SE1751286A1 - Defrosting cycle control - Google Patents

Abstract

The invention relates to An Air Handling Unit (AHU) 1 for a ventilation system in a building and a method for performing a defrosting operation on an AHU. The AHU 1 having a supply air channel 2 with an inlet 2a and an outlet 2b and an extract air channel 3 with an inlet 3a and an outlet 3b for guiding air to and from a building. The supply air channel 2 comprises a damper 4a and the extract air channel 3 comprises a damper 4b in order to control the air flow in the channels 3, 4. The supply air channel 2 and the extract air channel 3 are in a heat exchanging relation to each other via a heat recovery arrangement, e.g. a heat exchanger 5 and/or a heat pump 6. The AHU also comprises at least one fan 11a, 11b and an Electronic Control Unit, (ECU) 9. The ECU 9 is programmed to output a defrost cycle initiation signal in order to defrost the heat recovery arrangement, e.g. by to changing or heat the air flow in the AHU 1, according to a first predefined criteria. The ECU is further programmed to output a defrost cycle termination signal in order to end the defrost cycle when there is an indication of the defrosting being completed according to a second criteria.In order to optimize the defrost cycle time period the ECU 9 is programmed to change the first predefined criteria for output a defrost cycle initiation signal or the second criteria for output of a defrost cycle termination signal depending on the time for performing the defrost cycle such that the time period of the defrost cycle is prolonged respectively shortened when the defrost cycle time period is shorter respectively longer than a preferred defrost cycle time interval.

Description

TECHNICAL FIELD The present invention relates to an Air Handling Unit (AHU) for providing fresh air to a building. The invention is in particular directed to the feature of defrosting, or deicing, a heat recovery arrangement exchanging heat between the extract air flow and supply air flow in the AHU. The invention also relates to method for defrosting of an AHU.

BACKGROUND ln offices and other larger premises, there is often a need to be able to control the indoor climate separately in different parts of the premises or in individual rooms. An accurate local temperature and ventilation control is required to ensure a sufficient comfort level for the persons present in a building. Also heat generated by electric appliances and heat and exhaled air generated by persons inside the building have to be taken into account for the control. Together with the prevailing weather conditions, these factors have a large influence on the instantaneous demand for heating, cooling and ventilation capacity. Similar systems has lately been more commonly used also in smaller buildings, e.g. family houses, as there is a desire for better insulation and thus also a need for a forced change of air in these buildings. ln order to improve the energy efficiency of buildings using air conditioning systems with controlled air intake/air outlet is there a desire to lower heat losses by exhausting warm air to the outside and reduce heat losses by heat exchanging exhaust air with fresh air.

Different systems for controlling the indoor climate in buildings are known previously. For instance, US2004/0148950 A1 discloses an air-conditioning system for a building that comprises a hot water circuit, a cold water circuit and several local air conditioning units. Each of the local air conditioning units comprises a fan for blowing air into a space in the building, a heating coil connected to the hot water circuit and/or a cooling coil connected to the cold water circuit. At least one temperature control system allows control of the heating power of the heating coils and the cooling power of the cooling coils. A calorific energy management system is provided with a heat pump for transferring calorific energy from the cold water system to the hot water system, from the cold water system to the outdoor air and from the outdoor air to the hot water system. The system is capable of managing the calorific energy transfers by means of a three level control system so as to optimize the energy consumption. ln systems for controlling the indoor climate in buildings, it is common that at least one heat exchanger arranged in connection with the exhaust air duct forms part of a heat recovery arrangement. The heat exchanger is used for reducing the total energy consumption of the system by recovering a portion of the heat energy from the exhaust air before it is discharged. ln order to further improve the heat recovery system it may also be provided with an additional heat pump which also transfers heat from the exhaust air duct to the supply air duct. When used for heat recovery, the heat pump part in the exhaust air duct function as an evaporator in order to absorb heat and transfer heat via a heat carrying media to a condenser in the supply air channel where heat is released. This will increase the total heat flow from the exhaust air duct to the supply air duct and thus also increase the cooling effect in the exhaust air duct in certain conditions, frost or ice may form on the heat exchanger and/or the evaporator of the heat pump. The heat recovery system should then be controlled to perform a defrost cycle, during which the heat exchanger or evaporator is heated to melt the ice. The heat recovery system could of course only comprise a heat pump and no heat exchanger.

One way of performing defrosting (or deicing) of the heat exchanger or evaporator is to use the hot airflow leaving the building for heating and defrosting. ln this case is the cold flow usually restricted or completely shut off in the part of the heat exchanger which shall be defrosted. Such a system is for example described in EP 2 546 581 or WO 01/22021. However, there is always a loss in the energy efficiency during such a defrost cycle since the heat exchanger cannot be fully used for its normal purpose, that is to say heat recovery, which results in an undesirably high energy consumption. Hence, heat from the building to the outside will be lost to a higher degree during the defrost operation. ln WO 2016 /060 609 there is a method described which is particularly oriented to control the duration of defrost cycle for a heat exchanger having several heat exchanger portions in order to optimize the defrost operation. However, even though the system in WO 2016/060 609 may work perfectly well in the aspect described therein, there are still further measures which may be done to improve the defrosting operation generally used for a an Air Handling Unit (AHU) or Heating, Ventilation and Air Conditioning (HVAC) system comprising a heat exchanger or heat pump which may be subjected to ice formation.

DISCLOSURE OF THE INVENTION The object of the present invention is to provide an improved energy recovery and more efficient defrosting for an Air Handling Unit (AHU) to be used in a ventilation system in a building.

The AHU according to the invention comprises an extract air channel for guiding air from the house or building to the outside, i.e. a channel for exhausting air from a building to the outside. The extract air channel have at least one extract air inlet and in the general case is there only one extract air inlet which in turn may be connected to a main channel which in turn may be connected to a multitude of channels for extracting air from different parts of a building. However, the AHU may have more than one extract air inlet if desired. The extract air channel further comprises at least one extract air outlet for exhausting the flow of extract air from a building to the outside. lf desired, there could of course be further extract air outlets in the system, or connecting the extract air outlet to some kind of exhaust manifold.

The AHU further comprises a supply air channel for guiding supply air, or fresh air, from outdoor into a building or structure. The supply air channel is provided with at least one supply air inlet for intake of fresh air and at least one supply air outlet for delivering fresh air into a house or building, usually via a supply air channel system for distribution of the air in the building. There may of course be several supply air inlets or outlets as described above for the extract air channel. ln order to control the air flow in the AHU are there one or several supply air dampers and one or several extract air dampers located in the supply air channel respectively the extract air channel in order to control the air flow in the respective channels. The dampers may be located at the inlets and/or outlets to the main body of the AHU but may also be located at a distance from the air inlets or outlets in the AHU main body connected via a ducting arrangement.

The AHU is also provided with a heat recovery arrangement such that the heat in the exhausted extract air may be regained before it is discharged from the building. The heat recovery arrangement may for example be a heat exchanger and/or a heat pump. Such a heat exchanger can for example be a cross-flow or counter-flow heat exchanger (also called plate heat exchangers) or a heat exchanger wheel. Hence, the extract air channel and the supply air channel are designed to be in a heat exchanging relation to each other by the use of a heat recovery arrangement. lf a heat pump is used, it includes a first media phase change unit located in the supply air channel and a second media phase change unit located in the extract air channel in order to transfer heat between the supply air channel and the extract air channel. By media phase change unit is meant an evaporator, condenser or a unit being able to be used both as evaporator and condenser. ln case both units may be used as evaporator and condenser may the heat pump be designed to be able to operate in both normal mode and reversed mode, i.e. the heat pump may be used to transfer heat in either direction between the supply air channel and the extract air channel. This is particularly useful when there for example is a desire to reverse the heat flow in the heat pump from being in a heat recovery mode during normal operation in cold weather when there is a desire to use the heat pump to regain heat from the extract air stream to be in a reversed mode in case of an ice build up in the AHU. This may for example occur on the media phase change unit operating as evaporator in the extract air. By reversing the heat flow in the heat pump will the icy phase change unit start to work as a condenser and the heat pump will contribute to the defrosting of the icy surfaces of the media phase change unit.

The AHU further includes at least one fan in the supply air channel and/or the extract air channel in order to induce a flow of air in the AHU. To be noted, the fan and or the dampers described before must not be comprised in the same casing as the heat recovery arrangement in order to form part of the AHU. However, in general there is a fan located in the exhaust air channel as well as in the supply air channel within the same casing as the heat recovery arrangement.

The AHU also includes an Electronic Control Unit (ECU) in order to control the operation of the AHU, e.g. the heat recovery process and to perform defrost operations when needed. The ECU may be connected to various sensors and control devices and programmed to control the air treatment system in dependence of input from various sensors by providing output control signals to various control devices. The ECU may be a single entity or may comprise several entities so as to form an ECU, e.g. may sensor signals be processed by some kind of processor comprised in the sensor which thus forms part of the ECU.

The ECU is programmed to output a defrost cycle initiation signal during certain conditions. The defrost cycle initiation signal includes a control signal to a flow controller, such as an air damper or a fan, or a heating element in order to change or heat the air flow to operate in a defrost mode to defrost the heat recovery arrangement. The defrost cycle initiation signal will be outputted when there is an indication of an undesired high level of ice in a portion of the heat recovery arrangement. The signal will be sent according to a first predefined criterion, e.g. a pressure drop over some part thus indicating a more restricted flow path due to ice formation. The ECU is further programmed to send a defrost cycle termination signal in order to end the defrost cycle when there is an indication of the defrosting being completed according to a second criterion, e.g. that the pressure drop has decreased below a certain threshold.

There may also be other indications of an undesired high level of ice in the heat exchanger or heat pump, e.g. could the ice buildup in the heat exchanger be estimated from temperature measurements and/or humidity levels in the air. Concerning the problem related to frosting or ice buildup in the heat exchanger is this problem in general originating from exhausting hot and humid indoor air being cooled down to temperatures below zero and the condensing and freezing of moist in the exhaust air. Hence, if the system comprises means for estimating the humidity in the extract air leaving the building it may be calculated at what temperature the humidity in the air will condense and ice formation start. The absolute humidity in the extract air may for example be calculated from measuring the relative humidity and temperature of the extract air, e.g. in the extract air inlet, or by measuring the absolute humidity directly by any known means. When the absolute humidity is known, it may easily be derived at what temperature the moisture in the air will condense and there is a risk for ice formation in the heat exchanger. As is obvious, the temperature must be below zero degrees Celsius in order to have ice formation. Hence, the AHU may be provided with means for estimating the temperature in the heat exchanger portions. There are several relevant places where a thermometer may be located, e.g. outdoor, in the supply air inlet or in the extract air outlet downstream the heat recovery arrangement. The specific temperature measured which shall correspond to a risk for frosting of the heat exchanger may thus be dependent on where the temperature is measured. The outdoor temperature may be measured or a more accurate temperature measurement may for example be to measure the temperature by a thermometer in the coldest part of the heat exchanger and use the measurements therefrom as a more correct way for indicating frost risks. The AHU may thus be provided with means for estimating or measuring these parameters. lt may be possible to only use other parameters than pressure drop for deciding when to start a defrost operation. Regardless of which method that is used for estimating when to perform a defrost cycle, the level of ice bui|dup when it is desired to initiate a defrost operation may vary from case to case. Hence, the method may work for different ways of deciding when there is an undesired high level of ice bui|dup in the heat exchanger or heat pump setting different levels of ice bui|dup as acceptable limits before a signal is sent for initiating a defrost operation. However, since there are pressure sensors or means for estimating the pressure drop over the heat exchanger or heat pump present in the AHU anyway in many cases, is it in general suitable to use a limit of a measured or estimated pressure drop over a heat exchanger or a heat pump for triggering a defrost initiation signal.

Regardless of which method which is used for setting the defrost ignition and termination levels, there is a desire to control the defrost cycles efficiently. Even though defrosting or deicing operation of the AHU may be efficient is there generally a loss in the heat recovery efficiency, alternatively a reduced flow of supply air and/or increased energy consumption by the AHU from heating devices, during a defrost cycle. Hence, in whatever manner a defrost operation is performed, there is a desire to minimize the time when an AHU is operating in defrost a mode due to reduced energy efficiency, in particular there is a desire to end the defrost mode directly when the heat pump or heat exchanger is defrosted. However, it is also desired to assure that the selected subject to be defrosted will be completely defrosted in order to avoid frequent repetitions of defrosting cycles, which also influence the overall heat recovery efficiency in a negative way. ln order to provide energy efficient defrost operation could it be of importance to have defrost cycles being performed within certain time limits. Hence, the ECU could be programmed to change the first predefined criterion for outputting a defrost cycle initiation signal or the second criterion for outputting a defrost cycle termination signal depending on the time duration for performing the defrost cycle in order to adjust the next defrost cycle to more likely be performed within a prescribed time period.

Hence, the changed initiation signal or termination signal is amended such that the time period of the defrost cycle should have been increased if the changed first or changed second criterion had been used for the defrost cycle initiation signal respectively the defrost cycle termination signal when the defrost cycle is shorter than a first predefined time period. This would for example be the case if the initiation signal is based on a pressure drop value and the defrost cycle is shorter than desired why the initiation signal is changed to be outputted at a higher pressure drop. Likewise, the changed initiation signal or termination signal is amended such that the time period of the defrost cycle should have been decreased if the changed first or changed second criterion had been used for the defrost cycle initiation signal respectively the defrost cycle termination signal when the defrost cycle is longer than a second predefined time period. This would for example be the case if the initiation signal is based on a pressure drop value and the defrost cycle is longer than desired why the initiation signal is changed to be outputted at a lower pressure drop. lt is thus possible to shift either the defrost initiation signal and/or defrost termination signal for the defrost cycle. However, in general is the defrost termination signal set such that the defrost cycle will end when there has been an optimized defrosting operation of the AHU why it in most cases will be the first predefined criteria for the defrost cycle initiation signal which is changed in order to perform a defrost cycle within a desired time interval. ln general, there is a desire to control the defrosting operation such that it terminates as soon as the heat exchanger portion is defrosted but not before the defrosting is complete. ln order to optimize the overall efficiency of the AHU it may comprise an ECU which adapts its predefined settings for when to initiate a defrost cycle. The defrost cycle is in this case thought to have a desired target time or time interval. This means that the predefined limit of the criteria, e.g. a pressure drop, is increased respectively decreased when the defrost cycle is shorter than a first predefined time period respectively longer than a second predefined time period. ln this way, the ECU is adapting the starting criteria of a defrost cycle in order to adjust the duration of a defrost cycle to be within a desired time interval or targeting a desired time of a cycle. For example, if the defrost cycle is 20 percent longer than what is desired, the pressure drop limit will be lowered, e.g. by 20 percent. ln the next cycle, the cycle time is 5 percent shorter than what is desired and the next pressure drop limit will be set to 5 percent higher than the last pressure drop limit. This example is assuming, or using, a linear dependence between the defrost cycle time and the pressure drop value. This is most probably not correct but will most probably work good enough in order to slowly get closer to a desired value. The control system could also comprise some kind of learning function, e.g. to record relevant temperatures, humidity and supply air demand, in order to create a directory storing relevant pressure drop values depending on different conditions. Alternatively, or in addition to the above directory, there may be pressure drop curves versus time stored or made such that a relevant pressure drop value may be read from a curve suitable for the present conditions. ln order to use a pressure drop for deciding when to perform a defrost operation could the AHU be provided with a pressure indicating system in order measure the pressure drops in the AHU and verify if the first and second criteria are fulfilled.

The pressure indicating system could be designed to measure pressure drops in the extract air channel, e.g. by measuring the pressure in the extract air channel upstream respectively downstream of the heat recovery arrangement, e.g. a heat exchanger and/or a heat pump. lf a heat pump is used, it comprises a media phase change unit in the extract air channel as well as in the supply air channel. ln the system disclosed, the portion or part in the AHU which is most likely to be subject to ice formation is most probably the media phase change unit in the extract air channel when being used as evaporator, i.e. absorbing heat from the extract air. Concerning the problem related to frosting or ice buildup in the heat exchanger is this problem often originating from the hot indoor air, which often is rather humid, being cooled down to temperatures below zero and the condensing and freezing of moist in the air. lce on the heat exchanger or heat pump will decrease the heat exchange efficiency in the heat exchanger or heat pump, thus reducing the heat recovery, and also increase the pressure drop. Since the pressure drop over the heat exchanger or heat pump increases with ice growth is it possible to detect an icing condition in the heat exchanger or heat pump by measuring the pressure drop over the heat exchanger. Hence, the extract air channel may be provided with pressure indicating means or pressure sensors for estimating or measuring a pressure drop over the heat exchanger or heat pump in the extract air channel. The measurements may be made by measuring the absolute pressure in the extract air channel upstream respectively downstream of the heat exchanger or heat pump in order to calculate a pressure drop over the heat pump. Alternatively, the pressure drop may be measured directly by using a differential pressure sensor connected to a space in the extract air channel upstream the heat exchanger or heat pump via a first pressure communicating conduit and connected to a space in the extract air channel downstream the heat exchanger or heat pump via a second pressure communicating conduit such that a differential pressure over the heat exchanger or heat pump is measured by the differential pressure sensor.

The ECU could be connected to the pressure indicating means or pressure sensors in order to receive input signals from these sensors. The ECU is preferably also connected to flow controllers, e.g. the supply air damper and the extract air damper, in order to send control signals to these flow controllers. The ECU may also be programmed to output a defrost cycle initiation signal when there is an indication of an undesired high level of ice in the heat exchanger or heat pump system, e.g. from sensing an increased pressure drop somewhere in the AHU being above a defined limit. The ECU may be programmed to switch the function of the AHU from a normal heat recovery mode to a defrost mode and start a defrost cycle when the pressure indicating system estimates a pressure drop over the second media phase change unit above a first predefined limit. ln case a pressure drop limit is used by the ECU for initiating a defrost cycle is it suitable to program the ECU to switch off the defrost mode and end the defrost cycle when the pressure indicating system estimate a pressure drop below a second predefined limit. Using a heat pump arrangement, the ECU may be programmed to set said media phase change unit in the supply air channel to function as an condenser in order to release heat to the fresh intake air and the media phase change unit in the extract air channel to function as an evaporator absorbing heat from the exhaust air channel when the AHU is working in its normal heat recovery mode. When a defrost operation is desired could the ECU be programmed to set said the media phase change unit in the supply air channel to function as an evaporator in order to absorb heat from its surroundings and the media phase change unit in the extract air to function as a condenser in order to release heat to its surroundings when the AHU is working in its defrost mode. Hence, a reversible heat pump may be useful both for heat recovery and defrost operation. The defrost cycle initiation signal thus implies a possibility for various control signals to be sent from the ECU. The ECU could for example be programmed to send a control signal to flow controllers, e.g. one or several dampers, and change the flow channel configuration from admitting a large supply air flow when the AHU is in the heat recovery mode to reduce or restrict the supply air intake after a defrost initiation signal setting the AHU to be in a defrost mode. The defrost initiation signal preferably also changes the settings or positions of other dampers and other flow controllers. By restricting the flow of supply air by the supply air damper will cooling effect be reduced and accumulated ice start to defrost by the warm flow of exhaust air from the building in the extract air and supply air channels. ln addition, fans and possible by- pass dampers or recirculation dampers may be controlled.

The AHU may be further designed to include a short cut connection connecting the supply air channel with the extract air channel. The short cut connection could be designed such that it is connecting the supply air channel upstream of the heat recovery arrangement with the extract air channel downstream of the heat recovery arrangement. This means that if the short cut connection is opened may the extract air first flow through the heat recovery arrangement in the extract air channel where after the air may flow through the short cut connection and into the supply air channel and through the heat recovery arrangement before being recirculated into a building. The defrost initiation signal preferably changes the settings or positions of the short cut damper as well as other dampers. The ECU may be programmed to set the short cut damper in the short cut connection to be essentially closed when the AHU is operating in a heat recovery mode. When changing to a defrost mode, the short cut damper is changed to be essentially open. ln addition to change the position of the short cut damper when switching to a defrost mode could the ECU be programmed to switch the extract air damper and supply air damper from being set to essentially open when the AHU is in a heat recovery mode to set the extract air damper and supply air damper to be essentially closed when the AHU is working in a defrost mode. Alternatively, the ECU could be programmed to set the extract air damper and supply air damper to be essentially open when the AHU is in a heat recovery mode and to switch the extract air damper to be essentially closed while the supply air damper is maintained in an open position, partially or fully open, when the AHU is working in a defrost mode after defrost initiation signal.

By restricting the flow of supply air by the supply air damper will cooling effect be reduced and accumulated ice start to defrost by the warm flow of exhaust air from the building in the extract air and supply air channels. Hence, the flow has been controlled in order to defrost without the need for specific heating arrangement even though it may be possible to incorporate additional heating devices. 11 The invention also relates to a method for defrosting of an Air Handling Unit, AHU, for a ventilation system in a building, e.g. such an AHU as disclosed above. The AHU comprises a supply air channel for guiding supply air from the outdoor into a building and an extract air channel for guiding extract air from a building to the outside. The supply air channel and the extract air channel are in a heat exchanging relation to each other via a heat recovery arrangement, e.g. a heat exchanger and/or a heat pump. The method comprises the steps of: - outputting a defrost cycle initiation signal including a control signal to a flow controller, such as an air damper or a fan, or a heating element in order to change or heat the air flow to defrost said heat recovery arrangement when there is an indication of an undesired high level of ice in a portion of said heat recovery arrangement according to a first predefined criteria - Output a defrost cycle termination signal in order to end the defrost cycle when there is an indication of the defrosting being completed according to a second criteria. The method further comprises an adaptation for of the defrost cycle time period by including a step in which - the first predefined criteria for output a defrost cycle initiation signal or the second criteria for output of a defrost cycle termination signal depending on the time for performing the defrost cycle such that - the time period of the defrost cycle should have been increased if the changed first criterion or changed second criterion had been used for the defrost cycle initiation signal respectively the defrost cycle termination signal when the defrost cycle is shorter than a first predefined time period or - the time period of the defrost cycle should have been decreased if the changed first criterion or changed second criterion had been used for the defrost cycle initiation signal respectively the defrost cycle termination signal when the defrost cycle is longer than a second predefined time period. ln general, the time period for the defrost cycle corresponds to the amount of ice formation on the device to be defrosted. ln case there is need of a too long defrost cycle, it could indicate the formation of ice is so quick that the heat recovery on the AHU not is able to recover heat efficiently due to the ice formation. lf the criteria for initiating the defrost cycle is changed such that the defrost cycle is shorter will it most 12 certainly also imply a shorter heat recovery cycle thus reducing the time during ice formation will occur and thus stop the heat recovery cycle before too much ice is built up and the efficiency of the heat recovery mode is seriously affected. On the contrary, if the time for the defrost cycle is too short is it possible that the defrost cycle is starting before the ice formation contributes with any significant negative impact on the heat recovery thus causing the defrost cycle to start too early to be efficient for the overall energy efficiency. Hence, by changing the criteria for initiating and/or terminating the defrost cycle such that it will longer will most certainly allow more ice formation, but still without any serious impact on the heat transfer function, and also allow the AHU to be in its energy efficient heat recovery mode for a longer time.

According to one aspect of the method is the first predefined criteria for output a defrost cycle initiation signal a first, higher pressure drop limit and the second criteria for output of a defrost cycle terminating signal is a second, lower pressure drop limit. Hence, the pressure drop for initiating a defrost cycle is higher than the pressure drop for a terminating a defrost cycle. ln this case will these limits be changed in order to adapt the defrost cycle period to be within a desired time interval if the time period is too long or too short. ln case the defrost time period is too short may the limits be changed such that: - the first, higher pressure drop limit is increased and/or the second, lower pressure drop limit is decreased when the defrost cycle is shorter than a first predefined time period. For example, the threshold for starting the defrost cycle will be increased and thus more ice will be allowed to form before the defrost cycle is started and the defrost cycle time period will thus be longer since more ice needs to be melted. Alternatively, if the defrost time period is too long may the limits be changed such that: - the first, higher pressure drop limit is decreased and/or the second, lower pressure drop limit is increased when the defrost cycle is longer than a second predefined time period. For example, the threshold for starting the defrost cycle will be decreased and thus less ice will be allowed to form before the defrost cycle is started. This means that the defrost cycle may start before it is needed to perform a defrost operation for an efficient operation of the AHU there will be an excessive amount of defrost cycles performed over time thus decreasing the energy efficiency of the AHU. 13 BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described more in detail with reference to the appended drawings, where: Figure 1 discloses an Air Handling Unit (AHU) in heat recovery mode Figure 2 discloses the AHU from figure 1 being in defrost mode Figure 3 discloses the AHU from figure 1 being in an alternative defrost mode DETAILED DESCRIPTION ln figures 1-3 is disclosed an Air Handling Unit (AHU) 1 according to an embodiment of the invention. The AHU 1 comprises a supply air channel 2 having a supply air inlet 2a and a supply air outlet 2b for delivering fresh air to an air ventilation system in a building. The AHU further comprises an extract air channel 3 having an extract air inlet 3a receiving extract air, also commonly called exhaust air or indoor air, and guiding the extract air to the outside through an extract air outlet 3b. Hence, the left side of the AHU in the figures is intended to be connected to the surroundings or outdoor while the right side of the figures is intended to be connected to an air ventilation system in a building for distribution of fresh air to the building and extraction of exhaust air from the interior of the building. ln order to control the air flow in the supply air channel 2 is the AHU 1 provided with a supply air damper 4b in the supply air channel 2 close to the supply air outlet 2b. ln a similar way is the extract air channel 3 provided with an extract air damper 4b close to the extract air outlet 3b in order to control the flow of exhaust air in the extract air channel 3. The AHU is further provided with a heat exchanger 5, which for example may be a heat exchanger wheel, which is located to be in contact with and exchange heat between the flows of air in the supply air channel 2 and extract air channel 3. The AHU 1 also comprises a heat pump system 6 having a first media phase change unit 6a located in the supply air channel 2 and a second media phase change unit 6b located in the extract air channel 3. The heat pump system 6 transfers heat between the flow of air in the supply air channel 2 and extract air channel 3. Heat is transferred by using a heat transfer media which may evaporate and condense in the first and second media phase change units 6a, 6b. Hence, the media phase change units 6a, 6b may shift between working as evaporator and condenser such that the heat pump system 6 function as a reversible heat pump and may be controlled to transfer heat in either 14 direction, i.e. transfer heat to the air flow in the supply air channel 2 from the air flow in the extract air channel 3 or vice versa. As can be seen in the figures, the first and second media phase change units 6a, 6b are located downstream of the heat exchanger 5 in the supply air channel 2 as well as the extract air channel 3.

The AHU is further provided with a short cut connection 7 through which air may flow between the supply air channel 2 and extract air channel 3. The air flow through the shirt cut connection 7 is controlled by a short cut damper 8. The shortcut connection 7 connects the supply air channel 2 upstream of the heat exchanger 5 and the first media phase change unit 6a with the extract air channel 3 downstream of the heat exchanger 5 the second media phase change unit 6a.

The AHU 1 is also connected to an Electronic Control Unit (ECU) 9 which may be connected to relevant features in order to receive control inputs and measured data of relevant parameters or to output relevant control instructions to flow control devices. For example, the ECU may receive inputs such as measurements from pressure indicating means 10, thermometers (not shown), humidity sensors (not shown) or control inputs from an input unit or remote control. The ECU 9 may also send control outputs to flow controllers such as the supply air damper 4a, the extract air damper 4b, the short cut damper 8, a supply air fan 11a or an extract air fan 11b as well as control instructions to the heat pump system 6 or heat exchanger 5. Figures 1-3 disclose the same embodiment of the AHU 1 but disclosing the AHU 1 to be controlled differently in order to be in a heat recovery mode (figure 1) or in a defrost mode (figures 2 and 3).

Figure 1 thus discloses the AHU being in a heat recovery mode, i.e. a mode in which heat from the extract air in the extract air channel 3 is transferred to the air flowing into a building via the supply air channel 2. Heat is transferred via the heat exchanger 5 and the heat pump system 6. The heat pump system 6 is controlled by the ECU 9 such that the first media phase change unit 6a in the supply air channel 2 is working as a condenser and thus releasing heat to the surrounding air and the second media phase change unit 6b in the extract air channel 3 is working as an evaporator thus absorbing heat from the extract air. ln this heat recovery mode, according to figure 1, is the short cut connection 7 closed by the short cut valve 8 while both the supply air damper 4a and extract air damper 4b in the supply air channel 2 respectively extract air channel 3 are completely open. lf this mode is used during winter time, e.g. having an outdoor temperature below zero degrees, and wanting an indoor temperature of about 20 degrees, the indoor air leaving the building via the extract air channel 3 will first be cooled down in the heat exchanger 5, e.g. to a temperature close to zero degrees or even somewhat below, before continuing to the second media phase change unit 6b in the heat pump system 6. When the air comes in contact with the second media phase change unit 6b, which functions as an evaporator, and further cools the extract air to temperatures well below zero degrees Celsius, will the water content in the extract air condense and start to freeze on the cold surface of the second media phase change unit 6b. As the time passes will the layer of ice grow thicker on the second media phase change unit 6b and when sufficient time have passed will there be an ice layer which significantly decreases the heat exchange function in the heat pump and thus the overall efficiency of the AHU while also increasing the pressure drop over the second media phase change unit 6b. The initiation of a defrost cycle may for example be controlled by detecting the pressure drop over the second media phase change unit 6b by the pressure drop estimating means 10, which for example may be a pressure sensor upstream of respectively a pressure sensor downstream of the second media phase change unit 6b which thus may easily be used by the ECU to calculate the pressure drop and when the pressure drop is above a certain limit should there be a signal sent indicating a change to defrost mode should occur. Hence, when the pressure indicating system 10 indicates a pressure drop above a first predefined limit the ECU 9 is programmed to output a defrost cycle initiation signal. This defrost ignition signal may thus comprise control outputs from the ECU 9 causing the AHU 1 to change to defrost mode and be configured as described in figures 2 or 3. ln figure 2 is shown how a defrost mode or defrost cycle may be performed. The short cut connection 7 has been fully opened by the short cut damper 8 switching position and both the supply air damper 4a and extract air damper 4b in the supply air channel 2 respectively extract air channel 3 have been completely switched and are now completely closed. Hence, there will be no flow of fresh, supply air in the AHU in this mode disclosed in figure 2 but only recirculation of exhaust air entering through the extract channel inlet 3a, passing the heat exchanger 5 and the second media phase change unit 6b, now switched to function as evaporator in order to heat the surroundings. The air flow of exhaust air is thereafter guided to the supply air 16 channel 2 via the short cut connection 7 in order to flow through the heat exchanger once more but now on the supply air channel 2 side before the flow continues to the first media phase change unit 6b now working as evaporator in order to cold the air flow passing by.

This mode will thus enable a heating operation of the second media phase change unit 6b working both from the outside and inside. The exhaust air will heat on the outside by the relatively hot flow of exhaust air, which no longer will be significantly cooled in the heat exchanger 5 before reaching the second media phase change unit 6b, since there is no fresh, cool supply air entering the AHU 1. ln addition, the reversal of the heat pump system will heat the second media phase change unit 6b from the inside by condensing the cooling media inside. The defrost mode will continue until there is a control signal from the ECU indicating that the heat pump system 5 has been defrosted.

The defrost mode disclosed in figure 3 is similar to the one disclosed in figure 2 but in this case is the supply air damper 4a in the supply air inlet 2a open allowing cold supply air to enter. However, even if the supply air damper 4a is open will the flow of supply air into the supply air channel 2 be rather small since there is a recirculating flow. This defrost mode has the benefit of allowing a certain exchange of air in the building and some supply air to enter into the building also during defrost operation. However, such a configuration of the dampers as disclosed in figure 3 also has the drawback of prolonging the defrost operation in the general case since colder air will enter into the system and by flowing through the heat exchanger 5 thus cool down the heat exchanger somewhat which means that the extract air will be somewhat more cooled down in the heat exchanger before reaching the second media phase change unit 6b and also contribute to a colder flow reaching the first media phase change unit 6b and thus also somewhat lowering the efficiency of the heat pump system 6 in its operation as a heating device which heats the second media phase change unit 6b from the inside.

During the defrost operation as described in figure 2 or 3, ice will melt and the pressure drop will thus decrease. When the pressure indicating system 10 indicates a pressure drop below a second predefined limit the ECU 9 is programmed to output a defrost cycle termination signal including control outputs causing the system to return to its normal heat recovery mode as disclosed in figure 1. This defrost termination signal may thus comprise control outputs from the ECU 9 causing the 17 AHU 1 to change to normal heat recovery mode and be configured as described in figure 1. ln case the defrost cycle time period was longer than a predetermined time period limit, the first pressure drop limit for the next defrost cycle initiation signal could be decreased and if the defrost cycle time period was shorter than a predetermined time period limit, the first pressure drop limit for the next defrost cycle initiation signal could be increased.

The above embodiments only serve as an example of how the invention may be carried out. For example, the AHU need not to be provided with a recirculation function at all, neither is and heat pump necessary. ln addition, a defrost operation could for example be performed by reducing the flow of cold supply air from the outside. The AHU could also be provided with some electrical heating arrangement to improve the defrost operation.

Hence, the above described idea may function for a wide variety of AHUs having the need of a defrosting operation regardless of how the design of the heat recovery system is.

Claims (1)

1.Claims An Air Handling Unit, AHU, (1) for a ventilation system in a building, said AHU (1) having a supply air channel (2) comprising at least one supply air inlet (2a) for guiding supply air from the outdoor into a building through at least one supply air outlet (2b) and an extract air channel (3) comprising at least one extract air inlet (3a) for guiding extract airfrom a building to the outside through at least one extract air outlet (3b), said supply air channel (2) comprising a supply air damper (4a) in order to control the air flow in the supply air channel (2) and said extract air channel (3) comprising an extract air damper (4b) in order to control the air flow in the extract air channel (3), said supply air channel (2) and said extract air channel (3) being in a heat exchanging relation to each other via a heat recovery arrangement, e.g. a heat exchanger (5) and/or a heat pump (6), said air treatment system (1) further comprising at least one fan (11a, 11b) in order to induce a flow in said air treatment system (1) and an Electronic Control Unit, ECU, (9) programmed to output a defrost cycle initiation signal including a control signal to a flow controller, such as an air damper (4a, 4b) or a fan (11a, 11b), or a heating element in order to change or heat the air flow to defrost said heat recovery arrangement when there is an indication of an undesired high level of ice in a portion of said heat recovery arrangement according to a first predefined criteria and a defrost cycle termination signal in order to end the defrost cycle when there is an indication of the defrosting being completed according to a second criteria characterized in that said ECU (9) is further programmed to change the first predefined criteria for output a defrost cycle initiation signal or the second criteria for output of a defrost cycle termination signal depending on the time for performing the defrost cycle such that the time of the defrost cycle should have been increased respectively decreased if the changed first or changed second criteria had been used for the defrost cycle initiation signal respectively the defrost cycle termination signal when the defrost cycle is shorter than a first predefined time period respectively longer than a second predefined time period.An AHU (1) according to claim 1 characterized in that said first predefined criteria for the defrost cycle initiation signal is changed. An AHU (1) according to claim 1 or 2 characterized in that said AHU (1) is provided with a pressure indicating system (10) and the first and second criteria are pressure drops in the AHU (1 ). An AHU (1) according to claim 3 characterized in that the pressure indicating system (10) is designed to measure pressure drops in said extract air channel (2), e.g. by measuring the pressure in the extract air channel (3) upstream respectively downstream of the heat exchanger (5) or heat pump (6), An AHU according to any previous claim characterized in that the air flow through the air supply in|et (3a) is reduced or restricted during a defrost cycle. An AHU (1) according to any previous claim characterized in that said AHU comprises a short cut connection (7) connecting the supply air channel (2) with the extract air channel (3). An AHU according to claim 6 characterized in that the short cut connection (7) is connecting the supply air channel (2) upstream of the heat recovery arrangement, e.g. the heat exchanger (5) and the heat pump (6), with the extract air channel (4) downstream of the heat recovery arrangement. An AHU (1) according to any claim 6 or 7 characterized in that said ECU (9) is programmed to set a short cut damper (8) in the short cut channel (7) to be essentially closed when the AHU (1) is in a heat recovery mode and to set the short cut damper (8) to be essentially open when the AHU (1) is working in a defrost mode. A method for defrosting of an Air Handling Unit, AHU, (1) for a ventilation system in a building, said AHU (1) having a supply air channel (2) for guiding supply air from the outdoor into a building and an extract air channel (3) for guiding extract airfrom a building to the outside, said supply air channel (2) and said extract air channel (3) being in a heat exchanging relation to each other via a heat recovery arrangement, e.g. a heat exchanger (5) and/or a heat pump (6), said method comprising the steps of: - output a defrost cycle initiation signal including a control signal to a flow controller, such as an air damper (4a, 4b) or a fan (11a, 11b), or a heating element in order to change or heat the air flow to defrost said heat recovery arrangement when there is an indication of an undesired high level of ice in a portion of said heat recovery arrangement according to a first predefined criteria - Output a defrost cycle termination signal in order to end the defrost cycle when there is an indication of the defrosting being completed according to a second criteria characterized in that said method further comprising the step of - change the first predefined criteria for output a defrost cycle initiation signal or the second criteria for output of a defrost cycle termination signal depending on the time for performing the defrost cycle such that - the time period of the defrost cycle should have been increased if the changed first criterion or changed second criterion had been used for the defrost cycle initiation signal respectively the defrost cycle termination signal when the defrost cycle is shorter than a first predefined time period. or - the time period of the defrost cycle should have been decreased if the changed first criterion or changed second criterion had been used for the defrost cycle initiation signal respectively the defrost cycle termination signal when the defrost cycle is longer than a second predefined time period. 10. A method according to claim 9 characterized in that - said first predefined criteria for output a defrost cycle initiation signal is a first, higher pressure drop limit and said second criteria for output of a defrost cycle termination signal is a second, lower pressure drop limit and - the first, higher pressure drop limit is increased and/or the second, lower pressure drop limit is decreased when the defrost cycle is shorter than a first predefined time period. or - the first, higher pressure drop limit is decreased and/or the second, lower pressure drop limit is increased when the defrost cycle is longer than a second predefined time period.