Empirical Modeling of a Rolling-Piston Compressor Heat Pump for Predictive Control in Low-Lift Cooling

Abstract

Inverter-driven variable-capacity air conditioners, heat pumps, and chillers can provide energy-efficient cooling, particularly at part-load capacity. Varying the capacity of vapor compression systems enables operation at lower pressure ratios, or low lift, which fundamentally improves the coefficient of performance of the system by reducing the required compressor work while providing a similar cooling effect. This is illustrated in Figure 1, which shows conventional and low-lift vapor compression cycles for refrigerant R22. A cycle with a lower condensing temperature, higher evaporating temperature, and/or slower compressor speed (the low-lift cycle) can achieve a similar (or even greater) cooling effect (the area below the evaporation process line) to that of the conventional cycle (the area inside the cycle polygon) with less compressor work.

The use of radiant cooling, precooling of thermal energy storage, and predictive control of compressor speed, condenser flow rate, and evaporator flow rate allow for low-pressure ratio, or low-lift, chiller operation (Armstrong et al. 2009a, 2009b) to meet daily cooling loads in a near-optimal manner. The combination of these strategies will here be called low-lift cooling. Radiant cooling requires moderate chilled-water temperatures, around 60 [degrees]F to 65 [degrees]F (15.5 [degrees]C to 18.3 [degrees]C), rather than about 44 [degrees]F (6.6 [degrees]C) for conventional systems, allowing for higher evaporating temperatures and pressures. Pre-cooling thermal energy storage allows night-time operation of a heat pump or chiller, providing lower condensing temperatures and pressures