Project and Objectives

Air/water heat pumps are devices that efficiently cool, heat, and provide domestic hot water by recovering or rejecting energy from or to the ambient air. By reversing the cycle, they can heat or cool a water flow, allowing for dual functionality (heating/cooling) depending on the season or need. The new requirements set by the F-GAS regulation have made propane (R290) a highly advantageous refrigerant since heat pumps using this refrigerant are not affected by the regulation, which aims to phase down high GWP refrigerants. Propane, with a GWP of 3, is significantly less harmful to the environment and climate. Despite being classified as A3 flammable, its use today is not dangerous even for residential installations because the compressor is designed to prevent any leakage of refrigerant gas while the circuit is hermetically sealed, preventing water contamination in the heat exchanger. Additionally, compared to traditional refrigerants, propane allows for a reduced charge within the heat pump.

Plant modeling process:

The heat pump is dynamically simulated using Simcenter Amesim software, employing a 0D lumped parameter approach. All system components are simulated, including the piping, various valves that enable cycle inversion, and the control system that regulates the compressor speed and the expansion valve opening degree to achieve the desired superheating at the evaporator outlet.

The compressor and valves are modeled as static elements using algebraic equations since their response time is much higher compared to other elements in the system (heat exchangers, accumulators, etc.), which define the system’s dynamic response. The compressor model requires displacement and volumetric and isentropic efficiency curves. Alternatively, it is possible to input the polynomial parameters defined by AHRI Standard 540. The valves require the passage section size; alternatively, the characteristic curve linking flow rate to pressure loss can be loaded. The remaining system components are modeled through transient mass and energy balances, resulting in a system of differential equations. The heat exchangers require all necessary geometric information to evaluate passage sections, hydraulic diameters, and convective areas, in addition to the circuitry needed to assess the tube crossing order in the finned coil or the various passages’ crossing order in the plate exchanger.

In this way, the numerical model can also be used to evaluate the most efficient circuitry for the heat exchanger and observe how the machine’s behavior changes with variations in the circuitry or geometry. Finally, after selecting the appropriate formulation for the degree of vacuum, the software can calculate the mass of refrigerant within each component, allowing the user to observe the charge migration during various transients and determine the total refrigerant mass required for the circuit. The total refrigerant mass can also be set by the user, making it possible to optimize the refrigerant charge.

Steady-state simulation in heating/cooling mode

The first simulation was launched with the machine operating in heating mode: the air entering the finned coil is at 7°C dry bulb and 6°C wet bulb, while the water entering the plate heat exchanger is at a temperature of 30°C. The control system regulates the compressor’s rotational speed to ensure an outlet water temperature of 35°C, and the electronic expansion valve’s opening degree is adjusted to ensure a superheating of 5 K at the evaporator outlet. Given constant inlet fluid conditions, once stable working pressures are reached, the system attains a steady-state condition. Using the optimizer integrated in Amesim, a refrigerant charge of 2.1 kg was estimated to achieve 3 K of subcooling at the condenser outlet. Under these conditions, the heat exchanged at the condenser is 30 kW, while the power absorbed by the compressor is 6.8 kW, resulting in a COP of 4.4.

With the sketch animation function of the software, it is possible to verify the high-pressure and low-pressure sides of the system, as well as indicate the flow direction in the various branches of the circuit. Finally, it is also possible to create the log p-h diagram, which shows all the thermodynamic transformations of the heat pump.

After verifying the system’s performance in heating mode and determining the refrigerant charge, a steady-state simulation was run with the heat pump operating in cooling mode. In this case, the air conditions were changed to a dry bulb temperature of 35°C, while the water flow entering the plate evaporator was at a temperature of 12°C. The compressor speed is regulated to ensure an outlet water temperature of 7°C, and the valve opening degree is adjusted to achieve a superheating of 5 K at the plate heat exchanger outlet.

Under these conditions, the evaporator provides a power output of 22.1 kW, while the compressor absorbs a power of 7.7 kW, corresponding to an EER of 2.9.

Conclusions

BSIM evaluated the performance of a reversible air-to-water heat pump that uses R290 as the refrigerant. With the dimensions of all components of the system fixed, the objective was to verify whether the heat pump could provide the required thermal power under nominal conditions.

The system is equipped with a single scroll compressor whose rotational speed is regulated by an inverter. The water flow is heated (or cooled) via a plate heat exchanger, and the heat is finally absorbed or rejected to/from the environment through a finned coil with round tubes and rectangular fins.

In heating mode, performance was evaluated by simulating an ambient air temperature of 7°C (dry bulb)/6°C (wet bulb) and an inlet/outlet water temperature of 30/35°C. The refrigerant charge needed to ensure a subcooling of 3 K at the condenser outlet was estimated to be 2.1 kg. The power rejected at the plate condenser is 30 kW, corresponding to a COP of 4.4.

In cooling mode, with the charge determined in heating mode, performance was evaluated by simulating an ambient air temperature of 35°C and an inlet/outlet water temperature of 12/7°C. The power absorbed by the plate condenser is 22.1 kW, corresponding to an EER of 2.88.

With the layout and component dimensions fixed, the heat pump was demonstrated to be capable of delivering the required target power outputs.