Overview of Adhesives and Sealants in Renewable Energy. Abstract

Transcription

1 Overview of Adhesives and Sealants in Renewable Energy George T. Sivy Sika Corporation Stephenson Highway Madison Heights, MI Abstract Adhesives and sealants are integral materials in the manufacture of structures or components to harness most renewable energy sources. From structural epoxy or urethane adhesives to bond immense WTG (Wind Turbine Generator) composite blades, to low modulus PSA mastic tapes for sealing flexible thin film photovoltaic modules to roof membranes, adhesives and sealants have proven themselves capable of meeting the rigorous performance requirements for the harnessing assemblies and withstanding the harsh, long term service environments typically encountered. This paper provides an overview of the various adhesive and sealant types used in the solar and wind renewable energy sectors, and briefly discusses the biomass industry. The factors used to determine the suitability of each material for a given application, common test methods, and accepted industry standards and specifications are discussed. Copyright 2012 by Sika Corporation.

2 Contents 1. Introduction 3 2. Solar Photovoltaics (PV) Module Assembly Crystalline Silicon & Thin Film Concentrating Photovoltaics (CPV) Module Mounting Rack Systems Direct Mounting Building Integrated Photovoltaics (BIPV) Specifications & Approvals Solar Thermal Solar Collectors Specifications & Approvals Concentrating Solar Power (CSP) Specifications & Approvals Wind Towers Nacelles Rotors & Blades Specifications & Approvals Biomass Summary References 22 2

3 1. Introduction As the harvesting of energy from renewable sources continues to increase, so too does the use of adhesives and sealants. Renewable Energy Sources are those that are continually and naturally renewed. Basically, there are five such energy sources: Solar, Wind, Biomass, Geothermal, and Hydropower. Harnessing the energy from these sources for heat or electricity requires structures or components of varying complexity and sophistication, but common to all of these is the need to provide safe, consistent delivery of power over several decades at a cost that is competitive with fossil fuel sources. For the assembly of these structures, particularly for solar and wind, adhesives and sealants offer many advantages over conventional joining or sealing methods such as mechanical fasteners or welding or preformed gaskets. These include: No compromising substrate continuity and integrity with holes for screws or rivets. Adhesives allow for assemblies with flat profiles and no surface interruptions. A bonded mirror for example. Adhesives and sealants are easily applied from small cartridges or commercially available dispense machines. Most adhesives bond and seal. The ability to safely join dissimilar substrates such as glass to metal Stress from applied loads is distributed over much of the bonded area. This means reduction of high stress points as with mechanical fixation. Some adhesives compensate for movement of components due to thermal expansion differences of the joined substrates. This reduces the chance for buckling of large bonded surfaces. Liquid applied sealants eliminate the manual cutting and fitting as often needed for preformed gaskets Depending on the joining requirements, some sealants seal and bond, thereby eliminating mechanical fixation as required with preformed gaskets or rubber seals. To be fair, adhesives and liquid applied sealants do have some disadvantages. These include: Most systems require a dispense tool of sorts, so a capital equipment investment may be needed. Surface pretreatment may be necessary. Special training may be required for proper use. With the exception of some tapes, all adhesives and sealants require finite fixture and full cure times. These times can vary from seconds to days, depending on the chemistry. 3

4 Very fast curing systems that rely on application of energy from external sources, e.g. RF, induction cure, UV, require special equipment. Bonded or sealed joints may not be as readily serviceable as those mechanically joined. Shrinkage during cure, especially with solvent borne single component adhesives or sealants. Health and environmental concerns with chemicals as well as OSHA or other regulatory compliances. Lack of industry standards. Fortunately, most adhesive and sealant suppliers recognize the above limitations and have developed special sales programs and application practices that address most of them. For example, a supplier may offer a dispense equipment program in which the cost of the machine is worked into the price of the adhesive and amortized over several years. To address the finite cure times, many suppliers of structural adhesives also offer fast fixturing pressure sensitive tapes that can be used in conjunction with the adhesive to hold things together until functional or full cure is reached. With these new practices, the advantages of adhesives and liquid sealants far outweigh the disadvantages, and thus, the industry is enjoying somewhat steady growth in practically all industries, especially renewable energy. Actually the use of adhesives and liquid applied sealants in the renewable energy industry is not new. For example, the unmatched adhesion of silicone adhesives and sealants to glass has made them the preferred material for mounting flat panel photovoltaic modules into metal support frames for the past 30 years. In the following sections, the types of adhesives and sealants used in solar, wind, and biomass will be reviewed. Why and how the sealants are used will be discussed and any relevant governing bodies, standards, and specifications introduced. Much of this review will be on components and structures for conversion of solar and wind to electricity, but some mention will be made of the more interesting assemblies for direct heating, such as solar collectors, and generation of fuels such as from biomass. Structures used for combustion of fuels derived from renewable energy sources or those to generate electricity from steam, i.e. turbines, will not be discussed. 2. Solar In the most basic sense, all renewable energy sources could be called solar. As examples, consider the fact that the winds driving massive wind turbines come from solar heating of air, or that biomass comes from fermentation of vegetation that grew with light from the sun. However, this argument is best left to renewable purist, and for this section, assembly of structures for the direct conversion of sunlight to electricity or heat will be discussed. 4

5 Structures to harness the power of sunlight can be divided into two (2) categories: PV (Photovoltaic) and ST (Solar Thermal). Each of these can be divided into sub-categories. This is best shown in Figure 1. Figure 1. Assemblies for harnessing Solar Energy Numerous adhesive and sealants types and chemistries are used in the construction of ST and PV components. 2.1 Photovoltaics (PV) Module Assembly Crystalline Silicon & Thin Film In terms of bonded area, encapsulation films and backsheet laminating adhesives are by far the most prevalent in PV module manufacture. Consider the buildup of a generic crystalline silicon PV module as depicted in Figure 2. Figure 2. Crystalline Silicone PV simplified layers. Superstrate antireflective layer and cell contact layers are not shown. 5

6 EVA (Ethylene Vinyl Acetate) films have been the primary protective encapsulant material for several decades. Used primarily for crystalline silicon PV s and in some thin film constructions, EVA s success is due to very low moisture uptake coupled with outstanding thermal resistance. The use of PVB (Poly(vinyl butyral) encapsulant has been on the rise, especially for thin film PV s and Building Integrated PV s (BIPV s). Much of this growth is due to advances in PVB chemistry as well as decades of proven success in laminated glass construction In application, the electrically connected cells are sandwich between two sheets of the encapsulant and positioned on the superstrate. The backsheet is then applied and the assembly inserted into a laminator. Here, air is removed to apply uniform down pressure on the laminate. Heat is then applied, which melts the encapsulant to seal around the cells as well as join them to the superstrate and backsheet. The most common backsheet for the past 30 years or so has been a laminate of PET (Poly(ethylene terephthalate)) between two layers of PVF (Poly(vinyl fluoride)) film. Other laminates are also gaining popularity, for example PVF, PET, and EVA. In any case, adhesives are used to join the layers. Though formulation detail is proprietary, the adhesives are generically 2 component systems consisting of a solvent borne or solvent free polyester base and an isocyanate coreactant. These adhesive are characterized by good optical clarity, high bond strength, low moisture vapor transmission, and outstanding heat and chemical resistance. Application is by standard coating methods such as direct gravure followed by lamination. In one backsheet variation, the layers are directly extruded onto one another. In this way the adhesive is eliminated. Figure 3 shows generic crystalline silicon and thin film PV modules with the other various types of adhesives and sealants used. The key below the figures describes each. Figure 3. Generic Crystalline Silicon PV (left) and Thin Film PV (right) showing locations of adhesive and sealants used. 6

7 Adhesive & Sealant Key for Figure Bonding & Sealing of the modules into metal frames: 1 and 2 Component Silicones, Solid & Foam Acrylic PSA Tapes, 1 and 2 component STP s, butyls. 2. PV Module Edge sealing: PIB hotmelts (primary seal), 1 and 2 component Silicone, Acrylic Tape, or combinations. 3. Dielectric and Cosmetic Masking tape. Heat activated PSA tape. 4. Rail or Mounting Bonding: 1 and 2 Component Silicones or STP s, Solid & Foam Acrylic PSA Tapes. 5. Labels 6. Junction box bonding: 2 Component Silicone (potting as well), Epoxy, or Tape 7. Corner Bonding & Sealing: 2 Component Structural\Urethanes Epoxies, Acrylics, Welded, or Mechanically Fixed and Sealed with 1 Component Silicone or Acrylic, Anaerobic threadlockers. As can be seen, silicones and tapes might be found in just about any part of a module assembly. Both 1 and 2 component silicones systems are used, the selection of which depends on the performance requirements as well as the assembly process. For large volume manufacture, the 2 component systems have the fast cure time to allow for any post-bonding operations that would require movement of the parts. Cure of the 2 component systems can be accelerated with heat, but there may be some compromise in properties Neutral cure systems, either alkoxy or oxime, are generally preferred over the cheaper acetoxy cure systems because of corrosion concerns with the latter. Silicones adhesives and sealants generally have excellent adhesion to most PV substrates such as EVA films, PVF, polyesters, and glass. Primers are seldom required, but some type of cleaning may be needed. Silicone bonds to glass have outstanding UV resistance and, once optimized, are quite durable, even at temperature and humidity extremes. This environmental durability has been proven decade after decade in structural glazing applications around the world, so it is a small wonder that silicones are a natural choice for bonding PV modules. Silicone adhesives and sealants are elastomeric materials with low elastic and shear moduli. This allows them to accommodate high degrees of movement due to thermal expansion differences of joined components. This is particularly useful when bonding aluminum frames to the glass superstrate of crystalline silicone PV s or attaching mounting rails to the glass substrate of rigid thin film modules. Compared to other adhesives such as urethanes and hybrids, silicones often have lower tensile, peel, and shear strength, though they do retain a higher percentage of their strength at elevated temperatures. Most Pressure Sensitive Adhesive (PSA) foam and solid tapes used in solar applications have tensile, shear, and peel strength comparable to silicones. They can also accommodate significant 7

8 joint movement like silicones. However, unlike the silicones, some tapes undergo significant strength reduction at elevated temperatures. Adhesion of these tapes to most PV substrates is very good, but some end users believe that more rigorous pretreatment is required compared to silicones or other adhesives. Environmental durability is also good. The primary advantage of PSA tapes is their near immediate fixturing strength to facilitate high speed assembly. One component moisture cure polyurethane (PUR) and silane terminated polymer (STP) adhesives and sealants are sometimes used for frame and rail bonding. STP s include 1 component MS (Methoxy Silane Terminated) Polymer systems, silane terminated polyurethanes, and other similar chemistries. These systems are also referred to as hybrids. Generally speaking, PUR s and STP s offer higher strength than silicones, but are not as UV resistant or mechanically stable as temperatures increase. Cure speed is roughly similar to silicones, so in this regard there is no advantage over the near instant fixture of tapes. STP s generally have better primerless adhesion to many substrates than PUR s and show better UV resistance, though the latter is still inferior to that of silicones. Application of the adhesives or sealants, liquids or tapes, is generally with automated or semiautomated dispense equipment. Given the large number of modules produced in a short time, hand application would be impractical Concentrating Photovoltaics (CPV) Concentrating Photovoltaics or CPV s focus direct sunlight onto cells a fraction the size of regular photovoltaics. On the plus side, this technology allows for overall smaller cells. On the negative side, CPV modules do require special housing, an optical lens, and they do operate at higher temperature due to the focused light and therefore require some manner of cooling. Of course, adhesives and sealants are needed in many ways. A module housing is basically a box with a focusing lens attached at the top and a cooling heat sink attached at the bottom. Because of their good primerless adhesion to many substrates as well as proven resistance to harsh weathering conditions, 2 component STP s are often the choice for assembly of the metal or plastic boxes. Two types of lenses are used: PMMA (Poly(methyl methacrylate)) or glass. As expected, the preferred adhesives for joining and sealing glass lenses to the housings are 2 component silicones. However, in some designs a hot applied butyl is also used for greater moisture permeation resistance. Two component STP s are sometimes used, but a UV resistant primer may be needed on the glass. PMMA lenses are typically joined and sealed with a hot applied butyl mastic. 8

9 As mentioned before, CPV cells require cooling. To accomplish this, the cell is bonded into a heat sink material. Here, thermally conductive 2 component die attach adhesives are used. Epoxy and silicone chemistries are most common Module Mounting Rack Systems For this section, the term Module Mounting refers to 1) the attachment of modules onto rack systems to form panels, and 2) ground and roof mounting of the racks. Generally speaking, crystalline silicon and rigid thin film PV modules are usually mechanically fixed to metal rack systems. However, a few suppliers feel it is better for overall longevity of the modules to avoid stress concentrations from mechanical clamping and therefore choose adhesive bonding to join both framed and unframed modules to their rack systems. The preferred systems are 2 component silicones or STP s. At least one supplier offers (or did offer) a rack system for frameless crystalline silicon PV in which a small reinforcing structure is bonded directly to the back of the PV module. This assembly is then mechanically attached to an acceptor rack. The advantage of this system is mainly the elimination of the frames. The adhesive used is a 2 component STP. For roof mounting to form arrays, the racks are usually ballasted down or mechanically fixed to the decking. Single component silicones, PUR s, STP s, or solvent borne elastomeric caulks may be used to seal around any roof penetrations. Selection of a particular sealant depends on the type of roof being sealed, UV exposure, and time of installation. PUR and STP sealants are perhaps the most versatile in terms of adhesion to many substrates and tolerance to less than ideal application conditions, but they should not be used on asphalt roofs or where direct exposure to UV is possible. Silicones have the UV resistance and good adhesion, but they can be a bit sensitive to surface adsorbed moisture, so they are not the best year-round, all-purpose sealant choice [1]. Some solvent borne rubber based sealants are claimed to be compatible with asphalt roofing and tolerant of damp surfaces. However, these systems suffer from a high degree of cure shrinkage, which may manifest as leaks if not properly applied. Finally, it should be noted that non-penetrating ballasted systems are usually preferred, especially on single ply roofing systems. For ground mounting, the racks are mechanically fixed to or anchored into piers Direct Mounting Flexible thin film system technology is becoming more established amongst the other photovoltaic systems. As it is a lightweight solution, it is especially applicable for weight sensitive applications 9

10 like roofing. And consistent with this weight savings theme, flexible thin film PV s are often directly attached to existing or new roofs, thereby eliminating heavy racks and ballasting. For attachment and sealing to both metal and membrane roofs, low strength elastomeric PSA butyl tapes are the norm. Here the term butyl is not limited to those based on butyl rubber but includes those made with other elastomers such as ethylene propylene or styrene butadiene rubbers. Both crosslinked and uncrossliked formulations are used. While the main purpose of these tapes is to seal out air and moisture, they generally do have sufficient strength to resist typically encountered mechanical and wind loads. Basically, the tapes, sometimes called mastic tapes, are applied to the entire backside of the flexible module. This laminate is then applied to the roof decking. Several thin film PV manufacturers supply their modules with the tape preapplied in the factory, and the bonding or sealing side is covered with a silicone-coated release liner. These Peel and Stick modules are rolled up into a convenient coil for shipping. At the job site, the module is carefully unrolled as the release paper is slowly removed and the exposed mastic brought into contact with the roof decking. The module is then compressed down with a weighted roller. Other versions of this type of application use simple solvent borne PSA systems and bare PV modules. In these cases, the adhesive is first applied to the roofing deck in the exact area where the PV will be sealed. After drying or chemically curing, the module is then applied. A few thin film PV suppliers offer their modules prebonded to flexible membrane sections that can later be heat welded to the roof deck. Depending on the nature of the module and membrane section, prebonding can be done with heated activated EVA films or moisture cure reactive PUR hot melts or 2-component STP s. The EVA systems offer instant fixturing but require heating equipment. The moisture cure reactive hot melts also feature fast fixturing, but they do require moisture to cure, so the size of the laminate may be limited. The 2 component STP s have little initial fixture strength but do not require heaters or moisture for cure. Finally, framed crystalline silicon and rigid thin film PV modules are sometimes bonded directly to the tops of vertical standing seams of SSR (Standing Seam Roof) metal roofs. Both 1 and 2 component silicones are used Building Integrated Photovoltaics (BIPV) Building Integrated Photovoltaic (BIPV) is an area where architectural design meets ecological responsibility. BIPV s not only produce electricity, but they are also elements in the overall building envelope and aesthetic of new structures, often replacing parts of building elements such as roof decking, skylights, and facades. The systems available in the current market can be grouped into two main categories: facade systems and roofing systems. Facade systems include curtain wall and glazings. Roofing systems 10

11 include shingles, standing seam products, skylights, and flexible thin film PV pre-mounted to single ply roofing membranes. The three PV types, crystalline silicone, thin film rigid, and thin film flexible are used as or for BIPV s, and the basic PV construction is same as regular modules, and thus the adhesives and sealants used are the same. Most BIPV facades systems are custom made for each application, with needed modifications done to the PV frames or glass superstrate or substrates. For example, a PV module may be incorporated into an insulating glass unit (IGU) configuration as shown in Figure 4. Special colors may also be needed for the building aesthetic. BPIV systems are mounted to curtain wall elements or buildings in exactly the same way as regular glass, and this means structural 2 component silicones. Figure 4. BIPV glazing. The PV is the front pane of an IGU. For BIPV roof systems, several manufacturers offer PV shingles and tiles. Both are essentially small versions of flexible thin film modules with some modification to increase efficiency. For the most part, the base technologies regarding laminating or attachment of the PV modules into or on shingle design substrates are proprietary, but attachment to the roof is done with nails, just as with regular asphalt shingle. Flexible thin film PV s are also available premounted to rolls of SPR (Single Ply Roofing) membrane or sections of metal roofing sheets or pans. For SPR, the modules are sealed to the membranes with moisture cure reactive PUR hot melts and the edges sometimes sealed with a polyamide hot melt for extra moisture intrusion resistance. On metal roof sections, some suppliers use 2 components STP s to bond PV modules, while others use simple butyl mastic tapes. Selection of one chemistry over another depends on the type of array being assembled, slope, and the expected wind and mechanical loads Specifications & Approvals PV modules (including BIPV) are subject to the qualification and design standards devised by the International Electrotechnical Commission (IEC) and the Underwriters Laboratory (UL) [2]. BIPV s may also need to meet the standards of the element they are replacing. 11

12 The applicable standards to which whole modules and components are tested include: UL 1703 Flat-Plate Photovoltaic Modules and Panels IEC Crystalline silicon terrestrial photovoltaic (PV) modules Design qualification and type approval IEC Thin-film terrestrial photovoltaic (PV) modules Design qualification and type approval IEC Photovoltaic (PV) module safety qualification Part 1: Requirements for construction IEC Concentrator photovoltaic (CPV) modules and assemblies Design qualification and type approval Many finished PV Module assemblies are listed in UL Categories QIGU or QIGU2, Photovoltaic Modules and Panels, or Photovoltaic Modules and Panels Components, respectively. The sealants and adhesives used in the PV assembly or for mounting are included as recognized components. Adhesive and sealant manufacturers usually seek listing or certification for their solar assembly products in UL category QMFZ2, "Plastics Component," although this category is not specifically for PV s or PV components. Properties such as burn characteristics, Relative Thermal Index (RTI), and electric properties are given for certified products. All are determined per the UL specifications given in the category. Concurrently, UL Category QIHE2, "Photovoltaic Polymeric Materials Component, is specifically for adhesives and sealants and other materials used in the construction of PV s or PV components. The other materials include encapsulants, substrates or superstrates, and potting compounds. Many adhesive and sealant tapes for PV mounting are listed in UL category QOQW2, Polymeric Adhesive Systems, Electrical Equipment Component. Some adhesive and sealant suppliers may also show their products as IEC or or compliant or approved. For the former 2, approval usually means the adhesive or sealant was tested as part of a complete module system. In addition to the specification testing and the obvious validation of adhesion, the adhesive supplier must work with the manufacturer to define the required bondline dimensions for the application. These dimensions are calculated based on the known engineering properties and limits of the adhesive or sealant and how they can accommodate the anticipated wind, mechanical, and thermal loads as defined by the manufacturer. 12

13 2.2 Solar Thermal Solar Collectors Solar Collectors or Solar Thermal Collectors are assemblies that use the sun s energy to heat water or other fluids that are passed through tubes. The two main types are Flat Plate collectors and Evacuated Tube collectors. Figure 5 shows a drawing of a generic Flat Plate Collector. Both collector types may use 1 or 2 component Figure 5. A flat plate solar collector silicones or hot melt butyls to bond and seal the top glazing sheet to a frame. Other designs use solid rubber gaskets. The frame, typically foam insulated aluminum or plastic channels, may be assembled with 2 component acrylics, urethanes, or epoxies. If assembled with mechanical fasteners, anaerobic threadlockers may be employed. One or two component silicones may be used to seal tube manifolds in some Evacuated Tube designs Specifications & Approvals There is no single global certification body for Solar Collectors, but rather numerous ones over many countries [3]. The main bodies are Solar Keymark in Europe (an initiative of the European Standards Organizations CEN and CENELEC), Solar Rating & Certification Corporation (SRCC) in the USA, Golden Sun in China, and SAI in Australia. The standards for testing are also not harmonized but in general most certifications specify EN either in whole or in part. Note that EN is to be replaced by EN ISO Entire collector assemblies are tested for certification, but some manufacturers pretest and qualify adhesives and sealants separately. Generally they use the methods given in EN 12975, specifically retention of adhesion through environmental exposures Concentrating Solar Power (CSP) Simply put, CSP assemblies use mirrors to reflect and focus sun light onto a target that coverts it to heat. The mirrors are mounted on concentrator support structures to follow the sun. The entire assembly is referred to as a heliostat. See Figure 1 for the types of CSP concentrators. 13

14 For Linear CSP systems, the mirrors are large, slightly curved rectangles or parabolic trough shapes, and the targets or receivers, are long tubes through which a liquid flows. The liquid is an oil or molten salt which in turn is used to boil water to drive traditional steam turbines. Parabolic Trough systems have single receiver tubes that are positioned at each mirror s focal line; Linear Fresnel Reflector systems have one receiver tube positioned above several mirrors. As the name indicates, Dish CSP systems use parabolic mirrors much like the shape of a satellite TV dish. The mirrors are usually made up of smaller mirrors or facets of various shapes, for example wedges arranged on a dish shaped metal or composite support. The target is a thermal receiver which transfers the heat to an engine generator such as a Sterling Engine. A Power Tower system is basically a field of many flat heliostats that focus sunlight onto a receiver at the top of a tower. There, a heat-transfer fluid in a receiver is heated and used to generate steam. Some power towers use water as the heat-transfer fluid, others use molten salts. Adhesives in the area of CSP are used mainly to attach the reflecting mirrors to the supporting structure. In one parabolic trough design, as shown in Figure 6, a 2 component structural silicone is used to bond ceramic mounting pads with metal nut inserts onto the backs of the mirrors. They are then mechanically fixed to the steel concentrator support structure. The silicone will accommodate the slight movements between the mirror and substructure and thereby minimize any stress concentrations that could affect focus or damage the glass. Also, since many support Figure 6. Parabolic trough reflector. Mounting tabs are bonded to the mirrors with a 2 component silicone. structures are open in the back, an adhesive with superior UV resistance is needed since there is a chance for exposure. One and two component urethanes and STP s are also used to attach mirrors. One of the largest manufacturers of a Linear Fresnel system has used a PUR to bond their mirrors to a closed galvanized steel decking for many years in various locations around the world. In this application, the urethane has better decking characteristics than a silicone, specifically having more initial body so as not to squeeze out of the joint from the weight of the mirror before it cures. And since the galvanized steel decking is closed at the back, there is no concern with UV exposure. Application is generally with automated or semi-automated meter mix dispense equipment. Given the sheer size of the mirrors and structures, hand application would be impractical. 14

15 Other adhesives such as anaerobic thread lockers or retaining compounds may be found on some concentrator support structures. Some designs may employ structural adhesives to join tube or truss elements, but the most widely used systems rely on mechanical joining methods. The mirrors or mirror sections on dish reflectors are usually applied to the smooth metal or composite support with a double sided adhesive tape on a film carrier. Typically an acrylic adhesive is used and the carrier is polyester. Metalized silver films are a light weight alternative to glass mirrors. These films are backed with a high tack PSA adhesive for application to the dish structure Specifications & Approvals Each CSP manufacturer defines their own testing methods and acceptance criteria for adhesives and with the supplier validates the systems for the application. After a thorough review of the CSP assembly, strength requirements, and operating conditions, an adhesive or sealant is chosen and initial adhesion to all substrates tested to determine if pretreatments are needed. Depending on the application, prototypes or scale assemblies may then be built and subjected to wind load testing and accelerated aging conditions. The latter may include UV exposures, NSS (neutral salt spray), CASS (copper-accelerated acetic acid salt spray), elevated humidity, and temperature & humidity cycling. Concurrently, the adhesive itself might be tested through the same exposures. Some manufacturers specify testing of lap joints, other rely on simple bead adhesion or quick knife testing, a type of peel test. 3. Wind Wind is perhaps the earliest renewable energy source to be harnessed by man with machines. It is believed that windmills were first developed between 500 and 900 AD and were made with paddles made of bundled reeds [4]. Over the years, especially during the 1980 s and 1990 s, windmills evolved, getting bigger and more efficient, and adhesives and sealants have played a vital role in this evolution. Modern windmills, or Wind Turbine Generators (WTG), consist of 3 basic parts: the foundation and tower on which the wind turbine sits, the rotor assembly consisting of a hub and blades, and the nacelle which houses the equipment. Adhesives and sealants are used in each part as described and discussed in the following sections. It must be noted that these discussions are only for large scale horizontal axis WTG's. 15

16 3.1 Towers Towers can be made of steel or concrete, with the former being dominate. Steel towers are basically large hollow cylinders built up with smaller flanged sections. See Figure 7. The first section is bolted to the foundation, and each following section mechanically fixed to the previous one. Some manufacturers use PUR s or 1 component STPs to seal around the bolted seams; others simply use preformed rubber gaskets. Concrete towers are relatively new. Here precast Figure 7. WTG Segmented Steel Tower cylindrical segments are stacked up to form the tower, essentially like the steel construction. This process is sometime referred to as Segmental Concrete Tower Construction. Unlike steel segments, there are no bolting flanges, but rather, in between the segments, a 2 component epoxy resin is used around the perimeter, primarily as a sealant. Other designs are also used; but for all, the integrity of the towers comes from the compressive strength of the concrete and post-tensioning cables passing through holes down the entire length of the construction and secured at the top and bottom. 3.2 Nacelles Figure 8 shows a generic nacelle and where adhesives and sealants can be found. Figure 8. Generic Nacelle with locations of adhesives and sealants used 16

17 Adhesive & Sealant Key for Figure Nacelle Cover Panel Joining. Bolted and Sealed with 1 Component Urethanes or STP s or Silicones or Preformed Rubber Seals. 2. Other Seam Sealing and Through Hole Penetrations for Rails, Anemometers, Safety Eyebolts, Latches, etc. PUR or 1 Component STP s or Silicones. 3. Surface Finishing. 2 Component Urethane, 2 Component Epoxy, Acrylic PSA Tapes (Urethane), 4. Hatch and Brush Seals Bonding: Cyanoacrylates, 1 and 2 Component urethanes and STP s. 5. Threadlocking/sealing of motors, brakes, shafts and coupling, and haydraulic fittings. Sealing Gear box flanges. Retaining pitch & yaw bearings: Anaerobic Threadlockers, Retaining Compounds, and Sealants Sealants are used primarily on the nacelle cover, which is either metal or composite. Some have internal frames onto which wall panels are mechanically fastened or welded; others, particularly those made with composites, use a type of clam shell construction or smaller modular Figure 9. Typical nacelle panel joints. sections. In a majority of designs, the panels are bolted together and sealed, with coach joints (flange joints) or joggle laps being the most common joining configurations. See Figure 9. Sealants are typically PUR s, 1 component STP s, or silicones, but some manufacturers employ preformed rubber gaskets. PUR s and 1 component STP s are used to seal through hole penetrations as well. Nacelles of Off Shore WTG s tend to have more sealants, since they may be slightly pressurized to maintain a constant internal working atmosphere. Most of the adhesives used in nacelles are anaerobic threadlockers, and because of the large fasteners involved and the eventual need for service, slow curing, medium strength formulations are typical. 3.3 Rotors & Blades By far, the majority of the adhesives and sealants used in a WTG are in the blades. And of this, most is adhesives for blade assembly. For example, approximately kg ( lbs) of adhesive are used to bond the 2 halves of a single 50 m (164 ft) epoxy prepreg WTG blade. The size of a typical adhesive bead used is shown in Figure 9. There are two main blade designs successfully used today, Figure 9. Typical adhesive bead size used for blade bonding 17

18 specifically 1) those utilizing a box spar onto which the air foil shells are bonded, and 2) those utilizing structural shells joined through spar caps with shear webs for load transfer [5]. To realize these designs, there are two manufacturing methods used; namely, fiber reinforced resin prepregs (typically epoxy) for box spar design, and resin infusion for shear web design. Epoxy or polyester resins are used for the latter. Both types of blades are shown in Figure10 along with the various types of adhesives and sealants used. The key below the figure describes each. Figure 10. The 2 main large scale wind turbine blade types with the locations of adhesives and sealants Adhesive & Sealant Key for Figure Structural Bonding: 2 Component Epoxy, 2 Component Urethane, 2 Component Acrylics 2. Root Base Bulkhead Sealing: 2 Component STP, 2 Component Silicone, PUR. 3. Bolt or Metal Stud Fixing (for attachment to rotor hub): Mechanically fastened with T- bolts or bonded: 2 Component Urethane, 2 Component Epoxy, 2 Component Acrylic, Anaerobic Threadlockers or Retaining Compounds 4. Surface Finishing: 2 Component Urethane, 2 Component Epoxy, Acrylic PSA Tapes (Urethane), 5. Balancing Compounds: 2 Component Urethane, 2 Component Epoxy 6. External Lightening Protection: 2 Component Urethane, 2 Component Epoxy, 2 Component Acrylic. 7. Component Bonding, Vortex Shedding, Rain Deflectors (not shown): 2 Component Urethane, 2 Component Epoxy, 2 Component Acrylic 8. Components Bonding, Winglets (not shown): 2 Component Urethane. With the shear web design or resin infusion blades, structural integrity is strongly dependent on the bondline properties and hence the demands on the adhesive are high, particularly in terms of 18

19 mechanical properties and stability at elevated temperatures (see Section 3.4). With the box spar design or prepreg blades, stiffness and integrity are mainly provided by the spar, and thus there are lower demands on the adhesive. Three structural adhesive chemistries are used in blade assembly: 2 component epoxies, 2 component urethanes, and 2 component acrylics, generally based on MMA (methyl methacrylate) monomer. Two component urethanes are used primarily for prepreg blade assembly, and until recently, only 2 component epoxies have had the shear & peel strength and glass transition temperature (Tg) needed to meet the structural requirements for resin infusion blade assembly (see Section 3.4). Advancements in urethane chemistry have now resulted in 2 part systems that can compete with the epoxies. Furthermore, the urethanes are claimed to offer shorter and simpler cure cycles, lower exotherm during cure, and greater long term retention of properties, especially under dynamic loads. The acrylic adhesives are usually used for fiber reinforced polyester blades. In resin infusion blade production, the adhesive is first applied to the spar cap built into the bottom shell member, and the shear web(s) mated and held in place with a jig. Adhesives is dispensed onto the perimeter of the structural shell and onto the top of the shear web(s). The top shell is then mated and the blade heated to accelerate cure and achieve the needed Tg. The joint may also be glass tabbed for even greater strength. Application is done with meter mix equipment capable of large delivery rates, for example 11 to 19 liters/minute (3 to 5 gallons/minute). Bonding of prepreg blades is similar, but no special holding jigs are needed. 3.4 Specifications & Approvals GL (Germanischer Lloyd) Renewable Certification is the leading certification body in Wind Energy. For an adhesive to secure a GL certification for resin infusion blade assembly, it must meet the requirements as given in Rules for Classification and Construction II, Part 2 [6]. This section specifies the required testing and cured properties of structural adhesives for blade bonding. These requirements are summarized in Table 1 below. Table 1. Summary of GL Certification Testing for Structural Adhesives Property Test Method Requirement Tensile Lap Shear Strength DIN EN MPa Peel Resistance ISO N/mm Dimensional Stability Under Heat DIN EN ISO 75-2, Method A 65 C Long Duration Shear Tension Based on EN mm at 0.5 mm thickness Testing is carried out after 2 conditioning states: 24 hours at 23 C and 1000 hrs in 23 C distilled water. 19

20 There are no specifications for adhesives used in prepreg bonding, but most formulations nevertheless afford high shear and peel strengths, sometimes equal to or near that of adhesives for infusion blades. There are no generic GL Certifications for sealants, but some suppliers have developed special programs with GL for approval of their products. 4. Biomass Biomass energy, or "bioenergy" refers to energy from plants and other biological material. Brining wood is the simplest form. There are 3 biomass applications [7]. Biofuels Converting biomass into fuels for transportation. Bioproducts Converting biomass into chemicals for making plastics and other products that typically are made from petroleum Biopower Converting biomass into gaseous or liquid fuels that burn more efficiently to generate electricity. Converting is done by 1) burning biomasses, 2) co-firing or mixing with fossil fuels, 3) gasification at high temperatures and little oxygen, 4) pyrolysis, like co-firing but without oxygen, or 5) anaerobic biodigestion, perhaps the cleanest process in terms of reducing emissions. In this brief section, only anaerobic digestion will be discussed. Anaerobic digestion is the natural decay of biomass under anaerobic conditions to produce methane, which can be captured and used for power production. Digestion on a large scale is typically done in large tanks made of reinforced concrete, stainless steel, or glass lined or epoxy coated steel tanks. This is where adhesives and sealants are used. Concrete tanks are usually sealed at the base with PUR s to prevent leakage. The bolted seams and the bases of metal tanks are where most sealants are used. PUR s and 1 component STP s are preferred because they have been shown to resist the low ph environment in and above the tank. A proper formulation will have inert fillers such as talc. Sealants with carbonate fillers may not be compatible. 20

21 5. Summary Adhesives and sealants are critical elements in the manufacture of all components and constructions to harness renewable energy resources, particularly solar and wind. In PV (Photovoltaic) module manufacture, encapsulation films such as EVA and backsheet laminating adhesives such 2 component reactive urethanes are by far the most prevalent in terms of actual bond area. These sealants and adhesives form or help form the modules front line defense against the harsh operating environments the assemblies see in their lifetime. Silicones and tapes are used for frame bonding and stiffening as well as additional sealing. The high temperature resistance and UV stability of silicones and the fast, near instant fixture times of tapes makes each ideal for the demanding assembly and performance requirements for PV s. In CSP (Concentrating Solar Power), silicones, PUR s and STP s are used to bond and seal large mirrors to the underlying supporting structures. Such elastic adhesives and sealants have the strength required to sustain the mechanical, thermal, and wind loads these assemblies must endure. The monstrous blades of a WTG (Wind Turbine Generator) require an equally immense amount of adhesive. Here, only very high strength epoxy, urethane, or acrylic adhesives are used to give the blades their needed stiffness. Sealants are used on the blades, nacelles, and towers to smooth their surface, fill in minor cosmetic flaws, and keep water out. Anaerobic threadlockers and retaining compounds are used extensively on the numerous internal mechanical assemblies such as gear boxes and bearings. In Biomass, PUR s and single component STP s are used to seal the towering steel or concrete biodigesters in which methane is produced. These sealants must basically spend their entire life exposed to the harsh chemical environment created by the digestion process. The harvesting of energy from renewable sources will continue to grow in the years to come, and so too will the use of adhesives and sealants. 21

22 6. References 1. Fain, S. and Brearly, D. Pitched Roof Racking. Solar Pro. October-November (2008): Speer, Bethany K. "Solar PV Quality Assurance from the Developer's Perspective." 4 Nov National Renewable Energy Laboratory, Golden, CO. 17 April < 3. Fritzsche, Ulrich. Collector Certification Across the World Challenge or Opportunity. SME Thermal Berlin Dodge, Darrell M. The Illustrated History of Wind Power Development. TelosNet TelosNet, Littleton, CO. 01 May < 5. Tong, Wei. Wind Power Generation and Wind Turbine Design, Billerica, MA: WIT Press, Germanischer Lloyd (GL), Rules for Classification and Construction, II Materials and Welding, 2 Non-metallic Materials, 1 Fibre Reinforced Plastics and Bonding, Section 2 - Inspection and Testing of Fibre Composite Materials, 6. Adhesives. Germanischer Lloyd Aktiengesellschaft, Vorsetzen 35, Hamburg, Germany, Renewable Energy Basics: Biomass. 30 May National Renewable Energy Laboratory of The US Department of Energy. 30 May < 22