PV direct heat pump water heater?

[SajgZi]

I've been pondering the little point of use water heaters as a pre-heater. Don't recall the exact sizes but they're in the 13 gal range I believe. They have replacable heating elements. It would be super easy to take out the existing AC element and pop in a DC one matched to your panels.

Or if you don't want to have solar dedicated to one appliance (I don't like that idea either). Get a bench top power supply (300-600W, maybe 2x), and plug them into smart plugs on the AC side (I'm assuming you have an inverter). On the DC side they go to the water heating element. Do some python to automate the smart plugs (or use relays, etc. to control). Now you're cycling 300-600W on and off instead of 1500W. Should be a lot more friendly for your inverter.

I have thought about extending this with smart plugs and python to have a hierarchy of power needs. Pre-heating the water would probably come 3rd to last in importance. 2nd to last would be zeroing out my grid usage. Last would be space heating/waste heat. The order of the water heating/grid usage of course depends on the current price of natural gas (I have a natural gas water heater) as well as the current price of electricity (not on TOU yet).

 

[Shimmy]

I've been pondering the little point of use water heaters as a pre-heater.

Generally speaking, a point-of-use heater is going to be better as a final heater and not for pre-heat. For pre-heat you want a larger thermal mass-- more water storage.

 

[efficientPV]

Preheat tanks are a waste of money and energy, give me a nice tall tank.

 

[gotbeans]

I don't believe in dedicating solar panels to just one appliance.
When it's not running, that's wasted potential.
Price isn't as important as value.

yep I agree, these AC units etc directly to solar panels are cool and all but what about all of the other things that can be powered as well? lol..

 

[ksmithaz1]

Heat Pump HWH $2000. Regular HWH $600. 500W Solar panel $300. Four extra solar panels $1200 added to your system would easily cover the costs of running the regular HWH, and leave you some money left over, and some extra power, and we still are not counting the needs to heat the water in the heat pump unit, as it still uses SOME electricity. It would help marginally with managing demand, that's about it. Total waste unless you can't mount another panel. I mean the ROI can be low, it just should exist. Even Tim's condensate collection project could have an ROI if it lasts 100 years or so.

 

[Shimmy]

Heat Pump HWH $2000. Regular HWH $600. 500W Solar panel $300. Four extra solar panels $1200 added to your system would easily cover the costs of running the regular HWH, and leave you some money left over, and some extra power, and we still are not counting the needs to heat the water in the heat pump unit, as it still uses SOME electricity. It would help marginally with managing demand, that's about it. Total waste unless you can't mount another panel. I mean the ROI can be low, it just should exist. Even Tim's condensate collection project could have an ROI if it lasts 100 years or so.

FWIW, that depends on if you can make use of the heat removed from the space the heat pump water heater sits in.

 

[ksmithaz1]

FWIW, that depends on if you can make use of the heat removed from the space the heat pump water heater sits in.

A calorie is defined as the amount of energy required to raise 1cc of water 1 degree C. So Lets take a highball glass drop a few ice cubes in it say 100cc. Now lets take a shot of whisky and dump in in the glass, around 110 calories, and another 100cc of water. Sip it as the ice melts, somewhere along the line your body needs to raise the temperature of the 200cc's ice water from 0 to 30C so 30x200 = 6000 calories, so you should be able to sit around and drink scotch and water on the rocks all day and lose weight like mad.

This does not work. Well OK, the argument is seriously flawed for a number of reasons as well, but hey it SOUNDS good doesn't it?

I don't think your cool the room idea will work either. I doubt you could get much benefit from the cool side of the compressor on an HWH heat pump. Your active cycle times will be fairly short, and the compressor itself and fan are using electricity and generating even more waste heat. When you start talking $1400 more money per unit, I think it would be better spent on beefier inverters to match demand, or more panels and batteries, from which you could use the energy elsewhere.

On an average day my HWH uses maybe 8kwh. Now I grant you it's only two adults, add a couple of teenagers taking 3 showers a day, it goes up, significantly, higher on the weekends, so lets make it 10kwh for round figures. A heat pump hwh is only going to use 1/3 of that, so think of it in terms of an AC unit that is going to us a mere 10kwh a day, except you put the compressor inside and it kinda gets warm too. I doubt you could produce enough cool air to really notice, but at least you paid an exorbitant amount of money for it.

I'd make similar arguments for heat pump dryers. I mean if you use a crap-ton of hot water (commercial laundry or something), or dry ton's of clothes, perhaps. Just not seeing any cost benefit at all from my chair.

 

[gotbeans]

1000 calories = 1 Calorie which is why your math doesn't work. To heat that up is only 6 Calories
A food Calorie is kcal or kilocalorie

found this post, seems to be the best one (as it relates with what you said above):

The calorie and the food Calorie

 

[ksmithaz1]

1000 calories = 1 Calorie which is why your math doesn't work. To heat that up is only 6 Calories
A food Calorie is kcal or kilocalorie

found this post, seems to be the best one (as it relates with what you said above):

The calorie and the food Calorie

Yep, one of many fallacies in the argument. A food 'calorie' is a "kilo-calorie", but if you don't dig into the argument and take it on it's surface, it sure does sound like a good idea. I have this 18yro bottle of, . . . It seems like you could take a heat pump hot water heater, and drop it in a room and let it cool the room while it heats your water. Win-win right? The problem is your not extracting enough heat and you are generating waste heat, not to mention you have this big honkin' 40 gallon tank in the room. They don't make small ones because the overhead of the compressor. Now if you were a commercial hotel, on the grid, with some 100 gallon units in the laundry area that also fed guest, blah, blah it might make sense, cut on-peak costs, and the like.