Consider evaporative roof cooling to reduce your HVAC load
Cooling the energy crisis, from the top
To reduce the load on air conditioning systems, a growing number of companies are cooling the building from the top down using water
By Kyle Flora, The CoolRoof Co., Paducah, KY
|A large portion of a buildings summer heat load comes from the roof|
|Roof cooling can have a very short payback period|
|Roof cooling extends the life of a roof.|
The amount of air-conditioning required in a large facility (manufacturing, warehouses, etc.) is basically determined by people, equipment, windows, walls, lighting, square footage, and the roof.
These variables differ per application; however, there is one variable that consistently represents 25[en]40% of the buildings total heat load: the roof. Most industrial rooftops are horizontal. Because of this, the roof receives high amounts of the sun’s energy during summer days, often reaching temperatures of more than 165 F. This makes the roof serve as a heat exchanger for the entire facility.
Heat gain through a roof
The formula for heat gain through a roof is:
|q = U R A(CLTD)
q = heat gain through roof, Btu/hr
U R = conduction transfer function coefficient, Btu/hr-sq. ft- F
A = surface area of roof, sq. ft
CLTD = Cooling Load Temperature Difference , D T, F
A 100,000 sq. ft manufacturing facility has a metal roof with 2-in. of fiberglass insulation, 325 tons of mechanical cooling, and an inside design temperature of 78 F. The fiberglass has a U-value of 0.14. Using ASHRAE criteria, CLTD is determined to be 71.4 F.
q = (0.14)(100,000)(71.4) = 999,600 Btu/hr heat load.
Dividing this by 12,000 Btu/hr (one ton of air-conditioning) gives a dry solar roof heat load of 83.3 tons. That is over 25% of the building’s total heat load.
It is understood that a roof gets very hot in the summer and creates a tremendous amount of heat gain in a building, but this problem is too often accepted as a fact of life. There is a solution.
To reduce the load on air conditioning systems, a growing number of companies are cooling the building from the top down using water. Evaporation is a natural way of cooling.
When one gallon of water evaporates, it absorbs about 8,000 Btu’s. Evaporative roof cooling is based on this fundamental principle.
Roof cooling began over 60 years ago. Ponding, rather than evaporation, was the original concept. Although it had some drawbacks, the overall results were very satisfactory for this cooling concept. Ponding was good, but would not work on all roofs, and had too many negative effects on the roof. Engineers worked to change the concept of roof cooling from a ponding to an evaporative technique.
How roof cooling works
An evaporative roof cooling system is designed to reduce the temperature of a roof from 165 F to about 90 F. This cooling reverses the heat flow through the roof. Heat is now transferred out of the building through the roof. This effect creates an effect equivalent of R-7 roof insulation during the day. Rather than absorbing heat, as does insulation, the evaporation of water carries heat away from the building.
This evaporation is achieved by periodically misting a small amount of water onto the surface of the roof. Sensors, on the surface of the roof, continuously monitor the roof temperature. Once water is misted onto the roof’s surface, the system pauses, allowing the water to evaporate, reducing the temperature of the roof.
The system will not mist again until the temperature increases to a point in which more water is required for cooling. This precise control eliminates excessive water runoff while achieving maximum cooling. The no-clog/no-drip system is composed of few moving parts and requires little maintenance. UVRPVC water pipe rests in specially designed pipe supports that do not penetrate the roof membrane. Water is dispersed evenly across the roof using spray nozzles.
Take another look at the example above where all values remain the same except for D T. Reduce it to 5 F.
q = (0.14)(100,000)(5) = 70, 000 Btu/hr
That is a 93% saving. Roof cooling has reduced the building’s heat load by over 929,000 Btu/hr, which is equivalent to adding over 77 tons of mechanical air-conditioning. To install 77 tons of air-conditioning would cost approximately $1000 per ton installed, or $77,000, not to mention high monthly operating costs. Compare this to the estimated installed price for an evaporative roof cooling system of $48,500, with water being the main operating cost.
Energy savings for this example illustrate the benefit of roof cooling in relieving the energy crisis. 77 tons uses 100.1 kW based on 1.3 kW per ton. Using a $10.00 per kW demand charge on a 100% ratchet would amount to $1,001 per month in savings.
77 tons x 1.3 kWh per ton x 300 hours of operation per month (single shift) provides a monthly usage savings of 30,030 kWh. With a kWh cost of $0.11, this facility would enjoy a monthly usage savings of over $3,300.
If well water is available, the operating cost for a roof cooling system is next to zero. In this example, assume city water would be required. Since this is an evaporative cooling technique, no sewer charges are assessed. Assume the national average of $1.50 per 1000 gal. of water. Evaporative roof cooling normally uses approximately 2.8 gal. of water per square foot per month.
Monthly operating cost = 100,000 sq. ft x 2.8 gal./sq. ft. x $1.50/1000 gal. = $420
Assuming a 7-month year-of-operation of a roof cooling system, the total monthly savings are:
|Demand energy savings
Usage energy savings
Operating cost (water)
Total monthly savings
Based on these savings, a roof cooling system would have a payback of less than 13 months.
Initially with evaporative roof cooling there is concern for the roofing membrane. Although this concern is understandable, it is not accurate. In reality, the opposite is true. There are three major factors involved in roof deterioration: blisters, expansion and contraction, and solar shock.
Blisters form when roof temperatures exceed 150 F. During a 24-hr period, roof surface temperatures can fluctuate from 75 F at night to 165 F during the day. This fluctuation creates a continual expansion and contraction of the roofing membrane, which eventually damages the membrane at seams and flashings.
When the roof’s surface temperature is at a daytime norm of 165 F and an afternoon thunderstorm passes, the roof’s surface temperature can suddenly drop to 60[en]70 F. This sudden drop produces many problems for the roof, frequently tearing the roof open. By maintaining the roof’s surface temperature at or around 90 F, all three of these problems are virtually eliminated.
-Edited by Joseph L. Foszcz, Senior Editor,
The author is available to answer questions about roof cooling. Mr. Flora can be reached at 270-554-5053 or email@example.com . Additional information on roof cooling can be obtained at the web site cool-roof.com .
Bussmann, a division of Cooper Industries, is one of the world’s largest producers of circuit protection devices to protect electrical, electronic and automotive systems from circuit overload. The Goldsboro, NC facility has a molding shop (10,000 ft2) and an adjacent warehouse/shipping dock (14,000 ft2) that are covered by a standing seam, metal roof.
The temperature of this metal roof has been measured as high as 140 F. During the summer months, the heat generated by atmospheric conditions, combined with the interior heat generated by the molding operations, creates uncomfortable working conditions for plant personnel.
The interior heat load for the molding shop consists of 21 thermoplastic injection-molding presses. Each press has an injection barrel and screw that operates at a temperature between 500[en]750 F. The molds in each press can operate at up to 400 F. The hydraulic molding machine operates between 90[en]110 F. The combined heat generated by these operations is substantially more than the four 20-ton, rooftop, air-conditioning units can handle to keep internal building temperatures tolerable.
The heat load generated in the warehouse/shipping dock is mainly from trailers that are unloaded/loaded at the dock. The trailers arrive at the dock full of very hot air (especially during the summer months) which escapes into the building when the trailer doors are opened. Also, the opening of dock doors allows hot, humid, outside air to enter the building. The negative interior air pressure created by the exhaust systems of the manufacturing processes in the building is the main cause for this influx of hot air.
Between April and October plant personnel complained that the areas of the molding shop and warehouse/shipping dock were too hot. The plant Facilities Engineer investigated two options to address the problem: install additional air conditioning capacity or install an evaporative roof cooling system.
Additional air conditioning would result in additional electrical energy usage and additional usage of hazardous materials (HCFC R-22). The roof cooling system would use a minimum amount of electricity and no hazardous materials. The cost of installing additional air conditioning capacity to address the 22 tons of heat was estimated at $38,500. The installation cost of the roof cooling system was $14,000.
When the system was started for the first time, outside temperatures were reaching the mid-90’s. The system improved working conditions in the molding shop and shipping/warehouse dock. Roof cooling removed 22 tons of heat from the 24,000 sq. ft area during one summer season of April to October.
The system avoided the use of 40,000 kWh of electricity that would have been necessary if additional air conditioning units had been installed at a cost of $1,602. It also avoided the use of 171 kW of demand use electricity that would have been necessary if additional air conditioning units had been installed at a cost of $1,370. The roof cooling system provided a total annual energy savings of $2972.