GB2514098A

GB2514098A - Overheat protection system for solar thermal collectors

Info

Publication number: GB2514098A
Application number: GB1308099.9A
Authority: GB
Inventors: Samuel Gerard Bailey
Original assignee: Individual
Current assignee: Individual
Priority date: 2013-05-04
Filing date: 2013-05-04
Publication date: 2014-11-19
Also published as: GB201308099D0

Abstract

An overheat protection system 1 for solar thermal heat collector 2 comprises a heat exchanger 8 and a valve 5, when the valve is opened a convection current causes circulation of fluid through the collector and a heat sink allowing excess heat to dissipate. The valve may be a non-return valve and may operated manually, electrically or by a pressure differential in the system. The heat sink 8 and valve 5 may be integrated into the solar collector unit with the heat sink forming the rear face of the solar collector. The system is used in a system with a pump 6 and a primary heat load 7 to which the solar heat is transferred. The system may be operated to allow the heat sink 8 to dissipate heat from the primary heat load 7 by opening the valve 5 when the pump 6 is activated.

Description

OVERHEAT PROTECTION SYSTEM FOR SOLAR THERMAL

COLLECTORS

This invention relates to an overheat protection system suitable for solar thermal collectors.

Solar Thermal Collectors (Solar Panels) are used in solar heating systems for space heating and hot water heating. A solar thermal collector typically comprises a dark coloured collector surface, designed to heat up when incident solar radiation hits it, conduits for a heat transfer fluid to be heated by said collector surface, and often a layer of glazing to reduce heat loss to the surrounding air. It may be flat or tubular shaped. It works by absorbing solar radiation and using it to heat the heat transfer fluid which is then used as a means to transfer the heat to another medium. This could be a primary heat load, typically a hot water cylinder, hcat store, or heat cxchangcr, or to heat directly the hot water in domestic hot water (DHW) systems. There is also typically a temperature sensor and electronic confrofler which determines when the collector is receiving sufficient energy to transfer heat to the primary load, and a pump which is switched on to circulate the heat transfer fluid to transfer said heat energy from the collector to the primary load.

A typica.l solar therma.l system uses water or a water/antifreeze (typically glycol) mix as the heat transfer fluid. This is maintained at a pressure above atmospheric (this is known as a pressurised or unvented system), usually in the range 3-10 bar. This high pressure raises the boiling temperature of the heat transfer fluid, typically to around 130-160 Celsius. There is then a pressure release valve which would be set to release any excess fluid if a set pressure is exceeded, thus avoiding the system over-pressurising and failing in a potentially dangerous maimer.

Tf insufficient cooled heat transfer fluid passes through the solar panels to cool them, they can reach very high temperatures, exceeding 130-160 Celsius. This can exceed the boiling point of the heat transfer fluid (a process known as stagnation), causing the fluid to vaporise, and/or the pressure release valve to open.

Repeated boiling and recondensing of the heat transfer fluid has two disadvantages.

Firstly the system will require replenishing with fluid as fluid is lost through the pressure release valve. Secondly, the heat transfer fluid degrades, causing it to become more viscous and sludgy. This damages the system components (e.g. by sticking to valve sealing surfaces), and requires the fluid to be replaced periodically.

It is therefore beneficial to design the system to avoid stagnation. For DHW systems, a standard technique is to size the solar collectors such that they are unlikely to cause the domestic hot water cylinder to be overheated during the course of a day. They can therefore continue to dump heat to the hot water cylinder without risk of overheating.

However they often still stagnate if no hot water is drained from the tank to cool it, for example if the householders are absent.

A solution described in e.g. CA2601 162 is to permit thc heat transfer fluid to drain back from the collectors if the system detects an overheat event. However this requires that the system is open vented (i.e. not pressurised), and is not compatible with pressurised systems.

An alternative known in the art is to circulate the heat transfer fluid between the collector and a heat sink. The heat sink used is typically a heat dump radiator plumbed in to the system. The heat dump radiator could be a domestic style or other radiator, the shape of which is designed to transfer the heat from the fluid to the surrounding air, or a heat exchanger which transfers the heat to another medium. If the solar controller detects a potential overheating of the system and cannot dump more heat to the hot water cylindei it will pump the heat transfer fluid from the solar collector through the heat dump radiator. This will transfer heat from the fluid to the atmosphere or another heat sink, cooling the tluid before it is recirculated through the collector, thus cooling the collector and preventing stagnation. However this system is not passively safe. It requires that the controller is powered up, and actively circulates the fluid through the collector and heat dump radiator. If the control system or pump fails, or it does not have powcr, then the system will not function and stagnation can occur. Also the high temperature heat transfer fluid is passed through the pump, raising the temperature of the pump, and can damage said pump. Furthermore it requires complex plumbing and electronic valves to operate.

The invention described here is for a system which can automatically transfer heat from a solar collector to a heat sink without the need for electricity or other external power input, thus cooling the system and reducing the maximum temperature to which the heat transfer fluid is exposed. The system will still function in the event of the failure of either the power supply, controller, pump or other actively powered system components.

A specific embodiment of a solar thermal heating system comprising the overheat protection system of the invention is shown in Fig 1. It functions as follows: When solar radiation (1) is incident upon the inclined solar collector (2), the radiation heats the fluid in the collector pipe work (3). A temperature sensor (4) then indicates to a controller that heat is available. Under normal operation, if heat is desired by the primary heat load (7) then the circulation pump (6) is powered by the controller. This pumps fluid upwards through the collector pipe work (3), through the primary heat load (7) and then back to the collector. It also causes a reverse pressure differential across the Non Return Valve (NRV) (5), causing said non-return valve to close, preventing fluid flowing through the heat sink (8). Fluid is therefore circulated between the collector (2) and the load (7). heating the load.

If there is a controller, pump, sensor or system failure, or the primary heat load is already at its maximum allowable temperature, the pump (6) is not powered. Heat is then transferred from the collector to the heat sink as follows: When solar radiation is incident on the collector, the fluid in the collector (2) heats up, expands, and becomes less dense. This causes it to rise by convection. The fluid in the heat sink (8) is colder than in the collector (2), and is therefore denser and it sinks. A pressure differential therefore causes a convection current to form, flowing upwards in the collector (2) and downwards in the heat sink (8). The non-return valve is configured such that a zero or very low pressure differential will permit the fluid to flow from the top of the collector in to the top of the heat sink under convection only.

The fluid is cooled by passing through tile heat sink. Cooler fluid therefore enters the base of the solar collector. This lowers the temperature of the solar collector, reducing the risk of stagnation. in tests the design has lowered the peak temperature from 136'C to lOOt.

When the pump is powered, the flow is reversed, and a reverse pressure differential will cause the NRV to close and stop the fluid flow through the heat sink, so heat is transferred instead to the primary heat load (7).

All alternative configuration of the overheat protection system is shown in a solar thermal heating system in Figure 2. This functions the same as the system in Figure 1, but with the NRV located below the heat sink.

Thc invcntion thus provides an overheat protection system for a solar thermal heating system comprising a solar thermal collector, a valve and a heat sink, wherein the collector is inclined such that when the valve is open, a convection current is created between the collector and the heat sink, causing fluid to circulate between the collector and the heat sink, causing heat to be transferred from the fluid to heat sink, cooling the system.

What is meant by solar thermal heating system is anything that uses solar energy to heat a working fluid to provide heat input e.g. to create hot water for washing, swimming or to provide heat to heat buildings or industrial processes. The solar thermal collectors in the solar thermal heating system can be of any type known in the art.

What is meant by a heat sink is any device or system by which heat can be removed from the working fluid, for example a radiator or heat exchanger, or other method for transferring heat to another medium.

The heat sink can be any means of transferring heat from the heat transfer fluid to another medium e.g. a gas, a liquid, or a solid, e.g. air or water. For example a radiator for transferring heat from the transfer fluid to the surrounding air. The radiator may be finned or flat plate, or any other shape. Or the heat sink may be a heat exchanger which transfers heat to another fluid or another thermal mass.

In general, to lose heat the working fluid is passed through the heat sink and hcat is lost from the heat sink to another medium. The heat sink is thus preferably shaped to optimise heat transfer to the other medium (e.g. air) and the shape is any suitable shape. Preferably it is shaped to increase heat transfer to the other medium relative to heat transfer from a fiat surface. The heat sink could, for example, have protuberances, for example fins, or corrugations. Alternatively the heat sink may provide other means for exchanging heat to another media, for example vents to permit air flow.

Thermal collectors are described above. Examples of thermal collectors this overheat protection system could apply to are Vitosol 200-F, auroTherm VFIC 155, auroTherm vacuum tubcs and others.

By inclined it means at any angle from the horizontal. The angle of inclination of the collector can be any angle that permits convective flow, for example between 1 to 90 degrees from the horizontal e.g. any of 1-90, 10-80. 20-70, 30-60 or 40-50 degrees, or any angle between them, preferably 50-90, 60 -80 70 -80.

The figures and the discussions above refer to a non return valve but die valve of the invention is a valve of any type that can be opened when heat is not required for the primary load and closed when heat is required for the primary load. When the valve is open the convection current causes the fluid to pass through the heat sink, cooling the fluid before it returns to the collector. The fluid then cools the collector. When it is closed, the fluid cannot pass through the heat sink. If the fluid is then circulated by other means, e.g. a circulation pump which is present in the solar thermal heating system, it passes through the collector only and not through the heat sink. This prevents heat energy from the fluid from bcing lost to the heat sink when it is required to heat the primary load.

Said valve could be a non-return valve, arranged such that the pressure difference caused by convection when the collector is hotter than the heat sink will open the valve to permit convective flow between the collector and the heat sink. When a circulation pump (e.g. in the solar thermal heating system) is powered (e.g. when heat is required for the primary heat load) the pressure difference is reversed, closing the valve and preventing fluid flow through the heat sink, and stopping heat transfer.

The non-return valve is of any type that allows fluid to pass in one direction only.

Possible types known in the art are swing pattern, sprung, diaphragm, or other mechanisms to allow fluid to pass in one direction only.

Alternatively the valve can be one that can be opened or closed by the operator e.g. manually, or opened and closed electronically by an electronic controller. The valve is preferably closed when the circulation pump is pumping and open when it is off.

An electrically operated valve may be configured to be normally open such that convection flow occurs when power to the valve is off. This would cause the system to enter a state where heat is transferred to the heat sink when the power fails, reducing the risk of overheating.

The valve could alternatively be a manually operated valve. Such a valve could be opened or closed. Preferably the valve is opened for any period when heat was not required (e.g. during a period of absence by a householder who would then not require hot water).

The valve (e.g. NRV) can be located above or below the heat sink. A configuration in which the valve is above the heat sink is shown in Figure 1 and a configuration in which the valve is below the heat sink is shown in Figure 2. When the valve is below the heat sink the pressure differential across the valve during operation of any pump present (e.g. in the solar thermal heating system) is higher than in the configuration shown in Figure 1 as the fluid entering the valve will not have been subject to a pressure loss by passing through the heat sink prior to entering the valve (5). This reduces the risk that if the valve is an NRV, that it will not be eloscd by the pressure differential created by the pump.

In a possible embodiment the heat sink is integrated into the solar collector unit. The rear of the solar collector (e.g. the side not facing the sun) could have conduits running through it (in addition to tile ones designed to collect the absorbed soiar energy in the absorption face) to allow the working fluid to pass through. These would form the heat sink. These conduits could be pipe work in thermal contact with the rear face of the solar collector, or other shapes designed to transfer heat to a surrounding media such as the air.

In another possiblc embodiment, the valve could be integrated into the collector. This is shown in figure 3. The integrated collector and heat sink unit (9) comprises the collector, heat sink and valve.

In a possible embodiment, the heat sink is mounted such that there is a gap between the heat sink and whatever structure it is fixed to, permitting air to flow around the heat sink. The air gap can be at ieast 1,2, 3,4,5, 10,2050cm e.g. 1-50,2-40, 3-30, 4-10.

The non-return valve is arranged such that it will be open when convection flow is created when the collector is heated, and closed when a circulation pump (e.g. which is part of the solar thermal heating system) pumps against it In a fttrther embodiment, both the valve and the heat sink are integrated into the solar collector. This would comprise a single unit with a solar collector on the front face (e.g. the side facing the sun), plumbed internally in a circuit with said heat sink and valve. The valve could be of any type defined herein.

If said valve is electronic or manual, the valve may be opened whilst the pump is operated such the heat sink can be used to cool primary heat load and/or the rest of the system. The valve would be opened electronically or manually.

The invention also provides a solar thermal heating system which comprises the Overheat Protection (OHP) system as defined herein. In addition to the eomponcnts of the OHP system, the solar thermal heating system may comprise one or more of a pump, a primary heat load, a temperature sensor and a controller (e.g. an electronic controller).

The pump is a circulation pump of any type known in the art. When heat is required by the primary load, the pump can operate, circulating fluid between the collector and the primary load (e.g. in a clockwise direction in figure 1, or an upward flow in the collector). When said circulation pump is powered and a NRV is present, the pressure difference across the valve closes the non-return valve, preventing flow through the heat sink.

When cooling is required, the pump may operate in the opposite direction (anti clockwise in figure 1 i.e. downward flow in the collector), transfernng heat form the primary load to the collector and/or the heat sink. If the valve is a non return valve, to cool the primary heat load, a reversible pump, or a separate pump operating in the other direction, could be used to pump the heat transfer fluid in the opposite direction i.e. downwards through the collector and heat sink, causing the non-return valve to open when cooling of the primary load is required.

The temperature sensor and an electronic controller act to detect the temperature of the fluid in the solar thermal heating system and to control one or more other components of the heating system. For example if the temperature sensor detects that the fluid in the solar thermal heating system is too hot e.g. it reaches or exceeds a maximum allowable temperature, which is determined by the operator but which may, for example be about 100, 110, 120, 130, 140, ISO or 160°C, e.g. 100-160, 110-ISO, 120-140°C, the electronic controller acts such that the pump is switched off, enabling heat to be transferred to the heat sink as described elsewhere herein.

The primary heat load is the system which is intended to be heated, for example the hot water for washing or swimming, or the interior or fabric of a building, or any other suitable medium which requires heat.

The invention also provides an overheat protection system or solar thermal heating system as described herein with reference to the Figures.

The invention also provides a method for protecting a solar thermal heating system from overheating comprising operating the overheat protection system as described herein.

Example 1:

A test of the system has been performed and is described below.

4 flat panel collectors of type EWE T2 of nominal collector efficiency of 0.77, with a combined collector area of Sm2 were set up angled at 20 degrees to the vertical. They were installed at ground level to permit easy access to perform the test measurements.

The collectors were plumbed in circuit with a typical domestic radiator with nominal rating at 50K of l200Watts to act as the heat sink. A non retuni valve was fitted to the system as shown in figure 1. Additionally a manually operated ball cock valve was plumbed in series with the NRV to cut off the flow to simulate closing of the NRV.

Temperature sensors were fitted to the top of the outlet of the collector array and also the bottom of the dump radiator. They were connected to a data logger recording the temperature every 10 seconds.

In full sunshine, the ball cock valve was closed and the temperature at the top of the collector was recorded. When the temperature had reached a steady state temperature of 136°C (after approximately 10 mins), the balleock valve was opened. The temperature of the top of the collector array dropped to 103°C within 2 minutes and then fluctuated at (114+!-8°C). The temperature of the bottom of the radiator rose from 22C to 29C. The test was repeated by closing the valve. Again the peak temperature rose to 136°C, then \vhen the valve was opened, the temperature dropped and settled at approximately 100°C.

The temperature trace from the test is shown in Fig 4. This demonstrates that the system as specified can successfully lower the temperature of the tluid in the collectors to below the temperature at which stagnation is likely.

Claims

CLAIMS1. An overheat protection system for a solar thermal heating system comprising a solar thermal collector, a valve, and a hcat sink, wherein thc collector is inclined such that when the valve is open, a convection dulTent is created between the collector and the heat sink, causing fluid to circulate between the collector and the heat sink.
2. An overheat protection system as in claim I, where said valve is a non-return valve
3. A overheat protection system as in claim 2, wherein said non return valve is arranged such that the pressure difference caused by convection when the collector is hotter than the heat sink opcns thc valve and permits convcctivc flow between the collector and the heat sink.
4. A overheat protection system as in claim I, where said valve is electrically operated.
5. A overheat protection system as in claim 1, where said valve is operated manually.
6. An overheat protection system as in claim 4 or 5 wherein said valve is opened when heat is not required.
7. A overheat protection system as in any one of claims ito 5, where said valve andior heat sink are integrated into the solar collector unit.
S. A overheat protection system as in claim 7 where the rear face of the solar collector forms the heat sink.
9. A overheat protection system as in claim S where the integrated collector and heat sink unit is designed to be mounted above a surface such that it will have an air gap between the heat sink unit face and the structure to which it is mounted to allow' air movement.
10. A overheat protection system as in any one of claims ito 9 where the heat sink is shaped to increase the surface area of the heat sink to increase heat transfer to the air or other fluid.
11. An overheat protection system of any one of claims 1-12 which further comprises a pump and a primary heat load.
12. An overheat protection system of claim 13, wherein said valve is a non return valve, when said circulation pump is powered, the pressure difference across the valve closes the non-return valve, preventing flow through the heat sink.
13. An overheat protection system as in claim 11 where said valve can be opened whilst the pump is on to allow the heat sink to be used to cool a primaly heat load which is above its desired temperature.
14. A overheat protection system as in claim 11 where said non-return valve can be forced to open by reversing the fluid flow to cool the primary heat load.
15. A solar thermal heating system which comprises the overheat protection system of any one of claims 1-14.
16. A solar thermal heating system which comprises the overheat protection system of any one of claims 1-14, further comprising a temperature sensor and an electronic controller.
17. An overheat protection system or solar thermal heating system as described herein with reference to the Figures.
18. A method for protecting a solar thermal heating system from overheating comprising operating the overheat protection system of any one of claims I-17.