عملکرد سطوح خنک کننده درخشنده با توجه به مصرف انرژی و آسایش حرارتی

The intensive use of electrically driven vapour compressor air conditioning is accompanied by an analogous environmental impact and the imposition of a severe stress on the electric grid. Still, the decarbonisation of the latter, by the increased utilisation of renewable energy sources, and the implementation of smart grids could help in overcoming those issues, if a different approach in thermal energy storage and thermal comfort in the cooling of buildings can be achieved. Hydronic radiant building surfaces address those issues providing thermal comfort by cooling directly the building and the people, utilising high temperature cooling water, that increases the efficiency of cooling systems, while they present the feature of thermal energy storage embedded in their construction. The present study compares radiant and convection systems with respect to final energy consumption and thermal comfort in a test cell representing an office room. The results highlight the issues of proper control of radiant systems in order to take full advantage of their specific features and of the appropriate evaluation of thermal comfort conditions provided by those systems.

The quest for coolness in warm climates occupied people for thousands of years. Several ancient architectural practices (high thermal mass walls, high room ceilings, shading and natural cooling techniques) have been employed to adjust buildings to local climatic conditions, while several cultural practices developed to help people adjust (e.g. clothing). The natural, or passive, cooling techniques have therefore a centuries’ long record of being used. Mechanical cooling emerged only during the 19th century, but boosted by the electrification of cities it expanded to the current point where today it is considered to be the standard way of cooling. That wide availability and intense operation of electricity driven vapour compression cooling systems resulted in a twofold effect. It led people and buildings to loose or abandon their acclimatisation features and produced a significant environmental impact and a huge stress on electricity grids, especially during hot periods. However, if electricity generation is about to be proliferated by renewable energy sources’ (RES) contribution and transmitted and distributed through smart grids, a big challenge emerges: to successfully implement demand responsive schemes, that will force building operators to run their electricity driven cooling equipment when RES-generated electricity is available, in order to minimise the environmental impact and relieve the grid from peak mismatches between supply and demand. For this to be realised two issues should be resolved: how to provide uninterrupted cooling, when the availability of the respective equipment is intermittent following the availability of RES, and how to provide thermal comfort in an indoor environment where occupants do not have full control over time of their conditioning equipment.

Radiant building surfaces are suitable for buildings which feature a very good thermal insulation of their envelope and an adequate thermal mass. Their operation should be regulated by controlling the operative temperature and not the air temperature of indoor spaces. In moderate climates, or for specific types of buildings where cooling loads are generated mainly by solar or internal heat loads and not so much by very high air temperatures, radiant cooling can augment ventilation cooling, since it can provide comfort with higher indoor air temperatures. Under these prerequisites, radiant systems can be very effective cooling terminal units, utilising fairly high temperature cooling media and thus increasing the efficiency of the cooling plant's equipment. On the other hand, one cannot but notice that their limited capacity, together with their inability to handle latent cooling loads, does not allow them to ensure satisfactory thermal comfort conditions, at least according to deterministic comfort evaluation standards. Still, as radiant surfaces can be easily combined with fan coils, covering both latent and sensible cooling loads is possible, whilst ensuring excellent comfort conditions and low energy consumption. Furthermore, fan coils can also augment radiant surfaces during heating, since the start-up of radiant floors and roofs is relatively slow. Finally, and from a methodological point of view, when using radiant surfaces exclusively, and in particular radiant floors, adaptive models for naturally ventilated buildings are most suitable to evaluate the thermal comfort conditions, as the radiant systems are hardly noticed by the occupants.