How Does It Work? (The Heat Pump System, That Is)

Over the holidays I stayed away from social media, read quantum physics textbooks instead, and The Chief Engineer and I mulled over the fundamental questions of life, the universe and everything. Such as: How to explain our heat pump system?

Many blog postings were actually answers to questions, and am consolidating all these answers to frequently asked questions again in a list of such answers. However, this list has grown quickly.

An astute reader suggested to create an ‘animation’ of the gradual evolution of the system’s state. As I learned from discussions, one major confusion was related to the role of the solar collector and the fact that you have to factor in the history of the heat source: This is true for every heat pump system that uses a heat source that can be ‘depleted’, in contrast to a flow of ground water at a constant temperature for example. With the latter, the ‘state’ of the system only depends on the current ambient temperature, and you can explain it in a way not too different from pontificating on a wood or gas boiler.

One thing you have to accept though is how a heat pump as such works: I have given up to go into thermodynamical details, and I also think that the refigerator analogy is not helpful. So for this pragmatic introduction a heat pump is just a device that generates heating energy as an output, the input energy being electrical energy and heat energy extracted from a rather cold heat source somewhere near the building. For 8kW heating power you need about 2kW electrical energy and 6kW ambient energy. The ratio of 8kW and 2kW is called the coefficient of performance.

What the typical intro to heat pumps in physics textbooks does not point out is that the ambient heat source actually has to be able to deliver that input energyduring a whole heating season. There is no such thing as the infinite reservoir of energy usually depicted as a large box. Actually, the worse the performance of a heat pump is – the ratio of output heat energy and input electrical energy, the smaller are the demands on the heat source. The Chief Engineer has coined the term The Heat Source Paradox for this!

The lower the temperature of the heat source, the smaller the coefficient of performance is: So if you run an air source heat pump in mid-winter (using a big ventilator) then less energy is extracted from that air source than a geothermal heat pump would extract from ground. But if you build a geothermal heat source that’s too small in relation to a building’s heating demands, you see the same effect: Ground freezes, source temperature decreases, performance decreases, and you need more electrical energy and less ambient energy.

I am harping on the role of the heat source as the whole point of our ‘innovation’ is our special heat source that has two components, both of them being essential: An unglazed solar / air collector and an underground water / ice tank plus the surrounding ground. The collector allows to replenish the energy stored in the tank quickly, even in winter: Air temperature just needs to be some degrees warmer than the cold brine. The tank is a buffer: When no energy is harvested by the collector at ambient temperatures below 0°C, water freezes and releases latent heat. So you can call that an air heat pump with a huge, silent and mainentance-free ‘absorber’ plus a buffer that provides energy for periods of frost and that allows for storing all the energy you don’t need immediately. Ground does provide some energy as well, and I am planning to post about my related simulations.(*) It can be visualized as an extension of the ice / water energy storage into the surroundings. But the active volume or area of ground is smaller than for geothermal systems as most of the ambient energy actually comes from the solar / air collector: The critical months in our climate are Dec-Jan-Feb: Before and after, the collector would be sufficient as the only heat source. In the three ‘ice months’ water is typically frozen in the tank, but even then the collector provides for 75-80% of the ambient energy needed to drive the heat pump.(*)

(*) Edit: This post written in 2017 show how much energy is stored / exchanged by each component. An overview of essential numbers is given here; emphasis on the volume of ice – which is compared to simulations here.

Components are off-the-shelf products, actually rather simple and cheap ones, such as the most stupid, non-smart brine-water heat pump. What is special is 1) the arrangement of the heat exchanger in the water tank and 2) the custom control logic, that is programming of the control unit.

So here is finally the series of images of the system’s state, shown in a gallery and with captions: You can scroll down to see the series embedded in the post, or click on the first image to see an enlarged view and then click through the slide-show.

More information on the system (technical data, sizing) and measurement data since 2012 can be found in this documentation – updated every few months.

Information for German readers: This post contains the German version of this slide-show.

12 thoughts on “How Does It Work? (The Heat Pump System, That Is)

  1. It’s interesting that this informative post should surface now. As you may know, we may be building here in spring. Everyone is pushing heat pumps … and even heat pump hot water. What would you recommend as a reliable, easy to understand, easy to install, easy to fix, logical system? We would stay away, I think, from the seemingly complicated system you have (involving solar and water sink). We ‘d like something simple and low maintenance. D

    • The type of heat pumps that is marketed most by installers are air source heat pumps: It’s like AC for cooling ‘in reverse’ (and they are reversible AC / heat pump devices, especially for warmer climates or for heating in autumn and spring only). In this case the ‘source’ one big box with a ventilator outside the building, and there is no storage.
      The advantage is that you don’t have to drill or dig deep holes in order to build a source, so you might save investment costs (I say might as it depends on existing cavities or wells that might be re-used) and it may be ‘simplest’. The downside is that the performance factor of air heat pumps is lowest compared to all other systems as the temperature of the source – cold air – is lowest. Below a minimum temperature the system switches to a backup heating rod. So operating costs are highest. Since hot water is heated to higher temperatures than water (or air) used for space heating, performance of air heat pumps suffers especially if used as the only heating system.

      I would check field tests of real-live systems in your climate to compare expected differences in performance. Then, it is an economical decision after all: Pay more for electrical power every year and less for the initial investment or invest more and pay less every year. This calculation also depends on expected yearly heating energy: If you build a very low energy / nearly passive house with modest size in a not too cold climate and payback period should be short, the business case for a heat pump with low performance could look good.

      Our system is typically built by people who are interested in the technology, keen on DIY installation, and who want to optimize the systems in terms of performance as much as possible: So a lower performance factor is not considered acceptable, and investment costs are low due to low costs for contractors.

  2. The clarifications are great. We spent time over the break also looking more closely at various heat pump systems (0ne was a natural gas fueled heat pump used to produce electricity, with heat as a by-product.), and I realized I had some misconceptions about how they actually work. It helped to look more closely at home heating systems used in Germany, which helped us better understand how very different systems are from one country to the next (as you stated in comments above). I think I was still trying to make connections between our different localities. The drawings are also quite useful for quickly demonstrating the climate of your region, and seeing the differences which could either make your system compatible or not with other regions.

    • The gas-powered heat pumps are marketed especially in Germany (especially by one very large utility, as sort of a complete package – heating system and energy, if I recall correctly) although the technology is still rather new: It had recently been scaled down to private homes after it had been used in industry for a long time – using the ‘cooling / refrigerator side’ of a similar machine, by manufacturing plants that turned their waste heat into cooling power. I think this is due to the high costs of electrical power in Germany compared to the very low costs of natural gas – which seems to be related to the low costs of oil and fossil fuel in general and/or to investements in renewable energy (German ‘Energiewende’) and subsidies included in the costs of electricity. An electrical heat pump would need to have a seasonal perormance factor greater than 5 to be economical, whereas the rather low performance factor of a gas-powered heat pump is suffcient, given the current ratio of costs of gas versus electrical power.

      Thanks for the feedback on clarity, I have hoped we have finally found a way of explaining / ‘demonstrating’ it better. The temperatures are typical ones, showing a typical state of the system at point of time, and perhaps this is more helpful than showing daily averages as I did in most plots before.
      I had quoted the numbers of ice days in your place as sort of the worst case for sizing the ice/water tank – https://elkement.wordpress.com/2015/01/28/more-ice-exploring-spacetime-of-climate-and-weather/ – but I did not walk through the whole calculation using your numbers, also because I could only guess typical heating demands of a typical building in your place (in kWh/day heating energy).
      Maurice posted similar questions, some of them related to sizing the underground cistern in Canada, in the comments here (on the recent poem) – I have added my worst-case calculation given a long period of temperatures well below zero https://elkement.wordpress.com/2015/12/16/google-mediated-self-poetry-holiday-edition/#comment-35653

      In the very first discussion on sizing of the tank on my first post on this system – https://elkement.wordpress.com/2013/02/26/controlling-the-four-elements-or-why-heat-pumps-are-cool/ – I made a crude estimate in the comments, that the tank would need to be 2-3 times larger in Canada than here. If our record-breaking frost of 1963 was similar to your typical winter as I conjectured in the post about climate, then it would have to be rather 3 times as confirmed by simulations I did in the meantime, using German weather data for that extreme historical winter mentioned in the other post.

      • This is great information. We are looking around at different locations in Canada where ‘green’ energies might be more economical than traditional sources of fuel, especially those without access to natural gas. In some of these places the climate is more moderate, with different heating needs.
        Of course, one question that always comes up is what sort of government regulation might make innovation difficult, and what sort of monopolies exist that could potentially block DIY enthusiasts looking at becoming a start-up. Our interest remains on the hobby end for now, except for how it influences where I might choose some courses at school. We’ll take some more time with your link collections over the coming months. Thanks.

  3. This is great, Elkement. Right now, I have this image in my head with energy transfers between air, tank, soil and the heat sink (a.k.a. house). My house only has a heat sink and a CO2 factory. I’m almost ashamed to visited your site…! About time I turn towards energy solutions 🙂

    • Thanks – I read that in Switzerland 75% of heating systems installed per year are heat pumps. On the other hand in France (and if I recall correctly, Belgium) direct heating with electricity is popular due to the lower price of electricity (…nuclear power…). Correct?
      Reading historical accounts I am always baffled how culture and politics actually influence implementations of technical systems. In Austria, we have access to an abundance of renewable hydropower, and we have a strong ‘tradition’ in renewable energy – more solar thermal collectors have been installed per capita than in every other European country except Cyprus, driven by a strong DIY movement in the 1990s. Electrical power is still expensive compared to other countries, especially North America, but it is cheaper than in Germany. The region where I live is windy and in terms of yearly energy balance ‘autonomous’, using only wind power (In winter they buy hydro power). So using a heat pump (with a reasonable source) is both economical and ‘green’. Not a difficult decision to make 🙂
      Actually, the most important driver for us to build the system were the international gas crises in the last decade – a heat pump is the logical successor of a gas boiler if you want to same convenience (and dislike storage rooms for fuel).

      • You make all the right points. I live in France and many houses are heated by electricity. However, there are also a lot of houses heated by gas or oil stored in the garden. When I bought the house it had gas.

        We are seeing solar power in our area but mostly voltaic or else thermal for non-heating usage. The french government is always messing with incentives which means that you can’t build a business case for long term investment since an incentive can quickly become a burden. As a consequence people remain quite neutral in the willingness to innovate.

        Behind all of this, France takes the view that with is massive coverage from nuclear power stations, it has no strong motivation to stimulate alternative solutions.

        • I am actually a bit wary of governmental incentives – I always feel they are gamed and gamified more than it helps. I’ve seen too many ‘business models’ based on ripping off incentives, a whole industry sector of companies or non-profit organizations or combinations of those who have honed the skill of writing grants and perpetuating their ‘research’ forever.
          I am quite proud that we have funded our own research and declined options to participate in funded research projects. Actually, all that had made me reluctant to go ‘into’ that sector for some years as I don’t want to be lumped together with all the funded stuff.
          Ironically our system is profitable despite incentives (Of course it depends on payback time – it might not be profitable if you assume a high mobility, like selling your house and moving every few years): When I say it is economical, I mean market prices, not incentives for heat pumps or the like (which do exist but it’s harder to apply the standard bureaucracy to non-standard system as ours). Electrical power is rather expensive now and not following the oil price trend, because a large part of the costs are taxes and fees dedicated to support renewable energy initiatives … elsewhere. I am not sure if I like the idea so much that investing in wind power has become lucrative for professional investors because normal home owners that don’t have the money to invest in anything (like the stereotype grandma with a small house) have to pay a lot for electricty because of ‘green initiatives’.

          • Governments need a mindset change that will come with the Gen XYZ kids. Massively distributed power generation is on the way and it won’t work because of government control but through deregulation and private sector initiatives. It’s the like of Elon Musk that will show the way. No, correc that, the likes of you!

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