"Nightsky" System Cools Roof Tops, Saves Energy

"Nightsky" System Cools Roof Tops, Saves Energy

The All Weather Architectural Aluminum Window Co. in Vacaville, CA, uses an innovative night spray roof storage cooling system to cool its 585 m² office building. The system, called NightSky and manufactured by Integrated Comfort, Inc., provides chilled water to in-slab tubing and to fan coils. Water sprayed onto the office roof at night is chilled by the night air to approximately 10-15ºC (50-60Fº). This cooled water is drained to a storage tank and filtered. Whenever the water is warmer than a time-dependent temperature target, a chiller operates to reach the temperature target, and tank water circulates through tubing in the slab floor before entering the chiller. In midsummer months, the slab is cooled throughout the night roof spray cycle. Passive cooling delivery from the slab cools the building in parallel with the "active" chilled water fan coils. During occupancy hours, a ventilation fan coil delivers tempered outdoor air into the return air plenums of other fan coils, reducing blower energy use.

Energy-saving benefits offered by the system include increased efficiency due to night-roof spray cooling, efficient chiller cooling due to operation in cooler outdoor conditions, reduced blower heat output and energy use, and elimination of unnecessary latent cooling. These factors plus off-peak chiller operation substantially reduce peak electricity demand.

According to a study performed by the Davis Energy Group, Inc., the system saves more than 25,000 kWh annually and has a simple payback of 2.5 years. This system is most effective in arid climates with clear night skies.

Technical data

The NightSky roof spray system cools water below outdoor wet bulb temperatures. The 10-ton chiller installed in the building does not operate during the noon-to-6 p.m. hours and operates fewer than 350 hours per year. The slab floor cooled by water chilled on the roof typically remains 2-4ºF (-17 to -15ºC) below air temperature.

Energy data:

Table 1 compares cooling delivery and efficiency for the chiller and the night roof spray system for a week during which NightSky delivered two-thirds of the cooling. For many milder summer weeks [the average July 23-29 maximum outdoor temperature was 34.4ºC (94ºF)], the roof spray delivered all required cooling.

Table 1. Comparative cooling performance (July 23-29, 1998)
Cooling MJ (MBtu) Cooling % kWh EER system
Chiller	2848.5 (2.7)	33	252	10.7
NightSky 5697 (5.4)	67	92	58.7

Full-year savings simulations were based on monitoring data for the completed project. Base case duct inefficiencies were estimated at 15% to reflect long “base case” duct runs through unconditioned space. Actual duct losses were estimated to be 2%, since the actual ducts are located almost entirely in conditioned space, plus 3% losses for the storage tank and piping. Latent cooling was estimated to be 15% for the base case and zero for the actual system, which has no condensate drains and has never shown condensate in the pans. Table 2 shows electrical savings for a typical year. Data verify 87% summer peak demand reduction compared with the base case.

Table 2. Annual electric savings
Base case NightSky Savings Savings(%)
Annual energy
Cooling kWh*	19,587	5,250	14,337 73
Delivery kWh	17,074	5,733	11,341 66
Total kWh	36,661	10,983	25,678 70
Peak demand kW @ 40ºC (104ºF)	33.7	4.4	29.3	87
*Includes spray and chiller pumps.

The system also saves heating energy by eliminating duct and roof top unit cabinet losses, reducing thermal stratification, and reducing burner cycling. Countering the duct savings are piping losses and added losses from the heated slab. Simulation results for the base case show an annual heating energy use of 92,840 MJ (88 MBtu). Reducing losses, stratification, and cycling lowers the installed system gas consumption to 58,025 MJ (55 MBtu), but requires 190 kWh of pump operation (charged as “delivery kWh” in Table 2). Table 3 summarises the system's annual cost impacts.

Table 3. Annual cost impacts (in USD)
Cost category Base case NightSky Savings
Electricity
Energy 2,593 656 1,937
Demand 1,965 676 1,289
Cost category Base case NightSky Savings
Total	4,558	1,332 3,226
Natural gas	504	315 189
Water	—	20 -20
Totals	5,062 1,667 3,395
Annual cost savings — 67%

With the system, annual electricity consumption is about 10,900 kWh annually.

Economic data

The incremental cost of installing the NightSky system the facility was approximately USD 8,500 . Baseline cost for a conventional cooling system is around USD 43,400. Installing NightSky components increases that cost to around USD 51,900. Annual savings are about USD 3,390, creating a simple payback of about 2.5 years.

Contacts:

All Weather Architectural Aluminum

777 Aldridge Road

Vacaville, CA 95688

707-452-1600

707-452-1616 (fax)

http://www.allweathersweb.com

Davis Energy Group

123 C Street

Davis, CA 95616

916-753-1100

916-753-4125 (fax)

http://www.davisenergy.com

Night Sky Radiant Cooling System

WhiteCap™ Roof Spray Cooling

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