WO2003069324A1

WO2003069324A1 - A method and an apparatus for the detection of the presence of polymer in a wind turbine blade

Info

Publication number: WO2003069324A1
Application number: PCT/DK2003/000098
Authority: WO
Inventors: Jørgen DAHL VESTERGAARD
Original assignee: Lm Glasfiber A/S
Priority date: 2002-02-15
Filing date: 2003-02-14
Publication date: 2003-08-21
Also published as: DE10392293T5; AU2003208301A1

Abstract

Method and apparatus of inspecting the quality of a wind turbine blade which includes a blade shell (1,2) of fibre-reinforced polymer and areas to which fluid or viscous, curable polymeric compound (6, 7, 8, 10) has been supplied. The wind turbine blade is exposed to heat or cold, the temperature variations are measured over an area of the surface of the wind turbine blade and the parts of an area with insufficient presence of polymer are determined, the temperature thereof differing from that of the parts of an area containing polymer.

Description

Title: A method and an apparatus for the detection of the presence of polymer in a wind turbine blade.

Technical Field

The invention relates to a method according to the preamble of claim 1 and an apparatus according to the preamble of claim 14. For moulding and bonding parts fluid or viscous and curable polymeric compounds, ie more or less fluid compounds, are commonly used. Polyester and epoxy are examples of such compounds which are used for instance in connection with the manufacture of fibre-reinforced composite products. When a mould or a joint is to be filled with moulding material or an adhesive compound, it is often desirable subsequently to ensure that the entire mould cavity or glue joint cavity has been filled with the moulding material or the adhesive compound, as this has vital importance with respect to the finished product's strength, weight, mass distribution, etc.

Background Art

A known method of inspecting moulded or bonded parts is to knock on various posi- tions of the relevant areas and listen to the changes in the clang pattern. The method is usually performed manually, but equipment exists for striking a material and listen to the response. A thick solid part sounds different than the same part provided with for instance air inclusions. The method is unreliable in connection with thick composite materials and the assessment of the results is furthermore subjective and diffi- cult to substantiate.

Another method principle relates to ultrasound, which has been established as a de facto standard within the field of material inspection of composite components. An ultrasonic head is placed on top of the part and emits sound waves reverberating re- flections during their propagation through the material. The ultrasonic head has to be in contact with the component. This is usually ensured by applying water or gel to the surface of the component. Reflections arise when the sound wave passes a boundary layer between two different materials, eg from polyester to air (porosity), polyester to fibre glass, etc. These reflections are recorded either by the same ultrasonic head that emitted the sound waves or by another ultrasonic head arranged on the opposite side of the component. By recording the emission time and subsequently recording a sound wave, it is possible to calculate from which depth the particular reflection originates, the velocity of propagation of the sound through the component being calculated.

As a reflection occurs at each boundary layer, the result is based on a plurality of reflections, which makes it difficult to interpret the result. In practice this means that experts are needed to interpret the recorded results.

Yet another method is based on X-radiation. By X-raying a component, the differ- ences in density can be detected and a defined area containing air in a solid component may thus be determined. The images from a radiography are intuitively easy to interpret with respect to large defined errors. This method is encumbered by the risk of radiation and the precautionary measures necessary in connection therewith.

US 5.209.881 discloses a system for the continuous production of fibre-reinforced plastic panels, said system including a curing oven in which one or more infrared sensors are located for measuring the gelation temperature of the resin so as to control the production parameters.

WO 00/29836 discloses a system and a method of producing personal care articles, whereby said articles are bonded by means of a hot-melt adhesive or by ultrasonic sealing. In both of the above methods, the temperature of the bond exceeds that of the surrounding material and this heat may be detected by infrared sensors or by an infrared camera.

US 5.374.122 discloses a method and an apparatus for measuring the porosity in a non-metallic body, one side of the body being heated by means of a laser beam and the increase in temperature on the opposite side of the body being measured by means of an infrared sensor so as to determine the porosity of the body.

US 5.399.016 discloses a device and a method for measuring and monitoring the thickness of a shaped heated section, the heat of the section being scanned by means of an infrared camera.

Brief Description of the Invention

The object of the invention is to provide a new and effective method of inspecting the quality of wind turbine blades, which are moulded or bonded by means of a fluid or viscous and curable polymeric material.

According to the invention the wind turbine blade is characterised in that it is exposed heat or cold, that the temperature variations are measured over an area of the surface of the wind turbine blade and that the parts of the area with insufficient presence of polymer are determined, the temperature thereof differing from that of the parts of the area containing polymer. By means of such a method it may be deter- mined in a particularly simple and inexpensive manner whether polymeric material has been supplied to the intended areas of the turbine blade such that the intended strength of the blade is obtained. The wind turbine blade may be exposed to heat or cold in several ways. Having reached a first temperature the entire wind turbine blade may be transferred to an area having a different temperature, and the parts of an area with insufficient polymer presence may be determined based on the temperature measurements, the thermal conductivity, the heat capacity and thus the change in temperature in these parts differing from those in parts of the area containing polymer.

According to the invention the method is characterised in that the blade shell is made by vacuum infusion, in which a fluid curable polymeric material is injected into the fibre material.

According to an embodiment the insufficient presence of injected polymeric compound in the blade shell may be determined by measuring the temperature. Problems with air inclusions are experienced in blade shells made by vacuum infusion due the distribution of the polymeric compound at vacuum in the comparatively thin component.

The thickness of the blade may be at least 5 mm, preferably at least 10 mm.

According to the invention the blade shell may be formed of two blade shell halves being bonded along their edges by means of glue joints of a fluid or viscous and curable polymeric compound to form a wind turbine blade, parts of an area with insufficient presence of polymeric compound in the glue joints being determined by meas- uring the temperature. The glue joints in wind turbine blades can be very long and a correct filling of the joints in the longitudinal direction of the entire blade may thus prove difficult. By the method according to the invention a particularly simple and reliable quality inspection of such glue joints is obtained.

According to the invention the wind turbine blade may include one or more bracings, which by means of glue joints of a fluid or viscous and curable polymeric compound are bonded to and interconnect the inner faces of the two blade shell halves, parts of an area with insufficient presence of polymer in the glue joints being determined by measuring the temperature. Hitherto it has proved difficult or complicated to inspect such glue joints, as they are located in the closed blade and thus are not easily accessible.

According to a preferred embodiment of the invention the polymeric compound is an exothermic curing compound, the area being heated by the exothermic curing poly- meric compound during and/or after the curing process during which the compound generates heat, the parts of an area with insufficient presence of polymer and thus with insufficient heating being determined by measuring the temperature. This method is particularly simple in that exposure to external heat or cold is not required. During the measurement the wind turbine blade has a different surface temperature at these parts of an area due to the lack of exothermic heat generation and due to the slow heating of the area caused by the insufficient presence of polymeric compound therein.

According to another embodiment the wind turbine blade is moved from first surroundings having a first temperature, eg outdoors, to other surroundings having a different, higher temperature, eg. indoors, or vice versa, the parts of an area with insufficient presence of polymer being determined by measuring the temperature, the change in temperature in these parts of an area being faster due to a lower heat capacity and reduced thermal conductivity. This embodiment is particularly simple and may be performed at any stage after the manufacture of the blade, eg shortly after de- livery to the customer. The method may also be used when the wind turbine blade has been in operation for some time and optionally subsequent to a blade breakdown to ascertain whether the breakdown was caused by defective production.

According to an embodiment the polymer is an exothermic curing polymer, eg poly- ester or epoxy.

According to an embodiment the temperature variations over the area are detected by means of a temperature sensor, which is moved over the area. By using this embodiment one and the same temperature sensor may be used for detecting the whole area, whereby calibration of the temperature sensor in relation to other temperature sensors is avoided.

According to an optional embodiment the temperature variations over the area are detected by means of an array of closely spaced apart temperature sensors, which are located adjacent the area. As a result a very fast detection of the temperature profile may be carried out. The temperature sensor(s) may be formed of infrared sensors. It is not required that such sensors are in contact with the surface whose temperature is to be measured.

According to an embodiment of the method an infrared camera is used to detect the temperature variations over the area. Images taken by the infrared camera clearly show the temperature profile over the photographed area. This method is particularly quick and simple to carry out. The temperature profile over the surface of the whole blade may thus be detected by means of a single or a few images, a large number sensors or movable sensors travelling over the surface of the blade are thus not nee- ded.

The invention relates furthermore to an apparatus for carrying out said method, said apparatus including a holder member having a contact face and mounted such that the contact face may support the outer surface of the wind turbine blade, and being characterised in that it further includes temperature meters for detecting temperature variations over the outer surface of the wind turbine blade.

According to an embodiment the holder member is a mould part having a mould cavity for the manufacture of a blade shell half, the contact face being provided by the surface of the mould cavity. This apparatus is particularly suitable for inspecting the quality immediately after the manufacture of the wind turbine blade and is particularly suitable for use in connection with exothermic curing polymers, where the heat generated in the curing process is utilised to detect the insufficient presence of polymer.

In the apparatus according to the invention the temperature meters may include a track provided with a temperature sensor displaceable in the longitudinal direction of the track, said track being mounted thus on the apparatus that it may adopt a position in which the temperature sensor is able to detect the temperature variations over the outer surface of the wind turbine blade.

According to the invention the temperature sensor may be an infrared camera. According to another embodiment the temperature meters may include a track provided with a plurality of temperature sensors mounted in the longitudinal direction of the track, said track being mounted thus on the apparatus that it may adopt a position in which the temperature sensors are able to detect the temperature variations over the outer surface of the wind turbine blade.

According to the invention the temperature sensor may be formed of one or several infrared sensors.

According to an embodiment of the apparatus the holder member may be mounted on a rack provided with wheels such that the apparatus may be used for the transportation and storage of the wind turbine blade. This embodiment is particularly advantageous in connection with the above method, in which the wind turbine blade is moved from first surroundings having a first temperature, eg outdoors, to other surroundings having a different, higher temperature, eg. indoors, or vice versa, the blade being moved from the first surroundings to the other surroundings by means of the apparatus.

According to yet another embodiment the apparatus according to the invention may include an additional mould part having a mould cavity for producing another blade shell half, the track being mounted such on the apparatus that it may adopt a position in which the temperature sensor(s) may detect the temperature variations over the surface of the mould cavity of the additional mould part. The two blade shell halves, which are produced in individual mould parts, are typically bonded by bringing the two mould parts together. After the bonding, the heat generation causes a heating of the surface of the mould cavity of the additional mould part due to the curing of the exothermic curing polymer adhesive. Immediately after the bonding the additional mould part is removed, and the surface of the mould cavity of the additional mould part may be more readily accessible than the surface of the wind turbine blade. Brief Description of the Drawings The invention is explained in greater detail below with reference to embodiments of the invention, in which the invention is used for the quality inspection of glue joints in wind turbine blades, as illustrated in the drawings, in which

Fig. 1 is diagrammatic sectional view of a wind turbine blade, equipment being shown in a position in which it is able to read the temperature of the outer surface of the wind turbine blade,

Fig. 2 is an image of two well executed glue joints, said image being taken by an infrared camera,

Fig. 3 is an image taken by an infrared camera and showing defective glue joints,

Fig. 4 is a diagrammatic view of an apparatus according to the invention, and

Fig. 5 is a diagrammatic view of a section of the apparatus according to the invention during use of the apparatus.

Best Modes for Carrying Out the Invention

Wind turbine blades are typically made by means of two blade shell halves of fibre- reinforced polymer. When moulded the two halves are bonded along the edges thereof and via one or more bracings, which prior thereto have been glued onto the inner face of one the blade shell halves. The other blade shell half is then arranged on top of bracings and glued thereto and along the edges thereof. It is fairly simple to glue the bracings onto the first blade shell half, while a correct fixing of the bracings in the other blade shell half and along the edge may prove more difficult inter alia as the now closed structure impedes an internal inspection thereof, especially at the blade tip. In a blade of about 29 metres a grown man is only able to crawl about 10 metres into the blade from the blade root before the space becomes too narrow. Fig. 1 is a diagrammatic cross-sectional view through such a wind turbine blade, in which two bracings 3, 4 being U-shaped in a cross-sectional view, are glued onto the lower blade shell half 2 and a reinforcement element 5 also is glued onto the lower blade shell half 2 at the leading edge of the blade. Adhesive is then applied to the upper horizontal flanges of the two bracings 3, 4, onto a projecting portion of the reinforcement element 5 glued onto the lower shell half 2 and onto an area of the inner surface of the lower blade shell half 2 adjacent the trailing edge of the finished blade. The glue joints 6 between the upper horizontal flanges of the bracings 3, 4 and the inner face of the upper blade shell half 1 include an adhesive provided between the upper face of the flanges and the inner face of the blade shell half 1 and the adhesive filling 7 between the ends of the flanges and the inner face of the blade shell half 1. The blade shell halves 1, 2 are made in separate moulds and the blade shell halves 1,2 are joined by lowering the upper blade shell half 1 down onto the mould housing the lower blade shell half 2. At a point in time, the curing process of the adhesive is so far advanced that the mould part of the upper blade shell half 1 can be removed. At this stage the reaction heat of the adhesive has heated the bracings 3, 4 and the upper blade shell 1 to such a degree that this heat is sufficient to enable a temperature sensor to measure the temperature of the outer surface of the blade for detection of an incomplete adhesive filling. If adhesive-deficient areas exist, the corresponding areas of the outer surface of the blade are not heated to the same extent as the glue joint areas provided with sufficient adhesive. This type of quality inspection does not prolong the production time, as it may be carried out at a stage when the blade in some cases has to be arranged in the lower mould part and cure typically for about three to four hours before it can be removed from the mould. The upper mould part may be removed for instance after one hour's curing time such that the surface temperature of the upper blade shell half 1 may be measured.

Fig. 1 is a diagrammatic view of an infrared temperature sensor 9 arranged adjacent the outer surface of the upper blade shell half 1. In the shown position, the infrared temperature sensor 9 is able to measure the heating of the upper blade shell half surface, said heating being caused by the adhesive between the upper horizontal flange of one bracing 3 and the upper blade shell half 1.

The temperature sensor 9 is shown in form of an infrared sensor detecting the infrared thennal radiation emitted by the outer surface of the blade shell half 1. According to a particular embodiment the temperature sensor may, however, be formed of an infrared camera, which is advantageous in that comparatively large areas and optionally several glue joints may be detected at the same time. Fig. 1 illustrates such an infrared camera 11, the lens thereof facing the outer surface of the upper blade shell half 1.

Fig. 2 shows an image taken by an infrared camera and in which the two glue joints 6 are clearly visible and extending substantially continuously as light streaks, ie heated areas. It is also apparent that an effective adhesion has been obtained in the photographed area.

Fig. 3 shows an image corresponding to that shown in Fig. 2, the glue joints 6, however, being incomplete in that in the areas indicated by F, an insufficient adhesive amount has been provided on the lower face of the upper blade shell half 1 resulting in a defective bonding in these areas. In the top half of Fig. 3, four adjacent bracings are arranged on the inner face of the upper blade shell half 1 for which reason a single wide glue joint 13 is provided.

The glue joint has a nominal thickness of 4 mm, but may due to tolerances vary between 1 mm and 10 mm. Successful tests have been carried out with the polyester- based adhesive FI 184 from the company of Reichold. Other types of adhesives, eg based on epoxy or polyurethane, may also be used.

Tests have shown that in practice a 5 mm thick fϊbreglass-reinforced polyester body is impervious to infrared beams. Consequently the heat increase measured on the face of the body opposite the glue joint is caused by thermal conductivity through the body. Due to the heat capacity and the thickness of the body, the heating of the opposite face of the body is delayed in relation to the exothermic reaction in the glue.

For carrying out the method according to the invention an infrared camera of the type Flir Therma CAM PM 695 from the company of Ashtead Technology may be used. This camera has a minimum resolution of 0.08° at 30°C, which is sufficient for the manufacture of wind turbine blades in that tests have shown that the difference in temperature between a correct glue joint filling and an incomplete glue joint filling typically ranges between 1-2°C.

The thickness of the blade shell ranges typically between 5 and 100 mm. Such a shell thickness offers a comparatively high heat capacity which jointly with the thermal conductivity properties of the composite material results in a comparatively long-lasting temperature increase, for which reason a quality control may be per- formed even after several hours. Tests with a blade of 29 metres have shown that even after two hours a fully adequate quality control may be carried out. The quality control may thus be carried out at time at which it does not interfere with the other stages of production.

In case the camera 11 had been arranged further spaced apart from the blade than shown in Fig. 1, the glue joint 10 at the leading edge of the blade and the glue joint 8 at the trailing edge of the blade would have been visible in the same image as the glue joints 6.

In the shown embodiment the method according to the invention is used to check glue joints. The method according to the invention may, however, also be used to check the blade shells per se to ascertain whether these have been filled with resin (exothermic curing polymer). As mentioned above, the blade shells are typically made by vacuum infusion, in which fibre mats are arranged in the cavities of the mould parts subsequent to which a vacuum bag is arranged on top of the fibre mats. By creating vacuum (typically 80-90%) in the cavity between the inner face of the mould part and the vacuum bag resin is sucked into and fills the cavity containing the fibre material. In order to obtain the optimum distribution of resin, so-called distribution layers and distribution channels are often used between the vacuum bag and the fibre material. In this connection the positioning of the resin inlets and the vac- uum ducts is important. However, it is often difficult to ensure a complete distribution of the resin material on the entire mould cavity and so-called dry spots occur, ie. areas containing fibre material which are insufficiently impregnated with resin. The method according to the invention is particularly useful for checking the resin filling in that such dry spots are detectable as dark or light areas on the images taken by an infrared camera. The control may for instance be carried out upon completion of the filling process, the images being taken from the inner face of the blade, before or after removal of the vacuum bag.

Dry spots adjacent the outer face of the blade shell be may checked by photograph- ing the outer face of the blade shell after demoulding. Optionally the upper mould part can be lifted to allow photographing of upper blade shell half, subsequent to which the upper mould part is reclosed, the mould is then rotated 180° such that the same procedure can be carried out with respect to the lower blade shell half.

The glue joints 6 between the bracings 3, 4 and the lower blade shell half 2 and the joints between the reinforcement element 5 and the lower blade shell half 2 may also be photographed from the inner face of the blade shell half before the upper blade shell 1 is arranged thereon.

According to the invention the glue joint may be checked by measuring the temperature of the inner face of the mould part to be used in the manufacture of the blade shell. The surface of the mould part is thus heated during or after the curing process of the adhesive, as it abuts the outer face of the blade shell during the curing process. The quality of the glue joint may thus be inspected after the blade or the blade shell has been removed from the mould part, which is advantageous in relation to the production. Fig. 4 is a diagrammatic view of an apparatus for the manufacture of wind turbine blades, each of the two blade shell halves 1, 2 being moulded in a separate mould part 16, 17 by vacuum infusion. In the shown embodiment, the two mould parts 17, 18 are interconnected by means of a hinge 18 such that when the mould part 16 is ro- tated about the hinge 18 the upper mould shell half 1 can be arranged on top of the lower blade shell half 2 for joining thereof. The apparatus shown includes temperature meters in form of two tracks 14, which via legs 19 may be arranged either on the outer face of the upper blade shell half 1 or on the inner face 15 of the mould cavity of the mould part 16. An infrared temperature sensor 9 extends from each of the tracks 14, said sensor being displaceable in the longitudinal direction of the track so as to detect temperature variations along the outer face of the blade shell half 1 or the surface of the mould cavity 15. It has not been shown how the temperature meters are arranged on the outer face of the blade shell half 1 or the surface of the mould cavity 15, but this may be effected by means of a lever arm system, which optionally is remote-controlled.

Fig. 5, which shows a section along the line V-V in Fig. 1, is a diagrammatic view of a portion of the apparatus shown in Fig. 4 in operation. As illustrated by the dual arrow P, the infrared temperature sensor 9 may be displaced in the longitudinal direc- tion of the track 14. The non-hatched areas F of the glue joint 6 between the upper blade shell half 1 and the upper horizontal flange of a bracing 3 illustrate parts of the glue joint 6 with insufficient presence of adhesive or wherein an air gap exists between the adhesive and the inner face of the upper blade shell half. The outer face of the blade shell half 1 is thus heated in these areas, which is detected by the tempera- ture sensor 9. Instead of infrared temperature sensors, other sensor types may be deployed such as thermocouples. The thermocouples have to be in contact with the heated surface.

FI 184 made by the company of Reichold and based on polyester has been used as exothermic curing adhesive. The glue joint typically has a nominal thickness of 4 mm, but may vary between 1 mm and 10 mm. Such a glue joint thickness ensures that sufficient heat is generated during the curing process to allow a satisfactory measuring of the temperature variations on the outer face of the blade shell due to the thermal conductivity through the blade shell.

The method according to the invention is particularly suitable in connection with adhesive materials based on cold-setting polyester types, epoxy or polyurethane. Other adhesive types based on exothermic curing polymer may also be used.

The invention is not restricted to the above embodiments. The temperature meters of the apparatus may for instance be embedded in the mould parts or mounted on the back of the shell bodies defining the mould cavity of the mould parts.

The method according to the invention is not restricted to the above embodiments, but may also be used in other production methods, in which a curing polymer mate- rial is used, eg injection moulding.

As the described above the heat generation caused by the exothermic curing of the polymer has been utilized. According to another aspect of the invention, by exposing the wind turbine blade to an external cold or heat source the insufficient presence of polymer in the blade structure may be determined, eg by arranging a heater mat on the surface of the blade.

A subsequent measuring of the temperature shows that the temperature decreases more slowly at "dry spots" or at non-filled glue joints, the heat not being conducted away from these areas at a high rate.

In another method the wind turbine blade is arranged for a period of time in surroundings having a first temperature, eg outdoors, and subsequently is moved to surroundings having a different temperature, eg indoors, "dry spots" in the blade shell and/or non- filled areas of the glue joints being heated faster due to the reduced thermal conductivity. At these two methods it is naturally of no importance whether the used polymer is exothermic curing or not. The method may be used at any point in time, ie a long time after the manufacture of the blade.

List of reference numerals

1 Upper blade shell half

2 Lower blade shell half.

3 Bracing.

4 Bracing.

5 Reinforcement element

6 Glue joint

7 Glue joint

8 Glue joint

9 Temperature sensor

10 Glue joint

11 Infrared camera

12 Lens

13 Glue joint

14 Track

15 Upper surface of the mould cavity

16 Mould part

17 Mould part

18 Hinge

19 Leg

P Motion direction of the temperature sensor

Claims

1. Method of inspecting the quality of a wind turbine blade including a blade shell (1,2) of fibre-reinforced polymer and areas to which fluid or viscous, curable polymeric compound (6, 7, 8, 10) has been supplied, characterised in that the wind turbine blade is exposed to heat or cold, that the temperature variations are measured over an area of the surface of the wind turbine blade and that the parts (F) of an area with insufficient presence of polymer are determined, the temperature thereof differing from that of the parts of an area containing polymer.

2. Method according to claim 1, characterised in that the blade shell (1, 2) is made by vacuum infusion, in which a fluid curable polymeric material is injected into the fibre material.

3. Method according to claim 2, characterised in that the insufficient presence of injected polymeric compound in the blade shell (1,2) is determined by measuring the temperature.

4. Method according to one of the claims 1-3, characterised in that the thickness of the blade shell (1, 2) is at least 5 mm, preferably at least 10 mm.

5. Method according to one of the claims 1-4, characterised in that the blade shell is formed of two blade shell halves (1, 2) being bonded along their edges by means of glue joints (8, 10) of a fluid or viscous and curable polymeric compound to form a wind turbine blade, parts (F) of an area with insufficient presence of polymer in the glue joints (8, 10) being determined by measuring the temperature.

6. Method according to one of the preceding claims, characterised in that the wind turbine blade includes one or more bracings (3, 4), which by means of glue joints (6, 7) of a fluid or viscous and curable polymeric compound are bonded to and interconnect the inner faces of the two blade shell halves (1, 2), parts (F) of an area with insufficient presence of polymeric compound in the glue joints (6, 7) being determined by measuring the temperature.

7. Method according to one of the preceding claims, characterised in that the polymeric compound is exothermic curing, the area being heated by the exothermic curing polymeric compound during and/or after the curing process, during which the compound generates heat, the parts (F) of an area with insufficient presence of polymeric compound and thus with reduced heating being determined by measuring the temperature.

8. Method according to one of the claims 1-6, characterised in that the wind turbine blade is moved from first surroundings having a first temperature, eg outdoors, to other surroundings having a different, higher temperature, eg. indoors, or vice versa, and that the parts of an area with insufficient presence of polymer is deter- mined by measuring the temperature, the change in temperature in these parts of an area being faster due to the lower heat capacity and reduced thermal conductivity.

9. Method according to one or more of the preceding claims, characterised in that the polymer is an exothermic curing polymer, eg polyester or epoxy.

10. Method according to one of the claims 1-9, characterised in that the temperature variations over the area are detected by means of a temperature sensor (9), which is moved over the area.

11. Method according to one of the claims 1-9, characterised in that the temperature variations over the area are detected by means of an array of closely spaced apart temperature sensors which are located adjacent the area.

12. Method according to one of the claims 1-11, characterised in that infrared sensors are used as temperature sensors.

13. Method according to one of the claims 1-12, characterised in that an infrared camera (11) is used to detect the temperature variation over the area.

14. Apparatus for carrying out the method according one or more of the preceding claims, said apparatus including a holder member (17) provided with a contact face (15) and mounted such that the contact face (15) may support the outer surface of the wind turbine blade (1, 2), characterised in that the apparatus further includes temperature meters (9) for detecting the temperature variations over the outer surface of the wind turbine blade (1, 2).

15. Apparatus according to claim 14, characterised in that the holder member is a mould part (17) having a mould cavity for the manufacture of a blade shell half (2) and that the contact face is formed of the surface (15) of the mould cavity.

16. Apparatus according to claim 14 or 15, characterised in that the temperature meters include a track (14) provided with a temperature sensor (9) displaceable in the longitudinal direction of the track, said track being mounted such on the apparatus that it may adopt a position in which the temperature sensor (9) is able to detect the temperature variations over the outer surface of the wind turbine blade (1, 2).

17. Apparatus according to claim 16, characterised in that the temperature sensor is formed of an infrared camera.

18. Apparatus according to claim 14 or 15, characterised in that the temperature meters include a track provided with a plurality of temperature sensors mounted in the longitudinal direction of the track, said track being mounted such on the apparatus that it may adopt a position in which the temperature sensors are able to detect the temperature variations over the outer surface of the wind turbine blade (1, 2).

19. Apparatus according to claim 16 or 18, characterised in that the temperature sensor is formed of an infrared sensor (9).

20. Apparatus according to one of the claims 14-19, characterised in that the holder member (17) is mounted on a rack provided with wheels such that the apparatus may be used for the transportation and storage of the wind turbine blade.

21. Apparatus according to claim 15 and one of the claims 16-19, characterised in that it includes an additional mould part (16) having a mould cavity for producing another blade shell half (1), and that the track (14) is mounted such on the apparatus that it may adopt a position in which the temperature sensor(s)(9) may detect the temperature variations over the surface (15) of the mould cavity of the additional mould part (16).