SE542350C2

SE542350C2 - Air Handling Unit comprising an Electronic Control Unit for defrost cycle control

Info

Publication number: SE542350C2
Application number: SE1751287A
Authority: SE
Inventors: Anders Fransson; Andreas Gustavsson; Daniel Johansson; Daniel Oltegen; Magnus Ahl
Original assignee: Swegon Operations Ab
Priority date: 2017-10-17
Filing date: 2017-10-17
Publication date: 2020-04-14
Also published as: SE1751287A1; SE542350E

Abstract

The invention relates to an Air Handling Unit, AHU (1) for an air ventilation system for a building. The AHU is in particular designed to be able to perform an efficient defrost operation. The AHU 1 has 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 air from a building to the outside through at least one extract air outlet 3b. The supply air channel 2 comprises a supply air damper 4a in order to control the air flow in the supply air channel 2 and the extract air channel 3 comprises an extract air damper 4b in order to control the air flow in the extract air channel 3.The supply air channel 2 and the extract air channel 3 are configured to be in a heat exchanging relation to each other via a heat exchanger 5.The AHU 1 further comprises a heat pump 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 in order to transfer heat between the supply air channel 2 and the extract air channel 3. The AHU 1 further includes a short cut damper 8 which controls a flow in a shortcut connection 7 between the supply air channel 2 and the extract air channel 3. The shortcut connection 7 connects the supply air channel 2 upstream of the heat exchanger 5 with the extract air channel 3 downstream of the heat exchanger 5.

Description

TITLE Air Handling Unit comprising an Electronic Control Unit for defrost cycle control TECHNICAL FIELD The present invention relates to an air treatment system for providing fresh air to a building. The invention is in particular directed to the feature of defrosting, or deicing, a heat exchanger unit exchanging heat between the extract air flow and supply air flow in the air treatment system.

BACKGROUND In 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. In 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.

In 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 in forms part of a heat recovery system. 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, and can be 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.

In order to further improve the heat recovery the system may be provided with a 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 collect heat and transfer heat via heat carrying media to a condenser in the supply air channel. 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.

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. In 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.

It is therefore a desire for an improved control and device in order to provide a more efficient defrosting of a heat exchanger and a heat recovery system in the air treatment system in order to improve the overall energy efficiency of the system.

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 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. If 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 air treatment system 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. In order to control the air flow 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. 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 exchange function such that the heat in the exhausted extract air may be regained before it is discharged from the building. 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 one or several heat exchangers. In addition to the heat exchanger the AHU comprises a heat pump. The heat pump 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. In 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 comprises a shortcut connection between the supply air channel and the extract air channel. The shortcut connection connects the supply air channel upstream of the heat exchanger with the extract air channel downstream of the heat exchanger. The short cut connection is provided with a short cut damper which controls a flow of air between the supply air channel and the extract air channel. In normal mode, when the AHU is working for ventilating a building and provide fresh air to the building, is this channel normally closed such that there is no mixture of extract air and supply air. However, the opening of the damper and the admitting of extract air to the supply air channel via the shortcut connection could be used to improve a defrosting operation while also reducing or even completely preventing the exhaust of relatively hot air from a building to the surroundings without a proper heat regeneration of the heat content in the extract air due to the defrost operation of the AHU.

The kind of system described above is in particular useful in cold climates where a lot of heat may be regained by heat exchanging warm extract air from inside the building with cold supply air from the outside. In order to be able to recover as much heat as possible, it is desired to cool down the extract air exhausted as much as possible. There may thus be a risk for ice growth on the heat exchanger as a result of the cooling down of the extract air, which often is rather humid, in the heat exchanger when the temperature is below zero degrees Celsius. Ice 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 over the heat exchanger such that there is needed an increased force, and thus increased energy consumption, in order to provide a desired air flow through the heat exchanger.

A beneficial configuration of the AHU is that the first media phase change unit is located in the supply air channel downstream of the heat exchanger and said second media phase change unit is located in the extract air channel downstream of the heat exchanger. This means that when the AHU is set to work in heat recovery mode will the air flows in the respective channels, i.e. the supply air channel and extract air channels, first be heat exchanged and thereafter will the remaining heat in the extract air be used to further heat the supply air leaving the heat exchanger. The heat exchanger, e.g. a rotating heat exchanger wheel, is well adapted to efficiently recover heat when the temperature differences in the air flows are significant but may not work well when there are small temperature differences meanwhile a heat pump may regain heat also when the heat differences are small or even negative.

The AHU may be further designed such that the short cut connection is connecting the supply air channel upstream of the heat exchanger and the first media phase change unit with the extract air channel downstream of the heat exchanger and the second media phase change unit. This means that if the short cut connection is opened may the extract air first flow and pass the heat exchanger and the first media phase change unit in the extract air channel where after the air may flow through the short cut connection and into the supply air channel and pass the heat exchanger and the second media phase change unit before being recirculated into a building. In order to control the extract air flow and the supply air flow to allow them to pass in their respective channels during normal heat recovery mode and to recirculate the extract air as described above and being able to control the supply air to decrease or being shut off could the AHU be designed such that the extract air damper is located in the vicinity of the extract air outlet and the supply air damper is located in the vicinity of the supply air inlet.

In order to control the heat recovery process and to perform defrost operations when needed is the AHU preferably connected to an Electronic Control Unit (ECU). The ECU may be programmed to set said first media phase change unit to function as an condenser in order to release heat to its surroundings in the supply air channel and said second media phase change unit to function as an evaporator absorbing heat from its surroundings in the extract 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 first media phase change unit to function as an evaporator in order to absorb heat from its surroundings and said second media phase change unit 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 AHU is designed such that the extract air channel is provided with a pressure indicating system in order to estimate the pressure drop over the second media phase change unit. In the system disclosed above, the portion or part in the AHU which is most likely to be subject to ice formation is most probably the second media phase change unit when being located downstream of the heat exchanger in the extract air channel and 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. Ice 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 is 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.

As described above, the AHU may include an Electronic Control Unit (ECU) in order to control the system. 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 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 is 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. In 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 over the second media phase change unit below a second predefined limit.

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 originating from the hot indoor air being cooled down to temperatures below zero and the condensing and freezing of moist in the air. Hence, if the system comprises means for estimating the humidity in the extract air it may be calculated at what temperature the humidity in the air will condense. 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 exchanger portions. 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. To have some safety margin a measured outdoor temperature of zero degrees Celsius may be used since there is absolutely no chance that there will be any ice if the outdoor temperature is above zero degrees Celsius. A more specific measurement may for example be to measure the temperature by placing 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. It 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 buildup 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 buildup in the heat exchanger or heat pump setting different levels of ice buildup as acceptable limits before a signal is sent for initiating a defrost operation. However, since there are in many cases pressure sensors or means for estimating the pressure drop over the heat exchanger present anyway, is it in general suitable to use a limit of a measured or estimated pressure drop over a portion of a heat exchanger or a heat pump for triggering a defrost initiation signal.

The defrost cycle initiation signal implies various control signals to be sent from the ECU. The ECU could for example be programmed to send a control signal to a selected flow controller, e.g. the short cut damper, and change the short cut damper from being essentially closed when the AHU is in the heat recovery mode to be essentially open after a defrost initiation signal when the AHU is working in a defrost mode.

The defrost initiation signal preferably also changes the settings or positions of other dampers. In 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.

In order to be able to control the air flow in the AHU properly the AHU may comprise at least one fan in order to induce a flow in the AHU. The air treatment system must comprise at least for inducing a flow of air in the extract air channel and the supply air channel of the AHU. In general, there is a fan in the supply air channel and another fan in the extract air channel in the AHU. However, the fans in the AHU could be replaced for, or accompanied with, one or several fans at more remote locations, e.g. a large fan in a main supply air/extract air channel in a an air treatment system connected to the AHU.

Even though the above described arrangement and control actions performed by the ECU provides an efficient defrosting or deicing of the AHU is there always a loss in the heat recovery efficiency, alternatively a reduced flow of supply air and/or increased energy consumption by the AHU, during a defrost cycle. Many of the methods above suggest primarily a reduced flow of supply air during defrosting operation. 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, 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. Hence, 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. In order to optimize the overall efficiency of the AHU it comprises an ECU which adapts its predefined settings for when to initiate a defrost cycle. The ECU is programmed to initiate a defrost cycle depending on the pressure drop over the subject matter to be defrosted. The ECU is programmed to change the first predefined limit of the pressure drop for switching the function of the AHU from a normal heat recovery mode to a defrost mode, i.e. for initiating a defrost cycle. The change of the pressure drop limit depends on the time for performing 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 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. In 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. In 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.

The method may include further features as has been described for the system earlier, e.g. having defrost cycle initiation threshold limits, adaptive and reprogrammable threshold limits for the defrost cycle.

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 In 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. In 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. In 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 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.

In 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.

If 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.

In 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 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. In 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.

Claims

1. An Air Handling Unit, AHU, (1) for an air ventilation system for 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 air from 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 airflow 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 exchanger (5), said AHU (1) further comprising a heat pump (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) in order to transfer heat between the supply air channel (2) and the extract air channel (3) and a short cut damper (8) which controls a flow in a shortcut connection (7) between the supply air channel (2) and the extract air channel (3), said shortcut connection (7) connects the supply air channel (2) upstream of the heat exchanger (5) with the extract air channel (3) downstream of the heat exchanger (5), said extract air channel (3) is provided with a pressure indicating system (10) in order to estimate a pressure drop over the second media phase change unit (6b) and said AHU (1) is connected to an Electronic Control Unit (ECU) (9) which is programmed to switch the function of the AHU (1) from a normal heat recovery mode to a defrost mode and start a defrost cycle when the pressure indicating system (10) estimate a pressure drop over the second media phase change unit (6b) above a first predefined limit characterized in that said ECU (9) is programmed to change the first predefined limit of the pressure drop for switching the function of the AHU (1) from a normal heat recovery mode to a defrost mode depending on the time for performing a defrost cycle such that the predefined limit of the 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.

2. An AHU (1) according to claim 1 characterized in that said first media phase change unit (6a) is located in the supply air channel (2) downstream of the heat exchanger (5) and said second media phase change unit (6b) is located in the extract air channel (3) downstream of the heat exchanger (5).

3. An AHU (1) according to claim 1 or 2 characterized in that said short cut connection (7) is connecting 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) and the second media phase change unit (6b).

4. An AHU (1) according to any previous claim characterized in that said Electronic Control Unit, ECU, (9) is programmed to set said first media phase change unit (6a) to function as an condenser in order to release heat to its surroundings in the supply air channel (2) and said second media phase change unit (6b) to function as an evaporator absorbing heat from its surroundings in the extract air channel (3) when the AHU (1) is working in its normal heat recovery mode and to set said first media phase change unit (6a) to function as an evaporator in order to absorb heat from its surroundings and said second media phase change unit (6b) to function as an condenser in order to release heat to its surroundings when the AHU (1) is working in its defrost mode.

5. An AHU (1) according to any previous claim characterized in that said extract air damper (4b) is located in the vicinity of the extract air outlet (3b) and said supply air damper (4a) is located in the vicinity of the supply air inlet (2a).

6. An AHU (1) according to any previous claim characterized in that said Electronic Control Unit, ECU, (9) is programmed to set the short cut damper (8) 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.

7. An AHU (1) according to claim 6 characterized in that said ECU (9) is programmed to set the extract air damper (4b) and supply air damper (4a) to be essentially open when the AHU (1) is in a heat recovery mode and to set the extract air damper (4b) and supply air damper (4a) to be essentially closed when the AHU (1) is working in a defrost mode.

8. An AHU (1) according to claim 6 characterized in that said ECU (9) is programmed to set the extract air damper (4b) and supply air damper (4a) to be essentially open when the AHU (1) is in a heat recovery mode and to set the extract air damper (4b) to be essentially closed and the supply air damper (4a) to be partially or fully open when the AHU (1) is working in a defrost mode.

9. An AHU according to any previous claim characterized in that said AHU (1) further comprises at least one fan (11a, 11b) in order to induce a flow in the AHU (1).

10. An AHU (1) according to any of claims 1 to 4 characterized in that said ECU (9) is programmed to switch off the defrost mode and end the defrost cycle when the pressure indicating system (10) estimate a pressure drop over the second media phase change unit (6b) below a second predefined limit.