On 28 Nov 2005 12:06:58 -0800, wrote:

Peter Parry wrote:

The maximum amount of energy in ideal situations in the UK varies
from about 0.5kWh/m2 in the winter to 5kWh/m2 a day in the summer. A
solar hot water system has a total collection efficiency of around 30
to 70%. Let's assume the higher figure. That equates to 350W/hr of
energy per sq/m per day. For a typical 2sqm panel installation
that's 700W/hr a day.

theres a key problem right there, 2sqm. 2sqm is ok for summer, but not
winter. For winter one needs more area and more efficiency. Not 10x as
much though, as will be explained below.

More area = more cost = more CO2 used in manufacture and shipping
across the globe. The equation never balances.

Assume a daily household requirement of about 100L of water at 60degC

But recognise that that is not a necesarily true figure. Household
water consumption can vary widely depending on whether the householder
wants to conserve or not. Aeration, turning water off while soaping up
in the shower, and a shower or whole house drain heat exchanger will
all make a significant difference to hot water consumption.

Of course you can reduce water consumption dramatically. There is no
requirement to wash at all, it has little health benefit. I've
spent many weeks living on one or two litres of pretty disreputable
water a day with no harmful effect.

Whether you can convince Sun readers to become even more malodorous
is of course a different matter and when you start looking at trying
to add a waste water heat exchangers to an existing build the sums
get silly.

The specific heat
of water is 4.2Joules per gram per deg C so the energy required (in
kJ) is 4.2 x volume in litres x temperature rise in deg C. In our
example 4.2 x 100 x (60-10) = 21MJ or 5,800W/hr a day. In the winter
therefore the solar heater can provide no more than 12% of the
required energy. This is for an absolutely perfect site.

ONLY if you install an inadequate panel of course!

I'm not sure how you reach this conclusion - 70% overall efficiency
is beyond most systems. You can keep adding panels of course but the
economics stay the same.

And do nothing to
conserve energy use in the first place.
You might just as well install a half square metre panel and then
complain theres not enough sun for summer solar hot water.

Not really - just spend the money you were going to waste on solar
panels on more effective ways of conserving and gaining energy.

If the
panel isn't aligned due south it can drop by 50%.

one does not normally install them that far off, so thats not a real
world problem with installed solar systems.

One does if the roof is in the wrong direction as most are.

Heating water in the summer is simple - I've achieved higher
temperatures than 70degC with plastic bag. Heating adequate amounts
of water to acceptable temperatures all the year around is completely
beyond the capability of a modern commercial domestic solar heater.

Yes... but note how you slipped the word 'commercial' in there. You
might as well have said competent performance is beyond the ability of
any incompetent system.

Most people are not going to make their own - so it is the
performance of commercial systems which matters. I can make a solar
heating system which will give me hot water 365 days of the year. To
do so makes no economic sense whatsoever and no sense at all from an
energy point of view.

Compare Tony's figure of GBP15 energy saving per year with other
yearly energy savings you could make :-

Double Glazing - GBP82
Energy Efficient dishwasher - GBP13
Energy efficient Fridge Freezer - GBP35

one woud have to be upgrading from something fairly ancient to achieve
that sort of saving.

Not really, quite a few Fridge/Freezers of 5 years old or more can be
replaced by modern equivalents to give this sort of saving.

Replacement condensing boiler - GBP 256
(Figures from the Energy Saving Trust)

I dont see how a condensing boiler will save me the entire annual spend
on gas.

Did seem high I must admit.

The only
thing they do is help the crazy political game - if you move
manufacturing to China you "save" CO2 from the UK's "balance sheet".
You actually contribute far more CO2 to the world supply of course
but if it's CO2 made in China it doesn't matter and it doesn't count
in the fairyland of Kyoto - absolute insanity.

It does something very sensible. Selling solar DHW makes a market with
money available for businesses. This makes it more attractive, and
provides funds for a business to develop the technology further, and
that is what is needed today.

Funnily enough that was exactly what was said in about 1975. In the
meantime the "technology" (there really isn't that much involved) has
progressed minimally and costs have stayed high. Exactly the same has
happened in countries where solar energy has long been far more
attractive than it ever will be here. There is a well established
worldwide market in solar heating and it hasn't driven development or
cost reduction to any significant extent so this argument does seem
rather implausible.

You have to have a market for anyone to
invest funds, and you have to invest funds to develop better
technology. Its all part of the process.

The market has existed in many countries for centuries, you simply
can't develop what isn't there to develop. In countries with lots of
sun, solar water heating has existed for years and uses crude and
quite adequate technology. In countries like the UK nothing is going
to make up for thee fact there simply isn't adequate solar energy for
most houses for most of the year.


--
Peter Parry.
http://www.wpp.ltd.uk/