That's a fair question and I've not done the math. Does "overall cost" include long term usage - I suppose it does or it wouldn't be "total".
For example, I spent a bunch of money to install solar panels, house insulation, and soon a heat pump. They each have, respectively, about 9 year, 30 year and probably 15 year payback time at today's energy prices, so depending on lifespan and future energy prices it's an open question if it would've been cheaper just to stay as I was.
But there's the comfort factor (heat pump should regularize house temps) and security factor (still warm & cosy during a power cut). I'll allow myself a little feel-good factor of carbon emission reduction too. Maybe these can also apply to the question of using a heat-pump in the dirt-as-thermal-storage scenario here.
I suppose overall I'm of the mind we should be collectively treating carbon emissions as the highest priority and using a heat pump here might aid that?
The key insight is that the "cost of inefficiency" is proportional to the number of charge/discharge cycles in the storage system. That's because, for a given storage capacity, there is a cost from the extra energy thrown away at each cycle.
However, the cost incurred in building the storage system is independent of the number of charge/discharge cycles.
So: in comparing diurnal (daily) storage systems, with seasonal (up to yearly) storage systems, the relative importance of efficiency and capex are radically different. In the latter, capex can be 365 times more important than in the former.
For seasonal storage, one is strongly driven to minimize capex, even at the cost of making round trip efficiency worse.
For example, I spent a bunch of money to install solar panels, house insulation, and soon a heat pump. They each have, respectively, about 9 year, 30 year and probably 15 year payback time at today's energy prices, so depending on lifespan and future energy prices it's an open question if it would've been cheaper just to stay as I was.
But there's the comfort factor (heat pump should regularize house temps) and security factor (still warm & cosy during a power cut). I'll allow myself a little feel-good factor of carbon emission reduction too. Maybe these can also apply to the question of using a heat-pump in the dirt-as-thermal-storage scenario here.
I suppose overall I'm of the mind we should be collectively treating carbon emissions as the highest priority and using a heat pump here might aid that?