GB2485311A

GB2485311A - Hot water storage tank heating control unit and associated sensor arrangement

Info

Publication number: GB2485311A
Application number: GB1201884.2A
Authority: GB
Inventors: Jens-Otto Ravn Andersen; Steven Lose; Jan Nielsen; Sa Ren Stig Abildgaard
Original assignee: EC Power AS
Current assignee: EC Power AS
Priority date: 2007-07-11
Filing date: 2008-07-07
Publication date: 2012-05-09
Anticipated expiration: 2028-07-07
Also published as: GB0812402D0; RU2010103526A; RU2459154C2; RU2012101085A; WO2009007710A2; WO2009007710A3; EP2171359B1; EP3081866B1; GB2450983A; GB2485311B; EP3081866A1; GB0713475D0; GB201201884D0; GB2450983B; EP2171359A2

Abstract

Disclosed is a temperature sensor apparatus for a water storage apparatus 1 in which an amount of hot water 2 stored can be determined by the vertical position of a hot/cold water boundary 4. The apparatus comprises a plurality of temperature sensors 7 for installation at different levels in the water storage apparatus 1, and a control unit. The control unit is arranged select one of the temperature sensors 7 to indicate an amount of hot water stored in the tank and to optimise the amount of hot water stored by selecting a sensor at a different level based on a desired operating condition such as minimum heat storage required to meet hot water demand and/or maximum heat storage whilst enabling a certain level of cooling capacity. A corresponding method of controlling a hot water storage system is also disclosed.

Description

I

HEAT STORAGE

The present invention relates to heat storage, and in particular to hot water storage for use with a cogeneration unit.

In many circumstances it is desirable to store heat, for example as a means of providing heat or hot water for later use. Stored heat can be used to heat a building or the like. Heat storage systems are common in cogeneration situations, where both heat and power are produced by a single unit such as a combined heat and power (CHP) internal combustion engine or a fuel cell. The power could be in the form of mechanical power but is generally in the form of electrical power. The production of power produces waste' heat. This heat can sometimes be utilised as it is produced, but generally the demand for power does not correspond tot the demand for heat so that it is desirable to store the heat when demand for heat is low, and release it when demand for heat is high.

The heat is often stored as hot water, as it can be easily transported about a cogeneration system from storage to cool the cogeneration unit, and then back to storage. Conventional plumbing can be used, and the hot water can be utilised for washing and so on. It is however also possible to use oil or other liquids to store heat, and the discussion below should be taken to include such alternatives where replacing water with them would be possible. When using a liquid such as water to store heat, it is necessary to know when the maximum storage capacity has been reached. This is generally the point when the water in the storage reservoir (for example a storage tank or tanks) is all at a maximum temperature. There will also generally be other requirements, such as a minimum stored heat value to meet the expected demand for heat from storage.

Where cogeneration of heat and power is used, water used to cool the cogeneration unit is heated, and this heated water is then stored for later use. In this situation it is useful to know how much cooling capacity for the cogeneration unit is remaining, i.e. how much more storage capacity is available before the heat produced by the unit cannot be usefully stored, or before the cogeneration unit cannot be run due to a lack of cooling capacity. The ability to store heat makes the use of cogeneration efficient. If heat cannot be stored, then it may be more efficient to rely on an alternative power source, rather than using a cogeneration unit for power without being able to utilise the waste heat. Also, there is a minimum requirement for available cooling water to allow the cogeneration unit to be started up efficiently. If too little cooling water is present, then the cogeneration unit will not be able to run for a sufficient time to make if worthwhile starting it up, because there is an energy penalty involved in starting the unit.

In a known storage system, the hot water is stored at one end of a storage apparatus such as a tank or the like, and cold water is at the other end. For example in the simplest arrangement a single vertically orientated tank can store hot water at the top, with cold water at the bottom, due to the difference in density of water at different temperatures. As the water is heated, cold water passes out of the bottom of the tank, and hot water enters the top of the tank. Thus, the hot/cold boundary moves down the tank. It will be appreciated that there is some mixing of hot and cold water across the boundary, so there will not be a step change in temperature, but instead there will be some temperature gradient from hot to cold.

In such a system the assessment of heat stored is simplified by the use of temperature sensors that are located part way up the tank in order to indicate the parameters used to control the system. An upper sensor can be used to indicate a minimum required storage capacity, and a lower sensor can be used to indicate a minimum cooling capacity for allowing the cogeneration unit to run for a set minimum time. When the hotlcold boundary reaches the position of the sensor then the amount of heat stored is known based on the sensor position. As the hot/cold boundary is not a step change in temperature, it will be appreciate that in these systems there is some leeway for the threshold temperature of the sensor, which can be any selected point on the temperature gradient from the hot to the cold temperatures.

The desired parameters indicated by the sensors positions are referred to as T1 and T2, where T1 is the value for the minimum heat storage required and T2 is the value for the maximum heat storage, or equivalently the minimum cooling capacity, that will allow the operation of the cogeneration unit. Thus, the value of T1 should provide sufficient hot water to cover the peak heat consumption, and the value of T2 should allow the cogeneration unit to run for a certain minimum time, which can be determined depending upon the type of cogeneration unit and other properties of the system.

Generally, a temperature sensor is also present at the top of the hot water storage apparatus. This sensor, referred to as K0, shows when the storage is completely empty of hot water, or when the hot/cold boundary is near the top depending on the point on the temperature gradient from hot to cold that is used for the threshold value of K0. Thus, the sensor K0 can be used to trigger operation of the cogeneration unit to produce heat, i.e. if heat is required and the storage is empty. In some installations, a boiler is used in combination with a cogeneration unit, and in this case the sensor K0 can be used to trigger the boiler, i.e. if heat but not power is required. In addition, a further sensor may be present, designated K1. This sensor is typically positioned very close to K0, and is used to provide hysteresis in boiler control. Therefore, when the boiler is off and K0 is cold, the boiler can be turned on to supply heat to meet demand. If the boiler is on and the heat storage is filling up (for example if a power demand has triggered operation of the cogeneration unit), then the boiler can be turned off when K1 is hot, as heat can then be supplied by the cogeneration unit and/or from storage.

In general, the K1 sensor controls the boiler whereas the T1 sensor controls the cogeneration unit. Thus, the cogeneration unit will usually be running when T1 is cold, and the boiler might also be run should K0 also be cold. If K1 then turns hot, the boiler is turned off, and the cogeneration unit will be run until at least the point where T1 is hot, after which it is operated based on an optimal heat and power production regime.

Parameters such as K1, T1 and T2 are consequently very important to the efficient and effective operation of a water storage apparatus, and it is important that the sensors that detect these parameters are positioned appropriately.

Viewed from a first aspect, the present invention provides a temperature sensor apparatus for a water storage apparatus in which an amount of hot water stored can be determined by the vertical position of a hot/cold water boundary, the apparatus comprising: a plurality of temperature sensors for installation at different levels in the water storage apparatus, and a control unit, wherein the control unit is arranged to, in use, select one of the temperature sensors to indicate a hot water storage parameter and to optimise the value of the parameter by selecting a sensor at a different level based on a desired operating condition.

The hot water storage parameter may be T1 or T2, with the operating condition being selected accordingly. As will be appreciated, the value of the hot water storage parameters are set by the position of the sensors in the storage apparatus. The prior art systems require an installer to judge the required minimum stored heat and minimum cooling capacity for the particular system, and to physically position the sensors in the water storage apparatus accordingly. The optimum position will depend on the interaction of number of factors such as the type of system and the use to which it will be put. If a misjudgement is made, or if the use of the system changes then the result would be an inefficient system that cannot easily be adjusted, as access within the storage apparatus is required to move the sensors.

By the use of multiple sensors at different levels, which can be selected and re-selected by the control unit, the present invention avoids these difficulties and ensures that the optimal value of the parameter is set, and can also be adjusted later as required. The level of the sensor corresponds to its position in the water storage apparatus relative to the movement of the hotlcold boundary. In the simplest case, with a single tank, this will correspond to the height of the sensor. Where there are multiple tanks the sensor level will be based on the height of the sensor in each tank, and also the order of the tanks.

The optimisation and adjustment process can be automatically and easily controlled without the need for a skilled installer to be present. In this way, a number of physical temperature sensors are provided, and selected sensors are designated as virtual' sensors taking the role of T1 or T2 or other parameter.

There may be only two temperature sensors, which is the minimum value that allows some degree of optimisation of a parameter. Preferably however there are more than two temperature sensors. For example there may be four temperature sensors, or more. In the discussion below, reference is made to selecting a higher or lower level sensor in order to optimise the parameter value. This is to be taken to mean that a higher or lower sensor should be selected where available. Obviously if the highest sensor is being used, then it will not be possible to move to a higher level, and if the lowest sensor is being used, then it will not be possible to move to a lower level. Preferably the number of sensors is selected so that sensors can be placed at intervals extending from highest and lowest levels that will correspond to hot water storage levels above and below the highest and lowest values for the hot water storage parameter. The reference to higher and lower levels relates to the level of the hot/cold boundary, and hence not to an absolute height. For example, in a system with two adjacent and similar tanks, the lowermost sensor in the first tank will have a lower absolute height than sensors in the second tank, but the sensors in the second tank have a lower level in relation to the movement of the hot/cold boundary.

Where the parameter is T1, the desired operating condition may be that the amount of heat stored is sufficient to meet demand and there may be a further condition that the storage of too much excess heat should be avoided. Thus the control unit may be arranged to select a higher level sensor if too much heat is stored, and may be arranged to selected a lower level sensor if too little heat is stored. Assessment of this operating condition may be based on a determination of if the heat storage apparatus runs out of heat, or becomes close to running out of heat.

This determination may be carried out over a set time interval, preferably a day. A day corresponds to the smallest cycle of heat use for most installations.

In a preferred embodiment this is achieved by providing a temperature sensor for installation at the hot end of the storage apparatus, for example the top of a tank, corresponding to the sensor K0 discussed above, where if K0 goes cold, then a lower sensor is selected for T1, and if K0 shows no tendency to go cold, then a higher sensor is selected for T1.

A sensor in the tank may also be designated as K1, as discussed above. In this case, as an alternative to the above, the condition may be that if K0 goes cold, then a lower sensor is selected forT1, and if K1 shows no tendency to go cold, then a higher sensor is selected for T1.

Where K1 is used, this may be a parameter that is optimised by selecting an appropriate sensor as discussed above. Thus, the operating condition may be that it should take a least a predetermined time for the amount of hot water to reach K1 from empty (i.e. the time from K0 to K1). This might be to allow a boiler controlled as above to operate for at least a predetermined time, for example a minimum of 10 minutes, and hence avoid excessive switching of the boiler. The control unit may be arranged to select a lower level sensor for K1 if the time for the amount of hot water to reach K1 from empty is too short. There may also or alternatively be a maximum allowable time for the amount of hot water to reach K1 from empty, and the control unit may be arranged to select a higher level sensor for K1 if the time for the amount of hot water to reach K1 from empty is too long.

The control unit may be arranged to allow the user to specify that full storage is required at a certain time. For example a hotelier may require full storage in the morning so that there is no risk of running low on hot water when the majority of guests are showering or bathing. In this case, the control unit may, at the specified time, select a higher level sensor for K1 in order to release the boiler to operate concurrently with the cogeneration unit and thereby provide the maximum heat storage. The control unit may also allow an absolute minimum of stored heat, such that the user always has this minimum level, even at the expense of less efficient operation of the system.

Where the parameter is T2, the desired operating condition is that the amount of cooling capacity remaining when T2 is indicated by the selected temperature sensor is sufficient to allow a cogeneration unit connected to the heat storage apparatus to run for a desired time, i.e. where water is the heat storage medium a requirement that the amount of cold water in the heat storage apparatus is at a certain level. Thus, the control unit may be arranged such that if the time between T2 being indicated by the selected temperature sensor and the cooling capacity running out (i.e. the water storage apparatus being full of hot water) is less than a lower time limit, then a higher level sensor forT2 is selected, and if the time is above an upper time limit, then a lower level sensor for T2 is selected.

A parameter T3, which indicates when the storage is full, may be used to determine the time from T2 turning hot to the storage being full, alternatively, the time for the cooling capacity to run out may be extrapolated from the time for the hot/cold boundary to reach the next sensor, and the total number of sensors remaining before the heat storage is full. The parameter T3 may be indicated by a physical temperature sensor positioned at the water storage apparatus cold water outlet.

Preferably however T3 is an imaginary sensor (i.e. no physical temperature sensor is present) that is on, i.e. shows hot, when a valve for cooling the cogeneration unit is fully open and a demand for additional cooling has continued for a set time, for example 30 seconds. The T3 imaginary sensor is off again, i.e. shows cold, when the valve is not completely open anymore. Advantageously, this arrangement avoids the need for a physical sensor positioned at the point of maximum heat storage.

It will be appreciated that whilst the upper and lower time limits may be the same, it is preferred that there is some interval between the upper and lower time limits, in order to smooth the operation of the system. This smoothing consideration also applies to the amount of heat stored assessed for the optimisation ofT1, although this is to a lesser degree if the control of T1 is based on two sensors K0 and K1, as the use of two sensors provides a hysteretic effect. Thus, a hysteresis loop may be used in the control unit to avoid excessive switching and oscillation between sensors selected for T1 and/or T2. Such a hysteresis loop may be implemented in the control system in a conventional manner, and with T2 this can simply be a case of setting an appropriate difference between the upper and lower time limits.

Alternatively, in a preferred embodiment, control of the sensor selected for T2 is carried out based on the expected change cooling capacity if a higher or lower level sensor was selected. In this embodiment the current cooling capacity may be measured or extrapolated, and this is divided by the number of sensors beneath T2 to enable an expected change to be determined. A higher or lower level sensor is selected if the new sensor is expected to produce a cooling capacity that is closer to the required minimum cooling capacity, which may be sufficient capacity to produce 20 minutes of cooling. By way of example, if at least 20 minutes of cooling is required, the time from T2 hot' to T3 hot' is 24 minutes, and the time per sensor is 3 or 4 minutes (i.e. with 8 or 6 sensors below T2) then T2 can be moved down to the next physical sensor. However, T2 is not moved if the time per sensor is larger than the difference between the actual time and the desired time, i.e. the extrapolated time is never allowed to be less than the minimum. Hence, with a required time of 20 minutes and an actual time of 24 minutes, the sensor will not be moved if the time per sensor is greater than 4 minutes.

The sensor apparatus may optimise multiple parameters, for example both of T1 and T2, and in this case the control unit is arranged to select a sensor for each parameter and optimise the parameters by adjusting sensor level according to a predetermined condition for each parameter. For the optimisation of both T, and T2 it is clearly not allowable for T, to be below T2. Preferably, the control unit is arranged to give precedence for T2, such that if it is determined that T2 should move to a higher level sensor, and that sensor is currently T,, then T, is also moved to a higher level sensor as a consequence. In a similar way, if a K1 is used then T, preferably has precedence over K, such that T, will push' K, to a higher level if required. If movement of T, requires it then T,, as a virtual' sensor, may be allowed to be the same physical sensor as K1, with K0 hence having the upper sensor and both K, and T, occupying the next sensor down. If further upward movement ofT, is required, perhaps due to movement of T2, then K0, K, and T, may all be placed at the upper sensor.

Preferably, if K0, K, and T, were pushed to use the same physical sensor, then when T, moves away from 1<0, K, will automatically follow. As an example of the preferred arrangement, 1<0, K, and T, might be pushed to use the same physical sensor by movement of T2, which takes precedence. T2 will move away again when conditions require a different physical sensor for T2, and the movement of T2 leaves some freedom for T, to operate. Some time hereafter T, might be required to move down by one sensor, and when this occurs K, follows T, so that K0 has the upper physical sensor and K, and T, share the next physical sensor. At this point T2 may already have moved further down the storage apparatus. The next time that T, moves down a sensor, K, does not need to follow, as there is now room for it to have its own sensor. K, K, and T, are then placed on the upper three sensors. If T, moves further down, then K, has freedom to move if necessary.

Advantageously, the optimisation of T, and/or T2 enables the gap between these two sensors to be maintained at the maximum possible distance that is allowable for effective operation of the heat storage system. Where the heat storage system is used in combination with a cogeneration unit this provides the broadest possible range where the cogeneration unit can be operated without constraint and hence operation can be optimised for efficiency. This is because above T1 the cogeneration unit must operate to re-fill the storage to the required minimum and below T2 the cogeneration unit may have to run at less than the optimum load and/or stop to avoid running without any cooling. As noted above, it is also not efficient to start operation of a cogeneration unit when less than a certain amount of cooling is available, because there is an energy penalty associated with starting the cogeneration unit.

Similarly, optimisation of K1 enables a boiler or the like to be operated more efficiently to meet heat demands.

In a preferred embodiment, the control unit is arranged to allow selection of new sensors for K0, K1 and/or T2 based on a continuous assessment of operation of the heat storage apparatus with respect to the desired operating conditions. This allows continuous optimisation of these parameters. For T1, the control unit is preferably arrange to determine if a different sensor should be selected based on a full day of operation, i.e. T1 is restricted to move to a higher or lower sensor only once each day. This is because the minimum heat storage required, i.e. the desired operating condition for T1, is dependent on the maximum heat usage in the user's usage cycle, which will generally be a daily cycle. Thus, if K0 goes cold once or more during a day, then T1 should be moved to a lower sensor to provide more heat storage for the following day. An exception to this occurs in the case where T2 has precedence overT1, when movement ofT2 can require a movement ofT1 as discussed above. This movement can occur in addition to the usual daily cycle for T1.

When two or more parameters are to be optimised the number of sensors affects the degree of optimisation. Preferably the apparatus includes at least four sensors, more preferably at least eight sensors. In a preferred embodiment, sensors are provided in groups of four, each group having an associated control circuit.

An advantageous feature of the apparatus is that it is not necessary for any information regarding the temperature sensors to be provided to the control unit upon installation, as they can be arbitrarily selected before optimisation. In a preferred embodiment, the control unit is arranged to select an initial sensor for the or each hot water storage parameter based on the order in which the sensors heat up during operation of the water storage apparatus. Preferably, the temperature sensors include a temperature sensor for installation at the top of the storage apparatus, and the control unit is arranged to identify this sensor as the first sensor to heat up.

A preferred embodiment of the present invention provides a water storage apparatus in which an amount of hot water stored can be determined by the vertical position of a hot/cold water boundary, the water storage apparatus being fitted with a temperature sensor apparatus as discussed above.

Thus, the water storage apparatus includes a plurality of temperature sensors installed at different levels, and a control unit arranged to optimise a water storage parameter.

The water storage apparatus may be a water tank, or it may be a number of water tanks connected in series. The use of the sensor apparatus allows flexibility in the arrangement of the tanks, and parameters such as the remaining cooling capacity or the minimum required heat storage capacity can be optimised for multiple tanks by the same control unit that can optimise the same parameters in a single tank.

The control unit may advantageously be arranged to learn the location of temperature sensors using the order in which the various levels of the water storage apparatus, for example a tank or tanks, heat up or cool down, as well as being arranged to optimise parameters by adjusting the level of the sensor selected. For example, the sensor at the hot end of the storage system, which might be the top sensor K0 in a tank or in the first tank of multiple tanks, can be identified as being the first sensor to heat up. As noted above, where there are multiple tanks, a temperature sensor in a tank in series with a preceding tank, can be considered to be at a higher level than a sensor in the preceding tank, such that the sequence of sensor levels goes from the lowest sensor in the tank to the highest sensor in the preceding tank. There may be one or more groups of sensors in each tank, where the order of sensors in each group is known, but the order of the groups is unknown.

In this case, the system may be arranged to determine the order of the groups of sensors based on the order in which they detect a change in temperature.

Viewed from a second aspect, the present invention provides a method of controlling a water storage apparatus in which an amount of hot water stored can be determined by the vertical position of a hot/cold water boundary, the method comprising: using one of a plurality of temperature sensors to indicate a hot water storage parameter, the temperature sensors being at different levels in the water storage apparatus; and optimising the value of the parameter by selecting a sensor at a different level based on a desired operating condition.

As for the apparatus discussed above, the hot water storage parameter may be T1, K1 or T2, with the operating condition being selected accordingly, and the method including method steps corresponding to the actions of the control unit discussed above.

Thus, in a preferred embodiments the parameter is T1. In this case the method may include selecting a higher level sensor if to much heat is stored, and selecting a lower level sensor if too little heat is stored, This may be achieved by a step of determining if the heat storage apparatus runs out of heat, or is close to running out of heat.

In another preferred embodiment the parameter is T2, The method may include determining the time between T2 being indicated by the selected temperature sensor and the cooling capacity running out, and then selecting a higher level sensor for T2 if this is less than a lower time limit or selecting a lower level sensor for T2 if this is more than an upper time limit. As discussed above, the upper and lower time limits may be the same, but it is preferred that there is some interval between the upper and lower time limits.

The method may include selecting temperature sensors for multiple parameters, and optimising each of the multiple parameters. In a preferred embodiment the multiple parameters are T1 and T2, and in this case the step of selecting a sensor for T1 includes ensuring that the sensor is above the sensor selected for T2, with T2 being given precedence as discussed above.

Preferably, the method includes selecting an initial sensor for the or each hot water storage parameter based on the order in which the sensors heat up during operation of the water storage apparatus. Preferably, the temperature sensors include a temperature sensor at the top of the storage apparatus, and this sensor is identified as the first sensor to be heated.

The method may include operating a cogeneration unit, such as a CHP internal combustion engine based on the parameters indicated by the selected sensors. Thus, the cogeneration unit can be utilised efficiently according to the available heat storage and cooling capacity in the water storage apparatus.

View from a third aspect, the present invention provides a computer program product comprising instructions which when executed on a control unit in a temperature sensor apparatus will arrange the control unit to carry out a method discussed above in relation to the second aspect.

Preferably, the temperature sensor apparatus of this aspect is as discussed above in relation to the first aspect.

A temperature sensor apparatus and a method in which the sequence or location of some or all of the sensors can be determined automatically is considered to be inventive in its own right.

Therefore, viewed from a fourth aspect the present invention provides a temperature sensor apparatus for a water storage apparatus in which an amount of hot water stored can be determined by the vertical position of a hot/cold water boundary, the apparatus comprising: a plurality of temperature sensors for installation at different levels in the water storage apparatus, and a control unit, wherein the control unit is arranged to determine the relative locations of the sensors, in use, based on the order in which the sensors detect a temperature change during operation of the water storage apparatus.

Hence, upon installation of the temperature sensors, the control unit is not provided with information regarding the location of all the temperature sensors, i.e. upon installation the relative locations of at least some of the temperature sensors is unknown. Instead, the relative location of these sensors is determined based on information received by the control unit when the temperature sensors are initially used, i.e. when they measure a change in temperature. Thus, the system utilises the known behaviour of the hot/cold water boundary (generally a linear vertical movement) to enable the location of some or all of the sensors to be initially unknown, and to determine these locations during operation. This simplifies installation, as there is no requirement for certain sensors to be placed in certain locations, and no input to the control unit upon installation is required to enable the apparatus to be used effectively. In addition, this apparatus allows multiple tanks to be connected in series without the need for the sequence of the tanks to be programmed into a control unit upon installation.

The assessment of temperature sensor location can be carried out during a temperature change caused by either heating or cooling of the water, i.e. either as the heat storage apparatus is filled or empties of hot water. As noted above, there will be a gradient across the hot/cold boundary, and hence detection of a temperature change is preferably implemented by detecting when a preset threshold temperature is reached. It is preferable to detect location during heating, as this can be carried out by heating the tank from cold, with the sensor location then being known in relation to the empty state. If the order of cooling is used then, to obtain an absolute reference point, it is necessary to start at maximum heat storage capacity, which is likely to lead to unnecessary use of energy.

Preferably, the temperature sensors include a temperature sensor for installation at the top of the storage apparatus, and the control unit is arranged to identify this sensor as the first sensor to be heated.

The temperature sensors may be provided in groups, where the order of each sensor in the group of sensors is known, but the order of the groups of sensors in the storage apparatus is unknown. In this case, the control unit may be arranged to determine the relative sensor locations based on the order in which the groups of sensors heat up. In a preferred embodiment, sensors are provided in groups of four.

Each group of sensors may have a control circuit and preferably also an associated network connection. The control circuit and network connection may be arranged to enable the relative locations of the sensors within the group of sensors to other network devices, which may include the control unit.

In a preferred embodiment the temperature sensor apparatus is fitted in a water storage apparatus in which an amount of hot water stored can be determined by the vertical position of a hot/cold water boundary.

The temperature sensors may comprise sensors for installation in a water storage apparatus comprising multiple storage tanks. in this case, the control unit can advantageously determine the sequence of sensors across the multiple tanks in the same way as for a single tank. in this case the determination of relative sensor location may be simply the determination of relative sensor location in the heat storage apparatus by determining the sequence of the multiple tanks. Thus, there may be only one sensor or one group of sensors in each tank. Where there is more than one sensor or group of sensors in each tank, the system may also determine the order of these sensors in each tank.

The use of this system to determine the order of a series of tanks is of benefit when it is necessary to replace or add storage tanks to the heat storage apparatus.

The temperature sensor apparatus is preferably arranged to retain information relating to the known location of sensors connected to the apparatus, and to determine the relative position of new sensors added to the system based on the order in which the new sensors heat up or cool down relative to the known sensors.

In this regard, the control unit may include or be connected to a memory, which stores information that identifies sensor location when the location is or has been determined by the control unit.

Thus, for example, in a heat store with four heat storage tanks, the system may know the order of tanks one, two, three and four. if a new tank is added, which for convenience is fitted between tanks three and four, then, when the heat store is being filled the system will detect that a temperature sensor (or sensors) in the new tank heats up after the sensor(s) in tank three, and before the sensors in tank four.

Similarly, if a tank is removed and replaced, for example if tank two is removed, the order of the tanks in the new system is determined based on the order of heating (or indeed cooling) of the new (unknown) sensors relative to the known sensor locations in the remaining tanks one, three and four.

Advantageously, by retaining information about the location of sensors with positions which have already been determined, there may be no need to fully heat or cool the system to find the position of a new tank. For example, the position of sensors in a tank located between existing tanks one and two can be found without needing to fully heat tank two and subsequent tanks will not need heating at all.

Viewed from a fifth aspect, the present invention provides a method of controlling a water storage apparatus in which an amount of hot water stored can be determined by the vertical position of a hot/cold water boundary, the method comprising: heating the water in the storage apparatus and determining the relative location of temperature sensors installed at different levels in the water storage apparatus based on the order in which the sensors detect a change in temperature during operation of the water storage apparatus.

Preferably, the temperature sensors include a temperature sensor installed at the top of the storage apparatus, and this sensor is identified as the first sensor to be heated. There may be sensors installed across multiple tanks connected in series, and these can be identified in a similar manner, as they will still change temperature in a sequence corresponding to the amount of hot water stored. In this case, the different levels of the temperature sensors will be in sequence across the multiple tanks.

The method may include operating a cogeneration unit, such as a ChIP internal combustion engine, based on the parameters indicated by the selected sensors. The control of the cogeneration unit can be based on sensor information from the water storage apparatus without the need for prior knowledge of the sensor sequence.

The method may include features corresponding to the preferred features of the apparatus of the fourth aspect, as discussed above.

View from a sixth aspect, the present invention provides a computer program product comprising instructions which when executed on a control unit in a temperature sensor apparatus, will arrange the control unit to carry out the method discussed above in relation to the fifth aspect.

Preferably, the temperature sensor apparatus of this aspect is as discussed above in relation to the fourth aspect.

Preferred embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a schematic of a water tank with sensors, and Figure 2 is a schematic of a system using two tanks.

In the embodiment of Figure 1, a water storage system 10 consists of a tank 1, which is shown partially filled with hot water 2, with the remainder of the tank containing cold water 3. The hot and cold water is separated by a hot/cold boundary 4 as discussed above. The actual temperature of the hot water 2 and the cold water 3 will vary depending on the ambient temperature and the water heating apparatus.

The water heating apparatus, which is not shown here, can be a cogeneration unit, such as a CHP engine. A combination of a petrol fuelled CHP engine and a cylindrical hot water storage tank is commonly used to supply heat and power to a domestic building. Similar systems are used, on an appropriate scale, in industrial settings.

Hot water enters and leaves the tank 1 at the top, for example via pipes 5 or the like, the detail of which is conventional, and is not shown. Similarly, cold water enters and leaves the tank 1 at the bottom, for example via pipes 6 or the like.

Temperature sensors 7 are arranged at different levels in the tank. These sensors 7 can be put into the tank I during installation, or perhaps during a maintenance procedure. The sensors 7 are each connected to a control unit, which is not shown in Figure 1.

When the system is first operated the control unit detects when the sensors 7 heat up. Hot water 2 enters the top of the tank 1, and the tank I fills up with hot water 2 from the top downwards as the cogeneration unit is running. Thus, the first sensor to heat up is the uppermost sensor in the tank, and the control unit therefore identifies this as K0, which can be used as discussed above to indicate when the tank is empty of hot water, or is near to being empty. The level in the tank I of the other sensors 7 can then be determined by the sequence in which they heat up, which is indicative of the sequence in which the hot/cold boundary 4 reaches them. The sensors 7 can be provided in groups, where the order of each sensor 7 in the group is known. In this case the system uses the order in which sensors 7 heat up to identify the sequence of the groups of sensors 7.

The control unit can then select sensors 7 to use as an initial sensors for the parameters K1, T1 and T2. This selection can be based on an estimate using data of prior similar systems, or it could be a selection based on more simple criteria, such as selecting the sensor adjacent to K0 to be K1, the 2nd sensor from K0 to be T1, and the 6th sensor from K0 to be T2. The control unit can allow the technician who installs the system to indicate an initial set of sensors to use for the parameters.

During further use of the hot water stored in the tank I the control unit will optimise the sensors selected for T1 and T2 based on the measured operating conditions. As discussed above, T1 represents the minimum amount of hot water storage required and T2 is a parameter for the minimum amount of cooling capacity that can be remaining to allow the cogeneration unit to be run. For example, T2 can be selected to provide a minimum of 20 minutes run time for the cogeneration unit.

Thus, K3, K1, T1 and T2 are, in effect, virtual' sensors, which can be assigned to any of the physical temperature sensors 7.

If during the use of heat or hot water K0 has a tendency to go cold, then the control unit will select a lower level sensor forT1. This will lead to a lower position of the hot/cold boundary at T1, and consequently a greater amount of stored heat as the minimum required. Conversely, if excess heat is found to have been stored, ia if K1 shows no tendency to go cold, then a higher level sensor will be selected forT1, leading to a higher position of the hot/cold boundary at T1.

For T2, if the running time of the cogeneration unit between T2 being indicated by the selected temperature sensor and the cooling capacity running out is less than a lower time limit, for example 20 minutes then a higher level sensor for T2 will be selected, so that the cooling capacity at T2 is increased. Conversely, if the time is more than an upper time limit, or if a calculation of based on the time per sensor requires it, then a lower level sensor for T2 will be selected.

Figure 2 shows an example of an embodiment where the water storage apparatus 10 consists of multiple tanks, in this case two tanks 1 and 1'. The water storage apparatus is partially filled with hot water 2, such that the first tank I is partially full of hot water, with the remainder of the tank 1 containing cold water 3, and the second tank 1' is completely full of cold water 3. The hot and cold water is separated by a hot/cold boundary 4 as discussed above.

Hot water enters and leaves the water storage apparatus 10 at the top of the first tank 1, for example via pipes 5. Cold water enters and leaves the water storage apparatus 10 at the bottom of the tank 1', for example via pipes 6. The first tank 1 and second tank 1' are connected by a pipe 8, with the top of the second tank 1' joined to the bottom of the first tank 1.

Temperature sensors 7 are arranged at different levels in both of the tanks 1,1'. The sensors 7 are each in communication with a single control unit.

When the water storage apparatus 10 of Figure 2 is used, the first tank 1 heats up first, and so the sensor 7 at the top of the first tank 1 is identified as K0. The sensors 7 will then heat up in order of height in the first tank 1, followed by order of height in the second tank 1'. T1 and T2 could be identified as shown, and then optimised as discussed above. Thus, the present invention is able to deal with multiple tanks without any modification to the basic concept. It will be appreciated that further tanks could be added in series as well. Likewise, the size and capacity of the tanks is irrelevant, as the control unit and sensor arrangement will work with any capacity.

A parameter T3 is used to show when the water storage apparatus 10 has no more cooling capacity available, i.e. when it is full of hot water. This is an imaginary sensor that is on, i.e. shows hot, when the valve for cooling the cogeneration unit is fully open and a demand for additional cooling has continued for a set time, for example 30 seconds. The T3 imaginary sensor is off again, i.e. shows cold, when the valve is not completely open anymore.

The optimisation of T1 and T2 sensor position is carried out upon installation and first use, but is also repeated during operation. The sensor positions can therefore be adjusted with varying operating conditions, for example the minimum amount of heat storage required could be adjusted on a daily basis to follow seasonal variations in the demand for heat.

As well as optimisation of T1 and T2, the control unit can also make use of historical data concerning the usage of the water storage system to predict how it will operate. The sensor K0 gives data regarding the inlet temperature to the hot water end of the water storage apparatus 10. The final sensor can also be identified, for example by heating up the water storage apparatus 10 to its full capacity, and finding the final sensor to heat up. This sensor gives data regarding the outlet water temperature, which will generally be the cold water temperature and thus provides an indication of the temperature of the cooling water. The inlet and outlet temperature data at any particular time, as well as other data such as ambient temperature and cogeneration unit power output, can be analysed and compared with historical data.

The amount of cold water left in the tank is known based on the level indicated by the temperature sensors. The remaining cooling time for a particular power output and ambient temperature can then be determined. Similarly, the time required to run the cogeneration unit to fill the water storage apparatus with the minimum required hot water, i.e. the running time required to reach T1 can also be predicted.

Claims

CLAIMS: 1. A temperature sensor apparatus for a water storage apparatus in which an amount of hot water stored can be determined by the vertical position of a hot/cold water boundary, the apparatus comprising: a plurality of temperature sensors for installation at different levels in the water storage apparatus, and a control unit, wherein the control unit is arranged to, in use, select one of the temperature sensors to indicate a hot water storage parameter and to optimise the value of the parameter by selecting a sensor at a different level based on a desired operating condition.
2. An apparatus as claimed in claim 1, wherein the parameter is a value for minimum heat storage required, T1, and the desired operating condition is that the amount of heat stored is sufficient to meet a demand.
3. An apparatus as claimed in claim 2, wherein the control unit is arranged to select a higher level sensor if too much heat is stored.
4. An apparatus as claimed in claim 1 or 2, wherein the control unit is arranged to selected a lower level sensor if too little heat is stored.
5. An apparatus as claimed in claim 2, 3 or 4, wherein the control unit is arranged to assess the sufficiency of the amount of heat stored based on a determination of if the heat storage apparatus runs out of heat, or becomes close to running out of heat.
6. An apparatus as claimed in claim 5, wherein a temperature sensor is provided for installation at the hot end of the storage apparatus, and the control unit is arranged to detect if this sensor indicates that the storage apparatus runs out of heat, or becomes close to running out of heat.
7. An apparatus as claimed in claim 1, where the parameter is a value for maximum heat storage, T2, and the desired operating condition is that the amount of cooling capacity remaining in the heat storage unit is at a certain level.
8. An apparatus as claimed in claim 7, wherein the control unit is arranged such that if the time between T2 being indicated by the selected temperature sensor and the cooling capacity running out is less than a lower time limit, then a higher level sensor for T2 is selected.
9. An apparatus as claimed in claim 7 or 8, wherein the control unit is arranged such that if the time is above an upper time limit, then a lower level sensor for T2 is selected.
10. An apparatus as claimed in claim 1, wherein the control unit is arranged to optimise both of a parameter that is a value for minimum heat storage required, T1, and a parameter that is a value for maximum heat storage, T2, and the control unit is arranged to select a sensor for each of T1 and T2 and to optimise the parameters by adjusting sensor level according to a predetermined condition for each parameter.
11. An apparatus as claimed in claim 10, wherein the control unit is arranged to optimise T1 in accordance with any of claims 2 to 6, and/or to optimise T2 in accordance with any of claims 7 to 9.
12. An apparatus as claimed in claim 10 or 11, wherein the control unit is arranged such that the initial sensor selected for T1 is above the initial sensor selected for T2, and such that during the optimisation process, T1 remains above T2.
13. An apparatus as claimed in any preceding claim, wherein the control unit is arranged to select an initial sensor for the or each hot water storage parameter based on the order in which the sensors are heated during operation of the water storage apparatus.
14. An apparatus as claimed in any preceding claim, wherein the temperature sensors are fitted in a water storage apparatus in which an amount of hot water stored can be determined by the vertical position of a hot/cold water boundary.
15. A method of controlling a water storage apparatus in which an amount of hot water stored can be determined by the vertical position of a hot/cold water boundary, the method comprising: using one of a plurality of temperature sensors to indicate a hot water storage parameter, the temperature sensors being at different levels in the water storage apparatus; and optimising the value of the parameter by selecting a sensor at a different level based on a desired operating condition.
16. A method as claimed in claim 15, wherein the parameter is a value for minimum heat storage required, T1, and the desired operating condition is that the amount of heat stored is sufficient to meet demand.
17. A method as claimed in claim 16, comprising selecting a higher level sensor if too much heat is stored.
18. A method as claimed in claim 16 or 17, comprising selecting a lower level sensor if too little heat is stored.
19. A method as claimed in claim 16, 17 or 18, comprising assessing the sufficiency of the amount of heat stored the based on a determination of if the heat storage apparatus runs out of heat, or becomes close to running out of heat.
20. A method as claimed in claim 19, comprising providing a temperature sensor at the hot end of the storage apparatus, and detecting if this sensor indicates that the storage apparatus runs out of heat, or becomes close to running out of heat.
21. A method as claimed in claim 15, where the parameter is a value for maximum heat storage, T2, and the desired operating condition is that the amount of cooling capacity in the heat storage apparatus is at a certain level.
22. A method as claimed in claim 21, comprising selecting a higher level sensor for T2 if the time between T2 being indicated by the selected temperature sensor and the cooling capacity running out is less than a lower time limit.
23 A method as claimed in claim 21 or 22, comprising selecting a lower level sensor for T2 if the time is above an upper time limit.
24. A method as claimed in claim 15, comprising optimising both of a parameter that is a value for minimum heat storage required, T1, and a parameter that is a value for maximum heat storage, T2, by selecting a sensor for each of T1 and T2 and adjusting the sensor level according to a predetermined condition for each parameter.
25. A method as claimed in claim 24, comprising optimising T in accordance with any of claims 15 to 20, and/or optimising T2 in accordance with any of claims 21 to 23.
26. A method as claimed in claim 24 or 25, comprising selecting an initial sensor for T1 which is above the initial sensor selected for T2, and preventing T1 from being below T2 during the optimisation process.
27. A method as claimed in any of claims 15 to 26, comprising selecting an initial sensor for the or each hot water storage parameter based on the order in which the sensors are heated during operation of the water storage apparatus.
28. A method as claimed in any of claims 15 to 27, including operating a cogeneration unit, such as a CHP internal combustion engine, based on the parameters indicated by the selected sensors.
29. A computer program product comprising instructions that, when executed on a control unit in a temperature sensor apparatus, will arrange the control unit to carry a method as claimed in any of claims 15 to 28.