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A future is envisioned where buildings are assembled on-site from factory manufactured modular elements that integrate the smart technology needed to enable scalable, cost-effective solutions with autonomous, occupant-responsive, healthy, and sustainable features. The use of modular elements would mean that buildings are assembled rather than constructed on-site with better quality control, less material waste, and more predictable schedules. The use of manufactured building elements can enable more cost-effective integration of new sensors, embedded intelligence, networking, adaptive interfaces, renewable energy, energy recovery, comfort delivery, and resiliency technologies, making high-performance buildings more affordable. To explore and evaluate these modular and intelligent comfort delivery concepts and advanced approaches for interaction with occupants, a new human-building interaction laboratory (HBIL) has been designed and is under development. The facility has a modular construction layout with thermally active panels. The interior surface temperature of each panel can be individually controlled using a hydronic system. Such configuration allows us to emulate different climate zones and building type conditions and perform studies such as the effect of different active building surfaces on thermal comfort, localized comfort delivery, and occupant comfort control, among others. Moreover, each panel is reconfigurable to allow investigating different interior surface treatments for different visual and acoustic comfort conditions. In this paper, the overall design approach of the facility is presented. Furthermore, a prototype panel has been constructed to validate the design and assess the dynamic and steady-state thermal performance. Test results for the prototype panel are also presented here with a discussion on their agreement with design phase modeling results.

This paper presents a cost-effective enhancement of a trans-critical carbon dioxide (CO2) cycle in air-conditioning mode by utilizing a thermoelectric sub-cooler. It is well-documented that the cooling COP of the transcritical CO2 cycle decreases as the ambient air temperature significantly increases above the critical temperature of the refrigerant. A high gas cooler outlet temperature limits the enthalpy of evaporation so that the air-conditioning cooling performance is reduced. Sub-cooling is known as a mitigation method to this problem. However, adding a small-scale heat pump to a residential or light commercial air conditioner can be quite costly. Therefore, a thermoelectric solid-state sub-cooler is proposed. The thermoelectric cooling (TEC) devices utilized in small temperature differences ranging from 5 to 15 oC can be quite efficient since the intrinsic heat loss of the TEC by heat conduction in reverse direction of pumping heat is minimal. Based on the prior work, the optimum design for cost-per-performance shows that the cost for sub-cooling is dominated by the heat exchangers and it is not by the thermoelectric material itself. The TECs are compact and have a low thickness, which is in the range of a few mm. Hence the TEC modules can be integrated into the form factor of a plate heat exchanger. In this study, the cooling COP of the CO2 air conditioner is enhanced by approximately 12% using an optimally designed thermoelectric sub-cooler at an ambient temperature of 35 oC. This potential improvement is based on a figure-of-merit (ZT) of currently available thermoelectric materials (ZT~1). The seasonal primary energy efficiency and the cost performance of the optimized TE sub-cooled CO2 heat pump system will be presented in comparison to other compact sub-cooling technologies.

This paper presents the performance measurements of carbon dioxide two-stage reciprocating compressors using a hot-gas load stand. The data is used to characterize the compressors’ performances and evaluate the oil management between the two compressors. A compressor load stand has been redesigned to conduct the compressor performance tests. Two hermetic reciprocating compressors, including two oil separators, two intercooling fans and safety control valves, are connected to the load stand. A membrane tank filled with the working fluid is connected to the load stand, using an appropriate back pressure on the membrane to ensure a fixed intermediate pressure as the anchor point of the hot gas cycle. The first series of compressor tests was conducted for varying suction pressures and varying discharge pressures with different pressure ratios. In a second series, the running time was varied and the suction pressure was kept constant. Based on these measurements, the compressor performance indices, such as the volumetric and overall isentropic efficiencies, have been determined. Furthermore, compressor initial clearance factor can be calculated, based on available compressor maps, which is useful tool for future carbon dioxide system modeling. Also, a simulation model to predict the compressor performance has been developed. The entire process can be simulated to provide the compressor performance data for different working conditions. Finally, the compressor performance measurements are used to validate the model predictions.

The aim of the study presented in this paper is an experimental investigation of the oil return characteristics of alternative refrigerants for residential air conditioning systems using R22. Due to the phase-out of HCFC refrigerants, the production of R22 will be stopped by the year 2020. In some cases of vapor compression systems installed in the field, it will be necessary to replace the R22 with alternative drop-in refrigerants as a part of maintenance procedures. Since R22 system typically use mineral oils and HFC replacement refrigerants use POE or PAG oils. It may be necessary to change the type of oil used inside the system when retrofitting an existing R22 system with an alternative drop-in refrigerant. Changing the oil in a vapor compression system is an expensive proposition due to the required flushing and cleaning of the system before the new oil can be installed. Therefore, refrigerant manufacturers are searching for alternative refrigerants, which are compatible with mineral oils and have similar working performance and conditions as R22. The measurement of the oil return in the suction line helps to draw conclusions about the ability of the oil circulating back to the compressor. In this study, a unitary spilt-system air conditioner with a cooling capacity of 7 kW was equipped with an oil separator and oil measuring cylinder in the suction line. The experimental test setup was designed and built to measure the mass of oil returned to the compressor. Â The weight of the oil collected in the suction line using three different compositions of the alternative drop-in refrigerant R438 are compared to the mass of oil returned when using R22.

The multi-piston axial reciprocating compressor using the Sanderson-Rocker Arm Motion (S-RAM) mechanism is expected to have high volumetric efficiencies due to the application of a new type of seal to prevent the in-cylinder refrigeration gas leaking through the clearance between piston and cylinder wall. The stroke of the compressor can be controlled by adjusting the inclination angle between the connecting shaft and machine driving shaft. This allows the control of the delivered refrigerant mass flow rate to match the capacity requirement in the field. A comprehensive simulation model for a prototype reciprocating compressor using the S-RAM mechanism has been developed. The natural refrigerant carbon dioxide (CO2) is used as the working fluid. The comprehensive model is comprised of a kinematics model, compression process model, dynamics model and an overall energy balance model. In the kinematics model, the movement of the piston is given including its displacement, velocity and acceleration. It is found that the moving path of the center of the ball in the ball-socket joint is moving around a corresponding cylinder centerline with a ‘figure 8’ motion instead of moving along the cylinder centerline. In the compression process model, the system of governing equations is solved, which incorporates a valve sub-model, leakage sub-model and gas pulsation sub-model. The classical 4th order Runge-Kutta method and Broyden’s method are employed to solve the non-linear system of equations to find the in-cylinder refrigerant state (temperature, pressure) at each rotational angle of the machine driving shaft. The variations of suction and discharge valve movements with respect to driving shaft rotational angle are also given. The values of the cylinder wall temperature, the actual suction and discharge temperatures in the connecting pipes are required to initiate the solving of the compression process model. These temperatures are solved simultaneously by incorporating the overall energy balance model with the compression process model. A lumped temperature assumption is employed in the overall energy balance model to assume there is no temperature gradient in each compressor component at steady-state. The dynamics model, which focuses on the frictional power loss, uses the in-cylinder refrigerant pressure determined from the solution of the compression process model.Â

Hysteresis and nonlinearity exist in most valves that are utilized in building HVAC systems, such as in cooling or heating coil valves. The hysteresis mainly comes from the manufacturing clearance in an actuator gearbox and nonlinearity in the variation of the regulated flow with respect to control signal originates from valve design. The combination of valve hysteresis and nonlinearity poses significant difficulties in obtaining a satisfactory control performance when utilizing a conventional PI controller. This paper presents experimental testing results of a set of valves in an operating HVAC system serving an office building. The test valves include VAV reheat coil and AHU cooling coil valves provided by two main-stream manufacturers. Open-loop experiments were carried out that can be used to analyze the valve performance in terms of hysteresis and nonlinearity. Valve closed-loop performance was also investigated and the results showed that valves with significant hysteresis and nonlinear characteristics could lead to unsatisfactory supply air or zone air temperature controls with severe temperature oscillation and control chattering.Â

In dairy processes, there are significant simultaneous heating and cooling demands. A novel type of transcritical CO2 heat pump system is proposed, and its features and benefits are introduced. Bassed on the technical characteristics, primary energy-savings, and operating cost aspects, the CO2 heat pump system is simulated and compared to current heating and cooling systems used in dairy plants. The results show that the highest primary energy-saving rate of the CO2 heat pump is 51.5%. For fluid milk and cheese manufacturing processes, the primary energy-saving is 36.2% and 45.1%, respectively. In addition, the operating cost savings of fluid milk and cheese production are evaluated based on the cost structures in the states of Wisconsin, California and New York.

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.

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.

Research activities towards developing CO2 compressors have increased drastically during the last couple years. Since the transcritical CO2 cycle operates at much higher absolute pressures as compared to the conventional vapor compression cycles, it is necessary to develop new compressors or modify existing ones. In this paper, a novel positive-displacement oil-free CO2 compressor will be introduced. The compressor’s mechanical linkage system will be described. In addition, preliminary compressor test results will be presented, including volumetric efficiency, overall isentropic efficiency, discharge temperature and mass flow rate. The compressor was designed and manufactured to provide cooling capacities from 10 kW to 100 kW. The novel compressor design introduces a new low-friction drive mechanism. The displacement of the compressor can be mechanically varied while keeping a small constant head clearance. A test stand was constructed to map the compressor efficiency, mass flow rate, power consumption and discharge temperature. The test stand is based on a hot gas bypass design, where parts of the discharged refrigerant flow bypasses the condenser, whereas the other part of the flow changes phase as it flows through a condenser. The two streams are mixed to obtain the desired compressor super heat at the suction side of the compressor. The prototype compressor has been tested under different pressure ratios (5, 4, 3, 2, 1.6) and at different speeds (900 rpm, 1200 rpm, 1500 rpm, 1800 rpm).

The vapor compression cycle using refrigerant CFC (chlorofluorocarbon) and HCFC (hydrochlorofluorocarbon) is widely used in refrigeration, air-conditioning and water heating industries. Utilization of these gases are prohibited by the Montreal Protocol. The critical temperature of carbon dioxide (CO2) is much lower than that of the other refrigerants, namely 31.06°C. The gas cooling process occurs at a constant pressure but at variable temperatures takes the place of the condensation process that occurs at both constant pressure and temperature. Domestic type refrigerators use smaller compressor capacities between 100 W to 300 W. Data on CO2 (R744) refrigeration cooling cycle applications and calorimetric measurements are not available in the market and literature. Utilizing the experimental setup, our research group has developed a model for mass flow rate and compressor power of commercially available R744 compressors at different pressures and temperatures. In this article, experimental results and mathematical model calculations will be presented and discussed.