NL2027593B1

NL2027593B1 - Solar system for converting solar energy into heat

Info

Publication number: NL2027593B1
Application number: NL2027593A
Authority: NL
Inventors: Jacobus Louise Van Ruth Nico
Original assignee: Conico Valves B V
Priority date: 2021-02-18
Filing date: 2021-02-18
Publication date: 2022-09-14

Abstract

A solar system for converting solar energy into heat is described. The system comprises a solar collector for collecting solar energy; a heat storage container for storing a heat transfer fluid; and a piping circuit that comprises a supply line connecting the solar collector to an inlet of the heat storage container; a return line connecting an outlet of the heat storage container to the solar collector; a bypass line provided between the supply line and the return line; a circulation pump configured to circulate the heat transfer fluid through the piping circuit under the control of a controller; and a bypass-valve arranged for directing the heat transfer fluid from the supply line via the bypass line to the return line in a bypass state and from the supply line via the heat storage container to the return line in a storage container state, under the control of the controller. The piping circuit comprises a first supply line section that connects to the heat storage container via a thermal bridge which causes this first supply line section to heat up through natural convection and/or through conduction by heat originating from the heat storage container when not circulating the heat transfer fluid, which first supply line section is bounded by a second and third line section that both connect with the first supply line section and extend downwards in a vertical or downward sloped direction relative to the first supply line section, which causes the first supply line section to buffer the heat originating from the heat storage container when not circulating the heat transfer fluid.

Description

Solar system for converting solar energy into heat

FIELD OF THE INVENTION The invention relates to a solar thermal system for converting solar energy into heat, comprising a solar collector for collecting solar energy, a heat storage container for storing a heat transfer fluid, and a piping circuit for the heat transfer fluid, provided to connect the solar collector and the heat storage container. in between. The invention in particular relates to a solar thermal system having intrinsic frost protection and/or time of the day detection.

BACKGROUND Solar thermal systems are widely used to generate heat from solar radiation, with common applications such as for instance domestic hot water preparation and space heating of dwellings.

Solar thermal systems may use water as a heat transfer fluid, in which case the outdoor section of the heat transfer circuit, including the solar collector, typically needs to be protected against potential frost damage. In systems with pumped circulation, such frost protection may typically be achieved by circulating warm fluid through the system when the solar collector and/or outdoor temperature falls below a threshold temperature. Other solar thermal systems use alternative frost protection methods, such as the use of a glycol mixture as the heat transfer fluid in order to change the temperature of freezing. Pumped solar thermal typically consist of at least a solar collector, a heat reservoir, a circulation pump, and a piping circuit connecting these components to form a heat transfer circuit. Some pumped solar thermal systems also include a 3-way bypass valve, such as the system described in EP2009359, which allows the heat transfer fluid of the solar thermal system to be circulated through a bypass line without passing said fluid through the heat reservoir. This may be used for determining the temperature in different parts of the heat transfer circuit using a single temperature sensor, without potentially drawing heat from the heat reservoir.

SUMMARY OF THE INVENTION It would be desirable to provide a solar thermal system that may easily and automatically be protected against frost damage.

This and other aims are achieved by a solar thermal system according to claim 1. According to a first aspect of the invention there is provided a solar system for converting solar energy into heat, comprising: a) a solar collector for collecting solar energy; b) a heat storage container for storing a heat transfer fluid; c) a piping circuit, comprising - a supply line connecting the solar collector to an inlet of the heat storage container, - a retum line connecting an outlet of the heat storage container to the solar collector, - a bypass line provided between the supply line and the return line, - a circulation pump configured to circulate the heat transfer fluid through the piping circuit under the control of a controller, and - a bypass-valve arranged for directing the heat transfer fluid from the supply line via the bypass line to the return line in a bypass state and from the supply line via the heat storage container to the return line in a storage container state, under the control of the controller, wherein the piping circuit comprises a first supply line section that connects to the heat storage container via a thermal bridge which causes this first supply line section to heat up through natural convection and/or through conduction by heat originating from the heat storage container when not circulating the heat transfer fluid, which first supply line section is bounded by a second and third line section that both connect with the first supply line section and extend downwards in a vertical or downward sloped direction relative to the first supply Hne section, which causes the first supply line section to buffer the heat originating from the heat storage container when not circulating the heat transfer fluid.

The solar thermal system of the invention has a bypass line that bypasses the heat reservoir, with a bypass-valve (or bypass valves) that direct the flow of the heat transfer fluid either via the heat reservoir or via the bypass line. Typically, in normal operation of the solar thermal system the bypass-valve is in the bypass state, wherein a flow of the heat transfer fluid is directed from the supply line via the bypass line.

Heat reservoirs in pumped solar thermal systems often, particularly in the upper section of the heat reservoir, have the piping connected to the reservoir in such a way that at least a part of the piping runs in a downward direction, from the perspective of the reservoir. This is done to prevent heat loss from the reservoir through natural convection and flow effects inside the piping. In horizontal or upward sloping pipes a significant amount of heat is lost from hot fluid flowing away from the reservoir in the upper part of the horizontal pipe section, and cold fluid flowing simultaneously flowing back in the bottom part of the pipe section. This natural convection effect heats the piping section, which increases heat losses to the environment.

The invention makes use of the above insights by introducing a downward sloping piping section, often a vertical section, which stops the natural convection flow, since the warm fluid has a lower density than colder fluid and the gravity effect that drives the natural convection is stopped. The result is that any horizontal or upwards sloping section between the heat reservoir and the downward sloping piping section is heated by the reservoir to a high temperature relatively quickly, and will assume approximately the same temperature as the heat reservoir at the position of the piping connection, assuming the piping is adequately insulated and the horizontal or upward sloping section is preferably limited in length. In this way, the heat transfer coefficient from the reservoir to the horizontal or upward sloping piping section may be much higher than the heat transfer coefficient from the piping section to the ambient.

In the present invention, the heat transfer circuit comprises a heat buffering section configured for buffering heat supplied from the heat reservoir and a thermal bridge section configured for allowing a heat flow from the heat reservoir to the heat butfering section. The thermal bridge section provides that heat from the heat reservoir is suitably conducted to the heat buffering section, even when there is no circulation of the heat thermal fluid inside the heat transfer circuit.

The thermal bridge section may allow natural convection of the heat transfer fluid from the heat reservoir to the heat buffering section through the thermal bridge section. In an example, the thermal bridge section has a piping being substantially horizontally sloping and/or npwards sloping relative to the gravity direction. In alternative embodiments or additional embodiments, the thermal bridge section may allow heat conductivity through solid parts of the thermal bridge section. In an example, the thermal bridge section comprises a solid metal element, such as a metal rod, which conducts heat along the longitudinal direction of the thermal bridge section from the heat reservoir to the heat buffering section.

The heat buffering section is configured for buffering heat supplied from the heat reservoir. The heat buffering section is a part of the heat transfer circuit and, as such, supports transport of the heat transfer fluid along the heat transfer circuit. The heat buffering section may be at least partly composed of a piping being arranged substantially horizontally sloping and/or upwards sloping relative to the gravity direction. The heat buffering section according to the invention is confined along the heat transfer circuit, in particular at both ends of the heat buffering section, in order to maintain the buffered heat inside the heat buffering section, when there is no circulation of the heat thermal fluid inside the heat transfer circuit. The size and structure of the heat buffering section may be suitably selected to determine the amount of heat buffered in the heat buffering section and to restrict any excessive heat loss, when there is no circulation of the heat thermal fluid inside the heat transfer circuit.

In this invention in the bypass state the heat transfer fluid is transported through the heat buffering section by the circulation pump, in case the temperature of the heat transfer fluid runs below a predefined threshold value. The bypass state is the state of the bypass valve in which the heat transfer fluid is circulated from the supply line back to the solar collector via the bypass line without passing through the heat reservoir. The circulation of the heat transfer fluid through the heat buffering section picks up the, predetermined, buffered heat from the heat buffering section. The buffered heat is transported by the heat transfer fluid from the heat buffering section into the heat transfer circuit in order to maintain a temperature of the heat transfer fluid in the heat transfer circuit and/or in the solar collector above a frost temperature.

Additionally, due to the circulation flow, the temperature inside the heat buffering section may drop and the thermal bridge section provides a controlled heat flow to the heat buffering section, which may additionally be used to continue the circulation flow to maintain a temperature of the heat transfer fluid in the heat transfer circuit and/or in the solar collector above a frost temperature. A heat transfer rate to the heat buffering section is controlled by a temperature difference AT between the heat buffering section and the thermal bridge section. In this way, it can be controllably prevented that the heat transfer fluid inside the heat transfer circuit and/or in the solar collector freezes.

Thus, the invention can be used for multiple purposes, one being that the heat picked up indirectly from the heat reservoir can be used to protect the solar circuit against frost damage, with the benefit being that the heat delivered can be carefully dosed by periodically circulating the heat transfer circuit through the bypass line, and each time adding a relatively small amount of heat, which can be dispersed throughout the whole heat transfer circuit. The relatively small amount of heat can keep the heat transfer circuit at a temperature high enough to prevent frost damage, but not higher than necessary, thereby limiting the heat lost for frost protection to a minimum. In existing pumped solar thermal systems using water, heat is picked up directly from the heat reservoir, which makes it very difficult to control the amount of heat delivered for frost protection and still disperse this heat throughout the whole heat transfer circuit. This typically results in the heat transfer circuit receiving too much heat, which raises the heat transfer circuit's temperature to an unnecessarily high temperature, resulting in high heat losses to the ambient.

The bypass valve may be an electrically actuated valve, a thermostatic self-actuating 3-way valve that, that switches at a pre-set temperature level or any other suitable valve. The actuator may be any suitable actuator for controlling the bypass-valve, such as an electrically operated actuator based on a thermal element, that is electrically heated to actuate in one direction, and returns to a 5 default position when the electrical heating is interrupted, or a self-actuating actuator, which responds to a temperature difference of the temperature at the valve and the temperature in the heat reservoir. In an exemplary embodiment, the second and third line sections that bound the first supply line section are part of the supply line. The downward sloping parts act as blockage for natural convection flow of the thermal transfer fluid. In examples, the downward sloping part may continuously slope downwards with respect to the direction of gravity along its longitudinal length or the downward sloping part may intermittently slope downwards with respect to the direction of gravity along its longitudinal length. In any way, a downward height distance of the downward sloping part along its longitudinal direction may be suitably selected by the skilled person for blocking natural convection flow of the thermal transfer fluid along the line.

In another exemplary embodiment, the second line section is part of the supply line and the third line section is part of the of the bypass line.

In yet another embodiment, the first supply line section that is bounded by the second and third line sections extends in a horizontal or upward sloped direction. This improves the heat buffering.

The thermal bridge used in the solar thermal system of the invention connects the heat storage container with the first supply line section in which heat is buffered. The thermal bridge may be a separate item, such as a piece of pipe, provided between the heat storage container and the first supply line section. In another embodiment of the solar system the inlet comprises the thermal bridge.

In the solar system of the invention , the controller may be configured to protect the system against frost damage. According to an embodiment, the controller is configured to intermittently circulate the heat transfer fluid through the piping circuit in the bypass state causing the heat buffered in the first supply line section to circulate through the piping circuit. This effectively heats at least a part of the piping circuit without having to use heat that is stored in the heat storage reservoir. The heat provided may protect parts of the piping circuit against frost damage.

In an appropriate embodiment therefore, a solar thermal system is provided wherein the heat transferred from the heat storage container to the piping circuit, while in the bypass state, is used for frost protection.

Improved temperature control may be achieved by the following embodiments. In an embodiment, the controller is configured to circulate the heat transfer fluid through the piping circuit in the bypass state during intermittent first and second circulation cycles with a stationary period in between.

In another embodiment, the piping circuit comprises a first temperature sensor and the controller is configured to circulate the heat transfer fluid through the piping circuit in the bypass state when the temperature of the heat transfer fluid sensed by the first temperature sensor satisfies a first temperature criterion.

In yet another embodiment, the first temperature criterion comprises a minimum temperature sensed during the first or second circulation cycle, or during the stationary period, that is lower than a pre-set frost-protection temperature.

In another embodiment, the first temperature criterion comprises a maximum temperature sensed during the first or second circulation cycle, or during the stationary period, that is higher than a pre-set overheating-protection temperature.

In a further improved embodiment, the first temperature criterion comprises an average temperature sensed during the first or second circulation cycle, or during the stationary period, that is lower than the pre-set frost-protection temperature or higher than the pre-set overheating- protection temperature.

Yet another embodiment provides a solar thermal system wherein the controller is configured to circulate the heat transfer fluid through the piping circuit in the bypass state until the temperature of the heat transfer fluid sensed by the first temperature sensor stops satisfying the first temperature criterion with a deviation of the maximum, minimum or average temperature by a preset amount, such as 10% of said temperature.

A further improved embodiment provides a solar thermal system wherein the controller is configured to circulate the heat transfer fluid through the piping circuit in the bypass state at least during one circulation cycle when the temperature of the heat transfer fluid sensed by the first temperature sensor satisfies the first temperature criterion. Yet another embodiment relates to a solar thermal system wherein the second supply line section comprises a second temperature sensor configured to sense the temperature of the buffered heat. An efficient embodiment uses a common temperature sensor and is characterized in that the second temperature sensor is the first temperature sensor.

In other particular embodiments of the solar thermal system, the control unit is configured to determine a morning solar state of the solar system. In EP2009359, a difficulty may be to reliably know when it is morning and the system should start circulating to measure the temperature in the heat transfer circuit. The purpose of the present embodiments is to identify when the sun comes out.

i5 When the sun comes out, it heats the solar collector, which causes the heat transfer fluid to expand, which causes some flow through the said horizontal or upward sloping piping section, when an expansion vessel of the heat transfer circuit is positioned after said horizontal or upwards sloping piping section. This flow can be detected by cooling down in the downward sloping pipe close to the section of pipe of the heat buffering section that is horizontal or upward sloping, which is approximately at the temperature of the heat reservoir. By positioning a temperature sensor at a carefully chosen position along the downward sloping pipe, this effect can be used as a ‘morning detection” (which may also be referred to as detection of morning solar state) by the controller, in order to decide starting circulation and check the temperatures in the heat transfer circuit.

An embodiment of the invention therefore provides a solar thermal system, wherein the controller is configured to start circulating the heat transfer fluid when the second temperature sensor experiences a temperature change due to movement of the heat transfer fluid in the supply line while the circulation pump is not active, caused by expansion of the heat transfer fluid contained in the solar collector as the solar collector heats up through solar irradiation.

Preferably, the controller is configured to start circulating the heat transfer fluid through the piping circuit when the temperature of the heat transfer fluid sensed by the second temperature sensor satisfies a second temperature criterion.

In one exemplary embodiment, the second temperature criterion comprises a difference between a temperature sensed at a moment in time of the first circulation cycle and a temperature sensed at a same corresponding moment in time of the second circulation cycle that is larger than a pre-set temperature difference.

In a second exemplary embodiment, the second temperature criterion comprises a difference between a temperature gradient sensed at a moment in time of the first circulation cycle and a temperature gradient sensed at a same corresponding moment in time of the second circulation cycle that is larger than a pre-set temperature gradient difference.

The temperature difference preferably is a decrease.

It has further advantages to provide a solar thermal system according to an embodiment wherein the second temperature sensor is positioned in the second supply line section in close proximity to the first supply line section.

In another embodiment of the invention, a thermowell fitting is arranged at the top of the downward sloping part of the supply line, wherein the thermowell fitting is configured for holding the supply temperature sensor. The use of a thermowell fitting allows the supply temperature sensor to be surrounded by fluid on all sides, and may offer decreased response time and increased sensitivity of supply temperature sensor when compared to mounting on the outside of the piping. The materials used for the piping and the thermowell fitting may influence the sensitivity of the ‘morning detection” feature, particularly using a material with a relatively low thermal conductivity, such as stainless steel (low thermal conductivity compared to copper), for the downward sloping part piping is advantageous for creating a large temperature difference between the fluid surrounding the supply temperature sensor and the fluid just below that position, which increases sensitivity of the ‘morning detection’ feature.

Additionally, using materials with high heat conductivity, such as copper, for the piping of the heat buffering section, in particular the piping arranged substantially horizontally and/or upwards sloping relative to the gravity direction, and the thermowell fitting helps the fluid surrounding the supply temperature sensor to heat up relatively quickly after a periodic circulation, which is also advantageous for the ‘morning detection’ feature.

For improved proper functioning, a solar thermal system according to another embodiment may be provided wherein the heat storage container comprises a heat exchanger as part of the piping circuit.

Other useful embodiments of the invention relate to solar thermal systems wherein the bypass line further comprises a, preferably spring mounted, check valve in order to prevent natural convection flow from taking place through the piping circuit when the circulation pump is off, and the bypass line connects to the return line through a 3-way valve, or, alternatively, wherein the bypass line comprises a 2-way valve, the return line comprises another 2-way valve and the bypass line connects to the return line downstream from the other 2-way valve. Yet other embodiments that have been proven useful relate to a solar thermal system further comprising a second bypass-valve arranged for directing the heat transfer fluid from the supply line via the bypass line to the return line in a bypass state and from the supply line via the heat storage {5 container to the return line in a storage container state, wherein the second bypass-valve is positioned at the junction of the first supply line section and the bypass line. Another embodiment provides a solar thermal system wherein the bypass line connects to an outlet in a middle part of the heat storage container, wherein a first bypass line section that connects with the middle outlet of the heat storage container, which causes this bypass line section to heat up through natural convection and/or through conduction by heat originating from the heat storage container when not circulating the heat transfer fluid, and wherein a second bypass line section connects with the first bypass line section and extends downwards in a vertical or downward sloped direction relative to the first bypass line section, wherein a third bypass line section connects with an end of the first bypass line section opposite the outlet, and the third bypass line section also extends downward from the end in a vertical or downward sloped direction, which causes the first bypass line section to buffer the heat originating from the heat storage container when not circulating the heat transfer fluid.

Conveniently, the solar thermal system according to yet another embodiment may further comprise an expansion vessel in the return line. The thermal bridge section provides a thermal conductance between the heat reservoir and the heat buffering section which may be at least 1.5 W/K, preferably at least 2 W/K. This supports a fast supply of the heat from the heat reservoir to the heat buffering section. In a particular embodiment, the thermal bridge section provides a thermal conductance between the heat reservoir and the heat buffering section which is at most 10 W/K, preferably at most 5 W/K. In a particular embodiment, the piping of the thermal bridge section and/or the heat buffering section is composed of piping material having a thermal conductivity of at least 75 W/m.K, preferably at least 100 W/m.K. This corresponds to a relatively high thermal conductivity which enhances the heat transfer along the respective sections. In a particular embodiment, said downward sloping part of the bypass line and/or the supply line is composed of a piping material having a (relatively low) thermal conductivity of at most 50 W/m.K, preferably at most 25 W/m.K. This enhances a blocking of heat flow along the downward sloping part. In particular it may support the purpose of accurately determining the morning solar state, as explained above. In a particular embodiment, each of said downward sloping parts is configured for restricting heat transfer along the downward sloping part, in case the heat transfer fluid is held substantially stationary inside the respective line, wherein preferably the downward sloping parts jointly have a thermal conductance along its longitudinal length which is at most 1.0 W/K, preferably at most 0.6 W/K. The thermal conductance along its longitudinal length is determined in a state wherein the heat transfer fluid is held substantially stationary inside the respective line.

In a particular embodiment, the thermal conductance of the thermal bridge section, expressed in WIK, is at least two times the thermal conductance of the downward sloping parts, preferably is at least three times the thermal conductance of the downward sloping parts, more preferably is at least four times the thermal conductance of the downward sloping parts. This enables a buffering of the heat inside the heat buffering section, while the buffered heat is swiftly depleted by the thermal bridge section to the heat buffering section in case needed. In other useful embodiments, an outside temperature sensor may be arranged for measuring an outside temperature To indicative of a temperature of the heat transfer fluid in outside parts of the heat transfer circuit and/or in the solar collector, wherein the outside temperature sensor is operatively connected to the control unit. In a particular embodiment, a thermal self-actuating valve arranged to switch between a reservoir state and a bypass state in response to a temperature difference of the thermal transfer fluid inside the heat reservoir Tr and inside the first supply line section Ty may be provided. The thermal self- actuating valve is arranged for directing a flow of the heat transfer fluid from the first supply line section to the bypass line in the bypass state in response to Tr > Tg and is arranged for directing a flow of the heat transfer fluid from the tee section to the heat reservoir in the reservoir state in response to Tr < Tr.

In this embodiment, the thermal self-actuating valve contains the bypass-valve and the actuator for actuating the bypass-valve. In an embodiment, thermal self-actuating valve may comprise a first container, arranged inside the heat reservoir near the thermal bridge section, for adapting to the temperature inside the heat reservoir and a second container, arranged inside the tee section at the valve element for adapting to the temperature inside the tee section. For any further embodiments IO of said self-actuating valve mechanism of said thermal self-actuating valve, which can be used in combination with the invention, we specifically refer to EP2573392, which is incorporated by reference.

DEFINITIONS As used herein, the term “solar system” means a system configured for converting solar energy into heat. The solar system may also be referred to as solar thermal system.

As used herein, the term “solar collector” means a collector configured for absorbing solar energy, transfer the solar energy into heat and to transfer the heat into the heat transfer fluid of the solar system. The solar collector may also be referred to as solar thermal collector.

As used herein, the term “heat reservoir” means a reservoir, such as a storage tank, for storing a heat transfer fluid. In normal operation, the heat reservoir stores the heat transfer fluid in a heated state. The heat reservoir in this invention is connected to the solar collector via the heat transfer circuit.

As used herein, the term “heat transfer circuit” means lines, such as pipings or tubes, to transport the heat transfer fluid between the solar collector and the heat reservoir.

As used herein, the term “supply line” means the line of the heat transfer circuit, which transports the, preferably heated, heat transfer fluid from the solar collector to the heat reservoir. The direction of transportation of the heat transfer fluid is determined by the circulation pump of the heat transfer circuit.

As used herein, the term “return line” means the line of the heat transfer fluid circuit, which transports the heat transfer fluid from the heat reservoir to the solar collector. The direction of transportation of the heat transfer fluid is determined by the circulation pump of the heat transfer circuit.

As used herein, the term “frost temperature” means a temperature at which the heat transfer fluid starts to freeze.

BRIEF DESCRIPTION OF THE FIGURES The accompanying drawings are used to illustrate presently preferred non-limiting exemplary embodiments of devices of the present invention. The above and other advantages of the features and objects of the invention will become more apparent and the invention will be better understood from the following detailed description when read in conjunction with the accompanying drawings, in which: Figure 1 illustrates schematically a solar thermal system of an exemplary embodiment in accordance with the invention; Figure 2 illustrates schematically a solar thermal system of another exemplary embodiment in accordance with the invention; Figure 3a illustrates schematically a piping arrangement of an exemplary embodiment in accordance with the invention; Figure 3b illustrates schematically a piping arrangement of another exemplary embodiment in accordance with the invention; Figure 4 illustrates schematically a solar thermal system of yet another exemplary embodiment in accordance with the invention; Figure 5 illustrates schematically a piping and component arrangement of an exemplary embodiment in accordance with the invention; and Figure 6 illustrates exemplary graphs of temperature-time profiles in accordance with embodiments of the invention

DESCRIPTION OF EMBODIMENTS Figure 1 depicts a solar thermal system 100, comprising a solar collector 102, a heat reservoir 104, a circulating pump 106, a supply pipe 101, a return pipe 105, a thermal bridge 103, a 3-way valve 108, a check valve 110, a tee-section 112, a first temperature sensor 114, a second temperature sensor 116 and an expansion vessel 118. The piping of the heat transfer circuit comprises a first down-sloping piping section 120 and a second down-sloping piping section 122 that slope down from the point of view of hot fluid flowing away from the tank due to natural convection flow within a pipe, so that pipe sections 120 and 122 act as blockages for natural convection flow. The IO part of the heat transfer circuit connected to the thermal bridge 103, that is heated by the heat storage tank 104 through the thermal bridge 103 when there is no forced convection due to the pump being on, is the piping section between the down-sloping pipe sections 120 and 122, and this pipe section is referred to as heat buffering section 124. The solar system 100 also comprises an electronic controller 109, that obtains temperature data from the first and second temperature sensors 114, 116, and operated the circulation pump 106 and 3-way-valve 108.

The heat reservoir 104 may comprise a heat exchanger 126, in case the fluid in the heat reservoir is not suitable for circulating through the solar collector, for example if heat reservoir 104 contains potable water, unsuitable for circulating through a solar collector. In many cases heat storage 104 need not comprise a heat exchanger, for example when the fluid in the heat reservoir is process water, for example water used in a space heating circuit, or when the fluid is highly pure potable water, that is suitable for circulating through a solar collector. Whether or not heat reservoir 104 comprises a heat exchanger 126 or not does not alter the functionality of the system described below.

The thermal bridge 103 is designed to transfer heat from the heat reservoir 104 to the heat buffering section 124, through heat conduction or natural convection effects, or a combination of both.

Solar thermal system 100 may not use a temperature sensor in, or in close proximity to, solar collector 102, and may not have any temperature sensor to measure the outdoor ambient temperature, In order to measure the temperatures in different parts of the heat transfer circuit such as the solar collector 102 and the piping, particularly the outdoor sections of the piping, the controller of the solar thermal system can start the circulation pump 106 periodically to pump the circulate the fluid, so the controller can register the temperature of the fluid that passes the temperature sensors 114, 116. During this periodic circulation the 3-way valve is in the bypass position, which means the circulating fluid does not pass through the heat reservoir 104, but through the bypass piping section, which is the piping section comprising down-sloping piping section 122 and check valve 110. The periodic circulation time should be sufficient for the solar circuit in bypass mode to be circulated at least once around, the circulation time can be pre-set in the controller, or the controller may contain an algorithm for determining the ideal circulation time for the periodic circulation. At the end of the periodic circulation the controller uses an algorithm to decide either to switch off the pump, or to keep pumping to collect heat from the solar collector 102, and to charge the heat reservoir 104 once the temperature in the circuit exceeds the temperature in the heat reservoir, in which case 3-way valve 108 is switched to direct the IO circulation via heat reservoir 104. When the controller decides to switch off the circulation pump, an algorithm also determines the waiting period until the next periodic circulation. The decision whether to keep pumping, or to switch off the pump for a certain period of time, is based on the temperature observed during the periodic circulation, for example the maximum temperature observed, the minimum temperature observed and the average temperature observed. The controller can also take into account the change in maximum, minimum or average temperature observed, relative to the previous periodic circulation, to determine the waiting period until the next periodic circulation. The waiting period can range from as high as 2 to 4 hours, down towards 0 seconds (towards continuous circulation).

During each periodic circulation, the heat buffering section 124 is typically cooled down, since the circulation pump 106 circulates cold fluid from the piping and the solar collector 102 through this section that has been heated up by the heat reservoir 104 through natural convection and conduction. The heat taken from the heat buffering section heats up the heat transfer circuit, and this heat can be used to keep the heat transfer circuit above a minimum temperature to protect the heat transfer circuit from frost damage. For this purpose, the algorithm in the controller that determines the waiting period between circulation periods can start reducing the waiting period or extending the circulation period, based on minimum temperature observed during the periodic circulation and the change in average temperature and in minimum temperature relative to the previous periodic circulation, to keep the minimum temperature in the heat transfer circuit above a critical value. The waiting period can go down towards continuous flow when the conditions require, and if even with continuous flow the minimum temperature observed falls below a critical value, the controller can switch 3-way valve 108 to circulate via heat reservoir 104, which will switch the system to a conventional frost protection, drawing heat directly form the heat reservoir

104.

The diameter, length and slope of the heat buffering section 124, and the properties of the thermal bridge 103, should be predetermined based on the expected heat losses from the outdoor part of the solar circuit in frost conditions. These dimensions should be chosen so that in normally expected frost conditions the thermal bridge 103 can transfer sufficient heat through the natural convection process for frost protection. These dimensions should also take into account that not too much heat is transferred during periodic circulation, since this can lead to unnecessary heat loss during periodic circulation when there is no risk of frost damage, for this reason the heat buffering section 124 should be very small. Preferably these piping sections and tee-section 112 are prefabricated for a certain type of system in certain climatic conditions, to avoid errors during installation on-site.

In the solar thermal system 100 the first temperature sensor 114 is positioned in the first down- sloping piping section 120, in close proximity to the heat buffering section 124. The first and second down-sloping piping sections 120, 122 are typically heated up by conduction from the top down by the heat reservoir piping section 124 during the waiting period between periodic circulation, when the circulation pump 106 is off, and the 3-way valve 108 in the bypass position. This will cause the temperature of first temperature sensor 114 to increase over time in a predictable manner from the moment the circulation pump 106 switches off, with the rate of temperature increase typically being high shortly after circulation pump 106 switches off, and decreasing steadily as the temperature increases (illustrated in exemplary graphs, with reference to Figure 6). The rate of change of the temperature of first temperature sensor 114, while circulation pump 106 is off, is monitored by the controller 109, either continuously or at discreet points in time, and compared either to a pre-set pattern or to values observed during previous periods when the circulation pump 106 was off, for example the preceding period when the circulation pump 106 was off.. While the circulation pump 106 is off and the temperature of the first temperature sensor 114 increases, a temperature gradient develops in the first downward sloping piping 120, and if during the waiting period any flow, even a very small amount of flow, occurs through first downward sloping piping 120, the resulting movement of the liquid in downward sloping piping 120 will influence the rate by which the supply temperature changes over time, due to the temperature gradient present in downward sloping piping 120. This will cause the rate of change of the temperature of first temperature sensor 114 to deviate from the preset pattern, or from the rate of change during a previous period when the circulation pump 106 was off, which will be observed by the controller. If the deviation is more than a pre-set value, the controller can activate the circulation pump 106. For example, the controller 109 may determine the temperature 1 minute and 40 minutes after the pump was switched off, and determine the temperature difference, and this temperature difference can be compared to a previous period when the pump was off. If the temperature difference is more than 1 K lower than in the previous period, the circulation pump

106 can be started Alternatively, the controller 109 may monitor a rate of decrease of the first temperature sensor 114, if the rate of decrease is more than for example 2 K/hr the circulation pump 106 is activated.

The controller may use multiple such criteria that indicate deviation from the expected pattern.

Flow through down-sloping pipe sections 120, 122 can occur when solar collector 102 heat up, and the fluid contents of solar collector 102 expand.

The expanding fluid in solar collector will cause fluid to flow towards expansion vessel 118, for which the flow has to pass through first and second down-sloping pipe sections 120, 122. The flow through first down- sloping pipe section 120 caused by the expansion in solar collector 102 can be detected as described above.

Alternatively, first temperature sensor 114 may be installed in the second

IO downward sloping pipe section 122, any flow through the downward pipe section will in this case cause an increase in the observed temperature above an expected value.

In the case that first temperature is installed in the first downward sloping pipe section, it can also function as the supply temperature sensor of the solar system during normal operation.

The controller 109 can respond by starting circulation pump 106, so that the temperature in the heat transfer circuit can be checked.

This is particularly useful when the system is in a long waiting period between periodic circulations, so that it can detect the sun coming up, or out, during the waiting period, creating a ‘morning detection’ feature.

Over longer periods of time, for example exceeding one hour, the measured supply temperature can stop its increase over time and reach a plateau, where the rate of temperature change is 0, and may even decrease slowly, for example due to heat reservoir 104 cooling down due to heat loss to the ambient.

These effects can be accounted for in the pre-set pattern that the controller 109 has for the rate of change of the supply temperature during periods when circulation pump 106 is off.

In addition, the controller may also detect freezing of the circulation fluid in the collector or outdoor piping, since the decrease in density when water freezes (when circulation pump 106 is off) also generates expansion and flow through downward sloping pipe sections 120,122, similar to when the fluid heats up.

The use of a second supply temperature sensor positioned in down-sloping pipe sections 120, 122, positioned below the first temperature sensor 114, can increase the sensitivity of the detection of small amounts of flow, by allowing the temperature development at both positions to be compared to behaviour in the absence of any flow.

The material of down-sloping pipe section may be chosen to have relatively low thermal conductivity to increase the sensitivity of the flow detection, for example stainless steel instead of copper, particularly in the section below first temperature sensor 114 when it is installed in the first down sloping pipe section 120, or the section above first temperature sensor 114 when it is installed in the second down sloping pipe section 122. Using stainless steel as the main piping material of the first down-sloping piping section 120 also reduces heat loss from the heat reservoir pipe section 124 to the rest of the heat transfer circuit.

It will be understood by those familiar with the state of the art that the current hydraulic arrangement can be achieved also by using two 2-way valves instead of 3-way valve 108, and that also a valve with more than three ports may be used. 3-way valve 108 may comprise an electrically operated actuator based on thermal element, that is electrically heated to actuate in one direction,

and returns to a default position when the electrical heating is interrupted.

The check valve 110 is a spring mounted check valve, that prevents natural convection flow from taking place through the heat transfer circuit when circulation pump 106 is off.

When two 2-way valves are used instead of 3-way valve 108, check valve 110 will not be needed, since both 2-way valves can be closed simultaneously, blocking any natural convection flow when the circulation pump 106 is off.

For frost protection it is beneficial if the default position (the position when there is no power supplied) of 3-way valve 108 directs the flow via the heat reservoir 104, not via check valve 110, so that in case of a power failure, when the valve will automatically move to its default position, the heat transfer circuit is open via the heat reservoir 104, and check valve 110 is not part of the heat transfer circuit.

In the case that two 2-way valves are used, it is beneficial that the default position is the open position (normally open actuator). This will allow a thermosiphon flow to start up, driven by hot fluid in the heat reservoir 104 that wants to rise, due to the hotter fluid having a tower density than cool fluid, towards solar collector 102, which is typically positioned higher than heat reservoir 104. This thermosiphon can thereby act as a back-up frost protection in case of power failure.

Figure 2 depicts a solar thermal system 200, comprising a solar collector 202, a heat reservoir 204, a circulating pump 206, a supply pipe 201, a return pipe 205, a 3-way valve 208, a check valve 210, a tee-section 212, a first temperature sensor 214, a second temperature sensor 216 and an expansion vessel 218. The piping of the heat transfer circuit comprises a thermal bridge pipe section 203, a first down-sloping piping section 220 and a second down-sloping piping section 222 that slope down from the point of view of hot fluid flowing away from the tank due to natural convection flow within a pipe, so that pipe sections 220 and 222 act as blockages for natural convection flow.

The part of the heat transfer circuit connected to the thermal bridge pipe section

203, that is heated by the heat storage tank 204 through the thermal bridge pipe section 203 when there is no forced convection due to the pump being on, is the piping section between the down- sloping pipe sections 220 and 222, and this pipe section is referred to as heat buffering section 224. The solar system 200 also comprises an electronic controller 209, that obtains temperature data from the first and second temperature sensors 214, 216, and operated the circulation pump 206 and

3-way-valve 208.

The heat reservoir 204 may comprise a heat exchanger 226, in case the fluid in the heat reservoir is not suitable for circulating through the solar collector, for example if heat reservoir 204 contains potable water, unsuitable for circulating through a solar collector. In many cases heat storage 204 need not comprise a heat exchanger, for example when the fluid in the heat reservoir is process water, for example water used in a space heating circuit, or when the fluid is highly pure potable water, that is suitable for circulating through a solar collector. Whether or not heat reservoir 204 comprises a heat exchanger 226 or not does not alter the functionality of the system described below.

The thermal bridge pipe section 203 is designed to transfer heat from the heat reservoir 104 to the heat buffering section 124 while the circulation pump 206 is not active, through natural convection effects. While the circulation pump 206 is active the 3-way valve 208 may be in bypass position, in which case some heat is also transferred from the heat reservoir 104 to the heat buffering section 124, which is then immediately removed from the heat buffering section 124 due to the pumped circulation. While the circulation pump 206 is active the 3-way valve may direct the flow through the heat exchanger 226, in which case the thermal bridge pipe section 203 acts as part of the supply pipe.

Figure 3a depicts a piping arrangement 300, comprising a heat reservoir 302, a first pipe section 306, a second pipe section 308, and a third pipe section 310, a fourth pipe section 312 and a pipe bend 314. The first pipe section 306 provides the thermal bridge section. The heat buffering section contains the tee-section 304, the second pipe section 308, the third pipe section 310, and the fourth pipe section 312. All the components are depicted without an insulation shell, for clarity, whereas in reality all components are insulated. Figure 3a is a side-on cross-section detail view of the piping arrangement, showing detail of the piping arrangement in a solar thermal system such as solar thermal system 200, with reference to Figure 2, with the down direction the same as the direction of the force of gravity. The first pipe section 306, being the thermal bridge section, connects the heat reservoir 302 to tee-section 304 of the heat buffering section, and comprises a pipe section fixed to the heat reservoir 302, that protrudes its insulation shell, a coupling, and a pipe section from the coupling to tee-section 304. The second pipe section 308 is a horizontal pipe section connected to tee-section 304, and represents a part of the heat buffering section 224 with reference to Figure 2. The third pipe section 310 is a pipe section connected to the second pipe section 308, with pipe bend 314 between third pipe section 310 and second pipe section 308, so that third pipe section 310 runs downward from second pipe section 308, and may represent the first down-sloping pipe section 220, with reference to Figure 2. The fourth pipe section 312 is connected to tee-section 304, and runs downwards from tee-section 304, and may represent the second down-sloping pipe section 222, with reference to Figure 2. Figure 3a presents a detail view of piping sections similar to those presented in Figure 2, but in the absence of a temperature sensor, such as first temperature sensor 214, with reference to Figure 2.

In the case of using a piping arrangement 300 in system 200, with reference to Figure 2, the piping arrangement 300 is only used for frost protection, with reference to Figure 2. Figure 3b depicts a piping arrangement 350, comprising a heat reservoir 302, a tee-section 304, a first pipe section 306, a second pipe section 308, a third pipe section 310, a fourth pipe section 312, a thermowell fitting 352 and a temperature sensor 354. The first pipe section 306 provides the thermal bridge section. The heat buffering section contains the tee-section 304, the second pipe section 308, the third pipe section 310, the fourth pipe section 312,and includes the thermowell fitting 352. Figure 3b is a side-on cross-section detail view of the piping arrangement, showing detail of the piping arrangement in a solar thermal system such as solar thermal system 200, with reference to Figure 2, with the down direction the same as the direction of the force of gravity. The first pipe section 306 connects the heat reservoir 302 to tee-section 304. The second pipe section 308 is a horizontal pipe section connected to tee-section 304, and represents a part of the heat buffering section 224 with reference to Figure 2. The second pipe section 308 is connected to thermowell fitting 352, and thermowell fitting 352 is connected to the third piping section 310, which runs a downward direction from thermowell fitting 352, so that third piping section 310 may represent the first down-sloping pipe section 220, with reference to Figure 2. The fourth pipe section 312 is connected to tee-section 304, and runs downwards from tee-section 304, and may represent the second down-sloping pipe section 222, with reference to Figure 2. The temperature sensor 354 is positioned inside the thermowell of thermowell fitting 352, at the bottom of the thermowell, so that temperature sensor 354 is positioned below the second piping section 308, and near the top of the third pipe section 310, and this temperature sensor 354 may represent the first temperature sensor 214, with reference to Figure 2.

The use of a thermowell fitting 352 allows the temperature sensor 354 to be surrounded by fluid on all sides, and may offer decreased response time and increased sensitivity of temperature sensor 354 when compared to mounting on the outside of the piping. The materials used for the piping and the thermowell fitting can influence the sensitivity of the ‘morning detection’ feature, particularly using a material with a relatively low thermal conductivity, such as stainless steel (low thermal conductivity compared to copper), for the third pipe section 310 is advantageous for creating a large temperature difference between the fluid surrounding temperature sensor 354 and the fluid just below that position, which increases sensitivity of the ‘morning detection’ feature.

Using materials with high heat conductivity, such as copper, for the second pipe section 308 and the thermowell fitting 352 helps the fluid surrounding temperature sensor 354 heat up relatively quickly after a periodic circulation, which is also advantageous for the ‘morning detection’ feature.

Figure 4 depicts a solar thermal system 400 comprising a heat reservoir 402, a solar collector 404, a circulation pump 406, a supply pipe 401, a return pipe 405, a 3-way valve 408, a first 2-way valve 410, a first tee-section 412, a second 2-way valve 414, a second tee-section 415, a first temperature sensor 416, a second temperature sensor 418 and an expansion vessel 420. The 3-way IO valve 408 is connected to the heat reservoir 402 through the first thermal bridge section 403, running horizontally, or upwards, from thermal reservoir 402 towards 3-way valve 408, so that 3- way 408 is heated up through natural convection heat by the heat reservoir 402, when circulation pump 406 is off.

The piping of the heat transfer circuit comprises a first down-sloping piping section 422 and a second down-sloping piping section 424 that slope down from the point of view of hot fluid flowing away from the 3-way valve 408, so that pipe sections 422 and 424 act as blockages for natural convection flow. The part of the heat transfer circuit at the top connection to the reservoir that is heated by natural convection flow effects, when there is no forced convection due to the circulation pump 406 being active, are the piping sections between the down-sloping pipings 422 and 424 and the heat reservoir 402, including 3-way valve 408. The section of the heat transfer circuit between 3-way valve 408 and first down-sloping piping section 422, and including 3-way valve 408, is referred to as a first heat buffering section 426. The piping of the heat transfer circuit further comprises a third down-sloping piping section 428 and a fourth down-sloping piping section 430 at a lower connection to heat reservoir 402, creating a second heat buffering section 432 including first tee-section 412, which is heated with heat from the thermal reservoir 402, transferred through second thermal bridge section 411.

The first heat buffering section 426 enables the ‘morning detection’ feature of the controller, by monitoring the temperature development of first temperature sensor 316, in the same way as heat buffering section 224 enables the ‘morning detection’ feature in combination with first temperature sensor 214, with reference to Figure 2. The first heat buffering section 426 also provides heat for frost protection, in the same way as heat buffering section 224 does, with reference to Figure 2. The second heat buffering section 432 provides additional heat for frost protection, so that less heat for frost protection needs to be taken from the top of the heat reservoir 402 for frost protection.

Solar system 400 has one 3-way valve 408 and two 2-way valves 410, 4141, connected to second tee-section 415, which allow the fluid to pass through the top part of heat reservoir 402 or the bottom part of heat reservoir 402, or to bypass heat reservoir 402. This allows the solar system 400 to heat up the top part of heat reservoir 402 quickly when necessary, or to heat the whole tank from the bottom section. This feature can be particularly useful when reservoir 402 is used to provide heat for multiple applications, for example domestic hot water preparation and space heating, which may require different temperature levels, It also allows the solar system 400 to reserve a quantity of cool fluid in the bottom of the tank for the end of the afternoon, when solar intensity decreases and the solar collector can no Jonger efficiently generate heat at high temperature levels.

Figure 5 depicts a piping and component arrangement 500 in accordance with the solar system 400, with reference to Figure 4. The piping and component arrangement 500 comprises a heat reservoir 502 and a 3-way valve 504, which corresponds to the 3-way valve 408 in Figure 4, which is a self- actuating valve, described in EP2573392, that switches positions based on temperature difference between the temperature inside 3-way valve 504 and heat reservoir 502. The actuator of 3-way valve 504 protrudes into heat reservoir 502, so that it is exposed to both the temperature inside 3- way valve 504 and inside the heat reservoir 502, and can actuate based on the temperature difference, using the actuation principle described in EP2573392. 3-way valve 408 may also be a different type of valve, for example an electrically actuated valve, or a thermostatic self-actuating 3-way valve that, that switches at a pre-set temperature level. The piping and component arrangement further comprises a first down-sloping piping section 506 corresponding to first down-sloping piping 422 with reference to Figure 4, a second down-sloping piping section 508 corresponding to second down-sloping piping 424 with reference to Figure 4, and a first heat buffering section 510 corresponding to first heat buffering section 526 with reference to Figure 4. The thermal bridge 503 corresponds to the thermal bridge 403 with reference to Figure 4. The heat transfer of the thermal bridge 503 can consist of convective heat transfer, due to natural convection of a liquid, or conductive heat transfer, or a combination of both. The thermal bridge 503 can be open for liquid flow, comprising for example a pipe section, but may also be closed for liquid flow, for example when the thermal bridge 503 comprises components of a valve that closes the fluid channel through the thermal bridge 503 in situations when heat transfer through the thermal bridge section is still required. In this case at least in part of the thermal bridge section the only heat transfer is conductive, where in other parts of the thermal bridge section convective heat transfer may still take place. The thermal bridge 503 may also be permanently closed for flow,

serving only to transfer heat to the heat buffering section, without being used for forced convective heat transfer when the circulation pump is active.

Figure 6a depicts a graph of how the temperature of the temperature sensor used for ‘morning detection’ may develop over time, which can be first temperature sensor 214 with reference to Figure 2, temperature sensor 354 with reference to Figure 3 or first temperature sensor 416 with reference to Figure 4. The measured temperature of the sensor used for morning detection is named T1, The temperature of the thermal reservoir, for example thermal reservoir 204 with reference to Figure 2, at the position of the thermal bridge, for example thermal bridge 203 with reference to IO Figure 2, is named T(top-tank) in Figure 6a.

Timer periods when the circulation pump, for example circulation pump 206 with reference to Figure 2, is switched on are indicated by a grey background colour.

Figure 6a illustrates a typical expected behaviour during night-time, with T1 increasing when the circulation pump is off, as heat is transferred from the thermal reservoir via the thermal bridge to the thermal buffer, below which the temperature sensor is positioned.

The increase of T1 is predictable, and also repetitive from one rest period to the next.

Figure 6b illustrates a first method of ‘morning detection’, where the controller detects a change in the temperature change over time (dT {/dt). If the change in dT1/dt is above or below a pre-set threshold value, the controller will start the circulation pump.

Figure 6c illustrates a second method of ‘morning detection’, where the controller detects the temperature T1 at a pre-set point in time during each cycle t41=X, and compares this temperature T1(teyae=X)a to the temperature during the previous cycle Tl(t31.=X)n.1. If the difference between these temperatures T1({ ye. =X)n1 - TH{toye=X), is below a pre-set level, the controller will start the circulation pump.

Whilst the principles of the invention have been set out above in connection with specific embodiments, it is to be understood that this description is merely made by way of example and not as a limitation of the scope of protection which is determined by the appended claims.

Claims

CONCLUSIONS

A solar thermal system (100, 400) for converting solar energy into heat, the system comprising a) a solar collector (102, 404) for collecting solar energy; b) a heat storage vessel (104, 402) for storing a heat transfer fluid; c) a pipe circuit, comprising - a supply pipe (101, 120, 124, 121) connecting the solar collector {102, 402) to an inlet (111, 412) of the heat storage vessel (104, 402), - a return pipe (105, 405) connecting an outlet (107, 407) of the heat storage vessel {104, 402) to the solar collector (102, 402), - a bypass line (122, 430) provided between the supply line (121) and the return line (103, 405) ), - a circulating pump (106, 406) arranged to pass the heat transfer fluid through the conduit circuit under the control of a controller (109, 409), and - a bypass valve (108, 408) arranged to circulate the heat transfer fluid from the supply line (101, 120, 124, 121) through the bypass line (122, 430) to the return line (105, 405) in a bypass condition and from the supply line (101, 120, 124, 121) through the heat storage vessel ( 104, 402) to the return line (105, 405) in a storage vessel state, under the control of the controller (109, 409), wherein the duct circuit comprises a first supply duct section (124) connected to the heat storage vessel (104) via a thermal bridge (103) whereby said first supply duct section (124) is heated by natural convection and/or by conduction by heat from of the heat storage vessel (104) when the heat transfer fluid is not being circulated, said first supply line section (124) being bounded by a second and third line section (120, 121) both connected to the first supply line section (124) and extending downwardly in a vertically or downwardly inclined direction with respect to the first supply line section (124), whereby the first supply line section (124) buffers the heat from the heat storage vessel (104) when the heat transfer fluid is not circulated.

The solar system of claim 1, wherein the second and third conduit sections (120, 121) form part of the supply conduit.

The solar system of claim 1, wherein the second pipe section (220) is part of the supply pipe and the third pipe section (222) is part of the bypass pipe.

The solar thermal system (100) of any preceding claim, wherein the first supply line section (124) extends in a horizontal or slanting direction.

A solar system according to any one of the preceding claims, wherein the inlet 203 comprises the thermal bridge 103.

A solar thermal system (100) according to any one of the preceding claims, wherein the controller (109) is arranged to intermittently circulate the heat transfer fluid through the conduit circuit in the bypass condition whereby the heat buffered in the first supply conduit section (124) is circulated through the conduit circuit. .

A solar thermal system (100) according to any one of the preceding claims, wherein the heat supplied from the heat storage vessel (104) to the piping circuit in the bypass condition is used for frost protection.

A solar thermal system (100) according to any preceding claim, wherein the controller is arranged to circulate the heat transfer fluid through the conduit circuit in the bypass condition during intermittent first and second circulation cycles with a stationary period in between.

The solar thermal system (100) of any preceding claim, wherein the conduit circuit includes a first temperature sensor and the controller is arranged to circulate the heat transfer fluid through the conduit circuit in the bypass state when the temperature of the heat transfer fluid measured by the first temperature sensor is satisfactory. to a first temperature criterion.

The solar thermal system (100) of claim 9, wherein the first temperature criterion comprises a minimum temperature measured during the first or second circulation cycle or during the stationary period, which is lower than a predetermined freeze protection temperature.

The solar thermal system (100) of claim 9, wherein the controller is arranged to circulate the heat transfer fluid through the conduit circuit in the bypass state until the temperature of the heat transfer fluid measured by the first temperature sensor no longer meets the first temperature criterion with a deviation of the minimum temperature by a predetermined amount, such as 10% of said temperature.

The solar thermal system (100) of any one of claims 9-11, wherein the controller is configured to circulate the heat transfer fluid through the conduit circuit at least during one circulation cycle in the bypass state when the temperature of the heat transfer fluid measured by the first temperature sensor is at meets the first temperature criterion.

The solar thermal system (100) of any preceding claim, wherein the second supply line section (120) comprises a second temperature sensor (114) adapted to measure the temperature of the buffered heat.

The solar thermal system (100) of claim 13, wherein the second temperature sensor (114) is the first temperature sensor.

The solar thermal system (100) of claim 13 or 14, wherein the controller (109) is arranged to initiate circulation of the heat transfer fluid when the second temperature sensor (114) experiences a temperature change due to movement of the heat transfer fluid in the supply line (101, 10). 120, 124) while the circulation pump (106) is not active, caused by expansion of the heat transfer fluid contained in the solar collector (102) as the solar collector heats up by solar radiation.

The solar thermal system (100) of any of claims 13-15, wherein the controller is configured to initiate circulation of the heat transfer fluid through the conduit circuit when the temperature of the heat transfer fluid measured by the second temperature sensor (114) meets a second temperature criterion. satisfies.

The solar thermal system (100) of claim 16, wherein the second temperature criterion comprises a difference between a temperature measured at a time of the first circulation cycle and a temperature measured at a corresponding time of the second circulation cycle that is greater than a predetermined temperature difference.

The solar thermal system (100) of claim 16, wherein the second temperature criterion comprises a difference between a temperature gradient measured at a time instant of the first circulation cycle and a temperature gradient measured at a time instant of the second circulation cycle that is greater than a predetermined temperature gradient difference.

The solar thermal system (100) of claim 17, wherein the temperature difference is a decrease.

The solar thermal system (100) of any preceding claim, wherein the heat storage vessel (104) includes a heat exchanger (126) as part of the piping circuit.

The solar thermal system (100) of any of claims 13-20, wherein the second temperature sensor (114) in the second supply line section (120) is positioned in close proximity to the first supply line section (124).

A solar thermal system (100) according to any preceding claim, wherein the bypass conduit (122) further comprises a preferably spring-mounted check valve (110) to prevent natural convection flow through the conduit circuit when the circulation pump (106) is operated. is off, and the bypass line (122) is connected to the return line (105) by a 3-way valve (108).

The solar thermal system (100) of any preceding claim, wherein the bypass line (421) includes a 2-way valve (414), the return line (405) includes another 2-way valve (410), and the bypass line (421) is connected with the return line (405) downstream of the other 2-way valve (410).

A solar thermal system (400) as claimed in any preceding claim, further comprising a second bypass valve (408) configured to transfer heat transfer fluid from the supply line (401, 422, 426) through the bypass line (424) to the return line (405). ) in a bypass condition and from the supply line (401, 422, 426) through the heat storage vessel (402) to the return line (405) in a storage vessel condition, wherein the second bypass valve (408) is positioned at the intersection of the first supply line section (426) and the bypass line (424).

A solar thermal system (400) according to any one of the preceding claims, wherein the bypass conduit (421, 424) is connected to a jacket (411) in a center portion of the heat storage vessel (402), wherein a first bypass conduit section (432) is connected to the center outlet of the heat storage vessel (402) through which this bypass line section (432) heats by natural convection and/or through conduction of heat from the heat storage vessel (402) when the heat transfer fluid is not circulated, and wherein a second bypass line section (430) is connected to the first bypass conduit section (432) and extending downward in a vertical or sloping direction relative to the first bypass conduit section (432), wherein a third bypass conduit section (428) is connected to an end of the first bypass conduit section (432) opposite the outlet, and the third bypass line section (428) also extends downward from the end in a vertical or slant one direction, whereby the first bypass conduit section (442) buffers the heat from the heat storage vessel (402) when the heat transfer fluid is not being circulated.

The solar thermal system (100, 400) of any preceding claim, further comprising an expansion vessel (118, 420) in the return line (105, 405).