KPI For Calculating The Climate-potential Of Passive Cooling Systems

As reported by several international research publications, in recent years, in both emerging and industrialized countries, the energy consumption for space cooling and ventilation is rising quickly.

Nevertheless, even if the average efficiency of installed air-conditioning units is increasing, this trend is still remaining and concerns several causes as the separation between building design choices and climate conditions. The increase in the living standards and comfort expectation, the rise of internal heat gains, and the effect of local and global warming all are contributing factors. To face this challenge, the spread of passive and hybrid solutions for space cooling represents a valid alternative to reduce the consumption of electricity in the built environment.

An example of passive climate control (credit: Flickr)

Nevertheless, different from passive heating solutions, natural cooling systems do not refer to one principal energy source, but deal with a large set of possible bioclimatic solutions. Moreover, natural heat gains’ dissipation techniques, based on the use of thermal sinks (water, air, ground and night sky), are very site-specific. For this reason, a series of studies are under development at Politecnico di Torino, Department of Architecture and Design to define tools and atlas of applicability of passive cooling systems in different geo-climatic locations.

In a recent series of papers, Dr. Giacomo Chiesa, assistant professor of Architectural Technology at the Politecnico di Torino, has developed a series of indicators to define the applicability of several passive cooling solutions under different climate conditions. At present, controlled natural ventilation (comfort and night ventilation), direct evaporative cooling and heart-to-air heat exchangers were considered, while other papers of the author investigate the effect of climate on the summer and winter performance of other passive systems (e.g. sunspaces).

To start, a building was measured using recognized climate-related indicators of heating and cooling demand –  Heating Degree Days (HDD) and Cooling Degree Days or Hours (CDD or CDH). Considering different base temperatures, a “virtual residual” climate cooling/heating demand approach was defined and further compared with energy consumptions by using energy dynamic simulations on sample buildings.  These “residual” indexes are based on the effects that different passive ventilative systems have on the external air, treating it before building introduction.

Development Of Earth-To-Air Heat Exchangers

As an example, in a recently published paper presented at CISBAT 2017, the climate-potential of earth-to-air heat exchangers (EAHX) was discussed by introducing a tool to estimate the potential of such technology since early-building-design phases. EAHX can pre-treat the air both in winter and in summer increasing or reducing the temperature of the treated air-flow thanks to the ground temperature (sensible exchange). The treated temperature is used to calculate the residual HDD and CDH and to compare them with the original ones in different locations according to local typical meteorological conditions.

The EAHX-treated airflow temperature is function of the environmental inlet air (typical weather hourly conditions), the temperature of the ground at a given depth in the same hour (calculated adopting the Hadvig equation), which is influenced by the soil composition and the annual weather conditions, and the effectiveness of the EAHX system, which is a parameter here set on the average results of a long-term monitoring conducted during a previous research (see papers on Building and Environment; Energy Procedia).

Thanks to this approach, the potential applicability of EAHX can be calculated for early-design phases changing the depth of the system. An updated version of this tool is under development including also the possibility to vary some geometrical characteristics of the system and to analyze its applicability and potential in a larger set of locations.

Other climatic studies analysing the potential of controlled natural ventilation as a thermal sync were published on recent papers in “Energy and Buildings” and in “the International Journal of Ventilation”, while the effect of PDEC was analysed on papers published on “the International Journal of Ventilation”, a Springer book chapter, and for the conference SEB-16 (“Energy Procedia”). Furthermore, a comparison between these series of geo-climatic indicators was illustrated at the URBAN-CEQ conference in Genoa (September 2017).

Further researches are under development/reviewing in order to include additional technologies and different climate-data sources by also analyzing climate changes and urban heat island effects. The adoption of different climate database was in fact demonstrated to sensibly affect the results of simulations in the building context (see a paper presented at IBPC 2015).

This article is a compilation of papers by Dr. Chiesa and others. The recent study, Climate-potential of earth-to-air heat exchangers was published in the journal Energy Procedia.

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