Proper relative humidity (RH) control is critical to maintaining healthy and productive indoor environments in buildings. It is estimated that U.S. companies waste as much as $48 billion annually covering medical costs and $160 billion annually in lost productivity as a result of sick-building syndrome (Mumma, 2006). Mold remediation costs associated with poor RH control have been observed to top $1 million annually on military bases. Proper RH control minimizes the potential for indoor air quality problems and related sick-building illnesses while improving thermal comfort and productivity.
The current “industry standard” method to control RH and biological growth involves sub-cooling air to condense moisture out of the air, then reheating the same air that was just subcooled to reduce the RH of the air before it enters the space. This method has been around and used for over 100 years and is known to be very energy intensive due to the need for reheat. However, the reheat process is extremely important in dehumidification applications. The cold, 100% RH air leaving the air-handling units (AHUs) needs to be warmed up to eliminate the potential for surface condensation to occur in the space, and to eliminate condensation in the space, which is critical to the control of mold and biological growth.
The High Efficiency Dehumidification System (HEDS) is a patent-protected, proprietary energy recovery method designed to save more than 50% of the dehumidification-related cooling and heating plant energy in RH controlled environments while also eliminating the health, wellness, product, and productivity loss risks caused by poor RH control. HEDS is designed to be simple and easily maintainable, and to require knowledge of only basic Heating, Ventilating, and Air-Conditioning (HVAC) system operations. HEDS is designed to be scalable, from the smallest room level equipment to the largest central system equipment.
The objective of this project was to validate the performance of a new HVAC dehumidification technology designed to significantly reduce energy use associated with dehumidification, while improving indoor air quality and reducing potential for mold growth. Performance claims, installation costs, and maintenance impacts were investigated through the installation of two test units at Tinker Air Force Base (AFB), OK and Fort Bragg, NC. The primary objective of this part of the project was to evaluate the cost and performance of the HEDS technology, specifically by investigating performance claims, installation costs, and maintenance impacts through the installation of two test units at Tinker AFB, OK and Fort Bragg, NC.
The HEDS technology is very simple. It is comprised of a standard AHU built with a pair of deep, low face velocity heat transfer coils: a cooling coil and a cooling recovery coil. The first coil does the cooling and dehumidifying, the second coil uses the warm water leaving the cooling coil and does the reheating for RH control and cuts the loads on the chiller and boiler plants by using the low-quality recovered cooling energy to meet reheat loads. The result is a dehumidification system that is energy efficient, maintainable, and resilient.
Two test units were installed, a Variable Air Volume (VAV) system at Tinker Air Force Base (AFB), OK and a constant air volume (CAV) system at Fort Bragg, NC. This report summarizes the observed field performance results from more than 6 months of real world testing for both sites. Performance tests were conducted across a range of supply air dew point temperatures to emulate the needs of various building types in the U.S. Department of Defense (DoD), General Services Administration (GSA), VA, and Federal building portfolios.
For the constant volume system at Fort Bragg, the peak day cooling load savings was 18%, while the average cooling load reduction was 25%. For the VAV system at Tinker AFB, the peak day cooling load savings was 29%, while the average cooling load reduction was 28%. The peak load reductions effectively expand the capacity of the existing chilled water systems, enabling the chiller plants to serve more cooling loads with the installed capacity, or to be downsized in the future or for new construction projects. Both of these benefits can help to reduce capital costs.
Based on the results from the Fort Bragg, NC and Tinker AFB, OK HEDS Environmental Security Technology Certification Program (ESTCP) tests, the energy reclamation function of HEDS is able to significantly reduce the cooling load associated with dehumidification while completely eliminating the need for additional reheat energy to provide RH control in a variety of facility types. Cooling load savings range from 20 to 37% depending on the application, and the dehumidification-related heating energy savings associated with the reheat function at the AHU is 100% in all cases.
Note that the actual cooling energy percentage savings that will show up at the utility meter can be a much greater figure than the cooling load savings percentage. This is due to the non-linear relationship between energy use and load on modern variable speed equipment such as pumps, fans and chillers. For example, reducing the cooling load on chilled water pumps with variable speed drives by 20% typically results in electricity savings of around 40%.
The results from the two ESTCP test sites indicate that HEDS exceeded the energy savings targets by a significant amount. Chiller plant energy savings related to the dehumidification process varied between 32% for hospital-type applications with 24/7 cooling loads, to 64% for administrative type VAV cooling loads that only need conditioning 12/5, but that are typically run 24/7 during the dehumidification season in humid climates. Reheat energy savings related to the dehumidification process was 100% for the test sites.
A major barrier to acceptance of the HEDS is market skepticism with new technologies that claim high savings levels. More technology demonstration projects in different applications and third-party validations is needed to substantiate the savings shown in this demonstration. The simplicity of the HEDS design and operating strategy should help to overcome any reluctance to embrace this new technology. Note that the latest version of American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) 90.1 prescriptive energy codes explicitly disallows any form of simultaneous cooling and heating or reheating of air for RH control if the heat or reheat is not from a reclaimed or solar-thermal source. Since HEDS uses reclaimed energy for the reheat energy source, it will be a cost-effective solution to provide ASHRAE 90.1 compliance across a wide range of HVAC system sizes and types.
HEDS units are currently only available under license with one manufacturer, which can limit procurement options. Given that AHUs are mostly built to order using licensed technology or components, HEDS units will need to use the same market channels of mechanical product vendors, installers, and AHU manufacturers to achieve market scale. This will require further engineering support from the vendor networks, which requires training, education, and experience with HEDS systems.
Both demonstration sites had issues with failing chillers that led to high chilled water supply temperatures from the chiller plants. Even as chilled water supply temperatures rose as high as 60 °Fahrenheit (°F), both HEDS units were able to continue to provide dehumidification while reducing cooling loads by 16 to 30%. The cooling load saved by the HEDS unit was used by the other AHUs on the chilled water system, which provided added cooling to those spaces and led to improved comfort, productivity, health, and wellness, even when the chiller performance was sub-optimal. A HEDS installation can improve resiliency by doing more with less.
Throughout the demonstration, HEDS was shown to have the same, or slightly lower, maintenance needs than a normal AHU; thus the needs are significantly lower than other commercial dehumidification technologies.