Alan Cuthbertson

alan@diydoubleglaze.com.au

DECEMBER 2014

Double Glaze Matters

Montmorency Food

Swap

Sun 11th Jan,

10:30—11:30

Were St shops

Bring your home grown

produce to swap.

Franciscus Henri’s contribution.

Geo Thermal Power

I know X’mas has past, so a happy

New Year to everyone.

It has been a busy year for action on

Climate Change. While the Australian

government has been trying to take us

backwards, there at least some move-

ment in foreign governments for action.

However as soon as governments

seem to get serious, national interests

return to the fore. I sometimes wonder

if the world will ever work together to

solve the problem. The cartoon below

from Franciscus is all too accurate.

On the domestic front, I sense a

greater commitment from people that

we must take action, but maybe that is

just the circles I move it. Perhaps the

two sides are just becoming more po-

larized. It is disappointing that Climate

Change didn’t raise a mention in the

state elections.

I received this quote the other day

based on the recent call from the Pope

to take action “... hopefully the Pope

will trump the Abbott and the Bishop…”

And for all those who dislike Mr Abbott

and love Monty Python- https://www.youtube.com/watch?v=liU9NjuItYQ

..and for those that want to win at

Jenga, here is a Jenga Gun- http://www.youtube.com/watch?v=F9BmTmMEOhQ

Merry X’mas

PAGE 2

DOUBLE GLAZE

MATTERS

Solar Panels & Batter ies

The economics of solar panels and batteries is quiet

complex and difficult to understand. What I would

like to do is to explain it in a different way which

will hopefully clarify issues.

I will assume that power used in the home costs 30

cents per kwh and excess power can be sold back

to the grid for 9 cents per kwh. Let’s also assume a

usage profile in the house of 12 kwh per day (ie.5

kw for 24 hours a day), equally spread over the

day. I will also assume it costs $1,500 to install 1

KW of solar panels and it will generate 4 kwh per

day or 1,400 kwh per year.

The first .5 kw of panels is very cost effective. All of

the power will be used in the house so the annual

saving will be 700 kwh times 30 cents or $210. The

$750 cost of the panels is paid of in just over 3.5

years.

The next .5 kw of panels is also pretty good. 90%

of the power will be used in the house and 10% fed

back into the grid so the annual saving will drop to

$195. The $750 cost of the panels is paid of in 4

years.

After installing 5 kw of panels on the roof, the

next .5 kw of panels will be less cost effective.

Probably only 20% of the additional power will be

used in the house and 80% fed back into the grid

so the annual saving for these panels drops to $92.

The $750 cost of the panels is paid of in just over 8

years which is still pretty good.

At that stage though, is it better to use batteries?

Rather than sell the energy for 9 cents per kwh, is it

better to store it in a battery and use it at night

time and save 30 cents? The price for a 1 Kwh bat-

tery is around $750. Assuming we have spare

power each day, we can store 1 kwh of power dur-

ing the day and use it at night, a saving of 21

cents. If we do that every day of the year, we save

$55 a year, taking 15 years to pay off the batteries,

not a terribly good investment. Batteries would

have to halve in cost before it is more worthwhile

than investing in additional solar panels. Besides

costs of batteries, there is also the issue of how

long they last. I would expect 5 – 10 years, which is

much less than the life of panels.

Assuming you did install 10 kwh of batteries, the

economics of additional batteries gets worse. The

additional batteries will only be used when there

has not been sufficient solar power to charge the

first 10 kwh of batteries. This will only occur around

100 times a year, so each kwh battery is only re-

turning $21 a year.

Lets return to the solar panels. I assumed that

you can always sell your power back to the

electricity companies. At the moment this is

true since renewables account for only 6% of

the power generated. Even in peak production

they never reach 100% of the demand – al-

though recently this did occur in South Aus-

tralia which has a much higher percentage of

wind energy. If renewables reach 20% of pro-

duction, peak production will exceed demand

and you won’t be able to sell power to the

power companies. The 9 cents will drop to

zero cents, reducing the value of additional

panels. Of course this helps the case for bat-

teries, since the value increases from 21 to 30

cents.

This whole discussion highlights the problem

with renewable power. How to store excess

daytime power for later use. At the moment,

this is not a problem since the grid will always

soak it up. Beyond 20% renewable we need a

storage system, such as batteries or pumped

hydro to store the energy for use at night.

Probably the best solution is stored thermal,

where a tank of salt is heated during the day

and drives the power generator for 24 hours.

By spreading the usage over 24 hours you can

move to 60% of power from renewable.

Then it gets harder. You now need to store

the spare power generated in summer to use

in winter—or install 3 times the generation

capacity so that the power generated in winter

is sufficient . This is very expensive. Of course

you could reduce the annual variation by lo-

cating power stations near the equator, or

transfer power across the equator so summer

capacity in the southern hemisphere can fulfill

the northern hemisphere winter shortage.

Wind helps a bit, since it generates more in

winter than summer. However you need a

large national network so that wind shortage

in one state can be met by an excess in an-

other state. Beyond Zero believe that have

addressed this issue. I think they relied on

using Bio fuel in winter. Bio Fuel has the ad-

vantage that storing the fuel for 6 months is

quite feasible.

Other storage technologies could work. For

instance you could use electricity to create

hydrogen and store it for 6 months. Once you

have built the infrastructure, it is not a lot

more expensive to save it 6 months rather

than 24 hours—you just need some large

tanks to store the hydrogen.