Hibernation of a hot water system with solar water heater

[samy G]

Hello,

I would like to have the opinion of professionals. My family owns a house in a village in Greece, where temperatures do not fall below 5-10 degrees Celsius in winter. The house is located in a location where the water supply is not constant and with low pressure (which justifies the configuration of the plumbing system - detailed hereafter). In addition, the water is very hard. Here is the description of the system:

• A cold water tank supplies a pump / compressor that pressurizes the hot and cold water pipes.
• Hot water is generated in a hot water tank that is heated by two means
  • Electric resistance
  • A solar collector with heating liquid and its own dedicated circulation pump.
• A hot water circulator is also installed to circulate the hot water in the hot water loop.

I would like to avoid finding myself with a large electricity bill since the systems would be consuming electricity all winter and degrade the equipment.

What is the best way to put this equipment in "hibernation" and is it hard to put back in service at the beginning of the next season? Should I:

• Empty the tank, disconnect the pump/compressor, disconnect the hot water circulator, cover the heating panels and disconnect the panel circulator?

I am worried about bacteria proliferation and clogging of pump valves if the system is not emptied of it water.

Thank you for you reply
Kind regards,
Samy

 

[Brambles]

Instal an ion exchange water softener, and fit a timer to the immersion heater ( “electrical resistance”) for it to only operate to 60 degrees C for two hours each month.

I would be wary of trying to isolate a thermal solar system in a country with strong sunlight and high ambient temperatures.

 

[samy G]

Instal an ion exchange water softener, and fit a timer to the immersion heater ( “electrical resistance”) for it to only operate to 60 degrees C for two hours each month.

I would be wary of trying to isolate a thermal solar system in a country with strong sunlight and high ambient temperatures.

Hello Brambles and thanks for your reply. What is the purpose of the water softener for putting the system in hibernation?

Why would you be wary of covering the panels?

Does it mean that you would leave the panels operating and as thus the circulation pumps and compressor in operation?

Why do you prefer that method over emptying the system of its water.

Kind regards,
Samy

 

[Brambles]

Solar thermal systems (as opposed to solar pv) are intended for continuous use. They are capable of delivering very high hot water temperatures, which can shorten their life and boil the circulation fluid. To minimise this happening, the circulation system must remain in unimpeded operation to keep the circulating fluid temperature below 65 degrees C and preferably with a suitable heat dump. Once the circulation fluid has “boiled” it normally needs replacing. Bear in mind that the circulation system is under pressure so boiling occurs above 100 degrees C.

If you want to switch off the circulation system, the tubes should be removed, capped and placed vertically in a dark place. Covering on the roof is not sufficient, it does not prevent the tubes being heated by radiant heat, it could blow off, or be removed inadvertently with no adequate circulation system in place.

Most solar thermal systems (even in the dull UK) will experience boiling at some point in their life.

Whilst these systems look simple and straightforward, in an uncontrolled state they are capable of delivering much higher hot water temperatures than most conventional heating systems.

With respect to softening water, the thermal efficiency (and life) of the system will be improved / lengthened if soft water is used. The type of heat exchanger used (coil or tank in tank) also has an impact in this respect.

Without seeing a schematic layout of your system and the size of your hot water storage, I cannot easily comment on whether your pressurisation pump could be safely turned off for the winter.

In my experience domestic hot water and heating pumps and control valves et al have a longer life (and generally more trouble free) when they are in regular use.

 

[samy G]

Solar thermal systems (as opposed to solar pv) are intended for continuous use. They are capable of delivering very high hot water temperatures, which can shorten their life and boil the circulation fluid. To minimise this happening, the circulation system must remain in unimpeded operation to keep the circulating fluid temperature below 65 degrees C and preferably with a suitable heat dump. Once the circulation fluid has “boiled” it normally needs replacing. Bear in mind that the circulation system is under pressure so boiling occurs above 100 degrees C.

If you want to switch off the circulation system, the tubes should be removed, capped and placed vertically in a dark place. Covering on the roof is not sufficient, it does not prevent the tubes being heated by radiant heat, it could blow off, or be removed inadvertently with no adequate circulation system in place.

Most solar thermal systems (even in the dull UK) will experience boiling at some point in their life.

Whilst these systems look simple and straightforward, in an uncontrolled state they are capable of delivering much higher hot water temperatures than most conventional heating systems.

With respect to softening water, the thermal efficiency (and life) of the system will be improved / lengthened if soft water is used. The type of heat exchanger used (coil or tank in tank) also has an impact in this respect.

Without seeing a schematic layout of your system and the size of your hot water storage, I cannot easily comment on whether your pressurisation pump could be safely turned off for the winter.

In my experience domestic hot water and heating pumps and control valves et al have a longer life (and generally more trouble free) when they are in regular use.

Do you have some examples of panel covers that you are thinking about?

Thanks

 

[Brambles]

I would not recommend any form of cover for evacuated thermal tubes. There are suppliers in the US who manufacture and supply “curtain systems” to cover thermal solar panels. However, these are to reduce the output of the panel array in midsummer, not to shut it down.

We have used them on one oversized installation in the UK. Installed, but manually operated thermal curtains came out at around £135/m2 for a 68m2 thermal solar array. This particular array is ground not roof mounted. The curtains are to reduce the summer output by around 40%, when the hot water requirement is significantly reduced. It is supplying hot water to University College accommodation which is vacant from July to September.

They are quite difficult to instal. They need to be offset above the tubes by around 75mm to allow a free flow of ventilation. Consequently, they require a support structure, that is strong enough to resist wind forces, but slender enough not to restrict thermal output in the winter months.