Time-averaged single sided ventilation rates and thermal environment in cooling mode for a low energy retrofit envelope

Non-invasive, scalable, building retrofit solutions are very attractive deep renovation techniques to improve energy efficiency in existing buildings; this includes natural ventilation for cooling due to the low impact nature of the installation. However, a number of criteria that are important to natural ventilative cooling strategies can be substantially altered as a result of an external retrofit solution. This paper investigates this experimentally; it presents ventilation rate and internal thermal environment results from full scale testing of a modular, scalable, external low energy retrofit envelope solution applied to an existing 1970s precast concrete building in Ireland. Experimental results of time averaged single sided ventilation rates for three different ventilation opening configurations in a retrofitted office space during a warm and low wind summer period are analysed and compared to a single configuration control space. Results show that the highest time averaged ventilation rates were measured in the control space although a similar distribution was present in one retrofit opening configuration. Analysis of tracer concentration decay fluctuation profiles during tests suggest increased unsteady flow effects in the control space compared to all retrofit configurations. This is likely due to the different responses to turbulent diffusion processes and wind pressure fluctuations at the window opening compared to the louvred retrofit design. Zone thermal stratification and diurnal temperature variation within the control and retrofit spaces were measured during each ventilation rate test and also continuously for an extended period. Results show that vertical temperature differences have been substantially reduced following the retrofit works with all δTs values within recommended acceptable limits.The original pilot project works was supported through a grant from the Department of Education and Skills, Ireland

Energy aspects and ventilation of food retail buildings

Worldwide the food system is responsible for 33% of greenhouse gas emissions. It is estimated that by 2050, the total food production should be 70% more than current food production levels. In the UK, food chain is responsible for around 18% of final energy use and 20% of GHG emissions. Estimates indicate that energy savings of the order of 50% are achievable in food chains by appropriate technology changes in food production, processing, packaging, transportation, and consumption. Ventilation and infiltration account for a significant percentage of the energy use in food retail (supermarkets) and catering facilities such as restaurants and drink outlets. In addition, environmental conditions to maintain indoor air quality and comfort for the users with minimum energy use for such buildings are of primary importance for the business owners and designers. In particular, supermarkets and restaurants present design and operational challenges because the heating ventilation and air-conditioning system has some unique and diverse conditions that it must handle. This paper presents current information on energy use in food retail and catering facilities and continues by focusing on the role of ventilation strategies in food retail supermarkets. It presents the results of current studies in the UK where operational low carbon supermarkets are predicted to save 66% of CO2 emissions compared to a base case store. It shows that low energy ventilation strategies ranging from improved envelope air-tightness, natural ventilation components, reduction of specific fan power, ventilative cooling, novel refrigeration systems using CO2 combined with ventilation heat recovery and storage with phase change materials can lead to significant savings with attractive investment return

Cool roof technology in London: An experimental and modelling study

This is the post-print version of the final paper published in Journal of Energy and Buildings. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2011 Elsevier B.V.One of the primary reasons for the application of cool materials is their energy and associated environmental impact on the built environment. Cool materials are usually applied on the roof of buildings to reduce cooling energy demand. The relative benefits of this reduction depend on the construction of the building, external weather conditions and use of the building. This paper examines the impact from the application of a reflective paint on a flat roof in a naturally ventilated office building in the area of London, UK where the climate is moderate with high heating demand by buildings. The environmental conditions (internal/external air and surface temperatures) of the building were monitored before and after the application of the cool roof during the summer. It was found that internal temperatures were reduced after the application of the cool roof. The building was modelled using TRNSYS and the model was calibrated successfully using the measurements. A parametric analysis was carried out by varying the reflectivity and insulation of the roof and ventilation rate; the heating and cooling demand for a year was calculated using the Summer Design Year for London as the weather file. It was found that cooling demand is significantly reduced, heating demand is increased and the total energy savings vary between 1 and 8.5% relative to an albedo of 0.1 for the same conditions. In free floating (naturally ventilated) buildings summer comfort is improved but there is a penalty of increased heating energy during the winter. Thermal comfort can be improved by an average of 2.5 °C (operative temperature difference for a change of 0.5 in albedo) but heating demand could be increased by 10% for a ventilation rate of 2 air changes per hour. The results indicate that in the case of temperate climates the type, operation and thermal characteristics of the building should be considered carefully to determine potential benefits of the application of cool roof technology. For the examined case-study, it was found that a roof reflectivity of 0.6–0.7 is the optimum value to achieve energy savings in a cooled office, improve summer internal thermal conditions in a non-cooled office (albeit with some heating energy penalty). It indicates that it is a suitable strategy for refurbishment of existing offices to improve energy efficiency or internal environmental conditions in the summer and should be considered in the design of new offices together with other passive energy efficient strategies.Intelligent Energy Europe (IEE

Building envelope design for climate change mitigation: a case study of hotels in Greece

This paper presents results of a study of the impact of future climate change scenarios as developed by the Intergovernmental Panel on Climate Change and implemented in weather files for specific future time slices (2020, 2050 and 2080) for the three climatic regions of Greece on the design of the external envelope of a hotel building in Greece. The impact of climate change on the hotel is assessed via hourly simulations of a calibrated model developed using the software TRNSYS. Additionally, the paper aims to identify optimal refurbishment strategies, given the constraints of the existing case-study building when transposed to the three different climatic zones in Greece. Two modes of the hotel building were studied: ‘all year’ and ‘seasonally’ operated. It was found that different external envelope energy-efficient strategies can be applied depending on the climatic zone and whether the hotel is all-year or seasonally operated

London's urban heat island: Impact on current and future energy consumption in office buildings

This article is available open access and shared under a Creative Commons license: (http://creativecommons.org/licenses/by/3.0/). Copyright @ 2011 Elsevier B.V.This paper presents the results of a computational study on the energy consumption and related CO2 emissions for heating and cooling of an office building within the Urban Heat Island of London, currently and in the future. The study developed twenty weather files in an East-West axis through London; the weather files were constructed according to future climate change scenario for 2050 suitable for the UK which have been modified to represent specific locations within the London UHI based on measurements and predictions from a program developed for this purpose (LSSAT). The study simulated an office with typical construction, heat gains and operational patterns with an advanced thermal simulation program (IESVE). The predictions confirm that heating load decreases, cooling load and overheating hours increase as the office location moves from rural to urban sites and from present to future years. It is shown that internal heat gains are an important factor affecting energy performance and that night cooling using natural ventilation will have a beneficial effect at rural and city locations. As overheating will increase in the future, more buildings will use cooling; it is shown that this might lead to a five-fold increase of CO2 emission for city centre offices in London in 2050. The paper presents detailed results of the typical office placed on the East-West axis of the city, arguing the necessity to consider using weather files based on climate projections and urbanheat island for the design of currentbuildings to safeguard their efficiency in the future.EPSR

Non dimensional analysis and characterisation of driving forces for a single sided slot louvre ventilation system

Adopting natural ventilation as a low impact retrofit strategy for space cooling is attractive due to the cooling potential of untreated outdoor air for large periods of the extended cooling season, particularly in northern climates. Furthermore, it is important to characterise the performance of natural ventilation components in successfully transferring the cooling potential of outdoor air to the occupied zone. This paper presents an analysis of the results from 25 individual ventilation rate tests of a single sided slot louvre ventilation system installed in a low energy retrofit application and 13 tests from a pre-retrofit window opening, taken as a control space. Parameters permitting characterisation of different permutations for combined momentum and buoyancy driving forces during each test were also recorded, allowing an investigation of the existence of any underlying patterns as well as the relative effect of the different opening configurations. Analysis shows that different patterns emerge for the dominant driving forces depending on opening configuration in the slot louvre system. Owing to the primary airflow exchange mechanisms normally present, the transient evolution of the normalised tracer gas concentration during tests is analysed using the concentration fluctuation amplitude. The slot louvre ventilation system has led to steadier ventilation rates. Opening height and geometry are shown to have a significant effect on the net contribution from momentum driving forces and the fluctuation amplitude of the ventilation rate and this effect is wind direction dependent. Ventilation rates are shown to correlate well with fluctuation amplitude. The nature of the ventilation rate during tests for different wind directions is shown to vary depending on wind patterns at the building envelope

Quantifying the performance of a top-down natural ventilation windcatcher

Measurements and smoke tests show that the quadrants of a Windcatcher with a positive pressure across them act as supply ducts, while those with a negative pressure across them act as exhaust ducts. However, analysis of the side and leeward Cp values shows that they do not necessarily balance mass flow in and out of the Windcatcher, indicating that either the pressure in the supplied room drops or there is an amount of infiltration through the building fabric initiated by the Windcatcher. In order to better understand Windcatcher performance, a simple analytic model is developed that utilises experimental data to estimate the losses in the system. Two different scenarios are considered for the room adjoining the Windcatcher: (i) this room is perfectly sealed; and (ii) air infiltration is allowed into the room so that the pressure in the room remains atmospheric. Here, it is observed that, for those values of Cp reported for a square Windcatcher in the literature, the overall volume flow rate of air out of the room always exceeds that coming into the room. Based on this data, the analytic model may be used to estimate the losses in the Windcatcher, from which it is then straightforward to derive a simple relationship between the overall area of the Windcatcher and the volume flow rates into and out of the Windcatcher in order to predict Windcatcher performance for a given application

Cool materials in the urban built environment to mitigate heat islands: potential consequences for building ventilation

Urban warming, commonly referred to as the ‘Urban Heat Island’ phenomenon (UHI), is a well-established effect that affects cities all over the world. This occurs due to urban physical characteristics such as urban canyon geometry and vegetation, but mainly to its typical materials. The thermal properties of the materials used for the external walls and roofs of buildings, as well as pavements, can have a major influence on the surface temperature. As a consequence of increased temperature, the UHI has an effect on energy consumption for heating and cooling urban buildings. Cool materials are a cost effective, environmentally friendly and passive technique that uses a coating with high thermal emissivity and solar reflectance properties. At building scale, this technique is recognized for decreasing the amount of heat conducted through the surface and the solar thermal load of the building, reducing its energy requirements for cooling. At urban scale, this strategy contributes to improving the urban microclimate by lowering surface and air temperatures which, in turn, increases the potential for ventilative cooling in the buildings. The goal of this paper is to evaluate the impact of the use of cool materials on the thermal environment of urban spaces and how this can affect ventilative cooling for buildings. The cool materials were evaluated considering the application on roofs and pavements, and the Federal University of Mato Grosso campus, located in Cuiabá, Brazil was used as case study. The study was performed through computer simulations where the 3 scenarios (cool roof, cool pavement and reference scenario) were simulated for the climate of Cuiabá (Aw2 Köppen classification – Tropical wet and dry), considering winter and summer conditions. The methodology consists of three steps: a. preparatory stage (acquisition and compilation of climatic data and physical characteristics of the study area), b. numerical simulation and c. validation of the model and data calibration, for further comparative analysis. As a result, in the scenarios where cool materials were applied, significant differences were found both in the surface temperature and air temperature (height of the pedestrians), up to 7.02°C. The difference was more evident when used as cool pavement than when used as cool roof and this tendency varies in amplitude, considering the locality within the study area and the season analysed. These results allow us to infer that cool materials can increase the potential of the ventilation as strategy for cooling indoor environments, especially by means of stack ventilation which is benefited from greater temperature differences. Thus, it is believed that the wide use of such materials can significantly contribute not only to the mitigation of the heat island effect, but also to reduce overheating risk in buildings by increasing the effectiveness of ventilative cooling and thus reduce the need for air conditioning.The work is supported by the Capes Foundation (BEX: 1462/12-1) and National Council for Scientific and Technological Development (CNPq), Ministry of Education of Brazil

The London Heat Island – surface and air temperature measurements in a park and street gorges

This paper reports results from short-term tests carried out as part of a project to characterize the urban heat island in London. The investigations looked at air temperatures upstream and downstream of a park and the surface and air temperatures within street gorges. It was found that the air in the park was associated with lower mean (0.6°C [1.1°F] less) and peak temperatures (1.1°C [2.0°F] less) compared to residential or shopping streets on either side. The apparent cooling influence of the park extended downstream between 200 and 400 meters (200 to 400 yards). Measurements in four street gorges showed a wide variation in surface temperatures—up to 22°C (40°F)—although 5ºC to 10°C (9ºF to 18°F) was more typical. For a given façade, lighter surfaces were associated with lower temperatures, between 6ºC and 10°C (11ºF and 18°F) cooler. A strong relationship was found between mean gorge surface temperature and the gorge air temperature measured at 6 m (20 ft) (half-gorge height). This was true for both a sunny day and a cloudy day. The results suggest that significant reductions in air temperature may be possible by adjusting the albedo of urban surfaces

Air quality measured in a classroom served by roof mounted natural ventilation windcatchers

This study examines air quality measured in two classrooms in a UK school, which uses two different forms of natural ventilation, over an eight month period. The first classroom is an internal room that contains a top-down natural ventilation system known as a “Windcatcher”. The room also has a separate mechanical extract fan. The second classroom is ventilated using windows and doors that open to the outside. This study focuses on measuring the performance of a Windcatcher and reviews its potential to replace ventilation provided by conventional windows. Potential benefits of Windcatchers include the ability to provide night cooling without posing a security risks, and daytime ventilation without relying upon opening windows. The study will examine Windcatcher performance in terms of air quality delivered in the first room, and then compare results with measurements obtained for a room that uses conventional opening windows. The study will also review the effectiveness of Windcatchers in meeting the regulatory standards for naturally ventilated classrooms, as set out by the UK Government. The air quality measurements reported demonstrate that the classroom utilising a Windcatcher was able to meet the UK Government standards for carbon dioxide and temperature, while the classroom relying solely on windows failed to meet the carbon dioxide requirements. Furthermore, the study demonstrates that Windcatchers provide significant night cooling and increase air exchange rates. Windcatchers do, therefore, have a significant role to play in meeting ventilation requirements in schools