Photo: Pixabay

More and more energy for cooling

Due to the climate-induced rise in temperature, the cooling requirements of buildings will continue to increase. An extrapolation by Empa researchers based on data from the NEST building and future climate scenarios for Switzerland shows that this increase in energy demand for cooling is likely to be substantial and could have a strong impact on our future – electrified – energy system.

Due to climate change, the average temperature will rise in the coming decades. This should also lead to a significant increase in the number of so-called cooling degree days. These measure the number of hours that the ambient temperature is above a threshold at which a building needs to be cooled to keep the indoor temperature at a comfortable level. The rising values may lead to an increase in the installation of cooling appliances in private households. This could further increase the energy demand for cooling buildings, which is already set to increase due to climate change and population growth.

Head-to-head race between heating and cooling

In order to gain a better understanding of how large this increase will be in Switzerland, Empa researchers have analysed the heating and cooling requirements of the NEST research and innovation building. “Taking into account the ambient temperatures, we were able to extrapolate the future thermal energy demand of buildings based on the climate scenarios for Switzerland. In addition to climate change, we also took population growth and the increasing use of cooling devices into account,” explains Robin Mutschler, postdoc at Empa’s Urban Energy Systems Lab.

The results forecast a sharp increase in cooling energy demand: Assuming an extreme scenario in which the whole of Switzerland would be dependent on air conditioning, almost as much energy would be needed for cooling as for heating by the middle of the century. Expressed in figures, this corresponds to about 20 terawatt hours (TWh) per year for heating and 17.5 TWh for cooling. The required cooling energy was calculated independently of the technology: If this is provided by reversing a heat pump process, e.g. with COP 3 for cooling, the electricity demand for 17.5 TWh of cooling energy amounts to about 5.8 TWh.

The heating requirements of the occupied modules in the NEST building are comparable to those of a modern apartment building. The calculated figures are therefore representative if it is assumed that the average Swiss building stock corresponds to the NEST building. When this will be the case depends on the renovation rate. Even in a more moderate scenario, cooling demand in Switzerland will increase significantly. In this scenario, the researchers assume an additional energy demand of 5 TWh per year.

Strong influence on the Swiss energy system

The energy demand of Swiss buildings today accounts for around 40 percent of the total energy demand. The main part of this is accounted for by heating. This is likely to remain the case until at least the middle of the 21st century, but energy demand for cooling buildings is expected to increase sharply. If thermal energy is provided by heat pumps, which can also cool, this potentially has a strong impact on the overall energy system and in particular on electricity as an energy carrier.

At present, only a small proportion of Swiss households are thought to have air conditioning, but the number of households with heat pumps is increasing. The Empa researchers estimate that the number of households with cooling appliances could rise to over 50 percent due to the increase in cooling degree days. This increase could lead to significant peaks in demand on hot days. An additional energy demand of 5 TWh for cooling would correspond to about 2 percent of today’s electricity demand if cooling is done with heat pumps. In an extreme scenario, the demand for cooling could even reach 10 percent of today’s total electricity demand. However, this will not be evenly distributed throughout the year, but will correlate with hot periods, which may lead to peaks in demand. It is advantageous that the cooling requirement is relatively well covered by the electricity generated by photovoltaic systems. The impact of cooling residential buildings will be significantly higher compared to office buildings, as they account for about two thirds of the building area.

Based on these findings, it is clear to the researchers that these developments should be taken into account when constructing new buildings and that the possibilities, such as passive cooling, should be fully exploited. “Building architecture should no longer focus only on optimizing heat losses in winter, but also on reducing heat gains in summer,” Mutschler says. This could be achieved, for example, through urban planning measures for climate adaptation at neighbourhood level, the implementation of heat reduction programmes or the reduction of the proportion of glazing in buildings. “In addition, it is central that policymakers also address this development and investigate how best to meet the increasing demand for cooling energy while minimizing the impact on the future decarbonized energy system,” Mutschler said. A possible contribution to the cooling of buildings can be provided by district cooling systems, which have already been successfully implemented in Switzerland – for example in Geneva. Others are in the making, for example in Zug.