Heat pumps?

[nuru05]

I see a lot of threads talking about trying to get solar AC on a mono but for some reason no one is talking about heat pumps. What am I missing here? It seems like something that can cool 7,000 btu's worth of capacity while only consuming 500 watts would be pretty useful. Could a person not throw on an extra 800 watts of solar and have cooling during the worst (sunniest) parts of the day?

[HeinSdL]

But isn't AC a heat pump?

Through the use of a compressor the thermal energy in the boat is extracted and pumped into the surroundings (sea water). So a lot of thermal energy is transferred to the sea, much more than the energy consumed by the compressor.

When heating a house (basically by cooling the surroundings) the amount of energy pumped into the house is much greater than the energy consumed by the compressor. It's just that the compressor does nonetheless take up a lot of energy, making it tricky on a boat. (And tricky for any CO2 targets we may want to meet going forward as aircos are ramped up.)

In any case the whole process is also known as the Carnot process, allowing efficiencies of 500% which I used to think was odd, but actually makes sense.

[denverd0n]

I don't know of any heat pump, of any reasonable size, that can run on 500 watts. Maybe it is a matter of terminology. Are you talking about a sump cooler? That is, one that uses a cascade of water, with air flowing through it, for the cooling?

If so, those are great for cooling, and use very little electricity, but they really only work in a very dry climate. When I lived in Denver, lots of houses had them, and they were great. But you NEVER see them in Florida, because they just don't work with the high humidity.

Unfortunately, high humidity is more the rule than the exception in most coastal areas. You never see a sump cooler on a house in the Caribbean, for example.

If I am mistaken about what you are talking about, then I would be very interested in hearing some details. Like I said, I have never heard of a heat pump that only uses 500 watts of electricity.

[nuru05]

[QUOTE=denverd0n;3472555]I don't know of any heat pump, of any reasonable size, that can run on 500 watts. Maybe it is a matter of terminology. Are you talking about a sump cooler? That is, one that uses a cascade of water, with air flowing through it, for the cooling?

/QUOTE]



https://www.defender.com/product3.js...083&id=1848266

[flightlead404]

Originally Posted by denverd0n

I don't know of any heat pump, of any reasonable size, that can run on 500 watts. Maybe it is a matter of terminology. Are you talking about a sump cooler? That is, one that uses a cascade of water, with air flowing through it, for the cooling?

If so, those are great for cooling, and use very little electricity, but they really only work in a very dry climate. When I lived in Denver, lots of houses had them, and they were great. But you NEVER see them in Florida, because they just don't work with the high humidity.

Unfortunately, high humidity is more the rule than the exception in most coastal areas. You never see a sump cooler on a house in the Caribbean, for example.

If I am mistaken about what you are talking about, then I would be very interested in hearing some details. Like I said, I have never heard of a heat pump that only uses 500 watts of electricity.

You mean “swamp” cooler?

[fourlyons]

[QUOTE=nuru05;3472585]

Originally Posted by denverd0n

I don't know of any heat pump, of any reasonable size, that can run on 500 watts. Maybe it is a matter of terminology. Are you talking about a sump cooler? That is, one that uses a cascade of water, with air flowing through it, for the cooling?



/QUOTE]







https://www.defender.com/product3.js...083&id=1848266

NURU, that is 500 watts at 115 volts, not 12 volts. 4.6 amps at 115 volts, but more than 50 amps at 12 volts, that's 10x100 watt panels.

[nuru05]

[QUOTE=fourlyons;3472609]

Originally Posted by nuru05

NURU, that is 500 watts at 115 volts, not 12 volts. 4.6 amps at 115 volts, but more than 50 amps at 12 volts, that's 10x100 watt panels.

I must be missing something I thought a watt was a watt and you get watts by multiplying amps by volts. So 4.6 amps at 115 volts is about 500 watts which is "the same" amount of power as 50 amps at 12v.

So for example this panel https://www.emarineinc.com/lg-solar-...pv-module.html produces 10.8amps at 43v (minus loss) to get a rating of 360 watts


What am I missing?

[seandepagnier]

Originally Posted by denverd0n

I don't know of any heat pump, of any reasonable size, that can run on 500 watts. Maybe it is a matter of terminology. Are you talking about a sump cooler? That is, one that uses a cascade of water, with air flowing through it, for the cooling?

There are lots of ac/heatpumps down to 250watts (for backpacking although there is no way I would put one in my backpack and walk 5 miles) on aliexpress.

250 watt A/C is enough to heat/cool a tent (or 24ft boat) by perhaps 10 degrees.

[QUOTE=nuru05;3472643]

Originally Posted by fourlyons

I must be missing something I thought a watt was a watt and you get watts by multiplying amps by volts. So 4.6 amps at 115 volts is about 500 watts which is "the same" amount of power as 50 amps at 12v.

What you are missing is, that this is only under ideal circumstances. Most solar panels do not output anywhere near their rating which is at the equator at noon at 20C. This combination is also very rare.

It is also rare that people track the sun on both axes, and completely avoid any shade on their boats. It is also very difficult to produce power (to power a heat pump) in the winter, and on sunny days you are less likely to need heat, especially during daytime hours.

With this said, it may theoretically be viable on boats with a lot of solar (like solar-electric boats) otherwise designed for propulsion but having no need for such excessive power or battery storage at anchor. Such boats could conceivably block off a small portion of the boat, with very good insulation, as well as pump water (above freezing) if the water temperature exceeds air temperature to the heat pump and in this way provide some heating in the winter.


I have used wind turbine during storm (25 knot sustained) to cycle a space heater (400 watts) at 50% duty cycle and this heated a small boat by 10 degrees. So consider a large 2kw wind turbine raised in the rigging (used only at anchor) during stormy conditions could for sure heat the boat, and the heat pump if efficient and the boat well insulated this would be viable probably more so than solar at least in very cold weather but it really all depends on the weather conditions.

[rwidman]

An air conditioner is essentially a "heat pump", just hooked up in revers. Many marine air conditioners are "reverse cycle" meaning that they can be set to move heat out of or into the boat.

[fourlyons]

[QUOTE=nuru05;3472643]

Originally Posted by fourlyons

I must be missing something I thought a watt was a watt and you get watts by multiplying amps by volts. So 4.6 amps at 115 volts is about 500 watts which is "the same" amount of power as 50 amps at 12v.




So for example this panel https://www.emarineinc.com/lg-solar-...pv-module.html produces 10.8amps at 43v (minus loss) to get a rating of 360 watts





What am I missing?

Sorry, I was not very clear there; you are correct about watts being volts x amps. I was just thinking it's tough to produce 50 amps at 12 volts (or 500 watts) of solar on the average monohull. Plus of course enough for the other electrical needs. But maybe you have a large cat? Sean put it more clearly.

[denverd0n]

Well, I stand corrected. I had not seen a heat pump that small before. The question then is whether or not 6,000 BTU is enough for your boat.


And, yes, the kind of cooler I was talking about are most commonly called swamp coolers. Though I have heard them called, on occasion, sump coolers.