A gas meter for a biogas digester

Abstract

A smart biogas meter 300 for a biogas digester comprises: a venturi pipe component 310, connector pipes 332, 336a, 336b, a static pressure sensor 334, a differential pressure sensor 338, a printed cir

Description

Field of invention The present invention relates to a gas meter for a biogas digester, and a method for remotely monitoring a biogas digester. The gas meter may be a remote gas meter which is connected (via a communication means) to a server. Background Household biogas digesters often comprise small digester units used in the production of biogas from waste matter (e.g. agricultural waste, manure, sewage, or food waste). Produced biogas can be used in household appliances such as gas cookers, refrigerators and heaters, acting as an affordable and clean fuel. Such biogas digesters are particularly advantageous in remote rural locations (since, unlike alternative heat / energy sources, there is generally no need to transport in fuel). In rural operations, operators necessarily must often travel long distances to visit, monitor and maintain various biogas digester sites. Additionally, it may not be immediately apparent at each site that any problem has occurred with a digester system, so faults may go unnoticed for a significant period between maintenance visits. Biogas digesters are also often used in peri-urban and urban scenarios, though with typically less extreme advantages and disadvantages due to their more likely proximity to alternative fuel sources and sources of technical support. Summary of the invention Aspects and embodiments of the present invention are set out in the appended claims. These and other aspects and embodiments of the invention are also described herein. According to an aspect of the invention, there is provided a gas meter for a biogas digester, comprising: at least one pressure sensor for measuring the pressure of gas emitted by a biogas digester (optionally in an output pipe of a biogas digester); the at least one pressure sensor being separated from the emitted gas (while operating so as to measure the pressure of emitted gas). Said separation is a physical separation, such that the emitted gas does not contact the at least one pressure sensor. In other words, the at least one pressure sensor indirectly measures the pressure of gas output by the biogas digester, in that measurement is based on a measurement of the pressure of a fluid that is not the gas output by the biogas digester; preferably wherein said fluid is air. The at least one pressure sensor may thus be described as being isolated from the emitted gas. This may advantageously shield and protect the at least one pressure sensor from the gas emitted by the biogas sensor. The at least one pressure sensor is configured to measure air pressure (i.e. it is not generally a pressure sensor which is specifically configured for measurement of pressure in biogas). The gas meter is preferably used in a bypass configuration on an output pipe of a biogas digester. The static gas pressure / maximum gas pressure of the output of the biogas digester in operation is under 20 kPa, preferably under 15 kPa, more preferably under 10 kPA, still more preferably under 8 kPa (for a concrete dome digester) or under 2 kPA (for a bag digester). Preferably, the gas meter further comprises at least one fluid conduit (i.e. a tube or pipe) connecting the pressure sensor to an output of a biogas digester; wherein the at least one fluid conduit comprises means for separating the at least one pressure sensor from gas emitted by the biogas digester. Preferably, the means for separating comprises a liquid (or alternatively a gas) held in the at least one fluid conduit. Liquid may be held in the fluid conduit between air (proximate the pressure sensor) and the gas output from the biogas digester. In this manner gas pressure may be transmitted to the pressure sensor across the liquid but gas is prevented from directly interacting with the pressure sensor, which protects the sensor from any harmful and / or corrosive component gases. Preferably, the means for separating comprises a trap for retaining liquid. This trap may assist in presenting leakage of the fluid onto the pressure sensor or into the biogas output tube. Preferably the trap is a U-bend. This configuration may be convenient when the at least one conduit is used vertically, as the liquid remains within the U-bend. Preferably, the liquid is at least one of: a solution that is insoluble to H2S (and optionally air); and a solution that reacts with H2S in such a way as to produce an inert liquid (preferably a non-toxic and / or non-corrosive liquid), such as an alkaline solution such as calcium nitrate solution. The liquid may also be an alkali solution, and a solution in which H2S (and optionally air) is not dissolvable. These options may be effective in preventing H2S gas from reaching the pressure sensor. H2S is a corrosive gas which is commonly present in biogas. Preferably, the fluid conduit is formed of a material that is resistant to H2S, preferably Polyvinyl chloride (PVC) or Tygon E-3603. The fluid conduit may remain in contact with any H2S present in the biogas, and so use of a material resistant to the gas may reduce the corrosion damage. Preferably the at least one pressure sensor comprises a differential pressure sensor, more preferably connected to at least two fluid conduits. This may allow gas flow to be calculated using the recorded value of differential pressure. Preferably the gas meter further comprises a pipe component connecting the output of the biogas digester to further pipework, wherein the at least one (static) pressure sensor is connected to the pipe component (optionally via respective fluid conduits). Preferably, the differential pressure sensor is connected over a constriction (i.e. a restriction in width) in the pipe component, such that a venturi tube arrangement is formed. This arrangement may cause the gas pressure to drop within the constriction, and the differential pressure measured can be used to calculate flow rate. Use of a venturi tube may limit loss of pressure. The constriction is preferably formed as a cone having a convergence angle of between 30 and 60 degrees, preferably between 40 and 50 degrees, and more preferably 45 degrees. This angle may allow for the gas to flow in laminar flow (allowing the venturi / pipe component to work), while simplifying manufacture of the component (as compared to manufacturing the shallow angles used in typical venturi arrangements). The pipe component may further comprise a first chamber having a first diameter and a second chamber having a second diameter, the first and second chambers being joined by the construction, preferably wherein the first diameter is larger diameter than the second diameter. According to a further aspect, there is provided a venturi component, having a first chamber with a first diameter; a second chamber with a second diameter, the second diameter being smaller than the first diameter, and a cone (i.e. a shoulder region) connecting the first and second chambers, wherein a convergence angle of the cone is between 30 and 60 degrees, preferably between 40 and 50 degrees, and more preferably 45 degrees. This angle may allow for the gas to flow in laminar flow (allowing the venturi component to work), while simplifying manufacture of the component (as compared to manufacturing the shallow angles used in typical venturi arrangements). According to a further aspect, there is provided a pipe component (i.e. a fluid conduit) for a gas meter for a biogas digester, comprising a pipe with a constriction; the constriction being formed as a cone having a convergence angle of between 30 and 60 degrees, preferably between 40 and 50 degrees, and more preferably 45 degrees. At least two sensing apertures are provided in the pipe wall. Preferably, at least one sensing aperture is provided on each side of the constriction. Providing a single pipe component with connections for both the differential pressure and static pressure measurements may facilitate ease of use. The first diameter may be between 10 and 30 mm, preferably between 15 and 25 mm, more preferably between 19 and 21 mm, and still more preferably 20 mm. The second diameter may be between 4 and 12 mm, preferably between 6 and 10 mm, more preferably between 9 and 11 mm, and still preferably 8 mm. The length of the first chamber may be between 20 and 40 mm, preferably between 25 and 35mm, more preferably between 28 and 33 mm, still more preferably between 30 and 31 mm, and most preferably 30.7 mm. The length of the second chamber may be between 35 and 55 mm, preferably between 40 and 50 mm, more preferably between 43 and 47 mm, still more preferably between 45 and 46 mm, and most preferably 45.5 mm. The length of the cone (i.e. the height of the cone) may be between 4 and 8 mm, preferably between 5 and 7 mm, and more preferably 6 mm. The length of the venturi / pipe component may be between 70 and 95 mm, preferably between 80 and 85 mm, more preferably between 81 and 83 mm, and still more preferably 82.2 mm. Preferably, the pipe / venturi component further comprises struts which may improve the strength and structural integrity of the pipe component and which may limit distortion of the element. Preferably the gas meter further comprises a (further) static pressure sensor. This may allow the measurement of both static pressure and differential pressure across the constriction. Preferably the gas meter further comprises a processor configured to record the pressure of the gas as measured by the at least one pressure sensor, preferably in a local cache (optionally wherein the recorded pressure is saved in the cache either continuously or at intervals), more preferably wherein the processor and / or cache is provided on a printed circuit board. This may allow calculation and analysis locally at the site of the meter. Preferably the printed circuit board is configured to record the differential pressure and calculate the gas flow rate. Preferably, the processor is configured (optionally, to record the differential pressure and) to calculate the gas flow rate based on pressure data, and more preferably to calculate the gas consumption. The printed circuit board may record these data locally. Data may be recorded with a timestamp. Preferably the processor is configured to send data to a server via a communication network. This may allow the data to be monitored remotely and may facilitate sophisticated analysis on an external server. Optionally, the processor further comprises a transmitter in this regard. Preferably the gas meter further comprises a battery and a photovoltaic panel for charging the battery. This may facilitate the meter being self-sufficient and may decrease expenses incurred in running the meter. Preferably the gas meter further comprises a casing / housing for enclosing various components of the gas meter. The gas meter may further comprise a temperature sensor, which may be a thermistor. This may allow the calculations to be compensated for temperature, which may improve the accuracy of the calculated parameters. Preferably the gas meter further comprises an actuated valve, preferably for shutting off the output of the biogas digester. This may enable the gas flow to be switched on and off remotely. More preferably, the actuated valve is a latching actuated valve, which may minimize power requirements and improve safety. Preferably the actuated valve is configured such that the risk of a spark being caused is minimized, as this could cause the biogas to ignite. Preferably, the valve, when closed, is configured to open when a predetermined threshold gas pressure is detected; preferably wherein the gas meter notifies a user accordingly. This may allow the biogas digester to operate as a Pay As You Go system while avoiding unnecessary venting of gases. According to a further aspect, there is provided a method of using a biogas digester, comprising: providing a biogas digester; closing an outlet pipe of the digester using a valve; detecting a predetermined threshold (static) gas pressure in the digester; and opening the valve upon detecting the predetermined threshold gas pressure. The method may further comprise allowing gas in the digester to displace slurry in the digester; wherein slurry is outlet via a slurry outlet. The method may further comprise notifying a user to use the biogas. The valve may be an actuated valve and / or may be controlled remotely. According to a further aspect there is provided a gas meter for a biogas digester, comprising an actuated valve for shutting off the output of the biogas digester. According to a further aspect there is provided a gas meter for a biogas digester, comprising a flow rate sensor. This may allow metering to take place. The gas meter is preferably arranged to measure static pressure and differential pressure. The flow rate sensor is preferably a differential pressure sensor. The gas meter may comprise means for transmitting collected data to an external server, optionally for analysis. In general, performing flow-rate sensing for biogas digesters is challenging as digesters are low pressure systems and contain corrosive gasses, meaning there are very limited methodologies that will accurately measure biogas flow rate without compromising the gas flow rate (by increasing the pressure loss in the pipeline). The present invention provides an arrangement based on a venturi tube which may allow such accurate measurement with reduced pressure loss. According to a further aspect, there is provided a system comprising a plurality of biogas digesters having a respective plurality of gas meters as described herein; and a server. Optionally, the plurality of gas meters send data to the server. Optionally, the operation of the plurality of biogas digesters may be configured and / or controlled based on the data received at the server. The gas meter preferably comprises a pipe having a pipe constriction, a static pressure sensor, and a differential pressure sensor, the differential pressure sensor being connected to the pipe across the pipe constriction. The pipe constriction is preferably in a pipe component connecting the output of the biogas digester to further pipework. According to a further aspect, there is provided a method of using a gas meter for a biogas digester, comprising: providing at least one pressure sensor for measuring the pressure of gas emitted by a biogas digester (optionally in an output pipe of a biogas digester); separating the at least one pressure sensor from the emitted gas; and optionally using the at least one pressure sensor for measuring the pressure of gas emitted by a biogas digester. According to a further aspect, there is provided a method for remote monitoring of a biogas system, comprising: using a gas meter, monitoring at least one parameter of gas emitted from a biogas digester; recording the monitored at least one parameter locally on the gas meter; and transmitting data associated with the at least one parameter to a remote server. The gas meter is optionally a gas meter as described herein. The at least one parameter preferably comprises at least one of: static pressure, differential pressure, flow rate, and gas consumption. Preferably the method further comprises at the remote server, optionally using a trained model, detecting patterns in the received data, and generating at least one further parameter based on said determination. The method may comprise training the model (i.e. training pattern recognition software), optionally using machine learning algorithms. Detecting patterns may also involve using statistical analysis, preferably clustering algorithms. Preferably the pattern recognition comprises recognizing patterns related to fault scenarios. Fault scenarios may include a gas leak, reduced or unexpectedly high or low pressure, reduced or unexpectedly high or low flow, no flow, water in the pipeline, and / or any measurement of one or a combination of flow, static pressure and temperature that are irregular and not consistent with regular operation. Preferably, the software / model is also trained to recognize the patterns which indicate a fault is likely to be imminent. The method may comprise detecting anomalies in the data thereby to identify fault conditions. Preferably the method further comprises producing an alert in dependence on the generated further parameter; preferably wherein said alert prompts a user to take action (optionally via a user interface). This can alert the user that maintenance is required, for example if a slow leak is detected. Preferably the further parameter comprises at least one of the following parameters: gas usage, gas consumption, and feeding / consumption habits of a biogas digester. Gas usage may encompass usage habits by a particular gas appliance, for example a gas cooker. This may include gas consumption and / or cooking time. An indication of the corresponding number of bottles of liquid petroleum gas (LPG) or firewood or other fuel source this consumption would have required may be provided. The consumption data may be used for metering purposes for Pay As You Go (PAYG) financing models and / or to obtain Carbon Credits. Feeding habits of the digester may provide data which can be used to improve the running of a biogas digester. Preferably, the further parameter is an indication of a fault condition and / or a predicted future event. Preferably the further parameter is used to produce maintenance instructions. As the software is trained to recognize patterns and determine the status of the system, it can also provide tailored maintenance instructions for each scenario. This may include providing instructions that a pipe should be checked for leaks after a fault scenario has been detected which suggests that a leak has occurred or is likely to occur. Advantageously, this may prevent a system manager being required to travel between remote sites to check the equipment and to maintain it, unless such travel is necessary. An indication of preventative maintenance that could be performed before any fault has been occurred may be provided after the system has indicated that a fault might be imminent. Preferably, the method further comprises outputting data and / or analysis to a user interface. The data and / or analysis may be presented graphically and / or as values, which may provide the user with a clear overview of the parameters of the system remotely. Preferably, the method further comprises outputting maintenance instructions. The gas meter may be the gas meter as described herein. According to a further aspect, there is provided a method for remote monitoring of a biogas system, comprising: measuring at least one parameter; recording at least one parameter locally; and transmitting data on at least one parameter to a remote server; wherein the at least one parameter comprises at least one of: static pressure, differential pressure, flow rate; and gas consumption. According to a further aspect, there is provided a method for remote monitoring of a biogas system, comprising: receiving data from a gas meter; (optionally using a trained model for) detecting patterns in the received data, and generating at least one further parameter based on said determination. The data is preferably flow rate data. According to a further aspect, there is provided a method for remote monitoring of a biogas system, comprising: measuring static gas pressure and differential pressure, and transmitting the data to a remote server. Preferably, the method further comprises calculating flow rate. Preferably, the method further comprises calculating gas consumption. Preferably the method further comprises saving at least one parameter locally, the at least one parameter being at least one of: static gas pressure; differential gas pressure; flow rate and gas consumption. The at least one parameter may be saved along with a timestamp. In general terms, there is provided a smart biogas meter which is capable of measuring properties related to gas pressure and flow. The smart biogas meter is connected to a server configured to monitor a biogas digester remotely and perform analysis on the received data. This analysis can provide efficient oversight of biogas digesters and inform their preventative maintenance. As used herein, the term “biogas digester” is used to refer to a system which processes waste matter into biogas, preferably via anaerobic fermentation. The term “biogas digester” is used synonymously with the term “biogas plant”. As used herein, the term “biogas” is used to connote any gas produced by anaerobic fermentation of waste matter; preferably a gas that can be used as fuel. As used herein, the term “meter” is used to connote a measuring instrument. As used herein, the term “U-bend” is used to connote any pipe incorporating U-shaped curves or similar and encompasses S-shaped pipes and the like. Brief Description of Figures One or more aspects will now be described, by way of example only and with reference to the accompanying drawings having like-reference numerals, in which: Figure 1 illustrates an arrangement of a biogas digester incorporating a smart biogas meter according to the present invention; Figure 2 illustrates an embodiment of the smart biogas meter system according to the present invention; Figure 3 illustrates a perspective view of an embodiment of the pipe component; Figure 4 illustrates a schematic diagram of an embodiment of a connector pipe; and Figure 5 illustrates an example user interface reporting the readings of the smart biogas meter system. Detailed Description Figure 1 provides a schematic overview of the manner in which the smart biogas meter of the present invention is incorporated into a household biogas digester. Household biogas digesters use waste matter, performing anaerobic fermentation in the presence of methanogenic bacteria, to produce a methane-rich gas, which can be used for cooking and other household tasks such as electricity generation. This provides a clean alternative to conventional fuels such as kerosene or firewood and reduces greenhouse emissions and indoor air pollutants. The digester also promotes a circular economy by utilizing household waste to produce energy. The by-products can be further used as fertilizer. Once installed, a biogas digester therefore has relatively low running costs. It provides a clean, renewable and low-cost energy source, which is particularly beneficial to rural developing communities. Post-installation, there is a need to prevent breakdowns and malfunctions. The biogas digester 100 contains a slurry 180 of water and waste matter, which undergoes anaerobic fermentation to produce biogas typically containing methane (CH4), carbon dioxide (CO2) and hydrogen sulphide (H2S). Household digesters typically have a capacity (maximum volume of slurry and biogas) of around 4-10 m3, although community and commercial plants may have a capacity of 40 m3 or greater. The digester has a slurry inlet 120 and a slurry outlet 140. These facilitate the slurry 180 being easily inserted and removed, for example to then be reused as fertilizer. Covering the top of the digester 100 is a plastic bag 160 (or alternatively a concrete dome or floating plastic dome), which defines the chamber into which the biogas is released. The chamber is fluidly connected to a pipe 200, through which the biogas flows towards the use point (e.g. a gas cooker 400). Arrows provided in Figure 1 indicate the direction of this flow. Branching from the pipe 200, there is typically provided a gas pressure release U-bend 220 containing a volume of fluid. The fluid is typically water, occupying approximately 20 cm of the U-bend. Further along the pipe 200 in the direction of flow of the biogas is provided a main gas valve 240 to control the flow. The smart biogas meter 300 according to the present invention is located between the main gas valve 240 and the biogas plant 160, through it will be appreciated that the biogas meter could instead be located anywhere on the pipe 200, including between the main gas valve 240 and a gas appliance such as a gas cooker 400 (where such positioning may be particularly suitable for retrofitting the biogas meter 300). Figure 2 provides a schematic view of the smart biogas meter 300. The system comprises a pipe component 310 (which interfaces with the aforementioned pipe 200), connector pipes 332,336a,336b, a static pressure sensor 334, a differential pressure sensor 338, a printed circuit board 342 containing a GSM board, a battery 350 and a photovoltaic panel 360. The pipe component 310 is used to interface the main biogas meter 300 with the gas flowing through the pipe 200. The pipe component 310 is provided fluidly connected to the pipe 200 of the biogas plant, such that it is positioned within the flow path of the biogas. As biogas pressures are typically small (up to 2 kPa for a digester including a bag, or up to 8 kPa for a digester including a concrete dome), the system is configured to have a small form factor. The dimensions of the pipe component 310 are configured to ensure a wide range of differential pressure, while at the same time reducing pressure loss in the biogas pipe 200 and allowing laminar flow of the biogas. The pipe component 310 has threaded ends 312 to engage with the pipe 200 in a secure connection. The pipe component 310 is formed as a venturi tube arrangement comprising two chambers, a first chamber 314 having a larger (20 mm) diameter and a second chamber 318 having a smaller (8 mm) diameter. The direction of flow of gas is from the first chamber to the second chamber (this is indicated by an arrow in Figure 2). The chambers are arranged such that the biogas flows first through the first chamber 314, the diameter of the first chamber then reduces linearly to form a constriction in a shoulder region 316, and the gas then flows through the second chamber 318. The diameter of the shoulder 316 reduces linearly from the diameter of the first chamber 314 to the diameter of the second chamber 318. The shoulder region is formed as a cone having an angle of 45° relative to the sides of the first and second chamber (i.e. relative to the horizontal; referred to as the convergence angle), which is steeper than the angle used in a typical venturi arrangement. . The diameters of the first and second chambers 314,318 and the angle of incidence of the shoulder 316 are all precisely controlled dimensions used to calculate the flow rate from the differential pressure difference between the first and second chambers. Specifically, the total length of the pipe component 310 is 82.2 mm, where the length of the first chamber 314 is 30.7 mm (to the chamfered edge of the shoulder 316), the shoulder is 6 mm long, and the length of the second chamber 318 is 45.5 mm (to the chamfered edge of the shoulder 316). The pipe 200 has a (interior) diameter of 20 mm (or % inch) adjacent the first chamber 314, and a (interior) diameter of 15 mm (or% inch) adjacent the second chamber 318. In other words, the pipe 200 is formed of two differently sized pipe sections either side of the pipe component 310. A first upper outlet 324 is provided in the second chamber, which is connected to the static pressure sensor 334 via connector pipe (or fluid conduit) 332. Biogas may contain hydrogen sulphide (H2S), which is a poisonous and corrosive gas and can cause damage to the piping and components of the system. Therefore, the pipe 332 provides a means for separating the pressure sensor 334 from gas emitted by the biogas digester. The pressure sensor 334 therefore measures the gas pressure indirectly, based on a measurement of the pressure of a fluid that is not the gas output by the biogas digester. In the embodiment shown, a volume of liquid is placed within the connector pipe 332, which is configured in a U-bend arrangement to act as a trap to keep the fluid in place. Therefore, the pressure sensor 334 measures the pressure of the gas on the other side of the volume of water - typically this will be air. In this way, the sensor 334 does not interact directly with the biogas. The pipe connector 332 is formed from a material which is resistant to hydrogen sulphide, for example Polyvinyl chloride (PVC) or Tygon E-3603. Hydrogen sulphide can dissolve in water leading to the formation of sulphuric acid, which is a strongly corrosive acid. Therefore, the liquid provided within the pipe connector 334 is an alkali solution or a solution in which H2S is non-dissolvable. In a similar arrangement, two upper outlets 328a,328b are provided in the first chamber 314 and second chamber 318 respectively and are connected to two pipe connectors 336a,336b. These connectors also contain a means for separating the pressure sensor 338 from gas emitted by the biogas digester. In the embodiment illustrated, the pipe connectors 336a,336b are configured in the same manner as the first U-bend pipe described above, formed of an H2S resistant material and within which there is provided a liquid barrier against hydrogen sulphide gasses and sulphuric acid. U-bend pipes 336a and 336b connect the first and second chambers 314, 318 to a differential pressure sensor 338. As such, the differential pressure sensor is connected ‘across’ the shoulder 316, such that the pipe component 310 operates as a venturi arrangement. Static pressure sensor 334 is a piezoresistive pressure sensor, which provides a linear voltage output according to the pressure. The pressure sensor 334 is capable of measuring pressure between 0 to 10 kPa with a sensitivity in the voltage readouts of 5.5 mV / kPa. By way of example, an NXP™ MP12GP sensor may be used. Differential pressure sensor 338 is a CMOS sensor having a very high sensitivity to small pressure differences, as the small size of the pipe component 301 means there is only a small change in flow. It can perform pressure measurements in the range of -12.5 Pa to 125 Pa with a zero-point accuracy of 0.08 Pa and a span accuracy of 3% of the reading. A flow rate of up to 840 cc / s can be measured. Optionally, it is temperature compensated and calibrated on-chip. By way of example, a Sensiron™ SDP816-125 Pa (or alternatively 500 Pa, depending on the expected flow rate) differential pressure sensor may be used. Both the static pressure sensor 334 and differential pressure sensor 338 are able to function in the presence of a high water vapour and are formed from materials which have a reduced likelihood of being affected by corrosive gases such as epoxy, polybutylene terephthalate (PBT), glass, and silicone seals. The differential pressure readings are used to calculate the flow rate using standard equations fora venturi arrangement. Both the static pressure sensor 334 and the differential pressure sensor 338 reside in a casing 340 which acts to protect the circuit elements within. This casing 340 is formed of IP-rated 65 nylon and is in two parts which form a front and back portion. The front portion is removable in order to access the circuit elements within the casing. The back portion comprises screw holes through which screws may be passed to attach the casing to a wall. Within the casing 340 is provided a printed circuit board (PCB) 342 (also referred to as a processor), which is connected to a global system for mobile communication (GSM) network via a GSM board 344, which can accommodate a subscriber identity module (SIM) card. Alternatively or additionally, a low-frequency radio (LoRa) board and / or means for connecting via Bluetooth® to a user device (optionally where the user device has a GSM connection) may be provided. The PCB 342 is further connected to a battery 350 such as a lithium ion battery. By way of example, this may be a 3.7v 18650 lithium ion battery. The PCB 342 is further connected to a photovoltaic panel 360, which is positioned external to the casing by a user for optimal sunlight exposure. This may typically be a 3 W 6 V solar panel connected via an extended cable. The battery 350 is configured to have at least three days battery life (from a fully charged state) in the absence of sunshine. Figure 3 provides an external perspective view of an embodiment of the pipe component 310 (where reference numerals corresponding to those of Figure 2 have been used). The pipe component 310 is formed from a material resistant to H2S, such as a high density polyethylene (HDPE). The pipe component 310 comprises ribbed ends 312, which engage with the pipe 200 to fluidly connect the pipe component to the biogas digester. The ribbed ends 312 are configured to engage with the pipe 200, facilitating a gas-tight seal to avoid leakage of the biogas and to prevent any pressure loss at the join between the pipe 200 and the pipe component 310. Upper outlets 324,328a,328b have tapered pipe ends, which engage with the U-bend pipes 332,336a,336b to form a secure attachment without loss of pressure. There are further provided concentric shields 325,329a,329b around each outlet such that the ends of the U-bend pipes are accommodated in a recessed ring encircling the outlets 324,328a,328b. This can aid the seal of the outlets 324,328a,328b and U-bend pipes 332,336a,336b, and protect the join from any external damage caused for example by corrosion or accidental knocks. The shields are connected by struts 326 which run along the pipe component 310 longitudinally between the shields and increase the strength and structural integrity of the shielding arrangement. Further longitudinal struts 327 are provided along the length of the sides of the pipe component 310, configured to strengthen the arrangement and hinder distortion of the plastic. As discussed above, the dimensions of the pipe component 310 are precisely controlled in order to ensure the accuracy of the flow determination. The longitudinal struts 327 may assist in preventing distortion (for example due to torsional stresses orwarping) of the dimensions of the pipe component 310, which would render the flow calculations (using standard equations for a venturi arrangement) inaccurate as the calculations are calibrated according to the specific dimensions of each pipe component 310. The differential pressure sensor is configured to measure low flow rates. Accordingly, the pipe component 310 has a small form factor and minor distortions have a large relative effect on the dimensions, and as such the potential to have a large effect on the calculations. Figure 4 provides an illustrative arrangement of a pipe connector 332,328a,328b connecting the pipe component 310 and the sensors 334, 338. The pipe connector is arranged as a pipe comprising two substantially semi-circular U-bend curves, a lower curve 412 and an upper curve 414. Provided within the lower curve 412 is a volume of fluid 416. This arrangement is sometimes referred to as an ‘S-trap’. The fluid 416 provided is chosen to prevent any H2S or sulphuric acid which may be present in the biogas from reaching the sensors 334, 338 when the pipe connectors are used in an upright position, as previously mentioned. The fluid 416 may be replaced periodically during standard maintenance of the biogas digester, as otherwise an alkali and / or calcium nitrate solution may become fully neutralized, for example. A pressure tight join 418 is provided between the U-bend pipe 332,328a,328b and the sensor 334,338, and between the U-bend pipe 332,328a,328b and the pipe component 310 (connection not shown) so that an accurate measurement of the pressure within the pipe component 310 may be taken. Optionally, a coefficient of performance may be added into the pressure calculations to take into account any pressure loss caused by the liquid in the U-bends. When the present invention is implemented in connection with a biogas digester, the biogas produced flows through the sensor system 300 from the pipe 200. As the biogas passes from the first chamber 314 to the second chamber 318, the velocity of the gas increases and the pressure decreases. This change in pressure across the pipe component constriction is measured by the differential pressure sensor 338. The flow rate of the gas can be calculated from the differential pressure. Then, the flow can be integrated to determine the gas consumption. These calculations are performed locally on site by the meter 300. The static pressure of the gas as it passes through the second chamber 318 of the pipe component 310 is also measured by the static pressure sensor 334 (optionally, the static pressure before the constriction may also be calculated based on the static pressure measurements either on the meter or at the server). These data are transmitted via the PCB 342 and GSM board 344 to a remote smart biogas web-application server or other software system for analytics to be performed and presented via a user interface. Meters according to the present invention can be installed in a network of biogas digester systems to form a network of sensor nodes, which can be monitored remotely by a user at a central point. The network of sensor nodes can therefore cover remote locations. Once installed, the smart biogas meter 300 monitors and records the static pressure and gas flow of the biogas digester and performs some processing and pattern recognition in location. The smart biogas meter 300 itself records the static pressure and differential pressure and calculates the flow rate and gas consumption. Average pressure and average flow rate are calculated and recorded every minute, for the previous minute, and gas consumption is calculated and recorded every hour. This arrangement balances minimizing data transmission and data granularity. The more frequent recording of pressure and average flow rate enables more sophisticated analysis of trends. The meter stores these values (with a timestamp based on a clock of the PCB) locally in the PCB RAM (which is used as a buffer / cache), and sends them to the server once an hour. In this way the meter maintains tracking of the values even if the connection to the server goes down. Additionally, the smart biogas sensor 300 is configured to recognize locally from the static pressure reading when excess gas is being produced (which may lead to this excess gas being vented from the system), and optionally also to recognize other important patterns (indicative of major faults). A more sophisticated pattern recognition software system is provided on a cloud server. Using the recorded gas pressure and flow data overtime, complemented with additional contextual data (e.g. geography, feedstock, climate, income, agricultural practices, demography, family size, etc.) the software can recognize patterns in the received data from one or a plurality of connected biogas digestors related to system functions such as cooking time, digester status, carbon saved, digester feeding habits, and digester faults. Through the use of statistical analysis of the data (specifically, flow and / or static pressure, and sometimes including temperature against time), diagnostic, predictive and prescriptive analysis takes place and user habits can be identified. Diagnostic analysis includes (but is not limited to) anomaly analysis, for example using clustering algorithms, to identify certain fault scenarios. Predictive analysis uses the dataset to predict certain events, such as the gas is about to start venting based on known rules and or past outcomes. Prescriptive analysis then suggests actions for the user to take, based on the diagnostic and predictive analysis, for example warning the user to check the pipe for water build up or to check the pipe for leaks and turn off the main gas valve In an example of diagnostic analysis, a slow leak may be indicated by gas pressure failing to return to maximum after biogas use or a reduction in pressure without a corresponding flow being detected. Alternatively, a gradual decrease in pressure over a long period of time may indicate the digester is not working at optimum capacity. Through remotely monitoring the network of sensor nodes and recognizing such patterns, the software can encourage preventative maintenance. In addition, the software may further utilize other data-driven machine learning (ML) and / or artificial intelligence (Al) protocols, which may involve learning data trends for specific biogas plant types and predicting faults based on this learnt data set. The biogas plant digester system may be connected to one or more gas appliances such as a gas oven and hobs and simultaneously used to fuel heating systems. When a gas cooker is the one of the end appliances of the biogas digester system, available cooking time can be calculated from a direct correlation with flow rate. A sudden and significant increase in flow rate is an indication that the gas is being used. In particular, the use of biogas for cooking is typified by a sudden and steady loss of pressure in the digester for a fixed dome digester (though not a digester including a plastic bag, which tends to operate at a fixed pressure). The timespan over which such indications are measured provides the cooking time. It is important to know the type of digester used in the biogas plant so that typical pressure patterns can be determined to provide a reference typical pressure pattern. Additionally, information on the gas appliances used within the biogas plant, such as the number of burners on a hob, can be fed into the data input into the server to improve pattern recognition. For example, this helps to differentiate between the typical flow rate when the gas is being used for cooking and any changes in flow rate when a leak has occurred. The software can be configured to use the gas pressure, flow rate and consumption data to recognize a range of scenarios, some examples of which include: • Gas leaks • Changes in peak flow rate • Changes in gas pressure • Normal flow rate profiles • Abnormal flow rate profiles • Normal gas pressure profiles (for example during and after use) • Abnormal gas pressure profiles • Water present in pipes • No gas flow • Low or high gas flow • Underfeeding of digester • Overfeeding of digester • Methane venting into atmosphere A slow leak is likely to be characterized by a small flow (for example, <0.2 m3 / hour) detected for a sustained period of time (for example >30 s) in the case where the leak is down stream of the biogas meter. Where the leak is upstream of the meter, the leak may be characterized by a slow loss in static pressure without any corresponding flow rate, indicating that there is an undesirable flow out of the system somewhere else in the pipeline. If the system recognizes such a pattern, it may trigger an alarm that such a leak is likely to be present in the plant. If a previously achieved peak flow rate is not reached, this can also indicate a possible fault with the system. Therefore, an alarm may be triggered, for example, if the maximum measured flow rate is not within a set range of the previous month’s flow rate, such as 95%. Similarly, an alarm may be triggered if an unexpected static pressure is measured, for example not reaching a previous maximum pressure or within a range of a previous maximum pressure. A slow return to an optimum pressure after cooking events may also indicate a fault in the digester system. Furthermore, the server may be trained to recognize patterns which indicate the occurrence of water in the pipes (generally consisting of rapid fluctuations of static pressure and flow rate). An alarm may sound if there is no flow measured for a given period of time or the pattern of flow indicates that there is a fault in the system. The system may also output suggested appropriate actions and maintenance. The software system can also be trained to recognize patterns related to digester status. For example, if no flow is detected for a set period of time, for example two weeks, this may indicate the digester requires feeding. The flow characteristics measured after feeding can be used to provide insight into the digester feeding habits. The system can be trained to recognize patterns related to overfeeding and methane venting into atmosphere, such as the static pressure reaching a maximum pressure and then ‘flat-lining’, which indicates that gas is bubbling through the pressure relief U-bend 220 in the case of most bag digesters (typically at 20 mBar / 2kPa) or is venting through the slurry outlet 140 in the case of most fixed dome digesters (typically at 80 mBar / 8kPa) . This can provide feedback to the user as to the requirements and / or optimal maintenance of each digester. This could be provided, for example, by mobile phone. The system may be trained on such data and accordingly output recommendations for the optimal upkeep of each digester system. The time at which measurements are taken can also be recorded and fed into the pattern recognition software. Accordingly, the pattern recognition calculations can take into consideration the time of day at which the measurement patterns occur. For example, an alarm may be triggered if a change in flow rate is measured at an unexpected time, such as during the middle of the night. The outputs from the meter and the server (including alerts and alarms) are presented to the user via a user interface. This may take the form of a web-based interface, desktop-based interface, mobile-phone app or any other form of user interface. Figure 5 illustrates an embodiment of the user interface as a web application interface 500. The interface 500 displays the data both in a graphical form 510 and as numerical values 512. More than one digester may be added to a user’s account profile via the relevant button 514, so that a user can monitor several digesters at different locations simultaneously. There are provided buttons to view an overview of all the digesters monitored 516 and an overview of the user’s account 518. The type of data presented, for example static pressure or differential pressure, can -17- be determined by the user 520. Data is presented from a time period as chosen by the user 522; for example, in Figure 5, a time period of two weeks is illustrated. The user also has the option to specify the end time of the displayed data 524 so that, for example, a previous two-week period may be viewed as a comparison to recently collected data. As mentioned, data is collected by the meter every minute but is sent to the server once an hour. In addition, where a significant fault is detected on-device, such as a gas leak, the meter will immediately send the data to the server in order to promptly notify the user. The user is provided with the option to update the display interface, via a ‘refresh’ button 526. The options for flow rate data reported include current flow rate (in m3 / hour); a historic flow rate graph over user-definable periods (in m3 / hour); maximum flow rate within a time period specified by the user (in m3 / hour); basic statistical analytics of the flow rate graph such as median, mean, etc.; cooking time (in hours) within a user-defined time period; biogas consumption (in m3) over a time period as defined by the user; and a comparison to the corresponding volume of liquid petroleum gas (LPG) for example, and the associated number of bottles of a certain volume or weight / volume of wood fuel, this consumption would have required. The options for static pressure features reported include the current pressure of the digester (in kPa); a historic pressure graph over a time period as defined by the user (in kPa); basic statistical analytics over a user-defined time period, such as standard deviation of pressure (in kPa), minimum pressure (in kPa), maximum pressure (in kPa), pressure range (in kPa) etc. Carbon monitoring data can also be reported, including the cumulative carbon saved over a period determined by the user (e.g. as compared to the use of LPG), which may be the total lifetime of the digester. As data is recorded locally, this can be determined even if the system becomes disconnected periodically. Further analytics features which can be reported include an indication of digester status; alerts when the digester has a fault, for example when a slow leak is recognized, no flow over a given period of time, unexpected peak flow rate, unexpected static pressure range, flow rate at unexpected times, and any other pattern recognized to be abnormal; feeding habits of the digester; cooking and / or usage habits; and gas venting events. The user may download all the data as a .csv document for any given digester. The limited pattern recognition taking place on device operates in essentially the same way as described with reference to the server, albeit fewer patterns may be recognized. In an alternative, all processing may be performed on device and communicated directly to a user device (rather than being communicated via a server). Each smart biogas meter and associated software can be tailored to the particular requirements of each biogas digester and user. For example, in some biogas digesters harmful substances such as hydrogen sulphide may not be present or may be removed during the digester process. This may be achieved through air or oxygen dosing to the gas in combination with iron chloride dosing to the slurry or the addition of sulphide oxidizing microorganisms to the slurry. In these circumstances, inclusion of the U-bend shaped connector pipes 332,328a,328b is optional and standard piping may be used to connect the pipe component 310 and the sensors 334,338. Alternative arrangements of traps may be implemented in the pipe connectors in order to keep the fluid in place. For example, a P-trap, an integral trap, a running trap, a drum trap or any similar trap known to the skilled person may be used as an alternative. A temperature sensor such as a thermistor may optionally be included in the system to allow temperature measurement of the gas flow. The thermistor may be mounted on the end of a wire, and may be inserted into the pipeline 200 using a standard pipe fitting with a gas tight washer or gasket. As the temperature of the gas flow affects the pressure of the system, the measurement of temperature allows the gas pressure readings to be compensated for temperature. This results in the calculation of more accurate data on gas pressure and flow. A standard 10k negative temperature coefficient (NTC) thermistor or similar may be employed for this purpose. Temperature may preferably be recorded and sent once an hour to the server. An actuated valve may also be optionally integrated within the system, which may enable flow to be controlled remotely. The actuated valve is a latching type valve which is actuated by a pulse from a smart controller. The smart controller may be a separate element or may be part of the system software. The actuated valve is configured to withstand corrosive gases and is configured such that there is no chance of the valve creating a spark, as this could cause the biogas to ignite. The valve may be controlled to cause disruption (i.e. shut off) to the supply of gas, but also to prevent venting of excess biogas. Venting may be prevented by allowing gas to displace slurry out of the digester while the valve is closed. When the gas is at the point of venting, detected by the static pressure sensor 334, to atmosphere (typically at the maximum pressure, i.e. 2 kPA in the case of a bag digester and 8 kPa in the case of a concrete dome digester) the valve will open and the user may be notified to use the gas to avoid harmful global warming emissions to the environment. This may allow intermittent use (and so allow the system to operate as a Pay As You Go (PAYG) system) and encourage the user to keep paying for the service, while also reducing environmental impact due to unnecessary venting. A further optional addition is the inclusion of a piezo ignition system, which would automatically flare off the gas. It will be appreciated that the pipe component 310 could alternatively be suitably scaled for use with other sizes of pipes, in particular 40 mm (or 1.5 inch) and 25 mm (or 1 inch) diameter pipes (where the larger diameter pipe is used adjacent the first chamber 314, and the smaller diameter pipe is used adjacent the second chamber 318). For any size of pipe, the dimensions of the pipe component 310 should be configured to generate a differential pressure of between 0 and 500 Pa across the shoulder 316 (when used with a biogas digester), which is in the operating range of the differential pressure sensor 338 (which, as mentioned, is an ‘off-the-shelf’ unit) where a 500 Pa sensor is used. Optionally, the pipe component 310 may further include a tapered divergent cone (not shown) after the second chamber 318, having a divergence angle within the range 6 to 10°, in order to minimize disruption to the flow and any associated pressure loss. In such a case a commensurately sized pipe may be used. The embodiment described above utilizes GSM communications to connect the meter to the server. Alternatively, low-frequency radio (LoRa) data connectivity may be used in addition to, or instead of, GSM connectivity. This may be used for connecting biogas meters in locations where there is poor GSM coverage. An additional optional component is a WiFi® dongle, optionally with Bluetooth® capabilities. In this embodiment the smart biogas meter can be used as a WiFi® node, allowing local interfacing through the use of mobile phones or another portable controller. The real time biogas flow sensing of the present invention achieves improved accuracy metering. This facilitates the optional incorporation of the system into accurate Pay As You Go (PAYG) finance models and the monetization of climate change abatement benefit, for example Carbon Credits (CC). The dimensions of the pipe component and sensors can be used to calibrate the data outputs to correspond to Carbon Credits. Optionally, a user may amalgamate the output of a network of small biogas digestor projects to pool together resources to obtain Carbon Credits. The use of remote connectivity may facilitate this in a way such that small projects can benefit from offsets and credits in a manner they currently do not. In an alternative, one side of the differential pressure sensor is used to provide a static pressure measurement, and a separate static pressure sensor is not included in the gas meter. In a further alternative, the static pressure sensor may be provided upstream of the differential pressure sensor. In an alternative, a flow sensor that is not a differential pressure sensor is used in the gas meter. For example, a flow sensor using a float (i.e. a ‘rotometer’) may be used. It should be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention. Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination. Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

Claims

1, A biogas meter for a biogas digester, comprising:a pipe component for receiving a gas emitted by a biogas digester, comprising;a first chamber with a first diameter, anda second chamber with a second diameter, which is smaller than the first diameter,wherein the first chamber is separated from the second chamber by a constriction in the pipe component, andat least one differential pressure sensor, being connected to the first chamber and the second chamber by respective fluid conduits, for measuring the pressure difference between the first and second chamber based on measurements of the pressure of a gas that is not the gas emitted by the biogas digester;wherein the fluid conduits comprise means for separating the at least one differential pressure sensor from the gas emitted by the biogas digester,wherein the maximum gas pressure of the gas emitted by the biogas digester is under10kPa.

2. The biogas meter of Claim 1, wherein the means for separating comprises a liquid held in the at least one fluid conduit.

3. The biogas meter of Claim 1 or 2, wherein the means for separating comprises a trap for retaining liquid.

4. The biogas meter of any of Claims 2 to 3, wherein the liquid is at least one of: a solution that is insoluble to H2S; and a solution that reacts with H2S in such a way as to produce an inert liquid, such as an alkaline solution such as calcium nitrate solution.

5. The biogas meter of any preceding claim, wherein the fluid conduit is formed of a material that is resistant to H2S.

6. The biogas meter of any preceding claim, wherein the at least one differential pressure sensor is connected over the constriction in the pipe component, such that a venturi tube arrangement is formed.

7. The biogas meter of any preceding claim, wherein the constriction is formed as a cone having a convergence angle of between 30 and 60 degrees.

8. The biogas meter of any preceding claim, further comprising a static pressure sensor.

9. The biogas meter of any preceding claim, further comprising a processor configured to record the pressure of the gas as measured by the at least one differential pressure sensor and / or the static pressure sensor.

10. The biogas meter of Claim 9, wherein the processor is configured to calculate gas flow rate based on pressure data.

11. The biogas meter of Claim 9 or 10, further comprising a transmitter, wherein the processor is configured to transmit data to a server using the transmitter.

12. The biogas meter of any preceding claim, further comprising a temperature sensor.

13. The biogas meter of any preceding claim, further comprising an actuated valve for shutting off the output of the biogas digester.

14. The biogas meter of Claim 13, wherein the valve, when closed, is configured to open when a predetermined threshold gas pressure is detected.

15. A method for remotely monitoring a biogas digester, comprising:using a biogas meter according to any preceding claim, monitoring at least one parameter of gas emitted from a biogas digester;recording the monitored at least one parameter locally on the gas meter; and transmitting data associated with the at least one parameter to a remote server.

16. A method according to Claim 15, wherein the at least one parameter comprises at least one of: differential pressure, flow rate, and gas consumption.

17. The method of Claim 15 or 16, further comprising: at the remote server, detecting patterns in the received data, and generating at least one further parameter based on said determination.

18. The method of Claim 17, further comprising producing an alert in dependence on the generated further parameter.

19. The method of Claim 17 or 18, wherein the further parameter comprises at least one of: gas usage, gas consumption, and feeding / consumption habits of a biogas digester.

20. The method of any of Claims 17 to 19, wherein the further parameter is an indication of a fault condition and / or a predicted future event.

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