Patent ID: 12228253

DETAILED DESCRIPTION

Until recently, natural gas has only been delivered directly from gas fields via pipeline. High fuel costs and a limited supply of fossil fuel based natural gas have made the bulk transportation of Liquid Natural Gas (LNG) from remote sites that would normally be considered too far from its final point of use to be economically viable, for instance, via gas pipeline, common place. Specially designed transport ships holding very large cryogenic flasks are also now being used.

However, the carbon dioxide (CO2) produced when fossil fuel-based natural gas is burned still results in an overall increase in CO2emissions. By comparison, biogas is carbon neutral when combusted, and negative when compared to waste disposed by landfill. Unmanaged waste, for instance contained in existing landfill sites, causes the emission of high levels of bio-methane, which is a powerful greenhouse gas. In fact, it is as much as 25 times more damaging than the CO2that would be produced if it had been put through an anaerobic and combustion process. In the latter instance, the CO2by-product is itself a valuable resource that can be used, for example, to enhance growth of vegetable produce in greenhouses.

Bio-methane represents the ideal fuel for replacing fossil fuels, especially when considering that existing internal combustion engines can be converted to use bio methane, and that they would actually run at a higher chemical energy to mechanical energy conversion efficiency ratio. In addition, there are significant areas of currently unmanaged and unexploited green waste, such as that found in gardens, set aside land, roadside verges and parks, for example. However, the present inability to store methane as a liquid at room temperature and at relatively low pressure is currently restricting the use of such green waste as a replacement fuel. This represents a significant limitation for many applications such as de-centralized and remote power generation, as well as automotive transport.

Unlike fuels such as butane and propane, methane cannot presently be compressed into a liquid at room temperature and pressure using existing techniques. Consequently, it can only be stored as a liquid at reduced temperature with its boiling point at 1 bar of 112K (−161 C). Although useful quantities can be stored as a pressurised gas, current container technology restricts this to approximately 200 bar and in volumes of less than 100 liters. At this pressure, methane holds only approximately 30% of the energy of gasoline, thereby greatly reducing available range and storage capacity. Scaling the pressurized cylinders' volume does not provide a solution, as the required cylinder wall thickness scales with pressure, resulting in excessive weight and cost.

For temperate climates, the lack of compact storage systems and methods is particularly problematic for the generation of bio-methane from green waste. In temperate climate locations, the majority of green waste is present in summer months; however, most energy usage occurs during the winter months. Thus, for anaerobic digestion, the energy production cycle is out of phase with the seasonal energy demand, limiting its applicability as a sustainable renewable energy source.

If methane can be stored as a liquid under low pressure (equating to an energy density of gas stored at approximately 600 bar), then the energy density in terms of volume would converge to that of current fossil fuels. In terms of energy density as a function of mass, it would exceed current fossil fuels by 30%, having dramatic implications across a number of fields, such as aviation applications. This disclosure provides a solution to this problem. Certain of the embodiments described herein provide an intelligent, safe, scalable, self-contained, standalone, cryogenic, liquid methane storage facility. According to some embodiments, a standalone cryogenic methane storage facility is provide whereby the system can be left unattended with negligible risk of catastrophic failure or release of methane to the environment.

Cryogenic liquid storage systems are presently used for inert gases such as argon, nitrogen and helium. They usually consist of a sealed, vacuum insulated, cryogenic Dewar that is fitted with a pressure release valve that allows the excess boil off to vent to the atmosphere. Typical boil-off rates for industrial self-pressurizing Dewars are 1% of liquid a day.

As the gas inside a Dewar slowly warms, the pressure increases and eventually a pressure release valve opens, venting a small quantity of gas. This reduces the pressure, thereby closing the pressure release valve and ending the venting cycle. The pressure then builds and the cycle is repeated. This self-pressurized mechanism works well for inert non-flammable gases in non-confined spaces. However, for methane, a self-pressurized release mechanism on its own does not provide a safe or environmentally responsible solution. While liquid bio-methane is not flammable, gaseous bio-methane is not only flammable in concentrations of 5-15%, but is also a powerful greenhouse gas and should not be released to the atmosphere arbitrarily.

A solution to the above-identified self-pressurization problem can include the use of a mechanically or electrically powered cooling unit, such as a close cycle refrigerator coldhead coupled to a storage Dewar's main reservoir. The energy required to liquefy methane is much less (approximately 10%) of the chemical energy stored within the methane itself. This excess energy provides a convenient means of controlling the temperature of the main reservoir by using a self-limiting pressure release mechanism to ensure that boil-off is prevented and or controlled, and direct methane release to the environment is eliminated. The elimination of direct methane release limits the possibility of catastrophic over pressurization and or explosion due to ignition of a methane/air mixture.

According to particular embodiments, as the reservoir of liquid methane slowly warms and the pressure in the storage tank increases due to gaseous methane boiling off, the excess chemical energy stored in the boiled-off methane gas can be used by converting it to electrical energy via a power generator. This power generator could be, for instance, an internal combustion (IC) engine electrical generator or fuel cell. The excess electricity generated can be used to power a cooling unit, which can include a close cycle refrigeration cold head, to provide sufficient cooling to the main liquid methane reservoir, thereby reducing the pressure and preventing the subsequent boil-off of methane gas. The gaseous output from the combustion engine or fuel cell can be released to the environment as carbon neutral CO2and H2O, which provides no added risk or environmental issues.

Accordingly, a cycle that includes: (1) pressurization; (2) venting; (3) power generation; (4) cooling; and (5) de-pressurization is self-limiting. With intelligent control, for instance, via a microprocessor, the cycle can be used as the basis for long-term standalone storage capability of liquid methane. Catastrophic over pressurization of the main methane reservoir can be avoided without venting methane gas to the atmosphere. In addition, the storage period is dramatically increased as the excess chemical energy is used to self-cool the reservoir.

While certain embodiments are described using an internal combustion engine generator or fuel cell to produce a reservoir of electrical energy stored, for instance, in a battery, there are equivalent setups wherein an IC engine (or battery) instead powers a gas compressor which fills a pressurized gas reservoir, which in turn can be used to drive the cooling. For simplicity, the following description will primarily utilize electrically powered examples, but it should be noted that an equivalent gas driven or mechanically driven mode of operation is equally applicable.

In some embodiments, micro anaerobic digestion (micro-AD) units may be deployed at geographically remote locations and connected via network connections to a central hub and/or control station. These units may include, for example, one or more anaerobic digestion tanks and a cryogenic storage system. The units may be deployed, for instance, as illustrated inFIG.1, which shows an exemplary networked micro-AD system100. The system100includes a number of micro-AD units102,104which are connected to a central hub106. In some embodiments, each of the micro-AD units is configured to produce gaseous methane and store it in liquid form according to one or more of the techniques disclosed herein. Each of the micro-AD units may, for example, be configured to generate power using vented methane gas, which is in turn used to cool the liquid methane of their respective storage tanks. The system100also includes a controller108, which may be co-located with the hub106or at an independent, remote location.

Referring now toFIG.2,FIG.2illustrates a block diagram of a controller according to some embodiments. As shown inFIG.2, the controller108may include: a data processing system202, which may include one or more data processing devices each having one or more microprocessors and/or one or more circuits, such as an application specific integrated circuit (ASIC), Field-programmable gate arrays (FPGAs), etc; a data storage system204, which may include one or more computer-readable mediums, such as non-volatile storage devices and/or volatile storage devices (e.g., random access memory (RAM)); and a network interface206for connecting controller108to a network (e.g., an Internet Protocol (IP) network). The controller108may communicate with one or more of the micro-AD units102,014or the central hub106via the network connection. In some embodiments, the controller108may include a transceiver212and antenna210to communicate wirelessly with one or more of the micro-AD units102,104or the central hub106.

In embodiments where data processing system202includes a microprocessor, a computer program product is provided, which computer program product includes: computer readable program code (software), which implements a computer program, stored on a computer readable medium, such as, but not limited, to magnetic media (e.g., a hard disk), optical media (e.g., a DVD), memory devices (e.g., random access memory), etc. In some embodiments, computer readable program code is configured such that, when executed by data processing system202, the code causes the controller to perform the steps described herein (e.g., one or more steps shown in the flowcharts and/or described in connection withFIG.3). In other embodiments, controller108may be configured to perform steps described herein without the need for additional code. For example, data processing system202may consist merely of specialized hardware, such as one or more application-specific integrated circuits (ASICs). Hence, the features of the present disclosure described above may be implemented in hardware and/or software.

According to certain embodiments, the controller108can be configured to monitor aspects of the anaerobic digestion or storage/generation processes at the remote micro-AD units102,104. For instance, the controller108may monitor one or more of PH, temperature, gas production rate, and gas pressure through the use of sensors located on the anaerobic digestion tanks and storage systems. These sensors may be, for example, electrical, mechanical, and/or chemical and may be accessed either directly or indirectly wirelessly or via network connection to the units. The controller108may send one or more control/activation signals to the micro-AD units to control certain aspects of the anaerobic digestion or methane storage process. For instance, the controller108may send a signal to open one or more valves. These valves may be used, for example, in the anaerobic digestion process or in the storage/generation process. Signals sent by the controller108and received by units102,104may operate one or more components of the unit directly, such as valves, or can cause a microprocessor located on the units to perform certain functions indicated by the signal.

Referring toFIG.3, a process300for generating electricity is provided. In some embodiments, controller108is configured to cause a micro-AD unit, such as unit102, to produce electricity in accordance with process300.

In step310, the controller accesses electricity data, such as pricing data. For instance, controller108may access current electricity prices over the network. Additionally, electricity pricing data may be regularly downloaded onto data storage204. In some embodiments, the data is evaluated to determine whether it is an optimal time to produce electricity. The data may indicate, for example, that there is a peak demand for electricity and/or that prices are higher than average.

In step320, the controller evaluates the status of one or more micro-AD units to determine if it should be used for production. For instance, the controller108may evaluate micro-AD units102,104to determine whether one or both would be a suitable candidate for electrical power production. According to certain aspects, each of the micro-AD units102,104may be associated with a user. The status of a micro-AD unit may include, for example, historical usage data, historical production data, specifications of the unit, and user preferences regarding generation and storage volumes.

In step330, based on its evaluation of the status one or more units, the controller118selects a unit for production. For example, the controller may select a micro-AD unit whose status indicates that it has a large production capacity, but is currently only using a small amount of that capacity. In some embodiments, the controller may not select a micro-AD having a low methane storage volume.

In step340, the controller transmits a production command to the micro-AD unit. For instance, controller108can transmit a command to unit102that causes electrical power production. For instance, the controller118can transmit a signal to unit102instructing the unit to generate methane gas, power one or more electrical power generators located at the unit102with the generated methane gas, and provide the generated electrical power to an external power system, such as the mains electricity power grid.

In some embodiments, the controller may transmit a signal to the micro-AD to stop electrical power generation. In some embodiments, rather than using a stop command, the micro-AD102may be configured to produce electricity for a limited time, eliminating the need for stop command. In certain aspects, the amount of time or amount of energy produced may be set by the initial production command.

In some embodiments, the controller may receive a communication requesting electrical power production. For instance, the controller108may be in communication with one or more power companies and configured to receive on-demand power requests. Similarly, the micro-AD units may be in direct communication with one or more power companies, such that the power companies can directly initiate power production at one or more micro-AD locations.

According to certain aspects, the controller may include one or more databases, such as data storage204, that stores information regarding the users associated with the micro-ADs. For instance, the database may include accounts records regarding the amount of electricity generated by a unit and/or delivered to the power grid. In some embodiments, following the production of the electricity, a controller, such as controller108, updates a user's account to reflect the amount of electricity produced. In this manner, users can be accurately compensated for electricity production.

In some embodiments, electricity may be produced using methane that was created and stored several months in the past. For instance, methane gas may be created by the anaerobic digestion process during the warm summer months, cryogenically stored, and then used to produce power or heat during the colder winter months. In this manner, methane gas may be captured from a readily abundant source, such as fresh grass clippings, but used to create energy when there is peak demand, e.g., out of season from peak production. In certain aspects, due to the high efficiency of the disclosed systems, methane may be stored for several months or years before on-demand use.

Referring now toFIG.4,FIG.4illustrates a block diagram of a storage portion400of micro-AD unit102according to some embodiments.

Micro-AD unit102includes at least one inlet valve402. Methane gas or liquid methane gas may be introduced into storage vessel410via the inlet valve402. The storage vessel410may be, for instance, a Dewar. A cooling unit404is configured to cool the methane gas or liquid methane gas within the storage vessel410. The cooling unit may include, for instance, a closed cycle refrigerator406and closed cycle refrigeration head408. In certain aspects, when the head408is cooled to a temperature below the condensing temperature of methane gas, liquid methane condenses on the head408. In some embodiments, the head may be cooled to a temperature equal to or below 110 K. In certain instances, the head may be cooled to temperatures below 80 K; however, it is recognized that significant reduction in the temperature of head408increases the power consumption of the micro-AD unit102. The condensed liquid methane will run off head408into the main liquid methane reservoir412.

In some embodiments, mains electricity is available to the micro-AD unit102. For instance, a mains power line418may be attached to a power supply/inverter414, which is used to maintain a power storage device, such as battery420, at a maximum charge level. In certain aspects, a microcontroller416, which may include one or more processors, is also provided and attached to power line418. One of skill in the art will recognize that the power supply/inverter414and microcontroller416may be provided as a single unit, or alternatively, as independent devices. The unit may also include one or more transceivers and antennas connected to microcontroller416, for instance, to enable communication with external devices, such as a central controller, other micro-AD units, or user electronics.

One or more of the battery420and power supply/inverter414can be used to power cooling unit404, for instance, providing power to closed cycle refrigerator406to manage boil-off of the liquid methane of reservoir412. For example, the refrigerator406can be controlled to prevent any boil-off of the liquid methane of412. According to this embodiment, the storage period of the liquid methane may be indefinite, as the unit can be configured such that there is no pressurization occurring within reservoir412.

According to certain aspects, there may be an interruption of the mains electricity supply, in which case the battery420may be used to power the close cycle refrigerator406and head408head to manage (or even prevent) boil-off. It is recognized that if mains supply is unavailable for an extended period of time, the battery reserve will drop to its minimum charge level (according to type) at which point the battery420will stop powering the cooling unit404. As the main reservoir of liquid methane starts to slowly warm, the pressure in reservoir412will start to build as boil-off of the liquid methane occurs and excess gaseous methane is present in the space above (422) the liquid methane.

The micro-AD unit102may include a pressure valve424to release boil-off methane. For instance, the valve424may be configured to release boil-off at a preset level into a buffer reservoir426. The gaseous methane of the buffer reservoir may be used as fuel for one or more power generators of the micro-AD unit102. For example, when the buffer reservoir reaches maximum capacity, or any pre-determined level, a power generator428may be started and powered by the gaseous methane of buffer reservoir426. The power generator may be any source capable of providing sufficient electrical power to cooling unit404, such as an internal combustion (IC) generator or fuel cell. In certain aspects, the IC engine output power may be sized such that sufficient electrical power is generated to allow full charging of battery420while simultaneously providing sufficient power to the closed cycle refrigerator406. The generator428may include a CO2and/or H2O exhaust446.

Aspects of some embodiments provide that once the buffer reservoir has been depleted of gaseous methane, the power generator428will stop. If the battery420is fully charged, then this charge can be used to continue to power to the close cycle refrigerator406, for instance, to prevent boil-off. In certain aspects, if the battery420is not fully charged or if additional power is needed for one or more functions of the micro-AD unit102, the closed cycle refrigerator406can switched off to allow the subsequent build-up and release of additional methane to fuel the power generator428and continue the charging process and/or additional process of the unit102. Accordingly, methane gas is available “on demand” via controlled boil-off and release to the buffer reservoir426. This cycle may be repeated until the battery420is fully charged, mains power is restored, and/or the reservoir of liquid methane is exhausted. One of ordinary skill will recognize that micro-AD102can be fully self-sustainable without mains electricity.

In some embodiments, if the battery420is fully charged, but conditions require the generator428to continue to consume the methane boil-off, then excess power from the generator428can be used to power the closed cycle refrigeration head408to cool the main reservoir412to the minimum allowable temperature. This cooling can delay further boil-off and extend the hold period of the system, thereby avoiding any overcharging of the battery420.

According to certain aspects, the storage systems of this disclosure may be used to supply a continuous flow of gas or energy to an industrial or domestic plant (such as home heating) or an automotive application. In this instance, it may be necessary that micro-AD unit102is configured for the extraction of methane gas “on-demand.” This extraction may be through a main output valve430. Further, an internal heater432, such as a resistive heater, of the main reservoir412can be used to intentionally increase boil-off and increase Dewar pressure for release of either methane gas or liquid methane. Liquid methane can be released through a liquid methane take-off port434. Alternatively this heat could be supplied by reversing the closed cycle refrigerator406polarity such that it draws heat from the outside of the Dewar and transfers it to the main reservoir.

In addition to the natural self-limiting cycle described above, additional safeguards may be incorporated into the system. For instance, internet connectivity436can be incorporated to allow the microcontroller416to send a status update or alarm to the unit owner or supplier. This also allows for remote control or inspection of the system to be carried out by the owner or supplier. Additionally, while the main Dewar can utilize a vacuum for maximum insulating properties and minimal boil-off, an outer layer of solid insulation438, such as low density polystyrene or other insulating material, can be incorporated to limit the excess boil-off resulting from a sudden loss of vacuum. The generator428can be rated to readily accept all boil-off resulting from such a scenario with all energy diverted to the coldhead408or resistive ballast in the microcontroller416as appropriate.

In some embodiments, the micro-AD unit may be provided with additional safeguard such as a high pressure release valve440. Such a safeguard may be used, for instance, following a failure of the closed cycle refrigerator406and simultaneous failure of the generator428. The boil-off may flow to a flame containment vessel442, such as a metal gauze cavity. The pressure release valve440can be configured to trigger a mechanical igniter444to ignite the resulting boil-off converting the methane to CO2and H2O in a controlled flare.

Referring now toFIG.5, a flow500illustrating a process for storing liquid methane is provided.

In step510, methane gas is generated from a reservoir of liquid methane gas. The reservoir of liquid methane may be, for instance, the product of compressing biogas formed from anaerobic digestion. In some embodiments, the methane gas may be generated by activating a heater, such as heater432ofFIG.4, to cause boil-off of methane gas from the liquid reservoir. Generating may also include, for example, turning off or reducing a cooling unit, such as unit404ofFIG.4, or allowing methane gas to be naturally generated due to boil off of a reservoir of liquid methane.

In step520, the methane gas is vented. For instance, the methane gas may be vented from the storage unit containing the reservoir of liquid methane, such as storage unit410ofFIG.4. The gas may be vented through one or more valves and/or buffer stages to an electrical power generator.

In step530, the electrical power generator generates electrical power using the vented methane gas. For instance, the power generator may be an internal combustion (IC) engine configured to run on methane gas. Alternatively, the power generator may be a fuel cell. One of ordinary skill in the art will recognize that the present disclosure is not limited to IC engines and fuel cells, but rather, the devices and methods herein can be implemented using any power generator configured to run, at least in part, on methane gas.

According to certain embodiments, the method500may include receiving an activation signal, for instance, from a user or central controller. The venting and/or power generation of steps520and530may be responsive to this activation signal. For example, the activation signal may cause one or more controllable valves to open, thereby venting methane gas and providing fuel to a power generator. The activation signal may be received directly at one or more control devices, or, at a microcontroller located with the methane storage unit configured control its operation.

In step540, a cooling cycle is initiated. The cooling cycle may be initiated, for instance, by activating a cooling unit, such as unit404of theFIG.4. In some embodiments, the cooling unit may include a close cycle refrigerator and a cold head within the methane storage unit. The cooling unit causes the temperature of the reservoir of liquid methane to drop, thereby reducing the pressure in the storage unit. In certain respects, the cooling is powered by the electrical power generated in step530. The cooling may be powered directly by the power generator, or indirectly by the power generator through the use of a battery, such as battery520ofFIG.4. In some embodiments, the cooling cycle may be initiated by activating a compressor coupled to a heat exchanger, for instance, as illustrated inFIG.6. For example, compressor606may be powered by the electrical power produced in step530.

In some embodiments, process500may further include producing liquid or gaseous methane according to an anaerobic digestion process and transferring the methane to the storage unit. This anaerobic digestion process may use, for example, green vegetative feedstock such as grass clippings.

Referring now to system600ofFIG.6, in some embodiments, gas generated in an anaerobic digester602can be stored in a gas buffer604. The gas may be stored in the buffer604, for example, until a predetermined pressure is reached. A compressor606is started and the gas is circulated from the buffer through the compressor606. The compressor606may be started, for example, once the predetermined pressure in buffer604is reached. The compressed gas may then be passed to an optional cleaning stage608. The compressed gas is circulated to a storage tank610via inlet612that includes a heat exchanger614. In some embodiments, heat exchanger614may include finned heat exchanger tubing. According to certain aspects, the gas may be expanded through an orifice of heat exchanger614for cooling, or alternatively, a variable needle valve. After expansion, liquefied methane collects in the storage tank610, which may be, for example, a Dewar. The remaining gaseous methane, i.e., the non-liquefied methane, returns to gas buffer604via exit616. The system600may also include one or more control valves618to regulate pressure and control gas flow. The liquid methane may be removed from storage tank610as needed, for instance, via a decanter620.

According to some embodiments, anaerobic digestion may be performed using multiple tanks. Typically, anaerobic digestion requires the use of heat to initiate the process. Methanogens, which are microbes that digest feedstock such as grass, can be split into two categories based on the temperature ranges at which they function. These categories/ranges are referred to as “thermophilic” (approximately 45-70 C) and “mesophilic” (approximately 15-40 C). The thermophilic anaerobes are typically considered more difficult to sustain in a continuous process, although possible. Thermophilic anaerobes, however, are able to digest grass at a much faster rate (approximately twice as fast as mesophilic anaerobes) and can be sustained in a continuous process. In certain aspects, systems disclosed herein may be designed to operate between these two modes. For instance, a first thermophilic process can be used to break down as much of the feedstock (e.g., grass) as possible, while the remaining organic matter is then passed onto one or more additional tanks to finish of the digestion using a mesophilic process.

Referring now toFIG.7, an anaerobic digester700according to certain embodiments is illustrated. In some embodiments, the anaerobic digester ofFIG.7may be a part of micro-AD102and coupled to storage system400. For instance, digester700can provide the input to system400at inlet402. The anaerobic digester700consists of multiple tanks (702,704). The tanks may be relatively small, and the total number of tanks may be set according to a customers intended use and/or the amount of land that will be used to supply feedstock. In certain aspects, a PH gradient and temperature gradient can be maintained across the tanks. The digester700may further include an inlet706for receiving feedstock and a macerator708to mulch, agitate and/or separate components of the feedstock during anaerobic digestion.

In some embodiments, the gas output from each tank is controlled via a latching gas valve710,712. In certain aspects, the valve may be remotely controllable, for instance, via local or remote computer. If a quantity of substrate (e.g., partially digested feedstock) is required to be moved from one tank to the next, for instance, from tank702to tank704, the gas output of the sending tank702can be turned off using gas valve710. However, the gas output valve712of the receiving tank704is left open. The gas pressure in the sending tank702is then allowed to build up and as a result the substrate is forced though the outlet pipe714and into the receiving tank704. Once the substrate move has taken place, the gas pressure from the sending tank702is relieved to a point at which transfer stops. The gas pressure may then be maintained at this level to prevent re-syphoning of the substrate. In some embodiments, the pressure can be completely released to allow the levels of the tanks702,704to re-equalize.

In some embodiments, one tank of digester700is left intentionally empty. In certain embodiments, at least one tank is always left empty. This enables the complete movement of one tank load of substrate along the line of tanks. Accordingly, the entire state of a number of tanks can be moved one place to the left or right, leaving the state of the entire process intact. In this way, it is possible to introduce a completely new fill of feedstock, for instance, at the beginning of the stream. This movement of substrate can be achieved using the pressure generated by the anaerobes, for instance, as described above. If latching valves are used, the moving process uses a negligible amount of energy.

In some embodiments, the feedstock needs to pass through several different stages of digestion. For instance, it may pass from an initial aerobic hydrolysis/bacterial phase which breaks down insoluble polymers, such as carbohydrates, and makes them available for other bacteria. This phase also provides significant levels of heat which can be used to establish a rapid thermophilic phase, followed by a slower mesophilic phase used to digest the remaining material and produce the final digestate. The temperature of the substrate can be used to speed up or slow down the anaerobes digestion rate; however, if the PH moves outside a certain window, then the complete population can be killed, thereby halting the process entirely. According to certain aspects, the process is returned to an aerobic digestion state to compost any residual organic matter and remove any unpleasant odors before expelling the digestate, for instance, via an outlet valve716.

According to some embodiments, pressure in the final tank or “stage” of a multi-tank/stage anaerobic digester, such as digester700ofFIG.7, can be allowed to build up to a desired pressure. Pressure build-up can be controlled, for example, by adjusting one or more valves of the digester and/or increasing temperature. In certain aspects, the desired pressure is high enough that a compressor in subsequent processing stages is not required. For instance, if the pressure is allowed to build to a high enough level, for example between 2 and 30 bar, it may be possible to eliminate compressors that would otherwise be needed during cleaning or liquification, such as compressor808ofFIG.8or compressor606ofFIG.6. The specific pressure required, however, will depend on the configuration of one or more of the digester and subsequent stages. Referring toFIG.6, in some embodiments, the pressure in a final stage of anaerobic digester602can be allowed to increase such that naturally pressurized biogas is fed to heat exchanger614, which may include finned heat exchanger tubing. Similarly, with respect to the example ofFIG.8, naturally pressurized biogas may by passed through filter816onto heat exchanger812, which may also include finned heat exchanger tubing. Accordingly, the cleaning and/or liquification process can be effectively powered by the anaerobic digestion process itself and the associated microbes.

According to certain embodiments, a micro-AD102can be designed to operate using primarily a single type of feedstock, such as grass cuttings. By restricting the feedstock to a single source, such as grass cuttings, the chemistry required to achieve a continuous anaerobic digestion and methane production process is greatly simplified. When using a micro-AD configured for a single source, a continuous multistage process that mimics a ruminants digestive process may be implemented, which maximizes throughput and methane production. This allows for the use of very small digester tanks, which therefore allows the entire system to be placed inside a small visually inert enclosure.

Grass cuttings offer certain advantages over other feedstocks, such as animal slurry or food waste. For instance, the energy content per unit volume can be much higher, as grass is able to process and store approximately 6% of the sunlight that falls onto it as chemical energy, held in the form of sugars and starches. With respect to animal slurry, a ruminant such as a cow, horse or lamb for example, has extracted much of the available energy to drive its metabolism, grow and store fat. Additionally, the energy stored in a grass leaf is more readily extracted, being largely in the form of sugars and starches rather than lignin, a chemically inert material that is difficult for most microorganisms to digest. This offers the potential to achieve much higher conversion rates from the raw feedstock into methane. Grass is also abundantly available and can be harvested from domestic lawns, parks, golf courses and roadside verges etc.

In some embodiments, the exclusive use of grass cuttings also assists in biogas purification. The biogas that results from anaerobic digestion of grass cuttings consists primarily of methane (≈70%), carbon dioxide (≈30%) and a trace amount of hydrogen sulphide. The hydrogen sulphide can be removed, for example, by passing the biogas through a filter, such as a steel powder filter. The removal of the hydrogen sulphide limits the release of unpleasant odors. Once the hydrogen sulphide has been removed, the remaining biogas can be used directly to fuel a combustion engine without causing long term damage to the combustion engine. According to some embodiments, the carbon dioxide is simply passed through the engine and released by the exhaust.

However, in certain embodiments, the carbon dioxide should be removed. For instance, if the biogas is to be stored for later use the carbon dioxide should be removed. As carbon dioxide is itself a potentially useful and valuable byproduct, the carbon dioxide may be separated and stored in its solid cryogenic form. This can be carried out as part of the methane storage process described above, for instance inFIG.5, and in conjunction with the devices ofFIGS.4and7.

Referring toFIG.8, an exemplary CO2removal stage800is provided. The removal stage800includes an inlet802, and outlet804, and a CO2storage unit806. The stage800further includes a number of compressors (808,810) and heat exchangers (812,814), as well as an optional hydrogen sulphide filter816. The inlet802is connected to a biogas source. For instance, inlet802may be connected to anaerobic digester700illustrated inFIG.5to receive the biogas generated by the digester700. Outlet804may be coupled to a methane storage unit, such as storage system400of micro-AD unit102. For instance, outlet804may be connected directly to input valve802.

Referring toFIG.9, a process900for removing CO2from a methane-based biogas is shown. In step910, biogas comprised of at least methane and carbon dioxide is pressurized by a compressor. For instance, biogas may be passed through inlet802to compressor808ofFIG.8and pressurized by the compressor808. The biogas may also include hydrogen sulphide, in which case, it may be passed to a filter, such as filter816ofFIG.8.

In step920, the compressed biogas is passed through a heat exchanger, such as heat exchanger812ofFIG.8. In some embodiments, the heat exchanger may include a coiled finned tube enclosed by an outer gas shield. At the end of the heat exchanger, the gas passes through small orifice, which results in a rapid expansion of the gas. Due to a phenomenon referred to as the Joule Thomson effect, this rapid expansion results in a rapid cooling of the gas. The compressor, heat exchanger, and orifice can be designed to ensure that the cooling is sufficient to take the output gas below the solidification temperature of carbon dioxide (−78 C). In this instance, the carbon dioxide falls out as a form of CO2“snow” and can accumulate in a CO2storage vessel, such as storage unit806ofFIG.8. However, the methane, which liquefies at the much lower temperature (−161 C.) remains in a gaseous state and leaves the heat exchanger/storage vessel. For instance, the vessel may include a low pressure output from the heat exchanger to a methane storage system. The carbon dioxide storage vessel can be heavily insulated to allow the carbon dioxide to be stored in its solid form and sized so that the solid CO2can to be collected. It may be collected, for instance, by a service provider of a micro-AD unit. In some embodiments, a bypass interlock enables the vessel to be back purged to remove any remaining methane before being removal of the solid CO2.

In step930, the methane gas separated from the carbon dioxide in the sequestration vessel is passed through a second heat exchanger, such as heat exchanger814ofFIG.8. This heat exchanger is used to warm the gas prior to it being passed through to a second compressor phase, configured to liquefy the methane. The second heat exchanger effectively recovers some of “cold” that would otherwise be wasted. For instance, in the example ofFIG.8, warm methane may be passed at a low pressure from heat exchanger814to compressor810, and returned at a higher pressure.

In step940, the methane is passed to a methane storage unit, such as micro-AD102, for subsequent compression and storage in liquid form. High pressure methane may be passed, for instance, via an outlet such as outlet804ofFIG.8.

Referring now toFIG.10, a micro-AD installation1000is illustrated. In installation1000, a micro-AD unit1002, such as micro-AD unit102, is connected to a structure1004, such as an office building or home. The micro-AD unit1002and structure1004are connected via one or more power and data lines, which enable the unit1002to monitor physical characteristics of the structure1004, such as temperature. Depending on the structure1004′s demands, the micro-AD unit1002can provide power and/or heat to the structure1004. In some embodiments, for instance, micro-AD unit1002can function as a combined heating and power (CHP) unit, and provide to the structure1004heat produced during micro-AD unit1002′s storage operation and generation operation. In certain embodiments, the characteristics of structure1004may be monitored by a central controller, such as controller108. Data transfer to and from the central controller can use the structure1004′s existing internet connection. A secondary back up data transfer can be incorporated via an IP connection made using a mobile telephone network where available.

Referring now toFIG.11, a process1100for installing a micro-AD unit is provided. For instance, process1100may be used to install the micro-AD installation1000illustrated inFIG.10.

In step1110, an anaerobic digester is installed at an installation site, such as a home or business location. The anaerobic digester may be may be, for example, anaerobic digester700illustrated inFIG.7. According to certain embodiments, the anaerobic digester may be specially configured to generate methane gas from green waste feedstock. In some embodiments, the digester may be sized to accommodate a predetermined amount of the green waste generated at the installation site. For instance, the digester may include a sufficient number of tanks such that all of the green waste generated at the installation site during peak growing months can be digested on-site. Alternatively, the digester may be sized based on the power and/or heating needs of one or more structures at the installation site.

In step1120, a methane storage unit is installed. The methane storage unit may be, for example, the methane storage system400illustrated inFIG.4and connected to the anaerobic digester of step1110. The anaerobic digester can provided the methane to the storage unit. According to some embodiments, the storage unit includes one or more power generators configured to be powered by methane gas vented from the storage unit. For instance, the storage unit may include an internal combustion (IC) engine or fuel cell. According to certain aspects, the power generator may be a combined heat and power (CHP) unit, which simultaneously produces electricity and heat that may be captured and re-used. For instance, in the example installation1000ofFIG.10, heat may be created during power generation in micro-AD unit1002and delivered to structure1004. In certain embodiments, the heat generated can also be used to accelerate the anaerobic digestion process.

In step1130, the methane storage unit is connected to a local structure. For instance, in the example installation1000ofFIG.10, micro-AD1002is connected to a structure1004. In some embodiments, the structure1004may include one or more buildings such as homes and businesses. The connection may include, for example, one or more of power, data, and heat. In the instance of heat, heat created by a CHP co-located with the methane storage unit can be delivered to the structure via an insulated pipe. In some embodiments, the methane storage unit may be located within the structure itself, for instance, in the garage of a home. The methane storage unit may also be located near the home, for instance, within a100meter radius. However, it will be recognized by one of ordinary skill in the art that the storage unit may be located remotely from the structure, for example in excess of 1 kilometer, yet service the structure via the one or more connections. For example, a highly insulated pipe may be used to deliver heat to the structure over a large distance.

In step1140, a communications connection is established between the methane storage unit and a controller device, such as controller108. The connection may be established, for example, via a network connection over the Internet or wirelessly. In some embodiments, the controller is remotely located.

Referring now toFIG.12, a process1200for managing an anaerobic digestion network is provided. The anaerobic digestion network can include a plurality of anaerobic digestion units, such as anaerobic digester700. Additionally, the anaerobic digestion units may be coupled to methane storage units, such as system400illustrated inFIG.4. In some embodiments, the network may be configured as illustrated inFIG.1with a plurality of remote micro-AD units102,104connected to a central hub106and a controller108.

In step1210, one or more feedstock collections are scheduled. For instance, feedstock may be scheduled for collection from a first location and a second location. In some embodiments, a first micro-AD unit may be installed at the first location while a second micro-AD unit may be installed at the second unit. However, feedstock can be collected from locations that do not include micro-AD units, such as parks, public green spaces, or neighboring properties. In some embodiments, the collection scheduling is based at least in part on the one or more of the size of the respective micro-AD units or characteristics of their locations.

The process may include collecting the feedstock. In some embodiments, collecting includes performing a mowing service to collect green waste feedstock, such as grass or clover.

In step1220, feedstock is loaded into first and second anaerobic digesters. In some embodiments, the feedstock is loaded into one or more anaerobic digestion tanks configured to produce methane from green waste feedstock, which are connected to methane storage systems configured to cryogenically store the methane produced by the tanks. For instance, the feedstock can be loaded into anaerobic digester700ofFIG.7. According to certain aspects, the first anaerobic digester is located at a first location, while the second anaerobic digester is located at a second, geographically remote, location.

In optional step1230, excess feedstock is loaded into a central anaerobic digester. For instance, the excess feedstock can be loaded into a central anaerobic digester at central hub106in the example network ofFIG.1.

In step1240, one or more characteristics of the first and second anaerobic digestion units are monitored. For instance, a controller, such as controller108ofFIG.1, can be used to monitor one or more of PH, temperature, gas production rate, and gas pressure of the micro anaerobic digestion units. Additionally, the monitored characteristics may relate to energy production and usage. For instance, the characteristics may relate to the amount of electricity produced by an anaerobic digestion unit or used by a structure coupled to the anaerobic digestion.

In step1250, a control signal is issued to the first and/or second anaerobic digestion units. The control signal may be issued, for instance, by a controller such as controller108ofFIG.1. The control signal may be related to adjusting an operational characteristic of the anaerobic digestion tanks or methane storage unit, such as PH, temperature, gas production rate, and gas pressure. For instance, the control signal can operate one or more valves of either an anaerobic digestion tank or storage unit. In certain aspects, the control signal can initiate a safety procedure, such as emergency venting of methane gas due to pressure build-up. In some embodiments, the control signal causes the generation of electrical power, for instance, by causing the storage unit to vent methane gas, which can be used to run a power generator for electricity production.

It will be apparent to one of ordinary skill in the art that the disclosed systems and processes may be combined. For instance, one or more embodiments may be combined to from a complete anaerobic digestion management, methane gas production, and electricity generation process, which can be centrally controlled. This process can include scheduling and collection of the feedstock, as well as monitoring of electricity data to initiate production at an optimal time. For instance, as illustrated inFIG.13, an electricity generation process according to an embodiments can include scheduling and performing feedstock collection (step1310), loading the feedstock into a plurality of anaerobic digestion units (step1320), which are connected methane storage systems, evaluating electrical power data (step1330) to determine if electrical power should be produced, monitoring operational characteristics of micro-AD units of an anaerobic digestion network (step1340), selecting an appropriate micro-AD for production (step1350), and sending an activation signal to the selected micro-AD to produce electricity (step1360).

While various embodiments of the present disclosure are described herein, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.

Additionally, while the processes described above and illustrated in the drawings are shown as a sequence of steps, this was done solely for the sake of illustration. Accordingly, it is contemplated that some steps may be added, some steps may be omitted, the order of the steps may be re-arranged, and some steps may be performed in parallel.