Cooling alternatives to conventional air conditioning systems

thirdside July 21, 2015

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Radiant cooling has been utilized in commercial cooling settings for more than 20 years. A very active, very large airport — Suvarnabhumi International Airport in Thailand — has been successfully using radiant cooling for two decades. Radiant cooling works in much the same way that radiant heating works. Most people who have basements have experienced radiant cooling without even being aware of it. In older homes without concrete wall insulation, the ground temperatures below grade dictate the Mean Radiant Temperature (MRT) in a basement setting. When a person walks from upstairs to down into the basement, the body’s sensors sense this lower MRT, and it “feels” cooler than the upper floors, even though the air temperature is nearly the same.

A radiant ceiling installation.

The three major environmental factors that dictate human comfort are MRT, which is the average surface temperature of the surfaces surrounding our bodies, relative humidity, and air temperature. Maintaining all three of these in a radiant cooling application is different than what is required when using a conventional air conditioning system. By lowering the mean radiant temperature and controlling the humidity, good human comfort is achievable for cooling at air temperatures that are significantly higher than we typically see with a conventional air conditioning system. This is because RT is one of the primary factors that dictates human comfort. The air temperature and relative humidity can be perfect, but if the MRT is off in one direction or another most people will be uncomfortable. Control of the relative humidity in a radiantly cooled environment is not just a good idea, it is mandatory in order to avoid the production of condensation on the surfaces of the radiant panel.

Residentially speaking, this requires the use of window and door proximity switches, and if any of these openings are left open no radiant cooling is provided. If humidity is controlled and maintained between 30% and 60%, and the chilled water temperatures are kept between 50 and 70 degrees F, then condensation production is not possible under these conditions. The use of a dew point controller is also a minimum requirement whenever radiant cooling is to be employed.

In commercial settings, rarely are windows operable and doors are always on auto closers, so the production of condensation on the radiant sources is not an issue where humidity is being controlled between 30% and 60%.

In certain areas of the country, it is entirely possible to use the building’s irrigation system during the summer months to provide a coolth source for augmenting the cooling of a given structure utilizing radiant cooling. This will obviously require the addition of heat exchangers and pumps, but still represents a free source of cooling. Another potential source that is being exploited is “Night Sky Re-radiation Cooling.” This requires the placement of unglazed solar collectors on the roof of a dwelling to reject built up heat into the night sky. I have personally witnessed collector absorber temperatures that are as much as 20 degrees F. cooler than the lowest ambient temperature seen for the same period of time.

And, of course, there is always ground source heat pump systems, which have a proven track record of efficiency and reliability. There are some new variations of this age old system bubbling to the surface, and they are showing a huge potential as it pertains to radiant heating and cooling applications. (Thermal Battery Storage systems, for instance.) It is also entirely possible to utilize the ground source vertical bore fields as a cooling source under the right conditions without the need to fire a compressor. Remember, the typical approach temperatures for a radiant panel vary between 50 and 70 degrees F.

Having too much or too little much humidity will also significantly sway the human comfort factors, regardless of what is happening with the MRT and air temperature. In areas of high humidity, it is necessary to maintain the relative humidity between 30% and 60%. This not only provides better human comfort, but also has to do with the control of microbial bacteria, dust mites and preservation of fine wood products. In humid climates, it will be necessary to dry the air out. For make up air units, a method known as DOAS (Dedicated Outdoor Air Systems) is used to wring the humidity out of the incoming air before introducing it into the controlled environment. Internally generated humidity is controlled and removed through the use of refrigerant-based direct expansion dehumidification systems, where the room air is first run across an evaporator coil to remove excess humidity and then the condenser reheats the stream of air. This avoids overcooling the air and creating “cold 70” conditions.

Conversely, in areas with low humidity it may be necessary to introduce humidity into the controlled environment. This can be done with the use of direct evaporative cooling, or direct humidity injection systems. There are even some newer classifications of indirect evaporative cooling systems known as Maisotsenko M-Cycle cooling systems. This type of system uses an indirect heat exchanger to avoid the possible generation of humidity into the conditioned air stream, which in some cases can cause discomfort or mugginess.

In conventionally “air conditioned” systems, there is the need to not only cool down the air, but also to control the relative humidity within the controlled space. At times, the demand for air dehumidification causes the air conditioning system to over cool the air, resulting in more complaints about the air being excessively cold.

In a radiantly cooled controlled environment, the majority of the sensible cooling demand is handled by the control of the MRT surfaces. By allowing the radiant cooling system to handle the majority of the sensible demand, the amount of air movement required to handle the latent (humidity control) demand is significantly smaller. This can result in a significant decrease in cooling energy, resulting in energy savings of 50% or more depending upon the application.

It also opens up more “off peak” ice generation/storage options because the approach temperatures of the water used in radiant cooling systems typically vary between 50 and 70 degrees F. By shifting the electrical and mechanical energy consumption to “off peak” periods, additional monetary savings can be achieved.

Another area that is receiving major attention is the use and application of Phase Change Materials. This method is incorporated into the structure of the building, and in some cases uses free nighttime cooling or off peak electricity to “freeze” the phase change materials. As the materials absorb heat energy, they go through the reverse phase change and “melt,” absorbing significant amounts of thermal energy. They can reduce peak loading by as much as 30% in a hot climate. Although they do not and cannot address the latent energy loads, they do a fantastic job of addressing the sensible loads, which are the biggest load in most commercial buildings. These compounds can be custom tailored to operate at any temperature that is normally used to maintain comfort in a given structure.

Mark Eatherton is the executive director for the Radiant Professionals Alliance (RPA).