Ambient heat engine

[memoryman]

Peltier devices have a low efficiency (~10%?) and are expensive.

[Philip Hardcastle]

There are some free energy possibilities that can be explored, one that I think is overlooked is the temperature difference between the ground and the sky.


A reliable source of warm is in the ground, the deeper the better, and on clear days and nights the sky is a reliable cold.


The sky temperature in most places is about 1C (34F), though it can be much colder, and the ground temperature is typically 21C at modest depth, this gives a 20C differential. Such a temperature difference is the basis for OTEC (Ocean Thermal Energy Conversion).


For people living off grid and in desert locations a mix of solar and OTEC is ideal, but then the new 21-70 batteries make storing PV now cost effective

[gotoluc]

An ambient heat engine was something I pondering on for a while during the build of my 28 foot Solar electric catamaran house boat with 2.5Kw of solar panels on the roof to drive the 2 electric motors (one per pontoon) with about 2Kw/hr of lithium battery backup.


Here's what I was thinking of. Solar panels are heat collector which drives their efficiency down. I was thinking of adding solar pool roof heaters (tubing) at the back of the panels and pump glycol through them to copper tubing attached to the pontoons (below water line) to cool the fluid. I'm sure the gain in electrical power from the 2.5Kw array would be greater then what the pump would use.
I then thought, what if I add a heat pump between this heat source and sink. There could easily 30 degrees F or much greater temperature difference between the back of the solar panels and the sea water.
So the idea is, could we not use the electricity from the now cooled and better performing solar array to power the heat pump which would create steam and use steam engines to power the houseboat instead of electric motors. Could the steam engines possibly be a greater propulsion power then using the 2Kw directly to electric motors?


I'm no expert at heat pumps and refrigerants but I'm good at thinking up ideas like these


Please let me know your thoughts on this.


Regards


Luc

[Philip Hardcastle]

Hi Luc,


Your ideas are interesting, but alas the gains are not likely to be as good as you would hope.


A few things, we are all talking about heat engines etc, the hated Carnot limit is a guide we should all observe when considering the engineering of differential temperature heat engines. The formula is know to all and it says that if you have 20C delta then you can only get a maximum efficiency of about 6%, and probably only 4% when you take into account real world losses such as electric pumps.


That you have solar and li-ion is the way to go for value for money, and having a large towable pontoon deck with a large solar array would be a cool thing to do. Remember that long pontoons have less drag than short fat ones.


The 21-70 batteries I mentioned are predicted to drop to $100 per kWh, so that will be awesome.


Solar PV efficiency at say 15% is always going to beat a thermal alternative, as memoryman pointed out most peltier devices are low efficiency.


Whilst OTEC is low efficiency it relies upon having massive reservoirs of heat to work with, so even 5% efficiency makes sense, but in the case of a marine vessel you need maximum bang for the buck. Better to invest in the pontoon or some sort of PV wings that can be lowered in smooth conditions (perhaps with a simple outrider).


If you do want to consider thermal as a secondary generation matter the cost is likely to be much higher than that of more PV.


My view of solar sailing, and I am a keen observer having a desire to build a round the World vessel one day, is that the hulls of a catamaran can be super slippery through water and can be well filled with 21-70 batteries to have a capacity of say 1MWh, this coupled to a large foldout PV array of say 50kW would mean that by the time you have spent a few days in port you can make a 24 hour crossing to the next port or anchorage.


The issues of low drag seem to be best dealt with by the idea of having the hulls essentially open bottomed such that they trap air, the pumps needed to keep them filled needing less power than that needed to push ordinary hulls through the water. The obvious additional benefit being that the hulls are barnacle free.


The idea of 50kW arrays that can fold away in rough conditions seems sensible, and in smooth seas 50kW will give you good speed during the day. Depending on your desire to make long distance voyages you can of course run 24/7 at a lower speed and so match input energy to motor usage. I think the super slippery hulls and due care to make the above water areas aerodynamic could have a 24/7 cruiser maintaining 10 knots.


For people interested in getting a higher efficiency out of simple Stirling engines might I point out that the temperature of some collectors can get much higher than others, in fact one of the best solar collectors is black nickel, this can achieve an equilibrium temperature of over 200C, if in vacuum, and if such a high temperature is used than Carnot would predict about 40% efficiency (but say 30% real world - 2 x most PV).


Anyhow Luc, I hope my comments are of interest to you, happy sailing.

[gotoluc]

Hi Philip,

glad I caught your attention... thanks to your boating interests.

BTW, the catamaran solar powered houseboat was an all in one (not something I towed)
I chose a catamaran hull for best displacement characteristics (speed and efficiency)
The ideal is to navigate during sunlight at the speed (watts) the panels provide in real time. The batteries were just a backup in case you need a little extra power for maneuvering in and out of port.

So a 28 feet x 8 feet array of vacuum tubes with black nickle collector plates would do a better job then photovoltaic panels. Not a bad idea!
I guess parabolic concentrators (mirror polished stainless steel) could be used to reduce the amount of tubes and cost?

Interesting stuff!

Thanks for sharing

Luc