Over the summer of 2022, Sero and Kensa Heat Pumps are trialling a passive cooling system using the ground source heat pump (GSHP) fitted at Sero’s demonstration home at The Mill development in Cardiff.
The home had an existing Kensa Shoebox heat pump heating system which has successfully and efficiently heated the home for the past few years. In early summer 2022, the home was retrofitted with Kensa’s proposed passive cooling solution which uses a fan coil unit (FCU) to cool the home’s living room using water from the GSHP’s ground loop.
The cooling system has been up and running since the end of June 2022 and so far has exceeded expectations and proven very effective at both quickly cooling the room when the cooling is turned on, and keeping the room temperature low for extended periods.
Fortuitously, this trial coincided with the mid-July 2022 heatwave where Wales experienced record-breaking temperatures allowing us to stress test the system under some quite extreme conditions. The system performed admirably throughout the heatwave – on the 18th of July 2022, when Wales experienced its warmest day on record the outside temperature in Cardiff peaked at over 32°C but the home’s living room remained between 20°C and 21°C all day.
The green line in the chart below shows the rapid rate at which the system was able to cool the room once it was turned on at 8 am. Within 45 mins the system had brought the room down from 23.5° to the target temperature of 21°C.
Longer Term Cooling Performance
The 2022 summer featured two periods of unusually warm weather; mid-July and again in mid-August. While the media focussed on the record-breaking peak temperatures of the first of these periods, the second arguably presented more of a challenge. August may not have quite reached the peak temperatures of July, but the first half of the month presented a more sustained period of extremely warm weather.
The chart below compares outdoor temperature (blue) and the temperature of the living room (orange) at the times when the cooling system was enabled. The cooling system proved capable of consistently cooling the room to set-points above 20°C, even during the warmest days. The variations shown in the chart below were due to set-point changes, as we tested the system through a range of settings.
Across the whole of the summer, there was very little variation in the performance of the ground loop. The temperature of the water coming from the ground loop temperature remained stable and largely unaffected by outdoor temperature.
The chart below shows the daily average ground loop and outdoor temperature for the times when the cooling system was run. There is no direct correlation between ground loop temperature and outdoor temperature. During both the July and August heat waves, the ground loop was largely unaffected. Initial analysis may suggest that the number of days between significant rainfall has a larger bearing on ground loop temperature than the outdoor temperature, but more data and further analysis would be needed to say this with confidence.
Operating Energy, Carbon & Cost
The main benefit of a passive cooling system such as this over traditional “active” air conditioning systems is the energy efficiency. Passive systems use significantly less energy than active cooling systems and are therefore considerably cheaper to run, with lower CO2 emissions.
We have estimated that the system used at the trial has an energy demand of between 0.15kW and 0.2kW depending on how hard it is working to maintain temperature. Running costs largely depend on energy tariff and how often the cooling is used but we have estimated costs for a range of scenarios, at both the current highly elevated electricity tariffs (due to the energy crisis) and at what can be considered more normal rates for residential buildings.
We have also estimated how much CO2 would be emitted by the electricity used to run the cooling. The CO2 figures in the table below assume an average grid CO2 intensity of 0.174 kgCO2/kWh - roughly the average carbon intensity of national grid supplied electricity during summer 2022.
Using this same carbon intensity, we estimate that a similar home to the test home would normally have annual emissions of around 750 kg each year, and for a typical UK home, this is closer to 4,800 kg. This puts the addition of less than 14 kg for the cooling system into context showing how efficient it is, with minimal contribution to the home's carbon footprint.
What about PV Panels and Batteries?
The costs and CO2 emissions stated in the above table assume that the cooling system is solely supplied with electricity from the national grid. However, the test home at The Mill development also has a solar PV system on the roof, coupled with a battery.
PV systems generate most of their energy during the summer, coinciding with the lowest electricity demand from the home since the electrically powered heating system isn't needed. This home's battery is therefore usually very well charged during the summer and often excess PV-generated electricity must be exported to the grid as the home has no use for it and the battery is already fully charged.
For this test home, it's therefore the case that the carbon-free electricity that's generated by the PV panels should be able to power the cooling system for the vast majority of the time it's needed. This would reduce the CO2 emissions and costs listed in the above table significantly.