A water bottle for your boat?

Random thought....

I recently traveled to a tented camp lodge with my family. To combat the cool nights in unheated accommodations, the staff placed old-fashioned rubber hot water bottles with woolen covers in the beds. That not-only pre-warmed the beds, but the bottles were still putting out noticeable heat the next morning

That's a neat trick for your bunk regardless, but what about scaling it up? Passive and active solar heating systems for homes often use water to store heat. Why not rig a water tank on or under the cabin sole in a small boat as a heat storage medium charged by a solar collector and/or the cookstove?

Even without benefit of the sun, running the cookstove to heat the water then shutting it down to sleep soundly with just the radiant heat from the hot water and no worries of fire or carbon monoxide sounds pretty attractive.

Thoughts? Don't worry, I've got my flameproof underoos on.

 

That`s is a nice thought..we always take our old fashioned hot water bottles every where we go camping we do not believe in trying to be tough guys.

 

A waterbed with a solar heat collector ?

I think you are onto a great idea.

 

You could design a thermo-siphon to keep water flowing from the bed/reservoir to the collector/heater to constantly heat it. Of course the best thermal mass heat source is an iron diesel engine that has been run all day long......

 

True enough, though an unpowered thermo-siphon (natural convection pump, hot water rises, cool water sinks) works better with the collector located below the tank. Plus, I wonder if movement of the boat shaking and mixing the water might hurt the thermo-siphon effect.

I had in mind something mechanical, but simple, like a black-painted stainless tank in the cabin connected to a solar collector on the cabin top and powered by a little 12v pump connected to a solar PV panel. The motor would be connected to a couple of thermostats in series so that water would only be pumped through the collector if 1) ambient temperature was below, say, 70°F and 2) temperature in the collector above, say, 80°F. The key would be finding a low flow, low current draw, quiet 12v pump.

 

Salt water holds heat better than fresh - very salty water even more so.

Maybe a thick salty heat reservoir ?

 

That's an interesting notion, though I don't know where I'd find the numbers on salt vs. fresh water. If going that route, probably better to use a plastic tank as a hot saturated salt solution would likely be awfully corrosive. Maybe something like this, which is rated up to 180°F? That should be enough, but I might need another thermostat in the system to limit the temperature in the tank.

 

Yes, it would take some experimentation for sure. Its used on a big scale in heat ponds in Israel, but I have never heard of it being used on a small scale.

This guy takes the process to a new level
http://www.builditsolar.com/Experimental/PCM/DIYPhaseChangeMaterial.htm

a recipe for phase change solution " freezing at 26 C and melting at 30."
" ..... 1 liter stores 225 Btu ...."



I have been researching heat storage for my shed, where I have a huge old compressed air storage tank, that I was planning to fill with oil, heat it with sunlight, and run it through hydronic pipes for a heating system.

I got the inspiration when finding that a big lump of steel was still hot to touch two hours after the sun had gone down.

 

Hold on RW
I was reading the other day that Australia is moving at a rate of 60 to 70 cm a year towards Indonesia.

You'll soon be warm enough.

Poida

 

For what it's worth, I looked it up and while salty water has a higher boiling point, the salt actually reduces the specific heat capacity of the water. In other words, it requires less heat to raise the temperature of salty water one degree than it does to raise the fresh water one degree. So, aside from freezing issues depending on the climate, fresh water is actually better for this purpose.

 

Thanks for that followup Fred. I has a good look through my files for my source of information, but of course, couldn't find it.

So, I quickly googled stuff to refresh my understanding. It seems that its a contest between Specific Heat and Total Volumetric Capacity

eg.
Water has one of the highest thermal capacities
Heat capacity - 4.2 J/(cm³·K)
whereas concrete has about one third of that. On the other hand concrete can be heated to much higher temperatures – 1200 °C ...... and therefore has a much higher overall volumetric capacity.
http://en.wikipedia.org/wiki/Thermal_energy_storage


For saltwater, the boiling point is raised, and the freezing point is lowered. (~ -2  °C) The range depends on how much salt there is in the saline water. The boiling point of salt water will rise by about half a degree Celsius for every 30 grams of salt dissolved per kilogram (litre) of water. ( maximum 359 g/L )
The boiling point of salt water greatly depends on the amount of salt in the water. Typical salty seawater has a higher boiling point than water, and boils at 103 degrees


Given the lack of hard data, I can only assume that the increase in density and increased phase change range ( freezing/ boiling range ) was supposed to make the salt water a better storage solution due to total thermal capacity.

Either that, or my source was wrong ( or at my age, I misremembered )

I will keep looking, but any other info would be welcomed.









Odd information and links I will keep here for interest sake.

Sodium chloride, Melting point 801 °C


(kcal/kg oC) (kJ/kg K)
Salt 0.21 0.88
Sandstone 0.22 0.92

http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html


(kcal/kg oC) (kJ/kg K)
Water, fresh 4.19 1
Water, sea 36oF 3.93 0.938
http://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html

 

Thanks, RW. In this case, since we are dealing with relatively low temperatures and not looking to boil the water in the collector, I think fresh water has the edge.

One other thing that came to mind...on a boat with inside ballast, it might well be possible to use the ballast as thermal mass. For example, in a boat ballasted with scrap iron in concrete, you could lay the iron and pour the concrete over some rigid foam insulation in the bilge and run some hose through it to create a heat store a bit like a radiant floor. Less efficient by weight or volume as heat storage than water, of course, but if it's already there, who cares?

A radiant floor need not be very hot, in fact, it's uncomfortable if it get's too hot. With a wood cabin sole, I think heating the ballast to 80-90 °F or so would be enough to keep the cabin nice and toasty, though insulation would be key to keep the cold sea from sucking away all the heat.

 

Would increasing the insulation of the hull sides or the cabin roof have more benefit?

 

I'd probably need both for real cold-water, cold-weather sailing, just kicking around ideas for now. The boat I have in mind has a large, fixed 80-gallon water tank placed to serve as ballast and two flexible 20-gallon tanks. I could definitely see insulating the big one as a heat storage tank which would still provide emergency fresh water in a pinch. Maybe run some hose around the perimeter of the cabin as baseboard heating?