Measurement Data for Our Heat Pump System – Finally Translated Documentation

In an earlier post  I said

Although we have very innovative, and if I may say so, geeky / nerdy customers it is rather unlikely that we will plan heat pump systems in Australia via sending checklists or doing ‘remote support’ in the same way we work in IT projects.

OK – now we really got a question from a non-German speaker in a remote place who tried to make sense of our mostly German documents. Thus finally I really got started and translated the documentation of measurement data and systems parameters for our heat pump system.

That work sucked all the creativity and research capabilities out of me – so In this post I try to mix some of the diagrams presented in that document with replies to some FAQs.

We had a very warm winter and early spring here in Austria – this was the solar collector last month:

Solar Collector in March 2014. Beauty is in the eye of the beholder.

It is also reflected in the long-term measurements of ambient temperatures:

Ambient Temperature 2014-04, measurement data heat pump system LEO_2, punktwissen
Ambient air temperature in Eastern Austria. ‘Maximum’, ‘average’, and ‘minimum’ refers to one day, respectively.

Although I find that the collector is quite a cool decoration / replacement for a fence the typical question by visitors is (in addition to the question: Where can we install this so that nobody sees it?)

Can I use flat plate collectors?

Not really if the system should work in a performant way. Actually, those unglazed collectors have been picked deliberately, not because they are cheaper or lighter.

This system should replace any other fossil fuel powered system – we haven’t switched on our gas heater in two years now. Thus it has to harvest energy when it is really cold. Flat solar plate collectors are optimized for harvesting energy from solar radiation in summer; they are designed for minimum losses via convection of air.

Unglazed collectors are typically used for heating swimming pools as you can live with rather high convective losses here. But the highly efficient convective heat transfer is to our advantage in winter – then you gain energy even in the night if the temperature of the air is just a few degrees above the temperature of the brine flowing through the collector.

In summer you have more energy than you need anyway, so we don’t care about ‘convective losses’. Rather on the contrary: we are happy that we dont’ have to worry about high temperature making the brine decompose.

In addition the system is used for passive cooling in summer – that is, the temperature of the water tank (the ‘heat source’, then ‘cold source’) must not exceed a reasonable temperate which is well below the room temperature. This is also in line with the fact that there is a maximum heat source temperature the heat pump can deal with, specified by the manufacturer (about 20°C).

Energy Harvested by the Collector 2014-04, measurement data heat pump system LEO_2, punktwissen
Energy harvested by the collector. The total heating demand of the building is about 18.000 kWh per year, incl. hot water. Nearly all the energy needed is delivered to the water tank via the collector (and a minor part directly from ground). Collector power becomes negative if the system operates in cooling mode.

Can you explain BRIEFLY how the system works?

It is all about using a large tank of water as energy storage: The heat pump extracts heat and cools the water, then freezes it. Either the collector transfers heat to the tank in winter, or the floor heating system delivers heat to it in summer when the heater is actually a cooler.

Energy Stored in the Water Tank 2014-04, measurement data heat pump system LEO_2, punktwissen
Energy stored in the Water Tank. The 25m3 water tank corresponds to 430 kWh sensible heat – extracted when cooling water – and 2.300 kWh latent heat – extracted when freezing.

Anything else is the details of hydraulics and control – this is a screenshot of the online monitoring system (a different way to present the hydraulic design shown in the earlier post)

Online Diagram, Hydraulic-Setup. Heat pump system punktwissen.
Online monitoring diagram – sketch of the heat pump system showing measurement data. The water tank and the solar collector are the combined heat source of the heat pump. The heat pump works either in ‘space heating mode’ or ‘hot water heating mode’ and diverts the heating water to either circuit. Buffer storages are important for efficient control as the heat pump always operates at its maximum power.

Regarding the hydraulic design a question that comes up very often is about hot water heating:

You heat hot water indirectly by using a tank at 50°C? I don’t believe you that this is sufficient.

Believe me, it is. My very own very long and very hot showering – elementary showering as I call it – is a worst case test. The heat exchanger in this hygienic storage tank has an effective area of nearly 6m2 – that’s rather large, and this is crucial for a heat-pump-powered system.

The operating temperature of the heat pump should be kept as low as possible in order to obtain high coefficients of performance. Thus the temperature difference between tap water and heating water is rather low, and in order to compensate for that and still get reasonable heating powers the area of the heat exchanger should be big. The effective heating power of this heat exchanger is 12kW.

What’s the performance?

We proudly present:

 Monthly Performance Data 2014-04, measurement data heat pump system LEO_2, punktwissen
Heating Energy: Space heating and hot water. Total Electrical Energy: Heat pump, brine pump, heating circuit pump. Monthly Coefficient of Performance: Ratio of heating energy and electrical energy. The dotted line indicates the performance factor for the whole period covered in the diagram.
Solar Collector in April 2014






11 Comments Add yours

  1. M. Hatzel says:

    I like this post, especially the discussion on the difference of solar collectors (something I did not know) and, as always, I’m interested in how you are storing the energy. To my way of thinking, running a home off-grid and independent of service provider fees is a great financial investment for retirement planning.

    This morning our local paper ran a story on battery systems being tested with wind turbines here in Saskatchewan, which is exciting, given that most of our province’s energy comes from fossil fuels. We have a fairly hefty infrastructure cost to maintain the energy grid system (which is currently being updated to accommodate energy needs of larger industries–such as mining and oil field drilling–most of it coming from a remote location in the southeast corner of the province and then moved across province). I can’t help but wonder if this is an effective use of resources, or if there’s a better way. (Any rate, here’s a link to the newspaper article,

    1. elkement says:

      We are “unfortunately” just storing low-temperature thermal energy, and the system depends on electricity to “unlock” it with a heat pump.
      We might add photovoltaic modules in the future and thus create some of the power ourselves – wind turbines for home owners are not really an option here as our premises are too small… you would never get a permit to install one as your neighbors will not like it ;-)
      But we rather compare the dependence on electric power with the dependence on gas here. In the region where I live we can produce the electrical energy we need locally (though this is only correct when checking the yearly net energy balance), but re gas we depends on Russian gas. I can remember the last Ukrainian / Russian gas crisis in 2009 after which the Austrian major gas provider decided to build a large storage that will hold all gas needed in a year. There is actually a region in Austria that was living off the main grid until the 1970s (so they had their own, non-synchronized local grid) and their infrastructure does still reflect this. Today they want to utilize this and revert to this local generation of energy by the their small power plants. The problem is that you cannot simply cut off the connection to the grid even if you would be to produce the energy because all generators on the grid work in sync.

      I agree that storage is actually the major challenge today – AFAIK there are some projects in Japan that make use of gigantic batteries (sodium / light sulphur) but there are some other options to store electrical power generated by wind turbines, such as Power-to-Gas – here you could (in contrast to batteries) transport the energy stored by using the existing gas grid.

      I have once done a simulation on using a wind turbine and batteries for a mountain hut – that’s probably comparable to other dispersed settlements. The batteries you need to cover up for a few days without wind are quite huge and the batteries are the most expensive part of the system. But I fully agree – probably the best thing you can invest in before the next economic crisis causes hyper-inflation or whatever.

      1. M. Hatzel says:

        This is the research question of the century, how to store energy. I can’t help but pause and wonder, what will human life be like once the fossil fuels are gone (assuming we haven’t gone extinct first). This is all so very interesting.

  2. The engineering and physical aspects are out of my league but interesting none-the-less. We have chosen to heat exclusively with wood. My son-in-law tells me that the carbon footprint of that practice is just as bad as if I were burning propane or oil, and I cannot disagree. Renewables have always been of interest to me but I wonder about upkeep and maintenance over, say, a 20-25 year period? And also, your system requires electricity and, if I am not mistaken, requires a generator if the power should go out? Interesting analyses though. And your system requiring only a large water reserve and collector would mean a LOT LESS WORK than harvesting, cutting, and splitting massive amounts of wood every year. D

    1. elkement says:

      Good questions, Dave!
      Re the footprint:iit really depends on the scope of what to consider – doing this calculations is an arcane science and I feel the outcome often depends on biases and who paid for the research. E.g. do you take all the production steps for making the chainsaw into account?
      I live in a region if Austria that hosts some projects in making small villages nearly autonmous by using only local renewable energy sources – mainly a combination of biomass, wind and solar power. It is hardly commercially viable without governmental funding of the initial investments to build these powerplants, but as I understood the experts you can achieve a footprint lower than for fossil fuel given that wood is only processed locally and disctrict heating is used for villages (the latter is for keeping cost and complexity low for every home owner).

      Re maintenance: Solar thermal collectors (flat plate collectors are popular here for heating hot water) and photovoltaic panels are nearly maintenance-free. According to manufactuers it is the electronics (inverter) that will have to be replaced first – but this should be the same for every heating system. Heat pumps are rather long-lived, too (counting them as renewable energy, based on our supply with hydro or wind power). Wind power plants here “re-powered” only for commercial reasons – the turbines are sold to operators in other countries and larger new turbines are installed – but the foundations are theoretically only built to sustain a certain number of “cycles”.

      Re power outage: Our yearly average outage is about 15-30 minutes per utilities’ customer – but even if the outage is longer (several hours unplanned outage is exceptional) heat pump systems are built to sustain outages per design: Some utlities offer cheaper “heat pump rates”, that is: Power is switched off for 4 hours a day – any time. The way to achieve resilience is to use large buffers of heat, as a buffer storage tank or the volume of concrete of the house itself – in particular with floor heating large “concrete buffers” are used.

  3. Elke I am curious as to how it would perform in Newfoundland. We are at 47.6 n lat, get 1630 h if sunshine annually and gave a mean annual temp of 9 C.

    1. elkement says:

      (We are about at the same latitude :-))
      The essential parameter is: What is the longest period of time temperatures never rise above 0°C all the day and how much energy do you typically use for heating today on such days?
      It should always perform good as long as not all the water freezes because then the temperature of the heat source remains high enough to allow for a reasonable coefficient of performance – but this means the tank has to be built big enough.
      Even if temperatures are just a few degrees above 0°C the energy in the water tank gets refilled via the collector. But if it is too cold all the day, the collector cannot be used and all the energy required needs to be harvested via freezing the water in the tank.
      So as a very crude estimate the water tank needs to be as big as the equivalent of the heating energy used in these days (1m3 of water ~ 93kWh) – if the calculation would result in a tank the size of “100s of cubic meters” then I guess it not commercially or technically feasible.
      On the coldest day in the last two years here – about -9°C average temp. on that day – we needed about 130kWh heating energy per day. Thus our tank with 25m3 can sustain 17 such days in a row (Simulations showed that it would have been fully utilized once in the last 10 years).
      Of course a more detailed simulation would need to be done to check if the recovery periods between the very cold periods are sufficient to allow for melting some ice in the tank.
      But then it could still be economically reasonable to have an additional heater for extended periods of very cold days (wood burner e.g.) instead of making the tank so big that it can provide all the energy.

      1. We are in the ballpark. I consume an annual average of around 90 kWh and a winter max of around 190 kWh so it could be made to work with mods I think. On another but related topic. Electricity here is 0.11 canadian (around 0.07 euros) per kWh so I use it to heat the house. Wondering how that compares with your rate and how it affects relative viability of the system.

        1. elkement says:

          Wow – that’s cheap! If I re-calculate our price, taking into account fixed fees and taxes and costs of the transmission line operator, we end up with effectively ~0,18 Euros per kWh. So direct heating with electrical power is not an option as it would be too expensive.
          Here the options are mainly gas, oil, wood, pellets or heat pumps. “Multiplying” the energy using a heat pump with reasonable performance factor results in lower costs than heating with gas.
          Are geothermal heat pumps common in Canada? You could save even more money when using heat pumps – but I guess this is then a question of comparing operating costs and costs of investment. If I believe the clichés about the mobile North American workforce versus us rather sedentary Austrians – then I could understand that heat pumps probably never pay off as you move before break-even.

          1. Air sourced heat pumps are becoming increasingly common but the geothermal ones are extremely rare here. I should probably find out why but I am guessing it’s because we are on a fairly think, stable tectonic plate so you’d likely have to go a little deeper than normal to get it to work–but that’s just a guess. We have abundant running water and receive significantly more precipitation than we transpire. That means we are well-suited to hydro projects. And yes, we are terrifically lucky to have such cheap electricity. That said, our people have been getting very lax about it lately, demonstrated in two ways. (1) St. John’s gets most of its electricity from a thermal power generating station. It burns 6000 barrels of #6 per day and has a max output of a little under 500 MW. It is, to me, a disgrace. (2) we have currently underway a new hydro development which should give a little over 800 MW. It will likely result in an increase in rates, though, possibly to around 10 Euros and people around here are hopping mad about that increase–mostly unaware of the fact that those would still be competitive with North America.
            Elke–sometimes people make me angry :-) so willing to take the short but stupid and unsustainable road, ignoring the fact that the harder path is generally the best one in the long term.

          2. elkement says:

            Here also air sourced heat pumps are becoming more and more popular. I suspect the reason is simply that they are so much easier to install – the HVAC contractor does not need to work with somebody else who takes digs the ground. So these heat pumps are hyped a lot despite their lower performance factors… which don’t matter that much for modern “passive houses” or “plus energy houses” as they also get quite common here.
            “Geothermal” is actually a misnomer as most “geothermal” heat pumps here use ground loops layed out in about 1,5 meters below the surface. Deep drilled bore holes (about 100 meters typically) are also rare as you need special permits and a geologist’s expert report.

            What about offshore wind power in your place?

            I did a quick estimate of your power plant’s efficiency: 6000 barrels of oil per day and, assuming 11kWh/liter fuel value would result in:
            6000b/d x 159ltr/b x 11kWh/ltr / 24h/d ~ 437.000 kW = 437 MW
            As far as I know thermal power plants are more closer to an efficiency of about 30-40% (Carnot’s efficiency and some additional losses), probably increased to 60% in a Combined Cycle Gas Turbine power plant. But anyway I agree – burning fossil fuel to produce electricity is a disgrace as you always have so much heat left (you cannot outsmart thermodynamics) you either release into the environment, “heat a river”, etc.
            In Austria a lot of biomass-powered plants have been built in recent years, most due to abundant governmental subsidies (and many of them bankrupt now) – renewable on principle, but you still need to get rid of the remaining heat and so operators desperately try to find business cases, such as heating greenhouses or swimming pools… which, if you actually wanted to heat them would be powered better using free solar energy.
            Also geothermal power plants (real onles, requiring drilling down to several 1000 meters) suffer from the same issue of “what to do with the waste heat”.

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