Topic: Back of the envelope calculations

[Citizen Skeptic]

Yeah but even if the earth was a perfect sphere, the calculation is wrong. To do it properly you need to think concentric spheres assume the volume and solve for delta r.

The linear approximation is the same as the integral for all intents and purposes because 0.3mm is less than one billionth of the radius of the earth.

For the record the delta r calculation, assuming 42 gallon barrels and 6371Km earth radius, is about 0.0008 mm greater.

Yeah.  I wouldn't worry about that when the number of barrels of oil only has 2 significant figures.

and since the layer is so thin it's likely to get absorbed into the surface and decompose so the thickness would probably end up being zero shortly after the spill took place.

[Chew]

Inspired by this thread:

0.75% of the population is in prison
10% of US population are atheists
0.08% of the prison population are atheists

1 out of 0.75% x 0.08% / 10% = 16,666 atheists are in prison.

FTFY.

Bet you the survey methodology is different.   One counts only people who explicitly self-identify as "atheist" and the other counts everybody who says they lack a belief in god, or something.   So one of them is an underestimate because people don't properly know what "atheist" means.

Oh, shit. What an embarrassing typo. Thanks.

But of the people who know what atheist means, only 1 out of 16k is in prison!

[jwray]

energy released from burning oil * 7 billion = ~ the amount of energy in the substrates (including O2).

proton to electron mass ratio = 1836.15267 = the exact date when the colt revolver was patented (Feb 25, 1836)

[jwray]

Inspired by this thread:

0.75% of the population is in prison
10% of US population are atheists
0.08% of the prison population are atheists

1 out of 0.75% x 0.08% / 10% = 16,666 atheists are in prison.

FTFY.

Bet you the survey methodology is different.   One counts only people who explicitly self-identify as "atheist" and the other counts everybody who says they lack a belief in god, or something.   So one of them is an underestimate because people don't properly know what "atheist" means.

Oh, shit. What an embarrassing typo. Thanks.

But of the people who know what atheist means, only 1 out of 16k is in prison!

Not really,  because if the 10% survey only counted people who explicitly identified as "atheist", then you'd have a much smaller denominator.

[Chew]

In the winter my domicile uses 2.5 gallons of heating oil per day.
One gallon of heating oil has the energy equivalent of 40.8 kWh of electricity.
2.5 gallons x 40,800 Wh/gallon / 24 hours = 4250 watts.
A Radioisotope Thermoelectric Generator used on numerous spacecraft uses 7.8 kg of Pu-238 to generate 4400 watts of heat.
4250 / 4400 x 7.8 kg = 7.5 kg.

7.5 kg of Pu-238 would heat my house in the winter.

[Chew]

Jay once remarked on the podcast about solar cells for electric cars. What would be the benefit of having solar cells on a car roof in terms of a more commonly understood unit of measure such as gallons of gasoline?

Assuming:
You live in Phoenix, AZ which receives an average solar radiation of 5.51 kWh/m²/day on a horizontal surface (i.e. an array tilt of 0^o).
2 square meters are available on the rooftop.
The solar cells are 20% efficient.
Gasoline has an energy content of 33.7 kWh per gallon.


5.51 kWh/m²/day * 365 days/years * 20% * 2 m² / (33.7 kWh/gallon) = 23.9 gallons/year.

[Zytheran]

Jay once remarked on the podcast about solar cells for electric cars. What would be the benefit of having solar cells on a car roof in terms of a more commonly understood unit of measure such as gallons of gasoline?

Assuming:
You live in Phoenix, AZ which receives an average solar radiation of 5.51 kWh/m²/day on a horizontal surface (i.e. an array tilt of 0^o).
2 square meters are available on the rooftop.
The solar cells are 20% efficient.
Gasoline has an energy content of 33.7 kWh per gallon.


5.51 kWh/m²/day * 365 days/years * 20% * 2 m² / (33.7 kWh/gallon) = 23.9 gallons/year.

..and if gas is $3.50 per gallon in Phoenix then the saving is $83.65 per year.
As far as I can tell the price for those solar cells at current market rates without rebates would be $2000 to $4000.
This gives a simple ROI of 24 to 48 years.

[Citizen Skeptic]

Jay once remarked on the podcast about solar cells for electric cars. What would be the benefit of having solar cells on a car roof in terms of a more commonly understood unit of measure such as gallons of gasoline?

Assuming:
You live in Phoenix, AZ which receives an average solar radiation of 5.51 kWh/m²/day on a horizontal surface (i.e. an array tilt of 0^o).
2 square meters are available on the rooftop.
The solar cells are 20% efficient.
Gasoline has an energy content of 33.7 kWh per gallon.


5.51 kWh/m²/day * 365 days/years * 20% * 2 m² / (33.7 kWh/gallon) = 23.9 gallons/year.

..and if gas is $3.50 per gallon in Phoenix then the saving is $83.65 per year.
As far as I can tell the price for those solar cells at current market rates without rebates would be $2000 to $4000.
This gives a simple ROI of 24 to 48 years.

not many cars and no solar cells will last that long.

[Chew]

The New York Metropolitan Area has 7 million households.
The average electricity consumption per household is 24 kWh/day.
A 24 meter diameter, 9 meter high tank of molten salt can store 400 MWh of extractable energy. This tank has a volume of 4071.5 m³ and can provide electricity to 16,667 households.
The tank volume required to provide electricity for 7 million households is 1,734,461 m³. That is a tank 210 meters in diameter and 50 meters high.
Two tanks are normally required for liquid thermal storage systems, one hot and one cold.

This only considers residential electricity consumption. Industrial and commercial electricity consumption is about 3 times higher.

[Citizen Skeptic]

where does the energy to melt the salt come from? you'd need a pretty good size reflector of some sort.

[Chew]

where does the energy to melt the salt come from? you'd need a pretty good size reflector of some sort.

Look! Stop trying to confuse the issue!

 

[Citizen Skeptic]

where does the energy to melt the salt come from? you'd need a pretty good size reflector of some sort.

Look! Stop trying to confuse the issue!

 

lol. let me help...

said tank filled with salt would weigh about 500,000kg
salt has a specific heat of .21 kcal/kg/degC and a melting point of 800C
which works out to 80 million kcal to melt the salt
80 million kcal is 90,000 kwh which would require an area of 18,000 m3 for the collector

warning: i am math impaired

[Chew]

warning: i am math impaired

Boy, I'll say! 

The salt commonly used in solar thermal storage facilities is a 60/40 mix of sodium nitrate and potassium nitrate (combined density 2.2 g/cm³ or 2200 kg/m³). The salt in the tank would weigh 3,815,814,200 kg.

[Citizen Skeptic]

lol. I assumed table salt. 

I'll get back to you with a new calc.

[Chew]

lol. I assumed table salt. 

I'll get back to you with a new calc.

The salt is initially melted and kept in its liquid state. The temperature of the salt is raised in the daytime and lowers in the evening. Assume about a 100^o C temperature difference.