Radiant Cooling – Part 2
One of the most common questions asked about radiant cooling is “How do you deal with humidity in the air and the potential for condensation on the slab?” This probably leads to the most angst and uncertainty with engineers thinking about water damage, mould and safety issues. While this is an extremely critical design issue, it is one that is easily overcome with good design and controls.
A radiant cooling system will not deal with any latent heat removal (moisture in the air), so this must be handled by incorporating a separate air handling system to remove the latent moisture load in the space. The air handling system will usually also be required to provide ventilation and air quality control. Careful control of the humidity level must be maintained at all times in a radiant cooling system, to ensure that the dew point of the air does not get lower than the surface temperature of the slab or panel.
Controls will most often include multiple sensors embedded in the slab and air humidity sensors in the air space. Condensation issues can be avoided completely by constantly monitoring and controlling both humidity levels and slab temperatures. Important design criteria to follow include keeping floor surface temperatures no less than 66F, and a having a room set point temperatures in the range of 76F to 78F. The cooling capacity of the radiant cooling system is limited to about 12-15 btu/h/ft2. The air handling system must handle the balance of the cooling load. When radiant cooling is used in areas where solar gain will directly contact the floor, then the cooling capacity can increase substantially up to 25-32 Btu/h/ft2. This makes radiant cooling in the floor very effective in buildings with large glass facades. Because the radiant cooling slab will handle the sensible load, the size of the air handling system is reduced substantially compared to a traditional cooling system. Smaller fans, ductwork and distribution systems results in significant installation savings, as well as reduced electricaloperating costs. Smaller plenums can sometimes result in savings in building height, which makes architects happy.
Energy use comparison; conventional versus radiant (courtesy Lawrence Berkeley National Laboratory).
One of the best advantages that radiant cooling systems offer is the potential to lower energy consumption compared to conventional cooling systems. Research conducted by the Lawrence Berkeley National Laboratory in California, has shown significant energy savings potential for radiant cooling, with the amount depending on the climatic area. Their research has shown on average across the USA, savings are in the range of 30 per cent compared to conventional systems. Cool, humid regions might have savings of 17 per cent while hot, arid regions have savings of 42 per cent.
There are several reasons for reduced energy consumption, with much having to do with the benefits of hydronics in general, as well as the unique nature of radiant heat transfer. It is a much easier task to pump water than to blow air; to move the same amount of Btu’s the electrical consumption of a pump is much less than from a fan. Also the air handling component in a radiant cooling system is much smaller than conventional cooling systems, resulting in lower electrical energy consumption.
That large cool surface of a radiant cooling system provides a heat sink to draw heat away from our bodies. With the majority of heat being radiated away, there is a reduction in the amount that must be moved by convection. This allows people to feel comfortable at a higher air temperature set point. This, of course, converts into energy savings as well as better overall comfort. Another possibility for energy savings exists in high mass systems, where radiant cooling can shift some cooling to off-peak nighttime hours when electricity is cheaper.
Typically a radiant heating and cooling system will incorporate a condensing boiler for heating mode, and a chiller for cooling mode. A water-to-water geothermal heat pump with a reversing valve offers a natural synergy with radiant heating and cooling, as this device can provide both heating water in the winter and chilled water in the summer. Geothermal systems operate most efficiently when providing low heating water temperatures and moderate chilled water temperatures. This is precisely the range of operation that will be needed in a high mass radiant heating and cooling system, so the geothermal unit will operate at close to its maximum COP almost all year long.
Occupant comfort is another great benefit of radiant cooling systems. The large cool surface provides a heat sink to draw heat away from our bodies with a gentle cooling action creating a very comfortable, stable indoor climate. The volume of forced air flow is greatly reduced compared to convention air cooling systems. This reduces the potential for cold draughts, and also reduces dust and other allergens moving around. Radiant cooling systems operate very quietly, with a greatly reduces amount of noise from fans or blowers.
Can radiant cooling be used for residential homes? According to Leyte, “it is possible to do, but there are certainly more challenges and a higher level of risk. With the possibility that doors and windows can be left open, the control of humidity can become very difficult.”
Another limitation for residential homes is that the use of carpet and pad would not be recommended for radiant cooling. For these reasons there is a very limited market in Canada for residential systems, coming primarily from “creative customers that are up for the challenge” according to Leyte. There are pockets, such as the southwest U.S., where the hot, dry climate offers the greatest advantage for radiant cooling with a large portion of cooling being the removing sensible heat, not latent. In these areas residential radiant cooling can work very effectively.
So while the market for residential radiant cooling may not develop in Canada, the commercial market does present some great promise. With building codes pushing to lower energy consumption in buildings, and a growing trend towards new green building technologies, radiant heating and cooling systems should continue to grow in popularity for many buildings. With benefits like high efficiency, high comfort levels, quiet and healthy operation, and building integration, there is a lot to like about radiant heating and cooling systems.
Original Article: HPAC Magazine: https://www.hpacmag.com/features/radiant-cooling/
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