Increasing the efficiency of small scale CSP applications
By
Jenny Muirhead
on
Mar 14, 2013
An Interview with Dr Eduardo Zarza
Small scale and modular CSP projects (less than 10 MW) offer a
whole new potential market for CSP investment. Modular CSP has a number
of significant benefits: materials can be pre-packaged and assembled on
site reducing labour requirements; it is more adaptable to different
terrains than large scale CSP projects; it doesn’t have the same
permitting headaches associated with large scale CSP projects and,
relatively speaking, the upfront investment is lower meaning that
financing should be a more straight forward matter. And as industrial
applications, such as mining and enhanced oil recovery, become more of
an area of focus for CSP, modular CSP is likely to increase in traction.
However, there are a number of concerns facing small-scale CSP,
primarily in terms of the economies of scale and lower efficiency levels
when compared to large scale CSP plants.
Research by Dr Eduardo Zarza, Head of the CSP Research Unit
at Plataforma Solar de Almería (PSA) has shown that replacing synthetic
oil with Carbon Dioxide (CO2) as a working fluid can significantly
improve the efficiency of small scale CSP projects.
CSP Today Speaks to Dr Zarza about PSA research into Carbon Dioxide as a heat transfer fluid in small scale applications of CSP
.
CSP Today: Please tell me about PSA research into Carbon Dioxide as a heat transfer fluid in a CSP plant.
EZ: Our research at PSA has focused primarily on small scale
applications of CSP whereby the overall efficiency of the project is
improved by replacing the thermal oil with CO
2.
With CO
2 we could implement a Brayton Cycle with high efficiency
where a 5 MW plant withCO
2 could have similar efficiency to a 50 MW CSP
plant with oil as a transfer fluid. This to me is the biggest advantage.
Using CO
2 we could implement a Brayton cycle with a top temperature of
500˚C giving us a power block gross efficiency of about 40% - which is a
similar efficiency as the power block of a 50 MW thermal oil plant. The
parameters of the Brayton cycle required for this application have been
defined and the pre-feasibility study of such a Brayton cycle have been
very encouraging. However, a significant research and development
effort would be needed to develop at a commercial level the components
of this Brayton cycle.
If we implement a 5 MW CSP plant using thermal oil rather than CO
2 as a
transfer fluid then the maximum efficiency it could hope to achieve is
less than 30%. With CO
2 and the analyzed innovative Brayton cycle we
could meet market needs of small power plants with efficiency similar to
that of larger scale CSP plants.
CSP Today: Has this been considered only for modular CSP plants?
EZ: Yes. The main problem for CO
2 is the high pressure loss in the
solar field piping. If we have a large field with 50 or 100 km of
receiver pipes, the pressure loss with CO
2 would be unaffordable. This
means CO
2 is specifically suited to small scale CSP plants. Concerning
the impact on central receivers we have found with a theoretical study
that the coupling of the new Brayton cycle to a central receiver using
supercritical CO
2 could also led to a high efficiency for plants within
the power range of 2-5 MWe. However, we have only done experimental
research on a parabolic trough plant.
CSP Today: What is the impact of using CO2 on the cost of a CSP plant?
EZ: For a small plant (less than 10 MW), I think the CAPEX will be
slightly higher, but the efficiency would be significantly higher, so
there would be a clear benefit – the final cost of electricity would be
lower.
The maintenance cost would be similar to that of a plant using oil – the main benefit is the higher efficiency.
CSP Today: Is there an operational CSP plant using CO2 as an HTF?
EZ: Not on a commercial scale – there are only some small test
facilities. At the PSA we have a test facility with a nominal thermal
output of 400 kW. We have already achieved 510ºC in a parabolic trough
system – I think this may be the highest in the world using a compressed
gas in the receiver pipes. We have proven that carbon dioxide can be
used in a parabolic trough collector to produce thermal energy at 500ºC.
With such a high temperature the efficiency of such a high plant would
be about 40% when used in the innovative Brayton Cycle we have designed.
CSP Today: Why a Brayton Cycle as opposed to a Rankine Cycle?
EZ: A Rankine cycle needs higher power to achieve good efficiency,
because of the limits imposed by small steam turbines. With the new
Brayton cycle we can achieve a much higher efficiency at low power.
Brayton cycle is also more user-friendly. The engine of an aircraft is a
Brayton cycle: they have a turbine which expands the hot air which
gives them propulsion – these engines can easily start and shutdown. In a
Rankine cycle you need steam turbines, pump, cooling systems,
deaerators and so forth. You cannot start it in ten minutes – you need
at least half an hour.
CSP Today: Is a Rankine cycle more costly to use than a Brayton cycle, or does the cost depend on the scale of the plant?
EZ: I would say they are quite similar.
CSP Today: Are there any other areas you are researching with regards to CSP plant efficiency and cost reduction?
EZ: There are many possibilities. ESTELA (the European Solar
Electricity Association) has just published their strategic research
agenda which includes topics such as better receiver pipes, improved
thermal storage systems, new working fluids and so on. PSA, DLR and
other research institutes have collaborated to produce this agenda. The
list of potential areas for improvement is large. However, there is not a
single area which will lead to overall improvement. Rather the solution
lies in the accumulation of small improvements in many items – no one
item could achieve a 30% LCOE reduction. Better receiver pipes with
lower maintenance for parabolic troughs could be an improvement. Direct
steam generation can be also a significant step forward as replacing
thermal oil with water would lower the maintenance cost because of fire
risks and the auxiliary systems required by oil systems. There are also
companies investigating the technical features of using molten salts as a
working fluid.
Using molten salt would result in a lower maintenance cost because
the thermal storage medium used in parabolic trough plants these days is
molten salt. If molten salt is used in the solar field the working
fluid and the storage medium would be the same element – we wouldn’t
need heat exchangers in between. This is why many research and
development and private companies are researching the technical
capabilities of molten salt in trough collectors. The main technical
barrier is the high freezing point of 240ºC so at night time the plant
operator needs to be very careful to avoid freezing. A salt plug in the
solar field piping could be a disaster. There are also companies trying
to find new salts with a lower freezing point to avoid these maintenance
problems.
There are also companies which are researching increasing the maximum
working temperature of thermal oils. At the moment the maximum
temperature is 398˚C – above that and the oil starts to degrade. This
degradation increases the maintenance costs. This maximum temperature is
a significant barrier – the higher the temp in a Rankine cycle the
higher the efficiency. Efficiency is therefore limited by the working
temperature nowadays.
CSP Today: Are there other areas you are aware of that can reduce the cost of a CSP plant?
EZ: New working fluids need to be found to improve the CSP technology
as they allow higher temperatures and as a result efficiencies would be
higher. Efficiency and cost move in parallel. Any item improving
efficiency will reduce the energy cost.
Another area is the reduction of man power – in civil works and
assembly of components. If we develop automatic procedures – we could
significantly reduce the CAPEX of a CSP plant.
CSP Today: Is labour reduction affected by the size of the plant?
EZ: There is a clear scale up effect – the bigger the plant the lower the investment cost per KW.
Nowadays we already have parabolic trough collector designs with a
significant man power reduction of 25-30% less manpower than a collector
design from ten years ago. They are cheaper in assembly and production
and there is still room for further man power reduction.
CSP Today: How do you think CSP can achieve a better LCOE?
EZ: There are many ways. But at the end of the day it depends on three
major factors that can impact the cost of a CSP plant either reduced
CAPEX, increased efficiency and lower O&M costs – these are the most
important areas – any improvement applied to one of these areas will
reduce LCOE.
