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Turning waste into heat

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2013 Phys. Educ. 48 555

(http://iopscience.iop.org/0031-9120/48/5/F01)

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555P h y s i c s E d u c at i o nSeptember 2013

As part of the Peterborough Powerdown fortnight, students were asked to design a simple solar heater made from waste materials. The scenario was to provide hot water for an African village using only a waste heap; a picture of children rummaging on a tip was the backdrop to the event. In this case the waste materials were straws, rubber tubing and a used orange-squash container. In this article I describe how a simple solar heater can be con-structed from the above apparatus and used effec-tively to show the ideas behind solar heating.

The construction is based on the simple solar water heater I described in a previous article [1]. Black drinking straws, preferably matt black party straws with an articulated joint, were joined together using 50 mm segments of rubber tubing. The students were given 25 black straws, as well as a number of different coloured straws of the same diameter as distractors.

By joining the straws into a continuous pipe, a simple solar heating receptor can be constructed (see figure 2). There are a number of variables here that students can explore; the number of straws, the packing of the straws, the colour of the straws, the colour of the rubber tubing (some students coloured the tubing black), and the area of the straw receiver. Students also had access to waste materials such as different coloured card. The best teams used these as backing. We could have added an acetate sheet, but we were worried about these melting under the lamps used as the heat source.

Keeping the theme of an African village, which would not have access to a pump to circulate the water, the source of the water was gravity fed from the squash bottle. This was held in a clamp stand for convenience, and the rate of flow (another variable) was controlled by a number of clothes pegs clamped onto a small section of rubber tubing (see figure 1). In the actual event, the students were also given tub-ing clips to totally cut off the water supply. Using a small pump, such as that supplied by Mindsets [2], would allow a circulating system to be designed.

As mentioned, heat lamps were used to mimic the Sun. These used inexpensive 150 W reflector bulbs placed about 30 cm from the solar heaters,

Turning waste into heatS o l a r h e a t i n g

Figure 1. The assembled apparatus. The clothes peg adjusts the flow rate of the water.

Figure 2. The heater array. The straws are held down using masking tape.

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556 P h y s i c s E d u c at i o n September 2013

and proved very effective; on our first trial run we obtained water at 65 °C, which melted the straws. This has been tried using the Sun and is just as effective, though a larger heater array is required. Students were allowed 5 min heating with water in the straws before allowing the water to flow; during this period the water had to flow continuously. Most students achieved temperature changes of 10 °C or above, the limiting factor being the rate of water flow and the time under the lamp, which was deliberately cut short so the water did not become too hot.

This is a very cheap way in which to investigate solar heaters, the main cost being that of the straws. The lamps were a one-off purchase as there was insufficient sunlight in November. We allowed the students 2 h plus a working lunch to design, build and test the water heater. Each group had to provide 400 ml of hot water over a 5 min period, the hottest

water being the winner. The exercise provides stu-dents with a number of challenging issues as well as a chance to design, test and modify their ideas. It does need the equipment (the rubber tube diameter and straws) to be chosen carefully, but it can be very rewarding for the students as we found out on the day. The downside? Do be prepared to have mops on hand, it can get wet.

References[1] Kinchin J 2008 Turn scrap into a solar water

heater Phys. Educ. 43 352[2] www.mindsetsonline.co.uk/product_info.

php?products_id=1031 (BLC036)

John Kinchin and Dorothy Ball The King’s School, Peterborough and Jenna Hiley Peterborough City Council