|COP (Coefficient of Performance) = Q (Energy Ouput) / P (Power Input)|
Heat Sinks, Hope Rises
One winter, chillin' in Sitka Harbors, we noticed one boat that was pumping water, 24/7.
Normally this is a bad sign. Bilge pump on. Leak. Problems. So we flagged down a Harbor Dude, and wondered if he might not want to contact the owner.
"Oh, yeah", he said, "it's not a bilge pump. He's got some kinda heater going."
Heater??? OMG, it's gotta be a heat pump!!! I started to salivate and haven't stopped.
Later, I ran into the owner on a 20degF day. He confirmed that it was indeed "some kinda heat pump", and that it was keeping his boat at about 60degF. "The equivalent", he said, "of a 1500W heater." (!!!)
So here's the deal. Any mass above 0degKELVIN is a heat sink, seething with kinetic energy. Gaian (liquid) water temps on pelagic-scale heat sinks means there are boatloads of energy out there to 'harvest'.
The notion of extracting heat from a liquid that, should we jump in, might be freaking cold, is counter-intuitive (at least to me). In high school physics I struggled (in vain) against the notion that there is ample heat in a frozen lake to boil a pot of tea. But it's one of those weird and wonderful truths!
The source of this energy is the nuclear-fired Sun. No perpetual motion, folks... when the sun burns out, the free lunch counter is closed. Get over it. Energy is not being produced by the machine, merely extracted from an energy reservoir.
P is the amount of power (energy) invested to harvest latent energy from the fields of the sea. Q is the amount of energy harvested.
This is analogous to eating a big breakfast and gassing up a chainsaw (P) to cut wood. Burning that wood (releasing its latent energy as Q) returns more heat than you'd get burning the food and gas direct. The food and gas are investments against the energy stored in harvested wood. The Coefficient of Performance (COP) of the whole operation is Q / P. The bigger the COP, the better.
Heat pumps aren't magic, or even terribly exotic.
Your 'fridge pumps heat from the masses in its interior, and exhausts heat into its environment. Interior and environment are always tending toward equilibrium (insulation impedes this tendency). Heat pumps use energy to temporarily overpower this tendency.
Ditto a heat sink.
Heat may be pumped to or from a sink. Air conditioners pump heat from the inside of your home and exhaust it in the atmospheric heat sink. And we actually rely upon oceanic heat sinks for our very lives. No other known planet has open, liquid water on its surface. Think about it!
Commercially available units are 'air conditioners', primarily designed to cool cabin spaces:
Pump seawater in.
Raise it's temperature a few degrees using heat drawn from the boat's interior.
Pump seawater out, warming the environment (heat to heat sink).
Some (the ones that have me a'drool) have a reverse cycle, functioning as a heater:
Pump seawater in.
Lower it's temperature a few degrees, diverting that energy into the interior.
Pump seawater out, cooling the environment (heat from heat sink).
P runs the water pump, the heat pump (which raises or lowers water temperature, using refrigerant gas/fluids for the transfer) and a fan to circulate conditioned air.
Q is the thermal energy transferred to or from the water.
Mind your Ps and Qs... in other words, convert them to common units (Watts is convenient).
COP, or Q / P is a ratio (no units, since they cancel out), which currently approaches three... for every unit of energy invested, we harvest about three units. We could say Q = 3P. Our net gain is about 2P (we double our investment). As any banker will tell you, that's pretty good return!
One small, commercial unit made by WEBASTO is rated at 5000BTU/hr, pulling 4.4A on 115V. That translates to P = 506W/hr, Q = 1465W/hr, COP = 2.89 and change.
Downsides are, these units are large, spendy and not at all KISS. Those ratings are set for some supposed-to-be-average condition. For those of us who sail in below-average conditions, performance falls. The power draw is high enough to be prohibitive for cruisers off-the-grid.
Still, 'tis early in the game, and technologies are advancing which could lower P and raise Q, increasing COP. Costs should fall with market penetration and economies of scale.
In the near future, I would love to see some of Q converted to electricity and fed back into P... once kick-started, it could run on harvested energy (with correspondingly lower COP). Since the sink is virtually unlimited, there would be no limit to the system size (aside from practical considerations such as cost and footprint). When not being used as a cooler/heater, it would be in essence a generator.
NOTE: If our WEBASTO example, once up and running, converted heat to electricity (assuming 100% efficiency), then Q = Q-P, or 959W/hr, free and clear. This could go to heat, charging or output power nearly equivalent to a 1K generator. Inefficiencies will gobble some of this up, but it's better than a kick in the head!
A large-scale implementation is now working at the Marine Science Institute in Juneau, Alaska, The heat it supplies replaces 60,000 gallons of fuel per year! Click here to check it out!
Alaska has other projects underway and online around the state. If we can do it here...