GB2129169A - Thermal storage systems using liquid as a thermal store - Google Patents

Abstract

In a thermal storage or liquid heating system utilising a main heater or heaters 13, 14 at the bottom of a tank 10 energised from an off-peak power supply controlled by a logic unit 20 and switch 22 under the control of a temperature sensor 16, there are provided auxiliary heating means 25 with a 24-hour power supply 26 controlled by a temperature sensor 35 with a presettable control amplifier 36. The logic unit 20 ensures that the auxiliary supply is energised only outside off-peak periods when the return water temperature is below a predetermined value, when the tank top temperature is below a predetermined value and when a room temperature sensor calls for heat. <IMAGE>

Description

SPECIFICATION Thermal storage systems using liquid as a thermal store This invention relates to thermal storage systems and to the control of a liquid heating system such as for example a domestic thermal storage arrangement using liquid as the heat storage media.

In domestic installations it is well-known to use hot water in a thermally insulated tank for energy storage, the water being electrically heated, usually by an immersion heater during "off-peak" periods of the electricity supply. The hot water may be used for direct circulation in small bore or in microbore radiator systems. In domestic premises, the tank might typically hold 1 50 ga!lons and several heaters, typically each rated at 3 or 4 kw may be connected in parallel. The heaters are located at the bottom of the tank.

It is the common practice to effect temperature control using mechanical rod thermostats controlling the energisation of a contactor in the heater supply circuit Such mechanical thermostats however have a wide setting tolerance and, as a result, it is not normally possible to control the temperature to be near the boiling point of the liquid. The lower temperature reduces; the thermal storage capacity for a giyen size or tank.

:It is ,an object of the present invention to provide an improved thermal storage system and niore particularly a'control system for the heating of a liquid in a hot liquid storage tank.

According to one aspect of the present invention, in a thermal storage system having liquid forming thermal store in a tank and with at least one main heater at or near the bottom of the tank for energisation from an off-peak electricity supply and having at least one auxiliary electrically powered heater at or near the top of the tank for auxiliary heating and means for circulating liquid from and returning it to said tank, there is provided a control system including a first precision temperature sensor comprising a platinum resistance thermometer or a thermistor. or other solid state sensor located on the surface or in a pocket in the tank at or near the bottom thereof,.switching means for the main heater or heaters operative to complete the circuit for that heater when the sensor at or near the bottom of the tank indicates a temperature below its set value and logic control means for the auxiliary heater or heaters operative to complete a circuit for supplying power to that heater or heaters only in time periods when off-peak power supply to the main heaters is unavailable and only if the return water temperature sensed by a return water temperature sensor is below its set value and if a room temperature sensor is calling for heating and the top of the tank temperature sensed by the top of the tank thermostat is below its set value.

By using precision temperature sensors such as a platinum resistance sensor or a precision thermistor or other solid state sensor, it becomes possible to operate the system near to the boiling point of the liquid since such temperature sensors can be accurately set and have long period stability. Preferably each sensor has an associated electronic amplifier with a presettable temperature reference. It will be seen that the auxiliary heaters, which normally are only used in extreme adverse climatic conditions and which, for reasons of economy, must be used as little as possible, will only be switched on during peak periods of the supply and if there is a demand for heat from a storage system and if the return water tempeature shows that this demand cannot be met because of the low temperature of the water.

Preferably an overriding cut-out is provided for all the heaters responsive to a top limit temperature determined by said temperature sensor at or near the top of the tank or by another temperature sensor in the event of the water temperature reaching some predetermined upper limit.

Particularly if two or more main heaters are employed, failure of such a heater may not be detected and would result in premature operation of the auxiliary heater or heaters by reason of the thermal storage system not being fully charged during the off-peak period. It is preferred therefore to provide means for monitoring the operation of the or each main heater. This may be done by checking that the heater passes current when electrically energised. The monitoring may thus be effected by current sensing means arranged for sensing the current when the heater is connected to a supply for energisation and/or by feeding a low level current through the heater whilst it is non-operative. Conveniently a latched warning indicator is provided responsive to any failure of the heater detected by the monitoring means.

However, it may be preferred to monitor the operation of the or each main heater by checking whether or not the pre-set store temperature is reached during the off-peak period. This may be done using the aforementioned bottom temperature sensor and an indicator, e.g. a lamp indicator with latching means operative to indicate if the required temperature is not reached in the off-peak period.

In the following description, reference will be made to the accompanying drawings in which: Figure lisa diagram illustrating one embodiment of a domestic thermal storage system employing hot water storage, and Figure 2 shows a rnodification of the arrangement of Figure 1.

Referring to Figure 1, a tank 10, typically for holding about 1 50 million gallons of water and thermally lagged, constitutes a thermal storage system from which heat is withdrawn by circulating the water when required through a small bore or microbore radiator system. For this purpose, the tank 10 has a pipe 11 feeding water out to the radiators and a return pipe 1 2. Two or more main immersion heaters 13, 14 are provided in the bottom of the tank 10, these being connected in parallel for energisation from an off peak power supply source indicated diagrammatically at 1 5.During off-peak periods, temperature control is effected by means of a platinum resistance temperature sensor 1 6 or other precision temperature sensor located in a pocket at the bottom of the tank and connected via an electronic amplifier 1 7 having a control 18 for a pre-settable temperature reference signal.

The amplifier 1 7 is responsive to the difference between the reference temperature and the temperature sensed by sensor 16 and provides a signal to a logic unit 20, which includes means 21 energised from the off-peak power supply 1 for indicating time periods when the off-peak power supply is available and when it is not available.

The logic unit 20, in response to the output from the amplifier 17, during off-peak periods, closes a switch 22 in the supply to the heaters 13, 14 so that these heaters are energised during off-peak periods if the sensed temperature at sensor 1 6 falls below a predetermined reference value. The switch 22 may be a mechanical contactor or may be a solid state switching device.

The reference temperature for the amplifier 1 7 would normally be set to control the temperature to be near boiling point so as the ensure that during off-peak periods, the thermal storage system is charged substantially to the maximum thermal capacity of the tank.

At least one auxiliary heater 25 is positioned in the water near the top of the tank for use during peak periods in the event of the tank having discharged its useful heat energy. This heater is energised from an auxiliary heater supply 26 which is required to be available during peak periods. Power drawn from the supply 26 would therefore normally be charged at a much higher rate than power from the off-peak supply 1 5 and it is therefore desirable to minimise energy consumption from the supply 26.The supply to the heater 25 is controlled therefore by means of a switch 27 responsive to signals from the logic unit 20, this logic unit being arranged so that the heater 25 is only energised provided (a) the time is outside the time when the off-peak supply 1 5 is available, (b) the return water temperature from the heating system, as sensed by a temperature sensor 28 in the return water pipe, is below a preset value determined by a reference temperature control 29 on an amplifier 30, (c) a room temperature sensor 31 having a reference temperature setting means 32 is calling for heat and (d) the top of the tank temperature, sensed by a further temperature sensor 35 in a pocket in the tank 10 is below a set value determined by the setting of a control 33 on an amplifier 34.

The temperature sensor 35 is associated with a further amplifier 36 and reference temperature setting means 37 to provide, via the logic unit 20, an override cut-out signal for the main heaters 1 3, 14 and the auxiliary heater or heaters 25 in the event of the water temperature exceeding some predetermined upper limit.

The condition of each of the main heaters 13, 14 is monitored by a separate monitor, each monitor having a current sensor 40 and a monitoring circuit 41 including a latching unit responsive to any- sensed heater failure, and operating an indicator lamp 42. In the particular arrangement illustrated- the, condition of the heaters is monitored whilst they are energised by sensing the passage of current from-the power supply 1 5. It would alternatively be possible to monitor the condition of the heaters -whilst they were switched off by feeding a low level current through them in this conditiop.The latching arrangement in the monitors 4-1 would normally be arranged so that the latches carinot be reset by an externally accessible reset contrbl. The latch would normally not be reset until the heater or its associated wiring has been repaired.

The whole control system may conveniently be arranged in a wall mounting enclosure which may include an isolator to disconnect both off-peak and peak supplies.

In the arrangement of Figure 1 , the heater monitoring circuit monitors the possible failure of one or more bottom heaters. It may be preferred in some cases however to monitor the 'effect of the bottom heaters so that the monitoring will also cover possible malfunctions of the associated switch and drive logic controlling these heaters.

Such an arrangement is illustrated in. Figure 2 which shows a modification of the arrangement of Figure 1. In Figure 2, the same reference numerals are used as in Figure 1 for corresponding components and, in the following description, reference will me made only to the distinctive features of Figure 2.

In the arrangement of Figure 2, the bottom heaters 13, 1 4 are as before, only energised during off-peak periods but, to avoid having to provide two separate heavy duty cables to heaters in the supply tank (that is one with a 24-hour supply for the auxiliary heaters and one with an off-peak only supply for the main heaters 1 3, 14) both sets of heaters are fed from a 24-hour supply- 55; the switch 22 for the main heaters 13, 14 is controlled by the logic unit 20, which receives an enable signal from an off-peak supply 55, so that these heaters 13, 14 can only be energised during off-peak periods, as described with reference to Figure 1.

In Figure 2, the units 41 and 42 forming monitoring circuits responsive to the current to the bottom heaters are omitted. Instead, use is made of a latch unit 51 which is set by the application of off-peak supply to the heaters 13, 14 but is re-set by the bottom temperature sensor 28 and its associated amplifier circuit 30 if the predetermined temperature is reached during the off-peak period. The latch unit 51 controls a lamp indicator 52, which is labelled "low store", so that the lamp 52 is lit when the latch unit 51 is set.

If the required temperature is reached during the off-peak period, the latch unit 51 is re-set and lamp 52 is extinguished. If, however, the required temperature is not reached, the latch unit 51 remains "set" and the lamp 52 is lit. At the end of the off-peak period, if the latch unit is still "set", then the lamp 52 is caused to flash, by flasher means in unit 51, so as to provide a warning that some failure has occurred such that the required thermal charging has not been effected during the off-peak period. This arrangement thus not only gives a warning if there is a failure of one of the heaters 13, 14 but also if there is a malfunction of switch 22 or the associated drive logic.

Claims (9)

1. A thermal storage system having liquid forming a thermal store in a tank and with at least one main heater at or near the bottom of the tank for energisation from an off-peak electricity supply and having at least one auxiliary electrically powered heater at or near the top of the tank for auxiliary heating and means for circulating liquid from and returning it to said tank, wherein there is provided a control system including a first precision temperature sensor comprising a platinum resistance thermometer or a thermistor or other solid state sensor located on the surface or in a pocket in the tank at or near the bottom thereof, switching means for the main heater or heaters operative to complete the circuit for that heater when the sensor at or near the bottom of the tank indicates a temperature below its set value and logic control means for the auxiliary heater or heaters operative to complete a circuit for supplying power to that heater or heaters only in time periods when off-peak power supply to the main heaters is unavailable and only if the return water temperature sensed by a return water temperature sensor is below its set value and if a room temperature sensor is calling for heating and the top of the tank temperature sensed by the top of the tank sensor is below its set value.

2. A thermal storage system as claimed in claim 1 wherein each sensor has an associated electronic amplifier with a presettable temperature reference.

3. A thermal storage system as claimed in either claim 1 or claim 2 wherein an overriding cut-out is provided for all the heaters responsive to a top limit temperature determined by said temperature sensor at or near the top of the tank or by another temperature sensor in the event of the water temperature reaching some predetermined upper limit.

4. A thermal storage system as claimed in any of the preceding claims wherein means are provided for monitoring the operation of the or each main heater by checking that the heater passes current when electrically energised.

5. A thermal storage system as claimed in claim 4 wherein the monitoring means for each heater comprises current sensing means arranged for sensing the current when the heater is connected to a supply for energisation.

6. A thermal storage system as claimed in any of the preceding claims wherein the monitoring means for each heater comprises means for passing a low level current through the heater whilst the heater is non-operative.

7. A thermal storage system as claimed in any of claims 4 to 6 wherein a latched warning indicator is provided responsive to any failure of the heater detected by the monitoring means.

8. A thermal storage system as claimed in any of claims 1 to 3 wherein means are provided for monitoring the operation of the main heater or heaters comprising latching switch means operative to complete a circuit for energising an indicator when off-peak power is applied to the main heater or heaters with latch re-set means operatively controlled by a water temperature sensor to re-set the switch and extinguish the lamp if a predetermined temperature is reached during the off-peak period.

9. A thermal storage system as claimed in claim 8 wherein the indicator is a lamp and wherein flashing means are provided for the lamp operative to cause the lamp to flash if the latching switch means remains closed to complete the circuit for energising the lamp after the end of the off-peak period.

1 0. A thermal storage system substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.