Super efficient all-electric bus water heating

[WiredForStereo]

I've seen a couple all electric buses in videos, even without generators, just a generous helping of solar panels and lithium batteries.

I like this idea. I can do the solar and the batteries. I already have a generator I could use so I'll table that for the time being.

Heating and cooling should easily be handled with a mini-split.

The thing that I'm a little hung up on is water heating. I've seen a few small seven gallon electric water heaters on buses, but electric resistance heating is just about the worst option efficiency wise. In my current home, I have a heat pump water heater, which I love because it uses a ridiculously low amount of power ($5-$10 a month) and provides free air conditioning.

So, I've been looking for an option for heat pump water heating.

I have discovered a unit that looks like the outdoor unit of a mini-split, but it's a bit big maybe, and you'd still need to have a tank somewhere.

Has anybody come across anything like this?

[Bert06840]

I’m building a multi-source water heating system. Diesel, and electric, and engine heat. Not from a heat pump.

The basic physics is pretty simple. It takes 2.44 Wh to heat one gallon of water by 1 degree F with resistive heating. If you use 20 gallons a day (less than half of U.S. residential use per person), and your delta T is 80 degrees F (from 50 degrees to 130 degrees, for example), then you’ll need:

2.44 x 80 x 20 = about 4kWh. In summer, in the U.S., at reasonable latitudes, that means you need 800 Watts in solar to power it.

You could use a heat pump to produce heat for your water, say that it runs with an efficiency factor of two (your delta T is high...), you’re down to 2 kWh a day. It will also take a substantial investment.

In the winter (not a Florida winter) you’re in trouble though. To harvest enough sun to heat water, you’re gonna need an enormous array. WAY bigger than reasonably fits on a bus.

[WiredForStereo]

Originally Posted by Bert06840

I’m building a multi-source water heating system. Diesel, and electric, and engine heat. Not from a heat pump.

Thanks for the response, can you give me a quick rundown of your system?

[joeblack5]

Nyle water system used to make a separate heatpump based water heater for domestic hot water..it would not be hard to use a currently available heatpump based hot water heater and vent the cold air inside the bus as a/c or pull and vent outside during the heating season.

We use a sanden CO2 based heatpump designed for the floor heating in our house.
Many larger house heatpump have a domestic water heat exchanger that uses the superheat from the heatpump as a side product to heat water.

The heat is around you just have to capture it.


Here we are experimenting and using a short evacuated solar tube and aluminum and wood scrap found it the desert. It made a 2 cups of boiling water in 25 minutes, good for tea.
The other pic is us with a a solar thermal plant, Security was not impressed with our competition.


Good luck,
Johan

[Bert06840]

Originally Posted by joeblack5

Many larger house heatpump have a domestic water heat exchanger that uses the superheat from the heatpump as a side product to heat water.

The heat is around you just have to capture it.

What absolute temperature levels are we talking about? I’m mean the hot side of the pump. Can it generate 70C-80C? They would seem very high for a heat pump. I am trying to avoid having to make a dual-stage system. It’s already complicated enough

The other pic is us with a a solar thermal plant, Security was not impressed with our competition.

Haha, this is fantastic. What would you expect if someone is waving half a kilowatt death ray around? And to them it would seem you needed a length of quartz tube, too, conveniently available nearby

[joeblack5]

I think 140f- 60c with a cop of 2-3.
Sanden CO2 is Interesting.. look it up if you have time.

The superheat for domestic is in more or less conventional heatpump.

[Bert06840]

Originally Posted by joeblack5

I think 140f- 60c with a cop of 2-3.
Sanden CO2 is Interesting.. look it up ...

Yeah, I had not heard about this. Sounds indeed rather promising. A cop of ~5 for ~70 degrees F delta t is astonishing. Thanks for the pointer.

Maybe I’ll talk to them to see if they would sell me the pump without the tank, because 40 gallons is not practical for a bus.

[Bert06840]

Originally Posted by WiredForStereo

Thanks for the response, can you give me a quick rundown of your system?

Sure, I’ll sketch it out in a while. I need it for my records anyway.

My combined hydronic/electric water heater just showed up. I won’t use the electric element often, only as a load dump for my panels when the batteries are at 100% state of charge and I have excess electricity to burn.

[Bert06840]

I’ll make the coolant circuit part of our hydronic system, which later may even include the ability to harvest engine heat.

I don’t think that harvesting engine heat will be giving me too much efficiency gains, as we tend to stay a few days at a single place, so I’m in no special hurry to include that particular feature.

[WiredForStereo]

Originally Posted by Bert06840

I’ll make the coolant circuit part of our hydronic system, which later may even include the ability to harvest engine heat.

This is good stuff, I'm particularly interested in the shape and configuration of the water heater, that might even be able to fit in my belly bay. Most water heaters are vertical.

As far as harvesting engine heat, I like the idea, the problem is engine temperatures should easily pop your T&P valve, so I'll have to think that one through.

I would think retaining one or more of the stock heaters (perhaps altering the configuration and function) would be the most effective solution for space heating while the engine is running, and presumably under way. However, I just don't see much use to use engine heat for water heating because I don't think the shower will be in much use while we're moving.

I have been considering in-floor radiant heating though, because, I'm building the thing, why not? Putting that together with a diesel water heater to heat the thank and the loop sounds like a good idea for cold weather.

[Bert06840]

Originally Posted by WiredForStereo

As far as harvesting engine heat, I like the idea, the problem is engine temperatures should easily pop your T&P valve, so I'll have to think that one through.

You'd need to control it, of course. I'll be controlling it electronically, so that the water heater temp stays around 160 degrees F.

Surely there are also mechanical/passive valves that do the same. But I like electronics so that is how I'll skin that cat.

[Bert06840]

Originally Posted by WiredForStereo

Thanks for the response, can you give me a quick rundown of your system?

I did not have enough time today to finish it up, but here is a start of one part of the system.

I still need to add the sensors, the controls, and then - of course - the actual heat input sources.

[WiredForStereo]

Unpressurized intermediate tank, not a bad idea, if you have the space.

[JWes]

How much power exactly do these heat pumps that you're talking about use? Both the minisplit air conditioner heat pump, and the water heater heat pump.

[WiredForStereo]

Originally Posted by JWes

How much power exactly do these heat pumps that you're talking about use? Both the minisplit air conditioner heat pump, and the water heater heat pump.

IIRC, my heat pump water heater here at home uses 1.6 amps at 240 volts. That's a 40 gallon tank.

Assuming a COP of 3, that is equivalent to 1152 watts resistive. I think it operates more like 3.5 and possibly even higher because it pulls air from the hot moist bathroom while people are showering. It is slower to heat, but it is so efficient. Even with 5 of us (and occasionally my inlaws) it has never had problems keeping up.

In a bus, I think 20 gallons is pushing it, and probably more like the 11 gallon unit, I'll have to look at the specs.

Showers only on sunny days. Resistance type backup will be good to run off the generator or engine if necessary.

[WiredForStereo]

Originally Posted by JWes

How much power exactly do these heat pumps that you're talking about use? Both the minisplit air conditioner heat pump, and the water heater heat pump.

The SanCO2 system is more powerful, 4500w of heat, COP of 5, which works out to 3.75 amps at 240 volts.