"Why didn't I think of that?" you might ask yourself.
Actually you probably did!
And no doubt you've had many many different thoughts about how to innovate our way out of this mess of climate change, pollution, poverty, injustice and discomfort. So many great green ideas are floating around these days, and lots of them will probably work and work well.
The real question we often ask ourselves at Solar CITIES when we have such ideas is "why didn't we implement it?"
And when we reach that point, that half-way place between vision and reality, particularly when it comes to simple, obvious ideas, we feel a sudden urge to get empirical, and DO something.
Photo: A traditional ARTI India biogas digester wrapped in polyethylene pipe as a heat exchanger connected to the solar hot water system. Once insulated, this should keep the digester warm in the winter months.
This month, in Cairo, we stuck ourselves with the task of creating a cheap solar hot water system to improve our biogas digester's performance in the winter months (the mesophilic bacteria we use don't like temperatures below 20 C and all but shut down at 15 C).
At home in Germany we built a solar hot water system for our biogas digester out of an old steel radiator (painted black in a wooden box covered with glass) but these radiators, ubiquitous in Europe, don't exist in Egypt. And the normal Solar CITIES solar hot water systems we build in Cairo cost a lot because of the huge expense and difficulty of copper pipes (both the raw material and the welding). The marginal benefits of laying out a few thousand Egyptian pounds to heat the bacteria so they would increase their output seemed to outweigh the costs and impose an acceptance barrier for most Egyptian families who can't even afford solar hot water for their own bathing.
Was there a way to radically reduce the costs without sacrificing performance?
In California we are very familiar with black plastic heat-exchangers used for solar heating swimming pools; in the summer everything black gets hot, and water flowing through black plastic pipes is no exception. But these solar pool heaters are specialty products, factory molded to provide the maximum surface area, are far too expensive for the average Egyptian, and in any event do not work in the winter when exposure to the cold air and wind quickly removes all the heat. What we needed was something that would heat up quickly during the short winter days when the sun was out and transfer that heat to the water in an insulated tank to then flow into the heat exchanger around the biogas system. And it had to be cheap.
Some great papers by Iranian and Kuwaiti researchers in Tehran suggested that black polypropylene did in fact have good heat transfer properties and would be suitable for solar hot water systems, but we couldn't find a description of how to make it, and we found little published on the use of polyethylene. (See "An experimental evaluation of copper, steel and polypropylene tubes in solar water heaters with thermosyphonic flow" by M. R. Riazi1 , J. Razavi2, A. Sadeghi2 and A. Javaheri in Applied Solar Energy Volume 45, Number 1 / March, 2009). In any case, in Cairo we couldn't find suitable thin walled polypropylene. But we could find polyethylene drip irrigation pipe everywhere.
Bolstered by the graphs and data in Riazi et al., our decision was thus to try to simply get out and do something -- to go into the field and build a collector by hand, replacing the copper pipes in our normal Solar CITIES solar hot water system with the cheapest black polyethylene irrigation tubes we could find but otherwise keep the design exactly the same as we have always done things. This way, if the experiment failed, we could always salvage all the other parts (the galvanized box, the aluminum heat absorber, the glass, the polypropylene plumbing and fitting and the recycled plastic water tanks and float valves) and throw in the copper pipes and still have a solar hot water system (albeit a more expensive one).
This is part of the nature of the Solar CITIES ethos: when experimenting, try to make everything modular and re-usable (re-purposable) so that if one idea fails the net loss is low. Since we are very poor relative to Western Standards (yet relatively wealthy by the standards of the Egyptain poor) we are learning to understand how those less wealthy in this world think while having the flexibility to do experimentation.
Photo: Instead of being imprisoned by the old way of thinking, we've now learned that we can radically cut costs by replacing the expense and labor of using welded copper with plastic irrigation hose.
Photo: The Culhane's cousin Heni, visiting from Germany, shows how lightweight the new plastic heat exchanger is.
What we learned this month in Cairo is that a solar collector made from black polyethylene irrigation pipe inside the glass topped box instead of copper works very well indeed, producing hot water over 45 degrees Celsius even on cold (but bright sunny) days.
Photo: Comparison of a copper heat exchanger with a same sized polyethylene heat exchanger. The one on the left costs about 500 LE, the one on the right about 30 LE.
Photo: Two people can assemble the heat exchanger in less than an hour with scissors and screw drivers. No welding required!
Even better, such a system can be made by children, since it involves no welding. Instead the heat exchanger "shabaka" (matrix) can be made with scissors and a screw driver and hose clamps and plastic T's. This way, the students at the Sekem Environmental Science Center (with whom we are partnering) can learn how to build fully functional solar hot water systems that they can take home to their families, and, in the future, if they want to learn welding and go to the extra expense of buying copper, can simply replace the plastic shabaka with a copper one for improved performance and durability. In this way school kids can learn real skills without having to always build "toys" or "little models" that have no immediate practical value.
It is true that the plastic heat exchanger is relatively delicate and is prone to springing tiny leaks if not constructed properly (i.e. if the hose clamps aren't tightened properly and one isn't careful about putting stress on it during assembly). But the total cost of the plastic heat exchanger is merely 30 LE for each box, as opposed to 500 to 600 LE for the copper (425 for the copper itself, about 100 or more for the drilling and welding, to say nothing of the difficulty), and broken plastic T's can be fairly easily replaced. In any event, if it does leak a little it has no effect on the collector -- plastic doesn't rust or create a galvanic response that would degrade the aluminum absorber so no permanent damage is done while waiting for a repair. And when using the panel to heat a biogas digester, slight water dripping and lowered performance isn't a disaster.
The neat thing is that we've found this low-cost do-it-yourself solar heater works just fine for domestic water heating too -- a good entry level solar hot water system for those who want to get into the game and understand how to build and use solar energy infrastructure.
Photo: Solar CITIES Egypt Director Hanna Fathy and Solar CITIES co-founder T.H. Culhane stand in front of their irrigation-pipe solar heater and solar heated cold-season biogas digester system.
Photo: Fathy and Culhane assemble a traditional copper heat-exchanger next to the polyethylene system for comparison and performance testing. The goal is to see if we can abandon using copper altogether and still get good efficiencies during the winter months with a system that uses plastic pipes and plastic storage tanks.