I am planning to build my own off-grid home in the next year or two. I am trying to figure out, what would be better:
1) heat pumpwater heater with the cooling aspect cooling a food storage room
2) tankless electric water heater
3) Something you come up with (but no gas or propane involved)
While we are able to conserve our hot water usage we do wish to have frequent guests whose water usage we do not want to control too harshly.
Any numbers you have to help me sort this through would be much appreciated, including installation and energy costs. I feel like I just read stuff for an hour and didn't get much closer to knowing which would be better for me.
[Editing my reply because Larisa has a great, simple solar-direct excess-dump electric heating method.]
I'd say - use larisa's offgrid direct-electric method if you want to use electricity.
Generally speaking - any heating and cool takes a lot of energy. Heating water with electricity takes more electricity than you'd expect. Storing electricity off-grid wastes a lot of it and is expensive. So, I don't recommend storing electricity in batteries and then using it to heat water.
Folks at the Solar Living Institute are anticipation large improvements in electric batteries over the next few years (bc of electric cars, etc). They are recommending buying the cheapest batteries and switching. ...These performance improvements may or may not make sense as far as ecological externalities.
I use propane on-demand / tankless water heaters atm. I had a small one for a shower untill the whole region burned down, including my lil shower. I now have a big outdoor one for a offgrid radiant hot water system to heat a nursery. The good thing about it is the whole system minus the heater itsself can turn into a compost heated radiant system as soon as we build it/have a bunch of money... Tankless heaters have efficiency ratings. A good one will use >90somthing% of the propane/heat and a crappy one will use 80somthing% of the BTUs. So I recommend getting a good one.
We use a 12V electric heater element in a standard 10 gallon tank. It can either run as a divert load on the PV system or be manually switched on to get hot water when the system isn't diverting. The water gets heated to about 155*F, at which point we shut it off to avoid boiling it off (we have a digital thermometer placed against the mid point on the side of the tank under the insulation to monitor it). When diluted with cold water, that 10 gallon tank yields more hot water than its small size would when run normally, but you have to know that straight hot water from the tap is too intense. Normally we heat water by PV for at least 8-9 months of the year. In the winter months we have a 1/2" copper coil around our masonry stove's flue pipe that connects to the same tank. We don't have hot water 24/7 but work around weather and stove use and have plenty for our modest means. You can see more about our systems at http://geopathfinder.com/Solar-Electricity.html
seems like solar hot water is the elephant in the room. A direct way to make hot water that can possibly be set up with a thermosiphon to eliminate all moving parts--until you pump it to use it.
This is something where climate can be a factor that might limit it, and backups might include wood fired or compost heat, or even a regular electric or propane backup.
A couple medium sized hot water panels in zone 7 can produce so much heat in summer it becomes a challenge to handle it all.
Whatever you do though, your brief description needs some elaboration, certainly climate, but also site specifics, type of house, zoning, personal abilities to build and maintain, money, can all impact the design you come up with
We use a heat-pump water heater. It's just a standard water heater tank with an add-on heat pump unit that I bought off eBay. On average it uses about 1.2 kwh per day, for just my wife and I. When we had 2 of out grand kids living with us a couple years ago it was averaging closer to 2 kwh, mostly because our granddaughter takes long showers (teenagers, rolleyes)
During the summer it produces about 10,000 btus worth of cooling. Not a lot, but it's basically free.
I recently installed some duct work so it can pull warm air from under the roof during the winter which should reduce it's winter-time energy requirements somewhat.
I considered using a Solar water heater, but talked to a few local folks that had them first and changed my mind. Even though we live in southern Arizona, we still get some freezing weather during the winter, plus average daytime temps in winter are around 50F.
Freezing weather means that batch water heaters won't work during the winter, which means you need a system with anti-freeze, heat exchangers, pump(s), etc. The average power required to run those systems was around 500 wh a day, plus there was typically 3-4 weeks during the winter when they needed to use a backup heating system, which brought the average daily consumption up to around 800 wh.
However, the energy requirements were highest during the winter, when solar production is at it's lowest. Most of the people here use a standard electric water heater as a back up system and they were using 4,000-6,000 wh a day.
When you add in the energy value of the cooling the heat-pump provides during the summer, it was pretty much the same either way, with lower winter time energy requirements for the heat pump.
Final consideration was that the price of the Solar water heating systems (with the collectors, pumps, exchangers, etc.) started at around $5,000 and up, average price was around $7,000. That did not include installation.
For off-grid, tankless is pretty much out of the question. The daily energy requirements will be almost the same as a heat-pump, but the instantaneous POWER requirements are huge.
With an add-on type heat pump you could use the tank as a diversion load for your solar array and just run the heat pump as needed. Actually with some clever controls you could use a resistance coil for a diversion load when you expect to have lots of surplus power and run the heat-pump as a diversion load when you expect only small amounts of surplus. You get the best of both that way.
My opinions are barely worth the paper they are written on here, but hopefully they can spark some new ideas, or at least a different train of thought
I'm definitely an advocate for solar. We have a stainless low pressure tank on the roof with 30 evacuated tubes. As soon as the weather gets to 20c and sunny I have steam coming off of the roof and I have to cover some tubes with shade cloth.
The tank has four plugs on the bottom so I can have an inlet and outlet for a radiator to heat a room in the house and another inlet and outlet for a wetback to boost the water with a wood heater in winter.
We haven't hooked up the radiator or wetback yet.