C. U. Okujagu 1* and C. E. Okeke 2

department of Physics University of Port Harcourt, P. M.B 5323 Port Harcourt Nigeria.
department of Physics and Astronomy University of Nigeria Nsukka.

* Corresponding Author: info@okuiagu. com

Abstract

Overheating of the interior of building with highly glazed faqade can cause discomfort to persons living and working in such buildings in tropical climate. Solar and thermal control in buildings can be achieved either by conventional air conditioning or by non-conventional architectural and ventilation methods or by innovative switchable or selective glazing systems. Switchable and selective single layer thin films can offer a technically one-step option for the reduction of indoor temperature and maintenance of an acceptable luminous (daylighting) level within the building thereby creating both thermal and visual comfort within the building. Photo-optical and selectivity properties of Iron (Fe), Tin (Sn) and Manganese (Mn) Halide films show that these films are highly reflecting in the Near infrared (NIR), transmuting in the visible (VIS) while absorbing in the near ultraviolet (NUV). Hence the films act as optical shutters to both UV and IR radiations which could raise the indoor temperature of the buildings, while it is highly transmutting in the visible region and therefore can maintain acceptable level of daylight (luminosity) within the building. This will create comfortable indoor environment and could serve as Natural Air — Condition coating for buildings and cars.

Introduction

Solar radiation that is transmitted into buildings with highly glazed facade can lead to overheating of buildings interiors causing great discomfort to persons living and working in such buildings, especially in tropical (hot and humid) climate and an increase in energy demand for heating/cooling of such buildings [1-2]. Under such condition, the issues of Solar — thermal control for indoor thermal/visual comfort and energy efficiency for the building becomes very crucial [3-5]. Many researchers have suggested and developed various methods for creating comfort and /or reducing energy cost in such environment.

These solutions include the use of;

Conventional air conditioning and water cooling methods to reduce indoor thermal level. This method has a disadvantage of increasing energy consumption and hence cost.

Non-conventional approaches which incorporate heat reduction techniques into the architectural design and orientation of buildings (Solar Buildings), thus allowing the glazing to admit far less solar heat and glare into the building [5-10]. This may lead to complicated and sophisticated architectural/constructional technicalities and increased cost as well.

Various shading and ventilation devices such as blinds and louvers which may be fixed or motorized, natural or mechanical and passive or active/dynamic; all aimed at improving indoor thermal and visual comfort and contributing to the energy saving schemes[5-12]. Such

technologies will definitely lead to increased cost. In addition, the system may suffer from wear due to extreme metrological conditions and can also lead to overheating when they are poorly installed.

Double skinned facades supply air windows and solar chimneys of various designs to circulate air or to create an insulating air layer that is aimed at improving ventilation, solar shading and daylighting levels in buildings [12-30]. This will automatically raise the cost of achieving thermal comfort in the buildings.

Hybrid systems which combine a number of architectural designs and shading devices to create conducive indoor environment [17-31]. This approach is very cumbersome and may be complicated and expensive.

Although these methods have recorded various degrees of successes in reducing indoor temperature to appreciable limits, some of them actually cause reduction in daylighting levels in the buildings there by introducing another dimension into the problem of visual comfort. To redress this, effort has to be made to improve the daylighting level, thus leading to complicated architectural designs, sophisticated daylighting technologies and hybridization schemes that will definitely increase cost in most cases [32-41]. This means that conventional high performance glazing cannot fulfill the entire requirement concerning energy saving and improved indoor comfort. Therefore innovative solutions for glazing systems and transparent facades have to be developed in order to achieve the objective of thermal/visual comfort as well as energy efficiency and cost reduction. Up till the present, the main approaches that have been suggested, developed and manufactured in commercial quantities are: [7, 43-47].

Sun Protection Glasses (SPG’s).

Single and Multilayer Inorganic/Metellic Coatings.

Organically coated glasses and Plastics.

These glazing system and coatings act as either solar thermal control devices with static/fixed properties which are incapable of adjusting their properties according to the variable demands on heating, cooling and lighting load in a building apartment. For example, a low emittance (thermal control) coating is adequate during winter in reducing thermal losses, but will cause overheating in summer. On the other hand, solar control coating may be adequate for offering cooling comfort in summer, but may cause excessive heat loss in winter. In addition to this delimitation, some of these system may cut of vital wavelength for better visual comfort, hence effort has to be made to incorporate day lighting equipment (light guides) into the building which will increase the cost for the building [40-42].

Another highly innovative glazing approach is the use of chromic/tropic coatings with dynamic properties to achieve switchable/adjustable characteristics within a single glazing system [48-58]. These glazing systems are the so called “smart windows” which have gained popularity within the last few decades but which have not really hit the market stand in large commercial quantities [7]. Nevertheless, these innovative glasses seem to hold the future for transparent facades because they are not produced as external attachments to the glazing and facades systems but are integrated with the glazing itself as external coating on the glass, to achieve variable transmittance without the use of any control devices. Thus the glazing change their characteristic color (chromic) or transmission/reflection/scattering (tropic) properties in response to a critical temperature or luminosity transition values.

The different types of switchable coatings are:

Chromogenic glasses; [49-54, 56-58]. These change their color (from light to dark or vise versa) at predetermined critical transition temperature (thermochromic) or luminosity (photochromic) or electronic/ionic transition (electrochromic).

Tropogenic Glasses; [48, 58] These change their optical

(transmission/reflection)(scattering) properties (from highly transmutting to highly reflecting or from highly absorbing to highly transmutting) at certain predetermined critical transition temperature (thermotropic) or luminousity (phototropic) or electron transition (electrotropic). The production of smart windows with appropriate chromic (color) and tropic (transmission) characteristic require some sophisticated deposition or coating techniques which are expensive. Hence the product in the market so far has not reached optimum performance due to soaring production coast [7, 43]. As the search for these and other products for glazing and facade applications intensify, it is also pertinent to look at other products (coatings) which can be used to achieve reduction of indoor temperature and create comfortable daylighting level as well a cost reduction on heating/cooling and which can be produced with simple (complicated) technology.

This work is therefore a contribution to the search for such coatings and glazing systems and materials.