WO1989001729A1

WO1989001729A1 - Time delay circuit for heating appliances

Info

Publication number: WO1989001729A1
Application number: PCT/AU1988/000294
Authority: WO
Inventors: Douglas John Slack; Brendan Vincent Bourke
Original assignee: Rheem Australia Pty Ltd; Texas Instruments Australia Pty Ltd
Current assignee: Rheem Australia Pty Ltd; Texas Instruments Australia Pty Ltd
Priority date: 1987-08-07
Filing date: 1988-08-08
Publication date: 1989-02-23
Anticipated expiration: 1990-02-07

Abstract

Delayed reheat of off-peak electric heated storage appliances (12) such as water heaters, being aimed at more uniform demand on electrical generation capacity: time delay means (10) creates delay sympathetic to actual depletion of stored thermal energy at commencement of off-peak period and includes thermal energy sensor (14), bridge circuit (16), relaxation oscillator (18), multi-stage binary counter (20) and relay (22) which energises the heating cycle of appliance (12) when the maximum count of counter (20) has been reached. The sensor (14) is arranged to provide an output proportional to the thermal energy content of the reservoir (24). This ouptput controls the oscillator (18) and thereby provides a corresponding time delay during the off-peak period which is such that it permits, in the remainder of the off-peak period, the required thermal energy replenishment of the reservoir (24).

Description

TIME DELAY CIRCUIT FOR HEATING APPLIANCES

BACKGROUND

This invention relates to electrically powered appliances, of the types which are intended for operation, either partly or fully, using time-of-day controlled or interrupted energy supply. Such appliances are commonly termed "off-peak" types, because they are switched into connection with a community electricity distribution system only at times outside the periods of other peak demand on the system. These off-peak appliances absorb energy during the off-peak period, storing it in the form of heat for later use as required.

The most familiar examples include storage type hot water systems and space heaters of the type which store heat in large masses of cast iron or magnesite bricks for later room heating. The power rating of the heating elements in these appliances is intended to be sufficient to enable the heat storage medium in the appliance to be heated from its pre-existing temperature to the required maximum within one allocated off-peak period, which typically is of the order of eight hours. High power ratings apply to these elements, 3.6 or 4.8kW being common, in the case of water heaters and 6 k in space heaters.^'

One disadvantage of existing arrangements is experienced by electricity supply utilities and follows from the fact that there tends to be a significant fall in the total load from the community in the period 3 to 6 am each morning followed by a sudden increase from 6 to 8 am. Under these circumstances matching of generation capacity to load poses practical difficulties.

The fact that appreciable load in the period 10 pm to 6 am is comprised of off-peak water heating makes the 3 to 6 am demand fall-off not as marked as it would otherwise be, however there is a benefit to be gained if further reduction in the swings in load experienced can be realised. The present invention is intended to provide one means effective in achieving that end.

The present invention is aimed at assisting the matching of the off-peak load to generation capacity in the distribution system during and immediately following the off-peak period. Under existing arrangements the connected heating elements in the each off-peak appliance "population" all draw full power immediately supply is switched to them, regardless of whether this is necessary to enable any given individual appliance to achieve full replenishment of stored energy during the allocated off-peak period.

The alleviation of abrupt surges of demand on the electrical distribution system is thought to be conducive in reducing the instances of significant variations of the electrical parameters of the system (e g voltage drop and frequency "spikes"). The present invention provides a means to lessen this potential problem by tending to reduce the difference between the system load swing between 3 am and around 8 am on a typical heavy demand morning.

Various approaches tending to counteract the undesirable effects of excessive and/or sudden load changes on the electrical supply network are known but are not free of disadvantages. These approaches include installation of defences against voltage and frequency changes at the user's premises. Another approach is segmentation of the network into districts, coupled with the staggering of switching time district by district. The former approaches include "filter" or shut off devices to prevent frequency or voltage aberrations from damaging vulnerable equipment. The latter approach includes dividing the network up into sub-systems which may be separately triggered by a high frequency (typically 750 kHz) "pulse injections", which enables switching on off-peak devices by districts by use of a pulse sensitive switch fitted at each consumer's premises. The latter approach is referred to as "distributed" load control.

The former approach incurs costs without removing the problem, rather it seeks to protect against its effects. The latter approach tends to be not a universal choice in that it is very expensive to retrofit in established networks. The approach also has shortcomings because in practice it is necessary to stagger the switching of the separate districts to the extent that all supply districts can not always be suppl ied within the night period duri ng the preferred low demand time . Thi s can lead to off-peak power bei ng made avail able i n some di stri cts as late as 10 am, i n order that al l consumers woul d have been provided with their ful l off-peak time entitl ement. In thi s si tuati on certai n types of users would be enabled to qui ckly deplete thei r entire reservoi r and cause renewed l oad on the di stribution system vi a the off-peak demand route duri ng the already heavy morni ng peak demand period. Thi s i s contrary to the purpose of maki ng preferenti al ly pri ced off-peak tarri ffs avai lable to el ectri city consumers.

The present i nvention seeks to provide an option to assist i n overcomi ng these and other di sadvantages and accordi ngly proposes new and useful alternati ves to earl ier proposal s.

The present invention provides a sensor capable of sensing average thermal energy content σf a partly depleted thermal energy reservoir used in conjunction with a time delay circuit. When the invention is used in a significant .number of off-peak appliances in a community, the diversity of off-peak energy consumption within the community when considered over an extended period provides practical advantages in load smoothing. It is normal practice, for example, to install a hot water supply of the off-peak storage type to premises having the capacity to meet the highest anticipated consumption of hot water for those premises in any one day. It can readily be appreciated that many reasons result in variations between minimum and maximum daily consumption such as occupancy, weather and seasonal changes.

In this spec fication the term thermal energy is used in a broad sense. When it is stated in the specification that a sensor capable of sensing average thermal energy content it is strictly more correct to say that the sensor integrates the temperature profile of the heated storage vessel or body and thereby implicitly provides a means to relate the output of the sensor to the thermal energy content of the heated storage vessel or body. This assumes that the specific heat of the medium occupying the hot body and its mass are known quantities if in fact it is thermal energy per se (referred to a datum temperature) which is required. Otherwise it is a mean temperature from which a thermal energy content is linearly related which is available by the use of the sensor type as discussed in relation to the invention. The utility of the invention is unaffected by the actual definition of the quantity actually measured whether it be thermal energy, mean temperature, average temperature, heat content or some other related expression. For the purpose of characterising the invention the term thermal energy is adopted. A further interpretation which could be put upon the sensor function is that of providing an output from which, in a given class of installation, the quantity of time required to replenish the body to its intended or design full capacity thermal energy content is derivable.

SUMMARY

The invention consists of a time delay means for a heating appΗance comprising an electrical circuit used in conjunction with a heat energy storage reservoir having electrical heating means of a known power rating, in which the thermal energy capacity of the reservoir is replenishable daily during an allocated off-peak time interval of standardised duration, under an initiating on-off control exercised by an external _time-of-day controlled switching means separate from the said circuit characterised by a subservient control in which a sensor is arranged to provide an output related by a known proportionality to the thermal energy content of the reservoir, further characterised by means to transduce the output of the sensor to provide a time delay in the energisation of the electrical heating means over a portion of said time interval sufficient to enable a required thermal energy replenishment of the reservoir to be at least substantially concluded within the remainder of the said time interval.

The invention also consists of the method of switching a heating appliance hav ng a heat energy storage reservoir wherein a specific time interval for heating is periodically available, which method comprises determining a quantity related to the thermal energy content of the reservoir at the commencement of the said interval, and delaying the energisation of the heating means for a portion of the time interval sufficient to enable a required replenishment of thermal energy of the reservoir to be at least substantially concluded within the remai nder of the i nterval .

When used in a significant number of the total off-peak appliance population in a community the off-peak load will tend to fill in the electrical generation system load minimum in the 3 to 6 am period and will tend also to enable a much more smooth transition between the off-peak switch off and the immediately following continuous tarriff morning peak period.

DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms that may fall within its scope, one preferred form of the invention and variations thereof will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig 1 is a block diagram summary of the principle of the invention illustrating the application of the time delay means according to the invention to a storage water heater reservoir.

Fig 2 is a diagram showing schematically the time delay means in conjunction with a^* hot water storage reservoir or tank having a temperature sensor extending from top to bottom to sense tank mean temperature or thermal energy content.

Fig 3 is a circuit diagram showing a circuit of the time delay means according to the invention and showing the switching means.

Fig 4 is a timing generator circuit.

Fig 5 shows the timing generator wave forms.

Fig 6 shows two alternative water heater wiring diagrams a and b applicable when using the time delay circuit according to this invention. 1 Fig 7 shows the bridge amplifier circuit which is a portion of the time delay circuit.

Fig 8 shows a graph of peak electrical load on a large 5 metropolitan electrical supply network supplied by off-peak power wh ch is controlled using distributed off-peak load switching techniques, including a part of the graph showing how the present invention could influence the shape of the demand curve in a favourable manner. 0

Fig 9 shows a graph of peak electrical load on a large metropolitan electrical supply network supplied by off-peak power using a time clock controlled method of off-peak switching including a part of the graph showing how the present invention could influence the 5 shape of the demand curve in a favourable manner.

Fig 10 is a general layout view of a time delay means according to this invention.

0 DETAILED DESCRIPTION

With reference to Fig 1 showing generally the time delay means 10 enclosed by a dotted line, at the time off-peak power becomes available

^• to the appliance 12 the sensor 14 provides an energy dependant 5 resistance in one arm of a bridge 16, shown in more detail in Fig 7, the bridge excitation input being provided at a low potential and low current to avoid spurious electrical heating of the sensor. Any unbalance condition of the bridge 16 is directed via analog to digital conversion, to compare the resistance of the sensor 14 as signalled by 0 the unbalance potential of the bridge against a predetermined datum value. An output from the A/D converted bridge output is supplied to a relaxation oscillator 18 thence multi-stage binary counter 20 which is configured to delay switching of the relay 26 which energises the heating cycle of the storage reservoir 24 by the heating element 28 the 5 reheat being discontinued in the normal course of events by thermostat 30. The delay in switching is set to be related to the output of the sensor 14, as follows. When the sensor output in effect conveys that there has been a minimal depletion of energy from the reservoir 24 then the time delay imposed on the relay 22 is long, in the order of 4 to 8 hours. When at the other extreme the sensor output conveys that a very substantial or total depletion of energy from the reservoir 24 has occurred then the time delay imposed on the relay 22 is short, in the order of 15 minutes.

After the relay 22 has switched and the off-peak power is supplied to the heating element in the reservoir 24 the heating is maintained normally until the thermostat 30 interrupts the power supply.

Referring to Fig 2, the sensor 14 is a device providing an electrical output which is related in a known proportionality to the thermal energy content of a heat storage space with which the sensor is in distributed thermal contact. Preferably the sensor 14 is an elongate resistive element having a positive temperature coefficient of resistance. The resistive element may be co-extensive with the heat storage space, shown as a reservoir 24 and generally referred to as such, or it may be helically formed around it or submerged in it, but is arranged to sense the total volume so that the output is related to the time required to the time required to raise the average temperature to the required value. The graph on the right side of Fig 2 is related to the schematic draw ng of the water heater tank on its left, the delay times shown being examples only.

The sensor 14 comprises a length of nickel wire in good thermal contact with the external wall of the reservoir 24 such as a water heater tank. The wire so placed can monitor water temperature of the reservoi from top to bottom and manifest change in the average temperature or thermal energy content of the reservoir as a change in resistance of the wire. The significance of the top-to-bottom placement of the sensor wire is that although the reservoir can be expected normally to be temperature stratified the sensor still provides a single input to the time delay circuit to enable the circuit to correctly assess and apply the appropriate time delay for the particular situation.

The time delay means 10 shown i n Figs 1 and 2 detects the amount of hot water stored i n the reservoi r 24 when the off-peak power supply i s made avai lable, then delays the switch on time for the water heater accordi ngly, i .e. a reservoi r that does not have any hot water stored will be switched on almost immediately thus giving it maximum heat up time, whereas a reservoir that is nearly ful l of hot water will be delayed u to say six hours. Considered en masse, this wil l minimi se the surge when the off-peak power i s switched on and wi ll tend to distribute the load evenly over the entire off-peak period. The sensor output i s steplessly variable and the time delay i ncrements set are very smal l , further contributing to mi nimi sation of surge and more unifomity of demand duri ng the off-peak period. After the relay has switched and off-peak power i s supplied to the heati ng element 28 i n the appliance the heati ng i s mai ntai ned until the thermostat 30 i nterrupts the power supply or, in an unususal circumstance, i nvolvi ng a large draw-off of hot water very close to the end of the allocated off-peak period, the disconti nuation of the off-peak power woul d stop the reheat cycle.

The operation of the time delay circuit wi l be described in relation to Fig 3, which is a typical circuit diagram.

The sensor 14 is connected to a bridge 16 circuit that is connected to the input of a first operational amplifier 32. The bridge 16 is then balanced with the zero trim potentiometer 34 when the water in the reservoir 24 is at a known temperature. This known temperature can be ambient but there is an accuracy advantage in balancing for the temperature at which the water heater thermostat is set, approximately 70 degrees Celsius. This temperature is a known constant, whereas the ambient temperature of the inlet water may vary from 7 to 30 degrees Celsius depending on locality and season and is thus not ideal as a set point if greater accuracy is being sought. After setting, any difference between the average temperature of the tank and the set point will appear as an off-set to the first operational amplifier 32 and a proportional output from that amplifier will result. This output becomes a reference voltage for a second operational amplifier 36. Another input to to the second operational amplifier 36 is supplied from a constant current generator 38 made up of a third operational amplifier 40 and a first transistor 42. The constant current generator 38 charges the capacitor 44 up to a reference voltage set at positive input to the second operational amplifier 36. At this point the second operational amplifier 36 is turned on and the output is counted by a multistage binary counter 20 and at the same time the fourth operational amplifier 46 is turned on which in turn switches second transistor 48 on. The transistor 48 shunts the constant current generator 38 output to ground and also discharges the capacitor 44. The second operational amplifier 36 is then switched off and the cycle starts again. This part of the circuit acts as a relaxation oscillator 18, the output of which is counted by the binary counter 20. When the binary counter 20 reaches the maximum count it will turn on transistor 48, but this time it will remain latched on until the mains supply power is switched off. The output from the binary counter 20 will also switch on third transistor 50 which energises the optically coupled triac 52 to then complete the circuit for the thermal relay heater 54. Within a few seconds the thermal relay contacts 56 will close, completing the circuit to the water heater. The power switching is preferably by means of a snap action thermal relay 22 to avoid contact chatter. The choice of optically coupled triac device 52 i s- to avoid any chance of mains voltage connection between the time delay circuit and the water heater reservoir via the sensor attachment and as such is a fail-safe feature.

The normal temperature control thermostat 30 i s connected in series with the element which is connected at 28.

With reference to Figs 4 and 5, the chosen 14-stage bi nary counter wi ll always count the same number of pul ses (2 rai sed to the power of 14 whi ch i s 16384 pulses) before i t switches to provide an output. Therefore vari able time i s achi eved by varying the i nput pulse width . Thi s i s determi ned by the reference voltage set by the fi rst operati onal ampl ifi er 32 and the charge time of the capaci tor 44 as shown i n the timi ng generator ci rcuit di agram of Fig 4, the timi ng generator waveforms being shown in Fig 5.

It should be noted that the arrangement as described with respect to Figs 4 and 5 provides an advantage to the versatility of the invention as a whole insofar as it can cope with the unusual situation of a consumer who draws an appreciable quantity of stored hot water at say 2 am, that is, during the off-peak period while the particular appliance has running a long time delay based on minimal depletion at the, say 11 pm, normal off-peak commencement time. Because the input pulse width would be changed by the time delay would be continously reset until in effect a different and much shorter time delay would be applied by the circuit appropriate to the changed thermal energy content of the storage reservoir of the appliance.

Fig 6a shows what can be regarded as a normal connection arrangement of the time delay circuit of the invention applied to a water heater. Fig 6b shows an alternative connection whereby the time delay circuit is only powered when the heater is at a temperature below the thermostat 30 set point. The reason why this alternative connection is in some situations advantageous is that it can prevent bulk consumers of hot water taking advantage of the protraction of off-peak supply as is sometimes necessarily practised in supply networks using distributed load control. As was above indicated this can lead to off-peak power being made available in some districts as late as 10 am, in order that all consumers would have been provided with their full off-peak time entitlement. In this situation certain types of users would be enabled to quickly deplete their entire reservoir and cause renewed load on the distribution system via the off-peak demand route during the already heavy morning peak demand period. This is contrary to the purpose of making preferentially priced off-peak tarriffs available to electricity consumers. The alternative connection of Fig 6b effectively prevents this double ration of off-peak power being taken during a morning peak period where the above noted conditions apply.

With reference to Fig 7 the sensor 14 is one arm of the bridge 16 circuit that is connected to the input of operational amplifier 32. The bridge 16 is then balanced with the zero trim pot 34 when the water in the tank is at a known temperature. At the time off-peak power becomes available to the appliance the sensor provides an energy dependant resistance in one arm of a bridge the bridge excitation input being provided at a low potential and low current to avoid spurious electrical heating of the sensor. Any unbalance condition of the bridge is directed via analog to digital conversion, as an input to the relaxation oscillator 18, see Fig 1.

Fig 10 shows a suitable physical arrangement of the time delay means in relation to an existing control box 58 on a storage water heater appliance (not shown) indicating the power connections and general component arrangement, the time delay means as an enclosed unit being referred to by the numeral 60. The existing control box 58 is provided with an output terminal .pair 62 from which connection (not shown) of the tank sensor (see fig 2) to the remainder of the time delay means can be made.

USES AND ADVANTAGES

Referring to Figs 8 and 9, the unbroken line curve shows the existing electrical load on a large metropolitan electrical supply network for a 24-hour of maximum demand in winter. The broken line shows how the present invention would be beneficial during the off-peak period generally within the 9:00 pm to 7:00 am time interval, influencing the pattern of demand in a manner such as to reduce the overall cost of power for the 24 hour period. The two peaks visible around 10 to 11 pm in Fig 9 are caused by sudden load increase when off-peak power is supplied to the community using distributed load control, whereby off-peak power can be supplied district by district. The demand trough between 3 am and 6 am is of particular concern to the generating authority since it tends to dip below the base load capacity of thermal power stations in the grid; and so steam release or pumping of water to storage have been necessary recourses, albeit less than optimal ones. By means of the present invention the required number of hours to replenish each appliance is effectively converted into a time delay so that the load represented by the shaded area 64 would be converted into an equivalent load represented by the unshaded area 66 (both Figs 8 & 9) assuming that a significantly large number of off-peak appliances were to be installed in the community having the sensor and time delay circuit according this invention. Thus the undesirable sudden demand increase and the undesirable minimum will be effectively interchanged to provide a synergistic benefit.

Claims

1. A time delay means for a heating appliance comprising an electrical circuit used in conjunction with a heat energy storage reservoir having electrical heating means of a known power rating, in which the thermal energy capacity of the reservoir is replenishable daily during an allocated off-peak time interval of standardised duration, under an initiating on-off control exercised by an external time-of-day controlled switching means separate from the said circuit characterised by a subservient control in which a sensor is arranged to provide an output related by a known proportionality to the thermal energy content of the reservoir, further characterised by means to transduce the output of the sensor to provide a time delay in the energisation of the electrical heating means over a portion of said time interval sufficient to enable a required thermal energy replenishment of the reservoir to be at least substantially concluded within the remainder of the said time interval.

2. A time delay means for a heating appliance according to claim 1 wherein the sensor is a resistance member extending at least substantially from top to bottom of the heat storage volume of the reservoir and wherein the sensor is connected to a bridge circuit to provide a bridge output proportionally related to the thermal energy stored, and wherein the output of the bridge circuit delays the heating cycle of the reservoir consistent with the amount of heating required to achieve the thermal energy replenishment within the remainder of the interval.

3. A time delay means for a heating appliance according to claim 2 wherein the bridge output is directed to a first operational amplifier to generate a reference voltage for a second operational amplifier coupled to a constant current generator connected to charge a capacitor, and wherein the output of the second operational amplifier is directed to a relaxation oscillator arranged to cyclically discharge the capacitor until the maximum count of the multistage binary counter is reached, whereupon the heating of the heat storage reservoir proceeds until the earlier of either its full replenishment or the end 1 of the said time interval.

4. A time delay means for a heating appliance according to claim 3 wherein the heating means of the reservoir is actuated by a snap action

5 thermal relay energised through switching means under control of the multistage binary counter.

5. A time delay means for a heating appliance according to claim 4 wherein the said switching means for the snap action thermal relay

10 comprises an optically coupled device.

6. A time delay means for a heating appliance constructed and operable substantially as described and illustrated in the accompanying drawings.

15

7. The method of switching a heating appliance having a heat energy storage reservoir wherein a specific time interval for heating is periodically available, which method comprises determining a quantity related to the thermal energy content of the reservoir at the

20 commencement of the said interval, and delaying the energisation of the heating means for a portion of the time interval sufficient to enable a required replenishment of thermal energy of the reservoir to be at least substantially concluded within the remainder of the interval.

258. The method of claim 7 wherein the said quantity related to the thermal energy content at the commencement of the specific time interval is determined using a sensor to monitor the reservoir from at least sustantially top to bottom, using the sensor output as an input to a bridge circuit, and using the output of the bridge circuit to

30 determine the amount of energisation of the heating means required during the interval, and delaying the energisation of the heating means for a portion of the time interval sufficient to enable a required replenishment of thermal energy of the reservoir to be at least substantially concluded within the remainder of the interval.

35

9. The method of claim 8 wherei n a reference voltage i s generated for a second operational ampli fier from a fi rst operational ampl ifi er coupled to the bridge circuit to charge a capacitor and act as a relaxation oscillator and wherein the reference voltage is determined by a multi stage binary counter connected to output at a selected number of pulses and actuate a relay to actuate the heating means of the reservoir.

10. The method of switching a heating appliance such as a storage heater applied as described in the specification with reference to the accompanying drawings.