CHILLER ABSORPTION FOR THE CHENA HOT SPRINGS AURORA ICE MUSEUM
Gwen Holdmann, Chena Hot Springs Donald C. Erickson, Energy Concepts Company INTRODUCTION
Absorption chilling uses a refrigeration cycle that requires little or no electric input, because it does not employ a mechanical compressor. Instead, the refrigeration effect is produced through heating and cooling a circulating absorbent liquid. When a moderate temperature heat source is available and/or local electric costs are high, absorption refrigeration can be quite economical. The type of absorption chiller most familiar to the general public is the propane refrigerator, which is found in RVs or at remote homesteads. In fact, absorption chilling is not a new concept. The first simple ammonia/water absorption chiller was built by Ferdinand Carre in 1846 and marketed as an ice-making machine. Moderate temperature geothermal resources have long been recognized as promising heat sources to drive an absorption cycle, but the 230F minimum temperature classically considered necessary to make an air-cooled system cost effective encroached on temperature ranges which could similarly be used for power generation. For this reason, and because of increased maintenance issues when working with unique systems, no absorption chillers were in operation on geothermal sites between 1989 and 2004. Prior to 1989, the Oregon Institute of Technology used a lithium bromide/water absorption chiller for air conditioning buildings on campus.
Interior view of the Aurora Ice Museum.
ENERGY CONCEPTS COMPANY
Once the first Aurora Ice Museum melted, it was clear that in order to justify rebuilding the structure an alternative refrigeration system would be needed. The geothermal resource at Chena Hot Springs is 165F (74C), substantially lower than the minimum threshold generally considered for absorption systems. Fortunately, the owners of Chena were familiar with an absorption chiller operating in Kotzebue on the northwestern coast of Alaska, to make ice for the local fishing fleet. The Kotzebue chiller was powered by 165F (74C) jacket water from a diesel generator (coincidentally the identical temperature as the Chena geothermal resource). The system uses a three pressure ammonia/water absorption cycle, and had been in operation for 10 years. Chena contacted Energy Concepts Company, the manufacturer of the Kotzebue system, in late 2004, to order a similar unit for the Aurora Ice Museum. The order was placed in early January 2005, and installation was required by the end of March. This meant that there was no time for a shop test of the unit before shipping. It also meant that the unit had to be air shipped to meet the schedule, and hence compactness was at a premium.
THE AURORA ICE MUSEUM
In 2003, Chena Hot Springs Resort built the first Ice Museum in the United States. Chena is located 60 miles (100km) northeast of Fairbanks, Alaska, which is the traditional world capital of ice art. The Aurora Ice Museum was from its inception intended to be a year-round structure, something which had never before been attempted. Unfortunately, due to a number of reasons, the initial version of the Museum did not survive an unseasonably warm spring in 2004. One of the primary problems was immediately identified as the cost of refrigeration for the structure. A standard 200ton (700kW) Trane air-conditioning unit reprogrammed to circulate 300 gpm (19 l/s) of glycol was originally planned, but the cost of operating the unit was quickly deemed too high. Chena Hot Springs is located 30 miles (50km) from the nearest utility grid. Prior to the installation of a geothermal power plant in 2006, the cost of power generated onsite from a 200kW diesel generator was calculated to be 30 per kWh. The onsite generator not was large enough to power the 200-ton (700kW) Trane unit, and so a separate 500kW generator needed to be installed to start and operate it. The fuel cost alone for operating the Trane unit was over $500 per day. GHC BULLETIN, SEPTEMBER 2006
CHENA ABSORPTION CHILLER
Chena Hot Springs required significantly more refrigeration capacity than Kotzebue, and so Energy Concepts designed a unique three pressure system for the Ice Museum, using an ammonia-water absorption cycle. One significant advantage Chena had over other sites was a supply of readily available cold river water to cool the absorbers and condenser. Special heat exchangers are used throughout the system to give high performance in a very compact package. The components which contact external fluids (hot spring water, river water, chilled brine) are constructed of stainless steel. Other internal cycle components are carbon steel. The chilled brine which transfers heat out of the Ice Museum is a CaCl2 solution, concentrated to allow temperatures as low as -50F (-46C) (typical for winter temperatures at Chena). The brine circulates behind the Ice Museum and through an air handler, which cools an annular space in the Ice Museum between a thin inner wall and the external insulated wall. With 165F (74C) hot spring water (85 gpm (5.4 l/s)), and 40F (4C) (80 gpm (5.0 l/s)) river water, the brine is delivered at -20F (-29C) (55 gpm (3.5 l/s)) and the temperature in the ice hotel is maintained at a constant 24F (-4C). An additional system component is a 2000 gallon (7.56m3) brine storage tank, which acts as a giant fly wheel in the system. If the brine in the tank is kept below freezing, it can continue to provide cooling to the Ice Museum for one hour during shutdown of the absorption chiller for any reason. Additionally, defrost of the air handlers has not been perfected and is accomplished using hot water sprayed onto the coils. This heat can unbalance the absorption chiller, and therefore the tank allows the brine to circulate freely while allowing the air handlers to be cut out of the loop.
wise required power plant capacity. The capital cost of the Thermochiller is substantially less than the capital cost of those two displaced components. Therefore, even with ...