Peter Parry wrote:
On Mon, 28 Nov 2005 13:39:33 +0000, David Hansen
wrote:
So, even if your particular system does not collect adequate amounts
of energy that does not mean that all solar water heating systems
don't collect adequate amounts of energy.
There wasn't an adequate amount of energy to collect all the year
around. The technology had nothing to do with it. Again you show
this distaste for figures.
I agree with a lot of what you say, if not this bit. What you say
further down does not support this conclusion either. It only tells us
that your system, and many others with similar performance, arent upto
it for winter use.
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.
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.
and an input temperature of 10degC in the winter.
will be higher if a drain heat exchanger is used.
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!
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.
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.
If the panel isn't
cleaned frequently it can drop by another 5-10%.
In the summer the same collector can manage 7,000W/hr a day, more
than is needed and of course the excess cannot be stored.
it can and there are systems that do it, but I would fully agree it
isnt worth doing.
Each day
you throw away the equivalent of nearly two days worth of winter
energy gain.
a non problem
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.
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.
Energy efficient Fridge - GBP15
ditto
Upgrading ;loft insulation - GBP58
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.
Overall Solar water heating saves trivial amounts of energy and
money.
agreed! At least when applied to single domestic dwellings anyway.
In energy terms the cost of manufacture, shipping the major
components around the world and installation probably comfortably
exceed the useful energy they collect in their lifetimes. 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. 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.
I quite agree with your core message: solar domestic DHW is not
something to invest in today.
NT