What’s the best method for converting megajoules to heat?

September 30, 2013 | Filed under: Self Reliance | Posted by:

Question by Sedgy: What’s the best method for converting megajoules to heat?
I am building a solar heater to help with my heating costs over the winter. I recently asked a question on here about what kind of heat I could generate using this method. I got this response: “If you live in an area with strong sunshine for around six hours or more each day you could expect to collect around 10 MJ of heat for each square meter of Plexiglas surface.” I can’t find any applicable metric-to-standard conversion for this anywhere. Can anyone tell me what this means? Thanks.

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3 Responses to What’s the best method for converting megajoules to heat?

  1. 1 joule will heat 1 ml of water 1 degree C.

    edit
    sorry water has a heat capacity of 4.18J/gK
    so 4.18J will heat 1mL of water 1 degree
    so 10MJ will heat about 40 litres of water in your hotwater system so if you have a 200l hot water tank you will need at least 5m2 of plexiglass surface to get it hot

    M Perry
    September 30, 2013 at 4:38 pm
    Reply

  2. Energy has units of “joules” (named for James Prescott Joule, a famous physicist). The joule is the standard metric unit for energy.

    When you are talking about joules you ARE talking about heat, heat energy that is.

    Of course, a mega-joule is simply one million joules, and is a boatload of energy. I don’t have any idea how accurate this person’s assessment is of your situation.

    You can convert the heat to calories, or BTU, if either of those are more familiar to you.
    1 calorie = 4.18 joules
    1 BTU = 1055 joules
    1 megajoule = 947.8 BTU

    ========= Follow up ==========

    M. Perry is incorrect. One calorie will raise the temperature of 1 g (1 mL) of water by 1 C degree. It will take 4.18 joules to provide the same amount of thermal energy.

    pisgahchemist
    September 30, 2013 at 5:19 pm
    Reply

  3. Heat is energy. It flows from warm to cool areas. Kinetic energy of molecules is proportional to absolute temperature. So you have energy, and can convert it to another unit of energy or relate it to temperature.
    Wikipedia says the following: 1 kilowatt hour = 3.6 ×106 J (or 3.6 MJ)
    1 Kw hr/3.6MJ = ?/10MJ
    ? = 2.8 Kw hr for 6 hours in a day with 10MJ produced. I know nothing about electricity.
    http://en.wikipedia.org/wiki/Gigajoule#SI_multiples
    Me neither. It seems that the Btu and calorie are used in U.S though after some research. They are still related to SI though.

    Physics section would also be an excellent source, since I have not taken physics yet.
    ===================
    EXCELLENT job pigachemist. While I was researching while he was posting. I got the right answer. Yah! Convert 10 MJ to Btu or calories.

    ChemGuy
    September 30, 2013 at 5:58 pm
    Reply

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