PRIOR ART
[0002] Hydrogen is often obtained from hydrocarbons, for example by steam
reforming, which in light of the fight against climate change is no longer
politically desirable in many places. Therefore, to reduce the carbon dioxide
15 emissions, methods based on electrolysis, in particular of water, are increasingly
used industrially for hydrogen production.
[0003] Other substances which play a key role in the energy sector or chemical
industry can also be produced by electrolysis methods and thereby reduce the
20 emissions of climate-active gases. For example, synthesis gas can be produced
from carbon dioxide and water, and is conventionally produced by steam
reforming of fossil hydrocarbons. Electrolysis as a production method thus
enables renewable sources for these substances and can contribute to reducing the
carbon dioxide content in the atmosphere. For example, by electrolysis of carbon
25 dioxide, net-negative emissions of gases that contribute to climate warming are
possible.
[0004] Various approaches are possible here, for example electrolysis in the form
of an alkaline electrolysis (AEL) or an electrolysis on a proton exchange
30 membrane (PEM) or anion exchange membrane (AEM), which can all be used in
the form of low-temperature electrolysis, typically with operating temperatures of
below 60°C. High-temperature electrolysis methods, for example using solid
oxide electrolysis cells (SOEC), are also used for electrolysis, for example of
water and/or carbon dioxide.
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[0005] In principle, the following reactions occur during the electrolysis of water.
3
[0006] In the case of electrolysis with a PEM:
At the anode: H2O → ½ O2 + 2H+ + 2e-
At the cathode: 2e- + 2H+ → H2
5
[0007] In the case of electrolysis with an AEM:
At the anode: 2OH- → ½O2 + 2H2O + 2e-
At the cathode: 2e- + 2H2O → H2 + 2OH-
10 [0008] In the case of electrolysis with an SOEC:
At the anode: 2O2- → O2 + 4e-
At the cathode: H2O + 2e- → H2 + O2-
[0009] The above-mentioned electrolysis of carbon dioxide can also be carried
15 out as a low-temperature electrolysis on aqueous electrolytes. To put it generally,
the following reactions take place:
At the cathode: CO2 + 2e- + 2M+ + H2O → CO + 2MOH
At the anode: 2MOH → ½O2 + 2M+ +2e-
20 [0010] During the electrolysis of carbon dioxide, too, the presence of water in the
electrolyte solution partially results in the formation of hydrogen at the cathode in
accordance with:
2H2O + 2M+ + 2e- → H2 + 2MOH
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[0011] Due to their high dynamics, the mentioned low-temperature electrolysis
methods, in particular, are suitable for efficiently using renewable electrical
energy, which is frequently subject to strong supply fluctuations, and at the same
time to compensate for these supply fluctuations, which can additionally
30 contribute to stabilizing corresponding power grids.
[0012] The waste heat produced during electrolysis often goes unused, which has
an overall negative impact on the efficiency of the method. It is therefore
desirable to provide an improved electrolysis concept in which waste heat is
35 utilized as efficiently as possible.
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DISCLOSURE OF THE INVENTION
[0013] This object is achieved by methods and systems according to the
independent claims. Advantageous developments of the invention are the subject
5 matter of the dependent claims and of the following description.
[0014] The invention relates to a method for electrolytically producing at least
one product stream containing hydrogen, wherein a feed stream containing at least
water is subjected to electrolysis so as to obtain two extraction streams.
10 Downstream of the electrolysis, the two extraction streams are subjected to
separation so as to obtain the at least one product stream and two liquid fractions
containing water. At least one of the two liquid fractions is fed back at least in part
to the electrolysis. Upstream of the electrolysis, the feed stream is heated by
exchanging heat with at least one of the two extraction streams. The at least one
15 extraction stream from which heat is removed by means of the heat exchange is
subjected to additional cooling, the additional cooling taking place by using at
least one organic Rankine cycle or a Rankine cycle that uses an organic-chemical
heat transport medium. The electrolysis is thus operated at a higher temperature
level than is usually the case, because the cooling effect is lower as a result of the
20 feed pre-heating. This brings about an increase in efficiency when the electrolysis
is in operation. The higher temperature level of the electrolysis also produces the
effect that waste heat is produced at a higher temperature than usual. An organic
Rankine cycle can thus be used efficiently for waste heat recovery. This is not
economically viable with conventional systems due to the lower operating
25 temperatures of typically below 60°C.
[0015] The organic Rankine cycle (ORC) is based on a thermodynamic cycle
according to Clausius-Rankine. This process is in principle identical to a
conventional steam circuit in which water is evaporated by heating, the energy is
30 removed by the performance of work, in particular mechanical work, and the
steam is re-condensed in order to be fed back to the starting point of the cycle
process. In contrast, during the organic Rankine cycle, another, in particular
organic-chemical, working fluid which has a higher vapor pressure or lower
boiling point than water is used instead of water. The working temperatures can
35 thus be drastically reduced depending on the working fluid selected, so that even
waste heat at a relatively low temperature level can be used, for example, for
5
power generation by means of turbines. For high-temperature (HT) applications
(T ≥ 300°C), the efficiency of this process is up to 20%, in special cases up to
24%. The lower the working temperature, the lower the efficiency of the process.
For applications at medium process temperature (MT) (150°C ≥ T ≥ 110°C), the
efficiency for the conversion of heat into 5 electrical current is about 7% to 8%.
Low-temperature (LT) applications (110°C ≥ T ≥ 80°C) achieve an efficiency of
about 5%. Corresponding system components are offered by various companies.
Series production in particular for the use of smaller amounts of heat has resulted
in significant decreases in investment costs. For example, systems for using 1
10 MW heat are offered for generating 75 kW power, which equals an electrolysis
input power of approximately 4 MW direct current.
[0016] It is provided here that a suitable working fluid is selected for ORC
depending on the intended temperature range. They can include individual
15 organic-chemical compounds or mixtures of different compounds.

We Claim:
1. A method for electrolytically 5 producing at least one product stream
containing hydrogen, wherein a feed stream (1, 2) containing at least water is
subjected to electrolysis (E) so as to obtain two extraction streams (3, 4), wherein
the two extraction streams (3, 4) are subjected to separation (S1, S2) downstream
of the electrolysis (E) so as to obtain the at least one product stream (6, 7) and two
10 liquid fractions (2, 5) containing water, characterized in that the feed stream (1,
2) is heated upstream of the electrolysis (E) by exchanging heat with at least one
of the two extraction streams (3, 4), in that the at least one extraction stream (3)
from which heat is removed by means of the heat exchange is subjected to
additional cooling, and in that the additional cooling takes place by using at least
15 one organic Rankine cycle or at least one Rankine cycle that uses an organicchemical
heat transport medium (O).
2. The method according to claim 1, wherein the electrolysis (E) is operated
at an electrolysis temperature level and the separation (S1, S2) is operated at a
20 separation temperature level.
3. The method according to claim 2, wherein the electrolysis temperature
level is within a temperature range between 60°C and 200°C, preferably between
70°C and 150°C, particularly preferably between 80°C and 110°C, in particular
25 95°C.
4. The method according to claim 2, wherein the electrolysis temperature
level is within a temperature range between 300°C and 1000°C, preferably
between 500°C and 900°C, in particular 800°C.
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5. The method according to any one of claims 2 to 4, wherein the separation
temperature level is within a temperature range between 20°C and 100°C,
preferably between 25°C and 50°C, in particular 30°C.
6. The method according to any one of the 5 preceding claims, wherein at least
one of the two liquid fractions (2, 5) is fed back at least in part to the electrolysis
(E).
7. The method according to any one of the preceding claims, wherein the
10 feed stream (2) is fed into the electrolysis (E) by partially bypassing (8) the heat
exchange.
8. The method according to any one of the preceding claims, wherein waste
heat from the additional cooling is additionally used for desalinating a water
15 stream, in particular the fresh feed (1).
9. The method according to any one of the preceding claims, wherein the
additional cooling and/or the heat exchange (W) is applied to both extraction
streams (3, 4).
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10. The method according to any one of the preceding claims, wherein the
feed stream (1, 2) additionally contains carbon dioxide and the product stream
containing at least hydrogen additionally contains carbon monoxide.
25 11. A system for electrolytically producing at least one product stream
containing hydrogen, which system comprises at least one electrolysis unit (E), a
heat exchanger, an additional cooling unit and a separation unit (S1, S2), wherein
the at least one electrolysis unit (E) is configured to electrochemically convert a
feed stream (1, 2) containing at least water at least partially using electrical energy
30 so as to obtain two extraction streams (3, 4) containing gaseous electrolysis
products, wherein the at least one separation unit (S1, S2) is configured to
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separate the gaseous electrolysis products from liquid fractions (2, 5) contained in
the extraction streams (3, 4), characterized in that the at least one heat exchanger
is configured to heat the feed stream (1, 2) using at least one of the two extraction
streams (3), wherein the at least one extraction stream (3) is cooled, and in that the
cooling unit is configured to cool the at least 5 one extraction stream (3) cooled in
the heat exchanger at least using an organic Rankine cycle or a Rankine cycle that
uses an organic-chemical heat transport medium (O).
12. The system according to claim 11, further having means which are
10 configured to carry out a method according to any one of claims 1 to 10.