Newbie alert - solar thermal installation in the south of France

[mangocrazy]

Hi! My first post, so I'd like to introduce myself and ask some advice. I'm English, have a house in Sheffield in the UK and also a holiday home (fixer-upper) in the south of France (about 15-20kms from the Med, near Beziers).

Unsurprisingly, it's the house in France that has the solar installation(s) - I have solar thermal on the roof facing Southwest and solar PV on the roof facing Northeast. The solar thermal array consists of 30 x 58mm diameter tubes and the PV array is 12 panels approx 1.7m x 1m in size. The solar thermal got pride of place as it was installed first (September 2012) although that was only putting the tubes on the roof. They didn't get connected up and working until September 2023! At least it proves that tubes can sit on a roof unconnected and not come to any harm.

The tubes, mounting frame and hot water cylinder came as a kit which also contained a 230v pump, pressure vessel and control station. I really wasn't that enamoured of a mains voltage pump and all the attendant programming involved, so searched for a low voltage pump that could be driven by a small solar panel, and that is what is fitted now. I'm currently in France and last weekend we had some really warm, sunny conditions and the control station indicated a maximum of 67 degrees C for the water in the cylinder. I haven't used the thermostat in the DHW cylinder since the solar thermal has been operational.

When I leave the house to go back to the UK I just disconnect the pump from the solar panel and drain all the water out of the solar thermal pipework. That way there is no danger of freezing over winter or boiling in high summer if left unattended. I've also incorporated a central heating radiator that can be switched in or out via a system of ball valves to act as a heat dump if needed. I haven't had to use it yet.

The solar PV was installed in spring 2018 and has been quietly running the meter backwards ever since. It was almost an impulse purchase, as we just stopped off at a stall in a street market near us and got talking to the stall holders about the solar PV systems they were promoting. They came to our house, did a site survey and gave us a quote, which we accepted. I have the paperwork somewhere but it was around €8,000 for the 12 panels, DC/AC converters and installation. It just works, which is what we wanted.

I'm not sure how many other people use a low voltage, solar-driven pump for their solar thermal array, but I was hoping to tap into people's knowledge and experience. I'v noticed that in the morning the temperature of the water in the DHW cylinder decreases quite noticeably and only recovers when the sun has worked its way round by about midday. This is because the pump is active from quite early but the tubes don't produce much heat until later in the day, so I'm effectively pumping cool water into the cylinder and reducing the heat of the stored water.

Is there any way to delay the start of the solar-powered pump until such time as it will be feeding hot (or at least warm) water into the cylinder's indirect coils? I was wondering if there was a way of only passing DC electricity to the pump once it passed a certain voltage threshold? Would this be an option or is there a better way to achieve the same ends? I really want to keep the simplicity of the solar-powered pump but would like to tweak it to improve its overall efficiency.

Many thanks in advance.

 

[skyking1]

a simple thermostatic control will get you the water you want.
Put a mercury thermostat in the upper manifold housing of the solar tube collectors.
Set it so it makes the circuit when the that housing is greater than 40C
Turn this unit up to the upper limit, or simply modify the spring inside to suit your needs.
Use it to switch a relay at the pump. Off becomes on.
US Amazon example.

 

[Somethingobscure]

This may not be the kind of advice you want and may not be relevant as my experience is in Southern Spain , but I recently removed an evacuated tube solar thermal hot water systems (installed in 2009) in favour of expanding the solar PV and an adding air source heat pump, which provide both DHW and Heating but also cooling in the summer. We also recently added standard (emersion style) electric water heaters for summer as the ASHP can't simultaneously do cooling and heating.

 

[skyking1]

It's just off topic.

 

[mangocrazy]

a simple thermostatic control will get you the water you want.
Put a mercury thermostat in the upper manifold housing of the solar tube collectors.
Set it so it makes the circuit when the that housing is greater than 40C
Turn this unit up to the upper limit, or simply modify the spring inside to suit your needs.
Use it to switch a relay at the pump. Off becomes on.
US Amazon example.

Yes, I was thinking of a water temperature based solution as well as a voltage based one. Thanks for the link.

 

[skyking1]

do you understand how to wire it up?

 

[mangocrazy]

This may not be the kind of advice you want and may not be relevant as my experience is in Southern Spain , but I recently removed an evacuated tube solar thermal hot water systems (installed in 2009) in favour of expanding the solar PV and an adding air source heat pump, which provide both DHW and Heating but also cooling in the summer. We also recently added standard (emersion style) electric water heaters for summer as the ASHP can't simultaneously do cooling and heating.

No problem, glad to hear differing views. But I'm very happy with the arrangement we have at the moment just want to tweak it a bit. I still have unused real estate on both roofs so the addition of more solar PV is definitely in my plans.

 

[mangocrazy]

do you understand how to wire it up?

Not at the moment, but I'm sure I can find out.

 

[mangocrazy]

This the solar thermal array and small solar panel that drives the pump.

 

[skyking1]

cool,er,hot!
I would use a temperature system rather than voltage of the solar panel output to control things. You want a certain minimum temperature of water.
Imagine a clear cold morning, the panels start making good juice and you give your water tank a dose of that cold water because that is going to lag behind voltage considerably.

 

[mangocrazy]

Yeah, the more I think about it, the more I think a temperature based solution is the way to go. You're reacting to what is the current state, rather than trying to regulate an input.

 

[mangocrazy]

As a temporary solution to the problem (while I decide on a longer term one) I've put together a wooden box that fits snugly over the solar array that drives the pump. I cover the solar panel about half an hour before sunset and remove it around 10:30 am (this will change as the year progresses) and the pump is entirely inactive in that time period. I've done this for the last couple of nights and it has lessened the heat drop in the cylinder overnight, but not by as much as expected.

Before covering the solar panel I was seeing a drop of around 30 deg C overnight. With the solar panel covered and no pump activity early in the day I'm now seeing a drop of around 20 deg C overnight. All the flow and return (15mm copper) pipe work between collector and cylinder is lagged with high temperature 19mm thick insulation but the 22mm pipework to the cylinder is not lagged and this will be done ASAP. What kind of heat loss would you expect overnight from an insulated stainless steel 300 litre DHW cylinder?