# Blended energy systems



## Ross (May 9, 2002)

We use wood oil furnaces which is probably the most common blended alt energy/conventional energy systems used, mostly because it's tried and true. Adding solar to the hydronic part would be very doable, and a solar or wind genset to power the pumps and blowers is something we've been considering. What kind of blended systems do you currently use or plan to try?


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## JAK (Oct 15, 2005)

Electricity is currently cheap here, so I keep a 1500watt heater going through the winter and one tank oil to bring the house back up to temperature in the morning or after we have been out. I also try and snuggle up to the wife as much as possible.

Electricity and Oil will be going up in the next few years so I want to insulate around the slab foundation and but in a more efficient wood stove and water heater, with solar hot water in the summer. No plans on replacing the wife with a younger model, but I am not exactly sure what her plans are.


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## SolarGary (Sep 8, 2005)

JAK said:


> Electricity and Oil will be going up in the next few years so I want to insulate around the slab foundation and but in a more efficient wood stove and water heater, with solar hot water in the summer. No plans on replacing the wife with a younger model, but I am not exactly sure what her plans are.


Hi JAK,
Since you are thinking about solar hot water, I'd be interested in your (or anyones) thoughts on this scheme:

It uses a horizontal pond of water (about 150 gallons). The pond can be around 4' by 8' by 12 inchs (or so) deep. The pond is framed in wood, and lined with EPDM for the waterproof lining. It is insulated on sides and bottom with 2 inch extruded polystyrene foam.

Suspended in the pond is a coil of PE pipe (maybe 200 ft of 1.25 inch dia).
The cold water enters one end of this coil, and the other end is hooked to the cold water inlet on the house hot water tank. So the hot water tank gets water that is preheated by going through the coil.

Floating on top of the pond water is a 2 inch thick slab of rigid insulation with a sheet EPDM on top of it to absord solar heat. A small submersible pump sits in the pond below the floating slab. When the sun is out, the pump pumps pond water onto the top of the EPDM sheet; the water picks up heat from the EPDM, and then drains back down into the pond. When the sun is not out, the pump shuts down, and the insulation slab reduces heat loss from the pond.

The whole thing is glazed over with polycarbonate glazing that is sloped at about 30 degrees -- the sides of the glazing enclosure are reflectorized to get more of the lower winter sun reflected on the EPDM. A 2nd layer of film glazing sits just over the EPDM to provide more R and prevent water vapor from getting to the upper glazing.

The aims are:
Easy to build
All season (although performance in winter will be less)
Long life
Cheap (about $200)

Here is a pdf that describes describes prototype version 1, and explains the logic behind it:
http://www.builditsolar.com/Projects/HorzPondDHW/DHWPondProto.pdf

We are making some changes from this for prototype 2 -- here is progress so far on prototype 2:
http://www.builditsolar.com/Experimental/DHWPond/Prototype2.htm

The main changes are to slope the glazing to prevent muddy puddles and to get more winter sun.
Go to a single coil of larger diameter pipe. The coil holds enough water to meet most how water demands in the house just with the water in the coil -- it then has time to transfer heat from the pond into the coil before the next hot water demand occurs. This way you don't have to worry much about the heat exchange efficiency of the coil to the pond. The pictures at the link above show two coils, but I have decided to go with one larger diameter coil -- since heat transfer rate between pond and coil is not critical, I plan to pretty much take the coil as it comes from the supplier, and not try to spread out the coils as I did for the ones in the picture. 

Any thoughts would be appreciated. I'm working on this with Nick Pine, and we hope to get prototype 2 up an running this summer.

Gary


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## Ross (May 9, 2002)

Two thoughts Gary a small circulating pump just to move the pond water around the coils of piping and yeah I'd use copper. By moving the pond water you're going to keep the hottest water near the potable water piping and you'll avoid stratification in the "pond" although it's pretty shallow. I'd use copper, as it has better heat transfer properties and if you could find the kind with fins on the outside (as used in tankless coils in boilers) you might increase the heat transmision even more. Tankless coils are why your building inspectors shouldn't care about passing potable water through a coil in a stagnant pond though. Off to read some more.


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## Loumeister (Mar 31, 2006)

Gary,

This was great, I had been thinking about something like this as well... The comments about copper tubing are great for additional heat transfer. One question that I had was how long does it take to get to the 120 Deg temp? What is the starting temperature of the pool? The increased thermal transfer will cool the pond that much faster, and unless there is enough thermal mass it will cool very rapidly. Then again, if your just trying to heat water as it's going into the heater for a shower or laundry this is a great idea. 

I'm new to this forum, but it's been so great to see people exchangeing these great concepts.


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## JAK (Oct 15, 2005)

Interesting ideas?
I'm wondering about angle of the Sun. 
I'm wondering also about evaporation and concensation.
I'm wondering also if this is the beginings of a Solar Stirling Engine.


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## SolarGary (Sep 8, 2005)

Ross said:
 

> Two thoughts Gary a small circulating pump just to move the pond water around the coils of piping and yeah I'd use copper. By moving the pond water you're going to keep the hottest water near the potable water piping and you'll avoid stratification in the "pond" although it's pretty shallow. I'd use copper, as it has better heat transfer properties and if you could find the kind with fins on the outside (as used in tankless coils in boilers) you might increase the heat transmision even more. Tankless coils are why your building inspectors shouldn't care about passing potable water through a coil in a stagnant pond though. Off to read some more.


Hi Ross,

The copper with fins seems like a good idea -- I'll see what I can find out about that. I've seen them in the Slant Fin catalog. The fins might make a big difference in heat transfer rate. It seems like the fins I have seen are alumium -- wonder how they will do for corrosion? Maybe a sacrifical anode could be used to prevent corrosion?

The test on the first prototype shows that the output temperature is fine until you use all the water thats in the coil. After that the PE pipe coil does not transfer heat fast enough from the pond and the output temperature drops. 
Nick's idea to fix this is to make the coil of pipe hold enough water so that you could take a shower with just the water thats in the coil (about 10 or 12 gallons). This is a pretty cheap solution that (it seems to me) should work pretty well, as long a people wait a few minutes between showers to give the coil time to pick up heat from the pond between showers. At some point I plan to try a copper coil, and see if it transfers heat well enough so that it can pick up enough heat "on the fly" to work well. The copper tubing is much more expensive, so hopefully it won't take too much length. Copper would also have the advantage that people think of copper as being more durable and long lasting.
The keeping the pond mixed with the pump also sounds like a good idea. I can put temperature sensors in a few places and try a hot water draw with and without the pump on to see how much difference there is. It would be nice not to have to add a 2nd pump for this.
We have been wondering how the building code folks will view the PE pipe going through the hot pond, and whether they will think a double wall exchanger is needed. I can see arguments both ways.

Thanks,

Gary


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## SolarGary (Sep 8, 2005)

Loumeister said:


> Gary,
> 
> This was great, I had been thinking about something like this as well... The comments about copper tubing are great for additional heat transfer. One question that I had was how long does it take to get to the 120 Deg temp? What is the starting temperature of the pool? The increased thermal transfer will cool the pond that much faster, and unless there is enough thermal mass it will cool very rapidly. Then again, if your just trying to heat water as it's going into the heater for a shower or laundry this is a great idea.
> 
> I'm new to this forum, but it's been so great to see people exchangeing these great concepts.


Hi,

Its been a while since the test on the first prototype, but the fill water came from our well, which is usually around 50F. I think (but I'm not certain) that it warmed up to pretty close to the 120F area during the first day. This was one of those very clear and sunny winter days. 
To me, it seems like the 150 gallons (more or less) might perform pretty well, but like most solar water heaters, its not going to provide much help on cloudy winter days. Nick has visions of a larger version (perhaps contained in a greenhouse) with more water and more glazing that would supply nearly 100% of hot water needs year round. 
I am hoping that the 2nd prototype will provide a lot better understanding of how well this concept will work.
It does seem to me that solar hot water setups are too expensive for what they do, and that there has to be a simple way to do a good year round solar water heater inside of a few hundred dollars. Hopefully if this is not the one, then someone will figure out another way.



Thanks,

Gary


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## SolarGary (Sep 8, 2005)

JAK said:


> Interesting ideas?
> I'm wondering about angle of the Sun.
> I'm wondering also about evaporation and concensation.
> I'm wondering also if this is the beginings of a Solar Stirling Engine.


HI JAK,

I think the sun angle is going to limit winter performance, especially for higher latitudes. The 2nd prototype adds the reflector sides to help some in this area.
During the summer, I think that horizontal should work really well -- the tables say horz is actually better than a tilt at latitude for mid summer.
If it did 100% of the hot water demand in summer and 50% in winter I'd be very happy. But, maybe theres a way to do a better reflector?

The inner layer of film glazing (right over the floating insulation slab) is mostly there to keep condensation off the outer glazing, and prevent the loss in efficiency that would go with that. On proto 1 I used a layer of bubble wrap that was actually floating on the water (there was no floating insulation on proto 1). This worked well as far as controlling condensation on the polycarbonate glazing, but I don't think the bubble wrap is up to the job on a long term basis. It had fused itself to the black poly pond lining after a few weeks. Any ideas for a good long term inner glazing?

What I know about Sterling engines would fit in a thimble, but that sounds interesting -- maybe you could elaborate?

Thanks,

Gary


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## JAK (Oct 15, 2005)

Hi Gary,

Sounds neat. I think I will try something this summer. I am not sure how to cut down on evaporation/condensation. Maybe if there isn't too much space between the water and the twinwall there won't be as much water vapour to condense after you drain back down. I am also curious about whether its air tight and where the air goes when you drain up and down. I think the reflector idea is good because it allows you to keep it flat. You can drop the reflector down at night for insulation, though thats getting a little bothersome.

I am trying to find a link to a sterling engine I saw that was similar to your water heater. It would be interesting to combine the two with the sterling engine on top and the water heater capturing the rejected heat. I will keep looking.

Here it is:
http://www.stirlingengines.org.uk/sun/sola5.html
http://www.bsrsolar.com/core1-2.php3

Maybe the sterling concept could be used to pump the water around so you wouldn't need electricity or thermosyphon.


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## Ross (May 9, 2002)

Copper with aluminum fins is for radiators working in air, I've seen the copper pipe for inside boilers in plumbing stores but I'll bet its expensive. It has copepr fins. I really can't see why the buildign inspectors etc. would object to the pipe circulatign potable water through a hot water heat collector of dead water. Its how oil or gas fired boilers with a heatign loop inside them often work. Their water is black and full of anticorosion elements or even glycol. its still just single wall copper (probably type K) The water in the pipe is under higher pessure so there's not much risk of contamination. The only health risk might be from the pond growing bacteria but it should be too hot, often enough for that not to be a problem.


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## SolarGary (Sep 8, 2005)

JAK said:


> Hi Gary,
> 
> Sounds neat. I think I will try something this summer. I am not sure how to cut down on evaporation/condensation. Maybe if there isn't too much space between the water and the twinwall there won't be as much water vapour to condense after you drain back down. I am also curious about whether its air tight and where the air goes when you drain up and down. I think the reflector idea is good because it allows you to keep it flat. You can drop the reflector down at night for insulation, though thats getting a little bothersome.
> 
> ...



Hi JAK,
That would be great if you could have a go at one this summer.
I'm sure the design will progress a lot faster with more than one prototype in work.
I tested one version of the floating insulation slab that had a layer of EPDM on top of it. It had edge dams all the way around the periphery of the slab that basically allow the water to overflow back into the pond when it gets about 1/4 inch deep. The water was distributed to the top of the EPDM by a CPVC manifold that ran along the centerline of the insulation slab. This worked fairly well, but I think that a scheme where there are edge dams, but that the water enters near one corner and drains back into the pond near the other corner via a pipe whoose edge is set up about a quater inch above the EPDM might work better. This would eliminate the need for a gap between the insulation slab and the edge wall (which is needed on the other one to allow a place for the water going over the edge dams to drain back into the pond. In both cases there are a couple small drain holes through the insulation slabe and EPDM that are flush with the EPDM to allow the quarter inch thick layer of water to drain slowly back into the pond when the pump shuts down. 

I'm on the road for a few days -- will take a good look at the Sterling links when we get back home.

Gary


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## SolarGary (Sep 8, 2005)

Ross said:


> Copper with aluminum fins is for radiators working in air, I've seen the copper pipe for inside boilers in plumbing stores but I'll bet its expensive. It has copepr fins. I really can't see why the buildign inspectors etc. would object to the pipe circulatign potable water through a hot water heat collector of dead water. Its how oil or gas fired boilers with a heatign loop inside them often work. Their water is black and full of anticorosion elements or even glycol. its still just single wall copper (probably type K) The water in the pipe is under higher pessure so there's not much risk of contamination. The only health risk might be from the pond growing bacteria but it should be too hot, often enough for that not to be a problem.


Hi Ross,
I like the boiler analogy -- seems like it should make a good precident.

Nick thinks that a bit of copper sulphate(?) could be used to keep any growth from happening without being toxic. And, as you say it should get hot enough quite often to kill off any growth anyway.

Gary


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## Loumeister (Mar 31, 2006)

Gary,

Just a few other thoughts about this. After reading up on "solar ponds" it seems that there maight be a couple of relatively simple improvements. I don't kow how deep it would have to be to work properly, but the large solar ponds use a salt solution to stop the convection process that brings water to the surface to cool. The salt water is heavier and the heat just keeps building up in the lower layer. Just keep in mind that the solution would eventually have to be disopsed of, this could be a problem unless you let the water evaporate away when that time comes. Also, a black dye in the water might help with the heat absorbtion.


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## SolarGary (Sep 8, 2005)

Loumeister said:


> Gary,
> 
> Just a few other thoughts about this. After reading up on "solar ponds" it seems that there maight be a couple of relatively simple improvements. I don't kow how deep it would have to be to work properly, but the large solar ponds use a salt solution to stop the convection process that brings water to the surface to cool. The salt water is heavier and the heat just keeps building up in the lower layer. Just keep in mind that the solution would eventually have to be disopsed of, this could be a problem unless you let the water evaporate away when that time comes. Also, a black dye in the water might help with the heat absorbtion.


Hi,
I have read some about solar salt gradient ponds in the past. Seems like an interesting concept. One thing I found a bit discouraging about them is that a good percentage of the web sites that are associated with these ponds have note that says something like "pond temporarily offline while rebuilding gradiant" -- makes me think that it may be hard to keep a good gradient. Although, with the glazing on top, the salt gradient may tend to be more stable since the wind cannot interact with the top layer of pond water. 
I did see one lab study that did a gradient pond that was only a couple feet square, so, size may not be a bit issue.
Maybe this could be prototype version 3 

Gary


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