Drainback Solution against Pressurised System
Pressurised system:
The vast majority of Solar Thermal systems installed in Ireland are of the pressurised variety. The solar circuit is completely filled with a heat-transfer fluid, which is then pressurised to increase its boiling point. A circulating pump drives this fluid around the solar circuit, transferring heat from the solar collector to a storage tank. An expansion vessel is fitted to damp out changes in fluid volume caused by thermal expansion throughout the day, maintaining a steady system pressure.
Safety is provided in the form of a blow-off valve on the pump station, which, in the event of a system over pressurisation, will allow some of the heat-transfer fluid to escape to a catch tank.
Click here to view a detailed schematic of that type of system.
| Pressurised system | ||
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Features |
Advantages |
Benefits |
| System depends on pressure to be maintained. |
No gravity circuit required. | Panels can be at a lower level than storage cylinder. Flexibility in the piping routes. |
| Less complex pipework. | Requires less skill to install. | Lower the cost of installation. |
|
Features |
Disadvantages |
Consequences |
| Fully filled closed circuit, subject to “stagnation“. | Without a “heat dump” on hot days, the system is exposed to high temperatures which can damage the equipment. | More expensive and complex controls are required to regulate temperature and avoid stagnation. |
| During cold weather without protection, panels are at risk of freezing. | Requires anti-freeze. | |
| Requires Glycol (anti-freeze). | Anti-freeze gets affected by high temperature; it becomes acidic and causes corrosion. Anti-freeze loses its performance with time. |
User interaction, regular service and anti-freeze liquid replacement are required, which represent an extra cost for the customer. Less efficient heat transfer. |
| Requirement for “heat dump”. | Requires storage space. Can break. |
Extra cost. Loss of space. |
The Problems of Pressurisation:
Pressurisation of the solar circuit comes with its own drawbacks. The most notable drawback is Stagnation. The collector is always filled with fluid, even when the system is not actively circulating.
If the system settles into stagnation on a hot day, continued heat production in the collector will raise the temperature of the heat transfer fluid to the point where it begins to boil.
This has the potential to cause the system pressure to increase, ultimately causing the system blow-off valve to activate and dump solar fluid, leading to a loss in pressure when the system cools. Sustained high temperatures can also damage system components, while accelerating wear and tear.
Stagnation mitigation solutions such a heat dump do not entirely eliminate the risk. If the heat dump has not been correctly sized to match the power output of the solar collector then the system still has the potential to overheat. If the system should lose electrical power, there will be no pump to drive the circulation of the fluid, allowing the system to stagnate anyway.
Drainback system:
A drainback solar system consists of a circulating pump, solar panels, a drainback vessel, a controller and some form of thermal store. Nothing else is required.
Drainback functions on the same basic principal as a conventional pressurised system. A circulating pump pushes water up to a solar collector on the roof, where it absorbs heat from the collector, then carries it back down to the hot water store.
The difference between drainback, and pressurised solar systems is that, when the circulating pump shuts off, heat-transfer fluid is allowed to drain under gravity from the solar panels down into a dedicated drainback storage vessel. The solar circuit is unpressurised.
This drainback vessel is mounted such that the fill level is somewhere above the level of the circulating pump, ensuring the pump never runs dry and that no air can enter the circuit. A good gravity flow is also required between the solar panels and the drainback vessel to ensure they properly drain.
Click here to view a detailed schematic of that type of system.
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Drainback System |
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| Features | Advantages |
Benefits |
| Drainback vessel provides a “fail safe” system. | When the pump isn’t running, panels are empty, fluid is stored in the drainback vessel. |
No risk of damaging the system by over heating or freezing. No extra cost. Maintenance is simplified , customer intervention is not required. |
| System is not pressurised. | No pressurisation kit required when installing. | Reduces the amount of parts. Increase reliablity. Reduced install cost. |
| No expansion vessel required. | ||
| No Glycol (anti-freeze) required. | Water can be used as solar fluid. | More efficient heat transfer. Reduced install cost. No disposal cost. |
| Wall mounted dual-line pump station. | Rapid and easy install. | Time and money savings. |
|
Features |
Disadvantages |
Consequences |
| Drainback systems rely on gravity. | Gravity fall is required in pipework. Planning and careful installation required to ensure this. |
Does not suit every project. Higher skilled installers required. |
No overheat protection or anti-freeze is required.
The chief advantage of Drainback solar is that, when the circulating pump is not operating, the collector is dry. In summer, there is no fluid in the collector to boil. In winter, there is nothing in the collector to freeze. Thus, the system cannot enter stagnation and cannot freeze solid.
The requirement for a heat-dump and anti-freeze is eliminated, simplifying the installation. There is no longer a need for holiday recooling functions and frost-protection modes within the solar controller. The system is also passively safe – should the pump lose power, all fluid will drain from the collector to the drainback vessel as normal.
The lack of pressure minimises the potential for leaks. Pipe corrosion and seal degradation due to acidic antifreeze is eliminated, improving system reliability and robustness.
Drainback Solar is a simplified, safe, robust and reliable solution.
Currently, Greentherm are the only company in Ireland offering Drainback solar thermal systems. Geentherm systems are based around Veridian ‘Clearline’ solar panels. These are one of the few flat-plate panels that are specifically designed for the Irish climate. All Clearline panels and flashing come a BBA agreement cert, verifyingtheir suitability for use as a roofing material.
For more information on Viridian solar panels Click Here
