heating or boiling water

see also:

• bushwalking in Australia
• heaters for camping
• keeping warm when camping or hiking
• risk management when camping
• comparison of full fabric 2P and 3P tents for colder nights
• shelters and tents for car camping or alpine 4 season use
• tips for camping in the rain
• camping recipes
• cooking options when camping
• how to reduce your risks in lightning storms
• hot water and showers for campers
• wood stoves for camping
• electric induction stoves for camping

• water requires a surprising amount of heat energy to raise its temperature thanks to its high specific heat which is much higher than for metal such as cast iron or stainless steel
• this is also why it takes a lot of energy for your fridge to cool water down - best to put refrigerated water into your camp fridge or esky rather than room temperature water!
• the method of heating water will use the same amount of energy to irrespective of heat source (if you ignore heat losses such as wind blowing a gas flame away)

• this relates to:
  • how much water you need to heat
  • the initial water temperature and your desired temperature
  • the heat flux from your heat source
    • for electrical or gas burners, this directly relates to the wattage being used, but the total watt-hours of energy required will be similar (apart from inefficiencies)
  • the surface area of the water exposed to the heat source
  • how much are the heat losses due to evaporation, etc.

• change in temperature in °C per second = incident heat flux (kW/m²) x surface area exposed to heat source (m²) x pot thermal absorptivity (0.95 for black surfaces) x efficiency (usually 0.5-0.8 in open air) / (mass of water (kg) x specific heat (~4180 J/kg·K) )

• midday sun in summer = ~1 kW/m²
  • hence you can easily heat water for a shower in a black bag with large surface area to water volume ratio when left in the sun
• small campfire at 0.5m distance = 2-5 kW/m²
  • the closer the billy can to the flames, the faster it will heat up - remember inverse square rule - doubling the distance from the source results in one quarter the heat!
  • the wider the diameter of the pot, the faster it will heat up as surface area to volume is a factor
  • the blacker the pot, the more heat it absorbs and the faster will be the heating - hence cast iron beats stainless steel for larger cooking needs such as baking, roasts but the much heavier weight means the pot itself takes longer to heat up (and much longer to cool down)

• small gas hiking stove such as a Soto WindMaster stove
  • under ideal conditions, max. heat output is 11,000 BTU or 3kW
  • using thin, flat-bottom pots, boils 250mL water in ~ 1 minute with minimal breezes

• time to boil 500mL water from 20°C:
  • 2200W kettle takes 1.5 minutes and uses 58Wh = 11Wh per 100mL water boiled
  • portable 2000W Kickass induction stove:
    • at “400W” on cycles between 1160W (4.8A AC ~100A 12V DC), 450-600W and 4W taking 8.5min and using around 57Wh
    • at “2000W” on portable 2000W Kickass induction stove maintains relatively constant 1890W taking 100sec and using around 55Wh
  • inverter microwave at 1100W seems to be less efficient than kettle or induction stove and takes about 4 minutes = 70Wh