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Drainback to protect against overheating
By DamonHD, Section Heat Posted on Sun Dec 09, 2007 at 06:47:51 PM MST
If I want to oversize a solar thermal system for winter DHW/CH...

...then will using a drainback system be enough to protect against summer overheating damage?
(And maybe using an evacuated tube system to minimise the liquid capacity and thermal mass of the collector system to maximise efficiency...)
The problem here in the UK around London is that of course insolation is lowest in the winter (by about 4x lower than summer) when DHW/CH demand is highest (about 3x higher than in summer).
So a collector big enough to supply all the DHW/CH in winter is more than 10x oversized in summer. (This ratio can be reduced slightly by vertical panels, etc, but I wouldn't have that option in my current property.)
Many sites and people seem to take it as an article of faith that it is impossible to safely and reliably (eg without lots of user intervention) manage such a winter-sized system. There seems to be more than one opinion on OtherPower too.
Does someone here have experience of building a system that completely (or nearly so) covers DHW/CH in winter and deals with the huge overcapacity in summer (without using something like a swimming pool as a heat dump)? My aim would be to avoid a backup heat source for more than (say) 10 days per year.
Rgds
Damon

Drainback to protect against overheating | 25 comments (25 topical)

Re: Drainback to protect against overheating (3.00 / 0) (#1)
by GaryGary on Sun Dec 09, 2007 at 05:51:31 PM MST

Hi,
One approach to avoiding overheating in systems that are oversized for summer needs is to mount the collectors vertically.

The sun is high enough in the summer that vertical collectors can be stagnated without any problem (at least in my experience).

Vertical collectors do well in the winter with the low sun, and are inefficient in the summer with high sun -- which seems to be what you want.

If going vertical is not enough, a fixed overhang could be placed above the collectors to shade part of the collector for the high summer sun.

Gary
Gary gary@BuildItSolar.com www.BuildItSolar.com

Re: Drainback to protect against overheating (3.00 / 0) (#5)
by DamonHD on Mon Dec 10, 2007 at 02:46:21 AM MST

Hi Gary,

Yes, some of what you suggest would be great, and I'll be looking out for the opportunity for those in any new property. I've explicitly taken a note of the overhang idea!

At the moment I have the problem that my roof surfaces face east and west and that if I depart from my roof line and/or use the vertical surfaces I'll get into planning and covenant problems.

So I'm after inherently safe system against overheating, if possible.

Rgds

Damon

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#22)
by huntedheads on Wed Jan 02, 2008 at 04:50:36 PM MST

Dig trenches in your yard and install plastic tub, in the summer run your water there and put a garden over it. You can plant way sooner than all the neighbors and it will last longer into the winter!
Seen this somewhere? something about 12 foot sunflowers.

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#23)
by huntedheads on Wed Jan 02, 2008 at 04:52:14 PM MST

oops! that would be plastic tube!

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#2)
by fcfcfc on Sun Dec 09, 2007 at 06:32:36 PM MST

Hi: You mention vert is out so I would ask what is the steepest mounting angle you can use..??.. Based generally on the ratio's you give for heat availability in Summer vs. Winter, I think 10 days in Winter for backup only is not realistic. As an example, a house I did in Lehighton PA uses high efficiency evac tubes, 210 of them (7 - 30 tube units) and the collectors are mounted at 67 degs.. The system is extremely efficient and uses highly stratified storage which most systems do not. Anyway, this past 12 month period his oil usage was reduced about 67%. From late April to October he used very, very close to zero oil for hot water, literally a couple of gallons, which means the ~1/3 oil usage left was for backup space heating. If you consider 180 days for space heating, at 1/3 you are looking at 60 days backup usage equiv.. Once you hit about a 75% reduction in your backup usage, diminishing returns and huge system sizes set in to the point of not being practical in anyway. The more "clumpy" and extended the cloudy periods are, the worse it gets for very high solar fractions. In short, mount the units as steep as you can and shoot for a reasonable reduction in space heating load...

.....Bill

Re: Drainback to protect against overheating (3.00 / 0) (#6)
by DamonHD on Mon Dec 10, 2007 at 02:52:46 AM MST

Hi,

On a new property I'd go for 70* or vertical panels. My current house has a number of shortcomings for RE.

I hear what you say about 'clumpy' and my own measurements (by proxy, from my pilot PV system) suggests that 7 days' storage would be needed to carry through those.

Now a 7-day store in the house, or seasonal thermal store to cover the 33% you said still had to come from oil, is not totally impossible at all.

That actually gives me a sensible target to shoot for!

Rgds

Damon

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#8)
by DamonHD on Mon Dec 10, 2007 at 06:53:56 AM MST

Hi,

Further thoughts... Thanks again for your data point.

My plans are a mixture of theory (what might be possible in order to go zero or negative carbon), and more practical (what I might do at my current tiny house or if we move to somewhere close by larger).

Your 67% figure is quite interesting, ie 2/3rd of year-round DHW and CH from solar thermal, with the balance from another fuel source.

If I aimed for that with solar thermal with protection from overheating, then I could in principle (though expensively) make up some of the gap with solar PV and a plain ground-source heat-pump and/or a more moderate thermal store than I was planning before (eg 20kl rather than 60kl). A good CoP on the heat-pump might compensate for much of the relative inefficiency of PV compared to thermal for energy capture.

And the big potential advantage of solar PV is that I can safely just not use any excess, eg in summer, or even export it to the grid. Overheat is not an issue.

I'm trying to work through making some of these ideas a bit more practical on this page:

http://www.earth.org.uk/towards-a-LZC-home.html

though please bear with me since I'm still mid-thought, so maybe check by later today.

Rgds

Damon

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#9)
by fcfcfc on Mon Dec 10, 2007 at 10:00:45 AM MST

Hi: You mention Geo as a possible add in. Before we open up that can of worms, do you have any idea what your annual heating load is for space heating and DHW..??..
.....Bill

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#10)
by DamonHD on Mon Dec 10, 2007 at 10:24:02 AM MST

Hi,

Yes, at this house DHW (and cooking) gas use is ~10kWh/day, and CH (central/space heating) load for half the year is approx an additional 20kWh/day (and zero the rest of the time).

So total year-round DHW/CH demand is approx 7.3MWh/year.

But under the scheme that I've outlined here:

http://www.earth.org.uk/towards-a-LZC-home.html

the heat-pump would only need to be working flat-out to supply all the CH needs in mid-winter, as solar thermal would be contributing some or all DHW/CH needs at other times.

We can of course work on reducing the CH demand by increasing insulation, etc, and that will make GSHP more plausible.

I am assuming that if we move to a new place we can keep DHW/CH demand similar or even lower.

Rgds

Damon

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#11)
by fcfcfc on Mon Dec 10, 2007 at 12:10:44 PM MST

Hi: Ok, so at 20KWH that's ~70,000 Btu's per day average which means there are times in the Winter you will use 140,000 Btu's or higher in a day and less than 70,000 Btu's other times. Add in the DHW etc.. and you are around 100,000 Btus per day average. Last questions, do you have any idea what the sunshine percents are in the heating season for your location..??.. Second, you mention roof area <50M2 but what are the roof dimensions, ridge length and length down the slope of the east west roofs and roof pitch..??..

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#12)
by DamonHD on Mon Dec 10, 2007 at 01:27:09 PM MST

Hi,

For the current house it's probably hopeless, but I can tell you that PVGIS (http://re.jrc.ec.europa.eu/pvgis/apps3/pvest.php) suggests:
886Wh/m^2/day at about our roof pitch (36*) and 944Wh/m^2/day on a vertical surface in December south-facing, with year round averages of 3100Wh/m^2/day and 2180Wh/m^2/day.

I think that those numbers are roughly halved on east- and west- facing pitches, though tubular collectors may do better than that suggests because of their profile.

I don't know our exact roof pitch, but by eye 36* isn't far off.

The north-south ridge of the roof is roughly 5m long and the house front to back (ie east to west) is about 8m.

Rgds

Damon

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#13)
by fcfcfc on Mon Dec 10, 2007 at 03:05:27 PM MST

Hi: Boy, that is tough. You could run two down the East slope and two down the West slope facing true South but your roof is about 10 degs to steep, to much shading. You could only use that approach if you mounted all the way at the South end of the house so the shading problem goes away except the whole rest of the roof will then be in the shadows of the thermal units, not good for PV. That would give you 120 tubes which would yield about 100,000 to 120,000 Btu's on a perfect solar day, which would get you part way there. I suppose you could put some PV on the North end facing south but you would have to raise them up a bit for shading issues.. Not an easy install either way....

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#14)
by DamonHD on Mon Dec 10, 2007 at 03:54:32 PM MST

Hi again,

Interesting. Thanks for that.

I think the situation is not at all good for this house for real RE.

Really time to look for a better place I think... B^>

Rgds

Damon

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#15)
by eliyahu on Wed Dec 12, 2007 at 11:26:26 PM MST

" A good CoP on the heat-pump might compensate for much of the relative inefficiency of PV compared to thermal for energy capture."

One thing I have heard from people who have used heat pumps in Ontario anyway is that the c.o.p. doesn't reflect the energy needed to extract the heat. In other words if the ground temperature is 54* fahrenheit or 11* Celsius the pump has to run almost continually to extract the heat. Whereas if the heat pump had a source of 70* heat then the pump wouldn't have to run as much.

What size and type of thermal mass are you using for heat storage?

What I am trying to implement is that I have a cistern in my basement that will hold about a 1,000 gallons of water furthermore the cistern is built with a large amount of rock around it as there is a stone foundation. Being an old farmhouse I can't insulate under the house so I have an infinite heat sink. My plan is to in late summer use ground in my greenhouse on the south side of the house to start storing heat in the 4 feet of ground below the floor of the greenhouse and into the stone foundation wall. The floor of the greenhouse will have r20 and the ground around the perimeter of the greenhouse I will insulate down 4 feet. What I hope to do is store heat with embedded water pipes in the 6"of concrete and air pipes 6" perforated drainage using a small solar powered fan to send the heat in the greenhouse down in to the ground to bring the temperature of the ground up 20* going into the fall so that it acts like a big flywheel.

You might be able to do something similar to shorten your winter needs as well.

I really like drainback systems because water is the best storage medium. There is no risk of poisoning yourself in the case of a leak. And since the water in the closed drainback system can be distilled then you don't have to worry about build up of calcium etc in your system. The energy requirement for the pump can be very small. Some people can actually get a thermosiphon working that of course would be sweet. But this demands that the water storage is higher than collectors. Another idea is to look at the Thomason house or Solaris idea for collecting heat. He used water running down the metal roofing under glass as the collector being stored in the basement but as was discovered by the 70's back to the land pioneers warm moist air can be a breeding ground for mold which is why water is king.

Where I am, I have to supplement with wood heat. I have a boiler that heats the house and outbuildings that I need when it gets below -10 celsius 22* fahrenheit. What I want to do though is that the boiler can be undertasked by this and so want to capture any other heat from the wood being burned and store it in the cistern which then acts as a preheat for domestic water. And my thermal heat sink.

Hope this helps

Of all the alternative energy systems on the planet solar thermal has to be king. Of course that is what the earth is a big heat sink.

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#16)
by DamonHD on Thu Dec 13, 2007 at 01:03:22 AM MST

Hi,

Thanks!

I'm proposing a tank of up to 60,000l which would hold all our winter DHW/CH heat, though a mixed scheme might get away with 20,000l. And the surrounding soil would be good to use except that we might (I don't know) have relatively fast-moving groundwater not very far down from two nearby rivers.

http://www.earth.org.uk/milk-tanker-thermal-store.html

http://www.earth.org.uk/towards-a-LZC-home.html

My current house isn't really good for either scheme, unfortunately... So as we're looking to move anyway, it's an opportunity to find a more suitable location for RE too.

Rgds

Damon

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#17)
by eliyahu on Fri Dec 14, 2007 at 01:00:08 AM MST

Moving may prove to be best. Can you move to an apartment while you start from scratch and build? If you build an apartment can offer some life stability, especially since building is very stressful. Straw bale, tire houses and poured concrete forms sound really appealing from a design perspective. If you can build into the side of a hill then you've got thermal mass. Plan on a root cellar and passive solar greenhouse on south side. Buying raw land at least in my part of the world would save substantially in the long run. Though financing can be harder to secure.

A couple who live near me built a house out of tires. They were paid to take the tires and when they were done after 3-4 years of grueling work. They now have a house that will last that is warm in the winter and cool in the summer and NO mortgage.

Would be my plan if changing.

Looks like big economic storm is rolling in. They are trying to keep everybody happy spending while they look for assets that will escape massive depreciation. Reading the financial rags to see the roaring and tossing is mind boggling. This will keep the world wondering for quite a few years, and may provide a substantial melt-down in ability to access capital. Real-estate with big poorly built and insulated housing on it could depreciate dramatically. It sounds like that is now happening in the U.S. Looks like back to basics, as such houses that won't require a lot of external energy inputs will be smart.

There are housing developments in Alberta that have sunk large insulated storage tanks that use summer heat. Apparently they are working fabulously.

Hope this is helpful..

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#18)
by DamonHD on Fri Dec 14, 2007 at 01:27:35 AM MST

Hi,

I earn most of my living from one of the big US investment banks (announced its results yesterday). Everyone was glum, since they did bonuses the same day, but no one's jumping off the roof. I expect a little more market turbulence, but I think my contract is safe for now for example, and we have money in the bank, and no debt other than our mortgage.

Anyway, that means that I need to stay within commuting distance of London, and even finding land to build on looks tough.

So we're looking for the best existing property we can: one bedroom more than we have now, a bigger garden (for playing in and burying a thermal store!), a nice place to live, good doctors and schools for our daughter (turned 2 today!), easy commute to London by public transport (possibly with the assistance of a small electric car), and of course a nice south facing unshaded aspect (eg on the roof) for solar thermal/PV...

Rgds

Damon

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#19)
by DamonHD on Fri Dec 14, 2007 at 01:33:10 AM MST

Once recent development in the UK (and the rest of the EU) is that all houses for sale must show how energy-efficient they are) very soon, which will certainly be one of the aspects we'll be considering.

Rgds

Damon

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#20)
by eliyahu on Sun Dec 16, 2007 at 01:55:23 PM MST

Space is at a premium in Europe, frequent visitors to Canada from Europe have been known to make comments like they would see Halifax on Monday, Toronto, Montreal and Ottawa on tuesday and wednesday, Calgary on thursday and Vancouver for the weekend. I know from a short trip to France and the Netherlands that it took us 5 hours by train to go from Amsterdam to Paris. In Ontario 5 hour drive will get you from Ottawa to Toronto. It can take 3 days (8 hours) to get out of Ontario driving from Ottawa going west. I don't see cities as safe places. Most people are still convinced that they can pour toxic chemicals down the drain, coupled with a plethora of hormones and industrial solvents etc. So my inclination would be to get out of dodge. But that has it's costs and consequences. Adding a long commute doesn't seem wise. I would definitely want to be able to have a large garden and would choose land over building. How long would you have to drive to get out of London and hit country? Cheers

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#21)
by DamonHD on Mon Dec 17, 2007 at 01:26:04 AM MST

Very much depends what you call 'country' and 'London'!

From where I do most of my work in Canary Wharf over to the east of London, 30 miles or less could get you into relatively open space. It's certainly a line of investigation for me.

Rgds

Damon

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#3)
by jonas302 on Sun Dec 09, 2007 at 08:31:40 PM MST

assuming there would be several units would it work to just valve some off

Re: Drainback to protect against overheating (3.00 / 0) (#4)
by Volvo farmer on Sun Dec 09, 2007 at 09:35:02 PM MST

The fluid gets wery wery hot in a thermal panel if fluid is not flowing. Valves are no good, water boils, glycol degrades at high temps.

I think Gary's idea is the best. He's actually heating his house with solar thermal panels and should be listened to, IMO. I have also recently heard of a system that uses drainback AND a glycol solution with a heat exchanger. It sounded complex and redundant on first examination, but had the advantage of being absolutely freeze proof, plus the ability to drain down un-needed panels in the summer.

I've listened to more than a few people who have actually heated their home or DHW with thermal solar panels and Gary is not the first one to recommend vertical panels. I don't know why it's impossible in the OPs regard but it certainly is a perfect solution to his problem, if you ask me.

Volvo Farmer

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#7)
by DamonHD on Mon Dec 10, 2007 at 02:55:20 AM MST

Hi,

On a new property I would do vertical (or 70*+) panels like a shot.

I don't think it's going to be possible for two separate legal reasons on my current property, though it's looking more and more worthwhile to try.

Yes, Gary's practical experience is just what I'm after.

Rgds

Damon

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#24)
by sbotsford on Thu Jan 10, 2008 at 10:21:58 PM MST

I don't like the idea that the stagnation temp of the drained collector is very hot, then gets hit with cool water. Sounds like a good way to spring a leak.

In general, I think you will be better off to reduce the area of the collector in the summer. The overhang has been mentioned. Easier: Greenhouse supply companies sell shade cloth in various densities and colours. If you picked up a roll of 70% shade cloth in white, and covered your collectors for the summer season, I think you would do ok. While the stuff doesn't last forever, it's UV stabilized, and is good for a few years.

Re: Drainback to protect against overheating (3.00 / 0) (#25)
by DamonHD on Fri Jan 11, 2008 at 02:19:43 AM MST

Thanks!

Damon

[ Parent ]

Drainback to protect against overheating | 25 comments (25 topical)

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