WO2023233161A1

WO2023233161A1 - Thermal energy storage

Info

Publication number: WO2023233161A1
Application number: PCT/GB2023/051442
Authority: WO
Inventors: Jeremy Miller; Andrew Slaney; Andrew Smith
Original assignee: Remit Zero Ltd
Current assignee: Remit Zero Ltd
Priority date: 2022-06-01
Filing date: 2023-05-31
Publication date: 2023-12-07
Anticipated expiration: 2024-12-01
Also published as: GB202208148D0

Abstract

A thermal energy storage arrangement comprises a storage vessel for storing liquid, a heat exchanger, a variable-speed pump, a temperature sensor and a control system. The storage vessel comprises a vessel liquid inlet and a vessel liquid outlet. The heat exchanger comprises a primary liquid inlet and a primary liquid outlet, wherein the primary liquid inlet is connected to the vessel liquid outlet and the primary liquid outlet connected to the vessel liquid inlet so as to form a primary liquid circuit. The heat exchanger also comprises a secondary liquid inlet and a secondary liquid outlet for connecting to a secondary liquid circuit. The variable-speed pump pumps liquid through the primary liquid circuit. The temperature sensor detects the temperature of liquid within the secondary liquid circuit. The control system controls the pumping speed of the variable-speed pump based on an output of the temperature sensor.

Description

Thermal Energy Storage

BACKGROUND

Traditionally, central heating and/or hot water systems utilise a boiler which uses a heater, whether electric or gas, to heat water held therein and to pump that heated water to radiators distributed around a building in order to heat that building. However, such systems are typically inefficient and suffer from the possibility of all the heated water within the boiler being used up by a consumer, leaving them without hot water or heating thereafter until water within the boiler is heated to a suitable temperature again.

Countries are steadily increasing their reliance on renewable energy sources e.g. wind, solar, tidal, etc. While renewable energy sources are excellent from an environmental standpoint, they are notoriously inconsistent in their power output: solar power requires sunlight and therefore does not produce energy at night, wind power is highly dependent on the weather, etc. There therefore exists a substantial need for means of storing energy generated using renewable sources in order to provide consistent power to consumers at times when the sources themselves are not generating sufficient power. Lithium-ion batteries have been proposed as one such energy storage mechanism, though these are expensive to manufacture, degrade quickly, and even modern batteries are not fully efficient - they do not enable 100% of the energy input to the battery to be output again when desired, as a large proportion of the energy is lost as unwanted heat. Furthermore, large-scale lithium-ion batteries require cooling which further decreases their overall efficiency as the cooling itself draws power.

Flywheel-based energy storage systems have also been proposed which use a high-speed, low-resistance flywheel suspended in a vacuum to store energy in the form of rotational kinetic energy. Kinetic energy is provided to the flywheel using an electric motor, and extracted, when desired, using a generator. However, such systems are also not efficient, due to unavoidable energy losses to friction (i.e. heat) that are generated as a result of the flywheel’s motion. Most previously proposed energy storage solutions, like lithium-ion batteries and flywheel-based systems, take input in the form of electrical energy and generate output also in the form of electrical energy, though less than the energy initially input due to their inherent inefficiencies. For heating applications, that electrical energy output needs to then be converted into thermal energy, another process which does not offer 100% efficiency. The inefficiencies in storing and extracting energy, and then converting that energy into thermal energy, are therefore compounded in such solutions, reducing their overall efficiency.

The present invention aims to address at least some of the issues set out above.

SUMMARY OF THE INVENTION

When viewed from a first aspect, the invention provides a thermal energy storage arrangement comprising: a storage vessel for storing liquid, the storage vessel comprising a vessel liquid inlet and a vessel liquid outlet; a heat exchanger, comprising: a primary liquid inlet and a primary liquid outlet, wherein the primary liquid inlet is connected to the vessel liquid outlet and the primary liquid outlet connected to the vessel liquid inlet so as to form a primary liquid circuit; and a secondary liquid inlet and a secondary liquid outlet for connecting to a secondary liquid circuit; a variable-speed pump arranged to pump liquid through the primary liquid circuit; a temperature sensor arranged to detect a temperature of liquid within the secondary liquid circuit; and a control system configured to control a pumping speed of the variablespeed pump based on an output of the temperature sensor.

Thus it will be seen that in accordance with the present invention energy may be stored in a liquid-based thermal energy storage arrangement, as thermal energy of liquid (e.g. water) held in the storage vessel. This stored thermal energy may then be transferred from that liquid to liquid (e.g. water) within the secondary liquid circuit. The fact that the heat transfer medium is a liquid, rather than gas, enables a greater proportion of the storage vessel to be filled with liquid. As a result, the thermal energy storage arrangement provides a greater energy storage density than if gas has been used, as greater amounts of thermal energy can be stored in liquid than in gas.

Furthermore, the liquid-based arrangement reduces the overall volume of condensable gases (e.g. steam) stored within the storage vessel than if gas had been used. High temperature condensable gases like steam are inherently difficult to control, and thus introduce safety and reliability concerns particularly in applications such as domestic central heating or hot water. Reducing the volume of condensable gases stored in the storage vessel therefore advantageously increases the overall safety and reliability of the thermal energy storage arrangement whilst still providing the energy storage benefits set out above.

Furthermore, the Applicant has recognised that, by pumping liquid through the primary liquid circuit rather than e.g. steam, the variable-speed pump, in combination with the first temperature sensor and the control system allow the rate of thermal energy transfer from the primary liquid circuit to the secondary liquid, and thus the temperature of liquid within the secondary liquid circuit, to be controlled by varying the flow rate of liquid through primary liquid circuit. This advantageously provides a simple, cost-effective, efficient and reliable system for controlling the temperature of liquid within the secondary liquid circuit. This also enables the temperature of liquid within the secondary liquid circuit to be controlled precisely, without any risk of boiling occurring within the secondary liquid circuit.

The invention extends to a heating system comprising the thermal energy storage arrangement outlined above or below, and a secondary liquid circuit connected between the secondary liquid outlet and the secondary liquid inlet of the heat exchanger.

In a set of embodiments, the secondary liquid circuit comprises a heating system. It may comprise a domestic central heating system, or a heating system for a larger building (e.g. commercial or retail premises), comprising one or more radiators. Additionally or alternatively, it may comprise a hot water supply system. It may comprise a domestic hot water supply system comprising one or more outlets e.g. taps. In hot water supply systems the secondary circuit may comprise a further inlet for admission of water to replenish hot water drawn from the circuit.

In a set of embodiments, the heat exchanger is arranged to allow thermal energy to transfer from liquid within the primary liquid circuit to liquid within the secondary liquid circuit. In a set of embodiments, the liquid in the primary circuit is water. Liquid water has a particularly high specific heat capacity thus making it an ideal candidate for storing thermal energy. In a set of (not necessarily overlapping) embodiments the liquid in the secondary circuit is water. It will be appreciated that this is convenient and allows the secondary circuit to supply hot water directly in some embodiments.

The vessel liquid inlet may be located at a lower portion - e.g. at the bottom of the storage vessel when the vessel is positioned upright. The vessel liquid outlet may be located at a higher portion on the storage vessel when the vessel is positioned upright. This may ensure that higher temperature liquid located nearer to the top of the vessel passes through the heat exchanger thereby maximising the rate of heat transfer from liquid within the primary liquid circuit to liquid within the secondary liquid circuit. Furthermore, the vessel liquid outlet being located at a higher portion may provide an in-built safety system in the case of leakage from the vessel, since this may lead to the liquid level within the vessel falling below the level of the outlet thus automatically preventing liquid from flowing through the primary liquid circuit.

In a set of embodiments, the vessel liquid inlet and the vessel liquid outlet are coaxial. By having the vessel liquid inlet and outlet be coaxial in this manner, the overall number of penetrations into the storage vessel may be reduced, advantageously decreasing the overall cost to manufacture the vessel. Furthermore, cooler liquid flowing into the vessel liquid inlet may provide a cooling effect on the heated liquid flowing out of the vessel liquid outlet and thus advantageously help prevent boiling within the primary liquid circuit. The vessel liquid inlet may be located within the vessel liquid outlet. The vessel liquid inlet may be connected to a dip tube located inside the vessel that runs from the coaxial inlet and outlet to a lower portion of the storage vessel so that the liquid inlet and outlet can still effectively take place at different heights to promote mixing within the vessel. In a set of embodiments, the storage vessel is arranged to store liquid at pressures greater than atmospheric pressure. Pressurising the vessel in this manner increases the maximum temperature of liquid that can be stored therein without transitioning into a gaseous state. This advantageously increases the overall amount of thermal energy that can be stored within the storage vessel.

The storage vessel may be arranged to operate at any suitable pressure. For example, the storage vessel may be arranged to operate at pressures of up to 30 barg (equivalent to 31 bar absolute pressure at sea-level atmospheric pressure). In other embodiments, the storage vessel may be arranged to operate at pressures of less than or equal to 15 barg. However, some regulatory requirements define different categories of storage vessels for heating systems and stipulate the pressures that they can operate at for each category, as well as the manufacturing scrutiny requirements for each category. In a set of embodiments, the storage vessel is arranged to operate at pressures of less than or equal to 10 barg (equivalent to 11 bar absolute pressure at sea-level atmospheric pressure). This maximises the overall thermal energy storage capacity of the vessel while keeping it within a category in which a vessel suitable for domestic use can be manufactured cost-effectively. In yet further embodiments, the storage vessel may be arranged to operate at pressures of less than or equal to 7 barg, less than or equal to 6 barg, less than or equal to 5 barg, less than or equal to 4 barg, or less than or equal to 3 barg, depending on application.

By controlling the pressure at which the vessel operates, the thermal storage capacity of the vessel, which is dependent on its pressure, may be adjusted to suit the anticipated thermal load from the secondary liquid circuit by adjusting the pressure of the vessel. For example, where the secondary liquid circuit comprises a domestic central heating and/or hot water system, the anticipated load thereof may be lower during summer months and thus the energy storage capacity of the vessel may be reduced during this time by reducing the pressure at which it operates.

The principles outlined herein for the storage vessel may be applied to a vessel of any volume capacity. However, some regulatory requirements stipulate the maximum volume capacity of the storage vessel for each category. Thus, in a set of embodiments, the storage vessel has an overall volume capacity of less than or equal to 508 litres, advantageously maximising the overall thermal energy storage capacity of the vessel while keeping it within a category suitable for domestic use. The storage vessel may be arranged to store less than or equal to 508 litres of liquid therein. In other embodiments, the storage vessel may have an overall volume capacity of less than or equal to 1000 litres, less than or equal to 700 litres, less than or equal to 400 litres, less than or equal to 300 litres, or less than or equal to 200 litres, depending on application.

In a set of embodiments, the storage vessel is insulated. This advantageously decreases the rate of thermal energy transfer from the vessel to its surroundings, thus enabling thermal energy to be stored within the vessel for greater periods of time without significant losses.

In a set of embodiments, the storage vessel comprises an inner wall and an outer wall that are physically separated. The inner wall may be made from stainless steel or galvanised steel, and the outer wall may be made from glass reinforced plastic (GRP) or galvanised steel. In a set of embodiments, an insulating material is provided in an insulating region located between the inner wall and the outer wall, or between the exterior of the vessel and an external housing, which may comprise granular fill insulation. In other embodiments, a vacuum is provided between the inner wall and the outer wall in order to provide insulation.

In a set of embodiments, the inner wall is supported by one or more ring supports. The inner wall may be supported by a ring support made of an insulating material - e.g. wood or GRP. One or more further ring supports e.g. made of stainless steel may be connected to the first ring support in order to strengthen it.

The double-walled design outlined above is advantageous as it allows the level of insulation provided for the vessel to be adjustable during manufacture. This may be adjusted by: modifying the distance between the inner and outer walls; modifying the thickness of the insulation provided between the inner and outer walls; and/or modifying the type of insulation provided between the inner and outer walls (e.g. the quality thereof, the material(s) used, etc.). This may advantageously enable the rate of heat transfer from liquid within the vessel to its surroundings, through the wall(s) of the vessel, to be configured in dependence on the intended application. For example, for some applications it may be desirable for the amount of thermal energy transfer from the liquid within the vessel to its surroundings through the wall(s) of the vessel to be minimised, in order to maximise retention of thermal energy stored within the liquid - e.g. in applications where the vessel will be located outside. In other applications it may be desirable for some thermal energy transfer from the liquid within the vessel to its surroundings through the wall(s) of the vessel in order to heat the vessel’s direct surroundings - e.g. in applications where the vessel will be located within a room or domestic airing cupboard or commercial drying room.

The level of insulation provided for the vessel may be selectable by a consumer when ordering the vessel. The level of insulation may be selectable on a substantially continuous scale, or it may be selectable from a plurality of discrete categories.

In a set of embodiments, the storage vessel comprises a heater for heating liquid stored therein. The heater may comprise an electric heater. This may advantageously enable the heater to be powered by a renewable energy source. It may be powered directly by a directly connected renewable energy source, or it may be powered indirectly by a renewable energy source through a power grid. It may therefore allow greater utilisation of intermittently available renewable sources. The heater may be powered using mains electricity provided through a power grid irrespective of the source. Thus it will be seen that, in such embodiments, the storage vessel acts as a thermal ‘battery’ whereby electrical energy, preferably though not exclusively from renewable energy sources, is converted into thermal energy, using the heater, and transferred to liquid stored within the storage vessel. This provides an alternative energy storage mechanism to e.g. lithium-ion batteries. The Applicant has found that that storing thermal energy in this manner may be more efficient overall than other types of energy storage (e.g. lithium-ion batteries) for heating applications, and provide greater overall energy intensity (i.e. total energy storable per unit of floor space required). In a set of embodiments, the arrangement comprises means for measuring an amount of electrical energy provided to the heater. In embodiments where the vessel comprises an inner wall and an outer wall, the heater may comprise a flange for rigidly connecting to the inner wall.

The temperature sensor could be located anywhere in the secondary liquid circuit but in a set of embodiments the temperature sensor is arranged to detect a temperature of liquid as it flows out of the secondary liquid outlet of the heat exchanger. Embodiments where the temperature sensor is located within the heat exchanger may advantageously reduce installation times by requiring fewer individual components to be installed. Thus, the control system may be provided with information about the temperature of liquid leaving the heat exchanger, and control the pumping speed of the variable-speed pump accordingly.

In a set of embodiments, the secondary liquid circuit comprises a second temperature sensor arranged to detect the temperature of liquid downstream of the aforementioned (hereinafter “first”) temperature sensor. In a set of embodiments the second temperature sensor is arranged to detect the temperature of liquid as it flows into the secondary liquid inlet of the heat exchanger. In such embodiments, the control system is preferably configured to control the pumping speed of the variable-speed pump based additionally on an output of the second temperature sensor. Thus, the control system may be provided with information about the temperature of liquid leaving the secondary liquid circuit and entering the heat exchanger, and takes this into account when controlling the pumping speed of the variable-speed pump. This may provide the control system with a more comprehensive overview of the conditions within the secondary liquid circuit, and thus allow it to more precisely control the temperature of liquid within the secondary circuit.

In a set of embodiments the thermal energy storage arrangement comprises means for determining a flow rate of liquid in the secondary liquid circuit. The control system may be further configured to take account of the flow rate in controlling the pumping speed of the variable-speed pump, in addition to the output of the first temperature sensor and optionally the output of the second temperature sensor.

In a set of embodiments, the secondary liquid circuit comprises a secondary circuit pump arranged to pump liquid through the secondary liquid circuit including any components, e.g. radiators, located within the secondary liquid circuit. The pump curve of the secondary circuit pump may be known - i.e. the secondary circuit pump may exhibit a predictable relationship between the flow rate of liquid flowing out of the pump and one or more operational conditions of the pump - e.g. its head pressure, power consumption, voltage, etc. Thus, in such embodiments, the flow rate through the secondary liquid circuit may be determined based on one or more operational conditions of the secondary circuit pump.

While the flow rate through the secondary circuit may be estimated using the pump curve of the pump as outlined above, in a set of embodiments, the secondary liquid circuit comprises a flow sensor arranged to detect the flow rate of liquid in the secondary liquid circuit. In a set of embodiments flow sensor is arranged to detect the flow rate of liquid as it flows out of the secondary liquid outlet.

The control system may be configured to activate the variable-speed pump to pump liquid around the primary liquid circuit when the output of the flow sensor or secondary circuit pump indicates that water is flowing through the secondary liquid circuit e.g. due to central heating activation. Detecting the flow rate of liquid through the secondary liquid circuit using a flow sensor or the secondary circuit pump in this manner, as well as helping prevent cavitation within the heat exchanger, may increase the overall compatibility of the thermal energy storage arrangement with existing central heating systems, as it enables a conventional central heating pump to be used as the secondary circuit pump.

In embodiments where the secondary liquid circuit comprises the first and second temperature sensors, these, in combination with an estimate or measure of the flow rate through the secondary circuit, may provide the control system with the ability to estimate or measure the heat flux from the secondary liquid circuit to its surroundings (e.g. a building). Thus, in a set of embodiments, the control system is configured to estimate the heat flux from the secondary liquid circuit to its surroundings based on the output of the first temperature sensor, the output of the second temperature sensor, and the flow rate.

In a set of embodiments, the control system is configured to monitor, over time, the heat flux from the secondary liquid circuit to its surroundings, and thus estimate the total amount of thermal energy transferred from the secondary liquid circuit to its surroundings. This may advantageously enable the control circuit to effectively act as a heat meter for the secondary liquid circuit. In a set of embodiments, the control system is further configured to estimate the overall efficiency of the heating system comprising the storage vessel based on the outputs of the first and second temperature sensors, the flow rate, and a measure of the power supplied to the heater. This may enable the control system to estimate the overall efficiency of the electrical energy consumed to charge the storage vessel to the heat transferred to the surroundings of the secondary liquid circuit.

It will be appreciated of course that such analysis of heat flux and overall efficiency could equally be carried out remotely based on data gathered by the control system.

In a set of embodiments, the control system is configured to cause the variablespeed pump to pump liquid through the primary liquid circuit while charging the storage vessel - i.e. whilst increasing the temperature of the liquid in the storage vessel, e.g. using the heater - regardless of whether there is demand from the secondary liquid circuit, which may be detected using the flow sensor. This may advantageously improve mixing of liquid within the storage vessel while charging, thereby improving the overall efficiency with which thermal energy may be input to the storage vessel. This may also advantageously reduce or eliminate stratification within the vessel both during and after charging, as stratification can cause damage to the thermal energy storage arrangement and/or reduce the overall efficiency of the thermal energy storage arrangement.

In a set of embodiments, the storage vessel comprises a lid or inspection flange to access to the interior thereof for e.g. maintenance purposes. The lid or inspection flange may be hinged for ease of opening and closing, and may be flanged in order to enable a strong seal to be created between the interior and exterior of the vessel.

The heat exchanger may in some embodiments be located externally to and separately from the storage vessel. This may enable easy access to the heat exchanger for e.g. maintenance purposes. However, in some embodiments, the heat exchanger is rigidly attached to an exterior surface of the vessel. In other embodiments, where the vessel comprises a lid, the heat exchanger may be rigidly attached to an internal or external surface of the lid. In embodiments where the lid is flanged, the heat exchanger may hang from the flange of the lid. Having the heat exchanger physically proximate the vessel minimises the overall length of the primary liquid circuit, potentially increasing the maximum flow rate through the primary liquid circuit (thereby increasing the maximum rate of energy transfer from the primary circuit to the secondary circuit), and reducing the overall surface area within the primary circuit through which thermal energy may be lost from liquid held therein to the surroundings (thus increasing the overall energy retention of the thermal energy storage arrangement).

In a set of embodiments, the heat exchanger is arranged such that liquid flowing from the primary inlet to the primary outlet flows in substantially the same direction within the heat exchanger as liquid flowing from the secondary inlet to the secondary outlet. In other words, the heat exchanger may be arranged such that liquid flowing within the primary liquid circuit flows co-currently to liquid flowing within the secondary liquid circuit within the heat exchanger. This may advantageously help prevent liquid from boiling within the heat exchanger, both on the primary and secondary side, thereby increasing the overall longevity of the thermal energy storage arrangement by reducing negative effects associated with such boiling within the heat exchanger e.g. knocking, cavitation, etc.

In a set of embodiments, the primary liquid circuit further comprises one or more one-way valves arranged to allow liquid to flow from the vessel liquid outlet to the vessel liquid inlet and to prevent or inhibit liquid from flowing from the vessel liquid inlet to the vessel liquid outlet. This may ensure that liquid flowing through the primary liquid circuit may only flow in the correct direction through the heat exchanger (i.e. co-currently to liquid flowing through the secondary liquid circuit), thereby helping prevent liquid from boiling within the heat exchanger. Furthermore, the one-way valves may help prevent inversion - i.e. where a body of cooler liquid sits above a body of warmer liquid in a state of temporary stability within the storage vessel. It is desirable to avoid inversion as a small disturbance to a vessel in an inverted condition can cause a rapid recovery to an equilibrated condition - i.e. where a body of warmer liquid sits on top of a body of cooler liquid - which can cause a sudden and potentially damaging increase in pressure. Additionally, the one-way valves may ensure no liquid may flow from the vessel liquid inlet to the vessel liquid outlet through the primary liquid circuit, which may disrupt mixing processes within the vessel which may cause stratification.

In a set of embodiments, the storage vessel further comprises a venting outlet. The venting outlet may be located substantially at the top of the vessel when the vessel is positioned upright. The venting outlet may comprise a venting valve and/or a pressure sensor. The pressure sensor may be arranged to detect the internal pressure of the storage vessel. The control system may be configured to control the operation of the venting valve based on an output of the pressure sensor. Such embodiments may provide a convenient mechanism for the control system to control the pressure within the storage vessel by releasing gas through the venting outlet, when necessary, in order to counteract increasing pressure within the vessel due to the heating of liquid stored therein. The control system may be configured to control the operation of the venting valve such that the pressure within the storage vessel does not exceed 10 barg.

In a set of embodiments, the thermal energy storage arrangement is arranged, during operation, to store a volume of non-condensable gases (e.g. air) that maintain pressure within the storage vessel. In a set of embodiments, the control system is configured to control the operation of the venting valve in order to store a volume of non-condensable gases that maintain pressure within the storage vessel during operation. Maintaining a volume or charge of non-condensable gases in this manner may advantageously help prevent steam formation and thumping or cavitation within the storage vessel while charging, as the pressure exerted by the non-condensable gases may help prevent steam bubbles from forming within the liquid held within the storage vessel. Furthermore, maintaining a volume or charge of non-condensable gases may help ensure that the pressure within the storage vessel remains at or greater than atmospheric pressure, particularly as the temperature of liquid held within the storage vessel decreases, thereby advantageously helping prevent gases (e.g. air) being drawn into the storage vessel due to a negative pressure gradient between the vessel and its surroundings.

In a set of embodiments the vessel, e.g. the gas venting outlet, further comprises a pressure-relief safety valve arranged to release pressure within the storage vessel when it exceeds a predetermined value. This may advantageously prevent overpressurisation of the storage vessel, thereby increasing overall safety.

In a set of embodiments, the storage vessel comprises a single storage chamber for storing liquid and gases. In such embodiments, the thermal energy storage arrangement may further comprise an expansion vessel for storing noncondensable gases. The expansion vessel may be physically distinct from the storage vessel, and it may be connected to the venting outlet of the storage vessel. The expansion vessel may have a smaller volume capacity than the storage vessel.

In other embodiments, the storage vessel comprises a first chamber for storing liquid and a second chamber for storing non-condensable gases. In such embodiments, the venting outlet may feed out of the second chamber. The second chamber may be positioned above the first chamber when the storage vessel is positioned upright. The second chamber may be physically separated from the first chamber e.g. by a wall. A passageway and/or valve may be provided between the second chamber and the first chamber that allows gas stored within the first chamber to move to the second chamber. The second chamber may have a smaller volume capacity than the first chamber.

Providing an expansion vessel for storing non-condensable gases, or a second chamber within the storage vessel for storing non-condensable gases, may provide a location for the volume of non-condensable gases to be stored and thus help ensure that the storage vessel remains pressurised, providing the various benefits set out above, whilst also maximising the volume of liquid that can be stored within the storage vessel and therefore the overall thermal energy storage capacity thereof.

In a set of embodiments, the vessel liquid inlet is further connected to a mains water supply via a fill valve. In other embodiments, the storage vessel may comprise a second vessel liquid inlet connected to a mains water supply via a fill valve. The second vessel liquid inlet may be located substantially at the bottom of the vessel when the vessel is positioned upright. The operation of the fill valve may be controlled by the control system. This may enable the control system to control the mass of water held within the primary liquid circuit e.g. by causing additional water to flow into the storage vessel.

In a set of embodiments, the storage vessel is at least partially formed from stainless steel. Stainless steel makes a particularly suitable material for the storage vessel due to its anti-corrosion properties that increase the overall operational lifetime of the vessel.

Since liquid in the primary circuit cools as it passes through the heat exchanger, liquid leaving the heat exchanger at the primary liquid outlet, and thus entering the storage vessel at the vessel liquid inlet, may have a greater density than heated liquid contained in the vessel. This difference in density can cause liquid to flow through the primary liquid circuit independently of the pumping of the variablespeed pump - referred to herein as ‘density drive’. In a set of embodiments, the variable-speed pump comprises a variable-speed positive displacement pump. This may advantageously help reduce the effects of such density drive, particularly compared to e.g. a centrifugal pump, and thus enable greater control of the flow rate of liquid within the primary liquid circuit by the control system.

When viewed from a second aspect, the invention provides a method of operating a heating system comprising a thermal energy storage arrangement and a secondary liquid circuit, the thermal energy storage arrangement comprising: a storage vessel for storing liquid, the storage vessel comprising a vessel liquid inlet and a vessel liquid outlet; a heat exchanger, comprising: a primary liquid inlet and a primary liquid outlet, wherein the primary liquid inlet is connected to the vessel liquid outlet and the primary liquid outlet connected to the vessel liquid inlet so as to form a primary liquid circuit; and a secondary liquid inlet and a secondary liquid outlet connected to the secondary liquid circuit; a variable-speed pump arranged to pump liquid through the primary liquid circuit; and a temperature sensor arranged to detect a temperature of liquid within the secondary liquid outlet; wherein the method comprises controlling the pumping speed of the variable-speed pump based on an output of the temperature sensor

Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. Where reference is made to different embodiments or sets of embodiments, it should be understood that these are not necessarily distinct but may overlap.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 is a schematic diagram of a thermal energy storage arrangement in accordance with an embodiment of the invention;

Fig. 2 shows a table indicating example values of various conditions of water stored in a storage vessel forming part of the thermal energy storage arrangement of Fig. 1;

Fig. 3 shows a table indicating example values of various conditions of the storage vessel forming part of a thermal energy storage arrangement of Fig. 1;

Fig. 4 is a schematic diagram of a heating system comprising the thermal energy storage arrangement of Fig. 1 in accordance with an embodiment of the invention; and

Fig. 5 is a schematic diagram of a thermal energy storage arrangement in accordance with another embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic diagram of the thermal energy storage arrangement 2 in accordance with an embodiment of the invention. The arrangement 2 comprises a primary liquid circuit 4 and a secondary liquid circuit 6. A dotted line 8 represents the divide between the primary circuit 4 and the secondary circuit 6. As will be explained in more detail below, energy is transferred from liquid within the primary circuit 4 to liquid within the secondary circuit 6 via a heat exchanger 10 which, in this particular embodiment, is a conventional plate heat exchanger.

The primary circuit 4 comprises a thermal storage vessel 12 which includes a first chamber 14 for storing water and a second chamber 16 for storing gases. In this particular embodiment, the vessel 12 is made of stainless steel, though it will be appreciated that other appropriate materials could be readily envisaged.

While not shown in the embodiment in Fig. 1, in some embodiments the vessel 12 may comprise a lid to enable access to the interior thereof for e.g. maintenance purposes. The lid may be hinged for ease of opening and closing, and the lid may be flanged in order to create a strong seal between the interior and the exterior of the vessel 12.

A heater 18 comprising a heating element 20 that protrudes into the interior of the first chamber 14 is fixedly attached to the exterior of the storage vessel 12. The heater 18 is located near the bottom of the first chamber 14 in order to allow water contained within the first chamber 14 to mix effectively, when heated, through convection. In this particular embodiment, the heater 18 comprises a 6.5kW electric heater but any suitable type of heater could be used.

A primary circuit feed pipe 22 is connected between a vessel liquid outlet 24 of the first chamber 14 and a primary liquid inlet 26 of the heat exchanger 10. A first oneway valve 28 is provided within the primary circuit feed pipe 22. The vessel liquid outlet 24 of the first chamber 14 is located near to the top of the first chamber 14. A primary circuit return pipe 30 is connected between a primary liquid outlet 32 of the heat exchanger 10 and a vessel liquid inlet 34 of the first chamber 14. The vessel liquid inlet 34 of first chamber 14 is located at the bottom of the first chamber 14. A variable-speed pump 36 and a second one-way valve 38 are provided in line with the primary circuit return pipe 30 is provided within the primary circuit outlet pipe 30. A fill valve 40 is connected between the primary circuit outlet pipe 30 and a mains water supply 42. In some embodiments the variable-speed pump 36 comprises a variable-speed positive displacement pump, though a variable-speed centrifugal pump or other type of variable-speed pump could also be used.

A venting outlet comprises a venting pipe 46 connected between the second chamber 16 and a flue 50 which is located at a safe, outside location. A venting valve 52, a pressure switch 54, a pressure sensor 56 and a bursting disc 58 are provided along the venting pipe 46. The venting pipe 46 passes through a wall 60 in order to reach the flue 50. The pressure switch 54 and the pressure sensor 56 are located between the venting valve 52 and the vessel 12. The pressure switch 54 is arranged to actuate when pressure exceeds a predetermined value, and the pressure sensor 56 detects the pressure of the gas within the venting pipe 46 and therefore the vessel 12.

The secondary circuit 6 comprises a secondary circuit return pipe 64 and a secondary circuit feed pipe 66. The secondary circuit return pipe 64 is connected to the secondary liquid inlet 68 of the heat exchanger 10, and the secondary circuit feed pipe 66 is connected to the secondary liquid outlet 70 of the heat exchanger 10. A first, feed path, temperature sensor 76 and an optional flow sensor 78 are provided within the secondary circuit feed pipe 66. A second, return path temperature sensor 72 is provided within the secondary circuit return pipe 64, and a secondary pump 74 is provided at a point along the secondary circuit inlet pipe 64.

With additional reference to Fig. 4 there may be seen schematically the other components of an exemplary heating system 82 comprising the thermal energy storage arrangement 2 shown in Fig. 1 (itself depicted in schematic fashion in Fig. 4). In this example, the secondary liquid circuit 6 comprises a domestic central heating system and includes a first radiator 84 and a second radiator 86 connected between the feed and return sides of the secondary circuit. It will be appreciated that the principles outlined herein equally apply to other types of heating systems e.g. hot water supply systems, domestic hot water systems, etc. It will also be appreciated that while two radiators 84, 86 are shown in Fig. 4, the heating system 82 is not limited as such and may comprise any suitable number of radiators and/or additional components depending on application.

The heater 18 may be connected to a renewable energy source 88 (e.g. a solar panel, wind turbine, etc.) and/or to a power grid 90 (which may itself supply power from a remote renewable energy source. It will be appreciated that the heater 18 may be connected to any appropriate power sources depending on application.

Fig. 4 also shows the mains water supply 42 for re-filling the vessel 12 and the electronic control system 43 to which the heater 18, the variable-speed pump 36, the fill valve 40, the vent valve 52, the pressure switch 54, the pressure sensor 56, the secondary circuit pump 74, the inlet temperature sensor 72, the outlet temperature sensor 76 and the optional flow sensor 78 are all connected.

The direction of water travel through the secondary liquid circuit 6, when water is flowing, is indicated by the arrows 80. The secondary pump 74 controls the rate of water flow through the secondary liquid circuit 6. The inlet temperature sensor 72 detects the temperature of water within the secondary circuit inlet pipe 64 (i.e. before, or as, the water flows into the heat exchanger 10), the outlet temperature sensor 76 detects the temperature of water within the secondary circuit outlet pipe 66 (i.e. after, or as, the water flows out of the heat exchanger 10), and the optional flow sensor 78 detects the rate of water flow within the secondary circuit 6.

Operation of the heating system will now be described with reference to Figs. 1 and 4. First the first chamber 14 is filled by opening the fill valve 40, thereby allowing water from the mains water supply 42 to enter the first chamber 14 (and the rest of the primary circuit 4). Once the first chamber 14 has been filled with the correct volume of water, which may be determined by any appropriate means - e.g. weight sensors or water level sensors located within the vessel 12 (not shown) - the fill valve 40 is closed thus sealing the primary circuit 4 from the mains water supply 42.

Water held within the first chamber 14 is heated by the heater 18. Since the energy coming from the power sources 88, 90 may be directly or indirectly derived from renewable sources, it may only be available intermittently. However the heater 18 can be operated whenever the energy is available to gradually store energy in the vessel 12 in the form of heat in the water therein. The vessel 12 thus acts as a thermal battery, with water held within the first chamber 14 and non-condensable gases (e.g. air) held within the second chamber 16. Thermal energy is primarily though not exclusively stored within the water contained within the first chamber 14.

While charging the vessel 12 (i.e. heating the water contained in the first chamber 14), the variable speed pump 36 is used to pump hot water from the top of the first chamber 14 to the bottom, around the primary circuit 4. This advantageously mixes the water stored within the first chamber 14 thereby avoiding stratification and thus increasing efficiency and safety during charging. This also increases the maximum thermal energy that can be stored by the vessel 14, as stratification reduces the overall thermal energy that can be stored in a liquid.

In this particular embodiment, the first chamber 14 has an overall capacity of approximately 508 litres. Thus, the maximum volume of water that the vessel 12 stores in the first chamber 14, at the maximum operable pressure of 10.00 barg and when fully charged, is also approximately 508 litres. It will be appreciated however that the principles outlined herein are not limited to these specific capacities, and that other capacities could be readily envisaged.

When required, the thermal energy stored in the vessel 12 can be transferred to the secondary circuit 6 using the heat exchanger 10. In order to do this, heated water from the first chamber 14 is pumped by the pump 36 through the vessel outlet 24 to the inlet 26 on the primary side of the heat exchanger 10. The direction of water flow through the primary circuit 4 is indicated by the arrows 44. The variable-speed pump 36 controls the rate of water flow through the primary circuit 4, and the oneway valves 28, 38 prevent water flowing backwards within the primary circuit 4 (i.e. from the vessel liquid inlet 34 to the vessel liquid outlet 24 through the heat exchanger 10). These one-way valves advantageously help prevent inversion of liquid stored within the first chamber 14 which, if disturbed once formed, can cause a sudden and potentially damaging increase in pressure.

Water within the secondary circuit 6 is also pumped by the pump 74 through the secondary side of the heat exchanger 10. Water flows through the primary and secondary sides of the heat exchanger 10 co-currently (i.e. in the same direction), as can be seen from Fig. 1. Advantageously, this helps prevent water within the primary circuit 4 and water within the secondary circuit 6 from boiling, thereby increasing longevity of pipes and other components included in the primary and secondary circuits 4, 6 by causing fewer potentially damaging gas bubbles to form therein.

The heat exchanger 10 causes thermal energy stored in the heated water on the primary side to transfer to the water on the secondary side, thereby increasing the temperature of the water on the secondary side and decreasing the temperature of the water on the primary side. Cooler water at the primary outlet 32 of the heat exchanger 10 is then pumped back into the vessel inlet 34, and heated water at the secondary outlet 70 of the heat exchanger is pumped into the secondary circuit 6.

The direction of water flow within the secondary liquid circuit 6 is indicated by the arrows 80. Heated water flows from the secondary liquid outlet 70 of the heat exchanger 10 to the first radiator 84 which then transfers thermal energy to the radiator’s surroundings in order to heat a building. The water then flows to the second radiator 86 which functions in the same manner. The radiators 84, 86 may comprise one or more valves for activating, deactivating and/or adjusting their heat output. Cooler water flows from the second radiator 86 to the secondary liquid inlet 68 of the heat exchanger 10 to be heated again, completing the secondary circuit 6.

The temperature of the water within the secondary circuit outlet pipe 66, measured by the feed temperature sensor 76, is controlled by controlling the speed of the variable-speed pump 36, using the control system 43. Increasing the speed of the variable-speed pump 36 causes the rate of water flow through the primary circuit 4 to increase, which in turn increases the rate of heat transfer from the primary side of the heat exchanger 10 to the secondary side of the heat exchanger 10. This causes the temperature of the water within the secondary circuit outlet pipe 66 to increase accordingly. Similarly, decreasing the speed of the variable-speed pump 36 causes the rate of water flow through the primary circuit 4 to decrease, thus decreasing the rate of heat transfer from the primary side of the heat exchanger 10 to the secondary side of the heat exchanger 10.

When the thermal energy storage arrangement 2 is in operation and thus heat is being transferred from the primary circuit 4 to the secondary circuit 6, water leaving the primary liquid outlet 32 of the heat exchanger 10 is typically cooler than the water contained within the first chamber 14 of the vessel 12. Thus, the water entering the vessel 12 from the primary circuit return pipe 30 at the vessel liquid inlet 34 has a higher density than the heated water contained within the first chamber 14. This difference in density causes water to naturally flow within the primary circuit 4, independently of the pumping of the variable-speed pump 36 - i.e. there is a density drive. In order to help alleviate the effects of such density drive, the variable-speed pump 36 can be provided by a variable-speed positive displacement pump, rather than e.g. a centrifugal pump. This use of a variable- speed positive displacement pump can provide greater control over the flow rate of water within the primary circuit 4, and thus of the rate of heat transfer to the secondary circuit 6 via the heat exchanger 10, by the control system 43.

The speed of the variable-speed pump 36 is also influenced by the outputs of the second temperature sensor 72 and, optionally, the flow sensor 78. For example the variable-speed pump 36 might only be activated to pump water around the primary circuit 4 when the flow sensor 78 indicates that water is flowing around the secondary circuit 6, e.g. due to central heating demand, unless primary circuit flow is required during charging as explained above. Alternatively, the variable-speed pump 36 might only be activated when one or more monitored operational conditions of the secondary circuit pump 74 indicated that water is flowing around the secondary circuit 6. This advantageously helps prevent cavitation which can cause damage to both the primary and secondary circuits 4, 6. Furthermore, this advantageously enables the secondary pump 74 to be a conventional central heating pump, thereby decreasing the overall cost of the thermal energy storage arrangement 2 and increasing compatibility of the primary circuit 4 with existing secondary circuits 6 - e.g. older central heating systems. It will be appreciated however that the secondary pump could be suitable of type of pump.

The control system 43 monitors the feed temperature sensor 76, the return temperature sensor 72 and the flow rate of water through the secondary circuit 6 (either by monitoring one or more operational conditions of the secondary circuit pump 74 or using the optional flow sensor 78). Thus, the control system 43 is able to estimate the instantaneous heat flux from the secondary liquid circuit 6 to its surroundings using these measurements and, by monitoring this over time, it is also able to estimate the total amount of thermal energy transferred from the secondary liquid circuit to its surroundings. In this manner, the control system 43 can effectively act as a heat meter for the secondary circuit 6. Additionally, the control system 43 monitors the energy usage of the heater 18 over time, and is thus able to estimate the overall energy efficiency of the arrangement 2.

When in operation, the vessel 12 is pressurized. The pressure of the vessel 12 is controlled by the control system 43 using the heater 18. Heating the water contained within the first chamber 14 using the heater 18 causes the pressure within the vessel 12 to rise, as the water contained within the first chamber 14 and the non-condensable gases contained within the second chamber 16 undergo thermal expansion. Thus, the control system 43 activates the heater 18 in order to increase the pressure of the vessel 12, and deactivates the heater 18 when the pressure of the vessel 12 reaches a desired target value, determined based on the outputs of the pressure sensor 54 and/or the pressure switch 56. When the outputs of the pressure sensor 54 and pressure switch 56 indicated that the pressure of the vessel 12 has risen above a predetermined maximum value (which may be configured for safety purposes), the venting valve 52 is opened by the control system 43 thus allowing gas held within the second chamber 16 to exit via the flue 50 thus relieving pressure within the vessel 12. The direction of gas flow through the venting pathway, when gas is flowing out of the flue 50, is indicated by the arrows 62.

During operation, the control system 43 controls the venting valve 52 in order to store a volume of non-condensable gases within the second chamber 16 and, optionally, the first chamber 14, thereby pressurising the first chamber 14. This advantageously prevents steam formation and thumping or cavitation within the storage vessel 12, as the pressure exerted by the stored non-condensable gases helps prevent steam bubbles from forming within the liquid held in the vessel 12. Furthermore, this also helps ensure that the pressure within the storage vessel remains greater than atmospheric pressure, even as the temperature of the liquid stored therein decreases, thereby preventing gases being drawn into the storage vessel 12 due to a negative pressure gradient between the vessel 12 and its surroundings (which may damage the vessel 12).

The pressure of the vessel 12 can be controlled by the control system 43, using the heater 18, in dependence on the anticipated thermal load of the secondary liquid circuit 6. This allows less thermal energy to be stored within the vessel 12 to be stored at times of predicted low demand, thereby saving energy by reducing the rate of unwanted heat loss from the vessel 12 to its surroundings when high energy storage is not required. For example, there may be a lower anticipated demand from the secondary liquid circuit 6 during summer months, and the control system 43 may decrease the pressure of the vessel 12 accordingly. By maintaining the pressure of the vessel 12 at an appropriate value, and storing an appropriate mass of water within the first chamber 14, the water held within the first chamber 14 remains primarily liquid, even at high temperatures, as the water-steam mixture within the first chamber 14 becomes saturated. Thus, heated liquid water is pumped around the primary circuit 4 through the heat exchanger 10, rather than steam. This increases the level of control that can be provided by the thermal energy storage arrangement 2, as heat transfer from liquid water is less powerful than steam condensing and therefore easier to control. Thus, the rate of heat transfer from the primary circuit 4 to the secondary circuit 6 can be more easily controlled by pumping liquid water, rather than steam, through the primary circuit 4. Furthermore, pressurised liquid water is able to store greater amounts of thermal energy than steam due to its greater density and specific heat capacity. As a result, the vessel 12 is able to store greater overall amounts of thermal energy than with a mixture of heated water and steam.

In this particular embodiment, the vessel 12 operates at pressures up to 10.00 barg (11.00 bar absolute pressure). The vessel 12 is able to store more thermal energy at higher pressures than lower pressures, so it is generally desirable to operate the vessel 12 at close to the maximum operable pressure - i.e. 10.00 barg.

In this particular embodiment, the first chamber 14 and the second chamber 16 are shown as separate storage cavities within the vessel 12, with a wall provided therebetween. However, in other embodiments (see e.g. Fig. 5) the vessel could comprise a single storage chamber which stores both the water and the noncondensable gases, with the less dense non-condensable gases being stored at the top of the vessel. In such embodiments, the first chamber and the second chamber are merely portions of the same storage chamber within the vessel, and are not separated by a wall as in the embodiment shown in Fig. 1.

In Fig. 1 , the heat exchanger 10 is shown to be located externally to and separately from the vessel 12. Such embodiments advantageously enable the heat exchanger 10 to be easily accessed e.g. for maintenance purposes, and to be replaced or repaired independently from the vessel 12, if required. However, in other embodiments, the heat exchanger could be is rigidly attached to the exterior surface of the vessel. In some embodiments where the vessel comprises a lid, the heat exchanger may be rigidly attached to an interior or exterior surface of the lid in order to enable easy access to the heat exchanger e.g. for maintenance purposes. Alternatively, the heat exchanger may hang from the flange of the lid in embodiments where the lid is flanged. In such embodiments, by keeping the heat exchanger physically proximate the vessel, the maximum flow rate through the primary circuit 4 may be increased compared to embodiments where the heat exchanger 10 is located externally to and separately from the vessel, as the overall length of the primary circuit can be kept to a minimum. Additionally, this reduces the overall surface area of the primary circuit through which thermal energy may be lost from liquid held therein to its surroundings, thus increasing the overall energy retention of the thermal energy storage arrangement 2. Furthermore, such embodiments may advantageously enable the heat exchanger and the vessel to be sold and shipped as a complete unit for the convenience of both the consumer and the manufacturer.

As explained previously, in the embodiment shown in Fig. 1, the vessel 12 is arranged to operate at pressures of up to 10.00 barg. Specifically, the vessel 12 is operable at pressures between -0.84 barg (0.16 bar absolute pressure) and 10.00 barg (11.00 bar absolute pressure). The pressure at which the vessel 12 operates has a significant effect on various other operation conditions of the vessel 12. In particular, the operable pressure of the vessel 12 impacts various conditions of water stored within the first chamber 14, including its maximum temperature, maximum enthalpy, maximum useable enthalpy (i.e. the enthalpy stored that can be transferred from the water stored in the first chamber 14 to the water within the secondary circuit 6, via the heat exchanger 10), maximum energy stored, density and volume. Fig. 2 shows a table indicating example values of these conditions of the water stored in the first chamber 14 at various pressures between -0.84 barg (0.16 absolute pressure) and 10.00 barg (11.00 bar absolute pressure), for the vessel 12 shown in Fig. 1 which has a total volume of 508.021 litres and stores 448.4kg of water.

It can be seen from the table of Fig. 2 that the greater the pressure of the vessel 12, the greater the energy that can be stored in the water stored in the first chamber 14. It is therefore desirable to operate the vessel 12 at the maximum permitted pressure in order to maximise thermal energy storage. However, regulatory requirements put an upper limit on the maximum operable pressure of the vessel 12 to remain within a given category, and thus 10.00 barg is the maximum pressure that the vessel 12 is arranged to operate in this particular embodiment. When operating at 10.00 barg, the vessel 12 can store water therein at a temperature of 184.06 °C when fully charged.

The walls of the vessel 12, as distinct from its contents, also store some thermal energy. Fig. 3 shows a table indicating energy storage within the walls of the vessel 12 in embodiments where the vessel 12 is made of stainless steel, and the total energy stored within the vessel 12 including water stored therein, at various operating pressures. It can be seen from the table that the walls of the vessel 12 also store greater amounts of energy at higher pressures, and that the total energy stored by the vessel 12 increases as pressure increases.

Fig. 5 shows a thermal energy storage arrangement 2’ comprising a storage vessel 12’, according to another embodiment. The arrangement 2’ is connected in the same manner as that shown in Figs. 1 and 4, and like reference numerals (with prime suffix) are used to indicate similar features in Fig. 5. The arrangement 2’ also operates in substantially the same manner as that described with reference to Figs. 1 & 4. The arrangement 2’ thus comprises a vessel 12’ connected to a venting pipe 46’, a mains water supply 42’ via a locking fill valve 40’, and to a heat exchanger 10’ which is also connected to a secondary return pipe 64’ and a secondary feed pipe 68’. The heating system 2’ includes the same pumps, valves, sensors, etc. shown in Fig. 1 but these are not shown in Fig. 5 for the sake of simplicity.

The differences between the vessel 12’ shown in Fig. 5 and the vessel 12 shown in Fig. 1 will now be outlined in detail. Firstly, the vessel 12’ comprises a single chamber 14’ which contains both liquids and gases rather than the two-chamber arrangement shown in Fig. 1. The venting outlet 48’ is then connected to an optional expansion vessel 9T which stores non-condensable gases therein. Unlike the second chamber 16 of Fig. 1 , the optional expansion vessel 9T is physically distinct from the storage vessel 12’. The optional expansion vessel 9T is then connected to a flue (not shown) in the same manner as that shown in Fig. 1, via the same sensors, valves, etc. The control system 43’ (not shown) controls the venting valve (not shown) in order to maintain a volume of non-condensable gases within the expansion vessel 9T and thus pressure in the storage vessel 12’. Where the expansion vessel is not included, the venting outlet 48’ is connected to a flue in the same manner as that shown in Fig. 1.

The vessel 12’ further includes an inner wall 92’ and an outer wall 94’. The inner wall 92’ surrounds the chamber 14’, and an insulating region 96’ is located between the inner wall 92’ and the outer wall 94’. In this embodiment, the inner wall 92’ is made from stainless steel, the outer wall 94’ is made of glass reinforced plastic (GRP), and the insulating region 96’ is filled with high quality granular fill insulation though it will be appreciated that any appropriate insulation may be provided within the insulating region 96’, including but not limited to a vacuum, insulating foam, etc.

The insulating region 96’ helps reduce the loss of thermal energy from the liquid stored within the chamber 14’ to the surroundings of the vessel 12’. This is enhanced by the provision of the high-quality granular fill insulation provided within the insulating region 96’ in this particular embodiment. The distance between the inner wall 92’ and the outer wall 94’, and the insulation provided within the insulating region 96’, can be configured during manufacture to suit the application envisaged for the vessel 12’. For example, where the vessel 12’ is to be located in an environment where heat transfer to the surroundings of the vessel 12’ is undesirable (e.g. outside), a greater separation between the inner wall 92’ and the outer wall 94’ may be provided and/or higher-quality insulation may be provided within the insulating region 96’. Alternatively, where the vessel 12’ is to be located in an environment where heat transfer to the surroundings of the vessel 12’ is desirable (e.g. in a kitchen or domestic airing cupboard), a smaller separation between the inner wall 92’ and the outer wall 94’ may be provided and lower-quality insulation may be provided within the insulating region 96’.

The inner wall 92’ is separated from, and rigidly connected to, the outer wall 94’ by a supporting ring 98’ which is itself rigidly connected to two strengthening rings 98’ and 100’. It will be appreciated that the supporting ring 96’ and the strengthening rings 98’ and 100’ are continuous rings and follow the perimeter of the chamber 14’, but that Fig. 5 shows a cross-section of the vessel 12’ and therefore a cross-section of the supporting and strengthening rings 98’, 100’ and 102’. The heater 18’ is rigidly connected to the outer wall 94’ and includes a flange 19’ which is rigidly connected to the inner wall 92’. The supporting ring 98’ acts as a base for the chamber 14’. It holds the lower portion of the inner wall 92’, and therefore the chamber 14’, at an elevated position relative to the lower portion of the outer wall 94’, and thus allows insulation to be provided all the way around the inner wall 92’ The strengthening rings 100’ and 102’ are rigidly connected to the outer surface of the supporting ring 98’ using e.g. screws and provide additional support to the supporting ring 98’ by helping prevent it from being forced outward in a radial direction due to the weight of the chamber 14’ and the liquid held therein. In this embodiment, the supporting ring 98’ is made from a material with low thermal conductivity - e.g. Scot’s pine or GRP - in order to minimise heat transfer from the inner wall 92’ to the outer wall 94’. The strengthening rings 100’, 102’ are made of stainless steel due to its high tensile and compressive strength. Unlike the arrangement shown in Fig. 1, the vessel liquid outlet 24’ and the vessel liquid inlet 34’ in this embodiment are coaxial. The vessel liquid outlet 24’ is connected to the primary circuit feed pipe 22’ which is in turn connected to the primary liquid inlet 26’ of the heat exchanger 10’. The primary liquid outlet 32’ of the heat exchanger is connected to the primary circuit return pipe 30’. However, rather than feeding to the same inlet as the mains water supply 42’ at the bottom of the vessel 12’, the primary circuit return pipe 30’ instead feeds back into the primary circuit feed pipe 22’ and feeds into the chamber 14’ at the vessel liquid inlet 34’. The primary circuit return pipe 30’ is then connected to a dip tube 102’ which runs from the vessel liquid inlet 34’ to the bottom of the chamber 14’. In this particular embodiment, the vessel liquid outlet 24’ has an outer diameter of 28mm and the vessel liquid inlet 34’ has an outer diameter of 22mm.

The co-axial vessel liquid outlet 24’ and vessel liquid inlet 34’ provide two main advantages. Firstly, they reduce the total number of penetrations into the vessel 12’ from three as with the vessel 12 shown in Fig. 1 to two, thereby reducing the overall cost to manufacture the vessel 12’. Secondly, the cooler liquid within the primary circuit return pipe 30’ exhibits a slight cooling effect on the warmer liquid within the primary circuit feed pipe 22, thus helping to prevent boiling within the primary circuit 4’. Although various specific embodiments have been shown for illustrative purposes, it will be understood that these are not limiting on the scope of the invention. For example the principles of the invention are applicable to vessels operating at higher pressures or having larger capacities than those described, Furthermore although having a storage vessel divided into two chambers and having a separate expansion vessel are shown in different embodiments, these two features could be provided together.

Claims

1. A thermal energy storage arrangement comprising: a storage vessel for storing liquid, the storage vessel comprising a vessel liquid inlet and a vessel liquid outlet; a heat exchanger, comprising: a primary liquid inlet and a primary liquid outlet, wherein the primary liquid inlet is connected to the vessel liquid outlet and the primary liquid outlet connected to the vessel liquid inlet so as to form a primary liquid circuit; and a secondary liquid inlet and a secondary liquid outlet for connecting to a secondary liquid circuit; a variable-speed pump arranged to pump liquid through the primary liquid circuit; a temperature sensor arranged to detect a temperature of liquid within the secondary liquid circuit; and a control system configured to control a pumping speed of the variablespeed pump based on an output of the temperature sensor.

2. The thermal energy storage arrangement as claimed in claim 1, wherein the heat exchanger is arranged such that liquid flowing from the primary inlet to the primary outlet flows in substantially the same direction within the heat exchanger as liquid flowing from the secondary inlet to the secondary outlet.

3. The thermal energy storage arrangement as claimed in claim 1 or 2, wherein the storage vessel comprises an electric heater for heating liquid stored therein.

4. The thermal energy storage arrangement as claimed in claim 3 comprising means for measuring an amount of electrical energy supplied to the electric heater.

5. The thermal energy storage arrangement as claimed in any preceding claim, wherein the control system is configured to cause the variable-speed pump to pump liquid through the primary liquid circuit while charging the storage vessel.

6. The thermal energy storage arrangement as claimed in any preceding claim, wherein the primary liquid circuit comprises one or more one-way valves arranged to allow liquid to flow from the vessel liquid outlet to the vessel liquid inlet and to prevent or inhibit liquid from flowing from the vessel liquid inlet to the vessel liquid outlet.

7. The thermal energy storage arrangement as claimed in any preceding claim, wherein: the storage vessel is arranged to operate at pressures of less than or equal to 10 barg; and/or the storage vessel has an overall volume capacity of less than or equal to 508 litres.

8. The thermal energy storage arrangement as claimed in any preceding claim, wherein: the storage vessel comprises an inner wall and an outer wall that are physically separated; and an insulating material is provided in an insulating region located between the inner wall and the outer wall.

9. The thermal energy storage arrangement as claimed in any preceding claim, wherein: the temperature sensor is arranged to detect a temperature of liquid as it flows out of the secondary liquid outlet of the heat exchanger; the thermal energy storage arrangement comprises a second temperature sensor arranged to detect the temperature of liquid as it flows into the secondary liquid inlet of the heat exchanger; and the control system is configured to control the pumping speed of the variable-speed pump based additionally on an output of the second temperature sensor.

10. The thermal energy storage arrangement as claimed in any preceding claim comprising means for determining a flow rate of liquid in the secondary liquid circuit.

11. The thermal energy storage arrangement as claimed in claim 10, wherein the control system is configured to take account of said flow rate when controlling the pumping speed of the variable-speed pump.

12. The thermal energy storage arrangement as claimed in claim 10 or 11 , wherein the control system is configured to activate the variable-speed pump to pump liquid around the primary liquid circuit when liquid is flowing through the secondary liquid circuit.

13. The thermal energy storage arrangement as claimed in any of claims 10 to

12, comprising a or the second temperature sensor arranged to detect the temperature of liquid as it flows into the secondary liquid inlet of the heat exchanger, wherein the control system is configured to estimate the heat flux from the secondary liquid circuit to its surroundings based on the output of the first temperature sensor, the output of the second temperature sensor, and the flow rate.

14. The thermal energy storage arrangement as claimed in any of claims 10 to 13 wherein: the secondary liquid circuit comprises a secondary circuit pump arranged to pump liquid through the secondary liquid circuit; and the means for determining a flow rate comprises means to determine one or more operational conditions of the secondary circuit pump.

15. The thermal energy storage arrangement as claimed in any of claims 10 to

13, wherein the means for determining a flow rate comprises a flow sensor.

16. The thermal energy storage arrangement as claimed in any preceding claim, wherein the vessel liquid inlet and the vessel liquid outlet are coaxial and the vessel liquid inlet is connected to a dip tube located inside the vessel that runs from the coaxial inlet and outlet to a lower portion of the storage vessel.

17. The thermal energy storage arrangement as claimed in any preceding claim, wherein the storage vessel comprises a first chamber for storing liquid and a second chamber for storing gases, the second chamber being positioned above the first chamber when the storage vessel is positioned upright.

18. The thermal energy storage arrangement as claimed in any preceding claim, further comprising an expansion vessel for storing gases that is connected to the storage vessel.

19. The thermal energy storage arrangement as claimed in any preceding claim, wherein: the storage vessel comprises a pressure sensor arranged to detect the internal pressure of the storage vessel; the storage vessel comprises a venting outlet comprising a venting valve; and the control system is configured to control the operation of the venting valve based on an output of the pressure sensor.

20. The thermal energy storage arrangement as claimed in claim 19, wherein the control system is configured to control the operation of the venting valve in order to store a volume of non-condensable gases in order to maintain pressure within the storage vessel during operation.

21. The thermal energy storage arrangement as claimed in any preceding claim, wherein the variable-speed pump comprises a variable-speed positive displacement pump.

22. A heating system comprising the thermal energy storage arrangement as claimed in any preceding claim, and a secondary liquid circuit connected between the secondary liquid outlet and the secondary liquid inlet of the heat exchanger.

23. The heating system as claimed in claim 22, wherein the secondary liquid circuit comprises: a central heating system comprising one or more radiators; and/or a hot water supply system comprising one or more outlets.

24. A method of operating a heating system comprising a thermal energy storage arrangement and a secondary liquid circuit, the thermal energy storage arrangement comprising: a storage vessel for storing liquid, the storage vessel comprising a vessel liquid inlet and a vessel liquid outlet; a heat exchanger, comprising: a primary liquid inlet and a primary liquid outlet, wherein the primary liquid inlet is connected to the vessel liquid outlet and the primary liquid outlet connected to the vessel liquid inlet so as to form a primary liquid circuit; and a secondary liquid inlet and a secondary liquid outlet connected to the secondary liquid circuit; a variable-speed pump arranged to pump liquid through the primary liquid circuit; and a temperature sensor arranged to detect a temperature of liquid within the secondary liquid outlet; wherein the method comprises controlling the pumping speed of the variable-speed pump based on an output of the temperature sensor.