We haven’t seen much of it this winter yet.
I am talking both about the ice you would expect in winter and about the one created from extracting heat from a water tank – our heat pump system‘s heat source.
This winter does again disappoint; it seems we will not be able to generate Pannonia‘s largest ice cube in this season. This plot shows the growth of ice in the past three seasons, since the system went live in autumn 2012:
The water tank temperature is 20°C maximum. This is the maximum heat source temperature the heat pump can deal with, so the solar collector is hardly used in summer. Heat provided by ground is sufficient to provide the energy which is extracted from the tank on heating hot water.
This is the energy stored in the tank over time:
The specific heat of water is 1,16kWh per m3 – cooling down the 25m3 tank from 20°C to 0°C provides about 580kWh. Currently we need about 70kWh per day for space heating and hot water heating; the maximum in this season was about 100kWh per day so far. We had not seen ice before December in the past three seasons: Water does not freeze as long as as the energy provided by the solar collector replenishes the energy in the tank quickly enough.
The ice formation curves in the first figure show that the blue peaks always follow a cold spell of weather – a negative peak in the (green) ambient temperature. As soon as there is a positive peak the ice is quickly melted again. This year the latest green positive peak was quite pronounced – about 12°C average daily temperature; maximum temperatures were about 20°C in some regions in Austria.
But we try harder now to create a gigantic ice cube: On rebuilding the solar collector last summer a new feature has been added for research purposes – the effectively utilized area of the collector can be changed by letting brine only flow through a subset of the tubes.
Currently we use only the upper half of the area. There is hoarfrost on the pipes which are in use – as they are colder as energy is extracted from the flowing brine by the heat pump and / or by the water tank:
If this is still not sufficient to challenge the system we might turn off the collector permanently in February. 100kWh heating energy per day translates to 75kWh to be extracted by the heat pump (given a performance coefficient of about 4). The tank containing about 2.000 kWh would then be exhausted and completely frozen in 27 days.
Other plots and key performance data for each month and each season are detailed in our documentation of measurement data – this file contains two full seasons as per the writing of this blog post.
In the unlikely case somebody stumbles upon this post when searching for historical weather data for Austria: The English Annals page show the data in a format that is difficult to work with (you need an outdated browser), but CSV files can be downloaded from the German page with historical data. Pick daily data (Tagesauswertung) for the greatest level of detail.