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Measuring the Power and Efficiency of a Wood Gas Stove
using pfodApp™

by Matthew Ford 5th September 2016 (original 5th September 2016)
© Forward Computing and Control Pty. Ltd. NSW Australia
All rights reserved.

Jan used pfodApp to measure the efficiency of his Wood Gass Stove developed for Engineers Without Borders. Here is his description of that project.

Introduction

As part of a project for Engineers Without Borders I developed a Wood Gas Stove. This is a very efficient type of wood stove. I measured the power and fuel efficiency of the Wood Gas Stove using an Arduino in combination with the pfodApp. Before going into the technical set up a little background.

Wood Fires: An Unknown Global Issue - About 40% of the world population (3 billion people) cooks their warm meals on wood fire. Often this is a very inefficient traditional “three stone fire”. These fires burn with a lot of smoke, which is causing lung cancer. Each year an estimated 4 million people die because of this smoke. The wood consumption leads to deforestation. That has impact on the climate, but also has direct impact for the women collecting wood.

Improved Cooking - Because many of the wood fire users live in the “sun belt” of the world, the best solution would be to cook on solar energy. This is possible, but solar energy has limitations in availability and power. Therefore EWB developed a hybrid cooking device: a solar cooker, supported by a small Wood Gas Stove to kick start the cooking process or to cook when there is no sun at all.

The Wood Gas Stove Design - I was inspired by this YouTube movie. It is a compact design with a stainless steal cup. I will not go into the interesting technical details here. The only thing worth mentioning is that I have chosen for a fan driven Wood Gas Stove. The fan needs very little power (it runs for 10 hours on a small power bank). The power bank can also provide energy in the evening for LED lighting, replacing smoky oil lamps.

Measuring Power and Fuel Efficiency

For measuring the power you need to measure the increase of water temperature of a known weight of water. It measures the effective power, used to heat the water. Heat loss occurs and is influenced by factors like outside temperature and wind. The impact of these fluctuations can be neglected in this set up. The (effective) power (in J/s = W ) equals the specific heat of water times increase in temperature times weight of the water, divided by the time:
4200 (J/kg/K) * delta T (K) * water (kg) / time (s)

To measure the wood burning efficiency we need to know the wood consumption in gram per second. By dividing the power (J / s) by the burn rate (g / s) we get the J / g.
From literature we know the energy content of wood, which is about 18 kJ/g.

The Test Set Up using Arduino & pfodApp

Measuring the water temperature is done with a DS18B20 temperature sensor. These can be bought in a water proof lead for less than 5 Euro. The sensor is accurate to 0.1 Celsius. For the weight a 5 kg kitchen scale is used. These scales are available for less than 10 Euro. The interface with Arduino is made via a HX711. A HX711 sensor board costs a few Euro.

There is a lot of documentation on the internet how to connect these interfaces. For the temperature sensor you need to connect 3 wires and a resistor. For the scale interface you have to disconnect the 4 wires from the load element and solder them to the HX711 board. The HX711 board has 4 wires connected to the Arduino. To avoid the need for adding a LCD to monitor the measurements and a SD data logger to record the data I used the PFOD app. This way I could fit the electronics inside the housing of the kitchen scale. The Android smartphone provides the display and the storage for data logging.



The complete device built in the kitchen scale. In the middle the weight load element. Mid bottom the Arduino Nano, right top the HX711 and left top the terminal for two external temperature sensors (leaving the casing in the left top). On the board is also an internal DS18B20 sensor. The potentiometer (top middle) was used to log visual observations. On the right hand side the HC-05 BlueTooth module. The two push buttons of the scale (left side) and the RTC clock (left bottom) were not used in the final design.

The Project Results

Scale with Wood Gas Stove and pan with 2 litres of water and the data as presented by the pfodApp. The lower plot is the weight. The stove is put on the scale (500gm), the pan with water is added (plus 3000gm) and after that there is a hardly visible decrease in weight due to the burning of wood. The top part shows the linear increasing temperature of the water in the pan from 15 to 100 degrees Celsius.

The Measurement Results

The Wood Gas Stove delivered about 500 Watt into the water. This is enough to bring 1 litre of water in 12 minutes to the boil. The stove has been kept small so it can be integrated in a Solar Cooker. Larger stoves can provide more power. The combustion heat delivered to the water was about 6 kJ/g, an efficiency of about 33%. The efficiency of a traditional three stone fire is about 17%. The Wood Gas Stove halves the wood fuel consumption.

The Arduino code used is here. The code needs a number of support libraries, HX711 from https://github.com/bogde/HX711 and OneWire and DallasTemperature from https://www.hacktronics.com/Tutorials/arduino-1-wire-tutorial.html If you are interested in more information about the Wood Gas Stove please



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