Rooftop heat-pump water heater room
I am considering putting solar powered oversized heat pump water heaters on the roof of a townhouse building. They would provide hot water for plumbing and hydronic air handlers for winter heating. One rooftop room could hold several water heaters for multiple townhouses.
The exterior is EIFS and the interior is black cement board on the floors and walls. South facing windows let the sun warm the surfaces and make the water heater more efficient.
The question is, would something like this store enough heat during short daylight hours in the winter to be worth the cost assuming the building is properly sealed and insulated?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
I think it really depends on where you are located and how much winter sun you get. Around my neck of the woods (Toronto), unless you have a very large array, you don't get enough sun in Jan/Feb to be able to heat a house with electricity using a heat pump.
Most heat pump water heaters have a 4500btu heat pump, some like the Senden go up to 15000Btu, so you will not be doing much heating with that, except maybe a small passive house.
More realistic one would be to provide hot water, which if the glazing and solar PV is sized correctly, would be possible with your setup. The setup would still need to be connected to the grid in case there is a cloudy day. The setup would still be more expensive than a 2x solar panels and standard electric water tank though.
Would the cost of building a room and purchasing and plumbing all those HP water heaters offset the cost of a conventional Solar Hot Water system?
As Akos pointed out, it would be helpful to know your climate zone or geographical location. In general, the need for space heating correlates poorly with the times of winter sunlight, and in many parts of North America, there isn't much sunlight at all in November and December.
This doesn't sound like a good use of resources. Assuming you live somewhere where the local electric utility supports PV installation, you should take the money it would have cost you to build this room and to buy all those heat pump water heaters, and just use that money to buy PV modules.
Thanks for your responses. I am in Atlanta so we don't have to endure the cold temperatures as long as Toronto. We were looking at storing heat in hot water to reduce battery size. Looks like it is not as economical. I appreciate your thoughts!
Why does your building have a battery?
Installing the water heaters in a closed room creates a real possibility of overchilling the room. Once the solar heat runs out, the water heaters will continue to extract heat from the room regardless. If the water heaters don't have a low temperature lockout to switch to resistance heat, you will need one.