Technical Field

The invention relates to a thermal storage and supply apparatus configured to store energy in water to meet a thermal energy demand.

It is known to meet a thermal energy demand in various settings by converting one form of energy into thermal energy. For example, domestic boilers convert chemical energy into thermal energy for use in domestic heating and hot water systems. Conventional fuels for such systems have known disadvantages including carbon emissions and connectivity requirements.

It is known to provide energy storage systems to store energy when it is readily available or inexpensive for subsequent use, such as electric batteries or thermal storage heaters (also known as night storage heaters). However, such systems typically require expensive materials or (in the context of thermal storage heaters) may be inefficient by virtue of thermal losses.

According to a first aspect there is disclosed a thermal energy storage and supply apparatus comprising: a pressure vessel; a heat exchanger; a circulation path extending from a steam outlet of the pressure vessel to a condensate inlet of the pressure vessel, via the heat exchanger; the circulation path including a discharge path extending from the steam outlet of the pressure vessel to the heat exchanger, and a return path extending from the heat exchanger to the condensate inlet; wherein the pressure vessel is configured to store primary process water as saturated liquid water and steam at a storage pressure of at least 0.5 MPa; wherein the heat exchanger is configured to receive a flow of secondary water, and to transfer heat to the secondary water from steam received from the steam outlet; the apparatus further comprising: an electrically-powered heater configured to heat liquid primary process water stored in the pressure vessel to vary a storage pressure within the pressure vessel; a valve arrangement disposed along the circulation path for controlling flow of primary process water through the heat exchanger along the circulation path; and a controller configured to: control the heater to heat liquid process water within the pressure vessel to reach a peak storage pressure of at least 0.5MPa; wherein the controller and/or the valve arrangement is configured to: selectively control flow of primary process water through the heat exchanger using the valve arrangement, to meet a thermal energy demand associated with the flow of secondary water.

The valve arrangement may comprise one or more valves disposed along the circulation path.

Control to meet the thermal energy demand as defined herein may comprise control to maintain a target thermal condition associated with the flow of secondary water, for example maintaining a thermal parameter associated with the thermal energy demand or flow of secondary water (such as a discharge temperature from the heat exchanger) within a target range.

It may be that the controller and/or valve arrangement is configured to selectively permit a flow of primary process water through the heat exchanger to reduce the storage pressure from the peak storage pressure by at least 50% of the peak storage pressure. It may be that the apparatus comprises a common support structure supporting the pressure vessel and the heat exchanger. It may be that the apparatus comprises a common housing for the pressure vessel and the heat exchanger. It may be that the pressure vessel has a volume of no more than 750 litres. It may be that the pressure vessel has a volume of no more than 600 litres.

The thermal storage and supply apparatus may be configured so that steam discharged through the steam outlet is provided to the heat exchanger directly without passing through an intermediate steam accumulator. Otherwise, any steam accumulator or steam accumulators between the pressure vessel and the heat exchanger may have a total volume which is less than a volume of the pressure vessel.

The controller may be configured to operate the heater to heat the primary process water independently of selectively controlling a flow of primary process water. The controller may therefore operate the heater simultaneously with controlling (e.g. causing or permitting) a flow of primary process water through the heat exchanger, and may also operate the heater to heat the primary process water without controlling (e.g. causing or permitting) such a flow of primary process water through the heat exchanger, and may also control (e.g. cause or permit) such a flow of primary process water through the heat exchanger without operating the heater to heat the primary process water. It may be that the return path is configured to at least partially fill with a column of condensate condensed from steam at the heat exchanger, to drive a return flow of condensate to the pressure vessel.

It may be that the return path includes the primary heat exchange path, and it may be that the primary heat exchange path is configured to at least partially fill with the column of condensate.

When the primary heat exchange path is at least filled by the column of condensate, the heat exchanger may be defined as (at least partially flooded).

It may be that the heat exchanger is arranged with respect to the pressure vessel so that the return flow of condensate from the column of condensate is driven by gravity.

For example, the return flow may be caused (e.g. be established or maintained) by a pressure head in the column of condensate, which may be referred to herein as a pressure head effect (e.g. irrespective of any temperature difference between the column of condensate and the process water). It may be that the return flow is established or maintained when a fill level of the column of condensate (a condensate fill level) is relatively higher than a fill level in the pressure vessel (a pressure vessel fill level) by an offset (a level offset), and a pressure head as discussed in the present disclosure is considered to be related to the presence of such a level offset. It is considered that, in the absence of a temperature difference between the column of condensate and the process water in the pressure vessel, the return flow is caused (e.g. established or maintained) when the condensate fill level is relatively higher than the pressure vessel fill level by a positive offset. This assumes the absence of any pumping of the return flow. The magnitude of the offset may be a function of any pressure losses in the return path and/or in the circulation path.

Further, the return flow may be caused (e.g. be established or maintained) by a buoyancy effect owing to the column of condensate having a higher density than the process water in the pressure vessel, for example when the column of condensate is subcooled (i.e. at a temperature lower than a saturation temperature for the process water in the apparatus, which may be a saturation temperature corresponding to the prevailing storage pressure in the pressure vessel).

It will be appreciated that the return flow may be caused (e.g. be established or maintained) by a combination of a pressure head effect and a buoyancy effect as described herein.

It may be that the return path is configured to open into the pressure vessel at a relatively lower portion of the pressure vessel, to return condensate to the pressure vessel. Relatively lower may be in the 50% of the pressure vessel by height, in the 30%, in the 20%, or in the lower 10% of the pressure vessel.

The return path may comprise a dip tube that extends downwardly within the pressure vessel, opening into the pressure vessel at its lower end. Alternatively, the return path may extend downwardly outside of the pressure vessel, and extend through a wall of the pressure vessel at the lower portion of the pressure vessel.

It may be that the heat exchanger is disposed relatively higher than the pressure vessel, and/or relatively higher than a peak liquid fill level of the pressure vessel corresponding to at least 90% of the volume of the pressure vessel.

It may be that the heat exchanger is disposed relatively higher than the pressure vessel (or the peak liquid fill level) by virtue of all portions of a heat exchange path through the heat exchanger being relatively higher than the pressure vessel (or the peak liquid fill level).

It may be that the controller and/or valve arrangement is configured to selectively control the flow of primary process water through the heat exchanger to operate the apparatus in a continuous flow mode while meeting the thermal energy demand. In the continuous flow mode, a condensate fill level of the column of condensate may be maintained relative to a pressure vessel fill level of liquid primary process water in the pressure vessel, so as to maintain a circulating flow of condensate from the column of condensate to the pressure vessel.

In the continuous flow mode, the condensate fill level is maintained relative to the pressure vessel fill level based on a pressure equilibrium between the pressure vessel and the column of condensate (e.g. based on a pressure head associated with a level offset between the condensate fill level and the pressure vessel fill level, counteracting any pressure losses along the return path). This pressure equilibrium may be in contrast to a cyclical mode of operation in which the condensate fill level rises and falls without maintenance of a pressure equilibrium. For example, when the condensate fill level rises (in a cyclical mode of operation), this may be owing to closure of a valve to prevent return of condensate; and when the condensate fill level reduces, this may be to return towards a pressure equilibrium by draining of condensate, but the pressure equilibrium is not achieved until draining is complete, and a rate of draining may be limited by a flow restrictor in the return path.

Alternatively or additionally, the continuous flow mode may be defined as a mode in which a rate of heat transfer at the heat exchanger is regulated to meet and not exceed the thermal energy demand, such that the condensate fill level is maintained relative to a pressure vessel fill level based on a steady pressure equilibrium. It may be that the controller and/or valve arrangement is configured to selectively control the flow of primary process water through the heat exchanger based on a monitored parameter relating to the thermal energy demand.

It may be that the heat exchanger is arranged with respect to the pressure vessel to define a thermosiphon arrangement between the pressure vessel and the heat exchanger, whereby the column of condensate in the return path is subcooled for return to the condensate inlet.

By virtue of being subcooled, the column of subcooled water has a higher density than the process water for return to the pressure vessel, for example owing to buoyancy forces. When the water is subcooled, a condensate fill level of the column (of subcooled water) may be relatively higher, relatively lower or equal to a pressure vessel fill level, while maintaining a return flow. Maintenance of a return flow despite a relatively lower condensate fill level may be possible owing to the relatively higher density of water in the column. Any level offset between the fill level in the column and the fill level in the pressure vessel may at least partially depend on any pressure losses along the return path and/or circulation path.

It may be that the controller and/or the valve arrangement is configured to selectively control the flow of primary process water through the heat exchanger to maintain a subcooled state of the column of condensate, to drive the return flow of condensate.

It may be that the valve arrangement comprises: a valve at an upstream valve location along the discharge path; and/or a valve at a downstream valve location along the return path.

It may be that the controller and/or valve arrangement is configured to operate the apparatus in a continuous flow mode in which the or each valve of the valve arrangement along the circulation path is in an open state to permit circulation of the primary process water throughout the circulation path.

It may be that the valve arrangement comprises a variable control valve at the upstream valve location or at the downstream valve location, the variable control valve configured to variably restrict flow of the primary process fluid along the circulation path, wherein the controller and/or the valve arrangement is configured to operate the variable control valve to regulate a rate of heat transfer at the heat exchanger to meet the thermal energy demand.

The variable control valve may be configured to regulate the rate of heat transfer at the heat exchanger by regulating a condensate fill level within the heat exchanger. The controller and/or the valve arrangement may be configured to operate the variable control valve to regulate a rate of heat transfer at the heat exchanger to meet and not exceed the thermal energy demand, thereby operating the apparatus in a continuous flow mode in which a condensate fill level is maintained in pressure equilibrium with a pressure vessel fill level while maintaining a return flow of condensate to the pressure vessel.

It may be that the variable control valve is at the upstream valve location.

In other words, the variable control valve may be disposed on the discharge path. When the variable control valve is at the upstream valve location (i.e., disposed on the discharge path), the valve arrangement may further comprise a valve at the downstream valve location (i.e. disposed on the return path), for example to fully isolate the heat exchanger from the pressure vessel. For example, the valve at the downstream valve location may be a binary valve rather than a variable control valve.

It may be that the variable control valve is at the downstream valve location.

In other words, the variable control valve may be disposed along the return path. When the variable control valve is at the downstream valve location (i.e., disposed on the return path), the valve arrangement may further comprise a valve at the upstream valve location, for example to fully isolate the heat exchanger from the pressure vessel. For example, the valve at the upstream valve location may be a binary valve rather than a variable control valve.

It may be that a temperature-responsive valve of the valve arrangement is provided with a temperature-responsive device configured to be in thermal communication with the flow of secondary water, wherein the temperature-responsive valve is configured to control opening and closing of the temperature-responsive valve to regulate heat transfer at the heat exchanger to meet the thermal energy demand.

The temperature-responsive valve may be a or the variable control valve. The temperature-responsive valve may be configured to regulate the rate of heat transfer at the heat exchanger by regulating a condensate fill level within the heat exchanger.

It may be that the temperature-responsive device is a metering bulb (also known as a sensing bulb), for example mounted on or in a line conveying the flow of secondary water (e.g. downstream of the heat exchanger). When the temperature-responsive valve is a variable control valve, it may be configured (together with the temperature-responsive device) so that a decrease in the temperature of the flow of secondary water (e.g. downstream of the heat exchanger) causes the control valve to reduce a restriction to the flow of process water along the circulation path to promote heat transfer at the heat exchanger, and so that an increase in the temperature of the flow of secondary water (e.g. downstream of the heat exchanger) causes the control valve to increase a restriction to the flow of process water along the circulation path to reduce heat transfer at the heat exchanger. It may be that a variable control valve is defined above is an electronic control valve. It may be that the controller is configured to monitor a parameter associated with the thermal energy demand or associated with the flow of secondary water, and to responsively control the variable control valve to regulate the flow of primary process water along the circulation path to maintain the parameter within a target range.

It may be that the controller and/or valve arrangement is configured to operate the apparatus in a cyclical fill mode in which the valve arrangement is selectively controlled or operates to alternately cause a condensate fill level to rise and fall in the return path, to regulate a heat transfer rate at the heat exchanger to meet the thermal energy demand.

It may be that the electrically-powered heater and the pressure vessel are configured so that the apparatus has an absolute charging period defined as a dimensional ratio of the useable thermal energy of primary process water stored at the peak storage pressure, divided by a rated power of the heater. The useable thermal energy of primary process water is defined relative to a baseline minimum operating pressure. It may be that the absolute charging period is at least 2 hours, optionally at least 4 hours, at least 6 hours or at least 8 hours.

According to a second aspect there is disclosed a method of operating a thermal energy storage and supply apparatus in accordance with the first aspect, comprising: heating liquid primary process water in the pressure vessel using the electrically- powered heater; the controller or the valve arrangement selectively controlling flow of primary process water through the heat exchanger, using the valve arrangement, to meet a thermal energy demand associated with the flow of secondary water.

It may be that the method comprises: a charging period in which the storage pressure is raised from a minimum operating pressure to the peak storage pressure, the charging period being at least 2 hours, optionally at least 4 hours, at least 6 hours or at least 8 hours; and a depletion period subsequent to the charging period, in which flow of primary process water through the heat exchanger is caused or permitted to meet a thermal energy demand, whereby the storage pressure reduces by at least 50% from the peak storage pressure.

The method may comprise operating the apparatus in a continuous flow mode to meet the thermal energy demand. It may be that, during the continuous flow mode, a condensate fill level of a column of condensate is maintained relative to a pressure vessel fill level of liquid primary process water in the pressure vessel based on a steady pressure equilibrium, while maintaining a continuous return flow of condensate to the pressure vessel.

The method may comprise operating the apparatus in a cyclical fill mode; wherein, during the cyclical fill mode, the valve arrangement alternately causes a condensate fill level to rise and fall in the return path, to regulate a heat transfer rate at the heat exchanger to meet the thermal energy demand.

It may be that the flow of primary process water is subcooled at the heat exchanger.

It may be that there is a return flow of condensate from the heat exchanger to the condensate inlet, driven by: a pressure head associated with a level offset between a condensate fill level of condensate in the return path and a pressure vessel fill level; and/or a buoyancy force associated with subcooling of the condensate in the return path, whereby the return flow is established in the manner of a thermosiphon.

The controller(s) described herein may comprise a processor. The controller and/or the processor may comprise any suitable circuity to cause performance of the methods described herein and as illustrated in the drawings. The controller or processor may comprise: at least one application specific integrated circuit (ASIC); and/or at least one field programmable gate array (FPGA); and/or single or multi-processor architectures; and/or sequential (Von Neumann)/parallel architectures; and/or at least one programmable logic controllers (PLCs); and/or at least one microprocessor; and/or at least one microcontroller; and/or a central processing unit (CPU), to perform the methods and or stated functions for which the controller or processor is configured.

The controller may comprise or the processor may comprise or be in communication with one or more memories that store that data described herein, and/or that store machine readable instructions (e.g. software) for performing the processes and functions described herein (e.g. determinations of parameters and execution of control routines).

The memory may be any suitable non-transitory computer readable storage medium, data storage device or devices, and may comprise a hard disk and/or solid state memory (such as flash memory). In some examples, the computer readable instructions may be transferred to the memory via a wireless signal or via a wired signal. The memory may be permanent non-removable memory, or may be removable memory (such as a universal serial bus (USB) flash drive). The memory may store a computer program comprising computer readable instructions that, when read by a processor or controller, causes performance of the methods described herein, and/or as illustrated in the Figures. The computer program may be software or firmware, or be a combination of software and firmware.

Except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore, except where mutually exclusive, any feature described herein may be applied to any aspect and/or combined with any other feature described herein.

Brief Description of the Drawings

The invention will be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 schematically shows an example energy storage and supply apparatus;

Figure 2 is a flow diagram of a method of operating the apparatus of Figure 1 ;

Figure 3 schematically shows an operational profile of the method of Figure 2;

Figure 4a schematically shows an example valve arrangement for the energy storage and supply apparatus;

Figures 4b and 4c schematically show example conditions in a circulation path of the apparatus of Figure 4a;

Figure 5a schematically shows an example valve arrangement for the energy storage and supply apparatus;

Figures 5b and 5c schematically show example conditions in a circulation path of the apparatus of Figure 5a; and

Figure 6 shows an example energy storage and supply apparatus.

Detailed Description

Figure 1 shows a first example thermal storage and supply apparatus 100. The apparatus 100 comprises a pressure vessel 110, a heat exchanger 120 and a circulation path 130 between them.

The pressure vessel 110 is configured to store primary process water 10 as saturated liquid water 10 and saturated gaseous steam 12 at a storage pressure of at least 0.5MPa, for example at least 0.7MPa. The expression “process water” or “process fluid” as used herein relates to supply of water (or other fluid) which the apparatus 100 acts on (e.g. uses) in operation. As will be described further below, the apparatus is configured to store and circulate primary process water 10 in a closed loop. All pressure values provided in the following disclosure relate to absolute pressure, rather than gauge pressure (which relates to a pressure relative to atmospheric/ambient pressure).

The example pressure vessel 110 is shown in Figure 1 in a filled condition in which it is filled with primary process water as saturated liquid water and saturated gaseous steam, such that the water has a pressure vessel fill level 14 which is the interface between the phases (i.e. between the liquid water and steam). The pressure vessel 110 (and associated parts of a circulation path to be described below) may be charged (e.g. filled) so as to only contain primary process water or a functionally equivalent process fluid, and to be substantially free of non-condensable gases. In this example, the pressure vessel 110 is charged with an amount of primary process water (i.e. a mass of primary process water) corresponding to the pressure vessel fill level 14 being at 90% of the volume of the pressure vessel at a peak storage pressure (to be described later). As will be appreciated, the specific volumes of saturated liquid water and of steam are functions of pressure, and so even for a captive (i.e. trapped) mass of primary process water, the pressure vessel fill level will vary depending on the storage pressure.

The pressure vessel 110 comprises a condensate inlet 112 and a steam outlet 114. The steam outlet 114 is located towards an upper end of the pressure vessel 110, whereas in this example the condensate inlet 112 is located towards a lower end of the pressure vessel. The expression “towards an upper end” may correspond to the steam outlet 114 being within the upper 20% of a height range of the pressure vessel (by reference to its internal volume), for example the upper 10%, the upper 5%, or substantially at the extreme upper end of the pressure vessel. The expression “towards a lower end” may correspond to the condensate inlet 112 being in the lower 50% of the height range of the pressure vessel, for example the lower 30%, the lower 20%, or the lower 10% of the height range of the pressure vessel.

The apparatus 100 comprises an electrically powered heater 140 which is configured to heat liquid primary process water 10 stored in the pressure vessel 110, to vary a storage pressure within the pressure vessel as will be described below. As shown in Figure 1 , the heater 140 is disposed in or extends into the pressure vessel 110 to heat the liquid primary process water 10. In this example, the heater is disposed towards the lower end of the pressure vessel, which may correspond to the heater being in the lower 50% of the height range of the pressure vessel, for example the lower 30%, the lower 20%, or the lower 10% of the height range of the pressure vessel. A circulation path 130 extends from the steam outlet 112 to the condensate inlet, via a primary heat exchange path 134 within the heat exchanger 120. The circulation path 130 includes a discharge path 120 extending between the steam outlet 112 and the heat exchanger 120, the primary heat exchange path 134 and a return path 136. As described below, the return path 136 may be considered to include the primary heat exchange path 134.

The heat exchanger 120 is configured to receive a flow of secondary water or other secondary fluid (e.g. a secondary process fluid) along a secondary heat exchange path 124 of the heat exchanger, in this example from a secondary system which circulates the flow of secondary water to a thermal load (not shown). Figure 1 shows a secondary loop 150 which interfaces with and/or forms part of the secondary system, and is incorporated in the apparatus 100. For example, the secondary loop 150 may have terminals for connecting to the secondary system, for example at locations corresponding to the extension lines (waved lines) as shown in Figure 1. Further components of the secondary system 90 are not shown, as any unshown features are not relevant to the technical description herein, for example the particular configuration of the thermal load of the secondary system is not relevant to the operation of the apparatus 100. In this example, the secondary loop 150 forms part of a secondary circulation path of the secondary system. The secondary loop 150 includes the secondary heat exchange path 124, and a pump 152 disposed for driving the secondary fluid along the secondary heat exchange path 124 (e.g. along the secondary circulation path including the secondary heat exchange path 124).

The heat exchanger is configured to transfer heat from the primary heat exchange path 134 to the secondary heat exchange path 124, in particular from steam or condensate in the primary heat exchange path 134 to the secondary fluid in the secondary heat exchange path 124. The heat exchanger may be of any suitable type, for example a cross-flow, counter-flow or co-current flow heat exchanger, and of any suitable construction (e.g. brazed plate or tube and fin).

Figure 1 schematically illustrates a level of a lower extent 135 of the primary heat exchange path 134 (within the heat exchanger 120), and indicates a level offset 16 (i.e. a height offset or vertical offset) between the lower extent 135 of the primary heat exchange path 134 and the pressure vessel fill level 14.

Figure 1 shows two valve locations 142, 144 for one or more valves of a valve arrangement (to be described below). A valve may be provided at one of the valve locations 142, 144, or respective valves may be provided at each of the valve locations 142, 144. The valve locations shown include an upstream valve location 142 along the discharge path 132 and a downstream valve location 144 along the return path 136, between the heat exchanger 120 and the condensate inlet 112.

Figure 1 shows a controller 160 operatively coupled to control and monitoring components of the apparatus 100. In this example, the control and monitoring components include a valve of the valve arrangement (e.g. at one of the valve locations 142, 144), the heater 140, and the pump 152. Various different operating parameters of the apparatus may be monitored in use, for example by the controller 160.

In this example, the monitoring components of the apparatus include a pressure sensor 162 configured to monitor a storage pressure in the pressure vessel. In this example, the pressure sensor 162 is disposed (e.g. extends into) an upper portion of the pressure vessel, for example above a peak pressure vessel fill level as described herein, so as to monitor a pressure of steam in the pressure vessel. Pressure in the pressure vessel is generally a function of the pressure of the steam and a depth below the pressure vessel fill line. Any variance in a pressure distribution of the steam within the pressure vessel is considered to be significantly lower than a variance in a distribution of the pressure in the liquid water in the pressure vessel, owing to the greater density of the liquid water in the pressure vessel. Accordingly, for simplicity and accuracy of control, it is considered to be advantageous to monitor the steam pressure in order to determine a representative storage pressure of the pressure vessel. Nevertheless, a pressure sensor may be disposed, or a plurality of pressure sensors distributed, at any location(s) throughout the pressure vessel for monitoring a storage pressure of the pressure vessel.

In this example, the monitoring components include a level sensor 164 for monitoring a condensate fill level in the return path. In this example, the level sensor 164 is disposed along the primary heat exchange path 134, but in other examples may additionally or alternatively be disposed along portions of the return path 136 outside of the primary heat exchange path 134, or elsewhere. A level sensor 164 may be implemented by any suitable means, for example by temperature sensors distributed along the respective path(a temperature-based level sensor), or by use of pressure sensors (a pressurebased level sensor).

For a temperature-based level sensor, a distribution of temperature sensors at a plurality of temperature sensor locations may be used to determine a condensate fill level based on condensate generally being subcooled (even by a small amount, such as 0.1°C or more, for example or 0.5°C or 1°C or more). The controller 160 may determine a saturation temperature based on a pressure sensor reading local to the primary heat exchange path 134, and determine that the condensate fill level is at a position lower than each of the temperature sensor positions which correspond to temperature readings at saturation temperature, and to determine that the condensate fill level is at a position higher than each of the temperature sensor positions which correspond to temperature readings below saturation temperature.

For a pressure-based level sensor, there may be a first pressure sensor local to the primary heat exchange path 134 (for example in the discharge path 132 downstream of any valve and upstream of the primary heat exchange path 134), and a second pressure sensor downstream of the heat exchanger. For example, the second pressure sensor may be in the return path 136 upstream of the condensate inlet. The second pressure sensor may be upstream of any valve at the downstream valve location 144, such that any pressure losses associated with the valve occur downstream of the second pressure sensor with respect to the direction of flow along the return path to the condensate inlet. The controller 160 may determine a condensate fill level based on a gas pressure determined based on monitoring the first pressure sensor, and a liquid pressure determined based on monitoring the second pressure sensor. A difference between the two pressures corresponds to a pressure head in the return path, and thereby the condensate fill level can be determined. The determination may further be based on monitoring temperature at one or more positions in the return path to determine a specific volume or density of the condensate to be taken into account in the determination of the condensate fill level.

Similarly there may be a temperature-based level sensor and/or a pressure-based level sensor in the pressure vessel operating on the same principles as sit woet out above.

In this example, the monitoring components include a secondary fluid temperature sensor 166 configured to monitor a temperature to which the flow of secondary water is heated.

In other implementations, a level sensor 164 and/or a temperature sensor 166 may not be required, and the disclosure envisages other forms of monitoring and control as will be discussed further with respect to specific examples below.

The apparatus or any implementation of it may comprise further components, a detailed description of which is not relevant considering the functionality and effects which are relevant to the present disclosure. For example, the apparatus may comprise safety components such as a pressure relief valve in the circulation path, and suitable insulation for the pressure vessel and/or circulation path. The example apparatus of Figure 1 , as configured for use in a domestic setting, has a pressure vessel 110 which is configured to store primary process water as saturated liquid water and saturated liquid steam at a storage pressure of at least 0.5MPa, for example 0.8MPa, or throughout a range of 0.5MPa-1.5MPa, for example 0.5MPa- 1.1 MPa. In this particular example, the pressure vessel is configured to store the primary process water at a peak storage pressure of 0.8MPa, and has a volume of approximately 385 litres. A volume of the pressure vessel may be selected to comply with regulations, for example to provide a product of pressure and volume which does not exceed 3,000 bar.L (gauge), which is a regulatory limit in various regulations associated with equipment categories and safety regulations (including the UK regulation “Pressure Equipment (Safety) Regulations 2015”, and the EU directive 97/23/CE “Pressure Equipment Directive”). The conversion from absolute pressure in MPa to bar is well known.. For the example pressures given above, the pressure vessel may have a volume in the range of approximately 200-750 litres. However, in some examples for a domestic setting, higher pressures (and products of pressure and volume) may be used (e.g. outside of the category A.2 range indicated above), including pressures associated with an industrial implementation as discussed further below.

The apparatus of Figure 1 , by way of example, is configured for use in a domestic setting for providing thermal energy to a domestic heating and/or hot water system (i.e. as the secondary system circulating the flow of secondary water as described above). However, it will be appreciated that a thermal storage and supply apparatus as disclosed herein can be configured for any size of thermal load and installation, and is not limited to domestic applications or installations. In alternative settings, for example for industrial usage, the pressure vessel may be configured to store process water as saturated liquid water and steam at higher pressures, for example at a storage pressure of at least 1 MPa, at least 2MPa, or throughout a range of 0.5MPa-3.5MPa, 0.5MPa-3MPa, or 0.5MPa- 2.5MPa, or 0.5MPa-1.1MPa.

Prior to use, a charging process may be conducted in which the apparatus is charged with primary process water. As part of the charging process, non-condensable gases may be removed by a purging process as is known in the art, and the pressure vessel or fluid circulation path may be provided with a purge port for this process. Such a purging process may be conducted at elevated temperature and pressure (as is known in the art of de-aeration), as non-condensable gases initially contained within the primary process water and also on and in the surfaces of the components of the apparatus. An example method 200 of use of the apparatus will now be described, with reference to the example apparatus 100 of Figure 1 , the flow diagram of Figure 2 showing the method 200, and the example operating profile of Figure 3.

The apparatus 100 is provided charged (e.g. filled) with an amount of primary process water corresponding to a pressure vessel fill level 14 of 90% by volume of the pressure vessel at a peak storage pressure, for example 0.8MPa.

Descriptions below with respect to the controller 160 controlling an entity (e.g. to cause an effect) are to be understood as also disclosing that the controller is configured to control that entity (e.g. to cause that effect).

In block 202 of the method 200, the controller 160 controls the heater 142 to heat the liquid primary process water 10 in the pressure vessel. By heating the liquid process water 10, for example from a starting pressure and starting saturation temperature, the temperature of the liquid process water increases. The starting pressure and saturation temperature may be relatively low, for example lower than a temperature to which the secondary water is to be heated or maintained.

The pressure vessel 110 and the circulation path 130 define a closed circuit for the primary process water. As such, as the process water is heated, the storage pressure rises (e.g. in the absence of heat transfer at the heat exchanger 120 and any other counterbalancing heat losses that equal or outweigh the heat input). The rise in pressure in relation to temperature will approximately follow the saturation curve for water, with the temperature becoming substantially equal between the liquid primary process water and the steam (i.e. both being at substantially the same saturation temperature).

In this example, the controller controls the heater 140 to heat the liquid primary process water to cause the storage pressure to reach the peak storage pressure, which in this example is 0.8MPa. As noted above, when the storage pressure is at the peak storage pressure, the pressure vessel fill level may be at a predetermined level commensurate with the amount of process water with which the apparatus is charged. In this example this is 90% by volume of the pressure vessel.

The storage pressure being at the peak storage pressure is representative of a peak amount of thermal energy being stored in the process water. It is advantageous for the fill level to be relatively high in order to maximise an amount of thermal energy that can be stored. Liquid water has a higher specific heat than steam and so can store more thermal energy per unit volume. In block 204, the stored thermal energy is provided from the primary process water stored in the apparatus 100 to the secondary heat exchange path 124 (e.g. to the secondary fluid which circulates from the secondary system through the secondary heat exchange path). More briefly, the stored thermal energy is used or consumed by heat transfer at the heat exchanger 120.

In general, the heat transfer occurs by the primary process water provided to the primary heat exchange path 134 transferring heat to the secondary water in the secondary heat exchange path 124, which is generally provided at a lower temperature than the temperature of the primary process water. A flow of primary process water for heat exchange at the heat exchanger 120 (i.e. along the primary heat exchange path 134) is controlled by the valve arrangement, for example by one or more valves as the respective valve locations 142, 144. The or each valve may be configured to operate to control the flow (e.g. by opening and closing) responsive to a monitored parameter, or may be controlled by the controller 160 responsive to one or more monitored parameters, as will be described further below.

When the process water is provided to the heat exchanger 120 in the form of steam, the steam condenses at the heat exchanger 120 and thereby provides its latent heat of evaporation to the heat exchanger for heat transfer. As is known in the art, this provides an efficient and relatively fast rate of heat transfer, even without sensible cooling (i.e. without a temperature drop). Nevertheless, heat is also transferred when the process water is provided in the form of liquid water or condensate within the primary heat exchange path 134, which generally results in sensible cooling of the liquid water (or condensate) and a relatively slower rate of heat transfer.

In the absence of additional heating by the heater 140, heat transfer from the primary process water at the heat exchanger 120 generally causes the storage pressure in the pressure vessel to reduce. As the pressure reduces, the saturation temperature of the primary process water also reduces.

Heat transfer at the heat exchanger can be maintained to meet a thermal demand of the secondary system throughout an operating pressure range of the apparatus. The disclosure envisages that an operating pressure range extends from the peak storage pressure down to significantly lower pressures, including pressures below ambient or atmospheric pressure. An operating pressure range refers to a range of pressures at which the temperature of the primary process water is sufficient to provide heating the secondary water, and may therefore depend on a heating demand associated with the secondary water, for example a target temperature at which the secondary water is to be maintained. For example, at 0.02 MPa, the saturation temperature of water is approximately 60°C which may be suitable for providing heat transfer to secondary which is to be maintained at 55°C (for example), such as water for a domestic heating system. A lower limit of an operating pressure range of the apparatus may be no more than 0.02MPa, for example no more than 0.03MPa, no more than 0.05MPa, no more than 0.1 MPa or no more than 0.2MPa. These lower limits for the operating pressure range may be combined with any peak storage pressure as described herein to define an operational pressure range over which the apparatus is configured to operate, and over which the controller is configured to operate the apparatus to provide heat transfer to the secondary water (e.g. 0.02MPa-3.5MPa, 0.02MPa-2.5MPa, 0.02MPa-2MPA, 0.02MPa- 1MPa).

Although the flow diagram of Figure 2 shows block 204 below block 202, it is to be appreciated that the flow diagram does not imply a limited order of operation or use of the system. Heating of the primary process water (as in block 202), and heat exchange at the heat exchanger 120 (as in block 204) may be conducted separately, together, and in any sequence.

Figure 3 shows an example profile of operation of the apparatus 100 over a time period t. Figure 3 includes a profile of storage pressure over time, together with superposed blocks indicating whether heating of the primary process water (202) and/or heat exchange at the heat exchanger 120 (204) occurs. In the profile of operation the storage pressure gradually increases from a starting pressure as the primary process water is heated, to reach a peak storage pressure 302 (which in this example is 0.8MPa). As the primary process water is heated the storage pressure 302 crosses a threshold corresponding to a lower limit of an operating pressure range, which in this example is 0.02MPa.

As shown in Figure 3, the storage pressure reduces from the peak storage pressure 302 as heat is exchanged at the heat exchanger 120 to meet a thermal energy demand of the secondary system (204). Over time, the storage pressure reduces overall despite several phases in which additional heating (202) is conducted with the heater 140 to temporarily raise the storage pressure. As shown by the overlapping blocks 202, 204, at some phases of operation both heating with the heater 140, and heat transfer to the secondary system with the heat exchanger 120 occur at the same time, whereas at other times only one of these operations occurs. Towards the end of the example operating profile, the storage pressure is gradually reduced and passes below the lower limit of the operating range, at which point heat transfer to the secondary system at the heat exchanger 120 stops. The system may resume operation by once again raising the storage pressure as described above.

While heating using the heater 140 may occur at any time, the apparatus 100 permits electrical energy to be consumed (i.e. converted to thermal energy stored in the primary process water) when it is readily available. Electrical energy may be considered to be readily available when, for example, it is generated by a renewable energy source local to the apparatus 100, for example by a local solar or wind energy source. Electrical energy may be considered to be readily available when, for example, it can be purchased at a relatively lower cost considering a variable cost for the resource, for example a cost for supply from a grid (e.g. a local, state or international power grid) which varies during the course of a day depending. Such a cost may vary depending on supply and demand in the grid, or on predetermined costs set for respective times. The apparatus 100 permits electrical energy to be used when such a variable cost is relatively low, with the energy being converted to thermal energy as stored in the primary process water, for subsequent use as thermal energy in the secondary system. In the context of an example apparatus 100 used for heating a domestic heating system, it may be that electrical energy is generated local to the apparatus 100 by a local renewable resource at a time when there is little or no heating demand from the domestic heating system, or a variable cost of electricity from a grid tends to be relatively higher when there is a heating demand (e.g. during morning and early evening when a house is occupied), and relatively lower in periods when there is little of no heating demand (e.g. during the night). The apparatus 100 permits times when there is a heating demand to be decoupled from times when electrical energy is obtained, thereby permitting electrical energy to be obtained when it is readily available, as discussed above. For example, electrical energy may be used at night (e.g. from 20:00 to 05:00 hours) when its cost is relatively low to raise the storage pressure (e.g. to the peak storage pressure), and the stored thermal energy may be gradually discharged or depleted during periods when the secondary system has a heating demand (e.g. during daylight hours, such as from 0800-18:00 hours).

A relatively low power electrical heater may be provided for operation of the system, because the apparatus is not operated to rapidly store energy for immediate use, but is instead generally operated to store electrical energy as thermal energy over an extended period when the thermal energy is not required, and to later use the thermal energy. For example, an apparatus as disclosed herein, filled with primary process water as discussed above, may have a minimum charging period of at least 4 hours at the maximum rated heat output of the electrical heater, wherein the minimum charging period corresponds with raising the storage pressure through the operating range (e.g. 0.02MPa-3.5MPa, 0.02MPa-2.5MPa, 0.02M Pa-2 MPA, 0.02MPa-1MPa). The minimum charging period may be at least 6 hours or at least 8 hours. In other examples, the minimum charging period may be shorter, for example if a higher rated electrical heater is provided than necessary for such operation, or if it is anticipated that a large amount of electrical energy may become available for use or rapid storage as thermal energy. The minimum charging period may be defined and evaluated assuming substantially no thermal losses from the vessel during charging.

The power of the electrical heater may also be defined or limited by reference to an absolute charging period corresponding to the ratio of the useable thermal energy stored in the pressure vessel at peak storage pressure (e.g. 40 kWh) divided by the power rating of the electrical heater (e.g. 5 kW). The useable thermal energy may be determined by reference to a baseline minimum operating pressure or baseline saturation temperature for heat transfer, for example 0.02 MPa and 60°C. The useable thermal energy (in kWh) is the enthalpy difference (in kJ) between the stored primary process water at the peak storage pressure and the minimum operating pressure, divided by 3600 (to account for kJ being defined by reference to seconds, and kWh being defined by reference to hours). For example, the useable thermal energy may be 40 kWh, and the rating of the electrical heater may be 5 kW. This gives a more simple definition of a charging period (the “absolute charging period) defined by reference to the stored useable energy. Even for installations which have a lower operating pressure or saturation temperature than the example baseline values given above, a definition based on the baseline values may be calculated and provided. The absolute charging period (evaluated as a dimensional ratio) may be 8 hours or more, for example 6 hours or more, 4 hours or more, or 2 hours or more.

As noted above, a flow of primary process water to the heat exchanger 120 is controlled by the valve arrangement, to meet a thermal energy demand associated with the secondary system. The disclosure envisages various configurations of the valve arrangement to perform this function, and envisages operation of the apparatus in a continuous flow mode and a cyclical fill mode to control the amount of heat transfer at the heat exchanger 120, as will now be described.

A first example embodiment of the valve arrangement is schematically shown in Figure 4a, in which there is no valve at the upstream valve location 142 and there is a valve 444 at the downstream valve location 144. Figure 4a is a simplified illustration of the apparatus 100 of Figure 1 , having the same configuration except for with respect to the valve arrangement, for which a specific valve arrangement will now be described. Accordingly, like reference numerals are used between Figure 1 and Figure 4a and apply to each drawing respectively.

In the example of Figure 4a, the valve 444 is configured to move between open and closed positions, and is not controllable to settle at an intermediate position. This type of valve is referred to herein as a binary valve, and may be otherwise referred to as an on/off valve or an isolation valve. A suitable type of binary valve, by way of example, is a solenoid valve. A binary valve as referenced in this disclosure differs from a variable control valve which may be configured to move between and be maintained at any of a plurality of open positions corresponding to various flow restrictions, and a closed position.

Figure 4b schematically shows a portion of the circulation pathway 130 including part of the discharge path to the heat exchanger 120, the primary heat exchange path 134 and a part of the return path 136 (noting that the primary heat exchange path 134 may be considered to form part of the return path 136 as discussed above). Figure 4b and similar drawings herein will be referenced below to describe the effect of valve control, as follows.

When the downstream valve 444 is closed, the circulation pathway 130 is closed for flow from the steam outlet 114 to the condensate inlet 112. In operation of the apparatus with the downstream binary valve 444 closed, a column of condensate 402 is maintained or accumulates in the return path 136, and a fill level 135 of the column of condensate in the return path 136 may be such that it is located within or above the primary heat exchange path 134, such that the heat exchanger is partially or fully flooded. Use of the expression “flooded” herein relates only to the primary heat exchange path 134 and not the secondary heat exchange path 124. In this example, above the column of condensate 402 there is a region 404 of steam which is in fluid communication with pressure vessel so as to be at the storage pressure.

With the downstream valve 444 closed, the column of condensate 402 does not return to the pressure vessel via the condensate inlet, and so the fill level 135 rises without any pressure head effect acting to balance the condensate fill level 135 (of the column of condensate) relative to the pressure vessel fill level 14 (as will be described further below), and as such the level offset 16 can grow independently of the fill level 14. The column of condensate may be subcooled below the saturation temperature. Heat transfer at the heat exchanger may cause the condensate fill level 135 to rise above the primary heat transfer path 134 so that the heat exchanger 120 is completely flooded, as shown in Figure 4c.

As is noted above, heat transfer at the heat exchanger 120 is conducted at a relatively higher rate when the primary heat exchange path 134 is provided with steam as opposed to condensate. Accordingly, a rate of heat transfer at the heat exchanger may be dependent on the condensate fill level in the heat exchanger 134. For example, assuming comparable conditions on the secondary side of the heat exchanger 120, in particular an inlet temperature and flow rate of the secondary water, the rate of heat transfer is relatively higher when the heat exchanger is unflooded (i.e. the fill level 135 of the column of condensate is below the primary heat exchange path), and reduces as the heat exchanger is increasingly flooded (i.e. as the fill level of the column of condensate causes more of the primary heat exchange path 134 to be occupied with condensate).

As the column of condensate 402 is effectively trapped, further heat exchange with the secondary water causes subcooling of the primary process water, and as such a rate of heat transfer may progressively reduce. When there is no heating demand on the secondary side of the heat exchanger, an equilibrium may be reached over time where there is substantially no (or no) heat transfer from the column of condensate to the secondary side of the heat exchanger.

When the downstream valve 444 is opened, starting from a condition as described above with respect to Figure 4b or Figure 4c, a pressure head effect owing to the level offset 16 and/or buoyancy effects owing to a higher density of any subcooled condensate drives condensate in the column 402 to return to the pressure vessel via the condensate inlet. Return of the condensate under the action of buoyancy effects may be referred to herein as operation (of the apparatus) as a thermosiphon. The level offset 16 settles to an amount corresponding to there being a sufficient pressure head effect to overcome any pressure losses in the circulation path 130. As such, the level offset 16 will typically be positive such that the condensate fill level 135 is higher than the pressure vessel fill level, and the level offset 16 is proportional to pressure losses in the circulation path 130.

By opening the valve, the flooding in the heat exchanger reduces, such that more of the primary heat exchange path is exposed to steam and the heat transfer rate increases.

The example apparatus of Figure 4a may be operated in a continuous flow mode and a cyclical fill mode when there is a thermal energy demand. The controller 160 may control the pump 152 to circulate the secondary water when there is a thermal energy demand. In variant examples, there may be no pump associated with the secondary water as part of the apparatus 100, for example the secondary water may naturally circulate within the secondary system, or the secondary system may have a separate pump which is independent of the apparatus 100.

The thermal energy demand may correspond to a demand for thermal energy at a thermal load of the secondary system, and may be variable. The thermal energy demand and a determination of whether it is being met may be made by reference to a monitored parameter. The monitored parameter may be communicated to the controller 160, for example from a thermal load of the secondary system outside of the apparatus 100. The monitored parameter may be a parameter relating to the secondary water and determined by monitoring of the secondary water within the apparatus 100. For example, the monitored parameter may be a discharge temperature of the secondary water, as monitored by a temperature sensor 166 downstream of the secondary heat exchange path 134. However, it will be appreciated that a thermal energy demand may be determined by monitoring one or more other parameters relating to the secondary water (e.g. a flow rate, inlet temperature) or of the thermal load.

In the continuous flow mode, the downstream valve is open and a steady state condition is established wherein the heat transfer at the heat exchanger is maintained at a rate sufficient to meet a thermal energy demand associated with the secondary system. Operation in the continuous flow mode may correspond to operation at conditions on the primary and secondary sides of the heat exchanger which naturally balance. For example, the saturation temperature and condensate fill level 135 in the return path 136, together with the temperature of the secondary water, secondary water flow rate and thermal energy demand, may be such that the resulting heat transfer is sufficient to meet and not exceed the thermal energy demand while maintaining a circulating flow of primary process water in the circulation path 130. In the continuous flow mode, the condensate fill level 135 and associated level offset 16 cause a continuous flow of condensate returning to the pressure vessel via the condensate inlet 112, under action of a pressure head effect corresponding to the level offset 16 and/or a buoyancy effect associated with any subcooling of the condensate. As noted above, return of the condensate under the action of buoyancy effects may be referred to herein as operation as a thermosiphon. The condensate fill level 135 may be substantially constant or maintained within a target range in the continuous flow mode. The controller 160 may be configured to operate the apparatus in the continuous flow mode when the condensate fill level 135 is determined to be within a predetermined level range in the return path, and/or when a rate of change of the condensate fill level 135 is below a threshold value corresponding to substantially steady state operation. The controller 160 may determine (e.g. estimate) the condensate fill level 135 by any suitable means, for example by reference to an output of a level sensor 164 as described in and/or a model relating to a predicted level. For example, a fill level may be determined (e.g. estimated) by a model which determines an amount of heat transfer at the heat exchanger (e.g. by reference to monitored inlet and outlet temperatures of the secondary water) and a storage pressure and/or saturation temperature, and determines the corresponding fill level based on a relationship or database of values reflecting modelled or empirically determined conditions.

As will be appreciated, a rate of change of storage pressure corresponds to the rate of heat transfer at the heat exchanger (in the absence of any heating input using the electrical heater, or once any impact of such heating on the storage pressure is adjusted for), since the storage pressure reduces as steam is condensed. The controller may be configured to operate the apparatus in the continuous flow mode to maintain the rate of change of storage pressure at a rate corresponding to the thermal energy demand.

The controller 160 may be configured to adjust a thermal energy demand associated with the secondary water in order to maintain operation in the continuous flow mode.

In a cyclical fill mode, the condensate fill level 135 is permitted to vary owing to an imbalance between a heat transfer rate at the heat exchanger 120 and a thermal energy demand associated with the secondary system, with the downstream valve 444 being controlled to maintain a target parameter associated with the secondary system within a target range.

For example, a set point temperature for the secondary water (i.e. the temperature to which the secondary water is to be heated and maintained) may be specified by the controller. An example set point temperature is 55°C. The target parameter may be a monitored temperature of the secondary fluid (e.g. as monitored by a secondary fluid temperature sensor 166). A target temperature range may be specified around the set point temperature range, for example 50°C-60°C (e.g. based on a set point margin of 5°C).

The controller is configured to operate the downstream valve 444 to close when the monitored parameter reaches a limit of the target range corresponding to excessive heating (in this example the monitored temperature reaching the upper limit of the target temperature range), and to open when the monitored parameter reaches a limit of the target range corresponding to insufficient heating (in this example the monitored temperature reaching the lower limit of the target temperature range).

As described with above, when the downstream valve is closed 444 the condensate fill level will tend to increase (e.g., to increasingly flood the heat exchanger) and the rate of heat transfer will reduce. When the downstream valve 444 is opened the condensate fill level will tend to increase and the rate of heat transfer will increase.

Additionally or alternatively, the controller may monitor a rate of change of storage pressure (and optionally any heating input) to determine a rate of heat transfer, and control opening and closing of the valve to regulate the rate of heat transfer to meet the thermal energy demand. This may have the same impact on the raising and lowering of the condensate fill level as set out above.

The controller 160 is configured to operate the example apparatus 100 of Figure 4a in a no-demand condition in which there is no thermal energy demand associated with the secondary system. To avoid excessive heating of the secondary fluid in the secondary heat exchange path 134 of the heat exchanger when there is no thermal energy demand, the controller closes the downstream valve 444 (or maintains it closed), such that any residual heat transfer causes the column of condensate 402 to rise and become subcooled, thereby limiting heat transfer. The controller 160 may be configured to control an auxiliary pump 153 in a bypass line provided in parallel to the second heat exchange path 124, as shown in Figure 1 (and in Figures 4a, 5a) to circulate the secondary water around the secondary system despite in the no-demand condition, despite there being no thermal energy demand, to avoid excessive heat transfer to a portion of the secondary water.

In the example of Figure 4a, the apparatus 100 is configured for partial isolation of the heat exchanger in the no-demand configuration, by virtue of preventing fluid communication via the condensate inlet 112, but not limiting fluid communication via the steam outlet 114. Despite being in fluid communication with steam in the upper portion of the pressure vessel, heat transfer from the pressure vessel to the secondary side of the heat exchanger is limited in the partial isolation condition once a column of subcooled condensate is established, as heating between the interface of the subcooled condensate and the steam is over a limited interface area for heat transfer. Although the description above refers to the downstream valve 444 being controlled by the controller 160, the disclosure also envisages that the downstream valve 444 can be configured to automatically open and close based on a temperature of the secondary water (e.g., without separate processing of a controller). In particular, the downstream valve 444 may be provided with a sensing bulb in thermal communication with a discharge line from the secondary heat exchange path 134 (e.g., in a location corresponding to the temperature sensor 166 in Figure 1), and may be configured and calibrated so as to open and close corresponding to the respective temperature of the secondary water (e.g., in the manner of a thermostatic valve). The valve may be configured and calibrated so as to open and close corresponding to crossing a threshold temperature, or reaching upper and lower limits of a target temperature range as disclosed above.

While the above description of the example of Figure 4a refers to the downstream valve 444 as a binary valve, it may instead be a control valve (i.e., a variable control valve, such as a proportional control valve) configured to move between a plurality of open states to effect a variable flow restriction, and a closed state.

When the downstream valve 444 is a variable control valve, the controller 160 (or a dedicated controller of the valve) may be configured to control the valve to regulate heat transfer at the heat exchanger. Such control may have the effect of regulating and/or maintaining the condensate fill level 135 so as to maintain a continuous return flow of condensate to the pressure vessel under the action of pressure head and/or buoyancy effects (operation as a thermosiphon).

The controller may be configured to control the variable control valve based on monitoring one or more parameters corresponding to meeting the thermal energy demand. For example, the controller may be configured to monitor a condensate fill level based on a target fill level, with the target fill level being determined based on a relationship or model to correspond to meeting a thermal energy demand. The controller may be configured to monitor a rate of change of storage pressure, and optionally a rate of any heating using the electrical heater, to determine a rate of heat transfer at the heat exchanger. The controller may be configured to monitor a parameter relating to the thermal energy demand of the secondary system (for example a discharge temperature of the secondary water from the secondary heat exchange path as discussed above, or a parameter received from outside of the apparatus 100, such as from the thermal load of the secondary system). By varying the downstream variable control valve 444 to regulate heat transfer to meet the thermal energy demand, the controller may extend a portion of the operating map of the apparatus 100 in which it can operate in the continuous flow mode, and avoid operation in the cyclical fill mode.

Although the above description refers to there being no valve at the upstream valve location, in a variant example of the example of Figure 4a described above, there is additionally a valve at the upstream valve location 142. In the variant example, the valve is a binary valve as described above, but may equally be implemented as a variable control valve. In the variant example, the controller 160 is configured to control operation of the apparatus substantially as described above, but is additionally configured to close the upstream valve in the no-demand condition to fully isolate the heat exchanger from the pressure vessel (i.e. , isolating it both from the steam outlet 114 and the condensate inlet 112). The controller may be configured to, in the no-demand condition, first close the upstream valve while leaving the downstream valve 444 open, so as to permit steam and condensate in the return path 136 to condense and subcool below an isolation threshold temperature. The controller may be configured to subsequently close the downstream variable control valve 444 to fully isolate the heat exchanger. By first closing the upstream valve and permitting subcooling in the return path 136, a situation may be avoided in which high pressure and high temperature fluid is effectively trapped in the circulation path, which may lead to excessive pressure differences over the valves when the trapped fluid is further cooled (with corresponding condensation and pressure drop).

Figure 5a shows a further example implementation of the apparatus 100 of Figure 1 . As with Figure 4a, the example of Figure 5a differs from the apparatus 100 as described with respect to Figure 1 by virtue of features concerning the valve arrangement, and like components are indicated with like reference numerals. Again, Figure 5a is a simplified illustration of substantially the same apparatus 100 as illustrated in Figure 1.

In the example of Figure 5a, the valve arrangement comprises an upstream valve 442 at the upstream valve location, which is a variable control valve, and a downstream valve 444 which in this example is a binary valve (but in variant examples could equally be implemented as a variable control valve).

The apparatus 100 of Figure 5a is configured for operation as described above with respect to Figures 1-3, and the following further features are disclosed in relation to operation to meet the thermal energy demand associated with the secondary system. As with the example of Figure 1 , the controller 160 is configured to operate the apparatus in a continuous flow mode. In this example, the controller 160 is also configured to operate the apparatus in a cyclical fill mode.

As above, any description below with respect to the controller controlling an entity (e.g., to cause an effect) are to be understood as also disclosing that the controller is configured to control that entity (e.g., to cause that effect).

When the upstream variable control valve 442 is open (e.g., in a fully open state) and the downstream valve 442 is open, there may be substantially no pressure drop at the valve 442. As such, steam provided to the primary heat exchange path 134 from the steam outlet 114 and condensing to reject heat at the exchanger may accumulate in the return path 136 as a column of condensate 402 as described above with respect to the example of Figures 4a-4c. The condensate level 135, relative to the pressure vessel level 14 at an equilibrium condition, is a function of pressure head effects of the column of condensate 402 as counteracted by any pressure losses in the circulation path, and/or buoyancy effects that arise if the condensate is subcooled (thereby returning condensate to the pressure vessel as a thermosiphon).

As noted above with respect to the example of Figures 4a-4c, when the valve arrangement does not restrict flow along the circulation path 130 (i.e. , when the or each valve is in an open or fully open state), a rate of heat transfer at the heat exchanger 120 may be suitable to meet the thermal energy demand, or may result in excessive heat transfer.

In the example of Figure 5a, as further described with reference to Figures 5b and 5c below, the upstream valve 442 is operable to regulate flow of primary process water along the circulation path. The upstream valve 442 is configured to variably restrict steam flow from the steam outlet 114 to the heat exchanger 120. A partial restriction causes a pressure drop over the valve. Figure 5b schematically illustrates respective regions of the circulation path 130 when there is a partial restriction provided by the upstream valve 442, and when any downstream valve 444 is open.

A part 504 of the discharge path 132 upstream of the upstream variable control valve 442 is in fluid communication with the steam in the pressure vessel such that is at substantially the same pressure (i.e., the prevailing storage pressure). A part 506 of the discharge path downstream of the upstream variable control valve 442 comprises steam at an intermediate pressure lower than the storage pressure, by a pressure drop corresponding to the restriction provided by the valve 442. A column 402 of condensate in the return path 402 is in fluid communication with primary process water in the pressure vessel via the condensate inlet 112. The column 402 of condensate is also acted on by the steam in the part 506 at intermediate pressure.

In operation (e.g., at an equilibrium state), the condensate fill level 135 may be above the pressure vessel fill level 14 by a level offset 16 to provide a pressure head effect which counteracts the pressure drop to intermediate pressure and drives a flow of condensate to return to the pressure vessel via the condensate inlet. Further, where the condensate is subcooled, a buoyancy effect may act to drive condensate to return to the pressure vessel (e.g., by action of a thermosiphon).

In the specific example of Figure 5b, the condensate fill level 135 is below the primary heat exchange path 134 in the heat exchanger, and so it may be that the column of condensate is not subcooled, and may only be driven to return to the pressure vessel via the condensate inlet owing to a pressure head effect.

By way of example, Figure 5c illustrates the same portion of the circulation path in a condition corresponding to the upstream variable control valve 442 providing a relatively greater flow restriction. As compared with the condition in Figure 5b, the pressure in the part 504 of the discharge path 132 upstream of the valve 442 is substantially the same (e.g., at storage pressure). The intermediate pressure in the part 506 downstream of the upstream control valve 442 is relatively lower than the intermediate pressure in the condition of Figure 5b. As such, there is a different equilibrium condition in which the level offset 16 is relatively greater, to provide a sufficient pressure head effect to counteract the relatively lower intermediate pressure. This relatively greater level offset 16 and higher condensate fill level 135 is shown in Figure 5c. In this example, the primary heat exchange path 134 is partially flooded in this condition, and so the condensate may be subcooled to provide a buoyancy effect to drive a return flow of condensate via the condensate inlet 112 (e.g., acting as a thermosiphon).

In use, the controller 160 (or a controller of the valve 442) controls the upstream variable control valve 442 to meet the thermal energy demand, in particular by regulating a flow of primary process fluid from the steam outlet 114 to the heat exchanger. The discussion above (with reference the example of Figures 4a-4c) regarding the controller controlling the variable control valve based on monitoring one or more parameters corresponding to meeting the thermal energy demand applies equally to the example of Figures 5a-5c. Equally, the disclosure also envisages that the upstream control valve can be automatically controlled based on a temperature of the secondary water (e.g., without separate processing of a controller). In particular, the above description relating to the example provision of a sensing bulb in thermal communication with a discharge line from the secondary heat exchange path 134 applies equally to control of the variable control valve 442.

By controlling the upstream variable control valve 442 to meet the thermal energy demand, the apparatus 100 can be operated in the continuous flow mode as described above. The above description does not rely on operation of a downstream valve 442. However, the variable restriction and variable pressure loss along the circulation path can equally be provided by a downstream variable control valve 444, or by a combination of an upstream and a downstream variable control valve (albeit at added system expense and complexity).

With the configuration discussed above, the apparatus 100 may be operated in the continuous flow mode throughout an operating map of the apparatus (e.g., throughout a range of thermal energy demands and operating pressures), and as such the controller may not be configured to control the apparatus in a cyclical fill mode as described above. Nevertheless, in case of an equilibrium condition for operation in the continuous flow mode cannot be determined in operation, the controller 160 may be configured to operate the apparatus in the cyclical fill mode as described above, for example by control of the downstream valve 442.

In a no-demand condition, the upstream variable control valve 442 is closed (e.g., by a control action of the controller) to prevent flow of steam to the heat exchanger 120 for heat transfer. In this example, the downstream valve 442 (which is a binary valve in this example) is also closed to fully isolate the heat exchanger 120, and the two valves may be closed in sequence as described above.

However, in other examples, there may be no downstream valve 442 and as such the primary heat exchange path 134 of the heat exchanger may remain in fluid communication with the primary process water in the pressure vessel 110 via the return path 136 (thereby providing a partially isolated condition of the heat exchanger 120 as described above). In such an implementation of the apparatus 100, heat exchange at the heat exchanger 120 would cause steam to condense and form a column of subcooled water. The column of subcooled condensate may be prevented from returning to the pressure vessel in the absence of any replenishing supply of water from an upstream source. Accordingly, the column of condensate may effectively thermally isolate the heat exchanger from the process water in the pressure vessel. There may be a limited amount of buoyancy-driven bi-directional flow through the condensate inlet, but this may be sufficiently low to avoid excessive heating of secondary fluid in the secondary heat exchange path 124.

Among the example implementations of a thermal energy storage and supply apparatus described above, implementations using either one or more binary valves, and/or one or more variable control valves are disclosed. It is considered that control with a variable control valve may be preferable for establishing a continuous flow mode for all or a relatively larger part of an operating map of the apparatus, to provide better performance for meeting a thermal energy demand (e.g., maintaining a temperature of discharged secondary water closer to a set point). For example, control of the variable control valve may be implemented using a PID control method (proportional integral derivative control) or similar control method to provide consistent performance with respect to a thermal energy demand or target parameter. In contrast, implementations utilizing one or more binary valves may operate in a cyclical fill mode for a relatively larger part of the operating map, with consequently poorer performance in accurately meeting a set point. Nevertheless, binary valves may offer advantages, such as being less expensive and simpler to control.

An apparatus according to any of the examples disclosed herein may be configured so that, in the cyclical fill mode of operation, there is a flow restriction in the circulation path (e.g., at the downstream valve location or at the upstream valve location) to slow a rate at which an accumulated column of condensate is returned to the pressure vessel when the or each valve of the valve arrangement is opened. This may slow a rate at which the condensate fill level settles at an equilibrium state relative to the pressure vessel fill level (e.g., under the action of a pressure head effect and/or buoyancy effect as described above), which is referred to in the following discussion as draining of the column of condensate. Slowing the rate of draining may be desirable to slow a rate at which a heat transfer rate at the heat exchanger increases (e.g., owing to an increased portion of the primary heat exchange pathway being exposed to steam). This may therefore reduce a switching period of the cyclical fill mode (i.e. , a period over which it switches between accumulating and returning the column of condensate). Implementations of the apparatus may have a wide operating pressure range, and as such a flow restriction as described above may provide a first relative fast rate of draining use when the storage pressure is relatively high, and may provide a second relatively slow rate of draining when the storage pressure is relatively low. A further advantage of using a variable control valve for control of a cyclical fill mode is that a size of the restriction may be adjusted over the operating pressure range, and as such the draining rate may be decoupled from the prevailing storage pressure. Figure 6 shows a simplified illustration of the apparatus 100 as described with respect to any of the examples above, provided with a common support structure for the pressure vessel 102, which in this example is provided as a common housing with integrated support. In other examples, a common support structure (such as a structural support frame or bracket) may be provided independently of a common housing, and one or both of a common support structure and a common housing may be provided.