Most people are familiar with radiant cooling / heating systems. In fact, in some parts of the world they are such a common fixture of everyday life that people seldom even notice how essential they are to comfort. However, despite this, the myth that radiant cooling systems cannot be used in hot, humid climates persists.
In this blog, I will outline 3 easy steps to realize radiant cooling in climates that have high latent (humidity) and sensible (heat) loads.
1. Selecting a radiant cooling system
No surprise that the first step is to actually install a radiant cooling system. However, the reason I put it as step 1 is that there are a lot of considerations for the installation of the system if you want to realise radiant cooling in a hot, humid region. Among other things, the cooling load calculations are vital for the radiant cooling system requirements. The cooling load is the rate at which a cooling system or process must remove heat from a conditioned zone to maintain it at a constant dry bulb temperature and humidity. This has to factor in all sources of energy transfer, such as solar gain through windows. The average number of people in the space is also important, as it affects not only the surface area that needs to be cooled, but also the energy input into the system at any given moment from metabolic processes. Calculating and visualising the cooling load can be greatly assisted by technology such as building information modelling (BIM), EnergyPlus, IES Virtual Environment, IDA ICE, Esp-r, and TRNSYS. Other things to factor in are the pros and cons of different types of radiant systems. This includes choosing between high or low thermal mass systems. Within each of these categories there are further choices in terms of materials and construction. I will not go into details here, but the message it that there is a lot to consider when choosing and installing a radiant system.
2. Efficient dehumidification
Once the radiant system is selected and installed, the most important factor in a region with high outdoor relative humidity (high latent load) is efficient dehumidification. Any HVAC system that is providing cooling will have problems with condensation if the dew point is higher than the temperature of surfaces in the space. The first thing to do to achieve efficient humidity regulation is to control leakage in the building envelope. Unregulated opening and closing of doors and windows at all times can make controlling relative humidity difficult. The second, and most vital way of regulating relative humidity is installing a separate system that handles the latent energy load. Radiant cooling systems do not independently control the latent energy load, or the ventilation and indoor air quality (IAQ) requirements of the space. Therefore, a separate system has to be put in place to regulate these extra components. The best mechanism for doing this is coupling the radiant cooling system with a dedicated outdoor air system that efficiently controls ventilation, IAQ, and relative humidity in the space. The radiant cooling system can then be kept at a temperature just above the controlled dew point that optimally manages the thermal comfort of occupants, while never having condensation related issues. For more information see a previous blog on realising efficient dehumidification.
3. Energy recovery
The third step is to couple the radiant cooling system with an energy recovery device such as an enthalpy wheel. In a region that has high sensible and latent energy loads it is especially important to recycle as much energy as possible to lower the performance requirements of the radiant cooling components and the air handling units. An enthalpy wheel will enable recycling of the low latent and sensible loads of the internal spaces. This means that part of the dehumidification and cooling is done passively, enhancing the energetic efficiency of the whole HVAC system. Without an energy recovery device, dealing with the constantly high incurrent sensible and latent energy loads will be very energetically costly. Given the importance of controlling the latent load to enable a low dew point temperature, and thus avoid condensation issues, the installation of at least a desiccant wheel to recover latent energy is a minimum requirement in humid climates. As a side note, a radiant cooling system combined with a DOAS and an enthalpy wheel will be able to recover desired energy from the internal space, while providing 100 % fresh, outdoor air to the space. This avoids IAQ issues with all-air systems that recirculate air. Conclusions Realizing radiant cooling in a region with high latent and sensible loads requires a lot of considerations. However, the benefits are highly worth the effort. Studies over the last two decades have repeatedly shown the benefits of radiant cooling systems, especially when coupled with DOAS. To quote Niu et al. (2001), “In hot, humid climates like Hong Kong, chilled ceiling combined with desiccant cooling is a feasible, energy saving, and pro-human alternative to conventional HVAC systems”.