Technology of solar receivers manufacturing for the WINNDER tower.
Rugescu, Radu Dan ; Silivestru, Valentin ; Ionescu, Mircea Dan 等
Abstract: The manufacturing technology for the solar receiver of
the aeroacoustic wind tunnel WINNDER is described. The WINNDER project
was first developed through the ADDA program at the University
"Politehnica" of Bucharest under a limited CNCSIS grant. The
study had proved the new, efficient means of creating a sustained
airflow with very low turbulence and driving noise level. The novelty
consists in a driving system without any moving parts, like compressors
and fans. The additional improvement was promoted by using the solar
heat to boost the airflow. The method proved far more efficient than the
only known European endeavor to develop solar towers in Germany, due to
the combination of a solar concentrator and a low temperature-low
pressure solar receiver (heat exchanger). The design of the
receiver-exchanger, the main and most difficult part of the system, is
debated in comparison to the known solutions already developed within
other projects. Practical conclusions are derived.
Key words: Heat exchangers, solar receiver, manufacturing.
1. WINNDER PRINCIPLE
The acceleration of the air into the noiseless wind tunnel means
alternatively (1) to heat the fresh air from electrical radiators,
placed inside the airflow, (2) to heat it from a hot fluid through heat
exchangers similar to the domestic radiator or (3) directly from the
concentrated solar radiation. The solar direct heating raises the most
difficult problems, due to the very low adsorption capacity of the fresh
air in normal atmospheric conditions and to its low specific heat,
compared with liquids in general. An intermediary body in the form of a
large contact surface wall, swept by the flowing air and efficiently
heated by the sunlight must be used. Such heat exchange surfaces are
usually called solar receivers. The challenge of designing an efficient
solar receiver with its surface that able to retain most of the incident
solar wide spectrum radiation, with a very low albedo, and
simultaneously able to transfer its heat to the low speed flowing air is
what the research team is faced to solve. All these at air temperatures
as low as 200[degrees]C and ambient pressure, with total pressure losses
below 0.1 bar.
2. STATE OF THE ART IN SOLAR RECEIVERS
A series of former solutions for solar-air receivers are known
(Haeger, 1994), (Romero et al., 2004), (Romero et al., 2002), (Tellez et
al., 2004), still they belong to the high temperature class, involving
heat exchange surfaces that work at about 700[degrees]C or more. The
case study is of the SOLAIR power plant in Spain (Tellez et al., 2004)
with its ceramic Phoebes receiver (Fig. 1). As seen from the schematic
drawing, solar radiation (large arrows) is received on the frontal area
of the ceramic small size tubes (gray raster), closely packed to each
other. Along them both the recycled and fresh air are flowing (small
arrows) at quasi-atmospheric pressure, forced by a blower that controls
the flow through the receiver. The blower is driven by a fraction of the
energy produced in the power plant. The heart of the absorber is the
honeycomb structure made of recrystallized SiC with a normal open
porosity of 49.5%.
[FIGURE 1 OMITTED]
The 140-mm square ceramic modules with honeycomb structure are
fixed in the square part of the cups with cylindrical exit, also
ceramic. Due to the fact that the frontal area of the ceramic receiver
in the Plataforma Solar de Almeria (Spain) is planar, the heliostat array field is placed on a limited angular space behind the tower. The
CESA-1 tower itself works like a supporting structure to raise the
position of the receiver panel only, with no draught contribution at
all. The average flux density equals 0.5 MW/[m.sup.2]. The temperature
of the volumetric receiver honeycomb raises to almost 1000[degrees]C,
when no metal casing could be conveniently used, higher working
temperatures by the ceramic receiver resulting in a lower generating
cost, about 10% lower than the metal receiver system technology (Tellez
et al., 2004). The air leaves the absorber outlet at the pretty high
temperature of 700-750[degrees]C. The high surface temperature produces
a sensible return of radiation in a loss of quasi steady-state energy
efficiency. This raises to (72[+ or -]9)% at 750[degrees]C and to (74[+
or -]9)% at 700[degrees]C. The great uncertainty of 9% was due to the
accumulation of uncertainties in the measurement process. This is mainly
due to air mass flow and incident solar power measurements. Note that in
combustion and electric heat exchanger appliances for home and
industrial applications the efficiency of 98% of the thermal transfer is
usual. The sensible quantity of radiation return by reemission is
clearly visible in the picture of the Almeria setup in Fig. 2, where the
brilliant glance of the receiver is impressive. Along with the reduction
of the air temperature the efficiency of the receiver raises, although
in the SOLAIR-3000 type power plants lower air temperatures reduce the
efficiency of the steam generator and of the heat accumulator blocks and
is thus non-profitable. SOLAIR research shown estimated efficiencies of
up to 89% for outlet air temperatures in the range of 590-630[degrees]C
and mean incident solar fluxes of 310-370 kW/[m.sup.2].
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
3. WINNDER RECEIVER TECHNOLOGY
For the WINNDER project the above information is an additional
support as far as the optimal air temperatures at receiver outlet will
be of around 200[degrees]C, with similar difference to the receiver
walls like in the Almeria project. The problems of thermal transfer are
much different at these temperatures, optimal for the WINNDER concept
(Tache et al., 2006), (Rugescu, 2007), (Rugescu et al., 2007).
Besides its low working temperature range, the distinctive
characteristic of the receiver are the conflicting requirements
regarding a low drag and a high heat transfer coefficient. High transfer
rates could only be achieved when the speed of swiping air is high,
still in this case pressure losses go also high. Two versions are under
consideration, one with horizontal air circulation and the other with
vertical flow through the receiver walls. The vertical set-up is given
in Fig. 3 (Tache et al., 2006).
To enhance the thermal transfer from the walls to the fresh air the
application of deflecting buckles on the walls is envisaged (Rugescu,
2007). A similar technology was already applied in the air cooler built
at COMOTI manufacturing facility for a five staged industrial air
compressor (Fig. 4). Within this technology the buckling walls are
formed into a circular shell brazed between an inner cooper tube of 13x1
mm (outer diameter x thickness) and an outer tube of 28x1 mm, with a
common length of 450mm (Fig. 5). By contrast, the WINNDER heat exchanger
will use planar buckling in aluminum, upon the planar vertical walls
with the geometry in Fig. 6. Brazing with tin alloy is envisaged,
regarding the working temperatures of the receiver walls of up to
400[degrees]C.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
The pressure losses within the tubular COMOTI air cooler,
containing 226 evenly distributed longitudinal tubes, closely fit to
each other into a 750mm diameter block, are of between 0.1 and 0.2 bar,
barely depending on the surface state and smoothness. It was observed
for instance that the initial very clean surface of air contact produces
the minimal losses of 0.1 bar, while after a period of work of 200 hours
the pressure losses double, mainly due to the degradation of the surface
quality in contact with the flowing air. No detailed investigations
could have been made and this task is consequently transferred to the
WINNDER project, where pressure losses should reduce significantly.
[FIGURE 6 OMITTED]
4. CONCLUSIONS
A new manufacturing technology with tubular-shells, brazed with
buckling walls in aluminum is envisaged for WINNDER solar receivers. The
technology is simple and reliable, yet qualification tests are still
under development. This represents a Romanian contribution to the
renewable energy field.
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