Monday, September 21, 2009
Biogas Water Heating Trial 1
The first video clip shows the heating of 2 liters of water with biogas for 18 minutes. We see the full unthrottled six brick flame and the throttled flame.
The second video clip shows the spreadsheet calculations for determining how much of a smaller quantity of hot water must be added to a larger quantity of cold water to get bath temperature water.
Graph shows the temperature curve for heating 2 liters of water with tonights biogas using 6 bricks for pressure, but restricting the pressure using the stove valve . X-axis is time in minutes, Y-axis is temperature in degrees C. We will replicate this with the valve wide open, giving a higher flame but a shorter burn, to see if this makes much of a difference.
One of the questions I often get is, "cooking with biogas is all fine and good, but what about using it to heat bathing water? Does it produce enough?"
Tonight I started experimenting with this.
We had two sunny days in Germany this weekend with average daytime temperatures between 18 and 20 degrees and tonight were able to use the biogas for 18 minutes.
We decided to put 2 liters of water in a cooking pot (about what you would use to cook spaghetti) and see how hot it would get (water heating is a very energy intensive process). In 18 minutes it reached 84 degrees. It might have gotten hotter if we had remembered to cover the pot earlier -- we only put the cover on after 11 minutes, so we lost a lot of heat to the room!
Originally I wanted to see how long it would take to boil 2 liters of water for a pot of spaghetti using my biogas, but when I ran out of gas after 18 minutes having reached a temperature of 84 degrees -- not hot enough to finish the spaghetti -- rather than throw the water out I changed my plan and decided to see how much bath temperature water (between 32 and 40 degrees) this 2 liters of 84 degree water might give me.
I have created a table in Excel (note: you can use "Calc", OpenOffice.org's Open Source Spreadsheet if you don't want to spend the money on Excel!) called “how to get water to bath temperature” for the families I have been working with in Cairo. It uses the common physics formula for mass and temperature showing that the final or “total” Temperature of a body of water (Tt) is equal to an initial mass of water multiplied by its temperature plus the mass of a second body of water that is added to it multiplied by its temperature, divided by the sum of the two masses.
Tf = ((m1*T1)+(m2*T2))/(m1+m2)
Variables Values Calculations Formulas Description
I had previously calculated what it would take to heat 40 liters of water for a bath (the amount I usually use, which is twice what most under-capitalized Egyptians use in the "poor communities.") The formula showed that if you boiled 10 liters of water and added 30 liters of cold tap water you could take a 40 liter hot bath (at 38.5 C, hotter than your body temperature, so it would feel hot).
m1 30 30.0 (m2*(Tt-T2))/(T1-Tt) mass of water one, the bathtub
m2 10.5 10.5 (m1*(T1-Tt))/(Tt-T2) mass of water two, the water you are going to add
T1 17 17.0 ((Tt*(m1+m2)-m2*T2)/m1) temperature of water one, the bathtub
T2 100 99.9 (Tt*(m1+m2)-m1*T1)/m2 temperature of water two, the water you are boiling
Tt 38.5 38.5 ((m1*T1)+(m2*T2))/(m1+m2) Final total temperature desired
I algebraically manipulated the equation on the spreadsheet so that one could plug in any of the variables to get any of the other unknowns and demonstrated it to the families. We noticed that the average rule of thumb was that you can take a bucket of water at pipe temperature (average 17 degrees celsisus) and add roughly 1/3 of a bucket of boiling water and get it up to the required 38.5 degrees for a bath or clothes washing. This mathematical explanation satisfied the families, who claimed they already knew the principle of this, boiling about a third of the quantity of water on the stove to prepare a bath. This home grown appreciation of an alternative way to get hot washing water is not captured in most attribute tables. As has been reported for India, many residents in Cairo may be exercising a preference for effectively boiling water using biomass or low cost waste materials, defying the modernist assumption of a linear “energy ladder” (see Gupta, forthcoming, Amacher 1993, Barnes 2002, Arnold 2006, Pohekar 2006) Thus, the status quo, if properly explored, may be very revealing!
In our recent experiment shown in the video we heated 2 liters of water with biogas for 18 minutes and got it to 84 degrees.
By the time we went to add it to 5 liters of tap water (at 17 degrees) it had dropped to 82. The theoretical temperature we should have gotten according to the formula was 35.57 degrees:
where A2 is the cell with the final temperature, B2 contains the biogas heated mass, C2 the temperature reached, D2 the larger tap water mass, E2 the temperature of the cold tap water.
T final Mass 1 Temp 1 Mass 2 Temp 2
35.57 2 82 5 17
We recorded a fluctuating 33 to 32 degrees, still very comfortable for a bath, and attribute the difference to losses from pouring and mixing (losses to the air and to the walls of the bucket) and possible inaccuracies of the measuring device. The bottom line is that it was still plenty warm for a bath.
Given that we can get an hour to two hours of biogas from the 1000 liter digestors we build in Cairo, it should be no problem each day to heat enough bathing water for one or two or more people. The average Cairene in my sample used a 20 liter bastila for bathing and heating about 5 liters on the stove to about 90 degrees, mixing it with 15 liters of cold tap water at about 17 degrees, achieving roughly the same temperature as in our experiment. The formula shows it equaling 35.25 degrees C.
T final Mass 1 Temp 1 Mass 2 Temp 2
35.25 5 90 15 17
In 20 minutes of biogas heating we could have gotten the 2 liters up to 90 degrees. This implies that 50 minutes of heating would raise 5 liters to that temperature. On good biogas production days, even with 90 to 100 minutes of gas one should be able to double that amount and provide 2 20 liter baths.
Is it worth it? The India experience is that the biogas is better utilized for cooking, and most families who have biogas digesters still use propane bottles for heating water for bathing. Nonetheless people don't always eat at home, and it is useful to see if it is worth trying to heat bathing water with gas that, after all, came from kitchen scraps. My feeling is that it definitely is.
Further experiments have to be done to determine whether or not it is best to use the biogas to heat the whole 20 liters to bath temperature or heat a smaller quantity to near boiling and add it to 15 liters of cold tap water. There are pros and cons to both, and they are experienced daily by the enterprising Egyptians who heat their bath water on the stove using bottled gas (60 % of Manshiyet Nasser's Zabaleen and 25% of Darb Al Ahmar are in this category).
Placing 20 liters of water on a stove can break the stove and many elderly people and young people can not lift the 20 liters, particularly without spilling. On the other hand, the larger quantity of water never gets hot enough to be dangerous. 5 liters heated to near boiling is lighter in weight, so it can be heated much easier on the stove, but it is very dangerous to carry; spilling causes scalds that puts many people in the hospital every year. Heating 2 small batches of 2.5 liters (a spaghetti pot worth of water) would be safer, but while waiting for the second 2.5 liters to heat one would lose much of the heat in the first batch. If one has a two burner stove one could heat two batches of 2.5 liters simultaneously but many families have only one burner.
What will weigh in here is the efficiency of heating large versus small quantities of water, given that biogas is in limited supply each day. Certainly families using biogas can supplement with bottled gas when needed, but we would like to have the data for the times when fossil derived natural gas is either too expensive or is simply unavailable.