Energy demand for active mechanical space cooling is projected to double by 2050. Wider adoption of passive cooling systems can help reduce demand.
Globally, buildings consume 60 percent of available electricity, 18 percent of which is due to space cooling. Barring an alternative, electricity demand for space cooling is projected to double by 2050, driven by accelerating urbanization, primarily in hot and warm climates 
. This adds additional loads on the electric grid during peak use periods. To the extent the electricity to satisfy these demands is generated by nonrenewable means, increased mechanical space cooling results in an increase in carbon emissions, exacerbating the demand for cooling in the future.
Meeting space cooling loads with passive systems rather than active mechanical air conditioning systems can play two important roles. The first is reducing burdens on electrical grids, and the second is reducing or eliminating carbon emissions associated with energy generation due to the increased use of active mechanical cooling. However, this will require wider adoption of passive systems than is currently the case.
The basic principles of passive cooling have been understood and applied since ancient times, and there have been continuous innovations in the field, resulting in contemporary passive systems that are not only effective, but capable of being integrated into commercial and residential buildings. In spite of this, applications for passive cooling systems remain limited, due to widespread unfamiliarity with the systems and consequent misperceptions regarding their performance and operation.
2. Passive Cooling Principles
Effective passive cooling requires limiting space heat gains, then modulating any unavoidable gains, and finally dissipating sensible and latent heat gains without reliance on mechanical systems using three heat sinks: the atmosphere, the sky, and the earth 
. Cross and stack ventilation, wind towers, and downdraft evaporative cooling towers dissipate heat into the atmosphere through convection and evaporation. Roof ponds transfer heat into the night sky through radiation, and earth tubes as well as earth contact exchange heat with the ground through conduction 
3. Passive Downdraft Cooling Systems in Buildings
Heat transfer through direct evaporation of water is one of the oldest means of passive cooling, dating back to ancient Egypt around 2500 B.C. and then evolving to be integrated with the outlets of wind towers in the desert regions of Iran in the early tenth century 
. Innovations in wind tower design came in the 1980s when a rigid media direct evaporative cooling system was placed at the top of the tower, causing heavier, cooler air to drop downwards by gravity; hence, the design was named the passive downdraft evaporative cooling tower (PDECT) 
PDECT designs were subsequently refined through variations to the tower inlet cooling mechanisms which can be classified under three categories illustrated in Figure 1
. The first is direct or latent cooling which uses rigid media, shower heads, or misting nozzles. The second is indirect or sensible cooling which uses closed-loop chilled water pipes. The third is hybrid cooling which uses a combination of direct and indirect cooling 
Figure 1. PDECT operational principle and cooling mechanisms.
Studies have demonstrated that built PDECT examples are energy- and water-efficient, cost-effective, and thermally comfortable under most conditions 
. This is in contrast to widespread misperceptions regarding PDC performance and operation, and risk-averse reliance on more familiar active AC systems.
In the United States, more than 88 percent of residential building stock uses mechanical air conditioning systems for space cooling 
. Because mechanical AC systems are designed to maintain interior temperatures and relative humidity within very narrow ranges, a barrier to wider adoption of PDC systems may be the assumption that occupant comfort requires precise control beyond the performance capabilities of passive systems, and that passive downdraft cooling is incapable of providing reliable thermal comfort for occupants.
Most PDC systems use direct latent cooling, leading to the misperception that passive systems use substantially more water during their operation than comparable mechanical AC systems. This is also a barrier to PDC adoption, particularly in hot dry regions with limited water supplies. In fact, PDC has proven to be a competitive solution when water used in the generation of electricity to power mechanical AC systems is included 
4. Passive Downdraft Cooling System Performance Evaluation Methods
While effective research methods exist for evaluating the performance of PDC systems, most of these methods require considerable investments in terms of space, capital, and training. In addition, the availability of more sophisticated PDC systems, such as multi-stage hybrid systems, is constrained by a lack of applied research and development on downdraft cooling. Familiarity with PDC systems could be increased and widespread misperceptions addressed if performance evaluation methods were more cost-effective and accessible.
The five common research methods used in studies of PDC are computational fluid dynamics (CFD) simulations 
, full-scale prototypes tested outdoors 
or in a controlled environment 
, and reduced-scale prototypes tested outdoors 
or in a controlled environment 
Table 1 outlines the advantages and limitations of each method based on an analysis of recent studies. CFD simulations require an extended time to learn how to properly use the tool without providing the ability to understand operational issues. Full-scale prototype testing indoors and outdoors can evaluate system operational issues but requires large space, detailed construction knowledge, and higher budgets to set up. Reduced-scale prototypes can be quickly fabricated and provide insights into operational challenges; however, when they are tested outdoors, data collection timeframes are bound by ambient conditions.
Table 1. Advantages and limitations of the different PDC performance evaluation methods.
Reduced-scale prototype testing in a controlled environment addresses the limitations of the other four methods and provides for expanded research and development opportunities of PDC. This method allows for rapid quantitative testing of multiple design iterations as well as a qualitative understanding of operational issues that would occur at full scale. However, fully realizing the advantages of reduced-scale testing requires a controlled environment that is both affordable to build and capable of practically and reliably replicating environmental conditions year-round independent of ambient conditions. PDC reduced-scale prototype testing is typically conducted inside wind tunnels of various scales 
which are complex to build and commission. In addition, they require capital and space investments even greater than what is required for full-scale prototypes.