[ExI] EROEI

[Keith Henson]

On Wed, Apr 27, 2011 at 7:30 AM, Eugen Leitl <eugen at leitl.org> wrote:

snip

> EROEI of 40:1, and still climbing. Grid parity coming
> soon.

I don't think EROEI is a good metric in all cases.

For example, suppose you had a way to make energy that cost 80,000 kWh
to make a steady 1 kW but would do so for 100 years (some RTG might be
in this class).  At the end of 100 years, you have an EROEI of 10
times, but it takes ten years just to pay back the energy that was
invested in making it.

Easy oil fields have a very high EROEI, typically 300 to one.  Most of
the oil is pumped out in ten years, with the majority of it in the
early years, but even for linear production, they payback is 30 to one
in the first year or an energy payback time on the order of ten days.

The energy payback for solar cells may have come down by now, but it
was in the years not long ago (of course that depends on where you put
them too).

Aluminum is the most expensive construction material in terms of
releasing it from the oxide and it's less (13 kWh/kg) than the least
possible energy investment to get it to GEO (15 kWh/kg).  Since real
and projected energy cost are many times more than the minimum, most
of the energy investment in power satellites is in transport to GEO.

For a power satellite made with stuff brought up by elevator, 5 kg
(enough for a kW) will take 75 kWh to lift it.  The payback time is
just over *3 days.*  (6 dats if you count the energy needed to make
aluminum.)

Chemical rockets are around 2.5% efficient so the energy payback time
is 40 times that long or about 120 days.

For the laser part, it draws around a GW to send 60 t/h to GEO.
(Starting from a sub orbital boost by the Skylon.)  1 M kW/60,000kg is
17 kWh/kg

The Skylon phase burns 66807 kg of hydrogen per launch.  The energy
content for three per hour would be 14029470 kWh (at 70 kW/kg), or 233
kWh/kg.

Together, 250 kWh/kg, (6% efficient) so material for a kW of power
satellite would take 1250 kWh to lift--which gives an energy payback
time of around 52 days.

Worse than oil, but about ten times as good as ground solar or wind.

Keith