On the impact of a warming climate on the energy demand for cooling and summer comfort

Wolfgang Feist

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Assessment on cooling energy:

We start with climates with significant heating demand (like Central, Eastern and Northern Europe, Canada; let’s call these “Northern Climates”): If there is significant warming … say +3 K or more … it might not be possible to maintain comfort during future summer months in buildings without active cooling. That would happen even in relatively mild places like Berlin, Paris or London. In Beijing this is already the case.

Some engineers and architects are worried about this trend toward active cooling. But, it actually is not a big problem, IF we manage to improve building insulation (which we have to do anyway to reduce the heating demand, which will still remain to be the dominant issue with regard to the building’s energy related climate impact).

These are the reasons:

Without active cooling summer indoor temperatures will rise well above comfort levels (like in Napoli, New York and Beijing right now) because the nights will not cool down enough to initiate night cooling using opened windows. The only option left will be active cooling during large parts of the summer. In most of these climates it will not be extremely humid (not like in the tropics) and the actual cooling loads in well insulated buildings will still be relatively low – typically far below 15 kWh/(m²a) in such future “Northern climates”. The newly installed cooling systems (e.g. a split unit will do) are not expensive and, as has been confirmed by measurements, have quite good summer performances (seasonal energy efficiency ratings SEER) of at least 3 and up to 5 in practical use. So the total consumption will be in the range of 3 to 5 kWh/(m²a) of electricity running the active cooling during summer. PV systems will gain at least 100 kWh/(m²PVa) in such locations and most of the electricity produced will be in summer, at the time when active cooling is needed. Indeed, with increasing renewable supply, there will be high summer surplus electricity. So, these 3-5 kWh/(m²a) for additional cooling does not cause a headache.

Now let us have a look into “Southern Climates” (i.e. locations closer to the equator where active cooling is already needed under today’s climate conditions):

The energy demand for cooling will naturally increase under warmer climate conditions. The future demands will be somewhat in between 50 and 150 kWh/(m²a) useful cooling including dehumidification in a Passive House (around 50% less than in not energy efficient buildings!), depending on the climate (mainly driven by the humidity levels). The overall SEER here will be a bit less, around SEER=3, because of the even higher outdoor temperature. That results in 15 to 50 kWh/(m²a) electricity for the active cooling systems. Now, in these climates we typically also have higher solar gains: A typical PV system will be able to generate some 200 kWh/(m²PVa) and that still will fit relatively well to the times of the cooling demand. So, in order to cover that cooling demand, we would need some 0.1 to 0.25 m² of PV-area per m² of treated floor area of a conditioned space. That is the case, if we successfully meet the Passive House efficiency levels. Electricity generation by PV in such climates should be favoured, then Passive Houses can run on regionally generated electricity. The PV requirements will obviously be significantly higher for less efficient buildings and a regional renewable energy supply no longer feasible.

This is why it is so important to start building energy efficient buildings in new construction right now. Buildings have quite long lifespans – and modernising a mediocre building is not going to happen within 15 to 20 years after construction (it’s simply too expensive without a need for refurbishment). And it shows that it is important to introduce renewable (PV-based) electricity generation especially in the hot/dry and hot/humid areas of the planet.

Most of these buildings in the hot climates of emerging economies that will create the future cooling demand do not yet exist. So, in order to limit the future need for cooling and enable a sustainable energy supply these could still be built to Passive House standard; and, if necessary, there could also be a limitation on the floor area. One way would be, to have it mandatory, to invest in the PV-areas needed for any new building as a condition to get planning permission. You will then almost automatically try to have the building as efficient as possible, because available ground area (also for PV) will be the limiting and costly factor.

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