Thermal storage and water to water heat pump

[jeff5may]

So what is the question here? are you planning on building a custom heat pump? I am here to tell you it is entirely possible. The more planning that can be done before anything is built, the better your custom rig will operate. If you are going to try to run the thing directly off the solar panels and a battery bank with an inverter, you definitely want to go with a variable speed compressor. The single speed compressors need a big jolt of power to start, which limits what you can run that won't kill the inverter. Variable-speed compressors can be soft started, eliminating the startup spike. The decrease in peak demand equates to much more useful capacity.

IMHO, battery banks should only be used for emergency power, or if you are going to have a power source that is completely off-grid. The reason being: charging and discharging battery banks is horribly inefficient. On the way in (and out), parasitic losses are typically 10%. You must also oversize the battery bank as to not have large charge/discharge cycles. For regularly used banks, a 25% cycle is typical. So for 10KWH of storage, you need 40KWH worth of storage capacity. To get more depth of discharge, the AGM or lithium battery types are not economical. All of these factors lead to one conclusion: if you can run a grid-tied system, DO IT!

In your situation, I would definitely look into PAHS and include it in the design. The cistern water temperature could be regulated by the ground surrounding it with a small pump. It could also charge the ground for use a few months later. All the heat or cold that accumulates in the cistern could be spread out or drawn back in using your PV power, so basically free.