Patent Publication Number: US-2021185856-A1

Title: Mems-based cooling systems for closed and open devices

Description:
CROSS REFERENCE TO OTHER APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 63/079,448 entitled MEMS BASED COOLING SYSTEM FOR CLOSED DEVICES filed Sep. 16, 2020, and U.S. Provisional Patent Application No. 62/949,383 entitled AIRFLOW CONTROL SYSTEM IN PIEZOELECTRIC COOLING FOR DEVICES filed Dec. 17, 2019, both of which are incorporated herein by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     As computing devices grow in speed and computing power, the heat generated by the computing devices also increases. Various mechanisms have been proposed to address the generation of heat. Active devices, such as fans, may be used to drive air through large computing devise, such as laptop computers or desktop computers. Passive cooling devices, such as heat spreaders, may be used in smaller, mobile computing devices, such as smartphones, virtual reality devices and tablet computers. However, such active and passive devices may be unable to adequately cool both mobile devices such as smartphones and larger devices such as laptops and desktop computers. Consequently, additional cooling solutions for computing devices are desired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the invention are disclosed in the following detailed description and the accompanying drawings. 
         FIGS. 1A-1E  depict an embodiment of an active cooling system including a centrally anchored cooling element. 
         FIGS. 2A-2B  depict embodiments of cooling elements usable in active cooling systems including centrally anchored cooling elements. 
         FIGS. 3A-3B  depict embodiments of cooling elements usable in active cooling systems including centrally anchored cooling elements. 
         FIGS. 4A-4C  are diagrams depicting an embodiment of an active cooling system. 
         FIGS. 5A-5E  depict an embodiment of an active cooling system formed in a tile. 
         FIGS. 6A-6B  depict embodiments of an active cooling system used in devices. 
         FIGS. 7A-7B  depict embodiments of an active cooling system used in devices. 
         FIG. 8  depicts an embodiment of an active cooling system as used in a smart phone. 
         FIG. 9A-9B  depicts an embodiment of an active cooling system as used in a closed smartphone. 
         FIGS. 10A-10D  depict embodiments of and performance for smartphones with and without cooling systems. 
         FIGS. 11A-11C  depict performance for embodiments of notebooks with and without cooling systems. 
         FIGS. 12A-12D  depict performance for embodiments of notebooks with and without cooling systems. 
     
    
    
     DETAILED DESCRIPTION 
     The invention can be implemented in numerous ways, including as a process; an apparatus; a system; a composition of matter; a computer program product embodied on a computer readable storage medium; and/or a processor, such as a processor configured to execute instructions stored on and/or provided by a memory coupled to the processor. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. In general, the order of the steps of disclosed processes may be altered within the scope of the invention. Unless stated otherwise, a component such as a processor or a memory described as being configured to perform a task may be implemented as a general component that is temporarily configured to perform the task at a given time or a specific component that is manufactured to perform the task. As used herein, the term ‘processor’ refers to one or more devices, circuits, and/or processing cores configured to process data, such as computer program instructions. 
     A detailed description of one or more embodiments of the invention is provided below along with accompanying figures that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured. 
     As semiconductor devices become increasingly powerful, the heat generated during operations also grows. For example, processors for mobile devices such as smartphones, tablet computers, notebooks, and virtual reality devices can operate at high clock speeds, but produce a significant amount of heat. Because of the quantity of heat produced, processors may run at full speed only for a relatively short period of time. After this time expires, throttling (e.g. slowing of the processor&#39;s clock speed) occurs. Although throttling can reduce heat generation, it also adversely affects processor speed and, therefore, the performance of devices using the processors. As technology moves to  5 G and beyond, this issue is expected to be exacerbated. 
     Larger devices, such as laptop or desktop computers include electric fans that have rotating blades. The fan that can be energized in response to an increase in temperature of internal components. The fans drive air through the larger devices to cool internal components. However, such fans are typically too large for mobile devices such as smartphones or for thinner devices such as tablet computers. Fans also may have limited efficacy because of the boundary layer of air existing at the surface of the components, provide a limited airspeed for air flow across the hot surface desired to be cooled and may generate an excessive amount of noise. Passive cooling solutions may include components such as a heat spreader and a heat pipe or vapor chamber to transfer heat to a heat exchanger. Although a heat spreader somewhat mitigates the temperature increase at hot spots, the amount of heat produced in current and future devices may not be adequately addressed. Similarly, a heat pipe or vapor chamber may provide an insufficient amount of heat transfer to remove excessive heat generated. 
     Varying configurations of computing devices further complicate heat management. For example, computing devices such as laptops are frequently open to the external environment while other computing devices, such as smartphones, are generally closed to the external environment. Thus, active heat management solutions for open devices, such as fans, may be inappropriate for closed devices. A fan driving heated fluid from the inside of the computing device to the outside environment may be too large for closed computing devices such as smartphones and may provide limited fluid flow. In addition, the closed computing device has no outlet for the heated fluid even if the fan can be incorporated into the closed computing device. Thus, the thermal management provided by such an open-device mechanism may have limited efficacy. Even for open computing devices, the location of the inlet and/or outlet may be configured differently for different devices. For example, an outlet for fan-driven fluid flow in a laptop may be desired to be located away from the user&#39;s hands or other structures that may lie within the outflow of heated fluid. Such a configuration not only prevents the user&#39;s discomfort but also allows the fan to provide the desired cooling. Another mobile device having a different configuration may require the inlets and/or outlets to be configured differently, may reduce the efficacy of such heat management systems and may prevent the use of such heat management systems. Thus, mechanisms for improving cooling in computing devices are desired. 
     A system including an active cooling system is described. The active cooling system includes a cooling element in communication with a fluid and configured to use vibrational motion to direct a fluid toward a surface of heat-generating structure(s). Heat is transferred from the heat-generating structure to the fluid. The system is configured such that the fluid follows a path from the surface of the heat-generating structure(s) past a structure having a lower temperature than the surface of the heat-generating structure. The structure absorbs heat from the fluid. The structure is within the system and distal from the active cooling system. 
     In some embodiments, the system is configured such that the fluid follows the path from the surface of the heat-generating structure(s) past the structure and returns to the active cooling system. Thus, the system may be a closed system. The system may include at least one vent allowing fluid communication with an external environment. Thus, the system may be an open system. In such embodiments, system is configured such that the fluid follows the path from the surface of the heat-generating structure(s) past the structure and exits the system to the external environment through the vent(s). For example, the system may include an inlet and an outlet. Fluid may enter through the inlet, travel to the active cooling system and be driven toward the surface of the heat-generating structure. Fluid thus follows the path from the surface of the heat-generating structure(s) past the structure and through the outlet. 
     The cooling element of the active cooling system may include a first side and a second side. The first side is distal to the heat-generating structure(s) and in communication with the fluid. The second side is proximal to the heat-generating structure(s). The cooling element is configured to direct the fluid from the first side to the second side using the vibrational motion such that the fluid is driven toward the surface of the heat-generating structure(s). In some embodiments, the active cooling system further includes a support structure and the cooling element is selected from a centrally anchored cooling element and an edge anchored cooling element. The centrally anchored cooling element has a central region and a perimeter. The centrally anchored cooling element is supported by the support structure at the central region. At least a portion of the perimeter is unpinned. The edge anchored cooling element has a central portion and an edge. The edge anchored cooling element is supported by the support structure at the edge and has at least one aperture therein. The active cooling system may also include an orifice plate having orifice(s) therein. The orifice plate is disposed between the cooling element and the surface of the heat-generating structure(s). The cooling element is actuated to drive the fluid through the orifice(s). The fluid travels from the orifice(s) toward the surface of the heat-generating structure(s). In some embodiments, the active cooling system has a thickness of not more than two millimeters. 
     The heat-generating structure(s) may have characteristic power versus time curves. A curve may have a characteristic throttling point at a first time. The heat-generating structure(s) have actively cooled power versus time curve(s). These curves for active cooling may have an actively cooled throttling point at a second time. The first time is less than the second time. 
     In some embodiments, a system includes multiple cooling cells. Each of the cooling cells includes a cooling element in communication with a fluid. The cooling element is also configured to use vibrational motion to direct the fluid toward a surface of heat-generating structure(s) to extract heat from the heat-generating structure(s). The system is configured such that the fluid follows a path from the surface of the heat-generating structure(s) past a structure within the system, having a temperature lower than the surface the heat-generating structure(s) and distal from the active cooling system. The structure absorbs heat from the fluid. In some embodiments, the system is configured such that the fluid follows the path from the surface of the heat-generating structure(s) past the structure and returns to the active cooling system. In some embodiments, the system includes at least one vent allowing fluid communication with an external environment. In such embodiments, the system is configured such that the fluid follows the path from the surface of the heat-generating structure(s) past the structure and exits the system to the external environment through the at least one vent. 
     In some embodiments, the cooling element includes a first side and a second side. The first side is distal to the heat-generating structure(s) and in communication with the fluid. The second side is proximal to the heat-generating structure(s). The cooling element is configured to direct the fluid from the first side to the second side using the vibrational motion such that the fluid travels toward the surface of the at least one heat-generating structure. The cooling element may be selected from a centrally anchored cooling element and an edge pinned cooling element. In some embodiments, the active cooling system includes an orifice plate having orifice(s) therein. The orifice plate is disposed between the cooling element and the surface of the heat-generating structure(s). The cooling element is actuated to drive the fluid through the orifice(s). The fluid travels from the orifice(s) toward the surface of the heat-generating structure(s). In some embodiments, the active cooling system has a thickness of not more than two millimeters. 
     The heat-generating structure(s) may have characteristic power versus time curves. A curve may have a characteristic throttling point at a first time. The heat-generating structure(s) have actively cooled power versus time curve(s). These curves for active cooling may have an actively cooled throttling point at a second time. The first time is less than the second time. 
     A method is also described. The method includes driving a cooling element of an active cooling system to induce vibrational motion at a frequency. The cooling element is in communication with a fluid and configured to use the vibrational motion to direct the fluid toward a surface of heat-generating structure(s) to extract heat from the heat-generating structure(s). The method also includes directing the fluid to follow a path from the surface of the heat-generating structure(s) past a structure having a lower temperature than the surface of the heat-generating structure. The structure is within the system, distal from the active cooling system and absorbs heat from the fluid. In some embodiments the fluid is directed to follow the path from the surface of the heat-generating structure(s) past the component and returns to the active cooling system. In some embodiments, the fluid is directed to follow the path from the surface of the at least one heat-generating structure past the component and to exit the system to an external environment through at least one vent. 
       FIGS. 1A-1E  are diagrams depicting an exemplary embodiment of active cooling system  100  usable with heat-generating structure  102  and including a centrally anchored cooling element  120 . For clarity, only certain components are shown.  FIGS. 1A-1E  are not to scale. Although shown as symmetric, cooling system  100  need not be. 
     Cooling system  100  includes top plate  110  having vent  112  therein, cooling element  120 , orifice plate  130  having orifices  132  therein, support structure (or “anchor”)  160  and chambers  140  and  150  (collectively chamber  140 / 150 ) formed therein. Cooling element  120  is supported at its central region by anchor  160 . Regions of cooling element  120  closer to and including portions of the cooling element&#39;s perimeter (e.g. tip  121 ) vibrate when actuated. In some embodiments, tip  121  of cooling element  120  includes a portion of the perimeter furthest from anchor  160  and undergoes the largest deflection during actuation of cooling element  120 . For clarity, only one tip  121  of cooling element  120  is labeled in  FIG. 1A . 
       FIG. 1A  depicts cooling system  100  in a neutral position. Thus, cooling element  120  is shown as substantially flat. For in-phase operation, cooling element  120  is driven to vibrate between positions shown in  FIGS. 1B and 1C . This vibrational motion draws fluid (e.g. air) into vent  112 , through chambers  140  and  150  and out orifices  132  at high speed and/or flow rates. For example, the speed at which the fluid impinges on heat-generating structure  102  may be at least thirty meters per second. In some embodiments, the fluid is driven by cooling element  120  toward heat-generating structure  102  at a speed of at least forty-five meters per second. In some embodiments, the fluid is driven toward heat-generating structure  102  by cooling element  120  at speeds of at least sixty meters per second. Other speeds may be possible in some embodiments. Cooling system  100  is also configured so that little or no fluid is drawn back into chamber  140 / 150  through orifices  132  by the vibrational motion of cooling element  120 . 
     Heat-generating structure  102  is desired to be cooled by cooling system  100 . In some embodiments, heat-generating structure  102  generates heat. For example, heat-generating structure may be an integrated circuit. In some embodiments, heat-generating structure  102  is desired to be cooled but does not generate heat itself. Heat-generating structure  102  may conduct heat (e.g. from a nearby object that generates heat). For example, heat-generating structure  102  might be a heat spreader or a vapor chamber. Thus, heat-generating structure  102  may include semiconductor components(s) including individual integrated circuit components such as processors, other integrated circuit(s) and/or chip package(s); sensor(s); optical device(s); one or more batteries; other component(s) of an electronic device such as a computing device; heat spreaders; heat pipes; other electronic component(s) and/or other device(s) desired to be cooled. 
     The devices in which cooling system  100  is desired to be used may also have limited space in which to place a cooling system. For example, cooling system  100  may be used in computing devices. Such computing devices may include but are not limited to smartphones, tablet computers, laptop computers, tablets, two-in-one laptops, hand held gaming systems, digital cameras, virtual reality headsets, augmented reality headsets, mixed reality headsets and other devices that are thin. Cooling system  100  may be a micro-electro-mechanical system (MEMS) cooling system capable of residing within mobile computing devices and/or other devices having limited space in at least one dimension. For example, the total height of cooling system  100  (from the top of heat-generating structure  102  to the top of top plate  110 ) may be less than 2 millimeters. In some embodiments, the total height of cooling system  100  is not more than 1.5 millimeters. In some embodiments, this total height is not more than 1.1 millimeters. In some embodiments, the total height does not exceed one millimeter. In some embodiments, the total height does not exceed two hundred and fifty micrometers. Similarly, the distance between the bottom of orifice plate  130  and the top of heat-generating structure  102 , y, may be small. In some embodiments, y is at least two hundred micrometers and not more than one millimeter. In some embodiments, y is at least two hundred micrometers and not more than three hundred micrometers. Thus, cooling system  100  is usable computing devices and/or other devices having limited space in at least one dimension. However, nothing prevents the use of cooling system  100  in devices having fewer limitations on space and/or for purposes other than cooling. Although one cooling system  100  is shown (e.g. one cooling cell), multiple cooling systems  100  might be used in connection with heat-generating structure  102 . For example, a one or two-dimensional array of cooling cells might be utilized. 
     Cooling system  100  is in communication with a fluid used to cool heat-generating structure  102 . The fluid may be a gas or a liquid. For example, the fluid may be air. In some embodiments, the fluid includes fluid from outside of the device in which cooling system  100  resides (e.g. provided through external vents in the device). In some embodiments, the fluid circulates within the device in which cooling system resides (e.g. in an enclosed device). 
     Cooling element  120  can be considered to divide the interior of active cooling system  100  into top chamber  140  and bottom chamber  150 . Top chamber  140  is formed by cooling element  120 , the sides, and top plate  110 . Bottom chamber  150  is formed by orifice plate  130 , the sides, cooling element  120  and anchor  160 . Top chamber  140  and bottom chamber  150  are connected at the periphery of cooling element  120  and together form chamber  140 / 150  (e.g. an interior chamber of cooling system  100 ). 
     The size and configuration of top chamber  140  may be a function of the cell (cooling system  100 ) dimensions, cooling element  120  motion, and the frequency of operation. Top chamber  140  has a height, h 1 . The height of top chamber  140  may be selected to provide sufficient pressure to drive the fluid to bottom chamber  140  and through orifices  132  at the desired flow rate and/or speed. Top chamber  140  is also sufficiently tall that cooling element  120  does not contact top plate  140  when actuated. In some embodiments, the height of top chamber  140  is at least fifty micrometers and not more than five hundred micrometers. In some embodiments, top chamber  140  has a height of at least two hundred and not more than three hundred micrometers. 
     Bottom chamber  150  has a height, h 2 . In some embodiments, the height of bottom chamber  150  is sufficient to accommodate the motion of cooling element  120 . Thus, no portion of cooling element  120  contacts orifice plate  130  during normal operation. Bottom chamber  150  is generally smaller than top chamber  140  and may aid in reducing the backflow of fluid into orifices  132 . In some embodiments, the height of bottom chamber  150  is the maximum deflection of cooling element  120  plus at least five micrometers and not more than ten micrometers. In some embodiments, the deflection of cooling element  120  (e.g. the deflection of tip  121 ) has an amplitude of at least ten micrometers and not more than one hundred micrometers. In some such embodiments, the amplitude of deflection of cooling element  120  is at least ten micrometers and not more than sixty micrometers. However, the amplitude of deflection of cooling element  120  depends on factors such as the desired flow rate through cooling system  100  and the configuration of cooling system  100 . Thus, the height of bottom chamber  150  generally depends on the flow rate through and other components of cooling system  100 . 
     Top plate  110  includes vent  112  through which fluid may be drawn into cooling system  100 . Top vent  112  may have a size chosen based on the desired the acoustic pressure in chamber  140 . For example, in some embodiments, the width, w, of vent  112  is at least five hundred micrometers and not more than one thousand micrometers. In some embodiments, the width of vent  112  is at least two hundred fifty micrometers and not more than two thousand micrometers. In the embodiment shown, vent  112  is a centrally located aperture in top plate  110 . In other embodiments, vent  112  may be located elsewhere. For example, vent  112  may be closer to one of the edges of top plate  110 . Vent  112  may have a circular, rectangular or other shaped footprint. Although a single vent  112  is shown, multiple vents might be used. For example, vents may be offset toward the edges of top chamber  140  or be located on the side(s) of top chamber  140 . Although top plate  110  is shown as substantially flat, in some embodiments trenches and/or other structures may be provided in top plate  110  to modify the configuration of top chamber  140  and/or the region above top plate  110 . 
     Anchor (support structure)  160  supports cooling element  120  at the central portion of cooling element  120 . Thus, at least part of the perimeter of cooling element  120  is unpinned and free to vibrate. In some embodiments, anchor  160  extends along a central axis of cooling element  120  (e.g. perpendicular to the page in  FIGS. 1A-1E ). In such embodiments, portions of cooling element  120  that vibrate (e.g. including tip  121 ) move in a cantilevered fashion. Thus, portions of cooling element  120  may move in a manner analogous to the wings of a butterfly (i.e. in phase) and/or analogous to a seesaw (i.e. out of phase). Thus, the portions of cooling element  120  that vibrate in a cantilevered fashion do so in phase in some embodiments and out of phase in other embodiments. In some embodiments, anchor  160  does not extend along an axis of cooling element  120 . In such embodiments, all portions of the perimeter of cooling element  120  are free to vibrate (e.g. analogous to a jellyfish). In the embodiment shown, anchor  160  supports cooling element  120  from the bottom of cooling element  120 . In other embodiments, anchor  160  may support cooling element  120  in another manner. For example, anchor  160  may support cooling element  120  from the top (e.g. cooling element  120  hangs from anchor  160 ). In some embodiments, the width, a, of anchor  160  is at least 0.5 millimeters and not more than four millimeters. In some embodiments, the width of anchor  160  is at least two millimeters and not more than 2.5 millimeters. Anchor  160  may occupy at least ten percent and not more than fifty percent of cooling element  120 . 
     Cooling element  120  has a first side distal from heat-generating structure  102  and a second side proximate to heat-generating structure  102 . In the embodiment shown in FIGS.  1 A- 1 E, the first side of cooling element  120  is the top of cooling element  120  (closer to top plate  110 ) and the second side is the bottom of cooling element  120  (closer to orifice plate  130 ). Cooling element  120  is actuated to undergo vibrational motion as shown in  FIGS. 1A-1E . The vibrational motion of cooling element  120  drives fluid from the first side of cooling element  120  distal from heat-generating structure  102  (e.g. from top chamber  140 ) to a second side of cooling element  120  proximate to heat-generating structure  102  (e.g. to bottom chamber  150 ). The vibrational motion of cooling element  120  also draws fluid through vent  112  and into top chamber  140 ; forces fluid from top chamber  140  to bottom chamber  150 ; and drives fluid from bottom chamber  140  through orifices  132  of orifice plate  130 . 
     Cooling element  120  has a length, L, that depends upon the frequency at which cooling element  120  is desired to vibrate. In some embodiments, the length of cooling element  120  is at least four millimeters and not more than ten millimeters. In some such embodiments, cooling element  120  has a length of at least six millimeters and not more than eight millimeters. The depth of cooling element  120  (e.g. perpendicular to the plane shown in  FIGS. 1A-1E ) may vary from one fourth of L through twice L. For example, cooling element  120  may have the same depth as length. The thickness, t, of cooling element  120  may vary based upon the configuration of cooling element  120  and/or the frequency at which cooling element  120  is desired to be actuated. In some embodiments, the cooling element thickness is at least two hundred micrometers and not more than three hundred and fifty micrometers for cooling element  120  having a length of eight millimeters and driven at a frequency of at least twenty kilohertz and not more than twenty-five kilohertz. The length, C of chamber  140 / 150  is close to the length, L, of cooling element  120 . For example, in some embodiments, the distance, d, between the edge of cooling element  120  and the wall of chamber  140 / 50  is at least one hundred micrometers and not more than five hundred micrometers. In some embodiments, d is at least two hundred micrometers and not more than three hundred micrometers. 
     Cooling element  120  may be driven at a frequency that is at or near both the resonant frequency for an acoustic resonance of a pressure wave of the fluid in top chamber  140  and the resonant frequency for a structural resonance of cooling element  120 . The portion of cooling element  120  undergoing vibrational motion is driven at or near resonance (the “structural resonance”) of cooling element  120 . This portion of cooling element  120  undergoing vibration may be a cantilevered section in some embodiments. The frequency of vibration for structural resonance is termed the structural resonant frequency. Use of the structural resonant frequency in driving cooling element  112  reduces the power consumption of cooling system  100 . Cooling element  120  and top chamber  140  may also be configured such that this structural resonant frequency corresponds to a resonance in a pressure wave in the fluid being driven through top chamber  140  (the acoustic resonance of top chamber  140 ). The frequency of such a pressure wave is termed the acoustic resonant frequency. At acoustic resonance, a node in pressure occurs near vent  112  and an antinode in pressure occurs near the periphery of cooling system  100  (e.g. near tip  121  of cooling element  120  and near the connection between top chamber  140  and bottom chamber  150 ). The distance between these two regions is C/2. Thus, C/2=nλ/4, where λ is the acoustic wavelength for the fluid and n is odd (e.g. n=1, 3, 5, etc.). For the lowest order mode, C=λ/2. Because the length of chamber  140  (e.g. C) is close to the length of cooling element  120 , in some embodiments, it is also approximately true that L/2=nλ/4, where λ is the acoustic wavelength for the fluid and n is odd. Thus, the frequency at which cooling element  120  is driven, v, is at or near the structural resonant frequency for cooling element  120 . The frequency v is also at or near the acoustic resonant frequency for at least top chamber  140 . The acoustic resonant frequency of top chamber  140  generally varies less dramatically with parameters such as temperature and size than the structural resonant frequency of cooling element  120 . Consequently, in some embodiments, cooling element  120  may be driven at (or closer to) a structural resonant frequency than to the acoustic resonant frequency. 
     Orifice plate  130  has orifices  132  therein. Although a particular number and distribution of orifices  132  are shown, another number and/or another distribution may be used. A single orifice plate  130  is used for a single cooling system  100 . In other embodiments, multiple cooling systems  100  may share an orifice plate. For example, multiple cells  100  may be provided together in a desired configuration. In such embodiments, the cells  100  may be the same size and configuration or different size(s) and/or configuration(s). Orifices  132  are shown as having an axis oriented normal to a surface of heat-generating structure  102 . In other embodiments, the axis of one or more orifices  132  may be at another angle. For example, the angle of the axis may be selected from substantially zero degrees and a nonzero acute angle. Orifices  132  also have sidewalls that are substantially parallel to the normal to the surface of orifice plate  130 . In some, orifices may have sidewalls at a nonzero angle to the normal to the surface of orifice plate  130 . For example, orifices  132  may be cone-shaped. Further, although orifice place  130  is shown as substantially flat, in some embodiments, trenches and/or other structures may be provided in orifice plate  130  to modify the configuration of bottom chamber  150  and/or the region between orifice plate  130  and heat-generating structure  102 . 
     The size, distribution and locations of orifices  132  are chosen to control the flow rate of fluid driven to the surface of heat-generating structure  102 . The locations and configurations of orifices  132  may be configured to increase/maximize the fluid flow from bottom chamber  150  through orifices  132  to the jet channel (the region between the bottom of orifice plate  130  and the top of heat-generating structure  102 ). The locations and configurations of orifices  132  may also be selected to reduce/minimize the suction flow (e.g. back flow) from the jet channel through orifices  132 . For example, the locations of orifices are desired to be sufficiently far from tip  121  that suction in the upstroke of cooling element  120  (tip  121  moves away from orifice plate  13 ) that would pull fluid into bottom chamber  150  through orifices  132  is reduced. The locations of orifices are also desired to be sufficiently close to tip  121  that suction in the upstroke of cooling element  120  also allows a higher pressure from top chamber  140  to push fluid from top chamber  140  into bottom chamber  150 . In some embodiments, the ratio of the flow rate from top chamber  140  into bottom chamber  150  to the flow rate from the jet channel through orifices  132  in the upstroke (the “net flow ratio”) is greater than 2:1. In some embodiments, the net flow ratio is at least 85:15. In some embodiments, the net flow ratio is at least 90:10. In order to provide the desired pressure, flow rate, suction, and net flow ratio, orifices  132  are desired to be at least a distance, r 1 , from tip  121  and not more than a distance, r 2 , from tip  121  of cooling element  120 . In some embodiments r 1  is at least one hundred micrometers (e.g. r 1 &gt;100 μm) and r 2  is not more than one millimeter (e.g. r 2 &lt;1000 μm). In some embodiments, orifices  132  are at least two hundred micrometers from tip  121  of cooling element  120  (e.g. r 1 &gt;200 μm). In some such embodiments, orifices  132  are at least three hundred micrometers from tip  121  of cooling element  120  (e.g. r 1 &gt;300 μm). In some embodiments, orifices  132  have a width of at least one hundred micrometers and not more than five hundred micrometers. In some embodiments, orifices  132  have a width of at least two hundred micrometers and not more than three hundred micrometers. In some embodiments, the orifice separation, s, is at least one hundred micrometers and not more than one millimeter. In some such embodiments, the orifice separation is at least four hundred micrometers and not more than six hundred micrometers. In some embodiments, orifices  132  are also desired to occupy a particular fraction of the area of orifice plate  130 . For example, orifices  132  may cover at least five percent and not more than fifteen percent of the footprint of orifice plate  130  in order to achieve a desired flow rate of fluid through orifices  132 . In some embodiments, orifices  132  cover at least eight percent and not more than twelve percent of the footprint of orifice plate  130 . 
     In some embodiments, cooling element  120  is actuated using a piezoelectric. Thus, cooling element  120  may be a piezoelectric cooling element. Cooling element  120  may be driven by a piezoelectric that is mounted on or integrated into cooling element  120 . In some embodiments, cooling element  120  is driven in another manner including but not limited to providing a piezoelectric on another structure in cooling system  100 . Cooling element  120  and analogous cooling elements are referred to hereinafter as piezoelectric cooling element though it is possible that a mechanism other than a piezoelectric might be used to drive the cooling element. In some embodiments, cooling element  120  includes a piezoelectric layer on substrate. The substrate may be a stainless steel, Ni alloy and/or Hastelloy substrate. In some embodiments, piezoelectric layer includes multiple sublayers formed as thin films on the substrate. In other embodiments, the piezoelectric layer may be a bulk layer affixed to the substrate. Such a piezoelectric cooling element  120  also includes electrodes used to activate the piezoelectric. The substrate functions as an electrode in some embodiments. In other embodiments, a bottom electrode may be provided between the substrate and the piezoelectric layer. Other layers including but not limited to seed, capping, passivation or other layers might be included in piezoelectric cooling element. Thus, cooling element  120  may be actuated using a piezoelectric. 
     In some embodiments, cooling system  100  includes chimneys (not shown) or other ducting. Such ducting provides a path for heated fluid to flow away from heat-generating structure  102 . In some embodiments, ducting returns fluid to the side of top plate  110  distal from heat-generating structure  102 . In some embodiments, ducting may instead directed fluid away from heat-generating structure  102  in a direction parallel to heat-generating structure  102  or perpendicular to heat-generating structure  102  but in the opposite direction (e.g. toward the bottom of the page). For a device in which fluid external to the device is used in cooling system  100 , the ducting may channel the heated fluid to a vent. In such embodiments, additional fluid may be provided from an inlet vent. In embodiments, in which the device is enclosed, the ducting may provide a circuitous path back to the region near vent  112  and distal from heat-generating structure  102 . Such a path allows for the fluid to dissipate heat before being reused to cool heat-generating structure  102 . In other embodiments, ducting may be omitted or configured in another manner. Thus, the fluid is allowed to carry away heat from heat-generating structure  102 . 
     Operation of cooling system  100  is described in the context of  FIGS. 1A-1E . Although described in the context of particular pressures, gap sizes, and timing of flow, operation of cooling system  100  is not dependent upon the explanation herein.  FIGS. 1B-1C  depict in-phase operation of cooling system  100 . Referring to  FIG. 1B , cooling element  120  has been actuated so that its tip  121  moves away from top plate  110 .  FIG. 1B  can thus be considered to depict the end of a down stroke of cooling element  120 . Because of the vibrational motion of cooling element  120 , gap  152  for bottom chamber  150  has decreased in size and is shown as gap  152 B. Conversely, gap  142  for top chamber  140  has increased in size and is shown as gap  142 B. During the down stroke, a lower (e.g. minimum) pressure is developed at the periphery when cooling element  120  is at the neutral position. As the down stroke continues, bottom chamber  150  decreases in size and top chamber  140  increases in size as shown in  FIG. 1B . Thus, fluid is driven out of orifices  132  in a direction that is at or near perpendicular to the surface of orifice plate  130  and/or the top surface of heat-generating structure  102 . The fluid is driven from orifices  132  toward heat-generating structure  102  at a high speed, for example in excess of thirty-five meters per second. In some embodiments, the fluid then travels along the surface of heat-generating structure  102  and toward the periphery of heat-generating structure  102 , where the pressure is lower than near orifices  132 . Also in the down stroke, top chamber  140  increases in size and a lower pressure is present in top chamber  140 . As a result, fluid is drawn into top chamber  140  through vent  112 . The motion of the fluid into vent  112 , through orifices  132 , and along the surface of heat-generating structure  102  is shown by unlabeled arrows in  FIG. 1B . 
     Cooling element  120  is also actuated so that top  121  moves away from heat-generating structure  102  and toward top plate  110 .  FIG. 1C  can thus be considered to depict the end of an up stroke of cooling element  120 . Because of the motion of cooling element  120 , gap  142  has decreased in size and is shown as gap  142 C. Gap  152  has increased in size and is shown as gap  152 C. During the upstroke, a higher (e.g. maximum) pressure is developed at the periphery when cooling element  120  is at the neutral position. As the upstroke continues, bottom chamber  150  increases in size and top chamber  140  decreases in size as shown in  FIG. 1C . Thus, the fluid is driven from top chamber  140  (e.g. the periphery of chamber  140 / 150 ) to bottom chamber  150 . Thus, when tip  121  of cooling element  120  moves up, top chamber  140  serves as a nozzle for the entering fluid to speed up and be driven towards bottom chamber  150 . The motion of the fluid into bottom chamber  150  is shown by unlabeled arrows in  FIG. 1C . The location and configuration of cooling element  120  and orifices  132  are selected to reduce suction and, therefore, back flow of fluid from the jet channel (between heat-generating structure  102  and orifice plate  130 ) into orifices  132  during the upstroke. Thus, cooling system  100  is able to drive fluid from top chamber  140  to bottom chamber  150  without an undue amount of backflow of heated fluid from the jet channel entering bottom chamber  10 . 
     The motion between the positions shown in  FIGS. 1B and 1C  is repeated. Thus, cooling element  120  undergoes vibrational motion indicated in  FIGS. 1A-1C , drawing fluid through vent  112  from the distal side of top plate  110  into top chamber  140 ; transferring fluid from top chamber  140  to bottom chamber  150 ; and pushing the fluid through orifices  132  and toward heat-generating structure  102 . As discussed above, cooling element  120  is driven to vibrate at or near the structural resonant frequency of cooling element  120 . Further, the structural resonant frequency of cooling element  120  is configured to align with the acoustic resonance of the chamber  140 / 150 . The structural and acoustic resonant frequencies are generally chosen to be in the ultrasonic range. For example, the vibrational motion of cooling element  120  may be at frequencies from 15 kHz through 30 kHz. In some embodiments, cooling element  120  vibrates at a frequency/frequencies of at least 20 kHz and not more than 30 kHz. The structural resonant frequency of cooling element  120  is within ten percent of the acoustic resonant frequency of cooling system  100 . In some embodiments, the structural resonant frequency of cooling element  120  is within five percent of the acoustic resonant frequency of cooling system  100 . In some embodiments, the structural resonant frequency of cooling element  120  is within three percent of the acoustic resonant frequency of cooling system  100 . Consequently, efficiency and flow rate may be enhanced. However, other frequencies may be used. 
     Fluid driven toward heat-generating structure  102  may move substantially normal (perpendicular) to the top surface of heat-generating structure  102 . In some embodiments, the fluid motion may have a nonzero acute angle with respect to the normal to the top surface of heat-generating structure  102 . In either case, the fluid may thin and/or form apertures in the boundary layer of fluid at heat-generating structure  102 . As a result, transfer of heat from heat-generating structure  102  may be improved. The fluid deflects off of heat-generating structure  102 , traveling along the surface of heat-generating structure  102 . In some embodiments, the fluid moves in a direction substantially parallel to the top of heat-generating structure  102 . Thus, heat from heat-generating structure  102  may be extracted by the fluid. The fluid may exit the region between orifice plate  130  and heat-generating structure  102  at the edges of cooling system  100 . Chimneys or other ducting (not shown) at the edges of cooling system  100  allow fluid to be carried away from heat-generating structure  102 . In other embodiments, heated fluid may be transferred further from heat-generating structure  102  in another manner. The fluid may exchange the heat transferred from heat-generating structure  102  to another structure or to the ambient environment. Thus, fluid at the distal side of top plate  110  may remain relatively cool, allowing for the additional extraction of heat. In some embodiments, fluid is circulated, returning to distal side of top plate  110  after cooling. In other embodiments, heated fluid is carried away and replaced by new fluid at the distal side of cooling element  110 . As a result, heat-generating structure  102  may be cooled. 
       FIGS. 1D-1E  depict an embodiment of active cooling system  100  including centrally anchored cooling element  120  in which the cooling element is driven out-of-phase. More specifically, sections of cooling element  120  on opposite sides of anchor  160  (and thus on opposite sides of the central region of cooling element  120  that is supported by anchor  160 ) are driven to vibrate out-of-phase. In some embodiments, sections of cooling element  120  on opposite sides of anchor  160  are driven at or near one hundred and eighty degrees out-of-phase. Thus, one section of cooling element  120  vibrates toward top plate  110 , while the other section of cooling element  120  vibrates toward orifice plate  130 /heat-generating structure  102 . Movement of a section of cooling element  120  toward top plate  110  (an upstroke) drives fluid in top cavity  140  to bottom cavity  150  on that side of anchor  160 . Movement of a section of cooling element  120  toward orifice plate  130  drives fluid through orifices  132  and toward heat-generating structure  102 . Thus, fluid traveling at high speeds (e.g. speeds described with respect to in-phase operation) is alternately driven out of orifices  132  on opposing sides of anchor  160 . The movement of fluid is shown by unlabeled arrows in  FIGS. 1D and 1E . 
     The motion between the positions shown in  FIGS. 1D and 1E  is repeated. Thus, cooling element  120  undergoes vibrational motion indicated in  FIGS. 1A, 1D, and 1E , alternately drawing fluid through vent  112  from the distal side of top plate  110  into top chamber  140  for each side of cooling element  120 ; transferring fluid from each side of top chamber  140  to the corresponding side of bottom chamber  150 ; and pushing the fluid through orifices  132  on each side of anchor  160  and toward heat-generating structure  102 . As discussed above, cooling element  120  is driven to vibrate at or near the structural resonant frequency of cooling element  120 . Further, the structural resonant frequency of cooling element  120  is configured to align with the acoustic resonance of the chamber  140 / 150 . The structural and acoustic resonant frequencies are generally chosen to be in the ultrasonic range. For example, the vibrational motion of cooling element  120  may be at the frequencies described for in-phase vibration. The structural resonant frequency of cooling element  120  is within ten percent of the acoustic resonant frequency of cooling system  100 . In some embodiments, the structural resonant frequency of cooling element  120  is within five percent of the acoustic resonant frequency of cooling system  100 . In some embodiments, the structural resonant frequency of cooling element  120  is within three percent of the acoustic resonant frequency of cooling system  100 . Consequently, efficiency and flow rate may be enhanced. However, other frequencies may be used. 
     Fluid driven toward heat-generating structure  102  for out-of-phase vibration may move substantially normal (perpendicular) to the top surface of heat-generating structure  102 , in a manner analogous to that described above for in-phase operation. Similarly, chimneys or other ducting (not shown) at the edges of cooling system  100  allow fluid to be carried away from heat-generating structure  102 . In other embodiments, heated fluid may be transferred further from heat-generating structure  102  in another manner. The fluid may exchange the heat transferred from heat-generating structure  102  to another structure or to the ambient environment. Thus, fluid at the distal side of top plate  110  may remain relatively cool, allowing for the additional extraction of heat. In some embodiments, fluid is circulated, returning to distal side of top plate  110  after cooling. In other embodiments, heated fluid is carried away and replaced by new fluid at the distal side of cooling element  110 . As a result, heat-generating structure  102  may be cooled. 
     Using the cooling system  100  actuated for in-phase vibration or out-of-phase vibration, fluid drawn in through vent  112  and driven through orifices  132  may efficiently dissipate heat from heat-generating structure  102 . Because fluid impinges upon the heat-generating structure with sufficient speed (e.g. at least thirty meters per second) and in some embodiments substantially normal to the heat-generating structure, the boundary layer of fluid at the heat-generating structure may be thinned and/or partially removed. Consequently, heat transfer between heat-generating structure  102  and the moving fluid is improved. Because the heat-generating structure is more efficiently cooled, the corresponding integrated circuit may be run at higher speed and/or power for longer times. For example, if the heat-generating structure corresponds to a high-speed processor, such a processor may be run for longer times before throttling. Thus, performance of a device utilizing cooling system  100  may be improved. Further, cooling system  100  may be a MEMS device. Consequently, cooling systems  100  may be suitable for use in smaller and/or mobile devices, such as smart phones, other mobile phones, virtual reality headsets, tablets, two-in-one computers, wearables and handheld games, in which limited space is available. Performance of such devices may thus be improved. Because cooling element  120  may be vibrated at frequencies of 15 kHz or more, users may not hear any noise associated with actuation of cooling elements. If driven at or near structural and/or acoustic resonant frequencies, the power used in operating cooling systems may be significantly reduced. Cooling element  120  does not physically contact top plate  110  or orifice plate  130  during vibration. Thus, resonance of cooling element  120  may be more readily maintained. More specifically, physical contact between cooling element  120  and other structures disturbs the resonance conditions for cooling element  120 . Disturbing these conditions may drive cooling element  120  out of resonance. Thus, additional power would need to be used to maintain actuation of cooling element  120 . Further, the flow of fluid driven by cooling element  120  may decrease. These, issues are avoided through the use of pressure differentials and fluid flow as discussed above. The benefits of improved, quiet cooling may be achieved with limited additional power. Further, out-of-phase vibration of cooling element  120  allows the position of the center of mass of cooling element  100  to remain more stable. Although a torque is exerted on cooling element  120 , the force due to the motion of the center of mass is reduced or eliminated. As a result, vibrations due to the motion of cooling element  120  may be reduced. Moreover, efficiency of cooling system  100  may be improved through the use of out-of-phase vibrational motion for the two sides of cooling element  120 . Consequently, performance of devices incorporating the cooling system  100  may be improved. Further, cooling system  100  may be usable in other applications (e.g. with or without heat-generating structure  102 ) in which high fluid flows and/or velocities are desired. 
       FIGS. 2A-2B  depict plan views of embodiments of cooling systems  200 A and  200 B analogous to active cooling systems such as cooling system  100 .  FIGS. 2A and 2B  are not to scale. For simplicity, only portions of cooling elements  220 A and  220 B and anchors  260 A and  260 B, respectively, are shown. Cooling elements  220 A and  220 B are analogous to cooling element  120 . Thus, the sizes and/or materials used for cooling elements  220 A and/or  220 B may be analogous to those for cooling element  120 . Anchors (support structures)  260 A and  260 B are analogous to anchor  160  and are indicated by dashed lines. 
     For cooling elements  220 A and  220 B, anchors  260 A and  260 B are centrally located and extend along a central axis of cooling elements  220 A and  220 B, respectively. Thus, the cantilevered portions that are actuated to vibrate are to the right and left of anchors  260 A and  260 B. In some embodiments, cooling element(s)  220 A and/or  220 B are continuous structures, two portions of which are actuated (e.g. the cantilevered portions outside of anchors  260 A and  260 B). In some embodiments, cooling element(s)  220 A and/or  220 B include separate cantilevered portions each of which is attached to the anchors  260 A and  260 B, respectively, and actuated. Cantilevered portions of cooling elements  220 A and  220 B may thus be configured to vibrate in a manner analogous to the wings of a butterfly (in-phase) or to a seesaw (out-of-phase). In  FIGS. 2A and 2B , L is the length of the cooling element, analogous to that depicted in  FIGS. 1A-1E . Also in  FIGS. 2A and 2B , the depth, P, of cooling elements  220 A and  220 B is indicated. 
     Also shown by dotted lines in  FIGS. 2A-2B  are piezoelectric  223 . Piezoelectric  223  is used to actuate cooling elements  220 A and  220 B. In some embodiments, piezoelectric  223  may be located in another region and/or have a different configuration. Although described in the context of a piezoelectric, another mechanism for actuating cooling elements  260 A and  260 B can be utilized. Such other mechanisms may be at the locations of piezoelectric  223  or may be located elsewhere. In cooling element  260 A, piezoelectric  223  may be affixed to cantilevered portions or may be integrated into cooling element  220 A. Further, although piezoelectric  223  is shown as having particular shapes and sizes in  FIGS. 2A and 2B , other configurations may be used. 
     In the embodiment shown in  FIG. 2A , anchor  260 A extends the entire depth of cooling element  220 A. Thus, a portion of the perimeter of cooling element  260 A is pinned. The unpinned portions of the perimeter of cooling element  260 A are part of the cantilevered sections that undergo vibrational motion. In other embodiments, anchor need not extend the entire length of the central axis. In such embodiments, the entire perimeter of the cooling element is unpinned. However, such a cooling element still has cantilevered sections configured to vibrate in a manner described herein. For example, in  FIG. 2B , anchor  260 B does not extend to the perimeter of cooling element  220 B. Thus, the perimeter of cooling element  220 B is unpinned. However, anchor  260 B still extends along the central axis of cooling element  220 B. Cooling element  220 B is still actuated such that cantilevered portions vibrate (e.g. analogous to the wings of a butterfly). 
     Although cooling element  220  A is depicted as rectangular, cooling elements may have another shape. In some embodiments, corners of cooling element  220 A may be rounded. Cooling element  220 B of  FIG. 2B  has rounded cantilevered sections. Other shapes are possible. In the embodiment shown in  FIG. 2B , anchor  260 B is hollow and includes apertures  263 . In some embodiments, cooling element  220 B has aperture(s) in the region of anchor  260 B. In some embodiments, cooling element  220 B includes multiple portions such that aperture(s) exist in the region of anchor  260 B. As a result, fluid may be drawn through cooling element  220 B and through anchor  260 B. Thus, cooling element  220 B may be used in place of a top plate, such as top plate  110 . In such embodiments, apertures in cooling element  220 B and apertures  263  may function in an analogous manner to vent  112 . Further, although cooling elements  200 A and  200 B are depicted as being supported in a central region, in some embodiments, one cantilevered section of the cooling element  220 A and/or  220 B might be omitted. In such embodiments, cooling element  220 A and/or  220 B may be considered to be supported, or anchored, at or near one edge, while at least part of at least the opposing edge is free to undergo vibrational motion. In some such embodiments, the cooling element  220 A and/or  220 B may include a single cantilevered section that undergoes vibrational motion. 
       FIGS. 3A-3B  depict plan views of embodiments of cooling systems  300 A and  300 B analogous to active cooling systems such as cooling system  100 .  FIGS. 3A and 3B  are not to scale. For simplicity, only cooling elements  320 A and  320 B and anchors  360 A and  360 B, respectively, are shown. Cooling elements  320 A and  320 B are analogous to cooling element  120 . Thus, the sizes and/or materials used for cooling elements  320 A and/or  320 B may be analogous to those for cooling element  120 . Anchors  360 A and  360 B are analogous to anchor  160  and are indicated by dashed lines. 
     For cooling elements  320 A and  320 B, anchors  360 A and  360 B, respectively, are limited to a central region of cooling elements  320 A and  320 B, respectively. Thus, the regions surrounding anchors  360 A and  360 B undergo vibrational motion. Cooling elements  320 A and  320 B may thus be configured to vibrate in a manner analogous to a jellyfish or similar to the opening/closing of an umbrella. In some embodiments, the entire perimeter of cooling elements  320 A and  320 B vibrate in phase (e.g. all move up or down together). In other embodiments, portions of the perimeter of cooling elements  320 A and  320 B vibrate out of phase. In  FIGS. 3A and 3B , L is the length (e.g. diameter) of the cooling element, analogous to that depicted in  FIGS. 1A-1E . Although cooling elements  320 A and  320 B are depicted as circular, cooling elements may have another shape. Further, a piezoelectric (not shown in  FIGS. 3A-3B ) and/or other mechanism may be used to drive the vibrational motion of cooling elements  320 A and  320 B. 
     In the embodiment shown in  FIG. 3B , the anchor  360 B is hollow and has apertures  363 . In some embodiments, cooling element  320 B has aperture(s) in the region of anchor  360 B. In some embodiments, cooling element  320 B includes multiple portions such that aperture(s) exist in the region of anchor  360 B. As a result, fluid may be drawn through cooling element  320 B and through anchor  360 B. The fluid may exit through apertures  363 . Thus, cooling element  320 B may be used in place of a top plate, such as top plate  110 . In such embodiments, apertures in cooling element  320 B and apertures  363  may function in an analogous manner to vent  112 . 
     Cooling systems such as cooling system  100  can utilize cooling element(s)  220 A,  220 B,  320 A,  320 B and/or analogous cooling elements. Such cooling systems may also share the benefits of cooling system  100 . Cooling systems using cooling element(s)  220 A,  220 B,  320 A,  320 B and/or analogous cooling elements may more efficiently drive fluid toward heat-generating structures at high speeds. Consequently, heat transfer between the heat-generating structure and the moving fluid is improved. Because the heat-generating structure is more efficiently cooled, the corresponding device may exhibit improved operation, such as running at higher speed and/or power for longer times. Cooling systems employing cooling element(s)  220 A,  220 B,  320 A,  320 B and/or analogous cooling elements may be suitable for use in smaller and/or mobile devices in which limited space is available. Performance of such devices may thus be improved. Because cooling element(s)  220 A,  220 B,  320 A,  320 B and/or analogous cooling elements may be vibrated at frequencies of 15 kHz or more, users may not hear any noise associated with actuation of cooling elements. If driven at or near the acoustic and/or structural resonance frequencies for the cooling element(s)  220 A,  220 B,  320 A,  320 B and/or analogous cooling elements, the power used in operating cooling systems may be significantly reduced. Cooling element(s)  220 A,  220 B,  320 A,  320 B and/or analogous cooling elements may not physically contact the plates during use, allowing resonance to be more readily maintained. The benefits of improved, quiet cooling may be achieved with limited additional power. Consequently, performance of devices incorporating the cooling element(s)  220 A,  220 B,  320 A,  320 B and/or analogous cooling elements may be improved. 
     In some embodiments, the cooling element may be anchored at one or more edges instead of at its center. For example,  FIGS. 4A-4C  depict an embodiment of cooling system  400  in which the edges of the cooling elements are anchored. 
       FIGS. 4A-4C  are diagrams depicting an exemplary embodiment of active cooling system  400  usable with a heat-generating structure  402 . For clarity, only certain components are shown and  FIGS. 4A-4C  are not to scale. Cooling system  400  is used in connection with a heat-generating structure  402 . Although shown as symmetric, cooling system  400  need not be symmetric. 
     Cooling system  400  includes cooling elements  410  and  420 . Cooling system  400  also includes orifice plate  430  having orifices  432  therein, top chamber  440  and bottom chamber  450  that may be analogous to orifice plate  130  having orifices  132  therein, top chamber  140  and bottom chamber  150 . Also shown are optional chimneys  460  used to direct fluid. 
     Cooling element  410  has a first side distal from heat-generating structure  402  and a second side proximate to heat-generating structure  402 . The first side of cooling element  410  is the top of cooling element  410  and the second side is the bottom of cooling element  410 . Cooling element  410  also has a passive vent  412  therein. In the embodiment shown, passive vent  412  is a centrally located aperture in cooling element  410 . In other embodiments, passive vent  412  may be located elsewhere. For example, passive vent  412  may be closer to one of the edges of cooling element  410 . Passive vent  412  may have a circular, rectangular or other shaped footprint. Although one passive vent  412  is shown, multiple passive vents might be used. 
     Cooling element  420  is between cooling element  410  and heat-generating structure  402 . In the embodiment shown, cooling element  420  is also between cooling element  410  and orifice plate  430 . Cooling elements  410  and  420  are separated by gap  442  and form a top chamber  440 . A bottom chamber  450  is formed between cooling element  420  and orifice plate  430 . Cooling element  420  also has active vents  422  therein. In the embodiment shown, active vents  422  are apertures located away from the central region of cooling element  420 . In other embodiments, active vents  422  may be located elsewhere. For example, an active vent may be centrally located in cooling element  420 . Although two active vents  422  are shown, another number (e.g. one, three, etc.) might be present. In some embodiments, active vents  422  are positioned such that the active vents  422  are not aligned with passive vent  412 . Active vents  422  may have circular, rectangular or other shaped footprints. In some embodiments, a single cooling element  410  or  420  which does not include a vent may be used in lieu of two cooling elements. 
     In some embodiments, cooling system  400  includes chimneys  460 . Chimneys  460  provide a return path for heated fluid to flow away from heat-generating structure  402 . In some embodiments, chimneys  460  return fluid to the side of cooling element  410  distal from heat-generating structure  402 . In the embodiment shown, chimneys  460  direct heated fluid substantially perpendicular to heat-generating structure  402  and toward the side of cooling element  410  distal from heat-generating structure  402 . In other embodiments, chimneys  460  may be omitted or configured in another manner. For example, chimneys may instead directed fluid away from heat-generating structure  402  in a direction parallel to heat-generating structure  402  or perpendicular to heat-generating structure  402  but opposite to the direction shown (e.g. toward the bottom of the page). If multiple cooling systems  400  are provided in an array, each cooling system  400  may include chimneys, only cooling systems  400  at the edges may include chimneys, other ducting may be provided at the edges or other locations in the array to provide a path for heated fluid to flow and/or other mechanisms may be used to allow heated fluid to be removed from the region proximate to heat-generating structure  402 . 
       FIG. 4A  depicts cooling system  400  in a neutral position. Thus, cooling elements  410  and  420  are shown as substantially flat. In operation, piezoelectric cooling elements  410  and  420  are actuated to vibrate between positions shown in  FIGS. 4B and 4C . Piezoelectric cooling elements  410  and  420  are, therefore, piezoelectric actuators. Operation of cooling system  400  is described in the context of  FIGS. 4B and 4C . Referring to  FIG. 4B , piezoelectric cooling element  410  has been actuated to move away from (deform to be convex) heat-generating structure  402 , while piezoelectric cooling element  420  has been actuated to move toward (deform to be concave) heat-generating structure  402 . This configuration is referred to as the suction arrangement. Because of the vibrational motion of piezoelectric cooling elements  410  and  420 , gap  442  has increased in size and is shown as gap  442 A. For example, in some embodiments, gap  442  has a height of at least ten and not more than twenty micrometers in the neutral position ( FIG. 4A ). Gap  442 A may have a height of at least twenty and not more than thirty micrometers in the suction arrangement ( FIG. 4B ). Thus, top chamber  440  has increased in volume, while bottom chamber  450  has decreased in volume. In the suction arrangement, the flow resistance of passive vent  412  (passive suction flow resistance) is low. Consequently, the pressure at passive vent  412  is low. In contrast, the flow resistance of active vent  422  (active suction flow resistance) is high. Consequently, the pressure at active vent  422  is high. Because of the low passive suction flow resistance, fluid is drawn into top chamber  440  through passive vent  412 . This is shown by arrows in  FIG. 4B . However, fluid does not flow out of (or flows out to a limited extent) active vent  422  because of the high passive suction flow resistance. However, active vent  422  is not physically closed in this configuration. For example, active vent  422  is not in contact with orifice plate  430  in the suction arrangement. 
       FIG. 4C  depicts an expulsion arrangement. Piezoelectric cooling element  410  has been actuated to move toward (deform to be concave) heat-generating structure  402 , while piezoelectric cooling element  420  has been actuated to move away from (deform to be convex) heat-generating structure  402 . Because of the vibrational motion of piezoelectric cooling elements  410  and  420 , gap  442  has decreased in size and is shown as gap  442 B. For example, in some embodiments, gap  442  has a height of at least ten and not more than twenty micrometers in the neutral position ( FIG. 4A ). Gap  442 B has a height of at least five and not more than ten micrometers in the expulsion arrangement ( FIG. 4C ). Thus, top chamber  440  has decreased in volume, while bottom chamber  450  has increased in volume. In the expulsion arrangement, the flow resistance of passive vent  412  (passive expulsion flow resistance) is high. Consequently, the pressure at passive vent  412  is high. In contrast, the flow resistance of active vent  422  (active expulsion flow resistance) is low. Consequently, the pressure at active vent  422  is low. Because of the low active expulsion flow resistance, fluid is expelled from top chamber  440  through active vent  422 , into bottom chamber  450  and through orifices  432 . This is shown by arrows in  FIG. 4C . However, fluid does not flow out of (or flows out to a limited extent) passive vent  412  because of the high passive expulsion flow resistance. Thus, passive vent  412  is considered closed and active vent  422  is considered open in the expulsion arrangement. However passive vent  412  is not physically closed in this configuration. For example, passive vent  412  is not in contact with cooling element  420  in the expulsion arrangement. Gap  442 B does not have a zero length. 
     Virtual valves may be considered to be formed at or near active vent  422  and passive vent  412 . A virtual valve has a high, but not infinite, flow resistance when closed. Thus, a virtual valve does not physically block flow but instead uses a high flow resistance or high pressure to throttle or prevent flow. A virtual valve has a significantly lower flow resistance or pressure when open, allowing flow. In some embodiments, the ratio of flow resistances or pressures between closed and open for a virtual valve is at least three and not more than ten. Thus, active vent  422  and its virtual valve (“active virtual valve”) are considered closed in the suction arrangement because the flow resistance is sufficiently high that little or no fluid flows through active vent  422  in the suction arrangement. Passive vent  412  and its virtual valve (“passive virtual valve”) are considered open in the suction arrangement because the pressure or flow resistance is sufficiently low to allow fluid to be drawn in to top chamber  440  through passive vent  412 . In contrast, active vent  422  and active virtual valve are considered open in the expulsion arrangement because the pressure or flow resistance is sufficiently low to allow fluid to flow through active vent  422  and be driven out of orifices  432 . Passive vent  412  and passive virtual valve are considered closed in the expulsion arrangement because the pressure or flow resistance is sufficiently high that little to no fluid is drawn through passive vent  412  in the expulsion arrangement. 
     Due to the vibrational motion of cooling elements  410  and  420  (and the attendant decrease in gap  442 A/ 442 B from  FIG. 4B  to  FIG. 4C ), the fluid is drawn in to top chamber  440  and through orifices  432 . The motion of the fluid is shown by arrows through orifices  432 . The fluid may spread as it travels away from orifice plate  420 , as shown by dashed lines and arrows for some orifices  432  in  FIG. 4C . The fluid deflects off of heat-generating structure  402  and travels along the channel between heat-generating structure  402  and orifice plate  430 . 
     The motion between the positions shown in  FIGS. 4B and 4C  may be repeated. Thus, piezoelectric cooling elements  410  and  420  vibrate, drawing fluid through passive vent  412  from the distal side of cooling element  410 , into top chamber  440 , out of chamber  440  through active vent  422  and pushing the fluid through orifices  432  and toward heat-generating structure  402 . In some embodiments, the frequency/frequencies of vibration of cooling elements  410  and/or  420  are analogous to those of cooling element  120 . Further, in some embodiments, piezoelectric cooling element(s)  410  and/or  420  may be driven at or near the resonant frequency. The resonant frequencies of piezoelectric cooling element(s)  410  and  420  may also be desired to be close. In some embodiments, the resonant frequencies of piezoelectric cooling element(s)  410  and  420  are desired to be within one hundred Hertz. In some embodiments, feedback is used to maintain piezoelectric cooling element(s)  410  and/or  420  at or near resonance. In some embodiments, the resonant frequencies of cooling elements  410  and/or  420  are closely matched to the acoustic resonant frequencies of chamber(s)  440  and/or  450 . In some embodiments, the speed at which the fluid impinges on heat-generating structure  402  is in the ranges described herein for cooling system  100 . 
     As indicated in  FIG. 4C , the fluid driven toward heat-generating structure  402  may move substantially normal (perpendicular) to the top surface of heat-generating structure  402 . In other embodiments, the fluid motion may have a nonzero acute angle with respect to the normal to the top surface of heat-generating structure  402 . In either case, the fluid may thin and/or form apertures in the boundary layer of fluid at heat-generating structure  402 . The boundary layer in one case is indicated by the curved dotted lines at the top surface of heat-generating structure  402  in  FIG. 4C . As a result, transfer of heat from heat-generating structure  402  may be improved. The fluid deflects off of heat-generating structure  402 , traveling along the surface of heat-generating structure  402 . In some embodiments, the fluid moves in a direction substantially parallel to the top of heat-generating structure  402 . Thus, heat from heat-generating structure  402  may be extracted by the fluid. The fluid may exit the region between orifice plate  430  and heat-generating structure  402  at the edges of cooling system  400 . In the embodiment shown, chimneys  460  at the edges of cooling system  400  allow fluid to be carried away from heat-generating structure  402 . In other embodiments, heated fluid may be transferred further from heat-generating structure  402  in another manner. The fluid may return to the distal side of cooling elements  410  where the fluid may exchange the heat transferred from heat-generating structure  402  to another structure or to the ambient environment. The fluid may then be circulated through cooling system  400  to extract additional heat. In other embodiments, heated fluid is carried away and replaced by new fluid at the distal side of cooling element  410 . As a result, heat-generating structure  402  may be cooled. 
     Opening and closing of passive vent  412  (passive virtual valve) and active vent  422  (active virtual valve) to draw fluid into chamber  450  and expel fluid through orifices  432  is based upon dynamic changes to flow resistance. In some embodiments, the ratio of active suction flow resistance to active expulsion flow resistance is at least three. In some such embodiments, the ratio of active suction flow resistance to active expulsion flow resistance is not more than ten. In some embodiments, the ratio of passive expulsion flow resistance to passive suction flow resistance is at least three. In some such embodiments, the ratio of passive expulsion flow resistance to passive suction flow resistance is not more than ten. Thus, virtual valves corresponding to vents  410  and/or  420  may be opened and closed. These ratios of pressures may be considered to be due to the change in size of gap  442 / 442 A/ 442 B (e.g. five through thirty micrometers in some embodiments). In some embodiments, the difference in pressure between being open and closed is 0.1 atmosphere through 0.2 atmosphere. For example, the pressure at passive vent  412  in the suction arrangement may be at least 0.1 atmosphere and not more than 0.2 atmosphere less than the pressure at passive vent  412  in the expulsion arrangement. Similarly, the pressure at active vent  422  in the expulsion arrangement may be at least 0.1 atmosphere and not more than 0.2 atmosphere less than the pressure at active vent  422  in the suction arrangement. 
     Using the cooling system  400 , fluid may be drawn in through passive vent  412  (in the suction arrangement) and driven through active vent  422  and orifices  432  (in the expulsion arrangement). Thus, the fluid may efficiently dissipate heat from heat-generating structure  402  in a manner analogous to the fluid driven by cooling system  100 . Thus, performance of a device utilizing cooling system  400  may be improved. Further, cooling system  400  may be a MEMS device. Thus, cooling system  400  may small-having a total height not exceeding five hundred micrometers. Consequently, cooling systems  400  may be suitable for use in mobile devices, such as smart phones, other mobile phones, virtual reality headsets, tablets, two-in-one computers, wearables and handheld games, in which limited space is available. Active cooling system  400  may also be used in other compute devices. Because piezoelectric cooling element(s)  410  and/or  420  may be vibrated at ultrasonic frequencies, users may not hear any noise associated with actuation of cooling elements. If driven at or near resonance frequency for the first and second piezoelectric cooling element(s), the power used in operating cooling systems may be significantly reduced. 
       FIGS. 5A-5E  depict an embodiment of active cooling system  500  including multiple cooling cells configured as a tile, or array.  FIG. 5A  depicts a top view, while  FIGS. 5B-5E  depict side views.  FIGS. 5A-5E  are not to scale. Cooling system  500  includes four cooling cells  501 A,  501 B,  501 C and  501 D (collectively or generically  501 ), which are analogous to one or more of cooling systems described herein. More specifically, cooling cells  501  are analogous to cooling system  100 . In some embodiments, cooling cell(s)  501  may be analogous to cooling system  400  and/or another cooling system. Although four cooling cells  501  in a 2×2 configuration are shown, in some embodiments another number and/or another configuration of cooling cells  501  might be employed. In the embodiment shown, cooling cells  501  include shared top plate  510  having apertures  512 , cooling elements  520 , shared orifice plate  530  including orifices  532 , top chambers  540 , bottom chambers  550  and anchors (support structures)  560  that are analogous to top plate  110  having apertures  112 , cooling element  120 , orifice plate  130  having orifices  132 , top chamber  140 , bottom chamber  150  and anchor  160 . In some embodiments, cooling cells  501  may be fabricated together and separated, for example by cutting through top plate  510  and orifice plate  530 . Cooling elements  520  are driven out-of-phase (i.e. in a manner analogous to a seesaw). Further, as can be seen in  FIGS. 5B-5C  and  FIGS. 5D-5E  cooling element  520  in one cell is driven out-of-phase with cooling element(s)  520  in adjacent cell(s). In  FIGS. 5B-5C , cooling elements  520  in a row are driven out-of-phase. Thus, cooling element  520  in cell  501 A is out-of-phase with cooling element  520  in cell  501 B. Similarly, cooling element  520  in cell  501 C is out-of-phase with cooling element  520  in cell  501 D. In  FIGS. 5D-5E , cooling elements  520  in a column are driven out-of-phase. Thus, cooling element  520  in cell  501 A is out-of-phase with cooling element  520  in cell  501 C. Similarly, cooling element  520  in cell  501 B is out-of-phase with cooling element  520  in cell  501 D. By driving cooling elements  520  out-of-phase, vibrations in cooling system  500  may be reduced. 
     Cooling cells  501  of cooling system  500  function in an analogous manner to cooling system(s)  100 ,  400 , and/or an analogous cooling system. Consequently, the benefits described herein may be shared by cooling system  500 . Because cooling elements in nearby cells are driven out-of-phase, vibrations in cooling system  500  may be reduced. Because multiple cooling cells  501  are used, cooling system  500  may enjoy enhanced cooling capabilities. Further, multiples of individual cooling cells  501  and/or cooling system  500  may be combined in various fashions to obtain the desired footprint of cooling cells. 
       FIGS. 6A and 6B  depict cooling systems  600 A and  600 B used in closed device  610 A and open device  610 B. Referring to  FIG. 6A , active cooling system  600 A is used in closed device  610 A. Thus, the fluid (e.g. air) remains within the device. Closed device  610 A may be a mobile device such as a smartphones, tablet computer, notebook, virtual reality device, and/or other device. Active cooling system  600 A includes one or more cooling cell(s)  601 , tiles (formed of multiple cooling cells  601 ), and/or devices that are analogous to cooling cells, cooling systems and their components described herein. However, for clarity, some such structures are not shown. 
     Cooling cell(s)  601  are analogous to cooling cells  501 , cooling systems  100  and/or  400  and their components described herein. Thus, cooling cells  601  may include cooling element(s) driven to undergo vibrational motion for example at the structural and/or acoustic resonant frequency of the chamber(s) therein, an orifice plate and other structures. However, for clarity, some such structures are not shown. Cooling cell(s)  601  also drive fluid at the speeds described herein using the vibrational motion. Further, cooling cells  601  and, in some embodiments, active cooling system  600 A and/or  600 B have a low profile. For example, the total thickness of cooling cells  601  and/or active cooling systems  600 A and/or  600 B may not exceed three millimeters. In some embodiments, the total thickness of cooling cell(s)  601  and/or active cooling systems  600 A and/or  600 B may not exceed than two millimeters. 
     Heat-generating structure  602  may be an integrated circuit (e.g. a chip package) and/or other structure residing on substrate  670 . Heat-generating structure  602  is a source of heat and may be analogous to heat-generating structure  102  and/or  402  or a heat spreader. Although described as a single component, in some embodiments, multiple components may be present in heat-generating structure  602  and cooled by cooling system  600 A. For example, heat-generating structure  602  may include a heat spreader, vapor chamber, multiple integrated circuits and/or other mechanism for spreading and/or reducing heat in addition to the component(s) that are the source of heat. Substrate  670  may be a printed circuit board or other structure. Also shown are components  664 ,  666  and  667  which may be integrated circuits or other components. Mechanisms for mounting components  664 ,  666  and  667  are not shown. Although not shown, internal and/or external temperature sensors as well as other components including but not limited to a heat spreader might be employed. Cover  604  that encloses device  610 A is also shown. 
     Cooling system  600 A is attached in proximity to heat-generating structure  602 . For example, cooling system  600 A may be attached to a frame in proximity to heat-generating structure  602 . In some embodiments, a heat spreader, vapor chamber or other mechanism for spreading and/or reducing heat may be interposed between component  662  and cooling system  600 . A jet channel between an orifice plate and component  662 , corresponding heat spreader and/or other heat-generating structure may be maintained to allow fluid flow. The flow of fluid in  FIG. 6A  is depicted by unlabeled arrows. 
     Cooling system  600 A operates in a manner analogous to cooling systems described herein. As can be seen by arrows in  FIG. 6A , cooler fluid (e.g. air) near component  667  is drawn toward cooling system  600 A. Cooling system  600 A drives fluid from its distal to the proximal side Thus, fluid is driven toward and impinges on heat-generating structure  602 . Heat from heat-generating structure  602  is transferred to the fluid. The fluid exits the region near heat-generating structure  602  carrying away heat from heat-generating structure  602 . The fluid is driven through closed device  610 A to a region of device  610 A sufficiently distant that heat may be transferred to and dissipated by one or more structures along the path of fluid flow. Stated differently, the fluid follows a path such that the fluid exiting cooling system  600 A passes a one or more structures within device  610 A that have temperature(s) less than the surface of heat-generating component  602 . In some embodiments, these structure(s) have temperature(s) less than the fluid passing the structure(s). Thus, such structure(s) in device  610 A may absorb some heat from the fluid carrying heat from heat-generating structure  602 . In some embodiments, the fluid is driven to a region distal from cooling system  600 A before returning to the cooling system  600 A. In some embodiments, the fluid passes a sufficient number and/or configuration of lower temperature structure(s) that at least ninety percent of the heat transferred from heat-generating structure  602  is removed from the fluid. In some embodiments, the fluid passes a sufficient number and/or configuration of lower temperature structure(s) that at least eighty percent of the heat transferred from heat-generating structure  602  is removed from the fluid. In some embodiments, the fluid passes a sufficient number and/or configuration of lower temperature structure(s) that at least fifty percent of the heat transferred from heat-generating structure  602  is removed from the fluid at steady state operation of heat-generating structure  602 . Other fractions of the heat transferred from heat-generating structure  602  are removed from the fluid in some embodiments. In some embodiments, device  610 A includes ducting, grooves in substrate  670  and/or case  604  or other features used direct the fluid flow. In some embodiments, the fluid&#39;s path may include a heat sink or other mechanism for dissipating heat. For example, cover  604  may be used to dissipate heat. 
     For example, in device  610 A, fluid exiting the region of heat-generating structure  602  is driven past components  664  and  666  and along a portion of substrate  670 . Fluid is also shown as traveling along cover  604  and around component  667  before returning along components  666  and  664 . The fluid may then re-enter cooling cell  601  and be reused. Although not explicitly shown in  FIG. 6A  some fluid may flow between components  664  and  666  and between components  666  and  666 . Further, in some embodiments, fluid need not flow to the opposing end (e.g. past component  667 ) to be cooled as desired. Because cover  604  and/or one or more of components  664 ,  666  and  667  are cooler than heat-generating structure  602  and cooler than the fluid carrying heat from heat-generating structure  602 , heat in the fluid can be absorbed by one or more of component(s)  604 ,  664 ,  666 , and  667 . In some embodiments, the fraction of heat transferred to one or more of component(s)  604 ,  664 ,  666 , and  667  is as described above. Thus, the fluid returning to cooling cell  601  is cooler than the fluid leaving the region of heat-generating structure  602 . Cooling cell  601  drives the cooler fluid toward heat-generating structure  602 . Because the fluid has been cooled while traversing device  610 A, the fluid can absorb heat from heat-generating structure  602 . 
     Active cooling system  600 A may provide the benefits of cooling system(s)  100 ,  400 , and/or  500 . Thus, cooling system  600 A may more efficiently and quietly cool heat-generating structure  602  at lower power. Thus, performance of heat-generating structure  602  may be improved. Additional cooling systems (not shown) can be employed and/or cooling system  600 A can be increased in size, for example by adding more cells, to cool additional portions of the device, such as components  664  and/or  667 . Because active cooling system  600 A circulates heated fluid within closed device  610 A, heat from heat-generating structure  602  may be spread among various structures that may be better able to dissipate heat. Thus, performance of closed device  610 A may be improved. 
     In addition, active cooling system  600 A may also be used to mitigate issues related to hot spots on cover  604  of device  610 A. In conventional devices which do not use active cooling system  600 A, hotspots typically develop in the back portion of the cover, just above the integrated circuit or heat spreader (corresponding to the portion of cover  604  just above heat-generating structure  602 ) due to radiation and/or free convection. To reduce the temperatures at these hotspots, conventional devices typically increase the distance between the heat spreader and back cover (e.g. increase the distance between heat-generating structure  602  and cover  604 ), place holes above the heat spreader/integrated chip area (e.g. place holes in cover  604  above heat-generating structure  602 ) and try to use a fan system to pull air through these holes, or place conductive tape on the back cover near the heat spreader/integrated circuit (e.g. place conductive tape in the region of cover  604  near heat-generating structure  602 ) to conduct heat away from the hot spot to conduct the heat away from the hot spot. However, there may be limited ability to increase the distance between the back cover and the heat spreader/integrated circuit in a conventional system. Typically there is a high flow resistance in the region of the heat spreader/integrated circuit. Thus, a fan may have limited ability to generate a sufficient flow in this region to mitigate hot spots. Further, fans may be unable to be used in devices such as smartphones. Similarly, the use of conductive tape provides only a limited ability to spread heat across the cover. In contrast, when active cooling system  600 A is turned on, cooler fluid (e.g. air) flowing toward active cooling system  600 A can not only be used to cool heat-generating structure  602 , but also naturally cools the region of cover  604  near active cooling system  600 A. For example, in some embodiments, the region near the inlet of active cooling system  600 A may be at least twenty degrees Celsius cooler than in fluid exiting the region near heat-generating structure  602  for steady state operation of device  610 . In some embodiments, the region near the inlet of active cooling system  600 A may be at least thirty degrees Celsius cooler than in fluid exiting the region near heat-generating structure  602  for steady state operation of device  610 . In some embodiments, the region near the inlet of active cooling system  600 A may be up to thirty-five degrees Celsius cooler than in fluid exiting the region near heat-generating structure  602  for steady state operation of device  610 . For similar reasons, when system  600 A is on and used to cool heat-generating structure  602 , the external surface temperature of case  604  proximate to (e.g. directly aligned with) the inlet (e.g. vents analogous to vents  112  and/or  422 ) of active cooling system  600 A is decreased by at least five degrees Celsius. In some embodiments, the external surface temperature of case  604  proximate to the inlet of active cooling system  600 A is decreased by at least ten degrees Celsius when system  600 A is on and used to cool heat-generating structure  602 . Other (e.g. larger) decreases are possible in some embodiments. Thus, hot spots area on cover  604  may be reduced or eliminated. 
       FIG. 6B  depicts cooling system  600 B used in open device  610 B. Thus, the fluid (e.g. air) enters, travels through and exits the device. Cooling system  600 B and open device  610 B are analogous to cooling system  600 A and closed device  610 A, respectively. Open device  610 B may be a mobile device such as a smartphone, tablet computer, notebook, virtual reality device, and/or other computing device. Active cooling system  600 B includes one or more cooling cell(s)  601 , that are analogous to cooling cells  501  and  601 , cooling systems  100  and/or  400  and their components described herein. Thus, cooling cells  601  may include cooling element(s) driven to undergo vibrational motion for example at the structural and/or acoustic resonant frequency, an orifice plate and other structures. However, for clarity, some such structures are not shown. 
     Device  610 B is analogous to device  610 A. Consequently, analogous structures have similar labels. Device  610 B includes heat-generating structure  602  on substrate  670 , cover  604 , and additional components  664 ,  666  and  667  that are analogous to heat-generating structure  602 , substrate  670 , cover  604 , and additional components  664 ,  666 , and  667 , respectively of device  610 A. Mechanisms for mounting components  664 ,  666  and  667  are not shown. Although not shown, internal and/or external temperature sensors as well as other components might be employed. Cooling system  600 B is attached in proximity to heat-generating structure  602 . For example, cooling system  600 B may be attached to a frame in proximity to heat-generating structure  602 . A jet channel between an orifice plate and heat-generating structure  602 , corresponding heat spreader and/or other heat-generating structure may be maintained to allow fluid flow. The flow of fluid in  FIG. 6B  is depicted by unlabeled arrows. Vents  690  and  692  allow for fluid communication between the interior of device  610 B and the external environment. In the embodiment shown, vent  690  operates as inlet  690  and vent  692  operates as outlet  692 . 
     Cooling cell(s)  601  of cooling system  600 B operate in a manner analogous to cooling systems described herein. As can be seen by arrows in  FIG. 6B , cooler fluid (e.g. air) near cooling system  600 B is drawn toward cooling cell(s)  601 . In particular, fluid from inlet  690  travels toward cooling cell(s)  601 . Cooling cell(s)  601  drive fluid from the side distal side from heat-generating structure  602  to the side proximal to heat-generating structure  602 . Thus, fluid is driven toward and contacts heat-generating structure  602 . Heat is transferred to the fluid. The fluid flows along heat-generating structure  602  and exits the region near heat-generating structure  602 , carrying away heat from heat-generating structure  602 . Thus, fluid carrying heat from heat-generating structure  602  can be expelled from device  610 B and new fluid from the outside environment drawn in through inlet  690  to cool heat-generating structure  602 . Further, because the fluid is directed to outlet  692  that is distal from cooling cell(s)  601 , heat may be transferred to and dissipated by one or more structures along the path in a manner analogous to that described above for cooling system  600 A. For example, heat may be transferred to cover  604  and/or component(s)  664 ,  666  and  667  because structures  604 ,  664 ,  666 , and  667  may have a lower temperature than heat-generating structure  602  and/or a lower temperature than the fluid carrying heat from heat-generating structure  602 . Thus, the fluid may be cooled at least somewhat (i.e. may transfer heat to one or more structures  604 ,  664 ,  666  and/or  667 ) before exiting device  610 B. 
     In some embodiments, the fluid passes a sufficient number or configuration of lower temperature structure(s) and/or a sufficient distance that at least ninety percent of the heat transferred from heat-generating structure  602  is removed from the fluid. In some embodiments, the fluid passes a sufficient number and/or configuration of lower temperature structure(s) and/or a sufficient distance that at least eighty percent of the heat transferred from heat-generating structure  602  is removed from the fluid. In some embodiments, the fluid passes a sufficient number and/or configuration of lower temperature structure(s) and/or a sufficient distance that at least fifty percent of the heat transferred from heat-generating structure  602  is removed from the fluid at steady state operation of heat-generating structure  602 . Other amounts of heat may be dissipated in other embodiments. In some embodiments, device  610 B includes ducting, grooves in substrate  670  and/or case  604  or other features used direct the fluid flow. In some embodiments, the fluid&#39;s path may include a heat sink or other mechanism for dissipating heat. For example, cover  604  may be used to dissipate heat. Consequently, heat may be efficiently removed from device  610 B. 
     Active cooling system  600 B may share benefits of active cooling systems  100 ,  400 ,  500 , and/or  600 A. Further, because cool fluid form outside of device  610 B can be used to cool heat-generating structure  602 , heat management may be improved. Thus, performance of heat-generating structure  602  may be further improved. Additional cooling systems (not shown) can be employed and/or cooling system  600 B can be increased in size, for example by adding more cells, to cool additional portions of the device, such as components  664  and/or  667 . Further, fluid may be driven to an outlet  692  that is sufficiently distant that fluid is at least somewhat cooled prior to exiting device  610 B. Thus, outlets  692 , and other structures in device  610 B, may be placed where desired. Consequently, configuration of device  610 B may be more flexible. Because the fluid exiting device  610 B may be cooler than if vent  692  were placed close to heat-generating structure  602 , discomfort or injury to the user due to fluid flowing through outlet  692  may be mitigated or avoided. Further, hot spots on cover  604  may be reduced in temperature or eliminated. 
     Similarly,  FIGS. 7A-7B  depict active cooling systems  700 A and  700 B used in closed device  710 A and open device  710 B, respectively. Cooling systems  700 A and  700 B each includes one or more cooling cell(s), tiles, and/or devices that are analogous to cooling cells, cooling systems and their components described herein. For example, cooling systems  700 A and  700 B are analogous to one or more of active cooling systems  100 ,  400 ,  500 ,  600 A and/or  600 B. However, for clarity, some such structures are not shown. In particular, cooling system  700 A including cooling cells  701 , and closed device  710 A are analogous to active cooling system  600 A, cooling cell(s)  601  and device  610 A. Active cooling system  700 B, cooling cells  701  and open device  710 B are analogous to cooling system  600 A, cooling cell(s)  601 , and open device  610 B. Active cooling systems  700 A and  700 B each explicitly include multiple cooling cells  701 , of which only one is labeled in each drawing. 
     Heat generating structure  760  includes component  762  (e.g. a chip package) residing on substrate  770  and heat spreader  764 . Although described as a single component, in some embodiments, multiple components may be present in component  762 . Heat spreader  764  spreads heat from component  762  and is cooled by cooling system  700 A. Thus, in device  710 A, heat spreader  764  is explicitly interposed between the fluid driven by cooling system  700 A and the component  762  that generates heat. Substrate  770  may be a printed circuit board. Heat spreader  764  resides between cooling system  700 A and component  762 . Also shown are components  765 ,  766  and  767  which may be integrated circuits or other components. Mechanisms for mounting components  765 ,  766  and  767  are not shown. Cover  780  that encloses device  710 A is also shown. Cooling system  700 A is attached in proximity to component  762 . For example, cooling system  700 A may be attached to a frame in proximity to component  762 . A jet channel between an orifice plate and heat spreader  764  is thus present. The flow of fluid in  FIG. 7A  is depicted by unlabeled arrows. 
     Cooling system  700 A operates in a manner analogous to cooling systems described herein, particularly system  600 A. As can be seen by arrows in  FIG. 7A , cooler fluid (e.g. air) from between components  766  and  767  is drawn toward cooling system  700 A. Cooling system  700 A drives fluid from its distal to the proximal side and along heat spreader  764 . Thus, heat is transferred from heat spreader  764  to the fluid and carried along heat spreader  764 . Further, heat spreader  764  includes an aperture therein. The heated fluid exits the region near the edge of heat spreader  764 , carrying away heat from component  762  and heat spreader  764 . Device  710 A is configured such that the fluid travels through device  710 A to a region of device  710 A sufficiently distant that heat may be transferred to and dissipated by one or more structures along the path of fluid flow. Stated differently, the fluid follows a path such that the fluid exiting the region near heat spreader  764  passes one or more structures that have temperature(s) less than the surface of heat spreader  764 . In some embodiments, these structure(s) have temperature(s) less than the fluid passing the structure(s). Thus, such structure(s) in device  710 A may absorb some heat from the fluid carrying heat from heat spreader  764 . In some embodiments, the fluid is driven to a region of system  710 A distal from cooling system  700 A before returning to the cooling system  700 A. In some embodiments, the fluid passes a sufficient number and/or configuration of lower temperature structure(s) that at least ninety percent of the heat transferred from component  762  is removed from the fluid. In some embodiments, the fluid passes a sufficient number and/or configuration of lower temperature structure(s) that at least eighty percent of the heat transferred from component  762  is removed from the fluid. In some embodiments, the fluid passes a sufficient number and/or configuration of lower temperature structure(s) that at least fifty percent of the heat transferred from component  762  is removed from the fluid at steady state operation of component  762 . Other fractions of the heat transferred from heat-generating structure  760  are removed from the fluid in some embodiments. In some embodiments, device  710 A includes ducting, grooves in substrate  770  and/or case  780  or other features used direct the fluid flow. In some embodiments, the fluid&#39;s path may include a heat sink or other mechanism for dissipating heat. For example, cover  780  and/or other structures may be used to dissipate heat. 
     Active cooling system  700 A may provide the benefits of cooling system(s)  100 ,  400 ,  500 ,  600 A and/or  600 B. Thus, cooling system  700 A may more efficiently and quietly cool heat-generating structure  760  at lower power. Thus, performance of component  762  may be improved. Additional cooling systems (not shown) can be employed and/or cooling system  700 A can be increased in size, for example by adding more cells, to cool additional portions of the device, such as components  765 ,  766  and/or  767 . Because active cooling system  700 A circulates heated fluid within closed device  710 A, heat from heat-generating structure  660  may be spread among various structures that may be better able to dissipate heat. Thus, performance of closed device  710 A may be improved. Further, hot spots on case  780  may be reduced in temperature or eliminated. 
       FIG. 7B  depicts cooling system  700 B used in open device  710 B. Thus, the fluid (e.g. air) enters, travels through and exits the device. Cooling system  700 B and open device  710 B are analogous to cooling system  700 A and closed device  710 A, respectively. Open device  710 B may be a mobile device such as a smartphone, tablet computer, notebook, virtual reality device, and/or other computing device. Active cooling system  700 B includes multiple cooling cell  701 , that are analogous to cooling cells  501 ,  601  and  701 , cooling systems  100  and/or  400  and their components described herein. Thus, cooling cells  701  may include cooling element(s) driven to undergo vibrational motion for example at the structural and/or acoustic resonant frequency, an orifice plate and other structures. However, for clarity, some such structures are not shown. 
     Device  710 B is analogous to device  710 A. Consequently, analogous structures have similar labels. Device  710 B includes heat-generating structure  660  on substrate  670  that explicitly includes heat spreader  764  separate from component  762 . Also shown are additional components  765 ,  766  and  767  as well as cover  780 , which are analogous to those shown in  FIG. 7A . Although not shown, internal and/or external temperature sensors as well as other components might be employed. Cooling system  700 B is attached in proximity to heat-generating structure  760 . For example, cooling system  700 B may be attached to a frame in proximity to heat-generating structure  760 . A jet channel between an orifice plate and heat spreader  764  may be maintained to allow fluid flow. The flow of fluid in  FIG. 6B  is depicted by unlabeled arrows. Vents  790  and  792  allow for fluid communication between the interior of device  610 B and the external environment. In the embodiment shown, vent  790  operates as inlet  790  and vent  792  operates as outlet  792 . 
     Cooling cells  701  of cooling system  700 B operate in a manner analogous to cooling systems described herein. As can be seen by arrows in  FIG. 7B , cooler fluid (e.g. air) from inlet  790  is drawn toward cooling cells  701 . Cooling cells  701  drive fluid from the side distal side from heat spreader  764  to the side proximal to heat spreader  764 . Thus, fluid is driven toward and along the surface of heat spreader  764 . Heat is transferred to the fluid. The fluid exits the region near heat spreader  764 , carrying away heat. Because of the configuration of device  710 B, the fluid is directed toward outlet  792 . Thus, fluid carrying heat from heat-generating structure  760  can be expelled from device  710 B and new fluid from the outside environment drawn in through inlet  790  to cool heat-generating structure  760 . Because the fluid is directed to outlet  792  that is distal from cooling cells  701 , heat may be transferred to and dissipated by one or more structures along the path in a manner analogous to that described above for cooling system  700 A. For example, heat may be transferred to cover  704  and/or component(s)  765 ,  766  and  767  because structures  704 ,  765 ,  766 , and  767  may have a lower temperature than heat spreader  764  and/or a lower temperature than the fluid carrying heat from heat-generating structure  760 . Thus, the fluid may be cooled at least somewhat (i.e. may transfer heat to one or more structures  704 ,  765 ,  766  and/or  767 ) before exiting device  710 B. In some embodiments, the fluid passes a sufficient number or configuration of lower temperature structure(s) and/or a sufficient distance that the fractions of heat transferred from heat-generating structure  760  described herein are removed from the fluid. Consequently, heat may be efficiently removed from device  710 B. 
     Active cooling system  700 B may share benefits of active cooling systems  100 ,  400 ,  500 ,  600 A,  600 B, and/or  700 A. Further, because cool fluid form outside of device  710 B can be used to cool heat-generating structure  760 , heat management may be improved. Thus, performance of heat-generating structure  760  may be further improved. Additional cooling systems (not shown) can be employed and/or cooling system  700 B can be increased in size, for example by adding more cells, to cool additional portions of the device, such as components  765  and/or  767 . Further, fluid may be driven to an outlet  792  that is sufficiently distant that fluid is at least somewhat cooled prior to exiting device  710 B. Thus, outlets  792 , and other structures in device  610 B, may be placed where desired. Consequently, configuration of device  710 B may be more flexible. Because the fluid exiting device  710 B may be cooler than if vent  792  were placed close to heat-generating structure  760 , discomfort or injury to the user due to fluid flowing through outlet  792  may be mitigated or avoided. Further, hot spots on case  780  may be reduced in temperature or eliminated. 
       FIG. 8  depicts cooling system  800  used in smartphone  810 . Cooling system  800  includes one or more cooling cell(s), tiles, and/or devices that are analogous to cooling cells, cooling systems and their components described herein. However, for clarity, such structures are not shown. In the embodiment shown, smartphone  810  is closed. Thus, the fluid (i.e. air) used to cool heat-generating structures in smartphone  810  remains within smartphone  810 . Smartphone also includes housing  820  and midframe  840 . In other embodiments, smartphone  810  might include inlet and/outlet vents in order to allow for fluid communicating with the external environment. In some embodiments, multiple active cooling systems  800  might be deployed in smartphone  810 . For clarity, a portion of the top of smartphone  810  has been removed. 
     In operation, active cooling system  800  drives fluid toward the underlying heat-generating structure(s), such as processor(s). As can be seen by the direction of the unlabeled arrows in  FIG. 8 , heated fluid travels along smartphone  810 , past the battery. As the heated fluid traverses smartphone  810 , heat is transferred to other structure(s) having a lower temperature. For example, heat may be transferred to housing  820  and/or midframe  840 . Smartphone  810  is configured to allow for this fluid flow. For example, grooves or channels  820  (of which only one is labeled) may be provided in midframe  840  and/or housing  820  to direct the flow of fluid. Similarly, spaces  832  provide a path for fluid to exit the region near active cooling system  800 . Thus, a path for the flow of fluid through one or more heat-absorbing structures in device  810  is provided. At a region of smartphone  810  distal from cooling system  800  (the opposing end of smartphone  810  in the embodiment shown), cooled fluid begins its return path to cooling system  800 . Thus, the fluid circulates within smartphone  810 . 
     Thus, cooling system  800  may be used to cool smartphone  810 . This cooling may be more efficient and capable of transferring a greater amount of heat from heat-generating structures. Further, because cooling system  800  is thin, cooling system  800  may be integrated into smartphone  800  with little or no change in thickness. Performance of smartphone  800  may thus be improved. Further, hot spots on housing  820  may be reduced in temperature or eliminated. 
       FIGS. 9A-9B  depict smartphone  910  utilizing cooling system  900  and cooling system  900 . Cooling system  900  includes four cooling cells  901  that are analogous to cooling cells/cooling systems described herein. Also shown is optional ducting  903  used to control fluid flow around cooling cells  901 , flexible connector  904  and associated electronics  906 . In the embodiment shown, smartphone  910  is closed. Thus, the fluid (i.e. air) used to cool heat-generating structures in smartphone  910  remains within smartphone  910 . Smartphone  910  includes housing  920  having cover glass  922  and back cover  924 . Smartphone  910  also includes camera module  912 , motherboard  914 , processor  960  and heat spreader  962  which form a heat-generating structure, other electronic components  916 , battery  918 , and midframe  940 . 
     As can be seen in  FIG. 9A , cooling system  900  drives cooled fluid (solid unlabeled arrows) onto a heat-generating structure. In this embodiment, the heat-generating structure includes heat spreader  962  thermally coupled to processor  960  on motherboard  914 . Cooling system  900  is coupled to midframe  940  in this embodiment. In some embodiments, cooling system  900  may be connected to a different component. Fluid driven by cooling system  900  is driven toward processor  960  and heat spreader  962 , contacts and flows along heat spreader  962 , and extracts heat generated by processor  960 . The heated fluid (dashed unlabeled arrows) travels along midframe  940  and battery  918 . Back cover  924  may be used to dissipate heat. For example, heat in the fluid may be convected to back cover  924  and radiated out. Similarly, heat may also be transferred to midframe  940  and dissipated. Heat may be transferred to back cover  924  and midframe  940  because these structures are lower in temperature than the heated fluid. Grooves  941  (shown with dotted lines) in midframe  940  in the side of midframe  940  proximal to battery  918  and grooves  944  (shown with dotted lines) in back cover  924  may be used to control the flow of the fluid. In some embodiments, the grooves may be on the order of 0.2 millimeters through 0.4 millimeters (e.g. nominally 0.3 millimeters) deep. Thus, as the fluid flows along midframe  940  and back cover  924 , the fluid cools. This is indicated by unlabeled longer dashed arrows. Past battery  918 , the cooler fluid returns along cover glass  922 , where it continues to cool. Fluid may be directed in a space between midframe  940  and cover glass  922  and/or in grooves  942  (shown with dotted lines) in midframe  940 . In some embodiments, the space and/or grooves  942  may be on the order of 0.2 millimeters through 0.4 millimeters (e.g. nominally 0.3 millimeters) deep. The cooled fluid returns to cooling system  900 , where it can again be used to cool the processor and/or other components. 
     Thus, cooling system  900  may be used to cool smartphone  900 . The configuration of smartphone  900 , such as grooves and the placement of structures therein, may also aid in directing fluid flow and absorbing heat from the fluid. This cooling may be more efficient and capable of transferring a greater amount of heat from heat-generating structures. Further, because cooling system  900  is thin, cooling system  900  may be integrated into smartphone  900  with little or no change in thickness. Performance of smartphone  900  may thus be improved. Further, hot spots on cover  922  may be reduced in temperature or eliminated. 
       FIGS. 10A-10D  depict embodiments of and performance for smartphones with and without cooling systems such as are described herein. More specifically,  FIG. 10A  depicts smartphone  1000 A cooled using only a vapor chamber. Consequently, no active cooling system is used. Thus, fluid flow is minimal within smartphone  1000 A.  FIG. 10B  depicts an embodiment of closed smartphone  1000 B including active cooling system  1001 B as described herein. Thus, fluid (e.g. air) used in cooling the smartphone depicted in  FIG. 10B  remains within the smartphone. Smartphone  1000 B is configured such that fluid follows a path from the surface of the heat-generating structure(s) near cooling system  1001 B past structure(s) having lower temperature(s) than the surface of the heat-generating structure. The structure(s) absorb heat from the fluid. The fluid then returns to cooling system  1001 B. The path of the fluid may be seen by lines  1002 B of which only two are labeled. 
       FIG. 10C  depicts an embodiment of vented smartphone  1000 C including cooling system  1001 C as described herein. Vented smartphone  1000 C also includes inlets  1003 C and outlet  1004 C. Thus, for smartphone  1000 C depicted in  FIG. 10C , cooled air from outside of smartphone  1000 C may be used by the cooling system for cooling heat-generating structures and heated fluid vented from the smartphone. Smartphone  1000 C is configured such that cooler fluid enters via inlets  1003 C and is driven toward a heat-generating structure by active cooling system  1001 C, as described previously. The fluid extracts heat from the heat-generating structure and follows a path from the surface of the heat-generating structure(s) near cooling system  1001 C past structure(s) having lower temperature(s) than the surface of the heat-generating structure. The structure(s) absorb heat from the fluid. The fluid is directed out of vented smartphone  1000 C via outlet  1004 C. The path of the fluid may be seen by lines  1002 C of which only three are labeled. 
       FIG. 10D  is a graph including curves  1010 ,  1020  and  1030  indicating the power usable by the processors in the smartphones  1000 A,  1000 B and  1000 C, respectively, of  FIGS. 10A, 10B and 10C , respectively.  FIG. 10D  is for explanatory purposes only and is not intended to correspond to particular devices. As can be seen in  FIG. 10D , curve  1010  for smartphone  1000 A (no active cooling system described herein) utilizes lower power because of heating of the processor. Further, the processor commences throttling at point  1012  sooner and at a lower power. Curve  1020  indicates that the processor for closed smartphone  1000 B using cooling system  1001 B described herein can utilize (and dissipate) higher power. Further, the throttling at point  1022  occurs later and at a higher power than for curve  1010 . Curve  1030  indicates that a processor for vented smartphone  1000 C using a cooling system described herein may utilize even higher power and may resist throttling for longer. Thus, performance may be improved. Further, as described above, because of the size of the cooling system, the thickness of a smartphone utilizing the cooling system may remain unchanged. In addition, hot spots on the covers of smartphones  1000 B and  1000 C may be reduced in temperature or eliminated. 
     The cooling systems described herein may be used with other devices and achieve analogous performance improvements. For example,  FIGS. 11A-11C  compares performance for notebook computer  1100 A without active cooling as described herein (shown in  FIG. 11A ) and embodiments of notebook computer with an active cooling system described herein, such as vented notebook computer  1100 B. More specifically,  FIG. 11A  depicts notebook computer  1100 A that does not utilize a cooling system described herein. Although notebook computer  1100 A includes vents, fluid flow indicated by lines  1102 A is minimal.  FIG. 11B  depicts an embodiment of open (i.e. vented) notebook computer  1100 B including cooling system  1102 B as described herein. Thus, for notebook computer  1100 B, cooled air from outside of the notebook computer enters via inlets  1103 B, is directed toward cooling system  1001 B and may be used by the cooling system for cooling heat-generating structures. The heated fluid is directed toward outlet  1104 B. Thus, heated fluid is vented from notebook computer  1100 B. Fluid flow is shown by lines  1102 B, of which only three are labeled.  FIG. 11C  is a graph including curves  1110 ,  1120  and  1130  indicating the power usable by the processors in notebook computer  1100 A, an embodiment of a closed notebook computer (not shown in  FIGS. 11A-11C ) and vented notebook computer  1100 B, respectively. As can be seen in  FIG. 11D , curve  1110  for the notebook computer that does not use a cooling system described herein utilizes lower power because of heating of the processor. Curve  1120  indicates that the processor for a closed notebook computer using a cooling system described herein can utilize (and dissipate) higher power. Curve  1130  indicates that a processor for a vented notebook computer using a cooling system described herein may utilize even higher power and may resist throttling for longer. Thus, performance may be improved. Further, as described above, because of the size of the cooling system, the thickness of a notebook computer utilizing the cooling system may remain unchanged. In addition, hot spots on the covers of notebook  1100 B may be reduced in temperature or eliminated. 
     Similarly,  FIGS. 12A-12D  depict embodiments of and performance for a virtual reality device with and without cooling systems such as are described herein. More specifically,  FIG. 12A  depicts virtual reality device  1200 A that does not utilize active cooling as described herein. Thus, fluid flow is minimal within virtual reality device  1200 A.  FIG. 12B  depicts an embodiment of closed virtual reality device  1200 B including active cooling system  1201 B as described herein. Thus, fluid (e.g. air) used in cooling the virtual reality device  1200 B depicted in  FIG. 12B  remains within the virtual reality device. Virtual reality device  1200 B is configured such that fluid follows a path from the surface of the heat-generating structure(s) near cooling system  1201 B past structure(s) having lower temperature(s) than the surface of the heat-generating structure. The structure(s) absorb heat from the fluid. The fluid then returns to cooling system  1201 B. The path of the fluid may be seen by lines  1202 B of which only two are labeled. 
       FIG. 12C  depicts an embodiment of vented virtual reality device  1200 C including cooling system  1201 C as described herein. Vented virtual reality device  1200 C also includes inlet  1203 C and outlet  1204 C. Thus, for virtual reality device  1200 C depicted in  FIG. 12C , cooled air from outside of virtual reality device  1200 C may be used by the cooling system for cooling heat-generating structures and heated fluid vented from the virtual reality device. Vented virtual reality device  1200 C is configured such that cooler fluid enters via inlets  1203 C and is driven toward a heat-generating structure by active cooling system  1201 C, as described previously. The fluid extracts heat from the heat-generating structure and follows a path from the surface of the heat-generating structure(s) near cooling system  1201 C past structure(s) having lower temperature(s) than the surface of the heat-generating structure. The structure(s) absorb heat from the fluid. The fluid is directed out of vented virtual reality device  1200 C via outlet  1204 C. The path of the fluid may be seen by lines  1202 C of which only two are labeled. 
       FIG. 12D  is a graph including curves  1210 ,  1220  and  1230  indicating the power usable by virtual reality devices  1200 A,  1200 B and  1200 C, respectively.  FIG. 12D  is for explanatory purposes only and is not intended to correspond to particular devices. As can be seen in  FIG. 12D , curve  1210  for virtual reality device  1200 A (no active cooling system described herein) utilizes lower power because of heating of the processor. Further, the processor commences throttling at point  1212  sooner. Curve  1220  indicates that the processor for closed virtual reality device  1200 B using cooling system  1201 B described herein can utilize (and dissipate) higher power. Further, the throttling at point  1222  occurs later than for curve  1210 . Curve  1230  indicates that a processor for vented virtual reality device  1200 C using a cooling system described herein may utilize even higher power and may resist throttling for longer. Thus, performance may be improved. Further, as described above, because of the size of the cooling system, the thickness of a virtual reality device utilizing the cooling system may remain unchanged. In addition, hot spots on the covers of virtual reality devices  1200 A,  1200 B and  1200 C may be reduced in temperature or eliminated. 
     Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed embodiments are illustrative and not restrictive.