[XCSSA] Solar and Green events in SA

[xcssa@xcssa.org]

On May 15, 2007, at 11:17 PM, xcssa-admin@xcssa.org wrote:

> On Tuesday 15 May 2007 14:27, xcssa-admin@xcssa.org wrote:
> [...]
>> WRT solar water heat, I'd like to talk to more people about that,
>> such as that the Green event.   But based on the web research I've
>> done, it's not as much a clear win as it seems.  Among them, there's
>> a load balancing and "vacation" problem.  If you make the collectors
>> too big, it's going to be getting too hot.  Once all the water is at
>> maximum temperature, you have excess heat.
>
> Why not just have a simple bi-metal thermostat valve engaged heat  
> exchanger..
> Just like in the engine of a car.  The water hits like 180.. and  
> the thermo
> valve "pops open", giving a secondary route for passive cooling  
> through a
> convection, radiator/air cooler.  Simple.. passive.. closed loop 
> (s).  It
> could even be designbed to have a secondary higher temp "emergency  
> valve"
> that trips at 210 (well below boiling for pressurized, coolant  
> mixed water).
> And then.. your fail safe is a high temp/pressure blow valve..  Triple
> safety.. no active components.

The typical "indirect" solar water heater is microprocessor  
controlled, and does indeed do something like that.  My beef (or  
misinformed guess) is that that they don't take the "convection  
radiator" part all that seriously.  In several hours of web  
searching, I found only one example of a "heat dumping" device for a  
solar water heater, it it was a simple home-brew coil of tubing.  It  
didn't look adequate to me to deal with 1-2Kw of excess power on a  
continuous basis.  They seemed to be relying on (1) system "sizing",  
(2) vacation covering, and (3) overall random heat leakage to deal  
with the problem.  They also assume that the main water tank is OK  
with having its temperature vary over a wide range, from hot (say 140  
degrees F) to near boiling.  That appears to be a big part of the  
implied heat dumping strategy.  And then occasional pressure  
release.  Seemed like poor design to me.  With a good heat dumping or  
rejection strategy, you could keep the temperature within a  
relatively narrow range, increasing lifespan of all components, and  
not worry about system sizing or vacations.

BTW, I like the "hot tub" heat dumping strategy!  Assuming it's not  
in air conditioned space.

Then elsewhere I found some reliability studies.  One of the big cost  
reliability problems is main tank leakage from a history of large  
thermal cycling.  Connect the dots.   And typically, in order to  
withstand the temperature cycling, much higher quality tanks are used  
than in conventional water heaters.  Such as solid stainless steel.   
But it can break at the welds.  Other problems could be traced to  
heat abuse of the heat transfer glycol.  In an indirect system, the  
glycol flow shuts down thermostatically when heat isn't needed or  
available.  Then it cooks in the hot sun, getting partly solidified.   
Then that destroys the pump.  Pump failure is probably the single  
most common problem in an indirect system.

You can imagine that the "heat dumping" isn't typically a big selling  
point.  Typical consumer might wonder why heat needs to be dumped at  
all...it seems like a waste.  Also the question is where to put the  
heat dumper.  Probably outside, in the shade, and out of human/pet  
reach if you are seriously dissipating 2Kw at about 200 degrees F.  
But I didn't find any designs that did that.  In one diagram it was  
right next to the tank.

I'm spending thousands of dollars, I want a tank that's going to last  
for decades, and a pump that's going to last for at least one decade.

>
>
>> It's not to say that better water heating couldn't potentially be a
>> very big win.  The average person in the US uses nearly as much
>> energy heating water as powering their car.  Look at your electric
>> water heater and be amazed by the annual cost estimate.  And kW water
>> heating panels are a tiny fraction of the cost of photovoltaic.
>
> I think that's a good two pronged approach..
> Use a daylight only PV system (no costly/toxic batteries) feeding  
> juice back
> onto the grid during the daytime, coupled with a hydro-thermal  
> system to
> supplement/replace the hot water.  I bet that would but my eBill by  
> at least
> 75% each month. Hmmm...
>

Yeah, using both photovoltaic and solarthermal water heat is what is  
typically done in "demonstration homes."  Sometimes the solarthermal  
system also provides house heat when needed.

With enough money invested, you can have a negative monthly energy  
bill...at least in principle.

Charles