Circulating Solar Heated Water

Yes, it will work fine in most environments, problems can occur if the ambient temperature stays below freezing for an extended period though. Thermosiphon solar water heaters have been around for years especially in the tropics.

systems have existed for years that have a tank "below" the collector wired with a differential switch. when heat is available the pump starts and sends water to the collector to capture heat..all of this is wired in series with a high temp limit in the tank. allof this can be plumbed with pvc and the tank insulated with hard foam board. no special welding , wiring or controls are needed

If you search CR4 for DIY Solar water heater, you will probably find some designs done by DELL the Cat and discussion there about how it was fabricated and so on.

Good luck taking the step from "thought experiment" to "practical construction".

Thanks for your comments and links. It appears this may work.

I want a design that can be assembled using a wire welder, some thin guage steel sheet, and some plumbing.

Gavilan

As has already been noted, thermosyphon systems abound in the marketplace.

You suggest using a steel collector but make no mention of how you intend to protect that steel from corrosion.

Most collectors are fabricated from copper pipe with a few plastic ones out there.

One manufacturer that I am aware of uses a steel collector but it is configured as a closed system with the fluid in the collector being a Glycol mix and circulating through a separate jacket surrounding the storage tank.

You also need to consider the possibility of freezing which can rupture the collector.

The jacketed system overcomes this via the Glycol mix, others have a small valve that opens at low temperatures allowing some of the hot water to escape through the collector and so protecting it, still others have a small heating element that comes on at low ambient.

bwilko;

Thank you for your reply.

The outside of the collector plate will be protected by a very thin coat of black paint. When it rusts through from the inside - you recycle it. (Some number of years)

The thermal conductivity of plastic is too low to be an effective thermal transfer medium.

This is to be a simple design, assembled locally, and used in the tropics. Freezing is not an issue.

"Thermo-syphon" was a new term to me. I am not trying to invent anything here - just sharing a possible simple design an islander with a wire welder and some plumbing skills can build for local use; and wanting to avoid the need for an in line circulation pump.

Copper pipe is a great thermal conductor and resists corrosion - but its also VERY expensive. There is no need for the thermal transfer potential of the collector to exceed solar insolation rate.

As long as you're not trying to wash whites in the hot water, as the rust stains will be quite evident.

Plastic may well have a lower thermal conductivity, but as you have said, there is no need to exceed the insolation rate.

Although not for someone who wishes to fabricate with a wire welder, plastic is used for the purpose.

A coil of thin plastic plumbing pipe will handle the pressure, take up little space, be quicker to assemble than the steel, will have no rust problems, and will likely be far cheaper than either the copper or Steel alternatives.

As an aside, putting the collector under low iron glass will greatly enhance its ability by increasing thermal transfer and reducing wind losses.

It's already been noted that steel corrodes, and that copper is the material most likely to be in contact with water (and some collectors use aluminum fins to minimize the high cost of copper). Tankless/passive designs generally incorporate the tank into the collector, otherwise having a small volume of water in the collector risks overheating and self-destruction!

Regarding the value of the hot water provided every year: Try starting out with the current cost of heating water! Here in California (when the furnace is not running and there is no other natural-gas consuming appliance in my house), my natural-gas cost of heating water amounts to about $16-20 per month, for a family of 4. Thus I'm spending about $240 per year. If a solar water heater can save 2/3 of this cost of heating, the most value I can derive from a solar water heater is $160/year! The 2/3 figure depends not just on how much heat is collected, but also how long it can be stored. Also how many cloudy days are there in a given location? Finally, what is the temperature of the incoming water (and consider that it should be heated to >50C or 120 degrees F so the subsequent gas or electric heater does not have to work to hard to heat the water sufficiently to prevent bacteria from living in the tank).

Therefore the theoretical value of hot water that the collector CAN produce does not matter: the present value calculation starts from the current cost of heating! Over a 20 year payback period (not even considering interest rate for a more thorough financial calculation) I might save $3200. Can I install a hot water solar heater for this price? Last time I checked the price tag with installation was $8000.

Let me know if there is a really cheap and effective system out there. In my temperature zone is almost never freezes. Can I buy a passive system for under $3000?

Obviously the cost of electricity and available water heaters is going to be region specific, but just for an example. In 1987 at a cost of AUD1200 (which, at the then prevailing exchange rate, was about USD800) I installed an Australian made Solahart 300 litre jacketed close coupled system on my North facing roof (that's correct for Australia).

In the first year of operation with 2 teenage daughters still living at home, my total cost for water heating was a massive 11 cents.

Even allowing for inflation and maybe poorer weather in ensuing years, it would not be unreasonable to estimate a total cost for water heating over the last 25 years as being considerably less than, but let's say $300

That's an amortisation rate of 4:1. not many household appliances can claim that sort of payback.

The heater is still performing flawlessly, touch wood.

I am in Mumbai, India, and I use passive solar water heater system. This is not copper tube based but "Evacuated glass tube" system. The outer tubes are 57mm diameter and inner tubes are 47mm diameter. There are 13 such 2 meter long tubes to make 1 meter wide collector.The storage tank capacity is 125 litres.

I am using it for two years now is more than sufficient for bathe water for a family of 4.

The advantage of this unit is that it works well also on cloudy days(I doubt copper tube system would work). Since the evacuated tubes directly enter the storage tanks, the thermal gredient is less and hence more efficient(some where I had read that about 40% more efficient).

While the system cost me slightly less than $500, the copper finned tubes system was quoted at less than $600. Evacuated tube system is definitely more efficient and with lower cost.

Some great points Mark ! Natural Gas is a pretty cost effective means of heating water - especially if you are using an instant heat system. Its hard to compete with efficient centralized distribution systems here in the US.

Given your hot water heating costs of $20 per month; passive solar hot water heating may not be a viable alternative for you.

Although energy costs are quite low in California; they run anywhere from 48 cents to $1.08 per KWH on the islands of intended use. This changes the economics somewhat.

High tech evacuated systems may be over designed for tropical use; and certainly are not useful as simple self assembled systems.

Is it possible the $8,000 dollar figure might be whittled down a bit using appropriate technology and a local plumber for installation?

First, what is the pressure in this thin, flat, steel plate? Things might bulge pretty badly. Second, what is the value of the water you'll lose when it blows out from rust? In the tropics, the Israeli-style combined tank and plate thermosiphon system works really well. They're required on all houses, and are reasonable to build. http://www.emaxgreenenergy.com/faq-thermal.html

As Israeli user - these systems work approx. 8 years ,and mostly the boiler is corroded and have to change,while the heating solar panel lasts more than 15 years....but from time to time needs tubes rough cleaning (I've done it manually...)

The tubes are Copper/Aluminum coated !!

The typical price (without erection on site)-approx.;710-810 USD....

If you like approach :"CHROMAGEN" - they many international outlets

Have a good proved system !!

Some great points !

Would the water pressure in the rudimentary water distribution systems used exceed the yield stress of thin steel plate? It's worth doing the arithmetic, which I will do.

Intuition tells me that the yield stress of welded steel plate may be substantially higher than copper - I will check it out.

What material is used as the containment vessel for conventional hot water heaters?

The design is intended to be simple enough for self assembly.

Perhaps once the collector is welded some type of corrosion resistant paint can be circulated through the collector and allowed to dry.

Gavilan