Modeling Direct-Expansion Solar-Assisted Heat Pumps

Numerical models of solar assisted heat pumps were developed to evaluate the coefficient of performance in comparison to air-source heat pumps. The models considered solar assisted heat pumps with direct-expansion configurations, which passes the refrigerant directly through the solar collector by replacing the evaporator in the heat pump cycle. Initial modeling emphasized two parameters: Temperature increases of the refrigerant in the solar collector and degrees of superheating both individually increased the coefficient of performance. Two more models were developed and used to analyze the performance of direct-expansion solar assisted heat pumps utilizing solar and temperature data from the Schenectady, NY area. The first model showed that a collector with a tilt angle of 42.81° (the latitude of the location) provides more solar insolation than a collector tilted vertically (90°) throughout the entire year, but provided similar insolation amounts in the winter months. The second numerical model showed that the monthly daily average coefficient of performance decreases during the winter months and increases during warmer months, leading to analysis of the heat gained by the system. This showed that months that have similar coefficient of performance parameters gain heat through the ambient environment, the solar insolation, or a combination of both. The model showed that the month of March gained significant heat through solar insolation when the collector was tilted to latitude while October gained most of its heat through the ambient environment.