Controlling the Four Elements. Or: Why Heat Pumps Are Cool.

Despite my attempts to post mainly geeky and weird stuff peppered with (very often not down-to-earth) physics, I got involved in some serious discussions on renewable energy, sustainability, heat pumps, and the pleasures of Building Your Own Stuff. So I will describe what I am actually working on / playing with when I am not blogging, liking, sharing or tweeting.

Elkement is an amalgam of my first name, Elke, and one of my nicknames, The Subversive Element. I have a penchant for the words elementary and elemental, any puns comprising those, and I like geek gadgets exhibiting the periodic system of elements. It is straight-forward that I have to work on a heating system that utilizes The Four Elements: Earth, Wind, Air and Fire.

As usual, I first considered this an incredibly creative way to describe a heating system – until I discovered that zillions of companies in the HVAC business use the same analogy. Besides I like the philosophical, if not new age-y, connotation contrasting with down-to-earth engineering.

I (we) work on optimizing a heat pump system that uses these said elements, which actually is: An unconventional source of heat that makes the system different from geothermal, ground water or air source heat pumps.

This is a simplified schema. Please bear with me!

The Four Elements are indicated: A rather large (about 20 m3) tank filled with Water is heated using an unglazed solar collector.

This collector allows for harvesting energy from solar radiation (Fire), but – above all – from ambient Air via convection.

The tank is located below ground (Earth). Actually, in the prototype system the ‘tank’ is a former small cellar that once had been used to store potatoes and wine (This is common in the region where I live). The cellar had been lined with plastic sheets which makes it a pretty cheap tank. There is an average net flow of energy from the ground to the tank – it would be detrimental to insulate the bottom and the side walls of the tank.

Heat pump system utilizing a water (ice) tank as heat source (Copyright:
Heat pump system utilizing a water tank as heat source. Energy is harvested from ambient air, solar radiation, heat transfer from the ground, and from freezing water (Copyright:

The water tank constitutes the heat source: Instead of burying pipes in the garden (ground source loops), the heat exchanger pipes are immersed in the tank.

It is a single (simple) closed cycle:

Heat pump –> solar collector –> tank –> heat pump

The combination of the tank and the collector is the actual heat source of the heat pump.

Depending on the temperature difference between ambient air and the tank and on the heating demand of the building the controller decides whether the collector is used or circumvented and whether the heat pump is turned on.

The heat pump heats two different hot water tanks – one is for heating the tap water, the other one is for transferring heat to the room heating loops. A heat pump cannot be “dimmed” continuously to different output powers: It delivers heat at full power or it is off. Thus you need an intermediate storage that allows for gradual heat transfer to the heating loops.

There is an important 5th element (You have expected me to add a link like that, didn’t you?): Ice.

During the heating season about 75% of the heating energy is harvested from air and fire channeled through the solar collector, freezing of the water in the tank and heat transfer from the ground yields the remaining 25%.

But heat pumps are cool, and this setup allows for a simple way of passive (“free”) cooling: In summer the room heater becomes a cooler: The ‘Hot Water Tank for Room Heating’ becomes the heat exchanger that transfers heat from the room heating loops to the underground tank. The heat pump can still heat the tap water in parallel – actually this is beneficial for cooling!

The overall goals for this design have been:

  • Using a heat pump with a high seasonal performance factor, but avoiding the drilling of deep bore holes or having to turn the whole area of your garden into a huge pile of soil temporarily.
  • Using rather simple, state-of-the-art components that could be purchased by home owners in online shops and that would allow skilled Do-It-Yourself enthusiasts to built the system themselves.
  • Allowing for cooling without adding complicated components or needing air-condition (to American readers: AC in homes in uncommon in Europe).


Further reading: This was the prequel of the story (“How I started loving heat pumps more than IT.”)
Our German blog has more detailed technical information – I am not sure of we will ever provide the same level of details in English.

17 Comments Add yours

  1. M. Hatzel says:

    I can’t stop thinking about this system. In harvesting natural sources of energy, storing the energy is still problematic. My husband and I talked about the batteries that would be needed for a closed loop system of energy collection (i.e. not connected to the municipal grids where the system becomes the means of ‘storing’ energy) to maintain supply even when energy can’t be produced because of lack of sun or wind. But in your system the water tank becomes the battery… which is great, if you have space or a root cellar to convert (or even a water well?). So, I started thinking about the options, and I have to ask if this idea can be applied to municipal sewage systems which use lagoons for holding waste water. Can the lagoon be used as the water tank? I know that some agricultural applications are already in use for collecting the heat from composting livestock waste, especially in dairy. This would add another heat source… but would it be enough to keep the system efficient in cold regions?

    1. elkement says:

      You are right – the water is a seasonal storage of energy, and you could use any water tank in the broader sense, such as a pond. However, the water is going to freeze and this freezing should not interfere with some other purpose of the water storage, lagoon etc. Fish in a pond might not like it ;-) and I guess the waster water should flow. If the flowing waster water is warm enough (after extracting heat by the heat pump) to prevent freezing it would be an option.
      The decisive factors are: How many days in winter would the temperature of the air be lower than 0°C all the time? What are your typical heating demands on such days?
      At these times, no energy can be harvested from the ambient air and energy is gained from freezing water only. The tank would need to be as big as to cover these days by delivering energy from freezing water only. Since the temperature of the water is never (much) less then 0°C, the efficiency of the heat pump could still be high, but the tank would become rather big. If your heating demands would be similar to ours, in the worst case the tank would need to be 2-3 times as large – planning for 100% of the energy produced by freezing – which is very pessimistic.

  2. M. Hatzel says:

    I read your link to “Discovering Your Life Being Cliché” as I was reading this… and had to laugh. Then I emailed my husband this post. He’s spent years looking for a technology that he’s described as ‘like geothermal, but not, something like a solar powered water heater but not quite.’ (He has to translate out of technical language for me.) I’m looking forward to discussing this with him tonight. (Also, I think the solar collector has aesthetic appeal!)

    1. elkement says:

      Thanks, Michelle! “Aesthetic appeal of the solar collector”… You made my day :-D , in particular as I know your blog and your discerning remarks about buildings!

  3. bert0001 says:

    Could you use this on an ordinary heating system, or do you have to replace existing radiators by floor heating exchangers?

    1. elkement says:

      It really depends on the flow temperature of the heating loop – the seasonal performance factor would be considerably lower if you would run radiators with 60°C inlet temperature. But you might also run radiators at lower flow temperatures if the size (the surface) is large enough in relation to the heating demands.
      The “test building” mentioned in my reply to Matthew uses radiators with an inlet temperature of about 40°C.

      1. bert0001 says:

        Stuff to think about … I will have to invest within the next 5 to 10 years. At least the hot water loop, now running on electricity will have to be replaced by something less costly. A solar boiler seems to be a waste of investment, if I look at the amount of cloudy days …

        1. elkement says:

          With flat plate solar thermal collectors and a larger hot storage tank (~1000 Liters) you could typically cover about 60-70% of your yearly heating demands for hot water (Reasonable collector areas).
          100% solar heating would be possible using very large hot water tanks and collector sizes, but this is not economical.

  4. Matthew Rave says:

    Very nice. Two questions: what sort of efficiency do you get? And what percentage of your heating needs in the winter can be met by this? If you live in a cold place (like Poland or North Dakota) do you need to supplement the system with heating oil (say)?

    1. elkement says:

      In the region where I live and work (Eastern Austria, near the Hungarian border), such a system can meet 100% of your heating demands – room heating and tap water.
      Our prototype system is now working since November 2012 in a old, but renovated building that requires about 17.000kWh heating energy per year and that does have a combination of floor heating systems and conventional wall-mounted heaters (thus not fully optimized for heat pumps). Nevertheless, the seasonal performance factor up to now is 4,2 and we expect it to rise further until the end of the heating season due to the energy gained by the collector. Typical (measured) geothermal systems here yield an SPF of about 3,8.
      I have done simulations so far by using meteorological data of Austria (picking a “very cold season”). Since most of the energy is harvested from the ambient air, you would need to make the tank considerably larger to compensate for the lower temperatures by making more latent heat available. This winter for example the maximum volume of water frozen was about 6m3, in a typical cold season here the volume would peak at about 20m3. I would expect that covering the top 5-10% of the heating demands in colder regions with another system (here wood is popular) would make the system much more economical because you would need to compare the price of the additional fuel with the costs of making the system considerably bigger.

    1. elkement says:

      Thanks – simplicity was an important design goal indeed! The know-how is in the details of the heat exchanger and in the logic of the controller system.

  5. Straightforward enough! So … I guessing, in terms of efficiency, that this system breaks even when compared to traditional geothermal. The real question then is … which is cheaper to build and maintain in the long run – and which has a quicker payback on investment. I’m guess your system here would win out. Am I right? D

    1. elkement says:

      Thanks! Yes, it is true that the seasonal performance factor is higher than with geothermal systems (I am comparing our measurements to published results of field studies) because the inlet temperature – when the brine enters the evaporator of the heat pump – is higher. As for costs: It is much cheaper to build than vertical bore holes or ground water systems, but slightly more expensive than horizontal ground source loops. But the higher SPF would compensate for that after some years.
      Note that all I say is true for middle European climate, about 48° latitude, only several 100m above sea level. You could certainly make the tank and the collector bigger to meet higher heating demands, but I guess there is a maximum latitude / altitude ;-) where this system makes sense.

      1. Very cool indeed (no pun intended). D

        1. elkement says:

          A previous version of the header of this article was: “Why heat pumps are hot (or cool)” ;-)

          1. Great minds think alike!

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