Night sky radiant cooling (NSRC): 
What you need to know

Night sky radiant cooling (NSRC) has been something of a lost art, even though it has been documented and discussed by HVAC and solar heating professionals for many decades. For example, the 1993 ASHRAE handbook of fundamentals mentions it (“nocturnal radiative cooling”) and provides a map of the U.S. showing cooling potential around the country under typical comfort-cooling conditions. 

The New Standard solar hydronic combisystems we have been discussing in this column provide an ideal application for this technology to "plug in" and perform. Using this cooling resource can be as simple as turning on a circulator at night and off in the morning. Because it is so easy, I have been including the capability for NSRC cooling into virtually every flat-plate solar heating system I have designed in recent years. And, NSRC cooling control capability is built into every heating control system that we make here at SolarLogic. Its seasonal operation can be turned on or off by the user and is part of our standard room thermostat function.

Because this technology has been largely overlooked in the past, it has not been widely deployed, and its advantages and limitations are not always well understood. So, here is a list of six things that you should know about NSRC before you decide to include it into your next solar hydronic combisystem installation.

1. Flat-p late solar panels can be used for cooling. This fact has been widely known by solar heating designers dating back at least to the 1950s. In fact, this is probably the reason that the solar differential thermostat (dT) was invented in the first place, to control solar heat collectors. The most basic function of a dT thermostat is to prevent the solar circulator from running at night when a heat-storage water tank is typically warmer than the solar collector. If the water tank is warm and the collector is cool, heat can be rapidly removed from the tank and lost to the great outdoors simply by running the solar pump. At night, heat loss occurs mostly by thermal radiation from the face of the collector to the sky. The more intelligent dT controllers today use this condition as a cooling function to automatically cool the tank when the system is in danger of overheating.  

Generally any flat-plate panel employed as a solar heat collector, glazed or unglazed, may also be used to dissipate heat at night by thermal radiation. However, this is not true for vacuum tube collectors that use heat pipe technology inside the glass tubes. These cannot be used for cooling since the heat pipe is unable to transfer heat “in reverse.”

Glazed solar collectors, seen in Figure 48-1, tend to produce twice as much heat by day as unglazed panels of the same size and can produce reasonably high temperatures in colder climates.  

Unglazed panels, seen in Figure 70-3, tend to produce twice as much cooling as glazed panels but do not produce high temperatures and may not produce any useful heat in very cold weather when used as heat collectors. In the installation seen in Figure 70-3, we have used one bank of glazed panels for heat (not shown), and another bank of unglazed horizontal panels for cooling only in summer.

2. NSRC is not complicated or "high-tech.People often assume that NSRC is some other kind of “solar cooling” that requires PV panels, compressors, heat engines, high temperature concentrators or refrigeration fluids like Freon or Ammonia. Nothing could be further from the truth. The process is deceptively simple.

Warm hydronic fluid from the building is pumped through flat-plate solar panels at night and is cooled by radiant heat loss to the night sky. This cool fluid is circulated through the building to absorb heat and then back to the panels to be cooled. It is a very subtle means of cooling, but over the ten hours or so available overnight, a large amount of cooling can be accomplished. It is very effective for night time chilling of masonry floors that have radiant heat tubing embedded in them. It can also be used to chill water tanks for later use during the heat of the following day.

3. It makes better use of components you may already have. In our solar combisystems, we typically already have flatplate solar panels and masonry warm floors installed for space heating. But the solar panels are also capable of rejecting heat by thermal radiation to the night sky, and the masonry floors have a significant thermal storage capacity that can be chilled as well as heated. For only the cost of running the solar circulator pumps at night, we can use the components we already have to provide supplemental cooling. NSRC provides an intermittent cooling source in the same way that solar heat is an intermittent heating source. So, the objective is similar; store it when it’s available to use later when you need it. It makes sense to use what you already have any time there are real benefits. This has proven to be an added selling point when someone is considering the pros and cons of installing a solar combisystem.

4. It’s less like an air conditioner, more like an economizer cycle. An economizer cycle is often built into the controls of conventional air-conditioning systems. This allows cool outdoor air to be circulated directly through the air ducts at night, or whenever the outdoor air is cool enough to allow the conventional air-conditioner equipment to be shut off. Also, some building control systems allow cool night ventilation to "pre-cool" the mass of the building to prevent or at least delay the need for the conventional air conditioner during the following day.

Using NSRC, you can do with hydronics what is commonly done with cool night air described above. Radiantly cooled hydronic fluid is circulated through the concrete masonry radiant floors at night absorbing heat from the mass of the floor and storing “coolth” for the next day. Using the natural time delay inherent in a mass floor, NSRC offsets the need for conventional cooling, even though it may not always eliminate it.

Cooling the concrete mass causes the surface of the floor to become cooler than the other surfaces in the room. This causes the mean radiant temperature (MRT) of the room to drop. So, even though we are not chilling the room air directly, the occupant senses a cooling effect caused by the lower MRT. Controls must be provided to keep the temperatures in each room within the human comfort range, prevent conflict between heating and cooling jobs and also,  in more humid locations, to prevent dew-point condensation from occurring.

5. Radiant cooling can be utilized in non-desert locations. It is often (wrongly) assumed that NSRC is similar to evaporative cooling and will work only in a desert climate with clear skies. It is true that humidity and clouds tend to reduce the radiant cooling capability, but will not stop it completely in many U.S. climate locations. NSRC comfort cooling does not work when night conditions are both very hot and with high humidity (like Miami in summer) but does provide comfort cooling under a surprisingly wide range of climate conditions. For example, estimates shown in a previous column show that during the hottest time of the year, the NSRC cooling available in Los Angeles is about twice that of New York, and about 20 percent less than that of Denver. So, to gain the same amount of cooling in each climate you would need to double the size of the panel in New York and use a slightly smaller panel in Denver. This is not so different from how solar panels are sized in those climates for heating.

Keep in mind that we have been talking so far only about comfort cooling. NSRC panels can also be used to cool process fluid, electronic equipment and other non-human applications. When the need for cooling is at a higher temperature or can allow higher temperature swings, NSRC becomes even more effective and useful throughout more of the year.

6. There are real efficiency advantages. The electric efficiency of a NSRC system is very high. The coefficient of performance (COP) of these systems is at least twice that of a conventional air conditioner. The COP in cooling mode for the glazed system seen in Figure 48-1, for example, was estimated at 5.56 on a hot day in June and can be twice as high on hot days in the spring or fall. When these systems are carefully designed, the COP can be even higher.
Data from the unglazed system seen in Figure 70-3 shows, for example, the peak NSRC cooling recorded a week before the seasonal changeover was 160,000 BTUs overnight. The electrical data recorded that night shows that the cooling system consumed about 3.6 kWh to run the circulator pumps. That translates to an equivalentCOP of about 12.9 overnight. For every unit of energy consumed by the pumps, 12.9 units of useful cooling energy were delivered to the building.

In arid climates, there is the added advantage of a much needed water savings when compared to evaporative coolers, and also, water savings at the electric power plant. As an added bonus, when NSRC cooling prevents any conventional electrical or combustion fuel equipment from running, there is real savings in carbon emissions.

Conclusion

NSRC cooling can be accomplished using glazed flat-plate solar heat panels or (even better) using unglazed flat panels (often used to heat swimming pools). In many recent installations, we have included control settings that allow the warm floors to be cooled at night in summer by running the solar collectors backwards at night. Similar control functions can be programmed to dissipate heat at night from overheating water tanks when the stored heat is not being consumed. This cooling system operates at a very high COP. 

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed mechanical contractor in New Mexico. He is the chief technical officer for SolarLogic LLC in Santa Fe, New Mexico, where he is involved in development of solar heating control systems and design tools for solar heating professionals Visit www.solarlogicllc.com for more information.

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