Solar powered Preheater for Tankless Water Heater

Step 7 Mana from Heaven aka: Money from Sunlight

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Now go build one. It's not going to build itself! You can preheat water for any type of water heating system, gas or electric, tankless or with a tank, and it will translate into savings for you. Just be safe and get professional help if you have to in order to follow local codes and so forth. Yes, that was a disclaimer.

Remember, even if what you build is not enough to supply all your needs, that doesn't matter. If it just supplements what you have, then every bit of energy you harvest is money saved! It amounts to turning sunlight into money!

Oh, one other thing --- If the ambient outdoor temperature is higher than the temperature of the water coming out of your supply pipe, this thing will work even without sunlight. That's right... on a cloudy day or even at night! If your water comes out of the supply pipe at 55 degrees and it is a cloudy, but warm 80 degree day (or night), the water will still circulate until the holding tank is at 80 degrees. And feeding 80 degree water to your heater is better than feeding it 55 degree water!

Money saved - day AND night!

Q - "What if it gets cold at night? Won't that cool off the water in my holding tank?"

A- No. If the water in the collector is cooler than the water in your holding tank, then the cooler water will just sit there because it is heavier. Circulation will stop until the sun comes up and heats the water above the holding tank temperature. Perfect automatic control!

Just remember that hot water rises. If your regular heater has a tank, then it needs to be at the highest point in the system. The solar collector needs to be at the lowest point and the holding tank between the two.

If you use a tankless heater, then you only have to worry about having the collector lower than the holding tank and you don't have to worry about the height of the tankless. It could even be in your basement. Don't forget to insulate all your connecting pipes!

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Now what...

Well, I need automatic control of the mirrors.

I'm learning about heliostats. Heliostats are different from solar trackers. A solar tracker keeps something (like a pv panel) pointed at the sun. But we don't want to point a mirror at the sun.

A heliostat follows the sun and positions a mirror in such a way that it keeps the sun's reflection on one spot... your collection box. It is a bit more complicated than a solar tracking system because the mirror has to be pointed at an angle that exactly bisects the angle between the sun and the collection box and that angle is always changing. Also, PV panels are forgiving about the vertical tilt angle but a heliostat has to be accurate in the Z axis as well as the X in order to keep the reflection on one point.

I don't want a M$ Windows controlled system. I don't really want to have to learn to use a Pic controller if I don't have too. I want it to be powered by a small lead acid battery charged with a small pv panel.

I'm leaning towards a system that uses GaP (green) LED's as sensors which control a Full Bridge Motor Driver. But I'm still in the initial stages of that. One thing I have learned is that thinking about heliostats makes my brain hurt.

For those not familiar, LED's can convert light into electricity, similar to a photovoltaic cell. Gallium Phosphate (green) LED's do it best. Three green LED's wired in series and exposed to bright sunlight will produce a voltage high enough to switch TTL logic circuitry. Before you get any ideas, LED's are not a practical replacement for photovoltaic panels. They will produce a voltage but practically no current.

I just ordered a bag of 100 water clear green LED's and spent last night destroying/salvaging two defunct printers for motors, gears, etc.

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I hope you liked my first Instructable. Visit my project site: http://davetech23.tripod.com to see other diy projects I have documented. Also documented is my Electric Tankless Water Heater with performance observations if you are interested. Hey, it's free.

UPDATE August 8, 2009 - I'm still working on the heliostat but have been slowed down by the need to make a relatively complex circuit board for the controller. I got sidetracked building a tabletop cnc machine (a la Tom's Easy Mill) to make the pc board with. I'm almost to the point of making the board, waiting for engraving bits to arrive from the other side of the world.

My build of the mill and pretty much day-to-day progress is being documented at:
http://cerebralmeltdown.com/forum/index.php?board=4.0

If you don't mind, click a star up there and give us a rating. Thanks!

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Comments

[Unkel]

A most excellent presentation!

I've been toying with the idea of augmenting my current heating system with a solar hot water heater for some time now and have been wondering how I might make it happen.

I have an indirect system where a loop from my boiler goes into tank to heat the water. My boiler is about 85% efficient, and I'd love to improve that efficiency using solar.

Your project confirms several ideas I've been thinking about, so given time, money and materials, I just may be able to implement something.

Keep up the good work!
Unkel

[ravan]

Hi Dave,

Thanks for this instructable. Really inspiring :)

One thought: you might want to stress the fact that it is really important for anyone who wants to build a similar setup to keep the 'old' heating ON (either tank or tankless) and not be tempted to switch it OFF (or set it to a low temperature) and only use solar heated water. The warm water that reaches you shower must always have been heated to at least 60°C/140°F (!) to prevent the possible build-up of Legionella in the holding tank getting into your longs while showering.

And one question: what are your thoughts on leaving this collection box outside and connected during the winter in areas where it can freeze up to -20°C? Will ice in the pipes rupture them and cause big leakage or do you think the mass of the relatively warm water in the holding tank inside, by way of natural heat exchange through the water pipes (read: heat loss) prevent the still water in the pipes outside from freezing up?

Thanks for your thoughts!

[laughingcoyote]

Disabuse me if I'm wrong, but I though that I had read that Legionella resulted from hotels keeping the water at such high temperatures benevolent bacteria was killed off leaving the heat-tolerant Legionella bacteria without any competitors.

In my travels through Asia I saw water heated everywhere solely by solar, and Legionella was never an issue.

[ravan]

Laughingcoyote, I have no idea how the Legionella bacteria holds out in an environment populated with plenty of other (benevolent) kinds but the idea here is to use this pre-heater for normal tap water, which in most 'modern' countries, usually is relatively void bacteria. Not completely, but still.

So then the question is in what environment, in terms of temperature only (since there will be hardly any competition), the Legionella bacteria will thrive. The article on Wikipedia I referred to, confirms what I have been told about the subject: they thrive between 25 and 50 degrees Celcius but will be killed when the water reaches a temperature of about 60 degrees.

All cases of Legionella poisoning I have heard/read about involve three things: (1) water with a temperature within the correct range, that (2) has been still for long enough to let the bacteria propagate plus (3) a shower, garden sprinkler, fire hose, or fountain that sprays that water around so the bacteria can get airborne (living in tiny droplets) and reach fragile long tissue. Given these conditions, infection is likely.

Maybe the solar heated water in India is never used in showers with shower heads that spray fine enough droplets, or the water usually reaches above 60 degrees C, or the tanks are never so big that they keep water long enough to let Legionella build up...

[AsianSpanker]

Having been an engineer on a ship in Alaska for many years I can tell you. You don't want any water siting in a pipe at -20 C. Having had many pipes rupture even those that were valved off, because I didn't get them drained. If you live in an area that gets cold you need a draindown valve that empties the water out. And your system needs be designed to not hold pockets of water, like a manifold system rather than a single loop. A manifold system looks like a bunch of capital H's and water enters the bottom and is drawn from the top.

[laughingcoyote]

Or else use a closed loop that runs water with anti-freeze or other liquid through a heat exchanger in your water tank.

[Davetech]

Hi Ravan, Two good points. Actually, I had not even thought of the possibility of problems from bacterial buildup in the holding tank. I'm on chlorinated city supplied water. But someone getting their water from a well or using rain water would probably do best to read the Wikipedia link you posted and take appropriate precautions. In my Instructable, I only touched on the problem of freezing weather. In the picture on Step 5, there is a comment box that says I plan to convert over to a heat exchange system and use some kind of antifreeze solution when winter gets here. I got a couple of new but broken tank-type water heaters for free from my local plumbing supplier. They just had defective gas valves and I guess it was not worth the cost and hassle to return them to the manufacturer. They were just set out back and were free for the taking, otherwise they were destined for the recycler yard. I plan to take the tank out of one and put it in a 55 gallon drum. The drum would be well insulated and contain the antifreeze solution that circulates to the outside collection box while the heater tank is plumbed into the tankless heater supply line. I might use isopropyl alcohol/water as the antifreeze. To answer your question, no, the warm water in the holding tank will not protect the pipes in the collection box in the system I have built. It might be okay during the day with sunlight falling on the pipes, but on a very cold night or a very cold cloudy day, the circulation will stop and the temperature of the pipes will approach the outside ambient temperature. The pipes would need to be drained or some other measures taken to protect the pipes from freezing and bursting.

[ravan]

Hi Dave, thanks for your response.

About Legionella and chlorine: I understand (but am no expert on the subject) that the little critters are more resistant to chlorine than most other contaminants. Found a study on the subject on http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=239530 that might tell a more learned mind what the risk are. Obviously, the level of chlorination of your city tap water needs to be considered. Apart from that, it looks like they found that chlorinated water of a higher temperature (in the study 35 C which would normally be perfect for survival) Legionella dies quicker than in chlorinated water of lower temp (4C and 21C in the study)... So by your preheating method you seem to be actually reducing the risk of Legionella! If that is a correct in deduction of course... ;)

Sadly (in this respect) I am using non chlorinated water so by preheating, I am increasing the risk. By ensuring that my electric boiler tank always raises the water temperature well above 60C, I should be fine... I suppose this last stage heating (reaching over 60C) on night current only will be enough. Electric heating is relatively expensive so I would like to keep it limited to the reduced price of night time consumption.

About the freezing risk problem: I found a company in the UK, SolarTwin, that actually sells a somewhat similar system. The company claims their collectors do not need draining in winter since the pipes are made of some silicone rubber that can withstand expansion from freezing water. As soon as the sun comes back, the water will melt again and the system will be back functioning like before. As far as I can tell, their collector is designed for low pressure water systems, not high pressure city water... Maybe that allows for the use of more flexible tubes in the collector. I wonder how rigorous tubes must be for high water pressure and still be flexible enough to withstand 4% expansion when the water inside it freezes up. And if at all possible, what kind of tubes/material would suit the job.

Any thoughts on this last missing piece in the puzzle? If that is tackled, there would be no need for any heat exchange / antifreeze type solutions which would make it so much more expensive and difficult to install

:)

Allard

[Blackice504]

Hi Dave good project i am about to start something like this one thing i found is old fluro tubes around the pipes increase heat when you have the pipe in the middle of the tube and sealed with foil duck tape on each end you get like a 360 degree heating of the pipe plus a nice green house effect so it keeps more heat in on those cloudy days, from what i seen of your design it would not be too hard to add this.

This idea is on this site as well but he used pvc pipe so it would work alot better with those nice copper pipes.

[laughingcoyote]

Engineers in Gaviotas in Brazil (described in "Gaviotas: A Village to Change the World") used discarded fluro tubes to do the same thing. The engineers used some sort of reaction to oxidize the surface of the copper pipes to make them the very darkest black possible. They then found that the temperatures became so hot that the fluro tubes burst, so they created a vacuum inside of them. The result was that they could get the temperature much higher using their recycled rubbish than Japanese engineers using the latest and most expensive technology. Besides water heaters, they heated oil which they ran through copper-coil stoves used first to sterilize hospital equipment and later to process pine resin.

[tahoekid]

Hey Davetech, Nice instructable. I'm looking to build a similar solar water heater for the tankless gas heater for my pool. I was wondering how the copper t-junction worked to feed the hot water into the tankless heater. I'm concerned about plumbing the system so that it doesn't pop leaks and/or back up. Thanks!

[Davetech]

Hi Tahoekid, It is just a simple "T" inserted between the holding tank and the tankless heater. No valves or anything needed. The arrows show the convection flow. When water is pulled through the tankless heater, some will be pulled through the collection box as well as from the holding tank, but that should not matter. Hope this helps and good luck with your project.

[Davetech]

After some thought, there may be a better set up than that first diagram. It is the way my system is set up, for intermittent use, where the holding tank slowly warms up and only occasionally gets drawn from. But for a pool, I'm going to have to make some assumptions. Like the tankless heater will run for long periods? And its supply is not from the street, but from the pool? If so, you might not even benefit from having a holding tank. Your pool itself is your holding tank. This diagram might be a better setup for a pool. I'm not a pool man and I don't even play one on TV, but this is my best guess. If your tankless is getting its supply from the pool and just recirculating it, then it doesn't matter where the preheated water gets injected. Any calories you add to the water will make life a little easier for the tankless.

[ravan]

This seems to be a good starting point but there is one thing missing: a pump to circulate water to/from the collection box. At least for most pools because it is simply not feasible to place the collection box below the pool (usually below ground level already) so the heat collection also takes care of water flow. Two options spring to mind: 1. connect the collection box between the heater and pool so that the already installed pump does the work - or - 2. add a second pump that is driven by a small solar panel. This second option can be a simple setup since it would need no battery for solar power storage since there is only pump action needed when the sun shines anyway.

[Aldeas]

I wonder if it would be possible to mount the control circuit "eye" of a regular sun tracker in a shaded box on top of your collector, but keep the actual control motors on the reflective mirrors? Then your collector would "track" the reflection of the sun from the mirrors, keeping them turned toward your collector at all times. Just a thought, possibly a little less dizzying than the heliostat.

[redrok]

This is what I call an "In-Line" heliostat tracker.

There are other places you can place the sensors for In-Line trackers.

Very close to the mirror is the best place. It looks at the sun back through the mirror and attempts to keep the sun in the center of the mirror. When working it is stable.

However, there is another stable position. This is when the suns image is anyplace on the edge of the mirror.

There is another tracker type called the "Receiver Axis", or "Target Axis" tracking mount. It's a bit more complicated to describe. See:
http://www.redrok.com/heliolighting.htm#receiveraxis
The main advantage is it can't get confused because the tracker is always aimed directly at the sun. It is mounted on a small mechanical gear mechanism.

Duane

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[Davetech]

That sounds like a good approach, but I don't know how to re-aquire the target if clouds blocked it for a while. With more circuitry, I could stop it from searching while the cloud was blocking the sun, but then I'd have to re-aquire. I think I'm going to have to learn to use a Picaxe micro-controller. Ordered one last night and have started scaling the learning curve. Thanks for the idea!

[Davetech]

Update: I've found that the Picaxe is pretty easy to work with and is Fun! Now I'm leaning towards using your idea, by putting 4 sensors around the collector box; one above, one below, one to the right and one to the left. As the sun's reflection drifts off the box, a sensor will detect it and signal the microprocessor to run either the x axis or the z axis motor to un-light the sensor. Waiting for parts to arrive and working on permanent mirror mounts now.

[jimalee37]

Dave i really like the idea it seems easy enough for even me to try , but i am confused on how to plumb all of this together with the water heater. could you help with that. thanks jimmy

[Davetech]

Sure, just shoot me an instructable email and tell me about your heater system and I'll try to help you.

[Pindell]

My water heater is in the basement. I guess I'd have to keep the initial storage container (garbage can in this case) on the main floor in order to use thermal suction and gravity feed from near the top of the initial storage container to my electric water heater in the basement somehow, right? Or is there another way that you'd propose (preferably without using electric pumps, etc.)?

[Davetech]

Well, with that setup, the warmest water in your basement heater would convect up to your holding tank. Not what you want. Just remember that hot water rises. You would have to overcome the convection with some kind of pump. To take advantage of convection, your regular heater needs to be the highest item in the system, with the solar collector box at the lowest point and the holding tank in between the two.

[Davetech]

-Unless - your water heater in the basement was tankless. That would simplify things. The collector box could convect up to the holding tank on your first floor and just the water flow when you turned on a faucet would send the warm water from the holding tank down to your tankless heater in the basement.

[Davetech]

I have added that point to the instructable.