In search of an all plastic solar hot water system for domestic hot water

"As Providence would have it", in a recent ichat with Andy Posner, between Essen, Germany and Providence, Rhode Island, talking about the potential for building a solar hot water system entirely out of plastic to grace the roofs of the Zabaleen plastic recyclers in the slums of Cairo, we stumbled upon this diagram from a paper by J. Razavi^a, M. R. Riazi^{, , b} and M. Mahmoodi^a entitled "Rate of heat transfer in polypropylene tubes in solar water heaters" (Solar Energy Volume 74, Issue 6, June 2003, Pages 441-445). Razavi and Mahmoodi are doing their research in Tehran, Iran, while Mahmoodi collaborates from Kuwait, and they are proving that while the world is villifying Iranian energy scientists for pursuing nuclear power and Kuwaiti scientists for a monotheistic devotion to oil, some of the most innovative and hopeful work in renewable energy is being done in these two unlikely nations.

We at Solar CITIES want to applaud Razavi, Riazi and Mahmoodi, and thank them for providing yet another clue as to how we can improve our Zabaleen plastic systems.

We already proved we could use recycled 200 liter plastic surfactant barrels for both the cold water and hot water storage tanks in our systems, locally made Zahran plastic seals and fittings for tank connections and polypropylene pipes and fittings for all our plumbing work. And this past fall we proved we could make the boxes that house the absorber plates and copper pipes in our collectors out of recycled plastic shopping bags -- the same bags you see littering the streets of Cairo, jokingly called "the national bird of Egypt" because there are more of them flying around the few nature reserves than actual wild-life.

We found that the Egyptian Plastics Recycling company "PrimaPlast" purchases the bags that the Zabaleen gather, clean and shred, and melt them into panels 3 meters by 1.5 meters by 1 cm.




We purchase the panels directly from deputy chairman Rafik Nasralla at the factory in Madinat El-Badr, cut them up and make them into boxes and now we no longer have to use aluminum sheet (energy intensive and wasteful) or wood (in scarce supply in a desert country) for our boxes.

The next hurdle was to be "how do we replace the copper in the systems"? Copper is the most expensive part of our hand-made solar hot water systems, accounting for roughly 90 dollars per panel (180 dollars for a two panel system).

We were aware that solar pool heating systems use unglazed polypropylene, but thought that the polypropylene wouldn't perform well under glass and/or might not put out the kind of temperatures we need for DHW (domestic hot water). But we hadn't tried it, preferring to wait until the community accepted our systems before trying out another dubious innovation.

Gene Lin, our infrastructure engineering colleague at US AID who helped us get the $25,000 grant we are using to build over 30 solar roofs in the slums of Cairo, suggested we use steel pipe instead of copper, and we have had plans to build such a system. Now, thanks to Razavi, Riazi and Mahmood's work, we may be able to leapfrog that decision and go right to polypropylene for the heating collector system itself.

The authors say, " [Riazi and Razavi, 1997] compared the performance of polypropylene tubes with steel tubes through a set of experiments. It was found that polypropylene tubes may increase water temperature by 10 °C more than steel tubes, and as a result it was recommended that use of polypropylene tubes in solar water heating systems is preferable over steel tubes."

Their data is displayed as follows (Tin is the city water temp flowing in, To is the temperature going to the hot water tank, V is the velocity or flow rate in millliters per second)

Table 1. Set of experimental data

Note that at a flow rate of 15 milliliters per second we get an average output temp of 44.4 degrees (a nice temperature for a hot bath), definitely enough to fill a 10 liter bathing bucket in a little over 10 minutes. This means that over 5 hours of good sun, or 300 minutes, you could easily thermosiphon fill our whole 200 liter tank and then some!

Note also that, according to their diagram and calculations we eliminate the need for aluminum or tin absorber plates (in our current design, based on the solar collectors we observed at the Wadi Foods factory in Noubariya, we use 7 16 mm copper pipes in a grid 80 cm x 180 cm and fill the spaces between them with aluminum strips made into absorber fins that grab the copper pipes. This costs nearly 16 dollars per unit and takes time and labor to bend and tie the fins onto the pipes. In the Iranian/Kuwaiti system they use a densely packed grid of 36 plastic pipes, obviating the need for aluminum altogether:

"n is the number of tubes, ℓ and d are the length and inside diameter of tubes. In our calculations n=36, d=1.9 cm and ℓ=200 cm (A_i=42,955 cm²)."

This not only saves money for aluminum, but does away with the need for the cost and mess of black paint, and the potential for galvanic corrosion wherever the aluminum and copper touch and there is no paint separating them. It also eliminates the problem of getting good heat transfer by trying to get the absorber fins to touch the copper.

All in all the Razavi, Riazi and Mahmood experiments hold great promise toward reaching our holy grail of an all plastic solar hot water system.

If their data holds in our field tests this January we will have taken yet another step closer to bringing do-it-yourself renewable energy systems to the urban poor!

(Final note: there is a flaw in the diagram from Razavi et al's paper reprinted above: Please note that the arrows for numbers 1 and 2, main water supply and hot water outlet for use, are reversed; the main water supply arrow should be going INTO the tank, and the hot water OUTLET should be flowing out of the tank. Furthermore, according to our experiments and experience in Palestine, #3, the hot water inlet from the collector to tank, should ideally not be .28 m from the top of the tank, as shown in the diagram, but approximately .28 m from the bottom of the tank, just above #4, the cold water outlet from the tank to the collector. This, we have found, gives maximum performance in the winter months.)