The more we enjoyed our spring-like winter, the more we were worried if we will ever see much ice in our underground water tank this heating season.
So we did what I had announced – we switched off the solar collector completely: Since February 1st our heat pump has been extracting heat energy from the tank only.
Ice started to grow from the heat exchanger pipes into the bulk of the tank, our re-purposed root cellar. These images have been taken at Day 9; showing the part of the pipes above water level:
The density of ice is about 10% lower than the density of water, and the supporting construction needs to make sure that this home-grown iceberg does not start to float. The visible tip of a floating iceberg would be equivalent to those 10% of the total volume. If the whole bulk of ice has to remain under water, the water level increases gradually. There is a tiny heater in the right place, to prevent the formation of a continuous slab of ice that would trap the not-yet-frozen water.
From the change in water level we determine the volume of ice:
The volume of the tank is about 27m3, so it can hold about 25m3 of ice. Now a bit more than half of it is frozen. About 0,3m3 of ice has been created per day, and the rate is slowing down as the weather gets milder and milder. We need less than 70kWh heating energy per day, for space heating and hot water.
The water level has now raised above the top part of the tubes shown in the previous image – and some interesting new structures have emerged which are not directly related to heat pump operations (Image taken at Day 25):
If it rains heavily, some water trickles down into the former cellar which still uses the original ‘ceiling’ built decades ago. The rain water hits the bulk of ice and is frozen as well.
Next part of this story: High Score!
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More data: Our documentation of measurement data now covers two full seasons (since autumn 2012) plus this season until end of January.
elkemental Force 
I found something of interest for you on alternative energy/heating for homes. I have to put it in a post, and link back to some of your more recent posts. It’s all too exciting! I can hardly keep the secret until then.
OK :-) Seems I have a pingback now…clicking it: 3…2…1…
And here is the link to that great article – as a service to other readers ;-)
https://mhatzel.wordpress.com/2015/03/10/the-art-of-insulating-or-reviving-a-lost-prairie-innovation-energy-conservation/
So … when all of the heat has been removed from the tank … you’ll switch back to solar? And, at that point you’ll have a huge iceberg down there, won’t you? In that case, will you use the lowered temperatures to cool your place during the summer months?
Yes – when the tank is full frozen, the collector will be turned on again, and then solar energy will gradually melt the ice again. In case there is still ice in summer it will support cooling (using the floor heating loops for cooling). You can also cool in summer without having created so much ice before, just using the still rather cold water – in the past years we had cooling turned on for those 1-2 really hot weeks. Perhaps we test to switch in on on more often this year to test how much cooling energy we will need. Generally, there is not much need for heavy cooling in our climate, and AC in private homes is rather uncommon.
Ice bucket challenges seem lame next to an entire ice storage cellar challenge
I did hope somebody would say this :-)
They say hreat minds think alike. I hold only a very limited form of this proverb: great minds laugh about the same things. They laugh about small minds, which really do think alike