Adsorption chiller hits US, and its desiccant gel gets a warm reception.
Yes, water. Would you believe this is the only coolant in a big, powerful chiller? And how about a system that can take widely fluctuating heat inflow to drive an intriguing kind of cooling?
These are two prime selling points in an adsorption chiller now newly arriving from abroad, after two decades in Japan and one in Europe. If this epoch-making cooling technology continues to pan out here---as it seems to be doing, so far---then its process for water-based chilling could revolutionize combined cooling heating and power (CCHP) projects here, starting very soon.
The aqueous technology isn’t actually new except in the US, where it arrived a few years ago, a bit belatedly, from its place of origin in Japan. Nishiyodo Kuchoki Corp., of Kyoto, first developed the unique cooling process in the mid-1980s, and ever since has been implementing its NADAC-model chiller to scores of hospitals, hotels, major and minor manufacturers, processing plants, and assorted other facilities enticed by the almost natural appeal of watery adsorption chilling (note the “d”) as an alternative to the more usual toxic chemicals in heat-fired cooling.
Worldwide distribution of Nishiyodo’s invention, the Adsorption Chiller, has recently shifted to Houston, where availability within the US recently began under the production and patent licensee HIJC USA Inc. (Heat Integrated Joint Companies).
Currently, HIJC has two dealerships selling and supporting it---both in California. To date, five ADCs have been installed, all of them in that state, where high electric rates and other incentives have spurred a quest for power alternatives, but a mild climate makes it tough to utilize exhaust heat very well during warm months. Thus, the piping of generator exhaust to a reliable heat-fired chiller has tended to become absolutely critical to the viability of CCHP projects.
Absorption chillers (note the “b”), using assorted toxic chemicals and being fired by heat, have been performing for years, of course. However, as HIJC USA dealer Donald Pruss, CEO of CoGen Equipment Solutions of Carmel, CA, points out, heat-driven chemical chilling hasn’t always proven easy. One primary reason is that chemically based systems, as he points out, use “a toxic fluid for the refrigerant, and have a far shorter life-cycle period, simply because they use a corrosive liquid material.” He’s referring especially to the LiBr used in many absorption chillers, but similar problems can arise when using ammonia water or freon. For one thing, use of these imposes an extra burden for safe handling and eventual disposal, he notes, “every time you use them”—which tends to be often, and which requires a highly skilled, hazmat-qualified, and often hard-to-find mechanic. Another drawback, he believes, is that chemical systems provide a much narrower margin for error in operation and are relatively intolerant of real-world fluctuations in operating conditions. Corrosion and crystallization readily occur in these systems, he adds, in serious cases causing equipment to seize, “and then you’ve got to tear it all apart” to remove the damage. Lengthy, expensive downtime is likelier, and the added stress from chemicals and compressors shortens equipment life. Pruss has sworn off chemical systems and has been representing ADCs ever since.
Adsorption Chilling Trick is Evaporation
To appreciate this usability breakthrough, it will be helpful to understand how the process actually works. In refrigeration, the term adsorption refers to the collection of water vapor by a hydroscopic material like silica-gel. Such desiccants are common in dehumidifiers, in which they soak moisture from the air, thereby drying it; and afterwards, warmth is injected to regenerate the desiccant for re-use.
At the heart of an ADC are similar events occurring inside the packaged chiller. The essential ingredient here is what Nishiyodo calls a “permanent” silica gel positioned in the adsorbent chamber. Nishiyodo claims this will survive 30 years of constant regenerating. It produces chilling in a continuous cycle in the following way:
- an adsorbent chamber first adsorbs water vapor from an evaporator section, thereby cooling the inflowing water (of, say, 85†F) down about 44†F; simultaneously
- a second adsorbent chamber is heated with very hot (122†F to 194†F) water flowing through its heat exchanger to regenerate the adsorbent;
- the water vapor released from the adsorbent is condensed in a condenser section;
- the condensed water then returns to the evaporator section. This part of the cycle takes about seven minutes to complete, after which,
- a second adsorbent chamber switches over to adsorbing the water vapor, while the first chamber’s desiccant is being regenerated.
The end result is, as noted, very chilled water, all produced by the effect of evaporation. In lieu of a mechanical, electric-powered compressor, the only significant energy used to drive this entire process is, of course, the very hot recharge water.
Now comes perhaps the most exciting part from an energy-efficiency standpoint. In countless sites, opportunities abound for utilizing very cheap, recycled heat, and turning it into chilling. For example, industrial facilities that do processing---dairies, wineries, breweries, juicers, food plants of all types, paper mills, printing presses, chemical plants, plastics fabricators, rubber and cement plants, and so on---quite often use heat. Other good sources include steam boilers in hospitals, hotels, residential facilities, etc., where heat can be recycled and piped out as hot water. Still another great source is the searing exhaust from an onsite generator, be it a diesel, reciprocating engine, turbine, or fuel cell. Output from these typically ranges between 140†F and 212†F. Even solar hot water systems can be used to make cooling, as a recent application in Freiburg, Germany, has demonstrated (see below). All of this readily available heat can be squeezed out and turned into cooling, thanks to HIJC’s Adsorption Chiller processes using lower-temperature heat, from unsteady sources.
All of which could potentially become a tremendous boon to making CCHP payback numbers work. The new ADC promises to make viable countless more power projects than were feasible before.
Non-Chemical and Non-Mechanical: Comparative Advantages
And again, besides yielding these dramatic heat-utilization gains, the adsorber touts major benefits in ease-of-use and economizing, with all of these goodies stemming from the refrigerant being good old H2O.
First, in comparison with chemically based chilling, the water-evaporation method means, as Pruss notes, that there are no hazmats, no waste disposal issues, and no risks of crystallization. There’s virtually no potential damage from leaks, either; no periodic change-out of old chemicals and no pricey mechanic to pay. More positively, the water system promises a much longer lifespan and much lower overall lifetime cost. HIJC projects, in fact, that its chiller will last as long as 30 years. (However, as HIJC’s Web site notes, the product isn’t that old yet, so there’s no empirical data on longevity.) By comparison, chemical chillers in the real world tend to survive just seven to 10 years or so, says the company.
Other ADC Pluses
Few moving parts. - There’s no compressor, thereby eliminating a major source of complications in other systems; no high-pressure/high-voltage issues to deal with, no alignment, no surging, no vibration, no noise, and no compressor maintenance, oil changes and overhauls. Instead, chilling is achieved by a completely different process, making the operation much, much easier.
Another Gain Derived from Having No Compressor - Low electrical load. The 180-ton HIJC needs a mere 0.4 kW to run its automated controls, open a few valves, and power up two small pumps—one, a vacuum for non-condensable gases and running only minimally (at start-up and for one hour in every 40 hours of operating time); and the second being a water pump that comes on only briefly while unloading. Cumulatively, the sum is but a trickle compared to that of compressors.
Another Dividend - Much simpler maintenance, consisting of (1) periodically checking the vacuum pump oil and (2) replacing seals on the butterfly valves every three to five years.
Results of Simplified Design - Easy, automated operation. Startup is quick. Hit the on button, and chiller water will begin circulating in minutes. The events in the cycle follow a programmed seven-minute sequence. No skilled operator is needed, because everything is on “auto.” Controls are self-contained; there are no external temperature valves, no chemical tests to perform, nor other such elements to worry about as are customary with chemicals. Two modes of operation are selectable—standard and economy—determined by the hot-water supply volume and output needs. The combination of simplicity in design, minimal wear, and full automation adds up to greater durability, thereby allowing more reliable 24/7/365 operation, the company claims.
Another Touted Benefit - More stable output under varying conditions. A frequent challenge with exhaust-heat-fired chillers is the wide variability of heat output. An engineer must often figure out how to make the heat more constant, and how to control it. And it’s not unusual for a system to require considerable supplemental heat from boilers to drive the chiller. In fact, some hot-water-fired absorption chillers require minimum temperatures of a relatively high 185†F, thus requiring more fuel; and the cogen efficiency is seriously impaired. By comparison, the ADC reportedly can yield chilled water from a hot water inflow ranging from as little as 122†F up to 194†F—as is typical of the swings found in many heat exchangers. In the HIJC product line, the only consequence of a dip in temperature to the 120s is that the coefficient of performance (COP) will be reduced. Normal COP, achieved with high-efficiency heat exchangers, should come to .68 in economy mode or .75 in standard mode.
Similarly, the input conduit can tolerate widely variable hot-water flow rates---again, a situation that often occurs in waste-heat-recovery systems. Even if inflows fluctuate as much as plus-or-minus 50%, stable, chilled water will continue pumping forth.
That said, the ADC purportedly outputs 44†F chilling, assuming inlet water at 85†F and the availability of 194†F hot water to recharge the desiccant. In this model, five sizes are currently available, ranging from 25 tons to 180 tons.
Any disadvantages? Yes. The Adsorption Chiller currently costs more than a comparably sized, basic LiBr chiller. However, as HIJC’s North American sales manager Carl Moeller points out, “When needed three-way temperature control valves and back-up boilers are factored in, the Adsorption Chiller is competitive.” The value climbs even higher when you factor in the ease of start-up and maintenance, and the benefits listed above, he says.
So much for presentation. How does it work in the real world? Nishiyodo boasts scores of successful applications since 1985. Here are two---a unique, solar-powered, quasi-experimental applied research in Europe, and a recent award-winner in Silicon Valley.
University Hospital, Freiburg, Germany
Nishiyodo cracked the EU markets in the early 1990s, and reportedly scores of successful applications have followed. Perhaps the most intriguing—and first solar-driven adsorption chiller anywhere—was engineered and installed at the University of Freiburg, in a collaboration between the university’s engineering department and the nearby Fraunhofer-Institut für Solare Energiesysteme ISE, along with government funding. Results were later published by the institute’s Hans-Martin Henning and by Hendrik Glaser of Universitätsklinikum Freiburg Geschäftsbereich Technik.
In the design, 90 square meters (later increased to 170 square meters) of evacuated-tube solar heat collectors made by Sunda Solartechnik were attached to the flat rooftop to service the university’s hospital with natural warmth year-round. A significant factor in deciding to use this type of collector, notes the report, “was the easy mounting on the flat roof.”
Each individual pipe was angled to the south at 45 degrees (or, in the extension, at 30 degrees). A plate heat exchanger conveys the heat down to floor level, with the transfer also assisted by continuously adjustable pumps.
At the floor-level cooling plant, the heat is piped into the 70-kW adsorption chiller’s primary heat cycle. Supplementing this, a steam heat exchanger supports the chiller’s secondary heating cycle. Besides these two inputs, a heat storage or buffering system was rigged, allowing equalization and better control by means of various combinations of inputs and modes of operation. During wintertime, the same collection system can pre-heat outdoor fresh air for circulating through the hospital’s HVAC.
In summertime, the solar collectors have routinely succeeded in delivering “a remarkable fraction” of the recharger heat, the researchers found; their results were carefully documented and published in a chart-laden analysis. For one example of daily output, during several hours on June 26, 2001, the sun’s warmth actually exceeded the amount needed to recharge the desiccant gel. At other times that day, heat was readily supplemented with other efficient sources or store heat, making for an overall solar contribution for that day at nearly 80% of the total.
Further technical measurements showed, though, that the COP came to only 40%. This is about one-third less than the manufacturer’s claims for the given conditions (i.e., inflow water, cooling water, and chilled water output). Researchers concluded that this performance “should not be generalized,” and in fact, later modifications raise the COP to slightly above 50%.
Assorted custom-made hydraulics and controls were also developed. One especially critical element turned out to be the correct sizing and operation of the solar heat storage buffering system. These can greatly increase the heat utilization efficiency and assure proper evacuation of heat during the chilling cycle. A similar buffering system, ultimately measuring about two cubic meters in size, was developed to stabilize the chilled water, which is also highly desirable.
Experimenters varied adsorption and desorption cycle times under differing load conditions to determine the impact on COP. One conclusion: “During operation it turned out that it makes sense to provide very low temperature heat to the adsorption chiller, as long as the actual load requirement can be matched.”
Network Appliance, Sunnyvale, CA
In 2001, Network Appliance (NetApp) was rudely exposed to the vulnerability of its electrical supply during California’s energy deregulation fiasco. This $1.4 billion (current sales) IT business, with 32 offices worldwide, provides data storage, protection, backup, disaster recovery, and software and hardware products and services. Considering the critical nature of its power security needs, NetApp responded by ordering combined heat-fired cooling and power generation (CHP) for its headquarters building. Three prepackaged natural gas--fueled reciprocating engines from Carson City, NV--based cogen developer Hess Microgen were soon installed as a base-loading system (i.e., a primary power source). Each produces 350 kW, for a total of just over 1 MW.
As for the heating side, engine exhaust is being captured and piped in to activate a 300-ton LiBr absorption chiller purchased from Century, with output sufficient to carry most of the headquarters’ cooling load.
Installed in January 2002, after one year of operation the system was yielding “pretty good success,” NetApp’s director of facilities Dan Hoffman recalls.
With that positive experience under their belts, a year later NetApp needed a new data center, and again opted for CHP. With its storage floors filled with sensitive electronic equipment, this mission-critical site would require steady, reliable cooling and power. Capacity for 2 MW and 360 tons of chilling, available initially, made good economic sense, Hoffman notes. Additional space remained nearby “to grow that if needed,” he says, adding that his initial goal was “to start with 1 MW, minimum, of steady load, and 200 to 300 or so tons of steady chilled water needs.” The system was thus designed for flexible base-loading.
HIJC’s “alternative chiller,” as it was called, came into play at the recommendation of the project’s mechanical design-build contractor, Air Systems. When Hoffman heard water-based cooling described, he recalls, “it made a lot of sense,” and what stood out as especially attractive was the broad range of hot water inputs and output chilled water temps it could accommodate. Besides all of this, he adds, “it was a much better choice environmentally,” than the LiBr type. In fact, NetApp eventually received two environmental awards for investing in the project.
Cost comparisons showed that, although it was pricier to buy, the ADC would bring lower operating costs and a more positive cash flow. The price differential would be recovered in less than three years, at which point the ADC appeared to be more economical. And the icing on this deal was a hefty million-dollar rebate (offsetting the power portion only) from Pacific Gas & Electric, funded through the California Public Utilities Commission. Hoffman observes that $1 million “makes a very nice incentive.” NetApp’s previous CHP project in 2001 had netted $740,000 from CPUC. Not surprisingly, as Hoffman notes, rebates are a key factor in sizing generating equipment and, lacking the subsidy factor, helping the economics to work out differently.
The exotic technology itself was of some concern to him, but this was quickly allayed when he learned of Nishiyodo’s 20-year track record in Japan and Europe. Hoffman also had a chance to visit two other recently commissioned ADC units that HIJC had delivered nearby, at Mission Plastics in Ontario, CA, and at Sunkist in Tipton. According to Hoffman, very favorable reports indicated “no startup issues, no issues on installation, and they seemed to be very easy to operate and maintain.” From these site visits he also picked up some good design tips regarding fluctuating input and output temperatures and the layout of hot water storage tanks, pipe racking, and heat exchangers, which can be critical in evening out and balancing the various water temperatures.
Three Hess Microgen 375-kW reciprocating engines thus arrived to power-up NetApp’s new data center, prepackaged with primary heat exchangers, tanks, and a second exchanger used in separating the tank outflow—all of this to service hot water to three new 120-ton ADCs, running in parallel.
Commissioning (in autumn 2003) was routine, and after more than a year in service, all hardware has been working “with no problems,” Hoffman reports. Power output, at 1.125 MW, runs in parallel with the PG&E grid and handles the building’s entire electrical load. Control and monitoring of generators, chillers, pumps, temperatures, and flows is automated---the gauges and controls being integrated with building management systems via an intranet. Heat from the tailpipes warms the recharger water to the requisite 120†F to 190†F range, and the desiccant utilizes all the heat it can get. Maintenance and servicing needs “are minimal,” he says, consisting of a quick look. “Really, there’s virtually no maintenance.” What little labor there is can be done by the HVAC crew; instructions are in the manual, and there’s tech support by phone. Hoffman sums up, “We’re very happy with them.”