by Mike Hatfield
In most circles, when people talk about moving people up the energy ladder they are speaking of moving away from simpler traditional forms of energy, such as wood, to more concentrated forms such as natural gas. At Aprovecho we like to turn things around a bit and throw in a bit of science along the way. As you may know, Aprovecho has taken part in the global movement of helping to develop improved cook stoves for the 2.6 billion people who presently cook with biomass on traditional stoves and have to endure the many detrimental side effects of that cooking method. While many people talk of moving our 6 billion and growing population up the energy ladder, we are looking at alternatives for saving fuel and growing local forests that could allow people to indefinitely provide and have control over their own energy needs. As I write this I am presently in Sao Tome, a small island off the west coast of Africa where I am working on a project for the United Nations World Food Program. I am designing a stove to be built at schools and orphanages that will cook for 250-1000 children a day. This is one of the most exciting types of projects we work with, as the savings for each stove built are so great when compared to how many people they are serving. I am digressing a bit and you will have to wait for a future article to hear about how this trip turned out (shameless pitch to become a member and supporter of our organization). What I wanted to talk about was our own move down the energy ladder as we put solar water heating panels on our straw bale dorm and shut off our large propane tank.
At the Aprovecho Campus up to 15 students per semester come to study appropriate technology, organic gardening, and sustainable forestry (and yes, a shameless pitch for our educational program at www.aprovech.net). About 5 years ago we mostly cooked on propane in our dormitory. In my appropriate technology class I would usually suggest that people attempt to cook entirely on wood, using our outdoor kitchen and solar ovens in the summer and our cooking/heating stove in the winter. About 50% of the time it would succeed. Having the ease of just turning a knob instead of having to start a fire (as most people in the world do) often got the better of our students as the semester got busier. Then finally came the group of students that
challenged us to pull out the safety net of a propane stove. We removed the stove and have never looked back. As our students now study the technologies we are developing for third world situations, they are also gaining the day-to-day experience and motivation to design and build to the best of their abilities. Along with cooking on sun and wood I have also worked with the students to design and build low tech solutions to providing for our own hot water needs with solar and wood fired technologies. We have had now for some years the means for every student to use only wood or sun to heat their water for cooking and cleaning. Once again, having the convenience of an indoor shower in the strawbale dorm, powered by propane, was often too convenient to override the desire to fire up a stove or even to go outdoors to the simple solar shower we have built.
We have now installed a more conventional solar hot water heating system on the strawbale dorm. With the help of a grant from the Lane Workforce Partnership, South Lane School District, and the federal government, we were able to install a two panel system that should supply the majority of our hot water needs. Being Aprovecho, we wanted to take it a step further and make this an ongoing experiment to help people decide between some growing options for solar hot water heating.
With the help of Newt Loken and his company Solar Assist, based out of Eugene, we decided to install the two most popular types of panels on the market and set them up in a way that we are able to compare the difference in efficiency between the two in a variety of light conditions.
Flat Plate Collector vs. Evacuated Tube Collector:
A flat plate collector consists of a thin absorber sheet of thermally stable polymers, aluminum, and steel to which a black or photo absorbing coating is applied. Behind this is a grid or coil of fluid tubing, usually made of copper that is firmly attached to the absorber plate and treated with the same heat absorbing coating. This is placed on top of a bed of insulation within a weatherproof shallow box with plastic or glass glazing.
Fluid is circulated, using either the natural convection of a thermal siphon or an electric pump, through the tubing to remove the heat from the absorber and to transport it to an insulated water tank. This fluid can either be the same water that is used in the household (called a open loop) or a separate freeze protected liquid, such as propylene glycol, (called a closed loop) that will not freeze but must be run through a radiator in the water tank to transfer heat to the household water. If the open loop system is used, then an extra device for draining the panel in case of freeze (a mechanical temperature controlled drain back system or a very attentive person) must be installed. In the end it is a 6 of one ½ dozen of the other situation, and we have chosen to go with the closed loop system under advice of our solar installer.
I have read of an alternative to metal tube collectors, new polymer flat plate collectors that are now being produced in Europe. These have freeze-tolerant water channels made of silicone rubber instead of metal. By dispensing with a heat exchanger in these flat plate panels, temperatures need not be quite so high for the circulation system to be switched on. These direct circulation panels, whether polymer or otherwise, can be somewhat more efficient, particularly at low light levels. As of now, metal flat plate panels are the standard in the US and so these are what we used in our system.
Evacuated tube collectors are made of a series of modular tubes, mounted in parallel, whose number can be added to or reduced as hot water delivery needs change. The type of tube we are installing consists of an inner copper tube with a flat metal absorber plate that is about 2 inches wide attached to it. This sits inside a glass tube that is surrounded by a vacuum that is in turn surrounded by another tube of glass. The sunlight comes in, is turned into heat via the absorber plate, and is trapped (insulted) by the vacuum space. For a given absorber area, evacuated tubes are believed to maintain their efficiency over a wide range of ambient temperatures and heating requirements. The absorber area only occupied about 50% of the collector panel on early designs (as shown in diagram below), however this has changed as the technology has advanced to maximize the absorption area.
Both designs have made claims of better efficiency in different environments. It is our hope that with a controlled experiment with both types of collectors in an identical side-by-side configuration, we can add to the growing scientific backing or non-backing for these claims. I hope you will continue to follow our organization (and why not support us while you are at it) and read about the result of this and other experiments we are working on in future publications. For now from the mixed lingual Sao Tome Island…..ciao!!