Peter Parry wrote:
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.
Its really not that simple. 'More area = more cost' only if we use the
same kind of panels, and more of them. That isnt the way to go. Firstly
we need different types of panel in the one install to optimise
cost/return and performance. Some of these will be lower, and only some
vacuum tube.
Second, solar DHW badly needs much cheaper collectors than are common
today, cheap enough that much larger areas can be used at lower cost
than now. I've come up with an approach to do this, though I havent
built and tested them. In short these panels would be made from layers
of a long life thermoplastic film (there is of course more to it than
that), and would be attached by feeding strips under tiles/slates and
screwing in place. These can be much larger and cheaper than todays
glazed boxes or vacuum tubes.
Shipping plastic film round the globe, if one goes that route, is
remarkably cheap compared to shipping vacuum tubes or traditional flat
panels. A much larger panel contains far less embodied energy, less
cost, way less material, and takes up a fraction of the shipping
volume.
Of course you can reduce water consumption dramatically. There is no
requirement to wash at all,
There you go off on something neither sensible nor relevant.
and when you start looking at trying
to add a waste water heat exchangers to an existing build the sums
get silly.
The sums are more impressive for new builds, so thats where they'll
make inroads.
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.
To make a good system does not mean just adding more of the same
panels. And of course the figures arent the same with a system with
different cost and different payback.
just spend the money you were going to waste on solar
panels on more effective ways of conserving and gaining energy.
I agree with that. But the interest is almost exclusively with DHW for
some reason, and has been for decades. It has a market, and in time I
think it will pay, once the sleeping industry is roused by a competent
competitive team. And I think it will be at some point. Rising energy
costs will help as well.
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.
in bad alignment cases one does not install solar DHW. Not if sensible
anyway. Inappropriate systems may get installed today for a few buyers
with no real understanding of the figures involved, by companies
unconcerned by whether their product works, or how much the customer
loses. That tells us nothing much about solar technology of course.
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.
Then we're coming from 2 quite different places. I'm not interested in
todays commercial systems since theyre a waste of time and money. What
I am interested in is a newer generation of design that will both
perform and pay, and put todays valueless junk peddlers out of
business.
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.
using your design, I agree. Using any now popular commercial design, I
agree.
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.
I dont know any recent FFs that eat that plus the energy consumption of
a new one. I think you'll find that figure only true for faulty
machines and oddball cases.
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.
A jump from 65% to 91% is 26% more efficiency. So one's annual gas
spend would have to be fairly extreme to achieve that sort of saving.
These are not realistic figures, or even close.
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
If you review the patents granted since then I think you'll find there
are many new technologies. There have also been some significant trials
since then.
I dont evaluate solar technology by looking at junk commercial systems,
any more than I'd evaluate car technological progress by looking at the
latest Trabant, or medical progress by looking at quacks who can do a
little and charge a lot.
Solar tech still needs more progress. It has certainly progressed since
75, but it will take some time yet before the industry gets where I
want to see it get to. And its fair to say that DHW is not the hottest
area of solar research, since there are bigger fish to catch elsewhere.
and costs have stayed high.
For most systems, sure. The challenges are both to make systems that
work well, and to make systems that pay well. But again, the state of
commercial systems really has little to do with it. Good designs do not
mirror such systems.
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
Read some patents.
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.
There is more than enough insolation on lots of house roofs to heat DHW
with a 50% efficient system. You keep saying there isn't enough over 2
sqm, but I'm not sure that tells us a great deal, except that the
commercial systems you refer to are basically expensive novelty
products.
The fact that the roofing material behind the solar panels can be
cheaper is also a plus for new builds. Strips of fibre cement board
save time and money over tiles and slate. This isnt worth doing with 2
sqm, but can be with larger areas.
NT