US20110079216A1

US20110079216A1 - Hermetic primary circuit for thermal solar system

Info

Publication number: US20110079216A1
Application number: US12/889,006
Authority: US
Inventors: Gilbert Descy
Original assignee: European Solar Engr SA
Current assignee: European Solar Engineering Sa; European Solar Engr SA
Priority date: 2009-10-01
Filing date: 2010-09-23
Publication date: 2011-04-07
Also published as: ATE536518T1; EP2306116A1; EP2306116B1; CA2715208A1

Abstract

The present invention relates to a thermal solar installation with draining by gravity, wherein:

- the return circuit of the primary circuit (2) is subdivided into two conduits (25;26,27), a first conduit (26,27) that brings the heat-transfer liquid back to a low point of the buffer tank (31) when the pump (19) is operating and a second conduit (25) opening at a high point of said tank (31), and provided with an anti-return valve (20), which allows air to be returned to the collector (1) and hence the primary circuit (2) to be drained by gravity when the pump (19) no longer operates;
- the first conduit (26,27) is separated into a conduit (27) opening into the buffer tank (31) and a bypass conduit (28) opening into said supply circuit (30) downstream from the pump (19), and provided with a normally open valve (21) that closes as soon as the pump (19) operates;
- the buffer tank (31) is connected to the exchanger (4) through a connection conduit to the exchanger (14), the return conduit (29) connecting the exchanger (4) to the pump (19) and comprising an anti-return valve (22), being located upstream from the connection of the supply circuit (30) to the bypass circuit (28).

Description

FIELD OF THE INVENTION

The present invention relates to a system for producing thermal solar energy having a hermetically sealed primary circuit with drainage by gravity (drain-back) and with low flow rate (low flow) in order to achieve a significant temperature difference in the circuit.

TECHNOLOGICAL BACKGROUND AND STATE OF THE ART

The principles of the solar boiler (SB) and of the combined solar system (CSS) for producing hot water and heating (see FIG. 1) are known. Solar energy is transformed into heat by solar heat absorbers or collectors 1. They comprise an absorbing surface that captures direct and diffuse solar radiation and transmits this thermal energy to a heat-transfer fluid circulating in a circuit of tubes 2, 8 with a view to using the heat 3 (production of sanitary hot water, under-floor heating, etc.), through one or several heat exchangers 4. The heat-transfer fluid, for example water or anti-freeze, circulates in the primary circuit 2, isolated by at least one exchanger 4 of the secondary circuit 8 where the transported calories are used. The exchanger 4 may also be connected to a fossil-fuel boiler 6. The heat-transfer fluid may naturally circulate in the primary circuit (thermo siphon) or more generally in a forced way by means of a pump 5 (circulator). In this case, the intermittent operation of the pump 5 is driven by a control 7 operating for example on the basis of measurements provided by three temperature probes (Tc, Tin, Tb: respective temperatures of the solar collectors 1, of the inlet and outlet of the exchanger 4).
In order to compensate the expansion of the heat-transfer fluid and to ensure safety of the equipment, the primary circuit usually comprises an expansion tank provided with a safety valve, which is imperative if the circuit is closed. One refers to drainage by gravity (drain-back) when the solar collectors, generally located at a greater height than that of the primary circuit, are drained in the event that the pump stops. This may for example be achieved by fitting the pump with a normally open electromagnetic valve that is in bypass when the pump is at a standstill and closed when the pump is operating.
The design of this primary transfer circuit 2 is very important for optimizing the yield of the solar system, reducing the assembly costs and ensuring easy maintenance. Thus, it is important to protect the heat-transfer fluid in order to avoid both overheating and deterioration of its chemical composition. Indeed, the systems of the state of the art may have overpressures, are not hermetically sealed (presence of purging systems) or further have to be refilled.
Document U.S. Pat. No. 4,237,862 discloses a pressurized and closed solar heating system including a primary circuit communicating with a heat exchanger and containing a buffer tank filled with water to a determined level, before operation. When a certain temperature difference is reached between the solar collectors and the heat-exchanger tank, a controller triggers the pump. The air contained in the circuit then accumulates in the buffer tank, which lowers the water level in it. The latter is in communication with the return circuit of the collectors through a vertical drainage conduit ending at a low point of the tank and a bypass conduit ending at a high point of the tank. The latter conduit is mounted as a siphon device (or vacuum-breaking section), designed so that no siphoning occurs during the forced circulation of the heat-transfer fluid. When the pump stops, siphoning occurs and the air flows upwards towards the collectors through the aforementioned bypass conduit. This drains the collectors and the return conduit towards the tank, as well as the supply conduit, through the volutes of the pump, in reverse flow.
Document U.S. Pat. No. 4,336,792 discloses a closed solar-energy heating system including a primary circuit communicating with a heat exchanger and containing a storage tank allowing to drain the water found in the portions of the circuit exposed to frost (in particular the collectors) and to replace it with air, when there is no circulation of water. When water circulates, a bypass inflow tube of the storage tank allows the free flow of the total volume of water, with the extraction of the air and its carrying away into the upper portion of the storage tank. When the circulation stops, the pressure difference between the cold water column and the hot water column drains the water in the opposite direction by siphoning, with an up-flow of water into the tank and of air into the upper portion of the installation, and in particular into the collectors.
Document U.S. Pat. No. 4,027,821 discloses a solar-energy heating system in a closed circuit including a primary circuit communicating with a heat exchanger also acting as a storage tank and extended in its upper portion with an expansion chamber. When water circulates in the circuit, air builds up in the expansion chamber. The latter is connected to the outlet of the solar collectors by a ventilation conduit, the opening of which is achieved by controlled valves that open in the absence of fluid or in the case of a pressure reduction in the primary circuit, therefore when the pump no longer operates. In this case, the accumulated air returns towards the collectors and drains the installation by gravity, both in the supply line towards the collectors and in the outlet line of the collectors.
It can be seen that the different systems of the state of the art intended to ensure drainage by gravity for a solar-heating installation comprise for example passive siphon-circuits or ventilation circuits with controlled valves. The problems that may result from this are either a lack of reliability or the complexity of the installation, or a wear of the components such as the valves or excessive consumption of electric energy. Further, these systems often have a supply of outside air and are not necessarily hermetically sealed (e.g. vacuum breaker). There are therefore risks of corrosion.

AIMS OF THE INVENTION

The present invention aims to overcome the drawbacks of the state of the art.
In particular, the invention aims to ensure passive safety of the thermal solar installation by suppressing the overheating problems of the heat-transfer fluid and its deterioration.
The invention also aims to ensure complete emptying of the primary circuit by means of drainage by gravity in a hermetic circuit, without leaving subsisting stagnation areas.
The invention further aims to optimize the yield of the energy transfer and the cost of the system.

MAIN CHARACTERISTIC ELEMENTS OF THE INVENTION

A first aim of the present invention relates to a solar thermal installation with drainage by gravity, comprising at least:

- one absorbing solar collector that is raised relative to the remainder of the installation and in which a heat-transfer fluid circulates,
- one hermetic primary circuit, comprising a supply circuit and a return circuit towards and from the collector respectively, the primary circuit being provided with a pump as well as with a thermally insulated buffer tank and initially filled with heat-transfer liquid to an imposed level, having the function of a liquid/air separator,
- at least one heat exchanger and
- one secondary circuit transporting the recovered energy towards a location of use, wherein:
- the return circuit of the primary circuit is subdivided into two conduits, a first conduit that brings back the heat-transfer liquid to a low point of the buffer tank when the pump is operating and a second conduit opening at a high point of said tank, and provided with an anti-return valve, which allows air to be returned to the collector and hence the primary circuit to be drained by gravity, when the pump is no longer operating;
- the first conduit is separated into a conduit opening into the buffer tank and a bypass conduit (28) opening into said supply circuit downstream from the pump, and provided with a normally open valve that closes as soon as the pump operates;
- the buffer tank is connected to the exchanger through a connection conduit to the exchanger, the return conduit connecting the exchanger to the pump and comprising an anti-return valve found upstream from the connection of the supply circuit with the bypass circuit.

Exemplary embodiments of the invention further comprise one or several of the following features taken as a combination:

- the return conduit of the exchanger comprises a regulating valve with an integrated flow meter downstream from the pump;
- the valve found in the bypass conduit is an electromagnetic valve;
- the capacity of the buffer tank is adapted for receiving the whole volume of air initially contained in the primary circuit, while maintaining a permanent level of heat-transfer liquid in the low portion of said buffer tank;
- the conduit, opening at a low point of the buffer tank, is provided with a stop valve;
- the diameter of the conduits is selected to obtain an interfacial meniscus between air and liquid, such that the air pushes the whole liquid contained in the solar collector during drainage by gravity;
- the installation comprises a plurality of solar collectors positioned according to a series and parallel arrangement, such that the group hydraulic pressure loss of collectors connected in series and placed in parallel is predominant relative to the hydraulic pressure losses in the remainder of the primary circuit.

A second aim of the present invention relates to a method for implementing the installation described above, characterized by the following steps:

- the primary solar circuit is hermetically filled with heat-transfer fluid up to the imposed level of the buffer tank;
- the pump is started, sucks up the heat-transfer fluid and sends it into the solar collector;
- by a piston effect, the heat-transfer fluid comprising the air contained in the primary circuit, in particular in the collector, is sent through the first return conduit into the buffer tank, where the air is separated from the liquid by gravity and at the outlet of which the heat-transfer fluid essentially without any air is sent through the connection conduit towards the exchanger;
- upon returning from the exchanger, the heat-transfer liquid is sent back to the collector through the pump, the valve of the bypass circuit between the supply circuit and the tank being closed as long as the pump is operating and the installation starts to operate steadily;
- when the pump stops operating, the heat-transfer liquid stops circulating in the primary circuit and the air confined in the buffer tank flows upwards by overpressure through the conduit towards the collector, through the anti-return valve, which is then in the through-pass state;
- the upward flow of air causes the drainage by gravity of the primary circuit, the heat-transfer liquid returning to the buffer tank through the return conduit on the one hand and through the supply conduit, the bypass conduit and the return conduit on the other hand, the valve located in the bypass conduit being open.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1, already mentioned, schematically illustrates a thermal solar system as in the state of the art.

FIG. 2 illustrates a buffer tank piece of equipment for automatically managing the drainage by gravity of the liquid and the up-flow of the air, as in the invention.

FIG. 3 schematically illustrates a series-parallel connection self-regulating a uniform flow rate in the field of collectors by significant pressure losses in the collectors in series.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The present invention relates to a thermal solar system comprising a totally hermetic solar primary circuit. No leakage through a safety valve or air purge will be accepted in this installation in order to protect the circuit against any supply of atmospheric oxygen.
According to the invention, the primary circuit comprises a regulated device for draining by gravity, called Dynasol™, provided with a thermally insulated buffer tank 31 and initially filled with heat-transfer fluid to an imposed level 32. This level is visible and controllable through translucent piping 18 connected between two connections 16, 17 each provided with a stop valve (not shown).
The capacity of this buffer tank 31 will be selected so as to receive the whole volume of air initially contained in the primary circuit while always maintaining a permanent level of liquid 33 in the lower portion of the tank 31.
The volume of air of the circuit is at least that contained in the assembly of the solar collectors and of the piping for supplying these collectors, apart from the energy-dissipation equipment such as the heat exchangers.
The buffer tank 31 is hydraulically connected to the primary circuit as shown in the diagram of FIG. 2, so as to operate as a separator of liquid and of air, without permanently mixing the liquid with the air confined in the upper portion of the tank. This arrangement prevents dissolution of air in the liquid and its transfer into the circuit during circulation, which may be detrimental to the proper operation of the pump and to the energy performance of the solar collectors.
The device thus comprises two hydraulic connections 11,12 (for supply/return) to the circuit of the solar collectors located as for them in a raised position relative to the draining device.
The device also comprises two hydraulic connections 14,15 (for supply/return) to the circuit of use (i.e. the exchanger), the connection 14 being itself provided with a stop valve.
After the connection 12, the return circuit is separated into two pipings 25,26. The piping 25 is provided with an anti-return valve 20. The piping 26 runs as far as a T-shaped connection where it is separated into two pipings 27,28. The piping 27 is provided with a stop valve 13.
The buffer tank 31 is thus provided with an upward connection 25 allowing the air to flow up in the upper portion of the circuit and the liquid to be drained by gravity in the other portion of the circuit 26,27, towards this tank 31. The anti-return valve 20 prevents the up-flow of air during the circulation of the liquid. However, when the circulation of liquid stops, the presence of pressurized air in the tank 31 results in the pressure in the fluid return column of the collectors being greater than the pressure in the fluid column flowing towards the collectors. The result is that the air confined in the tank returns towards the collectors through the piping 25.
The device as in the invention also comprises in the supply circuit of the collectors, piping 29 connecting the connection 15 and the pump 19 provided with an anti-return valve 22. Further, piping 30 connects the valve 22 to a regulating valve 24 with an integrated flow meter 23.
When the solar circuit is triggered, the pump 19 sucks up the heat-transfer fluid and sends it towards the connection 11 and then towards the solar collectors. At the same time, the air comprised in the circuit is sent downwards through the connection 12. The air cannot travel through the piping 25 since the anti-return valve 20 blocks the passage of the fluid flowing downwards (air+liquid) and consequently, the air passes through the piping 26, and then through the piping 27 since the normally open electromagnetic valve 21 at the level of the conduit 18 is closed as soon as the pump operates.
The solar circuit is therefore built very simply so that, when the liquid is circulated by the pump, it acts as a piston causing the volume of air located in the collectors to be pushed into the return circuit as far as the buffer tank without leaving any residual air pockets in the circuit, which ensures good heat transfer and operational safety.
According to the invention, the flow rate of the primary circuit, ensured by an appropriate selection of the pump, is set to a value such that the liquid flow rate passing into the solar collectors is comprised under all temperature and viscosity circumstances, within a set value between 15 and 20 liters per hour and by square meter of solar collectors.
The circuit of the solar collectors is further built in such a way that the emptying of the liquid by draining by gravity is complete. Thus, the solar collectors will preferably have a coil-shaped continuous circuit designed so that no pocket may form a dead volume allowing the liquid to stagnate in any collector. The dimension of the tubes will be selected in order to allow the interfacial meniscus between air and liquid to be such that the air pushes all the liquid contained in the solar collectors during the draining.
According to a preferred embodiment of the invention, equidistribution of the global flow in the whole of the field of solar collectors will be ensured by a series (101) and parallel (102) arrangement of collectors such that the group hydraulic pressure loss of collectors connected in series and placed in parallel is by far predominant relative to the hydraulic pressure losses of the complementary portion of the primary solar circuit (FIG. 3). This arrangement of the solar collectors will constructively ensure uniform distribution of the flow in all the solar collectors in order to optimize the performance of the system.
The selection of the pump ensuring total flow will be such that it allows to overcome the total manometric height, namely the geometrical height as well as all the hydraulic losses of the primary circuit during its operation.
The pump will be selected so that the flow rate is maintained as constant as possible in order to maintain optimal flow and turbulence conditions at the level of all the heat exchangers, including the solar collectors.
The volume of air comprised in the buffer tank will ensure compensation for the thermal volume expansion of the liquid contained in the primary circuit. When it is closed after initial filling, the solar circuit is not pressurized, it is at atmospheric pressure in the initial state. However, an overpressure of the order of 50,000 Pa may possibly be generated in order to ensure pump suction pressure preventing cavitation. According to a particular embodiment, in addition to the tube for reading the level 18, the buffer tank 31 will also comprise in the upper portion a pressure gage and a normally closed valve that will allow to correct the volume of liquid if need be (not shown).
The present invention proposes a simple and reliable thermal solar installation with permanent draining by gravity. Unlike some installations of the state of the art, the buffer tank is not coupled with heat exchangers. The return of the heat-transfer liquid during the draining consequently occurs in a perfectly controlled way, along a short circuit, without any stagnation areas of the liquid, which guarantees optimal energy transfer and without passing again through the pump and the exchangers.

Key

1. Solar collector
2. Primary circuit
3. Use
4. Multiple exchanger
5. Pump (circulator)
6. Gas or fuel oil boiler
7. Control
8. Secondary circuit
11. Supply connection from primary circuit to collectors
12. Return connection from collectors to primary circuit
13. Stop valve
14. Connection of the primary circuit to the circuit of use
15. Return connection of the primary circuit from the circuit of use
16. Connection with stop valve
17. Connection with stop valve
18. Translucent piping
19. Pump
20. Anti-return valve
21. Controlled valve
22. Anti-return valve
23. Integrated flow meter
24. Regulating valve
25. Piping between the return circuit and the buffer tank (I)
26. Piping between the return circuit and the buffer tank (II)
27. Piping towards the buffer tank
28. Piping towards the collectors
29. Return piping from the location of use to the pump
30. Piping between the valve 22 and the regulating valve 24
31. Buffer tank
32. Imposed level of heat-transfer fluid
33. Minimal level of heat-transfer fluid

Claims

1. A thermal solar installation with draining by gravity, comprising at least:

one absorbing solar collector (1) raised relative to the remainder of the installation and in which a heat-transfer liquid circulates,

one hermetic primary circuit (2), comprising a supply circuit (15,29,30,11) and a return circuit (12,26,27, 25,14) towards and from the collector (1) respectively, the primary circuit being provided with a pump (5,19) as well as with a thermally insulated buffer tank (31) and initially filled with heat-transfer liquid at an imposed level (32), having the function of a liquid/air separator,

at least one heat exchanger (4) and

one secondary circuit (8) transporting the recovered energy towards a location of use (3), wherein:

the return circuit of the primary circuit (2) is subdivided into two conduits (25;26,27), a first conduit (26,27) that brings the heat-transfer liquid back to a low point of the buffer tank (31) when the pump (19) is operating and a second conduit (25) opening at a high point of said tank (31), and provided with an anti-return valve (20), which allows air to be returned to the collector (1) and hence the primary circuit (2) to be drained by gravity when the pump (19) no longer operates;

the first conduit (26,27) is separated into a conduit (27) opening into the buffer tank (31) and a bypass conduit (28) opening into said supply circuit (30) downstream from the pump (19), and provided with a normally open valve (21) that closes as soon as the pump (19) operates;

the buffer tank (31) is connected to the exchanger (4) through a connection conduit to the exchanger (14), the return conduit (29) connecting the exchanger (4) to the pump (19) and comprising an anti-return valve (22), being located upstream from the connection of the supply circuit (30) to the bypass circuit (28).

2. The installation as in claim 1, wherein the return conduit (29,30) of the exchanger (4) comprises a regulating valve (24) with an integrated flowmeter (23) downstream from the pump (19).

3. The installation as in claim 1, wherein the valve (21) located in the bypass conduit (28) is an electromagnetic valve.

4. The installation as in claim 1, wherein the capacity of the buffer tank (31) is adapted for receiving the whole volume of air initially contained in the primary circuit (2), while maintaining a permanent level of heat-transfer liquid (33) in the lower portion of said buffer tank (31).

5. The installation as in claim 1, wherein the conduit (27) opening at a low point of the buffer tank (31) is provided with a stop valve (13).

6. The installation as in claim 1, wherein the diameter of the conduits is selected so as to obtain an interfacial meniscus between air and liquid, such that the air pushes the whole liquid contained in the solar collector during drainage by gravity.

7. The installation as in claim 1, comprising a plurality of solar collectors (1) positioned according to a series (101) and parallel (102) arrangement, such that the group hydraulic pressure loss of collectors connected in series and placed in parallel is predominant relative to the hydraulic pressure losses in the remainder of the primary circuit.

8. A method for implementing the installation as in claim 1, characterized by the following steps:

the primary solar circuit (2) is hermetically filled with heat-transfer fluid up to the imposed level (32) of the buffer tank (31);

the pump (19) is started, sucks up the heat-transfer fluid and sends it into the solar collector (1);

by a piston effect, the heat-transfer fluid comprising the air contained in the primary circuit (2), in particular in the collector (1), is sent through the first return conduit (26,27) into the buffer tank (31), where the air is separated from the liquid by gravity and at the outlet of which the heat-transfer fluid essentially without any air is sent through the connection conduit (14) towards the exchanger (4);

upon returning from the exchanger (4), the heat-transfer liquid is sent back to the collector (1) through the pump (19), the valve (21) of the bypass circuit (28) between the supply circuit and the tank (31) being closed as long as the pump (19) is operating and the installation starts to operate steadily;

when the pump (19) stops operating, the heat-transfer liquid stops circulating in the primary circuit (2) and the air confined in the buffer tank (31) flows upwards by overpressure through the conduit (25) towards the collector (1), through the anti-return valve (20), which is then in the through-pass state;

the upward flow of air causes the drainage by gravity of the primary circuit, the heat-transfer liquid returning to the buffer tank (31) through the return conduit (26,27) on the one hand and through the supply conduit (30), the bypass conduit (28) and the return conduit (27) on the other hand, the valve (21) located in the bypass conduit (28) being open.