Homework 2

I did my corrections for this homework which was to figure out the expected natural gas usage for the month of January for the ETC. To be able to determine that I looked up the amount of heating degree days on http://www.degreedays.net/ for rohnert park in January which was 242. Since energy is equal to power X time I multiplied the UA product I found of 154.5 w/C by the amount of heating degree days.

The answer is 37,389 watts, however, watts is not energy so I converted the answer into BTU's by divinding it by 0.293 w since that is equivalent to 1 BTU.

The answer in BTU's is 127,607.

Since natural gas is measured in therms and 1 therm is equal 100,000 BTU, the expected natural gas usage for the month of January in the ETC is about 1.28 therms.

To find the surface area of the ETC I used the same length, width, and height measurements I used in the first homework which was:

length = 14 m

width = 9 m

height = 8 m

I then found the formula for the surface area of a rectangular prism: $$A = 2(h*l + w*h) + lw$$

The surface area is about 494 meters^2.

Since the R-value of 20 is in US units I decided to convert my area into feet to first determine the UA product in BTU/hour/F. There is about 3.3 feet in one meter so I multiplied that by 494 to convert my area.

The surface area in feet is equal to 1630 ft. We are also looking for the UA product so since U = 1/R I took 1/20 and got 0.05 BTU/hrft^2F

The UA product of the ETC in US units is 81.5 BTU/hr*F.

In order to find the answer in SI units I converted my answer of 81.5 BTU/hr*F to w/C.

81.5 BTU/hr*F $*$ 5C/9F $*$ 3600 sec/1 hr $*$ 1 BTU/1055 J = 154.5 w/C

The answer in metric units is 154.5 w/C.

I still don't think this is an accuarate model for mostly the same reasons I didn't think it was accuarate in the first homework. We didn't factor in the amount of heat that is absorbed by the ground which is probably more than the amount of heat absorbed by the walls. We also did not look at the infiltration of the air coming from the outside which affects the amount of heat that is kept inside. Again I also did not look at the entire area of the whole building which could still affect how much of our heat is really staying in one room.