Scenario - Solar off Grid - Heat Pump Water Heater and Wood Boiler

[fowlerrudi]

Hey folks, I have a scenario for you. I know you can help. I am still working away at our new home. It is totally off grid - all solar electric power and a big forklift battery.

Here is what I want to do. The topic is water heating. In the winter, I planned on originally running a loop through my boiler storage tank since that tank should always be hot in the winter. This is all fine and dandy. However, my solar array has been putting out way more power than I need, so after the battery is full in the morning - the charge goes into float mode and the panels shut down. So I could be making a ton of extra kilowatts if I had my relay set up to trigger an "opportunity load" that activates when the house has it's electrical loads satisfied and the battery is full.

What I want to do it put a heat pump water heater (such as the Accelera 300 (broken link removed to http://www.stiebel-eltron-usa.com/accelera.html) - or similar) to heat my water in heat pump mode only - and only when there is surplus energy. I think I can handle all this.

But what I am having trouble wrapping my brain around is what order to put things in. First pump my water through the heat pump water heater, then through the boiler storage tank loop (it may or may not be hot at this point, depending if there was excess energy from the sun) then pump it through a combi propane boiler designed to heat the entire house (as backup) and also designed to do DHW. The reason for the propane unit is so my primary heat for the house is propane (insurance purposes) and, if it is cloudy for a few days in the summer (when the wood boiler isn't fired) then the propane boiler will measure the incoming temp of the DHW and then top it up if need be. Ideally the propane boiler will never fire up for DHW, but it will if need be.

Does all of this make sense? I have yet to move into this new house, but my design will handle 25-30 Kwhr per day, and I only expect to use 10 kwhr. Also, if surplus energy is available, would it be possible to have the heat pump water heater contribute to the whole home heating by circulating through the 1000 gallon storage tank? Unpressurized storage.

I am really anticipating your responses! Thanks!

-Rudi

 

[tom in maine]

Hi Rudi,
A Geyser heat pump can directly heat a tank (Pressurized or un-pressurized) directly.
It has a built in heat exchanger and circulator pump which can circulate into the tank.
As for space heating, they are spec'ed as indoor units for DHW. They do perform well down to 45F and output 8000 btus/hr.
Then again, so are the Stiebel units and most other integral tank units.

 

[Floydian]

Hello Rudi,
Sounds like your PV system is way over sized for your needs! Hope you bought this with today's prices vs PV costs from a few years ago.

To me the HWHP doesn't make much sense because your really just redistributing heat that you have already created with your boiler. Wherever you locate the unit you will create a cooling load of about 5,800 btus/hr in the winter, Brrr. I guess if your boiler+storage are giving off plenty heat in an area you don't want it then some PV powered, wintertime AC might make sense......Nah, still don't like it.

Now, a mini split heat pump could be a better option to throw some excess power at for some shoulder season heating. The Mitsubishi units with Hyper Heat can provide the rated output down to 5,
and usable heat down to -13. So the smallest unit would draw 580 watt/hr to deliver 10,900 btus/hr. It could also provide some summertime cooling/dehumidification if needed.
Just thinking out loud here,

Noah

 

[tom in maine]

Good points. My usual installation for the Geyser is for DHW backup in summer.
It can be useful for small space heat backup during fringe months.In such situations, it is usually taking overheat from the house during the day and storing it for later use. At least that is the ideal situation.

I believe Nyle Systems will have a heat pump boiler available next year, which would operate as you desire.
And it will be extracting heat from outdoors, not inside the house.

 

[Floydian]

Tom,
Can you tell us more about the Nyle heat pump boiler? I am guessing it will compete with Daikin Altherma, providing air source heating and DHW? The Altherma seems nice but it sure is expensive, so it will be good to see more domestically made options available.

Thanks,
Noah

 

[tom in maine]

Hi Noah,
They had developed it about five or six years ago. I have been pushing them to introduce it.
There are apparently two versions, both air source, water heaters. One yields 140F and the other 180F.
They are being field tested this winter with Bangor Hydroelectric Company.
I would hope they show up within a year. COP's are in the 3-4 range.

If it is like the Geyser, they are robustly built.
That is all I know right now. They are in the field.

I will try to find out more.

 

[Clarkbug]

I know that the heat pump gives you more "oomph" due to the COP, but following the KISS principal....

Can you just install some electric resistance heaters in your boiler storage tank? This way you have a place to dump your extra kW, and you can heat the house and your DHW (assuming DHW will be heated from boiler storage)

Im curious to hear about the other devices you mentioned Tom, Ill need to replace my electric DHWH eventually...

 

[tom in maine]

Electric is simple and efficient and we have made electric resistance heated tanks. Unfortunately, here in Maine, at least, the cost of electricity is high.
The heat pump cuts that cost to 1/3 of electric resistance, making is pretty inexpensive. So far, the units I see from Nyle look to be built more to industrial standards than consumer grade.

I would suggest that for a small household, straight electric is not too bad. The heat pump cuts that cost to 1/3. And you get free dehumidification, if you need it.
We use them a lot with our tanks for summertime DHW.

 

[fowlerrudi]

The electric elements are the typical way to dump the excess solar power,,however as Tom says, I could get 3 times the heat with a heat pump. I could just put the elements into the big tank, but in the summer I would have to heat 1000 gallons of water in order to get the temp up, I guess a better idea would be a smaller tank for dhw, then when that is hot, dump to another element in the big tank. Also, how could I ensure that all the power that I make off the solar panels goes to a load I wonder? A heat pump water heater would certainly use a set amount of watts whereas typical elements would just use whatever were put into them. A question for a solar forum perhaps. I am not worried about the heat pump stealing a little heat from the boiler room, it's just a little more firewood in the winter and free cooling in the summer. The heat pump wouldn't really need to be running in the winter anyway since the boiler is producing the hot water. If it did run in the winter it wouldn't matter anyway since it is making the hear that it is producing, and electricity is free since it is surplus. What is the cost of a geyser? I would love to attempt a diagram of the entire system, what computer program do you guys use? Maybe I should leave it to the professionals...

 

[Clarkbug]

Thanks for the input guys.

I know that heat pumps are definitely more efficient, it just seems like they could be a pain when just trying to use as a dump for extra kW from the solar panels.

On a related note, I know a lot of the heat pump HW heaters utilize an electric element as backup, in cases of high demand or if the surrounding area is too chilly to make the setpoint temp. Could you just use that same element in the DHW tank for your kW dump? That way it could be used year round, and its a smaller volume to heat up. You would just need to have a means of making sure you could cut off the current if your DHW temps got too warm or you could blow your T&P relief...

 

[pdf27]

How would you throttle the heat pumps to only use the surplus power? Easy with electric resistance heating, not so sure you can do so with heat pumps without damaging them.

 

[tom in maine]

A Geyser runs around $1100. It pulls 700 watts peak. I have no clue about using the Geyser as a heat dump unless it is tied to a system that turns it on (overriding the aquastat) when a system needs to dump. That would be do-able.

 

[kopeck]

I hate to hijack a thread but....

Tom, would one of the Geysers have enough "umph" to keep a 820 gallon bank up to temp suitable for DHW during the summer?

Just thinking out loud, something like this or a stand alone heat pump DHW heater. I can dream, right?

K

 

[tom in maine]

Hi Matt,
I have one operating in the summertime on a 350g tank.
I think heating the tank to 120F works pretty well with the Geyser.
The heat loss is relatively low and in your case there is 820g of reserve.
The operational cost has been the same as the 80g Amtrol commercial tank I had that was foam insulated.

 

[kopeck]

I guess once it's up to temp, it probably doesn't make a huge difference if it's 100 gallons or a 1000 gallons does it? If any thing having the reserve would smooth over some recovery issues...

I'm interested. Making hot water with my oil boiler stinks. The stand alone heat pump hot water heaters are nice but kind of expensive at this point, plus it's just adding something else to my basement. It seems the Geyser might be the best of both worlds and allow me to supplement with solar if I wanted to down the road.

Something to think about later on for sure...

K