EDIT: I'm settling for a simpler system: no solar thermal panels; just a small 50 liter water heater, AC powered, run through the inverter. (So, practically no hot water in the winter time.)
I'm planning to build an off-grid summer house in southern Finland.
The plan includes:
electricity: PV panels + inverters + batteries, enough for a 2000 W induction stove (single burner).
water: a well for drinking/kitchen/shower water, an electric pump
heating: a body of water (or a non-toxic propylene glycol mixture) is heated in the heat tank. The water from the well would be heated in a coil of the tank, thus the water we use will never stay in the heater. The tank would be heated by solar thermal panels and/or a water heating fireplace. A small pump is needed for the thermal panels and another one the fireplace.
The cabin would mostly be used during the summertime (June to August), when the building doesn't need to be heated. For May and September-October, some heating will be required, and for the winter months, a lot of heating. The cabin is 70 m² (750 sq ft).
Since there is no external electricity and the outside temperature gets below 0°C (32°F) for several months, some measures must be taken. The lowest temperatures are -30°C (-22°F), which would mean 50 % glycol.
Batteries will survive only if they are fully charged for the winter.
The tank (and all pipes) must either be emptied for the winter, or it must contain non-freezing glycol mixture.
I think I now need to decide between these two options:
1) Fill the tank with water, and empty it for the wintertime. Heat it with solar heating panels.
- simple system
- the fireplace is not connected to the tank, so it’s easier to use it for heating the cabin in the winter (without the risk of overheating the tank)
- no possibility for water in the wintertime
- obligatory autumn maintenance
2) Fill the tank with propylene glycol. Heat it with a water heating fireplace, and possibly solar thermal panels.
- less maintenance?
- Possibility for occasional hot water even in the wintertime? (Must empty the water pipes every time, though)
- more expensive
- have to add radiators (using the same glycol) to be able to heat the cabin in the wintertime, distributing the heat from the fireplace
The second option raises some important questions:
Is 50/50 glycol too thick for the pumps? Can 50/50 transfer enough heat?
How often must the glycol be changed? I need 500 litres (130 gallons), so the expense is non-trivial.
Most important: is this feasible? Does anyone have this kind of setup? I found very little information on the subject.