Solar Electric Cooking

Part of Radically Inexpensive Solar Electricity for the Global Poor

Background

According to the World Health Organization, 3 billion people in the world cook with biomass and coal; consequently, 4 million people die from associated emissions. In many communities, biomass cooking has lead to deforestation and can cause harmful pollution to the environment. Women are threatened by sexual assault when they leave their communities to collect firewood or purchase coal. The purpose of our research is to minimize the environmental impact and health issues that arise from biomass cooking.

Our Solution

We’ve developed a $5 device similar to a hot water heater can be wired directly to a solar panel (no need for an inverter) and placed in any existing cooking pot in direct contact with food. This device can be made anywhere (likely in the village it will be used in) using extremely affordable and commonly found materials and tools, with no need for heavy machinery. Additionally, it can charge a phone or light an LED simultaneously while cooking.

We developed Insulated Solar Electric Cooking (ISEC) in 2015. See publications:
1) Insulated Solar Electric Cooking – Tomorrow’s Healthy Affordable Stoves? and associated video. A solar electric panel is directly connected to an electrical heating element, which cooks food. If the cooking chamber is insulated, a low power (~ 100 W) system costing less than $100 can slow cook a family’s meal in several hours.
2) A working paper on how we hope to bring solar electricity in Africa down to $0.02/kWh. We use a diode chain (explained below) instead of a resistive heater. This allows for us to utilize the solar electricity for charging appliances and cooking together. Using close to 100% of the solar electricity results in a very low cost of electricity in price per kWh used.
3) not a publication: Matt Walker’s Independent Study Report (Winter 2019) outlines our first baking implementations as well as general improvements in ISEC using diode heating with 1N5402 rectifier diodes purchased through TME (Transfer Multisort Elektronik),  factory specifications. We’ve recently conducted experiments using Grape solar panels. See factory specifications: GS-STAR-100W.
4) We’ve recently submitted the following research paper that will be made public here when it is published:
Hot Diodes!: Dirt Cheap Cooking and Electricity for the Global Poor? Gius, M. Walker, A. Li, N. Adams, R. Van Buskirk, P. Schwartz, Development Engineering, submitted June, 2019 
5) See the video showing how the use of phase change thermal storage allows an ISEC user to both cook after dark as well as dump the stored energy (of the melted phase change material) into the food in a short time at elevated power.
6) We separate the pot with the food in it from the heater in order to make cooking more convenient. Please see this video about how to bake with the sleeved ISEC
7) August, 2019, we built and tested ISEC as we plan a small company to build and disseminate ISECs in local communities. Please see this short video: Solar Cooking in Ghana.

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This work is part of our research toward radically inexpensive solar electricity. Please meet our research team. We intend to integrate ISEC with home electrical systems, in collaboration with Kuyere! in Malawi, and Aid Africa in Uganda. Additionally, this work has been addressed by group projects in Schwartz’s Appropriate Technology Classes in Spring of 2019, Fall of 2018, Spring of 2018 and another project, Fall of 2017, Spring of 2017 and another projectFall of 2016, Winter 2016, and Fall 2015 and another project. For a Cost Benefit Analysis of this project, click here.

Immersion Heaters

There are 3 main components to Immersion Heaters:

  • – electrical heating element
  • – insulation
  • – external casing

Electrical Heating Element

The heating element consists of a string of diodes (to see the history of this design, click here):

  1. Diodes effectively pull power from a solar panel because the voltage on a diode stays relatively constant with changes in current, in contrast to a resistive heater that can only be optimized for one time of day.
  2. A string of diodes can act as a crude voltage controller for house-hold appliances such as charging for cell phones and lights. 

The diode chain is inserted into a food safe enclosure with a composite as shown below.

The small scale diode chains we test with, shown below, consist of 5 diodes, a thermocouple attached to the hottest point of the diode, and a bimetallic thermostatic switch with shutoff temperature of 160°C.

Heaters are made of a diode chain inside of a food safe housing. Wires protruding are applied voltage, thermocouple wires, and voltage controlled output.

Diodes present a thermal challenge because they get as hot as NiCr wire, which often runs glowing red hot. We use rectifier diodes (1N5408) costing about $0.02 each that are rated up to 150°C. Although we have found that the diode functions up to about 300°C, they still easily reach that heat, so we must make an effort to keep them cool. The challenge is to develop an electrically insulating composite that is thermally conductive (see how we measure the thermal conductivity of composites). We are presently using concrete.

We add bimetallic thermostatic switches to our diode chains, which open the circuit when the switch itself reaches its temperature rating: in our case, 150°C.

Small scale heater of 5 parts MgO powder and 1 part Cement. Peaks and dips indicate where the thermal switch turns on and off.

Heater ran in near boiling water, water is keeping the diodes cool enough for the thermal switch to not turn on. Shows how the diodes heat up and maintain a constant temperature.

External Casing

For the surrounding pipe, we use aluminum because it is safe for cooking and easier to work with than stainless steel. With aluminum, we are able to heat treat, bend into a U-shape, or seal the end of the tube with a vice in a toothpaste-like manner. For more information on how we work with aluminum, click here.

Manufacturing & Videos

In addition to choosing materials that can be found easily and affordably almost anywhere in the world, we created a manufacturing process that allows anyone to build these heaters with no heavy machinery and only very basic handheld tools. This allows them to be made in the village they are used in, creating sustainable, local economies.

  • Manufacturing Process and material list by Spring 2018 Appropriate Technology Class
  • Instructional How-To-Build video by 2018 Summer Research Team, special thanks to Grant Bortz for film & edit
  • Promotional Video: Why Should You Care? by 2018 Summer Research Team, special thanks for Grant Bortz for film & editt: graphic and explanation of what it does picture of diode chain, insertion process, and finished product. Analysis of related costs. Temperatuink you should spin the “how we got here onto a separate page “development process and  lessons learned,” onto different page.