PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009

Displacement Ventilation and Passive Cooling Strategies

PAUL CAREW1, BERNARD BEKKER1

1PJCarew Consulting, Cape Town, South Africa

ABSTRACT: The publication of the ASHRAE “System Performance Evaluation and Design Guidelines for Displacement Ventilation” [0] has contributed to the wider acceptance of displacement ventilation (DV) as a ventilation strategy, by offering clear guidelines from an established organisation. A significant advantage of DVis that it lowers the supply air quantities required for cooling compared to conventional mixing ventilation (MV) at the same supply air temperature.This opens up opportunities for the use of passive (non refrigeration-cycle based) cooling sources, which typically are limited in supply air temperature and based on 100% fresh air supply when compared to conventional refrigeration-cycle based sources. This paper quantifies the impacts of using DVin comparison to MV on the peak capacity, size and humidity levels associated with the following passive cooling sources: evaporative cooling, two stage evaporative cooling, thermal stores and air to ground heat exchangers. Ageneric office building in Johannesburg, South Africa, is used as a model. The paper illustratesthe extent to which the use of DVexpands the ability of passive cooling strategies to serve spaces previously considered as having too high a heat load (when calculated using MV system guidelines). The paper however also recognises that passivecooling strategies are unlikely to be widely implemented until design guidelines exist from organizations similar to ASHRAE. Keywords: passive design strategies, displacement ventilation

INTRODUCTION Based on the authors’ experiences as consultants in the built environment, conventional Heating Ventilation and Air Conditioning (HVAC) engineers keep to using clear guidelines and calculations methods provided by established organisations. It is unlikely that unconventional strategies and systems will be incorporated in a project in the absence of such guidelines or methods, unless it is a showcase project with sufficient resources to adequately investigate alternatives.

Displacement ventilation (DV) can be seen as an example of this. While popular in Scandinavia [0], the American Society for Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) only recently released “System Performance Evaluation and Design Guidelines for Displacement Ventilation” [0], which provides generic DV sizing and design guidelines. This publication differs from existing supplier-specific DV design guidelines in that itprovides a simple ten-step guide to design the DV system [0]. While the authors have not found literature that documents the worldwide growth in the number of installed DV systems, since the ASHRAE publication was released they have found HVAC engineers in general to be more receptive to the use of DV for conventional projects.

The aim of this paper is to illustrate the impacts of using DV in comparison tomixing ventilation (MV) on

the peak capacity, size and humidity levels associated with various passive cooling strategies, informed by the ASHRAE DV guidelines. The intention is NOT to provide clear design guidelines for these passive strategies, but merely to demonstrate the implications of these strategies through quantitative analysis.

Through this analysis it is demonstrated that a number of passive cooling strategies, considered impractical for MV applications, can potentially be of benefit in building designs utilising DV.The comparison also highlights the risk of over-design when conventional MV guidelines are used to design cooling sources for DV applications. DISPLACEMENT VS. MIXED VENTILATION DV introduces air at low level and low velocity, and at high supply air temperature (Ts),typically around 18°C,when compared to conventional air conditioning systems which use MV. The air that is slightly cooler than the intended room temperature runs along the floor until it reaches a heat load (Fig. 1). The heat load induces a plume of warmer air that rises due to lower density. This induces stratification in room temperature with the occupied area of the room within comfort conditions and the space near the ceiling at higher temperature conditions. The air near the ceiling is continually exhausted to prevent a build up of warm air into the occupied zone.

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PLEA2009 - 2

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PLEA2009 - 2

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26th Conference on

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PLEA2009 - 2

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26th Conference on

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PLEA2009 - 2

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26th Conference on

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PLEA2009 - 2

ection on thof the season

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a variety of d length of the illustrates howegy can be u

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26th Conference on

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e – the storagct being the refor the store.

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ksman, L. (200esign Guidelin

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Energy LabIWEC weather

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y.gov/buildings/eon 22nd May 200vaporative cooli

Box 1943, Halfw

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