Replace solar water panels with solar electric + heat pump?

vilord

We live in central Massachusetts, where it is cold for at least y'know, half the year. The solar roof panels work when it is cold out if the sun beats down, but they don't do a lot. In the summer, they work great and significantly reduce our oil spend.
The system we have is a drain-back system that runs coolant, with a 100 gallon dual-coil indirect. It's about 11 years old. The tank itself is in great shape, as are the two 4'x8' roof panels.
We are getting quotes to install solar electric on the roof, and the 64 square feet of roof space that the solar water system takes up would give us a bunch more electric panels.
Would it make economic sense to rip out the solar water system, bypass the drainback pump, and hook up a standalone heat pump heat exchanger to the lower loop in our indirect tank? Do they even sell standalone non-tank heat pump water heaters anymore?
Thanks!

Jamie Hall

No, it wouldn't make either economic or environmental sense. Your solar water heater panels are very very close to 100% efficient at capturing the energy from the sun. (keep them clean!). A top end photovoltaic panel might capture 15 to 20 percent -- but that's top end, and new. They degrade.

Keep your drainback system. You can put photovoltaics on the rest of the roof, if you like -- not a problem.

The best use for solar energy is direct space heating with a properly designed structure. Next best is water heating (either for domestic hot water, as you are doing, or hydronic space heating). Well down the list is generating electricity...

jumper

Solar thermal doesn't save much even in sunny warm states with very high utility rates. But 64 square feet PV doesn't produce much either. So if the panels are truly in good shape .....

hot_rod

It seems like a bit too much tank for the two 4X8 collectors? Typically 2.5- 2 gallons of tank per square foot of collector. What is the second coil in the tank connected to?  I would run that system for another 10 years. I woukd guess in your climate you could expect a 40% solar fraction, offsetting 40% or more of your DHW needs
Depending on your DHW needs, average families spend around $400 per year for DHW.

vilord

2.5 gallons of tank per square foot of collector tells me that 100 gallons for 64 square feet might even be too small, not too big!

Second coil is connected to our oil boiler.

vilord

Sounds like we should a) leave it alone, and b) give the panels a good cleaning!

Solid_Fuel_Man

If I had a good working system that was saving me $$$ then there would be no way I'd remove it....

ethicalpaul

I installed a state-of-the-art phase-change solar tube water heater array on the roof of my old house. It had a nice controller that would decide when to circulate the loop fluid (with antifreeze), and a storage tank that fed into my water heater.

I watched it work correctly and I loved installing it, and I don’t regret it. But it was sold as having a 3-5 year payback, which was ludicrous. The key is what @hot_rod said above: hot water only costs a few hundred per year. The payoff never comes. 

The best way to have efficient affordable hot water is a heat pump heater with a tank (like the Rheem)