Sunday, 22 January 2012

BBQ Challenge Week

As the weather here really begins to heat up, I've noticed that our house can cope reasonably well, especially on the ground floor. The top floor can become uninhabitable in the evenings, but usually by around 9pm, with all the windows open, we can get back to outdoor temperature. (Although this week, it's forecast to hit 40C, so outdoor nighttime temperature is still probably going to be totally unlivable!)

Last week when I got home from work, I went ahead with a plan to cook a few things that involved the oven, and used it continuously for about an hour and a half. By the time I was finished cooking, I was drenched in sweat and tempted to put the AC on. I looked at the numbers and realised that over that time, our 2kW oven would have pumped 10 MJ of energy into the kitchen, enough to raise our whole open-plan area by 80 C, if of course we hadn't been temporarily confining its effects to the oven, and ventilating the area*.

So, this inspired me to plan our meals around the outdoor barbecue, where we will be able to dump all of our waste heat straight into the atmosphere. No better for the environment, but much more pleasant for us in the house!

I set a few rules to the challenge:
Super-husband gets started on the challenge!
  1. No use of the indoor oven, whatsoever;
  2. Hob use allowed for boiling, and making sauces, but no more than one burner at a time;
  3. Breadmaker allowed since we don't have an outdoor power point, and the BBQ oven temperature is too variable for bread (at my current skill level).
  4. Salads and other cold things of course allowed - it's more of a 'no oven' week than 'must barbecue' week :) But I like the latter name better.
Let's see how the week goes!

* For the interested reader, I calculated this back-of-the-envelope style mostly using unit analysis to get all the terms on the right sides. I knew I'd need the specific heat capacity of air, which I got from wikipedia, about 1000 J kg-1 K-1. Our ground floor has a volume of around 6m x 6m x 3m ~ 100m3, and air has a density of 1.225 kg m-3 at sea level, so our ground floor has roughly 125 kg of air in it (kinda cool to know that). Our oven is actually 2.5kW, but given that it wouldn't have to run at full blast for the full hour and a half, because the oven is fairly well-insulated, I generously knocked off 500W. The energy it input is power x time, = 2000 W (=J s-1) * (1.5 * 60 * 60)s ~ 10 000 000 J, i.e. 10 MJ. So, assuming all air is trapped, and all heat from the oven dissipates into said air, the temperature rise of the air in our kitchen can be calculated as:
10 000 000 J
1000 J kg-1 K-1 * 125 kg
which is 80K.

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