Air-conditioning (AC) is the single largest contributor to peak demand on U.S. electricity grids and is the primary cause of grid failures and blackouts. AC accounts for approximately 15% of all source energy used for electricity production in the United States alone (nearly 4 quadrillion British thermal units [Btu]), which results in the release of about 343 million tons of carbon dioxide into the atmosphere every year. Evaporative ACs can mitigate the environmental impacts and help meet Energy Independence and Security Act (EISA) 2007 and U.S. Department of Defense (DoD) energy policy goals by eliminating energy waste and reducing electricity demand.
The objective of this project was to demonstrate the capabilities of the high-performance multi-staged indirect evaporative cooling (IEC) technology and its ability to enhance energy efficiency and interior comfort in dry climates, while substantially reducing electric-peak demand.
The multi-staged IEC technology known as the Coolerado Cooler uses a thermodynamic cycle referred to as the Maisotsenko Cycle (or M-Cycle). It works by cooling both the primary (or product) air and the secondary (or working) air in a 20-stage process. Each stage contributes to cooling by combining multiple direct stages with a single indirect stage. The cumulative result is a lower supply air temperature than is possible with conventional evaporative cooling technologies, as the unit can achieve wet bulb effectiveness (WBE) of 90%-120%.
This project tested 24 cooling units in five commercial building types at Fort Carson, Colorado. In addition to these buildings, a stand-alone unit was installed at the wastewater treatment plant to test the technology’s ability to operate using gray water. The performance of the Coolerado technology was also evaluated in a retrofit scenario using the energy simulation software tools eQuest and EnergyPlus in three building types across six applicable climate zones (Phoenix, Arizona; Las Vegas, Nevada; Los Angeles, California; Albuquerque, New Mexico; Colorado Springs, Colorado; and Helena, Montana).
The units met all performance objectives other than the supply air temperature limit for select units, the water draw requirement, and maintainability (failure). The increased water draw was due to high water consumption settings in the Coolerado controls, which were modified near the end of the 2011 cooling season. These modifications reduced water consumption to levels that were slightly higher than the original performance metric and were around 3 gal/ton-h. The test unit that was operated on gray-water showed significant algae growth that rendered the unit inoperable within a span of 3 weeks.
The Coolerado units demonstrated the ability to operate with an average seasonal efficiency as low as 0.157 kW/ton (energy efficiency ratio [EER] = 76.4) when calculated as a function of the total cooling provided by the unit and as low as 0.262 kW/ton (EER = 45.8) when calculated as a function of building cooling, which is considerably better than the specified performance metric. The total installed costs, seasonal energy efficiency, energy use, and projected water consumption of the Coolerado units were used to compare the economics and performance to a code-minimum packaged rooftop unit (RTU) with an integrated energy efficiency ratio (IEER) of 12. Given the measured performance of the Coolerado units during the 2011 cooling season, the annual energy savings were estimated at 63.3% compared to a code-minimum RTU. The estimated simple payback was 7.62-41.8 years, depending on the facility that the unit was installed in when the maintenance costs were assumed to be equivalent to a packaged RTU.
The Coolerado technology can reduce energy use by 57%-92% relative to standard air-cooled, refrigeration-based AC units, depending on facility type, location, baseline heating, ventilating, and air-conditioning (HVAC) equipment, and technology application. The Coolerado technology has the best economics when applied to data centers, which had a positive net present value (NPV) in all climate zones. The quick service restaurant had favorable economics in Phoenix and unfavorable economics in Colorado Springs and the simple payback period (SPP) was better in both climate zones than the single-zone classroom. The single-zone classroom unit showed favorable economics in Phoenix and Las Vegas and unfavorable economics with payback periods of 52-345 years in Los Angeles, Albuquerque, Colorado Springs, and Helena.
The economics of this technology are extremely sensitive to operations and maintenance (O&M) costs; any increase or decrease in O&M costs has a significant impact on the economics of the installation. The analysis indicates that the Coolerado technology has the best economics as a retrofit technology when it is competing against smaller air-cooled AC systems with EERs of 8-12. DoD should target facility types with high internal loads and/or high ventilation rates that require year-round cooling. The data center application is the most cost-effective application in all five applicable climate regions. For common DoD spaces such as offices, warehouses, and other facilities with internal loads below 2 Watts/ft2, the system is not life-cycle cost effective if the building has an existing cooling system, unless it is installed in locations that require year-round cooling such as Phoenix or Las Vegas as an outside air pre-conditioner. The multistage IEC system should be considered in new construction and for facilities without existing AC systems in all five climate zones.