Keith Gawlik

A numerical and experimental investigation of low-conductivity unglazed, transpired solar air heaters

The performance of low-conductivity unglazed, transpired solar collectors was determined numerically and experimentally. The numerical work consisted of modelling flow conditions and plate geometries with the FLUENT computational fluid dynamics software and the experimental work utilized laboratory apparatus at the National Renewable Energy Laboratory. Good agreement was found between the numerical and experimental results. The results showed that for practical low-conductivity materials, performance differed little from the equivalent geometry in high-conductivity material.Copyright

Wind Heat Loss From Corrugated, Transpired Solar Collectors

Heat transfer from a perforated, sinusoidal plate with suction to air flowing over the plate, perpendicular to the corrugations, has been studied numerically and experimentally. This study used a numerical model, validated by wind tunnel tests and hot wire anemometer/resistance thermometer measurements, to determine the heat loss to the air stream over the plate as a function of wind speed, suction velocity, and plate geometry. Both attached and separated flow regimes were observed, and the criterion for flow attachment was determined to be Re V0,P ≥6.93 Re 0.5 U ∞ ,A . Correlations were developed for heat transfer to the air stream for each flow regime. For attached flow, the heat transfer can be represented as Nu att = Nu falat {1 +0.81(A/P) 0.5 }. For separated flow, the following correlation applies: Nu sep = 2.05(A/P) 1.40 Re 1.63 .

Self-repairing poly(phenylenesulfide) coatings in hydrothermal environments at 200 °C

Usefulness of hydraulic calcium aluminate (CA) fillers, containing monocalcium aluminate (CaO.Al2O3) and calcium bialuminate (CaO.2Al2O3) reactants as the major phases, in healing and repairing micro-sized cracks generated on the surfaces of poly(phenylenesulfide) (PPS) coating was evaluated by exposing the cleaved coatings to a simulated geothermal environment. CA fillers with a grain size of <40 μm were incorporated into the PPS coatings. The cleaved PPS coatings containing fillers then were exposed for up to 20 days in a 200 °C CO2-laden brine. The decalcification–hydration reactions of the CaO.Al2O3 and CaO.2Al2O3 reactants disclosed in the cracks led to the rapid growth of boehmite crystals, while the crystalline calcite phase formed by the carbonation of these reactants was leached out of cracks because of the formation of water-soluble calcium bicarbonate. During exposure for 24 h, the block-like boehmite crystals, ∼4 μm in size, densely filled and sealed the open cracks; this was reflected in an increase in pore resistance to two orders of its magnitude compared with that of cleaved coatings without fillers. Extending the exposure time to 20 days resulted in no change in pore resistance, suggesting that the sealing of the cracks by boehmite crystals played an essential role in reconstituting and restoring the function of the failed coatings as corrosion-preventing barrier. Therefore, CA-filled PPS coatings are able to self-heal and -repair cracks generated on the surfaces of coating films in hydrothermal environments.

High-performance polymer coatings for carbon steel heat exchanger tubes in geothermal environments

The most critical issue in developing thermal conductive coatings for the interior surfaces of heat exchanger tubes made from mild carbon steel (MCS), which are used in geothermal power plants at temperatures ranging from 110° to 89°C, is the deposition of scales. These scales, induced by the brine, chemically adhere to the coating surfaces. One of the major factors governing the formation of a strong interfacial bond at interfaces between the coatings and scales was the brine-promoted hydrothermal oxidation of the coatings. In seeking coating unsusceptible to hydrothermal oxidation, two semi-crystalline thermoplastic polymers, polyphenylenesulfide (PPS) and polytetrafluoroethylene (PTFE)-blended PPS, were applied as interior surface coatings to the zinc phosphated MCS tubes. The PPS coating surfaces suffered some oxidation caused by their chemical affinity for FeCl2 in geothermal brine. FeCl2-promoted oxidation of PPS surfaces not only incorporated more oxygen into them, generating a sulfide → sulfone → sulfonic acid conformational transformation within the PPS, but also caused the disintegration of PPS, yielding fragmental polychloroaryl compound and ferrous sulfate (FeSO4) derivatives. The FeSO4 reaction product formed at the interfaces between the scale and PPS coating was soluble in water, so that the coatings could be easily removed by highly pressurized water. The oxidation of PPS was considerably inhibited by blending PTFE into it, forming coating surface unsusceptible to hydrothermal oxidation reactions with hot brine. The major reason for such inhibition of oxidation was the formation of a chemically inert PTFE layer segregated from the PPS layer at the outermost surface site of the coating. Hence, the scale easily flaked off from the PTFE-blended PPS coating surfaces. This characteristic of surface was similar to that of the stainless steel surfaces. Nevertheless, both PPS and PTFE-blended PPS coatings can be classified as scale-free coatings.

Anti-silica fouling coatings in geothermal environments☆

Abstract An important key to a successful use of carbon steel-based heat exchanger tubes in geothermal binary-cycle power plants is to understand how to efficiently prevent the deposition on them of silica scales. These deposits are caused by the high sensitivity to silica of ferric oxide layer occupying the outermost surface sites of carbon steel. We evaluated the usefulness of two high-temperature performance coatings, polyphenylenesulfide (PPS) and polytetrafluoroethylene (PTFE)-blended PPS, in inhibiting silica scaling. To obtain this information, the coated steel panels were immersed for up to 7 days in 200 °C silica-rich brine. As a result, the surfaces of the unblended PPS coating underwent some degree of brine-induced oxidation. Although the amount of silica deposited was negligible, it was found that the sulfur–oxygen derivatives formed on the surfaces by oxidation make them susceptible to silica scaling. In contrast, the PTFE-blended PPS coating had a high potential as the anti-silica fouling barrier. The major reason for this was due to the segregation of anti-oxidant, hydrophobic PTFE top surface layer above a PPS layer in the coating.

Nanoscale boehmite filler for corrosion- and wear-resistant polyphenylenesulfide coatings

We evaluated the usefulness of nanoscale boehmite crystals as a filler for anti-wear and anti-corrosion polyphenylenesulfide (PPS) coatings exposed to a very harsh, 300°C corrosive geothermal environment. The boehmite fillers dispersed uniformly into the PPS coating, conferring two advanced properties: First, they reduced markedly the rate of blasting wear; second, they increased the PPSs glass transition temperature and thermal decomposition temperature. The wear rate of PPS surfaces was reduced three-fold when 5 wt% boehmite was incorporated into the PPS. During exposure for 15 days at 300°C, the PPS underwent hydrothermal oxidation, leading to the substitution of sulfide linkages by sulfite linkages. However, such molecular alteration did not significantly diminish the ability of the coating to protect carbon steel against corrosion. In fact, PPS coating filled with boehmite of ≤ 5 wt% adequately mitigated its corrosion in brine at 300°C. One concern in using this filler was that it absorbs brine. Thus, adding an excess amount of boehmite was detrimental to achieving maximum protection.

Milled carbon microfiber-reinforced poly(phenylenesulfide) coatings for abating corrosion of carbon steel

Effectiveness of milled carbon microfibers, 7.5 μm in diameter and 100-200 μm long, in increasing the thermal conductivity, tensile strength, and elongation of fiber-reinforced poly(phenylenesulfide) (PPS) composite coatings was investigated. Thermal conductivity depended on the fiber content of the composites; the maximum content (5 wt% fiber) was responsible for a 2.6-fold enhancement over that of non-reinforced ones. However, the most effective amount of fiber in improving the tensile strength and elongation of the composition coating films was 3 wt%, reflecting 5.2 times and 2.6 times improvements, respectively, compared with those of the non-reinforced ones. The composite coatings adequately protected the underlying carbon steel against corrosion in 250°C CO2-laden brine. The reason for their great hydrothermal stability was the conformational changes in the molecular structure of PPS, from sulfide bridging to sulfone bridging, which increased the thermal stability of the matrix.

Carbon fibre-reinforced poly(phenylenesulphide) composite coatings

To enhance the thermal conductivity and improve the mechanical properties of poly(phenylene sulphide) (PPS) that was used as a coating for carbon steel heat exchanger tubes, chopped carbon fibres, ∼7.5 μm diam. x ∼3 mm long were incorporated into the PPS matrix. All the experiments in this work were performed using steel panels instead of steel tubes. As a result, 0.5 wt% of fibres was the most effective amount of fibres in minimizing the rate of penetration of electrolytes through the composite coating films. Even though the steel panels coated with the 0.5 wt% fibre-reinforced PPS composites were exposed for 14 days in an autoclave containing 20,000 ppm CO2-laden 13 wt% NaCl solution at 200°C, the coatings remained intact. This suggested that the composite coatings not only adequately protect steel against corrosion in a wet, harsh, and hostile geothermal environment, but they also have great hydrothermal stability. In addition, the thermal conductivity of the bulk PPS rose ∼ 60% by adding 0.5 wt% fibre. The rough surface texture of the fibres provided good mechanical interlocking bonds with the PPS matrix. Such interfacial bonding can be interpreted as one of the factors governing the outstanding mechanical properties, such as tensile strength, tensile modulus, and elongation, of the multidirectional fibre-reinforced composite films.

A Model of Radiation-Induced Thermal Stratification in an Integral-Collector-Storage Tank

A finite difference model was developed that predicts the transient temperature profile of the water in an integral collector-storage unit during charge from solar radiation incident on the absorber. The model uses a forward difference technique. A Nu(Gr*) correlation for a flat plate in an infinite medium under prescribed uniform heat flux was used on the inside surface of the absorber to relate imposed flux to boundary layer mass flow and temperature. Nodal mass-energy balances were imposed to derive the overall mass flows and temperatures. Model results were compared to experimental and numerical data with good agreement.Copyright

Hydrothermal degradation study of phenolic polymer coatings by advanced analytical methods

Resolve-type phenolic polymer coatings were deposited on carbon steel panels, and then were exposed for 15 days to a simulated geothermal brine with pH 1.6 at 150°, 175°, and 200°C. The phenolic polymers were hydrothermally oxidized. To comprehensively understand the mechanisms of this oxidative degradation of the coatings, the modern analytical techniques of XPS, contact angle, TGA, SEM-EDX, and EIS were used in combination. The oxidative degradation of polymer took place in the three-step oxidation routes: first, the bridging methylene linkages in the network polymer structure were replaced by the benzlhydrol-type linkages; second, the benzlhydrol-type linkages were transformed into the benzophenone-type linkages; and finally, the C-C-C linkage in the benzophenone derivative ruptured to form salicylic acid derivatives as the ultimate degradation products. Hydrothermal temperatures of >175°C promoted the degree of such oxidative degradation, causing the coating surface to become susceptible to moisture, to absorb more brine, and also to allow corrosive electrolytes to permeate easily. Consequently, iron oxides as the corrosion products from the underlying steel were yielded at a critical interfacial zone between the coating and steel. The excessive growth of iron oxides led to the generation of internal stress-induced cracks in the coating film, thereby resulting in the failure of these corrosion-preventing barriers.