# Parabolic basket and tin can solar cooker

Project Coordinator: Bart Orlando

Student Engineers:Ginger Fletcher-Santillan and Ana Kolpin

## Introduction

The objective of this project is to create a solar cooker out of local invasive species and waste materials. We want to create a device that can pasteurize water and be an alternative to the use of fossil fuels for cooking food. Our criteria for evaluating the appropriateness of technology are: use of local materials, efficiency, durability, ease of use, ease of construction, cost, impact on environment, impact on culture/lifestyle, education or expertise needed for construction.

## Literature Review

### General Research on Parabolic Solar Cookers

Parabolic solar cookers are devices for preparing food that do not require fuel. They concentrate the sun’s rays to one focal point where a dark pot or pan can be used to cook with. There are many advantages and disadvantages to using a solar cooker. Some of the advantages are: they can be used in places where there is little available fuel, they do not pollute, the outside of the parabolic shape does not get hot, they can be made out of recycled and local materials and they do not cost anything to operate. Some disadvantages are: the area to cook on is only big enough for one pot, cooking can only be done on sunny days, cooking time can take longer than when using a conventional stove or oven, the cooker may need to be refocused as the sun shifts. (Working Group on Development Techniques 1990) Since parabolic solar cookers do not require fuel, they are being experimented with in many developing countries around the world as an alternative and supplement to cooking with fire. In India, a study was conducted to assess the utility of parabolic solar cookers in household use. The criteria for the assessment included ease of operation, initial cost, need for training, aesthetics, and overall safety. The overall conclusion of the study was that the parabolic solar cooker has a pretty high utility but in order for them to be commercialized, “The utility on technical, behavioral and commercial aspects should be improved.” (Pohekar, Ramachandran 2006). According to the Solar Cooker Review, a man in Vietnam has started producing solar cookers made out of local baskets. The cookers consist of a basin surrounded by insulation set in a basket made of local materials. The solar cookers are well suited to the central and northern provinces of Vietnam which receive 2-3000 hours of sunlight a year (Solar Cooker Review 2000).

### Parabolas and Paraboloids

The three dimensional parabola, the paraboloid, is the most efficient shape for solar cookers. Unlike box solar ovens, the parabolic shape has a central focal point where any light that hits the parabola’s walls will reflect into it. Using sunlight, the heat, along with the light, will reflect into that focal point and warm that spot. The cooking container should be located at the focal point so the maximum amount of heat will be used for cooking. With the general equation for a parabola being: $y=x^2$, the focal point can be found by taking a point on the curve, (x, y) and putting it into the equation: $a=x^2/4y$, with the variable, a as the focus (Stein, 1999).

### Himalaya Blackberry

The structure of the parabolic solar cooker will be made from the canes, or stems, of the locally invasive Himalaya blackberry. Canes will be harvested and dethorned so they can be woven into a parabolic basket shape. The Himalaya blackberry, Rubus procerus, is an evergreen that was brought to the United States from Europe in the early nineties (Everett, 1981). It has a vigorous trailing growth pattern and it is heavily thorned. Since the canes will be harvested during the winter months, they will be dried, sturdier, and easier to dethorn and manage. The Himalaya blackberry canes form parabolic curves, so when they dry and stiffen they will maintain the sturdy parabolic shape of the basket (Figure 1).

Although blackberries cannot withstand extreme climates and grow mostly during the springtime (Whatcom), the Himalaya has a rapid growth rate of around 20 to 50 feet per season (Bailey, 1961) causing it to spread and takeover areas. The Himalaya blackberry is an invasive, non-native species to Humboldt County, and California state government has provided funding for the removal of the species throughout regional and state parks (California Department of Parks and Recreation, 2005) so harvesting them for the basket will not have a negative environmental impact on the area.

### Humboldt County Weather

The amount of clear days in Humboldt County is the major inconvenience with local solar cooking. Eureka has around 78 completely clear days per year with the most during September and October and a monthly average of 6.5 clear days (Western Regional Climate Center). Although most days are not completely clear, there are still partly cloudy days where it will clear up long enough to cook something around midday to early afternoon.

## Design

### Materials and Tools

 Blackberry vines Pampas grass used bike inner tubes hemp twine thin aluminum rods wire tin can lids wire cutters gloves thick nail hammer can opener scissors utility knife large inner tube

### Construction

#### Shape and Structure

To figure out the shape of our solar cooker, we decided that we wanted the curve of the parabola to pass through the point (2,2) and that the apex of the basket, the top rim, should have a diameter of about 5 feet. In order to find the equation for the parabola that would pass through that point we needed to figure out the coefficient, a, of the general parabolic equation,

$y=ax^2$

To find the coefficient, a, we used the equation

$a=y/x^2$

and replaced the x and y with the point (2,2)

$a=2/2^2$

which gave us

$a=.5$

With the value of the coefficient, we were then able to insert different x values into the equation

$y=.5x^2$

and find the corresponding y values.

Fig 2: Chalk coordinate axis
x y
.5 .125
1 .5
1.5 1.125
2 2
2.5 3.125

We plotted those points on our coordinate axis and drew a line through them to give us the parabolic curve that we wanted.

Using that curve, we shaped the blackberry vines to fit that parabola and tied string from one side of the curved blackberry vine to the other in order to restrain the parabolic shape.

In order to create the three dimensional parabolic shape, the paraboloid, we lashed the vertices, the center bottom point, of the shaped blackberry vines together with used bike inner tubing. Since the bike inner tubing worked so well to hold the vines together, we thought it would work to weave most of the basket like that. However, when we started doing that, the inner tubing created too much force on the blackberry vines and moved them out of the parabolic shape.

As we noticed that the tighter the weave the more the vines would move out of shape, we decided that we needed large rings placed at certain points above the vertex to provide support for the structure and to keep it from warping. Different sizes of hula hoops seemed like a good idea for those rings at first, and they could have worked, but we did not want to buy new ones and we could not find any used ones. Then we went to the scrap metal yard in search of something we could use and we found long, light aluminum rods that were perfect for shaping into rings of any size. Along with the rods, we made a couple rings of thick wire for additional support.

#### Weaving

After lashing the metal rods and wire onto the blackberry vines for support, we thought we were ready to weave the Pampas grass through the vines but we soon realized that the gaps between our blackberry vines were too far apart for us to create a good weave. To fix that, we tied lengths of bike inner tubing from the top ring of our basket down to the vertex and in between each of the blackberry vines. Those extra spots that we created to weave grass through helped a lot, but it was still slow moving since we were only weaving one blade at a time. After a few tries, we discovered that we could weave handfuls of Pampas grass at one time just as easily, so that is how we finished weaving our parabolic solar cooker.

#### Reflective Surface

To give the parabolic cooker its necessary reflective surface, we gathered around 300 tin can lids to line the inside of the basket. We punched holes in the can lids so we could string them together in lines of 8-10 lids each. Then we tied these lengths of can lids to the basket using either hemp twine or twist ties. Since there were still many spaces on the basket uncovered by can lids, we gathered a bunch of large can lids and attached those individually to the basket.

### What We Would Do Differently

If we were to make the solar basket version 2.0 here are some changes we would make:

1. double up the blackberry runners so that the ribs of the basket are stronger
2. use another material instead of pampas grass because it causes lumps on the basket's surface, maybe something softer.
3. use all large can lids, of the shiniest quality
4. construct the rings that hold the parabolic shape out of a less flexible material than the aluminum and wire that we found.

APPROPRIATE TECHNOLOGY ENGINEERING 305 CLASS

FOCUS ON THE SUN !