The first field test of a G3 Rocket Stove used to heat a large tank of water to provide showers for a large family camp was held this summer in Dorset. The idea was to provide hot water from a solar panel, but due to the fickle nature of British weather, we wanted a wood-fired backup. But with up to 150 campers, half of them children - would it work?
When at camp a lot of time is spent heating water, mainly for cooking and washing up, with personal hygiene coming a distant third. There are a lot of children playing all day who need a good wash, but they are reluctant to use the cold shower even on a hot day.
This year we and the Bridport Renewable Energy Group were determined to provide at least a solar shower but with some modification we managed to provide a wood-fired backup in the form of a G3 Rocket Stove. The first things that came to mind were the safety issues: including rig stability and ensuring no-one was scolded. After that, usability: would people want and be able to use it, would the hot water be sufficient and who would tend the stove? We thought long and hard but to reach some of the solutions just had to try it out.
The system
A 110 litre insulated copper tank was built into a wooden structure, with a 28mm heat exchange coil within the tank. A flat plate solar collector was positioned in the sun, in front of and below the water tank to allow the hot water to transfer heat via thermosyphon. A modified G3 Rocket Stove was connected to the same coil on a convenient platform. The hot and cold water were both at the same pressure to facilitate mixing which was regulated by a thermostatic valve set to a warm shower temperature, set at 38C to ensure there was no possibility of scolding. A privacy enclosure and ground tarpaulin were set up around the shower to keep the ground dry to avoid the water tower being undermined.
The Rocket Stove.
A standard G3 Rocket Stove was adapted to have a heating coil secured with bolts where a cooking pot would normally be put. The hot gases were slowed by baffle plates in the chimney. Being a rocket stove, most of the smoke is burned giving much heat for little wood. Separating combustion from thermal transfer (meaning the flames don't touch the coil - only combusted gases) also increases the quality of combustion making it much more efficient than an open fire. The heat output of the stove is estimated to be 4kW, though a portion of this would be lost between the fire and the water coil.
The baffle plate sizing was a first guess and seems to be too restrictive as a lot of soot built up on the outside of the stove because it couldn't exit through the chimney readily enough. The photo shows another problem: overloading of the combustion chamber. This was because there was no one person tending the fire; it was kept going by whoever was passing/showering.
The main problem with tending the fire was that the ends would burn off in 15 minutes and unless someone was willing to sit by the stove and nudge the sticks in, the fire would go out. When people realised the value of the shower more tending was done by those nearby, and eventually an elastic self-feeding mechanism emerged (also pictured, if you look closely). This worked quite well but it did have an occasional tendency to launch burning sticks!
How well did it work?
This water heater was a great success. It worked throughout the camp, was easy to operate and was used widely buy campers of all ages. The Rocket Stove increased the amount of time hot water was available from previous years when no sun meant no hot shower: often just when you need hot water. The G3 was slightly undersized for the amount of water being heated in the timeframe, and a bigger stove would have done the job quicker, but this turned out to be a useful safety mechanism as it was difficult to boil the tank. The other advantage of being able to use such a compact stove was that it made the whole unit transportable in one journey.
Some more refinements are planned but the camp is hoping to acquire its own system for next year based on this design - borrowed from BREG this year.
