WO1988005160A1

WO1988005160A1 - Integrating temperature-averaging sensor

Info

Publication number: WO1988005160A1
Application number: PCT/AU1987/000445
Authority: WO
Inventors: Richard E. Collins; Bernard A. Pailthorpe; Brendan V. Bourke
Original assignee: Rheem Australia Limited; The University Of Sydney
Priority date: 1986-12-24
Filing date: 1987-12-24
Publication date: 1988-07-14
Also published as: AU608563B2; AU1102188A

Abstract

A temperature sensor comprising a temperature to electrical quantity transducer or array of interconnected transducers in thermal contact with a defined space such as a tank in which a non-isothermal temperature profile exists. Sensor may be arranged with respect to the shape of the space and its profile enabling single output of sensor to represent integral summation of temperature within the space, linearly interpretable for a given space as representing average temperature or thermal energy content of the space.

Description

INTEGRATING TEMPERATURE-AVERAGING' SENSOR

BACKGROUND

This invention relates to several forms of device capable of sensing thermal energy content of a space when the temperature within the space or the temperature of a medium contained within the space has a non-uniform profile. The non-isothermal profile of the space may for example be due to temperature induced density changes causing thermal stratification of the medium.

In such a space, bulk values such as the thermal energy content or the average or mean temperature or heat content, are not linearly related to a single point measurement of temperature at any one location either within the space or at the boundary of the space. It is often of much greater use to be able to measure or estimate these bulk values rather than one or more localised point values.

The invention enables estimation of such bulk values and is capable of application in achieving useful effects in a wide variety of situations. One principal application relates to hot water storage in tanks, with one ensuing application relating to providing for economies in the generation of electrical energy particularly for off-peak supplied electrical energy.

Further applications pertain to the fields of heating or temperature control, in rooms, in chemical process and reaction vessels, in steam condensers and in distillation columns. It can be applied also, in certain embodiments, for thermal energy content measurement in an enclosure having a non-uniform cross-sectional shape.

The invention is capable of one important useful application in the field of electrically heated off-peak heat energy storage appliances wherein means to measure the heat energy content as provided by this invention can be used in conjunction with signal processing means to control the undesirable effects of excessive or sudden loading in a community electricity supply network resulting at the beginning of an off-peak period when a large requirement to replenish depleted heat energy storage appliances in the community is met by substantially simultaneous switch ng of supply to at least significant portions of the total load. It can be used to significant advantage also to maximise the amount of lowest-generati on-cost electricity supplied to the community.

In a significant example of the use of the invention involving off-peak hot water systems, it is explained that at the completion of a heating cycle, all the water in a hot water storage tank is at approximately the same temperature. This occurs because the off-peak heating element and the single thermostat controlling it are located near the bottom of the tank and natural convection ensures that very small temperature gradients remain when the thermostat switches off the element to terminate the heating cycle. Subsequently, as some hot water is drawn from the tank, cold water enters at the bottom and the body of pold water and the body of hot water in the tank remain essent ally separate, that is, temperature stratified, subject to no intervening heat input. This stratificat on occurs because the density of hot water is significantly less than^" that of cold water and the tank is not > normally mixed by the simultaneous withdrawal and accompanying replenishment of water. As the interface between the two bodies of water approaches the top of the tank due to draw off of hot water from near the top, the useful thermal energy content in the tank nears exhaustion. The temperature of the water^' leaving the tank therefore remains approximately constant over substantially all of the draw off, finally dropping rapidly to close to the inlet temperature.

When the invention is applied to off-peak storage hot water systems, supplied with power on a daily basis during a def ned off-peak period, the energy sensor of the invention can be used to provide a delay in switching on of the heating element in the tank, in conjunction with suitable signal processing circuitry outside the scope of this specification. Normally, this delay would be applied when a shorter time than the full off-peak period would suffice to fully replenish the design thermal energy content of the tank. Therefore the invention can be of important benefit to a supplier of electricity to the community, who could by its use be able to progressively build up to peak load rather than other more difficult procedures and expensive investments to enable adequate balance between generation capacity and load on a day-to-day basis.

One method of estimation of thermal energy content in a non-isothermal hot water storage tank would be to distribute over the height o the tank a number of separate discrete temperature measuring devices such as thermocouples or thermistors at a number of points either inside the tank or, conveniently, in thermal contact with the external wall of the tank. Each temperature dependent output could then be connected separately to a measuring, readout, control device or other information processing system. This method is discusssed in Australian patent application 33728/84. The reading of each thermocouple or other discrete sensor could be averaged to give an indication of total thermal energy content stored. This method, however, would involve many wires emerging from the tank into the information processing system. In fact such a multiple parallel connected discrete sensor method would provide more infβrmation than is actually required. It is sufficient to know the total energy in the storage tank, which, as will be shown, for vessels of substantially uniform cross-sectional area is linearly related to the integral of the temperature profile from top to bottom of the tank. Therefore a sensor which integrates this temperature would provide adequate information for the require, purposes. The present invention provides a very much less expensive and more serviceable alternative to that proposed in patent application 33728/84 to enable effective achievement of an at least equivalent end result.

In many instances of general application of the present invention it is preferable if an average temperature or bulk thermal energy content, with respect to a chosen datum, of an enclosure or space be available. In such circumstances the present invention of an integrating thermal energy sensor enables,for example, more economical and reliable solutions to problems related to control of heat flux with respect to the space. For example, in applications such as room temperature control it is a cause of difficulty that a single temperature sensor is subject to unintended localised temperature influences which cause unintended control responses. In air conditioning applications the effects of solar radiation through windows is one such example, and another is the effects of localised heat sources wi hin a room, both of which tend to create control difficulties.

The invention also has useful application in solar hot water systems having a storage tank heated by solar energy backed up by electrical heating. In such systems the sensor can be coupled with a readout of energy content of a tank, located at a conveniently accessible point remote from the tank. This can be useful in indicating to a user of such a system whether it is necessary to take steps to initiate a boost heating of the tank at a particular time. Boost heating controlled in this manner is aimed at improving the solar contribution of the system.

In a principal form the invention is readily applicable to tall tanks such as th^'e cylindrical type very commonly used in domestic off-peak mains pressure hot water storage systems, such tanks having a capacity of 160 litres or more. Such tanks normally display distinct thermal stratification of the water in the tank and indeed, owe their effectiveness to that tendency in the context of off-peak once-per-day heat up.

When the invention is applied to enclosures defining interior spaces of other orientations or cross-sectional shape than those described as tall cylinders, alternative embodiments of the invention may be employed in order to relate the weighted summation of temperature sensed to be representative of the thermal energy content of the medium within the space.

In one form the invention relies on the. property that the electrical resistance of a conductor comprising for example a length of wire is dependent upon the volume resistivity of the material of the wire, its length and cross-sectional area and its effective temperature. When the wire passes through regi^'ons of variable and differing temperature and each increment of length of the wire is at least substantially isothermal with its immediate surroundings, the resistance measureable over the full length of the wire can be calibrated to be indicative of the integral summation of temperature of each increment of length of the wire. If the temperature of each increment of the wire is representative of a larger element of some hot medium substantially isothermal within that element then the resistance of the wire can be tranduced to be also indicative of the integral summation of temperature of the hot object and hence is indicative also of its thermal energy content related to a reference temperature.

In applying this to determining the thermal energy of a non-isothermal storage tank for hot water it is known that in such a tank the wall of the tank at any given height will tend to equilibrate at a temperature quite close to that of the contents at the same height, especially if the tank is externally heat insulated. It is also known that the temperature profile of the water occupying the tank space is related to vertical distance from the bottom (ortop) end. A resistance in the form of an elongate wire can be made to serve the purpose of indicating the energy content of the tank if for example arranged in thermal contact with the external wall of the tank and extending from between high and low points on the wall of the tank.

When using an elongate resistance wire it is known that within the temperature range involved that the resistance of the wire is substantially linearly variable with temperature. Electrically conductive materials display different temperature coefficients of resistance,these being tabulated in standard electrical engineering and other handbooks. It has been found that for many metals the change in resistance is sufficiently distinct to be readily measurable for the purpose of providing a useful measure of thermal energy content of the tank, enclosure, space or medium occupying the space or, the mean temperature.

The above considerations have been extended to include an alternative method of sensing thermal energy wherein a resistive element or, more generally, transducers of temperature to an electrically measurable output may be used. For example, a series connected set of thermistors can be distributed between high and low points of the tank, enclosure boundary wall or defined space to include a number of spaced-apart discrete locations intermediate the extremes of the space or at least the extremes of the non-isothermal extent of the space in cases where part of the space may be isothermal.

It has been found that several types of transducer 'device may be employed within the same inventive concept to achieve the advantages asserted for it, including some transducer types not normally associated with temperature sensing. Any elongate resistor or conductor or discrete semi -conductive devices such as diodes or thermistors, when series connected can be effectively used, so long as the resistance or leakage current measurable is temperature dependant.

The invention applies whether the temperature coefficient of the transducer device is positive or negative; that is, whether the electrical quantity measurable increases with increasing temperature or vice-versa. Only devices showing negligible temperature coefficient of output, including resistance, would be clearly unsuitable.

In this specification the term thermal energy is used in a broad sense. When it is stated in the specification that a sensor is 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 datum temperature) which s 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 types of 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.

When a series connected array of semi-conductors, such as silicon diodes, is used, the temperature output measurable need not be related to resistance, per se, since semi-conductors do not obey Ohm's Law, offering unequal resistance to forward and reverse current flow. The invention in its broadest sense uses the transduction by transducer means of a non-uniform temperature to a single electrical quantity representative of the integral summation of temperature of the space, as represented by the physical disposition of the transducer means in relation to both the physical confines of the space and profile of thermal zones within it.

SUMMARY

Accordingly, the invention consists broadly of an integrating thermal energy sensor capable of installation in conjunction with a space wherein the sensor comprises transducer means having temperature dependant output of 'an. electrically measurable quantity; and wherein the output of the transducer means is connectable to interface means for display of and/or for controlled response to the output; characterised in that the transducer means is adapted to be installed in thermal contact with the space to traverse linearly extensive non-i sothermal strata within the space thus to be capable of providing the output indicative of either the thermal energy content of the portion of the space traversed by the sensor when a known medium occupies the space or, its average temperature.

In a highly preferred form the energy sensor as above described is one in which the transducer means is an elongate electrical resistance. This form is of low cost and easily installed.

In a further preferred form the energy sensor as above described is one in which the transducer means is disposed in a tortuous or serpentine path. This enables improved accuracy when compared with a single length. In a further preferred alternative form the energy sensor as above descri ed is one in which the transducer means is disposed in a multi -branched path comprising parallel and series interconnections.

This enables the making of allowance for the variation in cross section of the space with respect to the direction of stratification of temperature within the space.

In a further preferred alternative form the energy sensor as above described is one in which the transducer means is a series connected array of discrete transducers of temperature to electrically measurable quantity. The significance of this is that the invention is applicable to transducer means other than elongate resistive types such as wire.

In a further preferred form the energy sensor as above described is one in which the discrete transducers comprise thermistors, semi-conductor diodes, thermostats or thermocouples. These forms of transducer provide workable embodiments of the invention.

In a further preferred form the energy sensor as above described i s one in which the transducer means as described in the previous paragraph is disposed in a multi-branched path comprising parallel and series interconnections; this again provides means for allowing for cross-sectional variation of the space.

In a further preferred form the energy sensor as above described is one in which the transducer means is disposed in distributed thermal contact with a boundary of the space and is thermally insulated from the surroundings to the space. The distributed thermal contact with the boundary enables the temperature variation of the boundary to be sensed wherever thermal contact is made and the thermal insulation from the surroundings enables the temperature of the sensor to more accurately represent the temperature of the boundary.

In a further preferred form the energy sensor as above described is one in which the transducer means is electrically isolated from an electrically conductive boundary of the space and is thermally insulated from the surroundings to the space. This is significant in any embodiment in which the boundary is electrically conductive in which case electrical isolation ensures that the transducer output is not affected by the conductivity of the boundary.

In a further preferred form the energy sensor as above described is one in which the transducer means is alternatively disposed in uniform thermal contact with a medium occupying the space. This is a useful alternative means of contacting the transducer means with the non-isothermal strata such as for example where the space is the interior of a vessel and it is not convenient or, would lead to inaccuracy to fix the transducer to the external wall of the vessel.

In a further preferred alternative form the energy sensor as above described is one in which the transducer means comprises discrete transducers applicable to the space in groupings of numbers and in disposition of the groupings such as to be indicative of a single reference temperature assignable to the contents of an inc emental stratum of the space multiplied by the capacity to store thermal energy possessed by the contents of that stratum to which the grouping and disposition is adapted to be applied. This is a useful embodiment in cases where the temperature profile is variable but generally predictable in form and the cross-sectional area and capacity is also variable.

In a further preferred alternative form the energy sensor as above described is one in which the transducer means is a resistive element having a variable cross-sectional area dimensioned to be adaptable for arrangement in thermal contact with the space and for traversing strata of variable capacity and wherein the change in variable cross-sectional area of the transducer is at least substantially inversely related to the capacity of each stratum which it is adapted to traverse thereby to be able to provide an output proportional to the thermal energy content or weighted average temperature of substantially the whole space as defined by the extent and arrangement of the. sensor with respect to the space. This is also a useful embodiment in cases where the temperature profile is variable but generally predictable in form and the cross-sectional area and capacity is also variable. In a further preferred form the energy sensor as above described is useable in association with a space defined by the interior of a vessel adapted for f lling with and heating of a medium.

In a further preferred form the energy sensor as above descri bed is useable in association with a vessel which is adapted for filling with of water and heating by electricity. Appl cation in the field of domestic storage hot water systems is considered a principal application of the invention.

In a further preferred form the energy sensor as above described is one in which the transducer means is adapted to traverse regions of a room subject to thermal stratification.

In a further preferred form the energy sensor as above descri bed is one in which the transducer means is adapted to traverse regions of a heat storage bank subject to thermal stratification. This and the previous paragraph describe two other applications of the invention.

_> In a further preferred form the energy sensor as above described is one in which the transducer means is disposed to traverse substantially the elevational extremes of the space or object to which it is adapted to be associated. This enables the commonly occurring vertical stratification of temperature in, for example, a tank filled with a medium to be allowed for in assessing the total thermal energy content of the tank.

DESCRIPTION OF THE DRAWINGS

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

Figure 1 is a diagrammatic part elevation-part cross section of a hot water storage tank to which one form of energy sensor according to the invention is shown schematically, affixed to the tank. Figure 2 is a graph showing resistance of an energy sensor according to the invention when applied to a 250 litre storage tank, as a function of energy draw-off from the tank, together with a schematic illustration of the energy sensor and its mode of attachment to the tank.

Figure 3 is a diagrammatic elevation view to a reduced scale of a hot water storage tank to which the integrating thermal energy sensor in one further form according to the invention, is applied.

Figure 4 is a diagrammatic elevation view to a reduced scale of a hot water storage tank to which several alternative embodiments of integrating thermal energy sensor according to the invention are ap pi i ed .

Figures 5(a) to (d) are a related family of diagrammatic perspective views to a reduced scale of sensors according to the invention applied to a hot water storage tank in a disposition as commonly mounted on roofs of buildings in the case of Fig 5(a), and in the case of Figs 5(b) to (d), showing several further alternative embodiments of sensors according to the invention applicable for installation in conjunction with a horizontally installed tank. Figure 5(e) illustrates division of the tank of Fig 5a into notional seg ental zones or strate appropriate to the temperature profile. Figs 5b, c and d are a flat development of sensors applicable to attachment to the cylindrical wall of a horizontally oriented tank.

Figure 6 is a cross-sectional elevation of the same type of hot water storage tank as in Figure 1 showing an alternative method of installing one form of integrating energy sensor in one form of conjunction with the interior space of the tank.

Figure 7 is a diagrammatic perspective view to a reduced scale of a room in a building showing several forms of integrating thermal energy sensor according to the invention, installed by attachment to the walls of the room in various orientations with respect to potential temperature profiles in the room space. Figure 8 is a part cut-away part cross-sectional elevation view of a sensible heat energy storage enclosure of the electrically heated type showing one form of integrating thermal energy sensor according to the invention installed in conjunction with the heat storage medium occupying the interior space of the enclosure.

DETAILED DESCRIPTION

Referring to Fig 1, an insulated hot water storage tank 10 is shown comprising a steel cylindrical tank 11, a heat insulation layer 12 partly cut away to show a resistive transducer element in the form of a length of wire 13 attached to the wall of the tank and extending from a top extreme point 14 to a bottom extreme point 15. The wire is electrically insulated from the tank by a thin dielectric layer 16, which is selected to provide a negligible thermal separation of wire 13 from the adjacent tank wall, especially in view of the protection from convective heat loss from the wire by virtue of the surrounding heat insulation layer 12.

The ends 14 and 15 of the wire are connected by leads 17 and 18 for connection to measuring and control circuitry - not shown - suitable for detecting the relatively small change in resistance of the wire indicating the heat energy content or mean temperature in the space enclosed by the tank.

By means of circuitry outside the scope of th s specification the value of the resistance of wire 13 can be measured and adapted to provide a delayed switching of at least one heating element (not shown) of the tank at the commencement of an off-peak time period, thus delaying, in conjunction with suitable timing circuitry, the switch-on time for individual hot water systems among a large population of off-peak appliances connected to the electricity supply network, by an amount in approximate proportion to the heat input required for the particular hot water tank to raise its thermal energy content to the desired extent. Alternatively, the resistance can be adapted to provide a readout of thermal energy content of the tank at a meter remote from the tank.

This provision can be applied to enable a solar hot water system to be operated at an improved solar contribution level.

Alternative embodiments include different arrangements for the resistor where the sensitivity is increased by attaching a resistor of increased length to the wall in an up and down or spiral path. A resistance path having the start and finish at adjacent points is preferred for convenient connection to associated circuitry.

With reference to Figure 2, in order to test the validity of the use of the invention, experiments were performed on two 250 litre hot water storage tanks of a form as depicted in the illustration. In a first tank a copper tube was inserted through a top central hole in the tank wall to pass along the cylindrical axis to a point immediately adjacent the bottom of the tank. A second copper tube was arranged parallel to the first but fixed in good thermal contact to the outside of the vertical metal wall of the tank. In both tubes thermocouples were arranged to be moveable up and down the tubes to selected heights above the lowest point of the tank. It was thus possible to measure the temperature at all points along both the axis and the outside vertical wall of the tank. The temperatures in both these sets of locations were measured for the tank when full of hot water and also when draw-off of known amounts of hot water had been made together with the simultaneous make-up of cold water accompanying draw-off. Under a variety of conditions it was confirmed that the temperature as measured on the outside of the tank wall was within 1 to 2 degrees Celsius of the temperature at the same height on the axis of the tank.

Figure 2 includes a graph of heat energy removed from the tank versus the resistance of a copper wire which was taped in good thermal contact to t vertycal wall of the second 250 litre tank in a second experiment. The copper wire used was 30 B and S gauge and it was run from the top of the vertical cylindrical wall of the tank to a point within about 20mm of the bottom of the same wall, traversing this distance 4 times. The resistance of the wire was in the order of 2 ohms. An approximately linear variation of resistance with respect to the thermal energy content of the tank was confirmed by the experiments. The very minor departures from linearity at the two extremes, when the tank is nearly full - or on the other hand, when nearly depleted of hot water - are due to the shape of the top and bottom of the tank.

Referring to Figure 3, a hot water storage tank 30 of capacity typically 160 litres to 350 litres is shown, omitting the features irrelevant to the present invention. The tank is of the mains pressurised type. Two forms of integrating thermal energy sensors 31 and 32 according to the invention are shown attached to the cylindrical wall 33 of the tank as will be discussed below. The tank 30 is tall in relation to its diameter and the convexity of the top end 34 or bottom end 35 has negligible influence on the total tank capacity as compared to a tank having substantially flat ends.

The sensor 31 in the form of a long wire is affixed to the vertical wall, the wire being arranged in two vertical legs so that the two ends of the wire 36 and 37 terminate close together near the bottom of the tank in order to be convenient for attachment to signal processing means (not shown). The sensors 31 or 32 in use carry electrical current typically of the order of 1 mi Hi amp at a potential of typically 10 volts.

The sensors are electrically insulated from the metallic wall of the tank by a thin dielectric layer or coating (not shown). The layer is as thin as possible to assist good thermal contact between the sensor wire and tank wall and to minimise the response time of the sensor, should the temperature profile change quickly. Th s enables each length increment of wire to be substantially isothermal with the increment of tank wall with which it is in contact. Thus an increment of wire length can be considered substantially isothermal with a horizontal slice of the tank interior space and the medium contained, that s, in this example, hot water.

The sensor 32 i s a longer wire than the sensor 31 and is applied to the tank wall in a part-spiral path with its upper extremity and lower extremity as the same height as the respectively corresponding extremity of sensor 31. The purpose of this method of fixing of the longer sensor wire 32 is for making a single length of sensor wire suitable for application to different length tanks. Thus a shorter tank such as one of 160L capacity would use a sensor affixed as is sensor 32, namely in a part spiral path, whereas a taller tank such as one of 350L capacity would have an identical length sensor fixed straight and vertical. After fixing to the tank, the sensor as used is encapsulated in heat insulation (not shown).

Referring to Figure 4, a similarly oriented tall hot water tank to that of Figure 3 is shown fitted with a variety of interconnected discrete temperature sensitive devices, illustrated symbolically. The top and bottom ends of the tank as shown for illustation purposes as both "minus" ends, that is convex to the tank interior. It is immaterial to the scope of this invention whether the ends are arranged as illustrated in either Figures 3 or 4 or whether both ends are "plus", that is, concave to the tank interior (not shown).

In Figure 4, the tank wall 40 has plural interconnected thermistors represented symbolically by dots 41. Return lead wire 42, thermally insulated from the tank wall connects one lead of the uppermost thermistor to a point adjacent one lead of the lowermost, so that connection points 43 and 44 are available side by side for connection to appropriate signal processing circuitry (not shown). Also shown attached to tank wall 40, for illustrative convenience, are alternative forms of series connected discrete devices suitable for exemplifying the present invention. Triangles 45 represent symbolically a row of thermocouples, also series connected analogously to thermistors 41, in either case arranged with substantially uniform spacing along the length of the vertical tank wall 40. Square symbols 46 correspondingly represent semi-conductor diodes.

The circle symbols 47 correspondingly represent small thermostats of the double throw type each one of which has a resistor of typically 5 ohms across one pair of terminals. Thus when each thermostat senses a temperature above its set point, conveniently 60C, the contacts switch open so that the thermostat has a resistance of 5 ohms across that pair of terminals.

When each thermostat senses a temperature below its set point, the contacts switch closed so that the thermostat has no resistance across the terminals. Thus, in this case, the signal processing means which would be connected across all the thermostats in series utilises a resistance change made up of a summation of zero and 5 ohms resistances dependant upon the switched state of the thermostats in the series.

The several types of transducer described in relation to Figure 4, namely thermistors, thermostats, diodes and thermocouples are each connected to appropriate signal processing circuitry (not shown). In each case an output would be available from the sensor enabling a reading of an electrical quantity such as resitance, current or voltage or a derivative quantity of those quantities able to be related to the temperature of the discrete device and hence the tank wall and hence the zone within the space which is represented by the positioning and spacing of the discrete devices.

Figure 5(a) shows a long storage tank mounted horizontally, as is commonplace for roof mounted solar hot water systems. The wall of the tank has applied one version of the integrating temperature sensor of the invention. In Figs 5(b), 5(c), and 5(d) are shown several alternative forms of sensor as will be further discussed below. In each case of Figs 5a to d the principle applies of using either an elongate temperature to resistance transducer as illustrated by Fig. 3 or, a number of discrete transducers of temperature to another conveniently measurable electrical quantity as illustrated by Fig. 4.

An important principle illustrated in Fig. 5a to d is that of using either a length of resistance wire (in contact with the tank wall) to be representative of the volume of a segment or zone of enclosed tank space or using another form of transducer to fulfil the same representation related to volume of a segment. The segments are in general bounded by the tank wall and two horizontal planes passing 1 through the (cylindrical) tank and in which an 'increment of resistance wire makes thermal contact with the increment of the tank wall defining the segment. Convenient forms of segmental division are shown in Fig

5(e), in which the horizontal tank is shown in endwise cross-section.

5 Above the horizontal axis x-x is shown a division into labelled zones or strata 1 to 6, formed notionally by the subtended angle as illustrated. The small volume of tank not included above the intersection of the 55 degree line has been ignored as it only represents approximately 4.5% of the total tank volume. Below x-x is

10 shown an alternative division into zones labelled 1A to 6A where the horizontal thicknesses "A" of each zone are equal again ignoring the insignificant lower extreme. The division into horizontal zones is appropriate for hot water tanks since the thermal stratification which is observable in such tanks is also into horizontal layers.

15

Fig. 5(c) illustrates using a different number of discrete transducers (of the types exemplified in relation to Fig. 4) in thermal contact with the different segments or zones of the horizontally mounted tank. Thus, for example, by placement of semi-conductor diodes

20 51 the volumes of the zones a's illustrated in Fig 5(e) ^"are accounted for by an output from the diode or diode group attached to its wall. Thus the output of the diode or diode group is proportional to the product of the temperature sensed by the diode or diode group and the volume of tank space within the segment to which. the diode or diode

25 group is attached.

Fig 5c also illustrates discrete lengths of resistance wire connected in parallel to form a transducer composed of lengths of wi e 55 having resistance varying between the end of the transducer for 0 association of lesser resistance portions with lesser zone volumes and higher resistance portions with higher zone volumes.

With reference to Fig 5(a) elongate resistance wire 52 is affixed to the tank in a serpentine path where each length of straight 5 resistance wire in the serpentine conceptually represents a reference level in a segmental zone of the tank. For a level of accuracy sufficient to the purpose, it is not of significance how the exact position of the horizontal lengths of serpentine specifically relate to the arbitrarily selected division into segments. It is possible that the spacing of each horizontal leg of the serpentine when projected onto a vertical plane will be chosen to be uniform from leg to leg, but this choice is arbitrary.

In Figure 5(b), sensor wire 53 is affixed to the tank such that it traces two parallel pairs of smooth curved paths from a point high on the tank wall and from a point low on the tank wall to a point at mid-height. The dimensions shown on Fig 5b relate to one example only in which the sensor as a whole wraps around a portion of a cylinder of 475 mm diameter and the opposite extremes of the sensor wire transducer subtend an included angle at the centre of the cylinder of 110 degrees (see Fig. 5e) .

In this arrangement the purpose of the path shape is analogous to the above described serpentine shape. This is that the horizontally aligned segments having a small volume near the top and bottom of the tank have a short length of the wire affixed to the wall of the segment and the large volume segments near the centre of the tank have a long length of wire attached to the wall of the respective segment. Thus the sensor wire has a steeper slope near the upper and lower extremities of the tank and a lesser slope near the centre.

As may be appreciated from Figs 5(a) to (e) in a horizontally installed cylindrical tank an appropriate path of resistor wire or an appropriate array of other transducers can perform the function of integrating the product of temperature and enclosed volume. The sensor wire or discrete device array can be arranged so as to provide an indication of the thermal energy content possessed by the medium occupying the internal space of the tank assuming a value can be ascribed to the specific heat of the medium occupying the space. In an analogous manner a combination of resistance or other transducer devices may be arranged with respect to any tank whose cross-sectional area varies in the same direction in which a temperature profile change exists in the internal space contained by the tank. Where such an effect is necessary or i s desired in view of the cross- sectional variation of the tank other techniques may be -used in place of the arrangement suggested by Fig 5c. As examples, a resistor of varying cross-section so that its resistance either increases or decreases along its length as required may be used. For this purpose etching of a wire can be a suitable way of forming portions of reduced cross-section and thus higher resistance per unit length with respect to unetched portions. Other methods such as varying the numbers of wires in portions of the overall length of the sensor may be adopted as shown by

55 of Fig 5c, or use of different wire materials series connected . together. For example in the case of the cylindrical tank in Figs 5 use of wire materials having temperature coefficients of resistance in the ratio of 1 to 3 is effective in enabling an adequate approximation of the effect of the higher volume of the mid-height zones with respect to the top and bottom zones, when used in the form of a semi -circumferenti ally extending band.

An alternative method available for use in producing a sensor for such' appli cations includes forming conductive coatings on a dielectric supporting film. For example, with reference to Fig 5(d), the coating ca^'n be formed having variation of width along its length so that the resistance varies in the required relation to the part of the non-uniform cross sectional area tank to which it is applied.

Thus the various forms of sensor illustrated in Figs 5a to 5d provide an indication of the integral summation of the temperature and associated volume of the segmental zone of the tank space, or of any medium occupying the segmental zone of the tank space with which the relevant portion or length of the respective sensor is in thermal contact.

Figure 6(a) shows a hot water storage tank 70 having a threaded hole 60 at the top end, through which is normally inserted an elongate anode 61 for inhibiting internal corrosion of the enamelled internal wall of the tank. Figure 6(b) shows the anode to a larger scale. Where it would be inconvenient to affix an elongate sensor wire to a vertical external wall of a tank, sensor wire 62 can be attached to the anode. To achieve this, two small holes 63 are formed through the anode end cap 64 through which the elongate resistor wire 62 is inserted and subsequently sealed. The wire 62 is retained at the bottom end of the anode, at least during insertion, by holding it in position with the aid of end cap 65, conveniently manufactured from polypropylene, the cap being initially a neat fit over the anode when the latter is installed. The cap 65 is weighted to retain it in the top-to-bottom extending position in the event that corrosion of the anode prevents it retaining the wire. The method of installation as shown in Figs 6a and

6b can be of particular convenience where it is desired to adapt this type of sensor to already installed water heater tanks. A preferred sensor for this use is light gauge copper wire coated with a thin dielectric layer of polyvinylidene fluoride, the latter being able to withstand prolonged hot water contact well.

Figure 7 depicts a room to be temperature controlled to which some forms of sensors according to the invention are attached to the walls. An elongate resistance wire 70 is fixed to a wall, running generally parallel to the floor. Sensor 71 shows a similar wire attached to the wall from a point approximately mid-height to a point running toward floor level. An alternative arrangement for fixing a resistance wire in a vertical direction is shown by 92. A discrete transducer device series 73 i s connected running in a line from a point high on the wall to a point low on the wall. These devices can be of the same type and used analogously to those shown in relation to the hot water tank shown in Fig 4. Depending upon the specific temperature control applications desired arrangements analogous to those shown in relation to Figs 5a to d can be adopted in the arangements of Fig 7 to provide additional weighting factor to particular levels of the room depending on thermal profiles pertaining in practice.

Referring to Fig 8 a cubical heat bank enclosure 80 is illustrated in part cross-section in which an arrangement of cast iron bricks adapted to be electrically resistance heated is installed in a bank with means to permit air to permeate through the bank, the bank being contained inside a heat insulated enclosure. The bank is heated using off-peak electrical energy. For space heating, air is fan induced into duct 81 and the air heated by passage through the bank before exiting through duct 82 to a room. Wire sensor 83 is affixed as shown to an external wall of the heat bank and covered by the heat insulation 84.

Connection points 85 at each end of the wire are for connection to suitable signal processing equipment, not shown. The heat storage bank as a whole is frequently unlikely to be isothermal throughout its mass at any particular moment. Therefore the use of a sensor of the type 83 is capable of providing a more useful and economical method of determining the amount of heat energy required to restore the heat bank to its full intended thermal capacity than would a single location temperature sensor. Thus, with the use of appropriate signal processing means the off-peak power required to reheat a heat bank may be timed to commence so as to ensure optimal control of the demand for off-peak power over the total period in which it is made available by the electricity supply authority.

In the foregoing descriptions it has been implied that a sensor which integrates the temperature profile gives a means to linearly relate the output to the thermal energy content of the enclosure. This assumes that the specific heat of the medium occupying the enclosure and its mass are known 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 types of 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 used.

In support of the foregoing implication, using the tall vertically oriented cylindrical tank of Figure 1, as an example, it can be shown mathematically that a sensor which integrates the temperature profile gives an output which is substantially linearly related to energy content of the tank, provided that the sensor .spans a major part of the full height of the tank. Applying calculus to a considered horizontal increment, dy, of the height of the tank in the cylindrical region:- Thermal energy = Area x density x specific x dy x [T(y)-T(0)]

(referred to of heat a datum temp, /₀ tank _/-

A A <-/^» where T(y) is the temperature of the layer of water at a height of y units above the bottom of the tank.

. _* 7 _/ I Wj^

since A is constant for most of the tank, the top and bottom ends having negligible volume compared to the remainder in a tall tank

Similarly, considering the resistivity of the sensor resistance wire as a function of height of the tank:-

Resistivity = Resistivity [ 1 + alpha <T(y)-T(0)>]

as a function at datum temp / _Q of temperature where alpha is the temperature coefficient at height y of resistivity and T(y) is the temperature of the resistance at height y Resistance dR = resistivity x dy/a of element where a is the between y and cross-section of the y+dy resistance wire

Thus it may be seen that both total thermal energy and resistance are linearly related to the integral of [T(y)-T(0)]dy over the height of the tank, the other factors in the analysis being substantially constants.

Of resistor materials in the form of wire, film or tape, many are considered suitable for the illustrated and cited applications. These

> include those of iron, aluminium, nickel or platinum. Preferred materials have a reasonably high resistivity and an adequate temperature coefficient of resistivity. For example, copper has a temperature coefficient of resistivity of .004 per degree C giving a total variation of resistance of about 20% between the applicable extremes of temperature in water heating applications, say 20 to 70 degrees. A generally suitable material for an elongate resistor has been found to be annealed mild steel wire in the diameter range 0.3 to 0.9 mτι, the thinner end of the range being especially preferred.

It has been found possible to measure the energy content in a hot water storage tank to an accuracy of about 5 to 10%. Long term stability, annealing effects on the resistor material and variation in wire dimension due to thermal expansion are not likely to detract from the accuracy of the sensor to any significant extent. USES AND ADVANTAGES

Other potential applications for the forms of sensor of this invention include those where the enclosure or space covers storage silos or vessels or swimming pools. In these cases the sensor would enable for example estimation of energy input necessary to effect a desired temperature change, enabling conceivably an optimal use of lower cost energy sources while available. Similar applications in concept include the preheating of large industrial process vessels during off-peak supply periods, the amount and timing of the preheating being controlled by the average existing temperature being determinable from a sensor of the type as disclosed in this specification. The invention is believed to provide an improved means of control for underfloor heating in factories and other buildings where in cold climates this form of heating is widely practised. In a diverse field, meteorology, it is believed the use of such sensors attached externally to tall structures would be useful in improving the accuracy of forecasting temperature changes.

Further variations and modifications may be made in the invention as above descr bed and as limited only by the following claims:

Claims

1. An integrating thermal energy sensor capable of installation in conjunction with a space wherein the sensor comprises transducer means having temperature dependant output of an electrically measurable quantity; and wherein the output of the transducer means is connectable to interface means for display of and/ or for controlled response to the output; characterised in that the transducer means is adapted to be installed in thermal contact with the space to traverse linearly extensive non-i sothermal strata within the space thus to be capable of providing the output indicative of either the thermal energy content of the portion of the space traversed by the sensor when a known medium occupies the space or, its average temperature.

2. An integrating thermal energy sensor as claimed in claim 1 in which the transducer means is an elongate electrical resistance.

3. An integrating thermal energy sensor as claimed in claim 2 in which the transducer means is disposed in a tortuous or serpentine path.

4. An integrating thermal energy sensor as claimed in claim 2 in which the transducer means is disposed in a multi-branched path comprising parallel and series interconnections

5. An integrating thermal energy sensor as claimed in claim 1 in which the transducer means is a series connected array of discrete transducers of temperature to electrically measurable quantity.

6. An in egrating thermal energy sensor as claimed in claim 5 in which the transducer means comprises thermistors, semi-conductor diodes, thermostats or thermocouples.

7. An integrating thermal energy sensor as claimed in claim 6 in which the transducer means is disposed in a multi-branched path comprising parallel and series interconnections.

8. An integrating thermal energy sensor as claimed in claim 1 in which the transducer means is disposed in distributed thermal contact with a boundary of the space and is thermally insulated from the surroundings to the space.

9. An integrating thermal energy sensor as claimed in claim 8 in which the transducer means is electrically isolated from an electrically conductive boundary of the space and is thermally insulated from the surroundings to the space.

10. An integrating thermal energy sensor as claimed in Claim 1 in which the transducer means is disposed in uniform thermal contact with a medium occupying the space.

11. An integrating thermal energy sensor as claimed in Claim 1 in which the transducer means comprises discrete transducers applicable to the space in groupings of numbers and in disposition of the groupings such as to be indicative of a single reference temperature assignable to the contents of an incremental stratum of the space multiplied by the capacity to store thermal energy possessed by the contents of that stratum to which the grouping and disposition is adapted to be applied.

12. An integrating thermal energy sensor as claimed in Claim 1 in which the transducer means s a resistive element having a variable cross- sectional area dimensioned to be adaptable for arrangement in thermal contact with the space and for traversing strata of variable capacity and wherein the change in variable cross-sectional area of the transducer is at least substantially inversely related to the capacity of each stratum which it is adapted to. traverse thereby to be able to provide an output proportional to the thermal energy content or weighted average temperature of substantially the whole space as defined by the extent and arrangement of the sensor with respect to the space.

13. An integrating thermal energy sensor as claimed in claim 1 when the space is defined by the interior of a vessel adapted for filling with and heating of a medium.

14. An integrating thermal energy sensor as claimed in claim 13 when the vessel is adapted for filling with of water and heating by electrici y.

15. An integrating thermal energy sensor as claimed in Claim 1 in which the transducer means is adapted to traverse the space, being regions of a room subject to thermal stratification.

16. An integrating thermal energy sensor as claimed in Claim 1 in which the transducer means is adapted to traverse the space, being regions of a heat storage bank subject to thermal stratification.

17. An integrating thermal energy sensor as claimed in any one of the previous claims in which the transducer means is disposed to traverse substantially the elevational extremes of the space to which it is adapted to be associated.

Dated this 23rd day of December 1987 RHEEM AUSTRALIA LIMITED THE UNIVERSITY OF SYDNEY