Is it optimum to only preheat DHW with solar?
John Allen
Typically solar is only used to "preheat" the water going into the more reliable standard water heater like this:
http://www.ips-solar.com/waterheating_500x282.gif
If I include the electrically supplemented container of hot water, there will be sometimes when I could warm up the other containers but do not since the solar water is not any hotter than the electrically supplemented container. OTOH if I do NOT include it, then there will be sometimes when I could heat it with solar energy but can not because it is not available for that. I could add a separate loop between containers for these times or add controllable valves but that seems like overkill.
I believe that most of the time when there is significant sunshine to be harvested, even my old flat plate collector can produce water at about 120, but that is assuming input temp at about 110, and near perfect alignment to the current angle of the sunshine. Evacuated tubes make this problem go away because they can make very hot water. A pump and controller capable of variable flow depending on the output temp needed will also solve the problem.
So many choices... The "obvious" thing to do is to fall back to the mainstream approach of just preheating the water, but I'm wondering if there is some opportunity for improvement.
What do you think?
ja
--
John A Allen
Tony Luck
on demand heater like the Bosch RP27PT (http://tinyurl.com/ynqmnp)
[though presumably you could use something far smaller with a large
tank of pre-heated water ... your needed temperature rise should
always
be far smaller than any of the examples given for that unit!]. There
are
natural gas and LPG versions too, but I might worry about whether they
would be good for a solar pre-heat setup where they don't actually
need
to come on for five consecutive months in the summer ... that is a lot
of time for dust and junk to build up.
Otherwise, if you are going to maintain a tankfull of water at usable
residential temperture (120F???) then your question revolves around
how many hours a day you can get hot enough water from you solar
collector to add energy to this tank. I suppose you might be able to
do some smart things in the control system ... e.g. if a lot of hot
water
is used in the morning (residents taking showers), then you could
delay
using the electric heater to warm the water in the tank because you
expect to be able to harvest solar power during the day.
Perhaps the answer to the question changes between summer and winter?
In this case a manual set of valves to be adjusted in Spring to apply
heat
to the hottest storage tank across summer, then reset in Autumn to
go back to pre-heat mode would be simpler and cheaper than machine
switchable valves under computer control.
-Tony
Joe Rich
like them, given one major draw back.
We used them for the volunteer showers in Buras, where we had many
volunteers taking showers (although, after a few weeks, showers became
more infrequent and the odors became more ripe - made me start
wondering what it was like living in medieval times). The provided
all the hot water we needed when we needed it. I do not believe that
a regular tank hot water system would have been able to handle the
peak load, unless it was quite large. for this appliction they were
great.
On the other hand, the on demand water heaters took propane. Assuming
gas sells for $0.085 per kilowatt hour, and propane is ~$3.00 /
gallon, there is a fair amount of disparity in the cost of operating
(This may be corrected with natural gas prices, though). From my
info, a gallon of propane is about 84.3KBTU, while a KWH is 3.4KBTU.
This is a ratio of about 25. A gallon of propane is about $3.00 and a
KWH is about $0.085, which is a ratio of 35. Therefore, not
accounting for efficiencies, it seems like it cost about a third more
to use on demand. Natural gas prices might correct for this, I am not
sure.
The other problem was that the computer controller board failed in the
on demand heater. This was not trivial, and cost about $250 to
replace (which we never did - we converted to a tank, which looted for
free). We believe the failure was caused by a power surge.
On demand heaters were running between about $800 and $1000 when I
checked last, depending upon the size. I believe the Bosch heater is
made to work with pre-heated water - I remember some advertisement
they had at the last SolFest.
John Allen
Joe Rich stopped by today and after consulting with him for a couple hours, I've decided to stick with the idea of using a MUCH smaller flow of electricity to heat the unpressurized storage tank if and when there is not enough sunshine to do the job. I'll add a simple time of day timer and size the electric heater to be able to give me three hot showers after running from 5 pm to 5 am.
Tony - I love your idea of manual bypass valves for seasonal adjustment. I had seen that in the literature for using a conventional pressurized storage heater for supplemental needs, but forgotten about it. In that design the pressurized backup heater is effectively mothballed during the summer. The bypass arrangement you suggest is a bit different in that it only isolates the glycol HX in the hottest tank, not the tap water HX. It solves the question I asked "Is it optimum to only preheat DHW with solar?" The answer is - Yes in crummy weather, no most of the time." I'll run the glycol loop through every container of storage water but nip it off with a valve when it would cool down my hottest container. I think this will be rare enough to do manually. If I neglect to remember, the consequences will just be some movement of heat from one tank to another, not actual wastage. I'll size the small electric heater for the worst case of having cooled down my hottest container to the temp of ground water ~60 F.
So what I'm left with is an "on demand" heater for tap water composed of copper coils running through one container of water heated by the sun, and supplemented by electricity if and when the sun does not do the job. There will likely be additional containers of water that are in series. All containers will have both the glycol and tap water heat exchangers. In a spell of really bad weather I'll bypass the hottest tank and only run the glycol through the "preheat" containers. In this case I'll feed the temperature of the hottest preheat container into the differential temperature controller rather than the temperature of the container with the supplemental electric heat available. Fortunately this is already implemented by my son Eric in the LAN accessible web interface to the Tini board. It takes about 15 seconds to access and change the sensor used.
I have not seen this design before so we may have come up with something new under the sun. A solar electric hybrid "on demand" water heater. Thank you both for your help.
Anybody see a flaw?
ja
John A Allen
John Allen
I imagine I can use a large aquarium immersion heater to provide auxiliary electric heat when needed. Alternatively I could use an industrial over the side immersion heater or a residential Point Of Use electric heater with a small circulating pump.
ja
John A Allen
Tony Luck
> I'm now dreaming of switching from my
> 55 gallon drums to large plywood boxes lined with Metacrylics "A layered,
> watertight blanket of elastomeric acrylics, encapsulating a tough stitch
> bonded polyester fabric. The result is a system which handles roof, deck,
> and building movement regardless of extreme temperature, thermoshock,
> earthquakes, hurricanes or floods."
I'd also been wondering about a "big box" storage tank ... my thoughts
on materials were on resin-fiberglass ... but the metacrylics sounds like
it might be easier to work with. I wonder how gracefully these materials
age in the environment we are planning to subject them to (the storage
tank doesn't sound any more hostile than the roof for which these were
designed ... perhaps less so as there is no UV-light to contend with). But
it would be good to have some guess at the expected lifetime (both to
evaluate the cost effectiveness, and for planning replacement before
failure ... a leak is going to result in a very wet basement).
-Tony
Joe Rich
application. I am sure they have seen almost everything.
Joe
On Mar 19, 6:26 pm, "Tony Luck" <tony.l...@gmail.com> wrote:
John Allen
ja
John A Allen