Abstract:
A gas accumulator for the storage of biogas, comprising three gas-impermeable membranes, the first of which at least partially defines a gas accumulation chamber and the third of which is attached to the second by means of a gas-impermeable seal, such that the second and third membranes together define a gas-impermeable pressurisation chamber, as well as comprising means for pressurising said pressurisation chamber and means for anchoring the gas accumulator to a support surface, characterised in that the anchoring means is configured to anchor only the second membrane to the support surface.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to membrane gas accumulators that can be used to collect and store bio-gas. 
       BACKGROUND TO THE INVENTION 
       [0002]    Membrane gas accumulators are commonly used to collect and store bio-gas released by the anaerobic respiration of various bacteria within sewage or other rotting organic matter 
         [0003]    Some existing gas accumulators consist of two membranes. The first of these is an inner gas-impermeable membrane which sits on the ground or on a support surface such as a concrete slab and forms at least part of a gas accumulation chamber. The membrane must therefore either be anchored to the ground in a gas-impermeable manner, or may form the complete gas accumulation chamber. 
         [0004]    The second membrane is an outer gas-impermeable membrane. This membrane at least partially encloses the first membrane, and protects it from external forces. The outer membrane must therefore be strong and durable enough to resist forces caused by wind and other adverse weather conditions. In the event of failure or decompression of the outer membrane, all external forces will be transferred to the inner membrane, and therefore the inner membrane must also be structurally able to withstand external forces. 
         [0005]    A pressurisation chamber is formed between the two membranes. The pressurisation chamber can be inflated or deflated to regulate the flow of gas into and out of the gas accumulation chamber. For example, the pressurisation chamber can be inflated to cause gas in the gas accumulation chamber to be released at a constant rate once the gas accumulation chamber is full. The pressurisation chamber has a mechanism for the introduction and extraction of an auxiliary gas, in order to maintain and adjust the pressure within the chamber. This gas is normally air, as it is cheap and readily available, though other gases can be used. 
         [0006]    In much of the prior art, edges of the membranes are attached to a base support surface to secure the membranes to the surface. The attachment formations used may have to be air and gas tight, and so are laborious and expensive to install. 
         [0007]    Existing membrane gas accumulators have both the first and second membranes anchored to a base support surface, typically using staples or clamp plates which engage with ropes, rods or directly with the fabric, clamping via a keder or bolt rope edge or similar. This means that installing the accumulator is time-consuming, as potentially two rows of staples or bolted clamp plates must be inserted and attached to the membranes around the entire circumference of the accumulator. 
         [0008]    Existing gas accumulators can also be expensive. Both the first and second membranes must be of a material that is strong enough to withstand the stresses of forces such as wind and bad weather. Putting the gas accumulation chamber under such stresses may also increase the risk of damage to the membrane, which may result in gas leaks. More material is also needed to create the attachments on both membranes. 
         [0009]    A variety of existing membrane gas accumulators has three membranes. The third membrane is attached in a gas-impermeable manner to at least the second membrane, and helps to define the pressurisation chamber. The pressurisation chamber is therefore not defined by the first and second membranes, but by the second and third membranes. 
         [0010]    This creates a layer of non-pressurised gas between the gas accumulation chamber and the pressurisation chamber. In other gas accumulators, any gas leak from the gas accumulation chamber would enter the pressurisation chamber, which would then contain a dangerous and potentially explosive mix of gases. In three-membrane accumulators, any gas escaping from the gas accumulation chamber enters the layer of air between the first and the third membranes, and can thereby escape the gas accumulator. Any gas leak can also be noticed quickly and easily, as a leak from the gas accumulation chamber cannot be compensated for by the pressurisation chamber, as can happen in models with two membranes if the two chambers are connected by the leak. 
         [0011]    Different means for monitoring the volume of the gas accumulation chamber are known in the art. In an existing embodiment, this monitoring means consists of an element that is suspended in the pressurisation chamber and partially rests on the third membrane. As the gas accumulation chamber fills, the third membrane takes some of the weight of the element. The change in tension due to the weight of the portion of the element that remains suspended is detected by a device outside the gas accumulator. 
         [0012]    Such a monitoring system does not connect the two membranes, and thus does not place additional stress on the membranes, thereby reducing the probability of damage. However, there is still some possibility of damage by the element to the first membrane. This monitoring system would also not work unless the second membrane can form a dome shape without the first membrane being full and providing pressure to hold the dome up. This method is therefore not suitable for all designs of gas accumulator. 
         [0013]    Known gas accumulators also have a problem with damage from rodents. Rodents such as mice and rats eat the PVC that the membranes are often made of, thereby causing damage and potential leaks. 
       SUMMARY OF THE INVENTION 
       [0014]    According to a first aspect of the invention there is provided a gas accumulator for the storage of bio-gas, comprising: a first gas-impermeable membrane, which at least partially defines a gas accumulation chamber; a second gas-impermeable membrane; a third gas-impermeable membrane, which is attached to the second membrane by means of a gas-impermeable seal such that the second and third membranes together define a gas-impermeable pressurisation chamber; means for pressurising said pressurisation chamber by introduction of an auxiliary gas; and means for anchoring the gas accumulator to a support surface; characterised in that the anchoring means is configured to anchor only the second membrane to the support surface. 
         [0015]    In the gas accumulator of the present invention the anchoring means only engages with the second membrane. This reduces the time needed to install the gas accumulator, compared to existing gas accumulators in which both membranes are anchored. By anchoring only the second membrane the first membrane need not be load bearing, and therefore can be made of a lighter and possibly cheaper material than in the prior art. Additionally less material may be needed to anchor only the second membrane, which may further reduce the cost of the gas accumulator in comparison to prior art systems. 
         [0016]    The anchoring means may comprise a staple which engages with a pole running through a fabric pocket attached to the second membrane, the staple having free ends which are received in the support surface. Such anchoring means are cost effective and do not take a long time to install. 
         [0017]    It is to be appreciated, however, that other anchoring means may be used, for example a series of D rings fixed to an outer surface of the second membrane via catenary webbing which itself is stitched to the third membrane to engage via a hook with an anchor ring that is secured in the base support surface. 
         [0018]    A base of the gas accumulation chamber may comprise the first membrane. This creates a gas-impermeable gas accumulation chamber, without the anchoring means having to provide a gas-impermeable seal. The anchoring means can therefore be mechanical, and can be cheap and fast to install. 
         [0019]    The gas accumulator may provide a layer of non-pressurised gas between the pressurisation chamber and the gas accumulation chamber. 
         [0020]    All three membranes are flexible and gas-impermeable, such that if gas escapes from the gas accumulation chamber, it enters this layer of non-pressurised gas rather than entering the pressurisation chamber, which is potentially dangerous. 
         [0021]    The pressurisation chamber may be provided with a valve for the introduction and extraction of the auxiliary gas. This allows control of the rate of gas being expelled from the gas accumulation chamber when it is full. 
         [0022]    The auxiliary gas preferably is air, as air is cheap and readily available, though other gases can be used. 
         [0023]    The gas accumulation chamber may comprise means for extracting and introducing gas, for example by means of a valve mounted in a flange on the first membrane. 
         [0024]    The gas accumulator may further comprise means for detecting the volume of gas in the gas accumulation chamber. 
         [0025]    The means for detecting the volume of gas in the gas accumulation chamber may comprise an ultra-sound level detector. This allows the volume of the gas accumulation chamber to be monitored without connecting the first and third membranes, thereby preventing stress on the first membrane and the possibility of damage. 
         [0026]    A flange may be provided at an apex of the second membrane of the gas accumulator in which the ultrasound level detector can be received. 
         [0027]    The first membrane may be made of a material which is lighter in weight than that of the second and third membranes. 
         [0028]    For example, the first membrane, which forms the non-structural gas accumulation chamber, may be made of laminated and calendered PVC and the second and third membranes, which are structural, may be made of Polyester reinforced PVC. 
         [0029]    The second membrane may extend beyond the point at which the fabric pockets are attached to the second membrane. 
         [0030]    In use of the gas accumulator, an edge of the second membrane may be disposed beneath the gas accumulation chamber. This impedes access to the first membrane to rodents, thereby protecting the first membrane from damage and potential gas leaks. 
         [0031]    A condensate drain may be provided towards the base of the gas accumulation chamber. This allows excess liquid to be drained from the gas accumulation chamber. 
         [0032]    The first membrane may be covered by a material having a lubricating and anti-static characteristic. This helps to prevent damage to the membrane due to friction when it contacts the third membrane. 
         [0033]    The material may be, polyurethane coated glass fibre fabric for example. 
         [0034]    The gas accumulator may be generally dome shaped when full. 
         [0035]    Alternatively, the gas accumulator may be generally cylindrical when full. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which: 
           [0037]      FIG. 1  is a cross-sectional view of a gas accumulator when the gas accumulation chamber is full; 
           [0038]      FIGS. 2  is a cross-sectional view of a section of a base edge of the gas accumulator; and 
           [0039]      FIG. 3  is a cross-sectional view of a section of a base edge of the gas accumulator showing alternative anchoring means. 
       
    
    
     DESCRIPTION 
       [0040]    Referring first to  FIG. 1 , a gas accumulator is shown generally at  10 , and includes a first, flexible and gas-impermeable membrane  12 , a second, outer, flexible and gas-impermeable membrane  14  and a third, intermediate flexible and gas-impermeable membrane  16 . 
         [0041]    The first membrane  12  has a base or floor portion and a roof portion and defines a gas accumulation chamber  18  which, when filled with gas, assumes a generally dome-like shape. 
         [0042]    The second membrane  14  fully encloses the first membrane  12 , also forming a generally dome-like shape. 
         [0043]    The third membrane  16  is attached to the second membrane  14  by means of a gas-impermeable seal and, with the second membrane, defines a pressurisation chamber  20 . 
         [0044]    Between the first membrane  12  and the third membrane  16  is a layer of non-pressurised gas  22 . Any gas escaping from the gas accumulation chamber  18  enters this layer  22  and can escape the gas accumulator  10 . 
         [0045]    As all three of the membranes  12 ,  14 ,  16  are flexible and gas-impermeable, if gas escapes from the gas accumulation chamber  18 , it enters this layer of non-pressurised gas rather than entering the pressurisation chamber  20 , which is potentially dangerous. This arrangement also allows any leaks to be detected quickly, as in the event of a leak from the gas accumulation chamber  18  the gas accumulation chamber  18  will deflate, causing the shape of the gas accumulator  10 , or at least the shape of the gas accumulation chamber  18 , to change. 
         [0046]    The first and third membranes  12 ,  16 , or sections thereof may be coated or otherwise provided with a low friction and anti-static material or with a material having a lubricating characteristic, for example, polyurethane coated glass fibre. This helps to prevent damage to both membranes  12 ,  16  due to friction when the coated sections are in contact with one another. 
         [0047]    The second membrane  14  is anchored to the base support surface  24  by anchoring means  26 , as is described in more detail below. 
         [0048]      FIG. 2  is a view of a section of a base edge of the gas accumulator  10 , showing how the gas accumulator  10  is anchored to the ground or to another support surface. A fabric pocket  34  is attached to the outside of the second membrane  14 , and a pole  36  runs through the fabric pocket  34 . This pole runs around the circumference of the gas accumulator  10  through additional fabric pockets  34  disposed around a perimeter of the second membrane  14  of the gas accumulator  10 , and is retained by staples  38  whose legs are embedded in the ground or the support surface, thereby anchoring the second membrane  14  to the base support surface  24 . 
         [0049]      FIG. 3  is a view of a section of a base edge of an alternative embodiment of the gas accumulator  10 . In the embodiment shown in  FIG. 3 , a webbing belt  40  is attached to the outside of the second membrane  14 . The webbing belt  40  runs around the circumference of the gas accumulator  10  generally describing a continuous catenary edge and incorporating, at intervals (which may be, for example,  1 . 5  metres), D rings  42 . The D rings  42  are attached to anchor rings  44  which are embedded in the ground or the support surface  24 , via clips  46 , thereby anchoring the second membrane  14  to the base support surface  24 . 
         [0050]    It will be appreciated from the description above that only the second membrane  14  is anchored to the ground or the support surface, and thus only the second membrane  14  must be load bearing. As the second membrane  14  completely encloses the first membrane  12 , the second membrane  14  provides support and protection for the first membrane  12 . 
         [0051]    As the first membrane  12  need not be load bearing, it can be made of a lighter weight material than the second and third membranes  14 ,  16 . For example, the first membrane  12 , which forms the non-structural accumulation chamber, can be made of laminated and calendered PVC, and the second and third membranes  14 ,  16 , which are structural, can be made of Polyester reinforced PVC. The material of the first membrane  12  may be less expensive than that of the second and third membranes  14 , 16 , due to its lighter weight and lower specification. 
         [0052]    As is shown in  FIG. 2 , the second membrane  14  extends beyond the point at which the fabric pockets  34  or webbing  40  are attached to the second membrane  14 , thereby defining an edge of the second membrane  14 , which in use of the gas accumulator  10  can be disposed beneath the gas accumulation chamber  18 , as shown in  FIGS. 2 and 3 . This helps to prevent damage to the first membrane  12  by rodents and other animals, thereby helping to prevent leaks from the gas accumulation chamber  18 . 
         [0053]    A condensate drain  28  is provided towards a base of the first membrane  12  and allows excess liquid to be drained from the gas accumulation chamber  18 . 
         [0054]    Auxiliary gas such as air is introduced into and extracted from the pressurisation chamber  20  through a valve  30 , which is provided in part of the second membrane  14 . 
         [0055]    Gas is introduced and extracted from the gas accumulation chamber  18  through a valve in a flange  32 , which is part of the first membrane  12 . 
         [0056]    A flange (not shown) may be provided towards an apex of the outer second membrane  14 , and may include a recess or other receiving formation for receiving means for detecting the volume of gas in the gas accumulation chamber  18 . In one example, the means for detecting the volume of gas in the gas accumulation chamber is an ultrasonic level detector, which can be used to monitor the volume of gas in the gas accumulation chamber  18  without connecting the first and third membranes  12 ,  16 , thereby preventing stress on the first membrane  12  and reducing the possibility of damage to the first or third membranes  12 ,  16 . 
         [0057]    In use of the gas accumulator  10 , gas to be stored is deposited in the gas accumulation chamber  18  by means of the valve  32 , until the gas accumulation chamber  18  is full. When gas is to be released from the gas accumulation chamber  18 , the valve  32  in the gas accumulation chamber  18  is opened and air or another auxiliary gas is pumped into the pressurisation chamber  20  via valve  30 , thereby pressurising the gas accumulation chamber  18  and causing gas to escape via the valve  32 . Air is preferred as the auxiliary gas as it is freely available, but it will be understood that other gases may be used if necessary or desired. 
         [0058]    The gas accumulation chamber  18  of the gas accumulator  10  described above and shown in  FIGS. 1 and 2  is defined by the floor and roof portions of the first membrane  12 . In other words, the first membrane  12  of the gas accumulator  10  shown above forms a gas impermeable dome for storing gas. 
         [0059]    Although the gas accumulator  10  described above and shown in  FIGS. 1 ,  2  and  3  is generally dome-shaped, it will be appreciated by those skilled in the art that the principles of the present invention are equally applicable to other shapes of gas accumulators, such as, for example, cylindrical or pyramid-shaped gas accumulators.