Patent Description:
Combi boilers have high heat outputs, where even small models can provide a heat output of <NUM> kW to provide domestic hot water (DHW), while the requirement to heat mains water from <NUM> to <NUM> is about <NUM> to <NUM> kW (for a typical D supply flow rate of <NUM>-<NUM> liters per minute). Thus, combi boilers can directly provide domestic hot water without a thermal energy storage (TES).

If a system to provide domestic hot water has no combi boiler, but only a residential heat pump with a heat output of approximately <NUM> to <NUM> kW, the residential heat pump alone is insufficient to heat domestic hot water (DHW) directly within the heat pump. Thus, in these systems, a TES, specifically a domestic hot water TES (DHW-TES), is required for providing domestic hot water. However, once the DHW-TES is fully discharged, the system cannot provide hot water anymore and it takes about two hours to fully recharge the DHW-TES again.

<CIT> discloses a preheat tank to which a heat exchanger is operatively coupled and which receives water from a distribution subsystem. A controller has a first mode in which fluid is routed through the heat exchanger to pass heat to the preheat tank and a second mode, in which the fluid is routed through an evaporator of a refrigerator to pass heat to the refrigerant. A water storage tank is coupled to the preheat tank to receive water therefrom and is coupled to a condenser of the refrigerator such that heat rejected by the condenser is passed to the contents of the water storage tank.

<CIT> discloses a domestic water tank pre-heating system to pre-heat domestic water within a domestic water tank and a water saver system is provided for limiting the wasting of clean but tepid water.

<CIT> discloses an auxiliary heat exchange unit used in conjunction with a hot water cylinder of a hot water supply system and which comprises a first tank, a second tank and a central tank.

<CIT> discloses a domestic hot water preheater operable to supply domestic hot water to a structure and/or to preheat a cold return of a space heating system.

In heat pump systems known in the prior art, the heat pump and domestic hot water are separated, because mains water enters the domestic hot water thermal energy storage device directly and is heated within the domestic hot water thermal energy storage device. Furthermore, in heat pump systems known in the prior art, heating of the domestic hot water thermal energy storage device is independent from domestic hot water demand and all thermal energy from the heat pump is directed to the domestic hot water thermal energy storage device for charging.

The problem with the system known in the prior art is that in cases in which the domestic hot water thermal energy storage device (DHW-TES) is fully discharged, the system cannot provide hot water anymore and it can take several hours until the DHW-TES is fully charged again, i.e. the systems cannot provide hot water for quite a long time, i.e. until the charge of the DHW-TES has reached a level suitable for heating mains water to a desired temperature. This leads to considerable downtimes in providing domestic hot water.

<CIT> discloses heating system for heating a building and for heating hot water, having a main heat source, a heat accumulator and an additional heat source, wherein the additional heat source is connected hydraulically in parallel with the heat accumulator.

<CIT> discloses a box-free type heat pump hot water device which comprises a heat pump circulating system and a waterway heating system, wherein the two systems are coupled through a heat accumulator and a reheating heat exchanger, wherein the system in designed such that tap water directly exchanges heat with a phase-change material in the heat to obtain hot water, wherein, when the hot water cannot reach a preset water temperature, an auxiliary heating through the reheating heat exchanger located downstream of the heat accumulator is performed.

Starting therefrom, it was the object of the present application to provide a system and a method which does not have the disadvantages of prior art systems and methods. Specifically, it should be possible with the system and method to provide hot water without downtimes in providing domestic hot water.

The object is solved by the device having the features of claim <NUM> and the method having the features of claim <NUM>. The dependent claims illustrate advantageous embodiments of the invention.

According to the invention, a system for providing domestic hot water is provided, comprising.

characterized in that the heat exchanger (<NUM>) is located upstream of the thermal energy storage device (<NUM>) and the controller is configured to control the heat pump based on a selected operation mode.

The system according to the invention allows the provision of providing domestic hot water without downtimes and allows an extension of the domestic hot water discharge capacity. The reason is that the system according to the present invention can switch between the first operation mode and second operation mode and thus link a heat pump operation with a degree of domestic hot water demand. If there is no demand of domestic hot water, the system can switch to the first operation mode. If there is a (high) demand for domestic hot water, the system can switch to the second operation mode.

In the first operation mode (i.e. when there is no demand for domestic hot water), the controller of the system controls the heat pump to provide thermal energy from the heat pump to the thermal energy storage device, but not to the heat exchanger, i.e. the controller effectuates a charging of the thermal energy storage device. In the second operation mode (i.e. when demand for domestic hot water is high), the controller of the system controls the heat pump to provide thermal energy from the heat pump to the heat exchanger of the system, i.e. the controller effectuates a heating of the mains water in the heat exchanger. This heating is a preheating of the mains water before it enters the thermal energy storage device of the system. Thus, the heat exchanger is located upstream of the thermal energy storage device in the system according to the invention (used in the method according to the invention).

It becomes clear that the additional heating of mains water by the heat exchanger in the second operation mode reduces the heating burden which is put on the thermal energy storage device, especially during periods of a high demand of domestic hot water, in this operation mode. In other words, a higher volume of domestic hot water at a target temperature can be provided or, put differently, domestic hot water at a target temperature can be provided for a longer period of time, thus abolishing downtimes.

Briefly, the second operation mode has the following advantages:.

The controller of the system can be configured to, in the second operation mode, allow heated mains water to flow from the heat exchanger into the thermal energy storage device. Preferably heated mains water is allowed to flow such that thermal energy of the thermal energy storage device is transferred to the heated mains water (= further heating of the mains water, e.g. if the thermal energy storage device is a PCM-TES) or such that thermal energy of the heated mains water is transferred to the thermal energy storage device (= charging of the thermal energy storage device, e.g. if the thermal energy storage device is a water storage tank).

The controller of the system can be configured to switch to the first operation mode if there is no active demand for domestic hot water and a state of charge of the thermal energy storage device is below a preset threshold.

Furthermore, the controller of the system can be configured to switch to the second operation mode if there is a high active demand for domestic hot water and the heat pump is running or not running, or if there is a low demand for domestic hot water and the heat pump is running.

The system can comprise a third operation mode in which no thermal energy is provided from the heat pump to the thermal energy storage device and to the heat exchanger. In the third operation mode, the system is configured to provide domestic hot water only by heat energy stored in the thermal energy storage device, i.e. only the thermal energy storage device is used for heating mains water to domestic hot water. The controller is preferably configured to switch to the third operation mode if there is a low active demand for domestic hot water and/or if a state of charge of the thermal energy storage device is at or above a preset threshold and the heat pump shall not be operated. In the third operation mode (when demand for domestic hot water is low), the controller of the system controls the heat pump to provide no thermal energy from the heat pump to the thermal energy storage device and no thermal energy of from the heat pump to the heat exchanger of the system, i.e. the controller effectuates that the mains water is only heated by thermal energy stored in the thermal energy storage device, which is thereby discharged.

The system can further comprise a fluid flow detection sensor which is suitable for detecting a volume flow of fluid from the thermal energy storage device to a domestic hot water discharge of the system.

Besides, the system can further comprise a resistance heater for heating water of the mains water supply. The controller is preferably configured to activate the resistance heater if there is a high active demand for domestic hot water. The resistance heater allows a further heating of the mains water and allows a quicker response to a heating demand than the heat pump, because the heat pump needs a moment to run at full capacity. The resistance heater can be located upstream or downstream of the thermal energy storage device. A location upstream of the thermal energy storage device has the advantage that the resistance heater can contribute to charging the thermal energy storage device (with thermal energy). Moreover, the resistance heater can be located upstream or downstream of the heat exchanger. Preferably, the resistance heater is located downstream of the heat exchanger and upstream of the thermal energy storage device. It is also possible that the resistance heater and the heat pump together provide warm potable water at temperatures that are not perceived as very cold by the user. This allows bypassing the thermal energy storage device to avoid its discharge or provides a further measure to avoid downtimes if the thermal energy storage device is fully discharged and there is still a high demand for domestic hot water.

The system can comprise a means for heating water of the mains water supply by energy recovered from waste water. Said means is preferably located upstream of the heat exchanger in the system.

The system can further comprise a state of charge device which is configured to determine the state of charge of the thermal energy storage device. Preferably, the controller is configured to control the heat pump based on a state of charge determined by the state of charge device. Moreover, the controller can be configured to switch to the first operation mode if the determined state of charge of the thermal energy storage device is below a preset threshold. Furthermore, the controller can be configured to switch to the second operation mode if the determined state of charge of the thermal energy storage device is below a preset threshold. Besides, the controller can be configured to switch to a third operation mode, in which no thermal energy is provided from the heat pump to the thermal energy storage device and to the heat exchanger, if the state of charge of the thermal energy storage device is at or above a preset threshold.

The controller can be configured to switch into the second operation mode if there is a need of a large amount of domestic hot water by a hot water consuming device. The hot water consuming device is preferably configured to communicate a need of a large amount of hot water to the controller. The hot water consuming device is particularly preferably selected from the group consisting of kitchen sink, bathtub, washing machine, dishwasher and combinations thereof.

Furthermore, the controller can be configured to switch into the second operation mode if a domestic hot water demand prediction predicts a need of a large amount of domestic hot water. Preferably, the controller is configured to perform said prediction.

Moreover, the controller can be configured to switch into the second operation mode if a volume flow from the thermal energy storage device to a domestic hot water discharge of the system falls above a certain threshold and the domestic hot water discharge of the system is associated to large domestic hot water discharges. The volume flow is preferably detected by a fluid flow detection sensor of the system which is configured to communicate a detected volume flow to the controller. The fluid flow detection sensor can be selected from the group consisting of flowmeter, pressure sensor, temperature sensor and combinations thereof.

Besides, the controller can be configured to switch into the second operation mode if a direct user input communicates a need of a large amount of domestic hot water. The direct user input is preferably detected by an input device of the system which is configured to communicate a need of a large amount of domestic hot water to the controller.

In a preferred embodiment, the thermal energy storage device is a phase change material thermal energy storage device (PCM-TES). The phase change material thermal energy storage device can comprise a heat exchanger embedded into it which promotes transfer of heat from the heat pump to the phase change material thermal energy storage device. A PCM-TES as thermal energy storage device is beneficial from an energy perspective. The PCM-TES stores most of its energy content by utilising the heat of fusion of the phase change material (PCM), meaning that almost all the energy is stored and released at the melting temperature of the PCM (e.g. <NUM>). Hence, water entering at a higher inlet temperature will lead to a lower discharge heat flow rate from the PCM to the water. The lower discharge rate increases the amount of water that can be extracted from the heat stored in the PCM at <NUM>.

Alternatively, the thermal energy storage device can be a water storage tank, preferably a water storage tank containing an encapsulated phase change material.

According to the invention, a method for providing domestic hot water is provided, comprising the steps.

With the inventive method, domestic hot water can be provided without downtimes and the domestic hot water discharge capacity can be extended.

In the method, the controller can be configured to, in the second operation mode, allow heated mains water to flow from the heat exchanger into the thermal energy storage device. Preferably the heated mains water is allowed to flow from the heat exchanger into the thermal energy storage device such that thermal energy of the thermal energy storage device is transferred to the heated mains water or thermal energy of the heated mains water is transferred to the thermal energy storage device.

In the method, the controller can be configured to switch to the first operation mode if there is no active demand for domestic hot water and a state of charge of the thermal energy storage device is below a preset threshold.

Furthermore, in the method, the controller can be configured to switch to the second operation mode if there is a high active demand for domestic hot water and the heat pump is running or not running, or if there is a low demand for domestic hot water and the heat pump is running.

The system provided in the method can comprise a third operation mode in which no thermal energy is provided from the heat pump to the thermal energy storage device and the heat exchanger. In the third operation mode, the system is configured to provide domestic hot water only by heat energy stored in the thermal energy storage device, i.e. only the thermal energy storage device is used for heating mains water to domestic hot water. The controller is preferably configured in the method to switch to the third operation mode if there is a low active demand for domestic hot water and/or if a state of charge of the thermal energy storage device is at or above a preset threshold and the heat pump shall not be operated.

The system which is provided in the method can further comprise a fluid flow detection sensor which is suitable for detecting a volume flow of fluid from the thermal energy storage device to a domestic hot water discharge of the system.

Moreover, the system which is provided in the method can further comprise a resistance heater for heating water of the mains water supply. The controller is preferably configured in the method to activate the resistance heater if there is a high active demand for domestic hot water. The resistance heater allows a further heating of the mains water and allows a quicker response to a heating demand than the heat pump, because the heat pump needs a moment to run at full capacity. The resistance heater can be located upstream or downstream of the thermal energy storage device. A location upstream of the thermal energy storage device has the advantage that the resistance heater can contribute to charging the thermal energy storage device (with thermal energy). Moreover, the resistance heater can be located upstream or downstream of the heat exchanger. Preferably, the resistance heater is located downstream of the heat exchanger and upstream of the thermal energy storage device. It is also possible that the resistance heater and the heat pump together provide warm potable water at temperatures that are not perceived as very cold by the user. This allows bypassing the thermal energy storage device to avoid its discharge or provides a further measure to avoid downtimes if the thermal energy storage device is fully discharged and there is still a high demand for domestic hot water.

The system which is provided in the method can comprise a means for heating water of the mains water supply by energy recovered from waste water. Said means is preferably located upstream of the heat exchanger in the system of the method.

The system which is provided in the method can further comprises a state of charge device which is configured to determine the state of charge of the thermal energy storage device. The controller is preferably configured in the method to control the heat pump based on a state of charge determined by the state of charge device. Moreover, the controller is preferably configured in the method to switch to the first operation mode if the determined state of charge of the thermal energy storage device is below a preset threshold. Furthermore, the controller is preferably configured in the method to switch to the second operation mode if the determined state of charge of the thermal energy storage device is below a preset threshold. Besides, the controller is preferably configured in the method to switch to a third operation mode, in which no thermal energy is provided from the heat pump to the thermal energy storage device and to the heat exchanger, if the determined state of charge of the thermal energy storage device is at or above a preset threshold.

The controller can be configured in the method to switch into the second operation mode if there is a need of a large amount of domestic hot water by a hot water consuming device. The hot water consuming device is preferably configured in the method to communicate a need of a large amount of hot water to the controller. The hot water consuming device can be selected from the group consisting of kitchen sink, bathtub, washing machine, dishwasher and combinations thereof.

Moreover, the controller can be configured in the method to switch into the second operation mode if a domestic hot water demand prediction predicts a need of a large amount of domestic hot water. Preferably, the controller is configured in the method to perform said prediction.

Furthermore, the controller can be configured in the method to switch into the second operation mode if a volume flow from the thermal energy storage device to a domestic hot water discharge of the system falls above a certain threshold and the domestic hot water discharge of the system is associated to large domestic hot water discharges. The volume flow is preferably detected by a fluid flow detection sensor of the system which is configured in the method to communicate a detected volume flow to the controller. The fluid flow detection can be selected from the group consisting of flowmeter, pressure sensor, temperature sensor and combinations thereof.

Besides, the controller can be configured in the method to switch into the second operation mode if a direct user input communicates a need of a large amount of domestic hot water. The direct user input is preferably detected by an input device of the system which is configured in the method to communicate a need of a large amount of domestic hot water to the controller.

In a preferred embodiment, the thermal energy storage device of the system provided in the method is a phase change material thermal energy storage device. The phase change material thermal energy storage device preferably comprises a heat exchanger embedded into it which promotes transfer of heat from the heat pump to the phase change material thermal energy storage device.

With reference to the following figures and examples, the subject according to the invention is intended to be explained in more detail without wishing to restrict said subject to the specific embodiments shown here. Among the following Figures and Examples, <FIG> and Example <NUM> do not show embodiments according to the invention, but are helpful to understand certain aspects thereof.

<FIG> illustrates schematically a simple rule-based control of the controller of a system and method according to the present invention. The controller is configured to determine whether the system should operate in a first operation mode (charging mode) or a second operation mode (preheating mode). In this embodiment, the controller is also configured to determine whether the system should operate in a third operation mode (Cold mains heated only by DHW-TES mode). Preheating may be activated when active demand for domestic hot water is high, or if active demand for hot water is low and the heat pump is already running. The controller may return to the first operation mode (charging mode) when a domestic hot water (DHW) demand has finished. In addition, the control activates DHW preheating mode, if a large DHW demand is detected or the state of charge (SOC) of the thermal energy storage device (TES) falls below a minimum threshold.

<FIG> illustrates schematically a first system according to the present invention. In this embodiment, the thermal energy storage device is a phase change material thermal energy storage device (PCM-TES). Domestic hot water exiting the mains water supply <NUM> flows in a fluid line through the heat exchanger <NUM> in which it can receive thermal energy from a fluid line exiting the heat pump <NUM>. On its way to the thermal energy storage device <NUM> (in this case: a PCM-TES) the preheated mains water can further be heated by a resistance heater <NUM>. In the thermal energy storage device, the preheated mains water is further heated to domestic hot water <NUM> which can flow to the kitchen sink <NUM> or to the bath tub <NUM>. The controller <NUM> of the system is configured to control the heat pump <NUM>. The illustrated system also comprises a state of charge device <NUM> configured to determine the state of charge of the thermal energy storage device <NUM> and a flow detection sensor <NUM>. For directing the flow of mains water, the system also comprises a switching valve <NUM> which allows switching the mains water to flow either through the heat exchanger <NUM> via the resistance heater <NUM> to the thermal energy storage device <NUM>, or to the local space heating system <NUM>. The heat exchanger <NUM>, resistance heater <NUM>, controller <NUM> and switching valve are comprised by an indoor unit <NUM>.

<FIG> illustrates schematically a further system according to the present invention in the first operation mode (charge mode). In this system, the thermal energy storage device <NUM> is a water storage tank. Domestic water exiting the mains water supply <NUM> flows in a fluid line through the heat pump <NUM> in which it can receive thermal energy from the heat pump <NUM>. In this first operation mode, there is no discharge from the thermal energy storage device <NUM> (see greater line thickness), i.e. no domestic hot water <NUM> is provided to the kitchen sink <NUM> and/or to the bath tub <NUM>. Moreover, no mains water enters the thermal energy storage device <NUM>, but a second pipe actively extracts water from the thermal energy storage device <NUM>, where it flows through heat exchanger <NUM> and then resistance heater <NUM>, before returning back to the thermal energy storage device. The controller <NUM> of the system is configured to control the heat pump <NUM>. The illustrated system also comprises a state of charge device <NUM> configured to determine the state of charge of the thermal energy storage device <NUM> and a flow detection sensor <NUM>. For directing the flow of mains water, the system also comprises a switching valve <NUM>. The heat exchanger <NUM>, resistance heater <NUM>, controller <NUM> and switching valve are comprised by a cylinder unit <NUM> for domestic hot water.

<FIG> illustrates schematically the system illustrated in <FIG>, but in the second operation mode (preheating water discharge mode). Domestic water exiting the mains water supply <NUM> flows in a fluid line through the heat exchanger <NUM> in which it can receive thermal energy from a fluid line exiting the heat pump <NUM>. On its way to the thermal energy storage device <NUM> (in this case: a water storage tank) the preheated mains water transfers heat to the thermal energy storage device <NUM>. Water heated in the thermal energy storage device <NUM> to domestic hot water <NUM> can flow to the kitchen sink <NUM> or to the bath tub <NUM>. The controller <NUM> of the system is configured to control the heat pump <NUM>. The illustrated system also comprises a state of charge device <NUM> configured to determine the state of charge of the thermal energy storage device <NUM> and a flow detection sensor <NUM>.

For directing the flow of mains water, the system also comprises a switching valve <NUM>. The heat exchanger <NUM>, resistance heater <NUM>, controller <NUM> and switching valve are be comprised by a cylinder unit <NUM> for domestic hot water.

<FIG> illustrates schematically a further system according to the present invention in the second operation mode (postheating water discharge mode). Some components of the system shown in <FIG> have been omitted for clarity reasons. In this embodiment, mains water exiting a thermal energy storage device <NUM> of the system is heated by the heat exchanger <NUM> which receives thermal energy from the heat pump <NUM> and is located downstream of the thermal energy storage device <NUM> in this case. This allows domestic hot water <NUM> at a target temperature to be provided to a kitchen sink <NUM> and/or a bath tub <NUM> even if the thermal energy storage device has a low thermal capacity or is fully discharged.

For example, a shower requires water flow rates of about <NUM> liters per minute (LPM). To supply the shower with DHW, cold mains water enters the heat exchanger at <NUM>, where it is heated by <NUM> kW from the heat pump to <NUM> in the heat exchanger, optionally also an additional <NUM> kW from a resistance heater, to <NUM>. The preheated mains water is then further heated in the DHW-TES from <NUM> to <NUM> outlet temperature.

This leads to a discharge rate of <NUM> kW from the DHW-TES, which is significantly lower than the <NUM> kW that would have been required without preheating mode.

Thus, the DHW volume that can be supplied by the DHW-TES is doubled if the heating mode (second operation mode) is applied. In addition, the shower is still supplied with warm water at <NUM> once the DHW-TES is fully discharged.

A first embodiment of a system and a method according to the invention is shown in <FIG>. In this embodiment, the thermal energy storage device is a PCM-TES.

The first operation mode represents a mode in which the heat pump transfers heat to the PCM-TES (preferably via transfer of heat to a heat exchanger embedded into the PCM-TES), but does not transfer heat to water flowing through the heat exchanger which establishes a thermal connection between a fluid line exiting the mains water supply and a fluid line exiting the heat pump.

The first operation mode can be activated if there is no active DHW demand and the state of phase (SOC) of the PCM-TES is below a threshold.

The second operation mode represents a mode in which the heat pump transfers heat to water flowing through the heat exchanger which establishes a thermal connection between a fluid line exiting the mains water supply and a fluid line exiting the heat pump. Mains water heated in the heat exchanger is then transported into the PCM-TES in which the mains water is further heated and acquires its final temperature.

As can be seen e.g. in <FIG>, mains water originating from the mains water supply will enter the heat exchanger (which establishes a thermal connection between a fluid line exiting the mains water supply and a fluid line exiting the heat pump) and will be heated in said heat exchanger by heat originating from the (fluid line of the primary circuit of the) heat pump. An optional resistance heater can further heat the mains water flow after exiting the heat exchanger. The preheated mains water enters the PCM-TES (preferably a heat exchanger embedded into the PCM-TES) at an intermediate temperature and is heated to its desired final temperature by the PCM-TES.

The second operation mode can be activated e.g. if.

The third operation mode represents a mode in which the heat pump transfers no heat to the PCM-TES and also no heat to the heat exchanger which establishes a thermal connection between a fluid line exiting the mains water supply and a fluid line exiting the heat pump. In this third operation mode, the heat pump is neither supplying thermal energy to the mains water (via the heat exchanger) nor to the PCM-TES (i.e. no charging of the PCM-TES occurs).

As can be seen e.g. in <FIG>, mains water originating from the mains water supply will enter the heat exchanger (which establishes a thermal connection between a fluid line exiting the mains water supply and a fluid line exiting the heat pump) and will not be heated in said heat exchanger. An optional resistance heater is switched off. The non-preheated mains water enters the PCM-TES (preferably a heat exchanger embedded into the PCM-TES) at its original temperature and is heated to its desired final temperature by the PCM-TES (alone).

Since all energy input to the mains water is provided by the PCM-TES, the third operation mode is advantageous if only small DHW discharges are required, e.g. discharges shorter than the start-up time needed by the heat pump. This configuration can further be advantageous, if other control decisions prohibit a heat pump DHW cycle at that moment, e.g. absence of inexpensive, renewable energy.

A second embodiment of the system and method according to the invention is shown in <FIG> and <FIG>. In this embodiment, the thermal energy storage device is a water storage tank.

The first operation mode is illustrated in <FIG> and represents a mode in which the heat pump transfers heat to the water storage tank, but does not transfer heat to mains water flowing through the heat exchanger which establishes a thermal connection between a fluid line exiting the mains water supply and a fluid line exiting the heat pump.

The first operation mode can be activated if there is no active DHW demand and the heating capacity of the water storage tank is below a threshold.

The second operation mode is illustrated in <FIG> and represents a mode in which the heat pump transfers heat to mains water flowing through the heat exchanger which establishes a thermal connection between a fluid line exiting the mains water supply and a fluid line exiting the heat pump. Mains water heated in the heat exchanger is then transported into the water storage tank in which the heated mains water transfers heat to the water storage tank.

As can be seen e.g. in <FIG>, mains water originating from the mains water supply will enter the heat exchanger (which establishes a thermal connection between a fluid line exiting the mains water supply and a fluid line exiting the heat pump) and will be heated in said heat exchanger by heat originating from the (fluid line of the primary circuit of the) heat pump. An optional resistance heater can further heat the mains water flow after exiting the heat exchanger. The preheated mains water enters the water storage tank and transfers heat to the water storage tank. The preheated mains water is transported into the water storage tank in a manner that it is not mixed. To this end, it is beneficial if the inlet for conducting preheated mains water into the water storage tank is located close to the bottom of the water storage tank, as shown in <FIG> and <FIG>. This helps to maintain the stratification, while allowing simultaneous re-charging of the tank.

The second operation mode can e.g. be activated (by the controller of the system) if.

The third operation mode represents a mode in which the heat pump transfers no heat to the water storage tank and also no heat to the heat exchanger which establishes a thermal connection between a fluid line exiting the mains water supply and a fluid line exiting the heat pump. In this third operation mode, the heat pump is neither supplying thermal energy to the mains water (via the heat exchanger) nor to the water storage tank (i.e. no charging of the water storage tank occurs).

Since all energy input to the mains water is provided by the water storage tank, the third operation mode is advantageous if only small DHW discharges are required, e.g. discharges shorter than the start-up time needed by the heat pump. This configuration can further be advantageous, if other control decisions prohibit a heat pump DHW cycle at that moment, e.g. absence of inexpensive, renewable energy.

In a third embodiment, the mains water is additionally heated by energy recovered from waste water. For example, the additional heating can be arranged before the mains water enters the heat exchanger and is further heated in the heat exchanger (not shown in the Figures).

In the fourth embodiment, mains water exiting a thermal energy storage device of the system is further heated by using the heat pump in the second operation mode (postheating water discharge mode).

This is possible with one single thermal energy storage device, but also with more than one thermal energy storage device in the system. In one example, the system comprises a first low temperature PCM-TES having a storage capacity at <NUM>-<NUM>, which is mainly used for space heating, and a second high temperature PCM-TES having a storage capacity of <NUM>-<NUM>.

Claim 1:
System for providing domestic hot water (<NUM>), comprising
a) a heat pump (<NUM>),
b) a mains water supply (<NUM>);
c) a heat exchanger (<NUM>) establishing a thermal connection between a fluid line exiting the mains water supply (<NUM>) and a fluid line exiting the heat pump (<NUM>);
d) a thermal energy storage device (<NUM>) connected to the heat pump (<NUM>);
e) a first operation mode in which thermal energy is provided from the heat pump (<NUM>) to the thermal energy storage device (<NUM>), but not to the heat exchanger (<NUM>);
f) a second operation mode in which thermal energy is provided from the heat pump (<NUM>) to the heat exchanger (<NUM>), and
g) a controller configured to select at least between the first operation mode and the second operation mode;
characterized in that the heat exchanger (<NUM>) is located upstream of the thermal energy storage device (<NUM>) and the controller is configured to control the heat pump (<NUM>) based on a selected operation mode.