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!
More data: Our documentation of measurement data now covers two full seasons (since autumn 2012) plus this season until end of January.