BACKGROUND
Various embodiments of the present invention generally relate to systems and methods for improving the heating efficiency and comfort of a heated space; in particular, to heating system component regulating, in order to improve heating system efficiency and effectiveness.
Heating in colder climates is primarily performed by central heating systems. Typically these central heating systems burn liquid fossil fuels, for example: heating oil, propane and natural gas. As the high economic and environmental costs of fossil fuel consumption have become realized, heating with alternative fuels has become more attractive.
Solid biomass fueled central heating systems are a promising alternative to fossil fuel central heating systems. Solid biomass fuel is renewable, less costly than most fossil fuels, local to North America and does not release long sequestered carbon into the atmosphere when burned. The use of solid biomass hydronic heating systems has become commonplace throughout Western Europe, and notably in Upper Austria and Scandinavia. Wood pellet fueled boilers are regularly used in commercial, municipal and residential heating applications in these regions. These European wood pellet boilers have varying levels of sophistication and amount of intervention required by the operator. The most sophisticated wood pellet boilers, exemplified well by the ÖkoFEN brand of boilers from ÖkoFEN Forschungs- und Entwicklungs G.m.b.H. of Niederkappel, Austria, require little more operator intervention than a fossil fuel boiler, and have efficiencies and features that rival those of the most technologically advanced fossil fuel boilers.
Some solid fuel burners, such as the burner incorporated on ÖkoFEN brand boilers, allow for modulated operation. Modulated operation allows the thermal power of the burner to vary, decreasing from the burner's maximum power output. Cord wood burners typically achieve modulated operation by choking a flow of air to the burner, limiting the combustion rate of the cord wood. Granular solid fuel burners, such as wood pellet and wood chip burners, typically limit the flow of air as well as a flow of fuel in order to modulate the thermal power output of the burner. Modulating wood pellet burners may vary the flow of air and fuel by decreasing the speed of a combustion air fan and reducing the speed of a fuel dosing auger. An example of a modulating burner is Janfire NH pellet burner, from Janfire AB of Åmål, Sweden. The Janfire NH has 7 modulation levels in a modulation range between a maximum heat output of 78,000 btu/hr and a minimum heat output of 10,000 btu/hr and a LOW modulation level of 2,000 btu/hr for maintaining operation of the burner when no additional heat is needed. The LOW modulation level on the Janfire NH keeps the burner operating longer, decreasing the number of ignition sequences required by the burner. The modulation level of the Janfire NH may be controlled by a signal voltage of 0-10V. Other solid fuel burners do not modulate and operate at a single thermal power at all times, like most fluid fueled heater burners. An example of a non-modulating burner is the PB-1525 from Pellergy LLC of Montpelier, Vt. The maximum thermal power of the PB-1525 is 120,000 BTU/hr, although it may be manually set a single thermal power anywhere in the range of about 60,000 BTU/hr-120,000 BTU/hr. The operation of the PB-1525 burner is controlled by a normally open connection, which when closed causes the PB-1525 to operate at the single power output. The normally open connection of the PB-1525 is designed to work with a thermostat, such that the burner operates when the thermostat senses a demand for heat.
Unlike fluid burning devices, solid fuel burners may take about 10 minutes to initiate operation. During a startup, a small fire is ignited, usually by a blast of hot air, above the auto-ignite temperature of the fuel, and stoked to a combustion level of the desired power. The blast of hot air is usually provided for by a glow plug and the combustion air fan of the burner. The startup usually results in incomplete combustion, and therefore results in inefficient consumption of fuel, and exhaust gases having high carbon monoxide levels and high particulate matter.
Forced hot air heating systems generally perform a large number of startups and cycles. This is because; current forced hot air heating system control methods operate, such that the burner on a forced hot air system only produces heat when a thermostat call is made. The thermostat call existing only when the space to be heated is below a thermostat setpoint. With the thermostat call the burner ignites and begins producing heat, heating air within a plenum. Once the air within the plenum is above a temperature such that it is warm enough to heat a space, a blower blows the air to the space. When the thermostat in the space is satisfied, meaning no longer providing a call for heat, the forced hot air burner stops generating heat. The lack of heat generation causes the air temperature within the plenum to drop. Once the air in the plenum drops below a certain temperature, typically just above room temperature, the blower stops blowing. This cycle repeats many times throughout a day during a heating season. Because of the solid fuel burner's poor performance during startup and long startup time, solid fuel burners have been poorly suited for use as a component in a forced hot air heating system.
The use of solid fuel burners in forced hot air heating systems may result in uncomfortable heating as solid fuel burner's typically have long startup times. As the burner ignites after a thermostat call the temperature of the space to be heated continues to drop further below the setpoint temperature. Even after ignition is achieved it typically takes solid fuel burners, such as wood pellet burners, longer to raise the plenum temperature than an oil or gas burner. The result is that the temperature of the space to be heated is much colder than the setpoint temperature before the air within the plenum reaches the desired temperature and heat is delivered. Often this drop in temperature is noticeable to occupants in the space.
U.S. Pat. No. 4,842,190 describes a wood pellet furnace control system that attempts to shorten the length of time for a plenum to heat. This reference fails to solve the problem of slow warming of the air within the plenum with the use of a solid fuel burner, as it only affects the speed of the heat transfer once the burner has ignited and fails to address the time taken during startup by the burner to ignite. U.S. Pat. No. 4,842,190 also does not address the poor efficiencies and emissions during the startup of a typical solid fuel burner.
The United States is heated primarily with forced hot air central heating systems. This is unlike Europe, which heats almost exclusively with hydronic central heating systems. The stated incompatibilities between current forced hot air heating systems and methods and solid fuel burning is a problem that prevents the benefits of solid fuel heating from being realized in the United States on a large scale. A solid fuel forced hot air heating system and method that allows for prolonged burner on-time and fewer startups is needed.
Additionally, the current methods of heating with forced hot air are criticized as being less comfortable than hydronic heating systems. Because of the ON-OFF heating of the forced hot air system it is often the case that the space being heated becomes too hot, when the forced hot air system is blowing hot air into the space and too cold before the forced hot air system begins to heat again. Oscillation about a desired temperature is made even more noticeable by a roaring noise that is present during the operation of the forced hot air system. When a fluid fueled forced hot system is running, the roaring noise may be created by the combustion of the fluid fuel. This roaring noise is transmitted throughout the space to be heated by ducting and the blowing of the air. A more comfortable means of heating with forced hot air is desired. Therefore, a forced hot air heating system that is quieter and more comfortable than what is currently achievable is desired.
For the foregoing reasons, there is a need for forced hot air heating system and method for the North American market that cleanly, efficiently, and comfortably heats using solid fuel.
SUMMARY OF THE INVENTION
In various embodiments, a system in accordance with the present invention facilitates forced hot air heating with solid fuel, such as wood pellets, with near immediate response to demands for heat in a space to be heated. This is achieved, in part, by sensing the temperature of a space to be heated, providing a space temperature feedback, and transporting an ambient air to the space to be heated in response to an air handler power signal. The air handler power signal is provided in response to the space temperature feedback, allowing the transportation of the ambient air to be immediately responsive to the temperature of the space to be heated. The ambient air having thermal energy transferred to it from exhaust gases resulting from the combustion of a solid fuel. The combustion of the solid fuel is provided for by a burner operating in response to a burner power signal. The transportation of the ambient air occurs immediately once a need for heat in the space to be heated is determined and ceases immediately once the need for heat is satisfied. Heating of the space to be heated is thus not directly dependent on the burner, instead it is only required that the ambient air being transported to the space to be heated is of an elevated temperature, such that it warms the space to be heated.
In order to provide immediate response to the need for heat, the ambient air must either be maintained at an elevated temperature or immediately heated by a thermal store. Thus in some embodiments, the temperature of the ambient air is sensed, providing an ambient temperature feedback and the burner power signal is provided in response to the ambient temperature feedback. Thus combustion, the resulting exhaust gases and thermal energy are provided in response to the temperature of the ambient air, rather than the temperature of space to be heated, which is the conventional method. The thermal energy being either stored in a thermal mass, for immediate transfer to the ambient air, or transferred directly to the ambient air for the exhaust gases allows the ambient air to be of an elevated temperature when transported to the space to be heated. Thus the generation of thermal energy is decoupled from the temperature of the space to be heated. With appropriate burner heat output sizing and the optional use of thermal compliance, maintaining an elevated temperature of the ambient air is efficient during periods of consistent heating needs, such as Maine in February. However, during shoulder seasons that have a periodic need for heat constantly maintaining an elevated ambient air temperature may be inefficient or result in overheating of the heating system.
In order to allow for safe and efficient, automatic, year-round heating some embodiments include a furnace regulator switching means. The furnace regulator switching means selectively grants control of the burner and air handler power signal to either a continuous operation furnace regulating means or a periodic operation furnace regulating means. Where the continuous operation furnace regulating means provides the burner power signal in response to the ambient temperature feedback, maintain the ambient air at an elevated temperature, such that the air handler power signal may be immediately responsive to the space temperature feedback. And, the periodic operation furnace regulating means provides the burner power signal in response to the space temperature feedback as is the case with conventional forced hot air heating systems. Control of the burner and air handler power signals may be selectively granted according to a number of variables, including: The presence of an overheat status in which the exhaust gases, ambient air or any other system component is found to be over a high limit setpoint temperature. A duration of time that the power signal has remained at a specific state, such as HIGH, ON, LOW, or OFF, exceeding a timeout value. A calculated heat load, representing the amount of thermal energy being provided over a determinable time exceeding a heat load threshold value. Or the outdoor temperature, as provided by an outdoor temperature feedback, being cold enough to warrant the granting of control to the continuous operation furnace regulating means or warm enough to grant control to the periodic operation furnace regulating means.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a block diagram of components of a forced hot air solid fuel heating system;
FIG. 2A shows examples of thermosensitive means;
FIG. 2B shows a block diagram of a PID controller;
FIG. 3 shows a block diagram of components of the forced hot air solid fuel heating system;
FIG. 4 shows thermal energy storage;
FIG. 5A shows a flow chart for a burner regulating means;
FIG. 5B shows a flow chart for an air handler regulating means;
FIG. 6A shows a flow chart for a furnace regulating means;
FIG. 6B shows a flow chart for a furnace regulator switching means;
FIG. 6C shows a flow chart for a burner regulating means for a continuous operation regulating means;
FIG. 6D shows a flow chart for an air handler regulating means for a continuous operation regulating means;
FIG. 6E shows a flow chart for a burner regulating means for a periodic operation regulating means;
FIG. 6F shows a flow chart for an air handler regulating means for a periodic operation regulating means;
FIG. 7 shows an axiomatic design decomposition of the solid fuel heating system;
FIG. 8 shows an example electrical circuit for amplification and smoothing; and
FIG. 9 shows the burner regulating means controlling a dosing auger and a combustion fan.
FIG. 10A shows a Temperature vs. Time plot for a prototype heating system comprising a periodic operation regulating means;
FIG. 10B shows a Temperature vs. Time plot of a heating system comprising a burner regulating means and an air handler regulating means;
DETAILED DESCRIPTION
FIG. 1 shows a burner that burns solid fuels to provide combustion resulting in exhaust gases. The burner may be fueled by wood pellets, wood chips, cord wood or any other type of solid fuel. The amount of thermal energy output by a non-modulating burner, such as the Pellergy 1525, is determined only by the amount of time the non-modulating burner is operating. The amount of thermal energy delivered by a modulating burner, such as the Janfire NH, is determined by the amount of time the modulating burner is operating as well as a modulation level of the modulating burner at any given time. The operation of the burner is controlled by a burner power signal. The burner power signal for the modulating burner provides the modulation level for the modulating burner to operate at. The burner power signal for the non-modulating burner provides an operational state for the non-modulating burner, either ON or OFF.
The burner is located proximate to a heat exchanger, so that thermal energy from the exhaust gases may be transferred by the heat exchanger. The heat exchanger is an air to air type heat exchanger having two or more sides, all sides being sealed from one another. The exhaust gases are contained adjacent to a hot side of the heat exchanger and an ambient air is contained adjacent to a warm side of the heat exchanger. The heat exchanger may be a shell and tube type heat exchanger, wherein one fluid, either exhaust gases or ambient air, is contained within one or more tubes and the other fluid is contained within a shell which surrounds the tubes.
An air handler is located in fluidic communication with the heat exchanger, such that operation of the air handler moves the ambient air toward a space to be heated. Examples of spaces to be heated include: residences, municipal buildings and business. Examples of air handlers include blowers, and dampeners. Air handlers may be variable or discrete in operation. A variable speed blower for example, allows for the ambient air to be transported to the space to be heated at various rates within a speed range. The amount of air delivered to the space to be heated by the variable speed blower is determined by the amount of time the variable speed blower is operating as well as the flow rate of the variable speed blower at any given time. The variable speed blower may be a multi-speed blower, such as a 3-speed furnace blower model #6BLR12FAP from Fantech, Inc. of Lenexa, Kans. The 6BLR12FAP may be run at three speeds LOW, MEDIUM, and HIGH. The 6BLR12FAP comprises a permanent split capacitor (PSC) motor. Variable speed blowers that comprise PSC motors and shaded pole motors may be controlled by a variable speed blower controller. Such variable speed blower controllers take as input a signal voltage and drive the variable speed blower at a speed proportional to the signal voltage. The variable speed blower controller allows the variable speed blower to be operated within a range of speeds rather than three discrete speeds. For example, the pairing of the 6BLR12FAP with a Nimbus—AC Fan Control, Model No. 240B7T00-F from Control Resources, Inc. of Littleton, Mass., allows for the blower to be operated variably over a range of speeds not just 3 discrete speeds. One of the benefits of the variable speed blower controllers, such as the Nimbus, is that it provides for the gradual ramping up and ramping down of air flow. Gradually increasing (and/or decreasing) the flow rate of air into an occupied space makes the blowers operation less noticeable to the occupants of the space. The variable speed blower controller may comprise TRIAC, variable frequency or AC to DC inverter technology in order to control the speed of the variable speed blower.
Alternatively, the air handler may be comprised by a single speed blower. The flow rate for the single speed blower is constant, and the amount of ambient air delivered to the space to be heated is controlled only by the amount of time the single speed blower is operating. The G-12 blower mated with a ¾ horsepower motor from Delhi Industries, Inc. of Brockville, Ontario Canada is an example of a single speed blower that may provide 100,000 BTU/hr to the space to be heated, given a typical ambient air temperature rise. The ambient air is typically directed toward the space to be heated through one or more supply ducts, which are in fluidic communication with the air handler, and from the supply ducts into the space to be heated through one or more supply outlets. The ambient air is typically returned to the heat exchanger by one or more return ducts.
An ambient thermosensitive means senses an ambient air temperature and provides an ambient temperature feedback. A space thermosensitive means senses a space to be heated temperature and provides a space temperature feedback. An ambient temperature setpoint and a space temperature setpoint are provided relative the ambient temperature feedback and the space temperature feedback.
The ambient temperature feedback and the space temperature feedback may be provided continuously. Examples of thermosensitive means that provide temperature feedback continuously are shown in FIG. 2A. The ambient temperature feedback and space temperature feedback may be provided continuously by a resistance temperature detector (RTD), a thermistor or a thermocouple. The thermocouple provides an output potential that correlates with a change in temperature. The RTD and the thermistor provide an output resistance that varies with temperature. The model# ATH100K1R25T70 from Analog Technologies, Inc. of Santa Clara, Calif. is an exemplary thermistor for measuring temperatures associated with the ambient air. By applying an input potential of a known value, for example a signal voltage such as 5V, to the thermistor or the RTD and measuring the resulting flow or potential over the sensor the output resistance of the thermistor or RTD and the correlating temperature may be found. Typically the thermistor or the RTD is a part of a thermistor circuit or an RTD circuit that comprises: a potential divider, a Wheatstone bridge, or an equivalent circuit. The thermistor circuit or the RTD circuit provides an output resistance through the use of one or more reference resistances, of known value, and the input potential. The output potential of the thermistor circuit or the RTD circuit varies according to the output resistance provided by the thermistor or RTD. The thermistor circuit, the RTD circuit or a thermocouple circuit may further comprise an analog to digital converter (ADC), providing for measurement of the output potential. The ADC converts the output potential to an output code, which is a digital number representative of the output potential. The precision of the ADC is measured by a resolution, which is the maximum number of the possible output codes over the measurable range of the output potentials. For example, the resolution of an 8-bit ADC allows for 2̂8 or 256 discrete values. An 8-bit ADC measuring over a 0-5V range is 5/(256−1), or roughly 20 mV. The resolution is equal to the smallest change in the output potential that is required to produce a change in the output code. In the case of a continuous thermosensitive means, such as those described above and as well as eqivalents such as a Silicon diode bandgap sensor circuit, the corresponding temperature setpoint will typically be an electronically or digitally coded value.
The ambient temperature feedback and space temperature feedback may alternatively function discretely. The thermosensitive means functioning discretely provide a temperature feedback that is relative to a setpoint temperature. The thermosensitive means functioning discretely provide the ambient temperature feedback or the space temperature feedback discretely, such that the ambient temperature feedback or the spacer temperature feedback is either TRUE or FALSE. The ambient temperature feedback or the space temperature feedback functioning discretely gages if the sensed temperature is higher or lower than the ambient temperature setpoint or the space temperature setpoint. Examples of thermosensitive means that function discretely include: a mechanical thermostat and a thermostatic switch. Generally the mechanical thermostat and the thermostatic switch employ the use of a bimetallic strip to sense the temperature. The bimetallic strip is comprised of two metal strips with differing rates of thermal expansion, fused together. As the temperature of the bimetallic strip changes, a first metal deforms more than a second metal causing the bimetallic strip to bend proportionally with the change of temperature. For example, the mechanical thermostat generally makes use of a bimetallic coil. Depending on the orientation of the bimetallic coil, the bimetallic coil may grow tighter or looser with increasing temperature. The change in the bimetallic coil results in an outside end of the bimetallic coil moving. A moving contact is attached to the moving end of the bimetallic coil. An electrical connection is made within the mechanical thermostat, between the moving contact and a static contact at a specified temperature. The electrical connection is often made when the temperature falls below the specified temperature. Typically the specified temperature of the mechanical thermostat is set by adjusting the location of the static contact. The thermostatic switch usually incorporates a bimetallic disk. The thermostatic switch is typically not adjustable, although thermostatic switches that allow for adjustment are available. The electrical connection is made within the thermostatic switch between a first contact at the center of the bimetallic disk and a second contact at the circumference of the bimetallic disk. At a specified transition temperature the bimetallic disk pops, inverting itself. The bimetallic disk once inverted creates (or breaks) the electrical connection with the first and second contact of the thermostatic switch. Other types of thermostatic switches may include a bellows with a fluid or a wax, of a known rate of thermal expansion. The bellows expands under increasing temperature causing a mechanical switching mechanism, such as a micro-switch, to create (or break) the electrical connection. In the case of discretely functioning thermosensitive means the corresponding temperature setpoint will typically be a mechanical arrangement that allows for electrical connection or a change in electrical continuity, at the transition temperature.
A burner regulating means is shown in FIG. 1. The burner regulating means shown in FIG. 1 provides a burner power signal in response to the ambient temperature feedback and the ambient temperature setpoint. The ambient temperature feedback is provided continuously as the coded output from the RTD circuit. The ambient temperature feedback may be provided as a direct measure of the temperature of the ambient air by sensing the temperature of the ambient air directly. It may also be practical, in certain situations, to provide the ambient temperature feedback through indirect measurement. For example, the ambient temperature feedback may be indirectly measured through the sensing of the temperature of the warm side of the heat exchanger, or another substance that is in or near thermodynamic equilibrium with the ambient air. The temperature of the substance in or near thermodynamic equilibrium with the ambient air may provide an indirect measure of the ambient air and the ambient air temperature feedback. The ambient temperature feedback may be a coded ambient setpoint. The coded ambient setpoint may be stored within a digital storage memory, such as FLASH, random access memory (RAM), or read only memory (ROM). The burner regulating means may comprise a proportional-integral-derivative (PID) burner controller, shown in FIG. 2B. The PID burner controller may be implemented through the use of a microcontroller (MCU) or a field programmable gate array (FPGA). The PID burner controller calculates an ambient temperature error, e(t), which is the difference between the ambient temperature setpoint and the ambient temperature feedback. The ambient temperature error may be calculated at regular intervals defined by a time constant. The current ambient temperature error is weighted by a proportional (P) factor creating a proportional variable. The sum of the ambient temperature errors over time is weighted by an integral (I) factor creating an integral variable. The difference between the previous ambient temperature error and the current ambient temperature error is divided by the time constant and weighted by a derivative (D) factor creating a derivative variable. The proportional variable, integral variable, and derivative variable are summed to create a control variable. The control variable is then used as the burner power signal for the modulating burner. Once tuned the PID burner controller will minimize the ambient temperature error, or the difference between the ambient temperature feedback and the ambient temperature setpoint, by adjusting the burner power signal.
An air handler regulating means provides the air handler power signal. The air handler regulating means shown in FIG. 1 comprises the mechanical thermostat located in the space to be heated. The static contact of the mechanical thermostat provides the space temperature setpoint. The moving contact of the mechanical thermostat provides the space temperature feedback. As the temperature of the space to be heated drops below the space temperature setpoint, the moving contact and the static contact of the mechanical thermostat form the electrical connection. The electrical connection may be used directly, to deliver the air handler power signal, as with an inline thermostat. Alternatively, the electrical connection of the mechanical thermostat may carry only a smaller control current that closes a relay, which delivers the air handler power signal. The signal speed blower shown in FIG. 1 operates when the mechanical thermostat determines that the temperature of the space to be heated is below the space temperature setpoint.
The ambient air setpoint in FIG. 1 may be in a range 40-95 degrees Celsius. This range is selected as it is an air temperature range that is sufficiently above room temperature so that air at this temperature would act to heat a space to be heated. The burner regulating means in FIG. 1 maintains the ambient air temperature in this range. When the temperature of the space to be heated is below the space temperature setpoint the air handler regulating means, which comprises the mechanical thermostat, operates the single speed blower and the ambient air is delivered into the space to be heated, warming it. When the air handler is operating and moving the ambient air past the heat exchanger, the temperature of the ambient air decreases and the burner regulating means increases the modulation level of the burner. As the space to be heated is heated to the space temperature setpoint, the air handler regulating means stops the single speed blower. The ambient air flow decreases, causing the ambient temperature feedback to increase, and the burner regulating means decreases the modulation level of the burner.
A similar embodiment of the present invention is shown in FIG. 3. The thermostatic switch is shown as the ambient thermosensitive means for providing the ambient temperature feedback. The thermostatic switch also provides the ambient temperature setpoint as the specified transition temperature of the thermostatic switch. The thermostatic switch may directly provide the burner power signal to the non-modulating burner, shown in FIG. 3. In the shown embodiment the thermostatic switch regulates the operation of the non-modulating burner in order to keep the temperature of the ambient air at or above the ambient temperature setpoint. Typically the ambient temperature setpoint is about 40 degrees Celsius, so that the non-modulating burner operates when the ambient air temperature falls below 40 degrees Celsius. A hysteriesis of the thermosensitive means defines an operation range proximate the setpoint temperature according to the current operational state. The hysteriesis of the thermostatic switch will cause the non-modulating burner, which is currently operating, to stop once the ambient temperature feedback is outside of the operation range. The operation range typically has an upper limit that is about 60 degrees Celsius. This operation range may alternatively be defined by the use of more than one thermostatic switch. Additionally, one or more high limit temperature sensors may be used to cease the operation of the burner through the burner power signal. These high limit temperature sensors may be located on the hot side or the warm side of the heat exchanger, in the exhaust gas or in the ambient air.
A digital thermostat shown in FIG. 3 employs the thermistor circuit to provide the space temperature feedback. The digital thermostat also has an input for the space temperature setpoint, allowing the selection of a desired temperature in the space to be heated. The microcontroller takes as input the space temperature feedback and the space temperature setpoint and provides the air handler power signal. The flow rate of the variable speed blower is controlled by the air handler power signal. The microcontroller shown in FIG. 3 may use PID controls or any other closed feedback control algorithm to provide the air handler power signal that determines the flow rate of the variable speed blower. As the burner will operate at times when the space to be heated is at or above the space to be heated temperature feedback, the heat exchanger or exhaust gases could potentially become dangerously hot before the ambient temperature feedback is greater than the ambient temperature setpoint. In order to prevent the heat exchanger or exhaust gases from overheating the variable speed blower may operate at a low flow rate. The low flow rate is generally defined as the flow rate produced when the variable speed blower is operating at 50% or less than the variable speed blower's maximum speed. The low flow rate moves enough ambient air away from the heat exchanger to allow more thermal energy to be transferred away from the exhaust gases, but does not move enough ambient air to raise the temperature of the space to be heated more than about 2° C. Also, the air handler means may include a dampener that allows the flow of ambient air past the heat exchanger, but not into the space to be heated at times when the temperature of the space to be heated is above the space temperature setpoint to prevent over heating of the heat exchanger or exhaust gases. As the air handler regulating means determines an increased need for heat in the space to be heated the flow rate of the variable speed blower is increased up to 100% speed.
Independent regulation of the ambient air temperature and the space to be heated temperature may require the incorporation of a thermal storage means for providing a thermal mass. The thermal mass may act as an energy buffer during times when the heat provided by the burner is not balanced by the heat provided to the space to be heated. Many solid fuel burners, such as the Janfire NH and the Pellergy 1525 must cease operation and purge the ash, which has accumulated in the burn chamber, periodically. An ash scrape mechanism related to the Janfire NH is the subject of U.S. Pat. No. 7,739,966. During this time the burner is not operating and no heat is being generated, however the space to be heated may still require heat. The thermal mass allows for the heat stored in it to be transferred to the ambient air and transported to the space to be heated when the burner is not operating. The thermal mass should be sized to provide for delivery of heat to the space to be heated throughout the off-time of a typical ash scrape cycle. Additionally at times when the space to be heated is not in need of heat, the thermal mass may accumulate the heat that is generated by the burner in order to prevent heat from being vented, and subsequently fuel from being unnecessarily consumed. FIG. 4 shows some examples of thermal storage means. The thermal mass may be provided through a wall thickness, or mass of the heat exchanger. The wall thickness when increased creates the thermal mass. The ambient air itself may also act as the thermal storage means. Recirculating the ambient air past the heat exchanger allows for a greater volume of ambient air for thermal energy storage. Although the heat capacity of air at standard room conditions is low, during recirculation, all surfaces that come into contact with the ambient air as it recirculates additionally act as the thermal storage means. Recirculation of the ambient air may be achieved through the use of the air handler and a recirculation dampener, which when closed directs the ambient air back to the air handler and when open directs the ambient air to the space to be heated.
FIG. 5A depicts the burner regulating means for providing the burner power signal configured for use with the non-modulating burner, such as the Pellergy 1525, and comprising thermosensitive means that function discretely. The ambient thermosensitive means shown in figure FIG. 5A is the thermostatic switch. The thermostatic switch may be a Klixon, open on rise, thermostat part number: M223L140050521 from Sensata Technologies, Inc. of Attleboro, Mass. The Klixon, open on rise, thermostat may open at 140° F. and close about 135° F. Optionally, the burner regulating means may include an exhaust gas high limit switch, such as a thermal safety switch part No. WMO-1 200 from Field Controls, LLC of Kinston, N.C. and/or a heat exchanger high limit switch. The heat exchanger high limit switch may be, for example, a SHL250 Thermostat Limit Control from Sealed Unit Parts Co., Inc. of Manasquani, N.J. The SHL250 may be automatically resetting. The SHL250 cuts out at 250° F. and cuts in at 210° F. If the exhaust gases are sensed by the exhaust gas high limit switch to be at a higher temperature than an exhaust gas high limit setpoint, then the burner power signal is interrupted and the exhaust gas high limit switch will need to be manually reset before the burner power signal will be uninterrupted. If the heat exchanger is sensed by the heat exchanger high limit switch to be at a temperature in excess of a heat exchanger high limit setpoint the burner power signal is interrupted. As the ambient air temperature feedback falls below the ambient air setpoint as sensed by the thermostatic switch, the burner power signal operates the non-modulating burner. The non-modulating burner once operating warms the ambient air until the ambient air is outside of the operational range causing the burner regulating means to interrupt the burner power signal and cease the non-modulating burner from operating. It is shown in FIG. 5A that the burner regulating means provides the burner power signal, generally in response to the ambient temperature feedback, as it is in response to the ambient temperature feedback that the burner power signal initiates the burner. The burner power signal may be interrupted or changed to an OFF signal in response to other feedbacks, such as with the exhaust gas high limit switch above, however the ON signal provided by the burner power signal requires the ambient temperature feedback be within the operational range, defined by the burner regulating means.
FIG. 5B shows the air handler regulating means for providing the air handler power signal, configured for use with the single speed blower and implemented through discrete mechanisms. The air handler power signal causes the operation of the single speed blower when the space temperature feedback is below the space temperature setpoint. The space temperature feedback may be provided for by the mechanical thermostat, for example a Honeywell Model # CT8K from Honeywell International, Inc. of Belmont, N.C., and a relay circuit. Whereby, the air handler regulating means provides the air handler power signal powering the single speed blower. Once the space to be heated has been sufficiently warm to overcome the hysteresis of the mechanical thermostat the air handler power signal will be interrupted and the single speed blower will cease to operate. Optionally, a minimum ambient temperature switch and a maximum ambient temperature switch may be located proximate the warm side of the heat exchanger to sense the temperature of the ambient air. If included in the air handler regulating means the minimum ambient temperature switch will interrupt the air handler power signal if the ambient air has a temperature lower than a minimum ambient temperature setpoint. The minimum ambient temperature switch may be a close on rise thermostat that closes at 90° F. such as SHF90 Thermostat Fan Control from Sealed Unit Parts Co., Inc. The purpose for the minimum ambient temperature switch is to prevent ambient air from being transported to the space to be heated, if the ambient air is too cool. If included in the air handler regulating means the maximum ambient temperature switch will interrupt the air handler power signal, and optionally the burner power signal if the ambient air has a temperature that is above a maximum ambient air setpoint. The maximum ambient air switch may be, for example a SHL200 Thermostat Limit Control from Sealed Unit Parts Co., Inc. The SHL200 is an open on rise type limit switch and cuts out at 200° F. The function of the maximum ambient temperature switch is to prevent ambient air from being transported to the space to be heated, if the ambient air may be unsafe to deliver to the space to be heated. The State of Maine requires that solid fuel burning furnaces have the functionality provided by the maximum ambient temperature switch. Specifically the Maine Solid Fuel Board requires: “Furnaces shall have a 250 degree Fahrenheit limit control installed in the supply plenum not more than ten (10) inches above the top surface of the heat exchanger. The limit control shall extend at least twelve (12) inches into the supply plenum.”
FIG. 6A depicts a furnace regulating means for selectively providing the burner power signal and the air handler power signal. The furnace regulating means may be software based and operate on the microcontroller. The furnace regulating means is comprised of three sub-functions: a continuous operation means and a periodic operation means, each of which provides the burner power signal and the air handler power signal and a furnace regulator switching means for selectively granting control of the burner power signal and the air handler power signal to either the continuous operation means or the periodic operating means. The continuous operation means for providing the burner power signal is provided in response to the ambient air temperature, and independent of the space temperature feedback. This allows for the burner to operate more continuously reducing the number of times the burner starts up. Thus, the continuous operation means is well suited when demand for heat is high, greater than 50% maximum heat output of the heating system. Alternatively, the periodic operation means for provides the burner power signal only during a thermostat call, which is provided for by the space to be heated temperature feedback. Thus, the periodic operation means will allow for heat to be generated by the burner only at times when the space to be heated requires heating and not operate the burner when the space to be heated does not require heat. The periodic operation means allows for reduced fuel consumption, and is well suited when demand for heat is moderate or low, or less than 50% of the maximum heat output of the heating system. The furnace regulator switching means, deterministically grants control based upon a perceived need for continuous operation, to either the continuous operation means or the periodic operation means.
An embodiment of the furnace regulator switching means for selectively granting control of the burner power signal and the air handler power signal is shown in FIG. 6B. The perceived need for continuous operation may be based on an outdoor temperature feedback. The outdoor temperature is sensed as input as the outdoor temperature feedback. A heat load is calculated as a total energy, the summation of the burner heat output beginning at an energy start time, divided by the difference between the current time and the energy start time. If the burner power signal is currently at a LOW power level and a previous burner power signal, defined as the last burner power signal, is not LOW, the current time is recorded as a low fire start time. If the burner power signal is currently at a HIGH power level, defined as 100% heat output for the burner, and the previous burner power signal is not HIGH, the current time is recorded as a high fire start time.
If the continuous operating means currently has control of the burner power signal and air handler power signal, the furnace regulating switching means decides based on one or more criteria if the continuous operating means should continue to have control according to the embodiment of the furnace regulator switching means shown in FIG. 6B. The continuous operation means loses control with the presence of one or more of the following conditions, which may or may not be determining factors in other embodiments of the invention: If the burner is currently operating at LOW and has been operating at LOW for longer than a burner low timeout value, if the outdoor temperature feedback is found to be above a maximum continuous outdoor temperature, if an overheat status is present, in which case the overheat status may be cleared, if the heat load is below a minimum continuous heat load value and the time the heat load is measured over is greater than a minimum heat load time. If conditions that cause loss of control by the continuous operation means are not present the furnace regulator switching means continues granting control to the continuous operation means. If conditions that cause loss of control by the continuous operation means are present, the total energy is reset to zero and the current time is recorded as the energy start time.
If the continuous operation means currently does not have control, the furnace regulator switching means shown in FIG. 6B decides based on one or more criteria if the continuous operating means should be granted control of the burner power signal and the air handler power signal. According to the embodiment of the furnace regulator switching means shown in FIG. 6B the continuous operation means are granted control under one or more of the following conditions, which may or may not be determining factors in other embodiments of the present invention: If the burner is currently operating at HIGH and has been operating at HIGH for longer than a burner high timeout value, if the outdoor temperature feedback is found to be below a minimum periodic outdoor temperature, if the heat load is above a maximum periodic heat load value and the time the heat load is measured over is greater than the minimum heat load time. If conditions that cause the grant of control to the continuous operation means are present, the total energy is reset to zero and the current time is recorded as the energy start time.
An embodiment of the continuous operation means provides the burner power signal and the air handler power signal according to the flow chart shown in FIG. 6C and FIG. 6D. The continuous operation means may optionally, as a safety precaution sense the temperature of the exhaust gases and the heat exchanger and compare them against the exhaust gas high limit temperature and the heat exchanger high limit temperature. If either the exhaust gas temperature or the heat exchanger temperature is found to be higher than its corresponding high limit temperature, the burner power signal sends an OFF signal to the burner and an overheat error is flagged. If there is no overheating of the exhaust gases or heat exchanger, or if those safety features are not present the burner power signal is provided in response to the ambient air temperature feedback. For example, the burner power signal provided for the modulating burner may be provided by the PID controller. Once the burner power signal has been provided the continuous operation means performs the logic to provide the air handler power signal. Optionally, the continuous operation means may check the temperature of the ambient air in order to ensure that it is within a range defined by the minimum ambient air temperature and the maximum ambient air temperature. If the temperature of the ambient air is found to be outside of the range defined by the minimum ambient air temperature and the maximum ambient air temperature the air handler signal sends an OFF signal to the air handler. If the temperature of the ambient air is found to be above the maximum ambient air temperature then the burner power signal is provided as an OFF signal to the burner. If the temperature of the ambient air is found to be with the range defined by the minimum ambient air temperature and the maximum ambient air temperature, the continuous operation means provides the air handler power signal in response to the space temperature feedback. The space temperature feedback may be provided by the mechanical thermostat. Such that, the air handler power signal is provided a HIGH signal when there is a thermostat call and a LOW signal when there is no thermostat call. The air handler power signal may be used to operate the variable speed blower, such that a LOW signal results in a small ambient air flow, typically less than <50% speed of the variable speed blower and the HIGH signal results in a large ambient air flow typically between 90% and 100% speed of the variable speed blower. The small ambient air flow may be defined as being large enough to remove heat from the heat exchanger, and prevent overheating while allowing the burner to continue to operate. The small ambient air flow may also be defined as being small enough to not raise the temperature of the space to be heated during an average heat load during a heating season.
An embodiment of the periodic control means provides the burner power signal and the air handler power signal according to the flow chart shown in FIG. 6E and FIG. 6F. The periodic control means may optionally, as a safety precaution sense the temperature of the exhaust gases and the heat exchanger and compare them against the exhaust gas high limit temperature and the heat exchanger high limit temperature. If either the exhaust gas temperature or the heat exchanger temperature is found to be higher than its corresponding high limit temperature, the burner power signal sends an OFF signal to the burner and an overheat error is flagged. If there is no overheating of the exhaust gases or heat exchanger, or if those safety features are not present the burner power signal is provided. The periodic control means provides the burner power signal operating the burner only when the space temperature feedback is lower than the space temperature setpoint. For example, the space temperature thermosensitive means may be provided for by the mechanical thermostat and the burner may be the modulating burner. In this case the periodic control means may provide the burner power signal based upon the ambient temperature feedback only once the mechanical thermostat is calling for heat. The periodic control means may provide the air handler power signal according to the ambient air temperature. If the temperature of the ambient air is found to be outside of the range defined by the minimum ambient air temperature and the maximum ambient air temperature the air handler signal sends an OFF signal to the air handler. If the temperature of the ambient air is found to be above the maximum ambient air temperature then the burner power signal is provided to send an OFF signal to the burner. If the temperature of the ambient air is found to be within the range defined by the minimum ambient air temperature and the maximum ambient air temperature, the periodic operation means provides a HIGH air handler power signal to the air handler. The air handler may be the variable speed blower in which case the speed of the blower may be ramped up and/or ramped down in order to improve comfort.
FIG. 7 is an axiomatic design decomposition of the invention. The decomposition shows a list of Functional Requirements (FR) to the left and a list of corresponding Design Parameters (DP) to the right. It can be seen from FIG. 7 that FR1 “Control the generation of heat” is an independent function from FR4 “Control the flow of ambient air toward the space to be heated”. Characterizing the present invention, the generation of heat is done as needed to maintain enough available heat, such that the ambient air may be warmed FR3. The generated heat may be stored FR2, by either the ambient air or other means prior to its delivery to the space to be heated. Delivery of the ambient air occurs as needed to maintain a set temperature in the space to be heated. Although procedural coupling does exists, for example heat may not be stored until FR2 or transfer to the ambient air FR3 before it has been generated FR1, the design is decoupled. Current solid fuel forced hot air systems are not decoupled as the heat is generated in response to the temperature of the space to be heated, preventing the generation of heat from occurring independently from DP4.1, which “sense(s) the temperature of the space to be heating.” Because of the decoupled design shown in FIG. 7 it is possible for the designer of the forced hot air system to setup a design equation that defines the heating of the space to be heated in terms defined in the design decomposition. The solving of this design equation allows for optimal performance:
Decoupling these functional requirements allows for a solid fuel hot air furnace that operates with longer burner ON times, more efficient combustion, and results in more comfortable heating. The space to be heated is maintained at the setpoint temperature without excessive oscillation about the setpoint. The heating of the space is often unnoticeable by the occupants, as the temperature of the space remains steady and the roar that often accompanies a fluid fueled burner is absent, with solid fuel burning appliances.
In an embodiment of the present invention, the microcontroller used to operate the regulating means described above may be an Arduino Mega 2560, which is an Open Source microcontroller for development. Controlling the modulation level of the Janfire NH burner may be achieved through the burner power signal provided for by one of a number of PWM Analog 0-5V analog outputs of the ATMega2560. An operational amplifier or equivalent means may be used to double the potential of the PWM 0-5V analog output, as shown in FIG. 8. An RC filter may be used to smooth the PWM 0-10V analog signal. The resolution of the analog outputs of the ATMega2560 is 8-bit. Therefore the 0-10V signal to the Janfire NH burner must be mapped to 0-255 digital bits.
The ATMega2560 may control a single speed blower through the use of a digital output and solid state relay, such as Crydom, Inc. model No. DC60S3 from Crydom of San Diego, Calif. The ATMega2560 may control a variable speed blower through the use of an AC fan controller, such as the Nimbus—AC Fan Control Model No. 240B7T00-F from Control Resources, Inc. of Littleton, Mass. The ATMega2560 may communicate to the Nimbus via a control signal, which has a variable potential of 0-10V. The Nimbus provides the air handler power signal to the variable speed blower proportionally to the control signal. The control signal may be output by via an analog output of the ATMega2560 through the schematic shown in FIG. 8 and equivalents thereof.
The ATMega2560 may control the operation of the burner through the PWM analog outputs, as shown in FIG. 9. The ATMega2560 may control the speed of the combustion fan, which is suited to deliver air for the burner. The ATMega2560 may control the amount of wood pellet fuel delivered to the burner through the use of the dosing auger. The dosing auger may include an auger position feedback that measures the position of the dosing auger allowing for a known amount of wood pellet fuel to be delivered. In the case that the dosing auger and combustion fan are directly controlled by the burner regulating means, the burner power signal comprises the signals operating both the dosing auger as well as the combustion fan.
The ATMega2560 may take as digital inputs: the mechanical thermostat and thermostatic switches and may take as analog inputs thermocouple circuits and RTD or thermistor circuits. The digital inputs detect a change in the continuity of the circuit attached (mechanical thermostat or thermoswitch) and change the value of a boolean variable to match the switches state. The analog inputs of the ATMega2560 are 8-bit and allow for mapping of the output potential to a 0-255 output code.
For functions that require timing the ATMega2560 may use the millis ( ) function to return the current time from an external 16 MHz oscillator. Digital storage on the ATMega2560 is provided for by 256 Kb of FLASH storage.
Software code for regulating a wood pellet forced hot air furnace is shown in Appendix A. The code in Appendix A may be run on an Arduino, such as ATMega2560, in order to provide the burner power signal and the air handler power signal according to an exemplary embodiment of the invention.
The Temperature vs. Time plots shown FIG. 10A-C illustrate the operation of a solid fuel forced hot-air heating system operating in accordance with various embodiments of the present invention. The system comprising a shell-tube type heat exchanger, a Janfire NH wood pellet burner, a blower, and an Arduino ATMega2560 running software illustrated in Appendix A. The system is configured to heat a room, the interior space of The God-Damned Jewel and Ideal Woman Tavern of Bethel, Me. The room is of approximately 150 cubic meters. Hot air is provided through a supply duct and return air is return to the system through a return duct. The system employs a thermostat in the room to provide a space temperature feedback and a space temperature setpoint. The system employs thermistors in thermal communication with the ambient air and the heat exchanger to provide an ambient temperature feedback. An ambient temperature setpoint is included within the code running on the Arduino.
FIG. 10A illustrates the initial cold-start start of the system. The initial start of the system demonstrates the periodic operation means outlined in FIG. 6D as well as the current control methods employed by common forced hot-air heating systems. It can be seen that it takes a substantial amount of time, ˜750 seconds, after the thermostat call for the ambient air within the heat exchanger to warm sufficiently for the blower to begin to transport the ambient air to the room. During this amount of time the room will continue to lose heat and drop lower below the space temperature setpoint set on the thermostat. The long delay between the thermostat call and the initial delivery of heat is due to the time required for the burner to initialize combustion and heat the heat exchanger and ambient air. It is shown in FIG. 10A to take the heating system nearly 3000 seconds for the room to reach the space temperature setpoint set on the thermostat and for the thermostat to be satisfied. It then takes another ˜750 seconds for the ambient air within the heat exchanger to cool causing the blower to stop transporting the ambient air (and heat) to the room. During this amount of time the room is continuing to be heated and the temperature of the room is exceeding the space temperature setpoint set on the room thermostat. The system operating in the periodic operation regulating means illustrated in FIG. 10A results in temperature swings within the room in excess of 10° F. above and below the temperature set at the thermostat when operated with outside temperatures approaching freezing. Temperature swings of this magnitude are noticeably uncomfortable for those occupying the room. Fortunately for the frequenters of the tavern, noticeable temperature swings, even at outdoor temperatures of −20° F., are eliminated with the employment of system regulating means in accordance with the present invention.
FIG. 10B illustrates the same system as FIG. 10A, but after the initial start and operating according to the burner and air handler regulating means detailed in FIG. 5A and FIG. 5B. At Zero seconds the ambient air is shown to have an elevated temperature of about the ambient air setpoint coded in the software running on the Arduino. For the system an air temperature above 100° F. and below 250° F. is coded in the software as effectively being about the ambient temperature setpoint. After ˜100 seconds the thermostat calls for heat, evidencing the space temperature feedback being significantly below the space temperature setpoint. This triggers the air handler power signal to immediately change state to an ON signal. The system may optionally vary the rate of the blower via the air handler power signal additionally base upon the temperature of the ambient air, such that full speed ambient air transport occurs at ambient air temperatures in excess of 150° F. and slower speed transport of the ambient air occurs when it is at lower temperatures. The function of the air handler is principally responsive to the space to be heated temperature through the thermostat. Before 500 seconds it can be seen that the ambient air temperature falls out of a range that is effectively about the ambient air setpoint. At this point the burner power signal responds by changing the burner to an ON state. At ˜750 seconds the room has reached the hysteresis temperature of the thermostat and the space temperature feedback changes reflecting no need for heat in the room. This triggers the blower to cease transporting ambient air to the room. The burner power signal does not change the state of the burner until the ambient air temperature exceeds the acceptable range that is effectively about the ambient temperature setpoint at ˜900 seconds. The heating system is shown in FIG. 10B to deliver heat immediately in response to a thermostat call and stop delivery of heat immediately when the thermostat is no longer calling. Additionally, the burner operates without direct dependence upon the temperature of the room. With sufficient thermal mass and the correct burner heat output, the burner may provide thermal energy with fewer stops and starts and more efficiently.
Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. For example, the use of software coded control algorithms, which do not perform PID calculations to provide the burner power signal or the air handler power signal. Also the use of the ambient air feedback and one or more other feedback signals, such as: an exhaust gas oxygen sensor feedback, a flame presence illumination sensor feedback, or an exhaust gas pressure sensor feedback. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.
Patent Application
20150362217
