I really can't help but think there might be a fundamental problem here. If heat loss in dirt is so incredibly slow, and we use a heat pump to extract that heat, then wouldn't it be equally slow to replace the heat extracted? (absent heat injection, i.e. reversing the heat pump's operation in summer)
Has anyone looked at the subsurface ground temperatures after days, weeks, months, even years of heat pump operation?
I do seem to remember seeing one article on the subject showing that after one winter the subsurface temperature had declined enough to materially affect the heat pump's COP. But the timescale didn't extend to multiple years.
In one of eight cases studied where heat flow is unidirectional (cooling load only) over a 20 year timescale the authors find:
"the mean ground temperature ... increased from 21.87 °C to 26.18 °C, ... . This significant rise could have a potential impact on the performance of the system in the later years of its operational life."
The other 7 cases showed weaker or negligible long term variation.
An additional graph shows COP variation over 15 years and the worst case shows a decline of perhaps 10% (just eyeballing it).
Surprisingly, some cases showed a long term improvement in heating COP - presumably the injection of summer heat into the soil made for warmer soil than just sunshine and natural diffusion?
Oh! You're right, I hadn't realized.
But... wouldn't it be more efficient with a heat pump and exploit the 2-4x COP?
Ok, maybe I get it now: heat pump wouldn't work at the high temps they are achieving.
Thx.
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.
Has anyone looked at the subsurface ground temperatures after days, weeks, months, even years of heat pump operation?
I do seem to remember seeing one article on the subject showing that after one winter the subsurface temperature had declined enough to materially affect the heat pump's COP. But the timescale didn't extend to multiple years.
edit: found this one: https://pangea.stanford.edu/ERE/db/GeoConf/papers/SGW/2024/M...
In one of eight cases studied where heat flow is unidirectional (cooling load only) over a 20 year timescale the authors find:
The other 7 cases showed weaker or negligible long term variation.An additional graph shows COP variation over 15 years and the worst case shows a decline of perhaps 10% (just eyeballing it).
Surprisingly, some cases showed a long term improvement in heating COP - presumably the injection of summer heat into the soil made for warmer soil than just sunshine and natural diffusion?
So my takeaway: "it depends." :-)