How much energy reaches the earth each day from the sun?

[gregcropper]

We are all worried about our future energy needs. How much energy reaches the sun each day, and how much do we (as a global user) consume? The obvious question I am posing is what percentage do we need to capture? We are using fuels which took millions of years to create and lay down, yet we will use them (oil, gas, coal etc) within a couple of hundred years
Thanks Greg

[Azalian]

The Sun provides approximately 100 watts per square foot or 27,878,400 watts per square mile.
This is equivalent to 274,000,000 gigawatt years or 8.2 milloin Quads of Btu energy per year.

The entire human race uses, in all forms, 400 Quads per year, so if collected well, gives us 20,000 times our present day needs.

[gregcropper]

Blow me down Azalian, that's amazing knowledge ! Thanks Greg

[wildwood]

A bit hard to guess what percentage but we do use a lot more of the sun's energy than is generally thought. Without the sun's warmth and light there would be no vegetation, thus no life on Earth.

Fuels are now made from cereals and all sorts of plants. This puts the cost of food out of range of the 3rd world countries, so more misery.

[gen2]

QUOTE: "How much energy reaches the sun each day?"

ANS: Almost none. The sun is illuminated by all the stars, but they are so distant, that the amount that reaches the sun is negligable compared with the output from the sun.

[MarkRae]

> The sun is illuminated by all the stars

Genius!

[vascop]

That's 100 watts per square foot when the area is oriented towards the sun, not per square foot of land area.
Take the UK for example. You need to make several corrections:
We need to compensate for the tilt between the sun and the land, which reduces the sun's intensity at midday to about 60% of its value at the equator, and we also lose more because it's not always midday. On a cloud-free day in March or September, the ratio of the average intensity to the midday intensity is about 32%. Finally we lose power because of cloud cover - In a typical UK location the sun shines during just 34% of the daylight hours.
Combining these factors andthe additional complication of the wobble of the seasons, reduces the energy on a south facing roof in Britain to about 10 Watts per square foot.

[vascop]

If this energy is captured by a thermal solar panel, and making the reasonable assumption that these panels would only be 50% efficient means you get 5 Watts per sq foot of thermal energy (water heating).
If you use photovoltaic cells to convert to electricity instead of heating water, then since typical PV cells are only 10% efficient and expensive ones about 20%, then you would get at best about 2 watts per square foot.

[vascop]

So, as you can see this means that Azalian's figures are optimistic to say the least.

[gregcropper]

Apologies..... it should have read 'How much energy reaches us from the sun each day' By way of mitigation, I'd had a couple of glasses of wine !
Greg

[vascop]

So multiply all my figures by 24/1000 and you will get Kilowatt-hours per day per square foot(kWh per day per sq ft):
So in the UK we receive 0.024 x 10 = 0.24 kWh per day per sq ft
Using thermal solar panels to heat water that's 0.12 kWh per day per sq ft
and using Photovoltaic cells to produce electricity that's about 0.05 kWh per day per sq ft

So if you have 100 sq foot covered with cells you will get 12 kWh per day and 5 kWh per day respectively.

[vascop]

by the way, since a 40 watt light bulb kept on all the time uses 1kWh per day, then
that's 12(twelve) 40 W light bulbs and 5(five) 40W light bulbs respectively.

[New Judge]

Well worth it, then!

[vascop]

It depends on what you mean by worth it. The heating consumption is about 40kWh per day per person (kWh/d/p), and the electrical consumption is 18 kWh/d/p (both these incidentally almost entirely provided by fossil fuels).
Since the thermal solar panel energy can only be used for heating then assuming that there are 2 people living in a house, that provides 12/80 % = 15% of the heating.
The PV panels will produce electricity, so they will provide 5/36 % = approx 15% of the electricity a 2-person house needs.
You can see that with a family of 2 kids say, this would shrink to 7%.

So you could save on your bills but you would not be anywhere near able to supply all your heating and electricity needs.

[vascop]

By the way the cost of PV cells is about 4 times the cost of solar thermal panels and they only deliver about half the energy of the thermal panels although it is higher grade energy (electricity). In a not-so-sunny country like the UK it's probably better to go for the thermal cells first. In sunnier countries the best solution is a combined system that uses PV cells to produce heating and electricity.