Same Procedure as Every Autumn: New Data for the Heat Pump System

October – time for updating documentation of the heat pump system again! Consolidated data are available in this PDF document.

In the last season there were no special experiments – like last year’s Ice Storage Challenge or using the wood stove. Winter was rather mild, so we needed only ~16.700kWh for space heating plus hot water heating. In the coldest season so far – 2012/13 – the equivalent energy value was ~19.700kWh. The house is located in Eastern Austria, has been built in the 1920s, and has 185m2 floor space since the last major renovation.

(More cross-cultural info:  I use thousands dots and decimal commas).

The seasonal performance factor was about 4,6 [kWh/kWh] – thus the electrical input energy was about 16.700kWh / 4,6 ~ 3.600kWh.

Note: Hot water heating is included and we use flat radiators requiring a higher water supply temperature than the floor heating loops in the new part of the house.

Heating season 2015/2016: Performance data for the 'ice-storage-/solar-powered' heat pump system

Red: Heating energy ‘produced’ by the heat pump – for space heating and hot water heating. Yellow: Electrical input energy. Green: Performance Factor = Ratio of these energies.

The difference of 16.700kWh – 3.600kWh = 13.100kWh was provided by ambient energy, extracted from our heat source – a combination of underground water/ice tank and an unglazed ribbed pipe solar/air collector.

The solar/air collector has delivered the greater part of the ambient energy, about 10.500kWh:

Heating season 2015/2016: Energy harvested from air by the collector versus heating-energy

Energy needed for heating per day (heat pump output) versus energy from the solar/air collector – the main part of the heat pump’s input energy. Negative collector energies indicate passive cooling periods in summer.

Peak Ice was 7 cubic meters, after one cold spell of weather in January:

Heating season 2015/2016: Temperature of ambient air, water tank (heat source) and volume of water frozen in the tank.

Ice is formed in the water tank when the energy from the collector is not sufficient to power the heat pump alone, when ambient air temperatures are close to 0°C.

Last autumn’s analysis on economics is still valid: Natural gas is three times as cheap as electricity but with a performance factor well above three heating costs with this system are lower than they would be with a gas boiler.

Is there anything that changed gradually during all these years and which does not primarily depend on climate? We reduced energy for hot tap water heating – having tweaked water heating schedule gradually: Water is heated up once per day and as late as possible, to avoid cooling off the hot storage tank during the night.

We have now started the fifth heating season. This marks also the fifth anniversary of the day we switched on the first ‘test’ version 1.0 of the system, one year before version 2.0.

It’s been about seven years since first numerical simulations, four years since I have been asked if I was serious in trading in IT security for heat pumps, and one year since I tweeted:


7 thoughts on “Same Procedure as Every Autumn: New Data for the Heat Pump System

  1. Hi Elke, I will have a closer look at your graphs and entire website but WOW it looks very professional and well detailed. When I read you average COP is 4.6 does this include the electrical energy used by the circulation pumps? And I may play the devil’s advocate : wouldn’t the result have been the same with horizontal ground loops, connected to the heatpump, instead of solar collectors and a water/ice buffer? Kind Regards,

    • Hi Olivier, thanks! The performance factor includes the brine cirulation pump and the pump between heat pump and hot water tanks, but not the circulation pumps for the radiators and floor heating loops.
      Of course I cannot answer your second question with 100% certainty as I cannot test another source under the same conditions 🙂 From comparing with field studies done for horizontal ground loops (e.g. by German Fraunhofer Institute, 2013, one could conclude that our system’s performs a bit better than an average horizontal collector – also given the fact that we use radiators with a higher supply temperature in part of the building.

  2. “Last year it was just a weird inside joke: Planning heat pump systems as we work with IT clients, ‘remote only’. Now we are doing it!”

    This is passion at work. And it works! I’m not sure how you measure this but your project always inspires me. Would ROI be the measure? Return On Information?

    • Thanks a lot for your comment – and your re-share on Twitter! Much appreciated!!

      Actually, we try not to measure any ‘management-style’ performance indicators. I think when we did this as a small department in a large corporation or as a true start-up funded by VC capital the funding entitites had been very disappointed by the slow progress 🙂 That’s why I like funding ourselves and not having to send quaterly reports to anybody I am dependent on ;->

      But Return on Information sounds like a great key number. On the other hand, I have been told repeatedley that visitors (of website and blog, mainly our German blog that focuses on that heat pump system) were surprised that we ‘provide so much information for free’. So this ROI would not be that impressive 🙂

      • I give a lot of things for free. I would rather my information gets out there than that I limit its distribution for profit reasons. Not that I mind profit but information momentum matters. more. You can always switch to a profit strategy at any time.

  3. So, what is the bottomline Elke? Is your system working for you economically? Are you producing more energy than you need, break even, or are you still needing to purchase from the electric company? If your system is generating an income … how long until the system pays for itself and your balance sheet runs into net positive territory?

    • I compare economics of the heat pump system to that of natural gas, as this is the logical competitor in terms of convenience and the heating system we had before. As I said in the post, our system is more economical than a gas boiler as long as the performance factor is greater than 3 – as gas is three times cheaper than electrical power. So it is important that we managed to obtain a rather high performance factor of well above 4 using our special heat source – higher than that of many other ground source heat pumps and much higher than that of air heat pumps.
      The linked post from last year shows that we save about 40% of heating costs compared to gas, that is about € 500 per year. In addition to those savings, we repaced fossil fuel, imported from other countries (plus associated political risks) with our local abundant sources of renewable energy (60-70% of Austrian electrical energy is from hydro power, and wind power is growing rapidly).

      I am only considering the heat pump system here, not the photovoltaic generator, so generation of energy is not included. The ‘free’ energy included here is only the energy from air, but you always need electrical power in addition to that to drive the heat pump.
      In winter, the daily energy you can harvest from photovoltaic panels (of somewhat typical size) is much much smaller than what the heat pump needs (given a somewhat typical house, in our climate) even with out high performance factor – so also batteries would not help.

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