Between June and August 2022, the United States experienced its third hottest summer on record, causing significant strain on air conditioners and increasing energy usage on utility grids to the point of failure. Additionally, during this time, many unairconditioned buildings became unsafe to work in. SkyCool Systems’ passive radiative cooling panels reject heat to the sky, with no input electricity, 365 days per year. When used with cooling systems that run year-round, the panels have the potential to save more than two-times the energy that could be generated with a solar panel for the same area. 

The primary objectives of this project were to: 

• Demonstrate the energy efficiency and cooling capacity improvements for existing cooling systems located at Department of Defense facilities.
• Demonstrate the seamless integration of SkyCool panels with existing heating, ventilation, and air conditioning equipment.
• Evaluate the value of SkyCool’s passive radiative cooling panels in different integration modes, while in a humid climate.

Radiative sky cooling occurs naturally because Earth’s atmosphere is partially transparent to infrared thermal radiation (the light wavelengths associated with heat). As a result, at night sky-facing surfaces emit more energy as thermal radiation to the sky than they receive from it. Radiative sky cooling is not a new concept. Its first recorded use was by ancient Persian civilizations, and they used it to make ice at night in the desert. Prior to this work, this effect was not observed during the day because the sun heats up all outdoor, sky-facing surfaces. However, this effect happens all the time and is most prominent on clear sky days and even occurs when clouds are present. 

SkyCool Systems has developed a rooftop cooling panel, which uses radiative cooling to improve the efficiency of air conditioning and refrigeration. The panels cool without evaporating water and only require the electricity to run a small circulating water pump. The radiative cooling effect from the panels occurs all day and is very well aligned with the 24/7 operation of refrigeration systems in supermarkets and cold storage facilities, and air conditioning systems in data centers. In typical operation, water-glycol is circulated through the panels in a closed loop, and the water-glycol is used to indirectly cool refrigerant after the condenser.

In the work, SkyCool’s panels were deployed at two sites at Fort Moore. At the first site, 10 panels were integrated with a dining facilities administration center (DFAC) refrigeration system. At the second site, 20 panels were connected with an air conditioning system in a data closet. For the DFAC system, the panels reduced the power demand by the refrigeration system by 15% at 90°F ambient and 22% over the entire year; and for the data closet, the panels reduced the net power demand by 20% reduction at 90°F and 51% reduction over the entire year, relative to the baseline cooling systems. The team estimates the energy saved over the year by these systems to be 22% and 42% respectively. While the percent savings are relatively high, the amount of energy saved per day for each system is small since both systems that were modified served small cooling loads (the DFAC cooling system is rated at 3.5 ton and the data closet cooling system 1.25 tons). For the DFAC system, the panels save about 14 kWh per day and the data closet system saves 8 kWh per day.

Due to the fact that the cooling systems were relatively small, and the install costs were high due to several issues, the payback in both sites is > 20 years. In general, these pilot projects show that in order for the deployment to have a sub-five-year payback, the systems need to have cooling loads of greater than 50 tons and need to run at least 80% of the time. The high runtimes will be true for datacenters, centralized cooling systems with high runtime, and commissary refrigeration systems and less likely to be applicable to distributed air conditioning systems in offices or dormitories. 

While the energy savings measured at both sites met or exceeded expectations, there were several challenges which were faced during the implementation of the cooling panels at these two sites. These include: 

• COVID related supply chain issues procuring relatively common materials like plumbing components and increasing the cost of shipping containers. 
• COVID related labor issues resulted in the need to work with contractors that were remote from this site. 
• Installation started during Thanksgiving and continued through the winter holidays leading to lots of travel to and from the site. 
• Two smaller sites that required one central location to store materials and it took time to transport materials from the storage site to the buildings. 
• Wrong rooftop drawings led to a pause in the installation.