Green Building Forum - Flat panels -vs- Evacuated tube... ...the showdown...

Flat panels -vs- Evacuated tube... ...the showdown...

welshboy — Wed, 14 Apr 2010 14:48:12 +0100

Posted By: Johan

I have 8m2 of FP and 750l of storage for those wondering...

Johan
Could you please tell us the typical temperatures reached in your 750l store in a month or so?
The reason I ask is that I am about to connect 6 X 2.5m panels to a 1100 litre store so a similar concept but I am wondering if there are enough panels. Your experience would be very much appreciated.
I will be routing the flow through a heat x to heat a 170l dhw store and then to the main store. I hope to run some underfloor heating directly from the big store and when the temperature of the store drops too low for the direct ufh to then extract the remaining heat via another heatx raising the temperature of the groundloop of our heat pump so it does not need to work as hard and is more efficient.
As far as the original debate FP v ET I am in the bangs for bucks corner and my unused panels cost me a little over Â£900. The installation will be drainback keeping everything simple.

Flat panels -vs- Evacuated tube... ...the showdown...

davidfreeborough — Wed, 14 Apr 2010 19:03:19 +0100

Posted By: welshboyI hope to run some underfloor heating directly from the big store and when the temperature of the store drops too low for the direct ufh to then extract the remaining heat via another heatx raising the temperature of the groundloop of our heat pump so it does not need to work as hard and is more efficient.

This is an interesting idea. I've not heard it mentioned before.

How does this work? Is there a heat exchanger on the GSHP's ground loop return fed from the 170l solar store? Does the GSHP's load loop feed the 1100l store or does it feed the underfloor heating directly? How is this controlled?

I guess the solar store becomes unuseable for underfloor heating below about 28 degrees. Is the idea to take the solar store down close to zero degrees? The GSHP's ground loop may operate close to or below zero degrees, have you thought about frost protection?

David

Flat panels -vs- Evacuated tube... ...the showdown...

welshboy — Wed, 14 Apr 2010 19:40:08 +0100

Whew Dave Questions !
I will explain the gshp first. Output goes through indirect coil in 170l store then goes direct to UFH downstairs and big cast iron radiators upstairs. This has been in operation for 18months.
Solar plan- Stress plan - about to be installed .
Flow is routed through a heatx which thermosyphons heat to 170l .This 170l tank is part of the dhw and takes to part of the space heating. After the solar flow has routed through the heatx it goes into the 1100l store.
Inside the 1100l store there is a heatx which routes via a pump to the house ufh and radiators.
Valves control which circuit heats the house and using timeslots on both the solar controller and the heatpump controller enables one only to run at a time.The heatpump will be seen by the solar controller as auxiliary heating required as determined by a sensor.
The heatpump has a groundloop as its primary heat source. As explained I intend routing this ground loop through a heatx ( GHTX) In winter this ghtx will probably be redundant but in the shoulder months when the 1100l store is not hot enough to use direct I can route the store heat via a controller sensor and pump through the ghtx raising the temp of the Groundloop.
Frost protection can be covered by the cheapo controller.
My groundloop is returning at 4c at the moment.
Hope it all works.

Flat panels -vs- Evacuated tube... ...the showdown...

davidfreeborough — Fri, 16 Apr 2010 13:18:12 +0100

Posted By: welshboySolar plan- Stress plan - about to be installed .
Flow is routed through a heatx which thermosyphons heat to 170l .This 170l tank is part of the dhw and takes to part of the space heating. After the solar flow has routed through the heatx it goes into the 1100l store.
Inside the 1100l store there is a heatx which routes via a pump to the house ufh and radiators.

I'm not a plumber, but don't you need a diverter valve across the solar connections to the 1100l store?

There's a limit to how much power the first heat exchanger can take out of the solar flow & there's a limit to how much power the solar panels can put into the solar flow. Normally the output of this heat exchanger would go back directly to the solar panels. So whatever temperature you get out of the heat exchanger becomes the inlet temperature to the solar panels. As the temperature in the 170l DHW store goes up, so does the inlet temperature to the solar panels. The solar panels then add 10-20 degrees to the flow & send it back to the 170l DHW store again until the target DHW temperature is reached.

If you connect the 1100l thermal store in series with the 170l DHW store's heat exchanger then the inlet temperature to the solar panels will be roughly equal to the temperature of the 1100l thermal store. In other words, unless you do something to switch the heat exchangers then your DHW temperature will be limited to 10-20 degrees above the 1100l thermal store temperature.

David

Flat panels -vs- Evacuated tube... ...the showdown...

bella — Fri, 16 Apr 2010 14:47:01 +0100

A French company called CLIPSOL manufacture and install solar systems across France. They use solid floors (12 cm deep) as the main heat store combined with a single pump, a 350ml tank and up to 10sqm of FP collectors. The controller (their own) determines whether the water from the collectors goes to floor or tank. Their system was rated highly by a European study (sorry for the moment forget name but the Editor was Austrian and the UK did not feature at all) that analysed performance of systems designed to heat space as well as water. The same study was also v. enthusiastic about drainback.

Flat panels -vs- Evacuated tube... ...the showdown...

welshboy — Fri, 16 Apr 2010 14:57:09 +0100

Posted By: davidfreeborough
Posted By: welshboySolar plan- Stress plan - about to be installed .
Flow is routed through a heatx which thermosyphons heat to 170l .This 170l tank is part of the dhw and takes to part of the space heating. After the solar flow has routed through the heatx it goes into the 1100l store.
Inside the 1100l store there is a heatx which routes via a pump to the house ufh and radiators.
I'm not a plumber, but don't you need a diverter valve across the solar connections to the 1100l store?

There's a limit to how much power the first heat exchanger can take out of the solar flow & there's a limit to how much power the solar panels can put into the solar flow. Normally the output of this heat exchanger would go back directly to the solar panels. So whatever temperature you get out of the heat exchanger becomes the inlet temperature to the solar panels. As the temperature in the 170l DHW store goes up, so does the inlet temperature to the solar panels. The solar panels then add 10-20 degrees to the flow & send it back to the 170l DHW store again until the target DHW temperature is reached.

If you connect the 1100l thermal store in series with the 170l DHW store's heat exchanger then the inlet temperature to the solar panels will be roughly equal to the temperature of the 1100l thermal store. In other words, unless you do something to switch the heat exchangers then your DHW temperature will be limited to 10-20 degrees above the 1100l thermal store temperature.

David

"If you connect the 1100l thermal store in series with the 170l DHW store's heat exchanger then the inlet temperature to the solar panels will be roughly equal to the temperature of the 1100l thermal store. In other words, unless you do something to switch the heat exchangers then your DHW temperature will be limited to 10-20 degrees above the 1100l thermal store temperature."

That is the way it is planned.

I did not say but the 170l store linked to the heatpump and planned solar acts as a preheat store for another 170l dhw store in series.Ie Hot out from preheat store is feed for dhw tank.
Also the preheat store 170l is the dishwasher and washing machine feed.
Works ok.

Flat panels -vs- Evacuated tube... ...the showdown...

davidfreeborough — Fri, 16 Apr 2010 15:35:27 +0100

Posted By: welshboy
Posted By: davidfreeboroughIf you connect the 1100l thermal store in series with the 170l DHW store's heat exchanger then the inlet temperature to the solar panels will be roughly equal to the temperature of the 1100l thermal store. In other words, unless you do something to switch the heat exchangers then your DHW temperature will be limited to 10-20 degrees above the 1100l thermal store temperature.

That is the way it is planned. I did not say but the 170l store linked to the heatpump and planned solar acts as a preheat store for another 170l dhw store in series.Ie Hot out from preheat store is feed for dhw tank.
Also the preheat store 170l is the dishwasher and washing machine feed. Works ok.

I thought you were in the process of installing solar, are you saying it works OK with the heat pump?

Even in summer, there aren't going to be many days where you can use solar thermal to heat 1440l of water to 65 degrees. So you'll end up paying for energy to heat the DHW to 65 degrees when there's energy in your system which could have done it for free.

It's usual with larger stores to prioritise heating the DHW by using multiple heat exchangers at different levels or diverting flow to different stores. The more sophisticated controllers provide support for this.

Have you thought about having a 3 way diverter valve so that the system starts by heating the 170l DHW store, when this is up to temperature the flow is diverted to the 170l preheat store & only when this is up to temperature the flow is diverted to the 1100l thermal store?

David

Flat panels -vs- Evacuated tube... ...the showdown...

evan — Fri, 16 Apr 2010 15:48:50 +0100

I'm going to use a diverter valve for both solid fuel and solar input - 120L DHW tank first, then switch over to a pair of 180L tanks coupled together which I hope will act as a buffer for the radiators.

However, you can arrange solar to heat your DHW tank to the acceptable temperature even if the panels are always fed with very low temperature water - just rig the controller to cycle the pump on only when the temperature at the panel reaches the target.

This isn't as efficient though, as the panels obviously work best when raising a large volume of water at a low temperature to a very slightly higher temperature :)

Flat panels -vs- Evacuated tube... ...the showdown...

crusoe — Sat, 17 Apr 2010 09:59:44 +0100

DF - good advice.

Evan...be interested to see your SF/solar diverting.

Flat panels -vs- Evacuated tube... ...the showdown...

welshboy — Sat, 17 Apr 2010 11:02:42 +0100

DF thanks- The heatpump - when on heats the preheat cylinder and that water goes to the dishwasher and washing machine working ok. This has reduced our electric bill as we are rural not on gas. It also reduces ( via cop)the paid for energy for the dhw.
Going to wait and see how the solar works as the thermosyphon should act as a control on overheating the preheat cylinder.
Just bought another 2 panels making just under 20 sq metres of flat plates.
Depending on how it performs I am considering the possibility of a single panel dedicated to the dhw cylinder.- have to see what temps the preheat cylinder gets to first tho.