it is furthermore
possible to segment
the pipeline system, each segment having at least two pipelines and each
segment being
assigned a drainage container. The segments are in this case selected so that
sufficiently
rapid drainage into the drainage container is possible and the total pipeline
length, in
particular of the drainage lines, can still be kept short enough. In such a
segment, for
example, it is possible first to drain the individual pipelines of the
pipeline system via
drainage valves respectively into a drainage line, combine the drainage lines
to form a
common manifold line and make this open into the drainage container. If
problems then
occur during drainage for example in one pipeline, the effect of this is that
any damage can
occur at most in the segment containing the pipeline or this segment cannot be
started up
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again without problems. The other segments, however, can still be operated
without
problems.
As an alternative or in addition to applying a pressurized gas to the pipeline
via the venting
5 valve, it is also possible respectively to evacuate the drainage
containers. In this case,
when the venting valves are opened, the air pressure leads to accelerated
drainage of the
pipelines into the drainage container. Evacuation of the drainage containers
has the further
advantage that rapid and reliable drainage is possible even if, for example
owing to an
electricity failure, sufficient pressurized gas is not available. Rapid
pressure-driven
10 drainage is possible in this case against ambient pressure when opening
the venting valve
to the environment.
Further, as an alternative or in addition it is possible that the pipeline is
routed in such a
way that he pipeline has a steep incline with a high hydrostatic potential
difference near
the drainage container. For this purpose it is possible, for example, to place
the drainage
container in a ground depression, for example with a depth of 2 to 5 m. In
this case a high
driving hydrostatic pressure is effective on the salt melt. It is possible to
prevent the
penetration and ascension of gases from the overlaying gas in the drainage
container in
opposition to the flow direction and drainage direction by a dipped insertion
of the salt melt
via a dip tube into the drainage container. To prevent a rupture of the liquid
column while
flowing, it is necessary that there is a pressure at each position of the
liquid column which
is higher than the vapor pressure of the salt melt. It is possible to set the
pressure in the
salt melt by a high flow resistance near the drainage container or in the dip
tube into the
drainage container. For this purpose, it is possible, for example, to install
baffles or
systems for a direction change, which have the additional advantage that
erosive corrosion
of the container walls is reduced.
When using the pipeline system in a solar field of a solar power plant,
particularly in a solar
field of a parabolic trough solar power plant or a Fresnel power plant, the
salt melt
preferably contains at least one nitrite or at least one nitrate of the alkali
metals or alkaline
earth metals. Preferred are nitrite or nitrate of sodium, potassium or
calcium, or any
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mixture of these salts. A mixture of sodium nitrate and potassium nitrate in a
ratio of 60:40
is particularly preferably used. Further particularly preferred is a mixture
of nitrite and
nitrate of potassium and sodium in any mixture, also denoted as nitrite salt.
Besides this
so-called solar salt, it is also possible to use any other salts with a high
melting point which
are suitable as a heat transfer medium. In the context of the present
invention, a high
melting point means a melting temperature of at least 100 C. It is furthermore
preferable
for the salt to be thermally stable even above temperatures of 470 C.
Exemplary embodiments of the invention are represented in the figures and will
be
explained in more detail in the description below.
Figure 1 shows a solar field of a parabolic trough solar power plant
having a drainage
container according to the prior art,
Figure 2 shows a solar loop of a solar power plant having a drainage device
according
to the invention,
Figure 3 shows a start section and an end section of a solar loop,
Figure 4 shows a solar field of a parabolic trough solar power plant having a
segmented
pipeline system.
Figure 1 shows a solar field of a parabolic trough solar power plant having a
drainage
container according to the prior art.
A solar field 1 of a parabolic trough solar power plant has a plurality of
solar loops 3. The
solar loops 3 are respectively formed with a pipeline 5 through which a heat
transfer
medium flows. According to the invention a salt melt, preferably solar salt,
that is to say a
mixture of potassium nitrate and sodium nitrate in a ratio of 40:60, or as a
eutectic with a
mixing ratio of 44:56, or nitrite salt is used as the heat transfer medium.
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In solar loops 3, the heat transfer medium is heated by means of incident
solar energy. To
this end, the pipelines 5 are enclosed segmentally by a glass tube. The space
between the
pipeline 5 and the glass tube 7 is evacuated. Below the glass tube 7, there is
furthermore
a parabolic trough in which incident sunlight is reflected and directed onto
the glass tube 7.
Owing to the radiation incident on the glass tube 7, heat is delivered to the
heat transfer
medium which flows through the pipeline 5, so that the heat transfer medium is
heated.
The heat transfer medium flowing through the pipelines 5 of the solar loops 3
flows into a
manifold 9, and from the manifold 9 on to a heat transfer medium outlet 11.
The heat
transfer medium flowing through the heat transfer medium outlet 11 is
conventionally fed
into a heat exchanger, where it releases heat to a steam circuit by which, for
example,
electricity generation turbines are driven. The cooled heat transfer medium
leaving the
heat exchanger is fed via a heat transfer medium inlet 13 into a distributor
15, and from the
distributor 15 into the pipelines 5 of the solar loops 3.
In order to be able to drain the pipelines of the solar power plant during
offline times, a
drainage container 17 is provided. The drainage container 17 is connected to
the
distributor 15 and the manifold 9. Via the manifold 9 and the distributor 15,
the salt melt
flows into the drainage container 17.
In order to prevent salt melt from flowing out and spreading uncontrolledly
into the
environment in the event of damage to the drainage container 17, the drainage
container
17 is preferably enclosed by a trough 19, the capacity of the trough 19
corresponding to
the volume of the drainage container 17.
Figure 2 represents by way of example a solar loop having a pipeline system
formed
according to the invention.
The solar loop 3 has a pipeline 5 which is configured essentially in the shape
of a U, and is
connected by one branch to the manifold 9 and by the second branch to the
distributor 15.
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The connections of the pipeline 5 to the manifold 9 and distributor 15 are
respectively
established via connecting pipes 21.
According to the invention, the pipeline 5 has a gradient inclined with
respect to the
horizontal. The gradient is preferably in the range of from 0 to 1%. In one
embodiment the
gradient is preferably in the range of from 0.1 to 0.5%, particularly
preferably in the range
of from 0.2 to 0.4%. In an alternative embodiment the gradient is in the range
of from 0 to
0.3%, preferably in the range of from 0.01 to 0.2%. The gradient of the
pipeline 5 in each
case extends from a venting valve 23 to a drainage valve 25. In the embodiment
represented here, each of the branches of the U-shaped pipeline 5 is connected
to a
drainage valve 25. The drainage valve 25 closes or opens a connection of the
pipeline 5
and the connecting pipe 21 to a drainage line 27. During normal operation, the
drainage
valve 25 is closed. The drainage lines 27 open into a drainage container 17,
which is
configured to be large enough so that it can receive all of the salt melt
contained in the
pipeline 5.
The drainage container 17 is equipped with a relief valve 29, which opens when
the
pipeline 5 is being drained. This avoids a pressure buildup in the drainage
container 17.
So that the drainage container 17 can be drained when required, it furthermore
has an
outlet valve 31.
The valves used, that is to say the venting valve 23, drainage valves 25,
relief valves 29
and outlet valve 31 may have any desired form. For example, it is possible to
use rotary
disk valves, disk valves, flap valves and cock valves. In the scope of the
present invention,
the term valve is also intended to include disk valves and flap valves which
can only be
switched between an open position and a closed position. It is, however,
preferable to use
valves with which the throughput can also be controlled, that is to say any
desired
alternative aperture cross section can be achieved besides the "open" and
"closed"
positions.
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14
During normal operation of the solar power plant, the venting valve 23 may
also be used
as a relief valve, for example in order to be able to remove inert gases from
the solar melt.
To this end, a phase separator 33 is preferably provided in addition to the
venting valve
23. In the phase separator 33, the gas is separated from the salt melt and can
then be
removed through the valve 23.
During normal operation, the venting valve 23 and the drainage valves 25 are
closed. The
salt melt flows from the distributor 15 into the pipeline 5 and is heated in
the receivers
formed by the glass tubes 7 and parabolic trough mirrors. The solar melt
heated in this
way then flows via the second connecting pipe 21 and the manifold 9 into a
heat
exchanger, where the heat is released to a connected steam circuit.
During a functional problem of the system or in the event of a power loss, for
example
owing to an electricity failure, or in case of intended drainage, the venting
valve 23 is
opened. At the same time, the manifold valve 35 and the distributor valve 37
are closed so
that salt melt can no longer pass from the manifold 9 or the distributor 15
via the
connecting pipes 21 into the pipeline 5. Furthermore, the drainage valves 25
are switched
so that the connection from the pipeline 5 into the drainage line 27 is
opened. Owing to the
gradient in the pipeline 5, the salt melt is drained from the pipeline 5 into
the drainage
container 17 via a dip tube 41 by being driven by the force of gravity. In
order to assist the
drainage process, it is possible to apply a pressurized gas to the venting
valve 23, so as to
expel the salt melt from the pipeline 5 into the drainage container by the
applied pressure.
In addition or as an alternative, it is also possible to evacuate the drainage
container 17 in
order to further accelerate the drainage process.
If the drainage container 17 is not evacuated, the relief valve 29 will be
opened in order for
gas contained in the drainage container 17 to be able to flow out during the
drainage
process, so that a pressure is not built up in the drainage container 17.
In order to start the solar loop up again after a drainage operation, the
relief valve 29 is
first closed. Subsequently, the drainage valves 25 are switched so that the
salt melt can
CA 02847724 2014-03-05
flow from the drainage container 17 back into the pipeline 5. After this, a
pressurized gas is
fed to the drainage container 17 via a venting valve 39. The pressurized gas
is in this
case, depending on the salt used, for example compressed air, synthetic air,
CO2-
scrubbed air or an inert gas, for example nitrogen. Compressed air can only be
used if no
5 chemical reaction of constituents of the air takes place with the
constituents of the salt.
By application of the pressurized gas through the venting valve 39 into the
drainage
container 17, a pressure is built up in the drainage container 17. The
pressure building up
drives the heat transfer medium contained in the container 17 through the dip
tube 41,
10 which works as a riser pipe, into the drainage lines 27, and from there
through the
drainage valves 25 back into the pipeline 5. The drainage valves 25 are in
this case
opened slowly at the start of the filling process. At the expected end of the
filling process,
the valves 25 are slowly closed again. The actual end of the filling process
is monitored by
means of the smallest flow, optionally in pulsed operation. The termination of
the filling is
15 triggered by using a phase detector 43 at the end of the dip tube 41.
When the filling
process is terminated, the venting valve 23 is closed. Furthermore, the
drainage valves 25
are also closed so that the flow can now pass from the pipeline 5 via the
connecting pipes
21 to the manifold 9 and the distributor 15. In order to resume operation, the
manifold
valve 35 and the distributor valve 37 are then also opened. Gas contained in
the pipeline is
entrained with the salt flow and removed by the inert gas separation which is
carried out by
the phase separator 33 and the venting valve 23.
If there is too much salt in the drainage container 17, the excess amount can
be delivered
into the salt circuit by applying pressurized gas via the venting valve 39 and
opening one
of the drainage valves 25 while, simultaneously, the manifold valve 35 or
distributor valve
37 are opened and the venting valve 23 is closed.
The rate at which the salt melt flows through the pipelines 9, 15, 21 and 5
can be
controlled by the degree of opening of the respective valves 35, 37.
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As an alternative to delivering the salt melt from the drainage container 17
by applying
compressed air, it is also possible to use an immersion pump. In addition, the
immersion
pump may also be used to apply compressed air.
The drainage valves 25 and the venting valve 23 are preferably formed as
valves with a
failsafe function and switched so that in the event of a functional problem
they are
respectively open, in order that the salt melt contained in the pipeline 5 can
flow out into
the drainage container 17. The filling and drainage, respectively, of a solar
loop 9 from and
into a drainage container 17 allows rapid filling and drainage of the solar
loops 3, so that
the line system can be drained in the evening and filled in the morning with
high functional
reliability.
An increase in the functional reliability can be achieved by providing a
suitable heating
system in the pipelines. For heating, for example, it is possible to lay a
heating element
inside the pipeline. In this case, the salt inside the pipeline is initially
melted on the heating
element and forms a channel through which molten salt can be transported away.
This will
prevent an excessive pressure from being exerted on the pipeline 5 owing to
the volume
expansion of the salt melt. A uniform temperature distribution along the
heating element
also leads to the salt melting simultaneously around the heating element over
the entire
length of the pipeline 5, so as also to form a channel through which salt melt
can flow and
the pressure can thus be equilibrated.
Overheating of the salt melt in the pipeline is prevented by using a
defocusing instrument
of safety grade for the collectors.
Figure 3 schematically represents the inlet end of a solar loop and its end
provided with
the venting valve.
In order that the solar power plant can always be operated optimally, the
individual
receivers are preferably arranged in a mobile fashion so that the parabolic
mirrors can
optimally capture the radiation energy of the sun. To this end, it must be
possible for the
CA 02847724 2014-03-05
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pipelines of the individual receivers to be swiveled. In order to make this
possible, flexible
lines 45 are installed between the mobile pipelines of the receivers and
statically installed
connections such as manifolds, distributors and the drainage line 27. The
flexible lines 45
are in this case configured so that they have a gradient from the venting
valve 23 to the
drainage line 27, such that the salt melt can flow out.
A second position of the pipelines is shown by dashes in Figure 3.
In the embodiment represented in Figure 3, the venting valve 23 and the
drainage line 27
are fixed and the pipelines lying between the drainage line 27 and the venting
valve 23 are
configured so that they can be swiveled. The swiveling is shown by arrow 47.
A solar field in which the pipeline system is segmented is represented in
Figure 4.
In the embodiment represented in Figure 4, 5 solar loops 3 are respectively
combined to
form a segment 49. Each segment 49 is assigned a drainage container 17, into
which the
drainage lines 27 of the respective solar loops 3 open. Here, the drainage
lines 27 of a
solar loop 3 are combined into a manifold line 51, which then opens into the
drainage
container 17. The size of the drainage container 17 is selected so that the
salt melt from all
the solar loops 3 of a segment 45 can be received by the drainage container
17. The
number of solar loops 3 which are assigned to a drainage container 17 is
selected so that
drainage of the entire solar field can be carried out within a predetermined
time. In this
case, it should be taken into account that the drainage time is commensurately
greater
when more solar loops 3 have to be drained to a container 17.
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List of References
1 solar field
3 solar loop
5 pipeline
7 glass tube
9 manifold
11 heat transfer medium outlet
13 heat transfer medium inlet
distributor
17 drainage container
19 trough
21 connecting pipe
15 23 venting valve
drainage valve
27 drainage line
29 relief valve
31 outlet valve
20 33 phase separator
manifold valve
37 distributor valve
39 venting valve
41 dip tube
25 43 phase detector
flexible line
47 swivel region
49 segment
51 manifold line
