Workshops

3.1. Education. We started by broadcasting a video of the local television named ‘Floresta en peligro’. In this documentary it is explained and showed all the threats for the forest of São Tomé, being the use of wood and charcoal for cooking one of the most important ones. Then I explained what a ‘mass balance’ and ‘energy balance’ are, what enters, what goes out, what reacts… This enabled me to go a bit further into sustainability and what this term means. I used examples as places or people to sustain my hypothesis. We talked a bit of some sources of energy and why some are going or may go to depletion. I explained some sources of renewable energies.

In terms of the workshops I briefly explained some of the concepts required to understand how the systems work; types of heat transmission (radiation, convection…), types of solar cookers using direct radiation (concentration of solar rays) or convection (ovens), fermentation and methane production with bacteria, short discussion of the law of ideal gasses (for building a barometer) and hydraulics (for building systems that contain liquid and gases and use this principles to pump one or the other). All of these concepts were explained in a way they can use them in their normal life, with accessible language, avoiding if possible technical words, playing games and putting them in real situations. The concepts of biogas production through bacteria-assisted-digestion were better understood from the basis of fermentation and wine production, which almost all of them knew. 

As for the general material requirements: screw-driver, hacksaw, rule(r), pens, drill, silicone-holder (syringe type) and hummer.

3.2. Solar energy. The systems that use solar radiation for heating-cooking can be divided attending to the main way they transfer the energy from the sun into the element (black-metallic-tube or -pan in, where we put the food or water): radiation, also called solar concentrators or panels; and radiation-convection, also called solar ovens. The materials required for the workshops are accessible in almost every country, proven that we were able find them here. Most part of the workshops do not require especial abilities except some carpentry, welding and mathematics (to design correctly) among others, in each case we will explain such requirements.

These workshops required little explanation for understanding the scientific principles in which they are based. As long as most people have experienced the difference from a sunny to a cloudy day, and how an oven works, it is straight forward to start designing and building. There is an exception nonetheless, which is the principle under the solar heater works: the water is warmed in the oven and then changes its density, therefore it is forced to flow upwards so that cold water (with lower density) flow downwards.

For solar cooking workshops it is required to have a black pan or one that can be painted in black.

3.2.1. Solar panel 1, the first design of a collector was based on one design proposed in the literature named CooKit (http://solarcooking.wikia.com/wiki/CooKit). The unit was easy to construct and use so as a starting point presented a number of benefits. 

Materials: cardboard (130x100cm), aluminium foil, glue, cutter and metallic-wire. 

Price: 120,000 dobras (€5.30).

Skills: none.

Procedure: We first drew the lines (described in the reference previously cited) on one side of the cardboard. On the other side of the cardboard we glued the aluminium foil to the surface taking care that the surface is as flat as possible. Using the cutter and the ruler we cut the perimeter of the collector. For the folding parts we did a small incision in the first layer of the carton (on the side without aluminium foil). The metallic wire is twisted to give cylindrical shape with a diameter enough to sustain a pan at 5-10 cm from the bottom. 

3.2.2. Solar panel 2, the second design was a parabolic collector of 1.6 m of diameter. It was important to emphasise with the students that the one we built is different from the antennas since the focus laid inside of the parabola. As a result the user needed to make much less rearmaments with the change in solar position (or shall I say the earth position?).

Materials: metallic-frame, cardboard (300x100cm), aluminium foil or panels (10 panels of 70x50cm), glue (pattex), ruler, cutter, metallic-wire and stapler-staples (used for carpentry).

Price: 1,000,000 dobras (€44.40)

Skills: As we aimed a robust and relatively big panel, it was hence required the abilities to wield, twist and cut metal. 

Procedure: A blacksmith built the metallic frame of the panel with the support for the pan. This structure is shown in Fig. 1. The outline of the panel was calculated using the following expression (Eq. (1)):

x² =4·a·y

where x and y are the horizontal and vertical position respectively while a is the focal length (a= 40 cm).

Once received the structure, we installed the cardboard with triangular shapes along the panel, making small perforations and using metallic-wires to fix them to the frame. The dimensions of the aluminium panels available were smaller than the cardboard, so that the triangular shapes of aluminium were smaller. We fixed the panels to the cardboard using first glue (pattex) and then the stapler. 

     Fig. 1. Scheme of the solar panel 2.

3.2.3. Solar oven. The oven was specifically designed for cooking coconut biscuits. The shape and size of such wafers yielded the final dimensions of the oven, as described in Fig. 2. 

Materials: cardboard (300x150cm or two boxes, one a bit smaller than the other), glass (dimensions in Fig. 2) aluminium foil, glue, ruler, metallic-panel painted in black, cutter, insulating material (hollow cardboard or some short of plastic), small rope (1.5 m) adhesive tape and silicone.

Price: 200,000 dobras (€8.90)

Skills: none

Procedure: I started by using a video of youtube to explain the procedure better to the students (http://www.youtube.com/watch?v=96qPRgmfbXc). 

We constructed the inner box, covering all its surfaces with aluminium foil, using glue. In the bottom we installed four bases of insulating material. Then we assembled the outer box making all of its faces except the bottom and the top of three layers; cardboard, insulating material and cardboard. The dimension of the hole of the outer box had to be such that the inner box (plus insulating material) was placed tight. Once placed both, we introduced the metallic-panel (it needn’t to be aluminium) and constructed the top of the box with cardboard, glass and silicone. We also assembled some extra reflectors made with cardboard, aluminium foil and rope on the top. 

     Fig. 2. Scheme of the solar oven.

3.2.4. Solar water-heater. The designed unit was made for heating 50 L of water, that is, for the communitarian showers. 

Materials: plastic container of 50 L, hose (id. 1 cm; length, 4 m), 2 clumps, cupper tube (id. 0.7 cm; length, 2 m), wood (30x2x200 cm), metallic-panel, insulator, studs, silicone, glass, ruler and pen. The dimensions of the metallic-panel and the glass are specified in Fig. 3. 

     Fig. 3. Scheme of the heating element (solar heater).

Price: 1,200,000 dobras (€53.30)

Skills: none.

Procedure: We twisted the copper tube giving it an S-shape, we assembled the wooden box using the studs (some carpenter skills and gadgets were required) and then the isolator plus the metallic-panel. We set the tube inside the box, making two holes for inlet and outlet. We fixed the glass using some extra wooden pieces and studs, shielding all the holes with silicone. 

We perforate three times the plastic container: the lowest one for the inlet of the heating element, the outlet of the heating element (15 cm above) and the outlet of the container (1 cm above the second). By doing so we allowed filling the heating element while avoiding empting it during normal use. 

We connected the container with the heating element with the hose, shielding the connections using silicone (for the container and also for the third hole) and the clamps (for the heating element).

3.3. Biogas. The principles of biogas production from manure are based on anaerobic digestion of bacteria. Although it has been investigated in detail previously we focused in the classes on explaining to the students the very concepts required to understand how it works. These concepts, as it was previously explained, were better understood talking about fermentation and human digestion. It is recommended for the introductory classes to put manure and water in a bottle (PET, 1.5 L) with a balloon on top prior the class (4 days in a tropical weather). This allows the teacher to show the gas expansion during the digestion of manure.

The material used was accessible with the sole exception of the tubular polyethylene used for the biggest digester. The abilities for these workshops involve only bit of mathematics to do the calculation of the dimensions of the digester.

3.3.1. Pilot-plant digester. As a first step we tried to prove the concept by a small pilot plant of 50 L, the unit consisted in three sections; digestion chamber, barometer (pressure gauge) and burner. Materials: plastic container of 50 L, hose (id. 1 cm; length, 1.5 m), gas-hose (id. 0.5 cm; length, 1 m), wood (30x2x100 cm), studs and silicone. 

Price: 500,000 dobras (€22.20)

Skills: none.

Procedure: We made two holes on top of the container, introducing en each the hoses specified before and shielding the connections with silicone. We twisted one of the hoses and gave it a U-shape, fixing it to the wood using the studs and filling it with some water. The other hose was specific for natural-gas connections and it was just plugged to a burner.

3.3.2. Kitchen-plant digester. For a complete description of the materials and procedures it is advised to consult the literature (http://www.bioenergylists.org/files/guia_biodigestor_jmh_final.pdf). The design scheme followed these criteria: We wanted to use 40 kg/day of manure, considering a dilution 1:3-4 in mass we had a total daily loading of 160-200 kg/day. In the literature there is not an agreement of the resident time, so the most conservative value was selected; 20 days for this tropical conditions. The total mass in the plant was calculated multiplying the daily loading by the residence time yielding 3200-4000 kg. Assuming that the volume of the liquid would be 75% of the total digester, the final volume was 4.3-5.3 m³. 

Materials: instruments for digging, tubular polyethylene (id. ~1.9 m; length, 14 m), rubber (recycled tyres), PVC tube (id. 15 cm; length, 2 m), connection male-male for transposing the walls of the digester (PVC), elbow connection (PVC), tee connection (PVC), hose (id. 1.3 cm; length 10 m depending in the distance digester-kitchen), 10 clumps, 1 ball-valve (metallic), 1 bottle (5 L, PET) and 1 burner. No special skills required.

Price: 1,600,000 dobras (€71.10)

Skills: some mathematics to calculate the dimensions of the hole and digester.

Procedure: the hole was made in first place (see Fig. 4), then we assembled the digester (see also Fig. 4); (1) make 2 layers of plastic, (2) make a hole for the exit of gases, (3) put the connection for crossing the walls of the digester plus the elbow and (4) tight the inlet and the outlet of manure-water using the rubber. We placed the digester in the hole and we leak-tested the unit using the exhaust gases of a car. Once leak-tight we introduced the mixture manure-water recollected several days in advance.

     Fig. 4. Scheme of the digester.