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 air temperature in Zagersdorf, 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 and 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. 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. 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 slightly different way to present the hydraulic design shown in the earlier post)
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:
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