Passive and Free Cooling of Buildings
Residential sectors in EU are responsible for 40% use of final energy and almost 50% of total final energy used for space heating and cooling. Although the energy demand for space cooling is not well known, it is estimated (Persson and Werner, 2015; HRE4 project, 2017) that currently 47 to 77 TWh per year of cooling energy is supplied, which is only 2 to 2.55% of total final energy demand in the residential sector. Nevertheless, in some states (e.g., Cyprus and Malta), space cooling contribute to almost half of the final energy demand in buildings, and in Greece and Spain the energy demand for space cooling is much above the EU average (HRE4 project, 2017). Meanwhile, the energy demand for heating will decrease due to more stringent energy efficiency of buildings requirements and global warming. It is expected that the energy demand for cooling will significantly rise in the future. There are several reasons for that, such as global warming, increased urban heat islands, demographic trends, and substantial increase of cooled buildings is expected. Although 80% of residential buildings are mechanical cooled in Cyprus and 65% in Malta, less than 5% of residential buildings are mechanical cooled in the EU member states in average.
Local climate conditions related to the needs for space cooling are expressed by cooling degree-hours (CDH) or cooling degree-days (CDD) expressed in К • hour or К • day per year. CDH and CDD are determined by the expression:
where 0e ^ is the outdoor air temperature of the i-th hour over the year, 0e,avg,j is the average daily outdoor air temperature of the j-th month over the year, and 0e baSe is the outdoor temperature at which the building doesn’t need cooling. The value of the so-called “base” temperature of 0e base of 18°C is commonly used (European Environment Agency, 2019). For the territory of the Europe, the CDD map is shown in Figure 15.1. Significant trends were noticed in the last three decades showing that the heating degree day (HDD) decreases, meanwhile CDD increases (Figure 15.2).
Electricity is the primary energy carrier that is used for compressor-driven mechanical cooling nowadays. In spite of the technological development that led to the improved energy efficiency of cooling aggregates (with coefficient of performance [COP] up to 8), the use of electricity has a large impact on the environment.
FIGURE 15.1 Cooling degree days for the territory of the Europe. (From Jakubcionis, M. and Carlsson, J., Energy Policy, 113, 223-231, 2018.)
FIGURE 15.2 Changes of HDD and CDD in Europe (European Environment Agency, Data and maps, Heating and cooling degree days, Prod-ID: IND-348_en, 2019); values are weighted by the population exposed to the climate changes.
More environmentally friendly cooling technologies include PV and thermal solar- driven space cooling. Nevertheless, the electricity demand can be decreased applying passive and free cooling techniques.
Meanwhile, the CDD gives appropriate insight in the energy demand for mechanical cooling of the building. They are not suitable for assessment of the potential of the passive and the free cooling, at least not for natural cooling by ventilation. In this case, it is more appropriate that meteorological data for the site are expressed by average daily outdoor air temperature 0eavg and daily amplitude of the outdoor air temperature 0e. An example for selected sites is presented in Figure 15.3 together with the conditions favored for the efficient passive and free cooling of the buildings by ventilation. Values for the period between June and August are shown.
It is expected that climate changes will have an impact on the efficiency of the passive and the free cooling of the buildings due to the decreased amount of environment cold and increased overheating of the buildings. With the following example, we want to show which of these changes, which otherwise has the opposite effect, will have a greater impact on the efficiency of natural cooling. Four estimated climate change scenarios (AIT, A IB, A1F1C, and Bl) were investigated by dynamic thermal response modeling of a reference building (Figure 15.30) using meteorological data in form of Test Reference Year (TRY) for The City of Ljubljana (Cfb Koppen-Geiger classification). TRY data was corrected (CTRY) using Fienkelstein- Schafer statistics (adapted from Vidrih and Medved, 2008): (1) moderate temperature increase (CTRY A - AIT)—average temperature rise of 1°C; (2) moderate temperature and solar-radiation energy increase (CTRY В ~ A IB)—average temperature rise of 1 °C and average solar radiation energy increase of 3%; (3) significant
FIGURE 15.3 Daily average outdoor air temperatures and daily amplitude of outdoor air temperature for selected sites; conditions suitable for passive and free cooling by ventilation are presented. (From Meteonorm. Global Meteorological Database for Solar Energy and Applied Climatology; Version 5.1: Edition 2005; Software and Data on CD-ROM; Meteotest AG, Bern, Switzerland, 2005.)
temperature increase (CTRY C ~ A1F1C)—average temperature rise of 3°C; and (4) significant temperature and solar-radiation energy increase (CTRY D ~ В1): average temperature rise of 3°C and average solar-radiation energy increase of 6%. Beside reference climate data (TRY), the measured data at the site from the year 2003 was analyzed because the outdoor air temperature was close to the most severe climate change scenario. Results of dynamic thermal response of the free running reference office (without mechanical cooling) are presented in Figure 15.4. The occupancy
FIGURE 15.4 Annual overheating hours of a reference free running office at different climate conditions. (Adapted from Vidrih, B. and Medved, S., In!. J. Energy Res., 32, 1016— 1029. 2008.) schedules and internal gains were assumed according to Vidrih and Medved (2008) and ISO 18523-1 (2016). It can be seen that the technique of the natural cooling will continue to be effective in spite of expected climate change.