Outdoor unit of air conditioner

Abstract

An outdoor unit of an air-conditioning apparatus is provided with a compressor configured to compress refrigerant, a heat exchanger through which refrigerant sent out from the compressor flows, a flow switching device configured to switch flow directions of refrigerant, a casing that houses the compressor, the heat exchanger, and the flow switching device, a bottom plate heater provided to a bottom plate of the casing and configured to heat water that falls from the heat exchanger, and a controller configured to control the compressor and the flow switching device and to perform a heating operation and a defrost operation in which frost that attaches to the heat exchanger during a heating operation is removed. The controller is configured to control the bottom plate heater according to outdoor air temperature and outdoor air humidity during the heating operation and the defrost operation.

Description

Technical Field

[0001] The present disclosure relates to an outdoor unit of an air-conditioning apparatus that performs a defrost operation.Background Art

[0002] In an outdoor unit of an air-conditioning apparatus, frost usually adheres to a heat exchanger in a case where a heating operation is continued in a low-temperature and high-humidity environment. For this reason, some known technology inhibits the growth of frost by attaching an electric heater to a sheet metal of a side surface of an outdoor unit (e.g., Patent Literature 1). In the outdoor unit described in Patent Literature 1, the provision of the electric heater prevents a vent hole, formed in the side surface of the outdoor unit, from being blocked by the grown frost, and thereby prevents a decrease in heat exchange efficiency.Citation ListPatent Literature

[0003] Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2014-214983Summary of InventionTechnical Problem

[0004] However, the outdoor unit disclosed in Patent Literature 1 does not prevent the refreezing of water drained from a heat exchanger to a bottom plate of the outdoor unit by a defrost operation. Here, by attaching the electric heater to the bottom plate and energizing the electric heater when outdoor air temperature is low, the freezing of water that has fallen from the heat exchanger is prevented. However, even if the electric heater attached to the bottom plate is energized when the outdoor air temperature is low, water does not necessarily accumulate on the bottom plate, and power may be wasted.

[0005] The present disclosure is made to address such problems described above and its object is to reduce the electric power consumption of a heater provided to a bottom plate in an outdoor unit of an air-conditioning apparatus.Solution to Problem

[0006] An outdoor unit of an air-conditioning apparatus according to an embodiment of the present disclosure is provided with a compressor configured to compress refrigerant, a heat exchanger through which refrigerant sent out from the compressor flows, a flow switching device configured to switch flow directions of refrigerant, a casing that houses the compressor, the heat exchanger, and the flow switching device, a bottom plate heater provided to a bottom plate of the casing and configured to heat water that falls from the heat exchanger, and a controller configured to control the compressor and the flow switching device and to perform a heating operation and a defrost operation in which frost that attaches to the heat exchanger during a heating operation is removed. The controller is configured to control the bottom plate heater according to outdoor air temperature and outdoor air humidity during the heating operation and the defrost operation.Advantageous Effects of Invention

[0007] The outdoor unit of an air-conditioning apparatus of an embodiment of the present disclosure controls the bottom plate heater according to the outdoor air temperature and the outdoor air humidity during a heating operation and a defrost operation. It is known that, in an environment where outdoor air is at low temperature and at low humidity, little frost forms on a heat exchanger during a heating operation and almost no water is drained from the heat exchanger to a bottom plate. Therefore, the outdoor unit of an air-conditioning apparatus of the present disclosure limits energization time to the heater provided to the bottom plate and thereby reduces electric power consumption.Brief Description of Drawings

[0008] [Fig. 1] Fig. 1 is a refrigerant circuit diagram that illustrates an air-conditioning apparatus according to Embodiment 1. [Fig. 2] Fig. 2 is an exploded perspective view that illustrates an outdoor unit according to Embodiment 1. [Fig. 3] Fig. 3 is a top view that illustrates the interior of the outdoor unit according to Embodiment 1. [Fig. 4] Fig. 4 is a diagram that illustrates a bottom plate and a bottom plate heater of the outdoor unit according to Embodiment 1. [Fig. 5] Fig. 5 is a functional block diagram that illustrates the outdoor unit according to Embodiment 1. [Fig. 6] Fig. 6 is a hardware configuration diagram that illustrates a controller according to Embodiment 1. [Fig. 7] Fig. 7 is a hardware configuration diagram that illustrates the controller according to Embodiment 1. [Fig. 8] Fig. 8 is a flowchart that illustrates an operation of the controller according to Embodiment 1. [Fig. 9] Fig. 9 is a flowchart that illustrates an operation of the controller according to Embodiment 1. Description of EmbodimentsEmbodiment 1

[0009] An air-conditioning apparatus 1 according to Embodiment 1 is described below with reference to drawings. Fig. 1 is a refrigerant circuit diagram that illustrates the air-conditioning apparatus 1 according to Embodiment 1. As illustrated in Fig. 1, the air-conditioning apparatus 1 has an outdoor unit 2 and an indoor unit 3. The outdoor unit 2 has a compressor 11, a flow switching device 12, a heat exchanger 13, an air-sending device 14, and an expansion device 15. The indoor unit 3 has a heat exchanger 16. The air-conditioning apparatus 1 performs, as operation modes, a cooling operation, a heating operation, and a defrost operation in which frost that attaches to the heat exchanger 13 during a heating operation is removed.

[0010] The compressor 11 sucks in refrigerant in a low-temperature and low-pressure state, compresses the sucked refrigerant into refrigerant in a high-temperature and high-pressure state, and discharges the refrigerant. The flow switching device 12 switches the flow directions of the refrigerant in the refrigerant circuit, and is, for example, a four-way valve. Note that the flow switching device 12 may also be a combination of any of two-way valves and three-way valves. The heat exchanger 13 exchanges heat between refrigerant and outdoor air and is, for example, a fin-and-tube heat exchanger. The heat exchanger 13 acts as a condenser during a cooling operation and as an evaporator during a heating operation. The air-sending device 14 is a device that sends outdoor air to the heat exchanger 13. The air-sending device 14 has a fan motor 14a, and an air volume is adjusted by controlling the rotational speed of the fan motor 14a. The expansion device 15 decompresses and expands refrigerant and is, for example, an electronic expansion valve.

[0011] The heat exchanger 16 exchanges heat between indoor air and refrigerant. The heat exchanger 16 acts as an evaporator during a cooling operation and as a condenser during a heating operation. Note that the indoor unit 3 may also have, for example, an air-sending device, such as a cross-flow fan, that sends indoor air to the heat exchanger 16.

[0012] The outdoor unit 2 has a refrigerant temperature sensor 31 and an outdoor air temperature sensor 32. The refrigerant temperature sensor 31 is attached closely to a pipe that connects the heat exchanger 13 and the expansion device 15 with each other. The refrigerant temperature sensor 31 measures the temperature of refrigerant that flows through the pipe and transmits a measurement result to a controller 21. In particular, the refrigerant temperature sensor 31 detects the temperature of refrigerant that flows into the heat exchanger 13 of the outdoor unit 2 during a heating operation and the temperature of refrigerant that passes through the heat exchanger 13 of the outdoor unit 2 during a defrost operation. The outdoor air temperature sensor 32 is provided inside a casing 40 of the outdoor unit 2, which is described later. The outdoor air temperature sensor 32 measures the temperature of outdoor air before the heat of the outdoor air is exchanged at the heat exchanger 13 and transmits a measurement result to the controller 21. Note that the controller 21 is described later.

[0013] Here, an operation of the air-conditioning apparatus 1 is described for each operation mode. First, a cooling operation is described below. In a cooling operation, the air-conditioning apparatus 1 switches the flow switching device 12 such that the discharge side of the compressor 11 and the heat exchanger 13 are connected with each other as illustrated in a solid line. In a cooling operation, refrigerant sucked into the compressor 11 is compressed by the compressor 11 and discharged as refrigerant in a high-temperature and high-pressure gas state. The refrigerant discharged from the compressor 11 in a high-temperature and high-pressure gas state passes through the flow switching device 12 and flows into the heat exchanger 13, which acts as a condenser. The refrigerant that flows into the heat exchanger 13 exchanges heat with outdoor air sent by the air-sending device 14, condenses, and liquefies. The refrigerant in a liquid state flows into the expansion device 15, where the refrigerant is depressurized and expanded and thus becomes refrigerant in a low-temperature and low-pressure two-phase gas-liquid state. The two-phase gas-liquid state refrigerant flows into the heat exchanger 16, which acts as an evaporator. The refrigerant that flows into the heat exchanger 16 exchanges heat with indoor air, evaporates, and gasifies. At this time, the indoor air is cooled and cooling is performed in a room. Then, the refrigerant that has evaporated and is in a low-temperature and low-pressure gas state passes through the flow switching device 12 and is sucked into the compressor 11.

[0014] Next, a heating operation is described below. In a heating operation, the air-conditioning apparatus 1 switches the flow switching device 12 such that the discharge side of the compressor 11 and the heat exchanger 16 are connected with each other as illustrated in a dashed line. In a heating operation, refrigerant sucked into the compressor 11 is compressed by the compressor 11 and discharged as refrigerant in a high-temperature and high-pressure gas state. The refrigerant discharged from the compressor 11 in a high-temperature and high-pressure gas state passes through the flow switching device 12 and flows into the heat exchanger 16, which acts as a condenser. The refrigerant that flows into the heat exchanger 16 exchanges heat with indoor air, condenses, and liquefies. At this time, the indoor air is heated and heating is performed in a room. The refrigerant in a liquid state flows into the expansion device 15, where the refrigerant is depressurized and expanded and thus becomes refrigerant in a low-temperature and low-pressure two-phase gas-liquid state. The two-phase gas-liquid state refrigerant flows into the heat exchanger 13, which acts as an evaporator. The refrigerant that flows into the heat exchanger 13 exchanges heat with outdoor air sent by the air-sending device 14, evaporates, and gasifies. Then, the refrigerant that has evaporated and is in a low-temperature and low-pressure gas state passes through the flow switching device 12 and is sucked into the compressor 11.

[0015] Subsequently, a defrost operation is described below. In a defrost operation, the air-conditioning apparatus 1 switches the flow switching device 12 as in a cooling operation such that the discharge side of the compressor 11 and the heat exchanger 13 are connected with each other as illustrated in the solid line. In a defrost operation, refrigerant sucked into the compressor 11 is compressed by the compressor 11 and discharged as refrigerant in a high-temperature and high-pressure gas state. The refrigerant discharged from the compressor 11 in a high-temperature and high-pressure gas state passes through the flow switching device 12 and flows into the heat exchanger 13. The refrigerant that flows into the heat exchanger 13 condenses and liquefies through heat exchange with frost that adheres to the heat exchanger 13. At this time, the frost, which adheres to the heat exchanger 13 during a heating operation, is removed. The flow of refrigerant thereafter is the same as in a cooling operation and its description is thus omitted.

[0016] Subsequently, the internal configuration of the outdoor unit 2 is described below with reference to Fig. 2 to Fig. 4. Fig. 2 is an exploded perspective view that illustrates the outdoor unit 2 according to Embodiment 1. As illustrated in Fig. 2, the outdoor unit 2 houses, inside the casing 40, the compressor 11, the flow switching device 12, the heat exchanger 13, the air-sending device 14, and the expansion device 15. The casing 40 serves as the outer contour of the outdoor unit 2 and is formed by a bottom plate 41, a front panel 42, a side surface portion 43, a rear surface portion 44, and a top panel 45. The bottom plate 41 serves as the bottom portion of the casing 40, on which the compressor 11, the heat exchanger 13, and the air-sending device 14 are placed. The front panel 42 serves as the front portion and a side portion of the casing 40 and has an opening that faces the air-sending device 14. Also, the front panel 42 has a fan cover 42a with a lattice structure that covers the opening. The side surface portion 43 is formed by a combination of a plurality of parts and serves as a side portion of the casing 40. The rear surface portion 44 is also formed by a combination of a plurality of parts and serves as the rear portion of the casing 40. The parts that form the rear surface portion 44 have a plurality of vents through which air that flows to the heat exchanger 13 passes. The top panel 45 serves as the top portion of the casing 40.

[0017] Fig. 3 is a top view that illustrates the interior of the outdoor unit 2 according to Embodiment 1. In Fig. 3, the parts of the casing 40 other than the bottom plate 41 and the air-sending device 14 are not illustrated. As illustrated in Fig. 3, the L-shaped heat exchanger 13 is located along the rear portion and a side portion of the casing 40. The outdoor air temperature sensor 32 is provided at a position based on the following first to fifth points, which are taken into account, to accurately measure outdoor air temperature.

[0018] First, as a first point, the outdoor air temperature sensor 32 is attached such that it is not in close contact with the top panel 45 so that the measurement results are not affected by heat conduction from the top panel 45. For this purpose, the outdoor air temperature sensor 32 is attached to, for example, the rear surface portion 44 or the top panel 45 with a post (not illustrated) or a dedicated holder (not illustrated) in between. As a second point, the outdoor air temperature sensor 32 is provided directly under the top panel 45 so that snow and other substances do not adhere and outdoor air temperature remains measurable. In other words, the outdoor air temperature sensor 32 is covered by the top panel 45 when the outdoor unit 2 is viewed from above.

[0019] As a third point, the outdoor air temperature sensor 32 is located as far away as possible from the heat exchanger 13 so that the measurement results are not affected by radiant heat from the heat exchanger 13. As a fourth point, the outdoor air temperature sensor 32 is located as far away as possible from the installation surface of the outdoor unit 2 so that the measurement results are not affected by geothermal heat. And, as a fifth point, the outdoor air temperature sensor 32 is provided upwind of the heat exchanger 13 to ensure that the measurement results are not affected by heat-exchanged air; in other words, the sensor measures the temperature of air before heat exchange.

[0020] Fig. 4 is a diagram that illustrates the bottom plate 41 and a bottom plate heater 51 of the outdoor unit 2 according to Embodiment 1. Fig. 4 is a diagram that illustrates the outdoor unit 2 viewed from above as in Fig. 3. However, to describe the bottom plate 41 and the bottom plate heater 51, the illustration of components other than the bottom plate 41 and the bottom plate heater 51 is omitted. As illustrated in Fig. 4, in the bottom plate 41, a plurality of drain holes 41a are formed. The drain holes 41a are openings through which water on the bottom plate 41 is discharged out of the outdoor unit 2. Note that, although the plurality of drain holes 41a are installed, one may also suffice. The bottom plate 41 has a recess and the plurality of drain holes 41a are located in the recess. Also, each of the drain holes 41a is surrounded by a slope that decreases in height toward the opening. A drainage path is thereby formed in the bottom plate 41 such that water flows smoothly out of the outdoor unit 2.

[0021] Also, as illustrated in Fig. 4, the bottom plate heater 51 is provided to the bottom plate 41. When the bottom plate heater 51 is energized, it generates heat. The bottom plate heater 51 prevents frost that adheres to the heat exchanger 13, melts during a heating operation and a defrost operation, and falls onto the bottom plate 41 from refreezing. The bottom plate heater 51 is provided directly below the heat exchanger 13 and is located in an annular shape. Specifically, the area on the bottom plate 41 where the bottom plate heater 51 is attached includes the area directly below the heat exchanger 13. In other words, the bottom plate heater 51 is located along the projected area of the heat exchanger 13 onto the bottom plate 41. This structure is designed to raise the temperature of drainage water from the heat exchanger 13 before the drainage water comes into contact with the bottom plate 41. Also, the bottom plate heater 51 is provided on the drainage path in the bottom plate 41. Furthermore, the bottom plate heater 51 is provided around the drain holes 41a.

[0022] The outdoor unit 2 has the controller 21. Fig. 5 is a functional block diagram that illustrates the outdoor unit 2 according to Embodiment 1. The controller 21 switches the connection directions of the flow switching device 12 and thereby performs a cooling operation or a heating operation in accordance with instructions for an operation mode input by a user via a remote control (not illustrated) provided to the indoor unit 3. Also, the controller 21 controls the operating frequency of the compressor 11, the rotational speed of the fan motor 14a, and the open-close state of the expansion device 15 such that the temperature in a room where the indoor unit 3 is provided satisfies a set temperature input by a user via the remote control. And, as illustrated in Fig. 5, the controller 21 controls an operation of the bottom plate heater 51 according to measurement results of the refrigerant temperature sensor 31 and the outdoor air temperature sensor 32.

[0023] Conditions for performing a defrost operation are described below. The controller 21 switches the operation mode from a heating operation to a defrost operation when a defrost start condition is satisfied during the heating operation. The defrost start condition is, for example, where a temperature indicated by a measurement result of the refrigerant temperature sensor 31 becomes lower than or equal to a first defrost temperature. The first defrost temperature is a temperature at which frost may form on the heat exchanger 13 in a case where outdoor air temperature is low, and is, for example, 0 degrees Celsius. Also, when a defrost end condition is satisfied during a defrost operation, the controller 21 switches the operation mode from the defrost operation to a heating operation. The defrost end condition is, for example, where a temperature indicated by a measurement result of the refrigerant temperature sensor 31 exceeds a second defrost temperature. The second defrost temperature is a temperature at which defrosting is sufficiently expected to be completed when the temperature of refrigerant that has passed through the heat exchanger 13 is set as a measurement target, and is, for example, 0 degrees Celsius.

[0024] Control of an operation of the bottom plate heater 51 is briefly described below. The controller 21 controls the bottom plate heater 51 according to outdoor air temperature and outdoor air humidity during a heating operation and a defrost operation. In particular, in a case where the controller 21 determines that outdoor air is at low temperature and at low humidity and outdoor air temperature does not significantly vary after an heating operation is first started, the controller 21 does not operate the bottom plate heater 51 during subsequent heating operations and operates the bottom plate heater 51 only during a defrost operation. The controller 21 judges outdoor air temperature lower than a first control temperature as a low temperature. The first control temperature is a temperature at which water may freeze on the bottom plate, and is, for example, 0 degrees Celsius. In other words, if the bottom plate heater 51 is not turned on when outdoor air temperature is lower than the first control temperature, water may freeze on the bottom plate 41.

[0025] Note that the controller 21 calculates, as outdoor air temperature, the average of measurement results for a predetermined time period in a state where, after a heating operation starts, the air-sending device 14 operates and air flows around the outdoor air temperature sensor 32. The predetermined time period is, for example, two or three minutes. The controller 21 monitors outdoor air temperature during a heating operation and measures outdoor air temperature in the same method at each determination timing.

[0026] Also in particular, the controller 21 determines whether or not outdoor air humidity is low according to time required for a defrost operation. In the following description, time required for a defrost operation is referred to as defrosting time. When outdoor air humidity is low, very little frost forms on the heat exchanger 13. In a case where defrosting time is less than a reference time, the controller 21 judges that outdoor air humidity is low. The reference time is a time, as defrosting time, during which a small amount of frost accumulation on the heat exchanger 13 is sufficiently expected, and is, for example, three minutes. Note that the reference time is set differently depending on the size of the heat exchanger 13. As described above, in Embodiment 1, the amount of frost accumulation on the heat exchanger 13 varies depending on outdoor air humidity. As a result, it may be said that defrosting time indirectly indicates the amount of frost accumulation and whether or not outdoor air is at low humidity is therefore determined according to the defrosting time.

[0027] Furthermore, control of an operation of the bottom plate heater 51 is described below in detail. During a heating operation, the controller 21 always keeps the bottom plate heater 51 turned off if outdoor air temperature is higher than or equal to the first control temperature. On the other hand, during a heating operation, the controller 21 keeps the bottom plate heater 51 turned on if outdoor air temperature is lower than the first control temperature.

[0028] In a case where outdoor air temperature is lower than the first control temperature and the bottom plate heater 51 is turned on, the controller 21 continuously keeps the bottom plate heater 51 turned on if outdoor air temperature is higher than or equal to a second control temperature. The second control temperature is lower than the first control temperature, and is, for example, -7 degrees Celsius. This is because the humidity in air is particularly high in the temperature range of -7 degrees Celsius to 0 degrees Celsius. In other words, in this temperature range, there is a high possibility of frost formation on the heat exchanger 13. On the other hand, if outdoor air temperature is lower than the second control temperature, the humidity of air is reduced, and very little frost may form on the heat exchanger 13. Therefore, in this case, the bottom plate heater 51 may be turned off in combination with other conditions.

[0029] In a case where outdoor air temperature is higher than or equal to the second control temperature and the bottom plate heater 51 is kept on, the controller 21 further continuously keeps the bottom plate heater 51 turned on if defrosting time exceeds the reference time. This is because, in a case where defrosting time is long, it is expected that outdoor air is not at low humidity and that the amount of frost accumulation on the heat exchanger 13 is large. If the bottom plate heater 51 is not kept on when the amount of frost accumulation on the heat exchanger 13 is large, water that accumulates on the bottom plate 41 may freeze during a heating operation after a defrost operation.

[0030] On the other hand, if defrosting time is less than or equal to the reference time, the controller 21 turns off the bottom plate heater 51 after an operation continuation time has elapsed after the defrost operation ends. The operation continuation time is time until the rotational speed of the fan motor 14a exceeds a predetermined rotational speed after a heating operation is resumed. When a heating operation is resumed after a defrost operation, the rotational speed of the fan motor 14a of the outdoor unit 2 is gradually increased as the rotational speed of the compressor 11 is gradually increased. As the rotational speed of the fan motor 14a increases, the amount of heat dissipation from the surface of the bottom plate heater 51 also increases and the ability to melt ice on the bottom plate 41 thus decreases. The operation continuation time is determined in light of these circumstances and with the aim of maintaining energy-saving performance and is, for example, five minutes. On the other hand, in a case where the rotational speed of the fan motor 14a is low, the airflow speed around the bottom plate heater 51 is low. This phenomenon results in a small amount of heat transfer by convection currents around the bottom plate heater 51 and a small amount of heat dissipation from the surface of the bottom plate heater 51. Note that the reason for gradually increasing the rotational speed of the compressor 11 at the start of a heating operation is to prevent failure caused by uneven temperature inside the compressor 11 due to a rapid increase in speed.

[0031] Even in a case where the operation continuation time has elapsed after a defrost operation and the bottom plate heater 51 is turned off, the controller 21 turns on the bottom plate heater 51 again if the variation of outdoor air temperature from the start of a heating operation is higher than or equal to a third control temperature. This is because, even after a heating operation is initially started, outdoor air temperature may significantly vary due to a factor such as rainfall, snowfall, and the cessation of rainfall or snowfall. The third control temperature is, for example, 2 degrees Celsius.

[0032] On the other hand, in a case where the operation continuation time has elapsed after a defrost operation and the bottom plate heater 51 is turned off and the variation of outdoor air temperature from the start of a heating operation is lower than the third control temperature, the bottom plate heater 51 is not operated during subsequent heating operations and the bottom plate heater 51 is operated only during a defrost operation. Note that the variation of outdoor air temperature is the absolute value of a difference obtained by subtracting outdoor air temperature measured at a control point from outdoor air temperature measured at the start of a heating operation that is initially performed. Also, the variation of outdoor air temperature may also be calculated according to the most recent heating operation rather than according to a heating operation that is initially performed.

[0033] Here, an example of the hardware configuration of the controller 21 is described. Fig. 6 is a hardware configuration diagram that illustrates the controller 21 according to Embodiment 1. In a case where the various functions of the controller 21 are executed by hardware, the controller 21 is formed by a processing circuit 61, as illustrated in Fig. 6. The processing circuit 61 corresponds to, for example, a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC, an FPGA, or a combination of these circuits.

[0034] Also, another example of the hardware configuration of the controller 21 is described below. Fig. 7 is a hardware configuration diagram that illustrates the controller 21 according to Embodiment 1. In a case where each function of the controller 21 is executed by software, the controller 21 is formed by a processor 62, such as a CPU, and a memory 63, as illustrated in Fig. 7. Fig. 7 illustrates that the processor 62 and the memory 63 are communicatively connected to each other via a bus 64. In a case where each function is executed by software, the functions of the controller 21 are implemented by software, firmware, or a combination of software and firmware. The software and firmware are written as programs and stored in the memory 63. The processor 62 reads and executes the program stored in the memory 63 and thus implements the functions of the controller 21.

[0035] As the memory 63, for example, a non-volatile semiconductor memory such as a ROM, a flash memory, an EPROM, and EEPROM is used. A volatile semiconductor memory of a RAM may also be used as the memory 63. Furthermore, a removable recording medium such as a magnetic disk, a flexible disk, an optical disk, a CD, an MD, and a DVD may also be used as the memory 63.

[0036] Here, an operation of the controller 21 is described with reference to Fig. 8. Fig. 8 is a flowchart that illustrates an operation of the controller 21 according to Embodiment 1. Here, control of the bottom plate heater 51 in a state where the air-conditioning apparatus 1 performs a heating operation is mainly described. First, the controller 21 determines whether or not outdoor air temperature is higher than or equal to the first control temperature (step S1). In a case where outdoor air temperature is higher than or equal to the first control temperature (YES in step S1), the controller 21 keeps the bottom plate heater 51 turned off until the outdoor air temperature drops below the first control temperature.

[0037] In a case where outdoor air temperature is lower than the first control temperature (NO in step S1), the controller 21 turns on the bottom plate heater 51 (step S3). Then, the controller 21 determines whether or not outdoor air temperature is higher than or equal to the second control temperature (step S4). In a case where outdoor air temperature is higher than or equal to the second control temperature (YES in step S4), the controller 21 performs a defrosting process (step S5).

[0038] Here, the defrosting process is described with reference to Fig. 9. Fig. 9 is a flowchart that illustrates an operation of the controller 21 according to Embodiment 1. Fig. 9 is a flowchart that illustrates the defrosting process illustrated in Fig. 8. First, the controller 21 determines whether or not the defrost start condition is satisfied (step S11). In a case where the defrost start condition is not satisfied (NO in step S11), the controller 21 does not perform the defrost operation and keeps the bottom plate heater 51 turned on until determination changes in step S1, S4 or S11. Then, the controller 21 determines whether or not the bottom plate heater 51 is turned on (step S12). In a case where the bottom plate heater 51 is turned off (NO in step S12), the bottom plate heater 51 is turned on (step S13) and a defrost operation is started (step S14).

[0039] The controller 21 determines whether or not the defrost end condition is satisfied every predetermined time (step S15). In a case where the defrost end condition is not satisfied (NO in step S15), the controller 21 continues the defrost operation until the defrost end condition is satisfied. In a case where the defrost end condition is satisfied (YES in step S15), the controller 21 ends the defrost operation (step S16) and the defrosting process ends. With reference back to Fig. 8, when the defrosting process in step S5 ends, the controller 21 keeps the bottom plate heater 51 turned on until determination changes in step S1 or S4 described above.

[0040] Also in a case where outdoor air temperature is lower than the second control temperature, the controller 21 performs the defrosting process (step S6). The defrosting process in step S6 itself is the same as the defrosting process described in step S5 and its description is thus omitted. After the defrosting process in step S6 ends, in a case where defrosting time exceeds the reference time (YES in step S7), the controller 21 judges that outdoor air is not at low humidity and keeps the bottom plate heater 51 turned on until determination changes in step S1, S4 or S7 described above.

[0041] In a case where defrosting time is less than or equal to the reference time (NO in step S7), the controller 21 judges that outdoor air is at low humidity and turns off the bottom plate heater 51 after the operation continuation time (step S8). The controller 21 then determines whether or not an outdoor air temperature variation is higher than or equal to the third control temperature (step S9). In a case where an outdoor air temperature variation is higher than or equal to the third control temperature (YES in step S9), the process from step S1 is repeated. In a case where an outdoor air temperature variation is lower than the third control temperature (NO in step S9), the controller 21 repeats the process from step S4 again. At this time, the controller 21 does not operate the bottom plate heater 51 during a heating operation and keeps the bottom plate heater 51 turned on only during a defrost operation until determination changes in step S1, S4, S7 or S9 described above.

[0042] As described above, the outdoor unit 2 of the air-conditioning apparatus 1 of Embodiment 1 controls the bottom plate heater 51 according to outdoor air temperature and outdoor air humidity during a heating operation and a defrost operation. In an environment where outdoor air is at low temperature and at low humidity, little frost forms on the heat exchanger 16 during a heating operation and, consequently, very little water drains from the heat exchanger 16 to the bottom plate 41. Therefore, the outdoor unit 2 of the air-conditioning apparatus 1 of the present disclosure limits energization time to the heater provided to the bottom plate 41 and thereby reduces electric power consumption.

[0043] Specifically, the controller 21 of Embodiment 1 operates the bottom plate heater 51 during a defrost operation, operates the bottom plate heater 51 during a heating operation in a case where the controller 21 determines that outdoor air is at low temperature, and stops the bottom plate heater 51 during a heating operation in a case where the controller 21 determines that outdoor air is at low temperature and at low humidity. As described above, the outdoor unit 2 of the air-conditioning apparatus 1 switches operation states of the bottom plate heater 51 and limits energization time during a heating operation and thereby reduces electric power consumption.

[0044] Furthermore, the controller 21 of Embodiment 1 operates the bottom plate heater 51 during a heating operation in a case where the controller 21 determines that outdoor air is lower than the first control temperature and higher than or equal to the second control temperature and stops the bottom plate heater 51 during a heating operation in a case where the controller 21 determines that outdoor air is lower than the second control temperature and is at low humidity. As described above, in a case where a large amount of frost accumulation is expected, the bottom plate heater 51 is operated during a heating operation so that water is prevented from freezing on the bottom plate 41.

[0045] Also, in Embodiment 1, whether or not outdoor air is at low humidity is determined solely by defrosting time, that is, the amount of frost accumulation on the heat exchanger 13 and a humidity sensor is thus not required. Therefore, compared to a case where a humidity sensor is used, the condition of the humidity of outdoor air is judged at a lower cost.

[0046] An embodiment of the present disclosure is described above. The present disclosure is not limited to the configuration of the embodiment described above and various modifications or combinations are possible within the scope of its technical concept. For example, the defrost start condition may also be that a predetermined time has elapsed from the start of the heating operation. Similarly, the defrost end condition may also be that a predetermined amount of time has elapsed from the start of the defrost operation.

[0047] Also, in Embodiment 1, a control method of the bottom plate heater 51 is described above with reference to the flowcharts illustrated in Fig. 8 and Fig. 9. However, the control method of the bottom plate heater 51 is not limited to any particular method. For example, any one or a plurality of processes in steps S4 and S5, S8, or S9 may also be omitted. Even in this case, in a case where the controller 21 at least determines that outdoor air is at low temperature and at low humidity according to a measurement result from the outdoor air temperature sensor 32 and time required for a defrost operation, the controller 21 does not operate the bottom plate heater 51 during a heating operation and operates the bottom plate heater 51 only during a defrost operation. In other words, the outdoor unit 2 of the air-conditioning apparatus 1 of Embodiment 1 does not operate the bottom plate heater 51 in a case where a small amount of frost accumulation on the heat exchanger 13 is determined. Therefore, energization time to the heater provided to the bottom plate 41 is limited, and electric power consumption is reduced.

[0048] Also, the operation continuation time described in step S9 illustrated in Fig. 8 may also be an arbitrary time set by a user. In this case, the user sets the operation continuation time according to factors such as the model of the air-conditioning apparatus 1 and the environment in which the air-conditioning apparatus 1 is installed. Note that the operation continuation time may also be set before the defrost operation is performed and may also be set after the defrost operation ends by visually observing the state of drainage from the heat exchanger 13.

[0049] Also, the operation continuation time may also be determined by providing a drainage sensor to the bottom plate 41. The drainage sensor is, for example, attached at the lowest position of the bottom plate 41 and detects whether or not the drainage sensor itself is immersed in water. The controller 21 communicates with the drainage sensor periodically after a defrost operation. When the controller 21 receives a signal from the drainage sensor that indicates that the drainage sensor is immersed in water and later receives a signal that indicates that the drainage sensor is no longer immersed in water, the controller 21 judges that drainage is complete. The controller 21 may also use time from the end of a defrost operation until drainage is completed as the operation continuation time.

[0050] Also, a humidity sensor may also be provided to the outdoor unit 2, and, according to a measurement result of the humidity sensor, determination may also be made on whether outdoor air is at low humidity. The location of the humidity sensor is determined, for example, according to the same points as those described for the outdoor air temperature sensor. The controller 21 determines that outdoor air is at low humidity in a case where the humidity of outdoor air measured by the humidity sensor is lower than or equal to a threshold value. The threshold value here is, for example, a degree at which a small amount of frost accumulation on the heat exchanger 13 is sufficiently expected, such as 20%. Note that it is also possible to determine whether or not outdoor air is at low humidity by taking into consideration both measurement results of the humidity sensor and defrosting time. In this case, it is possible to more accurately judge that outdoor air is at low humidity. However, judgment based on defrosting time may be omitted and it may also be possible to determine whether or not outdoor air is at low humidity according to a measurement result of the humidity sensor alone.Reference Signs List

[0051] 1: air-conditioning apparatus, 2: outdoor unit, 3: indoor unit, 11: compressor, 12: flow switching device, 13: heat exchanger, 14: air-sending device, 14a: fan motor, 15: expansion device, 16: heat exchanger, 21: controller, 31: refrigerant temperature sensor, 32: outdoor air temperature sensor, 40: casing, 41: bottom plate, 41a: drain hole, 42: front panel, 42a: fan cover, 43: side surface portion, 44: rear surface portion, 45: top panel, 51: bottom plate heater, 61: processing circuit, 62: processor, 63: memory, 64: bus

Claims

1. An outdoor unit of an air-conditioning apparatus comprising: a compressor configured to compress refrigerant; a heat exchanger through which refrigerant sent out from the compressor flows; a flow switching device configured to switch flow directions of refrigerant; a casing that houses the compressor, the heat exchanger, and the flow switching device; a bottom plate heater provided to a bottom plate of the casing and configured to heat water that falls from the heat exchanger; and a controller configured to control the compressor and the flow switching device and to perform a heating operation and a defrost operation in which frost that attaches to the heat exchanger during a heating operation is removed, the controller being configured to control the bottom plate heater according to outdoor air temperature and outdoor air humidity during the heating operation and the defrost operation.

2. The outdoor unit of an air-conditioning apparatus of claim 1, wherein, in a case where the controller determines that outdoor air is at low temperature and at low humidity, the controller is configured not to operate the bottom plate heater during the heating operation and to operate the bottom plate heater only during the defrost operation.

3. The outdoor unit of an air-conditioning apparatus of claim 1 or 2, wherein the controller is configured to operate the bottom plate heater during the defrost operation, to operate the bottom plate heater during the heating operation in a case where the controller determines that outdoor air is at low temperature, and to stop the bottom plate heater during the heating operation in a case where the controller determines that the outdoor air is at the low temperature and at low humidity.

4. The outdoor unit of an air-conditioning apparatus of claim 1, wherein, in a case where the controller determines that outdoor air is at low temperature and at low humidity and a temperature difference in outdoor air after the heating operation starts is lower than a preset threshold value, the controller is configured not to operate the bottom plate heater during the heating operation and to operate the bottom plate heater only during the defrost operation.

5. The outdoor unit of an air-conditioning apparatus of claim 1, wherein the controller is configured to operate the bottom plate heater during the defrost operation, to operate the bottom plate heater during the heating operation in a case where the controller determines that outdoor air is lower than a first control temperature and higher than or equal to a second control temperature, which is lower than the first temperature, and to stop the bottom plate heater during the heating operation in a case where the controller determines that the outdoor air is lower than the second control temperature and at low humidity.

6. The outdoor unit of an air-conditioning apparatus of any one of claims 1 to 4, wherein, in a case where outdoor air is lower than a first control temperature, the controller is configured to determine that the outdoor air temperature is low.

7. The outdoor unit of an air-conditioning apparatus of any one of claims 1 to 6, wherein, in a case where time required for the defrost operation is shorter than a reference time, the controller is configured to determine that the outdoor air humidity is low.

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