Mixed-mode ventilation

Abstract: The United Arab Emirates’ (UAE) hot climate, coupled with the extreme demographic and urban growth experienced over the past four decades, has shaped a built environment where energetic quality of construction has been superseded by the quantity of construction needed to support the country's growth. This development is further aggravated by the slow development of energetic building codes, as well as the subsidized cost of electricity. The result is that the UAE consistently leads the list of countries with the highest environmental footprint, and the electricity production required to drive building cooling constitutes the brunt of the emissions balance-sheet. The work presented here reviews primarily UAE-based research that addresses the effectiveness of passive building-envelope measures that reduce energy consumption. A number of measures have been developed in response to the increasing demands of emerging energy regulations, and include measures specific to the building envelope in the planning phase or as retrofit, including radiative, convective and conductive heat transfer through walls, windows, roof, as well as energy efficient natural ventilation techniques. This review is geographically restricted to the UAE as its development challenges are directly tied to its distinct economic growth pattern and specific legislation implemented to address energy efficiency. Results confirm the importance of the following factors for energy-optimized structures: building orientation, thermal insulation (which can generate in excess of 20% energy savings in particular in the residential context), appropriate glazing type and orientation in highly glazed office buildings (up to 55% energy savings reported), excessive light levels and glare, and natural ventilation, which can reduce energy consumption from a reported high of 30% in villas to up to 79% in a high rise office building using mixed mode ventilation.

Abstract: “Mixed-mode” refers to a hybrid approach to space conditioning that uses a combination of natural ventilation from operable windows (either manually or automatically controlled), and mechanical systems that provide air distribution and some form of cooling (air-conditioning, radiant cooling, etc) By utilizing mechanical cooling only when and where it is necessary to supplement the natural ventilation, a well-designed mixed-mode building offers the potential to improve the indoor environmental quality while minimizing the significant energy and operating costs of air-conditioning But there is limited information about the performance of mixed-mode buildings, particularly with regard to occupant satisfaction, and this can potentially be a powerful part of the argument to avoid or minimize the use of air-conditioning This paper describes the results of web-based surveys conducted in 12 mixed-mode buildings, in comparison to our overall benchmarking survey database of 370 buildings, with over 43,000 individual responses The survey focuses on seven areas of indoor environmental performance, including thermal comfort, air quality, acoustics, lighting, cleanliness, spatial layout, and office furnishings The data shows that only 11% of the 370 buildings, most of which have conventional air-conditioning systems, are meeting the intent of the thermal comfort standards to achieve 80% satisfaction in the buildings In comparison, the mixed-mode buildings are performing exceptionally well compared to the overall building stock, especially with regard to thermal comfort and air quality Among the mixed-mode buildings, the best performers were those that were in more moderate climates, were newer, had radiant cooling or mechanical ventilation only (instead of an air-cooled system), and allowed high degrees of direct user control without changeover window interlock systems