The Desiccant Debate Continues (January 2001)
Other commercial applications, such as office and retail spaces, remain a question mark. While the manufacturers say that desiccant systems can remove moisture from the increased outside air that is required in many commercial buildings, some critics say all that's necessary is a well-designed mechanical cooling system.
Is Mechanical Cooling Enough?George Smith, P.E., president, HCR Inc. (Lewistown, MT), is one of the more vocal critics of solid desiccant systems. He says that while certain food processing applications may benefit from a liquid desiccant system, in air conditioning applications, desiccant systems essentially never make sense, even in cases in which the latent heat load increases or is otherwise found to be greater than the latent heat capacity of the equipment installed.
"Nowhere in the literature - ASHRAE, DOE, GRI, engineering journals, or otherwise - is a gas-heat, reactivated desiccant system shown by calculation or by test to be superior energy-wise to its conventional, correctly designed counterpart," says Smith.
He further asserts that a properly designed mechanical or absorption refrigeration system for air conditioning applications provides the same dehumidification results as a properly designed desiccant-based refrigeration system. "The most important fact, though, is that the former does so at a fraction of the energy input required by the latter," notes Smith.
Part of the problem may stem from the fact there are few, if any, independent studies that compare the dehumidification effect and energy savings of a properly designed desiccant-based system with a properly designed mechanical cooling system. One engineer, who wishes to remain anonymous, says that manufacturers often compare desiccant systems with improperly designed mechanical cooling systems.
"If you're going to have to cool that air all the way down to the dewpoint that you want to maintain in the building, and you're going to do that with your cooling coil, that's very expensive to do. But a simple little heat pipe system allows you to momentarily remove your sensible heat, cool it down, then put that sensible heat right back in again," says the anonymous engineer.
This particular engineer believes that manufacturers often compare desiccant systems to conventional installations that are either improperly designed or don't have heat pipes. Therefore, studies can show that buildings are spending a lot of money on electricity for a mechanical cooling system to get the temperature down to the required dewpoint, overcool the drybulb temperature, and then heat it back up electrically. That can be very expensive.
Smith states that the calculation procedures of the "Psychrometric Processes" section of ASHRAE Manual GRP-158 and its underlying principles were taught to him in the early 1950s. Through using those principles, and by taking the correct amount of care, he has been able to design optimum efficiency, no humidity problem, air-drying, air conditioning systems, without a need for reheat or heat-reactivation of any sort.
He adds that many authors involved with desiccant-oriented publications do not employ the authority of GRP-158, or its later version, in their evaluations of desiccant-based systems. "This fact of not one instance where a desiccant-based system is compared against its correctly designed, conventional counterpart is most unsettling from a consulting engineering point of view. How might authors claim desiccant-based superiority in view of such an omission?"
Evert Osterman, Western regional manager for Desert Aire (Milwaukee) is also one who believes that mechanical cooling systems can take care of dehumidification in most situations, noting that when the dewpoint is above 50°F, a direct expansion (DX) system works just fine. If the dewpoint is below 40°, he believes a desiccant system can work well for humidity control, not temperature control. If the dewpoint needs to be between 40° and 50°, then it's necessary to look at the project individually.
"In other words, when the air is warmer and humid, use DX. The refrigeration system works very well under these conditions. When the temperature comes down or when the humidity comes down it is more difficult for a DX system to condense water out of the air, and a desiccant system will be the system of choice under these conditions, as it can pull water out of the air," says Osterman.
Not Necessarily SoA mechanical cooling system just can't provide the same conditions inside a space as a desiccant system can, says Chuck Campbell, vice president and general manager, Munters Corp. - Dehumidification Division (Selma, TX). "A conventional system overcools the air to a great extent to remove moisture under all but 'design conditions,' which occur a very small percentage of the time. Most of the time, the temperature will be lower and the humidity higher than design."
This is where the conventional system must reheat the air in order to achieve the desired indoor air condition, which results in higher energy costs. Campbell notes that a conventional system is not usually controlled by a humidistat, so the compressors cycle off during off-design conditions. "This significantly decreases the system's dehumidification capacity, often at the times when dehumidification is needed most. A desiccant system removes moisture independently of cooling to provide the required temperature and humidity levels," he says.
Jim Sand, program director for the Department of Energy (DOE) desiccant program at Oak Ridge National Laboratory (Oak Ridge, TN) states that conventional vapor compression systems use temperature to control the humidity, which can be a problem. "Desiccant systems allow you to have an independent system, so that humidity can be controlled to any desired level, and you can use the conventional system to control the temperature to any desired level. It gives you complete independent control of humidity," says Sand.
And even if a mechanical cooling system can properly take care of humidity, many engineers resort to oversizing a system, because they think that will provide proper dehumidification. Unfortunately, an oversized unit will cool the space very quickly, so when the space is cool, the unit shuts off, and all the moisture that condensed on that cooling coil then re-evaporates into the air.
The outside air is still being pumped into the building, so instead of getting better humidity control, you get far worse humidity control by oversizing the equipment. And you spent more money to do it.
"In general, commercial markets, where humidity moderation is the only thing that's necessary, oversizing the equipment is the reason that you don't get any humidity moderation. It is the problem," says Lew Harriman, director of research and development for Mason-Grant Consulting (Portsmouth, NH). "And that is so clear from all the literature and all the research and all the field tests. It's abundantly clear. But that doesn't stop an engineer from saying if a 5-lb hammer is good, then a 7-lb hammer is better."
The Expense FactorIn addition to stating that solid desiccant systems are virtually never needed, critics also point to the amount of money a desiccant system can cost - both in first cost and ongoing operational expenses. An active solid desiccant system often uses natural gas to remove moisture, while a mechanical cooling system uses electricity. The difference in these energy costs, as well as the particular application can dramatically affect operating costs.
Yes, it's true, notes Campbell, that the initial product cost of a desiccant system may be more than other technology for some applications. But he says if there is an initial cost premium, it will be paid back through energy savings - particularly when the technology is applied to supermarkets.
Unfortunately, notes Sand, it's often difficult to compare apples to apples, because the first cost for conventional systems is usually rated in tons of air conditioning, while the desiccant systems are usually rated per cfm, because they're meant as ventilation air pretreatment. "The costs don't merge well," he says.
Smith states adamantly that in addition to the higher first cost of desiccant-based systems, research performed at ORNL shows that desiccant-based systems require substantially more energy than mechanical cooling systems of correct design. "Where both sensible and latent cooling are required, energy wastefulness compared to a properly designed refrigeration system occurs in essentially all [desiccant] applications."
Because both gas and electricity are involved in applications utilizing desiccant systems, energy costs are a real concern. Some engineers say that with two different kinds of utilities involved, there's no way a desiccant system can make good economic sense. Harriman notes that in circumstances where humidity moderation is the goal, such as retail and office spaces, that may definitely be true. "But if humidity control is the goal, it may or may not be true, depending on the way the engineer designs the system, how badly they oversized the cooling system."
Campbell adds that while a mechanical cooling system may be less expensive to install and operate, an application may not be comfortable and may also have additional indirect costs from IAQ issues, even though the direct energy costs will be less. "This is because it costs energy to 'turn down the humidistat' just like it does to turn down the thermostat. If humidity is not controlled, the energy usage will be less." He says that by looking at a program like "BinMaker" (located at www.binmaker.com), engineers will be able to accurately estimate the energy costs of the different systems.
Other DisadvantagesIn addition to higher first cost and potentially higher operating costs, there are other disadvantages to desiccant systems, according to both critics and supporters of the technology. One of the biggest, according to many, is the fact that manufacturers are not yet large enough to handle all the requests for information.
As Harriman notes, engineers may not have any trouble getting support for target applications, such as supermarkets. However, people who would simply like to explore the technology may find the process more difficult. "Once you are working with one of their target applications, they will help you. They are not big enough to support general education about desiccants." he said.
Getting the equipment installed may be another sticking point with building owners, many of whom don't want to use gas, which is required for most active solid desiccant systems. Some are concerned about combustion problems and carbon monoxide issues and will not even consider the technology. In addition, desiccant systems are an air-moving technology, so the equipment tends to be big and bulky. Building owners may not want to give up the space for the equipment.
And, finally, once the equipment is installed, the maintenance and engineering staffs often know very little about desiccants and may have a tough time maintaining them. What usually happens is the in-house staff doesn't trust the technology, and they haven't been taught how to properly use or maintain it, so they're more apt to shut it down when things go wrong. They often don't know the right thing to do to fix it, and there usually isn't the local support to help them.
So the question remains - when should you consider desiccant systems? While these articles have shown that question can't be answered definitively, perhaps they will encourage independent organizations and/or universities to consider conducting studies between desiccant systems and conventional mechanical cooling systems so that more definitive answers can be obtained. ES
Putting The Issue On IceThe most interesting dilemma engineers may face right now in regards to when to use active solid desiccant systems is the ice rink. There is currently somewhat of a marketing battle taking place between desiccant system manufacturers and DX system manufacturers in the ice rink arena.
The main reason for the debate in this arena is that until now, no organization has established the right indoor conditions for an indoor ice rink. ASHRAE suggests indoor conditions from 40° to 60° at 80% relative humidity (rh). This indicates dewpoints from 35° to 55°. Many rink owners have selected 55° at 50% rh, or just under 40° dewpoint. Based on that selection, a desiccant system would be the system of choice for recirculating air inside the ice rink.
However, code requires the introduction of outside (ambient) air. This air, under full load conditions, is warm and humid in many parts of the country, and a desiccant system will have to be oversized to deal with the additional load, says Evert Osterman, Desert Aire (Milwaukee). "Considering the first cost and operation cost for a desiccant system, this choice would be incorrect. It appears to me that if the rink owner insists on indoor conditions of 55° at 50% rh, it might be much better to select a desiccant system for the recirculating air and a DX system to pretreat the ventilation air."
To ensure that the DX system can take care of the latent load, the engineer would have to start with the new ASHRAE weather data, which is based on the high wetbulb and the coincident drybulb temperatures. These numbers differ from the old weather data, which was based on the high drybulb and the coincident wetbulb temperatures. The new weather data reflect the highest latent load for the outside air. Using this information will define the correct latent load that needs to be taken care of. Once the correct load is known, sizing can easily be done in cooperation with the manufacturer of the air conditioning or dehumidification equipment.