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Achieving Energy Reductions with Evaporative Cooling

Facility managers looking to cost-effectively save energy can turn to a simple, time-honored idea.

January 4, 2013
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Energy prices are up—way up—and no one has been spared the pain of their endless meteoric rise. As a result, entire industries have been created to develop and market the latest energy-saving technologies to businesses around the globe.

It’s this backdrop that has set the stage for possibly the greatest challenge for facility maintenance managers: the desire to save energy, but the need to do so economically. Is this even possible? The answer is a definitive “yes.” But the technology wasn’t developed in a laboratory on the outskirts of Silicon Valley. The idea is nearly as old as time itself and employs concepts no more complicated than basic physics.

 

Evaporative Cooling: The Basics

The easiest way to understand evaporative cooling is to think of your own body. Whenever we work out or perform a strenuous activity in a warm environment, our bodies produce perspiration (moisture) on the surface of our skin. This perspiration then evaporates—a process which carries away the heat we have generated internally. The system allows us to maintain a normal core temperature despite the increased workload.

 

A Brief History

Evaporation and its cooling effects have been utilized by cultures dating back to ancient times. The Egyptians are widely considered to be the first to capture the benefit, fanning large porous clay jars with water to add moisture to the desert’s hot, dry air. Romans took things one step further, channeling water from the aqueducts through the walls of their homes to maintain a cooler indoor temperature.

Fast forward to the 1860s where soldiers fighting in the U.S. Civil War were frequently camped in crowded, sweltering tents for days at a time between battles. Since much of the war was fought in the South, it meant doing whatever necessary to survive the long and brutally hot summer months. Again, evaporation became the method of relief. Canvases soaked in water were placed over top of the tents, and the result was cooler, more tolerable living conditions. 

 

Evaporative Cooling’s Industrial Application

As the background information and history lesson suggest, the process of evaporation is one of the simplest and most effective methods for carrying heat away from a given structure—be it a Civil War tent or the human body. But the question becomes: How can we harness the cooling effects of evaporation in a large industrial setting on a continuous basis? The answer comes as no surprise: To lower the temperature inside a given structure, you must first prevent the penetration of radiant heat from its roof. The roof is the only surface of any building that is constantly exposed to the sun’s radiant heat, and therefore generates the greatest heat load—nearly 50 percent of the total heat for the entire structure. It stands to reason, then, if we eliminate the roof as a source of radiant heat, there can be an overall temperature reduction inside the building. This is where an evaporative roof cooling system comes in.

The cooling associated with evaporation, while effective, is only temporary. In order to achieve continuous cooling, there must be a system in place to automatically deliver a thin film of water to the roof in predetermined intervals. Notice the use of the phrase “thin film of water.” Ponding water on a roof is not only ineffective, but actually counterproductive. Any puddle will act as an insulator, trapping heat on the roof instead of carrying it away.

Generally, most evaporative cooling systems divide the roof into zones, each consisting of numerous spray heads. A control scheme (either software or programmable control box) activates each zone for a period of 15 to 20 seconds, spraying the aforementioned “thin film of water.” Once all zones have been adequately sprayed, a “dwell time” of 2 to 3 minutes is allowed for evaporation to take place. Then a new cycle begins again. This entire sequence is usually monitored by a thermostat wired into the control scheme, which only permits the system to activate if conditions exist to facilitate complete evaporation.  

 

How Can Roof Cooling Save Energy?

We’ve discussed exactly what evaporative cooling is and how it can be applied to a roof in an industrial setting. I’m sure by now you’re thinking, “That’s great, but how is this going to help my facility reduce its energy costs?” For the answer, let’s start by looking at a few basic facts:

  • The evaporation of one gallon of water will absorb 8,652 BTUs of heat energy.
  • One ton of air conditioning is the equivalent of 12,000 BTUs of energy.

 

Given the above statements, we see that it takes the evaporation of less than 1.5 gallons of water to provide the equivalent cooling effect of 1 ton of air conditioning. Since the cost of 1.5 gallons of water is considerably less than the cost to run 1 ton of air conditioning, can you begin to see the savings? Here’s the takeaway: Every ton of heat load removed by an evaporative roof cooling system is removing a ton of cooling load from your facility’s HVAC system.

What can this reduction in cooling load mean for your facility? First and foremost, it translates into direct energy savings that go straight to your bottom line. A reduced heat load results in an immediate reduction in A/C cycle time frequency and duration, and thus electrical consumption. For large industrial facilities that consume mass quantities of electrical power, the addition of an evaporative roof cooling system can also soften the blow of costly demand charges and ratchets imposed by many power companies.

 

Are There Any Other Benefits of a Cool Roof?

In addition to energy savings, the most obvious benefit of a cooler roof comes in the form of lower interior building temperatures. On an average summer day, the temperature on the surface of a dry roof can easily exceed 150 F. This can mean under roof temperatures of about 120 F, and a blistering 96 to 102 F at the working level. These can hardly be considered ideal working conditions if safety and efficiency are paramount. But what if we maintained the roof temperature all day at a reasonable range of 90 to 95 F? In short, interior building temperatures are reduced dramatically. Under roof temperatures would fall from 120 F to mid-80 F levels, while the working level would sustain low 80s.

By keeping the roof in this narrow temperature range through the course of the day, we find a secondary but equally important benefit of extended roof life. How is this possible? Every roof, regardless of material, experiences expansion and contraction due to changes in temperature—an effect commonly known as “cycling.” This daily movement places stress on every part of the roof, and over time can cause buckling, distortion and separation. The effects can be made worse by passing summer thunderstorms, which often produce wild temperature swings in a short period of time. Installing an evaporative roof cooling system can mitigate, or even completely eliminate, these temperature shocks—putting dollars back into your maintenance budget and prolonging the need for costly roof repair or replacement.

 

Yeah, But…What’s it Going to Cost Me?

This is without a doubt the million dollar question. When weighed against the cost and effectiveness of alternatives, as well as the continued costs of doing nothing, the response is “not as much as you’d think.” Why? A professionally installed, properly programmed evaporative roof cooling system can last for many years, requires very little continuous maintenance and its operating costs are minimal compared to a large HVAC system. Also, there is no decrease in system effectiveness over time.

Let’s look at an example to demonstrate cost savings and return on investment (payback): 100,000-square-foot facility with tar and gravel roof, 1 inch of fiberglass insulation with a U-factor of 0.3, 400 tons of A/C capacity, electric rate of $0.07/kWh and a cost of $1.50 for 1,000 gallons of water.

On any given summer day, an untreated tar and gravel roof can reach 160 F. With an evaporative cooling system, that temperature could easily be brought down to 90 F. Therefore, the heat load removed from the HVAC system is:

[100,000 x (160-90) x 0.3]/12,000 = 175 tons of heat load removed

It takes approximately 1.3 kW per hour to operate 1 ton of air conditioning, and on average it runs 2,100 hours per year. With that in mind, our projected yearly energy savings is as follows:

175 x 1.3 x 0.07 x 2,100 = $33,442.50 in energy savings

The evaporative cooling system uses about 0.1 gallon per square foot per day, meaning our system would cost $15 per day or $3,000 per year to run. This translates into a net annual savings of $30,442.50, and a payback of less than three years on an $85,000 system. A payback period of this length is well below the 3- to 3.5-year industry standard for an energy conservation system.

 

Putting It All Together

 The tide of high energy prices shows no sign of ebbing anytime soon—global unrest, waning supply, high demand and investor speculation will see to it. Businesses large and small, whether local or multinational, continue to face challenging head winds in dealing with this trend. However, we have seen that the solution need not be complicated or expensive. All it takes is rudimentary physics, some water and a little help from Mother Nature! 

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