CO2 HEAT PUMPS FOR COMMERCIAL BUILDING APPLICATIONS WITH SIMULTANEOUS HEATING AND COOLING DEMAND

Many commercial buildings, including data centers, hotels and hospitals, have a simultaneous heating and cooling demand depending on the season, occupation and auxiliary equipment. A data center on the Purdue University, West Lafayette campus is used as a case study. The electrical equipment in data centers produce heat, which must be removed to prevent the equipment temperature from rising to a certain level. With proper integration, this heat has the potential to be used as a cost-effective energy source for heating the building in which the data center resides or the near-by buildings. The proposed heat pump system utilizes carbon dioxide with global warming potential of 1, as the refrigerant. System simulations are carried out to determine the feasibility of the system for a 12-month period. In addition, energy, environmental and economic analyses are carried out to show the benefits of this alternative technology when compared to the conventional system currently installed in the facility. Primary energy savings of ~28% to ~61%, a payback period of 3 to 4.5 years and a decrease in the environmental impact value by ~36% makes this system an attractive option. The results are then extended to other commercial buildings

Analysis of a Data Center Using Liquid-Liquid CO2 Heat Pump for Simultaneous Cooling and Heating

Liquid–liquid CO2 heat pump systems are a promising technology for commercial building applications, which require simultaneous heating and cooling. This paper presents the investigation of a data center on the Purdue University, West Lafayette campus. The data center located in the Department of Mathematics is the most energy intensive data center on campus. The cooling load of the data center is approximately 750 kW/hour. The heating season in West Lafayette is 7 to 8 months and the heating load of the buildings is very high during the coldest months. The heating load of the Mathematics building can go to as high as 600 kW/hour during the coldest days of the year. To suffice this simultaneous cooling and heating demand, a liquid-liquid CO2 heat pump is proposed. Presently, the cooling load of the data center is met by eight electrically driven and four steam-driven chillers and the heating load is satisfied by two coal fired and two natural gas boilers. Simulations are performed to compare the proposed CO2 heat pump system with the present system. The assessment shows noteworthy fuel savings and reduction in the CO2 emissions with the system working with a coefficient of performance (COP) of 6.19. If the CO2 heat pump system is installed, 574.92m3/day of natural gas and 751.68 kg/day of coal could be saved on a cold day. The system has the potential to reduce CO2 emissions by 2980.76 kg/day

Experimental Study of a CO2 Thermal Battery for Simultaneous Cooling and Heating Applications

This paper presents experimental investigations of the dynamics of a transcritical CO2 heat pump system with two thermal storages for simultaneous cooling and heating application. The preliminary results of the thermal battery are provided using a small-scale test bed that shows the accelerated penetration of renewable energy sources for building heating and cooling applications. The experimental system consists of a CO2 heat pump system with a compressor of 3 kW (1.02x104 BTU/hr) cooling capacity and two water tanks. During operation, the compressor and expansion valve are considered quasi-static. Thermal sensors are located in each of the two tanks to monitor the temperature gradient of water along the vertical orientation of the tank which impacts the overall system performance. Experiments are carried out under different water circulation flow rates for both the gas cooler and the evaporator in the heat pump, as well as under various discharge pressure conditions controlled by different charging rates and expansion valve openings. The impacts of water circulation flow rate and valve opening are reported in an effort to find the optimum coefficient-of-performance (COP). The results show that increasing the water inlet temperature in the gas cooler raises the discharge pressure significantly and drops the COP, whereas increasing the water temperature of the evaporator raises the discharge pressure relatively moderately. Although a larger water flow rate enhances the heat exchanger capacity and system COP, a smaller water flow rate seems to be preferable to maintain the thermal profile of the water tanks and to provide a more stable COP. At higher gas cooler water inlet temperature, the COP tends to increase with closing expansion valve. In this particular setup, the best COP is found to be approximately 7.0 at a specific expansion valve opening and at a discharge pressure between 75 and 83 bars (1088 to 1204 psia). The heating COP negatively corresponds to the water temperature at the gas cooler inlet. Experiments suggest the need of a proper control strategy and a matched tank capacity design. Based on these results, a 20% power enhancement may be possible by controlling the hot and cold water flow rates