Abstract:
A fireplace appliance for warming room air without line electrical connection has a high efficiency thermoelectric generator having a heat-rejecting surface connected to a heat sink. The generator has a heat-receiving surface facing the site where a fireplace flame is to be located. In one embodiment the generator provides power to operate a fan that forces air through an air duct. The air duct has an inlet port receiving a flow of room air, and an outlet port. The heat sink is placed in the air duct where airflow generated by the fan moves across and cools the heat sink. The air heated by the heat sink flows to the room through the outlet port. One suitable material for the thermoelectric generator is a Bi—Te semiconductor. A number of options are shown that allow fan operation to commence properly while the appliance begins a cold start.

Description:
BACKGROUND OF THE INVENTION 
     Fireplaces have been a part of permanent dwellings since such dwellings were first built. In the early years before central heating was developed, fireplaces were an important source of the heat that warmed these dwellings and their occupants. However, after central heating became available, the greater convenience and efficiency of central heating relegated fireplaces to an esthetic function for the most part. 
     One long-standing problem with fireplaces is the inconvenience and mess of burning wood. It is relatively difficult to start a wood fire. Once that has been done, it is necessary to continuously add further wood to maintain the fire. It is not easy to shut down a wood fire. Instead the occupant must allow it to burn itself out, during which time cold air can flow down the flue, cooling the room air. Then, after waiting for the ashes to completely cool which may take a day or more, the occupant must remove and discard the ashes. This last is a dirty and tedious job. Ashes are dusty, and the fine particles drift throughout the room during ashes removal. 
     For these reasons, gas-fueled fireplaces are becoming more and more popular. They are easy to start and stop, and they produce little or no soot and essentially no ash. An artificial log or two provide a wood fireplace ambiance, and a hidden burner directs a flow of gas to feed the flame and to form a combustion site within the fireplace. 
     More recently fireplace appliances or inserts have been developed that substantially improve fireplace efficiency. These appliances include a heat exchanger receiving heat from the combustion site for warming room air. A circulating fan forces room air through the heat exchanger. One significant disadvantage of most of these inserts is that they require line electrical power to operate the circulating fan. Thus, they are inoperable during power outages, when they&#39;re frequently needed most. Secondly, particularly during installation in existing fireplaces, running line power to a fireplace is expensive. 
     Recent developments have addressed this problem to some extent. For example, U.S. Pat. No. 6,037,536 (Fraas) shows a fireplace insert using a panel of photovoltaic devices to convert infrared radiation energy to electrical energy. This design has the potential to provide a substantial amount of power, and more than enough to operate a circulating fan. However, the overall design may not be well suited for heating room air. And the photovoltaic devices may be expensive and require frequent cleaning for good efficiency. 
     Accordingly, there are good reasons to seek a different technical approach when the aim is improve the ability of a fireplace to heat a room. Thermoelectric devices such as thermopiles have been available for many years, used for example for sensing presence of pilot flame in a burner. The pilot flame produced sufficient heat to produce a current allowing a solenoid to hold a gas valve open. However, until recently, thermopiles produced power measured in the hundreds of milliwatts at most, which is much less than needed to operate a fan for drawing air from a room for heating using fireplace combustion. Further, these thermopiles had cylindrical shapes not well suited for the aesthetics of a fireplace. 
     Recently more efficient thermoelectric devices have been developed that are formed as a plate or layer, hereafter referred to as a thermoelectric layer. The thermoelectric layer has a heat-receiving surface facing in a first direction and a heat-rejecting surface facing generally in a direction opposite to the heat-receiving surface. One such device designated as the HZ-2 thermoelectric module is currently available from Hi-Z Technology, Inc., 7606 Miramar Rd., San Diego, Calif. 92126-4210. The HZ-2 device has a bismuth-tellurium semiconductor layer (hereafter Bi—Te layer) and is about 1.15 in. (2.9 cm.) square and 0.2 in. (0.5 cm.) thick. The HZ-2 device provides over 2 watts of electrical power when its heat-receiving and heat-rejecting surfaces are held at a 200 C. temperature difference. A number of HZ-2 modules can be combined to provide more power. Further discussions of this technology are found in U.S. Pat. Nos. 5,769,943; 5,610,366; and 5,747,728. 
     BRIEF DESCRIPTION OF THE INVENTION 
     We have developed an appliance for efficiently heating room air from the heat of a flame having a combustion site within a fireplace. The appliance is to be placed within the fireplace cavity. 
     The appliance includes an airflow path having an inlet duct for receiving room air and an outlet duct through which this air returns to the room, and has a heat exchange duct between the inlet and outlet ducts. The inlet, heat exchange, and outlet ducts collectively define or form the airflow path. 
     A fan is mounted within the airflow path to force room air through the airflow path from the inlet duct to the outlet duct and through the heat exchange duct. A motor is mechanically connected to operate the fan. 
     A thermoelectric generator is mounted to receive heat from the flame and to provide electrical power at an electrical terminal. A heat sink is mounted in the heat exchange duct and in heat exchanging relationship with the thermoelectric generator. 
     Air flowing through the heat exchange duct is heated by the heat sink. The airflow removes heat from the heat sink, thereby holding the heat sink cool relative to the temperature of the thermoelectric generator where the heat from the flame is received from the combustion site. 
     One version of this invention includes an electrical connection between the thermoelectric generator&#39;s electrical terminal and the motor. The motor receives electrical power from the thermoelectric generator and operates the fan. The fan causes airflow through the heat exchange duct, which cools the heat sink by heating the air. The heated air flows back into the room, thereby warming the room. 
     A preferred version of the invention includes a thermoelectric generator having thermoelectric material with a heat-receiving surface for mounting adjacent to the combustion site and a heat-rejecting surface in heat-transferring relation with the heat sink. 
     The thermoelectric generator may include a heat-receiving plate having a first surface to be mounted facing the combustion site, and a second surface oppositely facing from the first surface and in heat-transferring contact with the thermoelectric material&#39;s heat-receiving surface. The heat sink is in heat-transferring contact with the heat-rejecting surface of the thermoelectric material. 
     One problem that a commercial embodiment must address is the startup dynamics. After the flame first occurs, there will be little heat gradient between the heat-receiving and heat-rejecting surfaces of the thermoelectric generator. Accordingly, little power will be generated. If the heat-rejecting surface temperature rises quickly as the heat-receiving surface warms, the thermoelectric generator will produce little or no power. In this case, the fan may fail to operate, with the result that no cooling airflow across the heat sink occurs. The situation may lead to temperature runaway for the heat sink, with the fan failing to ever operate. 
     We have developed a number of solutions to this problem. One of these solutions comprises using a heat sink having a large thermal mass. As the heat is applied to the thermoelectric generator&#39;s heat-receiving surface, the large thermal mass of the heat sink keeps the heat-rejecting surface of the thermoelectric generator sufficiently cool to allow the fan to begin operating. After the fan begins to operate, the airflow will function to maintain the heat-rejecting surface at a sufficiently low temperature. 
     A load-reducing feature in the fan may be combined with the high thermal mass heat sink solution, or may be employed alone. Such a feature can in one embodiment comprise feathering or folding fan blades that provide limited airflow while feathered. Such blades require little torque to rotate. As the motor speed builds, centrifugal force causes the fan blades to deploy in an extended position which forces increased airflow through the ducts. 
     An alternative load-reducing feature may be a clutch for connecting the fan to the motor. Still another type of load-reducing feature may be a small auxiliary fan suitable only for partially cooling the heat sink but that operates on a relatively small amount of power while power is removed from the large main fan. Once the heat-receiving surface of the thermoelectric generator has heated sufficiently, enough electrical power is available to operate the large fan. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic side section outline view of one possible preferred embodiment of the invention, and includes load-reducing features for startup operation. 
     FIG. 2 is a cross sectional view of the heat sink shown in FIG.  1 . 
     FIG. 3 is an enlarged view of the cross sectional view of the heat sink shown in FIG.  2 . 
     FIG. 4 is a larger than scale view of a folding or feathering fan to function as a load-reducing feature for the embodiment of FIG.  1 . 
     FIG. 5 is a block diagram of a clutch connecting the fan to the motor to serve as a load-reducing feature. 
     FIG. 6 is a diagrammatic view of a fan system using both auxiliary and main fans. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the diagrammatic side section view of FIG. 1, a conventional fireplace  20  is shown in outline. Fireplace  20  has a combustion site  15  with a gas fireplace log  24  for supporting a flame  25 . A gas pipe  23  provides fuel for log  24 . In the conventional manner, log  24  simulates the appearance of a wood log. Log  24  has a series of holes through which gas from gas pipe  23  flows. A flue  21  conveys hot combustion gasses from the combustion site  15 . 
     Room air is circulated through an air duct comprising an inlet port  30 , a heat exchange path generally between  32  and  33 , and an outlet port  31 . Outlet port  31  of course allows combustion gasses to flow from combustion site  15  to flue  21 . Walls  27  and  28  define the heat exchange path  32 ,  33 . A motor  36  mounted on a symbolically shown bracket  35  drives a circulating fan  37 . Motor  36  may be mounted at any convenient location within the air duct. Arrows  45  show the general direction of air circulation. 
     A heat sink  40  is mounted in or forms a part of wall  28  and projects into heat exchange path  32 ,  33 . Referring next to FIGS. 2 and 3 as well as FIG. 1, heat sink  40  includes a plurality of fins or bars  42  that increase the exposed area available for convective heat transfer from heat sink  40  to adjacent airflow. In a typical design, there will be many more than the four fins  42  shown. The spaces or channels  43  between the individual fins  42  should preferably extend longitudinally in the direction of airflow in heat exchange path  32 ,  33 . We prefer that heat sink  40  be made of cast aluminum. Aluminum is relatively light and cheap, and next to copper and silver, is the best of the metal heat conductors. Aluminum also has quite good specific heat capacity, and this will be seen to be a potentially important advantage. 
     A thermoelectric generator  47  converts heat produced by flame  25  into electrical power through both radiation and convection. FIG. 3 shows the arrangement by which thermoelectric generator  47  is attached to heat sink  40 . Generator  47  has the general form of a plate or layer shown on edge in FIGS. 2 and 3. For convenience, we consider a generator  47  of this shape to comprise a thermoelectric layer. Generator  47  has a terminal  48  at which electrical power from generator  47  is provided to an electrical device. Generator  47  also has a heat-receiving surface  51  and a heat-rejecting surface  52 , each of which is shown in FIG. 3 on edge as a line. Generator  47  is attached to heat sink  40  in some way that places heat-rejecting surface  52  in good thermal contact with heat sink  40 . 
     We show a protective plate  44  in facing and adjacent relation to combustion site  15  for clamping generator  47  to heat sink  40 , although other means such as heat-resisting adhesives may also be used for this purpose. Plate  44  is normally the preferred solution since its ruggedness will provide mechanical protection for generator  47 . If plate  44  is made of aluminum, the thermal drop through plate  44  is minimal thereby leaving the efficiency of electrical generation relatively high. Further, we expect that surfaces facing combustion site  15  will become dirty over time. A dirty surface interposed between combustion site  15  and generator  47  may reduce the efficiency of electrical generation. A relatively thick (say 0.1 in. or 2.5 mm.) aluminum plate  44  provides mechanical protection against abrasion during cleaning. 
     Plate  44  will also reduce thermal shock when flame  25  is initiated. Plate  44  and generator  47  are held in place by cap screws  54  that thread into tapped holes in heat sink  40 . Depending on the particular design and the mechanical strength of generator  47 , screws  54  may tightly clamp plate  44 , generator  47 , and heat sink  40  to each other to form a good thermal contact, or may be tightened only sufficiently to hold plate  44 , generator  47 , and heat sink  40  all firmly in place. 
     Another suitable means to create a good thermal contact between the surfaces of generator  47  and the adjacent surfaces of plate  44  and heat sink  40  is to place silicone grease or other heat-conducting liquid between these two pairs of surfaces. Silicone grease has been used for decades in the electronics industry to aid heat transfer between electronic devices and heat sinks on which they are mounted. It is stable at high temperatures, is inexpensive and easy to apply, and conducts heat quite efficiently. Silicone grease creates good thermal contact without high flatness and smoothness on the surfaces involved, and hence may result in less costly manufacture. If silicone grease is used here, the manufacturer&#39;s specifications for application and clamping force must be observed to avoid both voids and forcing of the grease from the space between heat-rejecting surface  47  and heat sink  40 . 
     While generator  47  is shown as a single plate or layer, it may be formed as a number of separate modules that are electrically connected together and to terminal  48 . One advantage of such a structure is that by connecting the modules in series may provide higher output voltage which is often more compatible with existing designs available to use as motor  36 . If a number of modules comprise generator  47 , the use of plate  44  to clamp them into place is particularly convenient. 
     The Background section refers to the HZ-2 Bi—Te thermoelectric generator module. The HZ-2 module or a larger variation of it is suitable for use as generator  47 . 
     Conductor  38  carries electrical power provided at terminal  48  to a motor controller  39 . Controller  39  monitors the power level at terminal  48  and completes the connection between motor  36  and terminal  48  when the power is sufficient to operate fan  37 . Fan  37  draws air from the room through inlet duct  30  and forces this air through heat exchanger path  32 ,  33 . Air then returns to the room through outlet duct  31 , all as shown by arrows  45 . Air flows through channels  43  of heat sink  40 , thereby increasing its temperature and at the same time cooling heat sink  40 . As long as fan  37  continues to rotate at a normal speed, air flow through heat exchanger path  32 ,  33  continues to cool heat exchanger  40 , thereby maintaining a temperature difference between the sides  51  and  52  (FIG. 3) of heat exchanger  40 . 
     If desired, the air duct may include a heat exchanger portion  56  for carrying airflow to outlet port  31 . An external surface  57  of the heat exchanger portion  56  is positioned to allow the combustion gasses rising to flow into flue  21  to also flow across the external surface  57 . The hot combustion gasses further heat the room air flowing through the heat exchanger portion  56  thereby providing hotter room air to port  31 . The outlet duct heat exchanger should not cool the combustion gasses to the extent of affecting natural convective flow of combustion gasses through flue  21 . Since these flue gasses may sometimes be toxic, backflow into occupied quarters is undesirable. 
     One problem we attempt to solve with our invention is that of insufficient power to operate motor  36  during the time after flame  25  is first initiated. When flame is first established, the temperature drop across generator  47  is very small, resulting in little power at terminal  48  preventing motor  36  operation. As flame  25  begins to heat plate  44 , the temperature at heat-receiving surface  51  increases. It is possible that a substantial amount of heat generated during this startup phase can pass through generator  47  to heat-rejecting surface  52 . This has the potential to warm surface  52  and the adjacent volume of heat sink  40 , preventing a temperature drop across generator  47  adequate to operate motor  36 . If motor  36  cannot ever start operation, then heat sink  40  will not ever be sufficiently cool to establish a temperature drop allowing motor  36  operation. 
     We have a number of solutions for this problem. A first, and one compatible with other solutions to be shown, is to provide a heat sink  40  whose thermal mass is much larger than that of plate  44  and of generator  47 . A heat sink  40  whose mass near to heat-rejecting surface  52  is several times larger than the total mass of plate  44  will warm only slightly over the first few minutes after flame  25  startup. During this time, a temperature gradient across generation  47  that will provide sufficient power to operate motor  36  and fan  37  will become established. 
     In some situations a difference in mass between plate  44  and heat sink  47  may not be adequate to begin motor  35  operation during startup. One solution is an auxiliary motor-fan unit  49  mounted on bracket  46  to provide an air stream across heat sink  40  when operating. Motor-fan unit  49  should be capable of operating on substantially smaller power than motor  36  and fan  37  and yet provide adequate cooling for heat sink  40  until sufficient power to operate motor  36  and fan  37  is available. Controller  39  operated by power from generator  47  should disconnect motor  36  from generator  47  until power output from generator  47  is sufficient to operate motor  36 . 
     The operation of controller  39  may be electronic and depend on the voltage produced at terminal  48  to indicate the power available from generator  47 . Many types of thermopiles suitable to use as generator  47  produce a voltage across a suitably chosen resistor that accurately indicates the power available at any given time from generator  47 . In that case, controller  39  may monitor the voltage on conductor  38  and connect motor  36  only when sufficient power is available. We will disclose in connection with FIG. 6 another means to monitor power output from generator  47  while relying on an auxiliary motor-fan unit  49 . 
     FIG. 4 shows version of apparatus allowing motor  36  to start up with reduced power. Motor  36  has a shaft  67  carrying a folding or feathering fan blade unit  60 , shown partly feathered in FIG. 4, and significantly enlarged as well relative to the view of FIG.  1 . Blade unit  60  includes a pair of blades  70  and  71 , each of which is attached by a pivot pin  80  or  81  to a bracket unit  63  carried on the end of shaft  67 . In this embodiment, the axes of pins  80  and  81  are transverse to the axis of shaft  67 . Arrows  75  indicate the articulation that blades  70  and  71  can undergo while moving from feathered or folded to fully extended. A mechanical spring  73  urges the blades  70  and  71  into a folded position where the rotational inertia and air resistance is minimized. Blades  70  and  71  can rotate against spring  73  force into fully extended positions. 
     In the folded position, blades  70  and  71  may have a shape that propels a small amount of air through the heat exchange path  32 ,  33  and past heat sink  40 . Such a level of airflow must be adequate to cool heat sink  40  to a temperature that results in generation of adequate electrical power by generator  47  to operate motor  36  at a relatively low speed. Little aerodynamic drag from blades  70  and  71  is present because of the small active area of blades  70  and  71 . With increased electrical power applied to motor  36 , speed of shaft  67  increases. When shaft  67  speed reaches a predetermined level, centrifugal force increases to a level that causes blades  70  and  71  to begin to unfold and extend against the force of spring  73 . As blades  70  and  71  unfold, the volume rate of air flow through heat exchange path  32  increases to a level that will add measurable heat to the room as well as more efficiently cool heat sink  40 . 
     FIG. 5 shows yet another version of apparatus allowing motor  36  to start with less than normal power. Power from generator  47  is carried on conductor  38  to a clutch controller  84 . Power is also carried directly to motor  36 . Power from generator  47  must be adequate to operate controller  84  and a magnetic clutch  83  at some point before motor  36  can drive fan  37 . Clutch  83  adjusts the amount of torque transmitted from motor  36  to fan  37  responsive to a clutch control signal from controller  84  to prevent motor  36  from stalling. Recall that drag torque for fan  37  increases substantially as fan  37  speed increases. Controller  84  must control clutch  83  to transmit torque at a level that avoids stalling motor  36 . 
     Controller  84  measures the amount of power available from generator  47 . Generator  47  voltage is an indication of the level of power available at any instant from generator  47 . Controller  84  can monitor generator  47  voltage and when the voltage level indicates available power is above a predetermined level adequate to operate fan  37  at low speed, controller  80  provides a clutch control signal engaging the clutch  83  to transmit a sufficient level of torque to slowly rotate fan  37 . Fans generally, have very little aerodynamic resistance at low speed, so motor  36  can slowly rotate fan  37 . As airflow from fan  37  helps to keep heat sink  40  cool and plate  44  warms further, power from generator  47  to motor  36  increases. When power from generator  47  increases to a level sufficient to run fan  37  at full speed, controller  84  applies a clutch control signal sufficient to lock up clutch  83 . 
     In this way, motor  36  can be operated at the speed near its peak torque given the power available. If fan  37  were to be directly connected to motor  36 , fan  37  torque at that speed may be larger than the torque available. This will stall motor  36 , preventing any airflow generated by fan  37  rotation. As air continues to flow across heat sink  40 , and plate  44  continues to heat from flame  25 , the temperature differential across generator  47  will continue to increase. This increases power available from generator  47 . When available motor torque is adequate to rotate fan  37  with clutch  83  locked up, controller  80  provides a clutch control signal that locks clutch  83 . 
     FIG. 6 shows one version of a system using an auxiliary motor-fan unit  49 . Unit  49  must be chosen to operate on relatively low power, and provide sufficient airflow to cool heat sink  40  while the temperature differential across generator  47  is established. Unit  49  must also increase speed and consequently, airflow as well, with increasing power from generator  47 . 
     A sail or paddle  85  is mounted in the air stream generated by unit  49 . A mechanical linkage  87  cooperates with sail  85  to operate a motor switch  90  when airflow sensed by sail  85  reaches a predetermined level. Switch  90  controls flow of electrical power from conductor  38  and terminal  48  to motor  36 . This predetermined airflow level correlates with the power available from generator  47 . When switch  90  closes due to the level of airflow sensed by sail  85 , motor  36  begins operation. In this way, motor-fan unit  49  in cooperation with sail  85  and linkage  87  can sense the power available from generator  47 .