Going 'Green' with Hybrid HVAC

Poorly designed HVAC systems, high concentrations of occupants and rising energy costs often make indoor air quality barely adequate.

Germs, mold, bacteria and contaminants are quickly spread in densely populated spaces, and conventional HVAC systems designed for schools are not only energy hogs, but they do not provide optimal ventilation.

In fact, school HVAC systems frequently do not meet minimum ventilation standards set for IAQ by ASHRAE Standard 62.1-2001. One of the major weaknesses found in typical school HVAC systems is that they often recirculate a majority of the building’s air. Recirculation causes internally generated contaminants to become concentrated and spread to all areas served by the ventilation system. It becomes a hotbed of respiratory ills, possibly causing an increased onset of asthma.

IAQ can be improved in two ways — excellent filtration such as HEPA filters and/or bringing more outside air into the space to dilute contaminants. From an HVAC standpoint, the conundrum every administrator faces is how to temper or filter the air without creating greater energy expenditures. Conventional heating and cooling schemes employ energy-intensive components that require 100-percent virgin energy sources, boosting the fuel consumption for the facility. Many schools are looking for“green” solutions and believe the answer to lowering consumption and energy costs lies in high-efficiency equipment. Yet, simply using high-efficiency equipment does not fully solve the problem.

In reality, high-efficiency equipment only scratches the surface. True environmentally efficient systems must delve into total system design. A truly green system will not merely employ a more efficient chiller or boiler, it will significantly downsize (or even eliminate) the need for that equipment from the outset through the use of less energy-intensive components and energy recovery.

One unusually efficient system, is a hybrid, multifunction system designed by a school’s engineering team, mixing conventional and newer technologies. Its main component is an indirect evaporative cooler that also provides heat (energy) recovery in winter operations. The result is a dramatically more efficient system with lower energy consumption and significantly improved IAQ.

Developed by a Wisconsin engineering firm, Lentz Engineering Associates, this hybrid system uses 100 percent outside air, but overcomes the so-called energy penalty typically caused by doing so. Evaporative cooling technologies form the backbone of the system. This system exploits the indirect, rather than just the direct, evaporative process using an air-to-air, plate-and-frame heat exchanger to separate supply air from both the water used for evaporation, as well as the building exhaust used for energy recovery.

Two separate air streams flow over the heat exchanger plates, and water is sprayed on one side to create the evaporative cooling effect, yet it never comes in contact with the supply air. This method reduces moisture sent into the air during a conventional direct evaporative process. Its objective is to harvest energy from the expended air, lowering the amount of raw energy required.

A hybrid system uses other components whenever additional cooling or heating capacity is required. A typical chilled water or direct expansion (DX) system can be incorporated into the overall design to provide supplemental cooling or heating only when the demand is high.

Such a hybrid system incorporates a multifunctional concept, in which individual pieces of equipment serve multiple design objectives. For example, not only does the IDEC unit provide primary cooling and heating, but it also acts as the cooling tower for the chilled water system. The system also uses “amplification,” that is, multiple heat exchangers may be used to amplify cooling/heating energy for recovery. In addition, the hybrid system is designed to use recoverable thermal energy before tapping into virgin energy.

A hybrid system not only reduces energy consumption by a phenomenal rate, but has ancillary benefits. In fact, it is common for schools to shift from using minimum outside air to 100-percent outside air ventilation — and at the same time, reduce their energy expenditures. A typical retrofit using a hybrid design will reduce energy usage by about 30 percent. More staggeringly, fitting a new building with such a system may actually lower energy consumption by 70 percent.

Five years ago, Wausau West High School, in Wausau, Wis., designed and installed a hybrid system. The existing 292,000-sq.-ft. facility reduced its primary heating plant by 60 percent and its primary cooling plant by 92 percent. Overall gross energy use reductions for natural gas and electricity decreased by 38 percent and 28 percent, respectively. The school reports its gross energy costs were reduced by 29.3 percent. Total energy consumption went from 52 million BTUs to 34 million BTUs, a savings of approximately 18 million BTUs. This was with a HVAC system that was changed from minimum outside air ventilation to 100-percent outside air ventilation.

Three new schools in Howell Township, N.J., will each save more than $25,000 in yearly HVAC energy costs after installing a hybrid design and will soon become the first LEED silver-certified schools in the state. It has been reported that since the beginning of the current school year, absentee rates at these schools are approximately 60 percent lower than at other schools in the same district.

Unfortunately, most project teams design HVAC systems around cooling, not ventilation. Relatively new technologies that allow for the design and construction of highly efficient HVAC systems have been available for years, yet it seems project teams are reluctant to use them. It will take a change in overall thinking and engineering design strategies, but with more forethought about the basic HVAC process, not only can schools solve indoor air quality concerns, but also reduce energy consumption and lower energy costs as well.

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