Completed Demonstrations - Kennedy Space Center

New Energy-Reducing Heat Pipe Technology Demonstrated at Kennedy Space Center

Heat pipe heat exchangers were installed on three air handlers serving NASA's Life Sciences Support Facility in Hangar L at the Cape Canaveral Air Station, Kennedy Space Center, Cape Canaveral, Florida, in December 1996. The intent of these retrofits was to improve the dehumidification performance of the cooling systems, thus reducing the electric and steam energy required for reheating air, and reducing electric energy used by the chillers.

Space-conditioning needs established for hangar

Heat pipe precool and reheat principal: Air is passed through a precool coil that takes the air from 80 degrees to 70 degrees. At the main cooling coil, the temperature drops to 50 degrees which results in increased moisture removal. The air is then heated to 60 degress and is more dry and comfortable.

The heat pipe precool and reheat principle.

Space-conditioning needs were determined for the laboratory and office areas constructed inside the original hangar structure. The hangar is approximately 175 feet wide, 150 feet long, and 30 feet high, covering a ground floor area of about 26,000 square feet. There are approximately 20,000 square feet of air-conditioned space inside the hangar, including offices, labs, and clean rooms. There are very few windows and essentially no direct solar gains on conditioned spaces. Most of the cooling load is caused by internal gains from operating lights and equipment and from conditioning outside air for ventilation. Normal operating hours for personnel are between 0700 and 1700 hours. Much of the equipment and lighting operates 24 hours per day.

Three air handlers condition temperature and humidity

Three air-handling units (AHUs) serve most of the conditioned laboratory and office space. Air handler AH-1 is on the main floor of the hangar and supplies conditioned air to the clean rooms. This equipment operates with 100% outside air. The accompanying figure shows a schematic diagram of this air handler, which controls both temperature and relative humidity (RH). The outside air first passes through a precool coil before it enters the main cooling coil. The heat pipe was installed at the precool coil. The two coils are supplied with chilled water (in parallel) from a 110-ton chiller on the north side of the building. During periods of high latent loads, humidity is controlled by cooling the supply air below the point required to meet the sensible load, thus condensing enough moisture from the air to meet the required RH set point. Hot water supplied by a local oil-fired steam boiler is used for reheat in this system. If the RH in the conditioned space drops below the humidistat set point, steam from the boiler is injected in the air flow stream to humidify the air.

Heat pipe dehumidification process.

Air handlers AH-2 and AH-3 are located on the mezzanine level above the office spaces on the south side of the building. These AHUs serve a combination of offices and laboratory space. Chilled water is supplied by a 55-ton air-cooled chiller on the north side of the building. Humidity is controlled by subcooling the air flow to condense sufficient moisture from the air, and then reheating to maintain the dry-bulb temperature set point. Reheating is accomplished by electric heaters in the supply ducts to each zone. Steam can be injected in the supply ducts to humidify the air during rare periods of low humidity in cool weather.

New chiller system evaluated using before-and-after retrofit data

Using long-term data, energy use of the various systems was compared directly both before and after the heat pipe retrofits in order to assess actual energy savings. The preretrofit period extends from mid-August 1996 to mid-November 1996. The postretrofit period includes all of 1997. For the latter period, there is no reheat energy used during summer months (June through September) and significant reheat energy used during winter, spring, and late fall. (Note: there is no reheat data for air handler AHU-2 during parts of February and March of 1997 because of equipment problems.) This data set shows that there was much less reheat energy used after the new heat pipe system was installed for the period August through November.

It is not possible, however, to quantify the amount of annual energy savings attributed to the heat pipe retrofit from this data, partly because of the wide variation in reheat energy required for any given period. Also, there were significant changes that affected the performance of the air-handling systems that were not attributable to the heat pipe installations. These include a new condenser and compressor for the chiller serving two of the retrofitted air handlers, changes in supply air flow rates, and possible changes to the cooling coil control mechanism.

Evaluation parameters included simulation models of each HVAC system

The measured data were used to develop thermal models of each heating, ventilating, and air-conditioning (HVAC) system. These models were used to simulate the energy use of each HVAC system on an hourly basis, for a consistent set of operating conditions. This simulation will predict the annual energy use of each system with and without the changes attributed to the heat pipes (primarily, the new air flow rates and increased chiller capacity), thus yielding annual energy savings predictions for each of the three air handlers.

Because it uses 100% outdoor air taken from inside the hangar, AH-1's system has a very large latent load. As a result, the system does not adequately regulate humidity levels in the space. Even after the heat pipes were installed, humidity levels have sometimes reached 70% in the spaces. In addition, the heat pipe had no positive effect on lowering the overall sensible heat ratio (SHR) of the two cooling coils. As originally designed, this combination of cooling coils was able to achieve SHRs at, and even below, 0.4. Applying the heat pipe to the precool coil defeated the original design of having saturated air entering the main cooling coil and did not lead to the desired lower level of SHR. Nor did it result in any reheat or chiller energy savings.

AHU-2 uses chilled water from the 55-ton chiller, has a single cooling coil, and a fresh-air ratio of 13% ducted from outside the hangar. The system achieved very good RH control after the retrofit. The heat pipe clearly lowered the SHR by a significant amount, as much as 5% to 8% during the summer months. There was little change, however, in the space dry-bulb temperatures. Finally, reheat requirements are reduced by approximately 3,000 kilowatt-hours (kWh), while the cooling coil load increases 880 ton-hours. The net energy savings are approximately 1,600 kWh.

AHU-3, which also uses chilled water from the 55-ton chiller, has a single cooling coil and a fresh-air ratio of 15% taken from inside the hangar. This system achieved perhaps the best temperature and RH control in the space it serves, both before and after the heat pipe retrofit. With the heat pipe installed, the annual reheat energy would be predicted as 10,900 kWh, representing an annual savings of 51,600 kWh. In addition, the chiller can expect a reduced load of approximately 11,000 ton-hours per year.

The monitored data does not indicate that this new system saved a large amount of reheat energy during the summer months. The simulation, however, indicates that the increased cooling capacity, due to a new condenser and compressor, would have provided better comfort control at the expense of greatly increased reheat requirements. Nevertheless, applying both the heat pipe retrofit and chiller improvements to this particular system reduced energy use by nearly 20,000 kWh. The total savings for this system is approximately 70,000 kWh per year, and includes 18,000 kWh of chiller energy savings.

Demonstration indicates that heat pipe retrofit technology can yield significant energy savings

The analysis of the three heat pipe installations at the Kennedy Space Center reveals a wide range of possible results. The heat pipe installation on AHU-3 proved to be quite beneficial, whereas the installations on AHUs 1 and 2 led to little in the way of substantial energy savings. It is clear that the effects of a heat pipe retrofit on an existing air-conditioning system are complex, and that a thorough understanding of how the altered HVAC system operates is essential for proper design of a heat pipe retrofit.

Copies of the Kennedy Space Center demonstration report^(a) featured in this summary are available to DOE and DOE contractors from the Office of Scientific and Technical Information, P.O. Box 62, Oak Ridge, TN; prices available from (615) 576-8401.

Copies are available to the public from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Rd., Springfield, VA 22161.

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^(a)C.E. Hancock and Paul Reeves, "New Technology Demonstration Program Kennedy Space Center Hangar L Heat Pipe Project: Performance Evaluation Report," NREL/TP-710-24738, Golden, Colorado, March 1999.