July 2015 was the hottest July ever since meteorological data had been recorded in Austria (since 248 years). We had more than 38°C ambient air temperature at some days; so finally a chance to stress-test our heat pump system’s cooling option.
Heating versus cooling mode
In space heating ‘winter’ mode, the heat pump extracts heat from the heat source – a combination of underground water / ice tank and unglazed solar collector – and heats the bulk volume of the buffer storage tank. We have two heating circuits exchanging heat with this tank – one for the classical old radiators in ground floor, and one for the floor heating loops in the first floor – our repurposed attic.
The heat pump either heats the buffer tank for space heating, or the hygienic tank for hot tap water. (This posting has a plot with heating power versus time for both modes).
We heat hot tap water indirectly, using a hygienic storage tank with a large internal heat exchanger. Therefore we don’t need to fight legionella by heating to high temperatures, and we only need to heat the bulk volume of the tank to 50°C – which keeps the Coefficient of Performance high.
In summer, the still rather cold underground water tank can be used for cooling. Our floor heating loops become cooling loops and we simply use the cool water or ice in the underground tank for natural (‘passive’) cooling. So the heat pump can keep heating water – this is different from systems that turn an air-air heat pump into an air conditioner by reverting the cycle of the refrigerant.
Heating hot water in parallel to cooling is beneficial as the heat pump extracts heat from the underground tank and cools it further!
How we optimize cooling power this summer
Water tank temperature. You could tweak the control to keep the large ice cube as long as possible, but there is a the trade-off: The cooler the tank, the lower the heat pump’s performance factor in heating mode. This year we kept the tank at 8°C after ‘ice season’ as long as possible. To achieve this, the solar collector is bypassed if ambient temperature is ‘too high’. The temperature in the tank rose quickly in April – so our ice is long melted:
At the beginning of July the minimum inlet temperature in the floor loops was 17°C, determined by the dew point (monitored by our control system that controls the mixer accordingly); at the end of the month maximum daily ambient air temperatures were greater than 35°C, and the cooling water had about 21°C.
Room temperature. Cooling was activated only if the room temperature in the 1st floor was higher than 24°C – this allows for keeping as much cooling energy as possible for the really hot periods. We feel that 25°C in the office is absolutely OK as temperatures outside are more then 10°C higher.
Scheduling hot water heating. After the installation of our PV panels we set the hot water heating time slots to periods with high solar radiation – when you have more than 2 kW output power on cloudless days. So we utilized the solar energy generator in the most economic way and the heat pump supports cooling exactly when cooling is needed.
Using the collector for cooling in the night. If the ambient temperature drops to a value lower than the tank temperature, the solar collector can actually cool the tank!
Ventilation. I have been asked if we have forced ventilation, ductwork, and automated awnings etc. No, we haven’t – we just open all the windows during the night and ‘manually operated’ shades attached to the outside of the windows. We call them the Deflector Shields:
These are details for two typical hot days in July:
The cooling power is lower than so-called standard cooling load as defined in AC standards – the power required to keep the temperature at about 24°C in steady-state conditions, when ambient temperature would be 30°C and no shades are used. For our attic-office this standard cooling power would amount to more than 10 kW which is higher than the standard (worst case) heating load in winter.
Overall electrical energy balance
I have been asked for a comparison of the energy needed in the house, the heat pump in particular, and the energy delivered by the PV panels and fed in to the grid.
PV numbers in July were not much different from June’s – here is the overview on June and July, maximum PV power on cloudless days has decreased further due to the higher temperatures:
In July, our daily consumption slightly decreased to 9-10 kWh per day, the heat pump needs 1-2 kWh of that. The generator provides for 23 kWh per day,
Currently the weather forecast says, we will have more than 35°C each noon and 20-25° minimum in the night until end of this week. We might experience the utter depletion of our cooling energy storage before it will be replenished again on a rainy next weekend.
elkemental Force 
When I first read your post, I also thought you still had ice in the tank. This actually didn’t surprise me, as we had ice crystals lasting into early July in the sewer line clean-out in the sewage system at our farm. We had to break through it to clean the line after our first winter living on that property (this was way back, about 11 years ago, now). If I had commented sooner, I would have asked the same question Maurice posed, as to when you need to get rid of it to begin heat accumulation.
This past week we had temperatures in the mid 30 C range, and very humid. It is not uncommon here to have a couple nights during the hot phase of summer that the temperature remains too warm to effectively cool down the house. This usually happens over a one week period, temperatures building and leading to a couple of uncomfortable nights during which no one sleeps without some sort of cooling system. I think this was that week – I hope we don’t experience another one this year.
I also use window coverings to help maintain the coolness inside, and prefer not to run the AC unless necessary. I never thought to call them anything but that sissy decorator name, draperies. I will choose now to call them deflector shields instead. :)
Thanks for the comment, Michelle! I have now edited some references to ‘Ice Storage’ in the text to make it clearer. The German term (‘Eisspeicher’) has become a standard technical term and is used to denote the heat source as such (as compared to e.g. ‘ground loops’) and should not indicate that it contains ice all the time; so ‘ice storage’ is rather a shortcut for ‘underground water tank that contains water or a mixture of ice and water’. The plot titled ‘Temperatures and Performance Factors’ has a red arrow indicating the end of ice season. The temperature in the tank increased quickly to 8°C in April, so the ice is long gone.
True, if you would leave the tank alone, it could keep the ice for a long time, but we heat it with our solar collector – which can already deliver the energy needed for heating the house + melting the ice fast at the end of March :-)
Here, it has finally got cooler last night! As long as the temperature is below 20°C at some point of the night, it is fine, but those tropical nights are unpleasant.
I reread the graph ‘Temperatures and Performance Factors’ and see it now. I had paused to wonder about the incongruities of my resulting misunderstandings, as we sometimes use the cold air in the basement of our house to cool the other parts of the house, and thus avoid running air conditioning. The furnace fan (without the heating turned on) and ducting is used to deliver the air. The cool air in the basement is also a limited supply, and it works best by conserving it, too, with shading during the hot parts of the day and opening windows during the cooler parts of the night. It is more comfortable than using air conditioning systems, and much quieter than a clunking noisy machine. I need to think more about this.
WoW ! 38 deg, that’s over 100F. Highest temp on our patio this summer was 91F, in the tropics, and we are only about 300ft (91 metres) above sea level. Gets much hotter in the Mall car parks.
Yes, it is really exceptional. If I recall correctly we have “beaten” mediterranean countries. We are at about 200m above sea level – this is the lowlands in Eastern Austria close to the Hungarian border, part of the Pannonian Plain. It is typically hotter than in the rest of Austria.
Now–more questions. This time about the hot water for showers, dish-washing etc.I have a simple electric heater but have the thermostat turned back from it’s initial setting of 70C to 55C, for much the same reason as you have. It’s by no means the “greenest” way to heat water but it does get the job done. It’s a 160 l glass-lined tank wrapped with insulation and heated through a pair of coils, one in the bottom and one near the middle. Standard stuff. It has a sacrificial anode and, even though I change it out, the tank tends to get perforated every 6-7 years and I have to change it out. So–what about the makeup of your tank? What’s it constructed from, how large is it, and how well do you feel it meets your needs?
I’m intrigued to find that you still have significant ice even now. I imagine that at some time you will need to have it gone and allow the water to start warming up again in anticipation of the next heating season.
Side note: our weather has not been great. While last year we had a particularly warm summer, this year it’s the exact opposite. Today, thankfully, has been better.
One more interesting fact–in early July, eldest son started working at a new Nickel refining plant located about an hour from here. It uses a “hydromet” process. Very interesting. See here: http://www.vale.com/canada/EN/business/mining/nickel/vale-canada/long-harbour/Pages/Hydromet-Technology.aspx
No, no, we don’t have any ice since months! “Ice storage” is just sort of the “standard product name” for that type of tank, when used as a heat source.
Re our hot water tank – great question, thanks! We don’t heat tap water directly but just pump the same “old and dead” heating water round and round in the bulk volume of the tank. Then you don’t need a sacrifical anode and the walls don’t need to be glass-lined.
The heat exchanger surface needs to be rather big to allow for a high heating power (as the temperature is rather low). Our tank is a standard, maintenance-free 500 liters hygienic tank with walls made of steel (not stainless steel), with a large stainless steel heat exchanger coil – its surface being about 5m2.
The “performance” of the tank is great! I am (have been?) a fan of very long and very hot hot showers, but I did not notice any trade-off in convenience when when abandoned the gas boiler and its directly heated 100 liter tank. Rather the contrary – you can control the temperature much better in the new tank with two temperature sensors on top and bottom (compared to the “on/off” control of the gas heater), and the new tank allows for stratified heat storage, using the heat energy more efficiently.
Checked the link now – really interesting! I had once done contract research for the steel industry, and even back then I was amazed how “high-tech” metallurgy was.