Abstract:
An innovation is disclosed which relates to a wind turbine foundation. A circular foundation using fiber reinforced concrete has optional circular reinforcement rods. The foundation includes a vertical stanchion that rests in the bottom of an excavated hole and supports anchor bolts and reinforcement bars in a predetermined configuration while concrete is poured into the hole. All the necessary foundation materials can be combined in a simple and compact kit which can be shipped to a customer.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No. 60/900,287 which was filed on Feb. 8, 2007 and which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a wind turbine foundation. Specifically the invention relates to the design of a foundation kit for monopole towers used with small and residential-scale wind turbines. 
     BACKGROUND OF THE INVENTION 
     Wind turbines are renewable energy devices that are being deployed in greater numbers as awareness grows of fossil fuels&#39; disadvantages. The largest percentage of installed cost for wind turbines—certainly for small wind turbines—is attributable to towers and foundations. Development of innovative towers and foundations that have the potential for simplifying turbine installation and reducing cost, will promote more widespread deployment. Of particular interest are free-standing (no guy wires or ancillary supports) monopole towers. They are favored for their visual appeal and maintenance simplicity, but are more costly than guyed towers. 
     Traditional turbine foundations use a square cross section of the foundation, which is not an efficient shape. While the entire block contributes mass that is useful in resisting the applied overturning moment, the moment arm to the corners can not be assumed because there is no a priori assurance of the loading direction. The corners of the block are essentially wasted space and materials. It would be advantageous to provide a tower foundation created in a shape with no wasted space and materials, resulting in a lower cost. 
     Another aspect of prior art towers that adds to the cost of installation is having the entire top of the foundation above-ground. This requires extensive forms to create the desired shape and structure, necessitating more materials, design, and labor to install. It would be advantageous to provide a tower foundation which does not require extensive forms to create an above-ground portion. 
     These towers must have stringently engineered bases able to withstand the forces presented by the turbine and tower. The standard method is to use structurally reinforced concrete. Prior art used grids of rebar manually fastened together, set in concrete, to provide the structure and support. This method is very time consuming and costly. The possibility of error during the fabrication is also significant. It would be advantageous to provide a tower base which does not require tedious, time consuming, and costly assembly with possibility of error. 
     Prior art tower foundations are generally presented to the customer in the form of plans. The customer then must acquire the materials, cut them, and otherwise prepare them, before assembling the foundation. For people not familiar with the materials, or without the tools to properly manipulate them, this can be a very time-consuming process, with great room for error. It would be advantageous to provide a kit containing all necessary parts for the assembly of a tower foundation. 
     SUMMARY OF THE INVENTION 
     The disclosed invention provides an improved foundation for use with wind turbine monopole towers. The innovation accomplishes a significant reduction of material costs and construction labor. The foundation is located below grade, with only a small stub pier exposed, accomplished using a specific concrete forming method and longer anchor bolts. It enhances the use of plain structural concrete by using fiber additives to reduce concrete cracking. It is characterized by a preferred circular (or multi-sided polygonal) cross sectional shape and a stanchion to capture helically arranged reinforcement and precisely locate anchor bolts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages according to embodiments of the invention will be apparent from the following Detailed Description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a prior art square foundation using structurally reinforced concrete. 
         FIG. 2  shows a plan view of a prior art mat foundation. 
         FIG. 3  shows a plan view of prior art foundation forms. 
         FIG. 4  shows a section view of a circular improved foundation. 
         FIG. 5  shows a plan view of an improved foundation, detailing the forms 
         FIG. 6  shows a section view of an improved foundation detailing the forms 
         FIG. 7  shows the stanchion, templates, and anchor bolt cage 
         FIG. 8  shows blowout view of the assembly of the bolt cage. 
         FIG. 9  shows the foundation using a stanchion and reinforcement bars. 
         FIG. 10  shows the foundation using a stanchion, reinforcement bars and reinforcement using a steel cable 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventions disclosed herein entail improvements to wind turbine foundation design. 
     A prior art mat foundation  5 , is shown in  FIG. 1 . The cross-section, as shown in  FIG. 2 , is that of a square. A tower  7  is attached to the foundation  5 . The foundation width and depth are determined by engineering analyses, taking into consideration the applied loads (overturning moment, side shear and axial force), soil conditions, frost depth, anchor bolt configuration and other factors. Steel reinforcing bars  10  are placed orthogonally a certain distance apart so as to create a reinforcing grid  15   a  and  15   b  in both horizontal directions. Detail of the reinforcing bars  10  and the reinforcing grid  15   a  and  15   b  are shown in  FIG. 2 . The bars  10  are fastened together at their intersections using wire ties, as is typical of structurally reinforced concrete. One of the most tedious tasks in constructing the mat foundation  5  is tying the reinforcing bars  10  at their intersections. In a typical installation there are two 9×9 meshes of reinforcing bars  10  accounting for 162 intersections that must be fastened. The bars  10  themselves, being round in cross section, tend to roll about and are difficult to securely fasten. 
     One such reinforcing grid  15   a  (see  FIG. 1 ) is placed at the bottom of the excavation, usually several inches above the exposed soil. Placement details are governed by building codes aimed at avoiding corrosion (rust) of the steel from moisture infiltration through the concrete and reducing the likelihood of concrete cracking. The bottom reinforcing mesh  15   a  is placed on “chairs” (concrete spacer blocks) that elevate it above the soil. Placing the reinforcing grid  15   a  at the bottom of the foundation  5  is also difficult. Either the chairs  20  must be placed on the base of the excavation in a pattern that will support the reinforcing grid  15   a , or the chairs  20  must be attached to the reinforcing grid  15   a  itself before lowering into the excavation. 
     It is not possible to conveniently step into the excavation because the reinforcing grid  15   a  is too narrow to allow placement of a worker&#39;s foot in a grid space. Furthermore, experience shows that some of the chairs  20  inevitably break away, leaving concern that the reinforcing grid  15   a  will settle to the bottom of the excavation under the pressure of the concrete poured on top of it. 
     A second reinforcing grid  15   b  is placed near the top of the foundation by suspending it, usually with wires, from form boards  25 . Although not quite so onerous as the bottom reinforcing grid  15   a , placing the top reinforcing grid  15   b  is time consuming because it must be suspended by wires from the concrete forms  25 . For the baseline mat foundation  1  in the present application, the form boards  25  are also used to suspend an anchor-bolt cage  30 , which will later be used to bolt the turbine tower  7  to the foundation. 
     The baseline mat foundation has numerous problems that add to construction time and money. First, the concrete forms  25 , using multiple parts shown in  FIG. 3 , must be constructed, typically using standard dimensional lumber and various fasteners. To obtain the proper dimensions and ensure structural integrity, an effort of several hours is required in addition to the cost of materials. Either before or after the forms  25  are constructed, the foundation excavation must be laid out. Keeping the sides parallel and the layout square, and in cases where a particular alignment is desired, excavation layout can also be time consuming. 
     The present invention, an improved foundation  35  avoids the many negative aspects of the prior art mat foundation  5 . The first innovation, is the use of a circular or near-circular cross-section foundation  35  installed in a hole in the ground  100 . The hole  100   h  is defined by an open top  100   t , at least one side  100   s , and a bottom  100   b . The at least one side  100   s  extends from the open top  100 t to the bottom  100   b . As can be appreciated, the hole  100   h  is eventually filled with concrete  100 C as shown in  FIG. 4  and explained further herein. In  FIG. 4 , a tower (not shown) attaches to anchor bolts  40  which are part of an anchor bolt cage  30 . The anchor bolt cage  30  connects to a stanchion  50 , which supports the entire structure. Optionally, reinforcement bars  10  are attached to the stanchion  50 , and are optionally wrapped with a steel cable (not shown in this figure) to provide further reinforcement. Ropes  52  tied off to stakes  52   s  in the ground or other fastening as apparent to one skilled in the art are used to position the anchor bolt cage  30 , the weight of which is supported by a base on the stanchion  50 , this is detailed in reference to  FIG. 7 . 
     The first advantage of this approach is simplicity of layout. Installation requires identifying the desired tower location, placing a stake in the ground at that location, and use a string to circumscribe a circle of the desired excavation diameter. The extremities of the excavation are marked with additional stakes or spray paint. Digging the excavation by hand is possible. Typical excavation equipment can not dig a circular section, but a polygon is perfectly acceptable. In one possible configuration of the present invention, the polygon is a dodecagon (12 sided polygon) and the straight edges of the polygon are approximately 18 inches wide, resulting in a 72 inch circle circumscribing the polygon. Thus, a back hoe or excavator with an 18″ bucket could excavate the foundation in a series of straight lines. This method of using a circular or near circular cross-section foundation  35  requires fewer materials resulting in a lower cost. 
     The use of concrete forms  25  in prior art foundations requires a significant investment of materials cost and labor hours. Therefore, the present invention does not use extensive forms  25 . Forms  25  in the present invention are only used to support anchor bolt cage  30 , which extends above the top of the concrete. The concrete is poured to grade level at the extremities of the excavation, unless (a) the slope of the grade precludes this, or (b) edge forms are desired for aesthetic purposes. If forms  25  are used, they are extremely simple to construct and install, again simply laying out the circumference of a circle using thin plywood or steel (landscaping) edging. The improved foundation  35  uses few simple stringers to create a form structure  25 , as shown in  FIG. 5 . A cross-section detailing the form is shown in  FIG. 6 . 
     In another possible embodiment, the foundation of the present invention does not use any wood forms. In this embodiment, the excavation is filled with concrete to six inches below grade and then a stub pier surrounding the anchor bolts  40  is filled with concrete to two inches above grade. A section of SONOTUBE™ is used to form the stub pier. The SONOTUBE™ is attached to the anchor bolt templates with brackets. This approach eliminates the need for any wood forms at all. 
     A purpose of the improved foundation  35  is to support a wind turbine tower (not shown), which is bolted to the foundation  35  using anchor bolts  40  embedded in the concrete. Details of an anchor bolt cage  30  assembly are shown in  FIGS. 7 and 8 . The placement of anchor bolts  40  are shown in  FIG. 7 . Placing these anchor bolts  40  precisely, and maintaining them in a desired position while concrete is poured, is an important requirement of foundation construction. This is done by using an anchor bolt cage  30 , shown in  FIG. 7 . The precision is accomplished by fastening two anchor bolt templates  55  to the top of the stanchion  50 . The anchor bolt cage  30  is then wrapped with one or more circular supports  56  made of rebar or other appropriate material as would be apparent to one of ordinary skill in the art. The circular supports  56  are attached to the anchor bolts using wire ties (not shown) or other types of ties as would be apparent to one of ordinary skill in the art. To facilitate the placement of the anchor bolts  40  and facilitate stability during concrete pouring, the anchor bolts may be secured to the pipe of the stanchion using a stanchion brace  56   s . The stanchion brace  56   s  extends radially from the pipe. The distal ends of the arms  56   a  have bores  56   b , which may be a plurality of bores. The anchor bolts extend through one or more of the bores  56   b  and may be fitted with nuts such that the anchor bolts are coupled to the stanchion brace  56   s  to facilitate the placement of the anchor bolts  40 . This provides extra support to the structure. The entire structure is held upright by a base  58 . Base  58  is shown in an X-shape having extensions in a radial direction having a first dimension d 1  larger than a second dimension d 2  of the pipe. 
     Choosing ABS pipe for the stanchion  50  permits the use of standard ABS fittings to secure the templates  55 . The stanchion  50  may be formed of other materials or shapes in place of ABS pipe as would be apparent to one of ordinary skill in the art. The templates  55 , can be made from ½-inch-thick plywood or other materials such as plastic plate material or sheet metal. In the preferred embodiment, the templates  55  are formed from inch thick plywood. The templates  55  must be made of a material that is sufficiently stiff and sufficiently thick to provide a relatively rigid placement of anchor bolts  40 . An assembly sequence for the stanchion and anchor bolts are shown in  FIG. 8 . If the design of the anchor bolt cage  30  is modified from the embodiment described herein, the assembly sequence may differ from the description below. First, a female adapter  60   a  is placed on the top of the stanchion  50  with a threaded end facing up. Next, template  55   a  is secured to the stanchion  50  by threading a male adapter  65   a  through a hole in the center of the template and into the female adapter  60   a . A short section of ABS pipe  70  is inserted into a female end of the male adapter  65   a . A female adapter  60   b  is then placed on top of the short section of ABS pipe  70 . A second template  55   b  is secured to the stanchion  50  by threading a male adapter  65   b  through the template  55   b  and into the female adapter  60   b . Anchor bolts  40  are inserted through the two templates  55  with spacers  80  (can be made of short sections of ABS or PVC pipe) in between. Hexagonal nuts  85  ( FIG. 7 ) are fastened to the anchor bolts  40  at the top of the top template  55   b  and the bottom of the bottom template  55   a , thereby creating a rigid cage for the anchor bolts  40  resting atop the stanchion  50 . In a similar manner to that described above, rigid non-metallic (PVC) electrical conduit  90  is assembled to the templates  55  so as to provide for electrical wire pull after the concrete is poured. 
     The assembly in  FIG. 7  is necessary for anchor bolt placement, but it is also necessary to hold the entire assembly in place during the concrete pour. This can be done in the following manner (not shown in FIGURES) or using another method as determined by one skilled in the art. The stanchion  50  and anchor bolt cage  30  is first lowered into the excavation—this can be done manually by two or three people, depending upon the quantity and weight of reinforcement. Adjustable guy lines, ratchet straps or other tie-downs  52  (shown in  FIG. 4 ) are used to fasten four anchor bolts  40  to four stakes pounded into the ground. The bolts  40  are chosen to form two opposed pairs (such as north-south and east-west). A carpenter&#39;s level is placed on the top of two opposing anchor bolts  40  and the guy lines are adjusted to achieve level in that direction. The procedure is repeated for the two anchor bolts in the direction 90 degrees opposed to the first two. In this manner, the anchor bolts  40  can be leveled and secured prior to pouring concrete. 
     The entire structure described in the previous figures must be supported by concrete of some form. A foundation using only standard concrete is prone to cracking and other degradation. In prior art this was prevented by using rebar or other structural supports, as is described in  FIGS. 1-3 . This method has many expensive and tedious elements, as described above. It is possible to avoid the use of all structural reinforcement if fiber reinforced concrete is used. 
     Fiber reinforced concrete is available through companies such as Propex Concrete Systems under names such as Fibermesh®. Fibermesh® uses fibers which are evenly distributed through the mix of concrete, giving strength to the entire foundation, not just where structural reinforcement exists. The synthetic fibers add tensile strength and prevent cracking. Fibermesh® product is added directly to the concrete as it is mixed, making the use simple, efficient, and cost effective. Using fiber-reinforced concrete for a wind turbine tower, coupled with the above-described method of creating a bolt cage  30  presents an innovation that will help to further the use of wind turbines. 
     In some situations, fiber reinforced concrete may not be considered a strong enough option,  FIG. 9  and  FIG. 10  show methods which can be used to further reinforce the foundation.  FIG. 9  shows a stanchion  50  with holes (not shown) drilled in a helical pattern for insertion of reinforcement bars  10  that span the excavation. The diameter, length, spacing, number and material type of the reinforcing bars  10  are determined from structural analyses. As an example, suppose it was decided to use (12) #6 (¾ inch diameter) rebar spaced 30 degrees apart around a 3.5 inch diameter stanchion. The space between drilled hole centerlines located circumferentially around the stanchion would be equal to the circumference of the stanchion divided by twelve, or π(3.5)÷12=0.916 inches. The vertical separation between the drilled holes  95  could be determined by one of ordinary skill in the art, but to keep the helix as tight as possible, the vertical separation is taken to be the diameter of the reinforcement bars  10 . The reinforcement bars may be made of rebar, fiber bars, or other materials as determined by one of ordinary skill in the art. For situations where the materials are to be shipped, lighter material such as fiber bar may be desirable to lower shipping costs. 
     As an example implementation of the improved foundation  35 , the stanchion  50  may be constructed of black ABS plastic, which is inexpensive, readily available and easily fabricated. However, many other materials are possibilities, such as PVC plastic, cast polyurethane and injection molded plastic, and others as determined by one of ordinary skill in the art. The benefit of this design is the simplicity of the stanchion  50  supporting the reinforcement bars  10 . For instance, if 12 reinforcement bars  10  are inserted through the stanchion  40  creating 24 segments, reinforcement is distributed uniformly throughout the foundation  35  without the need for wire ties or “chairs” 30. This process can be completed very rapidly. 
     Use of the round or polygonal foundation shape and the stanchion  50  supporting reinforcement bars  10  serves to simplify assembly and distribute reinforcement uniformly around the foundation. However, this pattern may need amplification to ensure the structural integrity of the concrete and avoid troublesome cracking. Thus, steel cables  97  may be attached to the ends of the reinforcement bars  10  to add further structural integrity as shown in  FIG. 10 . The steel cables  97  are fastened with cable clamps (not shown) or other fasteners as determined by one of ordinary skill in the art. 
     The entire improved foundation  35  is designed for easy packaging and shipment as a kit. This allows the end user to purchase all the necessary materials in one place, not requiring costly time and energy to acquire all the necessary parts. The only materials besides the kit that must be procured are the concrete, and optionally some standard lumber if needed for form creation. All other parts are shipped in a convenient box complete with detailed installation instructions. This innovation will make construction of tower foundation cheaper, faster, and easier for both experienced installers and first-time users. This innovation ensures that proper materials will be used, reducing risk of foundation failures. This innovation allows the above-described innovations to be used by many people in need of a simple and strong foundation for mounting a wind turbine or other tower. 
     There are many useful embodiments of the disclosed invention not all of which have been described specifically in the preceding disclosure but will be evident to one skilled in the art.