Method for reducing energy consumption in a refrigerating machine, and refrigerating machine operating according to said method

Abstract: The method for controlling a motor compressor (3, 5) in a refrigerating machine envisages determining an optimal rate (f ott ) of operation of the motor compressor, which minimizes energy consumption of the machine, and causing said motor compressor to operate, during each turning-on cycle, alternatively at said optimal rate (f ott ) or at a higher rate (f max ) according to the required cooling rate.

Description: The present invention relates to a method and device for reducing energy consumption in compression refrigerating machines. The invention also relates to a refrigerating machine equipped with means for reducing energy consumption. Compression refrigerating machines comprise a circuit for a thermodynamic fluid, which performs a cycle of compression, cooling and possible condensation, expansion, heating, and subsequent compression. In brief, the fluid is compressed by a compressor and then cooled and condensed in a heat exchanger, which, either directly or indirectly, yields heat to the environment. The liquid thus cooled is expanded in an expansion valve or in any other suitable means, such as a turbine, for recovery of part of the energy. The expanded fluid, at a low temperature, is made to circulate in an exchanger inside the refrigerating machine, where it heats up absorbing heat from the compartment of the refrigerating machine that is to be kept at a low temperature. The fluid thus heated up is then sent back again to the compressor for a new thermal cycle. The compressor is operated by an electric motor and is switched on and off according to the temperature detected by a thermostat set in the refrigerated compartment of the refrigerating machine. The compressor is started up when the temperature inside the refrigerated compartment exceeds a maximum threshold and is turned off again when the temperature drops below a minimum threshold. If the refrigerated compartment remains closed and inside it there is no generation of heat (for example due to processes of fermentation of products stored in the compartment itself), switching-on of the compressor and the consequent extraction of heat from the refrigerated compartment has the sole function of compensating for penetration of heat from the outside to the inside of the refrigerated compartment through the walls and the apertures of the compartment. The switching-on time of the compressor depends upon the heat flow obtained with the refrigerating fluid and is thus a function, among other things, of the running rate (r.p.m.) of the motor that controls the compressor. A higher r.p.m. (i.e., a higher number of revs) entails a greater heat flow expressed in frigories per unit time from the refrigerated compartment towards the outside environment, and consequently a more timely restoration of the minimum temperature. When the refrigerated compartment is opened and re-closed, and a product is possibly inserted therein, which may give rise to the generation of thermal energy on account of phenomena of fermentation, the fluid that carries out thermodynamic conversion in the cooling circuit has to pump out the heat that has entered the refrigerated compartment owing to the opening thereof or the introduction of products, or else the heat that is generated inside the refrigerated compartment on account of the aforesaid fermentation phenomena. The time interval during which the motor that operates the compressor is on depends, in this case, both upon the r.p.m. of the motor and upon the amount of heat that enters the refrigerated compartment or is generated therein. In order to reduce energy consumption of refrigerating machines, the tendency, in the first place, is to improve thermal insulation, in order to minimize the penetration of heat inside the refrigerated compartment. EP-A-0490089 discloses a refrigerator compressor controlled with a method according to the preamble of claim 1, and according to which two operating frequencies are set. The lower frequency corresponds to the highest overall efficiency of the compressor under normal operating conditions of the refrigeration appliance associated with the compressor, while the other frequency is higher and is used under conditions of a higher refrigerating capacity demand. The purpose of the present invention is to provide a device and method for controlling the motor compressor of the refrigerating machine which will enable further reduction of energy consumption, modifying the modes of operation of the motor compressor. The above object and further objects and advantages, which will appear evident to persons skilled in the art from the ensuing text, are basically achieved by means of a method according to claim 1. In essence, according to the method of the present invention, it is envisaged to measure the duration of at least two successive time intervals of turning-off of the motor compressor, and to operate the motor compressor when it is turned back on after the second turning-off time interval at the optimal rate if the second turning-off time interval has a duration equal to or greater than that of the previous turning-off time interval, or at a higher rate if the second turning-off time interval has a duration lower than that of the previous turning-off time interval. In order to be able to implement the method on any machine without prior knowledge of the characteristics thereof, it is possible to envisage a learning cycle for determining the optimal operating rate of the motor compressor. During the learning cycle, the characteristics of the motor compressor are verified in order to identify the minimum operating rate below which there is a worsening of the efficiency of the motor compressor. In addition, the learning cycle can be used to identify the class to which the refrigerating machine belongs. The invention also relates to a refrigerating machine controlled according to the method illustrated above, as well as a control circuit for a motor compressor programmed according to said method. Further advantageous characteristics and embodiments of the invention are specified in the attached claims. A better understanding of the invention will be provided by the ensuing description and by the attached drawing, which illustrates a practical, non-limiting embodiment of the invention. More in particular, in the drawing: Fig. 1 is a schematic illustration of a compression refrigerating machine; Figs 2A, 2B, 3A and 3B are graphs of the power absorbed in time in various operating situations of the refrigerating machine; Figs 4A and 4B show a block diagram of the learning cycle; and Fig. 5 shows a block diagram of the regime operating cycle of the refrigerating machine. Fig. 1 is a very schematic illustration of a compression refrigerating machine in its main components. Designated by the reference number 1 is a motor compressor comprising an electric motor 3, supplied by the mains, indicated by the reference number 4, and a compressor 5. The compressor 5 is inserted in a refrigerating circuit comprising a heat exchanger 7 traversed by a coil 9, which forms the condenser and in which the fluid compressed by the compressor 5 is condensed and brought to the liquid state. Designated by the reference number 11 is an expansion valve, where the liquid undergoes expansion before passing through a vaporizer coil 13, in which the fluid is completely vaporized by absorbing heat from the refrigerated compartment 15 of the refrigerating machine. The motor compressor 1 is controlled by means of a programmable microprocessor control unit 17 connected to the supply of the motor 3 and to a temperature sensor or a thermostat 19 set inside the refrigerated compartment 15. The motor compressor 1 is operated cyclically according to the temperature detected by the sensor 19 and to the temperature that it is desired to maintain inside the refrigerated compartment 15. In practice, the motor compressor 1 is turned on when the temperature inside the refrigerated compartment 15 exceeds a maximum value T M and is kept turned on until, thanks to the extraction of heat by the refrigerating fluid, the temperature inside the refrigerated compartment 15 reaches a minimum value T m . The power absorbed by the motor compressor 1 during the turning-on cycle varies in time, decreasing from a maximum value to a minimum value. The diagram of Fig. 2A shows qualitatively the plot of the power absorbed as a function of time between an instant t 0 , in which the temperature T inside the refrigerated compartment 15 reaches the maximum value T M (and hence the motor compressor 1 is turned on), and an instant t a , in which the temperature T inside the refrigerated compartment 15 reaches the minimum value T m (and hence the motor compressor 1 is turned off). The value W M of the maximum power and the value W m of the minimum power absorbed by the motor compressor 1 during the turning-on cycle depend upon the speed of rotation of the motor compressor 1 itself, and hence upon the supply frequency of the motor 3. A higher speed of rotation corresponds to a higher rate of flow of the refrigerating fluid in the circuit, and consequently to a faster extraction of heat from the refrigerated compartment 15. Fig. 2B shows the same graph for a lower speed of rotation of the motor compressor 1. The values of maximum and minimum power absorbed by the motor compressor 1 are lower than those of the case illustrated in Fig. 2A. The instants of turning on and turning off of the motor compressor 1 are denoted by t 0 and t b . The rate of flow of the refrigerating fluid in the circuit is lower, and hence the turning-on time (t b - t 0 ) is higher than that of the example of Fig. 2A. The curve representing the power presents a slope that is less steep than the slope of Figure 2A. In both of the graphs of Figs 2A and 2B, the area under the curve representing the absorbed power (i.e., the integral of the curve itself) corresponds to the energy absorbed during the turning-on cycle or period. This area is approximately the same in the two cases, in so far as it corresponds to the same quantity of heat extracted from the refrigerated compartment 15. Consequently, for each turning-on cycle, the energy absorbed by the motor compressor 1 is substantially equal irrespective of the speed of rotation of the compressor, and hence of the supply frequency of the motor 3. However, as is clearly illustrated in the diagrams of Figs 3A and 3B, the mean energy absorbed by the machine in a period of time that comprises a plurality of turning-on cycles changes according to the speed of rotation of the motor compressor 1. The diagram of Fig. 3A shows, in a time interval (t 1 - t 0 ), three turning-on cycles of the motor compressor 1. Each turning-on cycle has a duration denoted by t on . Between one turning-off of the motor compressor 1 and a subsequent turning-on, there elapses a time interval t off which is constant if it is assumed that the refrigerated compartment 15 is not opened. In fact, according to this hypothesis the duration of turning-off depends exclusively upon the flow of heat through the walls delimiting the refrigerated compartment 15. The graph of Fig.3B shows, for the same time interval (t 1 - t 0 ), the situation in which the motor compressor 1 works at a lower rate. In the time interval considered, there fall two entire turning-on cycles corresponding to the cycle of Fig. 2B, whereas a third turning-on cycle starts just before the instant t 1 , this in so far as, whilst the turning-off interval is the same in the two cases, the duration of the turning-on cycle is longer in the second case than in the first case. Since, as has been noted above, the area under each power-absorption curve is the same in the two situations, when the motor compressor 1 works at a high rate (Fig.3A), in the time interval considered (t 1 - t 0 ) there is on the whole a greater energy absorption as compared to the absorption in the same time interval in the situation represented in Fig. 3B. In practice, if for the moment we neglect other factors which also affect the energy absorption and which will be dealt with in what follows, it is noted that the lower the rate of operation of the motor 3, and hence the lower the speed of rotation of the motor compressor 1, the lower the mean energy absorbed over time. It may thus be considered that a way to reduce energy consumption of the refrigerating machine is to work at a low rate of rotation of the motor 3. In the limit, the energy absorption is minimized when the speed of rotation of the motor compressor 1 is such as not to cause the machine ever to turn off, i.e., on the assumption the rate is set at a value such as to offset, by means of the heat extracted by the refrigeration fluid, the flow of heat from outside towards the inside of the refrigerated compartment 15 through the walls of the compartment itself. On the other hand, the foregoing discussion has been based upon the hypothesis that during the turning-off interval the refrigerated compartment 15 remains closed. In actual fact, the refrigerated compartment 15 may be opened to enable removal of a product or insertion of a new product to be preserved. If the motor compressor 1 were always controlled at the lowest rate, it would not be possible to obtain cooling or freezing of the new product inserted into the refrigerated compartment 15 nor extraction of the greater amount of heat that enters the refrigerated compartment 15 on account of opening of the access door. In addition, as is known to persons skilled in the sector, below a certain speed of rotation the compressor 5 presents lower levels of efficiency, and hence an energy loss. There thus exists a minimum rate of operation below which energy consumption increases on account of losses in the compressor 5. It is therefore necessary to take into account these factors and to control the motor compressor 1 in such a way as to reduce energy consumption by avoiding entry into the area of low efficiency of the compressor 5 and by maintaining the possibility of cooling rapidly the new product that is inserted into the refrigerated compartment 15. For the above purpose, it is in the first place necessary to establish what is the optimal minimum speed of operation of the compressor 5 that minimizes energy consumption. This speed changes from one machine to another, and it is expedient to provide a method of control that enables identification of this parameter without any prior knowledge of the characteristics of the motor compressor 1. It should moreover be considered that the turning-off time (denoted by t off in the diagrams of Figs 3A and 3B) depends upon the characteristics of the refrigerating machine and, more in particular, upon the class to which it belongs, which is indicative of the quality of insulation of its walls. Also this information may not be known a priori. Figs 4A and 4B are schematic representations, in the form of block diagrams, of a learning cycle that the program executed by the programmable control unit 17 can perform, for example, on occasion of the first turning-on of the machine (or whenever this may become necessary) in order to acquire the mechanical and thermodynamic characteristics of the system, and hence set the optimal parameters for operation when running the machine in an energy-saving mode. Whenever the learning cycle is executed, the control unit 17 can pass to management of the motor compressor 1 by means of a regime cycle, illustrated schematically in the form of a block diagram in Fig.5. The learning cycle illustrated in Figs 4A and 4B envisages a first cooling step and a second learning step. In greater detail, the learning cycle functions as described in what follows. The motor compressor 1 is turned on at a speed of rotation corresponding to the maximum supply frequency of the motor 3. This maximum frequency is denoted by f max . The working frequency of the motor 3, and hence in the final analysis, the speed of rotation of the motor compressor 1, is denoted in the block diagram by f. The motor compressor 1 is kept operating at the maximum working rate until the temperature inside the refrigerated compartment 15 reaches the minimum value T m , at which the motor compressor 1 is turned off. The motor compressor 1 remains off until, on account of the gradual penetration of heat from outside into the refrigerated compartment 15, the temperature T inside the latter again reaches the maximum value T M , at which there is a new turning-on of the motor compressor 1. Also in this step, the motor compressor 1 is sent into rotation at the maximum rate (f max ), at which there is the maximum rate of cooling. This second turning-on represents the start of the learning cycle proper. The learning cycle is an iterative cycle, and the iterations are counted by means of a counter, designated by N. During the learning cycle, measurements of time and energy must be made. More in particular, the control unit 17 must detect the energy absorbed during a turning-on cycle, this being denoted by EC in the diagram and corresponding to the area under the power-absorption curve illustrated in Fig. 2. In addition, the control unit 17 must determine the time duration of each turning-off interval between two consecutive operating cycles of the motor compressor 1. This time is denoted by t off . When the motor compressor 1 is turned on, the control unit 17 stores in memory the duration of the turning-off interval that has just concluded. This parameter is denoted in the diagram by (t off ) N-1 . Also the value of the energy absorbed in the previous turning-on cycle of the motor compressor 1 is stored in memory (or has been stored previously). This parameter is denoted by EC N-1 . When the motor compressor 1 is turned on, the counter N is increased. During the period in which the motor compressor 1 remains on, the control unit 17 calculates the energy absorbed during the cycle (EC N ) by sampling of the absorbed power. When the minimum temperature T m is reached in the refrigerated compartment 15, the motor compressor 1 is turned off, and the control unit 17 starts counting the turning-off time of the compressor. The energy absorbed during the N-th turning-on cycle is stored in memory as the parameter EC N , whereas the duration of the time interval during which the compressor 5 remains stationary after the N-th turning-on cycle is denoted by (t off ) N . Counting of the turning-off time (t off ) N ceases when the temperature T inside the refrigerated compartment 15 reaches again the maximum value T M , at which the motor compressor 1 must be turned on again. If this procedure is carried out on at least two successive cycles (N-1 and N), the control unit 17 will have available in memory two values corresponding to the durations of the turning-off periods of the compressor, denoted by (t off ) N-1 and (t off ) N . If the duration of the second turning-off period (i.e., the duration (t off ) N ) is equal to or higher than the duration of the preceding turning-off period (t off ) N-1 , this means that the refrigerated compartment 15 has not been opened during the second turning-off period. If, instead, the motor compressor 1 has remained off for a time shorter than the preceding cycle, this means that the refrigerated compartment 15 has been opened at least once. If it is assumed that the refrigerated compartment 15 has been opened at least once during the last turning-off period between two successive turning-on cycles, it is advisable, at the new turning-on, for the motor compressor 1 to be brought to its maximum speed to dissipate in as little time as possible the excess heat that has entered the refrigerated compartment 15. Consequently, as emerges from the first decision block represented in Fig. 4B, if (t off ) N < (t off ) N-1 the learning cycle envisages that the next turning-on of the motor compressor will take place once again at the maximum rate (f = f max ). If, instead, no heat has penetrated the refrigerated compartment 15 owing to its having been opened, it is possible to proceed to reducing the speed of operation of the motor compressor 1, i.e., to reducing the supply frequency of the motor 3, in a way compatible with the fact that (as has been said above) there exists a minimum rate below which the efficiency of the compressor drops, causing an increase in losses, and hence an increase in the energy absorbed for each turning-on cycle. According to the method represented schematically in the block diagram of Fig. 4 with the aim of identifying the optimal value of the rate of operation of the motor 3 that minimizes the mean energy absorbed in the time interval, a control is carried out on the energy absorbed in two successive turning-on cycles if and only if the preceding control has ascertained that the refrigerated compartment 15 has not been opened in the course of the last turning-off interval. The second decision block represented in Fig. 4B indicates that the control unit 17 carries out a comparison between the energy absorbed in the last two turning-on cycles, i.e., between the quantities EC N and EC N-1 . The behaviour of the system is substantially as described in what follows. If the energy absorbed in two consecutive turning-on cycles, which are characterized by two distinct operating rates, is higher in the second cycle (carried out at a lower rate of operation) than in the first cycle (carried out at a higher rate of operation), the value of the optimal rate of operation is identified as the one used in the first cycle. The above behaviour is represented schematically in the diagram of Fig. 4B by the second decision block, according to which if EC N-1 > EC N i.e., if in two consecutive turning-on cycles carried out at two different rates of operation there has not been any increase in energy absorption per cycle, the rate of operation (f) is decreased by a pre-set amount (Δf). The value of increase in the rate will be used for the new turning-on cycle of the motor compressor 1. If, instead, the energy absorbed in the last turning-on cycle is greater than that absorbed in the preceding turning-on cycle, this means that the current rate of operation (f) has dropped below the value beyond which there is a deterioration in the mechanical efficiency of the compressor 5. The system thus takes as optimal rate (f ott ) the one immediately higher than the current value. When the optimal value (f ott ) of the rate is defined, the control system has concluded the learning cycle and passes on to operating according to the regime operating cycle, schematically represented by the block diagram of Fig. 5. During regime operation of the machine, the control unit 17 keeps in memory the time duration of two successive turning-off periods, denoted in the diagram of Fig. 5 by (t off ) N-1 and (t off ) N . The motor compressor 1 is turned on whenever the temperature inside the refrigerated compartment 15 reaches the maximum temperature T M and is kept turned on up to the moment at which the temperature in the refrigerated compartment 15 reaches the minimum value T m . The speed of operation of the compressor, and hence the rate of cooling, is determined on the basis of a comparison between the durations of the last two turning-off intervals. If the duration of the last turning-off interval (t off ) N is greater than or equal to the duration of the penultimate turning-off interval (t off ) N-1 , this means that the refrigerated compartment 15 has not been opened, and hence that the motor compressor 1 can be made to operate at the minimum rate, i.e., at the optimal rate (f ott ) determined during the learning cycle. If, instead, the duration of the last turning-off interval (t off ) N is shorter than that of the preceding turning-off interval, this means that the refrigerated compartment 15 has been opened, and hence the system must proceed to a rapid cooling, thus imposing on the motor compressor 1 the need to work, in the new turning-on cycle, at the maximum rate (f max ), which enables rapid restoration of the conditions of minimum temperature T m , with extraction of the excess heat introduced into the refrigerated compartment 15 and deriving, for example, from the mere opening and re-closing of the compartment, or else from the introduction of new products that are to undergo refrigeration. It is understood that the drawing only illustrates a possible embodiment of the invention, which may vary in its embodiments and arrangements without thereby departing from the scope of the idea underlying the invention as defined by the claims. The possible presence of reference numbers in the attached claims has the sole purpose of facilitating reading thereof in the light of the foregoing description and in no way limits the scope of protection represented by the claims.

Claims: A method for controlling a motor compressor in a refrigerating machine, including the steps of determining an optimal rate (f ott ) of operation of the motor compressor, which minimizes energy consumption of the machine, and by operating said motor compressor, during each turning-on cycle, alternatively at said optimal rate (f ott ) or at a higher rate (f max ) according to the required cooling rate, characterised by measuring the duration of at least two successive turning-off intervals (t off ) of the motor compressor, and by operating the motor compressor when the latter is turned back on after the second turning-off time interval, at the optimal rate (f ott ) if the second turning-off time interval has a duration equal to or higher than that of the previous turning-off time interval, or at a higher rate (f max ) if the second turning-off time interval has a duration (t off ) lower than that (t off ) of the previous turning-off time interval. The method according to Claim 1, characterized by : determining the turning-off time interval ((t off ) N ) between one turning-off of the motor compressor and a subsequent turning-back-on of the motor compressor; comparing said time interval with the preceding time interval ((t off ) N-1 ) between the preceding turning-off and the preceding turning-back-on of the motor compressor; when the motor compressor is turned back on, operating the motor compressor at the minimum optimal rate (f ott ) if the turning-off time interval ((t off ) N ) is longer than or equal to the preceding turning-off time interval; or else at said higher rate (f max ) if the time interval ((t off ) N ) is shorter than the preceding turning-off time interval ((t off ) N-1 ). The method according to Claim 1, or 2, characterized by providing a learning cycle for determining the value of the optimal rate (f ott ) of operation of the motor compressor. The method according to Claim 3, characterized in that said learning cycle is carried out whenever the refrigerating machine is started. The method according to Claim 3 or 4, characterized by determining the minimum rate of operation of the motor compressor below which there is an increase in the energy losses of the motor compressor, and in that said minimum rate is assumed as optimal rate (f ott ). The method according to Claim 3, or 4, or 5, characterized in that during the learning cycle there is determined the class to which the refrigerating machine belongs by determining the duration of the turning-off period of the motor compressor. The method according to Claim 5 or 6, characterized by determining the energy (EC) absorbed by the motor compressor during a plurality of turning-on cycles of the motor compressor, reducing the rate of operation of the motor compressor between successive turning-on cycles until a rate (f ott ) is reached, below which the energy absorbed during a turning-on cycle increases on account of the losses in the motor compressor, said rate being assumed as optimal rate. The method according to Claim 7, characterized by reducing the rate of operation of the motor compressor between one turning-on cycle and the next only if, during the turning-on interval between said successive two turning-on cycles, the refrigerated compartment of the refrigerating machine has not been opened. The method according to Claim 8, characterized in that if, between one turning-on cycle and one second turning-on cycle, the refrigerated compartment of the refrigerating machine has been opened, at the second turning-on cycle, the motor compressor is made to operate at the maximum rate (f max ). The method according to Claim 9, characterized by verifying the possible occurrence of opening of the refrigerated compartment of the refrigerating machine, by comparing the duration of successive turning-off time intervals of the motor compressor. A refrigerating machine comprising a motor compressor and an electronic control circuit for controlling the motor compressor with a microprocessor and a control program, characterized in that said microprocessor is programmed to carry out a control method according to one or more of Claims 1 to 10. A control circuit for a compression refrigerating machine, comprising a microprocessor, characterized in that said microprocessor is programmed to carry out a control method according to one or more of Claims 1 to 10.

Inventors: BIANCHI ANDREA, CANOVA ANTONIO, MARTINI DAVID, BIANCHI, ANDREA, CANOVA, ANTONIO, MARTINI, DAVID
Applicants: MAGNETEK SPA, MAGNETEK S.P.A.
Application year: 2001
Main classification: F25B49
IPC codes: F25B49