When it gets hot

When it gets
The global power industry recognizes
the demand for clean and dependable
energy from renewable resources.
Concentrated solar power (CSP)
generation is a sustainable solution for
global energy needs, but it poses
extreme challenges to pump
technology. The energy of the sun is
used to heat up a fluid—the hotter the
fluid, the more efficient the power
generation. Sulzer has extensive
experience in solar power technologies
and has developed a new pump for the
hottest fluids.
Sulzer Technical Review 2/2014
Temperatures in modern steam power plants are getting hotter and hotter. The reason for that is simple:
Thermal efficiency depends on steam pressure and
temperature. Above a certain critical pressure, liquids
directly become steam without boiling. For water, this
happens at 374 °C and 220.6 bar. This is called supercritical steam. Operating a thermal power plant at
steam conditions above this critical point increases the
efficiency significantly. Supercritical steam has been
used in coal-fired power plants for a long time. Now
this trend is booming in concentrated solar power
(CSP) plants as well.
CSP plants concentrate solar radiation by using mirrors
or lenses to heat up a working fluid (see Sulzer Technical Review 1/2012). The heated fluid drives a steam
turbine to produce electricity. Today, the most widespread CSP technology is the parabolic trough system.
Long, trough-shaped mirrors concentrate sunlight
onto thermally efficient receiver tubes along the trough’s
focal line. These tubes are filled with a heat transfer
fluid, such as thermal oil, which is heated up by the
sunlight. In order to avoid a degradation of the thermal
oil, the temperatures need to be below 400 °C. A second type of CSP technology is the central tower
system. This technology, which does not have thermal
oil but molten salt as its primary heat transfer fluid,
allows even higher temperatures and reaches better
thermal efficiencies than parabolic trough plants.
Circular arrays of flat, orientable mirrors (called heliostats) concentrate sunlight onto a tower-mounted
thermal receiver that contains a heat transfer media.
Benefits and challenges of molten salts
Because of their high melting temperature and significant specific-heat capacity, molten salts are used in
CSP plants more and more today as the primary heat
transfer fluid and also for heat storage. The possibility
of storing heat with molten salts brings great benefits.
When a thermal storage reservoir of molten salts is integrated into a CSP plant, electricity can be generated
even after the sun goes down. This way, the operation
period can be extended by 6–8 hours. The molten-salt
system consists of two huge tanks—one for the hot
salt and one for the cold salt. Vertical pumps are
mounted in the tanks to circulate the molten salt. The
pumps can handle the temperature of 400 °C in the
parabolic trough systems quite well. Such temperatures are common for pumps in the hydrocarbon processing industry as well. But the working temperature
of central tower systems is in the range of 500–600 °C,
which is an extreme condition for a pump. There are
not many other fluids that are so hot.
Sulzer Technical Review 2/2014
Sulzer has developed a new molten-salt circulation
pump that meets these extreme requirements. It is a
vertical mixed-flow pump with medium-to-high capacity and head. The pump is called SJT (VCN) and is an
evolution of the existing VCN pumps. The VCN pumps
have already been applied successfully to molten-salt
heat storage in other industries. Now, Sulzer has
adapted the pump design especially for the needs of
the solar power industry.
Dealing with extremes
When dealing with extreme temperatures, one big
challenge is the material growth. As the molten-salt circulation pumps are installed in large tanks, they need
to be very long—usually in the range of 13–15 meters.
The thermal expansion of such a long pump during
operation is significant and has to be considered in the
pump design. A second issue is the sealing. Because
the molten salts are not compatible with any elastomer
materials, the pump has to be free of any elastomer
components. Therefore, it is not possible to seal the
shaft with mechanical seals, which all use elastomer
materials. The Sulzer engineers have developed a special sealing configuration by means of throttle bushing.
The fluid pressure is throttled, and the leakage is circulated internally back to the tank.
A further important topic that Sulzer addressed is the
risk of freezing. The molten salt freezes at 220 °C,
which means that when it comes to contact with the
environment, it becomes rock hard and blocks the
pumps immediately. To avoid any contact of the fluid
with the environment, the pump has to be selfdraining.
Sulzer started developing the SJT (VCN) pump for concentrated solar power five years ago and managed to
fulfill all the challenging requirements of this application.
Now the pump is ready for field testing. After that, it will
be available on the market. In addition to supplying
molten-salt circulation pumps, Sulzer supports the
solar power industry with a full product line of pumps.
These include pumps for feed water, hot water circulation, condensate extraction, and cooling water, as well
as main and auxiliary pumps for heat transfer oil.
Contact: Miguel Angel Rivas
[email protected]
The SJT (VCN) pump incorporates the hydraulics from
the SJT range. It is engineered to balance high efficiency,
low submergence, and net positive suction head (NPSH)
considerations. The maximum pressure is 60 bar
(870 psi), and the maximum temperature is 600 °C
(1100 °F).
Sulzer Technical Review 2/2014
Solar island
Power island
Solar receiver
Hot salt
Steam turbine
Hot salt
Steam generator
Heliostat field
Central solar tower
Cold salt
Cold salt
extraction pump
Cooling water
In central tower systems, heliostat fields heat up the molten salt. The molten salt is used as the primary heat transfer
fluid and is also used to store heat. Sulzer supports this process with pumps for feed water, condensate extraction,
cooling water, and molten salt circulation (hot salt pump, cold salt pump).
Miguel Angel Rivas
manages the Power
Portfolio at Pumps
Equipment and
explains the idea
behind the new SJT
(VCN) pump.
Central tower technology is a new and promising
approach in the solar power industry. What are the
key benefits?
I believe that central tower technology is the future of
concentrated solar power. Compared with parabolic
troughs, the thermal efficiency is much higher, and the
cost of installation is reasonably lower. In central
tower systems, you could produce supercritical steam,
which would make power generation more efficient
and would reduce the mirror surface that you need to
create the same amount of energy. A further advantage of the central tower system is that molten salts
are more environmentally friendly than the thermal oils
that circulate in the parabolic troughs. Thermal oil is an
aggressive liquid, which is potentially polluting and
flammable. By contrast, the molten salts are absolutely neutral.
A further advantage of molten salts is that they can
store energy. Why is that so important?
The time when you can collect heat from the sun is not
always the time of highest price or electricity
consumption. That’s why having the option of storing
the energy is very attractive. Then you can release the
energy in the hours of peak electricity use. The possibility of storing energy in the form of heat makes concentrated solar power competitive with photovoltaic
power. Photovoltaic systems have the advantage of
generating energy at lower investment costs but they
don’t have any storage options.
But pumping molten salts is an extreme challenge…
Yes, it’s like pumping melted rocks. The temperatures
are extreme, and the risk of blocking the pump with
solid particles is high. We developed the idea for our
new SJT (VCN) pump five years ago, but our experience with molten salts goes back much further. We
have manufactured pumps for molten salts for the past
45 years. Now, as new solar power technologies
emerge, we are launching a pump that is tailored perfectly to the needs of the solar power industry.
How is the central tower industry developing?
The first central tower plant using molten salts in
operation was Gemasolar in Spain. Now, many new
projects are underway—especially in the US, Israel,
Algeria, and South Africa. Many countries recognize
that it’s imperative to have renewable power generation in order to reduce the greenhouse effect. In regions
where there is high sun exposure, it’s obvious that
solar power is an attractive option. That’s why we
continually develop innovative solutions for the solar
power market.
Interview: Tünde Kirstein
Sulzer Technical Review 2/2014