Technology of solar receivers manufacturing for the WINNDER tower.
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]
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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|Author:||Rugescu, Radu Dan; Silivestru, Valentin; Ionescu, Mircea Dan|
|Publication:||Annals of DAAAM & Proceedings|
|Article Type:||Technical report|
|Date:||Jan 1, 2007|
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