This Federal Technology Alert discusses how to evaluate the cost-effectiveness of hybrid, two-wheel, desiccant-based cooling systems (TWDS), summarizes the desiccant dehumidification process, reviews field applications, and examines energy savings and other benefits.

Desiccant systems save energy by using low-grade thermal sources to remove moisture from the air, and to eliminate the overcooling and reheat step necessary in certain conventional vapor-compression cooling systems. In some applications of hybrid desiccant-based systems, the vapor-compression system can be replaced with less expensive direct or indirect evaporative cooling systems.

Hybrid systems can provide year-round comfort. The direct or indirect heaters used for reactivation can supply comfort heating, and the heat wheel can be used to recover energy all year.

These systems can be used in any building application. However, high initial costs typically limit the use of this technology.

The systems are extremely effective, and their use is well established in conditioning storage areas, ice arenas, hospital operating rooms, and supermarkets. Site-specific conditions and differing application requirements must be understood before the use of hybrid systems can be justified on economic grounds.

A detailed analysis is generally required to evaluate the cost effectiveness of these systems with a conventional cooling system. Typically, hybrid systems should be considered if the following conditions are met:

• Low indoor humidity (below 50°F dewpoint);

• High latent load fraction (greater than 25%);

• High fresh air intake (greater than 20%);

• High summertime electric demand and energy costs; and

• Low summertime gas costs.

About the technology

The continuous desiccant dehumidification process can be achieved in a number of ways, using liquid-spray tower, solid packed tower, rotating horizontal bed, multiple vertical bed, and rotating wheel.

The TWDS falls into the rotating wheel category. In such a system, the moist airstream, which has a high vapor pressure, passes through a rotating desiccant wheel. The desiccant, which has a low vapor pressure, adsorbs the moisture until the desiccant is saturated.

Next, the saturated portion of the wheel rotates into a hot airstream, which is forced through the wheel to remove the moisture from the desiccant. The dried desiccant is rotated back into the moist airstream, and the process repeats itself.

After it has been regenerated (dried), the desiccant is cooled to lower its vapor pressure.

As shown in Figure 1, a TWDS consists of a desiccant wheel, rotary heat exchanger (sometimes referred to as a sensible heat wheel), a supply fan, exhaust fan, and heat source for regenerating the desiccant.

The desiccant wheel is made of a finely divided desiccant material, usually silica gel, titanium silicates, or some type of zeolite (a mineral containing hydrous silicates).

The desiccant material is impregnated into a fibrous support structure that looks like corrugated cardboard that has been rolled into the shape of a wheel, or into a wheel-shaped rotor with a lightweight structural honeycomb core of artificially made, fire-retardant material.

The rotary heat exchanger, which exchanges (recovers) heat rather than moisture, resembles the desiccant wheel in appearance and design. Any form of thermal energy stream (usually at 180° or above) can be used to dry and regenerate the desiccant, including electric-resistance heaters, solar hot water coils, heat reclaim coils, and hot water or steam from boilers.

Most commercial applications use either direct- or indirect-fired natural gas burners.

The TWDS can control or lower humidity, but its ability to lower sensible heat is limited. Therefore, in most commercial applications the TWDS is supplemented with either a vapor-compression or an evaporative cooling system.

Although the technology has been proven in industrial environments, there is limited field test data to evaluate the long-term performance of the TWDS in commercial building applications.

The first cost of the hybrid desiccant system is generally higher than that of a conventional system, but this is offset by lower operating costs in certain applications.

The TWDS offers many other benefits in addition to operational cost savings.

Most commercial systems are designed to maximize the energy cost savings and minimize the initial cost. To optimize the benefits of hybrid systems, an understanding of the impacts of the technology, the load being served (latent vs. sensible), and the climate in which it operates, are all essential parameters.

Applications domain

They have been successful in those instances because there are no practical alternative processes that are capable of providing low moisture levels (less than 30° dewpoint), low microbial growth, or improved air quality.

In the residential and commercial building sectors, desiccant technology currently competes with the well-established, conventional vapor-compression technology.

Although no firm shipment numbers are available, the building sector has seen significant growth in installation of hybrid systems in the past few years. The applications where the benefits have been extensively demonstrated include dry storage spaces, ice arenas, most supermarket applications, military commissaries, hospital operating rooms, schools, fast-food restaurants, unheated warehouses, and as an add-on to existing air conditioning systems with inadequate dehumidification capacity.

The following would encourage increased guidance of hybrid systems and techniques for reducing the first cost of hybrid desiccant cooling systems:

• Performance documentation and confirmation, such as that provided by the Department of Energy (to encourage government facilities’ managers to objectively evaluate this technology for large- and medium-sized building projects);

• A development of design tools (such as a user-friendly computer program), to enable designers to easily evaluate economic tradeoffs and design these systems based on actual performance data; and

• Electric utility incentives.
  
  

  Energy savings

  The energy-saving mechanism of a TWDS can best be understood by comparing the dehumidification and cooling process of the conventional and desiccant-based systems.

  Both systems can be operated in various modes (recirculation, pure ventilation, and mixed). It is assumed that both systems take in 100% outdoor air.

  The following steps describe the psychrometric process for a hybrid desiccant dehumidification and supplemental cooling system. (The letters correspond to state points on the psychrometric chart in Figure 2.)

  Dehumidification:

  A. Intake — hot and humid outdoor air enters the desiccant wheel at point A on the psychrometric chart (Figure 2 [a]).

  A-B. Dehumidification — as the moisture from the outdoor air is removed by sorption, the heat generated when the water is sorbed (akin to condensation) remains in the airstream, increasing the airstream’s sensible load. There is a slight increase in the enthalpy (i.e., the energy content of the airstream increases) when latent heat is being converted into sensible heat.

  At state B, the air is hot and dry and cannot be directly used to cool the conditioned area.

  Cooling:

  B-C. Heat loss or post cooling — the dehumidified outdoor air enters the rotary heat wheel, where it exchanges heat with the exhaust (return) airstream from the conditioned space. In this process, the hot and dry outdoor air cools down, and the cold exhaust air is preheated for reactivating the desiccant wheel.

  C-D. Supplemental cooling — the air leaving the rotary heat wheel is colder than the air leaving the desiccant wheel, but further cooling is often required before it can enter the conditioned space. This can be achieved by using a conventional direct-expansion, vapor-compression cooling system.

  D-E. Space cooling load — the exhaust air leaving the conditioned space is at state E.

  Regeneration:

  E-H. Heat recovery — the exhaust airstream enters the rotary heat wheel, where it exchanges heat with the hot, dry air leaving the desiccant wheel. Part of the heat lost in step B-C is recovered from this process (Figure 2 [b]).

  H-I. Heat addition — the hot exhaust air is further heated to increase the vapor pressure at the desiccant.

  I-J. Reactivation — the hot exhaust airstream dries and reactivates the saturated desiccant.

  For comparing the above process with that of a conventional system, the following steps for cooling and dehumidification with a conventional vapor compression system are shown on the psychrometric chart (Figure 2 [a]).

  Sensible cooling:

  A. Intake — hot and humid outdoor air enters the evaporator coil of a conventional vapor-compression system at point A on the psychrometric chart (Figure 2 [a]).

  A-F. Sensible cooling — the hot and humid outdoor airstream is cooled until it reaches saturation. At this point the air is cold enough to be used in the conditioned space, but cannot be circulated because it is saturated with moisture. To remove moisture, the air must be cooled to below its dewpoint temperature.

  Latent cooling (dehumidification) and reheat:

  F-G. Dehumidification — the evaporator continues to cool the saturated airstream and condenses the moisture, further reducing the drybulb and the humidity. If the humidity requirement is low (less than 40 grains/lb of dry air), the air must be cooled to less than 43Þ in order to condense enough moisture. In this state, it is too cold to be recirculated to the conditioned space.

  G-D. Reheat — the cold, dry airstream is mixed with hot air or reheated to the desired circulation temperature (state D).

  D-E. Cooling load — the exhaust air leaves the conditioned space (state E).

  The psychrometric processes shown in Figure 2 highlight the differences in the way dehumidification is accomplished by the two systems. The amount of energy saved depends primarily on the ability of the hybrid system to shift part of the cooling load (dehumidification load) to a low-grade thermal source and to eliminate reheat (step G-D).

  The fan power is slightly increased because of increased air pressure drop through the desiccant and sensible wheels. The amount of energy saved and the reduction in electric demand depend on several factors that we do not have room to discuss in this article.