Serpentine Solar Water Heating (aidg) Print E-mail
Saturday, 07 January 2012 15:01

aidg-2The serpentine solar water heater was part of the original repertoire of technologies that AIDG established with XelaTeco in 2005.  It is a heating system for domestic hot water, in Guatemala the main application is for showers or baths.  There are two main parts that make up the heater: the collectors, where the water is heated by solar radiation, and the tank, where the hot water is stored.  The collectors are galvanized iron pipe bent into a serpentine shape, mounted on a sheet metal backing to absorb radiation, all beneath a glass cover to keep the radiation that enters from bouncing back into the atmosphere.


In a solar water heater such as this one with the tank and collectors separate, the water must constantly circulate between the tank and the collectors.  In many systems, this is achieved by means of a pump.  In the serpentine system, this is achieved by means of a thermosiphon – a natural convection current in which cold water goes down and hot water goes up.  The tank is placed above the collectors, and so the colder water at the bottom of the tank drains down the pipe to the bottom of the collectors.  As it enters the collectors it begins to heat up, and as it continues to heat up it begins to climb through the collectors, eventually returning to the tank and beginning the cycle again.

The Good

Most Guatemalan households heat their bathwater with either firewood or instant electric heaters.  The instant electric heaters are water heaters that are part of the shower head, and heat the water on demand as it passes.  They are powerful pieces of equipment, usually consuming between 4 and 6 kilowatts of electricity.  Use of these heaters represents a large proportion of the monthly electricity bill for the majority of people.

Solar water heating represents a viable option for people to save money over the long run by avoiding use of these electric heaters.  One alternative available in the Guatemalan market is a vacuum tube collector, a high performance solar heater using evacuated tube collectors.  The serpentine, while it heats less than the vacuum tube heater, is projected at about half the cost.  This puts the serpentine heater within the reach of a larger number of Guatemalans, giving them the opportunity to save money on their electricity bill and reduce electricity consumption from the largely petroleum powered Guatemalan electric grid.

The Bad

Although the serpentine heater is cheaper than the competition, without a creative financing plan it is still out of the financial reach of many Guatemalans.

The serpentine heater is currently not able to heat a large amount of water to bathing temperature on the cloudiest days.  This means that there is still a need to rely on electric heating at times, something that  many people surveyed have been reluctant to accept.  The vacuum tube heaters, on the other hand, are able to heat water even on the most cloudy days.

Installation of the serpentine heater is also somewhat complicated, especially on the sheet metal roofs that are common in Guatemala.  The roofs usually need to be reinforced to support the weight of the heater, and the placement of the tank in relation to the collectors means that the reinforcing needed is quite extensive.

The Bottom Line

In spite of its relatively high initial investment, the serpentine solar water heater is an attractive purchase for those who have the money to spend up front.  For a consumer outside of the Quetzaltenango municipal limits (where electricity is heavily subsidized), the serpentine heater could pay for itself in electricity savings in 3-4 years.  The minimum lifespan we have projected for the systems is 10 years, though it could be much longer.  This means that a user of the serpentine system is essentially guaranteed 6-7+ years of free hot water.

Current Model Base Specs:


Capacity: Approximately 120 liters or 30 gallons

Solar Collector Area: 3.4 square meters for a 3 collector system

Tank dimensions 890 high x 580mm diameter

Insulation material/thickness Terma-Flex insulation, a 1/4” thick insulation available in Guatemalan refrigeration shops

Cost (at time of writing) Q3350 – includes manufacturing labor and installation accessories, but not installation labor

Product Development History:

The serpentine water heater now has gone through 3 major revisions and the hands of six interns.

The first heaters that were constructed were based on a design found in the BACIBO foundation's ZigZag Collector Manual (for the complete reference, see our serpentine construction manual).  It quickly became apparent that although overall the design was solid, there were several drawbacks that would make it's application to the Guatemalan market difficult.  For one, the price was too high for the average Guatemalan consumer.  Second, the heating capacity was too low to make for an attractive payback period in replacement of the traditional water heaters.  Third, the casing did not stand up well to the tropical weather conditions present in Guatemala.

The first interns to work on further development of the design focused on improving the efficiency of the collector – improving how much water could be heated how fast with a given collector size.  There were limited results in this area.  The first modification to be tried was replacing the galvanized iron pipe with a copper pipe.  It was thought that the higher conductivity of copper and the relatively thin wall of the pipe would improve heat transfer from the sheet metal backing.  However, side by side testing showed only a 1-2 degree Celsius difference in water heating between copper and galvanized iron, certainly not enough to justify the additional investment required in using copper.

The second major modification attempted was the construction of a ladder style collector to replace the serpentine.  In this collector the same box and backing was used, but instead of a serpentine pipe carrying the water, two horizontal large diameter copper pipes -one at the bottom and one at the top- were connected with about 20 vertical smaller diameter pipes.  The objective was to create a greater heat transfer area for the water to pass through, thus possibly reducing the number of collectors necessary.  However, it was found that the fabrication was so labor intensive and difficult (leaks were abundant in the first prototype made) with the simple shop equipment that was used  that the potential benefits were offset by the added cost.

Several other modifications were tried, including different methods of attaching the pipe to the collector backing and different paint coatings, but none reached the goals of improving performance while maintaining  cost or else maintaining performance while dropping cost.


Tank design from the original Zigzag 35 gallon plastic barrel with inlet, used as inner tank


Collector Manual in AIDG redesign.

Better results were had in improving the tank design. Because hot water tanks are uncommon and expensive in Guatemala, the first tanks were made by tin soldering an inner and outer tank from galvanized sheet metal and filling the space between with fiberglass insulation.  This was a labor intensive process, and it was difficult to guarantee the seal of the inner tank.  Several different configurations with pre-made tanks and different insulations were tried, including an uninsulated 55 gallon drum (rusted, causing leaks, also didn't keep heat well), before settling on the current configuration with a plastic 35 gallon barrel as the inner tank and a 55 gallon drum as the outer tank.

Failure of the wooden frame called for in the original design in the collectors installed in Comunidad Nueva Alianza led to a successful redesign of the casing.  Even though the casing was painted, the extreme weather conditions common in the community led it to rot away, and the collectors completely fell apart when they were attempted to be moved during remodeling of the community's eco-hotel.  The casing is now made of galvanized sheet metal channeling, which is both lighter and more durable, at almost the same cost.


Wooden collector casing reproduced from the original Zigzag


Collector Manual


Collectors with frame /casing redesigned by AIDG: galvanized metal channel.

The latest work has been done by Ben Dana, who in the course of documenting the previous work has made further improvements to the tank and to the level control system.  The entire current system, including 3 collectors and a tank (enough to provide 3 hot showers daily) costs 3,359 Quetzales (around $400) to make.

In 2010 there were 5 serpentine water heaters installed by AIDG in Guatemala.  Two of these were installed for testing purposes, and two were early outreach projects.  One of these, at a community day care center in a community near to Xela, has been functioning well since 2007.  The technology has yet to enter serious commercialization, as some of the technical issues prevented it from being promoted early on by the business Xelateco, and their focus has since narrowed and switched to other technologies.  We do believe, however, that it has potential as a lower cost alternative to the vacuum tube solar heaters. In-depth market analysis would verify this.

There are four documents we are presenting in relation to this technology.  There is a design manual, in English and en Español.  The design manual outlines the different options that were considered; their merits in relation to the current design; a detailed presentation of the design, and suggestions for future work. There is also a fabrication manual, also in English and en Español. The fabrication manual is a step by step guide to manufacturing serpentine collectors and the tank.

Some of the work that could be done to continue improving upon the technology, for those of you that would like to give it a try:

  • Experimentation with the collector coating.  Both commercially available selective absorption coating and other types of simple black paint have been considered by AIDG.
  • An integrated tank and collector mounting system
  • Improved tank insulation

For further suggestions take a look at the design document.  Suggestions are also welcome from readers!

About AIDG:

One in 3 of us, roughly 2 billion people don't have basic services such as electricity, sanitation and clean drinking water. Access to these services is essential to breaking the cycle of poverty in developing countries. 
The Appropriate Infrastructure Development Group (AIDG) helps individuals and communities get affordable and environmentally sound access to electricity, sanitation and clean water. Its main focus is business incubation with entrepreneurs in the Appropriate Technology sector. This is its strategy to help people get technology that will better their health and improve their lives. However from 2005-2010 AIDG managed a significant Research and Development (R&D) programme on Appropriate Technology, in order to develop products for the market. 
These publications were produced by AIDG's Technology R&D programme. There are  Design documents and Fabrication manuals relating to the different technologies.The authors would like these technical resources to empower people everywhere to use the designs. The Design Manuals provide detailed specifications and assess the technological merits in comparison to similar designs. The Fabrication Manuals are practical guides on how to make the technology. The Technical R&D, photography and documentation took place in Guatemala; with the technologies having tested and put to use mainly in communities there.
These Manuals are available at AIDG tech publications. However, since its focus has shifted, AIDG has not had the resources to devote to disseminating the information. As a result the Burmese Environmental Working Group has agreed to host the documents as well. Ben Dana wrote the guides with editing by Steve Crowe. For further info, feedback on the documents, or to pass on further design developments on these designs, please email