Cooling Potential

I had an interesting discussion about the cooling potential of our heat pump system – in a climate warmer than ours.

Recently I’ve shown data for the past heating season, including also passive cooling performance:

After the heating season, tank temperature is limited to 10°C as long as possible – the collector is bypassed in the brine circuit (‘switched off’). But with the beginning of May, the tank temperature starts to rise though as the tank is heated by the surrounding ground.

Daily cooling energy hardly exceeds 20kWh, so the average cooling power is always well below 1kW. This is much lower than the design peak cooling load – the power you would need to cool the rooms to 20°C at noon on a hot in summer day (rather ~10kW for our house.)

The blue spikes are single dots for a few days, and they make the curve look more impressive than it really is: We could use about 600kWh of cooling energy – compared to about 15.000kWh for space heating. (Note that I am from Europe – I use decimal commas and thousands dots :-))

There are three ways of ‘harvesting cold’ with this system:

(1) When water in the hygienic storage tank (for domestic hot water) is heated up in summer, the heat pump extracts heat from the underground tank.

Per summer month the heat pump needs about 170kWh of input ambient energy from the cold tank – for producing an output heating energy of about 7kWh per day – 0,3kW on average for two persons, just in line with ‘standards’. This means that nearly all the passive cooling energy we used was ‘produced’ by heating hot water.

You can see the effect on the cooling power available during a hot day here (from this article on passive cooling in the hot summer of 2015)

Blue arrows indicate hot water heating time slots – for half an hour a cooling power of about 4kW was available. But for keeping the room temperature at somewhat bearable levels, it was crucial to cool ‘low-tech style’ – by opening the windows during the night (Vent)

(2) If nights in late spring and early summer are still cool, the underground tank can be cooled via the collector during the night.

In the last season we gained about ~170kWh in total in that way – so only as much as by one month of hot water heating. The effect also depends on control details: If you start cooling early in the season when you ‘actually do not really need it’ you can harvest more cold because of the higher temperature difference between tank and cold air.

(3) You keep the cold or ice you ‘create’ during the heating season.

The set point tank temperature for summer  is a trade-off between saving as much cooling energy as possible and keeping the Coefficient of Performance (COP) reasonably high also in summer – when the heat sink temperature is 50°C because the heat pump only heats hot tap water.

20°C is the maximum heat source temperature allowed by the heat pump vendor. The temperature difference to the set point of 10°C translates to about 300kWh (only) for 25m3 of water. But cold is also transferred to ground and thus the effective store of cold is larger than the tank itself.

What are the options to increase this seasonal storage of cold?

  • Turning the collector off earlier. To store as much ice as possible, the collector could even be turned off while still in space heating mode – as we did during the Ice Storage Challenge 2015.
  • Active cooling: The store of passive cooling energy is limited – our large tank only contains about 2.000kWh even if frozen completely; If more cooling energy is required, there has to be a cooling backup. Some brine/water heat pumps[#] have a 4-way-valve built into the refrigeration cycle, and the roles of evaporator and condenser can be reversed: The room is cooled and the tank is heated up. In contrast to passive cooling the luke-warm tank and the surrounding ground are useful. The cooling COP would be fantastic because of the low temperature difference between source and sink – it might actually be so high that you need special hydraulic precautions to limit it.

The earlier / the more often the collector is turned off to create ice for passive cooling, the worse the heating COP will be. On the other hand, the more cold you save, the more economic is cooling later:

  1. Because the active cooling COP (or EER[*]) will be higher and
  2. Because the total cooling COP summed over both cooling phases will be higher as no electrical input energy is needed for passive cooling – only circulation pumps.

([*] The COP is the ratio of output heating energy and electrical energy, and the EER – energy efficiency ratio – is the ratio of output cooling energy and electrical energy. Using kWh as the unit for all energies and assuming condenser and evaporator are completely ‘symmetrical’, the EER or a heat pump used ‘in reverse’ is its heating COP minus 1.)

So there would be four distinct ways / phases of running the system in a season:

  1. Standard heating using collector and tank. In a warmer climate, the tank might not even be frozen yet.
  2. Making ice: At end of the heating season the collector might be turned off to build up ice for passive cooling. In case of an ’emergency’ / unexpected cold spell of weather, the collector could be turned on intermittently.
  3. Passive cooling: After the end of the heating season, the underground tank cools the buffer tank (via its internal heat exchanger spirals that containing cool brine) which in turn cools the heating floor loops turned ‘cooling loops’.
  4. When passive cooling power is not sufficient anymore, active cooling could be turned on. The bulk volume of the buffer tank is cooled now directly with the heat pump, and waste heat is deposited in the underground tank and ground. This will also boost the underground heat sink just right to serve as the heat source again in the upcoming heating season.

In both cooling phases the collector could be turned on in colder nights to cool the tank. This will work much better in the active cooling phase – when the tank is likely to be warmer than the air in the night. Actually, night-time cooling might be the main function the collector would have in a warmer climate.

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[#] That seems to be valid mainly/only for domestic brine-water heat pumps from North American or Chinese vendors; they offer the reversing valve as a common option. European vendors rather offer a so called Active Cooling box, which is a cabinet that can be nearly as the heat pump itself. It contains a bunch of valves and heat exchangers that allow for ‘externally’ swapping the connections of condenser and evaporator to heat sink and source respectively.

Simulating Life-Forms (2): Cooling Energy

I found this comprehensive research report:
Energy Use in the Australian Residential Sector 1986–2020 (June 2008)
(several PDFs for download, click the link Energy Use… to display them)

There are many interesting results – and the level of detail is impressive: The authors modelled the energy used per appliance type, by e.g. factoring in how building types change slowly over time or by modelling the development of TV sets and their usage. Occupancy factors for buildings are determined from assumptions about typical usage profiles called Stay At Home, At Work or Night Owl.

I zoom in on simulating and predicting usage of air conditioning and thus cooling energy:

They went to great lengths to simulate the behavior of home owners to model operations of air conditioning and thus total cooling energy for a season, for a state or the whole country.

The authors investigated the official simulation software used for rating buildings (from …part2.pdf):

In the AccuRate software, once cooling is invoked the
program continues to assume that the occupant is willing to
tolerate less than optimal comfort conditions and will therefore terminate cooling if in the absence of such cooling the internal temperature would not rise above the summer neutral temperature noted in Table 57, + 2.5oC plus allowances for humidity and air movement as applicable. While this may be appropriate for rating purposes, it is considered to be an unlikely form of behaviour to be adopted by householders in the field and as such this assumption is likely to underestimate the potential space cooling demand. This theory is supported by the survey work undertaken by McGreggor in South Australia.

This confirms what I am saying all the time: The more modern a building is, or generally nowadays given ‘modern’ home owners’ requirements, the more important would it be to actually simulate humans’ behavior, on top of the physics and the control logic.

The research study also points out e.g. that AC usage has been on the rise, because units got affordable, modern houses are built with less focus on shading, and home owners demand higher standards of comfort. Ducted cooling systems that cover the cooling load of the whole house are being implemented, and they replace systems for cooling single zones only. Those ducted systems have a rated output cooling power greater than 10kW – so the authors (and it seems Australian governmental decision makers) are worried about the impact on the stability of the power grid on hot days [*].

Once AC had been turned on for the first time in the hot season, home owners don’t switch it off again when the theoretical ‘neutral’ summer temperature would be reached again, but they keep it on and try to maintain a lower temperature (22-23°C) that is about constant irrespective of temperature outside. So small differences in actual behavior cause huge error bars in total cooling energy for a season:

The impact of this resetting of the cooling thermostat operation was found to be significant. A comparison was undertaken between cooling loads determined using the AccuRate default thermostat settings and the modified settings as described above. A single-storey brick veneer detached dwelling with concrete slab on ground floor and ceiling insulation was used for the comparison. The comparison was undertaken in both the Adelaide and the Darwin climate zones. In Adelaide the modified settings produced an increased annual cooling load 64% higher than that using the AccuRate default settings.

The report also confirms my anecdotal evidence: In winter (colder regions) people heat rooms to higher temperatures than ‘expected’; in summer (warmer regions) people want to cool to a lower temperature:

This is perhaps not surprising, de Dear notes that: “preferred temperature for a particular building did not necessarily coincide with thermal neutrality, and this semantic discrepancy was most evident in HVAC buildings where preference was depressed below neutrality in warm climates and elevated above neutrality in cold climates (ie people preferred to feel cooler than neutral in warm climates, and warmer than neutral in cold climates)” (Richard de Dear et al 1997, P xi).

I noticed that the same people who (over-)heat their rooms to 24°C in winter might want to cool to 20°C in summer. In middle Europe AC in private homes has been uncommon, but I believe it is on the rise, too, also because home owners got accustomed to a certain level of cooling when they work in typical office buildings.

My conclusion is (yet again) that you cannot reliably ‘predict’ cooling energy. It’s already hard to do so for heating energy for low energy houses, but nearly impossible for cooling energy. All you can do – from a practical / system’s design perspective – is to make sure that there is an ‘infinite’ source of cooling energy available.

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[*] Edit: And it actually happenend in February 2017.

Having Survived the Hottest July Ever (Thanks, Natural Cooling!)

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.

Space heating with solar collector on, heat pump system punktwissen,

Space heating mode: The heat pump (1) heats the buffer tank (7), which in turn heats the heating circuits (only one circuit shown, each has its circuit pump and mixer control). Heat source: Solar collector (4) and water / ice storage (3) connected in a single brine circuit. The heat exchanger in the tank is built from the same ribbed pipes as the solar collector. If the ambient temperature is too low too allow for harvesting of energy the 3-way valve (5) makes the brine flow bypass the collector.

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.

Heating hot water, solar collector off, heat pump system punktwissen

Hot tap water heating mode: The flow of water heated by the heat pump is diverted to the hygienic storage tank (6). Otherwise, the heat source is used in the same way as for space heating. In this picture, the collector is ‘turned off’ – corresponding to heating water on e.g. a very cold winter evening.

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!

Space cooling while heating hot water, heat pump system punktwissen

Cooling mode: Via automated 3-way valve (9) brine is diverted to flow through the heat exchanger in the buffer tank (7). Water in the buffer tank is cooled down so water in the floor ‘heating’ / cooling loops. If the heat pump operates in parallel to heat hot tap water, it cools the brine.

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:

Temperatures and performance factors, July 2015

The red arrow indicates the end of the ice period; then the set temperature of the tank was 8°C (‘Ice storage tank’ is rather a common term denoting this type of heat source than indicating that it really contains ice all the time.) Green arrows indicate three spells of hot weather. The tank’s temperature increased gradually, being heating by the surrounding ground and by space cooling. At the beginning of August its temperature is close to 20°C, so cooling energy has nearly be used up completely.

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:

Ventilation: Night

Manually operated ventilation – to be shut off at sunrise. We had already 30°C air temperature at 08:00 AM on some days.

Deflector shields: Day

South-east deflector shields down. We feel there is still enough light in the (single large) room as we only activate the subset of shields facing the sun directly.

These are details for two typical hot days in July:

Temperature and cooling power for two days in July

The blue line exhibits the cooling power measured for the brine ‘cooling’ circuit. If the heat pump is off, cooling power is about 1 kW; during heat pump operations (blue arrows) 4 kW can be obtained. Night-time ventilation is crucial to keep room temperatures at reasonable levels.

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:

Daily energy balance, PV generation and self-consumption-2015-06-and-2015-07In 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.