JP7624902B2 - Liquid heating device and refrigerator - Google Patents

Description

The present invention relates to a liquid heating device that uses electric energy to heat liquid by Joule heat, and a refrigerator that uses the same.

There are several ways to heat a liquid, including burning a substance and transferring the heat generated to the liquid, and passing electricity through a conductor and transferring the Joule heat generated. When the liquid is water, the former is a gas water heater that boils water by burning gas, and the latter is a familiar example of an electric kettle. These methods can be considered indirect heating methods in the sense that they first heat something other than a liquid and then transfer the heat to the liquid. On the other hand, there are also methods that directly convert a form of energy other than heat into heat for the liquid. For example, a solar water heater heats water using the infrared rays contained in sunlight.

In most practical devices for heating liquids, an indirect heating method is used. As for direct heating methods, the aforementioned solar water heaters are relatively widely used in ordinary homes, but direct heating methods are undoubtedly in the overwhelming minority. As another example of a direct heating method, a technology (direct resistance heating technology) has long been known in which electricity is passed directly through a liquid and the liquid is directly heated by the generated Joule heat. This method has the characteristic of being extremely energy efficient, since when the liquid has a high electrical resistance compared to the electrical resistance of the wiring or electrodes, almost all of the work of the electricity is used to heat the liquid.

Indirect heating methods leave a hot heating element behind after the heated liquid is transferred, which can lead to the risk of heat diffusion to the surrounding area continuing for a long time and causing accidents due to contact with objects or people. Residual heat from a high-temperature heating element can shorten the lifespan of the equipment. There is also the risk that the heating element will continue to heat if the liquid runs out due to an unexpected event. Furthermore, long-term use can cause minerals (calcium and magnesium) contained in the liquid to precipitate and adhere to the heating surface. This reduces the efficiency of heat transfer from the heating element to the liquid, resulting in a decrease in the time required for overheating and power efficiency. Direct resistance heating has the advantage of being able to avoid these disadvantages.

Patent Document 1 discloses a method for heating a liquid by direct resistance heating technology using multiple electrode pairs. Since the electrical conductivity of a liquid changes depending on the electric carriers (mainly positively or negatively charged ions) contained in the liquid and the temperature of the liquid, multiple electrode pairs with different electrode spacings are prepared, and the electrodes are switched depending on the electrical conductivity of the liquid. If the current change during switching is large, a large amount of noise is generated in the power line to which the direct resistance heater is connected, which adversely affects devices connected to the same power system. For this reason, Patent Document 1 discloses an electrode spacing and control method that allows the current change to be smooth when switching electrodes by combining electrode spacings. In addition, in order to reduce the cost of the triac used as a switch during electrode switching as much as possible, a device is devised to minimize the maximum current capacity of the triac. Furthermore, since it is not possible to analytically design the electrodes to actually realize such devices, it is disclosed that the purpose is achieved using an optimization method, and that the electrode spacing design and minimization of the maximum current of the triac are necessary constraints for that purpose. In other words, Patent Document 1 presents a solution to the problems of noise generation and cost reduction during electrode switching that occur when multiple electrodes are used in direct resistance heating technology.

U.S. Pat. No. 7,817,906

Analytical Chemistry, Vol. 56, No. 7, pp. 1138-1142, (1984), “Temperature dependence and measurement of resistance of pure water”, Truman S. Light.

When heating a liquid for use, the time required for heating is important. Generally, the time required to heat a liquid is shorter the greater the thermal energy used for heating, whether indirectly or directly. In direct resistance heating technology, the energy of electricity is transferred to the liquid in the form of Joule heat, so a large amount of electrical energy can be used to shorten the heating time. On the other hand, the electrical resistivity of the liquid depends on the liquid temperature, and decreases as the temperature rises. In other words, the resistance between the electrodes decreases as the temperature rises. For this reason, with a general power supply that operates at a constant voltage or constant current, the Joule heat generated per unit time changes due to the lower resistance caused by an increase in the liquid temperature, and unless the power supply settings are appropriately selected, the time required for heating will be longer.

An example using specific numbers will be given. Figure 1 shows the calculations of the voltage 13 that can be applied to water when heating it using a commercial power source with a capacity of 100V and 15A, the resulting current 14 that flows, and the resulting power 15 that is applied to the water. Here, the electrical resistivity of the water is set to 50 Ωm at 25°C, and was calculated using the temperature dependence of the resistivity of water, which will be explained later. This value corresponds to the resistivity of ordinary tap water.

As shown in FIG. 1, the maximum voltage, maximum current, and maximum power that can be supplied from the power source are 100V, 15A, and 1500W. Maximum power can be supplied only when the maximum voltage and current are both at the maximum, and therefore, according to Ohm's law, this is only possible when the resistance of the load, i.e., the water, is 100/15 ≒ 6.67Ω. In region 11 where the water resistance is less than 6.67Ω, the maximum current of 15A flows at less than the maximum voltage of 100V, so the power supplied is less than the maximum power that can be supplied. Similarly, in region 12 where the water resistance is greater than 6.67Ω, the maximum current of 15A cannot flow even when the maximum voltage of 100V is applied, so the power supplied is less than the maximum power that can be supplied. In this way, the power supplied from the commercial power source is maximum only at the point where the water resistance is 6.67Ω, and only power less than the maximum power can be supplied in other regions.

As described above, when heating a liquid using direct resistance heating technology, the conditions under which the maximum power that can be supplied from the power source can be used for heating are limited. For this reason, if heating is performed without taking into account the temperature dependence of the electrical resistivity of the liquid to be heated, the power required for heating cannot be supplied to the liquid efficiently, and the time required for heating ends up being long.

Thus, in order to shorten the heating time with direct resistance heating technology, some ingenuity is required to supply as much power as possible from the power source to the liquid. However, no literature has been found that discloses such an ingenuity, and the issue of shortening the heating time remains. For example, Patent Document 1 discloses a direct resistance heating technology that uses multiple electrode pairs, but does not disclose a method for shortening the heating time or a heating method that suits the characteristics of the liquid.

A liquid heating device according to one embodiment of the present invention is a liquid heating device that heats a liquid having an initial temperature _Ti to a target temperature _Tf , and includes a container, an electrode pair disposed within the container, and an AC power source that passes a current through the liquid via the electrode pair immersed in the liquid, and the inter-electrode distance d of the electrode pair and the electrode area S through which the electrode pair passes a current through the liquid are determined based on the maximum current _Imax and maximum voltage _Vmax of the AC power source, and the electrical resistivity _ρ ( _Tm ) that the liquid can have at an intermediate temperature _Tm (( _Ti + _Tf )/2) between the initial temperature _Ti and the target temperature Tf.

The heating time can be shortened by determining the electrode specifications to satisfy the shortest heating conditions. It has also been confirmed that the effect on heating time of errors in the electrode pair size or the electrical resistivity of the liquid compared to the electrode specifications can be kept small.

Other issues, configurations, and advantages will become clear from the description of the embodiments below.

FIG. 1 is a diagram showing the relationship between resistance and power supply voltage, current, and power when water is heated by direct resistance heating. FIG. 1 is a diagram showing the temperature dependence of electrical resistivity of solutions having different impurity concentrations. FIG. 13 is a diagram (PT graph) showing the relationship between liquid temperature and power when water is heated by direct resistance heating. FIG. 1 is a diagram illustrating a configuration of a liquid heating device. FIG. 2 is an equivalent circuit diagram of the liquid heating device. FIG. 13 is a diagram showing the change in power and water temperature over time when the size of the electrode pair is set arbitrarily. FIG. 13 is a graph showing the changes in power and water temperature over time when the size of the electrode pair is optimized. FIG. 1 is a diagram illustrating a configuration of a liquid heating device. FIG. 1 is a diagram illustrating a configuration of a liquid heating device. FIG. 1 is a diagram illustrating a configuration of a liquid heating device. FIG. 13 is a diagram showing the changes in power and water temperature over time when the sizes of the two electrode pairs are optimized. FIG. 13 is a diagram showing the relationship between temperature and power when the sizes of N electrode pairs are optimized. FIG. 13 is a diagram showing the d/S value that is the optimal heating condition within the assumed range of resistivity. FIG. 13 is a diagram showing the relationship between the electrode spacing and the length of one side of a square electrode that provides optimal heating conditions. FIG. 13 is a diagram showing the relationship between the electrode spacing and the length of one side of a rectangular electrode that provides optimal heating conditions. FIG. 13 is a diagram showing the d/S value that is the optimal heating condition within the assumed range of resistivity. FIG. 13 is a diagram showing the relationship between the electrode spacing and the length of one side of a square electrode that provides optimal heating conditions when the liquid temperature is 45° C. or less. FIG. 13 is a diagram showing the relationship between the electrode spacing and the length of one side of a square electrode that provides optimal heating conditions when the liquid temperature is 45° C. or higher. FIG. 13 is a diagram showing the relationship between the electrode spacing and the length of one side of a rectangular electrode that provides optimal heating conditions when the liquid temperature is 45° C. or less. FIG. 13 is a diagram showing the relationship between the electrode spacing and the length of one side of a rectangular electrode that provides optimal heating conditions when the liquid temperature is 45° C. or higher. FIG. 13 is a diagram showing the relationship between temperature, time, and power when the sizes of N electrode pairs are optimized. FIG. 13 is a diagram showing the relationship between temperature, time, and energy when the sizes of N electrode pairs are optimized. FIG. 1 is a diagram showing an example of the appearance of a water server. FIG. 2 is a functional block diagram of the water server. FIG. 4 is a diagram showing an electrical configuration related to a heating unit. 4 is a timing chart of the heating operation of the water server. FIG. 4 is a diagram conceptually showing changes over time in power application to a heating unit. FIG. 4 is a diagram conceptually showing the change in water temperature in a heating section over time. FIG. 1 is a diagram showing an example of the appearance of a refrigerator having a water server function. FIG. 1 is a functional block diagram of a refrigerator with a water server function. FIG. 1 is a functional block diagram of a refrigerator with a water server function. 4 is a timing chart of the heating operation of the water server unit. 4 is a timing chart of the heating operation of the water server unit. 4 is a timing chart of the heating operation of the water server unit. 4 is a timing chart of the heating operation of the water server unit.

The inventors have noticed that there is almost no practical problem in expressing the temperature dependence of electrical resistivity of liquids, particularly liquids that are mainly composed of water, using a relatively simple approximation, an exponential function of temperature.

When a substance that dissolves in water is mixed into pure water, the electrical resistivity generally decreases. As a simple example, consider mixing impurities into ultrapure water. The impurities dissolved in ultrapure water are ionized, and the ions become carriers of electricity, facilitating electrical conduction and decreasing the electrical resistivity. Figure 2 shows the temperature dependence ρ(T) of the electrical resistivity of a solution in which a certain amount of impurity is mixed into ultrapure water. In a solution with a small amount of impurity, ρ(T) can be approximated with high accuracy by an exponential function whose exponent is a power function of temperature expressed with a finite number of terms (Non-Patent Document 1). In contrast, ρ(T) can also be approximated by the simpler (Equation 1), although accuracy is somewhat sacrificed.

Here, ρ ₀ and α are constants determined according to the amount of impurities. Each line in FIG. 2 shows the temperature dependence of the electrical resistivity expressed by (Equation 1) for each impurity concentration. It can be seen that the higher the impurity concentration, the lower the resistivity and the higher the accuracy of the approximation by (Equation 1). Normally, city water used as drinking water, so-called tap water, has a resistivity of about 10 to 200 Ωm, and in many cases about 50 to 100 Ωm. Furthermore, liquids that contain water as the main component are thought to have components other than water dissolved in the water as impurities, and therefore have a smaller resistivity than general tap water. For this reason, (Equation 1) can be used as a good approximation formula showing the temperature dependence of resistivity for tap water and liquids that mainly contain water, which have a relatively high impurity concentration and low electrical resistivity.

If the length of the electrodes is L, the height is H, and the electrode spacing is d, the resistance between the electrodes due to the liquid is expressed by (Equation 2).

The maximum voltage and current that the power source can supply with respect to the voltage V and current I between the electrodes are the maximum voltage _Vmax and the maximum current _Imax , respectively. The power P input to the liquid is calculated using Equation 4 or Equation 5, depending on the magnitude relationship of the resistance R between the electrodes relative to the resistance R' determined by Equation 3. Equation 4 applies when R>R', and Equation 5 applies when R<R'. Note that the electrode area S=L·H.

Here, as shown in FIG. 2, resistivity decreases with temperature rise, so α<0. In the rightmost sides of (Equation 4) and (Equation 5), in order to clearly indicate the sign of the exponent, α is expressed as an absolute value and then a sign is added. The transformation of (Equation 4) utilizes the fact that when the resistance R between the electrodes is larger than R', the voltage V is constant at the maximum voltage V _max (see FIG. 1), and a current I=V _max /R flows. Similarly, the transformation of (Equation 5) utilizes the fact that when the resistance R between the electrodes is smaller than R', the current I is constant at the maximum current I _max (see FIG. 1), and the voltage V=I _max ×R. If the power source and the liquid are determined, the maximum voltage V _max , maximum current I _max , constant α, and constant ρ ₀ are determined. Therefore, it is shown that the power P that can be applied to the liquid in both equations is determined by the temperature T of the liquid at that time.

3 (PT graph) is a graph that shows a schematic representation of the relationship between power P and liquid temperature T based on (Equation 4) and (Equation 5). When the liquid temperature T=T', where the resistance R=R', is T, the power P=maximum power _Pmax , and in both region I where the liquid temperature T<T' and region II where the liquid temperature T>T', the power P monotonically decreases as it moves away from the liquid temperature T'.

As explained below, the P-T graph has a symmetrical shape with respect to the liquid temperature T = T'. The power P-liquid temperature T relational equations corresponding to regions I and II, i.e. (Equation 4) and (Equation 5), are differentiated with respect to the liquid temperature T, resulting in (Equation 6) and (Equation 7).

Equations 6 and 7 show that the slopes of the graphs at points where the power P is equal in regions I and II have opposite signs and the same magnitude. In other words, the P-T graph has a symmetrical shape with respect to the liquid temperature T'.

The temperature rise of the liquid is proportional to the input energy E. If the proportionality constant is A and the liquid temperature T and energy E are each expressed as a function of time to clearly show the change over time, then T(t) = AE(t). Using this relationship, the time derivative of power P can be expressed as (Equation 8).

In addition, the transformation of the formula in (Equation 8) uses (Equation 6), (Equation 7) and the relationship of dE/dt=P. The sign - in (Equation 8) corresponds to region I in the P-T graph (FIG. 3), and the sign + corresponds to region II in the P-T graph (FIG. 3). From (Equation 8), it can be seen that in the graph of power P vs. time t (P-t graph), the slopes of the graph at points where the power P has the same value have the same magnitude but the opposite signs. In other words, the P-t graph is a graph that is symmetrical with respect to the time when the resistance R=R' (the time when the power P=the maximum power _Pmax ) and has a shape similar to the P-T graph shown in FIG. 3.

In order to heat in the shortest time, the energy used for heating should be input to the liquid in the shortest possible time. Since energy is an amount of electricity, its size corresponds to the area of the Pt graph. The area of the Pt graph from the time when heating of the liquid starts to the time when heating ends (the time when the liquid temperature reaches the target temperature) is the amount of electricity used for heating, so the heating conditions in the shortest time (called the shortest heating conditions) are conditions in which the maximum power P _max is input halfway between the heating start time and the heating end time.

In this way, the shortest heating conditions are determined so that the time interval between the time of the symmetry center in the P-t graph and the heating start time and the time interval between the time of the symmetry center in the P-t graph and the heating end time are equal, which means that the amount of power input is equal on both sides of the symmetry center time. Since the specific heat of a liquid mainly made of water is generally constant from room temperature to the boiling point, if the amount of input energy is the same, the amount of temperature rise is the same regardless of the starting temperature. Therefore, the amount of temperature change on both sides of the symmetry center time is also the same. In other words, the shortest heating conditions can be rephrased as a condition in which the maximum power P max is input when the liquid temperature at the heating start time is the initial temperature T _i and the target _temperature at the heating end time is T _f , and the temperature is the intermediate temperature T _m = (T _i + T _f )/2.

This condition can be met by specifying the electrode specifications. Under the shortest heating conditions, (Equation 2) to (Equation 9) hold.

Equation 9 can be transformed to obtain Equation 10. By using an electrode pair with a spacing d and electrode area S = L x H that satisfies Equation 10, the liquid can be heated in the shortest time possible.

Next, we consider the effect of error. In reality, the electrode area (L, H) and the electrode distance d are processed with a certain degree of error. Also, deviations from the design value of resistivity ρ due to variations and fluctuations in the liquid quality also result in errors. Such deviations from the optimal value cause the heating time to be longer than the shortest time. First, we evaluate how much the position of maximum power P _max differs due to electrode area errors in the P-T graph. Suppose the electrode area changes from S to S + ΔS. At this time, we assume that the liquid temperature at which maximum power P _max is obtained changes from T to T', and this can be written as (Equation 11).

The liquid temperatures T and T' can be calculated from (Equation 11), and the difference ΔT = T - T' can be calculated from (Equation 12).

Here, quadratic and higher terms of ΔT are ignored. In (Equation 11), S and d are in the numerator and denominator of the exponential function coefficient, respectively, so even if there is an error in the distance between the electrodes ((d → d + Δd)), the sign will change and the equation will have the same shape (ΔT ~ 1/α · Δd/d). From this result, we can see that the deviation in liquid temperature at maximum power P _max is about the same as the rate of dimensional change (first order).

In the same way, we evaluate how much error in the electrode area leads to an error in the power at each temperature. When the electrode area is S and satisfies the optimal condition (power P at each temperature at this time), and when it changes to S + ΔS and deviates from the optimal condition (power P' at each temperature at this time), the difference between the two, ΔP = P - P', is expressed by (Equation 13) in region II and (Equation 14) in region I.

Here, quadratic and higher terms of ΔS are ignored, and ΔS/S<1.

From the above, it can be seen that the power error at each temperature in both regions I and II is of the first order of the ratio of the error in the size of the electrode pair. Since the deviation amount ΔT explained earlier is also of the first order of the ratio of the error in the size of the electrode pair, the deviation from the optimal condition due to the error on both the vertical and horizontal axes of the P-T graph is of the first order of the ratio of the error. As mentioned above, since temperature and energy are proportional, it can be said that the energy error is also of the first order of the ratio of the error in the size of the electrode pair. On the other hand, energy is expressed as the product of power and time, and the time deviation due to the error corresponds to the time required to input additional energy increased by the error. Since energy and power are each of the first order of the error in the size of the electrode pair, it can be said that the time error is of the second order or lower. For example, if the size of the electrode pair changes by about 30%, the effect on the heating time is expected to be 9% or less, but it can be calculated that the actual effect will be an extension of the time by about 5%. Similarly, a 50% deviation in the size of the electrode pair is expected to lead to an extension of the heating time by 25% or less, and calculations show that it will be extended by about 10 to 15%.

How much the error needs to be reduced is a design issue, depending on how much time extension is allowed, but for example, if the error in the size of the electrode pair can be kept below 30%, the time delay will be a maximum of 10% at most, so this error value remains within a range that is generally acceptable for practical use. Here, we have described how to think about error when there is one electrode pair, but the same basic approach can be used when multiple electrode pairs are used. Also, with regard to water quality errors, small deviations that can be treated as errors can generally be overlooked.

In the first embodiment, a device for heating a liquid using one electrode pair will be described. FIG. 4A shows a schematic configuration of a liquid heating device for heating a liquid by direct resistance heating technology. An electrode pair 43 is installed in a container 41 and connected to a power source 44 via wiring 45. The container 41 contains a liquid to be heated so that the electrode pair 43 is completely immersed in the electrode pair 43. The container 41 is preferably made of polycarbonate, which has relatively high heat resistance. For the electrodes, for example, carbon formed by CIP (Cold Isostatic Pressing) is used. A hole is drilled in the carbon material and connected to the wiring 45 with a screw and a nut. At this time, the hole is countersunk on the side facing the electrode to eliminate the protrusion of the screw, so that the electrode and the screw do not come into contact with each other and cause a short circuit when the electrode spacing is narrow. The electrode spacing d is determined by sandwiching and fixing an insulator of a predetermined thickness between the electrode pairs. At this time, the insulator covers only a part of the electrode to ensure a current path between the electrode pair 43. In addition, the part sandwiching the insulator is excluded from the electrode length L and electrode height H because no current flows through it. In other words, the electrode area S refers to the area of the part of the electrode that can be passed through the liquid. The electrode pair 43 is placed on the bottom of the container 41. The electrodes may be placed so that they are inserted into the liquid surface from the top side of the container, but the electrode pair 43 should be designed to be inserted near the bottom of the container. If the electrode pair 43 is located near the liquid surface, the heated liquid will remain near the liquid surface, and it will take time for the temperature of the liquid near the bottom to rise. The wiring 45 is led out of the container through a small hole opened on the side of the container and connected to the power source 44. After leading out the wiring, the small hole in the container is filled with a water-resistant bond to prevent water leakage. The power source 44 uses an AC power source that can supply a maximum voltage of 100V, a maximum current of 5A, and a capacity of 500W. Although not shown in the drawing, a thermometer is inserted from the top of the container so that the liquid temperature can be measured at a position approximately halfway down the water depth.

Figure 4B is an equivalent circuit of Figure 4A. Electricity flows through the liquid between the electrode pair 43, and the electrical resistance is represented by resistor 42. The magnitude of resistor 42 changes depending on the temperature of the liquid, so it is represented as a variable resistor.

First, as a comparative example, the size of the electrodes was appropriately set to 5 cm in length and 10 cm in height. Using an electrode pair with an electrode gap of 2 mm, water at approximately 0°C was heated to 90°C. 500 mL of cooled water was placed in a container and heated by applying a voltage of 100 V between the electrodes. The water used had a resistivity of approximately 50 Ωm at 25°C. Figure 5 shows a plot of the water temperature and the power input against time. It took approximately 4,300 seconds for the water temperature to reach 90°C.

In contrast, the size of the electrode pair that is optimal for heating water from 0°C to 90°C is obtained according to (Equation 10). In this case, the intermediate temperature T _m =45°C, the maximum voltage V _max =100V, the maximum current I _max =5A, and d =2mm, so that (Equation 10) gives S = L·H = 5.7 cm ^2. Note that, as values for water with a resistivity of 50Ωm at 25°C, α = -2.83×10 ^-2 and ρ ₀ = 20.29 are used. The electrode length L = 5 cm, the electrode height H = 1.15 cm, and an electrode of approximately 1.1 cm was used. Figure 6 shows the change in power and water temperature over time in this case. It can be seen that it takes approximately 770 seconds to reach 90°C, and the maximum power is input at about half that time, about 380 seconds. In this way, by determining the size of the electrode pair based on the specifications that satisfy the shortest heating conditions of the liquid heating device, it is possible to significantly shorten the heating time compared to the case of using an electrode pair of any size.

While Figure 4A shows an embodiment in which the liquid is heated while stored in a container, it is also possible to heat the liquid while it is flowing. In this case, by treating the flow rate per unit time as a lump of liquid and applying a similar concept to the flowing liquid, it is possible to control the temperature of the flowing liquid at a specified position. In other words, it is also possible to configure the flowing liquid so that the liquid temperature at the outlet is heated to a specified target temperature. This is the same in the following examples.

In Example 2, a method for shortening the heating time using the direct resistance heating method will be described. In order to shorten the heating time, the power input to the liquid per unit time can be increased. An example using two electrode pairs is shown in Fig. 7A and Fig. 7B. In Fig. 7A, the electrode pair 71 and the electrode pair 72 are connected in series to an AC power source, and in Fig. 7B, the electrode pair 71 and the electrode pair 72 are connected in parallel to an AC power source. The electrode distance _d1 of the electrode pair 71 is set to d1, the electrode area S1 is set to _d1 , and the resistance at that time is set to _d1 . Similarly, the electrode distance d2 of the electrode pair 72 is set to _d2 , the electrode area _S2 is set to d2, and the resistance _R2 is set to _d2 . Here, if the resistance ratio q= _R1 / _R2 is defined, then it can be expressed as q= _d1S2 / _d2S1 from (Equation 2). Therefore, once the electrode specifications are determined, the value of the resistance ratio _q is determined to a certain value. In this case, the load seen from the power source (resistance of the liquid to be heated) is expressed by (Equation 15) in the case of series (FIG. 7A) and by (Equation 16) in the case of parallel (FIG. 7B).

In this way, in either case, heating is equivalent to using a single electrode pair with a combined resistance R determined by the constant q. Therefore, although there is more freedom in electrode design, the heating time cannot be shortened.

In contrast, in FIG. 7C, a switch 73 is provided to conduct electricity to either one of the electrode pairs 71 and 72 connected in parallel. In this case, one electrode pair heats from the initial temperature T _i to the switching temperature T _s at the time of switching, and the other electrode pair heats from the switching temperature T _s to the target temperature T _f . Therefore, for each electrode pair, the conditions for heating from the liquid temperature T _i to the liquid temperature T _s and from the liquid temperature T _s to the liquid temperature T _f in the shortest time can be set. This can be determined by the method disclosed in the first embodiment. The remaining question is where to set the switching temperature T _s , and the switching temperature T _s can be set to the intermediate temperature ((T _i + T _f )/2) between the initial temperature T _i and the target temperature T _f . This can be qualitatively explained as follows.

First, assume that the switching temperature _Ts is set near the initial temperature _Ti . This is almost the same as when there is one electrode pair. Conversely, the same is true when the switching temperature _Ts is set near the target temperature _Tf . However, by increasing the number of electrode pairs by one, the condition under which the maximum power can be input can also be increased by one, so the amount of power that can be input within a certain time can be increased. Therefore, in both cases, the time can be shortened slightly compared to when there is one electrode pair. Next, consider the case where the switching temperature _Ts is continuously changed from the initial temperature _Ti to the target temperature _Tf . When the switching temperature _Ts is slightly higher than the initial temperature _Ti, the time can be shortened compared to when there is one electrode pair, and when the switching temperature _Ts is slightly lower than the target temperature _Tf , the time can be shortened compared to when there is one electrode pair. Therefore, when the switching temperature _Ts is increased from the initial temperature _Ti to the target temperature _Tf , the amount of reduction in the heating time first increases, and then the amount of reduction in the heating time decreases as the temperature approaches the target temperature _Tf from a certain point. Symmetry shows that the shortening amount is maximum at a temperature exactly halfway between the initial temperature T _i and the target temperature T _f .

FIG. 8 shows the time change of temperature and power when the switch 73 is switched at a temperature exactly halfway between the initial temperature T _i and the target temperature T _f . FIG. 8 compares the case of one electrode pair and the case of two electrode pairs (the configuration of FIG. 7C) when heating 500 ml of water with a resistivity of 50 Ωm at 25° C. from an initial water temperature of 20° C. to a target water temperature of 100° C. The power source used is an AC power source capable of supplying a maximum voltage of 100 V, a maximum current of 15 A, and a capacity of 1500 W. The switching temperature T _s is 60° C., which is an intermediate temperature between 20° C. and 100° C. In the case of two electrode pairs, according to (Equation 10), if d/(L·H) is 0.204 from 20° C. to 60° C. and d/(L·H) is 0.632 from 60° C. to 100° C., the conditions for heating can be achieved in the shortest time. Therefore, the electrode spacing is set to d = 2 mm, the former electrodes are 9.9 cm square, and the latter electrodes are 5.6 cm square. By switching switch 73 at a water temperature of 60°C, the temperature reaches 100°C in approximately 150 seconds, as shown in waveform 81. On the other hand, when there is one electrode pair, the shortest condition is d/(L H) ≈ 0.414 from (Equation 10). Here, the water temperature rise when heating with d = 2 mm, L = 10 cm, and H = 5 cm is shown as waveform 82. It takes approximately 210 seconds to reach 100°C.

Waveforms 83 and 84 in Figure 8 compare the power applied when there are two electrode pairs and when there is one. It can be seen that when heating up to 60°C, two electrode pairs allow for greater power input, which contributes to shortening the time. In this way, it can be seen that by switching between two electrode pairs, heating can be accelerated compared to when there is one electrode pair.

It is easy to see that the time can be further reduced by increasing the number of electrode pairs to three or more. The minimum time conditions in this case can also be easily generalized, and the electrode conditions for the shortest heating time when N electrode pairs are used can be expressed by (Equation 17). The relationship between temperature and power in this case is shown in Figure 9.

That is, in the case where N electrode pairs are used to heat a liquid with an initial temperature T _min to a target temperature T _max , (T _max - _{T min} ) is equally divided into N, and each temperature range is designated as T ₁ , T ₂ , ..., T _N , one electrode pair is assigned to the temperature range T _i (1≦i≦N), and the conditions for the assigned electrode pair within that range to heat in the shortest time can be selected according to the method described in the first embodiment.

In the above examples, we have described a method for determining optimal electrode specifications, assuming that the liquid to be heated has a constant resistivity, although the value is arbitrary. However, optimal electrode specifications determined for a liquid with a specific resistivity value are not optimal electrode specifications for liquids with a different resistivity. In Example 3, we explain a method for determining the electrode configuration and specifications that can heat the liquid to be heated in roughly the shortest time within the expected range of electrical resistivity when there is a range of possible electrical resistivities.

In Example 3, the resistivity of the assumed liquid is the resistivity of tap water. The resistivity of tap water varies depending on the water quality, and even within Japan, for example, the value varies depending on the region. However, the resistivity at 25°C is generally in the range of 10 to 200 Ωm, so this range is assumed.

First, consider the case where there is one electrode pair. The initial temperature T _i =0°C, the target temperature T _f =90°C, the maximum voltage V _max =200V, and the maximum current I _max =15A are shown in FIG. 10, which shows the values of d/S that are optimal heating conditions for each value within the assumed range of resistivity. The values of parameters ρ ₀ and α in (Equation 1) that represent the temperature dependency of resistivity are also shown. FIG. 11 is a graph showing the relationship between the electrode spacing d and the length of one side of the electrode (√S) when the electrode shape is a square for each resistivity value based on the obtained d/S value. For example, when the electrode spacing d is 2 mm, the length of one side of the electrode is 2.9, 6.5, 9.2, 11.3, and 13 cm for resistivities of 10, 50, 100, 150, and 200 Ωm, respectively. Therefore, if these lengths are approximated as 3, 6, 9, 11, and 13 cm, respectively, an electrode consisting of 13 x 13 = 169 unit electrodes of 1 cm square can be prepared, which can provide an electrode that can heat water of a selected range of resistivity in approximately the shortest time. That is, the 1 cm square electrode and the power source are connected via a switch, and the switch is controlled so that the electrode dimensions are optimal according to the electrical resistivity. This makes it possible to provide an electrode configuration that can heat water in approximately the shortest time corresponding to the expected electrical resistivity. With this configuration, the total electrode area can be reduced compared to installing electrode pairs of each size. Note that instead of making both of the two electrodes that make up the electrode pair into unit electrodes, only one electrode can be made into a spread electrode, and the other electrode can be, in this example, an electrode of 13 x 13 cm square.

In this method, the optimal electrode pair size is determined by considering five resistivities in the expected range of 10 to 200 Ωm, so strictly speaking, the shortest time will not be obtained for resistivities outside of the five points. However, by selecting the closest resistivity from the five points, the delay from the shortest time can be suppressed to a level that does not pose a practical problem. As mentioned above, if the resistivity deviation is within about 30%, the time deviation can be suppressed to within approximately 5%. In addition, similar calculations are possible for resistance values other than the five selected points, so calculations can be performed for the resistivity most suitable for the liquid to be heated, and the switch can be controlled according to the optimal electrode pair size. This allows heating to be performed under conditions closer to the shortest heating conditions.

As an example, a range of resistivity was set here, and five representative values were chosen from that range for explanation. In addition, the distance between the electrodes was set to 2 mm, and the unit electrode was set to 1 cm square, but these values are all examples, and other values may be selected to configure the electrodes based on the above ideas.

Next, we will describe an example in which a rectangular electrode is used as a unit electrode shape other than a square. The basic idea is the same as that already described. For the value of d/S of the shortest heating condition shown in Figure 10, we need to consider values with different electrode lengths and heights. Here, we consider the length of one side of a rectangular electrode to be 3 cm as an example. Figure 12 shows a graph of the relationship between the distance between the electrodes and the length of the remaining side. If the electrode spacing is 2 mm, the optimal lengths for each resistivity are 2.84, 14.2, 28.4, 42.6, and 56.8 cm, respectively. In this case, since 2.84 cm is the greatest common denominator of the other values, a 2.48 cm x 3 cm electrode is used as the unit electrode, and 20 unit electrodes are prepared to cover the selected range of resistivity. Since the number of unit electrodes can be reduced compared to square electrodes, there is an advantage in that the number of switches and wiring length can be reduced. On the other hand, since the electrode area of the unit electrode is larger, there is a risk that the deviation from the shortest heating condition for water of the resistivity between the selected five representative resistivity values will be greater. In this case, instead of both of the two electrodes that make up the electrode pair being unit electrodes, only one of the electrodes can be a unit electrode, and the other electrode can be an electrode measuring 56.8 x 3 cm in this example.

Next, an example will be described in which the method of accelerating heating by switching the electrode pair according to the temperature range described in Example 2 is applied to the electrode configuration of Example 3. As in the previous example, the initial temperature T _i =0°C, the target temperature T _f =90°C, the maximum voltage V _max =200V, the maximum current I _max =15A, and the assumed range of resistivity of the liquid to be heated is also 10 to 200Ωm. Here, the electrode pair is switched once before the target temperature T _f =90°C is reached, and therefore the electrode pair switching temperature T _s =45°C. Figure 13 shows d/S that is the shortest heating condition for the two temperature ranges.

First, consider the case where the unit electrode shape is square. Figure 14A is a graph showing the relationship between the electrode spacing d and the length of one side of the square electrode (√S) for each resistivity when the liquid temperature is 45°C or less, based on the d/S value in Figure 13. Figure 14B is a similar graph for the temperature range of 45°C or more. If the electrode spacing d = 2 mm, the lengths of one side of the square electrode for resistivities of 10, 50, 100, 150, and 200 Ωm at 45°C or less are 4.01 (4), 8.97 (9), 12.7 (13), 15.5 (15), and 17.9 (18) cm, and at 45°C or more, they are 2.12 (2), 4.75 (5), 6.71 (7), 8.22 (8), and 9.49 (9) cm. The values in parentheses are approximate values. As above, if unit electrodes of 1 cm square are used, then by preparing 18 x 18 = 324 electrodes, it is possible to provide electrodes that meet the electrode specifications required for the entire range of resistivity from 10 to 200 Ωm, with a switching temperature of 45°C. By preparing electrodes of unit size, they can be used both above and below the electrode switching temperature, and as a result, the electrode area can be reduced compared to preparing electrodes for each temperature range.

Next, an example of a rectangular unit electrode shape will be described. One side of the rectangle is 3 cm. Figure 15A is a graph showing the relationship between the electrode spacing d and the length of the other side of the rectangular electrode for each resistivity when the liquid temperature is 45°C or less based on the d/S value in Figure 13. Figure 15B is a similar graph for a temperature range of 45°C or more. If the electrode spacing d = 2 mm, the length of the other side of the rectangle for resistivities of 10, 50, 100, 150, and 200 Ωm is 5.37, 26.8, 53.7, 80.5, and 107 cm below 45°C, and 1.50, 7.51, 15.0, 22.5, and 30.0 cm above 45°C. Since the minimum electrode length is the greatest common denominator of the other electrode lengths, 20 unit electrodes each of 5.37 cm x 3 cm and 1.50 cm x 3 cm should be prepared. In this case, the number of electrode pairs can be reduced compared to square electrodes, which has the advantage of reducing the amount of wiring and mounting fixtures required. On the other hand, there is a risk that the rate of deviation from the heating conditions for the shortest time will be large for liquids that exhibit resistivity values other than those shown as examples.

Above, we have explained the case where electrodes are switched during heating, but even in this case, the dimensions used in the explanation are merely examples and are not limited to those values.

In the fourth embodiment, a method of switching the electrode pairs during heating in the liquid heating device described in the second and third embodiments will be described. In the following description, the electrode pairs themselves will be switched, and the electrode area of the electrode pairs will be switched. FIG. 16A shows the state of power change when heating a liquid from an initial temperature T _min to a target temperature T _max in the shortest time using N electrode pairs. The heating start time is t ₀ =0, and the time to reach the target temperature T _max is t _N. It is assumed that the N electrode pairs used satisfy the shortest heating conditions under given conditions. Since N electrode pairs are used, the point at which the electrodes are switched is the temperature that is the boundary of the region that divides the initial temperature T _min and the target temperature T _max into N equal parts. In FIG. 16A, this corresponds to the N-1 boundary temperatures indicated by the liquid temperatures T _i ' (i = 1 to N-1). The initial temperature T _min is measured, and the target temperature T _max is input to the heating system as a set value, so that the switching temperature T _i ' can be calculated. Then, the liquid temperature is measured, the necessary electrode pair is selected, and the specified power is applied. The liquid temperature is then continuously measured, and when the liquid temperature reaches each switching temperature T _i ', the electrode pair is switched to continue heating, and when the target temperature T _max is reached, the application of power is stopped. According to this method, the electrode switching point is determined in advance from the target temperature setting and temperature measurement information, and the electrode pair switching point can be known based on the liquid temperature measurement value.

As another method, the electrode pair can be switched based on time. If the initial temperature T _min and the target temperature T _max are determined, the required amount of power can be calculated. Furthermore, if the resistivity of the liquid to be heated is determined, the power input pattern under the shortest heating conditions in each region obtained by dividing T _min and T _max into N equal parts is determined, so the switching time can also be calculated. Specifically, the interval (t _i -t _i-1 ) between each switching time is equal in each of the N equal parts. That is, as shown in FIG. 16A, the switching time t _i is synchronized with the switching temperature T _i which is the switching point when switching by temperature. Therefore, heating can be performed in the shortest time by switching the electrode pair at this constant time interval. In the case of switching based on temperature measurement, it is assumed that the temperature measurement is performed appropriately, that is, there is no unevenness in the liquid temperature, and if the liquid temperature is uneven, there is a possibility that the electrodes will be switched at an undesirable timing. In contrast, according to the switching method based on time, the electrode pair can be switched based on the time required to input the required power.

Next, a method of switching based on power will be described. As already mentioned, when heating in the shortest time, the condition was that the power input pattern be symmetrical with respect to the intermediate temperature ((T _min +T _max )/2) between the initial temperature T _min and the target temperature T _max . Even when the initial temperature T _min and the target temperature T _max are divided into N equal parts, the shortest heating condition is to make the power input pattern symmetrical with respect to the intermediate temperature of each of the N equal parts. For this reason, if the input power is measured and the measured power value for a selected electrode pair matches the input start power, it indicates that it is the timing to switch the electrode pair. As shown in FIG. 16A, the initial power value P _c for each of the N divided regions is constant. Therefore, the supply of power to a certain electrode pair is started, and when the supplied power value changes from increasing to decreasing and becomes equal to the initial power value P _c , the electrode pair to which power is input may be switched to the next electrode pair. This method also has the advantage of not being affected by the unevenness of the liquid temperature. On the other hand, if there is a design error in the electrode area or spacing, or if the resistivity of the liquid to be heated deviates from the expected value, the symmetry around the intermediate temperature _Tm will be shifted, and the timing of switching may deviate from the optimal timing. To address this, it is possible to set an allowable error and switch when the power value that would normally switch falls within the allowable error range.

Finally, a method of switching based on the amount of power will be described. The previously described switching method based on time was based on the fact that the amount of power required for heating is equal in each region obtained by dividing the initial temperature T _min and the target temperature T _max into N equal parts. Therefore, switching for heating in the shortest time is possible even by directly measuring the amount of power. FIG. 16B shows a graph in which the vertical axis of FIG. 16A is not power but power, i.e., energy. Since the amount of power is an integrated value of power, FIG. 16B is an integrated graph of FIG. 16A. Since the temperature rise in each of the N equal parts is equal, the power input is also equal. Therefore, the required amount of power can be calculated from the target temperature T _max , the initial temperature T _min , the amount of water (or the flow rate per unit time in some cases), and the number of divisions N. All that is left to do is to measure the power, calculate the amount of power from it, and switch the electrode pair when the amount of power reaches a predetermined amount. That is, heating is started from the energy of 0 J on the vertical axis of FIG. 16B, and when the required energy E ₁ is input, the electrode pair is switched, and when the energy reaches E ₂ , the electrode pair is switched again, and this cycle is continued until the energy reaches E _max . Here, (E _i -E _i-1 ) are all equal values, and if this value is ΔE, then E _i = i × ΔE (i = 1 to N). Another advantage of this method is that it is not necessary to worry about non-uniformity in the liquid temperature. In addition, even if there is an error in the electrode area or the distance between the electrodes, or if the resistivity of the liquid to be heated deviates from the expected value, there is also an advantage that the switching can be performed at the correct switching point that takes these effects into consideration without being affected by them.

Above, we have shown several methods for switching electrode pairs or the electrode area of electrode pairs, but in practice it is desirable to check the timing using multiple methods rather than choosing one method from among these. This allows for redundancy against malfunctions. Also, by looking at the methods in a combined manner, it is possible to detect device failures based on whether the timing of each method matches or does not match.

In the fifth embodiment, a water server is used as an example to show an example of a method for configuring a hot water supply system. FIG. 17A shows an example of the appearance of a water server. The front of the housing 1701 is provided with a display unit 1702, an input unit 1703, a hot water outlet 1704, and the like. The opening 1706 is provided with a cup holder for placing a cup 1705 that receives hot water from the hot water outlet 1704. Water is supplied to the server from a water supply mechanism 1707, and power is supplied from a power supply mechanism 1708. The display unit 1702 may be capable of displaying messages, such as a liquid crystal display, or may be capable of conveying messages by a pattern of lights turning on and off. The input unit 1703 is for specifying conditions such as the temperature and amount of hot water to be supplied, and for instructing the start of hot water supply, and may be a button type or a touch panel. It may be provided with a wireless input/output mechanism, and may be a method of instructing using a smartphone, etc. When a touch panel or a smartphone is used, the display unit 1702 and the input unit 1703 may be integrated. The water supply mechanism 1707 is connected to a water pipe or a tank. The power supply may be connected to the power supply mechanism 1708 via a socket or a circuit breaker.

An example of a functional block diagram of a water server is shown in FIG. 17B. The thick solid lines indicate the water path, the thin solid lines indicate the electrical path, and the dotted lines indicate the signal line. Water is supplied to the water server from a water supply mechanism 1720. The water supply mechanism 1720 is a water pipe or a tank for storing drinking water. The control unit 1730 reads information from each part of the water server and issues instructions to each part via a signal line. The control unit 1730 is equipped with a calculation processing mechanism and a memory mechanism to perform calculations using the read information and data stored in the device and to determine the contents of the instructions. The flow detection/control unit 1721 is a pressure gauge, a flow meter, or in some cases a pressure regulator. The first valve 1722 and the second valve 1724 open and close the water channel according to the command of the control unit 1730. The heating unit 1723 heats the liquid by direct resistance heating. The control unit 1730 can adjust the amount of liquid supplied to the heating unit 1723 by the flow rate detection/control unit 1721 and the first valve 1722. If the flow rate detection/control unit 1721 is a pressure gauge or a flow meter, the time for opening the first valve 1722 is adjusted according to the value. If the pressure gauge has a pressure adjustment mechanism or the flow meter has a variable flow rate, the time for opening the first valve 1722 can be made constant by setting them to appropriate values. The heating unit 1723 heats water according to given conditions. It is also desirable for the heating unit 1723 to be equipped with a thermometer for measuring the water temperature. The read water temperature is passed to the control unit 1730 as water temperature information. After heating is completed, hot water is supplied by opening the second valve 1724. There are several methods for supplying hot water, such as pumping hot water, pushing it out by pressure, and using gravity to allow the liquid to flow out naturally.

The power from the power source 1731 supplied to the water server is supplied to the heating unit 1723 via the power adjustment unit 1732 and the electrode selection unit 1733. The maximum voltage _Vmax and the maximum current _Imax are basically the maximum capacity that the power source 1731 can supply. For example, when power is supplied from a general household outlet, the maximum voltage _Vmax is 100V or 200V, and the maximum current _Imax is generally 15A, depending on the circuit breaker capacity of the distribution board. The control unit 1730 issues a command to the power adjustment unit 1732 to supply the required power, and also reads the supply power measured by the power adjustment unit 1732. The simplest configuration of the power adjustment unit 1732 is a switch that switches between supplying and cutting off the power. When power is supplied from an outlet or a circuit breaker, the power adjustment unit 1732 supplies a constant voltage to the heating unit 1723, which is the load, and a current corresponding to the size of the load flows. Other inexpensive power adjustment mechanisms include the so-called phase control method and cycle control (zero-cross control) method, which use power semiconductors such as thyristors. Compared to control using a simple switch, these methods have the advantage of making it easier to adjust the power. For example, if power usage in other devices increases, it is possible to control the power usage of the water server to keep the total power usage within a certain value. This feature is necessary when the water server is used in combination with other devices. It is also possible to control the power supply by intermittently switching the switch, but this has disadvantages such as the effect of noise on other devices and accelerated deterioration of the equipment due to surges.

It is desirable that the power adjustment unit 1732 can measure the voltage V applied to the heating unit 1723 and the current I flowing. The measured value of the power input by the power adjustment unit 1732 can be calculated from these values. In addition, if the voltage V and the current I can be measured, the resistivity of the liquid to be heated can be obtained. Since the area of the electrodes to be used can all be known in advance, the resistivity can be calculated from the current flowing when one electrode pair is selected from them and a predetermined voltage is applied, and the water temperature measured by the heating unit 1723. If the resistivity is obtained, specifications such as the size of the electrode pair according to the heating conditions are determined as described above, so there is an advantage that it is easier to handle a wide range of water qualities. On the other hand, with regard to water quality, since the range of resistivity is generally limited by region, the range of resistivity that can be handled may be limited according to the region where the device is installed. In addition, the water quality of city water may be measured before installation, and the range of handling may be customized for each device. In these cases, since the resistivity that needs to be handled is limited, the area of the electrodes attached to the heating unit 1723 can be limited, and the total area of the electrodes can be reduced.

Figure 17C shows the electrical configuration related to the heating unit 1723. Power supplied from a power source 1731 via an outlet, a circuit breaker, etc. is fed to an electrode selection unit 1733 via a power adjustment unit 1732. The electrode selection unit 1733 switches the electrodes according to instructions from a control unit 1730 according to the heating conditions of the liquid. Here, an example is shown in which the heating unit 1723 is composed of two electrode pairs (a first electrode pair 1711 and a second electrode pair 1712). Figure 17C shows an example in which the first electrode pair 1711 is selected. The switch that constitutes the electrode selection unit 1733 may be a mechanical switch or an electronic switch such as a photocoupler. Since power sources often have a large capacity, it is sufficient to select a switch that corresponds to that capacity.

FIG. 17D shows a timing chart of each signal. Assume that a request for hot water supply is made from the input unit 1703 at time _t1 . Based on this request, at time _t2, the display unit 1702 is commanded to display a message prompting input of the amount of hot water supply and the hot water supply temperature. After that, the hot water supply conditions inputted by the input unit 1703 are read, and the first valve 1722 is opened for the necessary time (between time _t3 and time _t4 ) to supply the specified amount of water to the heating unit 1723. The control unit 1730 reads the initial temperature of the water measured by the heating unit 1723, and determines the electrode switching pattern in combination with the specified hot water supply temperature. As explained in Example 4, the electrode switching can be determined not only based on the water temperature, but also based on time, power, power amount, etc. The electrode selection pattern, which is to energize which electrode and at what timing, is determined at this stage. In this example, there are two electrode pairs, and it is assumed that switching is performed once.

Fig. 17E shows the change over time in power application in this case, and Fig. 17F conceptually shows the change in temperature of the water in the heating unit 1723. When switching electrodes based on the water temperature, the control unit 1730 calculates an intermediate temperature _Tmid ( _Tmid = ( _Tmin + _Tmax )/2) between the measured initial temperature _Tmin and the instructed hot water temperature _Tmax , and compares this with temperature information measured by the heating unit 1723 during heating to determine the switching timing of the electrode selection unit 1733. When the water temperature reaches the intermediate temperature _Tmid , which is time _t6 on the timing chart, the electrode pair is switched.

When switching according to time, the timing of electrode switching is determined by adding the energy used to heat the water from the initial temperature _Tmin , hot water supply temperature _Tmax , and amount of hot water supply, and the energy not used to heat the water due to heat dissipation specific to the device, and calculating the time required for the device to input that energy from the value. The heat dissipation specific to the device is, so to speak, the efficiency of the device, and is determined by the design and configuration of the device, so it is a parameter that is determined in advance.

When switching electrodes by power, the power value when power application starts, i.e., the value of power P _i at time t ₅ in Fig. 17E, is stored, and the electrode pair is switched when the applied power value becomes P _i again, i.e., at time t ₆ in the figure. In this case, it is not necessary to determine a switching pattern in advance, and the electrode pair can be switched simply by monitoring the power value read by the control unit 1730 from the power adjustment unit 1732.

When determining the electrode switching pattern based on the amount of power, the control unit 1730 calculates the amount of power by integrating the power values read from the power adjustment unit 1732, and switches the electrode pairs based on this value. As in the case of time switching, the energy required for heating is calculated, and the electrode pairs are switched when the amount of power, i.e., the energy value, reaches half the value of the required energy.

The configuration example in Figure 17C is an example of a simple combination in which two electrode pairs are switched with one switch. As explained in Examples 2 and 3, when one electrode of an electrode pair is used in combination with multiple small electrodes, one switch can be connected to each small electrode, and the small electrodes can be switched ON and OFF.

According to the method and timing for switching the electrode pairs, the control unit 1730 outputs an electrode selection signal to the electrode selection unit 1733 and a power adjustment signal to the power adjustment unit 1732 to start applying power to the heating unit 1723. In the timing chart of FIG. 17D, power application starts at time _t5 . At the same time, a display signal for a message informing the display unit 1702 that heating is in progress is sent to the display unit 1702. After that, the electrode pair is switched at a predetermined stage. In this example, an electrode selection signal is sent to the electrode selection unit 1733 at time _t6 to select the second electrode pair 1712. After completing the supply of power according to a predetermined pattern, it stops. At this stage (time _t7 ), hot water of the desired temperature is prepared in the heating unit 1723. At time _t8 , a message is displayed on the display unit 1702 indicating that heating has been completed, and at the same time, the second valve 1724 is opened to supply hot water. After the hot water supply is completed, at time _t10 , the control unit 1730 outputs a signal to the display unit 1702 to instruct the display unit 1702 to display a completion message, and the message is displayed for a certain period of time, and the series of operations is completed.

In Example 6, an example is shown in which a hot water supply function using direct resistance heating technology is provided by combining it with another device. Here, a refrigerator is taken as an example of another device, and a refrigerator with a water server function is described. An example of the external appearance of the device is shown in FIG. 18. The refrigerator is provided with three refrigerator compartments 1802, 1803, and 1804 in a housing 1801. The refrigerator is further provided with a water server section 1805. The water server section 1805 is provided with an opening 1807 for receiving water supply, and a heating section 1808 using direct resistance heating inside. A heat insulating layer 1809 is provided to insulate the heating section 1808 and the path through which the hot water passes from the refrigerator compartment. The refrigerator is provided with a display/input section 1806 with a touch panel for display and input. In this example, it is used as the display/input section of the refrigerator.

Figures 19A and 19B show functional block diagrams of a refrigerator with a water server function. A refrigerator with a water server function 1901 is composed of a refrigeration unit 1902 that provides the refrigerator function and a water server unit 1903 that provides the water server function. Note that the configuration shown here is an example, and is not limited to this. The functional blocks of the refrigeration unit 1902 also have different configurations depending on the functions of the refrigerator. Furthermore, the functions of each unit are merely representative, and are not limited to this. For example, the door monitor does not have a control unit or a signal conversion unit, but the door monitor configuration itself includes a control mechanism. In contrast, it is possible to configure the door to be opened and closed as an ON/OFF switch, and the central control unit to read that information directly. Regarding power supply, the voltage and current capacity supplied by the power source of the motor power system such as the compressor and fan and the power source of the control system composed of electronic components are different, but these differences are not distinguished here.

The water server function can be roughly divided into two configurations depending on how the control unit that controls it is arranged. FIG. 19A shows an example of a configuration in which the control unit 1904 is arranged inside the water server unit 1903a, and is a configuration in which the water server configuration shown in FIG. 17B is added to the refrigerator as is. Communication with the refrigeration unit 1902 is limited to matters related to display and input, available power capacity and power supply, and interrupt signals to suppress power usage on the water server side when a large amount of power is used for a function on the refrigeration unit side. Therefore, most of the water server's operations can be controlled by confining them to the water server unit 1903a. This makes it relatively easy to add a water server function to an existing refrigerator. It is also relatively easy to add it to functions other than the refrigerator.

In contrast, FIG. 19B shows a configuration in which the functions of the control unit 1904 in the water server unit 1903a are integrated into the control unit (central control unit 1905) of the refrigeration unit 1902, and the water server unit 1903b does not have the control unit functions. In this configuration, the control for driving the water server is also performed by the central control unit 1905. In this case, since the water server unit 1903b and the refrigeration unit 1902 are controlled by a single control unit, the operations of both units can be integrated and controlled, making it easier to perform fine control. Also, since a control unit is not required for the water server unit 1903b, the number of parts can be reduced.

First, the flow of hot water supply in the configuration shown in Figure 19A will be briefly explained. When a request for hot water supply is made from the display and input function of the refrigerator, the central control unit 1905 notifies the request to the control unit 1904 of the water server unit 1903a. On receiving the request, the control unit 1904 requests the central control unit 1905 to display a message prompting the input of information necessary for heating, such as the amount of water and the hot water temperature. The central control unit 1905 displays this message and transmits the input information together with the available power estimated in consideration of the operating status of the refrigeration unit 1902 to the control unit 1904. The control unit 1904 supplies hot water using the method explained in Example 4 based on this information. Here, it has been written that after receiving a hot water supply request, some information is exchanged between the control unit 1904 and the central control unit 1905, and then the hot water supply conditions and available power are transmitted. However, since this is a standard procedure, the central control unit 1905 may independently display and input messages in response to the hot water supply request, and after collecting the necessary information, provide the information to the control unit 1904 all at once.

It is assumed that the power supply for this refrigerator with water server function is also supplied from an outlet. In this case, the maximum voltage V _max supplied to this device is 100V or 200V, and the maximum current I _max is also normally 15A. While this refrigerator is operating, the current consumption is measured, and when the water server unit 1903a is operated, the current value obtained by subtracting the current value used immediately before from the maximum current I _max (15A) is transmitted to the control unit 1904 of the water server unit 1903a as I _max (here, this is expressed as I _max ' to distinguish it). The water server unit controls the power supply capacity to have the maximum voltage V _max = 100V (or 200V) and the maximum current I _max = I _max ', so as to adjust the power in the power adjustment unit. If there are no continuing events occurring that would lead to an increase in temperature inside the cabinet, such as the opening and closing of the door, it is also possible to control the cooling function of the refrigeration unit 1902 to temporarily stop and increase the amount of power supplied to the water server unit 1903a as much as possible to shorten the heating time.

The control of hot water supply in the configuration of FIG. 19B is the same as in the configuration of FIG. 19B, except that the central control unit 1905 performs all the roles previously handled by the control unit 1904 of the water server unit 1903a.

The above is an example of control when no other event occurs while water server unit 1903 is heating water. However, in reality, it is considered that there are frequent cases where a sudden event also occurs in refrigeration unit 1902 while water server unit 1903 is consuming a large amount of power during heating operation, and the event consumes a large amount of power, making it impossible to supply power with the power supply capacity of maximum current I _max = I _max '. When such a sudden interrupt event that consumes a large amount of power occurs, the following is the procedure to be followed.

When defrosting, which is a typical example of a high power consumption event in the refrigeration unit 1902, overlaps with hot water supply in the water server unit 1903, it is possible to determine priorities in advance and control, for example, to interrupt defrosting and concentrate power on hot water supply. Here, as an example requiring more complex control, we will look at a case where the door is opened and closed as an interrupt process when the refrigerator is full of items and input is high.

Figures 20A-D show timing charts for events such as the opening and closing of the refrigerator door, the heating operation of the water server unit, and the associated cooling of the refrigerator compartment. Figure 20A shows an example in which interrupt processing is not required, in which no interrupt event (cooling operation associated with opening and closing of the door) occurs during heating in the water server unit. When the heating event of the water server unit ends, the refrigerator compartment adjacent to the heating unit is cooled. In the figure, this operation transition 2001 is indicated by an arrow. Since direct resistance heating directly heats the liquid, there is no heat transfer from the high-temperature heating element after hot water is supplied. However, the electrodes for passing electricity through the liquid must be immersed in the liquid, and the temperature of the electrodes at the immersed parts will be about the same as the heating temperature. For this reason, a small amount of heat will dissipate from the electrodes, so a heat insulating layer 1809 (see Figure 18) is placed between the heating unit and the refrigerator compartment, and the refrigerator compartment is cooled.

FIG. 20B shows a case where a refrigerator door opening event occurs at the timing indicated by time _t21 while the heater is operating. In this example, the heating operation ends at time _t22 before the door closing event occurs. In this case, first, the refrigerator compartment next to the heating unit is cooled in response to the end of the heating operation of the water server unit (operation transition 2002). A door closing event occurs while the cooling operation continues, and in response to this, the cooling operation of the refrigerator compartment whose door was open is started (operation transition 2003). In the figure, the timing chart is shown without distinguishing between the refrigerator compartment next to the heating unit and the refrigerator compartment where the door opening event occurred, but the two refrigerator compartments may be the same or different. The interruption operation from the refrigerator unit to the hot water supply unit is the same, and there is no difference in the operation timing of the event, so no distinction will be made hereafter.

20C shows a case where a door open event occurs at time _t23 while the water server unit is in a heating operation, and a door close event also occurs at time _t24 before the heating operation ends. When the door close event occurs, the target refrigerator compartment needs to be cooled, so the refrigerator unit 1902 attempts to start cooling, but since the water server unit 1903 is in operation, it does not perform the cooling operation and goes into a waiting state for cooling. After that, in response to the end of the hot water supply event at time _t25 , an operation transition 2004 occurs, and the target refrigerator compartment and the cooling compartment next to the heating unit are cooled. Note that cooling may be prioritized, hot water supply may be put into a waiting state, and hot water supply may be resumed after cooling is completed.

20D shows a case where a request for heating operation is made to the water server unit at time _t26 during a door opening event. Since the door opening event is in progress at time _t26 , the heating operation does not start even if a heating request is received, and the system goes into a waiting state. The door then closes, and cooling of the refrigerator compartment starts in response (operation transition 2005). In response to the end of cooling (operation transition 2006), heating operation starts at time _t27 . In reality, a hot water supply request is also made using display and input unit 1806 on the front of the refrigerator, so it is believed that a door opening event will rarely occur at the same time.

The present invention has been described above based on the embodiments. In the above embodiments, the maximum voltage _Vmax is 100 V or 200 V, but the present invention is not limited to these and any commercial AC power source used in any country or region can be used in the same manner as long as the commercial AC power source is used.

The above-mentioned embodiments have been described in detail to clearly explain the present invention, and are not necessarily limited to those having all of the configurations described. It is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. Furthermore, it is possible to add, delete, or replace part of the configuration of each embodiment with the same configuration or another configuration.

11, 12...area, 13...voltage, 14...current, 15...power, 41...container, 42...resistance, 43...electrode pair, 44...power source, 45...wiring, 71, 72...electrode pair, 73...switch, 81, 82, 83, 84...waveform, 1701...housing, 1702...display unit, 1703...input unit, 1704...hot water outlet, 1705...cup, 1706...opening, 1707...water supply mechanism, 1708...power supply mechanism, 1711...first electrode pair, 1712...second electrode pair, 1720...water supply mechanism, 1721...flow rate detection/control unit, 1722...first valve, 1723...heating unit, 1724...second valve, 1730...control unit, 1731...power source, 1732...power adjustment section, 1733...electrode selection section, 1801...casing, 1802, 1803, 1804...refrigerator compartment, 1805...water server section, 1806...display/input section, 1807...opening, 1808...heating section, 1809...insulating layer, 1901...refrigerator with water server function, 1902...refrigeration unit, 1903...water server unit, 1904...control unit, 1905...central control unit, 2001, 2002, 2003, 2004, 2005, 2006...operation transition.

Claims (12)

A liquid heating device for heating a liquid having an initial temperature T _i to a target temperature T _f , comprising:
A container;
an electrode pair disposed within the vessel;
an AC power source that applies a current to the liquid via the electrode pair immersed in the liquid;
The AC power source performs a constant current operation at a maximum current I _max or a constant voltage operation at a maximum voltage V _max depending on the resistance of the liquid,
When the inter-electrode distance of the electrode pair is d, the electrode area through which the electrode pair passes current through the liquid is S, and the electrical resistivity that the liquid can have at an intermediate temperature _Tm (( _Ti + _Tf )/2) between the initial temperature _Ti and the target temperature _Tf is ρ( _Tm ), the size of the electrode pair is expressed as the ideal values of the inter-electrode distance and electrode area of the electrode pair that satisfy (Equation 1):

A liquid heating device in which the error in the inter-electrode distance and the error in the electrode area of the electrode pair from the ideal values are each 30% or less . In claim 1,
The container has N (N≧2) electrode pairs,
The liquid heating device heats the liquid from a minimum temperature T _min to a maximum temperature T _max ;
The range from the minimum temperature T _min to the maximum temperature T _max is divided into N sections, and N electrode pairs are assigned to each section,
A liquid heating device in which, when the initial temperature _Ti is the initial temperature of the section assigned to the electrode pair, and the target temperature _Tf is the target temperature of the section assigned to the electrode pair, the size of each of the N electrode pairs is such that the error in the inter-electrode distance and the error in the electrode area from the ideal values of the electrode pair are each 30% or less . In claim 2 ,
A liquid heating device, wherein a temperature difference between an initial temperature and a target temperature in the section assigned to the electrode pair is (T _max - T _min )/N. In claim 2 ,
A control unit that switches the N electrode pairs according to the N sections,
The control unit is a liquid heating device that switches the electrode pairs based on at least one of the temperature of the liquid, the heating time of the liquid, the power input by the AC power source to the liquid, and the amount of power input by the AC power source to the liquid. In claim 1,
The electrode pair is capable of switching the electrode area,
Regarding the electrical resistivities of the plurality of liquids, the size of the electrode pair is set such that an error in the inter-electrode distance and an error in the electrode area from the ideal value of the electrode pair are each 30% or less ,
A liquid heating device in which the electrode area of the electrode pair is switched to an electrode area determined for a possible electrical resistivity of the liquid that is closest to the electrical resistivity of the liquid. In claim 1,
The electrode pair is capable of switching the electrode area,
The liquid heating device heats the liquid from a minimum temperature T _min to a maximum temperature T _max ;
The range from the minimum temperature T _min to the maximum temperature T _max is divided into N (N≧2) sections, and the electrode areas of the electrode pairs are switched according to the N sections;
A liquid heating device in which, when the initial temperature _Ti is the initial temperature for each of the N sections and the target temperature _Tf is the target temperature for each of the N sections, in each of the N sections, the size of the electrode pair is such that the error in the inter-electrode distance and the error in the electrode area from the ideal values of the electrode pair are each 30% or less . In claim 6 ,
A temperature difference between the initial temperature and the target temperature in each of the N sections is (T _max - T _min )/N. In claim 6 ,
a control unit that switches the electrode areas of the electrode pairs according to the N sections;
The control unit is a liquid heating device that switches the electrode area of the electrode pair based on at least one of the temperature of the liquid, the heating time of the liquid, the power input by the AC power source to the liquid, and the amount of power input by the AC power source to the liquid. a refrigeration unit having a plurality of refrigeration compartments;
A water server unit for heating the liquid;
an AC power supply that supplies power to the refrigeration unit and the water server unit and performs a constant current operation at a maximum current _Imax or a constant voltage operation at a maximum voltage _Vmax according to a load;
The water server unit includes:
a heating section including one or more electrode pairs for applying electrical current to the liquid;
an electrode selection unit for switching the electrode pair or an electrode area through which the electrode pair passes a current to the liquid;
a power adjustment unit that supplies power of a predetermined power capacity to the heating unit from the AC power source,
The water server unit heats the liquid from a minimum temperature T _min to a maximum temperature T _max ;
When the range from the minimum temperature T _min to the maximum temperature T _max is divided into N (N≧2) sections, and for an electrode pair switched by the electrode selection unit to heat the liquid in each of the N sections, the inter-electrode distance of the electrode pair is d, the electrode area through which the electrode pair passes current to the liquid is S, the electrical resistivity that the liquid can have at an intermediate temperature T _m ((T _i +T _f )/2) between the initial temperature T _i of the section and the target temperature T _f of the section is ρ(T _m ), and the maximum current that the power adjustment unit supplies to the heating unit is I _max ', the size of the electrode pair is the ideal value when the inter-electrode distance and electrode area of the electrode pair that satisfy (Equation 2) are:

The refrigerator , wherein the error in the inter-electrode distance and the error in the electrode area of the electrode pair from the ideal values are each 30% or less . In claim 9 ,
The temperature difference between the initial temperature and the target temperature in each of the N sections is (T _max - T _min )/N. In claim 9 ,
A control unit for controlling the electrode selection unit,
The control unit switches the electrode pair or the electrode area based on at least one of the temperature of the liquid, the heating time of the liquid, the power input to the liquid by the AC power source, and the amount of power input to the liquid by the AC power source. In claim 9 ,
A heat insulating layer is provided between the heating unit of the water server unit and a refrigerator compartment adjacent to the heating unit,
After the water server unit heats the liquid, the refrigeration unit cools the refrigeration compartment adjacent to the heating unit.

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